Selenium as a Structural Surrogate of Sulfur: Template-Assisted Assembly of Five Types of Tungsten–Iron–Sulfur/Selenium Clusters and the Structural Fate of Chalcogenide Reactants

Syntheses of five types of tungsten–iron–sulfur/selenium clusters, namely, incomplete cubanes, single cubanes, edge-bridged double cubanes (EBDCs), PN-type clusters, and double-cuboidal clusters, have been devised using the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)­WVIS3]1– [Tp* = tris­(3,5-dimethylpyrazolyl)­hydroborate(1−)], which in the assembly systems organizes Fe2+/3+ and sulfide/selenide into cuboidal [(Tp*)­WFe2S3] or cubane [(Tp*)­WFe3S3Q] (Q = S, Se) units. With appropriate terminal iron ligation, these units are capable of independent existence or may be transformed into higher-nuclearity species. Selenide is used as a surrogate for sulfide in cluster assembly in order to determine by X-ray structures the position occupied by an external chalcogenide nucleophile or an internal chalcogenide atom in the product clusters. Specific incorporation of selenide is demonstrated by the formation of [WFe3S3Se]2+/3+ cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W2Fe6S6Se2]2+ containing μ4-Se sites. Reaction of these species with HSe affords the PN-type cores [W2Fe6S6Se3]1+, in which selenide occupies μ6-Se and μ2-Se sites. The reaction of [(Tp*)­WS3]1–, FeCl2, and Na2Se yields the double-cuboidal [W2Fe4S6Se3]2+/0 core with μ2-Se and μ4-Se bridges. It is highly probable that in analogous sulfide-only assembly systems, external and internal sulfide reactants occupy corresponding positions in the cluster products. The results further demonstrate the viability of template-assisted cluster synthesis inasmuch as the reduced (Tp*)­WS3 unit is present in all of the clusters. Structures, zero-field Mössbauer data, and redox potentials are presented for each cluster type.