Polyoxometalate-Supported Bis(2,2'-bipyridine)mono(aqua)nickel(II) Coordination Complex: an Efficient Electrocatalyst for Water Oxidation.

Abstract

A polyoxometalate (POM)-supported nickel(II) coordination complex, [NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·3H2O (1; 2,2'-bpy = 2,2'-bipyridine), has been synthesized and structurally characterized. We could obtain a relatively better resolved structure from dried crystals of 1, NiII(2,2'-bpy)3]3[{NiII(2,2'-bpy)2(H2O)}{HCoIIWVI12O40}]2·H2O (D1). Because the title compound has been characterized with a {NiII(2,2'-bpy)2(H2O)}2+ fragment coordinated to the surface of the Keggin anion ([H(CoIIW12O40]5-) via a terminal oxo group of tungsten and the [NiII(2,2'-bpy)3]2+ coordination complex cation sitting as the lattice component in the concerned crystals, the electronic spectroscopy of compound 1 has been described by comparing its electronic spectral features with those of [NiII(2,2'-bpy)2(H2O)Cl]Cl, [NiII(2,2'-bpy)3]Cl2, and K6[CoIIW12O40]·6H2O. Most importantly, compound 1 can function as a heterogeneous and robust electrochemical water oxidation catalyst (WOC). To gain insights into the water oxidation (WO) protocol and to interpret the nature of the active catalyst, diverse electrochemical experiments have been conducted. The mode of action of the WOC during the electrochemical process is accounted for by confirmation that there was no formation/participation of metal oxide during various controlled experiments. It is found that the title compound acts as a true catalyst that has NiII (coordinated to POM surface) acting as the active catalytic center. It is also found to follow a proton-coupled electron-transfer pathway (two electrons and one proton) for WO catalysis with a high turnover frequency of 18.49 (mol of O2)(mol of NiII)-1 s-1.