Macrocyclic ligands selectivity

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. 

It is to be mentioned here that the coordination chemistry of macrocyclic ligands is covered in Volume 7. Therefore only selected systems having some direct relevance to the bioinorganic chemistry of copper will be discussed. Readers are recommended to consult review articles/ 3... 

Silver(I) complexes with macrocyclic ligands that possess O/N/S as donor atoms have been studied extensively 1474-1533 in Figure 16 a selection of some silver complexes with this type of ligands can be seen. 

Based on donor atom type, macrocyclic ligands can be considered to span two extreme types. First there are those systems which chiefly contain nitrogen, sulfur, phosphorus, and/or arsenic donors. These macrocycles tend to have considerable affinity for transition and other heavy metal ions they usually show much less tendency to form stable complexes with ions of the alkali and alkaline earth metals. The present discussion will be restricted to a consideration of a selection of such ligands and their complexes. 

Selected other metal ion systems. There have been a number of investigations of the reduction of iron macrocyclic ligand complexes. In one such study, the Fe(n) complex [FeL(CH3CN)2]2+ [where L = (292)] was shown to exhibit three reduction waves in acetonitrile (Rakowski Busch, 1973). Controlled-potential electrolysis at the first reduction plateau (—1.2 V) led to isolation of [FeL]+ for which the esr spectrum is typical of a low-spin Fe(i) system. The quasi-reversible Fe(i)/Fe(n) couple occurs at —0.69 V versus Ag/AgN03. 

Multidentate and macrocyclic ligands with thioether functions in combination with N- and/or O-donor groups are of interest in the development of new series of selective metal extractants e.g. for Ag(I).167... 

All these methods have found applications in theoretical considerations of numerous problems more or less directly related to solvent extraction. The MM calculated structures and strain energies of cobalt(III) amino acid complexes have been related to the experimental distribution of isomers, their thermodynamic stability, and some kinetic data connected with transition state energies [15]. The influence of steric strain upon chelate stability, the preference of metal ions for ligands forming five- and six-membered chelate rings, the conformational isomerism of macrocyclic ligands, and the size-match selectivity were analyzed [16] as well as the relation between ligand structures, coordination stereochemistry, and the thermodynamic properties of TM complexes [17]. 

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 Ni(II) complexes of 14-membered tetraaza macrocyclic ligands, cyclam, Lie, L18, and Lie show catalytic activity in H20 or aqueous MeCN. The total mole-for-mole yields of CO and H2 are ca. 1 in most cases. The [Ni(cyclam)]2+ complex is a very effective and selective catalyst for the electrochemical reduction of C02 to CO relative to the reduction of water to H2 in aqueous solution when it is adsorbed onto mercury. The CO/H2 product ratio is >100 for [Ni(cyclam)]Cl2 (79). It is suggested that the greater selectivity for the electroreduction of C02 compared with water is related to the size of the macrocyclic ligand... 

A variety of macrocycles with asymmetric centers have been reported, examples of which are shown in (57) to (66). Chiral discrimination has been observed in the study of thiolysis of activated ester bonds with tetracysteinyl[18]crown-6 (67), e.g. Gly-L-Phe reacts up to 80 times faster than Gly-D-Phe with this ligand.231 The chiral macrocyclic ligand (66) is also capable of enantiomeric discrimination, by assisting in the selective reduction of carbonyl compounds with high optical yields.232,233... 

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