A Self-Assembled Ionophore with Remarkable Cs+ Selectivity

Abstract

Nuclear waste management requires methods for radionuclide separation, and a major component of the waste is 137Cs.1,2 Due to its 30 year half-life, most of the 137Cs produced during the nuclear age still exists. Methods for 137Cs+ separation include precipitation as phosphotungstate salts, ion exchange chromatography, and extraction by ionophores.3 Highly selective ionophores are required to separate 137Cs+, since Na+ and K+ concentrations in nuclear waste are much greater than that of 137Cs+.3 Selective coordination of Cs+ (r ) 1.67 A) in the presence of Na+ (r ) 0.97 A) and K+ (r ) 1.33 A) is challenging. Because of their flexibility, crown ethers often have only modest Cs+ selectivities.4 More promising results have been obtained with rigid macrocycles,5 particularly the calix[4]arenecrowns.6-8 While the Cs+ selectivities of the calixarenecrowns are impressive, cation and ionophore recovery may prove difficult due to the stability of the ionophore-Cs+ complex. An alternative ionophore design uses hydrogen bonds to build self-assembled structures that coordinate ions.9-11 Cation binding affinity and selectivity may be achieved through cooperative assembly of the host. We have focused on 5′-(tertbutyldimethylsilyl)-2′,3′-O-isopropylidene isoguanosine (isoG) 1. IsoG 1 self-associates in organic solvents to form a stable tetramer, (isoG)4 2 (Scheme 1).11,12 Tetramer 2, with four oxygens in its central cavity, has a high affinity for cations. Isopropylidene 1 coordinates K+ to form (isoG)8-K 3, with a binding constant rivaling that of 18-c-6 derivatives.11b We proposed that the conformational rigidity of the isopropylidene facilitates self-association of isoG 1. Herein, we demonstrate that isoG’s sugar influences both the Cs+ affinity and Cs+/K+ selectivity of the self-assembled ionophore. Specifically, isopropylidene 1 forms a self-assembled ionophore with remarkable Cs+ selectivity.