Lanthanide homobimetallic triple-stranded helicates:: Insight into the self-assembly mechanism

The self-assembly mechanism leading to the exclusive formation of a triple-stranded bimetallic helicate upon reaction of Eu-III with a ditopic hexadentate ligand L bearing two carboxylate moieties has been fully elucidated in water for a wide range of [Eu](tot)/[L](tot) ratios. Using a fruitful combination of electrospray mass spectrometry, potentiometry, UV/Vis spectrophotometry, luminescence, and H-1 NMR spectroscopy, the final product Eu2L3 and the intermediate species EuL2 and Eu2L2 have been characterised. The presence of terminal carboxylates in L significantly reduces the electrostatic repulsions of the coordination sites in Eu2L2 and Eu2L3 compared with the corresponding complexes formed with analogous neutral ligands and thus increases the stability of the L-europium(III) complexes. Kinetic investigations carried out with an excess of L and with an excess of Eu-III, show that the self-assembly proceeds through either EuL2 or Eu2L, intermediates depending on the experimental conditions and leads to a pre-organized Eu2L2 complex by either a "braiding" or a "keystone" mechanism. In the last step, a fast and efficient wrapping of the third ligand strand leads to the target Eu2L3 helicate. The overall process is mainly governed by electrostatic interactions and proceeds via a key double stranded intermediate helicate Eu2L2. To the best of our knowledge, as a result of the fine-tuning of the coordination properties of L, we present one of the most efficient and cooperative metal/ligand systems for the spontaneous organization of a bimetallic triple-stranded structure. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004).