Polyoxometalate-based Multielectron-transfer Systems Restricted; Files Only

Abstract

Harnessing solar energy through multielectron-transfer processes remains a central challenge in renewable energy research. This thesis investigates the design and fundamental chemistry of polyoxometalate (POM)-based systems as molecular platforms for visible-light-driven catalysis that convert solar energy into chemical energy. This dissertation also focuses on the role of microenvironments in POM systems including counterions, solvent interactions, and internal transition-metal substitutions, which exert a major effect on the structural, electronic, and catalytic properties of POMs. Chapter 2 studies phosphotungstic acid in cyclic urea (CU) solvents which reveal the formation of novel, deeply-colored dimer POMs of formula [(CU)2H]3PW12O40 (where CU = DMI or DMPU), that undergo visible-light-induced inter-ion charge-transfer processes. These systems represent a new type of photoredox chemistry that involves electron donation from the [(CU)2H]+ cations to the polytungstate units, and expands the photochemical activity of traditionally UV-active POMs into the visible region. Chapter 3 is a comprehensive review of counterion–POM interactions, demonstrating that counterions can directly influence the electronic structures, optical spectra, and reactivities of POMs. Organometallic, organic, and protonated donor–POM systems are categorized to highlight common mechanistic features and knowledge gaps, establishing a framework for interpreting counterion-dependent POM chemistry. Finally, the fourth chapter is a synthetic and catalytic investigation of mixed-metal sandwich-type POMs, exemplified by [Mn2Ni2(PW9O34)2]10-, that provides molecular insights into the interactions between the POM microenvironments. These systems exhibit tunable redox properties and catalytic water oxidation activity, serving as soluble analogues of heterogeneous mixed-metal oxide water oxidation catalysts. Collectively, this latter work demonstrates that tuning POM microenvironments offers a powerful strategy for investigating multielectron-transfer pathways and advancing the development of molecular catalysts for solar-driven energy conversion.

Table of Contents

Chapter 1. Introduction to Polyoxometalate-based Materials   1

1.1 Overview of Polyoxometalates 1

1.2 Structure of Polyoxometalates 1

1.3 Functionalization of Polyoxometalates 6

1.4 Photochemical Properties of Polyoxometalates 7

1.5 Polyoxometalates Catalyzed Reactions 9

1.5.1 Polyoxometalates in Oxidation Reactions 9

1.5.2 Polyoxometalates in CO2 Photoreduction 11

1.6 Scope of Current Work 11

1.7 References 13

Chapter 2. Polyoxometalate-Based Visible-Light-Induced Intermolecular Charge Transfer Systems 26

2.1 Abstract 27

2.2 Introduction 27

2.3 Experimental 29

2.3.1 Materials and Methods 29

2.3.2 Preparation of the Protonated DMI Dimer Salt of [PW12O40]3- 30

2.3.3 Preparation of the Protonated DMPU Dimer Salt of [PW12O40]3- 31

2.3.4 X-ray Crystallography of 1-DMI and 1-DMPU 32

2.3.5 Electrochemistry 32

2.3.6 Continuous Illumination Conditions and Methods 33

2.3.7 Transition Absorption Spectroscopy Conditions 33

2.3.8 Computational Procedures 34

2.4 Results and Discussion 35

2.4.1 Synthesis and Characterization of the Non-covalent Charge Transfer Complexes, [(CU)2H]3PW12O40 35

2.4.2 Geometrical and Electronic Structures of 1-DMI and 1-DMPU 35

2.4.3 X-ray Crystallography of 1-DMI and 1-DMPU 36

2.4.4 Electronic Structure, Spectroscopy, Redox and Photoredox Chemistry of 1-DMI and 1-DMPU 41

2.4.5 [(CU)2H]+-PW12O403- (Inter-ion) Electronic Interactions in 1-DMI and 1-DMPU 48

2.4.6 Transient Absorption Spectroscopy (TAS) of 1-DMI 50

2.5 Conclusions 56

2.6 References 57

Chapter 3. POM counterions in electronic structure and reactivity 71

3.1 Abstract 72

3.2 Background and Introduction 72

3.3 Geometrical and Electronic Structures of POMs 75

3.4 Proton-counterion POM Salts 78

3.5 Conclusions 108

3.6 References 109

Chapter 4. Synthesis, Characterization, and Catalytic Water Oxidation Study of the Mixed Transition-Metal Sandwich-type Polyoxometalate 126

4.1 Introduction 127

4.2 Experimental 130

4.2.1 Materials and Methods 130

4.2.2 Electrochemistry 130

4.2.3 Synthesis and Characterization 131

4.2.3.1 TBA/Mn salt of [Mn2Ni2(PW9O34)2]10- (TBAMn2Ni2) 132

4.2.3.2 TMA Salt, Na2(TMA)8[Mn2Ni2(PW9O34)2] (TMAMn2Ni2) 133

4.2.3.3 Cesium salt of [Mn2Ni2(PW9O34)2]10- (CsMn2Ni2) 134

4.3 Results and Discussion 135

4.3.1 Synthesis and Crystallization of Mixed-Metal Sandwich POMs 135

4.3.2 Redox properties and catalytic water oxidation activity 137

4.4 Conclusion 143

4.5 References 144

About this Dissertation

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School	Laney Graduate School
Department	Chemistry
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Chemistry, Inorganic Chemistry, Physical
Keyword	Polyoxometalate Charge transfer
Committee Chair / Thesis Advisor	Craig L. Hill, Emory University
Committee Members	Brian Dyer, Emory University Cora E. MacBeth, Penn State Behrend

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