Kinetics and mechanism of complex formation: the reaction of nickel(II) with 1,4,7-triazacyclononane-N,N′,N″-triacetic acid

Conventional and multi wavelength stopped-flow spectrophotometry was used to study the kinetics of the reaction of Ni2+ ions with the hexadentate macrocyclic ligand TCTA (1,4,7- triazacyclononane-N,N′,N″-triacetic acid) to yield the complex Ni(TCTA)−. The experiments were carried out at variable pH (4.4–6.8) and variable nickel concentration in buffered aqueous solution at I=0.8 M (NaClO4) and 288 K. In the presence of an excess of nickel, the formation of the complex Ni(TCTA)− is first order in [TCTA]. The experimental rate constant, kabs, of the rate law obtained, rate = kabs[TCTA], depends on the nickel concentration and on the proton concentration according to kabs=kK[Ni2+]/(1 + K[Ni2+]) and k=kH/(1 + [H+]/KH), with K = 13±1 M−1, kH=154 ±15 s−1 and pKH=5.77±0.04. The value obtained for pKH corresponds to the pKa of the ligand species H2TCTA−. The results are in agreement with the mechanistic interpretation that; (i) nickel ions and the ligand species HTCTA2− equilibrate rapidly (equilibrium constant K) to form the intermediate Ni(HTCTA), which is subject to protonation (KH); (ii) Ni(TCTA)− is formed by slow first-order rearrangement (kH) of the intermediate Ni(HTCTA) under proton release. The experimental evidence describing the state of coordination of the intermediate Ni(HTCTA) is discussed.