Small Molecule Reactivity and Electron Transfer Studies of Multinuclear Cobalt Complexes

Abstract

The activation of small molecules (N2, O2, H2O, H2, CO2) is central to renewable energy conversion and storage technologies such as solar cells, fuel cells, and solar to liquid fuels conversions. These transformations involve the transfer of multiple protons and electrons (proton coupled electron transfer, PCET), and typically require transition metal catalysts. Understanding the mechanisms of these reactions and how and why different PCET pathways are operative under different conditions has been a key area of inorganic chemistry research in the last century. Two such reactions, the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) are particularly pertinent to sustainable energy technology – OER occurs at the anode of solar cells and ORR occurs at the cathode of fuel cells. These transformations require the transfer of four electrons and protons in addition to the formation of an O–O bond (OER) or the cleavage of an O–O bond (ORR). Recently, it was discovered that OER proceeds efficiently in neutral pH regimes on amorphous cobalt oxide thin films capped by phosphate or borate buffer species, collectively called cobalt oxygen evolving catalysts (Co-OECs). Our group and others have also developed molecular models composed of cobalt oxide clusters of varying size from Co2 to Co7, to model the active sites and various properties of the Co-OEC. Together, the Co-OEC and its molecular models provide an excellent platform with which to study mechanistic aspects of transition metal-mediated OER. The goal of this thesis is to explore unanswered questions of ET, PCET, and small molecule activation in the cobalt oxide cluster system. We begin by exploring edge site chemistry, first by using isotopic labeling techniques to probe the precise mechanism of the rate limiting O–O bond formation step, and second by studying the high valent Co(IV)2 site poised for turnover with in situ electrochemical spectroscopy of a model Co4O4 cubane compound. Next, we examine the conduction of protons and electrons through the bulk of the film using an interdigitated microelectrode array technique, and correlate conduction properties with intermediate range mesostructure and catalytic performance. These studies provide a thorough understanding of the Co-OEC’s ability to activate water and form a new O–O bond, so in the second section of the thesis we explore further related reactivity by the cobalt oxide cluster motif. To explore cobalt-catalyzed ORR, the microscopic reverse of OER, we synthesize an organic-soluble dicobalt molecule, test its ability to reduce O2, and characterize reaction intermediates along the O2 activation pathway. Lastly, we demonstrate that the surface oxyl radicals generated during OER can be used to activate organic molecules such as ethylene glycol, glucose, and lignin.