Metallocene
Organometallic coordination compounds in which one atom
of a transition metal such as iron, ruthenium or osmium is
bonded to and only to the face of two cyclopentadienyl [5(C5H5)] ligands which lie in parallel planes
Fe
Ferrocene: The First Metallocene
1
Homogeneous Catalysis
Considerations:
1. Mechanistic considerations better understood
using soluble systems
2. Catalyst requirements lower as the implicated
amounts are totally available for activity
3. No possibility of catalyst deactivation as a result
of polymer coating
4. Uniform molecular distribution as there is no or
marginal change in available catalyst
2
Breslow and Newberg observed that when an orange soln of
Cp2TiCl2 in toluene is treated with two moles of [Et2AlCl]2,
there is an immediate colour change to red and finally to blue.
The operative equilibrium was found to be:
3
Active Species
Rearrangement of Active Species and Propagation
4
Reaction of Cp2TiCl2 with Al2Cl6 (AlCl3)
Reaction of Cp2TiCl2 with [MeAlCl2]
Chloride abstraction by aluminum!!!
5
Generation of Ti(III): Abrupt
Colour Changes
Aluminum alkyls are reducing agents, and therefore a
reduction Ti(IV) to Ti(III) inevitably takes place if the two
components are brought together.
6
Polymerization and Reduction?
In Cp2TiCl2, as well as in Cp2TiEtCl, the titanium is present in
an approximately tetrahedral environment.
On complex formation with an aluminum alkyl, one of the
ligands of the Al also requires a place in the coordination
sphere of the Ti. We propose that this requirement forces
the Ti into an octahedral environment (only tetrahedral and
octahedral complexes of Ti have so far been reported).
7
By this procedure the Ti-Et bond comes under the transinfluence of the bridged aluminum and presumably suffers
weakening. This weakening is responsible for the two
phenomena, polymerization and reduction.
In the absence of ethylene only the reduction reaction has to
be taken into account. The octahedral complex has one
coordination site empty. A β-hydrogen atom of the ethyl
group of a second complex unit may occupy this site.
Subsequent transfer of this hydrogen to the other ethyl
group would lead to the formation of ethane and ethylene,
as has been observed experimentally. As a consequence the
titanium is reduced to Ti(III).
8
Under polymerization conditions, ethylene can coordinate
to Ti at the sixth, so far empty, coordination site.
Part of the electron density will be transferred from the
bonding orbital of the ethylene to the metal, thus weakening
the ethylene double bond and making it susceptible to
polymerization.
9
Kinetics of Ethylene Polymerization
Catalyzed by Cp2TiCl2 and [Me2AlCl]2
Results indicated a relationship
Rp = kp[C][m]
where Rp is the rate of polymerization, [C] and [m] are the
concentrations of propagating metal alkyl complex and
monomer, respectively.
Increase in Polymer Yield with Time
10
At a fixed temperature and monomer pressure, the polymer
molecular weight depends mainly upon the catalyst
concentration.
Kinetic expression for chain termination
-d[C]/dt = kt[C]2
Initiation was followed by using [(C14H3)2AlCl]2 and measuring
the increment of C14 in high polymers with time.
11
Variation of C14 activity in polymer
samples with time
12
Arrhenius Plots
13
Natta and Mazzanti Provided a
Closer Look
TiCl4 and PhAlCl2 when mixed results in the formation of an
equilibrium mixture consisting of (a) and (b)
14
Configuration of Active Species
15
It is difficult to distinguish whether the polymer chain grows
on the Al or Ti center. Hence a partial ionic dissociation takes
place as indicated in the mechanism below.
16
Cationic Zr(IV) Benzyl Complexes
Structure of 4
17
1c
18
RZrMe+ + AlMe3 = ?
Characterized Intermediate
Mechanism
19
Alk-1-yne Oligomerization
20
Catalyst Construction: Progress,
Challenges and Opportunities
21
Metallocene Synthesis
Cp2TiCl2/Cp2TiMe2
H
H
n-BuLi
_ Li+
TiCl4
Ti
Cl
Cl
MeMgBr
Ti
Me
Me
22
Cp2ZrCl2/Cp2Zr(CH2Ph)2
H
H
n-BuLi
_ Li+
ZrCl4
Zr
Cl
Cl
PhCH2MgBr
Zr
Ph
Ph
23
Cp*TiCl3/Cp*TiMe3
Cp*H
n-BuLi
TiCl4
Cp*Li
Cp*TiCl3
Me3SiCl
MeMgBr
Cp*SiMe3
Cp*TiMe3
= Cp*
24
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26
EBIH2
rac-(EBI)Zr(NMe2)2
27
28
29
H
SBIH2
30
31
32
33
34
Catalysts of Commercial
Importance
35
Dow Elastomers Business
NH2
n-BuLi
H
H
n-BuLi
MCl4
SiMe2Cl
n-BuLi
Me2SiCl2
NHLi
SiMe2NH
H
Cl
Me2Si
M
N
H
MeMgBr
Cl
Me
Me2Si
M
N
Me
M = Ti, Zr
Constrained Geometry Catalyst System
36
37
Autoclave for CGC Polymerizations
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Catalyst Structure-Polymer
Microstructure Relationship
39
Polymerization and Metallocene
Symmetry
Metallocenes have earned enormous attention as a clear correlation between metallocene symmetry and polymer stereochemistry is unambiguously established. In 2002 polymer
literature contained more papers on metallocenes than any
other subject.
The most studied ligands are Cp and substituted Cp, 1-indenyl
(Ind), 4,5,6,7-tetrahydro-1-indenyl (H4Ind) and 9-fluorenyl (Flu)
Indenyl
Tetrahydroindenyl
Fluorenyl
40
The metallocene initiators are termed single-site catalysts as
each metal center has the same coordination environment. The
resultant polymer has narrower distributions of molecular wt,
regiochemistry and stereochemistry.
Stereoselective polymerizations with high reaction rates occur
for metallocene catalysts that are both chiral and stereorigid.
Chiral and stereorigid metallocenes have appropriately substd
η5-Cp ligands that are linked together by a bridging group.
These are also referred to as ansa metallocenes. The
bridging groups may be CH2CH2, CH2, SiMe2 or CMe2. The
unbridged catalysts donot achieve highly stereoselective
polymerization as free rotation of the η5-Cp ligand results in
achiral environment at the active site. The bridge locks the
symmetry of the active sites.
41
The group 4 metallocene has two active sites (the two Rgroups on the metal). The stereoselectivity of each of the two
coordination sites on the metal may be enantioselective or
nonselective.
The relationship between the stereoselectivities of the two
active sites of a metallocene catalyst (homotopic, enantiotopic, diastereotopic) determines the type of stereocontrol (chain
end or site end).
Group 4 metallocenes have the following general geometry:
The angle between the ligands, β, is called bite
angle is in the range 60-75 deg. The metal is
pseudotetrahedral and  is in the range 115-125
deg.  is few degrees less than 90.
The plane of the two ligands are not parallel and
hence these are called bent metallocenes.
42
C2v Symmetry
Examples: unsubstituted bis Cp catalysts, Me2Si(Flu)ZrCl2
These catalysts are achiral, and both the coordination (active)
sites are chiral and homotopic.
Atactic polymer is formed with chain end control
43
C2 Symmetry
Examples: rac-Me2SiInd2ZrCl2
Meso fraction separated by fractional crystallization!
The two sites are equivalent (homotopic) and enantioselective
for the same monomer enantioface. As a result, there is
44
isoselective polymerization.
The steric environment at the active site determines which
enantioface of the incoming monomer is coordinated to the
transition metal. The chiral active site forces the propagating
polymer chain to assume an orientation that minimizes the
steric interaction with one of the η5-ligands and this results in
discrimination between two faces of the monomer. There is
precedence of catalyst site control as the mode of propagation.
The structural variables on the ligand plays an important role
in determining the course of polymerization by altering the bite
angle as a result the stereorigidity of the ligand is altered.
If the bite angle is too large, stereorigidity is lowered and the
degree of isotacticity decreases.
45
Interrelation between Structural Parameters
1. Ti metallocenes are less active and less stereoselective than Zr and Hf.
Zr metallocenes are most useful as these are most active in comparison
with their Ti and Hf analogues. These have been optimized by various
structural variations to yield very high stereoselectivity along with high
molecular weight. Hf metallocenes produce higher molecular weights but
not better stereoselectivities as compared to Zr analogues.
2. Substituents at the 3- and 4- positions of the Cp ring have maximum
effect in increasing activity, isoselectivity and molecular weight. Substituents at the 2- and 5- positions have a positive but lesser effect. The 6membered ring plays the role of 4- and 5- substituents in Ind and H4Ind
ligands.
46
H4 Ind ligands generally increase isoselectivity with some decrease in
activity.
3. The effect of bridge between ligands depends upon the type of the
ligand. The bite angle and stereorigidity are affected by the type of the
bridge depending upon the type of the ligand.
4. The presence of heteroatoms into the ligands via alkoxy or trisubstituted
amino groups generally deteriorates catalyst.
5. Bisfluorenyl zirconocenes generally are neither highly active nor
isoselective.
47
Cs Symmetry
Popular examples include:
(1)
(2)
(1) produces a highly atactic polymer even higher than best C2
metallocene.
(2) produces highly syndiotactic polymer
48
C1 Symmetry
Some popular examples include:
There are no elements of symmetry. Each site is in a chiral
environment. These exhibit a range of stereo specificity
depending on the choice of ligand.
49
Schematic Representation of Various classes of Polyolefins
Decreasing
stereoregularity
50
Non-Metallocene Synthesis
51
ORTEP diagram of [1,8-C10H6(NSiMe3)2]ZrCl2 dimer
52
53
ORTEP diagram of [1,3-C3H6(NSi(i-Pr)3)2]ZrCl2
54
55
NH2
NH2
1. n-BuLi
2. Me3SiCl
SiMe3
SiMe3
NH
N
NH
SiMe3
1. n-BuLi
2. TiCl4
Cl
Ti
N
Cl
SiMe3
SiMe3
N
MeMgBr
Me
Ti
N
Me
SiMe3
56
NH2
TiCl4
n-BuLi
NHLi
N
Cl
Ti
N
Cl
Br
Br
NH
HN
TMEDA
MeMgBr
N
Me
Ti
N
Me
57
NH2
Me3Si
NHLi
Me3SiCl
n-BuLi
1. n-BuLi
Me3Si
2. ZrCl4
Me3Si
N
N
NH
Cl
Zr
Cl
MeMgBr
Me3Si
Me3Si
N
Zr
Me
Me
N
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Status of Non-Metallocene Research
65
Example:
300 MHz Spectra in CD2Cl2
66
Performance Requirement
Driven Product Design Logic
67