Dicarboxylate Recognition by Two Macrobicyclic Receptors: Selectivity for Fumarate over Maleate

Two ditopic polyamine macrobicyclic compounds have been studied as receptors for the recognition of dicarboxylate anions of varying chain length in aqueous solution. One of the receptors consists of two tris­(2-aminoethyl)­amine-derived binding subunits separated by <i>p</i>-xylyl spacers, while the other is a heteroditopic compound, combining two different head units, a tren-derived and a 2,4,6-triethylbenzene-derived one, also separated by <i>p</i>-xylyl spacers. The acid–base behavior of the compounds as well as their binding ability with oxalate (oxa^2–), malonate (mal^2–), succinate (suc^2–), glutarate (glu^2–), maleate (male^2–) and fumarate (fum^2–) anions were studied by potentiometry at 298.2 K in aqueous solution and at ionic strength 0.10 M in KTsO. NMR studies were also performed to obtain structural information in solution on the supermolecules formed by association of the protonated macrobicycles with the dicarboxylate substrates. The results revealed that both compounds are able to form stable associations with the dianionic substrates in competitive aqueous solution, with unprecedented selectivity for fum^2– over other dicarboxylate competitors, including its <i>cis</i> isomer male^2–. In addition it was found that although the selectivity pattern is unaffected by the introduction of the 2,4,6-triethylbenzene head unit, the affinity toward dicarboxylates is significantly reduced. Therefore, the comparison between the binding behavior of these two receptors showed the effect of the increased rigidity and lipophilicity of the receptor with the 2,4,6-triethylbenzene head unit in the binding properties and the selectivity pattern