The Protonation of Unactivated Aromatic Hydrocarbons on Osmium(II): Stabilization of Arenium Cations via Unprecedented η2- and η3-Coordination

Complexes of the form [Os(NH₃)₅(L)](OTf)₂ (where L = an unactivated arene or polyaromatic hydrocarbon) are readily protonated by triflic acid (HOTf) to generate stable arenium, naphthalenium, and anthracenium cations. A series of substituted anisole complexes were also investigated. The metal stabilizes the hydrocarbon arenium system, in most cases, by coordinating the organic ligand in an η³ fashion. Where L = m-xylene, however, NMR data strongly suggest that the arenium ion is essentially dihapto-coordinated, where an allyl cation fragment remains uncoordinated. For the corresponding anisolium systems, NMR data indicate η²-coordination. It is likely that η² and η³ geometries represent limiting cases for a continuum of distorted allyl (pseudo-allyl) complexes. The pK_a values determined for these complexes are dramatically higher than those of the free arenium cations.