Jean-Claude Bunzli - The solution structure of supramolecular lanthanide triple helices revisited: application of crystal-field independent paramagnetic NMR techniques to mono- and di-metallic complexes

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      Publication Details (including relevant citation   information):

      Rigault,S., Piguet,C., Bunzli,J.C.G. Journal of the Chemical   Society-Dalton Transactions 2000 (13)   2045-2053

      Abstract: Pyridine-containing tridentate binding   units react with trivalent lanthanide ions, Ln(III), to give   C-3-symmetrical nine-coordinate triple helical complexes in which   subtle intramolecular interactions control the final structures.   While X-ray crystal structures allow some rationalisation in the   solid state, the access to solution structures by paramagnetic   NMR is limited by the unpredictable variation of the   crystal-field parameter along the lanthanide series. A recent   technique which considers two different nuclei within the same   complex overcomes this drawback and its application to triple   helical lanthanide complexes of increasing sophistication is   described. This new approach confirms the previously suggested   geometrical change occurring in the monometallic complexes   [Ln(L-2)(3)](3+) for the heavier lanthanides. Its use in the case   of the heterodimetallic triple-stranded helicates   [LnCo(L-4)(3)](6+) evidences a single isostructural series while   the separation of contact and pseudo-contact contributions   according to classical techniques gives intractable results.   Finally, new equations are derived for homodimetallic complexes   with large Ln ... Ln separation and they are applied to the   structural analysis of the triple-stranded helicates   [Ln(2)(L-5-2H)(3)]. The latter reveals that the dimetallic   edifices display a single solution structure along the complete   lanthanide series in contrast with a previous analysis   considering invariant crystal-field parameters. The scope and   limitations of this technique for supramolecular lanthanide   complexes is discussed together with specific effects resulting   in amplification of magnetic properties in polymetallic systems

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