Theoretical study of direct internal protonation of a [1.1.1]cryptand by proton carriers

Abstract

This study proposes a direct-protonation mechanism for the internal protonation of the [1.1.1]cryptand N(C₂H₄OC₂H₄)₃N as an alternative to the nitrogen-inversion mechanism. The [1.1.1]cryptand is one of the thermodynamically strongest and kinetically slowest bases. The apparent rate constant for internal protonation in [1.1.1]cryptands increases with increasing acidity along with a double sigmoid curve that contains a plateau and a maximum value. Nitrogen inversion for internal protonation is a unimolecular process caused by the umbrella inversion of the amine in the externally monoprotonated and diprotonated cryptands that are generated under different acidic conditions. This mechanism does not explain why the former inversion is slower than the latter. By contrast, direct internal protonation is a bimolecular reaction of the cryptand with various proton carriers. In this mechanism, the externally monoprotonated cryptand carries a proton through the opening between two amines inside the unprotonated cryptand under weakly acidic conditions, and the excess hydronium carries a proton inside the externally monoprotonated cryptand under strongly acidic conditions. The slower rate of the former protonation is due to the formation of less of the intermediate between the cryptand and the externally monoprotonated cryptand. This study found a solution to accelerate the internal protonation rate. The use of proton carriers with strong basicity (e.g. ammonia) suppresses the generation of the externally monoprotonated cryptand and allows rapid internal protonation without strong acids. This predicts that the cryptand can separate triton from tritiated water diluted with ammonia water or ammonium salt solutions by isotopic ion exchange.