On the thermal decomposition pathway of coordination compounds: synthesis, crystal structures and properties of new polymorphic CuI(2-ethylpyrazine) coordination compounds

Abstract

Five new coordination compounds were prepared at room temperature in solution, under solvothermal conditions at elevated temperatures or by thermal decomposition reactions. In the amine rich 1:2 compound [(CuI) 2(2-ethylpyrazine-N) 4] ( I) discrete molecular complexes are found that consist of (CuI) 2 dimers in which each copper atom is coordinated by two 2-ethylpyrazine ligands. The crystal structure of the 1:1 compound poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′) 2] ( II) contains (CuI) 2 dimers which are connected by the 2-ethylpyrazine ligands into layers parallel to (010). The second modification of a 1:1 compound poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′)-(2-ethylpyrazine-N)] ( III) consists of 8-membered CuI rings, which are connected by the 2-ethylpyrazine ligands into layers. In one modification of poly[(CuI) 2( μ 2-2-ethylpyrazine-N,N′)] ( IV) CuI double chains are connected by the ligands into layers parallel to (001). If the preparation is kinetically controlled, crystals of the second modification poly[(CuI) 2( μ 2-2-ethylpyrazine)-N,N′]] ( V) can be obtained. The crystal structure of V is very similar to that of compound IV, with the main difference that the orientation of the ethyl groups of the 2-ethylpyrazine ligands is different. The thermal reactivity of all compounds as well as the thermodynamic stability and the transformation behaviour of the polymorphic modifications were investigated using DTA-TG-MS and DSC measurements, temperature resolved X-ray powder diffraction as well as temperature dependent crystallisation experiments. The occurrence of the different phases is discussed and the results are compared with those obtained on the corresponding CuCl and CuBr compounds on the basis of preparative results and density-functional calculations.