Experimental and theoretical investigations of four 3d–4f butterfly single-molecule magnets

Abstract

The syntheses, structures, and characterization of four 3d–4f butterfly clusters are described. With different polyhydroxy Schiff-base ligands 2-(((2-hydroxy-3-methoxyphenyl)methylene)amino)-2-(hydroxymethyl)-1,3-propanediol (H₄L₁) and 2-(2,3-dihydroxpropyliminomethyl)-6-methoxyphenol (H₃L₂), three heterotetranuclear Ni^II₂Ln^III₂ complexes (Ni^II₂Dy^III₂-L₁ (1), Ni^II₂Tb^III₂-L₂ (2), Ni^II₂Dy^III₂-L₂ (3)) and one heterohexanuclear Co^III₂Dy^III₄ complex (4) were obtained. The three heterotetranuclear Ni^II₂Ln^III₂ complexes display a central planar butterfly topology. The heterohexanuclear complex was built from butterfly Co^III₂Dy^III₂ clusters and two Dy^III ions by the bridging of pivalate. The vertices of the body positions of the butterfly are occupied by transition metal ions in all four complexes. Magnetic analyses indicate that the complexes exhibit typical single-molecule magnet behaviour with anisotropy barriers of 33.7 cm⁻¹, 60.3 cm⁻¹, 39.6 cm⁻¹, and 18.4 cm⁻¹ for 1–4, respectively. Ab initio calculations were performed on these complexes, and the low lying electronic structure of each Ln^III (Ln = Dy, Tb) ion and the magnetic interactions were determined. It was found that the two Ln ions may have much more contribution to the total relaxation barrier through the stronger 3d–4f exchange couplings compared to weak Ln–Ln interactions.