Synthesis, thermodynamic properties and kinetics of the acid-catalyzed dissociation of nickel(II) diamido polyamino complexes

Abstract

The ligands 1,10-N,N′-bis(2-hydroxymethylbenzoyl)-1,4,7,10-tetraazadecane (L1) and 1,11-N,N′-bis(2-hydroxymethylbenzoyl)-1,4,8,11-tetraazaundecane (L2) have been synthesized. The stability constants of NiII complexes of ligands L1 and L2 have been studied at 25 °C using pH titrations. The kinetics of general acid (HCl, 0.04–2.34 mol dm−3) or buffer (DEPP or DESPEN, 0.05 mol dm−3, pH 4.83–5.72)-catalyzed dissociation of these NiII complexes have been investigated at 25 °C using a stopped-flow spectrophotometer. The ionic strength of solution was controlled at I = 2.34 mol dm−3 (KCl + HCl) and I = 0.1 mol dm−3 (KNO3, buffer), respectively. The kinetic dissociation of NiII complexes catalyzed by HCl obeys the equilibrium kobs = k1d + k2H[H+], whereas in buffer solution the observed rate constant kobs = kd + k1H[H+]. At pH 6. In the 4.8–5.7 pH range, the complexes dissociate mainly through a proton-assisted pathway.