Enhancement of TbIII-CuII Single-Molecule Magnet Performance through Structural Modification

We report a series of 3d-4f complexes {Ln(2)Cu(3)(H3L)(2)Xn} (X=OAc- Ln=Gd, Tb or X=NO3-, Ln=Gd, Tb, Dy, Ho, Er) using the 2,2'-(propane-1,3-diyldiimino)bis[2-hydroxylmethyl)propane-1,3-diol] (H6L) pro-ligand. All complexes, except that in which Ln=Gd, show slow magnetic relaxation in zero applied dc field. A remarkable improvement of the energy barrier to reorientation of the magnetisation in the {Tb2Cu3(H3L)(2)X-n} complexes is seen by changing the auxiliary ligands (X=OAc- for NO3-. This leads to the largest reported relaxation barrier in zero applied dc field for a Tb/Cu-based single-molecule magnet. Ab initio CASSCF calculations performed on mononuclear Tb-III models are employed to understand the increase in energy barrier and the calculations suggest that the difference stems from a change in the TbIII coordination environment (C-4v versus C-s).