Modeling the nitrogenase FeMo cofactor with high-spin Fe8S9X+ (X=N, C) clusters. Is the first step for N2 reduction to NH3 a concerted dihydrogen transfer?

A high-spin Fe(8)S(9)X(+) (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), C(2)H(2), and CO coordination to the Fe(8)S(9)X(+) cluster. After three protonation-reduction (PR) steps (modeled by addition of hydrogen atoms), N(2), C(2)H(2), 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 SH(2) 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 N(2). The identity of the central atom (nitrogen or carbon) has only a minor effect on the reaction steps.