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

Graphical Nickel is the answer: A noble-metal-free molecular system that comprises a Ni complex that bears internal bases, fluorescein as a cheap light harvester, and triethylamine, displays a turnover number up to 3230 in H 2 evolution in aqueous solutions. A Ni complex with internal bases that contain bipyridine-derived ligands, [Ni(L) 2 (H 2 O) 2 ](BF 4 ) 2 ([ 1 ](BF 4 ) 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 ](BF 4 ) 2 ([ 2 ](BF 4 ) 2 , L′=2-(2-pyridyl)quinoline), were synthesized and characterized. The electrochemical properties of these complexes were studied in CH 3 CN, H 2 O, and a mixture of EtOH/H 2 O. 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.