Advances in lanthanide cyclononatetraenyl chemistry

Cyclononatetraenyl (Cnt) is a nine-membered monoanionic aromatic ligand. Despite its early discovery in 1963, it has been rarely utilised in coordination chemistry, which is mainly due to its large diameter and easy skeletal rearrangement. Only in 2017, the first lanthanide Cnt complex was synthesised, marking the beginning of a new era in organolanthanide chemistry. Since then, the chemistry displayed by Cnt has expanded rapidly in lanthanide chemistry. Due to the ligand flexibility, both classical planar η⁹-Cnt ligands and heavily bent ones with lower hapticity were found in Ln coordination complexes. These novel structure motifs exhibit reactivity which differs significantly from traditional cyclopentadienyl (Cp) and cyclooctatetraenediide (Cot) complexes. Some of the obtained compounds also show interesting photoluminescence and magnetic properties. This review presents an overview of the synthesis, structure and reactivity of the growing family of lanthanide Cnt compounds.