Efficient electrocatalytic and photocatalytic hydrogen evolution using a linear trimeric thiolato complex of nickel

Increasing interest has been paid to the development of earth-abundant metal complexes as promising surrogates of noble-metal platinum and biological catalysts hydrogenases for catalyzing the hydrogen evolution reaction. In this study, we report on a molecular H₂-evolving catalyst based on a linear trimeric thiolato complex of nickel Ni₃(L^N2S2)₂ (L^N2S2 = N,N′-dimethyl-N-N′-bis(2-mecaptoethyl)-ethylenediaminato). Electrochemical studies showed that the trinuclear nickel complex Ni₃(L^N2S2)₂ can electrocatalyze hydrogen evolution from weakly acidic solutions with remarkable turnover frequencies (3495 s^?1 at ?1.98 V and 715 s^?1 at ?1.58 V vs SCE). An efficient noble-metal-free homogeneous photocatalytic system for hydrogen generation from water working under visible light irradiation was further constructed by using the target nickel complex as photocatalyst, fluorescein (Fl) as photosensitizer (PS), and triethylamine (TEA) as sacrificial electron donor. Our studies showed that Ni₃(L^N2S2)₂ can be used in purely aqueous solution and gave a turnover number (TON, vs catalyst) for H₂ evolution of 790, corresponding to a TOF 60 h^?1. The results show that multinuclear nickel(II) complexes are a promising new direction for molecular catalysts for the electro- and photoreduction of protons.