NMR study of intramolecular and intermolecular exchange processes in the Zn(II) ion-iminodiacetic acid system

Abstract

Rate constants for intramolecular and intermolecular exchange processes involving the 1:2 complex formed between Zn(II) ion and iminodiacetic acid (IDA) have been determined by complete line-shape fitting of nuclear magnetic resonance (NMR) spectra. Experimental data obtained from spectra recorded at different temperatures on solutions containing metal ion and ligand in the 1:2 concentration ratio enabled the kinetics and activation parameters to be investigated ( ΔH ≠ = 11 ± 2 kcal/mol; ΔS ≠ = −9 ± 6 e.u. at 25°C) for the intramolecular acetate scrambling processes. Intermolecular ligand exchange reactions have been studied at 35°C on solutions containing ligand to metal ion concentration ratios ranging from 2.5 to 4.0 and evidence has been found for the occurrence of second-order exchange processes between the complex and both the unprotonated and monoprotonated ligand anions, whose rate constants have been evaluated (1.9 ± 0.2) × 10 4 M −1 sec −1 and (1.5 ± 0.2) × 10 3 M −1 sec −1, respectively).