Chlorfenapyr Crystal Polymorphism and Insecticidal Activity

Abstract

Four crystalline polymorphs of the proinsecticide chlorfenapyr [4-bromo-2-(4-chlorophenyl)-1-ethoxymethyl-5-trifluoromethyl-1H-pyrrole-3-carbonitrile] have been identified and characterized by polarized light optical microscopy, differential scanning calorimetry, Raman spectroscopy, X-ray diffraction, and electron diffraction. Three of the four structures were considered polytypic. Chlorfenapyr polymorphs show similar lethality against fruit flies (Drosophila melanogaster) and mosquitoes (Anopheles quadrimaculatus) with the least stable polymorph showing slightly higher lethality. Similar activities may be expected to be consistent with structural similarities. Knockdown kinetics, however, depend on an internal metabolic activating step, which further complicates polymorph-dependent bioavailability.

Scheme 1. Chlorfenapyr, C₁₅H₁₁BrClF₃N₂O, M_w = 407.62 g/mol

Figure 1

Optical micrographs of chlorfenapyr polymorphs. Polarizers are crossed for images (A–E). (A) Form I was crystallized from the melt at 80 °C. (B) Form I crystallized from the melt at 23 °C. (C) Transformation of form I into form II/III starting from the edge of the slide at the lower left corner. (D) Feathery flakes of form IV grown from the melt and surrounded by spherulites of form I. (E) Spherulite form IV nucleated at 50 °C. (F) Crystal of form III grown from hexanes. (G) Needle of form II formed on the surface of a poly(ethylene) fiber. If not specified, the scale bars are 0.5 mm.

Figure 2

Linear growth rates of chlorfenapyr I (black circles) and IV (red triangles) from the melt. Significant scattering of points for form IV is related to its growth anisotropy.

Figure 3

DSC heating curves for chlorfenapyr polymorphs. Heating rate is 10 °C/min. Endotherms around 101 °C correspond to melting of form I. Endotherms around 88–96 °C correspond to II → I and III → I phase transformations and are likely to partial melting of forms II and III. An endotherm around 61–68 °C corresponds to IV → I phase transformation.

Figure 4

Raman spectra of chlorfenapyr polymorphs (A) and a fragment highlighting the differences between polymorphs (B).

Figure 5

Powder X-ray diffraction patterns for chlorfenapyr polymorphs. Forms I and IV were crystallized from the melt, while forms II and III were obtained from acetone and hexanes solutions, respectively.

Figure 6

2kl reciprocal space sections of 3D ED data from chlorfenapyr II and III. (A) Ordered form II, (B) ordered form III, (C) probable intergrowth of form II (large domain) and form III (small domain, arrow indicates the strongest reflection of form III with odd l index), (D) disordered form II where symmetry of the intensities follows the monoclinic symmetry (only vertical mirror symmetry), (E) disordered form II where symmetry of the intensities corresponds to orthorhombic symmetry (both vertical and horizontal mirror symmetry), and (F) simulation of (E) using 1:1 combination of smeared form II and III 2kl sections with induced orthorhombic symmetry.

Figure 7

TEM image of chlorfenapyr deposited from 0.8 wt % chloroform solution on a holey carbon grid and kept at room temperature for 12 min. The imaging was performed under cryogenic conditions. The areas marked with different colors correspond to the fast Fourier transforms (FFTs) marked with the same color. The boxed areas (blue, red, and green) present crystalline domains with FFT patterns that correspond to spacings of 4.46 ± 0.02 and 3.48 ± 0.02 Å. The area marked in yellow is vitrified ice without organic material showing no FFT pattern of chlorfenapyr. The blue area corresponds to a single crystal of form II, and the green and blue correspond to a mixture of forms II and III.

Figure 8

Crystal structures of chlorfenapyr polymorphs. Hydrogen atoms have been omitted for clarity.

Figure 9

KT₅₀ obtained for chlorfenapyr I and IV crystallized from the melt, chlorfenapyr II crystallized from acetone, chlorfenapyr IId crystallized from chloroform, and chlorfenapyr III as commercial form. Different bars correspond to different trials. (A) Fruit flies, Drosophila melanogaster. Bars marked with * correspond to the bioassay performed on the same day for which chlorfenapyr II and III were crystallized from acetone solution. Bars marked with # correspond to bioassay performed on the same day. (B) A. quadrimaculatus mosquitoes.