Synthesis of α-Dawson-type silicotungstate [α-Si2W18O62]8- and protonation and deprotonation inside the aperture through intramolecular hydrogen bonds.
The design of structurally well-defined anionic molecular metal-oxygen clusters, polyoxometalates (POMs), leads to inorganic receptors with unique and tunable properties. Herein, an α-Dawson-type silicotungstate, TBA8[α-Si2W18O62]⋅3 H2O (II) that possesses a -8 charge was successfully synthesized by dimerization of a trivacant lacunary α-Keggin-type silicotungstate TBA4H6[α-SiW9O34]⋅2 H2O (I) in an organic solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by means of intramolecular hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus-centered POM TBA6[α-P2W18O62]⋅H2O (III) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ3-oxygen atoms were increased by changing the central heteroatoms from P(5+) to Si(4+), thereby supporting the protonation of II. Additionally, II showed much higher catalytic performance for the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde than I and III.
Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan), Fax: (+81) 3-5841-7220.
Takuo
T
Minato
Kosuke
K
Suzuki
Keigo
K
Kamata
Noritaka
N
Mizuno
D018360:Crystallography, X-Ray; D019281:Dimerization; D006860:Hydrogen Bonding; D008958:Models, Molecular; D017646:Organosilicon Compounds; D011522:Protons; D011789:Quantum Theory; D017640:Silicates / Q000138:chemical synthesis / Q000737:chemistry*; D017973:Tungsten Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D000099294:Polyoxometalates
D017646:Organosilicon Compounds; D011522:Protons; D017640:Silicates; D017973:Tungsten Compounds; C102296:polyoxometalate I; D000099294:Polyoxometalates; C021746:silicotungstic acid
cage compounds; catalysts; hydrogen bonds; nanostructures; polyoxometalates