Electrochemical and Spectroscopic Studies of Oxidation of Biomass Mimics By Cobalt Complexes

Abstract

The reactivity of cobalt(II) complexes with molecular oxygen has long been a known and is a much-studied chemical mechanism. These interactions result in the formation of metal-dioxygen adducts that are responsible for numerous cobalt-catalyzed oxidations. In the case of 4-coordinate cobalt salen [Co(salen)] complexes, the formation of catalytically active, mononuclear, superoxo adducts in the presence of a secondary, N-donor ligands has been demonstrated [Co(salen)pyr-O2]. In batch reactions, these adducts are known to readily oxidize para-substituted phenolic compounds resulting in benzoquinone in high yield. Para-phenolic model compounds have been used to demonstrate the potential use of cobalt Schiff base complexes in the oxidation of lignin biomass. This work investigates the redox behavior of the Co(salen)pyr-O2­­ adduct as a potential recyclable electrocatalyst. Using traditional electrochemical techniques, the activity of the Co(salen)pyr-O2 adduct is evaluated as it applies to the oxidation of the substrate syringyl alcohol (4-(hydroxymethyl)-2,6-dimethoxy-phenol) in acetonitrile. Typical EC’ electrochemical behavior is reported showing a near linear relationship between substrate concentration and peak current density (Jp) up to 200 mV s-1. Electrochemical titration of catalytic amounts of Co(salen) with pyridine in the presence of excess oxygen and substrate indicate that the one-electron oxidation of Co(salen)pyr-O2H is reversible up to 2:1 pyridine to cobalt. However, both FTIR and EPR characterization of electrolysis experiments with Co(salen)pyr-O2 in the presence of excess substrate show evidence for the deactivation and/or degradation of the catalyst system after the two-hour mark indicating possible poor ligand stability under reaction conditions. Acknowledgements We gratefully acknowledge the NSF EPSCoR program, TN-SCORE, for support of this work.