Theoretical Elucidation of the Mechanism and Kinetic Experimental

Article pubs.acs.org/joc

Theoretical Elucidation of the Mechanism and Kinetic Experimental Phenomena on the Esterification of α‑Tocopherol with Succinic Anhydride: Catalysis of a Histidine Derivative vs an ImidazoliumBased Ionic Liquid Yaru Jing, Rongxiu Zhu, Chengbu Liu, and Dongju Zhang* Key Lab of Colloid and Interface Chemistry, Ministry of Education, Institute of Theoretical Chemistry, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: DFT calculations have been conducted to gain insight into the mechanism and kinetics of the esterification of α-tocopherol with succinic anhydride catalyzed by a histidine derivative or an imidazoliumbased ionic liquid (IL). The two catalytic reactions involve an intrinsically consistent molecular mechanism: a rate-determining, concerted nucleophilic substitution followed by a facile proton-transfer process. The histidine derivative or the IL anion is shown to play a decisive role, acting as a Brönsted base by abstracting the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated free energy barriers of two reactions (15.8 kcal/mol for the histamine-catalyzed reaction and 22.9 kcal/mol for the IL-catalyzed reaction) together with their respective characteristic features, the catalytic reaction with a catalytic amount of histamine vs the catalytic reaction with an excessed amount of the IL, rationalize well the experimentally observed kinetics that the former has faster initial rate but longer reaction time while the latter is initiated slowly but completed in a much shorter time. Scheme 1. Experimentally Reported Esterifications of αTocopherol with Succinic Anhydride Catalyzed by the Organocatalyst (Histamine) (a) and the Ionic Liquid Catalyst ([C5C1Im][NO3]) (b),36 Respectively

1. INTRODUCTION Vitamin E (VE), known as an elementary component of biological membranes, is one of the most major nature antioxidants and is identified as an essential nutrient.1−3 It refers to eight structurally similar compounds: α-, β-, γ-, and δtocopherols and α-, β-, γ-, and δ-tocotrienols,4−6 of which αtocopherol (Scheme 1) is the most biologically and chemically active.7−10 VE is oxidized easily in the presence of light, heat, air, and oxidizing agents.11,12 However, its various derivatives are shown to have high chemical stability13−15 and the same biological activity as VE.16,17 Therefore, VE is generally administered as its derivatives which have been emphasized as highly selective anticancer agents18−20 and antioxidant food additives.21−23 Esterified VE is one kind of the most widely used derivatives of VE in the food, cosmetic, and pharmaceutical fields.24−26 Thus, esterification of VE, as an important method to improve its stability, has received remarkable attention in medicine and food chemistry.27−29 Esterification of VE can be usually achieved by using either biological catalysts,2,30 such as lipase and esterase, or chemical catalysts,31,32 such as tertiary amine and pyridine. For example, VE succinate can be obtained in high yield from the enzymemediated procedure in aprotic polar solvents like dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF).5,33−35 A recent research by Tan’s group36 shows that the observed catalytic activity of several biocatalysts toward esterification of α© 2017 American Chemical Society

tocopherol (the most active isomer of VE) originates from the chemical catalysis of the histidine residue in proteins. On the basis of this important finding, Tan et al.36 designed two types of Received: August 19, 2017 Published: October 30, 2017 12267

DOI: 10.1021/acs.joc.7b02102 J. Org. Chem. 2017, 82, 12267−12275

Article

The Journal of Organic Chemistry

Scheme 2. Proposed Catalytic Cycles by Tan and Co-workers36 for the Esterification of α-Tocopherol with Succinic Anhydride Catalyzed by Histamine (Red Pathway) and [C5C1Im][NO3] Ionic Liquid (Blue Pathway), Respectively

dielectric property with ionic liquids, was used as solvent for the calculated IL-catalyzed reaction. It has indicated that 1,2-dichloroethane is suitable to describe the solvation of ionic liquids.40 It should be emphasized that for a complex system, a full potential energy surface (PES) search is only computationally feasible at a low level of theory. In this case, we did not carry out full PES search calculations to obtain the initial conformations of each supramolecular structure (intermediate or transition state) due to a vast number of free degrees involved in the present systems. Alternatively, most of these structures were initially obtained based on “best guesses” which contain various possible interactions between the catalyst and reactants, including H-bonding, electrostatic, and π−π stacking interactions, then fully optimized on the PES, and finally characterized through vibration analysis and intrinsic reactant coordinate (IRC) calculations.41 Such a method is widely used to search for intermediates and transition states involved in complex reaction systems.42,43 All stationary points have been identified as minima (zero imaginary frequencies) or transition states (one imaginary frequency). To refine the relative free energies, some key structures involved in the rate-determining step along each pathway were reoptimized using the solvation model density (SMD), which can simulate an IL environment more accurately,44 with the M06-2X functional, a high nonlocality functional parametrized only for nonmetals.39 As indicated by the calculated barriers shown in the following figures, both M06 and M06-2X functionals give substantially consistent conclusions. The population analysis has also been carried out using natural bond orbital (NBO) analysis45 under the Gaussian 09 package. Noncovalent interactions (NCIs) analyses were carried out using the Multiwfn program.46 All of the NCI isosurfaces and significant three-dimensional structures were visualized with VMD47 and CYLview48 softwares.

chemical catalysts containing an imidazole ring (the main structural motif of histidine), histidine-based organocatalysts and imidazolium-based ionic liquid (IL) catalysts, and tested their reactivity toward the esterification of α-tocopherol with succinic anhydride. Two representative examples of these esterification reactions are shown in Scheme 1, describing the organocatalyst (histamine)- and the IL ([C5C1Im][NO3])-catalyzed reactions, respectively. Both of them were found to give satisfactory yields of α-tocopheryl succinate. Scheme 2 shows the tentative catalytic mechanisms proposed by Tan et al.36 However, the feasibility of these mechanisms is still not confirmed by quantum chemistry calculations. More importantly, some interesting observations shown in Scheme 1 are not rationalized: the histamine-catalyzed reaction was initiated quickly but completed in a longer time; in contrast, the [C5C1Im][NO3]-catalyzed reaction was initiated with a much lower reaction rate (0.69 ± 0.12 vs 10.8 ± 0.2 × 10−2 min−1) but completed in a shorter time (1.5 vs 9 h). These facts encouraged us to perform a comparative calculation study on these two catalytic reactions. Interestingly, by carrying out density functional theory (DFT) calculations, we established intrinsically different mechanisms from Scheme 2 for both the histamine-catalyzed and the [C5C1Im][NO3]-catalyzed reactions, and also made a reasonable explanation of the observed kinetics of reactions by analyzing the different characteristic features of the two reactions.

2. COMPUTATIONAL DETAILS In the present study, the unreactive 4, 8, 12-trimethyltridecyl group in αtocopherol (Scheme 1) was replaced by propyl group. For the sake of simplicity, the simplified model compound is still referred to as “αtocopherol” in the following sections. All DFT calculations were performed at the M06/6-311G(d,p) level with the polarizable continuum model (PCM),37 as implemented in the Gaussian 09 software package.38 The M06 functional, which is parametrized for both transition metals and nonmetals, has been shown to be accurate enough for describing chemical reaction mechanisms, calculating barriers and reaction energies.39 For the calculated histamine-catalyzed reaction, DMF was used as solvent, whereas 1,2-dichloroethane, which has similar

3. RESULTS AND DISCUSSION The present calculations have identified three potential pathways for the histamine-catalyzed reaction, denoted as pathways IH, IIH, and IIIH, and five potential pathways for the IL-catalyzed reaction, denoted as IIL, IIIL, IIIIL, IVIL, and VIL. The calculated results at the M06/6-311G(d,p) level are summarized in Figures 1−6, where schematic geometries are consecutively numbered with a subscript “H” or “IL” and a superscript “I”, “II”, “III”, “IV”, or “V”, meaning the structures involved in the histamine- or IL12268


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Figure 1. Calculated energy profile (pathway IH) at the M06/6-311G(d,p) level with schematic structures (bond distances are in Å, and key geometrical parameters in transition states are highlighted in red for visual clarity) for the histamine-catalyzed reaction according to the mechanism proposed by Tan et al.36 Blue values indicate the refined geometrical parameters and relative free energies using the SMD model at the M06-2X/6-311g(d,p) level.

catalyzed reaction. The relative energies in each figure are given in both the Gibbs free energies and zero-point-energy corrected electronic energies (the detailed thermodynamic data for each species are given in Table S1 in the Supporting Information). In each pathway, the initial complex between the catalyst and reactants is set as a zero reference point of relative energy. In this case, all elementary steps do not involve the change of component number, resulting in low entropic effects comparing the electronic energy and Gibbs free energy. The following discussion on the reactivity uses the calculated relative Gibbs free energies, in which errors caused by possible “multiple cavities” (if any) for the supramolecular species nearly compensate for each other. 3.1. Histamine-Catalyzed Reaction. Mechanism Proposed in the Experiment. As indicated in Scheme 2, the experimentally proposed mechanism by Tan et al.36 involves imidazole catalysis, consisting of three main processes: (i) the deprotonated and protonated histamine molecules, two forms of histamine in DMSO or DMF, initially interact with succinic anhydride to form an acylimidazole-containing intermediate, (ii) the hydroxyl oxygen of α-tocopherol nucleophilically attacks the carbonyl carbon of the acylimidazole-containing fragment,

leading to an oxyanion-containing intermediate, and (iii) the product is formed via expelling the deprotonated histamine. The calculated energy profile is shown in Figure 1 (pathway IH). The reaction starts from the supramolecular complex 1IH among the histamine dimmer formed via intermolecular hydrogen binding, α-tocopherol and succinic anhydride. The N1 atom of one histamine molecule in 1IH bearing large negative charge (Figure S1) is ready to nucleophilically attack the anhydride carbonyl carbon atom. TS1IH is located as the transition state with a free energy barrier of 3.0 kcal/mol, leading to the acylimidazolecontaining intermediate 2IH. In this process, one histamine molecule acts as a nucleophile and the other plays a role of Brönsted base to promote the nucleophilic attack. Then, 2IH evolves to a more stable structure 3IH, where the protonated histamine interacts with the acylimidazole fragment. Although the substantial conformational rearrangement is observed for the transformation from 2IH to 3IH, it expected that this process is takes place easily because it mainly involves the reposition of weak intermolecular H-bonds. The following process is the proton transfer from N2 atom of the protonated histamine to O1 via transition state TS2IH, leading to intermediate 4IH, a trimolecular H-bonding complex among 12269


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Figure 2. Calculated energy profile (pathway IIH) at the M06/6-311G(d,p) level with schematic structures (bond distances are in Å, and key geometrical parameters in transition states are highlighted by red color for visual clarity) for the histamine-catalyzed reaction according to the mechanism proposed in this work. Blue values are the refined geometrical parameters and relative free energies using the SMD model at the M06-2X/6-311g(d,p) level.

phenolic oxygen of α-tocopherol on the carbonyl carbon of succinic anhydride with the assistance of a second histamine molecule which binds to succinic anhydride through H-bonding interaction. This process needs to overcome a free energy barrier of 15.8 kcal/mol, resulting in a transient structure 2IIH, which then evolves to a more stable intermediate 3IIH where the opened succinic anhydride ring extends to a linear chain with the protonated histamine binding to the more negatively charged oxygen atom of the carboxylic acid ion through H-bonding interaction. Finally, 3IIH converts to the product precursor 4IIH through transition state TS2IIH, in which the N1-proton returns to the negatively charged O1 atom. The calculated free energy barrier of the direct nucleophilic substitution pathway is 15.8 kcal/mol, which is much lower than that in Figure 1 (30.7 kcal/ mol). Note that throughout the pathway shown in Figure 2, the second histamine molecule seems to act as a spectator which only shows H-bonding interaction with the succinic anhydride. To ascertain the role of the histamine molecule played in the catalytic reaction, we performed calculations for the system without the presence of the second histamine molecule. The resulting energy profile is shown in Figure S2. The barrier of the reaction is calculated to be 19.5 kcal/mol, which is higher by 3.7 kcal/mol in energy than that shown in Figure 2 for the situation with the assistance of the second histamine molecule. Clearly, the second histamine molecule plays a substantial role for the histamine-catalyzed esterification of α-tocopherol with succinic anhydride. The NCI analysis shows that the second histamine molecule stabilized TS1IIH through both the H-bonding and π−π stacking interactions, as shown by Figure 3a.

the α-tocopherol, histamine, and acylimidazole. Subsequently, via intermolecular reposition, 4IH evolves into intermediate 5IH, where the histamine molecule is ready to abstract the hydroxyl hydrogen of the α-tocopherol while the α-tocopherol approaches the carbonyl carbon atom of acylimidazole. It should be noted that we failed to locate the assumed oxyanion-containing intermediate proposed by Tan et al.36 (Scheme 1). The final step of the reaction involves the nucleophilic attack of the hydroxyl oxygen of α-tocopherol on the carbonyl carbon of acylimidazole. TS3IH, with a relative energy of 24.5 kcal/mol, is identified as the transition state to realize the nucleophilic attack, in which the formation of O−C1 bond and the cleavage of C1− N1 bond occur simultaneously. The IRC calculation started from TS3IH results in intermediate 6IH, a transient conformer of the complex of the product with the zwitterionic dimer of histamine, which lies above the reaction entrance by 13.5 kcal/mol. 6IH easily evolves into the more stable structure, 7IH via the proton migration between two histamine units. From Figure 1, it is clear that the nucleophilic attack is the ratedetermining step, and the overall free energy barrier is 30.7 kcal/ mol, which is too high for the reaction to proceed under the mild conditions (50 °C and air pressure).36 Mechanism Identified from the Present Calculations. Alternatively, we located an energetically more favorable pathway (pathway IIH) for the histamine-catalyzed reaction (Figure 2). Different from the mechanism proposed by Tan et al., 36 this pathway does not involve the acylimidazole intermediate. The reaction occurs via a direct nucleophilic substitution process, as indicated by TS1IIH, where the N1 atom of histamine acts as the proton acceptor to assist the attack of the 12270


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Figure 3. Calculated weak interactions (a) and optimized geometries (b) for TS1IIH and TS1III H (some hydrogen atoms are omitted for clarity). Black and blue bond distances are from the M06/6-311G(d,p) and M06-2X/6-311G(d,p) calculations, respectively. Bond distances are in Å. Red values in square brackets are NBO charges (in e) calculated at the M06/6-311G(d,p) level.

Figure 4. Calculated energy profile (pathway IIIH) at the M06/6-311G(d,p) level with schematic structures (bond distances are in Å, and key geometrical parameters in transition states are highlighted by red color for visual clarity) for the rate-determining step of the histamine-catalyzed reaction with the presence of an acetic acid molecule. Blue values are the refined geometrical parameters and relative free energies after reoptimization using SMD model at the M06-2X/6-311g(d,p) level.

Effect of the Product on the Reactivity. It should be noted that the final product contains a carboxylic group, which is expected to have strong interaction with succinic anhydride. Therefore, we further considered the effect of the product on the reactivity. An acetic acid molecule was introduced into the reaction system to mimic the product effect, and the calculated result for the rate-determining step is given in Figure 4 (pathway IIIH). The free energy barrier is further reduced to 14.7 kcal/mol,

indicating that the presence of the product can further stabilize the rate-determining transition state. In this sense, this reaction can be referred as a self-assisted catalytic process, where the strong H-bonding interaction between the hydroxyl hydrogen of product and the carbonyl oxygen of succinic anhydride can effectively disperse the negative NBO charges on O1 and O2 (Figure 3b), stabilizing the rate-determining transition state. 12271


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Figure 5. Calculated energy profile (pathway IIL) at the M06/6-311G(d,p) level with schematic structures (bond distances are in Å, and key geometrical parameters in transition states are highlighted by red color for visual clarity) for the ionic liquid-catalyzed reaction according to the mechanism proposed by Tan et al.36 Blue values indicate the refined geometrical parameters and relative free energies using the SMD model at the M06-2X/6-311g(d,p) level. I resulting in an unstable carbene-containing intermediate 4IL which further potentially evolves into a less unstable intermediate 5IIL via reposition of the carbene and α-tocopherol units to favor the proton migration between them. Subsequently, the generated carbene abstracts the proton on the phenolic hydroxy of α-tocopherol via TS3IIL which lies above the entrance by 43.2 kcal/mol, resulting in regeneration of the catalyst cation, as shown in intermediates 6IIL. Via reposition of intermolecular Hbonds, 6IIL can evolves to a slightly stable intermediate 7IIL, where the deprotonated α-tocopherol is ready to perform the nucleophilic attack at C1. Subsequently, the reaction occurs via TS4IIL, resulting in formation of the product, as shown in structure 8IIL. Clearly, TS3IIL is the maximum on the reaction pathway; in other words, the overall barrier would be as high as 43.2 kcal/mol if the reaction proceeds according the mechanism shown in Scheme 2. This result is not compatible with the experimental observation that the esterification of α-tocopherol with succinic anhydride promoted by [C5C1Im][NO3] proceed smoothly at mild temperature (50 °C). The high energy demand along with this pathway can be understood by considering the basicity of the negatively charged O1 in intermediate 3IIL. In this intermediate, negative charge on O1 partially transfers to O2, weakening the ability of O1 to abstract the proton on C2 of [C5C1Im]+ cation to form N-heterocyclic carbene, a strong Brönsted base.49,50 Mechanism Identified from the Present Calculations. Alternatively, Figure 6 shows an energetically viable mechanism (pathway IIIL), which is similar to that for the histamine-catalyzed reaction discussed above. The reaction involves a synergetic II nucleophilic substitution process which occurs via TS1IL ,

3.2. Ionic Liquid Catalyzed Reaction. Mechanism Proposed in the Experimental Work. According to the mechanism proposed by Tan et al.36 (Scheme 2), the ILcatalyzed reaction consists of three steps. The first step involves the nucleophilic attack of the anion on the anhydride carbonyl with the assistance of the cation that donates the C2 proton to the oxygen atom of the succinic anhydride, forming an anion− anhydride intermediate. Subsequently, the hydroxyl oxygen of αtocopherol performs the nucleophilic attack on the carbonyl carbon of succinic anhydride to form an oxyanion-containing intermediate. The process benefits from the strong basicity of the deprotonated cation, a N-heterocyclic carbene,49,50 that accepts the proton so as to promote the nucleophilic attack and to reproduce the cation. Finally, the leaving of the anion gives the final product. Based on these conjectures, the calculated results are shown in Figure 5 (pathway IIL). It is noted that the mechanism details are somewhat different from those shown in Scheme 2. However, these processes are intrinsically similar to those involved in the histamine-catalyzed reaction (Figure 1). As shown in Figure 5, the reaction starts from the initial complex among succinic anhydride, α-tocopherol, and [C5C1Im][NO3]. Figure S3 shows the calculated four different initial complexes between the catalyst and reactants, and structure 1IIL is identified as the most stable one. The nucleophilic attack of NO3− anion on the carbonyl of succinic anhydride occurs via TS1IIL, overcoming a barrier of 23.1 kcal/mol in free energy. The generated intermediate 2IIL probably evolves into its less stable structure 3IIL via reposition of intermolecular H-bonds. Subsequently, the C2 proton of [C5C1Im]+ cation migrates to the negatively charged O1 atom via TS2IIL with a relative energy of 35.2 kcal/mol, 12272


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Figure 6. Calculated energy profile (pathway IIIL) at the M06/6-311G(d,p) level with schematic structures (bond distances are in Å and key geometrical parameters in transition states are highlighted by red color for visual clarity) for the [C5C1Im][NO3]-catalyzed reaction according to the mechanism proposed in this work. Blue values are the refined geometrical parameters and relative free energies using the SMD model at the M06-2X/6-311g(d,p) level.

optimized 3D geometries of several key structures of all calculated energy profiles are shown in Figure S8. Understanding the Kinetic Experimental Phenomena. According to the results above, we now try to understand the experimentally observed kinetic phenomena that the histaminecatalyzed reaction has faster initial rate but undergoes longer reaction time, while the [C5C1Im][NO3]-catalyzed reaction shows much slower initial rate but much less whole reaction time (Scheme 1). The different initial reaction rates can be rationalized by comparing the barriers involved in the two reactions. The estimated lowest barrier at the M06-2X/6311G(d,p) level is 20.7 kcal/mol for the histamine-catalyzed reaction (Figure 2) and 23.0 kcal/mol for the IL-catalyzed reaction (Figure 6). The difference of the free energy barriers for the two reactions is 2.3 kcal/mol, which is in good agreement with the corresponding quasi-experimental value, 1.8 kcal/mol, obtained from the Arrhenius equation based on the observed initial reaction rates (Scheme 1). The whole reaction time observed experimentally can be understood by considering characteristic features of the two reactions. For the histamine-catalyzed reaction, the amount of the histamine is much less than reactants, just like the situation in a conventional catalytic reaction, and thus, the reaction must undergo catalytic cycles over and over again. In contrast, for the [C5C1Im][NO3]-catalyzed reaction, the ionic liquid is excess, which plays a dual role of solvent and catalyst. In this situation, the reaction may be completed with only one cycle. Therefore, the whole reaction time for this catalytic reaction is shorter, although the reaction is slower initially. In other words, the observed different reactivity of the two reactions can be attributed to the different amount of catalyst.

followed by a proton-transfer process. First, the proton of the phenolic hydroxyl of α-tocopherol transfers to the negatively charged O atom of NO3− anion, the hydroxyl oxygen of αtocopherol nucleophilically attacks the C1 atom of succinic anhydride at the same time, and the cleavage of C1−O1 bond occurs simultaneously. The free energy barrier of this step is calculated to be 22.9 kcal/mol. The generated intermediate 2IIIL then can convert to a more stable intermediate 3IIIL by the extending of the opened succinic anhydride ring and the reposition of intermolecular H-bonds. Then 3IIIL evolves into the product precursor 4IIIL via the proton transfer from the HNO3 to O1. However, we failed to locate the assumed transition state for this proton transfer step. Figure S4 shows relaxed PES scan results of the proton transfer process from the HNO3 unit to the electronegative O1 in 3IIIL. It is found that the energy decreases monotonously with the proton migration, suggesting that this process is both kinetically and thermodynamically favorable. In addition, according to the results shown in Figures 2 and S2, the proton-transfer process from the protonated histamine to O1 atom involves either low barrier or no barrier; thus, the proton transfer from 3IIIL to 4IIIL is also considered to be very easy. Roles of the cation and anion of IL as well as the product for the IL-catalyzed reaction are discussed in the Supporting Information (Figures S5−S7). The main conclusions can be summarized as follows: (i) [C5C1Im]+ cation plays a substantial role, which attaches with the succinic anhydride via H-bonding interaction, delocalizing the accumulated negative charges on the oxygen atoms of succinic anhydride, (ii) [C5C1Im][NO3] shows the better catalytic performance than [C5C1Im]Cl due to the stronger alkalinity of NO3− anion than Cl− anion, and (iii) similar to the histamine-catalyzed reaction, the newly formed product can promote the IL-catalyzed reaction significantly. The 12273


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4. CONCLUSIONS With the aid of DFT calculations, an updated reaction model was proposed for the esterification of α-tocopherol with succinic anhydride catalyzed by the histidine derivative or the imidazolium-based ionic liquid. The reaction involves two elementary steps: the initial rate-determining nucleophilic substitution and the subsequent facile proton migration. In this reaction model, the histidine derivative, histamine, or the anion of the imidazolium ionic liquid is shown to play a decisive role, which acts a Brönsted base and abstracts the hydroxyl proton of α-tocopherol to favor the nucleophilic substitution of the hydroxyl oxygen of α-tocopherol on succinic anhydride. The calculated lowest barriers, 15.8 kcal/mol for the histaminecatalyzed reaction and 22.9 kcal/mol for the [C5C1Im][NO3]catalyzed reaction, are in good agreement with the observed distinctly different initial rates for the two reactions, 10.8 ± 0.2 vs 0.69 ± 0.12 (10−2 min−1). The very different reaction times, 9 h for the histamine-catalyzed reaction and 1.5 h for the [C5C1Im][NO3]-catalyzed reaction, is attributed to the different amount of catalyst. The reaction in the ionic liquid features an excess catalyst, making the reaction time is much shorter than that of the histamine-catalyzed reaction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02102. Calculated results and Cartesian coordinates of optimized intermediates and transition states with the imaginary vibrational frequencies (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86-531-88365833. Fax: +86-531-88564464. ORCID

Yaru Jing: 0000-0002-9025-9360 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was jointly supported by National Natural Science Foundation of China (No. 21433006, 21773139) and Shandong Provincial Natural Science Foundation of Shandong, China (No. 2014ZRE27295).

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NOTE ADDED AFTER ASAP PUBLICATION Structure corrections were made in the TOC/Abstract graphic, Schemes 1 and 2, and Figures 1, 2, 4, 5, and 6 in the version reposted on November 21, 2017.

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