Cobalt porphyrins electrochemical reduction

D. Behar, T. Dhanasekaran, P. Neta, C.M. Hosten, D. Ejeh, P. Hambright, and E. Fujita, Cobalt Porphyrin Catalyzed Reduction of CO. Radiation Chemical, Photochemical, and Electrochemical Studies, J. Phys. Chem. A, 102 (1998) 2870. 

Behar D, Dhanasekaran T, Neta P, Hosten CM, Ejeh D, Hambright P, FujitaE (1998) Cobalt porphyrin catalyzed reduction of CO2. Radiation chemical, photochemical, and electrochemical... 

Cobalt porphyrin derivatives were also reported129 to be active for electrochemical reduction of C02 to formic acid at an amalgamated Pt electrode. More recently, Becker et al have reported130 that Ag2+ and Pd2+ metalloporphyrins acted as homogeneous catalysts for C02 reduction in dry CH2C12 oxalic acid and H2 (its source was not clear) were produced, but no CO was detected. 

Cobaloxime(I) generated by the electrochemical reductions of cobaloxime(III), the most simple model of vitamin Bi2, has been shown to catalyze radical cyclization of bromoacetals.307 Cobalt(I) species electrogenerated from [ConTPP] also catalyze the reductive cleavage of alkyl halides. This catalyst is much less stable than vitamin Bi2 derivatives.296 It has, however, been applied in the carboxylation of benzyl chloride and butyl halides with C02.308 Heterogeneous catalysis of organohalides reduction has also been studied at cobalt porphyrin-film modified electrodes,275,3 9-311 which have potential application in the electrochemical sensing of pollutants. 

In summary, the electrochemical results indicate that the alkyl-metal bond-formation free energies range from 54 to 146 kJ mol-1 for iron porphyrins and from 84 to 159 kJ mol-1 for cobalt porphyrins. The maximum bond energies are for primary alkyl groups bonded to [(MeO)4TPP]Con and (OEP)Fen porphyrins. The porphyrin dianions [(porT)nFe and (porr)nCo ] facilitate the reduction of C02 to CO via the transient formation of a metal-carbon bond [(por7)M—C(0)0- — AGBF > 50 kJ mol-1 for iron porphyrins]. Thus, iron and cobalt porphyrins are especially effective electrocatalysts for the reduction of C02 ... 

A related series of mixed-metal face to face porphyrin dimers (192) has been studied by Collman et al.506 A motivation for obtaining these species has been their potential use as redox catalysts for such reactions as the four-electron reduction of 02 to H20 via H202. It was hoped that the orientation of two cofacial metalloporphyrins in a manner which permits the concerted interaction of both metals with dioxygen may promote the above redox reaction. Such a result was obtained for the Co11 /Co" dimer which is an effective catalyst for the reduction of dioxygen electrochemic-ally.507 However for most of the mixed-metal dimers, including a Con/Mnn species, the second metal was found to be catalytically inert with the redox behaviour of the dimer being similar to that of the monomeric cobalt porphyrin. However the nature of the second metal ion has some influence on the potential at which the cobalt centre is reduced. 

Irradiation of iron and cobalt porphyrins (13 Fe(TPP), 14 Co(TPP), H2TPP = 5,10,15,20-tetraphenyl-21/f,23/f-porphyrin) in the presence of triethylamine (TEA) using > 320-nm tight caused the photocatalytic reduction of CO2 [15, 16, 27-31]. When 13 was used as a photocatalyst, CO was detected with TNco = 70 after 180-h irradiation [15]. Formic acid was the main product when 14 was employed as a photocatalyst [16]. The reaction mechanism proposed on the basis of UV-vis absorption changes during photolysis and radiolysis, and electrochemical measurements are shown in Scheme 3. M (TPP) is reduced to M (TPP) by photoinduced electron transfer from TEA, which subsequently disproportionates to M°(TPP), the proposed catalyticaUy-active species. 

Electrocatalytic reductive coupling of aryl chlorides to afford biphenyls can be accomplished with dichloro(l,2-bis(di-propylphosphino)benzene)nickel(II) in yields as high as 96% with 2 mol % of the catalyst in polar, coordinating solvents (55). Similar couplings can also be achieved with nickel-2,2 -bipyridyl and Pd(PPh3)2Cl2 as catalysts (56, 57). Indirect electrochemical reduction of vicinal dibromides to alkenes occurs efficiently with iron and cobalt porphyrins as mediators (58). Vitamin B12 is a mediator for the indirect electrochemical reduction of a-halo acids (59). 

With Co-facial cobalt porphyrins (Figure 2.5) adsorbed on pyrolytic graphite both cobalt centers are electrochemically active . However if one of the cobalt centers is replaced by iron, this metal shows no redox activity. This cofacial porphyrin with two different metal centers shows catalytic activity for the reduction of O2 and the foot of the reduction is observed at a potential where presumably the Fe(III)/(II) should appear. The fact that the Fe center is electrochemically silent is not clear . 

Okada, T., M. Gokita, M. Yusada, and I. Sekine (1999). Oxygen reduction characteristics of heat-treated catalysts based on cobalt-porphyrin ion complexes. J. Electrochem. Soc. 145, 815-822. 

Cheng, S.-H. and Y.O. Sn (1994). Electrocatalysis of nitric oxide reduction by water-soluble cobalt porphyrin. Spectral and electrochemical studies. Inorg. Chem. 33, 5847-5854. 

Magdesieva, T.V., T. Yamamoto, D.A. Tryk, and A. Fujishima (2002). Electrochemical reduction of CO2 with transition metal phthalocyanine and porphyrin complexes supported on activated carbon fibers. J. Electrochem. Soc. 149(6), D89-D95. Atoguchi, T., A. Aramata, A. Kazusaka, and M. Enyo (1991). Cobalt (II)-tetraphenylporphyrin-pyridine complex fixed on a glassy carbon electrode and its prominent catalytic activity for reduction of carbon dioxide. J. Chem. Soc. Chem. Commun. 13, 156-157. 

When Ag and Pd porphyrins were employed as catalysts for the electrochemical reduction of CO2 in dichloromethane, H2, and oxalic acid were the detected products, Table 7.4, the latter being a rare product. Qiu and Sawyer reported that alkalyated iron and cobalt porphrins dianioins electrocatalytically reduce carbon dioxide to give CO and... 

Demel, R. and A.M. Eleonore (1999). Catalytic electrochemical reduction of 1,2-dibromocyclohexane derivative at carbon electrodes modified with cobalt porphyrin siloxane films. Acta Chemica Scandinavica 53(11), 1038-1042. 

One important characteristic of cobalt porphyrins is their ability to bind or react with small molecules, such as NO [27, 67, 70, 91, 93, 100], CO [36, 114, 115], O2 [314-320], or CO2 [321], and several studies have focused on the chemical and/or electrochemical reactivity of (P)Co toward these small molecules. The interaction of cobalt porphyrins with NO and the electrochemical properties of the resulting cobalt-nitrosyl porphyrins have been investigated by several research groups [7]. (TPP)Co(NO) exhibits two oxidations and three reductions at a microelectrode in CH2CI2 [90]. The NO group remains coordinated after electrooxidation and the initial electron abstraction from (TPP)Co(NO) was proposed to involve the porphyrin jr-ring system. Other electrode reactions were accompanied by a dissociation of NO from the compound and the site of electron transfer could not be determined. 

Electrocatalytic oxidations (mainly epox-idation) of alkenes by manganese porphyrins [77, 78] and a Schiff-base [79] and iron and cobalt porphyrins [78] have been achieved. Hydrogen peroxide or the superoxide ion (O ) was generated electrochem-ically by reduction of dioxygen in solvents containing an acid or acid anhydride, the metal compounds as catalysts, and olefins as substrates, in the presence or absence of an axial base. The reaction was believed to take place through the formation of a high valent metal 0x0 porphyrin, produced... 

Modification of the electrode started with academic studies on physical and chemical adsorption, i.e., with the appearance of fundamental researches on adsorption of different species on electrode surfaces, both under polarization and at open circuit potential [3]. The properties of similar chemically modified electrodes , in which the modifier consists of a monolayer of a variety of chemical species with different characteristics, possessing (or not) particular properties, were initially studied in a purely electrochemical context, aimed at the collection of fundamental physico-chemical data. A small group of electrochemists were among those involved in these basic studies, envisioning the perspectives opened by the novel systems. In the first, really fascinating, work with similar monomolecular layers, cobalt porphyrin and phthalocyanine, as well as deliberately synthesized dicobalt face-to-face porphyrins were adsorbed on Pt or C surfaces to catalyze molecular oxygen reduction [4]. However, similar systems were not always used or adequately tested in proper amperometric sensing by researchers more interested in electroanalysis dicobalt face-to-face porphirins still constitute a rare example of tailored materials for selective amperometric detection. 

Kaplan A, Soifer L, Eliyahu D, Korin E, Bettelheim A. Electrocatalytic activity towards oxygen reduction of electropolymerized cobalt porphyrin doped with ionic-liquid-functionalized graphene. J Electrochem Soc 2015 162 H481-5. 

Jiang L, Cui L, He X. Cobalt-porphyrin noncovalently functionalized grapheme as nonprecious-metal electrocatalyst for oxygen reduction reaction in an alkaline medium. J Solid State Electrochem 2015 19 497-506. 

Cobalt and nickel porphyrins have also been used for catalyzing the chemical dechlorination with a reductant, namely titanium(III) citrate or nanoscale Zero Valent Iron (nZVI), of e.g. atrazine, (2-chloro-4-(ethylamine)-6-(isopropylamine)-s-triazine), a widely used herbicide which is a persistent groundwater contaminant [38]. Nickel 5,10,15,20-tetrakis(l -methyl-4-pyridinium)porphyrintetra(p-toluene-sulfonate) (TMPyP) was activated by nZVI, while cobalt porphyrins (TMPyP,5,10,15,20-tetrakis(4-hydroxyphenyl)-21 H,23H-porphine-(TP(OH)P) and 4,4, 4",4 -(porphine-5,10,15,20-tetrayl)tetrakis (benzenesulfonic acid)-(TBSP)) were activated by titanium(lll) citrate as the electron donor. All these processes probably could be more efficient using electrochemical methods. 

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