Porphyrin-like macrocycles

Abstract Porphyrins and their analogues constitute one of the most important families of aromatic macrocycles. The present review discusses aromaticity of porphyrinoids, focusing mainly on non-expanded systems. The effect of structural modifications on the aromaticity-dependent properties of porphyrin-like macrocycles is described. It is shown that delocalization modes observed in porphyrinoids can be classified using a simple valence-bond approach. Aromaticity of porphyrinoids is further discussed as a function of tautomerism, coordination chemistry, and the oxidation state of the macrocycle. 

As noted above, the porphyrins fail to form stable complexes with gadolinium(III) [13,74,98]. Since Gd(III) is inherently more paramagnetic than Mn(III), it is intrinsically more attractive for use in MRI contrast applications [6]. In view of this, the authors proposed that a larger porphyrin-like macrocycle, capable of coordinating gadolinium(III) in a stable, nonlabile manner, could prove useful as an... 

The use of porphyrins and porphyrin-like macrocycles, namely the texaphyrins, which show good selectivity towards tumor cells, could be used to overcome the limitations of the current sensitizers, namely a lack of suitable biolocalization and a consequent high toxicity at the needed therapeutic doses [25]. The strengths and weaknesses of this approach are detailed below. 

The preparation and characterization of novel man-ganese(III) complexes of various porphyrin and porphyrin-likes macrocycles have continued to attract strong attention especially because of their importance in catalytical oxidation processes through the formation of a Mn(V)0 intermediate (see Section 6) and as model for metalloenzymes. In this line, an artificial enzyme formed through a directed assembly of a molecular square that encapsulated a Mn porphyrin has been prepared and investigated as a catalyst. In contrast to symmetrical binuclear bis(phenoxo) bridged macrocyclic Mn(III)Mn(III) complexes, unsymmetrical ones are rare. A new series of these kinds of carboxylate-free complexes has been described and their redox properties investigated. ... 

Several complexes of Sm, Eu and Gd with large porphyrin-like macrocycles have been prepared under the treatment of the reduced form of... 

Ibers used the Paal-Knorr furan synthesis to prepare a key intermediate for the synthesis of novel porphyrin-like aromatic macrocycles. Bis yrolyljfuran 27 was available in good yield via the acid catalyzed condensation of diketone 26. ... 

The chemical flexibility of the porphyrin-like metallomacrocycles has been used to vary the electronic properties of the molecular units and hence the resulting solid. This has been brought about by the choice of the basic ligand, the peripheral substituents and the central metal. The main variants on the porphyrin skeleton are shown in Figure 8. A large number of compounds have been obtained as conducting polycrystalline powders by oxidation of the parent metallo-macrocycle with iodine.90 Table 4 contains details of compounds which have been obtained as single crystals and have been extensively characterized and studied. 

Corrole is a tetrapyrrolic macrocycle with a direct link between two pyrrole rings. Lacking a C-20 meso carbon bridge it has a corrin like skeleton with double bonds involving porphyrin-like conjugation. It has an 18 electron rt system and hence aromatic character. The direct link between the A and D... 

Electrochemical reduction of Ni(taab)2+ [taab = (20), see p. 231] occurs in two one-electron steps, to complexes formulated as [Nim(taab)] + and [Niu(taab)]°. The relationship between the annulene taab and the two-electron reduction product, the porphyrin-like taab2-, is discussed.102 The preparation of macrocycles of type (87) by a template synthesis requires a minimum ring size of x = y = 3 and depends upon the strong complexing of the metal ion at the pH of the reaction, otherwise the metal hydroxide precipitates. The NiL(C104)2,nH20 (n = 0, 1, or 2) species have been prepared for x = 3, y = 4.479... 

The synthesis of a tripyrrane containing porphyrinogen-like macrocycle was reported by Sessler et al. in 1987 [59]. As discussed in the previous section (4.3), the first representative of this new class of expanded porphyrins (e.g. 115) was formed by the Schiff base condensation between a diformyltripyrrane 114 and... 

Some porphyrin-like aromatic macrocycles, in which one or two of the pyrrole nitrogen atoms have been replaced by oxygen or sulphur, have been studied 176>, and their n.m.r. shows the same characteristic deshielding of the peripheral protons (see 45, 46, 47 and 48, Table 9). 

Vanadium macrocycles could serve as geochemical tracers that link geo- and biosystems. Vanadyl porphyrins have been identified both in Precambrian shale as well as in extraterrestrial meteorites. Although there are reports of the presence of either vanadium or porphyrin-like molecules in many types of recent and ancient sediments, the nature of vanadium bonding in all these samples is not known. Once this knowledge is acquired, an insight into the structure of these natural resources will facilitate their intelligent applications in energy conversion and environmental control. 

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