Efficient separation of perrhenate as analogue to pertechnetate in nitric acid solution with a DOTA-tetraamide ligand: Solvent extraction, complexation and structure study

Abstract The two DOTA-tetraamide ligands, DOTAM-n-butyl (1,4,7,10-tetrakis[(N-n-butyl; carbamoyl)methyl]-1,4,7,10-tetraazacyclododecane) and DOTAM-benzyl (1,4,7,10- tetrakis[(N-benzylcarbamoyl)methyl]-1,4,7,10-tetraazacyclododecane), were synthesized and characterized by MS, NMR and FTIR spectra. In acid solution, DOTAM-n-butyl was liable to be protonated. FTIR, 1H NMR, ESI-MS spectra and X-ray diffraction analyses revealed that the nitrogen atoms of cyclen ring combined two protons to form a cation with two positive charges. The protonated DOTAM-n-butyl had strong extraction ability and good selectivity for ReO4−. It was extracted by means of the substitution of one ReO4− for one NO3− in the 1:2 complex of DOTAM-n-butyl with HNO3. The solvent extraction and X-ray crystallography demonstrated an anion-exchange extraction model. Meanwhile, X-ray crystal structure analyses of ligand complexes with HReO4 or HNO3 also indicated that ReO4− or NO3− ions were located in the gap between the upper and lower protonated ligands through multiple hydrogen bonds and electrostatic interactions.