CHALLENGES OF ENZYMATIC N-2 FIXATION - THE CHEMISTRY OF NITROGEN, HYDROGEN, AND NITROGEN HYDRIDES IN THE COORDINATION SPHERE OF TRANSITION-METAL SULFUR COMPLEXES

In the search for low molecular complexes modeling the reactivity of [ MS] oxidoreductases, a structure-function relationship for metal-sulfu r centers has been revealed. It shows that protonation (or alkylation) of thiolate donors in [Fe(CO)(2)('S-4')] or [Fe(CO)('NHS4')] strongly reduces the electron density at the Fe centers and drastically shifts the redox potentials in the anodic direction, but leaves the metal-su lfur distances unchanged, This sheds light on the strong coupling of p roton- and electron-transfer steps at [MS] centers and the resulting f acilitation of reduction processes. Verified and potential consequence s for the heterolytic activation of H-2 and the reduction of N-2 at [M S] centers under mild conditions are reported. Full characterization o f numerous intermediates of these processes obtained with [Rh('S-4')] and [Fe('NHS4')] fragments yields insight into the mechanisms of the r espective enzymatic processes, Coordinatively unsaturated hydride, thi ol hydride, and diazene complexes are described. The molecular structu re of [Rh(H)(PCy3)('S-bu(4)')] indicates an unprecedented [(CH)(2) ... H-Rh] interaction between CH and metal-hydride hydrogen atoms.