Synthesis, characterization and basicity properties of two new oxa-aza macrobicyclic receptors. Crystal structure of a ‘water cryptate’

Abstract

The synthesis and characterization of the two new oxa-aza macrobicycles 16,21 -dimethyl-4,7,10-trioxa-1,13,16,21-tetraazabicyclo[11.5.5]tricosane (C1) and 19,24-dimethyl-4,7,10,13-tetraoxa-1,16,19,24-tetraazabicyclo[14.5.5] hexacosane (C2) have been reported. Their proton transfer behaviour has been studied in aqueous solution by potentiometry (298.1 ± 0.1 K, I= 0.15 mol dm^–3), microcalorimetry and ¹H and ¹³C NMR spectroscopy. Both of them behave as diprotic bases. C1 and C2 behave as strong bases in the first protonation step and as moderate bases in the second one. Their basicity is due to a main enthalpic contribution. NMR experiments indicate that the nitrogen atoms involved in the protonation steps are the methylated ones. The crystal structure of [H₂C2·H₂O][ClO₄]₂[space group P2₁, a= 10.450(2), b= 11.391(2), c= 12.666(3)Å, β= 100.21(2)°, V= 1483.8(5)ų, Z= 2, R= 0.059 and R_w= 0.052] gives confidence to NMR results, confirming that in the [H₂C2·H₂O]²⁺ cation the protons are localized on the methylated nitrogen atoms. Furthermore, a water molecule is enclosed into the macrocyclic cavity, interacting via hydrogen bonds with both protonated nitrogens and three oxygens of the macrocyclic framework and originating a ‘water cryptate’.