Solvent-Free Polymorphism Control in a Covalent Mechanochemical Reaction

Four crystal modifications of a Schiff base derived from
2-hydroxy-1-naphthaldehyde
and 2-aminobenzonitrile have been obtained by conventional solution-based
methods. Three polymorphs out of four can be synthesized under solvent-free
conditions. Herein we report simultaneous solvent-free covalent synthesis
and polymorphism control of a single component system using mechanochemistry via neat grinding and seeding-assisted grinding, i.e. neat
grinding in the presence of seed crystals. We describe the important
role of seed crystals in directing the supramolecular organization
of the product of covalent solvent-free reaction toward the intended
polymorphic outcome. Single crystal X-ray analysis of the four crystal
forms revealed conformational differences in molecules. Polymorphs
display interesting and remarkably different molecular packing features via C–H···N and C–H···O
interactions: 1D chains in form I, a 2D-network in forms III and IV,
and a 3D-network in form II.