The O2 reduction reaction (ORR) catalysed by iron porphyrins with covalently attached pendant guanidine groups is reported.

Oxygen reduction reaction (ORR) is a complex 4e–/4H+ multistep process, and factors that determine the rate and selectivity of this reaction are a matter of contemporary interest. Synthetic iron porphyrin model complexes mimicking the active site of horseradish peroxidase (HRP) are shown to enhance the rate of ORR and 4H+/4e– selectivity irrespective of the rate of electron transfer from the electrode relative to unfunctionalized iron porphyrins. In operando spectroscopic analysis over self-assembled monolayer-modified electrodes allows characterization of reactive intermediates involved in ORR. Two key intermediate species of ORR, FeIII–OOH and FeIVO, are identified using 18O2 labeling depending on the nature of the pendant residue of these HRP mimics. The results indicate that the nature of the pendant distal moiety in the distal superstructure of iron porphyrin changes the distribution of the intermediate species under steady state relative to simple mononuclear porphyrins. Density functional theory calculations indicate that not only the presence of hydrogen bonding from the pendant groups but also its relative spatial orientation with respect to the proximal and distal oxygen atom of a FeIII–OOH intermediate species controls the selectivity of ORR.

Activation and reduction of O2 and H2O2 by synthetic and biosynthetic iron porphyrin models have proved to be a versatile platform for evaluating second-sphere effects deemed important in naturally occurring heme active sites. Advances in synthetic techniques have made it possible to install different functional groups around the porphyrin ligand, recreating artificial analogues of the proximal and distal sites encountered in the heme proteins. Using judicious choices of these substituents, several of the elegant second-sphere effects that are proposed to be important in the reactivity of key heme proteins have been evaluated under controlled environments, adding fundamental insight into the roles played by these weak interactions in nature. This review presents a detailed description of these efforts and how these have not only demystified these second-sphere effects but also how the knowledge obtained resulted in functional mimics of these heme enzymes.

The mononuclear copper hydroperoxo species (Cu(ii)–OOH) of Cu–Aβ is the active oxidant responsible for serotonin oxidation by Cu–Aβ in the presence of physiologically relevant oxidants like O2 and H2O2, which can potentially cause oxidative degradation of neurotransmitters, a marker of Alzheimer's disease.

Understanding the reactivity landscape for the activation of water till the formation of the O-O bond and O 2 release in molecular chemistry is a decisive step in guiding the elaboration of costeffective catalysts for the oxygen-evolving reaction (OER). Copper (II) complexes have recently caught the attention of chemists as catalysts for the 4e -/4H + water oxidation process. While a copper (IV) intermediate has been proposed as the reactive intermediate species, yet no spectroscopic signature has been reported so far. Copper (III) ligand radical species have also been formulated and supported by theoretical studies. We found, herein, that the reactivity sequence for the water oxidation with a family of Copper (II) o-phenylene bisoxamidate complexes are a function of the substitution pattern on the periphery of the aromatic ring. In-situ IR, EPR and rR spectroelectrochemical studies helped to sequence the elementary electrochemical and chemical events leading towards the O 2 formation selectively at the copper center. A copper (II) superoxide species is identified as the reactive intermediate, the stability of which governs the selectivity of 4eoxidation of water to molecular oxygen.

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Iron porphyrins with one or four tertiary amine groups in their second sphere are used to investigate the electrochemical O2 reduction reaction (ORR) in organic (homogeneous) and aqueous (heterogeneous) conditions. Both of these complexes show selective 4e–/4H+ reduction of oxygen to water at rates that are 2–3 orders of magnitude higher than those of iron tetraphenylporphyrin lacking these amines in the second sphere. In organic solvents, these amines get protonated, which leads to the lowering of overpotentials, and the rate of the ORR is enhanced almost 75,000 times relative to rates expected from the established scaling relationship for the ORR by iron porphyrins. In the aqueous medium, the same trend of higher ORR rates at a lower overpotential is observed. In situ resonance Raman data under heterogeneous aqueous conditions show that the presence of one amine group in the second sphere leads to a cleavage of the O–O bond in a FeIII-OOH intermediate as the rate-determining step (rds). The presence of four such amine groups enhances the rate of O–O bond cleavage such that this intermediate is no longer observed during the ORR; rather, the proton-coupled reduction of the FeIII-O2 – intermediate with a H/D isotope effect of 10.6 is the rds. These data clearly demonstrate changes in the rds of the electrochemical ORR depending on the nature of second-sphere residues and explain their deviation from linear scaling relationships.

The factors that control the rate and selectivity of 4e − /4H + O 2 reduction are important for efficient energy transformation as well as for understanding the terminal step of respiration in aerobic organisms. Inspired by the design of naturally occurring enzymes which are efficient catalysts for O 2 and H 2 O 2 reduction, several artificial systems have been generated where different second-sphere residues have been installed to enhance the rate and efficiency of the 4e − /4H + O 2 reduction. These include hydrogen-bonding residues like amines, carboxylates, ethers, amides, phenols, etc. In some cases, improvements in the catalysis were recorded, whereas in some cases improvements were marginal or nonexistent. In this work, we use an iron porphyrin complex with pendant 1,10-phenanthroline residues which show a pHdependent variation of the rate of the electrochemical O 2 reduction reaction (ORR) over 2 orders of magnitude. In-situ surface-enhanced resonance Raman spectroscopy reveals the presence of different intermediates at different pH's reflecting different rate-determining steps at different pH's. These data in conjunction with density functional theory calculations reveal that when the distal 1,10-phenanthroline is neutral it acts as a hydrogen-bond acceptor which stabilizes H 2 O (product) binding to the active Fe II state and retards the reaction. However, when the 1,10-phenanthroline is protonated, it acts as a hydrogen-bond donor which enhances O 2 reduction by stabilizing Fe III −O 2 .− and Fe III −OOH intermediates and activating the O−O bond for cleavage. On the basis of these data, general guidelines for controlling the different possible ratedetermining steps in the complex multistep 4e − /4H + ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.