Electrocatalytic hydrogen evolution by molecular Cu(II) catalysts

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 s1. 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 [Cu(L)(OAc)(HO)]+ reversible arrow [Cu(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 [Cu(L)(OAc)(OH)] on PCET generates [Cu(L)(OAc)(OH)]. The [Cu-H, in the presence of H in the presence of HO to complete the cycle.

Automated summary

Two mononuclear copper complexes were synthesized and characterized, and were found to exhibit good performance in electrochemical proton reduction and water reduction. Among them, complex 2' showed higher catalytic activity. Both complexes demonstrated comparable acid-base equilibria characteristics.