Proton cryptates. Kinetics and thermodynamics of protonation of the [1.1.1] macrobicyclic cryptand

Abstract

The (1.1.1) macrobicyclic cryptand, binds one or two protons either outside or inside its intramolecular cavity. The structures of the different mono- and diprotonated forms thus produced have been assigned on the basis of their spectral properties and their proton transfer rates. Five forms have been identified, io/sup +/, o/sup +/o/sup +/i,i/sup +/o/sup +/, and i/sup +/i/sup +/, and the thermodynamics and kinetics of their interconversion have been studied. External mono- and diprotonation or deuteration were studied in D/sub 2/O, acetone-d/sub 6/, and methanol-d/sub 4/. The protonation equilibrium with trifluoroacetic acid in acetone-d/sub 6/ was slow enough at 270 K to yield separate /sup 1/H NMR signals for the four sets of nonequivalent protons of the externally monoprotonated species, (1).H/sup +/. In H/sub 2/O and D/sub 2/O, the pK/sub al/ value for the externally monoprotonated species io/sup +/ is 7.1 +- 0.2, while pK/sub a2/ for the externally diprotonated species o/sup +/o/sup +/ is approx. = 1. The rates of proton transfer into and out of the cavity are very slow. The internally monoprotonated species, i/sup +/i, cannot be deprotonated unless the cryptand is destroyed. The rates of formation of (i/sup +/i) and (i/sup +/i/sup +/) in D/sub 2/O aremore » very slow, with activation energies of about 110 kJ mol/sup -1/ for each of the processes. The rate of the first process varies with pH. The rates for the second internal protonation and the removal of the second internal proton have been studied as a function of temperature. The rate of deprotonation of (i/sup +/i/sup +/) depends on the hydroxide ion concentration and also has an activation energy of about 110 kJ mol/sup -1/. The rate data provide a means for estimating the thermodyanmic stability of the various protonated species. Cryptand is thus a thermodynamically very strong and kinetically extremely slow base. The possible mechanisms for proton transfer into and out of the cavity are discussed.« less