Macrocyclic ligands cavity sizes

A metal ion encapsulated in the three-dimensional ligand cavity must be an acceptor bonded to the donor groups of all macrocycles forming the cage framework. This is time either when the metal ion size corresponds to the ligand cavity size or when the cavity can be transformed under the influence of the metal ion so that the distance between the central ion and the donor atoms of the cage is not over the sum of their ionic or covalent radii. In addition to the geometric parameters, the thermodynamic and kinetic stability of... 

The complexing abilities of thiacrown ethers and related macrocycles depend on several factors that have not been fully investigated. These factors include ligand cavity size, spatial distribution of binding sites within the ring, types of donor atoms, hard and soft acid-base rule , cation diameter, solvent, macrocycle conformation, and other factors . 

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 ... 

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-... 

With respect to the ring size, it has been stated that neither the redox potentials nor the half-lives of the Ni species are directly correlated to the cavity of the macrocyclic ligand, but the redox potentials are dependent on solvation effects.139 The effect of fused benzene rings and ring conformation has been monitored.140 In Ni complexes of fluorine-containing cyclams (25) the higher oxidation state becomes successively destabilized with respect to Ni, while the lower oxidation state (i.e., Ni1) becomes successively stabilized.141... 

The size and shape of the binding cavity that they define and their rigidity or flexibility are determined by the nature of the structural subunits making up the branches of the graphical representations. Their overall shape and appearance has also led to the coining of a number of so-called trivial names that refer to a particular aspect of the structure. Proposals have been made to name and define different types of macrocyclic ligands and molecular receptors [2.12]. Without being ex-... 

Apart from the different approaches to calculating hole sizes of macrocyclic ligands, there are also considerable differences in the force fields that have been used13,4,45,78,80,84 89]. Unfortunately, no comparative study that systematically analyzes the various methods and force fields is available. It is therefore worth noting that the computation of the cavity size of 12- to 16-membered tetraaza macrocycles with two very different models and force fields led to remarkably similar results (Table 8.2). 

Table 8.2. Cavity size for the tavu-III conformers of tetraaza macrocyclic ligands, calculated by molecular mechanics.

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