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Biomimetic Oxidation Reactions of a Naked Manganese(V)–Oxo Porphyrin Complex
Author(s) -
Lanucara Francesco,
Crestoni Maria Elisa
Publication year - 2011
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201101432
Subject(s) - chemistry , heteroatom , electrospray ionization , photochemistry , manganese , electron transfer , porphyrin , reactivity (psychology) , redox , hydride , ionic bonding , hydrogen atom , catalysis , radical , inorganic chemistry , medicinal chemistry , ion , hydrogen , organic chemistry , medicine , ring (chemistry) , pathology , alkyl , alternative medicine
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.