Cyclopentadienyl-, Indenyl- and Fluorenyl-Functionalized N-Heterocyclic Carbene Metal Complexes: Synthesis and Catalytic Applications

This microreview focuses on the preparation of metal complexes with N-heterocyclic carbene ligands linked to cyclopentadienyl (and related indenyl and fluorenyl) rings. Since the description of the first Ti and V complexes in 2006, the field has grown to afford a large number of new complexes, in which the metals range from early to late transition metals. The ligands that are now available not only include a wide set of cyclopentadienyl-, indenyl- and fluorenyl-NHCs with a variety of tethers, but may even present chirality. Several catalytic applications of the new complexes are described, including borrowing hydrogen processes (transfer hydrogenation, beta-alkylation of secondary alcohols, alkylation of amines with primary alcohols), hydrosilylation of aldehydes, epoxidation and hydroformylation of olefins, among others.