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-SiVIV2W10O36(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-SiVIV2W10O36(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-SiVIV2W10O37(μ-OH)3] (PyH·4, X = PyH; TBA·4, X = TBA). DFT calculations for [γ-SiVIV2W10O36(μ-OH)4]4– in water also support both the acidic
and basic nature of hydroxyl groups in 1.