| Abstract
| - Excitation spectroscopy of the 7F0 →5D0 transition of Eu3+ is used todetect and characterize a kinetic intermediatein the formation of a complex between Eu3+ and themacrocyclic ligand dtpa-dien(1,4,7-tris(carboxymethyl)-9,17-dioxo-1,4,7,10,13,16-hexaazacyclooctadecane).Both the long-lived intermediate and the final product,[Eu(Hdtpa-dien)(H2O)]+, are formedimmediately upon mixing the components, as evidenced by separate peaksinthe excitation spectrum. The transformation of the intermediate tothe final product, monitored by excitationspectroscopy, occurs by both proton-assisted and non-proton-assistedpathways. It is proposed that the intermediaterepresents a “blind alley” in the pathway to a finalnine-coordinate tricapped trigonal prismatic product.Theintermediate with an “up, down, up” configuration of carboxylatearms must undergo some decoordination toform the final complex. Molecular mechanics calculations andexcited state lifetimes suggest that the intermediateis eight-coordinate with one coordinated water molecule. Thestability constant of the final complex is found tobe log K = 14.11 ± 0.05.
- Upon mixing the macrocyclic ligand,dtpa-dien, with Eu3+ both the final product complex and along-lived intermediate are rapidly formed.7F0 → 5D0luminescence excitation spectroscopy of Eu3+ is used tofollow the time course of conversion of the intermediate into the finalproduct. The intermediate is characterized (eight-coordinate withone water molecule), and a detailed mechanism involving somedecoordination of the ligand on the way to the final product isproposed.
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