Synthetic models for the molybdenum/iron/sulfur cofactor of nitrogenase.

Abstract

This thesis describes the chemistry of Mo-3Fe-4S cubane complexes as possible structural and functional models of the Iron-Molybdenum Cofactor of Nitrogenase, which is instrumental for the catalytic reduction of atmospheric dinitrogen into ammonia. A new class of singly-bridged double cubane (SBDC) complexes was developed. These double cubane complexes are characterized by bidentate ligands as unsupported bridges between the Mo atoms of two $\rm\lbrack(C\sb6O\sb2Cl\sb4)MoFe\sb3S\sb4Cl\sb3\rbrack \sp{2-}$ sub-units; the bridging ligands investigated include hydrazine, pyrazine, bipyridine, ethylene diamine, and cyanide. The structures of the hydrazine and pyrazine singly-bridged double cubanes have been determined and are presented. The electronic interaction of the singly-bridged double cubanes was investigated as a function of bridging ligand. Only the cyanide-SBDC displayed significant intercubane coupling. The ability of the $\rm\lbrack(C\sb6O\sb2Cl\sb4)MoFe\sb3S\sb4Cl\sb3\rbrack\sp{2-}$ cubane complex to act as a catalyst for the reduction of hydrazine to ammonia has been investigated. Hydrazine single cubane complexes, $\rm\lbrack (C\sb6O\sb2Cl\sb4)MoFe\sb3S\sb4Cl\sb3(RN\sb2H\sb3)\rbrack \sp{2-}\ (R=H,\ CH\sb3,\ and\ C\sb6H\sb5),$ have been chemically reduced with cobaltocene and lutidine hydrochloride to produce ammonia and the corresponding primary amines. The catalytic reduction of large excesses of hydrazine and mono-substituted hydrazines has been achieved using these complexes in the presence of cobaltocene. As many as 150 catalytic turnovers have been observed. In addition, the catalytic reduction of several other nitrogenase substrates, protons, azide, nitrite, and hydroxylamine, is also presented. A model scheme for the pathway of electron transfer and substrate reduction has been developed from the available data. The activation and reduction of cyanide with a doubly-bridged double cubane complex (DBDC), $\rm\lbrack ((C\sb6O\sb2Cl\sb4)MoFe\sb3S\sb4Cl\sb2)\sb2(\mu$-CN)($\mu $-S)) $\sp{5-}$ is described. This unprecedented achievement is accomplished under very specific conditions; the presence of the doubly-bridged structure is a requirement. Spectroscopic evidence for the cleavage of the C-N bond of cyanide resulting in ammonia formation is presented, and relevance of the double cubane structure to the cofactor of nitrogenase is discussed.