Tropylium cation capsule of hydrogen-bonded tetraurea calix[4]arene dimers

Abstract

The interaction between tropylium salts and tetraurea calix[4]arene derivatives (such as 1 and 2) was studied in solution using 1D, 2D, diffusion, VT NMR and UV–visible spectroscopy. It was found that tropylium salts form charge transfer complexes with both the monomers and dimers of the tetraurea calix[4]arene derivatives depending on the experimental conditions. Compound 1 increases dramatically the solubility of tropylium salts in apolar solvents such as C₂D₄Cl₂, CDCl₃ and CD₂Cl₂ by forming the molecular capsule 1·C₇H₇⁺·1. In contrast to the benzene capsule of 1, in 1·C₇H₇⁺·1 the hydrogen bonds in the equatorial region that hold together the two parts of the dimer change their directionality faster than the NMR time scale (at 400 MHz) at temperatures higher than 298 K. Interestingly, the free energy barrier for this dynamic process at 298 K (ΔG^‡₂₉₈), depends on the nature of the counter-anion. Free energies of activation of 14.3 ± 0.2 kcal mol⁻¹ and 12.6 ± 0.2 kcal mol⁻¹ were found by total lineshape analysis for the dimeric capsules of C₇H₇⁺PF₆⁻ and C₇H₇⁺BF₄⁻, respectively. The affinity of the tropylium cation toward the dimer's cavity is much higher than that of neutral organic guests. Although exact quantitative values are not available due to the low solubility of tropylium salts in apolar solvents, a rough estimation in CD₂Cl₂ shows that the tropylium cation affinity is several orders of magnitude higher than that of benzene, which is known to be a good guest. These results show that once the steric requirements are met, electronic effects may serve as an additional driving force for the formation of such molecular capsules demonstrating the importance of cation–π interactions in such systems.