title: Synthesis, structure characterization, and reversible transformation of a cobalt salt of a dilacunary γ-Keggin silicotungstate and sandwich-type di- and tetracobalt-containing silicotungstate dimers.
abstract: A cobalt salt of a γ-Keggin dilacunary silicotungstate, {CoL5}2[γ-SiW10O34L2] [Co-SiW10; L = N,N-dimethylformamide (DMF) or H2O], could be synthesized by the cation-exchange reaction of TBA4[γ-H4SiW10O36] (TBA = tetra-n-butylammonium) with 2 equiv of Co(NO3)2 with respect to TBA4[γ-H4SiW10O36] in a mixed solvent of DMF and acetone (97% yield). Each Co-SiW10 was linked by water molecules via a hydrogen-bonding network. Besides Co-SiW10, various kinds of isostructural M-SiW10 could be synthesized via the same procedure as that for Co-SiW10 (M = Mn(2+), Fe(2+), Ni(2+), Cu(2+), Zn(2+), and Cd(2+)). By the reaction of Co-SiW10 with 1 equiv of TBA6[γ-H2SiW10O36] in acetone, a silicotungstate dimer pillared by two cobalt cations with a significantly slipped dimer configuration, TBA6[Co2(γ-H3SiW10O36)2]·3H2O (Co2), could be synthesized. By the reaction of Co-SiW10 with 3 equiv of TBAOH in acetone, a tetracobalt-containing sandwich-type silicotungstate, TBA6[{Co(H2O)}2(μ3-OH)2{Co(H2O)2}2(γ-H2SiW10O36)2]·5H2O (Co4), could be synthesized. Compound Co4 possessed the tetracobalt-oxygen core, [{Co(H2O)}2(μ3-OH)2{Co(H2O)2}2](6+), identical with those of previously reported Weakley-type sandwich polyoxometalates, [Co4(H2O)2(XM9O34)2](n-) (X = P(5+), Si(4+), Ge(4+), As(5+) or V(5+); M = Mo(6+) or W(6+)). The reversible transformation between these three compounds (Co-SiW10 ⇆ Co2, Co-SiW10 ⇆ Co4, and Co2 ⇆ Co4) took place by the addition and/or subtraction of required components in appropriate solvents, affording the desired products in high yields (71-93% yields).
authors.affiliation: Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Yuji
authors.initials: Y
authors.lastname: Kikukawa
authors.forename: Kosuke
authors.initials: K
authors.lastname: Suzuki
authors.forename: Kazuya
authors.initials: K
authors.lastname: Yamaguchi
authors.forename: Noritaka
authors.initials: N
authors.lastname: Mizuno
mesh_terms: D060326:Chemistry Techniques, Synthetic; D003035:Cobalt / Q000737:chemistry*; D019281:Dimerization; D008958:Models, Molecular; D008968:Molecular Conformation; D012825:Silicon / Q000737:chemistry*; D017973:Tungsten Compounds / Q000138:chemical synthesis* / Q000737:chemistry*
chemical_list: D017973:Tungsten Compounds; D003035:Cobalt; C045951:tungstate; D012825:Silicon