Two Stable Phosphorus-Containing Four-Membered Ring Radical Cations with Inverse Spin Density Distributions

Two phosphorus-containing four-membered ring radical cations 1(center dot+) and 2(center dot+) have been isolated and characterized by UV-vis absorption spectroscopy, electron paramagnetic resonance (EPR), and single-crystal X-ray diffraction. Compared with neutral molecules 1 and 2, radical 1(center dot+) has elongated P-P bonds and more pyramidalized phosphorus atoms, while shortened P-Min distances and larger angles around phosphorus centers are observed for 2(center dot+) EPR studies indicate that for 1(center dot+) spin density mainly resides on the exocyclic nitrogen atoms with very minor contribution from endocyclic phosphorus atoms, while the situation is opposite for 2(center dot+) Such an inverse spin density distribution is controlled by the exocyclic substituents, which is supported by DFT calculations.