MECHANISM OF LIGAND SUBSTITUTION IN AN IRIDIUM AMIDE COMPLEX

Reaction of Cp*IrPPh3Cl2 (Cp* = eta-5-C5Me5) with methyllithium gave Cp*IrPPh3Me2 (1a), which yielded Cp*IrPPh3(Me)(Cl) (2a) on treatment with anilinium hydrochloride. The reactions of complex 2a with silver acetate, benzylmagnesium chloride, and lithium anilide gave Cp*IrPPh3(Me)(OAc) (3a), Cp*IrPPh3(Me)(CH2Ph) (4a), and Cp*IrPPh3(Me)(NHPh) (5a), respectively. Cp*IrPPh3(Me)(OPh) (6a) was prepared from 2a and sodium phenoxide or from 3a and potassium phenoxide. Reaction of 2a with sodium ethoxide in ethanol gave Cp*IrPPh3(Me)(H) (7a). Treatment of 1a-6a with PPh2Me gave PPh3 and the corresponding Cp*Ir(PPh2Me)(Me)(X) compounds (X = Me, 1b; X = Cl, 2b; X = OAc, 3b; X = CH2Ph, 4b; X = NHPh, 5b; X = OPh, 6b). Complexes 1b-6b were prepared independently as for 1a-6a via Cp*Ir(PPh2Me)Cl2. The rate of the reaction of 5a and PPh2Me to give PPh3 and 5b does not depend on [PPh2Me]. On the basis of this result, the temperature dependence of the rate, and the lack of inhibition by PPh3, 5a is proposed to reversibly form the ring-slipped intermediate (eta-3-C5Me5)IrPPh3(Me)(NHPh) (8), which is stabilized by nitrogen lone pair to metal electron donation and trapped by PPh2Me to give the observed products. The large rate differences in the ligand substitutions of 1a-6a are rationalized in light of this mechanism.