A Noninnocent Cyclooctadiene (COD) in the Reaction of an “Ir(COD)(OAc)” Precursor with Imidazolium Salts S. M. Wahidur Rahaman, Shrabani Dinda, Arup Sinha, and Jitendra K. Bera* Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208016 India. * S Supporting Information 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-hetero- cyclic carbene (NHC) complexes [Ir(1-κ-4,5,6-η-C 8 H 12 )(κ 2 C,N- MesIPy)Br] (1) and [Ir(1-κ-4,5,6- η-C 8 H 12 )(κ 2 C, N-BnIN)Br] (2), respectively. In contrast, the methoxy analogue [Ir(COD)(μ- OMe)] 2 on reaction with MesIPy·HBr under identical con- ditions affords the Ir I -NHC complex [Ir(COD)(κ 2 C,N- MesIPy)Br]. Treatment of [Ir(COD)(κ 2 C,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 [ n Bu 4 N][BF 4 ]affords exclusively [Ir(bpy)(COD)][BF 4 ](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-κ-4,5,6- η-C 8 H 12 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-κ 1 ,η 2 -C 8 H 11 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. ■ INTRODUCTION The activation of C-H bonds is a process of immense signi ficance. 1 A clear understanding of C-H activation processes is central to the goal of improving the existing strategies for organic transformations 2 and of devising effective means to functionalize cheap and highly abundant hydrocarbons. 3 A wide array of metal complexes is reported to cleave C-H bonds, which suggests different activa- tion mechanisms. Depending on the electronic nature of the metal and the set of ligands in the active metal species, C-H activation processes are classified into different categories. 4 Oxidative addition (OA) is favored for electron-rich, low-valent transition metals capable of accommodating the increase in two-electron oxidation state and the change in geometry upon the formation of two new bonds. Transition metals that lack sufficient electron density for OA employ alternate mech- anisms: for example, σ-bond metathesis, 1,2-addition, elec- trophilic activation, and metalloradical activation. The distinc- tions between these processes are not always clear from the experimental details. Computational studies, however, often provide valuable insights in delineating the mechanism of the C-H cleavage. The C-H activation often involves an initial coordination of a C-H σ-bond to the metal. Experimental observation of σ complexes is difficult to achieve, although computational studies often encounter such species prior to the C-H cleavage process. Isolation of numerous agostic intermediates strongly indicates the existence of σ-complexes. 5 The subsequent C-H cleavage mechanism, however, differs depending on the nature of the active metal species. 4 For example, metal acetates cleave C-H bonds via an electrophilic mechanism. Fagnou 6 and others 7 have identified a concerted metalation-deprotonation (CMD) mechanism for directed C(sp 2 )-H and C(sp 3 )-H functionalization catalyzed by Pd(OAc) 2 . It involves C-H interaction with the metal via an agostic interaction followed by proton abstraction by the metal-bound base acetate. Cyclo- metalation reactions by [RuCl 2 (p-cymene)] 2 , [RhCl 2 Cp*] 2 , and [IrCl 2 Cp*] 2 in combination with an acetate have been sug- gested to proceed through a similar CMD mechanism. 8,9 Received: October 19, 2012 Published: December 27, 2012 Article pubs.acs.org/Organometallics © 2012 American Chemical Society 192 dx.doi.org/10.1021/om300982q | Organometallics 2013, 32, 192-201