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

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  Renaud,F., Piguet,C., Bernardinelli,G., Bunzli,J.C.G.,   Hopfgartner,G. Chemistry-A European Journal  1997 3 (10) 1646-1659

  Abstract: 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

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