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

Pyridine-containing tridentate binding units react with trivalent lanthanid e ions, Ln(III), to give C-3-symmetrical nine-coordinate triple helical com plexes in which subtle intramolecular interactions control the final struct ures. While X-ray crystal structures allow some rationalisation in the soli d state, the access to solution structures by paramagnetic NMR is limited b y the unpredictable variation of the crystal-field parameter along the lant hanide series. A recent technique which considers two different nuclei with in the same complex overcomes this drawback and its application to triple h elical lanthanide complexes of increasing sophistication is described. This new approach confirms the previously suggested geometrical change occurrin g in the monometallic complexes [Ln(L-2)(3)](3+) for the heavier lanthanide s. Its use in the case of the heterodimetallic triple-stranded helicates [L nCo(L-4)(3)](6+) evidences a single isostructural series while the separati on of contact and pseudo-contact contributions according to classical techn iques gives intractable results. Finally, new equations are derived for hom odimetallic 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 solut ion structure along the complete lanthanide series in contrast with a previ ous analysis considering invariant crystal-field parameters. The scope and limitations of this technique for supramolecular lanthanide complexes is di scussed together with specific effects resulting in amplification of magnet ic properties in polymetallic systems.