A novel extended covalent tripod for assembling triple-helical nine-co-ordinated lanthanide(III) podates: the rational design of a single conformer

The connection of a methyl group to the apical carbon atom of a semi-rigid extended tripod provides the novel podand 1,1,1-tris-{2-[2-(6-diethylcarbamoyl-pyridin-2-yl)-1-ethyl-1H-benzoimidazol-5-ylmethoxy]-ethyl}-ethane (L13). Reaction with lanthanide(III) in acetonitrile produces stable and dynamically inert C-3-symmetrical podates [Ln(L13)](3+) (Ln = La-Lu) in which Ln(III) is nine-co-ordinate in a pseudo-tricapped trigonal prismatic site. Electron-induced nuclear relaxation in the paramagnetic complex [Nd(L13)](3+) shows that the capping carbon atom adopts the exo conformation with the methyl group pointing outside the cavity of the podand. The crystal structure of [Eu(L13)](ClO4)(3) confirms the exclusive formation of the exo conformer, which contrasts with the mixture of two endo conformers reported for the non-methylated analogue. Structural analysis combined with photophysical data in [Ln(L13)](ClO4)(3) (Ln = La, Eu, Gd, Tb) point to negligible mechanical coupling mediated by the flexible four-atoms aliphatic chains between the apical carbon atom and the unsymmetrical tridentate binding units. However, the exo conformation of the tripod in [Eu(L13)](3+) removes some interactions between the high-frequency CH oscillators and the metal-centred excited electronic levels, thus leading to larger lifetime and improved quantum yield in solution.