A chloride capturing system via proton-induced structure transformation between opened- and closed-forms of dodecavanadates

Abstract

Chloride-incorporated dodecavanadates show two distinct structures of the monoprotonated-form [HV₁₂O₃₂(Cl)]⁴⁻ (closed-V12) with a spherical closed-structure and the opened-form [V₁₂O₃₂(Cl)]⁵⁻ (opened-V12). The reaction of closed-V12 with a stoichiometric amount of ethylenediamine drives the structure transformation reaction to opened-V12, quantitatively. From time dependent observations of ⁵¹V NMR, a tube-type intermediate [V₁₂O₃₂(Cl)]⁵⁻ (tube-V12) was observed in the transformation process. Isolation of the intermediate was achieved by the deprotonation reaction of closed-V12 with diethylamine, and the structure transformation was confirmed by using the isolated intermediate. The reverse transformation from opened-V12 to closed-V12 was also achieved by addition of trifluoroacetic acid. The geometrical difference between closed-V12 and opened-V12 is reflected in the reactivity difference to the external reagents, and this was demonstrated by examining the chloride removal reaction by using a silver cation. The incorporated chloride was preserved in the closed-V12 cage even in the presence of a silver cation. In contrast, the chloride in opened-V12 was removed as AgCl by the silver cation. In addition, by the reaction of chloride-free opened-V12 with a quantitative amount of {Et₄N}Cl retrieved opened-V12, showing the capability of opened-V12 to recapture a guest chloride in the cavity. This transformation between two isomeric dodecavanadate structures is regarded as the movement of a molecular mitt to catch a ball and secure it.