Polyoxometalates in Artificial Photosynthesis: Development of Chromophores and Water Oxidation Catalysts Open Access

Abstract

Polyoxometalates (POMs) have been gaining increasing interest in several research fields, especially in catalysis. The development of POM-based water-oxidation catalysts (WOCs) recently has brought unprecedented attention to POMs. The general focus of this thesis is to explore the structural features of POMs and their applications in artificial photosynthesis (AP); the specific focus is to develop POM-based chromophores and WOCs for AP.

The first part of thesis reports the synthesis, structures and reactivity of POM-supported metal carbonyl complexes. Dimeric structures based on [M(CO)3]⁺ (M = Re, Mn) and lacunary POM ligands, [X2W22O74(OH)2]^12- (X = Sb, Bi), [α-PW11O39]^7- and [α2-P2W17O61]^10-, have been prepared and fully characterized. They show "slipped-sandwich" and "twisted-sandwich" structures. Their electrochemical and photochemical properties have been investigated. This work has significantly expanded the area of hybrid POMs.

The second part focuses on the use of these hybrid POMs as chromophores for solar light conversion. A novel transition, metal-to-polyoxometalate charge-transfer (MPCT), is characterized for the first time. In complex [P4W35O124{Re(CO)3}2]^16-, the donor is an embedded Re(I) center and the acceptor is the all-inorganic d⁰ POM ligand, [α2-P2W17O61]^10-. The high visible-region absorptivity (ε470nm = 4000 M^-1cm^-1) results from a Re^I-to-POM CT transition. In complex [P2W17O61{Re(CO)3}3{ORb(H2O)}(μ3-OH)]^9-, the donor is a trinuclear Re(I) cluster, so MPCT transition in this system can be regarded as cluster-to-cluster CT. Ultrafast spectroscopy and DFT theoretical calculations confirm instantaneous (<50 fs) MPCT processes involving electron transfer from Re(I) center(s) to POM ligand(s).

The third part addresses the preparation and characterization of new cobalt(II) and tin(II) POMs. The structures and catalytic water oxidation activity of several Co(II) POMs based on [B-SbW9O33]^9- were investigated. Multiple Sn(II) POMs based on [B-SbW9O33]^9- and [α-XW9O34]^10- (X = Si, Ge) are the highest nuclearity Sn(II) POMs known to date. They crystallize as infinite three-dimensional all-inorganic frameworks with porous channels. These Sn(II) POMs have high absorptivity in the visible region, a feature characterized by spectroscopic techniques.

Table of Contents

Chapter 1 Introduction: The Frontiers of Polyoxometalate Chemistry .................................................... ................................................................................ 1
1.1 General Properties of Polyoxometalates ...................................................................................................................................................................... 2
1.2 Overview of POM Photochemistry .............................................................................................................................................................................. 5
1.3 Merging Organometallics with POM Chemistry .............................................................................................................................................................. 10
1.4 Transition-Metal-Substituted POMs with Applications in Catalytic Water Oxidation ............................................................................................................. 16
1.5 Metal-to-Metal Charge-Transfer Chromophores ........................................................................................................................................................... 21
1.6 Goal of This Work and Outline .................................................................................................................................................................................. 25
References ................................................................................................................................................................................................................ 27

Chapter 2 Metal Carbonyl Complexes Supported on Subvalent Sb^III/Bi^III-centered Polyoxoanions ............................................................................................... 35
2.1 Introduction ......................................................................................................................................................................................................... 36
2.2 Experimental ......................................................................................................................................................................................................... 37
2.2.1. Materials and Instrumentation .............................................................................................................................................................................. 37
2.2.2. Synthesis ......................................................................................................................................................................................................... 38
2.2.3. Crystallography .................................................................................................................................................................................................. 39
2.3 Results and Discussion............................................................................................................................................................................................. 42
2.3.1 Synthesis ........................................................................................................................................................................................................... 42
2.3.2 Structures........................................................................................................................................................................................................... 44
2.3.3 Spectroscopic and Computational Studies ................................................................................................................................................................. 46
2.4 Conclusions ........................................................................................................................................................................................................... 56
References ................................................................................................................................................................................................................. 56

Chapter 3 Mono-lacunary Polytungstate-supported Metal Carbonyl Complexes .......................................................................................................................... 59
3.1 Introduction ............................................................................................................................................................................................................ 60
3.2 Experimental ............................................................................................................................................................................................................ 61
3.2.1. Materials and Instrumentation ................................................................................................................................................................................. 61
3.2.2. Synthesis ............................................................................................................................................................................................................ 62
3.2.3. Crystallography .................................................................................................................................................................................................... 64
3.2.4. Electrochemistry ................................................................................................................................................................................................... 65
3.3 Experimental ............................................................................................................................................................................................................ 65
3.3.1 Synthesis and Crystal Structures .............................................................................................................................................................................. 65
3.3.2 Electrochemistry .................................................................................................................................................................................................... 77
3.4 Conclusions ............................................................................................................................................................................................................. 78
References ................................................................................................................................................................................................................... 79

Chapter 4 Spectroscopic and Computational Studies of a Metal-to-Polyoxometalate Charge Transfer Molecular Chromophore ............................................................. 87
4.1 Introduction ............................................................................................................................................................................................................ 88
4.2 Experimental ........................................................................................................................................................................................................... 90
4.2.1. Synthesis ............................................................................................................................................................................................................ 90
4.2.2. Transient Spectroscopic Studies .............................................................................................................................................................................. 91
4.3 Results and Discussion................................................................................................................................................................................................ 93
4.3.1. Steady-state Specstrocopy .................................................................................................................................................................................... 93
4.3.2. Computational Studies ............................................................................................................................................................................................ 95
4.3.3. Transient Spectral and Kinetic Analysis ...................................................................................................................................................................... 97
4.4 Conclusions .............................................................................................................................................................................................................. 102
References .................................................................................................................................................................................................................... 102

Chapter 5 An Inorganic Chromophore Based on a Molecular Oxide Supported Metal Carbonyl Cluster: [P2W17O61{Re(CO)3}3{ORb(H2O)}(μ3-OH)]^9- ................................. 106
5.1 Introduction ............................................................................................................................................................................................................. 107
5.2 Experimental ............................................................................................................................................................................................................. 108
5.2.1. Synthesis ............................................................................................................................................................................................................. 108
5.2.2 Crystallography ....................................................................................................................................................................................................... 111
5.2.3 Spectroelectrochemistry ........................................................................................................................................................................................... 113
5.2.4. Transient Spectral and Kinetic Analysis ....................................................................................................................................................................... 114
5.2.5. Computational Procedures and Modeling ...................................................................................................................................................................... 115
5.3 Results and Discussion.................................................................................................................................................................................................. 116
5.3.1. Structral studies ..................................................................................................................................................................................................... 116
5.3.2. Steady-state spectroscopy ...................................................................................................................................................................................... 120
5.3.3. Transient spectroscopy ............................................................................................................................................................................................ 123
5.3.4. Computational modeling of CO streching........................................................................................................................................................................ 131
5.4 Conclusion ................................................................................................................................................................................................................. 133
References ...................................................................................................................................................................................................................... 134

Chapter 6 Cobalt(II) and Tin(II) Complexes Based-on Trivacant Polyoxoanion Ligands: Synthesis, Structures, Physicochemistry and Catalytic Water Oxidation Activity ........ 139
6.1 Introduction ............................................................................................................................................................................................................... 140
6.2 Experimental ............................................................................................................................................................................................................... 142
6.2.1. Materials and Instrumentation .................................................................................................................................................................................... 142
6.2.1. Syntheses .............................................................................................................................................................................................................. 143
6.2.2. X-Ray crystallography ............................................................................................................................................................................................... 147
6.3 Results and Discussion................................................................................................................................................................................................... 148
6.3.1 Characterization of Cobalt(II) POMs .............................................................................................................................................................................. 148
6.3.2 Water-oxidation Activity of Na10[{Co(H2O)3}2{WO2}2(SbW9O33)2] .................................................................................................................................... 151
6.3.3 Characterization of Tin(II) POMs .................................................................................................................................................................................. 152
6.3.4 Spectroscopic Study of Tin(II) POMs ............................................................................................................................................................................ 157
6.4 Conclusions ................................................................................................................................................................................................................ 160
References ...................................................................................................................................................................................................................... 161

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School	Laney Graduate School
Department	Chemistry
Degree	PhD
Submission	Dissertation
Language	English
Research Field	Chemistry, Inorganic
Keyword	charge transfer artificial photosynthesis crystal structure metal carbonyl polyoxometalate transient spectroscopy
Committee Chair / Thesis Advisor	Hill, Craig L, Emory University
Committee Members	Lian, Tim, Emory University MacBeth, Cora E., Emory University

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