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

Abstract

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 B02, <Sz> or Cj values. However, this later method cannot provide quantitative values for Fi and Gi. 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 Fi, Gi and B02 parameters (L5 and L6), or with changes of the Fi and B02 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 [alpha] and [beta] parameters, it is able to detect small, unnoticed, structural changes in the complexes of L5 and L7. Clear structural changes, involving the Fi, Gi 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.