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

Etoxazole (C 21 H 23 F 2 NO 2 ) 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’ (C 19 H 25 F 2 NO 3 ), which has a readily protonated amine group, to give the R7H + cation, namely, 2-azaniumyl-2-(4- tert -butyl-2-ethoxyphenyl)ethyl 2,6-difluorobenzoate. This paper reports the crystal structures of two salts of R7H + : the bromide ( I : C 21 H 26 F 2 NO 3 + ·Br − ) and the 3-carboxyprop-2-enoate (hereafter fumarate) ( II : C 21 H 26 F 2 NO 3 + ·C 4 H 3 O 4 − ). Although the R7H + cations in I and II are chemically identical, the salts crystallize in substantially different structures. Crystals of I are tetragonal, space-group type P 4 2 1 c , whereas those of II are monoclinic, space-group type P 2 1 / n , each containing one cation–anion pair in their respective asymmetric units ( Z ′ = 1). The R7H + cation in both structures feature a central 2-azaniumyl linker connecting 4- tert -butyl-2-ethoxyphenyl and 2,6-difluorobenzoate 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 interactions, 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 intermolecular contacts in both structures are dominated by interactions involving hydrogen (93.4% in I , 92.2% in II ).