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 FeS 3 P coordination units and the formation of iron−sulfur clusters has been investigated. Reaction of [Fe 4 S 4 Cl 4 ] 2- with a small excess of PR 3 in acetonitrile/THF affords the cubane-type clusters [Fe 4 S 4 (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 electrochemically oxidized to [Fe 4 S 4 (PR 3 ) 4 ] 2+ and reduced to [Fe 4 S 4 (PR 3 ) 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 ([Fe 8 S 8 (PCy 3 ) 6 ]) or tetracubane ([Fe 16 S 16 (PR 3 ) 8 ]; R = Bu t, Pr i ) clusters. The [Fe 8 S 8 ] 0 dicubane core has two intercubane Fe−S bonds, defining an Fe 2 S 2 rhomb and affording a structure of overall idealized C 2 h symmetry. The tetracubane clusters consist of a cyclic array of four cubanes joined in four Fe 2 S 2 rhombs in a structure of overall D 4 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 [Fe 4 S 4 ] 1+/0 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°) favor tetrahedral FeS 3 P coordination sites. This has the effect of directing reactions away from the formation of Fe 6 S 6 (four trigonal pyramidal) and Fe 6 S 8 (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.