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*)­W^VIS₃]^1– [Tp* = tris­(3,5-dimethylpyrazolyl)­hydroborate(1−)], which in the assembly systems organizes Fe^2+/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 P^N-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.