Iron porphyrins as models of cytochrome c oxidase

Porphyrin Atropisomerism as a Molecular Engineering Tool in Medicinal Chemistry, Molecular Recognition, Supramolecular Assembly, and Catalysis

Porphyrin atropisomerism, which arises from restricted σ-bond rotation between the macrocycle and a sufficiently bulky substituent, was identified in 1969 by Gottwald and Ullman in 5,10,15,20-tetrakis(o-hydroxyphenyl)porphyrins. Henceforth, an entirely new field has emerged utilizing this transformative tool. This review strives to explain the consequences of atropisomerism in porphyrins, the methods which have been developed for their separation and analysis and present the diverse array of applications. Porphyrins alone possess intriguing properties and a structure which can be easily decorated and molded for a specific function. Therefore, atropisomerism serves as a transformative tool, making it possible to obtain even a specific molecular shape. Atropisomerism has been thoroughly exploited in catalysis and molecular recognition yet presents both challenges and opportunities in medicinal chemistry.

Second-Sphere Hydrogen-Bond Donors and Acceptors Affect the Rate and Selectivity of Electrochemical Oxygen Reduction by Iron Porphyrins Differently

The factors that control the rate and selectivity of 4e^-/4H⁺ O₂ 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 O₂ and H₂O₂ 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⁺ O₂ 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 pH-dependent variation of the rate of the electrochemical O₂ 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 H₂O (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 O₂ reduction by stabilizing Fe^III-O₂^.- 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 rate-determining steps in the complex multistep 4e^-/4H⁺ ORR are developed and a bioinspired principle-based design of an efficient electrochemical ORR is presented.

Second Sphere Effects on Oxygen Reduction and Peroxide Activation by Mononuclear Iron Porphyrins and Related Systems

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.

Tuning Oxygen Reduction Catalysis of Dinuclear Cobalt Polypyridyl Complexes by the Bridging Structure

The four-electron oxygen reduction reaction (4e^--ORR) is the mainstay in chemical energy conversion. Elucidation of factors influencing the catalyst's reaction rate and selectivity is important in the development of more active catalysts of 4e^--ORR. In this study, we investigated chemical and electrochemical 4e^--ORR catalyzed by Co₂(μ-O₂) complexes bridged by xanthene (1) and anthracene (3) and by a Co₂(OH)₂ complex bridged by anthraquinone (2). In the chemical ORR using Fe(CpMe)₂ as a reductant in acidic PhCN, we found that 1 showed the highest initial turnover frequency (TOF_init = 6.8 × 10² s^-1) and selectivity for 4e^--ORR (96%) in three complexes. The detailed kinetic analyses have revealed that the rate-determining steps (RDSs) in the catalytic cycles of 1-3 have the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ as an intermediate in common. In the only case that complex 1 was used as a catalyst, k_cat depended on proton concentration because the reaction rate of the O₂ addition to [Co^II₂(OH₂)₂]⁴⁺ was so fast as compared to that of the concerted PCET process of 1. Through X-ray, Raman, and electrochemical analyses and stoichiometric reactions, we found the face-to-face structure of 1 characterized by a slightly flexible xanthene was advantageous in capturing O₂ and stabilizing the Co₂(μ-O₂) structure, thus increasing both the reaction rate and selectivity for 4e^--ORR.

Oxygen reduction reaction catalyzed by overhanging carboxylic acid strapped iron porphyrins adsorbed on carbon nanotubes

A series of hybrid catalysts for the oxygen reduction reaction (ORR) has been investigated. They are composed of multi-wall carbon nanotubes (MWNTs) coated with iron strapped porphyrins. Two porphyrins have been probed; both are strapped with the same skeleton and differ only by the number of overhung carboxylic acid(s), either one or two. In this structure, the carboxylic acid group can act as a proton relay between the medium and the catalyst or as a polar group surrounding the dioxygen binding cavity. While the number of carboxylic acid group(s) does not exhibit a significant influence on the catalytic properties, the combination of both components–MWNTs and porphyrin–leads to a better catalytic activity than those of the nanotubes or the porphyrins taken separately.

Synergic effect on oxygen reduction reaction of strapped iron porphyrins polymerized around carbon nanotubes

Carbon nanotube-strapped iron porphyrin hybrids have been synthesized and their electrocatalytic activities for the oxygen reduction reaction have been investigated.

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Second sphere control of redox catalysis: selective reduction of O2 to O2- or H2O by an iron porphyrin catalyst

"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.

Electrocatalytic reduction of oxygen with phenanthroline-strapped porphyrins

The electrochemical behavior of three cytochrome c oxidase models has been investigated. All the models are derived from a phenanthroline-strapped, porphyrin framework that binds zinc(II) or iron(III) chloride in the porphyrin subunit, and copper(I) in the phenanthroline site. The iron complex and the bimetallic zinc(II) copper(I) complex of the parent ligand have been characterized by X-ray diffraction. One model consists of the parent structure on which C 12 alkyl chains have been added. This soluble model achieves electrochemical 2-electron reduction of oxygen in organic solvents without the addition of an exogenous axial base, and in the presence of an organic or inorganic source of protons. The two other models comprise the parent phenanthroline-strapped porphyrin framework, on which two pendant imidazoles have been incorporated. These models adsorbed on ring-disk electrodes with an edge-oriented, pyrolytic graphite (EOPG) disk and a platinum ring, efficiently catalyze the 4-electron reduction of oxygen in dioxygen saturated water at neutral pH.

Tripodal and/or picket porphyrins to mimic the cytochrome c oxidase activity

The efficiency of different series of tris-(2-aminoethyl)amine TREN-capped porphyrins as catalysts for the electroreduction of dioxygen has been studied after adsorption of the molecules at the surface of a graphite electrode. The influence of two structural features have been explored: the relative position of the two metals in the iron-copper bimetallic complexes and the nature of the benzyl groups of the tripod itself. These compounds have also been compared with picket porphyrins bearing aromatic amino functions, e.g. quinoline. The conclusions obtained in the case of TREN-capped catalysts cannot be extended to those bearing aromatic pickets.

Homogeneous catalytic O2 reduction to water by a cytochrome c oxidase model with trapping of intermediates and mechanistic insights

An efficient and selective four-electron plus four-proton (4e(-)/4H(+)) reduction of O(2) to water by decamethylferrocene and trifluoroacetic acid can be catalyzed by a synthetic analog of the heme a(3)/Cu(B) site in cytochrome c oxidase ((6)LFeCu) or its Cu-free version ((6)LFe) in acetone. A detailed mechanistic-kinetic study on the homogeneous catalytic system reveals spectroscopically detectable intermediates and that the rate-determining step changes from the O(2)-binding process at 25 °C room temperature (RT) to the O-O bond cleavage of a newly observed Fe(III)-OOH species at lower temperature (-60 °C). At RT, the rate of O(2)-binding to (6)LFeCu is significantly faster than that for (6)LFe, whereas the rates of the O-O bond cleavage of the Fe(III)-OOH species observed (-60 °C) with either the (6)LFeCu or (6)LFe catalyst are nearly the same. Thus, the role of the Cu ion is to assist the heme and lead to faster O(2)-binding at RT. However, the proximate Cu ion has no effect on the O-O bond cleavage of the Fe(III)-OOH species at low temperature.

Dioxygen binding of water-soluble iron(ii) porphyrins in phosphate buffer at room temperature

Two water-soluble tris(2-aminoethylamine) (tren) capped iron porphyrins were synthesized. The stability of their dioxygen adducts was studied in phosphate buffer, leading to half-life times around 7 min for the oxygenated species.

The Self-Ordering Properties of Novel Phthalocyanines with Out-of-Plane Alkyl Substituents

Two novel homologous series of phthalocyanines were prepared from 2,2-dialkylindane and 2,2-dialkyl-1,3-benzodioxole precursors. It was anticipated that attaching alkyl chains to five-membered rings, fused to the peripheral sites of the phthalocyanine ring, would result in the adoption of an out-of-plane configuration and thereby discourage cofacial aggregation, to provide an analogy with picket-fence porphyrins. This strategy proved partially successful. Some members of the series of phthalocyanines derived from 2,2-dialkyl-1,3-benzodioxoles, in which the alkyl chains are linked to the phthalocyanine via a cyclic ketal, form spin-coated thin films in which the phthalocyanine cores are perfectly isolated. This behaviour is associated with the formation of a disordered crystal that appears as a mesophase in the thermal profile of these materials. However, the phthalocyanines derived from 2,2-dialkylindanes display a columnar mesophase over a wide temperature range, with some liquid crystalline derivatives at ambient temperature. A single-crystal X-ray diffraction structure of the octahexyl derivative of this series shows how the columnar assembly accommodates the out-of-plane alkyl chains by tilting the macrocyclic plane of the phthalocyanine components with respect to the axis of the column. This study helps to emphasise the importance of both the steric and electronic effects of substituents on the packing behaviour of phthalocyanines in the condensed phase, and especially the role of electron-donating oxygen atoms directly attached to the ring.

Cobalt(IV) corroles as catalysts for the electroreduction of O2: Reactions of heterobimetallic dyads containing a face-to-face linked Fe(III) or Mn(III) porphyrin

A series of heterobinuclear cofacial porphyrin-corrole dyads containing a Co(IV) corrole linked by one of four different spacers in a face-to-face arrangement with an Fe(III) or Mn(III) porphyrin have been examined as catalysts for the electroreduction of O(2) to H(2)O and/or H(2)O(2) when adsorbed on the surface of a graphite electrode in air-saturated aqueous solutions containing 1M HClO(4). The examined compounds are represented as (PCY)M(III)ClCo(IV)Cl where P is a porphyrin dianion, C is a corrole trianion and Y is a biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or anthracene (A) spacer. The catalytic behavior of the seven investigated dyads in the two heterobimetallic (PCY)MClCoCl series of catalysts is compared on one hand to what was previously reported for related dyads with a single Co(III) corrole macrocycle linked to a free-base porphyrin with the same set of linking bridges, (PCY)H(2)Co, and on the other hand to dicobalt porphyrin-corrole dyads of the form (PCY)Co(2) which were shown to efficiently electrocatalyze the four electron reduction of O(2) at a graphite electrode in acid media. Comparisons between the four series of porphyrin-corrole dyads, (PCY)Co(2), (PCY)H(2)Co, (PCY)FeClCoCl and (PCY)MnClCoCl, show that in all cases the biscobalt dyads catalyze O(2) electroreduction at potentials more positive by an average 110mV as compared to the related series of compounds containing a Co(III) or Co(IV) corrole macrocycle linked to a free-base metalloporphyrin or a metalloporphyrin with an Fe(III) or Mn(III) central metal ion. The data indicates that the E(1/2) values where electrocatalysis is initiated is related to the initial site of electron transfer, which is the Co(III)/Co(II) porphyrin reduction process in the case of (PCY)Co(2) and the Co(IV)/Co(III) corrole reduction in the case of (PCY)MnClCoCl, (PCY)FeClCoCl and (PCY)H(2)Co. The overall data also suggests that the catalytically active form of the biscobalt dyad in (PCY)Co(2) contains a Co(II) porphyrin and a Co(IV) corrole.

O2 and CO Binding to Tetraaza-Tripodal-Capped Iron(II) Porphyrins

A series of tris(2-aminoethylamine) (tren) capped iron(II) porphyrins has been synthesized and characterized and their affinities for dioxygen and carbon monoxide measured. The X-ray structure of the basic scaffold with nickel inserted in the porphyrin is also reported. All the ligands differ by the nature of the group(s) attached to the secondary amine functions of the cap. These various substitutions were introduced to probe if a hydrogen bond with these secondary amine groups acting as the donor could rationalize the high affinity of these myoglobin models. This work clearly indicates that the cage structure of the tren predominates over all the other appended groups with the exception of p-nitrophenol.

Targeted Proton Delivery in the Catalyzed Reduction of Oxygen to Water by Bimetallic Pacman Porphyrins

A combined experimental and theoretical investigation of the role of proton delivery in determining O2 reduction pathways catalyzed by cofacial bisporphyrins is presented. A homologous family of dicobalt(II) Pacman porphyrins anchored by xanthene [Co2(DPX) (1) and Co2(DPXM) (3)] and dibenzofuran [Co2(DPD) (2) and Co2(DPDM) (4)] have been synthesized, characterized, and evaluated as catalysts for the direct four-proton, four-electron reduction of O2 to H2O. Structural analysis of the intramolecular diiron(III) mu-oxo complex Fe2O(DPXM) (5) and electrochemical measurements of 1-4 establish that Pacman derivatives bearing an aryl group trans to the spacer possess structural flexibilities and redox properties similar to those of their parent counterparts; however, these trans-aryl catalysts exhibit markedly reduced selectivities for the direct reduction of O2 to H2O over the two-proton, two-electron pathway to H2O2. Density functional theory calculations reveal that trans-aryl substitution results in inefficient proton delivery to O2-bound catalysts compared to unsubstituted congeners. In particular, the HOMO of [Co2(DPXM)(O2)]+ disfavors proton transfer to the bound oxygen species, funneling the O-O activation pathway to single-electron chemistry and the production of H2O2, whereas the HOMO of [Co2(DPX)(O2)]+ directs protonation to the [Co2O2] core to facilitate subsequent multielectron O-O bond activation to generate two molecules of H2O. Our findings highlight the importance of controlling both proton and electron inventories for specific O-O bond activation and offer a unified model for O-O bond activation within the clefts of bimetallic porphyrins.

Effect of electron availability on selectivity of O2 reduction by synthetic monometallic Fe porphyrins

Herein we report that biomimetic analogues of cytochrome c oxidase (CcO) couple reduction of O(2) to oxidation of a single-electron carrier, Ru(NH(3))(6)(2+), under steady-state catalytic turnover. Higher Ru(II) concentrations favor the 4-electron vs 2-electron O(2) reduction pathway. Our data indicate that the capacity of electrode-adsorbed Fe-only porphyrins to catalyze reduction of O(2) to H(2)O is due to high availability of electrons and is eliminated under the biologically relevant slow electron delivery.

The role of copper and protons in heme-copper oxidases: kinetic study of an engineered heme-copper center in myoglobin

To probe the role of copper and protons in heme-copper oxidase (HCO), we have performed kinetic studies on an engineered heme-copper center in sperm whale myoglobin (Leu-29 --> HisPhe-43 --> His, called Cu(B)Mb) that closely mimics the heme-copper center in HCO. In the absence of metal ions, the engineered Cu(B) center in Cu(B)Mb decreases the O(2) binding affinity of the heme. However, addition of Ag(I), a redox-inactive mimic of Cu(I), increases the O(2)-binding affinity. More importantly, copper ion in the Cu(B) center is essential for O(2) reduction, as no O(2) reduction can be observed in copper-free, Zn(II), or Ag(I) derivatives of Cu(B)Mb. Instead of producing a ferryl-heme as in HCO, the Cu(B)Mb generates verdoheme because the engineered Cu(B)Mb may lack a hydrogen bonding network that delivers protons to promote the heterolytic OO cleavage necessary for the formation of ferryl-heme. Reaction of oxidized Cu(B)Mb with H(2)O(2), a species equivalent in oxidation state to 2e(-), reduced O(2) but, possessing the extra protons, resulted in ferryl-heme formation, as in HCO. The results showed that the Cu(B) center plays a critical role in O(2) binding and reduction, and that proton delivery during the O(2) reduction is important to avoid heme degradation and to promote the HCO reaction.