Catalytic NH3 Oxidation to N2 by Hydrogen Atom Abstraction using a Low-valent Molybdenum Complex

Abstract

We report the syntheses and characterization of fac-[(CO)3Mo(PtBu2NPh2)(L)] (X = NCCH3 or, NH3) and demonstrate chemocatalytic NH3 oxidation to N2 using these complexes. We utilized stable organic radicals to abstract an H atom from a Mo−NH3 complex resulting in formation of a Mo−NH2 species that exhibits two diverging reaction pathways depending on the identity of the radical used during the H atom abstraction. The use of the less sterically bulky 2,4,6-tri-tert-butylphenoxyl radical results in an unexpected C−N coupling at the 4-position, generating an organic product 2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one and subsequently leads to diminished catalytic turnover for N2 formation. Switching to a radical with increased steric bulk at the 4-position, i.e. 2,6-di-tert-butyl-4-tritylphenoxyl radical, precludes C−N coupling and biases the system towards biomolecular coupling of two Mo−NH2 fragments to form a bridging hydrazine dimer [Mo−N2H4−Mo] en route to catalytic N2 generation. Shutting down the prominent side reaction increased N2 generation, affording up to 88 equiv N2 per Mo center. Density Functional Theory calculations were performed to assess the thermodynamics of the reaction steps of the proposed catalytic cycle to support the experimental results.