Activity of transition metal macrocycles

Estiu, G., A. Jubert, J. Molina, J. Costamagna, J. Canales, and J. Vargas (1995). Semiempirical SCF/CI interpretation of the origin of the catalytic activity of transition metal-macrocycle complexes. J. Mol. Struct Teochem. 330(1-3), 201-210. 

During the last two decades a variety of transition metal macrocycles have been shown to exhibit activity for O2 reduction In alkaline and acid media when adsorbed, chemically anchored or physically dispersed on electrode surface (Jf5). This class of compounds provides a unique opportunity to examine In detail some of the factors Involved In the activation and further reduction of 02 These would Include the effects associated with axial, peripheral... 

This chapter provides a critical review of transition metal macrocycles, both in intact and thermally activated forms, as electrocatalysts for dioxygen reduction in aqueous electrolytes. Fundamental aspects of electrocatalysis, oxygen reduction and transition metal macrocycles will be highlighted in this brief introduction, which should serve as background material for the subsequent more specialized sections. 

In alkaline and neutral solutions silver and carbon are also used as catalysts. In acid electrolytes carbon is not effective for O2 reduction. New ways for oxygen reduction catalysis have been offered via the interaction of O2 with transition metal complexes, as demonstrated for the face-to-face Co-Co-4 porphyrin and a number of transition metal macrocycles on earbon, graphite, or metal substrates. Heat treatment at 700-1200 K of macroeyeles such as cobalt tetramethoxyphenyl porphyrin (Co-TMPP) and Fe-(TMPP) improve the activity in alkaline and acid media, respectively. 

Shi Z, Zhang J. Density functional theory study of transitional metal macrocyclic complexes dioxygen-binding abilities and their catalytic activities toward oxygen reduction reaction. J Phys Chem C 2007 111 7084-90. 

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... 

Some of the transition metal macrocycles adsorbed on electrode surfaces are of special Interest because of their high catalytic activity for dloxygen reduction. The Interaction of the adsorbed macrocycles with the substrate and their orientation are of Importance In understanding the factors controlling their catalytic activity. In situ spectroscopic techniques which have been used to examine these electrocatalytlc layers Include visible reflectance spectroscopy surface enhanced and resonant Raman and Mossbauer effect spectroscopy. This paper Is focused principally on the cobalt and Iron phthalocyanlnes on silver and carbon electrode substrates. 

The first In situ MBS Investigation of molecules adsorbed on electrode surfaces was aimed primarily at assessing the feasibility of such measurements In systems of Interest to electrocatalysis (18). Iron phthalocyanlne, FePc, was chosen as a model system because of the availability of previous situ Mossbauer studies and Its Importance as a catalyst for O2 reduction. The results obtained have provided considerable Insight Into some of the factors which control the activity of FePc and perhaps other transition metal macrocycles for O2 reduction. These can be summarized as follows ... 

Pyrolyzed macrocycles. In 1976, approximately a decade after the first reports on macrocycle ORR activity, Jahnke et al. reported the improved activity and stability of pyrolyzed macrocycles (macrocycles heat treated in an inert environment).9 Since that time a large amount of work has been carried out on pyrolyzed Fe macrocycles,30,49,52,64-81 Co macrocycles,40-41,75-80,82-101 mixed Fe/Co macrocycles,102 104 other transition metal macrocycles,77,80,104,105 and non-metal macrocycles.75,85,86,99,106 Several reviews have also discussed the results of this body of literature.10-17,105 In general, there is agreement that Fe and Co starting materials... 

Oenyuk, B., Whiteford, J.A. and Stang, P.J. (1996) Design and study of synthetic chiral nanoscopic assemblies. Preparation and characterization of optically active hybrid, iodonium-transition-metal and all-transition-metal macrocyclic molecular squares. J. Am. Chem. Soc., 118 (35), 8221-8230. 

Catalysis can be broken down into a number of areas, depending on the substrate and the catalytic reaction. One of the prime areas of the initial effort in catalysis has been small molecule activation, such as oxygen with a number of transition metal ion macrocycles and carbon dioxide, the latter particularly with cobalt(I) and nickel(I) macrocycles. Once the polyanunonium macrocycles were found to be able to recognize substrates other than metal ions, other catalysis applications evolved. For example, phosphoryl transfer catalysis with simple polyanunonium macrocycles has become quite accessible. ... 

Systematic studies of the role of such factors as the nature of the metal center and the detailed structure of the chelating ring, particularly its peripheral functionalization, can afford valuable information toward unveiling structure-activity relationships for macrocycles as electrocatalysts for oxygen reduction. The following sub-sections describe some of the most salient aspects of a selected number of transition metal phthalocyanines and porphyrins, including the effects of redox and non-redox active substituents on the properties of Co porphyrins. 

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