Computational and synthetic studies on the cyclometallation reaction of dimethylbenzylamine with [IrCl2Cp*]2: role of the chelating base

The results of a joint computational and experimental study of the cyclometallation reactions of dimethylbenzylamine (DMBA-H) with [IrCl(2)Cp*](2) and a range of chelating bases are presented. With acetate, density functional theory calculations on the key intermediate, [Ir(DMBA-H)(kappa(2)-OAc)Cp](+), define a two-step C-H activation process involving initial kappa(2)-kappa(1) displacement of base to give an intermediate that is stabilized by internal H-bonding. Facile C-H bond cleavage then occurs via 'ambiphilic metal ligand activation' (AMLA). A similar pattern is computed for other carboxylates and bicarbonate, and in each case the ease of C-H activation is governed by the accessibility of the kappa(2)-kappa(1) base displacement step; thus, more weakly coordinating bases promote C-H activation. For triflate, [Ir(DMBA-H)(kappa(1)-CF(3)SO(3))Cp](+) is more stable than its kappa(2)-isomer and C-H activation proceeds with a barrier of only 3.8 kcal mol(-1). Experimental studies confirm that a range of carboxylates and triflate can effect cyclometallation; however, reactivity patterns are not consistent with the computed C-H activation barriers. Instead, the role of base in opening the [IrCl(2)Cp*](2) dimer and subsequent formation of the [Ir(DMBA-H)(base)Cp*](+) intermediates appears crucial. Calculations indicate these processes are far more favourable for acetate than for triflate.