Unraveling the Mechanism of N 2 O Reduction with B 2 pin 2 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.