Spectroscopic Characterisation of a Bio‐Inspired Ni‐Based Proton Reduction Catalyst Bearing a Pentadentate N2S3 Ligand with Improved Photocatalytic Activity.

Inspired by the sulfur‐rich environment found in active hydrogenase enzymes, a Ni‐based proton reduction catalyst with pentadentate N2S3 ligand was synthesised. When coupled with [Ru(bpy)3]2+ (bpy=2,2′‐bipyridine) as photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared with a homologous counterpart bearing a tetradentate N2S2 ligand. The mechanistic pathway of photoinduced hydrogen evolution was comprehensively analysed through optical transient absorption and time‐resolved X‐ray absorption spectroscopy, which revealed important electronic and structural changes in the catalytic system during photoirradiation. The NiII catalyst undergoes a photoinduced metal‐centred reduction to form a NiI intermediate with distorted square‐bipyramidal geometry. Further kinetic analyses revealed differences in charge‐separation dynamics between the pentadentate and tetradentate forms. [ABSTRACT FROM AUTHOR]