Computational Evaluation of Potential Molecular Catalysts for Nitrous Oxide Decomposition.

Nitrous oxide (N 2 O) 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 N 2 O 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 N 2 O splitting, which could aid in the development of more active and robust catalysts and provide deeper mechanistic insights: one Cu(I)-based, [(CF 3 O) 4 Al]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-N 2 O complex undergoes N-O bond cleavage to produce N 2 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 N 2 O is facilitated by an O -bound (F-L)Cu-O-N 2 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-(η 2 ,η 2 -O 2 )-Cu(F-L), which leads to dioxygen species (F-L)Cu(η 2 -O 2 ), that favorably dissociates O 2 . Key features of the DO/OER process for (POR)ClRu ( B - 1 ) include endergonic N 2 O coordination, facile N 2 evolution from LR'u-N 2 O-RuL to Cl(POR)RuO, moderate barrier coupling of Cl(POR)RuO to peroxo Cl(POR)Ru(O 2 )Ru(POR)Cl, and eventual O 2 dissociation from Cl(POR)Ru(η 1 -O 2 ), which is nearly thermoneutral. N 2 O decomposition promoted by (NTA)Ru(III) ( C-1 ) can proceed with exergonic N 2 O coordination, facile N 2 dissociation from (NTA)Ru-ON 2 or (NTA)Ru-N 2 O-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 N 2 O decomposition.