Single molecule magnetism in a family of mononuclear β-diketonate lanthanide(iii) complexes: rationalization of magnetic anisotropy in complexes of low symmetry

Abstract

The use of an amino-pyridyl substituted β-diketone, N-(2-pyridyl)-ketoacetamide (paaH), has allowed for the isolation of two new families of isostructural mononuclear lanthanide complexes with general formulae: [Ln(paaH*)₂(H₂O)₄][Cl]₃·2H₂O (Ln = Gd (1), Tb (2), Dy (3), Ho (4), Er (5) and Y (6)) and [Ln(paaH*)₂(NO₃)₂(MeOH)][NO₃] (Ln = Tb (7), Dy (8), Ho (9) and Er (10)). The dysprosium members of each family (3 and 8) show interesting slow magnetic relaxation features. Compound 3 displays Single Molecule Magnet (SMM) behaviour in zero DC field with an energy barrier to thermal relaxation of E_a = 177(4) K (123(2) cm⁻¹) with τ₀ = 2.5(8) × 10⁻⁷ s, while compound 8 shows slow relaxation of the magnetization under an optimum DC field of 0.2 T with an energy barrier to thermal relaxation of E_a = 64 K (44 cm⁻¹) with τ₀ = 6.2 × 10⁻⁷ s. Ab initio multiconfigurational calculations of the Complete Active Space type have been employed to elucidate the electronic and magnetic structure of the low-lying energy levels of compounds 2–5 and 8. The orientation of the anisotropic magnetic moments for compounds 2–5 are rationalized using a clear and succinct, chemically intuitive method based on the electrostatic repulsion of the aspherical electron density distributions of the lanthanides.