Nickel Complex with Internal Bases as Efficient Molecular Catalyst for Photochemical H2 Production

A Ni complex with internal bases that contain bipyridine-derived ligands, [Ni(L)(2)(H2O)(2)](BF4)(2) ([1](BF4)(2), L=2-(2-pyridyl)-1,8-naphthyridine), and a reference complex that bears analogous bipyridine-derived ligands but without an internal base, [Ni(L)(3)](BF4)(2) ([2](BF4)(2), L=2-(2-pyridyl)quinoline), were synthesized and characterized. The electrochemical properties of these complexes were studied in CH3CN, H2O, and a mixture of EtOH/H2O. The fluorescence spectroscopic studies suggest that both dynamic and the sphere-of-action static quenching exist in the fluorescein Fl(2-)/[1](2+) and Fl(2-)/[2](2+) systems. These noble-metal-free molecular systems were studied for photocatalytic H-2 generation. Under optimal conditions, the turnover number of H-2 evolution reaches 3230 based on [1](2+), whereas [2](2+) displays only approximately one third of the turnover of [1](2+). A plausible mechanism for the catalytic H-2 generation by [1](2+) is presented based on DFT calculations.