Basicity properties of a novel azaparacyclophane receptor and its acyclic precursor: a thermodynamic and structural approach
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)Å3, 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.