A Persistent N-H...N Ribbon Motif in the Crystal Structures of Triazine Herbicides

Abstract

Triazines have been used for many years as herbicides, but we now report for the first time the crystal structures of two ubiquitous exemplars, Simazine 1 (R = ethyl) and Atrazine 2 (R = i-propyl, 2 polymorphs), which were possible only with the Synchrotron diffraction measurements. Analysis of the crystal structures reveals infinite molecular ribbons (I) comprising centrosymmetric N-H..N hydrogen bonds, with weaker secondary C-H...Cl interactions. The hydrogen bonding constrains each molecule into a very specific conformation, yet the ribbon is conserved across 1 and both polymorphs of 2. Accommodation of the ribbon in each of the crystal structures results in changes in space group and some out-of-plane buckling in the ribbon. A search through the Cambridge Crystallographic Data showed N-H..N synthon is present in other members of the chlorotriazine family, forming a ribbon whenever the alkyl/aryl R groups can "solve" the packing problem to produce a hydrophobic domain parallel to the hydrogen bonds1. Such is the dominance of this ribbon motif that it may persist across a large range of chlorotriazines yet to be analyzed or synthesized.