Photochemical hydrogen production from water using the new photocatalyst [{(bpy)2Ru(dpp)}2RhBr2](PF6)5

The synthesis, electrochemical, and photochemical properties of the new photocatalyst [{(bpy)2Ru(dpp)}2RhBr2](PF6)5 (bpy = 2,2′-bipyridine, dpp = 2,3-bis(2-pyridyl)pyrazine) which couples two ruthenium light absorbing metal centers to a reactive rhodium center is reported. The substitution of the bromide in lieu of the chloride in the RhX3·nH2O starting material provides the bromo analog in high yield and purity. The basic chemical properties of the bromide complex are very similar to the chloride system [{(bpy)2Ru(dpp)}2RhCl2](PF6)5, being an efficient metal-to-ligand charge-transfer (MLCT) light absorber in the visible while possessing a lowest lying metal-to-metal charge-transfer (3MMCT) state. In the presence of an electron donor and visible light, [{(bpy)2Ru(dpp)}2RhBr2](PF6)5 undergoes excited state intramolecular electron transfer affording photoinitiated electron collection on the reactive rhodium center to generate the RhI complex, [{(bpy)2Ru(dpp)}2RhI](PF6)5. The new complex affords photocatalytic hydrogen production from water with Φ ≈ 0.01, generating higher hydrogen yields than the chloride analog, 10.9 versus 8.2 μmol H2 over a 4 h photolysis period.