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

Abstract Stopped flow spectrophotometry was used to study the complexation kinetics of the N-pendant arm macrocycle: N,N′,N″,N‴-tetrakis (2-carbamoylethyl)-1,4,8,11-tetraazacyclotetradecane (TCEC = L) reacting with two metal ions Hg(II) and Cu(II) in aqueous solutions at 25 °C and I = 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(II) 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 reactivities for the different ligand species $$ \left({k}_{{\mathrm{L}}_x\mathrm{H}}\right) $$ : $$ \mathrm{M}+{\mathrm{H}}_x\mathrm{L}\overset{k_{{\mathrm{H}}_x\mathrm{L}}}{\to }{\mathrm{MH}}_x\mathrm{L}\kern0.35em \left(\mathrm{M}=\mathrm{Hg}\left(\mathrm{II}\right),\mathrm{Cu}\left(\mathrm{II}\right)\right) $$ The rate constants calculated for the mono- and diprotonated species of TCEC were lower than those for the parent cyclam ligand by a factor of 102(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.