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Structural and DFT study of pyridine and imine nitrogen atoms as halogen bond acceptors: interplay of halogen bonds in the solid state

As importance of halogen bond in the solid state raises, more and more halogen-bonded cocrystals are synthesized and investigated. [1, 2] In many of them polytopic donors and acceptors are used, which often leads to the competition between different halogen bonds.[3] Further, halogen bonds are usually not isolated in the crystals, but rather coexist with other interactions, which leads to some yet unclear effects, mainly related to how one interaction affects the strength of the other.[4, 5] In order to study the competition of pyridine (Npy) and imine (Nim) nitrogen atoms as halogen bond acceptors, two bis(3- pyridyl)diimines were cocrystalized with three polytopichalogen bond donors: 1, 4- diiodotetrafluorobenzene, 1, 3- diiodotetrafluorobenzene and 1, 3, 5- triiodotrifluorobenzene (Figure 1). SCXRD measurements revealed that the pyridine nitrogen atom is the halogen bond acceptor in all six cocrystals, while imine nitrogen atom is acceptor in three of them. Also, in four cocrystals, the C– I∙∙∙I halogen bond, orthogonal to the C–I∙∙∙Npy halogen bond, was present, having as an acceptor the negative region of iodine atom. The cocrystals and the halogen bonds were characterized by means of quantum-chemical calculations, revealing that the C–I∙∙∙Npy is stronger than C–I∙∙∙Nim halogen bond, both in the gas phase and in the solid state. Also, observed differences in the supramolecular behavior of donors, as well as ambivalent (donor-acceptor) properties of iodine atom in the donor molecules, were elucidated, uncovering so far unseen antagonistic and agonistic couplings between multiple halogen bonds.

halogen-bonded cocrystals, halogen bonding, solution crystallization

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