Polar crystals in imines of 4-hydroxybenzohydrazide: a comparison between racemic and enantiomorphic crystals

Through the reaction of 4-hydroxybenzohydrazide with rac-3-methylcycplopentanone and R-(+)-3-methylcyclopentanone, racemic and enantiomorphic imines were obtained and characterized in the solid state. The racemic imine crystallizes either in the orthorhombic polar space group Pna21, and the crystal structure is polar and completely analogous to the structure of similar non-chiral imines we have recently studied, or in the monoclinic centrosymmetric space group P21/n. The enantiomorphic imine crystallizes in the monoclinic polar space group P21, but the crystal structure is actually non-polar and shows significant pseudo-centrosymmetry being quasi-isomorphous with the centrosymmetric structure of the racemic imine. The density of the enantiomorphic pseudocentric crystals is significantly higher than the polar orthorhombic racemic crystals and almost equal to the centrosymmetric racemic crystals.