Catalysts for Solar Fuels

1. 6.55 Å N5C-Ru Y43W 4.46 Å W7 3.77 Å Y8W 8.35 Å F40W 6.27 Å Y55W 6.98 Å W57 10.42 Å R60C-Re Thermodynamic Considerations Bimetallic pathway strongly favored under most conditions Strong acid and/or less positive E° (Co3+/2+) favor monometallic route Both can be competitive under intermediate conditions Ni(cyclam) TON Fujita, E.; Creutz, C.; Sutin, N. Brunschwig, B. S. Inorg. Chem.1993, 32, 2657-2662. Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc.2007, 129, 8988-8998. Kellett, R. M.; Spiro, T. G. Inorg. Chem.1985, 24, 2373-2377. Chao, T.-H.; Espenson, J. H. J. Am. Chem. Soc.1978, 100, 129-133. Aqueous Co(P) Electrocatalysts NSF Center for Chemical Innovation: CCI Solar Interdisciplinary collaboration focused on building and understanding a self-contained water splitting system powered by the sun as a source of clean, sustainable energy Selected Nonaqueous H2 Evolution Studies Aqueous electrocatalysis (>90% Faradaic H2 yield) at moderate overpotentials (<0.6 V) Catalysis likely occurs at CoII/I interface (limited mechanistic details reported) Several electronic absorptions in UV-visible region: oxidation state sensitive photoprobes Distance (C60 and C5) = 38.4 Å Catalysts for Solar Fuels Covalently tethered or adsorbed electrocatalyst on a light-absorbing nanostructured cathode stable to (moderately) reducing conditions Electrocatalytic H2 evolution occurs near Co2+/1+ couple Simulations and thermodynamics favor bimetallic pathway Ongoing Flash-Quench Spectroscopic Studies Kellet, R.; Spiro, T. G. Inorg. Chem. 1985, 24, 2373-2377. Reductive Quenching: [Ru(bpy)3]1+ from MeODMA J. L. Dempsey, J. R. Winkler, H. B. Gray manuscript in preparation. Hu, X.; Brunschwig, B. S.; Peters, J. C. J. Am. Chem. Soc.2007, 129, 8988-8998. Connolly, P.; Espenson, J. H. Inorg. Chem.1986, 25, 2684-2688. Spectroelectrochemistry: CoI(TMPyP)┐3+ in CH3CN Nanostructured anode or adsorbed thin film electrocatalyst stable to strongly oxidizing conditions Currently investigating timescale to confirm reduction at CoII site in [Co(TMPyP)]4+ Reductive quenching of [RuII(bpy)3]2+ promising for CoI(P) generation in situ Efforts to expand range of conditions to pH 5-8 continue Bulk photolysis experiments ongoing to confirm H2 evolution via homogeneous catalyst SEC-derived difference spectrum (blue; Pt mesh, 0.1 M TBAH in MeCN) UV-vis absorption spectrum of [CoII(TM4PyP)]4+ in MeCN (orange) K. E. Plass, M. A. Filler, J. M. Spurgeon, B. M. Kayes, S. Maldonado, B. S. Brunschwig, H. A. Atwater, N. S. Lewis Adv. Mater 2009, 21, 325-328 Juris; Balzani; Barigeletti; Campagna; Belser; von ZelewskyCoord. Chem. Rev.1988, 84, 85-277. Hoffman, M. Z.; Bolletta, F.; Moggi, L.; Hug, G. L. J. Phys. Chem. Ref. Data1989, 18, 219-543. Bryan D. Stubbert, Bert T. Lai, and Harry B. Gray Division of Chemistry and Chemical Engineering, California Institute of Technology Very Long Range Membrance Electron Transfer CO2 Reduction Catalysts: Very High h Roles of Ligand-N H–Bonding Non-rigidity and Preferred Conformations Two main isomerization routes: inversion at Ni–N< N–H deprotonation to Ni–N< (Ni2+ and Ni3+) Ni–N(R)< cleavage to Ni–N< (3° NL< and Ni1+) Goal: understand tryptophan electron transfer through OmpA as model membrane protein for PSII OmpA Photosystem II Pautsch, A.; Schulz, G. E. Nat. Struct. Biol.1998, 5, 1013-1017. Maimon, E.; Zilbermann, I.; Cohen, H.; Kost, D.; van Eldik, R.; Meyerstein, D. Eur. J. Inorg. Chem.2005, 4997. Soibinet, M.; Dechamps-Olivier, I.; Guillon, E.; Barbier, J.-P.; Aplincourt, M.; Chuburu, F.; Le Baccon, M.; Handel, H. Polyhedron2005, 24, 143-150. Ikeda, R.; Soneta, Y.; Miyamura, K. Inorg. Chem. Comm.2007, 10, 590-592. Currently moving towards heterobimetallic ligands to facilitate oxygen atom or hydroxyl group transfer in a two electron process Nelson, et al.Nat. Rev. Mol. Cell Bio. 2005, 6, 818. Babini, et al.J. Am. Chem. Soc.2000, 122, 4532. Winkler, et al.Pure Appl. Chem.1999, 71, 1753. Gray, et al.Annu. Rev. Biochem.1996, 65, 537. Single pendant arm donors afford similar results: diminished reactivity and minor decrease in overpotential Catalytic CO2 Reduction: [NiL4]2+ Fisher, B.; Eisenberg, R. J. Am. Chem. Soc.1980, 102, 7361. Bhugun, I.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc.1996, 118, 1769. Grodkowski, J.; Neta, P.; Fujita, E.; Mahammed, A.; Simkhovich, L.; Gross, Z. J. Phys. Chem. A2002, 106, 4772. Benson, E. E.; Kubiak, C. P.; Sathrum, A. J.; Smieja, J. M. et al. Chem. Soc. Rev.2009, in press. Targeting Cooperative C–O Cleavage Acknowledgements Team GCEP: Kyle M. Lancaster, Keiko Yokoyama, A. KatrineMuseth, Rose Bustos Bruce Brunschwig & Jay Winkler Jillian Dempsey & Lionel Cheruzel Gray and Lewis research groups Additional funding: OmpA ET Pathway: Follow the Hopping Hole CO Dehydrogenase Ni(h1-CO2) interaction H-bond stabilization Fe for “O” transfer Jeoung & DobbekScience2007, 318, 1461 >106 × CO2:H2O selectivity!! Ethylene-bridged bis(cyclam)Ni24+ lowers overpotential Modest gain possibly at the expense of selectivity Exogenous Lewis bases (e.g., pyridine) lower overpotential Reduced pyridinium catalysis not observed N-substitution induces conformational change: counterintuitive decrease in overpotential, but diminished reactivity Shih, C.; et al. (2008) Science320, 1760-1762. Electron transfer rates: hopping mechanism >> tunneling mechanism Tryptophan residues provide launch pads and landing sites OmpA has several well-positioned residues for long range ET ET dynamics investigated with time resolved laser flash photolysis Beley, M.; Cellis, J.-P.; Ruppert, R.; Sauvage, J.-P. J. Am. Chem. Soc.1986, 108, 7641 Flash-quench laser photolysis studies (pH 5) Data consistent with CoIP generation [Ru(bpy)3]2+ bleach convolutes data (480 nm) Plausible CO2 Reduction Mechanism at Ni