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Macrocyclic Hole Size - An Abused Concept

Consider a planar tetradentate macrocycle, and draw a best-fit circle through the centres of the four donor atoms (Fig. 6-25). The radius of this circle (from the centre of the circle to the centre of the donor atoms) may be considered to be dictated by two [Pg.159]

In a similar manner, nickel(li) has the correct ionic radius for the bonding cavity of the fourteen-membered ring, tetraazamacrocycle 6.28. The reaction of 6.29 with nickel(n) acetate in the presence of base gives the nickel(n) complex of 6.28 (Fig. 6-28). This is an example of a template reaction that involves a nucleophilic displacement as the ring-formation process. [Pg.161]

The pentadentate nitrogen donor ligand 6.30 has a hole size of about 0.7 A and we predict that we could use a first-row transition metal ion as the template for its synthesis. The macrocycle is best prepared by the condensation of 6.31 with glyoxal about a nickel(n) template. In this condensation, most other metal ions are ineffective as templates. [Pg.162]

In the preceding section, we used the correct matching of the size of the metal ion to the size of the macrocyclic cavity to select metal ions that would be effective as templates. We should now consider what the consequences will be if there is a gross discrepancy between the two parameters. One can envisage a number of possible consequences of template reactions if the metal ion used as the template is not the correct size for the desired macrocyclic product  [Pg.162]

Perhaps the simplest would be that the metal ion does not act as a template, and no reaction occurs  [Pg.162]


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