Cyclic iron tetra N-heterocyclic carbenes: synthesis, properties, reactivity, and catalysis

• Published in: Chemical Society reviews 2023-03, Vol.52 (6), p.2238-2277
• Main Authors: Schlachta, Tim P, Kühn, Fritz E
• Format: Article
• Language: English
• Subjects: Carbenes; Catalysis; Chemical synthesis; Epoxidation; Iron; Ligands; Organometallic compounds
• ISSN: 0306-0012; 1460-4744
• DOI: 10.1039/d2cs01064j

Cyclic iron tetracarbenes are an emerging class of macrocyclic iron N-heterocyclic carbene (NHC) complexes. They can be considered as an organometallic compound class inspired by their heme analogs, however, their electronic properties differ, e.g. due to the very strong σ-donation of the four combined NHCs in equatorial coordination. The ligand framework of iron tetracarbenes can be readily modified, allowing fine-tuning of the structural and electronic properties of the complexes. The properties of iron tetracarbene complexes are discussed quantitatively and correlations are established. The electronic nature of the tetracarbene ligand allows the isolation of uncommon iron( iii ) and iron( iv ) species and reveals a unique reactivity. Iron tetracarbenes are successfully applied in C-H activation, CO 2 reduction, aziridination and epoxidation catalysis and mechanisms as well as decomposition pathways are described. This review will help researchers evaluate the structural and electronic properties of their complexes and target their catalyst properties through ligand design. Cyclic iron tetracarbenes structurally resemble iron porphyrins, but the strong equatorial σ-donation results in a different electronic structure and reactivity.