Dimeric Ruthenium(II)-NNN Complex Catalysts Bearing a Pyrazolyl

Dimeric pincer-type ruthenium(II)-NNN complexes bearing an unsymmetrical pyrazolyl-pyridylamino-pyridine ligand were prepared and characterized by NMR...
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Dimeric Ruthenium(II)-NNN Complex Catalysts Bearing a PyrazolylPyridylamino-Pyridine Ligand for Transfer Hydrogenation of Ketones and Acceptorless Dehydrogenation of Alcohols Qingfu Wang,†,‡ Huining Chai,†,‡ and Zhengkun Yu*,†,§ †

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, PR China University of Chinese Academy of Sciences, Beijing 100049, PR China § State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, PR China ‡

S Supporting Information *

ABSTRACT: Dimeric pincer-type ruthenium(II)-NNN complexes bearing an unsymmetrical pyrazolyl-pyridylamino-pyridine ligand were prepared and characterized by NMR, elemental analysis, and X-ray single crystal structural determination. These complexes exhibited very high catalytic activity for both transfer hydrogenation of ketones and acceptorless dehydrogenation of secondary alcohols, achieving TOF values up to 1.9 × 106 h−1 in the transfer hydrogenation of ketones. The high catalytic activity of the Ru(II) complex catalysts is attributed to the presence of the unprotected NH functionality in the ligand and hemilabile unsymmetrical coordination environment around the central metal atoms in the complex.

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INTRODUCTION

tosylethylenediamine complexes and their later versions, that is, Ru(II) β-aminoalcohol complexes for asymmetrical transfer hydrogenation (ATH) of ketones.14 It is usually structurally required to form a XRu-NHR bond in these Ru(II) complexes. In this aspect, Baratta and co-workers reported highly active ruthenium(II) and osmium(II) NNC complex catalysts containing a NH2 group as the accelerating functionality to enhance the catalytic activity of the complex catalysts for the transfer hydrogenation of ketones and dehydrogenation of secondary alcohols.15 Morris et al. reported iron complexes containing a partially saturated amine(imine)-diphosphine ligand for asymmetric transfer hydrogenation of ketones under mild conditions.16 In general, mononuclear transitionmetal complex catalysts have been widely studied for this purpose, and a few efficient bimetallic transition-metal complex catalysts have only been recently reported.17 Acceptorless catalytic dehydrogenation of alcohols is an atom-economical approach to access carbonyl compounds, avoiding the use of stoichiometric amounts of oxidants. In addition, it finds potential applications in the field of organic hydrogen storage materials.18 Such dehydrogenation processes have recently been paid more and more attention due to the potential application in the field of hydrogen storage.18,19 During the ongoing investigation of transition-metal complex catalysts,20 we found that the NH functionality in a coordinative imidazolyl moiety of the tridentate NNN ligands could act as an effective acceleration functionality of the complex catalysts.21 Ruthenium(II) complexes bearing a

Highly active transition-metal complex catalysts are desperately needed in homogeneous catalysis and organic synthesis.1 Ligands usually play a crucial role in the catalytic processes by finely tuning the electronic and geometric properties of the complex catalysts. Recently, much attention has been paid to the construction of highly active transition-metal complex catalysts supported by a polydentate ligand containing a NH functionality in organometallic catalysis. As a strategy, a NHbearing moiety is usually introduced to the ligand to improve the catalytic activity of the resultant complex catalyst. It has been demonstrated that transition-metal complexes featuring an ancillary N−H functionality could exhibit highly catalytic activity for the hydrogenation of ketones,2 esters,3 nitriles,4 and other substrates.5 Dehydrogenation of alcohols,6 Nheterocycles,7 and amine-boranes8 were also reported by means of such transition-metal complex catalysts, with liberation of hydrogen gas as the only byproduct. One of the most successful examples of such complex catalysts is the [RuCl2(diphosphine)(diamine)] system developed by Noyori and co-workers.9 This Ru(II) catalyst system has shown great vitality in oganometallic catalysis for the last two decades. Very recently, Kempe et al. reported a series of transition-metal complex catalysts containing a triazine-based PNP ligand with ancillary NH functionalities. These complexes can exhibit highly catalytic activities for N-heterocycle synthesis,10 hydrogenation of ketones,11 and alkylation reactions.12 Transfer hydrogenation (TH) of ketones is considered an efficient alternative to hydrogenation of ketones for the production of alcohols by using isopropyl alcohol as the hydrogen donor.13 Noyori et al. developed Ru(II) N© XXXX American Chemical Society

Received: August 1, 2017

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DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Chart 1. Ruthenium(II)-NNN Complex Catalysts

Chart 2. Dipyridylamine-Based NNN Ligand

Scheme 1. Synthesis of Ligands 2 and Complexes 3a

a Legend: (i) 2 mol % Pd2(dba)3, 4 mol % dppf, 2 equiv of tBuONa, 1.5 equiv of 2-aminopyridine, toluene, reflux, 0.1 MPa N2, 8 h, 76%. (ii) 2 equiv of MeI, 2 equiv of NaH, THF, reflux, 6 h, 93%. (iii) 1 equiv of RuCl2(PPh3)3, toluene, reflux, 0.1 MPa N2, 3 h, 88% (3a); 8 h, 85% (3b).

pyrazoly-(1H-imidazolyl)-pyridine (A)21a,b or pyrazoly-(1Hpyrazolyl)-pyridine (B)21c ligand could exhibit an exceptionally high catalytic activity in the transfer hydrogenation of ketones, reaching a final TOF value up to 720 000 h−1 (Chart 1). Intrigued by the tautomerization of 2,2′-dipyridylamine (C) and the potential coordination modes of its anions (I-III) as a bidentate coordinative moiety to a metal (Chart 2), we reasonably envisioned that C might be applied to construct a versatile NNN ligand for establishing highly active transitionmetal complex catalysts. Herein, we report a well-defined dimeric Ru(II)-NNN complex catalyst bearing a pyrazolylpyridylamino-pyridine ligand for transfer hydrogenation of ketones and acceptorless dehydrogenation of secondary alcohols.

CH in 3a are shifted downfield by 1.81 and 0.42 ppm as compared to those of the free ligand (2a), respectively, suggesting that both the pyridyl and pyrazolyl moieties in 3a are coordinated to the metal center. The molecular structure of complex 3a was further determined by X-ray crystallographic studies (Figure 1). In the solid state, complex 3a features a dinuclear unit in the crystal lattice. Each ruthenium center is surrounded by the tridentate NNN ligand, one PPh3, and two cis-chlorides in a distorted-bipyramidal environment. The two ruthenium centers are bridged by the two chlorine atoms. The Ru(1)−Cl(1)−Ru(1A) and Cl(1)−Ru(1)−Cl(1A) angles are 99.91(4) and 80.09(4)°, respectively, suggesting that the two

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RESULTS AND DISCUSSION Synthesis of Ligands and Ru(II)-NNN Complexes. Ligand 2a was synthesized in 76% yield by palladium-catalyzed Buchwald−Hartwig coupling of 2-bromo-6-(3,5-dimethylpyrazol-1-yl)pyridine (1) with 2-aminopyridine. The N-methylated derivative of 2a, that is, 2b, was readily prepared by reacting 2a with iodomethane in the presence of NaH (Scheme 1). Reacting ligands 2 with equamolar amounts of RuCl2(PPh3)3 in refluxing toluene led to Ru(II)-NNN complexes 3a (88%) and 3b (85%), respectively. Recrystallization of complexes 3 from chloroform/n-hexane gave air- and moisture-stable yellow crystals. Characterization of Ru(II)-NNN Complexes 3. The NMR analyses of the complexes are consistent with their compositions. The 1H NMR signals of the NH and pyrazolyl

Figure 1. Molecular structure of complex 3a. B

DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX

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Organometallics chlorine atoms are cis positioned. The Ru(1)−Cl(1A) bond length is 2.5261(1) Å, which is longer than that (2.4546 Å) in the relevant mononuclear Ru(II)-NNN complexes reported by our group,20g suggesting that the metal centers in 3a are situated in a looser environment than that of the mononuclear metal center. Such a structural element usually enhances the catalytic activity of a transition-metal complex catalyst.17b,20a,b Transfer Hydrogenation of Ketones. Next, the catalytic behaviors of complexes 3 were comparatively investigated in a fashion similar to our previously reported procedures for TH reactions of ketones20 (Table 1). With 0.01 mol % dimeric

Table 2. Transfer Hydrogenation of Ketones Catalyzed by Complex 3aa

Table 1. Comparison of the Catalytic Activities of Complexes 3a

a Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst 3, 0.01 mol %; ketone/iPrOK/3 = 10000:20:1; 0.1 MPa N2, 82 °C. b Determined by GC analysis. cTurnover frequency (moles of ketone converted per mole of Ru per hour) at 50% conversion of the ketone. d Using 0.025 mol % 3.

complex catalyst 3, that is, with 0.02 mol % Ru loading in the catalytic system, ketone substrates were exclusively converted to the corresponding alcohol products in 96−99% yields over a period of 5−90 min. Complex 3a exhibited a much higher catalytic activity than that of complex 3b. In the cases of using propiophenone, cyclohexanone, and 2-octanone as the substrates, a higher catalyst loading of 3b (0.025 mol %) was required to complete the TH reactions (Table 1, entries 2, 5, and 6). These results have suggested that the NH functionality in the coordinative moiety of the polydentate ligand plays a crucial role in enhancing the TH reaction rates of ketones. Then, the catalytic activity of complex 3a was further explored in the transfer hydrogenation of various ketones (Table 2). With 0.01 mol % complex 3a as the catalyst, acetophenone was converted to 1-phenylethanol in 98% yield within 5 min, achieving a TOF value of 2.3 × 105 h−1 (Table 2, entry 1). Chloro-, bromo-, fluoro-, and trifluoromethyl substituents on the phenyl ring of the substituted acetophenones exhibited an acceleration effect on the TH reactions, whereas m-trifluoromethyl deteriorated the reaction efficiency (Table 2, entry 12). 4′-Bromoacetophenone accomplished the reaction to afford the alcohol product in 99% yield within 1 min, reaching the highest TOF value of 1.9 × 106 h−1 (Table 2,

a

Conditions: ketone, 2.0 mmol (0.1 M in 20 mL iPrOH); catalyst 3a, 0.01 mol %; ketone/iPrOK/3a = 10000:20:1; 0.1 MPa N2, 82 °C. b Determined by GC analysis. cTurnover frequency (moles of ketone converted per mole of Ru per hour) at 50% conversion of the ketone. C

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Table 3. Dehydrogenation of Alcohols Catalyzed by 3aa

entry 7). Acetophenones bearing an electron-donating 3′- or 4′methyl or 4′-methoxy substituent required a longer reaction time to achieve high conversions (Table 2, entries 15, 16, and 18). Aliphatic ketones were also efficiently applied for the TH transformations (Table 2, entries 20−22). Acceptorless catalytic dehydrogenation of alcohols is a straightforward route to access carbonyl compounds, avoiding the use of stoichiometric amounts of oxidants.19 The dehydrogenation reactions of secondary alcohols were conducted by using complex 3a (0.1 mol %) as the catalyst and tBuOK (10 mol %) as the base in refluxing toluene (Table 3). 1Phenylethanol was reacted to yield the corresponding ketone, that is, acetophenone, in 96% yield within 20 h (Table 3, entry 1). Aromatic secondary alcohols bearing various substituents exclusively gave the corresponding ketones over a period of 14−24 h (Table 3, entries 2−10). The reactions of sterically hindered diphenylmethanol and 2-naphthylmethanol also reacted effectively to form the corresponding ketones in excellent yields (Table 3, entries 11 and 12). By increasing the catalyst loading to 0.5 mol %, 1,2,3,4-tetrahydro-1-naphthol and 1-indanol could also be dehydrogenated, yielding the target products in 95% and 85% yields, respectively (Table 3, entries 13 and 14). In a similar fashion, aliphatic secondary cyclic and acyclic alcohols were dehydrogenated to give the corresponding ketones by using 0.5 mol % of the complex catalyst (Table 3, entries 15−19). Reaction Mechanism. The present transfer hydrogenation of ketones and dehydrogenative oxidation reactions of secondary alcohols may follow an inner-sphere mechanism.22 During the reaction, complex 3a initially interacts with the base to form a Ru(II) alkoxide which undergoes β-H elimination to form a RuH species that is presumably considered as the catalytically active species,23 although it was not successfully isolated by reacting 3a with the base in refluxing ethanol or isopropyl alcohol. Then, the in situ generated RuH species catalyzes reduction of the ketone or dehydrogenation of secondary alcohols via a possible inner-sphere pathway. It is noteworthy that the in situ proton NMR detection of the reaction mixture of the Ru(II) catalyst (3a) with potassium isopropoxide reveals a triplet at −10.25 ppm, suggesting formation of RuH species in the inseparable product mixture.

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CONCLUSIONS A versatile dimeric pincer-type Ru(II)-NNN complex bearing an unsymmetrical pyrazolyl-pyridylamino-pyridine ligand was successfully synthesized and exhibited excellent catalytic activity in the transfer hydrogenation of ketones and acceptorless dehydrogenation of secondary alcohols. Dipyridylamine may be applied as a NH-containing bidentate coordinative moiety for the construction of diverse polydentate ligands to access highly active transition-metal complex catalysts.

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EXPERIMENTAL SECTION

General Considerations. All the manipulations of air and/or moisture-sensitive compounds were carried out under a nitrogen atmosphere using the standard Schlenk techniques. The solvents were dried and distilled prior to use by the literature methods. The solvents were dried and distilled prior to use by the literature methods. 1H and 13 C{1H} NMR spectra were recorded on 400 MHz spectrometer and all chemical shift values refer to δTMS = 0.00 ppm, DMSO-d6 (δ(1H), 2.50 ppm; δ(13C), 39.52 ppm) and CD3OD-d4 (δ(1H), 3.31 ppm; δ(13C), 49.00 ppm). Elemental and HRMS analysis were achieved by the Analysis Center, Dalian University of Technology and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All the

a

Conditions: alcohol, 0.5 mmol (1.0 M in 0.5 mL toluene); catalyst 3a, 0.1 mol %, tBuOK, 10 mol %; 0.1 MPa N2, 110 °C. bDetermined by GC analysis. cUsing 0.5 mol % 3a.

chemical reagents were purchased from commercial sources and used as received unless otherwise indicated. D

DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX

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to afford 3b as a yellow crystalline solid (121 mg, 85%). Mp: >300 °C. 1 H NMR (CD3OD-d4, 400 MHz, 23 °C) δ 10.08 (d, J = 5.4 Hz, 2 H), 7.95 and 7.83 (t each, J = 7.2 and 8.2 Hz, 2:2 H), 7.66 (d, J = 8.2 Hz, 2 H), 7.25 and 7.09 (m each, 10:24 H), 6.79 (d, J = 8.2 Hz, 2 H), 6.39 (s, 2 H), 2.88, 2.87, and 2.21 (s each, 6:6:6 H). 13C{1H} NMR (CD3OD-d4, 100 MHz, 23 °C) δ 160.9, 158.6, 158.2, 153.9, and 146.0 (Cq each), 135.8, 134.4, 130.5, and 128.8 (d each, i-, o-, p-, m-C of PPh3), 155.6, 139.7, 139.3, 120.5, 115.9, 115.0, 112.1, 107.5, 42.4, 15.6, 14.2. 31P{1H} NMR (CD3OD-d4, 162 MHz, 23 °C) δ 58.7. IR (KBr pellets, cm−1): ν 3441, 3054, 1597, 1573, 1481, 1459, 1432, 1393, 1374, 1344, 1241, 1189, 1169, 1143, 1122, 1091, 1025, 997, 904, 779, 748, 696, 619, 529, 459, 426. Anal. Calcd for C68H64Cl4N10P2Ru2: C, 57.23; H, 4.52; N, 9.81. Found: C, 57.42; H, 4.55; N, 10.19. Typical Procedure for the Catalytic Transfer Hydrogenation of Ketones. The catalyst solution was prepared by dissolving complex 3a (14.0 mg, 0.01 mmol) in 2-propanol (50.0 mL). Under a nitrogen atmosphere, a mixture of a ketone (2.0 mmol), 1.0 mL of the catalyst solution (0.0002 mmol), and 2-propanol (18.6 mL) was stirred at 82 °C for 10 min. Then, 0.4 mL of 0.1 M iPrOK (0.04 mmol) solution in 2-propanol was introduced to initiate the reaction. At the stated time, 0.1 mL of the reaction mixture was sampled by a syringe and immediately diluted with 0.5 mL of 2-propanol precooled to 0 °C for GC analysis. After the reaction was complete, the reaction mixture was concentrated under reduced pressure and subject to purification by silica gel column chromatography to afford the alcohol product, which was identified by comparison with the authentic sample through NMR and GC analyses. Typical Procedure for the Catalytic Dehydrogenation of Alcohols. A mixture of an alcohol substrate (0.5 mmol), complex 3a (0.0005 mmol), and tBuOK (0.05 mmol) in toluene (0.5 mL) was refluxed with stirring under a nitrogen atmosphere in a system connected to a bubbler open to air for 20 h. After the reaction was complete, the reaction mixture was cooled and condensed under reduced pressure. The resultant residue was subject to purification by flash silica gel column chromatography to afford the corresponding alcohol product, which was identified by comparison with the authentic sample through the NMR and GC analyses.

X-ray Crystallographic Studies. The X-ray diffraction studies for complex 3a were carried out on a SMART APEX diffractometer with graphite-monochromated Mo Kα radiation (λ= 0.71073 Å). Cell parameters were obtained by global refinement of the positions of all collected reflections. Intensities were corrected for Lorentz and polarization effects and empirical absorption. The structures were solved by direct methods and refined by full-matrix least-squares on F2. All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions. Structure solution and refinement were performed by using the SHELXL-97 package. The Xray crystallographic files, in CIF format, are available from the Cambridge Crystallographic Data Centre on quoting the deposition number CCDC 1518680 for complex 3a. Copies of this information may be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 IEZ, UK (Fax: + 44−1223−336033; e-mail: [email protected] or http://www.ccdc.cam.ac.uk). Synthesis of 2a. Under a nitrogen atmosphere, a mixture of 2bromo-6-(3,5- dimethylpyrazol-1-yl)pyridine (1) (2.51 g, 10.0 mmol), 2-aminopyridine (1.41 g, 15.0 mmol), Pd2(dba)3 (183 mg, 0.2 mmol), dppf (222 mg, 0.4 mmol), and NaOt-Bu (1.92 g, 20.0 mmol) in 25 mL of toluene was stirred at 110 °C for 8 h. After cooling to ambient temperature, the mixture was filtered through a short pad of Celite and rinsed with 50 mL of CH2Cl2. The combined filtrate was concentrated under reduced pressure, and the resulting residue was purified by column chromatography on silica gel (eluent: petroleum ether (60−90 °C)/ethyl acetate = 5:1, v/v) to afford 2a as a white solid (2.01 g, 76%). Mp: 110−112 °C. 1H NMR (DMSO-d6, 400 MHz, 23 °C) δ 9.71 (s, 1 H, NH), 8.28 (dd, J = 4.8 and 1.1 Hz, 1 H), 7.78, 7.65, 7.24, and 6.86 (m each, 2:1:1:1 H), 7.56 (d, J = 8.4 Hz, 1 H), 6.04 (s, 1 H), 2.58 and 2.18 (s each, 3:3 H, pyrazolyl-CH3). 13C{1H} NMR (DMSOd6, 100 MHz, 23 °C) δ 154.2, 152.6, 151.1, 148.4, and 140.4 (Cq each), 147.4, 139.9, 137.5, 116.1, 112.0, 108.62, 108.57, 107.2, 14.2, 13.4. HRMS: calcd for C15H15N5 265.1327, found 265.1330. Synthesis of 2b. A mixture of 2a (265 mg, 1.0 mmol) and NaH (80 mg, 60% in mineral oil, 2.0 mmol) in 5 mL of THF was stirred at ambient temperature for 20 min. Iodomethane (284 mg, 2.0 mmol) was added, and the reaction mixture was heated to reflux for 6 h. After the mixture was cooled to ambient temperature, methanol (5 mL) was added to quench the reaction. All the volatiles were evaporated under reduced pressure, and the resultant residue was purified by column chromatography on silica gel (eluent: petroleum ether (60−90 °C)/ ethyl acetate = 10:1 v/v) to afford target product 2b as a yellow oil (382 mg, 93%). 1H NMR (DMSO-d6, 400 MHz, 23 °C) δ 8.26 (dd, J = 4.8 and 1.3 Hz, 1 H), 7.58 (m, 2 H), 7.23 (d, J = 7.8 Hz, 1 H), 7.16 (d, J = 8.3 Hz, 1 H), 6.91 (m, 2 H), 5.91 (s, 1 H), 3.41 (s, 3 H, N− CH3), 2.39 and 2.07 (s each, 3:3 H, pyrazolyl-CH3). 13C{1H} NMR (DMSO-d6, 100 MHz, 23 °C) δ 157.0, 155.4, 151.5, 148.6, and 140.4 (Cq each), 148.0, 139.6, 137.6, 118.0, 115.7, 109.0, 108.9, 106.3, 36.0, 14.6, 13.4. HRMS: calcd for C16H17N5 279.1484, found 279.1484. Synthesis of 3a. Under a nitrogen atmosphere, a mixture of RuCl2(PPh3)3 (192 mg, 0.2 mmol) and 2a (53 mg, 0.2 mmol) was refluxed with stirring for 3 h. After cooled to ambient temperature, the resulting mixture was filtered to afford a yellow powder, which was washed with ether (5 × 10 mL) and dried in vacuo. Recrystallization in chloroform/n-hexane (v/v = 1:3) gave complex 3a as red crystals (123 mg, 88%). Mp: >300 °C. 1H NMR (DMSO-d6, 400 MHz, 23 °C) δ 11.77 (s, 2 H), 9.26 (d, J = 6.0 Hz, 2 H), 7.66 (m, 4 H), 7.45 (d, J = 8.3 Hz, 2 H), 7.32 (m, 8 H), 7.19 (t, J = 6.8 Hz, 12 H), 7.11 (t, J = 8.8 Hz, 14 H), 6.68 (t, J = 6.6 Hz, 2 H), 6.46 (s, 2 H), 2.62 (s, 12 H). 13 C{1H} NMR (DMSO-d6, 100 MHz, 23 °C) δ 157.9, 151.1, 150.0, 149.5, and 144.1 (Cq each), 137.6, 132.7, and 128.1 (d each, i-, o-, m-C of PPh3), 129.7 (s, p-C of PPh3), 151.7, 131.3, 130.9, 116.2, 114.8, 114.4, 110.5, 103.4, 15.3, 14.7. 31P{1H} NMR (DMSO-d6, 162 MHz, 23 °C) δ 31.7. IR (KBr pellets, cm−1): ν 3426, 3193, 3048, 2917, 1951, 1645, 1597, 1579, 1536, 1485, 1436, 1384, 1363, 1283, 1246, 1165, 1090, 1024, 996, 859, 749, 699, 531, 499, 441. Anal. Calcd for C66H60Cl4N10P2Ru2·0.5H2O: C, 56.29; H, 4.37; N, 9.95. Found: C, 56.50; H, 4.41; N, 10.07. Synthesis of 3b. In a fashion similar to the synthesis of 3a, RuCl2(PPh3)3 (192 mg, 0.2 mmol) reacted with 2b (56 mg, 0.2 mmol)

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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.organomet.7b00587. NMR spectra of the compounds (PDF) X-ray crystallographic data for 3a (XYZ) Accession Codes

CCDC 1518680 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qingfu Wang: 0000-0002-2293-9081 Huining Chai: 0000-0001-8087-3458 Zhengkun Yu: 0000-0002-9908-0017 Notes

The authors declare no competing financial interest. E

DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (21672209).

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DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00587 Organometallics XXXX, XXX, XXX−XXX