Metal ions, functionalized macrocyclic

Fig. 18. Plot of change in complex stability, Alogf , that occurs for the pairs of ligands cryptand-2,2,2 and EN ( ), and 18-aneN204 and EN (O), as a function of metal ion radius (20). The diagram shows how neutral oxygen donors tend to stabilize the complexes of large relative to small metal ions in macrocyclic ligands. Data from Ref. (11).

Poly (macrocyclic) compounds. The analytical application of compounds such as crown polyethers and cryptands is based on their ability to function as ligands and form stable stoichiometric complexes with certain cations. Special importance is due to their preference for alkali metal ions which do not form complexes with many other ligands. A number of these compounds are commercially available and their properties and analytical applications have been described by Cheng et a/.11... 

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). 

The deprotection of the polyamine-thioethers 40a,b can also be accomplished in similar good yields with Li/naphthalene in THF (214). Cleavage of the thioether functions also commenced with [PdCl2(NCMe)2] in the case of 40b (216), and it is likely that the other macrobicycles will also undergo this metal-ion mediated cleavage reaction as well (Scheme 2). Note that this reaction cleaves only one of the two thioether bonds forming a macrocyclic ligand with one thiophenolate and one vinyl-thioether moiety, as... 

Finally, the presence of the substituents on the nitrogen donors also influences the reactivity of the corresponding complexes significantly. Thus, utilization of the permethylated derivative in place of H2L23 drastically alters the Lewis-acidity of the metal ions, that is the coligands in the [Ni2(L19)(L )]+ compounds become more polarized than in the [Ni2(L23)(L )]+ complexes. In the latter complexes, the Lewis-acidity of the divalent nickel(II) ions is in large part consumed for the polarization of the six secondary NH functions. This explains why only the complexes of the permethylated macrocycle are able to activate and transform small molecules such as H20 or C02 (210,239,241). 

The ability of macrocyclic ligands [58], such as crown ethers, to coordinate metal cations with different selectivities as a function of their cavity and metal ions diameter has been demonstrated... 

The highest flexibility for a variation of the functional group and the chains X and Y (i.e. the size of the rim of the lamp shade) will be realiad when the synthesis of 3 is convergent and modular (Scheme 1). Amide bonds can easily be formed in macrocyclizations [13], therefore macrocyclic diamines 7 and diacyl dichlorides 8 had to be prepared. For the synthesis of macrocyclic diamines 7, also a large number of reactions are known. However, in this case a reduction of a macrocyclic diamide could not be achieved [11]. Therefore, another route was used the formation of macrocyclic diimines 6 (bis-Schiff bases) followed by NaBH4 reduction to the macrocyclic diamines 7. This approach has the advantage that for the construction of macrocyclic diimines 6, the metal ion template effect [14] may be exploited. 

Some functional groups such as phenol, pyridyl, catechol, imidazole, and amine, which are attached to the carbon atoms of macrocyclic framework, also affect the coordination geometry of the metal ions (39-44). 

The unsubstituted, saturated aza macrocycles, the family of cyclic secondary amines, [ ]aneNm, are generally prepared by the method shown in Scheme 1. Some other types of aza macrocycle, particularly cyclic amides7 and macrocycles with N=CRCR=CRNH functions (see below), are also prepared conventionally. The preformed ligands are then reacted with the desired metal ion under appropriate conditions. 

Binuclear metal ion incorporation can also be achieved by attaching functionalized side chains to the macrocyclic framework (27).94 In (27) the soft phosphine portion can be thought of as the redox center, while the hard N202 ring provides a Lewis acid metal ion center. 

Enolase type activity is displayed in the efficient supramolecular catalysis of H/D exchange in malonate and pyruvate bound to macrocyclic polyamines [5.32]. Other processes that have been studied comprise for instance the catalysis of nucleophilic aromatic substitution by macrotricyclic quaternary ammonium receptors of type 21 [5.33], the asymmetric catalysis of Michael additions [5.34], the selective functionalization of doubly bound dicarboxylic acids [5.35] or the activation of reactions on substituted crown ethers by complexed metal ions [5.36]. 

Supramolecular metallocatalysts consist in principle of the combination of a recognition subunit (such as a macrocycle, a cyclodextrin, a cyclophane, etc.) that selects the substrate(s) and of a metal ion, bound to another subunit, that is the reactive site. Complexed metal ions presenting free coordination positions may present a variety of substrate activation and functionalization properties. Heterotopic coreceptors such as 70 bind simultaneously a substrate and a metal ion bringing them into proximity, thus potentially allowing reaction between them. 

The bouquet -type compounds 114 and 115 and related molecules thus form functional ion transfer entities. Further studies are required to ascertain the mechanism of transport and the effects of parameters such as membrane composition, nature of alkali metal ions and concentration of bouquet molecules on the transport rate in order to better describe the mode of action of these compounds. Ion transport by bouquet -type molecules related to 114 and bearing lateral macrocyclic units has been investigated [8.188]. 

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