A high-spin Fe8S9X+ (X=N, C) cluster is used to model the reduction of molecular nitrogen to ammonia by the nitrogenase FeMo cofactor at the B3LYP/6-311G(d,p)/ECP(Fe,SDD) level of theory. A total of seventy-three structures were optimized (including three transition state optimizations) to explore the structure and energetic of N-2, C2H2, and CO coordination to the Fe8S9X+ cluster. After three protonation-reduction (PR) steps (modeled by addition of hydrogen atoms), N-2, C2H2, and CO are predicted to bind to a Fe atom in the exo (cage does not open) position with binding energies of 7.6, 14.7, and 11.7 kcal/mol. With additional PR steps the coordination number of the core nitrogen atom is reduced from six to five and the bridging thiol group becomes a terminal SH2 group. The fifth and sixth PR steps occur on the core nitrogen and the open Fe site. Coordination of N-2 is enhanced after six PR steps to give an intermediate ideally suited for a concerted dihydrogen transfer from the Fe and core nitrogen atoms to the coordinated N2. The identity of the central atom (nitrogen or carbon) has only a minor effect on the reaction steps. (C) 2007 Wiley Periodicals, Inc.
