Size and shape trump charge in interactions of oxovanadates with self-assembled interfaces: application of continuous shape measure analysis to the decavanadate anion

Abstract

Polyoxometalates can serve as versatile catalysts in aqueous media. Solvation of these compounds is critical to their catalytic properties. In the studies reported here, we employ ⁵¹V NMR spectroscopy, dynamic light scattering and continuous shape measure analysis to probe the solvation of two polyoxometalates, decavanadate and monomolybdononavanadate, encapsulated in reverse micelles. The ⁵¹V NMR chemical shift reports on the protonation state of the oxometalate while its linewidth reveals the local environment sensed by the oxometalate in the reverse micelle. We have shown that placement of decavanadate in several protonation states in a microemulsion results in spectroscopic observation of the deprotonated [V₁₀O₂₈]⁶⁻ (V₁₀) molecule (Baruah, et al., J. Am. Chem. Soc., 2006, 128(39), 12758–12765). Previous studies have shown that the oxometalate requires at least 2–3 layers of water to stabilize it in the reverse micelles and present work shows that no differences are observed whether oxometalate has a charge of −5 or −6. The dynamic light scattering studies demonstrate that the size of the reverse micelles containing polyoxometalate does not change significantly upon loading. Finally, continuous shape measure (CShM) further describes the structural perturbation of V₁₀ as one of the V-atoms is replaced with a heteroatom. CShM analysis shows minor structural perturbation of the oxovanadate core with heteroatom replacements even though the charge and thus overall polarity of the anion changes. Similarly, the continuous shape measure analysis of protonated forms of V₁₀ show that the variation related to protonation of the V₁₀ anion does not significantly change the shapes of the different classes of vanadium atoms within the compact oxoanion even though the charge and thus overall polarity of the anion changes. Thus, the change in shape and steric interactions are very minor because any change in geometry is counteracted by changes elsewhere in the structure. This study represents the first application of shape measures to oxometalates. Because size and shape could both be critical for interaction with interfaces, two differently charged oxometalates are placed in a nanosized water droplet to investigate how they are solvated. Because little change was observed in the ⁵¹VNMR spectra and smaller changes were found in the size, we conclude that the shape was most important for this interaction and that changes in charge appeared to have a smaller impact on the system.