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

✍ Scribed by Michael L. Mckee

• Publisher: John Wiley and Sons
• Year: 2007
• Tongue: English
• Weight: 800 KB
• Volume: 28
• Category: Article
• ISSN: 0192-8651
• DOI: 10.1002/jcc.20635

✦ Synopsis

Abstract

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. © 2007 Wiley Periodicals, Inc. J Comput Chem 2007