DFT Study of Acceptorless Alcohol Dehydrogenation Mediated by

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DFT Study of Acceptorless Alcohol Dehydrogenation Mediated by Ruthenium Pincer Complexes: Ligand Tautomerization Governing Metal Ligand Cooperation Cheng Hou, Zhihan Zhang, Cunyuan Zhao,* and Zhuofeng Ke* MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China S Supporting Information *

ABSTRACT: Metal ligand cooperation (MLC) catalysis is a popular strategy to design highly efficient transition metal catalysts. In this presented theoretical study, we describe the key governing factor in the MLC mechanism, with the Szymczak’s NNN-Ru and the Milstein’s PNN-Ru complexes as two representative catalysts. Both the outer-sphere and inner-sphere mechanisms were investigated and compared. Our calculated result indicates that the PNN-Ru pincer catalyst will be restored to aromatic state during the catalytic cycle, which can be considered as the driving force to promote the MLC process. On the contrary, for the NNN-Ru catalyst, the MLC mechanism leads to an unfavored tautomerization in the pincer ligand, which explains the failure of the MLC mechanism in this system. Therefore, the strength of the driving force provided by the pincer ligand actually represents a prerequisite factor for MLC. Spectator ligands such as CO, PPh3, and hydride are important to ensure the catalyst follow a certain mechanism as well. We also evaluate the driving force of various bifunctional ligands by computational methods. Some proposed pincer ligands may have the potential to be the new pincer catalysts candidates. The presented study is expected to offer new insights for MLC catalysis and provide useful guideline for future catalyst design.

1. INTRODUCTION Metal ligand cooperation (MLC) catalysis is an intriguing strategy in the development of homogeneous transition metal catalysts.1 The definition of MLC refers to the catalytic mode in which both the metal center and the ligand participate in the bond formation and bond breaking processes. This interesting feature of MLC is quite different from the “classical” catalytic mode, in which the reaction mostly occurs on the metal center. Enhanced activity was observed when introducing the ancillary ligand to catalysts.2 The MLC catalysts can be divided into two main categories (Figure 1) according to different ligand cooperation modes.1c One is the MLC through metal−ligand bond. The well-known Noyori catalyst is a representative paradigm of the MLC catalysts activated by M−L bond (Figure 1A).2a−c Important catalysts can be classified into this kind,3 including the recent reports by Beller et al.3b,g and Morris et al.3c,d,i New type of ligands and using earth-abundant metals as the center are desired in the development of MLC catalysts. Recently, the M−L bond activation mode has been expanded to the ligands possessing a Lewis acid character.4 Another activation mode is known as the aromatization/dearomatization process. This mode is driven by the thermodynamic restoring force from a disrupted conjugated pincer ligand. During the catalytic process, the CC bond will become a proton accepting position, and the metal center will be responsible for accepting other incoming compound such as hydride (Figure © XXXX American Chemical Society

Figure 1. Two major types of metal ligand cooperation for H2 activation: (A) MLC via M−L mode; (B) MLC via (de)aromatization mode. Other ligands were omitted for clarity.

1B). Utilizing lutidine or picoline as the pincer ligand platform, Milstein et al. developed a series of catalysts,1a,c,e,g which can catalyze various reactions including bond activation, dehydrogenative coupling reactions, and catalytic hydrogenation/ dehydrogenation reactions. These PNP−M and PNN−M catalysts (M designates metal centers) present a unique type Received: March 24, 2016

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DOI: 10.1021/acs.inorgchem.6b00723 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Different Mechanistic Preferences of Two Ruthenium Pincer Catalysts

of MLC catalysts due to its high activity and this unique hydrogenation/dehydrogenation process. Many other catalytic systems based on the aromatization/ dearomatization process have been developed. Huang et al. developed a series of ruthenium complex based on the aminopyridine or imine ligands, which can mediate the transfer hydrogenation reaction and catalytic hydrogenation/dehydrogenation reaction effectively.1b,5 On the one hand, with a variety of pincer ligands, new metal center based on the earthabundant elements, such as Fe,3a,e,f,6 Ni,7 Co,3j,k,8 and Al,1d,9 has also become a new trend in developing MLC catalysts. On the other hand, computational studies have also been growing along with new catalysts developing.5a,10 Li and Wang et al. conducted computational studies of the PNP catalyst developed by Milstein et al. and verified that the bifunctional double hydrogen transfer mechanism is more favored than the innersphere β-H elimination mechanism, which has been supported by recent computational studies and experimental studies.10h,11 Yang performed a series of computational studies on the dehydrogenation reaction catalyzed by the pincer complexes and compared two activation modes, including the M−L bond mode and the aromatization/dearomatization mode.12 The hard work devoted by both experimental and theoretical chemists keep this area flourishing and thriving. However, with more and more catalysts developed, it is found that introducing the bifunctional ligand to the complex does not guarantee an MLC mechanism for the catalysis. In 2011, Morris et al. investigated the ketone hydrogenation catalyzed by ruthenium complex with a primary amine donor as the bifunctional ligand.13 Both computational and experimental studies suggest the outer-sphere MLC mechanism is not favored, which is mainly attributed to the diminished activity of the hydride by the metal center. The kinetics and isotopic studies support the conclusion, and this system is considered a rare example in which the catalyst with N−H group failed to undergo the MLC mechanism.13 Hanson et al. also reported that MLC mechanism is not necessary in the PNP-Co(II) system for transfer hydrogenation,3j which is observed in the later study by Jones et al.14 The later computational study performed by us supported their conclusion and illuminated the origin of the mechanistic preference: the d7 CoII center needs to overcome high distortion energy due to ligand field change in the transition state when operating outer-sphere MLC mechanism.15

Very recently, Szymczak et al. performed a detailed mechanistic study on the mechanistic preference between inner-sphere non-MLC mechanism and outer-sphere MLC mechanism.16 Using various experimental methods such as isotopic labeling and kinetics experiments, the dehydrogenation reaction is verified to occur via the inner-sphere β-H elimination mechanism, which is quite unique as compared with former studies (Scheme 1).10c,d,f,17 The elegant mechanistic study performed by Szymczak et al. and the interesting result of the mechanistic preference attracted our attention. Some questions naturally arise: Why does the NNN-Ru catalyst prefer the inner-sphere β-H elimination mechanism? What are the factors that govern a catalyst’s mechanistic preference? What is the origin for the mechanistic divergence between the NNN-Ru catalyst and other MLC catalysts? With these questions in mind, we performed theoretical study on the acceptorless alcohol dehydrogenation (AAD) reaction catalyzed by pincer ruthenium catalysts. In this present study, we intend to answer the questions raised above and provide an overview of the role of ligand in design and development of MLC catalysts.

2. COMPUTATIONAL DETAILS Although other spectator ligands of the catalysts will not be involved in bond forming/breaking process, they are important in the aspect of steric effect and electronic effect. Therefore, we chose to use the real catalysts rather than a simplified model to study the mechanism of pincer ruthenium catalyzed AAD reactions. All calculations were performed using Gaussian 09 D.01. program.18 To select the most appropriate functional for our calculation, we compared a series of functionals that are widely used in the computational studies relevant to catalytic hydrogenation and dehydrogenation reactions, and eventually we chose M06-L19 functional for this study (please see the Supporting Information, page S2). Geometry optimizations were performed at the M06-L/BSI level (BSI designates the basis set combination of SDD20 for 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. With M06-L/BSI geometries, the energy results were further refined by calculating single point energy at the M06-L/BSII level with larger basis set (BSII designates SDD for metal atom and 6-311++ G (d, p) for B


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Inorganic Chemistry nonmetal atoms). The solvation effect of toluene was simulated by the SMD continuum solvent mode.21 Since the calculation is based on ideal gas-phase model, the ignorance of the suppression effect of solvent on the translational and rotational freedoms of reactants will cause the overestimation of the entropic contribution. Therefore, we adopt the MHP scheme proposed by Martin, Hay, and Pratt, which is an approximate method to calculate entropy more accurately.22 This approach has been successfully applied to various catalytic systems and is found to produce reasonable results.5a,10a,c−e,j,k,23 On the basis of this method, a correction of 4.3 kcal/mol is applied when a component of the reaction changes (i.e., a reaction from m components to n components, the correction is (n − m) × 4.3 kcal/mol). For the evaluation of the driving force of pincer ligands in Figure 11, the reaction free energies are reported in kilocalories per mole at the M06-2X/BSII/SMD//M06-2X/BSI level of theory.24 The NICS(0) calculation is performed to evaluate the change in aromaticity during the ligand tautomerization reaction.25 The dummy atom is placed in the geometrical center in the multiring systems; the coordinate of the dummy atom is given by the Multiwfn program.26 The three-dimensional optimized structure figures in this paper were displayed by CYLview visualization program.24 Additional computational information and the Cartesian coordinates of the optimized structure are given in the Supporting Information.

Figure 2. The transition state A-TSiso-1 is located with the activation free energy of 20.4 kcal/mol. The isomer Aisomer-1-a

3. COMPUTATIONAL RESULTS 3.1. The Mechanistic Preference of N,N,N-Amide Ruthenium(II) Hydride Complex. Before exploring the detailed mechanism of the dehydrogenation process, it is necessary to discuss the possibility of the isomerization processes of the hydride complex, since the position of the hydride may have important influence on the catalysis and its mechanism. The proposed isomerization processes are shown in Scheme 2, and the optimized structures are depicted in

with the hydride at the axial position is lower in free energy as compared with A-1 (ΔG = −6.2 kcal/mol). The lower stability of A-1 is mainly attributed to the trans-influence of anionic nitrogen atom on the trans-site of the hydride. The transinfluence is revealed in the optimized structures (Figure 2). The Ru−N bond between Ru center and the anionic nitrogen atom is shortened by nearly 0.18 Å after the isomerization from A-1 to Aisomer-1-a. After the isomerization, the coordination of the phenethyl alcohol (Aisomer-2-a) is found to be very difficult with the free energy uphill to 14.7 kcal/mol. The following hydride− proton coupling transition state starts from Aisomer-2-a has a free energy of 31.5 kcal/mol (please see the Supporting Information, page S8). Accordingly, the steric effect between the substrate and the methyl group on the pincer ligand forbids the possibility of Aisomer-1-a as a starting point of the catalytic cycle. The further isomerization of the phosphine ligand is obviously impossible due to the strong steric effect between the pendent methyl group and PPh3 ligand. The transition state of the isomerization cannot even be located by calculations. The product of the isomerization process, Aisomer-1-b, is also uphill for 14.7 kcal/mol relative to Aisomer-1-a. Therefore, we can conclude that the following mechanistic exploration starts with the metal hydride complex A-1. With the initial understanding of the isomerization process, we started to explore the detailed mechanism using A-1 as the starting point. Two major scenarios were investigated, the outer-sphere MLC mechanism and the inner-sphere non-MLC mechanism, which were also proposed and discussed in the experimental mechanistic studies.16 The proposed pathways are shown in Scheme 3, and the optimized structures are depicted in Figure 3. The MLC mechanism refers to the metal ligand cooperation mechanism that the hydrogen atom of the αcarbon atom of phenethyl alcohol (PEA) will be transferred to ruthenium center as hydride. Meanwhile, the hydroxyl group of PEA will transfer the proton to the nitrogen atom of imine. The direct MLC mechanism via A-TS1O has an activation free energy of 41.5 kcal/mol. Previous relevant mechanistic studies

Figure 2. Optimized important intermediates and transition states for isomerization processes. Bond lengths are in angstroms. All C−H hydrogen atoms are omitted for clarity.

Scheme 2. Possible Isomerization Processes of the NNNRu(II) Catalysta

a

The free energies are reported in kilocalories per mole. C


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Inorganic Chemistry

Scheme 3. Outer-Sphere Metal Ligand Cooperation Mechanism and the Inner-Sphere Non-Metal Ligand Cooperation Mechanism for the NNN-Ru Catalysta

a

The relative free energies are given in kilocalories per mole. For clarity, selected parts of the ligand are shown in a light color.

Figure 3. Optimized important intermediates and transition states for the NNN-Ru(II) hydride complex. Bond lengths are in angstroms. All C−H hydrogen atoms are omitted for clarity.

have proposed the “proton shuttle” type of transition state, which involves a molecule of alcohol as a “proton bridge”. The proton shuttle transition state can significantly lower the activation free energy.5a,10d Herein, we also located the proton shuttle type of mechanism A-TS1O-PS (ΔG‡ = 25.5 kcal/mol), which is much lower than the direct MLC mechanism. The product A-2O is located with the free energy of 14.0 kcal/mol. The following process to generate the H2 is found to occur similarly to the hydride and proton transfer step. The direct

formation of H2 via A-TS2O is nearly 40.6 kcal/mol in free energy. The proton shuttle type of transition state, A-TS2O-PS, has a lower activation free energy of 25.7 kcal/mol. After the H2 release, the catalyst is regenerated, and the catalytic cycle restarts. As for the inner-sphere non-MLC mechanism, the substrate PEA will first coordinate to the ruthenium center (A-2I: ΔG = −1.1 kcal/mol). Since the hydride is on the trans-site of the anionic nitrogen atom, the metal hydride is very active and can D


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Inorganic Chemistry Scheme 4. Possible Isomerization Processes of the PNN-Ru Catalysta

a

The relative free energies are given in kilocalories per mole.

preference of NNN-Ru catalyst, we revisit this “classical” PNN-Ru system using the same substrate (PEA) to make a fair comparison with NNN-Ru complex, even though PNN-Ru has not been reported as for dehydrogenation reaction of secondary alcohols. Before the discussion of the overall mechanisms, it is necessary to discuss the possibility of isomerization. This issue should not be neglected when evaluating possible starting active species for the catalysis. We want to stress this issue because if the isomerization is easy to occur, it may lead to various coordination states for the catalyst, which can further result in potential active species and then give rise to other plausible mechanisms. For example, if the CO ligand is easy to isomerize, the hydride on the trans-position of the vacant site (marked in blue) may function as an active hydride to couple with the proton from the alcohol and release the H2. The proposed isomerization pathways are shown in Scheme 4, and the optimized structures are depicted in Figure 4. The carbonyl ligand can isomerize to the perpendicular position of the pincer planar via B-TSiso-1. However, this isomerization is found to be very difficult (ΔG‡ = 47.1 kcal/mol). The high energy penalty should be attributed to two major reasons. (1) The back-donation effect between the metal center and CO results in a high energy penalty for the isomerization via BTSiso-1. (2) The hydride on the trans-site possesses a strong trans-influence character. During the isomerization, the CO compound will move to the trans-site of metal hydride. The isomer Bisomer-1-a is highly unstable (ΔG = 27.1 kcal/mol). The following isomerization via B-TSiso-2 also has very high activation free energy (ΔG‡ = 31.9 kcal/mol). Therefore, the possibility of Bisomer-1-a (ΔG = 27.1 kcal/mol) and Bisomer-1-b (ΔG = 6.5 kcal/mol) as the starting point for the catalytic cycle can be ruled out. The dissociation of CO ligand was also considered. The scan calculation indicated the dissociation of CO is difficult to occur for PNN-Ru catalyst (please see Supporting Information page S11). Following mechanistic discussion will only focus on B-1 as the starting point.

directly couple with the proton of alcohol to generate H2. On the one hand, the H2 generation process via A-TS1I has an activation free energy of ΔG‡ = 11.3 kcal/mol. On the other hand, the proton shuttle type of transition state A-TS1I-PS is only 6.2 kcal/mol in free energy. After the quick release of H2 by the transient metal−H2 complex A-3I (ΔG = 1.9 kcal/mol), the Ru−alkoxide complex will undergo β-hydride elimination to give the final product acetophenone (ACP). The formation of metal alkoxide complex A-4I (ΔG = −7.7 kcal/mol) is found to be an exothermic process, since the transient metal−H2 complex is unstable with the trans-influence of the anionic nitrogen atom. The β-hydride elimination via A-TS2I (ΔG‡ = 11.5 kcal/mol) will yield the final product ACP. After the coordinated ACP releases, the catalyst A-1 is regenerated, and the catalytic cycle restarts. For the inner-sphere scenario, Suresh et al. also proposed another mechanism, in which the Ru(II) center was reduced to Ru(0).10g On the one hand, this mechanism is not accessible due to the high energy penalty for metal reduction (please see page S9 in the Supporting Information). On the other hand, the inner-sphere β-hydride elimination can also proceed in a ligand rotation manner. This pathway is also found not accessible for NNN-Ru system due to high distortion energy. We addressed these issues in the Supporting Information (please see pages S9 and S10). Our current calculated result is in good agreement with the conclusion by the experimental mechanistic studies,16 which suggests the inner-sphere βhydride elimination pathway is more plausible. The origin of the mechanistic preference will be explained in the Discussion Section. 3.2. The Mechanistic Preference of PNN-Ru(II) Catalyst. The “classical” PNN-Ru catalyst has been studied extensively both by computational methods and experimental methods.1a,g Li and Wang et al. verified the outer-sphere mechanism overwhelmed the inner-sphere elimination mechanism.10i,k Following studies on relative catalysts supported this conclusion.10d,f To reveal the origin of the mechanistic E


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Inorganic Chemistry

the NEt2 arm, the β-hydride elimination occurs to yield the product ACP via transition state B-TS3I (ΔG‡ = 17.0 kcal/ mol). After the dissociation of the coordinated ACP, the hydride is generally assumed to isomerize to the perpendicular position of the pincer ligand, and the NEt2 arm will coordinate back to the metal center. The generated metal hydride then couples with the proton on the benzyl group to yield H2 and regenerates the catalyst via transition state B-TS2O-PS. Current results indicate the outer-sphere mechanism is more favorable as compared with the inner-sphere mechanism. In the next section, we will discuss the different mechanistic preferences between the NNN-Ru system and PNN-Ru system.

4. DISCUSSION 4.1. Why Does the NNN-Ru System Fail in Metal− Ligand Cooperation? First of all, we will explain why the NNN-Ru system failed in MLC. For the MLC mechanism, the electron pair of the coordinated nitrogen anion will be transferred to form a CN bond and then push the electron pair of the CC bond or CN bond to accept the incoming proton. Although the mechanisms in the two systems are basically the same, the calculated activation free energy of the first dehydrogenation process is obviously different. For the NNN-Ru system (Scheme 6), the MLC mechanism (A-TS1OPS) needs to overcome an activation free energy of 25.5 kcal/ mol. In contrast, for the PNN-Ru system, the activation free energy of the first dehydrogenation step (B-TS1O-PS) is only 11.6 kcal/mol. As for the intermediate of MLC mechanism, the PNN-Ru system (ΔG = −6.2 kcal/mol) is also much more stable than NNN-Ru system (ΔG = 14.0 kcal/mol). The distinct difference behavior of the MLC mechanism for these two geometrically similar pincer catalysts can be originated from the tautomerization process of the neutralized pincer ligand.27 The calculated result of tautomerization is shown in Scheme 7. As we can see, the tautomerization of the NNN ligand in the NNN-Ru complex is uphill by 19.1 kcal/ mol. In contrast, the tautomerization of the PNN ligand in PNN-Ru complex is downhill by −10.7 kcal/mol. This is consistent with reaction free energy changes in their MLC transfer processes (Figure 6). We further checked the optimized structures of the pincer ligands. Even though the steric repulsion exists between two methyl groups, the ligand A1 mostly maintains a planar geometry (dihedral: ∠CNCN = −10.6°). However, after the tautomerization, the original conjugation is destroyed in ligand A-2, which is revealed by the highly twisted pincer ligand (dihedral: ∠CNCN = −51.7°; Figure 7 highlighted in green). We further confirm the aromaticity with the nucleus independent chemical shift index (NICS). The negative values of NICS(0) correspond to aromaticity, and the positive values correspond to antiaromaticity. The calculated results are consistent with the free energy change, which suggests the destabilization during the tautomerization in the NNN-Ru system (A-1 → A-2: 1.74 → 4.63). This well explains why MLC is not preferred in NNN-Ru system. Although the MLC mechanism of NNN-Ru system is nearly 14.1 kcal/mol higher than that of the PNN-Ru system, the overall free energy barrier of NNN-Ru system is only ∼3 kcal/ mol higher than that of the PNN-Ru system (Figure 8; A-1 → A-TS2O-PS vs B-2O → B-TS2O-PS), it is important to point out that the driving force from aromatization/dearomatization process could be a double-edged sword. On the positive aspect, the driving force leads to a relatively lower energy barrier by the

Figure 4. Optimized important intermediates and transition states for the NNN-Ru(II) hydride complex. Bond lengths are in angstroms. All C−H hydrogen atoms are omitted for clarity.

Since the calculation of isomerization has ruled out the nonMLC mechanism, both the outer-sphere mechanism and innersphere mechanism will undergo an MLC manner. The major pathways of PNN-Ru catalyst are depicted in Scheme 5, and some key optimized structures are depicted in Figure 5. The calculated free energy result is shown along with the structures. Our calculated results basically support the conclusion former mechanistic studies performed by Li and Wang,10k which suggested that the outer-sphere mechanism is more favored than inner-sphere mechanism. Similar to former discussion on the NNN-Ru catalyst, the proton shuttle type of transition state is also involved in our discussion. In former study, Li and Wang verified the outer-sphere MLC mechanism to be a stepwise process. However, it was found that the outer-sphere MLC can also occur via a concerted transfer manner. This phenomenon was also reported in their recent computational study,10d and it is considered reasonable since the different substrate PEA is used here. The transfer mode via proton shuttle type of transfer mode is slightly lower with an activation free energy of 11.6 kcal/mol. After the MLC process, the pyridine ligand is aromatized, and the complex becomes more stable (ΔG = −6.2 kcal/mol). The complex then undergoes the coupling reaction of the proton on the benzyl group of pyridine and the hydride on metal. The direct coupling via B-TS2O is found to be very difficult (ΔG‡ = 28.6 kcal/mol) due to the high distortion of the PNP ligand and high ring strain of the four-member ring transition state. However, the proton shuttle type of transition state B-TS2O-PS significantly lowers the energy barrier by releasing the ring strain (ΔG‡ = 16.5 kcal/mol), which is more plausible as the mechanism for the regeneration of the catalyst. As for the inner-sphere MLC mechanism, the substrate PEA will coordinate to the ruthenium center to give a coordination saturated complex B-2I (ΔG = 0.6 kcal/mol). Then the O−H bond of PEA will cleave to give the aromatized complex B-3I (ΔG = −7.0 kcal/mol). The direct cleavage via B-TS1I is calculated to be disfavored with an activation free energy of 16.7 kcal/mol compared with the proton shuttle mechanism BTS1I-PS (ΔG‡ = 6.1 kcal/mol). Then, the NEt2 arm needs to be dissociative to provide a vacant site for the β-hydride elimination. The isomerization (B-TS2I) will overcome the activation free energy of 14.0 kcal/mol. After the dissociation of F


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Inorganic Chemistry

Scheme 5. Outer-Sphere Metal Ligand Cooperation Pathway and Inner-Sphere Metal Ligand Cooperation Pathway for the PNN-Ru Catalysta

a

The relative free energies are given in kilocalories per mole. For clarity, selected parts of the ligand are shown in a light color.

Figure 5. Optimized important intermediates and transition states for the PNN-Ru catalyst. Bond lengths are in angstroms. All C−H hydrogen atoms except the bifunctional site are omitted for clarity. G


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Inorganic Chemistry Scheme 6. Comparison of Metal Ligand Cooperation Mechanism for the NNN-Ru and the PNN-Ru Catalystsa

a

The free energies are reported in kilocalories per mole.

Catalyst. Before we discuss the comparison of the inner-sphere mechanism, we would like to analyze the electronic structures of two catalysts. For both catalysts, a critical feature of the pincer ligand is that the nitrogen atom coordinated is an anion, and the pz orbital is filled with a lone pair of electrons. Therefore, the ligand is not only a σ-donor, but also a πdonor.28 The electron−electron repulsion between the filled pz orbital and the filled dxz orbital of Ru (II) will cause the instability of the pincer complex. Therefore, a stabilization strategy is to use additional π-acceptor ligand as a spectator to reduce the electron−electron repulsion. Two π-acceptor ligands, CO and PPh3, are used in each system (Figure 9). For the PNN-Ru catalyst, the inner-sphere mechanism still begins with the aromatization and dearomatization process, which leads to a thermodynamic sink (B-3I) in PES (Figure 8). To perform β-hydride elimination mechanism, the NEt2 arm needs to rotate to provide a vacant site. This process will result in an energy penalty due to the unsaturated intermediate. This dissociation process should be responsible for the high energy barrier of inner-sphere mechanism. From another perspective, the β-hydride elimination mechanism is generally considered to be promoted by the back bonding between metal center d orbital and the C−H σ* antibonding.29 The CO ligand can significantly lower the electron density of metal center and thus diminishes the interaction between metal center d orbital and the C−H σ* antibonding. Therefore, the β-hydride elimination mechanism must overcome a higher energy barrier because the metal center cannot well facilitate the C−H bond breaking process. The hydride on the trans-site of the alkoxide should also be a disadvantage factor, as the hydride has almost the strongest trans-influence among all the ligands. As for the NNN-Ru system, the hydride will directly couple with the proton on PEA. The hydride is on the trans-site of the

Scheme 7. Imine−Enamine Tautomerization of the NNN and the PNN Pincer Ligandsa

a

The free energies are reported in kilocalories per mole.

aromatic system restoration. On the negative aspect, the driving force leads to a stable intermediate complex, which could be a thermodynamic sink (B-2O) in the potential energy surface (PES). Thus, the following step in the catalytic cycle may have to overcome high energy barrier. In the PNN-Ru catalytic system, a metal hydride is initially designed at the trans-site of the incoming hydride, which can be seen as a strategy to keep the catalyst active for the next catalytic process. 4.2. The Comparison of Inner-Sphere β-H Elimination Mechanism for the NNN-Ru Catalyst and the PNN-Ru H


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Figure 6. Calculated PES of outer-sphere MLC mechanisms for the PNN-Ru and NNN-Ru catalysts. The relative free energies are given in kilocalories per mole.

Figure 7. Changes of molecular geometries during imine−enamine tautomerizations.

nitrogen anion; thus, this hydride is highly active for the deprotonation of PEA, which can be considered as one of the driving forces in the inner-sphere non-MLC mechanism for the NNN-Ru system. The vacant site is directly generated by the

Figure 9. Stabilization strategy using π-acceptor ligand in the NNN-Ru catalyst and the PNN-Ru catalyst.

Figure 8. Comparison of PESs of inner-sphere mechanism for the PNN-Ru system and the NNN-Ru system. The free energies are reported in kilocalories per mole. I


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Inorganic Chemistry H2 release process. On the one hand, when the β-hydride elimination occurring, the tridentate pincer ligand is still coordinated to the metal center. On the other hand, the PPh3 ligand has much weaker π-acidic character as compared with the CO ligand. Therefore, with an available vacant site, the inner-sphere β-hydride elimination in NNN-Ru system is more accessible compared to that in the PNN-Ru system. Above all the discussions, these two interesting systems establish two possible approaches and show the beauty of the catalyst designing strategy. From a theoretical perspective, a general picture of the comparison of two designing strategies for acceptorless alcohol dehydrogenation is summarized in Figure 10.

Figure 10. Comparison of inner-sphere non-MLC strategy and outersphere MLC strategy for designing acceptorless alcohol dehydrogenation catalyst.

We also replace the pyridine ligand to pyrimidine (L-9) and 1,3,5-triazine (L-10). This modification will slightly perturb the original pyridine, which leads to a smaller reaction free energy compared with L-1. This should be attributed to the better overlap of the p orbitals in the CC bond than in the CN bond, because the p orbital of nitrogen atom is different from the p orbital of carbon atom in energy. As for ligands L-11 and L-12, we aim at finding out how the driving force changes when the CC bond is bonded to electron-donating groups or electron-withdrawing groups, since the CC bond is responsible for accepting a proton. Therefore, we assume the pincer ligand with electron-donating group will have stronger driving force. The calculated result is consistent with our assumption. On the one hand, the reaction free energy of the pincer ligand with methoxyl group (L-11) is −11.1 kcal/mol. On the other hand, the pincer ligand with nitro group (L-12) is found to slightly positive in reaction free energy. This can be attributed to the stabilization of aromatization/dearomatization being offset by the energy penalty for losing the conjunction between the CN bond and nitro group. For the last four ligands, we modified the pincer ligand used in NNN-Ru system. The original indoline was replaced by pyrazole and indazole. The pincer ligand restored to an aromatic state with 10 π (indazole) or 6 π (pyrazole) electrons after tautomerization. Thus, the reaction free energy results are all negative (Figure 11; L-13 to L-16), which indicates these pincer ligands have the potential to perform the aromatization/dearomatization MLC catalysis. We choose ligand L-16 as a virtual designed catalyst and successfully locate the transition state for dehydrogenation of PEA. The activation free energy is only 13.7 kcal/mol (please see Figure 12). However, since a catalytic process is affected by various factors and the driving force could be a double-edged sword, the activities of catalysts bearing these ligands remain to be verified by experiments as final arbiters.

4.3. Evaluation of Pincer Ligands and Their Role in Metal Ligand Cooperation. On the basis of former discussion, we can use the tautomerization reaction of the pincer ligand to evaluate its driving force for the aromatization/ dearomatization process. We selected a series of pincer ligands to compare the influence caused by the electronic effects, and some of them (L-1, L-3, and L-4) have been verified to follow an MLC mechanism and modified the structures.1c For L1 to L4 (please see Figure 11), our major purpose is to compare the difference between CC bond to CN bond in the pincer ligand, which is responsible for accepting proton. The results suggest that the CC will have a stronger driving force than CN bond. The result is understandable, as the amine group has relatively smaller pKa due to the conjugative effect, as the amine group is bonded to the pyridine ring, which means the nitrogen atom easily loses a proton and difficultly acquires a proton. Therefore, tautomerization for CN bond is less exothermal as compared with CC bond situation. However, the reaction free energy of all the ligands in L-1 to L-4 are all negative, which indicates these pincer ligands can indeed function as the bifunctional ligand in metal ligand cooperation. On the basis of the NNN-Ru system, the tautomerization reaction free energy of several similar ligands (L-5 to L-8) are calculated and compared. Since these tautomerizations lead to antiaromatic intermediates, the reaction free energy values are all positive. The computational results suggest these ligands as innocent ligands.

5. CONCLUSION In this study, we have presented a detailed computational study to elucidate the different mechanistic preferences for the NNNRu system and the PNN-Ru system. The mechanism of acceptorless alcohol dehydrogenation reaction mediated by NNN-Ru catalyst is investigated with density functional theory method for the first time. Various isomerization possibilities are explored, and the most plausible active species is verified. For the overall mechanistic preference, both the inner-sphere nonMLC pathway and the outer-sphere MLC pathways are investigated and compared. The rate-determining step is located at the β-hydride elimination step (A-4I → A-TS2I: ΔG‡ = 19.2 kcal/mol). The result suggests that the innersphere β-hydride elimination pathway is more favored (by 6.5 kcal/mol) for the NNN-Ru catalyst, which is consistent with previous experimental mechanistic study.16 The unique origin of NNN-Ru catalyst’s mechanistic preference on the innersphere non-MLC mechanism is attributed to these reasons: (1) The outer-sphere MLC mechanism in the NNN-Ru system leads to an unstable tautomerization in the pincer ligand, which results in the high activation free energy. (2) The vacant site for inner-sphere β-hydride elimination is generated by the release of H2 without extra energy penalty, which makes the inner-sphere mechanism accessible. (3) The PPh3 has moderate π-acidic character and does not hinder the β-hydride elimination process. J


Article

Inorganic Chemistry

Figure 11. Tautomerization driving force evaluation of pincer ligands. The relative free energies are given in kilocalories per mole. NICS(0) value is dimensionless and shown along with the free energy change as reference data. The pincer ligands L-1,30 L-3,31 and L-45a have been verified to perform an MLC mechanism.

(1) The MLC mechanism in PNN-Ru system leads to a stable aromatic intermediate, which results in the low activation free energy. (2) The vacant site provided by ligand rotation has to pay extra energy penalty due to the unsaturation of the pincer complex. (3) Although the strong π-acceptor ligand CO can stabilize the pincer complex, it also diminishes the ability of metal center to facilitate the β-hydride elimination mechanism by decreasing the electron density of the metal center. Overall, the ligand tautomerization process on the bifunctional pincer ligand can be seen as the “engine” to provide driving force for the reaction, which is a key factor governing the possibility of MLC. However, the strong driving force could be a double-edged sword by leading the catalytic process to a thermodynamic sink in PES, which may deactivate the catalyst. Therefore, it is important to tune the ligands of the catalyst to ensure the catalyst’s efficiency properly along the whole catalytic cycle. Moreover, it is also necessary to select appropriate spectator ligands to force the catalyst to perform a certain mechanism, which can avoid unwanted side reactions and isomerizations. The inner-sphere mechanism is also not necessarily to be disfavored; the driving force coming from the metal center can function well by rational design of the catalyst. We expect this comparative study to be useful for future catalyst design.

Figure 12. Free energy profile of a computational modified catalyst. The relative free energies are given in kilocalories per mole.

As a comparison, we revisited the “classical” PNN-Ru catalyst system. The isomerizations of the catalyst were explored and compared for the first time. The calculated results indicate that the CO and metal hydride are unlikely to isomerize or dissociate, which ensures the occurrence of MLC mechanism. For the dehydrogenation step of PEA, the outer-sphere pathway (B-1 → B-TS1O-PS: ΔG‡ = 11.6 kcal/mol) is found to be more favorable than the inner-sphere mechanism (B-3I → B-TS3I: ΔG‡ = 24.0 kcal/mol) by ∼12 kcal/mol with respect to the PNN-Ru catalyst, which is consistent with former studies. The origin of the mechanistic preference on outer-sphere MLC mechanism in PNN-Ru can be attributed to these reasons: K


Article

Inorganic Chemistry

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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.6b00723. The comparison of different functionals and basis set, the absolute energies of all optimized structures, the hydride−proton coupling transition state starts from Aisomer-2-a, the RuII/Ru0 mechanism for NNN-Ru system, the bifunctional mechanism for NNN-Ru system via ligand rotation, the possibility dissociation of CO ligand and Cartesian coordinates of all optimized structures. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (Grant Nos. 21203256, 21473261, and 21373277), and the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2015A030306027). Computing facilities were supported in part by the Guangdong Province Key Laboratory of Computational Science, the Joint Funds of NSFC-Guangdong for Supercomputing Applications, and the National Supercomputing Center in Guangzhou.

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