Synthesis, crystal structure, thermal and luminescence properties of CuX(2,3-dimethylpyrazine) (X = Cl, Br, I) coordination polymers

Abstract

Three new coordination polymers based on CuI and 2,3-dimethylpyrazine (2,3-dmpyz) were prepared, structurally characterized and investigated for their thermal and luminescence properties. In the ligand rich 2 : 3 compound [(CuI)₂(2,3-dmpyz)₃] 1 (CuI)₂ dimers are found, which are connected by the N-donor ligands into chains, whereas in the structure of the 1 : 1 intermediate [(CuI)(2,3-dmpyz)] 2 (CuI)₄ tetramers are found, which are also connected into chains. The crystal structure of the ligand deficient 2 : 1 compound [(CuI)₂(2,3-dmpyz)] 3 is built up of CuI double chains, which are connected by the 2,3-dmpyz ligands into layers. Thermal decomposition of 1 results in its direct transformation into the ligand deficient compound 3, without the formation of the 1 : 1 compound 2 as an intermediate. A similar thermal reactivity is found for compound 2, which transforms into 3 on heating. Stirring of a crystalline suspension of pure 1 or 2 in acetonitrile, always leads to a transformation into the ligand deficient compound 3 indicating that compound 3 is the most stable of all the coordination polymers, whereas compounds 1 and 2 are metastable. The luminescence properties of the CuCl and CuI coordination polymers were investigated at 298 and 77K. It was observed that the emission maxima strongly depends on the nature of the halide atom and the composition and structure of the coordination polymers. In addition, several of these compounds show luminescence thermochromism. These results are compared with those obtained for the previously reported CuCl and CuBr(2,3-dimethylpyrazine) coordination polymers.