Investigations on the synthesis, structures, and properties of new copper(I) 2,3-dimethylpyrazine coordination compounds

Five new coordination compounds were prepared, structurally characterized, and investigated for their thermal properties. In the structure of the ligand-rich 4:9 compound, tetra(mu2-chloro)bis(mu2-2,3-dimethylpyrazine-N,N')tetrakis(2,3-dimethylpyrazine-N)tetracopper(I) tris(2,3-dimethylpyrazine)solvate (I), discrete complexes are formed by build up of two (CuCl-(2,3-dimethylpyrazine)2]2 dimers, which are connected by two 2,3-dimethylpyrazine ligands via mu-N,N' coordination. In the 1:1 compound polymu2-chloro-mu2-2,3-dimethylpyrazine-N,N'-copper(I)] (II), (CuCl)2 dimers are found, which are connected by the 2,3-dimethylpyrazine ligands into layers. For this composition, a second polymorphic modification was found (III), which exhibits a different topology of the coordination network and a different packing of the layers. In the most stable 3:2 compound catenatri(mu2-chloro)bis(mu2-2,3-dimethylpyrazine-N,N')tricopper(I)] (IV), six-membered rings of (CuCl)3 are found, which are connected by the 2,3-dimethylpyrazine ligands into chains. In the ligand-deficient 2:1 compound, polydi(mu3-chloro)(mu2-2,3-dimethylpyrazine-N,N')dicopper(I)] (V), CuCl double chains are found, which are connected by the 2,3-dimethylpyrazine ligands into layers. On heating, compound I transforms quantitatively into the 3:2 compound IV without the formation of II or III as intermediates. Compound IV is also obtained by heating either the 1:1 compound II or III. On further heating, the 3:2 compound IV loses additional ligands, forming the ligand-deficient 2:1 compound V, which then decomposes into CuCl. The stability, thermal reactivity, and the transition behavior of all compounds were investigated using different thermoanalytical methods. These results are compared with those previously reported for the structurally similar CuCl(2-ethylpyrazine) coordination compounds. The formation and the stability of the different compounds in solution were also investigated.