Y.Oliver Su

Electrocatalytic reduction of nitric oxide by water-soluble manganese porphyrins

Abstract Manganese porphyrins electrochemically catalyze the reduction of nitric oxide to hydroxylamine and ammonia in aqueous media. The reactions occurs via an ECE pathway. Demetallation and porphine ring reduction are the main factors of deactivation.

Electrochemical characterization and electrocatalysis of high valent manganese meso-tetrakis(N-methyl-2-pyridyl)porphyrin

Abstract Electrochemical oxidation of water-soluble manganese(III) meso -tetrakis( N -methyl-2-pyridyl)porphyrin (Mn III (2-TMPyP)) generates stable Mn IV and Mn V porphyrins. Speciation of various oxidation states of the porphyrin are characterized by spectroelectrochemical methods. The acid dissociation constants (p K a s) for Mn III (2-TMPyP)(H 2 O) 2 are 9.6 and 10.7, respectively. Spectroelectrochemical results of the one-electron oxidation of Mn III (2-TMPyP) exhibit different forms of oxomanganese(IV) porphyrin, depending on the pH of the solution and the applied potential. The p K a for OMn IV (2-TMPyP)(H 2 O) is 10.5. The axial oxygen atom ligated to the Mn(IV) center is protonated in acidic solution (p K a 3.4). Further one-electron oxidation generates dioxomanganese(V) porphyrin, (O) 2 Mn V (2-TMPyP), which is stable in alkaline solution at room temperature. No oxidation wave is observed in the cyclic voltammograms, indicating the slow heterogeneous electron transfer rate of these oxidation reactions. The electrogenerated dioxomanganese(V) porphyrin exhibits higher reactivity toward olefin oxidation than oxomanganese(IV) porphyrin in basic solutions.

Selective electrocatalysis of alkene oxidations in aqueous media. Electrochemical and spectral characterization of oxo-ferryl porphyrin, oxo-ferryl porphyrin radical cation and their reaction products with alkenes at room temperature

Iron(III) meso-tetrakis(3-sulfonatomesityl)porphyrin (FeIIITSMP) contains sterically hindered substituents at the periphery and is water-soluble. The oxidation potential and oxidized forms for FeTSMP vary with pH. In pH<4.0 solution, a one-electron porphine ring oxidation, which is pH independent, occurs first and is followed by a two-electron oxidation, which is pH dependent. The two-electron oxidation becomes two one-electron reactions as pH increases and one shifts to less positive potential to merge with the porphine ring oxidation. In 4.0<pH<6.0, consequently, the first oxidation is a two-electron process and the second oxidation is a one-electron step. Stable high-valent oxo-ferryl porphyrin radical cation was electrochemically generated in aqueous media at room temperature in this pH range. The oxidation pattern then becomes three one-electron steps as pH is higher than 8.0. The high-valent iron porphyrins exhibit different activities towards alkene oxidations in aqueous media. FeIIITSMP+ does not react with cyclopent-2-ene-1-acetic acid (1). OFeIVTSMP slowly reacts with (1) to form cyclopent-2-ene-4-one-1-acetic acid (2) in the presence of dioxygen while OFeIVTSMP+ reacts rapidly with (1) to form cyclopent-2,3-diol-1-acetic acid (3) in the absence of dioxygen. OFeIVTSMP+ reacts with FeIIITSMP conproportionally to form OFeIVTSMP, which in turn reacts with (1) to give (2).

Electrogenerated chemiluminescence of sterically hindered porphyrins in aqueous media

Abstract A sterically hindered water-soluble porphyrin, tetrakis(3-sulfonatomesityl)porphyrin (H2TSMP), could form stable radical cation in aqueous media after electrochemical one electron oxidation. The anodic oxidation of H2TSMP in the presence of tripropylamine or C2O42− as a coreactant in aqueous solution produces electrogenerated chemiluminescence (ECL) with maxima at 640 and 700 nm. The same emission spectrum of ECL and fluorescence indicates that the ECL emission is from the singlet state of H2TSMP. The annihilation reactions of ZnTSMP +and ZnTSMP −, which are generated electrochemically, in CH3CN+H2O (1:1) mixed solution results in an emission which is identical to the photoluminescence. Protection of the active sites against the nucleophilic attack of water or OH− by sterical hinderance is a successful strategy in designing new ECL-active compounds in aqueous media. Both ECL reaction mechanisms are proposed.

Electrocatalytic oxidation of styrene by a high valent ruthenium porphyrin cation radical

Abstract A sterically-hindered carbonylruthenium(II) porphyrin Ru II (CO)(TMP) (where TMP= meso -tetramesitylporphyrinato dianion) has been synthesized. Chemical oxidation of Ru II (CO)(TMP) by m -chloroperbenzoic acid ( m -CPBA) gives the dioxoruthenium(VI) porphyrin (Ru VI (O) 2 TMP). Cyclic voltammograms show that Ru VI (O) 2 TMP is reversibly oxidized at E 1/2 =+1.24 V in CH 2 Cl 2 . Thin-layer absorption spectra for oxidation of Ru VI (O) 2 TMP at +1.32 V indicates that the product is a porphyrin cation radical (Ru VI (O) 2 TMP + ). Electrogenerated Ru VI (O) 2 TMP + reacts selectively with styrene to give phenylacetaldehyde (96%) and benzaldehyde (4%). We report the first case of styrene oxidation by high valent ruthenium porphyrin under electrochemical conditions. An electrocatalytic oxidation reaction scheme is proposed.

Electrocatalytic oxygenation of alkenes by a sterically hindered water-soluble iron porphyrin in aqueous solutions

Iron(III) tetrakis(N-methyl-2-pyridyl)porphyrin (FeIII(2-TMPyP)) undergoes one-electron electro-oxidation to form O=FeIV(2-TMPyP) and O=FeIV(2-TMPyP)(OH) in pH 9.0 and 14.0 solutions respectively. A series of water-soluble carboxylic alkenes was used as substrates to investigate the catalytic reactivity and selectivity of the iron porphyrin. Both oxo-ferryl porphyrins oxidized alkenes via a pathway of allylic hydrogen abstraction. Molecular oxygen is also involved and the final product was identified to be an ene-one. The spectroelectrochemical method has indicated that the axial hydroxide ligand weakens the oxo-ferryl bond and facilitates the chemical reaction.

Spectral and electrocatalytic studies for unusual oxidation and reduction states of chromium porphyrins in aqueous solutions

Abstract Chromium(III) meso -tetrakis(3-sulfonatomesityl)porphyrin (Cr III TSMP) in aqueous solution exhibits two p K a s at 8.5 and 11.6, respectively. The redox reactions depend on the pH in the solution. In acidic solutions, the first oxidation at +0.875 V (versus Ag|AgCl) is a one-electron step corresponding to the porphyrin ring cation radical formation. The first two oxidations are separated in pH 9.0 solution. A possible internal electron transfer between the metal center and the porphyrin ring occurs in the pH 9.4–12.5 region. OCr IV TSMP is not reactive towards alkene oxidation while OCr IV TSMP +⋅ reacts with alkene to give enone at room temperature. The spectroelectrochemical results have been confirmed by chemical methods. The electrochemical reduction occurs at the porphyrin ring first and the second reduction is metal centered.

Electrocatalytic oxidation of alkenes by water-soluble manganese porphyrins in aqueous media: a comparison of the reaction products at different oxidation states

Manganese(III) tetrakis(N-methyl-4-pyridyl)porphine (MnTMPyP) is water-soluble and exhibits a reversible MnIII/II redox couple in acidic solutions. However, the MnIV/III couple cannot be seen by cyclic voltammetry in aqueous solutions of pH < 8. Spectroelectrochemical methods have shown that (H2O)MnIIITMPyP undergoes a one-electron oxidation with slow heterogeneous electron transfer rate to form OMnIITMPyP, which then oxidizes cyclopent-2-ene-1-acetic acid (1) catalytically to give cyclopent-2-ene-4-one-1-acetic acid (2). Further electrochemical oxidation of OMnIVTMPyP causes the deactivation of the catalyst, presumably due to porphine ring degradation. A sterically hindered porphyrin, manganese(III) tetrakis(sulfonatomesityl)porphine (MnTSMP), is first oxidized at the morphine ring and is stable as a radical cation (H2O)MnIIITSMP+. in pH < 2 solutions. In pH 8.5 buffer solutions, MnIIITSMP is oxidized totally by two electrons at Eappl = + 1.05 V. The electrogenerated OMnIVTSMP+. reacts rapidly with 1 to give 2 and cyclopent-2,3-diol-1-acetic acid (3). The reaction mechanisms are proposed.

Electrochemical and spectral characterization of stable iron(IV) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin in aqueous solution at room temperature

Stable iron(IV) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin has been generated electrochemically and characterized in aqueous solution at room temperature for the first time using the optically transparent thin-layer electrochemical method.