Synthesis and polymorphism of (4-ClpyH)₂[CuCl₄]: solid–gas and solid–solid reactions

Abstract

Reaction of blue crystalline solid trans-[CuCl2(4-Clpy)2] 1 (4-Clpy = 4-chloropyridine) with anhydrous HCl gas yields yellow crystalline salt (4-ClpyH)2[CuCl4] 2 with quantitative conversion. The reaction has been followed in situ by synchrotron powder X-ray diffraction using a specially designed gas-handling rig and demonstrates that the initial product is a C-centred monoclinic form of (4-ClpyH)2[CuCl4] (phase II), which then converts in the solid state to a primitive monoclinic form (phase I). The conversion has been quantified at each stage by mixed-phase Rietveld refinement of the powder diffraction data. The two polymorphic forms exhibit different patterns of N–H⋯Cl(Cu) hydrogen bonding and C–Cl⋯Cl(Cu) halogen bonding. Evidence from solution-phase crystallisations of 2 suggests that phase II is a kinetic product and phase I is the thermodynamic product. The salt 2 can also be prepared mechanochemically, by grinding for 2 min green-blue solid CuCl2·2H2O with white solid [4-ClpyH]Cl in a 1 : 2 stoichiometric ratio. The product was identified by powder diffraction as phase I of compound 2 in 97.5% yield, with the remaining material being compound 1.