Porphyrins and Tetrapyrrole Macrocycles

Examples such as this show the importance of alkali metal cation binding and transport in biochemistry, and a great deal of effort has been expended in supramolecular chemistry in attempts to understand natural cation binding and transport of the ionophore and channel type and to develop artificial systems capable of similar selectivities and reactivities. We will take a close look at many of these compounds in Chapter 3. 

Tetrapyrrole macrocycles such as haems and corrin exhibit a number of special features that make 

The underlying planar (or near-planar) ring system is very stable. 

As tetradentate chelate ligands that, after deprotonation, carry a single (corrin) or double negative charge, the tetrapyrrole can bind even highly labile metal ions. The complex can dissociate only if all metal ligand bonds are broken at the same time (cf. chelate and macrocyclic effects). 

In the ensuing sections we will examine some porphyrin and tetrapyrrole complexes in greater detail. 

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The biosynthesis of the tetrapyrrole macrocycle and its branches leading to haem and chlorophylls has been covered in detail in several reviews - - and will be concisely described in this section. Tetrapyrrole biosynthesis occurs entirely in the plastids and is composed of several enzymatic steps starting from 5-aminolevulinic acid (ALA), which is the key precursor of porphyrins and the source of their carbon and nitrogen. 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

Biologically essential tetrapyrrolic macrocycles of two main types exist the porphyrins, the prosthetic group of heme proteins, and the corrins, most representative among which is the coenzyme of Vitamin B12. 

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... 

Two new expanded porphyrins, that would perhaps best be classified as vinylogous porphyrins, have also been reported in the last several months. Corriu et al. have developed a facile synthesis of new tetrapyrrolic macrocyclic derivatives [241]. A notable difference between these new vinylogous porphyrins and those described above (Section 11) is the incorporation of pyridine, in one case, as part of the macrocycle. While these new expanded porphyrins are not aromatic, they may have great potential as ligands. Indeed, the formation of a bimetallic palladium complex was described. 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

Porphyrins and related tetrapyrrole macrocyclic ligands are one of the most prominent ligand types in coordination chemistry. Their occurrence in metallo-enzymes, their application as inorganic dyes and photosensitisers, their use as homogeneous catalysts, mainly for oxidation or oxygenation reactions, and their outstanding role in coordination chemistry is in general well documented. ... 

Corroles are also tetrapyrrolic macrocycles related to the cobalt-chelating corrin in vitamin B12 and have received considerable interest recently [217]. They are related to porphyrins by ring contraction. Corrolazines are derivative compounds obtained by aza substitution of corroles [218]. Related compounds are phthalocya-nines (tetrabenzotetraazaporphyrin) [219]. Although these are promising systems, so far they seem to have lower activity and selectivity than that of the corresponding porphyrins. 

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