Synthesis of functionalized cobaltacarboranes based on the closo-[(1,2-C 2B 9H 11) 2-3,3′-Co] − ion bearing polydentate ligands for separation of M 3+ cations from nuclear waste solutions. Electrochemical and liquid–liquid extraction study of selective transfer of M 3+ metal cations to an organic phase. Molecular structure of the closo-[(8-(2-CH 3OC 6H 4O)-(CH 2CH 2O) 2-1,2-C 2B 9H 10)-(1′,2′-C 2B 9H

Abstract

A new series of the boron substituted cobalt bis(dicarbollide)(1-) ion ( 1) derivatives of the general formula closo-[(8-X-(CH 2CH 2O) 2-1,2-C 2B 9H 10)(1′,2′-C 2B 9H 11)-3,3′-Co] − (X=1-O-2-CH 3OC 6H 4 ( 3), 1-O-2-C 6H 5CH 2C 6H 4 ( 4), 1-O-4-t-C 8H 17C 6H 4 ( 5), P(O)(OC 4H 9) 2 ( 6) and P(O)(OC 4H 9)(OH) ( 7) was prepared by ring cleavage of 8-dioxane- 1 derivative ( 2). The compounds were synthesized with the aim to develop a new class of extraction agents effective for liquid–liquid extraction of polyvalent cations, i.e. lanthanides and actinides from high level activity nuclear waste. All compounds were characterized by a combination of 11B NMR, 1H high field NMR methods, mass spectrometry with electrospray ionization, HPLC and other techniques. The molecular structure of the sodium salt of anion 3 was determined by single crystal X-ray diffraction analysis. Crystallographic results proved the sodium atom is tightly coordinated to five oxygen atoms of the spacer chain, the guaiacolyl terminal group X and one water molecule, and from the opposite side of the ligand plane the short B(8′)-HNa contact (2.26(3) Å) was found to be within bonding distance. The sodium cation is enveloped with an hydrophobic outer sphere composed of hydrophobic CH 2 and CH groups of the organic substituent and BH groups of cobalt bis(dicarbollide)(1-) cage. The complexation ability of all new anionic ligands for target metal cations was tested by means of cyclic voltammetry at the interface between two immiscible electrolyte solutions. The measurements confirmed stoichiometry of 1:3 (cation:ligand) for the Eu 3+ complex. Liquid–liquid extraction tests proved the extraction efficiency of anionic species for M 3+ cations, most of them being able to extract Eu 3+ only from slightly acidic solutions up to pH 2. The compounds allowed the use of low polar, less environmentally dangerous solvents, e.g. toluene in the liquid–liquid extraction process. Encouraging results were obtained for compound 7, for which the extraction coefficients from 1 M HNO 3 were found to be sufficiently high even in respect of possible technological applications.