Dynamic and structural NMR studies of cavitand-based coordination cages

The interionic structure, kinetic stability, and degree of anion encapsulation of coordination cages 1 were studied by PGSE, NOE, and EXSY NMR techniques. The rate constants for the formation/dissociation processes at 296 K were obtained independently via H-1-NOESY and F-19-NOESY experiments giving, respectively, k(obs) = 0.30 +/- 0.04 s(-1) in CDCl3 and k(obs) = 5.2 +/- 0.8 s(-1) in CD3NO2/CDC13 (7.1) mixture with the proton probe, and k(obs) = 0.33 +/- 0.06 s(-1) in CDCl3 and k(obs) = 5.0 +/- 0.8 s(-1) in CD3NO2/CDC13 (7/1 mixture) with the F-19 probe. PGSE experiments showed that in CDCl3 not only the encapsuled anion but also the external anions translate with the same rate as the cage. F-19,H-1-HOESY experiments indicated that an average of five external triflate anions are located in the equatorial sites close to the palladium moieties, while two of them approach the polar pockets formed by the alkyl chains. In a CD3NO2/CDC13 (7/1) mixture only one or two anions are in close proximity with the cage, while the others are solvated. In all the considered solvents (benzene, chloroform, methylene chloride, and nitromethane) the inclusion of a single unsolvated triflate anion in the cage is quantitative. F-19,H-1-HOESY experiments indicated that the charged guest head points toward one metal center. Therefore, while the ionic aggregation level and kinetic stability of coordination cages 1 are solvent dependent, anion encapsulation is not.