TRANSITION METAL CONTAINING SILICO- AND GERMANOTUNGSTATES
BY BASSEM S. BASSIL
A thesis submitted in partial fulfillment of the requirements for the degree
of Doctor of Philosophy in Chemistry
Approved, Thesis Committee:
Prof. Dr. Ulrich Kortz (chair, Jacobs University, Germany) Prof. Dr. Horst Elias (Technische Universität Darmstadt, Germany) Prof. Dr. Emmanuel Cadot (Université de Versailles, France) Dr. Michael H. Dickman (Jacobs University, Germany)
Date of defense: 27 th of June 2008 School of Engineering and Science
Abstract
Polyoxotungstates (POTs) are anionic tungsten-oxide clusters with a wide structural
variety and interesting properties. Thus, POTs exhibit potential applications in diverse areas such as catalysis, magnetism, bio- and nanotechnology, and materials science.
POTs are usually synthesized via condensation reactions in aqueous, acidic solution.
These reactions can be influenced by careful variation of the synthesis conditions, e.g. ratio and concentration of reagents, solvent, pH, counter cations, and temperature. Having identified the proper synthesis conditions, POTs usually form quickly in what is often described as a self- assembly process.
This work focuses on the interaction of the versatile dilacunary silicotungstate [γ-
8- SiW 10 O36 ] with different metal cations leading to novel compounds interesting for magnetic
and catalytic applications. Also this work reports two new lanthanide containing polytungstates,
a sandwich type silicotungstate and a dimeric-pentameric germanotungstate.
The 15-cobalt-substituted polyoxotungstate [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-
5- SiW 8O31 )3}] (1) has been characterized by single crystal XRD, elemental analysis, IR,
electrochemistry, magnetic measurements and EPR. Single-crystal X-ray analysis was carried
β out on Na 5[Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9( -SiW 8O31 )3}]·37H 2O, which crystallizes in the
hexagonal system, space group P6 3/m , with a = 19.8754(17) Å, b = 19.8754(17) Å, c = 22.344(4)
Å, α = 90 °, β = 90 °, γ = 120 °, and Z = 2. The trimeric polyanion 1 has a core of nine Co II ions
β - encapsulated by three unprecedented ( -SiW 8O31 ) fragments and two Cl ligands. The highly
β 17- symmetrical ( D3h ) assembly [Co 9Cl 2(OH) 3(H 2O) 9( -SiW 8O31 )3] is surrounded by six antenna-
II like Co (H 2O) 5 groups resulting in the satellite-like structure 1. Synthesis of 1 is accomplished in
2
II 8- a simple one-pot procedure by interaction of Co ions with [ γ-SiW 10 O36 ] in aqueous, acidic
NaCl medium (pH 5.4).
22- The 6-cobalt-substituted [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)}2] (2) has been characterized by IR and UV-vis spectroscopy, elemental analysis, magnetic studies, electrochemistry and gel filtration chromatography. Single-crystal X-ray analysis was carried out on K 10 Na 12 [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)}2]·49H2O ( KNa-2), which crystallizes
in the monoclinic system, space group P2 1/n , with a = 19.9466(8) Å, b = 24.6607(10) Å, c =
34.0978(13) Å, β = 102.175(1) °, and Z = 2. Polyanion 2 represents a novel class of asymmetric sandwich-type polyanions. It contains three cobalt ions which are encapsulated between an unprecedented (B-β-SiW 9O34 ) fragment and a ( B-β-SiW 8O31 ) unit. Polyanion 2 is composed of
two sandwich species via two Co-O-W bridges in the solid state and almost certainly also in
solution as based on gel filtration chromatography.
10- The new, monometal substituted silicotungstates [Mn(H 2O) 2(γ-SiW 10 O35 )2] (3),
10- 10- [Co(H 2O) 2(γ-SiW 10 O35 )2] (4) and [Ni(H 2O) 2(γ-SiW 10 O35 )2] (5) have been synthesized and isolated as the potassium salts K 10 [Mn(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-3), K 10 [Co(H 2O) 2(γ-
SiW 10 O35 )2]·8.25H 2O ( K-4) and K 10 [Ni(H 2O) 2(γ-SiW 10 O35 )2]·13.5H 2O ( K-5), which have been
characterized by IR spectroscopy, single-crystal X-ray diffraction, elemental analysis and cyclic
voltammetry. Polyanions 3-5 are composed of two ( γ-SiW 10 O36 ) units fused on one side via two
W-O-W’ bridges and on the other side by an octahedrally coordinated trans-MO 4(OH 2)2 transition metal fragment, resulting in a structure with C 2v point group symmetry. Anions 3-5
8- 2+ 2+ were synthesized by reaction of the dilacunary precursor [ γ-SiW 10 O36 ] with Mn , Co and
Ni 2+ ions, respectively, in 1M KCl solution at pH 4.5.
3
8- Reaction of ZrCl 4 with [γ-SiW 10 O36 ] in potassium acetate buffer results in two different
products depending on the reactant ratios. The trimeric species [Zr 6O2(OH) 4(H 2O) 3(β-
14- SiW 10 O37 )3] (6) consists of three β23 -SiW 10 O37 units linked by an unprecedented
Zr 6O2(OH) 4(H 2O) 3 cluster with C 1 point group symmetry. The dimeric species
10- [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] (7) consists of a β22 - and a β12 -SiW 10 O37 unit sandwiching a
Zr 4O2(OH) 2(H 2O) 4 cluster and has also C 1 symmetry.
We have also synthesized and structurally characterized the unprecedented peroxo-
18- zirconium(IV) containing [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] (8). Polyanion 8 comprises a cyclic 6- peroxo-6-zirconium core stabilized by three decatungstosilicate units.. Polyanion 8 represents the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units. The simple, one-pot synthesis procedure of 8 involving dropwise addition of aqueous hydrogen peroxide could represent a general procedure for incorporating peroxo groups into a large variety of transition metal and lanthanide containing POMs.
We have also synthesized and structurally characterized the mono-lanthanide containing
13- polyanions [Ln( β2-SiW 11 O39 )2] (Ln = La ( 9), Ce ( 10 ), Sm ( 11 ), Eu ( 12), Gd ( 13 ), Tb ( 14 ), Yb
(15 ), Lu ( 16 )). Synthesis was accomplished by reaction of the respective lanthanide ion with the
8- monolacunary, chiral, Keggin-type precursor [ β2-SiW 11 O39 ] in a 1:2 molar ratio in 1M KCl medium at pH 5. Polyanions 9-16 were isolated as potassium salts and then characterized by IR, single-crystal X-ray diffraction and elemental analysis. The structures of 9-16 are composed of
3+ 3+ an eight-coordinated Ln center sandwiched by two chiral ( β2-SiW 11 O39 ) units. Larger Ln ions
appear to favor an R,R (or S,S) configuration (point group C 2) of the Keggin units, with an increasing amount of R,S (or S,R) configuration (point group C 1) found in the solid state as the
4
Ln 3+ ion decreases in size. Evidence for this trend is also given by solution 183 W NMR results for
the diamagnetic La 3+ and Lu 3+ derivatives.
Finally, The gigantic 20-cerium(III) containing 100-tungstogermanate
56- III [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] (17 ) has been synthesized using Ce ions and [ α-
10- GeW 9O34 ] in aqueous acidic medium at pH 5.0. Polyanion 17 represents the first lanthanide substituted tungstogermanate and it is one of the largest polytungstates reported to date.
5
Acknowledgements
I thank Jacobs University, in the person of Dean Prof. Dr. Bernhard Kramer, for supporting this project by providing me a research opportunity and a stipend as a PhD student for three years. I also thank Prof. Ulrich Kortz for being a patient and guiding supervisor during my research in his group at Jacobs University. He also collected the crystal structure of some polyanions described here at Florida State University Chemistry Department (USA). I also wish to thank the entire Kortz group for all the help and in particular Dr. Michael H. Dickman for his
XRD work and guidance.
I would also like to express my gratitude and thanks Prof. Emmanuel Cadot and Prof.
Horst Elias, for accepting to part of my thesis and defense committee.
I also appreciate all the help from the undergraduate students Anca Tigan, Alexandra
Dumitriu, Shibani De Silva and Marie Asano in the experimental work done in the lab.
Last but not least I thank my parents for supporting me all the way till the top of the educational ladder.
6
Table of Contents
Chapter 1: General Introduction 15
I- Polyoxotungstates 16
II- Keggin Ion 20
III- Lacunary Keggin Silico- and Germanotungstates 24
8- A- [β2-SiW 11 O39 ] 26
8- B- [γ-SiW 10 O36 ] 28
10- C- A-[α-GeW 9O33 ] 30
IV- Nomenclature of β-dilacunary Keggin units 32
V- Instrumentation 34
8- Chapter 2: Results with [ γ-SiW 10 O36 ] 35
8- 2+ I- Interaction of [ γ-SiW 10 O36 ] with Co 36
A- Introduction 36
B- The Satellite-Shaped Co-15 Polyoxotungstates,
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 37
C- Cobalt Containing Silicotungstate Sandwich Dimer
22- [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)} 2] 43
D- Conclusion 51
7
12- II- Formation of the fused [γ-Si 2W20 O70 ] unit 53
A- Introduction 53
B- Transition Metal Containing Decatungstosilicate Dimer
10- 2+ 2+ 2+ [M(H 2O) 2(γ-SiW 10 O35 )2] (M = Mn , Co , Ni ) 54
C- Conclusion 61
8- 4+ III- Interaction of [ γ-SiW 10 O36 ] with Zr 63
A- Introduction 63
B- Synthesis and Structure of Asymmetric Zirconium-Substituted
14- Silicotungstates, [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] and
10- [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] 64
C- 6-Peroxo-6-Zirconium Crown Embedded in a Triangular Polyanion:
18- [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] 70
D- Conclusion 75
Chapter 3: Results with lanthanides 76
I- Introduction 77
8- II- Interaction of lanthanides with [ β2-SiW 11 O39 ] 79
A- Introduction 79
B- The Mono-Lanthanide Containing Silicotungstates
13- [Ln( β2-SiW 11 O39 )2] (Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu), a Synthetic
and Structural Investigation 79
C- Conclusion 94
10- III- Interaction of Cerium(III) with [A-α-GeW 9O34] 96
8
A- Introduction 96
56- B- The Tungstogermanate [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] :
A Polyoxometalate Containing 20 Cerium(III) Atoms 96
C- Conclusion 104
Chapter 4: Collaborative Work 106
I- Electrochemistry 107
A- Electrochemistry of
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 107
B- Electrochemistry of
22- [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)} 2] 107
C- Electrochemistry of
10- 2+ 2+ 2+ [M(H2O) 2(γ-SiW 10 O35 )2] (M = Mn , Co , Ni ) 108
18- D- Electrochemistry of [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] 108
II– Magnetism 109
A- Magnetism of
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 109
B- Magnetism of
22- [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)} 2] 109
10- Chapter 5: Preliminary Results with [α-SiW9O34 ] 110
10- I- Interaction of Mn(III) with [ α-SiW9O34 ] at basic pH 111
2- 10- II– Interaction of Ni(II) and CO 3 with [A-α-SiW9O34 ] 112
9
List of Figures
Fig. A.1.a. General scheme of POT formation……………………………………...17
Fig. A.1.b. Ball and Stick representation of different types of isopoly- and
heteropolytungstates..…..………………………………………………19
Fig. A.2.a. Top: top (right) and side (left) view of the polyhedral representation of the
Keggin ion with colored coding for the four ‘triads’. Each octahedron
represents a WO 6 group.
Bottom: Ball and stick representation of the Keggin ion with colored coding
for the different types of bonding oxygens………………………….….21
Fig. A.2.b. Scheme showing the formation of the different rotational isomers of the
Keggin ion starting from the parent alpha-isomer………………………23
Fig. A.3. Scheme of the synthetic pathway of the different plenary and lacunary
silicotungstates………………………………………………………….25
Fig. A.3.a. Structural relationship of the three monolacunary Keggin isomers
8- 8- 8- [β1-SiW 11 O39] , [ β2-SiW 11 O39 ] , and [ β3-SiW 11 O39 ] to the plenary
4- derivatives [ x-SiW 12 O40 ] (x = β, α)……………………………………27
8- Fig. A.3.b. Structural relationship of the dilacunary Unit [ γ-SiW 10 O36 ]
4- to the plenary derivatives [ x-SiW 12 O40 ] (x = γ, α)……………………29
10- Fig. A.3.c. Structural relationship of the trilacunary Unit A-[α-GeW 9O33 ]
4- to the plenary derivative [ a-GeW 12 O40 ] ……………………………… .31
Fig. A.4.a. Left: A Schlegel diagram taken from ref. 13 for the numbering of the tungsten
positions.
10
Right: projection of the diagram on a top view from the rotated triad of a beta-
Keggin unit, the oxygen where taken off for clarity……………………..33
Fig. B.1.a. Left: Polyhedral and ball and stick representation (side-view) of
[Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 5- (1).
Right: ball and stick representation (top-view) of (1)...... 40
Fig. B.1.b. Ball and stick representation (top-view) of the central cobalt-oxo
fragment in [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 5
(1)...... 42
Fig. B.1.c. Ball and stick presentation of the sandwich dimer
22- [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)}2] (2)………………45
Fig. B.1.d. Polyhedral and ball and stick presentation of the half-unit of 2 with the
11- molecular formula [Co 3(H 2O)( B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)]
(2a )………………………………………………………………………47
Fig. B.2.a. IR spectra of K 10 [Mn(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-3, upper),
K10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4, middle), and
K10 [Ni(H 2O) 2(γ-SiW 10 O35 )2].13.5H 2O ( K-5, lower)…………………… 57
Fig. B.2.b. Ball-and-stick (left) and polyhedral (right) representations of
10- 2+ 2+ 2+ [M(H 2O) 2(γ-SiW 10 O35 )2] (M = Mn , 3; Co , 4; Ni , 5)……………58
Fig. B.2.c. Coordination sphere with atomic labeling and bond lengths for the
incorporated transition metal centers in 3-5……………………………59
Fig. B.3.a. Left: Combined polyhedral/ball-and-stick representation of
14- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] (6).
11
Right: Ball-and-stick presentation of the Zr 6O2(OH) 4(H 2O) 3
cluster in 6……………………………………………………………….67
Fig. B.3.b. Left: Combined polyhedral/ball-and-stick representation of
10- [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] (7).
Right: Ball and stick presentation of the Zr 4O2(OH) 2(H2O) 4
cluster in 7...... 68
Fig. B.3.c. Combined polyhedral/ball-and-stick representation of
18- [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] ………………………...………….…..73
6+ Fig. B.3.d. Top view (left) and side view (right) on the central [Zr 6(O 2)6(OH) 6]
cluster in 8...... 74
Fig. C.1.a. Polyhedral (upper) and ball-and-stick (lower) representation of
13- [La( β2-SiW 11 O39 )2] (9)……………………………………………….86
Fig. C.1.b. This scheme highlights the inner coordination sphere of the lanthanide centers
in 9-16 . It also shows how the twist angle θ is determined. The subscript ‘e’
indicates that this oxygen belongs to an edge-sharing octahedron, the subscript
‘u’ indicates that this oxygen belongs to an unshared octahedron and the
subscript ‘t’ indicates that this oxygen belongs to an octahedron of the rotated
triad. The 2-fold rotation axis of 9 and 10 is also shown………………. 88
Fig. C.1.c. The two possible configuration isomers of 9-16 are shown and the respective
percentages for each with the corresponding twist angle θ. For clarity, the
right Keggin subunit was left unchanged in both representations……....89
Fig. C.1.d. Tungsten-183 NMR spectrum of K 13 [La( β2-SiW 11 O39 )2]·22H 2O ( K-9) in
H2O/D 2O at 293 K. The peaks with an asterisk represent most likely the
12
13- “all-alpha” isomer [La( α-SiW 11 O39 )2] due to partial
degradation of 9...... 93
Fig. C.2.a. Combined polyhedral/ball-and-stick representation of
56- [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] (17 )...... 100
28- Fig. C.2.b. Exploded view (left) of the half-unit [Ce 10 Ge 5W50 O188 (OH) 2(H 2O) 15 ] (17a ),
highlighting the arrangement of five equivalent “Ce 2GeW 10 ” Keggin building
blocks (shown in different colors for clarity) in 17 (right)……………101
Fig. C.2.c. Ball-and-stick representation of all Ce III centers in 17a with labeling scheme.
The figure also shows the inner coordination sphere of all Ce III ions and their
complete connectivity via WO 6 units, resulting in an asymmetrical
assembly……………………………..………………………………..103
8- 4 Fig. E.1.a. IR spectra of [ α-Mn 3SiW 9O37 (H 2O) 3] (18 ) and [ α-SiW 12 O40 ] ...... 111
Fig. E.1.b. Combined polyhedral and ball-and-stick representation of
8- [α-Mn 3SiW 9O37 (H 2O) 3] (18 )……………………………………….112
20- Fig. E.2.a. IR spectrum of [Ni(H 2O) 4(Ni 4(H 2O)O 3α-SiW 9O34 )2(CO 3)3] (19 )....113
Fig. E.2.b. Combined polyhedral and ball-and-stick representation of
8- [α-Mn 3SiW 9O37 (H 2O) 3] (19 )……………………………………….114
13
List of Tables
Table B.1.a. Crystal Data and Structure Refinement for
Na 5[Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}]·37H 2O (Na-1)…………………………………………………………………39 Table B.1.b. Crystal Data and Structure Refinement for
K10 Na 12 [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)} 2]·49H 2O (KNa -2)……………………………………………………………….46 Table B.2.a. Crystal data and structure refinement results for
K10 [Mn(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-3),
K10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4) and
K10 [Ni(H 2O) 2(γ-SiW 10 O35 )2]·13.5H 2O ( K-5)…………………………56 Table B.3.a. Crystal Data and Structure Refinement for
K14 [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3]·28.5H 2O (K-6) and
K10 [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2]·22H 2O ( K-7)…………………66 Table B.3.b. Crystal Data and Structure Refinement for
K18 [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3]⋅59H 2O (K-8) ………………………72 Table C.1.a. Crystal data and structure refinement for K-9 - K-16 ……………...83-84 13- Table C.1.b. Average Ln-O bond lengths in [Ln( β2-SiW 11 O39 )2] (Ln = La ( 9), Ce ( 10 ), Sm ( 11 ), Eu ( 12 ), Gd ( 13 ), Tb ( 14 ), Yb ( 15 ), Lu ( 16 ))………………………………………………………91 Table C.2.a. Crystal Data and Structure Refinement for
Cs 28 Na 28 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·~180H 2O (CsNa-17)...... 98
14
Chapter 1 General Introduction
15
I – Polyoxotungstates
Polyoxotungstates represent a subclass of polyoxoanions, better known as polyoxometalates (POMs). 1 POMs are best described as anionic metal-oxygen clusters where the metal centers are Group V and VI d-block transition metals in their highest oxidation states, namely V(V), Nb(V), Ta(V), W(VI) and Mo(VI). The ability of these metal centers to form polyoxoanions comes from the combination of adequate ionic radii and positive charges. POMs offer a wide family of inorganic compounds with multiple properties leading to a variety of possible applications, in fields such as catalysis, magnetism, nanotechnology and even medicine. 2
When POMs have tungsten as their metal ion of composition (also called addendum), they are called polyoxotungstates (POTs), or polytungstates. Polytungstates are the mostly studied and reported POMs in scientific literature to date. This comes from the fact that they are the most stable in solution between all POM species.
POTs form when [WO 6] octahedra are linked through corner or edge shared oxygen
bridges to form discrete anionic molecules, usually in aqueous solutions. A typical stepwise
formation of POTs in solution is initiated by the transition in solution of the coordination
2- environment of the tungsten(VI) from tetrahedral (in the tungstate ion [WO 4] ) to octahedral
when going to more acidic pH. This transition is simultaneously accompanied by
1 (a) Pope, M. T. Heteropoly and Isopoly Oxometalates ; Springer-Verlag: Berlin, 1983. (b) Pope, M. T.; Müller, A. Angew. Chem. Int. Ed. 1991 , 30 , 34. (c) Pope, M. T. Comp. Coord. Chem. II 2003 , 4, 635. (d) Hill, C. L. Comp. Coord. Chem. II 2003 , 4, 679. (i) Borrás-Almenar, J. J.; Coronado, E.; Müller, A.; Pope, M. T. Polyoxometalate Molecular Science ; Kluwer: Dordrecht, The Netherlands, 2004. 2 (a) Polyoxometalates: from Platonic Solids to Anti Retroviral Activity (Eds.: Pope, M. T.; Müller, A.), Kluwer, Dordrecht, 1994 . (b) Chem. Rev. 1998 , 98 , 1-389 (Special Thematic Issue on Polyoxometalates). (c) Polyoxometalate Chemistry: From Topology via Self-Assembly to Applications (Eds.: Pope, M. T.; Müller A.), Kluwer, Dordrecht, 2001 . (d) Polyoxometalate Chemistry for Nano-Composite Design (Eds.: Yamase, T.; Pope, M. T.), Kluwer, Dordrecht, 2002.
16
6- olygomerization of the [WO 6] octahedra through acidic condensation and release of water
molecules either through:
- Single condensation to form corner shared octahedra:
6- + 10- 2 [WO 6] + 2 H 3O [W2O11 ] + 3 H 2O
- Double condensation to form edge shared octahedra:
6- + 8- 2 [WO 6] + 4 H 3O [W2O10 ] + 6 H 2O
Although the possibility of face sharing is present, it is very unlikely since it leads to the
proximity of the two highly positive charges of the tungsten centers which renders the structure
unstable. The more acidic environment leads eventually to the formation of tungsten oxide WO 3,
also known as tungstic acid. See ‘Fig. A.1.a.’. Corner sharing
Low pH
Oxide Low pH POT s WO 3 Condensation formation
Low pH
2- 6- [WO 4] [WO 6]
Edge sharing
Fig. A.1.a. General scheme of POT formation
This oligomerization is not infinite. The presence of polytungstates as discrete anionic
entities in solution is mainly due to what is known as polarization of the peripheral tungsten
centers towards the outer ‘shell’ of the POTs. Tungsten(VI) cations present vacancies in their p
17 orbitals allowing for π back-bonding with terminal oxygens. Therefore, such double bonds occur
on the surface of the POTs thus rendering the terminal oxygen saturated and much less prone to
protonation, preventing the formation of more bridges with other octahedra. The driving force
for such polarization and the conditions under which it occurs are still unknown. In fact, the
mechanism of formation of POTs and of POMs in general is still not yet understood and falls
under the category of formation through ‘self assembly’.
Polytungstates are divided into two categories depending on their composition.
n- - Isopolytungstates, of general formula [WxOy] , contain only tungsten and oxygen atoms. As an example:
2- + 6- 7[WO 4] + 8H 3O [W 7O24 ] + 12H2O
2- Examples of known isopolytungstates include the Lindqvist-type ion [W 6O19 ] ,
6- 10- 1a paratungstate-A [W 7O24 ] (heptatungstate) and paratungstate-B [W12 O42 ] . See Fig. A.1.b.
n- - Heteropolytungstates, of general formula [X mWxOy] , enclose an additional heteroatom
X (or ‘heterogroup’ XO p) coordinated to surrounding edge- and/or corner-shared [WO 6] octahedral. The nature of X does not have any specific restriction and many of the elements of the periodic table are known to act as heteroatoms. The heterogroup can be tetrahedral, octahedral or even icosahedral. As an example of a formation equation of a silicotungstate:
2- + 2- 4- SiO 3 + 22H 3O + 12[WO 4] [SiW 12 O40 ] + 33H 2O
Examples of known heteropolytungstates include the Anderson-Evans anion which
n- contains an octahedral heterogroup, the Keggin anion [XW 12 O40 ] which contains a tetrahedral
n- heterogroup, and the Wells-Dawson anion [X 2W18 O62 ] which contains two tetrahedral heterogroups. 1a See Fig A.1.b.
18
10 - 2- 6- [W 12 O42 ] [W 6O19 ] [W 7O24 ]
n- [X 2W18 O62 ]
n- n- [XW 6O24 ] [XW 12 O40 ]
Fig. A.1.b. Ball and stick representation of different types of isopoly- and heteropolytungstates. Red balls: oxygens, Black balls: tungstens, Green balls: heteroatom
This thesis will be focusing on POTs synthesized from and containing Keggin-type units, which will be explained more in the following sections.
19
II – The Keggin Ion
The first Keggin ion, a phosphomolybdate, was reported by the Swedish Jöns
Jakob Berzelius in 1826 by reaction of ammonium molybdate with phosphoric acid. 3 Several speculation were made on the structure of that POM, notably by Alfred Werner and then by
Linus Pauling. It wasn’t until a century later, in 1933 that J.F. Keggin from the University of
Manchester performed powder X-ray diffraction on the free acid H3PW 12 O 40 and determined its structure, which is now known as the Keggin structure. 4
The Keggin ion, as seen in Fig. A.2.a. , is a highly symmetrical (T d point group)
heteropolyanion comprising a tetrahedral central heterogroup coordinated to four, equivalent
[M3O13 ] ‘triads’, which are corner-shared among each other through 12 µ2 oxygen bridges. Each
triad is three edge-shared octahedra through three µ2 oxygen bridges and a central common oxygen bridge which links the three octahedra to the heteroatom, making it µ4 bridging oxygen.
In addition, each of the twelve addenda metal atoms has a terminal π-bonded oxygen. This
double bond leads to the polarization of the addenda atoms within the octahedra making the
octahedra actually distorted. Having this distortion makes the M-µ4O bond, Trans to the terminal
M=O, significantly longer (around 10%) than the remaining four M-O bonds. IR spectroscopy studies of the Keggin ion revealed that the asymmetric vibration modes of the M-O bonds decrease in energy from terminal to corner sharing to edge sharing. 5
The Keggin ion can be more visualized as a central tetrahedral heterogroup which is
relatively loosely bound to a surrounding shell made up of a quasi-circular metal oxide cluster
3 Berzelius, J. Pogg. Ann. 1826 , 6, 369. 4 (a) Keggin, J. F. Nature 1933 , 131 , 908 (b) Keggin, J. F. Proc. Roy. Soc. A 1934 , 144 , 75. 5 Thouvenot, R.; Fournier, M.; Franck, R.; Rocchiccioli-Deltcheff, C. Inorg. Chem. 1984 , 23 , 598.
20
[M 12 O36 ]. In the case of Mo and W, this shell is neutral, resulting in the Keggin ion having the
charge of the heterogroup.
Fig. A.2.a. Top: top (right) and side (left) view of the polyhedral representation of the Keggin ion with colored coding for the four ‘triads’.
Each octahedron represents a WO 6 group. Bottom: ball and stick representation of the Keggin ion with colored coding for the different types of bonding oxygens.
Black balls: addenda atoms, green ball: heteroatom, blue balls: terminal oxygens, red balls: ‘triad’ µ2-oxygens, yellow balls: ‘triad’ µ4- oxygens, pink balls: corner shared, triad linking µ -oxygens. 2
21
The Keggin structure is not rigid and has five geometrical rotational isomers, also known as Baker-Figgis isomers. These isomers result from the rotation of each triad around its X-µ4O
axis by 60°. The parent isomer, with four equivalent triads, is referred to as alpha α-Keggin and
the remaining four isomers result from the rotation of one (beta β), two (gamma γ), three (delta
δ) or four (epsilon ε) triads by 60° each, see Fig A.2.b..The alpha and beta isomers are the most
stable from the five. The gamma isomer has two edge shared octahedra, the delta one has three
and the epsilon one has six. Therefore, it is natural for the structure to become less stable as we
go from alpha/beta to epsilon. The α, β and γ isomers of Keggin silicotungstates have been
successfully synthesized and characterized. 6 The delta isomer was found in the moiety of
18+ [Al 30 O8(OH) 56 (H 2O) 26 ] , a polyoxohydroxoaluminium cation cluster, as reported by Rowsell and Nazar.7 The group of Sécheresse reported the only epsilon Keggin isomer known to date, the
5- 8 lanthanum stabilized phosphomolybdate [ε-PMo 12 O36 (OH) 4{La(H 2O) 4}4] . This thesis will be discussing POTs having Keggin type silico- and germanotungstates as building blocks. These
POTs are not however synthesized from full (plenary) Keggin units, but from what are called lacunary units. When one or more octahedra are lost from POMs, they leave a vacancy thus creating a partially open POM structure or lacunary structure.
6 (a) Tézé, A.; Hervé, G. J. inorg. nucl. Chem. 1977 , 39 , 999. (b) Tézé, A.; Canny, J.; Gurban, L.; Thouvenot, R.; Hervé, G. Inorg. Chem. 1996, 35, 1001 7 Rowsell, J.; Nazar, L. F., J. Am. Chem. Soc. 2000, 122, 3777. 8 Mialane, P.; Dolbecq, A.; Lisnard, L.; Mallard, A.; Marrot, J.; Sécheresse, F. Angew. Chem. Int. Ed. 2002 , 41 , 2398.
22
α-Keggin
One Triad Four Triads Rotation Rotation
Three Triads Two Triads Rotation Rotation
ε-Keggin β-Keggin
γ-Keggin δ-Keggin
Fig. A.2.b. Scheme showing the formation of the different rotational isomers of the Keggin ion starting from the parent alpha-isomer.
Red octahedra: WO 6 groups, green octahedra: WO 6 groups belonging to a rotated triad, green ball: heteroatom.
23
III – Lacunary Keggin Silico- and Germanotungstates
A lacunary Keggin POT is usually formed when a plenary molecule starts losing one or more tungsten octahedra usually due to base hydrolysis. OH - can attack the POM framework
(depending on how stable that framework is at a specific pH) leading to loss of tungsten octahedra as tungstate and formation of the lacunary species. For example, a general silicotungstate hydrolysis reaction would be:
4- - n- [SiW 12 O40 ] + mOH [SiW 12-xO40-y] + xWO 4 + pH2O
If x=1, the lacunary unit will be called monolacunary, dilacunary for x=2, trilacunary for x=3 etc…
This section will be talking about the lacunary Keggin silico- and germanotungstates precursors that will be used for the synthesis of the reported POM structures throughout this thesis; these precursors are mono-, di- and tri-lacunary units. All the synthetic procedures for preparing the precursors were done according to the published procedure of Tézé and Hervé. 9
Figure A.3, which was postulated by Hervé and Tézé over the course of their work with silicotungstates, gives an overview of the synthetic pathway between the different plenary and lacunary silicotungstates in aqueous medium. Germanotungstates are expected to follow the same chemistry. In Figure A.3, solid arrows refer to formation due to acidic conditions, dashed arrows to partial/complete hydrolysis due to basic conditions, and doted arrows to in situ isomerization in solution.
9 Tézé, A.; Hervé, A. G. Inorganic Syntheses 1990 , Vol. 27.
24
2- Na 2WO 4 + Na 2SiO 3
10 - Α−α-SiW9O34
8- γγγ-SiW10 O36
10 - Α−β-SiW 9O34
8- 8- βββ222-SiW11 O39 ααα-SiW11 O39
8- 8- βββ333-SiW11 O39 βββ111-SiW11 O39
Fig. A.3. Scheme of the synthetic pathway of the different plenary and lacunary silicotungstates.
4- γγγ-SiW12 O40 4- 4- βββ-SiW12 O40 ααα-SiW12 O40 8- A- [βββ222-SiW11 O39 ]
Synthesis of K 8[βββ222-SiW11 O39 ].14H 2O
2- 2- + + 11[WO 4] + [SiO 3] + 16 H + 8K + 6H 2O K8[β2-SiW 11 O39 ].14H 2O
Sodium metasilicate (11 g, 50 mmol) is dissolved in 100 mL of water (Solution A).
Sodium tungstate (182 g, 0.55 mol) is dissolved in 300 mL of water in a separate 1-L beaker
containing a magnetic stirring bar. To this solution, 165 mL of 4 M HCl is added in 1-mL
portions over 10 min, with vigorous stirring (there is a local formation of hydrated tungstic acid
that slowly disappears). Then, solution A is poured into the tungstate solution, and the pH is
adjusted to between 5 and 6 by addition of 4 M HCl solution (~ 40mL). This pH is maintained by
addition of small amounts of 4 M HCl for 100 min. Solid potassium chloride (90 g) is then added
to the solution with gentle stirring. After 15 min, the precipitate is collected by filtering through a
sintered glass filter. Purification is achieved by dissolving the product in 850 mL of water. The
insoluble material is rapidly removed by filtration on a fine frit, and the salt is precipitated again by addition of solid KCl (80 g). The precipitate is separated by filtration, washed with 2 M potassium chloride solution (2 portions of 50 mL), and air dried. Yield 60 to 80 g (37-50%). IR data (KBr pellet) in cm -1: 988, 945, 875, 855, 805, 730, 610(sh), 530, 460(sh), 395 (sh), 360, and
325.
When the β-Keggin is formed, the symmetry is reduced to C 3v and the triads are not
equivalent anymore. Three positions exist from where the tungsten can be lost. The β3 position is
from the rotated, edge-shared ‘cap’ triad, the β2 position is from the ‘belt’ tungstens and β1 position id from the ‘base’ corner shared triad. Therefore the β monolacunary Keggin precursor
8- can have three isomers. From these three, and to date, [ β2-SiW11 O39 ] is the only chiral mono-
8- lacunary precursor, see figure A.3.a.. [β2-SiW 11 O39 ] was first reported in 1977 by Hervé and
26
Tézé who also performed polarographic and spectroscopic measurements on the monolacunary
unit. 6a However, to date, neither a crystal structure nor an NMR spectrum of the unit alone have
been reported, most probably due to its instability in solution leading to a conversion back to the
alpha isomer.
α-Keggin β1 Keggin Monolacunary
Loss of an octahedron from the base Rotation of β one triad 2 monolacunary Keggin
Loss of an octahedron from the belt
Loss of an β3 monolacunary octahedron from β-Keggin Keggin the rotated triad
Fig. A.3.a. 8- 8- 8- Structural relationship of the three monolacunary Keggin isomers [ β1-SiW 11 O39 ] , [ β2-SiW 11 O39 ] , and [ β3-SiW 11 O39 ] to the plenary 4- derivatives [x-SiW 12 O40 ] (x = β, α). Colors scheme for the red and green octahedra same as Fig. A.2.b., blue ball: Si.
27
8- B- [γγγ-SiW10 O36 ]
Synthesis of K 8[γγγ-SiW10 O36 ].12H 2O
8- 2- + - 2- [β2-SiW 11 O39 ] + 2CO 3 + 8K + 13H 2O K8[γ-SiW 10 O36 ].12H 2O + 2HCO 3 + [WO 4]
K8[β2-SiW 11 O39 ].14H 2O (15g, 5 mmol), synthesized as described above, is dissolved in
150 mL of water maintained at RT. Impurities in the K8[γ-SiW 10 O36 ] salt (mainly paratungstate)
give insoluble materials, which have to be removed rapidly by filtration on a fine frit or through
Celite ®. The pH of the solution is quickly adjusted to 8.8 (modified from the original procedure,
see below) by addition of a 2 M aqueous solution of K 2CO 3. The pH of the solution is kept at this
value by addition of the K 2CO 3 solution for exactly 16 min. the potassium salt of the γ-
decatungstosilicate is then precipitated by addition of solid potassium chloride (40g). The solid is
removed by filtering, washed with 1M KCl solution and air dried. Yield ~ 10g (70%). IR data
(KBr pellet) in cm -1: 989 (m), 941 (s), 905 (s), 865 (vs), 818 (vs), 740 (vs), 655 (sh), 553 (w),
528 (m), 478 (sh), 390 (sh), 360 (s), 328 (sh), 318 (m), and 303 (sh).
When two adjacent triads of the plenary Keggin are rotated each by 60° to form the gamma isomer, they form a pair of edge-shared octahedra between each other. This edge-shared pair becomes unstable and prone to be lost through basic hydrolysis thus forming the dilacunary
8- [γ-SiW 10 O36 ] unit, see Fig. A.3.b.. Experimentally, the dilacunary gamma unit is only known to
8- be synthesized in basic aqueous medium from [ β2-SiW 11 O39 ] . This pathway most probably involves the rotation of a second triad followed by a loss of an extra tungsten atom. Having [ β2-
8- 8- SiW 11 O39 ] as the only precursor for formation is understandable since [ β1-SiW 11 O39 ] and [ β3-
8- SiW 11 O39 ] prefer following the alternative pathway of trilacunary unit formation by loss of two corner shared octahedra for the former and the rest of the rotated triad for the latter. In 1986,
28
Hervé et al reported the synthesis and structural characterization of the γ dilacunary Keggin
silicotungstate. 10 They also performed solid and solution studies. The NMR spectrum gave three
peaks with a ration of 2:2:1, verifying the conservation of its C 2v symmetry in solution. In 2006,
Kortz et al reported an improved yield from the one published by Tézé in 1986 by using a
synthetic pH of 8.8 instead of 9.1. 11
8- Properties of [γ-SiW 10 O36 ] as well as synthesis, characterization and properties of its
transition metal substituted form have been reported by different groups. 12
γ-Keggin γ 8- α-Keggin [ -SiW 10 O36 ]
Rotation of two Loss of two
triads octahedra
Fig. A.3.b. γ 8- 4- γ α Structural relationship of the dilacunary Unit [ -SiW 10 O36 ] to the plenary derivatives [x-SiW 12 O40 ] (x = , ). Color scheme for the balls and red and green octahedra same as Fig. A.3.a., blue octahedra: edge shared WO 6 octahedra from the two rotated triads.
10 Canny, J.; Tézé, A.; Thouvenot, R.; Hervé Inorg. Chem. 1986, 25 , 2114 11 Nsouli, N.H.; Bassil, B.S.; Dickman, M.H.; Kortz; U.; Keita, B.; Nadjo, L. Inorg. Chem., 2006 , 45 , 3858. 12 (a) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi, S.; Mizuno, N. Science , 2003 , 300 , 964. (b) Canny, J.; Thowenot, R.; Tézé, A.; Hervé, G.; Leparulo-Loftus, M.; Pope, M.T. Inorg. Chem. , 1991 , 30 , 976. (c) Nakagawa, Y.; Uehara, K.; Mizuno, N. Inorg. Chem. , 2005 , 44 , 14. (d) Nakagawa, Y.; Uehara, K.; Mizuno, N. Inorg. Chem. , 2005 , 44 , 9068. (e) Mizuno, N.; Nakagawa, Y.; Yamaguchi, K. J. Mol. Catal. A: Chem ., 2006 , 251 , 286. (f) Nakagawa, Y; Kamata, K.; Kotani, M.; Yamaguchi, K.; N. Mizuno, Angew. Chem. Int. Ed. 2005 , 44 , 5136. (g) Wassermann, K.; Lunk, H.J.; Palm, R.; Fuchs, J.; Steinfeldt, N.; Stösser, R.; Pope, M.T. Inorg. Chem. , 1996 , 35 , 3273. (h) Nozaki, C.; Kiyoto, I.; Minai, Y.; Misono, M.; Mizuno, N. Inorg. Chem. , 1999 , 38 , 5724. (i) Mizuno, N.; Nozaki, C.; Kiyoto, I.; Misono, M. J. Am. Chem. Soc,. 1998 , 120 , 9267. (j) Nishiyama, Y.; Nakagawa, Y.; Mizuno, N.; Angew. Chem. Int. Ed. 2001 , 40 , 3639.
29
10- C- A-[ααα-GeW9O33 ]
Synthesis of Na 10 [ααα-GeW9O34 ].18H2O
2- + + 9[WO 4] + GeO 2 + 8H + 10Na + 14H 2O Na 10 [α-GeW 9O34 ].18H2O
Sodium tungstate (182 g, 0.55 mol) and germanium oxide (5.2 g, 50 mmol) are dissolved
in 200 mL of hot water (80-100°C) in a 1-L beaker containing a magnetic stirring bar. To this
solution is added dropwise 130 mL of 6M HCl in ~30 min with vigorous stirring. The solution is
boiled until the volume is ~300 mL. Unreacted silica is removed by filtration over a fine frit or
over Celite ® or by centrifugation. Anhydrous sodium carbonate (50g) is dissolved in 150 mL of
water in a separate beaker. This solution is slowly added to the first solution with gentle stirring.
A precipitate forms slowly. It is removed by filtering, using a sintered glass filter, after 1h. The
solid is stirred in 1L of 4 M NaCl solution and filtered again. It is then washed successively with
two 100 mL portions of ethanol and 100 mL of diethyl ether and dried under vacuum. Yield
~110g (85%). IR data (KBr pellet) in cm -1: 980, 938 (sh), 928, 861, 845 (sh), 800, 708, 543, 525,
486 (sh), 431, 369, 330.
4- Loss of three corner-shared WO 6 octahedra from the plenary [ α-GeW 12 O40 ] leads to
10- formation of the trilacunary A-[α-GeW 9O33 ] (A stands for loss of edge-shared octahedra while
B stands for loss of three edge-shared octahedra), see Fig. A.3.c.. This loss reduces the symmetry
of the Keggin unit to C 3v . The synthesis of this trilacunary germanotungstate was firstly reported
by Hervé and Tezé in 1977 alongside polarographic studies. 13 The synthesis involves direct formation of the trilacunary unit from germanium oxide and sodium tungstate salts, in two simultaneous steps. The first step is in situ formation of a more compact Keggin unit (plenary or monolacunary) followed by base hydrolysis with carbonate in solution.
13 Tézé, A.; Hervé, G. Inorg. Chem., 1977 , 16 , 2115.
30
Loss of three
octahedra
Fig. A.3.c. 10- 4- Structural relationship of the trilacunary Unit A-[α-GeW 9O33 ] to the plenary derivative [a-GeW12 O40 ] . Color scheme for the red octahedra same as Fig. A.3.b., blue octahedra: corner shared from the lost ‘bottom’ leading to the trilacunary unit.
31
IV – Nomenclature of β-dilacunary Keggin units.
Throughout the thesis, three POM structures will be encountered containing ( β-
10- SiW10 O37 ) units. This section explains the nomenclature used in numbering the vacancies of
these dilacunary polytungstate units.
In 1998, Jeanin published a review paper describing the method to number the tungsten
positions and hence the vacancies for the Keggin ion. 14 The numbering of the beta-Keggin unit has the following scheme, see Fig A.4.a. for a representative diagram:
1- The unit is viewed from the top of the rotated triad following the main C 3 axis.
2- The numbering has three ‘parts’: it starts from the ‘bottom’ three corner shared
tungstens, followed by the nine ‘central belt’ tungstens and ending with the three edge
shared tungstens of the rotated triad.
3- After numbering a ‘part’, the tungsten of the following ‘part’ closest to the one with
the lowest number is started with.
4- Deciding which Tungsten position is number ‘1’ and the direction of the numbering
for each ‘part’ (clockwise or counter-clockwise) is decided so that the first and second
vacancies have the lowest numbering possible.
This rather simple scheme gives rise to three isomers for the monolacunary beta-Keggin
unit: β(1), β(4) (chiral), and β(10) (also known as the β1, β2, and β3 Keggin ions respectively).
For the dilacunary beta-Keggin unit, 14 isomers can be found according to the diagram in Fig.
A.5.a.:
- For both vacancies in the corner shared ‘bottom’ triad ( β11 ): β(1,2).
14 Jeannin, Y.P. Chem. Rev. 1998 , 98, 51
32
- For a vacancy in the corner shared triad and one in the ‘belt’ ( β12 ): β(1,4) (chiral) ,
β(1,5) (chiral) and β(1,6) (chiral).
- For a vacancy in the corner shared triad and one in the rotated triad ( β13 ): β(1,10) and
β(1,11) (chiral).
- For both vacancies in the ‘belt’( β22 ): β(4,5), β(4,6) (chiral), β(4,7) (chiral) and β(4,9).
- For a vacancy in the ‘belt’ and one in the rotated triad ( β23 ): β(4,10) (chiral), β(4,11)
(chiral), β(4,12) (chiral).
- For both vacancies in the rotated triad ( β33 ): β(10,11)
9 4 10 1
8 5 3 2 12 11 7 6
Fig. A.4.a. Left: A Schlegel diagram taken from ref. 13 for the numbering of the tungsten positions. Right: projection of the diagram on a top view from the rotated triad of a beta-Keggin unit, the oxygen where taken off for clarity. Black balls: rotated triad tungstens, blue-gray balls: ‘belt’ tungstens, gray balls: corner shared ‘bottom’ tungstens, green ball: heteroatom.
33
V – Instrumentation.
Infrared spectra were recorded on KBr pellets using a Nicolet Avatar spectrophotometer.
NMR spectra were recorded on a 400 MHz JEOL ECX instrument.
All NMR spectra were recorded on a 400 MHz JEOL ECX instrument at room
temperature using H2O/D2O as the solvent. The 183 W NMR measurements were performed at
16.656 MHz in 10 mm tubes. Chemical shifts are reported with respect to 2 M Na 2WO 4, and the
chemical shifts downfield of the reference are reported as positive.
Thermogravimetric analyses were carried out on a TA Instruments SDT Q600
thermobalance with a 100 mL/min flow of nitrogen; the temperature was ramped from 20 to 800
°C at a rate of 2 °C/min.
All POM Figures were generated by Diamond Version 3.1e (copyright, Crystal Impact
GbR).
34
Chapter 2
8- Results with [γ-SiW 10 O36 ] All references are valid on date of publishing
35
8- 2+ I- Interaction of [ γ-SiW 10 O36 ] with Co
A- Introduction
Transition metal substituted Polyoxotungstates can also be of interest for their magnetic properties. 2 Structures that contain more than one paramagnetic transition metal ion in close proximity may exhibit exchange-coupled spins leading to large spin ground states. A common synthetic strategy for the preparation of transition metal substituted polyoxotungstates involves reaction of a transition metal ion with a lacunary POM. Within the class of transition-metal-
substituted POMs, the sandwich-type compounds represent the largest subclass. 15 To date the
Weakley-, Hervé-, Krebs-, and Knoth-type sandwich structures can be distinguished. Synthesis
of such compounds is usually accomplished by the reaction of a transition metal ion (e.g., Cu 2+ ,
2+ III 9- 10- Mn ) with the appropriate lacunary POM precursor (e.g., [As W9O33 ] , [GeW 9O34 ] , and
12- [P 2W15 O56 ] ). The tungsten-oxo framework is usually preserved in the product, if a stable lacunary POM is used. Our group has shown that this is not the case for the divacant
8- 8- decatungstosilicate, [γ-SiW 10 O36 ] . Reaction of [γ-SiW 10 O36 ] with a variety of first-row
12- transition metals has led to dimeric ([{ β-SiNi 2W10 O36 (OH) 2(H 2O)} 2] , [M 4(H 2O) 2(B-α-
12- 2+ 2+ 2+ 15- SiW 9O34 )2] (M = Mn , Cu , Zn )), trimeric ([( β2-SiW 11 MnO 38 OH) 3] ) and tetrameric ([{ β-
24- Ti 2SiW 10O39 }4] ) products and in all cases the gamma-tungsten-oxo-framework was not
15 Representative examples include the following and references therein: (a) Bi, L.-H.; Kortz, U. Inorg. Chem. 2004 , 43 , 7961. (b) Bi, L.-H.; Reicke, M.; Kortz, U.; Keita, B.; Nadjo, L.; Clark, R. J. Inorg. Chem. 2004 , 43 , 3915. (c) Kortz, U.; Nellutla, S.; Stowe, A. C.; Dalal, N. S.; Rauwald, U.; Danquah, W.; Ravot, D. Inorg. Chem. 2004 , 43 , 2308. (d) Kortz, U.; Nellutla, S.; Stowe, A. C.; Dalal, N. S.; van Tol, J.; Bassil, B. S. Inorg. Chem. 2004 , 43 , 144. (e) Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Anderson, T. M.; Hill, C. L. Inorg. Chem. 2004 , 43 , 3257. (f) Ritorto, M. D.; Anderson, T. M.; Neiwert, W. A.; Hill, C. L. Inorg. Chem. 2004 , 43 , 44. (g) Limanski, E. M.; Drewes, D.; Krebs, B. Z. Anorg. Allg. Chem. 2004 , 630 , 523. (h) Laronze, N.; Marrot, J.; Hervé, G. Inorg. Chem. 2003 , 42 , 5857. (i) Hussain, F.; Reicke, M.; Janowski, V.; de Silva, S.; Futuwi, J.; Kortz, U. C. R. Chim. 2005 , 8, 1045.
36
16 8- preserved. We can conclude that the dilacunary precursor [γ-SiW 10 O36 ] isomerizes easily in aqueous, acidic medium upon heating and in the presence of first-row transition metal ions, which can result in oligomeric products with unexpected structures. Therefore we decided to
2+ 8- investigate the system Co /[γ-SiW 10 O36 ] in some detail, as only one structurally characterized
cobalt(II) containing silicotungstate had been reported. 15h
B- The Satellite-Shaped Co-15 Polyoxotungstates,
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(βββ-SiW8O31 )3}]
Experimental Section
Synthesis. All reagents were used as purchased without further purification. The
dilacunary precursor K 8[γ-SiW 10 O36 ] was synthesized according to the published procedure.
Na 5[Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(βββ-SiW8O31 )3}]·37H 2O (Na-1): A 1.00 g (0.36 mmol) sample of K 8[γ-SiW 10 O36 ] was added with stirring to a solution of 1.14 g (4.75 mmol)
CoCl 2·6H 2O in 20 mL H 2O. After complete dissolution (~5 min.) the pH was adjusted to 5.5 by addition of 0.1 M NaOH. This solution was heated to 50 ºC for 30 min. and then cooled to room temperature and filtered. Slow evaporation at room temperature resulted in a purple/red crystalline product after 2-3 days that was filtered off and air-dried. Yield: 0.66 g (65%). IR:
995(m), 937(s), 891(sh), 852(s), 804(m), 750(sh), 695(s), 554(w), 534(w), 496(w) cm -1. Anal.
Calcd for Na 5-1: Na, 1.4; W, 52.1; Co, 10.4; Si, 1.0; Cl, 0.8. Found: Na, 1.4; W, 53.0; Co, 10.0;
Si, 1.1; Cl, 0.9.
16 (a) Kortz, U.; Jeannin, Y. P.; Tézé, A.; Hervé, G.; Isber, S. Inorg. Chem. 1999 , 38 , 3670. (b) Kortz, U.; Isber, S.; Dickman, M. H.; Ravot, D. Inorg. Chem. 2000 , 39 , 2915. (c) Kortz, U.; Matta, S. Inorg. Chem. 2001 , 40 , 815. (d) Hussain, F.; Bassil, B. S.; Bi, L.-H.; Reicke, M.; Kortz, U. Angew. Chem., Int. Ed. 2004 , 43 , 3485.
37
X-ray Crystallography. A red block of Na-1 with dimensions 0.15 x 0.05 x 0.03 mm 3 was mounted on a glass fiber for indexing and intensity data collection at 200 K on a Bruker D8
SMART APEX CCD single-crystal diffractometer using Mo K α radiation ( λ = 0.71073 Å). Of the 6502 unique reflections (2 θmax = 56.66 °), 6173 reflections (R int = 0.132) were considered observed ( I > 2 σ(I)). Direct methods were used to solve the structure and to locate the tungsten and cobalt atoms ( SHELXS-97 ). Then the remaining atoms were found from successive difference maps ( SHELXL-97 ). The final cycle of refinement, including the atomic coordinates,
anisotropic thermal parameters (W, Co, Si, Na and Cl atoms) and isotropic thermal parameters
(O atoms) converged at R = 0.081 and R w = 0.150 (I > 2 σ(I)). In the final difference map the
deepest hole was -3.359 eÅ -3 and the highest peak 5.603 eÅ -3. Routine Lorentz and polarization corrections were applied and an absorption correction was performed using the SADABS program.17 Crystallographic data are summarized in Table B.1.a..
Results and Discussion
II 8- Synthesis and Structure. Interaction of Co ions with [ γ-SiW 10 O36 ] in the ratio 12:1 in
a 1M NaCl medium (pH adjusted to 5.4) resulted in the novel, trimeric
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] (1), see Fig. B.1.a.. The core of polyanion 1 is
II composed of nine Co ions that are encapsulated by three unprecedented (β-SiW 8O31 ) fragments
- and two Cl ligands (see Figure B.1.b). This highly symmetrical ( D3h ) assembly
17- II [Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3] is surrounded by six antenna-like Co (H 2O) 5 groups, which are bound to terminal oxo groups, resulting in the title polyanion 1. Three of these antenna
17 (a)Sheldrick, G. M. SADABS ; University of Göttingen, Germany, 1996. (b) Sheldrick, G. M. SHELXS-97, Program for solution of crystal structures , University of Göttingen, Germany, 1997.
38 are each above and below 1 and they are parallel to each other and to the 3-fold rotation axis of
1. Therefore, the structure of 1 is truly unprecedented and could be described as satellite-like.
Table B.1.a. Crystal Data and Structure Refinement for Na 5[Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-
SiW 8O31 )3}]·37H 2O ( Na-1). ______
emp formula Cl 2Co 15 H155 Na 5O172 Si 3W24 fw 8475.2 space group (No.) P6 3/m (176) a (Å) 19.8754(17) b (Å) 19.8754(17) c (Å) 22.344(4) α (°) 90 β (°) 90 γ (°) 120 vol (Å 3) 7643.9(16) Z 2 temp. ( °C) -73 wavelength (Å) 0.71073 -3 dcalcd (Mg m ) 3.622 abs. coeff. (mm -1) 19.752 R [I > 2 σ(I)] a 0.081 b Rw (all data) 0.150 ______a b 2 2 2 2 2 1/2 R = ∑||Fo|-|Fc||/∑|Fo|. Rw = [∑w(Fo -Fc ) /∑w(Fo ) ] .
39
Fig. B.1.a. 5- Left: Combined polyhedral and ball-and stick-representation (side-view) of [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] (1). Right: ball-and-stick representation (top-view) of (1).
The polyhedra represent WO 6 (red), CoO 6 (violet). Color scheme for the balls: tungsten (black), cobalt (violet), silicon (blue), oxygen (red) and chlorine (yellow).
We believe that the six ‘outer’ Co II ions play an electrostatic stabilizing role by reducing
the charge of the core of 1 by 12 units from -17 to –5. The six capping Co II ions are attached via
only one covalent bond to the core of 1 (Co3-O2T 2.07(1) Å, W2-O2T 1.75(1) Å). Therefore
they are most likely substitution labile in solution. On the basis of structural arguments we
believe that the core of 1 could also exist independently, without the six outer cobalt ions. In this
case charge compensation could perhaps be achieved by protons or tightly bound alkali
40 counterions (e.g. K +). Alternatively, substitution of the nine ‘inner’ Co II ions by high-valent,
early transition metal ions (e.g. Ti IV , Zr IV , V V, Mo VI ) might lead to stable derivatives.
The title polyanion 1 represents only the second example of a structurally characterized
cobalt-substituted tungstosilicate. 14h Furthermore, 1 contains more paramagnetic transition metal centers than any other polyoxotungstate known to date. 18 At the same time it represents the first
discrete POM with a paramagnetic core and shell and it can also be viewed as a hybrid
polyoxoanion/coordination complex..
The core of polyanion 1 (see Figure B.1.b.) is most closely related to Weakley´s trimeric
16- 19 tungstophosphate [Co 9(OH) 3(H 2O) 6(HPO 4)2(PW 9O34 )3] . However, in this species the three
Keggin tungsten-oxo fragments are of the ( B-α-PW 9O34 ) type and the central nonacobalt cluster
2- is capped by a HPO 4 ligand above and below. The magnetic properties of
16- 20 [Co 9(OH) 3(H 2O) 6(HPO 4)2(PW 9O34 )3] have been investigated thoroughly by Coronado et al.
Also, Mialane et al reported on a similar copper-containing trimeric silicotungstate having azide as the central bridging ligand. 21
Bond valence sum calculations (BVS) on 1 indicate that all terminal ligands associated
II with Co ions are water molecules (6 associated with the Co9 core and 30 with the outer Co-
antenna). Furthermore, the three µ3-oxo within the Co 9 core of 1 are hydroxo groups (see Figure
B.1.b.). All of the above combined indicates a charge of –5 for 1. We were able to locate only three sodium counterions by X-ray diffraction, most likely due to disorder. However, the presence of the remaining two sodium ions was established by elemental analysis.
18 Mialane, P.; Dolbecq, A.; Marrot, J.; Rivière, E.; Sécheresse, F. Angew. Chem., Int. Ed. 2003 , 42 , 3523. 19 Weakley, T. J. R. J. Chem. Soc. Chem. Commun. 1984 , 1406. 20 Galán-Mascarós, J. R.; Gómez-García, C. J.; Borrás-Almenar, J. J.; Coronado, E. Adv. Mater. 1994 , 6, 221. 21 Mialane, P.; Dolbecq, A.; Marrot, J.; Rivière, E.; Sécheresse, F. Chem. Eur. J. , 2005 , 11 , 1771.
41
Co2'
Co1' Co1a'
Cl1' Co1a'' Cl1 Co1''
Co1a Co1 Co2'' Co2
Fig. B.1.b. Ball-and-stick representation (top-view) of the central cobalt-oxo fragment in [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] 5- (1). The color code is the same as in Fig. B.1.a..
As mentioned above, polyanion 1 is composed of three (β-SiW 8O31 ) fragments and to our
knowledge this fragment has never been observed before in polyoxoanion chemistry. We must
8- remember that 1 was synthesized starting from [ γ-SiW 10 O36 ] . Therefore, the mechanism of
formation of 1 must involve metal insertion, rotational isomerization ( γ-Keggin β-Keggin)
and loss of tungsten (SiW 10 SiW 8). It is not clear if 1 is formed from three monomeric Keggin
fragments in one condensation step or by gradual growth involving a dimeric intermediate.
42
Furthermore it is not clear at which point of formation of 1 the antenna-like, outer Co II ions come into play. The detailed mechanism of formation of POMs (including the largest ones known) via self-assembly remains one of the mysteries in inorganic chemistry.
C- Cobalt Containing Silicotungstate Sandwich Dimer [{Co 3(B-βββ-SiW9O33 (OH))( B-βββ-
22- SiW8O29 (OH) 2)} 2]
Experimental Section
Synthesis. All reagents were used as purchased without further purification. The
dilacunary precursor K 8[γ-SiW 10 O36 ] was synthesized according to the published procedure.
K10 Na 12 [{Co 3(B-βββ-SiW9O33 (OH))( B-βββ-SiW8O29 (OH) 2)}2]·49H 2O (KNa -2): A 0.19 g sample of CoCl 2·6H 2O (0.79 mmoles) was dissolved in 20 mL of a 0.5 M sodium acetate buffer
at pH 4.8 followed by addition of 1.0 g (0.36 mmoles) of K 8[γ-SiW 10 O36 ]. The solution was stirred for 30 min at 50 °C and then allowed to cool down to room temperature and filtered. Then
1 mL of 1 M KCl solution was added to the filtrate and the solution was allowed to evaporate at room temperature. After about 2 weeks purple crystals suitable for X-ray diffraction had formed
(yield 0.17 g, 38 %). It should be noted that even without addition of KCl solution crystals of
KNa -2 are formed. However, this process can be accelerated by addition of potassium ions.
Furthermore, the same polyanion 2 can be obtained if the above reaction is performed in water at
pH 4.8, keeping everything else the same. In fact, we discovered that 2 can be synthesized up to pH 7.0 as monitored by IR spectroscopy on precipitated solids. IR for KNa -2: 989(sh), 945(m),
885(s), 847(s), 789(m), 723(s), 619(sh), 534(w), 493(w) cm -1. Anal. Calcd for KNa -2: K, 3.8;
Na, 2.7; W, 60.4; Co, 3.4; Si, 1.1. Found: K, 3.7; Na, 3.0; W, 61.2; Co, 3.3; Si, 1.3.
43
X-ray Crystallography. A purple irregular block of KNa-2 with dimensions 0.14 x 0.10 x 0.06 mm 3 was mounted on a glass fiber for indexing and intensity data collection at 173 K on a
Bruker D8 SMART APEX CCD single-crystal diffractometer using Mo K α radiation ( λ =
0.71073 Å). Of the 40824 unique reflections (2 θmax = 56.66 °), 17543 reflections (R int = 0.161) were considered observed ( I > 2 σ(I)). Direct methods were used to solve the structure and to
locate the tungsten and cobalt atoms ( SHELXS-97 ). Then the remaining atoms were found from
successive difference maps ( SHELXL-97 ). The final cycle of refinement, including the atomic coordinates, anisotropic thermal parameters (W, Co, Si, Na and K atoms) and isotropic thermal parameters (O atoms) converged at R = 0.060 and R w = 0.145 (I > 2 σ(I)). In the final difference
map the deepest hole was -2.242 eÅ -3 and the highest peak 6.643 eÅ -3. Routine Lorentz and
polarization corrections were applied and an absorption correction was performed using the
SADABS program. Crystallographic data are summarized in Table B.2.a..
Results and discussion
2+ 8- Synthesis and structure. Reaction of Co ions with [γ-SiW 10 O36 ] in a ratio of 2:1 in acetate buffer medium (pH 4.8) at 50 °C resulted in the novel, dimeric sandwich assembly
22- [{Co 3(B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)}2] (2), see Figure B.1.c.. This polyanion is
composed of two equivalent sandwich fragments which are arranged almost orthogonally to each
other. Interestingly, each sandwich fragment is built up of two different Keggin units. One of
them is the ( B-β-SiW 8O34 ) fragment, which is the same as in 1. The other Keggin unit is the ( B-
β-SiW 9O34 ) fragment, which has never been observed before in polyanion chemistry. These two different Keggin units are held together by three cobalt(II) ions resulting in an asymmetric ( C1
44 symmetry) sandwich structure with the hypothetical formula [Co 3(H 2O)( B-β-SiW 9O33 (OH))( B-
11- β-SiW 8O29 (OH) 2)] (2a), see Figure B.1.d..
Fig. B.1.c. β β 22- Ball-and-stick presentation of the sandwich dimer [{Co 3(B- -SiW 9O33 (OH))( B- -SiW 8O29 (OH) 2)}2] (2). The color code is as follows: tungsten (black), cobalt (violet), silicon (blue) and oxygen (red).
45
Table B.1.b. Crystal Data and Structure Refinement for K10 Na 12 [{Co 3(B-β-SiW 9O33 (OH))( B-β-
SiW 8O29 (OH) 2)} 2]·49H 2O ( KNa -2).
emp formula Co 6H104 K10 Na 12 O179 Si 4W34 fw 10352.8 space group (No.) P2 1/n (14) a (Å) 19.9466(8) b (Å) 24.6607(10) c (Å) 34.0978(13) β (°) 102.1750(10) vol (Å 3) 16395.4(11) Z 4 temp. ( °C) -100 wavelength (Å) 0.71073 -3 dcalcd (Mg m ) 4.058 abs. coeff. (mm -1) 24.745 R [I > 2 σ(I)] a 0.057 b Rw (all data) 0.097 ______a b 2 2 2 2 2 1/2 R = ∑||Fo|-|Fc||/∑|Fo|. Rw = [ ∑w(Fo -Fc ) /∑w(Fo ) ] .
Our single crystal XRD results show that in the solid state two molecules of 2a are
connected via two equivalent Co-O-W’ bonds leading to the dimeric 2. The ‘outer’ cobalt ion of
one sandwich fragment is connected via an oxo-bridge to a belt-tungsten atom from a non-
rotated triad of the ( B-β-SiW 9O34 ) unit of the other sandwich fragment. Interestingly, in the case
of 2 always two identical enantiomers of 2a (dd or ll ) are linked resulting in a chiral assembly of
46
C2 symmetry. However, as both enantiomers are present in equal amounts in the solid state the polyanion crystallizes in a centrosymmetric space group ( P2 1/n ). We have evidence that 2 exists even in aqueous solution after redissolution of the solid material KNa -2 as based on gel chromatography and electrochemistry.
Fig. B.1.d. Combined polyhedral and ball-and-stick presentation of the half-unit of 2 with the molecular formula 11- [Co 3(H 2O)( B-β-SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)] (2a ).
The polyhedra represent WO 6 (red), CoO 6 (violet). The blue balls are Si.
The dimeric 2 is unprecedented in transition metal substituted POM chemistry. The only
other structurally characterized cobalt(II) containing silicotungstates known to date are Hervé´s
12- [K 2{Co(H 2O) 2}3(A-α-SiW 9O34 )2] and our Co-15 containing
47
5- 14h [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] (1). Nevertheless, there is one other class of polyanions that is structurally, but not compositionally similar to 2. These are the mixed valence
IV VI 11- heteropoly brown species [H 4BW 3W 17 O66 ] and the isopoly brown species
IV VI 11- 22 [H 6W 3W 17 O66 ] , both reported by Dickman et al. in 2000. These polyanions are also of the
asymmetric sandwich type, but they are composed of a ( B-α-XW 9O34 ) Keggin fragment and a
(β-W8O30 ) isopoly fragment. Furthermore, the composition of the central section of Dickman´s
heteropoly browns is very different from 2. Instead of three cobalt ions there are three reduced
WIV centers.
Bond valence sum calculations indicate that the dimeric sandwich assembly 2 is hexaprotonated. The oxygens O8C2, O147, O156, O25C, O273 and O324 are all monoprotonated (see Figure B.1.c.). In fact they are all µ2-hydroxo bridges and can be divided in three groups: the symmetry equivalent O8C2 and O25C are bridging between a cobalt and a tungsten center (Co2-O8C2-W8, Co4-O25C-W25) of the rotated triad within each of the two ( B-
β-SiW 9O34 ) fragments in 2. The oxygen atoms O156 and O273 are symmetry equivalent and so
are O147 and O324. They are all bridging between two tungsten centers (W15-O156-W16, W27-
O273-O33 and W14-O147-W17, W32-O324-W34) within each of the two ( B-β-SiW 8O31 )
fragments. The atoms W14, W15, W27 and W32 represent the four tungsten centers in 2 with two terminal oxo ligands. Interestingly this reduces the total negative charge of 2 to –22, which means that the monomeric sandwich fragment 2a would have exactly the same charge of –11 as
IV VI 11- IV VI 11- Dickman´s mixed valence clusters [H 4BW 3W 17 O66 ] and [H 6W 3W 17 O66 ] . We also learn that in 2 the ( B-β-SiW 8O31 ) fragment is more basic than the ( B-β-SiW 9O34 ) unit as in the former two W-O-W bridges are protonated, whereas in the latter only one Co-O-W bridge is
22 Dickman, M. H.; Ozeki, T.; Evans, H. T., Jr.; Rong, C.; Jameson, G.B.; Pope, M. T. J. Chem. Soc., Dalton Trans. 2000 , 149.
48 protonated. This means that it might be possible to extend the structure of 2 via condensation of
additional metal-oxo units onto these basic sites.
The structure of polyanion 2 can be considered as an intermediate during formation of the
n- well-known Weakley type sandwich structure [M 4(H 2O) 2(B-α-SiW 9O34 )2] . In 2000 our group
12- reported on the manganese(II), copper(II) and zinc(II) derivatives [M 4(H 2O) 2(B-α-SiW 9O34 )2]
2+ 2+ 2+ 8- (M = Mn , Cu , Zn ) and all three species were synthesized from [γ-SiW 10 O36 ] using
conditions very similar to 2.15b It must be remembered that formation of the Weakley dimer
8- (which contains B-α-SiW 9O34 units) from the [γ-SiW 10 O36 ] precursor requires isomerization
(gamma beta alpha) and loss of tungsten. Most likely the (B-β-SiW 9O34 ) and ( B-β-
SiW 8O31 ) fragments observed in 2 are intermediate transformation products of ( γ-SiW 10 O36 )
(B-α-SiW 9O34 ). Both fragments, which are probably very unstable as free, lacunary ions, are most likely trapped in the monomeric 2a first which then dimerizes to the more stable 2.
Interestingly we never isolated analogs of 2 or 2a for any other first-row transition metal besides
2+ 8- cobalt. This indicates that interaction of Co ions with [ γ-SiW 10 O36 ] is very different from the other divalent transition metal ions Mn 2+ , Cu 2+ and Zn 2+ . In fact, the behaviour of Co2+ with [ γ-
8- 2+ 2+ SiW 10 O36 ] in acidic, aqueous medium resembles somewhat that of Ni . For the system Ni /[ γ-
8- SiW 10 O36 ] we also did not observe formation of the Weakley dimer, but we rather isolated a
Keggin dimer structure containing the novel (β-SiW 10 O37 ) fragment, [{ β-
12- 15a SiNi 2W10 O36 (OH) 2(H 2O)} 2] .
It is also of interest to compare the structures and synthesis conditions of 2 and Co-15
5- containing [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] (1). Both species are cobalt(II)
containing silicotungstates and both contain the novel (B-β-SiW 8O31 ) fragment. Close inspection of the structural core of 1 reveals that it is composed of three fused (Co 3SiW 8) fragments, which
49 are also a building block of 2. Not surprisingly, polyanions 2 and
5- [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] have very similar synthesis conditions
regarding ratio and concentration of reagents, solvent, temperature, time of heating and pH.
However, what differ in both are two crucial elements: the counter ions and the presence/absence
5- of chloride ions. The Co-15 containing [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] forms only as a sodium salt and requires the presence of chloride ions. On the other hand, formation of
2 requires the presence of a relatively high concentration of potassium ions. As expected, we were also able to synthesize 2 in 1M KCl solution as based on IR. This observation reemphasizes the importance of counter cations and anions during the formation process of polyanions and not only for crystallization purposes. A full understanding and above all control of all parameters that have an impact on polyanion formation is still far away.
In summary, we suggest that the overall transformation ( γ-SiW 10 O36 ) (B-α-SiW 9O34 )
proceeds stepwise and involves the following intermediate fragments: (γ-SiW 10 O36 ) (β-
SiW 10 O38 )/( β-SiW 10 O37 ) (B-β-SiW 9O34 )/( B-β-SiW 8O31 ) (B-α-SiW 9O34 ). We notice that in
this transformation the number of lacunary sites increases, but in our opinion the main driving
force is the conversion from γ β α type rotational isomers. The most interesting and
8- probably the most important aspect of our work with the dilacunary precursor [ γ-SiW 10 O36 ] is
the fact that only specific 3d transition metal ions can stabilize the above Keggin fragments.
4+ 24- 2+ Specifically, Ti stabilizes ( β-SiW 10 O38 ) in [{ β-Ti 2SiW 10 O39 }4] , Ni stabilizes ( β-SiW 10 O37 )
12- 2+ in [{ β-SiNi 2W10 O36 (OH) 2(H 2O)} 2] , Co stabilizes ( B-β-SiW 9O34 )/( B-β-SiW 8O31 ) in 2 and ( B-
5- 2+ 2+ β-SiW 8O31 ) in [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3}] (1), and finally Mn , Cu and
2+ 12- 15 Zn all stabilize (B-α-SiW 9O34 ) in [M 4(H 2O) 2(B-α-SiW 9O34 )2] .
50
D- Conclusion
The 15-cobalt-substituted polyoxotungstate [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(β-
II SiW 8O31 )3}] 5- (1) has been synthesized by interaction of Co ions with the dilacunary [γ-
8- SiW 10 O36 ] in a simple one-pot procedure in aqueous, acidic NaCl medium. The trimeric 1 has a
II - core of nine Co ions encapsulated by three unprecedented (β-SiW 8O31 ) fragments and two Cl ligands. The highly symmetrical ( D3h ) assembly [Co 9Cl 2(OH) 3(H 2O) 9(β-SiW 8O31 )3]17- is
II surrounded by six antenna-like Co (H 2O) 5 groups resulting in the satellite-like structure 1.
Polyanion 1 represents only the second example of a structurally characterized cobalt-substituted tungstosilicate. Furthermore, 1 contains more paramagnetic transition metal centers than any other polyoxotungstate known to date. At the same time it represents the first discrete POM with a paramagnetic core and shell and it can also be viewed as a hybrid polyoxoanion/coordination complex. Besides its unique structure, the most interesting aspect of 1 is undoubtedly its
II enormous potential for surface functionalization. The six Co (H 2O) 5 antenna allow to incorporate a variety of mono-, di- and tridentate ligands leading to derivatives with interesting properties. Furthermore, it can be envisioned to substitute the six external cobalt ions by other paramagnetic as well as diamagnetic ions, resulting in isostructural polyanions with different redox and magnetic properties. Therefore, we believe that 1 is the parent compound of a novel and important subclass of transition metal substituted polyoxotungstates.
We have also prepared the cobalt containing silicotungstate sandwich dimer [{Co 3(B-β-
22- SiW 9O33 (OH))( B-β-SiW 8O29 (OH) 2)}2] (2) in a simple, one-pot reaction in aqueous, acidic medium. Polyanion 2 is composed of two asymmetric sandwich subunits which are each composed of three cobalt ions being encapsulated between an unprecedented (B-β-SiW 9O34 )
fragment and a ( B-β-SiW 8O31 ) fragment. Two of these sandwich subunits are linked via two Co-
51
O-W bridges in an approximately orthogonal fashion resulting in the novel polyanion assembly
2. We believe that the dimeric nature of 2 is maintained in solution as based on gel filtration chromatography.
The work presented here reinforces earlier results by our group highlighting the flexible
8- nature of the dilacunary precursor [γ− SiW 10 O36 ] precursor when it encounters different 3d
transition metal ions in aqueous solution. We have shown that such reactions can lead to
2+ 8- unprecedented and unexpected polyanion architectures. Also Co ions react with [ γ-SiW 10 O36 ] leading to novel polyanion structures, which cannot be obtained with any other transition metal ions.
Future synthetic work in this area will be focused on the discovery of other transition metal substituted silicotungstate structures with a potential for homogeneous and heterogeneous oxidation catalysis. We will focus on red/ox active 3d metals (e.g. Fe 3+ , Cr 3+ ) and also on 4d and
5d metals (e.g. Ru 3+ , Pd 2+ ). Of particular interest are open polyanion structures with accessible
8- transition metal centers. We believe that by using the [ γ-SiW 10 O36 ] precursor we have a good
chance of discovering fundamentally novel polyanion architectures. Finally, we are also
interested in preparing diamagnetic analogs of our novel compounds, in order to study their
solution properties by NMR in addition to electrochemistry.
52
12- II- Formation of the fused [γ-Si 2W20 O70 ] unit.
A- Introduction
Recently, fused lacunary silicotungstates have been reported by using the potassium salt
10- of the silicotungstate precursor A-α-[SiW 9O34 ] . In 2003 Hervé et al. reported on the "open
16- Wells-Dawson ion" [Si 2W18 O66 ] and the same authors also studied the reaction of K 10 [A-α-
23 SiW 9O34 ] with cobalt, manganese and zinc ions in aqueous media. The same dimeric, fused
10- (Si 2W18 O66 ) fragment was observed by Bi and Kortz in [Cu 5(OH) 4(H 2O) 2(A-α-SiW 9O33 )2] ,
2+ 10- which was synthesized by reaction of Cu ions with A-α-[SiW 9O34 ] in 0.5 M sodium acetate
24 buffer. Very recently, Hervé et al. reported on the fused tetramer [{[K(H 2O) 2]( µ-H2O)-
16- 16- [Li(H 2O) 2]} 2Si 4W36 O126 (H 2O) 4] , synthesized from [Si 2W18 O66 ] following a step-by-step
assembly process. 25
Our group has been working extensively on the interaction of the dilacunary silicotungstate
8- precursor γ-[SiW 10 O36 ] with transition metal ions. During the course of these studies we noticed an important effect of the type of alkali counter-cations (e.g. Na + vs K +) in the synthesis medium.
8- Therefore, we decided to continue our systematic investigation of the reactivity of [ γ-SiW 10 O36 ] with different transition metal ions as a function of the alkali metal ion type and concentration in the aqueous solvent.
Here we report on three discrete, molecular derivatives of a monosubstituted, dimeric
12- polyanion type containing the new [ γ-Si 2W20 O70 ] fragment.
23 Laronze, N.; Marrot, J.; Hervé, G. Chem. Commun., 2003, 2360. 24 Bi, L.-H; Kortz, U. Inorg. Chem., 2004, 43 , 7961. 25 Leclerc-Laronze, N.; Haouas, M. ; Marrot, J.; Taulelle, F. ; Hervé, G. Angew. Chem. Int. Ed., 2006, 43 , 139.
53
10- B- Transition Metal Containing Decatungstosilicate Dimer [M(H 2O) 2(γγγ-SiW10 O35 )2]
(M = Mn 2+ , Co 2+ , Ni 2+ )
Experimental Section
Synthesis. All reagents were used as purchased without further purification. The dilacunary precursor K 8[γ-SiW 10 O36 ] was synthesized according to the published procedure.
K10 [Mn(H 2O) 2(γγγ-SiW10 O35 )2]·8.25H 2O (K-3). To 20 mL of a 1M KCl solution were added simultaneously 0.078 g of MnCl 2·4H 2O (0.40 mmoles) and 1.0 g of K 8[γ-SiW 10 O36 ] (0.36 mmoles). The pH was then adjusted to 4.5 by dropwise addition of a 0.1 M HCl solution. The resulting mixture was heated to 50 °C for 30 min, and then allowed to cool to room temperature and filtered. Slow evaporation of the filtrate resulted in dark brown crystals (yield 0.40 g, 41%).
IR of K-3: 998(m), 950(m), 891(sh), 864(s), 773(s), 730(s), 614(w), 553(w), 530(w) cm -1. Anal.
Calcd for K-3: K, 7.1; W, 67.1; Mn, 1.0; Si, 1.0. Found: K, 6.6; W, 66.9; Mn, 1.1; Si, 1.2.
K10 [Co(H 2O) 2(γγγ-SiW10 O35 )2]·8.25H 2O (K-4). The synthesis procedure was identical to
K-3, but 0.095 g of CoCl 2·6H 2O (0.40 mmoles) was used. Slow evaporation of the filtrate
resulted in dark purple crystals (yield 0.40 g, 40%). IR of K-4: 999(m), 948(m), 897(sh), 866(s),
766(s), 730(s), 618(w), 552(w), 530(w) cm -1. Anal. Calcd for K-4: K, 7.1; W, 67.0; Co, 1.1; Si,
1.0. Found: K, 7.5; W, 66.2; Co, 1.3; Si, 1.1.
K10 [Ni(H 2O) 2(γγγ-SiW10 O35 )2].13.5H 2O (K-5). The synthesis procedure was identical to
K-3 and K-2 but 0.095 g of NiCl2.6H 2O (0.40 mmoles) was used. Slow evaporation of the filtrate resulted in bright green crystals (yield 0.45 g, 45%). IR of K-5: 1000(m), 948(m),
54
896(sh), 869(s), 793(s), 735(s), 619(w), 557(w), 534(w) cm -1. Anal. Calcd for K-5: K, 7.0; W,
65.9; Ni, 1.1; Si, 1.0. Found: K, 7.5; W, 66.6; Ni, 1.0; Si, 1.1.
X-Ray crystallography. A crystal of K-4 was mounted on a glass fiber and coated with
epoxy glue to retard decomposition for data collection at room temperature. Crystals of K-3 and
K-5 were mounted in a Hampton cryoloop using light oil for data collection at –100º C. Indexing
and data collection were performed using a Bruker D8 SMART APEX II CCD diffractometer
with kappa geometry and Mo-Kα radiation ( λ = 0.71073 Å). Compound K-4 was observed to have a unit cell with twice the volume of the other two compounds. A second crystal of K-4 was mounted and indexed to confirm this. Multiscan absorption corrections were performed using
SADABS. Routine Lorentz and polarization corrections were applied. Direct methods
(SHELXS97) solutions successfully located the W atoms, and successive Fourier syntheses
(SHELXL97) revealed the remaining atoms. Refinements were full-matrix least squares against
|F|2 using all data. All three structures contained more cation sites than could be fully occupied for the given charge of the anion, and the cations (and waters of hydration) were modeled with varying degrees of occupancy, a common situation for polyoxotungstate structures. In the final refinements, all non-disordered heavy atoms (W, K, Si, Co, Mn, Ni) were refined anisotropically while the O atoms and disordered cations were refined isotropically. No H atoms were included in the models. Crystallographic data for K-3, K-4 and K-5 are summarized in Table B.2.a.
55
Table B.2.a. Crystal data and structure refinement results for K10 [Mn(H 2O) 2(γ-
SiW 10 O35 )2]·8.25H 2O ( K-3), K 10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4) and K 10 [Ni(H 2O) 2(γ-
SiW 10 O35 )2]·13.5H 2O ( K-5).
K-3 K-4 K-5
Empirical Formula H20.5 K10 MnO 80.25Si 2W20 CoH 20.5 K10 O80.25 Si 2W20 H31 K9NiO 85.5 Si 2W20 M 5483.8 5487.8 5582.2 Crystal system Triclinic Triclinic Triclinic Space group (no.) P1 (2) P1 (2) P1 (2) a/Å 12.5742(12) 18.907(3) 12.5354(6) b/Å 18.9403(16) 22.688(3) 18.9293(9) c/Å 21.464(3) 24.795(4) 21.5454(19) α/° 107.577(4) 63.048(3) 107.780(2) β/° 101.054(4) 79.310(3) 101.753(2) γ/° 97.035(4) 71.193(3) 97.315(2) V/Å 3 4693.1(8) 8965(2) 4666.6(4) Z 2 4 2 T/ºC -100(2) 23(2) -100(2) λ/Å 0.71073 0.71073 0.71073 -3 Dc/Mg m 3.852 4.065 3.923 µ/mm -1 25.068 26.315 25.261 R[I<2 σ(I)] a 0.091 0.079 0.068 b Rw (all data) 0.256 0.275 0.218
a b 2 2 2 2 2 1/2 R = Σ|| Fo| - |Fc||/ Σ|Fo|. Rw = [ Σw(Fo – Fc ) /Σw(Fo ) ]
56
Results and Discussions
10- Synthesis and structure. The three new polyanions [M(H 2O)2(γ-SiW 10 O35 )2] (M =
2+ 2+ 2+ Mn , 3; Co , 4; Ni , 5) have been isolated as the potassium salts K 10 [Mn(H 2O) 2(γ-
SiW 10 O35 )2]·8.25H 2O ( K-3), K 10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4) and K 10 [Ni(H 2O) 2(γ-
SiW 10 O35 )2]·13.5H 2O ( K-5). K-3 and K-5 are isomorphous and polyanions 3-5 are isostructural,
which is reflected in their strikingly similar IR spectra (Fig B.2.a.). The anions 3-5 are composed
6- of two ( γ-SiW 10 O35 ) units fused on one side by two W-O-W µ-oxo bridges, resulting in a
12- [Si 2W20 O70 ] lacunary unit. The two (γ-SiW 10 O35 ) units are further joined by the 3d transition
2+ 2+ 2+ metal center M (Mn , Co , Ni ) on the other side, by a total of four W-O-M µ2-oxo bridges
(Fig B.2.b.). Polyanions 3-5 exhibit C 2v point group symmetry.
34 32 30
28 26
24 22
20 18
16
%T 14 12
10 8
6 4
2
0 -2
-4
1000 900 800 700 600 500 cm-1 Fig. B.2.a.
IR spectra of K 10 [Mn(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-3, upper), K 10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4, middle), and
K10 [Ni(H 2O) 2(γ-SiW 10 O35 )2].13.5H 2O ( K-5, lower).
57
Fig. B.2.b. γ 10- 2+ 2+ 2+ Ball-and-stick (left) and polyhedral (right) representations of [M(H 2O) 2( -SiW 10 O35 )2] (M = Mn , 3; Co , 4; Ni , 5). The balls represent W (black), O (red), Si (blue), M (yellow) and the polyhedra represent WO 6 (red), SiO 4, MO 4(H 2O) 2 (yellow).
The anions 3-5 are synthesized at identical conditions. They result from interaction of [ γ-
8- 2+ 2+ 2+ SiW 10 O36 ] with Mn , Co and Ni , respectively, in a 1:1 ratio in 1M KCl solution at pH 4.5.
Mild heating is used to speed up the reaction (50 °C for 30 min). It seems that the high
concentration of potassium ions stabilizes the dilacunary ( γ-SiW 10 O36 ) fragment, slowing down
8- or preventing isomerization and/or decomposition. Interestingly, reaction of [ γ-SiW 10 O36 ] with
M2+ in water at pH 4.5 followed by addition of sufficient solid KCl to obtain a 1M solution
resulted in a significantly lower yield compared to having enough KCl already present in
solution. Furthermore, the choice of pH is crucial. We noticed a decreasing yield with increasing
initial pH of the reaction. This probably indicates that at lower pH condensation of two [ γ-
58
8- 12- 3+ SiW 10 O36 ] anions to the fused [ γ-Si 2W20 O70 ] unit is favorable. Attempts to prepare the Cr ,
Fe 3+ , Cu 2+ and Zn 2+ analogues of 3-5 have not been successful. Furthermore, we have not been
12- able yet to isolate the hypothetical lacunary polyanion [ γ-Si 2W20 O70 ] . Our efforts so far have
4- resulted in the Keggin ion [SiW 12 O40 ] as based on IR.
Fig. B.2.c. Coordination sphere with atomic labeling and bond lengths for the incorporated transition metal centers in 3-5.
59
The transition metal ions incorporated in 3-5 exhibit a distorted octahedral coordination geometry with two terminal water ligands trans to each other. Protonation was estimated using bond valence sum calculations. Interestingly, the inner water molecule has a significantly longer
M-OH 2 bond than the outer one for the three polyanions: 2.321(17) Å vs. 2.217(17) Å for Mn-
OH 2 in 3, 2.250(18) Å vs. 2.19(2) Å for Co-OH2 in 4, and 2.193(14) vs. 2.062(14) for Ni-OH 2 in
5. There are no apparent steric, but electronic reasons for this. The inner water molecule is also
coordinated to a potassium cation near the belt of the anion. As expected, the four equatorial M-
O bond lengths decrease systematically from manganese to cobalt to nickel (Fig. B.2.c.). This
reflects nicely the decreasing ionic radius of M in the same sequence (Mn 2+ > Co 2+ > Ni 2+ ).
The potassium salts K-3, K-4 and K-5 all crystallize in the space group P-1 and in addition K-3 and K-5 are isomorphous. In contrast, K-4 has a unit cell twice as large as the others. There is no apparent reason why K-4 should crystallize differently from K-3 and K-5; we speculate that the two different structures observed are polymorphs, and that any of the compounds could have crystallized in either form. The two independent anions in K-4 are essentially the same. In particular, the Co-O bond lengths in the two coordination spheres are identical within error (Fig. B.2.c.). Table B.2.a. shows the crystallographic data of K-3, K-4 and
K-5.
Another fused, lacunary silicotungstate fragment is encountered in Bi and Kortz’s
10- 22 [Cu 5(OH) 4(H 2O) 2(A-α-SiW 9O33 )2] . The two Keggin subunits in this polyanion are also
16- linked via two W-O-W µ 2-oxo bridges. However, the lacunary fragment [Si 2W18 O66 ]
12- encapsulates five metal cations instead of one in the [Si 2W20 O70 ] subunit present in 3-5. This
difference in number of transition metal centers that can be fitted inside the cavity of the fused
60
16- polyanion can be rationalized by the availability of donor oxygens. Unlike [Si 2W18 O66 ] , which
12- has eight donor oxygens in its cavity, [Si 2W20 O70 ] has only four, thereby reducing the number
of possible coordinated metal centers.
Polyanions 3-5 are interesting both structurally and for their possible applications. These
12- anions represent the first examples containing the fused, dimeric, lacunary [Si 2W20 O70 ] unit in
16- POM chemistry, in analogy with Herve’s fused dimeric, lacunary [Si 2W18 O66 ] unit. In both
16- 12- fragments [Si 2W18 O66 ] and [Si 2W20 O70 ] the two hinge-like W-O-W linkages joining the two
Keggin units are constructed by pairs of edge-shared WO 6 octahedra (Fig. B.2.b.). The
8- possibility of complete fusion of two [ γ-SiW 10 O36 ] units forming a closed assembly (similar to the Wells-Dawson type POMs) is interesting, but has not been observed so far. In addition, the highly accessible transition metal center M with two, trans-related, labile aqua ligands can provide interesting catalytic activity to 3-5.
C – Conclusion
We have synthesized the new, monometal substituted silicotungstates [Mn(H 2O) 2(γ-
10- 10- 10- SiW 10 O35 )2] (3), [Co(H 2O) 2(γ-SiW 10 O35 )2] (4) and [Ni(H 2O) 2(γ-SiW 10 O35 )2] (5) in acidic
(pH 4.5) 1M KCl solution. The corresponding potassium salts K 10 [Mn(H 2O) 2(γ-
SiW 10 O35 )2]·8.25H 2O ( K-3), K 10 [Co(H 2O) 2(γ-SiW 10 O35 )2]·8.25H 2O ( K-4) and K 10 [Ni(H 2O) 2(γ-
SiW 10 O35 )2]·13.5H 2O ( K-5) have been characterized by IR spectroscopy, single-crystal X-ray
diffraction, elemental analysis and cyclic voltammetry. The isostructural polyanions 3-5 have C2v
12 point group symmetry and they contain the unprecedented dimeric lacunary unit [ γ-Si 2W20 O70 ] .
The incorporated transition metal ions have trans aqua ligands and are sterically accessible. The
presence of potassium ions facilitates formation of 3-5 and a high K + concentration in the
61 synthesis medium results in a better yield. We plan to investigate the catalytic activity of 3-5 in oxidation reactions. Furthermore, we will try to synthesize other transition metal derivatives of
3-5 with an emphasis on diamagnetic species for solution NMR studies. We believe that it might
8- also be possible to prepare the closed, dimeric all-tungsten derivative [ γ-Si 2W20 O68 ] in aqueous solution. Mizuno et al were able to synthesize the closed dimer in organic medium. 26
26 Yoshida, A.; Yoshimura, M.; Uehara, K.; Hikichi, S.; Mizuno, N. Angew. Chem., Int. Ed. 2006, 45 , 1956.
62
8- 4+ III- Interaction of [ γ-SiW 10 O36 ] with Zr
A- Introduction
Zirconium-containing POMs have been one of the least reported types so far in the literature, despite the fact that zirconium compounds have been useful as catalysts. Until recently, the number of
11- 27 zirconium-substituted POMs had been limited to Finke’s sandwich-dimer [Si 2W18 Zr 3O71 H3] .
However, over the last 5 years the area of Zr-POMs has experienced a renaissance as numerous
Keggin and Wells-Dawson based Zr-derivatives have been reported. 28 To date several
coordination complexes containing peroxo-Zr centers have been reported. A number of peroxo-
POMs in which the peroxo groups are usually linked to addenda atoms of the POM skeleton
have been reported. 29 To our knowledge there are no examples of structurally characterized
27 Finke, R. G.; Rapko, B.; Weakley, T. J. R. Inorg. Chem. 1989 , 28 , 1573 28 (a) Kikukawa, Y.; Yamaguchi, S.; Tsuchida, K.; Nakagawa, Y.; Uehara, K.; Yamaguchi, K.; Mizuno, N. J. Am. Chem. Soc. 2008 DOI: 10.1021/ja078313i (b) Errington, R. J.; Petkar, S. S.; Middleton, P. S.; McFarlane, W. J. Am. Chem. Soc. 2007 , 46 , 8502. (c) Sokolov, M. N.; Chubarova, E. V.; Peresypkina, E. V.; Virovets, A. V.; Fedin, V. P. Russ. Chem. Bull. 2007 , 56 , 220. (d) Boglio, C.; Micoine, K.; Remy, P.; Hasenknopf, B.; Thorimbert, S.; Lacote, E.; Malacria, M.; Afonso, C.; Tabet, J. C. Chem. Eur. J. 2007 , 13 , 5426. (e) Fang, X. K.; Hill, C. L. Angew. Chem. Int. Ed. 2007 , 46 , 3877. (f) Kholdeeva, O. A.; Maksimovskaya, R. I. J. Mol. Catal. A: Chem. 2007 , 262 , 7. (g) Kholdeeva, O. A.; Maksimov, G. M.; Maksimovskaya, R. I.; Vanina, M. P.; Trubitsina, T. A.; Naumov, D. Y.; Kolesov, B. A.; Antonova, N. S.; Carbo, J. J.; Poblet, J. M. Inorg. Chem. 2006 , 45 , 7224. (h) Carabineiro, H.; Villanneau, R.; Carrier, X.; Herson, P.; Lemos, F.; Ribeiro, F. R.; Proust, A.; Che, M. Inorg. Chem. 2006 , 45 , 1915. (i) Kato, C. N.; Shinohara, A.; Hayashi, K.; Nomiya, K. Inorg. Chem . 2006 , 45 , 8108. (j) Fang, X.; Anderson, T.; Hill, C. L. Angew. Chem. Int. Ed. 2005 , 44 , 3540. (k) Fang, X.; Anderson, T.; Hill, C. L. Chem. Commun. 2005 , 50448. (l) Villanneau, R.; Carabineiro, H.; Carrier, X.; Thouvenot, R.; Herson, P.; Lemos, F.; Ribeiro, F. R.; Che, M. J. Phys. Chem. B 2004 , 108 , 12465. (m) Gaunt, A. J.; May, I.; Collison, D.; Fox, O. D. Inorg. Chem. 2003 , 42 , 5049. (n) Gaunt, A. J.; May, I.; Collison, D.; Holman, K. T.; Pope, M. T. J. Mol. Struct. 2003 , 656 , 101. 29 (a) Venturello, C.; D’Aloisio, R.; Bart, J. C. J.; Ricci, M. J. Mol. Catal. 1985 , 32 , 107. (b) Hashimoto, M.; Iwamoto, T.; Ichida, H.; Sasaki, Y. Polyhedron 1991 , 10 , 649. (c) Yamase, T.; Ozeki, T.; Motomura, S. Bull. Chem. Soc. Jpn. 1992 , 65 , 1453. (d) Dickman, M. H.; Pope, M. T. Chem. Rev . 1994 , 94 , 569. (e) Salles, L.; Aubry, C.; Thouvenot, R.; Robert, F.; Dorémieux-Morin, C.; Chottard, G.; Ledon, H.; Jeannin, Y.; Brégeault, J.-M. Inorg. Chem . 1994 , 33 , 871. (f) Kim, G.–S.; Judd, D. A.; Hill, C. L.; Schinazi, R. F. J. Med. Chem. 1994 , 37 , 816. (g) Griffith, W. P.; Parkin, B. C.; White, A. J. P.; Williams, D. J. J. Chem. Soc., Dalton Trans. 1995 , 3131. (h) Ozeki, T.; Yamase, T. Bull. Chem. Soc. Jpn. 1997 , 70 , 2101. (i) Judd, D. A.; Chen, Q.; Campana, C. F.; Hill, C. L. J. Am. Chem. Soc. 1997, 119, 5461. (j) Server-Carrio, J.; Bas-Serra, J.; Gonzalez-Nunez, M. E.; Garcia-Gastaldi, A.; Jameson, G. B.; Baker, L. C. W.; Acerete, R. J. Am. Chem. Soc. 1999 , 121 , 977. (k) Suzuki, H.; Hashimoto, M.; Okeya, S. Eur. J. Inorg. Chem. 2004 , 2632. (l) Sakai, Y.; Kitakoga, Y.; Hayashi, K.; Yoza, K.; Nomiya, K. Eur. J.
63 peroxo-Zr. Therefore, we decided to investigate in detail the interaction of Zr 4+ ions with [ γ-
8- SiW 10 O36 ] in aqueous solution and in the presence of peroxide.
B- Synthesis and Structure of Asymmetric Zirconium-Substituted Silicotungstates,
14- 10- [Zr 6O2(OH) 4(H 2O) 3(βββ-SiW10 O37 )3] and [Zr 4O2(OH) 2(H 2O) 4(βββ-SiW10 O37 )2]
Experimental section
Synthesis. All reagents were used as purchased without further purification. The dilacunary precursor K 8[γ-SiW 10 O36 ] was synthesized according to the published procedure.
K14 [Zr 6O2(OH) 4(H 2O) 3(βββ-SiW10 O37 )3]·28.5H 2O (K-6). To 20mL of a 1M
CH 3COOH/CH 3COOK buffer at pH 4.8, 0.76 g of KCl were added. After complete dissolution,
0.74 g of ZrCl 4 (3.16 mmol) followed by 0.5 g (0.18 mmol) of K8[γ-SiW 10 O36 ] were added. The
solution was stirred for 30 min. at 50 ºC. Then it was cooled to room temperature and filtered.
Evaporation of the solvent at room temperature resulted in needle-like crystals of
K14 [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3]·28.5H 2O (K-6, 0.10 g, yield 18 %) after ~2 weeks, suitable for X-ray diffraction, FTIR spectroscopy, and elemental analysis. IR for K-6: 995(w),
-1 956(m), 889(s), 781(s), 668(w), 503 (w), 455(sh) cm . Anal. Calcd (Found) for K-6: K 6.0 (5.8),
Zr 6.0 (6.2), W 60.4 (59.3), Si 0.9 (1.1).
K10 [Zr 4O2(OH) 2(H 2O) 4(βββ-SiW10 O37 )2]·22H 2O (K-7). The synthesis procedure for polyanion 7 was identical to that of 6, but 0.37 g of ZrCl 4 (1.58 mmol) were used. Evaporation of
Inorg. Chem. 2004 , 4646. (m) Hussain, F.; Bassil, B. S.; Kortz, U.; Kholdeeva, O. A.; Timofeeva, M. N.; de Oliveira, P.; Keita, B.; Nadjo, L. Chem. Eur. J. 2007 , 13, 4733. (n) Schmidt, R.; Pausewang, G. Z. Anorg. Allg. Chem. 1986 , 535 , 135. (o) Chernyshev, B.N.; Didenko, N.A.; Bukvetskii, B.V.; Gerasimenko, A.V.; Kavun, V.Ya.; Sergienko, S.S. Zh. Neorg. Khim. 1989 , 34 , 1594. (p) Stanciu, C.; Jones, M. E.; Fanwick, P. E.; Abu-Omar, M. M. J. Am. Chem. Soc. 2007 , 129 , 12400.
64 the solvent at room temperature resulted in needle-like crystals of K 10 [Zr 4O2(OH) 2(H 2O) 4(β-
SiW 10 O37 )2]·22H 2O (K-2, 0.11 g, yield 20 %) after ~2 weeks, suitable for X-ray diffraction, FTIR spectroscopy, and elemental analysis. IR for K-7: 990(w), 951(m), 897(s), 891(sh), 785(s),
711(w), 684(w), 625 (w), 527(w) cm -1. Anal. Calcd (Found) for K-7: K 6.3 (6.0), Zr 5.9 (5.6), W
59.2 (59.8), Si 0.9 (1.2).
X-Ray Crystallography . Crystals of K-6 and K-7 were mounted in a Hampton cryoloop using light oil for data collection at low temperature. Indexing and data collection were performed using a Bruker X8 APEX II CCD diffractometer with kappa geometry and Mo-Kα radiation ( λ = 0.71073 Å). Data integration and routine processing was performed using the
SAINT software suite. Further data processing, including absorption corrections from equivalent
reflections, was performed using SADABS. Direct methods (SHELXS97) solutions successfully
located the W atoms, and successive Fourier syntheses (SHELXL97) revealed the remaining
atoms. Refinements were full-matrix least squares against F2 using all data. Cations and waters of hydration were modeled with varying degrees of occupancy, a common situation for polyoxotungstate structures. In the final refinements, all non-disordered heavy atoms (W, K, Si,
Zr) were refined anisotropically while the O atoms and some disordered cations were refined isotropically. No H atoms were included in the models. The crystallographic data are provided in
Table B.3.a..
Results and discussion
Synthesis and structure. The structures of 6 and 7 consist of ( β-SiW 10 O37 ) units
connected by a protonated zirconium(IV)-oxo cluster containing seven- and eight-coordinated
4+ Zr ions. All Zr-centers are linked by oxygen bridges to three different WO 6 octahedra and to a
65 silicate hetero group. It can be noticed that 6 and 7 are synthesized by reacting ZrCl 4 with [ γ-
8- + SiW 10 O36 ] in 1M CH 3COOH/CH 3COOK buffer with enough KCl added to bring the total K concentration to 1M. Excess Zr 4+ seems to be required for the reactions.
Table B.3.a. Crystal Data and Structure Refinement for K14 [Zr 6O2(OH) 4(H 2O) 3(β-
SiW 10 O37 )3]·28.5H 2O (K-6) and K10 [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2]·22H 2O ( K-7) ______
K-6 K-7 emp formula H67 K14 O148.5 Si 3W30 Zr 6 H54 K10 O104 Si 2W20 Zr 4 fw 9138.2 6207.6 space group P-1 Pna2 1 a (Å) 12.7321(9) 39.789(2) b (Å) 20.865(2) 19.7049(8) c (Å) 29.018(3) 12.9402(4) α (°) 98.802(3) 90 β (°) 94.962(4) 90 γ (°) 96.489(3) 90 vol (Å 3) 7527.0(12) 10145.6(7) Z 2 4 temp. ( °C) -100 -100 wavelength (Å) 0.71073 0.71073 -3 dcalcd (Mg m ) 3.933 4.03 abs. coeff. (mm -1) 23.62 23.51 R [I > 2 σ(I)] a 0.125 0.060 b Rw (all data) 0.183 0.082 ______a b 2 2 2 2 2 1/2 R = ∑||Fo|-|Fc||/∑|Fo|. Rw = [ ∑w(Fo -Fc ) /∑w(Fo ) ] .
4+ 8- The reaction of Zr ions with [ γ-SiW 10 O36 ] in a 16:1 ratio resulted in
14- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] (6). This polyanion consists of three ( β-SiW 10 O37 ) units
66
connected by a Zr 6O2(OH) 4(H 2O) 3 cluster, unprecedented in POM chemistry (see Figure B.3.a.).
In 6 there are three seven-coordinated Zr 4+ centers (Zr1, Zr2, Zr3) and three with coordination
number eight (Zr4, Zr5, Zr6), see Figure B.3.a. These two types of Zr 4+ ions are asymmetrically
distributed within the hexa-Zr cluster. The two outer zirconium ions Zr1 and Zr3 are seven-
coordinated, but Zr1 has two terminal water molecules while Zr3 has only one. Thus 6 has point
group symmetry C 1 and is therefore chiral. The three eight-coordinated zirconium ions Zr4, Zr5
and Zr6 are centrally linked via two µ3-hydroxo bridges (O4Z4 and O3Z5). Another µ3-oxo
bridge (O4Z5) links Zr5 and Zr6 to the outer Zr3. Then, Zr2 is linked via a µ3-oxo bridge (O6Z5)
to the two eight-coordinated Zr5 and Zr4, via a µ2-hydroxo bridge (O3Z2) to the outer Zr1, and
via another µ3-hydroxo bridge (O5Z4) to Zr1 and Zr4.
b c a O6Z3
Zr2 Zr5 Zr3 O6Z5 O3Z2 O2Z1 O4Z5 O4Z4 O5Z4 O3Z5 O5Z1 Zr4 Zr1 Zr6
Fig. B.3.a. 14- Left: Combined polyhedral/ball-and-stick representation of [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] (6).
The color code is as follows: WO 6 (red), WO 6 of the β-rotated triads (green), silicon (blue), zirconium (green) and oxygen (red).
Right: Ball -and -stick presentation of the Zr 6O2(OH) 4(H 2O) 3 cluster in 6. Oxygens O4Z4, O3Z5, O3Z2 and O5Z4 are mono -protonated.
67
O3Z Zr3 Zr4 O4Z1 O1Z3
O1Z O1ZA
O3Z2 O4Z2 O2Z Zr2 Zr1
Fig. B.3.b. β 10- Left: Combined polyhedral/ball-and-stick representation of [Zr 4O2(OH) 2(H 2O) 4( -SiW 10 O37 )2] (7). The color code is the same as in Figure B.3.a.
Right: Ball and stick presentation of the Zr 4O2(OH) 2(H2O) 4 cluster in 7. Oxygens O3Z2 and O4Z1 are mono-protonated.
10- The structure of [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] (7) corresponds to two β-SiW 10 O37
units sandwiching a Zr 4O2(OH) 2(H 2O) 4 cluster (see Figure B.3.b.). The two outer Zr ions Zr1 and
Zr3 have a coordination number of eight, while the two inner ones Zr2 and Zr4 have a
coordination number of seven (see Figure B.3.b.). Zr2 and Zr4 are connected by two µ3-oxo
bridges (O1Z3 and O4Z2), and at the same time they are connected to Zr1 and Zr3 by a µ2-
hydroxo bridge (O3Z2 and O4Z1, respectively). This zirconium cluster has a pseudo-inversion
18- center. In fact, it resembles the one present in Hill’s chiral [( α-P2W16 O59 )Zr 2(µ3-O)(C 4O5H3)] 2
68 where the water molecules and the µ2-hydroxo bridges replace the oxygens of the malate
25k ligand. Polyanion 7 has point group symmetry C 1 because the two ( β-SiW 10 O37 ) units are in
fact different isomers ( vide infra ). A few years ago Kortz et al. reported a structurally similar
12- tetra-nickel cluster incorporated in [{ β−SiW 10 Ni 2O36 (OH) 2(H 2O)} 2] , but in this polyanion the nickel(II) ions are six-coordinated. 15a Interestingly, this Ni-containing silicotungstate was also
8- synthesized with [ γ-SiW 10 O36 ] .
It can be noticed that 6 and 7 have the same ratio of zirconium ions to β-SiW 10 O37 units
4+ (2:1). However, 6 is synthesized using a Zr to β-SiW 10 O36 ratio of 16:1 whereas 2 is synthesized using a ratio of 8:1. Close inspection of the structures indicates that 6 cannot simply be considered as an extended derivative of 7 by grafting of an additional Zr 2(β-SiW 10 O37 ) unit,
for the following two reasons:
(i) The positions of the seven- and eight-coordinated Zr 4+ centers are different in 6 versus
7.
(ii) The β-SiW 10 O37 units in 6 and 7 represent different rotational isomers.
In Figures B.3.a. and B.3.b. the rotated W 3O13 triads are shown in green in order to highlight the isomerization of the respective β-SiW 10 O37 units. In 6 there are three β23 -SiW 10 O37 units (one WO 6 octahedron taken from the ‘belt’ and one from the rotated triad), more precisely
three β(4,10) -SiW 10 O37 . On the other hand, polyanion 7 is formed from a β22 - (both WO 6 octahedra taken from the belt), more precisely β(4,9) -SiW 10 O37 , and a β12 - (one WO 6 octahedron from the belt and one from the non-rotated triad) SiW 10 O37 unit, more precisely β(1,4) -SiW 10 O37 .
12- Kortz et al. have already encountered the β(4,10)-unit in [{ β−SiW 10 Ni 2O36 (OH) 2(H 2O)} 2] but
24- 15a,c another β12 -unit, β(1,10), in [{ β-Ti 2SiW 10 O39 }4] . The diversity found for these β-SiW 10 O37
69
8- isomers reiterates the metastable nature of the [γ-SiW 10 O36 ] precursor when interacting with
transition metal ions in solution.
C- 6-Peroxo-6-Zirconium Crown Embedded in a Triangular Polyanion:
18- [Zr 6(O 2)6(OH) 6(γγγ-SiW10 O36 )3]
Experimental section
Synthesis. All reagents were used as purchased without further purification. The dilacunary precursor K 8[γ-SiW 10 O36 ] was synthesized according to the published procedure.
K18 [Zr 6(O 2)6(OH) 6(γγγ-SiW10 O36 )3]⋅⋅⋅59H 2O (K-8): To 20 mL of a 1 M
CH 3COOH/CH 3COOK buffer at pH 4.8 was added 0.76 g of KCl. After complete dissolution,
0.37 g of ZrCl4 (1.60 mmol) followed by 0.50 g (0.18 mmol) of K 8[γ-SiW 10 O36 ] (prepared according to ref. 6) was added. The solution was stirred for 30 min at 50 °C. Then 1.0 mL (ca.
9.0 mmol) of 30% H2O2 (aqueous solution) was added. The synthesis solution turned turbid after addition of H 2O2, but with constant stirring at 50 °C overnight it cleared up eventually. Then the solution was cooled to room temperature and filtered. Evaporation of the solvent at room temperature resulted in needle-like crystals of K-8 suitable for X-ray diffraction after about 1 week (yield: 0.22 g, 37 %). IR of K-8 (in cm -1): 1019(m), 996(sh), 953(s), 913(m), 866(s),
793(s), 720(m), 689(w), 565(w), 537(w), 462(w), 449(m). Anal. Calcd (found) for K-8: K, 7.1
(6.9); Zr, 5.5 (5.2); W, 55.5 (56.4); Si, 0.9 (1.0).
X-Ray Crystallography . A plate-like crystal of K-8 was mounted in a Hampton
cryoloop using light oil for data collection at low temperature. Indexing and data collection were
performed using a Bruker X8 APEX II CCD diffractometer with kappa geometry and Mo-Kα
70 radiation ( λ = 0.71073 Å). Data integration and routine processing was performed using the
SAINT software suite. Further data processing, including absorption corrections from equivalent
reflections, was performed using SADABS. Direct methods (SHELXS97) solutions successfully
located the W atoms, and successive Fourier syntheses (SHELXL97) revealed the remaining
atoms. Refinements were full-matrix least squares against F2 using all data. Cations and waters of hydration were modeled with varying degrees of occupancy, a common situation for polyoxotungstate structures. In the final refinements, all non-disordered heavy atoms (W, K, Si,
Zr) were refined anisotropically while the O atoms and some disordered cations were refined isotropically. No H atoms were included in the models. The crystallographic data are provided in
Table B.3.b..
Results and Discussion
18- Synthesis and structure. The synthesis procedure of [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] 8
10- is identical to that of our dimeric [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] (7), except that some
aqueous H 2O2 (30%) was added to the reaction mixture. Also the procedure of our trimeric
14- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] (6) in the presence of H 2O2 resulted in 8, albeit in lower
yield. After addition of H 2O2 the synthesis solution became turbid, but with constant stirring at
50 °C it cleared up eventually. We crystallized 8 as the hydrated potassium salt
K18 [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3]⋅59H 2O (K-8). The number of crystal waters was determined by
TGA (to ca 253 °C). The TGA graph also showed another weight loss step up to ca 454 °C. We attribute this second step to loss of the peroxo groups, which leads to gradual decomposition of the compound.
71
8- Polyanion 8 is composed of three [γ-SiW 10 O36 ] units encapsulating the unprecedented
6+ [Zr 6(O 2)6(OH) 6] wheel (see Fig. B.3.c.). On the other hand, 8 can also be considered a cyclic assembly of three fused {Zr 2(O 2)2(OH) 2(γ-SiW 10 O36 )} monomers. The central peroxo-Zr crown is composed of 6 equivalent Zr IV centers which are all linked to each other by a side-on peroxo
unit and a hydroxo group. The presence of the latter was established by bond valence sum (BVS)
calculations.
Table B.3.b. Crystal Data and Structure Refinement for K18 [Zr 6(O 2)6(OH) 6(γ-
SiW 10 O36 )3]⋅59H 2O (K-8) ______
emp formula H124 K18 O185 Si 3W30 Zr 6 fw 9936.1
space group C2/c a (Å) 52.1923(14) b (Å) 17.6634(4) c (Å) 36.4505(8) α (°) 90 β (°) 93.573(1)° γ (°) 90 vol (Å 3) 33538.2(14) Z 8 temp. ( °C) -100 wavelength (Å) 0.71073 -3 dcalcd (Mg m ) 4.09 abs. coeff. (mm -1) 21.46 R [I > 2 σ(I)] a 0.075 b Rw (all data) 0.125 ______a b 2 2 2 2 2 1/2 R = ∑||Fo|-|Fc||/∑|Fo|. Rw = [ ∑w(Fo -Fc ) /∑w(Fo ) ] .
72
Fig. B.3.c. 18- Combined polyhedral/ball-and-stick representation of [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] .
The color code is as follows: WO 6 octahedra (red), silicon (blue), zirconium (green), hydroxo (yellow) and peroxo (pink).
Interestingly, the six bridging peroxo groups in 8 are all on one side of the ring (stabilized
in the solid state by a capping, 6-coordinated potassium ion, K ⋅⋅⋅O 2.69 – 2.98(2) Å) whereas the
six bridging hydroxo groups are on the other side, see Fig. B.3.c. In this respect 8 resembles a
73
coin, having two different sides. In 8 it is possible to distinguish the “peroxo face” (perhaps the
site of oxygen transfer) from the “hydroxo face” (perhaps the site of proton transfer). The Zr--O-1
bond lengths range from 2.15(2) Å to 2.22(2) Å, not significantly different from the Zr--O-2 bond
lengths which range from 2.09(2) Å to 2.21(2) Å. The peroxo group bond lengths, O--O, range
from 1.50(3) Å to 1.55(3) Å. The crystal structure of hydrogen peroxide published in 1951 noted
a O--O bond length of 1.49 (1) Å, comparable to the O--O bond lengths in 8.30 In 1989, a
Russian group reported a cyclic tri-Zr-peroxo coordination complex Zr 3F12 (O 2)3 with average O-
-O bond of 1.517(5) which also falls within the range of our observed peroxo O--O bond
lengths.27o
Fig. B.3.d. 6+ Top view (left) and side view (right) on the central [Zr 6(O 2)6(OH) 6] cluster in 8. The colour code of the balls is as follows: Zr (green), peroxo (pink), hydroxo (yellow), oxo (red), potassium (brown), and aqua (orange).
30 Abrahams, S. C.; Collin, R. L.; Lipscomb, W. N. Acta Crystallogr. 1951 ,4, 15.
74
Each zirconium ion is coordinated to the lacunary site of a decatungstosilicate fragment via two Zr-O(W) bonds to an edge-shared pair of WO 6 octahedra, resulting in a coordination number of 8. The peroxo groups in 8 are highly accessible which could be important for catalytic applications.
D- Conclusion
In summary, we have synthesized and structurally characterized the trimeric, hexa-Zr- containing silicotungstate 6 and the dimeric, tetra-Zr-containing silicotungstate 7. Both polyanions are chiral and represent the first representatives of zirconium-containing silicotungstates.
In future work we plan to study the solution properties of 6 and 7 by NMR and electrochemistry. Furthermore, the accessible nature of the outer zirconium centers in 6 and 7 via terminal water ligands may allow for binding of a variety of bidentate ligands. In this project we are particularly interested in amino acids for biological and in peroxide for catalytic applications.
As a follow-up, we have synthesized and structurally characterized the unprecedented
18- peroxo-zirconium containing 30-tungstosilicate [Zr 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] (8). Polyanion 8 represent the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units.
The simple, one-pot synthesis procedures of 8 involving dropwise addition of aqueous hydrogen peroxide could represent a general procedure for incorporating peroxo groups into a large variety of transition metal and lanthanide containing POMs.
75
Chapter 3 Results with lanthanides All references are valid on date of publishing
76
I- Introduction
Lanthanide ions, when incorporated into POM frameworks, show interesting properties in
luminescence, magnetism, and Lewis acid-type catalysis. Lanthanide-containing POMs have
been much less investigated than first row transition metal POMs. Due to their larger size
lanthanide ions have higher coordination numbers compared to 3d metals. Thus, lanthanides are
not fully inserted into the lacunary site(s) of vacant Keggin or Wells-Dawson type POM
precursors. Each lanthanide ion has several (usually 3-4) additional coordination sites available,
which can be used as linkers to one or more other lacunary POM units. This approach should in
theory result in polymeric or unusually large molecular POM assemblies. The latter is beautifully
31 30- demonstrated by Francesconi’s tetrameric [(PEu 2W10 O38 )4(W 3O14 )] , by Gouzerh´s
32 III 19- [Ce 3Sb 4W2O8(H 2O) 10 (SbW 9O33 )4] and in particular by Pope’s cyclic
76- 33 [As 12 Ce 16 (H 2O) 36 W148 O524 ] , the largest polyoxotungstate reported so far. Interestingly, the
last two polyanions could be synthesized without using any lacunary POM precursor.
Nevertheless, there are also several reports on studies of lanthanide ions with monolacunary Keggin and Wells-Dawson type POM precursors. In 1971, Peacock and Weakley first reported the synthesis and elemental analysis of sandwich species with different lanthanides and monolacunary Keggin and Wells-Dawson units, but no structural characterization was provided. 34 Since then, several groups have studied this ‘Weakley-Peacock’ species with the
n- general formula [LnL 2] , where Ln is the lanthanide ion and L is a monolacunary POM unit. In
11− 35 1990 Fedotov et al investigated the Keggin based [Ln(PW 11 O39 )2] by multinuclear NMR.
31 Howell, R.C.; Perez, F.G.; Jain, S.; Horrocks, W.D.W. Jr.; Rheingold, A.L.; Francesconi, L.C. Angew. Chem. Int. Ed. 2001 , 40 , 4031. 32 Xue, G. L.; Vaissermann, J.; Gouzerh, P. J. Clust. Sci. 2002 , 13 , 409 33 Wassermann, K.; Dickman, M.H.; Pope, M.T. Angew. Chem. Int. Ed. 1997 , 36, 1445. 34 (a) Peacock, R. D.; Weakley, T. J. R. J. Chem. Soc. A , 1971, 1836. (b) J. Chem. Soc. A , 1971, 1937. 35 Fedotov, M.; Pertsikov, B.; Kanovich, D. Polyhedron 1990 , 9, 1249.
77
Two Chinese groups reported crystal structures of the lanthanide-containing alpha-
13- 3+ 3+ 3+ 36,37 undecatungstosilicates [Ln( α-SiW 11 O39 )2] (Ln = Nd , Pr , Ce ). The Pope and
Sécheresse groups reported chain-like solid-state structures resulting from the interaction of
8- 38,39 40 lanthanide ions with the monolacunary silicotungstate [ α-SiW 11 O39 ] . Pope and
Francesconi 41 worked extensively on the interaction of lanthanides with the monolacunary
10- Wells-Dawson ions [x-A2W17 O61 ] (A = P, As; x = α1, α2).
Our group has also been interested in the interaction of lanthanide ions with lacunary
POMs. In 2003, we reported the ytterbium substituted tungstoarsenate(III)
7- [YbAs 2W20 O68 (H 2O) 3] and also the La-containing Wells-Dawson dimer
16- 3+ 42 [{La(CH 3COO)(H 2O) 2(α2-P2W17 O61 )} 2] with two acetates bridging the La ions. In the following year, Mialane demonstrated the same acetate bridging mode for the Keggin derivatives
12- III III 43 [( α-SiW 11 O39 Ln) 2(µ-CH 3COO) 2] (Ln = Gd , Yb ).
36 (a) Shan, Y.-K.; Liu, Z.-X. Sci. China 1991 , 34 , 313. (b) Shan, Y.-K.; Liu, Z.-X. Acta Chim. Sin. 1992 , 364. 37 Sun, R. Q.; Zhang, H. H.; Zhao, S. L.; Huang, C.-C.; Zheng, X. L. Chin. J. Struct. Chem. 2001 , 20 , 413. 38 Sadakane, M.; Dickman, M. H.; Pope, M. T Angew. Chem. Int. Ed. 2000 , 39 , 2914. 39 Mialane, P.; Lisnard, L.; Mallard, A.; Marrot, J.; Antic-Fidancev, E.; Aschehoug, P.; Vivien, D.; Sécheresse, F. Inorg. Chem. 2003 , 42 , 2102. 40 (a) Ostuni, A; Bachman, R. E.; Pope, M. T. J. Clust. Sci. 2003 , 14 , 431 (b) Sadakane, M.; Ostuni, A; Pope, M. T. Dalton Trans. 2002, 1, 63. (c) Ostuni, A ; Pope, M. T. Comptes C. R. Chim. 2000 3, 199. 41 (a) Boglio, C; Lenoble, G; Duhayon, C; Hasenknopf, B; Thouvenot, R; Zhang, C; Howell, R. C.; Burton-Pye, B. P.; Francesconi, L. C.; Lacote, E.; Thorimbert, S.; Malacria, M.; Afonso, C.; Tabet, J. C. Inorg. Chem. 2006 , 45, 1389. (b) Zhang, C; Howell, R. C.; McGregor D.; Bensaid, L.; Rahyab, S.; Nayshtut, M.; Lekperic, S.; Francesconi, L. C.; C. R. Chim. 2005, 8, 1035 (c) Zhang, C.; Howell, R. C.; Luo, Q. H.; Fieselmann, H. L.; Todaro, L. J.; Francesconi, L. C.; Inorg. Chem. 2005 , 44, 3569. (d) Zhang, C.; Howell, R. C.; Scotland, K. B.; Perez, F. G., Todaro, L.; Francesconi, L. C.; Inorg. Chem. 2004, 43, 7691. (e) Luo, Q. H.; Howell, R. C.; Bartis, J.; Dankova, M.; Horrocks, W. D.; Rheingold, A. L.; Francesconi, L. C.; Inorg. Chem. 2002, 41, 6112. (f) Luo, Q. H.; Howell, R. C.; Dankova, M.; Bartis, J.; Williams, C. W.; Horrocks, W. D.; Young, V. G.; Rheingold, A. L.; Francesconi, L. C.; Antonio, M. R.; Inorg. Chem. 2001, 40, 1894. (f) Bartis, J.; Dankova, M.; Lessmann, J. J.; Luo, Q. H.; Horrocks, W. D.; Francesconi, L. C.; Inorg. Chem. 1999, 38, 1042. (h) Antonio, M. R.; Soderholm, L.; Jennings, G.; Francesconi, L. C.; Dankova, M.; Bartis, J. J. Alloy. Compd. 1998 , 277 , 827. (i) Bartis, J.; Sukal, S.; Dankova, M.; Kraft, E.; Kronzon, R.; Blumenstein, M.; Francesconi, L. C. J. Chem. Soc., Dalton Trans., 1997 , 1937. (j) Bartis, J.; Dankova, M.; Blumenstein, M.; Francesconi, L. C.; J. Alloy. Compd. 1997 , 249 , 56. 42 a) Kortz, U.; Holzapfel, C.; Reicke, M. J. Mol. Struct. 2003 , 656, 93 b) Kortz, U. J. Clust. Sci. 2003 , 14 , 205. 43 Mialane, P., Dolbecq, A.; Rivière, E.; Marrot, J.; Sécheresse, F. Eur. J. Inorg. Chem. 2004 , 33.
78
8- II- Interaction of lanthanides with [β2-SiW 11 O39 ]
A- Introduction
To date no lanthanide containing polyanions based on the monolacunary, chiral [ β2-
8- SiW 11 O39 ] have been reported. As discussed above, the non-lacunary (plenary) α-Keggin is the most stable and ideally has T d symmetry, consisting of four triads of edge-shared octahedral
which are further connected by their corners. Rotating one of these triads by 60° results in the
formation of the β-Keggin isomer which reduces the symmetry to C 3v . The monolacunary β2 results from the loss of one tungsten octahedron adjacent to the rotated triad. This loss lowers the symmetry of the monolacunary species to C 1.
13- B- The Mono-Lanthanide Containing Silicotungstates [Ln( βββ2-SiW11 O39 )2] (Ln =
La, Ce, Sm, Eu, Gd, Tb, Yb, Lu), a Synthetic and Structural Investigation
Experimental Section
Synthesis. The monolacunary Keggin precursor K 8[β2-SiW 11 O39 ]·14H 2O was
synthesized according to the published procedure and identified in the solid state by FTIR. All
other reagents were used as purchased without further purification.
K13 [La( βββ222-SiW11 O39 )2]·22H 2O (K-9). To 20 mL of a 1 M KCl solution, 0.032 g (0.085
mmol) of LaCl 3·7H 2O and 0.500 g (0.155 mmol) of K 8[β2-SiW 11 O39 ] were added. The pH of the
solution was adjusted to 5.0 by addition of 0.1 M HCl solution dropwise. Then the solution was
stirred at 50 °C for 30 min and allowed to cool to room temperature. Filtration and slow
79 evaporation in an open container at room temperature resulted in colorless crystals after about 1 week. Yield 0.45 g, 91 %. IR data for K-9 in cm -1: 997(m), 970(m), 953(m), 940(m), 904(s),
893(s), 862(s), 832(m), 789(s), 762(sh), 722(s), 530(w). Anal. Calcd for K-9: K, 8.0; W, 63.3;
La, 2.2; Si, 0.9. Found: K, 8.2; W, 62.8; La, 2.0; Si, 1.1. Tungsten-183 NMR of K-9 in 2M NaCl in H 2O/D 2O at 298 K: -89.4, -103.3, -105.4, -114.5, -136.3, -147.1, -164.5, -173.6, -178.5, -
189.4, -196.7 ppm (all singlets).
K13 [Ce( βββ222-SiW11 O39 )2]·25.5H 2O (K-10). The same procedure as for K-9 was followed
-1 except for using 0.032 g of CeCl 3·7H 2O. Yield 0.44 g, 88 %. IR data for K-10 in cm : 998(m),
969(m), 952(m), 939(sh), 904(s), 884(s), 864(s), 832(m), 788(s), 723(s), 530(w). Anal. Calcd for
K-2: K, 7.9; W, 62.6; Ce, 2.2; Si, 0.9. Found: K, 7.9; W, 62.0; Ce, 2.4; Si, 1.0.
K13 [Sm( βββ222-SiW11 O39 )2]·29.5H 2O (K-11). The same procedure was followed using 0.031
-1 g of SmCl 3·6H 2O. Yield 0.46 g, 91 %. IR data for K-11 in cm : 1001(m), 969(m), 953(m),
939(sh), 908(s), 886(s), 867(s), 835(m), 793(s), 762(sh), 725(s), 532(w). Anal. Calcd for K-11 :
K, 7.8; W, 61.9; Sm, 2.3; Si, 0.9. Found: K, 7.4; W, 60.0; Sm, 1.8; Si, 1.1.
K13 [Eu( βββ222-SiW11 O39 )2]·25H 2O (K-12). The same procedure was followed using 0.031 g
-1 of EuCl 3·6H 2O. Yield 0.45 g, 90 %. IR data for K-12 in cm : 1002(m), 976(m), 953(m), 940(m),
908(s), 885(s), 868(s), 834(s), 793(s), 761(sh), 721(s), 530(w), 469(sh). Anal. Calcd for K-12 : K,
7.9; W, 62.6; Eu, 2.4; Si, 0.9. Found: K, 7.8; W, 62.0; Eu, 2.0; Si, 1.0.
K13 [Gd( βββ222-SiW11 O39 )2]·27H 2O (K-13). The same procedure was followed using 0.032 g
-1 of GdCl 3·6H 2O. Yield 0.46 g, 91 %. IR data for K-13 in cm : 1002(m), 976(sh), 953(m),
941(sh), 908(s), 885(s), 869(s), 836(m), 789(s), 764(sh), 723(s), 530(w), 467(sh). Anal. Calcd for
K-13 : K, 7.8; W, 62.2; Gd, 2.4; Si, 0.9. Found: K, 7.5; W, 62.2; Gd, 2.2; Si, 1.1.
80
K13 [Tb( βββ222-SiW11 O39 )2]·29.5H 2O (K-14). The same procedure was followed using 0.030
-1 g of Tb(CH 3COO) 3·H 2O. Yield 0.44 g, 87 %. IR data for K-14 in cm : 1006(m), 969(sh),
954(m), 943(sh), 910(s), 886(s), 869(s), 836(m), 793(s), 764(sh), 724(s), 530(w), 472(sh). Anal.
Calcd for K-14 : K, 7.8; W, 61.8; Tb, 2.4; Si, 0.9. Found: K, 8.0; W, 62.1; Tb, 2.3; Si, 1.1.
K13 [Yb( βββ222-SiW11 O39 )2]·26H 2O (K-15). The same procedure was followed using 0.038 g
-1 of Yb(NO 3)3·5H 2O. Yield 0.44 g, 87 %. IR data for K-15 in cm : 1005(m), 993(sh), 957(m),
906(sh), 887(s), 874(sh), 838(m), 791(s), 724(s), 532(w), 477(sh). Anal. Calcd for K-15 : K, 7.8;
W, 62.2; Yb, 2.7; Si, 0.9. Found: K, 7.6; W, 61.1; Yb, 2.4; Si, 1.2.
K13 [Lu( βββ222-SiW11 O39 )2]·27H 2O (K-16). The same procedure was followed using 0.031 g
-1 of LuCl 3·6H 2O. Yield 0.45 g, 89 %. IR data for K-16 in cm : 1006(m), 992(sh), 956(m),
906(sh), 887(s), 875(sh), 839(m), 791(s), 768(sh), 726(s), 532(w), 475(sh). Anal. Calcd for K-
16 : K, 7.7; W, 61.5; Lu, 2.7; Si, 0.9. Found: K, 7.9; W, 59.0; Lu, 2.7; Si, 1.0.
X-Ray Crystallography . Crystals were mounted in a Hampton cryoloop using light oil for data collection at low temperature. Indexing and data collection were performed using a
Bruker X8 APEX II CCD diffractometer with kappa geometry and Mo-Kα radiation ( λ =
0.71073 Å). Data integration and routine processing was performed using the SAINT software
suite. Further data processing, including absorption corrections from equivalent reflections, was
performed using SADABS. Direct methods (SHELXS97) solutions successfully located the W
atoms, and successive Fourier syntheses (SHELXL97) revealed the remaining atoms.
Refinements were full-matrix least squares against F2 using all data. Cations and waters of hydration were modeled with varying degrees of occupancy, a common situation for polyoxotungstate structures. In the final refinements, all non-disordered heavy atoms (W, K, Si,
Ln) were refined anisotropically while the O atoms and some disordered cations were refined
81 isotropically. No H atoms were included in the models. The crystallographic data are provided in
Table C.1.a..
82
Table C.1.a. Crystal data and structure refinement for K-9 - K-16 .
Empirical K13 [La(SiW 11O39 )2]·22H 2O K13 [Ce(SiW 11 O39 )2]·25.5H 2O K13 [Sm(SiW 11 O39 )2]·29.5H 2O K13 [Eu(SiW 11 O39 )2]·25H 2O Formula (K-9) (K-10 ) (K-11 ) (K-12 ) MW 6392.5 6456.8 6539.1 6459.7 Crystal Monoclinic Monoclinic Monoclinic Monoclinic system Space P2 1/c P2 1/c P2 1/c P2 1/c group A/Å 18.7300(13) 18.6232(13) 18.7442(5) 18.7596(5) B/Å 20.0384(11) 19.9618(12) 20.0470(5) 20.0516(5) C/Å 27.5289(19) 27.4783(13) 27.3945(5) 27.3865(5) α/° 90 90 90 90 β/° 96.966(4) 97.120(3) 97.439(1) 97.536(1) γ/° 90 90 90 90 V/Å 3 10255.9(12) 10136.4(10) 10207.3(4) 10212.7(4) Z 4 4 4 4 T/ºC -100 -100 -100 -100 λ/Å 0.71073 0.71073 0.71073 0.71073 -3 Dc/Mg m 3.956 3.962 3.923 3.980 µ/mm -1 25.458 25.761 25.700 25.722 R[I<2 σ(I)] a 0.063 0.068 0.085 0.068 Rw 0.197 0.222 0.270 0.202 (all data) b
83
Empirical K13 [Gd(SiW 11 O39 )2]·27H 2O K13 [Tb(SiW 11 O39 )2]·29.5H 2O K13 [Yb(SiW 11 O39 )2]·26H 2O K13 [Lu(SiW 11 O39 )2]·27H 2O Formula (K-13 ) (K-14 ) (K-15 ) (K-16 ) MW 6501.0 6547.7 6498.7 6518.7 Crystal Monoclinic Monoclinic Orthorhombic Orthorhombic system Space P2 1/c P2 1/c Pbca Pbca group (no.) a/Å 18.7494(10) 18.7191(6) 21.3738(5) 25.3783(9) b/Å 20.0610(9) 20.0259(5) 38.3275(10) 21.3325(6) c/Å 27.3796(14) 27.3498(6) 25.3172(7) 38.3131(14) α/° 90 90 90 90 β/° 97.586(3) 97.612(1) 90 90 γ/° 90 90 90 90 V/Å 3 10208.2(9) 10162.2(6) 20740.0(9) 20742.0(12) Z 4 4 8 8 T/ºC -100 -100 -110 -100 λ/Å 0.71073 0.71073 0.71073 0.71073 -3 Dc/Mg m 3.957 3.939 4.065 4.029 µ/mm -1 25.785 25.902 25.738 25.725 R[I<2 σ(I)] a 0.088 0.073 0.064 0.060 Rw (all 0.285 0.221 0.175 0.173 data) b
84
Results and discussion
Synthesis and Structure. We report here eight mono-lanthanide containing, dimeric
13- polyanions [Ln( β2-SiW 11 O39 )2] (Ln = La ( 9), Ce ( 10 ), Sm ( 11 ), Eu ( 12 ), Gd ( 13 ), Tb ( 14 ), Yb
(15 ), Lu ( 16 )). All derivatives 9-16 were prepared using the same synthetic procedure by reaction
of the respective lanthanide ion with the monolacunary, chiral, Keggin-type precursor [ β2-
8- SiW 11 O39 ] in a 1:2 molar ratio in 1 M KCl at pH 5. Polyanions 9-16 crystallize as hydrated
potassium salts K-9 – K-16 . K-9 – K-14 crystallize in the monoclinic space group P2 1/c, whereas K-15 and K-16 are isostructural and they crystallize in the orthorhombic space group
Pbca.
3+ Polyanions 9-16 are all composed of two chiral ( β2-SiW 11 O39 ) units sandwiching the Ln ion and at first sight they all seem to have the same structure. However, close examination of the bonding situation around the central lanthanide ion in 9-16 reveals that bond lengths and angles
and especially the configurations of the two Keggin units deserve special attention.
3+ Polyanion 9 is composed of two ( β2-SiW 11 O39 ) units sandwiching a La ion, which exhibits four La-O(W) bonds to each lacunary Keggin unit (see Fig. C.1.a.). Rotation of the undecatungstosilicate fragments by about 40° with respect to each other results in an idealized square-antiprismatic coordination geometry for La 3+ . It must be emphasized that 9 is composed
3+ of two chiral ( β2-SiW 11 O39 ) fragments so that the La ion is situated in a vacancy of the Keggin unit which is adjacent to the rotated triad. As a result, the lanthanum center is coordinated to each monolacunary Keggin fragment via two oxygens of the incomplete triad, one oxygen of the rotated triad and one oxygen of the adjacent, non-rotated triad (see Fig. C.1.a.). An alternative description is that the lanthanide center is coordinated to two oxygens from the belt, to one from the edge-shared (rotated) cap and to one from the corner-shared cap. The situation is similar for
85
the Ce-derivative 10 which differs only by a slightly (~1°) larger rotation angle between the two
Keggin units compared to 9.
Fig. C.1.a. 13- Polyhedral (upper) and ball-and-stick (lower) representation of [La( β2-SiW O ) ] (9). 11 39 2 The color code is as follows: tungsten (black), WO 6 octahedra (red, rotated triads are colored in green for clarity); oxygen (red); silicon β 13- (blue); La (grey). This structure is also representative for the other lanthanide derivatives [Ln( 2-SiW 11 O39 )2] (Ln = Ce ( 10 ), Sm ( 11 ), Eu ( 12 ), Gd ( 13 ), Tb ( 14 ), Yb ( 15 ), Lu ( 16 )) but they exhibit slightly different degrees of rotation of the two Keggin fragments with respect to each other (see text for details). In the polyhedral presentation, the rotated triad was colored green for clarification.
86
Polyanions 11 -14 have essentially the same molecular structure as 9 and 10 , but in the solid state they exhibit two different configurations. This can be described as configurational
disorder, where one of the two ( β2-SiW 11 O39 ) subunits has the position of the edge-shared and
corner-shared triads exchanged through a mirror plane containing the silicon hetero atom and the
five tungsten atoms of the ‘belt’ (see Fig. C.1.b.). We distinguish two configurations, The R,R or
S,S configuration (A) and the R,S or S,R configuration (B), The R and S define the relative
chirality of each lacunary Keggin ion with respect to the other. The point group of configuration
A is C 2 whereas the one of configuration B is C 1. For 11 we observe an A/B crystallographic disorder of 60% : 40% and for 12 -14 we observe essentially statistical (50% : 50%) A/B disorder. Here the positions of the oxygens bridging to the Ln 3+ ion are not disordered and as a result the local coordination environment of the lanthanide center is not changed.
Polyanions 9 and 10 both exhibit configuration A as 100% in the crystal studied, whereas
polyanions 15 and 16 both exhibit configuration B as 100% in the crystal studied. Our results
13- lead us to the conclusion that in [Ln( β2-SiW 11 O39 )2] the size of the lanthanide ion plays an important role regarding which configuration is favored. For early (large) lanthanide ions like
La 3+ and Ce 3+ configuration A is strongly favored, whereas for the lanthanide ions in the middle of the 4f period there is a gradual change away from configuration A to configuration B. Finally, the late (small) lanthanide ions Yb 3+ and Lu 3+ strongly favor configuration B.
We can also quantify the above observation by using the twist angle ‘ θ’ which measures
the relative orientation of the two rotated triads (one on each Keggin unit). Also, θ refers to the offset of the two squares built up from the respective four oxygens (O u, O t, O1 e, O2 e and O u’,
Ot’, O1 e’, O2 e’) at the lacunary site of each of the two Keggin units (see Fig. C.1.b.).
87
Fig. C.1.b. This scheme highlights the inner coordination sphere of the lanthanide centers in 9-16 . It also shows how the twist angle θ is determined. The subscript ‘e’ indicates that this oxygen belongs to an edge-sharing octahedron, the subscript ‘u’ indicates that this oxygen belongs to an unshared octahedron and the subscript ‘t’ indicates that this oxygen belongs to an octahedron of the rotated triad. The 2-fold rotation axis of 9 and 10 is also shown.
88
More precisely, θ describes the rotation angle between the two planes formed by the
following three atoms of each Keggin unit: the lanthanide center, its coordinated oxygen atom of
the rotated triad (edge-shared cap) and the coordinated oxygen of the corner-shared cap.
Francesconi et al. used the same method but different atoms to calculate the offset of the two
13- 38i Keggin halves for their studies on [Ln( α-SiW 11 O39 )2] . Figure C.1.c. shows the respective
twist angle(s) θ for 9-16 in the solid state.
Fig. C.1.c. The two possible configuration isomers of 9-16 are shown and the respective percentages for each with the corresponding twist angle θ. For clarity, the right Keggin subunit was left unchanged in both representations. The color-coding is the same as in Figure C.1.a.
89
Polyanion 15 shows an additional, subtle type of disorder in the solid state, which is best described as beta-alpha isomerization. In the crystal of K-15 studied, a small fraction of the dimeric polyanions 15 actually contained an ( α-SiW 11 O39 ) unit, i.e. a monolacunary Keggin fragment with no rotated triad. Only one of the two (SiW 11 O39 ) units in 15 exhibits this
phenomenon and only to a very small degree. We were able to model this disorder satisfactorily
by assigning partial occupancies of 0.9/0.1 to the six tungsten centers in question and therefore
the α−isomerization was found to account for only about 10% of the overall structure. This
observation suggests partial transformation of the “all-beta” isomer 7 to the (most likely more
13- stable) “all-alpha” isomer [Yb( α-SiW 11 O39 )2] in solution. It is very likely that this
13- transformation proceeds via the “alpha-beta-2” intermediate [Yb( α-SiW 11 O39 )( β2-SiW 11 O39 )] .
This reflects the well-known solution transformation pattern of the monolacunary silicotungstate
8- 8- 8- Keggin precursors: [ β2-SiW 11 O39 ] [β3-SiW 11 O39 ] [α-SiW 11 O39 ] as shown by Tézé and
Hervé. 44 These results are also fully consistent with our solution NMR studies on K-9 and K-16
(vide infra ).
In order to examine the expected decrease of Ln-O bond lengths with increasing atomic
number of Ln we also determined “average Ln-O bond lengths” for 9-16 . The results (see Table
C.1.b.) show the expected trend, which reflects the decreasing size from early to late Ln 3+ ions.
44 Contant, R.; Tézé, A. Inorg. Chem. 1985 , 24 , 4610.
90
13- Table C.1.b. Average Ln-O bond lengths in [Ln( β2-SiW 11 O39 )2] (Ln = La ( 9), Ce ( 10 ), Sm (11 ), Eu ( 12 ), Gd ( 13 ), Tb ( 14 ), Yb ( 15 ), Lu ( 16 )).
Ln Average Ln-O(W) bond lengths in Å La ( 9) 2.49 (3) Ce ( 10 ) 2.46 (2)
Sm ( 11 ) 2.41 (3) Eu ( 12 ) 2.40 (3) Gd ( 13 ) 2.40 (3) Tb ( 14 ) 2.37 (3) Yb ( 15 ) 2.33 (3) Lu ( 16 ) 2.321 (15)
We also performed 183 W NMR studies on the diamagnetic polyanions 9 and 16 in order
to learn more about the solution behavior of these species. Francesconi and coworkers have
studied the “all-alpha” derivatives of 9 and 16 .38i For our measurements we redissolved the maximum amount of solid K-9 and K-16 in H 2O/D 2O. For K-9 we discovered that after an
overnight, room temperature 183 W measurement the spectrum was identical to that reported by
13- Francesconi et al. for the “all-alpha” isomer [La( α-SiW 11 O39 )2] . In a second experiment using
identical conditions we monitored the evolution of the 183 W spectrum with time. We discovered
that already the first peaks which were visible after a few hours correspond to Francesconi´s
13- [La( α-SiW 11O39 )2] . Then we searched for a different solvent medium which could perhaps
stabilize 9 better than pure water and we identified a 2M NaCl, pH 5.5 solution as an appropriate
choice. At room temperature, overnight 183 W measurement on K-9 redissolved in 2M NaCl (pH
5.5) resulted in a spectrum with 11 peaks of about equal intensity (-89.4, -103.3, -105.4, -114.5, -
136.3, -147.1, -164.5, -173.6, -178.5, -189.4 and -196.7 ppm, see Fig. C.1.d.). This spectrum
91 corresponds to the C2 symmetry of 8 in the solid state (see Fig. C.1.a.). Careful inspection of the
spectrum reveals five additional peaks of lower intensity (-91.9, -136.8, -143.0, -166.3 and -
13- 200.0 ppm, see Fig. C.1.d., which we assign to the “all-alpha” isomer [La( α-SiW 11 O39 )2] . One
more peak is expected for this isomer, but it is most likely obscured by one of the other 11 peaks
of 9. Partial transformation of 9 to the “all-alpha” isomer during the overnight measurement is
not unexpected. In fact, increased heating resulted in a significantly faster transformation of 9 to
13- [La( α-SiW 11 O39 )2] accompanied by an increase of the five peaks and a decrease of the eleven
13- peaks due to 9. We conclude that the “all-alpha” isomer [La( α-SiW 11 O39 )2] is thermodynamically more stable than the beta-2 isomer 9. This is also in agreement with the work
8- 8- 41 of Tézé and Hervé who concluded that [ β2-SiW 11 O39 ] is less stable than [ α-SiW 11 O39 ] . As
8- indicated above, these authors showed that in aqueous solution [ β2-SiW 11 O39 ] transforms to [ α-
8- 8- SiW 11 O39 ] via the [ β3-SiW 11 O39 ] intermediate. We speculate that the transformation of the
13- [Ln( β2-SiW 11 O39 )2] species to the all-alpha isomer proceeds through the Tézé and Hervé mechanism by an initial dissociation of one lacunary Keggin followed by rearrangement to its
5- alpha isomer and re-coordination to the [Ln( β2-SiW 11 O39 )] fragment. This is consistent with our observation that high concentrations of alkali metal ions slow down the isomerization pathway
8- 8- 8- [β2-SiW 11 O39 ] [β3-SiW 11 O39 ] [α-SiW 11 O39 ] likely by occupying the lacunary site of
8- the dissociated [β2-SiW 11 O39 ] fragment.
The 183 W NMR studies on the Lu-derivative K-16 in 2M NaCl (pH 5.5) revealed around
22 peaks between –81.8 and –230.3 ppm. This is consistent with the observed C 1 symmetry in
the solid state.
92
* * * * *
-90 -100 -110 -120 -130 -140 -150 -160 -170 -180 -190 -200 ppm
Fig. C.1.d.
Tungsten-183 NMR spectrum of K 13 [La( β2-SiW 11 O39 )2]·22H 2O ( K-9) in H 2O/D 2O at 293 K. The peaks with an asterisk represent most 13- likely the “all-alpha” isomer [La( α-SiW 11 O39 )2] due to partial degradation of 9 (see text for details).
Furthermore, we performed thermogravimetric analyses (TGA) on all eight compounds
K-9 – K-16 under a N 2 atmosphere. These measurements allowed us to evaluate the thermal
stability of polyanions 9-16 in the solid state. By using this technique we could also determine
the number of water molecules of hydration. As expected, the thermograms of K-9 – K-16 look
all fairly similar. The compounds K-9 – K-16 start to lose crystal water almost instantaneously
upon heating from room temperature up to around 200 °C, after which there is no weight loss for
K-9 – K-15 up to the final temperature at 800 °C. Only the lutetium derivative K-16 exhibits a
93 second weight loss step at around 530 °C. Our TGA studies indicate that K-9 – K-15 dehydrate
completely and are thermally stable up to 800 °C. The degrees of hydration for K-9 – K-16 range from 22 to 29.5 water molecules.
C- Conclusion
The solution and solid state chemistry and reactivity of silicotungstates is well developed
8- in general, but the monolacunary [ β2-SiW 11 O39 ] has essentially been neglected as a polyanion
precursor. This is interesting, as this species represents a rare example of a lacunary POM which
8- is chiral. One reason for the lack of using [ β2-SiW 11 O39 ] as a precursor reagent is probably due to the fact that it is rather unstable, transforming easily to the beta-3- and alpha-derivatives in aqueous solution. In other words, it is difficult to preserve the beta-2 structure of this Keggin unit in the final polyanion products. Our work has shown that reaction with lanthanide ions allows the
(β2-SiW 11 O39 ) fragment to be stabilized in the form of the dimeric assembly [Ln( β2-
13- SiW 11 O39 )2] (Ln = La ( 9), Ce ( 10 ), Sm ( 11 ), Eu ( 12 ), Gd ( 13 ), Tb ( 14 ), Yb ( 15 ), Lu ( 16 )). To
8- our knowledge these are the first examples of POM derivatives based on [ β2-SiW 11 O39 ] as precursor and constituent building block. We have structurally characterized a total of eight polyanions including early, middle and late lanthanides. This number of derivatives allowed us to carefully evaluate the bonding situation (lengths, angles) around the lanthanide center, but most importantly the relative orientation of the chiral Keggin units with respect to each other as a function of the size of the lanthanide center. We discovered a pattern showing that one configuration is favored for early lanthanides and the other for late lanthanides with a gradual transition from one to the other throughout the lanthanide series. This phenomenon resulted in what we describe as configurational disorder in the solid state X-ray structures. Lastly, we also performed solution 183 W NMR studies on the two diamagnetic La- and Lu-derivatives 9 and 16 ,
94 which are consistent with the observed symmetries of the solid state structures. Moreover, the solvent medium plays an important role in the stabilization of the title compounds. A high alkali metal concentration seems to prolong the lifetime of the β-isomer as observed by NMR. In
conclusion, our work shows that unstable lacunary polyanion fragments can be stabilized by
lanthanide centers in solution and in the solid state.
95
10- III- Interaction of Cerium(III) with [α-GeW 9O34 ]
A- Introduction
Our group decided to focus on the interaction of the trilacunary polyanion precursor [ α-
10- GeW 9O34 ] , first reported by Hervé and Tézé in 1977 as stated in the general introduction, with
lanthanides. So far, no lanthanide containing derivative of this POM precursor has been reported.
10- The trilacunary nature of [ α-GeW 9O34 ] may allow for coordination with several lanthanide centers, perhaps resulting in the formation of very large POMs.
56- B- The Tungstogermanate [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] : A Polyoxometalate
Containing 20 Cerium(III) Atoms
Experimental Section
Synthesis. The trilacunary Keggin precursor Na 10 [GeW 9O34 ].18 H 2O was synthesized
according to the published procedure and identified in the solid state by FTIR. All other reagents
were used as purchased without further purification.
Cs 28 Na 28 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·~180H 2O (CsNa-17). A 0.064 g (0.17
mmols) sample of CeCl 3·7H 2O was dissolved in 20 mL of water at pH 5.0, followed by addition of 0.5 g (0.17 mmols) Na 10 [GeW 9O34 ]. This solution was heated to 50 °C for 30 minutes, allowed to cool to room temperature, and then was filtered. 0.5 mL of 1M CsCl solution were added to the solution which was allowed to evaporate in an open vial at room temperature. After about one week a yellow, crystalline product started to appear. Evaporation was allowed to
96 continue until the solution level had approached the solid product, which was filtered off and air- dried. Yield: 0.1 g (16%). IR: 1619(s), 941(m), 892(sh), 787(s), 752(sh), 702(sh), 671(w),
594(w), 521(w), 501(w) cm -1. Anal. Calcd for CsNa-17: Cs, 10.3; Na, 1.8; Ce, 7.8; W, 50.9; Ge,
2.0. Found: Cs, 9.1; Na, 2.2; Ce, 7.4; W, 52.0; Ge, 1.7. Polyanion 17 could also be synthesized in higher yield at analogous reaction conditions using aqueous 1M NaCl instead of water. The crystalline product Na 56 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·180H 2O (Na-17) was obtained after
two weeks, but the single crystals were not suitable for XRD. The identity of Na-17 was
established by comparison with the IR of CsNa-17 and the degree of hydration was determined
by TGA. Yield: 0.2 g (36%). IR: 1628(s), 937(m), 897(sh), 800(s), 765(sh), 714(sh), 625(w),
602(w), 518(w) cm -1.
X-Ray Crystallography . A yellow rod of CsNa-17 with dimensions 0.17 x 0.03 x 0.03 mm 3 was mounted in a Hampton cryoloop for indexing and intensity data collection at 173 K on
a Bruker D8 APEX II CCD single-crystal diffractometer using Mo K α radiation ( λ = 0.71073
Å). Of the 331397 reflections collected (2 θmax = 41.6 °, 99.4% complete), 27826 were unique
(R int = 0.278) and 16425 reflections were considered observed ( I > 2 σ(I)). Routine Lorentz and
polarization corrections were applied and an absorption correction was performed using the
SADABS program (Sheldrick, G. M.; Siemens Analytical X-ray Instrument Division: Madison,
WI, 1995). Direct methods were used to locate the tungsten and cerium atoms ( SHELXS-97 ).
Then the remaining atoms were found from successive Fourier maps ( SHELXL-97 ). The final cycle of refinement, including the atomic coordinates, anisotropic thermal parameters (W, Ce,
Ge, and Cs atoms) and isotropic thermal parameters (O and Na atoms) converged at R = 0.069 (I
-3 > 2 σ(I)) and R w = 0.212 (all data). In the final difference map the deepest hole was -2.85 eÅ
97
(0.74 Å from O34W) and the highest peak 3.98 eÅ -3 (0.88 Å from Cs5). Crystal data of CsNa-17 can be found in Table C.2.a.
Table C.2.a. Crystal Data and Structure Refinement for
Cs 28 Na 28 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·~180H 2O (CsNa-17). ______
emp formula Na 28Cs 28 Ce 20 Ge 10 W100 O590 H428 fw 36057.96
space group P21/n a (Å) 31.9413(18) b (Å) 24.3343(18) c (Å) 36.207(2) α (°) 90 β (°) 108.204(3) γ (°) 90 vol (Å 3) 7643.9(16) Z 2 temp. ( °C) -100 wavelength (Å) 0.71073 -3 dcalcd (Mg m ) 4.479 abs. coeff. (mm -1) 25.656 R [I > 2 σ(I)] a 0.069 b Rw (all data) 0.136 ______a b 2 2 2 2 2 1/2 R = ∑||Fo|-|Fc||/∑|Fo|. Rw = [ ∑w(Fo -Fc ) /∑w(Fo ) ] .
98
Results and Discussion
56- Synthesis and Structure. The Polyanion [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] (17) was
10- III synthesized by reaction of the trilacunary POM precursor [ α-GeW 9O34 ] with Ce ions in a 1:1
ratio in water at pH 5.0, see Fig. C.2.a.. The novel polyanion 17 was isolated as the mixed
cesium-sodium salt Cs 28 Na 28 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·~180H 2O (CsNa-17) which
crystallizes in the monoclinic space group P2 1/n. Polyanion 17 has a crystallographic inversion
center resulting in point group symmetry C i. The title polyanion could also be obtained as the sodium salt Na 56 [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ]·~180H 2O (Na-17) by using an aqueous 1M
NaCl solution instead of water as the solvent, but otherwise analogous reaction conditions.
Although these conditions resulted in a better yield, the crystals were not suitable for X-ray diffraction. However, the similarity of the IR spectra of the sodium and mixed cesium-sodium salts suggests that the same polyanion is present in both compounds.
Polyanion 17 can be described as a dimeric entity composed of two half-units of
28- [Ce 10 Ge 5W50 O188 (OH) 2(H 2O) 15 ] (17a) related by an inversion center, see Figure C.2.b.. The
two units 17a are linked to each other via long Ce-O(W) bridges (2.61(4) Å). This suggests that in solution, 17 might dissociate relatively easily providing two equivalents of 17a. Our preliminary 183 W NMR measurements on redissolved Na-17 (CsNa-17 was not soluble enough)
are not conclusive. We realize that NMR will most likely not allow us to conclude if 17 or 17a or both (in equilibrium) are actually present in solution. In order to resolve this issue, we are currently planning size-exclusion chromatography and electrochemistry experiments.
Furthermore, we will make use of laser-light scattering, which should also shed light on the
99
question whether 17 and/or 17a form the supramolecular, spherical agglomerates called
“blackberries” in solution. 45 All these results will be reported elsewhere.
Fig. C.2.a. 56- Combined polyhedral/ball-and-stick representation of [Ce20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] (17). The colour code is as follows: tungsten (black), cerium (orange), germanium (green) and oxygen (red). The WO octahedra are red and 6 the rotated triads are highlighted with green octahedra.
76- 30 Besides Pope’s spectacular, cyclic W 148 -POM [As 12 Ce 16 (H 2O) 36 W148 O524 ] and our
ball-shaped dimethyltin containing W 108 -POM [{Sn(CH 3)2-(H 2O)} 24 {Sn(CH3)2}12 (A-
36- 46 XW 9O34 )12 ] (X = P, As), the title polyanion 17 is the third-largest (in terms of the number of
W centers) polytungstate reported to date. The roughly dumbbell-shaped 17 has a maximum
dimension of about 4.2 nm.
45 Liu, G.; Liu, T.; Mal, S. S.; Kortz, U. J. Am. Chem. Soc. 2006 , 128 , 10103. 46 Kortz, U.; Hussain, F.; Reicke, M. Angew. Chem. Int. Ed. , 2005 , 44 , 3773.
100
Fig. C.2.b. 28- Exploded view (left) of the half-unit [Ce 10 Ge 5W50 O188 (OH) 2(H 2O) 15 ] (17a), highlighting the arrangement of five equivalent “Ce GeW ” Keggin building blocks (shown in different colors for clarity) in 17 (right). 2 10
Each half unit 17a comprises five { β(4,11)-GeW 10 O38 } units (i.e., β-Keggin fragments
that have lost a WO 6 group from position 4 in the belt and position 11 in the rotated triad,
according to the IUPAC rules for POMs) 13 linked asymmetrically by ten Ce 3+ centers, resulting
in an asymmetric assembly with C 1 symmetry (see Figure C.2.b.). The dilacunary {β(4,11)-
GeW 10 O38 } Keggin unit is unprecedented. Recently we reported the dimeric and trimeric
10- zirconium-containing tungstosilicates [Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] (6) and
14- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] (7) with lacunary Keggin fragments closely related to those
in 17. However, the Zr 4-POM is based on {β(1,4)-SiW10 O38 }and {β(4,9)-SiW10 O38 } units, and
101 the Zr 6-POM on {β(4,10)-SiW10 O38 } units. Close inspection of the structure of 17a indicates that essentially five dicerium-containing { β(4,11)-GeW 10 O38 } building blocks are linked to each
other. In other words, each of the dilacunary β(4,11)-Keggin units has two cerium ions coordinated to the respective vacant sites in the rotated triad and the belt.
The same linkage mode via a cerium ion incorporated in the Keggin ‘belt’ has been seen
13- previously in our lanthanide containing tungstosilicate family [Ln( β2-SiW 11 O39 )2] (Ln = La 9,
Ce 10, Sm 11 , Eu 12 , Gd 13 , Tb 14 , Yb 15 , Lu 16 ) .
These observations allow us to suggest a likely mechanism of formation for 1a via initial
formation of the monomeric building blocks ( β(4,11)-Ce 2GeW 10 O38 ) which then (most likely
gradually) self-assemble in an asymmetric fashion resulting in 17a. Then two enantiomers of 17a
(R,S or S,R) dimerize via two Ce-O(W’’) bonds, resulting in 17 (which has C i symmetry).
We have already pointed out above that 17 contains exclusively dilacunary “GeW 10 “
Keggin units, although the reaction was carried out with the trilacunary “GeW9” precursor. Gain
(and also loss) of tungsten during a reaction involving lacunary polytungstate precursors is not unprecedented in POM chemistry. Our group has observed this phenomenon before, e.g. for the
II 8- reaction of Mn ions with gamma-decatungstosilicate ([ γ-SiW 10 O36 ] ), resulting in the trimeric,
15- 16c cyclic, chiral [( β2-SiW 11 MnO 38 OH) 3] .
In order to find a more rational synthetic procedure for 1, we tried several other reactions.
8- 11 10- For example, using the [ γ-GeW 10 O36 ] precursor and also starting with [ α-GeW 9O34 ] and adding one equivalent of sodium tungstate. When using the same solvent, pH, etc. as for the original reaction, neither pathway resulted in 17 as based on IR spectroscopy.
102
Fig. C.2.c. III Ball-and-stick representation of all Ce centers in 17a with labeling scheme. The figure also shows the inner coordination sphere of all III Ce ions and their complete connectivity via WO 6 units, resulting in an asymmetrical assembly. The 15 terminal water ligands of the 10 ceriums are shown as green balls, the two bridging hydroxo groups are blue and the remaining oxygens are red. The weak interaction between Ce4 and the bridging oxo to Ce8 is shown as a dotted line.
We performed bond valence sum (BVS) calculations to test for the presence of hydroxo
groups in 17a . We identified two hydroxo bridges, one between W17 and Ce4 and one between
103
W38 and Ce8 (see Fig. C.2.c.). Therefore, a more detailed molecular formula of 17 is:
56- [{[Ce 10 (H 2O) 15 [β(4.11)-GeW 10 O37 (OH)] 2[β(4.11)-GeW 10 O38 ]3}2] . The total charge of 17 is therefore 56-, which is balanced equally by cesium and sodium ions in the solid state. We also performed TGA measurements on CsNa-17 and Na-17 from room temperature to 900 °C. These
results are fully consistent with the formulae provided above.
The 10 cerium(III) ions in 17a exhibit two coordination numbers: nine for Ce8 and Ce9, and eight for all others, with Ce—O distances ranging from 2.20(3) to 2.88(3). The cerium centers are linked either directly to each other via oxo or aqua bridges, or indirectly via O-W-O bridges, leading to an asymmetric assembly in 1a (see Figure C.2.c.). Eight of the ten cerium atoms are linked via O-bridges resulting in four pairs (Ce1 and Ce10, Ce2 and Ce3, Ce6 and
Ce7, and Ce8 and Ce9); while Ce4 has a very long (2.87(3) Å) interaction with a tungstate oxo atom bridging to Ce8, and Ce5 is “isolated,” bound only to tungstate oxygens or water not bridging to another Ce atom. There are no terminal water ligands on Ce2 and Ce9; Ce5 and Ce6 have one terminal water ligand each; and Ce3, Ce4, and Ce8 have two terminal water ligands each. The lanthanides Ce1 and Ce10 are bridged by a water molecule, the former having no terminal ligands and the latter having two extra terminal water ligands. This leaves Ce7, which has three terminal water ligands and is linked to a tungsten atom of the second half-unit of 1 via a long (2.61(4)Å) oxo bridge.
C- Conclusion
In summary, we have synthesized the large, dimeric, 20-cerium(III) containing 100-
56- tungstogermanate [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] (17). The title POM was synthesized using
104
10- III conventional reaction conditions from the trilacunary POM precursor [ α-GeW 9O34 ] and Ce ions. Polyanion 17 is one of the largest polytungstates reported to date and is interesting not only for its structure but also for its potential physical and catalytic properties. The detailed solution and red/ox behavior of 17 will be investigated in the near future by us and/or via collaborations in the areas of electrochemistry/electrocatalysis, laser light-scattering, and homogeneous/heterogeneous oxidation catalysis. Furthermore, the terminal water ligands on the cerium centers in 17 allow for ligand substitution, perhaps leading to interesting derivatives. Our work reemphasizes that lanthanides or other large cations are ideal linkers for lacunary POM fragments resulting in very large assemblies.
105
Chapter 4 Collaborative work
106
I – Electrochemistry
The electrochemical studies on polyanions 1-5 were performed by the group of Prof. L.
Nadjo from the Laboratoire de Chimie Physique, UMR 8000, CNRS, Equipe d’Electrochimie et
Photoe´lectrochimie, Université Paris-Sud, Bâtiment 420, 91405 Orsay Cedex, France. The
electrochemical studies on polyanion 8 were performed by the group of Prof L. Walder from the
Institute of Chemistry, University of Osnabrück, Barbarastrasse 7, 49069 Osnabrück, Germany.
For more detailed info please check Inorg. Chem. 2005 , 44 , 2659-2665, Inorg. Chem. , 2005 , 44 ,
9360-9368, Dalton Trans., 2006 , 4253-4259 and J. Am. Chem. Soc. 2008 , 130 , 6696-6697 in
Appendix.
5- A- Electrochemistry of [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(βββ-SiW8O31 )3}] (1)
Cyclic voltammetry of 1 was studied as a function of pH. Comparison of the current
intensities for redox processes of the CoII centers of 1 with free Co 2+ ions in solution suggests
strongly that the six outer Co 2+ ions remain attached to 1 in solution. The electrochemical properties of 1 allow to envision further investigations, starting with the search for appropriate solvent media for facile handling of 1. Finally, the anticipated stability of 1 in these media will
allow for detailed studies of its electrochemical and catalytic properties.
22- B- Electrochemistry of [{Co 3(B-βββ-SiW9O33 (OH))( B-βββ-SiW8O29 (OH) 2)} 2] (2)
In addition to the experimental proof of the dimeric structure of 2 in solution, several remarkable observations were made during the electrochemical characterization of this complex.
The two types of Co 2+ centers which can be distinguished in the structure of 2 are oxidized in separate waves, creating a mixed-valence species. These waves are well-behaved, with an
107 anodic-to-cathodic peak potential difference that suggests simultaneous one-electron oxidation of the centers belonging to the same group. Such reversibility of Co 2+ centers in multi-cobalt-
substituted POMs is uncommon and deserves emphasis. Furthermore, the stability of 2 and its
electrochemical behavior open up the way for catalytic and electrocatalytic applications, the
pathways of which might be amenable to simple interpretations.
10- 2+ 2+ 2+ C- Electrochemistry of [M(H 2O) 2(γγγ-SiW10 O35 )2] (M = Mn (3), Co (4), Ni (5))
Electrochemistry of 3, 4 and 5 demonstrates that two groups of waves exist: the W-waves
on the one hand and the redox patterns associated with the Co 2+ and theMn 2+ centers on the other hand. These two groups of waves are well-separated from each other. The behavior of the W- waves is clearly dominated by the acid–base properties of the reduced polyanion which depend on the type of incorporated transition metal ion. The corresponding pKa values turned out to be different, resulting in a facile merging of waves for 5 while those for 3 and 4 remain split. It is remarkable that the redox behavior of the Co center in polyanion 4 could be clearly detected. The most remarkable and promising process is the two-electron redox behavior of the Mn 2+ center in
3, accompanied by a conducting film deposition upon continuous cycling of the corresponding
wave.
18- D- Electrochemistry of [Zr 6(O 2)6(OH) 6(γγγ-SiW10 O36 )3] (8)
In order to check the stability of 8 in diluted solutions, UV-pH titration experiments were performed. The results show clearly that 8 is stable as it does not release the free ligand. In the CV measurements of 8, no prominent electrochemical signature for the Zrcore subunit was found, but the
Protonation equilibria 1 and 2 were confirmed:
+ Zr core L3Hn+1 ↔ Zr core L3Hn+ + H pKa ≈ 2.0 (1)
+ Zr core L3Hn+2 ↔ Zr core L3Hn+1 + H pKa ≈ 1.1 (2)
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II– Magnetism
The magnetic studies on polyanion 1 were performed by the group of Prof N. Dalal from the Department of Chemistry and Biochemistry, Florida State University and National High
Magnetic Field Laboratory and Center for Interdisciplinary Magnetic Resonance, Tallahassee,
Florida 32306-4390. The Magnetic studies on polyanion 2 were performed by the group of Prof
J. M. Clemente-Juan from the Instituto de Ciencia Molecular, Universidad de Valencia, C/Dr.
Moliner 50, 46100-Burjassot, Valencia, Spain. For more detailed info please check Inorg. Chem.
2005, 44 , 2659-2665 and Inorg. Chem. , 2005 , 44 , 9360-9368 in Appendix.
5- A- Magnetism of [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(βββ-SiW8O31 )3}] (1)
Magnetic studies of Na-1 reveal the presence of both ferromagnetic intra-trimer
interactions and antiferromagnetic inter-trimer interactions for the Co 9 core. A fully anisotropic
Ising model has been employed to describe the exchange-coupling and yields g = 2.42 ± 0.01, J 1
-1 -1 = 17.0 ± 1.5 cm , and J 2 = -13 ± 1 cm . Variable frequency EPR studies reveal an anisotropic
Kramer’s doublet.
22- B- Magnetism of [{Co 3(B-βββ-SiW9O33 (OH))( B-βββ-SiW8O29 (OH) 2)} 2] (2)
The low-temperature magnetic properties of 2 have also been studied and explained
according to an anisotropic exchange model. The magnitude of the ferromagnetic exchange
parameter is similar to those of other Co(II)- containing POMs. In the future, a detailed inelastic
neutron scattering investigation of KNa-2 is planned, which can help understand the anisotropic exchange interactions of the title polyanion 2 in more detail.
109
Chapter 5 Preliminary results with 10- [α-SiW 9O34 ]
110
10- I – Interaction of Mn(II) with [ α-SiW 9O34 ] at basic pH
2+ 10- Reaction of Mn with [α-SiW 9O34 ] in a 3:1 ration and in basic aqueous medium
8- resulted in the tri-manganese substituted Keggin polyanion [ α-Mn 3SiW 9O37(H 2O) 3] (18 ). The
IR spectrum of polyanion 18 is very similar of the IR of a plenary Keggin silicotungstate, with
especially the three peaks between 900 and 1000 cm -1 shifted to lower wavenumbers due to the
substitution by manganese; see Fig E.1.a.
82
80
78
76
74
72
70 %T
68
66
64
62
60
58
1100 1000 900 800 700 600 500 cm-1
Fig. E.1.a. 8- 4- IR spectra of [α-Mn 3SiW 9O37 (H 2O) 3] (18, blue) and [ α-SiW 12 O40 ] (red)
111
The crystal structure of 18 shows clearly the insertion of the manganese centers in the
positions of the corner shared triad through µ-oxo bridges, in addition to terminal aqua ligands, giving it a C 3v point group; see Fig E.1.b.. Work is still in progress for optimization of the synthetic procedure to give the purest and best yield. Future work on polyanion 18 will include
solid-state magnetic and solution electrochemical studies.
Fig. E.1.b. 8- Combined polyhedral and ball-and-stick representation of [α-Mn 3SiW 9O37 (H 2O) 3] (18 ). Color coding for octahedra: WO (red), for balls: Si (blue), Mn(brown), O(red), H O (pink) 6 2
2- 10- II– Interaction of Ni(II) and CO 3 with [A-α-SiW 9O34 ]
2+ 10- Reaction of Ni with [α-SiW 9O34 ] in a 3:1 ratio in the presence of carbonate resulted in the 9-nickel-three carbonate-containing silicotungstate [Ni(H 2O) 4(Ni 4(H 2O)O3α-
112
20- SiW 9O34)2(CO3)3] (19 ). The IR spectrum of polyanion 19 reveals two regions: the polyanion
region at ca 400-1100 cm -1 and the carbonate region at ca 1300-1700 cm -1, see Fig. E.2.a.. 65 60
55 50 45
%T 40 35 30
25
20
15
1600 1400 1200 1000 800 600 cm-1 Fig. E.2.a. IR spectrum of [Ni(H O) (Ni (H O)O α-SiW O ) (CO ) ]20- (19 ) 2 4 4 2 3 9 34 2 3 3
The structure of polyanion 19 comprises two tri-nickel substituted α-Keggin units,
sandwiching a Ni 2(CO 3)3 cluster. In addition, a nickel atom coordinates to the assembly through
a Ni-O-W bridge and a Ni-O(carbonate) bridge, leaving it with four terminal water ligands; see
Fig E.2.b.. With the extra nickel, Polyanion 19 has C 1 symmetry (C 2v without the nickel).Work is
still in progress for optimization of the synthetic procedure to give the purest and best yield.
Future work on polyanion 19 will include solid-state magnetic and solution electrochemical
studies.
113
Fig. E.2.b. 8- Combined polyhedral and ball-and-stick representation of [α-Mn 3SiW 9O37 (H 2O) 3] (19 ).
Color coding for octahedra: WO 6 (red), for balls: Si (blue), Ni(bright green), O(red), H 2O (pink), C(black)
114
Curriculum Vitae
BASSEM BASSIL Department of Chemistry Jacobs University Bremen, College Ring 6, D-28759, Bremen, Germany Email: [email protected] phone: +4917665196641
Personal Information:
Citizenship Lebanese Language skills English, French, Arabic (native) Gender Male Date of Birth 6th November, 1977
Qualifications
Dedicated researcher studied and worked at Jacobs University Bremen. Strong background in synthesis of inorganic metal clusters, in particular polyoxometalates. Have hands-on experience with various analytical instruments such as IR, TGA, NMR and single crystal X-ray diffraction. Articulate in communicating complex ideas to others, including teaching (tutorials and labs) in chemistry courses at Jacobs University Bremen, Germany. Professional activities include publishing research results in mainstream journals. Has good computer knowledge.
Highlights of Ph.D. Thesis
Jacobs University, Bremen, Germany 09/2005 to date Polyoxometalates are of interest because they exhibit tremendous structural variety and have potential applications in the fields of magnetism, photochemistry, catalysis, medicine and electrochemistry. The POM matrix can be seen as a diamagnetic host that could encapsulate and thereby isolate a cluster of transition metals. Magnetically, incorporation of more and more exchange-coupled paramagnetic transition-metal ions into these POM frameworks may result in molecules with high-spin ground states, large spin anisotropies, hysteresis etc. Such materials
115 are ultimately of technical interest in areas such as data storage materials and magnetic switches. These polyoxometalates are not common in today’s collection of magnetic molecules. They combine structural variety, nontrivial physics and a mechanism of formation commonly described as self-assembly. My thesis work is mainly concerned with derivatizing polyoxoanions by the attachment of diamagnetic and paramagnetic transition metal ions to the surface of the metal-oxo framework of the precursor. Synthesis of paramagnetic and diamagnetic metal substituted polyoxoanions is followed by characterization using analytical methods like IR, single crystal XRD, NMR ( 1H, 13 C, 183 W), elemental analysis and thermal analysis (TGA-DTA).
Instrumentations Skills
Hands-on experience in handling and usage of single crystal XRD (Smart Apex, Bruker), infrared spectroscopy, diffused reflectance infrared spectroscopy, thermal techniques, UV-visible spectroscopy, chromatographic techniques and multi-nuclear NMR spectroscopy.
Computer Skills
Diamond crystallographic software, MS Office, HTML, single crystal X-ray diffraction software (Apex2 and SHELXTL)
Education
PhD Jacobs University Bremen (2005-present)
Thesis title Transition metal substituted silico- and germanotungstates
Advisor Prof. Dr. Ulrich Kortz Professor of Chemistry, School of Engineering and Science, Jacobs University Bremen, Germany
M.Sc. Nanomolecular Sciences Jacobs University Bremen (2003-2005)
Thesis title Transition metal substituted silicotungstates
Advisor Prof. Dr. Ulrich Kortz Professor of Chemistry, School of Engineering and Science, Jacobs University Bremen, Germany
B. Sc Chemistry American University of Beirut, Beirut, Lebanon (1995- 1999)
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Research Experience
Masters Student, Nanomolecular Sciences, Jacobs University (2003-2005) Phd student, Department of Chemistry, Jacobs University (2005-present)
Worked under the supervision of Prof Ulrich Kortz on the synthesis and structural characterization of transition metal substituted silicotungstates and later on germanotungstates. The research work resulted publications in high-impact journals, see below for list of publications. Methods used in characterizations were FT-IR, solution NMR, single crystal X-ray diffraction and thermogravimetric analysis. Collaborations are also in progress with groups specializing in magnetochemistry and electrochemistry. Catalytic studies were also performed on the synthesized compounds.
Research assistant of Professor Ulrich Kortz at the American University of Beirut (2000- 2001)
The research assistantship work on synthesis of novel polyoxometalate structures and transition metal substituted polyoxometalates resulted in two publications in high impact journals, see below for the list of publications
Teaching Experience
Teaching assistant for the general chemistry and advanced inorganic chemistry lab courses at Jacobs University (2003-2006)
Assistant instructor for the general chemistry lab course at the American University of Beirut (2001 )
Awards, Fellowships and Scholarships
- Recipient of Jacobs University Fellowship for the PhD program. - Recipient of Jacobs University Scholarship for Masters studies in Nanomolecular Sciences. - Recipient of the DAAD award for academic achievement and social involvement.
Publications
III 26- - A Large, Novel Polyoxotungstate, [As 6W65 O217 (H 2O) 7] Kortz, U.; Savelieff, M. G., Bassil B. S.; Dickman, M. H. Angew. Chem. 2001 , 113 , 3488-3491, Angew. Chem. Int. Ed. Engl. , 2001 , 40 , 3384-3386.
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- Synthesis and Characterization of Iron(III)-Substituted, Dimeric Polyoxotungstates, n- III III IV IV [Fe 4(H 2O) 10 (ß-XW 9O33 )2] (n = 6, X = As , Sb ; n = 4, X = Se , Te ). Kortz, U.; Savelieff, M. G.; Bassil, B. S.; Keita, B.; Nadjo, L. Inorg. Chem. 2002 , 41 , 783-789.
- Structure and Magnetism of the Tetra-Copper(II) Substituted Heteropolyanion 8- [Cu 4K2(H 2O) 8(α-AsW 9O33 )2] Kortz, U.; Nellutla, S.; Stowe, A. C.; Dalal, N. S.; van Tol, J.; Bassil, B. S. Inorg. Chem. 2004, 43 , 144-154.
- Structural Control on the Nanomolecular Scale: Self-Assembly of The Polyoxotungstate Wheel 24- [{ β-Ti 2SiW 10 O39 }4] Hussain, F.; Bassil, B. S.; Bi, L.; Reicke, M.; Kortz, U. Angew. Chem. Int. Ed. 2004 , 43, 3485-3488.
- The Satellite-Shaped Co-15 Polyoxotungstate, [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(b- 5- SiW 8O31 )3}] Bassil, B. S.; Nellutla, S.; Kortz, U.; Stowe, A.C.; van Tol, J.; Dalal, N.S.; Keita, B.; Nadjo, L. Inorg. Chem. 2005 , 44 , 2659-2665.
- Cobalt Containing Silicotungstate Sandwich Dimer [{Co 3(B-ß-SiW 9O33 (OH))( B-ß- 22- SiW 8O29 OH) 2}2] Bassil, B. S.; Kortz, U.; Tigan, A. S.; Clemente-Juan, J.M.; Keita, B.; de Oliveira, P.; Nadjo, L. Inorg. Chem. , 2005 , 44 , 9360-9368.
- Synthesis and Structure of Asymmetric Zirconium-Substituted Silicotungstates, 14- 10- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] and Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] Bassil, B. S.; Dickman, M. H.; Kortz, U. Inorg. Chem., 2006, 45, 2394-2396.
8- - Synthesis and Structure of Dilacunary Decatungstogermanate, [ γ-GeW 10 O36 ] Nsouli, N.H.; Bassil, B.S.; Dickman, M.H.; Kortz; U.; Keita, B.; Nadjo, L. Inorg. Chem., 2006 , 45 , 3858-3860.
- Structural Characterization of Mono-Ruthenium Substituted Keggin-Type Silicotungstates Sadakane, M.; Tsukuma, D.; Dickman, M. H.; Bassil, B.S.; Kortz, U.; Higashijima, M.; Ueda ,W. Dalton Trans., 2006 , 4271-4276.
10- 2+ - Transition Metal Containing Decatungstosilicate Dimer [M(H 2O) 2(γ-SiW 10 O35 )2] (M = Mn , Co 2+ , Ni 2+ ) Bassil, B. S.; Dickman, M. H.; Reicke, M.; Kortz, U.; Keita, B.; Nadjo, L. Dalton Trans., 2006 , 4253-4259.
13- - The Mono-Lanthanide Containing Silicotungstates [Ln(b 2-SiW 11 O39 )2] (Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu), a Synthetic and Structural Investigation Bassil, B. S.; Dickman, M. H.; von der Kammer, B.; Kortz, U. Inorg. Chem., 2007 , 46, 2452-2458.
8- - Di-Titanium Containing 19-Tungstodiarsenate(III) [Ti 2(OH) 2As 2W19 O67 (H 2O)] : Synthesis, Structure, Electrochemistry and Oxidation Catalysis Hussain, F.; Bassil, B. S.; Kortz, U.; Kholdeeva, O. A.; Timofeeva, M. N.; de Oliveira, P.; Keita, B.; Nadjo, L. Chem. Eur. J., 2007 , 13, 4733-4742.
- Dimerization of Mono-Ruthenium(III) Containing Undecatungstosilicate [ α- III 5- SiW 11 O39 Ru (H 2O)] : Synthesis, Structure, Mechanism and Redox Studies Sadakane, M.;
118
Tsukuma, D.; Dickman, M. H.; Bassil, B. S.; Kortz, U.; Capron, M.; Ueda, W. Dalton Trans. , 2007 ,2833-2838.
56- - The 20-cerium(III) containing 100-tungstogermanate: [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] Bassil, B. S.; Dickman, M. H.; Römer, I.; von der Kammer, B.; Kortz, U. Angew. Chem. Int. Ed. 2007 , 46 , 6192 –619.
- 6-Peroxo-6-Zirconium Crown and its Hafnium-Analogue Embedded in a Triangular Polyanion: 18- [M 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] (M = Zr, Hf) Bassil, B. S.; Mal, S. S.; Dickman, M. H.; Kortz, U.; Oelrich, H.; Walder, L. J. Am. Chem. Soc. 2008 , 130 , 6696-6697.
Posters, Oral Presentations and Conference/Meetings attented.
- Poster presentation at Nano 2004, 7th International Conference on Nanostructured Materials, Wiesbaden, Germany, June 2004
- Poster presentation at the DFG-Priority Program "Molecular Magnetism", Bad Dürkheim, January 2006
- Poster and oral presentation at the 9th Northen-German Doctoral Student Colloquium of Inorganic Chemistry in Warnamunde, Germany, October 2006 . The oral presentation was entitled “Transition Metal Containing Silicotungstates”
- Poster presentation at the Meeting of COST Action D40: Innovative Catalysis - New Processes and Selectivities in Dublin, Ireland. May 2007
- Attended meetings for the COST Action D29: Sustainable/Green Chemistry and Chemical Technology in Rome, Italy and Paris, France.
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Appendix: 13 papers
- Structure and Magnetism of the Tetra-Copper(II) Substituted Heteropolyanion 8- [Cu 4K2(H 2O) 8(α-AsW 9O33 )2] Kortz, U.; Nellutla, S.; Stowe, A. C.; Dalal, N. S.; van Tol, J.; Bassil, B. S. Inorg. Chem. 2004, 43 , 144-154.
- Structural Control on the Nanomolecular Scale: Self-Assembly of The Polyoxotungstate Wheel 24- [{ β-Ti 2SiW 10 O39 }4] Hussain, F.; Bassil, B. S.; Bi, L.; Reicke, M.; Kortz, U. Angew. Chem. Int. Ed. 2004 , 43, 3485-3488.
- The Satellite-Shaped Co-15 Polyoxotungstate, [Co 6(H 2O) 30 {Co 9Cl 2(OH) 3(H 2O) 9(b- 5- SiW 8O31 )3}] Bassil, B. S.; Nellutla, S.; Kortz, U.; Stowe, A.C.; van Tol, J.; Dalal, N.S.; Keita, B.; Nadjo, L. Inorg. Chem. 2005 , 44 , 2659-2665.
- Cobalt Containing Silicotungstate Sandwich Dimer [{Co 3(B-ß-SiW 9O33 (OH))( B-ß- 22- SiW 8O29 OH) 2}2] Bassil, B. S.; Kortz, U.; Tigan, A. S.; Clemente-Juan, J.M.; Keita, B.; de Oliveira, P.; Nadjo, L. Inorg. Chem. , 2005 , 44 , 9360-9368.
- Synthesis and Structure of Asymmetric Zirconium-Substituted Silicotungstates, 14- 10- [Zr 6O2(OH) 4(H 2O) 3(β-SiW 10 O37 )3] and Zr 4O2(OH) 2(H 2O) 4(β-SiW 10 O37 )2] Bassil, B. S.; Dickman, M. H.; Kortz, U. Inorg. Chem., 2006, 45, 2394-2396.
8- - Synthesis and Structure of Dilacunary Decatungstogermanate, [ γ-GeW 10 O36 ] Nsouli, N.H.; Bassil, B.S.; Dickman, M.H.; Kortz; U.; Keita, B.; Nadjo, L. Inorg. Chem., 2006 , 45 , 3858-3860.
- Structural Characterization of Mono-Ruthenium Substituted Keggin-Type Silicotungstates Sadakane, M.; Tsukuma, D.; Dickman, M. H.; Bassil, B.S.; Kortz, U.; Higashijima, M.; Ueda ,W. Dalton Trans., 2006 , 4271-4276.
10- 2+ - Transition Metal Containing Decatungstosilicate Dimer [M(H 2O) 2(γ-SiW 10 O35 )2] (M = Mn , Co 2+ , Ni 2+ ) Bassil, B. S.; Dickman, M. H.; Reicke, M.; Kortz, U.; Keita, B.; Nadjo, L. Dalton Trans., 2006 , 4253-4259.
13- - The Mono-Lanthanide Containing Silicotungstates [Ln(b 2-SiW 11 O39 )2] (Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu), a Synthetic and Structural Investigation Bassil, B. S.; Dickman, M. H.; von der Kammer, B.; Kortz, U. Inorg. Chem., 2007 , 46, 2452-2458.
8- - Di-Titanium Containing 19-Tungstodiarsenate(III) [Ti 2(OH) 2As 2W19 O67 (H 2O)] : Synthesis, Structure, Electrochemistry and Oxidation Catalysis Hussain, F.; Bassil, B. S.; Kortz, U.; Kholdeeva, O. A.; Timofeeva, M. N.; de Oliveira, P.; Keita, B.; Nadjo, L. Chem. Eur. J., 2007 , 13, 4733-4742.
- Dimerization of Mono-Ruthenium(III) Containing Undecatungstosilicate [ α- III 5- SiW 11 O39 Ru (H 2O)] : Synthesis, Structure, Mechanism and Redox Studies Sadakane, M.; Tsukuma, D.; Dickman, M. H.; Bassil, B. S.; Kortz, U.; Capron, M.; Ueda, W. Dalton Trans. , 2007 ,2833-2838.
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56- - The 20-cerium(III) containing 100-tungstogermanate: [Ce 20 Ge 10 W100 O376 (OH) 4(H 2O) 30 ] Bassil, B. S.; Dickman, M. H.; Römer, I.; von der Kammer, B.; Kortz, U. Angew. Chem. Int. Ed. 2007 , 46 , 6192 –619.
- 6-Peroxo-6-Zirconium Crown and its Hafnium-Analogue Embedded in a Triangular Polyanion: 18- [M 6(O 2)6(OH) 6(γ-SiW 10 O36 )3] (M = Zr, Hf) Bassil, B. S.; Mal, S. S.; Dickman, M. H.; Kortz, U.; Oelrich, H.; Walder, L. J. Am. Chem. Soc. 2008 , 130 , 6696-6697.
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