Marija Zbačnik

Polymorphism control in the mechanochemical and solution-based synthesis of a thermochromic Schiff base

Three crystal forms of a thermochromic Schiff base, namely 1-{3-[(2-hydroxy-3-methoxy-benzylidene)-amino]-phenyl}-ethanone, derived from o-vanillin and 3-aminoacetophenone, were obtained by conventional solution-based methods. Two out of the three polymorphs were synthesized by mechanochemical syntheses, under solvent-free conditions. Herein, we report a study of the dependence of the composition of solvent, the crystallization temperature and impurities on the outcome of the synthesis of Schiff base polymorphs. We report the important role of seed crystals in directing the supramolecular organization of the product of a covalent solvent-free reaction towards the intended polymorphic outcome as well. All obtained polymorphs were investigated by means of thermal analysis, single crystal X-ray diffraction, ex situ and in situ powder X-ray diffraction and IR spectroscopy. The polymorphs display interesting and remarkably different molecular packing arrangements governed by C–H⋯O interactions leading to two-dimensional networks in forms I and III, and a three-dimensional networks in form II.

Ex situ and in situ monitoring of the syntheses of thermochromic Schiff bases

Schiff bases derived from o-hydroxy aldehydes were obtained by mechanochemical synthetic methods with a liquid or a paste present as an intermediate phase. The effectiveness of this procedure was monitored by means of ex situ PXRD and DSC, respectively. In the case of four Schiff bases, in situ PXRD studies were also conducted in order to monitor the synthesis of the product formed by merely putting the amine and the aldehyde in close contact. One of the o-hydroxy Schiff bases appears in two polymorphic forms. The monoclinic polymorph has been reported previously in the literature (herein obtained by the solution based method using methanol) while the synthesis and characterisation of the triclinic form is now reported (obtained by neat grinding or using acetonitrile during recrystallization). The reversible thermochromic properties in the solid state were also investigated in the temperature range from 298 to 110 K. The structural reasons of such behaviour were thoroughly reasoned.

The halogen bonding proclivity of the ortho-methoxy–hydroxy group in cocrystals of o-vanillin imines and diiodotetrafluoro-benzenes

In this work, we describe novel halogen bonded o-vanillin imine cocrystals. As cocrystal coformers, we selected perfluorinated diiodobenzenes, 1,2-, 1,3- and 1,4-diiodotetrafluorobenzene, as halogen bond donors. To explore the halogen bonding proclivity of the o-vanillin ortho-methoxy–hydroxy group, two imines were produced by condensation of o-vanillin with amines containing hydrophobic residues that have no additional potential halogen bond acceptor sites other than the o-vanillin oxygen atoms. We have prepared four cocrystals by both one-pot three-component solvent-free synthesis and the conventional solution-based method. The results of our work show that the ortho-methoxy–hydroxy group of the o-vanillin moiety is a good halogen bond acceptor and is capable of forming predictable halogen-bonded synthons. In the studied cocrystal structures, a wide range of varieties of this synthon were observed, including bonds with a single acceptor (either the methoxy O atom – in two structures, or the hydroxy O atom – in one), ‘slightly bifurcated’ bonds with a dominant (hydroxy) and ancillary (methoxy) acceptor, and a geometrically unusual asymmetrical bifurcated bond.