Structural and mechanistic studies of co-ordination compounds. Part 17. Preparation and acid hydrolysis of some cobalt(III) complexes containing the quadridentate macrocycle 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1] heptadeca-1 (17),2,11,13,15-pentaene: An extra-ordinary hydrolysis reaction

Abstract

The preparation and characterization of trans-[Co(HL1)X2]+ and [Co(L1)X]+ {HL1 = 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1] heptadeca-1 (17),2,11,13,15-pentaene L1 = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo-[11.3.1]heptadeca-1(17),2,11,13,15- pentaen-7-ylmethyl, X = Cl or Br} are described. The acid hydrolysis for the release of the first halide from trans-[Co(HL1)X2]+ is too fast to be followed by conventional techniques. The rates of release of the second halide from the corresponding aquahalogeno-complexes have been studied over a range of temperature. The immediate reaction product, [Co(L1)(OH2)]2+, for these two reactions has been shown by X-ray diffraction analysis on its chloro-analogue, [Co(L1)Cl][ClO4]·H2O, to be a distorted octahedral complex in which the N-methyl carbon, after losing one proton, is σ-bonded to the central cobalt(III) ion forming a localized three-membered ring. The mechanisms for these reactions are discussed. The observation that the kinetic ratio kCl: kBr is equal to 2.3 indicates that the softness of the central cobalt(III) ion is enhanced in this HL1 system as compared to that in other complexes of saturated macrocyclic amines.