The chemistry of molecular nitrogen and hydrogen at transition metal centers connects to a diverse set of fields that ranges from agriculture to alternative energy. Two examples from biology are instructive. The nitrogenase enzyme catalytically converts molecular nitrogen into ammonia with concomitant generation of a stoichiometric amount of H2. Despite decades of study, the site and geometry of N2 binding to the Fe7Mo-cofactor remains unresolved. Further, this transformation is inhibited by molecular hydrogen, and similarly, how H2 interacts with the active site cofactor is unknown. Hydrogenase enzymes, which catalytically (and reversibly) oxidize molecular hydrogen at both single- and multi-metallic Fe centers, are models for the energy-efficient storage of molecular hydrogen using abundant materials. The chemistry performed by both the nitrogenase and hydrogenase enzymes occurs at active sites characterized by paramagnetic metals. Further, given the particular difficulty characterizing hydrogen ligands by x-ray methods and the challenging technical requirements of neutron diffraction, Electron Nuclear DOuble Resonance (ENDOR) spectroscopy is uniquely suited to studying this chemistry by selectively probing the nitrogenous and hydrogenous-ligand environments.
Significant effort has been put forth to synthesize inorganic complexes that mimic the behavior/structure of these enzymes. The ultimate goal is to attain a comprehensive understanding of the enzyme reaction mechanisms, and to derive similarly efficient inorganic complexes that perform the same chemistry on an industrial scale. To this end, we present an ENDOR spectroscopy study of the magnetic properties of nitrogenous ligands interacting with mono- and multi-nuclear iron centers, and an investigation of the reaction of molecular hydrogen with two paramagnetic, Jahn-Teller unstable iron and molybdenum complexes. In the former case, we have obtained hyperfine and quadrupole measurements of nitrogenous ligands that can be used as spectral references for ENDOR measurements of nitrogenase intermediates trapped in the presence of ligands representing distinct stages of N2 reduction. In the latter, the 1H electron-nuclear hyperfine coupling has been used to determine the identity of the two, unique hydrogenous ligand structures, and in the process, has unambiguously characterized the first known example of a paramagnetic metal-dihydrogen complex. Finally, the kinetics of relaxation of nitrogenase trapped during turnover of N2 is presented.
|Advisor:||Hoffman, Brian M.|
|Commitee:||Rosenzweig, Amy C., Van Duyne, Richard P.|
|School Location:||United States -- Illinois|
|Source:||DAI-B 73/04, Dissertation Abstracts International|
|Subjects:||Inorganic chemistry, Physical chemistry|
|Keywords:||Electron nuclear double resonance, Electron paramagnetic resonance, Hydrogenase, Metalloenzymes, Nitrogenase, Spectroscopy|
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