Syntheses and Magnetic Properties of Stable Organic Triradicals with Quartet Ground States Consisting of Different Nitroxide Radicals

Abstract

Bis[p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] nitroxide (3) was synthesized via desilylation of O-protected diradical followed by autoxidation. By deoxgenation of 3, p-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)phenyl-p-(1-oxyl-3-oxide-4,4,5,5-tetramethylimidazolin-2-yl)phenyl nitroxide (4) and bis[p-(1-oxyl-4,4,5,5-tetramethylimidazolin-2-yl)phenyl] nitroxide (5) were prepared. X-ray analysis of a dark brown needle single crystal for 4 confirmed that 4 consisted of three different kinds of nitroxide radical centers, which showed three reversible redox waves at +0.42, +0.82, and +1.01 V (vs Ag/Ag+) in CH3CN solution. The temperature dependence of the molar magnetic susceptibilities for microcrystalline samples of triradicals 3−5 was measured by a SQUID susceptometer, showing the intramolecular ferromagnetic and intermolecular antiferromagnetic interactions. Experimental data were fitted to the equation derived from an asymmetric linear three-spin model (H = −2(J12S1·S2 + J23S2·S3)). The best fit parameters were J12/kB = J23/kB = 231 ± 4 K for 3, J12/kB = 349 ± 26 K, J23/kB = 130 ± 3 K for 4, and J12/kB = J23/kB = 127 ± 3 K for 5. Triradicals 3−5 thus have quartet ground states. These exchange coupling parameters are mutually compared in reference to their molecular structures.