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-triazacy clopentadecane-4,10,13-triacetic acid, 1, and 1,7-dioxa- 10,13-triadza cyclopentadecane-4,10,13-tripropionic acid, 2, have been studied by us ing stopped-flow and conventional spectrophotometry, The measurements were made at 25.0 +/- 0.1 degrees C in an aqueous solution of 0.1M (Na CIO4) ionic strength. The formation reaction takes place by rapid form ation of an intermediate complex (MM(2)L(+)) in which the metal ion i s 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 t he 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 concentra tion. 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 t he metal ions, the chelate ring size and rigidity of the ligands.