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), P-N-type clusters, and double-cuboidal clusters, have been devised using the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)(WS3)-S-VI](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*)WFe(3)S(3)Q] (Q = S, Se) units. With appropriate terminal iron ligation, these units are capable of independent existence or may be transformed into higher-nudearity 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 mu(4)-Se sites. Reaction of these species with HSe- affords the P-N-type cores [W2Fe6S6Se3](1+), in which selenide occupies mu(6)-Se and mu(2)-Se sites. The reaction of [(Tp*)WS3](1-), FeCl2, and Na2Se yields the double-cuboidal [W2Fe4S6Se3](2+/0) core with mu(2)-Se and mu(4)-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 Mossbauer data, and redox potentials are presented for each cluster type.