Efficient photocatalytic hydrogen production in a single-component system using Ru,Rh,Ru supramolecules containing 4,7-diphenyl-1,10-phenanthroline.

Light-to-chemical energy conversion schemes are of considerable interest in the quest for renewable, alternative energy resources. One approach to this issue is the photochemical reduction of H2O to H2. Converting solar energy to H2 fuel through H2O splitting is challenging as it involves separating charges to generate a potential at the molecular scale, collecting multiple reducing equivalents to provide for energetically feasible reactions, and breaking and forming chemical bonds. Molecular photocatalysts are attractive as the basic redox, spectroscopic, and photophysical properties can be tuned at the molecular level to understand system function using traditional techniques. Herein we report an efficient, high-turnover single-component system for H2O reduction to H2 with superior photocatalytic functioning. This system uses a single molecule to absorb light, generate charge separation, multiply reduce a metal center and deliver reducing equivalents to H2O to produce H2. Multi-component H2 photocatalysts provide the backdrop for the development of single-component photocatalysts. Multi-component photosystems incorporating separate molecular light absorbers (LA), electron relays, and catalysts have been developed. A photosystem comprising a [Ru(bpy)3] LA, [Rh(bpy)3] electron relay, Pt catalyst, and triethanolamine electron donor (ED) produces H2 from H2O with a turnover number (TON) of 80 (per Rh site) when irradiated at l = 450! 20 nm (bpy = 2,2’-bipyridine). Recently, multi-component systems have been reported that utilize a cyclometalated Ir LA with a Rh or Pt catalyst, as well as a Pt LA with a Co catalyst. Covalent coupling of multiple subunits whose individual acts lead to a complex function provides for supramolecular complexes acting as molecular devices. Photoinitiated electron collectors (PECs) are supramolecules that use photons to multiply reduce an electron collector (EC). The first device for PEC, [{(bpy)2Ru(dpb)}2IrCl2](PF6)5 (dpb = 2,3-bis(2-pyridyl)benzo-quinoxaline) collects reducing equivalents on the dpb p* orbitals functioning through electronic decoupling of the two Ru LA by the intervening Ir site. [(phen)2Ru(BL)Ru(phen)2](PF6)4 collects electrons on the BL p* orbitals using a remote acceptor decoupled from optical excitation (phen = 1,10-phenanthroline; BL = bridging ligand = tatpp, tatpq). [(bpy)2Ru(pbn)] couples a Ru LA to a NAD/NADH (NAD = nicotinamide adenine dinucleotide) model ligand that undergoes proton-coupled, twoelectron reduction to generate [(bpy)2Ru(pbnHH)] . These pioneering PEC systems store electrons on ligands but do not perform multi-electron photocatalysis or produce H2 fuel. The use of supramolecular architecture allows for the coupling of the light absorbers to the catalytically reactive metals, providing unique structural motifs functioning as single-component H2 photocatalysts. [2d,e] Rh-centered PECs [{(TL)2Ru(dpp)}2RhX2] 5+ (TL = terminal ligand = bpy, phen; X = Cl or Br; dpp = 2,3-bis(2-pyridyl)pyrazine) photocatalyze H2 from H2O with F" 0.01. Bimetallic [(TL)2Ru(dpp)RhCl2(TL’)] photoreduce but dimerize in the Rh form preventing photocatalysis. Photochemical H2 production from hydrohalic acids with a F" 0.01 and 27 turnovers per hour in the first 3 h use [Rh2 (dfpma)3(PPh3)(CO)] (dfpma = MeN(PF2)2), with a halogen atom trap. 10] Ru,Pt bimetallic systems photocatalyze H2 production from H2O. Ru,Pd bimetallic systems photochemically produce H2 providing 56 turnovers in ca. 30 h. Recent studies on Pt and Pd systems suggest that colloidal metal formation, due to Pt or Pd decomplexation, may be catalytically active. Znporphyrin LA coupled to a bio-inspired diiron catalyst photocatalyzes H2 production with a low turnover. The [(bpy)2Ru(L-pyr)Co(dmgBF2)2(OH2)] affords 56 turnovers in 4 h. The Ir-based system, [(ppy)2Ir(pyr)Co(dmgBF2)2(OH2)] + (ppy = 2-phenylpyridine), demonstrates 200 turnovers in 15 h. Generation of an Os,Rh bimetallic complex in situ provides 87 turnovers and F = 0.007. It is clear from initial studies that single-component systems that function as LA and catalysts for H2O reduction to H2 can be prepared and modification of molecular design can enhance functioning. Few systematic studies are undertaken and surprising results are often encountered. A stable, efficient singlecomponent system for H2 production remains elusive requiring sustained work in this field. Reported herein are [{(Ph2phen)2Ru(dpp)}2RhCl2](PF6)5 and [{(Ph2phen)2Ru(dpp)}2RhBr2](PF6)5 (Ph2phen = 4,7diphenyl-1,10-phenanthroline; Figure 1), two new supramolecules that undergo PEC and photocatalyze the reduction of H2O to H2 with substantially enhanced photocatalytic efficiency and high photocatalytic system turnover. This substantial enhancement in catalytic functioning through use of [*] T. A. White, S. L. H. Higgins, Dr. S. M. Arachchige, Prof. K. J. Brewer Department of Chemistry, Virginia Tech Blacksburg, VA 24061-0212 (USA) E-mail: kbrewer@vt.edu [**] Acknowledgements is made to the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, U.S. Department of Energy DE FG02-05ER15751 for their generous financial support of our research. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201105170.