Unusual Gd–nitronyl nitroxide antiferromagnetic coupling and slow magnetic relaxation in the corresponding Tb analogue

Abstract

The lanthanide–radical approach has been applied to construct three isomorphous mononuclear lanthanide complexes [Ln(hfac)₃(NIT2-PyOCH₃)] (Ln^III = Gd 1, Tb 2, Er 3) in which the nitronyl nitroxide radical NIT2-PyOCH₃ (NIT2-PyOCH₃ = 2-(3′-methoxy-2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) acts as a tridentate chelating ligand via the pyridyl nitrogen atom, the oxygen atom from the NO group and the oxygen atom from the methoxy group. The central lanthanide ions are nine-coordinated and their coordination geometry can be considered as an intermediate between muffin and spherical capped square antiprisms. Interestingly, an unusual antiferromagnetic interaction was observed between the Gd(III) and the coordinated nitronyl nitroxide radical in complex 1. Moreover, ac magnetic susceptibilities display frequency-dependent out-of-phase signals in the case of the anisotropic Tb complex, indicating magnetic relaxation behavior.