Syntheses of Polypyridyl Co Complexes and Application to Electrochemical H2 Evolution

Abstract

Platinum is an excellent metal for electrocatalytic H2 evolution because of its high current density and excellent activities. Nevertheless, the high-cost and low abundance of platinum are problematic. In this regard, the cost-effective and earth-abundant metals such as Fe, Mn, Ni, and Co have been studied for electrocatalytic and photocatalytic H2 production catalysts. In nature, the [FeFe]-hydrogenases are well known to catalyze both the reduction and oxidation of dihydrogen. One of the key features is an amine pendant, azadithiolate, which delivers proton from the nitrogen of amine pendant to Fe center efficiently. Here, we reported the six-coordinated polypyridyl cobalt complex, [NNN(NCH3)-Co(phen)(MeCN)](PF6)2 (where NNN = 2,6-bis((4-methylpiperazin-1-yl)methyl)pyridine) and phen = 1,10-phenanthroline), for electrocatalytic H2 evolution catalyst using acetic acid as proton source in dry acetonitrile. One of the key features of [NNN(NCH3)-Co(phen)(MeCN)](PF6)2 complex is amine pendants (1-methylpiperazine) as proton delivery shuttle, which transfers the proton from the nitrogen of amine pendant to cobalt center efficiently. The other one is polypyridyl system. We can simply tune the electronic properties of the cobalt center by attaching electron-donating or withdrawing groups to polypyridyl ligands. Electrochemical studies show that the nitrogen at the secondary coordination sphere is critical in operating the electrochemical H2 evolution catalyst. Density functional theory (DFT) is on-going to provide insight into the H2 evolution mechanism of our complex.