Electrochemical reduction porphyrins

The electronic absorption spectra of the products of one-electron electrochemical reduction of the iron(III) phenyl porphyrin complexes have characteristics of both iron(II) porphyrin and iron(III) porphyrin radical anion species, and an electronic structure involving both re.sonance forms Fe"(Por)Ph] and tFe "(Por—)Ph has been propo.sed. Chemical reduction of Fe(TPP)R to the iron(II) anion Fe(TPP)R) (R = Et or /7-Pr) was achieved using Li BHEt3 or K(BH(i-Bu)3 as the reductant in benzene/THF solution at room temperature in the dark. The resonances of the -propyl group in the F NMR spectrum of Fe(TPP)(rt-Pr) appear in the upfield positions (—0.5 to —6.0 ppm) expected for a diamagnetic porphyrin complex. This contrasts with the paramagnetic, 5 = 2 spin state observed... 

Iron porphyrins (containing TPP, picket fence porphyrin, or a basket handle porphyrin) catalyzed the electrochemical reduction of CO2 to CO at the Fe(I)/Fe(0) wave in DMF, although the catalyst was destroyed after a few cycles. Addition of a Lewis acid, for example Mg , dramatically improved the rate, the production of CO, and the stability of the catalyst. The mechanism was proposed to proceed by reaction of the reduced iron porphyrin Fe(Por)] with COi to form a carbene-type intermediate [Fe(Por)=C(0 )2, in which the presence of the Lewis acid facilitates C—O bond breaking. " The addition of a Bronsted acid (CF3CH2OH, n-PrOH or 2-pyrrolidone) also results in improved catalyst efficiency and lifetime, with turnover numbers up to. 750 per hour observed. ... 

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. 

FIGURE 4.3. Redox and chemical homogeneous catalysis of trans-1,2 dibromocyclohexane. a cyclic voltammetry in DMF of the direct electrochemical reduction at a glassy carbon electrode (top), of redox catalysis by fhiorenone (middle), of chemical catalysis by an iron(I) porphyrin, b catalysis rate constant as a function of the standard potential of the catalyst couple aromatic anion radicals, Fe(I), a Fe(0), Co(I), Ni(I) porphyrins. Adapted from Figures 3 and 4 of reference lb, with permission from the American Chemical Society. 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

The electrochemical reductions of oxo-, peroxo-, jj -azohenzene, and phenylimido Ti(IV) porphyrin complexes have been described, and the mechanism of the two-electron reduction of the peroxo-species to the 0X0 (compound) established [25, 26],... 

A famous use of electrochemical reduction of porphyrin systems is that used to convert a chlorophyll a derivative, chlorin-e6 trimethyl ester (64), into a chlorophyll b derivative, rhodin-g7 trimethyl ester (65) (71LA(749)109). Electrolysis of chlorin-e6 trimethyl ester gave the chlorin-phlorin which was photolyzed in dioxane/water to give the trans-d o (Scheme 7) this was simply transformed into rhodin-g7 trimethyl ester (65) to accomplish the first ever interconversion of a chlorophyll a series pigment into one of the companion b series. 

Busch et al.102 have reported the electrochemical reduction of complexes of tetra-aza[16]annulene (20), to the porphyrin-like dianion. The Con(taab)2+ ion can be... 

Electrochemical reduction of Ni(taab)2+ [taab = (20), see p. 231] occurs in two one-electron steps, to complexes formulated as [Nim(taab)] + and [Niu(taab)]°. The relationship between the annulene taab and the two-electron reduction product, the porphyrin-like taab2-, is discussed.102 The preparation of macrocycles of type (87) by a template synthesis requires a minimum ring size of x = y = 3 and depends upon the strong complexing of the metal ion at the pH of the reaction, otherwise the metal hydroxide precipitates. The NiL(C104)2,nH20 (n = 0, 1, or 2) species have been prepared for x = 3, y = 4.479... 

J. P. Collman, et al, Angew. Chem. Int. Ed. Engl. 1994,33, 1537 (Reduction of protons and oxidation by hydrogen, electrochemically by porphyrins reduction of N2 and catalysis). 

As potential models for the assimilatory nitrite reductases, electrochemical reduction of nitrosyliron(II) porphyrins (see Nitrosyl Complexes), PFeNO, yields first the Fe -NO complex, followed by two additional one-electron reductions. In the presence of phenols, the product of electrolysis is hydroxylamiue. " ... 

Bouwkamp-Wijnoltz, A.L. et ah. On active-site heterogeneity in pyrolyzed carbon-sup-ported iron porphyrin catalysts for the electrochemical reduction of oxygen an in situ Mdssbauer study, J. Phys. Chem. B, 106, 12993, 2002. 

Electrochemical reduction of 7 iron porphyrin dioxygen complexes leads to a species identified as [Fe(III)(porph)02 ] This species has also been reported to be formed on addition of superoxide to [Fe(porph)] ... 

The first Vm porphyrin species was isolated by electrochemical reduction of the corresponding non-oxo-Vlv porphyrin802 and shortly thereafter the first structure of a Vra porphyrin was reported (192).803 These studies were supplemented by the structural characterization of a related Vni phthalocyanine complex.804 Bis(pyridyl)ketone undergoes reductive coupling in the presence... 

For the first reduction the IR shifts point to a porphyrin-centred electron transfer. This is supported by further spectroscopy on the anion radical complexes [(Por)Ru(CO)(L)]. The observed EPR lines are narrow, unstructured, with g values around 2. The UV-Vis-NIR spectra of the radical anions are characterised by redshifted Soret bands of reduced intensity, a weak structured band system around 600 nm and weak broad absorptions around 800 or 900 nm (see Figure 4.15). Further support comes from resonance Raman investigations on [(OEP)Ru(CO)(THF)] for which the observed Raman bands fit perfectly to those of the [(OEP)VO] radical anion. There is some evidence that if the spectroelectrochemistry is not carried out in very aprotic and unpolar solvents or traces of water are present, the radical anionic complexes are readily transformed. This has been investigated for the [(OEP)Ru(CO)(L)] system, where the use of solvents like MeOH or nitriles for the electrochemical reduction leads to altered species with unreduced porphyrin ligands (see Figure 4.15)." ... 

On the electrode surface of CO formation metals Au, Ag, and Zn, CO2 is stabilized by adsorption both in aqueous and nonaqueous electrolytes. In nonaqueous media, a CO2 molecule reacts as a Lewis acid with adsorbed CO2 , and allows a C-O bond of the CO2 to be broken. This process forms CO(ad) and COs as postulated by Saveant and his coworkers for electrochemical reduction of CO2 catalysed by iron porphyrins. CO(ad) thus formed is easily desorbed, as shown in Fig. 11(2). 

Hammouche M, Lexa D, Momenteau M, Saveant JM (1991) Chemical catalysis of electrochemical reactions—homogeneous catalysis of the electrochemical reduction of carbon dioxide by iron(O) porphyrins—role of the addition of magnesium cations. J Am Chem Soc... 

Electrochemical reductions of the central MnUI ion have been performed with many different porphyrin and axial ligands. Some selected values are given in Table 4. The order of the for different macro-cyclic ligands is Phth [Pg.25]

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