Theoretical Study on Iridacycle and Rhodacycle Formation via C–H

Article pubs.acs.org/Organometallics

Theoretical Study on Iridacycle and Rhodacycle Formation via C−H Activation of Phenyl Imines Jing Li, Wei Hu, Yiming Peng, Yaomou Zhang, Junda Li, and Wenxu Zheng* College of Science, South China Agricultural University, Guangzhou 510642, People’s Republic of China S Supporting Information *

ABSTRACT: A computational study with the Becke3LYP DFT functional was carried out on the formation of iridacycles and rhodacycles through C−H activation of phenyl imines in methanol solvent. The whole catalytic pathway was proposed and verified, starting from the catalyst [Cp*MCl2]2 cleavage and ending with the cyclometalated complex. The five most important issues, namely, chloride dissociation and C−H activation precursor formation, aromatic C−H bond activation, the reaction rate difference between the Ir and Rh systems, the nature of regioselectivity, and the role of the protic solvent are discussed. The calculations indicate that the C−H bond activation by the transition metal iridium is kinetically and thermodynamically more favorable than that by rhodium, and the regioselectivity of the reaction has been determined both electronically and sterically.

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INTRODUCTION Cyclometalated complexes have been broadly investigated since the 1960s, owing to their numerous applications as chiral auxiliaries,1−4 as building blocks for molecular architectures of higher complexity,5−12 or as compounds with interesting mesogenic13 and luminescent properties,14−19 as well as their potential use in medicine and biology20−23 and in catalytic and synthetic processes.24−31 Generally, the synthesis of cyclometalated complexes is through C−H activation, and therefore this reaction was, and still is, considered as an important model for C−H activation of hydrocarbons by transition metals. The most widely used transition metal in cyclometalation is palladium, and usually it is straightforward to synthesize palladacycles, especially for five-membered metallacycles with nitrogen-containing ligands to direct the intramolecular C−H activation. However, high-yield processes that proceed under mild conditions have been elusive with palladium.32−35 To investigate the reactivities and mechanisms for metallacycle formation, Jones et al. synthesized a serial of iridacycles and rhodacycles and studied how stereoelectronic effects influenced the C−H activation step in metallacycle formation.36 Theoretically, Davies and co-workers investigated the C−H activation mechanism involved in both palladacycle and iridacycle formation.32,37 Their former research revealed that an agostic intermediate occurred in the two-step activation of Pd(OAc)2(DMBA-H) (DMBA-H is defined as dimethylbenzylamine), while their latter research mainly elaborated the C−H activation mechanism of arylamine catalyzed by iridium acetate. The results showed that an acetate-assisted H-transfer process involving a six-membered agostic transition state was the most accessible route, but no such agostic intermediate was found. On the basis of the experimental and theoretical research reported by Jones and Davies, we now wish to report our © 2014 American Chemical Society

calculations on the whole process of formation of iridacycles and rhodacycles. Our attention will be focused on the following important issues: chloride dissociation and C−H activation precursor formation, aromatic C−H bond activation, the reaction rate difference between the iridium and rhodium systems, the nature of regioselectivity, and the role of the protic solvent.

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COMPUTATIONAL DETAILS

The calculations were carried out using the Gaussian 09 programs,38 where density functional theory with Becke’s three-parameter nonlocal exchange functional along with the Lee−Yang−Parr nonlocal correlation functional (B3LYP)39,40 was employed. The 6-31g(d, p) basis set was applied for all the atoms except the metal and Cl atoms, for which the LANL2DZ basis set including a double-ζ valence basis set with the Hay and Wadt effective core potential (ECP) was used.41,42 In addition, an additional d polarization shell was added for Cl with an exponent of 0.640. The solvent effect of methanol (experimentally used) was considered by the polarized continuum model (PCM) model combined with the addition of the methanol molecule to the calculated system.43−45 Frequency analysis was used to confirm whether the structure is a minimum (with no imaginary frequency) or a transition state (only with one imaginary frequency) and to provide free energies at 298.15 K. Intrinsic reaction coordinate (IRC) analysis was applied to confirm that all stationary points are smoothly connected to each other.46,47 In addition, the atoms in molecules (AIM) theory of Bader, which is based on a topological analysis of the electron charge density, has been used to analyze the bonding characteristics of the products.48 The AIM theory has proved itself a valuable tool to conceptually define what an atom is and above all what a bond is in a quantum calculation of a molecular structure.49 The atoms in molecules methodology with the Received: August 19, 2013 Published: May 1, 2014 2150

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AIM 2000 program package50 was used here to describe the topological properties of the electronic charge density. Jones et al. had determined the crystal structure of the resultant iridacycles. As shown in the Supporting Information, our calculated structure showed a good agreement with the crystal structure (see Figure SI1 and Table SI1). In other words, the computational method employed in this paper is reliable. Furthermore, as the DFT functionals poorly describe dispersion effects, dispersion corrections for all free energies were estimated using the DFT-D3 program developed by Grimme and co-workers.51−54

to lie higher than nonsymmetrical products by 1.8 kcal/mol for the Ir complex and 5.2 kcal/mol for the Rh complex, which shows that the non-symmetrical path is more favorable than the symmetrical path for the cleavage of catalyst A. The nonsymmetrical cleavage of A decreases the energy of the system by 6.7 kcal/mol for the Ir complex and 4.0 kcal/mol for the Rh complex. Thus, all of the reaction pathways discussed below start from the non-symmetrical cleavage product B. This result is consistent with the species Jones observed in experiments36 upon dimer opening in the presence of acetate. Na+[Cp*MCl3]− would be expected to react further with NaOAc to generate NaCl plus additional B. Scheme 2 shows the proposed reaction pathways for the C− H activation precursor formation, and the energy profiles obtained at the B3LYP/6-31g(d,p) level of theory with the DFT-D3 corrections are shown in Figure 1. The optimized geometries of the intermediates and transition states involved in the mechanism are shown in the Supporting Information. 1.1. Pathway 1 (B → D → G). Jones et al. proposed a possible reaction mechanism (Scheme 2: pathway 1) on the basis of their experimental work.36 According to their proposed pathway, starting from B, complex D is formed through chloride dissociation. Jones et al. suggested that chloride dissociation was a reversible process. However, the calculation shows a large energy cost, with the energy difference between B and D being 20.9 kcal/mol for the Ir complex and 18.3 kcal/ mol for the Rh complex. This indicates that chloride dissociation in this step is very unfavorable, since another process (B → C) shows little energy cost. The reason for this is that with the loss of a chloride anion from the transition metal, the 18e neutral complex B changes to the 16e cation D, which is not an electronically stable structure for the Ir and Rh complexes. From the energy difference between B and D, it can be concluded that the formation of complex D from B needs to

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RESULTS AND DISCUSSION 1. C−H Activation Precursor Formation. At the beginning of the reaction, dimeric catalyst A cleaves to form a reactive monomeric species in the presence of NaOAc. The cleavage may proceed via two ways, namely symmetrical or non-symmetrical. Scheme 1 shows the proposed pathway for Scheme 1. Two Proposed Pathways for the Catalyst A Cleavage

the catalyst A cleavage. In the calculations, the halogen anion was combined with one MeOH solvent molecule through a hydrogen bond and Na+ was not considered. The symmetrical cleavage produces two identical [Cp*M(OAc)Cl2]− monomers, while the non-symmetrical cleavage produces [Cp*MCl3]− and B. The symmetrical products were calculated

Scheme 2. Proposed Reaction Pathways for the C−H Activation Precursor Formation with the Relative Gibbs Free Energy ΔGa

a

All values are given in kcal/mol. ΔG values for the Rh complex are given in parentheses. 2151

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Figure 1. Energy profiles for the formation of C−H activation precursor in methanol solvent. The calculated Gibbs free energies are given in kcal/ mol. Two major transition states are considered in each pathway. One is the chloride dissociation process, and the other is the coordination of phenyl imine to the metal center.

Figure 2. Optimized geometries for intermediates and the transition state involved in chloride dissociation with the aid of a methanol molecule.

substrate coordinates to the transition metal. From C to E, the energy of the system is decreased by 0.1 kcal/mol for the Ir complex while it is increased by 1.0 kcal/mol for the Rh complex. Finally, the chloride ligand dissociates from the metal center to form the C−H activation precursor F. The energy barrier for chloride dissociation is calculated to be 11.0 kcal/ mol for the Ir system and 9.7 kcal/mol for the Rh system. The overall energy barrier for the C−H activation precursor formation (B → TSE‑F) is 12.3 kcal/mol for the Ir complex and 14.9 kcal/mol for the Rh complex.

overcome a relatively high energy barrier of over 20 kcal/mol. Since the other pathways proposed for chloride dissociation have a dramatically lower energy barrier, the subsequent steps of pathway 1 will not be discussed here. 1.2. Pathway 2 (B → C → E → F → G). In this pathway, starting from complex B, intermediate C is formed via a metal− oxygen bond-breaking process. The energy of C is only increased by 1.5 kcal/mol for the Ir complex and 4.2 kcal/mol for the Rh complex in comparison with that of B. Then through the transition state TSC‑E with energy barriers of 3.9 and 2.7 kcal/mol for the Ir and Rh systems, respectively, phenyl imine 2152

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Scheme 3. Proposed Reaction Pathway for C−H Activation and Formation of Metallacycle Complex

1.3. Pathway 3 (B → C → X → X1 → X2 → F → G). Nguyen et al. proposed a protic solvent-assisted reaction, which involved both a direct protonation and a solvent-assisted route.55 Similarly, we proposed two MeOH-assisted routes (pathways 3 and 4). Therefore, we introduced a methanol molecule to attack C to form complex X. The energy difference between C and X was calculated to be 1.2 kcal/mol for the Ir complex and 2.8 kcal/mol for the Rh complex. In complex X, the distance between metal center and oxygen atom of methanol is 2.29 Å for the Ir complex and 2.30 Å for the Rh complex. Next, chloride dissociation caused by the formation of a hydrogen bond between chloride and the hydroxyl H of methanol occurs. The energy barrier for chloride dissociation is calculated to be 6.8 kcal/mol for the Ir system and 8.8 kcal/mol for the Rh system. To clarify the role of MeOH in chloride dissociation, the optimized structures of X, X1, and TSX‑X1 with some key geometric parameters are given in Figure 2. As it is similar to the Ir complex, the Rh complex is not shown in this figure. In the resultant intermediate X1, the Cl···H hydrogen bond length is found to be 1.88 Å for the Ir complex and 1.93 Å for the Rh complex, and the distance between the metal center and chloride is 4.27 Å for the Ir complex and 4.31 Å for the Rh complex. It is clear that the chloride is successfully removed from the metal center with a low energy barrier. The intermediate X1 offers an open site for the coordination of phenyl imine substrate or acetate ligand (pathway 3). The coordination of the substrate to the metal center only needs to overcome very low energy barriers of 3.2 and 2.1 kcal/mol for the Ir and Rh systems, respectively. From X1 to X2, the energy of the system is decreased by 1.7 kcal/mol for the Ir complex and 2.2 kcal/mol for the Rh complex. Finally, the C−H activation precursor F is formed after the dissociation of the methanol solvent ligand from complex X2. The overall energy barrier for the C−H activation precursor formation (B → TSX‑X1) is 9.5 kcal/mol for the Ir complex and 10.2 kcal/mol for the Rh complex. 1.4. Pathway 4 (B → C → X → X1 → Z → Za → Z1 → F → G). The chloride dissociation process in this pathway begins the same as that in pathway 3. After chloride dissociation, the open site in X1 is coordinated by a second acetate ligand to form intermediate Z. There is almost no energy change

involved in the formation of complex Z for both Ir and Rh systems. After the methanol solvent ligand departs from the metal center with the chloride, the phenyl imine substrate coordinates to the metal to form complex Z1 by overcoming energy barriers of 5.1 and 2.1 kcal/mol for the Ir and Rh systems, respectively. From Z to Z1, the energy of the system is decreased by 5.5 kcal/mol for the Ir complex and 5.4 kcal/mol for the Rh complex. Finally, the C−H activation precursor F is formed after the dissociation of the methanol-complexed acetate ligand from the complex Z1. The energy barrier for this step is calculated to be 10.4 kcal/mol for the Ir system and 12.2 kcal/mol for the Rh system. The overall energy barrier for the C−H activation precursor formation (B → TSZ1‑F) is 10.5 kcal/mol for the Ir complex and 15.3 kcal/mol for the Rh complex. From an energy point of view, pathway 3 is the most kinetically favorable for C−H activation precursor formation. It should be noted that the solvent molecule is very important in the reaction. 2. C−H Activation and Metallacycle Formation. Davies et al. proposed three pathways for the C−H activation in [Ir(DMBA-H)(OAc)Cp]+ and investigated them in detail.37 According to their study, a reaction mechanism for C−H activation in complex F and metallacycle formation was proposed (Scheme 3). Our calculations on the C−H activation step show a result similar with that of Davies et al. Here, we only discuss the six-membered-ring process, which is the most favorable. The other two pathways, four-membered-ring and oxidative addition processes, are illustrated in the Supporting Information (Figure SI2). The reaction free energies obtained at the B3LYP/631g(d,p) level of theory with the DFT-D3 corrections are shown in Figure 3. The optimized geometries of intermediates and transition states involved in the mechanism are shown in Figure 4. Before the C−H activation, F is transferred to the more stable intermediate G, in which two oxygen atoms of the acetate ligand coordinate to the metal center. Starting from G, the C−H activation needs to go through the six-membered-ring transition state TSG‑H with an energy barrier of 12.4 kcal/mol for the Ir system and 17.9 kcal/mol for the Rh system. As 2153

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bond is activated. For the Rh complex, similar results can be observed. Through the transition state, the phenyl H1 atom transfers to the acetate O1 atom to form complex H with a fivemembered ring and an acetic acid ligand. For the Ir complex, there is almost no energy change from G to H, whereas for the Rh complex, the energy is increased by 6.9 kcal/mol. After the displacement of CH3COOH by Cl− in H, the metallacycle complex I is formed. As shown in Figure 3, the formation of I causes the energy of the system to decrease dramatically.

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DISCUSSION

1. Reaction Pathway. As mentioned above, the whole reaction pathway for iridacycle and rhodacycle formations contains three steps: formation of a precursor for C−H activation, C−H activation, and metallacycle formation. According to the experimental conditions and calculated energy profiles, the most reasonable reaction pathway should be A → B → C → X → X1 → X2 → F → G → H → I. The energy profile for the pathway is shown in Figure 5. An overall barrier for C−H bond activation by the Ir complex is 19.7 kcal/mol from B to TSG‑H, and an overall barrier for C−H bond activation by the Rh complex is 26.2 kcal/mol from B′ to TSG‑H′.

Figure 3. Energy profiles for C−H activation and metallacycle formation: (a) for the Ir complex; (b) for the Rh complex. The calculated Gibbs free energies are given in kcal/mol.

shown in Figure 4 for the Ir complex, it is clear that the formation of the transition state TSG‑H lengthens the C1−H1 bond by 0.21 Å and shortens the O1−H1 and Ir−C1 bonds by 0.82 and 1.34 Å, respectively, which shows that the C1−H1

Figure 4. Optimized geometries for intermediates and transition states involved in the C−H activation of the six-membered-ring process. 2154

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cationic complex is formed. This is confirmed by observation of a slight energy increase upon formation of another neutral complex D2. However, D2 cannot be formed through M−Cl bond cleavage, since it should be generated via a process similar to that for formation of complex X1. From an energy point of view, loss of an acetate arm is more favorable than dissociation of an anion for complex B. Moreover, we examined the electronic and geometric structure of B. Since the acetate ligand is bidentate, the O− C−O bond angle is constrained to 120°, leading to a small O− M−O bond angle of 58.7° for the Ir complex and 59.3° for the Rh complex. The Cl−M−O bond angle is calculated to be 85.7° for the Ir complex and 88.3° for the Rh complex. Therefore, in the octahedral complex B, the orbital overlap between the Cl and the metal is more efficient than that between the oxygen and the metal. This can be verified by the fact that the M−O σ bond is found in the HOMO while the M−Cl σ bond is found in the HOMO-1 (see Figure 7), which implies that the M−Cl σ bond is energetically more stable than the M−O σ bond. Furthermore, the structure of B also shows that M−O−C−O forms a four-membered ring where the M− O bond can be broken easily owing to a high tension in strength of the ring. Thus, in the present reaction, the cleavage of the M−O bond occurs more easily than that of the M−Cl bond in complex B. 3. Chloride Dissociation. Jones et al. have proposed that chloride dissociation occurs in complex B and the resultant cation D is the species responsible for C−H activation. In our calculations, we studied three different pathways for chloride dissociation. The dissociation of chloride ligand from B needs to overcome a relatively high energy barrier of over 20 kcal/ mol. Chloride dissociation proceeding after the coordination of substrate to the metal center needs to overcome an energy barrier of 12.3 kcal/mol for the Ir system and 14.9 kcal/mol for the Rh system. With the aid of a methanol solvent molecule, chloride dissociation only needs to overcome an energy barrier of 9.5 kcal/mol for the Ir system and 10.2 kcal/mol for the Rh system. Therefore, chloride dissociation is achieved more easily with the aid of a methanol molecule. Consistent with the discussion in Bond Cleavage of M−Cl and M−O Bonds in Complex B, we find that the neutral complex X1 has the lowest energy among the chloride dissociation products. It should be noted that, although the other two chloride dissociation products are both cations, F is more stable than D. This is caused by the favorable coordination of the phenyl imine to the metal ion. 4. Reaction Rate Difference between Ir and Rh Systems. In the experiment, under the same reaction conditions, it was observed that reactions of [Cp*IrCl2]2 were 2−4× faster than the reactions of [Cp*RhCl2]2 with the same substrate. The calculation results reveal that the ratedetermining step of the reaction is C−H activation. The overall energy barrier for this step was calculated to be 19.7 kcal/mol for the Ir system but 26.2 kcal/mol for the Rh system. As shown in Figure 4, the C1−H1 bond stretches to 1.29 Å in TSG‑H vs 1.33 Å in TSG‑H′; simultaneously, the O1−H1 distance shortens to 1.39 Å in TSG‑H vs 1.31 Å in TSG‑H′, indicating that TSG‑H is an earlier transition state than TSG‑H′. Kinetically, the C−H activation in the Ir and Rh systems is mainly affected by the following three aspects: C−H dissociation, M−O dissociation, and M−C formation. In the present study, the C−H dissociation energy is identical for the two systems, owing to the same substrate being used in the

Figure 5. Energy profiles for the most reasonable reaction pathway.

2. Bond Cleavage of M−Cl and M−O Bonds in Complex B. To understand the reason the bond cleavage in complex B occurs at the M−O bond but not at the M−Cl bond, we compared the energy differences between B and the cleavage intermediates, as shown in Figure 6. According to

Figure 6. Optimized geometries with the Gibbs free energy relative to B for the intermediates generated from cleavage of B. (D1) a methanol molecule coordinates to the open site of D; (D2) CH3OH···Cl− coordinates to the open site of D.

Jones’ suggestion, the bond cleavage of B occurs at the M−Cl bond and the 16e cation D is formed. However, the calculation shows that the cleavage of B to D and Cl···HOCH3 increases the energy of the system by 20.9 kcal/mol for the Ir complex and 18.3 kcal/mol for the Rh complex. However, the energy of the system is only increased by 1.5 kcal/mol for the Ir complex and 4.2 kcal/mol for the Rh complex if the M−O bond of B is broken to form C. On consideration of the methanol solvent in the reaction, it seems that formation of the 18e cation D1 is more reasonable. Unfortunately, the formation of D1 also increases the energy of the system significantly. These results show that the energy of the system increases slightly as a neutral complex is formed, while it increases significantly as a 2155

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Figure 7. Spatial plots and the orbital energies of HOMO and HOMO-1 for complexes B and B′.

Table 1. NBO Atomic Charges for Some Key Atoms in I and I′ charge M

Cl

C

N

−QMQCl

−QMQC

−QMQN

0.165 (M = Ir) 0.112 (M = Rh)

−0.440 −0.480

−0.071 −0.018

−0.402 −0.397

0.073 0.054

0.012 0.002

0.066 0.044

experiment; therefore, the reaction rate difference for the two systems is determined by the other two factors. The M−O dissociation energy is calculated to be 1.3 kcal/mol for the Ir system and 4.8 kcal/mol for the Rh system, which implies that the Ir−O bond dissociates more easily than the Rh−O bond. The formation of the M−C bond is affected by the electrophilic property of the transition metal. A higher electrophilicity leads to an easier M−C bond formation. Since the electrophilicity of the Ir3+ ion is higher than that of the Rh3+ ion, an Ir−C bond would be formed more easily than a Rh−C bond. Therefore, as discussed above, C−H activation by the Ir complex is kinetically favorable in comparison to that by the Rh complex. Thermodynamically, the free energy is decreased by 8.4 kcal/ mol for the Ir system but only by 0.5 kcal/mol for the Rh system from complex B to I. As we know, the stability of a complex is proportional to the bond strengths. The bond strength in polarized M−L bonds results from a gain in covalent and ionic bonding energy.56,57 Thus, the bonding energy (EB) can be described as

E B ∝ E I + EC

(1)

E I ∝ −Q MQ L

(2)

EC ∝ ρb

(3)

Table 2. Electron Density (ρb) of the BCP for Some Key Bonds in I and I′ ρb (M = Ir) for I ρb (M = Rh) for I′

M−Cl

M−C

M−N

0.0616 0.0564

0.1412 0.1293

0.0933 0.0813

more stable than I′. As a result, the iridacycle is formed more thermodynamically favorably than the rhodacycle, owing to the fact that the Ir3+ center is more electrophilic than the Rh3+ center. 5. Nature of Regioselectivity. The reactions of iridacycle and rhodacycle formation reported by Jones et al. showed that the regioselectivity was extremely sensitive to steric effects, giving rise to a preference for isomers obtained by C−H activation para to the substituent.36 To understand the nature of the regioselectivity, we further explored the C−H activation of a series of meta-substituted phenyl imines (−CF3, −CH3, −OMe, and −F) for the Ir complex (see Scheme 4). The experimental and theoretical results are given in Table 3. According to the energy profile of the reaction illustrated in Figure 5, the regioselectivity is determined at the C−H activation step. The relative energies of the transition states and subsequent intermediates are compared. Experimentally, when −CF3, −CH3, and −OMe substituents attach to the phenyl-3R-benzaldimine, the C−H bond para to the substituent is activated more favorably to form isomer series a. However, when the −F substituent attaches to the phenyl-3-Rbenzaldimine, the C−H bond ortho to the substituent is activated more favorably to form isomer series b. Theoretically, as shown in Figure 8, it is clear that TSGa‑Ha and Ha have lower energies in comparison to TSGb‑Hb and Hb for −CH3, −CF3 and −OMe substituted systems owing to the steric repulsion. TSGb‑Hb and Hb have lower energies in comparison to TSGa‑Ha and Ha for the −F substituted system owing to the ortho effect of the fluoride ion, which could be due to hyperconjugation of the M−C bond with the C−F antibonding orbital.58,59 According to Table 3, the experimental results show that only one isomer is found in −CH3 and −CF3 substituted systems and mixtures of isomers are found in −F and −OMe substituted systems. The calculation results show that the energy of Hb is over 6.0 kcal/mol higher than that of Ha in

where EB is the bonding energy, EI is the ionic bonding energy, EC is the covalent bonding energy, QM is the atomic charge of the metal, QL is the atomic charge of the ligands Cl, C, and N, and ρb is the electron density of the bond critical point. Ionic bonding is greater when elements of high and opposite charge interact. Large differences in electronegativity favor strong ionic bonding. The NBO atomic charges for some key atoms in I and I′ are given in Table 1. Considering ionic bonding character, it is clear that all of the M−L bonds in the Ir complex are stronger than those in the Rh complex. Covalent bonding is greater when electron clouds overlap strongly. The electron density (ρb) of the bond critical point (BCP) reflects the degree of electron cloud overlap. ρb values of some key bonds in I and I′ are given in Table 2. Considering covalent bonding character, it is clear that all of the corresponding ρb values for the Ir complex I are larger than those for the Rh complex I′, giving strong evidence that I is 2156

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Scheme 4. Proposed Reaction Pathway for the C−H Activation of a Series of Meta-Substituted Phenyl Imines (−CF3, −CH3, −OMe, and −F) for the Ir Complex

6. Role of Protic Solvent. The experiment revealed that a protic solvent could accelerate the reaction. We selected methanol as the solvent in our calculations. Initially, we only considered the solvent effect by using the polarized continuum model (PCM) without addition of the methanol molecule to the system. Figure SI3 in the Supporting Information illustrates the corresponding calculated energy profile. In order to investigate whether specific solvation would affect the energy difference between B and D, we then added a methanol molecule to all of the calculated systems on the basis of the PCM model (pathways 1 and 4). The corresponding energy profile is shown in Figure 1. Comparison of the Figure SI3 (Supporting Information) and Figure 1 shows that the addition of methanol decreases the energies of all intermediates and transition states, which allows the reaction to proceed moderately. Furthermore, the energy barrier for chloride dissociation is decreased dramatically when the methanol molecule coordinates to the metal center to assist the loss of chloride ion (pathways 2 and 3).

Table 3. Regioselectivities in the Reactions of Phenyl-3-Rbenzaldimines with [Cp*IrCl2]2

I II III IV

R

(a:b)exptla

ΔGcalcd(TSb−TSa) (kcal/mol)b

ΔGcalcd(Hb−Ha) (kcal/mol)b

−CF3 −CH3 −OMe −F

a only a only 1:1.7 2.3:1

2.1 0.5 0.2 −2.9

6.4 6.3 1.5 −3.0

a

The experimental results from ref 36 to produce a or b. bThe energy difference calculated by DFT.

−CH3 and −CF3 substituted systems. This large energy difference gives isomer Ha as the only product. For the −OMe substituted system, since the steric repulsion of OMe is less than that of −CH3 and −CF3, there is only a small energy difference of 0.2 kcal/mol for the transition states and 1.5 kcal/ mol for the subsequent intermediates, which leads to a mixture of regioisomers. It should be noted that for the −F substituted system, although the mixture of regioisomers is also formed in the experiment, its regioselectivity is higher than that of −OMe substituted system. This is consistent with the calculated results that the energy difference for intermediate H of the −F substituted system is 1.5 kcal/mol higher than that for H of the −OMe substituted system. As mentioned above, it can be concluded that steric hindrance plays a crucial role in the reaction, and the regioselectivity of this reaction is mainly determined thermodynamically.

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CONCLUSION The mechanism of the formation of iridacycles and rhodacycles through C−H activation of phenyl imines in methanol solvent was theoretically studied with the aid of DFT calculations at the B3LYP level. A reasonable pathway for the reaction was proposed and verified. The calculation shows that the reaction contains three steps: formation of a precursor for C−H activation, C−H activation, and formation of a metallacyclic complex. Owing to its more electrophilic character, iridium can

Figure 8. Energy profiles for the C−H activation: (I) −CF3; (II) −CH3; (III) −OMe; (IV) −F. The calculated Gibbs free energies difference are given in kcal/mol. 2157

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undergo C−H activation both kinetically and thermodynamically better than rhodium. The regioselectivities of the reactions are determined in the C−H activation step. The calculations on the C−H activation of various meta-substituted phenyl imines (−CF3, −CH3, −OMe, −F) indicate that steric hindrance plays a crucial role in the reaction, and the regioselectivity of this reaction is mainly determined thermodynamically. Our calculations also show that the protic solvent is very important in accelerating the reaction process through decreasing the energy barrier and facilitating chloride dissociation.

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ASSOCIATED CONTENT

S Supporting Information *

Figures and tables giving the calculated structure compared with the crystal structure, optimized geometries for intermediates and transition states involved in the four-memberedring process and oxidation addition process, calculated energy profiles for pathways 1 and 4 (solvent effect is only considered by the PCM model), and Cartesian coordinates for all intermediates and transition states. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*W.Z.: e-mail, [email protected]; tel, +86 2085280325; fax, +86 2085285026. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the key Academic Program of the third phase “211 Project” of the South China Agricultural University, the National Science Foundation of China (21173088), and the Natural Science Foundation of Guangdong Province (s2012010010740).

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