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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)IrCl(2)](PF(6))(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)RhCl(2)](PF(6))(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)RhCl(2)](PF(6))(5), [{(bpy)(2)Ru(dpp)}(2)RhBr(2)](PF(6))(5), [{(phen)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5), [{(bpy)(2)Os(dpp)}(2)RhCl(2)](PF(6))(5), [{(tpy)RuCl(dpp)}(2)RhCl(2)](PF(6))(3), [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3), and [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(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)IrCl(2)](PF(6))(5) and [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(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 ((3)MLCT) and/or metal-to-metal charge-transfer ((3)MMCT) 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)IrCl(2)](PF(6))(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)RhCl(2)](PF(6))(3) is attributed to the lower-energy (3)MLCT 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.

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Photocatalytic generation of hydrogen from water is an integral part of the next generation clean fuel technologies. The conversion of solar energy into useful chemical energy is of great interest in contemporary investigations. The splitting of water is a multi-electron process involving the breaking and making of chemical bonds which requires multi-component photocatalytic systems. Supramolecular complexes [{(TL) 2 Ru(BL)} 2 RhX 2 ](Y) 5 (where TL ¼ terminal ligand, BL ¼ bridging ligand, X ¼ Cl À or Br À , and Y ¼ PF 6À or Br À ) have been synthesized and studied for their light absorbing, electrochemical and photocatalytic properties. The supramolecular complexes in this investigation are multi-component systems comprised of two ruthenium based light absorbers connected through bridging ligands to a central rhodium, which acts as an electron collecting center upon excitation. These complexes absorb light throughout the ultraviolet and visible regions of the solar spectrum. The supramolecular complexes possess ruthenium based highest occupied molecular orbitals (HOMO) and a rhodium based lowest unoccupied molecular orbital (LUMO). These molecular devices have been investigated and shown to function as photoinitiated electron collectors at the reactive rhodium metal center, and explored as photocatalysts to generate hydrogen from water in an aqueous solution in the presence of an electron donor.

Covalent coupling of Ru(II) light absorbers to a Rh(III) electron collecting site through polyazine bridging ligands affords photocatalytic production of H(2) in the presence of visible light and a sacrificial electron donor. A robust photocatalytic system displaying a high turnover of the photocatalyst has been developed using the photoinitiated electron collector [{(bpy)(2)Ru(dpp)}(2)RhBr(2)](5+) (bpy=2,2'-bipyridine; dpp=2,3-bis(2-pyridyl)pyrazine) and N,N-dimethylaniline in DMF/H(2)O. Studies have shown that increased [DMA], the headspace volume, and the use of DMF solvent improves the systems performance and stability providing mechanistic insight into the deactivation routes of the photocatalytic system. Photolysis of the system at 460 nm generates 20 mL of H(2) in 19.5 h with a maximum Φ=0.023 based on H(2) produced and an overall Φ=0.014 and 280 turnovers of the photocatalyst. The photocatalytic system also displays long-term photostability with 30 mL of H(2) generated and 420 turnovers in 50 h under the same conditions. Prolonged photolysis provides 820 mol H(2) per mole of catalyst.

The mixed-metal supramolecular complexes [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 (tpy = 2,2':6',2''-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were synthesized and characterized. These complexes contain ruthenium bridged by tppz to platinum centers to form stereochemically defined linear assemblies. X-ray crystallographic determinations of the two complexes confirm the identity of the metal complexes and reveal intermolecular interactions of the Pt sites in the solid state for [(tpy)Ru(tppz)PtCl](PF6)3 with a Pt...Pt distance of 3.3218(5) A. The (1)H NMR spectra show the expected splitting patterns characteristic of stereochemically defined mixed-metal systems and are assigned with the use of (1)H-(1)H COSY and NOESY. Electronic absorption spectroscopy displays intense ligand-based pi --> pi* transitions in the UV and MLCT transitions in the visible. Electrochemically [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 display reversible Ru (II/III) couples at 1.63 and 1.83 V versus Ag/AgCl, respectively. The complexes display very low potential tppz (0/-) and tppz(-/2-) couples, relative to their monometallic synthons, [(tpy)Ru(tppz)](PF6)2 and [Ru(tppz)2](PF6)2, consistent with the bridging coordination of the tppz ligand. The Ru(dpi) --> tppz(pi*) MLCT transitions are also red-shifted relative to the monometallic synthons occurring in the visible centered at 530 and 538 nm in CH3CN for [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4, respectively. The complex [(tpy)Ru(tppz)PtCl](PF6)3 displays a barely detectable emission from the Ru(dpi) --> tppz(pi*) (3)MLCT in CH 3CN solution at RT. In contrast, [ClPt(tppz)Ru(tppz)PtCl](PF6)4 displays an intense emission from the Ru(dpi) --> tppz(pi*) (3)MLCT state at RT with lambda max(em) = 754 nm and tau = 80 ns.

Coupling a reactive metal to light absorbers affords molecular devices for photoinitiated electron collection and photocatalytic conversion of substrates to fuels. A new Ru(II),Pt(II) tetrametallic supramolecule, [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6), and the trimetallic precursors, [{(phen)(2)Ru(dpp)}(2)RuCl(2)](PF(6))(4) and [{(phen)(2)Ru(dpp)}(2)Ru(dpq)](PF(6))(6), have been synthesized, and their redox, spectroscopic, spectroelectrochemical, photophysical and photocatalytic properties studied. They efficiently absorb UV and visible light. The electrochemistry of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6) suggests a lowest-lying terminal Ru→dpq charge-separated state that quenches the emission of the parent complex with non-unity population of the emissive (3)MLCT excited state. Photolysis of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](6+) at 470 nm with DMA gives multielectron reduction, storing electrons in a new manner on the central (dpp)(2)Ru(II)(dpq) moiety. Addition of H(2)O to the photolysis system produces 21 μmol of H(2) in 5 h, with 115 turnovers of the tetrametallic photocatalyst.