Calix[4]Arene Cavitands: A Solid State Study on the Interactions of their Aromatic Cavity with Neutral Organic Guests Characterised by Acid CH3 or CH2 Groups

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 calix[4]arenes 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 calix[4]arene complexes retrieved from the literature. To understand the effect of the acidity on these parameters, p-cyclohexylcalix[4]arene-biscrown-3 ⋑ CH3CN, p-tert-butylthiacalix[4]arene ⋑ CH3CN, p-tert-butylthiacalix[4]arene ⋑ CH3NO2, 1,3-dipropoxy-p-tert-butylcalix[4]arene ⋑ ClCH2CN and 1,3-dipropoxy-p-tert-butylcalix[4]arene ⋑ 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.