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Nondirected C−H Activation of Arenes with Cp*Ir(III) Acetate Complexes: An Experimental and Computational Study Daniel A. Frasco,† Sriparna Mukherjee,† Roger D. Sommer,† Cody M. Perry,† Nikola S. Lambic,† Khalil A. Abboud,‡ Elena Jakubikova,*,† and Elon A. Ison*,† †

Department of Chemistry, North Carolina State University and Eastman Chemical Company Center of Excellence, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United States ‡ Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: Combined experimental and computational studies have revealed factors that influence the nondirected C−H activation in Cp*Ir complexes that contain carboxylate ligands. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C−H bond of the substrate. A nonlinear Hammett plot was obtained to examine substituted arenes where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. Electron-donating substituents in the para position were shown to have a bigger impact on the C−H bond cleavage step, whereas electron-withdrawing substituents influenced the substrate-binding step. Although cleavage of the C−H bond was predicted to be more facile with arenes that contain substituents in the para position by DFT calculations, the cyclometalations of anisole and benzonitrile were observed experimentally. This suggests that these substituents, even though they are weakly directing, still result in cyclometalation because the barriers for activation at the ortho and para positions of arenes are comparable (24.3 and 26.5 kcal/mol, respectively). Incorporation of a weakly bound ligand was found to be necessary for facile reactivity. It is predicted by DFT calculations that the replacement of an oxygen atom with a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C−H activation, as the incorporation of formimidate and N-methylformimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol).

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INTRODUCTION The stoichiometric activation of strong C−H bonds in hydrocarbons by iridium complexes is well established.1 In recent years, Cp*Ir(III) complexes have been shown to be effective as catalysts for the activation and functionalization of aromatic compounds that contain directing groups.1e,2 For example, we have shown recently that [Cp*IrCl2]2 and its derivatives are effective at the catalytic ortho-C-H activation and functionalization of benzoic acids with alkynes and benzoquinone derivatives.3 In contrast, the activation and functionalization of arenes that do not contain a directing group are less well established.1b,4 Our lab has shown that Cp*Ir complexes can catalyze the H/D exchange of benzene with a variety of deuterated solvents.5 However, despite this report, there are very few examples of catalytic functionalization of unsubstituted arenes6 with Cp*Ir(III) catalysts. In recent years, ligand-assisted pathways have been proposed for the C−H activation of alkanes and arenes.7 These reactions may proceed via a four-membered or six-membered transition state where a ligand (typically containing a heteroatom) acts as an internal base and assists in the cleavage of the C−H bond. These reactions have been termed concerted-metalationdeprotonation (CMD) by Fagnou and co-workers, ambiphilic © XXXX American Chemical Society

metal ligand activation (AMLA) by Davies, Macgregor, and coworkers,1c,d,f,h and internal electrophilic substitution by Goddard, Periana, and co-workers.8 Given our prior results with the functionalization of arene substrates, we have recently become interested in the development of Cp*Ir(III) catalysts for the homocoupling of unsubstituted arenes.3 However, despite the extensive studies on C−H activation described above, there is a paucity of studies with this class of catalysts for the nondirected activation of arenes. In this work, we examine C−H activation of arenes of the form p-Z-C6H5 (where Z = electron-donating or electronwithdrawing group) with complexes of the form Cp*Ir(L)(X)2 (L = L-type ligand, X = carboxylate) (Scheme 1). Our goal was to understand the mechanism for C−H activation with Cp*Ir complexes experimentally and computationally and determine how substituents on the arene substrate, the L-type ligand, and the carboxylate ligand on the metal affect the barrier for C−H bond cleavage. The understanding obtained from these studies could eventually lead to the Received: April 16, 2016

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DOI: 10.1021/acs.organomet.6b00308 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. C−H Activation of Arenes with Cp*Ir(III) Complexes

rational design of new catalysts and a clear understanding of the factors that influence C−H activation in this class of complex.

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Figure 1. X-ray crystal structure of 2. Ellipsoids are at the 50% probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1−Cl1, 2.4082 (4); Ir1−Cl2, 2.4207 (4); Ir1−N1, 2.167 (2); N1−C14, 1.485 (2); N1−C11, 1.483 (3); Cl1−Ir1−Cl2, 87.92 (2); Cl1−Ir1−N1, 79.37 (5); Cl2−Ir1−N1, 82.54 (5); C14−N1−Ir1, 119.2 (1); C11−N1−Ir1, 115.8 (1).

RESULTS AND DISCUSSION Synthesis of Complexes. For this study, the synthesis of several new complexes was necessary. Complex 1, Cp*Ir(DMSO)(OAc)2, was synthesized as previously reported.3a In general, it was found that the addition of a nucleophilic ligand, L, to (Cp*IrCl2)2 resulted in the formation of Cp*IrCl2(L). For example, morpholine reacts with (Cp*IrCl2)2 to form complex 2 (Scheme 2). X-ray quality crystals were obtained by

Scheme 3. Synthesis of 4 and 5

Scheme 2. Synthesis of 2 and 3

are depicted in Scheme 4. In addition to 6, a minor amount (∼3%) of the diphenyl complex, Cp*Ir(Me2SO)(Ph)2, 7, was Scheme 4. Synthesis of 6

slow diffusion of pentane into a concentrated solution of 2 in dichloromethane (Figure 1). The geometry about the metal center can be described as a piano stool octahedral complex. The crystal structure confirmed that morpholine coordinates via the nitrogen to iridium rather than oxygen. Complex 2 was treated with silver acetate in benzene at room temperature to generate 3 (Scheme 2). Pyridine, dimethyl sulfide, and carbon monoxide also react with (Cp*IrCl2)2 to form the corresponding chloride complexes. However, stable acetate complexes with these ligands could not be obtained. The pyridine acetate complex specifically was unstable because of the lability of pyridine. We hypothesized that a more donating pyridine ligand would result in a stable complex, so 4tert-butylpyridine was considered. Treatment of Cp*Ir(t-BuPy)(Cl)2, 4, with silver acetate resulted in the new iridium acetate complex Cp*Ir(t-Bu-Py)(OAc)2, 5, in 87% yield (Scheme 3). Mechanism for the Nondirected C−H Activation of Benzene. Isolation of Cp*Ir(L)(Ph)(OAc). Cp*Ir(Me2SO)(OAc)2, 1, was examined for its reactivity with benzene to form the iridium monophenyl complex Cp*Ir(Me2SO)(Ph)(OAc), 6. After optimization, the best conditions for C−H activation

observed at long reaction times (48 or 72 h). Compound 6 was isolated in ∼60% yield9 and was characterized by 1H, 13C NMR and IR spectroscopy. X-ray quality crystals were obtained by layering pentane onto a concentrated solution of 6 in toluene and allowing the solution to stand at −20 °C for 3 days (Figure 2). The geometry about the metal center can be described as a piano stool octahedral complex. The asymmetric unit consisted of two chemically equivalent, but crystallographically independent, complexes that differed in the orientation of the acetate ligand. Examination of the Kinetics and Mechanism for the Nondirected C−H Activation of Benzene with 3. Kinetic and Mechanistic Studies. With the activation of benzene by 1 to form 6 established, the mechanism for the reaction was examined both experimentally and computationally. A time profile plot was constructed for the reaction of 1 with benzene to form 6 over the course of 53 h (Figure 3). Because KHCO3 was minimally soluble in benzene at 60 °C, it was not compatible for kinetic studies. Therefore, Et3N was used as it was previously shown to be effective as a base in similar C−H activation reactions and is soluble in benzene. A pseudo-firstB


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Organometallics

was found to have the following parameters: ΔH⧧ = 10.3 (1.0) kcal/mol, ΔS⧧ = −0.049 (0.003) kcal/mol·K, and ΔG⧧ = 26.6 (2.0) kcal/mol. Computational Studies. To further establish a mechanism for C−H activation of 1 to form 6, computations (DFT) were undertaken to examine possible reaction pathways. The calculated potential energy surfaces were used to model σbond metathesis, oxidative addition, and acetate-assisted pathways. In all three cases, C−H activation was the ratelimiting step of the reaction. The energies of activation (ΔE⧧) for the σ-bond metathesis, oxidative addition, and acetateassisted pathways were determined to be 51.9, 47.9, and 34.4 kcal/mol, respectively (see Figure 4 and the Supporting Information, Figures S13 and S14). This suggests that the acetate-assisted pathway is kinetically the most accessible mechanism for C−H activation. Figure 2. X-ray crystal structure of 6. Ellipsoids are at the 50% probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1A−S1A, 2.2685(8); Ir1A−O2A, 2.108(2); Ir1A−C5A, 2.066(3); S1A−Ir1A−O2A, 77.92(6); S1A− Ir1A−C5A, 88.11(8); O2A−Ir1A−C5A, 91.4(1).

Figure 3. Time profile plot of concentration of 6 vs time. The formation of 6 was determined over 53 h with 25 mM 1, 125 mM Et3N, and 1 mL of benzene at 60 °C. Results were obtained by 1H NMR analysis of the crude reaction mixture using mesitylene as an internal standard.

Figure 4. Acetate-assisted pathway. Relative electronic energies of the intermediates and transition states along the acetate-assisted pathway from 1 to 6 are shown in black, and the relative free energies of the intermediates and transition states are shown in red.

order rate constant of 6.6 (0.7) × 10−5 s−1 was obtained from this plot. These data revealed maximum formation of 6 at ∼7 h. The dependence on the concentration of 1, DMSO, and base was examined for the formation of 6 in neat benzene. The log− log plot of iridium dependence (slope = 0.96(0.11)) suggested a first-order dependence in iridium. The initial rate vs concentration of DMSO plot was also obtained. These data gave a negative slope of −0.27(0.01), indicating an inverse dependence on the rate of the formation of 6. The log−log plot of initial rate vs Et3N gave a slope of −0.08(0.04), indicating a zero-order dependence of base in the reaction. The kinetic isotope effect for the reaction was also examined using deuterated benzene. A KIE of kH/kD = 10.0 (0.6) was found, indicating a primary isotope effect for the C−H activation of benzene.6c,10 This would indicate that the cleavage of a C−H bond occurred in the rate-limiting step of the reaction. Activation parameters for the C−H activation of benzene were also determined. An Eyring plot was constructed for the reaction at 50, 60, 65, and 70 °C. At 60 °C, the reaction

Having shown that the acetate-assisted pathway results in the lowest activation energy (34.3 kcal/mol), the Gibbs free energy for this pathway was also calculated (Figure 4, in red). The use of a solvent model and triple-ζ basis sets along with the thermal corrections in the ΔG calculations significantly stabilized the energies of all the intermediates with respect to the energy of complex 1. In this pathway, 1 first gave two uphill intermediates, resulting from the loss of DMSO, followed by the addition of benzene to the reaction sphere to form Int-2a. C−H activation occurs from Int-2a with a free energy of activation of 28.2 kcal/mol, at 60 °C, which was within error of the experimentally determined value of 26.6 (2.0) kcal/mol. This TS leads to Int-3, which contains acetic acid, bound to iridium. Loss of acetic acid and coordination of DMSO lead to 6. The potential energy surface based on ΔG values was thus in good agreement with experiment and a good starting point for further calculations. Examination of the Electronic Effect of the Arene Substituent. In order to understand the influence of C


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Organometallics substituents on the arene substrate, we examined computationally the C−H activation of substituted arenes, where the arene ring was substituted at the para position. A Hammett plot was constructed (Figure 5) by plotting the change in the

Figure 5. Change in enthalpy of activation (ΔΔH⧧) for the acetateassisted pathway vs the Hammett parameter of substituent Y in parafunctionalized benzene. A linear fit with the correlation coefficient, R2, is shown for EDGs. No correlation was observed for EWGs. The change in the enthalpy of activation (ΔΔH⧧) is the enthalpy of activation of functionalized benzene relative to the unsubstituted benzene.

Figure 6. Relative free energy for intermediates and transition states of the acetate-assisted C−H activation pathway with substituted arenes.

enthalpy of activation ΔΔH⧧ against the Hammett parameter σ+ for 25 different arenes, where ΔΔH ⧧ (333 K) = ΔH ⧧(X) − ΔH ⧧(H)

and X is a substituent in the para position of a phenyl ring. The observed energy barrier for C−H activation was higher for benzene than with any functionalized arene. Surprisingly, the Hammett plot is nonlinear with two distinct regions: (a) a linear region (ρ = 1.67) from ca. σ+ = −2 to 0, suggesting that the rate of C−H activation increases as the substituent X becomes more electron-donating, and (b) a region above σ+ = 0, where there is no correlation as the substituent X is varied. Nonlinear Hammett plots are usually indicative of a change in mechanism as a given substituent is varied. To examine this further, the free energy surfaces for acetate-assisted C−H activation of para-substituted benzene with substituents NH2 and NO2 as well as unsubstituted benzene are depicted in Figure 6. These substituents were chosen because they belong to the two extremes of the range of Hammett parameters. As seen in Figure 6, the free energy of activation for TS 2a-3 decreases in both cases. However, this change is most pronounced for the electron-donating substituent, NH2 (ΔΔG⧧ = 2.6 kcal/mol compared to 0.9 kcal/mol for NO2). Differences in selected bond lengths of TS 2a-3 with EDGs and EWGs at the para position of benzene are depicted in Figure 7. The C−H bond of benzene in Figure 7a increases systematically as the σ+ parameter decreases, i.e., as the arene becomes more electron-donating. The increased bond length is consistent with an increased rate of C−H bond cleavage in the TS with electron-donating substituents and accounts for the trend observed in Figure 5. Similarly, the Ir−C bond lengths in TS 2a-3 are also depicted in Figure 7b. Shorter Ir−C bond lengths are observed with electron-donating substituents. This is consistent with the

Figure 7. Plot of selected bond lengths for TS 2a-3 vs σ+: (a) C−H bond lengths of benzene vs σ+, (b) Ir−C bond lengths vs σ+.

increased ability of EDGs to facilitate Ir−C bond formation. Thus, the TS for C−H activation involves simultaneous C−H bond cleavage and Ir−C bond formation and is facilitated by electron-donating groups consistent with a CMD mechanism. In contrast, electron-withdrawing groups influence the potential energy surface differently. As shown in Figure 6, the electron-withdrawing NO2 substituent has the most significant influence on the stability of Int-2a (ΔΔG = −1.2 kcal/mol, compared to 0.1 kcal/mol for the NH2 substituent). See the Supporting Information (Figure S16) showing the influence of substituent on Int-2a for a larger set of molecules. A comparison of the bond lengths in the optimized structures for Int-2a reveals details of the nature of this stabilization. D


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C−H bond functionalization is controlled by the substrate distortion energy, Class II, where C−H bond cleavage is controlled by interaction of the metal, and Class III, where C− H bond functionalization is controlled by both substrate distortion and interaction with the metal. Arenes are classified as Class III substrates in this system; thus, the results reported here and the general mechanism proposed in Scheme 5 are consistent with these classifications. We have shown here that subtle changes in the electronics of the Class III substrates (arenes) can have a profound effect on the mechanism and rate and C−H activation. Attempts at C−H Activation with Substituted Arenes. The computational studies described above suggest that the rate of C−H activation should be faster with both electron-withdrawing and electron-donating groups. In order to test this, we attempted the C−H activation of anisole and benzonitrile. However, as shown in Schemes 6 and 7, C−H activation with these substrates led to the cyclometalated complexes 8 and 9.

In Int-2a, the interaction between the arene and the lone pair on the κ1 acetate ligand increases systematically as the para substituent becomes electron-withdrawing. For example, as shown in Figure 8, the bond length of the O (acetate)−H

Figure 8. O−H bond lengths for Int-2a of selected para-functionalized benzenes with substituents NH2, H, and NO2.

(arene) bond decreases from 2.39 to 2.34 Å as the substituent changes from NH2 to NO2. The variation of O−H bond for Int-2a vs σ+ parameter is in the Supporting Information (Figure S17). This trend most likely reflects an increased electrostatic interaction between the acetate ligand and the C−H bond of the arene in the presence of an electron-withdrawing substituent. This interaction is weak and, as a result, is not expected to correlate strongly with the σ+ parameter, as has been observed. Thus, the overall mechanism for C−H activation by Cp*Ir(L)(OAc)2 complexes can be described by the two steps depicted in Scheme 5. In Step 1, an arene adduct is

Scheme 6. Attempted C−H Activation with Anisole

Scheme 7. Attempted C−H Activation with Benzonitrile

Scheme 5. Mechanism for C−H Activation Cp*Ir(L)(OAc)2

The thermal ellipsoid plots for 8 and 9 are depicted as piano stool complexes in Figures 9 and 10. Bond lengths and angles are typical for other Cp*Ir complexes. The isolation of complexes 8 and 9 suggests that, even for substrates with weak directing groups, ortho C−H activation is competitive

formed in a reversible process. In Step 2, C−H activation results in the formation of a phenyl complex and the loss of an acetic acid molecule. The substituents on benzene influence Steps 1 and 2 differently. EWGs have a bigger influence on Step 1, where the withdrawing nature of the substituent facilitates adduct formation. EDGs have a larger influence on Step 2, where the donating substituent facilitates C−H bond breaking. The effect seen here is reminiscent of a report by Gorelsky, Lapointe, and Fagnou, where, on the basis of distortion interaction analysis of the C−H bond cleavage for a variety of heteroarenes,6b,11 it was found that substrates for C−H activation can be divided into three classes: Class I, where

Figure 9. Thermal ellipsoid plot (50% ellipsoids) for 8. Selected bond lengths (Å) and angles (deg): Ir1−C11, 2.060(4); Ir1−C17, 2.087(4); Ir1−S1, 2.2212(9); C17−Ir1−S1, 90.55(12); C11−Ir1−C17, 78.59(15); C11−Ir1−S1, 88.75(10). E


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Organometallics Table 1. Effect of the Ancillary Ligand on the C−H Activation of Benzene

Figure 10. Thermal ellipsoid plot (50% ellipsoids) for 9. Selected bond lengths (Å) and angles (deg): Ir1−C11, 2.0588(13); Ir1−N1, 2.0608(12); Ir1−S1, 2.2340(3); C11−Ir1−N1, 77.69(5); C11−Ir1− S1, 89.38(3); N1−Ir1−S1, 87.30(4).

entrya

complex

L

1 2 3 4

1 3 5 10

DMSO morpholine 4-t-Bu-pyridine NHC

SMO (%)c 27 68 81 71

(4) (3) (6) (4)

product (%) 62 (3) NDb ND ND

a Ir complex (0.025 mmol) and KHCO3 (12.5 mg, 0.125 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred for 24 h at 60 °C. Yields are the average of at least three trials and were obtained by 1H NMR spectroscopy with an internal standard. Error is reported as the standard deviation of the mean. bNot detected. cSMO = starting material observed.

with the intermolecular nondirected C−H activation of the arene substrate. This is further confirmed by DFT calculations. As shown in Scheme 8, the free energy of activation for interand intramolecular C−H activation is comparable at reaction Scheme 8. Comparison of C−H Activation at Ortho and Para Positions of Anisole

Figure 11. Change in free energy (ΔG kcal/mol) involved in the dissociation of the ancillary ligand. Free energies were obtained from single-point calculations using the B3PW91-D3 functional with SDD+f for Ir and 6-311++G(d,p) for C, H, O, S in benzene (PCM model), and thermal corrections from optimized geometries at 60 °C.

conditions (ΔG⧧ = 26.5 and 24.3 kcal/mol, respectively). Thus, the data suggest that the presence of substituents that can act as directing groups (even weakly directing groups) results in C−H activation at the ortho position of the arene as opposed to C− H activation at the para position.12 Examination of The Effect of the L-Type Ligand. The synthesis of new Cp*Ir acetate complexes enabled the investigation of the affect of the ancillary ligand on C−H activation (Table 1).5a Only DMSO was found to be an effective ligand for benzene activation. The lack of reactivity with moderate to strongly binding ancillary ligands compared to DMSO can be explained by the inability of these ligands to dissociate. This hypothesis is confirmed by computational studies. In Figure 11, the change in free energy (ΔG) at 60 °C for the dissociation of the L-type ligand is depicted. Consistent with

the experimental observations, the dissociation of the DMSO ligand is the least endergonic (ΔG = 4.7 kcal/mol). These observations are consistent with the stronger binding of these ancillary ligands to the metal center. Examination of the Effect of the Carboxylate Ligand. In addition to examining the effect of the L-type ligand, we also investigated the effect of the carboxylate ligand on C−H activation. Complex 1 and the previously reported Cp*Ir(Me2SO)(O2CCF3)2, 11, which incorporates the electronwithdrawing trifluoroacetate ligand, were utilized.3a,13 In addition, Cp*Ir(Me2SO)(O2CtBu)2, 12, which contains the electron-donating tert-butyl carboxylate ligand, was synthesized from silver pivalate and Cp*Ir(Me2SO)Cl2, 13 (Scheme 9). Complexes 1, 11, 12, and 13 were analyzed (Table 2) for their ability to activate benzene. No reactivity was observed without a carboxylate ligand, as shown in entry 1. The F


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Organometallics Scheme 9. Formation of 12 from 13 with Silver Pivalate

Table 2. Examination of the Effect of Carboxylate Ligand on C−H Activation

entrya

X

SMR (%)d

PR (%)e

1 2c 3 4

Cl O2CCF3 O2CCH3 O2CtBu

90(7) ND 27(4) 51(7)

NDb ND 62(3) 54(4)

Figure 12. X-ray crystal structure of 12. Ellipsoids are at the 50% probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Ir1−S1, 2.3041 (4); Ir1−O2, 2.0711 (8); Ir1−O4, 2.105 (1); O2−C13, 1.299 (1); O4−C18, 1.294 (1); O3−C13, 1.230 (2); O5−C18, 1.225 (2); S1−Ir1−O2, 91.38 (2); S1−Ir1−O4, 99.28 (3); O2−Ir1−O4, 79.82 (3); Ir1−O2−C13, 123.53 (7); Ir1−O4−C18, 129.21 (8); O2−C13−O3, 125.5 (1); O4−C18−O5, 125.7 (1).

Table 3. DFT B3PW91-D3 Calculations for the C−H Activation of Benzene at 60 °C with Carboxylate and Formimidate Ligand

a

Ir complex (0.025 mmol) and KHCO3 (12.5 mg, 0.125 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred for 24 h at 60 °C. Yields are the average of at least three trials and were obtained by 1H NMR spectroscopy with an internal standard. Error is reported as the standard deviation of the mean bNot detected. c41(7)% of a new Cp*Ir complex formed. No reaction was observed when no base was added. dSMR = starting material recovered. ePR = product recovered.

trifluoroacetate ligands in 11 were also not effective for the C− H activation of benzene (entry 2). The acetate and pivalate ligands in 1 and 12, respectively, gave similar results (entries 3 and 4), which suggest that an electron-donating carboxylate ligand facilitates the C−H activation of benzene. X-ray quality crystals were obtained by stirring 12 in pentane, filtering the solution, and allowing the solution to sit at 0 °C for 2 days (Figure 12). The geometry about the metal center can be described as a piano stool octahedral complex. To explore this further, calculations were performed with a series of carboxylate ligands (1, 11, and 12) and formimidate ligands (X = CF3, 14; CH3, 15; t-Bu, 16), N-methylformimidate, 17. As shown in Table 3, while the free energies of activation of benzene at 60 °C (ΔG⧧) for 1 and 12 are similar, the barrier for activation with 13 is significantly higher (entry 1). These results are consistent with experimental observations. Interestingly, as shown in entries 2 and 3, replacement of an oxygen atom in the carboxylate ligand with nitrogen results in a significant lowering of the barrier for C−H activation of benzene. This impact on the barrier results from the increased basicity of the formimidate and N-methylformimidate ligands, which facilitates proton transfer to nitrogen in the transition state for C−H bond cleavage. These results suggest that altering the nature of the carboxylate ligand can have the most significant impact on altering the barrier for C−H activation with these complexes.

a

Calculations were performed with the B3PW91-D3 functional with SDD+f for Ir and 6-311++G(d,p) for C, H, O, S in benzene (PCM model), and thermal corrections from optimized geometries at 60 °C.

studies have revealed factors that influence C−H activation in this class of complexes. A two-step acetate-assisted pathway was shown to be operational where the first step involves substrate binding and the second step involves cleavage of the C−H bond of the substrate. Because of the two-step nature of the mechanism, substituents on the substrates affect the reaction differently. Electron-donating substituents in the para position have a bigger impact on the C−H bond cleavage step, whereas electron-withdrawing substituents influence the substrate-binding step. This is evident by a nonlinear Hammett plot, where a strong electronic dependence (ρ = 1.67) was observed for electron-donating groups, whereas no electronic dependence was observed for electron-withdrawing groups. The C−H bond

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CONCLUSIONS In this article, the mechanism for nondirected C−H activation with Cp*Ir complexes that contain carboxylate ligands was investigated. Combined experimental and computational G


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Organometallics

min, solvent was removed and ether was added to the crude solid. Filtering gave the product (254.4 mg, 94%) as a yellow solid. 1H NMR (CDCl3): δ = 8.84 (d, 2H, J = 7 Hz, Py), 7.32 (d, 2H, J = 7 Hz, Py), 1.57 (s, 15H, Cp*), 1.32 (s, 9H, tBu). 13C NMR (CDCl3): δ = 162.6 (CH), 152.8 (Cq), 122.7 (CH), 85.6 (Cq), 35.0 (Cq), 30.4 (CH3), 8.7 (CH3). Anal. Calcd for C19H28IrNCl2·0.5(H2O): C, 42.06; H, 5.39. Found: C, 42.32; H, 5.34. Cp*Ir(t-Bu-Py)(OAc)2, 5. Cp*Ir(tBu-Py)(Cl)2 (543.5 mg, 1.02 mmol) and silver acetate (425 mg, 2.54 mmol) were placed in a foilcovered round-bottom flask with 30 mL of dry benzene. The solution was stirred for 2 h at room temperature. The mixture was then filtered through Celite, and solvent was removed to give the product (512.7 mg, 87%) as a yellow solid. 1H NMR (CDCl3): δ = 9.06 (d, 2H, J = 5 Hz, Py), 7.30 (d, 2H, J = 5 Hz, Py), 2.10 (s, 6H, OAc), 1.51 (s, 15H, Cp*), 1.28 (s, 9H, tBu). 13C NMR (CDCl3): δ = 177.8 (Cq), 162.4 (Cq), 153.8 (CH), 123.0 (CH), 84.1 (Cq), 35.1 (Cq), 30.5 (CH3), 25.3 (CH3), 10.0 (CH3). IR (KBr): νCO 1566.9 cm−1. Anal. Calcd for C23H34IrNO4: C, 47.57; H, 5.90; N, 2.41. Found: C, 47.47; H, 6.07; N, 2.43. Cp*Ir(Me2SO)(Ph)(OAc), 6. Cp*Ir(Me2SO)(OAc)2 (200 mg, 0.38 mmol) and KHCO3 (1.9 g, 19 mmol) were placed in a 25 mL Teflonsealed reaction vessel with 8 mL of dry benzene. The solution was stirred for 48 h at 60 °C. After cooling, the solution was filtered through Celite with dry benzene and was extracted once with 5 mL of H2O. The solution was then stirred in sodium sulfate, and solvent was removed to give the product (124 mg, 60%) as a yellow solid. 1H NMR (CDCl3): δ = 7.57 (d, 2H, J = 8 Hz, Ph), 7.07 (m, 2H, Ph), 6.96 (m, 1H, Ph), 3.62 (s, 3H, Me2SO), 2.51 (s, 3H, Me2SO), 2.05 (s, 3H, OAc), 1.60 (s, 15H, Cp*). 13C NMR (CDCl3): δ = 177.5 (Cq), 142.5 (CH), 141.5 (Cq), 127.0 (CH), 122.5 (CH), 92.5 (Cq), 48.0 (CH3), 42.0 (CH3), 24.0 (CH3), 8.0 (CH3). IR (KBr): νCO 1623.8 cm−1. Anal. Calcd for C20H29IrO3S: C, 44.34; H, 5.40. Found: C, 44.05; H, 5.49. Cp*Ir(Me2SO)(CH2OC6H4), 8. Cp*Ir(Me2SO)(OAc)2 (100 mg, 0.191 mmol) and KHCO3 (100 mg, 1.00 mmol) were placed in a 3 mL Teflon-sealed reaction vessel with 1.5 mL of anisole. The reaction was placed in an oil bath and stirred at 60 °C for 24 h. After 24 h, the reaction was cooled to room temperature, diluted with dichloromethane, filtered through a short plug of Celite, and evaporated to dryness. The resulting residue was redissolved in dichloromethane and precipitated by the addition of pentane. The solid precipitate was collected by vacuum filtration to give the product (87.9 mg, 88% yield) as a yellow powder. X-ray quality crystals were obtained by slow diffusion of pentane into a concentrated solution of 8 in dichloromethane. 1H NMR (CDCl3): δ = 7.10 (d, 1H, J = 8 Hz, Ph), 6.80 (m, 1H, Ph), 6.54 (m, 2H, Ph), 5.82 (d, 1H, J = 7 Hz, CH), 5.55 (d, 1H, J = 7 Hz, CH), 2.85 (s, 3H, CH3), 2.77 (s, 3H, CH3), 1.82 (s, 15 H, Cp*). 13C NMR (CDCl3): δ = 136.8, 124.9, 119.5, 107.3, 95.8, 80.3, 61.8, 46.2, 42.4, 9.6, 9.2. IR (KBr thin film, cm−1) νC−O 1114. Elemental analysis, calcd for C19H27IrO2S ·0.5(H2O): C, 43.83; H, 5.42. Found C, 43.64; H, 5.42. Cp*Ir(Me2SO)(NHC(O)C6H4), 9. Cp*Ir(Me2SO)(OAc)2 (100 mg, 0.191 mmol) and KHCO3 (100 mg, 1.00 mmol) were placed in a 3 mL Teflon-sealed reaction vessel with dry benzonitrile (1.5 mL) as solvent. The reaction was placed in an oil bath and stirred at 60 °C for 24 h. After 24 h, the reaction was cooled to room temperature, diluted with dichloromethane, filtered through a short plug of Celite, and evaporated to dryness. The product was dissolved in dichloromethane and precipitated with pentane to give the product (107 mg, >99% yield) as a beige-colored solid. X-ray quality crystals were obtained by slow diffusion of pentane into a concentrated solution of 9 in dichloromethane. 1H NMR (CDCl3): δ = 7.52 (m, 1H, Ph), 7.43 (m, 1H, Ph), 7.07 (m, 2H, Ph), 4.52 (bs, 1H, NH), 2.74 (s, 3H, CH3), 2.58 (s, 3H, CH3), 1.76 (s, 15H, Cp*). 13C NMR (CDCl3): δ = 135.6, 133.0, 132.30, 129.3, 128.2, 123.8, 125.5, 94.8, 44.2, 43.0, 9.2. IR (KBr thin film, cm−1) νCO 1594, νC−N 1106. Elemental analysis, calcd for C19H28IrNO3S·(H2O): C, 42.05; H, 5.20. Found C, 42.05; H, 5.19. Cp*Ir(Me2SO)(O2CtBu)2, 12. Cp*Ir(Me2SO)Cl2 (200.0 mg, 0.41 mmol) and silver pivalate (217.3 mg, 1.04 mmol) were placed in a foilcovered round-bottom flask with 20 mL of dry benzene. The solution was stirred for 2 h at room temperature. The mixture was then filtered

is predicted to be cleaved more readily with arenes that contain electron-donating and electron-withdrawing substituents in the para position by DFT. However, substituents that are even weakly directing will result in cyclometalation since the barriers for C−H activation are competitive. Since the mechanism for C−H activation involves the dissociation of an L-type ligand from Cp*Ir(L)X, it was found that the incorporation of a weakly bound ligand is necessary for facile reactivity. In addition, it was found that electron-donating substituents on the carboxylate ligand lead to facile cleavage of the C−H bond in benzene. Further, it is predicted by DFT calculations that the replacement of a nitrogen atom in the carboxylate ligand would lead to a dramatic reduction in the barrier for C−H activation. Thus, the incorporation of formimidate and N-methylformimidate ligands leads to barriers of 23.4 and 21.7 kcal/mol, respectively. These values are significantly lower than the barrier calculated for the analogous acetate ligand (28.2 kcal/mol). The understanding obtained from these studies could eventually lead to the rational design of new catalysts that incorporate a Cp*Ir framework. The development of catalysts that incorporate formimidate and Nmethylformimidate ligands is currently underway in our laboratories.

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EXPERIMENTAL SECTION

General Considerations. IrCl3·3H2O was purchased from Pressure Chemical Company. (Cp*IrCl2)2,14 Cp*Ir(Me2SO)Cl2,3a Cp*Ir(Me2SO)(OAc)2,3a and Cp*Ir(Me2SO)(O2CCF3)13 were prepared as previously reported. All other reagents were purchased from commercial sources and used as received. All reactions were performed under air and using nondry solvents unless otherwise noted. 1H, 13C, and 19F spectra were obtained at room temperature on a Varian Mercury 400 MHz, a Varian Mercury 300 MHz spectrometer, or a Bruker 500 MHz spectrometer. Chemical shifts are listed in parts per million (ppm) and referenced to their residual protons or carbons of the deuterated solvents. Infrared spectra were obtained in KBr thin films on a JASCO FT/IR-4100 instrument. X-ray crystallography was performed at the X-ray Structural Facility of North Carolina State University by Dr. Roger Sommer and Nikola Lambic, and at The Center for X-ray Crystallography by Dr. Khalil Abboud of The University of Florida. Cp*Ir(HN(CH2CH2)2O)(Cl)2, 2. (Cp*IrCl2)2 (300.0 mg, 0.38 mmol) was placed in a round-bottom flask with 30 mL of THF, and morpholine (433 μL, 5.32 mmol) was added while stirring. After 30 min, solvent was removed and the solid was then crashed out with dichloromethane and hexanes. Filtering gave the product (350.2 mg, 96%) as a yellow solid. 1H NMR (CDCl3): δ = 3.85 (d, 2H, CH2), 3.17(d, 2H, CH2), 3.55 (m, 2H, CH2), 3.00 (m, 2H, CH2), 1.67 (s, 15H, Cp*) The N-H proton was not observed for this molecule. 13C NMR (CDCl3): δ = 84.5 (Cq), 68.3 (CH2), 51.0 (CH2), 8.7(CH3). IR (KBr): νCOC 1074.2 cm−1. Anal. Calcd for C14H24Cl2IrNO: C, 34.64; H, 4.98; N, 2.89. Found: C, 34.73; H, 5.09; N, 2.89. Cp*Ir(HN(CH2CH2)2O)(OAc)2, 3. Cp*Ir(HN(CH2CH2)2O)(Cl)2 (337.0 mg, 0.69 mmol) and silver acetate (298 mg, 1.73 mmol) were placed in a foil-covered round-bottom flask with 30 mL of dry benzene. The solution was stirred for 2 h at room temperature. The mixture was then filtered through Celite, and solvent was removed to give the product (282.0 mg, 77%) as a yellow solid. 1H NMR (CDCl3): δ = 6.97 (m, 1H, NH), 3.79 (d, 2H, CH2, J = 11 Hz), 3.56 (m, 2H, CH2), 3.12 (m, 2H, CH2), 3.01 (d, 2H, CH2, J = 11 Hz), 2.07 (s, 6H, OAc), 1.53 (s, 15H, Cp*). 13C NMR (CDCl3): δ = 178.0 (Cq), 82.0 (Cq), 68.0 (CH2), 50.5 (CH2), 24.5 (CH3), 8.5 (CH3). IR (KBr): νCO 1572.7 cm−1. Anal. Calcd for C18H30IrNO5: C, 40.59; H, 5.68; N, 2.63. Found: C, 40.33; H, 5.78; N, 2.64. Cp*Ir(t-Bu-Py)(Cl)2 4. (Cp*IrCl2)2 (200.0 mg, 0.25 mmol) was placed in a round-bottom flask with 25 mL of THF, and 4-tertbutylpyridine (514 μL, 3.51 mmol) was added while stirring. After 30 H


Article

Organometallics

is the percentage difference in length of the ith coordination bond of the optimized structure from the crystal structure.

through Celite, and solvent was removed to give the product (191.0 mg, 75%) as an orange solid. 1H NMR (CDCl3): δ = 2.65 (s, 6H, Me2SO), 1.66 (s, 15H, Cp*), 1.14 (s, 18H, O2CtBu). 13C NMR (CDCl3): δ = 179.5 (Cq), 82.2 (Cq), 40.4 (CH3), 39.8 (Cq), 27.5 (CH3), 9.2 (CH3). IR (KBr): νCO 1544.7 cm−1. Anal. Calcd for C22H39IrO5S·0.5(H2O): C, 42.84; H, 6.54. Found: C, 42.82; H, 6.35. General Procedure for the Activation of Benzene by 1. 1 (13 mg, 0.025 mmol) and base (0.125 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred for 24− 72 h at 60 °C. After cooling, solvent was removed and the crude reaction mixture was analyzed by 1H NMR spectroscopy with mesitylene as an internal standard. General Procedure for Monitoring the Activation of Benzene over Time. 1 (13 mg, 0.025 mmol) and Et3N (17.4 μL, 0.125 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred for 24 h at 60 °C. The reaction was set up multiple times and stopped at appropriate time points. After cooling, solvent was removed and the crude reaction mixture was analyzed by 1 H NMR spectroscopy with mesitylene as an internal standard. Initial Rates of Benzene Activation with DMSO. 1 (13 mg, 0.025 mmol) and Et3N (17.4 μL, 0.125 mmol) were added with DMSO (0.025−0.250 mmol) to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred at 60 °C in triplicate. The reaction was set up multiple times and stopped at appropriate time points. After cooling, solvent was removed and the crude reaction mixture was analyzed by 1H NMR spectroscopy with mesitylene as an internal standard. Initial Rates of Benzene Activation with Et3N. 1 (13 mg, 0.025 mmol) and varying amounts of Et3N (0.0625−0.1875 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred at 60 °C in triplicate. The reaction was set up multiple times and stopped at appropriate time points. After cooling, solvent was removed and the crude reaction mixture was analyzed by 1H NMR spectroscopy with mesitylene as an internal standard. Eyring Plot of Benzene Activation. 1 (13 mg, 0.025 mmol) and Et3N (17.4 μL, 0.125 mmol) were added to a 3 mL Teflon-sealed reaction vessel with 1 mL of benzene and stirred at 50, 60, 65, and 70 °C in triplicate. The reaction was set up multiple times and stopped at appropriate time points. After cooling, solvent was removed and the crude reaction mixture was analyzed by 1H NMR spectroscopy with mesitylene as an internal standard. Computational Methods. Structure Optimizations. Crystal structures of complexes 3, 14, and 15 were used as references to determine the most appropriate model chemistry for calculation of structures of intermediates and transition states along the reaction pathway. A variety of DFT functionals (BP86,15 BPW91, 16 B3LYP,15a,17 B3PW91,18 M06L,19 and M0620), basis sets for C, H, O, and S (6-31G*,21 6-311G*,22 6-31+G*21a,22,23), and basis sets for Ir (SDD,24 SDD+f25) were explored. The use of Grimme’s dispersion correction with and without the Becke and Johnson damping (GD326 and GD3BJ27) was also tested. The BPW91+D3/6-31G* (C, H, O, and S), SDD+f (Ir) model chemistry was determined to be the most appropriate for structure optimizations using an ultrafine grid (i.e., a pruned grid with 99 radial shells and 590 angular points per shell) in vacuum. Transition states were optimized by means of Berny optimization28 to a first-order saddle point. Gaussian 09, Revision D.01,29 was employed in all electronic structure calculations. The ability of a model to reproduce the crystal structure was determined by two methods:

(2) Error in the overall structure (RMSD) given by number of atoms

RMSD (Å) (overall structure) =

(di)2

number of atoms

where di is the displacement of the ith atom relative to the same atom in the crystal structure. Hydrogen atoms were excluded from this analysis. The immediate coordination environment around the central metal is more important for the reactivity of the complex than the periphery. Therefore, lower error in RMSE (vs RMSD) was given more importance for determining the most appropriate functional for estimating structure. Free Energy Calculations. The model chemistry for free energy calculations was decided by comparison of the calculated free energy of activation, ΔG⧧, for the lowest energy pathway to the experimental ΔG⧧ (26.7 ± 2 kcal/mol). The B3PW91 functional was used for all the free energy calculations. Use of the polarizable continuum model30 (PCM for benzene) and influence of the basis set choice for C, H, O, S on the calculated ΔG⧧ were explored. The 6-31G(d,p),21 6311G(d,p),22 6-311++G(d,p),23b,31 6-311+G(2df,p),22,23,32 6-311+ +G(2d,2p),22,23b,33 and 6-311++G(3df,2pd)22,23,32 basis sets were employed in the calculations. Finally, the free energies (ΔG) were obtained by single-point calculations on the previously optimized structures using the B3PW91D3 functional, SDD+f basis set for Ir, and 6-311++G(d,p) basis set for all other atoms. The PCM (benzene) was used to account for solvent effects. Thermal data were obtained at 333.15 K and 1 atm using unscaled vibrational frequencies from the fully optimized structures.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00308.

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Additional kinetic data; X-ray experimental data for 2, 6, 8, 9, and 12; full Gaussian reference (PDF) XYZ coordinates for computational data and chemical structure (XYZ) Crystallographic data for 2, 6, 8, 9, and 12 (CIF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.A.I.). *E-mail: [email protected] (E.J.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

(1) Error in the coordination environment (RMSE) formulated as

RMSE (%) (coordination environment) =

∑i

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∑i (ΔR i)2

ACKNOWLEDGMENTS Support for this work was provided by Eastman Chemical Company, under the Eastman Chemical Company − North Carolina State University Center of Excellence Agreement. K.A.A. wishes to acknowledge the National Science Foundation and the University of Florida for funding of the purchase of the X-ray equipment.

8

where

⎛ R optimized − R crystal ⎞ i ⎟⎟ × 100 ΔR i (%) = ⎜⎜ i R icrystal ⎠ ⎝ I


Article

Organometallics

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