Hybrid bilayer membrane: a platform to study the role of proton flux on the efficiency of oxygen reduction by a molecular electrocatalyst.

In this report, we present a novel platform to study proton-coupled electron transfer (PCET) by controlling the proton flux using an electrode-supported hybrid bilayer membrane (HBM). Oxygen reduction by an iron porphyrin was used as a model PCET reaction. The proton flux was controlled by incorporating an aliphatic proton carrier, decanoic acid, into the lipid layer of the HBM. Using this system, we observed a different catalytic behavior than obtained by simply changing the pH of the solution in the absence of an HBM.

Ferrocene embedded in an electrode-supported hybrid lipid bilayer membrane: a model system for electrocatalysis in a biomimetic environment.

An electrode-supported system in which ferrocene molecules are embedded in a hybrid bilayer membrane (HBM) has been prepared and characterized. The redox properties of the ferrocene molecules were studied by varying the lipid and alkanethiol building blocks of the HBM. The midpoint potential and electron transfer rate of the embedded ferrocene were found to be dependent on the hydrophobic nature of the electrolyte and the distance at which the ferrocene was positioned in the HBM relative to the electrode and the solution. Additionally, the ability of the lipid-embedded ferrocenium ions to oxidize solution phase ascorbic acid was evaluated and found to be dependent on the nature of the counterion.

Role of a distal pocket in the catalytic O2 reduction by cytochrome c oxidase models immobilized on interdigitated array electrodes.

Five iron porphyrins with different superstructures were immobilized on self-assembled-monolayer (SAM)-coated interdigitated-array (IDAs) gold-platinum electrodes. The selectivity of the catalysts i.e., limited formation of partially reduced oxygen species (PROS) in the electrocatalytic reduction of dioxygen, is a function of 2 rates: (i) the rate of electron transfer from the electrode to the catalyst, which is controlled by the length, and conjugation of the linker from the catalyst to the electrode and (ii) the rate of bound oxygen (superoxide) hydrolysis, which correlates with the presence of a water cluster in the gas-binding pocket influencing the rate of oxygen binding; these factors are controlled by the nature of the porphyrin superstructure. The structurally biomimetic Tris-imidazole model is the most selective.

Selective anodic desorption for assembly of different thiol monolayers on the individual electrodes of an array.

The close proximity of two individually addressable electrodes in an interdigitated array provides a unique platform for electrochemical study of multicatalytic processes. Here, we report a "plug-and-play" approach to control the underlying self-assembled monolayer and the electroactive species on each individually addressable electrode of an interdigitated array. The method presented here uses selective anodic desorption of a monolayer from one of the individually addressable electrodes and rapid formation of a different self-assembled monolayer on the freshly cleaned electrode. We illustrate this strategy by introducing variations in the length of the linker to the electroactive species in the self-assembled monolayer, which determines the rate of electron transfer. In order to separate the assembly of the monolayer from the choice of the electroactive species, we use CuI-catalyzed triazole formation ("click" chemistry) to covalently attach an acetylene-terminated electroactive species to an azide-terminated thiol monolayer selectively on each electrode. The resulting variations in the electron-transfer rate to surface-attached ferrocene and in the rate of catalytic oxidation of ascorbate by the ferrocenium/ferrocene couple demonstrate an application of this approach.

Interaction of nitric oxide with a functional model of cytochrome c oxidase.

Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a(3) should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a(3), thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

A cytochrome C oxidase model catalyzes oxygen to water reduction under rate-limiting electron flux.

We studied the selectivity of a functional model of cytochrome c oxidase's active site that mimics the coordination environment and relative locations of Fe(a3), Cu(B), and Tyr(244). To control electron flux, we covalently attached this model and analogs lacking copper and phenol onto self-assembled monolayer-coated gold electrodes. When the electron transfer rate was made rate limiting, both copper and phenol were required to enhance selective reduction of oxygen to water. This finding supports the hypothesis that, during steady-state turnover, the primary role of these redox centers is to rapidly provide all the electrons needed to reduce oxygen by four electrons, thus preventing the release of toxic partially reduced oxygen species.

Synthesis and characterization of Rh(III) corroles: unusual reactivity patterns observed during metalation reactions.

A new approach to the synthesis of Rh(III) corrole complexes is developed and an unusual activation of C-C and C-N bonds is disclosed.

Radical autoxidation and autogenous O2 evolution in manganese-porphyrin catalyzed alkane oxidations with chlorite.

A manganese porphyrin catalyst employing chlorite (ClO(2)(-)) as a "shunt" oxidant displays remarkable activity in alkane oxidation, oxidizing cyclohexane to cyclohexanol and cyclohexanone with >800 turnover numbers. The ketone is apparently formed without the intermediacy of alcohol and accounts for an unusually large fraction of the product ( approximately 40%). Radical scavenging experiments indicate that the alkane oxidation mechanism involves both carbon-centered and oxygen-centered radicals. The carbon-radical trap CBrCl(3) completely suppresses cyclohexanone formation and reduces cyclohexanol turnovers, while the oxygen-radical trap Ph(2)NH inhibits all oxidation until it is consumed. These observations are indicative of an autoxidation mechanism, a scenario further supported by TEMPO inhibition and (18)O(2) incorporation into products. However, similar cyclohexane oxidation activity occurs when air is excluded. This is explained by mass spectrometric and volumetric measurements showing catalyst-dependent O(2) evolution from the reaction mixture. The catalytic disproportionation of ClO(2)(-) into Cl(-) and O(2) provides sufficient O(2) to support an autoxidation mechanism. A two-path oxidation scheme is proposed to explain all of the experimental observations. The first pathway involves manganese-porphyrin catalyzed decomposition of ClO(2)(-) into both O(2) and an unidentified radical initiator, leading to classical autoxidation chemistry providing equal amounts of cyclohexanol and cyclohexanone. The second pathway is a "rebound" oxygenation involving a high-valent manganese-oxo intermediate, accounting for the excess of alcohol over ketone. This system highlights the importance of mechanistic studies in catalytic oxidations with highly reactive oxidants, and it is unusual in its ability to sustain autoxidation even under apparent exclusion of O(2).

Catalytic Activation of H(2) and C-H Bonds by Electron-Deficient Ruthenium(II) Porphyrins.

The Ru(2) and RuNi derivatives of 1,8-bis(10,15,20-trimesityl-5-porphyrinato)anthracene-a recently reported cofacial diporphyrin ligand comprising two hindered porphyrins spanned by an anthracene bridge-have been synthesized. Both Ru(2)(DPAHM) and RuNi(DPAHM) are extremely reactive species that apparently contain 14-electron Ru(II) centers and, as is the case for their monoporphyrin analog, (5,10,15,20-tetramesitylporphyrinato)ruthenium [Ru(TMP)], must be rigorously protected from oxygen, nitrogen, and other ligating agents. In addition, these electron-deficient Ru(II) porphyrins all appear to bind aromatic solvents such as benzene and toluene, the weakest ligating solvents in which these Ru(II) porphyrins have been found soluble. Ru(TMP) and its metallodiporphyrin analogs, Ru(2)(DPAHM) and RuNi(DPAHM), catalyze H(2)/D(2) exchange in benzene solution and as solids. When adsorbed on a particularly nonpolar carbon support, these Ru(II) porphyrins all manifest significant activity with respect to catalytic H(2)/D(2) exchange [approximately 40 turnovers s(-)(1), when normalized for Ru(II) content]. In addition, these molecules slowly catalyze the exchange of H(2) into deuterated aromatic hydrocarbons and, in the absence of solvent, the exchange of D(2) into CH(4). Kinetic studies of H(2)/D(2) exchange catalyzed by these Ru(II) porphyrins on carbon supports indicate that exchange is likely to be effected by one face of a single Ru(TMP) moiety. The activity of each supported catalyst was suppressed by the presence of ligands, either exogenous (CO irreversibly and N(2) reversibly) or from polar functionalities on the surface of the supporting matrix.