Macrocyclic effect kinetic

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. 

Consequences of unsaturation. Unsaturation in the macrocyclic ring may have major steric and electronic consequences for the nature of the ring. Extensive unsaturation will result in loss of flexibility with a corresponding restriction of the number of possible modes of coordination. Further, loss of flexibility tends to be reflected in an enhanced macrocyclic effect . For example, if the metal ion is contained in the macrocyclic cavity, the loss of flexibility reduces the possible pathways for ligand dissociation and this tends to increase the kinetic stability of the system. As explained in later chapters, enhanced thermodynamic stabilities will usually also result. 

Finally, a discussion of the kinetic features of the macrocyclic effect (the kinetic macrocyclic effect ) mentioned in Chapter 1 is deferred until the next chapter. 

The dissociation kinetics of macrocyclic complexes have received considerable attention, especially during investigations of the nature of the macrocyclic effect. Before discussing the dissociation of cyclic ligand species, it is of benefit to consider some aspects of the dissociation of open-chain ligand complexes. 

The formation and dissociation kinetics for the complexation of Cu, Zn, Co and Ni with the quadridentate l,4,8,ll-tetramethyl-l,4,8,ll-tetraazacyclotetradecane to give five-coordinate species have been reported.1203 The rate order (Cu > Zn > Co > Ni) is the same as that for H2O exchange, but the rates are much slower, probably owing to conformational changes occurring in the ligand. The dissociation is acid-catalyzed the five-coordinate species are found to be much less kinetically inert than four- or six-coordinate complexes. No macrocyclic effect was observed. 

Some of the important factors influencing these reactions are polarization effects, kinetic liability, thermodynamic stability, and stereochemistry of both the metal atom(s) and the reactants. In general, these syntheses are more metal ion specific than are complexations of the metal ion with a preformed macrocyclic ligand 45). 

Hydration enthalpy Stability or formation constant Overall and stepwise stability constants Chelate effect Macrocyclic effect Preorganization Equilibrium template effect Kinetic template effect Self-assembly... 

Interestingly, the K+ complex of 2.2.2 is approximately 10 more stable than the corresponding diazacrown complex. Such enhanced stabihty has been termed by Lehn the cryptate effect (which encompasses both enhanced kinetic as well as thermodynamic stability). The cryptate effect may be considered to be a special case of the well-known macrocyclic effect. In this context, it needs to be noted, however, that a number of complexes exist in which a metal ion is coordinated exo to the cryptand s cavity (or which only partially occupy the cavity) and, of course, the cryptate effect will be absent or greatly reduced in such cases. Similarly, complexes of 2 1 (metal cryptand) are known. For example, cryptand 2.1.1 forms a complex of type [Pb2(2.1.1)]" + for which it was postulated that one or both metals are bound exo to the cavity as clearly both lead ions are unable to simultaneously occupy the cryptand s central cavity. ... 

The effect of initiator type on the formation of cyclic oligomers manifests itself very markedly in the nonactivated anionic polymerization of CL [24]. When the process is initiated by the sodium salt of lactam, the formation of macrocycles is kinetically controlled the concentration of the cycles pronouncedly exceeds the equilibrium values, in similar fashion to cationic polymerization. In contrast, when the magnesium salts of CL or ethyl magnesium bromide are used as initiators, a considerable suppression of cyclization reactions was demonstrated for... 

Such modifications can be produced either in the kinetic aspects (proton transfer) or in the equilibrium constant. Both effects are mediated by intramolecular hydrogen bonds. For instance, Navarro et al. (93MI69) showed that the rate of proton transfer between the two nitrogen atoms of pyrazole (annular tautomerism) is considerably reduced in macrocycles containing oxygen or nitrogen atoms in the macroring. 

Hawthorne and co-workers have also produced a series of macrocyclic Lewis acid hosts called mercuracarborands (156, 157, and 158) (Fig. 84) with structures incorporating electron-withdrawing icosahedral carboranes and electrophilic mercury centers. They were synthesized by a kinetic halide ion template effect that afforded tetrameric cycles or cyclic trimers in the presence or absence of halide ion templates, respectively.163 These complexes, which can bind a variety of electron-rich guests, are ideal for catalytic and ion-sensing applications, as well as for the assembly of supramolecular architectures. 

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