Synthesis of Pincer Hydrido Ruthenium Olefin ... - ACS Publications

Jan 13, 2016 - Huaquan Fang,. †. Wubing Yao, Xuebing Leng, and Zheng Huang*. The State Key Laboratory of Organometallic Chemistry, Shanghai Institut...
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Synthesis of Pincer Hydrido Ruthenium Olefin Complexes for Catalytic Alkane Dehydrogenation Yuxuan Zhang,† Huaquan Fang,† Wubing Yao, Xuebing Leng, and Zheng Huang* The State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, 345 Lingling Road, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: A series of new hydrido Ru(II) olefin complexes supported by isopropyl-substituted pincer ligands have been synthesized and characterized. These complexes are thermally robust and active for catalytic transfer and acceptorless alkane dehydrogenation. Notably, the alkane dehydrogenation catalysts are tolerant of a number of polar functional species.

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INTRODUCTION Catalytic alkane dehydrogenation is an important transformation, because it converts low-value hydrocarbon feedstocks into alkenes, which are highly versatile synthetic intermediates. In industry, alkenes are primarily derived from alkanes through dehydrogenations over heterogeneous catalysts. During the past several decades, homogeneous catalytic alkane dehydrogenation has received tremendous attention because it overcomes intrinsic limitations associated with heterogeneous dehydrogenation, such as poor product selectivity and harsh reaction conditions (500−900 °C).1,2 Bis(phosphine)-based pincer (PCP)Ir complexes have proven to be highly active for alkane dehydrogenations.3 Related bis(phosphite)-based (POCOP)Ir complexes4 and pincer Ir complexes with metallocene,5 fused-ring,6 and triptycene7 backbones also exhibit high dehydrogenation activity. 8 Recently, our group reported a (PSCOP)Ir complex with a hybrid phosphinothious/phosphinite ligand that showed very high activity in the transfer dehydrogenation of cyclooctane (COA) and n-octane.9 Non-phosphine-based Ir complexes, such as bis(carbene)-based (CCC)10 and bis(arsenic)-based (AsOCOAs)11 Ir complexes, have also been developed, but they are less effective than the classical (PCP)Ir complexes for alkane dehydrogenations.12 Notably, the Ir-catalyzed dehydrogenation reactions require very clean starting materials, and the catalysts often suffer from an inability to tolerate functionalities.1a,13 While Ir catalysts have been intensely studied, examples of Ru-catalyzed alkane dehydrogenations are rare. Felkin et al. first reported in 1984 that Ru polyhydride catalyzes COA/tertbutylethylene (TBE) transfer dehydrogenation, giving a maximum of 55 turnovers (TOs) at 150 °C after 10 days.14 In 1999, Leitner and co-workers disclosed that bis(phosphine) Ru(II) bis(allyl) complexes are effective for acceptorless dehydrogenation of COA. The catalytic activities of these © XXXX American Chemical Society

complexes are very low due to facile ligand degradation under the dehydrogenation conditions (maximum TON values of 5 after 2 days).15 More recently, Roddick and co-workers reported transfer and acceptorless dehydrogenations of COA using a Ru(II) complex ligated by a strongly π accepting pincer ligand (CF3PCP).16,17 Its initial activity for COA/TBE transfer dehydrogenation is very high, but the catalyst fully decomposes after only 30 min at 200 °C, resulting in a limited total catalytic TO number (186). Interestingly, this catalyst is quite insensitive to added O2 and H2O.16a Ruthenium metal in general is less expensive than iridium metal (ca. 10 times less).18 Driven by our interest in developing more practical dehydrogenation catalysts and the potential advantages of using Ru catalysts that can tolerate heteroatoms and impurities from starting materials, here we report the preparation of hydrido Ru(II) olefin complexes supported by isopropyl-substituted pincer ligands. These new complexes are thermally stable at high temperatures for the transfer dehydrogenation of cyclic and linear alkanes. They are also active for the dehydrogenation of COA in the absence of a hydrogen acceptor. More importantly, the Ru catalysts are compatible with various polar functional groups.

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RESULTS AND DISCUSSION The synthesis of new isopropyl-substituted pincer Ru(II) complexes is outlined in Scheme 1.19 Treatment of ligand precursors iPrPOCOP-H (1a), p-C6F5-iPrPOCOP-H (1b), iPr PCP-H (1c), and iPrPSCOP-H (1d) with RuCl2(PPh3)3 in THF at 80 °C for 8 h formed the complexes (iPrPOCOP)RuCl(PPh3) (2a), (p-C6F5-iPrPOCOP)RuCl(PPh3) (2b), (iPrPCP)RuCl(PPh3) (2c), and (iPrPSCOP)RuCl(PPh3) (2d). Removal of the free PPh3 by neutral alumina column Received: October 31, 2015

A

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The bond distances and bond angles are consistent with those reported for the related phenyl-substituted pincer Ru(II) complex (PhPCP)RuCl(NBD).19c Reactions of complexes 3a−d with excess NaBH4 (10 equiv) in THF formed hydrido Ru(II) NBD complexes 4a−d in high isolated yields.19c The characteristic hydridic Ru−H resonances appear as triplets at ca. δ −12 ppm. The substitution of the chloride with hydride results in a downfield shift of 31P NMR resonances (e.g., δ 210.7 for 4a vs δ 192.2 for 3a). Complexes 4a,c were characterized by X-ray diffraction analysis. Both structures adopt a distorted-octahedral geometry, although the hydride in 4c could not be located (Figure 1). Note that the POCOP complex 4a has a wider P−Ru−P bond angle than the PCP analogue 4c (144.087(19) and 139.40(3)°, respectively), In contrast, the POCOP Ir complexes typically have more acute P−Ir−P bond angles than the corresponding PCP Ir complexes as a result of shorter bonds to the oxygen linker (C−O and O− P) and a wider bond angle (C−O−P) in comparison to the analogous bond lengths and angles of the PCP methylene carbons.1a,21 However, in the case of 4a,c, the phosphorus atoms in 4c deviate more significantly from the backbone plane than those in 4a (presumably due to the greater rigidity of the POCOP ligand in comparison to the PCP ligand), thus resulting in a more narrow P−Ru−P bond angle (Figure 1). The new Ru complexes were first tested for the benchmark catalytic transfer dehydrogenation of COA with TBE as the hydrogen acceptor.2a To evaluate the effect of the concentration of TBE on the activity, COA solutions of 4a (1.0 mM) with varying amounts of TBE were heated at 200 °C under Ar in sealed vessels. As shown in Figure 2, the reactions with 0.1,

Scheme 1. Synthesis of Pincer Ru(II) Hydrido Olefin Complexes

chromatography afforded the Ru complexes as dark green solids in high yields. Complexes 2a−d were fully characterized by NMR spectroscopy and elemental analysis. The 31P NMR spectrum of complex 2a shows a doublet at δ 168.4 for the pincer ligand and a triplet at δ 82.1 for the coordinated PPh3 with a mutual coupling of 32.8 Hz. Similarly, complexes 2b,c show two resonances, whereas the hybrid complex (iPrPSCOP)RuCl(PPh3) (2d) shows three resonances in the 31P NMR spectra. Additionally, 2a was characterized by X-ray diffraction analysis. The structure adopts a distorted-square-pyramidal geometry with PPh3 at the apical site (Figure 1).

Figure 2. TONs for 4a-catalyzed transfer dehydrogenation of COA/ TBE at 200 °C after 7 h as a function of the concentrations of TBE. Figure 1. Crystal structures of complexes 2a, 3a, and 4a,c. The key bond distances and angles are given in the Supporting Information.

0.2, and 0.3 M TBE gave nearly full conversion of TBE to TBA after 7 h, corresponding to TOs of 100, 198, and 279, respectively. The highest TO (7 h, 306) was obtained when 0.35 M TBE was used. Reactions with higher [TBE] resulted in lower TOs; the processes with 0.5, 0.6, and 0.8 M TBE gave TOs of 241, 213, and 137, respectively. The data suggest catalyst inhibition by TBE at high [TBE], which has been previously observed using related pincer Ir catalysts.3a,9 Accordingly, the COA/TBE transfer dehydrogenations with various Ru complexes were carried out using 0.35 M TBE. The results are summarized in Table 1. The POCOP complexes 4a,b and the PCP complex 4c exhibit good activity. However, the PSCOP analogue 4d is inactive.22 The more electron deficient complex 4b with a pentafluorophenyl substituent is more active than 4a: the initial turnover frequencies (TOFs;

We envisioned that the presence of PPh3 may inhibit the dehydrogenation activity; thus, we sought to substitute the coordinated PPh3 in 2a−d with a more labile olefin ligand. Using CuCl as a scavenger for PPh3,19c,20 reactions of 2a−d with norbornadiene (NBD) and CuCl (5 equiv each) formed the corresponding Ru(II) chloro NBD complexes 3a−d as orange solids in 58−72% isolated yields. The NMR spectra of these complexes show olefinic signals for the coordinated NBD and 31P resonances only for the pincer ligands, indicating the substitution of PPh3 with NBD. Moreover, the solid structure of 3a revealed an octahedral coordination geometry with two double bonds in NBD bonded to the Ru(II) center (Figure 1). B

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with cyclic alkanes (possibly due to catalyst inhibition by the dehydrogenation products, especially by α-olefins).9 Indeed, Ru-catalyzed transfer dehydrogenation of n-octane produced much lower TOs in comparison with reactions with COA. Thermolysis of n-octane solutions containing 2.0 mM Ru complex 4a−c and 0.4 M TBE at 200 °C gave 12−17 turnovers after 8 h. The regioselectivity for formation of α-olefin was low: 1-octene constituted 200 TOs after 8 h (entries 4 and 5). The dehydrogenation of diethyl ether was not detected under these conditions.23 Reactions using 0.5 M TBE in the presence of 10 and 100 equiv of acetone (relative to Ru) afforded 367 and 349 TOs after 8 h,

a

Values in parentheses are the percentages of 1-alkene relative to total alkene products. C

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of H2, β-hydride elimination from the resulting Ru(II) alkyl (G) would generate (PCP)RuH and alkene (Scheme 2b).

respectively (entries 7 and 8). A control experiment with 100 equiv of acetone, but with no TBE, was carried out. The reaction gave a TO number (1 h, 29 TOs) similar to that obtained in the acceptorless dehydrogenation (vide supra), suggesting that acetone did not act as a hydrogen acceptor in these reactions. The catalyst maintained good activity under a N2 atmosphere (entry 9) but significantly lost activity on exposure to air (entry 10). It should be noted that pincer Ir catalysts are very sensitive to N2 and polar functional groups.1a,3a,4a,13 Thus, despite the relatively low activity of the Ru catalysts in comparison to the classical pincer Ir catalysts, one important advantage of our Ru systems over Ir systems is that the former are likely less sensitive to polar functionalities and impurities from starting materials. Indeed, catalysis using COA and TBE without purification24 produced TONs comparable to those obtained with scrupulously pure reagents (entry 11 vs entry 1). Although experimental and theoretical studies are ongoing to gain insight into the mechanism of this pincer Ru-catalyzed alkane dehydrogenation, it is of interest to compare the Ru catalyst system to the well-established pincer Ir catalyst system (Scheme 2). A key step in Ir-catalyzed dehydrogenation

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CONCLUSION In summary, we have prepared a series of isopropyl-substituted pincer Ru(II) hydrido olefin complexes and studied their activity for alkane dehydrogenations. The relatively electron deficient complex 4b is particularly active for transfer and acceptorless dehydrogenation of alkanes. The effectiveness of these new pincer Ru complexes is attributed to their thermal stability at very high temperatures. The dehydrogenation reactions, which are tolerant of water, ketones, esters, ethers, and impurities from the starting materials, represent a remarkable advance in the development of practical dehydrogenation catalysts. The catalyst could find applications in latestage modification of natural products via dehydrogenation of heterocycles and alkyl chains containing polar functional groups.

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EXPERIMENTAL SECTION

General Materials and Methods. All manipulations were carried out in an argon-filled glovebox or under an atmosphere of dry argon using standard Schlenk techniques, unless stated otherwise. n-Hexane was purified by distillation over CaH2. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl prior to use. Diethyl ether, dichloromethane, n-pentane, and toluene were collected from a solvent purification system. Chlorodiisopropylphosphine (96%) was purchased from Acros and used as received. Cyclooctane (COA), tertbutylethylene (TBE), and n-octane were purchased from Alfa Aesar, dried over LiAlH4 overnight under an atmosphere of dry argon, and then distilled prior to use and stored in an argon atmosphere glovebox. NMR spectra were recorded on Varian Mercury 400 MHz and Agilent 400 MHz spectrometers. 1H NMR spectra were referenced to residual protio solvent peaks or the TMS signal (0 ppm), and 13C NMR spectra were referenced to the solvent resonance. 31P NMR chemical shifts were referenced to an external H3PO4 standard. 19F chemical shifts have not been referenced. Elemental analyses were performed at the Analytical Laboratory of the Shanghai Institute of Organic Chemistry (CAS). GC analysis was carried out with an Agilent 7890A gas chromatograph equipped with a flame-ionization detector. GC-MS analysis was carried out with an Agilent 7890A gas chromatograph coupled to an Agilent 5975C inert mass selective detector. iPrPOCOP-H (1a),28 p-C6F5-iPrPOCOP-H (1b),4b iPrPCP-H (1c),29 iPrPSCOP-H (1d),9a and RuCl2(PPh3)330 were prepared by literature procedures. Synthesis of Complex (iPrPOCOP)RuCl(PPh3) (2a).

Scheme 2. Proposed Mechanism for Pincer Ir- and RuCatalyzed Alkane Dehydrogenations

involves addition of the alkane C−H bond to the threecoordinate, 14-electron PCP Ir(I) fragment A.3c,4c,25 β-Hydride elimination from the resulting hydrido Ir(III) alkyl species B would then form the Ir(III) dihydride C and alkene (Scheme 2a). For the Ru system, the corresponding species for C−H bond activation appears to be the four-coordinate, 14-electron PCP hydrido Ru(II) species D.16a The alkane C−H bond cleavage by D could occur by oxidative addition, as for the pincer Ir(I)−Ir(III) redox couple, or by σ-bond metathesis. Currently we cannot distinguish the two pathways. However, it is worth noting that the d6 (PCP)RuH center is not isoelectronic with the d8 (PCP)Ir center. More importantly, Ru(II) complexes are not prone to two-electron oxidation,26 which makes oxidative addition of C−H bonds to D to form a discrete Ru(IV) intermediate F very unlikely. Thus, we propose that C−H bond activation proceeds by σ-bond metathesis through the Ru(II) transition state E.27 Following dissociation

A mixture of iPrPOCOP-H (1a; 1.50 g, 4.38 mmol) and RuCl2(PPh3)3 (3.82 g, 3.99 mmol) in 30 mL of dry THF was stirred at 80 °C for 8 h. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on neutral alumina, with n-hexane as eluent to remove PPh3, and then with THF as eluent to afford the product as a dark green solid (2.80 g, 95% yield). 1H NMR (400 MHz, C6D6): δ 7.83−7.78 (m, 6H, ortho-H of PPh3), 6.90 (m, 9H, meta-H and para-H of PPh3), 6.76 (t, 3JHH = 8.0 Hz, 1H, Ar-H), 6.57 (d, 3JHH = 8.0 Hz, 2H, Ar-H), 2.57−2.43 (m, 2H, P-CH), 1.75−1.62 (m, 2H, PCH), 1.28−1.00 (m, 24H, CH3 of iPr). 31P NMR (162 MHz, C6D6): δ 82.1 (t, 2JPP = 32.8 Hz, 1P, PPh3), 168.4 (d, 2JPP = 32.8 Hz, 2P, P of ligand). 13C NMR (101 MHz, C6D6): δ 166.9, 145.2, 136.3, 133.6, 129.3, 127.7, 124.5, 105.8, 31.2, 29.6, 19.8, 19.2, 17.5, 16.6. Anal. Calcd for C36H46ClO2P3Ru: C, 58.42; H, 6.26. Found: C, 58.35; H, 6.25. D

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129.9, 128.1, 121.6, 117.4, 107.1, 31.6, 29.9, 20.0, 19.2, 17.5, 16.8. Anal. Calcd for C42H45ClF5O2P3Ru: C, 55.66; H, 5.00. Found: C, 55.67; H, 5.25. Synthesis of Complex (p-C6F5-iPrPOCOP)RuCl(NBD) (3b).

A mixture of (iPrPOCOP)RuCl(PPh3) (2a; 2.80 g, 3.78 mmol), CuCl (1.93 g, 19.3 mmol), and NBD (1.93 mL, 19.0 mmol) in 30 mL of dry CH2Cl2 was stirred at room temperature for 12 h. The insoluble material was removed by filtration through a pad of Celite, the filtrate was collected, and the volatiles were removed through evaporation under vacuum. The residue was purified by column chromatography on neutral alumina with diethyl ether as eluent to afford the product as an orange solid (1.55 g, 72%). 1H NMR (400 MHz, CD2Cl2): δ 6.53 (t, 3JHH = 8.0 Hz, 1H, Ar-H), 6.36 (d, 3JHH = 8.0 Hz, 2H, Ar-H), 4.37 (s, 2H, vinylic NBD), 3.66 (s, 2H, P-CH), 3.63−3.54 (m, 2H, P-CH), 3.28−3.17 (m, 2H, CH of NBD), 2.20 (s, 2H, vinylic NBD), 1.81 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.72 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.37 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.25 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.17−1.10 (m, 2H, CH2 of NBD). 31P NMR (162 MHz, CD2Cl2): δ 192.2 (s, 2P). 13C NMR (101 MHz, CD2Cl2): δ 162.4, 142.2, 123.9, 104.8, 60.4, 56.8, 47.2, 43.0, 33.6, 33.1, 19.8, 19.3, 18.6, 18.0. Anal. Calcd for C25H39ClO2P2Ru: C, 52.67; H, 6.90. Found: C, 52.55; H, 7.02. Synthesis of Complex (iPrPOCOP)RuH(NBD) (4a).

A mixture of (p-C6F5-iPrPOCOP)RuCl(PPh3) (2b; 2.09 g, 2.31 mmol), CuCl (1.15 g, 11.62 mmol), and NBD (1.18 mL, 11.60 mmol) in 30 mL of dry CH2Cl2 was stirred at room temperature for 12 h. The insoluble material was removed by filtration through a pad of Celite, the filtrate was collected, and the volatiles were removed through evaporation under vacuum. The residue was purified by column chromatography on neutral alumina with diethyl ether as eluent to afford the product as an orange solid (0.90 g, 60%). 1H NMR (400 MHz, CD2Cl2): δ 6.47 (s, 2H, Ar-H), 4.45 (s, 2H, vinylic NBD), 3.69 (s, 2H, P-CH), 3.61 (m, 2H, P-CH), 3.27 (m, 2H, CH of NBD), 2.31 (s, 2H, vinylic NBD), 1.81−1.70 (m, 12H, CH3 of iPr), 1.38 (q, 6H, 3 JHH = 8.0 Hz, CH3 of iPr), 1.28 (q, 6 H, 3JHH = 8.0 Hz, CH3 of iPr), 1.15 (d, 2H, 3JHH = 8.0 Hz, CH2 of NBD). 31P NMR (162 MHz, CD2Cl2): δ 194.6 (s, 2P). 19F NMR (376 MHz, CD2Cl2): δ −142.0 (dd, 2F, 3JFF = 22.6 Hz and 4JFF = 7.5 Hz, ortho-F of C6F5), −156.6 (t, 1F, 3JFF = 22.6 Hz, para-F of C6F5), −162.1 (td, 2F, 3JFF = 22.6 Hz and 4 JFF = 7.5 Hz, meta-F of C6F5). 13C NMR (101 MHz, CD2Cl2): δ 162.7, 146.9, 144.9, 140.2, 138.4, 121.5, 116.8, 106.5, 61.6, 57.2, 47.4, 43.8, 33.9, 33.4, 20.0, 19.5, 18.7, 18.2. Anal. Calcd for C31H38ClF5O2P2Ru: C, 50.58; H, 5.20. Found: C, 50.44; H, 5.28. Synthesis of Complex (p-C6F5-iPrPOCOP)RuH(NBD) (4b).

A mixture of (iPrPOCOP)RuCl(NBD) (3a; 1.55 g, 2.72 mmol) and NaBH4 (1.03 g, 27.2 mmol) in 30 mL of dry THF was stirred at 80 °C for 4 h. After evaporation of the solvent under vacuum, the residue was dissolved in toluene and filtered through a pad of Celite. The filtrate was collected, and the volatiles were removed through evaporation under vacuum. The solid was washed with diethyl ether (9 mL) and dried in vacuo for 30 min to give an off-white solid as the product (1.37 g, 91%). 1H NMR (400 MHz, CD2Cl2): δ 6.46 (t, 1H, 3JHH = 8.0 Hz, Ar-H), 6.23 (d, 2H, 3JHH = 8.0 Hz, Ar-H), 3.79 (s, 2H, vinylic NBD), 3.64 (s, 2H, CH of iPr), 3.56−3.49 (m, 2H, CH of iPr), 3.32 (s, 2H, vinylic NBD), 2.62−2.57(m, 2H, CH of NBD), 1.70−1.62 (m, 12H, CH3 of iPr), 1.18−1.08 (m, 2H, CH2 of NBD), 1.06−1.00 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 0.81−0.75 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), −12.7 (t, 1H, 2JHP = 28.0 Hz, Ru-H). 31P NMR (162 MHz, CD2Cl2): δ 210.7 (s, 2P). 13C NMR (101 MHz, CD2Cl2): δ 162.1, 146.5, 122.5, 103.3, 60.2, 55.4, 49.1, 45.6, 35.8, 30.5, 19.4, 19.3, 18.2, 17.5. Anal. Calcd for C25H40O2P2Ru: C, 56.06; H, 7.53. Found: C, 56.37; H, 7.61. Synthesis of Complex (p-C6F5-iPrPOCOP)RuCl(PPh3) (2b).

A mixture of (p-C6F5-iPrPOCOP)RuCl(NBD) (3b; 0.90 g, 1.39 mmol) and NaBH4 (0.53 g, 14.01 mmol) in 30 mL of dry THF was stirred at 80 °C for 4 h. After evaporation of the solvent under vacuum, the residue was dissolved in toluene and filtered through a pad of Celite. The filtrate was collected, and the volatiles were removed through evaporation under vacuum. The solid was washed with diethyl ether (9 mL) and dried in vacuo for 30 min to give an off-white solid as the product (0.75 g, 88%). 1H NMR (400 MHz, CD2Cl2): δ 6.34 (s, 2H, Ar-H), 3.85 (s, 2H, vinylic NBD), 3.67 (s, 2H, P-CH), 3.56 (m, 2H, PCH), 3.42 (s, 2H, vinylic NBD), 2.64 (m, 2H, CH of NBD), 1.71− 1.63 (m, 12H, CH3 of iPr), 1.12 (q, 2H, 3JHH = 8.0 Hz, CH2 of NBD), 1.04 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 0.81 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), −12.73 (t, 1H, 2JHP = 28.0 Hz, Ru-H). 31P NMR (162 MHz, C6D6): δ 211.8 (s, 2P). 19F NMR (376 MHz, C6D6): δ −143.6 (dd, 2F, 3JFF = 22.6 Hz and 4JFF = 7.5 Hz, ortho-F of C6F5), −158.8 (t, 1F, 3JFF = 22.6 Hz, para-F of C6F5), −163.8 (td, 2F, 3JFF = 22.6 Hz and 4 JFF = 7.5 Hz, meta-F of C6F5). 13C NMR (101 MHz, CD2Cl2): δ 162.4, 152.1, 144.8, 139.8, 138.4, 119.7, 117.7, 105.0, 60.7, 56.4, 49.4, 46.8, 36.1, 30.8, 19.5, 18.4, 17.7. Anal. Calcd for C31H39F5O2P2Ru: C, 53.07; H, 5.60. Found: C, 52.96; H, 5.68. Synthesis of Complex (iPrPCP)RuCl(PPh3) (2c).

A mixture of p-C6F5-iPrPOCOP-H (1b; 1.50 g, 2.95 mmol) and RuCl2(PPh3)3 (2.57 g, 2.68 mmol) in 30 mL of dry THF was stirred at 80 °C for 8 h. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on neutral alumina, with n-hexane as eluent to remove PPh3, and then with THF as eluent to afford the product as a dark green solid (2.09 g, 86% yield). 1H NMR (400 MHz, CD2Cl2): δ 7.62 (t, 6H, ortho-H of PPh3), 7.20 (m, 9H, para-H and meta-H of PPh3), 6.31 (s, 2H, Ar-H), 2.49 (m, 2H, PCH), 1.64 (m, 2H, P-CH), 1.20−1.11 (m, 12H, CH3 of iPr), 1.08 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 0.88 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr). 31P NMR (162 MHz, CD2Cl2): δ 81.6 (t, 2JPP = 33.2 Hz, 1P, PPh3), 169.0 (d, 2JPP = 33.2 Hz, 2P, P of ligand). 19F NMR (376 MHz, CD2Cl2): δ −141.5 (dd, 2F, 3JFF = 22.6 Hz and 4JFF = 7.5 Hz, ortho-F of C6F5), −156.9 (t, 1F, 3JFF = 22.6 Hz, para-F of C6F5), −162.2 (td, 2F, 3JFF = 22.6 Hz and 4JFF = 7.5 Hz, meta-F of C6F5). 13C NMR (101 MHz, CD2Cl2): δ 166.6, 150.6, 144.8, 140.1, 138.4, 135.8, 133.4,

A mixture of iPrPCP-H (1c; 1.50 g, 4.43 mmol) and RuCl2(PPh3)3 (3.85 g, 4.02 mmol) in 30 mL of dry THF was stirred at 80 °C for 8 h. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on neutral alumina, with n-hexane as eluent to remove PPh3, and then with THF as eluent to afford the product as a dark green solid (2.69 g, 91% yield). 1H NMR (400 MHz, CD2Cl2): δ 7.43 (t, 6H, 3JHH = 8.0 Hz, ortho-H of PPh3), 7.28 (t, 3H, 3 JHH = 8.0 Hz, para-H of PPh3), 7.16 (t, 6H, 3JHH = 8.0 Hz, meta-H of PPh3), 6.79 (d, 2H, 3JHH = 8.0 Hz, Ar-H), 6.66 (t, 1H, 3JHH = 8.0 Hz, E

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Organometallics

CD2Cl2): δ 7.55 (t, 6H, 3JHH = 8.0 Hz, ortho-H of PPh3), 7.27 (t, 3H, 3 JHH = 8.0 Hz, para-H of PPh3), 7.16 (t, 6H, 3JHH = 8.0 Hz, meta-H of PPh3), 6.94 (d, 1H, 3JHH = 8.0 Hz, Ar-H), 6.60 (t, 1H, 3JHH = 8.0 Hz, Ar-H), 6.20 (d, 1H, 3JHH = 8.0 Hz, Ar-H), 2.96−2.92 (m, 1H, CH of iPr), 2.43−2.34 (m, 1H, CH of iPr), 1.69−1.58 (m, 2H, CH of iPr), 1.32 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 1.22−1.11 (m, 9H, CH3 of iPr), 1.06−1.01 (m, 6H, CH3 of iPr), 0.94 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 0.48 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr). 31P NMR (162 MHz, CD2Cl2): δ 168.7 (dd, 2JPP = 37.3 Hz and 2JPP = 299.7 Hz, 1P, P-O), 90.4 (dd, 2JPP = 26.7 Hz and 2JPP = 298.9 Hz, 1P, P-S), 76.4 (dd, 2 JPP = 26.7 Hz and 2JPP = 37.3 Hz, 1P, PPh3). 13C NMR (101 MHz, CD2Cl2): δ 135.9, 134.3, 134.0, 129.6, 129.1, 128.9, 127.6, 123.3, 117.1, 108.1, 32.3, 31.0, 30.2, 27.6, 20.8, 20.6, 19.2, 19.0, 18.5, 17.4, 16.4. Anal. Calcd for C36H46ClOP3RuS: C, 57.17; H, 6.13. Found: C, 57.00; H, 6.06. Synthesis of Complex (iPrPSCOP)RuCl(NBD) (3d).

Ar-H), 2.94 (dt, 2H, 2JHH = 16.0 Hz, 2JHP = 4.0 Hz, PCH2), 2.44−2.36 (m, 2H, CH of iPr), 1.92 (d, 2H, 2JHH = 16.0 Hz, PCH2), 1.37 (q, 6H, 3 JHH = 8.0 Hz, CH3 of iPr), 1.31−1.24 (m, 2H, CH of iPr), 1.08 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 0.91−0.85 (m, 12H, CH3 of iPr). 31P NMR (162 MHz, CD2Cl2): δ 80.6 (t, 2JPP = 32.4 Hz, 1P, PPh3), 43.7 (d, 2JPP = 32.4 Hz, 1P, P of ligand). 13C NMR (101 MHz, CD2Cl2): δ 152.7, 137.6, 134.8, 129.4, 128.5, 127.4, 122.0, 121.2, 35.9, 26.2, 25.8, 20.9, 20.1, 19.5, 18.0. Anal. Calcd for C38H50ClP3Ru: C, 61.99; H, 6.85. Found: C, 62.31; H, 6.95. Synthesis of Complex (iPrPCP)RuCl(NBD) (3c).

A mixture of (iPrPCP)RuCl(PPh3) (2c; 2.69 g, 3.66 mmol), CuCl (1.87 g, 18.7 mmol), and NBD (1.87 mL, 18.32 mmol) in 30 mL of dry CH2Cl2 was stirred at room temperature for 12 h. The insoluble material was removed by filtration through a pad of Celite, the filtrate was collected, and the volatiles were removed through evaporation under vacuum. The residue was purified by column chromatography on neutral alumina with diethyl ether as eluent to afford the product as an orange solid (1.35 g, 65%). 1H NMR (400 MHz, C6D6): δ 6.80 (d, 2H, 3JHH = 8.0 Hz, Ar-H), 6.56 (t, 1H, 3JHH = 8.0 Hz, Ar-H), 4.13 (s, 2H, vinylic NBD), 3.76 (dt, 2H, 2JHH = 16.0 Hz, 2JHP = 4.0 Hz, PCH2), 3.64 (s, 2H, P-CH), 3.32 (dt, 2H, 2JHH = 16.0 Hz, 2JHP = 4.0 Hz, P-CH2), 3.15−2.98 (m, 6H, P-CH, vinylic NBD, CH of NBD), 1.72 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.48 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1,27 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.18−1.11 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.06 (m, 2H, CH2 of NBD). 31P NMR (162 MHz, CD2Cl2): δ 64.5 (s). 13C NMR (101 MHz, CD2Cl2): δ 173.4, 147.5, 122.3, 120.5, 59.0, 58.0, 49.4, 46.1, 37.7, 28.8, 25.5, 21.6, 20.8, 20.5, 20.0. Anal. Calcd for C27H43ClP2Ru: C, 57.28; H, 7.66. Found: C, 57.26; H, 7.65. Synthesis of Complex (iPrPCP)RuH(NBD) (4c).

A mixture of (iPrPSCOP)RuCl(PPh3) (2d; 2.68 g, 3.55 mmol), CuCl (1.81 g, 18.1 mmol), and NBD (1.81 mL, 17.73 mmol) in 30 mL of dry CH2Cl2 was stirred at 50 °C for 12 h. The insoluble material was removed by filtration through a pad of Celite, the filtrate was collected, and the volatiles were removed through evaporation under vacuum. The residue was purified by column chromatography on neutral alumina with diethyl ether as eluent to afford the product as an orange solid (1.21 g, 58%). 1H NMR (400 MHz, CD2Cl2): δ 6.75 (d, 1H, 3JHH = 8.0 Hz, Ar-H), 6.47 (m, 2H, Ar-H), 4.48 (s, 1H, vinylic NBD), 4.17 (s, 1H, vinylic NBD), 3.75 (s, 1H, P-CH), 3.72−3.66 (m, 1H, P-CH), 3.59 (s, 1H, P-CH), 3.54−3.41 (m, 2H, P-CH and CH of NBD), 3.38−3.29 (m, 1H, CH of NBD), 2.92 (s, 1H, vinylic NBD), 2.33 (s, 1H, vinylic NBD), 1.76 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 1.71− 1.64 (m, 6H, CH3 of iPr), 1.60 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 1.43−1.35 (m, 9H, CH3 of iPr), 0.99 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 1.00 (t, 2H, 3JHH = 8.0 Hz, CH2 of NBD). 31P NMR (162 MHz, CD2Cl2): δ 187.1 (d, 2JPP = 217.9 Hz, 1P, P-O), 116.9 (d, 2JPP = 217.9 Hz, 1P, P-S). 13C NMR (101 MHz, CD2Cl2): δ 134.3, 129.6, 127.6, 123.4, 116.5, 107.8, 63.7, 63.0, 57.2, 47.7, 47.3, 46.7, 45.7, 34.4, 34.1, 31.8, 28.8, 21.2, 21.0, 19.8, 19.4, 19.0, 18.6, 18.2, 15.5. Anal. Calcd for C25H39ClOP2RuS: C, 51.23; H, 6.71. Found: C, 51.40; H, 6.66. Synthesis of Complex (iPrPSCOP)RuH(NBD) (4d).

A mixture of (iPrPCP)RuCl(NBD) (3c; 1.35 g, 2.38 mmol) and NaBH4 (0.90 g, 23.8 mmol) in 30 mL of dry THF was stirred at room temperature for 12 h. After evaporation of the solvent under vacuum, the residue was dissolved in toluene and filtered through a pad of Celite. The filtrate was collected, and the volatiles were removed through evaporation under vacuum. The solid was washed with diethyl ether (9 mL) and dried in vacuo for 30 min to give an off-white solid as the product (1.09 g, 86%). 1H NMR (400 MHz, CD2Cl2): δ 6.72 (d, 2H, 3JHH = 8.0 Hz, Ar-H), 6.44 (t, 1H, 3JHH = 8.0 Hz, Ar-H), 3.68 (s, 2H, vinylic NBD), 3.55 (s, 2H, P-CH), 3.41−3.34 (m, 4H, P-CH2 and P-CH), 3.21−3.13 (m, 2H, P-CH2), 2.93 (s, 2H, vinylic NBD), 2.25−2.07 (m, 2H, CH of NBD), 1.62−1.54 (m, 12H, CH3 of iPr), 1.10 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), 1.03 (s, 2H, CH2 of NBD), 0.72 (q, 6H, 3JHH = 8.0 Hz, CH3 of iPr), −11.95 (t, 2JHP = 28.0 Hz, 1H, Ru-H); 31P NMR (162 MHz, CD2Cl2): δ 84.0 (s). 13C NMR (101 MHz, CD2Cl2): δ 181.4, 146.5, 120.3, 119.8, 60.1, 49.3, 47.7, 47.1, 42.8, 31.8, 26.5, 21.2, 20.4, 19.0, 1.2. Anal. Calcd for C27H44P2Ru: C, 61.00; H, 8.34. Found: C, 61.13; H, 8.38. Synthesis of Complex (iPrPSCOP)RuCl(PPh3) (2d).

A mixture of (iPrPSCOP)RuCl(PPh3) (3d; 1.21 g, 2.06 mmol) and NaBH4 (0.78 g, 20.63 mmol) in 30 mL of dry THF was stirred at 80 °C for 4 h. After evaporation of the solvent under vacuum, the residue was dissolved in toluene and filtered through a pad of Celite. The filtrate was collected, and the volatiles were removed through evaporation under vacuum. The solid was washed with diethyl ether (9 mL) and dried in vacuo for 30 min to give an off-white solid as the product (1.08 g, 92%). 1H NMR (400 MHz, CD2Cl2): δ 6.71 (d, 1H, 3 JHH = 8.0 Hz, Ar-H), 6.46 (t, 1H, 3JHH = 8.0 Hz, Ar-H), 6.34 (d, 1H, 3 JHH = 8.0 Hz, Ar-H), 3.79 (s, 2H, vinylic NBD), 3.72 (s, 1H, P-CH), 3.71−3.64 (m, 1H, P-CH), 3.58 (s, 1H, P-CH), 3.49−3.39 (m, 3H, PCH and CH of NBD), 2.71−2.64 (m, 1H, vinylic NBD), 2.50−2.44 (m, 1H, vinylic NBD), 1.69−1.61 (m, 12H, CH3 of iPr), 1.14 (d, 3H, 3 JHH = 8.0 Hz, CH3 of iPr), 1.07 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 0.99 (dd, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 0.94 (d, 3H, 3JHH = 8.0 Hz, CH3 of iPr), 0.90 (d, 3JHH = 8.0 Hz, CH2 of NBD), −12.76 (t, 1H, 2JHP = 32.0 Hz, Ru-H). 31P NMR (162 MHz, CD2Cl2): δ 205.8 (d, 2JPP = 183.1 Hz, 1P, P-O), 135.7 (d, 2JPP = 183.1 Hz, 1P, P-S). 13C NMR (101 MHz, CD2Cl2): δ 133.5, 131.7, 129.2, 122.2, 115.1, 105.8, 60.5, 58.5, 56.6, 49.2, 48.8, 48.2, 48.1, 36.6, 33.7, 32.0, 31.3, 21.7, 21.5, 20.1, 19.5, 19.1, 18.6, 17.7, 15.5. Anal. Calcd for C25H40OP2RuS: C, 54.43; H, 7.31. Found: C, 54.73; H, 6.94.

A mixture of iPrPSCOP-H (1d; 1.50 g, 4.18 mmol) and RuCl2(PPh3)3 (3.63 g, 3.79 mmol) in 30 mL of dry THF was stirred at 80 °C for 8 h. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on neutral alumina, with n-hexane as eluent to remove PPh3, and then with THF as eluent to afford the product as a dark green solid (2.68 g, 85% yield). 1H NMR (400 MHz, F

DOI: 10.1021/acs.organomet.5b00912 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics General Procedure for Transfer Dehydrogenation of Cyclooctane with TBE. In an argon-filled glovebox, an oven-dried 5 mL thick-walled glass tube was charged with a ruthenium complex (1.0 mmol/L; 2.0 mg for 4a,c,d and 3.0 mg for 2b, 3b, and 4b), cyclooctane, and tert-butylethene (0.35 mol/L) as a hydrogen acceptor. The tube was sealed with a Teflon plug under an argon atmosphere, and the solution was stirred in a 200 °C oil bath. Periodically, the flask was removed from the bath and cooled to room temperature. An aliquot was removed from the flask and analyzed by GC. Turnover numbers were calculated from the ratio of the amount of tert-butylethane (TBA) produced against the catalyst amount added. General Procedure for Transfer Dehydrogenation of nOctane with TBE. In an argon-filled glovebox, an oven-dried 5 mL thick-walled glass tube was charged with a ruthenium complex (2.0 mmol/L; 3 mg for 4a,c and 4 mg for 4b), n-octane, and mesitylene (0.2 mol/L); tert-butylethene (0.4 mol/L) was then added as a hydrogen acceptor. The tube was sealed with a Teflon plug under an argon atmosphere, and the solution was stirred in a 200 °C oil bath. Periodically, the flask was removed from the bath and cooled to room temperature. An aliquot was removed from the flask and analyzed by GC. Acceptorless Dehydrogenation of Cyclooctane. In an argonfilled glovebox, an oven-dried 5 mL thick-walled glass tube was charged with ruthenium complex 4a,b (4.0 μmol) and 1.63 mL of COA (3000 equiv, 12 mmol). After a condenser was attached to the tube, the mixture was stirred at 200 °C under a constant argon flow. A three-way piece was placed on top of the condenser to remove the liberated H2 gas. After the reaction, the tube was cooled to room temperature and an aliquot was removed for GC analysis. Turnover numbers were calculated from the ratio of the amount of COE produced against the catalyst amount added. Alternatively, the reactions could be carried out in a sealed tube. In an argon-filled glovebox, an oven-dried 10 or 50 mL thick-walled glass tube was charged with the ruthenium complex 4b (2.0 mg, 2.72 μmmol, 1.0 mmol/L) and cyclooctane (2.7 mL, 20.0 mmol). The tube was sealed with a Teflon plug under an argon atmosphere, and the solution was stirred in a 200 °C oil bath. After the reaction, the flask was removed from the bath and cooled to room temperature. An aliquot was removed from the flask and analyzed by GC. Turnover numbers were calculated from the ratio of the amount of COE produced against the catalyst amount added.

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Science Foundation of China (21432011, 21422209), and the Science and Technology Commission of Shanghai Municipality (13JC1406900).

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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.5b00912. Crystallographic data and crystal structures for 2a, 3a, and 4a,c and procedures for alkane dehydrogenation (PDF) Crystallographic data for 2a (CCDC 1423966) (CIF) Crystallographic data for 3a (CCDC 1424059) (CIF) Crystallographic data for 4a (CCDC 1423967) (CIF) Crystallographic data for 4c (CCDC 1424058) (CIF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.H.: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (2015CB856600), the National Natural G

DOI: 10.1021/acs.organomet.5b00912 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.5b00912 Organometallics XXXX, XXX, XXX−XXX