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Synthesis, Reactivity, and Catalytic Transfer Hydrogenation Activity of Ruthenium Complexes Bearing NNN Tridentate Ligands: Influence of the Secondary Coordination Sphere Jing Shi,† Bowen Hu,† Xiangyang Chen,‡,∥ Shu Shang,† Danfeng Deng,† Yanan Sun,† Weiwei Shi,† Xinzheng Yang,*,‡ and Dafa Chen*,†,§ †

MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China ‡ Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China § State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China ∥ Institute of Chemistry, University of Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: By the introduction of −OH group(s) into different position(s) of 6-(pyridin-2-ylmethyl)-2,2′-bipyridine, several NNN-type ligands were synthesized and then introduced to ruthenium (Ru) centers by reactions with RuCl2(PPh3)3. In the presence of PPh3 or CO, these ruthenium complexes reacted with NH4PF6 in CH2Cl2 or CH3OH to give a series of ionic products 5−9. The reaction of Ru(L2)(PPh3)Cl2 (2) with CO generated a neutral complex [Ru(L2)(CO)Cl2] (10). In the presence of CH3ONa, 10 was further converted into complex [Ru(L2)(HOCH3)(CO)Cl] (11), in which there was a methanol molecule coordinating with ruthenium, as suggested by density functional theory calculations. The catalytic transfer hydrogenation activity of all of these new bifunctional metal−ligand complexes was tested. Dichloride complex 2 exhibits best activity, whereas carbonyl complexes 10 and 11 are efficient for selectively reducing 5-hexen-2-one, suggesting different hydrogenation mechanisms. The results reveal the dramatic influence for the reactivity and catalytic activity of the secondary coordination sphere in transition metal complexes.

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group.42−51 Recently, Szymczak et al. have developed a tridentate ruthenium catalyst bearing 6,6′-dihydroxy terpyridine (dhtp) as the ligand for the transfer hydrogenation of ketones (Figure 1b), and both of its activity and selectivity are better than similar ruthenium complexes incorporating 4,4′-dihydroxy terpyridine or terpyridine as the ligand.52,53 The results confirmed the importance of the 2-hydroxy group in the system.53 Kundu’s group then synthesized another similar ruthenium complex based on the ligand of 2-(2-pyridyl-2-ol)1,10-phenanthroline (Figure 1c), showing a final time-of-flight (TOF) of 2.40 × 103 h−1 for the transfer hydrogenation of acetophenone.54 Inspired by the structure of [Fe]-hydrogenase and Szymczak’s work, we have also synthesized several ruthenium catalysts with high activity for the transfer hydrogenation of ketones based on the ligand of 6-(2,2′-bipyridin-6-ylmethyl)pyridin-2-ol (L1) (Figure 2), which is more similar to the [Fe]hydrogenase active site than dhtp.55 For example, complex

INTRODUCTION

Metal−ligand cooperation has been receiving more and more attention in recent years.1−13 One of the strategies to design metal−ligand cooperative catalysts is to introduce a 2hydroxypyridyl moiety into the metal center.14−33 In the presence of a base, 2-hydroxypyridyl could be deprotonated to a pyridonate form with a CO bond, which directly affects the first coordination sphere of the metal center.14,15 For example, Fujita, Yamaguchi, and co-workers developed a series of iridium complexes bearing bidentate 2-hydroxypyridyl derivatives for dehydrogenative oxidation of alcohols,16−19 dehydrogenation of fused bicyclic N-heterocycles,20 dehydrogenative lactonization of diols,21 and hydrogen production from methanol−water solution.22 Himeda, Fujita, and co-workers utilized a bimetallic tetrahydroxybipyrimidine−iridium complex for the hydrogenation of CO2 to formate in aqueous solution under mild conditions.25 2-Hydroxypyridyl ligand was also found in [Fe]-hydrogenase. In its active site, Fe is coordinated with a special bidentate 2hydroxy-6-acylmethylenepyridyl (2-HO-6-CH2COC5H3N) derivative (Figure 1a).34−41 Both experimental and computational studies have indicated the essential role of the 2-hydroxy © 2017 American Chemical Society

Received: April 6, 2017 Accepted: July 3, 2017 Published: July 11, 2017 3406

DOI: 10.1021/acsomega.7b00410 ACS Omega 2017, 2, 3406−3416

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Figure 1. (a) Active site of [Fe]-hydrogenase; (b) Szymczak’s ruthenium complex; (c) Kundu’s ruthenium complex; and (d) ruthenium complex.

Figure 2. Designed ligands in this paper.

Ru(L1)(PPh3)Cl2 (1) (Figure 1d) exhibits a final TOF of 1.16 × 103 h−1 for the transfer hydrogenation of acetophenone.55 Because of the dramatic catalytic influence of the secondary coordination sphere in Szymczak’s systems,52−54,56,57 as an extension of our previous work, we designed a series of new ligands (L2−L4) by modification of L1 and introduced them to ruthenium centers (Figure 2). Herein, we report the synthesis, reactivity, and catalytic activity for ketone transfer hydrogenation of these ruthenium complexes supported by L1−L4.

methanol to give the product [(O−C5H3N−CH2−C5H3N− CH2−C5H4N−O)Ru2(CO)4(PPh3) (4) via deprotonations (Scheme 2). The infrared (IR) spectrum of 4 exhibits four Scheme 2. Synthesis of Complex 4

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RESULTS AND DISCUSSION Synthesis of Complexes 2−6. When 6′-(pyridin-2ylmethyl)-2,2′-bipyridin-6-ol (L2) and 6′-((6-hydroxypyridin2-yl)methyl)-2,2′-bipyridin-6-ol (L3 ) were treated with RuCl2(PPh3)3 in refluxing methanol, complexes Ru(L2)(PPh3)Cl2 (2) and Ru(L3)(PPh3)Cl2 (3) were obtained, respectively (Scheme 1). The proton nuclear magnetic resonance (1H

strong CO absorptions, suggesting different structures from 1− 3. From the X-ray single-crystal structure, we can see that 4 is a dinuclear ruthenium complex (Figure 3). One Ru atom coordinates with three pyridyl rings and two cis-CO groups, the other links with two cis-CO, one PPh3, and two O atoms, and the two Ru atoms are linked by a Ru−Ru bond. Different from 1−3, the three pyridyl groups in 4 are in facial configuration, and there is no O−H bond existing any more. The four CO ligands must be from the dehydrogenation of methanol.58 Complexes 2 and 3 were further treated with NH4PF6 in the presence of PPh3, and the products [Ru(L2)(PPh3)2Cl][PF6] (5) and [Ru(L3)(PPh3)2Cl][PF6] (6) were obtained (Scheme 3). 6 was not obtained in a pure form because there was always some unidentified impurities. When treated with excess of HCl, complexes 2 and 3 could be re-formed (Scheme 3). A similar transformation has been reported in our previous work.55 Catalysis. We investigated the catalytic activity of 2−5 for ketone transfer hydrogenation reactions. Acetophenone was chosen as the substrate, and iPrOH was chosen as the solvent (Table 1). The reactions were performed with a molar ratio of 200/10/1 for acetophenone/base/catalyst under N2. From entries 2−5, we can see that complex 2 is the most active catalyst in the presence of iPrOK, giving a 96% yield within 15 min. However, compared with 1, complex 2 still shows a little lower efficiency (entry 1). When NaOH, KOH, or tBuOK was chosen as the base, complex 2 also exhibited high activity (84− 92%), although the results were not as good as iPrOK (entries 6−8). If the reaction was carried out under air, the yield

Scheme 1. Synthesis of Complexes 2 and 3

NMR) spectrum of 2 in CD3OD shows two doublets for the −CH2− group at 4.10 and 3.57 ppm, indicating that the two protons are in different environments, which means the two Cl atoms linking with Ru are cis to each other. The phosphorus-31 nuclear magnetic resonance (31P NMR) spectrum of 2 shows one singlet for the PPh3 group at 56.5 ppm, comparable to that of 1.55 Both the 1H NMR and 31P NMR spectra of 3 are similar to those of 2. By using similar procedures as described above, no reactivity was found between 6,6′-(pyridine-2,6-diylbis(methylene))dipyridin-2-ol (L4) and RuCl2(PPh3)3. It might be due to the lower coordination ability of L4 caused by its high flexibility. Under a harsher condition, L4 reacted with RuCl2(PPh3)3 in 3407


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Table 1. Optimization of Reaction Conditions for the Transfer Hydrogenation of Acetophenone Catalyzed by Ru Complexes 1−5a

entry c

1 2 3 4 5 6 7 8 9d 10 11

base

time (min)

yield (%)b

PrOK PrOK i PrOK i PrOK i PrOK NaOH KOH t BuOK i PrOK i PrOK

15 15 15 15 15 15 15 15 15 15 15

98 96 28 26 81 84 84 92 13 0 0

cat. 1 2 3 4 5 2 2 2 2 2

i i

a

Figure 3. X-ray single-crystal structure of 4. The thermal ellipsoids are displayed at 30% probability. Hydrogen atoms have been omitted for clarity. Selected bond distances (angstrom) and angle (deg): Ru(1)− Ru(2), 2.7289(3); Ru(2)−N(1), 2.2085(17); Ru(2)−N(2), 2.2862(19); Ru(2)−N(3), 2.2150(18); Ru(1)−P(1), 2.4221(6); Ru(1)−O(1), 2.0850(16); Ru(1)−O(2), 2.0827(17); C(1)−O(1), 1.282(3); C(17)−O(2), 1.268(3); N(1)−Ru(2)−N(3), 98.77(7); N(1)−Ru(2)−N(2), 81.75(7); and N(2)−Ru(2)−N(3), 83.91(7).

General conditions: acetophenone, 2.0 mmol (2 M in 50mL iPrOH); acetophenone/base/cat. = 200/10/1; N2 (0.1 MPa); 82 °C. bAverage yield [gas chromatography (GC)] for two runs. cData from ref 55. d Reaction was carried out under air.

methyl group decreased the reactivity obviously (TOF: 128 h−1; Table 2, entry 11), which is consistent with the reported results.59−62 Benzophenone and 2-acetonaphthone reacted smoothly to give the desired product in 89−91% yields within 120 min (Table 2, entries 15 and 16). Aliphatic ketones, such as cyclohexanone, cyclopentanone, 2-heptanone, and 3-pentanone, showed moderate reactivity (TOF: 182−576 h−1; Table 2, entries 17−20). Two unsaturated aliphatic ketones, including 5-hexen-2-one and 6-methyl-5-hepten-2-one, were also tested. For 5-hexen-2one, both of the CO and CC bonds could be hydrogenated, and a mixture of 5-hexen-2-ol, 2-hexanone, and 2-hexanol was obtained, with the ratio of 1:0.22:0.63 in high conversion (94%) over 360 min (Table 2, entry 21). The low selectivity is similar to that catalyzed by 1 and suggests that the reactions might also follow an inner-sphere hydrogenation mechanism.55 Interestingly, only the carbonyl group was hydrogenated when 6-methyl-5-hepten-2-one was used as the starting material, and 6-methyl-5-hepten-2-ol was obtained in 94% yield over 180 min (Table 2, entry 22). It means that the chemoselectivity can be improved by the introduction of bulky groups into the terminal alkenyl carbon atom. Complexes 1−3 have similar structures; however, the catalytic activities of 1 and 2 are different from that of 3 for transfer hydrogenation of acetophenone (Table 1, entries 1−3). 1 shows a little higher efficiency than 2, and both 1 and 2 are much more active than 3. The results might be due to electronic differences between the ligands under the reaction conditions. To further understand the reasons, inspired by Szymczak’s work,53 we introduced the CO group into the Ru centers. The ligand fields can be evaluated by comparing the CO absorption bands in their infrared spectra. Synthesis of CO Derivatives 7−11. When complexes 1− 3 were treated with NH4PF6 in the presence of 1 atm of CO gas in CH3OH or CH2Cl2, products [Ru(L1)(PPh3)(CO)Cl][PF6] (7), [Ru(L2)(PPh3)(CO)Cl][PF6] (8), and [Ru(L3)(PPh3)(CO)Cl][PF6] (9) were obtained (Scheme 4). The 1H NMR

Scheme 3. Synthesis of Complexes 5 and 6

decreased to 13% (entry 9). Without a catalyst or base, no transformation was found (entries 10 and 11). It should be noticed that in the presence of 2, a trace of base could make the reaction occur; therefore, the reaction bottles must be cleaned completely to prevent base contamination. On the basis of the above results, iPrOK and complex 2 were chosen as the base and the catalyst, respectively, and a variety of ketones were applied as substrates for transfer hydrogenation reactions (Table 2). To guarantee the yields achieved were 90% or above (except entries 15 and 21 because of the relatively slow rates), the reaction times were optimized, and from GC results, it can be seen that no other products were formed besides the corresponding alcohols. For aryl ketones, the yields reached 89−96% within 15−180 min, with TOF values up to 768 h−1 (Table 2, entries 1−15). 1-Phenylethanol was obtained in 96% yield within 15 min through the reduction of acetophenone (TOF: 768 h−1; Table 2, entry 1). Halogen atoms and the ortho-methyl group in the phenyl ring did not influence the reaction rates obviously (TOF: 437−744 h−1; Table 2, entries 2−10), whereas para-methoxy and para-amino groups decreased the reactivity remarkably (TOF: 37−61 h−1; Table 2, entries 13 and 14). It should be noticed that a meta3408


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Table 2. Transfer Hydrogenation of Ketone Catalyzed by Complex 2a

Scheme 4. Synthesis of Complexes 7−9

(PPh3)(CO)Cl][PF6].53 8 and 9 have structures similar to that of 7, with CO absorptions at 1950 and 1998 cm−1, respectively. Interestingly, in the absence of NH4PF6, 2 reacted with CO in CH2Cl2 generating the complex [Ru(L2)(CO)Cl2] (10), through the replacement of PPh3 by CO (Scheme 5). By Scheme 5. Synthesis of Complexes 10 and 11

contrast, 1 and 3 reacted with CO, generating unidentified mixtures. The IR spectrum of 10 exhibits one CO absorption at 1981 cm−1. The X-ray crystal structure shows two cis chlorides in the solid state (Figure 4). The O(1)−Cl(2) distance (2.909 Å) in 10 reveals strong hydrogen bonding between the hydroxy

a

General conditions: ketone, 2.0 mmol (2 M in 50 mL iPrOH); ketone/iPrOK/cat. = 200/10/1; N2 (0.1 MPa); 82 °C. bAverage yield (GC) for two runs. cTOF value calculated with GC yield. dYield of 5hexen-2-ol. Note that other products were observed, see text. Figure 4. X-ray single-crystal structure of 10. The thermal ellipsoids are displayed at 30% probability. Hydrogen atoms [except H(1)] and solvents have been omitted for clarity. Selected bond distances (angstrom) and angle (deg): Ru(1)−N(1), 2.083(4); Ru(1)−N(2), 2.021(4); Ru(1)−N(3), 2.092(4); Ru(1)−Cl(1), 2.4556(13); Ru(1)− Cl(2), 2.4416(13); Ru(1)−C(17), 1.825(6); H(1)···Cl(2), 2.115; C(1)−O(1), 1.327(7); N(2)−Ru(1)−Cl(2), 175.14(13); N(1)− Ru(1)−N(3), 168.38(17); C(17)−Ru(1)−Cl(1), 178.70(18); and O(1)−H(1)−Cl(2), 163.19.

spectrum of 7 in d6-acetone shows two doublets for the −CH2− group at 4.02 and 3.43 ppm. The 31P NMR spectrum shows one singlet at 44.4 ppm for the PPh3 group and one multiplet at −143.9 ppm for the PF6− anion. The IR spectrum exhibits a strong CO absorption at 1968 cm−1. From the NMR and IR data, the molecular structure of 7 can be confirmed, as shown in Scheme 4, which is similar to that of [Ru(dhtp)3409


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reactivity dramatically, although the detailed reason is still unclear.53 As described by Szymczak and co-workers, an electron-rich ruthenium hydride species would facilitate insertion into the ketone substrate;53 hence, we started to compare their CO absorptions. The νCO data of the deprotonated 7−9 are 1952, 1926, and 1974 cm−1, revealing that the ligand field of the deprotonated L3 is the weakest, which makes the metal center of the deprotonated 9 have the lowest electron density and donates the least electrons into the CO π* orbitals. The results are consistent with the findings that 3 shows the lowest catalytic activity among 1−3. However, if only electronic effects are regarded, 2 should have exhibited higher activity than 1. It suggests that for complexes 1−3, the hydroxy position is also an important factor to influence the activity. Catalysis. Complexes 7−11 were also tested for the transfer hydrogenation reaction of acetophenone (Table 4). None of

group and Cl(2). The C(1)−O(1) distance [1.327(7) Å] is in the range of the C−O single bond. Complex 10 further reacted with CH3ONa to give product 11 (Scheme 5). The IR spectrum shows one CO absorption at 1946 cm−1 in the solid state. The 1H NMR spectrum exhibits two doublets at 4.90 and 4.47 ppm for the −CH2− group and one singlet at 3.33 ppm for the methoxy group. The 13C NMR exhibits one signal at 197.2 ppm for the terminal carbonyl group and two signals at 48.5 and 46.4 ppm for the −CH2− and methoxy groups, respectively. These data suggested that 11 was formed by replacing one Cl in 10 with a methoxy group. Although the molecular formula can be confirmed, the exact structure of 11 is still unknown. The stabilities of three possible isomers have been examined through density functional theory (DFT) calculations (further computational details are provided in the Supporting Information). The relative energies are shown in Table 3. Complex 11 is the most stable one, in which the hydroxyl proton moves to the oxygen in the methoxy group and forms a methanol molecule.

Table 4. Optimization of Reaction Conditions for the Transfer Hydrogenation of Acetophenone Catalyzed by Ru Complexes 7−11a

Table 3. Absolute and Relative Electronic Energies of the Possible Isomers of 11

complex

E [hartree]

ΔE [kcal mol−1]

11 11′ 11″

−1640.02282157 −1640.00533298 −1639.99432036

0.0 11.0 17.9

entry

cat.

time (min)

yield (%)b

TON

1 2 3 4 5 6 7 8 9 10

7 7 8 8 9 9 10 10 11 11

15 60 15 60 15 60 15 60 15 60

41 94 65 96 35 77 62 95 67 97

82 188 130 192 70 154 124 190 134 194

a

General conditions: acetophenone, 2.0 mmol (2 M in 50 mL PrOH); acetophenone/iPrOK/cat. = 200/10/1; N2 (0.1 MPa); 82 °C. b Average yield (GC) for two runs. i

The only structural difference among 7−9 is the NNN ligands, and all of these ligands contain the 6-(pyridin-2ylmethyl)-2,2′-bipyridine fragment; hence, they are good models for exploring the relationship between the ligand field strength and catalytic activity. In the catalytic process, ligands L1−L3 should be deprotonated to anionic forms in the first step; hence, the reactivity of 7−9 with the base was investigated. The reactions could be monitored by IR spectroscopy. Upon treatment of 7 with 4 or 10 equiv of i PrOK in dimethyl sulfoxide (DMSO) solution, the CO absorption shifted from 1968 to 1952 cm−1. The red-shifted phenomenon is attributed to enhanced electron density at the metal center upon ligand deprotonation, leading to greater donation into the CO π* orbitals. The decrease in νCO of 16 cm−1 is somewhat similar to that observed for a monocarbonyl copper complex featuring a single 2-hydroxypyridine group (Δν = −26 cm−1).63 Similarly, deprotonation of 8 shifted νCO from 1950 to 1926 cm−1 (Δν = −24 cm−1). Surprisingly, when 9 was treated with excess of iPrOK (4 or 10 equiv), the bathochromic shift in the magnitude of the CO band was only 24 cm−1 (from 1998 to 1974 cm−1), suggesting that only one of the two hydroxy groups was deprotonated.53 The shift data are much less than those observed upon double deprotonation of [Ru(dhtp)(PPh3)(CO)Cl][PF6] (Δν = −77 cm−1), indicating that the introduction of a −CH2− group into dhtp influences the

these complexes were as active as 2. Under the same condition as entry 2 in Table 1, all yields reached above 90% within 60 min except 9 that was used as the catalyst (Table 4, entries 2, 4, 6, 8, and 10). In 15 min, 7−9 provided 82, 130, and 70 turnovers (Table 4, entries 1, 3, and 5). The catalytic activity is in the order of 8, 7, and 9, consistent with that of the ligand fields. The results suggest that in the cases of 7−9, the electron densities in the Ru centers might play a dominant role for their activities. Probably with similar reasons, the reactivity of 8 with the π-acidic auxiliary ligand is lower than that of 5 (Table 1, entry 5).53 As discussed above, complex 2 is not efficient for selectively reducing 5-hexen-2-one (Table 2, entry 21). When 7−9 were applied for the same reaction, the reactivity was very low, with less than 20% conversion in 4 h (Figure 5). Whereas unexpectedly, 10 and 11 are both efficient for this reaction, with above 90% conversion in 5 h, yielding almost exclusively the carbonyl reduction product (Figure 5). The chemoselectivity data of 10 and 11 suggest that the reaction mechanism might follow an outer-sphere hydrogenation pathway, which is different from that of 2.52−54,64 Ruthenium complexes 1−9 all contain PPh3 ligands, which might be dissociated during the catalytic process, making their catalytic processes follow an outer-sphere hydrogenation pathway. There is no PPh3 in 10 and 11 and that might be one of the reasons that they facilitate the outer-sphere hydrogenation 3410


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analyses were performed on a PerkinElmer 240C analyzer. Mass spectra were recorded on an Agilent 6530 Accurate-Mass Quadrupole Time-of-Flight (Q-TOF) mass spectrometer, equipped with an electrospray ionization (ESI) source. X-ray diffraction studies were carried out in an Xcalibur E X-ray single-crystal diffractometer. Data collections were performed using four-circle kappa diffractometers equipped with chargecoupled device (CCD) detectors. Data were reduced and then corrected for absorption.65 Solution, refinement, and geometrical calculations for all crystal structures were performed by SHELXTL.66 All DFT calculations were performed in the Gaussian 09 suite of programs67 by using the M06 functional68 and an ultrafine integration grid (99 590) in conjugation with the all-electron 6-31++G(d,p) basis set69 for C, H, N, O, and Cl atoms. The Stuttgart relativistic effective core potential basis set is used for Ru (ECP28MWB).70,71 All three structures were fully optimized in the gas phase and verified as local minima through frequency calculations. L2−L4 were synthesized by using procedures similar to that described for L1, and complexes 2 and 3 were synthesized similar to 1.55 Synthesis of 2-Bromo-6-((pyridin-2-yl)methyl)pyridine. 2-Methyl-pyridine (3.7 g, 40.0 mmol) in 60 mL tetrahydrofuran (THF) was added drop by drop to a solution of n-BuLi (16.0 mL, 2.5 M, 40.0 mmol) at −78 °C. After 60 min of stirring at −20 °C, 2,6-dibromopyridine (4.7 g, 20.0 mmol) was added. The reaction mixture was refluxed for 4 h. Water (15 mL) was then added to quench the reaction. The water phase was extracted with CH2Cl2. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, v/v = 2:1) to give the product as a red liquid (1.86 g, 37%). High-resolution mass spectrometry (HRMS) (ESI): calcd for C11H9Br3N2 + H, 249.0027; found, 249.0029. 1H NMR (400 MHz, CDCl3, ppm): 8.55 (br s, 1H), 7.63 (t, J = 7.6 Hz, 1H), 7.45 (t, J = 7.6 Hz, 1H), 7.34−7.27 (m, 2H), 7.21 (d, J = 7.6 Hz, 1H), 7.15 (br s, 1H), 4.32 (s, 2H). 13C NMR (100 MHz, CDCl3, ppm): δ 160.7, 158.3, 149.2, 141.3, 138.6, 136.5, 125.6, 123.5, 122.2, 121.5, 46.5. Synthesis of 2-(6-Methoxypyridin-2-yl)-6-((pyridin-2yl)methyl)pyridine. A toluene solution (100 mL) containing 2-bromo-6-((pyridin-2-yl)methyl)pyridine (1.9 g, 7.0 mmol), 2-(tributylstannyl)-6-methoxypyridine (2.8 g, 7.0 mmol), LiCl (3.0 g, 70 mmol), and Pd(PPh3)4 (0.4 g, 0.35 mmol) was deoxygenated by bubbling N2 through it. The mixture was then refluxed under N2 for 2 weeks, and an aqueous solution (50 mL) of NaF (3.0 g, 70 mmol) was added to the solution at room temperature. The resultant mixture was further stirred for 30 min at room temperature. The insoluble solid was filtered off, and the filtrate was treated with a 5% Na2CO3 aqueous solution in a separating funnel. The organic layer was dried with anhydrous sodium sulfate, and the solvents were evaporated to dryness under reduced pressure. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, v/v = 2:1) to give the product as a yellow liquid (1.83 g, 94%). HRMS (ESI): calcd for C17H15N3O + H, 278.1293; found, 278.1285. 1H NMR (400 MHz, CDCl3, ppm): 8.57 (d, J = 5.2 Hz, 1H), 8.25 (d, J = 8.0 Hz, 1H), 8.04 (d, J = 8.0 Hz, 1H), 7.73−7.60 (m, 3H), 7.35 (d, J = 8.0 Hz, 1H), 7.23 (d, J = 8.0 Hz, 1H), 7.15 (m, 1H), 6.76 (d, J = 8.0 Hz, 1H), 4.42 (s, 2H), 4.03 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ 163.4, 159.7, 158.6, 155.6, 153.6, 149.2, 139.2, 137.2, 136.5, 123.7, 123.2, 121.4, 118.6, 113.8, 110.8, 53.1, 47.4. Synthesis of L2. 2-(6-Methoxypyridin-2-yl)-6-((pyridin-2yl)methyl)pyridine (1.5 g, 5.4 mmol) was added into HBr (15

Figure 5. Chemoselectivity of transfer hydrogenation for complexes 7−11. Conditions: 5-hexen-2-one, 2.0 mmol (2 M in 50 mL of i PrOH); 5-hexen-2-one/iPrOK/cat. = 200/10/1; N2 (0.1 MPa); 82 °C.

pathway. On the other hand, if methanol is replaced by iPrOH in 11, the resulting complex could be regarded as the intermediate when 10 was used as the catalyst and that is why the efficiencies of 10 and 11 were almost the same. Although no intermediate has been identified, to understand the reaction mechanism clearly, further experiments are needed.

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CONCLUSIONS In summary, we have synthesized a series of ruthenium complexes bearing NNN ligands with a 2-hydroxypyridyl fragment, and their catalytic activity for transfer hydrogenation of ketones was studied. In the presence of CO, the dichloride ruthenium(II) complexes 1−3 reacted with NH4PF6 in methanol or CH2Cl2 to give ionic products 7, 8, and 9. By comparing the νCO data of the deprotonated 7−9, it can be seen that the ligand field strengths for the deprotonated forms are in the order of L2, L1, and L3. The catalytic activity of 7−9 in the transfer hydrogenation of ketones is consistent with the order of their ligand fields, suggesting in these cases that the electron densities in the Ru centers might play a dominant role for their activities. By contrast, complex 2 shows better catalytic activity than 3, but it is not as active as 1, which means that for complexes 1−3, the hydroxy position is also important for the activity. In the absence of NH4PF6, the reaction of 2 with CO in CH2Cl2 gave a neutral product 10, which could further react with CH3ONa to form 11. DFT calculations suggest that there is a methanol molecule coordinating with ruthenium in 11. Both 10 and 11 are efficient for selectively reducing 5-hexen-2one to 5-hexen-2-ol, suggesting that the catalytic reactions might follow an outer-sphere hydrogenation pathway, similar to that of [Ru(dhtp)(PPh3)(CO)Cl][PF6] developed by Szymczak’s group.53 In fact, if methanol is replaced by iPrOH in 11, the resulting complex could be regarded as the intermediate when 10 was used as the catalyst. The results in this document are important for developing more active and chemoselective transfer hydrogenation catalysts.

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EXPERIMENTAL SECTION Schlenk line techniques were employed for all manipulations of air- and moisture-sensitive compounds. All solvents were distilled from appropriate drying agents under N2 before use. All reagents were purchased from commercial sources. The 1H, 31 P, and 13C NMR spectra were recorded on a Bruker Avance 400 spectrometer. The 1H NMR chemical shifts were referenced to the residual solvent as determined relative to Me4Si (δ = 0 ppm). The 31P{1H} chemical shifts were reported in ppm relative to external 85% H3PO4. The 13C{1H} chemical shifts were reported in ppm relative to the carbon resonance of CDCl3 (77.0 ppm) and d6-DMSO (39.43 ppm). Elemental 3411


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NH2NH2 (30.0 mL, 80% in water) was heated at 100 °C for 4 h. After cooling to room temperature, the solution was added to 20 mL of water, then neutralized by dilute HCl (1.0 M), and extracted with ethyl acetate. The organic phase was combined, dried over Na2SO4, and evaporated under vacuum. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, v/v = 4:1) to give the product as a yellow liquid (0.66 g, 50%). HRMS (ESI): calcd for C18H17N3O2 + H, 308.1399; found, 308.1397. 1H NMR (400 MHz, CDCl3, ppm): 8.24 (d, J = 8.0 Hz, 1H), 8.06 (d, J = 7.6 Hz, 1H), 7.69 (m, 2H), 7.48 (t, J = 8.0 Hz, 1H), 7.26 (d, J = 7.6 Hz, 1H), 6.84 (d, J = 7.2 Hz, 1H), 6.76 (d, J = 8.4 Hz, 1H), 6.58 (d, J = 8.4 Hz, 1H), 4.31 (s, 2H), 4.04 (s, 3H), 3.92 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ 163.6, 163.4, 158.9, 157.3, 155.4, 153.7, 139.2, 138.8, 136.9, 123.3, 118.5, 116.0, 113.8, 110.8, 107.9, 53.2, 53.1, 47.0. Synthesis of L3. By using a procedure similar to that described for L2, the reaction of 2-methoxy-6-((6-(6-methoxypyridin-2-yl)pyridin-2-yl)methyl)pyridine (4.0 g, 13.0 mmol) with HBr (40% in water, 40 mL) gave L3 as a milk white solid (3.0 g, 81%). mp: 257−261 °C. HRMS (ESI): calcd for C16H13N3O2 + H, 280.1086; found, 280.1091. 1H NMR (400 MHz, d6-DMSO, ppm): 8.45 (d, J = 8.0 Hz, 1H), 8.33 (t, J = 8.0 Hz, 1H), 8.01 (t, J = 8.0 Hz, 1H), 7.90 (d, J = 8.0 Hz, 1H), 7.83 (t, J = 8.0 Hz, 1H), 7.17 (br s, 1H), 6.91 (d, J = 8.0 Hz, 1H), 6.60 (m, 2H), 4.44 (s, 2H). A good 13C NMR could not be obtained because of its low solubility. Synthesis of Pyridine-2,6-diylbis((6-methoxypyridin2-yl)methanone). 2-Bromo-6-methoxy-pyridine (9.0 g, 48.0 mmol) in 25 mL of THF was added into a solution of n-BuLi (19.0 mL, 2.5 M, 48.0 mmol) in 60 mL of THF at −78 °C. After 30 min of stirring at the same temperature, 2,6pyridinedicarboxylic acid diethyl ester (5.40 g, 24 mmol) was added. The reaction mixture was stirred at −20 °C for 3 h. The solution was quenched with dilute HCl (1.0 M) and then neutralized with a solution of NaOH. The mixture was extracted with CH2Cl2, and the solvent was concentrated. The crude product was purified by column chromatography on silica gel (eluent: petroleum ether/ethyl acetate (v/v) = 2:1) to give the product as a white solid (3.54 g, 42%). mp: 106−108 °C. HRMS (ESI): calcd for C19H15N3O4 + H, 350.1141; found, 350.1147. 1H NMR (400 MHz, CDCl3, ppm): 8.22 (d, J = 8 Hz, 2H), 8.06 (t, J = 8 Hz, 1H), 7.80 (d, J = 8 Hz, 2H), 7.67 (t, J = 8 Hz, 2H), 6.93 (d, J = 8 Hz, 2H), 3.85 (s, 6H). 13C NMR (100 MHz, CDCl3, ppm): δ 191.5, 163.2, 153.9, 150.7, 138.7, 136.8, 126.8, 119.3, 115.0, 53.4. Synthesis of 2,6-Bis((6-methoxypyridin-2-yl)methyl)pyridine. By using a procedure similar to that described for 2methoxy-6-((6-(6-methoxypyridin-2-yl)pyridin-2-yl)methyl)pyridine, the reaction of pyridine-2,6-diylbis((6-methoxypyridin-2-yl)methanone) (3.54 g, 10 mmol) in 80 mL of ethylene glycol, NaOH (5.00 g), and NH2NH2 (80.0 mL, 80%) at 100 °C for 3 h gave the product as a yellow oil (2.6 g, 81%). HRMS (ESI): calcd for C19H19N3O2 + H, 322.1556; found, 322.1556. 1 H NMR (400 MHz, CDCl3, ppm): 7.51 (t, J = 8 Hz, 1H), 7.46 (t, J = 8 Hz, 2H), 7.12 (d, J = 8 Hz, 2H), 6.78 (d, J = 8 Hz, 2H), 6.57 (d, J = 8 Hz, 2H), 4.25 (s, 4H), 3.90 (s, 6H). 13C NMR (100 MHz, CDCl3, ppm): δ 163.5, 158.9, 157.1, 138.7, 136.4, 121.0, 115.8, 107.8, 53.0, 46.7. Synthesis of L4. By using a procedure similar to that described for L 2 and L 3, the reaction of 2,6-bis((6methoxypyridin-2-yl)methyl)pyridine (4.70 g, 14.6 mmol) with HBr (40% in water, 30 mL) at reflux for 3 h afforded a

mL, 40% in water), and the solution was heated at reflux for 3 h. After cooling to room temperature, a saturated NaOH/H2O solution was added slowly to neutralize it. The resulting solution was extracted with CH2Cl2 (3 × 100 mL). The combined organic extracts were dried over Na2SO4, filtered, and concentrated to afford L2 as a milk white solid (1.25 g, 88%). mp: 121−123 °C. HRMS (ESI): calcd for C16H13N3O + H, 264.1137; found, 264.1143. 1H NMR (400 MHz, CDCl3, ppm): 10.69 (br s, 1H), 8.54 (d, J = 4.4 Hz, 1H), 7.72 (t, J = 8.0 Hz, 1H), 7.70−7.62 (m, 2H), 7.45 (m, 1H), 7.35−7.29 (m, 2H), 7.15 (m, 1H), 6.77 (d, J = 6.8 Hz, 1H), 6.60 (d, J = 8.8 Hz, 1H), 4.36 (s, 2H). 13C NMR (100 Hz, d6-DMSO, ppm): δ 162.0, 159.1, 158.8, 149.0, 148.6, 144.1, 140.8, 138.0, 136.6, 124.3, 123.5, 121.6, 119.1, 118.1, 104.7, 46.0. Synthesis of Ethyl 6-(6-Methoxypyridin-2-yl)pyridine2-carboxylate. A toluene solution (400 mL) containing 6bromopyridine-2-carboxylic acid ethyl ester (8.1 g, 35.0 mmol), 2-(tributylstannyl)-6-methoxypyridine (14.0 g, 35.0 mmol), LiCl (15.0 g, 350 mmol), and Pd(PPh3)4 (1.2 g, 1.1 mmol) was deoxygenated by bubbling N2 through it. The mixture was then refluxed under N2 for 24 h, and an aqueous solution (150 mL) of NaF (15.0 g, 350 mmol) was added to the solution at room temperature. The resultant solution was further stirred for 30 min at room temperature. The insoluble solid was filtered off, and the filtrate was treated with a 5% Na2CO3 aqueous solution in a separating funnel. The organic layer was dried with anhydrous sodium sulfate, and the solvents were evaporated to dryness under reduced pressure. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, v/v = 10:1) to give the product as a yellow liquid (8.3 g, 91%). HRMS (ESI): calcd for C14H14N2O3 + H, 259.1083; found, 259.1087. 1H NMR (400 MHz, CDCl3, ppm): 8.59 (d, J = 8.0 Hz, 1H), 8.18 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.0 Hz, 1H), 7.93 (t, J = 8.0 Hz, 1H), 7.73 (t, J = 8.0 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 4.49 (q, 2H), 4.04 (s, 3H), 1.47 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ 165.3, 163.5, 156.2, 152.5, 147.7, 139.4, 137.6, 124.6, 123.9, 114.4, 111.6, 61.7, 53.2, 14.3. Synthesis of (6-Methoxypyridin-2-yl)(6-(6-methoxypyridin-2-yl)pyridin-2-yl)methanone. 2-Bromo-6-methoxy-pyridine (2.0 g, 11.0 mmol) in 60 mL THF was added drop by drop to a solution of n-BuLi (4.4 mL, 2.5 M, 11.0 mmol) at −78 °C. After 60 min of stirring at −78 °C, 6-(6methoxypyridin-2-yl)pyridine-2-carboxylate (2.9 g, 11.0 mmol) was added. The mixture was further stirred at −20 °C for 2.5 h. The solution was quenched with dilute HCl (1.0 M) and then neutralized with a solution of NaOH. The mixture was extracted with CH2Cl2, and the solvent was concentrated. The crude product was purified by column chromatography on silica gel (petroleum ether/ethyl acetate, v/v = 4:1) to give the product as a white solid (1.53 g, 44%). mp: 90−93 °C. HRMS (ESI): calcd for C18H15N3O3 + H, 322.1192; found, 322.1196. 1 H NMR (400 MHz, CDCl3, ppm): 8.64 (d, J = 8.0 Hz, 1H), 8.09 (d, J = 8.0 Hz, 1H), 8.01 (t, J = 8.0 Hz, 2H), 7.78 (m, 2H), 7.68 (t, J = 8.0 Hz, 1H), 6.98 (d, J = 6.4 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 4.06 (s, 3H), 3.89 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ 192.3, 163.4, 163.2, 155.1, 153.7, 152.6, 151.3, 139.3, 138.5, 137.1, 124.5, 123.0, 119.1, 114.7, 113.9, 111.4, 53.4, 53.1. Synthesis of 2-Methoxy-6-((6-(6-methoxypyridin-2yl)pyridin-2-yl)methyl)pyridine. A mixture of 2-methoxy6-((6-(6-methoxypyridin-2-yl)pyridin-2-yl)methyl)pyridine (1.4 g, 4.4 mmol), ethylene glycol (30 mL), NaOH (1.5 g), and 3412


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white solid of L4 (3.67 g, 85%). mp: 270−277 °C (dec). HRMS (ESI): calcd for C17H15N3O2 + H, 294.1243; found, 294.1245. 1 H NMR (400 MHz, d6-DMSO, ppm): 11.63 (brs, 2H), 7.74 (t, J = 8 Hz, 1H), 7.32 (t, J = 8 Hz, 2H), 7.22 (d, J = 8 Hz, 2H), 6.17 (d, J = 9 Hz, 2H), 5.94 (d, J = 6 Hz, 2H), 3.94 (s, 4H). 13C NMR (100 Hz, d6-DMSO, ppm): δ 163.4, 157.5, 147.5, 141.4, 138.3, 121.8, 117.5, 105.0, 31.2. Synthesis of 2. It was synthesized by using procedures similar to that described for 1 reported previously.55 Method a: A solution of L2 (0.17 g, 0.63 mmol) and RuCl2(PPh3)3 (0.60 g, 0.63 mmol) was refluxed in dried methanol (75 mL) under stirring for 24 h. The mixture was allowed to cool to room temperature, and the organic phase was evaporated under vacuum. The crude product was recrystallized with CH2Cl2/ diethyl ether to give 2 as a red powder (0.31 g, 71%). A red acicular crystal suitable for a single-crystal X-ray diffraction experiment was grown by vapor diffusion of diethyl ether into a dichloromethane solution of 2 at room temperature. Anal. calcd for C34H28Cl2N3OPRu: C, 58.54; H, 4.05; N, 6.02. Found: C, 58.55; H, 4.07; N, 6.00. 1H NMR (400 MHz, CD3OD, ppm): 9.98 (d, J = 6 Hz, 1H), 8.34 (d, J = 8 Hz, 1H), 8.09 (d, J = 8 Hz, 1H), 7.88 (m, 2H), 7.79 (t, J = 8 Hz, 1H), 7.53 (t, J = 8 Hz, 1H), 7.44 (d, J = 8 Hz, 1H), 7.25 (m, 3H), 7.19 (d, J = 8 Hz, 1H), 7.10−6.94 (m, 12H), 6.63 (d, J = 8 Hz, 1H), 4.10 (d, J = 16 Hz, 1H), 3.57 (d, J = 16 Hz, 1H). 31P NMR (162 MHz, CD3OD, ppm): 56.5. A satisfactory 13C NMR spectrum was not obtained because of its low solubility. Method b: 2 mL of HCl (1.0 mol/L) was added to a solution of 5 (0.53 g, 0.50 mmol) in MeOH (50 mL) drop by drop at room temperature. After 24 h, the red precipitate was collected, washed with copious amounts of ether, and dried under vacuum to provide 2 as a red powder (0.21 g, 60%). Synthesis of 3. It was synthesized by using procedures similar to that described for complex 2. Method a: The reaction of L3 (0.18 g, 0.63 mmol) with RuCl2(PPh3)3 (0.60 g, 0.63 mmol) in refluxing methanol (75 mL) for 24 h gave 3 as a red powder (0.43 g, 96%). mp: 246 °C (dec). Anal. calcd for C34H28Cl2N3O2PRu: C, 57.23; H, 3.96; N, 5.90. Found: C, 57.20; H, 3.95; N, 6.10. 1H NMR (400 MHz, CD3OD, ppm): 8.37 (d, J = 8 Hz, 1H), 8.17 (d, J = 8 Hz, 1H), 7.95 (t, J = 8 Hz, 1H), 7.82 (t, J = 8 Hz, 1H), 7.73 (t, J = 8 Hz, 1H), 7.33 (m, 3H), 7.19−7.05 (m, 14H), 7.00 (d, J = 8 Hz, 1H), 6.80 (d, J = 8 Hz, 1H), 6.62 (d, J = 8 Hz, 1H), 3.95 (d, J = 14 Hz, 1H), 3.48 (d, J = 14 Hz, 1H). 31P NMR (162 MHz, CD3OD, ppm): 55.3. 13 C NMR (100 MHz, DMSO, ppm): δ 170.6, 169.7, 162.1, 159.4, 156.9, 155.5, 140.2, 139.4, 135.7, 133.1, 132.8, 129.6, 127.6, 123.5, 121.2, 116.2, 115.4, 112.7, 110.5, 47.3. Method b: The reaction of 6 (0.53 g) with 2 mL of HCl (1.0 mol/L) in MeOH (50 mL) provided 3 as a red powder (0.31 g). Synthesis of 4. Complex 4 was obtained by the solvothermal method. A mixture of RuCl2(PPh3)3 (0.40 g, 0.42 mmol) and L4 (0.060 g, 0.21 mmol) in the ratio of 2:1 in 15 mL MeOH was sealed in a Teflon-lined stainless steel vessel (25 mL), heated at 150 °C for 24 h under autogenous pressure, then cooled to room temperature to give a yellow powder 4 (0.084 g, 46%). A yellow block crystal suitable for a singlecrystal X-ray diffraction experiment was grown by vapor diffusion of diethyl ether into a dichloromethane solution of 4 at room temperature. Anal. calcd for C39H28N3O6PRu2: C, 53.98; H, 3.25; N, 4.84. Found: C, 53.69; H, 3.45; N, 4.90. IR (νCO, KBr pellet, cm−1): 2008 (s), 1959 (s), 1936 (m), 1924 (s). 1H NMR (400 MHz, CDCl3, ppm): 7.70 (m, 7H), 7.42 (m, 9H), 7.36 (d, J = 8 Hz, 2H), 7.06 (t, J = 8 Hz, 2H), 6.28 (d, J =

8 Hz, 2H), 6.04 (d, J = 8 Hz, 2H), 4.59 (d, J = 14 Hz, 2H), 3.98 (d, J = 14 Hz, 2H). 31P NMR (162 MHz, CDCl3, ppm): 13.8. 13 C NMR (100 MHz, DMSO, ppm): δ 206.0, 205.5, 176.9, 176.8, 163.5, 158.6, 157.4, 152.0, 141.4, 139.0, 138.3, 133.6, 134.3, 134.0, 133.9, 133.0, 132.7, 132.5, 132.0, 131.9, 131.4, 131.0, 110.6, 129.3, 129.2, 128.9, 128.8, 123.6, 121.8, 114.0, 111.4, 48.1. Reaction of 2 with NH4PF6/PPh3 (Synthesis of 5). NH4PF6 (1.7 g, 10.43 mmol) was added into a solution of 2 (0.50 g, 0.72 mmol) and PPh3 (0.40 g, 1.53 mmol) in dried methanol (75 mL) under stirring. After 20 min, the precipitate was collected, washed with copious amounts of diethyl ether, and dried under vacuum to provide 5 as a red powder (0.45 g, 59%). Anal. calcd for C52H43ClF6N3OP3Ru: C, 58.41; H, 4.05; N, 3.93. Found: C, 58.32; H, 4.11; N, 3.92. 1H NMR (400 MHz, CDCl3, ppm): 11.33 (s, 1H), 9.44 (d, J = 4 Hz, 1H), 7.70 (m, 2H), 7.52−6.82 (m, 35H), 6.66 (t, J = 8 Hz, 1H), 6.37 (d, J = 8 Hz, 1H), 3.89 (s, 2H). 31P NMR (162 MHz, CDCl3, ppm): 21.9, −141.2. A satisfactory 13C NMR spectrum was not obtained because of its low solubility. Reaction of 3 with NH4PF6/PPh3 (Synthesis of 6). By using a procedure similar to that described for 5, the reaction of 3 (0.5 g, 0.70 mmol) with PPh3 (0.40 g, 1.53 mmol) and NH4PF6 (1.7 g, 10.43 mmol) in dried methanol (75 mL) provided a mixture of 6 and an unknown species as an orangered powder (0.50 g). 6: 1H NMR (400 MHz, CDCl3, ppm): 7.81 (m, 1H), 7.51−6.83 (m, 36H), 6.44 (d, J = 8 Hz, 1H), 5.98 (m, 1H), 3.77 (s, 2H). 31P NMR (162 MHz, CDCl3, ppm): 23.3, −141.1. A satisfactory 13C NMR spectrum was not obtained because of its low solubility. Synthesis of 7. A solution of 1 (0.75 g, 1.08 mmol) and NH4PF6 (1.7 g, 10.8 mmol) in methanol (80 mL) was purged with CO (1 atm) three times. After bubbling with CO gas for 24 h under stirring, the mixture turned from magenta to yellow. The organic solvent was evaporated under vacuum, and the crude product was purified by column chromatography on silica gel (eluent: dichloromethane/methanol, v/v = 5:1) to give a yellow powder 7 (0.70 g, 78%). Anal. calcd for C35H28ClF6N3O2P2Ru: C, 50.34; H, 3.38; N, 5.03. Found: C, 50.62; H, 3.39; N, 4.98. IR (νCO, in DMSO, cm−1): 1968 (s). 1 H NMR (400 MHz, d6-acetone, ppm): 8.63 (d, J = 8 Hz, 1H), 8.60 (d, J = 8 Hz, 1H), 8.27 (t, J = 8 Hz, 1H), 8.08 (m, 2H), 7.62 (d, J = 8 Hz, 1H), 7.46 (m, 3H), 7.39−7.17 (m, 15H), 6.58 (d, J = 5 Hz, 1H), 4.02 (d, J = 16 Hz, 1H), 3.43 (d, J = 16 Hz, 1H). 31P NMR (162 MHz, d6-acetone, ppm): 44.4, −141.1. 13 C NMR (100 MHz, DMSO, ppm): δ 207.0, 157.6, 156.9, 155.7, 154.1, 153.7, 141.3, 138.6, 139.0, 133.8, 133.7, 133.6, 131.1, 131.0, 132.5, 132.0, 131.9, 131.4, 130.1, 129.2, 129.1, 128.5, 127.5, 127.0, 126.3, 124.6, 122.3, 115.7, 111.1, 45.8. Synthesis of 8. A solution of 2 (0.50 g, 0.72 mmol) and NH4PF6 (1.2 g, 7.2 mmol) in CH2Cl2 (80 mL) was purged with CO (1 atm) three times. After bubbling with CO gas for 24 h under stirring, the mixture turned from magenta to yellow. The mixture was then filtered through Celite in the open atmosphere. The filtrate was added into a stirring solution of pentane (ca. 200 mL). The resulting yellow precipitate was collected on a sintered glass frit, washed with copious amounts of pentane, and then dried to give 8 as a bright yellow powder (0.50 g, 83%). Anal. calcd for C35H28ClF6N3O2P2Ru: C, 50.34; H, 3.38; N, 5.03. Found: C, 50.45; H, 3.38; N, 5.01. IR (νCO, in DMSO, cm−1): 1950 (s). 1H NMR (400 MHz, DMSO, ppm): 13.93 (s, 1H), 8.77 (d, J = 8 Hz, 1H), 8.73 (d, J = 8 Hz, 1H), 3413


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7.54 (t, J = 8 Hz, 1H), 7.10 (d, J = 8 Hz, 1H), 4.90 (d, J = 16 Hz, 1H), 4.47 (d, J = 16 Hz, 1H), 3.33 (s, 3H). 13C NMR (100 MHz, CDCl3, ppm): δ 197.2, 170.4, 160.9, 158.3, 157.1, 155.5, 154.3, 139.9, 139.0, 136.5, 124.9, 122.6, 122.2, 120.1, 116.7, 107.4, 48.5, 46.4. General Procedure for the Catalytic Transfer Hydrogenation of Ketones. The catalyst solution was prepared by dissolving complex 2 (69.7 mg, 0.10 mmol) in 2-propanol (10.0 mL), and the internal standard solution was prepared by dissolving dodecane (340 mg, 2.0 mmol) in 2-propanol (10.0 mL). Under an N2 atmosphere, the mixture of ketone (2.0 mmol), 1.0 mL of the catalyst solution (0.01 mmol), 1.0 mL of the internal standard solution (0.2 mmol), and 2-propanol (10.0 mL) was stirred at 82 °C for 10 min. Then, 0.2 mL of a 0.5 M iPrOK (0.10 mmol) solution in 2-propanol was introduced to initiate the reaction. At the stated time, 0.1 mL of the reaction mixture was sampled and immediately diluted with 0.5 mL of 2-propanol precooled to 0 °C for GC analysis. After the reaction was completed, the reaction mixture was condensed under reduced pressure and subjected to purification by flash silica gel column chromatography to afford the corresponding alcohol product, which was identified by comparison with the authentic sample through NMR and GC analyses.

8.33 (t, J = 8 Hz, 1H), 8.10 (d, J = 8 Hz, 1H), 7.85 (d, J = 4 Hz, 1H), 7.74 (t, J = 6 Hz, 1H), 7.67 (d, J = 8 Hz, 1H), 7.50−7.30 (m, 9H), 7.25 (t, J = 8 Hz, 1H), 7.07−7.00 (m, 8H), 4.27 (d, J = 16 Hz, 1H), 3.16 (d, J = 16 Hz, 1H). 31P NMR (162 MHz, CD3OD, ppm): 31.2, −141.2. 13C NMR (100 MHz, DMSO, ppm): δ 204.9, 167.3, 157.1, 156.8, 155.6, 154.3, 141.7, 141.5, 139.0, 133.1, 133.0, 131.6, 131.5, 131.0, 129.4, 129.3, 127.3, 126.6, 124.9, 122.6, 118.1, 110.6, 45.4. Synthesis of 9. A solution of 3 (0.75 g, 1.05 mmol) and NH4PF6 (1.7 g, 10.8 mmol) in methanol (80 mL) was purged with CO (1 atm) three times. After bubbling with CO gas for 24 h under stirring, the mixture turned from magenta to yellow. The organic solvent was evaporated under vacuum, and the crude product was purified by column chromatography on silica gel (eluent: dichloromethane/methanol, v/v = 5:1) to give a yellow power 9 (0.80 g, 50%). Anal. calcd for C35H28ClF6N3O3P2Ru: C, 49.39; H, 3.32; N, 4.94. Found: C, 49.30; H, 3.34; N, 4.98. IR (νCO, in DMSO, cm−1): 1998 (s). 1 H NMR (400 MHz, CD3OD, ppm): 8.56 (d, J = 8 Hz, 1H), 8.21 (t, J = 8 Hz, 1H), 8.12 (d, J = 8 Hz, 1H), 7.95 (t, J = 8 Hz, 1H), 7.82 (t, J = 8 Hz, 1H), 7.54−6.97 (m, 18H), 6.70 (d, J = 8 Hz, 1H), 4.14 (d, J = 16 Hz, 1H), 3.61 (d, J = 16 Hz, 1H). 31P NMR (162 MHz, CD3OD, ppm): 52.9, −141.2. 13C NMR (100 MHz, DMSO, ppm): δ 205.3, 168.0, 167.4, 157.3, 156.8, 155.2, 154.8, 141.1, 140.9, 140.8, 133.1, 133.0, 132.7, 132.3, 132.0, 131.9, 131.0, 129.3, 129.2, 128.8, 128.7, 125.8, 122.1, 117.7, 116.2, 112.5, 110.3, 46.0. Synthesis of 10. A solution of 2 (0.75 g, 1.07 mmol) in CH2Cl2 (80 mL) was purged with CO (1 atm) three times. After bubbling with CO gas for 24 h under stirring, the mixture turned from magenta to yellow. The organic solvent was evaporated under vacuum, and acetone (20 mL) was added to wash the residue. The resulting yellow precipitate was collected on a sintered glass frit, washed with copious amounts of pentane, and then dried to give 10 as a bright yellow powder (0.40 g; 80%). A yellow acicular crystal suitable for a singlecrystal X-ray diffraction experiment was grown by volatilization of dichloromethane and methanol solution of 10 at room temperature. Anal. calcd for C17H13Cl2N3O2Ru: C, 44.07; H, 2.83; N, 9.07. Found: C, 44.03; H, 2.60; N, 8.92. IR (KBr, cm−1): 3054, 1981 (CO), 1614, 1514, 1492, 1472, 1434, 1094, 697. 1H NMR (400 MHz, CD3CN, ppm): 11.57 (s, 1H), 9.42 (d, J = 6 Hz, 1H), 8.15 (d, J = 8 Hz, 1H), 8.03−7.90 (m, 3H), 7.85 (d, J = 8 Hz, 1H), 7.62 (d, J = 8 Hz, 2H), 7.42 (t, J = 8 Hz, 1H), 6.99 (d, J = 8 Hz, 1H), 4.64 (d, J = 16 Hz, 1H), 4.55 (d, J = 16 Hz, 1H). 13C NMR (100 MHz, CDCl3, ppm): δ 196.9, 167.4, 159.5, 157.0, 156.6, 155.4, 154.9, 141.4, 139.2, 139.0, 125.0, 124.8, 123.2, 121.6, 116.1, 113.3, 46.3. Synthesis of 11. To a solution of 10 (0.35 g, 0.76 mmol) in dried methanol (200 mL) was added sodium methoxide (0.041 g, 0.76 mmol). The mixture was stirred for 12 h at room temperature. The organic solvent was evaporated under vacuum, and acetone (10 mL) was added to wash the residue. The resulting yellow precipitate was collected on a sintered glass frit, washed with copious amounts of pentane, and then dried to give 11 as a bright yellow powder (0.30 g; 86%). Anal. calcd for C18H16ClN3O3Ru: C, 47.12; H, 3.51; N, 9.16. Found: C, 47.11; H, 3.52; N, 9.17. IR (KBr, cm−1): 3064, 2960, 2857, 1946 (CO), 1602, 1564, 1479, 1458, 1431, 1412, 1304, 1247, 1170, 1137, 1032, 1019, 987, 796. 1H NMR (400 MHz, d6DMSO, ppm): 11.47 (s, 1H), 9.30 (d, J = 6 Hz, 1H), 8.48 (d, J = 8 Hz, 1H), 8.18−8.14 (m, 2H), 8.09 (t, J = 8 Hz, 1H), 8.08− 8.02 (m, 1H), 7.79 (d, J = 8 Hz, 1H), 7.76 (d, J = 8 Hz, 1H),

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00410. Crystallographic details, IR and NMR spectra of the new compounds, and atomic coordinates of possible isomers of 11 (PDF) Crystallographic data for complexes 4 and 10 (CIF) IR spectra of 7−11 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.Y.). *E-mail: [email protected] (D.C.). ORCID

Xiangyang Chen: 0000-0002-6981-7022 Dafa Chen: 0000-0002-7650-4024 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 21672045, 21302028, 21673250, 21373228, and 21201049), the China Postdoctoral Science Foundation (2016M591519), and the “100-Talent Program” of the Chinese Academy of Sciences.

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