A Noninnocent Cyclooctadiene (COD) in the Reaction of an Ir(COD)(OAc) Precursor with Imidazolium Salts

The reactions between [Ir(COD)(mu-OAc)](2) and the functionalized imidazolium salt 1-mesityl-3-(pyrid-2-yl)imidazolium bromide (MesIPy center dot HBr) or 1-benzyl-3-(5,7-dimethylnaphthyrid-2-yl)imidazolium bromide (BnIN center dot HBr) at room temperature afford the COD-activated Ir-III-N-heterocyclic carbene (NHC) complexes [Ir(1-kappa-4,5,6-eta-C8H12)(kappa C-2,N-MesIPy)Br] (1) and [Ir(1-kappa-4,5,6-eta-C8H12)(kappa C-2,N-BnIN)Br] (2), respectively. In contrast, the methoxy analogue [Ir(COD)(mu-OMe)](2) on reaction with MesIPy center dot HBr under identical conditions affords the Ir-I-NHC complex [Ir(COD)(kappa C-2,N-MesIPy)Br]. Treatment of [Ir(COD)(kappa C-2,N-MesIPy)Br] with sodium acetate does not lead to COD activation. Further, use of 2,2'-bipyridine (bpy) with [Ir(COD)(mu-X)](2) (X = MeO or AcO) in the presence of [(Bu4N)-Bu-n][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 C-2-H and subsequent NHC metalation, metal oxidation from Ir-I to Ir-III, and 2e reduction of COD, effectively resulting in 1-kappa-4,5,6-eta-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 C-2-H oxidative addition to the metal followed by acetate-assisted allylic C-H activation (path A) and the reverse sequence, i.e., deprotonation of C-2-H by the acetate and subsequent allylic C-H oxidative addition to the metal (path B). The result is an Ir-III-NHC-hydride-kappa(1),eta(2)-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.