Facile access to the pnictocenium ions [Cp*ECl]+ (E = P, As) and [(Cp*)2P]+: chloride ion affinity of Al(OR(F))3.

The pnictocenium salts [Cp*PCl]+ [μCl]− (1 a), [Cp*PCl]+ [ClAl(OR(F))3]− (1 b), [Cp*AsCl]+ [ClAl(OR(F))3]− (2), and [(Cp*)2P]+ [μCl]− (3), in which Cp* = Me5C5, μCl = ((F)RO)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. The X-ray crystal structures of 1 a, 2, and 3 established that in the half as well as in the sandwich cations the Cp* rings are attached in an η2-fashion. By using one or two equivalents of the Lewis acid, the two new weakly coordinating anions [μCl]− and [ClAl(OR(F))3]− resulted. They also stabilize the highly reactive cations in PhF or 1,2-F2C6H4 solution at room temperature. The chloride ion affinities (CIAs) of a range of classical strong Lewis acids were also investigated. The calculations are based on a set of isodesmic BP86/SV(P) reactions and a non-isodesmic reference reaction assessed at the G3MP2 level.