title: Synthesis and reactions of cubane-type iron-sulfur-phosphine clusters, including soluble clusters of nuclearities 8 and 16.
abstract: A family of soluble, reduced iron-sulfur clusters with nuclearities 4, 8, and 16 having tertiary phosphine ligation and based on the Fe(4)S(4) cubane-type structural motif has been synthesized. The results of this investigation substantially extend and improve the results of our original work on iron-sulfur-phosphine clusters (Goh, C.; Segal, B. M.; Huang, J.; Long, J. R.; Holm, R. H. J. Am. Chem. Soc. 1996, 118, 11844). A general property of this cluster family is facile phosphine substitution. The clusters [Fe(4)S(4)(PR(3))(4)](+) are precursors to monosubstituted [Fe(4)S(4)(PR(3))(3)X] (X = Cl-, RS-), homoleptic [Fe(4)S(4)(SR)(4)](3-), and all-ferrous monocubanes [Fe(4)S(4)(PR(3))(4)] (R = Pr(i), Cy, Bu(t); generated in solution). In turn, [Fe(4)S(4)(PPr(i)()(3))(3)(SSiPh(3))] and [Fe(4)S(4)(PPr(i)(3))(4)] can be transformed into the dicubanes [Fe(8)S(8)(PPr(i)()(3))(4)(SSiPh(3))(2)] and [Fe(8)S(8)(PPr(i)((3))(6)], respectively. Further, the tetracubanes [Fe(16)S(16)(PR(3))(8)] are also accessible from [Fe(4)S(4)(PR(3))(4)] under different conditions. X-ray structures are described for [Fe(4)S(4)(PCy(3))(3)X] (X = Cl-, PhS-), [Fe(8)S(8)(PPr(i)(3))(4)(SSiPh(3))(2)], [Fe(8)S(8)(PPr(i)()(3))(6)], and [Fe(16)S(16)(PCy(3))(8)]. The monosubstituted clusters show different distortions of the [Fe(4)S(4)](+) cores from idealized cubic symmetry. The dicubanes possess edge-bridged double cubane structures with an Fe(2)(mu(4)-S)(2) bridge rhomb and idealized C(2)(h)() symmetry. The ready cleavage of these clusters into single cubanes is considered a probable consequence of strained bond angles at the mu(4)-S atoms. Tetracubanes contain four individual cubanes, each of which is implicated in two bridge rhombs so as to generate a cyclic structure of idealized D(4) symmetry. Redox properties and Mössbauer spectroscopic parameters are reported. The species [Fe(4)S(4)(PR(3))(4)] (in solution), [Fe(8)S(8)(PR(3))(6)], and [Fe(16)S(16)(PR(3))(8)] are the only synthetic all-ferrous clusters with tetrahedral iron sites that have been isolated. Their utility as precursors to other highly reduced iron-sulfur clusters is under investigation.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138, USA.
authors.forename: Hong-Cai
authors.initials: HC
authors.lastname: Zhou
authors.forename: R H
authors.initials: RH
authors.lastname: Holm
mesh_terms: D016948:Azotobacter vinelandii / Q000737:chemistry; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D007501:Iron / Q000737:chemistry*; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis* / Q000737:chemistry*; D010084:Oxidation-Reduction; D010720:Phosphines / Q000138:chemical synthesis* / Q000737:chemistry*; D015204:Spectroscopy, Mossbauer; D013237:Stereoisomerism; D013455:Sulfur / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D010720:Phosphines; D013455:Sulfur; D007501:Iron