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

Abstract

Stable phosphanylium salts of [P(NHBu^t)(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 Brønsted acid. Deprotonation initially gives the iminophosphane, P(NBu^t)(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.