Hydrogen Atom vs. Hydride Transfer in Cytochrome P450 Oxidations: A Combined Mass Spectrometry and Computational Study

Abstract

Biomimetic models of short‐lived enzymatic reaction intermediates can give useful insight into the properties and coordination chemistry of transition metal complexes. In this work, we investigate a high‐valent iron(IV)‐oxo porphyrin cation radical complex, namely [FeIV(O)(TPFPP+·)]+, for which TPFPP is the dianion of 5,10,15,20‐tetrakis(pentafluorophenyl) porphyrin. The [FeIV(O)(TPFPP+·)]+ ion was studied by ion‐molecule reactions in a Fourier transform ion cyclotron resonance mass spectrometer through reactivities with 1,3,5‐cycloheptatriene, 1,3‐cyclohexadiene and toluene. The different substrates give dramatic changes in reaction mechanism and efficiencies, whereby cycloheptatriene leads to hydride transfer, whereas cyclohexadiene and toluene react through hydrogen atom abstraction. Detailed computational studies point to major differences in ionization energy, as well as C–H bond energies of the substrates that influence the hydrogen atom abstraction vs. electron transfer pathways. The various variables that determine the pathways for hydride transfer vs. hydrogen atom transfer are elucidated and discussed.