Hydrogen atom versus hydride transfer in cytochrome P450 oxidations: A combined mass spectrometry and computational study

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+*)]+ where 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, while cyclohexadiene and toluene react via 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 versus electron transfer pathways. The various variables that determine the pathways for hydride transfer versus hydrogen atom transfer are elucidated and discussed.