Zirconium(IV) Complexes of Oxydiacetic Acid in Aqueous Solution and in the Solid State as Studied by Multinuclear NMR and X‐ray Crystallography

The structures of complexes of ZrIV and oxydiacetate (ODA2−) in aqueous solutions of pH 0–7 were investigated with the use of 1H, 13C, and 17O NMR spectroscopy. Equilibria of mononuclear [Zr(oda)]2+, [Zr(oda)2], and [Zr(oda)3]2− complexes have been observed. In all complexes ODA2− is bound in a tridentate fashion through the two carboxylate groups and the ether oxygen. No di- or oligonuclear species containing ODA2− were observed. An excess of free ZrIV remains in solution, probably as a result of weak electrostatic interactions between negatively charged Zr–ODA complexes or free ODA2− and a positively charged cyclic tetranuclear hydroxy zirconium complex. CP-MAS 13C NMR spectra of solid compounds isolated from the samples indicated that the structures of the [Zr(oda)2] and [Zr(oda)3]2− complexes in solution are similar to those in the solid state. This is corroborated by the single-crystal X-ray structure of Na2[Zr(oda)3]⋅5.5 H2O, which was obtained from a solution containing exclusively the [Zr(oda)3]2− complex. In this structure ZrIV is nine-coordinate with the three ODA2− ligands bound in a tricapped trigonal prismatic geometry. The negative charge of this [Zr(oda)3]2− complex is balanced by two Na+ ions, one of which is on a center of symmetry between Δ and Λ enantiomers of [Zr(oda)3]2−. This Na+ is octahedrally coordinated to six (non ZrIV-bound) carboxylate oxygen atoms of six different [Zr(oda)3]2− units.