On metal macrocycle

Aerobic oxidations on metal macrocycles encapsulated in zeolites... 

ORR on Metal Macrocycle Complex-Coated Carbon Electrodes 248... 

A variety of materials containing carbon and nitrogen, and in some cases transition metals, have been synthesized and tested for the ORR in alkaline media, and some of these non-precious catalysts have proven to be promising candidates to replace Pt and Ag in AFCs. One class of nonprecious N-M-C ORR catalyst is based on metal macrocycles. Some macrocycles, such as Fe phthalocyanine, exhibit high ORR activity in alkaline media, but such catalysts are subject to rapid deactivation [11]. In AMFC testing, Co phthalocyanine cathode catalysts have yielded performance as high as nearly 100 mW/cm in H2/O2, vs. around 130 mW/cm for Pt/C [12]. 

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. 

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. 

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... 

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. 

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 ... 

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... 

Breslow demonstrated the catalytic effect of having not only a Lewis acid zinc center but also an auxiliary catalytic center held in close, but non-binding, proximity. An imidazole or thiophenol (81) demonstrated increase in effectiveness in cyclization of a phosphate derivative.706,707 Pyridine pendents on this macrocycle gave a stability for zinc in line with the Irving Williams series for two pyridine and three pyridine pendents.708 A contrast was noted with other metals (Ni, Cu, Cd, Pb, Fe, and In), all of which increase in stability with three pendent arms as does zinc this was attributed to coordination preferences. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

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... 

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. ... 

Impetus was given to work in the field of selective cation complex-ation by the observation of Moore and Pressman (5) in 1964 that the macrocyclic antibiotic valinomycin is capable of actively transporting K+ across mitochondrial membranes. This observation has been confirmed and extended to numerous macrocyclic compounds. There is now an extensive literature on the selective complexation and transport of alkali metal ions by various macrocyclic compounds (e.g., valinomycin, mo-nactin, etc.) (2). From solution spectral (6) and crystal X-ray (7) studies we know that in these complexes the alkali metal cation is situated in the center of the inwardly oriented oxygen donor atoms. Similar results are found from X-ray studies of cyclic polyether complexes of alkali metal ions (8) and barium ion (9). These metal macrocyclic compound systems are especially noteworthy since they involve some of the few cases where alkali metal ions participate in complex ion formation in aqueous solution. 

Shi and Zhang studied dioxygen binding to a variety of transitional metal macrocycles and reported an 02 binding energy to the CoP and CoTPP of -10.8 and -9.9 kcal mol, respectively (202). Only the end-on structure was found to be stable. 

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

Work by K Travis Holman and Jerry Atwood at the University of Missouri, USA has resulted in a tricationic host 4.66 based on the macrocycle cyclotriveratrylele (CTV, Section 7.7), which exhibits a deep anion binding pocket surrounded by three metal centres. A guest PF6 anion fits neatly into the cavity, stabilised by C—H F interactions, which may be differentiated from symmetry-related, noninteracting protons on the other side of the metallated aryl ring by H NMR spectroscopy. The X-ray crystal structure of this material is shown in Figure 4.26. 

Instead of grafting a coordinating chromophore on the macrocyclic framework, which sometimes requires the recourse to relatively comphcated synthetic routes, another strategy was tested, which forms ternary complexes in situ. An example is pyrene acetic acid H34 (fig. 35) which reacts with [Ln(26d)(H20)2] to form kinetically labile ternary complexes [Ln(26d)(34)(H20)x], Ln = Nd, Yb (Faulkner et al., 2004). Metal-centered luminescence occurs upon pyrene excitation. However, according to lifetime measurements for Yb111, 0.72 and 2.52 ps in H2O and D2O, respectively, and the use of eq. (9a), q = 0.9, which suggests that a pyrene acetic acid molecule is coordinated to the metal center expelling one water molecule, but not the second one. That is, the ternary complex does not represent an improvement over [Yb(30a)]. 

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