Synthesis, Identification, and Reactivity Properties of Symmetrical MoFe(3)S(4) Double Cubanes with Fe-S-Fe and Fe-O-Fe Bridges.

Singly bridged double cubane clusters containing Fe(4)S(4) and/or MoFe(3)S(4) cores and linked by an Fe-S-Fe unit bear an interesting compositional similarity to the cofactor of nitrogenase and constitute a nearly unexplored class of clusters. We have expanded recently our investigation of the prototypic cluster {[(Meida)MoFe(3)S(4)Cl(2)](2)(&mgr;(2)-S)}(4)(-) (6) (Huang et al.; J. Am. Chem. Soc. 1997, 119, 8662) to other types {[(Meida)MoFe(3)S(4)L(2)](2)(&mgr;(2)-S)}(4)(-) (L = EtS(-), ArO(-) (8-10)) (Meida = N-methylimidodiacetate(2-)). Clusters are formed by coupling reactions of the single cubanes [(Meida)MoFe(3)S(4)L(3)](2)(-) (L = Cl(-) (1), ArO(-) (3-5) at their substitutionally labile iron sites with primarily Li(2)S. Using 2 NaOEt/H(2)O as an oxide source in Me(2)SO, the oxo-bridged clusters {[(Meida)MoFe(3)S(4)L(2)](2)(&mgr;(2)-O)}(4)(-) (L = Cl(-) (13), ArO(-) (14, 15)) have been prepared. All double cubanes were obtained as a mixture of as many as four diastereomers. Double cubane structures were established by NMR and electrochemical criteria. Both the sulfido- and oxo-bridged clusters undergo terminal ligand substitution reactions with bridge retention. Oxo bridges are much more susceptible than sulfido bridges to protic cleavage with water, (Et(3)NH)Cl, ArOH, and HS(-). Reaction of 13 with hydrosulfide affords a second route to 6. These and other results define a set of reactions that are regiospecific at either the terminal ligand or bridge sites and provide a basis for future reactivity manipulation of symmetrical and unsymmetrical bridged double cubanes.