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 tetr aethyl ammonium cation is included as a guest. The seam of hydrogen bonds C =O . . . (HN)(2)C=O in the equatorial region which holds the two calixarene counterparts together changes its directionality fast (at 25 degreesC), wh ile the dimer itself is kinetically stable on the NMR time scale. An energy barrier of AG 49.9 kJ mol(-1) (T-c 276 K) was estimated for this reorienta tion from variable-temperature (VT) NMR measurements. Lowering the temperat ure to about -50 degreesC restricts also the rotation of the encapsulated t etraethyl ammonium cation around a pseudo C-2-symmetry axis in the equatori al plane of the capsule, while its rotation around the C, axis is still fas t. As a result of this restriction two ethyl groups of the tetraethylammoni um cation point towards the "poles" of the capsule, while the other pair li es in the "equator" region. Variable-temperature H-1 NMR experiments led to a barrier of DeltaG* = 54.8 kJ mol(-1) (T-c 306 K) for the exchange of the se different ethyl groups. Studies with the ternary complex formed by a C-2 v-symmetrical tetraurea proved that both processes, reorientation of the hy drogen 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 a nd the interaction energy between the two tetraurea calixarenes is less fav orable by about 15 kcal mol(-1). This is overcompensated by an energy gain of about 36 kcal mol(-1) due to cation-a interactions. These results provid e a rationale to understand the high stability of the complex inspite of th e mobility of the hydrogen-bonded belt.