Ligand engineering of tetra N-heterocyclic carbenes for boosting catalytic aziridination

A comprehensive computational study on the underlying reactivity of iron tetra-NHC complexes for C₂ + N₁ aziridination catalysis is presented. A library of 18 unique iron tetra-NHC complexes was constructed, and a computational screening was performed on the reaction barriers associated with the rate-determining step (formation of an open chain radical intermediate). Thermodynamic barriers were computed along with a variety of steric and electronic properties, including the percentage of buried volume, orbital energies and ETS-NOCV analysis, which were used to identify key characteristics related to reactivity. The analysis performed in this study successfully identified key differences in tetracarbenes, such as linking groups (BMe₂ or CH₂) and the identity of the NHC groups (imidazole, imidazoline or benzimidazole) in terms of sterics, electronics and thermodynamics. Additionally, we have proposed two reaction pathways based on electronic structure arguments for the formation of the key open-chain radical intermediate. The first reaction pathway proceeds through a σ-hole channel where the Fe(IV)-imide intermediate evolves into Fe(III)-imidyl radical through electron donation into the antibonding σ* orbital, while the second involves a Fe(III)-imidyl radical formed through a π-hole channel (donation into π*). These pathways are consistent with the isoelectronic iron(IV)-oxo species for hydrogen atom abstraction mechanisms and they can be used as descriptors of the rate-determining step of the aziridination reaction.