Reactions involving change of ligand unsaturation

The study just described is in accordance with the observation that electrochemical reduction of the (highly conjugated) phthalocyanine (5) complex of Mn(n) also gives no evidence for the formation of a Mn(i) species (in contrast to the corresponding iron and cobalt complexes which, on reduction, yield Fe(i) and Co(i) products) (Lever, Minor Wilshire, 1981). 

As mentioned previously, a large number of redox reactions involving macrocyclic ligand complexes have resulted in discrete changes in the unsaturation pattern of a variety of macrocyclic systems. Chemical, electrochemical, and catalytic reactions have been widely used to change the level of unsaturation in such systems. Although the mechanisms of the majority of such transformations are not well understood, it is clear that the reactions tend to proceed via prior change in the oxidation state of the central metal ion. 

Oxidative dehydrogenations of many macrocyclic ligand complexes have now been documented. Typically, these reactions involve conversion of coordinated secondary amines to imine groups. 

In early studies, reaction of the Ni(n) complexes (59) and (60) of the trans and cis (diimine) isomers of the Curtis macrocycle with nitric acid yielded the tetraimine species (294) and (295), respectively. There is strong evidence that these reactions proceed via Ni(m) intermediates 

The reverse reaction, namely hydrogenation, has also frequently been used to decrease the degree of unsaturation present in macrocyclic systems - typically converting imine linkages to amine groups. Such hydrogenations have usually been performed catalytically (for example, using H2 in the presence of Raney nickel or a precious metal catalyst) or by means of chemical reductants such as sodium borohydride. 

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