THE MIXED-VALENCE DOUBLE-CUBANOID CLUSTER [FE8S12(BU(T)NC)(12)] - SYNTHESIS, STRUCTURE, AND EXCHANGE COUPLING OF A NEW STRUCTURAL ARRAY OF 4 FE(III) SITES

Reaction of the cubane cluster [Fe4S4(SEt)(2)((BuNC)-N-t)(6)] (7) and PhCH2SSSCH2Ph in benzene solution affords the cluster [Fe8S12((BuNC)-N -t)(12)] (6) in 60-74% yield. The structure of this cluster as [6] . 5 C(6)H(6) and that of [Fe4S5(SEt)(2)((BuNC)-N-t)(6)] (8, closely relate d to 7) were determined by X-ray diffraction. Cluster 6 contains two c ubanoid units Fe4S5 = Fe-4(mu(3)-S)(3)(mu(3):eta(2)eta(1)-S-2) bridged by two mu(2)-S ions and related by an inversion center. Certain featu res of the structure of 7 as revealed by 8 are retained in 6, includin g in each cubanoid unit two low-spin. distorted octahedral (FeS3)-S-II ((BuNC)-N-t)(3) and two distorted tetrahedral (FeC)-C-III(mu(3)-S)(3)( mu(2)-S) sites and certain dimensional similarities. The formal insert ion of an S(O) atom into a cubane to form a cubanoid unit disrupts one Fe-(mu(3)-S) bond and leads to dimensional changes such as to poise o ne iron atom in a stereochemistry and position favorable for the forma tion of an Fe-(mu(2)-S)-Fe bridge. The structures of iron-sulfur polyc ubanes are summarized; doubly sulfide-bridged double cubanes are unkno wn. In this family, cluster 6 is the only authenticated bis(mu(2)-S) s pecies and thus bears a distant relationship to the P clusters of nitr ogenase. Cluster 6, with four Fe(III) sites in the centrosymmetric arr ay [Fe-2(mu(2)-S)(2)](mu(2)-S)(2), presents an exchange coupling probl em not previously encountered in iron-sulfur compounds. A formal treat ment of the problem based on two coupling constants for intra- (J) and intercubanoid (J ') exchange is presented. It is shown that the there is no unique solution for J,J'. Reasonable values of these parameters based on different assumptions are considered. If a recently determin ed J value for coupling in a [Fe-2(mu(2)-S)(2)](2+) fragment of a cuba ne cluster is adopted, J = 280 cm(-1 )and J' = 445 cm(-1) for 6.