Polymorphism of 5-(pyridin-2-ylmethylene)-3-phenyl-2-methylthio-3,5-dihydro-4H-imidazole -4-one

The title compound, C16H13N3OS (1), exists in three polymorphic forms. Crystalline 1 undergoes an enantiotropic, first-order, k2 phase transition at 262.9(5) K with Delta H = 0.3(1) kJ mol(-1). Upon cooling below the transition temperature, the high temperature orthorhombic polymorph (Form I, space group Pbcm) transforms into a low temperature orthorhombic polymorph (Form II, space group Pbca) with a unit cell twice the size of that of the Form I. A molten 1 can be cooled in a controlled fashion to generate a monoclinic Form III of 1 with the unit cell size similar to that of Form I. Metastable Form III, once isolated, is indefinitely stable between 100 K and its melting point of 466 K. If crystals of Form III are in contact with seed crystals of Form I, a monotropic t2 first-order Form III -> Form I phase transition occurs upon heating with the onset between 420 and 448 K and Delta H = 1.7(4) kJ mol(-1). The most substantial differences among the molecular geometries of 1 in Forms I-III are observed in the position and tilt of the phenyl ring relative to the rest of the molecule. The packing in Form III is very different from those in the other polymorphs. DFT molecular geometry optimizations produce the following order of stable molecule configurations: Form II (most stable), Form I (0.50 kJ mol(-1)), Form III (2.81 kJ mol(-1)).