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 HNO3-H2SO4 nitrating mixture. In contrast to the more robust pentaamminecobalt(III) complexes, the nitrate imidazole ligand readily dissociates, providing an excellent synthetic route to a variety of nitroimidazoles. Pyridine coordinated to either Co(NH3)5(3+) or Co(en)2Cl2+ 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. Crystal data for bis(ethylenediamine)(benzimidazole)chlorocobalt(III) perchlorate, space group P1BAR, Z = 4, a = 8.168 (4) angstrom, b = 11.570 (4) angstrom, c = 21.162 (8) angstrom, alpha = 78.93 (2)-degrees, beta = 78.87 (2)-degrees, gamma = 69.24 (2)-degrees, and R = 6.67% for 3437 reflections; for bis(ethylenediamine)(pyridine)-chlorocobalt(III) chloride, space group P2(1)/n, a = 8.489 (3) angstrom, b = 14.021 (6) angstrom, c = 12.670 (6) angstrom, alpha = 90.0-degrees, beta = 91.63 (2)-degrees, gamma = 90.0-degrees, and R = 4.11% for 2185 reflections; for bis(ethylenediamine)(4-nitroimidazolato)chlorocobalt(III) chloride hydrate, space group P1BAR, a = 8.603 (5) angstrom, b = 7.358 (3) angstrom, c = 14.287 (7) angstrom, alpha = 95.82 (2)-degrees, beta = 86.79 (2)-degrees, gamma = 115.39 (2)-degrees, and R = 5.21% for 5903 reflections; for bis(ethylenediamine)(4-chloroimidazole)chlorocobalt(III) chloride, space group C2/c, Z = 8, a = 23.550 (6) angstrom, b = 11.805 (2) angstrom, c = 12.128 (3) angstrom, alpha = 90.0-degrees, beta = 115.92 (2)-degrees, gamma = 90.0-degrees, and R = 4.34% for 2377 reflections; and for (ethylenediamine)(4-nitrohistidinato-O,N,N')chlorocobalt(III) hydrate, space group P2(1).n, a = 8.544 (3) angstrom, b = 17.382 (5) angstrom, c = 10.256 (3) angstrom, alpha = 90.0-degrees, beta = 107.50 (2)-degrees, gamma = 90.0-degrees, and R = 5.60% for 2556 reflections, are reported.