Conformational changes in C_methyl-resorcinarene pyridine N -oxide inclusion complexes in the solid state

Aromatic N-oxides interact with C_methyl-resorcinarene resulting in marked changes in the conformation of the host resorcinarene. In the solid state, 2- and 3-methylpyridine N-oxides form pseudo-capsular 2 : 2 endo host-guest complexes with C_methyl-resorcinarene stabilized by C-H⋯π interactions. The C_methyl-resorcinarene·2-methylpyridine N-oxide complex has a C_4v crown conformation, while the C_methyl-resorcinarene·3-methylpyridine N-oxide complex has a slightly open C_2v boat conformation. On the contrary, other para-substituted and benzo-fused pyridine N-oxides form only exo complexes with C_methyl-resorcinarene. In the exo complexes, the asymmetry of the guest, conformational flexibility and preferred inter-host hydrogen bonding of C_methyl-resorcinarenes exclude endo complexation. All the exo complexes form robust 1-D hydrogen bonded chains between the host hydroxyl groups assisted by the guest N-O groups, resulting in a C_2v boat conformation.