Precursors to clusters with the topology of the PN cluster of nitrogenase:: Edge-bridged double cubane clusters [(Tp)2Mo2Fe6S8L4]z:: Synthesis, structures, and electron transfer series

Members of the cluster set [(Tp)(2)Mo2Fe6S8L4](z) contain the core unit M2Fe6(mu(3)-S)(6)(mu(4)-S)(2) in which two MoFe3S4 cubanes are coupled by two Fe-(mu(4)-S) interactions to form a centrosymmetric edge-bridged double cubane cluster. Some of these clusters are synthetic precursors to [(TP)(2)Mo2Fe6S9L2](3-), which possess the same core topology as the P-N cluster of nitrogenase. In this work, the existence of a three-member electron-transfer series of single cubanes [(Tp)MoFe3S4L3](z) (z = 3-, 2-, 1-) and a four-member series of double cubanes [(Tp)(2)Mo2Fe6S8L4](z) (Z = 4-, 3-, 2-, 1-) with L = F-, Cl-, N-3, PhS- is demonstrated by electrochemical methods, cluster synthesis, and X-ray structure determinations. The potential of the [4-/3-] couple is extremely low (<-1.5 V vs SCE in acetonitrile) such that the 4- state cannot be maintained in solution under normal anaerobic conditions. The chloride double cubane redox series was examined in detail. The members [(Tp)(2)Mo2Fe6S8Cl4](4-),(3-),(2-) were isolated and structurally characterized. The redox series includes the reversible steps [4-/3-] and [3-/2-]. Under oxidizing conditions, [(TP)(2)Mo2Fe6S8Cl4](2-) cleaves with the formation of single cubane [(Tp)MoFe3S4Cl3](1-). The quasireversible [2-/1-] couple is observed at more positive potentials than those of the single cubane redox step. Structure comparison of nine double cubanes suggests that significant dimensional changes pursuant to redox reactions are mainly confined to the Fe-2(mu(4)-S)(2) bridge rhomb. The synthesis and structure of [(Tp)(2)(Mo2Fe6S9F2H2O)-H-.](3-), a new topological analogue of the P-N cluster of nitrogenase, is described. (Tp = hydrotris(pyrazolyl)borate(1-)).