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

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      Publication Details (including relevant citation   information):

      Platas-Iglesias,C., Piguet,C., Andre,N., Bunzli,J.C.G.   Journal of the Chemical Society-Dalton Transactions  2001 (20) 3084-3091

      Abstract: A 4-aminophenylethynyl substituent has   been grafted into the 4-position of the pyridine ring in   dipicolinic acid to yield ligand L-4 the pK(a)'s of which are   8.77(5), 3.54(3), 2.58(1) and 1.01(3). Ligand L-4 reacts with   trivalent lanthanide ions to give stable triple-stranded helical   complexes in water (log beta (13) = 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(L-4   - 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 L-4 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

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