Paramagnetic NMR Lanthanide Induced Shifts for Extracting Solution Structures

This chapter focuses on the use and limitations of the model-free methods in axial paramagnetic supramolecular mono- and polymetallic lanthanide-containing complexes. A comprehensive survey of the systems with three- and fourfold symmetry, for which these techniques have been applied, is proposed together with a detailed discussion of the limitations of Bleaney's approach for the modelling of paramagnetic anisotropies and the detection of the structural changes along the lanthanide series. The structural changes are usually observed for flexible complexes, in which dynamic motions produce significant distortions whose magnitude is controlled by the strength of the R–ligand interactions ([R(L3)3]3+, [R(L7–2H)3]3-). On the other hand, the considerable rigidity imposed by podand ([R(L4)3]3+), non-covalent podand [RM(L5)3]5/6+, or macrobicyclic ([R(L6)3]3+) ligands ensures well-defined coordination sites and isostructurality along the complete lanthanide series.