Macrocyclic ligands, complexation thermodynamic properties

A very large number of synthetic, as well as many natural, macrocycles have now been studied in considerable depth. A major thrust of many of these studies has been to investigate the unusual properties frequently associated with cyclic ligand complexes. In particular, the investigation of spectral, electrochemical, structural, kinetic, and thermodynamic aspects of macrocyclic complex formation have all received considerable attention. 

It is important to note that, even when the coordination geometry prescribed by the macrocyclic cavity is ideal for the metal ion involved, unusual kinetic and thermodynamic properties may also be observed (relative to the corresponding open-chain ligand complex). For example, very often the macrocyclic complex will exhibit both enhanced thermodynamic and kinetic stabilities (kinetic stability occurs when there is a reluctance for the ligand to dissociate from its metal ion). These increased stabilities are a manifestation of what has been termed the macrocyclic effect - the multi-faceted origins of which will be discussed in detail in subsequent chapters. 

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]. 

In the crown ethers (18) the interactions between the ligand and metal ion are considered to be more electrostatic in nature, rather than the covalent binding observed for the transition metal complexes of the aza, thia, and phospha macrocycles. The thermodynamic properties of these macrocycles have been extensively studied, with numerous reviews covering complexation, selectivity, and structural aspects, some with extensive tables of thermodynamic data. Considerable efforts have been made to correlate the interrelationship between cavity size of the macrocycles and stability of alkali and alkaline earth metal complexes. From X-ray and CPK models, cavity radii are determined as 0.86-0.92A for 15-crown-5 (64), 1.34-1.43 A for 18-crown-6 (65), and about 1.7 A for 21-crown-7 (66). For complex formation between the alkali metal ions and 18-crown-6, the maximum stability... 

Thermodynamics plays a major role in helping the inorganic chemist to understand the effects that are important in determining the stability of complex metal ions in aqueous solution, including the properties of the complexing ligand that affect its ability to form the complex ion. Within the last decade, macrocyclic compounds have been studied extensively,2 in large part because of... 

The first example shows the synthesis of a C-C-bridged bis-macrocycle (Figure 2) [7]. The preparation of 1 is a condensation of a polyamine with a malonic ester derivative, in analogy to the procedure developed by Tabushi et al [8]. The tetraamide 1 is so insoluble that it precipitates and can be obtained practically pure from the reaction mixture. Its reduction to the octaamine takes place if the reaction with is done in diglyme (bis-(2-methoxyethyl)ether), in which 1 is partially soluble. The product 2 is an ideal ditopic ligand,since it has all the typical properties of 1,4,8,11-tetraazacyclotetradecane (cyclam), i. e. the thermodynamic and kinetical stability of its complexes and the C-C linkage between the two macrocyclic subunits does not reduce the coordination tendency of the amine nitrogens. 

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