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Competition between halogen bond donors in the solid state

Halogen bonds are intermolecular interactions between the partially positively charged part of a halogen atom bonded to an electron-withdrawing group, and a Lewis base. The strength of a halogen bond strongly depends on the chemical surroundings of the halogen atom (Metrangolo et al., 2016). In this study, we have prepared halogen bonded cocrystals of 4-benzoylpyridine (4bzpy) with octafluoro-1, 4-diiodobutane (ofib) and N-bromosaccharin (nbSac) by both liquid-assisted grinding (LAG) and conventional solution crystallization. Cocrystals have been characterised by powder and single crystal XRD. In the crystal structures of both cocrystals the molecules are connected via X∙∙∙N (X = I, Br) halogen bonds between the respective halogen bond donor and pyridine nitrogen atom. The I∙∙∙N distance of 2.795 Å in (4bzpy)2(ofib) is 20.8% less than the sum of the van der Waals (VDW) radii of nitrogen and iodine, 3.53 Å. The Br∙∙∙N distance in (4bzpy)(nbSac) is 2.140 Å, considerably less (37 %) than the sum of the VDW radii, while the N–Br bond of nbSac is elongated (2.024 Å) in comparison to that observed for the pure nbSac (1.835 Å), indicating charge transfer in halogen bonded complex and a partial covalent character of the halogen bond. In order to explore the competition between halogen bond donors in the solid state, newly prepared cocrystals (4bzpy)2(ofib) and (4bzpy)(nbSac) were subjected to LAG with select halogen bond donors: N-bromophthalimide (nbp), N-bromosuccinimide (nbs) and N-iodosuccinimide (nis). The obtained solid products were characterised by powder XRD and their patterns compared with the calculated ones of pure cocrystals to determine whether displacement occurred. Based on that, the hierarchy of halogen bond donors by strength was established and correlated to the lengths of halogen bonds in the crystal structures of cocrystals, as well as the energies of halogen bonds obtained computationally (wB97XD).

cocrystals ; halogen bond ; mechanochemistry ; supramolecular competition

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