Reversible Deprotonation and Protonation Behaviors of a Tetra-Protonated γ-Keggin Silicodecatungstate

The potentiometric titration of a γ-Keggin tetra-protonated silicodecatungstate, [γ-SiW10O34(H2O)2]4– (H4·I), with TBAOH (TBA = [(n-C4H9)4N]+) showed inflection points at 2 and 3 equiv of TBAOH. The 1H, 29Si, and 183W NMR data suggested that the in situ formation of tri-, doubly-, and monoprotonated silicodecatungstates, [γ-SiW10O34(OH)­(OH2)]5– (H3·I), [γ-SiW10O34(OH)2]6– (H2·I), and [γ-SiW10O35(OH)]7– (H·I), with C1, C2v, and C2 symmetries, respectively. Single crystals of TBA6·H2·I suitable for the X-ray structure analysis were successfully obtained and the anion part was a monomeric γ-Keggin divacant silicodecatungstate with two protonated bridging oxygen atoms. Compounds H3·I, H2·I, and H·I were reversibly monoprotonated to form H4·I, H3·I, and H2·I, respectively.