Nickel complexes based on BIAN ligands: transformation and catalysis on ethylene polymerization

Article abstract of DOI:10.1039/d1dt00649e

Treatment of bis(arylimino)acenaphthene (^ArBIAN) with Ni(COD)₂in toluene afforded^dmpBIANNi(COD) (2a, dmp = 2,6-Me₂C₆H₃) and^dippBIANNi(COD) (2b, dipp = 2,6-ⁱPrC₆H₃), respectively, in moderate yields. Complexes2aand2bcan be oxidized by a small amount of oxygen at low temperature leading to oxygen-bridged dinuclear Ni(ii) complexes (^dmpBIANNi)₂(μ-O)₂(4a) and (^dippBIANNi)₂(μ-O)₂(4b), respectively, as a purple powder. The reaction of^ArBIAN with 0.5 equiv of Ni(COD)₂or Ni(Ph₃P)₄gave bisligated complexes (^dmpBIAN)₂Ni (3a) and (^dippBIAN)₂Ni (3b), which can be considered as Ni(0) complexes supported by two neutral BIAN ligands. Oxidation of the bisligated nickel complexes3aand3bwith [Cp₂Fe][B(C₆F₅)₄] afforded cationic Ni(i) complexes [(^dmpBIAN)₂Ni][B(C₆F₅)₄] (5a) and [(^dippBIAN)₂Ni][B(C₆F₅)₄] (5b), respectively, in which the Ni(i) centre is chelated by two neutral Ar-BIAN ligands. These complexes were characterized by NMR and IR spectroscopy and DFT calculation, and the molecular structures of3b,4b, and5bwere well established by X-ray diffraction analysis. These complexes were evaluated as catalysts for ethylene polymerization in which2bshowed high activity in the presence of AlMe₃¹³C NMR analysis of polymers showed that the2b/AlMe₃catalytic system gave less-branched polymers when compared to that obtained with^dippBIANNiBr₂under the same conditions.

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Nickel complexes based on BIAN ligands:
transformation and catalysis on ethylene
Cite this: DOI: 10.1039/d1dt00649e
polymerization†
a
Shuyun Xu,‡a Xuemeng Chen,‡a Gen Luo b and Wei Gao
*
Treatment of bis(arylimino)acenaphthene (ArBIAN) with Ni(COD)2 in toluene afforded dmpBIANNi(COD)
(2a, dmp = 2,6-Me2C6H3) and dippBIANNi(COD) (2b, dipp = 2,6-iPrC6H3), respectively, in moderate yields.
Complexes 2a and 2b can be oxidized by a small amount of oxygen at low temperature leading to
oxygen-bridged dinuclear Ni(II) complexes (dmpBIANNi)2(μ-O)2 (4a) and (dippBIANNi)2(μ-O)2 (4b), respect-
ively, as a purple powder. The reaction of ArBIAN with 0.5 equiv of Ni(COD)2 or Ni(Ph3P)4 gave bisligated
complexes (dmpBIAN)2Ni (3a) and (dippBIAN)2Ni (3b), which can be considered as Ni(0) complexes sup-
ported by two neutral BIAN ligands. Oxidation of the bisligated nickel complexes 3a and 3b with
[Cp2Fe][B(C6F5)4] afforded cationic Ni(I) complexes [(dmpBIAN)2Ni][B(C6F5)4] (5a) and [(dippBIAN)2Ni][B
(C6F5)4] (5b), respectively, in which the Ni(I) centre is chelated by two neutral Ar-BIAN ligands. These com-
plexes were characterized by NMR and IR spectroscopy and DFT calculation, and the molecular structures
of 3b, 4b, and 5b were well established by X-ray diffraction analysis. These complexes were evaluated as
catalysts for ethylene polymerization in which 2b showed high activity in the presence of AlMe3. 13C NMR
analysis of polymers showed that the 2b/AlMe3 catalytic system gave less-branched polymers when com-
pared to that obtained with dippBIANNiBr2 under the same conditions.
Received 26th February 2021,
Accepted 19th April 2021
DOI: 10.1039/d1dt00649e
rsc.li/dalton
alkyls were experimentally detected by NMR in the reaction of
Brookhart-type catalysts with MAO at very low temperatures,5
1. Introduction
Diimine nickel complexes reported by Brookhart et al. in the intense purple colour was always observed when the solution
mid-1990s showed unique performance in ethylene polymeriz- was warmed to the real polymerization temperature suggesting
ation.1 One particularly attractive attribute of diimine-nickel/ the existence of other forms of nickel in the system.6 Petrovskii
MAO(alkylaluminium) catalytic systems is their ability to and co-workers have done some detailed research on
produce polymers with modulated branches and topology via a Brookhart-type catalytic systems using ESR spectroscopy and
“chain-walking” process. Thereafter, a large number of studies they found that the Ni(I) complex was formed during activation
focussing on improving the catalytic performance by modify- with MAO.7 Moreover, they revealed that treatment of Ni
ing the imine arms, as well as the backbones, were reported.2 (COD)2 with dippBIAN in the presence of MAO (methyl-
These investigations were generally based on a conventional alumoxane) in toluene afforded the Ni(I) species probably via
“Cossee mechanism”3 in which a cationic alkyl-nickel(II) che- an intramolecular charge transfer mechanism.6 Soshnikov and
lated by a neutral diimine ligand was proposed as the active co-workers argued that the Ni(I) in the catalytic systems is the
species. In such a mechanism, the diimine ligand was believed resting state of the catalysts, rather than the catalyst de-
to be innocent and only exerted a simple steric and electronic activation product.8 Recently, we demonstrated that
influence on the nickel centre.4 Although the cationic Ni(II) dippBIANNiCl(PPh3) in which the Ni(II) centre was chelated by a
radical dippBIAN behaved similar to dippBIANNiBr2 (1) in ethyl-
ene polymerization in the presence of alkylaluminium or
MAO.9a The dippBIAN radical was also detected in the reaction
of 1 with AlMe3 at room temperature.9 These results implied
that the dippBIAN ligand in 1 might be reduced to the anionic
radical during the activation with alkylaluminium and the
alkyl-nickel with the radical anionic dippBIAN ligand might be
the true active species for ethylene polymerization. More
recently, Long and co-workers disclosed that 1 can be reduced
aCollege of Chemistry, Jilin University, Changchun, China. E-mail: gw@jlu.edu.cn
bInstitutes of Physical Science and Information Technology, Anhui University, Hefei
230601, China. E-mail: Luogen@ahu.edu.cn
†X-ray crystallographic data and refinements for complexes 3b, 4b, and 5b in
CIF format, summary of crystallographic data. CCDC 2064828–2064830. For crys-
tallographic data in CIF or other electronic format see DOI: 10.1039/d1dt00649e
‡These authors contributed equally to this work.
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Synthetic procedures
Synthesis of complex 2a. A mixture of dmpBIAN (0.388 g,
1.0 mmol) and Ni(COD)2 (0.275 g, 1.0 mmol) in 20 mL of
toluene was stirred for 12 h at room temperature under a nitro-
gen atmosphere. Evaporation of the solvent to dryness gave a
red-violet powder which was washed with hexane (2–3 mL) and
1
dried in a vacuum (0.477 g, 82% yield). H NMR (C6D6): δ 7.53
Chart 1
(d, 2H, Ar–H), 7.24 (m, 6H, Ar–H), 7.02 (t, 2H, Ar–H), 6.73 (d,
2H, Ar–H), 3.94 (s, 4H, COD–CH), 2.48 (br, 4H, COD–CH2), 2.30
(s, 12H, Ar–CH3), 1.38 (s, 4H, COD–CH2). 13C NMR (100 MHz,
C6D6): 152.00 (NvC), 151.89 (Ar–ipso–C), 137.77 (Ar–C), 136.95
(Ar–C), 133.98 (Ar–C), 132.60 (Ar–C), 128.24 (Ar–C),128.07 (Ar–
C), 124.89 (Ar–C), 124.28 (Ar–C), 117.38 (Ar–C), 88.16 (COD–
CH), 30.51 (COD–CH2), 18.20 (Ar–CH3). Anal. calcd for
C36H36N2Ni (%): C, 77.85; H, 6.53; N, 5.04. Found: C, 78.01; H,
6.88; N, 4.67. IR(KBr): ν (cm−1) 3435m, 3070w, 3011w, 2929m,
1669m, 1633m, 1592s, 1530s, 1492m, 1466m, 1440m, 1421m,
1290w, 1258w, 1226w, 1206w, 1087w, 1033w, 924w, 831m,
780s, 767s.
Synthesis of complex 2b. Complex 2b was synthesized in the
same way described above for the synthesis of 2a, using
dippBIAN (0.500 g, 1.0 mmol) and Ni(COD)2 (0.275 g,
1.0 mmol). The product was isolated as a red-violet solid
(0.521 g, 75% yield). 1H NMR and 13C NMR spectroscopy
results are identical to the reported data. Anal. calcd for
C44H52N2Ni (%): C, 79.16; H, 7.85; N, 8.79. Found: C, 79.08; H,
7.80; N, 8.72. IR(KBr): ν (cm−1) 3453w, 3070w, 2964s, 2864m,
1660w, 1625w, 1588m, 1524w, 1460m, 1431m, 1383w, 1360w,
1326w, 1249w, 1188w, 1044w, 925w, 832m, 780m, 755m.
Synthesis of complex 3a. Complex 2a (0.291 g, 0.5 mmol) in
20 mL of toluene was stirred for 6 h at −50 °C under 1 atm
nitrogen atmosphere with a small amount of oxygen (N2/O2 =
20). Evaporation of the solvent to dryness gave 3a as a red-
violet powder (0.234 g, 98% yield). Anal. calcd for
by cobaltocene to Ni(I) species which can be further converted
to the nickel(II) complex with a radical ligand by the addition
of AlMe3. Remarkably, the reduced catalyst afforded polymers
with less branches than those with the neutral catalyst (1)
revealing that ligand-based redox chemistry plays an important
role in modulating the branching density and identity of poly-
mers (Chart 1).10
As mentioned above, in the diimine-nickel/alkylaluminium
(MAO) systems, both the diimine ligand and the nickel centre
are labile and prone to be reduced during activation with the
alkylaluminium reagent. The complexity of the system prohi-
bits more detailed studies on the mechanism. We envisioned
that the research base on some well-defined simple models,
such as BIAN ligated Ni(0) or Ni(I) complexes, may furnish
more direct insight into the understanding of the mechanism.
Herein we reported several nickel complexes bearing BIAN
ligands and their transformation. Their catalytic behaviour for
ethylene polymerization is also presented.
2. Experimental section
Materials and methods
All manipulations involving air and moisture-sensitive com- C58H54N4Ni2O2(%): C, 72.83; H, 5.69; N, 5.86. Found: C, 72.74;
pounds were carried out under an atmosphere of dried and H, 5.68; N, 5.78. IR(KBr): ν (cm−1) 3438m, 3071w, 3012w,
purified nitrogen using standard Schlenk and vacuum-line 2927m, 1670m, 1632m, 1592s, 1531s, 1491m, 1467m, 1438m,
techniques. Toluene and THF were dried over sodium metal 1420m, 1291w, 1260w, 1225w, 1205w, 1088w, 1033w, 926w,
and distilled under nitrogen. Hexane was dried over CaH2 and 832m, 781s, 768s.
distilled under nitrogen. Elemental analyses were performed
Synthesis of complex 3b. Complex 3b was synthesized in the
on a Varian EL microanalyzer. Infrared spectra were recorded same way described above for the synthesis of 3a, using 2b
using KBr disks with a Nicolet Avatar 360. NMR spectra were (0.347 g, 0.5 mmol) as a starting material. The product was iso-
obtained using Bruker 400 MHz instruments at room tempera- lated as a red-violet solid (0.286 g, 97% yield). Single crystals of
ture in CDCl3 or C6D6 solution for ligands and complexes. The 3b suitable for X-ray crystallography were obtained from a satu-
molecular weights of the polymer samples were determined at rated toluene solution by cooling. Anal. calcd for
150 °C using a PL-GPC 220 type high-temperature gel per- C74H86N4Ni2O2(%): C, 75.26; H, 7.34; N, 4.74. Found: C, 72.34;
meation chromatography system. 1,2,4-Trichlorobenzene was H, 7.26; N, 4.69. IR(KBr): ν (cm−1) 3453w, 3071w, 2963s,
employed as the solvent at a flow rate of 1.0 mL min−1. The 2865m, 1661w, 1624w, 1590m, 1522w, 1461m, 1430m, 1382w,
UV-vis spectrum was recorded on a Varian Cary 50 UV-vis ana- 1360w, 1325w, 1250w, 1189w, 1045w, 926w, 834m, 781m,
lyzer. Differential scanning calorimetry (DSC) measurements 753m.
were performed on a Netzsch DSC 204 instrument under a N2
atmosphere. The samples were heated at a rate of 10 °C min−1 (0.388 g, 1.0 mmol) and Ni(COD)2 (0.138 g, 0.5 mmol) in
and cooled at a rate of 10 °C min−1 dmpBIAN,11 dippBIAN,11 20 mL of toluene was stirred for 12 h at room temperature
dippBIANNiBr2 (1),1 dmpBIANNa12 and dippBIANNa12 were pre- under a nitrogen atmosphere. Evaporation of the solvent to
Synthesis of complex 4a. Method A: A mixture of dmpBIAN
.
pared according to the literature procedure.
dryness gave a blue powder. After filtering through Celite, the
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red-violet solid was washed with hexane (2–3 mL) and dried in 1.0 mmol) and 30 mL of toluene under N2, and the mixture
a vacuum. The product was isolated as a blue powder (0.293 g, was stirred vigorously until the dissolution of the powder was
70% yield). Method B: A toluene solution of dmpBIANNa complete (ca. 30 min) at −78 °C. Then, a mixture of
(0.338 g, 1.0 mmol) was added dropwise to a toluene (20 ml) dmpBIANNiBr2 (0.604 g, 1.0 mmol) and Na[(C6F5)4B] (0.702 g,
suspension of NiBr2(DME)2 (0.161 g, 0.5 mmol) at room temp- 1.0 mmol) was added under N2. The mixture was allowed to
erature and the mixture was stirred for 30 minutes. The reac- warm to ambient temperature and was stirred overnight.
tion mixture was filtered and the solvent was removed under Evaporation of the solvent gave a dark violet powder after filter-
reduced pressure. The residue was washed with hexane and ing through Celite which was washed with hexane and dried in
dried in a vacuum. The product was obtained as a blue powder a vacuum. The product was obtained as a dark violet powder.
(0.418 g, 82% yield). 1H NMR (400 MHz, C6D6) δ 7.84 (d, J = 8.4 Yield: 1.105 g, (73%). 11B NMR (400 MHz, THF-d8): δ −16.60 (s)
Hz, 4H, Ar–H), 7.01 (t, J = 6.9 Hz, 12H, Ar–H), 6.64 (d, J = 7.2 ppm. Anal. calcd for C80H48BF20N4Ni(%): C, 63.43; H, 3.19; N,
Hz, 4H, Ar–H), 6.43 (t, J = 7.7 Hz, 4H, Ar–H), 2.51 (s, 24H, Ar– 3.70. Found: C, 63.20; H, 3.13; N, 3.67. IR (KBr): ν (cm−1
)
CH3). 13C NMR (100 MHz, C6D6) 142.69 (NvC), 138.18 (Ar–C), 3069w, 2980w, 2916w, 1640w, 1602w, 1514m, 1465s, 1438m,
136.14 (Ar–C), 134.32 (Ar–C), 128.94 (Ar–C), 125.40 (Ar–C), 1415w, 1380w, 1292m, 1223w, 1203w, 1191w, 1138w, 1105w,
125.21 (Ar–C), 124.60 (Ar–C), 123.87 (Ar–C), 122.10 (Ar–C), 1084m, 1051w, 1033w, 980s, 953w, 841m, 830m, 773m, 756w.
120.67 (Ar–C), 32.42(Ar–CH3). Anal. calcd for C56H48N4Ni(%): μeff = 1.58μB (Evans method).
C, 80.48; H, 5.79; N, 6.70. Found: C, 80.23; H, 5.68; N, 6.67. IR
Synthesis of complex 5b. Method A: A THF solution of 4b
(KBr): ν (cm−1) 3063w, 3018w, 2917w, 2849w, 1672w, 1634w, (0.522 g, 1.0 mmol) was added to a THF (20 ml) solution of
1591m, 1494s, 1468m, 1436m, 1417m, 1375w, 1314w, 1257w, [Cp2Fe][B(C6F5)4] (0.221 g, 1.0 mmol) at room temperature and
1215w, 1195m, 1152w, 1082w, 1041w, 925w, 830w, 819m, the mixture was stirred for 30 minutes. The reaction mixture
768m.
was evaporated to dryness and the residue was recrystallized
toluene/hexane mixed solvent. The product was
Synthesis of complex 4b. Method A: Complex 4b was syn- with
a
thesized in a similar way described above for the synthesis of obtained as a violet solid. Yield: 1.105 g, (73%). Method B:
4a, using dippBIAN (0.500 g, 1.0 mmol) and Ni(PPh3)4 (0.554 g, Complex 5b was synthesized in the same way described above
0.5 mmol). The product was isolated as a blue solid. (0.413 g, for the synthesis of 5a, using dippBIANNa (0.673 g, 1.0 mmol),
78% yield). Method B: A toluene solution of dippBIANNa dippBIANNiBr2 (0.716 g, 1.0 mmol) and Na[(C6F5)4B] (0.702 g,
(0.520 g, 1.0 mmol) was added dropwise to a toluene (20 ml) 1.0 mmol). The product was obtained as a dark violet powder.
suspension of NiBr2(DME)2 (0.161 g, 0.5 mmol) at room temp- Yield: 1.442 g, (83%). Crystals of 5b suitable for X-ray structural
erature and the mixture was stirred for 30 minutes. The reac- determination were grown in toluene. 11B NMR (400 MHz,
tion mixture was filtered and the solvent was removed under THF-d8): δ −18.46 (s) ppm. Anal. calcd for C96H80BF20N4Ni(%):
reduced pressure. The residue was washed with hexane and C, 66.30; H, 4.64; N, 3.22. Found: C, 66.24; H, 4.58; N, 3.07. IR
dried in a vacuum. The product was obtained as a blue powder (KBr): ν (cm−1) 3441m, 3064w, 2964s, 2864m, 1646w, 1605w,
1
(0.418 g, 82% yield). H NMR (400 MHz, Tol-d8) δ 8.08 (d, J = 1511m, 1462s, 1433m, 1413w, 1386w, 1362w, 1315w, 1291m,
8.0 Hz, 4H, Ar–H), 7.29 (d, J = 7.1 Hz, 4H, Ar–H), 7.26–7.20 (m, 1271w, 1191w, 1086s, 1054w, 979s, 835m, 798w, 776m, 756m.
4H, Ar–H), 6.37 (t, J = 7.4 Hz, 4H, Ar–H), 6.34–6.23 (m, 4H, Ar– μeff = 1.47μB (Evans method).
H), 5.57 (d, J = 7.1 Hz, 4H, Ar–H), 4.12 (m, 4H, CHMe2), 3.21
Ethylene polymerization experiments
(m, 4H, CH(CH3)2), 1.79 (d, J = 6.0 Hz, 8H, CH(CH3)2), 1.00 (d,
J = 6.2 Hz, 8H, CH(CH3)2), 0.64 (d, J = 6.0 Hz, 8H, CH(CH3)2), The ethylene polymerization experiments were carried out as
0.11 (d, J = 6.1 Hz, 8H, CH(CH3)2). 13C NMR (100 MHz, C6D6): follows: a dry 100 mL steel autoclave with a magnetic stirrer
144.66 (NvC), 138.22 (Ar–C), 136.01 (Ar–C), 134.84 (Ar–C), was charged with 55 mL of toluene, thermostated at the
128.56 (Ar–C), 125.79 (Ar–C), 125.13 (Ar–C), 124.24 (Ar–C), desired temperature, and saturated with ethylene (1.0 atm).
123.73 (Ar–C), 122.91 (Ar–C), 119.81 (Ar–C), 30.12 (Ar–CHMe2), The polymerization reaction was started by the addition of a
29.01 (Ar–CHMe2), 28.17(Ar–CHMe2), 23.98 (Ar–CH(CH3)2), mixture of the catalyst and co-catalyst in toluene (5 mL) at the
23.32 (Ar–CH(CH3)2). Anal. calcd for C72H80N4Ni(%): C, 81.57; same time. The vessel was pressurized to 5 atm with ethylene
H, 7.61; N, 5.28. Found: C, 81.23; H, 7.85; N, 5.16. IR(KBr): ν immediately, and the pressure was maintained by continuous
(cm−1) 3458m, 3058w, 2964s, 2870m, 1663m, 1642w, 1590m, feeding of ethylene. The reaction mixture was stirred at the
1470s, 1433m, 1383w, 1362w, 1312w, 1250w, 1189w, 1112w, desired temperature for the desired period. The polymeriz-
1041w, 923m, 781m, 754m.
ation was then quenched by injecting an acidic ethanol solu-
Synthesis of complex 5a. Method A: A THF solution of 4a tion containing HCl (3 M). The polymer was collected by fil-
(0.52 g, 1.0 mmol) was added to a THF (20 ml) solution of tration, washed with water and ethanol, and dried to a con-
[Cp2Fe][B(C6F5)4] (0.22 g, 1.0 mmol) at room temperature and stant weight under vacuum.
the mixture was stirred for 30 minutes. The reaction mixture
was evaporated to dryness and the residue was recrystallized
Density functional theory (DFT) calculations
with
a toluene/hexane mixed solvent. The product was All calculations were performed with the Gaussian 16
obtained as a violet solid. Yield: 1.105 g, (73%). Method B: A program.13 The (U)OPBE functional,14 which often shows good
Schlenk flask was charged with dmpBIANNa (0.561 g, performance on accurate spin-state energies for transition-
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metal complexes,15,16 together with TZVP basis sets was used
for single-point calculations of X-ray structures. Several poss-
ible spin states were examined for each complex and the most
stable ground state was used for Mulliken spin population ana-
lysis. The symmetry of spatial orbitals (including the singlet
state) is allowed to be completely broken (broken symmetry)
for the search for the ground state.
Scheme 1 Reaction of the Ar-BIAN ligands with Ni(COD)2 in toluene.
Crystallography
The crystals were mounted on a glass fibre using the oil drop
method. Data obtained with the ω − 2θ scan mode were col-
lected on a Bruker SMART 1000 CCD diffractometer with
graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The
structures were solved using direct methods, while further
refinement with full-matrix least-squares on F2 was obtained
with the SHELXTL program package. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were introduced
at calculated positions with the displacement factors of the
host carbon atoms. The crystal structure files in cif format
were deposited in CCDC (2064828–2064830).†
respectively. It is worth noting that two resonances assignable
to the –CH2–group in COD were observed at 2.46–2.55 ppm
and 1.37–1.41 ppm, respectively, indicating that the two hydro-
gen atoms on the same carbon are in a different environment.
As shown in the molecular structure of 2b,15 the coordination
of dippBIAN results in a crowded environment at the nickel
centre and the repulsion between the aryl rings in the imine
group and the COD molecule restricts the free rotation of the
–CH2– units in COD. Based on their diamagnetism, complexes
2a and 2b can be tentatively assigned as a Ni(0) complex che-
lated by a neutral BIAN ligand and a COD molecule, despite
that the CvN bond (1.324(3)–1.336(3) Å) is slightly elongated
and the C–C bond (1.426(4)–1.428(4) Å) is somewhat con-
tracted when compared to the corresponding ones in neutral
dippBIANNiBr2.18 The deviations of the CvN and C–C bonds
from the values for a neutral BIAN ligand may be ascribed to a
strong back donation from the electron-rich Ni(0) centre to the
BIAN π orbitals. As expected, complexes 2a and 2b are EPR
silent in solution and solid-state, and no signal assignable to
the BIAN radical was detected. DFT calculation on 2b also
revealed a closed-shell dippBIAN ligand and no unpaired-elec-
tron in the ligand and the nickel centre was observed (see
Fig. 3) suggesting the absence of charge transfer from Ni(0) to
the ArBIAN ligand.
3. Results and discussion
The ArBIAN ligands (Ar = 2,6-Me2C6H3 (dmpBIAN), 2,6-iPr2C6H3
(
dippBIAN)) were efficiently synthesized according to the litera-
ture and well-identified with NMR spectroscopy.
Following our previous work on BIAN nickel complexes, we
intended to synthesize low-valent nickel complexes in the for-
mulation of ArBIANNi(COD). Such complexes feature a simple
composition and well-defined molecular-structure and there-
fore can be used as a valuable model for detailed investigation
of the catalytic reaction. Stephan and co-workers reported that
the reaction of dippBIAN with Ni(COD)2 in THF afforded
dippBIANNi(COD) (2b) in moderate yield,17 whereas, the reac-
tion of MesBIAN (Mes = 2,4,6-Me3C6H2) with equimolar Ni
(COD)2 in THF did not give the expected MesBIANNi(COD) but
furnished a COD-free bisligated nickel complex [MesBIAN]2Ni.
Likewise, we could not isolate dmpBIANNi(COD) (2a) from the
reaction of dmpBIAN with equimolar Ni(COD)2 in THF. The for-
mation of mono- or bis-ligated complexes from similar reac-
tions may be attributed to the differing steric demands of the
ligands. Besides, the solvent used in the reaction may also play
a vital role in the selectivity of the products. In our opinion,
the reaction of BIAN ligand with Ni(COD)2 in THF may be too
fast to stay at the monoligated stage when the less bulky
ligand was used. To this end, the less polar solvent toluene
was tested for the synthesis of ArBIANNi(COD). Experimentally,
the slow addition of ArBIAN ligands to the solution of Ni
(COD)2 in toluene at room temperature, followed by sub-
sequent work-up, afforded dmpBIANNi(COD) (2a) and
dippBIANNi(COD) (2b), respectively, as a red powder in moder-
ate to high yields (Scheme 1).
Complexes 2a and 2b are extremely sensitive to air. When
exposed to air, these complexes decomposed very rapidly
leading to free ligands and unidentified nickel oxides. But the
slow addition of a limited amount of dioxygen to the solution of
2a or 2b in toluene at low temperature (–78 °C) afforded the bis
(μ-oxo) dinuclear complexes 3a or 3b, respectively, in moderate
yields (Scheme 2). Complexes 3a and 3b are paramagnetic and
no informative NMR spectroscopy data were obtained. The
efficient magnetic moments (μeff) of 3a and 3b determined by
Complexes 2a and 2b are diamagnetic and can be well
characterized by NMR in which the resonance of CvCH of
COD was found at 4.20 ppm for 2a and 3.94 ppm for 2b, Scheme 2 Reaction of the ArBIANNi(COD) with dioxygen.
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Scheme 3 Synthesis of the bisligated ArBIAN nickel complexes.
netic and can be well characterized by NMR. 4a showed a
singlet resonance for Ar–Me in 1H NMR indicating that the
methyls in the ligands are equivalent whereas, 4b exhibited four
doublets in the high field suggesting that the isopropyl groups
in 4b are in different environments probably due to the bulky
ortho groups which inhibited their free rotations.
Fig. 1 Perspective view of 3b with thermal ellipsoids drawn at the 20%
probability level. Uncoordinated solvent molecules and hydrogen atoms
are omitted for clarity. The selected bond lengths (Å) and angles (°): Ni
(1)–N(1) 1.887(2), Ni(1)–N(2) 1.906(2), Ni(2)–N(3) 1.903(2), Ni(2)–N(4)
1.892(2), Ni(1)–O(1) 1.800(1) Ni(1)–O(2) 1.786(2) Ni(2)–O(1) 1.794(2) Ni
(2)–O(2) 1.800(2), Ni(1)–Ni(2) 2.6863(5); N(1)–Ni(1)–N(2) 81.57(10), N(3)–
Ni(1)–N(4) 81.44(10), N(1)–Ni(1)–N(4) 155.01(11), N(3)–Ni(1)–N(2) 154.99
(11).
Crystals of 4b suitable for X-ray diffraction analysis were
grown from mixed hexane/THF solution and the molecular
structure of 4b is depicted in Fig. 2 along with the selected
bond distances and angles in the caption. Complex 4b consists
of two N,N′-chelating dippBIAN ligands attached to a nickel
atom. The whole molecule is in a C2 symmetry having a two-
fold symmetry axis defined by the Ni atom and the centroids
of the C–C bond in the metallacycles. The bond N–Ni of 1.994
(2)–2.003(2) Å is slightly longer than that of 1.956(3)–1.972(3) Å
in 2b and that of 1.9592(13) Å in a similar bisligated nickel
complex (MesBIAN)Ni.17 Two dippBIAN ligands form a large di-
hedral angle of 36.272(56)° suggesting a twisted tetrahedral
geometry at the nickel centre. The connectivity in the two
dippBIAN ligands is almost identical with the bond lengths of
C–N and C–C in the chelating ring being 1.3305 Å (av) and
1.421(4) Å, respectively, suggesting the neutral nature of these
the Evans method19 are in the range of 2.77–2.84μB which are
typical of a d8 Ni(II) in a high-spin state. The identities of 3a
and 3b were well confirmed by elemental analysis and the mole-
cular structure of 3b was established by X-ray diffraction ana-
lysis (Fig. 1). In 3b, two dippBIANNi units are bridged by two O2−
ligands and the nickel atoms are in a distorted square-planar
geometry. The whole molecule is slightly twisted with the di-
hedral angle between two ligand planes (estimated with the C
and N atoms in the chelating ring of the ligands) being 32.53
(11)°. The bond distances of N–Ni (1.887(2)–1.906(2) Å) are
slightly shorter than those of 2.088(3)–2.110(3) Å in dimeric
dippBIANNiBr2.18 The Ni–Ni bond length of 2.6863(5) Å is very
comparable to that of 1.754(3) Å in the dimeric complex
20
(C10H9NNi(OOCCMe3)2)2
and that of 2.733(7) Å in [Ni{Ni
(NH2CH2CH2S)2}2]Cl2.21 But this distance is considerably longer
than that of 2.492 Å in metallic nickel suggesting a nonbonding
distance between the two nickel atoms.22
As expected, the reaction of dmpBIAN with 0.5 equiv of Ni
(COD)2 in THF afforded the bisligated complex (dmpBIAN)2Ni
(4a) in moderate yield. Interestingly, an attempt to synthesize
(
dippBIAN)2Ni (4b) by treating dippBIAN with 0.5 equiv of Ni
(COD)2 in THF or toluene failed and only the monoligated
complex 2b was obtained. The failure to obtain the bisligated
complex (dippBIAN)2Ni may probably be ascribed to the bulky
iPr groups in 2b which prohibit the substitution of the COD
molecule with the free dippBIAN ligand. Besides, the strong
coordination of the COD to the nickel in 2b may also preclude
the formation of the bisligated product. It is well-known that
the bulky phosphine Ph3P in Ni(Ph3P)4 is readily dissociated
and can be easily substituted by other ligands. So, Ni(Ph3P)4
was used as the nickel precursor for the preparation of 4b. As
expected, the reaction of dippBIAN with 0.5 equiv of Ni(Ph3P)4 in
toluene afforded the bisligated complex (dippBIAN)2Ni (4b) in
Fig. 2 Perspective view of 4b with thermal ellipsoids drawn at the 30%
probability level. Uncoordinated solvent molecules and hydrogen atoms
are omitted for clarity. The selected bond lengths (Å) and angles (°): Ni
(1)–N(1) 2.003(2), Ni(1)–N(2) 1.994(2), C(1)–N(1) 1.328(3), C(12)–N(2)
1.333(3), C(1)–C(12) 1.421(4); N(1)–Ni(1)–N(2) 82.56(8), N(2)–Ni(1)–N(2A)
moderate yield (Scheme 3). Complexes 4a and 4b are diamag- 148.52(12).
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Fig. 3 Mulliken spin population analysis of the most stable ground state
for 2b (closed-shell), 4b (closed-shell), and 5b (Ni: 0.92, N: 0.10) calcu-
lated at the level of (U)OPBE/TZVP. All hydrogen atoms were omitted for
clarity.
ligands. Thus, the whole complex can be tentatively described
as a Ni(0) centre ligated with two neutral dippBIAN ligands. It is
worth noting that a similar nickel complex (FArBIAN)2Ni (FAr =
3,5-(CF3)2C6H3) reported by Sundermeyer and co-workers was
described as a Ni(II) complex with two radical anionic BIAN
ligands.23 Complex (FArBIAN)2Ni is diamagnetic and EPR silent
probably due to the perfect antiferromagnetic coupling of the
two radicals with the Ni(II) centre. Therefore, the diamagnet-
ism and EPR cannot rule out the existence of the ligand
radical and Ni(I) species in 4a (4b). To obtain a deeper insight
into the structural nature of complex 4a (4b), density func-
tional theory (DFT) calculations were performed. The compu-
tational results suggested that in 4b the closed-shell singlet
state is the most ground state in which the Ni(0) atom is che-
lated by two neutral ligands (Fig. 3).
Fig. 4 Perspective view of 5b with thermal ellipsoids drawn at the 30%
probability level. Hydrogen atoms and [B(C6F5)4] are omitted for clarity.
The selected bond lengths (Å) and angles (°): Ni(1)–N(1) 2.047(3), Ni(1)–
N(2) 2.059(3), Ni(1)–N(3) 2.059(2), Ni(1)–N(4) 2.047(3), C(1)–N(1) 1.292
(4), C(12)–N(2) 1.228(4), C(37)–N(3) 1.293(4), C(48)–N(4) 1.304(4), C(1)–
C(12) 1.497(4), C(37)–C(48) 1.481(4); N(1)–Ni(1)–N(2) 81.57(10), N(3)–Ni
(1)–N(4) 81.44(10), N(1)–Ni(1)–N(4) 155.01(11), N(3)–Ni(1)–N(2) 154.99
(11).
ratio. Complexes 5a and 5b are paramagnetic and show broad-
ened signals in the NMR spectra. X-ray crystallography reveals
that 5b consists of a well-separated cation {[dippBIAN]2Ni}+and
[B(C6F5)4]− anion (Fig 4). In the cation part, the nickel centre is
in a tetrahedral geometry chelated by two dippBIAN ligands.
The metric parameters in the two dippBIAN ligands are almost
the same and the lengths of C–C and C–N in the chelating ring
are well comparable to those in 4b suggesting their neutral
nature. The DFT calculation revealed an unpaired electron dis-
persed on the nickel centre and no unpaired electron was
found in the ligand backbone. So, complex 5a and 5b can be
best described as a Ni(i) complex chelated by two neutral
ligands.
Given that the bisligated complexes 4a and 4b can be
described as Ni(0) complexes supported by two neutral ligands
and the fact that the Ni(I) might be the real active species for
ethylene polymerization as suggested by Petrovskii and co-
workers,7 we tried to synthesize the Ni(I) complex via an oxi-
dation reaction. The addition of [Cp2Fe][B(C6F5)4] to 4a or 4b
in toluene, after appropriate workup, afforded complexes
[
dmpBIANNi][B(C6F5)4] (5a) and dmpBIANNi][B(C6F5)4] (5b),
[
respectively, in low yields (Scheme 4). The existence of free
Cp2Fe in the reaction systems hindered the separation of 5a
(5b) from the mixtures via recrystallization. Alternatively, 5a Ethylene polymerization
and 5b can also be prepared in high yields by the reaction of
The well-defined nickel complexes were evaluated as precata-
ArBIANNa, ArBIANNiBr2 and Na[B(C6F5)4] in a 1 : 1 : 1 mole
lysts for ethylene polymerization. The polymerization reactions
were conducted in toluene using a 100 mL scale autoclave and
the preliminary results are summarized in Table 1. Blank tests
were also performed under the same experimental conditions,
using [Ni(COD)2] or AlMe3 solely, revealing inactivity under all
the tested conditions. Notably, almost no polymer was
obtained when these BIAN nickel complexes were used solely
to promote ethylene polymerization. However, upon activation
with 400 equiv of AlMe3, 2a and 2b showed moderate activities
in ethylene polymerization at 30 °C affording polymers with a
high molecular-weight. 2b is more active than 2a under the
same conditions probably due to the bulky substituents on the
ligands shielding the metal centre and preventing the catalyti-
cally active species from deactivation.24 It was found that the
activities are sensitive to the Al/Ni ratio (entries 2–6). The
activity of 2b increased with the increase of Al/Ni from 50 to
Scheme 4 Oxidation of the [Ar-BIAN]2Ni with [CP2Fe][B(C6F5)4].
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Table 1 Polymerization of ethylene under various conditionsa
Entry
Cat.
Tp (°C)
Activator
Yield (g)
Activityb
Mw c × 105
PDIc
Bd
Tm e (°C)
1
2
3
4
5
6
7
8
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
3a
3b
4a
4b
5a
5b
1
30
30
30
30
30
30
50
80
30
30
30
30
30
30
30
30
30
30
30
30
30
AlMe3(400)
AlMe3(400)
AlMe3(50)
AlMe3(100)
AlMe3(200)
AlMe3(500)
AlMe3(400)
AlMe3(400)
D-MAO(400)
AlEt3(400)
1.08
5.60
2.48
2.78
3.54
5.50
2.30
0.97
1.87
1.96
1.03
1.52
1.67
0.89
0.31
0.66
Trace
Trace
0.85
0.87
4.28
432
2240
1120
1112
1416
2200
920
388
748
784
412
608
668
356
124
264
—
4.0
3.7
2.2
2.9
2.8
3.4
2.1
1.6
12
6.0
3.7
4.4
3.6
6.5
3.6
2.5
—
2.1
1.6
1.5
1.3
1.7
2.3
1.4
1.3
1.1
1.2
1.5
1.2
1.4
2.0
2.2
1.4
—
10
37
38
37
39
35
n.d.
37
13
27
n.d.
39
n.d.
n.d.
n.d.
n.d.
—
129.0
126.6
128.9
127.2
126.8
127.1
125.1
91.0
127.9
109.4
125.6
115.0
124.7
115.9
131.7.
128.3
—
9
10
11
12
13
14
15
16
17
18
19
20
21
AliBu3(400)
AlEt2Cl(400)
Al2Et3Cl3(400)
AlEtCl2(400)
AlMe3(400)
AlMe3(400)
AlMe3(400)
AlMe3(400)
AlMe3(400)
AlMe3(400)
AlMe3(400)
—
—
—
—
n.d.
56
—
91.0
91.0
66.5
340
348
1712
1.8
1.6
0.9
1.2
1.3
1.5
70
a Polymerization conditions: Toluene (60 mL), cat (10 μmol), ethylene pressure, 5 bar, 15 min. b Activity in units of kg of PE mol(Ni)−1 h−1
.
c Determined by GPC relative to polystyrene standards. d Branching numbers per 1000 carbon atoms were determined by 13C NMR. e Melting
temperature determined by DSC.
400, while the further increase of the Al/Ni ratio to 500 showed initiation state of the polymerization. Further research about
no obvious effects on the productivities. The temperature was the detailed mechanism is underway.
found to play an important role in the polymerization. The
activity decreased dramatically from 2240 to 388 kg of PE mol
(Ni)−1 h−1 when the temperature went up from 30 to 80 °C. 4. Conclusions
This result may be ascribed to the instability of the catalytic
In summary, we have reported the synthesis and characteriz-
ation of several nickel complexes bearing BIAN ligands. The
species at high temperatures. Moreover, the activity of 2b
showed strong dependence on the aluminum catalysts used
reaction of the ligands with Ni(COD)2 afforded monoligated or
(entries 9–14) in which AlEt3, Al(iBu)3, and come chlorine con-
bisligated nickel complexes depending on the different reac-
taining alkylaluminum showed lower activities compared with
AlMe3. Great efforts were devoted to understanding the real
tant ratios. The nickel centre in these monoligated complexes
can be oxidized by dioxygen giving oxygen-bridged nickel com-
role of the alkylaluminum in the polymerization, but no con-
plexes. The bisligated (ArBIAN)2Ni can be oxidized to Ni(I) com-
clusive result has been obtained so far. We proposed that in
plexes, in which the Ni(I) centre was attached to two neutral
the initial stage of the polymerization, the alkylaluminum may
BIAN ligands. Upon activation with aluminium alkyls, these
facilitate the dissociation of the COD molecule from the metal
nickel complexes showed varied activities for ethylene
center giving an opening environment around the Ni(0)
polymerization, in which the monoligated COD nickel com-
center. Complexes 3a and 3b showed lower activities in ethyl-
ene polymerization and complexes 5a and 5b showed moder-
ate activities when activated with AlMe3. Interestingly, the
plexes are the most active catalyst with 400 equiv of AlMe3 as
the co-catalyst.
bisligated complexes 4a and 4b are essentially inert for ethyl-
ene polymerization, and almost no polymer was obtained
under the optimum conditions. Such phenomena may be
Conflicts of interest
ascribed to the tight chelation of the ligands which prohibits
There are no conflicts to declare.
the approach of the ethylene to the nickel center. The 13C
NMR analysis showed that the polymers produced with 2a/
AlMe3 have a lower degree of branching when compared with
those produced with 1/AlMe3 suggesting that the mechanisms
Acknowledgements
in the two systems are different. The exact mechanism for 2a/ We are thankful for the financial support from the
AlMe3 is not clear so far. We believe that an oxidative coupling National Natural Science Foundation of China for project No.
of two ethylene molecules at the nickel(0) center may be the 21574052.
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