Crowned porphyrin

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 . 

This crowned porphyrin can bind a transition metal ion and an alkali metal ion simultaneously. 

Fig. 16.4 Some examples of models of myoglobin. Only schematic stnjctures are shown—there are usually other heteroatoms (0, N) within the molecules but not shown here (a) a capped porphyrin, (b) a crowned porphyrin (c) picket fence porphyrin.

Chelation itself is sometimes useful in directing the course of synthesis. This is called the template effect (37). The presence of a suitable metal ion facihtates the preparation of the crown ethers, porphyrins, and similar heteroatom macrocycHc compounds. Coordination of the heteroatoms about the metal orients the end groups of the reactants for ring closure. The product is the chelate from which the metal may be removed by a suitable method. In other catalytic effects, reactive centers may be brought into close proximity, charge or bond strain effects may be created, or electron transfers may be made possible. 

There are, however, some crown type compounds which contain no structural feature except the thiophene subunit, and these deserve some comment here. This is especially true since one of these compounds was prepared very early in the history of crown compounds. Ahmed and Meth-Cohn were interested in sulfur analogs of the porphyrin ring system and prepared compound 7 in 1969 by the method shown in Eq. 

Calcium-binding proteins, 6, 564, 572, 596 intestinal, 6, 576 structure, 6, 573 Calcium carbonate calcium deposition as, 6, 597 Calcium complexes acetylacetone, 2, 372 amides, 2,164 amino acids, 3, 33 arsine oxides, 3, 9 biology, 6, 549 bipyridyl, 3, 13 crown ethers, 3, 39 dimethylphthalate, 3, 16 enzyme stabilization, 6, 549 hydrates, 3, 7 ionophores, 3, 66 malonic acid, 2, 444 peptides, 3, 33 phosphines, 3, 9 phthalocyanines, 2,863 porphyrins, 2, 820 proteins, 2, 770 pyridine oxide, 3,9 Schiff bases, 3, 29 urea, 3, 9... 

Fig. 3a,b. Template cyclization reactions of a crown ethers and b CPOs. The coordination bonds are illustrated by black arrows. In the crown ether synthesis, ethylene glycols coordinate toward the metal acting as the template (normal template reaction) however, the template coordinates to the incorporated metals of porphyrin In CPO synthesis (inverse-template reaction)... 

A methanofullerene derivative possessing an ammonium subunit has been prepared and subsequently shown to form a supramolecular complex with a porphyrin-crown ether conjugate <06T1979>. The synthesis and study of these fullerene-containing supramolecular photoactive devices have also been reported <06CRC1022>. 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

D Souza, F., et al., Self-assembled single-walled carbon nanotube-.Zinc-porphyrin hybrids through ammonium ion-crown ether interaction Construction and electron transfer. Chemistry - A European Journal, 2007.13(29) p. 8277-8284. 

B. Iron Complex of a Crown Ether-Porphyrin Conjugate Suitable for... 

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