Symmetries, pseudo-symmetries and single-molecule magnets

In recent years, one of the key frameworks for constructing lanthanide-based single-molecule magnets with high operational temperatures has been the so-called symmetry strategy. Under certain point-group symmetries the crystal-field operator corresponding to the molecular complex commutes with the angular momentum projection. This then leads to vanishing of several important transition magnetic moment matrix elements and to slower relaxation of magnetization. For example, local symmetries have been used to explain the relatively high effective energy barriers for the reversal of magnetization in several lanthanide complexes with pentagonal bipyramidal structure.[1] In the present work we will show by quantitative evaluation of the crystal-field using ab initio methods that the symmetry of the crystal-field operator does not match that of an ideal pentagonal bipyramidal environment. Furthermore, the best-performing single molecule magnets are based on dysprosium metallocene cations that do not have any obvious molecular point group symmetry.[2] We will discuss to what extent molecular point-group symmetry – exact or approximate – actually plays a role in determining the magnetic properties of lanthanide single-molecule magnets. We will propose that the magnetic properties are better described by considering the interaction between the ligands and the vacant lanthanide 5d oribtals, which then produces an indirect effect on the open 4f shell.

[1] J.-L. Liu, Y.-C. Chen, M.-L. Tong. Chem. Soc. Rev. 2018, 47, 2431–2453.
[2] F.-S. Guo, B. M. Day, Y.-C. Chen, M.-L. Tong, A. Mansikkamäki, R. A. Layfield. Science, 2018, 362, 1400–1403.