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

Abstract

Reaction of the macrocycle 3,6,9,17,20,23-hexaazatricyclo[23.3.1.1^11,15]triaconta-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 that correspond to the formation of mono- and bi-nuclear complexes. The rate constants for the two steps only differ by a factor of 7 and indicate that both metal ions are co-ordinated without significant interaction between them. However, the pH dependence of the rate constants suggests some kind of interaction of the buffering agent with the highly protonated forms of the macrocycle. 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 occurs in this case with two separate absorbance changes for the entry of both metal ions into the cavity of the macrocycle, and the analysis of the kinetic 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 H_xL^x+ (x = 4, 5 or 6), H₃CuL(ac)⁴⁺, H₂CuL(ac)³⁺, Cu₂L(ac)³⁺ and Cu₂L(ac)(OH)²⁺ have been determined from potentiometric titrations and their values used to separate the contributions from the different reaction pathways to the rate constants for complex formation. Although there are several contributions to the rate constant for the formation of mono- and bi-nuclear complexes, the analysis of kinetic data indicates that complex formation occurs essentially through reaction between those species that minimise the electrostatic repulsion. The rate constants for the reaction between Cu²⁺ and H₄L⁴⁺ (4.0 × 10⁴ dm³ mol^–1 s^–1) and between Cu(ac)₂ and H₂CuL(ac)³⁺ (1.2 × 10⁴ dm³ mol^–1 s^–1) are close to each other and also close to those previously reported for the formation of copper(II) complexes with the tetraprotonated forms of linear acyclic polyamines, which suggests that the macrocycle is flexible enough to make rapid any reorganisation step required for complex formation.

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