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

Abstract

Abstract The benzene radical anion (C 6 H 6 ) ●− , 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‐ i Pr 2 C 6 H 3 ; E = P, As, Sb; [B] = [B1] = B(C 6 F 5 ) 4 or [B2] = {3,5‐(CF 3 ) 2 C 6 H 3 }) featuring a central 7π‐electron planar C 4 E 2 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 C 4 E 2 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 C 4 E 2 ‐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) 2 E{E(TEMPO)}][B2] ( 4 ‐E) (E = P, Sb) and [(ADC) 2 P{P(SePh)}][B2] ( 5 ‐P), respectively.