NONCOVALENT INTERACTIONS IN HOST-GUEST COMPLEXES WITH FLUORINATED PHENYL COMPOUNDS

Complexation constants with the macrocyclic azoniacyclophane CP44 and phenyl guest compounds with at least four fluorine atoms or alternativ ely protons at the ring were obtained by NMR shift titrations in water , The fluorinated compounds show free energies of complexation which a re smaller by Delta Delta G=3.4-7.7 kJ mol(-1) in comparison with the protonated compounds, The NMR shifts induced upon 100% complexation (C IS values) were obtained simultaneously from non-linear least-squares fitting and indicate intra-cavity inclusion In all cases, The CIS valu es agree roughly with screening constants calculated from aromatic rin g current and linear electric field effects, the latter resulting from the permanent charges at the host compound, Molecular mechanics calcu lations (CHARMm) indicate that intracavity inclusion is possible with all compounds with negligible strain induced (<1 kJ mol(-1)) in the ma crocycle upon complexation, In contrast, alpha-cyclodextrin can accomm odate fluorinated phenyl compounds only at the rim of the cavity witho ut larger strain. Preliminary data with alpha-cyclodextrin, obtained b y competitive UV-visible titration with methyl orange, indicate again a smaller association free energy (Delta Delta G-1.-7 kJ mol(-1)) for pentafluorphenol compared with normal phenol as guest. (C) 1997 by Joh n Wiley & Sons, Ltd.