TRIMORPHISM OF 2,2'-DIPYRIDYLAMINE - STRUCTURES, PHASE-TRANSITIONS AND THERMODYNAMIC STABILITIES

Crystals of an unknown third crystalline modification of 2,2'-dipyridy lamine were obtained from acetone solution and their crystal structure was determined at both 150K and room temperature. In contrast to the known orthorhombic and triclinic forms, which contain hydrogen-bridged dimers, the molecules of the monoclinic polymorph are arranged in tet ramers. The crystallographic results for the monoclinic form are prese nted here, as well as a detailed comparison of crystal and molecular s tructures of the three polymorphs. Studies by differential thermal ana lysis (DTA) and optical microscopy, performed with single crystals, sh ow a transformation of the orthorhombic phase at similar to 323 K and of the monoclinic form at similar to 368 K. According to powder diffra ction studies, transformation of the low melting orthorhombic polymorp h results in a mixture of monoclinic and triclinic phases, whereas the monoclinic modification transforms into the triclinic phase just belo w its melting point at 368 K. The single crystals of both forms are de stroyed during the conversion and, therefore, in both cases a reconstr uctive transition via nucleation and growth should occur. The conditio ns for the crystallization of the distinct modifications and their rel ative thermodynamic stabilities are investigated in different solvents and at different temperatures. Independent of the solvent chosen, the orthorhombic form is the most stable below 263 K. In the range betwee n 263 and 313 K the monoclinic form appears to be thermodynamically ad vantageous and above that temperature, the stability order is changed in favour of the triclinic polymorph. Based on the experimental result s, a qualitative free energy-temperature diagram is provided.