Tuning facial–meridional isomerisation in monometallic nine-co-ordinate lanthanide complexes with unsymmetrical tridentate ligands

Abstract

The unsymmetrical tridentate benzimidazole–pyridine–carboxamide units in ligands L1–L4 react with trivalent lanthanides, Ln^III, to give the nine-co-ordinate triple-helical complexes [Ln(Li)₃]³⁺ (i = 1–4) existing as mixtures of C₃-symmetrical facial and C₁-symmetrical meridional isomers. Although the β₁₃ formation constants are 3–4 orders of magnitude smaller for these complexes than those found for the D₃-symmetrical analogues [Ln(Li)₃]³⁺ (i = 5–6) with symmetrical ligands, their formation at the millimolar scale is quantitative and the emission quantum yield of [Eu(L2)₃]³⁺ is significantly larger. The fac-[Ln(Li)₃]³⁺ ⇌ mer-[Ln(Li)₃]³⁺ (i = 1–4) isomerisation process in acetonitrile is slow enough for Ln = Lu^III to be quantified by ¹H NMR below room temperature. The separation of enthalpic and entropic contributions shows that the distribution of the facial and meridional isomers can be tuned by the judicious peripheral substitution of the ligands affecting the interstrand interactions. Molecular mechanics (MM) calculations suggest that one supplementary interstrand π-stacking interaction stabilises the meridional isomers, while the facial isomers benefit from more favourable electrostatic contributions. As a result of the mixture of facial and meridional isomers in solution, we were unable to obtain single crystals of 1 ∶ 3 complexes, but the X-ray crystal structures of their nine-co-ordinate precursors [Eu(L1)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₃H₅N)₂(H₂O) (6, C₄₅H₅₄EuF₉N₁₀O₁₃S₃, monoclinic, P2₁/c, Z = 4) and [Eu(L4)₂(CF₃SO₃)₂(H₂O)](CF₃SO₃)(C₄H₄O)_1.5 (7, C₅₁H₆₆EuF₉N₈O_15.5S₃, triclinic, P, Z = 2) provide crucial structural information on the binding mode of the unsymmetrical tridentate ligands.