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

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, Prⁱ), one-electron reduced over the initial cluster and possessing an S = ¹/₂ 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, Prⁱ) clusters. The [Fe₈S₈]⁰ dicubane core has two intercubane Fe−S bonds, defining an Fe₂S₂ rhomb and affording a structure of overall idealized C_2h 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.