Both CO and N2O are important, environmentally harmful industrial gases. The reaction of CO and N2O to produce CO2 and N2 has stimulated much research interest aimed at degradation of these two gases in a single step. Herein, we report an efficient CO oxidation by N2O catalyzed by a (PNN)Ru–H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N2O to the Ru–H bond to form a Ru–OH intermediate, followed by intramolecular OH attack on an adjacent CO ligand, forming CO2 and N2. Thus, the Ru–H bond of the catalyst plays a central role in facilitating the OAT from N2O to CO, providing an efficient and novel protocol for CO oxidation.

CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism

Herein, we report a Ni-catalyzed C(sp2)–H hydroxylation of aryl bromides with N2O as an oxygen-atom donor. The reaction is enabled by a 1,4-Ni translocation that results in ipso/ortho difunctionalized products. Regioselectivity and stereocontrol are dictated by a judicious choice of the ligand backbone, thus giving access to either carbonyl or phenol derivatives and offering an opportunity to repurpose hazardous substances en route to valuable oxygen-containing building blocks.

C(sp2)–H Hydroxylation via Catalytic 1,4-Ni Migration with N2O

Reduction of nitrous oxide (N2O) with H2 to N2 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 N2O under mild conditions (2 bar H2/N2O (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 N2O in tetrahydrofuran-d8 (THF-d8) initially produces deprotonated (at the NHC arm) species 5NHC, which readily reacts with H2 to regenerate the trihydride complex 4. However, prolonged exposure of 4 to N2O for 6 h yields the dinitrogen Ir(I) complex 7P, having a deprotonated (at the P-arm) pincer ligand. Complex 7P is a poor catalytic precursor in the N2O hydrogenation, pointing out to the formation of 7P as a catalyst deactivation pathway. Moreover, when the reaction of 4 with N2O is carried out in wet THF-d8, 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 N2O as the TOF-determining transition state (TDTS), with very similar reaction rates for the mechanisms involving either the NHC- or the P-CH2 pincer methylene linkers.

Catalytic Nitrous Oxide Reduction with H2 Mediated by Pincer Ir Complexes.

As the overall turnover-limiting step (TOLS) in the homogeneous conversion of N2O, the oxygen-atom transfer (OAT) from an N2O to an Ru-H complex to generate an N2 and Ru-OH complex has been comprehensively investigated by density functional theory (DFT) computations. Theoretical results show that the proton transfer from Ru-H to the terminal N of endo N2O is most favorable pathway, and the generation of N2 via OAT is accomplished by a three-step mechanism [N2O-insertion into the Ru-H bond (TS-1-2, 24.1 kcal mol−1), change of geometry of the formed (Z)-O-bound oxyldiazene intermediate (TS-2-3, 5.5 kcal mol−1), and generation of N2 from the proton transfer (TS-3-4, 26.6 kcal mol−1)]. The Gibbs free energy of activation (ΔG‡) of 29.0 kcal mol−1 for the overall turnover-limiting step (TOLS) is determined. With the participation of potentially existing traces of water in the THF solvent serving as a proton shuttle, the Gibbs free energy of activation in the generation of N2 (TS-3-4-OH2) decreases to 15.1 kcal mol−1 from 26.6 kcal mol−1 (TS-3-4). To explore the structure–activity relationship in the conversion of N2O to N2, the catalytic activities of a series of Ru-H complexes (C1–C10) are investigated. The excellent linear relationships (R2 > 0.91) between the computed hydricities (ΔGH−) and ΔG‡ of TS-3-4, between the computed hydricities (ΔGH−) and the ΔG‡ of TOLS, were obtained. The utilization of hydricity as a potential parameter to predict the activity is consistent with other reports, and the current results suggest a more electron-donating ligand could lead to a more active Ru-H catalyst.

/pdf/mechanistic-studies-of-oxygen-atom-transfer-oat-in-the-txvwb7t5.pdf

Mechanistic Studies of Oxygen-Atom Transfer (OAT) in the Homogeneous Conversion of N2O by Ru Pincer Complexes

The nitric oxide (NO) molecule is involved in a large number of biological routes. Thus, there is increasing interest in improving the understanding of the NO release mechanisms. One of the traditional NO release mechanisms involves (i) [Ru(NO)(NH3)5]3+ + e− → [Ru(NO)(NH3)5]2+ and (ii) [Ru(NO)(NH3)5]2+ + H2O → [Ru(H2O)(NH3)5]2+ + NO chemical reactions. Another possibility is associated with light irradiation: (iii) [Ru(NO)(NH3)5]3+ + H2O + hν → [Ru(H2O)(NH3)5]3+ + NO, aided by the Ru(dπ) → π*(NO) electronic transition, which decreases the π back-donation process in the Ru–NO chemical bond. The influence of the acid environment in which these chemical reactions typically occur experimentally has been explored in (iv) [Ru(NO)(NH3)5]2+ + H3O+ → [Ru(HNO)(NH3)5]3+ + H2O; and (v) [Ru(HNO)(NH3)5]3+ + H2O → [Ru(H2O)(NH3)5]3+ + HNO reactions. Reaction (v), supported by eight explicit water molecules, is the most propitious to occur. The HNO charge obtained from the atomic polar tensor scheme is close to zero. The methods of quantum theory of atoms in molecules and non-covalent interactions reveal that the HNO leaving group interacts with two water molecules through partially covalent or ionic chemical bonds. The HNO → NO conversion after the release from ruthenium molecules is thermodynamically feasible. The electronic spectrum of the structure [Ru(HNO)(NH3)5]3+ has, unlike the [Ru(NO)(NH3)5]3+ molecule, the Ru(dπ) → π*(NO) transition with an appropriate absorbance. Therefore, the proton increases the capability of ruthenium complexes to release nitric oxide after the chemical reduction reaction or the light-supported chemical reaction.

/pdf/how-does-the-acidic-milieu-interfere-in-the-capability-of-107fglc1uy.pdf

How does the acidic milieu interfere in the capability of ruthenium nitrosyl complexes to release nitric oxide

A DFT study on the mechanisms of hydrogenation and hydrosilylation of nitrous oxide catalyzed by a ruthenium PNP pincer complex

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A DFT study on the mechanisms of hydrogenation and hydrosilylation of nitrous oxide catalyzed by a ruthenium PNP pincer complex