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.