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 chlorophosphine precursor; (ii) displacement of chloride by the nucleofugic anion, OSO2CF3-; and (iii) protonation of an iminophosphine 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 η2 attachment of the C5Me5 ligand to phosphorous in the solid state. In solution, according to the results of NMR 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 coordination chemistry. In addition to activity as both Lewis acid and base, which is a common feature for phosphanylium ions, (4) is the first two-coordinate phosphorous cation which was shown to react as a Broensted acid. Deprotonation initially gives the iminophosphine, 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 coordination 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-phosphorous bonds.