Synthesis, crystal structure and thermal reactivity of [ZnX2(2-chloropyrazine)] (X = Cl, Br, I) coordination compounds

Reaction of zinc(II) halides with 2-chloropyrazine in different solvents leads to the formation of five new coordination compounds that contain either only 2-chloropyrazine or additional water molecules as donor ligands. In the ligand-rich 1:2 compound catena[bis(2-chloropyrazine-N)]di-mu-chlorozinc(II) (1) the zinc atom is coordinated by two 2-chloropyrazine ligands and four chlorine atoms in an octahedral fashion. The zinc atoms are connected by the chloride atoms forming linear chains. In the isotypic ligand-rich 1:2 compounds bis(2-chloropyrazine-N)dibromozinc(II) (2) and bis(2-chloropyrazine-N)diiodozinc(II) (3) discrete complexes are found in which each zinc atom is coordinated by two 2-chloropyrazine ligands and two halide atoms within distorted tetrahedra. The 1:1 compounds aqua-(2-chloropyrazine-N)di-bromozinc(II) (4) and aqua-(2-chloropyrazine-N) diiodozinc(II) (5) are also isotypic and form discrete complexes in which the zinc atoms are surrounded by two halide atoms, one 2-chloropyrazine ligand and one water molecule. Upon heating, compounds 1-5 form ligand-deficient 1:1 and 2:1 compounds of composition [ZnX2(2-chloropyrazine)] (X = halide) and [(ZnX2)(2)(2-chloropyrazine)]. X-ray powder diffraction shows that identical ligand-deficient intermediates are obtained on decomposition of either [ZnX2L2] (L = 2-chloropyrazine) or the [ZnX2L(H2O)] complexes. DFT calculations suggest that the formation of the [ZnX2L(H2O)] complex is energetically favoured for the heavier halide anions. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).