Ferric Heme Superoxide Reductive Transformations to Ferric Heme (Hydro)Peroxide Species: Spectroscopic Characterization and Thermodynamic Implications for H‐Atom Transfer (HAT)

Abstract

A new end‐on low‐spin ferric heme peroxide, [(PIm)FeIII−(O22−)]− (PIm‐P), and subsequently formed hydroperoxide species, [(PIm)FeIII−(OOH)] (PIm‐HP) are generated utilizing the iron‐porphyrinate PIm with its tethered axial base imidazolyl group. Measured thermodynamic parameters, the ferric heme superoxide [(PIm)FeIII−(O2⋅−)] (PIm‐S) reduction potential (E°′) and the PIm‐HP pKa value, lead to the finding of the OO−H bond‐dissociation free energy (BDFE) of PIm‐HP as 69.5 kcal mol−1 using a thermodynamic square scheme and Bordwell relationship. The results are validated by the observed oxidizing ability of PIm‐S via hydrogen‐atom transfer (HAT) compared to that of the F8 superoxide complex, [(F8)FeIII−(O2.−)] (S) (F8=tetrakis(2,6‐difluorophenyl)porphyrinate, without an internally appended axial base imidazolyl), as determined from reactivity comparison of superoxide complexes PIm‐S and S with the hydroxylamine (O‐H) substrates TEMPO‐H and ABNO‐H.

Hydrogen Atom Transfer Thermodynamics of Homologous Co(III)- and Mn(III)-Superoxo Complexes: The Effect of the Metal Spin State

“…Upon bubbling O 2 into a THF solution of 1-Co at −90 °C, the appearance of two characteristic absorption bands at 485 and Scheme 1. OO−H BDFEs of Biomimetic Metal-Hydroperoxo Complexes: (A) LCu(OOH), 7 (B) (P Im )Fe III (OOH), 8 (C) (P Ar )Fe III (OOH), 9,10 570 nm in the UV−vis spectrum signaled the generation of 2-Co, as the same features were found for the formation of 2′-Co (Figure S1). 15 Furthermore, 2-Co registers an almost identical EPR spectrum (Figure 2a) to 2′-Co, reflecting that 2-Co is composed of a low-spin (S Co = 0) Co III center interacting with a superoxo ligand as unequivocally identified for 2′-Co. 15 2-Co also performs a HAT reaction toward TEMPOH in THF at −90 °C to form a low-spin (S Co = 0) Co III -hydroperoxo complex, Co(BDP Br P)(OOH) (3-Co), and a TEMPO radical in 93% yield (Figures S4 and S5).…”

“…To the best of our knowledge, only a few large-scale industrial processes can realize this type of reactions, whereas such transformations have been frequently identified in the catalytic cycle of a diverse array of metalloenzymes. − In enzymatic processes, in situ generated metal-superoxo, -peroxo, and -oxo intermediates derived from O 2 activation by low-valent metal cofactors often employ hydrogen atom transfer (HAT) to oxidize their substrates. For instance, treatment of isopenicillin N synthase (IPNS) and myo-inositol oxygenase (MIOX) with O 2 was found to initially afford an Fe III -superoxo intermediate that is capable of performing HAT reactions to produce an Fe III -hydroperoxo species. , A range of metal-superoxo model compounds, such as LCu­(O 2 • ) (L, a bis­(arylcarboxamido)­pyridine ligand), (P Im )­Fe­(O 2 • ) (P Im , a porphyrinate ligand with an appended axial imidazolyl group), (P Ar )­Fe­(O 2 • ) (P Ar , a porphyrinate ligand in four varied derivatives), , [Cu 2 (XYLO)­(O 2 • )] 2+ (XYLO, a bis­(2-{2-pyridyl}­ethyl)­amine chelating ligand with a bridging phenolate moiety), L′Cu 2 (μ-O 2 • ) (L′, a tacn/pyrazolate hybrid ligand), and Co­(O 2 • )­(Me 3 TACN)­(S 2 SiMe 2 ), have been demonstrated to carry out HAT reactions and furnish metal-hydroperoxo complexes. Because the Gibbs free energy change of a HAT reaction can be estimated to be the difference of the X–H (X = C, N, O) bond dissociation free energy (BDFE) of the substrate relative to the OO–H BDFE of the hydroperoxo product, thermodynamically the occurrence of such a transformation requires that the latter value, as observed for all systems shown in Scheme , be greater than the former.…”

Proton-coupled electron transfer reactivities of electronically divergent heme superoxide intermediates: a kinetic, thermodynamic, and theoretical study

“…Finally, our thermochemical findings also parallel those recently reported for [(F8TPP)FeIII(O2−˙)] (p K a = 28.8; E ° = −1.17 V) and [(PIm)FeIII(O2−˙)] (p K a = 28.6; E ° = −1.33 V) by Karlin and coworkers ( Table 1 ), where similarly reversible protonation and reduction processes were observed. 26 …”

“…To the best of our knowledge, recent work by Karlin and coworkers is the only instance where proton-coupled electron transfer reactivities of heme superoxide intermediates have been shown, where H˙ abstraction from an exogenous, weak O–H bond substrate generates the corresponding heme hydroperoxo species in a single kinetic step. 26 …”

Synthetic heme dioxygen adducts: electronic structure and reactivity

“…Here the H-bonded preoriented geometry was the key for the unique reactivity showed by the ferric superoxide complex. to be 28.6 in THF for Fe-TPP, Fe-TMPP, Fe-F 16 TPP complexes, suggesting that FeO 2 complexes could potentially attack relatively weaker C-H bonds [74,83]. In fact, they were able to characterize ferric hydroperoxide intermediate using UV-vis absorption features during the reaction of ferric superoxide with TEMPO-H in THF at -80°C (Figure 10).…”