The two polymorphic forms of ( Z)-2-fluoro- N′-phenylbenzamidamide were obtained by room temperature crystallization of the synthesized compound (Fig. The structural and physical characterization of these crystal forms followed by analysis of their energy frameworks points to the occurrence of this unique phenomenon of ‘quasi-isostructural polymorphism’ in molecular crystals. 13 In the current study, it is observed that the polymorphs are stable at room temperature and have been obtained from different solvents. A detailed classification of isostructurality in molecular crystals is given in Kalman et al. With respect to the definition proposed by IUCr, “ two crystals are said to be isostructural if they have the same crystal structure, but not necessarily the same cell dimensions nor the same chemical composition”. The present case reported belongs to a distinct phenomenon of “quasi-isostructural polymorphism” with significant differences in the unit cell parameters. 12 The example reported represents a case of a change in the crystallographic symmetry with subtle changes in molecular packing and unit cell parameters. Recently, Coles and co-workers have reported the isostructural polymorphs of 3-chloro mandelic acid obtained by a temperature dependent phase transition. 11 Examples of such polymorphs having multiple Z′ in their crystal structures and formed under ambient conditions are extremely scarce in the literature. have reported nearly isostructural high Z′ polymorphs of ethynylbenzene generated by pressure freezing. Herein, we present a unique case of high Z′ polymorphs of ( Z)-2-fluoro- N′-phenylbenzamidamide that exhibit near-isostructurality and very similar features of packing and intermolecular interactions. Nangia and co-workers further reported the existence of four polymorphs and the nineteen crystallographically independent molecular conformations of 4,4-diphenyl-2,5-cyclohexadienone 10 which exhibit conformational polymorphism, conformational isomerism and concomitant polymorphism. Form I (space group: P2 1/ c and Z = 1) was stabilized via O–H⋯O hydrogen bonded chains and form II (space group: Cc and Z′ = 3) was stabilized via O–H⋯O hydrogen bonded chains between the three symmetry-independent molecules. 7 The polymorphic forms of liquid pentafluorophenol were obtained using in situ cryocrystallization 8,9 techniques under different kinetic conditions. 6 Desiraju and co-workers have reported the polymorphs of pentafluorophenol and trans-1,4-bis (phenylethyl) cyclohexane-1,4-diol. 5 Polymorphic structures in different crystalline environments can be compared on the basis of differences/similarities which exist in molecular conformation, packing arrangements and formation of different hydrogen bond motifs. 2–4 Thermodynamic and kinetic factors are very important during nucleation and crystal growth as such factors influence the final outcome of crystallization events leading to the formation of different morphologies with different molecular arrangements. Polymorphism 1 due to the presence of different numbers of symmetry-independent molecules in the asymmetric unit is of importance in the study of crystal structures and also in the area of crystal engineering. The results point to a unique class of ‘quasi-isostructural polymorphs’ which are nearly equi-energetic crystal structures exhibiting high degrees of similarity in physical properties. The energy framework analysis indicates 2D structural similarities in the interaction topologies of these crystalline forms. The polymorphs of ( Z)-2-fluoro- N′-phenyl benzamidamide with multiple Z′ produce quasi-isostructural supramolecular architectures, wherein C–H⋯F interaction plays a significant role.
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