Non-Covalent 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 alternatively protons at the ring were obtained by NMR shift titrations in water. The fluorinated compounds show free energies of complexation which are smaller by ΔΔG=3·4–7·7 kJ mol−1 in comparison with the protonated compounds. The NMR shifts induced upon 100% complexation (CIS values) were obtained simultaneously from non-linear least-squares fitting and indicate intra-cavity inclusion in all cases. The CIS values agree roughly with screening constants calculated from aromatic ring current and linear electric field effects, the latter resulting from the permanent charges at the host compound. Molecular mechanics calculations (CHARMm) indicate that intracavity inclusion is possible with all compounds with negligible strain induced (<1 kJ mol−1) in the macrocycle upon complexation. In contrast, α-cyclodextrin can accommodate fluorinated phenyl compounds only at the rim of the cavity without larger strain. Preliminary data with α-cyclodextrin, obtained by competitive UV–visible titration with methyl orange, indicate again a smaller association free energy (ΔΔG=1·-7 kJ mol−1) for pentafluorphenol compared with normal phenol as guest.