Nitrogen supramolecular chemistry

In this review, CPOs constructed by covalent bonds are mainly focused on however, stable coordination bonds comparable to the stability of the covalent bonds have potential for future enhanced molecular design of novel CPOs. One representative is the bond between pyridine-type nitrogen and metal, which is widely used in supramolecular chemistry, that is, the cyclic supramolecular formation reaction between pyridine-substituted porphyrin and metal salts (Fig. 6d) [27,28]. Palladium salts are frequently used as the metal salts. From the viewpoint of the hard and soft acid and base theory (HSAB), this N-Pd coordination bond is a well-balanced combination, because the bonds between nitrogen and other group X metals, N-Ni and Ni-Pt coordination bonds, are too weak and too strong to obtain the desired CPOs, respectively. For the former, the supramolecular architectures tend to dissociate into pieces in the solution state, and for the latter. 

The synthesis and properties of ruthenium phthalocyanines, RuPcs, have been well studied over the last 30 years, but only recently they have been rediscovered for their potential application in metallosupramolecular chemistry. Basically, RuPcs are different from ZnPcs in their tendency to form stronger complexes with basic sp nitrogen atoms (pyridine and imidazole) and in the possibility to form complexes on one side or on both sides of the macrocycle. Another interesting point of ruthenium phthalocyanines resides in the longer lifetime of their radical-ion-pair state when compared to that of zinc phthalocyanines. Thus, all things being equal, RuPcs display a richer potential for supramolecular chemistry than ZnPcs. 

Triazoles are an interesting class of heterocyclic units widely used in the discovery and modulation of drug candidates, development of new materials, supramolecular chemistry, design of new supported organocatalysts, and biotechnology area [55], Therefore, several elegant methods for the synthesis of this classic nitrogen heterocyclic compounds have been reported by 1,3-dipolar cycloaddition of azides with alkynes under thermal [56] conditions as well as copper catalysis. 

The final chapter, Heterocyclic Supramolecular Chemistry of Fullerenes and Carbon Nanotubes by N. Komatsu presents an extremely unique review that focuses on the noncovalent chemistry of fullerenes and carbon nanotubes with nitrogen- and/or oxygen-containing heterocyclic molecules such as porphyrin, DNA, protein, peptide, and carbohydrate. Not only exohedral but also endohedral fimctionahzation is reviewed, because the above guest molecules can interact with both faces of the carbon nanotubes. The hurdles in structural separation, nanofabrication, and bioappHcations of carbon nanotubes will hopefully be addressed by the supramolecular strategy. 

Recently, macrocyclic chiral compounds of crown ether or cyclamen type have been attracting wide interest. These compounds contain numerous heteroatoms in their molecules (mainly oxygen, sulfur, and nitrogen) and can find practical applications, for example, as chiral selectors [69,70] and chiral NMR discriminating agents [71]. Asymmetric substitution of two carbon atoms in the ring of crown ether or cyclamen can lead to many different optically active compounds useful in various branches of supramolecular chemistry. Such substitution can be accomplished with appropriate starting compounds that are optically active, for example, amino acids and polyhydroxy alcohols. 

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