Electrophilic C-H activation at {Cp*Ir}: Ancillary-ligand control of the mechanism of C-H activation

Density functional calculations on the low-temperature cyclometalation of dimethylbenzylamine with [IrCl₂Cp*]₂/NaOAc have characterized a novel electrophilic activation pathway for C-H bond activation. C-H activation occurs from [Ir(DMBA-H)(k²-OAc)Cp*]⁺, and OAc plays a central role in determining the barrier for reaction. Dissociation of the proximal OAc arm sets up a facile intramolecular deprotonation via a geometrically convenient six-membered transition state. Dissociation of the distal OAc arm, however, leads to a higher energy four-membered (σ-bond metathesis) transition state, while oxidative addition is even higher in energy. For this Ir³⁺ system, these three mechanisms appear to lie within a continuum in which the participation of the metal center and an H-accepting ancillary ligand are inversely related. The ability of the ancillary ligand to act as a proton acceptor is the key factor in determining which mechanism pertains.