Highly efficient photocatalytic hydrogen evolution from nickel quinolinethiolate complexes under visible light irradiation

Earth-abundant metal complexes have emerged as promising surrogates of platinum for catalyzing the hydrogen evolution reaction (HER). In this study, we report the design and synthesis of two novel nickel quinolinethiolate complexes, namely [Ni(Hqt)2(4, 4′-Z-2, 2′-bpy)] (Hqt = 8-quinolinethiol, Z = H [1] or CH3 [2], bpy = bipyridine). An efficient three-component photocatalytic homogeneous system for hydrogen generation working under visible light irradiation was constructed by using the target complexes as catalysts, triethylamine (TEA) as sacrificial electron donor and xanthene dyes as photosensitizer. We obtain turnover numbers (TON, vs. catalyst) for H2 evolution of 5923/7634 under the optimal conditions with 5.0 × 10−6 M complex 1/2 respectively, 1.0 × 10−3 M fluorescein and 5% (v/v) TEA at pH 12.3 in EtOH/H2O (1:1, v/v) mixture after 8 h irradiation (λ > 420 nm). We discuss the mechanism of H2 evolution in the homogeneous photocatalytic system based on fluorescence spectrum and cyclic voltammetry data. The novel nickel quinolinethiolate complexes displayed efficient hydrogen evolution in a noble-metal-free photocatalytic system under visible light. [Display omitted] •Two novel nickel quinolinethiolate complexes based photocatalysts (1, 2) is reported.•H2 evolution activity of 1, 2 reached 5923/7634 TON in the noble-metal-free system.•A H2 evolution mechanism of heterocoupling between NiH− and NH+ is proposed.•The elaborate nickel photocatalysts are efficient and stable.