Kinetic studies on the reactions of macrocyclic complexes: formation of mono- and bi-nuclear copper(II) complexes with a binucleating hexaazamacrocycle in slightly acidic solutions

Reaction of the macrocycle 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1(11,15)]t riaconta-1(29),11(30),12,14,25,27-hexaene (L) with Cu-II in borate-mannitol buffers at pH close to 5 occurred with two kinetically separated steps tha t correspond to the formation of mono- and bi-nuclear complexes The rate co nstants for the two steps only differ by a factor of 7 and indicate that bo th metal ions are co-ordinated without significant interaction between them . However, the pH dependence of the rate constants suggests some kind of in teraction of the buffering agent with the highly protonated forms of the ma crocycle. Since this interaction is difficult to analyse due to the complex composition of the buffer, the kinetics of reaction have also been studied in the presence of the simpler acetate buffers. Complex formation also occ urs in this case with two separate absorbance chances for the entry of both metal ions into the cavity of the macrocycle, and the analysis of the kine tic data is facilitated by a previous equilibrium study on the formation of ligand-acetate and Cu-L-acetate complexes. The stability constants of the species HxLx+ (x = 4, 5 or 6), H3CuL(ac)(4+), H2CuL(ac)(3+), Cu2L(ac)(3+) a nd Cu2L(ac)(OH)(2+) have been determined from potentiometric titrations and their values used to separate the contributions from the different reactio n pathways to the rate constants for complex formation. Although then are s everal contributions to the rate constant for the formation of mono- and bi -nuclear complexes, the analysis of kinetic data indicates that complex for mation occurs essentially through reaction between those species that minim ise the electrostatic repulsion. The rate constants for the reaction betwee n Cu2+ and H4L4+ (4.0 x 10(4) dm(3) mol(-1) s(-1)) and between Cu(ac)(2) an d H2CuL(ac)(3+) (1.2 x 10(4) dm(3) mol(-1) s(-1)) are close to each other a nd also close to those previously reported for the formation of copper(II) complexes with the tetraprotonated forms of linear acyclic polyamines, whic h suggests that the macrocycle is flexible enough to make rapid any reorgan isation step required for complex formation.