Innocent and Non-innocent Countercation Interactions in Transition Metal Oxidation Catalysts Open Access

Wieliczko, Marika (2017)

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Oxidation catalysis is ubiquitous in chemical transformations that are critical in modern society, and many industrially-relevant oxidation processes have analogues in biological systems. The effect of redox-inactive ions is essential in various biological processes and though some corollary to artificial systems is known, the effects of non-innocent cation interactions on transition metal oxidation catalysts remain poorly understood. Chapter 1 introduces the reader to the broad field of transition metal-catalyzed oxidation using specific examples of organic substrate oxidation, as in the monooxygenation of aromatic hydrocarbons, and inorganic substrate oxidation as in the conversion of water to dioxygen. These reactions illustrate the complementary ways in which oxidative catalysis is used to both extract energy in a downhill process, as well as capture and store energy in an energetically-uphill process. These reactions, which are catalyzed by transition metal centers, are the basis for the investigations which follow. Chapter 2 illustrates the influence of redox-inert Ca2+ ions on the oxidation state of a cobalt-centered coordination complex and explores its potential application in aromatic hydrocarbon oxidation. In Chapter 3, the specific binding modes of K+ cations to the active site of a tetraruthenium polyoxometalate water oxidation catalyst are demonstrated to significantly impact both redox and acid-base properties, which are critical aspects of the proton and electron transfer steps in the multi-step mechanism of dioxygen formation and evolution. In Chapter 4, the conversion of cobalt-based polyoxometalate water oxidation catalysts to hydrophobic forms using weakly interacting tetraalkylammonium cations facilitates detailed studies of these catalysts in non-aqueous media, where electrochemical, spectroscopic, stability and reactivity studies are conducted that are uninformative or not possible in bulk water. These studies yield unprecedented insights into the nature of transition metal oxidation catalysts whose reactivity and stability can be controlled by understanding the effects of both innocent and non-innocent cation interactions.

Table of Contents

Chapter 1

1.1 Introduction to Oxidation Catalysis. 2

1.1.1 Overview of Oxidation Catalysts. 2

1.1.2 Transition Metal-Oxo Complexes and Selectivity in Oxidation Catalysis. 3

1.2 Transition Metal Catalysts for Hydrocarbon Oxidation.. 6

1.2.1 Enzymatic Fe-catalyzed Hydrocarbon Oxidation by Cytochrome P450. 6

1.2.2 The Hock Process for Aromatic Hydrocarbon Oxidation in Chemical Industry. 7

1.3 Transition Metal Catalysts for Water Oxidation.. 1

1.3.1 Mn-catalyzed Water Oxidation in Photosynthesis. 1

1.3.2 Co and Ru Molecular Catalysts for Artificial Water Splitting.. 3

1.4 Non-innocent Ligands, Anions, and Cations in Oxidation Catalysis. 6

1.5 Hypothesis and Scope of this Work. 9

1.6 References. 12

Chapter 2. 20

2.1 Introduction: Activation of Transition Metal Complexes by Redox-inactive Cations. 21

2.1.1 Selective Monooxygenation of Aromatic Hydrocarbons. 21

2.1.2 Ligand Design for Accessing Mid- and Late-metal Oxo Species. 25

2.2 Experimental 28

2.2.1 General Considerations. 28

2.2.2 Ligand and Co(II) Complex Synthesis. 29

2.2.3 Catalytic Oxygenation of Benzene. 32

2.2.4 EPR and Magnetic Circular Dichroism Spectroscopy. 34

2.2.5 Resonance Raman Spectroscopy. 35

2.2.6 X-ray Crystallography. 36

2.3 Results and Discussion.. 36

2.3.1 Synthesis of Co(II) Complexes. 36

2.3.2 Reactivity of Complexes towards Oxygen Atom Transfer Agents. 37

2.3.3 Characterization of 1-O.. 42

2.3.4 Preliminary Reactivity Studies for Catalytic Monooxygenation of Benzene. 49

2.4 Conclusions. 53

2.6 References. 54

Chapter 3. 59

3.1 Introduction to Cations in Water Oxidation.. 60

3.1.1 Redox-inactive Cations in Photosynthesis and OEC Models. 60

3.1.2 Transition Metal-substituted Polyoxometalate WOCs. 61

3.1.3 Specific and Non-specific Cation Effects in POMs. 63

3.2 Experimental 65

3.2.1 General Considerations. 65

3.2.2 X-ray crystallography. 66

3.2.3 Electrochemical and Electrocatalytic Behavior 66

3.2.4 Titrations. 67

3.2.5 Stopped flow kinetics measurements. 67

3.3 Results and Discussion.. 67

3.3.1 Specific Binding of K+ in the X-ray crystal structures of Ru4POM... 67

3.3.2 Cation-specific Deviations in Acid-base Properties. 72

3.3.3 Effects of Cations on Kinetics of Oxidation.. 76

3.4 Conclusions. 78

3.5 References. 81

Chapter 4. 85

4.1 Introduction to Cobalt-catalyzed Water Oxidation.. 86

4.1.1 Heterogeneous Cobalt Catalysts for Water Oxidation.. 86

4.1.2 Homogeneous Cobalt Catalysts for Water Oxidation.. 87

4.1.3 Stability of Homogeneous Catalysts in Aqueous Media. 90

4.2 Experimental 92

4.2.1 General considerations. 92

4.2.2 Synthesis of POMs. 93

4.2.3 Conversion to hydrophobic salts. 94

4.2.4 X-ray crystallography. 97

4.2.5 Electrochemistry. 99

4.2.6 SEM-EDX Measurements. 99

4.3 Results and Discussion.. 100

4.3.1 Synthesis of Co4POMs and Conversion to Hydrophobic Forms. 100

4.3.2 Solid and Solution State Structures of Co4POMs. 107

4.3.3 Chemical and Electrochemical Oxidations of Co4P2W18 in MeCN.. 113

4.3.4 Dioxygen formation inhibition and pKa modulation in Co4P2W18 by MeCN.. 121

4.3.5 Stability and Reactivity of Co4V2W18 in Solution.. 125

4.3.6 Electrochemical Stability of Co4POMs in Anhydrous Media. 131

4.4 Conclusions. 137

4.5 References. 139

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