A route to a Keggin-Type α-[(XIIIO4)Mo12O35(OH)]4− anion through an Anderson-type [XIII(OH)6Mo6O18]3− anion: X = Ga

Abstract

From an aqueous Mo^VI–Ga^III–HCl system, a colourless complex was isolated as a K⁺ salt, which consists of a hexaprotonated Anderson-type [Ga(OH)₆Mo₆O₁₈]³⁻ anion. A yellow complex became kinetically stable by the presence of CH₃CN at concentrations of 30–40% (v/v). The X-ray structural analysis revealed that the yellow (NPrⁿ₄)₄[(GaO₄)Mo₁₂O₃₅(OH)] crystal contains an α-Keggin structure and the oxygen atom at an edge-shared contact is protonated. The formation conditions of the Keggin complex were elucidated in relation to those of the Anderson complex by a combined ⁷¹Ga NMR and voltammetric study. Evidence was obtained of a spontaneous conversion of [Ga(OH)₆Mo₆O₁₈]³⁻ to [(GaO₄)Mo₁₂O₃₅(OH)]⁴⁻ in the Mo^VI–Ga^III system.