Catalytic Nitrous Oxide Reduction with H 2 Mediated by Pincer Ir Complexes.

Reduction of nitrous oxide (N 2 O) with H 2 to N 2 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 N 2 O under mild conditions (2 bar H 2 /N 2 O (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 N 2 O in tetrahydrofuran- d 8 (THF- d 8 ) initially produces deprotonated (at the NHC arm) species 5 NHC , which readily reacts with H 2 to regenerate the trihydride complex 4 . However, prolonged exposure of 4 to N 2 O for 6 h yields the dinitrogen Ir(I) complex 7 P , having a deprotonated (at the P-arm) pincer ligand. Complex 7 P is a poor catalytic precursor in the N 2 O hydrogenation, pointing out to the formation of 7 P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N 2 O is carried out in wet THF- d 8 , 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 N 2 O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH 2 pincer methylene linkers.