Crystal structures of crown and aza-crown ether complexes with zirconium, hafnium, niobium, and tantalum fluorometallates

Abstract

A systematic X-ray diffraction study of the interaction products of Zr(IV), Hf(IV), Nb(V), and Ta(V) oxides (fluorides) with crown-ethers (CEs) in aqueous solutions of hydrofluoric acid is performed. It is shown that oxygen-containing CEs form oxonium complexes with [NbF6]s- and [TaF6]s- hexafluorometallate anions. In two systems, [cis-syn-cis-DCH18C6-H3O][TaF6] and [B18C6·H3O][TaF6], the phenomenon of supramolecular isomerism is found, which is caused by a change in the conformation of the macrocycle or by a partial redistribution of intermolecular hydrogen bonds. The use of aza-crown ethers as extractants made it possible to extract unique hydrolytically unstable anions, the products of incomplete fluorine substitution for oxygen atoms in the starting oxides in the form of crystalline complexes with a composition of [(HA15C5)2][Ta2F10O] and [(HA18C6·H2O)(A18C6·H2O)] [(H2O)Nb2F9O]. In [(18C6)(H7O3)2×(Hf2F10·2H2O)], [(HA18C6)(M2F10·2H2O)·(H3O)·H2O], and [(H2DA18C6) (M2F10·2H2O)·2H2O] (M=Zr, Hf) complexes, the metals are extracted in the form of identical (M2F10·2H2O)2s- anions with a similar topology. The performed study demonstrates that macrocyclic complexones are undoubtedly promising to extract Zr(IV), Hf(IV), Nb(V), and Ta(V) from fluorine-containing aqueous solutions.