Kinetics and mechanism of displacement of zinc(II) by copper (II) in complexes of saturated macrocyclic tetra-amines

Abstract

The kinetics of displacement of Zn²⁺ by Cu²⁺ from complexes of saturated 12- to 15-membered macrocyclic tetra-amines have been investigated in acetate-buffer solutions. Kinetic data support a common mechanism : [ZnL]²⁺+ MeCO₂H ⇌[Zn(O₂CMe)]⁺+ HL⁺(k₁, k_–1); HL⁺+[Cu(O₂CMe)]⁺→[CuL]²⁺+ MeCO₂H (k₂), where the dissociation of L from Zn²⁺ is the slowest step. Values for k₁ and k_–1/k₂ show good agreement with previous values obtained independently. The activation parameters for k₁ and k_–1 are discussed. The pre-dissociation mechanism is the same as that recently proposed for similar metal-exchange reactions in porphyrin complexes in dimethylformamide solutions.