Porphyrin-Type Macrocycles

So far, only one completely planar porphyrin-type macrocycle with boron atoms 29 has been reported (Fig. 9) [42]. This molecule has D4h-symmetry and is composed of four five-membered B2S3 rings that are linked through the 

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Fig. 9. Reaction of ChBNiPr with the dilithiated di-2-thienylhorane amine 27 gives the tetrathiaporphyrinogen 28. Compound 29 is a porphyrin-type macrocycle...

The use of porphyrinic ligands in polymeric systems allows their unique physio-chemical features to be integrated into two (2D)- or three-dimensional (3D) structures. As such, porphyrin or pc macrocycles have been extensively used to prepare polymers, usually via a radical polymerization reaction (85,86) and more recently via iterative Diels-Alder reactions (87-89). The resulting polymers have interesting materials and biological applications. For example, certain pc-based polymers have higher intrinsic conductivities and better catalytic activity than their parent monomers (90-92). The first example of a /jz-based polymer was reported in 1999 by Montalban et al. (36). These polymers were prepared by a ROMP of a norbor-nadiene substituted pz (Scheme 7, 34). This pz was the first example of polymerization of a porphyrinic macrocycle by a ROMP reaction, and it represents a new general route for the synthesis of polymeric porphyrinic-type macrocycles. 

While the majority of macrocycles formed by this type of Schiffbase condensation reaction are derived from pyridine containing fragments, considerable attention has also been devoted to the use of other heterocycles, including five membered ones, as the primary macrocyclic precursors. Although these latter ligands are for the most part not completely conjugated, they form an important group of expanded porphyrin-type macrocycles. It is for this reason that they are included in the present review,... 

With a tridentate ligand Au(terpy)Cl3.H20 has, in fact, AuCl(terpy)2"1" with weakly coordinated chloride and water while Au(terpy)Br(CN)2 has square pyramidal gold(III) the terpyridyl ligand is bidentate, occupying the axial and one basal position [124]. Macrocyclic complexes include the porphyrin complex Au(TPP)Cl (section 4.12.5) cyclam-type macrocyclic ligands have a very high affinity for gold(III) [125],... 

Since the initial disclosure of the basic corrin structure, there has been a considerable body of effort devoted toward the synthesis of macrocycles related to this chromophore that may be considered as being intermediates between porphyrin and corrin. These macrocycles, namely the dehydrocorrins (e.g., tetradehydrocor-rin 2.3) and the corroles (e.g., 2.4), represent interesting classes of contracted porphyrins that warrant specific mention here. The interest in these molecules derives in part from the fact that they could represent milestones along the biosynthetic pathway leading to vitamin B12. They are, however, also of interest from a non-biological perspective. Simply stated, this is because corrole-type macrocycles possess unique electronic and chemical characteristics, the study of which can help one to understand better the chemistry of all porphyrin analogs. 

In recent years, the desire to generate new expanded porphyrins has led to the preparation of ever larger macrocycles. Most of these have consisted of tetra-, penta-, and hexapyrrole-type macrocycles. However, a few higher order systems, expanded porphyrins containing more than six pyrrole-type subunits, are now known. It is a review of these systems, still limited to macrocycles containing eight and ten pyrrole-type subunits, that is the subject of the present chapter. 

The type of porphyrin macrocycle will influence both the half-wave potentials and the overall redox behavior of cobalt porphyrins [34,55,308,312-314]. Porphyrins with macrocycles, which vary substantially in basicity and planarity have been studied, including T(jc-X)PP, (CN)j TPP, BC cTPP,... 

AUcylidene porphyrins (including oxo-type porphyrinogens) occupy a special category since they are formed through synthetic modifications at both the periphery and the macro-cychc core and can have properties that depart significantly from those reported for the porphyrins. Several macrocycles... 

Formally, tetravalent cerium is long known to form stable sandwich complexes with tetrapyrrole-type macrocyclic ligands such as porphyrines, phthalocyanines, and 2,3,-naphthalocyanines (Scheme 12). For the rare earth sandwich complex series, virtually all the neutral complexes can be formulated as Ln° (ring-1 ) (ring-2 ), in which a trivalent rare earth metal center is sandwiched between a... 

A synthetic list of the photophysical and photochemical properties, which are typical of an efficient PDT photosensitizer, is shown in Table 146.1. Apparently, the first requirement is represented by the presence of intense absorption bands in the 700 to 800 nm wavelength region, which guarantees the uniform illumination of relatively large tissue volumes. To achieve this goal, selected modifications of the porphyrin-type tetrapyrroHc macrocycle were devised, including the following ... 

In other sections in this chapter, we have referred to a variety of macropolycyclic structures which are more elaborate than the simple three-stranded bicyclic cryptands. This includes bridged double-macrocycles " , in-out bicyclic amines and the macrotricyclic quaternary ammonium salts of Schmidtchen. In addition to these, there are two other types of compounds which deserve special note. The first of these is a stacked twin-ring cryptand, but it is a hybrid molecule rather than a double-cryptand . The species shown below as 20 is a crowned porphyrin, and was designed to provide a pair of metal cation binding sites similar to those which might be available in natural biological systems . 

The porphyrin ring system (the parent compound 1 is also known as porphin) consists of four pyrrole-type subunits joined by four methine ( = CH-) bridges to give a macrotetracycle. The macrocycle contains 227i-electrons from which 1871-electrons form a delocalized aromatic system according to Huckel s 4n + 2 rule for aromaticity. The aromaticity of the porphyrin determines the characteristic physical and chemical properties of this class of compounds. The aromatic character of porphyrins has been confirmed by determination of their heats of combustion.1"3 X-ray investigations4 of numerous porphyrins have shown the planarity of the nucleus which is a prerequisite for the aromatic character. 

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. 

The prevalence of the heme in O2 metabolism and the discovery in the 1960s that metallophthalocyanines adsorbed on graphite catalyze four-electron reduction of O2 have prompted intense interest in metaUoporphyrins as molecular electrocatalysts for the ORR. The technological motivation behind this work is the desire for a Pt-ffee cathodic catalyst for low temperature fuel cells. To date, three types of metaUoporphyrins have attracted most attention (i) simple porphyrins that are accessible within one or two steps and are typically available commercially (ii) cofacial porphyrins in which two porphyrin macrocycles are confined in an approximately stacked (face-to-face) geometry and (iii) biomimetic catalysts, which are highly elaborate porphyrins designed to reproduce the stereoelectronic properties of the 02-reducing site of cytochrome oxidase. 

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