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Halogen-bonded cocrystals of aromatic amines: the competition between halogen and hydrogen bonds

Halogen bonds that form between polarisable and electron-deficient halogen atoms (Br, I) and electron-rich Lewis bases have been utilised as alternatives to hydrogen bonds in the planned construction of crystal materials. Typical halogen bonds are similar to hydrogen bonds both in terms of length and directionality. In our studies of halogen-bonded cocrystals we focused on aromatic amines containing a potentially halogen bond acceptor groups: 4-aminoacetophenone (4aap), 4-aminobenzophenone (4ab), 3-aminopyridine (3ap), 5-amino-2-methoxypyridine (5a2mp), 3-aminobenzonitrile (3abn) and 4-nitroaniline (4noa). Such compounds, that contain good halogen bond acceptor species and good hydrogen bond donors, are significant from a crystal engineering standpoint, because they offer a possibility of constructing crystal structures containing both hydrogen and halogen bonds. As a good counterpart to amines, we selected 1, 4-diiodotetrafluorobenzene (tfib), a well-known commercially available halogen bond donor. A cursory search of the Cambridge Structural Database revealed that data on tfib halogen bonded to our specified acceptor sites (Figure 1) is very scarce. In this work we report the mechanochemical synthesis of halogen-bonded cocrystals via liquid-assisted grinding (LAG) of amine and tfib, i.e. by grinding solid reactants in the presence of a small quantity of liquid phase (acetonitrile). To observe mechanosynthesis, as well as to facilitate the characterisation of the new cocrystals by single-crystal X-ray diffraction, LAG experiments were accompanied by crystallization from the reactant solution. All reactants and products have been characterised by means of powder X-ray diffraction and differential scanning calorimetry. Crystal structure analysis revealed that (3abn)(tfib) and (3ap)(tfib) crystallize in a 1:1 ratio, while other four prepared cocrystals crystallize in a 2:1 ratio. To the best of our knowledge, the (4noa)2(tfib) cocrystal represents the first known cocrystal where tfib molecules form I∙∙∙O halogen bonds with nitro group oxygen atoms. Interestingly, crystal structure analysis of the (4aap)2(tfib) cocrystal demonstrated that the carbonyl oxygen atom is a good halogen bond acceptor, in contrast with the (4ab)2(tfib) cocrystal, where it preferentially forms strong N–H∙∙∙O hydrogen bonds, allowing only for the formation of I∙∙∙π halogen bonds. Furthermore, the amino group proved its potential to be a simultaneous hydrogen bond donor and halogen bond acceptor, as has been found out in the (3abn)(tfib) and (5a2mp)2(tfib) cocrystals. In (3ap)(tfib), unlike the other cocrystals, one of the tfib iodine atoms forms an expected I∙∙∙N halogen bond with a pyridine nitrogen atom, while the other atom participates in a I∙∙∙I halogen interaction with another tfib molecule. All halogen bonded complexes amine-tfib are connected into a 2D or 3D network via N–H∙∙∙X (X=O, F, N or I) hydrogen bonds.

1, 4-diiodotetrafluorobenzene ; aromatic amines ; halogen bond ; hydrogen bond

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