Transition Metal-Stabilized Arenium Cations:  Protonation of Arenes Dihapto-Coordinated to π-Basic Metal Fragments

A series of metal complexes was synthesized in which arenes were dihapto-coordinated to π-basic metal fragments having the general form {TpM(π-acid)(L)}, where Tp = hydridotris(pyrazolyl)borate, M = rhenium, molybdenum, or tungsten, π-acid = CO or NO⁺, and L = 1-methylimidazole, 1-butylimidazole, pyridine, or trimethylphosphine. The arene complexes were shown to be significantly more basic than the analogous pentaammineosmium(II) arene complexes and were protonated by moderate acids to give remarkably stable η² and η³ arenium cation complexes. A crystal structure of [TpRe(CO)(MeIm)(5,6-η²-2H-anisolium)](OTf) confirmed the η² coordination of the anisolium ligand, but suggests a weak long-range interaction between the metal and C1 of the anisolium.