LIGAND BASICITY AND RIGIDITY CONTROL FORMATION OF MACROCYCLIC POLYAMINO CARBOXYLATE COMPLEXES OF GADOLINIUM(III)

The formation reaction rates of some macrocyclic polyamino carboxylate complexes of gadolinium, GdL (where L is DO3A = 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, H3L, HP-DO3A = 10-(hydroxypropyl)-1,4,7,-10-tetraazacyclododecane-1,4,7-triacetic acid, H3L, and DO3MA = (1R,4R,7R)-alpha,alpha',alpha''-trimethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-triacetic acid, H3L), have been measured at 25.0 +/- 0.1-degrees-C and at a constant ionic strength of 1.0 (NaCl) by an indicator method. The formation reactions are first order in the limiting reagent (ligand) and nearly independent of the excess reagent (gadolinium ion). A mechanism of the formation of the gadolinium complexes involves the formation of a precursor (intermediate) complex, Gd(*HL), in an equilibrium step followed by its deprotonation and reorganization to the final product in the rate-determining step. The stability constants (log K(Gd(*HL))) of the intermediate have been determined from the kinetic data and the values are 8.9 (DO3A), 9.0 (HP-DO3A), and 10.7 (DO3MA). The nature of the intermediate is proposed in which the metal is coordinated to oxygens and at least one nitrogen of the ligand. Deprotonation and reorganization of the intermediate are specific-base assisted. The second-order rate constants (k(OH), M-1 s-1) for the reorganization of the intermediate, Gd(*HL) (L are given in the parentheses), are (2.1 +/- 0.1) X 10(7) (DO3A), (1.23 +/- 0.04) X 10(7) (HP-DO3A), and (7.2 +/- 0.3) x 10(4) (DO3MA), compared to the literature data (7.1 +/- 1) X 10(7) (NOTA) and (5.9 +/- 0.2) x 10(6) (DOTA). The specific-base assisted rate of reorganization of the intermediate, Gd(*HL), is correlated with the ligand strain energy and its first protonation constant. These observations lead us to conclude that the rate of reorganization of the intermediate is governed by the basicity and rigidity of the ligand.