(31)P NMR Chemical Shift Anisotropy in Paramagnetic Lanthanide Phosphide Complexes.

Lanthanide (Ln) magnetic resonance imaging and chiral shift reagents generally exploit (1)H NMR shifts, as paramagnetic broadening tends to preclude the use of heavier, less sensitive nuclei. Here, we report the solution and solid-state (31)P NMR shifts of an isostructural series of distorted trigonal bipyramidal Ln(III) tris-silylphosphide complexes, [Ln{P(SiMe(3))(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 (31)P NMR spectra for the Ln-bound P atoms for the 1-Ln family; in solution, 1-Nd showed the most downfield chemical shift (δ{(31)P} = 2570.14 ppm) and 1-Sm the most upfield value (δ{(31)P} = -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{(31)P} ≈ 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 (31)P NMR parameters. We find good agreement of experimental (1)H NMR chemical shifts with ab initio-calculated values for paramagnetic 1-Ln, while the shifts of heavier (13)C, (29)Si, and (31)P nuclei are not well-reproduced due to the current limitations of paramagnetic NMR calculations for nuclei with large contact shifts.