Metal macrocycle hole size

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... 

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... 

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). 

Despite such great interest, a question that is still unanswered is why modified tetrapyrrole ligands, like those found in factor F430 and vitamin B12, are employed by natural systems to carry out specific chemistry rather than the porphyrin ligand. A possible explanation that has been proposed [1, 2] is that these modified tetrapyrrole ligands exhibit different flexibility as compared with porphyrins. Another very important factor, and probably the most important one in the case of corroles and corrinoids, is the difference in hole size between the various macrocycles. The tetrapyrrole that would most efficiently perform a specific function would be the one with the proper hole size for the radius of the metal ion involved in the process. 

Figure 6-25. The hole size of a macrocyclic ligand defines the available bonding cavity for a metal ion. As a first step, a best-fit circle is drawn through the donor atoms, and the radius of this circle measured.

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. 

In many cases it is possible to utilise the hole size effects for the synthesis of specific types of macrocycle. Thus, a tetradentate macrocycle (6.33) is expected to be obtained from a template condensation of 2,6-diacetylpyridine with 1,5,9-triazanonane in the presence of small, first-row transition metal dications. The hole size of 6.33 closely matches the size of these metal ions. This is indeed what happens when Ni2+ (r = 0.8 A) is used as a template for the condensation and the nickel(n) complex of 6.33 is obtained in good yield (Fig. 6-32). However, when Ag+ (r = 1.0 A) is used as a template, the metal ion... 

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-... 

One of the more interesting hole size effects arises when the metal ion successfully acts as a template, but is labilised in the macrocyclic complex that is formed. The consequence of this is that the metal ion acts as a transient template. The metal ion may be viewed as pre-organising the reactants to form the macrocyclic products, but then finding itself in an unfavourable environment after the cyclisation. The effect is best observed when a small metal ion is used as a template for a reaction that can only give one product (or at least, only one likely product). What happens to the metal ion when it finds itself in an environment that does not match up to its co-ordination requirements The most useful consequence would be labilisation of the metal ion, with resultant demetallation and formation of the metal-free macrocycle. This would overcome one of the major disad-... 

Let us consider the [2+2] macrocyclic ligand 6.39, which is prepared by the non-template condensation of 1,2-diaminobenzene with 2,6-diformylpyridine. The hole size of this ligand is about 1.3 A, so we would expect it to be too large to bind first-row transition metal dications. As a matter of interest, the ligand binds K+, with an ionic radius of 1.38 A, more strongly than does the crown ether, 18-crown-6. In the absence of any che-... 

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