Supramolecular Organometallic Chemistry in Separations and Catalysis

A project supported by the U. S. Department of Energy

Principal Investigator: Thomas B. Rauchfuss

Coworkers:
James Lansing, B.S. University of Delaware
Chris Lekto, B.S. University of California-Riverside
Brian Manor, B.S. Kent State University
Mark Ringenberg, B.S. University of Pittsburgh
Aaron Royer, B.S. Indiana University
Todsapon Thananatthanachon, Ph.D. Washington University

Recent alumni:
Julie Boyer, Postdoctoral Associate, Brookhaven National Lab
Jonathan G. Breitzer, Assistant Professor, Fayetteville College
Stephen M. Contakes, Assistant Professor, Westmont College
Amanda L. Eckermann, Res. Asst. Prof, Northwestern Univ.
Zachariah Heiden, Ontario Postdoctoral Fellow, University of Toronto
Kevin Klausmeyer, Associate Professor, Baylor University
Matt Kuhlman, Research Scientist, GE
Swarnalatha Kokatam, Postdoctoral Associate, University of Basel
Didier Morvan, Research Scientist, UFP, Lyon, France
Robert F. Pafford, Research Scientist, Goodyear
Andrew C. Moreland, Research Scientist, Albemarle Catalysts
Maya Ramesh, with daughter
Daniel E. Schwarz, Research Scientist, Nalco
C. Matthew Whaley, Research Scientist, Dow Chemical Co.
Markus Wunder, Research Scientist, Ohmx

Overview.
In this project we apply organometallic chemistry to basic issues related to energy processing, catalysis, and environmental remediation. The era of examining simple unfunctionalized ligands attached to one or a few metals is mature, and growth is associated with the molecules of greater complexity and functionality, as embodied in the areas of supramolecular chemistry and nanoscience. Molecules are larger than before and expectations for selectivity and green-ness are high. Synthesis continues to play a major role, but is increasingly guided by information on weaker forces and by in situ techniques that probe transformations on the fly.

• Enzyme-Inspired Organometallics. Nature makes good use of organometallic chemistry for transformations of the energy-relevant substrates CO and H₂. For example, an iron acyl thiolate center defines the active site of the enzyme Hmd (methylenetetrahydromethanopterin dehydrogenase, see bottom left image in figure). This enzyme catalyzes the heterolytic activation of H₂ to effect reduction of a carbocationic substrate. Our progress in preparing models, indicated below, is highly versatile since the thioester can be generated in many forms.
  
  
  
• Non-innocent Ligands in Catalysis. Non-innocent describes ligands that undergo redox at potentials similar to those of the attached metals. Thus, both the metal and the ligand can undergo redox. Nature uses such ligands extensively, for example in O₂ activation. Coordination chemists have been interested in them for decades, but surprisingly little work has been aimed at their use in catalysis. We have developed a family of half-sandwich complexes of non-innocent ligands for use in activation of hydrogen and related small molecules. The ligands are derived from 2-aminophenol, which is a flexible scaffold.
• Second Coordination Sphere Control with 2-Pyridones. We are examining the role of 2-hydroxypyridines (2-pyridones) in catalysis and coordination chemistry. Pyridines are well known as ligands, and the presence of the 2-hydroxy group presents an easy and versatile method for influencing the second coordination sphere of the associated complex. We are also studying the species 2-hydroxypyridine-6-acetic acid, an analogue of cofactor GP, which is involved in the methanogenesis pathway. A family of coordination complexes have been prepared, and the main focus is on structural models for biological H₂-transfer catalysts.
• Metal-Amine-Hydrides as Prototype Catalysts for Fuel Cells. The very simple reaction O₂ + H₂ is both exothermic and slow in the absence of catalysts. We suggest that by elucidating pathways to accelerating this reaction, one might learn information that would inspire the design of new catalysts for fuel cells. Our main discovery is that Cp*Ir(NR₂)(NH₂R)H are excellent catalysts for this reaction. A key aspect of the catalysis is the participation of the N-H center in the hydrogenation of the dioxygen (See Figure).
  
  Figure: Optical spectra for the hydrogenation of O₂ by Cp*IrH(TsDPEN).
  
  
  
• Container molecules. Our container project is premised, broadly speaking, on the expectation that geometrically rigid, kinetically robust container molecules will increasingly become part of our technological culture. Machines of the future will rely on these molecules for plumbing and packaging. In this project we are preparing large organometallic cage compounds based on CN-linked multimetallic arrays. Well defined, kinetically robust, container molecules are difficult to prepare. Synthetic chemists are expected to elucidate design rules that will guide this effort. And simply being large is not enough, the molecules must be smart (functional) and in our hands at least, soluble. We discovered a templated method to induce condensation of CpCo(CN)₃^- and Cp*Ru(MeCN)₃⁺ to quantitatively produce the organometallic box [CpCo(CN)₃]₄[Cp*Ru]₄:
  
  
  This species display a pronounced affinity for Cs⁺ even in the presence of substantial excess of K⁺. ¹³⁷Cs⁺ in the presence of excess K⁺ presents an environmental problem associated with clean-up of nuclear wastes.

Representative Recent Publications

"Activation and Deactivation of Cp*Ir(TsDPEN) Hydrogenation Catalysts in Water" C. S. Letko, Z. M. Heiden, T. B. Rauchfuss, Eur. J. Inorg. Chem. 2009, 4927-4930.

"Oxidative Addition of Thioesters to Fe(0): Active-Site Models For Hmd, Nature’s Third Hydrogenase" A. M. Royer, T. B. Rauchfuss, D. L. Gray, Organometallics 2009, 28, 3618-3620.

"Proton-Assisted Activation of Dihydrogen: Mechanistic Aspects of Proton-Catalyzed Addition of H2 to Ru and Ir Amido Complexes," Z. M.Heiden, T. B. Rauchfuss, J. Am Chem. Soc. 2009, 131, 3593-3600.

"Redox-Activation of Alkene Ligands in Platinum Complexes with Non-innocent Ligands," J. L.Boyer, T. R. Cundari, N. J. DeYonker, T. B. Rauchfuss, S. R. Wilson, Inorg. Chem., 2009, 49, 638-645.

"Supramolecular Architectures Based on Organometallic Half-sandwich Complexes", T. B. Rauchfuss, K. Severin, in Molecular Nanostructures, J. Atwood, J. Steed, eds. Wiley-VCH, 2008, 179-201.

"Homogeneous Catalytic Hydrogenation of Dioxygen: a Step Toward an Organometallic Fuel Cell?" Z. M. Heiden, T. B. Rauchfuss, 2007, 129, 14303-14310.

"Cyanometallate Cages with Exchangeable Terminal Ligands", J. L. Boyer, H. Yao, M. L. Kuhlman, T. B. Rauchfuss, S. R. Wilson, Eur. J. Inorg. Chem. (invited paper for 10th anniversary issue) 2007, 2721-2728.

"Evolution of Organo-cyanometallate Cages: Supramolecular Architectures and New Cs⁺-specific Receptors", J.L. Boyer, M.L. Kuhlman and T.B. Rauchfuss, Accounts of Chemical Research, 2007, 40, 233-242.

"Redox-Switched Complexation/Decomplexation of K⁺ and Cs⁺ by Molecular Cyanometalate Boxes", J. L. Boyer, M. Ramesh, H. Yao, T. B. Rauchfuss, S. R. Wilson, J. Am. Chem. Soc. 2007, 129, 1931-1936.

"Proton-induced Lewis Acidity of Unsaturated Iridium Amides", Z. M. Heiden and T. B. Rauchfuss, J. Am. Chem. Soc. 2006, 128, 13048-13049.

"Competing H-H, S-S, and M-M Bond Formation in [Ru₄S₃(arene)₄]z Clusters" M. L. Kuhlman, T. B. Rauchfuss, Angew. Chem. Int. Ed. 2004, 43, 6742-6745.

"Synthesis and Characterization of the Hexagonal Prismatic Cage {THF[PhB(CN)₃]₆[Cp*Rh]₆}⁶⁺", M. L. Kuhlman, H. Yao, T. B. Rauchfuss, Chem. Commun. 2004, 1370-1.

"Hybrid Cluster-Cages Formed via Cyanometallate Condensation: CsCo₄Ru₆S₂(CN)₁₂, Co₄Ru₉S₆(CN)₉, and Rh₄Ru₉S₆(CN)₉ Frameworks", M. L. Kuhlman, T. B. Rauchfuss, Inorg. Chem. 2004, 43, 430-435.

"Research on Soluble Metal Sulfides: From Polysulfido Complexes to Functional Models for the Hydrogenases" T. B. Rauchfuss, Inorg. Chem. 2004, 43, 14-26.