Mechanism of N–H Bond Cleavage of Aniline by a Dearomatized PNP

Article pubs.acs.org/Organometallics

Mechanism of N−H Bond Cleavage of Aniline by a Dearomatized PNP-Pincer Type Phosphaalkene Complex of Iridium(I) Yung-Hung Chang,† Yumiko Nakajima,†,§ Hiromasa Tanaka,‡ Kazunari Yoshizawa,*,‡ and Fumiyuki Ozawa*,† †

International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan ‡ Institute for Materials Chemistry and Engineering, International Research Center for Molecular Systems, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: Detailed mechanistic investigations using kinetic and theoretical methods have been conducted for deprotonative N−H bond cleavage of p-YC6H4NH2 (Y = H, MeO, Me, Cl, Br, NO2) by [K(18-crown-6)][Ir(Cl)(PPEP*)] (1a) bearing a dearomatized PNP-pincer type phosphaalkene ligand (PPEP*) to afford [Ir(NHC6H4Y)(PPEP)] (2) with an aromatized ligand (PPEP). While 1a is in equilibrium with [K(18-crown-6)]Cl (3) and [Ir(PPEP*)] (4) in solution, the N−H bond cleavage proceeds via association of 1a with aniline, where the coordination of aniline to iridium is insignificant; instead, aniline is associated with PPEP* by hydrogen bonding. In contrast, the N−H bond cleavage of ammonia proceeds via the pentacoordinate intermediate [Ir(Cl)(NH3)(PPEP*)]. The difference between the N−H bond cleavage processes of aniline and ammonia is examined by DFT calculations.

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Scheme 1. N−H Bond Cleavage of Ammonia and Amines by a Dearomatized PNP-Pincer Type Phosphaalkene Complex of Iridium

INTRODUCTION Phosphaalkenes with a PC double bond possess an extremely low lying π* orbital around the phosphorus atom and thus serve as strong π acceptors toward transition metals.1 Recently, we have reported that this particular ligand property remarkably enhances the reactivity of a pyridine-based PNP-pincer complex of iridium toward deprotonative N−H bond cleavage of amines via metal−ligand cooperation.2,3 As shown in Scheme 1, K[Ir(Cl)(PPEP*)] (1), having a dearomatized PNP-pincer type phosphaalkene ligand (PPEP*), readily reacts with ammonia and amines (RNH2; R = H, n-C6H13, Ph) at room temperature to afford amido complexes 2a−c in quantitative yields, along with regeneration of the aromatic pyridine ring.4−6 The observed reactivity is clearly higher than that reported for [Ru(H)(CO)(PNP*)] and [Rh(L)(PNP*)], having a dearomatized PNP-pincer type phosphine ligand (PNP*).3a,b DFT calculations for the reaction of ammonia have revealed that N−H bond cleavage proceeds via the pentacoordinate intermediate [Ir(Cl)(NH3)(PPEP*)]−, which undergoes a proton shift from nitrogen to the vinylic carbon of PPEP*, and elimination of Cl− yields [Ir(NH2)(PPEP)] (2a). This process involves a remarkable increase in the electron density of iridium, which significantly destabilizes intermediate species. However, due to the strong π-accepting ability of the phosphaalkene unit, the electron density is effectively reduced by π back-donation, and therefore the N−H bond cleavage proceeds under very mild conditions. © 2014 American Chemical Society

This paper deals with the mechanism of N−H bond cleavage of aniline. Particular interest has been focused on the coordination behavior of 1. As mentioned above, DFT calculations have indicated an associative mechanism involving a Received: October 29, 2013 Published: January 24, 2014 715

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pentacoordinate intermediate. A similar mechanism has been proposed by Milstein et al. for the reaction of [Rh(L)(PNP*)] with aniline.3b In both cases, the starting complex is a 16-electron species with a d8 metal center, which is often very prone to undergo ligand exchange with nucleophilic amines to form amine adducts instead of amido complexes. This tendency is considered to be the primary cause of the difficulty of N−H bond activation by late transition metals.7 Thus, we have conducted detailed mechanistic investigations using 1a that can be isolated (Scheme 1).8 Herein, we present the first kinetic observations on the associative mechanism. DFT calculations reveal that the association of aniline is induced by hydrogen bonding and the coordination to iridium is insignificant even in the transition state of N−H bond cleavage.

Table 1. Effects of [K(18-crown-6)]Cl (3) on the Reaction of 1a with Anilinea entry

[3]add (M)b

104kobsd (s−1)

rc

1 2 3 4

0.017 0.034 0.051 0.085

2.90(9) 3.63(6) 3.89(7) 4.23(14)

0.995 0.999 0.999 0.997

a

Reaction conditions: [1a]0 = 0.0169 M, [PhNH2]0 = 0.169 M, in THF, at 5.0 °C. bConcentration of [K(18-crown-6)]Cl (3) added to the system. cCorrelation coefficient for least-squares calculations in linear regression analysis.

the reaction rate increases with increasing amounts of 3 but tends to be saturated at high concentrations of 3. A plot of 1/kobsd against 1/[3]add exhibited a good linear correlation: 1/kobsd = 22.4(5) × (1/[3]add) + 2113(19) (Figure 2).

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RESULTS AND DISCUSSION Reactions of [K(18-crown-6)][Ir(Cl)(PPEP*)] (1a) with Para-Substituted Anilines. Reactions of 1a with aniline derivatives proceeded at room temperature to afford anilido complexes 2c−h in quantitative yields, along with the byproduct [K(18-crown-6)]Cl (3). Complexes 2c−h were isolated from the reaction systems using 1 instead of 1a8 and were characterized by NMR spectroscopy and elemental analysis. Kinetic Examinations. Complex 1a (0.0169 M) was treated with aniline (0.169 M) in THF at 5.0 °C, and the conversion of 1a was followed at intervals by 31P{1H} NMR spectroscopy using a toluene-d8 solution of PPh3 sealed in a capillary tube as an internal standard for peak integration. Two sets of doublets arising from 1a (δ 234.6 and 18.9, 2JPP = 463 Hz) gradually decreased, to be replaced by the signals of anilido complex 2c (δ 206.0 and 24.3, 2JPP = 458 Hz), where no other signals were detected. Figure 1 (plot a) shows a pseudo-first-order

Figure 2. Plot of 1/kobsd against 1/[3]add for the reaction of 1a (0.0169 M) with aniline (0.169 M) in THF at 5.0 °C.

On the other hand, the reaction rate was linearly correlated with the concentration of aniline. Figure 3 presents a plot of log kobsd against log [PhNH2] in the concentration range 0.169− 0.422 M (10−25 equiv/1a), showing the following relation: log

Figure 1. First-order plots for the conversion of 1a (0.0169 M) in the reaction with aniline (0.169 M) in THF at 5.0 °C in the absence (plot a) or presence (plots b and c) of added [K(18-crown-6)]Cl (3).

plot for the conversion of 1a. The plot deviates from a straight line, indicating a gradual increase in the reaction rate (see also Figure S1 in the Supporting Information). This is probably due to the byproduct [K(18-crown-6)]Cl (3). Indeed, addition of 3 to the system significantly accelerated the reaction (Figure 1, plots b and c). Table 1 gives the rate constants observed at four different concentrations of added 3. The kobsd values were estimated by linear regression analysis of the first-order plots. It is seen that

Figure 3. Plot of log kobsd against log [PhNH2] for the reaction of 1a (0.0169 M) with aniline in THF in the presence of added 3 (0.051 M) at 0.0 °C. 716

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1 kobsd

=

K 1 + k[PhNH 2][3] k[PhNH 2]

(2)

k = 2.80 × 10−3 s−1 M−1, K = 0.0106 M. The negative entropy (ΔS⧧ = −14(3) eu) is consistent with an ordered transition state (i.e., TSNH in Scheme 2). Effects of Para Substituents. Next, we compared the reactivity of para-substituted anilines toward 1a at −10.0 °C. Table 2 summarizes the results. Electron-withdrawing substituents Table 2. Pseudo-First-Order Rates for the Reactions of 1a with p-Substituted Anilines in THF at −10.0 °Ca

Figure 4. Eyring plot for the reaction of 1a (0.0169 M) with aniline (0.169 M) in THF.

kobsd = 1.04(5) × log [PhNH2] − 2.84(3). Thus, the reaction is first order in the concentration of aniline. The Eyring plot for the rate constants observed at four different temperatures exhibited a good linear correlation (Figure 4), providing the following kinetic parameters: ΔH⧧ = 17.1(8) kcal mol−1, ΔS⧧ = −14(3) eu, and ΔG⧧ = 20.9 kcal mol−1 at 273 K. Moreover, a small deuterium isotope effect was observed with PhND2: kH/kD = 1.56 at 5.0 °C. These kinetic observations are consistent with the reaction process given in Scheme 2. Complex 1a is in equilibrium with [Ir(PPEP*)] (4) and 3 in solution. On the other hand, association of 1a with aniline causes N−H bond cleavage across the iridium and vinylic carbon atoms, giving 2c together with 3. When the interconversion between 1a and 4 + 3 is a rapid process, and the N−H bond cleavage serves as the ratedetermining step, the reaction rate is expressed by eq 1, where k[PhNH 2][3] d[2c] = k[PhNH 2][1a] = [Ir]total dt K + [3]

entry

p-YC6H4NH2

σp−(Y)b

pKac

104kobs (s−1)

1 2 3 4 5 6

p-MeOC6H4NH2 p-MeC6H4NH2 PhNH2 p-ClC6H4NH2 p-BrC6H4NH2 p-O2NC6H4NH2

−0.26 −0.17 0.00 0.19 0.25 1.27

32.5 31.7 30.6 29.4 29.1 20.9

0.0754(6) 0.137(1) 0.426(11) 9.47(12) 14.3(1) rapid

a Reaction conditions: [1a]0 = 0.0169 M, [aniline]0 = 0.169 M, in THF, at −10.0 °C. bData taken from ref 9. cData taken from ref 10.

remarkably accelerate the formation of anilido complexes. p-Nitroaniline is particularly reactive, and its reaction was completed in a few minutes even at −50 °C (entry 6). Figure 5 shows a plot of log (kY/kH) against the pKa values of aniline derivatives. It is known that the pKa values are proportional to the σp− values of para substituents.9,10 Although the number of data are limited, the plot may be divided into two regions. When aniline derivatives are acidic, the reactivity appears to depend simply on the pKa values. On the other hand, the reactivity of basic anilines (i.e., p-MeC6H4NH2 and p-MeOC6H4NH2) deviates from this tendency: i.e., the reactivity is higher than that expected from the pKa values. DFT Calculations. The mechanism of N−H bond cleavage of aniline by 1a was investigated by DFT calculations using the B3LYP-D functional.11 Geometry optimization and vibrational analysis were performed with the SDD and 6-31G(d) basis sets for iridium and the other atoms, respectively.12 Energy changes were evaluated by single-point calculations at the optimized geometries using the 6-311+G(d,p) basis set instead of the 6-31G(d) basis set. Solvation effects (THF) were taken into account using the polarizable continuum model (PCM).13

(1)

[Ir]total = [1a] + [4] at time t, K = [3][4]/[1a], and k stands for the rate constant for the conversion of 1a to 2c. Thus, the 1/kobsd values are correlated with the 1/[3] values by eq 2. Applying the slope and intercept values of Figure 2 to eq 2 results in the following rate and equilibrium constants at 5.0 °C:

Scheme 2. Proposed Mechanism for the Reaction of 1a with Aniline

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state (TS1a/5). Thereafter, the PhNH group of aniline gradually approaches iridium to yield 5. In step ii, elongation of the Ir−Cl bond in 5 induces the migration of the PhNH group from the apical position to the equatorial position. Complex 2c thus produced adopts a squareplanar configuration around iridium, and [K(18-crown-6)]Cl (3) is significantly distant from 2c (4.571 Å). Accordingly, 3 is completely eliminated from iridium. The transition state of step ii (TS5/2c) is located at E = 15.3 kcal mol−1, the value of which is lower than that of step i (TS1a/5, E = 16.8 kcal mol−1). Hence, the N−H bond cleavage is assigned to the rate-determining step, in accordance with the kinetic observations. The theoretical value of activation energy (Ea = 16.8 kcal mol−1) is in good agreement with the experimental value (ΔH⧧ = 17.1(8) kcal mol−1). Comparison of N−H Bond Cleavage Processes of PhNH2 and NH3. As described above, the coordination of aniline is not crucial for N−H bond cleavage. In contrast, ammonia is initially coordinated to iridium and then undergoes N−H bond cleavage via metal−ligand cooperation.2 Next, we examined the difference between these reaction systems using the Mayer bond order. Table 3 presents changes in the selected bond orders that are directly concerned with the N−H bond cleavage process. For both systems, a continuous loss of the N−H bond accompanied by the formation of a C−H bond is clearly demonstrated. On the other hand, the Ir−N bond orders are significantly different in the two systems. For the reaction with ammonia, a relatively large value (0.43) of the pentacoordinate intermediate [Ir(Cl)(NH3)(PPEP*)]− (6) corroborates the presence of a distinct bonding interaction between iridium and ammonia. The bond order of the Ir−N bond increases in the N−H bond cleavage process: 0.43 (6) → 0.69 (TS6/7) → 1.13 (7). The considerable change between 6 and TS6/7 (0.26) also supports the importance of the precoordination of ammonia in the N−H bond cleavage process. In contrast, for the reaction with aniline, the Ir−N bond order increases only by 0.06 from the precursor complex to the

Figure 5. Plot of log (kY/kH) against pKa values of para-substituted anilines. The data are taken from entries 1−5 in Table 2.

Figure 6 shows an energy diagram obtained for the whole reaction, which consists of two reaction steps: (i) deprotonative N−H bond cleavage across the iridium and vinylic carbon atoms to form the pentacoordinate amido intermediate [K(18-crown-6)][IrCl(NHPh)(PPEP)] (5) and (ii) elimination of [K(18-crown-6)]Cl (3) from 5 to give 2c. The most interesting finding is the absence of distinct coordination of aniline to iridium prior to N−H bond cleavage. Instead, aniline is associated with 1a by hydrogen bonding between the N−H bond and the vinylic carbon of PPEP*. The interatomic distance between C and H, which is 2.458 Å in the precursor complex (1a-PhNH2), is reduced to 1.279 Å in the transition state (TS1a/5), showing the occurrence of N···H···C type hydrogen bonding. At the same time, the N−H bond length is extended from 1.020 to 1.483 Å. On the other hand, the nitrogen atom of aniline remains far from iridium (3.432 Å) even in the transition

Figure 6. Energy profile and optimized structures for the reaction of 1a with aniline to afford 2c via deprotonative N−H bond cleavage. The given energies are relative to the value of 1a−PhNH2 in kcal mol−1. 718

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for ammonia. The Lewis acidity of the iridium center of 1a is significantly enhanced by π back-donation to the phosphaalkene part of the PPEP* ligand,2 and therefore the coordination effectively facilitates the deprotonation of amines. As a result, deprotonative N−H bond cleavage takes place even with basic amines such n-hexylamine and ammonia (pKa = 41) under very mild conditions.

Table 3. Mayer Bond Orders of Selected Bonds in the N−H Bond Cleavage of PhNH2 and NH3a description

N−H

C−H

Ir−N

0.06 0.53 0.87

0.14 0.20 0.80

0.05 0.47 0.87

0.43 0.69 1.13

PhNH2 1a-PhNH2 TS1a/5 5

0.77 0.33 0.01

6 TS6/7 7

0.74 0.34 0.00

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NH3

EXPERIMENTAL SECTION

General Considerations. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques and a glovebox. Nitrogen gas was dried by passing through a P2O5 column (Merck, SICAPENT). Toluene (Kanto, dehydrated), hexane, and Et2O (Wako, dehydrated) were used as received. THF and CH3CN were dried over sodium/benzophenone and calcium hydride, respectively, distilled, and stored over activated MS4A. [K(18-crown-6)][IrCl(PPEP*)] (1a)2 and [K(18-crown-6)]Cl (3)15 were prepared according to the literature. Other chemicals were purchased from commercial sources and used without purification. NMR spectra were recorded on a Bruker AVANCE 400 spectrometer (1H NMR, 400.13 MHz; 13C NMR, 100.62 MHz; 31P NMR, 161.98 MHz). Chemical shifts are reported in δ (ppm), referenced to 1H (residual) and 13C signals of deuterated solvents as internal standards or to the 31P signal of 85% H3PO4 as an external standard. Elemental analysis was performed by ICR Analytical Laboratory, Kyoto University. Preparation and Identification of [Ir(NHC6H4-p-Y)(PPEP)] (2d−h). The title compounds were prepared from K[IrCl(PPEP*)] instead of 1a8 according to the synthesis of 2c.2 A typical procedure is described for the p-chloroanilido complex 2f. A dark green crystalline solid of K[IrCl(PPEP*)], derived from [IrCl(PPEP)] (34 mg, 0.038 mmol) and tBuOK in Et2O, was placed in a 25 mL Schlenk tube and dissolved in THF (3 mL). A solution of p-chloroaniline (0.038 mmol) in THF (1 mL) was added, and the mixture was stirred for 2 h at room temperature. The solution instantly changed from dark green to brown, and a white solid of KCl gradually precipitated. Volatiles were removed under vacuum, and the residue was dissolved in hexane (1 mL) and the solution filtered through a Celite pad to remove KCl. The filtrate was evaporated under vacuum to give a dark brown solid, which was recrystallized from CH3CN to afford analytically pure 2f (25 mg, 0.026 mmol, 68%). Complexes 2d,e,g,h were similarly prepared using hexane (2d,e) or MeCN (2g,h) as a recrystallization solvent. Identification data are as follows. [Ir(NHC6H4-p-OMe)(PPEP)] (2d). 1H NMR (THF-d8, 25 °C): δ 8.43 (dd, JPH = 9.1 Hz, JPH = 3.5 Hz, 1H, PyCHP), 7.73 (td, JHH = 7.3 Hz, JPH = 3.1 Hz, 1H, Py), 7.63 (s, 1H, Ar), 7.55 (s, 1H, Ar), 7.53 (dd, JPH = 4.6 Hz, JPH = 1.7 Hz, 1H, Ar), 7.31 (m, 2H, Ar + NH), 6.85 (d, JHH = 8.2 Hz, 2H, Py), 6.77 (d, JHH = 7.0 Hz, 1H, Py), 5.98 (d, JHH = 8.9 Hz, 2H, Ar), 5.89 (d, JHH = 8.9 Hz, 2H, Ar), 4.18 (dd, JHH = 18.4 Hz, JPH = 10.3 Hz, 1H, PyCH2P), 4.13 (dd, JHH = 18.4 Hz, JPH = 9.9 Hz, 1H, PyCH2P), 3.45 (s, ArOCH3), 2.34 (dd, JHH = 14.9 Hz, JPH = 3.3 Hz, 1H, PCH2), 2.22 (dt, JHH = 14.9 Hz, JPH = 3.8 Hz, 1H, PCH2), 1.80 (s, 9H, CH3), 1.66 (s, 9H, CH3), 1.45 (s, 12H, CH3), 1.41 (s, 9H, CH3), 1.40 (s, 9H, CH3), 1.23 (s, 3H, CH3). 13C NMR (THF-d8, 25 °C): δ 165.1 (d, JPC = 5 Hz), 163.2 (dd, JPC = 6 and 3 Hz), 159.1 (dd, JPC = 14 and 5 Hz), 156.0 (d, JPC = 2 Hz), 155.8 (s), 154.5 (d, JPC = 2 Hz), 153.9 (s), 153.7 (d, JPC = 10 Hz), 152.7 (d, JPC = 2 Hz), 152.2 (d, JPC = 3 Hz), 137.4 (d, JPC = 47 Hz, PyCHP), 132.5 (dd, JPC = 18 and 3 Hz), 130.7 (dd, JPC = 34 and 6 Hz), 125.9 (s), 123.8 (d, JPC = 8 Hz), 123.0 (d, JPC = 7 Hz), 122.8 (d, JPC = 7 Hz), 122.7 (d, JPC = 26 Hz), 122.1 (s), 120.3 (d, JPC = 8 Hz), 114.5 (dd, JPC = 12 and 6 Hz), 112.9 (s), 55.9 (s), 50.7 (d, JPC = 29 Hz, PyCH2P), 44.6 (t, JPC = 4 Hz), 40.3 (d, JPC = 34 Hz), 39.9 (s), 39.8 (s), 38.3 (s), 36.1 (s), 36.0 (s), 34.7 (s), 34.3 (s), 33.6 (s), 32.9 (s), 31.9 (s), 31.8 (s), 31.1 (d, JPC = 9 Hz). 31P{1H} NMR (THF-d8, 25 °C): δ 196.6 (d, JPP = 459 Hz), 22.9 (d, JPP = 459 Hz). Anal. Calcd for C50H71IrN2OP2·0.5C6H14: C, 62.82; H, 7.76; N, 2.76. Found: C, 62.70; H, 7.97; N, 2.75. [Ir(NHC6H4-p-Me)(PPEP)] (2e). 1H NMR (THF-d8, 25 °C): δ 8.46 (dd, JPH = 9.4 Hz, JPH = 3.5 Hz, 1H, PyCHP), 7.75 (td, JHH = 7.4 Hz, JPH = 3.0 Hz, 1H, Py), 7.63 (s, 1H, Ar), 7.55 (s, 1H, Ar),

a

Structures for aniline are given in Figure 6, whereas those for ammonia are shown below. The substituents on phosphorus atoms and the countercation are omitted for simplicity.

transition state: 0.14 (1a-PhNH2) → 0.20 (TS1a/5). This data indicates that an acid−base interaction between the N−H bond and the vinylic carbon is predominant even in the transition state. Hence, the N−H bond cleavage of aniline can be regarded as a simple deprotonation process, rather than the metal−ligand cooperative activation of an N−H bond. The iridium center serves as an acceptor of the negatively charged PhNH group. The relatively high acidity of aniline (pKa = 30.6) could be the prime reason for this reaction process. On the other hand, ammonia (pKa = 41),14 which is much less acidic than aniline, would be incapable of undergoing a simple deprotonation process. In this case, the coordination to iridium, whose Lewis acidity is enhanced by π back-donation to PPEP*, should facilitate the deprotonation of ammonia, thereby enabling the N−H bond cleavage via metal−ligand cooperation. A similar mechanism is probably operative for basic anilines such as p-MeC6H4NH2 and p-MeOC6H4NH2; their reactivities are higher than those expected from the pKa values (Figure 5).

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CONCLUSIONS We have reported detailed examinations of the mechanism of N−H bond cleavage of aniline by 1a bearing a dearomatized PNP-pincer type phosphaalkene ligand (PPEP*). The kinetic data clearly indicate an associative reaction process. However, the coordination of aniline to iridium is unremarkable even in the transition state of N−H bond cleavage (TS1a/5). Instead, aniline is associated with 1a by hydrogen bonding between the N−H bond and the vinylic carbon of PPEP*. Thus, the N−H bond cleavage process of aniline is best described as a simple deprotonation process followed by the coordination of an anionic PhNH group to iridium. We consider that the relatively high acidity of aniline (pKa = 30.6) is the primary cause of this type of reaction process. Actually, the reactivity of acidic anilines, including p-O2NC6H4NH2 (pKa = 20.9), p-BrC6H4NH2 (pKa = 29.1), p-ClC6H4NH2 (pKa = 29.4), and PhNH2 (pKa = 30.6), depends simply on the pKa values (Figure 5). It is noteworthy that the synthesis of transition-metal amido complexes from dearomatized pincer complexes has so far been successful only with such relatively acidic amines.3a−c On the other hand, the N−H bond cleavage of less acidic amines probably requires precoordination to iridium, as evidenced 719

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the crystallization solvent. Anal. Calcd for C49H68BrIrN2P2·0.5C2H3N: C, 57.76; H, 6.74; N, 3.37. Found: C, 57.78; H, 6.84; N, 3.30. [Ir(NHC6H4-p-NO2)(PPEP)] (2h). 1H NMR (THF-d8, 25 °C): δ 8.69 (dd, JPH = 12.4 Hz, JPH = 3.8 Hz, 1H, PyCHP), 7.86 (td, JHH = 7.9 Hz, JPH = 2.6 Hz, 1H, Py), 7.65 (s, 1H, Ar), 7.58 (s, 1H, Ar), 7.57 (s, 1H, Ar), 7.34 (s, 1H, Ar), 7.30 (br d, JPH = 6.0 Hz, 1H, NH), 7.19 (d, JHH = 8.7 Hz, 2H, Ar), 7.07 (d, JHH = 7.0 Hz, 1H, Py), 7.00 (d, JHH = 7.9 Hz, 1H, Py), 5.87 (d, JHH = 8.7 Hz, 2H, Ar), 4.23 (dd, JHH = 18.4 Hz, JPH = 10.5 Hz, 1H, PyCH2P), 4.15 (dd, JHH = 18.4 Hz, JPH = 9.7 Hz, 1H, PyCH2P), 2.42 (dd, JHH = 15.0 Hz, JPH = 3.5 Hz, 1H, PCH2), 2.27 (dt, JHH = 15.0 Hz, JPH = 3.3 Hz, 1H, PCH2), 1.76 (s, 9H, CH3), 1.69 (s, 9H, CH3), 1.46 (s, 12H, CH3), 1.42 (s, 9H, CH3), 1.39 (s, 9H, CH3), 1.27 (s, 3H, CH3). 13C NMR (THF-d8, 25 °C): δ 167.1 (d, JPC = 5 Hz), 166.7 (dd, JPC = 6 and 3 Hz), 165.5 (s), 159.3 (dd, JPC = 15 and 4 Hz), 156.8 (s), 156.5 (s), 155.5 (d, JPC = 2 Hz), 153.8 (d, JPC = 10 Hz), 153.4 (d, JPC = 2 Hz), 143.4 (d, JPC = 50 Hz, PyCHP), 135.0 (s), 131.5 (s), 130.3 (dd, JPC = 18 and 5 Hz), 130.1 (dd, JPC = 36 and 7 Hz), 125.3 (s), 124.4 (d, JPC = 9 Hz), 123.5 (d, JPC = 7 Hz), 123.1 (d, JPC = 7 Hz), 121.5 (d, JPC = 26 Hz), 120.6 (d, JPC = 9 Hz), 117.4 (dd, JPC = 12 and 8 Hz), 117.3 (s), 50.9 (d, JPC = 28 Hz, PyCH2P), 44.7 (t, JPC = 4 Hz), 42.0 (d, JPC = 33 Hz), 39.6 (s), 38.1 (s), 36.1 (s), 36.0 (s), 34.4 (s), 34.0 (s), 33.8 (d, JPC = 2 Hz), 33.1 (s), 31.7 (s), 31.6 (s), 31.1 (d, JPC = 9 Hz). 31P{1H} NMR (THF-d8, 25 °C): δ 233.6 (d, JPP = 448 Hz), 23.0 (d, JPP = 448 Hz). Anal. Calcd for C49H68IrN3O2P2: C, 59.73; H, 6.96; N, 4.26. Found: C, 59.58; H, 6.87; N, 4.39. Kinetic Experiments. A typical procedure is as follows. A solution of [K(18-crown-6)][IrCl(PPEP*)] (1a; 10.0 mg, 0.00843 mmol) and [K(18-crown-6)]Cl (3; 2.9 mg, 0.0084 mmol) in THF (0.3 mL) was introduced into a screw cap NMR tube, and a capillary filled with a toluene-d8 solution of PPh3 (0.17 M) was inserted. A THF solution of aniline (10 μL, 8.43 M, 0.00843 mmol) was charged at −78 °C, and the total volume was adjusted to 0.50 mL with additional THF. The sample tube was placed in an NMR sample probe controlled to 5.0 ± 0.1 °C, and the conversion of 1a was monitored at intervals by 31 1 P{ H} NMR spectroscopy using the following marker signals: 1a, δ 234.6 and 18.9; PPh3, δ −4.7. DFT Calculations. Intermediates and transition structures on potential energy surfaces were searched by using the Gaussian 09 program.16 The PPEP* ligand and [K(18-crown-6)] were treated without any simplification. As a result, the whole system is neutral and the electronic ground state is the closed-shell singlet throughout the reaction. We adopted the B3LYP-D functional, which is the B3LYP hybrid functional11a−d combined with an empirical dispersion correction developed by Grimme.11e For optimization the SDD and 6-31G(d) basis sets were chosen for Ir and the other atoms, respectively.12 Systematic vibrational analyses were carried out for all reaction species to characterize stationary-point structures. An appropriate connection between a reactant and a product for each reaction step was confirmed by intrinsic reaction coordinate (IRC) calculations.17 Zero-point energy corrections were applied for energy changes (E) and activation energies (Ea) calculated for each reaction step.

7.52 (dd, JPH = 4.6 Hz, JPH = 1.7 Hz, 1H, Ar), 7.36 (br d, JPH = 5.9 Hz, 1H, NH), 7.31 (d, JPH = 1.5 Hz, 1H, Ar), 6.86 (d, JHH = 8.1 Hz, 1H, Py), 6.78 (d, JHH = 7.0 Hz, 1H, Py), 6.17 (d, JHH = 8.2 Hz, 2H, Ar), 5.86 (d, JHH = 8.2 Hz, 2H, Ar), 4.18 (dd, JHH = 18.1 Hz, JPH = 10.2 Hz, 1H, PyCH2P), 4.12 (dd, JHH = 18.1 Hz, JPH = 10.2 Hz, 1H, PyCH2P), 2.34 (dd, JHH = 14.8 Hz, JPH = 3.4 Hz, 1H, PCH2), 2.28 (dt, JHH = 14.8 Hz, JPH = 3.5 Hz, 1H, PCH2), 2.16 (s, 3H, ArCH3), 1.80 (s, 9H, CH3), 1.66 (s, 9H, CH3), 1.46 (s, 3H, CH3), 1.45 (s, 9H, CH3), 1.42 (s, 9H, CH3), 1.41 (s, 9H, CH3), 1.23 (s, 3H, CH3). 13C NMR (THF-d8, 25 °C): δ 165.2 (d, JPC = 5 Hz), 163.6 (dd, JPC = 6 and 3 Hz), 159.0 (dd, JPC = 14 and 5 Hz), 156.1 (d, JPC = 2 Hz), 155.9 (s), 155.8 (d, JPC = 3 Hz), 154.5 (d, JPC = 2 Hz), 153.7 (d, JPC = 10 Hz), 152.7 (d, JPC = 2 Hz), 137.8 (d, JPC = 47 Hz, PyCHP), 132.3 (dd, JPC = 18 and 3 Hz), 130.7 (dd, JPC = 34 and 6 Hz), 127.8 (s), 126.3 (s), 126.0 (d, JPC = 1 Hz), 123.8 (d, JPC = 9 Hz), 123.0 (d, JPC = 7 Hz), 122.9 (d, JPC = 7 Hz), 122.7 (d, JPC = 26 Hz), 121.3 (s), 120.1 (d, JPC = 8 Hz), 114.7 (dd, JPC = 12 and 6 Hz), 50.9 (d, JPC = 29 Hz, PyCH2P), 44.6 (t, JPC = 4 Hz), 40.4 (d, JPC = 34 Hz), 39.9 (s), 39.8 (s), 38.3 (s), 36.1 (s), 36.0 (s), 34.7 (s), 34.3 (s), 33.6 (s), 32.9 (s), 31.9 (s), 31.8 (s), 30.9 (d, JPC = 9 Hz), 20.1 (s). 31P{1H} NMR (THF-d8, 25 °C): δ 199.9 (d, JPP = 458 Hz), 22.6 (d, JPP = 458 Hz). Anal. Calcd for C50H71IrN2P2: C, 62.93; H, 7.50; N, 2.94. Found: C, 62.82; H, 7.64; N, 2.90. [Ir(NHC6H4-p-Cl)(PPEP)] (2f). 1H NMR (THF-d8, 25 °C): δ 8.55 (dd, JPH = 10.2 Hz, JPH = 3.6 Hz, 1H, PyCHP), 7.81 (td, JHH = 7.4 Hz, JPH = 2.8 Hz, 1H, Py), 7.65 (s, 1H, Ar), 7.57 (s, 1H, Ar), 7.53 (dd, JPH = 4.7 Hz, JPH = 1.6 Hz, 1H, Ar), 7.34 (s, 1H, Ar), 7.09 (br d, JPH = 6.2 Hz, 1H, NH), 6.89 (d, JHH = 7.9 Hz, 1H, Py), 6.85 (d, JHH = 7.0 Hz, 1H, Py), 6.24 (d, JHH = 8.7 Hz, 2H, Ar), 5.90 (d, JHH = 8.7 Hz, 2H, Ar), 4.20 (dd, JHH = 16.6 Hz, JPH = 10.4 Hz, 1H, PyCH2P), 4.14 (dd, JHH = 16.6 Hz, JPH = 10.2 Hz, 1H, PyCH2P), 2.41 (dd, JHH = 14.9 Hz, JPH = 3.3 Hz, 1H, PCH2), 2.28 (dt, JHH = 14.9 Hz, JPH = 2.6 Hz, 1H, PCH2), 1.78 (s, 9H, CH3), 1.66 (s, 9H, CH3), 1.47 (s, 3H, CH3), 1.44 (s, 9H, CH3), 1.42 (s, 9H, CH3), 1.41 (s, 9H, CH3), 1.29 (s, 3H, CH3). 13C NMR (THF-d8, 25 °C): δ 165.5 (d, JPC = 5 Hz), 164.4 (dd, JPC = 6 and 3 Hz), 159.2 (dd, JPC = 14 and 5 Hz), 157.3 (s), 156.2 (s), 156.1 (s), 154.9 (d, JPC = 2 Hz), 153.8 (d, JPC = 10 Hz), 153.0 (d, JPC = 2 Hz), 139.1 (d, JPC = 48 Hz, PyCHP), 131.9 (dd, JPC = 18 and 3 Hz), 130.4 (dd, JPC = 35 and 7 Hz), 127.6 (s), 127.0 (s), 124.0 (d, JPC = 8 Hz), 123.2 (d, JPC = 7 Hz), 123.1 (d, JPC = 7 Hz), 122.5 (d, JPC = 26 Hz), 121.5 (s), 120.6 (s), 120.3 (d, JPC = 8 Hz), 115.5 (dd, JPC = 12 and 6 Hz), 50.8 (d, JPC = 29 Hz, PyCH2P), 44.6 (t, JPC = 4 Hz), 40.9 (d, JPC = 34 Hz), 39.9 (s), 39.8 (s), 38.3 (s), 36.2 (s), 36.0 (s), 34.5 (s), 34.3 (s), 33.6 (d, JPC = 1 Hz), 32.9 (s), 31.8 (s), 31.7 (s), 31.0 (d, JPC = 9 Hz). 31P{1H} NMR (THF-d8, 25 °C): δ 209.0 (d, JPP = 453 Hz), 22.5 (d, JPP = 453 Hz). Anal. Calcd for C49H68ClIrN2P2: C, 60.38; H, 7.03; N, 2.87. Found: C, 60.44; H, 7.06; N, 2.99. [Ir(NHC6H4-p-Br)(PPEP)] (2g). 1H NMR (THF-d8, 25 °C): δ 8.56 (dd, JPH = 10.2 Hz, JPH = 3.6 Hz, 1H, PyCHP), 7.81 (td, JHH = 7.6 Hz, JPH = 2.4 Hz, 1H, Py), 7.65 (s, 1H, Ar), 7.57 (s, 1H, Ar), 7.54 (dd, JPH = 4.6 Hz, JPH = 1.7 Hz, 1H, Ar), 7.34 (s, 1H, Ar), 7.09 (br d, JPH = 7.0 Hz, 1H, NH), 6.90 (d, JHH = 8.0 Hz, 1H, Py), 6.86 (d, JHH = 7.2 Hz, 1H, Py), 6.35 (d, JHH = 8.8 Hz, 2H, Ar), 5.87 (d, JHH = 8.8 Hz, 2H, Ar), 4.21 (dd, JHH = 17.2 Hz, JPH = 10.8 Hz, 1H, PyCH2P), 4.14 (dd, JHH = 17.2 Hz, JPH = 10.0 Hz, 1H, PyCH2P), 2.41 (dd, JHH = 14.8 Hz, JPH = 3.2 Hz, 1H, PCH2), 2.27 (dt, JHH = 14.8 Hz, JPH = 3.3 Hz, 1H, PCH2), 1.79 (s, 9H, CH3), 1.67 (s, 9H, CH3), 1.47 (s, 3H, CH3), 1.45 (s, 9H, CH3), 1.42 (s, 9H, CH3), 1.41 (s, 9H, CH3), 1.29 (s, 3H, CH3). 13C NMR (THF-d8, 25 °C): δ 165.6 (d, JPC = 5 Hz), 164.5 (dd, JPC = 6 and 3 Hz), 159.1 (dd, JPC = 14 and 5 Hz), 157.8 (s), 156.2 (s), 156.1 (s), 154.9 (d, JPC = 2 Hz), 153.8 (d, JPC = 10 Hz), 153.0 (s), 139.2 (d, JPC = 48 Hz, PyCHP), 131.8 (dd, JPC = 18 and 3 Hz), 130.5 (dd, JPC = 35 and 7 Hz), 129.9 (s), 127.7 (s), 124.0 (d, JPC = 9 Hz), 123.2 (d, JPC = 7 Hz), 123.0 (d, JPC = 7 Hz), 122.5 (d, JPC = 26 Hz), 122.1 (s), 120.3 (d, JPC = 8 Hz), 115.6 (dd, JPC = 12 and 6 Hz), 107.6 (s), 50.8 (d, JPC = 29 Hz, PyCH2P), 44.6 (t, JPC = 4 Hz), 40.9 (d, JPC = 34 Hz), 39.9 (s), 39.8 (s), 38.3 (s), 36.2 (s), 36.0 (s), 34.5 (s), 34.3 (s), 33.6 (s), 32.9 (s), 31.9 (s), 31.8 (s), 31.0 (d, JPC = 9 Hz). 31 1 P{ H} NMR (THF-d8, 25 °C): δ 210.0 (d, JPP = 454 Hz), 22.8 (d, JPP = 454 Hz). This complex contained 0.5 of a molecule of MeCN as

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates of the optimized intermediates and transition structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: F.O., [email protected]; K.Y., kazunari@ms. ifoc.kyushu-u.ac.jp. Present Address §

National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan.

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Notes

(8) Isolated complex 1a was employed in kinetic runs to clarify the reaction stoichiometry. On the other hand, the use of 1, which was prepared in situ from [IrCl(PPEP)] and tBuOK, was crucial for the isolation of 2c−g because of the difficulty of separation of [K(18-crown-6)]Cl (3) from anilido complexes.2 (9) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 97, 165. (10) Bordwell, F. G.; Zhang, X.-M.; Cheng, J.-P. J. Org. Chem. 1993, 58, 6410. (11) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (c) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (e) Grimme, S. J. Comput. Chem. 2006, 27, 1782. (12) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (c) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (d) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (e) Francl, M. M.; Petro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (f) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123. (13) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (14) Bordwell, F. G.; Drucker, G. E.; Fried, H. E. J. Org. Chem. 1981, 46, 632. (15) Song, Y.; Jing, H.; Li, B.; Bai, D. Chem. Eur. J. 2011, 17, 8731. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision C.01; Gaussian, Inc., Wallingford, CT, 2010. (17) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (c) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a MEXT Grant-in-Aid for Scientific Research on Innovative Areas “Stimuli-responsive Chemical Species” (Nos. 24109014 (K.Y.) and 24109010 (F.O.)), a JSPS Grant-in-Aid for JSPS Fellows (No. 24·3950 (Y.C.)), and the MEXT Project of “Integrated Research on Chemical Synthesis”.

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REFERENCES

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