Polymorphs and Solvates of Pinoxaden: Crystal Structures and Phase Transformation Analysis

Solid form development in herbicides is a promising method to enhance herbicidal activity and minimize environmental impact. Pinoxaden (PXD) is a selective herbicide commonly used for the control of grass weeds in cereal crops, yet its solid form landscape remains unknown. Herein, four novel solid forms of PXD including two polymorphs (Form II and Form III) and two solvates (ethylene glycol solvate S_EG, acetic acid solvate S_AA) were discovered. The crystal structures of Form II and the two solvates were solved by single-crystal X-ray diffraction. The molecular conformations and packing patterns vary significantly due to the molecular flexibility of PXD and the incorporation of solvent molecules. All interactions in Form I and Form II are weak intermolecular forces, whereas hydrogen bonding between PXD and ethylene glycol or acetic acid results in the formation of trimeric and dimeric structures in S_EG and S_AA, respectively. Moreover, the stability and phase transformation relationships were investigated for these solid forms. These findings offer basic knowledge of PXD solid form landscape and highlight the potential for discovering additional solid forms of long-standing herbicides, paving the way for the development of desired solid forms with improved properties.