Electrophilic substitution reactions in heterocycles complexed to exchange-inert metal ions. (Ethylenediamine)cobalt(III) complexes of imidazole and histidine

(Ethylenediamine)cobalt(III) complexes of imidazole and histidine are nitrated under mild conditions with a standard HNO{sub 3}-H{sub 2}SO{sub 4} nitrating mixture. In contrast to the more robust pentaamminecobalt(III) complexes, the nitrated imidazole ligand readily dissociates, providing an excellent synthetic route to a variety of nitroimidazoles. Pyridine coordinated to either Co(NH{sub 3}){sub 5}{sup 3+} or Co(en){sub 2}Cl{sup 2+} is not nitrated, even under conditions that destroy the complex. The difference in reactivity is evidently a reflection of the Co(III) d electron interaction with the lowest unoccupied molecular orbital (LUMO) of the ligand. The nucleophilic displacement of a nitro group by chloride in a coordinated nitroimidazole is also observed.