Synthesis of ﾎｱ-Dawson-Type Silicotungstate [ﾎｱ-Si2W18O62]8- and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds.
英文:
Synthesis of ﾎｱ-Dawson-Type Silicotungstate [ﾎｱ-Si2W18O62]8- and Protonation and Deprotonation Inside the Aperture through Intramolecular Hydrogen Bonds.
The design of structurally well-defined anionic mol. metal-oxygen clusters, polyoxometalates (POMs), leads to inorg. receptors with unique and tunable properties. Herein, an ﾎｱ-Dawson-type silicotungstate, TBA8[ﾎｱ-Si2W18O62]3.H2O (II) that possesses a -8 charge was successfully synthesized by dimerization of a trivacant lacunary ﾎｱ-Keggin-type silicotungstate TBA4H6[ﾎｱ-SiW9O34]2.H2O (I) in an org. solvent. POM II could be reversibly protonated (in the presence of acid) and deprotonated (in the presence of base) inside the aperture by intramol. hydrogen bonds with retention of the POM structure. In contrast, the aperture of phosphorus-centered POM TBA6[ﾎｱ-P2W18O62].H2O (III) was not protonated inside the aperture. The d. functional theory (DFT) calcns. revealed that the basicities and charges of internal ﾎｼ3-oxygen atoms were increased by changing the central heteroatoms from P5+ to Si4+, thereby supporting the protonation of II. Addnl., II showed much higher catalytic performance for the Knoevenagel condensation of Et cyanoacetate with benzaldehyde than I and III. [on SciFinder(R)]
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