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Halogen-bonded cocrystals of an imine derived from N-aminomorpholine and 4-nitrobenzaldehyde: a playground for halogen bonding

Imines derived from N-aminomorpholine and aromatic aldehydes are potentially good building blocks in crystal engineering of halogen-bonded materials [1, 2]. Herein we have synthesized an imine (1) from 4- nitrobenzaldehyde and N-aminomorphline that has various functional groups which can act as halogen bond acceptor sites: morpholine oxygen atom, morpholine nitrogen atom, nitro group oxygen atom, imine nitrogen atom and the phenyl π-system [3]. For cocrystal synthesis, as halogen bond donors, we have selected perfluorinated compounds of different geometries based on iodine: 1, 4- diiodotetrafluorobenzene (14tfib) and 1, 3, 5- triiodotrifluorobenzene (135tfib). We obtained two cocrystals by mechanochemical and solution based synthesis. Products were characterized by means of powder X-ray diffraction, differential scanning calorimetry (DSC) and by single crystal X-ray diffraction. In the (1)2(14tfib) cocrystal only the bifurcated I···O2N halogen bond is present (d(I∙∙∙O) = 3.217 Å, (C–I∙∙∙O) = 169.4° ; d(I2∙∙∙Cl1) = 3.439 Å, (C–I∙∙∙O) = 153.2°), leading to the formation of a discrete halogen bonded complex. These discrete complexes are then connected into a layer through paired C– H···O hydrogen bonds, and through C–H···F contacts. Unexpectedly, the morpholinyl fragment does not participate in any directional supramolecular interactions. On the other hand, in the (1) (135tfib)2 cocrystal, a 1:2 stoichiometry of 1 and 135tfib is present (Figure 1). The two crystallographically independent 135tfib molecules exhibit different supramolecular bonding. The first 135tfib molecule participates in halogen bonding as a tritopic donor forming I···Nimine, I···Omorpholine and I···I halogen bonds (d(I∙∙∙N) = 3.288 Å, (C–I∙∙∙N) = 171.9° ; d(I∙∙∙O) = 3.066 Å, (C–I∙∙∙Cl) = 170.4° ; d(I∙∙∙I) = 3.904 Å, (C– I∙∙∙Cl) = 163.0°). The second crystallographically independent 135tfib molecule participates in halogen bonding as a ditopic donor, forming I···Onitro and I···Cπ halogen bonds (d(I∙∙∙O) = 2.941 Å, (C–I∙∙∙O) = 174.4° ; d(I∙∙∙C) = 3.381 Å, (C–I∙∙∙C) =172.1°)as well as being an acceptor in type II halogen bonding. The combination of the mentioned halogen bonds gives rise to an intricate 3-D network.

halogen bond, imine, 4-nitrobenzaldehyde, N-aminomorphline

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