31P NMR Chemical Shift Anisotropy in Paramagnetic Lanthanide Phosphide Complexes

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, [LnP(SiMe3)23(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 (δ31P = 2570.14 ppm) and 1-Sm the most upfield value (δ31P = −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 (Ω31P ≈ 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.