Trimorphism of 2,2'-dipyridylamine: structures, phase transitions and thermodynamic stabilities

Abstract

Crystals of an unknown third crystalline modification of 2,2'-dipyridylamine 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 tetramers. The crystallographic results for the monoclinic form are presented here, as well as a detailed comparison of crystal and molecular structures of the three polymorphs. Studies by differential thermal analysis (DTA) and optical microscopy, performed with single crystals, show a transformation of the orthorhombic phase at --~323 K and of the monoclinic form at --~368 K. According to powder diffraction studies, transformation of the low melting orthorhombic polymorph results in a mixture of monoclinic and triclinic phases, whereas the monoclinic modification transforms into the triclinic phase just below its melting point at 368 K. The single crystals of both forms are destroyed during the conversion and, therefore, in both cases a reconstructive transition via nucleation and growth should occur. The conditions for the crystallization of the distinct modifications and their relative thermodynamic stabilities are investigated in different solvents and at different temperatures. Independent of the solvent chosen, the orthorhombic form is the most stable below 263K. In the range between 263 and 313 K the monoclinic form appears to be thermodynamically advantageous and above that temperature, the stability order is changed in favour of the triclinic polymorph. Based on the experimental results, a qualitative free energy-temperature diagram is provided.

Synthesis, Crystal Structures, and Thermal and Thermodynamic Properties of Dimorphic Copper(I) Coordination Polymers

“…Besides these structural aspects of these phenomena also investigations on the transformation behavior and the kinetic and thermodynamic aspects are necessary in order to draw structure-property relationships. 17, [21][22][23][24] Recently, we have found polymorphic modifications in coordination polymers based on copper(I) halides or pseudohalides and multidendate N-donor ligands. 24 The structures of such coordination polymers are built up of different CuX substructures (X ) halide or pseudohalide) which are linked by the organic ligands to multidimensional coordination polymers.…”

Synthetic, structural and magnetic implications of introducing 2,2′-dipyridylamide to sodium-ferrate complexes

“…54 Being the simplest of the DPA through three N sites; one central amido N and two neutral pyridyl N atoms. 55 Rotation around the two Namido-C bonds allows for DPA to adopt three different conformations; syn/syn, syn/anti and anti/anti (Fig. 1).…”

Proton-transfer supramolecular salts based on proton sponge 2,2′-dipyridylamine

“…Proton sponge 2,2 0 -dipyridylamine According to the Cambridge Structural Data Base (CSD), different kinds of crystal structures related to 2,2 0 -dipyridylamine (dpa) are shown in Scheme 2. Two pyridyl nitrogen atoms in free dpa are on both sides of the central NH group and adopt a trans conformation, which may be caused by severe repulsion between two nitrogen lone electron pairs [49][50][51]. In the monoprotonated Scheme 1.…”