DFT Studies on the Reduction of Dinitrogen to Ammonia by a Thiolate-Bridged Diiron Complex as a Nitrogenase Mimic

Abstract

We have recently reported that the binuclear iron complexes Cp*Fe(μ-SR1)2(μ,η2-R2N═NH)FeCp* (R1 = Me, Et; R2 = Me, Ph; Cp* = η5-C5Me5) as novel models of nitrogenase could effectively catalyze the N–N bond cleavage of hydrazines, including NH2NH2 (J. Am. Chem. Soc. 2008, 130, 15250−15251). However, the mechanistic aspects involved in the catalytic cycle and the possibility of reducing N2 by these complexes have remained unexplored. In the present study, DFT has been applied for modeling the binding of Cp*Fe(μ-SEt)2FeCp* with N2 and the reduction of the N2 to two NH3 molecules at the diiron centers. The calculations of model system indicate that the hydrogenation (H+ + e–) energetically prefers to occur at the N atoms rather than the Fe or S atoms. The rate-determining step of the reduction of HN═NH could be the isomerization of a μ,η2-HN–NH2 moiety to a μ-HN–NH2 form with a NH-bridging feature, which occurred at the diiron centers. Such a transformation of the binding mode of HN–NH2 might be driven by unequal charge populations on the two Fe atoms. The results show an event of net electron transfer from the ancillary ligands to the Fe atoms and the NHNH2 moiety during the rate-determining step. In view of the experimental observations reported previously, the current computations suggest that the diiron complex Cp*Fe(μ-SEt)2FeCp* is possible to bind N2 and reduce it to NH3 via protonation/reduction. Such a reduction of N2 to NH3 at the diiron centers favorably occurs through the HNNH and HNNH2 forms rather than via the H2NNH2 unit.