Kinetics and mechanism of complex formation of a pendant arm macrocycle reacting with copper(II)‎

Abstract EN

Stopped flow spectrophotometry was used to study the complication kinetics of the Append ant arm macro cycle: A', A'',A'", A'"'-terraces (2-carbamoylethyl)-l, 4, 8, 1-tetraazacyclotetradec-ane (TCEC = L) reacting with two metal ions Hg(II) and Cu(II) in aqueous solutions at 25 °C and / = 0.2 M (Hg) and 0.5 M (Cu), respectively.

All the kinetic measurements were carried out under pseudo-first-order conditions in the pH ranges 4.0-5.2 and 5.0-6.6 for the Cu (II) / TCEC and Hg (II) / TCEC systems, respectively, with the total ligand concentration at least 10-fold excess of the total concentration of Cu (Il) and the total concentration of Hg (II) at least 10-fold excess of the total concentration of the ligand.

The complex formation is found to be a one-step second order reaction (first order in both the metal ion and the ligand).

The pH dependence was successfully analyzed in terms of the different reactivity's for the different ligand species :(Klhj) M + LHAA™* MLH, (M = Hg (II), Cu (II)) The rate constants calculated for the mono-and deprotonated species of TCEC were lower than those for the parent cyclamen ligand by a factor of 10"(Cu).

These results strongly indicate the noninvolvement of the pendant arms of TCEC in the complex formation reaction between the metal ion and the partially protonated ligand species and that the rate determining step is the first bound of the metal ion to the tertiary ring nitrogen for both ions.