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Research Summary
The research covered in our group is highly varied and ranges from understanding the roles that zinc plays in biological systems to the influence that "ansa bridges" exert on the chemistry of metallocene complexes.
The importance of the former topic is illustrated by the fact that zinc is a constituent of more than 300 enzymes. The active sites of these enzymes feature a zinc center attached to the protein backbone by three or four amino acid residues, the nature of which influences the specific function of the enzyme. In order to understand why different zinc enzymes utilize different amino acid residues at the active site, it is necessary to understand how and why the chemistry of zinc is modulated by its coordination environment. Answers to these questions are being provided by a study of synthetic analogues of zinc enzymes, i.e., small molecules that resemble the enzyme-active sites.
Metallocene complexes presently play prominent roles as reagents in organic syntheses and as catalysts for olefin polymerization. Significantly, the incorporation of an ansa bridge (which links the two cyclopentadienyl groups together) modulates the chemistry of metallocene derivatives, and we are presently ascertaining the reasons for the profound differences. As an illustration of the reactivity differences, we have demonstrated that a [Me2Si] linker promotes CH, CC, and CS cleavage reactions in molybdenocene chemistry.
In each case, the problems are studied by a combined experimental approach that involves synthesis, structural characterization by Xray diffraction, and a detailed study of reactivity and mechanism by using multinuclear NMR spectroscopy. In addition to experimental studies, further insight into the systems under investigation is provided by the use of computational methods. For example, we have used density functional theory calculations to explain why the WH bond energies in W(PMe3)4Hs2X2 are influenced by the nature of X, increasing in the sequence I < Br < Cl < F. This observation is of particular interest because MH bond energies are influenced in the opposite way in Ir(PPh3)2(CO)(X)H2 derivatives.
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“Hydride, Halide, Methyl, Carbonyl, and Chalcogenido Derivatives of Permethylmolybdenocene” by Jun Ho Shin, David G. Churchill, Brian M. Bridgewater, Keliang Pang, and Gerard Parkin,* Inorg. Chim. Acta, in press (invited contribution in honor of Brian James on the occasion of his 70th birthday)
“Thiolate Exchange in [TmR]ZnSR’ Complexes and Relevance to the Mechanisms of Action of Thiolate Alkylation Reactions Involving Zinc Enzymes and Proteins” by Jonathan G. Melnick, Guang Zhu, Daniela Buccella, and Gerard Parkin,* J. Inorg. Biochem., in press
“Valence , Oxidation Number and Formal Charge: Three Related but Fundamentally Different Concepts” by Gerard Parkin, J. Chem. Educ., in press
“Synthesis and Structural Characterization of [h3–B,S,S–B(mimR)3]Ir(CO)(PPh3)H (R = But, Ph) and [h4–B(mimBut)3]M(PPh3)Cl (M = Rh, Ir): Analysis of The Bonding in Metal Borane Compounds” by Victoria K. Landry, Jonathan G. Melnick, Daniela Buccella, Keliang Pang, Joseph C. Ulichny, and Gerard Parkin,* Inorg. Chem., in press
“Carbon–Hydrogen versus Carbon–Chalcogen Bond Cleavage of Furan, Thiophene and Selenophene by Ansa Molybdenocene Complexes” by David G. Churchill, Brian M. Bridgewater, Guang Zhu, Keliang Pang, and Gerard Parkin, Polyhedron, 25, 499-512 (2006)
“Synthesis and Structural Characterization of M(PMe3)3(O2CR)2(OH2)H2 (M = Mo, W): Aqua–Hydride Complexes of Molybdenum and Tungsten” by Guang Zhu and Gerard Parkin, Inorg. Chem., 44, 9637-9639 (2005)
“Synthesis and Structural Analysis of Bis(2–hydroxyphenyl)phenylamine, PhN(o–C6H4OH)2: Comparison with Tris(2-hydroxyphenyl)amine N(o–C6H4OH)3” by Bryte V. Kelly, Joseph M. Tanski, Mary Beth Anzovino and Gerard Parkin, J. Chem. Crystallogr., 35, 969-981 (2005)
“Intramolecular N–H•••S Hydrogen Bonding in the Zinc Thiolate Complex [TmPh]ZnSCH2C(O)NHPh: A Mechanistic Investigation of Thiolate Alkylation as Probed by Kinetics Studies and by Kinetic Isotope Effects” by Melissa M. Morlok, Kevin E. Janak, Guang Zhu, Duncan A. Quarless, and Gerard Parkin J. Am. Chem. Soc. , 127, 14039-14050 (2005)
“Reexamination of Lead(II) Coordination Preferences in Sulfur-Rich Sites: Implications for a Critical Mechanism of Lead Poisoning” by John S. Magyar, Tsu-Chien Weng, Charlotte M. Stern, David F. Dye, Brian W. Rous, John C. Payne, Brian M. Bridgewater, Ana Mijovilovich, Gerard Parkin, Jeffrey M. Zaleski, James E. Penner-Hahn, and Hilary Arnold Godwin J. Am. Chem. Soc., 127, 9495-9505 (2005)
“Multidentate Aryloxide and Oxo–Aryloxide Complexes of Antimony: Synthesis and Structural Characterization of [h4–N(o–C6H4O)3]Sb(OSMe2), {{[ h3–N(o–C6H4OH)(o–C6H4O)2]Sb}2(m2–O)}2 and {[h3–PhN(o–C6H4O)2]Sb}4(m?–O)2” by Joseph M. Tanski, Bryte V. Kelly and Gerard Parkin, Dalton Trans., 2442-2447 (2005)
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