Macrocycles 120° ligand

Conceptually the most simple syntheses of complex molecules involve the joining of structural units in which all functional groups and all asymmetric centres are preformed. This technique can usually only be applied to compounds in which these units are connected by —C—X— bonds rather than C—C. It is illustrated here by the standard syntheses of oligonucleotides, peptides, and polydentate macrocyclic ligands. 

The resulting macrocyclic ligand was then metallated with nickel(II) acetate. Hydride abstraction by the strongly electrophilic trityl cation and proton elimination resulted in the formation of carbon-carbon double bonds (T.J. Truex, 1972). 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

The template effects of potassium and lithium ions are responsible for the efficiency of the synthesis of macrocyclic ligands in 18-CROWN-6 and2,2.7,7,12,12,17,l 7-OCTAMETHYL-21,22,23,24-TETRAOXAPER-HYDROQUATERENE. 

As a final example we consider noncovalent molecular complex formation with the macrocyclic ligand a-cyclodextrin, a natural product consisting of six a-D-glucose units linked 1-4 to form a torus whose cavity is capable of including molecules the size of an aromatic ring. Table 4-3 gives some rate constants for this reaction, where L represents the cyclodextrin and S is the substrate ... 

The coordination chemistry of Pb with conventional ligands from groups 14-16 and with macrocyclic ligands has recently been reviewed. 

Macrocyclic effect and specific character of complex formation with rigid macrocyclic ligands such as porphyrins and phthalocyanins 97MI8. 

Novel electron-transfer reactions mediated by alkali metals complexed with crown ethers as macrocyclic ligands 98ACR55. 

Fig. 8-2. Schematic representation of the separation of enantiomers R and S using a supported chiral macrocyclic ligand host.

Macrocyclic ligands such as crown ethers have been widely used for metal ion extraction, the basis for metal ion selectivity being the structure and cavity size of the crown ether. The hydrophobicity of the ligand can be adjusted by attachment of alkyl or aromatic ligands to the crown. Impressive results have been obtained with dicyclohexano-18-crown-6 as an extractant for Sr in [RMIM][(CF3S02)2N] IL/aque-... 

This system has characteristic in ingeneous combination of a redox pump and the selective complexation of cation by the macrocyclic ligand. It must be noted that this new system is very promising from the point of view of extending the scope of the selection of the cation carriers, since any carrier can be employed so long as it has selectivity for a special cation and has enough stability toward redox system. 

Mononuclear complexes of palladium and platinum in the +3 oxidation state have only recently been unequivocally characterized [157]. The major advance has come in complexes with macrocyclic ligands such as 1,4,7-trithiacyclononane (ttcn) and 1,4,7-triazacyclononane (tacn) (Figure 3.96). 

Figure 3.96 Macrocyclic ligands used to stabilize palladium(III) and platinum(III).

With a tridentate ligand Au(terpy)Cl3.H20 has, in fact, AuCl(terpy)2"1" with weakly coordinated chloride and water while Au(terpy)Br(CN)2 has square pyramidal gold(III) the terpyridyl ligand is bidentate, occupying the axial and one basal position [124]. Macrocyclic complexes include the porphyrin complex Au(TPP)Cl (section 4.12.5) cyclam-type macrocyclic ligands have a very high affinity for gold(III) [125],... 

Macrocyclic Ligands (Corrins and Corroles) (see also Table 17.10)... 

Problems of complex formation with macrocyclic ligands. S. L. Davydova and N. A. Plate, Coord, Chem. Rev., 1975,16, 195-225 (130). 

Distinctive coordination chemistry and biological significance of complexes with macrocyclic ligands. D. H. Busch, Acc. Chem. Res., 1978,11, 392-400 (61). 

The spectra and structure of transition metal complexes with tetradentate macrocyclic ligands. K. B. Yatsimirskii and Y. D. Lampeka, Russ. Chem. Rev. (Engl. Transl), 1980, 49,1003-1020 (191). 

The specification of bonding cavities in macrocyclic ligands. K. Hendrick, P. A. Tasker and L. F. Lindoy, Prog. Inorg. Chem., 1985,33,1 (146). 

Rare earth complexes with neutral macrocyclic ligands. J. C. G. Bunzli and D. Wessner, Coord. Chem. Rev., 1984,60,191 (186). 

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