Synthesis, structural, and magnetic characterization of substituted benzoimidazole-l-yl N,N′-dioxides

Abstract

The crystal structures, EPR spectra and magnetic properties of the novel halogen- and cyano-substituted nitronyl nitroxide radicals 2-(2,6-dichlorophenyl)benzimidazolyl N,N′-dioxide, 6, 2-(2,6-difluorophenyl)benzimidazolyl N,N′-dioxide, 7, 2-(2-chloro-6-fluorophenyl)benzimidazolyl N,N′-dioxide, 8, 2-(2,3,6-trichlorophenyl)benzimidazolyl N,N′-dioxide, 9, 2-(2,3,4,5,6-pentafluorophenyl)benzimidazolyl N,N′-dioxide, 10, and 2-(3-cyanophenyl)benzimidazolyl N,N′-dioxide, 11, are reported. Compound 6 crystallizes in the triclinic crystal system in space group P. The molecules of 6 are arranged in pairs with short intermolecular distances between the NO groups. 7 crystallizes in two different modifications: polymorph α is orthorhombic, space group Pbca; polymorph β is monoclinic, space group P2₁/c. 8 crystallizes in two modifications: the α polymorph is monoclinic, space group P2₁/c; and the β polymorph is monoclinic, space group P2₁/n. 9 crystallizes in the monoclinic system, space group P2₁/c. 10 crystallizes in the monoclinic system, space group C2/c. The molecules of 10 are packed in pairs of two types that form a chain perpendicular to the c-axis. 11 crystallizes in the monoclinic crystal system in space group P2₁/c. The rotation angle between the two rings in compounds 6–10 is 54.2–76.7°. The rotation angle between the two rings is only 21.0° in 11 and it strongly affects the packing of the molecules that adopt the stacking mode. The magnetic measurements show that 6, 7, 10 and 11 exhibit large magnetic coupling. The best fitting with the experimental data for 6 and 11 was obtained using the Bleaney–Bowers singlet–triplet model plus the Curie–Weiss spin impurity (S = 1/2; H = −2JS₁·S₂) J/k_B = −84.2 K and θ_imp = 0.3 K and J/k_B = −95.3 K, θ_imp = 1.8 K, respectively. A Pade expression for 7 revealed J_intra/k_B = 66.0 K and zJ_inter/k_B = −14.0 K. Compound 10 shows evidence for large antiferromagnetic spin coupling (θ = −37.0 K Curie–Weiss model).