C-C versus C-H activation and versus agostic C-C interaction controlled by electron density at the metal center

Based on the PCN ligand 2, a remarkable degree of control over C-C versus C-H bond activation and versus formation of an agostic C-C complex was demonstrated by choice of cationic [Rh(CO)_n(C₂H ₄)_2-n] (n=0, 1, 2) precursors. Whereas reaction of 2 with [Rh(C₂H₄)₂(solv)_n]BF₄ results in exclusive C-C bond activation to yield product 5, reaction with the dicarbonyl precursor [Rh(CO)₂(solv)_n]BF₄ leads to formation of the C-H activated complex 9. The latter process is promoted by intramolecular deprotonation of the C-H bond by the hemilabile amine arm of the PCN ligand. The mixed monocarbonyl monoethylene Rh species [Rh(CO)(C ₂H₄)]BF₄ reacts with the PCN ligand 2 to give an agostic complex 7. The C-C activated complex 5 is easily converted to the C-H activated one (9) by reaction with CO; the reaction proceeds by a unique sequence of 1,2-metal-to-carbon methyl shift, agostic interaction, and C-H activation processes. Similarly, the C-C agostic complex 7 is converted to the same C-H activated product 9 by treatment with CO.