Selenium as a Structural Surrogate of Sulfur: Tern plate-Assisted Assembly of Five Types of Tungsten-lron-Sulfur/Selenium Clusters and the Structural Fate of Chalcogenide Reactants

摘要

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~*)W~(VI)S_3]~(1-) [Tp~* = tris(3,5-dimethylpyrazolyl)-hydroborate(1-)], which in the assembly systems organizes Fe~(2+/3+) and sulfide/selenide into cuboidal [(Tp~*)WFe_2S_3] or cubane [(Tp~*)WFe_3S_3Q] (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 [WFe_3S_3Se]~(2+/3+) cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W_2Fe_6S_6Se_2]~(2+) containing μ_4-Se sites. Reaction of these species with HSe~- affords the P~N-type cores [W_2Fe_6S_6Se_3]~(1+), in which selenide occupies μ_6-Se and μ_2Se sites. The reaction of [(Tp~*)WS_3]~(1-), FeCl_2 and Na_2Se yields the double-cuboidal [W_2Fe_4S_6Se_3]~(2+/0) core with μ_2-Se and μ_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~*)WS_3 unit is present in all of the clusters. Structures, zero-field Mossbauer data, and redox potentials are presented for each cluster type.