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Structural and DFT study of halogen bond in cocrystal of 1, 4-diiodotetrafluorobenzene and an imine derived from p-phenylenediamine and 3-pyridinecarboxyaldehyde

Understanding competition between short-range supramolecular interactions, such as hydrogen and halogen bonds, is of great importance for crystal engineering (Aakeröy et al., 2014 ; Ding et al., 2014). Although crystal stability does not depend only on these interactions, their energy relationship is usually a sufficient indicator as to which one of them will appear in the crystal. However, energies of hydrogen and halogen bonds in solid state have still not been accurately determined (unlike in gas phase) because influence of the crystal surroundings on these interactions cannot be easily taken into account and is for this reason not yet sufficiently understood. In this work we have synthesized 1:1 cocrystal of 1, 4-diiodotetrafluorobenzene (TFIB) and PDAPCA, an imine derived from p-phenylenediamine and 3-pyridinecarboxyaldehyde in molar ratio 1:2. Its crystal structure was determined by single-crystal X-ray diffraction. It crystalizes in P-1 space group with one formula unit per unit cell. In the structure, PDAPCA and TFIB molecules are alternately connected by C–I∙∙∙N halogen bonds in chains (d(N∙∙∙I)=2.897 Å, φ(C–I∙∙∙N)= 173.64°), and the adjacent chains interact only through weak (dispersion) interactions. In order to investigate the influence of surroundings in synthesized cocrystal on C–I∙∙∙N halogen bond energy, we have calculated energy of halogen bond between one TFIB and one PDAPCA molecule without surroundings and with various number of added vicinal molecules in the chain. All calculations were done in Gaussian 09 program employing the ωB97XD functional (Chai et al., 2008) on geometries obtained from the crystal structure. In all calculations counterpoise correction was included. The results show that halogen bond energy in this cocrystal differs from the one in the gas phase by 3.32 kJ/mol. It is large enough to make difference in the strength ratio (in the solid state in relation to the gas phase) of the two interactions with similar gas phase energies, which might intensely affect their competition in solid state and so dictate the crystal packing.

cocrystals ; halogen bond ; halogen bond energy

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