Magnetic Properties of Prolate Lanthanide Complexes: Synergy between Axial and Equatorial Ligands

The chemical design of single-molecule magnets is a vibrant field of research. Huge efforts in determining the key factors to slow down the relaxation time of the magnetization led to massive improvements in the blocking temperature. Even though several lanthanides are excellent magnetic anisotropy building blocks, the current literature is dominated by (pseudo)-axial Dy³⁺ complexes. Prolate ions such as Er³⁺ and Yb³⁺ exhibit on average relatively modest barriers and fast relaxation times. In this paper, we pinpoint how the combination of symmetry and ligand charge distribution affects the slow relaxation of prolate ions. In this paper, we synthesized and characterized two new lanthanide complexes, namely, [LnDOTA][CoF₂(py)₄] (Ln = Er³⁺ or Yb³⁺, DOTA = tetraazacyclododecane-<i>N</i>,<i>N</i>',<i>N</i>″,<i>N</i>‴-tetraacetate, py = pyridine). Such complexes were used to provide a proof of concept that apical ligands, typically considered detrimental in prolate ion complexes, can be exploited in combination with the other ligands' geometry to improve the performances of the system.