Reduction of Nitrogen Oxides by Hydrogen with Rh(I)-Pt(II) Olefin Complexes as Catalysts

The nitrogen oxides NO2, NO, and N2O are among the most potent air pollutants of the 21st century. A bimetallic Rh(I)-Pt(II) complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxicate these nitrogen oxides in the presence of hydrogen to water and dinitrogen as benign products. The catalytic reactions were performed at room temperature and low pressures (3 - 4 bar for combined nitrogen oxides and hydrogen gases). A turnover number (TON) of 587 for the reduction of nitrous oxide (N2O) to water and N2 was recorded, making these Rh(I)-Pt(II) complexes the best homogeneous catalysts for this reaction to date. Lower TONs were achieved in the conversion of nitric oxide (NO, TON = 38) or nitrogen dioxide (NO2; TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NOx were investigated by a combination of multi-nuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO2 proceeds stepwise, to first give NO and H2O, followed by the generation of N2O and H2O, which is then further converted to N2 and H2O. The nitrogen-nitrogen bond-forming step takes place in the conversion from NO to N2O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N2O22-) complex, which was detected as intermediate.