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

The La-III, Eu-III and Lu-III complexes with the potentially octadentate ne w 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. T he low-temperature limiting NMR spectra (H-1 and C-13) of the compounds sup port the presence in solution of one species with an approximately prismati c geometry for the lanthanum derivative and of two isomers with nearly squa re-prismatic and -antiprismatic geometries for the europium and the lutetiu m complexes. Both the dangling groups and the ethylenic moieties of the lig and are rigid in all the species at low temperature. The rigidity is lost a t higher temperatures yielding a dynamic behaviour which proceeds through b oth ring inversion and rearrangement of the pendant arms. The kinetic param eters for the ring inversion of the lanthanum derivative have been obtained from the temperature-dependent C-13 NMR spectra in D2O: k(298 K)=107 s(-1) ; DeltaH(double dagger)=61.4 kJ mol(-1); DeltaS(double dagger)=-0.4 J K-1 m ol(-1).