Imidazolin-2-iminato titanium complexes: Synthesis, structure and use in ethylene polymerization catalysis

Article abstract of DOI:10.1039/b511752f

The Staudinger reaction of the imidazolin-2-ylidenes, 1,3-di-tert- butylimidazolin-2-ylidene (1a), 1,3-diisopropylimidazolin-2-ylidene (1b), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1c), 1,3-bis(2,4,6- trimethylphenyl)imidazolin-2-ylidene (1d) an

Full text of DOI:10.1039/b511752f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Imidazolin-2-iminato titanium complexes: synthesis, structure and use in
ethylene polymerization catalysis
Matthias Tamm,*a So¨ren Randoll,a Eberhardt Herdtweck,b Nina Kleigrewe,c Gerald Kehr,c Gerhard Erkerc and
Bernhard Riegerd
Received 17th August 2005, Accepted 28th September 2005
First published as an Advance Article on the web 28th October 2005
DOI: 10.1039/b511752f
The Staudinger reaction of the imidazolin-2-ylidenes, 1,3-di-tert-butylimidazolin-2-ylidene (1a),
1,3-diisopropylimidazolin-2-ylidene (1b), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1c),
1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (1d) and 1,3-bis(2,6-diisopropylphenylimidazolin-
2-ylidene (1e), with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazolines
2a–e, which react with [(g-C5H5)TiCl3] to afford half-sandwich cyclopentadienyl titanium complexes of
the type [CpTi(L)Cl2] (3) (L = imidazolin-2-iminato ligand). Similarly, the reactions of 1,3-di-tert-
butyl-2-(trimethylsilylimino)imidazoline (2a) with [(g-tBuC5H4)TiCl3] results in the formation of
[(g-tBuC5H4)Ti(L)Cl2] (4) (L = 1,3-di-tert-butylimidazolin-2-imide). Bis(1,3-di-tert-butylimidazolin-
2-iminato)titanium dichloride (5) is obtained from the reaction of two eq. of 2a with TiCl4. Treatment
of 5 with methyllithium results in the formation of the corresponding dimethyl complex [L2Ti(CH3)2]
(6), whereas [CpTi(L)(CH3)2] (7) is similarly obtained from 3a. The molecular structures of 3a, 3b, 3c,
3e·C7H8, 4 and 7 are reported revealing linearly coordinated imidazolin-2-iminato ligands together with
very short Ti–N bond distances. All dichloro complexes (3a–e, 4 and 5) can be activated with
methylaluminoxane (MAO) to give active catalysts for ethylene homopolymerization. In most cases,
moderate to high activities are observed together with the formation of high (HMWPE) or even ultra
high molecular weight polyethylene (UHMWPE).
Introduction
Non-metallocene systems are nowadays playing a key role in the
quest for novel, highly efficient olefin polymerization catalysts.1
Among other systems, monocyclopentadienyl titanium complexes
of the type CpTi(L)X2, containing an additional monoanionic an-
cillary ligand L, represent an important class of precatalysts for the
homo- and copolymerization of olefins. Typical ligands L include
Fig. 1 Mesomeric structures for imidazolin-2-iminato ligands I.
p-donors such as aryloxides,2 ketimides,3 phosphoraneimides4 and
guanidinates,5 which can be regarded as monodentate analogues
to cyclopentadienyls, C5R5, due to their capability to act as 2r,4p-
silylated 2-iminoimidazolines,9 which can be directly used for
electron donors. We have recently established a general method
for the synthesis of related imidazolin-2-iminato ligands,6,7 which
can be described by the two limiting resonance structures IA and
IB (Fig. 1), indicating that the ability of the imidazolium ring to
effectively stabilize a positive charge should lead to highly basic
ligands with a strong electron donating capacity.
Our preparative route is based on the observation that stable
carbenes of the imidazolin-2-ylidene type undergo a Staudinger
reaction8 upon treatment with trimethylsilyl azide to furnish N-
the synthesis of transition-metal imidazolin-2-iminato complexes
from metal halides or oxides, respectively.6,7 Previous work10 in
this area is rather limited and had been confined to the use of the
ligand precursor 2-imino-1,3-dimethylimidazoline obtained by a
multi-step protocol from 2-aminoimidazole.11 In contrast, the co-
ordination chemistry of related phosphoraneiminato ligands has
been extensively studied producing a large number of structurally
diverse main group and transition-metal complexes.12 Since it can
be envisaged that the chemistry of imidazolin-2-iminato ligands
could develop into an equally flourishing area of research, we
have started a program to study their coordination chemistry as
well as their application as ancillary ligands for the design and
preparation of novel homogeneous catalysts. In this regard, we
wish to present herein the preparation of titanium complexes of
the type CpTi(L)X2 and L2TiX2 (L = imidazolin-2-imide, X =
Cl, CH3) and their use in ethylene polymerization catalysis. A
preliminary report on the synthesis of a limited number of such
titanium complexes has been published previously.6
aInstitut fu¨r Anorganische und Analytische Chemie, Technische Universita¨t
Carolo-Wilhelmina, Hagenring 30, D-38106, Braunschweig, Germany.
E-mail: matthias.tamm@tu-bs.de; Fax: +49 (251) 391-5309; Tel: +49
(251) 391-5387
bDepartment Chemie, Technische Universita¨t Mu¨nchen, Lichtenbergstr. 4,
D-85747, Garching, Germany
cOrganisch-Chemisches Institut, Westfa¨lische Wilhelms-Universita¨t, Cor-
rensstr. 40, D-48149, Mu¨nster, Germany
dAnorganische Chemie II, Universita¨t Ulm, Albert Einstein Allee 11, D-
89069, Ulm, Germany
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Preparation and characterization of cyclopentadienyl-titanium
imidazolin-2-iminato complexes
Results and discussion
Preparation and characterization of N-silylated
2-iminoimidazolines
Silylated iminophosphosphoranes have been widely used for com-
plexation reactions with various metal halides or oxides to yield
phosphoraneiminato complexes together with the elimination of
trialkylsilyl halides or hexaalkyldisiloxanes, respectively.12 Accord-
ingly, half-sandwich complexes of the type CpTi(L)Cl2 (3) can be
isolated as red crystalline solids in high yields from the reaction
of [(g-C5H5)TiCl3] with the 2-(trimethylsilylimino)imidazolines 2
in toluene overnight (Scheme 1). In general, desilylation and
coordination to the titanium atom does not have a significant
impact on the resonances observed for the hydrogen and carbon
atoms of the heterocyclic imidazoline moiety. For the N-alkylated
derivatives3a–c, the Cp hydrogen resonances are observed at about
6.5 ppm, whereas the N-arylated derivatives 3d and 3e exhibit the
corresponding resonances at higher field at about 6.0 ppm due to
shielding by the aryl substituents. The Cp carbon resonances for
all complexes 3 are found in the range between 114 and 116 ppm.
The molecular structure of 3a has been previously reported.6 To
compare the structural changes upon variation of the imidazoline
substituents, the structures of 3b, 3c and 3e·C7H8 have additionally
been established by X-ray diffraction analyses. The structural
parameters are assembled in Table 1, and ORTEP presentations
are depicted in Fig. 2 (3b, 3c) and Fig. 3 (3e), respectively. In
As previously described,6,7 the 2-(trimethylsilylimino)imidazolines
2 are generally accessible from the reaction of the imidazolin-2-
ylidenes 1 with trimethylsilyl azide in boiling toluene (Scheme 1).
2a (R = tBu, Rꢀ = H), 2d (R = Mes, Rꢀ = H) and 2e (R =
Dipp, Rꢀ = H) are obtained as pale yellow solids, which can
efficiently be recrystallized from hexane. In contrast, brownish oils
are isolated from the reactions of the N-diisopropylimidazolin-2-
ylidenes 1b and 1c with Me3SiN3, and the resulting N-silylated
imines 2b (R = iPr, Rꢀ = H) and 2c (R = iPr, Rꢀ = CH3)
can be purified by bulb-to-bulb distillation at 180 ◦C/9 mbar.
The conversion of the carbenes bearing 4,5-hydrogen atoms (1a,
1
1b, 1d, 1e) can easily be followed by H NMR spectroscopy as
pronounced high-field shifts of about −0.70 ppm are observed for
the resonances of the NCH hydrogen atoms upon formation of the
corresponding imines. For the conversion of 1,3-diisopropyl-4,5-
dimethylimidazolin-2-ylidene (1c) into the corresponding imine
2c on the other hand, a marked low-field shift of the septet
CH resonance from 3.95 to 4.61 ppm is indicative of product
formation. In contrast, this resonance remains almost unchanged
when 1,3-diisopropylimidazolin-2-ylidene (1b) is employed. In the
13C NMR spectra of all imines 2, the resonances of the former
carbene carbon atoms are found around 140 ppm, which is
approximately 90 ppm up-field from the corresponding resonances
in the free carbenes 1. According to their 1H and 13C NMR spectra,
the imines 2 exhibit pseudo-C2v symmetry in solution implying that
rotation around the N1–C1 axis is fast on the NMR time scale.
It should be noted that the C1–N1–Si angle in 2a was found to
be close to linearity [169.3(2)◦];6,7 depending on the substitution
pattern, however, sign◦ificantly smaller angles can be observed, e.g.
C1–N1–Si = 147.2(1) in 2d.13
Fig. 2 ORTEP drawings of 3b (top) and 3c (bottom) with thermal
Scheme 1 Mes = 2,4,6-trimethylphenyl, Dipp = 2,6-diisopropylphenyl.
460 | Dalton Trans., 2006, 459–467
ellipsoids drawn at 50% probability.
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to the steric requirements of both the cyclopentadienyl and of
the respective imidazolin-2-iminato ligand, the angles at titanium
between the Cp centroid and the nitrogen atom N1 are significantly
larger than the corresponding Cp–Ti–Cl angles or the N–Ti–Cl
and Cl–Ti–Cl angles, respectively (Table 1). In 3a, the molecule
resides on a crystallographic mirror plane, and the five-membered
imidazoline ring consequently adopts a perfectly perpendicular
orientation toward the pseudo mirror plane containing N1, Ti
and the centroid of the cyclopentadienyl ring (dihedral angle =
90◦).6 In contrast, 3b (61.7◦), 3c (75.4◦) and 3e (52.4◦) exhibit sig-
nificantly smaller torsion angles, and these considerable deviations
from regular horizontal conformations suggest that mainly crystal
packing forces account for the orientation of the imidazolin-2-
iminato ligands. Further evidence for a relatively shallow potential
energy surface of complexes 3 with respect to the orientation of the
imidazoline moiety stems from temperature-dependent 1H NMR
spectroscopy, since a stable horizontal conformation would render
the isopropyl methyl groups in 3b and 3c diastereotopic. In the
temperature range between −80 and 20 ◦C, however, only one
isopropyl CH3 hydrogen resonance can be observed revealing that
rotation about the Ti–N1–C1 axis is fast on the NMR time-scale
even at low temperature.
˚
Short Ti–N1 distances ranging from 1.765(3) A in 3a to
Fig. 3 ORTEP drawing of 3e in 3e·C7H8 with thermal ellipsoids drawn
˚
1.778(2) A in 3e are observed together with almost linear
at 50% probability.
geometries about the N1 atom, which is indicative of efficient
ligand-to-metal p-donation confirming that the ligand can be
regarded as a six-electron donor. Thereby, the isopropyl derivatives
all cases, a three-legged piano-stool structure is observed with
the titanium atom adopting a pseudo-tetrahedral geometry. Due
◦
˚
Table 1 Selected bond lengths (A) and angles ( )
3a
3b
3c
3e·C7H8
4
7
Ti–Cpav
Ti–Cpct
Ti–Cprange
Ti–N1
Ti–Cl1
Ti–Cl2
Ti–CMe1
Ti–CMe2
N1–C1
N2–C1
N2–C2
C2–C3
N3–C3
N3–C1
2.372
2.064
2.361
2.045
2.355
2.048
2.374
2.058
2.400
2.085
2.405
2.092
2.338(5)–2.414(3) 2.336(3)–2.381(2) 2.333(2)–2.374(4) 2.334(2)–2.419(3) 2.327(3)–2.501(2) 2.383(2)–2.428(2)
1.765(3)
1.768(2)
2.3118(6)
2.3184(6)
1.762(2)
2.3187(7)
2.3028(7)
1.778(2)
2.3088(7)
2.2977(8)
1.774(2)
2.3197(8)
2.3199(8)
1.805(1)
2.3253(6) × 2b
2.161(2)
2.161(2)
1.310(2)
1.389(2)
1.387(2)
1.333(2)
1.388(2)
1.386(2)
1.332(4)
1.373(2)
1.382(3)
1.331(3)b
1.324(2)
1.366(2)
1.389(2)
1.342(3)
1.385(2)
1.361(2)
1.324(2)
1.366(2)
1.399(3)
1.349(3)
1.393(2)
1.357(3)
1.309(2)
1.371(2)
1.394(2)
1.333(3)
1.391(2)
1.366(2)
1.331(3)
1.375(3)
1.383(3)
1.335(4)
1.389(3)
1.375(3)
Ti–N1–C1
N1–Ti–Cl1
N1–Ti–Cl2
170.7(2)
163.3(1)
105.61(5)
102.02(5)
164.6(1)
103.09(6)
102.93(5)
175.8(1)
104.17(5)
104.15(5)
172.6(2)
101.37(7)
103.33(7)
176.0(1)
103.47(5) × 2b
N1–Ti–CMe1
N1–Ti–CMe2
N1–C1–N2
CpCt–Ti–N1
C1–N2–C2
103.63(7)
104.73(7)
127.5(1)
124.35
108.8(1)
108.5(2)
126.9(1)
121.37
108.7(2)
108.2(2)
102.10(3)b
112.16 × 2b
128.0(2)
119.66
108.7(1)
107.9(2)
99.22(2)
113.92
126.2(2)
118.02
109.0(2)
107.1(2)
104.04(3)
111.92
128.9(2)
119.72
109.4(2)
107.6(2)
100.49(3)
113.04
126.8(2)
122.83
108.5(2)
108.4(2)
102.29(3)
112.89
N2–C2–C3
Cl1–Ti–Cl2
CpCt–Ti–Cl1
CpCt–Ti–Cl2
CMe1–Ti–CMe2
CpCt–Ti–CMe1
CpCt–Ti–CMe2
Interplanar anglea
113.87
115.20
113.11
111.73
95.92(8)
112.77
111.45
89.8
90
61.7
75.4
52.4
89.3
a Angle between the least-squares planes containing the heterocycle (C1, C2, C3, N2, N3) and Ti, N1 and the Cp centroid, respectively. b Molecule 3a
resides on a crystallographic mirror plane.
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3b [163.3(1)◦] and 3c [164.6(1)◦] exhibit the smallest Ti–N1–
C1 angles, whereas considerably larger angles are produced by
the sterically more demanding ligands in 3a [170.7(2)◦] and 3e
[175.8(1)◦]. In agreement with a strong contribution of canonical
form IB to the ground-state electronic structure of the imidazolin-
2-iminato ligands in complexes 3 (Fig. 1), the observation of short
Ti–N1 distances goes along with a pronounced elongation of the
corresponding N1–C1 distances, which in the case of 3a [1.332(4)
for the corresponding complex 3a. To evaluate structural changes
upon the introduction of the sterically demanding tert-butyl group,
crystals of complex 4 were subjected to an X-ray diffraction
analysis, and the molecular structure is shown in Fig. 4. To
minimize steric interaction, the imidazolin-2-iminato ligand and
the tert-butyl Cp substituent adopt a transoid orientation, and
an almost perfect horizontal conformation is observed for the
heterocyclic five-membered ring with respect to the pseudo-mirror
plane containing N1, Ti and the centroid of the cyclopentadienyl
ring (dihedral angle = 89.3◦). Due to the introduction of an
additional tert-butyl group, the cyclopentadienyl ring in 4 is
coordinated in a more asymmetric fashion than in 3a, and
˚
˚
A], 3b and 3c [both 1.324(2) A] adopt an intermediate position
2
between the values expected for a typical N–Csp single (1.38
14
˚
˚
A) and double bond (1.28 A), respectively. Consequently, the
shortest N1–C1 distance is observed for the diarylated derivative
˚
˚
3e [1.309(2) A], which at the same time exhibits the longest Ti–N1
the Ti–C distances range from 2.327(3) to 2.501(2) A with
bond length (vide supra, Table 1).
the largest separation being observed between titanium and the
quarternary ipso-carbon atom. Apart from the differences in Cp
coordination, the bond distances and angles involving Ti, N1 and
the two chlorine atoms as well as the structural parameters of the
imidazolin-2-iminato ligand are very similar to those established
for 3a (Table 1).6
Comparable structural parameters about the titanium atom
have been observed for complexes of the type CpTi(NPR3)Cl2 (R =
cyclo-C6H11, iPr, tBu) containing strong p-donating trialkylphos-
phoraneiminato ligands with Ti–N distances ranging between
4a
˚
1.750(3) and 1.775(11) A. The structural characterization of a
closely related guanidinate complex with a saturated 1,3-diphenyl-
4,5-dihydroimidazolin-2-iminato ligand reveals a similarly short
˚
but nevertheless notably longer Ti–N distance [1.792(2) A] and a
stronger deviation from a linear Ti–N–C orientation [152.9(2)◦].5
These findings indicate that hydration and saturation of the C–
C double bond leads to a decreased p-electron release capability
in comparison with the unsaturated imidazolin-2-iminato ligands
presented in our study. In addition, it should be noted that ketimide
=
complexes of the type CpTi(N CR2)Cl2, which have also been
employed in ethylene homopolymerization,3 exhibit much longer
Ti–N together with shorter N–C distances, e. g. Ti–N = 1.872(4)
ꢀ
ꢀ
˚
˚
=
A and N–C = 1.267(6) A in CpTi(N CRR )Cl2 (R = nBu, R =
tBu).3d
Finally, it should be noted that the Ti–N distances in the
structurally characterized complexes 3 reported herein fall in the
same range as the Ti–O separations in aryloxide complexes of
the type CpTi(OAr)Cl2 despite the larger size of the nitrogen
atom. This further supports the strong p-donating ability of
imidazolin-2-iminato ligands, which can be regarded as highly
p-basic aryloxide analogues.2l,m,15
Since variations of the substitution pattern not only of the
monodentate but also of the cyclopentadienyl ancillary ligand
often have a great impact on the level of catalytic activity,1–4 we
have synthesized complex 4 bearing a tert-butylcyclopentadienyl
(tBuC5H4) ligand in addition to the cyclopentadienyl-titanium
complexes 3 (Scheme 2). 4 could be readily obtained in high
Fig. 4 ORTEP drawing of 4 with thermal ellipsoids drawn at 50%
probability.
Preparation and characterization of bis(imidazolin-2-iminato)
titanium complexes and alkylation reactions
5
yield from the reaction of [(g -tBuC5H4)TiCl3] with 2a in hexane
overnight. Apart from the fact that additional 1H and 13C
NMR resonances are observed for the substituted Cp ring, the
spectroscopic characteristics of 4 are very similar to those observed
As described previously,6 bis(1,3-di-tert-butylimidazolin-2-
iminato)titanium dichloride
5 can be obtained in almost
quantitative yield as an orange crystalline solid from the reaction
of TiCl4 with two eq. of the silylated imine 2a in boiling toluene
for 48 h (Scheme 3). 5 could subsequently be converted into the
yellow dimethyl complex 6 by treatment with methyllithium in
diethyl ether. The alkylation can be easily followed by NMR
spectroscopy, since characteristic CH3 resonances are observed at
1.10 and 42.1 ppm in the 1H and 13C NMR spectra, respectively.
The molecular structure of 6 was additionally established by
X-ray diffraction analysis, and an ORTEP presentation is shown
in Fig. 5. The complex exhibits the expected pseudo-tetrahedral
geometry about titanium, and the Ti–N–C angles are close
Scheme 2
462 | Dalton Trans., 2006, 459–467
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Scheme 4
of the metal-coordinated methyl groups are observed at 0.74
and 27.9 ppm, respectively, significantly upfield from the
corresponding resonances found for the [L2Ti(CH3)2] analogue
6. To evaluate structural changes upon methylation, 7 was
characterized crystallographically as well, and the molecular
˚
structure is presented in Fig. 6. The Ti–C [both 2.161(2) A] and
˚
Ti–N separations [1.805(1) A] fall in the expected ranges and
are similar to those in 6. The imidazolin-2-iminato ligand is
linearly bound [Ti–N1–C1 = 176.0(1)◦], and the five-membered
heterocycle adopts an almost perfect horizontal conformation
with a torsion angle of 89.8◦ (Table 1). Comparison with the
structural data of the corresponding dichloro complex 3a reveals
Scheme 3
˚
a longer Ti–N1 separation [1.805(1) vs. 1.765(3) A] together
˚
with a shorter N1–2C1 separation [1.310(2) versus 1.332(4) A],
which indicates that methylation weakens the interaction between
titanium and the imidazolin-2-iminato ligand. The successful
preparation of complexes 6 and 7 implies that similar dimethyl
derivatives might be involved as intermediate pre-catalysts in the
homopolymerization of ethylene upon activation of complexes of
type 3 and 4 with methylalumoxane (vide infra).
Fig. 5 ORTEP drawing of 6 with thermal ellips◦oids drawn at 50%
˚
probability. Selected bond lengths (A) and angles ( ): Ti–C23 2.114(2),
Ti–C24 2.110(2), Ti–N1 1.819(1), Ti–N4 1.828(1), N1–C1 1.300(2),
N2–C1 1.388(2), N2–C2 1.388(2), C2–C3 1.325(3), N3–C3 1.390(2)
N3–C1 1.383(2), N4–C12 1.299(2), N5–C12 1.388(2), N5–C13 1.396(2),
C13–C14 1.312(3), N6–C14 1.381(2), N6–C12 1.390(2); Ti–N1–C1
176.1(1), N1–Ti–C23 105.75(8), N1–Ti–C24 109.69(7), N1–C1–N2
128.1(1), C1–N2–C2 108.9(1), N2–C2–C3 108.6(2), Ti–N4–C12 171.9(1),
N4–Ti–C23 109.81(8), N4–Ti–C24 109.65(8), N4–C12–N5 127.2(2),
C12–N5–C13 108.1(2), N5–C13–C14 108.9(2), N1–Ti–N4 116.36(6).
Fig. 6 ORTEP drawing of 7 with thermal ellipsoids drawn at 50%
probability.
to linearity [Ti–N1–C1 = 176.1(1)◦, Ti–N4–C12 = 171.9(1)◦].
˚
The Ti–C distances of 2.114(2) and 2.110(2) A as well as the
˚
Ti–N distances of 1.819(1) and 1.828(1) A are very similar to
Ethylene polymerization catalysis
those observed for the closely related bis(phosphoraneiminato)
˚
complex [(tBu3PN)2Ti(CH3)2] [Ti–C = 2.121(3), 2.129(3) A;
In order to compare their efficiency in olefin polymerization
catalysis, the dichloro complexes 3a–e, 4 and 5 were tested for
ethylene homopolymerization. Upon activation with methyla-
luminoxane (MAO), all compounds give active catalysts, and
the results are summarized in Table 2. The properties of the
isolated polyethylene (PE) were determined by means of DSC
and GPC techniques. Melting points of the polyethylene samples
4c
˚
Ti–N = 1.824(2), 1.830(2) A].
As
a
representative example for the half-sandwich
mono(imidazolin-2-iminato) complexes of the type CpTi(L)Cl2
(3), 3a was also dialkylated by treatment with methyllithium
to afford complex 7 as an orange crystalline solid in almost
quantitative yield (Scheme 4). The H and 13C NMR resonances
1
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Table 2 Ethylene polymerisation dataa
Pre-catalyst (M) m (pre-cat.)/mg; n (pre-cat.)/mmol Al/Tib
Reaction time/min Yield PE/g Activityc
Mp/◦C Mw
PDIe,f
f
3a (378.18)
20; 0.053
20; 0.053
20; 0.057
20; 0.053
20; 0.040
20; 0.034
20; 0.034
20; 0.046
20; 0.046
20; 0.046
20; 0.046
20; 0.039
122/1
122/1
113/1
122/1
162/1
189/1
189/1
140/1
140/1
140/1
140/1
164/1
15
15
60
50
60
30
30
15
10
5
10.8
10.0
3.0
2.6
4.4
4.5
3.6
13.4
7.1
6.5
5.0
0.2
408
378
26
30
55
132
106
583
463
848
652
8
126
—
960 000
1 200 000
2 400 000d
2 700 000
3 000 000
1 200 000
750 000
600 000
450 000
280 000
2.2
2.6
2.4
2.4
2.6
5.1g
4.5g
2.7
2.0
2.2
2.1
3b (350.12)
3c (378.18)
3d (502.32)
3e (586.48)
120
127
131
126
127
126
128
129
—
4 (434.28)
5
20
500 000
5 (507.38)
123
1 800 000d 16.7h,i
a Reactions in toluene solution at 25 ◦C, 2 bar ethene. b 3.5 mL of a 10% [m/m] solution of MAO in toluene (EURECEN AL 5100-10-toluene from
Crompton GmbH, Bergkamen; q = 0.89 g cm−3) with an overall Al concentration between 4.60 and 5.60% [m/m]. c Activity = g(PE) mmol(pre-cat.)−1
d
1
bar(ethene)−1 h−1
.
13C{ H} (1,2,4-trichlorobenzene–d6-benzene, 350 K): d 30.1 ppm. e The molecular weight distributions were determined by Prof.
B. Rieger (Ulm); 1,2,4-trichlorobenzene at 145 ◦C. f PDI = Mw/Mn. g Bimodal distribution. h Multimodal distribution. i Highly asymmetric curve
progression.
were found to be in the range 117–131 ◦C. Comparison of
the results obtained for complexes 3a–3c indicates that bulky
substituents on the imidazoline nitrogen atoms are beneficial for
the catalytic activity, which was found to be high for the tert-
butyl derivative 3a and moderate for the isopropyl derivatives
3b and 3c.1a,c Furthermore, substitution in 4,5-position of the
imidazoline ring does not seem to have a significant impact on
the activity. GPC data for the PE derived from complexes 3b
and 3c show the formation of high molecular weight PE with
Mw in the range of 2 500 000 g mol−1 and polydispersities of 2.4.
The catalysts derived from the aryl substituted compounds exhibit
moderate (3d) to high activities (3e).1a,c In the case of 3d, linear
polyethylene was obtained with an ultra high molecular weight
of Mw = 3 000 000 g mol−1 (UHMWPE) and a polydispersity of
2.6. The introduction of a sterically demanding substituent on
the cyclopentadienyl ring in compound 4 significantly improved
the activity in comparison to the catalytic system 3a/MAO. The
catalyst derived from 4 represents the most active system within
this series with an activity of up to 848 g (PE) mmol−1 (pre-
cat.) h−1 bar−1. For the non-metallocene complex 5, only a low
activity was observed, but also with this system, PE with high
molecular weight (Mw = 1 800 000) was obtained. A limited
activity in ethylene polymerisation was also reported for the
structurally related phosphoraneiminato complex (tBu3PN)2TiCl2
in combination with MAO as an activator.4c
The relatively narrow polydispersities of the PE derived from
complexes 3a–d and from 4 indicate the operation of single-site
catalysts. In contrast, bimodal or even multimodal molecular
weight distributions are observed for the polymers obtained by
use of 3e and 5, respectively, which might reveal the presence
of more than one active site. Although the mechanism of pre-
catalyst activation by MAO can not be fully explained at this
stage, the possibility to isolate stable dimethyl complexes such as 6
and 7 suggests the formation of similar dialkyl intermediates upon
reaction with the MAO. It remains to be investigated whether sub-
sequent methyl abstraction affords the presumed active catalysts
of type [CpTi(L)Me]+ or leads to the additional formation of other
species. To further elucidate the pathway of catalyst generation,
future work in this field will focus on the use of single-component
activators such as the borane B(C6F5)3, or the trityl and anilinium
salts [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4], respectively, for
the activation of imidazolin-2-iminato titanium dialkyl complexes.
Experimental
All operations were performed in an atmosphere of dry argon
by using Schlenk and vacuum techniques. All solvents were
1
purified by standard methods and distilled prior to use. H and
13C NMR spectra were recorded on JEOL-GX 270 (270 MHz)
and JEOL-GX 400 (400 MHz) instruments. The syntheses of
compounds 3a and 5 have been previously communicated.6 for
reason of comparison their characterization is also included
5
5
here. [(g -C5H5)TiCl3],16 [(g -tBuC5H4)TiCl3],17 the imidazolin-2-
ylidenes 1a–e18,19 and the 2-(trimethylsilylimino)imidazolines 2a,
2c and 2d7 were prepared according to literature procedures.
Preparations
General procedure for the preparation of 2-(trimethylsilylimino)-
imidazolines 2. A solution of the respective imidazolin-2-ylidene
1 (10 mmol) in toluene (20 mL) was treated dropwise with
trimethylsilyl azide (14 mmol in case of 1a and 1c–e; 10 mmol
in case of 1b) at ambient temperature, and the resulting reaction
mixture was subsequently heated in boiling toluene for 72 h.
Filtration and evaporation of the solvent afforded the imines as
yellowish solids (2a, 2d and 2e) or as a brownish oils (2b and 2c),
respectively, of which the latter can be purified by bulb to bulb
distillation at 180 ◦C/9 mbar.
2a. Yield: 88% (Found: C, 62.63; H, 11.09; N, 15.57%. Calc.
for C14H29N3Si: C, 62.86; H, 10.93; N, 15.71%); dH (400 MHz,
C6D6) 6.03 (2 H, s, NCH), 1.36 (18 H, s, CCH3) and 0.52 (9 H, s,
SiCH3); dC (100.52 MHz, C6D6) 139.7 (NCN), 107.5 (NCH), 54.2
(NCMe), 28.1 (CCH3) and 4.4 (SiCH3).
2b. Yield: 60%; dH (400 MHz, C6D6) 5.89 (2H, s, NCH), 4.40
(2H, sept, CHMe), 0.98 (12 H, d, CCH3) and 0.43 (9 H, s,
SiCH3); dC (100.52 MHz, C6D6) 143.2 (NCN), 106.8 (NCH), 44.3
(CHMe), 21.9 (CHMe) and 4.5 (SiCH3). It should be noted that
the reaction of 2b with Me3SiN3 is accompanied by the formation
464 | Dalton Trans., 2006, 459–467
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©

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of the disilylated product 1,3-diisopropyl-2-(trimethylsilylimino)-
4-trimethylsilylimidazoline, for which the following 1H NMR
spectroscopic data were obtained: dH (400 MHz, C6D6) 6.18 (1H, s,
NCH), 4.27 (1H, sept, CHMe), 3.95 (1H, sept, CHMe), 1.54 (6 H,
d, CCH3), 0.97 (6 H, d, CCH3), 0.44 (9 H, s, SiCH3) and 0.15 (9 H,
s, SiCH3).
2c. Yield: 92% (Found: C, 62.55; H, 11.10; N, 15.34%. Calc. for
C14H29N3Si: C, 62.86; H, 10.93; N 15.71%); dH (400 MHz, C6D6)
4.59 (2H, sept, CHMe), 1.71 (6H, s, CH3), 1.18 (12 H, d, CH3)
and 0.44 (9 H, s, SiCH3); dC (100.52 MHz, C6D6) 144.4 (NCN),
113.9 (NCMe), 45.0 (CHMe), 21.3 (CHMe), 10.0 (NCMe) and
4.3 (SiCH3).
2d. Yield: 80% (Found: C, 72.55; H, 8.16; N, 10.58%. Calc. for
C24H31N3Si: C, 73.61; H, 8.49; N, 10.73%); dH (400 MHz, C6D6)
6.78 (4H, s, m-H), 5.76 (2H, s, NCH), 2.21 (12H, s, o-CH3), 2.10
(6 H, s, p-CH3) and −0.09 (9 H, s, SiCH3). dC (100.52, C6D6) 140.7
(NCN), 137.6 (ipso-C), 136.2 (p-CMe), 134.5 (o-CMe), 128.9 (m-
CH), 112.1 (CH), 20.8 (p-CMe), 18.0 (o-CMe) and 3.1 (SiCH3).
2e. Yield: 94% (Found: C, 76.13; H, 9.37; N, 8.83%. C30H45N3Si
requires C, 75.73; H 9.53; N, 8.83%); dH (400 MHz, C6D6) 7.21
(4H, m, m-H), 7.14 (2H, s, p-H), 5.95 (2H, s, NCH), 3.18 (4H,
sept, CHMe), 1.38 (12H, d, CH3), 1.20 (12 H, d, CH3) and −0.16
(9 H, s, SiCH3). dC (67.93 MHz, C6D6) 148.0 (o-C), 141.3 (NCN),
135.2 (ipso-C), 129.4 (p-CH), 123.9 (m-CH), 113.8 (CH), 28.9
(CHMe), 24.4 (CHMe), 23.5 (CHMe) and 3.5 (SiCH3).
3e. Yield: 100% (Found: C, 64.25; H, 7.09; N, 6.62%.
C32H41Cl2N3Ti requires C, 65.53; H 7.05; N, 7.16%); dH (400 MHz,
C6D6) 7.23 (4H, m, m-CH), 7.14 (2H, s, p-CH), 5.94 (5H, s,
C5H5), 5.77 (2H, s, NCH), 2.95 (4H, sept, CHMe), 1.49 (12H,
d, CH3) and 1.07 (12 H, d, CH3). dC (67.93 MHz, C6D6) 146.7
(o-C), 143.3 (NCN), 132.5 (ipso-C), 130.9 (p-CH), 124.6 (m-CH),
115.5 (NCH), 115.0 (C5H5), 29.3 (CHMe), 24.7 (CHMe) and 23.4
(CHMe).
Preparation of [(g-tBuC5H4)Ti(L)Cl2] (4) (L = 1,3-di-tert-
butylimidazolin-2-imide). To a solution of 206 mg (0.75 mmol)
of [(g-tBuC5H4)TiCl3] in hexane (10 mL) was added a solution
of 200 mg (0.75 mmol) of 2a in hexane (8 mL). The reaction
mixture was stirred overnight. The solvent was evaporated to
yield 291 mg of complex 4 (0.67 mmol, 89%) (Found: C, 55.52;
H, 7.91; N, 9.52%. C20H33Cl2N3Ti requires C, 55.31; H, 7.66; N,
9.68%); dH (400 MHz, C6D6) 6.51 (2H, t, C5H4CMe3), 6.26 (2H, t,
C5H4CMe3), 5.71 (2H, s, NCH), 1.60 (9H, s, C5H4CCH3) and 1.36
(18 H, s, CH3); dC (67.93 MHz, C6D6) 161.4 (C4H4CCMe3), 115.7
(CHCCMe3), 111.4 (CHCHCCMe3), 109.2 (NCH), 58.6 (CMe),
33.5 (C5H4CMe3), 31.5 (C5H4CMe3) and 29.5 (CMe); the NCN
resonance could not be detected.
Preparation of [L2TiCl2] (5) (L = 1,3-di-tert-butylimidazolin-2-
imide). To a solution of 266 mg (1.40 mmol) of TiCl4 in toluene
(5 mL) was added a solution of 750 mg (2.80 mmol) of 2a in
toluene (10 mL). This mixture was heated to 110 ◦C for 48 h.
Orange crystalline 5 was isolated by filtration and dried under
vacuum (613 mg, 1.20 mmol, 86%) (Found: C; 51.66, H; 7.99, N;
16.08%. Calc. for C22H40Cl2N6Ti: C, 52.08; H, 7.95; N, 16.56%);
dH (400 MHz, C6D6) 5.82 (4H, s, NCH) and 1.60 (36H, s, CH3);
dC (100.52 MHz, C6D6) 142.9 (NCN), 107.7 (NCH), 57.0 (CMe)
and 28.8 (CMe).
General procedure for the preparation of complexes [(g-
C5H5)Ti(L)Cl2] (3) (L = imidazolin-2-iminato ligand). These
compounds were prepared employing very similar procedures, and
thus only one representative example is detailed. To a solution of
615 mg (2.80 mmol) of [(g-C5H5)TiCl3] in toluene (13 mL) was
added at room temperature a solution of 750 mg (2.80 mmol)
of 2a in toluene (10 mL). The reaction mixture was stirred
overnight, and the solvent was evaporated. The residue was washed
several times with hexane and dried in vacuo.
3a. Yield: 1.03 g, 97% (Found: C, 50,18; H, 6.54; N, 10,73%.
Calc. for C16H25Cl2N3Ti: C, 50.82; H, 6.66; N, 11.11%); dH
(400 MHz, C6D6) 6.42 (5 H, s, C5H5), 5.69 (2H, s, NCH) and
1.33 (18 H, s, CH3); dC (100.52, CDCl3) 146.6 (s, NCN), 115.6
(C5H5), 109.2 (NCH), 59.0 (CMe) and 29.8 (CMe).
3b. Yield: 99% (Found: C, 47.88; H, 6.09; N, 11.46%.
C14H21Cl2N3Ti requires C, 48.02; H, 6,05; N, 12.00%); dH
(400 MHz, C6D6) 6.44 (5H, s, C5H5), 5.52 (2H, s, NCH), 4.66 (2H,
sept, CHMe) and 0.86 (12 H, d, CH3); dC (100.52 MHz, C6D6)
114.6 (C5H5), 108.9 (NCH), 47.3 (CHMe) and 22.0 (CHMe); the
NCN resonance could not be detected.
3c. Yield: 89% (Found: C, 50.26; H, 6.69; N, 10.83%.
C16H25Cl2N3Ti requires C, 50,81; H, 6,66; N, 11.11%); dH
(400 MHz, C6D6) 6.46 (5H, s, C5H5), 4.70 (2H, m, CHMe), 1.32
(6H, s, NCCH3) and 1.10 (12 H, d, CH3); dC (100.52 MHz, C6D6)
116.7 (NCMe), 114.3 (C5H5), 47.6 (CHMe), 21.3 (CHMe) and 9.1
(NCMe); the NCN resonance could not be detected.
Preparation of [L2Ti(CH3)2] (6) (L = 1,3-di-tert-butylimidazolin-
2-iminato ligand). 116 mg (0.23 mmol) of 5 were suspended in
diethyl ether (6 mL) and 0.29 mL (0.46 mmol) of a 1.6 M solution
of MeLi in diethyl ether was added by a syringe. The mixture was
stirred at room temperature for 45 min, whereupon the colour
changed from red to yellow. The solvent was removed under
reduced pressure. The solid residue was taken up in 5 mL of toluene
to give a cloudy, bright yellow suspension, which was filtered. The
solvent was removed under vacuum yielding 93 mg (0.20 mmol,
87%) of 6 as a shiny yellow solid (Found: C, 61.81; H, 9.76; N,
17.29%. C24H46N6Ti requires C, 61.78; H, 9.94; N, 18.01%); dH
(400 MHz, C6D6) 5.97 (4H, s, NCH), 1.63 (36H, s, CH3) and
1.10 (6H, s, TiCH3); dC (100.52 MHz, C6D6) 140.5 (NCN), 106.8
(NCH), 56.0 (NCMe), 42.1 (TiCH3) and 28.4 (CCH3).
Preparation of [(g-C5H5)Ti(L)(CH3)2] (7) (L = 1,3-di-tert-
butylimidazolin-2-iminato ligand). To a suspension of 405 mg
(1.07 mmol) of 3a in toluene (15 mL) was added at room
temperature 1.34 ml (2.14 mmol) of a 1.6 M solution of MeLi
in diethyl ether. The colour of the solution immediately turned
from red to yellow. After stirring for 16 h, the reaction mixture
was filtered through Celite to give a clear orange solution. Removal
of all volatiles gave 340 mg (1.01 mmol, 94%) of 7 as an orange
solid (Found: C, 62.47; H, 9.09; N, 11.82%. C18H31N3Ti requires
C, 64.08; H, 9.26; N, 12.46%); dH (400 MHz, C6D6) 6.42 (5H, s,
C5H5), 5.90 (2H, s, NCH), 1.38 (18H, s, CH3) and 0.74 (6H, s,
TiCH3); dC (67.93 MHz, C6D6) 111.4 (C5H5), 107.6 (NCH), 44.3
3d. Yield: 87% (Found: C, 60.62; H, 5.95; N, 7.94%.
C26H29Cl2N3Ti requires C, 62.17; H, 5.82; N, 8.36%); dH (400 MHz,
C6D6) 6.79 (4H, s, m-H), 5.97 (5H, s, C5H5), 5.41 (2H, s, NCH), 2.16
(12H, s, o-CH3) and 2.08 (6 H, s, p-CH3). dC (100.52 MHz, C6D6)
142.9 (NCN), 139.7 (ipso-C), 136.2 (p-CMe), 132.0 (o-CMe), 129.6
(m-CH), 114.5 (C5H5), 114.1 (NCH), 21.0 (p-CMe) and 17.9 (o-
CMe).
This journal is
The Royal Society of Chemistry 2006
Dalton Trans., 2006, 459–467 | 465
©

View Article Online
(CMe), 28.5 (CMe) and 27.9 (TiCH3); the NCN resonance could
not be detected.
funnel, was charged with 200 mL of toluene and saturated with
ethene (2 bar) for 10 min at 25 C, controlled by the flow meter.
◦
The catalyst was dissolved in 3.5 mL of a 10% [m/m] solution
of MAO in toluene (q = 0.89 g cm−3) and injected into the
autoclave using the dried dropping funnel. The polymerization
reaction was stopped by quenching with 20 mL of aqueous
HCl/methanol (1 : 1 v/v). The resulting polymer was collected by
filtration, washed subsequently with HCl, water, tetrahydrofuran,
and acetone, and dried at 80 ◦C in vacuo overnight. Melting points
of the polyethylene samples were measured using a DSC 2010
Single-crystal X-ray crystal structure determinations
General procedure. Preliminary examination and data collec-
tion were carried out on a kappa-CCD device (NONIUS MACH3)
with an Oxford Cryosystems cooling system at the window of a
rotating anode (NONIUS FR591) with graphite-monochromated
˚
Mo-Ka radiation (k = 0.71073 A). Raw data were corrected for
Lorentz polarization, and, arising from the scaling procedure,
1
TA instrument, 13C{ H} NMR spectra were recorded on a AC
for latent decay and absorption effects. Full-matrix least-squares
200 P Bruker spectrometer at 350 K and the molecular weight
determination was performed using HT-GPC (Waters Alliance
GPC 2000), 1,2,4-trichlorobenzene; 145 ◦C; universal calibration
to polystyrene and relative to polyethylene standards).
ꢀ
refinements were carried out by minimizing w(Fo − Fc )2. The
final difference-Fourier map showed no striking features.
CCDC reference numbers 281530 (3b), 281531 (3c), 281532 (3e),
281533 (4), 281534 (6) and 281535 (7).
2
2
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b511752f
Acknowledgements
3b. C14H21Cl2N3Ti, Mr = 350.11, red fragment (0.38 × 0.46 ×
0.69 mm), monoclinic, space group C2/c (no. 15), a = 17.8609(2),
This work was financially supported by the Deutsche Forschungs-
gemeinschaft (DFG Ta 189-6/1).
◦
˚
b = 8.3732(1), c = 23.1179(2) A, b = 97.3494(3) , V = 3428.95(6)
3
−3
A , Z = 8, Dc = 1.356 g cm , F000 = 1456, l = 0.805 mm−1, T =
˚
References
123 K, R1 = 0.0298 [Io > 2r(Io)], wR2 = 0.0791 [all data], GOF =
−3
˚
1.034, Demin/max = +0.54/−0.25 e A .
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3c. C16H25Cl2N3Ti, Mr = 378.16, red needle (0.23 × 0.25 ×
0.66 mm), orthorhombic, space group Pbca (no. 61), a =
˚
18.0659(2), b = 11.2730(1), c = 18.2918(2) A, V = 3725.25(7)
3
−3
A , Z = 8, Dc = 1.349 g cm , F000 = 1584, l = 0.746 mm−1, T =
˚
123 K, R1 = 0.0346 [Io > 2r(Io)], wR2 = 0.0848 [all data], GOF =
−3
˚
1.029, Demin/max = +0.43/−0.44 e A .
3e·C7H8. C39H49Cl2N3Ti·C7H8, Mr = 678.58, red needle (0.43 ×
0.46 × 1.07 mm), monoclinic, space group P21/n (no. 14), a =
◦
˚
12.8096 (1), b = 18.5630(2), c = 15.5127(2) A, b = 90.8894(5) ,
3
−3
˚
V = 3688.24(7) A , Z = 4, Dc = 1.222 g cm , F000 = 1440, l =
0.407 mm−1, T = 173 K, R1 = 0.0444 [Io > 2r(Io)], wR2 = 0.0934
−3
˚
[all data], GOF = 1.120, Demin/max = +0.30/−0.26 e A .
4. C20H33Cl2N3Ti, Mr = 434.26, red needle (0.08 × 0.10 ×
0.51 mm), monoclinic, space group P21/c (no. 14), a = 8.8141(1),
◦
˚
b = 17.2213(2), c = 15.1835(2) A, b = 104.3295(5) , V = 2233.00(5)
3
−3
A , Z = 4, Dc = 1.292 g cm , F000 = 920, l = 0.632 mm−1, T =
˚
173 K, R1 = 0.0445 [Io > 2r(Io)], wR2 = 0.0916 [all data], GOF =
−3
˚
1.110, Demin/max = +0.74/−0.25 e A .
6. C24H46N6Ti, Mr = 466.54, yellow plate (0.08 × 0.25 ×
0.31 mm), orthorhombic, space group Pbca (no. 61), a =
˚
18.4995(1), b = 16.2737(1), c = 18.6379(1) A, V = 5611.04(5)
3
−3
A , Z = 8, Dc = 1.105 g cm , F000 = 2032, l = 0.325 mm−1, T =
˚
173 K, R1 = 0.0358 [Io > 2r(Io)], wR2 = 0.0984 [all data], GOF =
−3
˚
1.053, Demin/max = +0.32/−0.25 e A .
7. C18H31N3Ti, Mr = 337.33, orange fragment (0.28 × 0.36 ×
0.46 mm), monoclinic, space group P21/n (no. 14), a = 11.0252(1),
◦
˚
b = 10.1497(1), c = 16.7456(2) A, b = 90.3793(5) , V = 1873.83(3)
3
−3
A , Z = 4, Dc = 1.196 g cm , F000 = 728, l = 0.458 mm−1, T =
˚
123 K, R1 = 0.0311 [Io > 2r(Io)], wR2 = 0.0796 [all data], GOF =
−3
˚
1.024, Demin/max = +0.24/−0.31 e A .
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Ethene polymerization reactions
A Bu¨chi laboratory autoclave BEP 280, equipped with a BPC
Bu¨chi pressflow gas controller and a pressure-proofed dropping
466 | Dalton Trans., 2006, 459–467
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