Catalytic Ammonia Oxidation to Dinitrogen by Hydrogen Atom Abstraction

Abstract

Catalysts for the oxidation of NH3 are critical for the utilization of NH3 as a large‐scale energy carrier. Molecular catalysts capable of oxidizing NH3 to N2 are rare. This report describes the use of [Cp*Ru(PtBu2NPh2)(15NH3)][BArF4], (PtBu2NPh2=1,5‐di(phenylaza)‐3,7‐di(tert‐butylphospha)cyclooctane; ArF=3,5‐(CF3)2C6H3), to catalytically oxidize NH3 to dinitrogen under ambient conditions. The cleavage of six N−H bonds and the formation of an N≡N bond was achieved by coupling H+ and e− transfers as net hydrogen atom abstraction (HAA) steps using the 2,4,6‐tri‐tert‐butylphenoxyl radical (tBu3ArO.) as the H atom acceptor. Employing an excess of tBu3ArO. under 1 atm of NH3 gas at 23 °C resulted in up to ten turnovers. Nitrogen isotopic (15N) labeling studies provide initial mechanistic information suggesting a monometallic pathway during the N⋅⋅⋅N bond‐forming step in the catalytic cycle.

Electrocatalytic Ammonia Oxidation Mediated by a Polypyridyl Iron Catalyst

“…The measured H 2 :N 2 ratio was 2.6:1, in good agreement with the 3:1 ratio expected for NH 3 . When isotopically enriched 15 NH 4 OTf and 15 NH 3 were employed, only 30 N 2 was observed by GC-MS, confirming ammonia to be the source of detected N 2 (see SI for details).…”

“…14 Catalytic ammonia oxidation via HAA has also been recently demonstrated using a ruthenium complex and 2,4,6-tri(tertbutyl)phenoxyl radical by Mock and co-workers (Chart 1D). 15 Similarly, Nishibayashi and co-workers recently reported catalytic chemical oxidation of ammonia, again using a ruthenium system (Chart 1E). 16 Given the paucity of data available describing electrocatalytic ammonia oxidation and the fact that biologically relevant firstrow metals such as iron have yet to be described for this process, we targeted the study of promising candidate iron (electro)catalysts as models well suited to mechanistic interrogation.…”

Computational Mechanistic Study of Electro-Oxidation of Ammonia to N2 by Homogenous Ruthenium and Iron Complexes

“…These results are in agreement with the work of Mock et al for NH 3 oxidation by [Cp*Ru(P tBu 2 N Ph 2 )(NH x )] + (x = 0−2) complexes. 41 The BDFEs for N−H bonds steps reduced significantly after nucleophilic attack of ammonia to form a hydrazine intermediate. The lowest BDFE is for producing [L n M II (N 2 )] from [L n M III (NNH)], the sixth HAT, which is only 4.4, 4.7, and 5.5 kcal/mol for Ru-b, Ru-a, and Fe-b complexes, respectively.…”

“…In their study, HAA was the pathway for cleavage of the N−H bond of ammonia. 41 To the best of our knowledge, there has not been reported a comprehensive study of the full catalytic cycle of ammonia to N 2 by ruthenium homogeneous complexes. Thus, in this work, motivated by the work of Habibzadeh et al, 31 a series of ruthenium polypyridyl complexes were modeled (Figure 2a) by density functional theory (DFT) to explore the mechanism of the catalytic oxidation of ammonia to dinitrogen.…”

Manganese-Catalyzed Ammonia Oxidation into Dinitrogen

“…5 In the same year, Mock and co-worker demonstrated catalytic ammonia oxidation reactions via hydrogen atom transfer using a (pentamethylcyclopentadienyl)ruthenium complex, where a phenoxyl radical derivative worked as a hydrogen atom acceptor. 6 At the same time, independently, our group realized a catalytic ammonia oxidation using an oxidant and a base in the presence of a ruthenium complex bearing a bipyridine dicarboxylate ligand. 7 In these reaction systems, the use of ruthenium catalysts was essential to promote the ammonia oxidation reaction effectively.…”