Basicity properties of a novel azaparacyclophane receptor and its acyclic precursor: a thermodynamic and structural approach

Abstract

The protonation behaviour of the aza-p-cyclophane receptor 1,4,7,16,19,22-hexamethyl-1,4,7,16,19,22-hexaaza[9.9]paracyclophane (L1) and its acyclic precursor 4,4′-N-methyliminobis(ethylene-N-methyliminomethylene)dibenzoic acid (H₂L2) has been studied in aqueous solution by means of potentiometric, calorimetric and ¹H and ¹³C NMR techniques. L1 behaves as a pentaprotic base. NMR experiments have allowed the determination of the stepwise protonation sites. Considering the [H₄L1]⁴⁺ species, the acidic protons occupy alternate positions in the macrocycle, separated by an unprotonated amino group. The crystal structure of [H₄L1](ClO₄)₄(space group Pbca, a= 16.103(6), b= 22.34(2), c= 23.625(8)Å; V= 8499(9)ų, Z= 8, R= 0.0637) confirms the NMR data, showing the four protons located on the amino groups adjacent to the aromatic rings. Two ClO₄^– anions interact, via hydrogen bonds, with the protonated nitrogens of the macrocycle. L1 is characterized by two N₃ binding subunits, each of them interacting with one perchlorate ion. As far as H₂L2 is concerned, calorimetric and NMR data allow the proton sites to be determined. In the H₂L2 species two protons are located on two amino groups, while the two carboxylate groups are unprotonated, giving rise to an ionic structure.