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-pi 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 superset of CH3CN, p-teut-butylthiacalix[4]arene superset of CH3CN, p-tert-butylthiacalix[4]arene superset of CH3NO2, 1,3-dipropoxy-p-tert-butylcalix [4]arene ClCH2CN and 1,3-dipropoxy-p-tert-butylcalix [4] arene superset of CH2(CN)(2) complexes were prepared, crystallised and investigated in the solid state. CH3X guests are bound preferentially by hosts having a C-4 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 C-2v 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-pi 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-pi 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.