The family of macrobicyclic ligands called vaulted cyclidenes was designed to model the ternary complex formed by cytochrome P-450 in its reaction cycle, thereby establishing new structural features for coordination catalysts. Thus, it is necessary for these complexes to simultaneously bind dioxygen by coordination to a chelated metal ion and bind an organic molecule by host–guest complexing using hydrophobic interactions. We report here a detailed study of the host–guest complexing behavior of the vaulted cyclidene complexes. The crystal structure is reported for an example of the host molecules used in these studies. The copper(II) complexes of the vaulted cyclidene ligands have been prepared in order to study inclusion complexing by NMR relaxation techniques. Extensive experimental justification has been provided for the calculation of distances from the relaxation rates. Three FT-NMR spectrometers operating at different frequencies gave essentially identical results. Outer-sphere effects were shown to be small. Temperature dependence studies showed that the relaxation process is not chemical-exchange-limited. The conditions for saturation of the host–guest equilibrium were determined experimentally. Structures having very small or no cavities show, by counterexamples, the effects of the permanent void. This unusually well-controlled application of a well-known, sometimes maligned technique has yielded remarkable structural information about the inclusion complexes that exist in solution. Alcohols and phenols enter into the inclusion complex formation in a regiospecific manner. They extend their hydrophobic components into the cavity of the vaulted cyclidene complex, leaving their hydrophilic OH groups immersed in the solvent sheath that surrounds the host ion. The cavity is adequate to completely enclose the hydrocarbon moieties of all the alcohols studied with the result that their OH groups are, in all cases, at about the same distance from the copper atom. Phenols assume the same orientation but are too large to be engulfed by the small cavity. Solvent data are consistent with the expectation that a single water molecule is coordinated on the side of the copper opposite that of the cavity. These results support the credibility of the vaulted cyclidene systems as ternary complex models.
ASJC Scopus subject areas
- Colloid and Surface Chemistry