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

Reduction of nitrous oxide (NO) with H to N 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 (-) are shown to catalyze the hydrogenation of NO under mild conditions (2 bar H/NO (1:1), 30 °C). Among the tested catalysts, the Ir complex , 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 at 30 °C, and 16.4 h at 55 °C). Insights into the reaction mechanism with have been obtained through NMR spectroscopy. Thus, reaction of with NO in tetrahydrofuran- (THF-) initially produces deprotonated (at the NHC arm) species , which readily reacts with H to regenerate the trihydride complex . However, prolonged exposure of to NO for 6 h yields the dinitrogen Ir(I) complex , having a deprotonated (at the P-arm) pincer ligand. Complex is a poor catalytic precursor in the NO hydrogenation, pointing out to the formation of as a catalyst deactivation pathway. Moreover, when the reaction of with NO is carried out in wet THF-, formation of a new species, which has been assigned to the hydroxo species , 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 as the TOF-determining intermediate (TDI) and the transfer of an apical hydrido ligand to the terminal nitrogen atom of NO as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH pincer methylene linkers.