Macrocyclic ligands ring size

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 ... 

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... 

Complexation and determination of formation constants has been studied with the tetramethyl-cyclam (l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane) ligand.686 A series of zinc N4 donor macrocyclic complexes with different ring sizes were synthesized and characterized for comparison with cadmium derivatives.687... 

Tetraza-macrocycles of the right ring size are expected to give very high inplane ligand field strengths. Fe(II) complexes based on such ligands are therefore expected to be either low-spin or spin-crossover. Busch and his coworkers [17] have synthesized six coordinate iron(II) complexes [Fe (tet-a)X2], where tet-a is one of the isomers of hexamethyl cyclam, a 14-membered tetraza-macrocycle (8). 

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]. 

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