Macrocycles Porphyrins and Phthalocyanines

Not a peak this was the shortest investigated. See text for wavelength dependence of S in this region. See [158] for definitions of R and R for other values of N. 

When the periphery of the tetrapyrrolic cycle bears electron withdrawing groups, as in mac.2, instead of alkyl substituents, as in mac.l, the cross section in the B band region increases further (5 = 1.6 x 10 at 770 nm for mac.2), because of a decrease in AE and an increase in Me/, and the cross section was found to be proportional to the Hammett constant of the substituents [152]. 

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Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

The development of such a reaction proceeding under mild conditions is a technological challenge constituting one of the key points for the finalizing of efficient and low cost fuel cells. The catalytic properties of macrocyclic complexes like porphyrins and phthalocyanines for the reduction of molecular oxygen have been well known for four decades350,351 and numerous papers are devoted to this area. Here only some relevant and recent work in this field is described. 

At the time of Griffith s review (2), no macrocyclic complexes of Os had been reported except for the porphyrin and phthalocyanine com-... 

However, the changes in environment which occurred with the change from a reductive to an oxidative atmosphere rendered iron sulfide-based redox systems inconvenient, as they were very sensitive to (irreversible) oxidation. We saw in earlier chapters the facile formation of porphyrin and phthalocyanines from relatively simple precursors, and these systems were adopted for the final steps of electron transfer in oxidative conditions. The occurrence of iron centres in planar tetradentate macrocycles is ubiquitous, and metalloproteins containing such features are involved in almost every aspect of electron transfer and dioxygen metabolism. A typical example is seen in the electron transfer protein cytochrome c (Fig. 10-10). 

In addition to the ligands above, considerable attention is given to more complex ligand systems [4,5] aromatic and heteroaromatic compounds (heteroarenes) (i.e., five- or six-member cyclic structures with delocalized 7i-bonds in the ring containing, besides carbon atoms, either N, P, As, O, S, Se, or Te compounds [6-8]), various chelate-forming compounds, such as macrocyclic crown-ethers, cryptands, porphyrins, and phthalocyanines. 

There is a particularly extensive and rich coordination chemistry associated with iron(II) N donor macrocycles. The coordination chemistry of the biologically important iron porphyrin complexes has been of interest since the classic studies of Fischer, and over recent years there has been a resurgence of work on these and also on a wide range of related synthetic macrocyclic complexes. This section concentrates on their coordination chemistry, and where appropriate highlights enhanced ligand reactivity specifically induced by the iron(II) centre. First saturated ligands are discussed and then the unsaturated systems, with the particularly well-studied porphyrins and phthalocyanines being dealt with in separate subsections. 

Iron and Ru porphyrins and phthalocyanines , which are models of biological systems, react with CO at an axial position. Solvent molecules can be involved in the exchange see also ref. 23. Other macrocyclic ligands, e.g., tetradyidines, behave similarly. 

Cobalt porphyrins and phthalocyanines, nickel(II) macrocyclic polyamines, and other metal centers have been reported to catalyze the electroreduction of nitrous oxide according to N20 + 2111 I 2e —-N2 + H20. Kinetic experiments suggest the presence of N20-metal adducts in the rate-determining step.123,124... 

The macrocycle phthalocyanine contains 8 N atoms, but usually only the four N-atoms on the inner side of the cycle are able to coordinate. In fact, in most cases the synthesis of phthalocyanine is realized in the presence of a metal ion as the template. It is also possible to attach various substituents on the phthalocyanine macrocycle. As for porphyrin, when coordinating to a metal ion, the H-atoms of the two NH groups on the inner side of the phthalocyanine cycle are replaced. The incorporation of metal porphyrin and phthalocyanine complexes into porous crystals has been gaining increasing interest. The properties of the complexes located in zeolite channels or cages are usually different from those of the compounds in solution, and they may find applications in areas such as catalysis, photochemistry, electrochemistry, and biomimetics. 

Additional structural possibilities in Sections 3-5 include complex salts, with 2 3 or 1 2 stoichiometry for phenazine and TCNQ, and many polyiodides. Phenazines thus hardly fit into convenient isostructural families that facilitate systematic investigations. The organic donors related to tetrathiafulvalene (TTF) and macrocyclic organometallic donors involving porphyrins and phthalocyanines have been notably successful in producing isostructural series. The more numerous TCNQ salts and substituted phenazines display instead structural variety, although isostructural pairs undoubtedly exist. Now differences rather than similarities become interesting. 

In contrast to the large number of studies concerned with porphyrins and phthalocyanines, other macrocyclic ligand compounds of Mn" have received somewhat less attention. In part, this... 

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