In search for mononuclear helical lanthanide building blocks with predetermined properties: Triple-stranded helical complexes with N,N,N',N'-tetraethylpyridine-2,6-dicarboxamide

The ligand N,N,N',N'-tetra-ethylpyridine-2,6-dicarboxamide (L-9) reacts with trivalent lanthanide ions (Ln(III)) to give stable mononuclear triple-stranded helical complexes [Ln(L-9)(3)](3?(divided by)) (Ln = La to Lu). The crystal and molecular structures of [La(L-9)(3)](CIO4)(3) . 2.5C(2)H(5)CN (a) and [Eu(L-9)(3)](TfO)(3) . 2THF (9) show that the three ligand strands are each meridionally tricoordinated to produce a pseudo-tricapped trigonal prismatic arrangement of the nine donor atoms (six amide O and three pyridine N) around Ln(III). The distortions in the La-III coordination sphere of 8 are more significant than for Eu-III in 9, and the photophysical studies on Eu-doped (2%) La, Gd, and Lu complexes confirm a better structural match of L-9 for the heavy Ln(III) ions, The separation of contact and pseudo-contact contributions to the induced lanthanide paramagnetic NMR shifts in [Ln(L-9)(3)](3 divided by) shows that the triple-helical structure is maintained in acetonitrile, but a minor structural change relative to that observed in the solid state occurs between Tb-III and Er-III leading to two distinct isostructural series for Ln = La-Tb and Ln = Er-Lu. The origin of this effect together with its consequences for the dynamic helical (P reversible arrow M) interconversion and stability of [Ln(L-9)(3)](3 divided by) in solution are discussed, A detailed investigation of the emission properties of [Ln(L-9)(3)](3 divided by) (Ln = Eu, Tb) shows that mixed pyridine-carboxamide ligands can be used to simultaneously optimize the structural and photophysical properties in mononuclear triple-helical lanthanide building blocks.