NMR T1-Relaxation Measurements on Paramagnetic Organolanthanides: An Alternative Tool for Structure Determination in Solution
Résumé
1H NMR investigations were conducted on four paramagnetic organolanthanides all bearing the tetraisopropylcyclopentadienyl Cp4i ligand (Cp4i = HC5iPr4) in order to verify whether or not interactions observed in the solid state were maintained in solution. In some cases, variable temperature experiments were necessary to enhance the resolution and determine the best conditions of study. 1D NMR spectrum could be interpreted in every case. Complementary 2D COSY experiments allowed the full attribution of the signals. T1 (1H) relaxation measurements were determined for all the paramagnetic complexes at the most suitable temperature, and compared to those of the diamagnetic KCp4i. The same tendency was observed, with particular features concerning the isopropyl groups. Among the four methyl groups, one exhibits a much higher T1 value, one a much lower and the two others are intermediate. This was interpreted as the result of a privilegied conformation of the Cp4i ligand: the two a-isopropyl groups take a spatial orientation with one methyl in exo position, opposite to the metal, whereas the methyl groups of the two b-isopropyl are quite equidistant from the metal. Whatever the nature of the metal (Nd, Sm), the oxidation state (SmII, SmIII), or the temperature (298, 363 K), this conformation is retained. The structure in solution seems to be different from that previously determined in the solid state.
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