Theoretical study of the stepwise protonation of the dioxo manganese(V) porphyrin.

The molecular structural and electronic properties of the complexes dioxo (1), oxo-hydroxo (2), oxo-aqua (3), and bishydroxo-manganese(V) porphyrin (4) in their ground states are studied with the first principle density functional calculations and the natural bond orbital (NBO) analyses. One Mn(V)-O bond is strengthened in the order 1 < 2 < 3. In 3, the Mn(V) atom moves out of the porphyrin plane, hindering the coordination of water molecule. Furthermore, the NBO analyses for 3 show that there is no dative bonds or strong interactions between Mn(V) atom and water molecule. The thermodynamic analyses for protonation processes, the natural atomic population analyses, and the characteristics of the bonding between Mn(V) and O atoms indicate that the O atom of the hydroxyl group in 2 is a stronger base site for proton attack.