Lanthanide triple helical complexes with a chiral ligand derived from 2,6-pyridinedicarboxylic acid†

The ligand 3-[2,6-bis(diethylcarbamoyl)pyridine-4-yl]-N-(tert-butoxycarbonyl)alanine methyl ester (L4) bearing a chiral group in the 4-position of the pyridine ring has been synthesised and its interaction with trivalent lanthanide ions studied. L4 yields stable [Ln(L4)3]3+ complexes in acetonitrile, with logβ3 in the range 19–20. The specific rotary dispersion of the complexes is about ten times as large as that of the ligand alone, consistent with triple helical structures in solution. NMR data show the presence of only one time-averaged species in acetonitrile solution with a trigonal symmetry, that is, the helical P ⇌ M interconversion is fast on the NMR time scale. Circularly polarised luminescence on the Eu and Tb triple helical complexes displays weak effects, pointing to only a small diastereomeric excess in solution. High resolution luminescence spectra of the Eu complex in the solid state reveal a local symmetry derived from a trigonal arrangement around the metal ion and the long 5D0 lifetime (1.58 ms) is indicative of the absence of water molecules bound in the inner co-ordination sphere. Energy transfer processes in the luminescent Eu and Tb triple helical complexes are discussed.