Interactions in crystals .110. The dimorphism of (2-pyridyl)(2-pyrimidyl)amine

Preceding structural studies of 2-aminopyridine derivatives prove that they form polymorphic H-pair-bonded dimers, and, therefore, (2-pyridyl)(2-pyrimidyl)amine has been crystallized under different conditions. Two different monoclinic modifications are characterized by their crystal structures of the same space group P2(1)/c: form I exhibits the well known herringbone motif and form II the sandwich-herringbone one. Both modifications contain almost identical planar, H-pair-bridged molecular dimers. Crystals of form I are readily obtained from any solvent, whereas those of form II result either on fast evaporation of a Et(2)O solution or by resolidification of the melt. Differential thermal analysis (DTA) and crystallization experiments at different temperatures prove, for both modifications, no transitions in the temperature range between -25 degrees and the melting point at 152 degrees and, therefore, the system should be monotropic. The structural and thermodynamic aspects of the (2-pyridyl)(2-pyrimidyl)amine dimorphism are discussed in comparison to those of the trimorphism observed for di(2-pyridyl)amine.