SusChEM: Water Oxidation by Homogeneous Manganese Catalysts

Indiana University, Bloomington IN

Investigators

Abstract

In this project funded by the Chemical Synthesis Program of the Chemistry Division, Professor Jeremy M. Smith, Department of Chemistry and Biochemistry, Indiana University, investigates how well-defined manganese catalysts convert water into oxygen. Nature has learned to convert water to oxygen using visible light. This process is called photosynthesis and uses manganese in a complicated biological protein. In this project, smaller managanese compounds are being studied both to provide basic information on photosynthetic oxygen formation and to provide new catalysts, based on inexpensive, abundant metals, for use in the laboratory or by the chemical industry. An important part of the project involves the training of students in bioinorganic and energy chemistry. This project utilizes synthetic, spectroscopic and physical studies to investigate the mechanism of water oxidation by manganese macrocyclic complexes. The modular catalyst design allows for the introduction of functional groups, enabling systematic studies into the effect of proton and hydrogen atom transport as well as metal cofactors on catalyst performance. A family of manganese pyridinophane compounds serve as functional models for the oxygen evolving complex of Photosystem II. This work sets out to: (1) experimentally validate a computationally-proposed mechanism for water oxidation by these complexes, (2) determine the impact of pyridinophane stereoelectronic properties on catalytic selectivity and activity, and (3) delineate the effect of the second coordination sphere on catalytic activity, including the presence of proton and hydrogen atom relays, and redox-innocent metals. Spectroscopic, reactivity and mechanistic investigations of these species provide insights relevant to the biological water oxidation mechanism, particularly the key O-O bond forming step. Compounds are characterized by standard physical methods of X-ray diffraction, NMR, EPR, and IR, while catalytic behavior is investigated by a combination of electrochemical measurements and standard mechanistic probes.