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 it s acyclic precursor iminobis(ethylene-N-methyliminomethylene)dibenzoic acid (H(2)L2) has been studied in aqueous solution by means of potent iometric, calorimetric and H-1 and C-13 NMR techniques. L1 behaves as a pentaprotic base. NMR experiments have allowed the determination of the stepwise protonation sites. Considering the [H(4)L1](4+) species, the acidic protons occupy alternate positions in the macrocycle, separ ated by an unprotonated amino group. The crystal structure of [H(4)L1] (CIO4)(4) (space group Pbca, a = 16.103(6), b = 22.34(2), c = 23.625(8 ) Angstrom V = 8499(9) Angstrom(3), Z = 8, R = 0.0637) confirms the NM R data, showing the four protons located on the amino groups adjacent to the aromatic rings. Two CIO4- anions interact, via hydrogen bonds, with the protonated nitrogens of the macrocycle. L1 is characterized b y two N-3 binding subunits, each of them interacting with one perchlor ate ion. As far as H(2)L2 is concerned, calorimetric and NMR data allo w the proton sites to be determined. In the H(2)L2 species two protons are located on two amino groups, while the two carboxylate groups are unprotonated, giving rise to an ionic structure.