An Isolable Triarylphosphine Radical Cation Electronically Stabilized by Through-Space Radical Delocalization

Synthesis and characterization of a thermally stable triarylphosphine radical cation, [P(8-Br-C₁₀H₆)₃][BArF₂₄] ([1][BArF₂₄], BArF₂₄ = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate), enabled by stabilization through peri-bromo-substituted naphthalenes, are described. Unlike previously reported phosphine radical cations that rely on sterically bulky substituents for stabilization, our approach leverages electronic stabilization via "through-space" radical delocalization. Single-crystal X-ray diffraction of [1][BArF₂₄] reveals a tricapped tetrahedral geometry, resulting from the spatial proximity of the three bromine atoms to the phosphorus center, differentiated from the trigonal planar geometry observed in the previously reported triarylphosphine radical cations with sterically bulky substituents. EPR spectroscopy shows an isotropic signal with hyperfine couplings to both the phosphorus and the three bromine atoms, indicating spin delocalization over these four atoms and consequent formation of a four-center, seven-electron (4c-7e) bond. DFT computational studies further support the through-space radical delocalization mechanism, revealing that the HOMO of 1 exhibits antibonding character between the phosphorus center and the three adjacent Br atoms, distinct from common triarylphosphines.