POLYCUBANE CLUSTERS - SYNTHESIS OF [FE4S4(PR(3))(4)](1-I) AND [FE4S4]0 CORE AGGREGATION UPON LOSS OF PHOSPHINE(,0) (R=BU(T),CY,PR)

The influence of tertiary phosphines on the stability of FeS3P coordin ation units and the formation of iron-sulfur clusters has been investi gated. Reaction of [Fe4S4Cl4](2-) with a small excess of PR(3) in acet onitrile/THF affords the cubane-type clusters [Fe4S4(PR(3))(4)](1+) (R = Cy, Bu(t), Pr-i), one-electron reduced over the initial cluster and possessing an S = 1/2 ground state. These clusters may be electrochem ically oxidized to [Fe4S4(PR(3))(4)](2+) and reduced to [Fe4S4(PR(3))( 4)], which can also be generated in solution by chemical reduction. Th e neutral clusters upon standing in solution lose phosphine and aggreg ate to form dicubane ([Fe8S8(PCy(3))(6)]) or tetracubane ([Fe16S16(PR( 3))(8)]; R = Bu(t), Pr-i) clusters. The [Fe8S8](0) dicubane core has t wo intercubane Fe-S bonds, defining an Fe2S2 rhomb and affording a str ucture of overall idealized C-2h symmetry. The tetracubane clusters co nsist of a cyclic array of four cubanes joined in four Fe2S2 rhombs in a structure of overall D-4 symmetry, and present a new structural mot if in Fe-S cluster chemistry. Tertiary phosphines impose two significa nt features on this cluster chemistry. These ligands significantly sta bilize the [Fe4S4](1+10) core oxidation levels compared to the case of conventional [Fe(4)S(4)L(4)](3-,4-) clusters (L = monoanion). Ligands with cone angles exceeding that of PEt(3) (132 degrees) favor tetrahe dral FeS3P coordination sites. This has the effect of directing reacti ons away from the formation of Fe6S6 (four trigonal pyramidal) and Fe6 S8 (six square pyramidal) clusters having the indicated sites which ar e disfavored by large cone angles. Structural principles governing pol ycubane clusters together with a brief enumeration of stereochemically feasible polycubanes are presented and discussed.