In methanotrophic bacteria, methane monooxygenase enzymes (MMOs) are responsible for directly and selectively oxidizing methane to methanol; the active site of the enzyme contains a diiron(IV) bis-µ-oxo species. In the present work, inspired by the nature, different bimetallic-oxo complexes were modeled with density functional theory to calculate complete catalytic cycles of methane conversion to methanol (C–H activation and oxy-insertion reactions). We focused our attention on mid – late, Earth-abundant 3d transition metals using N,O- (oxypiridinate) supporting ligands to elucidate important trends. To explore the potential of various bimetallic catalyst candidates, two series of bimetallic complexes were investigated: (1) CoMb, where Mb = Fe, Co or Ni, and (2) MaCo, where Ma = Fe, Co or Ni. Per previous research on M-M complexes, the B3LYP-D3/6-311+G(d,p)/SMD-acetone level of theory was selected for the calculations. The plausible pathways for the reactions were surveyed by computing the barriers and thermodynamics of the reactions. Two possibilities for C–H bond scission by M-M-oxo complexes were calculated: (1) hydrogen atom abstraction (HAA) -radical rebound (RR), and (2) [2 + 2] addition in which the C–H bond is cleaved by the active metal-oxo, via a kite-shaped, four-centered TS. This reaction is followed by formation of a methane adduct and subsequent formation of methanol. Pyridine-N-oxide (PyO) was used as a model oxygen atom transfer reagent to generate the bimetallic-oxo complexes to close the catalytic loop. In general, the thermodynamics of the reactions were calculated to be very exergonic with reasonable barrier free energies in which the HAA pathway was found to be more plausible as compared to the [2+2] mechanism. Also, all complex models are more stable in their high spin multiplicities.

DFT modeling of the complete catalytic cycle of methane-to-methanol via Earth-abundant late 3d bimetallic complexes

2019 Spring National Meeting

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