Dimeric capsules of tetraurea calix[4]arenes. MD simulations and X-ray structure, a comparison

Abstract

The single crystal X-ray structure of a homodimer of a tetra(tolylurea) calix[4]arene including a tetraethylammonium cation as guest shows an expansion of the capsule and a distortion of its shape, in comparison to the structure of a similar dimer with an encapsulated benzene molecule. Thus, only 8 of 16 possible hydrogen bonds are present in the hydrogen bonded belt holding together the two hemispheres. The encapsulated cation is disordered over two equivalent positions with two methyl groups pointing to the equator, while two methyl groups pointing to the poles form CH–π interactions with the inner surfaces of the calixarene cavities. MD simulations are in agreement with the distorted X-ray structure for a short simulation time of 1–2 ns with a given orientation of the included cation and approach for longer simulation times (9 ns) the fourfold symmetry found by ¹H NMR spectroscopy in solution.