Paramagnetic NMR Analysis of Substituted Biscyclooctatetraene Lanthanide Complexes

Cyclooctatetraene
derivatives (COTR) with two carbocycles
attached to the central ring have been used as dianionic ligands for
the synthesis of double-decker complexes of the type [(COTR)2M]−. Two ligand derivatives were combined
with diamagnetic Y3+ and with the five paramagnetic lanthanide
ions from Tb3+ to Tm3+. The more complex substitution
pattern in comparison to the parent ligand COT or the popular bis-trimethylsilyl
derivative allows a sufficient number of signals to be obtained for
a comprehensive paramagnetic NMR analysis. The anionic double-decker
complexes gave well-resolved NMR spectra where almost all 1H and 13C NMR signals could be detected and assigned.
With these data, it was possible to separate the two main contributions
to the paramagnetic shift (pseudocontact and Fermi contact shifts,
respectively) and to determine the magnetic anisotropy of the lanthanide
ions in their ligand fields. We can easily obtain the sign and the
magnitude of the anisotropy of the magnetic susceptibility, which
is itself strongly related to the energy barrier for spin reversal
in single-molecule magnets. Our results confirm that Bleaney factors
are inadequate descriptors for magnetic anisotropy in these lanthanide
complexes.