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

Article abstract of DOI:10.1016/j.jorganchem.2017.03.006

Mononuclear neutral nickel catalyst Ni¹ and binuclear complexes Ni² and Ni³ based on rigid 9,9-dimethylxanthene 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.

Full text of DOI:10.1016/j.jorganchem.2017.03.006

Journal of Organometallic Chemistry 836-837 (2017) 34e43
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
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
Mononuclear neutral nickel catalyst Ni1 and binuclear complexes Ni2 and Ni3 based on rigid 9,9-
dimethylxanthene frameworks featuring short NiꢀNi 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 hyper-
branched 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.
Article history:
Received 24 November 2016
Received in revised form
27 February 2017
Accepted 4 March 2017
Available online 7 March 2017
Keywords:
Bimetallic catalysts
Olefin (co)polymerization
Neutral nickel catalysts
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
[17,31e33], have been adapted into several binuclear late transition
metal catalysts as well [12,18], some of which exhibit superior
Inspired by the preeminent catalytic properties of some en-
zymes [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 de-
velopments of olefin polymerization catalysts based on both early
and late transition metals [11,14e18], multinuclear olefin poly-
merization 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 ge-
ometry or aryloxy-iminato-type polymerization catalysts have
been shown to enhance both the molecular weight in ethylene
polymerization and the comonomer incorporation for the copoly-
merization of ethylene with various comonomers [22e30].
catalytic properties in ethylene polymerizations and co-
polymerizations relative to their mononuclear counterparts
[7,34e46]. Marks [41] introduced binuclear 2,7-diimino-1,8-
dioxynaphthalene 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 bisphe-
noxyiminato 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 sys-
tem exhibits higher efficiency than corresponding mononuclear
analogue in copolymerization of ethylene with terminal dienes and
some unsaturated esters.
The phenoxyiminato systems, as a well-studied catalyst family
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
* Corresponding author.
** Corresponding author.
E-mail addresses: muhongliang@ciac.ac.cn (H.-L. Mu), ysli@tju.edu.cn (Y.-S. Li).
http://dx.doi.org/10.1016/j.jorganchem.2017.03.006
0022-328X/© 2017 Elsevier B.V. All rights reserved.

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
35
analyzing the correlation between binuclear effects and complex
2.3. Synthesis of ligands L1ꢀ3
frameworks, designing binuclear complexes with a fixed metal-
metal distance and keeping the two metal atoms in close prox-
imity would be more conducive to synergistic action. Herein, we
report the design and synthesis of phenoxy-imine binuclear cata-
lysts based on the rigid 9,9-dimethylxanthene backbone, and
explore their application in ethylene (co)polymerization. Binuclear
catalyst Ni2 shows strikingly different polymerization characteris-
tics as compared with mononuclear Ni1 and gives distinct poly-
meric products.
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 or-
ange 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 hydro-
chloric acid (80 mL) was added. After stirring at room temperature
for 2 h, the aqueous layer was separated and extracted with ethyl
acetate (3 ꢂ 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)
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-dimethylforma-
mide (DMF) were purified by an MBraun solvent purification sys-
tem (SPS), and pyridine was distilled from sodium/benzophenone
ketyl under nitrogen prior to use. n-BuLi (2.4 M), 3,4-2H-dihy-
dropyran (3,4-DHP), 9,9-dimethylxanthene, trimethyl chlorosilane
(TMSCl), trimethyl borate were purchased from commercial ven-
dors and directly used without purification. N,N,N0,N0-Tetrame-
thylethylenediamine (tmeda) and 2,6-diisopropylaniline (DIPA)
were distilled after drying via calcium hydride for two days. Com-
plex (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 calo-
rimeter at a heating rate of 10 ꢁC/min. The weight-average molec-
ular 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).
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 su-
perfluous 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 ꢂ 100 mL), saturated so-
dium bicarbonate (2 ꢂ 100 mL), brine (2 ꢂ 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
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 polymeri-
zation. 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.

36
W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
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
CH3). 19F NMR (376 MHz, CDCl3)
CDCl3)
(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.
d
ꢀ62.86. 13C NMR (101 MHz,
d
169.54 (CH¼N), 149.82 (Ar), 146.10 (Ar), 141.12 (Ar), 138.97
(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,
2.4. Synthesis of nickel complexes Ni1ꢀNi3
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.
2.4.1. 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%
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),
yield as dark red solid. 1H NMR (400 MHz, C6D6)
d
8.84 (d, J ¼ 5.0 Hz,
7.04 (d, J ¼ 8.3 Hz, 2H, ArH), 1.65 (s, 6H, C(CH3)2). 13C NMR
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,
(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
Si(CH3)3), ꢀ0.76 (s, 3H, NiCH3). 13C NMR (101 MHz, C6D6)
d 166.19
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)
(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.
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)
2.4.2. Complex Ni2
The nickel methyl pyridine complex Ni2 was prepared in
excellent yields by adding dropwise the toluene solution of (pyr-
idine)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 fil-
trated 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
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
(2.0 mL) with hexane (6.0 mL). 1H NMR (400 MHz, C6D6)
d 8.52 (d,
same procedure as that of L1, except that 2d was used in place of 1d.
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
1H 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.
(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.
2.3.7. 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%
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)
yield. 1H NMR (400 MHz, CDCl3)
d
11.68 (s, 2H, OH), 8.01 (s, 2H,
d
8.18 (s, 8H, ArH), 8.04 (d, J ¼ 5.5 Hz, 4H, PyH), 7.77 (s, 4H, ArH),
CH¼N), 7.84 (s, 8H, ArH), 7.79 (s, 4H, ArH), 7.55e7.43 (m, 6H, 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,
6.78 (d, J ¼ 8.3 Hz, 2H, ArH), 6.63 (d, J ¼ 8.3 Hz, 2H, ArH), 1.54 (s, 6H,

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
37
4H), 1.24 (s, 6H, C(CH3)2), ꢀ0.97 (s, 6H, NiCH3). 19F NMR (376 MHz,
trimethylchlorosilane, 1.0 equiv B(OMe)3 and 3.0 equiv 35%
hydrogen peroxide. 4-Hydroxy-5-trimethylsilyl- 9,9-
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.
dimethylxanthene (1b) was obtained after further treated with
dilute hydrochloric acid in one-pot. Then, 3-formyl-4-hydroxy-5-
trimethylsilyl-9,9-dimethylxanthene (1d) was synthesized via hy-
droxyl 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 syn-
thesize ligand L1, L2 was obtained in 42% overall yield. Similarly,
ligand L3 was synthesized by condensing 2d with substituted ter-
phenyl amine, which was conveniently prepared by Suzuki
coupling of 2,6-dibromoaniline with 3,5-bis(trifluoromethyl)ben-
zeneboronic acid.
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
diffractometer with
u
scan mode (186 K) on a Bruker Smart APEX
CCD detector using Mo radiation
a
Ka
(
l
¼ 0.71073 Å). Absorption corrections were performed using the
3.2. Synthesis of nickel complexes
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.
According to the literature, the nickel methyl pyridine com-
plexes Ni1¡Ni3 were prepared in excellent yields by adding drop-
wise toluene solutions of ligands L1¡L3 to the toluene solution of
(pyridine)2Ni(Me)2 [47] with vigorous stirring at room tempera-
ture, 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 co-
ordination. 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.
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 tetra-
methylethylenediamine (tmeda) in hexane, reacted with 1.0 equiv
Scheme 1. Synthesis of the ligands based on 9,9-dimethylxanthene.

38
W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
Scheme 2. Synthesis of mono- and binuclear nickel complexes.
3.3. Structure of nickel complexes
pyridines trans to the N-aryl groups (Figs. 1e3), just as the struc-
tures of previously reported phenoxyiminato nickel complexes
[49]. Both bond lengths and angles of Ni1 bear a striking resem-
blance to similar salicylaldiminato Ni(II) catalysts with pyridines as
auxiliary ligands [51]. As for complex Ni2, the NiꢀNi 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,
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 crystallo-
graphic data together with the collection and refinement parame-
ters 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
prominently indicative of a
appropriate NiꢀNi distance together with the rigidity of the ligand
p p stacking interaction [52]. The
ꢀ
in Ni2 makes intramolecular cooperativity possible [44,53]. In Ni3,
Table 1
Crystal data and structure refinement for complexes Ni1¡Ni3.
Ni1
Ni2
Ni3
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C37H46N2NiO2Si
637.56
monoclinic
P2(1)/C
13.7619(13)
22.633(2)
11.9038(11)
90.00
110.554(2)
90.00
C53H62N4Ni2O3
920.49
monoclinic
P2(1)/n
17.5790(17)
8.0600(8)
33.615(3)
90.00
98.939(2)
90.00
C73H46F24N4Ni2O3
1600.56
triclinic
Pꢀ1
14.078(3)
15.015(3)
18.012(3)
65.914(4)
86.690(3)
82.784(4)
3448.3(12)
1.542
b (Å)
c (Å)
a
b
g
(ꢁ)
(ꢁ)
(ꢁ)
V (Å3), Z
3471.7(6), 4
1.220
0.627
4705.0(8), 4
1.299
0.847
Densitycalcd (Mg/m3)
Absorption coefficient (mmꢀ1
F (000)
)
0.662
1616
1360
1952
Crystal size (mm)
q
Reflections collected
0.36 ꢂ 0.30 ꢂ 0.18
1.80 to 23.27
14614
0.35 ꢂ 0.26 ꢂ 0.20
1.41 to 23.26
22861
0.38 ꢂ 0.38 ꢂ 0.20
1.46 to 26.04
18618
range for data collection (ꢁ)
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
4984 (Rint ¼ 0.0649)
4984/0/388
1.003
6745 (Rint ¼ 0.0988)
6745/0/559
1.014
13210(Rint ¼ 0.0315)
13210/19/974
1.047
Final R indices [I > 2
s
(I)]: R1, wR2
0.0511, 0.1154
0.398, ꢀ0.260
0.0637, 0.1344
0.410, ꢀ0.292
0.0762, 0.2421
1.846,ꢀ0.605
rmax and min, e/Å3

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
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 ꢂ 105 g of PE molꢀ1
Niꢀ1 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 single-
site behavior. But for Ni2, the broad range (Mw/Mn ¼ 5.2e8.5) im-
plies 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
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):
Ni1ꢀN1 ¼ 1.887(3), Ni1ꢀN2 ¼ 1.908(3), Ni1ꢀO1 ¼ 1.914(3), Ni1ꢀC27 ¼ 1.928(4),
N1ꢀNi1ꢀN2
¼
163.63(13), N1ꢀNi1ꢀO1
¼
94.15(12), N2ꢀNi1ꢀO1
¼
87.87(12),
N1ꢀNi1ꢀC27 ¼ 93.47(15), N2ꢀNi1ꢀC27 ¼ 87.63(15), O1ꢀNi1ꢀC27 ¼ 167.46(14).
distinct difference between the Mw's (Mw1
w2 z 250000) is observed.
This phenomenon reminds us a possible incomplete initiation
z
10000,
M
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 coordina-
tion 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 poly-
mers with lower Mw1 markedly increased (Fig. 4), which was in
good agreement with more intermediate Ni2-B generated in the
catalytic system.
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):
Ni1ꢀO1 ¼ 1.881(3), Ni1ꢀN1 ¼ 1.914(4), Ni1ꢀC52 ¼ 1.927(5), Ni1ꢀN3 ¼ 1.928(4),
Ni2ꢀN2 ¼ 1.908(4), Ni2ꢀN4 ¼ 1.923(5), Ni2ꢀC53 ¼ 1.926(5), Ni2ꢀO2 ¼ 1.927(3),
O1ꢀNi1ꢀN1
O1ꢀNi1ꢀN3
N2ꢀNi2ꢀN4
N2ꢀNi2ꢀO2
¼
¼
¼
¼
91.39(16), O1ꢀNi1ꢀC52
85.41(16), N1ꢀNi1ꢀN3
¼
172.4(2), N1ꢀNi1ꢀC52
172.31(18), C52ꢀNi1ꢀN3
¼
93.0(2),
91.0(2),
90.4(2),
173.1(2),
¼
¼
Additionally, a more sterically congested complex Ni3 was syn-
thesized 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), conse-
quently 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 sub-
stituent 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 poly-
merization 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 ꢂ higher than that afforded
177.01(18), N2ꢀNi2ꢀC53
¼
90.9(2), N4ꢀNi2ꢀC53
¼
90.24(17), N4ꢀNi2ꢀO2
¼
88.81(17), C53ꢀNi2ꢀO2 ¼
C8ꢀO2ꢀNi2 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 hin-
dered terphenyl amine, which facilitates the dissociation of addi-
tional 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

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):
1.972(5),
Ni2¡N2
¼ 1.889(4), Ni2ꢀO2 ¼ 1.898(3), Ni2ꢀN4 ¼ 1.928(4), Ni2ꢀC7 ¼ 1.953(5), Ni1ꢀO1 ¼ 1.888(3), Ni1ꢀN1 ¼ 1.906(4), Ni1ꢀN3 ¼ 1.929(4), Ni1ꢀC17 ¼
N2ꢀNi2ꢀO2 ¼ 92.33(15), N2ꢀNi2ꢀN4 ¼ 170.71(19), O2ꢀNi2ꢀN4 ¼ 84.99(16), N2ꢀNi2ꢀC7 ¼ 94.18(19), O2ꢀNi2ꢀC7 ¼ 171.60(18), N4ꢀNi2ꢀC7 ¼ 89.4(2), O1ꢀNi1ꢀN1 ¼ 92.85(15),
O1ꢀNi1ꢀN3 ¼ 85.71(16), N1ꢀNi1ꢀN3 ¼ 160.71(18), O1ꢀNi1ꢀC17 ¼ 162.9(2), N1ꢀNi1ꢀC17 ¼ 94.5(2), N3ꢀNi1ꢀC17 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 informa-
tion on the influences of binuclear effects, we have explored and
compared the polymer microstructures originated from mono- and
binuclear catalysts.
sec-butyl, and predominant methyl and ꢃ 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 S1ꢀS4. 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
Intriguingly, 13C NMR analysis of the polymer produced by Ni1 at
63 ꢁC reveals a variety of branch types, including ethyl, n-propyl,
Table 2
Ethylene polymerization using Ni1¡Ni3 based on 9,9-dimethylxanthene frameworks.a
Activityb
Mw
Mw/Mn
Tm
c
c
d
Entry
Catalyst
Pressure
(atm)
Temperature
(ꢁC)
Yield
(g)
(ꢁC)
1
2
3
4
5
6
7
8
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
1300
900
800
1.3
1.2
1.1
e
67
58
57
e
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
800
900
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
53
60
65
1000
75300
23500
19300
17500
14900
32000
36900
31700
11300
4800
3600
122
117
108
107
106
119
120
n.d.
109
97
9
10
11
12
13
14
15e
16
17
18
89
a
Reaction conditions: toluene, 100 mL; nickel, 10
In the unit of 105 g PE molꢀ1 Niꢀ1 hꢀ1
Determined by GPC vs polystyrene standards.
Determined by DSC.
20 mmol of Ni(cod)2 as a cocatalyst.
mmol; 20 min.
b
c
.
d
e

W.-W. Li et al. / Journal of Organometallic Chemistry 836-837 (2017) 34e43
41
M
= 10000
Ni2
w1
Ni2+Ni(cod)2
M
= 250000
w2
3
4
5
6
Log
M
w
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 phenom-
enon 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].
13
Fig. 5. C 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
13
Fig. 6. C 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
The polymerization of ethylene in the presence of 1,7-OD cata-
lyzed by Ni1 yields negligible polymer. In sharp contrast to the
mononuclear analogue, binuclear Ni2 yields copolymers at the ac-
tivity of 3.4 ꢂ 104 g polymer molꢀ1Niꢀ1hꢀ1, giving pendant vinyl
groups rather than cyclic structures. 1H NMR spectrum showed that
the intensity ratio of the signals corresponding to the vinylene
Ethylene copolymerization using Ni1¡Ni2 complexes.a
Entry Catalyst Comonomer Yield Activityb XM
c
d
d
e
Mw
Mw/Mn Tm
(equiv)
(g)
(ꢁC)
1
2
3
4
5
6
7
8f
9f
Ni1
Ni2
Ni1
Ni2
Ni1
Ni2
Ni2
Ni1
Ni2
e
e
0.99 29.7
1.21 36.3
e
e
800
14900 5.2
1.2
53
106
57
105
e
1,5-HD/1000 0.37
1,5-HD/1000 0.30
1,7-OD/500
1,7-OD/500
1,7-OD/1000 0.072 0.72
UAME/1000
UAME/1000 0.012 0.06
3.7
3.0
e
0.3 900
1.1 4100
1.2
1.6
e
(ꢀCH¼CHꢀ) 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.
13C NMR analysis of the copolymer (Table 2, entry 7) confirmed the
absence of cyclization (Fig. S5). However, the reason why mono-
and binuclear catalysts showed such a big difference in catalytic
activity here is currently not known.
e
e
e
0.34
3.4
0.27 23800 4.2
0.96 14000 3.9
115
108
e
e
e
e
e
e
3.1 20800 3.1
105
a
b
c
Reaction conditions: toluene, 100 mL; nickel, 20
In the unit of 104 g PE molꢀ1 Niꢀ1 hꢀ1
Comonomer incorporations determined by13C NMR for entries 4, 6, 7, by 1H
m
mol; 63 ꢁC; 5 atm; 30 min.
.
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 pro-
mote the copolymerization reaction (Table 3, entry 9). 1H NMR
NMR for entry 9.
d
Determined by GPC vs polystyrene standards.
Determined by DSC.
Reaction time was 60 min.
e
f
NMR spectroscopy. With respect to binuclear catalyst Ni2, a mod-
signals (Fig. 9) at
d
¼ 3.68 ppm and
d
¼ 2.33 ppm, assigned
erate activity (3.0 ꢂ 104 g mol-1 Niꢀ1 hꢀ1) was observed. In the 13
C
to ꢀCOOCH3 and ꢀCH2COOꢀ, 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.
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 five-
membered rings connected at their 1 and 3 positions to the poly-
mer 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
enhanced
catalytic
activity
for
ethylene/UAME
1
Fig. 8. H 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
[12] R. Gao, W.H. Sun, C. Redshaw, Catal. Sci. Technol. 3 (2013) 1172e1179.
43
copolymerization can be explained by the inhibition mechanism
ꢀ
[13] P. Buchwalter, J. Rose, P. Braunstein, Chem. Rev. 115 (2015) 28e126.
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 be-
tween the two nickel centers, the mechanism of which will
certainly merit further investigation.
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4. Conclusions
Herein we cover the design, synthesis and characterization of a
couple of binuclear neutral nickel catalysts based on 9,9-
dimethylxanthene frameworks as well as their applications in
ethylene (co)polymerization. By contrast experiment, marked dif-
ferences in polymerization characteristics are observed. Under
optimal conditions, binuclear catalyst Ni2 is ~2.2 ꢂ more active than
Ni1 for ethylene polymerization and produces polyethylene that
has a 15 times higher Mw with a bimodal molecular weight dis-
tribution. Ni1 generates polyethylene with hyperbranched struc-
ture, 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 negli-
gible 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.
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Acknowledgement
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Financial support by National Natural Science Foundation of
China (NO. 21304087 and 21274144) was gratefully acknowledged.
ꢀ
ꢀ
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Appendix A. Supplementary data
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