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.

A comprehensive test set of epoxidation rate constants for iron(iv)–oxo porphyrin cation radical complexes

“…Note here that ligand addition is typical for the reactivity of tetracoordinate [Fe III (TPFPP)] + ions. 12 a This finding provides circumstantial evidence for an ion population of the same elemental composition as [Fe IV (O)(TPFPP + ˙)] + , that may correspond to an isomeric structure oxidized on the porphyrin ligand, designated as [Fe III (TPFPP-O)] + .…”

“…In recent work, Crestoni, Fornarini and co-workers have succeeded in trapping and characterizing the Cpd I analogues of iron and manganese porphyrin complexes and studied their reactivity with a selection of substrates. 12 Thus, the [Mn V (O)(TPFPP)] + complex (TPFPP = meso -tetrakis (pentafluorophenyl)porphinato dianion) was found to react with model substrates through oxygen atom transfer (OAT), electron transfer (ET), hydride transfer and ligand addition. However, no direct hydrogen atom transfer (HAT) with any tested substrate took place.…”

Biomimetic Reactivity of Oxygen-Derived Manganese and Iron Porphyrinoid Complexes

“…138 The Mn V (O)(TPFPP) complex was also found to oxidize p -substituted thioanisoles and perform N-dealkylation of N,N -dialkylamines. 147 The sulfoxidation reaction rates were found to depend on the electronics of the substrate. A negative linear correlation between the Hammett σ-constant of the p -substituted thioanisole and the log of the rate constants could be obtained.…”

Probing the exposure of the phosphate group in modified amino acids and peptides by ion-molecule reactions with triethoxyborane in Fourier transform ion cyclotron resonance mass spectrometry

“…The gas phase represents an invaluable medium to explore the conformational properties of biomolecules, as the intrinsic chemical and physical properties of these species can be assessed without the interference of solvent molecules, aggregation phenomena and counter ions. Thus, gas‐phase ion‐molecule reactions have provided a wealth of knowledge as to the structure and reactivity of naked species of biological interest. Proton transfer and hydrogen/deuterium exchange reactions have been used to assess structural properties of proteins and peptides in the gas phase.…”