Ligand-Engineered Square-Planar Nickel(II) Complexes with NNS Tridentate Ligands

The design and development of innovative, cost-effective, and efficient electrocatalysts for the oxygen evolution reaction (OER) in an alkaline medium are essential for reducing the overpotential and thereby minimizing the energy requirement during water electrolysis. Herein, we report the synthesis and characterizations of four differently substituted hydrazine-1-carbothioamide ligands derived from 2-acetylpyridine: N-(aryl)-2-(1-(pyridin-2-yl)ethylidene)hydrazine-1-carbothioamides, where aryl = -3,5-dichlorophenyl (HdClPhHCT), -4-nitrophenyl (HNPhHCT), -4-methylphenyl (HMePhHCT), and -4-methoxyphenyl (HMeOPhHCT), and their corresponding nickel(II) complexes, [Ni(dCl)Cl], [Ni(N)Cl], [Ni(Me)Cl], and [Ni(MeO)Cl], using various spectroscopic techniques, single-crystal X-ray diffraction, and cyclic voltammetry (CV). Furthermore, these complexes were employed as electrocatalysts for the OER under alkaline conditions. Among the four nickel complexes, notably, the [Ni(dCl)Cl] film demonstrated remarkable efficiency, requiring only 320 mV of overpotential to attain a current density of 10 mA cm^-2. Its OER performance is thought to be enhanced by a square-planar coordination geometry and an increase in the Ni oxidation state. Investigations conducted before and after through FT-IR, Raman, UV-visible, PXRD, XPS, SEM, and TEM show that a part of the molecular complex undergoes activation to form the active catalyst Ni(O)OH, and the interface of [Ni(dCl)Cl]/Ni(O)OH acts as the true catalyst.