Host-guest complexes of conformationally flexible -hexyl-2-bromoresorcinarene and aromatic -oxides: solid-state, solution and computational studies.

Host-guest complexes of -hexyl-2-bromoresorcinarene (BrC6) with twelve potential aromatic -oxide guests were studied using single crystal X-ray diffraction analysis and H NMR spectroscopy. In the solid state, of the nine obtained X-ray crystal structures, eight were consistent with the formation of BrC6--oxide complexes. The lone exception was from the association between 4-phenylpyridine oxide and BrC6, in that case the host forms a self-inclusion complex. BrC6, as opposed to more rigid previously studied -ethyl-2-bromoresorcinarene and -propyl-2-bromoresorcinarene, undergoes remarkable cavity conformational changes to host different -oxide guests through C-H···π interactions. In solution phase CDOD/CDCl (1:1 v/v), all twelve -oxide guests form complexes according to H NMR; however, in more polar CDOD/DMSO- (9:1 v/v), only three -oxides with electron-donating groups form solution-phase complexes with BrC6. In solid-state studies, 3-methylpyridine -oxide+BrC6 crystallises with both the upper- and lower-rim BrC6 cavities occupied by -oxide guests. Computational DFT-based studies support that lower-rim long hexyl chains provide the additional stability required for this ditopic behaviour. The lower-rim cavity, far from being a neutral hydrophobic environment, is a highly polarizable electrostatically positive surface, aiding in the binding of polar guests such as -oxides.