Crystal structures and Hirshfeld surface comparison of fumarate and bromide salts of the etoxazole metabolite designated R7 (aminium ions).

Etoxazole (C21H23F2NO2) is a fluorinated insecticide/acaricide that acts as an inhibitor of chitin biosynthesis. Its degradation products include several persistent environmental contaminants, including a metabolite designated 'R7' (C19H25F2NO3), which has a readily protonated amine group, to give the R7H+ cation, namely, 2-aza-niumyl-2-(4-tert-butyl-2-eth-oxy-phen-yl)ethyl 2,6-di-fluoro-benzoate. This paper reports the crystal structures of two salts of R7H+: the bromide (I: C21H26F2NO3 +·Br-) and the 3-carb-oxy-prop-2-enoate (hereafter fumarate) (II: C21H26F2NO3 +·C4H3O4 -). Although the R7H+ cations in I and II are chemically identical, the salts crystallize in substanti-ally different structures. Crystals of I are tetra-gonal, space-group type P421 c, whereas those of II are monoclinic, space-group type P21/n, each containing one cation-anion pair in their respective asymmetric units (Z' = 1). The R7H+ cation in both structures feature a central 2-aza-niumyl linker connecting 4-tert-butyl-2-eth-oxy-phenyl and 2,6-di-fluoro-benzoate rings. Cation conformation is primarily governed by different torsional degrees of freedom, leading to distinct overall conformations. The crystal packing in I is driven by N-H⋯Br hydrogen bonds, π-π and C-H⋯π inter-actions, which arrange the cations into columns parallel to the crystallographic c-axis. In contrast, the packing in II consists of N-H⋯O hydrogen bonds to adjacent fumarate anions, forming extended hydrogen-bonded chains. The fumarate anions are sandwiched between R7H+ cations, placing the fumarate double bond between aromatic rings. Hirshfeld-surface analysis shows that inter-molecular contacts in both structures are dominated by inter-actions involving hydrogen (93.4% in I, 92.2% in II).