Reduction of Nitrogen Oxides by Hydrogen with Rhodium(I)-Platinum(II) Olefin Complexes as Catalysts.

The nitrogen oxides NO , NO, and N O are among the most potent air pollutants of the 21 century. A bimetallic Rh -Pt complex containing an especially designed multidentate phosphine olefin ligand is capable of catalytically detoxifying these nitrogen oxides in the presence of hydrogen to form 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 (N O) to water and N was recorded, making these Rh -Pt 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 (NO , TON of 8). These unprecedented homogeneously catalyzed hydrogenation reactions of NO were investigated by a combination of multinuclear NMR techniques and DFT calculations, which provide insight into a possible reaction mechanism. The hydrogenation of NO proceeds stepwise, to first give NO and H O, followed by the generation of N O and H O, which is then further converted to N and H O. The nitrogen-nitrogen bond-forming step takes place in the conversion from NO to N O and involves reductive dimerization of NO at a rhodium center to give a hyponitrite (N O ) complex, which was detected as an intermediate.