An Isolable Triarylphosphine\nRadical Cation Electronically\nStabilized by Through-Space Radical Delocalization

Synthesis and characterization of a thermally stable\ntriarylphosphine\nradical cation, [P(8-Br-C<sub>10</sub>H<sub>6</sub>)<sub>3</sub>][BArF<sub>24</sub>] ([<b>1</b>][BArF<sub>24</sub>], BArF<sub>24</sub> = tetrakis(3,5-bis(trifluoromethyl)­phenyl)borate), enabled\nby stabilization through <i>peri</i>-bromo-substituted naphthalenes,\nare described. Unlike previously reported phosphine radical cations\nthat rely on sterically bulky substituents for stabilization, our\napproach leverages electronic stabilization via “through-space”\nradical delocalization. Single-crystal X-ray diffraction of [<b>1</b>][BArF<sub>24</sub>] reveals a tricapped tetrahedral geometry,\nresulting from the spatial proximity of the three bromine atoms to\nthe phosphorus center, differentiated from the trigonal planar geometry\nobserved in the previously reported triarylphosphine radical cations\nwith sterically bulky substituents. EPR spectroscopy shows an isotropic\nsignal with hyperfine couplings to both the phosphorus and the three\nbromine atoms, indicating spin delocalization over these four atoms\nand consequent formation of a four-center, seven-electron (4c-7e)\nbond. DFT computational studies further support the through-space\nradical delocalization mechanism, revealing that the HOMO of <b>1</b> exhibits antibonding character between the phosphorus center\nand the three adjacent Br atoms, distinct from common triarylphosphines.