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Polyethylene glycol-functionalized N,P-ligands enhance catalytic properties of cobalt complexes for the hydrosilylation reaction
⁎Corresponding authors. jiayun1980@hznu.edu.cn (Jiayun Li), jjpeng@hznu.edu.cn (Jiajian Peng)
-
Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A sequence of polyethylene glycol phosphine ligands with N donor were synthesized and coordinated with Co(BF4)2 to catalyze the hydrosilylation of olefins and silanes. When the amount of catalyst is 0.5 mol% and the temperature is 90 °C, the reaction time is 5 h, the catalytic effect is the best. The catalytic cycle performance was also studied, and it was found that after 8 times, its yield could still reach>90 %.
Keywords
Polyethylene glycol phosphine ligands with N donor
Cobalt complexes
Hydrosilylation
1 Introduction
Silicon products are widely used in transportation, medicine, building materials, light industry and other industries, and have application into all aspects of life. (Nakajima and Shimada, 2015; Obligacion and Chirik, 2018) Hydrosilylation is the most common process used to prepare organosilicon products, and transition metal catalysts are often utilized to promote this process. (Sun and Deng, 2016) At present, platinum complexes have been successfully used on an industrial scale. (Lewis and Stein., J., Gao, Y., Colborn, R. E., Hutchins, G., , 1997) However, they have some weaknesses, such as limited reserves, poor recyclability toxicity and high cost,. Recently, researchers have shifted their focus toward finding economical, environmentally friendly, and recyclable non-precious metal catalysts. In 1965, Chalk and Harrod discovered that the hydrosilylation of terminal olefins could be carried out in the presence of Co2(CO)8 forming the anti-Markovnikov product. (Chalk and Harrod, 1965) Subsequently, It has been reported that the hydrosilylation of olefins could be catalyzed using many cobalt salts. (Cheng et al., 2019; Gao et al., 2018; Liu and Deng, 2017; Wang et al., 2017; Lee, 2017; Mo et al., 2013; Cheng et al., 2017) In 2016, Huang et al. reported that the hydrosilylation could be catalyzed using a sequence of [PNN]-pincer metal-complexes. Furthermore, the anti-Markovnikov addition products were synthesized using iron-complexes, while the Markovnikov addition products were obtained using cobalt-complexes. (Du et al., 2016) In 2020, Xie et al. reported that the Markovnikov addition products of the hydrosilylation of aryl olefins could be obtained using [CNC]-pincer cobalt hydride complexes with high selectivity, while the anti-Markovnikov products were obtained when alkyl olefins were used as the substrate. (Xie et al., 2020) In 2020, Xie and co-workers reported that the hydrosilylation could be reacted in the presence of Co(PMe3)3Cl under solvent-free conditions. The Markovnikov products were synthesized during the hydrosilylation of aryl olefins, while the anti-Markovnikov products were obtained when using alkyl olefins. (Xie et al., 2021; Schuster et al., 2016; Raya et al., 2017; Gorczyński et al., 2016).
This study has the following advantages when compared with the results previously reported in the literature: (1) low catalyst loading, (2) mild reaction conditions and short reaction time, (3) high yield and selectivity for the β-addition product, and (4) good recyclability with the β-addition product obtained in > 90% yield upon repeated use.
2 Experimantal section
2.1 Main instruments and reagents
Ultra Shield type nuclear magnetic resonance spectrometer (400 MHz, Bruker, Germany); 5977B gas-mass spectrometer (Agilent); 700 Fourier Infrared spectrometer (Nicolet Complay).
Methoxypolyethylene glycol 350 (mPEG350), methoxypolyethylene glycol 500 (mPEG500), methoxypolyethylene glycol 750 (mPEG750), methoxypolyethylene glycol 2000 (mPEG2000), pyridine (99.9%), methanesulfonyl chloride (99.9%), N-butylamine (98%), tert-butylamine (98%), N-propylamine (99%), octylamine (99%), benzylamine (99%), aniline (99.5%), 2-aminopyridine (98%), 2-aminomethylpyridine (99%), 4-methoxyaniline (99%), di-tert-butyl phosphorus chloride (98%), dicyclohexyl phosphorus chloride(98%), diisopropyl phosphine (98%), diisobutyl phosphine (97%), diphenylphosphine (99%).
2.2 Synthesis of polyethylene glycol functionlized N, P ligands
The experimental procedure using mPEG350 as an example is shown Scheme 1. First, under an argon atmosphere, mPEG350 (0.1 mol), 40 mL of distilled toluene, and pyridine (0.1 mol) were added to a 250 mL three-necked flask. Next, methanesulfonyl chloride (0.1 mol) was added dropwise to the mixture heated at 88 °C and kept to react for 30 h. After cooling to room temperature, concentrated hydrochloric acid solution (concentrated hydrochloric acid (8 mL) and water (16 mL)) was added to the resulting mixture. A solid precipitate appears, which disappears upon stirring. The aqueous layer extracted with toluene and the mixture was separated. The combined organic layers dried with Na2SO4. The toluene was removed via distillation under reduced pressure to give mPEG350-OMs. The mPEG350-OMs (14.0 mmol, 6.00 g) was added dropwise to n-butylamine (20.0 mmol, 1.46 g) and reacted for 5 h at 50 °C. The excess n-butylamine was removed to give mPEG350-NHC4H9. Argon gas was bubbled into 30 mL of distilled toluene for 10 min. Under an argon atmosphere, 4.26 g (10 mmol) of mPEG350-NHC4H9, 30 mL of degassed distilled toluene, and 0.45 g (15.0 mmol, 50% excess) of 37% formaldehyde solution was added to a dried 100 mL two-necked flask and heated at 60–63 °C. After the temperature increased, diphenylphosphine (10.0 mmol, 1.86 g) was added to the reaction mixture and stirred for 4 h. After cooling to room temperature, the resulting mixture was separated and the target product was obtained.
Synthesis of Polyethylene glycol-functionalized N,P-ligands.
L1a: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.51 – 7.35 (m, Ph, 10H), 3.65 [d, J = 4.6 Hz, (OCH2CH2)n, 50H], 3.38 (s, O-CH3, 3H), 2.37 (d, J = 10.2 Hz,3NCH2, 6H), 1.28 (d, J = 14.7 Hz,CH2, 2H), 1.23 – 1.10 (m, CH2, 2H), 0.86 – 0.77 (m, CH2Me, 3H).13C NMR (101 MHz, CDCl3) δ (ppm): 138.32, 133.32 – 132.89, 129.21 – 128.16, 71.91, 71.33, 70.70 – 70.42, 59.03, 28.71 – 28.46, 21.48, 14.02. 31P NMR (162 MHz, CDCl3) δ (ppm): −28.72. IR (KBr disc, cm−1): 2869, 1434, 1298, 1098, 1026, 742, 696. Yield: 85 %.
L2a: 1H NMR (400 MHz, CDCl3) δ(ppm): 7.75 – 7.70 (m, Ph, 6H), 7.51 (t, J = 7.6, 3.3 Hz, Ph, 4H), 3.59 [d, J = 4.8 Hz, (OCH2CH2)n, 92H], 3.31 (s, O-CH3, 3H), 2.45 (t, J = 7.5 Hz, NCH2, 2H), 1.94 (d, J = 5.8 Hz, CH2, 2H), 0.85 (t, J = 29.7, 6.9 Hz, CH2, 2H), 0.65 (t, J = 7.3 Hz, CH3, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 133.31 – 132.88, 131.55 – 131.16, 129.00 – 128.22, 71.91, 71.33, 70.85 – 70.34, 59.03, 29.51 – 28.16, 20.18, 14.42 – 13.69. 31P NMR (162 MHz, CDCl3) δ (ppm): −28.75. IR (KBr disc, cm−1): 2868, 1435, 1248, 1096, 1026, 732, 693. Yield: 87 %.
L3a: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.73 (dd, J = 10.8, 3.2 Hz, Ph, 6H), 7.43 (d, J = 2.1 Hz, Ph, 4H), 3.58 [d, J = 5.2 Hz, (OCH2CH2)n, 166H], 3.31 (s, O-CH3, 3H), 2.19 – 2.11 (m, CH2, 2H), 1.95 (s, CH2, 2H), 0.90 – 0.71 (m, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 135.57, 133.06, 128.97 – 128.33, 71.90, 71.32, 71.06 – 70.16, 62.64, 59.02, 42.73. 31P NMR (162 MHz, CDCl3) δ (ppm): −19.53. IR (KBr disc, cm−1): 2867, 1435, 1249, 1094, 1026, 743, 697. Yield: 91.0 %.
L4a: 1H NMR (400 MHz, CDCl3) δ(ppm): 7.74 – 7.70 (m, Ph, 6H), 7.44 (dd, J = 7.6, 2.8 Hz, Ph, 4H), 3.58 [d, J = 5.7 Hz, (OCH2CH2)n, 646H], 3.31 (s, O-CH3, 3H), 2.29 (s, NCH2, 2H), 2.20 – 2.09 (m, CH2, 2H), 1.94 (d, J = 6.4 Hz, CH2, 2H), 1.57 (s, CH2, 2H), 0.81 (t, J = 6.8 Hz, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ(ppm): 133.07, 131.37, 128.67, 71.89 (s), 71.31 (s), 70.57, 63.17 – 61.95, 59.00, 30.01 – 29.06, 22.41 – 21.68, 14.54 – 13.58. 31P NMR (162 MHz, CDCl3) δ(ppm): −28.73. IR (KBr disc, cm−1): 2868, 1435, 1249, 1094, 1026, 743, 697. Yield: 90 %.
L1b: 1H NMR (400 MHz, CDCl3) δ(ppm): 7.52 – 7.35 (m, Ph, 6H), 7.21 (ddd, J = 15.5, 11.9, 6.7 Hz, Ph, 4H), 3.72 – 3.61 [m, (OCH2CH2)n, 36H], 3.39 (s, O-CH3, 3H), 2.22 (d, J = 15.3 Hz, 3NCH2, 6H), 1.27 (s, CH2, 2H), 0.96 – 0.69 (m, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ(ppm): 137.87, 133.27 – 132.97, 129.12 – 128.13, 71.93, 71.35, 70.58, 62.63, 59.05, 21.48. 31P NMR (162 MHz, CDCl3) δ(ppm): −19.69. IR (KBr disc, cm−1): 2870, 1436, 1298, 1097, 1026, 744, 696. Yield: 89 %.
L1c: 1H NMR (400 MHz, CDCl3) δ(ppm): 7.76 – 7.71 (m, Ph, 6H), 7.53 (td, J = 7.6, 3.4 Hz, Ph, 4H), 3.59 [d, J = 3.3 Hz, (OCH2CH2)n, 36H], 3.31 (s, O-CH3, 3H), 2.29 (s, 2NCH2, 4H), 1.25 (d, J = 51.9 Hz, 3CH2Me, 9H). 13C NMR (101 MHz, CDCl3) δ(ppm): 137.84, 133.36 – 132.91, 129.09 – 128.13, 71.92, 71.34, 70.57, 59.03, 54.89 – 54.73, 54.33 – 54.18, 21.47. 31P NMR (162 MHz, CDCl3) δ (ppm): −19.61. IR (KBr disc, cm−1): 2870, 1436, 1362, 1298, 1190, 1097, 1025, 743, 696. Yield: 81 %.
L1d: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.76 – 7.55 (m, Ph, 6H), 7.33 – 7.17 (m, Ph, 4H), 3.62 – 3.56 [m, (OCH2CH2)n, 36H], 3.32 (d, J = 6.0 Hz, O-CH3, 3H), 1.30 – 1.03 (m, 6CH2, 12H), 0.80 (s, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 135.11, 133.31 – 132.93, 128.63, 71.89, 71.31, 70.67 – 70.45, 59.00, 19.84, 13.30. 31P NMR (162 MHz, CDCl3) δ (ppm): --29.01. IR (KBr disc, cm−1): 2858, 1435, 1298, 1101, 1024, 742, 696. Yield: 76 %.
L1e: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.39 – 7.15 (m, Ph, 15H), 3.73 – 3.61 [m, (OCH2CH2)n, 28H], 3.41 (s, O-CH3, 3H), 2.38 (s, 2NCH2, 4H). 13C NMR (101 MHz, CDCl3) δ (ppm): 138.38, 137.93, 133.13, 128.96 – 128.03, 127.02, 71.96, 71.38, 70.61, 60.65, 59.07, 58.29, 57.04. 31P NMR (162 MHz, CDCl3) δ (ppm): −28.48. IR (KBr disc, cm−1): 2869, 1568, 1503, 1434, 1248, 1098, 1025, 849, 741, 696. Yield: 86 %.
L1f: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.74 – 7.69 (m, Ph, 4H), 7.59 (dd, J = 7.5, 1.7 Hz, Ph, 3H), 7.50 (td, J = 7.6, 3.3 Hz, Ph, 4H), 7.27 – 7.18 (m, Ph, 4H), 4.57 (s, NCH2, 2H), 3.62 – 3.56 [m, (OCH2CH2)n, 68H], 3.31 (s, O-CH3, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 135.15, 134.03, 133.18, 131.37, 130.31, 128.77, 71.92, 71.34, 70.57, 62.77, 59.05. 31P NMR (162 MHz, CDCl3) δ (ppm): −27.29. IR (KBr disc, cm−1): 2869, 1569, 1503, 1435, 1298, 1097, 1026, 851, 743, 697. Yield: 89 %.
L1g: 1H NMR (400 MHz, CDCl3) δ (ppm): 8.53 – 8.35 (m, N = CH–, 1H), 7.76 – 7.73 (m, N = CH–, 1H), 7.50 – 7.38 (m, Ph, 6H), 7.31(s,N = CH–, 1H), 7.25 (dd, J = 7.2, 1.8 Hz, N = CH–, 1H), 7.21 – 7.15 (m, Ph, 4H), 4.35 – 4.30 (m, NCH2, 2H), 3.59 (td, J = 5.8, 2.8 Hz, (OCH2CH2)n, 50H), 3.31 (s, O-CH3, 3H), 2.29 (s, NCH2, 2H). 13C NMR (101 MHz, CDCl3) δ (ppm): 137.84, 133.17, 131.43, 130.40, 128.60, 71.94, 71.36, 70.60, 59.07, 53.48. 31P NMR (162 MHz, CDCl3) δ (ppm): −27.86. IR (KBr disc, cm−1): 2869, 1588, 1568, 1434, 1298, 1099, 1025, 742, 696. Yield: 80 %.
L1h: 1H NMR (400 MHz, CDCl3) δ (ppm): 8.01 (d, J = 4.3 Hz, N = CH–, 1H), 7.53 (d, J = 7.5 Hz, N = CH–, 1H), 7.42 – 7.39 (m, Ph, 6H), 7.24 (d, J = 6.5 Hz, Ph, 4H), 6.79 (t, J = 6.2 Hz, N = CH–, 1H), 6.54 – 6.49 (m, N = CH–, 1H), 4.15–3.97(m,NCH2, 2H), 3.58 [dd, J = 5.3, 4.3 Hz, (OCH2CH2)n, 52H], 3.31 (s, O-CH3, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 146.76, 137.11, 132.93, 128.82, 113.24, 107.61, 71.94, 71.36, 70.60, 59.07, 53.49. 31P NMR (162 MHz, CDCl3) δ (ppm): –23.53. IR (KBr disc, cm−1): 2869, 1596, 1435, 1245, 1096, 1025, 740, 695. Yield: 76 %.
L1i: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.47 – 7.41 (m, Ph, 6H), 7.04 (t, J = 6.3 Hz, Ph, 4H), 6.68 (d, J = 2.2 Hz, N-Ph-O, 2H), 6.61 – 6.54 (m, N-Ph-O, 2H), 3.69 (d, J = 3.2 Hz, O-CH3, 3H), 3.60 – 3.57 [m, (OCH2CH2)n, 50H], 3.31(s,O-CH3, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 156.08, 137.60, 133.16, 128.56, 114.51, 71.94, 71.36, 70.60, 59.07, 55.57, 53.48. 31P NMR (162 MHz, CDCl3) δ (ppm): −27.55. IR (KBr disc, cm−1): 2869, 1509, 1440, 1430, 1243, 1099, 1035, 849, 742, 695. Yield: 83 %.
L1j: 1H NMR (400 MHz, CDCl3) δ (ppm): 3.59 (d, J = 3.4 Hz, (OCH2CH2)n, 50H), 3.31 (s, O-CH3, 3H), 1.48 – 1.42 (m, CH2, 2H), 1.34 (t, J = 9.3 Hz, 6CH3, 18H), 1.19 (d, J = 4.7 Hz, CH2, 2H), 1.16 (d, J = 2.8 Hz, CH2, 2H), 1.06 – 1.00 (m, CH3, 3H).13C NMR (101 MHz, CDCl3) δ (ppm): 71.91, 71.34, 70.57, 59.03, 42.72, 35.11, 34.56, 27.04, 26.55, 22.80. 31P NMR (162 MHz, CDCl3) δ (ppm): 59.89. IR (KBr disc, cm−1): 2869, 1461, 1350, 1243, 1102, 1025, 751, 663. Yield: 73 %.
L1k: 1H NMR (400 MHz, CDCl3) δ (ppm): 7.03 – 6.96 (m, P-Ph-O, 4H), 6.82 – 6.78 (m, P-Ph-O, 4H), 3.78 (dd, J = 11.6, 5.1 Hz, 2O-CH3, 6H), 3.69 – 3.66 (m, (OCH2CH2)n, 42H), 3.40 (d, J = 2.5 Hz,O-CH3, 3H), 1.89 (d, J = 42.2 Hz, CH2, 2H), 1.72 – 1.36 (m, CH2, 2H), 0.99 – 0.84 (m, CH3, 3H).13C NMR (101 MHz, CDCl3) δ (ppm): 152.93, 150.83, 133.26, 132.19, 116.07, 114.59, 71.90, 71.32, 70.54, 59.00, 55.73, 55.32, 29.81, 20.45, 14.23. 31P NMR (162 MHz, CDCl3) δ (ppm): 54.52 (s). IR (KBr disc, cm−1): 2869, 1511, 1456, 1435, 1246, 1098, 1029, 828, 734, 664. Yield: 75 %.
L1l: 1H NMR (400 MHz, CDCl3) δ (ppm): 3.59 (d, J = 3.6 Hz, (OCH2CH2)n, 42H), 3.31 (s, O-CH3, 3H), 1.16 (d, J = 16.8 Hz, 2Cy, 20H), 1.11 (d, J = 2.5 Hz, CH2, 2H), 1.05 (d, J = 2.7 Hz, CH2, 2H), 0.96 – 0.93 (m, CH2, 2H), 0.82 – 0.75 (m, CH3, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 71.86, 71.28, 70.51, 58.92, 43.00, 30.08, 26.81, 22.61, 19.66, 14.06. 31P NMR (162 MHz, CDCl3) δ (ppm): 29.30. IR (KBr disc, cm−1): 2922, 2869, 1456, 1257, 1088, 1025, 731, 662. Yield: 71 %.
L1m: 1H NMR (400 MHz, CDCl3) δ (ppm): 3.58 (q, J = 5.2 Hz, (OCH2CH2)n, 36H), 3.31 (s, O-CH3, 3H), 2.85 (d, J = 30.6 Hz, NCH2, 2H), 2.30 – 1.99 (m, CH2, 2H), 1.37 (dd, J = 15.4, 7.3 Hz, CH2, 2H), 1.17 (td, J = 14.7, 73 Hz, 2CHMe2, 12H), 0.85–0.75 (m, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 71.75, 71.18, 70.59, 58.81, 56.03, 42.62, 36.37, 31.28, 23.69, 15.37 13.96. 31P NMR (162 MHz, CDCl3) δ (ppm): 57.02 (s). IR (KBr disc, cm−1): 2869, 1097, 1025, 662. Yield: 74 %.
L1n: 1H NMR (400 MHz, CDCl3) δ (ppm): 3.64 – 3.51 (m, (OCH2CH2)n, 36H), 3.31 (d, J = 5.7 Hz, O-CH3, 3H), 2.86 (d, J = 30.7 Hz, NCH2, 2H), 2.14 – 2.01 (m, CH2, 2H), 1.71 – 1.53 (m,2CH2, 4H), 1.05 – 0.98 (m, 2CHMe2, 12H), 0.89 – 0.77 (m, CH2Me, 3H). 13C NMR (101 MHz, CDCl3) δ (ppm): 71.76, 71.19, 70.39, 58.83, 58.56, 42.63, 36.40, 35.01, 31.30, 30.66, 24.63, 23.13, 18.92, 13.80. 31P NMR (162 MHz, CDCl3) δ (ppm): −34.95. IR (KBr disc, cm−1): 2869, 1098, 1028, 660. Yield: 76 %.
2.3 Catalytic hydrosilylation of alkenes
The typical hydrosilylation reaction procedure was as follows. The requisite amounts of metal salt and ligand were placed in a 10 mL glass tube. Under an argon atmosphere, an appropriate amount of distilled tetrahydrofuran was added to the flask and the mixture stirred for 48 h at room temperature. The solvent was removed in vacuo and the resulting residue washed three times with anhydrous ether and concentrated in vacuo to obtain the metal/polyethylene glycol functionalized N, P ligand complexes. The olefin (1.0 mmol) and silane (1.0 mmol) were then added and the tube was sealed. The mixture was heated to the required temperature and the hydrosilylation reaction was allowed to proceed for 5 h under constant stirring. When the reaction was complete, the product phase was separated from the catalyst by decantation and the olefin conversion and the selectivity of adduct formation were determined and quantified by GC analysis on an Agilent 7890B apparatus (See Scheme 2).
Hydrosilylation.
The catalyst was recharged with fresh alkene and silane for the next catalytic run.
3 Results and discussion
3.1 Effects of different metal salts/L1a
The effects of complexes formed by different metal salts with ligand L1a and their catalytic performance in the hydrosilylation of diphenylsilane with 1-octene were researched, and the results were showed in Table 1. It can be seen that only the hydrogenation products and β-addition products were detected with no α-addition products formed in the reaction. The Ti and Zn complexes exhibited no catalytic activity (entry 6–7, Table 1). The Cr and Mn complexes exhibited low catalytic activity (entry 3–4, Table 1). The conversions observed for the Ni and Fe complexes were similar and the selectivities was close to 100% (entry 2 and 5, Table 1). The catalytic performance and selectivity of the cobalt complex were the highest with a conversion of > 95% and selectivity close to 100% (entry 1, Table 1). Reaction conditions:1-octene 1 mmol, Ph2SiH2 1.1 mmol, MCln 0.5 mol ％of olefin, MCln: L1a = 1:2, 90 °C, 5 h.
Entry
Catalyst
Conv.（％）(based on 1-octene)
Select（％）
β
α
Octane
1
CoCl2/L1a
96.3
99.7
/
0.3
2
FeCl2/L1a
92.6
99.7
/
0.3
3
CrCl3/L1a
61.8
99.6
/
0.4
4
MnCl2/L1a
87.3
99.8
/
0.2
5
NiCl2/L1a
90.3
99.7
/
0.3
6
ZnCl2/L1a
/
/
/
/
7
TiCl4/L1a
/
/
/
/
3.2 The effects of catalyst formed using L1a and different cobalt salts
A series of cobalt salt precursors were coordinated with ligand L1a and the effect of the different anions on the catalytic properties of their corresponding cobalt complexes during the hydrosilylation reaction of diphenylsilane with1-octene studied. The results were showed in Table 2. The selectivity toward the β-addition product was > 95% when the cobalt complexes bearing different anions were used to catalyze the hydrosilylation reaction. The conversions using CoBr2/L1a and Co(C10H7COO)2/L1a were low (62.0% and 58.3%, respectively) (entry 4–5, Table 2). The conversions using Co(CH3COO)2/L1a, CoCl2/L1a, and Co(BF4)2/L1a were > 95%.; the highest conversion was achieved using Co(BF4)2/L1a (entry 1–3, Table 2). Subsequently, Co(BF4)2/L1a was selected as the most active catalyst for our subsequent experiments. Reaction conditions:1-octene 1 mmol, Ph2SiH2 1.1 mmol, CoX2 0.5 mol％of olefin, 90 °C, 5 h.
Entry
Catalyst
Conv.（％）
Select（％）
β
α
Octane
1
Co(BF4)2/L1a
98.7
99.7
/
0.3
2
CoCl2/L1a
96.3
99.3
/
0.7
3
Co(CH3COO)2/L1a
97.4
99.7
/
0.3
4
CoBr2/L1a
62.0
96.4
/
3.6
5
Co(C10H7COO)2/L1a
58.3
99.4
/
0.6
3.3 The effects of different polyethylene glycol-functionalized N, P-ligands
A series of polyethylene glycol-functionalized N, P-ligands and Co(BF4)2 were used to catalyze the hydrosilylation of diphenylsilane with1-octene and the results were showed in Table 3. L1a–L1i have different groups on the N atom (entry1-9, Table 3), L1j–L1n have different groups on the P atom (entry 10–14, Table 3), and L2a–L4a have different chain lengths of polyethylene glycol (entry 15–17, Table 3). The conversion of 1-octene using L1h was slightly lower than those obtained using the other ligands (entry 8, Table 3). This may be attributed to the interaction formed between the pyridine ring and N atom making the electron density shift from the P atom, reducing the conversion; the conversion of 1-octene using L1l was also lower than the other ligands (entry 12, Table 3). The cyclohexyl group has two conformations: chair- and boat-type. The two conformations constantly change upon heating, which causes steric hindrance, which decreases the conversion. When comparing the data obtained for L1a–L4a (entry 1, 15–17, Table 3), it was found that the chain length of polyethylene glycol does not have a significant effect on the conversion and selectivity of the hydrosilylation reaction Fig. 1. Reaction conditions: 1-octene 1 mmol, Ph2SiH2 1.1 mmol, catalyst 0.5 mol％of olefin, 90 °C, 5 h.
Entry
Ligand
Conv.(%)
Select(%)
β
α
Octene
1
L1a
98.7
99.7
/
0.3
2
L1b
92.9
99.1
/
0.9
3
L1c
91.9
98.1
/
1.9
4
L1d
94.7
99.1
/
0.9
5
L1e
92.4
99.3
/
0.7
6
L1f
94.5
99.3
/
0.7
7
L1g
93.2
98.4
/
1.6
8
L1h
87.0
98.6
/
1.4
9
L1i
95.8
99.7
/
0.3
10
L1j
98.6
99.7
/
0.3
11
L1k
99.1
99.7
/
0.3
12
L1l
91.8
98.9
/
1.1
13
L1m
97.1
97.9
/
2.1
14
L1n
99.2
99.7
/
0.3
15
L2a
95.4
99.4
/
0.6
16
L3a
95.7
99.3
/
0.7
17
L4a
98.2
98.8
/
1.2
Polyethylene glycol-functionalized N, P-ligands.
3.4 The effects of the reaction temperature and time on the hydrosilylation reaction
The Co(BF4)2/L1a complex was used to catalyze the hydrosilylation of diphenylsilane with 1-octene to research effects of the reaction temperature and the reaction time, and the results were shown in Fig. 2. The reaction temperature had an effect on the reaction. When the reaction temperature was 70 °C, the conversion only reached 60 % even when a prolonged reaction time was used. When the temperature was 80 °C, the conversion increased, but it was still < 90 %. When the temperature was 90 or 100 °C, the conversion gradually increased with time. After 5 h, the conversion was close to 100%. Therefore, it was determined that the optimal reaction temperature was 90 °C and the optimal reaction time was 5 h (See Fig. 2).
Effects of Temperature and Time on the Hydrosilylation Reaction.
3.5 The effects of the coordination of L1a and Co(BF4)2 in different ratios on the hydrosilylation reaction
The ratio of L1a and Co(BF4)2 was changed to explore its effect on the hydrosilylation reaction and the results were shown in Fig. 3. Upon changing the ligand to cobalt salt ratio, the selectivity of the β-addition product was close to 100%; when the ratio of L1a to Co(BF4)2 was 1:2,1:1 or 3:1, the conversion was < 90%; and when the ratio was 1:2, the conversion was as high as 98.7%. Therefore, it was determined that the optimal ratio of ligand to cobalt salt was 2:1 (See Fig. 3).
Effects of Coordination of L1a and Co(BF4)2 in Different Ratios on the Hydrosilylation.
3.6 The effects of the catalyst loading on the hydrosilylation reaction
Five sets of catalyst loading experiments were designed to explore the optimal amount of catalyst required in the hydrosilylation reaction, and the results shown in Fig. 4. When the catalyst loading was 0.5 mol% with respect to the olefin, the conversion reaches its highest value. Therefore, it was determined that the optimal catalyst loading was 0.5 mol% (See Fig. 4) .
Effects of Catalyst Addition on the Hydrosilylation.
3.7 Co(BF4)2/L1a catalyzes the hydrosilylation of different silanes and different olefins
Co(BF4)2/L1a was used to catalyze the hydrosilylation of various silanes and olefins, and the results were shown in Table 4. For linear olefins, the yield of the target product decreased as the carbon chain length increased, which could be attributed to the longer carbon chain inhibiting the interactions formed between the catalyst and substrate. Co(BF4)2/L1a exhibits good catalytic activity toward styrene and its derivatives, such as 4-fluorostyrene, 4-chlorostyrene, and 4-methoxystyrene. The yields obtained when using styrene bearing different electronegative substituents were not significantly different, indicating that the catalyst exhibits good functional group tolerance. In addition, Co(BF4)2/L1a also catalyzes the reaction of allyl acetate, allyl glycidyl ether, and diphenyl silane, which give the desired products in good yield. Reaction conditions: olefins 1.0 mmol, silanes 1.1 mmol, 90 °C, 5 h, cat 0.5 mol%, based on olefins.
98.4% yield
62.4% yield
29.4% yield
93.2% yield
87.9% yield
84.2% yield
91.6% yield
71.5% yield
83.5% yield
95.7% yield
69.7% yield
3.8 Recycling performance of the catalysts
The recycling performance of Co(BF4)2/L4a was investigated. After the catalytic system was recycled 8 times, the product yield can still reach > 90% and the selectivity toward the β-addition product remained unchanged (See Fig. 5).
Reusability of Catalyst.
4 Conclusions
A series of polyethylene glycol-functionalized N, P-ligands have been synthesized and coordinated with Co(BF4)2 and their corresponding complexes used to catalyze the hydrosilylation of silanes with olefins. The optimal catalytic effect was achieved using 0.5 mol% of catalyst at 90 °C over 5 h. The catalytic cycling performance has also been studied and it was found that the product yield was > 90% after eight cycles.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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