Cobalt(III) Corroles as Electrocatalysts for O2 Reduction: Reactivity of a Monocorrole, Biscorroles, and Porphyrin–Corrole Dyads (original) (raw)
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Journal of the American Chemical Society, 2005
Three series of cobalt(III) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me4Ph5Cor)Co, one a face-to-face biscorrole linked by an anthracene (A), biphenylene (B), 9,9-dimethylxanthene (X), dibenzofuran (O) or dibenzothiophene (S) bridge, (BCY)Co 2 (with Y) A, B, X, O or S), and one a face-to-face bismacrocyclic complex, (PCY)Co2, containing a Co(II) porphyrin and a Co(III) corrole also linked by one of the above rigid spacers (Y) A, B, X, or O). Cyclic voltammetry and rotating ring-disk electrode voltammetry were both used to examine the catalytic activity of the cobalt complexes in acid media. The mixed valent Co(II)/Co(III) complexes, (PCY)Co2, and the biscorrole complexes, (BCY)Co2, which contain two Co(III) ions in their air-stable forms, all provide a direct four-electron pathway for the reduction of O2 to H2O in aqueous acidic electrolyte when adsorbed on a graphite electrode, with the most efficient process being observed in the case of the complexes having an anthracene spacer. A relatively small amount of hydrogen peroxide was detected at the ring electrode in the vicinity of E1/2 which was located at 0.47 V vs SCE for (PCA)Co2 and 0.39 V vs SCE for (BCA)Co2. The cobalt(III) monocorrole (Me4Ph5Cor)Co also catalyzes the electroreduction of dioxygen at E1/2) 0.38 V with the final products being an approximate 50% mixture of H2O2 and H2O.
Journal of Inorganic Biochemistry, 2006
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 1 M 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 110 mV 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.
2010
We apply first principles computational techniques to analyze the two-electron, multistep, electrochemical reduction of CO 2 to CO in water using cobalt porphyrin as a catalyst. Density functional theory calculations with hybrid functionals and dielectric continuum solvation are used to determine the steps at which electrons are added. This information is corroborated with ab initio molecular dynamics simulations in an explicit aqueous environment which reveal the critical role of water in stabilizing a key intermediate formed by CO 2 bound to cobalt. By use of potential of mean force calculations, the intermediate is found to spontaneously accept a proton to form a carboxylate acid group at pH < 9.0, and the subsequent cleavage of a C-OH bond to form CO is exothermic and associated with a small free energy barrier. These predictions suggest that the proposed reaction mechanism is viable if electron transfer to the catalyst is sufficiently fast. The variation in cobalt ion charge and spin states during bond breaking, DFT+U treatment of cobalt 3d orbitals, and the need for computing electrochemical potentials are emphasized. † MS 1415,
Shaping the Electrocatalytic Performance of Metal Complexes for CO 2 Reduction
ChemElectroChem, 2021
The mass scale catalytic transformation of carbon dioxide (CO2) into reduced forms of carbon is an imperative to address the ever-increasing anthropogenic emission. Understanding the mechanistic routes leading to the multi-electron-proton conversion of CO2 provides handles for chemists to overcome the kinetically and thermodynamically hard challenges and further optimize these processes. Through extensive electrochemical investigations, Prof. J-M. Savéant and coworkers have made accessible to chemists invaluable electro-analytical tools to address and position the electrocatalytic performance of molecular catalysts grounded on a theoretical basis. Furthermore, he has bequeathed lessons to future generations on ways to improve the catalytic activity and on the electrocatalytic zone we must target. As a tribute to his accomplishments, we recall here a few aspects on the tuning of iron porphyrin catalysts by playing on electronic effects, proton delivery, hydrogen bonding and electrostatic interactions and its implications to other catalytic systems.
The Journal of Physical Chemistry A, 1998
Several cobalt porphyrins (CoP) have been reduced by radiation chemical, photochemical, and electrochemical methods, in aqueous and organic solvents. In aqueous solutions, the Co I P state is stable at high pH but is shorter lived in neutral and acidic solutions. Stable Co I P is also observed in organic solvents and is unreactive toward CO 2. One-electron reduction of Co I P leads to formation of a species that is observed as a transient intermediate by pulse radiolysis in aqueous solutions and as a stable product following reduction by Na in tetrahydrofuran solutions. The spectrum of this species is not the characteristic spectrum of a metalloporphyrin π-radical anion and is ascribed to Co 0 P. This species binds and reduces CO 2. Catalytic formation of CO and HCO 2is confirmed by photochemical experiments in acetonitrile solutions containing triethylamine as a reductive quencher. Catalytic reduction of CO 2 is also confirmed by cyclic voltammetry in acetonitrile and butyronitrile solutions and is shown to occur at the potential at which Co I P is reduced to Co 0 P. As compared with CoTPP, fluorinated derivatives are reduced, and catalyze CO 2 reduction, at less negative potentials.