Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes

Accepted Manuscript Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes Wubing Yao, Xiangqing Jia, Xuebing Leng, Alan S. Goldman, Maurice Brookhart, Zheng Huang PII: DOI: Reference:

S0277-5387(16)00138-8 http://dx.doi.org/10.1016/j.poly.2016.02.044 POLY 11860

To appear in:

Polyhedron

Received Date: Accepted Date:

19 January 2016 26 February 2016

Please cite this article as: W. Yao, X. Jia, X. Leng, A.S. Goldman, M. Brookhart, Z. Huang, Catalytic alkane transferdehydrogenation by PSCOP iridium pincer complexes, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly. 2016.02.044

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Catalytic alkane transfer-dehydrogenation by PSCOP iridium pincer complexes Wubing Yaoa, Xiangqing Jiaa, Xuebing Lenga, Alan S. Goldmanb, Maurice Brookhartc,*, and Zheng Huanga, * a

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,

Chinese Academy of Science, 345 Lingling Road, Shanghai 200032, China b

Department of Chemistry and Chemical Biology, Rutgers University, Piscataway, NJ 08854,

United States c

Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North

Carolina 27599-3290, United States

On the occasion of his 80th birthday, this article is dedicated to Professor Malcolm Green who has repeatedly made path breaking and enduring contributions to organometallic and inorganic chemistry over more than five decades.

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ABSTRACT. A series of new (tBu2PSCOPR2)IrHCl iridium complexes with ‘hybrid’ phosphinothious-phosphinite PSCOP ligands ([tBu2PSCOPR2 = 1-(SPtBu2)-3-(OPR2)-C6H4], R = tBu, 4a, R = Cy, 4b, R = iPr, 4c, and R = Et, 4d) have been synthesized and characterized. Treatment of complexes 4a-d with sodium tert-butyloxide generates the active species for catalytic transfer-dehydrogenation of cyclooctane (COA) or n-octane using tert-butylethylene (TBE) as hydrogen acceptor to form cyclooctene (COE) or octenes, respectively. The catalytic activity of these complexes and the product selectivity in alkane dehydrogenation is greatly influenced by the steric properties of the pincer ligand. In general, the less sterically bulky complex exhibits higher catalytic activity than the more hindered complex. Among the new (PSCOP)Ir-type complexes, the least crowded complex (tBu2PSCOPEt2)IrHCl 4d is most active for n-octane/TBE transfer-dehydrogenation. The relatively crowded, less active, complexes (tBu2PSCOPtBu2)IrHCl (4a) and (tBu2PSCOPCy2)IrHCl (4b) exhibit high regioselectivity for α-olefin formation at the early stages of the reaction. Keywords: alkane · alkene · dehydrogenation · homogeneous catalysis · iridium 1. Introduction Alkane dehydrogenation is a reaction with important potential applications. This transformation converts abundant, inexpensive saturated hydrocarbon feedstocks into olefins that are useful synthetic intermediates. Industrially, heterogeneous catalytic alkane dehydrogenation is carried out on enormous scales via hydrocarbon cracking and reforming processes. However, these processes typically operate at very high temperatures (> 500˚ C), resulting in low energy efficiency and poor product selectivities[1]. In this respect, the development of efficient homogeneous systems for selective catalytic alkane dehydrogenation under mild conditions is of great interest.

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Chart 1. Structures of (PCP)Ir, (POCOP)Ir, and (PSCOP)Ir Complexes

tBu2 P

PtBu2

Ir H

H

O tBu2P

Ir H Cl

O PtBu2

S iPr2P

Ir H Cl

O PiPr2

(tBu2PCPtBu2)IrH2

(tBu2POCOPtBu2)IrHCl

(iPr2PSCOPiPr2)IrHCl

1

2

3

Since the discoveries of homogenous alkane dehydrogenations by Crabtree and Felkin in the 1980s[2-4], numerous soluble molecular dehydrogenation catalysts have been developed[5-8]. Among them, the robust iridium complexes of pincer ligands have proven to be most effective. Chart 1 shows three examples of pincer iridium systems. The bis(phosphine)-based pincer (PCP)Ir complex 1 has been extensively investigated by the Kaska, Jensen, and Goldman groups[9-17], while the bis(phosphinite)-based pincer (POCOP)Ir complex 2 was mainly explored by the Brookhart group[18-21]. These and other relevant pincer-type iridium catalyst systems can effect alkane dehydrogenation with or without a hydrogen acceptor, typically at temperatures in the range of 150-200˚ C[22-35]. Recyclable, highly active solid-supported pincer iridium complexes have also been reported for alkane transfer-dehydrogenations[36]. In addition, the pincer iridium catalysts have been applied to various types of alkane transformations, including alkane metathesis via tandem alkane dehydrogenation and olefin metathesis[37-39], conversion of nalkanes to alkylaromatics via dehydroaromatization[40, 41], and regioselective conversion of nalkanes to linear alkylsilanes via Ir-catalyzed alkane dehydrogenation and Fe-catalyzed tandem olefin isomerization and hydrosilylation[42]. Although significant progress has been achieved in the field of homogeneous alkane dehydrogenation in the past two decades, there is room for improvement of known catalytic systems with respect to catalytic activity and product selectivity in dehydrogenation of linear alkanes[5]. Early studies on pincer-type Ir complexes have shown that the nature of the linkers connecting the P atoms and the aryl backbone[19, 43] can affect the productivity and regioselectivity in alkane dehydrogenation. In this context, very recently we reported that an isopropyl-substituted (PSCOP)Ir complex (3) with a hybrid phosphinothious-phosphinite ligand (see Chart 1), upon activation with NaOtBu, is highly active for transfer-dehydrogenation of

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cyclooctane (COA) and n-octane[44]. Herein we report the synthesis and characterization of a series of (PSCOP)Ir complexes bearing tBu groups on the phosphinothious arm and various alkyl substituents on the phosphinite arm. The steric demands of the pincer ligands have a large impact on the catalytic performance of these complexes in transfer-dehydrogenation reactions: the complexes bearing small phosphino-substituents are more active than those with bulky substituents for transfer-dehydrogenation of both cyclic and linear alkanes, while the crowded complexes offer higher regioselectivity for α-olefin formation in dehydrogenation of the linear alkane, octane. 2. Experimental General Considerations. All manipulations were carried out using standard Schlenk, high vacuum, and glovebox techniques. THF, hexane, pentane and toluene were distilled from sodium, then degassed by three freeze-pump-thaw cycles and stored in an argon atmosphere glovebox. Cyclooctane (COA) (99%), tert-butylethylene (TBE) (98.5%), n-octane (99%) were purchased from Aldrich and dried over LiAlH4 overnight, then distilled under vacuum and stored in an argon atmosphere glovebox prior to use. NaH (95%), dicyclohexylchlorophosphine (98+%), and 3-hydroxythiophenol (96%) were purchased from TCI and used as received. Di-tertbutylchlorophosphine (96%), chlorodiethylphosphine (95%), and chlorodiisopropylphosphine (96%) were purchased from Acros and used as received. Triethylamine was dried over LiAlH4 overnight, then distilled under vacuum and stored in an argon atmosphere glovebox prior to use. All other reagents and solvents mentioned in this text were purchased from commercial sources. [Ir(COD)Cl]2[45], (tBu4PCP)IrH2 (1)[9], and (tBu4POCOP)IrHCl (2)[19] were prepared according to previously reported procedures. NMR spectra were obtained on Aglient 400, Varian 400 MHz, and Varian 300 MHz instruments. 1H NMR spectra were referenced to residual protio solvent peaks or TMS (0 ppm) and 13C NMR spectra were referenced to the solvent resonance. 31P NMR chemical shifts were referenced to an external H3PO4 standard. 1H NMR data are recorded as follows: chemical shift (δ, ppm), multiplicity (s = singlet, d = doublet, t = triplet, m = multiplet or unresolved, br = broad singlet, coupling constant (s) in Hz, integration). 13C NMR data are reported in terms of chemical shift (δ, ppm). Elemental analyses and high resolution mass spectroscopy (HRMS) were carried out by the Analytical Laboratory of Shanghai Institute of Organic Chemistry (CAS).

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GC analysis was acquired on Agilent 7890A gas chromatograph equipped with a flameionization detector. GC-MS analysis was performed on Agilent 7890A gas chromatograph coupled to an Agilent 5975C inert mass selective detector. General procedures for transfer-dehydrogenation of COA with TBE In an argon-filled glovebox, the appropriate complex (RPSCOPR’)IrHCl (4a-d) (3 µmol) was dissolved in a solution of COA and TBE (3000 equiv. each relative to iridium, 9.0 mmol). Sodium tert-butoxide (1.5 equiv. with respect to iridium, 4.5 µmol) was then added to the solution. The solution was transferred into a 10 mL thick-wall glass tube. The tube was sealed and heated in a preheated oil-bath at 200 °C. At regular intervals, the tube was cooled to the room temperature and the sample was analyzed by gas chromatography. General procedures for transfer-dehydrogenation of n-octane with TBE In an argon-filled glovebox, the appropriate complex (R2PSCOPR’2)IrHCl (4a-d) (3 µmol) was dissolved in a solution of n-octane (5728 equiv. with respect to iridium, 17.2 mmol) and TBE (500 equiv. each relative to iridium, 1.5 mmol). Sodium tert-butoxide (1.5 equiv. with respect to iridium, 4.5 µmol) was then added to the solution. The solution was transferred into a 10 mL thick-wall glass tube. The tube was sealed and heated in a preheated oil-bath at 200 °C. At regular intervals, the tube was cooled to the room temperature and the sample was analyzed by gas chromatography.

S tBu2P

OH

Synthesis of 1-(SPtBu2)-3-(OH)-C6H4. Triethylamine (11.1 mmol, 1.12 g) was added to a solution of 3-hydroxythiophenol (7.84 mmol, 0.99 g) in 30 mL of THF. The mixture was heated at 70 °C for 1 h, di-tert-butylchlorophosphine (8.69 mmol, 1.57 g) was then added to the reaction mixture. The mixture was stirred at room temperature for 1 h. After evaporation of the solvent under high vacuum, the residue was dissolved in 30 mL of toluene and the extract was cannula transferred and filtered through a pad of Celite. After evaporation of the solvent under high vacuum, the flask was heated to 70˚ C for 1 h to remove the residual amounts of di-tertbutylchlorophosphine. The product was obtained as waxy solid. Yield, 70% (1.48 g, 5.49 mmol). 1

H NMR (300 MHz, CDCl3) δ 7.62 (s, 1H), 7.41 (d, 2JHH = 6.0 Hz, 1H), 7.06 (t, 2JHH = 6.0 Hz,

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1H), 6.93 (d, 2JHH = 9.0 Hz, 1H), 1.34 (d, 2JHH = 12.0 Hz, 18H). 31P NMR (121 MHz, CDCl3): δ 80.7. 13C{1H} NMR (101MHz, C6D6): δ 157.5, 138.9, 129.7, 122.6, 118.3, 113.7, 35.0, 29.6. HRMS (ESI), m/z calculated for C14H23OPS (M+): 270.1207, found: 270.1220.

S tBu2P

O PtBu2

Synthesis of ligand tBu2PSCOPtBu2. A suspension of NaH (11.0 mmol, 0.26 g) in 10 mL of THF was slowly added via syringe to a solution of the 3-hydroxythiophenol (5.00 mmol, 0.63 g) in 30 mL of THF (caution: hydrogen evolution). The mixture was stirred at room temperature for 1 h, then di-tert-butylchlorophosphine (10 mmol, 1.80 g) was added via syringe, and the mixture was stirred at room temperature for an additional 1 h. After evaporation of the solvent under high vacuum, the residue was dissolved in hexane (50 mL), and the extract was cannula transferred and filtered through a pad of Celite. After evaporation of the solvent under high vacuum, the flask was heated to 70˚ C for 1 h to remove the residual amounts of di-tert-butylchlorophosphine. The crude product was obtained as pale yellow viscous oil and exhibit >95% purity by NMR (see Supporting Information). The ligand was used without further purification. Yield: 73% (1.51 g, 3.65 mmol). 1H NMR (400 MHz, C6D6): δ 7.87-7.85 (m, 1H), 7.37-7.34 (m, 1H), 7.07-7.03 (m, 1H), 6.98 (t, 2JHH = 8.0 Hz, 1H), 1.23 (d, 2JHH = 12.0 Hz, 18H), 1.10 (d, 2JHH = 12.0 Hz, 18H ). 31

P{1H} NMR (162 MHz, C6D6) δ 154.4, 82.1. 13C{1H} NMR (101 MHz, C6D6) 160.5, 139.5,

129.8, 124.3, 121.2, 116.4, 35.8, 35.3, 29.9, 27.6. HRMS (ESI), m/z calculated for C22H40OP2S (M+): 414.2275, found: 414.2278. General procedure for synthesis of ligands tBu2PSCOPR2 (R = Cy, iPr, Et) A suspension of NaH (6.00 mmol, 0.14 g) in 10 mL of THF was slowly added via syringe to a solution of 1-(SPtBu2)-3-(OH)-C6H4 (5.0 mmol, 1.35 g) in 30 mL of THF (caution: hydrogen evolution). The mixture was stirred for 1 h at room temperature, then dialkylchlorophosphine (5.0 mmol) was added via syringe and the mixture was stirred at room temperature for additional 1 h. After evaporation of the solvent under high vacuum completely, the residue was dissolved in pentane (50 mL) and the extract was cannula transferred and filtered through a pad of Celite. After evaporation of the volatiles, the crude products were obtained as colorless or white oils and exhibit >95% or ca. 95% purity by NMR (see Supporting Information). The ligands were used without further purification.

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S tBu2P

O PCy2

Synthesis of ligand

tBu2

PSCOPCy2. Colorless oil, yield: 74%, >95% purity. 1H NMR (400 MHz,

C6D6) δ 7.87 (s, 1H), 7.37 (d, 2J HH = 8.0 Hz, 1H), 7.06 (d, 2J HH = 4.0 Hz, 1H), 6.97-7.01 (m, 1H), 1.92-1.95 (m, 2H), 1.75-1.64 (m, 8H), 1.58 (s, 2H), 1.50-1.44 (m, 2J HH = 12.0 Hz, 2H), 1.24 (d, 2

J HH = 12.0 Hz, 22H), 1.13 (t, 2J HH = 8.0 Hz, 4H). 31P{1H} NMR (162 MHz, C6D6) δ 143.0, 80.9.

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C{1H} NMR (101 MHz, C6D6) δ 160.4, 139.4, 129.7, 124.3, 121.3, 116.5, 38.5, 34.3, 29.8,

28.4, 27.3, 27.1. HRMS (MALDI/DHB), m/z calculated for C26H45OP2S (M+), 467.2649, found: 467.2660.

S tBu2P

O PiPr2

Synthesis of ligand

tBu2

PSCOPiPr2. White viscous oil, yield: 65%, >95% purity. 1H NMR (400

MHz, CDCl3) δ 7.79 (t, 2JHH = 4.0 Hz, 1H), 7.32 (d, 2JHH = 4.0 Hz, 1H), 7.01-6.94 (m, 2H), 7.07(d, 2

JHH = 16.0 Hz, 1H), 1.80-1.68 (m, 2H), 1.22 (d, 2JHH = 12.0 Hz, 18H), 1.11 (d, 2JHH = 8.0 Hz, 3H),

1.09 (d, 2JHH = 8.0 Hz, 3H), 0.98 (d, 2JHH = 8.0 Hz, 3H), 0.94 (d, 2JHH = 8.0 Hz, 3H). 31P{1H} NMR (162 MHz, C6D6) δ 148.7, 81.7. 13C{1H} NMR(101MHz, C6D6) δ 160.1, 139.4, 129.7, 124.5 121.3, 116.5, 35.3, 29.9, 28.7, 17.9 35.3, 17.1. HRMS (EI), m/z calculated for C20H36OP2S: 386.1962 (M+), found: 386.1960.

S tBu2P

O PEt2

Synthesis of ligand tBu2PSCOPEt2. Colorless oil, yield: 70%, ca. 95% purity. 1H NMR (400MHz, CDCl3) δ 7.31 (s, 1H), 7.19 (d, 2JHH = 8.0Hz, 1H), 7.10-7.14 (m, 1H), 6.86 (d, 2JHH = 8.0 Hz, 1H), 1.83-1.76 (m, 2H), 1.69-1.61 (m, 2H), 1.29 (d, 2JHH = 12.0 Hz, 18H), 1.19-1.11 (m, 6H). 31P{1H} NMR (162 MHz, CDCl3) δ 140.9, 84.6.

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C {1H} NMR (101 MHz, CDCl3) δ 158.4, 138.9,

124.7, 121.4, 116.3, 35.4, 29.8, 25.2, 7.9. HRMS (EI), m/z calculated for C18H32OP2S (M+), 358.1649, found: 358.1645. General procedure for synthesis of complexes (tBu2PSCOPR2)IrHCl 4a-4d

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A rubber septum-capped Schlenk flask was charged with 1 equiv of [Ir(COD)Cl]2 and 2.0 equiv of the PSCOP ligand. Toluene (10 mL) was added via syringe, and the solution was stirred in an oil bath at 120 °C for several hours. The volatiles were then removed under high vacuum. The residue was washed with pentane and dried under high vacuum to yield the pure product.

S tBu2P

Ir H Cl

O PtBu2

Synthesis of complex (tBu2PSCOPtBu2)IrHCl 4a. Following the general procedure, treatment of 242 mg (0.36 mmol) of [Ir(COD)Cl]2 with 298 mg (0.72 mmol) of tBu2PSCOPtBu2 for 10 h at 120 °C in 10 mL toluene afforded 402 mg (0.63 mmol, 87%) of 4a as a dark red solid. 1H NMR (400 MHz, C6D6): δ 7.13 (t, 2JHH = 8.0 Hz, 1H), 6.69 (d, 2JHH = 4.0 Hz, 2H), 1.47-1.22 (m, 36H), 41.05 (t, 2JPH = 14.0 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6): δ 164.5 (dd, 2JPH = 22.7 Hz, 2JPP = 358.0 Hz), 107.4 (dd,

2

JPH = 22.7 Hz, 2JPP = 335.3 Hz).13C {1H} NMR (101 MHz, C6D6) δ

167.8, 153.8, 132.9, 125.3, 116.7, 107.9, 43.2, 42.7, 39.7, 39.5, 29.5, 27.7. Elemental analysis, calcd for C22H40ClIrOP2S (642.23): C, 41.14, H, 6.28; Found: C, 40.83, H, 6.26.

S tBu2P

Ir H Cl

O PCy2

Synthesis of Complexes (tBu2PSCOPCy2)IrHCl 4b. Following the general procedure, treatment of 188 mg (0.28 mmol) of [Ir(COD)Cl]2 with 262 mg (0.56 mmol) of

tBu2

PSCOPCy2 for 10 h at

120 °C in 10 mL toluene afforded 305 mg (0.44 mmol, 79%) of 4b as a dark red solid. 1H NMR (400 MHz, C6D6): δ 7.15-7.14 (m, 1H), 6.73-6.71 (m, 2H), 2.64 (t, 2JHH = 12.0 Hz, 1H), 2.292.20 (m, 1H), 1.81-1.72 (m, 7H), 1.59-1.49 (m, 6H), 1.43 (d, 2JHH = 16.0 Hz, 9 H), 1.38 (d, 2JHH = 12.0 Hz, 9 H), -39.50 (dd, 2JPH = 12.0 Hz, 2JPH = 10.0 Hz, 1H).

31

P{1H} NMR (162 MHz,

C6D6): δ 156.2 (dd, 2JPP = 346.7 Hz, 2JPH = 14.6 Hz), 110.5 (dd, 2JPP = 351.5 Hz, 2JPH = 11.3 Hz). 13

C{1H} NMR (101 MHz, C6D6) δ 167.1, 153.6, 131.6, 125.2, 116.8, 108.0, 43.5, 39.8 , 39.7,

39.6, 32.0, 29.5, 27.7, 27.5, 26.9, 26.6, 26.4, 26.3, 26.0, 23.1, 14.4. Elemental analysis, calcd for C26H44ClIrOP2S (694.31): C, 44.98, H, 6.39; Found: C, 45.02, H, 6.46.

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S tBu2P

Ir H Cl

O PiPr2

Synthesis of Complexes (tBu2PSCOPiPr2)IrHCl 4c. Following the general procedure, treatment of 262 mg (0.39 mmol) of [Ir(COD)Cl]2 and 301 mg (0.78 mmol) of

tBu2

PSCOPiPr2 for 10 h at

120 °C in 10 mL toluene afforded 381 mg (0.62 mmol, 80%) of 4c as a red solid. 1H NMR (300 MHz, CDCl3): δ 7.16-7.14 (m, 1H), 6.73-6.67 (m, 2H), 2.66-2.59 (m, 1H), 2.26-2.18 (m, 1H), 1.44 (d, 2JHH = 12.0 Hz, 9H), 1.38 (d, 2JHH = 12.0 Hz, 9H), 1.25-1.14 (m, 6H), 1.11-1.02 (m, 6H), -39.40 (dd, 2JPH = 12.0 Hz, 2JPH = 10.0 Hz, 1H). 31P {1H} NMR (162 MHz, C6D6): δ 163.0 (dd, 2

JPH = 16.2 Hz, 2JPP = 343.4 Hz), 110.7 (dd, 2JPH = 11.3 Hz, 2JPP = 348.3 Hz).

13

C{1H} NMR

(101 MHz, C6D6): δ 166.9, 153.3, 131.1, 125.2, 116.9, 108.0, 43.5, 39.6, 29.8, 29.5, 28.9, 17.5, 17.3, 16.5. Elemental analysis, calcd for C20H36ClIrOP2S (614.18): C, 39.11, H, 5.91; Found: C, 39.15, H, 5.87.

Synthesis of Complexes (tBu2PSCOPEt2)IrHCl 4d. A Schlenk flask with a Teflon plug was charged with 235 mg (0.35 mmol) of [Ir(COD)Cl]2, 301 mg (0.84 mmol) of tBu2PSCOPEt2, and 10 mL of toluene. The solution was degassed by three pump-freeze-thaw cycles and then flask was charged with an atmosphere of H2. The flask was sealed and the solution stirred for 4 h at 120 °C. The volatiles were then removed under high vacuum. The residue was dissolved in pentane. Recrystallization of the crude product afforded 336 mg (0.57 mmol, 82%) of pure product 4d as dark red solid. 1H NMR (400 MHz, CDCl3) δ 7.02 (d, 2JHH = 8.0 Hz, 1H), 6.76 (t, 2JHH = 8.0 Hz, 1H), 6.50 (d, 2JHH = 8.0 Hz, 1H), 2.48-2.43 (m, 1H), 2.36-2.28 (m, 1H), 2.27-2.18 (m, 2H), 1.47 (d, 2JHH = 4.0 Hz, 9H), 1.50 (d, 2JHH = 4.0 Hz, 9H), 1.26-1.21 (m, 6H), -39.59 (dd, 2JPH = 14.0 Hz, 2

JPH = 10.0 Hz, 1H). 31P{1H} NMR (162 MHz, C6D6) δ 150.7 (d, 2JPP = 380.7 Hz), 107.6 (d, 2JPP

= 380.7 Hz). 13C{1H} NMR (75 MHz, CDCl3) δ 166.4, 153.4, 131.2, 125.3, 116.9, 108.2, 43.9, 37.5, 29.4, 25.3, 25.1, 24.3, 24.0. Elemental analysis, calcd for C18H32ClIrOP2S (586.13): C, 36.88, H, 5.50; Found: C, 36.87, H, 5.75.

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S tBu2P

Ir CO

O PtBu2

Synthesis of (tBu2PSCOPtBu2)Ir(CO) 6. 327 mg (0.51 mmol) of 4a and 54 mg (0.56 mmol) of NaOtBu were dissolved in nitrogen-free benzene in a rubber septum capped Schlenk flask while purging the flask with hydrogen. The solution was stirred for 4 h at the room temperature under an atmosphere of hydrogen. The color of the solution turned to pale orange. Then the solution was purged with CO. The solution was stirred overnight and the color of the solution turned to yellow slowly. Then the solution was cannula transferred and filtered through a pad of Celite. The volatiles were evaporated under high vacuum to give 274 mg (0.43 mmol, 85%) of the product 4a as yellow solid. 1H NMR (400 MHz, CDCl3) δ 7.05 (d, 2JHH = 8.0 Hz, 1H), 6.82 (t, 2

JHH = 8.0 Hz, 1H), 6.66 (d, 2JHH = 8.0 Hz, 1H), 1.50 (d, 2JHH = 16.0 Hz, 18H), 1.38 (d, 2JHH = 12.0

Hz, 18H). 31P{1H} NMR (162 MHz, CDCl3) δ 188.5 (d, 2JPP = 304.6 Hz), 125.8 (d, 2JPP = 304.6 Hz). 13C{1H} NMR (101 MHz, CDCl3) δ 196.5, 169.2, 167.6, 158.6, 127.5, 114.3, 106.6, 41.4, 41.2, 29.7, 28.5. IR (pentane, cm-1): 1946.4 (νCO). Elemental analysis, calcd for C23H39Cl IrO2P2S (633.78): C, 43.59, H, 6.20; Found: C, 43.36; H, 6.24. 3. Results and discussion Synthesis and Characterization of tBu2PSCOPR2 ligands and (tBu2PSCOPR2)IrHCl Complexes. The synthesis of phosphinothious/phosphinite PSCOP ligands and (PSCOP)IrHCl complexes (4a-d) is outlined in Scheme 1. The ligands were conveniently prepared from the readily available m-mercaptophenol. Deprotonation of m-mercaptophenol with 2.1 equiv of NaH, followed by diphosphorylation with di-tert-butylchlorophosphine afforded ligand tBu2PSCOPtBu2 in 73% yield. Ligands tBu2PSCOPR2 (R = Cy, iPr, Et) bearing different alkyl substituents on two P atoms were synthesized in two sequential steps. Using triethylamine as the base, monophosphorylation of m-mercaptophenol with di-tert-butylchlorophosphine generated the phosphinothious intermediate, which then underwent the second phosphorylation with the corresponding dialkylchlorophosphine using NaH as the base to form ligands tBu2PSCOPR2 in 6574% yield.

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Iridium complexes 4a-c were obtained in 79-87% yields through metalation of the PSCOP ligands with [Ir(COD)Cl]2 in toluene under reflux. Complex 4d was prepared in 82% yield by reaction of [Ir(COD)Cl]2 with tBu2PSCOPEt2 in the presence of an atmosphere of H2. Complexes 4a-c are not very sensitive to air and moisture, thus can be purified by washing with alkane solvents under air. In contrast, complex 4d is moisture-sensitive, and was purified through recrystallization in a glovebox. Scheme 1. Synthesis of PSCOP Ligands and the Corresponding Iridium Complexes 4a-d. 1) 2.1 equiv NaH THF, RT, 1 h

HS

S O OH 2) 2.1 equiv tBu PCl 2 PtBu2 tBu2P THF, RT, 1 h tBu2PSCOPtBu2, 73% 1) 1.4 equiv Et3N THF, 70 °C, 1 h 2) 1.1 equiv tBu2PCl THF, RT, 1 h 1) 1.2 equiv NaH THF, RT, 1 h

S tBu2P

OH

S tBu2P

O PR2

R = tBu, Cy, iPr, Et

S tBu2P

O PR2

2) 1 equiv R2PCl THF, RT, 1 h R = Cy, tBu2PSCOPCy2, 74% R = iPr, tBu2PSCOPiPr2, 65% R = Et, tBu2PSCOPEt2 70%

0.5 equiv [Ir(COD)Cl]2 toluene refluxing, 4-10 h

S tBu2P

O PR2

Ir H Cl R = tBu, 4a, 87% R = Cy, 4b, 79% R = iPr, 4c, 80% R = Et, 4d, 82%

All complexes 4a-d were characterized by NMR spectroscopy and elemental analysis. The 31

P{1H} NMR spectra of complexes 4a-d exhibits an AB pattern of the two non-equivalent P

nuclei. In the 1H NMR spectra, the characteristic hydridic IrH resonances appear as a triplet in the range of -35.50 (4b) and -41.05 (4a), similar to that expected for a five-coordinate Ir(III) hydrido chloride species with the hydride trans to a vacant coordination site. In addition, complexes 4a, 4b, and 4d were characterized by single-crystal X-ray diffraction (Figure 1). Note

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that the hydrides in the crystal structures of 4a and 4d could not be located. The selected bond distances and angles for complexes 4a, 4b, and 4d are listed in Table 1. Complexes 4a and 4b adopt a square pyramidal geometry with the apical site occupied by the hydride. Notably, the solid state structure of the least sterically hindered complex 4d adopts a chloride-bridged dinuclear structure. However, we consider complex 4d as having a mononuclear structure in solution because it has only one pair of AB pattern resonance in the 31P NMR spectrum and one hydride signal at -39.62 ppm in the 1H NMR spectrum, consistent with a pentacoordinate mononuclear Ir(III) species[46]. Both Ir atoms in the X-ray structure of dimeric 4d appear to have unoccupied coordination sites where in fact the hydrides are presumably located; these sites are not positioned where the hydrides could be bridging. In particular the “vacant” sixth coordination site of Ir2 in 4d is trans to the bridging chloride.

Figure 1. ORTEP plot of complexes 4a, 4b, 4d, and 2 with thermal ellipsoids shown at 50% probability. The hydrides in 4a and 4d could not be located.

12

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Steric and Electronic Properties of PSCOP Iridium Complexes. The only structural difference between (tBu2PSCOPtBu2)IrHCl (4a), (tBu2POCOPtBu2)IrHCl (2), and (tBu2PCPtBu2)IrHCl (1) are the linkers between aryl backbone and the P atoms. Thus, it is of interest to assess the influence of the linkers on the steric properties of the iridium pincer complexes by comparing the molecular structures of these isostructural complexes. The single crystal structure of 1 has been previously reported[16]; for comparison the structure of complex 2 was determined crystallographically in this study (Figure 1). Table 1. Selected Bond Distances (Å) and Angles (deg) for complexes 4a, 4c, 4d, and 2. 4a

4c

4d

2

Ir1-C17 2.019(3),

Ir1-C1 2.018(4),

Ir1-C1 2.022(7), Ir1-P1 2.2573(19)

Ir1-C10 2.011(5),

Ir1-P1 2.2986(8),

Ir1-P1 2.3102(12),

Ir1-P2 2.3224(18), Ir1-Cl1, 2.4066(19),

Ir1-P1 2.2971(12),

Ir1-P2 2.2980(8),

Ir1-P2 2.2563(13),

P2-S1, 2106(3), P1-O1, 1.643(6),

Ir1-P2 2.2928(12),

Ir1-Cl1 2.4149(8),

Ir1-Cl1 2.3970(12),

Ir2-C19 2.046(6), Ir2-P3, 2.2588(17),

Ir1-Cl1 2.4041(12),

C18-S1 1.772(7),

C2-S1 1.763(5),

Ir2-P4, 2.3321(17), Ir2-Cl2 2.4562(17),

C11-O9 1.399(5),

C22-O1 1.396(9),

C6-O1 1.408(5),

Ir2-Cl1 2.5980(19), P4-S2 2.107(2),

C15-O16 1.394(5),

P1-S1 2.123(3),

P1-S1 2.1085(18),

P3-O2 1.637(5)

P1-O9 1.662(3),

P2-O1 1.670(7)

P2-O1 1.638(4)

P1-Ir1-P2, 168.32(3),

P1-Ir1-P2 169.03(4),

P1-Ir1-P2 169.10(7), P1-Ir1-C1 81.2(2),

P1-Ir1-P2 160.06(4),

P1-Ir1-C17 84.50(9),

P1-Ir1-C1 87.78(13),

P2-Ir1-C1 88.2(2), C1-Ir1-Cl1 175.4(2),

P1-Ir1-C10 80.13(14),

P2-Ir1-C17 83.83(9),

P2-Ir1-C1 81.40(13)

C2-S1-P2, 100.7(3), C6-O1-P1 115.4(5),

P2-Ir1-C10 80.01(14),

C17-Ir1-Cl1

C1-Ir1-Cl1

S1-P2-Ir1, 104.45(9), O1-P1-Ir1 104.7(2);

C10-Ir1-Cl1

172.97(9),

169.94(12),

P3-Ir2-P4 162.13(6), P3-Ir2-C19

179.11(14),

C17-C18-S1

C2-S1-P1 99.78(16),

81.14(18),

C11-O9-P1 115.5(3),

118.9(4),

C6-O1-P2 115.0(3)

P4-Ir2-C19 85.67(18), C19-Ir1-Cl2

C15-O16-P2 115.5(3),

C17-C22-O1

S1-P1-Ir1 105.08(6),

176.52(18),

O9-P1-Ir1 104.40(12),

120.9(6),

O1-P2-Ir1 104.76(13)

C24-S2-P4 98.7(2), C20-O2-P3 116.3(4),

O16-P2-Ir1 104.45(12)

P2-O16 1.662(3)

S1-P1-Ir1 106.46(7),

S2-P4-Ir2 105.10(9), O2-P3-Ir2 104.5(2),

O1-P2-Ir1 103.2(3)

Ir1-Cl1-Ir2 153.93(10)

As shown in Table 1, complex 4a with the hybrid PSCOP ligand has a P–Ir–P angle of 168.32(3)˚, which is significantly greater than that for the POCOP analogue 2 (160.06(4)˚). The S-linker in PSCOP with a covalent radius of 1.05 Å is much larger than the O-linker in POCOP

13

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with a covalent radius of 0.66 Å[47]. The longer S–C (1.772(7) Å) and S–P (2.123(3) Å) bonds in 4a compared to O–C (1.39 Å) and O–P (1.66) Å) bonds in 2 (Table 1), combined with the larger size of the S-linker versus O, results in the phosphine substituents being “pushed forward” in 4a. Consequently, the gap between the trans-di-tert-butyl phosphino groups of 4a is smaller than that of 2. The P–Ir–P angle in complex 4a (168.32(3)˚) is even larger than that in the PCP complex 1 (164.27(4)˚).[16] The data indicate that for those iridium complexes containing the same phosphino-substituents, the degree of crowding follows the trend, (PSCOP)Ir > (PCP)Ir > (POCOP)Ir.

S tBu2P

Ir H Cl 4a

NaOtBu O CO PtBu2 toluene

S tBu2P

Ir CO 6, 88%

O (1) PtBu2

One possible method for evaluation of the electronic property of the pincer Ir complexes is the

νCO stretching frequencies of the corresponding carbonyl complexes. Complex 6, (tBu2PSCOPtBu2)Ir(CO), was obtained in 88% yield by reaction of 4a with NaOtBu in the presence of CO (eq 1). The νCO stretching frequency of complexes 6 in a pentane solution (1954 cm-1) is slightly blue-shifted from that of (tBu2POCOPtBu2)Ir(CO) (1949 cm-1),[18] suggesting that the (PSCOP)Ir fragment is slightly more electron-deficient than the (POCOP)Ir fragment. However, it has been shown that electrostatic factors also, independently, affect the νCO stretching frequency which is therefore not a definitive measure of electron density at the Ir center[14]. Catalytic transfer-dehydrogenation of COA using complexes 4a-d. The PSCOP iridium complexes were first tested as precatalysts for transfer-dehydrogenation of COA with tertbutylethylene (TBE). COA-to-TBE hydrogen transfer is considered a “benchmark” reaction, and is thermodynamically favorable by ~6 kcal/mol. With the PCP iridium complex 1 as the catalyst, TONs up to 1000 can be achieved at 200° C, but inhibition of catalysis by TBE requires portionwise addition of the hydrogen acceptor to the reaction mixture[9]. The POCOP iridium system is less subject to inhibition by TBE; with a COA:TBE:catalyst ratio of 3300:3300:1, the best POCOP catalyst gives 2200 TOs after 40 h at 200 °C[19]. The iPr-substituted (PSCOP)Ir

14

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complex (3) appears to be more active than complexes 1 and 2 for COA/TBE transferdehydrogenation; TONs up to 5900 have been obtained after 15 h at 200 °C with a COA:TBE:catalyst ratio of 6000:6000:1[44]. However, to assess the effect of the linkers on the catalytic activity, it is more useful to compare complex 4a to 1 and 2 because these all contain tBu substituents on the P atoms. Table 2. TONs for the Transfer-Dehydrogenation of COA/TBE Catalyzed by Complexes 4a-d in the Presence of NaOtBu.a 1.3 mM [Ir] 2.0 mM NaOtBu

+

a

+

200° C

TBA

COA 3.9 M

TBE 3.9 M

COE

time

4a

4b

4c

4d

10 min

69

56

607

472

30 min

100

109

679

503

1h

101

139

694

505

4h

102

164

707

509

8h

106

164

732

519

Average of three runs, based on conversion of TBE determined by GC.

Upon activation with NaOtBu, all PSCOP iridium complexes 4a-d are active for COA/TBE transfer-dehydrogenation, but their catalytic activities vary substantially. A system containing 1.3 mM iridium precatalyst, 2.0 mM NaOtBu, 3.9 M COA, and 3.9 M TBE (3000 equiv relative to Ir), was heated at 200 °C under argon in a sealed vessel. The results are summarized in Table 2. The most sterically hindered complex, (tBu2PSCOPtBu2)IrHCl (4a), bearing tBu groups on both P atoms exhibits low activity. After 8 h, catalysis by 4a converted only 106 equiv of TBE to TBA. This result, together with the data obtained in COA/TBE transfer-dehydrogenation reactions using 1 and 2 as the catalyst precursors (vide supra)[9, 19, 44], indicate that the sterically more congested PSCOP complex is much less efficient than the tBu-substituted POCOP and PCP analogues.

15

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Reducing the size of the phosphino-substituents has an advantageous effect on the catalytic activity. Complex (tBu2PSCOPCy2)IrHCl 4b with cyclohexyl substituents on one P atom shows very slightly enhanced activity relative to 4a, giving 164 TOs after 8 h. Complex (tBu2PSCOPiPr2)IrHCl 4c provides a more active catalyst than 4a and 4b; dehydrogenation with 4c gave 732 TOs after 8 h (entry 3). However, the catalytic activity does not increase by using the least hindered analogue 4d, (tBu2PSCOPEt2)IrHCl. The run with 4d gave 519 TOs after 8 h. The solid-state structure of 4d reveals that the iridium species with the tBu2PSCOPEt2 ligand tends to form a dinuclear cluster (vide supra). Thus we attribute the reduced catalytic activity of complex 4d relative to 4c to facile catalytic deactivation via formation of clusters. Notably a similar decomposition pathway has been reported for a sterically undemanding complex (tBuMePCPtBuMe)IrH4[15]. It should also be noted that more than 95% of overall TONs occur within the first 4 hours for catalysis with all the PSCOP complexes investigated. To summarize the results in this section, the catalytic activity of PSCOP complexes in COA/TBE transferdehydrogenation follows the trend: (iPr2PSCOPiPr2)IrHCl (3) > (tBu2PSCOPiPr2)IrHCl (4c) > (tBu2PSCOPEt2)IrHCl (4d) > (tBu2PSCOPCy2)IrHCl (4b) > (tBu2PSCOPtBu2)IrHCl (4a). Catalytic transfer-dehydrogenation of n-octane using complexes 4a-d. Next, the PSCOP iridium catalysts were tested for transfer-dehydrogenation of linear alkane. A system containing the catalyst precursor (1.0 mM), NaOtBu (1.5 mM), TBE (0.5 M, 500 equiv relative to Ir) in noctane was heated at 200° C under argon. The results, as summarized in Table 3, are compared to those using the PCP complex, (tBu2PCPtBu2)IrH2 (1), POCOP complex, (tBu2POCOPtBu2)IrHCl (2), and the iPr-substituted PSCOP complex (iPr2PSCOPiPr2)IrHCl (3). Consistent with the catalytic activity observed in COA/TBE transfer-dehydrogenation reactions, the more sterically demanding complexes (tBu2PSCOPtBu2)IrHCl (4a) and (tBu2PSCOPCy2)IrHCl (4b) are less effective than (tBu2PSCOPiPr2)IrHCl (4c) and (tBu2PSCOPEt2)IrHCl (4d) for dehydrogenation of n-octane. Runs with 4a and 4b gave only 32 and 40 TOs after 60 min at 200° C, respectively (entries 1 and 2). A comparison between 4a and the closely related complexes 1 (60 min, 135 TOs) and 2 (60 min, 143 TOs), also bearing tBu substituents, reveals that the (PSCOP)Ir complex is much less efficient than the POCOP and PCP systems (entries 5 and 6)[44]. The reaction with (tBu2PSCOPiPr2)IrHCl (4c) gave 277 TOs after 60 min (entry 3). To our satisfaction, the least crowded complex (tBu2PSCOPEt2)IrHCl 4d is markedly active, giving an initial turnover frequency of 21 min-1 with a total TO number of 475 after 60 min (entry 4). As a comparison, the activity of 16

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

4d is slightly lower than the highly active precatalyst (iPr2PSCOPiPr2)IrHCl (3) (entry 7, 60 min, 500 TOs), but much higher than PSCOP complexes 4a-c. For n-octane/TBE transferdehydrogenation reactions, the catalytic activity of the new PSCOP complexes is correlated to the steric properties of the pincer ligands and follows the trend: (iPr2PSCOPiPr2)IrHCl (3) > (tBu2PSCOPEt2)IrHCl (4d) > (tBu2PSCOPiPr2)IrHCl (4c) > (tBu2PSCOPCy2)IrHCl (4b) > (tBu2PSCOPtBu2)IrHCl (4a). Table 3. Transfer-dehydrogenation of n-octane/TBE Catalyzed by complexes 4a-d at 200 °C.a

n-octane + TBE

1.0 mM [Ir] 1.5 mM NaOtBu 200 °C

0.5 M

Entry

Cat.

octenes

TONsa

1-oct

(min) 5 10 30 60

Time

+

TBA

%b

trans-2

cis-2

6 11 25 32

[mM] 5 6 10 11

83 55 40 34

[mM] 1 5 10 14

[mM] 0 0 4 6

1

4a

2

4b

5 10 30 60

7 10 35 40

5 6 14 15

71 60 40 38

1 3 9 12

1 1 6 8

3

4c

5 10 30 60

65 89 213 277

21 22 22 22

32 24 10 8

23 34 79 98

13 19 42 50

4

4d

5 10 30 60

106 212 448 475

19 20 18 12

18 9 4 3

39 78 149 116

23 42 71 43

5c,d

1

5 10 30 60

69 92 120 135

35 39 37 27

51 42 31 20

20 32 48 64

10 16 23 27

6d

2

5 10 30 60

19 26 83 143

5 5 6 7

26 19 7 5

9 12 31 47

4 5 14 21

17

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65

7d

3

5 10 30 60

114 192 495 500

38 37 13 10

33 19 3 2

44 77 137 110

20 37 65 50

a

Average of three runs, based on conversion of TBE determined by GC. b The fraction of 1octene relative to the total octenes. c No NaOtBu was added. d Data obtained from Ref [12]. The selective formation of terminal olefin via alkane dehydrogenation is a goal of great importance. Transfer-dehydrogenation of n-octane using bulky complexes 4a and 4b gave high regioselectivity for 1-octene formation at the early stages of the reactions. For example, after 5 min at 200° C, the terminal olefin (5 mM) obtained in the run with 4a constitutes 83% of total octenes (entry 1). The combined 1-octene (11 mM) and 2-octenes (20 mM) observed in the reaction with 4a after 60 min comprises >95% of total octenes, indicating catalytic isomerization of 2-octenes to 3- or 4-octenes is much slower than isomerization of 1-octene to 2octenes. The more active and less hindered PSCOP precatalysts 4c and 4d afforded low fractions of 1-octene (entries 3 and 4). For instance, the reactions with 4d gave 18% 1-octene (19 mM) after 106 turnovers in 5 min, and 3% of 1-octene (12 mM) after 475 turnovers in 60 min (entry 4). A substantial amount of 3- and 4-octenes was formed in this case, implying very rapid olefin isomerization concomitant with transfer-dehydrogenation. 4. Conclusions Using readily available m-mercaptophenol as the building block, we have prepared a series of phosphinothious-phosphinite (PSCOP)Ir pincer complexes with varying steric properties. The Sfor-O substitution of the pincer ligand leads to a sterically more hindered, and electronically more deficient iridium center in comparison with the parent (POCOP)Ir system. We also demonstrated that replacement of tBu groups with sterically less congested units has a substantial favorable impact on catalytic transfer-dehydrogenation activity.

ASSOCIATED CONTENT Author information Corresponding Author

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*E-mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. Acknowledgements We gratefully acknowledge the financial support from the National Basic Research Program of China (2015CB856600), the National Natural Science Foundation of China (21432011, 21422209), and the National Science Foundation as part of the Center for Enabling New Technologies through Catalysis (CENTC), Phase II Renewal, CHE-1205189. Appendix A. Supplementary data CCDC 1444862-1444865 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected].

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New (tBu2PSCOPR2)IrHCl iridium complexes ligated by hybrid phosphinothious-phosphinite PSCOP ligands have been synthesized and characterized. The steric properties of the pincer ligands prove to have a marked impact on catalytic activities of these complexes in transfer-dehydrogenation of cyclic and linear alkanes.

New (tBu2PSCOPR2)IrHCl iridium complexes ligated by hybrid phosphinothious-phosphinite PSCOP ligands have been synthesized and characterized. The steric properties of the pincer ligands prove to have a marked impact on catalytic activities of these complexes in transfer-dehydrogenation of cyclic and linear alkanes.