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 1 H NMR spectroscopy, the final product Eu 2 L 3 and the intermediate species EuL 2 and Eu 2 L 2 have been characterised. The presence of terminal carboxylates in L significantly reduces the electrostatic repulsions of the coordination sites in Eu 2 L 2 and Eu 2 L 3 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 EuL 2 or Eu 2 L intermediates depending on the experimental conditions and leads to a pre‐organized Eu 2 L 2 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 2 L 3 helicate. The overall process is mainly governed by electrostatic interactions and proceeds via a key double stranded intermediate helicate Eu 2 L 2 . 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)