English

• Shell higher olefin process(SHOP)-type catalysts^[1] (Scheme 1, A) as the oldest neutral nickel catalysts have been successfully commercialized for the synthesis of linear α-olefins. However, no noticeable improvement has been made in this field until Grubbs et al.^[2-3] covered the phenoxyiminato neutral nickel catalysts(Scheme 1, B). These complexes, with bulky groups at the ortho position of the phenoxy moiety, generate high relative molecular mass polymers with excellent activities comparable to the classic metallocenes, and show substantial tolerance toward various polar additives. Since then, nickel catalysts^[4-10] have attracted much attention, and the electronic and steric effects of the auxiliary ligands have been extensively studied. Neutral nickel(Ⅱ) catalysts containing highly electron-withdrawing groups —CF₃ and —COOR^[11](Scheme 1, C) afford low relative molecular mass highly linear polyethylene(PE) with very high activity. Mecking group^[12-13] reported the vital role of electron-donating groups in neutral nickel polymerization catalysts, which are responsible for the resultant hyperbranched ethylene oligomers. Generally speaking, electron-donating groups would enhance tolerance of the catalysts toward polar groups by increasing the electron density of nickel center, and facilitate polymerization initiation by promoting the dissociation of additional stabilizing ligand. As an electron-donating group, RCOO— has been rarely applied for nickel catalysts in olefin polymerization^[14]. Thus, we designed a series of catalysts bearing RCOO— groups(Scheme 1, D) to study their properties in ethylene polymerization. Considering the vital role of effective blockage of the axial positions of nickel center, nickel complexes with 2, 6-(3, 5-tBu₂C₆H₃)₂C₆H₃NH₂ was also synthesized.

  图Scheme 1 Typical neutral nickel catalysts for olefin polymerization Scheme1. Typical neutral nickel catalysts for olefin polymerization

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  Moreover, —OC(O)—R—COO— as excellent bridges for binuclear catalyst^[15] have gained great success in various catalytic reactions^[16-19]. We thus also designed and synthesized novel binuclear neutral nickel catalyst Ni³ bearing OC(O)—R—COO—(R=o-C₆H₄) linkage to address their catalytic properties. Without any cocatalysts, all these complexes displayed very high activities up to 1.8×10⁶ g of PE mol^-1·Ni^-1·h^-1 even at low ethylene pressure(5×10⁵ Pa), which are among the highest catalytic activities for phenoxyiminato neutral nickel catalysts in ethylene polymerization. These catalysts demonstrated good polar monomer tolerance, and were capable to catalyze the copolymerization of ethylene with 1, 5-hexadiene, 1, 7-octadiene, 6-bromo-1-hexene and methyl 10-undecenoate, yielding copolymers with moderate relative molecular mass at moderate activities.

  1   Experimentals

  1.1   Instruments and Reagents

  All manipulations of air-and/or moisture-sensitive compounds were carried out using standard Schlenk techniques or in a Etelux lab2000 glovebox under a dry nitrogen atmosphere. Toluene, n-hexane, diethyl ether, dimethylformamide(DMF), and methylene dichloride were purified by an MBraun solvent purification system(SPS). Pyridine and N, N, N-triethylamine was distilled from sodium/benzophenone ketyl under nitrogen prior to use. BBr₃, MeI, K₂CO₃, CH₃COOH, CH₃COONa, p-toluenesulfonate, 4-dimethylaminopyridine, benzoyl chloride and phthaloyl dichloride were purchased from Beijing Chemical Works and directly used without purification. 1, 5-Hexadiene, 1, 7-octadiene, and methyl 10-undecenoate were distilled after drying via calcium hydride for two days. These monomers were purchased from J&K Chemical. 2, 5-Dimethoxy-3-phenylbenzaldehyde^[20], 2, 6-(3, 5-tBu₂C₆H₃)₂C₆H₃NH₂^[13], and (pyridine)₂Ni(CH₃)₂^[21] were synthesized according to reported literature. Ethylene(99.999%) was purchased from Changchun Juyang Corporation and was used without further purification.

  The nuclear magnetic resonance(NMR) spectra of polyethylene samples were all recorded on a Varian Unity 400 MHz spectrometer(Bruker Corporation) with o-dichlorobenzene-d₄ or 1, 1, 2, 2-tetrachloroethane-d₂ as the solvent at 110 ℃. All ¹H and ¹³C NMR spectra of small organic and organometallic compounds were obtained on a Bruker 300 MHz spectrometer, a Varian Unity 400 MHz spectrometer or a Bruker 500 MHz spectrometer at ambient temperature with CDCl₃, C₆D₆ or dimethylsulfoxide-d₆(DMSO-d₆) as the solvent. The DSC measurements were performed on a Perkin-Elmer Pyris 1 differential scanning calorimeter at a heating rate of 20 ℃/min. The mass-average relative molecular mass(M_w) and the polydispersity index(PDI) of polyethylene samples were determined via high-temperature gel permeation chromatography(GPC) in which 1, 2, 4-trichlorobenzene was used as mobile phase at a flow rate of 1.0 mL/min. The calibration was made by the polystyrene standard Easi-Cal PS-1(PL Ltd., Agilen, Santa Clara, California, USA).

  1.2   Synthesis of Compounds b~d

  3-phenyl-2, 5-dihydroxy-benzaldehyde(b): In a flame dried two necked flask equipped with a pressure equalizing dropping funnel, 3-phenyl-2, 5-dimethoxy-benzaldehyde (a) (2.2 g, 9.0 mol) was dissolved in dry dichloromethane (DCM, 20 mL). The resulting solution was cooled to -78 ℃ and a solution of BBr₃ in DCM(2 mol/L, 18 mL) was slowly added to the solution via the pressure equalizing dropping funnel in 20 min. The resulting solution was maintained at -78 ℃ for 40 min, then was allowed to warm up to room temperature and left with stirring overnight under a nitrogen atmosphere. After 10 hours, the solution was quenched with ice to neutralise the excess BBr₃, and the DCM was removed under reduced pressure. The resulting mixture was extracted with ethyl acetate (EtOAc, 50 mL×3) and the combined organic extracts were washed twice with brine (50 mL×2), dried (MgSO₄), and filtered. The solvent was removed in vacuo yielding 3-phenyl-2, 5-dihydroxy-benzaldehyde(1.7 g, 89%) as yellow powder. ¹H NMR(400 MHz, DMSO-d₆), δ:10.65(s, 1H), 10.02(s, 1H), 9.51(s, 1H), 7.54(d, J=7.0 Hz, 2H), 7.44(t, J=7.4 Hz, 2H), 7.36(t, J=7.3 Hz, 1H), 7.13(d, J=3.1 Hz, 1H), 7.09(d, J=3.1 Hz, 1H). ¹³C NMR(101 MHz, DMSO-d₆), δ:196.89, 150.64, 150.28, 136.46, 130.78, 129.08, 128.24, 127.44, 125.26, 121.85, 116.67.

  [(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-2, 5-(OH)₂-C₆H₂)](c): To an ethanol(5 mL) solution of compound b(0.63 g, 1.9 mmol) was added a catalytic amount of pyridinium p-toluenesulfonate(p-TSA) and 1.2 stoichiometric amount of 2, 6-diisopropylaniline. The mixture was stirred for 12 hours at room temperature. The yellow solid that precipitated was filtered, washed with cold ethanol and dried to afford the Schiff base in 99% yield. ¹H NMR(400 MHz, CDCl₃), δ:13.25(s, 1H, OH), 8.30(s, 1H, ArH), 7.67(d, J=7.4 Hz, 2H, ArH), 7.47(t, J=7.6 Hz, 2H, ArH), 7.37(t, J=7.0 Hz, 1H, ArH), 7.20(s, 3H, ArH), 7.08(s, 1H, ArH), 5.46(s, 1H, OH), 3.02(hept, J=6.8 Hz, 2H, CH(CH₃)₂), 1.18(d, J=6.9 Hz, 12H, CH(CH₃)₂). ¹³C NMR(101 MHz, CDCl₃), δ:166.59, 152.73, 139.23, 137.01, 131.14, 129.44, 128.53, 127.76, 126.00, 123.49, 118.53, 116.88, 28.30, 23.73.

  [(2, 6-(3, 5-^tBu₂C₆H₃)₂C₆H₃)－N＝C(H)(3-Ph-2, 5-(OH)₂-C₆H₂)](d): Using the same method for synthesizing compound c, reaction of compound b with 2, 6-(3, 5-tBu₂C₆H₃)₂C₆H₃NH₂ gave rise to compound d as yellow powder in 90% yield. ¹H NMR(500 MHz, DMSO-d₆), δ:12.49(s, 1H), 8.98(s, 1H), 8.08(s, 1H), 7.46(d, J=7.8 Hz, 2H), 7.42~7.33(m, 5H), 7.29(d, J=7.1 Hz, 1H), 7.26(t, J=1.7 Hz, 2H), 7.19(d, J=1.7 Hz, 4H), 6.77(d, J=2.8 Hz, 1H), 6.37(d, J=3.0 Hz, 1H), 1.19(s, 36H). ¹³C NMR(126 MHz, DMSO-d₆), δ:169.97, 150.51, 149.93, 148.87, 145.13, 138.02, 137.43, 134.81, 129.63, 129.09, 128.77, 127.92, 126.89, 125.73, 124.16, 121.24, 120.24, 118.86, 116.72, 34.43, 31.07.

  1.3   Synthesis of ligands L⁰~L³

  [(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-2-(OH)-C₆H₃)](L⁰): Ligand L⁰ was synthesized according to the reported literature^[3]. ¹H NMR(500 MHz, CDCl₃), δ:8.37(s, 1H), 7.71(d, J=8.3 Hz, 2H), 7.53(d, J=7.6 Hz, 1H), 7.48(t, J=7.6 Hz, 2H), 7.37(t, J=7.1 Hz, 2H), 7.20(s, 3H), 7.06(t, J=7.6 Hz, 1H), 3.02 (hept, J=6.8 Hz, 2H), 1.18(d, J=6.9 Hz, 12H).

  [(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-5-PhCOO-2-(OH)-C₆H₂)](L¹)    General procedure for the preparation of ligands L¹~L³. The appropriate acid chloride(1.0 mmol) was added to a solution of compounds c or d (1.1 stoichiometric amount for L¹ and L², 2.2 stoichiometric amount for L³), 4-dimethylaminepyridine(0.2 mmol), and triethylamine(3.0 mmol) in CH₂Cl₂(10 mL) at room temperature under N₂ atmosphere. After complete consumption of the acid chloride detected by TLC, the mixture was concentrated under reduced pressure and purified by silica gel chromatography. Ligand L¹ was afforded as yellow powder in 84% yield. ¹H NMR(500 MHz, CDCl₃), δ:8.34(s, 1H), 8.22(dd, J=8.3, 1.2 Hz, 2H), 7.73(d, J=7.1 Hz, 2H), 7.65(t, J=7.5 Hz, 1H), 7.53(t, J=7.8 Hz, 2H), 7.47(t, J=7.6 Hz, 2H), 7.40~7.35(m, 2H), 7.25(s, 1H), 7.19(s, 3H), 3.01(hept, J=6.8 Hz, 2H), 1.18(d, J=6.9 Hz, 12H). ¹³C NMR(126 MHz, CDCl₃), δ:166.34, 165.79, 156.65, 145.92, 142.84, 138.90, 136.76, 133.91, 131.41, 130.34, 129.55, 129.45, 128.79, 128.43, 127.83, 127.68, 125.84, 123.79, 123.45, 118.88, 28.27, 23.73.

  [(2, 6-(3, 5-^tBu₂C₆H₃)₂C₆H₃－N＝C(H)(3-Ph-5-PhCOO-2-(OH)-C₆H₂)](L²): Ligand L² was afforded in 73% yield. ¹H NMR(300 MHz, CDCl₃), δ:8.14(d, J=7.3 Hz, 2H), 7.92(s, 1H), 7.62(t, J=7.4 Hz, 1H), 7.49(t, J=7.4 Hz, 6H), 7.35~7.39(m, 4H), 7.31(s, 2H), 7.23(d, J=1.6 Hz, 4H), 7.16(d, J=2.8 Hz, 1H), 6.60(d, J=2.8 Hz, 1H), 1.25(s, 36H). ¹³C NMR(126 MHz, CDCl₃), δ:168.20, 165.30, 156.27, 150.76, 145.19, 142.35, 138.48, 137.02, 135.94, 133.70, 130.89, 130.22, 130.04, 129.64, 129.39, 128.70, 128.15, 127.49, 127.11, 125.98, 124.57, 123.07, 121.09, 119.08, 34.99, 31.51.

  [C₆H₄(COO)₂-{(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-5-yl-2-(OH)-C₆H₂)}₂](L³): Ligand L³ was afforded in 41% yield. ¹H NMR(500 MHz, CDCl₃), δ:8.27(s, 2H), 8.02(dd, J=5.7, 3.3 Hz, 2H), 7.73(dd, J=5.7, 3.3 Hz, 2H), 7.57(d, J=7.0 Hz, 2H), 7.38(dd, J=8.9, 6.1 Hz, 6H), 7.31(t, J=7.4 Hz, 2H), 7.28(d, J=2.9 Hz, 2H), 7.19(s, 6H), 2.96(hept, J=6.8 Hz, 4H), 1.14(d, J=6.9 Hz, 24H). ¹³C NMR(101 MHz, DMSO-d₆), δ:167.83, 165.93, 156.01, 145.22, 142.19, 138.10, 135.92, 132.63, 130.63, 129.99, 129.62, 128.95, 128.15, 127.56, 127.12, 125.71, 124.31, 123.20, 118.82, 27.75, 23.23.

  1.4   Synthesis of nickel complexes Ni⁰~Ni³

  [[(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-2-O-C₆H₃)-κ²-N, O]Ni(CH₃)(pyridine)](Ni⁰): According to the literature^[22], the nickel methyl pyridine complex Ni⁰ was prepared in excellent yield by adding dropwise the toluene solution of (pyridine)₂NiMe₂(0.295 g, 1.2 mmol) to toluene solution of ligand L⁰(0.36 g, 1.0 mmol) with vigorous stirring at room temperature. The mixture was stirred at room temperature to give a dark red solution as the reaction proceeded. After 6 hours, the mixture was filtrated to remove nickel black, and the filtrate was directly taken to dryness, affording complex Ni⁰ in 95% yield as dark red solid. ¹H NMR(500 MHz, C₆D₆), δ:8.48(s, 2H), 7.61(s, 1H), 7.44(s, 3H), 7.08~7.16(m, 7H), 6.59(d, J=40.9 Hz, 2H), 6.18(s, 2H), 4.23(hept, J=6.8 Hz, 2H), 1.53(s, 6H), 1.11(s, 6H), -0.68(s, 3H, NiCH₃). ¹³C NMR(126 MHz, C₆D₆), δ:166.59, 165.34, 151.97, 150.29, 141.21, 140.81, 135.51, 134.62, 133.91, 133.44, 129.97, 128.35, 127.51, 126.59, 123.65, 123.00, 120.73, 114.13, 28.60, 24.97, 23.23, -7.30(NiCH₃). Anal. Calcd for C₃₁H₃₄N₂NiO:C 73.11, H 6.73, N 5.50. Found:C 73.09, H 6.76, N 5.48.

  [[(2, 6-ⁱPr₂-C₆H₃)－N＝C(H)(3-Ph-5-PhCOO-2-O-C₆H₂)-κ²-N, O]Ni(CH₃)(pyridine)](Ni¹): Using the same method for synthesizing Ni⁰, complex Ni¹ was obtained in 88% yield as yellow powder. ¹H NMR(500 MHz, C₆D₆), δ:8.50(dd, J=6.4, 1.5 Hz, 2H), 8.31~8.26 (m, 1H), 7.53(s, 1H), 7.42(d, J=3.0 Hz, 1H), 7.37(dd, J=8.2, 1.1 Hz, 2H), 7.19~7.01(m, 6H), 6.93(t, J=7.6 Hz, 2H), 6.88(d, J=3.0 Hz, 1H), 6.64(tt, J=7.6, 1.6 Hz, 1H), 6.19(ddd, J=7.5, 5.3, 1.3 Hz, 2H), 4.21(hept, J=6.8 Hz, 2H), 1.56(d, J=6.9 Hz, 6H), 1.09(d, J=6.8 Hz, 6H), -0.65(s, 3H, NiCH₃). ¹³C NMR(101 MHz, CDCl₃), δ:166.04, 165.50, 163.40, 151.91, 150.13, 141.10, 139.82, 139.34, 135.55, 134.06, 133.15, 130.88, 130.33, 129.94, 128.82, 128.66, 127.47, 126.62, 126.08, 124.21, 123.62, 123.02, 119.60, 28.59, 24.92, 23.22, -7.23(NiCH₃). Anal. Calcd for C₃₈H₃₈N₂NiO₃:C 72.51, H 6.09, N 4.45. Found:C 72.50, H 6.12, N 4.43.

  [[(2, 6-(3, 5-^tBu₂C₆H₃)₂C₆H₃－N＝C(H)(3-Ph-5-PhCOO-2-C₆H₂)-κ²-N, O]Ni(CH₃)(pyridine)](Ni²): Using the same method for synthesizing complexes Ni⁰, Ni² was afforded in 91% yield as red powder. ¹H NMR(500 MHz, C₆D₆), δ:8.21~8.16(m, 4H), 7.87(d, J=1.8 Hz, 4H), 7.65(t, J=1.7 Hz, 2H), 7.55(d, J=7.6 Hz, 2H), 7.34(dd, J=8.2, 1.2 Hz, 2H), 7.29 ~7.25(m, 2H), 7.22(t, J=7.6 Hz, 1H), 7.11(d, J=7.4 Hz, 1H), 7.05(dd, J=13.7, 6.6 Hz, 3H), 6.95(t, J=7.5 Hz, 2H), 6.65(t, J=7.6 Hz, 1H), 6.54(d, J=3.1 Hz, 1H), 6.29~6.25 (m, 2H), 1.45(s, 36H), -0.54(s, 3H, NiCH₃). ¹³C NMR(101 MHz, CDCl₃), δ:168.49, 165.23, 162.75, 151.52, 151.09, 150.89, 139.98, 139.91, 138.75, 137.45, 135.35, 133.27, 132.98, 130.98, 130.27, 129.87, 129.68, 128.58, 127.33, 126.36, 125.90, 124.12, 122.65, 121.04, 120.20, 35.28, 31.78, -7.96(NiCH₃). Anal. Calcd for C₆₀H₆₆N₂NiO₃:C 78.17, H 7.22, N 3.04. Found:C 78.18, H 7.20, N 3.05.

  [C₆H₄(COO)₂-{[(2, 6-ⁱPr₂-C₆H₃N＝CH)(3-Ph-5-yl-2-O-C₆H₂)-κ²-N, O]Ni(CH₃)(pyridine)](Ni³): The nickel methyl pyridine complex Ni³ was prepared by adding dropwise the toluene solution of (pyridine)₂NiMe₂(0.15 g, 0.6 mmol) to toluene solution of ligand L³(0.36 g, 0.25 mmol) with vigorous stirring at room temperature. The mixture was stirred at room temperature for 6 hours, then the mixture was filtrated to remove nickel black, and the filtrate was directly taken to dryness, affording complex Ni³ in 70% yield as yellow powder. ¹H NMR(400 MHz, C₆D₆), δ:8.52(d, J=5.4 Hz, 4H), 7.81~7.75(m, 2H), 7.53(d, J=2.9 Hz, 2H), 7.41(s, 2H), 7.25(d, J=7.6 Hz, 4H), 7.14~6.93(m, 16H), 6.65(t, J=7.2 Hz, 2H), 6.27~6.16(m, 4H), 4.15(hept, J=6.8 Hz, 4H), 1.57(d, J=6.8 Hz, 12H), 1.04(d, J=6.8 Hz, 12H), -0.66(s, 6H, NiCH₃). ¹³C NMR(126 MHz, C₆D₆), δ:165.67, 165.05, 162.50, 150.96, 149.08, 140.10, 138.68, 138.35, 134.58, 133.10, 131.88, 130.35, 128.90, 128.34, 127.57, 126.55, 125.01, 124.70, 123.25, 122.59, 122.07, 118.68, 27.60, 23.95, 22.33, -8.07(NiCH₃). Anal. Calcd for C₇₀H₇₀N₄Ni₂O₆:C 71.21, H 5.98, N 4.75. Found:C 71.18, H 5.61, N 4.73.

  1.5   X-ray Structure Determination

  Single crystals suitable for X-ray diffraction analysis were grown within one day after layering a solution of these complexes in toluene with n-hexane in a glove box. The intensity data were collected with the ω scan mode(186 K) on a Bruker Smart APEX diffractometer with a CCD detector using MoKα radiation(λ=0.071073 nm). Absorption corrections were performed using the SADABS program. The crystal structures were solved using the SHELXTL program and refined using full matrix least-squares techniques. The positions of hydrogen atoms were calculated theoretically and included in the final cycles of refinement in a riding model along with attached carbons. Crystal data collection and refinement details are given in Table S1(see Supporting Information).

  1.6   Procedure for Ethylene Polymerization

  A 200 mL autoclave was heated under vacuum to 140 ℃ for 4 hours and was then cooled to the desired reaction temperature. The vessel was purged three times with ethylene and was charged with 30 mL toluene under ethylene pressure. A solution of nickel catalyst in 10 mL toluene was injected into the reactor. The reaction apparatus was then filled with ethylene and pressurized to the prescribed ethylene pressure immediately. The mixture was stirred for prescribed time for ethylene polymerization. After reaction, magnetic stirring was stopped, the reactor was vented, and the polymerization mixture was poured into 200 mL ethanol. The solid polymer was filtered, washed with ethanol several times, and dried to constant mass under vacuum.

  1.7   Procedure for Ethylene Copolymerization

  A 200 mL autoclave was heated under vacuum to 140 ℃ for 4 hours and was then cooled to the desired reaction temperature. The vessel was purged three times with ethylene and was charged with toluene under ethylene pressure. A solution of commoner was injected into the reactor before a toluene solution of nickel catalyst was added by using a dry syringe. The total reaction volume was 40 mL. The reaction apparatus was then filled with ethylene and pressurized to the prescribed ethylene pressure immediately. The mixture was stirred for 90 min under prescribed temperature. After reaction, magnetic stirring was stopped, the reactor was vented, and the polymerization mixture was poured into ethanol. The solid polymer was filtered, washed with ethanol several times, and dried to constant mass under vacuum.

  2   Results and Discussion

  2.1   Synthesis of Ligands and Complexes

  2, 5-Dimethoxy-3-phenylbenzaldehyde(a) was synthesized at excellent yield from commercially available 2-hydroxy-5-methoxybenzaldehyde according to reported literature^{[20, 23]}. The key intermediate 3-phenyl-2, 5-dihydroxy-benzaldehyde(b) was obtained after deprotection of compound a by BBr₃(Scheme S1, see Supporting Information). Condensation of compound b with 2, 6-iPr₂C₆H₃NH₂ or 2, 6-(3, 5-tBu₂C₆H₃)₂C₆H₃NH₂ gave rise to compounds c and d, respectively(Scheme S1).

  Ligands L¹~L³ were synthesized by the reaction of compounds c or d with corresponding acyl chlorides(Scheme 2). Using the procedure reported by Mecking et al.^[22], complexes Ni⁰~Ni³ were prepared by reaction of corresponding ligands with (pyridine)₂Ni(Me)₂ in 70%~95% yields(see Scheme 2). All these nickel complexes were characterized by ¹H NMR, ¹³C NMR, and elemental analysis. Single crystals of Ni⁰, Ni¹ and Ni² were grown within one day after layering a solution of these complexes in toluene with n-hexane(1/3, volume ratio) and the structures of these complexes were determined by X-ray diffraction analysis(Fig. 1~Fig. 3). The coordination geometry at the nickel center is slightly distorted square planar, and the methyl groups are located trans. to the oxygen atoms. Obviously, the coordination environment of Ni²(see Fig. 3) was more congested than that of Ni¹(Fig. 2). Due to the presence of bulkier substituted terphenyl imine moiety, the axial positions of the nickel center can be effectively shielded.

  图Scheme 2 Synthesis of ligands and complexes Scheme2. Synthesis of ligands and complexes

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  图1 Molecular structure of complex Ni⁰ Figure1. Molecular structure of complex Ni⁰

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  图2 Molecular structure of complex Ni¹ Figure2. Molecular structure of complex Ni¹

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  图3 Molecular structure of complex Ni² Figure3. Molecular structure of complex Ni²

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  2.2   Ethylene polymerization

  Ethylene polymerization trials were performed to determine the effects of polymerization temperature, reaction time, and additive triphenylphosphine (PPh₃)(Table 1). All these nickel complexes(Ni⁰~Ni³) studied here can generate polyethylenes without cocatalysts such as expensive bis(1, 5-cyclooctadiene)nickel(0) Ni(COD)₂ and B(C₆F₅)₃. Relative molecular mass distributions of obtained polymers between 1.6 and 2.2 indicate that all these complexes were well-behaved single-site catalysts. Generally speaking, the relative molecular mass of polyethylenes produced by Ni⁰~Ni³ decreased at elevated temperatures due to more extensive chain transfer reactions. Accordingly, the T_m values of polymers decreased at elevated temperatures as well.

  Table 1.  Ethylene polymerization using neutral nickel catalysts Ni⁰~Ni^3a

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  Entry	Catalyst	Temperature/℃	Yield/g	Activity b	Mwc/(kg·mol-1)	Mw/Mnc	Tmd/℃
  1	Ni0	40	0.19	1.5	45.4	2.2	114
  2	Ni0	50	0.72	5.8	25.2	2.3	103
  3	Ni0	60	1.23	9.8	15.1	2.2	96
  4	Ni0	70	0.96	7.7	10.1	2.1	87
  5	Ni1	40	0.22	1.8	51.7	2.2	115
  6	Ni1	50	1.31	10.5	19.4	2.2	97
  7	Ni1	60	0.82	6.6	11.6	2.2	89
  8	Ni1	70	0.53	4.2	8.6	2.1	75
  9e	Ni1	50	2.24	6.0	17.0	2.2	92
  10f	Ni1	50	3.07	4.1	17.0	2.1	92
  11g	Ni1	50	0.016	0.13	4.9	1.8	-
  12	Ni2	40	1.21	9.7	11.6	2.1	81
  13	Ni2	50	1.91	15.3	6.9	2.0	68
  14	Ni2	60	2.24	17.9	4.2	2.3	-
  15	Ni2	70	1.75	14.0	3.5	1.1	-
  16	Ni3	40	0.17	1.4	70.3	2.5	124
  17	Ni3	50	1.11	8.9	27.3	2.4	99
  18	Ni3	60	0.61	4.9	19.3	2.2	93
  19	Ni3	70	trace	-	-	-	-
  20e	Ni3	50	2.52	3.4	16.6	2.1	91
  21f	Ni3	50	2.82	1.9	18.0	2.1	93
  22g	Ni3	50	0.021	0.17	5.4	1.9	-
  a.Polymerizations conditions:toluene, 40 mL; nickel, 5(mol; ethylene, 5×105 Pa; 15 min; b.in the unit of 105 g PE mol-1·Ni-1·h-1; c.determined by GPC vs polystyrene standards; d.determined by DSC; e.reaction time: 45 min; f.reaction time:90 min; g.25(mol PPh3 as additives.

  Compared with Ni⁰, catalyst Ni¹ with PhCOO-remote from the nickel center exhibited higher activity at low reaction temperature(40 and 50 ℃), implying an easier initiation process. This may result from the weaker coordination of pyridine ligand to the less oxophilic nickel center due to the presence of electron-donating group PhCOO—. Catalyst Ni¹ yields polyethylene with M_w=1.9×10⁴ and 61 branches with the activity of 1.0×10⁶ g of PE mol^-1·Ni^-1·h^-1 at 50 ℃(entry 6, Table 1).

  After introducing bulky terphenyl moiety, catalyst Ni² generated PE with M_w=6900 and 81 branches per 1000 carbon atoms(C^-1) at 1.5×10⁶ g PE mol^-1·Ni^-1·h^-1 at 50 ℃(entry 13, Table 1). With activities up to 1.8×10⁶ g of PE mol^-1·Ni^-1·h^-1 under 5×10⁵ Pa ethylene pressure at 60 ℃(entry 14, Table 1), Ni² is among the most active phenoxyiminato nickel catalysts for ethylene homopolymerization^{[3, 13, 22, 24-27]}. The activity of Ni² has doubled relative to that of Ni¹, which is ascribed to the bulky substituted N-terphenyl moiety playing a vital role in preventing the forming of bis-ligated complex^[28]. However, the amorphous polymer M_w obtained by Ni² at 60 ℃ was much lower than that by Ni¹ under the same reaction conditions(4200 vs 11600, entry 14 vs 7, Table 1). Electron-donating tert-butyl groups are responsible for the lower M_w due to extensive chain transfer reaction, in good agreement with the findings by Mecking group^[12], that the electronic characteristics of the remote substituents of terphenyl moieties govern the polymerization behavior.

  Binuclear Ni³ shows similar optimal activity relative to mononuclear Ni¹(ca. 1.0×10⁶ g of PE mol^-1·Ni^-1·h^-1)(entry 6 vs 17, Table 1). Compared with mononuclear Ni¹, binuclear catalyst Ni³ enhanced the M_w of polymer. This is consistent with the reported result of literatures^[29-30] that binuclear catalysts enhanced polymer relative molecular mass. Catalyst Ni³ demonstrates slightly lower activity at low temperature than mononuclear Ni¹, and generates only trace amount of polymer at elevated temperature(70 ℃, entry 19, Table 1), indicating an inferior thermal stability of binuclear catalyst Ni³ to mononuclear Ni¹. It was reported by Grubbs et al. that bis-ligated nickel species was isolated as the main deactivation product when ligand frameworks were not sufficiently bulky^[31]. We thus proposed that inactive bis-ligated complex was more readily generated for binuclear complex Ni³ in which two nickel centers were close to each other, especially at elevated temperatures^[32]. The drop of the catalytic activity with polymerization time may partly reflect the stability of catalysts, we thus conducted ethylene polymerization for different polymerization time using catalysts Ni¹ and Ni³. From 45 to 90 min, a 32% decrease in catalytic activity for Ni¹ was observed and the value for binuclear Ni³ was 44%. This indicated a more serious catalyst deactivation with reaction time for binuclear Ni³, which was similar to the effect of reaction temperature.

  To further investigate the different performances of these catalysts, extra PPh₃ was added to the catalytic systems during the ethylene polymerization. In the presence of 5 stoichiometric amount of PPh₃, the activity of mononuclear Ni¹ was reduced to 1/80th of that without any additive due to the competitive coordination reaction of ethylene and PPh₃^[33]. For binuclear Ni³, the activity decreased sharply as well, but the activity in the presence of 5 stoichiometric amount of PPh₃ was still 1/50th of the original value. The better tolerance toward PPh₃ additives of the binuclear catalyst may be resulted from the close position of the nickel centers, which would prohibit simultaneous inhibition of the two metal centers by bulky PPh₃ ligands^[34-35].

  We also studied the microstructures of polyethylenes produced by these catalysts through ¹³C NMR. All these polymers obtained by Ni⁰~Ni³ at 50 ℃ possess similar branching pattern(Fig. 4). According to the ¹³C NMR, polymers generated by Ni¹ contain main methyl branches and minor ethyl branches^[36].

  图4 ¹³C NMR spectrum of PE obtained by Ni¹ at 50 ℃(entry 4, Table 1) Figure4. ¹³C NMR spectrum of PE obtained by Ni¹ at 50 ℃(entry 4, Table 1)

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  2.3   Ethylene Copolymerization

  The typical results of ethylene copolymerization catalyzed by Ni¹~Ni³ with 1, 5-hexadiene, 1, 7-octadiene, 6-bromo-1-hexene and methyl 10-undecenoate were summarized in Table 2. Both Ni¹ and Ni³ could catalyze ethylene with 1, 5-hexdiene at moderate activities(1.8~2.0×10⁴ g of PE mol^-1·Ni^-1·h^-1), but the incorporation of the comonomer was low. In stark contrast, 1, 7-octadiene was efficiently incorporated into the polyethylenes by using Ni¹ and Ni³. The resulting copolymers with pendant double bonds were clearly evidenced by the significant increase of external double bonds versus polyethylenes by ¹H NMR measurement (Fig.S1, see Supporting Information). Additionally, binuclear Ni³ demonstrated slightly higher activity and polymer M_w than Ni¹(entry 9 vs 2, Table 2).

  Table 2.  Ethylene copolymerization using Ni¹-Ni³ complexes^a

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  Entry	Catalyst	Comonomer	Yield/mg	Activityb	Mwc/(kg·mol-1)	Mw/Mnc	Tmd/℃	Xce
  1	Ni1		260	17.7	16.3	1.9	93	nd
  2	Ni1		230	15.3	16.6	2.0	94	nd
  3	Ni1		61	4.1	20.5	1.9	103	0.92
  4	Ni1		150	10.0	15.4	1.8	94	0.82
  5 f	Ni1		49	3.3	16.5	2.2	97	1.01
  6	Ni2		18	1.2	6.0	1.7	-	0.16
  7	Ni2		101	6.7	6.7	1.7	-	0.25
  8	Ni3		301	20.1	14.9	2.0	92	nd
  9	Ni3		252	16.8	17.6	1.7	94	nd
  10	Ni3		72	4.8	16.1	1.9	96	0.91
  11	Ni3		182	12.1	16.5	1.9	95	0.64
  12 f	Ni3		62	4.1	16.8	2.1	93	1.10
  a.Polymerizations conditions:toluene, 40 mL; nickel, 10 (mol; ethylene, 5×105 Pa; 90 min, 600 stoichiometric amount of comonomer; b.in the unit of 103 g PE mol-1·Ni-1·h-1; c.determined by GPC vs. polystyrene standards; d.determined by DSC; e.incorporation ratio was calculated according to 1H NMR spectra; f.1000 stoichiometric amount of comonomer.

  6-Bromo-1-hexene as a polar comonomer, could be efficiently incorporated into the copolymer backbone by Ni¹~Ni³ (entry 3, 6, 10, Table 2) when copolymerizing with ethylene. According to the ¹H NMR spectrum(Fig. 5) of copolymer obtained by Ni¹ at 50 ℃, the signal at δ 3.24 is clearly assigned to the —CH₂Br group^[37]. The catalytic activity was much lower than that for ethylene homopolymerization, which is well consistent with that reported in literature ^[31]. The incorporation ratio by Ni¹ is up to four times higher than that by Ni² (entry 3 vs 6, Table 2). This may originate from the highly sterically hindered imine moiety of the nickel center in Ni².

  图5 ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 3, Table 2) Figure5. ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 3, Table 2)

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  Complexes Ni¹~Ni³ also could efficiently initiate the copolymerization of ethylene with the methyl 10-undecenoate. ¹H NMR signals(Fig. 6) at 3.49 and 2.16, assigned to —COOCH₃ and —CH₂COO—, respectively, highlighted that methyl 10-undecenoate was effectively incorporated into the polymer backbone^[38]. When the comonomer in the feed was increased from 600 to 1000 stoichiometric amount, Ni¹ catalyzed the copolymerization with sharply decreased activity and higher comonomer incorporation(1.10%(molar fraction) vs 0.64%(molar fraction))(entry 5 vs 4, Table 2). In this aspect, binuclear catalyst Ni³ displayed similar tendency(entry 13, 14, Table 2). When compared with Ni² bearing bulky imine moieties, the mononuclear Ni¹ yielded copolymer with higher relative molecular mass(15400 vs 6700) and comonomer incorporation(0.82%(molar fraction)) vs 0.25%(molar fraction)). The bulky imine motif effectively shielded the axial positions of the nickel center, went against the coordination of comonomer with active center, and resulted in the sharply decreased incorporation ratio.

  图6 ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 4, Table 2) Figure6. ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 4, Table 2)

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  3   Conclusions

  We have successfully synthesized and characterized a series of neutral nickel complexes containing benzoyloxy groups. Without any cocatalyst, they can promote ethylene polymerization at very high activities generating polyethylenes with moderate relative molecular mass. For catalyst Ni¹, the electron-donating PhCOO— group promoted initiation of catalyst Ni¹, showes better activity at low temperature compared to catalyst Ni⁰. Introducing bulky 2, 6-(3, 5-(t-Bu)₂C₆H₃)₂C₆H₃N-moiety, the activities of catalyst Ni² up to 1.8×10⁶ g of PE mol^-1·Ni^-1·h^-1(at 5×10⁵ Pa ethylene) are among the highest values using phenoxyimina to neutral nickel catalysts. However, lower relative molecular mass polymer was obtained due to the electron-donating 3, 5-(t-Bu)₂C₆H₃ groups. The microstructures of the polymers generated by these catalysts contain main methyl branches and minor ethyl branches. Compared with catalyst Ni¹, binuclear catalyst Ni³ produced polyethylene with higher M_w at similar activity and demonstrated better tolerance toward PPh₃. These neutral nickel catalysts are also capable of copolymerizing ethylene with 1, 7-octadiene and polar comonomers such as 6-bromo-1-hexene and methyl-10-undecenoate to give various functional polyolefins

  Supporting Information [Synthesis of important intermediates; Crystal data and structure refinement for complexes Ni⁰, Ni¹ and Ni²; ¹H NMR spectra of homo-and copolymers] is available free of charge at http://yyhx.ciac.jl.cn.

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• 

  Scheme 1  Typical neutral nickel catalysts for olefin polymerization

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  Scheme 2  Synthesis of ligands and complexes

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  Figure 1  Molecular structure of complex Ni⁰

  Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity. Selected bond distances(nm) and angles(°):Ni1—N1=0.1888(2), Ni1—N2=0.1900(2), Ni1—O1=0.1922(2), Ni1—C6=0.1933(3), N1—Ni1—N2=171.08(9), N1—Ni1—O1=93.08(8), N1—Ni1—C6=9342(11), N2—Ni1—O1=86.31(8), N2—Ni1—C6=88.42(11), O1—Ni1—C6=170.11(12)

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  Figure 2  Molecular structure of complex Ni¹

  Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity. Selected bond distances(nm) and angles(°):Ni1—N1=0.1901(5), Ni1—N2=0.1915(5), Ni1—O1=0.1907(4), Ni1—C13=0.1921(6), N1—Ni1—N2=176.2(2), N1—Ni1—O1=92.82(18), N1—Ni1—C13=93.2(2), N2—Ni1—O1=85.82(19), N2—Ni1—C13=88.3(2), O1—Ni1—C13=173.2(2)

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  Figure 3  Molecular structure of complex Ni²

  Thermal ellipsoids are drawn at the 30% probability level, and H atoms are omitted for clarity. Selected bond distances(nm) and angles(°):Ni1—N1=0.1886(2), Ni1—N2=0.1904(3), Ni1—O1=0.1907(2), Ni1—C35=0.1935(3), N1—Ni1—N2=168.47(11), N1—Ni1—O1=93.87(10), N1—Ni1—C35=95.02(13), N2—Ni1—O1=85.55(11), N2—Ni1—C35=88.21(13), O1—Ni1—C35=164.62(13)

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  Figure 4  ¹³C NMR spectrum of PE obtained by Ni¹ at 50 ℃(entry 4, Table 1)

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  Figure 5  ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 3, Table 2)

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  Figure 6  ¹H NMR spectrum(400 MHz, o-dichlorobenzene-d₄, 110 ℃) of copolymer obtained by Ni¹ at 50 ℃(entry 4, Table 2)

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  Table 1.  Ethylene polymerization using neutral nickel catalysts Ni⁰~Ni^3a

  Entry	Catalyst	Temperature/℃	Yield/g	Activity b	Mwc/(kg·mol-1)	Mw/Mnc	Tmd/℃
  1	Ni0	40	0.19	1.5	45.4	2.2	114
  2	Ni0	50	0.72	5.8	25.2	2.3	103
  3	Ni0	60	1.23	9.8	15.1	2.2	96
  4	Ni0	70	0.96	7.7	10.1	2.1	87
  5	Ni1	40	0.22	1.8	51.7	2.2	115
  6	Ni1	50	1.31	10.5	19.4	2.2	97
  7	Ni1	60	0.82	6.6	11.6	2.2	89
  8	Ni1	70	0.53	4.2	8.6	2.1	75
  9e	Ni1	50	2.24	6.0	17.0	2.2	92
  10f	Ni1	50	3.07	4.1	17.0	2.1	92
  11g	Ni1	50	0.016	0.13	4.9	1.8	-
  12	Ni2	40	1.21	9.7	11.6	2.1	81
  13	Ni2	50	1.91	15.3	6.9	2.0	68
  14	Ni2	60	2.24	17.9	4.2	2.3	-
  15	Ni2	70	1.75	14.0	3.5	1.1	-
  16	Ni3	40	0.17	1.4	70.3	2.5	124
  17	Ni3	50	1.11	8.9	27.3	2.4	99
  18	Ni3	60	0.61	4.9	19.3	2.2	93
  19	Ni3	70	trace	-	-	-	-
  20e	Ni3	50	2.52	3.4	16.6	2.1	91
  21f	Ni3	50	2.82	1.9	18.0	2.1	93
  22g	Ni3	50	0.021	0.17	5.4	1.9	-
  a.Polymerizations conditions:toluene, 40 mL; nickel, 5(mol; ethylene, 5×105 Pa; 15 min; b.in the unit of 105 g PE mol-1·Ni-1·h-1; c.determined by GPC vs polystyrene standards; d.determined by DSC; e.reaction time: 45 min; f.reaction time:90 min; g.25(mol PPh3 as additives.

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  Table 2.  Ethylene copolymerization using Ni¹-Ni³ complexes^a

  Entry	Catalyst	Comonomer	Yield/mg	Activityb	Mwc/(kg·mol-1)	Mw/Mnc	Tmd/℃	Xce
  1	Ni1		260	17.7	16.3	1.9	93	nd
  2	Ni1		230	15.3	16.6	2.0	94	nd
  3	Ni1		61	4.1	20.5	1.9	103	0.92
  4	Ni1		150	10.0	15.4	1.8	94	0.82
  5 f	Ni1		49	3.3	16.5	2.2	97	1.01
  6	Ni2		18	1.2	6.0	1.7	-	0.16
  7	Ni2		101	6.7	6.7	1.7	-	0.25
  8	Ni3		301	20.1	14.9	2.0	92	nd
  9	Ni3		252	16.8	17.6	1.7	94	nd
  10	Ni3		72	4.8	16.1	1.9	96	0.91
  11	Ni3		182	12.1	16.5	1.9	95	0.64
  12 f	Ni3		62	4.1	16.8	2.1	93	1.10
  a.Polymerizations conditions:toluene, 40 mL; nickel, 10 (mol; ethylene, 5×105 Pa; 90 min, 600 stoichiometric amount of comonomer; b.in the unit of 103 g PE mol-1·Ni-1·h-1; c.determined by GPC vs. polystyrene standards; d.determined by DSC; e.incorporation ratio was calculated according to 1H NMR spectra; f.1000 stoichiometric amount of comonomer.

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