Macrocycle hole size

Neither of these problems occurs with approaches based on molecular mechanics calculations. The best fit of a metal ion M to a macrocyclic ligand L is related to the minimum in the sum of all M-L bonding interaction terms (Eq. 8.2). 

Apart from the different approaches to calculating hole sizes of macrocyclic ligands, there are also considerable differences in the force fields that have been used13,4,45,78,80,84 89]. Unfortunately, no comparative study that systematically analyzes the various methods and force fields is available. It is therefore worth noting that the computation of the cavity size of 12- to 16-membered tetraaza macrocycles with two very different models and force fields led to remarkably similar results (Table 8.2). 

One problem that has only rarely been appreciated is the fact that with unsym-metrical ligands the metal-ligand distances for each bond have to be varied within specific and different limits. An illustrative example is the hole size calculation of the hexaamine ligand rra/M-diammac which, with many first row transition metal ions, exhibits significantly different metal-ligand distances for the primary and secondary amines [65 182l As shown in Fig. 9.5 there are significant differences in 

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Factors influencing the macrocyclic hole size. The hole size of a macrocyclic ligand is a fundamental structural parameter which will usually influence, to a large degree, the properties of resultant metal complexes relative to those of the corresponding non-cyclic ligands. The large number of X-ray diffraction studies now complete for macrocyclic systems makes it possible to define many of the parameters which affect hole size... 

Figure 9.4 Geometrical determination of the macrocycle hole size[263].

A major focus in the study of mixed metal ion systems has been to examine metal ion discrimination. In particular, two specific mechanisms can be attributed to metal ion discrimination macrocyclic hole size and what Lindoy has termed as a dislocation mechaiusm. The key to this... 

In fact, even in his earliest works Pedersen found that primary ammonium compounds do form complexes with crowns whereas secondary, tertiary, and quaternary ammonium ions do not [45,46]. For the first time, it was ascribed to the mismatch of macrocyclic hole size and groups other than NH.f. 

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