silicon and tungstate

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).

Topics: Chemistry Techniques, Synthetic; Cobalt; Dimerization; Models, Molecular; Molecular Conformation; Silicon; Tungsten Compounds

We have synthesized and structurally characterized the monolanthanide-containing polyanions [Ln(beta2-SiW11O39)2]13- (Ln = La (1), Ce (2), Sm (3), Eu (4), Gd (5), Tb (6), Yb (7), Lu (8)). Synthesis was accomplished by reaction of the respective lanthanide ion with the monolacunary Keggin-type precursor [beta2-SiW11O39]8- in a 1:2 molar ratio in 1 M KCl medium at pH 5. Polyanions 1-8 were isolated as potassium salts and then characterized by IR, single-crystal X-ray diffraction, and elemental as well as thermogravimetric analysis. The structures of 1-8 are composed of an eight-coordinated Ln3+ center sandwiched by two chiral (beta2-SiW11O39) units. Large Ln3+ ions appear to favor an (R,R) (or (S,S)) configuration (point group C2) of the Keggin units, with an increasing amount of (R,S) (or (S,R)) configuration (point group C1) found in the solid state as the Ln3+ ion decreases in size. This trend is also supported by solution 183W NMR results for the diamagnetic La3+ and Lu3+ derivatives.

Topics: Isomerism; Lanthanoid Series Elements; Magnetic Resonance Spectroscopy; Models, Molecular; Molecular Conformation; Silicon; Tungsten Compounds

Unsymmetrical functionalization of a discrete alpha-{SiW12O40} Keggin with two heterometallic building blocks, a Cu(II) dimer and a Cu(II) monomer, results in the formation of [{Cu2(O2CMe)2(5,5'-dimethyl-2,2'-bipy)2}{Cu(5,5'-dimethyl-2,2'-bipy)2}SiW12O40] that displays interesting physical properties.

Topics: Copper; Magnetics; Models, Molecular; Organometallic Compounds; Powder Diffraction; Silicon; Surface Properties; Temperature; Tungsten Compounds

Complex formation between metMb and three heteropolytungstates, offering various sizes and charges, has been studied in the pH range 6-8. 1:1 complexes are formed with (KAs4W40O140)27- and (NaSb9W21O86)18-, with an association constant of 10(6) and 4 x 10(5) M-1 respectively, at pH 7.3 and 10 mM ionic strength. Resulting structural changes of the metMb moiety have been investigated by absorption, CD and EPR spectroscopies. Besides an acid-denatured-like form, obtained at a pH as high as 6.5 with the largest polyanion, the formation of an hemichrome is generally observed. It can be reduced to the hemochrome, whereas the complexation of deoxyMb by the polytungstates leaves the deoxy structure unaltered.

Topics: Antimony; Arsenic; Circular Dichroism; Electron Spin Resonance Spectroscopy; Hydrogen-Ion Concentration; Metmyoglobin; Protein Conformation; Silicon; Spectrophotometry; Tungsten; Tungsten Compounds