ic030259t_si_001.cif (283.02 kB)

Structural Conversions of Molybdenum−Iron−Sulfur Edge-Bridged Double Cubanes and PN-Type Clusters Topologically Related to the Nitrogenase P-Cluster

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posted on 26.01.2004, 00:00 by Yugen Zhang, R. H. Holm
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)2Mo2Fe6S8(PEt3)4] sustains terminal ligand substitution with retention of the Mo2Fe63-S)64-S)2 core structure and rearrangement to the Mo2Fe62-S)23-S)66-S) topology of the nitrogenase PN cluster upon reaction with certain nucleophiles. Four distinct processes for the conversion of double cubanes to PN-type clusters are documented, affording the products [(Tp)2Mo2Fe6S9(SR)2]3-, [(Tp)2Mo2Fe6S8(OMe)3]3-, and [(Tp)2Mo2Fe6S7(OMe)4]2-. In the latter clusters, two methoxides are terminal ligands and one or two are μ2-bridging ligands. The reverse transformation of a PN-type cluster to an edge-bridged double cubane has been demonstrated by the reaction of [(Tp)2Mo2Fe6S8(OMe)3]3- with Me3SiX to afford [(Tp)2Mo2Fe6S8X4]2- (X = Cl-, Br-). Edge-bridged double cubanes have been obtained in the oxidation states [Mo2Fe6S8]2+,3+,4+. The stable oxidation state of PN-type clusters is [Mo2Fe6S9]+. Structures of five double cubanes and four PN-type clusters are reported. The PN-type clusters are synthetic representations of the biologically unique topology of the native PN cluster. Best-fit superpositions of the native and synthetic cluster cores gives weighted rms deviations in atom positions of 0.20−0.38 Å. 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 PN 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−).)

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