Two-dimensional hydrogen-bonded polymers in the crystal structures of the ammonium salts of phenoxyacetic acid, (4-fluorophenoxy)acetic acid and (4-chloro-2-methylphenoxy)acetic acid

Abstract

The structures of the ammonium salts of phenoxyacetic acid, NH4+·C8H6O3−, (I), (4-fluorophenoxy)acetic acid, NH4+·C8H5FO3−, (II), and the herbicidally active (4-chloro-2-methylphenoxy)acetic acid (MCPA), NH4+·C9H8ClO3−·0.5H2O, (III) have been determined. All have two-dimensional layered structures based on inter-species ammonium N—H...O hydrogen-bonding associations, which give core substructures consisting primarily of conjoined cyclic motifs. The crystals of (I) and (II) are isomorphous with the core comprisingR12(5),R12(4) and centrosymmetricR42(8) ring motifs, giving two-dimensional layers lying parallel to (100). In (III), the water molecule of solvation lies on a crystallographic twofold rotation axis and bridges two carboxyl O atoms in anR44(12) hydrogen-bonded motif, creating twoR43(10) rings, which together with a conjoined centrosymmetricR42(8) ring incorporating both ammonium cations, generate two-dimensional layers lying parallel to (100). No π–π ring associations are present in any of the structures.