No abstract

No abstract

A series of dimeric metalloporphyrin molecules has been synthesized in which the two porphyrin rings are constrained to lie parallel to one another by two amide bridges of varying length that link the rings together. These cofacial metalloporphyrins have been applied to the surface of graphite electrodes and tested for catalytic activity toward the electroreduction of dioxygen to water in aqueous acidic electrolytes. All molecules tested exhibited some catalytic activity, but hydrogen peroxide rather than water was the chief reduction product. However, the dicobalt cofacial porphyrin linked by four-atom bridges produced a catalyzed reduction almost exclusively to water and at exceptionally positive potentials. Rotating ring-disk voltammetric measurements were employed to diagnose the electrode reaction pathway and a possible mechanism for the observed catalysis is suggested. The results seem to demonstrate the participation of two metal centers in controlling the course of a multiple-electron process.

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.

No abstract

Three series of cobalt(III) corroles were tested as catalysts for the electroreduction of dioxygen to water. One was a simple monocorrole represented as (Me(4)Ph(5)Cor)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)Co(2), 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)Co(2), and the biscorrole complexes, (BCY)Co(2), which contain two Co(III) ions in their air-stable forms, all provide a direct four-electron pathway for the reduction of O(2) to H(2)O 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 E(1/2) which was located at 0.47 V vs SCE for (PCA)Co(2) and 0.39 V vs SCE for (BCA)Co(2). The cobalt(III) monocorrole (Me(4)Ph(5)Cor)Co also catalyzes the electroreduction of dioxygen at E(1/2) = 0.38 V with the final products being an approximate 50% mixture of H(2)O(2) and H(2)O.

Dicobalt(II) cofacial bisporphyrins anchored by dibenzofuran (DPD) and xanthene (DPX) are efficient electrocatalysts for the four-electron reduction of oxygen to water despite their ca. 4 Å difference in metal-metal distances, suggesting that the considerable longitudinal 'Pac-Man' flexibility of the pillared platforms is the origin for the similar catalytic reactivity of these structurally disparate systems.Enzymatic systems are remarkable in their ability to accommodate the large range of motion required for the binding and catalysis of small molecules. In many cases, the kinetic steps of the processes involved are ultimately predicated on conformational changes of the active site upon substrate binding, activation, and/or product release. An outstanding example is the binding and biological reduction of dioxygen to water by cytochrome c oxidase (CcO). 1 The critical O-O bond cleavage chemistry is mediated by a flexible, dinuclear iron-heme/ copper (Fe a3 /Cu B ) assembly. 1,2 Nevertheless, the pursuit of structural and functional models for O 2 activation have emphasized, for the most part, bimetallic reaction centers poised within well-defined, rigid pockets. [3][4][5][6][7] For example, pillared cofacial dicobalt bisporphyrins bridged by anthracene (DPA) and biphenylene (DPB) 8-11 impair ring slippage, and as a result, these complexes efficiently electrocatalyze the direct fourelectron reduction of oxygen to water (as opposed to the twoelectron pathway involving peroxide) with little structural reorganization of juxtaposed subunits. Can efficient oxygenactivation chemistry be preserved when this cofacial structural motif exhibits a large range of motion? To address this issue, we have developed methods for the facile assembly of new cofacial bisporphyrins, incorporating dibenzofuran (DPD) 12 or xanthene (DPX) 13 pillars that exhibit variable pocket sizes with minimal lateral displacements. Herein, we report that dicobalt(II) complexes of both DPD and DPX efficiently mediate the direct four-electron reduction of oxygen to water despite a ca. 4 Å difference in their metal-metal distances (as determined from their X-ray crystal structures), suggesting that the longitudinal 'Pac-Man' flexibility of these molecular clefts allows the designed binding pocket to structurally accommodate reaction intermediates during multielectron catalysis.Co 2 (DPD) 1 was obtained in excellent yield (91%) from reaction of the corresponding free base bisporphyrin with CoCl 2 and 2,6-lutidine. † Crystals suitable for X-ray diffraction studies were grown from dichloromethane-methanol solutions. ‡ The structure of 1 (Fig. 1) shows that two methanol solvent molecules are coordinated inside the bisporphyrin pocket to the two cobalt(II) centers. In order to accommodate the two exogeneous ligands, the DPD framework opens its 'bite' considerably. The interplanar angle between the two macrocycles is 56.5°, resulting in metal-metal (8.624 Å) and centerto-center (8.874 Å) distances that are markedly larger than found in Zn 2 (DPD) (d Zn-Zn = ...

Four cobalt porphyrins were adsorbed on graphite electrodes and used to catalyze the electroreduction of O2. The two porphyrins without substituent groups in the meso positions of the porphyrin ring operated at the most positive potentials and catalyzed the reduction of O2 to both H2O2 and H2O, but the H2O did not result from significant reduction of H2O2. The porphyrins containing meso substituents catalyzed only the reduction of O2 to H2O2. The catalysts that accomplish the four-electron reduction of O2 are argued to consist of dimeric (or higher oligomeric) forms of the adsorbed porphyrins. The present results and those of two recent related studies 1,2 indicate that the presence of only hydrogen or small alkyl groups in the meso positions of porphyrin rings facilitates the spontaneous formation of van der Waals dimers with greater catalytic activity for the reduction of O2 by four electrons. Such cobalt porphyrins were also found to be unusually active catalysts for the electro-oxidation of H2O2.