Porphyrin complexes metal, reduction

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 reduction by these porphyrins. At present, it remains to be seen if the strategy of coordinating metal complexes on the periphery of a metalloporphyrin can be exploited in the rational design of new ORR catalysts. 

Studies on the electrocatalytic activity of metal porphyrins are limited in comparison with those on other classes of macrocyclic metal complex. Among the few porphyrin complexes tested, cobalt porphyrins have been demonstrated to be efficient electrocatalysts for the reduction of C02 to CO... 

The organometallic complexes with d-metals are considered as promising electrocatalysts for oxygen electroreduction in air-metal electrochemical cells. Obviously, the first idea was to employ the catalytic mechanism of the oxygen reduction with porphyrin-like metal complexes [1] found in living beings (Figure 1). 

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

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. 

Oxidation reactions are not limited to those that occur at a carbon centre. The perfluorinated Ni(F-acac)2-benzene-CgFi7Br system described above was also active for the oxidation of sulfides to sulfoxides and sulfones [28], A sacrificial aldehyde is required as co-reductant, but the reaction may be tuned by changing the quantity of this aldehyde. If 1.6 equivalents of aldehyde are used, the sulfoxide is obtained, whereas higher quantities (5 equivalents) lead to sulfones. Fluorous-soluble transition metal porphyrin complexes also catalyse the oxidation of sulfides in the presence of oxygen and 2,2-dimethylpropanal [29],... 

Fe(edta) is sensitive to O2 but is a useful gentle reductant which, for example, unlike many reductants, does not attack the porphyrin ring in metal-porphyrin complexes. For the reactions of Co(tpps) and Co(tap) + with (excess) Fe(edta) , predict the rate vs [Fe(edta) ] profile, and the effect of ionic strength on any second-order rate constants for the two reactions. 

It was reported that cobalt-tetraphenylporphyrin complex (CoTPP) coated on an electrode catalyzes electrocatalytic proton reduction,215 but the activity was not very high. We have found that metal porphyrins and metal phtahlocyanines when incorporated into a polymer membrane coated on an electrode show high activity in electrocatalytic proton reduction to produce H2.22,235 Some data are summarized in Table 19.2. It was shown that this catalyst is more active than a conventional platinum base electrode. 

The porphyrin complexes of ruthenium and osmium display a rich oxidation-reduction chemistry. Oxidation states +2, +3 +4, and + 6 are well documented. The scope of states that can be realised at the metal is restricted by the fact that the tetrapyrrole ligands (P)2 themselves can be oxidized or reduced to radicals (P )-1 or (P )-3, respectively, at potentials about + 0.7 or - 2.0 V. 

The CV curves obtained for carbons with preadsorbed copper shown in Figs. 45 (curves b, b, c, c ) and 46 (a-a")) exhibit only slight peaks of the Cu(II)/Cu(I) couple and broad waves due to the redox reaction of surface carbon functionalities (.see Section IV). However, preadsorbed copper enhances the peaks of the redox process in bulk solution (especially the anodic peaks for D—H and D—Ox samples), as can be seen in Fig. 46 (curves c-c"). The low electrochemical activity of samples with preadsorbed copper species observed in neutral solution is the result of partial desorption (ion exchange with Na ) of copper as well as the formation of an imperfect metalic layer (microcrystallites). Deactivation of the carbon electrode as a result of spontaneous reduction of metal ions (silver) was observed earlier [279,280]. The increase in anodic peaks for D—H and D—Ox modified samples with preadsorbed copper suggests that in spite of electrochemical inactivity, the surface copper species facilitate electron transfer reactions between the carbon electrode and the ionic form at the electrode-solution interface. The fact that the electrochemical activity of the D—N sample is lowest indicates the formation of strong complexes between ad.sorbed cations and surface nitrogen-containing functionalities (similar to porphyrin) [281]. Between —0.35 V and -1-0.80 V, copper (II) in the porphyrin complex (carbon electrode modifier) is not reduced, so there can be no reoxidation peak of copper (0) [281]. 

The alkylzirconium(m) octaethylporphyrin complex, (OEP)ZrCH2SiMe3 1, was prepared from the dialkylzirconium(rv) complex by reduction with H2 (1 atm) in toluene at 20 °C (Scheme 1). This reaction therefore appears to be a rather rare example of the chemical reduction of Zr(rv) to Zr(m) by H2. The structure of 1 was elucidated by single crystal X-ray diffraction and has a Zr-C bond length of 2.216(8) A. Although this complex formally contains zirconium in oxidation state hi, careful consideration of the structural and spectroscopic data led the authors to conclude that this was an overly simplistic view. At 77 K, an EPR signal typical of a metal-centered radical was observed, while no signal was detected at 293 K. The UV/Vis spectrum of 1 contains bands typical of a porphyrin anion. The electronic structure of 1 is therefore better described as a combination of two resonance forms a Zr(m) metal-based radical, and a zwitterionic form with a positively charged Zr(iv) center and a porphyrin radical anion. 

---