Dicarboxylate Recognition Properties of a Dinuclear Copper(II) Cryptate

A ditopic polyamine macrobicyclic compound with biphenylmethane spacers was prepared, and its dinuclear copper­(II) complex was studied as a receptor for the recognition of dicarboxylate anions of varying chain length in H₂O/MeOH (50:50 (v/v)) solution. The acid–base behavior of the compound, the stability constants of its complexes with Cu²⁺ ion, and the association constants of the copper­(II) cryptate with succinate (suc^2–), glutarate (glu^2–), adipate (adi^2–), and pimelate (pim^2–) were determined by potentiometry at 298.2 ± 0.1 K in H₂O/MeOH (50:50 (v/v)) and at ionic strength 0.10 ± 0.01 M in KNO₃. The association constants of the same cryptate as receptor for aromatic dicarboxylate substrates, such as phthalate (ph^2–), isophthalate (iph^2–), and terephthalate (tph^2–), were determined through competition experiments by spectrophotometry in the UV region. Remarkably high values of association constants in the range of 7.34–10.01 log units were found that are, to the best of our knowledge, the highest values of association constants reported for the binding of dicarboxylate anions in aqueous solution. A very well defined peak of selectivity was observed with the binding constant values increasing with the chain length and reaching the maximum for substrates with four carbon atoms between the carboxylate groups. Single-crystal X-ray diffraction determinations of the cascade complexes with adi^2– and tph^2– assisted in the understanding of the selectivity of the cryptate toward these substrates. The Hirshfeld surface analyses of both cascade complexes suggest that the establishment of several van der Waals interactions between the substrates and the walls of the receptor also contributes to the stability of the associations