Dithiolato-Bridged Nickel(II) Salicylcysteamine Complexes as Robust Proton Reduction Electrocatalysts: Cyclic...

A series of Schiff-base nickel(II) complexes were prepared from the reaction of nickel(II) acetate with N-salicylcysteamine [HO-C₆H₄-CH═N(CH₂)₂SH] ligands. These complexes were analyzed to be dimeric nickel complexes containing two bridging thiolato ligands. Using cyclic voltammetry, they were found to be efficient homogeneous proton reduction electrocatalysts when acetic acid was used as the proton source in acetonitrile. Catalysis was triggered upon electrochemical reduction of the nickel complex. In particular, rate constants (k_obs) in the range of 10⁴ s^-1 at moderate overpotentials of 0.5-0.6 V were achieved when chloro- or bromo-containing nickel complexes were used. Combined with the experimental data, density functional theory calculations lent support to an ECEC mechanism, with the first electrochemical reduction step contributing significantly to the rate-determining step.