When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study

Dec 7, 2016 - The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the a...
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When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study on the Mechanistic Preference in Ru-PNNH Pincer Complex Catalyzed Dehydrogenative Coupling Reaction Cheng Hou, Jingxing Jiang, Yinwu Li, Cunyuan Zhao,* and Zhuofeng Ke* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: Metal ligand cooperation (MLC) plays an important role in the development of homogeneous catalysts. Two major MLC modes have generally been proposed, known as the M-L bond mode and the (de)aromatization mode. To reveal the role of the dual potential functional sites on the MLC process, we present a detailed mechanistic study on a noveldesigned Ru-PNNH complex possessing dual potential MLC functional sites for the M-L bond mode and the (de)aromatization mode, respectively. Our results indicate that the Ru-PNNH complex prefers the M-L mode exclusively during different stages of the catalytic cycle. The unusual double deprotonation process and the mechanistic preference are rationalized. The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the amido group. The origin of the unexpected exclusive mechanistic preference on the M-L bond mechanism is due to the conjugation effect of the amido group, which stabilizes the dearomatized complex and diminishes the driving force of the (de)aromatization mode. This study highlights the pivotal role of the ligand’s electronic effect on the MLC mechanism and should provide valuable information for the development of highly efficient bifunctional catalysts. KEYWORDS: metal ligand cooperation, ruthenium, steric effect, conjugation effect, bifunctional

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INTRODUCTION The desire of developing efficient synthetic methods and the high concern of green chemistry encourage chemists to design novel catalysts with high activity and atom economy under environmentally benign conditions.1 Metal ligand cooperation (MLC) catalysis is one of the most promising strategies for catalyst design in recent years.2 MLC catalysis refers to the catalytic mode in which both the metal center and the ligand of the catalyst cooperate in bond formation and breaking processes. The catalyst which operates via MLC catalysis is usually referred to as a bifunctional catalyst.2c Generally, these MLC catalysts can be categorized into two major modes: the M-L bond mode (Figure 1A) and the (de)aromatization mode (Figure 1B).2c For the M-L bond mode, metal-amine/amide bifunctional systems are the most widely studied, for example, the well-known Noyori catalyst.3 The other mode is through the reversible aromatization−dearomatization process on the pincer ligand (Figure 1 B). With the aid of base, the CH2 group on the ortho-position of the pyridine is deprotonated and the complex loses its aromaticity. Then, the dearomatized complex can promote chemical reactions using the thermodynamic restoring force. This mode was first discovered by Milstein et al., and a family of catalysts which perform the (de)aromatization mechanism has been designed.2c,e,f,4 Various © 2016 American Chemical Society

Figure 1. Comparison of two MLC modes: (A) MLC via the M-L mode; (B) MLC via the (de)aromatization mode.

chemical reactions using this MLC (de)aromatization mode were developed, such as bond activation, 5 (de)hydrogenation,2e,4a,6 dehydrogenative coupling,7 and even energy related reactions.4b Along with the experimental developReceived: September 1, 2016 Revised: November 30, 2016 Published: December 7, 2016 786

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Figure 2. Unusual double deprotonation process of the Ru-PNNH system. PNNH designates 2-(di-tert-butylphosphinomethyl)-6-(tertbutylaminomethyl)pyridine.

the unusual N-arm double deprotonation lead to during the catalytic process? Therefore, we undertake a theoretical study on the Ru-PNNH system to answer these questions raised above. We expect this study can inspire future MLC catalyst design.

ments, theoretical studies also emerged and provided useful information on the mechanistic details. Wang et al. conducted systematic mechanistic studies on the PNN pincer complexes, and they pointed out the bifunctional double hydrogen transfer (BDHT) mechanism is more favored than the inner-sphere βelimination mechanism in the dehydrogenative coupling reaction.8 Hall et al. summarized different catalysts’ behaviors when operating the (de)aromatization mechanism.9 Yang et al. performed systematic studies on both the aliphatic and aromatized pincer-type complexes, which catalyze the (de)hydrogenation reaction and dehydrogenative coupling reaction.10 Our group has also carried out computational studies on the unique inner-sphere mechanistic preference of the cobalt pincer type catalyst developed by Hanson et al. and the ruthenium pincer complex developed by Szymczak et al.11 More recently, a general mechanistic picture for a Lewis acidic bifunctional catalyst has also been proposed by us.12 These theoretical studies unfold the diversity of metal ligand cooperation catalysis. As both the M-L mode and the (de)aromatization mode are effective strategies for preparing a bifunctional catalyst, a question naturally arises: what will happen if the catalyst has potential for two MLC modes? Recently, Milstein et al. has prepared a new pincer type catalyst, which combines two MLC modes into one catalyst.7a The purpose of this design strategy is to let the catalyst choose the appropriate MLC mode when confronting different catalytic stages. The catalyst can efficiently catalyze acceptorless dehydrogenative coupling of a primary alcohol and hydrogenation of esters under mild conditions. Structurally, the [RuII(PNNH) (CO)(H)(Cl)] pincer complex possesses two arms. The arm with the phosphorus ligand (tBu2P) is referred to as the P-arm, and the arm with the amine group (tBuNH) is referred to as the N-arm. An intriguing phenomenon observed in the experimental study is the double deprotonation of the pincer ligand with the aid of excess base, KOtBu, which means both the amine group and the CH2 on the N-arm are deprotonated (Figure 2). The structure of the doubly deprotonated complex [RuII(PNN2−) (CO)(H)]¯ confirmed by X-ray possesses two active sites, the N-site and the C-site, which correspond to the M-L bond MLC mode and the (de)aromatization MLC mode, respectively. This is significantly different from the previous catalysts, such as the Ru-PNN-Et2 complex,7b which is only deprotonated on the P-arm. Enhanced activity was observed compared with the catalyst with only one MLC active site. The novel structure of the complex and the high efficiency attract our attention. Although unprecedented features have also been achieved, important mechanistic details still remain unclear. (1) Which mechanistic scenario does the catalyst actually choose when encountering different reaction stages? (2) Why does the deprotonation occur on the N-arm instead of the P-arm in the Ru-PNNH catalyst? (3) And what consequence will

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COMPUTATIONAL DETAILS

All calculations were performed using the Gaussian 09 D.01 program.13 In order to select the most appropriate functional for our calculations, we compared a series of DFT functionals which are widely used in computational studies relevant to catalytic hydrogenation and dehydrogenation reactions, and eventually we employ M06-L14 as the functional for this study (Please see the Supporting Information page S2). Geometry optimizations were carried out at the M06-L/BSI level of theory. BSI designates the basis set combination of SDD15 for the metal atom and 6-31G(d, p) for nonmetal atoms. Frequency analysis calculations were performed to characterize the structures to be the minima (no imaginary frequency) or transition states (one imaginary frequency). Transition states were verified by intrinsic reaction coordinate (IRC) calculations. As the double deprotonated active species is a monoanionic ruthenium complex, the free energy results were further refined by calculating the single point energy at the M0614,16/BSII level of theory with a larger basis set at the M06-L/BSI geometries. BSII designates AUG-cc-pVTZ-PP17 for the metal atom and 6-311++G(d,p) for nonmetal atoms. The solvation effect of toluene was simulated by the SMD continuum solvent mode.18 The reliability of the computational method is checked by comparing with the experimental reaction enthalpy in the database (Please see page S2 in the Supporting Information).19 Important structures on the potential energy surfaces (PESs) are further optimized in SMD continuum solvent mode (Please see page S14 in the Supporting Information). The 3D optimized structural figures in this paper were displayed by the CYLview visualization program.20 NBO analysis was performed using NBO Version 3.1 as implemented in the Gaussian 09 package.21 The molecular orbitals and natural bond orbitals were depicted by the IboView software.22 Additional computational information and the Cartesian coordinates of the optimized structures are given in the Supporting Information.

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RESULTS Overall Catalytic Cycle. For the dehydrogenative coupling reaction, the whole catalytic cycle generally involves three major stages (Please see Scheme 1): (1) the dehydrogenation of primary alcohol, which furnishes the aldehyde; (2) the coupling of primary alcohol and aldehyde; (3) the final dehydrogenation of hemiacetal to yield ester as product. With two potential active sites, the mechanism becomes more complicated than those of traditional pincer-type catalysts. Since both the amido ligand and the CC bond on the N-arm are able to serve as functional site to promote the MLC process, two major possible reaction pathways are taken into consideration. The N-site pathway corresponds to the M-L bond mechanism, and the C-site pathway corresponds to the (de)aromatization mechanism. As for the nomenclature, the “IM” tag designates the intermediate during the catalytic process, whereas the TS tag designates the 787

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of the inner-sphere mechanism and find it less likely to occur due to the high activation free energy (nearly ∼40 kcal/mol; please see the Supporting Information, page S8). Therefore, the following discussion will mainly focus on the more feasible bifunctional mechanism during each stage. The dehydrogenation of the primary alcohol will follow the bifunctional catalytic manner, also known as the bifunctional double hydrogen transfer (BDHT) mechanism.8b In the BDHT mechanism, the metal will become the hydride acceptor and the bifunctional ligand will accept the incoming proton. In this case, the Ru-PNNH catalyst possesses two bifunctional sites on the ligand, which leads to two bifunctional mechanisms. We summarized the reaction pathways and calculated results in Scheme 2. The key intermediates and transition states of stage I are depicted in Figure 3. We first inspect the pathway via the N-site. The BDHT dehydrogenation process occurs via a stepwise manner. The proton transfers to the bifunctional site on the ligand and then the hydride is transferred to the Ru center. The activation free energy of TS1N-a is only 5.3 kcal/mol, which is lower than that of the proton shuttle type TS1N-PS-a (ΔG‡ = 7.7 kcal/mol). This dehydrogenation furnishes a transient intermediate IM-2N with high free energy (7.3 kcal/mol) on the PES. Although the free energy of IM-2N is slightly higher than that of TS1N-a, the electronic energy of TS1N-a is still 1.3 kcal/mol higher than that of IM-2N. The hydride is then transferred to the Ru center via TS1N-b (ΔG‡ = 14.1 kcal/mol) and gives the dihydride complex IM-3N (10.2 kcal/mol). As for the pathway via the C-site, the proton shuttle type transition state TS1C-PS-a (ΔG‡ = 12.1 kcal/mol) is significantly lower compared with TS1C-a of the direct BDHT mechanism (ΔG‡ = 22.9 kcal/mol). According to relevant studies,23 the proton shuttle can significantly lower the activation free energy by releasing the ring strain of the BDHT mechanism. This phenomenon is also observed in proton shuttle type TS1C-PS-a and TS1C-a. However, the large eight-membered ring can also cause unwanted constraints when the bifunctional site is close to

Scheme 1. Simplified Illustration of the Three Stages for the Dehydrogenative Coupling of a Primary Alcohol

transition state. For the reaction pathway schemes, the subscript “L” represents “N” or “C”, as the metal ligand cooperation are basically the same for both of the two active sites. The subscripts in each label in the free energy profile are classified as “N” or “C”, which designates the N-site pathway or C-site pathway, respectively. The doubly deprotonated complex [RuII(PNN2−)(CO)(H)]¯ is chosen as the starting point 1, which is negatively charged. The small organic substrates and products along the catalytic process are neutral. Initiated from starting point 1, all the species, including intermediates and transition states in different catalytic stages, are negatively charged. Stage I. The Dehydrogenation of BnOH. The catalytic cycle begins with the dehydrogenation of primary alcohol (Stage I). We choose benzyl alcohol (BnOH) as the substrate, which is used in the original experimental study.7a One issue that has to be noted is that the dehydrogenation process may also occur via an inner-sphere mechanism. We calculate the activation free energy

Scheme 2. Possible Dehydrogenation Pathways in the Dehydrogenation of BnOH (Stage I)a

a

The free energy results are reported in kcal/mol. BAL is short for benzaldehyde. The subscript “L” represents “N” or “C”. 788

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Figure 3. Important optimized intermediates and transition states for Ru-PNNH catalyst catalyzed dehydrogenation of BnOH. Bond lengths are in angstroms. C−H hydrogen atoms except for the bifunctional site are omitted for clarity.

Scheme 3. Alkoxide Mechanism for the Coupling Reaction of Aldehyde and Alcohol (Stage II)a

a

The free energies are reported in kcal/mol. BAL is short for benzaldehyde. The subscript “L” represents “N” or “C”.

energy of the interconversion via the three-membered ring TSiso1 is too high (ΔG‡ = 44.5 kcal/mol), and the transition state TSiso-PS-1 of the proton shuttle mechanism cannot even be located (Please see page S9 in the Supporting Information). The given intermediates IM-3N and IM-3C can regenerate active

the Ru center. The intermediate IM-2C is located very high (20.2 kcal/mol) on the PES. The subsequent hydride transfer step (ΔG‡ = 23.0 kcal/mol) yields the dihydride complex IM-3C (14.7 kcal/mol). We also investigated the possibility of the interconversion between IM-3N and IM-3C. The activation free 789

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ACS Catalysis Scheme 4. Aldehyde Coordination Mechanism for the Coupling Reaction of Aldehyde and Alcohol (Stage II)a

a

The free energies are reported in kcal/mol. BAL is short for benzaldehyde. The subscript “L” represents “N” or “C”.

proton shuttle TS3N-PS (ΔG‡ = 14.3 kcal/mol). However, the transition states via C-site pathways are much higher (TS3C-PS, 26.1 kcal/mol; TS3C, 35.3 kcal/mol). We also evaluate the possibility of the interconversion between IM-4N and IM-4C, and the transition state is infeasible (ΔG‡ = 49.9 kcal/mol). The high activation free energy forbids the isomerization of IM-4N and IM-4C (Please see page S9 in the Supporting Information). The given two intermediates IM-4N and IM-4C undergo the coupling reaction via a concerted manner, respectively. For the N-site pathway, the direct coupling TS4N is 20.3 kcal/mol in free energy. The proton-shuttle type TS4N-PS (ΔG‡ = 34.1 kcal/ mol) is much higher due to the steric effect. As for the C-site pathway, the direct coupling (TS4C: ΔG‡ = 32.0 kcal/mol) and proton shuttle coupling (TS4C-PS: ΔG‡ = 28.8 kcal/mol) are less plausible due to higher activation free energies. Another plausible coupling mechanism is also probed computationally. For this mechanism (Scheme 4), the aldehyde will first be activated by a Lewis acid Ru center to form IM4L-b. With the aid of the bifunctional site, the coupling will occur at the same time as deprotonation of the alcohol (TS4L-b). After the coupling step, the yielded metal alkoxide (IM4L-C) can undergo an isomerization reaction for the final dehydrogenation step. The proton shuttle type transition state is also taken into consideration. The reaction pathways and free energy profiles are shown in Scheme 4. As we can see, the coupling step via this mechanism is significantly lowered. The coordination of aldehyde on the metal center increases its electrophilicity, and the deprotonation by the bifunctional site increases the nucleophilicity of the alcohol. Therefore, the coupling reaction via TS4N-b is 19.2 kcal/mol in free energy. As for the bifunctional

species 1 by releasing a molecule of H2. Four possible transition states were evaluated for the regeneration of complex 1. The calculated results indicate the proton shuttle type TS2N-PS via the N-site (ΔG‡ = 21.4 kcal/mol) is more favored than the other three transition states in this case. This result can be rationalized by the constraints of the four-membered ring TS2N (ΔG‡ = 33.2 kcal/mol). The proton shuttle type TS2N-PS can release H2 without too much constraint, as the H2 is a small molecule. The high activation free energies of the pathways via the C-site (TS2C, 34.2 kcal/mol, and TS2C-PS, 39.1 kcal/mol) are understandable because a dearomatized complex is produced along these pathways. Therefore, we can conclude that the M-L bond mode prevails in stage I. Stage II. The Coupling of Aldehyde and Alcohol. During this stage, the aldehyde and the alcohol will couple to yield hemiacetal. First, we investigated the possibility of the direct coupling via the nonmetal catalyzed mechanism. The direct coupling mechanism via a 4-membered ring TScouple is 46.8 kcal/ mol in activation free energy. The alcohol assisted direct coupling (TScouple-PS) is 39.1 kcal/mol in activation free energy. The calculated result indicates that the coupling reaction without catalyst should be ruled out due to the high activation free energy (Please see page S10 in the Supporting Information). Therefore, two types of coupling are proposed and investigated. The first coupling mechanism consisted of two steps (Scheme 3): the O− H bond cleavage and the C−O bond coupling step. Similar to the discussions in stage I, four transition states are proposed for the cleavage of the O−H bond on two functional sites (with or without proton shuttle). For the N-site pathways, the direct cleavage TS3N (ΔG‡ = 15.5 kcal/mol) is slightly higher than the 790

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Figure 4. Important optimized intermediates and transition states for the coupling of aldehyde and alcohol. Bond lengths are in angstroms. C−H hydrogen atoms except the bifunctional site are omitted for clarity.

Figure 5. Important optimized intermediates and transition states for the dehydrogenation of hemiacetal. Bond lengths are in angstroms. C−H hydrogen atoms, except the bifunctional site, are omitted for clarity.

site preference, the coupling via this mechanism still chooses to occur through the N-site pathway, as the activation free energy barrier of the C-cite pathway is much higher (ΔG‡ = 29.8 kcal/ mol). After the coupling step, the formation of hemiacetal from IM-4N-C or IM-4C-C is found to be difficult (Please see Supporting Information page S12). Therefore, the intermediate

IM-4L-C (IM-4N-C or IM-4C-C) will undergo an isomerization to IM-5L (IM-5N or IM-5C) and then proceed to the final dehydrogenation reaction. The aldehyde coordination coupling mechanism is slightly more favorable (∼1 kcal/mol) than the alkoxide coupling mechanism. As for the mechanistic preference, the M-L bond mode is found to prevail again in both of the two 791

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ACS Catalysis Scheme 5. Mechanism of the Dehydrogenation of Hemiacetal (Stage III)a”

a

The free energies are reported in kcal/mol. BnBzO is short for benzyl benzoate. The subscript “L” represents “N”or “C.

Figure 6. Comparison of different deprotonation preferences in Ru-PNN-Et2 and Ru-PNNH catalysts.

coupling mechanisms. Some key intermediates and transition states of stage II are depicted in Figure 4 and Figure 5. Stage III. The Dehydrogenation of Hemiacetal. The dehydrogenation of hemiacetal is mechanistically similar to stage I, only with a different substrate (Scheme 5). For the M-L mode,

starting from a transient intermediate IM-5N (14.1 kcal/mol), the hydride is transferred to the Ru center via TS5N-b (ΔG‡ = 21.0 kcal/mol). As for the C-site pathway, the hydride transfer step has to overcome an activation free energy of 32.7 kcal/mol (TS5C-b). After the dehydrogenation of hemiacetal, the given 792

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ACS Catalysis Scheme 6. Tautomerization of the PNN-Et2 Ligand and PNNH Pincer Liganda

a

The free energies are reported in kcal/mol.

tization process by the back-donation from the CC π electron to the P−C σ* bond. In order to provide quantitative evidence, we performed the second order perturbation theory analysis using the natural bond orbital (NBO) method. The second order perturbation theory analysis can give the stabilization energy of the interaction between donor orbital and acceptor orbital. A larger E(2) energy corresponds to a greater interaction between two orbitals. With respect to P-arm deprotonation, the length of the P−C bond increases by 0.02 Å and the E(2) energy from the CC π bond to the C−P σ* bond is 5.56 kcal/mol. As for the Narm deprotonation situation, the length of the C−N bond increases by 0.01 Å and the second perturbation energy (from the CC π bond to the C−N σ* bond) is only 1.27 kcal/mol. The relatively stronger π-acidic character of P(tBu)2 may account for the thermodynamic favor of the P-arm deprotonation. The unusual N-arm deprotonation preference of the RuPNNH system is due to similar important factors. The first one is that the steric hindrance on the N-arm is obviously smaller for Ru-PNNH catalyst. With only one bulky tert-butyl group, the CH2 on the N-arm is more exposed when encountering base. In contrast, the P-arm possesses two tert-butyl groups, which can lead to a high activation barrier during the deprotonation, as the base (KOtBu) also has a bulky group. The second reason is the conjugation effect of the amido group. After deprotonation of CH2, the N atom of the amido becomes sp2 hybridized, as the amido is bonded to a conjugation system. Although the dearomatization process destabilizes the complex, the electron delocalization on the pincer plane is enhanced with a larger conjugated system on the N-arm. Thereby, the complex is stabilized by the conjugation effect of the sp2 amido group. We can directly visualize the difference of the conjugation effect between the N-arm deprotonation and P-arm deprotonation processes by viewing the Kohn−Sham molecular orbital (MO) of the complexes (Please see Figure 6). After deprotonation on the N-arm, the MO is highly delocalized on the dearomatized ring, the CC double bond, and the amido group. However, the MO of P-arm deprotonation is mainly localized on the CC double bond and the dearomatized ring. Therefore, the N-arm deprotonation leads to a larger conjugated system. The origin of thermodynamic favor of N-arm CH2 deprotonation is mainly attributed to the conjugation effect of the amido group. The Influence of the Amido Conjugation Effect on the (De)aromatization Mechanism in Ru-PNNH Catalyst. In order to illuminate the influence of the amido conjugation effect

IM-3N (9.8 kcal/mol) and IM-3C (14.3 kcal/mol) complexes lead to the regeneration of active species 1. The result is similar to the regeneration process in stage I; the hydrogen release process proceeds via transition state TS2N-PS. After the catalyst regenerated, the catalytic cycle restarted. As two coupling mechanisms are close to each other in activation free energy, we also summarized the free energy profile starting with the dehydrogenation of the hemiacetal (Please see page S13 in the Supporting Information). Some key intermediates and transition states are depicted in Figure 5. With the detailed exploration of the catalytic mechanisms, we can see the DFT results indicate the catalytic cycle via the M-L bond mode is more favorable than the (de)aromatization mode in each catalytic stage. The proton shuttle mechanism does not guarantee to lower the activation free energy, as the addition of an extra molecule of alcohol is disfavored in aspects of entropy and constraint. As H2 can be seen as an insoluble gas in most solvents, it is important to note that the H2 release process plays an important role as a driving force in the real catalytic process. We roughly estimate the influence of H2 release, which leads to a more favorable process thermodynamically (Please see page S17 in the Supporting Information). The following Discussion section will focus on the rationalization of the calculated results and answer the questions raised in the Introduction section.

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DISCUSSION The Rationalization of the N-Arm Double Deprotonation in Ru-PNNH Catalyst. In order to understand the unusual N-arm deprotonation of the Ru-PNNH system, we first discuss the P-arm deprotonation preference of “classical” Ru-PNN-Et2 catalyst. The first reason is that the steric hindrance on the N-arm is larger than the P-arm in Ru-PNN-Et2 catalyst (Please see Figure 6). A crucial feature of the PNN-Et2 pincer ligand is that the C−N bond on the N-arm is only 1.48 Å and the C−P bond on the P-arm is 1.86 Å. Therefore, the CH2 group of the N-arm is closer to the bulky NEt2 group. As the C−N bond is easy to rotate, the bulky NEt2 group can shield the CH2 and thereby prevents the CH2 from being deprotonated. In contrast, with a longer P−C bond, the P(tBu)2 group does not shield the CH2 much. Therefore, the base may choose to promote the deprotonation on the P-arm. This steric effect is responsible for the kinetically favored P-arm deprotonation. The second reason for the P-arm deprotonation is that the P(tBu)2 has moderate π-acidic character,24 which can stabilize the dearoma793

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Figure 7. Different steric effects in the coupling step catalyzed by two pincer complexes.

system and stabilizes the active intermediate indirectly. We further calculate the coupling step catalyzed by two complexes. The calculated results are consistent with our assumption. For two possible coupling mechanisms, the alkoxide mechanism and the aldehyde coordination mechanism, the coupling reaction via the N-arm is 20.3 and 19.2 kcal/mol for Ru-PNNH catalyst, respectively. On the contrary, the coupling reaction catalyzed via the P-arm of Ru-PNN-Et2 catalyst is 35.4 and 29.0 kcal/mol, respectively. The steric difference can be directly seen from the structures of the transition state. The advantage of the doubly deprotonated species in the Ru-PNNH system is that it avoids the unwanted steric hindrance during the metal ligand cooperation process and stabilizes the active species.

on the (de)aromatization mechanism, we used the tautomerization reaction of the pincer ligand to estimate the dehydrogenation reaction. We also involved the “classical” [RuII(PNN¯Et2)(CO)(H)] pincer complex (labeled as Et-1 in Scheme 6) into this discussion as a comparison (Please see Scheme 6). For the ligand in the [RuII(PNN2−) (CO)(H)]¯ complex (labeled as 1 in Scheme 6), the free energy change of N-arm deprotonation is positive (ΔG = 5.5 kcal/mol), which indicates the aromatized pincer ligand is not favorable thermodynamically. On the other hand, the tautomerization free energy is −7.7 kcal/mol in the PNN-Et2 ligand when the pincer ligand is restored to the aromatic state. We further calculated the dehydrogenation mechanism of alcohol catalyzed by the classical Ru-PNN-Et2 system (Please see page S11 in the Supporting Information). The DFT results indicate the (de)aromatization mechanism operates well with a lower activation free energy (ΔG‡ = 20.1 kcal/mol) in the Ru-PNN-Et2 catalytic system. More importantly, the free energy result of the final product (Et-IM3C: ΔG = 3.6 kcal/mol) is much lower than IM3C (ΔG = 14.7 kcal/mol). By comparing the tautomerization free energy of the pincer ligands and the reaction free energy change in the dehydrogenation process, we can see the driving force of the (de)aromatization mechanism is indeed weakened, which can account for the disfavor of the (de)aromatization mechanism in the Ru-PNNH system. Origin of the of Enhanced Activity Ru-PNNH Catalyst. Compared with the classical Ru-PNN-Et2 catalyst,7b the RuPNNH catalyst can catalyze the dehydrogenative coupling reaction under exceedingly mild conditions (only 35 °C), which suggested the activity of the Ru-PNNH catalyst was significantly enhanced. The advantage of Ru-PNNH can be rationalized from the aspect of steric hindrance. As for the previous Ru-PNN-Et2 catalyst, the metal ligand cooperation process operates on the Parm. The phosphorus ligand possesses two pendent tbutyl groups with a rigid conformation. Therefore, the steric effect on the Parm can hinder the metal ligand cooperation process, especially when encountering multiple substrates. For Ru-PNNH catalyst, the steric effect on the N-arm is clearly less (Please see Figure 7). In addition, even though the driving force of the (de)aromatization mechanism of the N-arm is sacrificed, the deprotonation of CH2 on the N-arm leads to a conjugation

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CONCLUSION In summary, we have performed a systematic mechanistic study on the dehydrogenative coupling of a primary alcohol reaction catalyzed by the Ru-PNNH catalyst. The coupling reaction consists of three major steps, including the dehydrogenation of primary alcohol (Stage I), the coupling of aldehyde and alcohol (Stage II), and the final dehydrogenation to generate the ester as product (Stage III). The DFT results indicate that the catalytic coupling reaction proceeds through the MLC mechanism via the M-L bond mode exclusively in all three stages, even though M-L bond mode and (de)aromatization mode are both presented in the active species. By comparing with the classical Ru-PNN-Et2 catalyst, the unusual preference of CH2 deprotonation on the N-arm of the Ru-PNNH catalyst is rationalized, which can be mainly attributed to two reasons: (1) the steric hindrance of the N-arm is smaller than the P-arm; (2) the conjugation effect of the amido group stabilizes the dearomatized complex. The unexpected exclusive mechanistic preference of the Ru-PNNH catalyst on the M-L bond mode is illuminated. Due to the conjugation of amido group, the Ru-PNNH catalyst gains extra stabilization energy during the deprotonation process. Therefore, the driving force of the (de)aromatization mechanism is diminished in the RuPNNH catalyst, which leads to the exclusive mechanistic preference of the M-L bond mode. The M-L mechanism avoids the unwanted steric effect during the MLC process, and this 794

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Research Article

ACS Catalysis

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accounts for the enhanced activity of the Ru-PNNH catalyst after the double deprotonation. This study reveals that both the steric hindrance and the π-conjugation effect of the pincer ligand can have tremendous impact on the catalytic mechanism and activity. We believe these mechanistic insights are inspiring for future bifunctional catalyst design.

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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/acscatal.6b02505. The comparison of different functionals, the absolute energies of all optimized structures, the inner-sphere mechanism, the interconversion of the intermediate, the coupling reaction via a nonmetal catalyzed mechanism, the dehydrogenative coupling catalyzed by the Ru-PNN-Et2 complex, the release of hemiacetal after the coupling mechanism, the dehydrogenation of hemiacetal, the free energy profile of reoptimization in the SMD model, the comparison of the substitution effect, the calculation of the double deprotonation process, the influence of H2 release, and Cartesian coordinates of all optimized structures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Zhuofeng Ke: 0000-0001-9064-8051 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21373277, 21473261, and 21673301), the Guangdong Provincial Natural Science Foundation (No. 2015A030313185), and the Guangdong Natural Science Funds for Distinguished Young Scholars (No. 2015A030306027). Computing facilities were supported in part by the Guangdong Province Key Laboratory of Computational Science and the Guangdong Province Computational Science Innovative Research Team, the Joint Funds of NSFCGuangdong for Supercomputing Applications, and the National Supercomputing Center in Guangzhou.

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REFERENCES

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DOI: 10.1021/acscatal.6b02505 ACS Catal. 2017, 7, 786−795