Stereognostic coordination chemistry .4. The design and synthesis of a selective uranyl ion complexant

A new approach to ligand design for the sequestration of metal-ore cations has been called stereognostic coordination chemistry, in that the ligand incorporates a traditional Lewis base coordination to the metal center and a hydrogen bond donor to interact with the oxo group. This paper reports the synthesis of ligands that are more rigid and sterically predisposed to bind the targeted UO22+ cation. These are the tripod ligands tris-N,N,N''-[2-(2-carboxy-phenoxy) ethyl]-1,4,7-triazacyclononane bis-hydrochloride (ETAC . 2HCl) and tris-N,N',N''-[2-(2-carboxy-4-decyl-phenoxy) ethyl]-1,4,7-triazacyclononane tris-hydrochloride (DETAC . 3HCl), which chelate uranyl with a tris-carboxylate coordination sphere and provide a hydrogen bond donor through a protonated amine on the triazacyclononane macrocycle to interact with one uranyl oxo atom. Structural models predict that upon uranyl binding the hydrogen bond donor must point directly towards the oxo atom, enforcing a stereognostic interaction. Both ETAC and DETAC chelate the uranyl ion; DETAC is a powerful extractant and will quantitatively extract uranyl into an organic phase at pH 1.9 and above. The extraction coefficient is estimated to be 10(14) in neutral aqueous conditions. Vibrational spectra of O-18 labeled UO22+ have been used to probe the stereognostic coordination to uranyl utilizing hydrogen bonding.