Electrocatalytic production of hydrogen using nickel complexes with tridentate N3 ligands

Abstract

The octahedrally coordinated nickel(II) complexes of the type [Ni(L)2](ClO4)2 (1, 2) have been isolated and characterised, where L is (pyridin-2-ylmethyl)amine (L1, 1) and 1-(1-methyl-1H-imidazol-2-yl)-N-(pyridin-2-ylmethyl)methanamine (L2, 2). Further, the electrochemical hydrogen evolution reaction (HER) was performed in acetonitrile and proved that NiI is an active redox centre in strong acids like p-toluenesulfonic acid. Unlike the analogous Ni complexes, the present complexes exhibit higher catalytic rates (∼ 104 M−1s−1) at relatively low overpotential (600 mV) and show significant TONs (1, 15; 2, 32). In the presence of weak acid like benzoic acid, electrocatalysis occurs only from the formal Ni0 state at the expense of reduced ligand backbone at the higher cathodic potential. In striking contrast, the overpotentials even decrease further (∼300 mV) by using weak acid compared to the TsOH. The differences in the HER activity arise mainly from the pyridine and imidazole donors in 1 and 2, respectively.