Synthesis and Reactions of a Lewis Acid

The Phosphocenium-Salts

Proofing the classification as a Lewis Superacid we showed that PhF→Al(OR^F)₃ abstracts fluoride also from unusual sources like [SbF₆]^–.^[1] Now we were interested to test, if this Lewis acid abstracts as well chloride from Cp*ECl₂ and (Cp*)₂ECl. The new pnictocenium salts [Cp*PCl]⁺[μCl]^– (1a), [Cp*PCl]⁺[ClAl(OR^F)₃]^– (1b), [Cp*AsCl]+[ClAl(OR^F)₃]– (2, figure 2), and [(Cp*)₂P]⁺[μCl]^– (figure 3) with μCl = (^FRO)₃Al-Cl-Al(OR^F)₃ and OR^F = OC(CF₃)₃ were prepared by halide abstraction from the respective halopnictines with the Lewis superacid PhF→Al(OR^F)₃.^[1,2]

In all X-ray crystal structures of the here reported half- or sandwich cations the Cp* rings are attached in an η²-fashion (figures 2a and 3a). The two new weakly coordinating anions (WCAs) [μCl]^– and [ClAl(OR^F)₃]^– are formed by using one or two equivalents of the Lewis superacid PhF→Al(OR^F)₃. These WCAs stabilize the highly reactive cations even in solution (PhF or 1,2-F₂C₆H₄) at room temperature.

Figure 2. a) Solid-state structure of [Cp*AsCl]⁺[ClAl(OR^F)₃]^– (2). Thermal ellipsoids of the cation are shown at the 50% probability level. The cationic structure of the phophocenium species is analogous, except for the absence of a stronger cation-chlorine contact. b) Hirshfeld surface of the cation of 2 showing the delocalized nature of the positive charge.

Figure 3. a) Cationic section of the solid-state structure of [(Cp*)₂P]⁺. b) Delocalized Hirshfeld surface.

With the Lewis Superacid we were able to generate the pnictocenium cations [Cp*ECl]+ (E = P, As) and [(Cp*)₂P]⁺ as so far unknown η²-bound Cp* complexes of the main-group elements P and As by halide abstraction. With [(Cp*)₂P]⁺ we openly report on the last missing bissandwich cation of the heavier pentele elements. The Synthesis of 1a and 2 opens the new field of the halo-substituted pnictocenium ions [Cp*EX]⁺.

The experimentals nicely agree with the reported CIA values, proving the high potential of the Lewis superacid PhF→Al(OR^F)₃ to form [ClAl(OR^F)₃]^– or [μCl]^– as very weakly coordinating novel counterions. The strong Lewis acidity and the non-oxidizing character of PhF→Al(OR^F)₃ suggest to employ the superacid in processes, where highly reactive, but low valent cations have to be stabilized.^[2]

Literature

[1] Lutz O. Müller, Daniel Himmel, Julia Stauffer, Gunther Steinfeld, John Slattery, Gustavo Santiso-Quiñones, Volker Brecht, Ingo Krossing* (2008): „Simple Access to the Non-Oxidizing Lewis Superacid Ph F→Al(OR^F)3 (R^F = C(CF₃)₃).”, Angew. Chem. 120, 7772-7776.

[2] Anne Kraft, Jennifer Beck, Ingo Krossing: „Facile Access to the Pnictocenium Ions [Cp*ECl]+ (E = P, As) and [Cp*₂P]+. Chloride Ion Affinity of Al(OR^F)₃.“ Chem. Eur. J. In Press