Conformational Change May Alter the Apparent Hole Size

3 Conformational Change May Alter the Apparent Hole Size 

We saw in Fig. 6-30 the conversion of ethylene oxide to crown ethers upon reaction with appropriate metal salts, and demonstrated that the hole sizes of the products corresponded to the ionic radius of the template ion. However, lest we become over-confident, it should be pointed out that the major product from the reaction of ethylene oxide with caesium salts (r = 1.67 A) is not the expected 21-crown-7 with a hole size of about 1.7 A) but 18-crown-6 (hole size, 1.4 A) (Fig. 6-34). The reason for this lies in the structure of the complex formed. We have always assumed that the metal ion will try to lie in the middle of the bonding cavity of the macrocycle. There is no real reason why this should be. Caesium could form a complex with 21-crown-7 in which all of the oxygen atoms lie approximately planar with the metal in the centre of the cavity. It is also apparent that caesium could not occupy the middle of the cavity in 18-crown-6. However, a different type of complex can be formed with 18-crown-6, in which a caesium ion is sandwiched bet- 

However, the problem arises from our measurement of the hole size of 6.35. Certainly the hole size of the ligand in a planar configuration is too small for the nickel(n), but it is possible for the ligand to fold to adopt a cisoid configuration. It is in this form that the ligand is co-ordinated to the nickel the metal is octahedral, with the two remaining coordination sites occupied by bromide ions and with near optimal Ni-N and Ni-S distances (Fig. 6-37). 

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