Crystalline Aryliminophosphinyl Radical: A Phosphorus Congener of Aryldiazenyl Radicals

Aryldiazenyl radicals (Ar-N═N·), bearing a terminal dinitrogen unit, are important reactive intermediates in organic transformations but typically short-lived due to rapid nitrogen extrusion. Herein, we report the synthesis, and structural and spectroscopic characterization of a robust aryliminophosphinyl radical Ar-N═P·, stabilized by a sterically demanding aryl substituent. It represents the first isolable heavier congener of aryldiazenyl radicals and the first phosphorus-centered radical featuring a pseudomonosubstituted phosphorus atom. Electron paramagnetic resonance spectroscopy and theoretical calculations revealed that the spin density is primarily localized on the phosphorus center with partial delocalization onto the nitrogen atom. Furthermore, we investigated its chemical property, revealing its phosphorus-centered radical reactivity and N-P unit associated redox reactivity. Notably, reduction of the radical with potassium graphite yielded an unprecedented anionic iminophosphanorcaradiene salt.