Theoretical and Experimental Studies on Reaction Mechanism for

Jun 1, 2010 - Department of Chemical System Engineering and Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hon...
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J. Phys. Chem. C 2010, 114, 10873–10880

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Theoretical and Experimental Studies on Reaction Mechanism for Aerobic Alcohol Oxidation by Supported Ruthenium Hydroxide Catalysts Fumiya Nikaidou,† Hiroshi Ushiyama,*,† Kazuya Yamaguchi,‡ Koichi Yamashita,† and Noritaka Mizuno‡ Department of Chemical System Engineering and Department of Applied Chemistry, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan ReceiVed: February 24, 2010; ReVised Manuscript ReceiVed: May 20, 2010

The experimentally proposed reaction mechanism for the aerobic alcohol oxidation by supported ruthenium hydroxide catalysts (Ru(OH)x/support, support ) TiO2 or Al2O3) is theoretically investigated by means of ab initio quantum chemistry calculations with model catalysts “Ru(OH)3(OH2)3” and “RuCl3(OH2)3” for Ru(OH)x/ support and RuClx/support, respectively. The experimentally proposed alcoholate formation and β-hydride elimination steps can be verified. In the case of 2-butanol (as a model substrate), the calculated activation energy for the alcoholate formation step with Ru(OH)3(OH2)3 (27.7 kJ mol-1) is much smaller than that with RuCl3(OH2)3 (123.2 kJ mol-1), showing that the alcoholate formation with Ru(OH)x/support much more easily proceeds than that with RuClx/support. The Ru(OH)x/support catalysts possess both Lewis acid (Ru center) and Brønsted base (OH- species) sites on the same metal site. Therefore, the alcoholate formation step can be promoted by the “concerted actiVation” of an alcohol by the Lewis acid (electron transfer from an alcohol to Ru) and Brønsted base (electron transfer from OH- to a hydroxyl proton) sites on Ru(OH)x/support. For the reaction of the hydride species with O2, the coordination of the electron-donating ligands (in particular, an alcohol and OH2) to form the six-coordinated ruthenium monohydride (Ru-H) species is a key to promote the O2 insertion to the hydride species. The electron donation from the ligands to the hydride species can make the Ru-H bond weaker, resulting in lowering the activation energy for the O2 insertion step. Finally, the alcoholate or hydroxide species is regenerated with the formation of H2O2, and the catalytic cycle is completed. Introduction The oxidation of alcohols is of paramount importance in organic syntheses as well as industries because of the wide use of the products (carbonyl compounds) as important intermediates for medicines, agricultural chemicals, and fragrances.1 Due to the increasing environmental concerns,2 many efforts have been made to develop oxidation systems (especially heterogeneous systems3) using environmentally benign O2 (or air) as a sole oxidant. Until now, many efficient noble metal-based catalysts have been developed for the selective oxidation of alcohols with O2.4 Also, some of the present authors have reported that supported ruthenium hydroxide catalysts (Ru(OH)x/support, support ) TiO2 or Al2O3) can act as efficient reusable heterogeneous catalysts for the aerobic oxidation of various kinds of structurally diverse alcohols (see Table S1, Supporting Information, for the synthetic scope of the Ru(OH)x/TiO2catalyzed aerobic alcohol oxidation).5 Besides the aerobic alcohol oxidation, many functional group transformations also efficiently proceeded with Ru(OH)x/support.6 The outstanding catalytic performance of Ru(OH)x/support is likely attributed to the presence of coordinatively unsaturated Ru centers (Lewis acid sites)7 and basic hydroxide groups (Brønsted base sites).6f On the basis of the several experimental pieces of evidence, i.e., (i) negligible effect of radical scavengers, (ii) faster oxidation of primary alcohols in the presence of secondary ones, * To whom correspondence should be addressed. Phone/fax: +81-3-58417270. E-mail: [email protected]. † Department of Chemical System Engineering. ‡ Department of Applied Chemistry.

(iii) negative Hammett F+ value (-0.47), (iv) high catalytic activities for hydrogen transfer reactions, and (v) 1: 1 (H2O produced: product) and 1:2 (O2 uptake: product) stoichiometries (see the Supporting Information for the experimental details),5 some of the present authors have proposed the reaction mechanism for the Ru(OH)x/support-catalyzed aerobic alcohol oxidation (Figure 1). In the catalytic cycle, the ruthenium alcoholate species is initially formed via the ligand exchange between an alcohol and the ruthenium hydroxide species on Ru(OH)x/support (step 1 in Figure 1). Then, the β-hydride elimination proceeds to afford the corresponding carbonyl compound and the ruthenium monohydride (Ru-H) species (step 2 in Figure 1). Finally, the hydroxide species is regenerated by the reaction of the Ru-H species with O2 (step 3 in Figure 1).8 However, direct experimental evidence for step 3, for example, the detection of the ruthenium hydroperoxide species (Ru-OOH) and H2O2, still has not been obtained because the hydroperoxide species and H2O2 are easily decomposed under catalytic reaction conditions.9 Quantum chemistry calculations are very useful in obtaining detailed information on the reaction mechanism. However, the calculations on heterogeneous catalysis still have been very difficult because heterogeneous catalysts are huge systems and the local structures of active sites are not clearly known in many cases.10 Several sizes and geometries for heterogeneous catalysts should be examined, resulting in the high cost of computational calculations.10 Thus, the utilization of appropriate model catalysts with suitable sizes is one of the most important subjects for quantum chemistry calculations on heterogeneous catalysis.10

10.1021/jp101692j  2010 American Chemical Society Published on Web 06/01/2010

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Figure 1. An experimentally proposed reaction mechanism for the present Ru(OH)x/support-catalyzed aerobic alcohol oxidation (see the Supporting Information).

As for the aerobic alcohol oxidation with Ru(OH)x/support catalysts prepared with different supports (three different TiO2 supports and an Al2O3 support, see the Supporting Information), the reaction rates for the rate-determining β-hydride elimination step monotonically increased with the decrease in the coordination numbers (CNs) of nearest-neighbor Ru atoms (determined by EXAFS analyses, Table S2, Supporting Information),5c suggesting that the monomeric ruthenium hydroxide species are effectiVe for the oxidation and that the effects of supports can be negligible. In addition, it was confirmed by XPS and ESR analyses that the (average) oxidation state of ruthenium species in Ru(OH)x/support was +3 throughout the catalytic alcohol oxidation.5 On the basis of this experimental evidence, we can propose a very simple “Ru(OH)3(OH2)3” as a model catalyst for Ru(OH)x/support (see the later section). In this paper, we initially examined the validity of the proposed model catalyst “Ru(OH)3(OH2)3” by the comparison between calculated activation energies and experimentally determined rate constants for the oxidation of benzyl alcohol and its para-substituted derivatives. Next, ab initio quantum chemistry calculations at the density functional theory (DFT) level were carried out with the model catalyst in order to verify the above-mentioned experimentally proposed reaction mechanism (Figure 1). We could clarify the several new mechanistic insights, especially for the role of the ruthenium hydroxide species (“concerted actiVation” of an alcohol) and the reaction of the Ru-H species with O2 (step 3 in Figure 1), by quantum chemistry calculations with the model catalyst. Experimental Section Computational Method. Ab initio quantum chemistry calculations at the DFT level were performed with the Gaussian 03 program.11 The “Ru(OH)3(OH2)3” and “RuCl3(OH2)3” species were used as model catalysts for Ru(OH)x/support and RuClx/ support, respectively. Benzyl alcohol and its para-substituted derivatives were used as substrates to confirm the validity of the model catalyst. 2-Butanol (aliphatic alcohol) was mainly

Nikaidou et al. used as a model substrate to examine the reaction mechanism for the Ru(OH)x/support-catalyzed aerobic alcohol oxidation. The calculations for the other kinds of alcohols such as benzyl alcohol (benzylic alcohol) and 3-buten-2-ol (allylic alcohol) were also carried out. The B3LYP hybrid functional (Becke’s threeparameter functional with the nonlocal correlation provided by the correlation functional of Lee et al.), the LanL2TZf12 basis set for the Ru atom, and the 6-311G(d,2p) basis set for the other atoms were used for the calculations. Electronic charges of ligands were examined by the natural bonding orbital (NBO) charges.13 Thedoubletstateswereassumedinallthecatalyst-substrate complexes. The transition state structures were searched by numerically estimating the matrix of second-order energy derivatives at every optimization step and by requiring exactly one eigenvalue of this matrix to be negative. The Cartesian coordinates of the optimized and the transition state structures are summarized in the Supporting Information. Catalyst Preparation and Alcohol Oxidation. Ru(OH)x/ support and RuClx/support catalysts were prepared according to the reported procedures (see the Supporting Information).5,6 The catalytic aerobic alcohol oxidations were carried out as follows. A suspension of the catalyst in toluene was stirred for 5 min. Then, an alcohol was added and O2 was passed through the suspension. The mixture was stirred (800 rpm) at the reaction temperature under 1 atm of O2. The alcohol conversions and product yields were periodically determined by GC analysis (see the Supporting Information for more detail). Results and Discussion Validity of the Model Catalyst “Ru(OH)3(OH2)3”. As mentioned in the Introduction, the monomeric ruthenium hydroxide species were effective for the present Ru(OH)x/ support-catalyzed aerobic alcohol oxidation and the effects of supports could be negligible (Table S2, Supporting Information).5c It was confirmed by XPS and ESR analyses that the (average) oxidation state of ruthenium species in Ru(OH)x/ support was +3 throughout the catalytic alcohol oxidation.5 The curve-fitting analyses for the EXAFS spectra of Ru(OH)x/support showed that the (average) CNs of the “first Ru-O shell” of Ru(OH)x/support were “six” (Table S3, Supporting Information). From the above experimental results, we here propose a very simple monomeric “Ru(OH)3(OH2)3” species as a model catalyst for Ru(OH)x/support. In the model catalyst, three aqua (OH2) ligands are used as capping ligands to fix both the CN (to 6) and the electronic charge of the Ru atom (to +3). Before examining the reaction mechanism by means of quantum chemistry calculations, we first examined the validity of the proposed model catalyst “Ru(OH)3(OH2)3”. The geometry of the proposed model catalyst was fully optimized. The optimized structure is shown in Figure 2a. The distances between Ru and O atoms of the hydroxyl groups were in the range of 1.93-2.11 Å and are consistent with those determined by the EXAFS analysis of Ru(OH)x/support (2.01-2.03 Å, Table S3, Supporting Information).6f The O-H bond lengths of the three OH2 ligands of the model catalyst were fixed throughout the calculations in order to avoid the hydrogen atom transfer from the OH2 ligands to hydroxyl groups. The distances between Ru atom and O atoms of the OH2 ligands in the model catalyst and catalyst-substrate complexes were in the range of 2.14-2.28 Å. On the basis of several experimental pieces of evidence, i.e., (i) large kinetic isotope effects (kH/kD ) 4.9-5.3) for the oxidation of R-deuterio-p-methylbenzyl alcohol and (ii) independence of reaction rates of the partial pressure of O2 (>0.5

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Figure 2. Optimized structures of (a) Ru(OH)3(OH2)3 and (b) the ruthenium alcoholate and (c) the transition state structure of the β-hydride elimination. Benzyl alcohol (or its para-substituted derivatives) was used as a model substrate. The blue, red, gray, and white spheres indicate the Ru, O, C, and H atoms, respectively.

Figure 4. (a) Schematic diagram and (b) optimized and transition state structures for the alcoholate formation and β-hydride elimination steps for the oxidation of 2-butanol with Ru(OH)3(OH2)3. The blue, red, gray, and white spheres indicate the Ru, O, C, and H atoms, respectively.

Figure 3. Relationship between the experimentally determined rate constants (ln kx) with Ru(OH)x/support and the calculated Ea(p-X) values.

atm) (see the Supporting Information), the rate-determining step for the present Ru(OH)x/support-catalyzed aerobic alcohol oxidation is found to be step 2 in Figure 1 (β-hydride elimination).5 Thus, we first examined the validity of the proposed model catalyst “Ru(OH)3(OH2)3” by comparison between “calculated actiVation energies” for the rate-determining β-hydride elimination and “experimentally determined rate constants” for the oxidation of benzyl alcohol and its parasubstituted derivatives (p-OCH3, p-CH3, p-Cl, and p-NO2). Here, we defined the activation energies for the β-hydride elimination of para-substituted benzyl alcohol derivatives (Ea(p-X)) as the energy differences between the alcoholate species (Figure 2b) and the corresponding transition states (Figure 2c). In Figure 3, the experimentally determined rate constants (ln kX) for the oxidation of para-substituted benzyl alcohol derivatives with Ru(OH)x/support are plotted against the calculated Ea(p-X) values. The experimentally determined ln kX values linearly decreased with an increased in the calculated Ea(p-X) values, showing that the quantum chemistry calculations with Ru(OH)3(OH2)3 can describe the differences for the oxidation of the para-substituted benzyl alcohol derivatives. In addition, the rate-determining step for the (theoretical) Ru(OH)3(OH2)3catalyzed alcohol oxidation was found to be the β-hydride elimination step (rather than the alcoholate formation step, see the later section), which agrees with experimental results for the Ru(OH)x/support-catalyzed alcohol oxidation (see the Supporting Information). All these results show that Ru(OH)3(OH2)3

is a suitable model for Ru(OH)x/support catalysts. Therefore, the detailed catalytic reaction mechanism for the Ru(OH)x/ support-catalyzed aerobic alcohol oxidation is hereafter examined by quantum chemistry calculations with Ru(OH)3(OH2)3 as a model catalyst. Quantum Chemistry Calculations for the Alcoholate Formation and β-Hydride Elimination. The quantum chemistry calculations for the alcoholate formation (step 1 in Figure 1) and β-hydride elimination (step 2 in Figure 1) steps were initially carried out with 2-butanol as a model substrate. These steps can be divided into the following elementally reactions (Figure 4a): (i) the elimination of one OH2 ligand from 1 (1 f 2), (ii) the coordination of 2-butanol to the Lewis acid site on 2 (2 f 3), (iii) the alcoholate formation via the transition state TS4 (3 f 5), (iv) the elimination of one OH2 ligand from 5 (5 f 6), and (v) the β-hydride elimination via the transition state TS7 (6 f 8). Figure 4b shows the optimized and the transition state structures in the reaction steps. The atoms that play important roles in the alcoholate formation and β-hydride elimination steps are labeled. The bond lengths of Ru-O1, O1-H1, O2-H1, C-O1, C-H2, and Ru-H2 along the reaction path are listed in Table 1. The Ru-O1 bond length in 5 (alcoholate species) was 2.08 Å and corresponded to the single bond. The C-O1 bond length in 6 was 1.42 Å (single bond) and decreased to 1.23 Å in 8 (double bond) via the β-hydride elimination. Figure 5 shows the energy diagram along the reaction path described in Figure 4. We defined the activation energy for the reaction i f j (Ea(ifj)) as the energy difference between the corresponding transition state and the intermediate i. The activation energies for the alcoholate formation via TS4 (Ea(3f5), Figure 5a) and β-hydride elimination via TS7 (Ea(6f8), Figure 5b) were calculated to be 27.7 and 77.2 kJ mol-1, respectively. The experimentally determined apparent activation energy for the oxidation of 2-butanol with Ru(OH)x/

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TABLE 1: Selected Bond Lengths (in Å) of Optimized and Transition State Structures for the Alcoholate Formation and β-Hydride Eliminationa Ru-O1 O1-H1 O2-H1 C-O1 C-H2 Ru-H2 a

3

TS4

5

6

TS7

8

2.23 0.98 1.98

2.16 1.23 1.21

2.08 1.83 0.99

1.99

2.15

2.17

1.42 1.10 3.12

1.27 1.94 1.66

1.23 3.10 1.63

9

TABLE 2: Energies of the Alcoholate Formation and β-Hydride Elimination Steps for 2-Butanol (Aliphatic), Benzyl Alcohol (Benzylic), and 3-Buten-2-ol (Allylic) Calculated with Ru(OH)3(OH2)3a

1.57

The atom labeling is shown in Figure 4.

a

Figure 5. Energy diagrams of (a) the alcoholate formation and (b) β-hydride elimination steps for 2-butanol calculated with Ru(OH)3(OH2)3.

TiO2 was 72.7 kJ mol-1 (Arrhenius plots, Figure S1, Supporting Information). The activation energies for backward reactions of the β-hydride elimination (hydride readdition, Ea(8f6)) and alcoholate formation (Ea(5f3)) were calculated to be 38.5 and 19.8 kJ mol-1, respectively, and were smaller than Ea(6f8). This means that the racemization of 2-butanol (secondary alcohol) can easily proceed in the absence of an oxidant (O2). Indeed, the racemization of various kinds of secondary alcohols efficiently proceeded in the presence of Ru(OH)x/support catalysts under anaerobic conditions (Table S4, Supporting Information).6c,f The calculations for the alcoholate formation and β-hydride elimination steps were also carried out with other kinds of alcohols such as benzyl alcohol (benzylic alcohol) and 3-buten2-ol (allylic alcohol). The results are summarized in Table 2. The activation energies for the β-hydride elimination steps were larger than those for the alcoholate formation steps for all the alcohol substrates examined (aliphatic, benzylic, and allylic alcohols). The above calculation results for the alcoholate formation and β-hydride elimination steps were consistent with the experimental results. Thus, these experimentally proposed steps could be verified by the quantum chemistry calculations. Role of Ruthenium Hydroxide Species. The RuClx/support catalyst was prepared in acetone without base treatment (support ) TiO2, see the Supporting Information for the catalyst preparation). In the case of RuClx/support, the ruthenium chloride species (not the hydroxide species) were highly dispersed on TiO2. For the oxidation of 1-phenylethanol, no reaction proceeded with RuClx/support under the conditions described in eq 1 (99% yield of acetophenone, eq 2), showing that the generation of the “ruthenium hydroxide species” on the surface of supports is very important to achieve high catalytic activity.14

To clarify the role of the ruthenium hydroxide species, the calculations for the alcoholate formation step were carried out for 2-butanol with the model catalyst “RuCl3(OH2)3” (as a model for RuClx/support, Figure 6a). The activation energy for the alcoholate formation via the transition state TS4Cl (Ea(3Clf5Cl)) was calculated to be 123.2 kJ mol-1 (Figure 6b) and was much larger than Ea(3f5) (27.7 kJ mol-1), showing that the alcoholate formation with Ru(OH)x/support (Ru(OH)3(OH2)3) much more easily proceeds than that with RuClx/support (RuCl3(OH2)3). All these experimental and calculation results strongly indicate that the creation of highly dispersed ruthenium hydroxide species on appropriate supports (Ru(OH)x/support catalysts) is very important to initiate the alcohol oxidation (i.e., alcoholate formation). The Ru(OH)x/support catalysts possess both Lewis acid (Ru center) and Brønsted base (OH- species) sites on the same metal site (see the structure of hydroxide species 2 in Figure 4). The alcoholate formation step can be promoted by the “concerted actiVation” of an alcohol by the Lewis acid (electron transfer from an alcohol to Ru) and Brønsted base (electron transfer form OH- to a hydroxyl proton) sites as shown in TS4 in Figure 4. Therefore, the Ru(OH)x/support catalysts do not need additives such as bases to promote the reactions.5,6 In contrast, bases such as NaOH, Na2CO3, KOH, K2CO3, and CsCO3 should be intrinsically required for the activation of alcohols with common metal (chloride) complexes.15

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Figure 7. (a) Schematic diagram and (b) optimized and transition state structures for the reaction of the Ru-H species with O2 along the reaction path 9 f 12. The blue, red, gray, and white spheres indicate the Ru, O, C, and H atoms, respectively.

TABLE 3: Selected Bond Lengths (in Å) of Optimized and Transition State Structures for the Reaction of Ru-H Speciesa

Figure 6. (a) Optimized and transition state structures (3Cl: RuCl3(OH2)2 species coordinated with 2-butanol; TS4Cl: transition state structure for the alcoholate formation; 5Cl: alcoholate species) and (b) energy diagram for the alcoholate formation step for 2-butanol calculated with RuCl3(OH2)3 along 3Cl f 5Cl (red lines indicate the results with Ru(OH)3(OH2)3 along 3 f 5, see Figure 5a). The blue, red, gray, white, and green spheres indicate the Ru, O, C, H, and Cl atoms, respectively.

Quantum Chemistry Calculations for the Reaction of the Hydride Species with O2. Step 3 in Figure 1 is proposed for the reaction of the Ru-H species with O2.5,8 At present, the direct experimental evidence for step 3, for example, the detection of the Ru-OOH species and H2O2, still have not been obtained because the Ru-OOH species and H2O2 are easily decomposed under catalytic reaction conditions.9 It is wellknown that O2 can react with transition metal hydride (M-H) complexes (in both aqueous and organic solvents) to afford the metal hydroperoxo (M-OOH) complexes16 and that the reactions are intrinsically exothermic.8e,16e To date, various kinds of M-OOH complexes have been directly observed and/or isolated for the reaction of M-H complexes with O2.16 Several paths have been proposed for the formation of M-OOH complexes,8c,e,16 for example, (i) direct O2 insertion to the M-H bond, (ii) reductive elimination of ligands followed by the reaction with O2 to form the η2-peroxo species (as intermediates), (iii) direct oxidative addition of O2, and (iv) free radical path (Figure S2, Supporting Information). It was experimentally confirmed that the radical intermediates were not involved in the present aerobic alcohol oxidation (see the Supporting Information). Thus, path iv can be excluded. Among the possible paths shown in Figure S2 (Supporting Information), the direct O2 insertion path is likely most plausible for the formation of hydroperoxide species with Ru(OH)x/support. Therefore, we hereafter examined several direct O2 insertion paths. Initially, the calculations for the formation of the hydroperoxide species were carried out according to the following reaction path (Figure 7a): (i) the adsorption of O2 to the fivecoordinated Ru-H species 9 (9 f 10), (ii) followed by the O2 insertion to form the Ru-OOH species 12 via TS11 (10 f 12). Figure 7b shows the optimized and the transition state structures. The bond lengths of Ru-H2, Ru-O3, O3-O4, and

9

10

TS11

12

1.57

1.57 4.09 1.21 3.55

1.76 2.04 1.34 1.35

3.13 1.91 1.47 0.97

8

10a

TS11a

13a

1.63

1.63 4.31 1.21 3.77 2.17

1.64 2.06 1.34 1.53 2.21

3.55 1.95 1.47 0.96 2.17

Ru-H2 Ru-O3 O3-O4 O4-H2

Ru-H2 Ru-O6 O5-O6 O5-H2 Ru-O1

Ru-H2 Ru-O9 O8-O9 O8-H2 Ru-O7 O7-H3 O9-H3

Ru-H2 Ru-O12 O11-O12 O11-H2 Ru-O10 O10-H4 O12-H4 a

2.17 9b

10b

TS11b

13b

1.66

1.66 4.26 1.21 3.26 2.26

1.67 2.05 1.35 1.45 2.32

3.58 2.01 1.46 0.96 2.25 0.97 2.32

2.26

9c

10c

TS11c

13c

1.62

1.62 4.20 1.21 3.61 2.27

1.70 2.07 1.34 1.41 2.33

3.43 1.96 1.45 0.97 2.29 0.99 3.31

2.27

TS14b

15b

2.15

2.27

2.17 1.28 1.16

1.98 2.86 0.97

TS14c

15c

2.15

2.26

2.18 1.27 1.17

1.99 2.72 0.97

The atom labeling is shown in Figures 7, 9, and 10.

O4-H2 are important to discuss the structural changes and are summarized in Table 3. The O3dO4 double bond in 10 (O2 molecule, 1.21 Å) changed to a single bond in 12 via the O2 insertion step. This is likely because the OdO bond strength in the O2 molecule is weakened by the electron donation from the model catalyst. The energy diagram along the reaction path 10 f 12 is summarized in Figure 8a (shown with black lines). The activation energy for the O2 insertion step via TS11 (Ea(10f12)) was calculated to be 49.8 kJ mol-1. The Ea(10f12) value was larger than those of the hydride readdition steps (Ea(8f6) ) 38.5 kJ mol-1 for 2-butanol, 6.3 kJ mol-1 for benzyl alcohol, and 40.3 kJ mol-1 for 3-buten-2-ol), suggesting that the hydride readdition easily proceeds rather than the O2 insertion along the reaction path 9 f 12. However, the calculation results along the reaction path 9 f 12 were

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Figure 8. Energy diagrams for (a) the O2 insertion to Ru-H species and (b) the H2O2 formation steps.

inconsistent with the experimental facts that the reaction of the Ru-H species with O2 (catalytic alcohol oxidation) preferentially proceeded rather than the hydride readdition (racemization) in the presence of O2 (Figure S3, Supporting Information). Therefore, other paths for the O2 insertion should be considered. Next, the calculations for the O2 insertion were carried out according to the following paths, using the Ru-H species with 2-butanone (8), 2-butanol (9b), and OH2 (9c) as ligands (Figure 9a): (i) the adsorption of O2 to Ru-H species (8 f 10a, 9b f 10b, or 9c f 10c), (ii) followed by the O2 insertion (10a f 13a, 10b f 13b, or 10c f 13c). In contrast with the reaction path along 9 f 12, the Ru atoms are six-coordinated along the reaction paths 8 f 13a, 9b f 13b, and 9c f 13c. Figure 9b shows the optimized and the transition state structures in the reaction steps. In the same way as the reaction path 9 f 12, the O-O double bonds (O2 molecule) changed to the single bonds (1.45-1.47 Å) via the transition states in all paths (Table 3). The energy diagram along these reaction paths is shown in Figure 8a (green lines for 8 f 13a, red lines for 9b f 13b, and blue lines for 9c f 13c). The activation energies Ea(10af13a), Ea(10bf13b), and Ea(10cf13c) were calculated to be 40.6, 15.9, and 20.1 kJ mol-1, respectively, and were lower than Ea(10f12) (49.8 kJ mol-1). In particular, the Ea(10bf13b) and Ea(10cf13c) values were significantly lower than Ea(8f6), showing that the ligands (especially an alcohol and OH2) would play an important role in the O2 insertion step. Thus, the O2 insertion likely proceeds along the reaction paths 9b f 13b and 9c f 13c rather than 9 f 12.17 The natural bonding orbital (NBO) charges13 of the ligands within 8 (2-butanone), 9b (2-butanol), and 9c (OH2) were +0.13, +0.16, and +0.10, respectively. The positive NBO charges of these ligands can be interpreted in terms of the electron donation from these ligands to the hydride species. Indeed, the Ru-H bond lengths in 8, 9b, and 9c were 1.63, 1.66, and 1.62 Å, respectively, and were longer than that in the five-coordinated

Figure 9. (a) Schematic diagrams and (b) optimized and transition state structures for the reaction of Ru-H species with O2 along the reaction paths 8 f 13a, 9b f 13b, and 9c f 13c. The blue, red, gray, and white spheres indicate the Ru, O, C, and H atoms, respectively.

Reaction Mechanism for Aerobic Alcohol Oxidation

J. Phys. Chem. C, Vol. 114, No. 24, 2010 10879 Acknowledgment. This work was supported in part by the Global COE Program (Chemistry Innovation through Cooperation of Science and Engineering) and Grants-in-Aid for Scientific Researches from Ministry of Education, Culture, Sports, Science and Technology. Supporting Information Available: Additional experimental section, additional results and discussion (experimentally proposed reaction mechanism), additional references, Cartesian coordinates of the optimized structures, and Tables S1-S4 and Figures S1-S7 as detailed in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 10. (a) Schematic diagrams and (b) optimized and transition state structures for the H2O2 formation step along the reaction paths 13b f 6 and 13c f 2. The blue, red, gray, and white spheres indicate the Ru, O, C, and H atoms, respectively.

9 without these ligands (1.57 Å). Therefore, the Ru-H bond would become weaker by the electron donation from these ligands. As a result, the activation energies for the O2 insertion to the Ru-H species became lower in the presence of the electron-donating ligands in comparison with that in the absence. Finally, the alcoholate species 6 or hydroxide species 2 is regenerated with the formation of H2O29 along the reaction path 13b f 6 or 13c f 2, which can be divided into the following elementary steps: (i) the H2O2 formation via TS14b or TS14c (13b f 15b or 13c f 15c), (ii) followed by the elimination of H2O2 to regenerate 6 or 2 (15b f 6 or 15c f 2) (Figure 10). The activation energies for the H2O2 formation steps via TS14b (Ea(13bf15b)) and TS14c (Ea(13cf15c)) were calculated to be 27.1 and 39.4 kJ mol-1, respectively (Figure 8b). Thus, the regeneration of 6 or 2 can easily take place along the reaction path 13b f 6 or 13c f 2, and the catalytic cycle is completed. Conclusion The experimentally proposed reaction mechanism for the Ru(OH)x/support-catalyzed aerobic alcohol oxidation was theoretically investigated by means of quantum chemistry calculations. On the basis of experimental results, a very simple model catalyst “Ru(OH)3(OH2)3” could be proposed for Ru(OH)x/ support catalysts. By using the Ru(OH)3(OH2)3 model catalyst, the experimentally proposed alcoholate formation and β-hydride elimination steps could be verified. The alcoholate formation step was found to be promoted by the “concerted actiVation” of an alcohol by the Lewis acid and Brønsted bases on Ru(OH)x/ support. In addition, it was found that the coordination of an alcohol or OH2 to form the six-coordinated Ru-H species would play an important role in the reaction of the hydride species with O2 to form the Ru-OOH species. The activation energy for the formation of the Ru-OOH species became lower by the electron donation from these ligands. After the Ru-OOH species were formed, the ligand exchange between the hydroperoxide species and an alcohol or water regenerated the alcoholate or hydroxide species with the formation of H2O2.

(1) (a) Sheldon, R. A.; Kochi, J. K. Metal Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (b) Hill, C. L. In AdVances in Oxygenated Processes; Baumstark, A. L., Ed.; JAI Press: London, UK, 1988; Vol. 1, pp 1-30. (c) Hudlicky, M. Oxidations in Organic Chemistry; ACS Monograph Series; American Chemical Society: Washington, DC, 1990. (d) Arends, I. W. C. E.; Sheldon, R. A. In Modern Oxidation Methods; Ba¨ckvall, J.-E., Ed.; Wiley-VCH: Weinheim, 2004, pp 83-118. (e) Murahashi, S.-I.; Komiya, N. In Modern Oxidation Methods; Ba¨ckvall, J.-E., Ed.; Wiley-VCH: Weinheim, Germany, 2004; pp 83-118. (2) (a) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: London, UK, 1998. (b) Sheldon, R. A. Green Chem. 2000, 2, G1. (c) Anastas, P. T.; Bartlett, L. B.; Kirchhoff, M. M.; Williamson, T. C. Catal. Today 2000, 55, 11. (3) (a) Sheldon, R. A.; van Bekkum, H. Fine Chemical through Heterogeneous Catalysis; Wiley: Weinheim, Germany, 2001. (b) Centi, G.; Cavani, F.; Trifiro, F. SelectiVe Oxidation by Heterogeneous Catalysis; Kluwer: New York, 2001. (c) Warren, B.; Oyama, S. T. Heterogeneous Hydrocarbon Oxidation; American Chemical Society: Washington, D.C., 1996. (d) Bhan, A.; Iglesia, E. Acc. Chem. Res. 2008, 41, 559. (4) (a) Sheldon, R. A.; Arends, I. W. C. E.; Dijisman, D. Catal. Today 2000, 57, 157. (b) Mallat, T.; Baiker, A. Chem. ReV. 2004, 104, 3037. (c) Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Chem. Asian J. 2008, 3, 196. (5) (a) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 41, 4538. (b) Yamaguchi, K.; Mizuno, N. Chem.sEur. J. 2003, 9, 4353. (c) Yamaguchi, K.; Kim, J. W.; He, J. L.; Mizuno, N. J. Catal. 2009, 268, 343. (6) (a) Yamaguchi, K.; Matsushita, M.; Mizuno, N. Angew. Chem., Int. Ed. 2004, 43, 1576. (b) Kamata, K.; Kasai, J.; Yamaguchi, K.; Mizuno, N. Org. Lett. 2004, 6, 3577. (c) Yamaguchi, K.; Koike, T.; Kotani, M.; Matsushita, M.; Shinachi, S.; Mizuno, N. Chem.sEur. J. 2005, 11, 6574. (d) Matsushita, M.; Kamata, K.; Yamaguchi, K.; Mizuno, N. J. Am. Chem. Soc. 2005, 127, 6632. (e) Kim, J. W.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2008, 47, 9246. (f) Yamaguchi, K.; Koike, T.; Kim, J. W.; Ogasawara, Y.; Mizuno, N. Chem.sEur. J. 2008, 14, 11480. (g) He, J. L.; Kim, J. W.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2009, 48, 9889. (7) (a) Madhavaram, H.; Idriss, H.; Wendt, S.; Kim, Y. D.; Knapp, M.; Over, H.; Assmann, J.; Loffler, E.; Muhler, M. J. Catal. 2001, 202, 296. (b) Kim, Y. D.; Seitsonen, A. P.; Wendt, S.; Wang, J.; Fan, C.; Jacobi, K.; Over, H.; Ertl, G. J. Phys. Chem. B 2001, 105, 3752. (8) The hydroperoxide (peroxide) paths have been proposed for the metal-catalyzed aerobic alcohol oxidation. See: (a) Yamaguchi, K.; Mori, K.; Mizugaki, T.; Ebitani, K.; Kaneda, K. J. Am. Chem. Soc. 2000, 122, 7144. (b) Zhan, B.-Z.; White, M. A.; Sham, T.-K.; Pincock, J. A.; Doucet, R. J.; Rao, K. V. R.; Robertson, K. N.; Cameron, T. S. J. Am. Chem. Soc. 2003, 125, 2195. (c) Konnick, M. M.; Stahl, S. S. J. Am. Chem. Soc. 2008, 130, 5753. (d) Jiang, B.; Feng, Y.; Ison, E. A. J. Am. Chem. Soc. 2008, 130, 14462. (e) Popp, B. V.; Stahl, S. S. Chem.sEur. J. 2009, 15, 2915. (9) While the formation of peroxide species (e.g., Ru-OOH and H2O2) is suggested for step 3 in Figure 1, the filtrate for the oxidation of benzyl alcohol showed a negative peroxide test (using Quantofix test stick, detection limit: 1 mg/L H2O2). Further, it was confirmed by a separate experiment that H2O2 was very rapidly decomposed to O2 and H2O by the presence of Ru(OH)x/support catalysts. (10) van Santen, R. A.; Neurock, M. Molecular Heterogeneous Catalysis; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2006. (11) Frisch, M. J., et al. Gaussian 03, Revision D.02; Gaussian, Inc., Wallingford, CT, 2004. (12) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (13) Glendening, E. D.; Reed, A. E.; Carpenter, J. E.; Weinhold, F. NBO, Version 3.1.

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(14) Under the same reaction conditions, Ru(OH)x/support gave a quantitative yield of acetophenone (see Table S1 in the Supporting Information). (15) (a) Ba¨ckvall, J.-E.; Chowdhury, R. L.; Karlsson, U. J. Chem. Soc., Chem. Commun. 1991, 473. (b) Trost, B. M.; Kulawiec, R. J. Tetrahedron Lett. 1991, 32, 3039. (c) Zassinovich, G.; Mestroni, G.; Gladiali, S. Chem. ReV. 1992, 92, 1051. (d) Trost, B. M.; Kulawiec, R. J. J. Am. Chem. Soc. 1993, 115, 2027. (e) Hanyu, A.; Takezaki, E.; Sakaguchi, S.; Ishii, Y. Tetrahedron Lett. 1998, 39, 5557. (f) Slugovc, C.; Ruba, E.; Schmid, R.; Kirchner, K. Organometallics 1999, 18, 4230. (g) Marko´, I. E.; Gautier, A.; Tsukazaki, M.; Llobet, A.; Plantalech-Mir, E.; Urch, C. J.; Brown, S. M. Angew. Chem., Int. Ed. 1999, 38, 1960. (h) Aranyos, A.; Csjernyik, G.; Szabo´, K. J.; Ba¨ckvall, J.-E. Chem. Commun. 1999, 351. (i) Martı´n-Matute, B.; Boga´r, K.; Edin, M.; Kaynak, F. B.; Ba¨ckvall, J.-E. Chem.sEur. J. 2005, 11, 5833. (j) Ito, M.; Kitahara, S.; Ikariya, T. J. Am. Chem. Soc. 2005, 127, 6172. (k) Cadierno, V.; Francos, J.; Gimeno, J.; Nebra, N. Chem. Commun. 2007, 2536. (16) (a) Bakac, A. J. Am. Chem. Soc. 1997, 119, 10726. (b) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 11900. (c) Thyagarajan, S.; Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. Chem. Commun. 2001, 2198. (d) Denney, M. C.; Smythe, N. A.; Cetto, K. L.; Kemp, R. A.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 2508. (e) Keith, J. M.; Muller, R. P.; Kemp, R. A.; Goldberg, K. I.; Goddard, W. A., III; Oxgaard, J. Inorg. Chem. 2006, 45, 9631. (f) Look, J. L.; Wick, D. D.; Mayer, J. M.; Goldberg, K. I. Inorg. Chem. 2009, 48, 1356.

Nikaidou et al. (17) When the Ru(OH)x/support-catalyzed MPV-type reduction of acetophenone was carried out with 2-deuterio-2-propanol (hydrogen source), the R-deuterium of 2-deuterio-2-propanol was transferred to the carbonyl carbon of acetophenone to afford racemic 1-deuterio-1-phenylethanol.6c In addition, when the racemization of a mixture of (S)-1-deuterio-1-(ptolyl)ethanol (deuterium labeled alcohol) and (R)-1-phelylethanol (unlabeled alcohol) was carried out, the scrambling of the deuterium between the two racemized alcohols proceeded (eq 3). All these results suggest that the carbonyl compound produced does not remain coordinate to the Ru center after the β-elimination has taken place (step 8 f 9 in Figure 4)

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