Heme-FeIII Superoxide, Peroxide and Hydroperoxide Thermodynamic Relationships: FeIII-O2•- Complex H-Atom Abstraction Reactivity.

Abstract

Establishing redox and thermodynamic relationships between metal-ion-bound O2 and its reduced (and protonated) derivatives is critically important for a full understanding of (bio)chemical processes involving dioxygen processing. Here, a ferric heme peroxide complex, [(F8)FeIII-(O22-)]- (P) (F8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F8)FeIII-(O2•-)] (S), are shown to be redox interconvertible. Using Cr(η-C6H6)2, an equilibrium state where S and P are present is established in tetrahydrofuran (THF) at -80 °C, allowing determination of the reduction potential of S as -1.17 V vs Fc+/0. P could be protonated with 2,6-lutidinium triflate, yielding the low-spin ferric hydroperoxide species, [(F8)FeIII-(OOH)] (HP). Partial conversion of HP back to P using a derivatized phosphazene base gave a P/HP equilibrium mixture, leading to the determination of pKa = 28.8 for HP (THF, -80 °C). With the measured reduction potential and pKa, the O-H bond dissociation free energy (BDFE) of hydroperoxide species HP was calculated to be 73.5 kcal/mol, employing the thermodynamic square scheme and Bordwell relationship. This calculated O-H BDFE of HP, in fact, lines up with an experimental demonstration of the oxidizing ability of S via hydrogen atom transfer (HAT) from TEMPO-H (2,2,6,6-tetramethylpiperdine-N-hydroxide, BDFE = 66.5 kcal/mol in THF), forming the hydroperoxide species HP and TEMPO radical. Kinetic studies carried out with TEMPO-H(D) reveal second-order behavior, kH = 0.5, kD = 0.08 M-1 s-1 (THF, -80 °C); thus, the hydrogen/deuterium kinetic isotope effect (KIE) = 6, consistent with H-atom abstraction by S being the rate-determining step. This appears to be the first case where experimentally derived thermodynamics lead to a ferric heme hydroperoxide OO-H BDFE determination, that FeIII-OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.