Iron Half‐Sandwich Complexes for Catalysis

Abstract

As the most abundant transition metal, iron is well‐known for its rich complex chemistry and variety of oxidation states. In recent years, iron salts such as Fe II/III halides and acetylacetonate complexes have been employed as (pre‐)catalysts in a variety of chemical transformations, including those mediated by Lewis acid activation of functional groups, redox‐catalysis, and cross‐coupling reactions. However, the absence of stable ligand spheres or ligand lability limits the application of these “naked” salts and complexes, with the properties of the catalytically active species being predominantly determined by donor functionalities of solvent, substrate, and additives in the reaction. Improvements in catalyst design arise from introduction of multidentate pyridine‐ and other N‐based ligand designs, many of which, however, still require high catalyst loadings. The iron half‐sandwich motif, offers a robustly bonded cyclic ligand, most commonly η 5 ‐cyclopentadienyl (Cp − ), which occupies half of the available coordination sites. Additional ligands complete the complex design and allow tuning of the electronic, steric, and geometric properties of the active species. This article provides an overview of catalytic processes in which iron half‐sandwich complexes play a key role. Rather than the synthesis of the iron complexes or the substrate scope, focus is put on the properties of the active species and mechanistic aspects, to provide an overview of the rich chemistry that the half‐sandwich motif offers.

Organometallic Ionic Plastic Crystals Incorporating Cationic Half-Sandwich Complexes

“…In addition to sandwich complexes, half-sandwich complexes represent another class of substantial organometallic complexes. Their ligand variation has led to structurally and chemically diverse complexes with high versatility as reactants and catalysts. − Integrating these complexes into IPCs may introduce novel chemical functionalities to IPC systems. However, this approach presents challenges owing to their low-symmetry structures.…”