Theoretical study of the coordination behavior of formate and formamidoximate with dioxovanadium(v) cation: implications for selectivity towards uranyl

Abstract

Poly(acrylamidoxime)-based fibers bearing random mixtures of carboxylate and amidoxime groups are the most widely utilized materials for extracting uranium from seawater. However, the competition between uranyl (UO2(2+)) and vanadium ions poses a significant challenge to the industrial mining of uranium from seawater using the current generation of adsorbents. To design more selective adsorbents, a detailed understanding of how major competing ions interact with carboxylate and amidoxime ligands is required. In this work, we employ density functional theory (DFT) and wave-function methods to investigate potential binding motifs of the dioxovanadium ion, VO2(+), with water, formate, and formamidoximate ligands. Employing higher level of theory calculations (CCSD(T)) resolve the existing controversy between the experimental results and previous DFT calculations for the structure of the hydrated VO2(+) ion. Consistent with the EXAFS data, CCSD(T) calculations predict higher stability of the distorted octahedral geometry of VO2(+)(H2O)4 compared to the five-coordinate complex with a single water molecule in the second hydration shell, while all seven tested DFT methods yield the reverse stability of the two conformations. Analysis of the relative stabilities of formate-VO2(+) complexes indicates that both monodentate and bidentate forms may coexist in thermodynamic equilibrium in solution. Investigations of VO2(+) coordination with the formamidoximate anion has revealed the existence of seven possible binding motifs, four of which are within ∼4.0 kcal mol(-1) of each other. Calculations establish that the most stable binding motif entails the coordination of oxime oxygen and amide nitrogen atoms via a tautomeric rearrangement of amidoxime to imino hydroxylamine. The difference in the most stable VO2(+) and UO2(2+) binding conformation has important implications for the design of more selective UO2(2+) ligands.

Origin of the unusually strong and selective binding of vanadium by polyamidoximes in seawater

“…A similar E 0 is identified in the V(V) metal oxide standard, Na 3 VO 4 , particularly when compared to V(IV) and V(III) species. Although anticipated from the literature 32 and previous computational work 33 – 35 , this is the first instance the + 5 oxidation state is experimentally confirmed for adsorbent-bound V.…”

Structural and computational characterization of a bridging zwitterionic-amidoxime uranyl complex

“…Previous computational evaluations of AO-uranyl complexes have largely focused on mononuclear structures with binding modes in Figure 1A-C. In contrast, evaluations of zwitterionic AO species are limited to a few computational papers evaluating AO tautomerization on uranium 53 and vanadium 54,55 binding as well as energetics of AO tautomerization. 56,57 Together these studies suggest AO tautomerization, before or after complexation, influences the binding mode and selectivity of AO based materials.…”

Solvation of the vanadate ion in seawater conditions from molecular dynamics simulations

“…Although a number of recent studies have shed important light on speciation of uranyl in different aqueous environments, [13][14][15][16] [17] little is known about speciation of vanadium species in seawater [12,[18][19][20][21]. Equilibrium complexation of HVO4 2has been previously determined from speciation calculations using free ion concentrations modeled for seawater and calculated stability contestants at the appropriate ionic strength [20].…”

“…They found that the aqueous environment induced a conformational change on the two VOH moieties from C2 symmetry to Cs symmetry and an exchange of water molecules between the first solvation sphere of H2VO4and the solution. Recent work with coupled cluster theory examined coordination number and prominent binding modes of water and other ligands to vanadyl (VO2 + ) [19]. From a single-crystal structure, Rao and coworkers have found a unique binding mode of vanadium to amidoxime-type ligand, where the V-O bonds are sequentially displaced by two amidoximate oxygens [23].…”

“…From a single-crystal structure, Rao and coworkers have found a unique binding mode of vanadium to amidoxime-type ligand, where the V-O bonds are sequentially displaced by two amidoximate oxygens [23]. In addition, the oxovanadium (IV) and dioxovanadium (V) complexation with an amidoximate ligand has been studied computationally [18][19].…”