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(μ2-chloro)bis(μ2-2,3-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
μ-N,N‘ coordination. In the 1:1 compound poly[μ2-chloro-μ2-2,3-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 catena[tri(μ2-chloro)bis(μ2-2,3-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, poly[di(μ3-chloro)(μ2-2,3-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.