Water Serving as Cocatalyst for the Highly Efficient Homogeneously Catalyzed Conversion of N2O/H2 Mixtures with Optimized Rhodium NHC Complexes

Abstract

Nitrous oxide (N₂O) is a valuable oxygen/nitrogen transfer reagent but reactions with N₂O are challenging due to its inertness. Consequently, it accumulates in the atmosphere, and because it is both an ozone-depleting reagent and potent green-house gas, effective mitigation methods become important. This study presents rhodium(I) amido bis(olefin) N-heterocyclic carbene complexes [Rh(trop₂N)(NHC)] as robust homogeneous catalysts for the direct hydrogenation of N₂O. Kinetic experiments and DFT calculations show that increasing π-acidity of the NHC and the presence of water significantly enhance the catalytic efficiency. The rhodium-amido bond facilitates cooperative H₂-cleavage and oxygen atom transfer from N₂O. The catalyst is regenerated with the loss of water from the hydroxide intermediate [Rh(OH)(trop₂NH)(NHC)], which forms a dimeric complex with a central bridging hydrated hydroxide ligand [H₃O₂]⁻. Water facilitates dinitrogen (N₂) loss from an intermediate containing an oxy-diazene ligand [H─N═N─O]⁻. The optimized catalyst achieves a TON > 230 000 and TOF > 1300 h⁻¹.