Synthesis, Crystal Structures, and Magnetic Properties of New Hexanuclear Mn^III ₂Ln^III ₄ Complexes: SMM Behavior of the Terbium(III) Analogue

This investigation reports the syntheses, crystal structures, and magnetic properties of a series of isomorphous hexanuclear Mn^III ₂Ln^III ₄ compounds of composition [Mn^III ₂Ln^III ₄L¹ ₂L² ₂(µ₃-OH)₄(µ_1,3-Pivalate)₆(NO₃)₂]·2CH₃CN {Ln = Tb (1), Dy (2), Ho (3), Er (4)}, where H₂L¹ is the Schiff base ligand that is obtained by [1+1] condensation of 3-methoxysalicylaldehyde (HL²) with 2-amino-2-methyl-1-propanol. While the O(phenoxo)N(imine)O(alkoxo) pocket of the ligand [L¹]^2– binds a Mn^III ion, the other pocket, O(methoxy)O(phenoxo), remains noncoordinated. In contrast, each of the O(methoxy)O(phenoxo) and O(phenoxo)O(aldehyde) sites of [L²]^– is occupied by a Ln^III ion. Additionally, there are four µ₃-hydroxo, six µ_1,3-pivalate, and two chelating nitrate ligands, stabilizing the hexanuclear cores. Magnetic susceptibility (2–300 K) and magnetization (M) data reveal significant anisotropy in 1–4. Detailed ac susceptibility measurements have revealed single molecule magnet (SMM) behavior in the Tb^III analogue with U_eff = 11.12 cm^–1 and τ₀ = 1.04 × 10^–7 s under a 1000 Oe dc field and U_eff = 10.42 cm^–1 and τ₀ = 1.62 × 10^–7 s under a 3500 Oe dc field, while the other three analogues do not show slow relaxation of magnetization.