Halogen protected cobalt bis(dicarbollide) ions with covalently bonded CMPO functions as anionic extractants for trivalent lanthanide/actinide partitioning

Halogen protected CMPO derivatives of the cobalt bis(dicarbollide) ion (1−) were synthesized with the aim to develop efficient extraction agents with increased stability for liquid–liquid extraction of lanthanides and actinides from high-level activity nuclear waste. New halogen protected compounds of the general formulation [3,3′-Co(III)-(8-CMPO-(CH2-CH2O)2-1,2-C2B9H10)(8′-X-1′,2′-C2B9H10)]− (where X = Cl, Br, I; CMPO = Ph2P(O)-CH2C(O)(t-C8H17)N-) (4a–c), were prepared and studied for Ln(III)/An(III) group extraction. These compounds were accessible via halogenated zwitterionic amino derivatives [3,3′-Co(III)-(8-t-C8H17NH2-(CH2-CH2O)2-1,2-C2B9H10)(8′-X-1′,2′-C2B9H10)] (3a–c), prepared by several different synthetic methods. All the compounds were characterized by a combination of 11B NMR, 1H high field NMR, ESI-MS, HPLC and other methods. The effect of the halogen in the structures of 4a–c on the extraction properties is outlined. In general, these compounds preserve good extraction efficiency and exhibit a better stability. On the other hand, the character of the halogenated ion 1− seems to prevail in these compounds and their best extraction efficiency was achieved in more polar auxiliary solvents such as nitro-, chloro-solvents or a laurylonitrile–dodecane mixture than in mixtures of hexyl methyl ketone (HMK) and dodecane (D) applied as the optimum solution for extraction with the parent compound 4, which contains no halogen in the structure. The reason for this behaviour can be seen in the substitution of the polarized B(8′)–Hδ− bond, which may contribute to complexation or protonation at the B(8)–O-site, smoothing thus the solubility of 4 in low polar solvents.