Slow magnetic relaxation in lanthanide complexes with chelating imino nitroxide radicals

Two lanthanide-radical complexes [Tb(hfac)3(IM-2py)] (1) and [Tb(hfac)3(IM-2thz)] (2) (hfac = hexafluoroacetylacetonate; IM-2py = 2-(2′-pyridyl)-4,4,5,5-tetramethyl- imidazoline-1-oxyl; IM-2thz = 2-(2′-thiazolyl)-4,4,5,5-tetramethylimidazoline-1-oxyl) have been synthesized. Single crystal X-ray diffraction analysis reveals that complexes 1 and 2 are mononuclear complexes in which the IM-2py/IM-2thz radical is coordinated to the Tb(III) ion in a chelating way. The similar magnetic behavior is observed for two compounds through direct current magnetic measurements, implying antiferromagnetic coupling between Tb(III) and imino nitroxide radical. Alternating current magnetic susceptibility studies show that both complexes exhibit frequency-dependent out-of-phase signals indicative of a slow relaxation of magnetization.