Synthesis and catalysis of di- and tetranuclear metal sandwich-type silicotungstates [(γ-SiW10O36)2M 2(μ-OH)2]10- and [(γ-SiW 10O36)2M44-O) (μ-OH)6]8- (M = Zr or Hf)

Yuji Kikukawa, Syuhei Yamaguchi, Kazutaka Tsuchida, Yoshinao Nakagawa, Kazuhiro Uehara, Kazuya Yamaguchi, Noritaka Mizuno

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The di- and tetranuclear metal sandwich-type silicotungstates of Cs 10[(γ-SiW10O36)2{Zr(H 2O)}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-SiW10O 36)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), Cs 8[(γ-SiW10O36)2{Zr(H 2O)}44-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-SiW10O 36)2{Hf(H2O)}44-O) (μ-OH)6]·23H2O (Hf4, tetragonal, P4 1212 (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-SiW 10O36] 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 [M44-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, [M44-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 [M 44-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.

Original languageEnglish
Pages (from-to)5472-5478
Number of pages7
JournalJournal of the American Chemical Society
Issue number16
Publication statusPublished - 2008 Apr 23


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