Establishing redox and thermodynamic relationships between metal-ion-bound O 2 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, [(F 8 )Fe III -(O 2 2− )] − (P) (F 8 = tetrakis(2,6-difluorophenyl)porphyrinate), and a superoxide complex, [(F 8 )Fe III -(O 2•− )] (S), are shown to be redox interconvertible. Using Cr(η-C 6 H 6 ) 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 lowspin ferric hydroperoxide species, [(F 8 )Fe III -(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 pK a = 28.8 for HP (THF, −80 °C). With the measured reduction potential and pK a , 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 (2,2,6,6tetramethylpiperdine-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, k H = 0.5, k D = 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 Fe III -OOH species being formed via HAT reactivity of the partner ferric heme superoxide complex.

A series of ferrous-heme 2,6-dimethylphenyl isocyanide (DIMPI) and ferrous-heme mononitrosyl complexes have been synthesized and characterized. The heme portion of the complexes studied is varied with respect to the nature of the axial ligand, including complexes, where it is covalently tethered to the porphyrinate periphery. Reduced heme complexes, [(F8)FeII], [(PPy) FeII], [(PIm)FeII], and [(PImH)FeII], where F8 = tetrakis(2,6-difluorophenyl)-porphyrinate and PPy, PIm, and PImH are partially fluorinated tetraaryl porphyrinates with covalently appended axial base pyridyl/imidazolyl or histamine moieties, were employed; PImH is a new construct. Room temperature addition of DIMPI to these iron(II) complexes affords the bis-isocyanide species [(F8)FeII-(DIMPI)2] in the case of [(F8)FeII], while for the other hemes, mono-DIMPI compounds are obtained, [(PPy)FeII-(DIMPI)] [(2)-DIMPI], [(PIm)FeII-(DIMPI)] [(3)-DIMPI], and [(PImH) FeII-(DIMPI)] [(4)-DIMPI]. The structures of complexes (3)-DIMPI and (4)-DIMPI have been determined by single crystal X-ray crystallography, where interesting H…F(porphryinate aryl group) interactions are observed. 19F-NMR spectra determined for these complexes suggest that H…F(porphyrinate aryl groups) attractions also occur in solution, the H atom coming either from the DIMPI methyl groups or from a porphyinate axial base imidazole or porphyrinate pyrrole. Similarly, we have used nitrogen monoxide to generate ferrous-nitrosyl complexes, a five-coordinate species for F8, [(F8)FeII-(NO)], or low-spin six-coordinate compounds [(PPy)FeII-(NO)], [(PIm)FeII-(NO)], and [(PImH)FeII-(NO)]. The DIMPI and mononitrosyl complexes have also been characterized using UV–Vis, IR, 1H-NMR, and EPR spectroscopies.

The focus of this study is in the description of synthetic heme/copper/O 2 chemistry employing a hemecontaining binucleating ligand which provides a tridentate chelate for copper ion binding. The addition of O 2 (−80 °C, tetrahydrofuran (THF) solvent) to the reduced heme compound (P ImH )Fe II (1), gives the oxy-heme adduct, formally a heme−superoxide complex Fe III −(O 2•− ) (2) (resonance Raman spectroscopy (rR):Simple warming of 2 to room temperature regenerates reduced complex 1; this reaction is reversible, as followed by UV−vis spectroscopy. Complex 2 is electron paramagnetic resonance (EPR)-silent and exhibits upfield-shifted pyrrole resonances (δ 9.12 ppm) in 2 H NMR spectroscopy, indicative of a six-coordinate low-spin heme. The coordination of the tethered imidazolyl arm to the heme−superoxide complex as an axial base ligand is suggested. We also report the new fully reduced heme−copper complex [(P ImH )Fe II Cu I ] + (3), where the copper ion is bound to the tethered tridentate portion of P ImH . This reacts with O 2 to give a distinctive low-temperature-stable, highspin (S = 2, overall) peroxo-bridged complex [(P ImH )Fe III −(O 2 2− )−Cu II ] + (3a): λ max , 420 (Soret), 545, 565 nm; δ pyrr , 93 ppm; ν O−O , 799 cm −1 (Δ 18 O 2 , −48 cm −1 ); ν Fe−O , 524 cm −1 (Δ 18 O 2 , −23 cm −1 ). To 3a, the addition of dicyclohexylimidazole (DCHIm), which serves as a heme axial base, leads to low-spin (S = 0 overall) species complex [(DCHIm)(P ImH )Fe III − (O 2 2− )−Cu II ] + (3b): λ max , 425 (Soret), 538 nm; δ pyrr , 10.2 ppm; ν O−O , 817 cm −1 (Δ 18 O 2 , −55 cm −1 ); ν Fe−O , 610 cm −1 (Δ 18 O 2 , −26 cm −1). These investigations into the characterization of the O 2 -adducts from (P ImH )Fe II (1) with/without additional copper chelation advance our understanding of the dioxygen reactivity of heme-only and heme/Cu-ligand heterobinuclear system, thus potentially relevant to O 2 reduction in heme−copper oxidases or fuel-cell chemistry.

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.

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.