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

Two phosphorus-containing four-membered
ring radical cations 1•+ and 2•+ 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•+ has elongated
P–P bonds and more pyramidalized phosphorus atoms, while shortened
P–Nring distances and larger angles around phosphorus
centers are observed for 2•+. EPR studies
indicate that for 1•+ spin density
mainly resides on the exocyclic nitrogen atoms with very minor contribution
from endocyclic phosphorus atoms, while the situation is opposite
for 2•+. Such an inverse spin density
distribution is controlled by the exocyclic substituents, which is
supported by DFT calculations.