An S = 1/2 Iron Complex Featuring N2, Thiolate, and Hydride Ligands: Reductive Elimination of H2 and Relevant Thermochemical Fe-H Parameters.

Believed to accumulate on the Fe sites of the FeMo-cofactor (FeMoco) of MoFe-nitrogenase under turnover, strongly donating hydrides have been proposed to facilitate N2 binding to Fe and may also participate in the hydrogen evolution process concomitant to nitrogen fixation. Here, we report the synthesis and characterization of a thiolate-coordinated FeIII(H)(N2) complex, which releases H2 upon warming to yield an FeII-N2-FeII complex. Bimolecular reductive elimination of H2 from metal hydrides is pertinent to the hydrogen evolution processes of both enzymes and electrocatalysts, but well-defined examples are uncommon and usually observed from diamagnetic second- and third-row transition metals. Kinetic data obtained on the HER of this ferric hydride species are consistent with a bimolecular reductive elimination pathway, arising from cleavage of the Fe-H bond with a computationally determined BDFE of 55.6 kcal/mol.

Structure and Reactivity of an Asymmetric Synthetic Mimic of Nitrogenase Cofactor.

The Mo nitrogenase catalyzes the ambient reduction of N2 to NH3 at its M-cluster site. A complex metallocofactor with a core composition of [MoFe7 S9 C], the M-cluster, can be extracted from the protein scaffold and used to facilitate the catalytic reduction of CN- , CO, and CO2 into hydrocarbons in the isolated state. Herein, we report the synthesis, structure, and reactivity of an asymmetric M-cluster analogue with a core composition of [MoFe5 S9 ]. This analogue, referred to as the Mo-cluster, is the first synthetic example of an M-cluster mimic with Fe and Mo positioned at opposite ends of the cluster. Moreover, the ability of the Mo-cluster to reduce C1 substrates to hydrocarbons suggests the feasibility of developing nitrogenase-based biomimetic approaches to recycle C1  waste into fuel products.

In vitro synthesis of the iron-molybdenum cofactor of nitrogenase from iron, sulfur, molybdenum, and homocitrate using purified proteins.

Biological nitrogen fixation, the conversion of atmospheric N2 to NH3, is an essential process in the global biogeochemical cycle of nitrogen that supports life on Earth. Most of the biological nitrogen fixation is catalyzed by the molybdenum nitrogenase, which contains at its active site one of the most complex metal cofactors known to date, the iron-molybdenum cofactor (FeMo-co). FeMo-co is composed of 7Fe, 9S, Mo, R-homocitrate, and one unidentified light atom. Here we demonstrate the complete in vitro synthesis of FeMo-co from Fe(2+), S(2-), MoO4(2-), and R-homocitrate using only purified Nif proteins. This synthesis provides direct biochemical support to the current model of FeMo-co biosynthesis. A minimal in vitro system, containing NifB, NifEN, and NifH proteins, together with Fe(2+), S(2-), MoO4(2-), R-homocitrate, S-adenosyl methionine, and Mg-ATP, is sufficient for the synthesis of FeMo-co and the activation of apo-dinitrogenase under anaerobic-reducing conditions. This in vitro system also provides a biochemical approach to further study the function of accessory proteins involved in nitrogenase maturation (as shown here for NifX and NafY). The significance of these findings in the understanding of the complete FeMo-co biosynthetic pathway and in the study of other complex Fe-S cluster biosyntheses is discussed.