Liquid-Assisted Mechanochemical Synthesis of Cesium Copper(II) Halogenides

A series of liquid-assisted inorganic mechanochemical reactions yielding orthorhombic cesium copper(II) halogenides of general composition Cs2CuCl2X2 (X = Cl or Br), monitored in situ by synchrotron powder X-ray diffraction (PXRD), is reported. The observed reaction proceeds through a common intermediate phase CsCuCl2X without any evidence of a polymorphic transition. However, liquid additives can drastically influence the kinetics and reaction outcome, especially in the case of strongly coordinating liquids, which leads to stable or metastable solvates, completely changing the mechanistic pathway. In this respect, three reaction pathways are identified. First, noninteracting, nonpolar, or slightly polar liquids convert the reactants into products simply through a CsCuCl2X intermediate. On the other hand, highly polar liquids with free electron pairs form CuCl2 solvates, which partially disable the consumption of CsX, leading to a mixture of solvate and higher halogenides Cs3Cu2Cl4X3. Other solvate-forming liquids extremely accelerate the reaction through localized solvation, and formation of the final product practically takes place in solution. Thus, the liquid additives act directionally on the mechanistic pathway. The observed considerably different roles of solvate-forming species with respect to organic mechanochemical systems indicate a nongenerality of the role of liquids for all types of mechanochemical reactions.