Amphiprotic properties of a bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type silicodecatungstate containing two different kinds of hydroxyl moieties.

A bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type silicodecatungstate, (TBA)4[γ-SiV(IV)2W10O36(μ-OH)4] (1), possesses two different kinds of hydroxyl groups and can work as an amphiprotic species to accept and donate proton(s). Dehydrative condensation reactions of 1 with methanol and formic acid proceed on more basic hydroxyl groups between two vanadium atoms without the deprotonation of more acidic hydroxides between two tungsten atoms to form (TBA)4[γ-SiV(IV)2W10O36(μ-OH)3(μ-OR)] (2·R, R = Me, Et, Pr; 3, R = C(O)H), showing Brønsted base properties of the hydroxyl groups between two vanadium atoms. On the other hand, the hydroxyl groups between tungsten atoms exhibit Brønsted acid properties and react with pyridine (Py) and TBAOH to form (TBA)4X[γ-SiV(IV)2W10O37(μ-OH)3] (PyH·4, X = PyH; TBA·4, X = TBA). DFT calculations for [γ-SiV(IV)2W10O36(μ-OH)4](4-) in water also support both the acidic and basic nature of hydroxyl groups in 1.