Lanthanide Organometallics as Single-Molecule Magnets

Abstract

Innovative synthetic chemistry has underpinned many important advances in molecular magnetism, particularly so with the development of single-molecule magnets (SMMs). Recently, the organometallic approach to SMMs has provided a series of eye-catching materials based on certain lanthanides that have re-energised a mature field of magnetism research. This chapter summarises the main highlights and shows that three lanthanides – terbium, dysprosium and erbium – and two ligands, cyclopentadienyl and cyclo-octatetraenyl, have played pivotal roles. The chapter considers the lanthanides in terms of conceptually simple models of 4f electronic structure and spin–orbit coupling and their relationship with the popular oblate and prolate depictions of electron density. For organisational purposes, the chapter is loosely divided by ligand hapticity, beginning with a review of η⁵-cyclopentadienyl compounds of dysprosium, from the discovery of the first organometallic SMM in 2010 to a series of cationic dysprosium metallocenes and radical-bridged SMMs that currently define the state of the art. Ingenious combinations of the η⁸-cyclo-octatetraenyl ligand with erbium, and the SMM properties of the ensuing compounds, are described. Less widely used organometallic ligands such as η⁶-arene and η⁷-cycloheptatrienyl are also considered, as are heteroaromatic ligands in which a carbon atom is replaced by an isolobal fragment based on, e.g., boron or phosphorus.

Organometallic chemistry has provided a valuable approach to the design of lanthanide SMMs that complements the impressive achievements made with Werner-type coordination chemistry. Important challenges remain to be surmounted, and the main message is that if SMMs are to achieve their potential in the arena of device technology then there is a clear need for more research into this fascinating family of magnetic materials.