Heteroleptic and Homoleptic Iron(III) Complexes with a Tris(N‐Heterocyclic Carbene) Borate Ligand: Synthesis, Characterization, and Catalytic Application

Heteroleptic and homoleptic iron(III) complexes supported by a tris(N‐heterocyclic carbene) borate ligand have been prepared and crystallographically characterized. The strong electron‐donating character of the tris(carbene) donor was revealed by UV‐vis absorption spectroscopy and electrochemical measurements combined with quantum chemical calculations. The catalytic activity of each complex was evaluated in cyclohexane oxidation reaction using meta‐chloroperoxybenzoic acid (=mCPBA) as an oxidant, and both complexes show high catalytic activity and selectivity with TON=∼350 and A/(K+L)=8–10. Mechanistic studies suggested that radical‐chain processes are involved in the reaction due to mCPBA acting as a one‐electron oxidant, concomitant with the pathway of metal‐based reactive species. Moreover, it was found that the homoleptic and heteroleptic complexes differed significantly in the involvement of metal‐based active species, with the homoleptic complex exhibiting more metal‐based reactions. Heteroleptic and homoleptic iron(III) complexes with tris(N‐heterocyclic carbene) borate ligands were synthesized and fully characterized. Both complexes showed high catalytic activity and alcohol selectivity in the cyclohexane oxidation reaction. Mechanistic investigation revealed that the catalysis of the homoleptic complex involves more metal‐based active species.