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

The combination of LnIII ions (GdIII, TbIII or DyIII) 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 LnIII 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 (D3h). 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 LnIII 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 (D4d). 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 LnIII 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.