Topochemical effect in thermal E–Zisomerization of azodioxides in solid state

Abstract

The thermally induced E-Z isomerization of the benzeneazodioxide derivatives is studied in the solid polycrystalline state. The reactions are studied from the kinetic and mechanistic aspects. In contrast to the solution, where isomerizations of azodioxides occur through the formation of corresponding nitroso monomers, the topochemical effect in solid state lead to the mechanism that includes formation of the "torsional" transition state, as it is the case in other known cis-trans isomerizations. It is proposed that both the reactions in solid state, the nitroso monomer-dimer equilibrium, and isomerization about the azodioxide N¼N bond tentatively involve similar transition states.

Aromatic C-Nitroso Compounds and Their Dimers: A Model for Probing the Reaction Mechanisms in Crystalline Molecular Solids

“…Most of the examples of Z-E isomerizations in solid state known from the literature are reactions triggered by UV or VIS irradiation. The observed Z-E interconversion of m-halogenonitrosobenzene azodioxides follows the first order kinetics with the activation energy of 126.2 ± 3.0 kJ·mol −1 [29].…”

“…The model, which we have developed is based on the reactions of the nitrosobenzene derivatives in the crystal phase [13,[25][26][27][28][29][30][31]. Such a system is simple because it includes the formation and/or breaking of only one chemical bond between two nitrogen atoms during the dimerization of C-nitroso compounds to the Z-or E-azodioxy dimers, or their subsequent dissociation to the starting monomers (Scheme 1) [32][33][34][35][36][37][38][39].…”

“…Monomeric molecules can be prepared within the crystalline solids by photodissociation of azodioxy dimers. Photolysis of different azodioxides to the corresponding monomers has been observed in crystals when they are cooled to cryogenic temperatures (10-12 K), and irradiated by UV light (254 nm) [26,[29][30][31]40]. The process can be followed even visually because the monomers are coloured blue or green, while most of the azodioxides are colourless or yellow.…”

“…Evidently, the gaseous monomers of m-halogenonitrosobenzenes dimerize immediately after the contact with the cold surface. Surprisingly, at room temperature, the Z-isomer spontaneously rearranges to the E-isomer in crystal phase [29]. The reaction is interesting because it is one of the very rare examples of the thermally-induced Z-E interconversion in the crystal phase.…”

“…It is very difficult to recognize and to determine the type and the number of crystal deformations on the • and *, respectively) and by (b) XRPD at 298 K (X-ray diffraction patterns at the beginning of the reaction are shown in blue and after 18 h in red). Reproduced with permission from [29]. Copyright 2016 Wiley-VCH.…”