Enzymatic and catalytic reduction of dinitrogen to ammonia: Density functional theory characterization of alternative molybdenum active sites
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We used density functional calculations to model dinitrogen reduction by a FeMo cofactor containing a central nitrogen atom and by a Mo-based catalyst. Plausible intermediates, reaction pathways, and relative energetics in the enzymatic and catalytic reduction of N-2 to ammonia at a single Mo center are explored. Calculations indicate that the binding of N-2, to the Mo atom and the subsequent multiple proton-electron transfer to dinitrogen and its protonated species involved in the conversion of N-2, are feasible energetically. In the reduction of N-2, the Mo atom experiences a cycled oxidation state from Mo(IV) to Mo(VI) by nitrogenase and from Mo(III) to Mo(VI) by the molybdenum catalyst, respectively, tuning the gradual reduction of N-2. Such a wide range of oxidation states exhibited by the Mo center is crucial for the gradual reduction process via successive proton-electron transfer. Present results suggest that the Mo atom in the N-centered FeMo cofactor is a likely alternative active site for dinitrogen binding and reduction under mild conditions once there is an empty site available at the Mo site. (c) 2005 Wiley Periodicals, Inc.