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

Abstract

The results of a joint computational and experimental study of the cyclometallation reactions of dimethylbenzylamine (DMBA-H) with [IrCl₂Cp*]₂ and a range of chelating bases are presented. With acetate, density functional theory calculations on the key intermediate, [Ir(DMBA-H)(κ²-OAc)Cp]⁺, define a two-step C–H activation process involving initial κ²–κ¹ 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 κ²–κ¹ base displacement step; thus, more weakly coordinating bases promote C–H activation. For triflate, [Ir(DMBA-H)(κ¹-CF₃SO₃)Cp]⁺ is more stable than its κ²-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.