Bond-Shift Rearrangement in Solid Li3P7(Monoglyme)3: A P MAS NMR Study

The ³¹P MAS NMR spectrum of solid Li₃P₇(monoglyme)₃ has been reinvestigated over a wide temperature range (−70 to +77°C) and under conditions of better resolution (Larmor frequency of 162 MHz and spinning rate of 30 kHz) than previously measured (121 MHz and 13 kHz). At low temperatures three spinning sideband (ssb) manifolds are observed: a singlet (centered at −45 ppm relative to 85% H₃PO₄) due to the apical atom (A) of the P₇-cage trianion; a 1 : 1 : 1 triplet (at −110, −117, and −124.5 ppm) due to the negatively charged equatorial (E) atoms, and a one to two doublet (at −161 and −168.5 ppm) due to the basal (B) atoms. These results are consistent with the P₇ cage having nearly, but not perfect, C_3v symmetry. The compound appears to be well ordered in the solid state with very little structural dispersity. On heating, the NMR lines broaden and eventually coalesce into a single ssb manifold. This behavior is ascribed to bond-shift rearrangement similar to the Cope rearrangement in bullvalene. A MAS 2D exchange experiment and a quantitative analysis of the 1D NMR lineshapes indicate that, unlike in solution where the rearrangement involves a single bond shift at a time, in the solid the process involves a succession of two bond shifts: The first leads to an intermediate species in which the rearranged P₇ cage is inverted, while in the subsequent step a second bond shift takes place that also restores the original orientation of the cage in the lattice. The overall effect of the double bond shift is equivalent to cyclic permutation of the phosphorus atoms within the five member rings of the P₇-cage. The quantitative analysis of the dynamic lineshapes shows that this cyclic permutation proceeds at a different rate in one ring (k_d¹) than in the other two (k_d^2,3). The kinetic parameters for these processes are E_a¹=18.7 kJ/mol, E_a^2,3=58.0 kJ/mol, k_d¹(17°C)=k_d^2,3(17°C)=10⁴ s⁻¹. No indications for independent threefold molecular jumps of the P₇ cage were found.