Benzyl cation stabilized by metal complexation. relative stability of coordinated methylene arenium, π-benzylic, and σ-benzylic structures

Benzyl cations are highly reactive compounds involved as intermediates in various chemical and biochemical processes. In this work metal coordination was utilized to stabilize different coordinated modes of benzyl cations, including methylene arenium (MA), π-benzyl, and σ-benzyl complexes. Two bidentate ligand frameworks, diphenylphosphinoethane (dppe) and di-tert- butylphosphinopropane (dtpp), were studied. η²-Coordination to Pd(II) allows for the characterization and studies of the reactivity of the otherwise unobserved methylene arenium species under ambient conditions. The relative stability and electronic structure of the three forms of the coordinated benzyl molecule, η²-MA, η¹-σ- benzylic, and η³- π-benzylic, were investigated experimentally and computationally. The MA and π-benzylic structures are preferred in the absence of counteranions, while the dtpp bulky ligand contributes to stabilization of the methylene arenium form. Counteranions have a significant influence on the relative stability. The triflate anion stabilizes the σ-benzylic form upon coordination to the metal center or the methylene arenium form as a result of compensation of positive charge on the MA ring. Use of the noncoordinating BArF counteranion promotes conversion to the π-benzylic form.