Encapsulation of labile trivalent lanthanides into a homobimetallic chromium(III)-containing triple-stranded helicate. Synthesis, characterization, and divergent intramolecular energy transfers

The segmental bidentate–tridentate–bidentate ligand L2 reacts with MII (M = Cr, Zn) and LnIII (Ln = La, Eu, Gd, Tb, Lu) to give the heterotrimetallic triple-stranded helicates [MLnM(L2)3]7+. For M = ZnII, the isolated complexes ZnLnZn(L2)37 (Ln = Eu, Tb) display only lanthanide-centred luminescence arising from the pseudo-tricapped trigonal prismatic LnN9 coordination site. For M = CrII, rapid air oxidation provides CrIII and leads to the isolation of inert CrLnCr(L2)39 (Ln = Eu, Tb) complexes, in which divergent intramolecular Ln Cr energy transfers can be evidenced. Taking [ZnEuZn(L2)3]7+ as a luminescent standard for Eu-centred emission, a quantitative treatment of the energy migration processes indicates that the rate constant characterizing the Eu Cr energy transfer is more efficient in the trimetallic system, than in the analogous simple bimetallic edifice. Particular attention is focused on potential control of directional energy transfer processes in Cr–Ln pairs.