KINETICS OF FORMATION AND DISSOCIATION OF THE 1,4,7-TRIAZACYCLONONONANE-N,N',N''-TRIACETATE COMPLEXES OF CERIUM(III), GADOLINIUM(III), AND ERBIUM(III) IONS

The formation rate of the NOTA complexes of lanthanide(III) ions (Ln3+: Ce3+, Gd3+, and Er3+) was found to be slow enough between pH 4.5 and 5.8 to be followed by conventional spectrophotometry. The reactions take place by rapid formation of a monoprotonated intermediate, which deprotonates in a slow, rate-determining step. Rearrangement of the intermediate to form the stable complex is catalyzed by OH-ions. Exchange reactions occurring between the Ln(NOTA) complexes and Cu2+or Zn2+ions take place by spontaneous as well as proton-assisted dissociation of the complexes. The rates of both the spontaneous and proton-catalyzed dissociation decrease significantly with the smaller radii lanthanide cations while formation rates do not differ appreciably with cation size. Thus, the increase in stability constants of the Ln(NOTA) complexes with increasing atomic number of lanthanides mainly results from a decrease in dissociation rates. © 1990, American Chemical Society. All rights reserved.