An attempt to describe, at a molecular level, solid-solid transformations between copper-(II) oxinates of known structure is presented. On the basis of experimental data and by modeling the molecular movements, it is shown that the dehydration of the stable dihydrated form (β) takes place with the transmission of structural information from the parent to the daughter phase, so that a coherence is maintained. After the departure of water molecules through channels, the resulting unstable new anhydrous material (NAM) reorganizes rapidly toward one of the two closest energy minima: X′ or β′, depending on whether an adduct is present or not in the parent structure. Polymorphic transitions from the anhydrous metastable forms (β′, γ ′, X′) to the stable form (β′′) are described by a nucleation and growth mechanism, with a complete loss of the structural information contained in the parent phase. Both proposed mechanisms (continuous and cooperative processes for the dehydration, and nucleation and growth mechanism for the polymorphic transitions) are consistent with available experimental results: crystal structures, routes of preparation, evolution of the crystal size distribution (CSD), specific surfaces, SEM photographs, and DSC results. An extension of this study together with several other examples led to a unified model for the dehydration mechanism of molecular crystals being proposed. Four decisive topological, energetic, and physical criteria are proposed. The mechanisms are related to the possible filiation of structural information and fall into two categories: class I mechanisms are associated with the absence and class II mechanisms correspond to the presence of structural filiation. Each class is divided into several subclasses according to the process of release of water molecules (cooperative or destructive) and to the eventual process of reorganization (cooperative or through a nucleation and growth process).
A series of Eu(III) and Tb(III) clusters as well as their Y(III) analogues with increasing nuclearities of 5, 8 and 9 have been synthesised using beta-diketonate ligands with decreasing steric hindrance. Their molecular structures have been established from X-ray diffraction on single crystals for most clusters and studied by luminescence and Raman spectroscopy. The Raman spectra have distinctive patterns for each nuclearity in accordance with their crystal structure. The luminescence spectra of the Eu(III) and Tb(III) clusters also show distinctive features.
The polymorphic behavior of racemic and enantiopure diprophylline (DPL), a chiral derivative of theophylline marketed as a racemic solid, has been investigated by combining differential scanning calorimetry, powder X-ray diffraction, hot-stage microscopy and single-crystal X-ray experiments. The pure enantiomers were obtained by a chemical synthesis route, and additionally an enantioselective crystallization procedure was developed. The binary phase diagram between the DPL enantiomers was constructed and revealed a double polymorphism (i.e., polymorphism both of the racemic mixture and of the pure enantiomer). The study of the various equilibria in this highly unusual phase diagram revealed a complex situation since mixtures of DPL enantiomers can crystallize either as a stable racemic compound, a metastable conglomerate, or two distinct metastable solid solutions. Crystal structure analysis revealed that the DPL molecules adopt different conformations in the crystal forms suggesting that the conformational degrees of freedom of the substituent that carries the only two H-bond donor groups might be related to the versatile crystallization behavior of DPL. The control of these equilibria and the use of a suitable solvent allowed the design of an efficient protocol for the preparative resolution of racemic DPL via preferential crystallization. Therefore, the resolution of DPL enantiomers despite the existence of a racemic compound stable at any temperature demonstrates that the detection of a stable conglomerate is not mandatory for the implementation of preferential crystallization.
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