Metalloporphyrins electrochemical reduction

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. 

The limited extent of charge transfer interaction between the exciplex components in the nonpolar benzene solutions is Indicated by plots of the logarithm of kq, the observed rate constant for the quenching of the metalloporphyrin triplet, versus the quencher electrochemical reduction potential. Although the slopes of such plots were linear for the nitroaromatics and for the chloro compounds, the slopes differed for the two groups of... 

Di(porphyrinato)rhodium(II) [Rh(Por)]2 described by OgoshP Wayland , Coil-man, and Kadish are the only known metalloporphyrin dimers containing one single metal-metal bond. Oxidative cleavage (Eq. 24) or thermal homolytic cleavage (Eq. 25) of the Rh-H bond of Rh(OEP)H can produce the [Rh(OEP)]2 dimer The Rh" dimer may also be formed by electrochemical reduction of some Rh" porphyrin complexes (See Sect. D III 4). 

Sawaguchi, T., T. Itabashi, T. Matsue, and I. Uchida (1990). Electrochemical reduction of oxygen by metalloporphyrin ion-complexes with heat-treatment. J. Elec-troanal. Chem. 279, 219-230. 

Akiba investigated the electrochemical behavior of a variety of phosphorus octaethylporphyrin derivatives all compounds showing a single reversible oxidation wave [91]. The absolute difference in potential between the first ring-centered oxidation and reduction varies from 2.19 to 2.36 V in dichloromethane. These values are within the range of the HOMO-LUMO gap observed for most metalloporphyrins. 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

In the most important series of polymers of this type, the metallotetraphenylporphyrins, a metalloporphyrin ring bears four substituted phenylene groups X, as is shown in 7.19. The metals M in the structure are typically iron, cobalt, or nickel cations, and the substituents on the phenylene groups include -NH2, -NR2, and -OH. These polymers are generally insoluble. Some have been prepared by electro-oxidative polymerizations in the form of electroactive films on electrode surfaces.79 The cobalt-metallated polymer is of particular interest since it is an electrocatalyst for the reduction of dioxygen. Films of poly(trisbipyridine)-metal complexes also have interesting electrochemical properties, in particular electrochromism and electrical conductivity.78 The closely related polymer, poly(2-vinylpyridine), also forms metal complexes, for example with copper(II) chloride.80... 

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. 

E25.17 Electrocatalysts are compounds that are capable of reducing the kinetic barrier for electrochemical reactions (barrier known as overpotential). While platinum is the most efficient electrocatalyst for accelerating oxygen reduction at the fuel cell cathode, it is expensive (recall Section 25.18 Electrocatalysis). Current research is focused on the efficiency of a platinum monolayer by placing it on a stable metal or alloy clusters your book mentions the use of the alloy PtsN. An example would be a platinum monolayer fuel-cell anode electrocatalyst, which consists of ruthenium nanoparticles with a sub-monolayer of platinum. Other areas of research include using tethered metalloporphyrin complexes for oxygen activation and subsequent reduction. 

SnIv. These metalloporphyrins are extremely acid-stable and produce a normal electronic spectrum [Snlv OEP (OAc)2 576 (19,100) 538 (19,100) 411 (403,000)] [Parnemann (139)]. The electrochemical redox pattern probably does not involve a SnIV Sn11 reduction. It follows that the porphyrin ligand is reduced, not the SnIV central ion [Fuhrhop (74)]. 

Figure 6.4.3 Plot of oxidation and reduction potentials of various metalloporphyri-nates. Metalloporphyrins that easily form cation radicals with weak oxidants cannot be chemically reduced (e.g., the Zn- and Mg-porphyrinates). Metalloporphyrins that easily form anion radicals upon reduction [e.g., Sn(IV)], on the other hand, cannot be chemically oxidized. Only electrochemical reactions are possible at potentials far above 1 V or far below -1 V...

Richoux, M. C., Neta, P, (1986). One- and two-electron reduction of metalloporphyrins, radiation chemical, photochemical, and electrochemical studies. Kinetics of the decay of 7C-radical anions, J. Am. Chem. Soc., 90 2462. 

The interest in these highly reduced states stems from their potential use as catalysts for the electrochemical or photochemical reduction of COj to The mechanism involves binding of COj as an axial ligand to the reduced metalloporphyrin followed by two-electron transfer. Fe and Co porphyrins do not react with CO2 but when they are reduced to the M° oxidation state they react rapidly. 

M.C. Richoux, P. Neta, A. Harriman, S. Baral, and P. Hambright, One- and Two-Electron Reduction of Metalloporphyrins. Radiation Chemical, Photochemical, and Electrochemical Studies. Kinetics of the Decay of x-Radical Anions, J. Phys. Chem., 90 (1986) 2462. 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

Currently available amperometric and voltammetric porphyrinic sensors for detection of electroactive analytes are based on their electrochemical oxidation or reduction on polymeric conductive films of metalloporphyrins. If the current generated during the process is linearly proportional to the concentration of an andyte, the current can be used as an analytic signal. This current can be measured in either the... 

The electrochemistry of [Th(Por)(OH)2]3 (Por=OEP,TPP) is of particular interest as they contain three redox active metalloporphyrin units (Kadish et al. 1988). The cyclic voltammogram of the OEP complex recorded in THE at -72 C shows three reversible one-electron reduction couples at -1.49, -1.70, and -1.87 V vs. SCE. As the temperature rises to room temperature, the third reduction becomes irreversible, and it has been shown that it involves a one-electron transfer followed by a fast chemical reaction (probably dissociation) and an additional one or more electron reduction (an electrochemical ECE-type mechanism). The UV-Vis spectrum of the electroreduced species [Th(OEP)(OH)2]3 shows absorption bands at 411, 456, and 799 nm, and its ESR spectrum displays a signal at g=2.003, both of which are characteristic of a porphyrin ti radical anion. Further one-electron reduction doubles the molar absorptivities of the absorption bands at 456 and 799 nm, indicating that the second reduction is also based on porphyrin. The TPP analog [Th(TPP)(OH)2]3 also exhibits three reversible one-electron reductions at -1.13, -1.27, and -1.36V at -55 C, which are shifted by 360-510mV relative to the respective processes for [Th(OEP)(OH)2]3 at —72 C. Three additional irreversible reductions at —1.76, -2.00, and —2.10 V are also observed for this complex when the potential is scanned to -2.20 V which may be due to the formation of dianions localized on each of the three porphyrin units. Spectroelectrochemical data also indicate that the initial three reductions occur at porphyrin based orbitals. 

Okunola AO, Nagaiah TC, Chen X, Eckhard K, Schuhmaim BM (2009) Visualization of local electrocatalytic activity of metalloporphyrins towards oxygen reduction by means of redox competition scanning electrochemical microscopy (RC-SECM). Electrochim Acta 54(22) 4971 978... 

For example, <7-bonded porphyrins with Rh, P, As, or Sb central ions undergo reversible oxidations and reductions at the 7T-conjugated macrocycle and there is little effect of the cr-bonded axial ligand on the porphyrin electrochemical behavior [12, 21]. However, derivatives with Al, Ga, In or T1 central metals all undergo a rapid cleavage of the metal-carbon bond after a one-electron oxidation of the compound. In contrast, the Fe, Ru, and Co cr-bonded metalloporphyrins (see further sections of the review) will undergo a metal-centered... 

Porphyrin-nitrosyl complexes with six other metal ions are also known, and all but one of which has been electrochemically investigated. These are Ru [69, 73, 94-96], Os [5], Rh[97], Cr [98], Mo [99] and Mn [100]. Some nitrosyl metalloporphyrins can be reversibly reduced or oxidized by one or two electrons without loss of the NO ligand and this generally occurs when the electrode reactions involve the 7T-conjugated macrocycle in the case of a metal-centered reduction or oxidation, however, the electron-transfer reactions will most often be accompanied by a loss of the NO ligand, resulting in an irreversible oxidation as shown in Fig. 7 for the case of (TPP)Cr(NO) and (TPP)Mn(NO) in CH2CI2. 

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