Magnetic anisotropy of endohedral lanthanide ions: paramagnetic NMR study of MSc2N@C80-Ih with M running through the whole 4f row

Abstract

Paramagnetic and variable temperature ¹³C and ⁴⁵Sc nuclear magnetic resonance studies are performed for nitride clusterfullerenes MSc₂N@C₈₀ with icosahedral I_h(7) carbon cage, where M runs through all lanthanides forming nitride clusters. The influence of the endohedral lanthanide ions on the NMR spectral pattern is carefully followed, and dramatic differences are found in peak positions and line widths. Thus, ¹³C lines broaden from 0.01–0.02 ppm in diamagnetic MSc₂N@C₈₀ molecules (M = La, Y, Lu) to several ppm in TbSc₂N@C₈₀ and DySc₂N@C₈₀. Direction of the paramagnetic shift depends on the shape of the 4f electron density in corresponding lanthanide ions. In TmSc₂N@C₈₀ and ErSc₂N@C₈₀ with prolate 4f-density of lanthanide ions, ¹³C signals are shifted down-field, whereas ⁴⁵Sc peaks are shifted up-field versus diamagnetic values. In all other MSc₂N@C₈₀ molecules lanthanide ions have oblate-shaped 4f electron density, and the lanthanide-induced shift is negative for ¹³C and positive for ⁴⁵Sc peaks. Analysis of the pseudocontact and contact contributions to chemical shifts revealed that the pseudocontact term dominates both in ¹³C and ⁴⁵Sc NMR spectra, although contact shifts for ¹³C signals are also considerable. Point charge computations of the ligand field splitting are performed to explain experimental results, and showed reasonable agreement with experimental pseudocontact shifts. Nitrogen atom bearing large negative charge and located close to the lanthanide ion results in large magnetic anisotropy of lanthanide ions in nitride clusterfullerenes with quasi-uniaxial ligand field.