Developing Redox-Active Ligands That Incorporate Intramolecular Hydrogen Bonding: Novel Reactivity of High-Spin Co(II) Complexes Open Access

Leidy, Michelle (Spring 2018)

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Redox-active ligands have the potential to allow inexpensive and environmentally benign 1st row transition metals to mimic 2nd and 3rd row transition metal chemistry, as well as provide opportunities to explore new chemistry. Our research group has been developing catalysts for O-atom transfer, C-H amination, and aerobic deformylation using high-spin CoII ions supported by a redox-active NNN-pincer ligand. A new variant of this parent catalyst, H5LUrea, has been synthesized that incorporates hydrogen-bond donating groups into this pincer ligand in order to modulate reactivity. The hydrogen-bond donors are expected to help stabilize higher oxidation states and negatively charged exogenous ligands. The H5LUrea ligand and its subsequent CoII complex can be synthesized and can be studied using a variety of methods. Exogenous ligands, such as O2, were added to the complex to see if they would bind, and if they did, what kind of reactivity they would have compared to its non-H-bonding analogue. Upon addition of O2, it formed a highly oxidized, five-coordinate species, which differs drastically compared to the previously reported complex that instead activated the O2. This reaction has been monitored using absorbtion spectroscopy, and the product was crystallized and characterized by X-ray crystallography, NMR, and IR. The effect of H-bonding in the ligand of this CoII complex is demonstrated by its difference in O2 reactivity. The five-coordinate species is a new and unusual finding, which may help achieve more challenging oxidations in the future.

Table of Contents

Abstract iv

List of Figures vii

List of Schemes viii

List of Tables ix

Chapter 1 General Introduction 1

Redox Active Ligands 2

Oxidation Using 1st Row Transition Metals and Redox-Active Ligands 8

References 12

Chapter 2 Incorporation of Hydrogen Bond Donors into the Ligand Backbone 14

Introduction 15

Results and Discussion 16

Conclusions 27

Experimental 27

References 31

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