Supramolecular Recognition of Heteropairs of Lanthanide Ions:  A Step toward Self-Assembled Bifunctional Probes

Three unsymmetrical ditopic hexadentate ligands coded for the recognition of trivalent lanthanide ions have been synthesized, L^AB, L^AC, and L^BC, where A represents a benzimidazole−pyridine−benzimidazole coordination unit, B a benzimidazole−pyridine−carboxamide one, and C a benzimidazole−pyridine−carboxylic acid moiety. Under stoichiometric 2:3 (Ln:L) conditions, these ligands self-assemble with lanthanide ions to yield triple-stranded bimetallic helicates having a sizable stability in acetonitrile:  log β₂₃ values for Eu are equal to 23.9 ± 0.5 (L^AB), 23.3 ± 0.7 (deprotonated L^AC), and 29.8 ± 0.5 (deprotonated L^BC). The crystal structure of the EuEu helicate with L^AB shows 9-coordinate metal ions and an HHH (H stands for head) configuration of the helically wrapped ligand strands. In the presence of equimolar quantities of Ln and Ln‘ ions, L^AB displays a remarkable predisposition to form HHH-heterobimetallic edifices, as proved both in the solid state by the crystal structures of the LaEu, LaTb, PrEr, and PrLu helicates and in solution by NMR spectroscopy. In all cases, the benzimidazole−pyridine−carboxamide units of the three ligands are bound to the smaller lanthanide ion, a fact further ascertained by high-resolution luminescence data on LaEu and by ¹H NMR. Analysis of the lanthanide-induced ¹H NMR shifts and of the spin−lattice relaxation times of the [LnLu(L^AB)₃]⁶⁺ series (Ln = Ce, Pr, Nd, Sm, Eu) demonstrates the isostructural nature of the complexes in solution and that the crystal structure of LaTb is a good model for the solution structure. The selectivity of L^AB for heteropairs of Ln^III ions increases with increasing difference in ionic radius, resulting in 70% of the heterobimetallic species for Δr_i = 0.1 Å and up to 90% for LaLu (Δr_i = 0.18 Å), and corresponding to Δ(ΔG) in the range 3−10 kJ·mol^-1. The origins of this stabilization are discussed in terms of the donor properties of the coordinating units and of the preferential formation of HHH isomers.