Probing Bare High-Valent Transition Oxo–Metal Complexes: An Electrospray Ionization Fourier Transform Ion Cyclotron Resonance Study of Reactive Intermediates

Abstract

Functional models of the Compound I intermediate of monooxygenase heme enzymes, namely [(TPFPP)(*+)Fe(IV)=O](+) and [(TPFPP)Mn(V)=O](+) (TPFPP = meso-tetrakis (pentafluorophenyl)porphyrinato dianion), are obtained as bare species by electrospray ionization from solutions of appropriate precursors and their reactivity is investigated in the gas-phase. By an alternative approach involving the reaction of a gaseous oxidant, the naked core of Compound I, [(PP-IX)(*+)Fe(IV)=O](+) (PP-IX = protoporphyrin IX dianion) has been produced as well. This achievement, unprecedented in studies run in solution, is now made possible working in the gas-phase. The long lifetime ensured by the dilute gas-phase allows to reveal both structural details and elementary steps of the catalytic activity of these high-valent oxo-metal intermediates. Depending on the features of the oxo-metal complex, ionic products are formed with neutral substrates involving: (i) addition, (ii) oxygen atom transfer, (iii) formal hydride transfer. In contrast, ionic products indicative of a net initial hydrogen atom transfer event are never observed. The reaction pathways of these ultimate catalytic intermediates void of any trans axial ligand, counterion, solvent or protein environment are thus elucidated.