Transferability of Ligand Field Parameters in a Family of 3d-4f M₂Ln₂ Butterfly Single-Molecule Magnets

Lanthanide-based single-molecule magnets are attractive candidates for applications in quantum information processing or data storage. The low symmetry ligand field environment often encountered in these complexes, combined with a sparsity of information in the common magnetic susceptibility and magnetization data recorded on powder samples, leads to massive overparametrization. This limits the application of ligand field models, such as the point charge or angular overlap models, for describing the data. In this work, the radical approach is taken to consider these models as black boxes, whereby they suspend their chemical or physical significance. Instead, the transferability of parameters across a series of structurally related compounds is enforced to achieve a reduced parametrization. This methodology is applied to four members of the isostructural series of [M₂^IIILn₂^III(μ₃-OH)₂(pmide)₂(p-Me-PhCO₂)₆]·2MeCN (pmideH₂ = N-(2-pyridylmethyl)-iminodiethanol) butterfly complexes, with M = Al, Fe, and Ln = Er, Dy. It is shown that the powder magnetic susceptibility and magnetization data can be described simultaneously with remarkable accuracy using only two parameters for characterizing the lanthanide ligand fields in all four compounds. Interestingly, the resulting parametrization and its values lie within the range of chemical intuition despite the black box procedure used to obtain them. Implications of this approach are discussed.