³¹P NMR Chemical Shift Anisotropy in Paramagnetic Lanthanide Phosphide Complexes

Area of Science:

• Inorganic Chemistry
• Nuclear Magnetic Resonance (NMR) Spectroscopy
• Lanthanide Chemistry

Background:

• Lanthanide (Ln) NMR typically relies on 1H NMR shifts due to paramagnetic broadening affecting heavier nuclei.
• Chiral shift reagents and magnetic resonance imaging commonly utilize 1H NMR shifts.
• Paramagnetic broadening in lanthanide complexes often limits the use of less sensitive nuclei like 31P.

Purpose of the Study:

• To investigate the solution and solid-state 31P NMR shifts in a series of isostructural lanthanide tris-silylphosphide complexes.
• To challenge the assumption that paramagnetic broadening precludes the use of 31P NMR in lanthanide complexes.
• To characterize the chemical shift anisotropies (CSAs) in the solid state and compare experimental and computational data.

Main Methods:

• Synthesis and characterization of [Ln{P(SiMe3)2}3(THF)2] (1-Ln) complexes for Ln = La, Ce, Pr, Nd, Sm.
• Multinuclear NMR spectroscopy (1H, 31P, 13C, 29Si), EPR, ATR-IR, UV-vis-NIR spectroscopy, and SQUID magnetometry.
• Magic angle spinning NMR for solid-state CSA determination and Density Functional Theory (DFT) calculations.

Main Results:

• Observed paramagnetically broadened 31P NMR spectra for the lanthanide-bound phosphorus atoms in the 1-Ln family.
• Demonstrated significant downfield shift for 1-Nd (2570.14 ppm) and upfield shift for 1-Sm (-259.21 ppm) in solution.
• Determined the largest CSA for solid 1-Pr (approx. 2000 ppm), linked to paramagnetism and phosphorus atom pyramidalization.

Conclusions:

• The study successfully observed and analyzed 31P NMR shifts in lanthanide tris-silylphosphide complexes, breaking prior assumptions.
• Experimental 1H NMR shifts showed good agreement with ab initio calculations for paramagnetic complexes.
• Limitations in current paramagnetic NMR calculations were noted for heavier nuclei (13C, 29Si, 31P) with large contact shifts.