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

A new end‐on low‐spin ferric heme peroxide, [(P Im )Fe III −(O 2 2− )] − ( P Im ‐ P ), and subsequently formed hydroperoxide species, [(P Im )Fe III −(OOH)] ( P Im ‐ HP ) are generated utilizing the iron‐porphyrinate P Im with its tethered axial base imidazolyl group. Measured thermodynamic parameters, the ferric heme superoxide [(P Im )Fe III −(O 2 ⋅− )] ( P Im ‐ S ) reduction potential ( E °′) and the P Im ‐ HP p K a value, lead to the finding of the OO−H bond‐dissociation free energy (BDFE) of P Im ‐ 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 P Im ‐ S via hydrogen‐atom transfer (HAT) compared to that of the F 8 superoxide complex, [(F 8 )Fe III −(O 2 .− )] ( S ) (F 8 =tetrakis(2,6‐difluorophenyl)porphyrinate, without an internally appended axial base imidazolyl), as determined from reactivity comparison of superoxide complexes P Im ‐ S and S with the hydroxylamine (O‐H) substrates TEMPO‐H and ABNO‐H.