Polymorphism of (H2mela)2[CuCl5]Cl (mela = melamine): structures, transformation and magnetic properties

Abstract

Two polymorphs of (H₂mela)₂[CuCl₅]Cl (mela = melamine) prepared by solution diffusion methods have been characterized structurally and magnetically. The kinetic phase in the monoclinic space groupP2/c was first crystallized. When kept in the mother solution, the kinetic phase gradually dissolved, while the thermodynamically stable phase in the monoclinic space groupP2₁/m appeared and became the final product. This transformation process was monitored by powder X-ray diffraction. Structural analysis revealed that the two polymorphs consist of alternatively arranged negative layers of [CuCl₅³⁻]_n chains and discrete non-coordination Cl⁻ anions and positive layers of H-bonded dimers of [H₂mela²⁺]₂, assembled though H-bonds in the lattices. The difference between the two structures lies in the packing and H-bonding patterns in the solids. The transformation associated changes in structures mainly referred to H-bonding are discussed. Both compounds are antiferromagnetic chains. Thermal stability and IR spectroscopic properties were also studied.