Synthesis and kinetic behaviour of lanthanide(III) complexes with the mixed pendant-arm macrocyclic ligand 1,7-bis(carboxymethyl)-4,10-bis(1-methylimidazol-2-ylmethyl)-1,4,7,10-tetraazacyclododecane

Abstract

The La^III, Eu^III and Lu^III complexes with the potentially octadentate new ligand 1,7-bis(carboxymethyl)-4,10-bis(1-methylimidazol-2-ylmethyl)-1,4,7,10-tetraazacyclododecane (H₂L) which bears two pairs of different dangling groups in trans positions on the cyclen skeleton, have been synthesized. The low-temperature limiting NMR spectra (¹H and ¹³C) of the compounds support the presence in solution of one species with an approximately prismatic geometry for the lanthanum derivative and of two isomers with nearly square-prismatic and -antiprismatic geometries for the europium and the lutetium complexes. Both the dangling groups and the ethylenic moieties of the ligand are rigid in all the species at low temperature. The rigidity is lost at higher temperatures yielding a dynamic behaviour which proceeds through both ring inversion and rearrangement of the pendant arms. The kinetic parameters for the ring inversion of the lanthanum derivative have been obtained from the temperature-dependent ¹³C NMR spectra in D₂O: k(298 K) = 107 s⁻¹; ΔH^‡ = 61.4 kJ mol⁻¹; ΔS^‡ = −0.4 J K⁻¹ mol⁻¹.