Mechanism of the Facile Nitrous Oxide fixation by Homogeneous Ruthenium Hydride Pincer Catalysts

Abstract

Solving ozone depletion and climate change problems require the development of effective methods for sustainably curbing them. With this aim, Milstein and coworkers developed a PNP pincer ruthenium catalyst for the homogeneous hydrogenation of nitrous oxide (N2O), an ozone-depleting substance and the third most important greenhouse gas, to generate dinitrogen and water as resultant products. The mechanism of this promising transformation was unveiled by means of experiments together with Density Functional Theory (DFT) calculations, which inspired Milstein and coworkers to use similar (PNN)Ru-H pincer catalysts for the reduction of N2O by CO to produce N2 and CO2. The use of the latter type of catalysts resulted in the proposition of a new reaction protocol and allowed to work under milder conditions. Here we describe the detailed mechanism of the last transformation catalyzed by a (PNN)Ru−H catalyst by means of DFT calculations, and not only this, but we also discover the way to block undesired parasitic reactions. Apart from that, we have explored a new evolution of this family of catalysts to go beyond previous experimental outcomes. The mechanism consists in a cascade of easy steps, starting from an insertion of the N2O oxygen into the Ru-H bond generating a hydroxo intermediate and releasing N2, and ending with a β-hydride elimination to form CO2 and regenerate the catalyst. The whole process occurs in a facile way with the exception of two steps: the formation of the hydroxyl ligand and the final β-hydride elimination to form CO2. However, the energy barriers of these two steps are not the bottleneck of the catalysis, but rather the easiness of the pyridyl group bonded to Ru to isomerize by C-H activation. We propose to solve this drawback by tuning the PNN ligand to block the pyridyl free rotation