Abstract
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)Mn^V^O]^+^; 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)Mn^IV^OH]^+^, is never detected. A thermochemical threshold for the formation of the product radical cation allows an evaluation of the electron‐transfer ability of a Mn^V^‐oxo complex, yielding a lower limit of 7.85 eV for the ionization energy of gaseous [(tpfpp)Mn^IV^O]. 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)Fe^IV^OH]^+^ and [(tpfpp)Mn^IV^OH]^+^, has been investigated with selected reference bases, revealing a comparatively higher basicity for the ferryl, [(tpfpp)Fe^IV^O], with respect to the manganyl, [(tpfpp)Mn^IV^O], unit. Finally, the neat association reaction of 2 has been studied with various ligands showing that harder ligands are more strongly bound.