Various mechanisms for the cyclometalation of dimethylbenzylamine by palladium acetate have been studied by DFT calculations. Contrary to previous suggestions, the rate-limiting step is the electrophilic attack of the palladium on an ortho arene C-H bond to form an agostic complex rather than a Wheland intermediate. The cyclometalated product is then formed by intramolecular deprotonation by acetate via a six-membered transition state; this step has almost no activation barrier.

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)(kappa2-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 (sigma-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.

Reaction of a pyrrole imine with [IrCl 2 Cp*] 2 /NaOAc leads to N-H actiVation in preference to C-H actiVation at the pyrrole; howeVer, with the N-methylated ligand C-H actiVation occurs. Density functional calculations show that N-H bond actiVation is both kinetically and thermodynamically preferred to C-H actiVation. Both reactions occur with relatiVely low energy barriers by an electrophilic agostic interaction with the metal with simultaneous intramolecular hydrogen bonding with acetate leading to deprotonation Via a six-membered transition state.