Modulating spin dynamics of LnIII-radical complexes by using different coligands

Abstract

The combination of Ln^III ions (Gd^III, Tb^III or Dy^III) with a triazole nitronyl nitroxide radical results in six novel 2p–4f compounds, namely, [Ln₂(hfac)₆(MeTrzNIT)(H₂O)₂]·1/2CH₂Cl₂ (Ln = Gd(1), Tb(2), Dy(3); hfac = hexafluoroacetylacetone; MeTrzNIT = 2-[3-(5-methyl-l,2,4-triazolyl)]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide and [Ln(Phtfac)₃(MeTrzNIT)]₂·C₇H₁₄·3H₂O (Ln = Gd(4), Tb(5), Dy(6); Phtfac = 4,4,4-trifluoro-1-phenylbutane-1,3-dione). Single crystal X-ray diffraction studies revealed that compounds 1–3 are binuclear isostructural complexes, in which one MeTrzNIT molecule acts as a double-bridging ligand coordinated to two Ln^III ions through its two NO groups and two nitrogen atoms of the triazole ring. In 1–3, the coordination number around the lanthanide ion is nine, and the polyhedron is a 4,4,4-tricapped trigonal prism (D_3h). While the larger steric hindrance of the Ph- group than CF₃- in the Phtfac ligand induces complexes 4–6 to be mononuclear bi-spin compounds, in which central Ln^III ions are coordinated by three Phtfac and one bidentate MeTrzNIT radical. The coordination number around the lanthanide ion in 4–6 is eight, and the polyhedron is in a square antiprism geometry (D_4d). Compounds 3 and 5 were found to exhibit slow relaxation of the magnetization, suggesting single-molecule magnet (SMM) behavior, while no ac signal is noticed for compounds 2 and 6. The different magnetic relaxation behaviours between 2 and 5, or between 3 and 6, are due to the different crystal structures around the Ln^III ions and the magnetic interaction. It is demonstrated that the β-diketonate coligand may play an important role in determining the spin dynamics for the lanthanide-radical system.