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 p-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 p-xylyl spacers. The acid–base behavior of the compounds as
well as their binding ability with oxalate (oxa2–), malonate (mal2–), succinate (suc2–), glutarate (glu2–), maleate (male2–) and fumarate (fum2–) 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 fum2– over other dicarboxylate competitors,
including its cis isomer male2–. 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.