Comparison of crystal field dependent and independent methods to analyse lanthanide induced NMR shifts in axially symmetric complexes. Part I. Systems with a C₃ symmetry axis

A critical analysis of the lanthanide induced paramagnetic shift (LIS) data for several series of Ln³⁺ complexes of C₃ symmetry in terms of structural changes, crystal-field effects and/or variation of hyperfine constants along the lanthanide series was undertaken using a combination of the two-nuclei and three-nuclei techniques together with the classical one-nucleus technique. The crystal-field independent two-nuclei technique to study the isostructurality of a series of lanthanide complexes, is usefully complemented by the three nuclei shift ratio method, which is based exclusively on the experimental shift data, requiring no knowledge of B ₀², 〈S_z〉 or C_j values. However, this later method cannot provide quantitative values for F_i and G_i. The combined use of the three methods was found to be a powerful analytical tool of the solution structure of lanthanide complexes. Isostructurality of whole series of complexes, either with no change of the F_i, G_i and B₀² parameters (L ⁵ and L⁶), or with changes of the F_i and B ₀² parameters (L⁷ and L⁸), is clearly defined by the combination of the two first methods. In these cases, the three-nuclei method sometimes fully supports such an isostructurality (L⁶, L⁸), but in other cases, due to the high structural sensitivity of its α and β parameters, it is able to detect small, unnoticed, structural changes in the complexes of L⁵ and L ⁷. Clear structural changes, involving the F_i, G _i and B₀² parameters, are observed for the series of complexes of (L⁹), where the three methods agree, involving hydration and carboxylate coordination changes. More subtle structural changes, involving the internal dynamics of the bound ligands, are proposed in other cases (L¹-L⁴). These could also result from a magnification, by the present graphical analysis, of the breaks expected from the gradual structural changes along the series due to the lanthanide contraction.