Synthesis and Reactions of a Lewis Acid
Proofing the classification as a Lewis Superacid we showed that PhF→Al(ORF)3 abstracts fluoride also from unusual sources like [SbF6]–. 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(ORF)3]– (1b), [Cp*AsCl]+[ClAl(ORF)3]– (2, figure 2), and [(Cp*)2P]+[μCl]– (figure 3) with μCl = (FRO)3Al-Cl-Al(ORF)3 and ORF = OC(CF3)3 were prepared by halide abstraction from the respective halopnictines with the Lewis superacid PhF→Al(ORF)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(ORF)3]– are formed by using one or two equivalents of the Lewis superacid PhF→Al(ORF)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(ORF)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(ORF)3 to form [ClAl(ORF)3]– or [μCl]– as very weakly coordinating novel counterions. The strong Lewis acidity and the non-oxidizing character of PhF→Al(ORF)3 suggest to employ the superacid in processes, where highly reactive, but low valent cations have to be stabilized.
 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(ORF)3 (RF = C(CF3)3).”, Angew. Chem. 120, 7772-7776.
 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(ORF)3.“ Chem. Eur. J. In Press