Bond-shift rearrangement in solid Li(3)P(7)(monoglyme)(3): a (31)P MAS NMR study

Area of Science:

• Solid-state inorganic chemistry
• Materials science
• Nuclear Magnetic Resonance spectroscopy

Background:

• The structure and dynamics of polyanionic clusters are crucial for understanding materials properties.
• Previous studies on Li3P7(monoglyme)3 were limited by lower resolution NMR.
• Understanding solid-state molecular rearrangements provides insights into reaction mechanisms and material stability.

Purpose of the Study:

• To reinvestigate the solid-state (31)P MAS NMR spectrum of Li3P7(monoglyme)3 with enhanced resolution.
• To elucidate the temperature-dependent dynamic behavior of the P(7) cage in the solid state.
• To compare the solid-state rearrangement mechanism with known solution-phase dynamics.

Main Methods:

• High-resolution (31)P MAS NMR spectroscopy over a wide temperature range (-70 to +77 °C).
• Variable temperature experiments at 162 MHz Larmor frequency and ~30 kHz spinning rate.
• MAS 2D exchange NMR experiments and quantitative 1D NMR lineshape analysis.

Main Results:

• At low temperatures, distinct NMR signals attributed to apical, equatorial, and basal phosphorus atoms of the P(7) cage were observed, indicating near C(3v) symmetry.
• Upon heating, NMR lines broadened and coalesced, indicative of a dynamic rearrangement process.
• Solid-state rearrangement involves a two-step bond shift mechanism, leading to inverted P(7) cages and cyclic permutations of phosphorus atoms, with differing rates in the cage's rings.

Conclusions:

• The P(7) cage in solid Li3P7(monoglyme)3 undergoes a unique double bond-shift rearrangement, distinct from single bond shifts observed in solution.
• The dynamic process involves sequential bond shifts leading to cyclic permutations of phosphorus atoms within the P(7) cage.
• Kinetic parameters reveal different rearrangement rates for different rings within the P(7) cage, with no evidence of independent threefold molecular jumps.