Unraveling the Mechanism of N₂O Reduction with B₂pin₂ Catalyzed by (NHC)Cu(Bpin)

The reduction of N₂O to N₂ with bis(pinacolato)diboron (B₂pin₂) catalyzed by N-heterocyclic carbene copper-boryl complexes represents a rare example of mild-temperature greenhouse gas mitigation, yet the mechanism underlying this transformation has remained elusive. Here, we reveal through density functional theory (DFT) calculations that the reaction of N₂O with the catalyst (NHC)Cu(Bpin), which is rate-determining, proceeds via a concerted (3 + 2)-like cyclic elimination mechanism. The asymmetric dipolar electronic structure of N₂O enables its simultaneous coordination to the Cu and B centers within the Cu-B moiety, with the terminal nitrogen binding to Cu and the terminal oxygen coordinating to the Lewis acidic boron center to form a five-membered ring arrangement. Electronically, electrons from the occupied Cu d/Cu-B σ hybrid orbital are donated into the empty N═N π* orbital, while the oxygen lone pair engages the vacant boron p-orbital. This five-membered ring arrangement corresponds to a transition state that facilitates N₂ extrusion and formation of a Cu-O-B species, thereby bypassing the insertion pathway previously established for isoelectronic CO₂ reduction. The overall barrier for this rate-determining process is 2.8 kcal/mol higher than that for CO₂ reduction, which explains why N₂O reduction requires room temperature, whereas CO₂ reduction proceeds under subambient conditions.