Computational studies on C-H activation reactions at Iridum, Rhodium and Ruthenium

Abstract

Density Functional Theory (DFT) calculations have been carried out to study the factors that affect the cyclometallation reactions of the model system [Ir(-Cp)(dmba−H)(2- RCO2)]+ (R= CH3, CF3, CCl3, OH and Ph), as well as the triflate analogue. The limiting step is, in all cases, the dissociation of one arm of the chelating base and in most cases a 1-intermediate was located. The transition state for the subsequent C−H activation exhibits short MC−H and OH interactions which combine to allow an easy hydrogen transfer. The combination of these two factors leads to a new term Ambiphilic Metal Ligand Activation (AMLA) to describe these C−H activation processes. The above study was extended to [M(ring)(dmba−H)(2-OAc)] systems, (where M(ring) = {Rh(-Cp)}+, {Ru(-C6H6)}+ and {Ru(-Cp)}). Cationic systems have very similar activation energies (E‡), although small variations in the overall energy were computed. These effects were rationalized in terms of the strengths of the M−C and M−O bonds formed and broken in the reaction. The neutral systems gave a lower E‡ although the products were less stable. In addition, the substitution of the dmba−H ligand for related imine or amide substrates shows that these species also readily undergo facile cyclometallation. Finally, the intermolecular C−H activation of benzene by [Ir(-Cp)(PH3)(2-OAc)]+ and the incorporation of this step into a catalytic cycle for the hydroarylation of ethene was assessed. The rate-limiting step is associated with the alkene insertion step (E‡ = 16.7 kcal/mol), while the C−H activation is slightly more accessible. Therefore, this model appears to be a promising target for catalysis.