742 Organometallics 2010, 29, 742–756 DOI: 10.1021/om900036j Benzene C-H Bond Activation in Carboxylic Acids Catalyzed by O-Donor Iridium(III) Complexes: An Experimental and Density Functional Study Steven M. Bischof,§ Daniel H. Ess,§,† Steven K. Meier,# Jonas Oxgaard,† Robert J. Nielsen,† Gaurav Bhalla,# William A. Goddard, III,*,† and Roy A. Periana*,§ §Department of Chemistry, The Scripps Energy Laboratories, The Scripps Research Institute, Jupiter, Florida 33458, #Department of Chemistry, Loker Hydrocarbon Institute, University of Southern California, Los Angeles, California 90089, and †Materials and Process Simulation Center, Beckman Institute (139-74), Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received January 15, 2009 3 The mechanism of benzene C-H bond activation by [Ir(μ-acac-O,O,C )(acac-O,O)(OAc)]2 (4)and 3 [Ir(μ-acac-O,O,C )(acac-O,O)(TFA)]2 (5) complexes (acac=acetylacetonato, OAc=acetate, and TFA= trifluoroacetate) was studied experimentally and theoretically. Hydrogen-deuterium (H/D) exchange ‡ ‡ between benzene and CD3COOD solvent catalyzed by 4 (ΔH =28.3(1.1 kcal/mol, ΔS =3.9(3.0 cal K-1 mol-1) results in a monotonic increase of all benzene isotopologues, suggesting that once benzene coordinates to the iridium center, there are multiple H/D exchange events prior to benzene dissociation. B3LYP density functional theory (DFT) calculations reveal that this benzene isotopologue pattern is due to a rate-determining step that involves acetate ligand dissociation and benzene coordination, which is then followed by heterolytic C-H bond cleavage to generate an iridium-phenyl intermediate. A synthesized iridium-phenyl intermediate was also shown to be competent for H/D exchange, giving similar rates to the proposed catalytic systems. This mechanism nicely explains why hydroarylation between benzene and alkenes is suppressed in the presence of acetic acid when catalyzed by [Ir(μ-acac-O,O,C3)(acac-O,O)(acac- 3 C )]2 (3) (Matsumoto et al. J. Am. Chem. Soc. 2000, 122, 7414). Benzene H/D exchange in CF3COOD solvent catalyzed by 5 (ΔH‡=15.3 ( 3.5 kcal/mol, ΔS‡=-30.0 ( 5.1 cal K-1 mol-1) results in significantly elevated H/D exchange rates and the formation of only a single benzene isotopologue, (C6H5D). DFT cal- culations show that this is due to a change in the rate-determining step. Now equilibrium between coordi- nated and uncoordinated benzene precedes a single rate-determining heterolytic C-H bond cleavage step. 1. Introduction bonds offers a new paradigm for transition metal mediated C-H bond activation. Previously, we reported that the 1 In contrast to classic oxidative addition and σ-bond iridium(III) acetylacetonato (acac) alkoxide and hydroxo 2 3 metathesis mechanisms, the recent discovery of 1,2-addi- complexes 1a and 1b promote stoichiometric benzene C-H 4 5 tion or substitution mechanisms across metal-heteroatom bond activation to generate methanol or water and the corresponding iridium-phenyl complexes (2a,b) by an inter- *Corresponding author. E-mail: [email protected]; wag@ nal substitution mechanism (IS, Scheme 1a).6 Recent density wag.caltech.edu. (1) (a) Bergman, R. G. Nature 2007, 446, 391. (b) Arndtsen, B. A.; functional theory (DFT) and energy decomposition cal- Bergman, R. G. Science 1995, 270, 1970. culations have revealed that the IS transition state involves (2) (a) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. the interplay between an electrophilic iridium center that (b) DeYonker, N. J.; Foley, N. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. 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H.; Ganesh, S. K.; Meier, S. K.; Ziatdinov, V. R.; Mironov, O.; (6) (a) Tenn, W. J., III; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Oxgaard, J.; Gonzales, J.; Goddard, W. A., III; Periana, R. A. J. Mol. Catal. Goddard, W. A., III; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172. A: Chem. 2006, 251, 8. (e) Crabtree, R. H. J. Organomet. Chem. 2004, 689, (b) Tenn, W. J., III; Young, K. J. H.; Oxgaard, J.; Nielsen, R. J.; Goddard, 4083. (f) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471. W. A., III; Periana, R. A. Organometallics 2006, 25, 5173. pubs.acs.org/Organometallics Published on Web 01/26/2010 r 2010 American Chemical Society Article Organometallics, Vol. 29, No. 4, 2010 743 Scheme 1. (a) Benzene C-H Bond Activation by (acac-O, Scheme 2. (a) Hydroarylation Catalyst 3, (b) Benzene-Alkene 2 O)2Ir(OR)(Pyridine)[acac-O,O = K -O,O-acetylacetonato, Hydroarylation Catalyzed by 3, (c) Benzene-Solvent H/D R=HorMe] and (b) Transition State for an Internal Exchange Catalyzed by 3 Substitution Mechanism facilitates iridium-carbon bond formation and a nucleophi- lic OR group that utilizes a lone pair to bond with hydrogen calculations have suggested that a coordinated acetate group (Scheme 1b).7 can assist in C-H bond cleavage via a substitution mecha- Benzene C-H bond activation by the (acac-O,O) Ir(R)(L) 2 nism akin to the IS mechanism (Scheme 2c, acetate-assisted motif8 also occurs during the hydroarylation reaction between transition state).7a,11 A similar transition state was also lo- benzene and alkenes catalyzed by the dinuclear [Ir(μ-acac-O, 3 3 9 cated for intramolecular cyclometalation of dimethylbenzyla- O,C )(acac-O,O)(acac-C )] complex 3 (Scheme 2a,b). Pre- þ 2 mine by the [CpIr(OAc)] complex (Cp = cyclopentadienyl, vious DFT studies have shown that the C-H bond activation OAc = acetate).12 Also, we have previously shown that an in this reaction occurs via an oxidative hydrogen migration10 acetate-assisted mechanism is likely operative for H/D ex- (OHM) transition state (Scheme 2b).5a The OHM transition change between benzene and trifluoroacetic acid (HTFA) state involves exchange of an iridium-R bond for an iri- 11d 2 catalyzed by K[Pt(pic)(TFA)2] (pic = κ -N,O-picolinate, dium-phenyl bond without forming an oxidative addition - - TOF=1.2 Â 10 2 s 1 at 70 °C) and (NNC)PtCl13 (NNC = intermediate (Scheme 2b). 0 0 κ3-6-phenyl-4,4 -di-tert-butyl-2,2 -bipyridine, TOF = 8.18 Â In the presence of acetic acid, benzene-alkene hydroary- - - 10 3 s 1 at 180 °C) and H/D exchange between methane and lation is severely suppressed; however, catalytic hydrogen- trifluoroacetic acid catalyzed by an (NNC)Ir(TFA)2 pincer deuterium (H/D) exchange between deuterated acetic acid and -2 -1 14 - complex (TOF=2.12 Â 10 s at 180 °C). benzene occurs with a turnover frequency (TOF) of 7.6 s 1 at Here we report an experimental and computational 160 °C(Scheme2c).9a This observation is intriguing because (B3LYP density functional) study of the mechanism of a new acetate/acetic acid complex is likely formed that does catalytic H/D exchange between benzene and carboxylic not catalyze hydroarylation, which requires both C-H bond acid solvents using (acac-O,O) Ir(R)(L).15 Among the most activation and alkene insertion reactions. Previous DFT 2 important questions to answer are the following: (1) What is the mechanism and rate-determining step for H/D exchange, (7) (a) Ess, D. H.; Bischof, S. M.; Oxgaard, J.; Periana, R. A.; benzene coordination, or C-H bond cleavage? (2) What Goddard, W. A., III. Organometallics 2008, 27, 6440.