Hole size mismatch

As the flexibility of the macrocycle increases, then mismatch hole-size effects are expected to be moderated. In any case, as discussed in Chapter 1, a metal ion which is too large for the cavity may be associated with folding of a flexible macrocycle thereby allowing normal metal-ligand bond distances to be achieved. However, this is not always the case, and a number of examples of unfolded macrocyclic complexes containing compressed metal-donor distances are known (Henrick, Tasker Lin-doy, 1985). 

The operation of the mismatch effects may be seen to best advantage when a range of products is possible from a single reactant or set of reactants. The reaction of ethylene oxide with metal salts results in the formation of crown ethers (Fig. 6-30). Obviously, a whole range of different cyclic oligomers and acyclic polymers could be formed from ethylene oxide. If we specifically wanted to obtain 18-crown-6, with a hole size of about 1.4 A, we would expect to use a potassium ion as template (r = 1.38 A). In fact, 18-crown-6 is obtained in good yield from the reaction of ethylene oxide with potassium tetra-fluoroboratc. In contrast, if we wanted 12-crown-4, with a hole size of about 0.8 A, it... 

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. 

Now let us assume that our pin is made of thermoplastic and it is injection molded. So, we know it cannot be a pure cylinder with a perfect diameter, as there is going to be some amount of draft added somewhere—regardless of how we orient the part in the mold—and perhaps even some mold mismatch. Then we have a hole in another part, which is thermoformed, or perhaps rotationally molded. That hole is not going to be a perfectly round hole (or straight), no matter how much we may want it to be. Now, we have to establish size targets for the pin and for the hole, evaluate the tolerances for a typical production scenario, and then do a tolerance analysis to ensure that everything always fits. 

---