Role of hydrogen bonds in benzimidazole-based organic magnetic materials: Crystal scaffolding or exchange linkers?

The highly stable nitroxide radicals 5(6)-chloro-2-(N-tert-buty2-N-aminoxyl )benzimidazole (4) and 5,6-dimethyl-2-(N-tert-butyl-N-aminoxyl)benzimidazol e (5) were synthesized and characterized by electron spin resonance spectro scopy, crystallography, and magnetic susceptibility. Both crystallize as di meric pairs with N-H hydrogen bond donors and N-O accepters. This behavior contrasts to that of analogous 2-(N-tert-butyl-N-aminoxyl)benzimidazole (3) , which forms hydrogen-bonded chains (Ferrer, J. R.; Lahti, P. M.; George, C.; Antorrena, G.; Palacio, F. Chem. Mater. 1999, 11, 2205-2210), Susceptib ility analysis shows dimeric antiferromagnetic exchange coupling with J/k = -22 K for 4 and -24 K for 5. Density functional theory (DFT) computations support a N-O to N-O throughspace antiferromagnetic exchange mechanism as t he major contributor to the overall magnetic behavior of 4 and 5. A spin-po larization exchange coupling mechanism involving a cyclic exchange path usi ng the -N-H . . .O-N hydrogen bonds in the dimers is expected to yield ferr omagnetic coupling between dimers, in contradiction to the experimental and DFT results. The hydrogen bonds in 4 and 5 thus act more as structural sca ffolding to hold the radicals in proximity rather than as electronic exchan ge linkers.