On the preparation of coordination polymers by controlled thermal decomposition: synthesis, crystal structures, and thermal properties of zinc halide pyrazine coordination compounds

The five zinc(II) halide pyrazine coordination compounds poly-bis(mu2-pyrazine)-dichloro-zinc(II) (I), poly-(mu2-pyrazine-N,N')-dichloro-zinc(II) (II), poly-bis(mu2-pyrazine-N,N')-dibromo-zinc(II) (III), catena-(mu2-pyrazine-N,N')-dibromo-zinc(II) (IV), and catena-(mu-pyrazine)-diiodo-zinc(II) (V) were prepared by the reaction of ZnX2 (X = Cl, Br, I) with pyrazine in acetonitrile. In the crystal structure of compound I, the zinc atoms are coordinated by two chlorine atoms and two pyrazine ligands within distorted tetrahedra. The zinc atoms are linked by the N-donor ligands into layers. The crystal structure of compound III is very similar to that of compound I. The structure of compound III was originally reported in space group Ccca with similar a and b axes, but it was proved that the correct space group is I4/mmm. Ligand-poor compound V is isotypic to compound IV, in which ZnX2 units (X = Br, I) are connected by the pyrazine ligands into chains. It was originally reported in the noncentrosymmetric space group P2(1), but we found that the correct space group is P2(1)/m. If ligand-rich 1:2 compounds I and III are heated in a thermobalance, different mass steps are observed. We have proven that in the first step, ligand-poor compounds II and IV are formed in quantitative yields. On further heating, a second mass step occurs that leads to the formation of two new compounds of composition (ZnCl2)2(pyrazine) (VI) and (ZnBr2)2(pyrazine) (VII). However, the mass step is not well-resolved. and the new compounds are not phase-pure after the thermal event. If ligand-poor 1:1 compound V is investigated by thermogravimetry, a not-well-resolved single mass step is observed in which new ligand-poor 2:1 compound (ZnI2)2(pyrazine) (VIII) is formed. On further heating, all 2:1 compounds lose their remaining ligands and transform into the pure zinc(II) halides.