Synthesis and Reactions of Cubane-Type Iron−Sulfur−Phosphine Clusters, Including Soluble Clusters of Nuclearities 8 and 16

A family of soluble, reduced iron−sulfur clusters with nuclearities 4, 8, and 16 having tertiary phosphine ligation
and based on the Fe4S4 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 [Fe4S4(PR3)4]+ are precursors to monosubstituted [Fe4S4(PR3)3X]
(X = Cl-, RS-), homoleptic [Fe4S4(SR)4]3-, and all-ferrous monocubanes [Fe4S4(PR3)4] (R = Pri, Cy, But; generated
in solution). In turn, [Fe4S4(PPri3)3(SSiPh3)] and [Fe4S4(PPri3)4] can be transformed into the dicubanes [Fe8S8(PPri3)4(SSiPh3)2] and [Fe8S8(PPri3)6], respectively. Further, the tetracubanes [Fe16S16(PR3)8] are also accessible
from [Fe4S4(PR3)4] under different conditions. X-ray structures are described for [Fe4S4(PCy3)3X] (X = Cl-, PhS-),
[Fe8S8(PPri3)4(SSiPh3)2], [Fe8S8(PPri3)6], and [Fe16S16(PCy3)8]. The monosubstituted clusters show different distortions
of the [Fe4S4]+ cores from idealized cubic symmetry. The dicubanes possess edge-bridged double cubane structures
with an Fe2(μ4-S)2 bridge rhomb and idealized C2h symmetry. The ready cleavage of these clusters into single
cubanes is considered a probable consequence of strained bond angles at the μ4-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 D4 symmetry. Redox properties and Mössbauer spectroscopic parameters are reported. The species
[Fe4S4(PR3)4] (in solution), [Fe8S8(PR3)6], and [Fe16S16(PR3)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.