Mimicking nitrogenase

Abstract

In seeking to mimic the hydrogenation of N(2) to NH(3) as effected under mild conditions by the enzyme nitrogenase, three classes of known metal sulfide clusters that resemble the NFe(7)MoS(9) core of FeMo-co, the active site of nitrogenase, have been assessed theoretically. The assessment has been made in the context of the previously proposed mechanism for nitrogenase, in which protons are relayed to FeMo-co, where, as hydrogen atoms accumulated on Fe and S atoms, they transfer to bound N(2) and subsequent intermediates in a critical sequence of intramolecular hydrogenations, probably accelerated by H atom tunneling. The three model systems possess the X(c)Fe(4)S(4) face which is the key active site of FeMo-co (X is most probably N in FeMo-co, and is S in the models). The most promising functional models are based on clusters M1, {(tpb)Mo(mu(3)-S)(3)Fe(2)(Fe-L)S(c)(mu-S)(2)(Fe-L)Fe(2)(mu(3)-S)(3)Mo(tpb)} [tpb = tris(1-pyrazolyl)hydroborate], for which syntheses are well developed. The assessment is based on the ability of the models to mimic the intermediates in the FeMo-co mechanism, as determined by density functional simulations. The elaborations of M1 required to mimic the FeMo-co behaviour are described. These include modification of the tpb ligands to control the coordination at the Fe atoms, to provide for the proton relay functionality, and to prevent unwanted reactivity at other Fe and S atoms. Literature references with prescriptions for synthesis of the predicted homogeneous catalysts are provided. Further, in view of the similarities between the model systems and the P-cluster of nitrogenase, it is speculated that the P-cluster could be a relic catalytic site for N(2) reduction.