FORMATION AND DISSOCIATION KINETICS OF TETRAAZA-CROWN-ALKANOIC ACID COMPLEXES OF CERIUM(III)

The formation and dissociation rates of Ce3+ Complexes of the 1,4,7,10 -tetraaza-13,16-dioxacyclooctadecane-N,N '',N'''-tetraacetic acid (1), ,4,7,10-tetraaza-13,16-dioxacyclooctadecane-N,N',N '',N'''-tetramethy lacetic acid (2), and ,4,7,10-tetraaza-13,16-dioxacyclooctadecane-N,N' ,N '',N'''-tetrapropionic acid (3) have been measured by the use of st opped-now spectrophotometry. Observations were made at 25.0+/-0.1 degr ees C and at an ionic strength of 0.10 M NaClO4,. The complexation of Ce3+ ion with 1 and 2 proceeds through the formation of an intermediat e complex (CeH3L2+) in which the Ce(3+ )ion is incompletely coordinat ed. This may then lead to be a final product in the rate-determining s tep. Between pH 4.76 and 5.76, the diprotonated (H2L2-) from is reveal ed to be a kinetically active species despite of its low concentration . The stability constants (logK((CeH3L2+))) and specific water-assist ed rate constants (k(OH)) of intermediate complexes have been determin ed from the kinetic data. The dissociation reactions of Ce3+ complexes of 1, 2, and 3 were investigated with Cu2+ ions as a scavenger in ace tate buffer. All complexes exhibit acid-independent and acid-catalyzed contributions. The effect of buffer and Cu2+ concentration on the dis sociation rate has also been investigated. The ligand effect on the di ssociation rate of Ce3+ complexes is discussed in terms of the side-pe ndant arms and the chelate ring sizes of the ligands.