Electronic structure determination of model complexes of [Mo-3Fe-4S] clusters and method development of in situ reductive amination using amine-boranes
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Date
2014
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Montana State University - Bozeman, College of Letters & Science
Abstract
In nature, bacteria are able to convert inert nitrogen gas to ammonia using the iron molybdenum-cofactor of the nitrogenase enzyme. Even though the crystal structure and the magnetic coupling of the iron molybdenum-cofactor cluster have been elucidated, the bonding site(s) and the mechanism of the reduction are not understood. The reactivity towards nitrogenase substrates of the iron molybdenum-cofactor from synthetic [Mo-3Fe-4S] biomimetic models has previously been examined, but these models are limited to reducing N,N single and double bonds, protons, acetylene, and acetonitrile. The reactivity of these clusters is dictated by their electronic structure, which is tuned by the chelating ligands. To study the ligand donations, a simpler cluster with a homoleptic ligand environment, the MoFe 3S 4(S 2CNEt 2) 5 complex, was synthesized and analyzed with S K-edge X-ray absorption spectroscopy. The S K-edge is convoluted due to the different sulfur environments and only plausible assignments could be inferred based on Slater's rules for effective oxidation states. Iron sulfide bonds have been characterized using S K-edge X-ray absorption spectroscopy, however, little is known regarding molybdenum sulfide bonding. The tetrahedral MoS 4 2- compound was utilized to understand the Mo-S bonding with X-ray absorption spectroscopy in conjunction with computational methods. The terminal sulfides of MoS 4 2- donate approximately five electrons to the formally +6 charge on the Mo center. Since the charge delocalization of transition metal dithiocarbamate complexes have not been studied extensively, the experimental electronic structure of Zn(II), Cu(II), Fe(II), Fe(III), and Mo(IV) dithiocarbamate complexes were examined. The dithiocarbamate ligands exhibit classical bonding in which the ionic character increases with increasing positive charge on the metal. Piecing these models together, the MoFe 3S 4(S 2CNET 2) 5 cluster contains a covalent [Mo-3Fe-4S] core with ionic dithiocarbamate ligands surrounding and stabilizing the cluster. The last part of this thesis is the development of a new method for reductive amination of dimethylhydrazones to dimethylhydrazines using amine-boranes. Conventionally, the C=N bond is reduced with toxic or less selective reducing agents. A potent, and easy to handle tert-butylamine borane was used and prepared in situ towards the synthesis of dimethylhydrazines containing different functionalities in high yields.