31P NMR Chemical Shift Anisotropy in Paramagnetic Lanthanide Phosphide Complexes

Abstract

Lanthanide (Ln) magnetic resonance imaging and chiral shift reagents generally exploit ¹H NMR shifts, as paramagnetic broadening tends to preclude the use of heavier, less sensitive nuclei. Here, we report the solution and solid-state ³¹P NMR shifts of an isostructural series of distorted trigonal bipyramidal Ln(III) tris-silylphosphide complexes, [Ln{P(SiMe₃)₂}₃(THF)₂] (1-Ln; Ln = La, Ce, Pr, Nd, Sm); 1-Ln was also characterized by elemental analysis; single-crystal and powder X-ray diffraction; multinuclear NMR, EPR, ATR-IR, and UV–vis-NIR spectroscopy; and SQUID magnetometry. Breaking assumptions, we observed paramagnetically broadened ³¹P NMR spectra for the Ln-bound P atoms for the 1-Ln family; in solution, 1-Nd showed the most downfield chemical shift (δ{³¹P} = 2570.14 ppm) and 1-Sm the most upfield value (δ{³¹P} = −259.21 ppm). We determined the span of the chemical shift anisotropies (CSAs) for solid 1-Ln using magic angle spinning NMR spectroscopy; the CSA was largest for 1-Pr (Ω{³¹P} ≈ 2000 ppm), consistent with a combination of paramagnetism and the relatively large differences in pyramidalization of the three P atoms in the solid-state. Density functional theory calculations for 1-La were in excellent agreement with the experimentally determined ³¹P NMR parameters. We find good agreement of experimental ¹H NMR chemical shifts with ab initio-calculated values for paramagnetic 1-Ln, while the shifts of heavier ¹³C, ²⁹Si, and ³¹P nuclei are not well-reproduced due to the current limitations of paramagnetic NMR calculations for nuclei with large contact shifts.