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4.1.2 Diketene PSM.................................................................................................... 23
4.1.3 Diketene PSM after activation in cyclohexane............................................... 23
4.1.4 Diketene PSM after activation in diethyl ether– Compound 2 .................... 23
4.1.5 KOtBu PSM – Compound 3............................................................................. 23
4.1.6 ZnEt2 PSM – Compound 4 .............................................................................. 23
4.1.7 [Rh2(CO)4(μ-Cl)2] PSM – Compound 5 .......................................................... 24
4.2 Analytical procedures ............................................................................................ 24
4.2.1 Powder X-ray diffraction (PXRD).................................................................. 24
4.2.2 Single crystal X-ray diffraction (XRD)........................................................... 24
4.2.3 1H NMR ............................................................................................................. 24
4.2.4 Electron dispersive X-ray spectroscopy (EDX) ............................................. 24
5. Synthesis of a MOF for PSM reactions ................................................................ 24
5.1 Synthesis of DMOF-1-NH2 (compound 1).......................................................... 24
5.2 PXRD analysis of compound 1 ............................................................................ 25
5.3 1H NMR analysis of compound 1......................................................................... 26
6. Treatment of compound 1 with diketene ............................................................. 26
7.1 Activation of compound 1 in cyclohexane.......................................................... 28
7.2 Activation of compound 1 in dry diethyl ether.................................................... 29
8. Treatment of compound 1 with diketene after activation................................ 30
8.1 Compound 1 activated in cyclohexane............................................................... 30
8.2 Compound 1 activated in dry diethyl ether......................................................... 31
8.2.1 PXRD analysis of compound 2 ........................................................................ 32
8.2.2 1H NMR analysis of compound 2 .................................................................... 32
8.2.3 Elemental analysis of compound 2.................................................................. 33
8.2.4 Single crystal XRD analysis............................................................................. 33
8.4 Summary of reactions with diketene ................................................................... 43
9. PSMetallation of compound 2 using KOtBu........................................................ 44
9.1 PXRD analysis of compound 3 ............................................................................ 44
9.2 1H NMR analysis of compound 3......................................................................... 45
9.3 Single crystal XRD analysis.................................................................................. 46
9.4 Elemental analysis ................................................................................................. 46
9.5 SEM/EDX analysis................................................................................................. 47
9.6 Summary of KOtBu reactions............................................................................... 49
10. PSMetalation of compound 2 using ZnEt2 ........................................................ 50
10.1 PXRD analysis...................................................................................................... 50](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-4-320.jpg)
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10.2 1H NMR analysis .................................................................................................. 51
10.3 Elemental analysis............................................................................................... 52
10.4 Summary of reaction with ZnEt2 ........................................................................ 52
11. PSMetalation of compound 2 using [Rh2(μ-Cl)2(CO)4] ................................... 53
11.1 PXRD analysis...................................................................................................... 54
11.2 1H NMR analysis .................................................................................................. 54
11.3 EDX/SEM analysis............................................................................................... 55
11.4 IR analysis............................................................................................................. 57
11.5 Summary of reaction with [Rh2(μ-Cl)2(CO)4].................................................... 58
12. Conclusions .............................................................................................................. 59
13. Future work................................................................................................................ 60
Acknowledgements........................................................................................................ 60
References........................................................................................................................ 61](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-5-320.jpg)
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Abbreviations
Whilst every effort has been made to ensure that abbreviations have been explained in the
main body of the report, they have been included here as a reference list.
bdc: benzene-1,4-dicarboxylate
H2-bdc: Terephthalic acid (benzene-1,4-dicarboxylic acid)
bdc-NH2: 2-aminobenzene-1,4-dicarboxylate
dabco: 1,4-diazabicyclo[2.2.2]octane
dkn: ketoamide PSM tag
btc: benzene-1,3,5-tricarboxylate
MOF: Metal organic framework
IRMOF: Iso-reticular metal organic framework
PXRD: Powder X-ray diffraction
EDX: Energy dispersive X-ray spectroscopy
NMR: Nuclear magnetic resonance
MIL: Material Institute Lavoisier
ZIF: Zeolitic imidazolate framework
PSM: Postsynthetic modification
PSMet: Postsynthetic metalation
UiO: Universitetet I Oslo
SBU: Secondary building unit
DMOF: dabco metal organic framework
UMCM: University of Michigan Crystalline Material](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-6-320.jpg)
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Abstract
A series of tandem postsynthetic modification (PSM) reactions were carried out on the
metal-organic framework [Zn(bdc-NH2)2(dabco)] (compound 1, DMOF-1-NH2) at the
amino site on the 2-amino-benzene-1,4-dicarboxylate linker molecules in the MOF.
Studies to activate compound 1 prior to postsynthetic modification were performed. These
were solvent exchanges using cyclohexane or diethyl ether. These reactions were followed
using powder X-ray diffraction and thermogravimetric analysis and it was found that
activation in diethyl ether was most successful for the first PSM reaction. Covalent
postsynthetic modification of compound 1 using diketene was successfully achieved with
69% conversion to the ketoamide tagged analogue (compound 2) and was characterised by
single crystal X-ray diffraction. A decomposition product with the formula [Zn(bdc-
NH2)(DMF)] (compound 1a) was formed after the addition of diketene with its structure
deduced by single crystal diffraction. There was evidence to show that dative postsynthetic
modification of the ketoamide tag had been achieved upon treatment of compound 2 with
KOt
Bu, ZnEt2 or [Rh2(μ-Cl)2(CO)4]. Powder X-ray diffraction studies showed that
compound 2 had changed upon treatment with these metal species and electron dispersive
X-ray spectroscopy showed incorporation of the metals in the product for the potassium
and rhodium based reagents (compounds 3 and 5 respectively).](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-7-320.jpg)
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1. Metal-organic frameworks (MOFs)
1.1 What is a MOF?
1.1.1 Terminology in MOF chemistry
The term metal-organic framework has been defined differently in the past with terms such
as coordination polymer and hybrid organic-inorganic materials often being used
interchangeably despite them having their own specific meanings[1][2]. Yaghi’s group
described a metal-organic framework on the first usage of the term in 1995 as a material
analogous to other microporous materials such as zeolites with open frameworks and
extended channel systems[3]. Others have offered narrower definitions. For example,
O’Keeffe wrote that he uses the term MOF when referring to frameworks built from
linking polyatomic clusters (or secondary building units) with strong bonds. He also stated
in this definition that MOFs do not include coordination polymers[4]. O’Keeffe also
highlights the point mentioned above that the term coordination polymer is sometimes used
instead of MOF and vice versa. A review by Corma et al.[5] describes a coordination
polymer as being formed from extended networks of metal ions and multidentate organic
ligands. It also states that this encompasses crystalline and amorphous materials and that
MOFs are a special group of coordination polymers. This implies that MOFs are a subclass
of coordination polymers.
It can be seen that it is difficult to assign a single clear definition to the term MOF
which is probably the reason that a lot of the literature begins by defining their own. As
such, an IUPAC ‘task group’ was set up to provide guidelines on terminology and
nomenclature. In 2012, they published provisional recommendations with some of the
important definitions summarised in Table 1[6]. These definitions will be adhered to in this
report.
Table 1. IUPAC proposed definitions for MOF chemistry[6]
Term Definition
Coordination Polymer A coordination compound continuously extending in 1, 2 or 3 dimensions
through coordination bonds
Coordination Network A coordination compound extending, through coordination bonds, in 1
dimension, but with cross-links between two or more individual chains,
loops or spiro-links, or a coordination compound extending through
coordination bonds in 2 or 3 dimensions
Metal-Organic Framework Metal-Organic Framework, abbreviated to MOF, is a coordination polymer
(or alternatively coordination network) with an open framework containing
potential voids
1.1.2 Structure of MOFs
MOFs are materials made from, as the name suggests organic and inorganic units. The
inorganic units can be metal ions (known as primary building units) or more complex
aggregates with different geometries to that of the individual metal ions (known as
secondary building units or SBUs)[7]. The organic sections are known as organic linkers
which are at their simplest just spacers between metals or SBUs and at their most complex,
absolutely crucial to chemistry and properties of the resulting MOF. An important factor
when choosing organic linkers is to select one that will withstand the reaction conditions
and keep its functionality in the resulting framework[8].](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-8-320.jpg)
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By considering some simple coordination chemistry principles, the structure of
MOFs can be understood. Coordination polymers and networks offer a useful comparison
to understand the structure of MOFs. This is because they have similar structures to MOFs
and the principles that govern their structure are also applicable to MOFs. The shape of
ligands can dictate the geometry of some coordination compounds. For example, the linear
ligand 4,4’-bipyridyl (4,4’-bipy) will coordinate to Ag+
to form a one-dimensional
coordination polymer as shown in Figure 1[9].
Figure 1. Structure of one-dimensional [Ag(4,4’-bipy)]+
coordination polymer[9]
Further to this, a ligand with a different denticity such as the tridentate tris-(-4-
cyanophenyl)amine (TCPA) will form a two-dimensional honeycomb like coordination
polymer with Ag+
as seen in Figure 2[10].
Figure 2. Reaction scheme to show a silver two-dimensional coordination polymer formed with the
tridentate TCPA ligand. The ligands on the right hand side structure have been simplified for clarity.
As well as ligand shape, preferred metal geometry can dictate the shape of a
framework. For example, Cu(I) tends to coordinate in a tetrahedral geometry and it has
been shown to form three-dimensional coordination polymers with structures analogous to
diamond when reacted with 4,4’-bipy. The framework is cationic due to the charge on the
copper and is stabilised by a [PF6]-
counterion. Unlike the C-C bonds found in diamond,
M-bipy-M units are present. A schematic representation of this structure is shown in figure
3 with the balls representing the metal and the struts the 4,4’-bipy ligand[11].
Figure 3. Schematic representation of a three-dimensional coordination network containing Cu(I) and 4,4’-
bipy.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-9-320.jpg)
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The previous examples begin to show how choice of ligand and metal can
determine the dimensionality and to a certain extent, the geometry of a structure. However,
they are all of coordination polymers or networks. IUPAC tells us that the difference
between these and MOFs is that MOFs possess the potential for porosity. The above
examples do not possess this potential because they are all cationic structures that must be
stabilised with counterions that often fill any potential pore.
Consequently, the development of MOFs has relied largely on the use of anionic
organic linkers so that a neutral framework results. A particularly common class of anionic
organic linker is carboxylates[12][7]. As well as being anionic, carboxylates can bridge
between metal centres which allows the formation of SBUs rather than the nodes in a
framework being single metal centres. Figure 4 shows how four carboxylate linkers bridge
two metal atoms to form what is known as a ‘paddle wheel’ SBU where the carboxylates
extend to give a roughly square planar geometry even though the metal itself may not
adopt this geometry[13]. The groups extending from the carboxylates can be any number
of groups that could be chosen based on their functionality for example.
Figure 4. Paddle wheel SBU formed from COOR carboxylate ligands showing that the SBU adopts an
approximately square planar geometry.
Although carboxylates account for most of the anionic linkers used in MOF
chemistry, another frequently used class of linkers are polydentate N-donor ligands. Like
carboxylates, their anionic nature allows them to produce neutral frameworks. For example
zeolitic imidazolate framework-8 (ZIF-8) utilises 2-methylimidazole (MeIM) linkers that
are connected to tetrahedral Zn(II) ions with the formula [Zn(MeIM)2]. The resulting
framework which is shown in Figure 5, has large pores (~11.6 Å) with smaller hexagonal
pore openings (~3.4 Å)[14].
Figure 5. Structure of ZIF-8 with hexagonal pore opening marked by white hexagon[15].](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-10-320.jpg)
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1.1.3 Reticular synthesis
Traditionally, solid state materials have been synthesised using methods that offer little
control over the character of the product. This is due largely to the fact that the starting
materials do not retain their structure during the reaction; which in turn produces a product
with little to no resemblance between the reactants and products.
Reticular synthesis provides a synthetic route where a target framework can be
constructed from rigid molecular ‘building blocks’ (SBUs) that maintain their structure
throughout the synthesis[16]. An important part of reticular synthesis is having an
understanding of the target framework and choosing appropriate starting materials for its
assembly[7]. As discussed in section 1.1.2, considering simple coordination ideas such as
ligand shape and preferred metal orientation, allows the starting materials to be chosen for
a particular framework.
It is this that has led to the recognition of a smaller number of general MOF
structures or topologies that are common to a whole group of MOFs with similar
geometries and symmetry[17]. There has been work, particularly by Wells[18] to try and
enumerate and describe topologies of solid state materials, but an approach for MOFs
based on tiling that has been used by numerous groups[19][20] is particularly useful in
understanding them. The tiles are a series of generalised polyhedra (cages) that fit together
in three dimensions to generate an entire structure based on the net (or reticulate) of a
particular topology. A well understood example of this approach is that of the Kelvin
structure (also known as sodalite in zeolite chemistry). In this case, the cages that fit
together are truncated octahedra where the vertices of the structure represent either silicon
or aluminium atoms (T atoms). The T atoms form TO4 tetrahedra that link together with
the edges of the cages being T-O-T linkages[19]. The way in which these cages form a
three dimensional framework is illustrated in Figure 6.
Figure 6. Schematic to show how truncated octahedral cages or ‘tiles’ (left) fit together to form a three
dimensional sodalite structure (right)[19]
ZIF-8 which is shown in Figure 5 also has the sodalite structure which
demonstrates that one framework type can apply to many MOFs.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-11-320.jpg)
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An extension of reticular synthesis is the concept of isoreticular MOFs. These are
MOFs that have different organic linkers with the same denticity and bonding to the metal.
This produces a framework with the same topology as when using the unfunctionalised
linker and is known as an isoreticular MOF (IRMOF)[16]. Figure 7 shows a series of
IRMOFs to illustrate that the gross structure remains the same regardless of the organic
linker molecule used.
Figure 7. IRMOF structures with a)bdc, b)cyclobutylbenzene, c)naphthalene-2,6-dicarboxylate linkers and
Zn4O(CO2)6
-
SBUs represented by the blue tetrahedra[21]
By using reticular synthesis, a vast number of theoretical MOF frameworks can be
produced by simply varying metal and liker type. This has led to an explosion in the
number of new MOFs with over 20,000 structures having been reported since the early
2000s[12].
1.2 Synthesis of MOFs
The synthesis of MOFs can be divided into two main categories; solution phase which is
by far the most common and widely used, and solid state. However, solid state synthesis
has the big disadvantage that it is difficult to obtain single crystals as it usually involves
some kind of grinding process which crushes the crystals that form. This makes it more
difficult to perform single crystal X-ray diffraction analysis as the crystals are often too
small which can lead to poor diffraction and cause poor quality data to be collected. This is
an issue when structures of new materials needs to be determined. MOF syntheses can be
divided further with a summary of these methods given in Figure 8.
Figure 8. Summary of MOF preparations[22]
The Figure shows that by using the same basic materials, MOFs can be prepared in a
number of ways by varying the temperature, time and energy type.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-12-320.jpg)
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1.2.1 Solvothermal synthesis
By far the most commonly used synthetic method is solvothermal synthesis (also known as
hydrothermal if water is the solvent used). A solvothermal process is defined as a reaction
between precursor materials in a sealed vessel in the presence of a solvent where the
reaction temperature is higher than of the boiling point of the solvent used[23]. Organic
solvents with high boiling points such as dimethylformamide (DMF), acetonitrile and
acetone are commonly used. Strictly speaking, many MOF syntheses that are referred to as
solvothermal actually use temperatures below the boiling point of the solvent. For
example, the synthesis of DMOF-1 ([Zn2(bdc-NH2)2(dabco)]), is carried out in DMF at
110 °C[24] but DMF boils at 152 °C.
Solvothermal synthesis can be considered as a Lewis acid-base reaction where the
metal centre acts as an acid and the basic solvent deprotonates the organic linker’s acid
precursor so that the resulting linker acts as a base. It is important that the solvent is a weak
base as this keeps the concentration of the linker low which favours slow crystal growth.
This often leads to the formation of large, good quality crystals suitable for single crystal
analysis[25]. Conventional synthesis of MOFs using solvothermal methods typically use
stoichiometric ratios of reagents in solvent. Self-assembly of the MOF then results upon
heating. This is usually followed by an activation process to remove any solvent remaining
in the pores. This process is summarised in Figure 9.
Figure 9. Schematic representation of solvothermal synthesis[26]
The main advantage of solvothermal synthesis is that good quality crystals can be
grown due to slow crystal growth. This allows single crystal and powder X-ray diffraction
analysis. However, the main disadvantage of solvothermal synthesis is that solvents such
as DMF tend to be toxic and often gets trapped inside the pores of the framework. This
limits their applications, because if the trapped solvents cannot be removed, the structures
are not truly porous. Also solvothermal synthesis can be cost inefficient as the waste
solvents are expensive to dispose of. Hence why there is a need for activation procedures.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-13-320.jpg)
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1.2.2 Microwave and ultrasound assisted synthesis
Microwave and ultrasound assisted synthesis of MOFs are essentially variations on
solvothermal methods. The aim of using microwave or ultrasound energy is to reduce
MOF preparation time whilst maintaining crystal quality.
Microwave assisted synthesis provides a rapid synthesis of MOFs where the crystal
size is usually much smaller than when using conventional heating methods. For example,
the iron based MOF, MIL-53 ([Fe(OH)(bdc)], where bdc is benzene-1,4-dicarboxylate) is
shown in Figure 10 to have smaller crystals when produced with microwave assistance
compared to conventional solvothermal heating methods[27].
Figure 10. SEM images of MIL-53 produced with microwave assistance (left) and using conventional
heating methods (right)[27]
Ultrasound assisted or sonochemical methods see the precursor materials undergo a
chemical change upon the application of ultrasound radiation. This causes localised areas
of extreme temperature and pressure which can promote formation of nucleation sites and
hence considerably reduce crystallation time[22]. This reduced crystallisation time again
generally reduces the crystal size compared to solvothermal methods. For instance, Li et al.
were able to produce the three-dimensional MOF, [Cu3(btc)2] (HKUST-1, btc is benzene-
1,3,5-tricarboxylate) by sonochemical synthesis with crystals in the 10-200 nm range
wheras they produced the same MOF using conventional solvothermal methods in the
range 10-30 μm[28]. However, there has been progress in maintaining a similar crystal size
using ultrasound. For example, Son et al. produced [Zn4O(bdc)3] (MOF-5) crystals in the
5-25μm range with ultrasound which is similar to the sizes they achieved when using
normal solvothermal synthesis[29].
1.2.3 Solvent free synthesis
Solvent free or solid state synthesis uses no (or very little) solvent to form MOFs. This
makes it a cleaner, more environmentally friendly method than solvothermal methods.
Instead of heating in the presence of a solvent, solvent free methods use mechanical force
to achieve formation of MOFs. Typically, reagents are ground together in a ball mill to
produce a product and this method has even been shown to produce three dimensional
MOFs[30]. Although, to date, this method has only been used to make a limited number of
MOFs.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-14-320.jpg)
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The disadvantage of solvent free synthesis is that, like microwave and
sonochemical syntheses, solvent free methods tend to produce samples with small crystal
size typically in the microcrystalline range as shown by PXRD and SEM analysis[8]. This
makes solving the structure of new frameworks difficult as single crystal data cannot
usually be collected.
1.2.4 Activation procedures
An important stage in the preparation of MOFs is activating them for use in potential
applications. One of the issues with solvothermal synthesis in particular is that solvent used
in the synthesis tends to get trapped in the pores of the frameworks. Since the porosity of
MOFs is often key to their uses, it is important that the solvent is removed prior to their
use.
Common methods to remove solvents from the pores include guest solvent
exchange for a more volatile solvent molecule that is easier to remove and heating to drive
out the solvent. Even freeze drying which first exchanges the solvent for benzene that is
then sublimed under vacuum to avoid the liquid to gas phase transition has been used to
activate MOFs[31].
Historically, the problem with activation techniques such as heating under vacuum
is that upon removal of the solvent, there is a collapse of the framework[13].
1.3 Characterisation of MOFs
1.3.1 X-ray crystallography
The primary and arguably most important technique used in the characterisation of new
MOFs is X-ray crystallography. This is because MOFs tend to be crystalline and
depending on the synthetic route used, large single crystals are usually obtained, hence
making them good candidates for X-ray analysis. Both single crystal and powder X-ray
crystallography can be used for the characterisation of MOFs.
Single crystal X-ray diffraction is used to determine the position of atoms within
unit cells and hence determine the overall crystal structure. However, this technique is only
used to look at the structure of a single crystal so is not representative of the sample as a
whole. As such, powder X-ray diffraction (PXRD) can be used to determine the bulk
structure of a sample. By analysing many crystals, an average structure is determined.
PXRD patterns can be quickly compared to powder patterns calculated from single crystal
data to identify any differences.
Differences in experimental and calculated powder patterns can be useful in a
number of ways. Most importantly, it allows the user to see changes in the unit cell or
space group of the sample. This is an indication that a structural change has taken place. It
also allows the user to quickly see if there are likely to be multiple phases in the bulk
sample. This is often shown by the presence or disappearance of peaks that aren’t in the
calculated pattern.
Another advantage of PXRD is that it is a quick and easy method to perform with
patterns usually taken in less than an hour whereas single crystal data acquisition typically
takes up to 24 hours.
However, PXRD cannot usually be used for structure determination by itself which
it is so often reported in conjunction with other characterisation methods.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-15-320.jpg)
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1.3.3 1H NMR
It is useful to be able to characterise the organic linker sections of MOFs as they can play a
key part in a MOF’s reactivity or properties. One method to do this is using 1
H NMR
spectroscopy. Samples are digested in acid to convert the linkers into the corresponding
acids. Hence the linkers can be characterised when not in a solid framework.
1
H NMR spectroscopy can show whether a ligand has remained intact in the
framework. It can also be used to calculate conversion rates is the ligand is modified or
exchanged after synthesis of the MOF.
The obvious disadvantage to using 1
H NMR as a characterisation technique for
MOFs is that it is a destructive technique. Digesting the MOFs to dissolve the organic
linker means that the framework is destroyed and cannot be recovered.
1.3.4 Elemental analysis
Elemental analysis gives the percentage of certain elements in a sample (usually carbon,
hydrogen and nitrogen). For MOF chemistry, elemental analysis can tell if the correct
amount of each element is present in a sample for the proposed structure which can further
confirm a framework structure.
The disadvantages of elemental analysis are that it provides no structural
information about a sample and in MOF chemistry, it sometimes isn’t very accurate. This
is because solvent is often trapped in the pores of a framework and will affect the ratio of
elements in a sample.
1.3.5 Electron dispersive X-ray spectroscopy (EDX)
EDX allows the surface elemental composition of a crystal to be determined. EDX causes
excitation of an electron from a particular part of a sample with the use of a beam of
charged particles (either electrons or protons). When the excited electron returns to its
ground state, it releases radiation in the form of X-rays. The emitted X-rays can be
measured. As each element has a different atomic structure, the energy of the emitted X-
rays is characteristic of that element. Hence, the elemental composition of a sample can be
determined.
The beam of charged particles used for excitation of electrons typically only
penetrates a sample by a few microns which is why only the surface composition can be
reliably determined.
1.4 Applications
One of the reasons that MOFs have become such a popular area of interest in the past few
decades is because of their potential to be used in a vast range of applications. These
include industrially and environmentally important processes such as gas storage and
separation, in addition to medical applications such as drug delivery.
1.4.1 Gas storage
With the move away from petroleum based fuels to using cleaner fossil fuels like natural
gas (largely comprised of methane), it is essential that effective storage solutions are
developed. Traditionally, gases have been stored in high pressure tanks but this method is
very expensive and difficult to use for small scale applications such as car fuel tanks[32].](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-16-320.jpg)
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An interesting example of MOF technology being used for gas storage is the
Mercedes-Benz F125 research vehicle. This utilises MOFs for hydrogen storage as they
have large surface areas (up to 10,000 m2
g-1
) which can store large amounts of hydrogen
(up to 80 bar) at lower pressures than an equivalent fuel reservoir without MOFs
present[33].
From an environmental point of view, the rising atmospheric CO2 levels is an area
of great interest, as the consequences of high CO2 levels poses a threat to all living
things[34]. As such, high surface area MOFs like MOF-210 have been targeted as potential
CO2 capture materials. MOF-210 is formed from biphenyl-4,4′-dicarboxylate (BPDC) and
4,4',4''-(benzene-1,3,5-triyl-tris (ethyne-2,1-diyl))tribenzoate (BTE) ligands with the
empirical formula [Zn4O(BTE)4/3(BPDC)] has one of the largest known surface areas
(10450 m2
g-1
). It has a CO2 uptake of 2400 mg g-1
(74.2 wt%) which is higher than that of
other MOFs with smaller surface areas like MOF-177(60 wt%) which has the empirical
formula [Zn4O(BTB)2] (where BTB is 4,4',4''-benzene-1,3,5-triyl- tribenzoate)[22].
1.4.2 Gas separation
Gas separation or selective gas adsorption is an application closely related to gas storage,
except that it relies on the pore size and the affinity for the target gases from the MOFs. An
early example of gas separation using MOFs is that of MOF-508 ([Zn(bdc)(4,4’-bipy)1/2])
which was used to separate various alkanes including compounds such as n-pentane and n-
hexane which are important in the process of petroleum refining[35]. This study filled a
chromatography column with MOF-508 and the following GC measurements showed clear
separation of the gases. Figure 11 shows that n-pentane and n-hexane have clearly different
retention times. In this study, the reason for the difference in retention time of the gases
was due to the difference in van der Waals forces the gases had with the MOF surfaces. It
showed that longer alkyl chains had greater van der Waals forces with the MOF and hence
took longer to travel the length of the column, thereby increasing their retention times.
Figure 11. Gas chromatogram to show different retention times of gases separated by MOF-508[35].
1.4.3 Catalysis
MOFs can produce highly porous structures but with the advantage that there are
many combinations of metals and ligands possible so the variety in framework structure is
much greater. This makes them good candidates for use in a variety of catalysis reactions.
It is important that a MOF for use in catalysis has active catalytic sites. These can be
generated in two ways. Firstly, the metal centre can act as a Lewis acid catalyst (after
removal of solvent from pores) [36]. An example of this is the catalysis of aldol reactions](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-17-320.jpg)
![13
using the [Cu3(btc)2(H2O)3] framework[37]. The MOF acted as a catalyst in the green
synthesis of pyrimidine chalcones via aldol condensation of ketones and
formylpyrimidines. Using the MOF gave a very high yield and easy isolation of the
product which reduced waste products, hence making it an eco-friendly synthesis. The
reaction is shown in Figure 12.
Figure 12. Aldol condensation of ketones with formylpyrimidines to form pyrmidine chalcones using
[Cu3(btc)2] as a catalyst.[37]
Secondly, functional groups on the framework (usually the organic linker) can form
active catalytic sites. The advantage of this method is that the catalytic sites are well
ordered and predictable due to the repeating nature of the framework structure[38]. This
type of MOF catalysis is pictured in Figure 13.
Figure 13. Schematic to show functionalised organic linkers acting as active catalytic sites[36].
MOFs containing these types of catalytic site have been shown to act catalytically
for certain Knoevenagel reactions[38] as shown in Figure 14. In this study, [Cd(4-
btapa)2(NO3)2] (4-btapa = 1,3,5-Benzene Tricarboxylic Acid Tris[N-(4-pyridyl)amide])was
effective at catalysing the condensation of malonitrile with benzaldehyde but not with
larger reagents such as ethyl cyanoacetate, and cyano-acetic acid tert-butyl ester. As such,
it was suggested that the reaction occurred inside the pores of the MOF and not on the
surface of the crystals.
Figure 14. Knoevenagel reactions of benzaldehyde with cyano reagents catalysed by [Cd(4-
btapa)2(NO3)2][38].](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-18-320.jpg)
![14
1.4.4 Drug delivery
Drug delivery is still a relatively new area of research for MOFs but there is some evidence
to suggest that they could be effective drug delivery agents. The need arises for drug
delivery materials as it is important to ensure that the drug is delivered to the correct
location in the body; in the correct quantity and released over the correct amount of time.
MOFs possess several characteristics that allow this to occur whereas drug molecules tend
to be broken down in the digestive system.
One example of the use of MOFs as drug delivery materials is the use of zeolitic
imidazolate framework-8, [Zn(MeIM)2] (ZIF-8) in the delivery of anticancer drug 5-
fluorouracil (5-FU). The research carried out by Sun and co-workers[39] was inspired by
the chemical stability of ZIF-8 in water and aqueous sodium hydroxide and the fact that it
readily dissolves in acidic solution. It is known that tumour tissues are more acidic (~pH 5)
than normal tissue and blood (~pH 7.4). It was thought that ZIF-8 could be used as a drug
delivery system as it would only decompose and release the drug at the tumour site and
hence reduce drug release during transportation in the circulatory system. As well as
increasing the amount of drug released at the tumour site, this method could reduce some
side effects of the drug caused by its release elsewhere in the body. As ZIF-8 is highly
porous (~11.4 Å pores[14]), it was able to absorb considerably more of the 5-FU than other
delivery system candidates and the study showed that at pH 7.4, it retained considerably
more 5-FU in its pores than at pH 5 where nearly all of the drug was released. Figure 15
shows the 5-FU release over time at different pH levels and it confirms that drug release
occurs much faster at lower pH 5 than pH 7.4.
Figure 15. 5-FU release from ZIF-8 over time at pH 5 (black) and pH 7.4 (red)[39]](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-19-320.jpg)
![15
2. Postsynthetic modification of MOFs
2.1 Introduction
One of the main attractions of MOFs is that their ability to form permanently porous
crystalline structures allows them to be used in a wide range of applications; some of
which are discussed in section 1.4. The properties of a particular framework and hence the
applications that it can be used for are dependent on the nature of the pore and by
introducing functional groups onto the pore surface, the properties can be tuned[40].
Typically, fuctionalisation of MOFs has been carried out presynthetically by
changing the organic linker for an analogous version with a simple functional group on it.
Common examples used amine or halide substituted polycarboxylate ligands[7]. This
method of introducing functionality into the pores of frameworks is summarised in Figure
16. The disadvantage with this method of functionalisation is that many functionalised
MOFs cannot be prepared in this way as their structure or may not be tolerant of the
reaction conditions.
Figure 16. Schematic representation of presynthetic method of MOF functionalisation[41]
Hence the process of functionalising MOFs known as postsynthetic modification
(PSM) has been developed. The idea of PSM was originally put forward by Hoskins and
Robson in the 1990s[42] but the term didn’t become widely used until the mid-2000s when
it was introduced by Wang and Cohen[43]. It is defined as
“the chemical modification of a framework after it has been synthesised”[41]
In essence, it allows a group or ‘tag’ to be added to the surface of the pore
heterogeneously once the framework has already been formed without compromising the
overall framework structure and the process can attain MOFs not available by direct
synthesis.
2.2 Types of PSM reaction
The field of MOF PSM has become widely studied in recent years[41][44], so it is useful
to split up PSM reactions into different categories. Burrows has split PSM reactions into
three convenient categories which will be discussed in the following sections[40] although
others have covered the same material in largely similar categories[45]. These categories
are:
Covalent PSM – Alteration of the linker ligand
Dative PSM – Addition of a metal to the linker
Inorganic PSM – A change in the SBU](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-20-320.jpg)
![16
2.2.1 Covalent PSM
Covalent PSM is the process of transforming the organic linker section of the framework
upon addition of a reagent. The earliest example was carried out by Wang and Cohen and
showed conversion of the amino tagged benzene-1,4-dicarboxylate (bdc) ligand in
IRMOF-3 ([Zn4O(bdc-NH2)3]) to the corresponding amide when treated with acetic
anhydride[43]. Figure 17 gives a schematic representation of the reaction on the bdc-NH2
linker in IRMOF-3.
Figure 17. Reaction scheme showing synthesis of amine tagged IRMOF-3 followed by covalent PSM with
acetic anhydride.
PSM of MOFs with amine tags is one of the most widely studied area of covalent
PSM to date and the bdc-NH2 (2-amino-benzene-1,4-dicarboxylate) ligand has proven to
be a good PSM ready ligand in many framework topologies. For example, Cohen et
al.[46] have shown how three MOFs; IRMOF-3, DMOF-1-NH2 ([Zn(bdc-NH2)2(dabco)])
and UMCM-1-NH2 ([Zn(bdc-NH2)(btb)4/3]); all allow conversion to the amide when
treated with anhydride; despite them having very different structures. bdc-NH2 also tends
to be a commonly used linker as its acid precursor, 2-amino-terephthalic acid is
commercially available. The reaction schemes for the reaction with anhydride to DMOF-
1-NH2 and UMCM-1-NH2 are shown below in Figure 18.
Figure 18. Reaction schemes showing that addition of anhydride group is possible on a range of MOF
topologies.[45]
An interesting observation that has been made when performing PSM reactions
with amine tags and anhydrides, is that the alkyl chain length on the anhydride species
affects the conversion of the amine to the corresponding amide. This was first noticed for
the zinc based MOF, IRMOF-3[47]. The study performed the PSM on IRMOF-3 using ten](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-21-320.jpg)
![17
anhydrides with alkyl chain lengths varying from one to eighteen carbon atoms. 1
H NMR
of the digested ligands after PSM with the anhydride were collected to show the degree of
conversion to the amide. The conversion rated ranged from ~99% down to ~7%. The
results are shown in Table 2.
Table 2. Conversion rates of amine to amide on IRMOF-3 using anhydride reagents with varying alkyl
chain lengths.
Alkyl chain length (no. of carbons) Percentage conversion to amide
1 ~99
2 ~99
3 98±3
4 96±3
5 90±3
6 80±5
8 46±7
12 32±5
15 20±1
18 7±1
A separate study by the same group also expanded this to DMOF-1-NH2 and
UMCM-1-NH2 frameworks[46]. Both MOFs showed similar trends to IRMOF-3 in
decreasing percentage conversion with increasing chain length. This is shown by the chart
in Figure 19. This study also investigated the effects of branched alkyl groups on
conversion to the amide. As with increasing chain length, increased branching decreases
the conversion rates.
Figure 19. Conversion of amine group to amide using anhydride reagents of varying chain length and
branching in IRMOF-3, DMOF-1-NH2 and UMCM-1-NH2. (n= no. of carbons in alkyl chain)[46]](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-22-320.jpg)
![18
These studies postulated that the main reason for the decrease in conversion were
largely due to steric clash with the MOF framework. As shown by Figure 18, relative
conversion followed the trend UMCM-1-NH2>IRMOF-3>DMOF-1-NH2. This
corresponds with the pore apertures (UMCM-1-NH2: ~32 Å>IRMOF-3: ~17 Å>DMOF-1-
NH2: ~7 Å)[48] of the MOFs so it can be seen that the smaller pores can only
accommodate smaller alkyl chains.
Another common functional group that has been widely studied in covalent PSM
chemistry is aldehydes. Burrows et al. demonstrated the conversion of aldehyde tagged 2-
formyl-biphenyl-4,4’-dicarboxylate ligand in a zinc based MOF to the corresponding
hydrazone using 2,4-dinitrophenylhydrazine[49]. Conversion of the aldehyde tag was
shown qualitatively by a change in the crystal colour as well as in the crystal structure.
Aldehyde tagged zeolitic imidazolate frameworks (ZIFs) have also proved to be
successful PSM candidates. This is due to the fact that they tend to be more stable than
most MOFs and hence can be used under as greater range of conditions[50]. For example,
the [Zn(ica)2], ZIF-90 (ica =imidazolate-2-carboxyaldehyde), contains an aldehyde tag
that can undergo PSM that a carboxylate linker zinc MOF such as IRMOF-3 could
not[51].
However, there has also been some work conducted on MOFs containing metals
other than zinc, which tend to be more chemically robust frameworks and hence allow
different PSM reactions to be performed. For example, MIL-53(Al)-NH2 which also
contains the bdc-NH2 linker has been shown to undergo a range of PSM reactions under
harsh conditions that would certainly degrade MOFs such as IRMOF-3. These reactions
included treatment with formic acid to produce the corresponding formylated MOF and
even treatment with diophosgene or thiophosgene to produce the corresponding isocyanate
and isothiocyanate tagged MOFs respectively[41].
2.2.2 Dative PSM
Dative PSM which is also sometimes referred to as coordinative[45] is the process of
coordinating a metal centre to the organic linker sections of a framework[40]. These
reactions occur when a MOF contains free ligand sites. These sites are typically
heteroatoms such as pyridyl nitrogens or simple functional groups such as alcohols.
For example, it was shown by Long et al. that the two free pyridyl sites in
[Al(OH)(bpydc)], MOF-253 (bpydc = 2,2′-bipyridine-5,5′-dicarboxylate) could chelate in
a bidentate fashion with PdCl2 and Cu(BF4)[52]. A schematic representation of this
chelation reaction is shown in Figure 20. The study showed that the selectivity for CO2
adsorption over N2 for the copper containing sample was over four times larger than in the
non metalated MOF which supports the use of MOFs as potentially important catalytic
materials.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-23-320.jpg)
![19
Figure 20. Dative PSM showing the chelation of PdCl2 to free pyridyl sites in MOF-253[40]
An example of dative PSM using hydroxy groups was performed by Lin et al. who
used [Cd3Cl6L3] (where L = -6,6′-dichloro-2,2′-dihydroxy-1,1′-binaphthyl-4,4′-bipyridine)
[53]. The pyridyl nitrogen atoms in this MOF bond to the cadmium centres to form the
framework structure. The two alcohol groups on the linker were showed to coordinate to
Ti(OPri
)4 as shown in Figure 21.
Figure 21. Coordination of Ti(OPri
)2 to alcohol functional groups on a cadmium based MOF[40]
The study showed that the reaction product in Figure 20 could be used as a heterogeneous
chiral catalyst for the addition of diethylzinc to aromatic aldehydes to afford secondary
alcohols. Importantly, the alcohols produced are chiral and in very high enantiomeric
excess, comparable to that obtained when using a homogeneous catalyst but with the
advantage that heterogeneous catalysts can be easily separated from the products.
2.2.3 Inorganic PSM
Inorganic PSM is the process of modification of the SBU or the way in which it interacts
with the linker. This can occur in a number of ways including substitution of linker or
terminal ligands connected to the SBU, or change in oxidation state of the metal centre.
Substitution of a linker or terminal ligand can occur because many MOFs have
labile ligands in the structure. These are often solvent molecules coordinated to the SBU.
These ligands can be removed during activation procedures to remove solvent from the
MOF to form open coordination sites, which is why this type of PSM reaction is often
performed in two stages. The first stage is removal of the labile ligand and the second is
addition of a new ligand to the open coordination site. The first example of this was](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-24-320.jpg)
![20
performed using the MOF, HKUST-1 ([Cu3(btc)2(H2O)3] where btc=1,3,5-
benzenetricarboxylate) which contains water molecules as coordinated ligands. It was
shown that heating of the MOF removed the coordinated water molecules and upon
treatment with pyridine (py), the open coordination sited in the activated MOF were filled
to produce [Cu3(btc)2(py)3][54].
This type of inorganic PSM has been shown to improve the gas adsorption
properties of MOFs. An example of this, by Rosseinsky et al.[55] again looks at the
exchange of coordinated water molecules in HKUST-1. The study incorporates 4-
(methylamino)-pyridine (map) into the MOF resulting in a product with the formula
[Cu3(btc)2(4-map)x(H2O)3−x]. It was demonstrated that the resulting MOF could adsorb
NO reversibly by reacting the 4-map ligand to form N-diazenium diolates. The parent
framework (HKUST-1) irreversibly binds NO into any open metal coordination sites. A
schematic representation of the reaction is shown in Figure 22.
Figure 22. Schematic representation of the inorganic PSM reaction of HKUST-1 with 4-map followed by
adsorption of NO[55].
Another type of inorganic PSM involves changing the oxidation state of the metal
in a MOF. [VO(bdc)] (MIL-47(V)) is an interesting example of a MOF that has been
shown to undergo PSM reactions of this type. Fischer’s group[56] showed that by reaction
with cobaltocene, [Co(η5
-C5H5)2], MIL-47(V) forms [Co(η5
-C5H5)2][VO(bdc)]2. In this
reaction, the cobaltocene is oxidised to cobaltocenium and the vanadium in the MOF is in
a 1:1 ratio of V(III) and V(IV). A subsequent study by Clet et al.[57] on MIL-47(V)
synthesised the framework with varying ratios of V(III) and V(IV). The results from this
study were that MIL-47(V) containing only V(III) produced a flexible framework
isostructural to that of the MIL-53 framework whereas when only V(IV) was present, a
rigid structure was produced.
2.3 Tandem PSM reactions
An important concept in PSM chemistry of MOFs is that of tandem PSM reactions. These
could be any of the types of PSM reactions described in the previous sections but
performed sequentially. There are many reasons why tandem PSM reactions may be
performed. Two common reasons are either to introduce many functionalities into a MOF
or to further modify a tag that was incorporated via a PSM reaction.
Wang and Cohen were the first to perform tandem PSM reactions. Using IRMOF-3
which has already been shown to be susceptible to PSM reactions, they successfully
incorporated two new functionalities into the framework whilst maintaining the framework
structure[58]. The study treated IRMOF-3 first with crotonic anhydride followed by acetic
anhydride to produce a framework containing both groups as shown in a schematic
representation in Figure 23.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-25-320.jpg)
![21
Figure 23. Schematic representation to show IRMOF-3 (left) treated with crotonic anhydride followed by
acetic anhydride (right) [58].
A follow up study by Cohen’s group extended this research on IRMOF-3 to work
out the maximum number of functionalities that could be incorporated into the
framework[59]. They successfully treated IRMOF-3 with four reagents sequentially;
namely, decanoic anhydride, allyl isocyanate, propylisocyanate and crotonic anhydride.
These tandem PSM reactions are shown in Figure 24.
Figure 24. Schematic representation of the tandem PSM reactions of IRMOF-3 with decanoic anhydride,
allyl isocyanate, propylisocyanate and crotonic anhydride[41].
Another type of tandem PSM reaction involves a first reaction to introduce a tag
into a framework followed by a second reaction to modify the tag. There have been many
examples of this type of tandem PSM that involve incorporation of a metal into the PSM
tag. These reactions are of particular interest as they allow vacant metal coordination sites
to be introduced into a MOF that is not on the SBU. Hupp et al. used a diol containing
MOF that they termed DO-MOF for a two stage tandem PSM reaction[60]. The MOF was
formed from Zn2+
centres with 1,2,4,5-tetrakis(4-carboxyphenyl) benzene (TCPB) and
meso-1,2-bis- (4-pyridyl)-1,2-ethanediol (DPG) linkers and had a molecular formula of
[Zn2(TCPB)(DPG)]. In the first stage of the reaction, [Zn2(TCPB)(DPG)] underwent a
covalent reaction with succinic anyhydride to produce a framework containing carboxylic
acid groups. The second stage of the reaction was a dative PSM that coordinated Cu2+
ions
to the carboxylic acid groups. The reaction scheme is shown in Figure 25.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-26-320.jpg)
![23
4. Experimental
4.1 Syntheses and activation procedures
4.1.1 DMOF-1-NH2 – Compound 1
The method for synthesising the MOF structure has been adapted from a well established
literature procedure[24]. Zn(NO3)2·6H2O (0.149 g), dabco (0.028 g) and H2-bdc-NH2
(0.091 g) were placed in glass microwave vials with 8 ml of DMF which were then sealed.
The vials were sonicated to fully dissolve reagents and heated at 120 °C for three days. The
resulting crystals were washed with three 5 ml washings of DMF. Samples were stored in
sealed vials under an inert nitrogen environment in 8 ml DMF until activation.
4.1.2 Diketene PSM
0.2 ml of diketene was added to samples of compound 1 stored in 8 ml DMF in sealed
vials. After three days and under a flow of nitrogen, the solvent was removed and replaced
with fresh dry DMF until samples were needed for analysis.
4.1.3 Diketene PSM after activation in cyclohexane
DMF was removed from the vials containing compound 1. This was replaced with 5 ml of
cyclohexane. Samples in sealed vials were heated at 50°C in an oil bath for one week. The
solvent was then removed from the vials and replaced with 5 ml of dry THF. This was
followed by the addition of 0.2 ml diketene. After three days, the mother liquor was
removed and replaced with 5 ml dry THF. Samples were stored under THF until analysis
of them.
4.1.4 Diketene PSM after activation in diethyl ether– Compound 2
DMF was removed from vials storing compound 1 whilst maintaining an inert environment
with nitrogen. 5 ml of dry Et2O was injected into the vials through a septum. For the
subsequent two days, Et2O was removed and replaces with fresh 5 ml portions all of which
was carried out under an inert nitrogen environment. After three washings with dry Et2O,
the diethyl ether was removed from the vials and replaced with 5 ml dry THF. Whilst still
under a flow of nitrogen, 0.2 ml diketene was injected into the vials containing sample 1
and left for three days. After three days, the mother liquor in the vials was removed and
replaced with 5 ml fresh dry THF, with the flow of nitrogen being maintained throughout.
Compound 2 was stored in dry THF in the sealed vials until analysis.
4.1.5 KOtBu PSM – Compound 3
Solid KOt
Bu was added in excess to compound 2 stored in 5 ml dry THF. Upon sealing the
vial, it was purged with a flow of nitrogen to maintain an inert atmosphere. Three days
after the addition of KOt
Bu, the mother liquor in the vial was removed using a syringe.
Following this, 5 ml dry THF was injected into the vial containing compound 3 where it
was stored until analysis.
4.1.6 ZnEt2 PSM – Compound 4
0.2 ml of ZnEt2 (1 M solution in hexane) was injected into samples of compound 2 stored
in sealed vials in 5 ml of dry THF. This was carried out under a flow of nitrogen. After
three days, the mother liquor was removed. The samples of compound 4 were then injected
with 5 ml dry THF under a flow of nitrogen where they were stored in sealed vials until
analysis.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-28-320.jpg)
![24
4.1.7 [Rh2(CO)4(μ-Cl)2] PSM – Compound 5
A stock solution of [Rh2(CO)4(μ-Cl)2] in dry THF (0.250 g in 5 ml) was made. Samples of
compound 2 stored under 5 ml dry THF were injected with 1 ml of the stock solution.
After three days, the mother liquor was removed and replaced with a 5 ml portion of fresh
dry THF.
4.2 Analytical procedures
4.2.1 Powder X-ray diffraction (PXRD)
For PXRD analysis, samples were washed with THF and air dried. Samples were then
crushed onto grease coated slides ready for data collection on a spinning plate. All PXRD
data was collected on a Bruker D8 diffractometer. Data was normalised using the program
EVA before being exported to and plotted in MS Excel 2013.
4.2.2 Single crystal X-ray diffraction (XRD)
Samples were not dried before being suspended in crystallographic oil on a glass slide.
Single crystal XRD data was collected on an Agilent Supernova diffractometer. Structures
were solved using SHELXS and refined using SHELXL. Structure pictures were created
using the programs X-seed and POV-ray.
4.2.3 1H NMR
Roughly 5 mg of samples were digested in 0.2 ml trifluoroacetic acid (TFA) which was
subsequently evaporated using a rotary evaporator. Samples were then dissolved in 0.7 ml
DMSO for data collection. Spectra were recorded on a Bruker Avance 300 MHz
spectrometer at 298 K.
4.2.4 Electron dispersive X-ray spectroscopy (EDX)
Samples were washed with THF and air dried prior to being fixed to sample holders with
adhesive tape and stored under vacuum overnight.
5. Synthesis of a MOF for PSM reactions
5.1 Synthesis of DMOF-1-NH2 (compound 1)
The first stage of this project was to synthesise a MOF for performing PSM reactions on.
The zinc based MOF, DMOF-1-NH2 (compound 1) with the formula [Zn(bdc-
NH2)2(dabco)] was chosen as there has already been a significant amount of work on the
framework that shows it is a suitable candidate for PSM reactions. This is discussed in
section 3 of this report. DMOF-1-NH2 is formed using Zn(NO3)2·6H2O as a source of
metal. The organic linker precursors that form DMOF-1-NH2 are 2-amino-1,4-
benzenedicarboxylic acid (H2-bdc-NH2) and 1,4-diazabicyclo[2.2.2]octane (dabco). The
exact synthesis is adapted from a literature preparation and can be found in section 4.1.1.
Figure 27 shows the reaction scheme for the formation of DMOF-1-NH2.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-29-320.jpg)
![25
Figure 27. Reaction scheme for the synthesis of DMOF-1-NH2
The reaction resulted in the formation of pale yellow-brown needle like crystals. To
confirm that the correct product was produced, samples were analysed using PXRD and 1
H
NMR spectroscopy.
5.2 PXRD analysis of compound 1
The experimental PXRD pattern of compound 1 and a calculated pattern from a previously
reported crystal structure[61] are shown in Figure 28. The patterns closely match one
another so it is reasonable to assume that the product of the reaction was compound 1
(DMOF-1-NH2). The experimental pattern is shifted to slightly higher angles than the
calculated powder pattern. This is likely suggesting a change in unit cell parameters due to
the fact that the calculated pattern is taken from single crystal data collected at 150K
whereas the experimental powder pattern was collected at 298K.
Figure 28. Calculated PXRD pattern for DMOF-1-NH2 vs. experimental pattern of compound 1 confirming
that compound 1 is DMOF-1-NH2.
0
5000
10000
15000
20000
25000
5 15 25 35 45 55
Intensity/arb.units
2θ / °
Calculated DMOF-1-NH2 Compound 1](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-30-320.jpg)
![26
5.3 1H NMR analysis of compound 1
To further confirm that the product of the reaction was compound 1, a proton NMR
spectrum was collected following digestion in TFA and is shown in Figure 29.
Figure 29. 1
H NMR spectrum of compound 1 (DMOF-1-NH2).
The peaks in red boxes can be assigned to DMF that remained in the pores after
synthesis. Doublets at 7.05 ppm and 7.75 ppm are assigned to the two aromatic protons on
the opposite side of the phenyl ring to the amine group. The remaining aromatic proton on
the phenyl ring corresponds to the peak at 7.40 ppm. The other ligand expected in the
product was dabco. A singlet would be expected for dabco and according to the literature
this should appear ~2.8 ppm[62]. Thus it was suggested that the peak in the spectrum at
this chemical shift assigned to DMF, overlapped with the peak for the dabco ligand.
The 1
H NMR spectrum combined with the PXRD pattern confirms that DMOF-1-
NH2 (compound 1) can be reliably reproduced and that a MOF of known composition had
been produced for use in PSM reactions.
6. Treatment of compound 1 with diketene
The second stage of this project was incorporating a PSM tag to the amino groups on the
bdc-NH2 ligands in compound 1. As discussed in section 2.2.1, amine groups are a
commonly used functionality for covalent PSM reactions which is one of the reasons that
compound 1 was chosen for this work. Diketene (C4H4O2) was chosen as a reagent as it
was hoped to produce a β-amidoketone group analogous to an acac ligand (with the
shorthand notation of ‘dkn’) on the phenyl ring of bdc linkers in compound 1. The
proposed reaction scheme for this reaction is shown in Figure 30 with the product having
the molecular formula [Zn2(bdc-dkn)2(dabco)] assuming the PSM reaction goes to
completion.
b a c](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-31-320.jpg)
![28
7. Activation of Compound 1
Compound 1 was activated prior to being treated with diketene. This was necessary
because as discussed in section 6, the proposed ketoamide tagged structure did not form
upon treatment of compound 1 with diketene. It was suggested that this could have been
due to solvent remaining in the pores of compound 1. In this case, DMF which has a high
boiling point (152 °C) and low volatility was the solvent used in the synthesis of
compound 1 and has been shown to remain in the pores of MOFs [26]. This could prevent
the diketene being able to access the inside of the pores and thus prevent it forming the
desired product. In an effort to remove the DMF from compound 1, two activation
procedures were tested.
7.1 Activation of compound 1 in cyclohexane
Samples of compound 1 had their solvent exchanged from DMF to cyclohexane and were
heated to 50 °C in sealed vials. This procedure is based on those previously reported for
Fujita crystalline flask materials [63]. This procedure was followed over time using PXRD
to see how long compound 1 would remain stable. This was because it was thought that the
longer the activation procedure was, the greater amount of DMF that would be removed
from the pores of compound 1 and hence increase the chances of the amino groups being
converted to the ketoamide PSM tag. Figure 32 shows the PXRD patterns for compound 1
activated in cyclohexane over time.
Figure 32. Powder patterns of compound 1 activated with cyclohexane at 50 °C over a period of two weeks.
0
20000
40000
60000
80000
100000
5 15 25 35 45 55
Intensity/arb.units
2θ / °
1 week 2 weeks 0 days 1 day 2 days 3 days](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-33-320.jpg)
![33
The spectrum shows that the ketoamide tagged MOF had been formed during the
reaction. Peaks at 9.00 ppm, 8.04 ppm and 7.68 ppm can be assigned to protons on the bdc
ligand containing the PSM tag. However, the spectrum also shows that some of the
unreacted bdc-NH2 linker remained in the product. This is shown by the presence of peaks
assigned to the bdc-NH2 linker at 7.76 ppm, 7.38 ppm and 7.02 ppm. An estimated
conversion rate from bdc-NH2 to bdc-dkn can be calculated using the ratio of each species
respective peak integrals. The two doublets at 7.68 ppm and 7.76 ppm for example have a
ratio of 2.37:1.06. This equates to ~69% conversion to the bdc-dkn ligand. This gives
compound 2 an average molecular formula of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)]
rather than [Zn2(bdc-dkn)2(dabco)] in the proposed product structure.
Singlets at 7.95 ppm, 2.90 ppm and 2.47 ppm represent DMF which was probably
trapped in the pores of the framework prior to digestion for 1
H NMR analysis. Singlets at
3.70 ppm and 3.15 ppm can be attributed to the CH2 (‘keto’) and CH (‘enol’) protons of the
ketoamide tag and at 2.70 ppm and 2.07 ppm for the terminal methyl protons. The reason
each of these sets of protons have two resonances is due to keto-enol tautomerism. The
ketoamide tag can exist in either the ketone or enol form as shown in Figure 38 which puts
the protons in different environments hence causing the different shifts.
Figure 38. Schematic representation showing the tautomers of the diketene PSM tag when coordinated to the
bdc linker section of compound 2.
8.2.3 Elemental analysis of compound 2
Compound 2 shows 69% conversion to the bdc-dkn ligand and has an expected
composition of: C – 45.94%, H – 3.86%, N – 7.91%. The empirical percentages were
measured to be: C – 47.02%, H – 4.09%, N – 7.42%. The difference in percentages is
likely due to left over solvent in the in the sample but they are similar enough to suggest
that compound 2 contained the ketoamide tag proposed in Figure 28
8.2.4 Single crystal XRD analysis
Single crystal data were collected on compound 2 in an effort to obtain conclusive
structural evidence that the ketoamide tag was incorporated into the MOF. The crystal
description of compound 2 is given below.
Crystal structure description of compound 2
Pale yellow needle like crystals of compound 2 with a calculated molecular formula of
[Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] were analysed using single crystal X-ray
diffraction, with the crystal data summarised in Table 3.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-38-320.jpg)
![34
Table 3. Crystal data for [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)]
Compound number 2
Empirical formula C13.64H13.64N2O5.32Zn
Formula weight 356.07
Temperature / K 150.00(10)
Crystal system monoclinic
Space group I2/a
a/Å 19.2194(8)
b/Å 14.7996(9)
c/Å 15.4061(8)
α/° 90
β/° 99.640(4)
γ/° 90
Volume/Å3
4320.2(4)
Z 8
ρcalcg/cm3
1.095
Crystal size/mm3
0.131 × 0.099 × 0.075
Radiation Cu Kα (λ = 1.54184/Å)
2θ range for data collection/° 7.58 to 146.632
Final R indexes [I>=2σ (I)] R1 = 0.0914, wR2 = 0.2772
Final R indexes [all data] R1 = 0.1164, wR2 = 0.3005
The asymmetric unit of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] consists of a Zn(II) centre
connected to one dabco ligand and either one 2-amino-1,4-benzenedicarboxylate ligand or
one 2-amino-1,4-benzenedicarboxylate ligand containing the ketoamide PSM tag (bdc-
dkn). The dabco carbon atoms exhibited disorder over two positions with a 50:50 ratio.
The –NH2 and –dkn groups on the bdc ligands are disordered over two positions on the
phenyl ring with a 50:50 ratio. Both the bdc-NH2 and bdc-dkn ligands are present in the
molecular formula as conversion from the –NH2 to the –dkn group was shown to be ~69%
by 1
H NMR. The site occupancy of the bdc nitrogen atoms is 0.5 as they are common to
both the –NH2 and –dkn groups. The ketoamide tags appear to be almost parallel to the
phenyl rings of the bdc ligands but protrude slightly into the centre of the pore. Electron
density for the ketoamide PSM tag becomes increasingly diffuse the further away from the
phenyl ring core. This results in the final C(O)CH3 group of the tag not being seen in the
crystal structure. The crystal structure of the asymmetric unit is shown in Figure 39.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-39-320.jpg)
![35
Figure 39. Part of the Crystal structure of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] with unprimed labelled
atoms forming the asymmetric unit. All other atoms including those denoted by a primed label are generated
by symmetry. Zn1’ related to Zn1 by the symmetry operation -x, 1-y,-z; O2’ related to O2 by the symmetry
operation -x, 1-y,-z; O3’ related to O3 by the symmetry operator -x, y-½, ½-z; and O4’ related to O4 by the
symmetry operation x, 3/2-y, z-1/2. All Hydrogen atoms have been omitted for clarity.
The structure shows that the zinc adopts a square pyramidal geometry with four
carboxylate oxygens forming the base plane; and the dabco ligand coordinated axially. Cis
O-Zn-O bond angles between the bdc carboxylate groups are in the range of 87.3(2)° to
89.2(2)°. N-Zn-O bond angles between the dabco and bdc ligands are in the range of
97.12(18)° to 103.90(17)°.
Zn1-O bond distances to the carboxylate oxygen atoms are all very similar and
within the range of 2.032(4) Å to 2.045(4) Å and the Zn1-N1 distance is 2.068(4) Å. The
Zn1-Zn1’ distance is 2.9662(11) Å which is significantly longer than the value for metallic
zinc (2.48 Å). This coupled with the fact that Zn(II) is a d10
metal ion means that a Zn-Zn
bond is not likely to be present.
The asymmetric units link together to form a paddlewheel structure which forms
square grids that are connected in a third dimension via the dabco ligands. Figure 40 shows
the square pores of the structure with the edges of the square grids curving alternately
inwards and outwards along the pore axis.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-40-320.jpg)
![36
Figure 40. A section of the framework structure viewed along the crystallographic a axis showing the square
pores with the edges curving in alternate directions. Hydrogen atoms are omitted for clarity.
There is significant pore space in the framework as shown by the space filling
model in Figure 41. Residual electron density in the framework was assessed using the
PLATON SQUEEZE algorithm. It was concluded that the residual electron density in the
structure pertained to the missing PSM tag atoms rather than any guest solvent molecules.
Figure 41. Space filling model of the [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] framework viewed along the
crystallographic a axis showing significant pore space.
The single crystal structure shows that compound 2 contained the ketoamide PSM
tag; albeit with only 69% conversion from compound 1. A possible reason for incomplete
conversion to the PSM tag could be due to DMF remaining in the pore even after solvent
exchange with diethyl ether which may have prevented diketene accessing some amine
groups.
Work carried out on this system by Dr William Gee showed conversion from
compound 1 to the bdc-dkn PSM tag at stoichiometric levels (as assessed by 1
H NMR
spectroscopy). It was observed that the crystal structure showed a slightly different](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-41-320.jpg)
![37
structure. All except the last carbon of the ketoamide PSM tag was found
crystallographically which is more than was found in the crystal data for compound 2.
Figure 42 shows the asymmetric unit of the structure with complete conversion to bdc-dkn.
Figure 42. Part of the crystal structure of [Zn2(bdc-dkn)2(dabco)] produced by Dr William Gee. All
unlabelled atoms have been generated by symmetry and hydrogen atoms are omitted for clarity.
The same general structure to that of compound 2 was observed. However, the
ketoamide PSM tags appear to be parallel to the phenyl rings whereas in compound 2, they
are slightly offset from the plane of the ring. The entire ketoamide tag was solved
crystallographically; and as with compound 2, the electron density gets more diffuse the
further away from the phenyl ring core it gets. This structure has a space group of C2/m
which is different to that of compound 2. This suggests that there is some flexibility in the
framework as the change in space group shows a change in symmetry and unit cell
parameters. The unit cell parameters for the structure in Figure 42 are given in Table 4.
Table 4. Unit cell parameters for [Zn2(bdc-dkn)2(dabco)]
a / Å 14.298
b / Å 16.325
c / Å 9.557
α / ° 90
β / ° 100.7
γ / ° 90
Volume / Å3
2191.93
The unit cell parameters are significantly different to those for compound 2 which
confirms that there has been a change in structure. The structure is viewed along the
crystallographic-c axis in Figure 40 and can be compared to the structure in Figure 43.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-42-320.jpg)
![38
Figure 43. Section of the [Zn2(bdc-dkn)2(dabco)] framework viewed along the crystallographic-c axis.
Hydrogen atoms have been omitted for clarity.
Figure 43 shows that the ketoamide tags protrude into the pore cavities which
didn’t appear to occur to such an obvious extent in compound 2. Unlike compound 2, the
edges of the pores don’t curve alternately inwards and outwards along the pore axis. The
space filled view of this framework is displayed in Figure 44.
Figure 44. Section of the [Zn2(bdc-dkn)2(dabco)] framework viewed along the crystallographic-c axis.
Hydrogen atoms have been omitted for clarity.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-43-320.jpg)
![39
It can be seen that the framework has some pore space, although it appears to be
less than in Figure 41. This is because the entire ketoamide tag has been solved
crystallographically and they protrude further into the pores in this framework.
It can be concluded from the crystal structures that differing amounts of conversion
of the bdc-NH2 linker to the bdc-dkn linker influence the structure of the framework.
Single crystal X-ray diffraction analysis was also performed on the brown-orange
block like crystals found in the product. They were found to have the formula
[Zn(bdc-NH2)(DMF)] and the full crystal description is given below.
[Zn(bdc-NH2)(DMF)] crystal structure
Large pale orange crystals of [Zn(bdc-NH2)(DMF)] (compound 1a) were analysed using
single crystal X-ray diffraction, with the crystal data summarised in Table 5.
Table 5. Crystal data for [Zn(bdc-NH2)(DMF)]
Compound number 1a
Empirical formula C11H12N2O5Zn
Formula weigh 317.60
Temperature/K 150.00(10)
Crystal system monoclinic
Space group P21/c
a/Å 7.9963(2)
b/Å 15.0643(3)
c/Å 11.0454(3)
α/° 90
β/° 111.160(3)
γ/° 90
Volume/Å3
1240.80(7)
Z 4
ρcalcg/cm3
1.700
Crystal size/mm3
0.102 × 0.092 × 0.074
Radiation CuKα (λ = 1.54184 / Å)
2θ range for data collection/° 10.404 to 146.594
Final R indexes [I>=2σ (I)] R1 = 0.0451, wR2 = 0.1138
Final R indexes [all data] R1 = 0.0535, wR2 = 0.1202](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-44-320.jpg)
![40
The asymmetric unit of [Zn(bdc-NH2)(DMF)] consists of a Zn(II) centre connected to one
2-amino-1,4-benzenedicarboxylate ligand and one DMF ligand. The DMF is disordered
over two positions with all atoms except O5 in a ratio of 70:30. The amino group on the
dicarboxylate ligand has N1:N1A disorder with a ratio of 55:45. The structure of the
asymmetric unit is shown in Figure 45.
Figure 45. Crystal structure of [Zn(bdc-NH2)(DMF)] with atoms labelled without a prime from the
asymmetric unit. All other atoms including those denoted with a prime are generated by symmetry. Zn1’
related to Zn1 by the symmetry operation -x, 1-y,-z, O2’ related to O2 by the symmetry operation -x, 1-y,-z,
O3’ related to O3 by the symmetry operator 1-x, ½+y, ½-z and O4’ related to O4 by the symmetry operation
x-1, ½-y, z-1/2. All Hydrogen atoms have been omitted for clarity.
The crystal structure shows that zinc centre adopts a square pyramidal geometry
with the four oxygens atoms of the carboxylate ligands forming the base plane and the
DMF ligand coordinated axially. Cis-O-Zn-O angles are in the range of 87.52(15)° to
88.54(14)° and in the range of 100.64(14)° and 102.03(13) ° between DMF and bdc-NH2
ligands.
The Zn1-O1 distance to the NH2-bdc ligand is slightly longer at 2.008(2) Å than the
Zn1-O5 distance of 1.982(3) Å. The Zn1-Zn1’ distance is 2.9339(8) Å which is
significantly longer than the value for metallic zinc (2.48 Å). As such, a Zn-Zn bond is not
likely in this structure, which is to be expected for a d10
metal ion such as Zn(II).](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-45-320.jpg)
![41
The asymmetric units link together to form a paddlewheel structure which forms
two-dimensional sheets with square channels as shown in Figure 46.
Figure 46. Two-dimensional sheet of [Zn(bdc-NH2)(DMF)] viewed along the crystallographic a axis.
The DMF ligands act as capping groups and prevent the sheet structure growing in a third
dimension. A small section of the framework shows this in Figure 47.
Figure 47. A section of a sheet in the framework of [Zn(NH2-bdc)(DMF)] showing the DMF ligands acting
as capping groups.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-46-320.jpg)
![42
The two-dimensional sheets pack in a staggered fashion with each sheet being slightly
offset relative to the one above and the one below it. In effect, the sheets stack in an
efficient manner such that the DMF ligands interdigitate with one another. This is
illustrated in Figure 48.
Figure 48. Two one dimensional sections of [Zn(bdc-NH2)(DMF)] showing that stacking occurs in a
staggered fashion with due to the protruding DMF ligands.
The staggered sheets of [Zn(bdc-NH2)(DMF)] result in a non-porous structure which is
clearly illustrated by a space filling model of the atoms. This is shown in Figure 49 and it
can be clearly seen that there is minimal pore space between the two-dimensional sheets of
the structure as the square channels start to become occupied by the sheets above and
below them.
Figure 49. Space filling model of stacked sheets of [Zn(bdc-NH2)(DMF)] shown along the a-crystallographic
axis, showing lack of void space in the square channels of the two-dimensional sheets.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-47-320.jpg)
![43
It was suggested that [Zn(bdc-NH2)(DMF)] was essentially a decomposition
product of the DMOF-1-NH2 framework and was likely formed upon treatment of
compound 1 with diketene as it didn’t appear to be present in the samples of compound 1
that were produced for this study.
8.4 Summary of reactions with diketene
In conclusion, it was found that of the three reactions of compound 1 with diketene;
activation in dry diethyl ether followed by treatment with diketene was the best route to
produce compound 2 containing the –dkn PSM tag.
Primarly, the crystal structure of compound 2 shows structurally that compound 1
was probably successfully postsynthetically modified to incorporate the ketoamide PSM
tag. Although, as part of the –dkn tag could not be resolved in the crystal structure, the
PXRD and 1
H NMR spectroscopy data were used to support the conclusion that it was
produced.
However, the product of the reaction appeared to be contain a mixture of crystal
morphologies, and a second phase was identified (compound 1a). The structure of the
second phase was confirmed by single crystal XRD analysis and was suggested to be a
decomposition product that formed upon treatment of compound 1 with diketene. After the
identification of compound 1a, it was postulated that the products of the reactions of
compound 1 with diketene following activation in cyclohexane or without activation were
likely to be compound 1a. To confirm this, the calculated PXRD pattern of compound 1a
was compared those of the products of the reactions mentioned. This is shown in Figure
50.
Figure 50. PXRD patterns of compound 1a, and those of compound 1 treated with diketene after activation
in cyclohexane and without activation.
It can be seen that the blue and orange patterns share a large number of peaks. This
suggests that the product of the reaction of compound 1 treated with diketene after
activation in cyclohexane was likely to have contained some compound 1a alongside the
compound 2 produced.
0
5000
10000
15000
20000
25000
30000
35000
5 15 25 35 45 55
intensity/arb.units
2θ / °
Calculated compound 1a
Compound 1 activated in cyclohexane treated with diketene
Compound 1 treated with diketene without activation](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-48-320.jpg)
![44
9. PSMetallation of compound 2 using KOt
Bu
The final stage of this project aimed to chelate a metal between the two carbonyl groups of
the ketoamide PSM tag. It was hoped that by doing so, the products would be suited for
applications in catalysis. There have already been several examples of coordinating metals
to MOFs that show catalytic activity[38]. The first metal tried for this project was
potassium. KOt
Bu was added in excess to a sample of compound 2 (with full details given
in section 4) and was chosen as a reagent for the reaction for a number of reasons. As a
basic reagent, it would deprotonate the central carbon atom of the –dkn tag to make it more
susceptible to chelation with a K+
ion. Also, potassium is a small metal with an atomic
radius of just 2.27 Å. Theoretically, this makes it small enough to enter the pores of the
DMOF-1-NH2 framework which has a pore aperture of approximately 7.5×7.5 Å2
[24] and
hence it should be able to react with all available –dkn tags. The target product of the
reaction (compound 3) contains a modified PSM tag with the notation –dknK and the
proposed scheme for this reaction is shown in Figure 51.
Figure 51. Proposed reaction scheme for compound 2 with KOt
Bu to form compound 3 with the molecular
formula [Zn2(dabco)(bdc-dknK)2]. Only the bdc section of the frameworks is shown.
Visual inspection of the product under an optical microscope showed that the
product of the reaction contained some light yellow needle like crystals similar to those of
compound 2. There were also some brown-orange crystals that were not needle like which
were expected to be compound 1a due to their very similar appearance.
9.1 PXRD analysis of compound 3
PXRD analysis was performed on the product of the reaction shown in Figure 52. The
pattern is compared to that of compound 1 and compound 2.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-49-320.jpg)
![50
10. PSMetalation of compound 2 using ZnEt2
Another metal that was reacted with compound 2 was zinc from ZnEt2. Zinc was chosen
because it was thought that as the MOF already contained zinc, that any chance of
transmetalation would be reduced. Also, unlike potassium that could not be seen in the
crystal structure of the previous reaction (section 9), it was hoped that zinc would be. This
is because it is a heavier element with more electron density than potassium. The proposed
product of the reaction contains a modified ketoamide PSM tag containing a coordinated
zinc atom and ethyl group with the notation –dknZn and molecular formula
[Zn2(dabco)(bdc-dknZn)2]. The proposed reaction of compound 2 with ZnEt2 is shown below
in Figure 59.
Figure 59. Proposed reaction scheme for compound 2 with ZnEt2 to produce compound 4 with the formula
[Zn2(dabco)(bdc-dknZn)2]. Only the bdc sections of the framework is shown.
Upon visual inspection of the product (compound 4), it appeared to contain the
same needle like crystals as in compound 2 but with a very slight colour change to a light
yellow. There did also appear to be some less needle like, orange-brown crystals that were
likely to be compound 1a based on their visual similarity with known compound 1a.
10.1 PXRD analysis
A PXRD pattern of the product of the reaction was collected and is shown compared to the
patterns for compounds 1 and 2 in Figure 60.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-55-320.jpg)
![53
11. PSMetalation of compound 2 using [Rh2(μ-Cl)2(CO)4]
The final metal that was reacted with compound 2 in this project was rhodium using Di-μ-
chloro-tetracarbonyldirhodium(I) as the reagent. Rhodium was chosen because it is a large,
heavy metal with a significant amount of electron density. Thus it was hoped that unlike
potassium and zinc, it would be seen crystallographically. Also, as one of the aims of the
project was to produce structures that have the potential to be used in catalytic
applications. Rhodium has long been used in catalysis so it was thought to be a good
candidate. The proposed product of the reaction (compound 5) contained a modified
ketoamide PSM tag with Rh(CO)2 chelated between the carbonyl groups with the notation
–dknRh(CO)2. The targeted product had the formula [Zn2(bdc-dknRh(CO)2)2(dabco)] and
the reaction scheme is shown in Figure 62.
Figure 62. Proposed reaction scheme for compound 2 with [Rh2(μ-Cl)2(CO)4]. Only the bdc sections of the
frameworks are shown.
Upon visual inspection under an optical microscope, there was a colour change in
the product. The light yellow crystals of compound 2 had a very dark red tint which was
the same colour as the rhodium reagent used. The product appeared to be a mixture. There
were still some oblong needle like crystals with a similar morphology to those found in
compound 2. There were also some darker orange crystals which were thought to be
compound 1a based on visual inspection compared to known compound 1a crystals.
Finally, there were some small round pieces of amorphous looking product. The product of
this reaction had a much lower proportion of the needle like product than when potassium
and zinc were used to treat compound 2. A reason for this could be because the by-product
of the proposed reaction would be hydrogen chloride. It has been shown that some MOFs
are unstable in acidic environments[41], so it was possible that the framework was partially
decomposed by this and hence reduced the proportion of needle like crystals in the
product.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-58-320.jpg)
![54
11.1 PXRD analysis
PXRD analysis of the product of the reaction was compared to the PXRD patterns of
compounds 1 and 2 as well as the pattern for [Rh2(μ-Cl)2(CO)4] in Figure 63. By
comparing the pattern for compound 5 to the pattern for the rhodium reagent, it was used
to determine whether any of the reagent was present in the product of the reaction.
Figure 63. PXRD patterns of compound 1 (blue), compound 2 (orange), [Rh2(μ-Cl)2(CO)4] (grey) and
compound 2 treated with [Rh2(μ-Cl)2(CO)4] (yellow).
The pattern of compound 5 (yellow) is significantly different to that of compounds 1 and 2.
There is a peak shift to higher angle of the peaks at ~8° and ~17° when compared to
compounds 1 and 2 which indicates a change in unit cell parameters. There also appears to
be a peak at ~47° which isn’t present in compounds 1 or 2. This could be an indication that
a new phase had formed. As the pattern for the product does not correspond with that of
the reagent (grey), this shows that there isn’t any unreacted reagent in the product. It is
worth noting that the peak intensities in the pattern for the product of the reaction are
generally lower and broader than those of compound 2. This indicated reduced crystallinity
of the product which is supported by the idea that the hydrogen chloride by-product of the
reaction could have caused partial decomposition of the MOF. These observations could
suggest that a reaction had taken place and that the product was not likely to contain
compounds 1 or 2 or any unreacted rhodium reagent. Further analysis was performed on
compound 5 to determine whether rhodium was incorporated into the ketoamide PSM tag.
11.2 1H NMR analysis
As with the other samples of compound 2 treated with metals, a proton NMR spectrum was
collected to see what percentage of bdc-dkn linker was in the sample. The same splitting
pattern to that of compound 2 was observed and is shown in Figure 64.
0
20000
40000
60000
80000
100000
120000
5 15 25 35 45 55
Intensity/arb.units
2θ / °
Compound 1 Compound 2 Rh2(μ-Cl)2(CO)4 Compound 5](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-59-320.jpg)
![55
Figure 64. Section of 1
H NMR spectrum compound 5 digested in TFA.
Using the doublets as with the other metalation reactions (7.70 ppm and 7.75 ppm),
the ratio of bdc-NH2 to bdc-dkn was calculated. A peak integral ratio of 1.94:1.13 (bdc-
dkn:bdc-NH2) shows ~63% bdc-dkn in the sample. This is less than the amount in
compound 2 before treatement with [Rh2(μ-Cl)2(CO)4]. The very large broad hump
~6.5 ppm is likely due to acid remaining in the sample. This could be from the TFA used
to digest the MOF. This hump makes the accuracy of the peak integrals questionable,
especially for the peaks closer to it. As such, the percentage of each ligand in the sample is
likely to be approximate.
11.3 EDX/SEM analysis
EDX analysis was performed on the product of the reaction. Figure 65 shows an SEM
image of the product. It can be seen that there are several types of crystals present as
described in section 11.
Figure 65. SEM image of the product of the reaction at 190× magnification. Sites of interest where EDX
spectra were collected are highlighted by red circles.
1
2
3
a bc e
f
d
g](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-60-320.jpg)
![56
Interestingly, the oblong needle like crystals in the photo which are most like those
found in compound 2 are much smaller in the product than before the reaction. This can be
seen by comparing Figures 54 and 65. Both figures are at 190× magnification yet the
crystals in Figure 65 all appear to be smaller and shorter. This is evidence that the MOF is
likely to have been degraded during the reaction. The sites where EDX spectra were
collected are denoted by red circles in Figure 65. Site one was thought most likely to be
compound 5 as the morphology of the crystal was most similar to that of compound 2. The
EDX spectrum for site one is shown in Figure 66.
Figure 66. EDX spectrum of site one of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4].
The spectrum shows the presence of rhodium in the sample. This was a good
indication that rhodium had been incorporated into the product and suggests that
compound 5 had rhodium inocorporated on the surface of the crystals. However, because
there are several crystal morphologies present in the product, other spectra were collected.
The spectrum for site two is shown in Figure 67.
Figure 67. EDX spectrum of site two of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4].
The crystal that site two was taken from had an appearance closer to that of
compound 1a and there were many more crystals of this type in the product than in
compound 2 before treatment with [Rh2(μ-Cl)2(CO)4]. However, the spectrum still shows
the presence of rhodium, albeit at what appears to be a lower level than at site one. This
could suggest that the crystal either was not compound 1a or perhaps there was some left
over reagent on the surface of the crystals. Finally, a spectrum was collected at site three
which was on what appeared to be a small, round, pieces of product. The spectrum is
shown in Figure 68.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-61-320.jpg)
![57
Figure 68. EDX spectrum of site three of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4].
The spectrum shows that rhodium was present at site three. It also shows that the
other elements expected in compound 5 are present. However, it is unlikely that product at
site three is compound 5 because it doesn’t appear to be crystalline and it is unlikely that
crystals of compound 2 would change that significantly upon treatment with [Rh2(μ-
Cl)2(CO)4].
Overall, there is a strong suggestion that there was rhodium in the product of the
reaction. However, as the data could not provide accurate elemental compositions, it is
unclear whether any of the products formed were the target product. Also, as EDX only
measures composition at the surface of the crystals of product, it is impossible to tell
whether the rhodium converted the ketoamide tag just on the surface of the crystals as was
suggested when compound 2 was treated with both KOt
Bu or ZnEt2. As each of the spectra
above show chlorine peaks that overlap with the rhodium peak at ~2.7 keV, it is possible
that some unreacted [Rh2(μ-Cl)2(CO)4] could have remained in the product.
11.4 IR analysis
Because the proposed structure of compound 5 sees –Rh(CO)2 chelated between the
carbonyl groups of the ketoamide PSM tag, it was expected that a characteristic inorganic
CO stretch would be seen in the IR spectrum of the product.
The IR spectrum of the product of the reaction showed strong sharp peaks at
2009 cm-1
and 2068 cm-1
which were attributed to the symmetric and asymmetric CO
stretches of the proposed structure of compound 5. However, it was possible that the
product could contain some unreacted rhodium reagent. To determine whether this was the
case, and IR spectrum was also collected on a sample of [Rh2(μ-Cl)2(CO)4]. This showed
strong peaks at 2034 cm-1
and 2090 cm-1
. These were assigned to the symmetric and
asymmetric CO stretches and were consistent with previously reported values in the
literature[64]. Therefore, it was deduced that the product of the reaction did not contain
[Rh2(μ-Cl)2(CO)4]. This is evidence to show that compound 5 contains inorganic carbonyl
groups which could be from the Rh(CO)2 group chelated to the ketoamide tag.
An interesting comparison that was also made was between the spectra of the
product of the reaction and [Rh(acac)(CO)2]. This is because the proposed structure of
compound 5 and [Rh(acac)(CO)2] are similar. This is shown in Figure 69. Essentially, the
diketene PSM tag in compound 5 acts as an acac ligand that is anchored to the MOF via an
amide link.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-62-320.jpg)
![58
Figure 69. bdc-dknRh(CO)2 section of compound 5 (left) and [Rh(acac)(CO)2] (right).
Therefore, it would be expected that the CO stretches for the two compounds would
be similar but different. The reported stretching frequencies for [Rh(acac)(CO)2] are:
νco – 2083 cm-1
(symmetric) and 2015 cm-1
(asymmetric)[65]. The difference in frequencies
is due to different amounts of π-back-bonding in each of the compounds. The frequencies
for the product of the reaction are lower than that which shows that the CO bond is weaker
than in [Rh(acac)(CO)2]. This is also a convincing piece of evidence that compound 5 was
produced during the reaction.
11.5 Summary of reaction with [Rh2(μ-Cl)2(CO)4]
It is likely that compound 5 was formed by reacting compound 2 with [Rh2(μ-Cl)2(CO)4].
A number of analyses seemed to confirm this. As with the other metalation reactions, the
PXRD pattern confirmed that a reaction had taken place.
SEM analysis showed that there appeared to be several types of substances present
in the product. EDX analysis subsequently showed that these samples all contained some
rhodium which was a strong indication that at least some of the compound 5 contained
rhodium. This was further supported by the IR data which showed clearly that there was
CO in the product as proposed in the structure of compound 5.
However, as with the other metalation reactions performed on compound 2, the
formation of compound 5 was not structurally confirmed as initial XRD experiments could
not resolve rhodium in the crystal structure. This could be due to the reaction only taking
place on the surface of the crystals or. The preliminary single crystal XRD experiments
also showed that there was a significant amount of compound 1a in the product as several
of the crystals analysed had the same unit cell as compound 1a. This further suggests that
compound 1a is essentially a decomposition product of the DMOF-1 type framework.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-63-320.jpg)
![59
12. Conclusions
This project led to the successful reproduction of the zinc-based metal organic framework,
compound 1, [Zn(bdc-NH2)2(dabco)] (DMOF-1-NH2) from the literature. Postsynthetic
modification of compound 1 using diketene was successful in producing a product with the
average molecular fomula of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] (compound 2). Only
69% of the bdc-NH2 linkers in compound 1 were converted to the bdc-dkn linkers as
shown by 1
H NMR spectroscopy. The second stage of the tandem postsynthetic
modifications studied in this project showed promising evidence that metals were
coordinated to the ketoamide PSM tags on the surfaces of the crystals of compound 3, 4
and 5; particularly potassium and rhodium (compounds 3 and 5 respectively).
When treated with diketene, compound 1 that hadn’t been activated proved
unsuccessful in producing compound 2. From this, it was deduced that conversion from
compound 1 to compound 2 would be benefitted from activating compound 1 prior to
PSM.
It was observed that by using a different activation procedures prior to postynthetic
modification with diketene affected the product that was produced. Activation of
compound 1 in cyclohexane failed to produce compound 2 and instead the product
appeared to be composed primarily of a decomposition product; [Zn(bdc-NH2)2(DMF)]
(compound 1a) which was characterised crystallographically. Compound 1a was formed
from two dimensional sheets with DMF ligands acting as capping groups. The sheets
interdigitated to form a structure with very little pore space. It was found that activation of
compound 1 in diethylether was successful in producing compound 2 which was
characterised crystallographically. This method did also produce some compound 1a
although to a much lower degree, and in this study, only 69% conversion from compound
1 to compound 2 was achieved. However, it was chosen to be the most appropriate
activation procedure.
The second stage of the tandem PSM reactions studied in this project involved
dative PSM of metal ions and metal containing fragments to the diketene PSM tag in
compound 2. There were fairly strong indications that coordination of potassium to the
diketene tag to produce compound 3 was successful. PXRD and elemental analysis proved
useful in gaining information that compound 2 had been changed upon treatment with
KOt
Bu. The presence of potassium and hence production of compound 3 was indicated by
EDX analysis. Similarly, treatment of compound 2 with [Rh2(μ-Cl)2(CO)4] offered some
convincing evidence that compound 5 was produced. Again, PXRD and EDX showed that
a reaction had taken place and that rhodium was incorporated into the product. IR
spectroscopy supported the formation of compound 5 as the characteristic CO stretching
frequencies are different to that of the rhodium reagent and similar rhodium complexes.
There was less evidence to show that zinc had been successfully chelated into the diketene
tag. Not necessarily because it was less likely to do so, but it proved difficult to
characterise as it is the same metal as in the framework structure. Thus EDX couldn’t be
used to show the formation of compound 4 unlike for compounds 3 and 5.
Unfortunately, crystallographic evidence for the formation of compounds 3, 4 and 5
was not obtained. Thus, further work would be needed to conclusively prove their
formation.](https://image.slidesharecdn.com/a8eb7862-11cb-4ccb-bbb5-7c238343f1fa-160704101547/85/OWEN_GLEDHILL_pdf-64-320.jpg)
OWEN_GLEDHILL_pdf
- 1. Department of Chemistry CH40165: Chemistryresearch project (MChem with IT) TANDEM POSTSYNTHETIC MODIFICATION OF THE METAL-ORGANIC FRAMEWORK DMOF-1-NH2 USING DIKETENE AND A RANGE OF METAL CONTAINING REAGENTS Owen J. Gledhill Registration number: 129086299 Supervisors: Prof. Andrew D. Burrows and Dr Mary F. Mahon
- 2. Plagiarism Declaration Please completethe following declaration by adding your name and the date. If handing in a printed copy, please sign this page. Submission of an electronic copy of this report through e.g. Moodle indicates that you agree with the statement I certify that I have read and understood the entry in the Student Handbook for the Department of Chemistry on Cheating and Plagiarism and that all material in this assignment is my own work, except where I have indicated with appropriate references. I agree that, in line with Regulation 15.3(e), if requested I will submit an electronic copy of this work for submission to a Plagiarism Detection Service for quality assurance purposes. Student Name Owen Gledhill Student Signature ________________ Date: ______________20/04/16_____
- 3. i Table of contents Abbreviations.................................................................................................................... 1 Abstract............................................................................................................................... 2 1. Metal-organic frameworks (MOFs) .......................................................................... 3 1.1 What is a MOF?........................................................................................................ 3 1.1.1 Terminology in MOF chemistry........................................................................ 3 1.1.2 Structure of MOFs ............................................................................................. 3 1.1.3 Reticular synthesis.............................................................................................. 6 1.2 Synthesis of MOFs................................................................................................... 7 1.2.1 Solvothermal synthesis....................................................................................... 8 1.2.2 Microwave and ultrasound assisted synthesis ................................................. 9 1.2.3 Solvent free synthesis ......................................................................................... 9 1.2.4 Activation procedures ...................................................................................... 10 1.3 Characterisation of MOFs..................................................................................... 10 1.3.1 X-ray crystallography ...................................................................................... 10 1.3.3 1H NMR ............................................................................................................. 11 1.3.4 Elemental analysis ............................................................................................ 11 1.3.5 Electron dispersive X-ray spectroscopy (EDX) ............................................. 11 1.4 Applications............................................................................................................. 11 1.4.1 Gas storage........................................................................................................ 11 1.4.2 Gas separation .................................................................................................. 12 1.4.3 Catalysis............................................................................................................. 12 1.4.4 Drug delivery .................................................................................................... 14 2. Postsynthetic modification of MOFs .................................................................... 15 2.1 Introduction.............................................................................................................. 15 2.2 Types of PSM reaction.......................................................................................... 15 2.2.1 Covalent PSM ................................................................................................... 16 2.2.2 Dative PSM........................................................................................................ 18 2.2.3 Inorganic PSM.................................................................................................. 19 2.3 Tandem PSM reactions......................................................................................... 20 3. Project aims................................................................................................................. 22 4. Experimental................................................................................................................ 23 4.1 Syntheses and activation procedures................................................................. 23 4.1.1 DMOF-1-NH2 – Compound 1 .......................................................................... 23
- 4. ii 4.1.2 Diketene PSM....................................................................................................23 4.1.3 Diketene PSM after activation in cyclohexane............................................... 23 4.1.4 Diketene PSM after activation in diethyl ether– Compound 2 .................... 23 4.1.5 KOtBu PSM – Compound 3............................................................................. 23 4.1.6 ZnEt2 PSM – Compound 4 .............................................................................. 23 4.1.7 [Rh2(CO)4(μ-Cl)2] PSM – Compound 5 .......................................................... 24 4.2 Analytical procedures ............................................................................................ 24 4.2.1 Powder X-ray diffraction (PXRD).................................................................. 24 4.2.2 Single crystal X-ray diffraction (XRD)........................................................... 24 4.2.3 1H NMR ............................................................................................................. 24 4.2.4 Electron dispersive X-ray spectroscopy (EDX) ............................................. 24 5. Synthesis of a MOF for PSM reactions ................................................................ 24 5.1 Synthesis of DMOF-1-NH2 (compound 1).......................................................... 24 5.2 PXRD analysis of compound 1 ............................................................................ 25 5.3 1H NMR analysis of compound 1......................................................................... 26 6. Treatment of compound 1 with diketene ............................................................. 26 7.1 Activation of compound 1 in cyclohexane.......................................................... 28 7.2 Activation of compound 1 in dry diethyl ether.................................................... 29 8. Treatment of compound 1 with diketene after activation................................ 30 8.1 Compound 1 activated in cyclohexane............................................................... 30 8.2 Compound 1 activated in dry diethyl ether......................................................... 31 8.2.1 PXRD analysis of compound 2 ........................................................................ 32 8.2.2 1H NMR analysis of compound 2 .................................................................... 32 8.2.3 Elemental analysis of compound 2.................................................................. 33 8.2.4 Single crystal XRD analysis............................................................................. 33 8.4 Summary of reactions with diketene ................................................................... 43 9. PSMetallation of compound 2 using KOtBu........................................................ 44 9.1 PXRD analysis of compound 3 ............................................................................ 44 9.2 1H NMR analysis of compound 3......................................................................... 45 9.3 Single crystal XRD analysis.................................................................................. 46 9.4 Elemental analysis ................................................................................................. 46 9.5 SEM/EDX analysis................................................................................................. 47 9.6 Summary of KOtBu reactions............................................................................... 49 10. PSMetalation of compound 2 using ZnEt2 ........................................................ 50 10.1 PXRD analysis...................................................................................................... 50
- 5. iii 10.2 1H NMRanalysis .................................................................................................. 51 10.3 Elemental analysis............................................................................................... 52 10.4 Summary of reaction with ZnEt2 ........................................................................ 52 11. PSMetalation of compound 2 using [Rh2(μ-Cl)2(CO)4] ................................... 53 11.1 PXRD analysis...................................................................................................... 54 11.2 1H NMR analysis .................................................................................................. 54 11.3 EDX/SEM analysis............................................................................................... 55 11.4 IR analysis............................................................................................................. 57 11.5 Summary of reaction with [Rh2(μ-Cl)2(CO)4].................................................... 58 12. Conclusions .............................................................................................................. 59 13. Future work................................................................................................................ 60 Acknowledgements........................................................................................................ 60 References........................................................................................................................ 61
- 6. 1 Abbreviations Whilst every efforthas been made to ensure that abbreviations have been explained in the main body of the report, they have been included here as a reference list. bdc: benzene-1,4-dicarboxylate H2-bdc: Terephthalic acid (benzene-1,4-dicarboxylic acid) bdc-NH2: 2-aminobenzene-1,4-dicarboxylate dabco: 1,4-diazabicyclo[2.2.2]octane dkn: ketoamide PSM tag btc: benzene-1,3,5-tricarboxylate MOF: Metal organic framework IRMOF: Iso-reticular metal organic framework PXRD: Powder X-ray diffraction EDX: Energy dispersive X-ray spectroscopy NMR: Nuclear magnetic resonance MIL: Material Institute Lavoisier ZIF: Zeolitic imidazolate framework PSM: Postsynthetic modification PSMet: Postsynthetic metalation UiO: Universitetet I Oslo SBU: Secondary building unit DMOF: dabco metal organic framework UMCM: University of Michigan Crystalline Material
- 7. 2 Abstract A series oftandem postsynthetic modification (PSM) reactions were carried out on the metal-organic framework [Zn(bdc-NH2)2(dabco)] (compound 1, DMOF-1-NH2) at the amino site on the 2-amino-benzene-1,4-dicarboxylate linker molecules in the MOF. Studies to activate compound 1 prior to postsynthetic modification were performed. These were solvent exchanges using cyclohexane or diethyl ether. These reactions were followed using powder X-ray diffraction and thermogravimetric analysis and it was found that activation in diethyl ether was most successful for the first PSM reaction. Covalent postsynthetic modification of compound 1 using diketene was successfully achieved with 69% conversion to the ketoamide tagged analogue (compound 2) and was characterised by single crystal X-ray diffraction. A decomposition product with the formula [Zn(bdc- NH2)(DMF)] (compound 1a) was formed after the addition of diketene with its structure deduced by single crystal diffraction. There was evidence to show that dative postsynthetic modification of the ketoamide tag had been achieved upon treatment of compound 2 with KOt Bu, ZnEt2 or [Rh2(μ-Cl)2(CO)4]. Powder X-ray diffraction studies showed that compound 2 had changed upon treatment with these metal species and electron dispersive X-ray spectroscopy showed incorporation of the metals in the product for the potassium and rhodium based reagents (compounds 3 and 5 respectively).
- 8. 3 1. Metal-organic frameworks(MOFs) 1.1 What is a MOF? 1.1.1 Terminology in MOF chemistry The term metal-organic framework has been defined differently in the past with terms such as coordination polymer and hybrid organic-inorganic materials often being used interchangeably despite them having their own specific meanings[1][2]. Yaghi’s group described a metal-organic framework on the first usage of the term in 1995 as a material analogous to other microporous materials such as zeolites with open frameworks and extended channel systems[3]. Others have offered narrower definitions. For example, O’Keeffe wrote that he uses the term MOF when referring to frameworks built from linking polyatomic clusters (or secondary building units) with strong bonds. He also stated in this definition that MOFs do not include coordination polymers[4]. O’Keeffe also highlights the point mentioned above that the term coordination polymer is sometimes used instead of MOF and vice versa. A review by Corma et al.[5] describes a coordination polymer as being formed from extended networks of metal ions and multidentate organic ligands. It also states that this encompasses crystalline and amorphous materials and that MOFs are a special group of coordination polymers. This implies that MOFs are a subclass of coordination polymers. It can be seen that it is difficult to assign a single clear definition to the term MOF which is probably the reason that a lot of the literature begins by defining their own. As such, an IUPAC ‘task group’ was set up to provide guidelines on terminology and nomenclature. In 2012, they published provisional recommendations with some of the important definitions summarised in Table 1[6]. These definitions will be adhered to in this report. Table 1. IUPAC proposed definitions for MOF chemistry[6] Term Definition Coordination Polymer A coordination compound continuously extending in 1, 2 or 3 dimensions through coordination bonds Coordination Network A coordination compound extending, through coordination bonds, in 1 dimension, but with cross-links between two or more individual chains, loops or spiro-links, or a coordination compound extending through coordination bonds in 2 or 3 dimensions Metal-Organic Framework Metal-Organic Framework, abbreviated to MOF, is a coordination polymer (or alternatively coordination network) with an open framework containing potential voids 1.1.2 Structure of MOFs MOFs are materials made from, as the name suggests organic and inorganic units. The inorganic units can be metal ions (known as primary building units) or more complex aggregates with different geometries to that of the individual metal ions (known as secondary building units or SBUs)[7]. The organic sections are known as organic linkers which are at their simplest just spacers between metals or SBUs and at their most complex, absolutely crucial to chemistry and properties of the resulting MOF. An important factor when choosing organic linkers is to select one that will withstand the reaction conditions and keep its functionality in the resulting framework[8].
- 9. 4 By considering somesimple coordination chemistry principles, the structure of MOFs can be understood. Coordination polymers and networks offer a useful comparison to understand the structure of MOFs. This is because they have similar structures to MOFs and the principles that govern their structure are also applicable to MOFs. The shape of ligands can dictate the geometry of some coordination compounds. For example, the linear ligand 4,4’-bipyridyl (4,4’-bipy) will coordinate to Ag+ to form a one-dimensional coordination polymer as shown in Figure 1[9]. Figure 1. Structure of one-dimensional [Ag(4,4’-bipy)]+ coordination polymer[9] Further to this, a ligand with a different denticity such as the tridentate tris-(-4- cyanophenyl)amine (TCPA) will form a two-dimensional honeycomb like coordination polymer with Ag+ as seen in Figure 2[10]. Figure 2. Reaction scheme to show a silver two-dimensional coordination polymer formed with the tridentate TCPA ligand. The ligands on the right hand side structure have been simplified for clarity. As well as ligand shape, preferred metal geometry can dictate the shape of a framework. For example, Cu(I) tends to coordinate in a tetrahedral geometry and it has been shown to form three-dimensional coordination polymers with structures analogous to diamond when reacted with 4,4’-bipy. The framework is cationic due to the charge on the copper and is stabilised by a [PF6]- counterion. Unlike the C-C bonds found in diamond, M-bipy-M units are present. A schematic representation of this structure is shown in figure 3 with the balls representing the metal and the struts the 4,4’-bipy ligand[11]. Figure 3. Schematic representation of a three-dimensional coordination network containing Cu(I) and 4,4’- bipy.
- 10. 5 The previous examplesbegin to show how choice of ligand and metal can determine the dimensionality and to a certain extent, the geometry of a structure. However, they are all of coordination polymers or networks. IUPAC tells us that the difference between these and MOFs is that MOFs possess the potential for porosity. The above examples do not possess this potential because they are all cationic structures that must be stabilised with counterions that often fill any potential pore. Consequently, the development of MOFs has relied largely on the use of anionic organic linkers so that a neutral framework results. A particularly common class of anionic organic linker is carboxylates[12][7]. As well as being anionic, carboxylates can bridge between metal centres which allows the formation of SBUs rather than the nodes in a framework being single metal centres. Figure 4 shows how four carboxylate linkers bridge two metal atoms to form what is known as a ‘paddle wheel’ SBU where the carboxylates extend to give a roughly square planar geometry even though the metal itself may not adopt this geometry[13]. The groups extending from the carboxylates can be any number of groups that could be chosen based on their functionality for example. Figure 4. Paddle wheel SBU formed from COOR carboxylate ligands showing that the SBU adopts an approximately square planar geometry. Although carboxylates account for most of the anionic linkers used in MOF chemistry, another frequently used class of linkers are polydentate N-donor ligands. Like carboxylates, their anionic nature allows them to produce neutral frameworks. For example zeolitic imidazolate framework-8 (ZIF-8) utilises 2-methylimidazole (MeIM) linkers that are connected to tetrahedral Zn(II) ions with the formula [Zn(MeIM)2]. The resulting framework which is shown in Figure 5, has large pores (~11.6 Å) with smaller hexagonal pore openings (~3.4 Å)[14]. Figure 5. Structure of ZIF-8 with hexagonal pore opening marked by white hexagon[15].
- 11. 6 1.1.3 Reticular synthesis Traditionally,solid state materials have been synthesised using methods that offer little control over the character of the product. This is due largely to the fact that the starting materials do not retain their structure during the reaction; which in turn produces a product with little to no resemblance between the reactants and products. Reticular synthesis provides a synthetic route where a target framework can be constructed from rigid molecular ‘building blocks’ (SBUs) that maintain their structure throughout the synthesis[16]. An important part of reticular synthesis is having an understanding of the target framework and choosing appropriate starting materials for its assembly[7]. As discussed in section 1.1.2, considering simple coordination ideas such as ligand shape and preferred metal orientation, allows the starting materials to be chosen for a particular framework. It is this that has led to the recognition of a smaller number of general MOF structures or topologies that are common to a whole group of MOFs with similar geometries and symmetry[17]. There has been work, particularly by Wells[18] to try and enumerate and describe topologies of solid state materials, but an approach for MOFs based on tiling that has been used by numerous groups[19][20] is particularly useful in understanding them. The tiles are a series of generalised polyhedra (cages) that fit together in three dimensions to generate an entire structure based on the net (or reticulate) of a particular topology. A well understood example of this approach is that of the Kelvin structure (also known as sodalite in zeolite chemistry). In this case, the cages that fit together are truncated octahedra where the vertices of the structure represent either silicon or aluminium atoms (T atoms). The T atoms form TO4 tetrahedra that link together with the edges of the cages being T-O-T linkages[19]. The way in which these cages form a three dimensional framework is illustrated in Figure 6. Figure 6. Schematic to show how truncated octahedral cages or ‘tiles’ (left) fit together to form a three dimensional sodalite structure (right)[19] ZIF-8 which is shown in Figure 5 also has the sodalite structure which demonstrates that one framework type can apply to many MOFs.
- 12. 7 An extension ofreticular synthesis is the concept of isoreticular MOFs. These are MOFs that have different organic linkers with the same denticity and bonding to the metal. This produces a framework with the same topology as when using the unfunctionalised linker and is known as an isoreticular MOF (IRMOF)[16]. Figure 7 shows a series of IRMOFs to illustrate that the gross structure remains the same regardless of the organic linker molecule used. Figure 7. IRMOF structures with a)bdc, b)cyclobutylbenzene, c)naphthalene-2,6-dicarboxylate linkers and Zn4O(CO2)6 - SBUs represented by the blue tetrahedra[21] By using reticular synthesis, a vast number of theoretical MOF frameworks can be produced by simply varying metal and liker type. This has led to an explosion in the number of new MOFs with over 20,000 structures having been reported since the early 2000s[12]. 1.2 Synthesis of MOFs The synthesis of MOFs can be divided into two main categories; solution phase which is by far the most common and widely used, and solid state. However, solid state synthesis has the big disadvantage that it is difficult to obtain single crystals as it usually involves some kind of grinding process which crushes the crystals that form. This makes it more difficult to perform single crystal X-ray diffraction analysis as the crystals are often too small which can lead to poor diffraction and cause poor quality data to be collected. This is an issue when structures of new materials needs to be determined. MOF syntheses can be divided further with a summary of these methods given in Figure 8. Figure 8. Summary of MOF preparations[22] The Figure shows that by using the same basic materials, MOFs can be prepared in a number of ways by varying the temperature, time and energy type.
- 13. 8 1.2.1 Solvothermal synthesis Byfar the most commonly used synthetic method is solvothermal synthesis (also known as hydrothermal if water is the solvent used). A solvothermal process is defined as a reaction between precursor materials in a sealed vessel in the presence of a solvent where the reaction temperature is higher than of the boiling point of the solvent used[23]. Organic solvents with high boiling points such as dimethylformamide (DMF), acetonitrile and acetone are commonly used. Strictly speaking, many MOF syntheses that are referred to as solvothermal actually use temperatures below the boiling point of the solvent. For example, the synthesis of DMOF-1 ([Zn2(bdc-NH2)2(dabco)]), is carried out in DMF at 110 °C[24] but DMF boils at 152 °C. Solvothermal synthesis can be considered as a Lewis acid-base reaction where the metal centre acts as an acid and the basic solvent deprotonates the organic linker’s acid precursor so that the resulting linker acts as a base. It is important that the solvent is a weak base as this keeps the concentration of the linker low which favours slow crystal growth. This often leads to the formation of large, good quality crystals suitable for single crystal analysis[25]. Conventional synthesis of MOFs using solvothermal methods typically use stoichiometric ratios of reagents in solvent. Self-assembly of the MOF then results upon heating. This is usually followed by an activation process to remove any solvent remaining in the pores. This process is summarised in Figure 9. Figure 9. Schematic representation of solvothermal synthesis[26] The main advantage of solvothermal synthesis is that good quality crystals can be grown due to slow crystal growth. This allows single crystal and powder X-ray diffraction analysis. However, the main disadvantage of solvothermal synthesis is that solvents such as DMF tend to be toxic and often gets trapped inside the pores of the framework. This limits their applications, because if the trapped solvents cannot be removed, the structures are not truly porous. Also solvothermal synthesis can be cost inefficient as the waste solvents are expensive to dispose of. Hence why there is a need for activation procedures.
- 14. 9 1.2.2 Microwave andultrasound assisted synthesis Microwave and ultrasound assisted synthesis of MOFs are essentially variations on solvothermal methods. The aim of using microwave or ultrasound energy is to reduce MOF preparation time whilst maintaining crystal quality. Microwave assisted synthesis provides a rapid synthesis of MOFs where the crystal size is usually much smaller than when using conventional heating methods. For example, the iron based MOF, MIL-53 ([Fe(OH)(bdc)], where bdc is benzene-1,4-dicarboxylate) is shown in Figure 10 to have smaller crystals when produced with microwave assistance compared to conventional solvothermal heating methods[27]. Figure 10. SEM images of MIL-53 produced with microwave assistance (left) and using conventional heating methods (right)[27] Ultrasound assisted or sonochemical methods see the precursor materials undergo a chemical change upon the application of ultrasound radiation. This causes localised areas of extreme temperature and pressure which can promote formation of nucleation sites and hence considerably reduce crystallation time[22]. This reduced crystallisation time again generally reduces the crystal size compared to solvothermal methods. For instance, Li et al. were able to produce the three-dimensional MOF, [Cu3(btc)2] (HKUST-1, btc is benzene- 1,3,5-tricarboxylate) by sonochemical synthesis with crystals in the 10-200 nm range wheras they produced the same MOF using conventional solvothermal methods in the range 10-30 μm[28]. However, there has been progress in maintaining a similar crystal size using ultrasound. For example, Son et al. produced [Zn4O(bdc)3] (MOF-5) crystals in the 5-25μm range with ultrasound which is similar to the sizes they achieved when using normal solvothermal synthesis[29]. 1.2.3 Solvent free synthesis Solvent free or solid state synthesis uses no (or very little) solvent to form MOFs. This makes it a cleaner, more environmentally friendly method than solvothermal methods. Instead of heating in the presence of a solvent, solvent free methods use mechanical force to achieve formation of MOFs. Typically, reagents are ground together in a ball mill to produce a product and this method has even been shown to produce three dimensional MOFs[30]. Although, to date, this method has only been used to make a limited number of MOFs.
- 15. 10 The disadvantage ofsolvent free synthesis is that, like microwave and sonochemical syntheses, solvent free methods tend to produce samples with small crystal size typically in the microcrystalline range as shown by PXRD and SEM analysis[8]. This makes solving the structure of new frameworks difficult as single crystal data cannot usually be collected. 1.2.4 Activation procedures An important stage in the preparation of MOFs is activating them for use in potential applications. One of the issues with solvothermal synthesis in particular is that solvent used in the synthesis tends to get trapped in the pores of the frameworks. Since the porosity of MOFs is often key to their uses, it is important that the solvent is removed prior to their use. Common methods to remove solvents from the pores include guest solvent exchange for a more volatile solvent molecule that is easier to remove and heating to drive out the solvent. Even freeze drying which first exchanges the solvent for benzene that is then sublimed under vacuum to avoid the liquid to gas phase transition has been used to activate MOFs[31]. Historically, the problem with activation techniques such as heating under vacuum is that upon removal of the solvent, there is a collapse of the framework[13]. 1.3 Characterisation of MOFs 1.3.1 X-ray crystallography The primary and arguably most important technique used in the characterisation of new MOFs is X-ray crystallography. This is because MOFs tend to be crystalline and depending on the synthetic route used, large single crystals are usually obtained, hence making them good candidates for X-ray analysis. Both single crystal and powder X-ray crystallography can be used for the characterisation of MOFs. Single crystal X-ray diffraction is used to determine the position of atoms within unit cells and hence determine the overall crystal structure. However, this technique is only used to look at the structure of a single crystal so is not representative of the sample as a whole. As such, powder X-ray diffraction (PXRD) can be used to determine the bulk structure of a sample. By analysing many crystals, an average structure is determined. PXRD patterns can be quickly compared to powder patterns calculated from single crystal data to identify any differences. Differences in experimental and calculated powder patterns can be useful in a number of ways. Most importantly, it allows the user to see changes in the unit cell or space group of the sample. This is an indication that a structural change has taken place. It also allows the user to quickly see if there are likely to be multiple phases in the bulk sample. This is often shown by the presence or disappearance of peaks that aren’t in the calculated pattern. Another advantage of PXRD is that it is a quick and easy method to perform with patterns usually taken in less than an hour whereas single crystal data acquisition typically takes up to 24 hours. However, PXRD cannot usually be used for structure determination by itself which it is so often reported in conjunction with other characterisation methods.
- 16. 11 1.3.3 1H NMR Itis useful to be able to characterise the organic linker sections of MOFs as they can play a key part in a MOF’s reactivity or properties. One method to do this is using 1 H NMR spectroscopy. Samples are digested in acid to convert the linkers into the corresponding acids. Hence the linkers can be characterised when not in a solid framework. 1 H NMR spectroscopy can show whether a ligand has remained intact in the framework. It can also be used to calculate conversion rates is the ligand is modified or exchanged after synthesis of the MOF. The obvious disadvantage to using 1 H NMR as a characterisation technique for MOFs is that it is a destructive technique. Digesting the MOFs to dissolve the organic linker means that the framework is destroyed and cannot be recovered. 1.3.4 Elemental analysis Elemental analysis gives the percentage of certain elements in a sample (usually carbon, hydrogen and nitrogen). For MOF chemistry, elemental analysis can tell if the correct amount of each element is present in a sample for the proposed structure which can further confirm a framework structure. The disadvantages of elemental analysis are that it provides no structural information about a sample and in MOF chemistry, it sometimes isn’t very accurate. This is because solvent is often trapped in the pores of a framework and will affect the ratio of elements in a sample. 1.3.5 Electron dispersive X-ray spectroscopy (EDX) EDX allows the surface elemental composition of a crystal to be determined. EDX causes excitation of an electron from a particular part of a sample with the use of a beam of charged particles (either electrons or protons). When the excited electron returns to its ground state, it releases radiation in the form of X-rays. The emitted X-rays can be measured. As each element has a different atomic structure, the energy of the emitted X- rays is characteristic of that element. Hence, the elemental composition of a sample can be determined. The beam of charged particles used for excitation of electrons typically only penetrates a sample by a few microns which is why only the surface composition can be reliably determined. 1.4 Applications One of the reasons that MOFs have become such a popular area of interest in the past few decades is because of their potential to be used in a vast range of applications. These include industrially and environmentally important processes such as gas storage and separation, in addition to medical applications such as drug delivery. 1.4.1 Gas storage With the move away from petroleum based fuels to using cleaner fossil fuels like natural gas (largely comprised of methane), it is essential that effective storage solutions are developed. Traditionally, gases have been stored in high pressure tanks but this method is very expensive and difficult to use for small scale applications such as car fuel tanks[32].
- 17. 12 An interesting exampleof MOF technology being used for gas storage is the Mercedes-Benz F125 research vehicle. This utilises MOFs for hydrogen storage as they have large surface areas (up to 10,000 m2 g-1 ) which can store large amounts of hydrogen (up to 80 bar) at lower pressures than an equivalent fuel reservoir without MOFs present[33]. From an environmental point of view, the rising atmospheric CO2 levels is an area of great interest, as the consequences of high CO2 levels poses a threat to all living things[34]. As such, high surface area MOFs like MOF-210 have been targeted as potential CO2 capture materials. MOF-210 is formed from biphenyl-4,4′-dicarboxylate (BPDC) and 4,4',4''-(benzene-1,3,5-triyl-tris (ethyne-2,1-diyl))tribenzoate (BTE) ligands with the empirical formula [Zn4O(BTE)4/3(BPDC)] has one of the largest known surface areas (10450 m2 g-1 ). It has a CO2 uptake of 2400 mg g-1 (74.2 wt%) which is higher than that of other MOFs with smaller surface areas like MOF-177(60 wt%) which has the empirical formula [Zn4O(BTB)2] (where BTB is 4,4',4''-benzene-1,3,5-triyl- tribenzoate)[22]. 1.4.2 Gas separation Gas separation or selective gas adsorption is an application closely related to gas storage, except that it relies on the pore size and the affinity for the target gases from the MOFs. An early example of gas separation using MOFs is that of MOF-508 ([Zn(bdc)(4,4’-bipy)1/2]) which was used to separate various alkanes including compounds such as n-pentane and n- hexane which are important in the process of petroleum refining[35]. This study filled a chromatography column with MOF-508 and the following GC measurements showed clear separation of the gases. Figure 11 shows that n-pentane and n-hexane have clearly different retention times. In this study, the reason for the difference in retention time of the gases was due to the difference in van der Waals forces the gases had with the MOF surfaces. It showed that longer alkyl chains had greater van der Waals forces with the MOF and hence took longer to travel the length of the column, thereby increasing their retention times. Figure 11. Gas chromatogram to show different retention times of gases separated by MOF-508[35]. 1.4.3 Catalysis MOFs can produce highly porous structures but with the advantage that there are many combinations of metals and ligands possible so the variety in framework structure is much greater. This makes them good candidates for use in a variety of catalysis reactions. It is important that a MOF for use in catalysis has active catalytic sites. These can be generated in two ways. Firstly, the metal centre can act as a Lewis acid catalyst (after removal of solvent from pores) [36]. An example of this is the catalysis of aldol reactions
- 18. 13 using the [Cu3(btc)2(H2O)3]framework[37]. The MOF acted as a catalyst in the green synthesis of pyrimidine chalcones via aldol condensation of ketones and formylpyrimidines. Using the MOF gave a very high yield and easy isolation of the product which reduced waste products, hence making it an eco-friendly synthesis. The reaction is shown in Figure 12. Figure 12. Aldol condensation of ketones with formylpyrimidines to form pyrmidine chalcones using [Cu3(btc)2] as a catalyst.[37] Secondly, functional groups on the framework (usually the organic linker) can form active catalytic sites. The advantage of this method is that the catalytic sites are well ordered and predictable due to the repeating nature of the framework structure[38]. This type of MOF catalysis is pictured in Figure 13. Figure 13. Schematic to show functionalised organic linkers acting as active catalytic sites[36]. MOFs containing these types of catalytic site have been shown to act catalytically for certain Knoevenagel reactions[38] as shown in Figure 14. In this study, [Cd(4- btapa)2(NO3)2] (4-btapa = 1,3,5-Benzene Tricarboxylic Acid Tris[N-(4-pyridyl)amide])was effective at catalysing the condensation of malonitrile with benzaldehyde but not with larger reagents such as ethyl cyanoacetate, and cyano-acetic acid tert-butyl ester. As such, it was suggested that the reaction occurred inside the pores of the MOF and not on the surface of the crystals. Figure 14. Knoevenagel reactions of benzaldehyde with cyano reagents catalysed by [Cd(4- btapa)2(NO3)2][38].
- 19. 14 1.4.4 Drug delivery Drugdelivery is still a relatively new area of research for MOFs but there is some evidence to suggest that they could be effective drug delivery agents. The need arises for drug delivery materials as it is important to ensure that the drug is delivered to the correct location in the body; in the correct quantity and released over the correct amount of time. MOFs possess several characteristics that allow this to occur whereas drug molecules tend to be broken down in the digestive system. One example of the use of MOFs as drug delivery materials is the use of zeolitic imidazolate framework-8, [Zn(MeIM)2] (ZIF-8) in the delivery of anticancer drug 5- fluorouracil (5-FU). The research carried out by Sun and co-workers[39] was inspired by the chemical stability of ZIF-8 in water and aqueous sodium hydroxide and the fact that it readily dissolves in acidic solution. It is known that tumour tissues are more acidic (~pH 5) than normal tissue and blood (~pH 7.4). It was thought that ZIF-8 could be used as a drug delivery system as it would only decompose and release the drug at the tumour site and hence reduce drug release during transportation in the circulatory system. As well as increasing the amount of drug released at the tumour site, this method could reduce some side effects of the drug caused by its release elsewhere in the body. As ZIF-8 is highly porous (~11.4 Å pores[14]), it was able to absorb considerably more of the 5-FU than other delivery system candidates and the study showed that at pH 7.4, it retained considerably more 5-FU in its pores than at pH 5 where nearly all of the drug was released. Figure 15 shows the 5-FU release over time at different pH levels and it confirms that drug release occurs much faster at lower pH 5 than pH 7.4. Figure 15. 5-FU release from ZIF-8 over time at pH 5 (black) and pH 7.4 (red)[39]
- 20. 15 2. Postsynthetic modificationof MOFs 2.1 Introduction One of the main attractions of MOFs is that their ability to form permanently porous crystalline structures allows them to be used in a wide range of applications; some of which are discussed in section 1.4. The properties of a particular framework and hence the applications that it can be used for are dependent on the nature of the pore and by introducing functional groups onto the pore surface, the properties can be tuned[40]. Typically, fuctionalisation of MOFs has been carried out presynthetically by changing the organic linker for an analogous version with a simple functional group on it. Common examples used amine or halide substituted polycarboxylate ligands[7]. This method of introducing functionality into the pores of frameworks is summarised in Figure 16. The disadvantage with this method of functionalisation is that many functionalised MOFs cannot be prepared in this way as their structure or may not be tolerant of the reaction conditions. Figure 16. Schematic representation of presynthetic method of MOF functionalisation[41] Hence the process of functionalising MOFs known as postsynthetic modification (PSM) has been developed. The idea of PSM was originally put forward by Hoskins and Robson in the 1990s[42] but the term didn’t become widely used until the mid-2000s when it was introduced by Wang and Cohen[43]. It is defined as “the chemical modification of a framework after it has been synthesised”[41] In essence, it allows a group or ‘tag’ to be added to the surface of the pore heterogeneously once the framework has already been formed without compromising the overall framework structure and the process can attain MOFs not available by direct synthesis. 2.2 Types of PSM reaction The field of MOF PSM has become widely studied in recent years[41][44], so it is useful to split up PSM reactions into different categories. Burrows has split PSM reactions into three convenient categories which will be discussed in the following sections[40] although others have covered the same material in largely similar categories[45]. These categories are: Covalent PSM – Alteration of the linker ligand Dative PSM – Addition of a metal to the linker Inorganic PSM – A change in the SBU
- 21. 16 2.2.1 Covalent PSM CovalentPSM is the process of transforming the organic linker section of the framework upon addition of a reagent. The earliest example was carried out by Wang and Cohen and showed conversion of the amino tagged benzene-1,4-dicarboxylate (bdc) ligand in IRMOF-3 ([Zn4O(bdc-NH2)3]) to the corresponding amide when treated with acetic anhydride[43]. Figure 17 gives a schematic representation of the reaction on the bdc-NH2 linker in IRMOF-3. Figure 17. Reaction scheme showing synthesis of amine tagged IRMOF-3 followed by covalent PSM with acetic anhydride. PSM of MOFs with amine tags is one of the most widely studied area of covalent PSM to date and the bdc-NH2 (2-amino-benzene-1,4-dicarboxylate) ligand has proven to be a good PSM ready ligand in many framework topologies. For example, Cohen et al.[46] have shown how three MOFs; IRMOF-3, DMOF-1-NH2 ([Zn(bdc-NH2)2(dabco)]) and UMCM-1-NH2 ([Zn(bdc-NH2)(btb)4/3]); all allow conversion to the amide when treated with anhydride; despite them having very different structures. bdc-NH2 also tends to be a commonly used linker as its acid precursor, 2-amino-terephthalic acid is commercially available. The reaction schemes for the reaction with anhydride to DMOF- 1-NH2 and UMCM-1-NH2 are shown below in Figure 18. Figure 18. Reaction schemes showing that addition of anhydride group is possible on a range of MOF topologies.[45] An interesting observation that has been made when performing PSM reactions with amine tags and anhydrides, is that the alkyl chain length on the anhydride species affects the conversion of the amine to the corresponding amide. This was first noticed for the zinc based MOF, IRMOF-3[47]. The study performed the PSM on IRMOF-3 using ten
- 22. 17 anhydrides with alkylchain lengths varying from one to eighteen carbon atoms. 1 H NMR of the digested ligands after PSM with the anhydride were collected to show the degree of conversion to the amide. The conversion rated ranged from ~99% down to ~7%. The results are shown in Table 2. Table 2. Conversion rates of amine to amide on IRMOF-3 using anhydride reagents with varying alkyl chain lengths. Alkyl chain length (no. of carbons) Percentage conversion to amide 1 ~99 2 ~99 3 98±3 4 96±3 5 90±3 6 80±5 8 46±7 12 32±5 15 20±1 18 7±1 A separate study by the same group also expanded this to DMOF-1-NH2 and UMCM-1-NH2 frameworks[46]. Both MOFs showed similar trends to IRMOF-3 in decreasing percentage conversion with increasing chain length. This is shown by the chart in Figure 19. This study also investigated the effects of branched alkyl groups on conversion to the amide. As with increasing chain length, increased branching decreases the conversion rates. Figure 19. Conversion of amine group to amide using anhydride reagents of varying chain length and branching in IRMOF-3, DMOF-1-NH2 and UMCM-1-NH2. (n= no. of carbons in alkyl chain)[46]
- 23. 18 These studies postulatedthat the main reason for the decrease in conversion were largely due to steric clash with the MOF framework. As shown by Figure 18, relative conversion followed the trend UMCM-1-NH2>IRMOF-3>DMOF-1-NH2. This corresponds with the pore apertures (UMCM-1-NH2: ~32 Å>IRMOF-3: ~17 Å>DMOF-1- NH2: ~7 Å)[48] of the MOFs so it can be seen that the smaller pores can only accommodate smaller alkyl chains. Another common functional group that has been widely studied in covalent PSM chemistry is aldehydes. Burrows et al. demonstrated the conversion of aldehyde tagged 2- formyl-biphenyl-4,4’-dicarboxylate ligand in a zinc based MOF to the corresponding hydrazone using 2,4-dinitrophenylhydrazine[49]. Conversion of the aldehyde tag was shown qualitatively by a change in the crystal colour as well as in the crystal structure. Aldehyde tagged zeolitic imidazolate frameworks (ZIFs) have also proved to be successful PSM candidates. This is due to the fact that they tend to be more stable than most MOFs and hence can be used under as greater range of conditions[50]. For example, the [Zn(ica)2], ZIF-90 (ica =imidazolate-2-carboxyaldehyde), contains an aldehyde tag that can undergo PSM that a carboxylate linker zinc MOF such as IRMOF-3 could not[51]. However, there has also been some work conducted on MOFs containing metals other than zinc, which tend to be more chemically robust frameworks and hence allow different PSM reactions to be performed. For example, MIL-53(Al)-NH2 which also contains the bdc-NH2 linker has been shown to undergo a range of PSM reactions under harsh conditions that would certainly degrade MOFs such as IRMOF-3. These reactions included treatment with formic acid to produce the corresponding formylated MOF and even treatment with diophosgene or thiophosgene to produce the corresponding isocyanate and isothiocyanate tagged MOFs respectively[41]. 2.2.2 Dative PSM Dative PSM which is also sometimes referred to as coordinative[45] is the process of coordinating a metal centre to the organic linker sections of a framework[40]. These reactions occur when a MOF contains free ligand sites. These sites are typically heteroatoms such as pyridyl nitrogens or simple functional groups such as alcohols. For example, it was shown by Long et al. that the two free pyridyl sites in [Al(OH)(bpydc)], MOF-253 (bpydc = 2,2′-bipyridine-5,5′-dicarboxylate) could chelate in a bidentate fashion with PdCl2 and Cu(BF4)[52]. A schematic representation of this chelation reaction is shown in Figure 20. The study showed that the selectivity for CO2 adsorption over N2 for the copper containing sample was over four times larger than in the non metalated MOF which supports the use of MOFs as potentially important catalytic materials.
- 24. 19 Figure 20. DativePSM showing the chelation of PdCl2 to free pyridyl sites in MOF-253[40] An example of dative PSM using hydroxy groups was performed by Lin et al. who used [Cd3Cl6L3] (where L = -6,6′-dichloro-2,2′-dihydroxy-1,1′-binaphthyl-4,4′-bipyridine) [53]. The pyridyl nitrogen atoms in this MOF bond to the cadmium centres to form the framework structure. The two alcohol groups on the linker were showed to coordinate to Ti(OPri )4 as shown in Figure 21. Figure 21. Coordination of Ti(OPri )2 to alcohol functional groups on a cadmium based MOF[40] The study showed that the reaction product in Figure 20 could be used as a heterogeneous chiral catalyst for the addition of diethylzinc to aromatic aldehydes to afford secondary alcohols. Importantly, the alcohols produced are chiral and in very high enantiomeric excess, comparable to that obtained when using a homogeneous catalyst but with the advantage that heterogeneous catalysts can be easily separated from the products. 2.2.3 Inorganic PSM Inorganic PSM is the process of modification of the SBU or the way in which it interacts with the linker. This can occur in a number of ways including substitution of linker or terminal ligands connected to the SBU, or change in oxidation state of the metal centre. Substitution of a linker or terminal ligand can occur because many MOFs have labile ligands in the structure. These are often solvent molecules coordinated to the SBU. These ligands can be removed during activation procedures to remove solvent from the MOF to form open coordination sites, which is why this type of PSM reaction is often performed in two stages. The first stage is removal of the labile ligand and the second is addition of a new ligand to the open coordination site. The first example of this was
- 25. 20 performed using theMOF, HKUST-1 ([Cu3(btc)2(H2O)3] where btc=1,3,5- benzenetricarboxylate) which contains water molecules as coordinated ligands. It was shown that heating of the MOF removed the coordinated water molecules and upon treatment with pyridine (py), the open coordination sited in the activated MOF were filled to produce [Cu3(btc)2(py)3][54]. This type of inorganic PSM has been shown to improve the gas adsorption properties of MOFs. An example of this, by Rosseinsky et al.[55] again looks at the exchange of coordinated water molecules in HKUST-1. The study incorporates 4- (methylamino)-pyridine (map) into the MOF resulting in a product with the formula [Cu3(btc)2(4-map)x(H2O)3−x]. It was demonstrated that the resulting MOF could adsorb NO reversibly by reacting the 4-map ligand to form N-diazenium diolates. The parent framework (HKUST-1) irreversibly binds NO into any open metal coordination sites. A schematic representation of the reaction is shown in Figure 22. Figure 22. Schematic representation of the inorganic PSM reaction of HKUST-1 with 4-map followed by adsorption of NO[55]. Another type of inorganic PSM involves changing the oxidation state of the metal in a MOF. [VO(bdc)] (MIL-47(V)) is an interesting example of a MOF that has been shown to undergo PSM reactions of this type. Fischer’s group[56] showed that by reaction with cobaltocene, [Co(η5 -C5H5)2], MIL-47(V) forms [Co(η5 -C5H5)2][VO(bdc)]2. In this reaction, the cobaltocene is oxidised to cobaltocenium and the vanadium in the MOF is in a 1:1 ratio of V(III) and V(IV). A subsequent study by Clet et al.[57] on MIL-47(V) synthesised the framework with varying ratios of V(III) and V(IV). The results from this study were that MIL-47(V) containing only V(III) produced a flexible framework isostructural to that of the MIL-53 framework whereas when only V(IV) was present, a rigid structure was produced. 2.3 Tandem PSM reactions An important concept in PSM chemistry of MOFs is that of tandem PSM reactions. These could be any of the types of PSM reactions described in the previous sections but performed sequentially. There are many reasons why tandem PSM reactions may be performed. Two common reasons are either to introduce many functionalities into a MOF or to further modify a tag that was incorporated via a PSM reaction. Wang and Cohen were the first to perform tandem PSM reactions. Using IRMOF-3 which has already been shown to be susceptible to PSM reactions, they successfully incorporated two new functionalities into the framework whilst maintaining the framework structure[58]. The study treated IRMOF-3 first with crotonic anhydride followed by acetic anhydride to produce a framework containing both groups as shown in a schematic representation in Figure 23.
- 26. 21 Figure 23. Schematicrepresentation to show IRMOF-3 (left) treated with crotonic anhydride followed by acetic anhydride (right) [58]. A follow up study by Cohen’s group extended this research on IRMOF-3 to work out the maximum number of functionalities that could be incorporated into the framework[59]. They successfully treated IRMOF-3 with four reagents sequentially; namely, decanoic anhydride, allyl isocyanate, propylisocyanate and crotonic anhydride. These tandem PSM reactions are shown in Figure 24. Figure 24. Schematic representation of the tandem PSM reactions of IRMOF-3 with decanoic anhydride, allyl isocyanate, propylisocyanate and crotonic anhydride[41]. Another type of tandem PSM reaction involves a first reaction to introduce a tag into a framework followed by a second reaction to modify the tag. There have been many examples of this type of tandem PSM that involve incorporation of a metal into the PSM tag. These reactions are of particular interest as they allow vacant metal coordination sites to be introduced into a MOF that is not on the SBU. Hupp et al. used a diol containing MOF that they termed DO-MOF for a two stage tandem PSM reaction[60]. The MOF was formed from Zn2+ centres with 1,2,4,5-tetrakis(4-carboxyphenyl) benzene (TCPB) and meso-1,2-bis- (4-pyridyl)-1,2-ethanediol (DPG) linkers and had a molecular formula of [Zn2(TCPB)(DPG)]. In the first stage of the reaction, [Zn2(TCPB)(DPG)] underwent a covalent reaction with succinic anyhydride to produce a framework containing carboxylic acid groups. The second stage of the reaction was a dative PSM that coordinated Cu2+ ions to the carboxylic acid groups. The reaction scheme is shown in Figure 25.
- 27. 22 Figure 25. Schematicrepresentation of the tandem PSM of DO-MOF (left) with succinic anhydride (middle) followed by complexation with Cu2+ ions (right). 3. Project aims As discussed in the previous sections, there has been a large amount of research into the postsynthetic modification of MOFs containing 2-amino-1,4-benzene dicarboxylate linkers (bdc-NH2). There have been numerous examples of alkylating the amino group using a variety of anhydrides across a range of frameworks. There has also been work particularly in recent years to incorporate metals into frameworks. This project aimed to continue work into modifying amino groups on carboxylate linkers in DMOF-1-NH2 with different PSM tags. The work in this project focussed on a series of two stage tandem PSM reactions. Before these reactions, the project aimed to optimise the activation of DMOF-1-NH2 to ensure successful PSM. The first stage of the tandem reactions aimed to convert the amine group on bdc-NH2 linkers into a β-keto-amide PSM tag using diketene as the reagent. The second stage of the tandem reactions aimed to chelate a metal or metal containing group between the two carbonyl groups on the ketoamide PSM tag. Another objective of the work was to use a range of techniques to characterise the MOFs. These techniques included X-ray crystallography for structural elucidation of the MOFs, 1 H NMR spectroscopy for identification and conversion rates of linkers and EDX for determining the surface composition of the products. Figure 26 shows the reaction scheme for the desired postsynthetic modification reactions. Figure 26. Reaction scheme for the proposed postsynthetic modification reactions of the bdc linker in DMOF-1-NH2.
- 28. 23 4. Experimental 4.1 Synthesesand activation procedures 4.1.1 DMOF-1-NH2 – Compound 1 The method for synthesising the MOF structure has been adapted from a well established literature procedure[24]. Zn(NO3)2·6H2O (0.149 g), dabco (0.028 g) and H2-bdc-NH2 (0.091 g) were placed in glass microwave vials with 8 ml of DMF which were then sealed. The vials were sonicated to fully dissolve reagents and heated at 120 °C for three days. The resulting crystals were washed with three 5 ml washings of DMF. Samples were stored in sealed vials under an inert nitrogen environment in 8 ml DMF until activation. 4.1.2 Diketene PSM 0.2 ml of diketene was added to samples of compound 1 stored in 8 ml DMF in sealed vials. After three days and under a flow of nitrogen, the solvent was removed and replaced with fresh dry DMF until samples were needed for analysis. 4.1.3 Diketene PSM after activation in cyclohexane DMF was removed from the vials containing compound 1. This was replaced with 5 ml of cyclohexane. Samples in sealed vials were heated at 50°C in an oil bath for one week. The solvent was then removed from the vials and replaced with 5 ml of dry THF. This was followed by the addition of 0.2 ml diketene. After three days, the mother liquor was removed and replaced with 5 ml dry THF. Samples were stored under THF until analysis of them. 4.1.4 Diketene PSM after activation in diethyl ether– Compound 2 DMF was removed from vials storing compound 1 whilst maintaining an inert environment with nitrogen. 5 ml of dry Et2O was injected into the vials through a septum. For the subsequent two days, Et2O was removed and replaces with fresh 5 ml portions all of which was carried out under an inert nitrogen environment. After three washings with dry Et2O, the diethyl ether was removed from the vials and replaced with 5 ml dry THF. Whilst still under a flow of nitrogen, 0.2 ml diketene was injected into the vials containing sample 1 and left for three days. After three days, the mother liquor in the vials was removed and replaced with 5 ml fresh dry THF, with the flow of nitrogen being maintained throughout. Compound 2 was stored in dry THF in the sealed vials until analysis. 4.1.5 KOtBu PSM – Compound 3 Solid KOt Bu was added in excess to compound 2 stored in 5 ml dry THF. Upon sealing the vial, it was purged with a flow of nitrogen to maintain an inert atmosphere. Three days after the addition of KOt Bu, the mother liquor in the vial was removed using a syringe. Following this, 5 ml dry THF was injected into the vial containing compound 3 where it was stored until analysis. 4.1.6 ZnEt2 PSM – Compound 4 0.2 ml of ZnEt2 (1 M solution in hexane) was injected into samples of compound 2 stored in sealed vials in 5 ml of dry THF. This was carried out under a flow of nitrogen. After three days, the mother liquor was removed. The samples of compound 4 were then injected with 5 ml dry THF under a flow of nitrogen where they were stored in sealed vials until analysis.
- 29. 24 4.1.7 [Rh2(CO)4(μ-Cl)2] PSM– Compound 5 A stock solution of [Rh2(CO)4(μ-Cl)2] in dry THF (0.250 g in 5 ml) was made. Samples of compound 2 stored under 5 ml dry THF were injected with 1 ml of the stock solution. After three days, the mother liquor was removed and replaced with a 5 ml portion of fresh dry THF. 4.2 Analytical procedures 4.2.1 Powder X-ray diffraction (PXRD) For PXRD analysis, samples were washed with THF and air dried. Samples were then crushed onto grease coated slides ready for data collection on a spinning plate. All PXRD data was collected on a Bruker D8 diffractometer. Data was normalised using the program EVA before being exported to and plotted in MS Excel 2013. 4.2.2 Single crystal X-ray diffraction (XRD) Samples were not dried before being suspended in crystallographic oil on a glass slide. Single crystal XRD data was collected on an Agilent Supernova diffractometer. Structures were solved using SHELXS and refined using SHELXL. Structure pictures were created using the programs X-seed and POV-ray. 4.2.3 1H NMR Roughly 5 mg of samples were digested in 0.2 ml trifluoroacetic acid (TFA) which was subsequently evaporated using a rotary evaporator. Samples were then dissolved in 0.7 ml DMSO for data collection. Spectra were recorded on a Bruker Avance 300 MHz spectrometer at 298 K. 4.2.4 Electron dispersive X-ray spectroscopy (EDX) Samples were washed with THF and air dried prior to being fixed to sample holders with adhesive tape and stored under vacuum overnight. 5. Synthesis of a MOF for PSM reactions 5.1 Synthesis of DMOF-1-NH2 (compound 1) The first stage of this project was to synthesise a MOF for performing PSM reactions on. The zinc based MOF, DMOF-1-NH2 (compound 1) with the formula [Zn(bdc- NH2)2(dabco)] was chosen as there has already been a significant amount of work on the framework that shows it is a suitable candidate for PSM reactions. This is discussed in section 3 of this report. DMOF-1-NH2 is formed using Zn(NO3)2·6H2O as a source of metal. The organic linker precursors that form DMOF-1-NH2 are 2-amino-1,4- benzenedicarboxylic acid (H2-bdc-NH2) and 1,4-diazabicyclo[2.2.2]octane (dabco). The exact synthesis is adapted from a literature preparation and can be found in section 4.1.1. Figure 27 shows the reaction scheme for the formation of DMOF-1-NH2.
- 30. 25 Figure 27. Reactionscheme for the synthesis of DMOF-1-NH2 The reaction resulted in the formation of pale yellow-brown needle like crystals. To confirm that the correct product was produced, samples were analysed using PXRD and 1 H NMR spectroscopy. 5.2 PXRD analysis of compound 1 The experimental PXRD pattern of compound 1 and a calculated pattern from a previously reported crystal structure[61] are shown in Figure 28. The patterns closely match one another so it is reasonable to assume that the product of the reaction was compound 1 (DMOF-1-NH2). The experimental pattern is shifted to slightly higher angles than the calculated powder pattern. This is likely suggesting a change in unit cell parameters due to the fact that the calculated pattern is taken from single crystal data collected at 150K whereas the experimental powder pattern was collected at 298K. Figure 28. Calculated PXRD pattern for DMOF-1-NH2 vs. experimental pattern of compound 1 confirming that compound 1 is DMOF-1-NH2. 0 5000 10000 15000 20000 25000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Calculated DMOF-1-NH2 Compound 1
- 31. 26 5.3 1H NMRanalysis of compound 1 To further confirm that the product of the reaction was compound 1, a proton NMR spectrum was collected following digestion in TFA and is shown in Figure 29. Figure 29. 1 H NMR spectrum of compound 1 (DMOF-1-NH2). The peaks in red boxes can be assigned to DMF that remained in the pores after synthesis. Doublets at 7.05 ppm and 7.75 ppm are assigned to the two aromatic protons on the opposite side of the phenyl ring to the amine group. The remaining aromatic proton on the phenyl ring corresponds to the peak at 7.40 ppm. The other ligand expected in the product was dabco. A singlet would be expected for dabco and according to the literature this should appear ~2.8 ppm[62]. Thus it was suggested that the peak in the spectrum at this chemical shift assigned to DMF, overlapped with the peak for the dabco ligand. The 1 H NMR spectrum combined with the PXRD pattern confirms that DMOF-1- NH2 (compound 1) can be reliably reproduced and that a MOF of known composition had been produced for use in PSM reactions. 6. Treatment of compound 1 with diketene The second stage of this project was incorporating a PSM tag to the amino groups on the bdc-NH2 ligands in compound 1. As discussed in section 2.2.1, amine groups are a commonly used functionality for covalent PSM reactions which is one of the reasons that compound 1 was chosen for this work. Diketene (C4H4O2) was chosen as a reagent as it was hoped to produce a β-amidoketone group analogous to an acac ligand (with the shorthand notation of ‘dkn’) on the phenyl ring of bdc linkers in compound 1. The proposed reaction scheme for this reaction is shown in Figure 30 with the product having the molecular formula [Zn2(bdc-dkn)2(dabco)] assuming the PSM reaction goes to completion. b a c
- 32. 27 Figure 30. Reactionscheme for the PSM of compound 1 using diketene to produce compound 2. Only the bdc section of the frameworks is shown. Samples of compound 1 were treated with diketene with the experimental procedure detailed in section 4.1.2 Visual inspection of the resulting product of this reaction showed some white amorphous looking product surrounded by very fine needle like crystals with a murky grey colour. This suggested that perhaps the proposed product in Figure 30 had not been produced. To confirm this, a PXRD pattern was collected on the product of the reaction and compared to the pattern for compound 1. These are shown in Figure 31. Figure 31. Comparison of PXRD patterns for compound 1 and the product of the reaction of compound 1 with diketene. Figure 31 clearly shows that the product of the reaction is not compound 1. Because it is very different, it is enough to suggest that the framework from compound 1 had not been maintained. This is because none of the peaks from the pattern of compounds 1 are common to the pattern for the product of the reaction (shown in grey). There are also a number of extra peaks in the pattern of the product which further support the fact that it is not the proposed product containing the ketoamide tag. 0 2000 4000 6000 8000 10000 12000 14000 16000 18000 20000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Compound 1 Compound 1 treated with diketene
- 33. 28 7. Activation ofCompound 1 Compound 1 was activated prior to being treated with diketene. This was necessary because as discussed in section 6, the proposed ketoamide tagged structure did not form upon treatment of compound 1 with diketene. It was suggested that this could have been due to solvent remaining in the pores of compound 1. In this case, DMF which has a high boiling point (152 °C) and low volatility was the solvent used in the synthesis of compound 1 and has been shown to remain in the pores of MOFs [26]. This could prevent the diketene being able to access the inside of the pores and thus prevent it forming the desired product. In an effort to remove the DMF from compound 1, two activation procedures were tested. 7.1 Activation of compound 1 in cyclohexane Samples of compound 1 had their solvent exchanged from DMF to cyclohexane and were heated to 50 °C in sealed vials. This procedure is based on those previously reported for Fujita crystalline flask materials [63]. This procedure was followed over time using PXRD to see how long compound 1 would remain stable. This was because it was thought that the longer the activation procedure was, the greater amount of DMF that would be removed from the pores of compound 1 and hence increase the chances of the amino groups being converted to the ketoamide PSM tag. Figure 32 shows the PXRD patterns for compound 1 activated in cyclohexane over time. Figure 32. Powder patterns of compound 1 activated with cyclohexane at 50 °C over a period of two weeks. 0 20000 40000 60000 80000 100000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° 1 week 2 weeks 0 days 1 day 2 days 3 days
- 34. 29 The powder X-raydiffraction pattern immediately after the exchange of DMF for cyclohexane (0 days) is consistent with that of compound 1 (Figure 18), so we can be sure that addition of cyclohexane doesn’t affect the framework. The patterns collected at 1 day, 2 days, 3 days and 1 week after exchange with cyclohexane also remain consistent with the pattern for compound 1. As such, we can be sure that compound 1 is stable in cyclohexane at 50 °C for up to one week. However, the pattern at two weeks is very different to the others. The peaks at ~9° and ~17° are shifted to higher angles and the peaks between these angles no longer appear in the pattern. This shows that at some point between one and two weeks in cyclohexane, compound 1 degrades so it would not be suitable for treatment with diketene. From this data, it was decided that the optimum time for activation prior to treating compound 1 with diketene using this procedure was one week. 7.2 Activation of compound 1 in dry diethyl ether The second method of activating compound 1 was by performing a solvent exchange with dry diethyl ether at room temperature. Diethyl ether was chosen as a replacement solvent because it is much more volatile and hence will leave the pores of the framework in compound 1 more easily than DMF. Diethyl ether is also soluble in DMF which also suggests that a solvent exchange could occur. The solvent exchange was followed using thermogravimetric analysis (TGA). This technique measures the weight loss of a sample as a function of temperature. Figure 33 shows the TGA plot for compound 1 before solvent exchange has occurred (DMF in pores). Figure 33. TGA plot for compound 1 before solvent exchange. Carried out under a flow of argon gas. Figure 33 shows two significant periods of mass loss in a narrow temperature window. The first of which is between ~130 °C to ~190 °C. This mass loss can probably be attributed to the loss of DMF from the sample as the boiling point of DMF (152 °C) falls within this region. This confirms that DMF does indeed remain in compound 1 after it has been synthesised. The second mass loss at ~250 °C is likely due to the thermal decomposition of the framework structure. A second TGA plot was collected after the solvent had been exchanged for dry diethyl ether and is shown in Figure 34. 3.50 4.50 5.50 6.50 7.50 8.50 9.50 30 80 130 180 230 280 330 380 430 480 Massofsample/mg Furnace temperature / °C
- 35. 30 Figure 34. TGAplot of compound 1 after solvent exchange with dry diethyl ether. Carried out under a flow of argon gas. Detection of sharp mass loss due to diethyl ether was difficult to do with a great degree of accuracy because TGA is performed under a flow of argon to prevent oxides forming upon degradation of sample. This causes most of the diethyl ether to evaporate before heating commences. However, the first sharp mass loss at ~85 °C can probably be attributed to the loss of diethyl ether. Similarly to Figure 29, the second sharp weight loss at ~250 °C is due to the thermal decomposition of compound 1. Importantly though, there is no sharp mass loss around the boiling point of DMF in Figure 33 which suggests that DMF was successfully exchanged for diethyl ether as expected. 8. Treatment of compound 1 with diketene after activation 8.1 Compound 1 activated in cyclohexane After activation in cyclohexane for one week, cyclohexane was swapped with dry THF as diketene isn’t soluble in cylcohexane. Subsequently, 0.2 ml diketene was added to compound 1 and left to react for three days at room temperature. Full experimental details are given in section 4. This reaction resulted in the formation of brown-orange crystals that were less needle like than those found in compound 1. Powder X-ray analysis was performed on the product of the reaction with the pattern compared to that of compound 1 in Figure 35. 1.60 2.10 2.60 3.10 3.60 4.10 4.60 5.10 30 80 130 180 230 280 330 380 430 480 Massofsample/mg Furnace temperature / °C
- 36. 31 Figure 35. PXRDpatterns of compound 1 (blue) and the product of the reaction of compound 1 activated in cyclohexane and treated with diketene (orange). It is clear that the patterns do not match and as there are few similarities between them, the product of the reaction is unlikely to be compound 2. However, some of the peak positions from compound 1 are found in the product at ~8.3°, 18.2° and ~25°. This could be an indication that a compound with a similar framework structure to that of compound 1 remained in the product of the reaction. Overall, it was not clear that compound 2 had been formed, nor that the product was a mixture of compound 1 and something else. Thus, it was concluded that activation of compound 1 in cyclohexane was not a suitable method to produce the ketoamide analogue of compound 1. As such, a second activation procedure using diethyl ether was attempted. 8.2 Compound 1 activated in dry diethyl ether Compound 1 that had been activated in dry diethyl ether was treated with diketene with the experimental procedure detailed in section 4. The product of the reaction was designated to be compound 2. The crystals of compound 2 had changed from those of compound 1 to a more yellow shade. However, there also appeared to be a second type of crystal present in the product that had a darker orange-brown colour. The crystals were analysed using PXRD, 1 H NMR spectroscopy, single crystal XRD and elemental analysis. 0 20000 40000 60000 80000 100000 120000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Compound 1 Compound 1 treated with diketene
- 37. 32 8.2.1 PXRD analysisof compound 2 The PXRD pattern of compound 2 is compared to the pattern for compound 1 in Figure 36. There are some subtle differences between the patterns. The two strong peaks at ~8° and ~16° are shifted to slightly higher angles and there appears to be a shoulder developing on the peak at ~16° for the sample treated with diketene. The relative intensities of some of the peaks appear to have reduced for the sample treated with diketene. These difference is the PXRD patterns are an indication that a reaction had taken place. Figure 34. PXRD patterns of compound 1 (blue) and compound 1 treated with diketene (blue). 8.2.2 1H NMR analysis of compound 2 A 1 H NMR spectrum of compound 2 digested in TFA was collected in order to identify the organic linkers present in compound 2. It was also used to calculate the degree of conversion from the bdc-NH2 group in compound 1 to the bdc-dkn PSM tag in compound 2. A section of the spectrum associated with the bdc ligands is shown in Figure 37. Figure 37. Section of the 1 H NMR spectrum of the product of the reaction of compound 1 treated with diketene. The individual peaks are assigned by the letters above them. 0 20000 40000 60000 80000 100000 120000 5 15 25 35 45 55 Intensity/arb.units 2θ / °Compound 1 Compound 1 treated with diketene b c b a e b f b d b
- 38. 33 The spectrum showsthat the ketoamide tagged MOF had been formed during the reaction. Peaks at 9.00 ppm, 8.04 ppm and 7.68 ppm can be assigned to protons on the bdc ligand containing the PSM tag. However, the spectrum also shows that some of the unreacted bdc-NH2 linker remained in the product. This is shown by the presence of peaks assigned to the bdc-NH2 linker at 7.76 ppm, 7.38 ppm and 7.02 ppm. An estimated conversion rate from bdc-NH2 to bdc-dkn can be calculated using the ratio of each species respective peak integrals. The two doublets at 7.68 ppm and 7.76 ppm for example have a ratio of 2.37:1.06. This equates to ~69% conversion to the bdc-dkn ligand. This gives compound 2 an average molecular formula of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] rather than [Zn2(bdc-dkn)2(dabco)] in the proposed product structure. Singlets at 7.95 ppm, 2.90 ppm and 2.47 ppm represent DMF which was probably trapped in the pores of the framework prior to digestion for 1 H NMR analysis. Singlets at 3.70 ppm and 3.15 ppm can be attributed to the CH2 (‘keto’) and CH (‘enol’) protons of the ketoamide tag and at 2.70 ppm and 2.07 ppm for the terminal methyl protons. The reason each of these sets of protons have two resonances is due to keto-enol tautomerism. The ketoamide tag can exist in either the ketone or enol form as shown in Figure 38 which puts the protons in different environments hence causing the different shifts. Figure 38. Schematic representation showing the tautomers of the diketene PSM tag when coordinated to the bdc linker section of compound 2. 8.2.3 Elemental analysis of compound 2 Compound 2 shows 69% conversion to the bdc-dkn ligand and has an expected composition of: C – 45.94%, H – 3.86%, N – 7.91%. The empirical percentages were measured to be: C – 47.02%, H – 4.09%, N – 7.42%. The difference in percentages is likely due to left over solvent in the in the sample but they are similar enough to suggest that compound 2 contained the ketoamide tag proposed in Figure 28 8.2.4 Single crystal XRD analysis Single crystal data were collected on compound 2 in an effort to obtain conclusive structural evidence that the ketoamide tag was incorporated into the MOF. The crystal description of compound 2 is given below. Crystal structure description of compound 2 Pale yellow needle like crystals of compound 2 with a calculated molecular formula of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] were analysed using single crystal X-ray diffraction, with the crystal data summarised in Table 3.
- 39. 34 Table 3. Crystaldata for [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] Compound number 2 Empirical formula C13.64H13.64N2O5.32Zn Formula weight 356.07 Temperature / K 150.00(10) Crystal system monoclinic Space group I2/a a/Å 19.2194(8) b/Å 14.7996(9) c/Å 15.4061(8) α/° 90 β/° 99.640(4) γ/° 90 Volume/Å3 4320.2(4) Z 8 ρcalcg/cm3 1.095 Crystal size/mm3 0.131 × 0.099 × 0.075 Radiation Cu Kα (λ = 1.54184/Å) 2θ range for data collection/° 7.58 to 146.632 Final R indexes [I>=2σ (I)] R1 = 0.0914, wR2 = 0.2772 Final R indexes [all data] R1 = 0.1164, wR2 = 0.3005 The asymmetric unit of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] consists of a Zn(II) centre connected to one dabco ligand and either one 2-amino-1,4-benzenedicarboxylate ligand or one 2-amino-1,4-benzenedicarboxylate ligand containing the ketoamide PSM tag (bdc- dkn). The dabco carbon atoms exhibited disorder over two positions with a 50:50 ratio. The –NH2 and –dkn groups on the bdc ligands are disordered over two positions on the phenyl ring with a 50:50 ratio. Both the bdc-NH2 and bdc-dkn ligands are present in the molecular formula as conversion from the –NH2 to the –dkn group was shown to be ~69% by 1 H NMR. The site occupancy of the bdc nitrogen atoms is 0.5 as they are common to both the –NH2 and –dkn groups. The ketoamide tags appear to be almost parallel to the phenyl rings of the bdc ligands but protrude slightly into the centre of the pore. Electron density for the ketoamide PSM tag becomes increasingly diffuse the further away from the phenyl ring core. This results in the final C(O)CH3 group of the tag not being seen in the crystal structure. The crystal structure of the asymmetric unit is shown in Figure 39.
- 40. 35 Figure 39. Partof the Crystal structure of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] with unprimed labelled atoms forming the asymmetric unit. All other atoms including those denoted by a primed label are generated by symmetry. Zn1’ related to Zn1 by the symmetry operation -x, 1-y,-z; O2’ related to O2 by the symmetry operation -x, 1-y,-z; O3’ related to O3 by the symmetry operator -x, y-½, ½-z; and O4’ related to O4 by the symmetry operation x, 3/2-y, z-1/2. All Hydrogen atoms have been omitted for clarity. The structure shows that the zinc adopts a square pyramidal geometry with four carboxylate oxygens forming the base plane; and the dabco ligand coordinated axially. Cis O-Zn-O bond angles between the bdc carboxylate groups are in the range of 87.3(2)° to 89.2(2)°. N-Zn-O bond angles between the dabco and bdc ligands are in the range of 97.12(18)° to 103.90(17)°. Zn1-O bond distances to the carboxylate oxygen atoms are all very similar and within the range of 2.032(4) Å to 2.045(4) Å and the Zn1-N1 distance is 2.068(4) Å. The Zn1-Zn1’ distance is 2.9662(11) Å which is significantly longer than the value for metallic zinc (2.48 Å). This coupled with the fact that Zn(II) is a d10 metal ion means that a Zn-Zn bond is not likely to be present. The asymmetric units link together to form a paddlewheel structure which forms square grids that are connected in a third dimension via the dabco ligands. Figure 40 shows the square pores of the structure with the edges of the square grids curving alternately inwards and outwards along the pore axis.
- 41. 36 Figure 40. Asection of the framework structure viewed along the crystallographic a axis showing the square pores with the edges curving in alternate directions. Hydrogen atoms are omitted for clarity. There is significant pore space in the framework as shown by the space filling model in Figure 41. Residual electron density in the framework was assessed using the PLATON SQUEEZE algorithm. It was concluded that the residual electron density in the structure pertained to the missing PSM tag atoms rather than any guest solvent molecules. Figure 41. Space filling model of the [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] framework viewed along the crystallographic a axis showing significant pore space. The single crystal structure shows that compound 2 contained the ketoamide PSM tag; albeit with only 69% conversion from compound 1. A possible reason for incomplete conversion to the PSM tag could be due to DMF remaining in the pore even after solvent exchange with diethyl ether which may have prevented diketene accessing some amine groups. Work carried out on this system by Dr William Gee showed conversion from compound 1 to the bdc-dkn PSM tag at stoichiometric levels (as assessed by 1 H NMR spectroscopy). It was observed that the crystal structure showed a slightly different
- 42. 37 structure. All exceptthe last carbon of the ketoamide PSM tag was found crystallographically which is more than was found in the crystal data for compound 2. Figure 42 shows the asymmetric unit of the structure with complete conversion to bdc-dkn. Figure 42. Part of the crystal structure of [Zn2(bdc-dkn)2(dabco)] produced by Dr William Gee. All unlabelled atoms have been generated by symmetry and hydrogen atoms are omitted for clarity. The same general structure to that of compound 2 was observed. However, the ketoamide PSM tags appear to be parallel to the phenyl rings whereas in compound 2, they are slightly offset from the plane of the ring. The entire ketoamide tag was solved crystallographically; and as with compound 2, the electron density gets more diffuse the further away from the phenyl ring core it gets. This structure has a space group of C2/m which is different to that of compound 2. This suggests that there is some flexibility in the framework as the change in space group shows a change in symmetry and unit cell parameters. The unit cell parameters for the structure in Figure 42 are given in Table 4. Table 4. Unit cell parameters for [Zn2(bdc-dkn)2(dabco)] a / Å 14.298 b / Å 16.325 c / Å 9.557 α / ° 90 β / ° 100.7 γ / ° 90 Volume / Å3 2191.93 The unit cell parameters are significantly different to those for compound 2 which confirms that there has been a change in structure. The structure is viewed along the crystallographic-c axis in Figure 40 and can be compared to the structure in Figure 43.
- 43. 38 Figure 43. Sectionof the [Zn2(bdc-dkn)2(dabco)] framework viewed along the crystallographic-c axis. Hydrogen atoms have been omitted for clarity. Figure 43 shows that the ketoamide tags protrude into the pore cavities which didn’t appear to occur to such an obvious extent in compound 2. Unlike compound 2, the edges of the pores don’t curve alternately inwards and outwards along the pore axis. The space filled view of this framework is displayed in Figure 44. Figure 44. Section of the [Zn2(bdc-dkn)2(dabco)] framework viewed along the crystallographic-c axis. Hydrogen atoms have been omitted for clarity.
- 44. 39 It can beseen that the framework has some pore space, although it appears to be less than in Figure 41. This is because the entire ketoamide tag has been solved crystallographically and they protrude further into the pores in this framework. It can be concluded from the crystal structures that differing amounts of conversion of the bdc-NH2 linker to the bdc-dkn linker influence the structure of the framework. Single crystal X-ray diffraction analysis was also performed on the brown-orange block like crystals found in the product. They were found to have the formula [Zn(bdc-NH2)(DMF)] and the full crystal description is given below. [Zn(bdc-NH2)(DMF)] crystal structure Large pale orange crystals of [Zn(bdc-NH2)(DMF)] (compound 1a) were analysed using single crystal X-ray diffraction, with the crystal data summarised in Table 5. Table 5. Crystal data for [Zn(bdc-NH2)(DMF)] Compound number 1a Empirical formula C11H12N2O5Zn Formula weigh 317.60 Temperature/K 150.00(10) Crystal system monoclinic Space group P21/c a/Å 7.9963(2) b/Å 15.0643(3) c/Å 11.0454(3) α/° 90 β/° 111.160(3) γ/° 90 Volume/Å3 1240.80(7) Z 4 ρcalcg/cm3 1.700 Crystal size/mm3 0.102 × 0.092 × 0.074 Radiation CuKα (λ = 1.54184 / Å) 2θ range for data collection/° 10.404 to 146.594 Final R indexes [I>=2σ (I)] R1 = 0.0451, wR2 = 0.1138 Final R indexes [all data] R1 = 0.0535, wR2 = 0.1202
- 45. 40 The asymmetric unitof [Zn(bdc-NH2)(DMF)] consists of a Zn(II) centre connected to one 2-amino-1,4-benzenedicarboxylate ligand and one DMF ligand. The DMF is disordered over two positions with all atoms except O5 in a ratio of 70:30. The amino group on the dicarboxylate ligand has N1:N1A disorder with a ratio of 55:45. The structure of the asymmetric unit is shown in Figure 45. Figure 45. Crystal structure of [Zn(bdc-NH2)(DMF)] with atoms labelled without a prime from the asymmetric unit. All other atoms including those denoted with a prime are generated by symmetry. Zn1’ related to Zn1 by the symmetry operation -x, 1-y,-z, O2’ related to O2 by the symmetry operation -x, 1-y,-z, O3’ related to O3 by the symmetry operator 1-x, ½+y, ½-z and O4’ related to O4 by the symmetry operation x-1, ½-y, z-1/2. All Hydrogen atoms have been omitted for clarity. The crystal structure shows that zinc centre adopts a square pyramidal geometry with the four oxygens atoms of the carboxylate ligands forming the base plane and the DMF ligand coordinated axially. Cis-O-Zn-O angles are in the range of 87.52(15)° to 88.54(14)° and in the range of 100.64(14)° and 102.03(13) ° between DMF and bdc-NH2 ligands. The Zn1-O1 distance to the NH2-bdc ligand is slightly longer at 2.008(2) Å than the Zn1-O5 distance of 1.982(3) Å. The Zn1-Zn1’ distance is 2.9339(8) Å which is significantly longer than the value for metallic zinc (2.48 Å). As such, a Zn-Zn bond is not likely in this structure, which is to be expected for a d10 metal ion such as Zn(II).
- 46. 41 The asymmetric unitslink together to form a paddlewheel structure which forms two-dimensional sheets with square channels as shown in Figure 46. Figure 46. Two-dimensional sheet of [Zn(bdc-NH2)(DMF)] viewed along the crystallographic a axis. The DMF ligands act as capping groups and prevent the sheet structure growing in a third dimension. A small section of the framework shows this in Figure 47. Figure 47. A section of a sheet in the framework of [Zn(NH2-bdc)(DMF)] showing the DMF ligands acting as capping groups.
- 47. 42 The two-dimensional sheetspack in a staggered fashion with each sheet being slightly offset relative to the one above and the one below it. In effect, the sheets stack in an efficient manner such that the DMF ligands interdigitate with one another. This is illustrated in Figure 48. Figure 48. Two one dimensional sections of [Zn(bdc-NH2)(DMF)] showing that stacking occurs in a staggered fashion with due to the protruding DMF ligands. The staggered sheets of [Zn(bdc-NH2)(DMF)] result in a non-porous structure which is clearly illustrated by a space filling model of the atoms. This is shown in Figure 49 and it can be clearly seen that there is minimal pore space between the two-dimensional sheets of the structure as the square channels start to become occupied by the sheets above and below them. Figure 49. Space filling model of stacked sheets of [Zn(bdc-NH2)(DMF)] shown along the a-crystallographic axis, showing lack of void space in the square channels of the two-dimensional sheets.
- 48. 43 It was suggestedthat [Zn(bdc-NH2)(DMF)] was essentially a decomposition product of the DMOF-1-NH2 framework and was likely formed upon treatment of compound 1 with diketene as it didn’t appear to be present in the samples of compound 1 that were produced for this study. 8.4 Summary of reactions with diketene In conclusion, it was found that of the three reactions of compound 1 with diketene; activation in dry diethyl ether followed by treatment with diketene was the best route to produce compound 2 containing the –dkn PSM tag. Primarly, the crystal structure of compound 2 shows structurally that compound 1 was probably successfully postsynthetically modified to incorporate the ketoamide PSM tag. Although, as part of the –dkn tag could not be resolved in the crystal structure, the PXRD and 1 H NMR spectroscopy data were used to support the conclusion that it was produced. However, the product of the reaction appeared to be contain a mixture of crystal morphologies, and a second phase was identified (compound 1a). The structure of the second phase was confirmed by single crystal XRD analysis and was suggested to be a decomposition product that formed upon treatment of compound 1 with diketene. After the identification of compound 1a, it was postulated that the products of the reactions of compound 1 with diketene following activation in cyclohexane or without activation were likely to be compound 1a. To confirm this, the calculated PXRD pattern of compound 1a was compared those of the products of the reactions mentioned. This is shown in Figure 50. Figure 50. PXRD patterns of compound 1a, and those of compound 1 treated with diketene after activation in cyclohexane and without activation. It can be seen that the blue and orange patterns share a large number of peaks. This suggests that the product of the reaction of compound 1 treated with diketene after activation in cyclohexane was likely to have contained some compound 1a alongside the compound 2 produced. 0 5000 10000 15000 20000 25000 30000 35000 5 15 25 35 45 55 intensity/arb.units 2θ / ° Calculated compound 1a Compound 1 activated in cyclohexane treated with diketene Compound 1 treated with diketene without activation
- 49. 44 9. PSMetallation ofcompound 2 using KOt Bu The final stage of this project aimed to chelate a metal between the two carbonyl groups of the ketoamide PSM tag. It was hoped that by doing so, the products would be suited for applications in catalysis. There have already been several examples of coordinating metals to MOFs that show catalytic activity[38]. The first metal tried for this project was potassium. KOt Bu was added in excess to a sample of compound 2 (with full details given in section 4) and was chosen as a reagent for the reaction for a number of reasons. As a basic reagent, it would deprotonate the central carbon atom of the –dkn tag to make it more susceptible to chelation with a K+ ion. Also, potassium is a small metal with an atomic radius of just 2.27 Å. Theoretically, this makes it small enough to enter the pores of the DMOF-1-NH2 framework which has a pore aperture of approximately 7.5×7.5 Å2 [24] and hence it should be able to react with all available –dkn tags. The target product of the reaction (compound 3) contains a modified PSM tag with the notation –dknK and the proposed scheme for this reaction is shown in Figure 51. Figure 51. Proposed reaction scheme for compound 2 with KOt Bu to form compound 3 with the molecular formula [Zn2(dabco)(bdc-dknK)2]. Only the bdc section of the frameworks is shown. Visual inspection of the product under an optical microscope showed that the product of the reaction contained some light yellow needle like crystals similar to those of compound 2. There were also some brown-orange crystals that were not needle like which were expected to be compound 1a due to their very similar appearance. 9.1 PXRD analysis of compound 3 PXRD analysis was performed on the product of the reaction shown in Figure 52. The pattern is compared to that of compound 1 and compound 2.
- 50. 45 Figure 52. PXRDpatterns for compound 1, compound 2 and compound 2 treated with KOt Bu Although similar, there are some subtle differences in the patterns. For example, the pattern for compound 2 when treated with KOt Bu shows peak broadening at ~8° and ~17°. Arguably, the peak at ~17° shows signs of peak splitting which could indicate the formation of a new phase. However, the peak intensities appear to be reduced compared to the patterns of compounds 1 and 2. This could indicate a reduction in crystallinity which may be due to the KOt Bu degrading the crystals. 9.2 1H NMR analysis of compound 3 A proton NMR spectrum was collected on compound 3. When digested in acid, it is likely that any metal chelated as proposed in compound 3 will be removed. Therefore it was expected that the splitting pattern would be the same as that of compound 2. However, the spectrum allowed us to see whether the percentage of compound 2 in the product remains at 69% as discussed in section 8.2.2. This would show whether treatment with KOt Bu caused degradation of the MOF. A section of the spectrum is shown in Figure 53. Figure 53. Section of the 1 H NMR spectrum of the product of compound 2 treated with KOt Bu. The spectrum shows the same splitting pattern to that of compound 2 (Figure 34) which confirms that the proportion of the ketoamide tag was the same before and after the 0 20000 40000 60000 80000 100000 120000 140000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Compound 1 Compound 2 Compound 2 treated with KOtBu
- 51. 46 reaction with KOt Bu.Using the ratios of peak integrals 0.44:1.07, a conversion of ~70% was observed. This is consistent with that of the spectrum of compound 2 which shows that treatment of compound 2 with KOt Bu did not appear to affect the diketene PSM tag. 9.3 Single crystal XRD analysis Single crystal X-ray diffraction data were collected on the product of the reaction to assess whether potassium was incorporated to produce compound 3. The sample produced crystals with the parameters shown in Table 6. Table 6. Unit cell parameters for the product of reaction of compound 2 treated with KOt Bu. a / Å 19.1720(8) b / Å 14.7454(12) c / Å 15.4064(11) α / Å 90.0 β / Å 100.047(5) λ / Å 90.0 These parameters are similar to those of compound 2. Upon initial inspection of the structure, neither the diketene tag nor the potassium chelated between the carbonyl groups was shown which implies that the crystal analysed may have been unreacted compound 2. It is also worth bearing in mind that potassium may have been incorporated into the product but as such a light element, it has very little electron density and any diffraction from it may not be visible. As the results from single crystal XRD data were inconclusive, further analysis of the product of the product of the reaction was performed to try and identify potassium in the sample. It is also worth noting that, XRD screening experiments on some of the crystals of product found unit cells concurrent with that of compound 1a which showed that it was present in the product as expected from visual inspection. 9.4 Elemental analysis Elemental analysis was performed on the product of the reaction. Assuming that compound 2 used for the reaction contained only –dkn tags on 69% of the possible sites as confirmed by proton NMR (section 8.2.2), the expected composition for compound 3 was: C – 42.99%, H – 3.52%, N – 7.46%. The measured composition was: C – 39.94%, H – 3.72%, N – 7.92%. The hydrogen and nitrogen percentages are very close to one another, however, there is a significantly lower carbon percentage in the measured composition. As with compound 2, the difference in composition may be due to solvent trapped inside the pores of the framework. It could also be due to unreacted KOt Bu or compound 1a in the sample used for the analysis. Obviously, the limitation with elemental analysis is that it only provides carbon, hydrogen and nitrogen percentages. Therefore, any conclusions drawn about the structure of the product are merely indicative.
- 52. 47 9.5 SEM/EDX analysis SEMand EDX data was collected on the product of compound 2 treated with KOt Bu. EDX analysis was used to identify the elements present on the surface of the product of the reaction. SEM was used to visually compare the morphologies of the crystals of compound 2 and the product of compound 2 treated with KOt Bu. SEM images were collected at 10 kV in a high vacuum of 10 Pa. Figure 54 shows crystals of compound 2 at 190× magnification. Figure 54. SEM image of compound 2 crystals coated in gold at 190× magnification. The crystals of compound 2 are long oblong shaped crystals that are fairly uniform in size. The cracks on the surface of the crystals is due to the drying and evacuation of samples before being placed into the scanning electron microscope. Figure 55 is an SEM image of compound 3 at 190× magnification. Figure 55. SEM image of compound 2 treated with KOt Bu at 190× magnification.
- 53. 48 There are somemarked differences in the appearance of the crystals in the above figure. There appears to be several substances in the product. The long oblong crystals are expected to be compound 3 assuming that a reaction had occurred. Because treatment with KOt Bu is a PSM reaction, the shape of the crystals wouldn’t be expected to change. The spheres shown in Figure 50 are an unexpected by-product. It was suggested that they formed when the sample was evacuated overnight prior to SEM analysis as they were not observed under an optical microscope beforehand. Figure 56 shows what appears to be the spheres protruding from within the crystals and remained unidentified. Figure 56. SEM image boundary between oblong crystals and ‘amorphous’ spheres of compound 2 treated with KOt Bu at 2500× magnification. The EDX measurements taken on compound 2 were used as a reference for the product of the reaction. Spectra were taken at a range of locations in the sample to gain a representative view of the composition of the product as a whole. The EDX spectrum for compound 2 is shown in Figure 57. Figure 57. EDX spectrum of compound 2.
- 54. 49 The spectrum showsthe presence of zinc, carbon, nitrogen and oxygen on the surface of the crystal. These are all expected elements in compound 2. Hydrogen is not observed as hydrogen peaks tend to get buried under the carbon peaks very close to 0 keV. The EDX spectrum for the oblong crystals in the product of the reaction is given in Figure 58. Figure 58. EDX spectrum of compound 2 treated with KOt Bu. The peaks at ~0.3 keV, ~3.3 keV and ~3.6 keV shows the presence of potassium on the surface of the product as well as all the other expected elements. This was evidence to suggest that compound 3 contained potassium on the surface of the crystals. The limitation of the EDX data though, was that it couldn’t provide an accurate percentage composition of the elements on the surface of the crystals measured. Therefore, a comparison between this and the expected percentages couldn’t be made and the above data can only be used qualitatively. Thus, the potassium in the spectrum could be attributed to any unreacted KOt Bu left in the sample. 9.6 Summary of KOtBu reactions When considering whether the reaction of compound 2 with KOt Bu was successful, there was evidence to suggest that potassium was incorporated at least onto the surface of the crystals of compound 3. This was based on the PXRD pattern being different to that of compound 2 and the EDX analysis showing the presence of potassium in the product. However, the lack of a single crystal structure of compound 3 does cast some doubt on this. It was suggested that a possible reason for observing potassium in the EDX spectrum but not the crystal structure could be due to potassium only chelating between the carbonyl groups on the ketoamide tag on the outside of crystals. This theory might also explain why the expected elemental analysis percentages do not agree with the expected percentages in compound 3.
- 55. 50 10. PSMetalation ofcompound 2 using ZnEt2 Another metal that was reacted with compound 2 was zinc from ZnEt2. Zinc was chosen because it was thought that as the MOF already contained zinc, that any chance of transmetalation would be reduced. Also, unlike potassium that could not be seen in the crystal structure of the previous reaction (section 9), it was hoped that zinc would be. This is because it is a heavier element with more electron density than potassium. The proposed product of the reaction contains a modified ketoamide PSM tag containing a coordinated zinc atom and ethyl group with the notation –dknZn and molecular formula [Zn2(dabco)(bdc-dknZn)2]. The proposed reaction of compound 2 with ZnEt2 is shown below in Figure 59. Figure 59. Proposed reaction scheme for compound 2 with ZnEt2 to produce compound 4 with the formula [Zn2(dabco)(bdc-dknZn)2]. Only the bdc sections of the framework is shown. Upon visual inspection of the product (compound 4), it appeared to contain the same needle like crystals as in compound 2 but with a very slight colour change to a light yellow. There did also appear to be some less needle like, orange-brown crystals that were likely to be compound 1a based on their visual similarity with known compound 1a. 10.1 PXRD analysis A PXRD pattern of the product of the reaction was collected and is shown compared to the patterns for compounds 1 and 2 in Figure 60.
- 56. 51 Figure 60. PXRDpatterns of compound 1 (blue), compound 2 (orange) and compound 4 (yellow). As with the reaction of compound 2 with KOt Bu, there were differences between the pattern of compound 2 and the pattern of the product of the reaction. The most obvious difference is that the relative intensity of the peaks in the yellow pattern are considerably lower than in the other two patterns; especially for the peaks at ~8.8° and ~17°. This could indicate a reduction in crystallinity which could be due to the reactive ZnEt2 decomposing the framework structure. There is also the beginning of what appears to be peak splitting at ~17.2° in the yellow pattern. At ~13.3° and between ~28-30° of the yellow pattern, there are a series of small peaks that are not in either the patterns for compound 1 or compound 2. The peak splitting and new peaks could suggest the formation of a new phase. Lastly, there is a peak shift to higher angles in the yellow pattern. This is most noticeable for the most intense peaks at ~8.4° and ~17°. This indicates a change in the lattice parameters. Further analysis of compound 4 was performed to identify the incorporation of Zn into the ketoamide PSM tag. 10.2 1H NMR analysis A proton NMR spectrum was collected on the product of the reaction in order to determine whether the ketoamide tag remained intact after treatment of compound 2 with ZnEt2. Figure 61 shows a section of the spectrum associated with the aromatic protons on the bdc linkers. Figure 61. Aromatic section of the 1 H NMR spectrum for the product of the reaction of compound 2 with ZnEt2 after digestion in trifluoroacetic acid. 0 20000 40000 60000 80000 100000 120000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Compound 1 Compound 2 Compound 4
- 57. 52 The same splittingpattern to that of compound 2 is observed for the product of the reaction which shows that the ketoamide tag remains intact in the product of this reaction. The doublets at 7.69 ppm and 7.76 ppm with an integral ratio of 2.33:1.04 were used to calculate the percentage of bdc-NH2 and bdc-dkn present in the sample. The sample contained bdc groups with 69% conversion to the ketoamide tagged linker which concurs with that found in compound 2. This suggested that treatment with ZnEt2 did not appear to degrade the organic linkers that were functionalised. 10.3 Elemental analysis Elemental analysis was performed on the product of the reaction. Assuming percentage conversion to the ketoamide PSM tag in the first stage of the tandem reaction was 69%, the expected elemental composition of the produce should have been: C – 41.28%, H – 3.39, N – 7.19%. The measured composition was: C – 45.30%, H – 4.40%, N – 6.15%. The experimental and theoretical values for each of the elements are widely different. This suggests that compound 4 may not have contained the zinc between the ketoamide carbonyl groups, or at least not been the only product formed. There could also be a number of other reasons why the elemental compositions were different from one another. There could be solvent trapped in the pores of the framework which would affect the percentage composition of a sample. Also, the reaction proposed only involves chelation of a zinc atom and an ethyl group between the two carbonyl groups of the ketoamide groups throughout the crystal. However, there is every possibility that this may not be the case. It is also worth considering diethylzinc itself. As a highly reactive reagent, there is the possibility that it reacts too rapidly to get to the inside of the pores of the crystals of compound 2. This would cause compound 4 to only contain chelated zinc on the surface of the crystals. 10.4 Summary of reaction with ZnEt2 Whilst it was hoped that by aiming to chelate zinc into the diketene tag of compound 2 would be likely due to reduced possibility of transmetalation with the zinc in the framework, this actually made the product difficult to characterise. As zinc is the metal in the framework, chelation of zinc meant that techniques such as EDX could not be used to analyse the product as zinc would be seen in the spectrum. Preliminary single crystal analysis showed a unit cell with parameters consistent with those of compound 2 but as when KOt Bu was used, no metal and only the first carbon of the diketene tag were seen crystallographically. To conclude, the reaction of compound 2 with diethylzinc showed only limited evidence to suggest that compound 4 contained zinc chelated into the ketoamide PSM tag. The PXRD pattern was perhaps the most useful piece of evidence as this showed that a reaction had taken place and the 1 H NMR data showed that diethyl zinc did not appear to affect the diketene PSM tag. However, there was unfortunately no structural evidence to show that zinc had chelated to the ketoamide PSM tag.
- 58. 53 11. PSMetalation ofcompound 2 using [Rh2(μ-Cl)2(CO)4] The final metal that was reacted with compound 2 in this project was rhodium using Di-μ- chloro-tetracarbonyldirhodium(I) as the reagent. Rhodium was chosen because it is a large, heavy metal with a significant amount of electron density. Thus it was hoped that unlike potassium and zinc, it would be seen crystallographically. Also, as one of the aims of the project was to produce structures that have the potential to be used in catalytic applications. Rhodium has long been used in catalysis so it was thought to be a good candidate. The proposed product of the reaction (compound 5) contained a modified ketoamide PSM tag with Rh(CO)2 chelated between the carbonyl groups with the notation –dknRh(CO)2. The targeted product had the formula [Zn2(bdc-dknRh(CO)2)2(dabco)] and the reaction scheme is shown in Figure 62. Figure 62. Proposed reaction scheme for compound 2 with [Rh2(μ-Cl)2(CO)4]. Only the bdc sections of the frameworks are shown. Upon visual inspection under an optical microscope, there was a colour change in the product. The light yellow crystals of compound 2 had a very dark red tint which was the same colour as the rhodium reagent used. The product appeared to be a mixture. There were still some oblong needle like crystals with a similar morphology to those found in compound 2. There were also some darker orange crystals which were thought to be compound 1a based on visual inspection compared to known compound 1a crystals. Finally, there were some small round pieces of amorphous looking product. The product of this reaction had a much lower proportion of the needle like product than when potassium and zinc were used to treat compound 2. A reason for this could be because the by-product of the proposed reaction would be hydrogen chloride. It has been shown that some MOFs are unstable in acidic environments[41], so it was possible that the framework was partially decomposed by this and hence reduced the proportion of needle like crystals in the product.
- 59. 54 11.1 PXRD analysis PXRDanalysis of the product of the reaction was compared to the PXRD patterns of compounds 1 and 2 as well as the pattern for [Rh2(μ-Cl)2(CO)4] in Figure 63. By comparing the pattern for compound 5 to the pattern for the rhodium reagent, it was used to determine whether any of the reagent was present in the product of the reaction. Figure 63. PXRD patterns of compound 1 (blue), compound 2 (orange), [Rh2(μ-Cl)2(CO)4] (grey) and compound 2 treated with [Rh2(μ-Cl)2(CO)4] (yellow). The pattern of compound 5 (yellow) is significantly different to that of compounds 1 and 2. There is a peak shift to higher angle of the peaks at ~8° and ~17° when compared to compounds 1 and 2 which indicates a change in unit cell parameters. There also appears to be a peak at ~47° which isn’t present in compounds 1 or 2. This could be an indication that a new phase had formed. As the pattern for the product does not correspond with that of the reagent (grey), this shows that there isn’t any unreacted reagent in the product. It is worth noting that the peak intensities in the pattern for the product of the reaction are generally lower and broader than those of compound 2. This indicated reduced crystallinity of the product which is supported by the idea that the hydrogen chloride by-product of the reaction could have caused partial decomposition of the MOF. These observations could suggest that a reaction had taken place and that the product was not likely to contain compounds 1 or 2 or any unreacted rhodium reagent. Further analysis was performed on compound 5 to determine whether rhodium was incorporated into the ketoamide PSM tag. 11.2 1H NMR analysis As with the other samples of compound 2 treated with metals, a proton NMR spectrum was collected to see what percentage of bdc-dkn linker was in the sample. The same splitting pattern to that of compound 2 was observed and is shown in Figure 64. 0 20000 40000 60000 80000 100000 120000 5 15 25 35 45 55 Intensity/arb.units 2θ / ° Compound 1 Compound 2 Rh2(μ-Cl)2(CO)4 Compound 5
- 60. 55 Figure 64. Sectionof 1 H NMR spectrum compound 5 digested in TFA. Using the doublets as with the other metalation reactions (7.70 ppm and 7.75 ppm), the ratio of bdc-NH2 to bdc-dkn was calculated. A peak integral ratio of 1.94:1.13 (bdc- dkn:bdc-NH2) shows ~63% bdc-dkn in the sample. This is less than the amount in compound 2 before treatement with [Rh2(μ-Cl)2(CO)4]. The very large broad hump ~6.5 ppm is likely due to acid remaining in the sample. This could be from the TFA used to digest the MOF. This hump makes the accuracy of the peak integrals questionable, especially for the peaks closer to it. As such, the percentage of each ligand in the sample is likely to be approximate. 11.3 EDX/SEM analysis EDX analysis was performed on the product of the reaction. Figure 65 shows an SEM image of the product. It can be seen that there are several types of crystals present as described in section 11. Figure 65. SEM image of the product of the reaction at 190× magnification. Sites of interest where EDX spectra were collected are highlighted by red circles. 1 2 3 a bc e f d g
- 61. 56 Interestingly, the oblongneedle like crystals in the photo which are most like those found in compound 2 are much smaller in the product than before the reaction. This can be seen by comparing Figures 54 and 65. Both figures are at 190× magnification yet the crystals in Figure 65 all appear to be smaller and shorter. This is evidence that the MOF is likely to have been degraded during the reaction. The sites where EDX spectra were collected are denoted by red circles in Figure 65. Site one was thought most likely to be compound 5 as the morphology of the crystal was most similar to that of compound 2. The EDX spectrum for site one is shown in Figure 66. Figure 66. EDX spectrum of site one of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4]. The spectrum shows the presence of rhodium in the sample. This was a good indication that rhodium had been incorporated into the product and suggests that compound 5 had rhodium inocorporated on the surface of the crystals. However, because there are several crystal morphologies present in the product, other spectra were collected. The spectrum for site two is shown in Figure 67. Figure 67. EDX spectrum of site two of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4]. The crystal that site two was taken from had an appearance closer to that of compound 1a and there were many more crystals of this type in the product than in compound 2 before treatment with [Rh2(μ-Cl)2(CO)4]. However, the spectrum still shows the presence of rhodium, albeit at what appears to be a lower level than at site one. This could suggest that the crystal either was not compound 1a or perhaps there was some left over reagent on the surface of the crystals. Finally, a spectrum was collected at site three which was on what appeared to be a small, round, pieces of product. The spectrum is shown in Figure 68.
- 62. 57 Figure 68. EDXspectrum of site three of the product of the reaction of compound 2 with [Rh2(μ-Cl)2(CO)4]. The spectrum shows that rhodium was present at site three. It also shows that the other elements expected in compound 5 are present. However, it is unlikely that product at site three is compound 5 because it doesn’t appear to be crystalline and it is unlikely that crystals of compound 2 would change that significantly upon treatment with [Rh2(μ- Cl)2(CO)4]. Overall, there is a strong suggestion that there was rhodium in the product of the reaction. However, as the data could not provide accurate elemental compositions, it is unclear whether any of the products formed were the target product. Also, as EDX only measures composition at the surface of the crystals of product, it is impossible to tell whether the rhodium converted the ketoamide tag just on the surface of the crystals as was suggested when compound 2 was treated with both KOt Bu or ZnEt2. As each of the spectra above show chlorine peaks that overlap with the rhodium peak at ~2.7 keV, it is possible that some unreacted [Rh2(μ-Cl)2(CO)4] could have remained in the product. 11.4 IR analysis Because the proposed structure of compound 5 sees –Rh(CO)2 chelated between the carbonyl groups of the ketoamide PSM tag, it was expected that a characteristic inorganic CO stretch would be seen in the IR spectrum of the product. The IR spectrum of the product of the reaction showed strong sharp peaks at 2009 cm-1 and 2068 cm-1 which were attributed to the symmetric and asymmetric CO stretches of the proposed structure of compound 5. However, it was possible that the product could contain some unreacted rhodium reagent. To determine whether this was the case, and IR spectrum was also collected on a sample of [Rh2(μ-Cl)2(CO)4]. This showed strong peaks at 2034 cm-1 and 2090 cm-1 . These were assigned to the symmetric and asymmetric CO stretches and were consistent with previously reported values in the literature[64]. Therefore, it was deduced that the product of the reaction did not contain [Rh2(μ-Cl)2(CO)4]. This is evidence to show that compound 5 contains inorganic carbonyl groups which could be from the Rh(CO)2 group chelated to the ketoamide tag. An interesting comparison that was also made was between the spectra of the product of the reaction and [Rh(acac)(CO)2]. This is because the proposed structure of compound 5 and [Rh(acac)(CO)2] are similar. This is shown in Figure 69. Essentially, the diketene PSM tag in compound 5 acts as an acac ligand that is anchored to the MOF via an amide link.
- 63. 58 Figure 69. bdc-dknRh(CO)2section of compound 5 (left) and [Rh(acac)(CO)2] (right). Therefore, it would be expected that the CO stretches for the two compounds would be similar but different. The reported stretching frequencies for [Rh(acac)(CO)2] are: νco – 2083 cm-1 (symmetric) and 2015 cm-1 (asymmetric)[65]. The difference in frequencies is due to different amounts of π-back-bonding in each of the compounds. The frequencies for the product of the reaction are lower than that which shows that the CO bond is weaker than in [Rh(acac)(CO)2]. This is also a convincing piece of evidence that compound 5 was produced during the reaction. 11.5 Summary of reaction with [Rh2(μ-Cl)2(CO)4] It is likely that compound 5 was formed by reacting compound 2 with [Rh2(μ-Cl)2(CO)4]. A number of analyses seemed to confirm this. As with the other metalation reactions, the PXRD pattern confirmed that a reaction had taken place. SEM analysis showed that there appeared to be several types of substances present in the product. EDX analysis subsequently showed that these samples all contained some rhodium which was a strong indication that at least some of the compound 5 contained rhodium. This was further supported by the IR data which showed clearly that there was CO in the product as proposed in the structure of compound 5. However, as with the other metalation reactions performed on compound 2, the formation of compound 5 was not structurally confirmed as initial XRD experiments could not resolve rhodium in the crystal structure. This could be due to the reaction only taking place on the surface of the crystals or. The preliminary single crystal XRD experiments also showed that there was a significant amount of compound 1a in the product as several of the crystals analysed had the same unit cell as compound 1a. This further suggests that compound 1a is essentially a decomposition product of the DMOF-1 type framework.
- 64. 59 12. Conclusions This projectled to the successful reproduction of the zinc-based metal organic framework, compound 1, [Zn(bdc-NH2)2(dabco)] (DMOF-1-NH2) from the literature. Postsynthetic modification of compound 1 using diketene was successful in producing a product with the average molecular fomula of [Zn2(bdc-dkn)1.32(bdc-NH2)0.68(dabco)] (compound 2). Only 69% of the bdc-NH2 linkers in compound 1 were converted to the bdc-dkn linkers as shown by 1 H NMR spectroscopy. The second stage of the tandem postsynthetic modifications studied in this project showed promising evidence that metals were coordinated to the ketoamide PSM tags on the surfaces of the crystals of compound 3, 4 and 5; particularly potassium and rhodium (compounds 3 and 5 respectively). When treated with diketene, compound 1 that hadn’t been activated proved unsuccessful in producing compound 2. From this, it was deduced that conversion from compound 1 to compound 2 would be benefitted from activating compound 1 prior to PSM. It was observed that by using a different activation procedures prior to postynthetic modification with diketene affected the product that was produced. Activation of compound 1 in cyclohexane failed to produce compound 2 and instead the product appeared to be composed primarily of a decomposition product; [Zn(bdc-NH2)2(DMF)] (compound 1a) which was characterised crystallographically. Compound 1a was formed from two dimensional sheets with DMF ligands acting as capping groups. The sheets interdigitated to form a structure with very little pore space. It was found that activation of compound 1 in diethylether was successful in producing compound 2 which was characterised crystallographically. This method did also produce some compound 1a although to a much lower degree, and in this study, only 69% conversion from compound 1 to compound 2 was achieved. However, it was chosen to be the most appropriate activation procedure. The second stage of the tandem PSM reactions studied in this project involved dative PSM of metal ions and metal containing fragments to the diketene PSM tag in compound 2. There were fairly strong indications that coordination of potassium to the diketene tag to produce compound 3 was successful. PXRD and elemental analysis proved useful in gaining information that compound 2 had been changed upon treatment with KOt Bu. The presence of potassium and hence production of compound 3 was indicated by EDX analysis. Similarly, treatment of compound 2 with [Rh2(μ-Cl)2(CO)4] offered some convincing evidence that compound 5 was produced. Again, PXRD and EDX showed that a reaction had taken place and that rhodium was incorporated into the product. IR spectroscopy supported the formation of compound 5 as the characteristic CO stretching frequencies are different to that of the rhodium reagent and similar rhodium complexes. There was less evidence to show that zinc had been successfully chelated into the diketene tag. Not necessarily because it was less likely to do so, but it proved difficult to characterise as it is the same metal as in the framework structure. Thus EDX couldn’t be used to show the formation of compound 4 unlike for compounds 3 and 5. Unfortunately, crystallographic evidence for the formation of compounds 3, 4 and 5 was not obtained. Thus, further work would be needed to conclusively prove their formation.
- 65. 60 13. Future work Thisproject showed promise in the field of tandem PSM reactions with an objective of chelation of a metal into a PSM tag. However, there were areas of the project that would have benefitted from further work. Initially, it would be worth optimising the activation procedure of the MOF prior to treatment with diketene. Dr William Gee in the Burrows group showed that it was possible to get stoichiometric conversion from the bdc-NH2 linker in compound 1 to the bdc-dkn linker. Crystallographic studies showed that at complete conversion, the whole bdc-dkn tag could be resolved crystallographically. Therefore, a study into different diketene loading would also be appropriate. As one of the difficulties of this project was obtaining good crystallographic data due to poor crystal quality. Therefore, altering the synthetic methods for the metalation reactions would be useful. Ideally, they would be changed with an aim to grow larger crystals that may have a better chance of diffracting well. This in turn may allow the identification of metals chelated to the ketoamide tag. During the project, it was suggested that the PSM reactions may only occur on the surface of crystals. EDX on a cross section of a crystal may offer some elucidation on this. By cleaving a crystal after a PSM reaction and collecting EDX spectra on the centre of the crystal as well as the surface would show if there was a different chemical composition on the surface compared to the interior of the crystal. Another method that may show whether PSM had occurred only on the surface is measuring the surface area of the crystals. This could be done by collecting isotherms and calculating BET surface areas of the MOFs. If the PSM reactions had occurred inside the crystals as well as the surface, a smaller surface area would be expected. This is because the PSM groups would occupy some of the pore space within the MOF crystals. It would also be interesting to perform these types of PSM reactions on different metal-organic frameworks. For example, studying MOF systems that still contain amino functionalised linkers but with much larger pores such as UMCM-1 (32 Å pores) may be more successful for these PSM reactions. The larger pores may allow the PSM groups to get access to the amino groups more easily that in DMOF-1-NH2 (compound 1). From a chemical stability perspective, it may also be useful to study some more chemically robust MOFs such as the MIL-53 series of MOFs because they may be able to withstand harsher reaction conditions. Acknowledgements I would like to thank my supervisors: Professor Andrew Burrows and Dr Mary Mahon for their help and support throughout this project. I would also like to thank the rest of the Burrows group and in particular Dr William Gee for day to day guidance throughout this research project.
- 66. 61 References 1 B. Moultonand M. J. Zaworotko, Curr. Opin. Solid State Mater. Sci., 2002, 6, 117– 123. 2 P. Hagrman, D. Hagrman and J. Zubieta, Angew. Chem. Int. Ed. Engl., 1999, 38, 2638–2684. 3 O. M. Yaghi and H. Li, J. Am. Chem. Soc., 1995, 117, 10401–10402. 4 M. O’Keeffe, Chem. Soc. Rev., 2009, 38, 1215–7. 5 A. Corma, H. García and F. X. Llabrés i Xamena, Chem. Rev., 2010, 110, 4606–55. 6 S. R. Batten, N. R. Champness, X.-M. Chen, J. Garcia-Martinez, S. Kitagawa, L. Öhrström, M. O’Keeffe, M. P. Suh and J. Reedijk, CrystEngComm, 2012, 14, 3001. 7 M. Eddaoudi, D. B. Moler, H. Li, B. Chen, T. M. Reineke, M. O’Keeffe and O. M. Yaghi, Acc. Chem. Res., 2001, 34, 319–330. 8 J. L. C. Rowsell and O. M. Yaghi, Micropor. Mesopor. Mater., 2004, 73, 3–14. 9 J.-J. Jiang, X.-P. Li, X.-L. Zhang, B.-S. Kang and C.-Y. Su, CrystEngComm, 2005, 7, 603. 10 J. Ni, K.-J. Wei, Y. Liu, X.-C. Huang and D. Li, Cryst. Growth Des., 2010, 10, 3964–3976. 11 O. M. Yaghi, D. A. Richardson, G. Li, C. E. Davis and T. . Groy, Mat. Res. Soc. Symp. Proc., 1995, 371, 15–19. 12 H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444. 13 J. Kim, B. Chen, T. M. Reineke, H. Li, M. Eddaoudi, D. B. Moler, M. O’Keeffe and O. M. Yaghi, J. Am. Chem. Soc., 2001, 123, 8239–8247. 14 K. S. Park, Z. Ni, A. P. Côté, J. Y. Choi, R. Huang, F. J. Uribe-Romo, H. K. Chae, M. O’Keeffe and O. M. Yaghi, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 10186–91. 15 O. Gledhill, University of Bath, 2015. 16 O. M. Yaghi, M. O’Keeffe, N. W. Ockwig, H. K. Chae, M. Eddaoudi and J. Kim, Nature, 2003, 423, 705–14. 17 M. O’Keeffe and O. M. Yaghi, Chem. Rev., 2012, 112, 675–702. 18 A. F. Wells, Structural Inorganic Chemistry, Oxford University Press, New York, 5th edn., 1984. 19 M. O’Keeffe, Nature, 1999, 400, 617–618. 20 O. D. Friedrichs, A. W. M. Dress, D. H. Huson, J. Klinowski and A. L. Mackay, Nature, 1999, 400, 644–647. 21 N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M. O’Keeffe and O. M. Yaghi, Science, 2003, 300, 1127–9. 22 C. Dey, T. Kundu, B. P. Biswal, A. Mallick and R. Banerjee, Acta Crystallogr. B.
- 67. 62 Struct. Sci. Cryst.Eng. Mater., 2014, 70, 3–10. 23 G. Demazeau, Z.Naturforsch, 2010, 65b, 999–1006. 24 D. N. Dybtsev, H. Chun and K. Kim, Angew. Chem. Int. Ed. Engl., 2004, 43, 5033– 6. 25 F. Gándara and T. D. Bennett, IUCrJ, 2014, 1, 563–70. 26 Y.-R. Lee, J. Kim and W.-S. Ahn, Korean J. Chem. Eng., 2013, 30, 1667–1680. 27 E. Haque, N. A. Khan, J. H. Park and S. H. Jhung, Chemistry, 2010, 16, 1046–52. 28 Z.-Q. Li, L.-G. Qiu, T. Xu, Y. Wu, W. Wang, Z.-Y. Wu and X. Jiang, Mater. Lett., 2009, 63, 78–80. 29 W.-J. Son, J. Kim, J. Kim and W.-S. Ahn, Chem. Commun. (Camb)., 2008, 6336–8. 30 A. Pichon, A. Lazuen-Garay and S. L. James, CrystEngComm, 2006, 8, 211. 31 J. E. Mondloch, O. Karagiaridi, O. K. Farha and J. T. Hupp, CrystEngComm, 2013, 15, 9258. 32 R. J. Kuppler, D. J. Timmons, Q.-R. Fang, J.-R. Li, T. A. Makal, M. D. Young, D. Yuan, D. Zhao, W. Zhuang and H.-C. Zhou, Coord. Chem. Rev., 2009, 253, 3042– 3066. 33 J. Clark, http://www.emercedesbenz.com/autos/mercedes-benz/concept- vehicles/mercedes-benz-f125-research-vehicle-technology/, 2011, Date accessed: 27/01/16. 34 K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald, E. D. Bloch, Z. R. Herm, T.-H. Bae and J. R. Long, Chem. Rev., 2012, 112, 724–81. 35 B. Chen, C. Liang, J. Yang, D. S. Contreras, Y. L. Clancy, E. B. Lobkovsky, O. M. Yaghi and S. Dai, Angew. Chemie, 2006, 118, 1418–1421. 36 L. Ma and W. Lin, Top. Curr. Chem., 2010, 293, 175–205. 37 N. B. Pathan, A. M. Rahatgaonkar and M. S. Chorghade, Catal. Commun., 2011, 12, 1170–1176. 38 S. Hasegawa, S. Horike, R. Matsuda, S. Furukawa, K. Mochizuki, Y. Kinoshita and S. Kitagawa, J. Am. Chem. Soc., 2007, 129, 2607–14. 39 C.-Y. Sun, C. Qin, X.-L. Wang, G.-S. Yang, K.-Z. Shao, Y.-Q. Lan, Z.-M. Su, P. Huang, C.-G. Wang and E.-B. Wang, Dalton Trans., 2012, 41, 6906–9. 40 A. D. Burrows, in Metal-Organic Framework Materials, eds. L. R. MacGillivray and C. M. Lukehart, John Wiley & Sons, 2014, pp. 195–218. 41 K. K. Tanabe and S. M. Cohen, Chem. Soc. Rev., 2011, 40, 498–519. 42 B. F. Hoskins and R. Robson, J. Am. Chem. Soc., 1990, 112, 1546–1554. 43 Z. Wang and S. M. Cohen, J. Am. Chem. Soc., 2007, 129, 12368–12369. 44 J. D. Evans, C. J. Sumby and C. J. Doonan, Chem. Soc. Rev., 2014, 43, 5933–51. 45 Z. Wang and S. M. Cohen, Chem. Soc. Rev., 2009, 38, 1315–29.
- 68. 63 46 Z. Wang,K. K. Tanabe and S. M. Cohen, Inorg. Chem., 2009, 48, 296–306. 47 K. K. Tanabe, Z. Wang and S. M. Cohen, J. Am. Chem. Soc., 2008, 130, 8508–17. 48 S. M. Cohen, Chem. Rev., 2012, 112, 970–1000. 49 A. D. Burrows, C. G. Frost, M. F. Mahon and C. Richardson, Angew. Chem. Int. Ed. Engl., 2008, 47, 8482–6. 50 A. Phan, C. J. Doonan, F. J. Uribe-Romo, C. B. Knobler, M. O’Keeffe and O. M. Yaghi, Acc. Chem. Res., 2010, 43, 58–67. 51 A. Huang, W. Dou and J. Caro, J. Am. Chem. Soc., 2010, 132, 15562–4. 52 E. D. Bloch, D. Britt, C. Lee, C. J. Doonan, F. J. Uribe-Romo, H. Furukawa, J. R. Long and O. M. Yaghi, J. Am. Chem. Soc., 2010, 132, 14382–4. 53 C.-D. Wu, A. Hu, L. Zhang and W. Lin, J. Am. Chem. Soc., 2005, 127, 8940–1. 54 S. S. Chui, Science, 1999, 283, 1148–1150. 55 M. J. Ingleson, R. Heck, J. A. Gould and M. J. Rosseinsky, Inorg. Chem., 2009, 48, 9986–8. 56 M. Meilikhov, K. Yusenko, A. Torrisi, B. Jee, C. Mellot-Draznieks, A. Pöppl and R. A. Fischer, Angew. Chem. Int. Ed. Engl., 2010, 49, 6212–5. 57 H. Leclerc, T. Devic, S. Devautour-Vinot, P. Bazin, N. Audebrand, G. Férey, M. Daturi, A. Vimont and G. Clet, J. Phys. Chem. C, 2011, 115, 19828–19840. 58 Z. Wang and S. M. Cohen, Angew. Chem. Int. Ed. Engl., 2008, 47, 4699–4702. 59 S. J. Garibay, Z. Wang, K. K. Tanabe and S. M. Cohen, Inorg. Chem., 2009, 48, 7341–9. 60 T. Gadzikwa, O. K. Farha, K. L. Mulfort, J. T. Hupp and S. T. Nguyen, Chem. Commun., 2009, 3720–2. 61 K. Hirai, S. Furukawa, M. Kondo, M. Meilikhov, Y. Sakata, O. Sakata and S. Kitagawa, Chem. Commun., 2012, 48, 6472–6474. 62 R. I. Yousef, B. Walfort, T. Rüffer, C. Wagner, H. Schmidt, R. Herzog and D. Steinborn, J. Organomet. Chem., 2005, 690, 1178–1191. 63 Y. Inokuma, S. Yoshioka, J. Ariyoshi, T. Arai and M. Fujita, Nat. Protoc., 2014, 9, 246–52. 64 J. Zakzeski, S. Burton, A. Behn, M. Head-Gordon and A. T. Bell, Phys. Chem. Chem. Phys., 2009, 11, 9903–11. 65 T. P. Dougherty, W. T. Grubbs and E. J. Heilweil, J. Phys. Chem., 1994, 98, 9396– 9399.