Intramolecular hydrogen-bonding in a cobalt aqua complex and electrochemical water oxidation activity

Abstract

Water oxidation is catalysed in Nature by a redox cofactor embedded in a hydrogen-bonded network designed to orchestrate proton transfer throughout the challenging 4 electron reaction. In order to mimic aspects of this microenvironment, [CoL^DMA(CH₃CN)₂][BF₄]₂ (2) was synthesized, where L^DMA is a dipyridyldiamine ligand with two dimethylamine bases in the secondary coordination sphere. Structural characterization of the corresponding aqua complexes establish hydrogen bonding between the bound water and pendant base(s). Cyclic voltammetry of [CoL^DMA(CH₃CN)₂][BF₄]₂ (2) reveals enhanced oxidative current upon titration with water and controlled potential electrolysis confirms evolution of O₂. The related complex [CoL^H(CH₃CN)₂][BF₄]₂ (1), which has the same primary coordination environment as 2 but lacks pendant bases, is inactive. The structural and electrochemical studies illustrate the role positioned proton relays can play in promoting redox reactivity.