Chiral Materials and Water Oxidation Catalysts from Transition-Metal-Substituted Polyoxometalates Open Access
Hou, Yu (2010)
Abstract
Polyoxometalates (POMs) are of great importance to both fundamental studies and practical applications, particularly, catalysis. The focus of this thesis is to explore the structural features of polyoxometalates that are closely related to their catalytic properties.
Attempting to synthesize chiral POMs using a lacunary Keggin ligand produced a chiral POM, [Hf(PW11O39)2] 10- without chiral organic molecules in the structure. Compound [Hf(PW11O39)2] 10- crystallizes in a chiral space group as a conglomerate of two enantiomerically pure crystals in the absence of any chiral source.
Efforts toward synthesizing a ruthenium-containing POM water oxidation catalyst yielded a diruthenium substituted polytungtosilicate, Cs6[{Ru2O2(OH2)2}(γ-SiW10O36)]·25H2O. Attempts to grow crystals of this compound were unsuccessful. Instead, the polyoxoanion, [RuIV 4O4(OH)2(H2O)4(γ-SiW10O36)2] 10-, was obtained by other group members and shown to be capable of oxidizing water to dioxygen in the presence of [Ru(bpy)3]3+ as an oxidant in buffered aqueous solution at ca. pH 7. In order to elucidate the mechanism of water oxidation, one-electron-oxidized form with a formula H2Ce2.5K(NH4)0.5[{RuVRuIV 3O6(OH2)4}(γ-SiW10O36)2]·33H2O was isolated and characterized by single X-ray crystallography, elemental analysis, infrared spectroscopy, UV-vis spectroscopy, and cyclic voltammetry. Its properties are also compared to those of K2Rb8[RuIV 4O4(OH)2(H2O)4(γ-SiW10O36)2]·25H2O. In addition, the effect of Li+, Na+ and K+ cations on the cyclic voltammograms are also presented in this thesis. In order to obtain less expensive and more efficient water oxidation based on abundant metal elements, polytungstophosphates Na2M2(PW9O34)2 12 - (M = Co, Ni, Mn and Zn) were obtained. Dissolving Na2M2(PW9O34)2 12- into 1M aqueous LiCl forms Li2M2(PW9O34)2 12- (M = Co, Ni, Mn and Zn). The use of high valent manganese starting materials produces a new polyoxometalate, Na13[MnIII(HPW7O28)2]⋅39H2O, which exhibits a novel sandwich structure. It also demonstrates that heptatungstate can ligate a 3d metal and represents a rare case of polyoxometalate-based sandwich complex with a single bridging metal.
Table of Contents
Table of Contents
Chapter 1 Introduction: Structures and Features of Typical Plenary and Lacunary Polyoxometalates...1
1.1 Overview of Polyoxometalates...2
1.2 Plenary and Lacunary Keggin POMs...3
1.3 Plenary and Lacunary Wells-Dawson POMs...12
1.4 Catalytic Applications of POMs...17
1.5 Goal of This Work and Outline...22
Chapter 2 Breaking Symmetry: Spontaneous Resolution of a Polyoxometalate...33
2.1 Introduction...34
2.2 Experimental...37
2.2.1 General Methods and Materials...37
2.2.2 Synthesis...37
2.2.3 Solid-state CD spectroscopy...38
2.2.4 Single X-ray Crystallography...38
2.3 Results and Discussion...41
2.3.1 Synthesis...41
2.3.2 Structure...41
2.3.3 Solid State Circular Dichroism...45
2.3.4 Characterizations in Solid state and Solution...46
2.4 Conclusions...50
Chapter 3 Syntheses and Characterizations of Diruthenium and Tetraruthenium Containing Polytungstosilicates...61
3.1 Introduction...62
3.2 Experimental...64
3.2.1 General Methods and Materials...64
3.2.2 Synthesis of 3.1...65
3.2.3 Synthesis of 3.2...66
3.2.4 X-Ray Crystallography...66
3.2.5 Effect of Li+, Na+ and K+
Cations on Reduction Potentials of 3.3...67
3.3 Results and Discussion...69
3.3.1 Synthesis...69
3.3.2 X-ray Crystal Structures of 3.2...69
3.3.3 Electrochemistry of 3.1 and 3.2...71
3.3.4 Electronic spectroscopy of 3.1 and 3.2...73
3.3.5 Effect of pH and Alkali Metal Cations on the Electrochemistry
of 3.3...74
3.4 Conclusions...78
Chapter 4 A New Family of Sandwich-Type Polytungstophosphates Containing Two Types of Metals in the Central Belt...85
4.1 Introduction...86
4.2 Experimental...88
4.2.1 General Methods and Materials...88
4.2.2 Synthesis...89
4.2.3 Lithium-Sodium Exchange Experiments...91
4.2.4 X-Ray Crystallography...92
4.3 Results and Discussion...96
4.3.1 Structures...96
4.3.2 IR Characterization...100
4.3.3 31P NMR Characterization...100
4.3.4 Magnetic Susceptibility...110
4.4 Conclusions...116
Chapter 5 A New Polyoxometalate Structural Class: MnIII(HPW7O28)213-...126
5.1 Introduction...127
5.2 Experimental...128
5.2.1 General Methods and Materials...128
5.2.2 Synthesis...128
5.2.3 X-Ray Crystallography...129
5.3 Results and Discussion...135
5.3.1 Synthesis...135
5.3.2 Structure...135
5.3.3 IR Characterization...137
5.3.4 Cation Exchange of 5.1...138
5.3.5 Solution Chemistry of 5.1...138
5.3.6 Magnetic Properties of 5.1...145
5.4 Conclusions...146
List of Figures
Chapter 1
Figure 1.1 The five Baker-Figgis isomers of the
Keggin anion
(XO4)W12O36n- in
polyhedral representation...4
Figure 1.2 Monovacant Keggin polytungstates and their coordination
chemistry...5
Figure 1.3 Trivacant Keggin polytungstates and their coordination
chemistry...8
Figure 1.4 Divacant Keggin polytungstates and their coordination
chemistry...11
Figure 1.5 Monovacant Wells-Dawson polytungstates and their
coordination chemistry...14
Figure 1.6 Lacunary Wells-Dawson polytungstates species...15
Figure 1.7 Trivacant Wells-Dawson polytungstates and their
coordination chemistry...16
Figure 1.8 X-Ray structure of
WRuZn2(H2O)2(ZnW9O34)2]11-
and
[RuIV4O4(OH)2(H2O)4(γ-SiW10O36)2]10-...19
Figure 1.9 X-Ray structure of
[Co4(H2O)2(α-PW9O34)2]10-...21
Chapter 2
Figure 2.1 Structure (50% probability
ellipsoids) of
[Hf(PW11O39)2]10-, 2.1,
in DMA2.1...42
Figure 2.2 Packing of enantiopure
[Hf(PW11O39)2]10-, 2.1,
in the unit cell...43
Figure 2.3 Solid state CD spectra of two enantiomeric crystals of
[Hf(PW11O39)2]10-, 2.1,
in KBr...46
Figure 2.4 The solid state 31P NMR of DMA2.1...47
Figure 2.5 31P NMR of THA2.1 in CDCl3
solution...47
Figure 2.6 The IR spectrum of DMA2.1...48
Figure 2.7 The IR spectrum of THA2.1...48
Figure 2.8 31P NMR of DMA2.1 in D2O
solution...49
Figure 2.9 The thermogravimetric analysis (TGA) curve of
DMA2.1...51
Chapter 3
Figure 3.1 X-Ray structure of 3.2...70
Figure 3.2 Cyclic voltammograms of 0.7 mM 3.3 (black curve) and 0.7
mM 3.2 (red curve) in 0.1 M HCl. Scan rate 25 mV/s...72
Figure 3.3 Cyclic voltammogram of 0.7 mM 3.1 in 0.1M HCl (freshly
prepared solution). Scan rate, 25 mV/s...72
Figure 3.4 UV-Vis spectra of 3.1 and 3.3 in 0.1M HCl...73
Figure 3.5 UV-vis spectra of 3.3 with different oxidation states in
0.1 M H2SO4 for 3.2...73
Figure 3.6 The UV-Vis spectra of 3.2 (black) and 3.3 (red) in 0.1 M
HNO3...74
Figure 3.7 Cyclic voltammograms of 0.7 mM of 3.2 in 0.1 M HCl at pH
1.0 (black curve) and in the presence of 0.58 M KCl (red
curve)...76
Figure 3.8 Cyclic voltammograms of 0.7 mM of 3.2 in 0.2 M lithium
sulfate buffer at pH 2.0 (black curve) and in the presence of 0.58
M KCl (red curve)...77
Figure 3.9 Cyclic voltammograms of 0.7 mM of 3.3 in 20 mM potassium
phosphate buffer at pH 7.2 in the presence of 0.1 M (black curve)
and 0.4 M KCl (red curve)...77
Figure 3.10 IR of 3.1, 3.2 and 3.3...78
Chapter 4
Figure 4.1 (a) X-ray structure of the polyanions
in
[M'2M2(PW9O34)2]12-.
(b) The connection motif of the metal atoms between two
B-a-PW9O349- units...99
Figure 4.2 IR spectra of 4.1-4.4...101
Figure 4.3 IR spectra of 4.1a-4.4a...101
Figure 4.4 (a) Diamagnetic 31P NMR spectrum of the
decomposition products of 4.1a in D2O (b)Diamagnetic
31P NMR spectrum of the decomposition products of 4.1 in
D2O...102
Figure 4.5 31P NMR spectrum of 4.4a 1day after
dissolving in 1M LiCl solution...103
Figure 4.6 31P NMR spectrum of 4.1a 2h after dissolving
in 1M LiCl (D2O)...104
Figure 4.7 31P NMR spectrum of 4.2a 2h after dissolving
in 1M LiCl (D2O)...105
Figure 4.8 31P NMR spectrum of 4.2a 7 days after
dissolving in 1M LiCl (D2O)...105
Figure 4.9 31P NMR spectrum of 4.1a 7 days after
dissolving in 1M LiCl (D2O)...106
Figure 4.10 Time profile of electronic absorption spectra of
ca.6.2×10-6 M 4.1a in 1M LiCl at 25 °C. The
kinetics at 206 nm is shown in the inset...106
Figure 4.11 Time profile of electronic absorption spectra of ca.
5.6×10-6 M 4.2a in 1M LiCl at 25 °C. The
kinetics at 206 nm is shown in the inset...107
Figure 4.12 Time profile of electronic absorption spectra of ca.
6.4×10-6 M 4.3a in 1M LiCl at 25 °C. The
kinetics at 207 nm is shown in the inset...107
Figure 4.13 Time profile of electronic absorption spectra of ca.
7.2×10-6 M 4.4a in 1M LiCl at 25 °C. The
kinetics at 206 nm is shown in the inset...108
Figure 4.14 31P NMR spectra of aqueous solutions of (a)
4.1 (b) 4.1a (c) 4.1
(in the presence of 2.8mM LiCl) at 277K; (d) 4.1 (e) 4. 1a (f)
4.1
(in the presence of 2.8mM LiCl) at 286K; (g) 4.1 (h) 4.1a (i)
4.1
(in the presence of 2.8mM LiCl) at 296K...109
Figure 4.15 31P NMR spectra of aqueous solutions of (a)
4.2 (b) 4.2a and (c) 4.2
(in the presence of 2.8mM LiCl) at 277K; (d) 4.2 (e) 4.2a (f)
4.2
(in the presence of 2.8mM LiCl) at 286K; (g) 4.2 (h) 4.2a (i)
4.2
(in the presence of 2.8mM LiCl) at 296K. 109
Figure 4.16 31P NMR spectra of (a) 4.4 and (b) 4.4a in D2O at 277K;
(c) 4.4 and (d) 4.4a in D2O at 286K; and (e) 4.4 and (f) 4.4a in
D2O at 296K...110
Figure 4.17 Temperature dependence of χmT for
4.1...111
Figure 4.18 Temperature dependence of 1/χm for 4.1.
The solid line is the best fit...112
Figure 4.19 Temperature dependence of χmT for
4.2...113
Figure 4.20 Temperature dependence of 1/χm for 4.2.
The solid line is the best fit...114
Figure 4.21 Temperature dependence of χmT for
4.3...114
Figure 4.22 Temperature dependence of 1/cm for 4.3. The solid line
is the best fit...115
Chapter 5
Figure 5.1 (a) An ADP plot (30% probability
ellipsoids) of
[MnIII(HPW7O28)2]13-
in 5.1 with atom numbering. (b) Polyanion 1 in polyhedral notation,
showing the C2h symmetry...136
Figure 5.2 IR spectra of 5.1...137
Figure 5.3 Time profile of electronic absorption spectra of 2.4 mM
5.1 in water at 25 °C showing the decay of 5.1. For clarity,
not all spectra are shown in this figure. The fitting at 484 nm is
shown in the inset...140
Figure 5.4 31P NMR of 5.1 in D2O 10 min after
dissolution...140
Figure 5.5 (a)31P NMR of 5.1 in D2O 2h after
dissolution with time (paramagnetic acquisition parameters) (b)
31P NMR of 5.1 in D2O 2h after dissolution
with time (diamagnetic acquisition parameters)...141
Figure 5.6 (a)31P NMR of 5.1 in D2O 4h after
dissolution with time (paramagnetic acquisition parameters) (b)
31P NMR of 5.1 in D2O 4h after dissolution
with time (diamagnetic acquisition parameters)...142
Figure 5.7 (a)31P NMR of 5.1 in D2O 24h after
dissolution with time (paramagnetic acquisition parameters) (b)
31P NMR of 5.1 in D2O 24h after dissolution
with time (diamagnetic acquisition parameters)...143
Figure 5.8 IR spectra of Mn-containing
polytungstophosphates...144
Figure 5.9 Temperature dependence of cm for 5.1. Inset: temperature
dependence of 1/cm for 5.1. The solid lines are the best fits to
the Curie-Weiss law...145
List of Schemes
Chapter 1
Scheme 1.1 Water oxidation catalyzed by
[RuIV4O4(OH)2(H2O)4(γ-SiW10O36)2]10-
using [Ru(bpy)3]3+ as oxidant...20
Scheme 1.2 Light-induced catalytic water oxidation by
[RuIV4O4(OH)2(H2O)4(γ-SiW10O36)2]10-
using [Ru(bpy)3]2+ as a photosensitizer and
persulfate as a sacrificial electron acceptor...20
Chapter 2
Scheme 2.1 The side view of the coordination mode of hafnium atom in one enantiomer through the coordinated ZrIV ions...45
List of Tables
Chapter 2
Table 2.1 Crystal data and structure refinement
for DMA2.1...40
Table 2.2 Selected bond lengths [Å] and angles [deg] for
DMA2.1...52
Chapter 3
Table 3.1 Crystal data and refinement parameters
for the X-ray structure of 3.3...56
Table 3.2 Potentials of anodic (Ea) and cathodic (Ec)
peaks in cyclic voltammograms of 0.7 mM 3.2...56
Chapter 4
Table 4.1 Crystal data and structure refinement
for 4.1, 4.2, 4.2a, 4.4a and 4.3...94
Table 4.2 Selected metal-oxygen bond lengths [Å]...95
Table 4.3 31P NMR data for the
[M'2M2(PW9O34)2]12-
species...95
Chapter 5
Table 5.1 Crystal data and structure refinement
for 5.1...131
Table 5.2 Selected bond lengths [Å] and angles [deg] for
5.1...132
Table 5.3 Charge density (negative charge/total atoms) on
polyoxoanions...139
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