Phosphorus-31 NMR studies of stabilized phosphorus ylids in the solid state

Area of Science:

• Solid-state chemistry
• Nuclear Magnetic Resonance (NMR) spectroscopy
• Organophosphorus chemistry

Background:

• Phosphorus ylids are versatile reagents in organic synthesis.
• Previous solution-state NMR studies suggested an invariant isotropic chemical shift for phosphorus ylids.
• Understanding the electronic structure of ylids is crucial for predicting their reactivity.

Purpose of the Study:

• To investigate the solid-state NMR properties of stabilized phosphorus ylids.
• To determine the principal components of the Phosphorus-31 chemical shift tensor.
• To correlate NMR parameters with valence bond resonance structures and electronic properties.

Main Methods:

• Acquisition of Phosphorus-31 (31P) powder NMR spectra.
• Utilizing high-resolution Magic Angle Spinning (MAS) NMR.
• Employing high-power proton decoupling and cross-polarization techniques.
• Analysis of 31P NMR line shapes to determine chemical shift anisotropy and tensor components.

Main Results:

• Observed variations in chemical shift anisotropy and tensor components for stabilized phosphorus ylids.
• Demonstrated that the invariant isotropic chemical shift in solution is due to a fortuitous cancellation of opposing changes in tensor components (δ11 and δ33).
• Analyzed the 31P dipolar NMR powder spectrum to determine the orientation of the chemical shift tensor relative to the 31P-13C dipolar vector.

Conclusions:

• The isotropic chemical shift of phosphorus ylids in solution is not a reliable indicator of electronic structure.
• Solid-state NMR provides a more accurate method to probe the electronic properties of phosphorus ylids.
• The findings necessitate a re-evaluation of previous interpretations based on solution-state NMR data.