Entropy-controlled solvolytic dissociation kinetics of lanthanide(III) complexes with polyaminocarboxylates in aqueous solutions

The factors involved in the formation of an inert complex in terms of solvolysis reaction have been studied for lanthanide (III)-acyclic polyaminocarboxylate complexes, as the basis for kinetically controlled selectivity used in analytical methodologies such as HPLC and HPCE. The rate constants for solvolysis and acid-assisted dissociation processes of the lanthanide complexes were determined in a batch system through metal-and ligand-exchange reactions. The reagents used were 8-amino-2-[(2-amino-5-methylphenoxy) methyl]-6-methoxyquinoline-N,N,N′,N′-tetraacetic acid (Quin2) and O,O′-bis(2-aminophenyl)ethylene glycol-N,N,N′,N′-tetraacetic acid (BAPTA) as octadentate ligands and trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CyDTA) as a hexadentate ligand. It has been found that the rate constants for solvolysis vary from 5.7 × 10^-3 s^-1 (La³⁺) to 1.7 × 10^-6 s^-1 (LU³⁺) depending on the ionic radii of Ln(III) ions for the Quin2 complexes, while no such monotonic dependence was observed for the BAPTA complexes. Among the parameters of activation, it is worth noting that there is a considerably large negative entropy of activation, of up to -250 J mol^-1 K^-1, and it is this which is responsible for the inertness of the Ln-polyaminocarboxylate complexes. Our data suggest that multiple ligation of the ligand in favor of the large coordination number of Ln(III) ions is of key importance for formation of the negative entropy of activation, in addition to the basicity of the ligand which also plays a significant role in the slow dissociation kinetics of the Ln(III) complexes.