Synthesis of Iridium(III) Carboxamides via the Bimetallic Reaction between Cp(PMe3)IrPh(OH) and [Cp(PMe3)Ir(Ph)NCR]

Reaction of Cp*(PMe3)IrPh(OH) (1) with nitriles is undetectably slow in benzene solution at room temperature. However, in the presence of Cp*(PMe3)IrPh(OTf) (2) (OTf = O3SCF3), the reaction is strongly catalyzed, leading to iridium(III) carboxamides Cp*(PMe3)IrPh[NHC(O)R] (6a−d) [R = C6H4CH3 (6a), C6H5 (6b), C6H4CF3 (6c), CH3 (6d)]. We propose that these transformations occur by initial displacement of the trifluoromethanesulfonate (“triflate”) anion of 2 by a molecule of nitrile, leading to a nitrile-substituted iridium cation, [Cp*(PMe3)IrPh(NCR)]+ (10). Following this, the nucleophilic hydroxide group of 1 attacks the (activated) nitrile molecule bound in 10, leading (after proton transfer) to the iridium carboxamide complex. In the case of nitriles possessing hydrogens α to the cyano group, competitive loss of one of these protons is observed, leading to iridium C-bound cyanoenolates such as Cp*(PMe3)(Ph)Ir(CH2CN) (7). Protonolysis of carboxamides 6a−d with HCl yields Cp*(PMe3)IrPh(Cl) (9) and the free amides. A pronounced solvent effect is observed when the reaction between 1 and nitriles catalyzed by 2 is carried out in THF solution. The basic hydroxide ligand of 1 induces an overall dehydration/cyclization reaction of the coordinated aromatic nitrile. For example, the reaction of 1 with p-trifluorotolunitrile and a catalytic amount of 2 leads to the formation of 6c, water, [Ph(PMe3)Ir[C5Me4CH2C(C6H4CF3)N]] (12), and [Ph(PMe3)Ir(C5Me4CH2C(C6H4CF3)NH)]OTf (13). A mechanism to explain the formation of both 12 and 13 and the role each compound plays in the formation of the iridium carboxamides is proposed.