Halogen trimer-mediated hexagonal host framework of 2,4,6-tris(4-halophenoxy)-1,3,5-triazine. Supramolecular isomerism from hexagonal channel (X = Cl, Br) to cage structure (X = I)

Abstract

The hexagonal layer structure of host atoms in chloro, bromo, and iodo derivatives of 2,4,6-tris(4-halophenoxy)-1,3,5-triazine, X-POT, is stabilized by a cyclic and cooperative halogen trimer (X···X) synthon. The X···X distance is ~3.5 Å in isostructural channel inclusion adducts of Cl-POT and Br-POT, whereas I···I is ~3.8 Å in cage structures of I-POT with aromatic and hydrophobic guests. X-ray crystal structures of I-POT with mesitylene, collidine, tribromomesitylene, triiodomesitylene, hexachlorobenzene, hexafluorobenzene, 1-methylnaphthalene, CH₂Cl₂, CH₂Br₂, and CH₂I₂ guests in R₃ space group are reported. Host molecules are fully ordered in these isostructural clathrates, whereas guest atoms are disordered except for C₆Cl₆ and C₆F₆. The guest molecule resides in a penta-decker sandwich surrounded by double layers of iodo trimer and triazine ring. Supramolecular isomerism from channel to cage framework and the persistent crystallization of trigonal X-POT molecules in high-symmetry host networks (space groups P6₃/m, P6₃, R₃) are discussed in crystal engineering terms: halogen trimer synthon, C_3i-Piedfort unit, weak C-H···O/N interactions, changes in size/polarizability of halogen atom, and CSDSymmetry statistics. Br-POT crystallizes in a channel or cage lattice depending on the guest species. Guest release from the cage framework occurs at a higher temperature compared to the channel structure in thermal gravimetric analysis, suggesting applications in organic zeolites. This study illustrates several aspects of crystal engineering from the understanding of intermolecular interactions to the design of crystal structures and their utility as functional solids.