The Structure of Cis -2,2′-Azopyridine in the Solid State

Descriptions of azoheteroaromatic compounds in their cis -isomeric form in the solid state are almost non-existent. Cis -2,2′-azopyridine is a known compound which structure differentiates from the trans -isomer based on spectroscopic evidence (UV–Vis absorption, 1 H, and 13 C NMR spectroscopy) and with studies regarding its stability, thermal reversion to the trans -isomer, and dipole moment in solution. However, the structure of this compound in the solid state has been unresolved. Herein, we present the crystal structure of cis -2,2′-azopyridine that forms red prismatic crystals in the monoclinic space group C2/c. The unit cell parameters are: a = 23.485 (4), b = 5.9241 (12), c = 21.656 (4) Å, and V = 2717.1 (9) Å 3 with 12 molecules per unit cell. Molecules are arranged as two helical conformers ( P and M ) with C–N=N–C torsion angles and aromatic ring planes’ angles comparable to those reported for cis -azobenzene. Graphical Abstract The rare structural determination of an azoheteroaromatic cis -isomer of 2-azopyridine that was synthesized, isolated and crystallized is presented. X-ray structure analysis reveals that the molecule forms dimeric complexes containing both M - and P -helical conformers as well as infinite stacked columns in the solid state through a combination of C–H⋯N hydrogen bonds, C–H⋯π interactions and lone pair⋯π contacts.