Acid-catalysed hydrolysis kinetics of carbonato cobalt(III) complexes with nonlabile tripodal quadridentate ligands

摘要

Acid-catalysed hydrolysis reaction velocity was studied by the stopped-flow method at 25 degrees C, ionic strength (I) of 2.0 (NaClO4) and [H+] = 0.02-1.0 M (M = mol dm(-3)) for seven cobalt(III) bidentate carbonato complexes [CoL(O2CO)](n+) with a uninegative or a dinegative tripodal quadridentate ligand L of fac(N)-5-dptma, mer(N)-5-dptma, fac(N)-1-dtma, mer(N)-i-dtma, mev(N)-i-2,3-ma, cis(O)-aeida and trans(O)-aeida where 5-dptma stands for N,N-bis(3-aminopropyl)glycinato, i-dtma for N,N-bis(2-aminoethyl)glycinato, i-2,3-ma for N-(2-aminoethyl)-N-(3-aminopropyl)glycinato, and aeida for N-(2-aminoethyl)-N-(carboxymethyl)glycinato. The time course of the absorbance of the acidic aqueous solution of the reactant complex was followed at relevant wavelength in the UV region and was analysed according to a program for the consecutive first-order reactions with two experimental rate constants k(fast) and k(slow). In the cases of L = fac(N)-5-dptma, mer(N)-i-dtma and trans(O)-aeida, both the k(fast) and the k(slow) increase with the increasing acidity of the solution. These results are incompatible with the current mechanism predicting one acidity dependent and the other acidity independent rate constants. Therefore, we have assumed additionally the deprotonation equilibrium from the intermediate complex cis-[CoL(OH2)(O2COH)](n+1) giving rise to an inert monodentate carbonato complex cis-[CoL(OH2)(O2CO)](n+) and derived from the acidity dependences of the k(fast) and the k(slow) the relevant equilibrium and rate constants. In the case of L = mer(N)-5-dptma, the k(slow) changes slightly with the acidity attaining a flat maximum around [H+] = 0.2 M, which we have interpreted by assuming an amphoteric character of the mer(N)-[Co(5-dptma)(OH2)(O2COH)](+). (C) 1998 Elsevier Science Ltd. All rights reserved. [References: 12]