An "All-Terrain" Ligand Design Strategy: Phosphorus-Functionalized Nickel Thiosemicarbazones for Hydrogen Evolution in Organic and Aqueous Media

Abstract

We report the synthesis and characterization of a series of mononuclear nickel thiosemicarbazone complexes bearing phosphorus-functionalized phenyl groups designed to improve their solubility and tune their catalytic properties for hydrogen evolution. NiTSC-PO(OEt)₂ and NiTSC-PO(OH)₂, which carry diethyl phosphonate and phosphonic acid groups, respectively, at the para position of the ligand phenyl rings, were investigated in dimethylformamide using trifluoroacetic acid as the proton source. NiTSC-PO(OH)₂ exhibited notable catalytic activity in aqueous acetate buffer and its photocatalytic performance was further evaluated in combination with nitrogen-doped carbon dots as photosensitizers and a mixture of tris(2-carboxyethyl)phosphine/ascorbic acid as sacrificial electron donor. The two complexes display complementary performances: NiTSC-PO(OEt)2 is the more active electrocatalyst in organic medium, whereas NiTSC-PO(OH)2 operates in water and under photocatalytic conditions. Ligand functionalization thus influences catalytic activity through both solubility and electronic effects, showing that a single substitution strategy can provide, within the same catalyst family, members suited to different reaction media.