title: Biomimetic oxidation reactions of a naked manganese(V)-oxo porphyrin complex.
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.
authors: Dipartimento di Chimica e Tecnologie del Farmaco, Università di Roma La Sapienza, P.le A. Moro 5, 00185, Roma, Italy.
authors: Francesco
authors: F
authors: Lanucara
authors: Maria Elisa
authors: ME
authors: Crestoni
mesh_terms: D032701:Biomimetics; D002384:Catalysis; D004583:Electrons; D005740:Gases / Q000737:chemistry*; D007700:Kinetics; D008024:Ligands; D008345:Manganese / Q000737:chemistry*; D008665:Metalloporphyrins / Q000737:chemistry*; D015394:Molecular Structure; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry*
chemical_list: D005740:Gases; D008024:Ligands; D008665:Metalloporphyrins; D011166:Porphyrins; D008345:Manganese