STRENGTH OF MOLECULAR COMPLEXATION OF APOLAR SOLUTES IN WATER AND IN ORGANIC-SOLVENTS IS PREDICTABLE BY LINEAR FREE-ENERGY RELATIONSHIPS - A GENERAL-MODEL FOR SOLVATION EFFECTS ON APOLAR BINDING

The stability of an inclusion complex formed by a macrobicyclic cyclophane host and pyrene was studied in water and 17 organic solvents covering the entire polarity range. Complexation strength decreases steadily for the series of solvents beginning with water (–ΔG° = 9.4 kcal·mol–1 at T = 303 K) and continuing to nonaqueous polar protic solvents, to dipolar aprotic solvents, and finally to apolar solvents, e.g, carbon disulfide (–ΔG° = 1.3 kcal·mol–1). This large difference in binding strength results from solvation effects. The empirical solvent polarity parameter ET(30) is very useful for predicting and rationalizing, in terms of a linear free energy relationship, the strength of apolar host-guest complexation in different solvents. While the most stable complexes form in water, strong binding is also observed in formamide and in small alcohols. The free energy of complexation in 2,2,2-trifluoroethanol is measured as –ΔG° = 7.8 kcal-mol–1, and this solvent comes closest to water in its ability to promote apolar binding processes. A general model of solvation effects on apolar complexation is presented. Binding is strongest in solvents with low molecular polarizability and with high cohesive interactions. The most stable complexes of apolar substrates form in water since solvent cohesive interactions are very large and water molecules possess the lowest molecular polarizability of all solvent molecules. The role of water in apolar complexation processes can be rationalized completely on the basis of its physical properties. © 1990, American Chemical Society. All rights reserved.