Macrocyclic effect origins

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. 

The presence of the cyclic backbone in ligands of this type makes a substantial contribution to their metal-ion complexing ability even though coordination involves donors which are not directly incorporated in the ring fragment. The origins of the enhanced stability of the metal-containing species may be considered to reflect the operation of an indirect macrocyclic effect (see Chapter 6) in these systems. 

The thermodynamic origins of the enhanced stabilities of macrocyclic ligands over their acyclic counterparts have been the subject of considerable debate since the term macrocyclic effect was first coined.83 Comparison of thermodynamic data for the several metal ion complexes of the [18]crown-6 and its acyclic counterpart are shown in Table 1. Enthalpy contributions to stabilization appear strongest for the K+ complex, while entropic contributions are stronger for the Na+ complex. Undoubtedly, the factors responsible for the thermodynamics will vary according to ion size, charge, solvation effects and structural preference. Hence, a single definable source of the macrocyclic effect is, in these systems at least, probably nonexistent. 

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... 

The macrocyclic effect is evident in the increase of 4.09 units in log K. It can be seen that the overall effect is of both enthalpic and entropic origin. The relative importance of these two contributions varies from case to case. 

It is not easy to generahze about the origins of the macrocyclic effect. In considering comparable open- and closed-chain complexes such as 6.22 and 6.23, entropy factors tend, in most cases, to favour the formation of the macrocyclic complex. However, the enthalpy term does not always favour the macrocyclic complex, although the value of AG° (i.e. the ultimate arbiter) always favours the formation of the macrocycle. We shall consider the formation of macrocyclic compounds further in Chapter 10. 

In terms of thermodynamics, the role of entropy versus enthalpy has been hotly debated. Log K and thermodynamic values are provided in Table 2. As can be seen from the thermodynamic parameters provided in the table, it becomes evident that the macrocyclic effect is primarily enthalpic in origin. This is dependent, however, on comparing systems without steric strain, which naturally adds other mitigating factors to the thermodynamics. 

It is particularly interesting to compare the thermodynamic aspects of supramolecular chemistry with transition metal chemistry (Tables 2 and 6). If similar chemistry holds, the macrocyclic effect should be primarily enthalpic in origin. As can be seen from Table 6, the A AH) values show that the effect is primarily enthalpic for Na" ", Cs+, Ba +, and Pb +, ranging from negative teens for the... 

The position and number of anchoring carboxylic groups strongly influence the DSSC performance. This effect originates from different orientations of the MP macrocycle with respect to the MO substrate plane incurred by the nature of the MP—MO binding (Scheme 12). This position affects efficiency of the electron injection to metal oxide and the charge recombination rate. 

It is interesting to note that although the first examples of template effects were observed in nitrogen macrocycles (see chapter 2) no template effect appears to operate in the synthesis of 72. Richman and Atkins note this in their original report . The authors replaced the sodium cation with tetramethylammonium cations and still obtained greater than 50% yield of tetra-N-tosyl-72. Shaw considered this problem and suggested that because of the bulky N-tosyl groups, .. . the loss of internal entropy on cyclization is small He offered this as an explanation for the apparent lack of a template effect in the cyclization. 

Omori T (1996) Substitution Reactions of Technetium Compounds. 176 253-274 Oscarson S (1997) Synthesis of Oligosaccharides of Bacterial Origin Containing Heptoses, Uronic Acids and Fructofuranoses as Synthetic Challengers. 186 171-202 Ostrowicky A, Koepp E, Vogtle F (1991) The Vesium Effect Synthesis of Medio- and Macrocyclic Compounds. 161 37-68 Otsuji Y,see Mizuno K (1994) 169 301-346 Pdlinkd I, see Tasi G (1995) 174 45-72... 

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