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

The use of an amino-pyridyl substituted beta-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)][CI](3)center dot 2H(2)O (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 tau(0) = 2.5(8) x 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 tau(0) = 6.2 x 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.