Porphyrin-calixarene

Recently, crown-related macrocycles have been applied to new supramolecular systems such as porphyrins, calixarenes, catenanes, and rotaxanes, and so on. To cover the widespread area in limited pages, this chapter mainly focuses on the syntheses and functions of crown-related macrocycles having 10-membered rings or larger with one or more O and S atoms. Therefore, the standard arrangement in 12 sections could not be applied in this chapter related to that of CHEC-II(1996) (Chapters 9.29 and 9.31). 

Calixarenes are cyclic oligomers obtained by condensation reactions between para-t-butyl phenol and formaldehyde. By judicious choice of base, reaction temperature, and reaction time, calixarenes having different ring sizes can be prepared in good yield. Calixarenes are like crowns in that they are preorganized complexants, yet, unlike crowns, they can be readily synthesized in large quantities. Unlike porphyrins, calixarenes are not fully conjugated, and the three-dimensional structure leads to cavities. 

In fact, traditional oxygen carrier molecules are dissolved in organic solvents. Only very few works focus on water-soluble carriers, such as Co-histidine which, however, suffers from very short lifetimes [28]. Therefore, a new type of water-soluble of Co-porphyrin with oxygen affinity was developed by Fiamrnengo et al. [43] for the PBM membrane. This novel carrier was based on the assembly of porphyrins with calixarenes. The self-assembly was driven by electrostatic interactions. The oxygen was assumed to complex inside the cavity of the porphyrin-calixarene assembly. Furthermore, the Co of the porphyrin was shielded from the other side by complexation with a nitrogenous base (1-methylimidazole or caffeine). 

Similar to calix[n]arenes, porphyrins have been known to be one of the support pillars of supramolecular chemistry attributing suitable photoactive and electroactive properties to the molecular structures designed around them for building artificial molecular devices. Thus, various metallated and free base porphyrin-calixarene assemblies could afford attractive scaffolds for application in the areas of multipoint molecular recognition, receptors, host-guest chemistry, catalysis and photoinduced electron transfers. 

J. COMBINATIONS OF PORPHYRIN AND OTHER HOSTS—PORPHYRIN-CALIXARENE, PORPHYRIN-CYCLODEXTRIN, PORPHYRIN-STEROIDS... 

Metalated container molecules can be viewed as a class of compounds that have one or more active metal coordination sites anchored within or next to a molecular cavity (Fig. 2). A range of host systems is capable of forming such structures. The majority of these compounds represent macrocyclic molecules and steri-cally demanding tripod ligands, as for instance calixarenes (42), cyclodextrins (43,44), and trispyrazolylborates (45-48), respectively. In the following, selected types of metalated container molecules and their properties are briefly discussed and where appropriate the foundation papers from relevant earlier work are included. Porphyrin-based hosts and coordination cages with encapsulated metal complexes have been reviewed previously (49-53) and, therefore, only the most recent examples will be described. Thereafter, our work in this field is reported. 

Similarly, a series of hydroxy-terminated poly(ether) dendrimers, 35, with a single carborane nucleus at their core, were observed to have water solubilities comparable to that of chloroacetic acid, or D,L-valine. Again this is in direct contrast to the starting carborane nucleus which is insoluble in aqueous solutions and permitted the use of 35 in neutron capture therapy. Similar effects have also been observed with dendrimers containing calixarenes [66] and porphyrins [67] as the central units. 

The first of this class of receptor reported was a cobalticenium-based macrocycle that complexed bromide ions, with shifts of up to 45 mV in the position of the redox peaks of the cobalt moieties [96]. Other species have been used as active receptors including calixarene-based compounds [97] and porphyrins [98]. 

Besides these classical aromatics and polyaromatic hydrocarbons, other very important classes or arene molecules are porphyrins [60, 61], phthalocyanins [61, 62], porphycens [63], calixarenes [64], resorcarenes [64], cydophanes [47], dendrimers [65], elementa-arenes [66], organometallic arene (hexahapto) [67], benzyne (dihapto), and aryl- and benzyl (monohapto) complexes [68], inorganic pyridine and polypyridine complexes [69], fullerenes [70, 71], and... 

Chiral calix[4]arene podands were made using A-benzyl histidine methyl ester.33 These histidyl calixarenes 12a,b were studied in complexation experiments with Co(H), but no use was made of their chirality. The same is true for a chiral calix[4]arene capped tetraphenyl porphyrin which is C4-symmetrical due to the four L-alanine derived linkers.34... 

Fig. 42a, b The calixarene-porphyrin assembly a with an energy minimized structure showing the encapsulation of methanol b. Reprinted from [121] with permission from Elsevier Science... 

By the end of this chapter you should be familiar with some of the more frequently encountered co-ligands in organometallic chemistry. These include phosphines, nitrosyls, (spectator) cyclo-pentadienyls, carbaboranes, calixarenes, silsesquioxanes, cor-rins and porphyrins... 

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