Catalytic Multielectron Reduction of Hydrazine to Ammonia and Acetylene to Ethylene with Clusters That Contain the MFe₃S₄ Cores (M = Mo, V)

Abstract

Clusters with the [MFe 3 S 4 ] n+ core, (M = Mo, n=3; M = V, n=2). are used as catalysts for the reduction of substrates relevant to nitrogenase function. Substrates such as hydrazine and acetylene, are catalytically reduced by (NEt4)2[(Cl4-cat)(CH3CN)MoFe3S 4 Cl 3 ], I, to ammonia and ethylene respectively, in the presence of added protons and reducing equivalents. Hydrazine also is catalytically reduced by the (NEt4)[(DMF)3VFe3S4Cl 3 ] cubane under similar conditions. Catalysis in excess of 100 turnovers (for hydrazine reduction) and in excess of 15 turnovers (for acetylene reduction) has been observed over a period of 24 hours. Kinetic studies of the acetylene reduction reaction have been carried out. Considerable evidence has been amassed which directly implicates the Mo and V atoms as the primary catalytic sites. The reduction of hydrazine is accelerated in the presence of carboxylate ligands bound to the Mo atom in I and this effect is interpreted in terms of a proton delivery shuttle involving the carboxylate group and the substrate during reduction. The possible role of the homocitrate ligand in the nitrogen cofactor is analyzed in terms of these findings.In recent single crystal X-ray structure determinations of the Fe-Mo protein of nitrogenase from Clostridium pasteurianum (7,2) and Azotobacter vinelandii (2) the structure of the Fe/Mo/S site has been revealed to near atomic resolution. This site catalyzes the biological reduction of dinitrogen (3) to ammonia under ambient temperature and pressure and its exact role and mechanism of action have been subject to intense interest. The Fe/Mo/S cluster contains two cuboidal subunits, Fe4S3 and MoFe^, bridged by three S 2 ' ions. The cluster is anchored to the protein matrix by a cysteinyl residue coordinated to an Fe atom at one end of the cluster and by an imidazole group from a histidine residue that is bound to the Mo atom at the other end of the cluster, Fig. 1. The Mo atom also is coordinated by a homocitrate molecule that serves as a bidentate chelate. Unusual structural features include the unprecedented trigonal planar coordination geometry for the six μ-S-bridged iron atoms, and the unusually short Fe-Fe distances across the two subclusters (2.5-2.6Â).

The Chemistry and Biochemistry of Vanadium and the Biological Activities Exerted by Vanadium Compounds

“…631,[651][652][653] Important new clusters that have been reported include two vanadium clusters with the VS 3 (132), are capable of catalyzing the reduction of hydrazine by cobaltocene/lutidine hydrochloride. 651,[653][654][655] Reduction of hydrazine to NH 3 is commonly used to document the reactivity of these clusters and for the cuboidal clusters (130)(131)(132)(133)(134)(135); the coordination of the hydrazine to the V-atom appears to be critical to its reduction. 630,651,653,655 In contrast to the complexes (129-135) described above, [(HBpz) 3 VFe 3 S 4 Cl 3 ] 2-(136) 653 does not catalyze hydrazine reduction.…”

Car–Parrinello Molecular Dynamics Study of the Initial Dinitrogen Reduction Step in Sellmann‐Type Nitrogenase Model Complexes

“…Apparently, protonation (path 2) of the sulfur donors should be considered as an essential and most promising initial step during the reduction of the dinuclear N 2 complex. [9,10,[16][17][18][19][20][21][22][23][24][25][26][27][28][29] It is the purpose of this study to investigate this first protonation reduction step in order to obtain a microscopic picture of a possible mechanism of a two-electron reduction of protonated Sellmann-type complexes. A detailed understanding of the reaction mechanism should yield suggestions for a particular ligand sphere for sulfur-rich metal complexes designed to stimulate nitrogen activation.…”

The discovery of Mo(III) in FeMoco: reuniting enzyme and model chemistry

“…8 ). In studies by Coucouvanis et al [ 84 , 85 ] [MoFe 3 S 4 ] 3+ cubanes (as well as [VFe 3 S 4 ] 2+ [ 86 ]) were found to be active catalysts for the reduction of hydrazine to ammonia [ 87 , 88 ], acetylene to ethylene [ 89 ] and cis -dimethyldiazene to methylamine [ 90 ]. These studies were conducted using cobaltocene and 2,6-lutidinium chloride as external sources of electrons and protons.…”