Synthesis, Crystal Structures, and Thermal Properties of New [ZnX2(2,5‐dimethylpyrazine)] (X = Cl, Br, I) Coordination Compounds

Abstract

AbstractEight new halidozinc(II) coordination compounds with 2,5‐dimethylpyrazine as a ligand were prepared. The reaction of ZnX2 (X = Cl, Br, I) and 2,5‐dimethylpyrazine in methanol results in the formation of catena‐(μ2‐2,5‐dimethylpyrazine‐N,N′)dichloridozinc(II) (1), catena‐(μ2‐2,5‐dimethylpyrazine‐N,N′)dibromidozinc(II) (3I and 3II), catena‐(μ2‐2,5‐dimethylpyrazine‐N,N′)diiodidozinc(II) (6), and diiodidobis(2,5‐dimethylpyrazine‐N)zinc(II) (7). In addition, in a solvent‐free reaction of ZnX2 (X = Cl, Br) and 2,5‐dimethylpyrazine,the ligand‐rich compounds bis(dichlorido)bis(2,5‐dimethylpyrazine‐N)(μ2‐2,5‐dimethylpyrazine‐N,N′)dizinc(II) (2), bis(dibromido)bis(2,5‐dimethylpyrazine‐N)(μ2‐2,5‐dimethylpyrazine‐N,N′)dizinc(II) (4), and dibromidobis(2,5‐dimethylpyrazine‐N)zinc(II) 2,5‐dimethylpyrazine solvate (5) were obtained. In the crystal structures of all compounds the zinc atoms are each coordinated by two halide atoms and two 2,5‐dimethylpyrazine ligands within a distorted tetrahedron. The zinc atoms in the ligand‐deficient 1:1 (1:1 = molar ratio of metal salt to ligand) compounds 1 and 6 and in the two polymorphic modifications 3I and 3II are linked by the N‐donor ligands into chains. In contrast, the crystal structure of the 2:3 compounds 2 and 4 are built up of oligomeric discrete [(ZnX2)2(2,3‐dimethylpyrazine)3] units in which each ZnX2 (X = Cl, Br) unit is coordinated by two halide atoms and one bridging and one terminal 2,5‐dimethylpyrazine ligand. The structures of the ligand‐rich 1:2 diiodido compound 7 and the 1:3 dibromido compound 5 consist of discrete monomeric tetrahedral building blocks. In the thermal decomposition reactions of the ligand‐rich compounds 2, 5, and 7, different mass steps are observed. In the first step, the ligand‐deficient 1:1 compounds 1 and 6 are formed in quantitative yields from 2 and 7, respectively, and 5 yields a mixture of 3I and 3II, and the second mass step leads to the formation of new ligand‐deficient 2:1 compounds. On further heating of the compounds, the remaining ligands are discharged. This results in the formation of the zinc(II) halides, which vaporize on further heating. Solvent‐mediated conversion experiments in methanol show that, irrespective of the ratio of zinc(II) halide and ligand, only the 1:1 compounds 1 and 3I are formed and therefore represent the thermodynamically most stable compounds at room temperature. For ZnI2 the 1:2 compound 7 is obtained at ratios of zinc(II) iodide to ligand larger than 1:3, whereas at lower ratios the 1:1 compound 6 is always obtained. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)