A New Quinoline-based Cobalt(II) Catalyst Capable of Bifunctional Water Splitting

Abstract

We report on a new water-soluble cobalt(II) complex capable of water splitting bifunctionality, i.e., water reduction and water oxidation. The species [CoII(LQpy)H2O]ClO4 (1), where LQpy is the deprotonated form of the new tripodal ligand N1,N1-bis(pyridin-2-ylmethyl)-N2-(quinolin-8-yl)benzene-1,2-diamine, HLQpy, was developed aiming to replace an oxidation prone methylene group by a sturdy and redox stable quinoline. The molecular and electronic structures of 1 were evaluated by multiple spectroscopic, spectrometric, electrochemical and computational methods, and detailed pre- and post-catalytic studies were conducted to acertain the molecular nature of the conversions. Complex 1 performs water reduction at a low onset overpotential (η) of 0.65 V at pH 7, reaching TON3h 2,900 (TOF 970 h−1) and TON18h 12,100 (TOF 672 h−1) with up to 98% Faradaic efficiency (FE). Species 1 also promotes water oxidation at η = 0.34 V under pH 8, achieving TON3h 193 (TOF 64 h−1) at 84% FE. Experimental and DFT results enabled us to propose reaction intermediates and mechanisms.