Inter and intra-phase conformerism in two calix [4]arenes

Calix [4]arenes are macrocyclic compounds which can assume four main conformations (cone, partial cone, 1,2-alternate, and 1,3-alternate) driving their desired host-guest ability. Here we report a new polymorph of 25,26,27,28-tetrakis(methoxycarbonylmethoxy)calix [4]arene, which is present with two similar crystallographically independent molecules. These molecules are cone-shaped, in contrast to the 1,3-alternate conformation found in the previously known crystal form of the same compound. Our DFT calculations have also pointed out that the literature 1,3-alternate conformation is more stable than the cone one in the gas phase. Therefore, the higher energy cone conformation is assisted in the solid state by C–H … π contacts, since these interactions occur only in the polymorph described here. We also reported in this study the simultaneous occurrence of two conformations in the crystal lattice of 5,11,17,23-tetra(p-fluoroaminobenzylidene)-25,26,27,28-tetrabutoxycalix [4]arene. Pinched cone conformations are assumed through either π … π or C–H … π intramolecular contacts between phenyl rings of the upper rim substituents. These non-covalent contacts are responsible for pointing two alternate phenyl rings from the annulus and their interacted upper rim substituents inwards the cone cavity. DFT calculations reveal that these conformations have very similar energies, helping us to understand the conformerism phenomenon into the elucidated solid state form. Such conformerism found either in two distinct solid state forms or into a same one shows the high conformational complexity of simple calixarenes, which can be decisive to their desired host-guest properties.