Subtle Changes to Peripheral Ligands Enable High Turnover Numbers for Photocatalytic Hydrogen Generation with Supramolecular Photocatalysts

The photo catalytic generation of hydrogen (H-2) from protons by two cyclometalated, ruthenium-platinum polypyridyl,complexes, [Rn(bpy)(2)(2,5-bpp)PtIS](2+) (1),and [Ru(dceb)(2)(2,5-bpp)PtIS](2+) (2) [where bpy = 2.2'-bipyridine, 2,5-bpp = 2,2',5',2 ''-terpyridine, dceb = 4,4',-di(carboxyethyl)bipyridine, and S = solvent], is reported. Turnover numbers (TONs) for H-2 generation were increased by nearly an order of magnitude by the introduction of carboxyethyl ester units,i.e., from 80 for 1P to 650 for 2P after 6 h of irradiation, with an early turnover frequency (TOF)- increasing from 15 to 200 h(-1). The TON and TOF values for 2P are among the highest reported to date for supramolecuular photocatalysts., The increase correlates with stabilization of the excited states localized the peripheral ligands of the light harvesting Ru-II center.