A noninnocent cyclooctadiene (COD) in the reaction of an “Ir(COD)(OAc)” precursor with imidazolium salts

The reactions between [Ir(COD)(μ-OAc)]2 and the functionalized imidazolium salt 1-mesityl-3-(pyrid-2-yl)imidazolium bromide (MesIPy·HBr) or 1-benzyl-3-(5,7-dimethylnaphthyrid-2-yl)imidazolium bromide (BnIN·HBr) at room temperature afford the COD-activated IrIII–N-heterocyclic carbene (NHC) complexes [Ir(1-κ-4,5,6-η-C8H12)(κ2C,N-MesIPy)Br] (1) and [Ir(1-κ-4,5,6-η-C8H12)(κ2C,N-BnIN)Br] (2), respectively. In contrast, the methoxy analogue [Ir(COD)(μ-OMe)]2 on reaction with MesIPy·HBr under identical conditions affords the IrI–NHC complex [Ir(COD)(κ2C,N-MesIPy)Br]. Treatment of [Ir(COD)(κ2C,N-MesIPy)Br] with sodium acetate does not lead to COD activation. Further, use of 2,2′-bipyridine (bpy) with [Ir(COD)(μ-X)]2 (X = MeO or AcO) in the presence of [nBu4N][BF4] affords exclusively [Ir(bpy)(COD)][BF4] (3). Metal-bound acetate is shown to be an essential promoter for activation of the COD allylic C–H bond. An examination of products reveals the following transformations of the precursor components: cleavage of the imidazolium C2–H and subsequent NHC metalation, metal oxidation from IrI to IrIII, and 2e reduction of COD, effectively resulting in 1-κ-4,5,6-η-C8H12 coordination to the metal. Mechanistic investigation at the DFT/B3LYP level of theory strongly suggests that NHC metalation precedes COD allylic C–H activation. Two distinct pathways have been examined which involve initial C2–H oxidative addition to the metal followed by acetate-assisted allylic C–H activation (path A) and the reverse sequence, i.e., deprotonation of C2–H by the acetate and subsequent allylic C–H oxidative addition to the metal (path B). The result is an IrIII–NHC–hydride−κ1, η2-C8H11 complex. Subsequent intramolecular insertion of the COD double bond into the metal–hydride bond followed by isomerization gives the final product. An acetate-assisted facile COD allylic C–H bond activation, in comparison to oxidative addition of the same to Ir, makes path A the favored pathway. This work thus raises questions about the innocence of COD, especially when metal acetates are used for the synthesis of NHC complexes from the corresponding imidazolium salts