Tris-Silanide f-Block Complexes: Insights into Paramagnetic Influence on NMR Chemical Shifts

The paramagnetism of f-block ions has been exploited in chiral shift reagents and magnetic resonance imaging, but these applications tend to focus on ¹H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) ²⁹Si NMR study of an isostructural series of locally D_3h-symmetric early f-block metal(III) tris-hypersilanide complexes, [M{Si(SiMe₃)₃}₃(THF)₂] (1-M; M = La, Ce, Pr, Nd, U); 1-M were also characterized by single crystal and powder X-ray diffraction, EPR, ATR-IR, and UV–vis–NIR spectroscopies, SQUID magnetometry, and elemental analysis. Only one SiMe₃ signal was observed in the ²⁹Si ssNMR spectra of 1-M, while two SiMe₃ signals were seen in solution ²⁹Si NMR spectra of 1-La and 1-Ce. This is attributed to dynamic averaging of the SiMe₃ groups in 1-M in the solid state due to free rotation of the M–Si bonds and dissociation of THF from 1-M in solution to give the locally C_3v-symmetric complexes [M{Si(SiMe₃)₃}₃(THF)_n] (n = 0 or 1), which show restricted rotation of M–Si bonds on the NMR time scale. Density functional theory and complete active space self-consistent field spin–orbit calculations were performed on 1-M and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental ²⁹Si NMR data for diamagnetic 1-La, suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe₃ groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound ²⁹Si nuclei could only be reproduced for diamagnetic 1-La, showing the current limitations of pNMR calculations for larger nuclei.