Enhanced Homogeneous Photocatalytic Hydrogen Evolution in a Binuclear Bio-Inspired Ni-Ni Complex Bearing Phenanthroline and Sulfidophenolate Ligands.

The prominence of binuclear, bimetallic catalysts underlines the need for the design and development of diverse bifunctional ligand frameworks that exhibit tunable electronic and structural properties. Such strategies enable metal-metal and ligand-metal cooperation towards catalytic applications, improve catalytic activity, and are essential for advancing multi-electron transfers for catalytic application. In this work we present the synthesis, crystal structure, and photocatalytic properties of a binuclear Ni(II) complex, [Ni(1,10-phenanthroline)(2-sulfidophenolate)] (1). Complex 1 crystallizes in the centrosymmetric triclinic system (P-1) showing extensive intra- and inter- non-coordinated interactions. 1 is employed as a catalyst for light driven hydrogen evolution. Its catalytic efficiency in a noble-metal-free photo-driven system using fluorescein as photosensitizer and triethanolamine as the electron donor, reaches TON 2900, threefold the efficiency of the corresponding homoleptic mononuclear complex [Ni(2-sulfidophenolate)]. Efficiency rises up to 9000 TONs when thioglycolic-coated CdTe quantum dots are used as photosensitizers in the presence of ascorbic acid at pH 4.5. UV-Vis spectroscopy, dynamic light scattering techniques, and Hg-poisoning measurements reveal that 1 maintains its molecular structure during catalysis. Electrochemical studies in DMF with TFA as the proton source were also performed for the elucidation of the mechanism of its catalytic action and its stability, suggesting that the proximity of two nickel ions plays a part in the increased catalytic activity, facilitating hydrogen evolution.