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.