1. School of Engineering and Science
Department of Chemistry
Prof. Dr. Ulrich Kortz
Bachelor Thesis
Titanium-Containing Tungstoarsenates
Written and Submitted by
Akina Carey
on
May 24, 2013
Matr.-No: 20329249
Supervised by
Prof. Dr. Ulrich Kortz
Lab supervision by
Dr. Bassem Bassil
Submitted in support of the degree
Bachelor of Science in Chemistry
2. 1
Abstract
Polyoxometalates (POMs) are polyanions consisting of early transition metals, such as W6+
,
Mo6+
, and V5+
, in high oxidation states. Polyoxotungstates (POTs) are a subclass of POMs
which are formed by self-assembly of tungstate ions, usually in aqueous medium. When
arsenic is added as the heteroatom to the POTs, they are referred to as tungstoarsenates.
POM chemistry has been a research topic of interest due to its potential applications in a
range of fields such as catalysis, magnetism, medicine and materials science.(1) (2)
There are several different structural types of heteropolytungstates, one being the Keggin
anion with the general formula of [XW12O40]m-
. Two tri-lacunary α-Keggin species can be
synthesized, A-α, [A-α-XM9O34]p-
, and B-α, [B-α-XM9O34]q-
. The B-α tri-lacunary species is of
particular interest, in particular when a lone pair is present instead of terminal oxygen
resulting in a molecular formula [B-α-XM9O33]s-
.
By finding a recrystallization procedure of the crude salt [As2W19O67(H2O)]14-
,
K10Na4[As2W19O67(H2O)]∙24H2O was produced. This recrystallization using a 1M NaOAc buffer
solution at pH 6 allowed for a crystal structure and 183
W solution NMR to be performed, which
could not previously been done. The recrystallized salt showed a stabilization of potassium
cations within the cavity of the ‘pacman’ structure of [As2W19O67(H2O)]14-
as well as the
presence of sodium cations surrounding the compound, which is necessary for the
recrystallization.
Using the recrystallized salt, K10Na4[As2W19O67(H2O)]∙24H2O, we were able to produce the
corresponding titanium containing product, K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O in a 1M KCl
solution. This structure is identical to the previously reported
Cs8[Ti2(OH)2As2W19O67(H2O)]∙18H2O.(3)
. This optimized synthesis allowed for a higher yield and
3. 2
purer final product which could also dissolve in H2O at pH 2 to allow for 183
W solution NMR to
be performed. IR Spectroscopy, TGA, single-crystal XRD, and 183
W solution NMR were done on
both the {As2W19} recryst. and titanium-containing product for characterization.
While investigating the optimal reaction conditions to allow for pure {Ti2As2W19} to form, a
hexameric titanium-containing product formed using the same reaction procedure as the K-
salt of {Ti2As2W19}, however in 1M NaCl instead of 1M KCl. Investigations are still being
conducted regarding this product.
We are also further investigating the formation of a proposed intermediate when using the
TBA salt of {Ti2As2W19} as a catalyst to activate H2O2. (4)
This structure has been confirmed,
however further investigation is required before a full explanation can be proposed. We are
also awaiting elemental analysis results for {As2W19} recryst. and K- and TBA- salts of
{Ti2As2W19}.
4. 3
Table of Contents
Abstract ...................................................................................................................1
Table of Contents....................................................................................................3
List of Figures ..........................................................................................................4
List of Instrumentation...........................................................................................4
List of Abbreviations...............................................................................................5
Introduction ............................................................................................................6
Historical Background ......................................................................................6
Structural Background .....................................................................................7
Keggin Structure (isomers and lacunary species)..........................................9
Motivation and Objective ....................................................................................12
Experimental Procedure.......................................................................................15
Synthesis of [As2W19O67(H2O)]14- (12)
..............................................................15
Recrystallization of K14[As2W19O67(H2O)] as K10Na4[As2W19O67(H2O)]∙24H2O
.........................................................................................................................15
Synthesis of K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O(3)
.....................................15
Synthesis of the TBA salt of [Ti2(OH)2As2W19O67(H2O)]8- (3)
.........................16
Discussion..............................................................................................................17
K10Na4[As2W19O67(H2O)]∙24H2O ....................................................................17
K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O ..............................................................21
{Ti10As6W59}.....................................................................................................26
Figure 13: IR comparison for the K-salt of {Ti2As2W19} and {Ti10As6W59}..27
Figure 14: Crystal Structure of the {Ti10As6W59}..........................................28
Conclusion .............................................................................................................30
Appendix................................................................................................................32
Appendix A: Table of Conducted Experiments............................................32
Appendix B: TGA measurement and comparison between
{As2W19} recryst. washed and not-washed samples.............33
Appendix C: NMR for {As2W19} recryst. .......................................................34
Appendix D: TGA measurement of K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O...35
Bibliography ..........................................................................................................36
Acknowledgements..............................................................................................38
Affidavit ..................................................................Error! Bookmark not defined.
5. 4
List of Figures
Figure 1: a) Anderson-Evans anion b) Wells-Dawson anion c) Keggin anion.......................... 8
Figure 2: Baker-Figgis isomers with respective symmetry.....................................................10
Figure 3: Formation of lacunary α-Keggin anions...................................................................11
Figure 4: Structural Representation of {As2W19} and {Ti2As2W19} ........................................13
Figure 5: Outline of the project ................................................................................................ 14
Figure 6: IR spectra comparison of the crude and recrystallized salts of {As2W19} .............17
Figure 7: Symmetry and structure of K10Na4[As2W19O67(H2O)] .............................................19
Figure 8: NMR explanation of {As2W19} recryst......................................................................21
Figure 9: IR spectra comparison between the Cs- and K-salt of {Ti2As2W19} .......................22
Figure 10: Symmetry and Structure of {Ti2As2W19}................................................................ 23
Figure 11 NMR for the K-salt of {Ti2As2W19} ...........................................................................24
Figure 12: IR comparison of the TBA- and K-salt of {Ti2As2W19}............................................25
Figure 13: IR comparison for the K-salt of {Ti2As2W19} and {Ti10As6W59}............................. 27
Figure 14: Crystal Structure of the {Ti10As6W59} .....................................................................28
List of Instrumentation
Nicolet Avatar 370 FT-IR Spectrophotometer using KBr pellets
JEOL 400 MHz ECX Nuclear Magnetic Resonance Instrument
Bruker D8 SMART APEX II CCD Diffractometer
TA Instruments SDT Q600 Thermobalance
7. 6
Introduction
Historical Background
Polyoxometalates (POMs) are a subset of metal oxide clusters which are of interest due to
their range of physical properties and structural abilities. (2)
The first POM was synthesized in
1826 by Jöns Jakob Berzelius, a Swedish chemist. (5)
He discovered that when adding
ammonium molybdate to phosphoric acid, a yellow precipitate formed leading to the now
known ammonium 12-molybdophosphate, (NH4)3PMo12O40. (6)
This discovery was later
enhanced upon when its structure was investigated by Svanberg and Struve in 1848. In 1862,
the next significant step came when Marignac discovered tungstosilicic acids and their
corresponding salts (7)
. With this discovery, Werner created his coordination theory to explain
the compositions and structures of heteropolyanions. (1)
His understanding and theory
resulted in him receiving a Nobel Prize in Chemistry in 1913 for his contributions. This
achievement later became the foundations of modern coordination chemistry. Werner’s
achievements were later advanced by Miolati and Pizzighelli in 1908 and even further by
Rosenheim. From this, the Miolati-Rosenheim theory was introduced stating that
heteropolyacids were based on 6-coordinate heteroatoms with a MO4
2-
or M2O7
2-
anions as
ligands or bridging groups.
This was criticized in 1929 by Pauling when he recognized that the ionic radii of Mo6+
and W6+
were suitable for an octahedral coordination by corner oxygen. Pauling suggested that each of
the MO6 (M= Mo, W) encapsulated a central tetrahedron, XO4 (X=heteroatom), for a 12:1
complex. This gave a more accurate explanation for the observed basicity than what would be
observed using the Miolati-Rosenheim theory.
8. 7
Pauling’s criticism was later corrected by Keggin when he revealed the structure of the
polyanion H3[PW12O40]∙5H2O by use of XRD. The crystal structure indicated that the octahedral
structure by WO6 was actually due to the linkage of both corner- and edge-sharing between
octahedra. (8)
Reports of isomorphous complexes of the “Keggin ion” followed this correction,
resulting in the reports of the structures of Evans-Anderson, Lidqvist, and Wells-Dawson.(1)
Structural Background
POMs are synthesized using aqueous solutions. This results in the condensation of octahedral
MO6 units which are linked via three types of sharing; edge, corner, and less frequently face.(3)
POMs are usually composed of early transition metals MO6 (M=W6+
,Mo6+
, V5+
, Nb5+
,Ta5+
) with
heteroatoms XO4 (X=P, Si, et). They form a structurally distinct class of complexes based
mainly on a quasi-octahedrally coordinated metal atom. Since an octahedral coordination is
formed by the metal atom, the maximum coordination number of these atoms needs to be six
to fulfill the structural requirements. In addition to the coordination, the early transition metal
atoms which can be used in such structures are limited based on their ionic radius and charge,
and the ability to accept pπ electrons from oxygen to form stable dπ-pπ M-O bonds. This,
therefore, reduces the number of metal atoms which can be used. There are no such
restrictions of the heteroatom. (1)
POMs are generally classified as polyanions which can be separated into two sub-categories:
iso- and hetero-polyanions. Isopolyanions consist only of the addenda atom, M, in its highest
oxidation state and bridged via oxygen, while, heteropolyanions contain a heteroatom, X.
9. 8
These polyanions are formulated as follows:
Isopolyanions: [MmOy]p-
M=Mo, W, V, Nb, Ta
Heteropolyanions: [XxMnOy]q-
(x ≤ m)
If tungsten is the addendum, the anion is referred to as being a Polyoxotungstate (POT). POTs
are of particular importance for this work. They are known for their high stability in solution
and are one of the most studied heteropolyanions in POM chemistry. Common structures of
POTs which can be observed are the Anderson-Evans anion [XW6O24]m-
, Wells-Dawson anion
[X2W18O62]m-
, and the Keggin anion [XW12O40]m-
(Figure 1).(1)
Since this thesis focuses on the synthesis and characterization of arsenic- and titanium-
containing POTs using the lacunary Keggin anions as a building block, the structures of the
Keggin anions will be discussed in further detail below.
Figure 1: a) Anderson-Evans anion b) Wells-Dawson anion c) Keggin anion
10. 9
Keggin Structure (isomers and lacunary species)
The Keggin anion [XW12O40]m-
is named after its discoverer James Fargher Keggin. While being
a doctoral student at the University of Manchester, he was the sole author of a publication
stating that he determined the structure of the free 12-tungstophosphoric acid by using
powder XRD. (9)
This was formed by the condensation of [MO6] octahedra while in the
presence of tetrahedrally shaped heterooxoanions [XO4].
The Keggin structure consists of the tetrahedron-shaped core surrounded by twelve [MO6]
octahedra. These twelve octahedra are grouped together in four groups of three [MO6]
octahedra, referred to as a triad. Each octahedron in the triad is linked by oxygen resulting in
edge sharing within each triad. Each triad is linked by corner-shared oxygen. The four triads
are also linked to the central tetrahedron by corner sharing. The Keggin structure, in the
idealized form, should have point-group symmetry of Td.(10)
The Keggin anion has five rotational isomers which can be obtained by rotating each triad by
60°. These can also be referred to as the Baker-Figgis isomers. (11)
The α-isomer is designated
to the original structure with no-rotated triads; the β-, γ-, and δ- isomers have one, two, or
three rotated triads respectively; and finally the ε-isomer has all four triads rotated (Figure 2).
11. 10
As well as having isomers of the Keggin structure, vacant, or more commonly referred to as
lacunary, species can be obtained from the α-, β-, and γ isomers. This can be done by
removing a number of [MO6] octahedra usually by base hydrolysis. (1)
Since the α-isomer is the
main focus of this thesis, the lacunary species of the α-isomer will be explained.
A lacunary species is commonly described based on the number of octahedron which is
removed; when one, two, or three octahedra are removed, they are referred to as the mono-,
di-, and tri-lacunary species, respectively. When one octahedron is removed, the resulting
mono-lacunary species has a formula of [α-XM11O39]m-
. There are two possibilities when
removing a triad; either by corner-shared octahedra [A-α-XM9O34]p-
, or by edge-shared
octahedra [B-α-XM9O34]q-
. Typically the B-α lacunary anion has one terminal oxygen which can
be quite reactive, however there is the possibility that a lone-pair is present instead (Figure 3).
This species is called a lone-pair containing B-α lacunary Keggin anion, [B-α-XM9O33]s-
. The
(blue: non-rotated triad; red:
rotated triad)
Figure 2: Baker-Figgis isomers with respective
symmetry
12. 11
latter is the building block of one of the reactants used in this thesis as well as part of the main
structure of the titanium-containing polyoxotungstate (Figure 4).
Detailed structural description of the reactant is discussed in the course of the thesis.
Figure 3: Formation of lacunary α-Keggin
anions
13. 12
Motivation and Objective
In the last few hundred years, the development and characterization of POMs have increased
dramatically ranging from not only the number but also the size of the structures. Titanium-
containing POMs have been increasingly observed and studied since the development of a
microporous titanium-silicalite TS-1 species in the 1908s by the Enichem group. (4)
Since then,
the interest of titanium-containing POMs has increased. They are of particular interest not
only for their structural aspect but of also their application in single-site titanium catalysis.(3)
The Kortz group took this into consideration, and in 2007, synthesized and characterized
Cs8[Ti2(OH)2As2W19O67(H2O)] by using [As2W19O67(H2O)]14-
as a reactant (Figure 4). In
collaboration with the Boreskov Institute of Catalysis in Russia and the Laboratoire de Chimie
Physique in France, the electrochemical and catalytic properties were also studied.(3)
{As2W19}
was used due to its capability to host two titanium atoms within its open cavity as well as the
presence of the unique linking tungsten bound to an external aqua and internal oxo ligands.
When titanium is present in the structure, there are terminal hydroxo ligands on the titanium
centers. This is the where the catalysis occurs since these hydroxo groups are highly reactive.
14. 13
There have been many publications by the Kortz group along with various collaborators,
mainly with collaborations in Russia, on the catalytic properties using the tetra-n-
butylammonium (TBA) salt of [Ti2(OH)2As2W19O67(H2O)]8-
. TBA was used due to the catalytic
work being done in organic solvents. (3) (4)
The goal of my research project was to determine a method to recrystallize {As2W19} in order
to obtain a crystal structure as well as a pure product, create more efficient synthesis and
further characterize for the previously reported {Ti2As2W19}, and synthesize and characterize
its pure TBA salt (Figure 5). To determine the best synthetic procedure, different solutions as
well as the use of crude versus recrystallized salt of {As2W19}, if possible, and ratios between
reactant and TiOSO4 starting material will be investigated.
a) [As2W19O67(H2O)]14-
b) of [Ti2(OH)2As2W19O67(H2O)]8-
Blue polyhedra: WO6; Blue: W; Yellow: As; Green: Titanium; Red: Oxygen
Figure 4: Structural Representation of {As2W19} and {Ti2As2W19}
16. 15
Experimental Procedure
Synthesis of [As2W19O67(H2O)]14- (12)
As2O3 (0.89g, 4.5mmol), Na2WO4∙2H2O (18.8g, 57mmol), and KCl (0.67g, 9.0mmol) were
added to 50mL H2O at 80°C with constant stirring. After dissolution, the pH was adjusted to
6.3 by adding 12M HCl dropwise. The solution was kept at 80°C with stirring for 10 minutes
and afterwards allowed to cool to room temperature and filtered. After filtration, KCl (15g,
201mmol) was added and solution stirred for 15 minutes. A white precipitate was formed,
isolated by filtration, and dried at 80°C.
Recrystallization of K14[As2W19O67(H2O)] as K10Na4[As2W19O67(H2O)]∙24H2O
10g {As2W19} was added to 200mL NaOAc buffer solution at pH 6 with constant stirring. The
solution was left to stir for 1 hour before being filtered. The resulting solution was left to
crystallize. After a few weeks, the formed crystals were then collected and air dried.
Synthesis of K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O(3)
TiOSO4 (0.14g, 0.88mmol, Merk) was dissolved in 20mL 1M KCl( 1.5g, 20.1mmol, D-D H2O) at
room temperature with constant stirring followed by the addition of {As2W19} recryst. (1.05g,
0.20mmol). The pH was then adjusted to 2 using 6M HCl. The solution was then heated to
80°C for 1 hour, cooled to room temperature, and filtered. The solution was left in a closed
vial overnight for precipitation. The following day, the solution was separated into two vials
and allowed to crystallize.
17. 16
Synthesis of the TBA salt of [Ti2(OH)2As2W19O67(H2O)]8- (3)
1g of the K-salt of {Ti2As2W19} (1g, 0.18mmol) was added to TBABr (1g, 3.1mmol) in 10mL of
D-D H2O at pH 2. The mixture was stirred for 5 to 10 minutes to allow for complete conversion
and precipitation. The final product was collected by filtration and dried in an oven at 80°C
overnight.
*See Appendix A for full outline of reactions done.
18. 17
Discussion
K10Na4[As2W19O67(H2O)]∙24H2O
[As2W19O67(H2O)]14-
was synthesized by reacting As2O3 (4.5mmol), Na2WO4∙2H2O (57mmol),
and KCl (9.0mmol) in deionized H2O at 80°C, pH adjusted to 6.3, and KCl was added to
precipitate the product. The white material was then collected, dried in an oven, then
confirmed using IR.
This crude salt product was then recrystallized using 1M NaOAc at pH 6, which allowed for a
pure salt to be collected. The two salts were checked using IR to determine whether the
recrystallized salt actually was the same as synthesized (Figure 6).
When comparing the two IR spectra for the crude and recrystallized salts, it can be seen that
the two spectra are quasi-identical fingerprints of each other. After determining that the
recrystallized salt still corresponds to the intended crude salt, single-crystal XRD confirmed the
‘pacman’-resembling di-lacunary polyanion. There are two B-α-{AsW9} subunits connected by
a center linking WO(H2O), with the oxo group facing to the interior and hydroxo group
Purple: crude salt Red: recrystallized salt
Figure 6: IR spectra comparison of the crude and recrystallized salts of {As2W19}
19. 18
exterior. The lone pair two B-α-{AsW9} subunits repel the two units slightly away from each
other creating the ‘pacman’ shape. This allows only for the smaller oxo group (compared to
the bulkier aqua ligand) from the linking tungsten center to be on the interior of the structure.
The {As2W19} structure has a 2-fold rotational axis going through the O-W-H2O group. There
are also two mirror planes containing the rotational axis and perpendicular to each other. This
results in the overall structure having a C2V point group symmetry (Figure 7a). The crystal
structure also showed potassium cations surrounding the structure as well as being present
within the cavity between the two {As2W9} subunits (Figure 7b). It can be proposed that they
are present to help stabilize the cavity between the subunits. Sodium cations are also present,
however only surrounding the structure, proposing that they are only needed to help in the
recrystallization of the product. The exact number of Na and K cations present can currently
only be estimated until the results of elemental analysis have arrived.
20. 19
TGA was then performed to determine the number of crystal waters present in the sample.
Two measurements were done, one using the pure sample and second using a sample that
had been washed with ethanol. This was done to ensure that no acetate was left from the
recrystallization processes. The material was heated to 400°C and an 8.25% weight loss was
observed. Little to no change occurred between the two samples indicating that acetate was
not present. A small shift occurred however this was due to a recalibration of the instrument
between the measurements, see Appendix B for spectrum. After the measurement, the
estimated formula for the recrystallized salt, using 10-K and 4-Na estimate in the sample, was
K10Na4[As2W19O67(H2O)]∙24H2O.
Blue octahedra: WO6; Yellow: As; Red: O; Pink: K; Dark blue: Na
Figure 7: Symmetry and structure of K10Na4[As2W19O67(H2O)∙18H2O]
a) Symmetry of {As2W19} recryst. (cations are removed)
b) Crystal structure of {As2W19} recryst.
21. 20
183
W solution NMR of {As2W19} recryst. was also done to observed solution stability. The
measurement was performed in 1M NaOAc pH 6 as solvent. It was expected that six peaks in a
4:4:4:4:2:1 ratio would be present in the spectrum, however only five peaks are
distinguishable. The peaks of relative intensity 4, located at chemical shifts around -96ppm, -
102ppm, -130ppm, and -142ppm, shown using the purple arrows in Figure 8, correspond to
the tungsten atoms in the octahedral numbered 1, 2, 3, and 4, also shown in Figure 8. It must
be kept in mind that the assignments in Figure 8 correspond to one subunit of the total
structure. The peak at -71ppm, indicated with the red arrow, would correspond to the central
linking tungsten since the environment surrounding this tungsten is different compared to the
others. The peak of intensity 2 corresponding to the tungsten in 5, indicated by orange arrow,
cannot be seen due to the signal being lost in the noise. There is a presence of possible three
peaks; however this cannot be differentiated from the background noise. The signal-to-noise
ratio is relatively poor due to the incomplete dissolution in the solvent, resulting in a reduced
concentration. High concentration is usually needed since the natural abundance of 183
W is
only 14.4%. (13)
LiOAc was also tried as a solvent, however no peaks were observed. Other
solvents to improve the signal-to-noise ratio are currently being investigated. See Appendix C
for the NMR Spectrum alone, without explanation.
22. 21
K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O
In order to determine the optimal synthesis to cleanly and effectively produce {Ti2As2W19},
various paths were investigated. The first investigation was using different TiOSO4-to-reactant
ratio; mainly 4:1 and 2:1. Secondly, two different TiOSO4 sources were used; namely Merck
and Riedel-de-Haen. Finally, different reaction solutions were investigated, in particular D-D
H2O, 1M KCl, 1M NH4Cl, and 1M NaCl. Appendix A lists the different conditions explored. After
optimization of the conditions, it was concluded that in D-D H2O, the 2:1 ratio gave the
desired product {Ti2As2W19}.The Cs and K-salts of {Ti2As2W19} were effectively crystallized
with the respective addition of Cs+
and K+
counter-cations. Moreover, 1M KCl and 1M NaCl
solutions were also investigated and showed promising results. Using 1M KCl and a 4:1 ratio,
Figure 8: NMR explanation of {As2W19} recryst.
23. 22
the desired product was obtained, however in as a K-salt instead of the previously reported
Cs-salt. To check whether the product was the desired one, a comparison between the Cs-
and K-salts were done using IR Spectroscopy (Figure 9).
When overlaying the two spectra, it can be seen that the spectra are very similar. The peaks
are generally in the same position with some minor exceptions which can be explained by the
presence of potassium instead of cesium.
Once it was determined that the intended product was produced, single-crystal XRD was done
to determine the crystal structure. The crystal structure revealed a sandwich-like structure
consisting of the two B-α-{AsW9} subunits, with a WO(H2O) central linkage (H2O facing
exterior, oxo group facing interior of the main structure). Two TiO4(OH) groups are present in
a square pyramidal coordination in between the two B-α-{AsW9} subunits. The OH terminal
Red: Cs-salt Blue: K-salt
Figure 9: IR spectra comparison between the Cs- and K-salt of {Ti2As2W19}
24. 23
groups on the titanium are facing ‘outwards’ of the structure (Figure 10b). The structure
contains a 2-fold rotation axis going through the O-W-H2O axis, as well as having two mirror
planes, both perpendicular and parallel to the rotation axis, therefore giving the structure C2V
point group symmetry (Figure 10a).
To insure that a collection of the pure product was achieved, the yellow crystals were
harvested regularly. After the initial growth in 2-4 days, the crystals were removed, dried and
collected. They were collected on a day-to-day basis and each collection was monitored with
IR Spectroscopy.
a) Symmetry of {Ti2As2W19}
b) Crystal structure of {Ti2As2W19}
Blue octahedra: WO6; Yellow: As; Red: O Blue: W Green: Ti
Figure 10: Symmetry and Structure of {Ti2As2W19}
25. 24
TGA was done of the K-salt to determine the number of crystal waters in the sample. The
measurement was performed at 400°C and a 6.36% weight loss was recorded. The full
formula for the K-salt of {Ti2As2W19} was found to be K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O. See
Appendix D for TGA spectrum.
In order to determine solution stability, 183
W solution NMR was done on the K-salt in D-D H2O
at pH 2 (pH of the synthesis). Since the point group symmetry of {Ti2As2W19} is the same as
the one of {As2W19}, namely C2v, the expected number of peaks should remain at 6, which is
what was observed (Figure 11). However, the peaks observed have an intensity ratio of
4:4:4:4:2:2. The two lower intensity peaks should correspond to the two octahedra in the two
subunits and the central linking tungsten. The peak at -120ppm corresponds to the former,
and the peak at -30ppm would correspond to the latter. The reason for the peak being higher
than expected could not be determined so far.
Figure 11 183
W NMR for the K-salt of {Ti2As2W19}
26. 25
As previously mentioned, titanium-containing POMs are of particular interest due to their
catalytic properties. The collaboration with the Boreskov Institute of Catalysis in Russia
showed this importance. (3) (4)
The TBA-salt of {Ti2As2W19} was therefore needed to perform
catalytic reactions in organic medium. In order to prepare this, the K-salt and TBABr were
added together in D-D H2O at pH 2, and the precipitate was collected and dried in the oven.
The IR spectrum of the TBA salt was collected and compared to the one of the K-salt in order
to verify that the structural motive of {Ti2As2W19} is still intact.
As seen in Figure 12, the peaks are relatively corresponding to each other, with the exception
of the addition of peaks for the TBA-salt at wavelengths 1050, 800, and 480 cm-1
. These peaks
can be explained by the presence of TBA in the product, rather than K. 183
W solution NMR was
Red: TBA salt of {Ti2As2W19}
Blue: K-salt of {Ti2As2W19}
Figure 12: IR comparison of the TBA- and K-salt of {Ti2As2W19}
27. 26
also performed on the TBA salt in acetonitrile, however the results were inconclusive. Further
investigations to allow for conclusive peaks are pending. In order to confirm the composition
of the TBA salt, the sample was sent for elemental analysis.
{Ti10As6W59}
As part of optimizing the synthesis for {Ti2As2W19}, different reaction solutions (D-D with
counter-cation, 1M KCl, 1M NH4Cl, 1M NaCl) were used. As previously mentioned, 1M KCl
proved to produce optimal yields and purity. It was expected that each solution would
produce a derivative of the previously reported Cs8[Ti2(OH)2As2W19O67(H2O)]. However, when
1M NaCl was used, an unexpected compound exhibiting a different structure and composition
was isolated. The reaction was performed in the same manner as for the K-salt; reacting
TiOSO4 and {As2W19} recryst. in a 4:1 ratio in 1M NaCl (instead of 1M KCl). Once it was
apparent that there was a formation of a product, IR Spectroscopy was done to compare the
sample to that of the K-salt.
28. 27
It can be seen in Figure 13, that the IR comparison between the K-salt and the 1M NaCl
product shows differences, with the additional peaks at 800cm-1
, 630cm-1
, and 430cm-1
for 1M
NaCl. These alterations would not solely account for the presence of sodium in the known
structure since there are too many differences between the two spectra, but rather for
substantial structural differences.
To understand completely the cause for this alteration, single-crystal XRD was performed to
determine the crystal structure of this unknown compound. The resulting structure does not
form a sandwich structure as seen in {Ti2As2W19}.
Red: K-salt of {Ti2As2W19}
Purple: {Ti10As6W59}
Figure 13: IR comparison for the K-salt of {Ti2As2W19} and {Ti10As6W59}
29. 28
The structure reveals an S-shaped structure containing two subunits (Figure 14). There are six
{AsW9} units (distributed equally between two subunits), four linking tungsten, and one
central tungsten. The central tungsten, like the {As2W19} reactant and {Ti2As2W19} compound,
has oxo and aqua ligands. This tungsten can either be shifted towards the oxo ligand or the
aqua ligand, resulting in distortion and shown as two separate tungstens in the crystal
structure, each with 50% occupancy.
Five titanium atoms are located in each subunit, three having square pyramidal coordination
and two having octahedral coordination. The former three are sandwiched between the
a) Side view b) full view
Blue octahedra: WO6 ; Red octahedra: linking W; Green: Ti
Blue: W; Yellow: As; Red: O
Figure 14: Crystal Structure of the {Ti10As6W59}
30. 29
{AsW9} units, and the latter located located in the center of the three {AsW9} units, with one
having three terminal aqua ligands. These five titanium atoms are directly linked together,
three being linearly aligned and two linked in a perpendicular fashion to the aligned atoms.
The short-hand formula has been determined as {Ti10As6W59}. The exact number of oxygen
atoms cannot be deduced at this time.
The structure has a 2-fold rotation axis containing the central tungsten as well as a single
mirror plane running perpendicular to the rotation axis resulting in C2h point group symmetry.
On the other hand, the structure of {Ti10As6W59} can be compared to the chair conformation
of hexane, with each {AsW9} unit sitting on the relative carbon center. Further
characterizations, such as determining the exact molecular formula, solution stability, and
other properties, are planned.
31. 30
Conclusion
Starting from the crude salt of [As2W19O67(H2O)]14-
, the recrystallized salt,
K10Na4[As2W19O67(H2O)], was successfully made in 1M NaOAc pH 6 buffer solution. Using IR
spectroscopy and single crystal XRD, the structure of recrystallized salt was confirmed to be
indeed that of the crude product. The resulting crystal structure showed a ‘pacman’ structure
containing two B-α-{AsW9} subunits connected by a center linking WO(H2O), with the oxo
group facing to the interior and the aqua group to the exterior. It was also shown that
potassium cations help stabilize the cavity of the structure, and that sodium cations, which
surround the exterior of the structure, are present to allow for recrystallization. 183
W solution
NMR was also performed showing that the relative structure stays intact in solution.
Elemental analysis results are pending.
The ability to recrystallize the reactant removes any impurities which might prevent or hinder
the formation of the required {Ti2As2W19} product. After experimenting with different TiOSO4
sources, types of reaction solutions, and reactant ratios, the optimal conditions to produce a
pure product with a relatively high yield is using a 4:1 Ti-to-precursor ratio, using Merck as
source of titanium, and in 1M KCl solution at pH 2.0. The product of the optimized synthesis
was characterized by single-crystal XRD showing a sandwich structure containing the {As2W19}
structure with two Ti atoms located in the cavity. 183
W solution NMR was also performed and
the spectrum was found to correspond to the structure; elemental analysis results are
pending.
Following the determination of the optimal synthesis of {Ti2As2W19}, the corresponding TBA-
salt was also produced in the context of oxidation catalysis in organic medium. Further
32. 31
investigation is necessary to obtain a conclusive 183
W solution NMR spectrum. Elemental
analysis results are pending.
In addition, a novel compound using 1M NaCl as reaction medium was obtained, and single-
crystal XRD was done to determine the structure. The novel polyanion, titled {Ti10As6W59},
corresponds to six {AsW9} units arranged in a cyclic fashion and encapsulating 10 Ti4+
cation,
with five bridging tungsten groups. Further investigations regarding TGA, elemental analysis,
and applications will need to be investigated further, as well as sent to collaborators for
further studies. The synthesis of {Ti10As6W59} opens the door for new strategies in the
synthesis of Ti4+
-containing tungstoarsenates and POMs in general.
37. 36
Bibliography
1. Pope, Michael T.; Heteropoly and Isopoly Oxometalates.; s.l. : Springer-Verlag; 1983.
2. Long, D.-L., Tsunashima, R. and Cronin, L. (2010), Polyoxometalates: Building Blocks for
Functional Nanoscale Systems. Angew. Chem. Int. Ed., 49: 1736–1758.
3. Hussain, F., Bassil, Bassem S., Kortz, U., Kholdeeva, Oxana A., Timofeeva, Maria N., de
Oliveira, P., Keita, B. and Nadjo, L. (2007), Dititanium-Containing 19-Tungstodiarsenate(III)
[Ti2(OH)2As2W19O67(H2O)]8−
: Synthesis, Structure, Electrochemistry, and Oxidation
Catalysis. Chem. Eur. J., 13: 4733–4742.
4. Kholdeeva, O. A., Donoeva, B. G., Trubitsina, T. A., Al-Kadamany, G. and Kortz, U. (2009),
Unique Catalytic Performance of the Polyoxometalate [Ti2(OH)2As2W19O67(H2O)]8–
: The
Role of 5-Coordinated Titanium in H2O2 Activation. Eur. J. Inorg. Chem., 2009: 5134–5141.
5. "Jöns Jakob Berzelius." Homepage of the Chemical Heritage Foundation. Chemical Heritage
Foundation, n.d. 26 Mar. 2013.
6. Berzelius, J. 1826, Poggend. Ann. Phys. Chem, Vol. 6, pp. 369-380.
7. Marignac, C. 1862, C.R. Acad. Sci., Vol. 55, p. 888.
8. Keggin, J.F. 1933, Nature, Vol. 131, p. 908.
9. Pope, M. T. (2013), Happy Birthday Keggin Structure!. Eur. J. Inorg. Chem., 2013: 1561.
10. Keggin, J.F. 131;13, 1933 : s.n., Nature, p. 968;351.
11. Baker, LCW and Figgis, JS. (1970), A New Fundamental Type of Inorganic Complex: Hybrid
between Heteropoly and Conventional Coordination Complexes. Possibilities for
Geometrical Isomerisms in 11-, 12-, 17-, and 18-Heteropoly Derivatives.12, J. Am. Chem.
Soc., 1970, 92, 3794-3797.
38. 37
12. Kortz, U., Savelieff, M. G., Bassil, B. S. and Dickman, M. H. (2001), A Large, Novel
Polyoxotungstate: [As(III)
6W65O217(H2O)7]26−
. Angew. Chem. Int. Ed., 40: 3384–3386.
13. Rankin, David W.H., Ebsworth, E.A.V and Cradock, Stephen. Structural Methods in
Inorganic Chemistry. 2nd. s.l. : Blackwell Scientific Publications, 1991.
39. 38
Acknowledgements
I would like to acknowledge Jacobs University for allowing me to perform my BSc in Chemistry
at the institution. I also give thanks to Prof. Ulrich Kortz for allowing me to perform my project
work in his research lab. I would also like to thank Dr. Bassem Bassil for supervising me
throughout the project by giving me background information and directing me with
procedures and topics, as well as doing the XRD measurements. I would like to thank Dr. Rami
Al-Oweini for helping create some of the diagrams as well as performing the NMR
measurements. Finally, I would like to thank all members of the research group for helping me
throughout my project work, either by advising me on various topics or proper
instrumentation usage.
![1
Abstract
Polyoxometalates (POMs) are polyanions consisting of early transition metals, such as W6+
,
Mo6+
, and V5+
, in high oxidation states. Polyoxotungstates (POTs) are a subclass of POMs
which are formed by self-assembly of tungstate ions, usually in aqueous medium. When
arsenic is added as the heteroatom to the POTs, they are referred to as tungstoarsenates.
POM chemistry has been a research topic of interest due to its potential applications in a
range of fields such as catalysis, magnetism, medicine and materials science.(1) (2)
There are several different structural types of heteropolytungstates, one being the Keggin
anion with the general formula of [XW12O40]m-
. Two tri-lacunary α-Keggin species can be
synthesized, A-α, [A-α-XM9O34]p-
, and B-α, [B-α-XM9O34]q-
. The B-α tri-lacunary species is of
particular interest, in particular when a lone pair is present instead of terminal oxygen
resulting in a molecular formula [B-α-XM9O33]s-
.
By finding a recrystallization procedure of the crude salt [As2W19O67(H2O)]14-
,
K10Na4[As2W19O67(H2O)]∙24H2O was produced. This recrystallization using a 1M NaOAc buffer
solution at pH 6 allowed for a crystal structure and 183
W solution NMR to be performed, which
could not previously been done. The recrystallized salt showed a stabilization of potassium
cations within the cavity of the ‘pacman’ structure of [As2W19O67(H2O)]14-
as well as the
presence of sodium cations surrounding the compound, which is necessary for the
recrystallization.
Using the recrystallized salt, K10Na4[As2W19O67(H2O)]∙24H2O, we were able to produce the
corresponding titanium containing product, K8[Ti2(OH)2As2W19O67(H2O)]∙18H2O in a 1M KCl
solution. This structure is identical to the previously reported
Cs8[Ti2(OH)2As2W19O67(H2O)]∙18H2O.(3)
. This optimized synthesis allowed for a higher yield and](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)