Sterically Encumbered Iridium Bis(N-heterocyclic carbene

Communication pubs.acs.org/Organometallics

Sterically Encumbered Iridium Bis(N-heterocyclic carbene) Complexes: Air-Stable 14-Electron Cations and Facile Degenerate C− H Activation Nicholas Phillips,† Johnny Rowles,† Michael J. Kelly,† Ian Riddlestone,† Nicholas H. Rees,† Athanasia Dervisi,‡ Ian A. Fallis,‡ and Simon Aldridge*,† †

Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford OX1 3QR, U.K. School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, U.K.

‡

S Supporting Information *

ABSTRACT: Cationic Ir(III) systems supported by a bis(expanded NHC) framework and featuring both agostic C−H and cis alkyl/hydride ligand sets have been targeted by protonation of the corresponding bis(alkyl) hydride complexes. Remarkably, the steric shielding afforded by the NHC substituents is such that these and related putative 14-electron cations are air and moisture stable. In solution, degenerate fluxional exchange is brought about by reversible σ-bond activation within the agostic alkyl C(sp3)−H bond; a non-dissociative mechanism is implied by the activation parameters ΔH⧧ = 8.8(0.4) kcal mol−1 and ΔS⧧ = −12.2(1.7) eu. undergo degenerate fluxional exchange and offer, through VTNMR, a platform on which to investigate reversible activation of a coordinated C(sp3)−H bond.11,12 The reaction of either of the free carbenes 6-Mes or 7-Mes with [Ir(coe)2Cl]2 in thf leads to the coordination of two NHC ligands and to the activation of a single methyl C−H bond within each (Scheme 1). The resulting complexes Ir(6-Mes′)2H (1a) and Ir(7-Mes′)2H (1b) possess effective C2 symmetry in solution (three o-Me and two p-Me 1H NMR signals) and give rise to a high-field resonance integrating to one hydride ligand (at δH −37.54 and −31.76 ppm, respectively). This double activation chemistry is in contrast with the behavior of the less strongly donating IMes ligand, which generates M(IMes)(IMes′)HCl (M = Rh, Ir) by oxidative addition of a single CH bond.7a,13 Optimization of the reaction conditions for 1a/1b occurs at an NHC:Ir ratio of 3:1, consistent with the additional elimination of HCl being driven by the formation of the hydrochloride salts [(6-Mes)H]Cl/[(7-Mes)H]Cl. Further characterization of both 1a and 1b in the solid state has been achieved by microanalysis and X-ray crystallography, with the latter (Figure 1) confirming a square-pyramidal coordination geometry at the iridium center, featuring pairs of trans NHC and trans tethered benzyl ligands and an apical hydride. Reaction of 1a or 1b with the conjugate acid of a very weakly basic anion, such as [H(OEt2)2][BArf4] (Brookhart’s acid),14 leads to the protonation of a single benzylic donor and to the formation of the putative 14-electron systems [Ir(6-Mes′)(6Mes)H][BArf4] ([2a][BArf4]) and [Ir(7-Mes′)(7-Mes)H] [BArf4] ([2b][BArf4]) (Scheme 1).15−17 In both cases, NMR data in CD2Cl2 solution imply that the cation is fluxional at

T

he selective functionalization of unactivated C−H bonds remains one of the primary synthetic challenges in organometallic chemistry.1 Alkyl C−H bond cleavage by oxidative addition at late-transition-metal centers and the role of alkane σ complexes therein have therefore been the subject of a number of seminal studies.2 Isolated σ complexes of this type, however, remain relatively few in number,3−5 with entropically less disfavored tethered (“agostic”) systems representing a convenient alternative platform on which to study parameters relating to the activation of alkyl C−H bonds.6 Group 9 complexes have featured heavily in such studies, and N-heterocyclic carbene (NHC) scaffolds have been shown to support a number of examples of intramolecular alkyl C−H coordination/activation,7,8 reflecting the strongly σ-donating properties of this ligand class.9 While existing studies have typically focused on five-membered heterocycles, saturated expanded ring systems (Chart 1) are known to be even Chart 1. NHC Ligands Relevant to the Current Study

stronger donors and (on the basis of more obtuse N−C−N angles) to offer increased steric shielding of reactive metal centers.10 With this in mind we have targeted bis(expanded NHC) ancillary ligand sets in the design of highly unsaturated group 9 systems capable of E−H bond activation. In doing so, we have gained access to cationic complexes containing both agostic C−H and cis alkyl/hydride ligand sets. These systems © 2012 American Chemical Society

Received: November 6, 2012 Published: November 28, 2012 8075

dx.doi.org/10.1021/om301060h | Organometallics 2012, 31, 8075−8078

Organometallics

Communication

In the case of [2b][BArf4], the presence of this agostic interaction in the solid state can be confirmed crystallographically (Figure 2), being signaled by Ir(1)···H(412) and

Scheme 1. Syntheses and Protonation of Ir(6-Mes′)2H (1a) and Ir(7-Mes′)2H (1b) and the Contrasting Behavior of IMesa

Figure 2. Structures of [2a][BArf4] (left) and [2b][BArf4]·1/2CH2Cl2 (right) in the solid state. Key bond lengths (Å) and angles (deg): for [2a][BArf4], Ir(1)−C(2) = 2.050(3), Ir(1)−C(26) = 2.060(3), Ir(1)− C(10) = 2.065(4), Ir(1)−H(11) = 1.50, Ir(1)···C(32) = 2.512(3), C(2)−Ir(1)−C(26) = 174.7(1); for [2b][BArf4], Ir(1)−C(2) = 2.063(8), Ir(1)−C(6) = 2.072(6), Ir(1)−C(27) = 2.051(7), Ir(1)− H(11) = 1.66, Ir(1)···C(41) = 2.546(8), Ir(1)···H(412) = 1.98, C(6)− Ir(1)−C(27) = 177.7(3).

a

Key reagents and conditions: (i) NHC = 6-Mes or 7-Mes, [Ir(coe)2Cl]2 (0.17 equiv of dimer), thf, 20 °C, 12 h, 30−50% isolated yield; (ii) [H(OEt2)2][BArf4] (1.00 equiv), fluorobenzene, 20 °C, 30 min, 60−90% isolated yield; (iii) HCl (1.00 equiv), Et2O, 20 °C, 15 min, 90% isolated yield; (iv) NHC = IMes, [Ir(coe)2Cl]2, thf, as per ref 12.

Ir(1)···C(41) contacts of 1.98 and 2.546(8) Å, respectively.6 Interestingly, [2a]+despite giving rise to a pattern of NMR signals in solution which is essentially identical with that of [2b]+reveals an alternative motif for the stabilization of the unsaturated iridium center in the solid state. In this case, the predominant secondary contact is not with the NHC methyl substituent but with the mesityl π system itself (Figure 2). Thus, a short Ir−Cipso contact of 2.512(3) Å is observed (for C(32)),18 while the closest approach of a pendant methyl group to the metal center results in an Ir···C contact of >3.3 Å. Clearly the energetic differences associated with the coordination of different secondary donors at iridium in [2a]+ and [2b]+ are relatively minor. Of interest from the perspective of CH activation are the barriers associated with the fluxional processes occurring for [2a]+ and [2b]+ in dichloromethane solution. In each case, degenerate ground-state structures interconvert by exchange between the agostic CH bond and the cis benzyl/hydride ligands (Scheme 2). Variable-temperature NMR measurements

Figure 1. Structures of 1a (left) and 1b (right) in the solid state (one of two molecules in the asymmetric unit shown in each case). Here and in Figures 2 and 3, most H atoms are omitted and mesityl Me groups are shown in wireframe format for clarity; thermal ellipsoids are set at the 40% probability level. H atoms were located in the difference Fourier map and refined isotropically. Key bond lengths (Å) and angles (deg): for 1a, Ir(26)−H(261) = 1.50, Ir(26)−C(27) = 2.015(7), Ir(26)−C(54) = 2.038(7), Ir(26)−C(35) = 2.15(1), Ir(26)−C(76) = 2.16(1), C(27)−Ir(26)−C(54) = 179.4(3), C(76)−Ir(26)−C(35) = 165.0(3); for 1b, Ir(1)−H(11)= 1.42, Ir(1)−C(2) = 2.029(8), Ir(1)−C(27) = 2.029(8), Ir(1)−C(45) = 2.19(1), Ir(1)−C(20) = 2.13(1), C(2)−Ir(1)−C(27) = 177.5(3), C(45)−Ir(1)−C(20) = 163.4(4).

Scheme 2. Fluxional Interconversion between Degenerate Ground-State Structures in [2a]+

room temperature (e.g., three mesityl o-Me and two p-Me signals), with the expected C1 symmetry skeleton being apparent only at low temperatures (T < −20 and −35 °C, respectively). Distinct resonances for the iridium-bound H and CH2 groups are resolved only in the low-temperature limit (at δH −31.06 and 1.65/2.32 ppm for [2a]+ and −28.59 and 1.83/ 2.38 ppm for [2b]+); for both cations an upfield-shifted methyl resonance (at δH 0.61 or −0.24 ppm, respectively) is also apparent at low temperature, consistent with the stabilization of the unsaturated iridium center by an additional agostic interaction.6

for [2a]+ indicate that this exchange process involves only two of the eight o-Me groups; i.e., no exchange occurs either between the pair of methyl groups within a given mesityl substituent or between mesityl groups within each NHC ligand. Line shape analysis enables exchange rates to be determined at temperatures in the range −25 to −55 °C, from which an Eyring plot yields values for ΔH⧧ and ΔS⧧ of 8.8(0.4) kcal mol−1 and −12.2(1.7) eu, respectively.19 This fluxional 8076

dx.doi.org/10.1021/om301060h | Organometallics 2012, 31, 8075−8078

Organometallics

Communication

these [L2IrX2]+ systems. Perhaps more remarkable still, the extent of this shielding is such that [2a]+/[2b]+ and [3a]+/ [3b]+ are stable (even in chlorocarbon solution) to exposure to air and moisture over a period of >10 days.23

exchange involves net oxidative addition of the agostic benzyl C−H bond and complementary “mirror-image” reductive elimination. Mechanistically, the negative entropy of activation would appear to preclude a dissociative mechanism involving an early C−H reductive elimination step, being more consistent with associative activation involving significant oxidative cleavage of the agostic C−H bond. By means of comparison, the values determined by Bergman and co-workers for the conversion of rhodium alkane complexes to the corresponding alkyl hydrides range from 2.7 ± 1 to 4.6 ± 0.4 kcal mol−1 (for ΔH⧧) and −7 ± 6 to −16 ± 5 eu (for ΔS⧧).4c Associative activation in a related aryl ortho-metalation process has also been proposed on the basis of observed trends in rate as a function of ancillary ligand.11a Alternatively, our data could also be interpreted in terms of a more concerted process based, for example, around a σ-CAM mechanism.20,21 Given the degenerate (i.e., non-productive) bond activation processes occurring for [2a]+/[2b]+ in solution, attempts have been made to examine the reaction pathways resulting from the displacement of the agostic C−H bond by other σ ligands. Consistently, exposure of either cation to dihydrogen results in rapid net H−H oxidative addition/C−H reductive elimination and the formation of systems of the type [Ir(NHC)2(H)2]+ ([3a]+/[3b]+, each as the [BArf4]− salt). In both cases spectroscopic data in solution and crystallographic studies in the solid state indicate that the eliminated C−H bond is not retained agostically and that the 14-electron metal center is stabilized instead by interactions with the flanking aryl π rings, similar to those observed for solid [2b][BArf4] (Figure 3).22

■

ASSOCIATED CONTENT

S Supporting Information *

Text giving characterization data for 1a, 1b, [2b][BArf4], [3a][BArf4], [3b][BArf4], and Ir(6-Mes)(6-Mes′)H(Cl) and CIF files giving data for all crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*[email protected]; Tel: +44 1865 285201; Fax: +44 1865 272690; Web: http://users.ox.ac.uk/∼quee1989. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the EPSRC for funding (studentship for N.P.) and for access to the National Mass Spectrometry Service Centre, Swansea. We thank Prof. R. N. Perutz (University of York) for helpful discussions.

■

REFERENCES

(1) For a collection of pertinent recent reviews, see: Acc. Chem. Res. 2012, 45 (6), 777−958 on “C−H functionalization”. Notable papers include: (a) Gaillard, S.; Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45, 778−787. (b) Colby, D. A.; Tsai, A. S.; Bergman, R. G. Acc. Chem. Res. 2012, 45, 814−825. (c) Hartwig, J. F. Acc. Chem. Res. 2012, 45, 864−873. (d) Hashiguchi, B. B.; Bischof, S. M.; Konnick, M. M.; Periana, R. A. Acc. Chem. Res. 2012, 45, 885−898. (e) Boisvert, L.; Goldberg, K. I. Acc. Chem. Res. 2012, 45, 899−910. (f) Roizen, J. L.; Harvey, M. E.; Du Bois, J. Acc. Chem. Res. 2012, 45, 911−922. (g) Neufeldt, S. R.; Sanford, M. S. Acc. Chem. Res. 2012, 45, 936−946. (h) Haibach, M. C.; Kundu, S.; Brookhart, M.; Goldman, A. S. Acc. Chem. Res. 2012, 45, 947−958. See also: Chem. Rev. 2010, 110 (2), 575−1211 on “selective functionalization of C−H bonds”. (2) For review articles see, for example: (a) Crabtree, R. H. Chem. Rev. 1985, 85, 245−269. (b) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. H.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154−162. (c) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879−2932. (d) Jones, W. D. Acc. Chem. Res. 2003, 36, 140−146. (e) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507−514. (f) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083−4091. (3) For reviews of relevant σ complexes see, for example: (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes: Structure, Theory and Reactivity; Kluwer Academic/Plenum Publishers: New York, 2001. (b) Hall, C.; Perutz, R. N. Chem. Rev. 1996, 96, 3125−3146. (c) Cowan, A. J.; George, M. W. Coord. Chem. Rev. 2008, 252, 2504− 2511. (d) Proc. Natl. Acad. Sci. U.S.A. 2007, 104 (17), 6899−6973 (coordination of saturated molecules special issue). (4) For key references relating to the spectroscopic characterization of alkane complexes, see: (a) Perutz, R. N.; Turner, J. J. J. Am. Chem. Soc. 1975, 97, 4791−4800. (b) Hermann, H.; Grevels, F. W.; Henne, A.; Schaffner, K. J. Phys. Chem. 1982, 68, 5151−5154. (c) MacNamara, B. K.; Yeston, J. S.; Bergman, R. G.; Moore, C. B. J. Am. Chem. Soc. 1999, 121, 6437−6443. (d) Ball, G. E.; Brookes, C. M.; Cowan, A. J.; Darwish, T. A.; George, M. W.; Kawanami, H. K.; Portius, P.; Rourke, J. P. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6927−6933. (e) Cowan, A. J.; Portius, P.; Kawanami, H. K.; Jina, O. S.; Grills, D. C.; Sun, X.-Z.; McMaster, J.; George, M. W. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6933−6938. (f) Lawes, D. J.; Darwish, T. A.; Clark, T.; Harper, J. B.; Ball, G. E. Angew. Chem., Int. Ed. 2006, 45, 4486−4490. (g) Bernskoetter, W. H.; Schauer, C. K.; Goldberg, K. I.; Brookhart,

Figure 3. Structures of [3a][BArf4] (left) and [3b][BArf4] (right) in the solid state. Key bond lengths (Å) and angles (deg): for [3a][BArf4], Ir(1)−C(2) = 2.046(6), Ir(1)−C(26) = 2.036(6), Ir(1)−H(11) = 1.47, Ir(1)−H(12) = 1.43, Ir(1)···C(17) = 2.876(5), Ir(1)···C(41) = 2.814(5), C(2)−Ir(1)−C(26) = 176.0(2); for [3b][BArf4], Ir(1)−C(2) = 2.053(6), Ir(1)−C(27) = 2.042(6), Ir(1)−H(11) = 1.85, Ir(1)−H(12) = 1.58, Ir(1)···C(18) = 2.732(7), Ir(1)···C(43) = 2.755(5), C(2)−Ir(1)−C(27) = 174.8(2).

[3a]+ and [3b]+ can thus be described as weakly π-stabilized Ir(III) dihydrides, as gauged by Ir(1)···Cipso distances of 2.814(5), 2.876(5) Å and 2.732(7), 2.755(5) Å and by sharp hydride signals at δH −43.57 and −41.68 ppm, respectively. Notably, the interaction of the π system with the metal center in each case results in significant canting of the carbene heterocycle to one side, as manifested by disparate Ir−C−N angles (e.g., for [3a][BArf4]: ∠Ir(1)−C(2)−N(3) = 128.3(4)°, ∠Ir(1)−C(2)−N(16) = 113.9(4)°). The ability of the expanded ring 6-Mes and 7-Mes NHC ligands to provide peripheral steric shielding of reactive metal centers, and if necessary secondary ligation via agostic CH bonds or arene π interactions, is apparent from the chemistry of 8077

dx.doi.org/10.1021/om301060h | Organometallics 2012, 31, 8075−8078

Organometallics

Communication

fluorobenzene. After 30 min the volatiles were removed in vacuo to yield a pale orange solid, which was washed with hexanes (3 × 20 cm3) and dried in vacuo for 2 h; the pale orange crude product was thus isolated in 88% yield (0.18 g, 0.11 mol). Crystals suitable for X-ray diffraction were obtained by layering a solution in dichloromethane with hexanes and storage at −30 °C. Spectroscopic data for [2a][BArf4]: 1 H NMR (CD2Cl2, 300 MHz, 298 K) δH 7.73 (8H, s, o-CH [BArf4]−), 7.56 (4H, s, p-CH [BArf4]−), 7.04, 7.03 (each 2H, s, m-CH Mes), 6.92 (2H, br, s, m-CH Mes′), 6.70 (2H, s, meta-CH Mes′), 3.34 (2H, m, NCH2), 3.24 (4H, m, NCH2), 2.89 (2H, m, NCH2), 2.36 (6H, s, pCH3 Mes), 2.31 (6H, s, p-CH3 Mes′), 2.26 (6H, s, o-CH3 Mes), 2.02 (6H, s, o-CH3 Mes′), 1.88 (4H, br, NCH2CH2), rapidly exchanging IrCH2, agostic Me and IrH not observed, see the Supporting Information for the limiting low-temperature spectrum; 13C NMR (CD2Cl2, 126 MHz, 298 K) signals due to cation: δC 188.3 (NCN), 141.8 (ipso-C Mes), 138.6 (p-C Mes), 138.5 (p-C Mes′), 136.6, 136.5 (o-C Mes), 134.9 (o-C Mes′), 134.2 (ipso-C Mes′), 131.8, 130.8 (mCH Mes′), 130.4 (o-C Mes′), 130.1, 129.5 (m-C Mes), 48.5, 46.7 (NCH2), 21.5 (NCH2CH2), 21.1 (p-CH3 Mes), 21.0 (p-CH3 Mes′), 19.0 (o-CH3 Mes′), 18.4, 18.0 (o-CH3 Mes), 1.1 (IrCH2), signals due to [BArf4]− anion : δC 162.1 (q, JCB = 49.6 Hz, ipso-C), 135.2 (br, oCH), 129.2 (q, 2JCF = 31.4 Hz, m-C), 125.0 (q, 1JCF = 270.5 Hz, CF3), 117.8 (m, p-CH), see the Supporting Information for the limiting lowtemperature spectrum; 11B NMR (CD2Cl2, 96 MHz, 298 K) δB −6.7 ([BArf4]−); 19F NMR (CD2Cl2, 282 MHz, 298 K) δ −62.9 (CF3); MS (ESI +ve) m/z 833.4, M +, 100%; accurate mass calcd for [C44H56N4191Ir]+ 831.4105, measd 831.4125. Anal. Calcd: C, 53.81; H, 4.04; N, 3.30. Found: C, 53.36; H, 3.83; N, 2.95. Crystallographic data for [2a][BArf4]: C76H68BF24IrN4, Mr = 1696.39, triclinic, P1,̅ a = 15.1100(3) Å, b = 17.1158(4) Å, c = 17.3974(4) Å, α = 63.004(2)°, β = 66.1775(19)°, γ = 75.0989(19)°, V = 3652.43(16) Å3, Z = 2, ρc = 1.542 Mg m−3, T = 150 K, λ = 1.54184 Å, 38689 reflections collected, 15191 independent (R(int) = 0.028), which were used in all calculations, R1 = 0.0333, wR2 = 0.0808 for observed unique reflections (F2 > 2σ(F2)) and R1 = 0.0351, wR2 = 0.0829 for all unique reflections, maximum and minimum residual electron densities 2.36 and −1.55 e Å−3, CSD reference 900459. See the Supporting Information for spectroscopic and crystallographic data for [2b] [BArf4]. (18) For comparison, Ir−C distances of 2.170(5)−2.428(7) Å have been measured for η6 π complexes involving the binding of the pendant Dipp substituents of IPr to cationic Ir(III) centers8d (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). (19) Data for [2b]+ are consistent with a value for ΔG⧧ which is very similar to that for [2a]+ (11.3 vs 12.3 kcal mol−1 at coalescence). (20) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578−2592. (21) A concerted mechanism of a similar type has been proposed to account for hydride exchange in Fe(H)2(H2)(PEtPh2)3: Van Der Sluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton., K. G. J. Am. Chem. Soc. 1990, 112, 4831−4841. (22) For examples of systems of the type [Ir(NHC)2(H)2]+ stabilized by agostic interactions with NHC-tethered CH bonds, see refs 7b−e. For related phosphine systems see: Cooper, A. C.; Clot, E.; Huffman, J. C.; Streib, W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 97−106. (23) No change in the 1H NMR spectrum of a solution of [2a] [BArf4] or [2b][BArf4] in bench (i.e., wet) chloroform-d exposed to air was seen over a period of >10 days. Moreover, single crystals obtained from such a solution revealed the same composition by X-ray diffraction as samples obtained prior to exposure. Similar behavior was observed for [3a][BArf4] and [3b][BArf4].

M. Science 2009, 326, 553−556. (h) Young, R. D.; Lawes, D. J.; Hill, A. F.; Ball, G. E. J. Am. Chem. Soc. 2012, 134, 8294−8297. (5) For crystallographically characterized systems featuring metal− alkane interactions, see: (a) Evans, D. R.; Drovetskaya, T.; Bau, R.; Reed, C. A.; Boyd, P. D. W. J. Am. Chem. Soc. 1997, 119, 3633−3634. (b) Castro-Rodriguez, I.; Nakai, H.; Gantzel, P.; Zakharov, L. N.; Rheingold, A. L.; Meyer, K. J. Am. Chem. Soc. 2003, 125, 15734− 15735. (c) Pike, S. D.; Thompson, A. L.; Algara, A. G.; Appeley, D. C.; Macgregor, S. A.; Weller, A. S. Science 2012, 337, 1648−1651. (6) For reviews of agostic interactions see, for example: (a) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250, 395−408. (b) Brookhart, M.; Green, M. L. H.; Wong, L.-L. Prog. Inorg. Chem. 1988, 36, 1−124. (c) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789−805. (d) Brookhart, M.; Green, M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6908−6914. (7) For relevant examples, see: (a) Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics 2000, 19, 1194−1197. (b) Dorta, R.; Stevens, E. D.; Nolan, S. P. J. Am. Chem. Soc. 2004, 126, 5054−5055. (c) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan, S. P. Angew. Chem., Int. Ed. 2005, 44, 2512−2515. (d) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516−3526. (e) Tang, C. Y.; Thompson, A. L.; Aldridge, S. J. Am. Chem. Soc. 2010, 132, 10578−10591. (f) Navarro, J.; Torres, O.; Martín, M.; Sola, E. J. Am. Chem. Soc. 2011, 133, 9738−9740. (8) For examples of the dehydrogenation of alkyl substituents pendant to NHC ligands see: (a) Prinz, M.; Grosche, M.; Herdtweck, E.; Herrmann, W. A. Organometallics 2000, 19, 1692. (b) Dible, B. R.; Sigman, M. S.; Arif, A. M. Inorg. Chem. 2005, 44, 3774. (c) Tang, C. Y.; Smith, W.; Vidovic, D.; Thompson, A. L.; Chaplin, A. B.; Aldridge, S. Organometallics 2009, 28, 3059−3066. (d) Tang, C. Y.; Lednik, J.; Vidovic, D.; Thompson, A. L.; Aldridge., S. Chem. Commun. 2011, 47, 2523−2525. (9) See, for example: (a) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39−92. (b) Hahn, F. E. Angew. Chem., Int. Ed. 2006, 45, 1348−1352. (c) Diez-Gonzalez, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612−3676. (10) (a) Iglesias, M.; Beetstra, D. J.; Knight, J. C.; Ooi, L.-L.; Stasch, A.; Coles, S.; Male, L.; Hursthouse, M. B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Organometallics 2008, 27, 3279−3289. (b) glesias, M.; Beetstra, D. J.; Kariuki, B.; Cavell, K. J.; Dervisi, A.; Fallis, I. A. Eur. J. Inorg. Chem. 2009, 1913−1919. (c) Dunsford, J. J.; Cavell, K. J.; Kariuki, B. M. Organometallics 2012, 31, 4118−4121. (11) For a related degenerate aryl ortho-metalation process see: (a) Albéniz, A. C.; Schulte, G.; Crabtree, R. H. Organometallics 1993, 11, 242−249. For systems featuring an agostic sp3 (alkyl) CH bond in equilibrium with the corresponding alkyl hydride see, for example: (b) Buccella, D.; Parkin, G. J. Am. Chem. Soc. 2006, 128, 16358− 16364. (c) Rybtchinski, B.; Cohen, R.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 11041−11050. (d) Verat, A. Y.; Pink, M.; Fan, H.; Tomaszewski, J.; Caulton, K. G. Organometallics 2008, 27, 166−168. (e) Sewell, L. J.; Chaplin, A. B.; Abdalla, J. A. B.; Weller, A. S. Dalton Trans. 2010, 39, 7437−7439. (12) For a degenerate C−C activation process see: Brayshaw, S. K.; Sceats, E. L.; Green, J. C.; Weller, A. S. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 6921−6926. (13) Tang, C. Y.; Smith, W.; Thompson, A. L.; Vidovic, D.; Aldridge, S. Angew. Chem., Int. Ed. 2011, 50, 1359−1362. (14) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992, 11, 3920−3922. (15) Reaction with an acid which features a more coordinating conjugate base (such as HCl) yields the neutral chloro complex Ir(6Mes)(6-Mes′)H(Cl) (see the Supporting Information). (16) Monitoring of the protonation of 1a at low temperature (−78 °C) revealed no evidence for initial reaction at any site other than the iridium benzylic linkages. (17) [2a][BArf4] and [2b][BArf4]: the two compounds were synthesized in a similar manner, exemplified here for [2a][BArf4]. A solution of [H(OEt2)2][BArf4] (0.12 g, 0.12 mmol) in fluorobenzene was added to a stirred solution of 1a (0.10 g, 0.12 mmol) also in 8078

dx.doi.org/10.1021/om301060h | Organometallics 2012, 31, 8075−8078