Electronic structure and magnetic properties of rare-earth organometallic sandwich compounds

Molecular compounds of the rare-earth elements in which the metal is bound between two π-bonded aromatic ligands are termed sandwich complexes, with the description being extended to closely related systems containing one such ligand (half-sandwich complexes) and, frequently, more than two such ligands. Sandwich compounds have been a cornerstone of rare-earth organometallic chemistry for several decades, from the early pioneering studies describing their synthesis and basic physico-chemical properties to their contemporary applications in synthesis, catalysis and small-molecule activation. In this chapter, we focus on the electronic structure, oxidations states and magnetic properties of rare-earth sandwich compounds, setting the key advances in the context of earlier milestones in the field. While we place emphasis on rare-earth organometallic physics, the indispensable link with innovative synthetic chemistry is also highlighted. The pivotal role played by cyclopentadienyl (Cp) ligands in rare-earth organometallic chemistry is showcased through sandwich complexes that contain the full series of lanthanides in their divalent form. The coverage of unusual oxidations states incorporates the only rare-earth compounds to contain formally zerovalent metals. We consider compounds in which dysprosium is sandwiched between two cyclopentadienyl ligands, which represent the current state-of-the-art in single-molecule magnetism, a phenomenon in which magnetic hysteresis is observed in a molecular material. Extending the single-molecule magnet design principles to erbium shows that another well-established ligand in f-element chemistry—cyclooctatetraene (COT)—has adopted a new role in the design of magnetic materials. Finally, a new type of rare-earth sandwich compound based on cyclononatetraenyl (Cn) ligands is presented, along with its photophysical properties. The overarching vision of the chapter is to demonstrate that rare-earth sandwich chemistry has a remarkable ability to re-invent itself and, despite the maturity of the field, also has considerable potential for development in the future.