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 (O 2 ) 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 O 2 is more accessible because of the axial ligand, facilitating the binding of O 2 . Moreover, the axial ligands lengthen the O−O bond of oxy-metalloporphyrins and make the bound O 2 negatively charged with a spin population between that of 3 O 2 and O 2 − . Therefore, the coordination of the axial ligand makes O 2 more active than free O 2, triggering the catalytic oxidation processes. Finally, the larger the electronic density of the bound nitrogen atom (N L ) of the axial ligand is, the more active the binding O 2 becomes.