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 solvo lysis reaction have been studied for lanthanide(III)-acyclic polyaminocarbo xylate complexes, as the basis for kinetically controlled selectivity used in analytical methodologies such as HPLC and HPCE. The rate constants for s olvolysis and acid-assisted dissociation processes of the lanthanide comple xes were determined in a batch system through metal- and ligand-exchange re actions. The reagents used were 8-amino-2-[(2-amino-5-methyllphenoxy)methyl ]-6-methylquinoline-N,N,N ' ,N '- tetraacetic acid (Quin2) and O,O ' -bis(2 -aminophenyl)ethylene glycol-N,N,N ' ,N ' -tetraacetic acid (BAPTA) as octa dentate ligands and trans-1,2-diaminocyclohexane-N,N,N ' ,N ' -tetraacetic acid (BAPTA) as a hexadentate ligand. It has been found that the rate const ants for solvolysis vary front 5.7 x 10(-3) s(-1) (La3+) to 1.7 x 10(-6) s( -1) (Lu3+) depending on the ionic radii of Ln(III) ions for the Quin2 compl exes, while no such monotonic dependence was observed for the BAPTA complex es. 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-polyam inocarboxylate complexes. Our data suggest that multiple ligation of the li gand in favor of the large coordination number of Ln(III) ions is of key im portance for formation of the negative entropy of activation, in addition t o the basicity of-the ligand which also plays a significant role in the slo w dissociation kinetics of the Ln(III) complexes.