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Imines derived from 2-hydroxy-1-naphthaldehyde as halogen bond acceptors

It has recently been shown that imines derived from ortho-vanillin and aromatic amines containing halogen bond acceptor moieties can form a variety of bonding motifs with halogen bond donors [1]. Since we have previously shown different bonding capabilities of select aromatic amines based on their substituent groups [2], our next goal was to investigate motif retention in a more robust building blocks. We synthesized a series of imines derived from 2-hydroxy-1-naphthaldehyde (n), and performed mechanochemical and solution cocrystallization experiments with 1, 4- diiodotetrafluorobenzene (tfib). Eight aromatic amines have been selected for imine syntheses: 3- aminoacetophenone (3aa), 4-aminoacetophenone (4aa), 3-aminobenzonitrile (3abn), 4- aminobenzophenone (4ab), 4-aminobenzonitrile (4abn), 4-nitroaniline (4noa), 3-aminopyridine (3ap) and 5-amino-2-methoxypyridine (5a2mp). Mechanochemical, powder X-ray diffraction and DSC experiments have shown that of our eight Schiff bases, only those with acetophenone or pyridyl fragments participate in cocrystal formation. Single crystal X-ray diffraction experiments have shown that, as expected [3], pyridine nitrogen atoms in the (n3ap)2(tfib) cocrystal form strong halogen bonds with tfib (d(I∙∙∙N) = 2.92 Å, angle(C–I∙∙∙N) = 175°). On the other hand, in the (n5a2mp)(tfib) cocrystal, halogen bonds are formed instead between tfib and methoxy oxygen groups (d(I∙∙∙O) = 3.15 Å, angle(C–I∙∙∙O) = 167°) and between tfib and the π-system of the naphthaldehyde fragment (d(I∙∙∙C) = 3.44 Å, angle(C–I∙∙∙C) = 166°).

cocrystals, halogen bonding, imines, mechanochemistry

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