Metal ions, functionalized macrocyclic system

The ligand reaction step may occur either with the template metal still intact or may take place after removal of the metal ion from the ring. As already mentioned, many of the Schiff-base macrocycles are unstable in the absence of a coordinated metal ion. However, for such systems, it has often been possible to hydrogenate the coordinated imine functions directly. The resulting saturated ligands will not be subject to the hydrolytic degradation which occurs for the imine precursors in the absence of their metal ion. 

The location of the induced unsaturation in the macrocyclic system is metal-ion dependent. This is illustrated by the examples given in Figure 8.2. In the Fe(n) complex, the imine functions form as conjugated pairs (Dabrowiak, Lovecchio, Goedken Busch, 1972 Goedken Busch, 1972) - such a-diimine species have long been known to have a special affinity for Fe(ii). In contrast, Ni(n) promotes formation of a product in which the respective imine functions are in electronically isolated positions (Curtis, 1968 1974). 

Despite such great interest, a question that is still unanswered is why modified tetrapyrrole ligands, like those found in factor F430 and vitamin B12, are employed by natural systems to carry out specific chemistry rather than the porphyrin ligand. A possible explanation that has been proposed [1, 2] is that these modified tetrapyrrole ligands exhibit different flexibility as compared with porphyrins. Another very important factor, and probably the most important one in the case of corroles and corrinoids, is the difference in hole size between the various macrocycles. The tetrapyrrole that would most efficiently perform a specific function would be the one with the proper hole size for the radius of the metal ion involved in the process. 

Schwabacher et al. (1972) prepared a cyclophane-type structure with metal ions coordinated into the walls of the macrocycle (70). This system was first reported in 1992 and shown to transport neutral aromatic hydrocarbons through an aqueous membrane. Even though this receptor has a net anionic charge, it has been illustrated to bind indole and naphthalene units functionalized with... 

Considerable attention has been given to the synthesis of compounds in which a single mono- or multinuclear coordination unit is at the center of a dendrimer array. These systems have been prepared by the construction of ligands with pendant dendritic wedges with subsequent convergent assembly upon coordination of a metal ion at the center or by covalent synthesis of an appropriate dendrimer-functionalized macrocyclic core. The primary interest for the study of such compounds lies in the very unusual, usually hydrophobic, environment in which the metal(s) at the center find themselves. This environment has been likened to that at the active site of a metalloenzyme and numerous studies have dealt with the catalytic and biomimetic activity of such conjugates. A general introduction to the area and the concept of chemistry within dendrimers is available. " The term dendrizyme has been used to describe these systems. 

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