Proton-Reduction Reaction Catalyzed by Homoleptic Nickel-bis-1,2-dithiolate Complexes: Experimental and Theoretical Mechanistic Investigations

A series of homoleptic monoanionic nickel dithiolene complexes [Ni(bdt)(2)](NBu4), [Ni(tdt)(2)](NBu4), and [Ni(mnt)(2)](NBu4) containing the ligands benzene-1,2-dithiolate (bdt(2-)), toluene-3,4-dithiolate (tdt(2-)), and maleonitriledithiolate (mnt(2-)), respectively, were employed as electrocatalysts in the hydrogen-evolution reaction with trifluoroacetic acid as the proton source in acetonitrile. All complexes are active catalysts with TONs reaching 113, 158, and 6 for [Ni(bdt)(2)](NBu4), [Ni(tdt)(2)](NBu4), and [Ni(mnt)(2)](NBu4), respectively. The Faradaic yield for the hydrogen evolution reaction reaches 88% for 2(-), which also displays the minimal overpotential requirement value (467 mV) within the series. Two pathways for H-2 evolution can be hypothesized that differ in the sequence of protonation and reduction steps. DFT calculations are in agreement with experimental data and indicate that protonation at sulfur follows the reduction to the dianion. Hydrogen evolves from the direduced-diprotonated form via a highly distorted nickel hydride intermediate. The effects of acid strength and concentration in the hydrogen-evolving mechanism are also discussed.