Visible light assisted hydrogen generation from complete decomposition of hyd...
Tedros A Balema Senior Project-INDEPENDENT RESEARCH
1. Investigation of Cyclometalated
Palladium(II) Complexes using NNC Pincer
Imine Ligands
Senior Project Submitted to
The Division of Science, Mathematics and Computing
of Bard College
Tedros Addisalem Balema
Annandale-on-Hudson, New York
April 2014
2. 2
Acknowledgements
I would like to thank my advisor Dr. Craig Anderson for his assistance and guidance in this
project. I would like to thank Emily McLaughlin for being a constant source of answers and
her patience during the school year. I would also like to thank Team Chem a.k.a “Craig’s
Crew” for being there with me to the end. Good luck with all your future endeavours. I would
also like to thank the rest of my friends for their encouragement and support in my years at
Bard. I dedicate this project to my family, they have shaped the person I am today and
without them I would never have gotten this far.
3. 3
Abstract
Palladium is one of the many transition metals that are used to activate C-H and C-X bonds.
Imine NNC pincer ligands and imine NC ligands were reacted with palladium sources to
form palladium(II) complexes. These complexes were subject to various characterisation
methods including NMR, LC-MS, and X-Ray crystallography. The successful formation of
the palladium(II) products suggested two probable mechanisms: concerted oxidative addition
and sigma bond metathesis. It was also noticeable that the palladium(II) complexes all
demonstrated a certain degree of regioselectivity in their syntheses. C-X bonds that formed
6-membered rings were selectively activated over C-H bonds to form the more frequently
observed 5-membered rings; in addition sp2
C-H bonds were selectively activated over sp3
C-
H bonds. The synthesis of palladium(IV) were also attempted, unfortunately no such complex
was isolated and characterised; however in one case a ligand substitution reaction was
reported.
4. 4
Table of Contents
Abstract ............................................................................................................................... 3
I. Introduction..................................................................................................................... 5
II. Results & Discussion .................................................................................................... 13
III. Conclusion & Future Work........................................................................................ 26
IV. References ................................................................................................................... 29
V. Experimental ................................................................................................................ 31
Synthesis of Starting Materials ........................................................................................ 31
Synthesis of Ligands ........................................................................................................ 32
Synthesis of Palladium(II) Complexes.............................................................................. 35
Attempted Synthesis of Palladium(IV) Complexes............................................................ 39
Recrystallisations ............................................................................................................ 40
Appendix A: 1
H NMR & 13
C NMR Spectra of Starting Materials.................................. 43
Appendix B: 1
H NMR & 13
C NMR Spectra of Ligands................................................... 48
Appendix C: IR Spectra of Ligands ................................................................................. 63
Appendix D: 1
H NMR, 13
C NMR & 31
P NMR Spectra of Palladium(II) Complexes ..... 71
Appendix E: LC-MS Spectra of Palladium(II) Complexes ............................................. 90
Appendix F: Crystal Structures ....................................................................................... 94
5. 5
I. Introduction
In nature we see a wide array of hydrocarbons that contain C-H bonds such as alkanes; this
very common yet immensely significant bond is a consequence of its lack of selectivity once
activated. This presence of strong and localized C–C and C–H bonds due to the molecules
having no empty orbitals of low energy, or having filled orbitals of high energy, that could
readily participate in a chemical reaction.1
The main reaction that C-H bonds can participate
in, however, is combustion reactions that tend to produce either carbon dioxide or carbon
monoxide (depending on the level of oxygen present) and water among other side products
(i.e. radicals). These common compounds do not necessarily have much significance when it
comes to synthesising valuable specialty compounds using C-H bonds. As a result, chemists
have tried a myriad of methods to not just simply activate the bonds but to do selectively and
also to functionalise these hydrocarbons into more useful compounds and reagents.2
Transition metals have been used in promoting C-H activation for many years.
Although the use of transition metals in reactions, Fenton’s reagent (hydroxylation) and
mercury salts (direct mercuration), involving hydrocarbons and other C-H compounds goes
back to the late nineteenth century;3
The first known instance of selective C-H activation goes
back to 1962. Chatt used a Ru(0)-diphosphine complex in the preparation of hydrido-
complexes of ruthenium(II).4
From there the list of transition metals capable of C-H
activation have been increased including Ru, Ir, W, Pt, Rh and Pd.3
It was during the 1960s that reactions involving the cyclometalation (i.e., the cleavage
of a C-H bond in a metal-coordinated phosphine or amine ligand) in the formation of cyclic
rings with aromatic compounds were performed. It was shown that palladium (II) derivatives
induce the oxidative coupling of arenes and the arylation of alkenes (the Fujiwara reaction).5,6
Meanwhile Shilov witnessed the first case of C-H activation through an oxidative addition
pathway using Pt(IV) as the oxidant and Pt(II) as the catalyst during the 70s.3
Since then
6. 6
Periana has made Shilov reactions more efficient by using a series of methane conversions
through Hg(II), Pt(II) and Pd(II) salt catalysts.7
As mentioned before, palladium is one of the transition metals that are able to
participate in C-H bond activation reactions. Although palladium exists in multiple oxidation
states, the palladium(0) and the palladium(II) states dominate in terms of usefulness in
organic methods meanwhile palladium(IV) has been becoming increasingly useful.8
As the
even-numbered oxidation states of palladium increases we find increased stability which can
be rationalised by palladium’s unlikeness to undergo one-electron or radical processes hence
two-electron oxidation or reduction are highly favoured.9
Due to palladium’s ability to
undergo reversible 2 electron processes has made it a highly effective catalyst since with each
different oxidation state (i.e. 0, +2, and +4) we observe different sets of chemical properties
and application. As a result transformations such as alcohol oxidation and cycloisomerization
are performed by Pd(II) while cross-couplings and olefin hydrogenation are usually
performed by Pd(0). 9
In the case of palladium(IV) complexes, although various complexes had been
proposed and a few pentafluorophenylpalladium(IV) complexes have been isolated, it wasn’t
until 1986 that an organopalladium(IV) complex had been fully characterised by Byers et
al.10
In the synthesis of the organopalladium(IV) complexes, oxidative addition reactions with
organohalides with palladium(II) complexes were used. In their study they found that,
mechanistically, the oxidative addition of MeI and PhCH2Br to PdMe2(bpy) and
PdMe2(phen) are consistent with the occurrence of the classical SN2 mechanism, involving
Pd(II) as the nucleophile.11
The kinetic studies verified this assertion by using 'H NMR
spectra at low temperatures allow detection of cationic intermediates to observe
PdMe2(NMe2CH2CH2NMe2) reacts with methyl triflate in CD3CN to form
[PdMe3(NMe2CH2CH2NMe2(NCCD3)]+
OSO2CF3
-
.10,12
However it is important to note that in
7. 7
comparison to platinum chemistry, the cations are fluxional and this is a demonstration of
palladium(IV)’s greater lability, which would also make such complexes favourable towards
reductive elimination.10
It is also important to note that this SN2 approach in palladium(IV)
synthesis is not the established standard and such syntheses are open to other mechanistic
approaches.
Another mechanistic approach in synthesising palladium(IV) complexes is σ-bond
metathesis; which also has been applied to palladium(II) chemistry. This mechanism, σ-bond
metathesis, is considered as the predominant pathway when C–H bond activation is
accomplished with electron-poor metal centres such as high-valent early transition metals.13
This mechanism (in a modified form) has also been proposed for late transition metals. In
Perutz’s model it is proposed that due to the significant electron density at late transition
metal centres, it is the metal that assists in stabilizing the organometallic complex alongside
the ligands.14
Thus in palladium’s case this becomes highly plausible, because first of all, as
palladium increases in oxidation state its electron density would increases due to the presence
of the donor electron ligand. This then creates a stabilising effect in which σ-bonds are
activated and formed between the ligands and the metal centre.
Imine ligands are known to be effective electron donors due to nitrogen having a
“hard” nature. This “hard” nature allows for the nitrogen to coordinate with the metal (in this
case palladium) via a σ bond. Since palladium is more electrophilic than platinum and
demonstrates reactive properties while in its even numbered oxidation states, the coordination
of nitrogen or donation of an electron pair to palladium should be quite favourable. The imine
functionality also uses the nitrogen’s π orbitals, making it a poor π-acceptor; hence very little
back-bonding interaction is able to occur further justifying the effectiveness of an imine
ligand coordinating to a palladium centre. When palladium(II) complexes are formed, the
palladium centre is electron rich. This allows the palladium to be reactive and also act as a
8. 8
nucleophile in SN2 reactions to could potentially cause palladium(IV) complexes to be
formed; should the palladium(II) be reacted in an oxidative addition process with reagents
such as MeI or PhCH2Br.10
On a side note, the presence of aromatic rings increases the
presence of π bonds in the system adding an increasingly electron rich environment.
Chelating ligands have been known for quite some time to greatly influence the
stability and reactivity in transition metal complexes. Among the various types of chelations
that chemists have been experimenting with in the past decades, terdentate pincer ligands
stand out for their ability to ensure high stability and enhanced reactivities to many transition
metal complexes.15
Ever since their initial synthesis by Shaw and van Koten in the 1970s,
pincer ligand complexes have been shown to display rare or unusual structural/bonding
features.16,17
In the case of NNC ligands, we can easily expect highly stable complexes. This
assertion comes from the observation that NNC ligands display “hard” N-based donor
moieties, which would imply that such ligands could not only stabilise the lower oxidation
states of palladium (0, +2) but also stabilise the higher ones (+4). With the imine
functionality combined with such chelation and aromaticity of the attached phenyl/naphtyl
groups, NNC ligands could potentially produce highly stable and functional palladium(II) and
palladium(IV) complexes.
In the formation of the palladium(II) complexes, which are the precursors to the
synthesis of palladium(IV) complexes, the first set of reactions will consist of
cyclometalations. Looking at the electronic design behind aromatic ligands with imine
functionality, it should be reasonable for such reactions to occur. Cyclometalation is a
transition metal-mediated activation of a C-X bond (X being Br, Cl, I, or H) to form a metal-
ligand cycle with a new metal-carbon σ bond.18
There are various factors that dictate how a
cyclometalation reaction may or may not proceed. These factors include: the substituents,
ring size, electronics of the C-R bond and the electronics of the newly formed ring.
9. 9
Substituents used in cyclometalation are important because if the substituent has
molecules located at the ortho- position, it could either promote or block cyclometalation
from occurring at that position resulting in orthometalation.13
Ring size is also another factor
to consider since there is somewhat of an observance that there is a greater preference to form
5-membered rings, versus other numbered rings, as it provides greater electronic stability in
the complex.13
Electronics of the C-R bond, aromatic Csp2-H bonds tends to be favoured over
aryl Csp3-H bond activation due to the greater kinetic lability of aromatic protons versus
protons in other hydrocarbons (i.e. olefins, alkanes).13
Finally the electronics of the ring itself
or endo/exo preference, generally there tends to be a greater preference of the endo-ring over
the exo-ring.13
The endo-ring structure in this project consists of the metal centre, an imine
and an aromatic ring; which would be more energetically favoured since the ring is stabilised
by the resonance of the aromaticity of the naphthyl group.
In the cyclometalation mechanism, a weak coordinating ligand in the metal precursor
is usually replaced by the donor site, labelled E, of the potentially cyclometalated ligand. The
effectiveness of E is determined by its basicity and steric effects; however most E donor
groups tend to follow the hard-soft acid-base principles described by Pearson.19
With this
principle in mind, soft transition metals (i.e. the platinum group metals) should favour
bonding to phosphines and sulfides as “soft” Lewis bases. However there are exceptions
where “hard” bases can bond effectively with “soft” platinum group metals, such as amine
donors bonding with palladium(II).20
Even though such hard-soft mismatches can produce
successful cyclometalations it has also been observed that there tends to be difficulties in
regioselectivity.13
Hence with these ideas in mind, the first thing that happens is that the initial ligand
coordinates to the metal source making complex A (figure 1, shown below). In many cases
donor group E can substitute weakly and moderately bonded ligands, increasing stability
10. 10
affording complex B. Next an intermediate before C-H activation, complex C, is formed
through by decoordinating of a ligand from complex A or a donor site from complex B. At
this stage the most important factor is note is the M-E bond strength. The strength of the M-E
bond depends on both electronic and steric factors or on the nature of the donor group E
either way one of those factors should dictate the stability of the M-E bond. Should the bond
be too strong or stable, ligand dissociation from complex B becomes much more difficult
making the production of complex C unlikely. Meanwhile should the M-E bond be too weak,
the reaction equilibrium could shift back to the starting materials hence no complex C is
formed.
Figure 1, transmetalation scheme using E-donor groups13
With this background knowledge and proposed effects of imine and NNC ligands the
characterisation and determination of successful syntheses of the palladium(II) and
palladium(IV) complexes may go forward. In our studies the main variables identified
included reaction conditions, structural identity of the complexes made and their ability to be
converted from a palladium(II) into a palladium(IV) complex.
Reaction conditions are significant considering that changing factors such as the
solvent, reaction time, or reaction temperature can determine whether or not one can isolated
11. 11
the desired product. For example, heating a reaction for too long could lead to undesirable
consequences and could lead to the production of extra and/or unwanted products or the
complete decomposition of produced complex. Solvents can be of some significance since
solvents do have interactions with complexes and different solvents could have different
effects usually depending on its particle size and polarity.
The structural identity of the complex is important since it enables us to determine
whether or not the cyclometalations have been performed successfully. This means
characterising the ligands to be used and looking at the structure of the given ligand. Whether
the ligands will undergo C-X or C-H activation and if it does undergo C-H activation which
C-H bond is preferred.
There are several methods of characterisation that shall be used to determine the
identities of products; 1
H NMR, 13
C NMR, LC-MS, X-ray diffraction (whenever suitable
crystals are successfully obtained) and elemental analyses.
NMR spectroscopy can easily help us determine whether or not a ligand and metal
(palladium) has successfully cyclometalated by observing shifts in the basic ligand
“backbone” which are highlighted by phase shifts in the imine position. LC-MS can verify
the presence of the cyclometalated ring since the main ion would possess both the m/z values
of the palladium centre and the chelated ligand backbone as one value while other smaller
ligands tend to detach themselves from the metal centre. However X-ray crystal diffraction,
would prove to be a much more effective way in determining structural information since it
can display not just the 3D structure of the product but also bond lengths and angles. With
proper structures present it could give better insight in the mechanistic manner of such
syntheses. Ligands used are shown below in figure 2.
12. 12
Figure 2, Ligands to be used
In the proposed sample reaction schemes, shown below in scheme 1, the chelation of
the ligand in question is very important. With NNC chelated ligands, a palladium(II) source
shall be used (i.e. palladium(II) acetate) in the cyclometalation reaction; which should be
appropriate due to the given electronics and stability behind such a chelation. Meanwhile the
NC chelated ligands will have to be reacted with a palladium(0) source
Tetrakis(triphenylphosphine)palladium(0)) due in part to the lack of donor electrons. Since
the mechanism in the production of palladium(IV) products is not well known, methyl iodide
will be used a basis to test whether or not an oxidative addition like mechanism occurs.
Scheme 1, proposed reactions with NNC and NC chelated ligands
13. 13
II. Results & Discussion
One of premises of the mechanism behind the product formation of palladium(II) complexes
was through transmetalation where a weak coordinating ligand in the metal precursor is
replaced by the donor site, labelled E, of the potentially cyclometalated ligand. Another
premise was a sigma bond metathesis mechanism. It is important to note that although we
have final products, final products do not necessarily prove mechanistic approaches. The only
sure way of verifying any mechanistic approach would be through the application of kinetic
studies or isolation. Do to the practical limitations, we shall assume both mechanistic
approaches in the formation of palladium(II) are applicable and in the presence of the data try
to ascertain as to which mechanism would be more likely according to the appropriate
products and reagents. Figure 3, below, shows the palladium(II) cyclometalated products:
Figure 3, the cyclometalated palladium(II) products
14. 14
Compounds TB-01, [Pd(I)(C15H17N2)], and TB-02, [Pd(Br)(C15H17N2)], are made
from halogenated ligands with NNC chelation and palladium(II) acetate as the source of the
transition metal centre. Since these compounds possess halide groups on the naphtyl ring, the
most clear mechanistic path would be the activation of the C-X, X being the halide, bond.
When looking at the reaction from a sigma bond metathesis perspective we see that in the
very first step we have a direct addition of electrons from N^N chelation, which does fall
within the characteristics typical of “hard” N-based donor. This is then followed by the
activation of the C-X bond, this activation is further aided by the fact the acetates are good
leaving groups and negative ligands thus the bonding is not as strong.21
As a result, the metal centre is able to accept another source of electrons to replace
and remove the presence of the weak ligands for greater stability. The metathesis mechanism
is also encouraged by the presence of the naphtyl ring on the ligand; this provides greater
stability and also makes the C-X bond stand out further from the highly stable naphtyl ring.
Once the C-X bond is activated, there still is the issue of the weak acetate ligand still bonded
to the metal centre while there is a presence of halide ions in the vicinity the complex would
benefit electronically by replacing the acetate with a halide, hence a direct substitution
occurs. As it is with all concerted oxidative addition processes, the nucleophile forms a
transition state with the substrate. Since intermediate states are stable enough to be isolated
and the palladium centre in this case is far too electron-rich the weakest bonded-ligands are
released until a stable complex is formed; resulting in the removal of the acetate ligands. The
possibility of sigma bond metathesis mechanism also becomes less likely as we consider that
C-X bond are much more active versus C-H bonds and sigma bond metathesis requires C-H
activation not C-X.22
The concerted oxidative mechanism is shown below in scheme 2.
15. 15
Scheme 2, concerted oxidative addition cyclometalation in the formation of the Pd(II) complex with
halogenated ligands resulting in products TB-01 and TB-02. “X” denotes a halide
Looking at the LC-M/S data for TB-01 and TB-02, both of which have a mass-to-
charge ratio value of 331m/z, this matches the value of the parent ion minus the halide ligand
(bromide or iodide). This mass-to-charge ratio verifies the successful formation of a 6-
membered metallocyclic ring, which is the desired outcome of a cyclometalation reaction. In
the LC-MS spectra it is also noticeable that the parent ion did not break apart from the metal
centre unlike the smaller halide ligands.
With the aid of X-ray crystallography, this structure was verified in the case of TB-01
(shown in figure4). It is important to notice that the 6-membered metallocyclic ring is on the
same plane as the naphtyl ring, and also the methyl groups off the terminal nitrogen of the
NNC chelation are at an axial position to the plan of the rings. This may suggest some
physical/steric contribution which may contribute to the outcome of its synthesis.
16. 16
Figure 4, ORTEP of TB-01
When given the non-halogenated NNC complex, TB-06 [Pd(OAc)(C16H19N2)], it
would appear that a sigma bond metathesis mechanism is the most probable. This comes
from the observation that C-H and not C-X activation does occur, which is a required for
sigma bond metathesis. Thus in scheme 3, this would be the most likely mechanism.
Scheme 3, sigma bond metathesis mechanism in the formation of the Pd(II) complex TB-06
The LC-MS verifies the successful formation of a palladium 6-membered
metallocyclic ring with a mass-to-charge ratio value of 345 m/z minus the acetate ligand. The
structure was even further analysed using X-ray crystallography yielding an ORTEP image
17. 17
(figure 5), confirming the structure of TB-06 and a verification of a successful C-H
activation reaction mechanism.
Figure 5, ORTEP image of TB-06
Another aromatic ligand to consider as well are thiophenes, however the focus was
narrowed down to observe C-H activation (hence no halo-thiophenes). In the formation of
TB-07 [Pd(OAc)(C9H14N2S)], sigma bond metathesis would be most likely mechanism.
Concerted oxidative addition mechanism is highly unlikely since there no presence of C-X
bonds. At the same time due to the size of the thiophene ring and its bonding in relation to the
palladium centre, there would be too much steric hindrance to favour any concerted oxidative
addition transition state formation. The mechanism for the formation of TB-07 is shown
below in scheme 4.
18. 18
Scheme 4, sigma bond metathesis mechanism in the formation of the Pd(II) complex TB-07
The sigma bond metathesis and concerted oxidative addition mechanism question
doesn’t only apply for NNC chelated ligands but also other chelation, the CN ligands. When
we look at the halogenated complexes TB-03 [Pd(I)(PPh3)(C20H19N)] and TB-04
[Pd(Br)(PPh3)(C20H19N)], the possibility of a oxidative addition is favourable. The
introduction of the large donor ligand in the presence of neutral triphenylphosphine ligands
would cause the overall complex to possess a greater positive charge thus attracting the
electron density of the adjacent C-X bond (X being a halide). At the same time,
triphenylphosphine is a very stable leaving group making it easier to remove and later on
dissociate from the X-
ion resulting in a direct substitution.
At the same time, the positions of the two triphenylphosphine ligands on the metal
centre do not really have any preference or hindrance with respect to each other there will be
a lack of selectivity resulting in the formation of diastereomers. The possibility of a sigma
bond metathesis is very low since a halide and a phosphine would have to eliminate together
and sigma bond metathesis does not change the oxidation number of the palladium centre.
The transition states assumes that there should be some kind of steric crowding which
would force the triphenylphosphine ligands to be pushed out until structural stability is
19. 19
achieved. Just like in the sigma bond metathesis there does not appear to be a significant
preferential position either triphenylphosphine ligands hence resulting in cis/trans
diastereomers, shown below in scheme 5.
Scheme 5, concerted oxidative addition cyclometalation in the formation of the Pd(II) complex with
halogenated ligands resulting in products TB-03 and TB-04. “X” denotes a halide
This observation was confirmed by the NMR spectra, where in the 1
H NMR, There
were clear pairs of peaks of similar nature at each significant phase shift of complex TB-03:
the imine (peaks c, 8.84 and 9.73 ppm), the tertiary carbon (peaks b, 3.15 and 4.20 ppm) and
the methyl group (peaks a, 1.40 and 1.74 ppm). This observation is also further confirmed by
the 31
P NMR where there were two distinct phosphorus peaks at 29.43 ppm and 75.01 ppm.
The possibility of the peaks being misinterpreted or as either free-floating triphenylphosphate
(-6.00 ppm) or its oxide (23.00 ppm) was eliminated since the given peaks do not coincide
within acceptable range of the literature values;23
Both spectra are shown in the figure 6
below.
20. 20
Figure 6, 1
H NMR of TB-03 (top) and 31
P NMR of TB-03 (bottom). The two sets of peaks represent the two
diastereomers
Similar observations were also made in the spectra of TB-04. In which the 1
H NMR
showed the presence of diastereomers. However unlike the TB-03 are not close to equal, one
diasteromer clear is in greater abundance compared to the other since we don’t see the other
21. 21
diasteromer in both the 31
P NMR (the spectrum show only one peak at 74.97 ppm) and the
13
C NMR. The data would suggest that there may be a little more favouritism in the bromide
reaction where although both are formed, one is favoured over the other perhaps due to
position/steric reasons. The 1
H NMR of TB-04 is shown below in figure 7.
Figure 7, 1
H NMR of TB-04 the two sets of peaks represent the two diastereomers
In the case of TB-05 [Pd(PPh3)2(C20H19N)]+
, the results become more ambiguous.
Although the 1
H NMR and the 13
C NMR verify the imine ligand back bone, it would appear
that the complex possesses two symmetrical phosphine ligands; verified by a large intense
peak at 29.03 ppm. When the structure for this complex was proposed, it would appear that it
has a palladium(I) centre. Although an oxidation state of 1 is not common for a palladium
complex, it might be possible that it could exist but not as a neutral complex: it would have to
exist as an ion, however the counter ion is not known.22
Another question would be what mechanism could have formed this compound; if it
exists as an ion it could mean that it most likely an isolated intermediate (shown in figure 8
22. 22
below). This is probable because given the reaction conditions in its synthesis: the solvent
was toluene (non-polar); the reagents were ligand LR (which is a non-halogenated NC
chelated ligand) and tetrakis(triphenylphosphine)palladium(0). From this assemblage of
conditions it is unlikely to form a neutral palladium complex since there is no negative ligand
present.
Figure 8, TB-05 most likely structure as an intermediate ion
In all these reactions so far, it is noticeable that we a sense of regioselectivity judging
from the products given. Looking at the given ligands it is noticeable that we have two
possibilities for C-H activation in the forming of TB-05, TB-06, and TB-07. The possibilities
include sp2
position that would result in a 5-membered cyclometalated ring or a sp3
position
that would result in a 6-membered ring. Regardless of the ligand, all showed the latter option
especially when the possibility of activating the adjacent hydrogen is eliminated (in the case
of non-halogenated ligands) with the presence of a methyl group. At the same time,
halogenated ligands also show that sp3
C-X bonds that form 6-membered rings were
selectively active versus sp3
C-H bonds. This would suggest that the preference of activation
in these reactions depend heavily on the nature of the naphtyl ring and not as much on the
imine chelation part (labelled R). Figure 9 better illustrates this observation.
23. 23
Figure 9, Regioselectivity of the cyclometalation reactions for the halogenated rings (left) and non-halogenated
rings (right), note that sites of activation forms the 6-membered metallocyclic rings
It was observed that in both sets of chelated compounds we can see that there is a
hierarchy in the reactions depending on the structure. It was observed that ligands with an
iodide tended to produce larger yields and purer complexes when compared to their bromide
counterparts. Since the possibility of a concerted oxidative addition type reaction mechanism
is there, it would make sense that the iodide is more effective since iodide is a much better
nucleophile and leaving group compared to bromide. The brominated products tended to
show very low yields and difficulty in purification despite the use of multiple techniques such
as washing with cold (0˚C) diethyl ether, and recrystallisation. The most effective complexes
in terms of yield and purity tended to be the complexes that were able to go through C-H
activation. Since these syntheses involved C-H activation, it would appear that the sigma
bond metathesis mechanism route would be the most likely (since there is no presence of a
nucleophile).
Using the complexes that had the largest yields and purity, converting them from a
palladium(II) complex to palladium(IV) was attempted. Using the approach of an oxidative
reaction, complexes TB-01, TB-03, TB-05 and TB-06 were reacted with methyl iodide to
observe at least a direct addition process, shown in scheme 6:
24. 24
Scheme 6, Attempted palladium(IV) syntheses, each complex was attempted with both a stirr and a reflux
method
What was observed was that in the synthesis of TB-08 was that the complex TB-01
completed decomposed in both long term stirring and considerable reflux time suggesting
that either a conversion to a palladium(IV) complex is possible as a transition state before
decomposition; or that the reagent (methyl iodide) may not be the most appropriate in this
case since TB-01 has an iodide present as a ligand maintain the charge balance of the
complex. As a result the presence of two iodides in the complex would not necessarily be
favourable due the palladium centre being overly electron rich and also the issue of steric
crowding causing the complex to be more unstable.
In the synthesis of TB-10 and TB-11, it was quite clear that these two complexes
couldn’t possibly be isolated. Besides the observation that phosphines are neutral ligands and
causes a sizable steric hindrance, similar to TB-08, the complex wouldn’t favourably
accomodate the physical insertion of both or either an extra iodide or a methyl group.
25. 25
In TB-09 case, it would appear that a completely different product was observed. It
would appear that the there was a subsitution reaction occur where the acetate ligand was
replaced with an iodide ligand, as shown in scheme 7 below:
Scheme 7, the substitution reaction of TB-06 in TB-09.5
This substitution may go through a SN2 mechanism. This mechanism becomes more likely
when we consider that acetate is a good leaving group, iodide is a good nucleophile and the
side product methyl acetate is produced. In the proton NMR this structure is confirmed by the
presence of a methyl acetate peak at 10.94 ppm and slight changes in phase shifts in the
significant peaks in the complex thus affirming the NNC imine “back bone”. Thus when
looking at the formation of the palladium(IV) forms of palladium(II) complexes perhaps a
wider range of ligands/ligand-sources should be considered that would not only satisfy steric
conditions but also electronic balances perhaps hypervalent iodides could be applicable.
26. 26
III. Conclusion & Future Work
After observing and analysing the data for the products of the syntheses of palladium(II)
complexes this can give an insight to the potential mechanisms that such reactions go
through. In looking at halogenated NNC chelated complexes, TB-01 and TB-02, the
mechanistic approach proposed is the concerted oxidative addition approach. This coming
from the presence of a nucleophile (the halide) and also the bond activated is not C-H but
rather C-X, which is a bond that is much easier to activate. At the same time the quantity of
the yield and the purity of the products seem to depend on the halogen in question which is
also directly related to the halide’s nature as a nucleophile, i.e. iodide is a better nucleophile
than bromide.
In the case of the C-H activated NNC chelated complexes, TB-06 and TB-07, the
sigma bond metathesis mechanism is the most likely. The absence of a nucleophile increases
this possibility. At the same confirmation of their structures through NMR and X-ray
crystallography further affirms this notion. The yields of these reactions may not be
necessarily as large as the yields on the iodide variation but the purity of the complexes were
higher. However in the case of TB-07, the presence of side products is significant which also
suggests that there may also be other reaction mechanism on a smaller scale perhaps due to
the thiophene based ligand.
Moving on to the halogenated imine complexes, TB-03 and TB-04, the most likely
mechanism for their syntheses would be a concerted oxidative addition considering that a
sigma bond metathesis does not change a palladium centre’s oxidation states while a
oxidative addition increases it from 0 to 2. The isolation of the diastereomers would be
considered difficult whether it is through crystallisation for X-ray diffraction analysis or even
analysis through LC-MS since distinguishing between the two diastereomers becomes more
27. 27
troublesome since the would-be result would contain the same parent ions and also physically
it would be difficult to form a proper crystal for X-ray diffraction.
Meanwhile for TB-05, it is still not clear what the full structure is however I propose
that this complex (which goes through C-H activation in its synthesis) could be an
intermediate until the presence of a stronger ligand is present. The proposed structure with
two phosphine ligands and the imine ligand would make this complex have a palladium(I)
centre, which in itself highly unlikely. Thus making this complex an ideal intermediate since
phosphines could easily be replaced by other ligands such as acetates or even halides.
In all cases, regioselectivity was observed by the sp2
activation of C-X and C-H bonds
on the naphtyl rings. This was observed by formation the 6-membered cyclometalated ring.
The regioselectivity can be controlled by either having a halide present on the ligand or by
eliminating competitive C-H bonds by substituting their positions with methyl groups.
For the palladium(IV) syntheses, the mechanisms are not clear. One reason is that a
palladium(IV) product was unable to be isolated for characterisation hence in all but one case
the mechanistic approach eludes this project. The exception was in the synthesis of TB-09
from TB-06. Instead of a palladium(IV) product we have a different palladium(II) product
from what we started with, TB-09.5. This product suggests very strongly for a substitution
type reaction since the acetate ligand was replaced with the iodide from the methyl iodide
reagent.
With all these experimental observations in mind this project would be a positive
candidate for further expansion. For thiophene imine ligands further study should be done on
the halogenated forms of the thiophene ring and also the different position of the sulphur on
the thiophene ring which may pose some electronic significance in product formation. For the
naphtyl imine ligands different chelations could be explored such as benzyl amines, since the
28. 28
chelation off the imine group of the ligands would pose both a steric and an electronic issue
in the synthesis of such complexes. At the same time adding and substituting different
functionalities could be another approach to increase the range of palladium(II) products.
With characterisation, applications for these complexes would have to be considered; perhaps
applying the complexes to catalysis experiments so see if these complexes have some use in
facilitating transmetalation reactions or in organic synthesis.
Palladium(IV) complexes remain to be a challenge and hence further studies would
have to performed to ascertain what conditions should any palladium(II) complex be set at.
Perhaps the reagents used to increase the oxidation state of the palladium should be
considerably stronger perhaps the application of hypervalent iodides could be used. The
substitution reaction observed in TB-9.5 is also an interesting spring board, to figure out what
ligands could be replaced in a already prepared palladium(II) complex could perhaps provide
a more favourable condition for the synthesis or conversion to palladium(IV).
Overall the experiments have provided positive results and further study should be
encouraged, especially long-term kinetic studies that could pin-point the mechanistic
approach in the palladium(II) syntheses.
29. 29
IV. References
1. Bergman, R. G. Nature 2007, 446, 391-393.
2. Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507-514.
3. Chatt, J.; Davidson, J. M. J. Chem. Soc. 1965, 843.
4. Shilov, A. E.; Shul'pin, G. B. Chem. Rev. 1997, 97, 2879-2932.
5. van Helden, R.; Verberg, G. Recl. Trav. Chim. Pays-Bas 1965, 84, 1263.
6. Fujiwara, Y.; Moritani, I.; Danno, S.; et al. J. Am. Chem. Soc. 1969, 91, 7166.
7. Crabtree R. H. Chem. 2004, 689, 4083.
8. Crabtree, R. H. The Organometallic Chemistry of the Transition Metals; Wiley: Hoboken,
2005.
9. Negishi, E. Handbook of Organopalladium Chemistry for Organic Synthesis; Wiley-
Interscience: New York, 2002.
10. Byers, P. K.; Canty, A. J.; Skelton, B. W.;White, A. H. J. Chem. SOC.Chem. Commun.,
1986, 1722.
11. Byers, P. K.; Canty, A .J.; Crespo, M.; Puddephatt, R J.; Scott, J. D. Organometallics,
1988, 7, 1363.
12. de Graaf, W. ; Boersrna, J.; Srneets, W. J. J.; Spek, A. L.; van Koten, G. Organometallics,
1989, 8, 2907.
13. Albrecht, M. Chem. Rev., 2010, 110, 576.
14. Perutz, R. N.; Sabo-Etienne, S. Angew. Chem. Int. Ed. 2007, 46, 2578.
15. Zargarian, D; Castonguay, A; Spasyuk, D. M. Topics in Organometallic Chemistry, 2012,
40, 131-173.
16. Moulton, C. J.; Shaw, B. L. Dalton Trans. 1976. 1020-1024.
17. van Koten. G.; Jastrzebski J. T.; Noltes, B. H; ; Spek J. G.; Schoone, A. L. J Organomet
Chem, 1978, 148, 233-245.
30. 30
18. Bruce, M. I. Angew. Chem. Int. Ed., 2003, 16, 2, 73-86.
19. Pearson, R. G. Chemical Hardness; Wiley-VCH: Weinheim, Germany, 1997.
20. Cope, A. C.; Friedrich, E. C. J. Am. Chem. Soc. 1968, 90, 909.
21. Cheung H.; Tanke, R. S.; Torrence, G.P. Ullmann's Encyclopaedia of Industrial
Chemistry; Wiley-VCH: Weinheim, 2005.
22. Granell, J; Martínez, M.; Dalton Trans., 2012, 41, 11243
23. NMR Notes. A Guide to NMR Reference Compounds: NMR Reference Compounds for
31
P Spectra. http://www.nmrnotes.org/NMRPages/refcomps.html
(accessed April 7, 2014).
24. Kurono, N.; Honda, E; Komatsu, F, Orito, K; Tokuda, M. Tetrahedron, 2004, 60, 1761.
31. 31
V. Experimental
The solvents and reactants used in the following reported synthesis were purchased from
Sigma Aldrich and Biogene Organics unless otherwise noted. The compounds were
characterised using NMR spectroscopy and LC-MS. The LC-MS spectra were performed at
Bard College using Varian 212 LC chromatography pump and Varian 500-MS. NMR spectra
were performed at Bard College using Varian MR-400 MHz spectrometer (1
H, 400 MHz;
13
C, 100 MHz) and referenced to CDCl3 (1
H, 13
C, and 31
P) and (CD3)2CO (13
C). The σ values
are given in ppm and J values are given in Hz. Abbreviations used: s = singlet; d = doublet; t
= triplet; m = multiplet; q = quarter; NMR labelling as shown below:
Synthesis of Starting Materials24
8-bromo-1-naphthaldehyde was obtained by stirring 8-bromonaphthalen-1-yl methanol (500
mg, 3.20 mmol) in a solution of methylene chloride (15 mL), PCC (682 mg, 3.20 mmol), and
silica gel (1.400 g) for four hours at room temperature, producing an orange solid. The
residue was then extracted with diethyl ether, washed twice with water and brine, dried over
magnesium sulphate, resulting in a white solid. 1
H NMR (400 MHz, CDCl3): δ = {7.39 [t,
3
J(H-H)= 7.8, 1H, Hc
]; 7.57 [t, 3
J(H-H)=7.9, 1H, Hf
]; 7.88-7.93 [m, 3H, Hg,d,e
]; 8.01 [dd,
3J(H-H)=8.2, 4J(H-H)=1.3, 1H, Hb
], aromatics}; 11.44 [s, 3H, Ha
]. 13
C NMR (100 MHz,
CDCl3): δ = 26.57 [Ca
]; {125.56; 126.97; 128.31; 128.70; 130.03; 130.42; 133.17, 133.48;
134.18; 135.91 aromatics}; 192.50 [Ch
].
8-iodo-1-naphthaldehyde was obtained by stirring 8-iodonaphthalen-1-yl methanol (500 mg,
1.80 mmol) in a solution of methylene chloride (15 mL), PCC (570 mg, 2.60 mmol), and
silica gel (1.170 g) for four hours at room temperature, producing a dark orange solid. The
solid was extracted with diethyl ether, washed twice with water and brine, dried over
magnesium sulphate, resulting in a pale yellow solid. 1
H NMR (400 MHz, CDCl3): δ = {7.23
32. 32
[t, 3
J(H-H)= 7.8, 1H, Hc
]; 7.54 [t, 3
J(H-H)=7.8, 1H, Hf
]; 7.88 [dd, 3
J(H-H) =1.4, 1H, Hg
];
7.93 [dd, 3
J(H-H)=8.2, 4
J(H-H)=1.0, 1H, Hd
]; 7.97 [dd, 3
J(H-H)=8.2, 4
J(H-H)=1.4, 1H, He
];
8.27 [dd, 3
J(H-H)=7.4, 4
J(H-H)=1.2, 1H, Hb
], aromatics}; 11.70 [s, 3H, Ha
]. 13
C NMR (100
MHz, CDCl3): δ = 89.67 [Ca
]; {125.88; 127.29; 129.33; 130.03; 133.17; 133.87, 135.20;
135.60; 136.22; 141.08 aromatics}; 191.48 [Ch
].
Figure 10, labelled protons and carbons of 8-iodo-1-naphthaldehyde and 8-bromo-1-naphthaldehyde
Synthesis of Ligands
Ligand [C15H17N2Br], LE, was obtained by stirring N, N –dimethylethylenediamine (21.0
mg, 0.238 mmol) and 8-bromo-1-naphthyladehyde (50.0 mg, 0.213 mmol) together in
methylene chloride (15 mL) at room temperature for one hour. The solvent was removed
resulting in a light brown oil. Yield: 37.0 mg (57.0%). [C15H17N2Br], LE. 1
H NMR (400
MHz, CDCl3): δ = 2.35 [s, 6H, Ha
]; 2.74 [t, 2H, Hb
]; 3.81 [t, 2H, Hc
]; {7.28-8.00, 6H,
aromatics}; 9.59 [s, 1H, Hd
]. 13
C NMR (100 MHz, CDCl3): δ = 45.73 [Ca
]; 49.51 [Cc
];
59.55[Cb
]; {126.17; 126.24; 128.95; 129.71; 130.98; 132.99, aromatics}; 163.88 [Cd
]. FTIR
(neat) 2895, 1676 (cm-1
).
Ligand [C19H16BrN], LF, was obtained by combining 8-bromo-1-naphthaldehyde (32.9 mg,
0.140 mmol), S-(-)-α-methylbenzylamine (21.6 mg, 0.178 mmol) and stirring the mixture in
methylene chloride (10 mL) at room temperature for 90 minutes. The solvent was removed
resulting in a pale brown oil. Yield: 42.5 mg (70.5 %). [C19H16BrN], LF. 1
H NMR (400
33. 33
MHz, CDCl3): δ = 1.69 [d, 3H, Ha
]; 4.68 [q, 1H, Hb
]; {7.27-9.03, 11H, aromatics}; 9.64 [s,
1H, Hc
]. 13
C NMR (100 MHz, CDCl3): δ = 24.12 [Ca
]; 70.20 [Cb
]; {126.50; 126.70; 127.32;
127.45; 128.91; 129.40; 130.48; 131.42; 133.40; 134.33; 134.17; 135.56; 136.29; 145.18,
aromatics}; 162.09 [Cc
]. FTIR (neat) 2966, 1633 (cm-1
).
Ligand [C15H17N2I], LG, was obtained by combining N, N –dimethylethylenediamine (25.0
mg, 0. 283 mmol) and 8-iodo-1-naphthyladehyde (78.0 mg, 0.277 mmol) and stirring the
mixture in methylene chloride (15 mL) at room temperature for one hour. The solvent was
removed resulting in a light brown oil. Yield: 78.0 mg (80.1%). [C15H17N2I], LG. 1
H NMR
(400 MHz, CDCl3): δ = 2.37 [s, 3H, Ha
]; 2.80 [m, 2H, Hb
]; 3.84 [t, 2H, Hc
]; {7.10-8.28, 6H,
aromatics}; 9.82 [s, 1H, Hd
]. 13
C NMR(100 MHz, CDCl3): δ = 45.78 [Ca
]; 59.30 [Cb
];
59.59[Cc
]; {126.02; 126.87; 129.68; 129.78; 131.39; 132.69; 135.45; 141.20, aromatics};
163.24 [Cd
]. FTIR (neat) 2816, 1634 (cm-1
).
Ligand [C19H16IN], LH, was obtained by combining 8-iodo-1-naphthaldehyde (100.0 mg,
0.355 mmol), S-(-)-α-methylbenzylamine (47.9 mg, 0.394 mmol) and stirring the mixture in
methylene chloride (15 mL) at room temperature for ninety minutes. The solvent was
removed resulting in a pale brown oil. Yield: 126.2 mg (92.4%). [C19H16IN], LH. 1
H NMR
(400 MHz, CDCl3): δ = 1.77 [d, 3H, Ha
]; 4.72 [q, 1H, Hb
]; {7.07-8.21, 11H, aromatics}; 9.85
[s, 1H, Hc
]. 1
H NMR (400 MHz, CDCl3): δ = 1.70 [d, Ha
]; 4.68 [q, Hb
]; {7.27-9.03, 11H,
aromatics}; 9.64 [s, 1H, Hc
]. 13
C NMR (100 MHz, (CD3)2CO): δ = 23.58 [Ca
]; 69.64 [Cb
];
{126.23; 126.64; 126.74; 128.24; 129.20; 129.84; 130.98; 133.13; 135.10; 136.05; 145.30,
aromatics}; 160.50 [Cb
]. FTIR (neat) 2966, 1630 (cm-1
).
Ligand [C16H20N2], LQ, was obtained by combining N,N-dimethylethylenediamine (332.6
mg, 3.77 mmol) and 642.2 mg 2-methyl-1-napthaldehyde (516.0, 3.78 mmol) in methylene
chloride (15 mL) stirred for 40 minutes then refluxed for 15 minutes. The solvent was
34. 34
removed, and the mixture returned to solution in hexanes (6 mL) and stirred for 2 hours. The
solvent was removed resulting in pale orange oil. Yield: 693.4 mg (76.5 %). [C16H20N2], LQ.
1
H NMR (400 MHz, CDCl3): δ = 2.37 [s, 6H, Ha
]; 2.57 [s, 3H, He
]; 2.76 [t, 2H, Hc
]; 3.91 [t,
2H, Hb
]; {7.32-8.49, 6H, aromatics}, 8.94 [s, 1H, Hd
]. 13
C NMR (100 MHz, CDCl3): δ =
20.66 [Ce
]; 46.24 [Ca
]; 60.52[Cb
]; 61.33 [Cc
]; {125.33; 125.52; 127.18; 128.54; 129.46;
129.81; 130.80; 132.72; 135.91; 162.00, aromatics}; 161.97 [Cd
]. FTIR (neat) 2815, 1678
(cm-1
).
Ligand [C20H19N], LR, was obtained by combining 2-methyl-1-naphthaldehyde (140.4 mg,
0.825 mmol) and S-(-)-α-methylbenzylamine (140.5 mg, 1.16 mmol) in methylene chloride
(15 mL) and refluxing for 2 hours. The solvent was removed resulting in colourless oil.
Yield: 226.3 mg (71.4 %). [C20H19N], LR. 1
H NMR (400 MHz, CDCl3): δ = 1.72 [d, 3H,
Ha
]; 2.56 [s, 3H, Hd
]; 4.70 [q, 1H, Hb
]; {7.31-8.50, 11H, aromatics}; 9.06 [s, 1H, Hc
]. 13
C
NMR (100 MHz, CDCl3): δ = 20.80 [Cd
]; 25.97 [Ca
]; 72.06 [Cb
]; {125.22; 125.56; 127.17;
127.21; 127.39; 128.62; 129.00; 129.56; 129.89; 130.78; 132.05; 132.80; 136.06; 145.53,
aromatics}; 159.64 [Cc
]. FTIR (neat) 2976, 1638 (cm-1
).
Ligand [C9H14N2S], LW, was obtained by stirring N, N –dimethylethylenediamine (39.0 mg,
0.450 mmol) and 3-thiophene carboxaldehyde (50.0 mg, 0.450 mmol) together in methylene
chloride (15 mL) at room temperature for three hours. The solvent was removed resulting in
brown oil. Yield: 75.2 mg (91.0 %). [C9H14N2S], LW. 1
H NMR (400 MHz, CDCl3): δ = 2.25
[s, 6H, Ha
]; 2.56 [m, 2H, Hb
]; 3.63 [q, 2H, Hc
]; 7.46 [d, J= 8, 1H, Hf
]; 7.53[s, 1H, He
];
7.54[d, J = 4; 1H, Hg
]; 8.25 [s, 1H, Hd
]. 13
C NMR(100 MHz, CDCl3): δ = 45.74 [Ca
]; 59.87
[Cb
]; 59.89 [Cc
]; 125.55 [Ce
]; 126.91 [Cg
]; 128.52 [Cf
]; 140.52 [Ch
]; 155.92 [Cd
]. FTIR
(neat) 2848, 1639 (cm-1
).
35. 35
Figure 11, labelled protons and carbons of the ligands
Synthesis of Palladium(II) Complexes
Compound [Pd(I)(C15H17N2)], TB-01, was obtained from refluxing excess palladium (II)
acetate (31.0 mg, 0.138 mmol) and ligand LG (48.8 mg, 0.138 mmol) in toluene for 12 hours.
The solvent was removed producing a yellow oil. The product was washed and triturated in
ice-cold diethyl ether yielding a yellow solid. Yield: 21.0 mg (68%). [Pd(I)(C15H17N2)], TB-
01. 1
H NMR (400 MHz, CDCl3): δ = 2.83 [s, 6H, Ha
]; 3.97 [m, 2H, Hb
]; 4.03 [q, 2H, Hc
];
{7.16-8.17, 6H, aromatics}; 9.12 [s, 1H, Hd
]. 13
C NMR(100 MHz, (CD3)2CO): δ = 49.68
[Ca
]; 60.63 [Cb
]; 61.13 [Cc
]; {123.10; 125.08; 125.45; 128.81; 133.52; 135.37; 136.94;
138.85; 141.51; 149.88, aromatics}; 160.60 [Cd
]. E.S.I., (Parent Ion)-I m/z = 331.
Compound [Pd(Br)(C15H17N2)], TB-02, was obtained from refluxing palladium (II) acetate
(57.0 mg, 0.254 mmols) and LE (77.7mg, 255 mmols) in toluene for 12 hours. The solvent
36. 36
was removed producing a red oil. The product was washed and triturated in ice-cold diethyl
ether yielding a red solid. Yield: 15 mg (14%). [Pd(Br)(C15H17N2)], TB-02. 1
H NMR (400
MHz, CDCl3): δ = 2.82 [s, 6H, Ha
]; 4.02 [m, 2H, Hb
]; 4.09 [q, 2H, Hc
]; {7.18-8.29, 6H,
aromatics}; 8.84 [s, 1H, Hd
]. 13
C NMR(100 MHz, (CD3)2CO): δ = 49.32 [Ca
]; 60.97 [Cb
];
60.98 [Cc
]; {123.39; 125.26; 125.67; 128.39; 133.52; 135.37; 137.17; 138.85; 139.60;
144.82, aromatics}; 160.75 [Cd
]. E.S.I., (Parent Ion)-Br m/z = 331.
Compound [Pd(I)(PPh3)(C20H19N)], TB-03, was obtained from refluxing
tetrakis(triphenylphosphine)palladium(0) (90.3 mg, 0.0779 mmols) and LH (30.0 mg, 0.0779
mmols) in toluene for 36 hours. The solvent was removed producing a dark brown film. The
product was washed and triturated in ice-cold diethyl ether yielding a light-brown solid.
Yield: 28.4 mg, 49%. [Pd(I)(PPh3)(C20H19N)], TB-03. 1
H NMR (400 MHz, CDCl3): δ = 1.40
[s, 3H, Ha
]; 3.15 [s, 1H, Hb
]; 4.09 [q, 2H, Hc
]; {7.18-8.29, 6H, aromatics}; 8.84 [s, 1H, Hd
].
13
C NMR(100 MHz, (CD3)2CO): δ = 24.85 [Ca
]; 69.99 [Cb
]; {124.54; 125.25; 125.84;
126.58; 127.60; 127.95; 128.30; 128.33; 129.00; 129.23; 129.99; 131.00; 131.70; 132.72;
135.60; 141.42, aromatics}; 160.44 [Cc
]. 31
P NMR (162 MHz, CDCl3): δ = 29.43.
Compound [C15H17BrN2Pd], TB-04, was obtained from refluxing
tetrakis(triphenylphosphine)palladium(0) (96.0 mg, 0.0831 mmols) and LF (30.0 mg, 0.0888
mmols) in toluene for 24 hours. The solvent was removed producing a brown oil. The
product was washed and triturated in ice-cold diethyl ether yielding a dark brown solid.
Yield: 9.06 mg, 16%. [C15H17BrN2Pd], TB-04. 1
H NMR (400 MHz, CDCl3): δ = 2.37 [s, 6H,
Ha
]; 4.05 [m, 2H, Hb
]; 4.64 [q, 2H, Hc
]; {7.16-7.92, 6H, aromatics}; 9.01 [s, 1H, Hd
]. 13
C
NMR(100 MHz, (CD3)2CO): δ = 51.03 [Ca
]; 55.49 [Cb
]; 60.04 [Cc
]; {124.15; 125.88; 126.97;
127.99; 128.75; 131.05; 131.75; 133.17; 134.89; 141.89, aromatics}; 59.54 [Cd
]. 31
P NMR
(162 MHz, CDCl3): δ = 74.97.
37. 37
Compound [Pd(PPh3)2(C20H19N)], TB-05, was obtained from refluxing
tetrakis(triphenylphosphine)palladium(0) (127 mg, 0.110 mmols) and LR (30.1 mg, 0.110
mmols) in toluene for 12 hours producing. The solvent was removed a dark-brown solid. The
product was washed and triturated in ice-cold diethyl ether yielding a light-brown solid.
Yield: 63.1 mg, 58%. [Pd(PPh)2(C20H19N)], TB-05. 1
H NMR (400 MHz, CDCl3): δ = 1.69 [s,
3H, Hd
]; 2.51 [s, 1H, Ha
]; 4.67 [q, 1H, Hb
]; {7.18-8.29, 6H, aromatics}; 9.04 [s, 1H, Hc
]. 13
C
NMR (100 MHz, CD3Cl3): δ = 19.68 [Cd
]; 25.48 [Ca
]; 71.40 [Cc
]; {124.54; 125.25; 125.88;
126.58; 127.60; 127.99; 128.31; 128.39; 129.01; 129.33; 130.03; 131.05; 131.75; 132.77;
135.60; 145.42, aromatics}; 159.03 [Cc
]. 31
P NMR (162 MHz, CDCl3): δ = 29.03.
Compound [Pd(OAc)(C16H19N2)], TB-06, was obtained from refluxing palladium (II) acetate
(70.0 mg, 0.332 mmols) and LQ (74.3 mg, 0.331 mmols) in toluene for 12 hours. The solvent
was removed producing brown oil. The product was washed and triturated in ice-cold diethyl
ether yielding a yellow solid. Yield: 20.0 mg (15%). [Pd(OAc)(C16H19N2)], TB-06. 1
H NMR
(400 MHz, CDCl3): δ = 2.67 [s, 3H, He
]; 2.75 [s, 6H, Ha
]; 4.02 [m, 2H, Hb
]; 4.09 [q, 2H, Hc
];
{7.18-7.90, 5H, aromatics}; 8.61 [s, 1H, Hd
]. 13
C NMR (100 MHz, CDCl3): δ = 21.03 [Ce
];
49.32 [Ca
]; 60.52 [Cb
]; 61.82 [Cc
]; {124.98; 126.68; 127.57; 129.11; 129.40; 131.47; 136.43;
136.96; 138.26; 141.39, aromatics}; 156.91 [Cd
]. E.S.I., (Parent Ion) -OAc m/z = 345.
Compound [Pd(OAc)(C9H14N2S)], TB-07, was obtained from refluxing palladium (II) acetate
(36.9 mg, 0.165 mmols) and LW (30.0 mg, 0.165 mmols) in toluene for 12 hours. The
solvent was removed producing dark-red oil. The product was washed and triturated in ice-
cold diethyl ether yielding a red solid. Yield: 18.1 mg, 39%. [Pd(OAc)(C9H14N2S)], TB-07.
1
H NMR (400 MHz, CDCl3): δ = 2.70 [s, 6H, Ha
]; 3.70 [m, 2H, Hb
]; 3.79 [q, 2H, Hc
]; 7.04
[dd, J = 8, 2H, He/f
]; 8.31 [s, 1H, Hd
]. 13
C NMR (100 MHz, CDCl3): δ = 45.66 [Ca1
]; 48.26
[Ca
]; 59.98 [Cb
]; 64.05 [Cc
]; 124.90 [Cg
]; 125.88 [Ce
]; 126.09 [Cf
]; 128.26 [Ch
]; 156.20 [Cd
].
39. 39
Attempted Synthesis of Palladium(IV) Complexes
Compound [Pd(I)2(Me)(C9H14N2S)], TB-08, the synthesis was attempted by refluxing TB-01
(10.1 mg, 0.0221 mmols) and excess methyl iodide (6.00 mg, 0.0441 mmols) in toluene for
12 hours. The solvent was removed producing a dark brown solid. The product was washed
and triturated in ice-cold diethyl ether yielding a yellow solid. 1
H NMR Spectra showed
decomposition of the complex TB-01. The synthesis was also attempted by stirring TB-01
(10.0 mg, 0.0220 mmols) and methyl iodide (3.10 mg, 0.0220 mmols) in toluene at room
temperature for 48 hours. 1
H NMR Spectra showed slight decomposition of the complex TB-
01 but overall no reaction.
Compound [Pd(OAc)(I)(Me)(C16H19N2)], TB-09, the synthesis was attempted by refluxing
TB-06 (30.1 mg, 0.0742 mmols) and excess methyl iodide (10.0 mg, 0.0742 mmols) in
toluene for 12 hours. The solvent was removed producing a red-brown solid. The product was
washed and triturated in ice-cold diethyl ether yielding an orange solid. Yield: 15.2 mg
(43%), however it proved to be a different compound, TB-09.5 [Pd(I)(C16H19N2)], 1
H NMR
(400 MHz, CDCl3): δ = 2.81 [s, 6H, Ha
]; 3.91 [m, 2H, Hb
]; 4.04 [q, 2H, Hc
]; {7.10-7.93, 6H,
aromatics}; 8.96 [s, 1H, Hd
]. The synthesis was also attempted by stirring TB-06 (15.0 mg,
0.0371 mmols) and methyl iodide (5.01 mg, 0.0371 mmols) in toluene at room temperature
for 72 hours. The 1
H NMR spectra showed no overall reaction.
Compound [Pd(OAc)(I)(Me)(C16H19N2)], TB-09, the synthesis was attempted by refluxing
TB-06 (30.1 mg, 0.0742 mmols) and excess methyl iodide (10.0 mg, 0.0742 mmols) in
toluene for 12 hours, producing a red-brown solid. The product was washed and triturated in
ice-cold diethyl ether yielding an orange solid. Yield: 15.2 mg (43%), however it proved to
be a different compound, TB-09.5 [Pd(I)(C16H19N2)], 1
H NMR (400 MHz, CDCl3): δ = 2.73
[s, 3H, He
]; 2.81 [s, 6H, Ha
], 3.91 [m, 2H, Hb
]; 4.04 [q, 2H, Hc
]; {7.10-7.93, 6H, aromatics};
40. 40
8.96 [s, 1H, Hd
]. The synthesis was also attempted by stirring TB-06 (15.0 mg, 0.0371
mmols) and methyl iodide (5.01 mg, 0.0371 mmols) in toluene at room temperature for 72
hours. The 1
H NMR spectra showed no reaction.
[Pd(I)(OAc)(Me)(PPh3)(C20H19N)], TB-10, the synthesis was attempted by refluxing TB-03
(15.1 mg, 0.0200 mmols) and methyl iodide (2.80 mg, 0.0200 mmols) in toluene for 12
hours. The solvent was removed producing a yellow solid. The product was washed and
triturated in ice-cold diethyl ether yielding a dark yellow solid. The 1
H NMR spectra showed
complete decomposition of TB-03. The synthesis was also attempted by stirring TB-03 (30.0
mg, 0.0401 mmols) and methyl iodide (5.60 mg, 0.0400 mmols) in toluene for 120 hours.
The 1
H NMR spectra showed significant decomposition of TB-03, but no desired product.
[Pd(I)(Me)(PPh3)2(C20H19N)], TB-11, the synthesis was attempted by refluxing TB-05 (20.0
mg, 0.0225 mmols) and methyl iodide (3.00 mg, 0.0225 mmols) in toluene for 12 hours. The
solvent was removed producing a dark brown solid. The product was washed and triturated in
ice-cold diethyl ether yielding a brown solid. The 1
H NMR spectra showed complete
decomposition of TB-05. The synthesis was also attempted by stirring TB-05 (30.0 mg,
0.0331 mmols) and excess methyl iodide (5.00 mg, 0.0400 mmols) in toluene for 72 hours.
The 1
H NMR spectra showed slight decomposition of TB-05, but no desired product.
Recrystallisations
TB-01: Trial 1: approximately 3 mg of TB-01 were dissolved in a minimal amount of
acetone and layered on top with cold diethyl ether. The system was left at room temperature
and after 5 days, crystals were formed. Crystals were sent for X-Ray diffraction studies,
however no crystal structure was refined.
TB-01: Trial 2: approximately 4 mg of TB-01 were dissolved in a minimal amount of
chloroform and placed in a 1-dram vial. The vial was placed in a scintillation vial which was
41. 41
filled with 5 mL of pentane. The system was left at room temperature and after 3 days,
crystals were formed. Crystals were sent for X-Ray diffraction studies and the structure was
determined and refined.
TB-02: Trial 1: approximately 2 mg of TB-02 were dissolved in a minimal amount of
chloroform and layered on top with pentane. The system was left at room temperature and
after 4 days, crystals were formed. Suitable crystals for X-Ray diffraction studies were not
available.
TB-02: Trial 2: approximately 4 mg of TB-02 were dissolved in a minimal amount of
chloroform and placed in a 1-dram vial. The vial was placed in a scintillation vial which was
filled with 5 mL of diethyl ether. The system was left at room temperature and after 4 days,
crystals were formed. Crystals have yet to be sent for X-Ray diffraction studies.
TB-03: approximately 3 mg of TB-03 were dissolved in a minimal amount of chloroform
and placed in a 1-dram vial. The vial was placed in a scintillation vial which was filled with 5
mL of diethyl ether. The system was left at room temperature and after 4 days, powder was
formed instead of crystals. The fine powder was dissolved for NMR spectral analysis.
TB-05: approximately 3 mg of TB-03 were dissolved in a minimal amount of chloroform
and placed in a 1-dram vial. The vial was placed in a scintillation vial which was filled with 5
mL of diethyl ether. The system was left at room temperature and after 3 days, powder was
formed instead of crystals. The fine powder was dissolved for NMR spectral analysis.
TB-06: approximately 3 mg of TB-06 were dissolved in a minimal amount of acetone and
layered on top with diethyl ether. The system was left at room temperature and after 3 days,
crystals were formed. Crystals were sent for X-Ray diffraction studies, crystal structure was
determined and refined.
42. 42
TB-07: Trial 1: approximately 3 mg of TB-07 were dissolved in a minimal amount of
chloroform and layered on top with diethyl ether. The system was left at room temperature
and after 2 days, crystals were formed. Suitable crystals for X-Ray diffraction studies were
not available.
TB-07: Trial 2: approximately 4 mg of TB-07 were dissolved in a minimal amount of
chloroform and placed in a 1-dram vial. The vial was placed in a scintillation vial which was
filled with 5 mL of diethyl ether. The system was left at room temperature and after 3 days,
crystals were formed. Crystals have yet to be sent for X-Ray diffraction studies.
