Effect of substituents on the stability of some lanthanum phenolate ion pairs

摘要

The objects of the present work were, firstly, to investigate the effect of substituents on the stability of ion pairs of the La[3+]-ion with some phenolate anions, and, secondly, to test the applicability of the spectrophotometric method in determining the association constants of such ion pairs. The first objective requires a knowledge of the dissociation constants of the phenols and the association constants of the respective lanthanum ion pairs. Most of the thermodynamic dissociation constants of the weak acids used in the present work have been determined with high accuracy. However, as there was some uncertainty regarding the reliability of the published data for 3- and 5-nitro salicylaldehyde, the thermodynamic dissociation constants of these acids have been redetermined, using a modified form of the spectrophotometric method of Ernst and Menashi. Experimental evidence for ion association in the lanthanum phenolate solutions was obtained by comparing the absorption curves recorded for two series of solutions, one of which contains the phenol only and the other,both lanthanum and the phenol. For the purpose of determining the association constants of the ion pairs studied in the present work, one new optimization and three new iteration procedures based solely on spectrophotometric measurements, were developed. By these means, the association constants of six lanthanum salicylate, three lanthanum/ salicylaldehyde and four lanthanum benzoate ion pairs have been determined. It has been found that a rough linear correlation exists between the stability of the metal complexes and that of the corresponding proton complexes; the results for the lanthanum salicylates and benzoates falling on one straight line and the results for lanthanum/salicylaldehyde ion pairs falling on another. The plots of the logarithm of the association constants for these ion pairs, against the reciprocal of the Bjerrum distance of closest approach have been found to give very good straight lines, one for the lanthanum salicylates and benzoates and another for the lanthanum/ salicylaldehyde ion pairs. It has been shown that the lanthanum salicylates involve only the carboxylate group of the anion and that, ion-dipole interaction between the La[3+]-ion and the -OH group is negligible. However, on the other hand, ion-dipole interaction between the La[3+]-ion and the aldehyde group plays' some part in the stabilization of the La[3+]-/salicylaldehyde ion pairs.