Equilibria and kinetics of complex formation between zinc(II), lead(II), and cadmium(II), and 12-, 13-, 14-, and 15-membered macrocyclic tetra-amines

Abstract

Polarographic methods have been used to determine the equilibria and kinetics of reaction of Zn^II, Pb^II, and Cd^II with 12- to 15-membered macrocyclic tetra-amines including 1,4,7,10-tetra-azacyclododecane (L¹), 1,4,7,10-tetra-azacyclotridecane (L²), 1,4,8,11-tetra-azacyclotetradecane (L³), and 1,4,8,12-tetra-azacyciopentadecane (L⁴) in acetate buffer solutions. Unlike the copper(II) system. the stability constants of the zinc complexes hardly change with macrocyclic ring size : log K_ZnL= 16.2, 15.6, 15.5, and 15.0 for L¹, L², L³, and L⁴, respectively (I0.20 mol dm^–3 and 25 °C). The much larger Cd^II and Pb^II form complexes only with L¹: log K_CdL 14.3 and log K_PbL, 15.9. The 10³–10⁵ times greater stabilities of the macrocyclic complexes compared with the related complexes of linear terra-amines are all due to favourable entropy changes, regardless of the metal-ion size. The rate law for complex formation in acetate buffers is d[ML]/dt=k_H[M(O₂CMe)⁺][HL⁺]+k_2H[M(O₂CMe)⁺][H₂L²⁺] for all the metal ions. A comparison with reactions of Cu^II and L¹–L⁴ shows that the values of k_H parallel the well established rates of water exchange of the aquametal ions.