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 MC−H and OH 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.