Synthesis and Catalysis of Di- and Tetranuclear Metal Sandwich-Type Silicotungstates [(γ-SiW10O36)2M2(μ-OH)2]10− an
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- 作者：Yuji Kikukawa ; Syuhei Yamaguchi ; Kazutaka Tsuchida ; Yoshinao Nakagawa ; Kazuhiro Uehara ; Kazuya Yamaguchi ; Noritaka Mizuno
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：April 23, 2008
- 年：2008
- 卷：130
- 期：16
- 页码：5472 - 5478
- 全文大小：1462K
- 年卷期：v.130,no.16(April 23, 2008)
- ISSN：1520-5126
文摘
The di- and tetranuclear metal sandwich-type silicotungstates of Cs10[(γ-SiW10O36)2{Zr(H2O)}2(μ-OH)2]·18H2O (Zr2, monoclinic, C2/c (No. 15), a = 25.3315(8) Å, b = 22.6699(7) Å, c = 18.5533(6) Å, β = 123.9000(12)°, V = 8843.3(5) Å3, Z = 4), Cs10[(γ-SiW10O36)2{Hf(H2O)}2(μ-OH)2]·17H2O (Hf2, monoclinic, space group C2/c (No. 15), a = 25.3847(16) Å, b = 22.6121(14) Å, c = 18.8703(11) Å, β = 124.046(3)°, V = 8974.9(9) Å3, Z = 4), Cs8[(γ-SiW10O36)2{Zr(H2O)}4(μ4-O)(μ-OH)6]·26H2O (Zr4, tetragonal, P41212 (No. 92), a = 12.67370(10) Å, c = 61.6213(8) Å, V = 9897.78(17) Å3, Z = 4), and Cs8[(γ-SiW10O36)2{Hf(H2O)}4(μ4-O)(μ-OH)6]·23H2O (Hf4, tetragonal, P41212 (No. 92), a = 12.68130(10) Å, c = 61.5483(9) Å, V = 9897.91(18) Å3, Z = 4) were obtained as single crystals suitable for X-ray crystallographic analyses by the reaction of a dilacunary γ-Keggin silicotungstate K8[γ-SiW10O36] with ZrOCl2·8H2O or HfOCl2·8H2O. These dimeric polyoxometalates consisted of two [γ-SiW10O36]8− units sandwiching metal–oxygen clusters such as [M2(μ-OH)2]6+ and [M4(μ4-O)(μ-OH)6]8+ (M = Zr or Hf). The dinuclear zirconium and hafnium complexes Zr2 and Hf2 were isostructural. The equatorially placed two metal atoms in Zr2 and Hf2 were linked by two μ-OH ligands and each metal was bound to four oxygen atoms of two [γ-SiW10O36]8− units. The tertanuclear zirconium and hafnium complexes Zr4 and Hf4 were isostructural and consisted of the adamantanoid cages with a tetracoordinated oxygen atom in the middle, [M4(μ4-O)(μ-OH)6]8+ (M = Zr or Hf). Each metal atom in Zr4 and Hf4 was linked by three μ-OH ligands and bound to two oxygen atoms of the [γ-SiW10O36]8− unit. The tetra-nuclear zirconium and hafnium complexes showed catalytic activity for the intramolecular cyclization of (+)-citronellal to isopulegols without formation of byproducts resulting from etherification and dehydration. A lacunary silicotungstate [γ-SiW10O34(H2O)2]4− was inactive, and the isomer ratio of isopulegols in the presence of MOCl2·8H2O (M = Zr or Hf) were much different from that in the presence of tetranuclear complexes, suggesting that the [M4(μ4-O)(μ-OH)6]8+ core incorporated into the POM frameworks acts as an active site for the present cyclization. On the other hand, the reaction hardly proceeded in the presence of dinuclear zirconium and hafnium complexes under the same conditions. The much less activity is possibly explained by the steric repulsion from the POM frameworks in the dinuclear complexes.