Dissociation kinetics of 1,7-diaza-4,10-,13-trioxacyclopentadecane-N,N'-diacetic acid complexes of lanthanides

The dissociation kinetics of 1,7-diaza-4,10,13-trioxacyclopentadecane-N,N'-diacetic acid (K21DA) complexes of lanthanide(III) ions were studied in acetate-acetic acid buffer medium, over the acid concentration range of 8.4 x 10/sup -6/-2.5 x 10/sup -4/ M and at a constant ionic strength of 0.1 M (LiClO/sub 4/). Copper(II) was used as the scavenger of free ligand, and the rates of dissociation of these complexes have been found to be independent of (Cu/sup 2 +/). All the complexes exhibit acid-independent and acid-dependent pathways. Lighter lanthanide complexes display a first-order dependence upon (H/sup +/) in the pH range studied. The complexes of heavier lanthanides show (H/sup +/) dependence at low acid concentrations but become acid-independent at high acid concentrations. Influence of acetate content in the buffer and total electrolyte concentration on the rate of dissociation has also been investigated. The observed rate constants for erbium, ytterbium, and lutetium complexes do not show a significant dependence on acetate concentration, but lanthanum and europium complexes do exhibit a first-order dependence on (acetate). All the complexes under study respond similarly with change in electrolyte concentration; i.e., the rate constants decrease with increase in (electrolyte). Activation parameters for both self-dissociation and acid-catalyzed dissociation pathways have been obtained for lanthanum, europium, erbium, and lutetium complexes, from the temperature dependence of rate constants in the 15-45 /sup 0/C range. The results are compared with those of the lanthanide-polyamino polycarboxylate systems, and possible mechanisms are discussed. 41 references, 4 figures, 6 tables.