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 0 atoms indicate that the O atom of the hydroxyl group in 2 is a stronger base site for proton attack.