On the Existence in Solution of Doubly Thiolate-Bridged Double Cubanes Containing the Heterometal [MFe3S4] Core (M = Mo, W)

Abstract

Clusters of the general type {[MFe3S4(SR)2(cat)]2(μ 2-SR)2}4- (cat = catecholate) have previously been shown to contain two heterometal cubane-type cores [MFe3S4]3+ (M = Mo, W) linked by two Fe-S(R)-M bridges such that the cluster has idealized centrosymmetry. These clusters resemble the P-clusters of nitrogenase in that both have two μ2-SR bridges between cubane (or cuboidal) halves of the cluster. When dissolved in a strongly coordinating solvent or treated with a suitable ligand L1-,0, the bridges are cleaved to afford the single cubanes [MFe3S4(SR)3(cat)L/(solv)]2-,3-. Because of their relation to P-clusters, the stability and electron transfer properties of the double cubanes have been examined in solution. 1H NMR spectroscopic and cyclic voltammetric properties of acetonitrile solutions prepared from Et4N+ salts of clusters with R = Et and M = Mo, W are entirely consistent with the presence of bridged double cubanes in solution. Key observations include different chemical shifts of bridging and terminal ethanethiolate ligands and two successive oxidations separated by 150-230 mV. Reduction occurs with twice the diffusion current of the oxidations and results in cleavage to two single cubanes. Bridge strength increases in the order R = Ph < Et (consistent with basicities of the thiolate anions) and M = Mo < W. This work has produced the first demonstration that doubly thiolate-bridged double cubanes of any description remain intact in solution. The synthesis of Fe4S4 double cubanes with bis(μ2-SR) bridges persists as an objective if actual analogues of the P-cluster are to be obtained.