STEREOGNOSTIC COORDINATION CHEMISTRY .1. THE DESIGN AND SYNTHESIS OF CHELATORS FOR THE URANYL-ION

A new approach to the molecular recognition of metal oxo cations is introduced based on a ligand design strategy that provides at least one hydrogen bond donor for interaction with oxo group(s) as well as conventional electron pair donor ligands for coordination to the metal center. This concept of stereognostic coordination of oxo metal ions is exemplified in the design of four tripodal ligands-tris[2-(2-carboxyphenoxy)ethyl]amine [NEB], tris[3-(2-carboxyphenoxy)propyl]amine [NPB], tris[3-(2-carboxynaphthyl-3-oxy)propyl]amine [NPN], and tris[3-(2-carboxy-4-octadecylphenoxy)propyl]amine [NPodB]-for sequestration of the uranyl ion. The ligands NEB, NPB, and NPN form 1:1 complexes with UO22+. The bidentate coordination of carboxyl groups of these compounds is indicated by the infrared spectra, which offer some support for the presence of a hydrogen bond to the uranyl group. Mass spectral data corroborate CPK model predictions that more than five intervening atoms between the tertiary nitrogen atom and the carboxylate groups arc required for metal ion incorporation and monomeric complex formation. Solvent extractions of aqueous UO22+ into chloroform solutions of the ligands have shown them to be powerful extractants. In the case of the very hydrophobic ligand NPodB the stoichiometry of the complexation reaction is shown to be a 1:1 UO2/ligand complex formed by the release of 3 protons. The extraction is quantitative at pH 2.5, and an effective extraction coefficient of about 10(11) is estimated for neutral aqueous solutions of UO22+.