Spectroscopic Characterization of a Hydroperoxo–Heme Intermediate: Conversion of a Side-On Peroxo to an End-On Hydroperoxo Complex

Axial Ligand Coordination Tuning of the Electrocatalytic Activity of Iron Porphyrin Electrografted onto Carbon Nanotubes for the Oxygen Reduction Reaction

The oxygen reduction reaction (ORR) is essential in many life processes and energy conversion systems. It is desirable to design transition metal molecular catalysts inspired by enzymatic oxygen activation/reduction processes as an alternative to noble-metal-Pt-based ORR electrocatalysts, especially in view point of fuel cell commercialization. We have fabricated bio-inspired molecular catalysts electrografted onto multiwalled carbon nanotubes (MWCNTs) in which 5,10,15,20-tetra(pentafluorophenyl) iron porphyrin (iron porphyrin FeF₂₀ TPP) is coordinated with covalently electrografted axial ligands varying from thiophene to imidazole on the MWCNTs' surface. The catalysts' electrocatalytic activity varied with the axial coordination environment (i. e., S-thiophene, N-imidazole, and O-carboxylate); the imidazole-coordinated catalyst MWCNTs-Im-FeF₂₀ TPP exhibited the highest ORR activity among the prepared catalysts. When MWCNT-Im-FeF₂₀ TPP was loaded onto the cathode of a zinc-air battery, an open-cell voltage (OCV) of 1.35 V and a maximum power density (P_max ) of 110 mW cm^-2 were achieved; this was higher than those of MWCNTs-Thi-FeF₂₀ TPP (OCV=1.30 V, P_max =100 mW cm^-2 ) and MWCNTs-Ox-FeF₂₀ TPP (OCV=1.28 V, P_max =86 mW cm^-2 ) and comparable with a commercial Pt/C catalyst (OCV=1.45 V, P_max =120 mW cm^-2 ) under similar experimental conditions. This study provides a time-saving method to prepare covalently immobilized molecular electrocatalysts on carbon-based materials with structure-performance correlation that is also applicable to the design of other electrografted catalysts for energy conversion.

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, [(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.

Integrated and Binder-Free Air Cathodes of Co3 Fe7 Nanoalloy and Co5.47 N Encapsulated in Nitrogen-Doped Carbon Foam with Superior Oxygen Reduction Activity in Flexible Aluminum-Air Batteries

All-solid-sate Al-air batteries with features of high theoretical energy density, low cost, and environmental-friendliness are promising as power sources for next-generation flexible and wearable electronics. However, the sluggish oxygen reduction reaction (ORR) and poor interfacial contact in air cathodes cause unsatisfied performance. Herein, a free-standing Co3 Fe7 nanoalloy and Co5.47 N encapsulated in 3D nitrogen-doped carbon foam (Co3 Fe7 @Co5.47 N/NCF) is prepared as an additive-free and integrated air cathode for flexible Al-air batteries in both alkaline and neutral electrolytes. The Co3 Fe7 @Co5.47 N/NCF outperforms commercial platinum/carbon (Pt/C) toward ORR with an onset potential of 1.02 V and a positive half-wave potential of 0.92 V in an alkaline electrolyte (0.59 V in sodium chloride solution), which is ascribed to the unique interfacial structure between Co3 Fe7 and Co5.47 N supported by 3D N-doped carbon foam to facilitate fast electron and mass transfer. The high ORR performance is also supported by in-situ electrochemical Raman spectra and density functional theory calculation. Furthermore, the fabricated Al-air battery displays good flexibility and delivers a power density of 199.6 mW cm-2 , and the binder-free and integrated cathode shows better discharge performance than the traditionally slurry casting cathode. This work demonstrates a facile and efficient approach to develop integrated air cathode for metal-air batteries.

Probing the Activity of Iron Peroxo Porphyrin Intermediates in the Reaction Layer during the Electrochemical Reductive Activation of O2

Herein we report the first example of using scanning electrochemical microscopy (SECM) to quantitatively analyze O₂ reductive activation in organic media catalyzed by three different Fe porphyrins. For each porphyrin, SECM can provide in one single experiment the redox potential of various intermediates, the association constant of Fe^II with O₂ , and the pK_a of the Fe^III (OOH^- )/ Fe^III (OO^2- ) couple. The results obtained can contribute to a further understanding of the parameters controlling the catalytic efficiency of the Fe porphyrin towards O₂ activation and reduction.

Differences and comparisons of the properties and reactivities of iron(III)-hydroperoxo complexes with saturated coordination sphere

Heme and nonheme monoxygenases and dioxygenases catalyze important oxygen atom transfer reactions to substrates in the body. It is now well established that the cytochrome P450 enzymes react through the formation of a high-valent iron(IV)-oxo heme cation radical. Its precursor in the catalytic cycle, the iron(III)-hydroperoxo complex, was tested for catalytic activity and found to be a sluggish oxidant of hydroxylation, epoxidation and sulfoxidation reactions. In a recent twist of events, evidence has emerged of several nonheme iron(III)-hydroperoxo complexes that appear to react with substrates via oxygen atom transfer processes. Although it was not clear from these studies whether the iron(III)-hydroperoxo reacted directly with substrates or that an initial O-O bond cleavage preceded the reaction. Clearly, the catalytic activity of heme and nonheme iron(III)-hydroperoxo complexes is substantially different, but the origins of this are still poorly understood and warrant a detailed analysis. In this work, an extensive computational analysis of aromatic hydroxylation by biomimetic nonheme and heme iron systems is presented, starting from an iron(III)-hydroperoxo complex with pentadentate ligand system (L5(2)). Direct C-O bond formation by an iron(III)-hydroperoxo complex is investigated, as well as the initial heterolytic and homolytic bond cleavage of the hydroperoxo group. The calculations show that [(L5(2))Fe(III)(OOH)](2+) should be able to initiate an aromatic hydroxylation process, although a low-energy homolytic cleavage pathway is only slightly higher in energy. A detailed valence bond and thermochemical analysis rationalizes the differences in chemical reactivity of heme and nonheme iron(III)-hydroperoxo and show that the main reason for this particular nonheme complex to be reactive comes from the fact that they homolytically split the O-O bond, whereas a heterolytic O-O bond breaking in heme iron(III)-hydroperoxo is found.

Synergetic adsorption and catalytic oxidation performance originating from leafy graphite nanosheet anchored iron(ii) phthalocyanine nanorods for efficient organic dye degradation

Leafy graphite nanosheet anchored iron(ii) phthalocyanine nanorods (FePc@LGNS) were facilely synthesized without using a complex covalent anchoring procedure.