Computational Evaluation of Potential Molecular Catalysts for Nitrous Oxide Decomposition.

Nitrous oxide (NO) is a potent greenhouse gas (GHG) with limited use as a mild anesthetic and underdeveloped reactivity. Nitrous oxide splitting (decomposition) is critical to its mitigation as a GHG. Although heterogeneous catalysts for NO decomposition have been developed, highly efficient, long-lived solid catalysts are still needed, and the details of the catalytic pathways are not well understood. Reported herein is a computational evaluation of three potential molecular (homogeneous) catalysts for NO splitting, which could aid in the development of more active and robust catalysts and provide deeper mechanistic insights: one Cu(I)-based, [(CFO)Al]Cu (), and two Ru(III)-based, Cl(POR)Ru () and (NTA)Ru () (POR = porphyrin, NTA = nitrilotriacetate). The structures and energetic viability of potential intermediates and key transition states are evaluated according to a two-stage reaction pathway: (A) deoxygenation (DO), during which a metal-NO complex undergoes N-O bond cleavage to produce N and a metal-oxo species and (B) (di)oxygen evolution (OER), in which the metal-oxo species dimerizes to a dimetal-peroxo complex, followed by conversion to a metal-dioxygen species from which dioxygen dissociates. For the (F-L)Cu(I) activator (), deoxygenation of NO is facilitated by an -bound (F-L)Cu-O-N or better by a bimetallic ,-bonded, (F-L)Cu-NNO-Cu(F-L) complex; the resulting copper-oxyl (F-L)Cu-O is converted exergonically to (F-L)Cu-(η,η-O)-Cu(F-L), which leads to dioxygen species (F-L)Cu(η-O), that favorably dissociates O. Key features of the DO/OER process for (POR)ClRu (-) include endergonic NO coordination, facile N evolution from LR'u-NO-RuL to Cl(POR)RuO, moderate barrier coupling of Cl(POR)RuO to peroxo Cl(POR)Ru(O)Ru(POR)Cl, and eventual O dissociation from Cl(POR)Ru(η-O), which is nearly thermoneutral. NO decomposition promoted by (NTA)Ru(III) () can proceed with exergonic NO coordination, facile N dissociation from (NTA)Ru-ON or (NTA)Ru-NO-Ru(NTA) to form (NTA)Ru-O; dimerization of the (NTA)Ru-oxo species is facile to produce (NTA)Ru-O-O-Ru(NTA), and subsequent OE from the peroxo species is moderately endergonic. Considering the overall energetics, (F-L)Cu and Cl(POR)Ru derivatives are deemed the best candidates for promoting facile NO decomposition.