Interchangeability and Disorder in the Solid-State Structures of Two Wall Calix[4]pyrroles Equipped with Iodine and Ethynyl para-Substituents

Herein, the synthesis and X-ray structures of three alpha,beta two wall aryl-extended calix[4]pyrroles having either identical (symmetrically substituted) or different (non-symmetrically substituted) meso-aryl substituents (aryl=4-ethynylphenyl and 4-iodophenyl) are reported. The X-ray structure of the co-crystal formed by the two symmetrically substituted calix[4]pyrroles is also described. In the solid state, all studied alpha,beta-calix[4]pyrroles exhibit a 1,3-alternate conformation with two co-crystallized acetonitrile solvent molecules H-bonded to adjacent cis-pyrrole rings. Remarkably, the 1,3-conformer of the non-symmetrically substituted iodophenyl/ethynylphenyl compound is intrinsically chiral. The two enantiomers are present in the average asymmetric unit in a 65 : 35 occupancy ratio displaying a head-to-tail directional disorder. This is due to the functional complementarity and the isosteric and isoelectronic properties of the para-substituents: iodo and ethynyl. That is, the negative belt of iodine is similar to the negative pi-system of the C equivalent to C triple bond and the sigma-hole in the iodine atom is similar to the positive proton at the C equivalent to C-H group.

Automated summary

This study reports the synthesis and X-ray structures of three alpha,beta two wall aryl-extended calix[4]pyrroles with either identical or different meso-aryl substituents. The solid-state structures of the compounds show a 1,3-alternate conformation and the presence of co-crystallized acetonitrile solvent molecules. The non-symmetrically substituted compound exhibits inherent chirality and a head-to-tail directional disorder due to the functional complementarity of the para-substituents.