title: 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.
abstract: 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(2)S(3)] 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-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(3)S(3)Se](2+/3+) cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W(2)Fe(6)S(6)Se(2)](2+) containing μ(4)-Se sites. Reaction of these species with HSe(-) affords the P(N)-type cores [W(2)Fe(6)S(6)Se(3)](1+), in which selenide occupies μ(6)-Se and μ(2)-Se sites. The reaction of [(Tp*)WS(3)](1-), FeCl(2), and Na(2)Se yields the double-cuboidal [W(2)Fe(4)S(6)Se(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 Mössbauer data, and redox potentials are presented for each cluster type.
authors.affiliation: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: Bo
authors.initials: B
authors.lastname: Zheng
authors.forename: Xu-Dong
authors.initials: XD
authors.lastname: Chen
authors.forename: Shao-Liang
authors.initials: SL
authors.lastname: Zheng
authors.forename: R H
authors.initials: RH
authors.lastname: Holm
mesh_terms: D002621:Chemistry / Q000379:methods; D018360:Crystallography, X-Ray / Q000379:methods; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008670:Metals / Q000737:chemistry; D008956:Models, Chemical; D008958:Models, Molecular; D008968:Molecular Conformation; D012643:Selenium / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry*; D014414:Tungsten / Q000737:chemistry*
chemical_list: D008024:Ligands; D008670:Metals; D013440:Sulfides; D013455:Sulfur; D007501:Iron; D012643:Selenium; D014414:Tungsten