Complex stability, macrocyclic carriers

For the alkali metal cations, the stability (14) and permeability (43) sequences for dicyclohexyl-18-crown-6 have been found to be the same (K+ > Rb+ > Cs+ > Na+ > Li+). Thus, a direct relationship exists between the ability of a macrocyclic compound to complex a particular cation (as measured by the log K value for complex formation) and its influence on the biological transport of that cation. Furthermore, it would appear that the biological ion-transport mechanism may in part be due to the complexation properties of the macrocyclic carrier molecules. This subject with respect to cyclic antibiotics has been treated extensively by Si wow and co-workers (2). 

Cryptands of type 7-9 and derivatives thereof carry alkali cations [6.4], even under conditions where natural or synthetic macrocycles are inefficient. The selec-tivities observed depend on the structure of the ligand, the nature of the cation and the type of cotransported counteranion. Designed structural changes allow the transformation of a cation receptor into a cation carrier [6.1, 6.4]. The results obtained with cryptands indicated that there was an optimal complex stability and phase-transfer equilibrium for highest transport rates. Combined with data for various other carriers and cations, they give a bell-shaped dependence of transport rates on extraction equilibrium (Fig. 11), with low rates for too small or too large... 

Figure 2.10 Relationship between carrier (macrocycle)-cation complex stability and solvents dielectric constant e for several solvents. From Ref. [98] with permission.

The stability of the host-guest complex is significantly affected by the nature and composition of the solvent in which the processes occur. This is an important factor when the reaction is carried out in aqueous medium, where hydrophobic interactions mainly contribute to the energy of complex formation due to an increase in its stability. In this regard, the use of water-soluble macrocycles with hydrophobic cavities as components of water-soluble metal complexes in two-phase catalytic systems is of particular interest [38,39]. The catalyst, soluble in the aqueous phase, can be easily separated from the water-insoluble reaction products and reused. It should be emphasized that the activity of conventional catalysts is very low for the reactions involving substrates poorly soluble in water. Due to the formation of water-soluble inclusion host-guest complexes, the macrocyclic receptors not only influence the activity and selectivity of the reaction, but also perform the function of interfacial substrate carrier in aqueous phase. 

Complex stability for macrocyclic carriers is determined by a number of factors including donor atom type, macrocyclic ring size, chelate ring size, pendant group characteristics, steric considerations, and degree of preorganization. Because complex stability is so crucial to carrier effectiveness, we examine a few of these factors below. 

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. 

A number of substances have been discovered in the last thirty years with a macrocyclic structure (i.e. with ten or more ring members), polar ring interior and non-polar exterior. These substances form complexes with univalent (sometimes divalent) cations, especially with alkali metal ions, with a stability that is very dependent on the individual ionic sort. They mediate transport of ions through the lipid membranes of cells and cell organelles, whence the origin of the term ion-carrier (ionophore). They ion-specifically uncouple oxidative phosphorylation in mitochondria, which led to their discovery in the 1950s. This property is also connected with their antibiotic action. Furthermore, they produce a membrane potential on both thin lipid and thick membranes. 

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

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