Synthesis and Characterization of Sulfur-Voided Cubanes. Structural Analogues for the MoFe3S3 Subunit in the Nitrogenase Cofactor

• Published in: Journal of the American Chemical Society 2002-01, Vol.124 (2), p.216-224
• Main Authors: Coucouvanis, Dimitri, Han, Jaehong, Moon, Namdoo
• Format: Article
• Language: English
• Subjects: Carbonyl and nitrosyl compounds, and their complexes; Carbonyl and nitrosyl metals; Chemistry; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Inorganic chemistry and origins of life; Inorganic compounds; Physics; Preparations and properties; Structure of solids and liquids; crystallography; Structure of specific crystalline solids
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja0110832

A new class of Mo/Fe/S clusters with the MoFe3S3 core has been synthesized in attempts to model the FeMo-cofactor in nitrogenase. These clusters are obtained in reactions of the (Cl4-cat)2Mo2Fe6S8(PR3)6 [R = Et (I), nPr (II)] clusters with CO. The new clusters include those preliminarily reported:  (Cl4-cat)MoFe3S3(PEt3)2(CO)6 (III), (Cl4-cat)(O)MoFe3S3(PEt3)3(CO)5 (IV), (Cl4-cat)(Pyr)MoFe3S3(PEt3)2(CO)6 (VI), and (Cl4-cat)(Pyr)MoFe3S3(PnPr3)3(CO)4 (VIII). In addition the new (Cl4-cat)(O)MoFe3S3(PnPr3)3(CO)5 cluster (IVa), the (Cl4-cat)(O)MoFe3S3(PEt3)2(CO)6cluster (V), the (Cl4-cat)(O)MoFe3S3(PnPr3)2(CO)6 cluster (Va), the (Cl4-cat)(Pyr)MoFe3S3(PnPr3)2(CO)6 cluster (VIa), and the (Cl4-cat)(PnPr3)MoFe3S3(PnPr3)2(CO)6 cluster (VII) also are reported. Clusters III−VIII have been structurally and spectroscopically characterized. EPR, zero-field 57Fe-Mössbauer spectroscopic characterizations, and magnetic susceptibility measurements have been used for a tentative assignment of the electronic and oxidation states of the MoFe3S3 sulfur-voided cuboidal clusters. A structural comparison of the clusters with the MoFe3S3 subunit of the FeMo-cofactor has led to the suggestion that the storage of reducing equivalents into M−M bonds, and their use in the reduction of substrates, may occur with the FeMo-cofactor, which also appears to have M−M bonding. On the basis of this argument, a possible N2-binding and reduction mechanism on the FeMoco-cofactor is proposed.