To gain information on CH-π aromatic interactions involved in the formation of host-guest adducts, the geometrical parameters which define the solid state structures of the complexes of calixarenes in the cone conformation with guests having acid CH3 or CH2 groups have been studied. Most of the data have been obtained from the CH3CN and CH2Cl2 calixarene complexes retrieved from the literature. To understand the effect of the acidity on these parameters, p-cyclohexylcalixarene-biscrown-3 ⋑ CH3CN, p-tert-butylthiacalixarene ⋑ CH3CN, p-tert-butylthiacalixarene ⋑ CH3NO2, 1,3-dipropoxy-p-tert-butylcalixarene ⋑ ClCH2CN and 1,3-dipropoxy-p-tert-butylcalixarene ⋑ CH2(CN)2 complexes were prepared, crystallised and investigated in the solid state. CH3X guests are bound preferentially by hosts having a C4 symmetry. The interaction is directional, but it is independent from the basicity of the host and acidity of the guest, indicating that classic hydrogen bond do not play a major role. On the contrary CH2XY guests find the best matching with hosts having a C2v symmetry, interacting specifically with two diametrical aromatic rings. These interactions are directional and show a correlation between the acidity of the guest and the CH-π aromatic distance, thus supporting a stronger contribution of “classic” hydrogen bond in these latter complexes. These results are in agreement with the hypothesis that CH-π aromatic interactions derive from the superimposition of different types of intermolecular forces, whose contribution depends on several factors as the nature of the interacting partners.
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