Kinetics of the reaction of copper(II) with meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane

Abstract

Copper(II) reacts with meso-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane (L¹) in aqueous solution (pH 4–6) to give [CuL¹]²⁺(blue) with λ_max. 620 nm, At higher pH (ca. 9), [CuL¹]²⁺(blue) is slowly converted into [CuL¹]²⁺(violet) with λ_max. 520 nm. The kinetics of formation of the blue complex have been studied at 25 °C and I= 0.1 mol dm^–3 in both acetate and piperazine-NN′-bis(ethane-2-sulphonic acid) buffers. In the absence of acetate, Rate =k₁[Cu²⁺][HL¹⁺] but in acetate buffers Rate =k1[Cu²⁺][HL¹⁺]+k₂[Cu(O₂CMe)][HL¹⁺]+k₃[Cu(O₂CMe)⁺][H₂L^{1 2+}], with k₁= 3.6 × 10³ dm³ mol^–1 s^–1, k₂= 1.8 × 10⁴ dm³ mol^–1 s^–1, and k₃= 1.5 × 10^–2dm³ mol^–1 s^–1. Possible mechanisms for the reaction are considered.