Surface ligands influence the selectivity of cation uptake in polyoxovanadate–alkoxide clusters

Abstract

The selective uptake of lithium ions is of great interest for chemists and engineers because of the numerous uses of this element for energy storage and other applications. However, increasing demand requires improved strategies for the extraction of this element from mixtures containing high concentrations of alkaline impurities. Here, we study solution phase interactions of lithium, sodium, and potassium cations with polyoxovanadate-alkoxide clusters, [V₆O₇(OR)₁₂] (R = CH₃, C₃H₇, C₅H₁₁), using square wave voltammetry and cyclic voltammetry. In all cases, the most reducing event of the cluster shifts anodically as the ionic radius of the cation decreases, indicating increased stability of the reduced cluster and further suggesting that these assemblies might be useful for the selective uptake of Li⁺. Exploring the consequence of ligand length, we found that the short-chain cluster, [V₆O₇(OCH₃)₁₂], irreversibly binds Li⁺ in the presence of excess potassium (K⁺) and exhibits an electrochemical response in titration experiments similar to that observed upon the addition of Li⁺ to the POV–alkoxide in the presence of non-coordinating tetrabutylammonium ions. However, in the presence of excess sodium (Na⁺), the cluster showed only a modest preference for lithium, with exchange between sodium and lithium ions governed by equilibrium. Extending these studies to [V₆O₇(OC₅H₁₁)₁₂], we found that the presence of the pentyl ligands allows the assembly to irreversibly bind Li⁺ in the presence of Na⁺ or K⁺. The change in mechanism caused by surface functionalization of the clusters increases the differential binding affinity for more compact cations, translating to improved selectivity for Li⁺ uptake in these molecular assemblies.