Organolanthanide complexes with slow magnetic relaxation – RelaxMax

The methods include synthetic methods, routine characterization of organometallic compounds as well as magnetism and in particular the study of magnetic relaxation as well as theoretical calculations at different levels of calculations.

Cnt and Cot ligands (cyclooctatrienyl and cyclononatetraenyl, respectively) were synthesized and were able to vary the ratio of isomers for the Cnt. Routine characterizations (NMR, X-ray, UV-Vis) were carried out for the compounds synthesized. The synthesis of trisCnt (Cnt) 3Ln type complexes was carried out for the Sm to Lu ions. Theoretical calculations (including new RASSCF type methods) and magnetism measurements were carried out. The synthesis of heteroleptic compounds of Ln (Cnt) (Cot) type was carried out for compounds from Sm to Lu. the synthesis of lanthanide ions from La to Nd. Magnetism measurements and calculations in particular allowed the analysis of the electronic structure, while the analysis of the structure revealed a shift in the coordination of the Cnt ligand. In addition, the complex of (Cot) 2Tm2- was synthesized with counter-cations of different nature and its magnetism properties were studied and compared to the results of theoretical calculations. In addition to a remarkable agreement between experiment and theory, this highly original compound turned out to be the first example of a complex behaving like a molecule-magnet without an applied field.

Work on divalent lanthanides and Cot and Cnt ligands is continuing in synthesis with promising preliminary results. They will be backed by theoretical calculations and measurements of magnetism. Then the ICP partner will be able to study, from a computational point of view, the coordination and isomerization dynamics of the Cnt ligand, which has already been demonstrated during the first studies.

1. Angew. Chem. Int. Ed. 2021, 60, 6042 (DOI: 10.1002/anie.202015428)
2. Chem. Eur. J. 2021, accepted article (DOI : 10.1002/chem.202101599 )

Rare earth metal ions are strategic elements for the decarbonisation of energy. The technologies that contain these as a resource vary from wind turbines to hybrid electrical motors, mostly because of their performing magnetic properties. Thus, as non-renewable feedstock, the magnet industry developed around them will need to rationalize their use. Important questions are how to miniaturize the magnets and how to recycle them. A good understanding of their chemistry will greatly help to tackle these problems: the study of their physical chemistry allows the rationalization of their magnetic properties, first step to envision their enhancement, while a good knowledge of their dynamics in solution with various environment (ligands and solvents), helps designing adapted method for their recycling. The overall methodology proposed herein will come to enforce these great challenges in both aspects: synthetic methodology in organometallic chemistry for the specific design of performing single-molecule magnets (SMMs) with high blocking temperature.
We aim at using an underused large monoanionic ligand: the cyclononatetraenyl (Cnt) ligand that is able to undergo hapticity switches depending upon various conditions and constrains. Due to its monoanionic nature, it is adapted with low-valent lanthanides but also with trivalent lanthanide in combination with the well-known dianionic cyclooctatetraenyl (Cot) ligand. We envision synthesizing a collection of organometallic lanthanides compounds with various lanthanides elements at different oxidation states and with various geometries, thanks to the large size of the cnt ligand, which allow hapticity switches (WP1). The synthetic methodology will use original approaches such as solvent-induced isomerization and solvent-induced hapticity switches. The use of these new methods and the study of the dynamics of the Cnt ligand must be rationalized by adapted characterizations and theoretical studies (WP2). An important challenge resides in the beneficial use of the experimental methodology to push the theoretical methodology and vice versa. The specific design of these lanthanide compounds with various geometries depending upon the metal center will allow the enhancement of their magnetic properties and in particular their magnetic relaxation properties. The challenge is to study and rationalize how (i) the ligand geometry and (ii) the metal physical properties influence the slow magnetic relaxation in the aim to increase the blocking temperature to reach the liquid nitrogen temperature (77 K) and above (WP4). To accomplish this deed, a very good knowledge of the electronic structure is essential. Spectroscopic (luminescence, EPR) and theoretical tools will be confronted. For these molecules with large number of electrons, large spin-orbit coupling and strong relativistic effects, the ab initio theory is very challenging and the development of an adapted methodology (WP3) is needed.