-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 H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) Si NMR study of an isostructural series of locally -symmetric early f-block metal(III) -hypersilanide complexes, [M{Si(SiMe)}(THF)] (; M = La, Ce, Pr, Nd, U); 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 SiMe signal was observed in the Si ssNMR spectra of , while two SiMe signals were seen in solution Si NMR spectra of and . This is attributed to dynamic averaging of the SiMe groups in in the solid state due to free rotation of the M-Si bonds and dissociation of THF from in solution to give the locally -symmetric complexes [M{Si(SiMe)}(THF) ] ( = 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 and desolvated solution species to model paramagnetic NMR shifts. We find excellent agreement of experimental Si NMR data for diamagnetic , suggesting = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe groups for (M = Pr and Nd); the NMR shifts for metal-bound Si nuclei could only be reproduced for diamagnetic , showing the current limitations of pNMR calculations for larger nuclei.