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

Abstract

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 Ir^III–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 Ir^I–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 Ir^I to Ir^III, 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 Ir^III–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.