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

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.

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.

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.