4-Cyanopyridine, a versatile mono- and bidentate ligand. Crystal structures of related coordination polymers determined by X-ray powder diffraction

Abstract

4-Cyanopyridine (4-CNpy, also known as isonicotinonitrile) can act as a monodentate ligand in transition metal complexes via the pyridine nitrogen atom (N_py), or as a bidentate ligand via both nitrogen atoms (N_py and N_CN), resulting in a linear bridge between two metal atoms. Seven new polymeric transition metal compounds, [CuCl₂(4-CNpy)]_n (1b), [MnCl₂(4-CNpy)]_n (2b), [NiCl₂(4-CNpy)₂]_n (3a), [NiCl₂(4-CNpy)]_n (3b), [CoBr₂(4-CNpy)₂]_n (4a), [NiBr₂(4-CNpy)₂]_n (5a) and [NiBr₂(4-CNpy)]_n (5b), are reported. Compounds 1b, 2b, 3b and 5b were obtained from the corresponding [M(II)X₂(4-CNpy)₂]_n compounds by careful thermal decomposition under controlled conditions. Compounds 3a, 4a and 5a were synthesized from 4-cyanopyridine and transition metal halides. For all compounds, IR-spectroscopy was used to distinguish between mono- or bi-coordination of the 4-cyanopyridine ligand: in bi-coordinated compounds the asymmetric stretching vibrations of the cyano group v_as(CN) are shifted to higher frequencies. All crystal structures were determined from X-ray powder diffraction data. Compounds 1b, 3a, 4a and 5a consist of polymeric chains of octahedra (double chains for 1b; single chains for 3a, 4a and 5a), in which 4-cyanopyridine acts as a monodentate ligand via the N_py atom. Compound 1b exists in two polymorphs, a triclinic (α-1b) and a monoclinic phase (β-1b); both exhibit a strong Jahn–Teller distortion of the CuCl₅N-octahedra. On the other hand, 2b, 3b and 5b exhibit layered structures, in which the 4-cyanopyridine acts as a bidentate ligand. Both nitrogen atoms coordinate to metal atoms, resulting in a linear M(II)-py-CN–M(II) bridge. These are the first examples of Mn and Ni compounds in which two 3d metal atoms are connected by a 4-cyanopyridine bridge. Due to the linearity of this bridge, 4-cyanopyridine lends itself to the construction of new metal–organic frameworks.