9,9-Dimethylxanthene-based binuclear phenoxy-imine neutral nickel(II) catalysts for ethylene homo- and copolymerization

Journal of Organometallic Chemistry 836-837 (2017) 34e43

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9,9-Dimethylxanthene-based binuclear phenoxy-imine neutral nickel(II) catalysts for ethylene homo- and copolymerization Wei-Wei Li a, b, Hong-Liang Mu a, *, Jing-Yu Liu a, Yue-Sheng Li c, ** a

State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Chinese Academy of Sciences, Beijing 100049, China c School of Material Science and Engineering, Tianjin University, Tianjin 300072, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 24 November 2016 Received in revised form 27 February 2017 Accepted 4 March 2017 Available online 7 March 2017

Mononuclear neutral nickel catalyst Ni1 and binuclear complexes Ni2 and Ni3 based on rigid 9,9dimethylxanthene frameworks featuring short Ni
Ni distances were synthesized, characterized and applied in ethylene (co)polymerization. Binuclear catalyst Ni2 exhibited higher activity in ethylene polymerization than corresponding mononuclear Ni1, and produce higher molecular weight polymer with a bimodal molecular weight distribution. Catalyst Ni2 is remarkably thermally robust, and still displaying promising activity at 93  C. The polymer microstructure produced by Ni1 reveals a hyperbranched structure with a variety of branch types, while Ni2 product features methyl branches primarily. In the presence of comonomer 1,5-hexadiene, 1,7-octadiene, and methyl 10-undecenoate, mononuclear Ni1 suffers from either low catalytic activity or poor incorporation efficiency, while binuclear catalyst Ni2 effectively enchained these comonomers into the polymer chain, giving copolymer with unique microstructures. © 2017 Elsevier B.V. All rights reserved.

Keywords: Bimetallic catalysts Olefin (co)polymerization Neutral nickel catalysts

1. Introduction Inspired by the preeminent catalytic properties of some enzymes [1e5], in which two or more metal centers are positioned in the vicinity, numerous catalysts including multimetallic centers in close proximity have been synthesized and investigated for various chemical transformations [6e13]. With the rapidly increasing developments of olefin polymerization catalysts based on both early and late transition metals [11,14e18], multinuclear olefin polymerization catalysts are also expected to show exceptional catalytic behaviors, and tremendous efforts have been devoted to this field [19e21]. Cooperative effects in bimetallic group 4 constrained geometry or aryloxy-iminato-type polymerization catalysts have been shown to enhance both the molecular weight in ethylene polymerization and the comonomer incorporation for the copolymerization of ethylene with various comonomers [22e30]. The phenoxyiminato systems, as a well-studied catalyst family

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H.-L. Mu), [email protected] (Y.-S. Li). http://dx.doi.org/10.1016/j.jorganchem.2017.03.006 0022-328X/© 2017 Elsevier B.V. All rights reserved.

[17,31e33], have been adapted into several binuclear late transition metal catalysts as well [12,18], some of which exhibit superior catalytic properties in ethylene polymerizations and copolymerizations relative to their mononuclear counterparts [7,34e46]. Marks [41] introduced binuclear 2,7-diimino-1,8dioxynaphthalene Ni(II) catalysts, and found that a proximate catalytically active Ni site substantially increases activity, and comonomer incorporation in ethylene copolymerization. Agapie [43e45] synthesized a series of rigid terphenyl dinickel bisphenoxyiminato complexes, leading to the superior tolerance toward amine additives for the copolymerization of ethylene and amino olefins. Recently, Osakada and Takeuchi [46] reported a binuclear nickel complex with a double-decker structure, the binuclear system exhibits higher efficiency than corresponding mononuclear analogue in copolymerization of ethylene with terminal dienes and some unsaturated esters. Generally speaking, only a few binuclear phenoxyiminato nickel systems have unambiguously demonstrated the expected binuclear effects. We have previously synthesized binuclear neutral nickel catalyst [((2,6-iPr2C6H3N)CH)C6H3ONi(PPh3)Ph]2, the binuclear complex showed higher catalytic activity and longer lifetime versus their mononuclear analogue in ethylene polymerization [34]. By

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analyzing the correlation between binuclear effects and complex frameworks, designing binuclear complexes with a fixed metalmetal distance and keeping the two metal atoms in close proximity would be more conducive to synergistic action. Herein, we report the design and synthesis of phenoxy-imine binuclear catalysts based on the rigid 9,9-dimethylxanthene backbone, and explore their application in ethylene (co)polymerization. Binuclear catalyst Ni2 shows strikingly different polymerization characteristics as compared with mononuclear Ni1 and gives distinct polymeric products. 2. Experimental 2.1. General procedures and materials All manipulations of air- and/or moisture-sensitive compounds were carried out using standard Schlenk and glovebox techniques under a dry nitrogen atmosphere. Toluene, n-hexane, diethyl ether, tetrahydrofuran, and methylene dichloride, N,N-dimethylformamide (DMF) were purified by an MBraun solvent purification system (SPS), and pyridine was distilled from sodium/benzophenone ketyl under nitrogen prior to use. n-BuLi (2.4 M), 3,4-2H-dihydropyran (3,4-DHP), 9,9-dimethylxanthene, trimethyl chlorosilane (TMSCl), trimethyl borate were purchased from commercial vendors and directly used without purification. N,N,N0 ,N0 -Tetramethylethylenediamine (tmeda) and 2,6-diisopropylaniline (DIPA) were distilled after drying via calcium hydride for two days. Complex (pyridine)2Ni(CH3)2 was synthesized by a modified literature procedure [47] and stored at
30  C in a glovebox prior to use. Commercially available ethylene of 99.99% purity was directly used without further purification. The NMR spectra of polyethylene samples were all recorded on a Varian Unity 400 MHz spectrometer with o-dichlorobenzene-d4 or 1,1,2,2-tetrachloroethane-d2 as the solvent at 120  C or 60  C. All 1H and 13C NMR spectra of small organic and organometallic compounds were obtained on a Bruker 300 MHz spectrometer or a Varian Unity 400 MHz spectrometer at ambient temperature with CDCl3, C6D6 or dimethylsulfoxide-d6 (DMSO-d6) as the solvent. Elemental analyses were performed on an enlemental Vario EL spectrometer. The DSC measurements were performed on a Perkin-Elmer Pyris 1 differential scanning calorimeter at a heating rate of 10  C/min. The weight-average molecular weight (Mw) and the polydispersity index (PDI) of polyethylene samples were determined via high-temperature 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). 2.2. Procedure for ethylene (co)polymerization A 200 mL autoclave was heated under vacuum to 140  C for 4 h and was then cooled to the desired reaction temperature. The vessel was purged three times with ethylene and was charged with 90 mL toluene under vacuum. Then, a solution of catalyst in toluene (10 mL) was added by using a dry syringe for ethylene polymerization. For ethylene copolymerization, a solution of commoner was injected into the reactor before a solution of catalyst in toluene (10 mL) was added by using a dry syringe. The total reaction volume was 100 mL. The reaction apparatus was then filled with ethylene and pressurized to the prescribed ethylene pressure immediately. The mixture was stirred for 20 min for ethylene polymerization, and copolymerization for 30 min or 60 min under prescribed temperature. After reaction, the stirring motor 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 weight under vacuum.

35

2.3. Synthesis of ligands L1
3 2.3.1. 4-Hydroxy-5-trimethylsilyl-9,9-dimethylxanthene (1b) To a pale yellow solution of 9,9-dimethylxanthene (4.2 g, 20.0 mmol) and tmeda (6.0 mL, 40.0 mmol) in dry diethyl ether (40.0 mL) was added n-butyllithium (2.4 M) in n-hexane (18.0 mL, 43.2 mmol) dropwise at
78  C under nitrogen. The resulting orange suspension was stirred at room temperature for 8 h. The suspension was cooled to
78  C, and then trimethyl chlorosilane (1.8 mL, 20.8 mmol) in hexane was added dropwise. After stirring overnight at room temperature, trimethyl borate (3.0 mL, 27 mmol) was added to the suspension cooled to
78  C. The reaction mixture was allowed to warm to room temperature and stirred for 4 h. The solvents were removed under reduced pressure. To the residue, cooled to 0  C, 35% hydrogen peroxide (6.0 mL, 60.0 mmol) in THF (50 mL) was added dropwise. The mixture was stirred at room temperature for 4 h. 2 M (1 M ¼ 1 mol/L) aqueous hydrochloric acid (80 mL) was added. After stirring at room temperature for 2 h, the aqueous layer was separated and extracted with ethyl acetate (3  50 mL). The organic layer and extracts were combined and washed with 1 M aqueous hydrochloric acid and brine. The resulting light yellow solution was dried over Na2SO4 and then concentrated to dryness. Analytically pure sample was afforded as white solid in 52% yield by silica gel column chromatography using hexane-AcOEt (10/1 (v/v)) as eluent. 1H NMR (400 MHz, DMSO-d6) d 9.46 (s, 1H, OH), 7.52 (dd, J ¼ 7.8, 1.6 Hz, 1H, ArH), 7.26 (dd, J ¼ 8.0, 1.6 Hz, 1H, ArH), 7.07 (t, J ¼ 7.2, 1H, ArH), 6.91 (dd, J ¼ 7.9, 1.7 Hz, 1H, ArH), 6.86 (t, J ¼ 7.8 Hz, 1H, ArH), 6.75 (dd, J ¼ 7.6, 1.7 Hz, 1H, ArH), 1.54 (s, 6H, C(CH3)2), 0.37 (s, 9H, Si(CH3)3). 13C NMR (101 MHz, DMSO-d6) d 154.32 (ArOH), 145.28 (Ar), 138.74 (Ar), 132.61 (Ar), 130.81 (Ar), 128.68 (Ar), 127.74 (Ar), 126.12 (Ar), 122.81 (Ar), 122.50 (Ar), 115.73 (Ar), 113.54 (Ar), 33.75 (C(CH3)2), 31.99 (C(CH3)2),
1.02 (Si(CH3)3). Anal. Calcd for C18H22O2Si: C, 72.44; H, 7.43. Found: C, 72.35; H, 7.55. 2.3.2. 4,5-Dihydroxy-9,9-dimethylxanthene (2b) 2b was synthesized from 9,9-dimethylxanthene according to literature [48]. Analytically pure sample was afforded in 83% yield by silica gel column chromatography using hexane-AcOEt (3/1 (v/ v)) as eluent. 1H NMR (400 MHz, DMSO-d6) d 9.14 (s, 2H), 7.04e6.88 (m, 4H), 6.82e6.67 (m, 2H), 1.54 (s, 6H, CH3). 2.3.3. 3-Formyl-4-hydroxy-5-trimethylsilyl-9,9-dimethylxanthene (1d) 1b (2.9 g, 12.0 mmol) was dissolved in ethyl acetate (10 mL) and catalytic amount of pyridinium p-toluenesulfonate (p-TSA) was added at room temperature. 3,4-Dihydro-2H-pyran (DHP) (4.2 g, 50.0 mmol) in ethyl acetate (10 mL) was added dropwise to the solution at 0  C. The reaction was stirred at RT overnight. The superfluous DHP and ethyl acetate were removed under reduced pressure. Ethyl acetate (50 mL) was added to dilute the residue. The organic layer was washed with water (2  100 mL), saturated sodium bicarbonate (2  100 mL), brine (2  100 mL) and dried over MgSO4. Analytically pure DHP ether was afforded in 94% yield as colorless oil by silica gel column. n-BuLi in hexane (5.0 mL, 12.0 mmol) was added to the DHP ether (3.56 g, 10.2 mmol) in dry Et2O (30 mL) at 0  C under nitrogen and the mixture was stirred overnight to afford a white suspension. This suspension was then cooled to
78  C and DMF (1.0 mL, 13.0 mmol) was added. After 5 min, the mixture was placed in a water bath and stirred for an additional hour. The reaction was quenched with saturated aqueous NH4Cl. The crude product was extracted with Et2O and the solvent was removed under reduced pressure. Concentrated aqueous HCl (ca. 3 mL) was added to the residue (the o-formylated pyranyl ether) in THF (15 mL) at room

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temperature. The mixture was stirred for 1 h and then diluted with H2O. The product was extracted with ethyl acetate and washed with saturated NaHCO3. The organic layer was dried over Mg2SO4. 1d was afforded as yellowish powder in 82% yield after purification by flash column chromatography (hexane/EtOAc ¼ 2/1). 1H NMR (400 MHz, DMSO-d6) d 11.12 (s, 1H, OH), 10.04 (s, 1H, CHO), 7.57 (dd, J ¼ 7.8, 1.5 Hz, 1H, ArH), 7.49 (d, J ¼ 8.0, 1H, ArH), 7.29 (dd, J ¼ 7.1, 1.5 Hz, 1H, ArH), 7.21 (d, J ¼ 8.4, 1H, ArH), 7.13 (t, J ¼ 7.4 Hz, 1H, ArH), 1.59 (s, 6H, C(CH3)2), 0.37 (s, 9H, Si(CCH3)3). 13C NMR (101 MHz, DMSO-d6) d 196.27 (CHO), 153.47 (Ar), 148.96 (Ar), 138.72 (Ar), 137.53 (Ar), 133.06 (Ar), 128.03 (Ar), 127.75 (Ar), 126.30 (Ar), 125.86 (Ar), 123.56 (Ar), 120.11 (Ar), 117.01 (Ar), 34.49 (C(CH3)2), 31.70 (C(CH3)2),
1.00 (Si(CH3)2). Anal. Calcd for C19H22O3Si: C, 69.90; H, 6.79; Found: C, 69.80; H, 6.84. 2.3.4. 3,6-Diformyl-4,5-dihydroxy-9,9-dimethylxanthene (2d) 2d was afforded in 63% yield as yellow powder after purification by flash column chromatography (hexane/EtOAc ¼ 2/1) by using the similar method of synthesizing 1d. 1H NMR (400 MHz, CDCl3) d 11.18 (s, 2H, OH), 9.89 (s, 2H, CHO), 7.32 (d, J ¼ 8.3 Hz, 2H, ArH), 7.04 (d, J ¼ 8.3 Hz, 2H, ArH), 1.65 (s, 6H, C(CH3)2). 13C NMR (101 MHz, CDCl3) d 195.89 (ArCHO), 150.42 (Ar), 138.67 (Ar), 136.98 (Ar), 126.98 (Ar), 119.66 (Ar), 116.50 (Ar), 35.71 (C(CH3)2), 31.44 (C(CH3)2). Anal. Calcd for C17H14O5: C, 68.45; H, 4.73. Found: C, 68.38; H, 4.78. 2.3.5. Ligand L1 To an ethanol (5 ml) solution of 1d (0.63 g, 1.9 mmol) was added a catalytic amount of pyridinium p-toluenesulfonate (p-TSA) and excess 2,6-diisopropylaniline. The mixture was stirred for 12 h at room temperature. The yellow solid that precipitated was filtered, washed with cold ethanol and dried to afford the ligand. Ligand L1 was isolated as yellow solid in 79% yield. 1H NMR (400 MHz, CDCl3) d 13.17 (s, 1H, OH), 8.30 (s, 1H, CH¼N), 7.46 (dd, J ¼ 7.8, 1.6 Hz, 1H, ArH), 7.37 (dd, J ¼ 7.2, 1.6 Hz, 1H, ArH), 7.22e7.17 (m, 3H, ArH), 7.15e7.06 (m, 2H, ArH), 7.03e6.98 (m, 1H, ArH), 3.03 (sept, J ¼ 6.8 Hz, 2H, CH(CH3)2), 1.67 (s, 6H, C(CH3)2), 1.18 (d, J ¼ 6.9 Hz, 12H, CH(CH3)2), 0.47 (s, 9H, Si(CH3)3). 13C NMR (101 MHz, CDCl3) d 166.48 (C¼N), 154.78 (Ar), 150.35 (Ar), 146.50 (Ar), 140.53 (Ar), 138.94 (Ar), 134.85 (Ar), 133.33 (Ar), 128.33 (Ar), 127.90 (Ar), 127.40 (Ar), 125.54 (Ar), 125.39 (Ar), 123.28 (Ar), 123.21 (Ar), 117.34 (Ar), 115.43 (Ar), 34.87 (C(CH3)2), 32.17 (C(CH3)2), 28.24 (CH(CH3)2), 23.77 (CH(CH3)2),
0.90 (Si(CH3)3). Anal. Calcd for C31H39NO2Si: C, 76.65; H, 8.09; N, 2.88. Found: C, 76.60; H, 8.14; N, 2.87. 2.3.6. Ligand L2 Ligand L2 was isolated as yellow solid in 81% yield using the same procedure as that of L1, except that 2d was used in place of 1d. 1 H NMR (400 MHz, CDCl3) d 13.53 (s, 2H, OH), 8.32 (s, 2H, CH¼N), 7.20 (m, 6H, ArH), 7.14 (d, J ¼ 8.3 Hz, 2H, ArH), 7.03 (d, J ¼ 8.3 Hz, 2H, ArH), 3.05 (sept, J ¼ 6.9 Hz, 4H, CH(CH3)2), 1.73 (s, 6H, C(CH3)2), 1.18 (d, J ¼ 6.9 Hz, 24H, CH(CH3)2). 13C NMR (101 MHz, CDCl3) d 166.29 (CH¼N), 150.45 (Ar), 146.25 (Ar), 139.21 (Ar), 138.98 (Ar), 133.71 (Ar), 125.68 (Ar), 123.39 (Ar), 117.31 (Ar), 115.51 (Ar), 35.26 (C(CH3)2), 32.08 (C(CH3)2), 28.27 (CH(CH3)2), 23.66 (CH(CH3)2). Anal. Calcd for C41H48N2O3: C, 79.83; H, 7.84; N, 4.54. Found: C, 79.80; H, 7.88; N, 4.51. 2.3.7. Ligand L3 2,6-[3,5-(CF3)2C6H3]2C6H3NH2 was synthesized according to reported literature [49]. Condensation of 2d with 2,6-[3,5(CF3)2C6H3]2C6H3NH2 in refluxing toluene gave rise to L3 in 92% yield. 1H NMR (400 MHz, CDCl3) d 11.68 (s, 2H, OH), 8.01 (s, 2H, CH¼N), 7.84 (s, 8H, ArH), 7.79 (s, 4H, ArH), 7.55e7.43 (m, 6H, ArH), 6.78 (d, J ¼ 8.3 Hz, 2H, ArH), 6.63 (d, J ¼ 8.3 Hz, 2H, ArH), 1.54 (s, 6H,

CH3). 19F NMR (376 MHz, CDCl3) d
62.86. 13C NMR (101 MHz, CDCl3) d 169.54 (CH¼N), 149.82 (Ar), 146.10 (Ar), 141.12 (Ar), 138.97 (Ar), 134.68 (Ar), 132.20 (dd, J ¼ 41.2, 25.8 Hz, CCF3), 131.47 (Ar), 130.18 (Ar), 126.56 (Ar), 125.31 (Ar), 124.60 (Ar), 121.89 (Ar), 121.29 (Ar), 116.66 (Ar), 115.62 (Ar), 35.27 (C(CH3)2), 31.35 (C(CH3)2). Anal. Calcd for C61H32F24N2O3: C, 56.49; H, 2.49; N, 2.16. Found: C, 56.43; H, 2.52; N, 2.14. 2.4. Synthesis of nickel complexes Ni1
Ni3 2.4.1. Complex Ni1 According to the literature [50], the nickel methyl pyridine complex Ni1 was prepared in excellent yields by adding dropwise the toluene solution of (pyridine)2NiMe2 (0.295 g, 1.2 mmol) to toluene solution of ligand L1 (0.49 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 h, the mixture was filtrated to remove nickel black, and the filtrate was directly taken to dryness, affording complex Ni1 in 95% yield as dark red solid. 1H NMR (400 MHz, C6D6) d 8.84 (d, J ¼ 5.0 Hz, 2H, PyH), 7.58 (s, 1H, CH¼N), 7.35 (dd, J ¼ 7.0, 1.2 Hz, 1H, ArH), 7.27 (dd, J ¼ 7.7, 1.3 Hz, 1H, ArH), 7.15e7.08 (m, 3H, ArH), 7.02 (t, J ¼ 7.4 Hz, 1H, ArH), 6.71 (d, J ¼ 8.5 Hz, 1H, ArH), 6.65 (t, J ¼ 7.6 Hz, 1H, ArH), 6.48 (d, 1H, ArH), 6.39 (t, J ¼ 7.2 Hz, 2H, ArH), 4.22 (sept, J ¼ 6.8 Hz, 1H, CH(CH3)2), 1.53 (d, J ¼ 6.9 Hz, 6H, CH(CH3)2), 1.50 (s, 6H, C(CH3)2), 1.14 (d, J ¼ 6.8 Hz, 6H, CH(CH3)2), 0.08 (s, 9H, Si(CH3)3),
0.76 (s, 3H, NiCH3). 13C NMR (101 MHz, C6D6) d 166.19 (C¼N), 158.75 (Ar), 156.61 (Ar), 152.45 (Ar), 150.94 (Ar), 143.85 (Ar), 141.71 (Ar), 135.72 (Ar), 133.34 (Ar), 133.18 (Ar), 129.61 (Ar), 126.87 (Ar), 124.38 (Ar), 123.98 (Ar), 123.28 (Ar), 119.07 (Ar), 110.67 (Ar), 35.16 (C(CH3)2), 32.22 (C(CH3)2), 28.92 (CH(CH3)2), 25.29 (CH(CH3)2), 23.59 (CH(CH3)2),
0.33 (Si(CH3)3),
5.45 (NiCH3). Anal. Calcd for C37H46N2NiO2Si: C, 69.70; H, 7.27; N, 4.39. Found: C, 69.81; H, 7.20; N, 4.32. 2.4.2. Complex Ni2 The nickel methyl pyridine complex Ni2 was prepared in excellent yields by adding dropwise the toluene solution of (pyridine)2NiMe2 (0.29 g, 1.2 mmol) to toluene solution of ligand L2 (0.32 g, 0.52 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 h, the mixture was filtrated to remove nickel black, and the filtrate was directly taken to dryness, affording complex Ni2 as dark red solid. The purified product was obtained as dark red crystal in 90% yield within one day after layering a solution of the crude product in toluene (2.0 mL) with hexane (6.0 mL). 1H NMR (400 MHz, C6D6) d 8.52 (d, J ¼ 5.4 Hz, 4H, PyH), 7.45 (s, 2H, CH¼N), 7.16 (s, 6H, ArH), 6.80 (t, J ¼ 7.5 Hz, 2H, PyH), 6.73 (d, J ¼ 8.5 Hz, 2H, ArH), 6.58 (t, J ¼ 6.7 Hz, 4H, ArH), 6.49 (d, J ¼ 8.5 Hz, 2H, ArH), 4.36 (sept, J ¼ 6.9 Hz, 4H, CH(CH3)2), 1.63 (d, J ¼ 6.9 Hz, 12H, CH(CH3)2), 1.60 (s, 6H, C(CH3)2), 1.16 (d, J ¼ 6.8 Hz, 12H, CH(CH3)2),
0.72 (s, 6H, NiCH3). 13C NMR (101 MHz, C6D6) d 165.40 (ArCH¼N), 158.36 (Ar), 152.36 (Ar), 150.26 (Ar), 143.51 (Ar), 141.32 (Ar), 135.56 (Ar), 131.82 (Ar), 129.37 (Ar), 126.52 (Ar), 125.97 (Ar), 123.69 (Ar), 119.15 (Ar), 110.54 (Ar), 35.48 (C(CH3)2), 31.49 (C(CH3)2), 28.72 (CH(CH3)2), 25.04 (CH(CH3)2), 23.54 (CH(CH3)2),
8.31 (NiCH3). Anal. Calcd for: C53H62N4Ni2O3: C, 69.16; H, 6.79; N, 6.09. Found: C, 68.30; H, 6.75; N, 6.00. 2.4.3. Complex Ni3 According to the similar procedure synthesizing Ni2, Ni3 was obtained as dark red crystal in 91% yield. 1H NMR (400 MHz, C6D6) d 8.18 (s, 8H, ArH), 8.04 (d, J ¼ 5.5 Hz, 4H, PyH), 7.77 (s, 4H, ArH), 6.96 (dt, J ¼ 12.2, 5.8 Hz, 6H, ArH), 6.77 (t, J ¼ 7.4 Hz, 2H, ArH), 6.70 (s, 2H, ArH), 6.31 (t, J ¼ 6.5 Hz, 4H, ArH), 6.21 (dd, J ¼ 22.2, 8.6 Hz,

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4H), 1.24 (s, 6H, C(CH3)2),
0.97 (s, 6H, NiCH3). 19F NMR (376 MHz, C6D6) d
62.56. 13C NMR (101 MHz, C6D6) d 167.05 (CH¼N), 158.23 (Ar), 151.18 (Ar), 150.51 (Ar), 142.35 (Ar), 141.93 (Ar), 135.14 (Ar), 133.53 (Ar), 131.60 (dd, J ¼ 57.8, 42.0 Hz, CCF3), 131.57 (Ar), 130.61 (Ar), 126.15 (Ar), 126.15 (Ar), 125.04 (Ar), 124.80 (Ar), 122.93 (Ar), 122.33 (Ar), 120.77 (Ar), 117.55 (Ar), 111.05 (Ar), 34.79 (C(CH3)2), 30.81 (C(CH3)2),
8.95 (NiCH3). Anal. Calcd for: C73H46F24N4Ni2O3: C, 54.78; H, 2.90; N, 3.50. Found: C, 54.90; H, 2.83; N, 3.45. 2.5. X-ray crystallography 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 u scan mode (186 K) on a Bruker Smart APEX diffractometer with a CCD detector using Mo Ka radiation (l ¼ 0.71073 Å). 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. 3. Results and discussion 3.1. Synthesis of ligands The trimethylsilyl group in ligand L1 was introduced to mimic the steric effect in binuclear ligand L2. 9,9-Dimethylxanthene was lithiated with 2.0 equiv n-BuLi in the presence of 2.0 equiv tetramethylethylenediamine (tmeda) in hexane, reacted with 1.0 equiv

37

trimethylchlorosilane, 1.0 equiv B(OMe)3 and 3.0 equiv 35% hydrogen peroxide. 4-Hydroxy-5-trimethylsilyl9,9dimethylxanthene (1b) was obtained after further treated with dilute hydrochloric acid in one-pot. Then, 3-formyl-4-hydroxy-5trimethylsilyl-9,9-dimethylxanthene (1d) was synthesized via hydroxyl protection, formylation, and deprotection. Condensation of 1d with 2,6-diisopropylamine gave rise to the ligand L1 in 32% overall yield (Scheme 1). 4,5-Dihydroxy-9,9-dimethylxanthene (2b) was prepared via a modified procedure according to the literature [48]. Subsequently, adopting the similar route to synthesize ligand L1, L2 was obtained in 42% overall yield. Similarly, ligand L3 was synthesized by condensing 2d with substituted terphenyl amine, which was conveniently prepared by Suzuki coupling of 2,6-dibromoaniline with 3,5-bis(trifluoromethyl)benzeneboronic acid. 3.2. Synthesis of nickel complexes According to the literature, the nickel methyl pyridine complexes Ni1¡Ni3 were prepared in excellent yields by adding dropwise toluene solutions of ligands L1¡L3 to the toluene solution of (pyridine)2Ni(Me)2 [47] with vigorous stirring at room temperature, respectively (Scheme 2) [50]. All these complexes were fully characterized by 1H and 13C NMR spectra and elemental analysis. In the 1H NMR spectra of the nickel methyl complexes Ni1-Ni3, the splitting of CH(CH3)2 into two doublets unveils the successful coordination. The signals corresponding to these methyl groups bound to the nickel centers were found in the upfield region of
0.59 and
0.97 ppm. In their 13C NMR spectra, resonances between
5.45 and
8.95 ppm are assigned to these methyl groups.

Scheme 1. Synthesis of the ligands based on 9,9-dimethylxanthene.

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Scheme 2. Synthesis of mono- and binuclear nickel complexes.

3.3. Structure of nickel complexes To further confirm the structures of these complexes, single crystals of Ni1¡Ni3 suitable for X-ray diffraction analysis were grown within one day after layering a solution of these complexes in toluene with n-hexane (1/3, v/v), respectively. The crystallographic data together with the collection and refinement parameters are summarized in Table 1. In the solid state, these complexes adopt a slightly distorted square planar coordination geometry, and the methyl groups are located trans to the oxygen atoms while

pyridines trans to the N-aryl groups (Figs. 1e3), just as the structures of previously reported phenoxyiminato nickel complexes [49]. Both bond lengths and angles of Ni1 bear a striking resemblance to similar salicylaldiminato Ni(II) catalysts with pyridines as auxiliary ligands [51]. As for complex Ni2, the Ni
Ni distance is 6.92 Å, similar with the value (7.1 Å) reported by Agapie [44]. The distance between the two parallel planes of pyridines is ca 3.5 Å, prominently indicative of a p
p stacking interaction [52]. The appropriate Ni
Ni distance together with the rigidity of the ligand in Ni2 makes intramolecular cooperativity possible [44,53]. In Ni3,

Table 1 Crystal data and structure refinement for complexes Ni1¡Ni3.

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a ( ) b ( ) g ( ) V (Å3), Z Densitycalcd (Mg/m3) Absorption coefficient (mm
1) F (000) Crystal size (mm) q range for data collection ( ) Reflections collected Independent reflections Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2s (I)]: R1, wR2 rmax and min, e/Å3

Ni1

Ni2

Ni3

C37H46N2NiO2Si 637.56 monoclinic P2(1)/C 13.7619(13) 22.633(2) 11.9038(11) 90.00 110.554(2) 90.00 3471.7(6), 4 1.220 0.627 1360 0.36  0.30  0.18 1.80 to 23.27 14614 4984 (Rint ¼ 0.0649) 4984/0/388 1.003 0.0511, 0.1154 0.398,
0.260

C53H62N4Ni2O3 920.49 monoclinic P2(1)/n 17.5790(17) 8.0600(8) 33.615(3) 90.00 98.939(2) 90.00 4705.0(8), 4 1.299 0.847 1952 0.35  0.26  0.20 1.41 to 23.26 22861 6745 (Rint ¼ 0.0988) 6745/0/559 1.014 0.0637, 0.1344 0.410,
0.292

C73H46F24N4Ni2O3 1600.56 triclinic P
1 14.078(3) 15.015(3) 18.012(3) 65.914(4) 86.690(3) 82.784(4) 3448.3(12) 1.542 0.662 1616 0.38  0.38  0.20 1.46 to 26.04 18618 13210(Rint ¼ 0.0315) 13210/19/974 1.047 0.0762, 0.2421 1.846,
0.605

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43

Fig. 1. Molecular structures of Ni1 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni1
N1 ¼ 1.887(3), Ni1
N2 ¼ 1.908(3), Ni1
O1 ¼ 1.914(3), Ni1
C27 ¼ 1.928(4), N1
Ni1
N2 ¼ 163.63(13), N1
Ni1
O1 ¼ 94.15(12), N2
Ni1
O1 ¼ 87.87(12), N1
Ni1
C27 ¼ 93.47(15), N2
Ni1
C27 ¼ 87.63(15), O1
Ni1
C27 ¼ 167.46(14).

Fig. 2. Molecular structures of Ni2 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni1
O1 ¼ 1.881(3), Ni1
N1 ¼ 1.914(4), Ni1
C52 ¼ 1.927(5), Ni1
N3 ¼ 1.928(4), Ni2
N2 ¼ 1.908(4), Ni2
N4 ¼ 1.923(5), Ni2
C53 ¼ 1.926(5), Ni2
O2 ¼ 1.927(3), O1
Ni1
N1 ¼ 91.39(16), O1
Ni1
C52 ¼ 172.4(2), N1
Ni1
C52 ¼ 93.0(2), O1
Ni1
N3 ¼ 85.41(16), N1
Ni1
N3 ¼ 172.31(18), C52
Ni1
N3 ¼ 91.0(2), N2
Ni2
N4 ¼ 177.01(18), N2
Ni2
C53 ¼ 90.9(2), N4
Ni2
C53 ¼ 90.4(2), N2
Ni2
O2 ¼ 90.24(17), N4
Ni2
O2 ¼ 88.81(17), C53
Ni2
O2 ¼ 173.1(2), C8
O2
Ni2 ¼ 118.9(3).

the spacing of two Ni atoms is the same as that in Ni2, but the metal centers are congested remarkably after introducing sterically hindered terphenyl amine, which facilitates the dissociation of additional stabilizing ligands [31].

3.4. Ethylene homopolymerization All these complexes (Ni1¡Ni3) studied here were capable of producing polyethylenes as single-component catalysts (Table 2). Ethylene polymerizations were conducted under different reaction temperature and/or ethylene pressure, and distinct polymerization characteristics in terms of catalytic activity, polymer molecular weight, and melting temperature of the polymer were observed using these mono- and binuclear catalysts. It was known that the salicylaldiminato neutral nickel catalysts reported by Grubbs are highly active for ethylene polymerization

39

under moderate temperature, but suffer from serious deactivation under elevated temperatures (above 80  C) [32]. We note that binuclear complex Ni2 exhibits ~16 times higher activity than Ni1 at 83  C (Table 2, entry 10 vs entry 3). Catalyst Ni2 still availably produces polyethylene at high activity of 1.62  105 g of PE mol
1 Ni
1 h-1 at 93  C, while Ni1 decomposes to yield a black nickel precipitate and trace amount of polyethylene (Table 2, entry 11 vs entry 4). These results strongly illustrate a marked improvement in catalyst thermal stability for the binuclear Ni2 versus mononuclear Ni1. Considering that the formation of bisligated nickel species is an important deactivation pathway for neutral nickel catalysts, this may indicate a lower tendency in the formation of bisligated nickel species for Ni2 complex. Under optimal conditions (10 atm ethylene, 63  C), complex Ni2 is two times more active than Ni1 for ethylene polymerization and produces polyethylene that has ~15 times higher Mw under the optimal condition (Table 2, entry 9 vs entry 2). Molecular weight distributions (MWDs) for ethylene polymerization with Ni1 range from 1.1 to 1.3, indicative of singlesite behavior. But for Ni2, the broad range (Mw/Mn ¼ 5.2e8.5) implies the presence of multi-site active species, which was consistent with the result of GPC analysis. GPC traces of the polymer show a bimodal molecular weight distribution (Fig. 4), and a extremely distinct difference between the Mw's (Mw1 z 10000, Mw2 z 250000) is observed. This phenomenon reminds us a possible incomplete initiation process for the binuclear system, especially considering the low steric profile of the catalyst. Without the addition of any cocatalyst, one of the two Ni centers in complex Ni2 may keep intact, while the other could be successfully initiated to generate the active species (Scheme 3, path A). Under this circumstance, for the initiated active center, the other Ni center plays a role of the bulky group, which suppresses chain transfer processes and results in the formation of high-molecular-weight (Mw2) polymers. Alternatively, both the two Ni centers are initiated to generate the intermediate Ni2-B, in which the two active centers possess less sterically congested coordination environment when compared with that in Ni2-A. The smaller steric hindrance, which is in favor of chain transfer, is responsible for the formation of lower molecular weight (Mw1) polymers. To verify this assumption and gain further insight into the mechanism, Ni(cod)2 was used as a cocatalyst to sequester pyridine [54,55]. With the addition of 2 equiv of Ni(cod)2, the percentage of polymers with lower Mw1 markedly increased (Fig. 4), which was in good agreement with more intermediate Ni2-B generated in the catalytic system. Additionally, a more sterically congested complex Ni3 was synthesized by introducing sterically hindered 2,6-diaryl aniline that is beneficial to the dissociation of pyridine. The molecular weight distribution of the resulting polymers ranges between 1.8 and 1.9, indicative of a single-site nature of the catalytic system. This means that both Ni centers were initiated (Scheme 3, path B), consequently leading to the formation of single-site active center Ni3-B. These results further evidence that the proposed mechanism in Scheme 3 was reasonable. Interestingly, the molecular weight of polymer produced by Ni3 with bulky imine substituents was lower when compared with that of Ni2. This may indicate that the substituent in the ortho position of the phenoxy moiety is more effective than imine moieties in producing high molecular weight polymer, or that the electron-withdrawing group accelerates chain transfer process. Then, question arises whether the striking differences in polymerization characteristics between mono- and dinuclear catalysts completely derived from the aforementioned steric factor. After examining the GPC data of the polymer produced by Ni2, however, we found that even the lower polymer Mw1 (z10000), not to mention Mw2 (z250000), was still ~8  higher than that afforded

40

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43

Fig. 3. Solid-state structure of Ni3 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ni2¡N2 ¼ 1.889(4), Ni2
O2 ¼ 1.898(3), Ni2
N4 ¼ 1.928(4), Ni2
C7 ¼ 1.953(5), Ni1
O1 ¼ 1.888(3), Ni1
N1 ¼ 1.906(4), Ni1
N3 ¼ 1.929(4), Ni1
C17 ¼ 1.972(5), N2
Ni2
O2 ¼ 92.33(15), N2
Ni2
N4 ¼ 170.71(19), O2
Ni2
N4 ¼ 84.99(16), N2
Ni2
C7 ¼ 94.18(19), O2
Ni2
C7 ¼ 171.60(18), N4
Ni2
C7 ¼ 89.4(2), O1
Ni1
N1 ¼ 92.85(15), O1
Ni1
N3 ¼ 85.71(16), N1
Ni1
N3 ¼ 160.71(18), O1
Ni1
C17 ¼ 162.9(2), N1
Ni1
C17 ¼ 94.5(2), N3
Ni1
C17 ¼ 92.3(2).

by mononuclear counterpart Ni1 (Mw ¼ 1300). By comparing the proposed structure of species Ni2-B with mononuclear Ni1, it is likely that the interactions between the two metal centers confined in proximity by the rigid ligand did play a vital role in enhancing the molecular weight of the polymer product. To gain more information on the influences of binuclear effects, we have explored and compared the polymer microstructures originated from mono- and binuclear catalysts. Intriguingly, 13C NMR analysis of the polymer produced by Ni1 at 63  C reveals a variety of branch types, including ethyl, n-propyl,

sec-butyl, and predominant methyl and  C4 linear branches (Fig. 5), which is consistent with extensive chain walking [56]. Assignments are numbered according to references [57,58]. Chain ends are assigned with S1
S4. Branches are labeled as xBy, where y is the branch length and x is the carbon, starting from the methyl end with 1. The methine groups for the different branch lengths are labeled with *By. A and B are the methyl groups of a sec-butyl branch. The presence of amounts of sec-butyl groups as the simplest and detectable form of branch-on-branch indicates hyperbranched structures, which are very rare in olefin

Table 2 Ethylene polymerization using Ni1¡Ni3 based on 9,9-dimethylxanthene frameworks.a Entry

Catalyst

Pressure (atm)

Temperature ( C)

Yield (g)

Activityb

Mwc

Mw/Mnc

Tmd ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15e 16 17 18

Ni1 Ni1 Ni1 Ni1 Ni1 Ni1 Ni1 Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni2 Ni3 Ni3 Ni3

10 10 10 10 5 20 30 10 10 10 10 5 20 30 10 10 10 10

43 63 83 93 63 63 63 43 63 83 93 63 63 63 43 43 63 80

0.34 1.02 0.10 trace 0.99 1.91 2.28 0.39 2.20 1.59 0.54 1.21 2.21 3.36 1.28 0.50 1.58 1.22

1.02 3.06 0.30 e 2.97 5.73 6.84 1.17 6.60 4.77 1.62 3.63 6.63 10.1 3.84 1.50 4.74 3.66

1300 900 800 e 800 900 1000 75300 23500 19300 17500 14900 32000 36900 31700 11300 4800 3600

1.3 1.2 1.1 e 1.2 1.2 1.3 8.5 5.4 5.2 5.6 5.2 7.5 7.9 5.1 1.9 1.8 1.8

67 58 57 e 53 60 65 122 117 108 107 106 119 120 n.d. 109 97 89

a b c d e

Reaction conditions: toluene, 100 mL; nickel, 10 mmol; 20 min. In the unit of 105 g PE mol
1 Ni
1 h
1. Determined by GPC vs polystyrene standards. Determined by DSC. 20 mmol of Ni(cod)2 as a cocatalyst.

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43

Mw1 = 10000

41

2

Ni 2 Ni +Ni(cod)2

Mw2 = 250000

3

4

Log Mw

5

6

Fig. 4. GPC profiles of the polyethylenes obtained by Ni2 and Ni2 þ Ni(cod)2 at 43  C.

coordination polymerization [58e60]. However, the Ni2 product has major methyl branches and only low amounts of ethyl branches (Fig. 6). Due to the multi-site nature of catalyst Ni2, this phenomenon should be considered separately. For the high molecular weight part, the sterically encumbered environment may affect the moderately branched polyethylene with shorter branches like previously reported neutral nickel catalysts bearing bulky groups in the ortho position of the phenoxy moiety. For low molecular weight part, it is likely that the adjacent Ni centers are responsible for the marked differences, which is also supported by previous reports in which binuclear effects are beneficial to enhance Mw and to form short branches especially methyl branching [21,41].

Fig. 5. 13C NMR spectrum (400 MHz, o-C6D4Cl2, 120  C) of polymer (Table 2, entry 2) obtained by Ni1 under 10 atm ethylene pressure at 63  C.

3.5. Ethylene copolymerization The typical results of ethylene copolymerization catalyzed by Ni1¡Ni2 with 1,5-hexadiene (1,5-HD), 1,7-octadiene (1,7-OD) and undecylenic acid methyl ester (UAME) are summarized in Table 2. Copolymerizations of ethylene with 1,5-HD were performed at ethylene pressure 5.0 atm with 1000 equiv of 1,5-HD at 63  C (Table 3, entry 4). As has been reported earlier, copolymerization of ethylene with non-conjugated dienes can generate different structures on the polymer backbone, such as cycles, branches and crosslinks. In order to gain more insight into the copolymerization, the microstructure of the copolymers was investigated with 13C

Fig. 6. 13C NMR spectrum (400 MHz, o-C6D4Cl2, 120  C) of polymer (Table 2, entry 9) obtained by Ni2 under 10 atm ethylene pressure at 63  C.

Scheme 3. Possible mechanism for the different molecular weight distribution of the polyethylene produced by binuclear catalysts Ni2 and Ni3.

42

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43

Table 3 Ethylene copolymerization using Ni1¡Ni2 complexes.a Entry Catalyst Comonomer (equiv) 1 2 3 4 5 6 7 8f 9f

Ni1 Ni2 Ni1 Ni2 Ni1 Ni2 Ni2 Ni1 Ni2

e e 1,5-HD/1000 1,5-HD/1000 1,7-OD/500 1,7-OD/500 1,7-OD/1000 UAME/1000 UAME/1000

Yield (g)

Activityb XMc

0.99 1.21 0.37 0.30 e 0.34 0.072 e 0.012

29.7 36.3 3.7 3.0 e 3.4 0.72 e 0.06

e e 0.3 1.1 e 0.27 0.96 e 3.1

M wd

Mw/Mnd Tme ( C)

800 14900 900 4100 e 23800 14000 e 20800

1.2 5.2 1.2 1.6 e 4.2 3.9 e 3.1

53 106 57 105 e 115 108 e 105

Reaction conditions: toluene, 100 mL; nickel, 20 mmol; 63  C; 5 atm; 30 min. In the unit of 104 g PE mol
1 Ni
1 h
1. c Comonomer incorporations determined by13C NMR for entries 4, 6, 7, by 1H NMR for entry 9. d Determined by GPC vs polystyrene standards. e Determined by DSC. f Reaction time was 60 min. a

b

NMR spectroscopy. With respect to binuclear catalyst Ni2, a moderate activity (3.0  104 g mol-1 Ni
1 h
1) was observed. In the 13C NMR spectra (Fig. 7) of samples obtained by Ni2, the signals not observed in the spectrum of polyethylene are clearly assigned to cyclic structures [61e64]. The similar ratio of internal to terminal double bonds in 1H NMR spectra for the copolymer and polyethylene suggests that negligible 1,5-HD was incorporated to form butene branches (Figs. S1e2). What is more, the good solubility of the polymers argues that crosslinked structure was not present in the copolymers as well. These information combined with the 13C NMR results strongly supports that 1,5-HD is incorporated into the polymer as fivemembered rings connected at their 1 and 3 positions to the polymer backbone, which is rare for phenoxyininato neutral nickel catalysts. The ratio of trans and cis conformers of the rings is ca. 2.5, and the total incorporation is about 1.1 mol % according to 13C NMR (Fig. S3). As for Ni1, the catalytic activity was similar to binuclear catalyst Ni2 (Table 3, entry 3 vs entry 4), but the polymer 13C NMR spectrum (Fig. S4) showed only very weak signals for 1,5-HD incorporation as five-membered-ring structures, corresponding to an incorporation less than 0.3 mol%.

The polymerization of ethylene in the presence of 1,7-OD catalyzed by Ni1 yields negligible polymer. In sharp contrast to the mononuclear analogue, binuclear Ni2 yields copolymers at the activity of 3.4  104 g polymer mol
1Ni
1h
1, giving pendant vinyl groups rather than cyclic structures. 1H NMR spectrum showed that the intensity ratio of the signals corresponding to the vinylene (
CH¼CH
) and vinyl (¼CH2) groups at d ¼ 5.48 and 5.01 ppm, respectively, varied from 1:0.25, via 1:0.42, to 1:1.21 (Fig. 8), which definitely illustrated 1,7-OD was incorporated into the polyethylene backbone to yield the repeating unit with a pendant double bonds. 13 C NMR analysis of the copolymer (Table 2, entry 7) confirmed the absence of cyclization (Fig. S5). However, the reason why monoand binuclear catalysts showed such a big difference in catalytic activity here is currently not known. When it comes to copolymerization of ethylene with polar monomer UAME, the difference between mono- and binuclear catalysts was even more apparent. Catalyst Ni1 deactivated and did not generate any polymer in the presence of this comonomer (Table 3, entry 8). Nevertheless, binuclear Ni2 could availably promote the copolymerization reaction (Table 3, entry 9). 1H NMR signals (Fig. 9) at d ¼ 3.68 ppm and d ¼ 2.33 ppm, assigned to
COOCH3 and
CH2COO
, respectively, highlight that UAME was effectively incorporated into the polymer backbone, and the incorporation ratio is 3.1 mol%. According to the copolymerization results of ethylene with comonomers, binuclear catalyst Ni2 shows more prominent properties than mononuclear catalyst Ni1, at least in aspect of catalytic activity and co-monomer incorporating ratio. The enhanced catalytic activity for ethylene/UAME

Fig. 8. 1H NMR spectra (400 MHz, o-C6D4Cl2, 120  C) of polymers produced by Ni2. (a) Table 3, entry 2; (b) Table 3, entry 6; (c) Table 3, entry 7.

Fig. 7. 13C NMR spectra of 1) polyethylene (Table 3, entry 4) and 2) E/1,5-HD copolymer (Table 3, entry 2) obtained by Ni2.

Fig. 9. 1H NMR spectrum (1,1,2,2-tetrachloroethane-d2, 120  C) of ethylene/UAME copolymers produced by Ni2 (Table 3, entry 9).

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43

copolymerization can be explained by the inhibition mechanism proposed by Agapie [45] However, the different performance of Ni2 and Ni1 in ethylene homopolymerization and ethylene/diolefins copolymerization is probably connected with the interaction between the two nickel centers, the mechanism of which will certainly merit further investigation. 4. Conclusions Herein we cover the design, synthesis and characterization of a couple of binuclear neutral nickel catalysts based on 9,9dimethylxanthene frameworks as well as their applications in ethylene (co)polymerization. By contrast experiment, marked differences in polymerization characteristics are observed. Under optimal conditions, binuclear catalyst Ni2 is ~2.2  more active than Ni1 for ethylene polymerization and produces polyethylene that has a 15 times higher Mw with a bimodal molecular weight distribution. Ni1 generates polyethylene with hyperbranched structure, while Ni2 product features major methyl branches. Ni1 achieves very limited comonomer insertion in copolymerizing ethylene with 1,5-HD, while binuclear Ni2 incorporates it into the polyethylene backbone as five-membered rings. Ni1 shows negligible activity for the copolymerization of ethylene with 1,7-OD, while Ni2 gives copolymers with pendant double bonds in good activity. Ni1 deactivates in the presence of polar monomer UAME, while Ni2 effectively promotes the copolymerization. Our results indicate a possible synergistic work of the two nickel centers positioned in close proximity, leading to distinct catalytic property between the binuclear catalyst and its mononuclear counterpart. Further mechanism investigations on the interaction between the two metal centers are in progress. Acknowledgement

[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]

Financial support by National Natural Science Foundation of China (NO. 21304087 and 21274144) was gratefully acknowledged.

[47]

Appendix A. Supplementary data

[48] [49]

Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jorganchem.2017.03.006.

[50]

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