4-Cyanopyridine complexes [MX2(4-CNpy)x]n (with X = Cl, Br and x = 1, 2): crystal structures, thermal properties and a comparison with [MX2(3-CNpy)x]n complexes

Abstract

Eleven new complexes [MX₂(4-CNpy)_x]_(n) with M = Mn, Fe, Co, Ni, Cu, and Zn, X = Cl and Br, 4-CNpy = 4-cyanopyridine, and x = 1 and 2 have been prepared and their structures were characterized by X-ray powder diffraction (XRPD). The reaction of the transition metal halides MX₂ (M = Mn, Fe, Co, Ni, Cu and X = Cl, Br) with 4-CNpy leads to the formation of coordination compounds with the composition [MX₂(4-CNpy)₂]_n first. In all these compounds, the metal atoms are octahedrally coordinated by four halogen atoms and two 4-CNpy ligands. The halogen atoms bridge the metal atoms, leading to infinite [MX₂]_n chains. The 4-CNpy molecules coordinate through their pyridine N atoms (N_py) and form lateral “wings” on the [MX₂]_n chains. The manganese compound [MnBr₂(4-CNpy)₂]_n shows polymorphism with a fully reversible phase transition. The thermal behaviour of compounds [MX₂(4-CNpy)₂]_(n) was investigated by a combined differential thermal and thermogravimetric analysis (DTA–TG). DTA results show that thermal annealing of [MX₂(4-CNpy)₂]_n at around 240–260 °C leads to compounds of composition [MX₂(4-CNpy)₁]_n (M = Mn, Fe, Co and X = Cl, Br). In these compounds, 4-CNpy acts as a bidentate ligand: the N_py and N_CN atoms both coordinate to the metal atoms, building (nearly) linear M-py-CN–M bridges between the [MX₂]_n chains. This results in two-dimensional (2D) networks, which form the layer structures of the [MX₂(4-CNpy)₁]_n compounds. In all bromo compounds [MBr₂(4-CNpy)₁]_n, the 4-CNpy bridges exhibit a head-to-tail-disorder with an inverted orientation of the 4-CNpy molecules between the metal atoms. Further heating of [MX₂(4-CNpy)₁]_n leads to [MnBr₂(4-CNpy)_1/2]_n, [FeCl₂(4-CNpy)_1/3]_n, [CoCl₂(4-CNpy)_1/3]_n and [CoBr₂(4-CNpy)_1/3]_n but their structures could not be determined yet. Additionally, the zinc compound [ZnBr₂(4-CNpy)₂] was prepared. In its both polymorphs, the Zn atoms are tetrahedrally coordinated by two bromine atoms and two 4-CNpy ligands (via the N_py atoms) to form discrete complexes. Upon heating to 240 °C, the Zn compound decomposes to ZnBr₂ without forming intermediate phases. In all compounds [MX₂(4-CNpy)_x]_(n), the coordination behavior of the 4-CNpy ligand was investigated by infrared (IR) spectroscopy. A comparison with the structures of the corresponding 3-CNpy compounds is given. This article aims at emphasizing the versatile ability of cyanopyridine to act as a mono- or bidentate ligand to build structures with isolated complexes, chains, double chains and planar or wavy 2D networks.