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(μ2-pyrazine)-dichloro-zinc(II) (I), poly-(μ2-pyrazine-N,N‘)-dichloro-zinc(II) (II), poly-bis(μ2-pyrazine-N,N‘)-dibromo-zinc(II) (III), catena-(μ2-pyrazine-N,N‘)-dibromo-zinc(II)
(IV), and catena-(μ-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 P21, but we found that the
correct space group is P21/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.