NMR T1‐Relaxation Measurements on Paramagnetic Organolanthanides:An Alternative Tool for Structure Determination in Solution

Abstract

Abstract1H NMR investigations were conducted on four paramagnetic organolanthanides, all bearing the tetraisopropylcyclopentadienyl ligand Cp4i (HC5iPr4) in order to verify whether or not interactions observed in the solid state are maintained in solution. In some cases variable‐temperature experiments were necessary to enhance the resolution and determine the best conditions for the study. The 1D NMR spectrum could be interpreted in every case. Complementary 2D COSY experiments allowed the full attribution of the signals. T1 (1H) relaxation values were determined for all the paramagnetic complexes at the most suitable temperature, and compared with 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 and one a much lower value; the two others are intermediate. This was interpreted as the result of a privileged conformation of the Cp4i ligand: the two α‐isopropyl groups take up a spatial orientation with one methyl group in the exo position, opposite to the metal atom, whereas the methyl groups of the two β‐isopropyl groups are equidistant from the metal atom. 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. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)