engleski

SOLVENT-FREE ORGANIC SYNTHESIS AND SOLVENT-FREE CONTROLLABLE POLYMORPHS: CONCOMITANT AND CONFORMATIONAL POLYMORPHISM IN (Z)-2-[2-HYDROXY-1- NAPHTHYL)METHYLENEAMINO]BENZONITRILE

Four crystal forms of the Schiff base derived from 2-hydroxy-1-naphthaldehyde (napht) and 2-aminobenzonitrile (abn) have been prepared by conventional solution-based method and their relationship was investigated by IR spectroscopy, thermal analysis and also by powder and single crystal X-ray diffraction. Forms I and IV both convert into Form III at high temperature and they are enantiotropically related. Melting of Forms III and II is observed at 197 and 192 °C, respectively. All four polymorphs crystallise concomitantly in solvent mixture of octanol/dichloromethane. We have also synthesized the Schiff base and its three polymorphs by solvent-free method. At temperature of 30 to 35 °C, 2-hydroxy-1- naphthaldehyde and 2-aminobenzonitrile react in a liquid eutectic formed by neat grinding providing Form I, while below melting temperature of the eutectic, i.e. 18 to 22 °C, the same materials appear to react without the formation of liquid phase providing Form II. Form III was obtained by neat grinding of reactants and seeding at 20 °C. The attempts to prepare Forms I and IV by solvent- free grinding/seeding experiments have been unsuccessful. This is probably due to the stability of these two forms under reaction conditions and the fact that both polymorphs can be converted into Form II by grinding in the presence of a small drop of acetonitrile or simply by standing in the atmosphere of acetonitrile vapor. Single crystal X-ray analysis of all forms revealed conformational differences in the Schiff base molecule and from the molecular geometry consideration the enol-imino tautomer was detected. Polymorphs display interesting and remarkably different molecular packing features via C—H•••N and C—H•••O interactions: 1D chains in Form I, 2D-network in Form III and IV and 3D- network in Form II.

Solvent-free organic synthesis; Polymorphism

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