Compensation of steric demand by cation–π interactions, cobaltocenium cations as guests in tetraurea calix[4]arene dimers

Abstract

The affinities of ferrocene (2) and the cobaltocenium cation (3⁺+), which have roughly the same size and differ in their charge, towards the inner cavity of the dimeric capsule formed by tetraurea calix[4]arene (1) were studied in C₂D₄Cl₂ solutions. While 3⁺+, which occupies more than 75% of the internal volume of the dimer, is readily encapsulated this is not the case for 2. This is probably due to cation–π interactions, which operate only between 3⁺+ and the aromatic rings of the calix[4]arene dimer. We found that the affinity of the cobaltocenium cation is higher than that of the tropylium cation (4⁺) and is only 2–3 times less than that of the tetraethylammoniun cation (5⁺). From the variable temperature ¹H NMR spectra of this capsule, the free energy of activation at 298 K (ΔG^‡_298K) for the reorientation of the hydrogen bonded belt between the two parts of the dimer could be determined by total line shape analysis for the aromatic protons of the calixarene. The value of 14.3 ± 0.2 kcal mol⁻¹ for the dimeric capsules of 3⁺+PF₆⁻ is very similar to the free activation energy found for dimeric capsules of 1 with 4⁺PF₆⁻ and 5⁺PF₆⁻in C₂D₄Cl₂. It becomes significantly lower, if PF₆⁻ is replaced by BF₄⁻. We also found that ten times more DMSO is needed to disrupt the capsule 1·3⁺+·1 than the corresponding 1·1 dimer containing benzene as guest. This demonstrates again the importance of the cation–π interactions for the stability of such hydrogen-bonded dimeric capsules.