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Halogen Bonded Cocrystals of Two Tautomers of the Imine Derived from 2-hydroxy-1- naphthaldehyde and 3-aminopyridine

Crystal engineering of imine compounds remains an interesting research field because imines can contain various functional groups that are potential halogen or hydrogen bond acceptors. Keto-enol tautomerism is also present in imines that have a hydroxyl group located adjacent to the C=N bond [1]. In most cases, only one imine tautomer can be obtained in the solid state, but there have also been studies on the possibility of using supramolecular interactions as a guiding tool for obtaining specific tautomer forms [2, 3]. In this work we report the first known halogen bonded cocrystals of equal composition but with different tautomers as building blocks. We have studied the cocrystallization of an imine (n3ap) derived from 2-hydroxy-1-naphthaldehyde and 3-aminopyridine with the halogen bond donor 1, 3-diiodotetrafluorobenzene (13tfib). Pure n3ap crystallizes in the enol-imino tautomer form [4]. Cocrystallization experiments resulted in the concomitant formation of two types of crystals, yellow needles and yellow plates. Single crystal X-ray diffraction has shown that the obtained crystals correspond to two stoichiometrically equivalent cocrystals of n3ap and 13tfib, but differ in the n3ap tautomer form present. In the (keto-n3ap) (13tfib) cocrystal, I∙∙∙Npyridyl and I∙∙∙Ocarbonyl halogen bonds are formed, while I∙∙∙Npyridyl and I∙∙∙π halogen bonds are formed in the (enol-n3ap)(13tfib) cocrystal. Lattice energy calculations on the periodic DFT- optimised structures of both cocrystals showed that (keto-n3ap)(13tfib) should be the more stable cocrystal form, as it is lower in energy by 85.6 kJ mol–1. This is consistent with (keto-n3ap)(13tfib) being the only observed mechanochemical product, its predominance in the original crystallization experiments, and the failure of repeat crystallizations aimed at obtaining the (enol-n3ap)(13tfib) cocrystal in bulk. In vacuo quantum chemical calculations have also been performed, and the results showed that the relative acceptor strength in n3ap falls in the series Npyridyl ≈ Oketo > Oenol > π-system.

cocrystals ; halogen bonding ; imines ; tautomerism

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