Synthesis, Structure, and Magnetic Properties of a New Family of Tetra-nuclear {Mn2IIILn2}(Ln = Dy, Gd, Tb, Ho) Clusters With an Arch-Type Topology: Single-Molecule Magnetism Behavior in the Dysprosium and Terbium Analogues

Sequential reaction of Mn­(II) and lanthanide­(III) salts with a new multidentate ligand, 2,2′-(2-hydroxy-3-methoxy-5-methylbenzylazanediyl)­diethanol (LH3), containing two flexible ethanolic arms, one phenolic oxygen, and a methoxy group afforded heterometallic tetranuclear complexes [Mn2Dy2(LH)4(μ-OAc)2]­(NO3)2·2CH3OH·3H2O (1), [Mn2Gd2(LH)4(μ-OAc)2]­(NO3)2·2CH3OH·3H2O (2), [Mn2Tb2(LH)4(μ-OAc)2]­(NO3)2·2H2O·2CH3OH·Et2O (3), and [Mn2Ho2(LH)4(μ-OAc)2]­Cl2·5CH3OH (4). All of these dicationic complexes possess an arch-like structural topology containing a central MnIII–Ln–Ln–MnIII core. The two central lanthanide ions are connected via two phenolate oxygen atoms. The remaining ligand manifold assists in linking the central lanthanide ions with the peripheral Mn­(III) ions. Four doubly deprotonated LH2– chelating ligands are involved in stabilizing the tetranuclear assembly. A magnetochemical analysis reveals that single-ion effects dominate the observed susceptibility data for all compounds, with comparably weak Ln···Ln and very weak Ln···Mn­(III) couplings. The axial, approximately square-antiprismatic coordination environment of the Ln3+ ions in 1–4 causes pronounced zero-field splitting for Tb3+, Dy3+, and Ho3+. For 1 and 3, the onset of a slowing down of the magnetic relaxation was observed at temperatures below approximately 5 K (1) and 13 K (3) in frequency-dependent alternating current (AC) susceptibility measurements, yielding effective relaxation energy barriers of ΔE = 16.8 cm–1 (1) and 33.8 cm–1 (3).