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.