LAUSR

Abstract Two mononuclear copper complexes [Cu(QCl-tpy)(OAc)(Cl)] (1′), and [Cu(4Ql-tpy)(OAc)(OH2)]Cl (2′) (QCl-tpy corresponds to 2-choloro-3-(2,6-di(pyridin-2-yl)pyridine-4-yl) quinoline and 4Ql-tpy represents 4-(2,6-di(pyridin-2-yl)pyridin-4-yl)quinoline) have been synthesized and characterized using various analytical techniques like HRMS, EPR, single-crystal XRD, CV etc. Both the complexes were employed for electrochemical proton reduction in 95:5 DMF/H2O (v/v) in the presence of AcOH as a proton source. Among the two complexes, 2′ exhibits higher catalytic activity for proton reduction and achieves a TOF value of 1682 s−1. The complexes also performed electrochemical water reduction in 0.1 M phosphate buffer at environmentally benign pH 7.0, with lower overpotential than that of a blank solution by 400 mV and 350 mV for complex (1′) and (2′) respectively. The acid-base equilibria study in Britton- Robinson buffer predicts that both the complexes show comparable pKa value around 4.4, which is for [CuII(L)(OAc)(H2O)]+ ⇆ [CuII(L)(OAc)(OH)] + H+, where L is QCl-tpy or 4Ql-tpy. The Nyquist plot shows that the Heyrovsky step is the rate-determining step in both the catalysts. As a result, both the complexes follow the same mechanistic cycle, wherein the deprotonated species [CuII(L)(OAc)(OH)] on PCET generates [Cu0(L)(OAc)(OH2)]-. The [Cu0(L)(OAc)(OH2)]- converts into the CuII-H, in the presence of H+ and further CuII-H evolves H2 in the presence of H2O to complete the cycle.