Magnetic anisotropy of endohedral lanthanide ions: paramagnetic NMR study of MSc(2)N@C(80)-I (h) with M running through the whole 4f row

Paramagnetic and variable temperature (13)C and (45)Sc nuclear magnetic resonance studies are performed for nitride clusterfullerenes MSc(2)N@C(80) 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, (13)C lines broaden from 0.01–0.02 ppm in diamagnetic MSc(2)N@C(80) molecules (M = La, Y, Lu) to several ppm in TbSc(2)N@C(80) and DySc(2)N@C(80). Direction of the paramagnetic shift depends on the shape of the 4f electron density in corresponding lanthanide ions. In TmSc(2)N@C(80) and ErSc(2)N@C(80) with prolate 4f-density of lanthanide ions, (13)C signals are shifted down-field, whereas (45)Sc peaks are shifted up-field versus diamagnetic values. In all other MSc(2)N@C(80) molecules lanthanide ions have oblate-shaped 4f electron density, and the lanthanide-induced shift is negative for (13)C and positive for (45)Sc peaks. Analysis of the pseudocontact and contact contributions to chemical shifts revealed that the pseudocontact term dominates both in (13)C and (45)Sc NMR spectra, although contact shifts for (13)C 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.