Formation of η² C-H agostic rhodium arene complexes and their relevance to electrophilic bond activation

Reaction of the ligand 1,3-bis((di-tert-butylphosphino)methyl)benzene (1a) with the [RhCO]⁺ fragment in THF resulted in clean formation of the crystallographically characterized his-chelated complex 2a which contains an η² agostic Rh C-H bond. Both the NMR data and the X-ray crystal structure show strong interaction between the metal center and the agostic C-H bond, which results in high acidity of the agostic proton. Reaction of 2a with a weak organic base (NEt₃ or collidine) affords the known cyclometalated complex 3. Reaction of the new ligand 1,3-bis((di-tert- butylphosphino)methyl)-4,5,6-trimethoxybenzene (1b) with the [RhCO]⁺ fragment in THF gives the analogous to 2a agostic complex 2b. Analysis of the NMR data and the reactivity of both 2a and 2b showed that there is very little, if any, contribution of a metal arenium structure. This interpretation is supported by B3LYP/LANL2DZ density functional calculations on model compounds. Thus, deprotonation of η² aromatic C-H agostic complexes can be proposed as an alternative route to electrophilic metalation of aromatic compounds.