Comparison of crystal field dependent and independent methods to analyse lanthanide induced NMR shifts in axially symmetric complexes. Part I. Systems with a C-3 symmetry axis

摘要

A critical analysis of the lanthanide induced paramagnetic shift (LIS) data for several series of Ln(3+) complexes of C-3 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-0(2), 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 B-0(2) parameters (L-5 and L-6), or with changes of the F-i and B-0(2) parameters (L-7 and L-8), is clearly defined by the combination of the two first methods. In these cases, the three-nuclei method sometimes fully supports such an isostructurality (L-6, L-8), 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 L-5 and L-7. Clear structural changes, involving the F-i G(i) and B-0(2) parameters, are observed for the series of complexes of (L-9), 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 (L-1-L-4). 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. (C) 2003 Elsevier B.V. All rights reserved. [References: 67]