Formation kinetics of triaza-crown-alkanoic acid complexes of first-row transition metal(II) ions

The complex formation reactions of divalent metal ions of Co2+, Ni2+, CU2+ and Zn2+ with the macrocyclic ligands 1,7-dioxa- 4,10,13-triazacyclopentadecane-4,10,13-triacetic acid, 1, and 1,7-dioxa- 4,10,13-triadzacyclopentadecane-4,10,13-tripropionic acid, 2, have been studied by using stopped-flow and conventional spectrophotometry, The measurements were made at 25.0 +/- 0.1 degrees C in an aqueous solution of 0.1M (NaCIO4) ionic strength. The formation reaction takes place by rapid formation of an intermediate complex (MM(2)L(+)*) in which the metal ion is incompletely coordinated. This may then proceed to the final product in the rate-determining step, The stability constants (logK(MH2L)+*) and the second order rate constants (k(OH)) for the rearrangement of the intermediate complex have been determined from the kinetic data, In the pH range examined, the monoprotonated (HL(2-)) form of ligands 1 and 2 is the kinetically active species despite of their low concentration. The structure of the intermediate is proposed In which the metal ions are coordinated to oxygens and one nitrogen of ligands 1 and 2. These observations are also discussed in terms of the ionic radii of the metal ions, the chelate ring size and rigidity of the ligands.