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 formrng a localized three-membered ring. The mechanisms for these reactions are discussed. The observation that the kinetic ratio k_Cl : k_Br 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.