Synthesis and crystal structures of Zn(ii) and Co(ii) coordination compounds with ortho substituted pyridineligands: two structure types and polymorphism in the region of their coexistence

Abstract

Reaction of cobalt(II) thiocyanate or zinc(II) thiocyanate with 2-methylpyridine, 2-bromopyridine and 2-chloropyridine at room-temperature lead to tetrahedral complexes of composition M(NCS)₂L₂ (M = Co, Zn; 1: L = 2-methylpyridine, 2: L = 2-bromopyridine, 3: L = 2-chloropyridine). For all compounds, X-ray diffraction showed that the M²⁺ cations are coordinated by two terminal N-bonded thiocyanato anions and two substituted pyridine ligands in a distorted tetrahedral geometry. In all cases, complexes derived from cobalt and zinc with the same ligands are isomorphous. Eight crystalline solids were studied, which belong to only two different structure types: two polymorphs were obtained for the compounds with 2-chloropyridine: 3-Mα is isotypic with the 2-methylpyridine complexes 1-M and crystallizes in the orthorhombic space group P2₁2₁2₁, whereas 3-Mβ and the isotypic 2-bromopyridine complexes 2-M adopt space group Pbcn. The structure types differ with respect to molecular conformation and packing: the complexes crystallizing in the former type are less distorted with respect to metal coordination; in the latter structure type, the angle subtended by the two ligands is by far the largest in the coordination sphere. P2₁2₁2₁ allows for a more efficient intermolecular arrangement, whereas the complex geometry in Pbcn features shorter non-bonding distances between the ortho substituents of the pyridine ligands and comes closer to the gas phase geometry. DSC measurements gave no hints of a transformation between these modifications but from a melt of 3-Coα, 3-Coβ was obtained. Solvent mediated conversion experiments revealed that 3-Mα represents the thermodynamically stable form at room-temperature whereas 3-Mβ is metastable, in agreement with its lower density. Therefore, the thermodynamically stable phases with 2-chloropyridine as ligand adopt the crystal structure of the compounds with 2-methylpyridine, whereas the metastable forms are isotypic to the compounds with 2-bromopyridine.