Complexation equilibria involving salts in non-aqueous solvents: ion pairing and activity considerations

Complexation of anions, cations and even ion pairs is now an active area of investigation in supramolecular chemistry; unfortunately it is an area fraught with complications when these processes are examined in low polarity organic media. Using a pseudorotaxane complex as an example, apparent K_a2 values (=[complex]/{[salt]_o?[complex]}{[host]_o?[complex]}) for pseudorotaxane formation from dibenzylammonium salts ( 2 ‐X) and dibenzo‐[24]crown‐8 ( 1 , DB24C8) in CDCl₃/CD₃CN 3:2 vary with concentration. This is attributable to the fact that the salt is ion paired, but the complex is not. We report an equilibrium model that explicitly includes ion pair dissociation and is based upon activities rather than molar concentrations for study of such processes in non‐aqueous media. Proper analysis requires both a dissociation constant, K_ipd, for the salt and a binding constant for interaction of the free cation 2 ⁺ with the host, K_a5; K_a5 for pseudorotaxane complexation is independent of the counterion (500 M ^?1), a result of the complex existing in solution as a free cation, but K_ipd values for the salts vary by nearly two orders of magnitude from trifluoroacetate to tosylate to tetrafluoroborate to hexafluorophosphate anions. The activity coefficients depend on the nature of the predominant ions present, whether the pseudorotaxane or the ions from the salt, and also strongly on the molar concentrations; activity coefficients as low as 0.2 are observed, emphasizing the magnitude of their effect. Based on this type of analysis, a method for precise determination of relative binding constants, K_a5, for multiple hosts with a given guest is described. However, while the incorporation of activity coefficients is clearly necessary, it removes the ability to predict from the equilibrium constants the effects of concentration on the extent of binding, which can only be determined experimentally. This has serious implications for study of all such complexation processes in low polarity media.