Synthesis, crystal structures and thermal properties of new copper(I)halide 2-ethylpyrazine coordination polymers: the influence of solid-state kinetics on product formation

Three new copper(I) coordination polymers were prepared by the reaction of copper(I) chloride with 2-ethylpyrazine in water at room temperature or under solvothermal conditions. In polyCuCl(μ₂-2-ethylpyrazine-N,N′)] (I), “zig-zag”-like CuCl chains are present, which are connected by the 2-ethylpyrazine ligand to a three-dimensional network. In comparison in catenaCu₃Cl₃(μ₂-2-ethylpyrazine-N,N′)₂] (II) six-membered Cu₃Cl₃ rings occur, which are connected to chains by the organic ligands. In polyCu₂Cl₂(μ₂-2-ethylpyrazine-N,N′)] (III), CuCl double chains are found, which are linked by the ligands to form sheets. The thermal behaviour of the different compounds was investigated using simultaneous thermogravimetry, differential thermoanalysis and mass spectroscopy as well as temperature-dependent X-ray powder diffraction. Two mass steps are found upon heating compound I in a thermobalance with 1°C/min, where the first corresponds to the transformation into compound III, and the second to the loss of the remaining ligands under formation of CuCl. If the heating rate is increased to 16°C/min, compound II is formed as an intermediate in a consecutive reaction. Therefore, the product formation depends on the actual heating rate, which shows that the solid-state kinetics plays an important role in such thermal reactions.