Vanadium-Iron-Sulfur Clusters Containing the Cubane-type [VFe3S4] Core Unit: Synthesis of a Cluster with the Topology of the PN Cluster of Nitrogenase

A synthetic method affording a topological analogue of the electron-transfer P-cluster of nitrogenase (Fe8S7((mu)2-SCys)2) in the PN state has been devised, based in part on our previous development of cubane-type VFe3S4 clusters (Hauser, C.; Bill, E.; Holm, R. H. Inorg. Chem. 2002, 41, 1615-1624). The cluster [(Tp)VFe3S4Cl3]2- (1) is converted to [(Tp)VFe3S4(PR3)3]1+ (R = Et (2), Bu (3)) by reaction with R3P. The phosphine ligands are readily substituted, leading to [(Tp)VFe3S4(SR)3]2- (R = Ph (4), H (5)). Reduction of 2 or 3 with cobaltocene produces the edge-bridged double cubanes [(Tp)2V2Fe6S8(PR3)4] (R = Et (6), Bu (7)), which are readily converted to [(Tp)2V2Fe6S8(SPh)4]4- (8). The structures of clusters 3-5 and 8 were proven crystallographically. Cluster 8 has the double-cubane structure previously shown for 6, in which two cubane units are bridged by two Fe-((mu)4-S) bonds. 57Fe isomer shifts are consistent with the formulation [VFe2.33+3S4]2+ for the single cubanes and the all-ferrous description 2[VFe2+3S4]1+ for the double cubanes. Reaction of 6 with 4 equiv of (Et4N)(HS) in acetonitrile results in the insertion of sulfide with concomitant structural rearrangement and the formation of [(Tp)2V2Fe6S9(SH)2]4- (10), obtained in ca. 50% yield as the Et4N+ salt. The cluster has C2 symmetry, with two allferrous VFe3S4 fragments bridged by a common (mu)6-S atom and two (mu)2-S atoms that simulate the bridging atoms in the two Fe((mu)2-SCys)-Fe bridges of the PN cluster. The bridge pattern V2Fe6((mu)2-S)2((mu)3-S)6((mu)6-S) and cluster shape match those of the native cluster. A best-fit superposition of the cores of 10 and the PN cluster affords a weighted rms deviation in atom positions of 0.33 (angstrom). Cluster 10 and [(Tp)2Mo2Fe6S9(SH)2]3-, prepared by a related route (Zhang, Y.; Holm, R. H. J. Am. Chem. Soc. 2003, 125, 3910-3920), demonstrate that the topology of the PN cluster can be achieved in molecular form in the absence of protein structure (Tp = tris(pyrazolyl)hydroborate).