Theoretical investigation of the gas-phase Mn+- and Co+-catalyzed oxidation of benzene by N2O

Abstract

The gas-phase Mn⁺- and Co⁺-mediated oxidation of benzene by N₂O has been theoretically investigated using density functional theory. The geometries and energies of all the stationary points involved are located. Two different oxidation mechanisms, i.e., mediated by M⁺(benzene) and MO⁺, are taken into account. In the former catalytic cycle, benzene initially coordinates to the metal ion affording the M⁺(C₆H₆) adduct (M = Mn or Co), then N₂O coordinates to the nascent benzene complex and gets activated by the metal to yield (C₆H₆)M⁺O(N₂). After releasing a molecular nitrogen, through the non-radical and/or O-insertion pathways, the system would be oxidized to phenol and regenerates the active catalyst M⁺. This catalytic mechanism is energetically favourable, explaining the efficient Mn⁺- and Co⁺-catalyzed benzene hydroxylation observed in ion cyclotron resonance (ICR) experiments [J. Am. Chem. Soc., 1994, 116, 9565–9570]. For the alternative MO⁺-mediated oxidation mechanism, spin inversion as well as high energy barrier in the course of the N–O activation imply low reaction efficiency of the ground-state reactants, according with the ICR experiment finding that MO⁺ was formed from exited M⁺*, thus both Mn⁺ and Co⁺ are unable to work as a catalyst in this case.