Computational Evaluation of Potential Molecular Catalysts for Nitrous Oxide Decomposition

Nitrous oxide (N2O) 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 N2O 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 N2O splitting, which could aid in the development of more active and robust catalysts and provide deeper mechanistic insights: one Cu(I)-based, [(CF3O)4Al]Cu (A-1), and two Ru(III)-based, Cl(POR)Ru (B-1) and (NTA)Ru (C-1) (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–N2O complex undergoes N–O bond cleavage to produce N2 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 (A-1), deoxygenation of N2O is facilitated by an O-bound (F–L)Cu–O–N2 or better by a bimetallic N,O-bonded, (F–L)Cu–NNO–Cu(F–L) complex; the resulting copper–oxyl (F–L)Cu–O is converted exergonically to (F–L)Cu–(η2,η2-O2)–Cu(F–L), which leads to dioxygen species (F–L)Cu(η2-O2), that favorably dissociates O2. Key features of the DO/OER process for (POR)ClRu (B-1) include endergonic N2O coordination, facile N2 evolution from LR′u–N2O–RuL to Cl(POR)RuO, moderate barrier coupling of Cl(POR)RuO to peroxo Cl(POR)Ru(O2)Ru(POR)Cl, and eventual O2 dissociation from Cl(POR)Ru(η1-O2), which is nearly thermoneutral. N2O decomposition promoted by (NTA)Ru(III) (C-1) can proceed with exergonic N2O coordination, facile N2 dissociation from (NTA)Ru–ON2 or (NTA)Ru–N2O–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 N2O decomposition.