Metalloporphyrin−Dioxygen Interactions and the Effects of Neutral Axial Ligands

We present here results of a series of density functional theory studies on models of metalloporphyrins and address the influence of nitrogen-containing neutral axial ligands on the binding process of dioxygen (O2) to metalloporphyrins. There are two closely lying spin states for metalloporphyrins, namely, triplet and quintet. However, the ground state for oxy-metalloporphyrins is the open-shell singlet. DFT calculations indicate that back-donation to O2 is more accessible because of the axial ligand, facilitating the binding of O2. Moreover, the axial ligands lengthen the O−O bond of oxy-metalloporphyrins and make the bound O2 negatively charged with a spin population between that of 3O2 and O2−. Therefore, the coordination of the axial ligand makes O2 more active than free O2, triggering the catalytic oxidation processes. Finally, the larger the electronic density of the bound nitrogen atom (NL) of the axial ligand is, the more active the binding O2 becomes.