Metal–Substrate Cooperation Mechanism for Dehydrogenative

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Metal−Substrate Cooperation Mechanism for Dehydrogenative Amidation Catalyzed by a PNN-Ru Catalyst Longfei Li,†,§ Ming Lei,*,† Li Liu,*,†,‡ Yaoming Xie,§ and Henry F. Schaefer, III*,§ State Key Laboratory of Chemical Resource Engineering, Institute of Materia Medica, College of Science and ‡Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing 100029, PR China § Center for Computational Quantum Chemistry, University of Georgia, Athens, Georgia 30602, United States Downloaded via UNIV OF NEW ENGLAND on July 17, 2018 at 01:04:09 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

†

S Supporting Information *

ABSTRACT: The pyridine-based PNN ruthenium pincer complex (PNN)Ru(CO)(H) can catalyze the well-known dehydrogenative amidation reaction, but the mechanism is not fully understood. In this work, we find there exists an alternative metal−substrate cooperation mechanism in this reaction system, which is more favorable than the aromatization−dearomatization mechanism. The possible reaction of the excess base t-BuO− with catalyst species (PNN)Ru(CO)(H) is studied, indicating t-BuO− is able to facilitate the ligand substitution and enhance catalytic activities. With the bifunctional Ru−N moiety, the iminesubstituted species (PN)(imine)Ru(CO)(H) 5 could serve as an alternative catalytic species and efficiently facilitate some elementary steps such as the hydrogen transfer, hydrogen elimination, and C−N coupling. Meanwhile, the C−N coupling step proceeds via the split of aldehydic C−H bond across the Ru(II)−imine bond, which results in an amide bond directly. The hemiaminal is uninvolved in the C−N coupling process. Finally, the formation of linear peptides and cyclic dipeptides are unveiled by the newly proposed mechanism. The metal−substrate cooperation could widely exist in transition metal catalyst systems with a large influence on the reaction activity.

1. INTRODUCTION The development of safe and renewable energy resources realizing green chemistry is highly desirable.1−3 With three times the energy content of gasoline, hydrogen would be an ideal candidate for a future energy supply without pollutants.4,5 However, storing sufficient hydrogen is a tremendous technical challenge due to energy security concerns.6−8 One intriguing possibility is “liquid organic hydrogen carriers” (LOHCs) for high-capacity hydrogen storage.9 New mechanistic pathways, unique substrate scope, and new reactivity are significant to the development of LOHCs.10,11 Recently, pincer complexes have been suggested to be efficient for the acceptorless dehydrogenation of LOHCs.12−15 In 2013, Beller and co-workers first achieved an aqueous-phase methanol dehydrogenation under basic conditions.16 Through the cooperation of the Ru−N unit in the aliphatic PNP pincer ruthenium complex (PNP)Ru(CO)H, methanol could react with water to produce hydrogen and carbon dioxide at low temperatures. Theoretical studies suggested the methanol substrate could assist the hydrogen elimination with a lower energy barrier.17−19 This state-of-the-art dehydrogenation process took a great step toward the hydrogen economy. Noyori and co-workers pioneered bifunctional catalysts with metal−ligand cooperation in the inner coordination sphere.20 © XXXX American Chemical Society

Through secondary coordination sphere interactions, pendent hydroxyl group in the aromatic ligands of metal complexes can also serve as proton-responsive substituents and facilitate the hydrogenation or dehydrogenation reactions.21 Fujita and coworkers developed a Cp*Ir catalyst, which exhibited high catalytic activity for the dehydrogenation of a variety of primary and secondary alcohols into aldehydes and ketones.22 Mechanistic investigations revealed that the catalytically active species is a hydrido iridium complex with a functional C,Nchelate ligand. Shvo’s catalyst, a cyclopentadienone-ligated ruthenium complex, is useful for hydrogenation and dehydrogenation reactions.23 The redox activity is distributed between the metal center and a cyclopentadienone ligand. Milstein and co-workers have made another outstanding progress in this context and found acceptorless dehydrogenative amidation reaction catalyzed by the pyridine-based pincer complex (PNN)Ru(CO)(H) (1).24 Involving the amide bond formation, the reaction has attracted tremendous interests from organic chemistry, biochemistry, and medicinal chemistry.25 Many theoretical and experimental studies have been performed and demonstrated the dehydrogenative amidation Received: March 4, 2018

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DOI: 10.1021/acs.inorgchem.8b00563 Inorg. Chem. XXXX, XXX, XXX−XXX

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based on the formation of hemiaminal molecule are reviewed by Li and Hall as shown in Scheme 3a,b.36 In 2017, Gusev discovered the alcohol-substituted catalytic intermediate can lead to a low-energy, outersphere catalytic pathway to form a hemiaminal bounded intermediate (shown in Scheme 3c), in a study named “Rethinking the dehydrogenative amide synthesis”.40 In 2011, Zeng and Guan first reported the direct polyamidation from diols and diamines using the Milstein’s (PNN)Ru(CO)(H) catalyst, as shown in Scheme 4a.41 Surprisingly, they reported the linear polyamide products to be cationic. We infer that anionic metal complexes could be generated in the reaction system. In 2015, Milstein and coworkers used 2-aminoethanol (AE) as substrates for the dehydrogenative amidation reaction, in which AE can be converted into the mixed products of cyclic dipeptides and linear peptides at 110 °C.42 When 1.2 or 2.4 equiv of base (KOtBu) relative to catalyst is applied, which is considered to be used for generating the actual catalyst, (PNN)Ru(CO)(H) 1, in situ from the precatalyst (PNN)Ru(H)(CO)(Cl). The amine (−NEt2) arm of the PNN ligand is observed to be substituted by the substrate AE. For the dehydrogenative amidation reaction, Milstein proposed a mechanism which involves the dissociation of the amine arm, but important details for the hydrogen elimination and C−N coupling steps are not suggested. In the dehydrogenative amidation reaction system,42 the amine substrate may react with the pyridine-based (PNN)Ru(CO)(H) complex and provide a Ru−N bifunctional moiety in the presence of base. Can the generated bifunctional moiety catalyze the dehydrogenative amidation reaction? Is the hemiaminal really formed in the dehydrogenative amidation reaction? Could the metal−substrate cooperation widely exist in the metal catalyst systems and influence the reaction rate? In order to address these issues, we began a systematic study of the dehydrogenative amidation reaction with AE substrates.

reaction to be consists of an alcohol dehydrogenation cycle and a C−N coupling dehydrogenation cycle, as shown in Scheme 1.26−30 However, the important details for the key hydrogen elimination and C−N coupling steps remain to be the hot topic of discussion. Scheme 1. Milstein and Co-Workers Developed a Dehydrogenative Amidation Reaction Catalyzed by the Pyridine-Based Pincer Complex (PNN)Ru(CO)(H)24

The aromatization−dearomatization mechanism is generally accepted for the hydrogen addition into pyridine-based (PNN)Ru(CO)(H) complex.31−33 However, high-energy barriers have been reported for the hydrogen elimination step (A3 via TSA3−1 to 1, in Scheme 2).34 In 2013, Yang Scheme 2. Aromatization−Dearomatization Mechanism for the First Alcohol Dehydrogenation Cycle31−33

2. COMPUTATIONAL METHODS In accordance with our previous studies,43−46 geometries optimizations were carried out with the DFT ωB97X-D/BS-I47 level of theory using the Gaussian 09 program.48 BSI denotes combination of the LANL2DZ49 basis set for Ru, and the 6-31++G** basis sets for 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 BSII, the SDD50 basis set was used for Ru and 6-311++G(2d, 2p) for all the other atoms. All the structures were fully optimized in 1,4-dioxane solvent (experimental conditions) using the Solution Model based on Density (SMD).51,52 All transition states were confirmed to exhibit only one imaginary frequency by calculating vibrational frequencies. All Gibbs free energies were computed at 298.15K. The energies discussed in the present paper are free energies relative to (PNN)Ru(CO)(H) (structure 1). The Cartesian coordinates of all optimized structures are reported in the Supporting Information. The natural bond orbital (NBO) analyses were performed using NBO 3.1,53 as implemented in the Gaussian 09 package.

predicted almost no catalytic activity for the pyridine-based (PNN)Ru(CO)(H) complex in the dehydrogenation of ethanol under mild conditions, because of the high energy for the transition state TSA3−1.35 Hall and co-workers36 raised a critical question, “aromatization−dearomatization or not”, and examined various theoretical methods, including 20 DFT functionals and the CCSD and CCSD(T) methods. They also predicted a high-energy barrier (about 30 kcal/mol) for the hydrogen elimination step. In addition, the alcohol-assisted hydrogen elimination in the aromatization−dearomatization mechanism has been ruled out since it experiences an even higher energy barrier than the direct hydrogen elimination.37,38 Recently, Boer suggested the aromatization−dearomatization mechanism was not necessarily the most favorable pathway for the H2 elimination catalyzed by pyridine-based pincer complex.39 For the key C−N coupling step in the second C−N coupling dehydrogenation cycle, two mechanisms (the alcohol catalyzed mechanism and aromatization−dearomatization mechanism)

3. RESULTS AND DISCUSSION We first investigate the ligand substitution process (1 → 5) in section 3.1. Then, the dehydrogenative amidation reaction consisted of two reaction cycles catalyzed by 5 are investigated. The first alcohol dehydrogenation cycle is shown in Figure 1 and is discussed in section 3.2, which includes hydrogen transfer (5 → 7 → 10) and hydrogen elimination (10 → 5) B

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Scheme 3. Mechanisms Based on the Formation of the Hemiaminal for the Second C−N Coupling Dehydrogenation Cycle36,40

Scheme 4. Catalyzed Polyamidation Reactionsa

a

(a) Zeng and Guan developed polyamidation with the (PNN)Ru(CO)(H) catalyst.41 (b) Milstein et al. developed polyamidation with (PNN)Ru(H)(CO)(Cl) catalyst.42

OtBu− with the (PNN)Ru(CO)(H) catalyst. As shown in Figure 2, the OtBu− can coordinate into the axial site of the metal Ru(II) in complex 1 to form 2, which is slightly exothermic, by 1.5 kcal/mol. Furthermore, the OtBu− ligand in 2 repels the coordination of the donor −NEt2 arm in the PNN ligand, with a longer Ru−N bond of 2.388 Å (compared with

steps. The second C−N coupling dehydrogenation cycle, including C−N coupling and hydrogen elimination steps, will be discussed in section 3.3. 3.1. Conversion of Species 1 into Catalytic Species 5. The excess base (KOtBu) was applied in the reaction system.42 First, we investigated the possible reaction of the excess base C

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Figure 1. Alcohol dehydrogenation cycle for 2-aminoethanol (AE) in the metal−substrate cooperation mechanism catalyzed by 5.

Figure 2. Gibbs free profile (in kcal/mol) for the reaction between starting species 1 and catalytic species 5.

Figure 3. Gibbs free energy profiles (in kcal/mol) for the hydrogen transfer step.

of 2, making that process (2 → 3) exothermic by 2.1 kcal/mol. For comparison, the direct ligand substitution in 1 to provide

2.261 Å in 1). This brings about the possibility for the amine groups (−NH2) of the AE molecule to replace the −NEt2 arm D

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Figure 4. Gibbs free energy profiles (in kcal/mol) for the bifunctional hydrogen elimination step. (The black line stands for path 1; the pink line stands for path 2.)

neutral 4 without −OtBu is highly endothermic by 17.4 kcal/ mol. For intermediate 3, a facile O−H bond activation across the Ru−N bond proceeds via the transition state TS3, which can produce isomer 3b. The energy barrier for this step is trivial, only 0.6 kcal/mol. After the release of the t-BuOH, species 5 is generated with an energy of −9.0 kcal/mol relative to 1. The addition of a proton from t-BuOH to the unsaturated carbon in 5 leads to the aromatization of the central pyridine ring of the PNN ligand in neutral complex 6, which is an isomer of 4. However, this step is difficult due to the high energy of 6. It should be emphasized that the Ru(II) oxidation state is retained throughout the process, and the formal negative charge on complex 5 is located in the nitrogen atom of the imine ligand with its NBO charge of −0.86. It seems that the excess base plays a role in facilitating the ligand substitution, and anionic species 5 can be generated facilely with low-energy barriers. It should also be noted that the concentration of AE and 2-aminoacetaldehyde (AA), which are essential to properly determine the Gibbs energy profile,54−56 also have an important role in the dehydrogenative amidation. Having the cooperation of the Ru(II) with the strong basic imine ligand, structure 5 could serve as a highly efficient donor−acceptor bifunctional catalytic intermediate to facilitate the key hydrogen transfer, hydrogen elimination and C−N coupling steps, as will be discussed below. 3.2. Alcohol Dehydrogenation Cycle Catalyzed by 5. 3.2.1. Hydrogen Transfer Step. The details of the hydrogen transfer step are discussed here. As shown in Figure 3, the AE molecule, as a hydrogen donor, could coordinate to the outer coordination sphere of Ru(II)−imine complex 5 to form intermediate 7. Due to the strong basic imine-substituted ligand in intermediate 7, there exists strong Nδ−---Hδ+−O attractive interactions, and this coordination process is exothermic by 0.6 kcal/mol. Subsequently, the two hydrogen atoms of the alcohol moiety in AE are transferred in a stepwise fashion. This begins with passage over the energy barrier (2.1

kcal/mol) of the pericyclic six-membered transition state TS7−8, where the proton Hδ+ in 7 transfers to the imine ligand to form 8. Through the second pericyclic six-membered transition state TS8−9, the hydride Hδ− transfers to the Ru center to form 9 with an energy barrier of 5.8 kcal/mol. The free energy of 9 is −0.1 kcal/mol relative to starting material 1. An aldehydic product AA with a polarized Cδ+Oδ− group is released from 9. The hydrogen transfer step (from 5 to 10) is endothermic by 9.6 kcal/mol. Note that although the Gibbs free energies of transition states TS7−8 and TS8−9 appear lower than those of intermediates 8 and 9, respectively, the potential energies for these transition states are higher than those of 8 and 9 on the potential energy surface. With the hydridic-protonic Nδ−−Hδ+---Hδ−−Ruδ+ interaction, transdihydride complex 10 could be used for a hydrogen elimination step as shown below. 3.2.2. Hydrogen Elimination Step. We find three possible pathways based on the metal−substrate cooperation for the hydrogen elimination step, and then make a comparison with the aromatization−dearomatization mechanism. Path 1 (black line in Figure 4) proceeds via the cooperation of the aminesubstituted ligand and the Ru(II) center. The amine proton Hδ+ in 10 transfers to the adjacent hydride Hδ− on the Ru to form a H−H bond in TS10−5, which is 31.2 (= 21.6 + 9.6) kcal/mol relative to intermediate 7. After the release of H2, 5 with the bifunctional Ru(II)−imine moiety is regenerated. The catalytic cycle of the alcohol dehydrogenation reaction is complete, and the total endothermicity is 10.8 (= 1.8 + 9.0) kcal/mol. Path 2 (pink line in Figure 4) proceeds via the cooperation of hydroxyl-substituted ligand with the Ru(II) center. Since the amine-substituted ligand (−NH2) in 10 could dissociate and coordinate easily, the −NH2 ligand linked to Ru(II) in 10 can be replaced by the −OH group in the AE substrate, which results in isomer 10a with a polarized bifunctional Oδ−−Hδ+--Hδ−−Ruδ+ moiety. Due to a weaker coordination interaction of the −OH group with the Ru(II) center, 10a lies higher in E

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Figure 5. Gibbs free energy profiles (in kcal/mol) for the alcohol-assisted hydrogen elimination (path 3).

energy than 10 by 11.0 kcal/mol. However, structure 10a could proceed to H2 elimination more easily, with the energy barrier of TS10−5a being 26.8 kcal/mol (= 17.2 + 9.6). The NBO charges for axial proton Hδ+ on the amine ligand and hydride Hδ− on Ru in TS10−5a are 0.31 and −0.11, while those in TS10−5 are 0.24 and −0.05 respectively. Therefore, the lower energy barrier of path 2 than that of path 1 could be associated with a stronger protonic−hydridic interaction in TS10−5a. After that, 5a, an isomer of 5, is generated. Through a hydrogen exchange sequence (5a → 5b → TS5b-5 → 5) shown in Figure 4, catalytic species 5 could be regenerated to complete the alcohol dehydrogenation catalytic cycle. Whether alcohol molecules could facilitate the hydrogen elimination step?36 We investigate the issue in path 3 (shown in Figure 5). Species 10c, an isomer of species 10, could bond an alcohol molecule through two hydrogen bonds (O−Hδ+--Oδ−−Hδ+---Hδ−−Ru). The methanol model is used in the computation for the alcohol molecules. Despite the entropy loss, generated intramolecular intermediate 10d is only 0.5 kcal/mol higher than 10 in Gibbs free energy because of the two strong hydrogen bonds. Subsequently, the hydrogen atom of hydroxyl can transfer to the hydride on Ru via an unprecedented transition state TS10d−e, in which three hydrogen bonds (N−Hδ+(O−Hδ+)---Oδ−−Hδ+---Hδ−−Ru) exist. The energy barrier for TS10d−e is 24.3 (= 14.7 + 9.6) kcal/mol relative to species 7. Then, intramolecular intermediate 10e is formed, which is followed by a hydrogen transfer sequence (10e → 10f → 5) to regenerate catalytic species 5. The aromatization−dearomatization mechanism for the hydrogen elimination step is also studied by us, in which the dissociation of amine arm is not involved. The hydrogen transfer steps (1 → A3) are shown in the Supporting Information. The H2 elimination proceeds via the transition state TSA3−4 with a high-energy barrier of 36.2 kcal/mol (see Figure 6). In contrast, paths 1−3 through the metal−substrate cooperation are more favorable for the H2 elimination step with lower energy barriers and higher activities, which partly stem from the presence of excess OtBu− anion. Since the countercation K+ may interact with the anionic Ru complex, there would exist small deviation for the activity of OtBu−. In the absence of OtBu− anion, the mechanisms become different. 3.3. C−N Coupling Dehydrogenation Cycle Catalyzed by 5. For the C−N coupling dehydrogenation cycle, a new

Figure 6. Gibbs free energy profiles (in kcal/mol) for the hydrogen elimination in the aromatization−dearomatization mechanism.

insight based on the metal−substrate cooperation is provided here. The C−N coupling step proceeds through the split of aldehydic C−H bond (red color in Figure 7) across the Ru(II)−imine bond, rather than the formation of hemiaminal. As a result, an amide bond (−CO−NH−) is formed in the trans-dihydride complex (12cis or 12trans). Two pathways may exist in the C−N coupling step: The path R leads to a trans amide bond; the path S leads to a cis amide bond. In the following, the formation of linear peptides and cyclic dipeptides will be unveiled. The aldehydic C−H bond can split across the Ru(II)−imine bond in a stepwise fashion as shown in Figure 8. First, the electronegative N atom in imine ligand of 5 attacks the weakly electrophilic carbonyl group in AA. Two enantiomers, 11R and 11S, are generated via the transition states TS5−11R and TS5− 11S, respectively. Meanwhile, the aldehydic C−H bond (red color in Figure 8) is weakened, with the C−H distance increasing from 1.109 Å in AA to 1.195 Å in 11R or to 1.171 Å in 11S. Then, the hydride transfers from the carbon atom to the Ru(II) center, with a trans or cis amide bond formed in 12trans or 12cis. Both structures, 12trans or 12cis, are stable, and the stabilization energy associated with the pN−πCO* hyperF

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process based on the metal−substrate cooperation mechanism is more favorable. Furthermore, 12trans leads to the linear peptide products with n − 1 trans amide bonds, while 12cis leads to cyclic dipeptide products with two cis amide bonds. In path R, 12trans isomerizes to another trans-dihydride complex 10(n=2) through a replacement of the AE(n=2)-substituted ligand (purple line in Figure 8). This isomerization is exothermic by 12.4 kcal/mol. With the bifunctional Nδ−−Hδ+---Hδ−−Ruδ+ moiety, hydrogen donor 10(n=2) can undergo a further hydrogen elimination procedure (paths 1−3 in section 3.2.2). 5(n=2) with the bifunctional Ru−imine moiety is formed, which can serve as a catalytic intermediate for the next reaction cycle. Repetitive C−N coupling dehydrogenation cycles in path R will result in a linear peptides ligand in 10n. Finally, through a ligand substitution in 10n by an AE molecule, the linear peptides products could be released with the regeneration of 10, at which point the chain propagation process stops. In path S, the cis-type peptide ligand of 12cis is directly substituted by an AE molecule to regenerate 10, which is exothermic by 8.8 kcal/mol. Released peptide P1cis could lead to the cyclic dipeptides product through a self-coupling dehydrogenation between the amine and hydroxyl end-groups as shown in Figure S1. It should be noted the total Gibbs reaction energy for the dehydrogenative amidation reaction in the solution is moderately positive (shown in Figure S22). However, the experimental success of this reaction arises from the vaporization of H2 from solution into gas phase, which is associated with the change of transitional entropy.

Figure 7. Metal−substrate cooperation mechanism for the C−N coupling dehydrogenation cycle catalyzed by 5.

conjugation in the amide bond is estimated as 37 kcal/mol by second-order perturbation theory. The energy barriers for the transition states TS11R−12trans and TS11S−12cis are 1.2 and 0.7 kcal/mol, respectively, which suggest the hydride transfer step is easy. It should be noted that intermediates 11R and 12trans (path R) are predicted to be lower in energy than 11S and 12cis (path S) by 3.0 and 6.9 kcal/mol, respectively. This stability of 11R and 12trans may result from an intermolecular H-bonding effect between the amide group and hydroxy endgroup. The energy barriers for the C−N coupling step are 8.8 kcal/ mol for path R (TS5−11R relative to 5) and 9.1 kcal/mol for path S (TS5−11S relative to 5). In contrast, the C−N coupling steps through the alcohol catalyzed mechanism and aromatization−dearomatization mechanism experience high-energy barriers of 23.2 and 24.1 kcal/mol, respectively (shown in the Supporting Information). Therefore, the C−N coupling

4. CONCLUSIONS The present paper unveils the existing “metal−substrate cooperation” in the dehydrogenative amidation reaction catalyzed by the pyridine-based PNN ruthenium pincer complexes. In addition to generating actual catalyst 1 in situ from the precatalyst, the excess base (t-BuO−) can facilitate the ligand substitution and enhance catalytic activities. Through the cooperation of the Ru(II) with the strong basic imine-

Figure 8. Gibbs free energy profiles (in kcal/mol) for the N−C coupling reaction. The black line stands for path S; the purple line stands for the path R. Red isolates the critical H and C atoms; n stands for the number of polymerized monomers. G

DOI: 10.1021/acs.inorgchem.8b00563 Inorg. Chem. XXXX, XXX, XXX−XXX

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substituted ligand, structure 5 could serve as a donor−acceptor bifunctional catalytic intermediate to facilitate the dehydrogenative amidation reaction with extremely high activity. In the first alcohol dehydrogenation cycle, we found there exist three possible pathways for the hydrogen elimination, which are more favorable than the aromatization−dearomatization mechanism. The lower energy barriers for hydrogen elimination are associated with the flexible coordination of substrate-substituted ligand. In the second cycle, the key C−N coupling step can be achieved by an unprecedented heterolytic split of the aldehydic C−H bond across the Ru(II)−imine bond. It also suggests the hemiaminal could not form in the C−N coupling dehydrogenation cycle. The formation of linear peptides and cyclic dipeptides are finally unveiled. The trans amide bond is generated in path R and leads to the linear peptides product; while the cis amide bond is generated in path S and leads to the linear peptides product. Metal−substrate cooperation as a novel paradigm may be found in more metal catalyst systems, with influencing the reaction activity largely. It also enlightens us that although supported by innocent ligands some metal complexes will probably exhibit the bifunctional catalytic functions because some substrates like amine could substitute a weak ligand to provide a reactive bifunctional ligand. Further experimental and theoretical studies about metal−substrate cooperation will help discover new catalyst systems and reactions in the future.

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REFERENCES

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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.inorgchem.8b00563.

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Optimized geometries and energies of all stationary points along reaction pathways, imaginary frequencies of transition states (PDF)

AUTHOR INFORMATION

Corresponding Authors

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

Ming Lei: 0000-0001-5765-9664 Henry F. Schaefer, III: 0000-0003-0252-2083 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 Nos. 21672018 and 21373023), Beijing Municipal Natural Science Foundation (Grant No. 2162029), BUCT Fund for Disciplines Construction and Development (Project No. XK1527), and China Scholarship Council (No. 201606880007). We thank the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (second phase) for providing part of the computational resources. The research at the Center for Computational Quantum Chemistry was supported by the U.S. National Science Foundation, Grant CHE-1661604. H

DOI: 10.1021/acs.inorgchem.8b00563 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b00563 Inorg. Chem. XXXX, XXX, XXX−XXX