Phthalocyanines transition metal macrocyclic

A mild triple catalytic system consisting of Pd(OAc)2, hydroquinone, and a transition metal macrocycle (for example, iron phthalocyanine) was reported [243]. The catalytic effect is carried out by the interaction of Pd(II) with the substrate and the acquisition of two electrons, which are further transferred to the benzoquinone that is reduced to hydroquinone. The hydroquinone is then reorganized to benzoquinone by the 02/metal macrocycle system. The following types of transformations were carried out in mild conditions using the developed system 1,4-oxidation of conjugated dienes, oxidation of terminal olefins to methyl ketones, and allylic oxidation. 

Their calculations demonstrated that dioxygen-binding abilities of the transition metal macrocyclic complexes are determined by central metal, ligand, and substituents. For cobalt phthalocyanine systems, electron-donating substituents increase... 

The first-principles DFT calculations of ORR on various M-N4 macrocyclic complexes have been carried out by several research groups [93, 168-173]. Through comparative study of O2 dissociation on different metalloporphyrins (MnP, FeP, CoP, NiP), the trends of the activation barriers for the O2 dissociation with respect to LUMO-HOMO characters of these metalloporphyrins have been discussed by Tsuda et al. [172]. FeP is demonstrated to be the best one due to the large d electrons contribution to the LUMO-HOMO level of the FeP and the stable Fe-O bond [172]. Shi and Zhang performed the DFT calculation on the O2 adsorption on various iron and cobalt porphyrins and phthalocyanines [171]. The catalytic activities of the transition metal macrocyclic complexes were positively related with... 

Theoretical approaches have in addition predicted that the substituents on the macrocyclic rings can also affect ORR catalytic activity. Co-phthalocyanine complexes with electron donating substituents should show improved ORR catalytic activity because the substituents can increase flie binding energy between O2 and the metal center(s) [55]. Calculation indicates fliat the catalytic activity of the transition metal macrocyclic complexes is due to flie partial electron transition between the filled d, dy, and empty r/ 2 orbitals of the transition metals, and the... 

Thermal treatment might lead to phase ehange in the transition metal macrocyclic complexes. Using XRD measurements, Baranton [58] found that received iron phthalocyanine was under a phase and that after being heat-treated at 450 °C it was under p phase. 

In 1964, Jasinski reported his pioneering work on using cobalt phthalocyanine, adsorbed on carbon and nickel eleetrodes, as a promising catalyst for the reduction of oxygen [8]. Following Jasinski s work, many other transition metal macrocyclic N4-complexes, including porphyrins, phthalocyanines, and tetraazannulenes, were also explored. The transition metals evaluated inelude Mn, Ru, Pd, Pt, Ir, Cr, Ni, Cu, Zn, Mo, Al, Sn, Sb, Ga, Na, Ag, vanadyl ion, as well as uranyl ion. All of these compounds show a certain level of eleetroeatalytie aetivity towards the ORR [6, 9]. 

This class of PC catalysts has also been extensively studied as a potential substitute for Pt as they are low cost, methanol tolerant and have reasonably good activity and remarkable selectivity toward ORR [194]. They normally catalyze a direct 4e reduction of O2 to )deld water. The major drawback of this kind of catalyst is of low stability in acidic media [195]. However, when the catalyst is heat treated, the activity and stability of transition metal macrocycle complex (TMMC) are improved significantly [194]. The molecules of TMMC have a square planar structure with the metal ion symmetrically surrounded by four nitrogen atoms these nitrogen atoms are from each member of the ring systems which, in turn, are connected by carbon atoms (porphyrins) or nitrogen atoms (phthalocyanines). 

A number of transition metal macrocycles have been shown to promote the rates of oxygen reduction when adsorbed on a variety of carbon surfaces. Attention has been mainly focused on phthalocyanines and porphyrins containing iron and cobalt centers, as their activity in certain cases has been found to be comparable to that of platinum. Essential to the understanding of the mechanism by which these compounds catalyze the reduction of O2 is the description of the interactions, not only with the reactant, but also with the substrate. In situ techniques can provide much of this needed information, and indeed a number of such methods have been used in connection with this type of system. One of the first illustrations of the use of Mossbauer... 

Perspectives for fabrication of improved oxygen electrodes at a low cost have been offered by non-noble, transition metal catalysts, although their intrinsic catalytic activity and stability are lower in comparison with those of Pt and Pt-alloys. The vast majority of these materials comprise (1) macrocyclic metal transition complexes of the N4-type having Fe or Co as the central metal ion, i.e., porphyrins, phthalocyanines, and tetraazaannulenes [6-8] (2) transition metal carbides, nitrides, and oxides (e.g., FeCjc, TaOjcNy, MnOx) and (3) transition metal chalcogenide cluster compounds based on Chevrel phases, and Ru-based cluster/amorphous systems that contain chalcogen elements, mostly selenium. 

The simple porphyrin category includes macrocycles that are accessible synthetically in one or few steps and are often available commercially. In such metallopor-phyrins, one or both axial coordinahon sites of the metal are occupied by ligands whose identity is often unknown and cannot be controlled, which complicates mechanistic interpretation of the electrocatalytic results. Metal complexes of simple porphyrins and porphyrinoids (phthalocyanines, corroles, etc.) have been studied extensively as electrocatalysts for the ORR since the inihal report by Jasinsky on catalysis of O2 reduction in 25% KOH by Co phthalocyanine [Jasinsky, 1964]. Complexes of all hrst-row transition metals and many from the second and third rows have been examined for ORR catalysis. Of aU simple metalloporphyrins, Ir(OEP) (OEP = octaethylporphyrin Fig. 18.9) appears to be the best catalyst, but it has been little studied and its catalytic behavior appears to be quite distinct from that other metaUoporphyrins [CoUman et al., 1994]. Among the first-row transition metals, Fe and Co porphyrins appear to be most active, followed by Mn [Deronzier and Moutet, 2003] and Cr. Because of the importance of hemes in aerobic metabolism, the mechanism of ORR catalysis by Fe porphyrins is probably understood best among all metalloporphyrin catalysts. 

Transition metal compounds, such as organic macrocycles, are known to be good electrocatalysts for oxygen reduction. Furthermore, they are inactive for alcohol oxidation. Different phthalocyanines and porphyrins of iron and cobalt were thus dispersed in an electron-conducting polymer (polyaniline, polypyrrole) acting as a conducting matrix, either in the form of a tetrasulfonated counter anion or linked to... 

Phthalocyanine complexes are organic macrocycles with 18 7t-electrons, structurally resembling the naturally-occuring porphyrins complexes [1-3], Electrodes modified with transition metal (notably Fe, Co, Mn, Ni) phthalocyanine (MPc, Fig.l) complexes have continued to generate immense research interests because of their well-established electrocatalytic properties [3-6],... 

Schiff base macrocycles and phthalocyanines are readily prepared and pre-date crown ethers and cryptands but are more suitable for binding transition metals or softer main group ions. 

In such a large subject, this article can only focus on certain aspects, namely those that involve complexation with inorganic substrates. We only consider the synthetic macrocycles, with emphasis on transition metal complexation. Aza, oxa, and, to a lesser extent, thia and phospha macrocycles are also covered. The naturally occurring porphyrins, corrins, corphins, chlorins, and phthalocyanins, as well as the cyclodextrins, are not included. Because of the general complexity of macrocychc systems and the resulting complicated systematic names, commonly used abbreviations or simplified names will be employed. This review will encompass the synthesis, thermodynamics, stmcture, and applications of macrocychc ligands. 

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