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

Abstract

Pyridine-containing tridentate binding units react with trivalent lanthanide ions, Ln^III, to give C₃-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(L²)₃]³⁺ for the heavier lanthanides. Its use in the case of the heterodimetallic triple-stranded helicates [LnCo(L⁴)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 [Ln₂(L⁵ − 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.