Tert‐Butyl‐calix[4]arenes Substituted at the Narrow Rim with Cobalt Bis(dicarbollide)(1–) and CMPO Groups – New and Efficient Extractants for Lanthanides and Actinides

Abstract

AbstractCalix[4]arene derivatives bearing two residues A(–) derived from cobalt bis(dicarbollide)(1–) (1) and two CMPO groups B at their narrow rim were synthesized from tBu‐calix[4]arene in four steps. The first step involved the preparation of tBu‐calix[4]arene diether derivatives with appropriate precursors for amino groups (mostly nitriles 3). These were O‐alkylated through ring‐opening reactions with the zwitterionic dioxane derivative [(8‐O(CH2CH2)2O‐1,2‐C2B9H10)‐(1′,2′‐C2B9H11)‐3,3′‐Co]0 (10) to produce ionic nitrile derivatives 4. Reduction of the nitrile groups with BH3·SMe2 (or deprotection in the case of the corresponding phthalimido or Boc derivatives 8) led to a series of diamines 5a–f, which were subsequently converted into the corresponding CMPO derivatives 6a–f by acylation with p‐nitrophenyl(diphenylphosphoryl) acetate. Pure cone conformers were isolated in moderate to excellent yields after the second or third reaction step, although the presence of other conformers in the reaction mixtures was sometimes observed by NMR spectroscopy and HPLC, and in one case a di‐A(–)‐di‐B derivative 6c* in the 1,3‐alternate conformation was prepared. The novel ionic ligands 6a–f showed dramatically enhanced extraction abilities for trivalent actinides and lanthanides in relation to the previously reported simple covalent combination of one A(–) with one B group or to various synergistic mixtures of calixarenes substituted exclusively with two A(–) or two B moieties. A 1:1 mixture of calix[4]arenes containing four A(–) anions (13) and four CMPO functions (14), however, showed similarly high extraction efficiency, comparable to that of the best calixarenes 6b–d with mixed substitution. Complexation studies with 6c for La3+, Eu3+ and Yb3+ in methanol carried out by UV spectrophotometry and microcalorimetry revealed the formation of 1:1 and 1:2 metal‐to‐ligand complexes with log β values ranging from 9.6 to 11.7.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)