Slow relaxation of the magnetization observed in mononuclear Ln–radical compounds with D4d geometry configurations

Abstract

The combination of Ln^III ions (Gd^III, Tb^III or Dy^III) and a pyrazole nitronyl nitroxide radical results in three isomorphous complexes, namely, [Ln(hfac)₃(NIT-Pyz)]₂ (Ln = Gd(1), Tb(2), Dy(3); hfac = hexafluoroacetylacetone; NIT-Pyz = 2-{3-pyrazolyl}-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide). Single crystal X-ray diffraction studies revealed that all of them are composed of two crystallographically independent mononuclear systems, in which the central Ln^III ions are coordinated by three hfac and one bidentate chelating NIT-Pyz radical. The central Ln^III ions are all in square antiprism geometry (D_4d) polyhedron configurations. Based on the spin Hamiltonian calculations, there exist antiferromagnetic couplings in the Gd^III-NIT radical system in complex 1. Complexes 2 and 3 show frequency-dependent out-of-phase signals in a zero field indicating single-molecule magnetic behavior. Moreover, Tb's complex (2) shows a single thermal relaxation process with an energy barrier of 26 K. For Dy's complex (3), the Orbach and Raman processes both contribute to the magnetic relaxation behaviors.