New Advances in the One-Dimensional Coordination Polymer Copper(II) Alkanoates Series: Monotropic Polymorphism and Mesomorphism

The polymorphism in the copper­(II) alkanoates, recently discovered for one member, has been thoroughly studied for the whole series, from 3 to 16 C atoms. Three polymorphic phases have been found, all of them sharing the same molecular unit, the paddle-wheel, which grows forming a 1D coordination polymer or catena. The three polymorphs are defined by a different packing of these catenae and a specific arrangement of the alkyl chains. Ten new crystal structures of those compounds have been solved by high resolution powder diffraction and presented in this paper. The polymorphism in this series has been found to be monotropic and is responsible for the complex thermal behavior observed. The most characteristic feature, the endothermic–exothermic effect, has been explained for the first time in these compounds by a combination of data from differential scanning calorimetry (in normal and modulated modes), powder X-ray diffraction and Fourier transform infrared spectroscopy. These techniques, together with small-angle X-ray scattering and optical microscopy, were used to analyze the hexagonal columnar discotic liquid crystal phase of copper­(II) alkanoates. Thus, new information has been found in the packing and stacking of the discs formed by the paddle-wheel units, also maintained in the mesophase.