Two Octanuclear {Cu4Ln4} (Ln = Dy or Tb) Complexes with a Butterfly-Shaped Unit Exhibiting Zero-Field Single-Molecule Magnet Behavior.

Abstract

One-pot reaction of an asymmetrical acylhydrazone ligand H3L with Ln(ClO4)3 and CuCl2 in MeOH and MeCN solvents resulted in two novel octanuclear complexes, [Cu4Dy4L4Cl6(CH3OH)8(H2O)4]·Cl2(CH3OH)9(H2O)3 (1) and [Cu4Tb4L4Cl6(CH3OH)2(H2O)10]·Cl2(H2O)x (2). Single-crystal X-ray diffraction studies revealed that these two complexes are isostructural and can be viewed as being built from two {Cu2Ln2L2} (Ln = Dy or Tb) butterfly-shaped units. Direct current magnetic susceptibilities and field-dependent magnetization measurements demonstrated the presence of strong ferromagnetic interaction between CuII and LnIII and magnetic anisotropy. Furthermore, alternating current (ac) magnetic measurements illustrated that these two complexes showed temperature- and frequency-dependent signals in the out-of-phase ac susceptibility under zero applied field, which are typical features of the slow relaxation of the magnetization for 1 and 2. The effective energy barrier (Ueff) for 1 was 54 K, which is one of the highest Ueff values yet reported for CuII/LnIII single-molecule magnets.