Lanthanide triple-stranded helical complexes with a substituted 2,6-pyridinedicarboxylate

Abstract

A 4-aminophenylethynyl substituent has been grafted into the 4-position of the pyridine ring in dipicolinic acid to yield ligand L⁴ the pK_a's of which are 8.77(5), 3.54(3), 2.58(1) and 1.01(3). Ligand L⁴ reacts with trivalent lanthanide ions to give stable triple-stranded helical complexes in water (logβ₁₃ = 13.7(1) for Eu). The separation of the contact and pseudocontact contributions to the NMR paramagnetic shifts according to the crystal field independent method points to a single axial structure for [Ln(L⁴ − 2H)₃]³⁻ along the series Nd–Yb. The solution structure generated by using the dipolar shifts compares well with that reported for the Eu^III tris(dipicolinate), with a co-ordination polyhedron which can be described as a distorted tricapped trigonal prism. This is confirmed by a photophysical study of the Eu^III complex both in solution and in the solid state. The triplet state of co-ordinated L⁴ lies at low energy (around 18 000 cm⁻¹ in water) resulting in a relatively poor sensitisation of Eu^III (quantum yield ca. 0.1% in methanol, lower in water) but in a sizable energy transfer to Yb^III.