C-H-CENTER-DOT-CENTER-DOT-CENTER-DOT-PI HYDROGEN-BONDING BETWEEN ELECTRON-RICH BENZENE RINGS AND POLARIZED C-H BONDS - SELECTIVITY IN THE COMPLEXATION OF HIGHLY HYDROPHILIC GUEST MOLECULES WITH CALIX[4]RESORCARENE HOSTS IN WATER

Tetrasulfonate derivatives of calix[4]resorcarene (calix[4]arene derived from resorcinol) (1) form 1:1 complexes with highly hydrophilic guest molecules, such as ethers, alcohols, ketones, and sulfoxides, in water. The affinities of three types of the guests increase in the order CH3-X-CH3<(-CH2CH2-X-)(2)<CH3-X-CH2CH2-X-CH3 (X=O, CH(OH), C=O, or S=O), reflecting the importance of multiple host-guest interactions. The binding constants (K) with respect to X increase in the order O<CH(OH)<C=O<S=O or CK(OK)<O<C=O<S=O. As for the effects of substituents Y on 2-C of the benzene rings of the host, both 1b (Y=CH3) and 1c (Y=OH) exhibit higher binding capabilities than does the parent host 1a (Y=H). Thus, the present complexation is promoted by electron-withdrawing residue (X) in the guests and electron-donating substituents (Y) in the host. The binding of CH3-X-CH3 (X=C=O or S=O) to hosts 1a-c is characterized by favorable enthalpy changes and unfavorable entropy changes. These results, coupled with NMR data, indicate that the driving force of the present complexation is a C-H ...pi interaction between C-H bonds of a guest as soft acids and benzene rings of the host as soft bases.