Polycubane Clusters:  Synthesis of [Fe₄S₄(PR₃)₄]^1+,0 (R = Bu^t, Cy, Prⁱ) and [Fe₄S₄]⁰ Core Aggregation upon Loss of Phosphine

Abstract

The influence of tertiary phosphines on the stability of FeS3P coordination units and the formation of iron-sulfur clusters has been investigated. Reaction of [Fe4S4Cl4]2- with a small excess of PR3 in acetonitrile/THF affords the cubane-type clusters [Fe4S4(PR3)4]1+ (R = Cy, Bu(t), Pr(i)), one-electron reduced over the initial cluster and possessing an S = 1/4 ground state. These clusters may be electrochemically oxidized to [Fe4S4(PR3)4]2+ and reduced to [Fe4S4(PR3)4], which can also be generated in solution by chemical reduction. The neutral clusters upon standing in solution lose phosphine and aggregate to form dicubane ([Fe8S8(PCy3)6]) or tetracubane ([Fe16S16(PR3)8]; R = Bu(t), Pr(i) clusters. The [Fe8S8]0 dicubane core has two intercubane Fe-S bonds, defining an Fe2S2 rhomb and affording a structure of overall idealized C2(h) symmetry. The tetracubane clusters consist of a cyclic array of four cubanes joined in four Fe2S2 rhombs in a structure of overall D4 symmetry, and present a new structural motif in Fe-S cluster chemistry. Tertiary phosphines impose two significant features on this cluster chemistry. These ligands significantly stabilize the [Fe4S4](1+/0) core oxidation levels compared to the case of conventional [Fe4S4L4]3-,4- clusters (L = monoanion). Ligands with cone angles exceeding that of PEt3 (132°) favor tetrahedral FeS3P coordination sites. This has the effect of directing reactions away from the formation of Fe6S6 (four trigonal pyramidal) and Fe6S8 (six square pyramidal) clusters having the indicated sites which are disfavored by large cone angles. Structural principles governing polycubane clusters together with a brief enumeration of stereochemically feasible polycubanes are presented and discussed.