Light-driven hydrogen evolution with a nickel thiosemicarbazone redox catalyst featuring Ni⋯H interactions under basic conditions

Abstract

The photocatalytic hydrogen evolution inspired by the highly evolved, finely tuned molecular photo-synthetic systems in nature represents an important process in sustainable solar energy conversion for the near future. By incorporating a phosphine donor within a thiosemicarbazone moiety, a new proton reduction catalyst Ni−thioP, featuring Ni⋯H interactions was synthesized and structurally characterized. Single crystal structure analysis revealed that the C–S, C–N and N–N bond lengths were all within the normal range of the single and double bonds, suggesting the extensive electron delocalization over the ligand skeleton. The presence of Ni⋯H interactions relative to the amide group coupled with the easy proton migration pathway involving thioamide/thiolate exchange suggested that the thiosemicarbazone complexes could serve as promising candidates for proton reduction. Luminescence titrations exhibited that Ni−thioP served as efficient luminescent quenchers for the photosensitizer Fl, providing the possibilities for the excited state of Fl to activate these catalysts for proton reduction. The direct generation of hydrogen was achieved by carrying out the photolysis of a solution containing fluorescein as the photosensitizer, and triethylamine as the sacrificial and the redox catalysts. Ni−thioP exhibited high activity with a turnover number (TON) of 8000 moles of H₂ per mole of the catalyst after 24 hours and an initial TOF larger than 500 moles of H₂ per catalyst per hour. To further investigate the potential mechanism for proton reduction, calculations were also performed using density functional theory.