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]<INF>tot</INF>/[L]<INF>tot</INF> ratios. Using a fruitful combination of electrospray mass spectrometry, potentiometry, UV/Vis spectrophotometry, luminescence, and ¹H NMR spectroscopy, the final product Eu<INF>2</INF>L<INF>3</INF> and the intermediate species EuL<INF>2</INF> and Eu<INF>2</INF>L<INF>2</INF> have been characterised. The presence of terminal carboxylates in L significantly reduces the electrostatic repulsions of the coordination sites in Eu<INF>2</INF>L<INF>2</INF> and Eu<INF>2</INF>L<INF>3</INF> compared with the corresponding complexes formed with analogous neutral ligands and thus increases the stability of the L-europium(<SC>III</SC>) 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 EuL<INF>2</INF> or Eu<INF>2</INF>L intermediates depending on the experimental conditions and leads to a pre-organized Eu<INF>2</INF>L<INF>2</INF> 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 Eu<INF>2</INF>L<INF>3</INF> helicate. The overall process is mainly governed by electrostatic interactions and proceeds via a key double stranded intermediate helicate Eu<INF>2</INF>L<INF>2</INF>. 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. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)