DFT Mechanistic Study on Alkene Hydrogenation Catalysis of Iron

Sep 11, 2017 - In the catalytic cycle, 3triplet at triplet state plays a role of active species, and the hydrogenation at triplet state is more favora...
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DFT Mechanistic Study on Alkene Hydrogenation Catalysis of Iron Metallaboratrane: Characteristic Features of Iron Species Longfei Li,† Ming Lei,*,† and Shigeyoshi Sakaki*,‡ †

State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science, Beijing University of Chemical Technology, Beijing 100029, PR China ‡ Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan S Supporting Information *

ABSTRACT: The variable coordination geometries, multiple spin states, and high density of states of first row transition metals offer a new frontier in the catalytic chemistry. A DFT study has been performed in order to unveil these characteristic features of iron metallaboratrane complex in alkene hydrogenation. A detailed spin-state analysis reveals there exist two minimum energy crossing points in the formation of (TPB)(μ-H)Fe(H) 3triplet from (TPB)Fe(N2) 1triplet. In the catalytic cycle, 3triplet at triplet state plays a role of active species, and the hydrogenation at triplet state is more favorable than that at singlet state. The dissociation of phosphine arm of TPB ligand from Fe center occurs easily in the triplet state, because the antibonding dσ is singly occupied in 3triplet. However, a nondissociative pathway without any phosphine ligand dissociation is not likely to occur. The product is formed via the σ-bond metathesis between a dihydrogen molecule and a Fe-styryl moiety. The usual direct reductive elimination involving bridging hydride (μ-H) is very difficult because the μ-H is strongly bonded with Fe and B atoms.

1. INTRODUCTION Hydrogenation of unsaturated hydrocarbons, one of the most broadly employed reactions in the transformation of alkenes, occurs via several common processes such as oxidative addition of dihydrogen molecule (H2), alkene insertion into a metal− hydride bond, and reductive elimination.1 Most transition metal catalysts for the hydrogenation of unsaturated hydrocarbon consist of precious metals such as palladium, rhodium, iridium, and ruthenium elements because they are reactive for these elementary steps.2 Due to the high cost, toxicity, and potential depletion of precious metals, many chemists are trying to develop earth-abundant metal catalysts with easily available and biorelevant advantages for homogeneous catalysis in the past decade.3 It has been a hot topic in the field of catalysis to realize the concept of green chemistry and atomic economy.4 Homogeneous catalysts based on earth-abundant metals are becoming an alternative to precious metal catalysts in a wide range of catalytic transformation reactions.5 However, in the case of earth-abundant first row transition metal complexes such as iron (Fe), cobalt (Co), and nickel (Ni),6 the alkene hydrogenation reaction occurs through alternative pathways, as will be discussed below. In this regard, several pioneering chemists have made excellent efforts to develop iron-based catalysts to facilitate multielectron reduction of small molecule.10 Iron carbonyl complex (Fe(CO)5) with strong field ligand is well-known for catalyzing alkene hydrogenation.11 The photodissociation or © XXXX American Chemical Society

thermal dissociation of carbonyl group is key for its reactivity, and the generated coordinatively unsaturated tri- and tetracarbonyliron species can mediate alkene hydrogenation through several elementary steps such as ethylene coordination, oxidative addition of hydrogen, hydrogen transfer, and reductive elimination of alkane. This catalytic cycle is the same as that reported for precious metals. A significant breakthrough was achieved by Chirik et al. by developing more effective weak field ligands for iron catalyst, and he said “...the variable coordination geometries, multiple spin states, and high density of states of first row transition metals offer a new frontier for the rational manipulation of electronic structure as applied to catalytic chemistry.”12 Chirik et al. reported an unusual high-spin d8 square pyramidal iron(0) bis(dinitrogen) complex (iPrPDI)Fe(N2)2 with a pyridinediimine (iPrPDI) ligand (a in Scheme 1) could serve as a precatalyst for the hydrogenation and hydrosilylation of alkenes and alkynes under mild conditions.7 The catalytic activity of (iPrPDI)Fe(N2)2 for alkene hydrogenation surpasses those of well-established precious metal catalysts such as Wilkinson’s catalyst and Pd/C catalyst. However, the extremely large reactivity to both oxygen and moisture complicates its preparation and use. In 2015, Zhu et al. reported a new method for in situ generation of precatalyst LFe(II)Cl2 (L = Received: June 15, 2017

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center and the electron-deficient borane ligand. As shown in Scheme 2, two different pathways were proposed for the release of ethylbenzene: One is direct reductive elimination involving bridging hydride (μ-H) (path a), and another is the σ-bond metathesis between dihydrogen molecule and Fe-styryl moiety (path b).15 Though the proposed nondissociative mechanism in Scheme 2 is reasonable, we address several important questions: (i) Fe is known to exhibit the spin-state complexity, but it has not been considered at all in this mechanism. Also, the corresponding influence of spin state on chemical reactivity is unclear. (ii) The conformational flexibility plays a key role in the stability, reactivity, and catalytic activity of metal complexes.16 However, the coordination flexibility of the TPB ligand has not been discussed at all; for instance, we must consider the possibility that one phosphine moiety of TPB dissociates from the Fe to provide a coordination site for alkene to facilitate the catalytic reaction. (iii) The role of the borane ligand is unclear, i.e., whether it could facilitate the reductive release, despite of its crucial role in H2 activation. Theoretical answers to these questions and understanding of the catalysis are useful for making further development of catalytic chemistry of Fe complexes.

Scheme 1. Iron Complexes for Alkene Hydrogenation Reactionsa

a

Chirik’s (iPrPDI)Fe(N2)2 complex (a),7 Zhu’s LFeCl2 complex (b),3 Peters’s [PhBPiPr3]Fe-R complex (c),8 and (TPB)Fe(N2) complexes (d).9

2. COMPUTATIONAL DETAILS AND MODELS In accordance with previous computational studies on hydrogenation and dehydrogenation reactions catalyzed by transition-metal complexes,17 all calculations were carried out by DFT method with the ωB97X-D18 using Gaussian 09 program in this study.19 Geometry optimization in the gas phase uses basis set system BS-I, where a double-ζ basis set (Lanl2dz) with the effective core potentials of Hay and Wadt was employed for Fe20 and 6-31G* basis sets were used for all the other atoms. Single-point calculations were performed to present better electronic energy with the ωB97X-D and a large basis set system BS-II, using the ωB97X-D/BS-I optimized geometries. In BS-II, the effective core potentials and the basis set by the Stuttgart− Dresden−Bonn group21 were used for Fe and 6-311++G** for all the other atoms. The solvent effect was evaluated using the conductor-like polarizable continuum model (CPCM),22 i.e., the ωB97X-D (with PCM)/BS-II//ωB97X-D/BS-I was employed for presenting final electronic energy. Thermal correction and entropy contribution to the Gibbs free energy were taken from the frequency calculations with the ωB97X-D/BS-I. The realistic model complex (TPB)Fe(N2), 1triplet, was used in the calculations. All transition states were confirmed to exhibit only one

phosphine-bipyridine) (b in Scheme 1) with phosphinebipyridine ligand for alkene hydrogenation, which is inexpensive and readily available.3 Since 2004, Peters and co-workers have developed a family of coordinatively unsaturated pseudotetrahedral iron(II) precursor supported by tris(phosphino)borate (TPB) ligand, [PhBPiPr3]Fe-R (c in Scheme 1), which could serve as a precatalyst for alkene hydrogenation under ambient conditions.8 The borane ligand plays interesting roles in facilitating H−H σ-bond activation; for instance, Zeng and Sakaki theoretically investigated the H2 activation by nickel metallaboratrane and reported large charge transfer from the H2 moiety to the Ni−B moiety.13 Peters et al. also reported a (TPB)Fe(N2) complex (d in Scheme 1), known as a catalyst for N2 reduction to NH3,9 could also catalyze alkene hydrogenation.14 They disclosed that (TPB)Fe(N2) 1 reacts with H2 at room temperature to afford (TPB)(μ-H)Fe(N2)(H) 2 through the cooperative functions of the electron-rich iron

Scheme 2. Non-Dissociative Alkene Hydrogenation Mechanism Catalyzed by (TPB)Fe(N2) Complex Proposed by Peters et al.14

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Scheme 3. Mechanism of the Hydrogenation of Styrene Catalyzed by Iron Metallaboratrane (TPB)(μ-H)Fe(H) (3triplet)

imaginary frequency by calculating vibrational frequencies. The energies relative to those of 1triplet were employed for discussion. All intermediates and transition states in this reaction at singlet/triplet spin state have been considered. In addition, the MECPs (minimum energy crossing points) were located by the method of Koga and Morokuma,23 using the code developed by Harvey and co-workers24 at the ωB97X-D level. The natural bond orbital (NBO) calculations were performed using NBO 3.1,25 as implemented in the Gaussian 09 package.19

3. RESULTS AND DISCUSSION The hydrogenolysis of precatalyst (TPB)Fe(N 2 ) 1 is investigated first in order to elucidate what spin state intermediates take, and whether spin crossover occurs in this reaction or not. The superscript of 1singlet (or 1triplet) denotes intermediate 1 at singlet (or triplet) state. The present DFT study discloses that the reaction of 1triplet with H2 produces (TPB)(μ-H)Fe(H), 3triplet, as will be seen below. As shown in Scheme 3, the dissociative pathway A and nondissociative pathway B were investigated to unveil the coordination flexibility of the TPB ligand and the influence of spin state on chemical reactivity. There are two possibilities in the dissociative pathway: pathway A1 with recoordination of phosphine arm (9triplet → 12triplet) and pathway A2 without recoordination of phosphine arm (9triplet → 10triplet). Finally, the possibility of participation of μ-H in reductive elimination step is discussed. The calculated structural parameters of precatalyst 2 at singlet state were compared with experimental data (see Figure 1). The difference in bond length is small, within 0.04 Å, which suggests the reliability of the calculation method used in this work. Only the mainframe of the real complex at stationary point along reaction pathway is presented hereafter, where substituents such as phenyl, i-Pr, and other groups are omitted for clarity. 3.1. Generation of Catalytic Species (TPB)(μ-H)Fe(H) (3triplet) from Precatalyst (TPB)Fe(N2) (1triplet). As shown in Figures 2 and 3, 1 and 3 with trigonal bipyramidal structure both have triplet states which are much more stable than their singlet states by 29.4 and 21.0 kcal/mol, respectively, indicating

Figure 1. Structural comparison of the catalyst precursor (TPB)(μH)Fe(N2)(H) (2) at singlet state. The optimized and experimental geometrical parameters are listed in plain and italic text, respectively (bond lengths are in Å).

that 16-electron iron complexes 1 and 3 have triplet ground states. However, 18-electron complex 2, which was isolated in the experiment,14 has a singlet ground state, and the triplet state is much less stable by 36.3 kcal/mol. The singlet ground state is not surprising because 2 is an 18-electron complex with a sixcoordinated structure (Figure 3). These results show that spin crossover should occur going from 1triplet to 2singlet. Mecp1 was calculated at 35.4 and 34.5 kcal/mol in the Gibbs free energy for singlet and triplet states, respectively, indicating that the conversion of 1 to 2 is slow. The H−H σ-bond cleavage occurs without barrier going from Mecp1 to 2singlet, which was confirmed by geometry optimization from Mecp1. (On the triplet surface, the dihydrogen molecule dissociates from the Fe center without barrier, while on the singlet surface, the optimization reaches 2singlet.) Also, the spin crossover occurs going from 2singlet to 3triplet, in which the dinitrogen ligand of 2singlet dissociates from the Fe center. Mecp1 going from 1triplet to 2singlet was located when dihydrogen molecule approaches the Fe center of 1triplet. In Mecp1, the H−H distance is not elongated very much (0.773 Å; Figure 3). This geometry indicates that the spin crossover from triplet to singlet is induced by the approach of H2 molecule to Fe. The spinC

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is elongated to 2.045 Å, and the Fe−H1 distance decreases to 1.528 Å. Further coordination of H2 molecule with the Fe atom of 3triplet could form dihydrogen complexes 5triplet/6triplet, but both are less stable than 3triplet (shown in Figures S2 and S3). Because 3 is an important active species as will be seen below, herein we analyze the natural bond orbitals (NBOs) of 3singlet and 3triplet, as shown in Figure 4, to elucidate the reason why it has a triplet ground state. In 3singlet, three nonbonding NBOs were found around FeII. They correspond to dxy, dxz, and dyz orbitals and their occupation numbers are 1.974, 1.970, and 1.947 e, respectively. Two NBOs, mainly consisting of the dx2−y2 and dz2, have antibonding character of Fe−P and Fe−H, the occupation numbers of which are 0.162 and 0.267 e, respectively. In 3triplet, the ϕ4(dx2−y2−λdπ) becomes singly occupied, and its occupation number is 0.981e. Because this ϕ4 is empty in the singlet state, the P3 lone pair orbital of phosphine can form bonding interaction with this dx2−y2, leading to the formation of rather short Fe−P3 bond. However, it becomes singly occupied in the triplet state (Figure 4), which is not favorable for the formation of the coordinate bond with the P3 lone pair. As a result, the Fe−P3 distance is elongated in 3triplet. Simultaneously, ϕ4 becomes nearly nonbonding in 3triplet because the overlap between the P3 lone pair and the dx2‑y2 becomes small. The presence of such two MOs at similar energy, as well as the large d−d exchange interaction, is the reason for the stability of the triplet state. It should also be noted as an important feature that in the triplet state the phosphine (P3) easily dissociates from the Fe center because ϕ4(dx2−y2−λdπ) is singly occupied in the triplet. This is important in the catalytic cycle because the P3 dissociation easily occurs to provide empty coordination site, as will be shown below. 3.2. Dissociative Pathways A1 and A2. We will discuss first dissociative pathway A1 (Scheme 3). As shown in Figure 5, the reaction on the triplet state is much more favorable than that on the singlet state. The structures along the pathway of styrene hydrogenation at the triplet state are shown in Figure 6. The first step (phosphine dissociation) occurs via transition

Figure 2. Gibbs free energy profile (in kcal/mol) for the generation of (TPB)(μ-H)Fe(H) (3triplet) from (TPB)Fe(N2) (1triplet). The Gibbs free energies are out of parentheses, and the electronic energies without ZPE correction are in parentheses.

crossing electronic energy barrier from 1triplet to 2singlet is 23.8 kcal/mol. Mecp2 on the pathway going from 2singlet to 3triplet is located at 17.9 and 17.2 kcal/mol in the Gibbs free energy for singlet and triplet states relative to 1triplet. In Mecp2, the Fe−N2

Figure 3. Optimized geometries of intermediates involved in the generation of (TPB)(μ-H)Fe(H) 3 from (TPB)Fe(N2) 1 at singlet and triplet spin states (phenyl groups, i-Pr groups, etc., are omitted for clarity; distances are in Å). D

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state TS3−7triplet to afford intermediate 7triplet, the Gibbs activation energy of which is 14.3 kcal/mol. Then, styrene coordinates with the Fe center of 7triplet at the vacant site in the equatorial plane to provide intermediate 8triplet. This styrene coordination step is moderately exergonic by 2.3 kcal/mol. In the hydrogen transfer step (or the styrene insertion step), apical hydride of 8triplet is transferred to the C2 atom of the styrene CC double bond to afford Fe-styryl intermediate 9triplet via transition state TS8−9triplet; this step corresponds to [1,2]styrene insertion. In this transition state, the H1−C2 length decreases from 2.34 to1.18 Å. The Gibbs activation energy for this hydrogen transfer step is only 2.5 kcal/mol, which indicates this step is very easy. We investigated another styrene [2,1]insertion but it needs larger activation barrier; see the Supporting Information. As shown in Figure 6, 9triplet involves an agostic interaction between the H1 of the styryl ligand and the Fe center. This agostic interaction is generally weak and could be easily broken. Therefore, the styryl ligand in 9triplet moves to the apical position with breaking the agostic interaction. Simultaneously, the free phosphine arm recoordinates with the Fe center because one empty coordination site appears at the apical position. As a result, five-coordinated intermediate 12triplet is formed. The Gibbs activation energy for TS9−12triplet is very small (0.4 kcal/mol). 12triplet undergoes dihydrogen coordination (12triplet → 13triplet) at the equatorial site to form dihydrogen intermediate 13triplet with the Gibbs activation energy of 20.8 kcal/mol. Then, σ-bond metathesis between the dihydrogen and styryl ligands proceeds via transition state TS13triplet to release the product and regenerate the active species 3triplet. The Gibbs activation energy of this pathway is calculated to be 24.6 kcal/mol relative to 12triplet. The total exergonicity of the whole catalytic cycle is large (35.6 kcal/ mol). Though the process from 13triplet to 3triplet via TS13triplet is understood to be metathesis, the transition state TS13triplet has an interesting structure, because one H atom is moving from the other H atom of dihydrogen molecule toward the C

Figure 4. NBOs of in 3singlet (a) and 3triplet (b). The occupancy values and energy values are listed below the orbitals, respectively. Energy is in eV and population is in e.

Figure 5. Gibbs free energy (ΔG) profile for the hydrogenation of styrene. Black line represents Gibbs free energy change (kcal/mol) on the singlet state, and purple line is that on the triplet state. E

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Figure 6. Optimized structures along the hydrogenation of styrene at triplet state. The phenyl groups, i-Pr groups, and so on are omitted for clarity. Distances are in Å.

3.3. Nondissociative Pathway B. Here, we investigated pathway B, which proceeds without any phosphine dissociation. First, we tried to optimize styrene coordination with the Fe in 3triplet on the triplet surface. However, we could not locate such intermediate. Styrene coordinates with the Fe atom at the apical position on the singlet surface to afford (TPB)(μH)Fe(H)(styryl) 11singlet, as shown in Figure 7; see the Supporting Information (page S7) for discussion of the reaction. Though 11singlet is less stable than 3singlet by 9.5 kcal/mol, it is very unstable (30.5 kcal/mol) relative to 3triplet. The structure of 11singlet having the Fe−C1 and Fe−C2 bonds (2.25 and 2.46 Å) at the position trans to the B−H group is not stable because of the strong trans-influence of the B−H group. Also, the equatorial site of 11singlet cannot be used for the coordination of styrene because of its congested space. In the hydrogen transfer step, the hydride H1 in the equatorial plane is transferred to the C1 of the styrene CC double bond via transition state TS11−12singlet with the Gibbs activation energy of 6.1 kcal/mol to form trigonal bipyramidal intermediate (TPB)(μ-H)Fe(styryl) 12singlet. The energy of 12singlet is much higher than that of 12triplet by 26.6 kcal/mol. The σ-bond metathesis between the dihydrogen and styryl ligands occurs in the pathway A1 with the Gibbs activation energy of 24.6 kcal/mol. Since the Gibbs activation energy for TS11-12singlet (36.6 kcal/mol relative to that of 3triplet) is very large, the hydrogenation without phosphine ligand dissociation is not likely. 3.4. Could μ-H Participate in a Reductive Elimination Step? One more question is whether the μ-H of the Fe−H−B moiety of 12triplet participates in the formation of ethylbenzene product or not. In one possible pathway, the μ-H of the Fe−

atom of the styryl ligand with keeping interaction with Fe atom. These features indicate that this process can be viewed as an Htransfer reaction with help by the metal. This TS13triplet differs from the traditional transition state of metathesis which has a four-center structure involving transition metal. The similar transition states were previously reported in the theoretical studies of alkene hydrogenation by Ir and Co complexes.26 In 13triplet, H2 molecule coordinates with the Fe atom in the equatorial plane. When the phosphine arm coordinates again with the Fe, another possible coordination occurs at the apical position of 9triplet; this is pathway A2. As shown in Figure 6, one H2 molecule approaches the Fe center of 9triplet along the apical direction to afford dihydrogen complex (TPB)(μ-H)Fe(styryl)(H2) 10triplet. Then, one hydrogen atom of the apical dihydrogen ligand is transferred to the C1 of the styryl ligand via transition state TS10triplet. This reaction is understood to be σ-bond metathesis. Finally, ethylbenzene is released, and one arm of the TPB ligand coordinates again with the Fe center to regenerate simultaneously active species 3triplet. The Gibbs activation energy going from 9triplet to the product is very small (4.3 kcal/mol), and the highest energy TS10triplet is at 7.5 kcal/ mol above that of 1triplet. Though the conversion from 9triplet to 12triplet in pathway A1 occurs with nearly no energy barrier and 12triplet is very stable (Figure 5), the σ-bond metathesis between the H2 on the equatorial site and the styryl at the apical site needs large Gibbs activation energy of 24.6 kcal/mol in the pathway A1. Because the highest energy TS13triplet of the pathway A1 is 8.1 kcal/mol above that of 1triplet and as stable as TS10triplet of pathway A2, both pathways A1 and A2 are possible. F

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Figure 8. Gibbs free energy (ΔG in kcal/mol) profile of the reductive elimination step with μ-H.

Figure 7. (a) Gibbs free energy (ΔG in kcal/mol) profile of the styrene coordinating with the Fe center at the apical position of 3singlet to lead to 12singlet in pathway B. (b) Optimized geometries at the pathway. The phenyl, i-Pr, and other groups are omitted for clarity; distances are in Å.

state. The dissociation of phosphine arm of TPB ligand from the Fe center is crucial for the catalytic reaction (pathway A), and it easily occurs because the antibonding dσ is singly occupied in 3triplet. In other words, the high spin state of Fe facilitates phosphine dissociation which is necessary for styrene coordination. The σ-bond metathesis between the H2 on the equatorial site and the styryl at the apical site is ratedetermining in the pathway A1, with the Gibbs activation energy of 24.6 kcal/mol. In contrast, σ-bond metathesis between the H2 at the apical site and the styryl on the equatorial site is very easy in the pathway A2, in which the phosphine dissociation step is rate-determining with the Gibbs activation energy of 14.3 kcal/mol. In the nondissociative pathway B, the coordination of styrene can not occur on the triplet surface. Though styrene can coordinate with the Fe on the singlet surface, styrene complex 11singlet is very unstable with high energy. Nondissociative pathway B is ruled out. Another important result is that the direct reductive elimination from 12triplet using bridging μ-H cannot occur because the μ-H is very stable in the Fe−H−B moiety. This is the reason why the product releasing step occurs via the σ-bond metathesis. On the basis of the features summarized above, it is concluded that the triplet spin state of the Fe center is important for the hydrogenation catalysis of this Fe complex. Because the closed-shell singlet is in general a ground state of 4d and 5d transition metal complexes, this type of catalysis based on triplet state is one characteristic feature of 3d transition metal complex. Also, the coordination flexibility of TPB is another important reason for the catalytic activity. It is realized in the 3d metal complex because the ligand

H−B moiety of 12triplet moves to the equatorial position to generate intermediate 14triplet (Figure 8). However, 14triplet is much more unstable than 12triplet by 35.4 kcal/mol, as shown in Figure 8. The reductive elimination occurs between the equatorial hydride and styryl ligands via transition state TS14triplet to release ethylbenzene with formation of 4triplet, as shown in Figure S5. Though the reductive elimination occurs with small Gibbs activation energy of 2.9 kcal/mol relative to 14triplet, the highly endergonic process of 12triplet to 14triplet clearly indicates that the μ-H cannot move from its bridging site to the equatorial site. Because the activation energy for the reductive elimination is not large relative to 14triplet, the presence of high-energy intermediate 14triplet is the most important reason why the μ-H does not participate in the reaction. As going from 12triplet to 14triplet, one B−(μ-H) covalent bond is broken, the Fe−B coordinate bond is formed, and the Fe−(μ-H) bond is converted to the terminal Fe−H bond. It is likely that the strong B−(μ-H) bond breaking is the reason for large endergonicity of the process going from 12triplet to 14triplet.

4. CONCLUSIONS A detailed and comprehensive theoretical study unveiled the characteristic features of iron species using DFT method. One of the important features is that spin-crossover occurs at Mecp1 and Mecp2 when going from 1triplet to 3triplet. All species in catalytic cycle were proved to have a high-spin triplet groundG

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Organometallics

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coordination is generally stronger in 4d and 5d metals than in 3d metal. These features clearly show the good performance of 3d metal as catalyst, which is different from those of 4d and 5d metals.

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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.7b00457. Electronic energies, Gibbs free energies, calculated imaginary frequencies of transistion states, potential energy curves, frontier orbitals, optimized geometries of intermediates, structures of 16−18, and bond orders (PDF) Cartesian coordinates (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

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

Ming Lei: 0000-0001-5765-9664 Shigeyoshi Sakaki: 0000-0002-1783-3282 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was in part supported by the National Natural Science Foundation of China (Grant No. 21672018, 2161101308 and 21373023), Beijing Municipal Natural Science Foundation (Grant No. 2162029), the Fundamental Research Funds for the Central Universities of China (Grant No. PYCC1708), and China Scholarship Council (No. 201606880007). We thank the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) for providing part of the computational resources. We are also thankful to the computational facility at the Institute of Molecular Science, Okazaki, Japan. S.S. is thankful to the Ministry of Education, Culture, Sports, and Science (Grants-in Aid for Scientific Research; JP15H03770) and Japan Science and Technology Cooperation (CREST ‘Establishment of Molecular Technology towards the Creation of New Functions’ Area).

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DOI: 10.1021/acs.organomet.7b00457 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00457 Organometallics XXXX, XXX, XXX−XXX