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 nriethoxy group afforded heterometallic tetranuclear complexes [Mn2Dy2(LH)(4)(mu-OAc)(2)]-(NO3)(2)center dot 2CH(3)OH center dot 3H(2)O (1), [Mn2Gd2(LH)(4)(mu-OAc)(2)]-(NO3)(2)center dot 2CH(3)OH center dot 3H(2)O (2), [Mn2Tb2(LH)(4)(mu-OAc)(2)]-(NO3)(2)center dot 2H(2)O center dot 2CH(3)OH center dot Et2O (3), and [Mn2Ho2(LH)(4)(mu-OAc)(2)]-Cl-2 center dot 5CH(3)OH (4). All of these dicationic complexes possess an arch-like structural topology containing a central Mn-III-Ln-Ln-Mn-III 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 center dot center dot center dot Ln and very weak Ln center dot center dot center dot Mn(III) couplings The axial, approximately square-antiprismatic coordination environment of the Ln(3+) 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 Delta E = 16.8 cm(-1) (1) and 33.8 cm(-1) (3).