KINETICS OF THE FORMATION OF MACROCYCLIC POLYAMINOCARBOXYLATE LIGAND COMPLEXES - A LASER-EXCITED LUMINESCENCE STUDY OF THE EU3-DTPA-DIEN SYSTEM()

Excitation spectroscopy of the F-7(0) --> D-5(0) transition of Eu3+ is used to detect and characterize a kinetic intermediate in the formati on of a complex between Eu3+ and the macrocyclic ligand dtpa-dien 9,17 -dioxo-1,4,7,10,13,16-hexaazacyclooctadecane). Both the long-lived int ermediate and the final product, [Eu(Hdtpa-dien)(H2O)](+), are formed immediately upon mixing the components, as evidenced by separate peaks in the excitation spectrum. The transformation of the intermediate to the final product, monitored by excitation spectroscopy, occurs by bo th proton-assisted and non-proton-assisted pathways. It is proposed th at the intermediate represents a ''blind alley'' in the pathway to a f inal nine-coordinate tricapped trigonal prismatic product. The interme diate with an ''up, down, up'' configuration of carboxylate arms must undergo some decoordination to form the final complex. Molecular mecha nics calculations and excited state lifetimes suggest that the interme diate is eight-coordinate with one coordinated water molecule. The sta bility constant of the final complex is found to be log K = 14.11 +/- 0.05.