The nickel-containing hydrogenases found in a variety of methanogenic and acetogenic bacteria have been shown to be characterized by electron-rich iron sulfur clusters, (i.e., a reducing environment), to be usually air- and sometimes light-sensitive, and capable of interacting directly with H2. The characteristics described are consistent with Ni in a soft-donor ligand environment familiar to the catalytic processes studied by organometallic chemists. A synthesis and mechanistic approach will be taken to examine reactions of import to bioinorganic processes which might plausibly be explained by the intermediacy of nickel hydride species. To be included are studies of (1. The synthesis of 5-coordinate nickel in a coordination environment of sulfur- and nitrogen-donor ligands, capable of supporting oxidation states of I, II and III, as well as Ni-H. Unique macrocyclic ligands based on diazacyclooctane, DACO, modified by sulfur-containing pendant arms, and possessing a sterically protected site have been prepared in preliminary work. A thorough characterization of redox chemistry and hydride/eta2-H2 binding/reactivity capability is proposed. (2. Nickel catalysis of CH4 elimination from MeSCH2CH2SH as a S-demethylation analogue of methyl-coenzyme M in order to provide a reaction model which involves electron-rich hydrogens (metal hydrides or metal hydrosulfides) as nucleophiles in the metalbound S-CH3 cleavage process. (3. The heterolytic activation of dihydrogen in the presence of sulfur containing heterobimetallic complexes of Ni. The synthesis and characterization of new discrete heterobimetallics of Ni and early transition metallocenal thiolates as well as a new Fe/Ni tetrametallic based on BMEDACO will be used in investigations of heterolytic dihydrogen cleavage/activation mechanisms as a model of the expected Ni(mu-SR)Fe sites in hydrogenase. An examination of hydride and small molecule binding by recently prepared is also proposed. (4. Studies of the mechanism of H2 release from known hydrido-thiolate complexes of Fe(II) and Ni(II) and the possibility of useful hydride transfer or hydrogenation chemistry resulting will also be approached. A successful outcome of the above work would permit the beginning of a critical assessment of the likelihood of metal hydrides as a mechanistic requirement in hydrogenase and methanogenic bacteria. Fundamental knowledge in the area of heterolytic cleavage of dihydrogen, removal of alkyl groups from metal bound sulfur, and the small molecule binding ability of Ni, hetero-ligated by both hard and soft donor sites, will be advanced.
