Ethylene/allyl monomer cooligomerization by nickel/phosphine-sulfonate catalysts

Article abstract of DOI:10.1039/c2dt31771k

Nickel(II) complexes bearing a phosphine-sulfonate ligand, [(R ₂PC₆H₄SO₃)NiMe(2,6-lutidine)] (R = cyclohexyl, 2-MeOC₆H₄, and 2-[2′,6′-(MeO) ₂C₆H₃]C₆H₄, were synthesized and applied as catalysts for the cooligomerizations of ethylene and allyl monomers.

Full text of DOI:10.1039/c2dt31771k

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COMMUNICATION
Ethylene/allyl monomer cooligomerization by nickel/phosphine–sulfonate
catalysts†‡
Shingo Ito, Yusuke Ota and Kyoko Nozaki*
Received 2nd August 2012, Accepted 21st September 2012
DOI: 10.1039/c2dt31771k
Herein we report a new strategy for the synthesis of nickel/phos-
phine–sulfonate complexes by a convenient single-pot procedure
and the application of these complexes as catalysts for the cooli-
gomerization of ethylene and allyl monomers CH2vCHCH2FG
(3a, FG = OAc; 3b, FG = OSiMe3; 3c, FG = NHBoc; 3d, FG =
Cl; 3e, FG = SiMe3) (Scheme 1).17
Nickel phosphine–sulfonate complex 2a was prepared as
shown in Scheme 2. First, the reaction of Ni(cod)2 with alkyl-
phosphonium–sulfonate 1a in the presence of 2,6-lutidine, fol-
lowed by treatment with allyl chloride, generated a nickel
chloride complex by allyl chloride insertion followed by β-Cl
elimination. The nickel chloride complex was not isolated but
Nickel(II) complexes bearing a phosphine–sulfonate ligand,
[(R2PC6H4SO3)NiMe(2,6-lutidine)] (R cyclohexyl, 2-
MeOC6H4, and 2-[2′,6′-(MeO)2C6H3]C6H4, were synthesized
and applied as catalysts for the cooligomerizations of ethyl-
ene and allyl monomers.
=
Late transition metal-catalysed copolymerization of ethylene and
polar vinyl monomers is an attractive method for the synthesis of
functionalized polyethylenes.1,2 The discovery that such copoly-
merizations can be achieved using palladium catalysts bearing an
α-diimine ligand3 or a phosphine–sulfonate ligand1e,4 represents
a major advance towards this end. Chemists have explored
alternative transition metal catalysts that can also achieve high
catalytic activity, polymer molecular weight, and comonomer
incorporation ratio, but are more attractive for industrial appli-
cations because of their relative cost and availability.1e In this
regard, nickel is an attractive candidate. In some cases, nickel
catalysts have been reported to exhibit higher activity5,6 and
produce higher molecular-weight polymers7 than palladium cata-
lysts in ethylene homopolymerization. However, there are few
successful copolymerizations with polar vinyl monomers cata-
lysed by nickel complexes due to low compatibility with polar
functional groups.8–11 It is also noteworthy that nickel-catalysed
copolymerization of polar vinyl monomers using state-of-the-art
phosphine–sulfonate ligands has not yet been achieved.
Reports on the synthesis of nickel complexes bearing a phos-
phine–sulfonate ligand are few. A series of nickel η3-allyl7,12 and
η3-benzyl13 complexes were prepared by the reaction of a phos-
phine–sulfonate with the corresponding nickel precursors, but
they exhibited low catalytic activity for ethylene polymerization.
It was also reported that the reaction of a phosphine–sulfonate
with NiPhCl(PPh3)2 7,13,14 or the reaction of a phosphonium–sul-
fonate with Me2Ni(tmeda)15,16 in the presence of another ligand
L forms [P–O]NiRL (R = Ph or Me; L = pyridine or tmeda).
Scheme 1 Cooligomerization of ethylene and allyl monomers 3a–e by
nickel/phosphine–sulfonate complex 2.
Department of Chemistry and Biotechnology, Graduate School of
Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo
113-8656, Japan. E-mail: nozaki@chembio.t.u-tokyo.ac.jp;
Fax: (+81) 3-5841-7263; Tel: (+81) 3-5841-7261
†Dedicated to Professor David Cole-Hamilton on the occasion of his
retirement and for his outstanding contribution to transition metal
catalysis.
‡Electronic supplementary information (ESI) available: Experimental
procedures and spectra for new complexes and polymers. CCDC 894586
(for complex 2a). For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31771k
Scheme 2 Synthesis of nickel/alkylphosphine–sulfonate complex 2a.
Dalton Trans., 2012, 41, 13807–13809 | 13807
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treated with Me3Al, leading to the Ni methyl lutidine complex
2a in 52% overall yield. The complex 2a was characterized by
1H, 13C, and 31P NMR and ESI-MS analyses. X-ray crystallo-
graphic analysis confirmed that the structure of 2a, shown in
Fig. 1, is analogous to the corresponding palladium complex
reported previously.2c,18 Other nickel/arylphosphine–sulfonate
complexes 2b and 2c were prepared by a modified literature pro-
cedure.15 Complex 2c was not isolated as a pure compound due
to its instability, and therefore was used as a mixture for
cooligomerizations.
The nickel complexes 2a–2c were applied as catalysts for
ethylene oligomerization (Table 1, entries 1 and 12) and ethyl-
ene/allyl monomer cooligomerization (entries 2–11). Homo-
oligomerization of ethylene catalysed by 2a in CH2Cl2 (entry 1)
produced linear oligoethylenes of Mn = 600 with a small amount
of methyl and longer alkyl branches (ca. 3 per 1000C).19,20
Although the cooligomerization of ethylene and allyl acetate
(3a) at 80 °C gave no oligomers (entry 2), the reaction at lower
temperatures (30 °C and 0 °C) led to formation of the ethylene/
3a cooligomers with Mn values of up to 1000 (entries 3 and 4).
However, the catalytic activity significantly dropped at −20 °C
(entry 5). The low activity of 2b (entry 6) and 2c (entry 7),
which contain ortho-substituted arylphosphine substituents,
suggests that alkyl substitution on the phosphine is an important
factor for copolymerizations conducted with nickel/phosphine–
sulfonate catalysts. The cooligomerizations of other allyl mono-
mers 3b–3e were carried out using complex 2a (entries 8–11).
The reaction with allyl silyl ether (3b) gave comparable results
to allyl acetate (3a) in terms of molecular weight and incorpor-
ation ratio (entry 8). Protected allylamine 3c (entry 9) and allyl
chloride 3d (entry 10) did not afford the corresponding cooligo-
mers. In contrast, cooligomerization with allylsilane 3e21 exhib-
ited high activity to give a cooligomer with 0.75% incorporation
of 3e (entry 11).
1
Analyses of the cooligomers by H and 13C NMR revealed
linear oligoethylene backbones with low methyl branching (ca.
3 per 1000C) and –CH2FG groups attached to the main chain,
which is in accordance with complexes obtained using Pd/phos-
phine–sulfonate catalysts.17 Successive insertion of 3 was not
observed. Saturated alkyl groups were observed at the initiation
chain end, and terminal olefins were observed at the main ter-
mination chain end.22 Additionally, small amounts of 2-alkene
chain ends, presumably generated by isomerization from term-
inal alkene chain ends by nickel species,23 were observed in the
13C NMR spectrum. The results suggest that the cooligomeriza-
tion was initiated by the insertion of ethylene into the Ni–Me
bond of 2 and terminated via β-H elimination after ethylene
insertion or β-OAc elimination after 2,1-insertion of allyl acetate.
The Ni–hydride species formed in situ by β-H elimination is
competent to initiate a new oligomer chain.
A possible mechanism for the catalyst deactivation in the pres-
ence of allyl acetate is shown in Scheme 3. We confirmed that
the deactivation does not originate from the intermolecular
σ-coordination of the ester group of allyl acetate to the nickel
center (path [a]), because the ethylene homooligomerization in
the presence of propyl acetate instead of allyl acetate exhibited
Fig. 1 X-ray structure of nickel complex 2a bearing a dicyclohexyl-
phosphine–sulfonate ligand at 50% probability. Hydrogen atoms are
omitted for clarity. Selected bond lengths (Å) and angles (°): Ni1–P1,
2.1714(18); Ni1–O1, 1.972(4); Ni1–C1, 1.944(5); Ni1–N1, 1.951(4);
P1–Pd1–O1, 94.64(12).
Table 1 Cooligomerization of ethylene and allyl monomers 3a–e by nickel/phosphine–sulfonate complex 2a
CH2Cl2 Temp. Yieldb Activity
Alkyl branchd Incorp.e
(g mmol−1 h−1
)
Mn c (103) Mw/Mn
(1000C)
(%)
c
Entry Catalyst Comonomer (mL)
(mL)
(°C)
(g)
1
2
3
4
5
6
7
8
2a
2a
2a
2a
2a
2b
2c
2a
2a
2a
2a
2a
—
15
12
12
12
12
12
12
30
80
30
0
−20
30
30
30
30
30
30
30
2.07
Trace
0.072 1.2
0.058 1.0
Trace
Trace
Trace
34.6
—
0.6
—
0.9
1.0
—
—
—
0.8
0.6
—
1.9
—
1.9
2.0
—
—
—
1.9
1.9
—
3.4
—
2.1
ndf
—
—
—
0.24
0.19
—
—
—
0.20
<0.01
—
CH2vCHCH2OAc (3a) (3)
3a (3)
3a (3)
3a (3)
3a (3)
3a (3)
—
—
—
—
—
CH2vCHCH2OSiMe3 (3b) (3) 12
CH2vCHCH2NHBoc (3c) (3) 12
CH2vCHCH2Cl (3d) (3)
CH2vCHCH2SiMe3 (3e) (3)
—
0.038 0.6
ndf
ndf
—
9
0.12
Trace
2.93
1.69
2.0
—
48.8
28.2
10
11
12g
12
12
12
0.8
0.7
2.0
2.3
2.8
—
0.75
—
a Cooligomerization of allyl monomer 3 with ethylene (3.0 MPa) by nickel complex 2 (0.02 mmol) in CH2Cl2 was performed in a 50 mL autoclave,
unless otherwise noted. b Yield was determined after precipitation with MeOH. c Molecular weight measured by SEC with Tosoh TSKgel GMHHR-H
(S)HT columns using polystyrene as an internal standard and corrected by universal calibration. d Number of alkyl branches per 1000 carbons was
1
f
determined by quantitative 13C NMR analysis. e Molar ratio of incorporated allyl monomers was determined by H NMR analyses. Not determined.
g The polymerization was performed in the presence of propyl acetate (3 mL).
13808 | Dalton Trans., 2012, 41, 13807–13809
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Scheme 3 A possible mechanism for deactivation of nickel/phos-
phine–sulfonate catalysts in the presence of allyl acetate. [a] Intermole-
cular σ-coordination of the ester group of allyl acetate. [b] 2,1-insertion
of allyl acetate. [c] β-OAc elimination. [d] Disproportionation.
8 For Ni/α-diimine complexes, see: (a) L. Johnson, A. Bennett, K. Dobbs,
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27, 4711–4723.
10 For [P–O]Ni and [P–N]Ni complexes, see: L. Wang, E. Hauptman,
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group at a remote position, see: (a) U. Klabunde and S. D. Ittel, J. Mol.
Catal., 1987, 41, 123–134; (b) T. R. Younkin, E. F. Conner,
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similar catalytic activity (compare entries 1 and 12). We propose
that the deactivation occurs instead by the formation of complex
C from 2,1-insertion of allyl acetate into the nickel–alkyl bond
(path [b]), followed by β-OAc elimination from C to form
nickel–acetate D (path [c]). However, disproportionation (path
[d]) to form bis-ligated complex E, which is known to be NMR
silent, cannot be ruled out.14
In conclusion, we have synthesized novel nickel complexes
bearing a phosphine–sulfonate complex and investigated their
catalytic activity for the cooligomerization of ethylene and allyl
monomers. The copolymerization with allyl acetate (3a), allyl
silyl ether 3b, and allylsilane 3e gave the corresponding cooligo-
mers. The present study represents the first example of nickel/
phosphine–sulfonate-catalysed ethylene/allyl monomer cooligo-
merization. Further studies will focus on improving the comono-
mer incorporation ratios.
This work was supported by the Funding Program for Next
Generation World-Leading Researchers, Green Innovation and
the Global COE Program “Chemistry Innovation through
Cooperation of Science and Engineering” from MEXT/JSPS,
Japan.
18 CCDC-894586 for complex 2a contains the supplementary crystallo-
graphic data for this paper.
19 The results are in good accordance with data in the related reference. See
ref. 15.
20 The use of toluene as a solvent leads to essentially the same results.
21 (a) J. Liu and K. Nomura, Macromolecules, 2008, 41, 1070–1072;
(b) D.-J. Byun, S.-M. Shin, C. J. Han and S. Y. Kim, Polym. Bull., 1999,
43, 333–340. For review, see: (c) M. J. Yanjarappa and S. Sivaram,
Prog. Polym. Sci., 2002, 27, 1347–1398, and references cited therein.
22 See the ESI‡ for details.
Notes and references
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(b) A. Sen and S. Borkar, J. Organomet. Chem., 2007, 692, 3291–3299;
(c) A. Berkefeld and S. Mecking, Angew. Chem., Int. Ed., 2008, 47,
2538–2542; (d) E. Y.-X. Chen, Chem. Rev., 2009, 109, 5157–5214;
23 M. Kanazawa, S. Ito and K. Nozaki, Organometallics, 2011, 31, 6029–
6032.
This journal is © The Royal Society of Chemistry 2012
Dalton Trans., 2012, 41, 13807–13809 | 13809