Study on the synthesis and molecular recognition of new receptors for selective complexation of carboxylic acids

A new synthetic method based on the synthesis of unsymmetrical thioureas followed by double S-alkylation reaction by xylylene dibromides was used to obtain isothiouronium receptors. Their binding abilities to acetate, succinate and maleate anions were evaluated by UV-vis spectroscopic titrations in such solvents as water, acetonitrile, methanol and mixtures of acetonitrile/methanol (1:1, v/v). For simple receptor 4 with one isothiouronium group, no selectivity was observed in the complexation of the anions studied. Receptors (R) 5a-c with two thiouronium groups are able to form with all the anions studied (A) not only stable complexes of 1:1 stoichiometry but also other possessing structure of the type AnRm. The most reliable values of stability constants are for systems of the type maleate anion-receptor 5 and acetate-receptor 5b. However, the best selectivity in the mixed solvents is demonstrated by anion-5c system. The study indicates also that particularly 5c is preferred as a chemosensor for the maleate anion. The obtained results suggest that subtle changes in the receptor structure lead to different binding modes towards anions.