Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si2W18O62]8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds |
| |
Authors: | Takuo Minato Dr Kosuke Suzuki Dr Keigo Kamata Prof?Dr Noritaka Mizuno |
| |
Institution: | 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 |
| |
Abstract: | 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, TBA8α‐Si2W18O62] ? 3 H2O ( II ) that possesses a ?8 charge was successfully synthesized by dimerization of a trivacant lacunary α‐Keggin‐type silicotungstate TBA4H6α‐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 TBA6α‐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 P5+ to Si4+, 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 . |
| |
Keywords: | cage compounds catalysts hydrogen bonds nanostructures polyoxometalates |
|
|