In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Triple‐stranded Helical Complexes with N,N,N',N'‐tetraethylpyridine‐2,6‐dicarboxamide

Abstract

AbstractThe ligand N,N,N',N'‐tetra‐ethylpyridine‐2,6‐dicarboxamide (L9) reacts with trivalent lanthanide ions (LnIII) to give stable mononuclear triple‐stranded helical complexes [Ln(L9)3]3+ (Ln = La to Lu). The crystal and molecular structures of [La(L9)3](ClO4)3.2.5C2H5CN (8) and [Eu(L9)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 LnIII. The distortions in the LaIII coordination sphere of 8 are more significant than for EuIII in 9, and the photophysical studies on Eu‐doped (2%) La, Gd, and Lu complexes confirm a better structural match of L9 for the heavy LnIII ions. The separation of contact and pseudo‐contact contributions to the induced lanthanide paramagnetic NMR shifts in [Ln(L9)3]3+ 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 TbIII and ErIII 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→M) interconversion and stability of [Ln(L9)3]3+ in solution are discussed. A detailed investigation of the emission properties of [Ln(L9)3]3+ (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.