Macrocycles chelate effect

Macrocycle/Chelate Effects and Enthalpic, Entropic Contributions. 76... 

This kind of argument may also be invoked for some Cr(III)-azamacrocyclic complexes. In this regard, it is noted that the macrocycle effect (which is essentially an enhanced chelate effect) stabilizes metal ions entrapped in the cavity formed by macrocyclic organic ligands... 

Cdbbiness and Margerum (35) suggest the term "macrocydic effect to describe the enhanced stability of a Mn+-macrocyclic chelate complex over that of the corresponding M +-non-macrocyclic chelate complex. Their data showing enhanced stability by a macrocydic complex are given in Table 10. 

The most important reaction of this type is the formation of imine bonds and Schiff bases. For example, salicylaldehyde and a variety of primary amines undergo reaction to yield the related imines, which can be used as ligands in the formation of metal complexes. However, it is often more desirable to prepare such metal complexes directly by reaction of the amine and the aldehyde in the presence of the metal ion, rather than preform the imine.113 As shown in Scheme 31, imine formation is a reversible process and isolation of the metal complex results from its stability, which in turn controls the equilibrium. It is possible, and quite likely, that prior coordination of the salicylaldehyde to the metal ion results in activation of the carbonyl carbon to amine nucleophilic attack. But it would be impossible for a precoordinated amine to act as a nucleophile and consequently no kinetic template effect could be involved. Numerous macrocyclic chelate systems have been prepared by means of imine bond formation (see Section 61.1.2.1). In mechanistic terms, the whole multistep process could occur without any geometrical influence on the part of the metal ion, which could merely act to stabilize the macrocycle in complex formation. On the other hand,... 

It should now be possible to determine whether the macrocyclic effect is entropic or enthalpic in origin. Initial investigations were made on transition metal complexes and most workers had a prejudice towards an entropic origin, similar to that of the chelate effect. More recently, it has become apparent that there is no single cause to which the... 

Chapter 3) in which the ether oxygen atoms play the same role as the polar water molecules, although the complex is stabilised by the chelate effect and the effects of macrocyclic preorganisation. The oxygen lone pairs are attracted to the cation positive charge. 

Hossain, M. A. Kang, S. O. Powell, D. Bowman-James, K. A new class of amide/quatemized amine macrocycles and die chelate effect, Inorg. Chem. 2003, 42, 1397-1399. 

Remarkably, enhanced binding by the chelate effect is occasionally seen not in free energies of complexation the association constants of e.g. amino acids with sulfonatocalix[4]arenes are quite close the ones observed with phenol-4-sulfonic acid as single host unit Only AH shows an advantage of the macrocycle with AH = 30 compared to 1 kJ/mol, compensated by a much smaller entropy disadvantage with the smaller monomeric host, which changes from e.g. TAS = -12 kJ/mol to favorable +12 kJ/mol.[34]... 

The importance of the chelate effect combined with the construction of multidentate ligands is well known in lanthanide chemistry. This is expressed in the rich coordination chemistry of / -diketonates [88] or complexes with Schiff bases [89] and macrocyclic polyethers [90] where lanthanide cations achieve steric saturation by high coordination numbers. Entrapment of the cation in a macrocyclic cavity results in greater complex stability. However, simply functionalized ligands such as ethanolamines can also supply a suitable ligand sphere [91-93],... 

We also investigated chelate binding by dimers of a synthetic hydrophobic macrocycle, in place ofthecyclodextrins [187]. In the systems examined the chelate effect was weaker than that seen with the cyclodextrin dimers. We also studied the strong binding of cholesterol by some cyclodextrin dimers and a cyclodextrin polymer, and saw that the large sterol could occupy parts of two binding cavities [188]. 

Chelate effect Macrocyclic effect Template effect... 

Chelating fluorophores and cbromophorcs incorporated on ion-selective macrocycles provide effective signal transduction of ion... 

There are also systematic trends in stability constants when frve-memberedpolyamine rings are replaced by six-membered rings, or when linear polyamine ligands are replaced by macrocyclic ligands of a similar strain see Chelate Effect). These effects can be well accounted for in terms of enthalpy contributions to the formation constant. ... 

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