Reduction Of Molecular Oxygen Research Articles

Electrochemical behavior of a number of bispyridyl-substituted porphyrins and their electrocatalytic activity in molecular oxygen reduction reaction

The investigation of electrochemical properties of active layers containing 5,15-bis(pyrid-3-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphine, [5,15-bis(pyrid-3-yl)-3,7,13,17-tetramethyl-2,8,12, 18-tetraethylporphinato]cobalt(II), bispyridine[5,15-bis(pyrid-3-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(III), [5,15-bis(pyrid-4-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(II) and bispyridine[5,15-bis(pyrid-4-yl)-3,7,13,17-tetramethyl-2,8,12,18-tetraethylporphinato]cobalt(III) in 0.1 M KOH aqueous solution has been carried out by cyclic voltammetry. The potential ranges of redox processes related to the transformation of the porphyrin macrocycle, pyridyl substituents and central metal have been established. The electrochemical parameters of oxygen electroreduction reaction (the half-wave potential — E[Formula: see text](O[Formula: see text]), the maximum potential — E[Formula: see text](O[Formula: see text]), the current density — j, the activation energy — Eact) have been determined. It was shown that all porphyrins exhibit the catalytic activity. The electroreduction process of oxygen on electrode with the porphyrin-ligand occurs via 2-electron mechanism to give hydrogen peroxide ion HO2-, and on ones with the cobalt porphyrins — via parallel-sequential pathway including 2-electron and 4-electron processes to give HO2-, OH-. The presence of pyridine axial ligands in Co-complexes leads to a significant increase of the electrocatalytic activity in molecular oxygen reduction reaction.

Stimulation by pro-apoptotic valinomycin of cytosolic NADH/cytochrome c electron transport pathway—Effect of SH reagents

Intrinsic and extrinsic apoptosis are both characterised by the presence of cytochrome c (cyto-c) in the cytosol. We present data on the extra-mitochondrial NADH oxidation catalysed by exogenous (cytosolic) cyto-c, as a possible answer to the paradox of apoptosis being an energy-dependent program but characterized by the impairment of the respiratory chain. The reduction of molecular oxygen induced by the cytosolic NADH/cyto-c pathway is coupled to the generation of an electrochemical proton gradient available for ATP synthesis. Original findings show that SH reagents inhibit the NADH/cyto-c system with a conformational change mechanism. The mitochondrial integrity-test of sulfite oxidase unequivocally demonstrates that this enzyme (120kDa) can be released outside but exogenous cyto-c (12.5kDa) does not permeate into mitochondria. Valinomycin at 2nM stimulates both the energy-dependent reversible mitochondrial swelling and the NADH/cyto-c oxidation pathway. The pro-apoptotic activity of valinomycin, as well as to the dissipation of membrane potential, can be also ascribed to the increased activity of the NADH/cyto-c oxidation pathway useful as an additional source of energy for apoptosis. It can be speculated that the activation of the NADH/cyto-c system coupled to valinomycin-induced mitochondrial osmotic swelling may represent a strategy to activate apoptosis in confined solid tumours.

Product formation from phenolic compounds removal by laccases: A review

Lignolytic enzymes are very promising tools for biotechnological applications in water treatment. Among them, laccases (EC 1.10.3.2: p-diphenol:dioxygen oxidoreductase) are versatile enzymes with the ability to oxidize a wide range of aromatic and non-aromatic compounds (polyphenols, aryl diamines, anilines, methoxy substituted phenols, organic/inorganic metal compounds, etc.). Laccases have confirmed their potential for the degradation of recalcitrant residual phenolic compounds in water. Typically, oxidation of a laccase substrate leads to the formation of free radical with the concomitant loss of one electron and reduction of molecular oxygen into water. The formed radicals can react via non-enzymatic reactions i.e. oxidative condensation, and oxidative phenolic coupling or oxidative coupling, leading to the formation of soluble or non-soluble reaction products, several of them of polymeric nature. As many studies have highlighted the generation of products potentially more toxic than the parent molecules with the use of tertiary treatments such as advanced oxidative processes (AOPs) applied in water treatment plans, the identification of reaction products generated by the degradation of phenolic contaminants in water by laccases, and their potential toxicity, have to be investigated. This review summarizes and discuss progress made this last decades in identification of products and their risk assessment regarding their potential toxicity for a list of the most studied recalcitrant phenolic pollutants in water, such as dyes, bisphenol A, triclosan, nonylphenol, natural and synthetic hormones, etc.

(Invited) Energetic Analysis of Interactions in Electron Transfer Complex Between Cytochrome C and Cytochrome C oxidase

The electron transfer (ET) reactions in mitochondrial and bacterial respiratory chains are essential processes for energy transduction in cells. In the respiratory chain of mitochondria, the electrons to reduce molecular oxygen at cytochrome c oxidase (CcO) are donated from a small hemoprotein, cytochrome c (Cyt c), and Cyt c is thought to form an ET complex with CcO to promote the ET reaction from the heme iron in Cyt c to the CuA site in CcO. While the reduction of molecular oxygen to water molecules requires four electrons, Cyt c can carry only one electron, implying that Cyt c repetitively associates with and dissociates from CcO and suggesting that the specific interprotein interactions between Cyt c and CcO regulate the binding affinity and the ET rate from Cyt c to CcO. The amino acid sequence and isoelectric point of Cyt c suggests that many positively charged residues are located on the protein surface, as confirmed by solving the three-dimensional structures of Cyt c 1,2, allowing us to speculate that the electrostatic interactions contribute to the formation of the ET complex between Cyt c and CcO. The contributions of electrostatic interactions are supported by a previous docking simulation between Cyt c and CcO3, but the simulation study on the binding of a PDZ domain, a common structural domain found in signaling proteins, to the target peptide showed that the hydrophobic interactions are the major thermodynamic factors stabilizing the protein complex and that the thermodynamic contribution of the electrostatic interactions to the stability of the protein complex was negligible4. The thermodynamic contributions of hydrophobic and electrostatic interactions to the formation of the ET complex between Cyt c and CcO, therefore, remain controversial and detailed experimental characterization of the interprotein interactions in the ET complex has been quite limited. Recently, however, we successfully identified the interaction site for CcO on Cyt c using NMR spectroscopy5. Our NMR analysis using chemical shift perturbations clearly indicated positively charged residues including several Lys residues (Lys5, Lys7, Lys8, Lys13, Lys79, Lys86, Lys87, and Lys88), located in the interaction site for CcO on Cyt c, as we expected. In addition to the positively charged residues, several negatively charged residues such as Glu4, Glu89, Glu90, and Asp93 and some hydrophobic residues such as Ile9, Ile11, Met12, and Ile81 are also present in the interaction site for CcO, indicating that both electrostatic and hydrophobic interactions would contribute to ET complex formation between CcO and Cyt. While we have successfully determined the interaction site of Cyt c for CcO, the NMR measurements can be applied only for ET complexes such as oxidized Cyt c – fully oxidized CcO or reduced Cyt c – fully reduced CcO complexes, where the ET reaction does not occur. Furthermore, our previous NMR analysis revealed the CcO interaction site on Cyt c, but no information about the Cyt c interaction site on CcO has been obtained, and a detailed analysis of the interactions such as the energetic components stabilizing the ET complex, particularly transiently formed ET complexes where the ET reaction from Cyt c to CcO is induced under turnover conditions, has not yet been conducted. Based on the structural information about the CcO interaction site on Cyt c 5, here we constructed the ET complexes between Cyt c and CcO by performing docking simulation. We also mutated the amino acid residues located in the CcO interaction site on Cyt c, and determined the Michaelis constant, K m, for the ET reaction from Cyt c to CcO by monitoring the oxidation of reduced Cyt c by CcO. The predicted protein complexes were computationally validated by the K m values for wild-type and mutant Cyt c and a reasonable complex structure under turnover conditions was selected. Together with the mutational effects on the ET reaction and the predicted ET complex, the interactions essential for the formation of the Cyt c – CcO complex under turnover conditions for the ET reaction were energetically characterized. (1) Banci, L.; Bertini, I.; Gray, H. B.; Luchinat, C.; Reddig, T.; Rosato, A.; Turano, P. Biochemistry 1997, 36, 9867 (2) Banci, L.; Bertini, I.; Huber, J. G.; Spyroulias, G. A.; Turano, P. J. Bioinorg. Chem. 1999, 4, 21 (3) Flock, D.; Helms, V. Proteins 2002, 47, 75. (4) Basdevant, N.; Weinstein, H.; Ceruso, M. J. Am. Chem. Soc. 2006, 128, 12766. (5) Sakamoto, K.; Kamiya, M.; Imai, M.; Shinzawa-Itoh, K.; Uchida, T.; Kawano, K.; Yoshikawa, S.; Ishimori, K. Proc. Natl. Acad. Sci. USA 2011, 108, 12271.

Role of oxidative stress in Alzheimer's disease.

Alzheimer's disease (AD) is the most common cause of disability in individuals aged >65 years worldwide. AD is characterized by the abnormal deposition of amyloid β (Aβ) peptide, and intracellular accumulation of neurofibrillary tangles of hyperphosphorylated τ protein and dementia. The neurotoxic oligomer Aβ peptide, which is the neuropathological diagnostic criterion of the disease, together with τ protein, are mediators of the neurodegeneration that is among the main causative factors. However, these phenomena are mainly initiated and enhanced by oxidative stress, a process referring to an imbalance between antioxidants and oxidants in favour of oxidants. This imbalance can occur as a result of increased free radicals or a decrease in antioxidant defense, free radicals being a species that contains one or more unpaired electrons in its outer shell. The major source of potent free radicals is the reduction of molecular oxygen in water, that initially yields the superoxide radical, which produces hydrogen peroxide by the addition of an electron. The reduction of hydrogen peroxide produces highly reactive hydroxyl radicals, termed reactive oxygen species (ROS) that can react with lipids, proteins, nucleic acids, and other molecules and may also alter their structures and functions. Thus, tissues and organs, particularly the brain, a vulnerable organ, are affected by ROS due to its composition. The brain is largely composed of easily oxidizable lipids while featuring a high oxygen consumption rate. The current review examined the role of oxidative stress in AD.

Corrosion of Aluminum in Contact with Oxidized Titanium and Zirconium

We study the corrosion of alloys based on titanium, aluminum, zirconium, and iron in solutions of sodium chloride. The corrosion rate of aluminum significantly increases in contact with zirconium, titanium, and stainless steel. It is shown that the kinetics of aluminum corrosion is controlled by the cathodic reaction of reduction of molecular oxygen. It is also demonstrated that the corrosion rate of aluminum outside the contact zone is equal to the rate observed in contact with oxidized zirconium or titanium. The protective properties of oxide coatings on titanium and zirconium obtained by the method of microarc oxidation and high-temperature oxidation are investigated by the method of impedance spectroscopy.

Reduction of molecular oxygen on the surface of transition metal complex oxide

Ab initio quantum‐chemical calculations have been carried out for the electronic structures of molecular clusters and adsorption complexes to model physical and chemical adsorption of molecular oxygen on the surface of nickel‐cobalt and iron‐cobalt mixed oxides (spinels). The quantum‐chemical calculations were done using the Hartree‐Fock‐Roothan self‐consistent field MO LCAO formalism in the 6–31 basis set of Gaussian functions. It was shown that the value of adsorption energy of hydrogen peroxide influences the mechanism of oxygen reduction: at lower adsorption energies oxygen is reduced by the two‐electron mechanism, which is followed by desorption of the peroxide molecule formed. At higher adsorption energies the reduction of oxygen occurs via the four‐electron mechanism up to water formation. An explanation was proposed that the catalytic reaction of oxygen reduction proceeds on average via a 3.7‐electron mechanism, which is confirmed experimentally.

Exploring Oxidation State Dependent Conformational Changes of Cytochrome C on Cardiolipin Containing Liposomes

¬A combination of circular dichroism, absorption and fluorescence spectroscopies were used to study the binding of oxidized and reduced cytochrome c to cardiolipin (CL) containing liposomes with different CL content as a function of cardiolipin concentration. Recently, our group provided evidence for a two-step binding process of the oxidized (ferricytochrome) state to CL containing liposomes. The second step of binding converts a native-like to a more unfolded structure in which M80 is replaced either by a lysine of a histidine as axial ligand. The conversion into this more unfolded conformation was found to be partially inhibited in the presence of NaCl. Currently we are investigating whether the initial binding and/or the equilibrium between the two membrane bound conformers depends on pH. Our work with the reduced (ferrocytochrome) state has revealed a dependence on NaCl for stability. In the absence of NaCl and at aerobic environment, the protein becomes oxidized upon binding to CL containing liposomes. When NaCl is added to the solution, it stabilizes the reduced state on the liposome surface and inhibits the auto-oxidation. At anaerobic conditions and in the absence of NaCl, the protein undergoes a notable conformational change, but clearly maintains its reduced state. This shows that the obtained oxidation of the protein involves the reduction of molecular oxygen. We are in the process of exploring the pH dependence of the underlying change of the protein's redox potential.

Molecular electrocatalysis of oxygen reduction by iron(II) phthalocyanine at the liquid/liquid interface

Liquid/liquid interface electrochemistry has manifested itself as a good approach to study oxygen reduction reaction (ORR) and ORR catalyzed by various catalysts. In this work, we investigated the ORR catalyzed by iron(II) phthalocyanine (FePc), which has a structure similarity to the heme group of O2-binding proteins and reducing enzymes, at the polarized water/1,2-dichloroethane interface. Using the four-electrode cyclic voltammetry and the shake-flask biphasic reaction under chemically controlled polarization, it was demonstrated that FePc could catalyze the molecular oxygen reduction by lipophilic electron donors, such as 1,1′-dimethylferrocene (DFc) or tetrathiafulvalene (TTF), at the heterogeneous phase boundary. The overall process essentially can be equivalent to an interfacial proton transfer coupled ORR, which proceeds preferentially via a four electron reduction pathway to produce mainly H2O with only minority of H2O2 (less than 5%). The catalytic activity of FePc was compared with all previously studied porphyrins and phthalocyanines. The reaction mechanism was also analyzed, in which a hydroperoxo intermediate was probably involved.

Synthesis and electrochemistry of cobalt tetrabutanotriarylcorroles. Highly selective electrocatalysts for two-electron reduction of dioxygen in acidic and basic media

Three [Formula: see text],[Formula: see text]-tetrabutano-substituted cobalt(III) triarylcorroles were synthesized and characterized by UV-vis, 1H NMR spectroscopy and mass spectrometry as well as electrochemistry and spectroelectrochemistry. The examined compounds are represented as butano-(YPh)3CorCo(PPh[Formula: see text], where Cor represents the core of the corrole and Y is a CH3, H or Cl group on the para-position of each meso-phenyl ring of the macrocycle. Each corrole undergoes two stepwise cobalt-centered reductions leading to formation of Co(II) and Co(I) derivatives in CH2Cl2 containing 0.1 M TBAP. Three reversible one-electron oxidations were observed within potential, ranging from 0.16 to 1.40 V in CH2Cl2. The first oxidation generates a mixture of Co(III) [Formula: see text]-cation radical and Co(IV), while the second oxidation is metal-centered to give the Co(IV) corrole. Each cobalt corrole was examined as a catalyst for electroreduction of O2 when coated on an edge-plane pyrrolytic graphite electrode in acidic and basic solutions. The results indicate that the cobalt tetrabutanotriarylcorroles can act as selective catalysts for the 2e reduction of molecular oxygen to give H2O2 as the final product under the given solution conditions.

N-Doped graphene frameworks with superhigh surface area: excellent electrocatalytic performance for oxygen reduction.

N-Doped carbon materials are promising candidates as alternative catalysts to noble metals in promoting the oxygen reduction reaction (ORR) in fuel cells. However, methods to further reduce the ORR overpotential and improve related kinetics remain to be developed. This study reports that N-doped graphene frameworks (NGFs) synthesized from the rapid pyrolysis of solid glycine particles in the presence of sodium carbonate, display an extremely large specific surface area (1760 m(2) g(-1)) and a graphitic-N-dominant C-N configuration. The NGFs can efficiently catalyze the electrochemical reduction of molecular oxygen into water following a 4e pathway, with a low overpotential (0.98 V of onset potential vs. RHE), very high kinetic limiting current density (16.06 mA cm(-2)), and turnover frequency (121 s(-1)), much better than the commercial Pt/C catalyst.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C-N bond formation to the reduction of molecular oxygen (O2). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of CuII-semiquinone radical intermediates, which enable dehydrogenative carbon-heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C-H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C-N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

Bioelectrochemical Study of ThermostablePycnoporus sanguineusCS43 Laccase Bioelectrodes Based on Pyrolytic Carbon Nanofibers for Bioelectrocatalytic O2Reduction

In this study, the bioelectrocatalytic reduction of molecular oxygen by two highly thermostable laccase isoforms from a native strain of Pycnoporus sanguineus CS43 were evaluated and compared to commercially available laccase from Trametes versicolor (TvL). The laccase isoforms (LAC1 and LAC2) and TvL laccase were immobilized by orientation onto anthracene-modified multiwalled carbon nanotubes (AC-MWCNT), which were subsequently immobilized onto carbon nanofiber mat electrodes fabricated using a carbon MEMS (C-MEMS) process. The performances of the isoforms were evaluated at differing pHs, temperatures, and with various inhibitors under hydrodynamic and hydrostatic conditions. Both LAC1 and LAC2 had onset potentials of over +650 mV vs Ag/AgCl at pH 4.0, which are among the highest reported to date for any laccase bioelectrode. High current densities were also obtained, producing 825 ± 88 μA/cm2 and 1220 ± 106 μA/cm2 with LAC1 and LAC2, respectively. The bioelectrodes also demonstrated remarkable operation...

Incorporation of copper ions into crystals of T2 copper-depleted laccase from Botrytis aclada.

Laccases belong to the class of multicopper oxidases catalyzing the oxidation of phenols accompanied by the reduction of molecular oxygen to water without the formation of hydrogen peroxide. The activity of laccases depends on the number of Cu atoms per enzyme molecule. The structure of type 2 copper-depleted laccase from Botrytis aclada has been solved previously. With the aim of obtaining the structure of the native form of the enzyme, crystals of the depleted laccase were soaked in Cu(+)- and Cu(2+)-containing solutions. Copper ions were found to be incorporated into the active site only when Cu(+) was used. A comparative analysis of the native and depleted forms of the enzymes was performed.

Activation of molecular oxygen for the oxidation of 2-mercaptoethanol: A kinetic and mechanistic approach

Reduction of molecular oxygen by 2-mercaptoethanol in the presence of cobalt(II)histidine is described. Cobalt(II)histidine complex forms dinuclear 2:1 (Co:O2) complex with molecular oxygen (μ-dioxytetrakis(histidinato)dicobalt(II)). The molecular oxygen did not directly react with 2-mercaptoethanol, but, reacted in the form of an adduct with cobalt(II)histidine complex. During the course of the reaction the molecular oxygen is reduced to hydroxide ion while the cobalt(II)histidine complex was oxidized to cobalt(III)histidine while 2-mercaptoethanol is reduced into 2,2′-dithiodiethanol. The decrease in absorbance for the dioxygen complex was monitored spectrophotometrically and was observed that the values of rate constant increased with the increase in [2-mercaptoethanol]. The values of kobs also increased with increasing [NaOH]. Thus, from the results of these studies, mechanism of the reaction has been proposed. In addition, the values of various equilibrium constants and rate constants were also determined using nonlinear least square techniques.

A study of corrosion stability of 50PtCoCr/C cathode catalyst

The corrosion stability (under the chemical effect of acid environment and cycling of electrode potential) of 50 wt % PtCoCr/C trimetallic cathode catalyst has been studied. It is proposed that the dominant mode of degradation is dissolution of platinum nanoparticles and their redeposition on a surface. The values of activation energy of electrolytic reduction of molecular oxygen on 50 wt % PtCoCr/C catalysts, as well as the activation energy of its corrosion in 0.5 M H2SO4 solution, have been determined. The high corrosion stability of PtCoCr/C catalyst is attributed to the lower extent of filling of platinum surface with oxygen-containing particles (Q O/2Q H), which constitute the initial stage of platinum dissolution. It has been shown that the decrease in mass activity in electrolytic reduction of O2 during cycling of potential at 20°C up to 4000 cycles is 15%, which is significantly higher than for the monoplatinum system. The above-proposed mechanism of catalyst degradation in model experiments can lead to permanent loss of Pt ions in electrolyte bulk. In the case of measurements in a fuel cell (FC), a degradation mechanism described as dissolution of platinum nanoparticles and their redeposition on surface can be accompanied (as a consequence of a low amount of electrolyte in the FC) with redeposition of platinum particles in polymeric electrolyte.

Au/BiOCl heterojunction within mesoporous silica shell as stable plasmonic photocatalyst for efficient organic pollutants decomposition under visible light

A new mesoporous silica protected plasmonic photocatalyst, Au/BiOCl@mSiO2, was prepared by a modified AcHE method and a subsequent UV light induced photodeposition process. The surfactant-free heterojunction allows the electrons spontaneously flow from Au to nearby BiOCl surface, leading to the accumulation of positive charges on Au surface, and negative charges on Bi species under visible light. Au/BiOCl@mSiO2 exhibits high visible light photocatalytic efficiency in complete oxidation of aqueous formaldehyde and Rhodamin B. We showed that a positive relationship exists between the LSPR effect and rate enhancements, and leads to a hypothesis that the metallic Au LSPR enhances the photocatalytic rates on nearby semiconductors by transferring energetic electrons to BiOCl and increasing the steady-state concentration of active OH species by a multi-electron reduction of molecular oxygen. The OH species is the main oxidant in photocatalytic transformations, whose intensity is greatly enhanced in the dye-involving systems due to the synergetic effect between LSPR and dye sensitization processes. In addition, the mesoporous SiO2 shell not only inhibits the over growth of BiOCl nanocrystals within the silica frameworks, but also protects the dissolution of chloride or Au species into aqueous solution, which ultimately makes the Au/BiOCl@mSiO2 catalysts rather stable during photocatalysis.

Modulating charge transport in semiconductor photocatalysts by spatial deposition of reduced graphene oxide and platinum

Spatially controlled deposits of reduced graphene oxide (rGO) and Pt on TiO2 were fabricated and assessed for the photocatalytic degradation of a range of organic compounds including carboxylic acid (oxalic acid), alcohols (tert-butanol, isopropanol), and halogenated aromatics (2,4-dichlorophenoxy acetic acid, 2,4-D). By selectively depositing Pt onto rGO that is attached to TiO2 (Pt–rGO/TiO2) or rGO onto Pt/TiO2 (rGO/Pt–TiO2), and by comparing with Pt/TiO2 and rGO/TiO2, we established the actual roles of Pt and rGO in photocatalysis. The deposition of Pt on TiO2, which formed the Schottky barrier at the interface, was found to be much more efficient in enhancing photocharge separation than rGO attachment, which merely extracts the photoelectrons interfacially. Both Pt and rGO show catalytic effects in the surface electron transfer. For the same Pt and rGO loading, the most efficient configuration was achieved by rGO/Pt–TiO2 since the rGO interfacially extracts the photoelectrons from the Pt sink, while providing additional sites for the reduction of molecular oxygen. The complex relationships between the physicochemical properties of the composites and the photocatalytic efficiencies of direct hole oxidation (oxalic acid), indirect oxidation by hydroxyl radicals (isopropanol and tert-butanol), and dearomatization and dechlorination (2,4-D) were established. Based on this, further enhancement of the photocatalytic activity of the composite photocatalysts was achieved by adding and optimizing the loading of Pt onto the rGO component of the rGO/Pt–TiO2 to enhance electron transfer on rGO. As a result, photocatalytic efficiency 3.5-fold that of the optimized Pt/TiO2 was achieved. For the same amount of Pt loading in Pt/TiO2, a detrimental effect was measured.

Cooperative Effects in Copper Polyamido­amine Dendrimer Complexes Catalyzing the Reduction of Molecular Oxygen

AbstractGeneration‐dependent cooperation was observed in the reduction of molecular oxygen by a series of Cu2+ complexes of pyridine‐modified polyamidoamine (PAMAM) dendrimers. The cooperative behavior of the metal ions was examined by preparing (G2–G6)‐dendri‐PAMAM‐(Py)n and testing the dendrimers for their ability to reduce oxygen catalytically in the presence of several Cu2+ ion concentrations. Spectrophotometric measurements during the reactive metallic ion titrations revealed that the maximum numbers of reactive Cu2+ ions available for O2 reduction were 4.8, 7.6, 22.2, 57.3, and 81.7 in each of the second‐ to sixth‐generation (G2–G6) dendrimer complexes, respectively. An analysis revealed that each Cu2+ ion worked independently as an active center in the G2 to G4 dendrimer complexes. By contrast, pairs of Cu2+ ion active species were present in the G5 and G6 dendrimer complexes. A kojic acid titration indicated that the copper ions were proximal and formed a bicopper active center in the G5 and G6 dendrimer complexes. Catalytic reactions with catechol as a reducing agent further demonstrated the activity of the dinuclear copper center in the G5 and G6 dendrimer complexes. The present findings suggest that generations of dendrimer–metal complexes can alter their catalytic behavior.

Biomimetic O2 Reduction at MN4 Catalysts Adsorbed on Carbon Nanotubes and on Pyridine Grafted Carbon Nanotubes

Metallophthalocyanines and metalloporphyrins are well known electrocatalysts for the reduction of O2(ORR) and have been extensively investigated in order o achieve a better understanding of the fundamentals of ORR at modified electrodes and with the final scope of replacing Pt in fuel cells. However, heat-treated MNx catalysts are more active and stable than intact MN4 complexes for ORR. In previous studies, we have established that the formal redox potential is a good reactivity index for the electrocatalytic activity of surface-confined MN4 complexes (MN4 complexes) toward the reduction of molecular oxygen [1], and this is also valid for pyrolyzed materials [2]. Those experiments were performed with the MN4 macrocyclic adsorbed directly on the edge plane graphite electrode [1]. The results show a linear, maybe a semi-volcano correlation where the catalytic activity of the complex increases when the potential of the redox couple associated to the catalytic process increases [1]. According to those results the best strategy to increase the catalytic activity of the complexes is to tune the redox potential oft he complex or catalyst to more positive values by the chemical modification of the ligand or the environment around the the site [1,2]. In a recent study, Cho and co-workers [3] found higher catalytic activity than Pt for electrodes modified with Fe-phthalocyanine with an axial ligand anchored on single walled carbon nanotubes [2]. Fundamental studies of MN macrocyclic complexes adsorbed on carbon nanotubes (CNTs) and on pyridine modified CNTs as catalysts for the ORR are missing. Preliminary studies in basic media show that when MN4 macrocyclic complexes are adsorbed on CNTs (single walled and double walled) modified electrodes higher catalytic current can be obtained (almost two orders of magnitude increment) and similar half-volcano trend reported for MN4 macrocyclic complexes is found for these hybrid electrodes where the activity increases as the formal potential of the metal catalyst redox center is shifted to more positive values were obtained [3]. When CNTs are modified with 4-aminopyridine aryl diazonium salts and then with axially ligated phthalocyanines even higher activities are observed but volcano correlations are distorted by the effect of the axial ligand. We have observed this enhancement of he activity by the action of an axial ligand on phthalocyanines anchored on gold [4]. The axial ligand mimics the action of natural occurring ORR catalyst cytochrome c oxidase that contains an active site of a Fe(II)-porphyrin which has a 5-coordinated structure with an axial ligand on the back side [3]. Ackowledgements. This work was funded by Fondecyt Projects 1140199, 3130538 and 3150271 and to Dicyt-USACH.