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 1 H NMR shifts as paramagnetic broadening makes less sensitive nuclei more difficult to study. Here we report a solution and solid-state (ss) 29 Si NMR study of an isostructural series of locally D 3 h -symmetric early f-block metal(III) tris -hypersilanide complexes, [M{Si(SiMe 3 ) 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 SiMe 3 signal was observed in the 29 Si ssNMR spectra of 1-M , while two SiMe 3 signals were seen in solution 29 Si NMR spectra of 1-La and 1-Ce . This is attributed to dynamic averaging of the SiMe 3 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 3 v -symmetric complexes [M{Si(SiMe 3 ) 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 29 Si NMR data for diamagnetic 1-La , suggesting n = 1 in solution and reasonable agreement of calculated paramagnetic shifts of SiMe 3 groups for 1-M (M = Pr and Nd); the NMR shifts for metal-bound 29 Si nuclei could only be reproduced for diamagnetic 1-La , showing the current limitations of pNMR calculations for larger nuclei.