Catalytic Nitrous Oxide Reduction with H₂ Mediated by Pincer Ir Complexes

Reduction of nitrous oxide (N₂O) with H₂ to N₂ and water is an attractive process for the decomposition of this greenhouse gas to environmentally benign species. Herein, a series of iridium complexes based on proton-responsive pincer ligands (1-4) are shown to catalyze the hydrogenation of N₂O under mild conditions (2 bar H₂/N₂O (1:1), 30 °C). Among the tested catalysts, the Ir complex 4, based on a lutidine-derived CNP pincer ligand having nonequivalent phosphine and N-heterocyclic carbene (NHC) side donors, gave rise to the highest catalytic activity (turnover frequency (TOF) = 11.9 h^-1 at 30 °C, and 16.4 h^-1 at 55 °C). Insights into the reaction mechanism with 4 have been obtained through NMR spectroscopy. Thus, reaction of 4 with N₂O in tetrahydrofuran-d₈ (THF-d₈) initially produces deprotonated (at the NHC arm) species 5, which readily reacts with H₂ to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N₂O for 6 h yields the dinitrogen Ir(I) complex 7, having a deprotonated (at the P-arm) pincer ligand. Complex 7 is a poor catalytic precursor in the N₂O hydrogenation, pointing out to the formation of 7 as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N₂O is carried out in wet THF-d₈, formation of a new species, which has been assigned to the hydroxo species 8, is observed. Finally, taking into account the experimental results, density functional theory (DFT) calculations were performed to get information on the catalytic cycle steps. Calculations are in agreement with 4 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of N₂O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH₂ pincer methylene linkers.