A tetraradical tetraanion is a stable product of 4-electron reduction of cone-5,11,17,23-tetranitro-25,26,27,28-tetrapropoxycalix[4]arene in nonaqueous dimethyl-formamide (DMF) [1]. It represents an electrochemically generated ligand which is able to coordinate all alkali metal ions. The existence of these complexes was recently proved by DC-polarography, cyclic voltammetry and UV-VIS spectroelectrochemistry. Observed stoichiometries are 2:1 (for Li⁺, Na⁺, Cs⁺), 3:1 (Rb⁺, Cs⁺) and 4:1 (K⁺). The complexes are kinetic labile and they have character of ion pairs between negatively charged nitroradical group and metal cations. The „normalized“ values of the stability constants were compared with stability constants of ionic pairs of alkali metals with nitrite anions. About three orders higher values for calix[4]arene radicals can be explained by a „structural vector“. Four charged and oriented nitroradicals on calix-scaffold as donor groups form considerably higher „field for electrostatic interaction“ than randomly oriented simple anions. The same type of interaction was revealed also for cone-5,11,17,23-tetranitro-25,26,27,28-tetrakis(ethoxycarbonylmethoxy)calix[4]arene. This compound was originally suspicious of binding alkali metal ions by ester groups on the lower rim, however, the experimental data exclude this possibility.

Reference

[1] A. Liška, P. Vojtíšek, A.J. Fry, J. Ludvík, Electrochemical and Quantum Chemical Investigation of Tetranitrocalix[4]arenes: Molecules with Multiple Redox Centers, J. Org. Chem. 78 (2013) 10651-10656.