Hexanuclear Ln₆ L₆ Complex Formation by Using an Unsymmetric Ligand

Multinuclear, self-assembled lanthanide complexes present clear opportunities as sensors and imaging agents. Despite the widely acknowledged potential of this class of supramolecule, synthetic and characterization challenges continue to limit systematic studies into their self-assembly restricting the number and variety of lanthanide architectures reported relative to their transition metal counterparts. Here we present the first study evaluating the effect of ligand backbone symmetry on multinuclear lanthanide complex self-assembly. Replacement of a symmetric ethylene linker with an unsymmetric amide at the center of a homoditopic ligand governs formation of an unusual Ln₆ L₆ complex with coordinatively unsaturated metal centers. The choice of triflate as a counterion, and the effect of ionic radii are shown to be critical for formation of the Ln₆ L₆ complex. The atypical Ln₆ L₆ architecture is characterized using a combination of mass spectrometry, luminescence, DOSY NMR and EPR spectroscopy measurements. Luminescence experiments support clear differences between comparable Eu₆ L₆ and Eu₂ L₃ complexes, with relatively short luminescent lifetimes and low quantum yields observed for the Eu₆ L₆ structure indicative of non-radiative decay processes. Synthesis of the Gd₆ L₆ analogue allows three distinct Gd⋯Gd distance measurements to be extracted using homo-RIDME EPR experiments.