Thioethers macrocyclic

Many other cyclic and noncyclic organic carriers with remarkable ion selectivities have been used successfiilly as active hosts of various liquid membrane electrodes. These include the 14-crown-4-ether for lithium (30) 16-crown-5 derivatives for sodium bis-benzo-18-crown-6 ether for cesium the ionophore ETH 1001 [(R,R)-AA -bisd l-ethoxycarbonyl)undecyl-A,yVl-4,5-tctramcthyl-3,6-dioxaoctancdiamide] for calcium the natural macrocyclics nonactin and monensin for ammonia and sodium (31), respectively the ionophore ETH 1117 for magnesium calixarene derivatives for sodium (32) and macrocyclic thioethers for mercury and silver (33). 

Monodentate thioether ligands are relatively poor ligands towards Ni11. However, several Ni11 complexes with polydentate thioether ligands are known, where the coordination capability is enhanced by chelation. In particular, complexes with macrocyclic thioether ligands, which are discussed in Sections 6.3.4.10.l(iii) and 6.3.4.10.3, are very stable. 

To gain insight into electron-transfer kinetics of copper(II/I) complexes of macrocyclic thioethers,446,447 Rorabacher and co-workers reported448,4 7 structures of five complexes (complexes (548)-(552)). 

Macrocyclic thioethers which contain a carborane backbone have also been prepared and the silver(I) complexes obtained. The reaction of AgN03 with the sulfur-containing macrocyclic ligand... 

Figure 1 Some examples of macrocyclic thioether ligands...

Remote cyclization of AT-substituted phthalimides has been widely employed in the synthesis of macrocycles. Thioether groups are particularly useful as electron-donors in this process. The phthalimides (221), for example, have been converted in this way into crown ether analogues (222) and (223) in... 

Another impetus for the study of the coordination chemistry of crown thioethers stems from the role of thioether binding in biological systems such as d-biotin (involving tetrahydrothiophene) (145, 208) and blue copper proteins such as plastocyanin and azurin (involving methionine) (4,13, 73,109,124,185). The binding of Cu(II) and Cu(I) centers to macrocyclic thioethers has led to a greater understanding of Cu-S(thioether) interactions and the stereochemical preferences of these metal centers (91, 95, 99,121,180,181). 

The binding of cyclic thioethers to metal centers has also led to the isolation of complexes in which the coordinative properties of the ligand do not lit the stereochemical preferences of the metal ion(s) (188), Thus, a series of macrocyclic thioether complexes incorporating unusual stereochemistries and/or oxidation states has been generated (188). This is linked to the biological activity of the blue copper proteins and model systems in which the coordination geometry about Cu(II) is strained [in an entatic state (.212,221)] such that the Cu(II)/(I) couple occurs at a particularly positive potential that is, the Cud) state is stabilized. The ability of cyclic thioethers to modify their coordination properties is inherent in this approach (76,108,111). 

The low affinity of sulfur for alkali metal ions, however, renders template effects of less consequence in the synthesis of polythia macrocycles. Thus, the competition between cycli-zation and linear polymerization is more statistically defined, with cyclization kinetically favored only at high dilution 64,65,66). Consequently, most of the synthetic methods for the synthesis of polythia rings involve high-dilution techniques coupled with relatively long reaction times. Historically, the study of the coordination chemistry of macrocyclic thioethers has been hindered by difficulties in the synthesis of the free ligands. The synthesis of [BJaneSa, first reported by Ochrymowycz and co-workers in 1977 101), illustrates this well. 

It is well known that sulfide ligands form stable complexes with soft metal cations [30] and numerous complexes of macrocyclic thioethers with various metal salts have been prepared and characterized [31]. The iron atom of ferrocene is also basic and therefore affords a potential coordination site as evidenced by derivatives of ferrocene that are known to form complexes with certain metal salts [32]. Thus,... 

A range of the macrocyclic thioethers whose coordination chemistry has been most studied in recent years is illustrated in Figure 1, and references to their synthesis are given in Table 1. 

Synthesis of some sulfone-linked paracyclophanes from macrocyclic thioethers... 

Scheme 5 Synthesis of macrocyclic thioether sulfones and their oxidation to sulfone-linked paracyclophanes.

Macrocyclic thioether design by molecular modeling, G. A. Forsyth and J. C. Lockhart, Supramol. Chem., 1994, 4, 17. 

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