Abstract:

In recent years, there has been an intense effort to develop renewable alternatives to fossil fuels for meeting the ever-increasing global energy need. Molecular dihydrogen (H₂) is the ideal energy carrier for the 21st century because it has high energy density and its combustion releases only water, and electrocatalysis is a powerful tool for its wide use. Developing new H₂-evolving molecular electrocatalysts with cheap and earth-abundant elements is highly desirable. Among all kinds of H₂-generating catalysts, [NiFe]-hydrogenases (H₂ases) have the active site featuring a redox-active {Ni(cysteinate)₄} center bridged through two of its cysteine residues to a redox-inactive {Fe(CN₂)(CO)} moiety. As a class of known natural enzymes, [NiFe]-H₂ases are promising candidates because they have inexpensive nickel and/or iron atoms at the active sites and can catalyze the reversible reduction of H⁺ to H₂ with high efficiency comparable to the noble-metal platinum. However, the catalytic behaviors of most artificial H₂ases-like active sites are usually inhibited by the existence of a small amount of O₂, which strongly limit their practical application. As such, it is attractive to develop new analogues of enzyme active sites to address this issue. On the other hand, [NiFeSe]-H₂ases, which are obtained by the introduction of Se into [NiFe]-H₂ases, have exceptional properties conducive for H₂ production, such as high H₂ generation performance, marginal inhibition by H₂, and high tolerance to O₂. The mechanistic understanding of [NiFeSe]-H₂ases function guides the design and synthesis of Se-substituted Ni-based molecular catalysts, and selection of suitable bio-inspired catalysts enables applications in catalysis for hydrogen evolution reaction (HER). In this contribution, six bio-inspired neutral nickel-based complexes (2a–2c, 3a–3b, 4) with diselenolate derivatives and diphosphine ligands have been prepared and structurally characterized. These complexes are important in the function of [NiFeSe]-hydrogenase models toward their application as electrocatalysts for the HER. The substituent effects of diselenolate and diphosphine ligands on the catalytic activities of hydrogen production by these nickel(Ⅱ) complexes are studied experimentally. When using a glassy carbon electrode, all the complexes are efficient electrocatalysts for H₂ production with different turnover frequencies (TOFs) of 12182 s^-1 (2a), 15385 s^-1 (2b), 20359 s^-1 (2c), 106 s^-1 (3a), 794 s^-1 (3b), 13580 s^-1 (4). The present results indicate that the nickel(Ⅱ) complex 2c ligated by a 4, 5-dimethyl-1, 2-benzenediselenolate and 1, 1'-bis(diphenylphosphino)ferrocene ligand, shows the highest efficiency, which surpasses the activity of a previously dppf-supported nickel(Ⅱ) 1, 2-benzenediselenolate with a TOF of 7838 s^-1. We believe that our results will encourage the development of the design of highly efficient Ni-based selenolate molecular catalysts.

Key words: Electrocatalytic hydrogen evolution, Nickel-based complex, Molecular catalysis, Diselenolate derivative, Diphosphine ligand