Theoretical Study of Oxovanadium(IV) Complexation with Formamidoximate: Implications for the Design of Uranyl-Selective Adsorbents

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Poly(acrylamidoxime) fibers are the current state-of-the-art adsorbent for mining uranium from seawater. However, the competition between uranyl (UO 2 2+ ) and vanadium ions poses a challenge to mining on the industrial scale. In this work, we employ density functional theory and coupled-cluster methods in the restricted formalism to investigate potential binding motifs of the oxovanadium(IV) ion (VO 2+ ) with the formamidoximate ligand. Consistent with experimental extended X-ray absorption fine structure data, the hydrated six-coordinate complex is predicted to be preferred over the hydrated five-coordinate complex. Our investigation of formamidoximate–VO 2+ complexes universally identified the most stable binding motif formed by chelating a tautomerically rearranged imino hydroxylamine via the imino nitrogen and hydroxylamine oxygen. The alternative binding motifs for amidoxime chelation via a nonrearranged tautomer and η 2 coordination are found to be ∼11 kcal/mol less stable. Natural bond orbital analysis was performed to understand the nature of the interactions in the VO 2+ complexes. The difference in the most stable VO 2+ and UO 2 2+ binding conformation has important implications for the design of more selective UO 2 2+ ligands.