Regulation of magnetic relaxation behavior by replacing 3d transition metal ions in [M2Dy2] complexes containing two different organic chelating ligands

Abstract

Four tetranuclear 3d–4f complexes, namely [Fe₂Ln₂(L)₂(teaH)₂(Cl)₂](NO₃)₂·4CH₃CN (H₂L = N1,N3-bis(3-methoxysalicylidene)diethylenetriamine, teaH₃ = triethanolamine, Ln = Dy for 1 and Ln = Gd for 1′) and [Co₂Ln₂(L)₂(pdm)₂(CH₃COO)₂(CH₃OH)₂](NO₃)₂·xCH₃OH·yH₂O (pdmH₂ = 2,6-pyridinedimethanol, Ln = Dy, x = 5 and y = 2.5 for 2 and Ln = Gd, x = 6 and y = 1.5 for 2′), have been reported. Two Fe^III and two Dy^III in 1 formed a zigzag Fe1–Dy1–Dy1a–Fe1a arrangement with a Fe1–Dy1–Dy1a angle of 105.328(3)°. However, in contrast to 1, two Co^III and two Dy^III ions in 2 formed a more linear Co1–Dy1–Dy1a–Co1a arrangement with a Co1–Dy1–Dy1a angle of 141.86(2)°. Additionally, two Dy^III ions in 1 are eight-coordinated with a triangular dodecahedron geometry, while two Dy^III ions in 2 adopt nine-coordination with a muffin geometry. Magnetic studies revealed slow magnetic relaxation behavior for 1, with an energy barrier E_a of 6.9 K. For 2, single molecule magnet behavior was presented under a zero dc field with an effective energy barrier U_eff of 64.0(9) K. Ab initio calculations for 1 and 2 indicate that compared to 2, complex 1 has a larger transversal magnetic moment of its ground Kramers doublets (KD) and a larger value of the tunnelling parameter (Δ_t) for the exchanged coupled ground state, which may result in poor single molecule magnet behavior for 1.