Electronic and relativistic contributions to ion-pairing in polyoxometalate model systems

Abstract

<p> Ion pairs and solubility related to ion-pairing in water influence many processes in nature and in synthesis including efficient drug delivery, contaminant transport in the environment, and self-assembly of materials in water. Ion pairs are difficult to observe spectroscopically because they generally do not persist unless extreme solution conditions are applied.&nbsp; Here we demonstrate two advanced techniques coupled with computational studies that quantify the persistence of ion-pairs in simple solutions and offer explanations for observed solubility trends. The system of study, (TMA,Cs)₈[M₆O₁₉] (M=Nb,Ta), is a set of unique polyoxometalate salts whose water solubility increases with increasing ion-pairing, contrary to most ionic salts. The techniques employed to characterize Cs⁺ association with [M₆O₁₉]^8- and related clusters in simple aqueous media are ¹³³Cs NMR (nuclear magnetic resonance) quadrupolar relaxation rate and PDF (pair distribution function) from X-ray scattering. The NMR measurements consistently showed more extensive ion-pairing of Cs⁺ with the Ta-analogue than the Nb-analogue, although the electrostatics of the ions should be identical.&nbsp;&nbsp; Computational studies also ascertained more persistent Cs⁺-[Ta₆O₁₉] ion pairs than Cs⁺-[Nb₆O₁₉] ion pairs, and bond energy decomposition analyses determined relativistic effects to be the differentiating factor.&nbsp; These distinctions are likely responsible for many of the unexplained differences between aqueous Nb and Ta chemistry, while they are so similar in the solid-state. The X-ray scattering studies show atomic level detail of this ion-association that has not been prior observed, enabling confidence in our structures for calculations of Cs-cluster association energies. Moreover, detailed NMR studies allow quantification of the number of Cs⁺ associated with a single [Nb₆O₁₉]^8- or [Ta₆O₁₉]^8- anion which agrees with the PDF analyses.&nbsp;</p>