Molybdenum−Iron Sulfide-Bridged Double Cubanes

Abstract

A new class of clusters, sulfide-bridged double cubanes containing the units MFe3(μ3-S)4 (M = Fe, Mo), has been investigated as possible synthetic precursors to the iron−molybdenum cofactor (FeMoco) of nitrogenase. Clusters containing the symmetric core structures [Fe4Q4-Q-Fe4Q4]2+ (11−13, Q = S, Se) have been prepared by the coupling of separate cubane clusters ([Fe4S4Cl4]2-, [Fe4Q4(LS3)Cl]2-, where LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzenate(3−)). Similarly, the cuboidal cluster [VFe4S6(PEt3)4Cl] was coupled to form [VFe4S6(PEt3)4]2S (14). The cluster [(Fe4S4Cl3)2S]4- (11) is the first structurally proven example of the class (Challen, P. R.; Koo, S.-M.; Dunham, W. R.; Coucouvanis, D. J. Am. Chem. Soc. 1990, 112, 2455). Bridged structures have also been established crystallographically for {[Fe4Se4(LS3)]2Se}4- (13) and 14. Other criteria for identification of this structure developed using 11−14 are coupled redox processes in cyclic voltammetry and the detection of intact double cubane ions by electrospray mass spectrometry. For the coupling of heterometal cubanes, the Mo site was protected by chelation with Meida (N-methylimidodiacetate(2−)) as in [(Meida)MoFe3S4Cl3]2- (6), thereby directing the bridging reaction to the Fe sites. The structures of two [VFe3S4]2+ cubane clusters containing the tricoordinate (Meida)V fragment are reported. The reaction system 6/Li2S afforded {[(Meida)MoFe3S4Cl2]2S}4- (15). The symmetrical sulfide-bridged double cubane structure of 15 has been established by the electrochemical and mass spectrometric criteria and by the existence of the four isomers consistent with this structure. The equimolar reaction system 6/[Fe4S4Cl4]2-/Li2S produced a mixture of {[(Meida)MoFe3S4Cl2]S(Fe4S4Cl3)}4- (17), 15, and 11. Cluster 17 is also formed in the system 15/[Fe4S4Cl4]2-. The unsymmetrical sulfide-bridged double cubane structure of 17 was established by mass spectrometry and detection of the two isomers consistent with this structure. In the first reaction system, the product mole ratio 17:15 ≈ 3:1 is explained in terms of differential steric hindrance of conformations arising from rotation around the Fe−S−Fe bridge. The core composition of double cubane 15 (Mo2Fe6S9) approaches that of FeMoco (MoFe7S9). The core composition of 17 is exactly the same as FeMoco; 17 is the first synthetic cluster with this property.