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Synthesis of α‐Dawson‐Type Silicotungstate [α‐Si 2 W 18 O 62 ] 8− and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds
Author(s) -
Minato Takuo,
Suzuki Kosuke,
Kamata Keigo,
Mizuno Noritaka
Publication year - 2014
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201400390
Subject(s) - protonation , chemistry , deprotonation , intramolecular force , hydrogen bond , heteroatom , density functional theory , inorganic chemistry , crystallography , medicinal chemistry , stereochemistry , computational chemistry , molecule , ion , ring (chemistry) , organic chemistry
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, TBA 8 [α‐Si 2 W 18 O 62 ] ⋅ 3 H 2 O ( II ) that possesses a −8 charge was successfully synthesized by dimerization of a trivacant lacunary α‐Keggin‐type silicotungstate TBA 4 H 6 [α‐SiW 9 O 34 ] ⋅ 2 H 2 O ( 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 TBA 6 [α‐P 2 W 18 O 62 ]⋅H 2 O ( III ) was not protonated inside the aperture. The density functional theory (DFT) calculations revealed that the basicities and charges of internal μ 3 ‐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 .