Proton reduction by a Ni(Ⅱ) catalyst and foot-of-the wave analysis for H_2 evolution

摘要

A nickel based molecular catalyst [Ni(QCl-tPY)(2)]Cl-2 center dot 7H(2)O (where QCl-tpy = 2-choloro-3-(2,6-di (pyridin-2yl)pyridine-4-yl) quinoline) has been synthesized, characterized by single crystal XRD and other spectroscopic techniques. The complex [Ni-II(QCl-tpy)(2)](2+) has also been employed for the electrocatalytic proton reduction in DMF/H2O (95:5 v/v) using trifluor acetic acid (TFA) as the proton source. It exhibits a reasonably efficient catalytic ability towards proton reduction under organic media. Compared to the parent [Ni-II(tpy)(2)](2+) during the electro-catalysis, the complex [Ni-II(QCl-tpy)(2)](2+) behaves as a better catalyst in terms of higher catalytic current and 180 mV of lower overpotential as well. It is expected due to the presence of 2-chloroqinoline moiety in the terpyridine framework. The rate of H-2 evolution was analysed with the use of Foot-of-the Wave Analysis (FOWA) method. The complex shows a TOF of 3.68 s(-1) as obtained from Foot-of-the Wave Analysis (FOWA) at the scan rate 100 mVs(-1) for 1.0 mM [Ni-II(QCl-tpy)(2)](2+) complex. The acid base equilibria reveals the dechelation followed by protonation at one of the coordinated pyridine rings of the QCl-tpy ligand. There could be a pendant base effect towards hydrogen evolution due to dechelated pyridine ring of the coordinated QCl-tpy ligand, which acts as a proton relay. Based on the spectroscopic evidence and electrochemical studies a plausible mechanism for the reduction of proton to H-2 has been proposed. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.