An iron porphyrin catalyst with four electron donor groups is reported. The porphyrin ligand bears a distal hydrogen bonding pocket which inverts the normal axial ligand binding selectivity exhibited by porphyrins bearing sterically crowded distal structures. This catalyst specifically reduces O(2) by four electrons under both fast and slow electron fluxes at pH 7.

"Click" reaction has been utilized to synthesize porphyrin ligands possessing distal superstructures functionalized with ferrocenes, carboxylic acid esters, and phenols. Both structural and spectroscopic evidence indicate that hydrogen bonding interaction between the triazole residues resulting from the "click" reaction promotes axial ligand binding into the sterically demanding distal pocket in preference to the open proximal side. An iron porphyrin complex with four ferrocene groups is found to bind O(2) and quantitatively reduce it by one electron to O(2)(-) in apolar organic solvents. However the same complex electro-catalytically reduces O(2) by four electrons to H(2)O in aqueous medium under fast, moderate, and slow electron fluxes. This selectivity for O(2) reduction is governed by the reduction potential of the electron transfer site (i.e., ferrocene) which in turn is governed by the solvent. This catalyst mimics control of catalysis of an enzyme active site by a second sphere electron transfer residue which is often encountered in naturally occurring metallo-enzymes.

An iron porphyrin catalyst bearing four ferrocenes and a hydrogen bonding distal pocket is found to catalyze 4e(-)/4H(+) oxygen reduction reaction (ORR) in organic solvent under homogeneous conditions in the presence of 2-3 equiv of Trifluoromethanesulphonic acid. Absorption spectroscopy, electron paramagnetic resonance (EPR), and resonance Raman data along with H2O2 assay indicate that one out of the four electrons necessary to reduce O2 to H2O is donated by the ferrous porphyrin while three are donated by the distal ferrocene residues. The same catalyst shows 4e(-)/4H(+) reduction of O2 in an aqueous medium, under heterogeneous conditions, over a wide range of pH. Both the selectivity and the rate of ORR are found to be pH independent in an aqueous medium. The ORR proceeds via a proton transfer followed by electron transfer (PET) step in an organic medium and while a 2e(-)/1H(+) proton coupled electron transfer (PCET) step determines the electrochemical potential of ORR in an aqueous medium.

High-valent nonheme Fe IV −oxido species are key intermediates in biological oxidation, and their properties are proposed to be influenced by the unique microenvironments present in protein active sites. Microenvironments are regulated by noncovalent interactions, such as hydrogen bonds (H-bonds) and electrostatic interactions; however, there is little quantitative information about how these interactions affect crucial properties of high valent metal−oxido complexes. To address this knowledge gap, we introduced a series of Fe IV −oxido complexes that have the same S = 2 spin ground state as those found in nature and then systematically probed the effects of noncovalent interactions on their electronic, structural, and vibrational properties. The key design feature that provides access to these complexes is the new tripodal ligand [poat] 3− , which contains phosphinic amido groups. An important structural aspect of [Fe IV poat(O)] − is the inclusion of an auxiliary site capable of binding a Lewis acid (LA II ); we used this unique feature to further modulate the electrostatic environment around the Fe−oxido unit. Experimentally, studies confirmed that H-bonds and LA II s can interact directly with the oxido ligand in Fe IV −oxido complexes, which weakens the FeO bond and has an impact on the electronic structure. We found that relatively large vibrational changes in the Fe−oxido unit correlate with small structural changes that could be difficult to measure, especially within a protein active site. Our work demonstrates the important role of noncovalent interactions on the properties of metal complexes, and that these interactions need to be considered when developing effective oxidants.

We present a mixed experimental/theoretical determination of the bond strengths and redox potentials that define the ground state thermodynamics for C-H bond activation in cytochrome P450 catalysis. Using redox titrations with [Ir(IV)Cl 6 ] 2− we have determined the compound II/ ferric (or Fe(IV)OH/Fe(III)OH 2) couple and its associated D(O-H) Ferric bond strength in CYP158. Knowledge of this potential as well as the compound II/ferric (or Fe(IV)O/Fe(III)OH) reduction potential in horseradish peroxidase and the two-electron compound I/ferric (or Fe(IV)O(Por •)/ Fe(III)OH 2 (Por)) reduction potential in aromatic peroxidase has allowed us to gauge the accuracy of theoretically determined bond strengths. Using the restricted open shell (ROS) method as proposed by Wright and coworkers, we have obtained O-H bond strengths and associated redox potentials for charge-neutral H-atom-reductions of these iron(IV)-hydroxo and −oxo porphyrin species that are within 1 kcal/mol of experimentally determined values, suggesting that the ROS method may provide accurate values for the P450-II O-H bond strength and P450-I reduction potential. The efforts detailed here indicate the ground state thermodynamics of C-H bond activation in P450 are best described as follows: E 0' Comp-I = 1.22 V (at pH 7, vs. NHE) with D(O-H) Comp-II = 95 kcal/mol, and E 0' Comp-II = 0.99 V (at pH 7, vs. NHE) with D(O-H) Ferric = 90 kcal/ mol.

A hydrogen bonded heme-Fe(III)-O(2)(-) adduct is stabilized and characterized using resonance Raman and EPR spectroscopy. The low O-O vibrations of this complex are quite different from those reported for other heme-Fe(III)-O(2)(-) adducts.

The protonation state of the iron­(IV) oxo (or ferryl) form of ascorbate peroxidase compound II (APX-II) is a subject of debate. It has been reported that this intermediate is best described as an iron­(IV) hydroxide species. Neutron diffraction data obtained from putative APX-II crystals indicate a protonated oxygenic ligand at 1.88 Å from the heme iron. This finding, if correct, would be unprecedented. A basic iron­(IV) oxo species has yet to be spectroscopically observed in a histidine-ligated heme enzyme. The importance of ferryl basicity lies in its connection to our fundamental understanding of C–H bond activation. Basic ferryl species have been proposed to facilitate the oxidation of inert C–H bonds, reactions that are unknown for histidine-ligated hemes enzymes. To provide further insight into the protonation status of APX-II, we examined the intermediate using a combination of Mössbauer and X-ray absorption spectroscopies. Our data indicate that APX-II is an iron­(IV) oxo species with an Fe–O bond distance of 1.68 Å, a K-edge pre-edge absorption of 18 units, and Mössbauer parameters of ΔE q = 1.65 mm/s and δ = 0.03 mm/s.

Considering the importance of water splitting as the best solution for clean and renewable energy, the worldwide efforts for development of increasingly active molecular water oxidation catalysts must be accompanied by studies that focus on elucidating the mode of actions and catalytic pathways. One crucial challenge remains the elucidation of the factors that determine the selectivity of water oxidation by the desired 4e–/4H+ pathway that leads to O2 rather than by 2e–/2H+ to H2O2. We now show that water oxidation with the cobalt–corrole CoBr8 as electrocatalyst affords H2O2 as the main product in homogeneous solutions, while heterogeneous water oxidation by the same catalyst leads exclusively to oxygen. Experimental and computation-based investigations of the species formed during the process uncover the formation of a Co­(III)–superoxide intermediate and its preceding high-valent Co–oxyl complex. The competition between the base-catalyzed hydrolysis of Co­(III)–hydroperoxide [Co­(III)–OOH]− to release H2O2 and the electrochemical oxidation of the same to release O2 via [Co­(III)–O2 •]− is identified as the key step determining the selectivity of water oxidation.