Half-sandwich chromium(III) complexes bearing β-ketoiminato and β-diketiminate ligands as catalysts for ethylene polymerization

Article abstract of DOI:10.1039/b820798b

Half-sandwich chromium(iii) complexes bearing β-ketoiminato and β-diketiminate ligands were synthesized and employed as catalysts for ethylene polymerization in the presence of triethylaluminium.

Full text of DOI:10.1039/b820798b

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Half-sandwich chromium(III) complexes bearing b-ketoiminato and
b-diketiminate ligands as catalysts for ethylene polymerization†
Yuan-Biao Huang and Guo-Xin Jin*
Received 20th November 2008, Accepted 25th November 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b820798b
Half-sandwich chromium(III) complexes bearing b-keto-
iminato and b-diketiminate ligands were synthesized and
employed as catalysts for ethylene polymerization in the
presence of triethylaluminium.
Since b-ketoiminato and b-diketiminate ligands, whose steric
and electronic properties can be easily modified, not only to have
stronger p-bonding tendencies to the same acceptor than salicy-
laldiminato ligands but also are able to stabilize metal complexes.12
b-Ketoiminato and b-diketiminate ligands are also the most ubiq-
uitous chelating ligands in coordination chemistry.2a,13 Especially,
the non-metallocene titanium and nickel complexes based on
b-ketoiminato and b-diketiminate ligands used for olefin poly-
merization have received wide interest in academic and industry
fields.2a,12–15 Recently, Smith’s group reported that half-sandwich
Cr(II) complexes containing large steric bulk b-diketiminate can
provide a platform for controlled radical polymerization of vinyl
acetate.8b Herein we report the half-metallocene chromium(III)
complexes bearing b-ketoiminato and b-diketiminate ligands,
which can be used as ethylene polymerization catalysts to produce
high molecular weight polyethylene with good catalytic activity
when activated with a small amount of triethylaluminium.
The chromium-based heterogeneous catalysts of Phillips and
Union Carbide systems (Cr/SiO2), are widely used in the large-
scale industrial production of polyolefins.1 The relatively ill-
defined nature of these paramagnetic olefin polymerisation cata-
lysts with respect to coordination environment and oxidation state,
however, have prompted considerable efforts to synthesize well
defined homogeneous single-site Cr-based catalysts.2 Group IV
metallocene catalysts are homogeneous single-site catalysts which
need a large amount of high cost cocatalyst such as MAO
(methylaluminoxane) or B(C6F5)3, hence it also promotes to
search for new polymerization catalyst.3 Activation of the non-
Cp (cyclopentadienyl) based chromium precatalysts such as those
bearing b-diketiminate ligands and salicylaldiminato ligands with
MAO leads generally to much lower activities compared to
those upon activation with alkylaluminium chlorides.2a,4 This
observation may indicate that the propagating centres are neutral,
rather than cationic alkyls, since an important mode of action
of MAO is the abstraction of an alkyl anion from the metal
precatalyst to give a cationic active site.2a Cp-based homogeneous
chromium catalysts, appearing to be close structural models of
the active site proposed for the Union Carbide heterogeneous
catalyst,1,5 have attracted intensive research interest.1,5–11 Although
it was shown that CpCr(acac)Br in the presence of aluminium
alkyls is active for the catalytic polymerization of ethylene,5a up to
now only a few half-sandwich type chromium catalysts have been
reported. Complexes [Cp*CrL2R]+A- (L = py, 1/2dppe, THF,
MeCN; R = Me, Et; A = PF6, BPh4; Cp* = pentamethylcy-
clopentadienyl) have been reported to exhibit catalytic activity
for ethylene polymerization.6 A Cp*Cr(acac)Cl–Et3Al system was
found to show a catalytic activity of 4.2 ¥ 104 g PE (polyethylene)
per mol Cr per h under 50 atm of ethylene.5b Recently, half-
sandwich salicylaldiminato chromium(III) catalyst, Cp*Cr[2,4-
The blue-green or green color half-sandwich Cr(III) complexes
1a–e and 2 were synthesized in 40–60% yields by the reactions
of CpCr(THF)Cl2 with the corresponding lithium salts of the
b-ketoiminato and b-diketiminate ligands in THF at -78 ◦C
(Scheme 1).‡ Attempts to use a similar salt metathesis reaction
to synthesize N-2,6-diisopropylphenyl-substituted b-diketiminate
chromium(III) complex was not successful, which may be due to the
bulky sterically hindered nature of the ((2,6-iPr2C6H3)NCMe)2CH
ligand.16 These Cr(III) complexes were characterized by IR and
1
elemental analysis. HNMR analyses indicate the paramagnetic
character of the Cr(III) complexes.1 The crystals of 1c and 2
suited for X-ray diffraction were obtained by recrystallization
from CH2Cl2–hexane solution at low temperature. The molecular
t
t
i
=
Bu2-6-(CH NR)–C6H2O]Cl/AlR¢3 [R = Bu, Ph, 2,6- Pr2C6H3;
i
R¢ = Me, Et, Bu] were reported and they can catalyze ethylene
polymerization to produce high molecular weight PE.11
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Advanced Materials Laboratory, Fudan Univer-
sity, Shanghai, 200 433, China. E-mail: gxjin@fudan.edu.cn; Fax: +86-21-
65641740; Tel: +86-21-65643776
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference numbers 705206 (1c) and 705207 (2). For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/b820798b
Scheme 1 Synthesis of complexes 1a–1e and 2.
Dalton Trans., 2009, 767–769 | 767
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t
=
structures of 1c and 2 along with selected bond lengths and angles
are depicted in Fig. 1.
chromium(III) complexes Cp*Cr[2,4- Bu2-6-(CH NR)–C6H2O]Cl
(5.5–83.7◦ and 48.2–56.5◦).11 The 2,6-diisopropylphenyl ring was
steeply inclined (110.8◦) to the plane formed by the Cr, O and N
atoms. In complex 2, The six-membered chelating ring Cr(1)–
N(1)–C(2)–C(3)–C(4)–N(2) almost locates in the same plane
˚
(Cr(1) deviates by 0.0815 A from the plane formed by the b-
diketiminate skeleton). The dihedral angles of the Cp ring with the
planes formed by the b-diketiminate skeleton and through Cr(1),
N(1) and N(2) atoms were 45.8◦ and 49.2◦, respectively. And the
dihedral angles of the plane formed by Cr(1), N(1) and N(2) atoms
with the two Ph rings were 70.2◦ and 78.9◦, respectively.
Notably, all the complexes are inactive for ethylene polymer-
ization in the presence of MAO. However, interestingly, upon
activation with a small amount of AlEt3, the blue-green or green
complexes 1a–1e and 2 changed to light pink and showed good
activity for ethylene polymerization under 1 bar, producing high
molecular weight (93–112 ¥ 104 g mol-1) PE. A summary of the
ethylene polymerization tests for 1a–1e and 2 is shown in Table 1.
13C NMR spectra (ESI†) of the PE samples showed that the
polymer produced by the chromium catalysts are almost linear
and have no branches. The melt transition temperatures (Tm)
of the obtained PE are in the range of 133.1–145.0 ◦C, and the
crystallinity of these polymers is in the range of 45.8–63.8%. The
b-ketoiminato catalysts 1a–1e (0.65–1.53 ¥ 105 g PE per mol Cr
per h) showed higher activities than the b-diketiminate complex
2 (0.09 ¥ 105 g PE per mol Cr per h). The reason may be the
bulky b-diketiminate was unfavourable for ethylene insertion.2a
And also the catalytic activities of the b-ketoiminato complexes
decrease with the increasing of the steric hindrance on the nitrogen
atom of the b-ketoiminato ligand (1a > 1b > 1c). These results
may be relevant to the dihedral angles of the Cp ring with the
b-ketoiminato skeleton, which indicated that the large sterically
opened degree in front of the chromium atom in the complexes was
favourable for ethylene access.11 The strong electron withdrawing
trifluoro methyl group introduced in the b-ketoiminato skeleton
enhanced the activity for the ethylene polymerization (1d > 1a;
1e > 1c), which appeared in the nickel and titanium complexes
for ethylene polymerization and norbornene polymerization.14
The catalytic activities decrease with increase of the temperature
(-15 to 60 ◦C), which indicated that the system was unstable at high
temperature. The highest catalytic activity of the b-ketoiminato
Fig. 1 Thermal ellipsoid plot of complexes 1c (a) and 2 (b) (30%
probability thermal ellipsoids). Selected bond lengths [A] and angles
˚
[◦]: 1c, Cr(1)–O(1) = 1.922(3), Cr(1)–N(1) = 2.042(4), Cr(1)–Cl(1) =
2.293(2), C(2)–O(1) = 1.272(6), C(4)–N(1) = 1.301(6), C(2)–(3) = 1.350(6),
C(3)–C(4) = 1.409(6), O(1)–Cr(1)–N(1) = 89.98(15), O(1)–Cr(1)–Cl(1) =
93.25(12), N(1)–Cr(1)–Cl(1) = 95.64(12). 2, Cr(1)–N(1) = 2.001(3),
Cr(1)–N(2)
=
1.995(3), Cr(1)–Cl(1)
=
2.3145(13), C(2)–N(1)
=
1.332(4), C(4)–N(2) = 1.345(4), C(2)–(3) = 1.386(5), C(3)–C(4) =
1.373(5), N(1)–Cr(1)–N(2) = 90.80(13), N(1)–Cr(1)–Cl(1) = 97.53(9),
N(2)–Cr(1)–Cl(1) = 96.62(9).
X-Ray crystallographic analysis shows that complex 1c and 2
have similar configurations (Fig. 1(a, b)), although there exist
two crystallographic independent molecules in the b-diketiminate
based complex 2. Both of the complexes 1c and 2 adopt
three-legged piano stool geometry with a pseudo-octahedral
coordination environment, containing a b-ketoiminato or b-
diketiminate ligand, a Cp ligand and chloride which were
similar to the Cp*Cr(acac)Cl and half-sandwich salicylaldim-
t
=
inato chromium(III) complexes Cp*Cr[2,4- Bu2-6-(CH NR)–
C6H2O]Cl.5b,11 Most of the metal–ligand bond lengths in the
complexes 1c and 2, including the Cp–Cr, Cr–Cl, and Cr–O and
Cp–N bond lengths, are close to that in some salicylaldiminato4c,11
and b-diketiminate4e chromium complexes and cyclopentadienyl
chromium complexes.5,16 In complex 1c, the chromium atom
˚
deviates by 0.6981 A from the plane formed by the b-ketoiminato
skeleton which was oriented almost orthogonal to the Cp ring
(98.4◦). The dihedral angle of the Cp ring and the plane
through Cr, O and N atoms was 127.9◦. Both of the two
dihedral angles were larger than that of the salicylaldiminato
Table 1 Results of ethylene polymerisation runs using catalysts 1a–e and 2a
Activityb
Mn
Tmd/◦C
Xc (%)
c
e
Run
Catalyst (4 mmol)
AlEt3 (mmol/equiv.)
Yield/g
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
2
1d
1d
1d
1d
1d
1d
100/25
100/25
100/25
100/25
100/25
100/25
40/10
200/50
400/100
100/25
100/25
100/25
0.19
0.17
0.13
0.24
0.21
0.02
Trace
0.20
0.19
0.28
0.30
Trace
0.94
0.85
0.65
1.21
1.06
0.09
—
1.03
0.95
1.42
1.53
—
104
101
99
108
105
95
—
96
93
134.9
134.1
132.0
135.4
134.3
135.5
—
133.7
133.1
145.0
143.8
—
50.9
50.1
63.8
50.7
55.4
49.6
—
45.8
60.1
54.6
62.9
—
10f
11g
12h
103
112
—
a Polymerization conditions: solvent 50 mL tol◦uene, temperature 25 ◦C, ethylene pressure 1 bar, time 30 min. b Units of 105 ◦g PE per mol Cr per h.
c Measured in decahydronaphthalene at 135 C, Units of 104 g mol-1. d Determined by DSC at a heating rate of 10 C min-1. e Crystallinity
◦
Xc = DHf/DHf◦, DHf = 273 J g-1 for completely crystalline PE. f Temperature 0 ◦C. g Temperature -15 ◦C. h Temperature 60 ◦C.
768 | Dalton Trans., 2009, 767–769
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complexes (1.53 ¥ 105 g PE per mol Cr per h, 1 bar) was lower
than that of the half-sandwich salicylaldiminato chromium(III)
complexes (4.04 ¥ 106 g PE per mol Cr per h-1, 5 bar),11 but higher
than Cp*Cr(acac)Cl/Et3Al system under more mild conditions.5b
When Al/Cr ratio was ca. 10 only trace of PE was obtained.
However, when the Al/Cr ratio reached 25 the highest catalytic
activity of the complex 1d was observed. When the Al/Cr ratio
further increases the activity decreases, which was similar to
to about 5 mL, and mixed with hexane (25 ml). Cooling to -30 ◦C to
give blue-green or green solid or crystals. Crystal data for complex 1c:
¯
C22H29ClCrNO, M = 410.91, triclinic, space group P1, a = 8.976(6), b =
◦
◦
◦
˚
9.965(6), c = 12.294(8) A, a = 83.284(9) , b = 81.187(9) , g = 81.543(10) ,
3
-3
˚
V = 1069.8(12) A , Dc = 1.276 g cm (Z = 2), R1 = 0.0551 (I > 2s(I))
and wR2 = 0.1513 (all data), reflections collected/unique = 4472/3714,
Rint = 0.0749. For complex 2: C22H22ClCrN2, M = 401.87, monoclinic,
˚
space group P21/c, a = 14.033(5), b = 23.567(8), c = 12.120(4) A, b =
◦
90.023(5) , V = 4008(2) A , Dc = 1.332 g cm-3 (Z = 8), R1 = 0.0454
3
˚
(I > 2s(I)) and wR2 = 0.1041 (all data), reflections collected/unique =
16489/3525, Rint = 0.0666. All the data were collected at 293(2) K on a
Bruker Smart APEX CCD diffractometer with graphite-monochromated
3
the Cp*Cr(C6F5)(h -Bz)/Et3Al system8a and half-sandwich type
salicylaldiminato chromium(III) catalyst system.11 Although we
have not been able to confirm the exact nature of the catalytic
active species, according to the results of the activity decrease
as the Al/Cr ratio increases and as previously suggested for
˚
Mo-Ka radiation (l = 0.71073 A). The structures were solved by direct
methods and subsequently refined on F2 by using full-matrix least-squares
techniques (SHELXL), absorption corrections were applied to the data.
The non-hydrogen atoms were refined anisotropically, and hydrogen atoms
were located at calculated positions. CCDC 705206 (1c) and 705207 (2).
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b820798b
3
the Cp*Cr(C6F5)(h -Bz)/Et3Al system8a and half-sandwich type
salicylaldiminato chromium(III) catalyst system11, we proposed
that the b-ketoiminato and b-diketiminate chromium system
may be through formation of a heterobimetallic chromium–
aluminium intermediate (Scheme 2). Ethylene can coordinate with
a heterobimetallic intermediate to form half-sandwich chromium–
ethylene mononuclear complex which is the active site. The
effect of trialkylaluminium concentration on the catalytic activity
suggests an equilibrium between the bridged heterobimetallic
chromium–aluminium complex and the ethylene insertion to form
mononuclear chromium active center, which promotes formation
of the catalytic cycle for ethylene polymerization.
1 K. H. Theopold, Eur. J. Inorg. Chem., 1998, 15.
2 (a) V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283;
(b) M. Enders, Macroml. Symp., 2006, 236, 38.
3 (a) Y. X. Chen and T. Marks, Chem. Rev., 2000, 100, 1391; (b) E.
Quintanilla, F. D. Lena and P. Chen, Chem. Commun., 2006, 4309.
4 (a) V. C. Gibson, P. J. Maddox, C. Newton, C. Redshaw, G. A. Solan,
A. J. P. White and D. J. Williams, Chem. Commun., 1998, 1651; (b) V.
C. Gibson, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White and
D. J. Williams, Eur. J. Inorg. Chem., 2001, 1895; (c) V. C. Gibson, S.
Mastroianni, C. Newton, C. Redshaw, G. A. Solan, A. J. P. White and D.
J. Williams, Dalton Trans., 2000, 1969; (d) V. C. Gibson, C. Newton, C.
Redshaw, G. A. Solan, A. J. P. White and D. J. Williams, Dalton Trans.,
1999, 827; (e) L. A. MacAdams, W.-K. Kim, L. M. Liable-Sands, I. A.
Guzei, A. L. Rheingold and K. H. Theopold, Organometallics, 2002,
21, 952.
5 (a) Y. Tajima, K. Tani and S. Yuguchi, J. Polym. Sci. Polym. Lett., 1965,
3B, 529; (b) O. Heinemann, P. W. Joly, C. Kruger and G. P. J. Verhovnik,
J. Organomet. Chem., 1998, 553, 477; (c) A. J. Gallant, K. M. Smith
and B. O. Patrick, Chem. Commun., 2002, 2914.
6 (a) B. J. Thomas and K. H. Theopold, J. Am. Chem. Soc., 1988, 110,
5902; (b) B. J. Thomas, S. K. Noh, G. K. Schulte, S. C. Sendlinger and
K. H. Theopold, J. Am. Chem. Soc., 1991, 113, 893.
7 (a) G. Bhandari, Y. Kim, J. M. McFarland, A. L. Rheingold and K.
H. Theopold, Organometallics, 1995, 14, 738; (b) Y. Liang, G. P. A.
Yap, A. L. Rheingold and K. H. Theopold, Organometallics, 1996, 15,
5284; (c) R. A. Heintz, S. Leelasubcharoen, L. M. Liable-Sands, A. L.
Rheingold and K. H. Theopold, Organometallics, 1998, 17, 5477; (d) A.
Dohring, J. Gohre, P. W. Jolly, B. Kryger, J. Rust and G. P. J. Verhovnik,
Organometallics, 2000, 19, 388.
8 (a) G. Mani and F. P. Gabbai, Angew. Chem. Int. Ed., 2004, 43, 2263;
(b) Y. Champouret, U. Baisch, R. Poli, l. Tang, J. L. Conway and K.
M. Smith, Angew. Chem. Int. Ed., 2008, 47, 6069.
9 J. S. Rogers and G. C. Bazan, Chem. Commun., 2000, 1209.
10 K. Ogata, Y. Nakayama and H. Yasuda, J. Polym. Sci. A: Polym. Chem.,
2002, 40, 2759.
11 T. Q. Xu, Y. Mu, W. Gao, J. G. Ni, L. Ye and Y. C. Tao, J. Am. Chem.
Soc., 2007, 129, 2236.
12 (a) D. Zhang, G.-X. Jin, L.-H. Weng and F. Wang, Organometallics,
2004, 23, 3270; (b) D. Zhang, G.-X. Jin and N. H. Hu, Chem. Commun.,
2002, 574.
Scheme 2 Heterobimetallic chromium–aluminium complex of type for
ethylene polymerization (L = ligands).
In summary, novel half-sandwich b-ketoiminato and b-
diketiminate chromium(III) complexes 1a–1e and 2 were synthe-
sized and employed as catalysts for the ethylene polymerization to
produce high molecular weight PE with good activity under mild
conditions in the presence of a small amount of AlEt3. Further
investigations exploring higher activity catalysts of this type and
possibilities copolymerization ethylene with other olefins using
these types of catalysts are underway.
Acknowledgements
This work was supported by the National Science Foundation
of China (20531020, 20721063, 20771028), by Shanghai Lead-
ing Academic Discipline project (B108), by the National Basic
Research Program of China (2005CB623800) and by Shanghai
Science and Technology Committee (08dj1400100, 08dj2270500).
13 (a) L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031; (b) S. J. Geier and D. W. Stephan, Chem. Commun., 2008,
2779.
Notes and references
14 For examples: (a) H.-Y. Wang, J. Zhang, X. Meng and G.-X. Jin, J.
Organomet. Chem., 2006, 691, 1275; (b) Y. Li, H. Gao and Q. Wu, J.
Polym. Sci. A: Polym. Chem, 2007, 46, 93; (c) W.-P. Ye, J. Zhan, L.
Pan, N.-H. Hu and Y.-S. Li, Organometallics, 2008, 27, 3642; (d) B. M.
Boardman, J. M. Valderrama, F. Mun˜oz, G. Wu, G. C. Bazan and R.
Rojas, Organometallics, 2008, 27, 1671; (e) S.-M. Yu and S. Mecking,
J. Am. Chem. Soc., 2008, 130, 13204.
15 L. A. MacAdams, G. P. Buffone, C. D. Incarvito, A. L. Rheingold and
K. H. Theopold, J. Am. Chem. Soc., 2005, 127, 1082.
16 J. C. Doherty, K. H. D. Ballem, B. O. Patrick and K. M. Smith,
Organometallics, 2004, 23, 1487.
‡ General synthesis procedures of complex 1a–d and 2: a solution of n-BuLi
(1.6M, 0.28 mL, 0.36 mmol) in hexane was added dropwise to a stirred
solution of ligand L1a–L1d and L2 (0.35 mmol) in THF (10 mL) at -78 ◦C.
The mixture was slowly warmed to room temperature and stirred for 6 h,
then was channeled at -78 ◦C to a solution of CpCr(THF)Cl2 which
was prepared by the reaction of CrCl3(THF)3 (0.14 g, 0.36 mmol) and
CpNa(THF)0.38 (0.041 g, 0.36 mmol) in THF (15 mL). The mixture was
slowly warmed to room temperature and continuously stirred overnight at
room temperature. A blue-green or green solution was obtained and the
solvent was removed under vacuum. After the residual solid was solved in
CH2Cl2 and filtered to remove inorganic salts, the filtrate was concentrated
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The Royal Society of Chemistry 2009
Dalton Trans., 2009, 767–769 | 769
©