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 ₂Cp*]₂ and a range of chelating bases are presented. With acetate, density functional theory calculations on the key intermediate, [Ir(DMBA-H)(?²-OAc)Cp]⁺, define a two-step C-H activation process involving initial ?²-?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 ?²-?¹ base displacement step; thus, more weakly coordinating bases promote C-H activation. For triflate, [Ir(DMBA-H)(?¹-CF₃SO₃)Cp]⁺ is more stable than its ?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 [IrCl₂Cp*] 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. © The Royal Society of Chemistry 2009.