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University of Groningen
Non Flory-Schulz ethene oligomerization with titanium-based catalysts
Deckers, Patrick Jozef Wilhelmus
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Deckers, P. J. W. (2002). Non Flory-Schulz ethene oligomerization with titanium-based catalysts
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General introduction
1.1
Introduction
Catalysis is the key to many chemical transformations. Catalytic processes are
responsible for about 80% of all chemicals produced, and form the basis of nearly all
processes with a throughput of more than 10,000 ton per annum1. For successful
industrial implementation of a catalyst certain prerequisites have to be fulfilled. The
ideal catalyst has to combine high efficiency (i.e. effective use of starting materials,
and minimal waste emission2), high selectivity (i.e. optimal conversion to the desired
product3), and high total turnover (i.e. amount of product formed per given amount
of catalyst) with durability (e.g. low toxicity4) and low overhead expenditure (i.e.
cheap catalyst, and little maintenance). Understanding how catalyst structure and
properties can affect these parameters, combined with chemical curiosity, is and will
be the driving force for the future improvement and development of catalysis.
Mutual efforts from industry and academia have made that the field of catalysis has
grown from mere trial-and-error catalyst development to judicious design based on
the rationalization of previous knowledge and theoretical background (when
possible)5.
1.2
Transition metal catalysis
Transition metals are at the core of a wide range of catalyst systems6. In
comparison with main group metals, transition metals have more orbitals available
for interactions, and with different symmetry (d-orbitals). The possibility to
distribute its valence electrons in nine valence shell orbitals that can interact with
other groups simultaneously, allows the formation of both σ- and π-bonds with, for
example, reactive substrates. Additionally, transition metals can combine specific
functions, e.g. Lewis acidity, substrate activation, redox chemistry and polar bonds.
This diversity is a key factor in imparting catalytic properties to transition metals and
their complexes. The ability to accommodate inert spectator ligands, in addition to
reactive moieties, forms the basis for innovative transition metal catalyst design.
These ancillary ligands primarily control the configuration and conformation of
reactive complexed intermediates, and thus catalyst selectivity. Variation of the
properties, either steric or electronic or both, of such ligands offers the opportunity to
tune catalyst properties. The wide range of possible spectator ligands, metal
1
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
oxidation states and coordination numbers provides a near infinite array of potential
catalysts.
The unique character of the metal-carbon bond is another important element of
(transition) metal-based catalysts, that has been exploited in many catalytic (and
stoichiometric) metal-mediated transformations7. Carbon is more electronegative
than any (transition) metal and, hence, metal-carbon bonds principally have a
polarity Mδ+-Cδ-, making the carbon atom susceptible to electrophilic, and the metal
center to nucleophilic attack. The polarity of the M-C bond can be easily tuned by
(1) the choice of metal, (2) the oxidation state of that metal, and (3) the properties of
its spectator ligands. The wide range of available organometallic metal-alkyl species
offers a broad spectrum of reactivity with various substrates to allow interesting
opportunities for C-C, C-H, C-O, C-N, and C-X bond formation.
A very important application of C-C bond formation with organometallic catalysts
is the polymerization and copolymerization of olefins, and, closely related, the
oligomerization of olefins. Both processes will be discussed in more detail in the
following sections.
1.3
Olefin polymerization
1.3.1 Polyolefins
In 1955 Ziegler found that the polymerization of ethene to high molecular weight
linear polyethene could be achieved with a TiCl4-AlClEt2 catalyst system8. At about
the same time Natta showed that it is possible to stereoselectively polymerize
propene to isotactic polypropene with a closely related catalyst system9. Shortly after
these findings, mixtures of CrO3 and SiO2 were reported to effectively polymerize
ethene to HDPE (High Density Polyethene, ‘Phillips process’)10. Since these initial
discoveries almost half a century ago, the quest for new catalysts which allow the
cheaper manufacture of existing polyolefins, or afford new and/or better olefin
polymers has never ceased11. The production of polyolefins has witnessed a 100%
increase in production volume over the past decade only12. The growth has recently
slowed down at an estimated worldwide annual production of 80 million tons (in
2000) for the olefin polymerization industry. With the current state of the art in
polyolefin production13, the properties of polyolefins can be varied within wide
limits to provide not just substitutes but alternatives to currently available polymers,
such as polyvinylchloride or polyacetates. The accessible range of stereo- and
regioregularities and molecular weights of polyolefins allows for the manufacture of
tailored polyolefinic materials with predetermined properties (e.g hardness,
toughness, stiffness or transparency).
Stereoselective homopolymerization of α-olefins (e.g. propene, 1-hexene and
styrene) affords a wide range of polymers (isotactic, syndiotactic, hemiisotactic and
block polymers), each with typical properties. Isotactic polypropene is a stiff but
brittle polymer, in contrast to syndiotactic polypropene which is a robust transparent
material14.
2
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Also copolymers are of great practical interest, and their total production volume
matches that of homopolymers15. Flexible linear polyethene with short-chain
branches (Linear Low Density Polyethene, LLDPE) is obtained by copolymerization
of ethene with C4-C8 α-olefins16, and is predominantly applied in packaging and as
thin films. Copolymerization of propene with ethene yields materials with lowered
crystallinity17 to give access to EP-rubbers and heterophasic materials18.
Copolymerization of ethene or propene with cyclic monomers (e.g. cyclopentene or
norbornene) affords amorphous highly transparent materials that can be used in
specialty optical and medical applications19.
Terpolymerization of ethene, propene and dienes yields EPDM (Ethene Propene
Diene Monomer) elastomers with high resistance to light and solvents20. Equally
interesting are catalysts with living polymerization characteristics, which should be
able to allow (elastomeric) block copolymers21.
1.3.2 Mechanisms of olefin polymerization and active species
In classical heterogeneous Ziegler-Natta catalysts8,9 polymerization takes place at
the dislocations and edges of TiCl3 crystals22. Cossee and Arlman proposed a
mechanism of polymer chain growth by cis-insertion of the olefin into a Ti-C bond
(Scheme 1)23, which is now generally accepted, and has been supported convincingly
by fundamental studies (vide infra).
Et
Cl
Cl
Ti
Cl
Et
AlEt3
Al
Cl
Cl
Cl Cl
Ti
Et Cl
Et
Cl
- AlClEt2
Cl Cl
Cl
C2 H4
Ti
Cl Cl
.
.
Et
Et Cl
CH2
Cl
Ti
CH2
Cl Cl
Cl
Ti
Cl
CH2
CH2
Cl Cl
Cl
Cl
Ti CH2
Cl Cl
CH2
Et
to Ti center
Scheme 1:
Cossee-Arlman mechanism for polymerization
Based on Natta's early ideas about the role of chiral surface sites in the formation of
isotactic polyolefins24, models were proposed to explain the induction of
stereoregular polymer growth by the chiral environment of the catalytic centers on
the crystal edges25. In Figure 1 the incoming propene adopts the enantiofacial
orientation which places the methyl substituent trans to the polymer chain at the
3
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
incipient C-C bond (left; the Cl* atom determines the orientation of the growing
polymer chain, trans to Cl*). The cis orientation (right) is disfavored.
P
P
y
.
.
x
z
Ti
Cl
Model for the stereospecific polymerization of propene at a chiral
titanium center on TiCl3 edges
Figure 1:
Due to the non-uniformity of the active sites in heterogeneous catalysts, and the
limited experimental access to structural details there is no direct proof for the
structures proposed. Homogeneous olefin polymerization catalysts should eventually
allow more direct observations of the catalytically active species, and hence, on the
mechanism of chain growth and stereocontrol26.
Shortly after the synthesis of the first group 4 metallocenes27, the use of these
compounds as polymerization catalysts was investigated28. Mixtures of Cp2TiCl2 and
AlClEt2 were found to polymerize ethene with moderate activity, but propene and
higher α-olefins were not polymerized at all. Breslow and coworkers29 and Chien30
proposed that ethene polymerization proceeds via olefin insertion into the Ti-C bond
of a Cp2Ti-R electron-deficient species. Initially, ligand exchange between
aluminum and titanium gives Cp2Ti(Cl)Et. Coordination of the Lewis acidic
aluminum to the chloride (Ti-Cl) polarizes the Tiδ+-Clδ- bond to create an electrondeficient titanium Cp2Tiδ+R species, which readily inserts ethene (Scheme 2). This
mechanism incorporates several features of the Cossee-Arlman mechanism: (a) the
Lewis acidity of the metal center, and (b) the mutual cis orientation of the metalalkyl bond and the incoming monomer.
δ-
δ-
.
δ+
Cl AlRCl2
Ti
R
Scheme 2:
4
Cl
C2H4
δ+
AlRCl2
CH2
Ti
R
CH2
δ-
Cl AlRCl2
δ+
Ti
CH2
R
CH2
Proposed pathway for the polymerization of ethene by Cp2TiRCl⋅AlCl2R via cis-insertion
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
There was considerable debate about the nature of the active species. Various
research groups advocated bimetallic halide-bridged titanium-aluminum
intermediates as the active species involved31. Crystal structures containing both
titanium and aluminum32 were obtained from reaction mixtures, but they are not
conclusive as such since they represent degradation products. The possibility of the
involvement of a cationic titanium active species [Cp2TiR]+ was suggested by Shilov
and coworkers33, but the idea of ionic intermediates did not find widespread
acceptance as the active species in olefin polymerization.
Interest in the cationic intermediate was revived by the discovery of Eisch and
coworkers that Ph-C≡C-SiMe3 reacts with Cp2TiCl2 in the presence of AlMeCl2 to
give a cationic titanium vinyl complex [Cp2Ti-C(Ph)=C(Me)SiMe3]+, formally the
product of alkyne insertion in the Ti-C bond of [Cp2TiMe]+ 34. A year later Jordan
and coworkers reported the synthesis of the ionic species [Cp2ZrMe(L)][BPh4]35.
This system is active in ethene polymerization, and provided the first experimental
evidence that well-defined, cationic metallocene species are capable of olefin
polymerization in the absence of aluminum alkyl activators. This and related
findings36 established (alkyl)metallocene cations as crucial intermediates in
homogeneous polymerization catalysis, and they are now fully accepted as the
catalytically active species.
1.3.3 Activation of metallocene catalysts
The moderate productivity of homogeneous catalyst systems in ethene
polymerization almost meant the end of research into and development of
homogeneous olefin polymerization catalysts. In the early 1980's, this drawback was
overcome by the discovery of methylaluminoxanes (MAO) by Sinn and Kaminsky37.
Although water is a poison for Ziegler-Natta catalysts, addition of small amounts of
water to the catalyst system may dramatically enhance the activity. This water-effect
has been observed for the system Cp2TiEtCl/AlEtCl238, and also for the normally
inactive Cp2TiCl2/AlMe2Cl system39. Sinn and Kaminsky observed that the normally
inactive halogen-free Cp2ZrMe2/AlMe3 system could also be activated tremendously
by addition of water37. The presumed formation of methylaluminoxane (MAO) by
partial hydrolysis of AlMe3, to give oligomeric mixtures of [MeAlO]n, was
supported by its direct synthesis. Activation of Cp2ZrR2 (R = Cl, Me) with the
isolated MAO led to exceedingly active catalysts for ethene polymerization.
Furthermore, metallocene catalysts activated with MAO were capable of
polymerizing propene and higher α-olefins in contrast to aluminum halide activated
systems40.
MAO is supposed to initially methylate the metal-halide bonds to give a Cp2MMe2
species (Scheme 3, reaction 1). Some of the aluminum centers in MAO are assumed
to have a high tendency to abstract a methyl anion from these
(dimethyl)metallocenes to give [Cp2MMe]+ species (Scheme 3, reaction 1), that are
most likely stabilized by the [Me-MAO]- counteranion. These coordinative contacts
are appropriately weak to give way, in the presence of (substituted) olefins, to olefin5
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
separated ion pairs that are required for polymer chain growth. Since its discovery,
MAO has become the most commonly used activator for catalytic olefin
polymerization.
The realization that a weakly coordinating anion is crucial for high catalytic olefin
polymerization activity led to the development of other (non-aluminum-based)
activators capable of generating weakly coordinating anions. The catalyst systems
thus generated were called ‘single site’ metallocene catalysts. Large anions as
[B(C6H4R)]- (R = H41, 3-Et36g, 4-F42) and carboranes, such as [C2B9H12]- 36g,43, were
used as counterions, but they showed fairly strong interactions44 with the [Cp2MMe]+
cations. Although active in ethene polymerization, most of these catalyst systems did
not polymerize propene or higher α-olefins. A breakthrough was the introduction of
the very weakly coordinating45 perfluorinated tetraphenylborate, [B(C6F5)4]-, as a
counterion46.
.
M Cl
Cl
MAO
M Me
Me
[PhNMe2][B(C6F5)4]
MAO
M Me
M Me B(C F )
6 5 4
[Me-MAO]-
(1)
+ PhNMe2 + MeH
(2)
.
M Me
Me
M Me
Me
Scheme 3:
M Me
Me
[Ph3C][B(C6F5)4]
B(C6F5)3
M Me B(C F )
6 5 4
M Me
Me B(C6F5)3
+ Ph3CMe
(3)
(4)
Different routes for the activation of metallocene precursors for olefin
polymerization
Reaction of Cp2ZrMe2 with [PhNMe2H][B(C6F5)4] afforded ethene polymerization
catalysts with exceptionally high activities and gave the first active ‘single site’
zirconocene catalysts for propene and higher α-olefin polymerization (Scheme 3,
reaction 2). To avoid the presence of the amine, alkyl-anion transfer with
[Ph3C][B(C6F5)4] could be used to generate the same ion pair (Scheme 3, reaction
3)47.
6
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
As an alternative to the generation of cationic complexes, the strong Lewis acid
B(C6F3)3 could be used to abstract a methyl anion from Cp2MMe2 to give
[Cp2MMe][MeB(C6F5)3] (Scheme 3, reaction 4)48. Residual coordinative contacts
between the cationic metal center and the anion via the abstracted methyl group have
been identified48bc,49, but appear weak enough to allow olefin coordination to yield
highly active polymerization catalysts.
An excellent review on cocatalysts for metal-catalyzed olefin polymerization was
recently written by Marks and Chen50.
1.3.4 Development of stereoselective metallocene catalysts
The use of MAO and perfluorophenyl boron species as activator in metallocene
catalysis gave access to active propene (and higher α-olefin) polymerization
catalysts. As Natta discovered, heterogeneous polymerization catalysts, formed from
TiCl3 and AlClxEt3-x, can polymerize propene with high stereoselectivity (to isotactic
polypropene)9. In general, simple homogeneous metallocene catalysts, e.g.
Cp2MMe2/MAO, afford atactic polypropene51. Designing ligands that allow one
enantiofacial coordination of the incoming monomer over the other52, Ewen and
Brintzinger reported the first homogeneous metallocene precatalysts for
stereoselective propene polymerization (Figure 2)51,53. Due to conformational
constraints the ethylenebis(indenyl) and ethylenebis(tetrahydroindenyl) ligands give
the rac form of the corresponding so-called ansa-metallocenes54 chiral structures
with C2 symmetry that are contained under catalysis conditions (note: the meso form
is Cs symmetric and affords atactic polypropene).
.
M Cl
Cl
X
M Cl
Cl
.
MM = Ti, Zr
X = C2H4, SiMe2
Figure 2:
Structures of rac-(en)(thind)2MCl2 and rac-(X)(ind)2MCl2
Stereocontrol in these systems is similar as proposed for heterogeneous ZieglerNatta catalysts (vide supra)25. The indenyl* ligand determines the orientation of the
growing polymer chain, and the incoming monomer adopts the enantiofacial
orientation (trans) that minimizes the steric interactions between the polymer chain
and the monomer (Figure 3)55. Via this ligand-induced stereoselectivity, these ansabisindenyl group 4 catalysts give isotactic polypropene.
7
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
P
P
y
.
.
x
z
Ti
Cl
Figure 3:
Stereocontrol in heterogeneous and homogeneous polymerization
catalysts
The stereoselectivity of these catalysts could not compete with that of
heterogeneous Ziegler-Natta catalysts56, especially at elevated reaction temperatures,
but the introduction of α- and β-substituents on the indenyl ligand has afforded
homogeneous catalysts with similar stereoselectivity and molecular weight as
heterogeneous Ziegler-Natta catalysts, albeit much more active57.
The structure-properties relationships in metallocene catalysts, and the underlying
mechanisms have been extensively investigated58, and have led to homogeneous
metallocene catalysts for syndiotactic59, hemiisotactic60 and stereoblock61
polypropene.
1.3.5 Chain termination processes
Two main mechanisms62 (Scheme 4) for chain termination in metallocenecatalyzed olefin polymerization are generally assumed: β-H transfer to metal57bc and
β-H transfer to monomer63. When β-H transfer to monomer prevails, the rate of
chain termination increases with increasing olefin concentration, but so does
insertion, and hence, molecular weights will be independent on monomer
concentration. For β-H transfer to metal, the rate of chain termination is olefin
independent, and molecular weights will increase with increasing monomer
concentration.
Another way to release the growing polymer chain is via chain transfer to an added
transfer agent. Transfer agents can be used to control the molecular weight of the
polymer. The most common transfer agent for that purpose is molecular hydrogen
that has been successfully employed in Ziegler-Natta catalysis64.
Other, but less frequently occuring, chain release processes are β-methyl transfer65
(quite common for metallocene catalysts with highly substituted cyclopentadienyl
ligands) and transalkylation to the aluminium cocatalyst66 (in catalyst systems with
high Al/metal ratio and low productivity).
8
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
H
M
M
CH2CHR(P)
H
CH2
H
M
CR(P)
CR(P)
M
- CH2=CR(P)
H2C
β-H transfer to metal
+ CH2=CHR
.
CH2CH2R
.
M
- CH2=CR(P)
β-H transfer to monomer
H2C
H2C
M
CHR
M
CH2CHR(P)
Scheme 4:
CHR
H
CR(P)
H2C
CH2CH2R
M
CR(P)
H2C
Chain termination via β-H transfer to metal and to monomer
1.3.6 Alternative ligand systems and group 4 catalysts
Group 4 polymerization catalyst research has mainly been focused on metallocenes,
CpR2MCl2. Nevertheless, numerous efforts have been made to explore the potential
of other ligands and catalyst systems. Nowadays a wide range of non-metallocene
group 4 catalysts has been prepared and tested in catalytic olefin polymerization
(Figure 4)13d.
One particular modification of the cyclopentadienyl ligands can be achieved by
insertion of a BY unit into the Cp ring to afford monoanionic boratobenzene ligands,
(C5R5BY)-. The corresponding zirconium complexes (A) are highly active in ethene
polymerization when activated with MAO67. By changing the Y substituent on boron
the relative rates of propagation and β-H elimination can be controlled and a range of
polymer molecular weights is available with these versatile catalyst systems68. Also
substitution of a CH unit for a P atom has been investigated by various research
groups69. The resulting catalysts (B) show activities comparable to metallocene
dichloride species.
For monocyclopentadienyl group 4 complexes (C) moderate activities have been
reported70, especially for the syndiospecific polymerization of styrene with
[CpRTiR'2]+ species. An important class of olefin polymerization catalysts is formed
by the so-called ‘constrained geometry catalysts’ (‘CGC’, D) developed by Dow and
Exxon71, based on Cp-amido ligands72. The MAO activated procatalysts are highly
active in homopolymerization of olefins as well as in copolymerization of ethene and
higher α-olefins73. Various groups have investigated the scope of the Cp-amido
framework and its effect on catalysis74. Replacing the NR group by an oxygen atom
9
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
affords Cp-alkoxide ligands that have also been successfully applied as ligands for
olefin polymerization active group 4 metal species (E)75.
B Y
R
R
P
Zr Cl
Cl
B Y
Zr Cl
Cl
P
A
Cl
B
Cl
N
Cl
O
C
R'
N
N
M
N
Cl
Cl
N
M
N
R N
Cl
Cl
M
R N
N
N
Ar
F
Cl
S
Cl
R
I
N
M
Cl
R''
O
Cl
R''
O
K
M
R'
R
Cl
Cl
N
O
R'
Figure 4:
Cl M Cl
N
N
O
R
J
Cl
Cl
N
R'
R
O
N
H
R'
M
.
R
R'
G
O
M Cl
Cl
E
D
Ar
.
M Cl
Cl
Si
M
R
L
Non-metallocene group 4 procatalysts for olefin polymerization
The high activity of the ‘constrained geometry catalysts’ prompted the development
of other catalysts containing amide or other nitrogen based ligands. Monodentate
amido76 and bidentate bisamido77 (F) group 4 complexes have been reported. Olefin
polymerization activities for the bidentate complexes vary significantly and depend
strongly on the chelate ring size and the nitrogen substituents78. Also tridentate
bisamido ligand systems containing a third neutral additional donor atom (N, O)
have been prepared and introduced on group 4 metals (G). The resulting catalysts
again vary strongly in olefin polymerization activities79. The combination of an
amido and an imine function gives a monoanionic six-electron donor ligand,
isostructural and isoelectronic with a cyclopentadienyl ligand. These so-called
10
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
amidinate ligands (H) yield group 4 catalysts with moderate activity in olefin
polymerization80. Amidinate ligands have also been employed in combination with
other ligands such as Cp81 and amine ligands82. The latter systems show remarkably
high activity for propene and 1-butene polymerization. Also β-diketimates, formally
a higher homolog of amidinate ligands, have been introduced on group 4 metals (I),
but only give moderately active catalysts83.
Besides nitrogen-based ligands, oxygen donor ligands have been used in group 4
olefin polymerization catalysis. Chelating bisphenoxide ligands (J) have given
moderately to highly active catalysts for olefin homopolymerization84 and
copolymerization of α-olefins with ethene85. Bisalkoxide ligands with one (K)86 or
more (L)11d,87 additional donor atoms (N, O, S) have been shown to improve the
olefin polymerization activity dramatically.
1.3.7 Group 3 metal and lanthanide catalysts
Neutral group 3 (Sc, Y) and lanthanide alkyl complexes are isoelectronic with
cationic group 4 alkyl complexes. This analogy has been used in the design of group
3 and lanthanide catalysts for olefin polymerization. The potential advantage would
be that they could be active as such, and would not require addition of a cocatalyst.
Ligand development for organoscandium and organoyttrium olefin polymerization
catalysts has been following the general trends of group 4 catalysts, although the
former are much less well explored88. Both scandium65c,72,89 and yttrium90 complexes
containing Cp-based ligands have been prepared, but they exhibit low to moderate
ethene polymerization activity. For yttrium also benzamidinate ligands have been
employed affording catalysts with very low activity91.
R
N
N
Sc
Ph
Figure 5:
R
Y
N
M
N
N
SiMe3
N
Cationic group 3 catalysts for olefin polymerization
Lanthanide-based olefin polymerization catalysts have invariably been stabilized
with substituted Cp analogs. The catalysts have been reported to show at best
moderate activity92, although high initial activities have been observed93 (note:
[Cp*2LnH]2 is the ethene polymerization catalyst with the highest initial activity
reported).
11
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Recent investigations in this area have afforded highly active cationic group 3 and
lanthanide catalyst systems94, such as [(η5-C5Me4CH2CH2NMe2)Sc(CH2Ph)]+
(Figure 5, M) and {[N,N’-R2-tacn-N’’-CH2CH2N(t-Bu)]Y(CH2SiMe3)}+ (N, R = Me,
i-Pr; tacn = 1,4,7-triazacyclononane).
1.3.8 Late transition metal catalysts for olefin polymerization
The impetus for late transition metal95 olefin polymerization catalysis is their
decreased oxophilicity and greater functional group tolerance. These features would
make such catalysts ideal for copolymerization of olefins with polar comonomers.
The current advances in late transition metal chemistry have been recently
reviewed96. This section will be limited to a discussion of the most important catalyst
systems (Figure 6).
Group 6 heterogeneous catalysts play an important role in the commercial
manufacture of polyethene. The Phillips catalyst consists of a silica support treated
with CrO3, which is subsequently reduced to an, as yet elusive, low-valent chromium
species10. This system is extremely active and offers the advantage of not requiring a
cocatalyst. Homogeneous chromium catalysts have recently been reviewed97. The
most active homogeneous chromium catalyst thus far is the Cp-amino Cr(III)
compound (O) reported by Jolly and coworkers98. Recently, BASF patented some
highly active chromium systems based on a substituted neutral 1,3,5triazacyclohexane (tac, P) ligand on Cr(III)99. By varying the substitution pattern on
the tac-ligand the catalyst system can shuttle between a polymerization100 or
trimerization101 catalyst.
R
N
R N
N
Cr
X
X
N
R'
N
R''
X
R' X
R''
X R'
M = Fe, Co
Q
.
Ar
Ph
Ph
N
Ni R'
L
O
P
Ph
O
Ni Ph
L
Ar
R
N
M
R
N
Ar
R
R
Figure 6:
12
R
N
M
X
X
R
N
R
Cr
P
O
.
R'
S
X
Y
M = Ni, Pd
T
Late transition metal procatalysts for olefin polymerization
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Few active group 8 and 9 catalysts have been reported. The most active
polymerization catalysts are iron(II) species stabilized with a 2,6-bis(imino)pyridine
ligand (Q) with highly substituted aryl substituents on the imino nitrogens102. By
decreasing the size of the ortho substituents the system can be converted into a very
active olefin oligomerization catalyst102b,103 The corresponding cobalt(II) compound
is the most active group 9 catalyst reported thus far, but its activity is a magnitude
smaller than that of the iron(II) compound.
Group 10 catalysts with anionic [P,O] ligands (R), well-known for the Shell Higher
Olefin Process (see section 1.4.3)104, can polymerize ethene under certain
conditions105. Crucial to the switch from oligomerization to polymerization is the
removal of the phosphine ligand, or the use of more labile stabilizing ligands, such as
pyridine or ylides. Recently, Grubbs reported nickel complexes with anionic [N,O]
ligands (S) that are highly active in homopolymerization of ethene and
copolymerization of ethene with polar comonomers11c. Another important class of
group 10 polymerization catalysts is formed by nickel(II) and palladium(II)
complexes with bulky diimine ligands (T)106. These catalysts are very versatile in
their polymerization behavior. Block polymers107, chain-end-controlled isotactic
polypropene108 and ethene copolymers with polar comonomers109 belong to the
possibilities. The diimine group 10 systems have been subject of both theoretical110
and experimental111 investigations.
1.4
Ethene oligomerization to higher linear α-olefins
1.4.1 Linear α-olefins
Apart from ethene and propene, higher (C4-C20) linear α-olefins have become
increasingly interesting as monomer112 (to give poly-α-olefins) and as comonomer113
(to form LLDPE16 and HDPE114) in catalytic olefin polymerization. Besides the use
as polyolefin building blocks, higher linear α-olefins find their main applications as
starting material for plasticizers (C6-C10) and surfactants (C10-C20).
The C6-C8 α-olefins can be hydrocarboxylated to give predominantly the linear
carboxylic acids, which can be used as additives for lubricants together with their
esters. Hydroformylation of C6-C10 α-olefins affords odd-numbered linear primary
alcohols. They can, among others, be converted to polyvinylchloride (PVC)
plasticizers.
Epoxidation of C10-C12 α-olefins opens a route to bifunctional derivatives or
ethoxylates as nonionic surfactants. The C14-C16 α-olefins can be alkylated with
phenols to yield alkylphenols, which can be employed as surfactants as well as
lubricant oil additives.
Several processes are known to obtain α-olefins: wax-cracking of paraffins,
oligomerization of ethene, dehydrogenation of paraffins, dimerization and metathesis
of olefins, dehydration of alcohols and electrolysis of straight-chain carboxylic acids.
Especially the second process, oligomerization of ethene115, has been industrially
13
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
utilized to manufacture large amounts of linear α-olefins in the C4-C20 range due to
the abundance of ethene and the high product quality. In 1999, the worldwide annual
production of higher α-olefins was about 2.5 million tons116. The increasing need for
α-olefins, especially as comonomer (C4-C8), causes the market still to expand, as
illustrated by expansion plans of all major α-olefin producing companies.
1.4.2 Aluminum-catalyzed ethene oligomerization
The first ethene oligomerization process was developed by Ziegler (Alfen process)
in the early 1950's. During extensive studies on organoaluminum compounds Ziegler
discovered the Aufbaureaktion117, a series of sequential repetitive olefin insertions
into the Al-C bond (Scheme 5, equation 1), under high ethene pressure (100-400 bar)
and moderate temperatures (100-150 °C). Relevant for the production of linear αolefins was the observation that the insertion of ethene into an Al-H bond is
reversible (Scheme 5, equation 2)118, indicating that olefin elimination can occur. By
varying pressure, temperature and reaction time Ziegler was able to induce
displacement of the olefin (Verdrängung) affording triethyl aluminum (AlEt3) and
three equivalents of α-olefins (Scheme 5, equation 3)119 at high temperature (250400 °C) and low ethene pressure (10 bar).
.
(
)n
AlEt3 + 3n CH2=CH2
Al
Al-H + RCH=CH2
Al-CH2CH2R
Al
(
)n
+ 3 CH2=CH2
3
Scheme 5:
AlEt3 + 3
(1)
3
(2)
(
)n-1
.
(3)
Key steps in aluminum catalyzed ethene oligomerization
Unfortunately, the separation of the α-olefins from the AlEt3 is extremely difficult,
since the boiling points of AlEt3 and 1-dodecene are close together, and most
industrially interesting product mixtures contain significant quantities of 1-dodecene.
The problem is often referred to as the Ziegler dilemma, since his group never found
a commercially acceptable solution to this problem.
Under the right conditions, e.g. at 200-250 °C, chain-growth and displacement can
be carried out simultaneously120. The AlEt3 is continuously regenerated via
displacement while chain-growth is proceeding. Small amounts of AlEt3 (0.4 wt%)
are needed which are hydrolyzed after reaction to facilitate separation. By reinsertion
of the displaced higher α-olefins β-branched α-olefins are produced, but by utilizing
14
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
high ethene pressure (250 bar) formation of these branched olefins can be
suppressed. This principle forms the basis of the Gulf/Chevron120,121 process, which
is responsible for about 15% of the annual worldwide production116.
In contrast to the Gulf/Chevron process, Ethyl Corporation122 uses a two-stage (one
catalytic, one stoichiometric) α-olefin synthesis to overcome separation problems.
The first stage is comparable to the Gulf process yielding predominantly C4-C10 αolefins. These olefins are transalkylated with long-chain aluminum alkyls, liberating
the higher α-olefins, whereas via chain-growth the remaining short-chain branched
aluminum alkyls are converted into long-chain branched analogs in the
stoichiometric part of the process. Via this two-stage α-olefin synthesis, Ethyl
Corporation furnishes about 30% of the annual worldwide production of α-olefins116.
1.4.3 The Shell Higher Olefin Process (SHOP)
The most important discovery made by Ziegler in 1952117a,119 was the observation
that in the presence of nickel salts the alkylaluminum-catalyzed Aufbaureaktion is
directed to yield mainly butenes. This phenomenon is often referred to as the ‘nickel
effect’123. These findings initiated an intensive investigation into organonickel
chemistry. Wilke and coworkers were the first to observe the ligand effects on
selectivity in nickel-catalyzed reactions124. At Shell, ligand variations were extended
to bidentate [P,O] chelates (Figure 7)125, and are now the basis for the Shell Higher
Olefin Process (SHOP)104,115, which accounts for about 35% of the annual worldwide
production of α-olefins made via ethene oligomerization116. The SHOP-process is
one of the larger industrial applications of homogeneous catalysis. In Table 1, the
product distribution of α-olefin products manufactured via SHOP is compared with
both aluminum-based processes (vide supra) and wax-cracking.
P
.
R
Ni
.
L
O
Figure 7:
Generalized structure of SHOP catalyst precursors
Table 1:
Comparison of product distributions (wt%) of C6-C18 α-olefins126
α-Olefins
Branched olefins
Paraffins
Dienes
Monoolefins
Wax-cracking
83-89
3-12
1-2
3-6
92-95
Gulf/Chevron
91-97
2-8
1.4
99
Ethyl
63-98
2-29
0.1-0.8
>99
SHOP
96-98
1-3
0.1
99.9
15
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
1.4.4 Neutral nickel(II) complexes for ethene oligomerization
Nickel(II)-based catalysts with monoanionic [P,O] chelating ligands for the
oligomerization of ethene have been extensively studied127,128, most notably by Keim
and coworkers. It turned out that the [P,O] chelating ligand controls the selectivity of
the ethene oligomerization127a, while the other ligands stabilize the complex. The
importance of the chelate interactions on selectivity, was elegantly illustrated by
Braunstein and coworkers127h. By inducing hydrogen bond interactions with the
oxygen atom of the [P,O] ligand, β-H elimination became dominant over
propagation, thus affording lower α-olefins.
Ph
Ph
P
Ph
Ni
Ph
O
P
CH2=CH2 / 70 °C
PPh3
Ph
P
Ni
O
F3C
F3C
C
Figure 8:
.
Ph
Ph
P
O
PPh3
O
B
Ph
Ph
H
Ni
- styrene
A
.
Ph
Ph
H
Ni
O
PCy3
D
Evidence for nickel(II) hydride species
It is largely accepted that the active species in ethene oligomerization is a nickel(II)
hydride. The mechanism for the hydride formation is supported by the reaction of the
nickel(II)-phenyl species (A) with ethene to yield the corresponding hydride
intermediate (B) and styrene (Figure 8, top)127d, which could be detected by GC. In
the case of complex C, in situ NMR studies indicated the existence of a nickel
hydride127c, whereas the stable nickel hydride D could be isolated and
characterized127d. Multiple ethene insertions into the Ni-H bond (followed by β-H
elimination) gives a Flory-Schulz distribution of linear α-olefins (Scheme 6).
Over the years many other monoanionic chelating ligands, e.g. [P,N]129, [P,S]130,
[S,O]131, [N,O]132, [As,O]133,[O,O]134, [N,N]135 and [S,S]136 chelates, have been
investigated, mainly by Keim, Braunstein and Cavell.
16
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
P
R
Ni
L
O
CH2=CH2
P
Ni H
O
P
P
.
`Rn
Ni
O
Ni
O
Rn
P
n-2 CH2=CH2
Scheme 6:
CH2=CH2
.
CH2=CH2
Ni
O
Postulated catalytic cycle for Ni-catalyzed ethene oligomerization
1.4.5 Cationic group 10 ethene oligomerization catalysts
In general, the neutral catalytically active species require high reaction
temperatures and pressure for the oligomerization of ethene. SHOP operates
typically at 80-120 °C and 70-140 bar. Since the late 1960's, cationic Ni(II) allyl
complexes were known to oligomerize and dimerize ethene137. Later on, also
cationic Pd(II)138 complexes (as well as PdCl2139) were found to oligomerize ethene.
Keim and Tkatchenko found that neutral [P,O] chelates, which afford cationic nickel
complexes140, allow ethene oligomerization at much lower temperature and
pressure141. Similar palladium compounds were found to dimerize ethene to
butenes142. This discovery led to development of group 10 complexes with neutral
chelating ligands, such as [N,N] (diimine143, pyridylimine144, and diamine145) and
[P,N]146, to provide highly active ethene oligomerization catalysts.
17
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
1.4.6 Group 8 and 9 based catalysts for ethene oligomerization
Very few active group 8 and 9 oligomerization catalysts have been reported in the
literature, in contrast to the well-documented wealth of group 10 catalysts. In the
early days of homogeneous catalysis, simple transition metal salts, such as
RhCl3139bc,147, were studied and found to be active in the dimerization of ethene to
butenes. For the Co(acac)3/AlEt3 system a selectivity of up to 99.5% 1-butene was
reported148. While the organometallic chemistry developed, new well-defined
hydride and alkyl species of group 8149 and group 9150 were tested in catalytic ethene
oligomerization. Only dimerization activity was found, indicating fast chain transfer
in these catalyst systems. Until recently, the only active group 8 or 9 catalyst (Figure
9, U) for ethene oligomerization was reported by Braca and Sbrana151 albeit with low
activity.
Cl
.
R'
CO
Ru
CO
CO
Figure 9:
R
N
N
R'
N
.
M
R''
U
R
R' X
R''
X R'
M = Fe, Co
Q
Group 8 and 9 ethene oligomerization catalysts
In 1998, Brookhart103 and Gibson102b reported highly active catalysts for ethene
oligomerization based on iron and a neutral tridentate 2,6-bis(arylimino)pyridine
ligand (Q). By varying the substitution pattern, especially on the ortho position, of
the aryl groups the activity and selectivity can be tuned152. Analogous Co systems
have been reported to show similar reactivity but less active153.
1.4.7 Early transition metal catalyzed ethene oligomerization
In the late 1950's, titanium precursors, e.g. RnTiCl4-n and Ti(OR)4, were modified
with organohalide aluminum compounds, RnAlCl3-n, and found to be active in ethene
oligomerization154 and dimerization155. The active species and the mechanistic
aspects of these titanium catalysts are not well-understood156. Similar to ethene
polymerization (see section 1.3.2), an empty coordination site at an electrondeficient titanium center, and a titanium-alkyl bond are fundamental requirements
for ethene oligomerization, but different proposals have been put forth concerning
the structure of the Ziegler-Natta catalyst active centers (Figure 10). They consist of
either bimetallic systems157, in which titanium is bonded to aluminum through halide
or halide-alkyl bridges, but also monometallic titanium systems23,158, and truly ionic
species159. Both the bimetallic160 and monometallic161 systems are backed up by
18
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
theoretical calculations. Recently however, it has been suggested that active species
similar to those found in polymerization systems are present (vide infra, see section
1.3). Chain termination in these systems is presumed to occur via β-H transfer from
the oligomer chain to coordinated ethene (see Scheme 4)162. Reduction of Ti(IV)
species to Ti(III) deactivates the oligomerization catalyst, and generates catalytically
active polymerization centers (see section 1.3.2).
Cln
Ti
P
M
P
Ti
Ti
.
X
Cl
Al
.
Cl
P
Ti
Figure 10:
Al
P
Cl Ti
Cl
Cl
P
Bi- and monometallic types of active centers
The TiR4/XnAlR'3-n systems can be improved by addition of ancillary ligands, such
as ketones, amines and phosphines. These additives increase the selectivity to linear
α-olefins163 by increasing the electron density on titanium, which allows ethene to
coordinate dominantly. The incorporation of higher α-olefins is inhibited and, in
consequence, suppresses the production of branched olefins. In addition to these
electronic effects, steric effects of these ligands are able to increase the linearity of
the olefins formed.
In the early 1970's, comparable zirconium/aluminum systems were tested in
catalytic ethene oligomerization164. The zirconium catalysts are, in general, more
active and more selective than the titanium catalysts. However, a constant ethene
supply is necessary to reach high selectivities to linear α-olefins. As an advantage to
the titanium catalysts, catalyst deactivation via reduction of zirconium does not seem
to occur115a. Deactivation of the zirconium species is assumed to proceed via
disproportionation of two Zr-alkyl species to give inactive bimetallic Zr-alkyl-Zr
species and alkane. Similar to titanium-based systems, donor ligands improve the
performance of the zirconium/aluminum catalysts. These zirconium/aluminum/donor
ligand catalyst systems were used by Idemitsu in a relatively small industrial process
for α-olefin manufacture165.
The role of the added ligands, like the nature of the active species, is subject of
debate. After the initial Idemitsu patents appeared, a vast number of donor ligands
have been used, most notably by Idemitsu itself, in zirconium-based ethene
oligomerization catalysts166. Based on the similar activities for ZrCl4⋅ester adducts
19
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
and Zr(CH2Ph)4 in ethene oligomerization studies, Young reasoned that the ancillary
ligands primarily serve to solubilize the ZrCl4 to allow rapid interaction between the
zirconium and the aluminum cocatalyst167. However, recently zirconium-based
systems (Figure 11) have been prepared in which catalyst activity168 and product
distribution169 can be controlled by steric and electronic ligand variation suggesting
active participation of the ancillary ligand.
A number of zirconocene-based catalysts have been found active in ethene
oligomerization68,170, including cationic species formed in situ in the absence of
aluminum cocatalyst171. The latter observation argues that an active cationic species,
similar to that suggested for polymerization catalysts, is highly likely.
R'
B OEt
.
Zr Cl
Cl
B OEt
N
Zr Cl
Cl
N
N
R''
X
X
R''
N
X = O, S
V
Figure 11:
W
Zr
R
Cl
.
Cl
R
R'
X
Zirconium-based ethene oligomerization catalysts
1.4.8 Inorganic heterogeneous catalysts for ethene oligomerization
Materials such as nickel oxide and nickel salts supported on silica and/or alumina
have been reported to be active catalysts for the oligomerization of ethene172. The
activities and selectivities of these catalyst systems are strongly affected by product
adsorption (on the support), and by isomerization and cooligomerization to produce
branched olefins. Increasing acid strength of the silica-alumina support achieves
higher activity per nickel site173. Nickel oxide on an alumina support modified with
sulfates proved to be a highly active and selective catalyst for ethene
oligomerization174, and variations of this system have been thoroughly investigated
in the last five years175.
Also nickel-exchanged zeolites176 as well as zeolites as such177 have been tested for
ethene oligomerization activity. But these systems show poor selectivity to linear αolefins. Recently, Jacobs and coworkers reported the successful use of zeolites as
support for active and selective metallocene-based oligomerization catalysts178.
1.4.9 Selective olefin oligomerization
All ethene oligomerization catalyst systems described in the foregoing part of this
section afford a distribution of linear α-olefins (Flory-Schulz or Poisson). Very few
oligomerization catalysts for selective olefin oligomerization, to a single olefin,
except dimerization (of ethene or α-olefins), have been reported179, some of which
20
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
have been employed industrially, most notably in the DIMERSOL process180.
Dimerization can either occur via an insertion-type pathway, i.e. two insertions
followed by β-H elimination181, or via a metallacyclic pathway involving oxidative
coupling182 (Scheme 7).
1-butene
M H
M
M
M H
β-H
.
.
1-butene
M
[M]
Scheme 7:
M
β-H or
1,3-H shift
[M]
Different pathways for ethene dimerization to 1-butene
A similar metallacyclic pathway is proposed for the selective ethene trimerization
to 1-hexene by chromium-based catalyst systems (mixtures of Cr salts with
alkylaluminum compounds, and added ligands, especially imidazoles, or
coordinating solvents, such as 1,2-dimethoxyethane)183. A low-valent Cr species
couples two ethene molecules to generate a chromacyclopentane, which
subsequently inserts a third ethene molecule to give a chromacycloheptane (Scheme
8)184. From the latter, 1-hexene is formed via β-H elimination (or 1,5-H shift) and
reductive elimination to regenerate the low-valent Cr species98. Recently,
triazacyclohexane-based chromium precursors activated with MAO101, TaMe2Cl3185,
and (arene)2VX (X = Cl, Br, [BF4]-)186 systems were also found to effectively affect
this unique and intriguing reaction, presumably via the same pathway.
1-hexene
2 ethene
[Cr]
Cr
.
Cr
H
.
Cr
Cr
ethene
Scheme 8:
Proposed catalytic cycle for selective ethene trimerization by Cr-based
catalysts
21
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
1.5
Switching from ethene polymerization to oligomerization activity
In the previous two sections, 1.3 and 1.4, we have encountered various systems that
can be tuned to either ethene polymerization or oligomerization activity (1) by
controlling the reaction conditions or (2) via subtle, easily accessible ligand
variations. For example, Ziegler discovered that the system TiCl4-AlClEt2
polymerizes ethene to high molecular weight polyethene8, but by controlling the
reaction conditions (temperature, ethene pressure) the same system can be switched
to a moderately active ethene oligomerization154 or dimerization catalyst155.
Ligand variation, potentially, offers a more elegant route to govern interconversion
between polymerization and oligomerization active species. The key to a successful
switch is control over the relative rates of chain propagation (kp) and chain
termination (kt). Increasing kt over kp will lower the molecular weight of the polymer,
and eventually lead to the production of α-olefins. Recently, a range of transition
metal catalysts have been developed that readily alternate between ethene
polymerization and oligomerization activity by changing the steric or electronic
properties of the ancillary ligand (Figure 12).
The diimine Ni(II) and Pd(II) catalysts (A) can be easily tuned by the ortho
substituents on the aryl rings. Bulky substituents, such as iso-propyl, effectively
block the axial positions110, and suppress the chain termination transition state, to
afford polyethene106. The sterically undemanding ortho methyl groups are not
adequately bulky, and β-H elimination can readily take place to give a distribution of
α-olefins144. The ortho aryl substituents in the bis(imino)pyridyliron(II)187 and
cobalt(II) complexes (B) exert a similar steric control on the rate of chain
termination, and thus on product formation102,103.
The substituents on the 1,3,5-triazacyclohexane ligand in the tac-chromium systems
(C) presumably perform steric control in reverse fashion. Linear alkyl chains (C1C12) on the nitrogen atoms lead to moderately active ethene polymerization
catalysts100. Substituents on the 2-position of the alkyl chain increase the steric
congestion at the metal center and convert the catalyst into a selective ethene
trimerization catalyst (note: α-olefins are exclusively trimerized, also with straight
chain alkyl nitrogen substituents101). In these systems, activity can also be tuned by
the
reaction
conditions.
Similar
effects
are
observed
in
(η5C5H4CH2CH2PR2)CrCl2/MAO ethene oligomerization systems188, in which the
molecular weight of the α-olefins produced decreases with increasing steric bulk of
the phosphor substituents189.
As a last example, the relative rates of chain propagation and chain termination can
be controlled by electronic substituent effects (although steric effects cannot be
completely excluded). By changing the boron substituent of di-iso-propylamine to
ethoxide (or phenyl) in boratobenzene zirconium complexes (D), the rate of β-H
elimination increases, which results in the formation of lower molecular weight
oligomers68 instead of polyethene67.
22
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Oligomerization catalysts
R
R
N
N
Polymerization catalysts
R
R
N
N
M
M
Y X
Y X
N
.
N
N
N
M
X
R
N
Cl
N
(= R)
R N
Cl
Cl
B OEt
Zr Cl
Cl
B OEt
1.6
B (M = Fe, Co)
R
Cr
Figure 12:
.
N
X X
R'
N
Cl
M
X
N
R N
A (M = Ni, Pd)
N
C
Cr
Cl
Cl
B N(i-Pr)2
Zr Cl
Cl
B N(i-Pr)
D
2
Effects of ligand variation on ethene conversion characteristics
Aim of this investigation
Group 10 compounds with anionic [P,O] ligands are well-known to oligomerize
ethene and play a central role in SHOP104. There have also been reports that SHOPtype oligomerization catalysts can polymerize ethene under certain conditions105.
Crucial to the formation of high molecular weight polymers rather than oligomers is
the use of labile groups L, such as pyridine or ylides, or the effective removal of
phosphorus-based L groups190, such as PPh3. In other words, the catalytic activity of
the Ni(II) species can be controlled by the nature of the neutral ancillary group L.
23
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
Ph
Polymerization
Oligomerization
pyridine, ylide
PPh3, PCy3
Ph
P
Ph
O
Ni Ph
L
L
.
.
M
Me
L
C4H8S, toluene
PhOMe, PhNMe2
L
Figure 13:
Reactivity control via neutral π-donor ancillary ligands
Similarly, the presence of weak donor ligands, such as aromatic ethers and amines,
can convert group 4 propene polymerization catalysts [Cp2MMe]+ into
oligomerization catalysts, albeit with a significant reduction in activity, since these
donor ligands effectively compete with the olefinic substrate for the coordination site
required for chain growth36e,f.
We became interested in the possibilities to tune the properties (activity, selectivity
and stability) of catalytically active species by neutral donor moieties. We decided to
combine this idea with the concept of ligand-controlled catalyst reactivity, and set
out to design ancillary ligands substituted with neutral donor-functionalized side
chains that provide the opportunity of intramolecular coordination. The chemistry of
cyclopentadienyl complexes of this type191 (e.g. Cp-N192, Cp-O193, Cp-P, Cp-As and
Cp-S194) and their potential application as catalysts present a rapidly growing area of
research. As the cyclopentadienyl-based catalyst systems of interest, we choose
cationic monocyclopentadienyl titanium species, [CpTiR2]+.
These half-sandwich titanium complexes show interesting catalytic activity, most
notably in the syndiospecific polymerization of styrene. Ishihara and coworkers were
the first to report that syndiotactic polystyrene, a new macromolecule at that time195,
could be produced with high stereoregularity and yield under conventional
conditions using half-sandwich titanium compounds activated with MAO196.
Syndiotactic polystyrene has become an important engineering plastic since197, and
many publications and patents have appeared on the subject198. Various
monocyclopentadienyl titanium complexes with chelating Cp ligands with neutral
oxygen199 and nitrogen200 donor moieties have been prepared (Figure 14), and the
effect of the π-donor on catalyst activity was studied. The presence of the tethered
amino groups has clear, but not spectacular, effects on olefin polymerization
reactivity of these complexes (lower activity and stereoselectivity for styrene
polymerization, and higher activity in ethene and propene polymerization, compared
to unsubstituted CpTiCl3).
24
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
R
N
. Cl Ti
O
Cl
Cl
Figure 14:
N
Cl Ti
Cl
Cl
Cl Ti
Cl
Cl
.
Half-sandwich titanium compounds with intramolecular π-donor
groups
Recently, the catalyst [Cp*TiMe2][MeB(C6F5)3] was found to display interesting
olefin polymerization activity with ethene and propene in toluene. Bochmann and
coworkers reported the living polymerization of propene to high molecular weight
atactic polypropene201, and Pellecchia and coworkers showed that polyethene
produced by this catalyst contains considerable amounts of n-butyl side chains202. It
was suggested that the catalyst is partly converted to a species that trimerizes ethene
to 1-hexene, which is then incorporated into the polymer. The latter observation is
very interesting, because it indicates that [CpTiR2]+ species can be transformed,
from polymerization into oligomerization (i.c. trimerization) catalysts under certain
conditions.
We sought ways of stabilizing electron-deficient half-sandwich titanium cations in
order to study the catalytically active species, but also their reactive properties. To
this purpose, we introduced (neutral) aromatic substituents on the cyclopentadienyl
ligand, which have the potential of acting as weakly and reversibly coordinating
ligands to the titanium center. The labile character of aromatic coordination to
titanium has been previously demonstrated. Baird and coworkers showed that
[Cp*MMe2]+ cations (M = Ti, Zr, and Hf) can bind arene solvents (benzene, toluene,
xylenes, mesitylene, styrene) reversibly203, and that this interaction is significantly
weaker for Ti than for Zr and Hf204. The feasibility of intramolecular arene
coordination was suggested by observations of Chien and Rausch on the (η5C5Me4CH2CH2Ph)TiMe3/[Ph3C][B(C6F5)4] system, where NMR evidence was
obtained for η6-coordination of the arene moiety in the thermally labile cationic
dimethyl species205.
For our studies on cyclopentadienyl-arene systems, we decided to use benzylsubstituted Cp ligands of general formula [C5H3R-CR’2-Ar]-, which can be easily
prepared by reaction of the appropriate 3-R-6,6-R’2-fulvene with aryl lithium to
afford lithium salts of the desired ancillary cyclopentadienyl ligands. The wide range
of suitable fulvenes and aryl lithium salts gives access to a readily available library
of ancillary ligands. Introduction of the ligands on titanium(IV) and subsequent
derivatization are straightforward. During the course of this investigation, other
groups reported the preparation of related bis-206 and monocyclopentadienyl207 group
4 metal complexes and their properties in catalytic olefin polymerization208.
25
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
1.7
Survey of the thesis
In the foregoing part of Chapter 1, olefin polymerization and ethene
oligomerization with transition metal-based catalysts is discussed. A synopsis of
catalyst systems that can switch from olefin polymerization to oligomerization
activity is given together with the aim of the investigation described herein.
In Chapter 2, the synthesis and characterization of (η5-C5H4CMe2Ar)TiR3 (Ar =
Ph, 3,5-Me2C6H3; R = Cl, Me, CH2Ph, CH2CMe3, CH2SiMe3) complexes are
described. In addition, the thermolysis of the trialkyl complexes (η5C5H4CMe2Ar)Ti(CH2R’)3 (R’ = Ph, CMe3, SiMe3), leading to ortho cyclometalation
of the ancillary cyclopentadienyl ligand, and a kinetic study of this process are
presented. The thermal decomposition shows simple first-order kinetics in the
starting material. The respective rates of thermolysis for the different alkyl
compounds show some aberrations from previously reported data for similar
compounds, and a tentative explanation is provided. Deuterium labeling studies
indicate a pathway involving an alkylidene intermediate, and the selectivity of
alkylidene formation and the subsequent C-H addition are discussed.
Chapter 3 covers the generation of the cationic complexes from the neutral alkyl
precursors activated with B(C6F5)3 and [Ph3C][B(C6F5)4] to give initially cationic
ansa-[(η5,η6-C5H4CMe2Ar)TiR2]+ species (Ar = Ph, 3,5-Me2C6H3; R = Me, CH2Ph).
For [(η5,η6-C5H4CMe2-3,5-Me2C6H3)TiR2][RB(C6F3)3] in bromobenzene, thermal
decomposition gives a dimeric Ti(III) dicationic species. The pathway involves
titanium-boron-alkyl scrambling, reduction from Ti(IV) to Ti(III) and solvent C-Br
bond activation. The Ti(III) dimer is the first structurally characterized example of
ansa-η5-cyclopentadienyl-η6-arene ligand coordination. The cationic species [(η5,η6C5H4CMe2Ph)Ti(CH2Ph)2]+ displays rapid ortho cyclometalation of the ligand at
ambient temperature to yield [(η5,η1-C5H4CMe2C6H4)Ti(CH2Ph)]+. Deuterium
labeling studies show that in the cations ortho cyclometalation proceeds via direct σbond metathesis, in contrast to the pathway via an alkylidene intermediate identified
for the neutral species.
Subsequently (Chapter 4), the catalytic olefin conversion properties of the cationic
dimethyl species, described in the previous chapter, are investigated. Interestingly,
ethene is found to be trimerized with high selectivity to 1-hexene. No significant
polymerization activity for propene and styrene is observed. The catalyst system (η5C5H4CMe2Ph)TiCl3/MAO is tested under various conditions in selective ethene
trimerization. This catalyst shows >95% trimerization selectivity affording mainly 1hexene and a C10 fraction (cotrimers of ethene and 1-hexene, mainly 5-methylnon-1ene). The effect of the aryl group (Ar = Ph, 4-MeC6H4, 3,5-Me2C6H3) on activity and
selectivity is discussed (extra aromatic methyl groups lead to a decrease in activity),
together with the influence of solvent and activator. The selective ethene
trimerization can be explained by assuming a mechanism involving
titanacycloalkane intermediates. Oxidative coupling of two ethene molecules to a
formal Ti(II) intermediate gives a titanacyclopentane. Subsequent insertion of ethene
26
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
followed by β-H elimination affords 1-hexene and regenerates the Ti(II) species. The
presence of the aryl group attached to the cyclopentadienyl ligand is deemed crucial
in the formation of the species active in trimerization.
In Chapter 5, the effect of ligand variations on the activity and the selectivity of the
selective ethene trimerization process is presented. The results indicate that the
bridging moiety between the cyclopentadienyl and arene group is of crucial
importance for selective ethene trimerization, since only CR2 groups (R ≠ H) afford
trimerization catalysts (the most active with a cyclohexyl-based bridge [(CH2)5]C).
Additional substituents on the cyclopentadienyl ligand (CMe3, SiMe3) can improve
both activity and selectivity. The substituents are also effective in increasing the 5methylnon-1-ene content in the C10 fraction, giving a higher overall α-olefin content.
The effect of the bridging group combined with that of cyclopentadienyl substituent
is found to be roughly additive, and leads to activity improvements of 61% for the
catalyst {η5-(3-SiMe3)C5H3C[(CH2)5]Ph}TiCl3/MAO. Surprisingly, combinations of
cyclopentadienyl and aryl substituents, the latter previously found to decrease
activity, give very active catalysts with a 93% improved activity for [η5-(3SiMe3)C5H3CMe2-3,5-Me2C6H3]TiCl3/MAO with respect to its unsubstituted analog.
The introduction of a second pendant arene group on the cyclopentadienyl ring
eventually gives the catalyst system [η5-C5H3-1,3-(CMe2Ph)2]TiCl3/MAO, which,
interestingly, displays a constant trimerization activity in time over at least 2 h.
1.8
References and notes
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(7)
27
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(8)
(9)
(10)
(11)
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(22)
28
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Stang, P.J.), Wiley-VCH, Weinheim, 1998, (f) Transition Metals for Organic Synthesis (Eds.
Beller, M., Bolm, C.), VCH, Weinheim, 1998, (g) Li, J.J., Gribble, G.W., Palladium in
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Busico, V., Corradini, P., De Rosa, C., Di Benedetto, E., Eur. Polym. J. 1985, 21, 239, (c)
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General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(23)
(24)
(25)
(26)
(27)
(28)
(29)
(30)
(31)
(32)
(33)
(34)
(35)
(36)
(37)
(38)
(39)
(40)
(41)
(a) Cossee, P., J. Catal. 1964, 3, 80, (b) Arlman, E.J., J. Catal. 1964, 3, 89, (c) Arlman, E.J.,
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Natta, G., Pino, P., Mazzanti, G., Lanzo, R., Chim. Ind. 1957, 39, 1032
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P., Barone, V., Fusco, R., Guerra, G., J. Catal. 1982, 77, 32, (d) Corradini, P., Barone, V.,
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(a) Breslow, D.S., Newburg, N.R., J. Am. Chem. Soc. 1959, 81, 81, (b) Long, W.P., Breslow,
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Reichert, K.H., Angew. Makromol. Chem. 1970, 13, 177, (e) Henrici-Olivé, G., Olivé, S.,
Angew. Chem. Int. Ed. Engl. 1967, 6, 790, (f) Henrici-Olivé, G., Olivé, S., J. Organomet.
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29
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(42)
(43)
(44)
(45)
(46)
(47)
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(52)
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(54)
(55)
(56)
(57)
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(59)
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(61)
30
Horton, A.D., Orpen, A.G., Organometallics 1991, 10, 3910
(a) Turner, H.W., EP 277 004 (1988) to Exxon, (b) Turner, H.W., Hlatky, G.G., EP 277 003
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Bochmann, M., Angew. Chem. Int. Ed. Engl. 1992, 31, 1181
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General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(62)
(63)
(64)
(65)
(66)
(67)
(68)
(69)
(70)
(71)
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31
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
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(89)
(90)
32
Swenson, D.C., Jordan, R.F., Organometallics 1997, 16, 1392, (j) Jäger, F., Roesky, H.W.,
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General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(91)
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Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
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Glockner, P.W., Keim, W., Mason, R.F., US 3,647,914 (1972) to Shell Development
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34
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(126)
(127)
(128)
(129)
(130)
(131)
(132)
(133)
(134)
(135)
(136)
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35
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
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36
General Introduction
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37
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
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(182) For example, see: Ziegler, K., Martin, H., US 2,943,125 (1954) to Studienges. Kohle
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(188) The catalyst system (η5-C5H4CH2CH2SMe)CrCl2/MAO was also reported to oligomerize
ethene, see: Döhring, A., Göhre, J., Jolly, P.W., Kryger, B., Rust, J., Verhovnik, G.P.J.,
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(189) Döhring, A., Jensen, V.R., Jolly, P.W., Thiel, W., Weber, J.C., Organometallics 2001, 20,
2234
(190) PR3 can be effectively removed by phosphane scavengers, such as [Ni(cod)2] (cod = 1,5cyclooctadiene).
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Ballard, D.G.H., Van Linden, P.W., Makromol. Chem. 1972, 154, 177, (b) Ballard, D.G.H.,
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1994
38
General Introduction
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
(198) (a) Ishihara, N., EP 210 615 (1986) to Idemitsu Kosan Co., (b) Ishihara, N., Kuramoto, M.,
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23
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39
Chapter 1
▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬
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40