Investigating proton coupled electron transfers in oxovanadium and dimeric copper-oxo systems: The importance of intrinsic barriers and reaction driving force.

摘要

This work describes reactions of vanadium and copper compounds as models for Proton Coupled Electron Transfer (PCET) processes involving metal-oxo complexes. In the first chapter, reactions that interconvert vanadium(IV) oxo-hydroxo complexes [VIVO(OH)(R2bpy)2]BF 4 (1a-c) [a: R = tBu, b: R = Me, c: R = H] and vanadium(V) dioxo complexes [VVO2(R2bpy)2]BF 4 (2a-c) are discussed. Complex 2a abstracts hydrogen atoms from organic substrates with weak O-H and C-H bonds at surprisingly slow rates. The net hydrogen-atom pseudo-self-exchange also occurs slowly, and DFT calculations show that this is due to the unusually short, strong vanadium-oxo bonds in these compounds, which lead to a high intrinsic barrier. In the second chapter, the oxidation of 1a-c by O2 in THF and THF/CH3CN to give the oxoperoxovanadium compounds [V VO(O2)(R2bpy)2]BF4 ( 3a-c) is reported. The mechanism of formation of 3a-c is complex and may occur via initial reduction of O2 at vanadium(IV) to give a superoxo-vanadium(V) intermediate, autoxidation of the THF solvent, or both. 3a is also slowly reduced to 1a by organic hydrogen atom donors.;Chapters three and four describe development of dimeric copper(II) bis(hydroxo) compounds as mechanistically well defined catalysts for water oxidation. In chapter three, compatible combinations of one-electron oxidants with organic bases, which act as net acceptors of a hydrogen atom, are described. The driving force provided by these reagents can be understood in terms of the redox potential of the oxidant and pKa of the base used. Copper(II) bis(hydroxo) dimers having neutral alkylamine ligands were reacted with many combinations of oxidant and base under a variety of conditions, but the desired oxidized copper products were not observed. In chapter four, anionic beta-diketiminate ligands are substituted in the bis(hydroxo) dimers to promote oxidation of the copper center. After again screening a variety of reactions, an unidentified orange solid, formed during attempted synthesis of copper(II) bis(hydroxo) dimer, appears to give the desired copper(III) bis(mu-oxo) intermediate when reacted with a variety of oxidant and base combinations.