NMR and Crystallographic Studies of a p-Sulfonatocalix[4] arene-Guest Complex

The ¹H and ¹³C NMR spectra of a complex formed from tetrasodium p-sulfonatocalix[4]arene (I₄) and trimelhylanilinium chloride (2) in D₂O showed that 2 is bound to the cavity of cone-shaped I₄: in the acidic pH region (pD 0.4) the phenyl moiety resides in the cavity, whereas in the neutral pH region (pD 7.3) both the trimethylammonium and the phenyl moiety are included nonspecifically in the cavity. The difference was accounted for by the change in the electron density in the benzene π-systems induced by the dissociation of OH groups. The complex was crystallized from acidic aqueous solution. The solid-state ¹³C NMR (CP-MAS) supports the inclusion of the phenyl moiety in the cavity. The X-ray crystallographic studies established that (i) 1₄ adopts a cone conformation, (ii) I₄ and 2 form a host-guest-type complex with the phenyl moiety inserted into the cavity, (iii) not only the hydrophobic force but also the electrostatic force operates for guest inclusion, and (iv) the overall structure is similar to that of clays, I₄ molecules being arranged into bilayers. These findings serve as a useful cross-link between solution complexes and solid-state complexes.