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

摘要

Syntheses of five types of tungsten-iron-sulfur/selenium clusters–incomplete cubanes, single cubanes, edge-bridged double cubanes (EBDCs), P^N-type clusters, and double-cuboidal clusters–have been devised based on the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)W^VIS3]¹⁻, which in the assembly systems organizes Fe^II,III and sulfide/selenide into cuboidal [(Tp*)WFe2S3] or cubane [(Tp*)WFe3S3Q] 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 product clusters. Specific incorporation of selenide is demonstrated by 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. Reaction of [(Tp*)WS₃]¹⁻, FeCl₂, and Na₂Se results in 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 cluster products. The results further demonstrate the viability of template-assisted cluster synthesis inasmuch as the reduced (Tp*)WS₃ unit is present in all clusters. Structures, zero-field Mössbauer data, and redox potentials are presented for all cluster types. (Tp* = tris(pyrazolyl)hydroborate(1−))