Annulated Radical Cations with a C ₄ E ₂ ‐Core (E = P, As, Sb): Stable Pnictogen Analogs of Elusive Aryl Radical Anions of Birch Reduction Reactions

The benzene radical anion (C₆H₆)^●-, possessing a 7π-electron count, is a crucial intermediate in the Birch reduction and has been extensively studied both experimentally and theoretically. Herein, we report the tricyclic pnictogen radical cations [(ADC)E]₂[B], [2-E][B] (ADC = PhC{N(Dipp)C}₂; Dipp = 2,6-iPr₂C₆H₃; E = P, As, Sb; [B] = [B1] = B(C₆F₅)₄ or [B2] = {3,5-(CF₃)₂C₆H₃}) featuring a central 7π-electron planar C₄E₂ ring embedded between two 1,3-imidazole-based anionic dicarbene (ADC) frameworks, as crystalline solids. [2-E][B] are prepared by one-electron (1e) oxidations of the corresponding base-stabilized cyclic bis-pnictinidene compounds [(ADC)E]₂ (1-E). Calculated spin densities (for E = P 76%, As 80%, Sb 88%) and EPR measurements suggest that [2-E]^●+ are pnictogen-centered radicals and stabilized by the delocalization of the unpaired electron over the C₄E₂ ring. The EPR spectra of [2-E][B] exhibit characteristic signals for S = ½ systems with clear hyperfine coupling constants for the two similar P, As, or Sb nuclei. Further 1e-oxidations of [2-E][B] result in the dicationic 6π-electron C₄E₂-species [(ADC)E]₂[B]₂ [3-E][B]₂, which readily undergo comproportionations with 1-E to afford [2-E][B]. The radical reactivity of [2-E][B2] is shown with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) and diphenyl diselenide (PhSeSePh) in yielding compounds [(ADC)₂E{E(TEMPO)}][B2] (4-E) (E = P, Sb) and [(ADC)₂P{P(SePh)}][B2] (5-P), respectively.