Modeling the nitrogenase FeMo cofactor with high‐spin Fe 8 S 9 X + (XN, C) clusters. Is the first step for N 2 reduction to NH 3 a concerted dihydrogen transfer?

A high‐spin Fe 8 S 9 X + (XN, 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. © 2007 Wiley Periodicals, Inc. J Comput Chem 2007