Aromatic CâH Ï-Bond Activation by Ni0, Pd0, and Pt0 Alkene

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Aromatic C−H σ‑Bond Activation by Ni0, Pd0, and Pt0 Alkene Complexes: Concerted Oxidative Addition to Metal vs Ligand-toLigand H Transfer Mechanism Shuwei Tang,†,‡ Odile Eisenstein,§ Yoshiaki Nakao,∥ and Shigeyoshi Sakaki*,† †

Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan Institute of Functional Material Chemistry, Faculty of Chemistry, Northeast Normal University, Changchun 130024, People’s Republic of China § Institute Charles Gerhardt, UMR 5253 CNRS-UM-ENSCM, Université de Montpellier, cc1501, 34095 Montpellier, France ∥ Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: C−H σ-bond activation of arene (represented here by benzene) by the Ni0 propene complex Ni0(IMes)(C3H6) (IMes = 1,3-dimesitylimidazol-2-ylidene), which is an important elementary step in Ni-catalyzed hydroarylation of unactivated alkene with arene, was investigated by DFT calculations. In the Ni0 complex, the C−H activation occurs through a ligand-to-ligand H transfer mechanism to yield NiII(IMes)(C3H7)(Ph) (C3H7 = propyl; Ph = phenyl). In Pd0 and Pt0 analogues, the activation occurs through concerted oxidative addition of the C−H bond to the metal. Analysis of the electron redistribution during the C−H activation highlights the difference between the two mechanisms. In the ligand-toligand H transfer, charge transfer (CT) occurs from the metal to the benzene. However, the atomic population of the transferring H remains almost constant, suggesting that different CT simultaneously occurs from the transferring H to the LUMO of propene. The electron redistribution contrasts significantly with that found for Pd0 and Pt0, in which CT occurs only from the metal to the benzene. Preference for ligand-to-ligand H transfer over concerted oxidative addition in the Ni0 complex is shown to be due to the smaller atomic radius of Ni in comparison to those of Pd and Pt and the smaller NiII−H bond energy relative to the PdII−H and PtII−H energies. Interestingly, the bulky ligand accelerates the ligand-to-ligand H transfer in the Ni0 complex by decreasing the distance between the coordinated benzene and alkene substrates. Thus, the Gibbs activation energy (ΔG°⧧) decreases in the case of cyclic-alkylaminocarbene with bulky substituents (CACC-K3), while the ΔG°⧧ values are similar in XPhos, IMes, and nonsubstituted cyclic alkylaminocarbene (CAAC-K0). An electron-withdrawing substituent on the arene accelerates the C−H activation by favoring the metal to arene CT.

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Ni0 carbene complex with a Lewis acid catalyzed alkylation of N,N-dimethylbenzamide with unactivated alkene via C−H activation of the arene group gave good to excellent yields and interesting para selectivity.22,23 The way the Ni0 complex performs C−H activation of arenes has been previously analyzed by computational studies;15,19 DFT calculations revealed that the reaction occurs via H transfer from the arene to the Ni-coordinated alkene or alkyne followed by reductive elimination, as shown in Scheme 2. This C−H activation reaction, denoted ligand-to-ligand H transfer,15 is different from the usual concerted oxidative addition to lowvalent metal atom. Even though these studies have established the principal features of this reaction, further studies are needed to highlight additional aspects of this reaction.

INTRODUCTION Transition-metal-catalyzed functionalization of arene via C−H activation has attracted considerable interest because functional groups are introduced into the aromatic ring by generating new C−C and C−N bonds, as reviewed recently.1−8 Among these synthetic transformations, the hydroarylation of unactivated alkenes and alkynes through direct C−H activation of arenes by a transition metal is one of the most attractive reactions for preparing alkylarenes and alkenylarenes.9−21 For instance, Nakao and co-workers reported that monophosphine Ni0 complex was capable of performing hydroarylation of unactivated alkynes with high yield, as shown in Scheme 1A.10 Hartwig and co-workers successfully used an IPr Ni0 complex (IPr = (1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro2H-imidazol-2-ylidene)) for hydroarylation of an unactivated alkene under mild conditions to afford a mixture of linear and branched alkylarenes,19 as shown in Scheme 1B. Very recently, Nakao and co-workers reported that a combined system of a © XXXX American Chemical Society

Received: April 6, 2017

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DOI: 10.1021/acs.organomet.7b00256 Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Ni-Catalyzed Alkenylation and Alkylation of Arenes

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COMPUTATIONAL DETAILS AND MODELS

DFT calculations were carried out with the M06 functional50 and BS-I and BS-II choices for effective core potentials (ECPs) and basis sets. This level of calculation has given good results in terms of structures and energies in recent studies of Ni0-catalyzed reactions.51,52 For BS-I, the metal atoms were described by the LANL2DZ ECPs and the associated basis sets53 and for all other atoms by 6-31G(d) basis sets. This BS-I was used for geometry optimization. For BS-II, the metal atoms were described with the SDD ECPs and associated basis sets54,55 and all other atoms by 6-311G(d). A single p polarization function was added to the active H atom (Ha hereafter) of the C−H bond to be cleaved. This BS-II was employed for single-point calculations to evaluate better potential energy, using the optimized geometry determined with BS-I. Test calculations carried out with the use of the B3LYP-D3 functional and optimization with BS-II gave similar results, notably in energy patterns (see Table S1 in the Supporting Information). Solvation effects of toluene were added in single-point calculations using the conductor-like polarizable continuum model (CPCM)56−58 with the Pauling radii. We present the results from the calculations at the M06/BS-II//M06/BS-I level corrected by the solvent effect. Electron population analysis was made, according to Weinhold et al.59−63 using NBO version 3. Normal mode analysis was carried out at the M06/BS-I level for all stationary structures to assign their nature (minimum or transition state). The intrinsic reaction coordinate (IRC) calculations were performed at the same level to check if the transition state (TS) connects the reactant and product (or intermediate). Thermal correction and entropy contributions were evaluated at the M06/BSI level for T = 298.15 K and p = 1 atm. All of these calculations were performed with the Gaussian09 program.64 The discussion here is presented on the basis of the Gibbs energy, where the translation entropy was corrected by the method of Whitesides and co-workers65 as in our previous works.51,52,66−69 However, this approximation has no real effect on the results, since all reactions have the same molecularity. Benzene, propene, and IMes (1,3-dimesitiylimidazol-2-ylidene) were used as models of arene, alkene, and ligand, respectively. Investigation of the ligand effect was carried out by replacing IMes with carbodicarbenes such as C(NHC) 2 and C(NHC-Me) 2,

The concerted oxidative addition of a σ bond to a Ni0 atom has been analyzed in many experimental and theoretical studies.24−49 Thus, we focus here on the ligand-to-ligand H transfer and address the following points. (i) Why is the ligandto-ligand H transfer preferred over the usual concerted oxidative addition of C−H bond in the case of Ni0 catalysts? (ii) Is the ligand-to-ligand H transfer also preferred over the concerted oxidative addition to the Pd0 and Pt0 analogues? (iii) What factors favor the ligand-to-ligand H transfer relative to the concerted oxidative addition to metal? To answer these questions, we investigated the C−H activation reaction of a model arene, i.e. benzene, with Ni0-, Pd0-, and Pt0-propene complexes, using density functional theory (DFT). In addition, we investigated the effects of ligand and substituent of benzene. An understanding of these features will contribute to better knowledge of the C−H activation in the catalytic functionalization of arenes.

Scheme 2. Proposed Reaction Mechanism for Hydroarylation of Alkene19

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Organometallics Scheme 3. Ligands Examined in This Work

Figure 1. Gibbs energy profile (at 298.15 K, in kcal/mol) for the Ni(IMes)-promoted C−H σ-bond activation of benzene. Several hydrogen atoms in the extrema are omitted for clarity. The bond lengths are given in Å. C1, C2, C3, and Ha are shown in 3-Ni. trimethylphosphine (PMe3), 2-dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl (X-Phos), and cyclic alkylaminocarbenes such as CAAC-K0 and CAAC-K3 (Scheme 3). Electron-rich and electrondeficient arenes modeled by aniline and nitrobenzene were also considered.

structure to accommodate the steric bulk of the three ligands. The CC double bond is considerably elongated (1.399 Å) in comparison to that (1.330 Å) in free propene, consistent with strong back-donation from the Ni to the propene. Ni0(IMes)(C3H6)(C6H6) (2-Ni), in which one of the propenes is substituted by benzene, is 14.5 kcal/mol above 1-Ni. In 2-Ni, benzene is η2-coordinated with the Ni through a somewhat elongated C−C bond (1.420 Å) in comparison to that (1.392 Å) in free benzene. The Ni−C1 bond (1.948 Å) becomes slightly longer in 2-Ni than in 1-Ni (1.933 Å). C−H activation cannot occur in the π complex 2-Ni, but 2Ni is easily converted to 3-Ni with a C−H bond coordinated to the Ni. The transition state TS2/3-Ni from the π-bonded to the C−H-bonded benzene has a ΔG°⧧ value only 1.2 kcal/mol above that of 2-Ni. In 3-Ni, the C3−Ha bond (1.106 Å) of the

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RESULTS AND DISCUSSION C−H Activation of Benzene by Ni0(IMes)(propene) Complex. The bis-propene adduct Ni0(IMes)(C3H6)2 (1-Ni; C3H6 = propene) is more stable than the propene-arene adduct Ni0(IMes)(C3H6)(C6H6) (2-Ni), as shown in Figure 1; 1-Ni is taken as a reference for energy hereafter. 1-Ni is a typical threecoordinated trigonal d10 complex in which the CC double bonds of the two propenes, the coordinated C1IMes of IMes, and the Ni are almost coplanar (Figure 1); the subscript “IMes” will be omitted for brevity hereafter. This complex has a Y-shaped C


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Organometallics

Figure 2. Gibbs energy profile (at 298 K, in kcal/mol) for the Pt(IMes)-promoted C−H σ-bond activation of benzene. Several hydrogen atoms in the extrema are omitted for clarity. The bond lengths are given in Å. C1, C2, C3, and Ha are shown in 4-Pt.

Considering that the oxidative additions of phenyl chloride, benzyl chloride, and benzonitrile to a Ni0 complex have been experimentally reported and theoretically analyzed,24−49 we tried to locate a related NiII hydride phenyl propene complex where the H(hydride) is cis to the phenyl group but failed to optimize such a structure. This agrees with previous works.15,19 C−H σ-Bond Activation by Pd0 and Pt0 Analogues. Calculations similar to those done with the Ni0 complexes were carried out with Pd0 and Pt0 analogues, with the results shown in Figure S1 in the Supporting Information and Figure 2, respectively. Though the geometrical features of Pt0(IMes)(C3H6)(C6H6) and the Pd0 analogue are similar to those calculated with the Ni0 analogue, bond distances to the metal are longer with Pd0 and Pt0 than with Ni0, which has importance as will be shown later; for instance, the Pt−C1 and Pd−C1 bond distances in 1-Pt and 1-Pd are longer (by 0.115 and 0.157 Å, respectively) than the corresponding Ni−C1 bond of 1-Ni. Likewise, the averages of two M−Cpropene distances are 2.007, 2.216, and 2.163 Å for 1-Ni, 1-Pd, and 1-Pt, respectively. The η2-coordinated benzene complexes 2-Pt/2-Pd and the C−H-coordinated complexes 3-Pt/3-Pd are found as minima. The intermediates 3-Pt and 3-Pd are below 2-Pt and 2-Pd, respectively. In 3-Pt and 3-Pd, the C3−Ha bond of benzene is weakly bonded to the metal center through the Ha only; the C3−Ha bond is less elongated in 3-Pt and 3-Pd (1.094 and 1.095 Å, respectively) than in 3-Ni, suggesting its weaker activation for the two heavier metals. This is consistent with the longer distance between Pt/Pd and the benzene in 3-Pt and 3Pd than in 3-Ni. The C−H σ-bond activation occurs through the transition state TS3/4-Pt to yield the PtII hydride phenyl propene intermediate PtII(IMes)(Ha)(Ph)(C3H6) 4-Pt. In TS3/4-Pt, the Pt−Ha and Pt−C3 bond distances are 1.663 and 2.159 Å, respectively, which are longer than those of TS3/4-Ni. The C3− Ha distance is shorter than the C2−Ha distance, and the latter is

benzene is moderately elongated relative to that (1.089 Å) of free benzene. In TS2/3-Ni, the Ni coordinates to the C3−Ha bond, with distances R(Ni−C3) and R(Ni−Ha) of 2.677 and 2.966 Å, respectively. The o-C (ortho to C3) is close to Ni with a distance of 2.285 Å, indicating that the Ni interacts with the benzene in an η3-manner. Geometry optimization starting from TS2/3-Ni confirms that this transition state connects two intermediates through counterclockwise and clockwise rotations of the benzene ring (Scheme S1 in the Supporting Information). Starting from 3-Ni, the C3−Ha bond of the benzene is cleaved by Ha transfer from the benzene to the coordinated propene to yield the NiII propyl phenyl intermediate NiII(IMes)(C3H5)(Ph) (4-Ni; C3H5 = propyl, Ph = C6H5) through a transition state TS3/4-Ni. The rather high ΔG°⧧ value of 30.8 kcal/mol relative to 1-Ni is consistent with the high reaction temperature (150 °C).23 In TS3/4-Ni, the Ni, the CC of propene, the migrating Ha, and C3 of benzene are essentially coplanar. The Ha atom is 1.767 Å from the C3, 1.488 Å from the Ni, and 1.675 Å from the closer C2 atom of the propene CC double bond, suggesting that the Ha migrates from the benzene to the propene, while still interacting with Ni. The rather short Ni−C3 distance of 1.951 Å indicates that the Ni−Ph bond is almost formed in TS3/4-Ni. This TS3/4-Ni is essentially the same as that reported in earlier works with ethylene instead of propene.15,19 The d8 intermediate 4-Ni with a typical T-shaped structure and trans propyl and phenyl ligands is similar to that reported in earlier theoretical70 and experimental studies.71 The IMes ligand is trans to an empty coordination site occupied by an agostic interaction of the C2−Ha bond of the propyl group (R(C2−Ha) = 1.186 Å, R(Ni−Ha) = 1.689 Å). The Ni−C3 distance (1.947 Å) is similar to that in TS3/4-Ni. The 4-Ni intermediate is 5.2 kcal/mol below TS3/4-Ni but 25.6 kcal/mol above 1-Ni; note that the final reductive elimination occurs with a negative ΔG° value relative to 1-Ni. D


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Figure 3. Optimized structures of metal hydride phenyl complexes (metal = Ni, Pd, Pt). The structure of the Ni complex was optimized by assuming the C1NiC2, C1NiC4, and C2NiHa angles to be the same as those of the Pd complex.

Scheme 4. Schematic Representation of Potential Energy Surfaces for Transition State in the C−H Activation Process

considerably longer than in TS3/4-Ni, indicating that the Ha remains far from the propene in TS3/4-Pt. TS3/4-Pt is a transition state typical of a concerted C−H oxidative addition. Similar results are obtained for Pd with a slightly larger Gibbs activation energy, in agreement with a more facile oxidative addition to Pt0 than to Pd0 (Figure S1 in the Supporting Information) due to the lower Pd 4d orbital energy in comparison to the Pt 5d.72,73 Thus, the C−H activation mechanism obtained for Ni0 is unique to this metal. However, these different mechanisms are associated with rather similar Gibbs activation energies (around 30 kcal/mol for Ni0 and Pt0 and slightly larger (34 kcal/mol) for Pd0). Following the C−H activation step, propene inserts into the Pt−H bond through the transition state TS4/5-Pt to yield the PtII propyl phenyl complex PtII(IMes)(C3H7)(C6H5) (5-Pt). This step occurs with negligible variation in the Gibbs energy. In TS4/5-Pt, the long C2−Ha bond distance (1.571 Å) and the moderately elongated Pt−Ha distance (1.697 Å) suggest a reactant-like transition state, structurally similar to 4-Pt. In 5Pt, the rather long C2−Ha bond distance (1.185 Å) and the rather short Pt−Ha distance (1.935 Å) indicate the presence of an agostic interaction with the C2−Ha bond of the propyl group, as obtained in 4-Ni. 5-Pt is at slightly higher energy than

4-Pt and at much higher energy than 1-Pt. Similar results are found in the Pd0 complex but with a slightly higher barrier for the propene insertion step (ΔG°⧧ around 4 kcal/mol relative to 4-Pd; see Figure S1 in the Supporting Information). In summary, the C−H bond activation of benzene occurs through the usual oxidative addition to the metal center in the case of Pd0 and Pt0 alkene complexes but through the ligand-toligand H transfer mechanism only in the case of Ni0 alkene complex. Analysis of the C−H Activation with the Ni0 and Pt0/ Pd0 Complexes. The results presented above show that the isoelectronic Ni0 and Pd0/Pt0 alkene complexes bearing the same IMes ligand have different modes to activate an arene C− H bond. To understand the reason(s) for these differences, we start by investigating the optimized structure of the metal hydride complex MII(Ha)(C3H6)(Ph)(IMes) (M = Ni, Pd, Pt). Since the NiII hydride phenyl complex is not a minimum, as mentioned previously, an enforced minimum was obtained by partial optimization with C1NiC2, C1NiC4, and C2NiHa angles fixed to the values in the PdII complex (see Figure 3 for atom labeling). As presented in Figure 3, the M−Ha distances are 1.465, 1.605, and 1.627 Å in the NiII, PdII, and PtII complexes, respectively, which is related to the increasing atomic radius Ni E


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C2···C3 distance and thus a long RC distance between the bottoms of PES#1 and PES#2, as shown in Scheme 4B. These features indicate that the C−H activation occurs preferably by passing from PES#1 to PES#3 and from PES#3 to PES#2; in other words, the PdII−H and PtII−H species can be formed as a stable intermediate. These calculations and qualitative analysis give a rational explanation for why (i) the concerted oxidative addition occurs with Pt0(IMes)(C3H6)(C6H6) and the Pd0 analogue and (ii) the ligand-to-ligand H transfer occurs with Ni0(IMes)(C3H6)(C6H6). The longer distance between the ligands and heavier metal and the greater stability of the metal−hydride bond favor the formation of a metal hydride intermediate for Pd and Pt. The compact size of the Ni complex and the small Ni−H bond energy facilitate the ligand-to-ligand H transfer, avoiding going through the formation of a NiII−H intermediate (see a further discussion of the metal atomic size on page S13 in the Supporting Information). Orbital Interactions in the Ligand-to-Ligand H Transfer Mechanism. The important orbital interaction in the usual concerted oxidative addition to metal center has been discussed in detail.74−84 However, the orbital interaction in the ligand-toligand H transfer has been less discussed.15 It has in fact been presented for the Si−H σ-bond activation of silane by a ZrII ethylene complex.85,86 The HOMO at the transition state is shown in Figure 4. The metal d orbital participates in all TSs of Ni, Pd, and Pt reaction systems. However, the contribution of the Ha atom is different in this orbital among the three metals; the Ha atom does not interact with the Ni d orbital despite the rather short Ni−Ha distance mentioned above, while Ha interacts with Pd and Pt d orbitals. Thus, the Ha interacts with Pd and Pt but not with Ni. To achieve a better understanding of the electron reorganization in the reaction, a fragment molecular orbital analysis was carried out.87−89 This analysis was successfully used in various organometallic reactions.90−94 The electron populations in representative MOs of M(IMes), propene, and benzene and those of Ha and Ni 3d orbitals were evaluated. Scheme 6 and Figure 5 show these MOs and their population changes (in units of |e|) against the C3−Ha distance, respectively; Figure S2 in the Supporting Information shows the population changes in the Pd reaction system. We discuss first the easier case of the reaction with Pt. The Pt 5d orbital population decreases during the C−H activation step (Figure 5B). It is notable that the Ha atomic population increases in TS3/4-Pt (unlike in TS3/4-Ni) and the benzene LUMO (π*) has a higher electron population in TS3/4-Pt than in TS3/4-Ni. An increase in the benzene LUMO population leads to an increase in the Ha atomic population because the LUMO of the distorted benzene contains some antibonding C−Ha character, as will be shown below. These population changes indicate that

< Pd < Pt. The M−C2 and M−C3 bond distances also increase in the same order. Simultaneously, the C2MC3 angle and the C2···C3 nonbonding distance increase in the order Ni < Pd < Pt. These results show that the Ha atom is farther away from both the phenyl ring and the propene for Pd and Pt in comparison to Ni. In the C−H σ-bond activation step, the C3−Ha bond of the benzene is cleaved and either the M−Ha or C2−Ha bond of the propyl group is formed. Therefore, three potential energy surfaces (PESs) participate in the reaction along a reaction coordinate that is qualitatively described by the cleavage of the C3−Ha bond in the benzene and the formation of either the M−Ha bond or the C2−Ha bond with the propene, as shown in Scheme 4; one is PES#1 involving the benzene C3−Ha bond, the second is PES#2 involving the C2−Ha distance of the propyl which corresponds to the C−H distance between Ha and C2 of the propene, and the third is PES#3 involving the M−Ha distance in MII(IMes)(H)(Ph)(C3H6). While only qualitative, these PESs contribute to assess the origin of the difference between the three metals. A reasonable hypothesis is that PES#3 is related to the homolytic M−H bond dissociation energy (BDE). The Ni−H, Pd−H, and Pt−H BDEs, calculated in MII(IMes)(Ph)(H)(C3H6) from structures shown in Figure 3, are 60.5, 69.2, and 79.3 kcal/mol, respectively (Scheme 5). This means that the bottom of Scheme 5. Equation Employed for Evaluating M−H Bond Dissociation Energy (BDE)

PES#3 for the NiII complex is higher than those for the PdII and PtII complexes. Another important factor is the shorter C2···C3 distance between propene and benzene for the Ni complex than for the Pd and Pt complexes. This is shown qualitatively by a short distance, along the reaction coordinate (RC), between the bottoms of PES#1 and PES#2 (see justification of the choice of C2···C3 as the reaction coordinate on page S13 in the Supporting Information). These features indicate that the bottom of PES#1 is closer (along the RC) to that of PES#2 and the bottom of PES#3 is at higher energy for Ni than for Pd and Pt, as shown in Scheme 4A. This qualitative scheme accounts for the fact that C−H activation occurs by direct passage from PES#1 to PES#2 without passing via PES#3 in the case of the Ni complex: i.e., without forming the NiII hydride phenyl species. For the Pd and Pt complexes, PES#3 exists at lower energy and rather long M−C2 and M−C3 distances yield a long

Figure 4. HOMO of the transition state for the C−H activation by M(IMes)(C3H6)(C6H6) (M = Ni, Pd, Pt). F


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that the CT occurs from the Ni 3d to the benzene LUMO as for Pt. However, the Ha atomic population changes little in the reaction, which differs from the significant increase in the Ha atomic population observed with Pt. The electron population of the propene HOMO (π) moderately decreases but that of its LUMO increases, suggesting that the propene directly participates in the C−H σ-bond cleavage. The CTs from Ni and the Ha to the propene can both increase the propene LUMO population. However, the back-donation from the Ni to the propene LUMO already present in Ni0(IMes)(C3H6)(C6H6) has no reason to increase further. In addition the benzene LUMO becomes populated but the Ha atomic population remains essentially constant. This is evidence for the electron density on Ha to be transferred directly to the propene, contributing to the formation of the C2−Ha bond of the propyl group. The formations of the propyl and phenyl in 4-Ni indicate that the formal Ni oxidation state changes from 0 to +II in this reaction. This is consistent with the significant decrease in the Ni 3d orbital population, which is the same as that in the Pt 5d orbital population by the concerted oxidative addition. When these population changes are considered, the ligand-to-ligand H transfer reaction can be viewed as the oxidative addition of a benzene C−H bond to the entire Ni0−propene moiety; a similar oxidative addition to a metal−ligand moiety has been briefly discussed recently.77 This description of the C−H activation is an alternative to the description in term of σ-bond metathesis proposed earlier.15 The earlier description implied that the formal oxidation state of Ni was NiII in 1-Ni and 2-Ni as in 3-Ni. However, the decrease in the electron density at Ni upon going from 3-Ni to the product is better described by an oxidative addition to the entire metal−propene moiety. Effects of Ligand of Ni0 and Substituent of Arene on the C−H Bond Activation. In this section, we investigate the ligand effects on this C−H activation reaction. As shown in

Scheme 6. Important Orbitals in the Transition State of C− H Activation

the charge transfer (CT) occurs from the Pt 5d orbital to the benzene LUMO, as discussed previously.74−84 The electron population of the HOMO (π) of propene moderately decreases with Pt but that of the LUMO (π*) does not vary much, unlike the case for Ni, which is consistent with a propene that does not participate directly in the C−H activation (see a discussion of the propene HOMO and LUMO populations on page S9 in the Supporting Information). Similar results are obtained with Pd0 (Figure S2). The Ni 3d orbital population considerably decreases, as in the case of the Pt 5d orbital population (Figure 5A). The electron population of the benzene LUMO increases, indicating

Figure 5. Electron population (in units of |e|) for (A) several important MOs, Ha, and metal d orbital in the C−H activation process of Ni0(IMes)(C3H6)(C6H6) and (B) those in the C−H activation process of the Pt0 analogue. Note that Ni-3d and Pt-5d are the sums of orbital populations of five d orbital components. G


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Figure 6. Activation barrier (in kcal/mol) and TS structure in the C−H activation as a function of the ligand coordinated to Ni0. Some of hydrogen atoms are omitted for clarity. The bond lengths are given in Å. Values in parentheses give the lone pair orbital energy of the ligand. ΔG°⧧ is defined as the difference in the Gibbs energies between the transition state and Ni0(IMes)(C3H6)2.

activation. The other parameter is the C2···C3 distance in M0(L)(C3H6)(C6H6), which decreases when the ligand size increases since the propene and the benzene become closer by getting away from L. The two correlations are qualitatively functional (see pages S14−S16 in the Supporting Information for further details), but the overall influence of these two parameters is small on the activation energy for C−H activation with the exception of CAAC-K3, where the very large bulk significantly decreases the C2···C3 distance and the lone pair orbital energy is not very low. This is an interesting consequence of the presence of a bulky ligand, but it should

Figure 6 and Figure S3 in the Supporting Information, there is essentially no influence of the nature of the neutral ligand on the Gibbs activation energy of the reaction (relative to Ni0(L)(C3H6)2; L = neutral ligand shown in Scheme 3) with the exception of CAAC-K3, where considerable lowering of the Gibbs activation energy by around 6 kcal/mol was obtained. To have a clear insight into the role of the ligand, a linear and nonlinear correlations between ΔG°⧧ and two parameters that could characterize the ligand were considered. One of the parameters is the energy level of the lone pair that interacts with the metal; a higher energy lone pair favors the C−H H


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formation of the propyl group. The CT to the benzene LUMO contributes to weakening of the C−H bond through mixing of the phenyl π* and the C−H σ* antibonding orbitals in a nonplanar benzene in the transition state. As a result, an electron-withdrawing group on benzene decreases the activation energy of the C−H activation step by lowering the LUMO energy. This ligand-to-ligand H transfer was originally described as a metathesis reaction because the metal was viewed as being oxidized to NiII in the reactant. The population analysis suggests an alternative description in term of oxidative addition of the C−H bond to the Ni0-alkene moiety. The reality is probably a mix of these two limit descriptions. Since a short distance between the alkene and the arene and a high-energy lone pair orbital of the ligand are important factors in this reaction, it appears that a bulky ligand such as CAAC-K3 coordinated to Ni could facilitate the reaction by pushing the arene and alkene toward each other if the steric bulk of L does not prevent the coordination of the two substrates.

be kept in mind that a very bulky ligand could disfavor the entry of the arene in the coordination sphere. No attempt was made to explore this further by considering additional parameters,95 due to the overall rather constant value of the activation energy for the C−H activation. The last factor to be investigated is the effect of the arene substituent. For this purpose, aniline (with an electrondonating NH2 group) and nitrobenzene (with an electronwithdrawing NO2 group) were employed in the calculation in place of benzene. As shown in Figure S4 in the Supporting Information, the C−H activation has a lower ΔG°⧧ value with an electron-withdrawing substituent (ΔG°⧧ values of 33.7, 30.8, and 27.8 kcal/mol for nitrobenzene, benzene, and aniline, respectively). The larger CT to the electron-poor arene occurs as well to the π* of the ring and the C−H σ* orbital due to the nonplanarity of the arene at the transition state (Scheme S2 in the Supporting Information shows the π*−σ* mixing in the LUMO). In conclusion, bulky ligands with strongly donating power (measured by the energy of the lone pair involved in a coordinate bond to the metal) and the introduction of an electron-withdrawing substituent on the arene could contribute to accelerate the C−H bond activation occurring without formation of a hydride intermediate.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00256. Effects of basis sets and functional, schematic representation for arene clockwise and counterclockwise rotations, geometry and Gibbs energy changes in the C−H cleavage process by the Pd0 analogue, electron population changes of important orbitals in Pd(NHC) complexes, discussions of the C2···C3 distance as a reaction coordinate, importance of atomic size, and relationship of the Gibbs activation energy with the C2··· C3 distance and the lone pair orbital energy of the ligand, and ligand effects on the Gibbs activation energy, geometry, and the Gibbs energy changes in the C−H activation of nitrobenzene and aniline by Ni(CACC-K3) (PDF) Cartesian coordinates of all stationary points located in this work (XYZ)

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CONCLUSIONS We have investigated with DFT calculations the C−H bond activation of benzene by Ni0-, Pd0-, and Pt0-propene complexes. The C−H activation by the Ni0 complex occurs via a ligand-toligand H transfer mechanism without generating a NiII hydride phenyl intermediate. This result parallels previous studies with other neutral ligands at the metal,15,19 suggesting that this mechanism is possible in other Ni0 arene alkene complexes. In the corresponding Pd0 and Pt0 complexes, the C−H activation occurs through the usual concerted oxidative addition to a metal center. There is thus a significant difference in C−H bond activation mechanism among these M0 complexes despite their similarities (d10 electron configuration, identical coordinated ligand, and globally similar structures for reactants and products). The difference in these reaction mechanisms was discussed in terms of three potential energy surfaces associated with M−H, Cbenzene−H, and Cpropyl−H bond distance variations, respectively. In the case of the Ni0 complex, the compact size decreases the distance between the benzene and the propene in the intermediate and the transition state. In addition, the M−H potential energy surface exists at high energy because of the weakness of the Ni−H bond. These characteristics make it preferable to perform C−H activation through the ligand-toligand H transfer mechanism without formation of a NiIIhydride intermediate. The situation is different for the Pt0 and Pd0 analogues because the M−H potential energy surface is lower (stronger Pd−H and Pt−H bonds) and the distance between the benzene and the propene is greater. These features favor the C−H bond activation via concerted oxidative addition with the formation of a hydride intermediate. An analysis of the electron reorganization indicates that the Ni 3d orbital population decreases and the electron populations of propene and benzene LUMOs increase, but the Ha atomic population changes little during the C−H activation step. Overall, the electron density at the Ni is transferred to the benzene LUMO and simultaneously CT occurs from Ha to the propene LUMO. The latter CT leads to the C2−Ha bond

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.S.: [email protected]. ORCID

Odile Eisenstein: 0000-0001-5056-0311 Yoshiaki Nakao: 0000-0003-4864-3761 Shigeyoshi Sakaki: 0000-0002-1783-3282 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Ministry of Education, Culture, Science, and Sports through Grants-in Aid for Scientific Research (B) (No. 15H03770), a Grant-inAid for Science Research on Innovative Areas (Grant 15H00940, Stimuli-responsive Chemical Species), and CREST “Establishment of Molecular Technology towards the Creation of New Functions” Area (JPMJCR14L3). Some of this work was supported by a project commissioned by the Ministry of Economy, Trade, and Industry through “New I


Article

Organometallics

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Energy and Industrial Technology Development Organization (NEDO)”. We are also grateful to the computational facility at the Institute of Molecular Science, Okazaki, Japan.

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