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

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.