Two stable phosphorus-containing four-membered ring radical cations with inverse spin density distributions

Area of Science:

• Inorganic Chemistry
• Organophosphorus Chemistry
• Radical Chemistry

Background:

• Phosphorus-containing four-membered rings are of interest due to their unique reactivity.
• Radical cations offer distinct electronic and structural properties compared to their neutral counterparts.

Purpose of the Study:

• To isolate and characterize novel phosphorus-containing four-membered ring radical cations.
• To investigate the influence of exocyclic substituents on the structure and spin density distribution of these radical cations.

Main Methods:

• UV-vis absorption spectroscopy
• Electron Paramagnetic Resonance (EPR) spectroscopy
• Single-crystal X-ray diffraction
• Density Functional Theory (DFT) calculations

Main Results:

• Two radical cations, 1(•+) and 2(•+), were successfully isolated and characterized.
• Radical 1(•+) exhibited elongated P-P bonds and pyramidalized phosphorus atoms, while 2(•+) showed shortened P-Nring distances.
• EPR studies revealed an inverse spin density distribution: 1(•+) had spin density on exocyclic nitrogens, whereas 2(•+) had it on endocyclic phosphorus atoms.

Conclusions:

• The exocyclic substituents play a crucial role in controlling the spin density distribution in these phosphorus-containing radical cations.
• DFT calculations support the observed inverse spin density distributions, highlighting the substituent effect.