Prediction of non-centrosymmetric packing for 1,3-disubstituted nitro aromatics. Crystal and molecular structure of 3-hydroxy-6-(3'-nitro)-phenylazopyridine

Abstract

The crystal and molecular structure of 3-hydroxy-6-(3'-nitro)-phenylazopyridine is reported. The crystals are non-centrosymmetric, Pna2₁, Z = 4, a = 16.522(3), b = 8.402(2), c = 7.945(1) Å, and the structure refined to an R-value of 0.046 on 1086 non-zero reflections. The molecule exists as the hydroxyazo tautomer and is intermolecularly O-H … N hydrogen bonded in the crystal. The title compound is one of several 1,3-disubstituted benzenes which adopt non-centrosymmetric packing, a necessary prerequisite for nonlinear second harmonic generation. Analysis of nearly 600 nitroaromatic crystal structures retrieved from the Cambridge Structural Database shows that 1,3-disubstitution significantly increases the probability of non-centrosymmetric space group adoption when compared to 1,2 or 1,4-disubstitution. Similar though less pronounced trends are observed for trisubstituted and higher derivatives.