Acceptorless Alkane Dehydrogenation Catalyzed by Iridium CCC

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Acceptorless Alkane Dehydrogenation Catalyzed by Iridium CCCPincer Complexes Anthony R. Chianese,* Myles J. Drance, Kelsey H. Jensen, Samuel P. McCollom, Nevin Yusufova, Sarah E. Shaner, Dimitar Y. Shopov, and Jennifer A. Tendler Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States S Supporting Information *

ABSTRACT: Iridium complexes of CCC-pincer bis-N-heterocyclic carbenes, including a newly synthesized trifluoromethylsubstituted complex, were examined as catalysts for the acceptorless dehydrogenation of cyclooctane and n-undecane. Up to 103 turnovers were observed for the dehydrogenation of cyclooctane, and up to 97 turnovers were observed for the dehydrogenation of n-undecane. The catalysts showed high initial turnover frequencies, followed by a gradual loss of activity over 24 h. Experiments indicate that this loss of activity is due to catalyst decomposition rather than product inhibition. Stoichiometric reactivity was investigated for the precatalysts, focusing on the synthesis of dihydride and trihydride complexes as well as the dissociation and addition of neutral ligands.

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transfer dehydrogenation (up to 12 turnovers),8b but more active (up to 68 turnovers) for the acceptorless dehydrogenation of cyclooctane.8a A substantial steric effect was observed: mesityl- and 3,5-di-tert-butylphenyl-substituted variants were active, while 3,5-xylyl and 2,6-diisopropylphenyl-substituted variants were inactive. Herein, we report more detailed studies of the acceptorless dehydrogenation of alkanes, catalyzed by CCC-Ir complexes. Time-course studies show fast initial catalysis, which slows substantially over 24 h; the falloff arises from catalyst decomposition rather than product inhibition. We describe the stoichiometric reactivity of the CCC-Ir precatalysts, focusing on neutral ligand dissociation/addition and reaction with the hydride donor LiHBEt3. We also report the synthesis and characterization of an electronically modified, CF3-substituted catalyst.

INTRODUCTION The selective, catalytic dehydrogenation of alkanes to give alkenes is a longstanding challenge in synthetic chemistry.1 Since the initial discoveries by Crabtree2 and Felkin3 more than 30 years ago, significant progress has been made. In 1996, Kaska and Jensen reported that an iridium complex of a bulky PCP-pincer ligand was especially active for the transfer of hydrogen from cyclooctane to tert-butylethylene.4 Through steric and electronic modification of the PCP ligand, turnover numbers in the thousands have been observed for the iridiumcatalyzed transfer dehydrogenation of alkanes employing a sacrificial alkene acceptor.5 The acceptorless dehydrogenation of alkanes, where strongly endothermic hydrogen removal is driven by continuously purging a refluxing solution with inert gas, has also been achieved with high turnover numbers.5b,d,6 Though the PCP-iridium systems are by far the most active for alkane dehydrogenation, it is noteworthy that a strongly electron-withdrawing PCP-Ru catalyst has shown promise, giving nearly 200 turnovers for the transfer dehydrogenation of cyclooctane.7 Recently, we8 and others9 have begun developing the chemistry of iridium complexes containing monoanionic CCC-pincer bis-N-heterocyclic carbene ligands. We observed that our CCC-Ir complexes were weakly active for alkane © XXXX American Chemical Society

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RESULTS AND DISCUSSION Synthesis and Characterization of a CF3-Substituted Precatalyst. Substitution of the para position of the aryl fragment in PCP-pincer ligands has previously been shown to have a substantial effect on the rate of iridium-catalyzed alkane Received: July 3, 2013

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Scheme 1. Synthesis of CF3-Substituted Precatalyst10

Figure 1. ORTEP diagram of 1-CF3Mes, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity; the iridium-bound hydride was not located. Selected metric data (bond lengths in Å and angles in deg): Ir(1)−C(16), 1.958(3); Ir(1)−C(6), 2.026(3); Ir(1)−C(19), 2.030(3); Ir(1)−Cl(2), 2.5012(7); Ir(1)−N(3), 2.101(2); C(19)−Ir(1)−C(16), 78.36(11); C(6)−Ir(1)−C(16), 78.70(11); C(6)−Ir(1)−C(19), 156.75(11).

dehydrogenation.5a By analogy with our previously described synthetic route,8a we successfully synthesized an electronically modified precatalyst as shown in Scheme 1. Palladium-catalyzed Buchwald−Hartwig coupling followed by cyclization in acidic triethylorthoformate gave the bis-benzimidazolium salt in good yield. Metalation was accomplished by heating the bisbenzimidazolium salt, [Ir(cod)Cl]2, and excess triethylamine in acetonitrile. The precatalyst 1-CF3Mes was purified by flash chromatography on silica gel followed by recrystallization. By 1H and 13C NMR spectroscopy, 1-CF3Mes appears to have Cs symmetry, and its structure is analogous to that previously observed for the non-CF3-substituted parent

compound.8a The two mesityl-substituted benzimidazol-2ylidene fragments are chemically equivalent. The nonequivalence of the two ortho-methyl groups and meta-hydrogens on each mesityl ring indicates that rotation about the C−N bond is restricted. The iridium-bound hydride appears at −22.7 ppm, and the presence of an acetonitrile molecule in the coordination sphere is apparent in both the 1H and 13C NMR spectra. NOESY indicates that the acetonitrile ligand is trans to the aryl fragment of the pincer ligand, while the hydride is cis to the aryl fragment. The structure of 1-CF3Mes was confirmed by X-ray crystallography. Diffraction-quality crystals were obtained by B

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ation of cyclooctane. Control experiments indicated that NaOtBu was needed to produce active catalysts; presumably its function is to promote the reductive elimination of HCl,13 generating a reactive Ir(I) intermediate capable of entering the catalytic cycle14 via C−H oxidative addition. 1-Mes and 1CF3Mes were similarly active, giving 103 and 84 turnovers after 22 h, respectively. In contrast, 1-dtbp was less active, producing only 35 turnovers in 22 h. Due to the challenges inherent in controlling the vigorousness of the reflux and the argon flow rate, acceptorless dehydrogenation experiments are challenging to reproduce precisely. Each cyclooctane dehydrogenation experiment was repeated twice, and the average relative standard deviation in the final TON across the three iridium complexes was 29%. This implies that the activities of 1-Mes and 1-CF3Mes are indistinguishable, but that 1-dtbp is clearly less active. Cyclodecane (bp 201 °C) is employed more frequently in studies of acceptorless dehydrogenation. Due to its higher boiling point, significantly greater rates and turnover numbers (up to 3050)15 have been observed for the acceptorless dehydrogenation of cyclodecane catalyzed by PCP-Ir catalysts. Despite this precedent, we found 1-Mes to be similarly active for the acceptorless dehydrogenation of cyclooctane and cyclodecane. After 22 h at reflux, 109 turnovers of cyclodecenes were produced, with a nearly constant cis:trans ratio of 1:5 (Figure 4).

recrystallization from CH2Cl2/pentane. The ORTEP diagram is presented in Figure 1. Overall, the structural parameters are nearly identical to the non-CF3-substituted parent compound.8a The coordination geometry at iridium is octahedral, distorted by the small bite angles in the CCC-pincer ligand. The hydride ligand was not located. Acceptorless Dehydrogenation of Cyclic Alkanes. The precatalysts employed in this study are shown in Figure 2. In

Figure 2. Precatalysts examined for acceptorless dehydrogenation of alkanes.

addition to the newly synthesized 1-CF3Mes, five previously synthesized CCC-Ir complexes8 were tested. In all of our trials, we found 1-dipp, 2-tBu, and 2-Ad to be inactive for alkane dehydrogenation. We found that 1-Mes, 1-CF3Mes, and 1dtpb, when activated with NaOtBu, formed active catalysts for the acceptorless dehydrogenation of a variety of alkanes. Cyclooctane (bp 149 °C) is a common model substrate for alkane dehydrogenation catalysis, due to its unusually low enthalpy of dehydrogenation (+24.3 kcal/mol)11 and the formation of only one product, cis-cyclooctene. Although TONs in the thousands have been recorded for transfer dehydrogenation of cyclooctane,5a,c,d,6b to our knowledge the highest TON for its acceptorless dehydrogenation is 190 after 120 h, catalyzed by a tert-butyl-substituted PCP-Ir complex.12 Figure 3 shows plots of catalytic turnover for the dehydrogen-

Figure 4. Time course plot for acceptorless dehydrogenation of cyclodecane. Reaction conditions: cyclodecane (3.0 mL, 19 mmol), 1Mes (1.5 μmol), NaOtBu (7.5 μmol), refluxing under a stream of Ar.

Test for Product Inhibition. The approximately 100 turnovers observed for cyclooctane dehydrogenation represent a conversion of only 1.3%. In order to determine whether the falloff in activity arises from catalyst decomposition or inhibition by the buildup of product alkene, we compared the dehydrogenation of cyclooctane catalyzed by 1-Mes/NaOtBu in the presence or absence of 0.40 mmol of added ciscyclooctene, equivalent to 100 catalytic turnovers. Nearly identical reactivity was observed, indicating that product inhibition is not the major source of the loss of activity. Cyclooctane Dehydrogenation under Different Atmospheres. Alkane dehydrogenation catalyzed by di-tertbutylphosphino-substituted PCP-Ir complexes has been reported to be strongly inhibited by dinitrogen, due at least in part to strong binding of N2 to the PCP-Ir fragment.16 In contrast, a bis-trifluoromethylphosphino-substituted PCP-Ir complex showed no such inhibition, giving the same reactivity under an Ar or N2 atmosphere.6b We compared the activity of

Figure 3. Time course plots for acceptorless cyclooctane dehydrogenation. Reaction conditions: cyclooctane (8.0 mL, 30 mmol), iridium complex (4.0 μmol), NaOtBu (20 μmol), refluxing under a stream of Ar. C

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1-Mes for cyclooctane dehydrogenation under a flow of argon, dinitrogen, and air. Catalyst activity was indistinguishable under nitrogen and argon. Under air, almost all catalyst activity was destroyed: small amounts of cyclooctene (up to 5 turnovers) were observed along with other unidentified products, but the time course was not reproducible. On the suggestion of a reviewer, we tested the reversibility of the catalyst deactivation under air by running the reaction under air for 1 h, then switching the atmosphere to argon for the remaining 19 h. In this experiment, catalyst activity was not restored, indicating that oxygen irreversibly deactivates the catalyst. Acceptorless Dehydrogenation of n-Undecane. The dehydrogenation of linear alkanes is particularly challenging, especially with respect to product selectivity. PCP-Ir catalysts have shown kinetic selectivity for terminal alkenes, which erodes with longer reaction times due to competing alkene isomerization catalysis.5d,15,17 In trials of PCP-Ir-catalyzed acceptorless dehydrogenation of linear alkanes, up to 108 turnovers were observed after 24 h for n-undecane dehydrogenation (bp 196 °C),15 and up to 71 turnovers were observed after 72 h for n-dodecane (bp 216 °C).18 Our CCC-Ir catalysts show comparable activity for the acceptorless dehydrogenation of n-undecane, as depicted in Figure 5. Notably, 1-dtpb, which

fast alkene isomerization catalysis we have documented for these systems.8b Reaction with Hydride Donors. Although in situ activation of Ir(III) hydridochloride complexes with NaOtBu has been successfully employed to generate active catalysts for alkane dehydrogenation,5a one-component catalysts consisting of dihydride or tetrahydride complexes have been more commonly employed. Notably, selective formation of 1-alkenes via dehydrogenation of n-alkanes has been observed only using one-component catalysts.5d,15,17 PCP-Ir-HCl complexes have been converted to dihydride, trihydride, and tetrahydride complexes using LiEt3BH/H2,5d,17b,19 NaH/H2,20 and NaOtBu/H2.20b By addition of LiEt3BH or NaEt3BH to 1-Mes or 2-Ad, we observed the formation of compounds tentatively assigned by 1 H NMR spectroscopy as the dihydride complexes 3-Mes and 3-Ad and the anionic trihydride complexes 4-Mes and 4-Ad (Scheme 2). Treatment of a dilute solution of 1-Mes in benzene-d6 with 1.2 equiv of LiEt3BH gave nearly complete conversion to 3-Mes, while addition of 5 equiv of LiEt3BH primarily gave 4-Mes. Addition of a large excess of NaEt3BH to a THF-d8 solution of 2-Ad gave clean formation of 4-Ad. Attempts to add 1 equiv of LiEt3BH to 2-Ad in THF-d8 gave only mixtures of the reactant 2-Ad and the trihydride 4-Ad, presumably due to the low solubility of 2-Ad. However, addition of an excess of LiEt3BH to a suspension of 2-Ad in benzene gave nearly complete conversion to the dihydride 3Ad; presumably its low solubility prevents further reaction to form 4-Ad in benzene. Attempts to isolate samples of the dihydrides 3-Mes and 3-Ad for use as one-component catalysts were unsuccessful, apparently due to decomposition under vacuum. By 1H NMR, the trihydride species 4-Mes and 4-Ad occupy the expected octahedral geometry, showing C2v symmetry in solution. The hydride ligands appear in a 1:2 ratio. In both cases, the NHC’s N-substituents act as stereochemical reporters: equivalent mesityl ortho-methyl groups (4-Mes) or equivalent adamantyl methylene hydrogens (4-Ad) indicate the existence of a mirror plane coincident with the plane of the CCC-pincer backbone, consistent with the structures depicted in Scheme 2. The dihydride species show an interesting structural dichotomy, the origin of which is not obvious. Compound 3Mes possesses C2v symmetry in solution: only one hydride resonance is observed, and the four ortho-methyl groups on the mesityl N-substituents are chemically equivalent. This is consistent with the trans arrangement of hydride ligands shown in Scheme 2. On the suggestion of a reviewer, we recorded the 1H NMR spectrum at temperatures between 22 and −75 °C to examine the possibility that the symmetric spectrum observed at room temperature for 3-Mes might result from a rapidly exchanging complex of lower symmetry. The spectrum was unchanged over this temperature range, indicating that the most likely structure is a trans-dihydride. In contrast, two distinct hydride resonances are observed at room temperature for 3-Ad. The observation of diastereotopic adamantyl methylene hydrogens beta to nitrogen supports the less symmetrical geometry shown in Scheme 2. It is not clear why this geometry would be favored, but one possibility is that THF binds reversibly to the site trans to hydride in the observed isomer of 3-Ad, but is too bulky to bind in the site trans to Caryl, as would be required in a trans-dihydride isomer.

Figure 5. Time course plots for acceptorless n-undecane dehydrogenation. Reaction conditions: n-undecane (8.0 mL, 38 mmol), iridium complex (4.0 μmol), NaOtBu (20 μmol), refluxing under a stream of Ar.

showed the lowest activity for cyclooctane dehydrogenation, is the most active for n-undecane, giving 97 turnovers after 22 h. Each n-undecane dehydrogenation experiment was repeated twice, and the average relative standard deviation in the final TON across the three iridium complexes was 9%. Therefore, the greater activity observed for 1-dtbp as compared to 1-Mes and 1-CF3Mes can be expressed with a moderate amount of confidence. It is noteworthy that 1-dtbp remains active for the entire course of the reaction here, whereas it loses activity more rapidly in the dehydrogenation of cyclooctane, which occurs at a lower reflux temperature; we cannot rationalize this observation with confidence. In contrast to PCP-Ir catalysts, which produced a mixture of undecenes containing 44−56% 1undecene after 8 h via acceptorless dehydrogenation of nundecane,5d our catalysts produce only internal undecene isomers at all time-points. This result is unsurprising given the D

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Scheme 2. Reaction of CCC-Ir Hydridochloride Complexes with LiEt3BH or NaEt3BH

Ligand Dissociation/Addition Reactions. As we have previously described,8b CCC-Ir hydridochloride complexes have been isolated either as six-coordinate, octahedral, acetonitrile-solvated species or as five-coordinate, squarepyramidal species, depending on the shape and steric bulk of the N-substituents (see Figure 2). Braunstein9b,c and Hollis9a have also observed that CCC-pincer complexes with small (nbutyl) N-substituents tend to favor the formation of sixcoordinate, octahedral iridium complexes. Because catalytic competence likely depends on the accessibility of the coordination site trans to the aryl fragment of the pincer ligand, we decided to examine the reactivity of 1-Mes and 2-Ad for ligand dissociation and ligand addition, respectively. Upon treatment with up to 500 equiv of acetonitrile or pyridine in CD2Cl2, the 1H NMR spectrum of 2-Ad shows no change, indicating that even weak binding of those ligands is not detectable. To assess whether a five-coordinate structure is accessible for 1-Mes, a sample was heated to 200 °C under vacuum overnight. The resulting brown solid was completely insoluble in noncoordinating solvents, but dissolved readily in CH2Cl2 to give yellow 5-Mes, where the acetonitrile ligand has been replaced by dichloromethane (Scheme 3). The dichloromethane adduct 5-Mes has been characterized spectroscopically and crystallographically (Figure 6). The formation of 5-Mes provides strong evidence that acetonitrile was removed by heating in a vacuum. Although the composition of the intermediate brown solid has not been determined, its

Figure 6. ORTEP diagram of 5-Mes, showing 30% probability ellipsoids. Hydrogen atoms are omitted for clarity; the iridium-bound hydride was not located. Selected metric data (bond lengths in Å and angles in deg): Ir(1)−C(6), 2.028(6); Ir(1)−C(9), 1.954(6); Ir(1)− C(12), 2.037(6); Ir(1)−Cl(2), 2.4842(15); Ir(1)−Cl(3), 2.5328(16); Cl(3)−C(4), 1.787(7); C(4)−Cl(5), 1.768(7); C(12)−Ir(1)−C(9), 79.2(2); C(6)−Ir(1)−C(9), 78.6(2); Ir(1)−Cl(3)−C(4), 109.3(2); Cl(3)−C(4)−Cl(5), 109.8(4).

Scheme 3. Synthesis of the Dichloromethane Adduct 5-Mes

insolubility in noncoordinating solvents likely indicates a polymeric structure. Solution characterization of 5-Mes by NMR spectroscopy was performed in CD2Cl2, in which 5-Mes is highly soluble. In CD2Cl2, no separate resonance for iridium-bound CH2Cl2 was observed, consistent with fast exchange with the solvent. Addition of 1 drop of acetonitrile to a CD2Cl2 solution of 5Mes results in rapid, complete conversion to 1-Mes. When 5Mes is dissolved in benzene-d6, a sharp 2H singlet at 4.35 ppm is observed, corresponding to bound dichloromethane (free E

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dichloromethane appears at 4.27 ppm in benzene-d6).21 When 2 equiv of dichloromethane is added to this solution, one sharp signal for dichloromethane is observed at 4.32 ppm, the signals for the iridium complex shift slightly, and the hydride resonance is not observed. The hydride signal is likely broadened into the baseline; heating to 80 °C did not allow its observation. This indicates that free and bound dichloromethane exchange rapidly on the NMR time scale at room temperature and that 5-Mes is likely in rapid exchange with another, unidentified complex under these conditions, potentially an adduct of benzene or adventitious water. We are aware of only two published crystal structures containing neutral dichloromethane bound to iridium; both are Ir(III) complexes. The Ir−Cl distance of 2.5328 Å in 5-Mes can be compared with Ir−Cl distances of 2.462 Å in Cp*Ir(PMe3)(CH 3 )(CH 2 Cl 2 ) 22 and 2.550 Å in IrH 2 (PR 3 ) 2 (OTf)(CH2Cl2).23 In both the previously known examples, the bound dichloromethane is significantly distorted. The C−Cl bonds proximal to iridium are 1.820 and 1.830 Å, respectively, while the C−Cl bonds distal to iridium are 1.730 and 1.695 Å, respectively. In 5-Mes, this distortion is much less pronounced: the proximal C−Cl bond is 1.787 Å, while the distal C−Cl bond is 1.768 Å. As this distortion is likely induced by πdonation from a filled d-orbital on iridium into the C−Cl σ*orbital, it appears that the iridium atom in 5-Mes is less π-basic than in the previously described complexes.

Iridium Complex 1-CF3Mes. The bis-benzimidazolium salt shown in Scheme 1 (1.77 mmol, 1.22 g), [Ir(cod)Cl]2 (0.872 mmol, 596 mg), and triethylamine (53 mmol, 7.4 mL) were added to an oven-dried, medium-walled pressure vessel in the glovebox. The mixture was stirred at 100 °C for 24 h behind a blast shield and then allowed to cool to room temperature. The flask was opened to air, the solvent was evaporated, and the residue was purified by chromatography on silica gel, eluting with a gradient of 0−20% ethyl acetate in dichloromethane. After removing the solvent under vacuum, the complex was analytically pure. Yield: 1.08 g, 61%. Recrystallization for catalytic trials was performed at room temperature in an argon-filled glovebox by layering a solution in dichloromethane with pentane. 1H NMR (CD2Cl2): δ 8.20 (d, 2H, 3JHH = 8.1 Hz); 7.89 Hz (s, 2H); 7.49 (t, 2H, 3JHH = 7.6 Hz), 7.32 (t, 2H, 3JHH = 7.7 Hz), 7.07 (s, 2H); 7.02 (s, 2H); 6.98 (d, 2H, 3JHH = 7.9 Hz); 2.35 (s, 6H); 2.22 (s, 6H); 1.96 (s, 6H); 1.45 (s, 3H); −22.7 (s, 1H). 13C NMR: δ 186.52; 152.87; 146.82; 139.18; 138.66; 137.53; 135.98; 133.31; 132.64; 129.45; 128.86; 126.08 (q, 1 JCF = 271.4 Hz); 124.08; 123.51; 123.44 (q, 2JCF = 31.6 Hz); 116.56; 111.39; 111.06; 105.17 (q, 3JCF = 4.1 Hz); 21.27; 18.38; 18.08; 2.91. 19 F NMR: δ −60.3. Anal. Calcd for C41H36ClF3IrN5: C, 55.74; H, 4.11; N, 7.93. Found: C, 55.53; H, 4.00; N, 7.88. Acceptorless Dehydrogenation of Cyclooctane. An ovendried 25 mL round-bottom flask was charged with an iridium complex (4.0 μmol), sodium tert-butoxide (20 μmol, 1.9 mg), and a stir-bar. A reflux condenser was attached, and the system was evacuated and backfilled with argon three times. Degassed cyclooctane (59 mmol, 8.0 mL) was added via syringe. The solution was refluxed while stirring and purging continuously with argon for 22 h. Aliquots were removed for analysis via cannula and analyzed by 1H NMR in CDCl3 (100 μL reaction mixture in 700 μL of CDCl3), comparing the integration of cyclooctene product with that of the cyclooctane reactant. The results in Figure 3 represent an average of two runs for each iridium complex. Acceptorless Dehydrogenation of Cyclodecane. The procedure was analogous to that for cyclooctane, using 1-Mes (1.22 mg, 1.5 μmol), sodium tert-butoxide (7.5 μmol, 0.7 mg), and degassed cyclodecane (19 mmol, 3.0 mL). Acceptorless Dehydrogenation of n-Undecane. The procedure was analogous to that for cyclooctane, using an iridium complex (4.0 μmol), sodium tert-butoxide (20 μmol, 1.9 mg), and degassed nundecane (38 mmol, 8.0 mL). Iridium Complex 3-Mes. Compound 1-Mes (1.0 mg, 1.2 μmol) was dissolved in 1 mL of benzene-d6 in the glovebox, and LiEt3BH (1.0 M in THF, 1.5 μL, 1.5 μmol) was added. The solution was transferred to a J-Young NMR tube. 1H NMR (C6D6): δ 8.04 (d, 2H, 3 JHH = 8.1 Hz); 7.82 (d, 2H, 3JHH = 7.8 Hz); 7.52 (t, 1H, 3JHH = 7.8 Hz); 7.21 (t, 3JHH = 7.3 Hz); 7.07 (t, 2H, 3JHH = 7.6 Hz); 6.86 (d, 2H, 3 JHH = 7.9 Hz); 6.75 (s, 4H); 2.26 (s, 12H); 2.11 (s, 6H); 0.94 (s, 3H); −9.21 (s, 2H). Iridium Complex 3-Ad. Compound 2-Ad (2.5 mg, 3.1 μmol) was suspended in 1 mL of benzene-d6 in the glovebox, and LiEt3BH (1.0 M in THF, 25 μL, 25 μmol) was added. The yellow suspension was stirred at room temperature overnight, during which time the yellow undissolved solid became white. The mixture was filtered, and the solid residue was washed with a small amount of benzene-d6, dissolved in THF-d8, and transferred to a J-Young NMR tube. 1H NMR (THF-d8): δ 8.32 (d, 2H, 3JHH = 8.0 Hz); 7.81 (d, 2H, 3JHH = 7.9 Hz); 7.28−7.33 (m, 3H); 7.19 (t, 2H, 3JHH = 7.3 Hz); 2.45 (d, 6H, 2JHH = 12.5 Hz); 2.27 (d, 6H, 2JHH = 12.5 Hz); 1.72 (s, 6H); 1.59 (d, 6H, 2JHH = 11.3 Hz); −8.15 (s, 1H); −16.41 (s, 1H). The 6H resonance at ∼1.80 ppm for the adamantyl CH2 group delta to nitrogen was overlapped with the THF signal. Iridium Complex 4-Mes. Compound 1-Mes (5.0 mg, 6.2 μmol) was suspended in 1 mL of benzene-d6 in the glovebox, and LiEt3BH (1.0 M in THF, 31 μL, 31 μmol) was added. The solution was transferred to a J-Young NMR tube. 1H NMR (C6D6): δ 8.06 (d, 3JHH = 8.1 Hz); 7.90 (d, 2H, 3JHH = 7.8 Hz); 7.58 (t, 1H, 3JHH = 7.8 Hz); 7.22 (t, 2H, 3JHH = 8.1 Hz); 7.04 (t, 2H, 3JHH = 7.9 Hz); 6.94 (s, 4H); 6.80 (d, 2H, 3JHH = 8.1 Hz); 2.32 (s, 6H); 2.17 (s, 12H); −12.15 (t, 1H, 2JHH = 5.8 Hz); −13.17 (d, 2H, 2JHH = 5.8 Hz).

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SUMMARY We have synthesized a new CF3-substituted CCC-pincer ligand and its acetonitrile-solvated iridium hydridochloride complex, 1-CF3Mes. We have studied the acceptorless dehydrogenation of alkanes catalyzed by 1-CF3Mes and previously reported CCC-Ir complexes. Three of these complexes, 1-CF3Mes, 1Mes, and 1-dtbp, are active for acceptorless alkane dehydrogenation when activated by NaOtBu, giving up to 103 catalytic turnovers. Rapid initial catalysis followed by a falloff in rate over 24 h is due to catalyst decomposition. Potentially useful CCCIr dihydride complexes 3-Mes and 3-Ad were synthesized by reduction of hydridochloride precursors, but instability under vacuum prevented their isolation for catalytic trials. A study of neutral ligand substitution on 1-Mes and ligand addition to 2Ad underscores the substantial effect of the varying steric bulk of the N-substituents. For the six-coordinate 1-Mes, removal of the acetonitrile ligand was accomplished by heating under vacuum to give a material that is likely polymeric, but cleanly gives the solvent adduct 5-Mes on dissolution in dichloromethane. In contrast, the five-coordinate 2-Ad is resistant to addition of a neutral ligand trans to Caryl, due to the increased steric bulk of the adamantyl N-substituent as opposed to mesityl.

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EXPERIMENTAL SECTION

General Methods. [Ir(cod)Cl]2 was prepared as previously described.24 All other materials were commercially available and were used as received, unless otherwise noted. Solvents were purified by sparging with argon and passing through columns of activated alumina, using an MBraun solvent purification system. Flash chromatography using solvent gradients was performed using a Combiflash RF system. NMR spectra were recorded at room temperature on a Bruker spectrometer operating at 400 MHz (1H NMR), 100 MHz (13C NMR), and 376 MHz (19F NMR). Elemental analyses were performed by Robertson Microlit (Madison, NJ, USA). Images of NMR spectra, along with procedures for ligand synthesis and characterization, are given in the Supporting Information. F

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Iridium Complex 4-Ad. Compound 2-Ad (7.0 mg, 8.7 μmol) was suspended in 0.75 mL of THF-d8 in the glovebox, and NaEt3BH (23 mg, 170 μmol) was added. The mixture was stirred at 25 °C until clear and then transferred to a J-Young NMR tube. 1H NMR (THF-d8): δ 8.28 (d, 2H, 3JHH = 8.2 Hz); 8.10 (d, 2H, 3JHH = 8.6 Hz); 7.83 (d, 2H, 3 JHH = 7.8 Hz); 7.18 (t, 2H, 3JHH = 7.8 Hz); 7.14 (t, 1H, 3JHH = 7.8 Hz); 7.04 (t, 2H, 3JHH = 7.8 Hz); 3.57 (s, 12H); 2.29 (s, 6H); 2.22 (d, 6H, 2JHH = 11.9 Hz); 1.76 (d, 6H, 2JHH = 11.9 Hz); −7.80 (t, 1H, 2JHH = 3.2 Hz); −12.28 (d, 2H, 2JHH = 3.2 Hz). The resonance at 3.57 ppm for the adamantyl CH2 groups beta to nitrogen was overlapped with the residual THF-d8 signal. Iridium Complex 5-Mes. Compound 1-Mes (25 mg, 31 μmol) was heated under vacuum (10 mTorr) at 200 °C for 16 h, resulting in the formation of a brown solid. This solid was recrystallized under argon by layering a dichloromethane solution with pentane. Yield: 16 mg, 60%. 1H NMR (C6D6): δ 7.74 (d, 2H, 3JHH = 8.4 Hz); 7.45 (d, 2H, 3JHH = 8.0 Hz); 7.26 (t, 1H, 3JHH = 8.0 Hz); 7.07 (t, 2H, 3JHH = 7.9 Hz); 6.92 (t, 2H, 3JHH = 7.9 Hz); 6.82 (s, 2H); 6.71 (d, 2H, 3JHH = 8.0 Hz); 6.50 (s, 2H); 4.35 (s, 2H); 2.31 (s, 6H); 2.04 (s, 6H); 1.88 (s, 6H). The iridium-bound hydride was not observed in C6D6. 1H NMR (CD2Cl2): δ 8.18 (d, 2H, 3JHH = 8.4 Hz); 7.68 (d, 2H, 3JHH = 8.0 Hz); 7.47 (t, 2H, 3JHH = 7.6 Hz); 7.34 (t, 1H, 3JHH = 8.0 Hz); 7.31 (t, 2H, 3 JHH = 7.7 Hz); 7.07 (s, 2H); 7.05 (s, 2H); 6.98 (d, 2H, 3JHH = 8.0 Hz); 2.39 (s, 6H); 2.18 (s, 6H); 1.97 (s, 6H); −22.68 (s, 1H). 13C NMR (CD2Cl2): δ 188.76; 145.80; 140.29; 137.62; 137.39; 136.16; 132.80; 132.62; 129.73; 129.52; 124.05; 123.37; 121.94; 111.47; 111.16; 108.42; 21.34; 18.17; 18.04. The iridium-bound Caryl was not observed, possibly due to broadening caused by exchange of the CD2Cl2 ligand. Anal. Calcd for C39H36Cl3IrN4: C, 54.51; H, 4.22; N, 6.52. Found: C, 56.90; H, 4.52; N, 6.95. Repeated analyses gave similar results, which may result from partial loss of dichloromethane upon storage under vacuum. X-ray Crystallography, General Methods. Structure determinations were performed on an Oxford Diffraction Gemini-R diffractometer, using Mo Kα radiation (1-CF3Mes) or Cu Kα radiation (5-Mes). Single crystals were mounted on Hampton Research Cryoloops using Paratone-N oil. Unit cell determination, data collection and reduction, and empirical absorption correction were performed using the CrysAlisPro software package.25 Direct methods structure solution was accomplished using SIR92,26 and fullmatrix least-squares refinement was carried out using CRYSTALS.27 All non-hydrogen atoms were refined anisotropically. Unless otherwise noted, hydrogen atoms were placed in calculated positions, and their positions were initially refined using distance and angle restraints. All hydrogen positions were fixed in place for the final refinement cycles. X-ray Structure Determination of 1-CF3Mes. X-ray quality crystals of 1-CF3Mes were grown by layering a dichloromethane solution with pentane. Minor conformational disorder was present for the CF3 group, but accounting for this disorder did not improve the agreement. Highly disordered solvent was present; correction for this residual density was performed using the option SQUEEZE in the program package PLATON.28 A total of 83 electrons per unit cell were removed, from a total potentially solvent-accessible void of 568.0 Å3. The iridium-bound hydride was not located in the difference map. X-ray Structure Determination of 5-Mes. X-ray quality crystals of 5-Mes were grown by layering a dichloromethane solution with pentane. Highly disordered solvent was present; correction for this residual density was performed using the option SQUEEZE in the program package PLATON.28 A total of 104 electrons per unit cell were removed, from a total potentially solvent-accessible void of 357.3 Å3.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation (Grant No. CHE1057792) for financial support. REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

CIF file giving crystallographic data for complexes 1-CF3Mes and 5-Mes. Complete experimental procedures for ligand synthesis and characterization and images of NMR spectra for all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org. G

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