Structural Diversity of Tetra- <i>O</i> -ethyl- <i>p</i> - <i>tert-</i> butylcalix[4]arene and a Desolvation-Induced Symmetry-Breaking Single-Crystal to Single-Crystal Transformation with Z′ Doubling

The crystal structures of three distinct conformers of tetra-O-ethyl-p-tert-butylcalix[4]arene have been revealed, namely cone (1), partial cone (2a and 2b), and 1,2-alternate (3a, 3b, 3c). The polymorphs 2a and 2b differ in whether tert-butyl or ethoxy groups engage in the intermolecular C–H···π contacts that dominate the packing. Two solvates of the 1,2-alternate conformer, 3a and 3c, were obtained from acetonitrile and chloroform, respectively. The acetonitrile molecules were removed in a single-crystal to single-crystal fashion from 3a, resulting in the solvent-free phase 3b. This irreversible transition causes the loss of the inversion center present in the solvated phase (monoclinic, P21/c), connected with a doubling of Z′, and generates a noncentrosymmetric lattice (orthorhombic, P212121). Moreover, desolvation of the chloroform solvate (3c), crystallizing in a triclinic system, leads to the same desolvated 3b phase, indicating the stability of the latter. The studies on the polymorphs as well as the desolvation-induced transformations were supported by investigations on bulk material, applying PXRD and TGA/DTA. Furthermore, a crystal structure containing two conformers, the partial cone and 1,2-alternate conformation, was also revealed (4).