Mechanism of several solid-solid transformations between dihydrated and anhydrous copper(II) 8-hydroxyquinolinates. Proposition for a unified model for the dehydration of molecular crystals

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 (beta) 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 beta', depending on whether an adduct is present or not in the parent structure. Polymorphic transitions from the anhydrous metastable forms (beta', gamma', X') to the stable form (beta'') 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).