Design Considerations for a System for Photocatalytic Hydrogen Production from Water Employing Mixed-Metal Photochemical Molecular Devices for Photoinitiated Electron Collection

Supramolecular complexes coupling Ru(II) or Os(II) polyazine light absorbers through bridging ligands to Rh(III) or Ir(III) allow the study of factors impacting photoinitiated electron collection and multielectron water reduction to produce hydrogen. The [{(bpy)(2)Ru(dpb)}(2)IrCl2](PF6)(5) system represents the first photoinitiated electron collector in a molecular system (bpy = 2,2'-bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline). The [{(bpy)(2)Ru(dpp)}(2)-RhCl2](PF6)(5) system represents the first photoinitiated electron collector that affords photochemical hydrogen production from water in the presence of an electron donor, N,N-dimethylaniline (dpp = 2,3-bis(2-pyridyl)pyrazine). The complexes [{(bpy)(2)Ru(dpp)}(2)RhCl2](PF6)(5), [{(bpy)(2)Ru(dpp)}(2)RhBr2](PF6)(5), [{(phen)(2)Ru(dpp)}(2)RhCl2](PF6)(5), [{(bpy)(2)Os(dpp)}(2)RhCl2]-(PF6)(5), [{(tpy)RuCl(dpp)}(2)RhCl2](PF6)(3), [{(tpy)OsCl(dpp)}(2)RhCl2](PF6)(3), and [{(bpy)(2)Ru(dpb)}(2)IrCl2](PF6)(5) are herein evaluated with respect to their functioning as hydrogen photocatalysts (tpy = 2,2':6',2 ''-terpyridine, phen = 1,10-phenanthroline). With the exceptions of [{(bpy)(2)Ru(dpb)}(2)IrCl2](PF6)(5) and [{(tpy)OsCl(dpp)}(2)RhCl2](PF6)(3), all other complexes demonstrate photocatalytic activity. The functioning systems possess a rhodium localized lowest unoccupied molecular orbital that serves as the site of electron collection and a metal-to-ligand charge-transfer ((MLCT)-M-3) and/or metal-to-metal charge-transfer ((MMCT)-M-3) excited-state with sufficient driving force for excited-state reduction by the electron donor. The lack of photocatalytic activity by [{(bpy)(2)Ru(dpb)}(2)IrCl2](PF6)(5), although photoinitiated electron collection occurs, establishes the significance of the rhodium center in the photocatalytic system. The lack Of photocatalytic activity of [{(tpy)OsCl(dpp)}(2)RhCl2](PF6)(3) is attributed to the lower-energy (MLCT)-M-3 state that does not possess sufficient driving force for excited-state reduction by the electron donor. The variation of electron donor showed the photocatalysis efficiency to decrease in the order N,N-dimethylaniline > triethylamine > triethanolamine. The general design considerations for development of supramolecular assemblies that function as water reduction photocatalysts are discussed.