Metal macrocycles porphyrins

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

Considerable progress has been made recently In the development of In situ spectroscopic techniques applicable to the study of transition metal macrocycles adsorbed at submonolayer coverages onto electrode surfaces. These have been aimed at gaining Insight into the nature of the Interactions of these compounds with the surface and with 02 Most of the attention In the authors laboratory has been focused on Fe- and Co-TsPc, although some preliminary results have already been obtained for some Iron and cobalt porphyrins. The main conclusions obtained from these Investigations will be outlined In the following sections. 

Dloxygen reduction electrocatalysis by metal macrocycles adsorbed on or bound to electrodes has been an Important area of Investigation (23 ) and has achieved a substantial molecular sophistication in terms of structured design of the macrocyclic catalysts (2A). Since there have been few other electrochemical studies of polymeric porphyrin films, we elected to inspect the dloxygen electrocatalytic efficacy of films of electropolymerized cobalt tetraphenylporphyrins. All the films exhibited some activity, to differing extents, with films of the cobalt tetra(o-aminophenylporphyrin) being the most active (2-4). Curiously, this compound, both as a monomer In solution and as an electropolymerized film, also exhibited two electrochemical waves... 

Most of the catalysts employed in PEM and direct methanol fuel cells, DMFCs, are based on Pt, as discussed above. However, when used as cathode catalysts in DMFCs, Pt containing catalysts can become poisoned by methanol that crosses over from the anode. Thus, considerable effort has been invested in the search for both methanol resistant membranes and cathode catalysts that are tolerant to methanol. Two classes of catalysts have been shown to exhibit oxygen reduction catalysis and methanol resistance, ruthenium chalcogen based catalysts " " and metal macrocycle complexes, such as porphyrins or phthalocyanines. ... 

The porphyrin macrocycle is an ampholyte with two pyrrolenine (=N—) nitrogen atoms capable of accepting protons, and two NH groups capable of deprotonation. The most useful scheme for assigning pK values is due to Phillips (60MI30700) in this the metal-free porphyrin is abbreviated PH2, the dianion P2 and the dication PH42+ ... 

Recent advances using macrocycles, such as supported metal-phthalocyamine and metal-tetraphenyl-porphyrine, pioneered by Jasinski [178] and pursued by Zagal and others [179-181] show significant promise, although significant optimization is necessary to reach practical current densities. 

For molecular electrocatalysts otherwise, and especially transition metal macrocycles, the electrocatalytic activity is often modified by subtle structural and electronic factors spanning the entire mechanistic spectrum, that is, from strict four-electron reduction, as for the much publicized cofacial di-cobalt porphyrin, in which the distance between the Co centers was set at about 4 A [12], to strict two-electron reduction, as in the monomeric (single ring) Co(II) 4,4, 4",4" -tetrasulfophthalo-cyanine (CoTsPc) [20] and Co(II) 5,10,15,20-tetraphenyl porphyrin (CoTPP) [21]. Not surprisingly, nature has evolved highly specific enzymes for oxygen transport, oxygen reduction to water, superoxide dismutation and peroxide decomposition. 

The thin layer of transition metal macrocycles attached to carbon generally lack long-term stability in concentrated acid and alkaline solutions. This drawback can be overcome by thermal treatment at 450-900°C for cobalt tetramethoxy phenyl porphyrin (Co-TMPP) [65]. Under these conditions, the Co-TMPP is substantially degraded to cobaltous oxide. Pyrolyzed layers involve high-area carbonaceous materials with a significant surface nitrogen and the transition metals as small oxide and metallic particles dispersed on the high-area substrate. These layers catalyze peroxide elimination in alkahne solutions. 

Metal-free porphyrins can undergo several steps of reduction and oxidation at the macrocyclic ring Tr-system. Metalloporphyrins may undergo reduction and oxidation reactions both at the porphyrin Tr-system and at the central metal ion. The site and rate of such redox reactions strongly depend on the porphyrin structure, the nature of the central metal ion, and the environment. Many of the fundamental reactions of porphyrins and metalloporphyrins have been studied by radiation chemical methods these studies are reviewed in this chapter. 

The porphyrin macrocycle contains conjugated double bonds that form an 18-membered Tr-electron system. This Tr-system can accept several electrons in a stepwise manner. The addition of one electron forms a porphyrin Tr-radical anion. Such Tr-radical anions have been prepared from metalloporphyrins and from metal free porphyrins and related compounds by irradiation in various environments. 

The identification of petroporphyrins rests heavily on UV-visible and mass spectroscopy. In the former, the region of the UV-visible spectrum that gives the most information are the Q bands, between 480-700 nm. As was explained in Chapter 3, the Q band structure is a powerful indicator of the substitution pattern around the porphyrin macrocyclic nucleus. The pattern of Q band intensities that goes IV>I>II>III, is indicative of a phyllo-type substitution pattern, typically displayed by DPEP. As a vanadyl complex, the Q band structure collapses to two bands. So, once a pure sample of VODPEP has been obtained, mass spectroscopy gives an accurate molecular mass. Acid work-up, followed by neutralisation, generates a metal-free porphyrin, whose UV-visible spectrum shows the typical DPEP Q band structure, while the mass spectrum confirms the loss of the V=0 unit. 

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