Self-assembly mechanism of a bimetallic europium triple-stranded helicate

We report the self-assembly process of a supramolecular edifice based on the coordination of europium(Ill) by a ditopic strand L bearing tridentate bis(benzimidazolyl)pyridine subunits. Varying the metal/ligand ratio and using a fruitful combination of electrospray mass spectrometry and absorption spectrophotometry, we characterized three major complexes (EuL2, EU2L2, and Eu2L3) in acetonitrile. Kinetic investigations showed an alternative braiding and keystone mechanism leading to Eu2L3. The formation mechanism of the dinuclear triple-stranded helicate, which is mainly governed by electrostatic interactions, goes via the side-by-side Eu2L2 intermediate. Our thermodynamic and kinetic data allow the prediction of the apparent magic self-assembly of Eu2L3 which is fast and efficient only under a strict set of conditions.