Molecular motions within self-assembled dimeric capsules with tetraethylammonium cations as guest

Hydrogen-bonded, dimeric capsules of calix[4]arenes substituted at the wide rim by four urea functions show unprecedented dynamic features when a tetraethylammonium cation is included as a guest. The seam of hydrogen bonds C=O...(HN)2C=O in the equatorial region which holds the two calixarene counterparts together changes its directionality fast (at 25 degrees C), while the dimer itself is kinetically stable on the NMR time scale. An energy barrier of deltaG++ = 49.9 kJmol(-1) (Tc 276 K) was estimated for this reorientation from variable-temperature (VT) NMR measurements. Lowering the temperature to about -50 degrees C restricts also the rotation of the encapsulated tetraethylammonium cation around a pseudo C2-symmetry axis in the equatorial plane of the capsule, while its rotation around the C4 axis is still fast. As a result of this restriction two ethyl groups of the tetraethylammonium cation point towards the "poles" of the capsule, while the other pair lies in the "equator" region. Variable-temperature 1H NMR experiments led to a barrier of deltaG++=54.8kJ mol(-1) (Tc 306 K) for the exchange of these different ethyl groups. Studies with the ternary complex formed by a C2v-symmetrical tetraurea proved that both processes, reorientation of the hydrogen bonds and rotation of the guest, take place independently. Molecular dynamics simulations suggest that the capsule is strongly expanded by the larger tetraethylammonium cation in comparison with benzene as guest. Thus, on average only one N-H...O hydrogen bond is formed per urea function and the interaction energy between the two tetraurea calixarenes is less favorable by about 15 kcal mol(-1). This is overcompensated by an energy gain of about 36 kcal mol(-1) due to cation-pi interactions. These results provide a rationale to understand the high stability of the complex inspite of the mobility of the hydrogen-bonded belt.