Structural conversions of molybdenum-iron-sulfur edge-bridged double cubanes and P(n)-type clusters topologically related to the nitrogenase P-cluster

Abstract

Edge-bridged Mo-Fe-S double cubanes are versatile precursors for the synthesis of other clusters of the same nuclearity. Thus, the double cubane [(Tp)(2)Mo(2)Fe(6)S(8)(PEt(3))(4)] sustains terminal ligand substitution with retention of the Mo(2)Fe(6)(micro(3)-S)(6)(micro(4)-S)(2) core structure and rearrangement to the Mo(2)Fe(6)(micro(2)-S)(2)(micro(3)-S)(6)(micro(6)-S) topology of the nitrogenase P(N) cluster upon reaction with certain nucleophiles. Four distinct processes for the conversion of double cubanes to P(N)-type clusters are documented, affording the products [(Tp)(2)Mo(2)Fe(6)S(9)(SR)(2)](3)(-), [(Tp)(2)Mo(2)Fe(6)S(8)(OMe)(3)](3)(-), and [(Tp)(2)Mo(2)Fe(6)S(7)(OMe)(4)](2)(-). In the latter clusters, two methoxides are terminal ligands and one or two are micro(2)-bridging ligands. The reverse transformation of a P(N)-type cluster to an edge-bridged double cubane has been demonstrated by the reaction of [(Tp)(2)Mo(2)Fe(6)S(8)(OMe)(3)](3)(-) with Me(3)SiX to afford [(Tp)(2)Mo(2)Fe(6)S(8)X(4)](2)(-) (X = Cl(-), Br(-)). Edge-bridged double cubanes have been obtained in the oxidation states [Mo(2)Fe(6)S(8)](2+,3+,4+). The stable oxidation state of P(N)-type clusters is [Mo(2)Fe(6)S(9)](+). Structures of five double cubanes and four P(N)-type clusters are reported. The P(N)-type clusters are synthetic representations of the biologically unique topology of the native P(N) cluster. Best-fit superpositions of the native and synthetic cluster cores gives weighted rms deviations in atom positions of 0.20-0.38 A. This study and an earlier investigation (Zhang, Y.; Holm, R. H. J. Am. Chem. Soc. 2003, 125, 3910-3920) provide a comprehensive account of the synthesis of structural analogues of the native P(N) cluster and provide the basis for continuing investigation of the synthesis of weak-field Mo-Fe-S clusters related to nitrogenase. (Tp = tris(pyrazolyl)hydroborate(1-).)