X-ray diffraction, DFT, spectroscopic study and insecticidal activity of (3-cyano-1-(2,6-dichloro-4-(trifluoromethyl)phenyl)-4-((trifluoromethyl)sulfinyl)-1H-pyrazol-5-yl)(2-(triethylammonio)acetyl)amide inner salt

Abstract

In the title compound, C₂₀H₁₉Cl₂F₆N₅O₂S, an inner salt derivative of fipronil was synthesized and characterized by X-ray diffraction, ¹H NMR, ¹³C NMR, IR, ESI-MS, EI-HRMS. The crystal is monoclinic, space group P2₁/n, with a = 15.1655(7) Å, b = 15.0465(7) Å, c = 21.0713(9) Å, V = 4754.7(4) ų and Z = 8 (at 173(2) K), and its phenyl pyrazole ring together with bond C-N-O form a big π bond while bonds C-N-C and N-C-H form an inner salt molecular. Crystal stacking scheme indicates the crystal consists of two different molecules. The two molecules are nonplanar with the torsion angles between the pyrazole rings and benzene rings of −99.7(7)° and 88.1(7)°, respectively, and linked by intermolecular C-H⋯O, C-H⋯F hydrogen bonds. By DFT calculations, molecular electrostatic potential clearly shows that the formation of hydrogen bonding interaction, which F5⋯H′42 and F6⋯H′41 are intermolecular hydrogen bonds, and F7⋯H44 is an intramolecular hydrogen bond in the crystal structure, is between the positive and negative regions. In addition, molecular geometry optimized by DFT methods is in good agreement with the experimental values. In the optimized structure, the O-S, S-C, C-F and C-Cl bonds are slightly longer in comparison with those in the crystal. The intermolecular interaction energy for the dimmer was calculated to be 4.96 kcal mol⁻¹. The results reveal that the two monomers are slightly combined with each other through two weak F⋯H hydrogen bonds. The compound and fipronil exhibited high insecticidal activity against the third instar larvae of Asian Corn Borer (Ostrinia furnacalis (Guenée)) at 48 h after treatment with LC₅₀ values of 4.17 μg/mL (r ² = 0.9974) and 5.36 μg/mL (r ² = 0.9970), respectively.