VANADIUM-IRON-SULFUR AND MOLYBDENUM-IRON-SULFUR CLUSTER CHEMISTRY (NITROGENASE)

The synthesis, structure, and properties of the VFe(,3)S(,4) single cubane cluster VFe(,3)S(,4)Cl(,3)(DMF)(,3) ('1-) (8) are described. Structural, spectroscopic, and electrochemical features of 8 establish that its VFe(,3)S(,4) ('2+) core is nearly isostructural with, and electronically similar to, the MoFe(,3)S(,4) ('3+) core of MoFe(,3)S(,4) single cubane clusters.

VFe(,3)S(,4)Cl(,3)(DMF)(,3) ('1-) (8) possesses labile ligands at all terminal metal binding sites. Terminal ligand substitution reactions with L = Me(,2)SO, MeCN, SR('-), bipy (bipyridine), edt (1,2-ethanedithiolate), and dmpe (bis(1,2-dimethylphosphino)ethane) are examined. In the absence of potential bridging ligands, terminal ligand substitution proceeds with retention of the VFe(,3)S(,4) single cubane core. Ligand binding properties of 8 are compared with those of isoelectronic MoFe(,3)S(,4) cubane clusters.

Derivative clusters VFe(,3)S(,4)(S-p-C(,6)H(,4)Me)(,3)(DMF)(,3) ('1-) (14), VFe(,3)S(,4)Cl(,3)(bipy(DMF) ('1-) (15), and VFe(,3)S(,4)Cl(,3)(dmpe)(MeCN) ('1-) (17) were isolated, and characterized. The structures of clusters 15 and 17 were determined by x-ray crystallography. Solution and solid state properties of 14, 15, and 17 establish that ligand substitution occurs with retention of the single cubane VFe(,3)S(,4) core, and that clusters 14, 15, and 17 are electronically similar to VFe(,3)S(,4)Cl(,3)(DMF)(,3) ('1-) (8). The electron-transfer properties of 8, 14, 15, and 17 were examined, and found to differ from those of isoelectronic MoFe(,3)S(,4) clusters; this difference may reflect differences in the mean Fe oxidation level.

Ethane-1,2-dithiolate reacts with VFe(,3)S(,4)Cl(,3)(MeCN)(,3) ('1-) (18a) to afford the centrosymmetric edt-bridged double cubane cluster V(,2)Fe(,6)S(,8)Cl(,4)(edt)(,2))('4-) (16). Cluster 16 was isolated, and its structure was determined by x-ray crystallography. The bridge of 16 remains intact in coordinating solvents.

The synthesis, structure, and properties of the low-valent Fe-Mo-S cluster MoFe(,3)S(,6)(CO)(,6) ('2-) (28a) which contains an unprecedented Fe(,2)((mu)(,3)-S)(,2)Fe((mu)(,2)-S)(,2)MoS(,2) ('2-) core and CO supporting ligands, are described. The synthesis and properties of related clusters WFe(,3)S(,6)(CO)(,6) ('2-) (28b), and VFe(,6)S(,8)(CO)(,12) ('3-) (34) are also described. Oxidative removal of the nonphysiological CO ligands in 28a with RSSR induces a dramatic core rearrangement affording the double cubane cluster Mo(,2)Fe(,6)S(,8)(SR)(,6)((mu)(,2)-S(,2))(,2) ('4-) (R = p-C(,6)H(,4)Cl, 37; R = p-C(,6)H(,4)Br, 38), which contain a potentially reactive ((mu)(,2)-(eta)('3)-S(,2))(,2) bridge. The structure of cluster 38 was determined by X-ray crystallography. Their redox activity and persulfide bridge structures made 37 and 38 attractive as potential cluster sites for substrate activation and reduction. (Abstract shortened with permission of author.)