Electrophilic Reactions of Co-ordinated Pyridines

In general, free ligand pyridines show a great reluctance to take part in electrophilic substitution reactions. Forcing conditions are frequently required, and low yields and specificity are normally observed. In principle, co-ordination to a metal ion capable of back-donation should increase the tendency for electrophilic attack, since back-donation results in an increase in n electron density. 

A similar pattern of reactivity is observed in quinoline complexes and very detailed studies have confirmed that 8-hydroxyquinolinesulphonic acid undergoes extremely rapid halogenation at the 7-position in the presence of metal ions (Fig. 8-39). Once again, the site of attack is the same as in the free ligand. Similar reactions of 8-hydroxy quinoline complexes occur with a variety of electrophiles. 

The electrophilic reactions of co-ordinated 1,10-phenanthrolines are not always as simple as might be expected. Thus, the nitration of cobalt(m) 1,10-phenanthroline complexes yields 5-nitro-1,10-phenanthroline derivatives at low temperature, but prolonged reaction in hot solution leads to further reaction and oxidation of the ligand to give excellent yields of 1,10-phenanthroline-5,6-quinone complexes (Fig. 8-40). Even after the formation of the quinone, the complexes may exhibit further reaction. For example, reaction of the l,10-phenanthroline-5,6-quinone complexes with base results in the formation of a complex of 2,2 -bipyridine-3,3 -dicarboxylic acid (Fig. 8-41) 

The formation of this dicarboxylic acid derivative is of some interest. The free ligand quinone is known to undergo a benzilic acid rearrangement with concomitant ring contraction, followed by decarboxylation to yield a diazafluoreneone on treatment with base (Fig. 8-42). 

The oxidation of the metal complexes of l,10-phenanthroline-5,6-quinone is thought to proceed in a similar manner, with the first step being a benzilic acid rearrangement. Rearrangements of this type may also be followed directly in nickel(u) and cobalt(m) complexes of 2,2 -pyridil. The first step of the reaction involves nucleophilic attack on an O-bonded carbonyl group to form a hydrate, followed by a benzilic acid rearrangement. In this case, the benzilic acid rearrangement products may be isolated as metal complexes (Fig. 8-43). 

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