Macrocycles, transition metal, oxygen

Oxygen Reduction on Macrocyclic Transition Metal Complexes on Graphite and Carbon Surfaces... 

This triple catalytic system allows the aerobic oxidation via a multistep electron transfer involving three redox systems Pdll)/Pd(0) - benzoquinone/hydroquinone -MLox/ML, where ML is an oxygen activating macrocyclic transition metal complex. 

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. 

In Situ Spectroscopic Studies of Oxygen Electrocatalysts Involving Transition Metal Macrocycles... 

Iliev I., Gamburzev S., Kaisheva A., Gas-diffusion electrodes with transition metal macrocyclic catalysts for electrochemical reduction of oxygen, Proceedings of the 31 ISE Meeting 1980 Sept. 22 - 26, Venice, Italy, Vol. I, p. 286-288. 

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... 

The soft donor atoms, phosphorus, arsenic and sulfur, potentially act as stabilizing influences to more polarizable substrates. These heteroatoms have been less extensively studied than the nitrogen and oxygen analogs. Of the three, the thioether macrocycles are the most common, and complexes of many of the first row as well as the heavier transition metals have been reported.43... 

Polyaza-, polythia-ligands. Recognition of transition metal ions. Replacing the oxygen sites with nitrogen or sulphur yields macrocycles and cryptands that show marked preference for transition metal ions and may also allow highly selective complexation of toxic heavy metals such as cadmium, lead and mercury [2.41-2.44, A.14]. 

We see that for the (alkali metal + oxygen macrocycle) complexes, charge and relative size of the ion play an important part in determining the stability of the complex. However, for the (transition metal + aza- or thia-substituted macrocycle) complexes, the nature of the bonding seems to be the important effect. Sten Ahrland16 has used a classification scheme for metal ion acceptors that helps us understand this difference. He designates the metal ion as either hard or soft. The characteristics that determine the assignment are as follows. 

There are many other reactions devised for oxygen transfer such as various oxidants combined with transition metals with or without macrocyclic hosts and industrial specific epoxidations using oxygen, to name just a few. 

Besides the ferrocene crown ether ligands described earlier, a variety of related macrocyclic ferrocenophanes containing various sulfur, oxygen, and nitrogen heteroatoms have been reported by a number of groups (110-115) (Scheme 21). These ligands complex transition metal guest cations such as Ag(I), Cu(I), and Pd(II) (116,117). Of particular... 

Transition-metal systems with macrocyclic nitrogen and oxygen donors are more analogous to biological 02-oxidation centers than the more organometallic type systems considered thus far. Studies on the macrocyclic ligand systems follow earlier ones such as Udenfriend s systems (e.g., Fe(II)-EDTA), and the use of other transition-metal salts, sometimes with added ligands (43, 113). 

Mossbauer spectroscopy may be important and useful when applied to electrodes which contain ferromagnetic components. It is basically an in situ tool which provides valuable information on possible orientation and oxidation states of ferromagnetic species in the electrodes as a function of the electrochemical process and the potential applied. For example, electrodes for oxygen reduction may be highly catalytic when containing macrocycles with transition metal cations such as Fez+, Niz+, Coz+ [89,90], A typical apparatus for this technique is described in Ref. 91. 

Template effects play an important role in most of the cyclizations leading to alloxygen macrocycles. Usually, alkali or alkaline earth metal cations serve as a template ion. Transition metal cations are useful in the syntheses of macrocyclic polyamines. The template effect is less pronounced in the case when formation of mixed nitrogen/ oxygen macrocycles are formed in comparison with their all-oxygen analogues. 

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