Crown ether substituted cobalta bis(dicarbollide) ions as selective extraction agents for removal of Cs+ and Sr2+ from nuclear waste

Abstract

A series of crown ether derivatives of the cobalta bis(dicarbollide) closo-[(1,2-C2B9H11)2-3,3′-Co]− (1) anion was prepared using two synthetic strategies. The first method employs dioxane ring cleavage of the closo-[{8-(C4H8O2)-1,2-C2B9H10}-3,3′-Co-(1′,2′-C2B9H11)]0 zwitterionic species with sodium oxymethyl crowns, producing a series of anions with spacer bonded crown ether moieties varying in crown ether ring size. Three compounds of this group; closo-[{8-(15-crown-5-CH2O)-(CH2CH2O)2-1,2-C2B9H10}-3,3′-Co-(1′,2′-C2B9H11)]− (2), closo-[{8-(18-crown-6-CH2O)-(CH2CH2O)2-1,2-C2B9H10}-3,3′-Co-(1′,2′-C2B9H11)]− (3) and [{8-(21-crown-7-CH2O)-(CH2CH2O)2-1,2-C2B9H10}-3,3′-Co-(1′,2′-C2B9H11)]− (4) were prepared in high yields. The second approach involved reaction of deprotonated closo-[8,8′-(OH-1,2-C2B9H12)2-3,3′-Co]− with p-toluenesulfonyl esters derived from hydroxymethyl crown ethers. Only compounds bearing one crown ether ring resulted in preparative amount from this reaction, the [(8-{(15-crown-5-CH2O)-1,2-C2B9H10}-3,3′-Co-{8′-(HO)-1′,2′-C2B9H10}]− (5) and [{8-(21-crown-7-CH2O)-1,2-C2B9H10}-3,3′-Co-(8′-HO-1′,2′-C2B9H10)]− (6). All compounds were characterized by high field 1H and 11B NMR methods, HPLC and MS, with electrospray ionization, and the molecular structure of the Cs+ complex of species 5 was determined by single crystal X-ray diffraction analysis. These compounds represent the first examples where oxygen crown ethers were successfully bonded to the cage of anion 1. The scope and limitations of the use of these compounds in the liquid–liquid extraction of Cs+ and Sr2+ from nuclear waste were studied, including comparison of extraction efficiency and selectivity of the covalently bonded species under study with synergistic mixtures of hexabrominated anion 1 with crown ethers. As a rule, crown ethers linked to the cobalta bis(dicarbollide) cage display distribution coefficients for Cs+ and Sr2+ comparable to the crown ethers mixed with the hexabromoderivative of 1, however, they were found to exhibit better selectivity with respect to Na+ cation.