Mn-O-O Electron Spin Flip Mechanism Triggered by the Visible-Light Irradiation for the Generation of an Active Mn(V)-Oxo Complex from O2: Insight from Density Functional Calculations

We reveal a new activation mechanism of O-2 by electron spin flip for the formation of a high-valent Mn(V)-oxo corrolazine complex under O-2 atmosphere and visible-light irradiation conditions. Ab initio calculations show that a loosely associated triplet complex from a quintet manganese corrolazine with a triplet O-2 is formed initially in reaction. The subsequent strong bonding interactions between dioxygen with the manganese corrolazine, triggered by the visible-light irradiation, give rise to an increase in 4 orbital splitting energy and drive the spin flip of electrons located at Mn and their rearrangement among the frontier orbitals, yielding the radical-like [Mn]-O-O species with high reactivity, which can easily abstract the hydrogen of the substrate to form the high-valent Mn(V)-oxo active oxidant through the following rebound reaction. Here the coupling interactions of d(xz) and d(yz) of Mn with the corresponding pi(3)(x)(z2) and pi(3)(yz2) orbitals of O-2 may form two new extending pi bonds, pi(4)(xz3): and pi(4)(yz3). Such an increase of antibonding pi electrons in the Mn-O-O moiety may further activate the Mn-bound dioxygen and facilitate the generation of the monomeric Mn(V)-oxo complex through selective oxidation of alkanes to alcohols by the photoinduced radical-like [Mn]-O-O species.