Synthesis, structure, and chemical reactivity of a stable C5Me5-substituted phosphanylium ion: (pentamethylcyclopentadienyl)(t-butylamino)phosphanylium tetrachloroaluminate

Abstract

Stable phosphanylium salts of [P(NHBut)(C5Me5)]+(4) were obtained via different routes, viz.(i) Al2Cl6-promoted halide abstraction from a chlorophosphane precursor, (ii) displacement of chloride by the nucleofugic anion, OSO2CF3–, and (iii) protonation of an iminophosphane precursor. A crystalline product was isolated in case of the tetrachloroaluminate of (4), and its structure was investigated by X-ray diffractometry. The results confirm the presence of discrete cations, featuring η2attachment of the C5Me5 ligand to phosphorus in the solid state. In solution, according to the results of n.m.r. spectroscopic studies, the cation exhibits a fluxional structure with all five ring atoms becoming equivalent. Investigations of the chemical reactivity of (4) include acid–base reactions and studies of the co-ordination chemistry. In addition to activity as both Lewis acid and base, which is a common feature for phosphanylium ions, (4) is the first two-co-ordinate phosphorus cation which was shown to react as a Bronsted acid. Deprotonation initially gives the iminophosphane, P(NBut)(C5Me5), which further reacts with (4) to yield a polycyclic cation, (14), the structure of which was determined by X-ray diffraction. Reactions of (4) with transition metals involve oxidative addition of complex metal hydrides and co-ordination to reactive metal centres to give cationic complexes which are isolobal to transition metal carbene complexes. No evidence was obtained in these reactions to indicate any activation of the C5Me5–phosphorus bonds.