Rearrangement of Symmetrical Dicubane Clusters into Topological Analogues of the P Cluster of Nitrogenase: Nature's Choice?

Reaction schemes have been developed that lead to clusters having the topology of the PN cluster of nitrogenase. The single cubane clusters [(Tp)MFe3S4Cl3]z (M = Mo, z = 1−; M = V, z = 2−) react with PEt3 to give [(Tp)MFe3S4(PEt3)3]1+, which are reduced to the neutral edge-bridged double cubanes [(Tp)2M2Fe6S8(PEt3)4] with highly reduced (2[MFe3S4]1+) cores. Reaction of these clusters in acetonitrile with (Et4N)(HS) results in the formation of [(Tp)2Mo2Fe6S9(SH)2]3- and [(Tp)2V2Fe6S9(SH)2]4-. X-ray structures of the Et4N+ salts reveal the bridging pattern M2Fe6(μ2-S)2(μ3-S)6(μ6-S) in which two cuboidal MFe3(μ3-S)3 units share the common bridge atom μ6-S and are externally bridged by two μ2-S atoms. The M sites possess trigonal octahedral, and the Fe sites, distorted tetrahedral coordination. Hydrosulfide ligands and sulfide atoms simulate terminal cysteinate ligation and μ2 bridges, respectively, in the protein-bound cluster Fe8S7(μ2-SCys)2(SCys)4. The synthetic clusters have the same bridging pattern as the PN cluster and approach congruency with it. These clusters are the first molecular topological analogues of a PN cluster. Like the latter, they are substantially reduced (majority of Fe(II)).