An integrated approach to the study of the recognition of guests containing CH3 and CH2 acidic groups by differently rigidified cone p-tert-butylcalix[4]arene derivatives

The efficiency and selectivity of calix[4] arenes, having different rigidities, in recognizing CH3X and CH2XY guests in apolar media have been investigated by H-1 NMR spectroscopy. 1,3-Dipropoxy-p-tert-butylcalix[4] arene (1) turns out to be less selective than the comparatively more rigid biscrown-3-p-tert-butylcalix[4] arene (2). To obtain further information on these recognition processes, both calorimetric and structural studies have been performed in solution and in the solid state, respectively. The calorimetric study shows that the inclusion process is enthalpically driven and entropically unfavoured in all cases. The favourable enthalpic contribution mainly results from specific host-guest CH-pi interactions. Methyl substituted guests show a less unfavourable entropic contribution. Solid state structural determinations of the complexes help in explaining these results.