Lead(II) alkanoates, from hexanoate to dodecanoate, have been analyzed by means of XRD, optical microscopy, DSC, FTIR, and electric spectroscopy. Four different phases have been identified, corresponding to the three thermal transitions measured by DSC:  two of them solid (crystal and “intermediate” phases), and another two fluid (neat phase and isotropic liquid). Powder crystal XRD data indicate that the samples present a bilayered structure. The analysis of the (00l) spacing dependence with temperature in the three ordered phases strongly points to the intermediate phase to be a rotator phase. Optical microscopy and FTIR versus temperature also confirm a structural change from the crystal to the intermediate phase and its solid-state nature. Electrical conductivity maps the thermal transitions of the samples and shows a high ionic conductivity in the intermediate phase, which does not depend much on the carbon chain length. The high conductivity values (3 orders of magnitude higher in comparison with that of the ordered crystal at room temperature) obtained for the intermediate phase gave a further support to the existence of a rotator mesophase in the lead(II) alkanoate series.

A new third polymorph of copper(II) decanoate has been found by two methods: from the crystallization of the salt− acid solution and from a specific thermal treatment of the polymorph obtained in n-heptane. The new polymorph, whose crystal structure has been solved for the first time, is the most stable from the thermodynamic point of view. It presents a bilayer structure, but with two different types of paddle-wheels catenae with opposite orientations. This polymorphism as well as the temperature vs composition phase diagram of the system, [(1 − x) C 9 H 21 CO 2 H + (x) (C 9 H 21 CO 2 ) 2 Cu], were solved by differential scanning calorimetry, Fourier transform infrared spectroscopy, Xray diffraction, pair distribution function, and optical microscopy. The singularities of the phase diagram are (a) a Krafft-like process of molecular association of the copper(II) decanoate into the acid solution, at about T = 361.5 K and x = 0.022 (forming salt-acid adducts of 1:1 stoichiometry); (b) a fusion transition as an invariant (T = 379.1 K) of the solid salt in excess from the copper(II) decanoate crystal phase to the discotic liquid crystal, forming homeotropic developable domains (seen for the first time in these systems) of the pure salt into the saturated acid solution. These domains are seen not only in the copper(II) decanoate but also in other members of the series, at high salt concentration in the corresponding acid solution, and allow identification of the hexagonal columnar discotic structure of the mesophase (with the exception of the copper(II) butanoate, where the tetragonal domains with tilted disks, paddle-wheels, were seen in the same salt−acid region).

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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.