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-butoxycarbony l)alanine methyl ester (L-4) bearing a chiral group in the 4-position of th e pyridine ring has been synthesised and its interaction with trivalent lan thanide ions studied. L-4 yields stable [Ln(L-4)(3)](3+) Complexes in aceto nitrile, with log beta (3) in the range 19-20. The specific rotary dispersi on 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 reversible arrowM 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 spect ra of the Eu complex in the solid state reveal a local symmetry derived fro m a trigonal arrangement around the metal ion and the long D-5(0) lifetime (1.58 ms) is indicative of the absence of water molecules bound in the inne r co-ordination sphere. Energy transfer processes in the luminescent Eu and Tb triple helical complexes are discussed.