Enhancement of Tb(III)–Cu(II) Single‐Molecule Magnet Performance through Structural Modification

We report a series of 3d–4f complexes {Ln(2)Cu(3)(H(3)L)(2)X(n)} (X=OAc(−), Ln=Gd, Tb or X=NO(3) (−), Ln=Gd, Tb, Dy, Ho, Er) using the 2,2′‐(propane‐1,3‐diyldiimino)bis[2‐(hydroxylmethyl)propane‐1,3‐diol] (H(6)L) 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 {Tb(2)Cu(3)(H(3)L)(2)X(n)} complexes is seen by changing the auxiliary ligands (X=OAc(−) for NO(3) (−)). 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 Tb(III) coordination environment (C (4v) versus C(s)).