Biomimetic Oxidation Reactions of a Naked Manganese(V)-Oxo Porphyrin Complex

The intrinsic reactivity of a manganese(V)-oxo porphyrin complex, a typically fleeting intermediate in catalytic oxidation reactions in solution, has been elucidated in a study focused on its gas-phase ion-chemistry. The naked high-valent Mn-V-oxo porphyrin intermediate 1 ([(tpfpp)(MnO)-O-V](+); tpfpp=meso-tetrakis(pentafluorophenyl) porphinato dianion), has been obtained by controlled treatment of [(tpfpp)Mn-III]Cl (2-Cl) with iodosylbenzene in methanol, delivered in the gas phase by electrospray ionization and assayed by FT-ICR mass spectrometry. A direct kinetic study of the reaction with selected substrates, each containing a heteroatom X (X = S, N, P) including amines, sulfides, and phosphites, was thus performed. Ionic products arising from electron transfer (ET), hydride transfer (HT), oxygen-atom transfer (OAT), and formal addition (Add) may be observed, with a predominance of two-electron processes, whereas the product of hydrogen-atom transfer (HAT), [(tpfpp)(MnOH)-O-IV](+), is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electrontransfer ability of a Mn-V-oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)(MnO)-O-IV]. Linear free-energy analyses of the reactions of para-substituted N, N-dimethylanilines and thioanisoles indicate that a considerable amount of positive charge is developed on the heteroatom in the oxidation transition state. Substrates endowed with different heteroatoms, but similar ionization energy display a comparable reaction efficiency, consistent with a mechanism initiated by ET. For the first time, the kinetic acidity of putative hydroxo intermediates playing a role in catalytic oxidations, [(tpfpp)(FeOH)-O-IV](+) and [(tpfpp)(MnOH)-O-IV](+), has been investigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)(FeO)-O-IV], with respect to the manganyl, [(tpfpp)(MnO)-O-IV], unit. Finally, the neat association reaction of 2 has been studied with various ligands showing that harder ligands are more strongly bound.