Lanthanide triple helical complexes with a chiral bis(benzimidazole)pyridine derivative

The ligand neopentyl 2,6-bis[(1-methylbenzimidazol-2-yl)]pyridine-4-carboxylate (L12) has been synthesised to test the effect of the chiral neopentyl ester group in the 4-position of the pyridine ring on (i) the helical wrapping, (ii) the diastereomeric induction and (iii) the thermodynamic and photophysical properties of the [Ln(L12)3]3+ complexes. The crystal structure of ligand L12 shows the expected trans-trans conformation of the tridentate binding unit. The ligand forms stable 1:3 complexes in anhydrous acetonitrile (log3 in the range 17.3-19.0, logK3 in the range 2.9-4.6). The triple helical structure in solution is responsible for the four times larger specific rotary dispersion measured in the complexes. Circularly polarised luminescence of the Eu triple helical complex displays a weak effect, suggesting a small diastereomeric excess in solution. Ligand L12 appears to favour a 3*-to-Ln energy transfer process for Eu, but temperature-dependent nonradiative processes lead to a very small quantum yield. High-resolution luminescence spectra indicate that the Eu complex has a distorted D3 local symmetry at the metal ion site