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

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(1) over bar. The molecules of 6 are arranged in pairs with short intermolecular distances between the NO groups. 7 crystallizes in two different modifications: polymorph alpha is orthorhombic, space group Pbca; polymorph beta is monoclinic, space group P2(1)/c. 8 crystallizes in two modifications: the alpha polymorph is monoclinic, space group P2(1)/c; and the beta polymorph is monoclinic, space group P2(1)/n. 9 crystallizes in the monoclinic system, space group P2(1)/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 P2(1)/c. The rotation angle between the two rings in compounds 6-10 is 54.2-76.7degrees. The rotation angle between the two rings is only 21.0degrees 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 = -2J S-1.S-2) J/k(B) = -84.2 K and theta(imp) = 0.3 K and J/k(B) = -95.3 K, theta(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 (theta = -37.0 K Curie-Weiss model).