Formation of eta(2) 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 (la) with the [RhCO](+) fragment in THF resulted in clean formation of the cryst allographically characterized bis-chelated complex 2a which contains an eta (2) 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 comp lex 3. Reaction of the new ligand 1,3-bis((di-tert-butylphosphino)methyl)-4 ,5,6-trimethoxybenzene (Ib) with the [RhCO](+) fragment in THF gives the an alogous to 2a agostic complex 2b. Analysis of the NMR data and the reactivi ty 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/LA NL2DZ density functional calculations on model compounds. Thus, deprotonati on of eta(2) aromatic C-H agostic complexes can be proposed as an alternati ve route,:to electrophilic metalation of aromatic compounds.