Isolation, Structural and Physical Characterization as well as Reactivity of Persistent Acenium Radical Cation Salts

The unsubstituted acenium radical cations (ARCs) are extremely sensitive and were hitherto only studied in situ, i. e. in the gas phase, as dilute solutions in strong acids or by matrix isolation spectroscopy at about 10 K. In this study, room temperature stable ARC salts with the weakly coordinating anion [F{Al(OR^F )₃ }₂ ]^- (OR^F =-OC(CF₃ )₃ ) supported by the weakly coordinating solvent 1,2,3,4-tetrafluorobenzene (TFB) were prepared and structurally, electrochemically and spectroscopically characterized. Reaction of the neutral acenes with Ag⁺ [F{Al(OR^F )₃ }₂ ]^- led, non-innocent,^[54] to intermediate [Ag₂ (acene)₂ ]²⁺ complexes, which decompose over time to Ag⁰ and the corresponding (impure) ARC salts. By contrast, direct deelectronation with the recently developed innocent^[54] deelectronator radical cation salt [anthracene^Hal ]^+⋅ [F{Al(OR^F )₃ }₂ ]^- led to phase-pure products [acene]^+⋅ [F{Al(OR^F )₃ }₂ ]^- (anthracene^Hal =9,10-dichlorooctafluoroanthracene; acene=anthra-, tetra-, pentacene). For the first time, a homogenous set of spectroscopic data on analytically pure ARC salts was obtained. In addition, cyclovoltammetric measurements of the acenes connected the potentials in solution with those in the gas-phase. Hence, the data complement the existing isolated gas-phase, strong acid or matrix isolation studies. A first entry to follow-up chemistry of the acenium radical cations as ligand forming oxidizers was demonstrated by reaction with $1/2$ Co₂ (CO)₈ giving [Co(anthracene)(CO)₂ ]⁺ .