group 4 metal complexes with ferrocenyl amidinates · anal calcd calculated elemental analysis bn...
TRANSCRIPT
Group 4 Metal Complexes with Ferrocenyl Amidinates
by
Kanwarpal Multani
A thesis submitted in conformity with the requirements for the degree of Master of Science
Graduate Department of Chemistry University of Toronto
© Copyright by Kanwarpal Multani 2010
ii
Group 4 Metal Complexes with Ferrocenyl Amidinates
Kanwarpal Multani
Master of Science
Graduate Department of Chemistry University of Toronto
2010
Abstract
Bis(amidinate) dichloride complexes of the type M(L)2Cl2 (M=Zr, 2a; M=Ti, 2b;
L=CyNC(C5H5FeC5H4)NCy) were synthesized by treating 2 equiv of ferrocenyl amidine,
H(L), with M(NMe2)2Cl2 (M=Ti, Zr.2THF). Half sandwich mono(amidinate) complexes,
Cp’ZrLCl2 (Cp’=Cp, 2c; Cp’=Cp*, 2d), were prepared by the reaction of Cp’ZrCl3 with 1
equiv of Li(L). The dialkyl complexes, M(L)2Me2 (M=Zr, 3a; M=Ti, 3b), CpZr(L)(CH2Ph)2
(3c) and Cp*Zr(L)Me2 (3d) were prepared by treatment of the dichloride complexes (2a-
2d) with an appropriate alkylating agent. The dichloride complexes (2a-2d) activated
with MAO, and dialkyl complexes (3a-3d) activated with B(C6F5)3 and [Ph3C][B(C6F5)4]
show low to moderate ethylene polymerization activities. Cyclic voltammetry studies on
the metal complexes containing ferrocenyl amidinates reveals quasi reversible oxidation
and reduction waves for the ferrocene/ferrocenium couple.
iii
Acknowledgments
I would like to acknowledge a number of people for their support over the past
little while. First and foremost, I would to thank my research advisor Prof. Doug Stephan
for his ideas, encouragement and enthusiasm.
Thanks to all of the past and present Stephan group members with whom I have
worked closely. Particularly, Dr. Alberto Ramos for teaching me basic laboratory skills
when I initially started. A special thank you to Dr. Zach Heiden for his extensive help on
cyclic voltammetry and solving crystal structures. Also, thanks to Meghan Dureen and
Dr. Edwin Otten for editing this thesis. Lastly, I would like to thank my parents for all of
their contributions.
SABIC Corporation is acknowledged for providing the financial support.
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Table of Contents Page
Acknowledgments ............................................................................................................. iii
Table of Contents ............................................................................................................. iv
List of Tables .................................................................................................................... vi
List of Figures................................................................................................................... vii
List of Schemes ................................................................................................................ ix
List of Abbreviations and Symbols .................................................................................... x
Compound Numbering Scheme ....................................................................................... xi
Chapter 1: Introduction ...................................................................................................... 1
1.1 Background ......................................................................................................... 1
1.1.1 Features of Successful Catalysts .................................................................... 2
1.1.1.1 Catalyst Activity ........................................................................................ 2
1.1.1.2 Polymer Properties .................................................................................. 2
1.2 Proposed Mechanism of Polymerization ............................................................ 3
1.2.1 Activation ......................................................................................................... 4
1.2.2 Propagation ..................................................................................................... 5
1.2.3 Termination ..................................................................................................... 6
1.3 Ethylene Polymerization Catalysts ..................................................................... 8
1.3.1 Early Catalysts ................................................................................................ 8
1.3.2 Metallocene Based Catalysts .......................................................................... 9
1.3.3 Post Metallocene Catalysts ........................................................................... 11
1.3.4 Amidinate Based Olefin Polymerization Catalysts ........................................ 12
1.4 Amidinates ........................................................................................................ 13
1.4.1 Modes of Coordination .................................................................................. 14
1.4.2 Synthesis of Metal-Amidinates ...................................................................... 15
1.5 Scope of Thesis ................................................................................................ 17
Chapter 2: Synthesis, Characterization and Ethylene Polymerization Activity of Group 4
Ferrocenyl Amidinate Complexes ................................................................................... 18
2.1 Introduction ....................................................................................................... 18
v
2.2 Results and Discussion .................................................................................... 20
2.2.1 Synthesis and Characterization .................................................................... 20
2.2.1.1 Synthesis of Amidine and Amidinate Derivatives ................................... 20
2.2.1.2 Synthesis of Group 4 Metal Dichloride Complexes ................................ 25
2.2.1.3 Synthesis of Group 4 Metal Dialkyl Complexes ..................................... 36
2.2.1.4 Synthesis of Metal Monoalkyl Cationic Complexes ............................... 43
2.2.2 Electrochemical Study of Dichloride and Dialkyl Complexes ........................ 49
2.2.3 Polymerization Results .................................................................................. 55
2.2.3.1 Ethylene Polymerization using Dichloride Precatalysts ......................... 55
2.2.3.2 Ethylene Polymerization using Dialkyl Precatalysts ............................... 58
2.2.4 Summary ....................................................................................................... 60
Chapter 3: Experimental Details .................................................................................... 62
3.1 General Considerations .................................................................................... 62
3.1.1 Solvents ........................................................................................................ 62
3.1.2 Materials ........................................................................................................ 62
3.2 Instrumentation ................................................................................................. 63
3.2.1 Nuclear Magnetic Resonance Spectroscopy (NMR) ..................................... 63
3.2.2 Electrochemistry ............................................................................................ 63
3.2.3 Other Instrumentation ................................................................................... 63
3.3 Synthesis and Characterization ........................................................................ 64
3.3.1 Reagents and Starting Materials ................................................................... 64
3.3.2 Organic and Organometallic Syntheses ........................................................ 64
3.4 Polymerization Protocol .................................................................................... 74
3.4.1 Schlenk Line Polymerization ......................................................................... 74
3.5 Crystallography ................................................................................................. 75
3.5.1 X-ray Data Collection and Reduction ............................................................ 75
3.5.2 Structure Solution and Refinement ............................................................... 75
References ...................................................................................................................... 79
vi
List of Tables Table 1.1. Ranking the effectiveness of the catalyst based on the activity. ..................... 2 Table 2.2. Selected bond distances (Å) and bond angles (°) for
[iPrNC(Fc)NiPr]H, 1b. ...................................................................................................... 24 Table 2.3. Selected bond distances (Å) and bond angles (°) for
[CyNC(Fc)NCy]SiMe3, 1c… ............................................................................................ 25 Table 2.4. Selected bond distances (Å) and bond angles (°) for
[CyNC(Fc)NCy]2ZrCl2, 2a and [CyNC(Fc)NCy]2TiCl2, 2b. ............................................... 31 Table 2.5. Selected bond distances (Å) and bond angles (°) for
CpZr[CyNC(Fc)NCy]Cl2, 2c, and for Cp*Zr[CyNC(Fc)NCy]Cl2, 2d… .............................. 35 Table 2.6. Selected bond distances (Å) and bond angles (°) for
[CyNC(Fc)NCy]2ZrMe2, 3a. ............................................................................................. 38 Table 2.7. Selected bond distances (Å) and bond angles (°) for
CpZr[CyNC(Fc)NCy](CH2Ph)2, 3c... ................................................................................ 42 Table 2.8. Selected bond distances (Å) and bond angles (°) for
[CyNHC(Fc)NHCy][B(C6F5)4]. .......................................................................................... 47 Table 2.9. Summary of the cyclic voltammetric data on the dichloride complexes, 2a-2d.
........................................................................................................................................ 49 Table 2.10. Summary of the cyclic voltammetric data on the dialkyl complexes, 3a, 3c-
3d. ................................................................................................................................... 53 Table 2.11. Ethylene polymerization results of dichloride precursors using MAO as the
cocatalyst ........................................................................................................................ 56 Table 2.12. Ethylene polymerization results of dialkyl precursors activated with B(C6F5)3
or [Ph3C][B(C6F5)4]. ......................................................................................................... 58
Table 3.1. Table of crystallographic parameters for [iPrNC(Fc)NiPr]H (1b),
[CyNC(Fc)NCy]SiMe3 (1c) and [CyNC(Fc)NCy]2ZrCl2 (2a). ............................................ 76 Table 3.2. Table of crystallographic parameters for [CyNC(Fc)NCy]2TiCl2 (2b),
CpZr[CyNC(Fc)NCy]Cl2 (2c) and Cp*Zr[CyNC(Fc)NCy]Cl2 (2d). .................................... 77 Table 3.3. Table of crystallographic parameters for [CyNC(Fc)NCy]2ZrMe2 (3a),
CpZr[CyNC(Fc)NCy](CH2Ph)2 (3c) and [CyNHC(Fc)NHCy][B(C6F5)4] (6). ..................... 78
vii
List of Figures Figure 1.1. Representation of (a) isotactic polymers, (b) syndiotactic polymers and (c)
atactic polymers. ............................................................................................................... 3
Figure 1.2. Propagation step of polymerization involves olefin coordination followed by
insertion............................................................................................................................. 6
Figure 1.3. Agostic interaction in propagation step of polymerization. ............................ 6
Figure 1.4. Several chain termination pathways. (a) β-hydride transfer to metal (b) β
hydride transfer to monomer (c) β-methyl elimination (d) chain transfer to aluminum. ..... 7
Figure 1.5. Examples of ansa-zirconocene catalysts controlling the steroselectivity of
the resulting polymer. ...................................................................................................... 10
Figure 1.6. General structure of constrained geometry catalysts. ................................. 11
Figure 1.7. General structure of (a) McConville’s diamide catalysts (b) phosphinimide
based catalysts (c) FI catalysts based on Group 4 salicylaldiminide complexes. ........... 12
Figure 1.8. Active amidinate based olefin polymerization catalysts (a)
bis(benzamidinate) complexes (b) acetamidinate complexes (c) guanidinate complexes
(d) bis(iminophosphonamide) based complexes . ........................................................... 13
Figure 1.9. Resonance structures of amidinates. .......................................................... 13
Figure 1.10. Ligands isoelectronic to amidinates: (a) guanidinates (b) carboxalates (c)
triazenates....................................................................................................................... 14
Figure 1.11. Commonly observed modes of coordination: (a) chelating bidentate (b)
bridging (c) monodentate. ............................................................................................... 15
Figure 2.1. Active olefin polymerization catalysts containing ferrocene based ligands:
(a) bis(amino)ferrocenyl ligand (b) ferrocenyl dimethylsilyl substituted zirconocenes (c)
ferrocenyl substituted phosphinimine ligand. .................................................................. 19 Figure 2.2. Molecular structure of [iPrNC(Fc)NiPr]H, 1b and [CyNC(Fc)NCy]SiMe3, 1c.
........................................................................................................................................ 24 Figure 2.3. 1H NMR spectrum of 2a from 4.65-3.45 ppm shows splitting of substituted
Cp protons and broadening of the cyclohexyl protons at low temperature. .................... 28 Figure 2.4. Molecular structure of [CyNC(Fc)NCy]2ZrCl2, 2a. ....................................... 30 Figure 2.5. Molecular structure of [CyNC(Fc)NCy]2TiCl2, 2b. ....................................... 31 Figure 2.6. Molecular structure of CpZr[CyNC(Fc)NCy]Cl2, 2c. .................................... 34 Figure 2.7. Molecular structure of Cp*Zr[CyNC(Fc)NCy]Cl2, 2d. .................................. 34 Figure 2.8. Molecular structure of [CyNC(Fc)NCy]2ZrMe2, 3a. ...................................... 38
viii
Figure 2.9. Molecular structure of CpZr[CyNC(Fc)NCy](CH2Ph)2, 3c. .......................... 41 Figure 2.10. Molecular structure of [CyNHC(Fc)NHCy][B(C6F5)4]. ................................ 46 Figure 2.11. Cyclic voltammogram of 2a.. ..................................................................... 50 Figure 2.12. Cyclic voltammogram of 2c. ...................................................................... 51 Figure 2.13. Cyclic voltammogram of 3a.. ..................................................................... 54 Figure 2.14. Cyclic voltammogram of 3c. ...................................................................... 54
ix
List of Schemes Scheme 1.1. Activation of metallocene precatalysts by several routes to prepare the
proposed active catalyst. ................................................................................................... 4
Scheme 1.2. Synthesis of N,N,N’-tris(trimethylsilyl)benzamidine .................................. 15
Scheme 1.3. Common routes for synthesis of metal amidinates. (a) elimination of
trimethylsilyl chloride (b) amine elimination (c) carbodiimide insertion (d) salt metathesis.
........................................................................................................................................ 16
Scheme 2.1. Synthesis of ferrocenyl amidine ............................................................... 21
Scheme 2.3. Synthesis of ferrocenyl amidine with isopropyl substituents on the
nitrogen. .......................................................................................................................... 22
Scheme 2.4. Synthesis of trimethylsilyl substituted ferrocenyl amidine, 1c. ................. 23
Scheme 2.5. Synthesis of titanium and zirconium bis(amidinate) complexes, 2a and 2b.
........................................................................................................................................ 26
Scheme 2.6. Rapid interconversion of 2a at room temperature. ................................... 28
Scheme 2.7. Synthesis of half sandwich mono(amidinate) zirconium complexes, 2c and
2d, via salt metathesis. ................................................................................................... 33
Scheme 2.8. Attempted synthesis of half sandwich mono(amidinate) titanium
complexes via salt metathesis was unsuccessful. ......................................................... 36
Scheme 2.9. Synthesis of half sandwich zirconium mono(amidinate) dimethyl complex,
which subsequently undergoes ligand redistribution. ...................................................... 40
Scheme 2.10. Synthesis of cationic monomethyl zirconium bis(amidinate) complexes
paired with MeB(C6F5)3- or B(C6F5)4
- anions, 4a and 5a, respectively. ............................ 45
Scheme 2.11. Synthesis of cationic monomethyl zirconium mono(amidinate) complex
paired with MeB(C6F5)3- or B(C6F5)4
- anions, 4d and 5d, respectively. ............................ 46
Scheme 2.12. Synthesis of cationic zirconium mono(amidinate) complex, 4c, paired
with MeB(C6F5)3- anion and possible modes of benzyl coordination. .............................. 48
x
List of Abbreviations and Symbols Ǻ Ǻngstrom, 10-10 m δ chemical shift Anal Calcd calculated elemental analysis Bn benzyl br broad Cp cyclopentadienyl Cp* pentamethylcyclopentadienyl Cy cyclohexyl d doublet DCM dichloromethane e.s.d estimated standard deviation Fc ferrocenyl FcLi monolithioferrocene FcLi2 dilithioferrocene Hz Hertz i-Pr isopropyl J symbol for coupling constant m multiplet Me methyl mL milliliter(s) min minute(s) mmol millimole(s) ORTEP Oak Ridge thermal ellipsoid plot Ph phenyl ppm parts per million s singlet t-Bu tertiary butyl THF tetrahydrofuran TMS trimethylsilyl
xi
Compound Numbering Scheme 1a [CyNC(Fc)NCy]H
1b [iPrNC(Fc)NiPr]H
1c [CyNC(Fc)NCy]SiMe3
1d Li[CyNC(Fc)NCy]
2a [CyNC(Fc)NCy]2ZrCl2
2b [CyNC(Fc)NCy]2TiCl2
2c CpZr[CyNC(Fc)NCy]Cl2
2d Cp*Zr[CyNC(Fc)NCy]Cl2
3a [CyNC(Fc)NCy]2ZrMe2
3b [CyNC(Fc)NCy]2TiMe2
3c CpZr[CyNC(Fc)NCy](CH2Ph)2
3d Cp*Zr[CyNC(Fc)NCy]Me2
4a [{CyNC(Fc)NCy}2ZrMe][MeB(C6F5)3]
4b [{CyNC(Fc)NCy}2TiMe][MeB(C6F5)3]
4c [CpZr{CyNC(Fc)NCy}CH2Ph][PhCH2B(C6F5)3]
4d [Cp*Zr{CyNC(Fc)NCy}Me][MeB(C6F5)3]
5a [{CyNC(Fc)NCy}2ZrMe][B(C6F5)4]
5b [{CyNC(Fc)NCy}2TiMe][B(C6F5)4]
5c [CpZr{CyNC(Fc)NCy}CH2Ph][B(C6F5)4]
5d [Cp*Zr{CyNC(Fc)NCy}Me][B(C6F5)4]
1
Chapter 1 Introduction
1.1 Background
Over the past several decades, plastic has slowly replaced alternative materials
such as paper, glass and metal. Plastics are high molecular weight synthetic polymers,
which exhibit a wide range of properties such as density, crystallinity, tensile strength
and elasticity based on their microstructure. Small changes in the chemical makeup can
lead to a significant difference in the properties of the resulting polymer.
Many different polymers are commercially produced for a range of applications.
Polyethylene is the most commonly used synthetic polymer, with an annual production
reaching approximately 80 million metric tons, followed by polyvinylchloride and
polypropylene.1 It was originally discovered in 1933 by Imperial Chemical Industries in
United Kingdom. These synthetic polymers can be classified into different categories
based on physical properties such as density, molecular weight and extent of branching
in the polymer. The two major classes of polyethylene are high density polyethylene
(HDPE) and low density polyethylene (LDPE). HDPE is linear straight chain
polyethylene with low degree of branching, whereas, LDPE has a high degree of short
and long chain branching.
Present, industrial and academic research is focused on developing new types of
polymers with unique properties such as biodegradability, aesthetics and ease of
processability.2 Although these new polymers may revolutionize the polymer industry,
polyethylene continues to be the most widely used polymer. As a result, an industrial
interest still remains in the development and commercialization of new ethylene
polymerization catalysts, and improving the catalytic activities of known catalysts.
2
1.1.1 Features of Successful Catalysts
There are several criteria used to measure the effectiveness of polymerization
catalysts such as activity, stereoregularity and molecular weight distribution of the
resulting polymer. Very few catalysts make it to the commercial stage as there are
additional requirements including the effectiveness of catalyst under industrial conditions
and patent position.
1.1.1.1 Catalyst Activity
Gibson developed a scale that can be used to rank the catalyst activity
quantitatively (Table 1.1).3 The catalyst activity or productivity can be calculated using
the following formula (Eq. 1.1).
pressure monomer time tionpolymeriza mass catalystmass polymer Activity
××= (Eq. 1.1)
In relevant literature, activities are most commonly reported in g.(mmol.hr.bar)-1 or
g.(mmol.hr.atm)-1. There are several factors that contribute to the activity value during
polymerization such as stirring rate, temperature, size of vessel, type of activator,
solvent scavenger, solvent, and order of precatalyst and activator addition.
Table 1.1. Ranking the effectiveness of the catalyst based on the activity.3 Rating Activity, g.(mmol.hr.atm)-1
Very low < 1 Low 1 – 10
Moderate 10 – 100 High 100 – 1000
Very high > 1000
1.1.1.2 Polymer Properties
An important characteristic of the resulting polymer is the average molecular
weight distribution determined through gel permeation chromatography (GPC). The
molecular weight distribution curve shows the relative amounts of polymer of different
3
molecular weights in a given sample. The polydispersity index (PDI) is calculated as the
ratio of the weight average molecular weight ( wM ) to the number average molecular
weight ( nM ) and illustrates this distribution.
∑∑=
i
iin
NMN
M (Eq. 1.2)
ii
iiw
MNMN
M∑∑=
2
(Eq. 1.3)
n
w
MMPDI =
(Eq. 1.4)
where Ni is the total number of molecules with a molecular weight of Mi. Single site
catalysts produce a narrow molecular weight distribution with PDI of approximately 2.
Stereoregularity of the resulting polymer is also an important consideration and
the type of catalyst used for polymerization of higher olefins will define this feature. An
isotactic polymer has substituents on the same side of the polymer, syndiotactic
polymers have substituents on alternate sides, and in atactic polymers the substituents
are placed randomly on the chain (Figure 1.1).
Figure 1.1. Representation of (a) isotactic polymers, (b) syndiotactic polymers and (c) atactic polymers.
1.2 Proposed Mechanism of Polymerization
The Cossee-Arlman mechanism is the commonly accepted mechanism for
polymerization of olefins by Group 4 catalysts.4,5 This process involves three main
stages of activation, propagation and chain termination.
4
1.2.1 Activation
The catalytically active species is proposed to be a cationic alkyl complex
stabilized by ancillary ligands with a vacant coordination site for olefin binding.6,7 This
cationic species exist as a cation-anion pair, which is generated from dichloride or dialkyl
precatalysts. Generation of this species requires an alkyl anion or halide abstraction by
a strong Lewis acid, which is often a cocatalyst in polymerization. The cocatalyst is
usually the source of the counteranion to the metal cation.
Scheme 1.1. Activation of metallocene precatalysts by several routes to prepare the proposed active catalyst.
There are several common routes for creating the cationic species. One method
involves the use of methylaluminoxane (MAO) as the cocatalyst. MAO is proposed to
methylate the dichloride precatalysts, followed by an abstraction of a methyl anion to
produce a mono methyl cationic species (Scheme 1.1, a).8,9 This cationic species is
stabilized by the counteranion, [Me-MAO]-. MAO is synthesized through controlled
hydrolysis of trimethylaluminum. Although its exact structure is unknown, it is agreed
that it exists as a mixture of oligomeric structures with a general formula of [Me(Al)O]x.10
This activator is used in large excess (500-1000 equivalents of precatalyst) and often
5
represents a major cost in production of polyolefins. MAO also functions as a solvent
scavenger to remove any catalyst poison such as oxygen, water, or sulfur compounds.
MAO is the most commonly used cocatalyst in the industry but other cocatalysts,
such as perfluorinated substituted phenyl boranes and borates, have also emerged
(Scheme 1.1, b-d). These borane activators follow the criteria required for highly active
catalysts, mainly the formation of bulky counter anions and weakly coordinating pairs to
allow olefin binding during polymerization. The borane activators are usually added in
stoichiometric amounts relative to the precatalyst as opposed to MAO, which is added in
large excess. The borate anions were found to be superior over MAO anions in the
elucidation of mechanistic details of olefin polymerization. B(C6F5)3, a strong Lewis acid,
can be used to abstract a methyl anion forming a MeB(C6F5)3- anion (Scheme 1.1, b).
Other commonly used activators include [Ph3C][B(C6F5)4] and [PhNHMe2][B(C6F5)4]
(Scheme 1.1, c-d). [Ph3C][B(C6F5)4] is usually preferred since it releases an unreactive
hydrocarbon, whereas, [PhNHMe2][B(C6F5)4] releases an amine, which may deactivate
the catalyst through coordination to the metal center.
1.2.2 Propagation
After the generation of catalytically active species, the second step of olefin
polymerization involves coordination of the olefin to the metal center donating the π
electron density to the Lewis acidic metal center (Figure 1.2). This step is followed by a
4-membered intermediate and olefin insertion into the metal-carbon bond of the growing
chain. This results in a free coordination site and an alkyl chain bound to the metal
center. The process repeats resulting in a growing polymeric alkyl chain. After each
insertion, the alkyl chain and vacant coordination site alternate. Based on the ligand
framework surrounding the metal center, the direction of olefin binding may be favoured
differently at the alternating coordination sites and this phenomenon gives rise to
6
stereoselective polymers in higher olefins. Green and Brookhart modified this
mechanism and proposed that the insertion step may be facilitated by an agostic
interaction between the metal center and the hydrogen on the alpha carbon of the
polymer chain (Figure 1.3).11,12
Figure 1.2. Propagation step of polymerization involves olefin coordination followed by insertion.
Figure 1.3. Agostic interaction in propagation step of polymerization.
1.2.3 Termination
The last step of polymerization stops incorporation of new monomers and
terminates the polymer chain (Figure 1.4).13 One method is via a β-hydride elimination
in which the hydride from the polymer chain is transferred onto the metal center resulting
in a polymer chain with an olefin end (Figure 1.4, a).14 The alkene insertion may take
place restarting the propagation step to generate another polymer chain. An alternative
termination pathway involves hydride transfer to an incoming bound olefin resulting in an
alkyl chain bound to transition metal and polymer chain with olefin end group (Figure 1.4,
7
b).15 After the polymer chain is ejected, growth of a new chain starts. β-hydride transfer
to the monomer is the dominant pathway under normal experimental conditions and
results in a new alkyl bound chain without the formation of metal hydride species.
Figure 1.4. Several chain termination pathways. (a) β-hydride transfer to metal (b) β hydride transfer to monomer (c) β-methyl elimination (d) chain transfer to aluminum.
8
β-methyl elimination involves the transfer of a methyl group to the metal center
(Figure 1.4, c). This termination method is quite rare but important in propylene
polymerization using Cp*2MIVCl2 (M = Ti, Zr) complexes.9 Chain termination by
transalkylation to an aluminum center is also possible (Figure 1.4, d). This mechanism
is highly dependent on Al:metal ratio and comonomer concentration. Lastly, chain
transfer agents, such as molecular hydrogen, may be added to terminate the polymer
chain and allowing specific control over polymer molecular weight. Other chain
termination methods may involve irreversible deactivation of the catalyst.
1.3 Ethylene Polymerization Catalysts
The commercial production of polyethylene began in the 1930’s using a free
radical polymerization process. This method requires harsh conditions using high
temperatures (approximately 300°C) and pressures (~2000 atm) producing highly
branched low density polyethylene with limited applications.16 Over the years, new
catalytic processes have been developed that operate under mild conditions and
broadened the range of polymers that are available in the market.
1.3.1 Early Catalysts
In the 1950’s, Ziegler and Natta demonstrated that transition metal based
catalysts could aid in polyethylene production. Ziegler discovered that a heterogeneous
mixture of transition metal halides, such as TiCl4, combined with alkyl aluminum based
cocatalysts effectively produced high density polyethylene.17 Around the same time,
Natta showed that linear isotactic, crystalline polypropene could be produced using
similar heterogeneous systems. These discoveries were a major breakthrough as
stereoselective polymers could now be produced at lower temperatures (often 20-50°C)
and pressures (often 8-10 atm) compared to previous methods.18 Later, Hogan and
9
Banks marked the commercial production of polyethylene at low pressures using
CrO3/SiO2 systems and this method still accounts for a majority of the worldwide
polyethylene production.19 Modern heterogeneous Ziegler-Natta catalysts with MgCl2
supports have high activities for polypropene production showing excellent
stereoselectivity for the formation of isotactic polymers.15
1.3.2 Metallocene Based Catalysts
With the success of Ziegler and Natta heterogeneous systems, many questions
such as the exact role of the catalytic surface, the detailed mechanism of polymerization
and the introduction of stereoregularity into the polymer chain were beginning to be
asked. However, a fundamental problem with the heterogeneous catalysts was that they
exhibited low activities due to the limited access to the active sites within the solid. An
improvement to this system was the discovery of first homogenous catalyst by Natta and
Breslow, who independently reported that titanium metallocenes (Cp2TiCl2) along with a
dialkyl aluminum chloride cocatalyst polymerized ethylene.20,21 Although, these catalysts
initially showed low activity, their homogeneous nature shed light on the mechanism and
kinetics of olefin polymerization by Ziegler Natta catalysts.
The success of metallocene catalysts is also due to the remarkable discovery by
Kaminsky and Sinn, who reported that zirconium dimethyl metallocenes show a much
higher activity when initiated with methylaluminoxane (MAO) instead of alkyl aluminum
reagents.22 The increased activity by MAO was observed for other systems (e.g.
Cp2TiEtCl/AlEtCl2) and previously inactive systems.23,24 Moreover, these catalysts also
produced polymers with narrow molecular weight distribution.
Koppl outlines several advantages of homogenous metallocene catalysts.25 The
metallocene catalysts are more active than traditional heterogeneous Ziegler Natta
catalysts. The increase in activity is attributed to their homogenous and uniform nature
10
compared to heterogeneous systems. Also, homogeneous systems contain a higher
number of active sites relative to their mass since every metal is available to act as a
catalyst. Homogeneous catalysts act as single site catalysts which aids in the
production of highly uniform polymers with narrow molecular weight distributions. The
catalysts can be easily altered to produce polymers with evenly distributed long chain
and short chain branching on the polymer chain, features desirable for production of new
polymers. The catalyst active site can be changed by modifying the cyclopentadienyl
ligand system surrounding the metal center resulting in polymers with different molecular
weights, branching and stereospecificity.
An excellent control of stereospecificity achieved by metallocene catalysts is
exemplified by the ansa-zirconocene catalysts. Activation of C2 symmetric η5-
indenylzirconcences and Cs-symmetric η5-fluorenylzirconocenes with MAO, yields
isotactic or syndiotactic polypropylene, respectively (Figure 1.5).26,27 Hence, the
changes in symmetry, steric and electronic properties of the Cp ring in addition to the
“locked” orientation of the zirconocene result in dramatic changes in the microstructure
of the polymer mainly the tacticity.
Figure 1.5. Examples of ansa-zirconocene catalysts controlling the stereoselectivity of the resulting polymer.
11
1.3.3 Post Metallocene Catalysts
New ligand designs that mimic the metallocene catalysts have been the major
driving force of research in the past two decades. Okuda reported highly active olefin
polymerization catalysts based on Group 4 complexes with ansa-bridged
cyclopentadienyl amido ligand (Figure 1.6), commonly known as constrained geometry
catalysts (CGCs). 28 CGCs have been studied extensively experimentally and
theoretically and are patented by Dow Chemical Company and ExxonMobil
Corporation.29,30,31 In comparison to metallocenes, CGCs are more stable at high
temperatures and show higher incorporation of α-olefin monomers in copolymerizations.
The copolymerization is facilitated by the open nature of the catalyst and lower tendency
of the growing polymeric chain to undergo chain transfer reactions.32 The coordination
sphere in CGC is less crowded considering that the Cpcentroid-M-N bite angle is generally
smaller by 25-30°C compared to Cpcentroid-M-Cpcentroid in metallocenes. 33 Lastly, the
polymers generated by CGC have narrow molecular weight distribution with long chain
branches, which aids in processing of the polymers and is not observed in metallocene
based catalysts.3
Figure 1.6. General structure of constrained geometry catalysts.
McConville and coworkers reported chelating diamide complexes of titanium
(Figure 1.7, a), capable of living polymerization of α-olefins and also show high activity in
1-hexene polymerization catalysis.34,35,36 Fenokishi-Imin Haiishi (FI) catalysts, based on
unsymmetrical phenoxy-imine chelating ligand on titanium and zirconium complexes
12
(Figure 1.7, c), show high catalytic activity for polymerization of ethylene (20 times
higher than Cp2ZrCl2), 1-hexene or ethylene-propylene and living polymerization of
ethylene.37,38,39 Stephan and coworkers reported phosphinimide based titanium
complexes (Figure 1.7, b) with high ethylene polymerization activities under industrially
relevant conditions.40,41,42 The success of the phosphinimide catalyst is due to its steric
and electronic similarity to the Cp ligand.
Figure 1.7. General structure of (a) McConville’s diamide catalysts (b) phosphinimide based catalysts (c) FI catalysts based on Group 4 salicylaldiminide complexes.
1.3.4 Amidinate Based Olefin Polymerization Catalysts
Group 4 complexes containing amidinates as ancillary ligands are active olefin
polymerization catalysts and have been widely studied in the past (Figure 1.8). These
complexes show moderate polymerization activities and excellent stereoselectivity for
polymerization of higher olefins. The bis(benzamidinate) complexes (Figure 1.8, a) and
half sandwich mono(benzamidinate) complexes have been reported to polymerize
propylene and styrene with moderate activities. Specifically, the zirconium
bis(benzamidinate) complexes polymerize propylene in a highly stereoregular manner,
which can be modulated by propylene pressure to produce either atactic or isotactic
polypropylene.43 Sita and coworkers have extensively studied half sandwich
acetamidinate based zirconium complexes (Figure 1.8, b), which promote stereospecific
and living polymerization of 1-hexene forming isotactic, high molecular weight
13
polymers.44 Guanidinate based complexes (Figure 1.8, c) are structurally similar to
amidinates and some derivatives show high ethylene polymerization activity.45 Lastly,
Collins reported excellent ethylene polymerization activity of 1400 g.(mmol.hr.atm)-1 using
bis(iminophosphonamide) based complexes (Figure 1.8, d).46
Figure 1.8. Active amidinate based olefin polymerization catalysts (a) bis(benzamidinate) complexes (b) acetamidinate complexes (c) guanidinate complexes (d) bis(iminophosphonamide) based complexes .
1.4 Amidinates
Amidinates are four electron donor, bidentate, anionic ligands with a general
formula of RNC(R’)NR (Figure 1.9) and coordinate to a metal center through the two
nitrogen atoms.47 These versatile ligands can be sterically and electronically modified by
varying the substituents. The substituents on the nitrogen can be tuned to change the
steric properties and control the mode of binding to a metal center. The substituent on
the bridging carbon mainly affects the electronic properties of the ligand.
Figure 1.9. Resonance structures of amidinates.
Amidinates are closely related to other types of ligands such as guanidinates,
where the substituent at the bridging carbon is a nitrogen donor (Figure 1.10).
14
Guanidinates are more electron donating than amidinates as the additional nitrogen can
bear 1 or 2 substituents and also donate electron density through π bonding.
Amidinates are isoelectronic to carboxylates but the additional substituents on the
nitrogen in amidinates can be used to modify the steric and electronic properties.
Figure 1.10. Ligands isoelectronic to amidinates: (a) guanidinates (b) carboxalates (c) triazenates.
1.4.1 Modes of Coordination
Amidinate complexes have been reported for most transition metals and some
main group metals. There are several frequently observed modes of coordination to the
metal center controlled by the substituents on the nitrogen and bridging carbon (Figure
1.11). The chelating metallacycle form is the most commonly observed mode followed
by the bridging mode, which is found in dinuclear species with multiple metal-metal
bonds. Sterically bulky substituents usually favour the chelating mode instead of the
bridging mode since large substituents on the nitrogen and bridging carbon will position
the lone pair of electrons on the nitrogen atoms closer together converging towards a
single metal center. In contrast, small substituents will result in more parallel location of
the lone pairs favouring the formation of the bridging mode. Lastly, the monodentate
form is much rare but has been observed in complexes with sterically bulky substituents.
15
Figure 1.11. Commonly observed modes of coordination: (a) chelating bidentate (b) bridging (c) monodentate.
1.4.2 Synthesis of Metal-Amidinates
There are several common routes for the synthesis of metal amidinate
complexes. Since, amidinates are salts of neutral amidines, few methods initially involve
synthesis of the corresponding amidines. One common method introduced by Sanger
involves the amidine synthesis by reacting benzonitrile with LiN(SiMe3)2 followed by
trimethylsilyl chloride (Scheme 1.2).48
Scheme 1.2. Synthesis of N,N,N’-tris(trimethylsilyl)benzamidine.48
Amidinate containing complexes can be synthesized from the corresponding
amidines. Reacting trimethylsilyl substituted amidine with metal halide forms the
amidinate complexes with the loss of trimethylsilyl chloride (Scheme 1.3, a). Second
common method involves the reaction of protonated amidine with a metal amido species
in an amine elimination reaction where the loss of secondary amine drives the reaction
(Scheme 1.3, b). The success of the amidinate ligand is also due to its ease of
synthesis using commercially available carbodiimides. Carbodiimide insertion into
metal-nitrogen or metal-carbon bond occurs readily with organolithium reagents or metal
complexes (Scheme 1.3, c). Lastly, deprotonation of amidines with alkali metals
16
followed by salt metathesis with metal halide precursors (Scheme 1.3, d) is also a
common strategy for the synthesis of metal complexes.
Due to the widespread success of the metallocene complexes as olefin
polymerization catalysts, it is interesting to compare the amidinates to cyclopentadienyl
ligands. The steric bulkiness of amidinates is ranked between Cp and Cp*.49,50 In terms
of electron donation to the metal center, amidinates are less electron donating compared
to Cp derivatives.51 Although electron rich ligands are desirable to stabilize the cationic
metal center during polymerization, electron deficient bis(indenyl) based zirconium
complexes have been reported for increased stereoselectivity in propylene
polymerization.52 Hence, amidinates are good alternatives to cyclopentadienyl ligands
for use in olefin polymerization catalysts.
Scheme 1.3. Common routes for synthesis of metal amidinates: (a) elimination of trimethylsilyl chloride (b) amine elimination (c) carbodiimide insertion (d) salt metathesis.
17
1.5 Scope of Thesis
Group 4 complexes containing amidinates show low to moderate olefin
polymerization activities and good stereoselectivity in polymerization of higher olefins.
Amidinates can be sterically and electronically tuned by modifying the substituents on
the nitrogen and the bridging carbon. In ferrocenyl amidinates, the placement of
sterically bulky ferrocenyl substituent at the bridging carbon offers the potential for redox
tunable polymerization catalysts and electronic cooperative interaction with iron center
during polymerization. The work in this thesis includes synthesis and characterization of
novel titanium and zirconium dichloride and dialkyl complexes containing ferrocenyl
amidinates. Ethylene polymerization activity of these complexes was evaluated after
activation with MAO, B(C6F5)3 and [Ph3C][B(C6F5)4] cocatalysts.
18
Chapter 2 Synthesis, Characterization and Ethylene
Polymerization Activity of Group 4 Ferrocenyl Amidinate Complexes
2 Heading 2
2.1 Introduction
Derivatives of ferrocene have received enormous attention in the past as ligands
in catalysis, polymeric materials and biomolecules.53,54 The wide use of ferrocenes is
primarily due to their high stability, low cost and versatile synthesis. Moreover, the steric
bulk, rigid structure, and well behaved redox chemistry of ferrocene makes it an
excellent substituent for ancillary ligands in olefin polymerization catalysts.
Group 4 complexes containing ferrocene based ligands are active olefin
polymerization catalysts and offer a number of advantages over conventional organic
ligands (Figure 2.1).55,56,57 Arnold and coworkers have prepared titanium and zirconium
complexes with the bis(amino)ferrocenyl ligand (Figure 2.1, a), and were able to
demonstrate high ethylene polymerization activity for the zirconium complex upon
activation with [Ph3C][B(C6F5)4].58,59 Recently, Erker and coworkers have prepared
bis(ferrocene-saliminato) group 4 metal complexes which show low to moderate activity
upon activation with MAO.60,61
Incorporation of ferrocene in the ligand backbone of a precatalyst allows one to
apply the reversible redox properties of iron for the synthesis of redox tunable
catalysts.62 Modulation of the oxidation state of iron can potentially alter the electron
density on the active metal center and affect the catalyst activity, comonomer
incorporation and stereoregularity of the resulting polymer. Gibson has prepared
titanium complexes containing ferrocenyl substituted bis(iminophenoxide) ligands and
19
reported significantly different conversions for ring-opening polymerization of rac-lactide
by FeIII ferrocenium species in comparison to the corresponding FeII species.63
Figure 2.1. Active olefin polymerization catalysts containing ferrocene based ligands: (a) bis(amino)ferrocenyl ligand58,59,64 (b) ferrocenyl dimethylsilyl substituted zirconocenes65 (c) ferrocenyl substituted phosphinimine ligand.66
Thus far, the study of redox tunable olefin polymerization catalysts has been
limited. The FeIII ferrocenium derivative of the bis(amino)ferrocenyl zirconium complex
(Figure 2.1, a) has been isolated and shown to be an active catalyst.64 Recently, Gibson
and coworkers have reported very similar ethylene polymerization activities by FeII and
their corresponding FeIII derivatives based on ferrocenyl substituted bis(imino)pyridyl
complexes upon activation with MAO.67
Heterobimetallic complexes containing the ferrocenyl fragment and group 4
metals also have the potential for electronic cooperative effects involving electron
donation from the electron rich iron to the electrophilic cationic group 4 metal center
during polymerization.68 Heterobimetallic complexes containing group 4 metals and a
ferrocenyl group, often as a ligand substituent linked by a carbon chain, have been
shown to be active catalysts.69 Mukaiyama and coworkers reported higher ethylene
polymerization activity and higher stereoselectivity in cyclopolymerization of 1,5-
hexadiene using ferrocenyldimethylsilyl substituted zirconocene precatalysts (Figure 2.1,
b) compared to Cp2ZrCl2.65,68,70
20
Group 4 complexes containing amidinates as ancillary ligands are active
catalysts for stereoselective olefin polymerization since amidinates are sterically and
electronically similar to cyclopentadienyl ligands. Based on the preceding discussion,
the introduction of a ferrocenyl group as an electron donating substituent at the central
carbon of the amidinate backbone, allows the synthesis of novel ferrocene substituted
amidinates. Moreover, in-plane orientation of the substituted Cp ring of ferrocene
relative to the amidinate NCN plane may offer the possibility of additional electron
donation by pi bonding.
In this work, titanium and zirconium bis(amidinate) and half sandwich zirconium
mono(amidinate) dichloride and dialkyl complexes were synthesized. The reactivity of
the dialkyl complexes with Lewis acids was investigated. The metal complexes were
evaluated as ethylene polymerization catalysts and the stability of the corresponding
complexes containing FeIII derivatives was investigated using cyclic voltammetry.
2.2 Results and Discussion
2.2.1 Synthesis and Characterization
2.2.1.1 Synthesis of Amidine and Amidinate Derivatives
N,N’-dicyclohexyl ferrocenyl amidine (1a) was synthesized by a one-pot
procedure reported by Arnold and Hagadorn.71 A pentane solution of tBuLi was added to
a THF/hexanes solution of ferrocene resulting primarily in the formation of
monolithioferrocene (Scheme 2.1). Subsequently, this solution was treated with
dicyclohexyl carbodiimide followed by hydrolysis with water. Sublimation of the crude
reaction mixture was required to remove the unreacted ferrocene and recrystallization
from hexanes was performed to selectively remove the ferrocene linked bis-amidine side
product.72
21
Scheme 2.1. Synthesis of ferrocenyl amidine.71
The original procedure by Hagadorn and Arnold reported a large scale synthesis
of 1a starting with 100 g of ferrocene but the reaction was scaled down to 30% and
optimized for the yield. The conditions used for the lithiation of ferrocene were the major
factor contributing to the conversion to 1a. Kagan and coworkers reported the effect of
several conditions such as the solvent, type of organolithium reagent used for
deprotonation, rate of reagent addition and reaction time on the relative ratio of
ferrocene, monolithioferrocene (FcLi) and dilithioferrocene (FcLi2) in solution.73 Based
on this work, conditions were selected to maximize FcLi formation and minimize the
formation of undesirable FcLi2. This resulted in less formation of ferrocene linked
bis(amidine) and 1a was obtained in 61% isolated yield.
In the previous one-pot procedure, the moderate yields are partly due to the
decomposition of 1a during the sublimation at 65°C required to remove the unreacted
ferrocene. Attempts to purify 1a by column chromatography were unsuccessful as the
product decomposed on silica and alumina. Instead, the intermediate FcLi was
selectively isolated as reported by Zanello and the sublimation was no longer required.74
Similar to the previous method, the isolated FcLi was reacted with dicyclohexyl
carbodiimide and hydrolyzed with water. This resulted in an overall yield of 85% starting
22
from FcLi and 68% yield starting from ferrocene, an improvement from the previous
method.
The synthesis of N,N’-diisopropyl ferrocenyl amidine (1b) was performed by
slightly modified procedure by starting from bromoferrocene. 1b was synthesized by
treatment of bromoferrocene with nBuLi, subsequently with diisopropyl carbodiimide and
followed by hydrolysis with water resulting in 70% isolated yield (Scheme 2.2). The 1H
NMR spectrum of 1b is consistent with the structure where the two triplet resonances at
3.93 ppm and 4.24 ppm are observed for the substituted Cp ring of ferrocene and a
sharp singlet at 3.96 ppm for the unsubstituted Cp ring. Also, 1b shows two broad
resonances and two doublets for the methine and methyl of isopropyl groups,
respectively, consistent with unsymmetrical nature of the structure.
Scheme 2.2. Synthesis of ferrocenyl amidine with isopropyl substituents on the nitrogen.
In summary, three similar methods were used to prepare the ferrocenyl amidine
derivatives, 1a and 1b. The first method was a one pot synthesis resulting in moderate
yields. The second method resulted in slightly higher yields but involved isolation of the
pyrophoric FcLi intermediate. The third method involved use of commercially available
monosubstituted ferrocene source and in situ generation of FcLi.
Ferrocenyl amidines 1a and 1b were easily synthesized since diisopropyl and
dicyclohexyl carbodiimide readily undergo insertion into carbon-lithium bond in FcLi.
23
However, attempts to synthesize ferrocenyl amidine with a -SiMe3 substituent on the
nitrogen have been reported to be unsuccessful using similar strategy of
bis(trimethylsilyl) carbodiimide insertion into FcLi due to the lower electrophilicity of the
carbodiimide.75,76 The cyclohexyl amidine derivative (1a) was used for the synthesis of
metal complexes since the more bulky cyclohexyl group provides greater steric
protection of the nitrogen atom in comparison to the isopropyl group. Protection of the
nitrogen atom is an important requirement for a polymerization catalyst since it is
susceptible to attack by Lewis acidic cocatalysts.
Lithium ferrocenyl amidinate (1d) was prepared by deprotonating 1a with MeLi in
hexanes by modified literature procedure.71 The synthesis of titanium complexes can be
achieved by Me3SiCl elimination instead of more common routes such as salt
metathesis. Therefore, mixed -SiMe3 and cyclohexyl substituted amidine (1c) was also
synthesized by treatment of lithium ferrocenyl amidinate with trimethysilylchloride
(Scheme 2.3). The 1H NMR spectrum of 1c is consistent with the structure where the
SiMe3 resonance appears at 0.46 ppm.
Scheme 2.3. Synthesis of trimethylsilyl substituted ferrocenyl amidine, 1c.
A crystal structure determination resulted in the molecular structure of 1b
depicted in Figure 2.2 and the selected bond distances and angles of 1b are shown in
Table 2.2. The structure contains localized CN single and double bonds with a distance
24
of 1.362(2) Å and 1.283(2) Å for N1-C7 and N2-C7, respectively. The Cp plane of the
ferrocene intersects the NCN plane at 41.1(3)°.
The molecular structure of one of the two independent molecules in the
asymmetric part of the unit cell of 1c and the selected bond distances and angles are
shown in Figure 2.2 and Table 2.3, respectively. Similar to 1b, localized single and
double CN bond distances are also observed. The Cp plane of the ferrocene intersects
the NCN plane at 15.2(5)°.
Figure 2.2. Molecular structure of [iPrNC(Fc)NiPr]H, 1b and [CyNC(Fc)NCy]SiMe3, 1c. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
Table 2.2. Selected bond distances (Å) and bond angles (°) for [iPrNC(Fc)NiPr]H, 1b. N1-C7 1.362(2) N1-C7-N2 119.8(2) N2-C7 1.283(2) N2-C7-C8 127.7(2) C7-C8 1.498(3) N1-C7-C8 112.5(2) C5-N1 1.454(2) C7-N2-C2 121.1(2) C2-N2 1.462(2) C7-N1-C5 123.7(2)
25
Table 2.3. Selected bond distances (Å) and bond angles (°) for [CyNC(Fc)NCy]SiMe3, 1c.
N3-C1 1.434(2) N3-C1-N4 126.1(1) N4-C1 1.276(2) N3-C1-C17 116.70 (1) N3-C2 1.486(2) N4-C1-C17 117.20(1) N3-Si4 1.742(1) C9-N4-C1 121.11(2) N4-C9 1.462(2) C2-N3-C1 114.24(1) C1-C17 1.487(2) C2-N3-Si4 126.82(1) Si4-N3-C1 118.94(1)
2.2.1.2 Synthesis of Group 4 Metal Dichloride Complexes
The synthesis of group 4 metal bis(amidinate) complexes was initially attempted
using M(NR2)4 (M = Ti, Zr; R = Me, Et) precursors. Reacting Ti(NEt2)4 with 2 equiv of 1a
resulted in no reaction at room temperature or at higher temperatures (80°C). However,
2 equiv of 1a reacted with Zr(NMe2)4(THF)2 at elevated temperatures (~80°C) but did not
selectively result in the bis(amidinate) complex. The 1H NMR spectrum is consistent
with the formation of mono, bis- and tris- amidinate products. Zirconium complexes
containing tris(amidinates) are known in the literature, even though the steric bulkiness
may suggest otherwise.77
Bis(amidinate) complexes were successfully prepared by treatment of
M(NMe2)2Cl2 (M = Zr⋅2THF; M = Ti; Scheme 2.4) with 2.05 equiv of 1a after refluxing in
toluene. The zirconium bis(amidinate) dichloride (2a) was isolated as an orange solid in
high yields (85%) by trituration of the resulting reaction mixture with hexanes followed by
subsequent washings with hexanes and ether. Hexane washings are required to
completely remove the unreacted amidine, 1a. The titanium bis(amidinate) dichloride
(2b) was isolated in 77% yield as a purple solid using the same workup procedure as for
2a. In this case, a color change from dark orange to purple was observed after refluxing
overnight in toluene.
26
Scheme 2.4. Synthesis of titanium and zirconium bis(amidinate) complexes, 2a and 2b.
The 1H NMR spectra are consistent with the formation of 2a and 2b. The
assignments of the NMR data were assisted by dept135 and HSQC experiments. Sharp
resonances for the unsubstituted Cp ring of the ferrocene for both bis(amidinate)
complexes are similar at ca. 4.29 ppm. These resonances are shifted downfield by 0.3
ppm compared to the protonated amidine, which is consistent with the decreased
electron density in the ligand caused by withdrawal of electron density by the group 4
metal center. The resonances for substituted Cp rings and the alpha proton of the
cyclohexyl ring are observed in the 4.6-4.0 ppm region. Broad resonances are observed
in 2.5-1.0 ppm region for the axial and equatorial protons of the cyclohexyl groups.
The NMR spectra of 2a and 2b show different fluxional behaviour in solution, a
feature commonly observed with other bis(amidinate) complexes.78,79,80 The C2
symmetry of the bis(amidinate) complexes results in two types of nitrogens depending
on their position (either cis or trans) relative to the chlorides. In the NMR spectra, the
resonances observed for the cyclohexyl groups reflect the nitrogen environment.
Specifically, the resonance for the alpha proton of the cyclohexyl group in 2a shows a
multiplet at 4.12 ppm integrating to 4 protons, whereas, 2b shows two multiplets at 4.41
ppm and 3.99 ppm integrating to 2 protons each. In addition, the alpha carbon of the
cyclohexyl group in the 13C NMR spectra of 2a shows a single resonance at 57 ppm,
27
however, 2b shows two resonances at 61 ppm and 59 ppm. The two separate signals
for alpha protons and alpha carbons in 2b are as expected and show that there are two
unique cyclohexyl groups depending on their attachment to the cis or trans nitrogens.
However, in 2a all of the cyclohexyl groups are equivalent, as each of the two amidinate
ligands are interconverting at room temperature (Scheme 2.5). In fact, Xue and
coworkers also reported this same difference in fluxional behavior based on the metal
center in bis(amidinate) complexes of the type, M(NMe2)2(CyNC(Me)NCy)2 (M = Zr; Ti).80
The size of the metal center has been found to influence this behaviour, as a smaller
titanium center restricts this fluxionality, whereas the larger zirconium center allows the
ligand to readily interconvert. Secondly, Richeson and coworkers have proposed that
bulky substituents on the central carbon decreases the chance of this interconversion.81
This trend is evident in the titanium bis(amidinate) complexes since
Ti[iPrNC(NMe2)NiPr]2Cl2 with a less bulky NMe2 does show fluxional behaviour82,
whereas, 2b with a bulky ferrocene at the central carbon does not.
The interconversion of the amidinate ligands in 2a may proceed through a
dissociative pathway or an internal twist mechanism as observed for other bis- chelating
octahedral complexes. The dissociative pathway has been observed but the Bailar-twist
mechanism involving a trigonal prismatic intermediate has been commonly reported for
the ligand interconversion in similar bis(amidinate) and bis(ketenimine) group 4 metal
complexes based on the activation entropy and enthalpy calculations.83,78,80,84
28
Scheme 2.5. Rapid interconversion of 2a at room temperature.
An attempt was made to study this behaviour further by low temperature 1H
NMR. Upon cooling a CD2Cl2 solution of 2a to 200K, the 1H NMR spectrum reveals
broadening of the alpha proton resonance and splitting of the resonance corresponding
to the protons on the substituted Cp ring (Figure 2.3). The splitting is likely caused by
the restricted rotation of the ferrocene moiety on the amidinate backbone at low
temperatures and this was also observed for 2b. The 1H NMR spectra do not show any
major changes from 298K to 230K and broadening of the alpha proton resonance starts
to occur noticeably at 230K. The kinetic parameters for the interconversion were not
determined as the slow exchange limit could not be reached.
Figure 2.3. 1H NMR spectrum of 2a from 4.65-3.45 ppm shows splitting of substituted Cp protons and broadening of the cyclohexyl protons at low temperature.
298K
230K
200K
29
The molecular structures of 2a and 2b are shown in Figure 2.4 and Figure 2.5,
respectively, with the selected bond distances and angles in Table 2.4. The metal
center is in a pseudo octahedral environment surrounded by 4 nitrogen atoms from the
amidinate ligands and two chloride ligands. The bidentate amidinate ligands are
positioned cis to each other. The bonding parameters within the two amidinate ligands
are not significantly different. The metal center, two nitrogen atoms and the central
carbon of the amidinate ligand lie in a single plane. The four nitrogens can be divided
into two types based on their relative orientation to each other, cis-NCy (N1, N4) and
trans-NCy (N2, N3) groups. In complex 2b, the Ti-N bond distance in cis-NCy groups
(2.079(2) Å, 2.103(2) Å) are longer than trans-NCy groups (2.041(2) Å, 2.037(2) Å) due
to the trans effect of two chlorides as observed in other group 4 bis(amidinate)
complexes.79 This trend is not evident for Zr-N bond distances in 2a. There are no
significant differences in Calpha-cyclohexyl-N bond distance between cis-NCy and trans-NCy
groups in both complexes.
The amidinate bite angle in 2b (63.94(8)°, 63.76(8)°) is larger than the bite angle
in 2a (60.02(8)°, 59.86(8)°). The charge delocalization and partial double bond
character in the amidinate NCN backbone is evident by the C-N bond distances in 2a
and 2b which range from 1.323(3) Å to 1.350(4) Å. The M-N-Calpha-cyclohexyl bond angle
reflects the steric crowding around the metal center. The average M-N-Calpha-cyclohexyl
angle in 2a is 139.4° and is smaller than the average angle in [CyNC(Me)NCy]2ZrCl2
(142.4°). This shows that the bulky ferrocenyl group pushes the cyclohexyl groups
closer to the metal center compared to the methyl group in acetamidinate complexes.
The M-Cl bond distances in 2a (2.4311(7) Å, 2.4413(9) Å) are longer than 2b
(2.3069(8) Å, 2.3204(7) Å). The Cl1-M-Cl2 bond angle in both complexes is similar
(93.92(3)° for 2a; 93.84(3)° for 2b) and smaller compared to Cp2ZrCl2 (97.1°).85 The
Cl1-M-Cl2 angle in 2a is similar compared to [CyNC(Me)NCy]2ZrCl2 (93.1(1)°) and
30
significantly different compared to [Me3SiNC(Ph)NSiMe3]2ZrCl2 (103.71°).79,86 Richeson
describes the bis(amidinate) complexes as pseudotetrahedral to compare them to
structurally similar metallocene complexes.79 In this description, two of the vertices are
defined as the vectors that bisect the amidinate ligand at the central carbons and the
other two vertices as the Zr-Cl vectors.87 Using this description, Camidinate-M-Camidinate
angle is similar in both complexes (115.46° for 2a; 115.85° for 2b) and significantly
smaller compared to Cpcentroid-Zr-Cpcentroid angle of 134° in Cp2ZrCl2.
Figure 2.4. Molecular structure of [CyNC(Fc)NCy]2ZrCl2, 2a. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
31
Figure 2.5. Molecular structure of [CyNC(Fc)NCy]2TiCl2, 2b. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
Table 2.4. Selected bond distances (Å) and bond angles (°) for [CyNC(Fc)NCy]2ZrCl2, 2a and [CyNC(Fc)NCy]2TiCl2, 2b.
2a (M = Zr) 2b (M = Ti) M-Cl1 2.4311(7) 2.3069(8) M-Cl2 2.4413(9) 2.3204(7) M-N1 2.189(2) 2.079(2) M-N2 2.209(2) 2.041(2) M-N3 2.186(2) 2.037(2) M-N4 2.238(3) 2.103(2) C1-N1 1.339(4) 1.329(3) C1-N2 1.338(3) 1.347(3) C1-C15 1.487(4) 1.484(4) C2-N3 1.350(4) 1.363(3) C2-N4 1.328(3) 1.323(3) C2-C37 1.477(4) 1.475(4) N1-M-N2 60.02(8) 63.94(8) N3-M-N4 59.86(8) 63.76(8) Cl1-M-Cl2 93.92(3) 93.84(3) N1-C1-N2 110.51(24) 109.25(22) N3-C2-N4 111.04(25) 109.03(22) M-N1-C3 137.85(19) 139.48(16) M-N2-C9 140.10(18) 138.73(16) M-N3-C25 137.06(19) 137.91(16)
32
2a (M = Zr) 2b (M = Ti) M-N4-C31 142.39(19) 142.19(17) Dihedral Angle: N1-C1-N2, Cpferrocene 48.2(3) 47.9(2) N3-C2-N4, Cpferrocene 38.2(3) 36.1(2)
The zirconium mono(amidinate) half sandwich complexes were synthesized by
salt metathesis between Cp’ZrCl3 (Cp’ = Cp; Cp*) and 1 equiv of 1d in THF solvent to
afford Cp zirconium mono(amidinate) (2c) and Cp* zirconium mono(amidinate) (2d)
dichloride complexes in 60% and 55% isolated yields, respectively (Scheme 2.6). It is
noteworthy to mention that initial attempts for synthesis of these complexes using
sodium amidinate salt, Na[CyNC(Fc)NCy], were unsuccessful. The two half sandwich
complexes show similar solubilities except 2d shows a greater solubility in hexanes than
2c. The half sandwich mono(amidinate) complexes are known to undergo ligand flipping
at the metal center at room temperature which was investigated with unsymmetrical
amidinates by Sita and coworkers.88
The 1H NMR spectra of 2c and 2d show the resonances for the unsubstituted Cp
ring of ferrocene at ca. 4.30 ppm, which is shifted downfield from the protonated
amidine, 1d. Only one resonance for the alpha proton of the cyclohexyl ring is observed
due to the CS symmetry of the complex. Furthermore, this resonance appears as a
triplet of triplets due to difference in coupling between the neighbouring axial and
equatorial protons on the cyclohexyl ring. The resonances for protons of the Cp ring and
the methyl groups of Cp* ring bound to the zirconium are shifted downfield compared to
Cp’ZrCl3.
33
Scheme 2.6. Synthesis of half sandwich mono(amidinate) zirconium complexes, 2c and 2d, via salt metathesis.
The molecular structure of 2c and 2d are shown in Figure 2.6 and Figure 2.7,
respectively, with the selected bond distances and angles in Table 2.5. Complex 2d
contains two molecules in the asymmetric unit cell. Both complexes display a
pseudotetrahedral geometry around the zirconium center. Complex 2c shows
asymmetric binding to the metal center based on the different Zr1-N1 (2.224(1) Å) and
Zr1-N2 (2.110(1) Å) bond distances. However, 2d shows symmetric binding since the
Zr1-N1(2.215(3) Å) and Zr1-N2 (2.217(3) Å) bond distances are identical. The C6-N1
and C6-N2 bond distances in the amidinate backbone in 2c and 2d are indicative of
partial double bond character and the amidinate ring forms a four membered nearly
planar metallacycle. The amidinate bite angle in 2c (59.99(4)°) and 2d (60.18(9)°) are
very similar. The ferrocene Cp plane is tilted (52.09(15)° for 2c and 52.39(11)° for 2d)
relative to the amidinate NCN plane indicating no π conjugation.
34
Figure 2.6. Molecular structure of CpZr[CyNC(Fc)NCy]Cl2, 2c. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
Figure 2.7. Molecular structure of Cp*Zr[CyNC(Fc)NCy]Cl2, 2d. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
35
Table 2.5. Selected bond distances (Å) and bond angles (°) for CpZr[CyNC(Fc)NCy]Cl2, 2c, and for Cp*Zr[CyNC(Fc)NCy]Cl2, 2d.
2c (Cp’ = Cp) 2d (Cp’ = Cp*) Zr1-Cl1 2.4404(4) 2.4426(9) Zr1-Cl2 2.4375(4) 2.4245(9) Zr1-Cp’centroid 2.1986(8) 2.211(1) Zr1-N1 2.224(1) 2.215(3) Zr1-N2 2.110(1) 2.217(3) C6-C19 1.487(2) 1.485(4) C6-N1 1.341(2) 1.334(4) C6-N2 1.341(2) 1.343(4) Cl1-Zr1-Cl2 90.65(2) 89.80(3) N1-Zr1-N2 59.99(4) 60.18(9) Cl2-Zr1-N1 89.44(3) 91.56(7) Cl1-Zr1-N2 89.41(3) 87.69(7) Zr1-N1-C13 140.81(9) 139.13(20) Zr1-N2-C7 140.91(10) 143.28(20) Zr1-N1-C6 93.90(9) 93.97(19) Zr1-N2-C6 94.99(8) 93.62(19) N1-C6-N2 111.07(12) 112.18(27) Cp’centroid-Zr1-N1 105.99(4) 109.20(8) Cp’centroid-Zr1-N2 114.20(4) 114.05(8) Dihedral Angle: N1-C6-N2, Cpferrocene 52.09(15) 52.39(11)
The synthesis of similar titanium mono(amidinate) complexes was found to be
unsuccessful using several different strategies. Salt metathesis of Cp’TiCl3 (Cp’ = Cp;
Cp*) with Li[CyNC(Fc)NCy], Li[CyNC(Fc)NCy].Et2O or Na[CyNC(Fc)NCy] did not result
in a single major product after attempting at several different conditions (Scheme 2.7).
The 1H NMR spectrum of the reaction mixture with CpTiCl3 shows two major peaks at
6.55 ppm and 6.10 ppm along with other minor peaks in this region corresponding to the
protons of the Cp ring bound to titanium. Similarly, for Cp*TiCl3 the reaction mixture
shows two major peaks at 2.08 ppm and 1.94 ppm, and other minor peaks in this region
for the methyl groups of the Cp* ring. The ferrocene Cp region from 4.0 to 4.5 ppm is
unclear due to presence of overlapping multiplets. Based on these results, the reaction
is not selective towards monosubstituted product and there is also formation of
36
bis(amidinate) half sandwich product. Further evidence is needed to confirm this
hypothesis.
An alternative synthesis was attempted by reacting the trimethylsilyl substituted
amidine with Cp’TiCl3 to form the amidinate complex with trimethylsilyl chloride
elimination but no reaction was observed at room temperature. Upon heating the
reaction mixture, the 1H NMR spectrum showed decomposition of amidine and a similar
product distribution as previous attempt by salt metathesis.
Scheme 2.7. Attempted synthesis of half sandwich mono(amidinate) titanium complexes via salt metathesis was unsuccessful.
2.2.1.3 Synthesis of Group 4 Metal Dialkyl Complexes
The dialkyl complexes (LnMR2, R = Me, CH2Ph) can be prepared from the
dichloride complexes (LnMCl2) by treatment with 2 equiv of alkyllithium or Grignard
reagents. The zirconium bis(amidinate) dimethyl complex, 3a, was prepared by
treatment of a toluene solution of 2a with 2 equiv of MeLi in 75% isolated yield. The 1H
and 13C NMR spectra show the Zr-Me resonance at 1.12 ppm and 45 ppm, respectively.
The resonance for the unsubstituted Cp ring of the ferrocenyl group in 3a is shifted
upfield shift from 4.30 ppm to 4.15 ppm relative to 2a since methyl groups are more
electron donating than chlorides, resulting in less electron withdrawal from the ligand by
the group 4 metal center. The 1H and 13C NMR spectra show a single resonance for
37
alpha proton and alpha carbon of the cyclohexyl group indicating interconversion of the
amidinate ligand as observed for the dichloride complex.
The molecular structure of 3a is shown in Figure 2.8 with the selected bond
distances and angles in Table 2.6. Complex 3a, structurally similar to 2a, shows a
monomeric pseudo octahedral geometry, where the metal center, two nitrogen atoms of
the amidinate ligand and the central carbon lie in a single plane. The four nitrogen
atoms can be divided into two types based on their relative orientation to each other, cis-
NCy (N1, N4) and trans-NCy (N2, N3) groups. The Zr-N bond distances in cis-NCy
groups (2.246(5) Å, 2.235(5) Å) are shorter than trans-NCy groups (2.273(4) Å,
2.262(5) Å). The Zr-N bond distances in 3a are longer than the bond distances in 2a
since methyl groups are more electron donating than chlorides.
The C47-Zr-C48 bond angle in 3a (88.22(21)°) is smaller than the Cl1-Zr-Cl2
bond angle in 2a (93.84(3)°). The C47-Zr-C48 angle is also smaller compared to other
dimethyl complexes such as [CyNC(Me)NCy]2ZrMe2 (92.4(3)°) and Cp2ZrMe2 (95.6°).79,89
The amidinate bite angle in 3a (58.49(16)°, 58.45(17)°) is smaller than the bite angle in
2a (60.02(8)°, 58.86(8)°). The charge delocalization and partial double bond character
in the amidinate NCN backbone is evident by the average C-N bond distances of 1.33 Å
in 3a.
38
Figure 2.8. Molecular structure of [CyNC(Fc)NCy]2ZrMe2, 3a. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
Table 2.6. Selected bond distances (Å) and bond angles (°) for [CyNC(Fc)NCy]2ZrMe2, 3a.
Zr1-C47 2.269(6) N1-Zr1-N2 58.49(16) Zr1-C48 2.277(6) N3- Zr1-N4 58.45(17) Zr1-N1 2.246(5) C47- Zr1-C48 88.22(21) Zr1-N2 2.273(4) N1-C1-N2 111.66(49) Zr1-N3 2.262(5) N3-C2-N4 111.43(50) Zr1-N4 2.235(5) C1-N1 1.338(6) Zr1-N1-C3 138.41(35) C1-N2 1.330(6) Zr1-N2-C9 140.86(33) C1-C15 1.488(8) Zr1-N3-C25 141.56(35) C2-N3 1.335(7) Zr1-N4-C31 136.78(36) C2-N4 1.322(7) C2-C37 1.492(8)
Similarly, treatment of the half sandwich Cp* zirconium mono(amidinate)
dichloride complex, 2d, with 2 equiv of MeLi in toluene afforded the dimethyl complex,
3d, in 71 % isolated yield. The NMR spectrum is consistent with the structure where the
Zr-Me resonance appears at -0.21 ppm and 47 ppm in the 1H and 13C NMR spectra,
respectively. The resonances for the methyl groups of Cp* ring and unsubstituted Cp
ring of the ferrocenyl group are shifted upfield compared to the dichloride complex. It is
39
noteworthy that the synthesis of the dialkyl complexes (3a and 3d) in THF solvent
instead of toluene often resulted in significantly lower isolated yields due to the formation
of unidentified paramagnetic side products. In fact, Qi and coworkers have isolated and
characterized THF ring opened product with titanium bis(guanidinate) complexes.90
Attempts to synthesize the dimethyl complex of 2c lead to unexpected reactivity.
Treatment of 2c with 2 equiv of MeLi or MeMgBr resulted in the dimethyl complex, which
subsequently undergoes disproportionation to form dimethyl zirconocene and zirconium
bis(amidinate) dimethyl complex, 3a (Scheme 2.8). This result was surprising at first,
but this type of reactivity has been observed previously for other half sandwich zirconium
mono(amidinate) complexes.91,45 Green and coworkers observed that treatment of
CpZr(amid)Cl2 (amid = Me3SiNC(Ph)NSiMe3) with 2 equiv of MeLi shows a 1:1 mixture
of Cp2ZrMe2 and Zr(amid)2Me2.73 They proposed that ligand redistribution is catalyzed
by excess MeLi and careful addition of 2 equiv of alkyl lithium resulted in no ligand
redistribution. However, in alkylation of 2c, if more than 1 equiv of MeLi was added, the
formation of Cp2ZrMe2 was always observed. In addition, Bergman and coworkers also
observed that CpZr(guan)Cl2 (guan = iPrNC(NMe2)NiPr) treated with 2 equiv of MeLi
formed Cp2ZrMe2 and Zr(guan)2Me2; CpZr(guan)Me2 was isolated by treatment of the
dichloride complex with 2 equiv of MeMgBr in 61% yield.45 However, in this work,
alkylation with MeMgBr also resulted in ligand redistribution and significant formation of
Cp2ZrMe2.
40
Scheme 2.8. Synthesis of half sandwich zirconium mono(amidinate) dimethyl complex, which subsequently undergoes ligand redistribution.
The dibenzyl derivative of 2c was instead synthesized. Initially, 2 equiv of benzyl
Grignard reagent was reacted with 2c under dilute conditions, however, this method
afforded the dibenzyl complex in low yields (~10%) due to the formation of other
unidentified products. The synthesis was improved using 2 equiv of KCH2Ph, which
resulted in significantly higher isolated yields (60%). The difference in yields by varying
the alkylating agent has been observed before in the alkylation of iron amidinates by
Hessen and coworkers.92 They postulate that Mg2+ cations may compete with the metal
center and ligand redistribution may result in the formation of magnesium amidinate
complexes. Complex 3c is stable and does not undergo loss of toluene to form a
metallacycle as observed for other dibenzyl complexes.93
The 1H NMR spectrum of 3c shows that the benzylic methylene groups are
diastereotopic and appear as a two sets of doublets as observed for Cp2Zr(CH2Ph)2.94
The resonance for Cp bound to zirconium appears at 6.01 ppm and is shifted upfield
41
relative to the dichloride complex. Molecular structure of 3c is shown in Figure 2.9 with
the selected bond distances and angles in Table 2.7. Both benzyl groups are pointed
away from the ancillary ligand and show η1 coordination to the metal center based on Zr-
C6-C7ipso (123.65(49)°) and Zr-C13-C14ipso (117.54(47)°) bond angles. This indicates
that the zirconium center in 3c is not electron deficient contrary to that observed
previously for other mono(amidinate) tribenzyl zirconium complexes, which show η2
coordination.87 The distances between the ipso carbons of the benzyl group and
zirconium are 3.419 Å (Zr1-C7) and 3.284 Å (Zr1-C14). The Zr-N bond distance
(2.269(5) Å, 2.268(6) Å) in 3c are longer than Zr-N bond distances in the corresponding
dichloride complex (2.224(1) Å, 2.110(1) Å).
Figure 2.9. Molecular structure of CpZr[CyNC(Fc)NCy](CH2Ph)2, 3c. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
42
Table 2.7. Selected bond distances (Å) and bond angles (°) for CpZr[CyNC(Fc)NCy](CH2Ph)2, 3c.
Zr1-C6 2.351(7) N1-Zr1-N2 58.73(20) Zr1-C13 2.291(7) C6- Zr1-C13 93.41(26) Zr1-N1 2.269(5) N1-C20-N2 111.72(53) Zr1-N2 2.268(6) Zr1-C6-C7 123.65(49) C20-N1 1.334(9) Zr1-C13-C14 117.54(47) C20-N2 1.354(8) Zr1-N1-C37 142.36(43) C20-C27 1.511(8) Zr1-N2-C21 142.98(38) C21-N2 1.485(10) N2-Zr1-C6 79.87(21) C37-N1 1.461(8) N1-Zr1-C13 96.80(22) C6-C7 1.501(8) C13-C14 1.520(10)
Alkylation of titanium bis(amidinate) dichloride (2b) was initially attempted using
MeLi, PhCH2MgBr and KCH2Ph in toluene or ether at -78°C. The reaction mixtures
turned to black or dark brown colour after a few minutes. These reactions were
performed in the exclusion of light due to the light sensitive nature of dialkyl titanocenes.
Extraction with hexanes afforded a mixture of unidentified products based on the
overlapping multiplets in the Cp region (4.5-4.0 ppm) and cyclohexyl region (1.0-2.5
ppm) in the 1H NMR spectrum. Several sharp resonances between 1.9-2.3 ppm region
are observed corresponding to the methyl bound to titanium.
After several attempts, the reaction of 2b with MeMgBr in toluene under dilute
conditions at -78°C afforded the dimethyl complex (3b) in low yields (32%) of 80%-90%
purity. Complex 3b is thermally unstable and light sensitive. Upon the addition of
MeMgBr, a color change is observed from pink to dark yellow. The 1H NMR spectrum
shows two triplet resonances at 4.47 ppm and 4.36 ppm, and a sharp singlet at 4.26
ppm for the substituted and unsubstituted ferrocene Cp rings, respectively. A sharp
singlet at 1.23 ppm is observed for the methyl groups bound to titanium.
The mechanistic details of decomposition of 3b were not investigated, however,
the 1H NMR spectrum after 1 day at room temperature in the presence of light in d8-
43
toluene shows the formation of methane, ferrocene and protonated ligand. The thermal
instability of 3b is not surprising since Cp2TiMe2 also shows similar characteristics,
although, titanium bis(amidinate) dimethyl complexes have been isolated previously. It
is known that the decomposition of Cp2TiMe2 proceeds autocatalytically in the solid state
and in solution over the period of few weeks under a nitrogen atmosphere in the dark.95
The major decomposition products include cyclopentadiene and methane proceeding
through a Cp2Ti=CH2 intermediate. The solvent dependent decomposition of Cp2TiR2
may also proceed through a homolytic Ti-R bond cleavage resulting in free radicals
which further catalyze the decomposition.96,97
2.2.1.4 Synthesis of Metal Monoalkyl Cationic Complexes
During polymerization trials, an activator is added along with the precatalyst to
generate a monoalkyl cationic complex paired with a weakly coordinated anion.
Investigating this reactivity of the dialkyl complexes with activators may provide clues to
the exact role of the iron center during polymerization. In the Mukaiyama catalysts
(ferrocenyldimethylsilyl substituted zirconocenes), electron donation from iron to cationic
zirconium center is postulated for the high ethylene polymerization activity compared to
Cp2ZrCl2. Further evidence for interaction of iron was obtained by Arnold and
coworkers, who observed short Fe-Ti bond distance in solid state structure of cationic
titanium complexes supported by chelating bis(amino)ferrocenyl ligand paired with
CH3B(C6F5)3- anion.98
Although treating the dialkyl complexes with perfluorophenyl substituted boranes
normally results in an ion pair, unexpected product formation is not uncommon.
Decomposition pathways often include cation-anion ligand redistribution, aryl transfer,
fluoride abstraction, intramolecular and/or intermolecular C-H activation.99,100 Recently,
Stephan and coworkers have shown that [Cp2ZrMe][MeB(C6F5)3] reacts with ferrocene
44
and undergoes C-H activation of the ferrocene Cp ring while losing methane to form
highly stable cationic complexes.101 In the ferrocenyl amidinate complexes, there is
possibility of intermolecular ferrocene C-H activation to form oligomeric structures.
Hence, it is wise to characterize the cationic complexes in solution but isolation of these
complexes can often be difficult due to the formation of insoluble clathrate-like oils.
The dialkyl complexes (3a-3d) were treated with a stoichiometric amount of a
strong Lewis acid, B(C6F5)3 or [Ph3C][B(C6F5)4], in C6D5Br to yield cationic monoalkyl
species (4a-4d, 5a-5d). These species were only characterized in solution by 1H, 11B
and 19F NMR spectroscopy. Repeated attempts to isolate the ion pairs were
unsuccessful resulting in viscous oils.
Abstraction of a methyl group from 3a with B(C6F5)3 and [Ph3C][B(C6F5)]4 in
C6D5Br proceeded cleanly to form 4a and 5a, respectively (Scheme 2.9). The 1H NMR
spectra of 4a and 5a show broad overlapping resonances in the 4.0-4.5 ppm and 2.0-0.6
ppm region corresponding to the resonances for the protons on the Cp rings and the
cyclohexyl group, respectively. The broad peaks are due to the rapid interconversion of
the ligands in the 5 coordinate complex, where cooling the sample down to -25°C still
shows broad peaks in the 1H NMR spectrum. For 4a, the 11B NMR spectrum is
consistent with the formation of MeB(C6F5)3- as it contains a sharp resonance at -14.4
ppm compared to the neutral tri-coordinated B(C6F5)3, which contains a broad resonance
at 58 ppm. The relative difference in the resonances of the meta and para fluorines,
known as the meta-para gap, in the 19F NMR spectrum allows one to determine whether
the ion pair is a solvent separated ion pair (values less than 3 ppm) or contact ion pair
(values of 3-6ppm).102 The 19F spectrum shows broad signals and the meta-para gap is
indicative of a contact ion pair given the value of 3.07 ppm for 4a.
45
Scheme 2.9. Synthesis of cationic monomethyl zirconium bis(amidinate) complexes paired with MeB(C6F5)3
- or B(C6F5)4- anions, 4a and 5a, respectively.
Similarly, the characterization of ion pairs derived from the analogous titanium
bis(amidinate) complexes was attempted. The 1H NMR was difficult to interpret due to
the impurities in the starting dimethyl complex and the broad nature of peaks in a 5
coordinate complex. For the reaction of the dimethyl complex, 3b, with B(C6F5)3, the 11B
NMR spectrum shows a sharp signal at -14.5 ppm and the 19F NMR spectrum shows a
meta-para gap of 2.40 ppm consistent with formation of a solvent separated ion pair.
The abstraction of a methyl group from 3d using B(C6F5)3 and [Ph3C][B(C6F5)4]
occurs cleanly in C6D5Br to form 4d and 5d, respectively (Scheme 2.10). For 4d, the 1H
NMR spectrum shows resonance of unsubstituted ferrocene Cp ring and zirconium
bound Cp* at 4.10 and 1.90 ppm, respectively, and both values are shifted upfield from
the neutral complex. The 11B NMR spectrum of 4d shows a sharp signal at -14.4 ppm
consistent with the formation of MeB(C6F5)3. The meta-para gap of 2.53 ppm for 4d on
the 19F NMR spectrum indicates a solvent separated ion pair. The 1H NMR spectrum of
5d is similar to 4d with the exception of methyl bound to zirconium which appears at
1.13 ppm.
46
Scheme 2.10. Synthesis of cationic monomethyl zirconium mono(amidinate) complex paired with MeB(C6F5)3
- or B(C6F5)4- anions, 4d and 5d, respectively.
An attempt to crystallize 5d by layering bromobenzene solution with pentane at
-30°C resulted in an unexpected product after several weeks. The crystal structure
(Figure 2.10) shows the formation of a protonated amidine ion pair
[CyNHC(Fc)NHCy][B(C6F5)4]. In fact, when the solution was monitored by NMR
spectroscopy for 5 days, it did not show any signs of decomposition. The formation of
the product and source of protons is not clear.
Figure 2.10. Molecular structure of [CyNHC(Fc)NHCy][B(C6F5)4]. Thermal ellipsoids are shown at 50% probability. Hydrogen atoms have been omitted for clarity.
47
Table 2.8. Selected bond distances (Å) and bond angles (°) for [CyNHC(Fc)NHCy][B(C6F5)4].
N1-C30 1.331(6) N1-C30-N2 122.10(47) N2-C30 1.315(7) N1-C30-C29 116.45(45) N2-C37 1.484(6) N2-C30-C29 121.45(44) N1-C31 1.464(6) C30-N2-C37 128.04(44) C30-C29 1.471(7) C30-N1-C31 129.36(44)
The abstraction of a benzyl group from 3c using B(C6F5)3 in C6D5Br produces a
solvent separated ion pair based on the meta-para gap of 2.8 ppm on the 19F spectrum
(Scheme 2.11). The Zr-CH2 signal is observed at 2.90 ppm downfield from the neutral
dialkyl complex. The sharp signal at -12.2 ppm in the 11B NMR spectrum shows the
formation of PhCH2B(C6F5)3-. Based on the upfield resonance of the ortho proton of the
coordinated benzyl group at 6.49 ppm in the 1H NMR spectrum, the benzyl group is most
likely coordinated by η2 bonding to stabilize the cationic center. The relative shift of the
benzyl ortho protons is diagnostic since ortho protons shift upfield depending on the
amount of pi bonding between the metal center and the aryl group.93 The implications of
the mode of coordination during polymerization are discussed later.
48
Scheme 2.11. Synthesis of cationic zirconium mono(amidinate) complex, 4c, paired with MeB(C6F5)3
- anion and possible modes of benzyl coordination.
Similarly, the abstraction of a benzyl group from 3c with [Ph3C][B(C6F5)4] leads to
the formation of the ion pair, 5c, and 1 equiv of 1,1,1,2-tetraphenylethane. The 1H NMR
spectrum shows signals of CH2 groups belonging to the benzyl group bound to zirconium
and tetraphenyl ethane at 2.89 ppm and 3.84 ppm, respectively.
In summary, the ion pairs derived from 3a-3d were characterized in solution by
NMR spectroscopy. They do not undergo any unexpected reactivity or intermolecular
49
ferrocene C-H activation. Decomposition of 4c and 5c was observed in solution after 24
hours but other ion pairs were stable for few days.
2.2.2 Electrochemical Study of Dichloride and Dialkyl Complexes
Cyclic voltammetry studies were conducted to evaluate the stability of the
oxidized FeIII containing species. These experiments were done in dry acetonitrile or
dichloromethane solutions employing [nBu4N]+[PF6]- or [nBu4N]+[BF4]- as the supporting
electrolyte. All potentials described herein are referenced to FeII/FeIII couple of
ferrocene.
Table 2.9. Summary of the cyclic voltammetric data on the dichloride complexes, 2a-2d.a E1/2 - Peak 1
(mV) E1/2 - Peak 2
(mV) 2a 439 -455 2bb 351 -487 2c 430 -483 2d
437 -557e
2ac 391 -400 2dc 402 -399e 2ad 391 -520e 2dd 371 -555e
aConditions: 10-3 M complex solution in dichloromethane, 10-1 M [nBu4N]+[PF6]- as supporting electrolyte and scan rate of 250mV⋅s-1 unless otherwise specified. b2b also shows an additional irreversible reduction at -1574 mV. cAcetonitrile was used as the solvent. d[nBu4N]+[BF4]- was used as the supporting electrolyte. eIrreversible redox couples, Epa is reported.
The cyclic voltammograms of the dichloride complexes (2a-2d) show a quasi-
reversible oxidation of FeII in dichloromethane solution using [nBu4N]+[PF6]- as the
electrolyte (Table 2.9). The voltammograms also show a second redox couple at E1/2
ranging from -400mV to -487mV (Figure 2.11 for 2a; Figure 2.12 for 2c). Complex 2b
shows an irreversible reduction at -1574 mV corresponding to the reduction of titanium
from TiIV to TiIII as commonly observed with other titanium complexes. The E1/2 of
50
FeII/FeIII redox couple in 2a, 2c and 2d is similar at ca. 430 mV. The E1/2 of 2b is 88 mV
lower and ∆Ep is broader by 134 mV in comparison to 2a. The metal complexes show
the iron oxidation at a higher potential compared to the protonated amidine, 1a, which
undergoes a quasi-reversible one electron oxidation at E1/2 of 310 mV.76 This is
expected since the metal withdraws electron density from the amidinate rendering the
iron center more electrophilic and more difficult to oxidize. The quasi-reversibility of FeII
oxidation is indicated by the larger separation of anodic and cathodic peak potentials in
comparison to the ferrocene standard under the same conditions for all complexes.
Moreover, the cathodic to anodic peak ratio is significantly lower in amidinate complexes
compared to the reversible FeII oxidation in the ferrocene standard. This loss of current
in the return wave suggests that the oxidized FeIII species undergo a subsequent
chemical reaction before the reduction back to FeII.
Figure 2.11. Cyclic voltammogram of 2a. Conditions: 10-3 M complex solution in dichloromethane, 10-1 M [nBu4N]+[PF6]- as supporting electrolyte and scan rate of 50mV⋅s-1 (orange curve) and 250mV⋅s-1 (blue curve).
-45
-35
-25
-15
-5
5
15
25
-800-300200700Potential (mV) vs. Fc/Fc+
Cur
rent
(A)
250 mV/s 50 mV/s
51
Figure 2.12. Cyclic voltammogram of 2c. Conditions: 10-3 M complex solution in dichloromethane, 10-3 M [nBu4N]+[PF6]- as supporting electrolyte and scan rate of 50mV⋅s-1 (orange curve) and 250mV⋅s-1 (blue curve).
The lack of stability of the ferrocenium species may be due to an attack by Lewis
basic substituents, solvent or supporting electrolyte.103 Electrochemical conditions were
varied by changing the solvent and electrolyte to evaluate the effect on reversibility of
the FeII/FeIII redox couple and compare the E1/2 of FeII oxidation to the standard
conditions of dichloromethane solution using [nBu4N]+[PF6]- electrolyte. Acetonitrile was
considered since it may coordinate to stabilize the FeIII center and suppress any
chemical reactivity of the oxidized species. Using acetonitrile with [nBu4N]+[PF6]- as the
electrolyte still resulted in quasi-reversible FeII/FeIII redox couple and the E1/2 of FeII
oxidation is 35 and 48 mV lower compared to the standard conditions for 2a and 2d,
respectively. Secondly, [nBu4N]+[BF4]- was used as the alternate electrolyte in
dichloromethane solution since changing the counter anion may stabilize the oxidized
product. Similarly, these conditions also resulted in quasi-reversible FeII/FeIII redox
couple and the E1/2 of FeII oxidation is 48 mV and 66 mV lower compared to the standard
conditions for 2a and 2d, respectively. In both cases, the cathodic to anodic peak ratio
-35
-25
-15
-5
5
15
-680-180320820Potential (mV) vs. Fc/Fc+
Cur
rent
(A)
250 mV/s 50 mV/s
52
was similar to the previously observed ratio in dichloromethane solution using
[nBu4N]+[PF6]- electrolyte.
A closer look at the voltammogram of 2a (Figure 2.11) reveals that the oxidation
wave for the FeII/FeIII redox couple at ca. 430 mV is not symmetrical in comparison to the
return reduction wave. In fact, lowering the scan rate to 50 mV.s-1 reveals that it is
overlapping with another redox couple. For the bis(amidinate) complexes, this may be
justified by assuming that the oxidation of the first ferrocene moiety is followed by slightly
higher potential required to oxidize the second ferrocene moiety resulting in two
overlapping redox couples. However, most complexes containing multiple ferrocenyl
systems are usually independent and undergo oxidation at the same potential.104
Moreover, similar voltammograms showing overlapping redox couples were also
obtained for half sandwich zirconium mono(amidinate) complexes (Figure 2.12 for 2c)
with only one ferrocene moiety. These results were unexpected since dichloride
zirconocenes only undergo reduction at -1700mV vs. SCE, and the introduction of a
ferrocene substituent in the ligand should only result in one additional redox couple
corresponding to the FeII oxidation.105 This suggests that perhaps the overlapping redox
event may correspond to oxidation at the nitrogen atom since other amidines are known
to undergo oxidation.106 Additional evidence is needed to support this hypothesis.
Secondly, the voltammograms show a second couple at ca. -450 mV as quasi-reversible
or irreversible depending on the complex, scan rate and the electrochemical conditions.
Moreover, this peak only appears after the oxidation and reduction of the iron center and
could be associated with the side products from that redox event.
It is worth evaluating the stability of the dimethyl complexes containing oxidized
FeIII species since the dimethyl complexes can be activated by non-aluminum based
cocatalysts. Gibson has reported that in bis(imino)pyridyl based complexes containing a
ferrocenyl substituent, the oxidized FeIII species show the same ethylene polymerization
53
activity as the corresponding FeII species because MAO reduces the FeIII to FeII during
polymerization.67 The dimethyl complexes exhibit two quasi reversible couples at ca.
430 mV and 100 mV (Table 2.10). The redox couple at ca. 430 mV corresponds to the
FeII oxidation in the amidinate ligand. Complex 3a (Figure 2.13) and 3c (Figure 2.14)
shows similar E1/2 as their corresponding dichloride complexes, whereas, 3d shows
lower E1/2 by 100 mV compared to 2d. The reduced E1/2 in 3d is expected since methyl
groups are more electron donating than chlorides causing an increase in electron
density at the zirconium center. As a result, the iron center is less electrophilic resulting
in a complex that is easier to oxidize. The second couple may correspond to the
oxidation at the nitrogen and is distinctly separated compared to the dichloride
complexes where it was overlapping. However, chemical oxidation needs to be
attempted to confirm this hypothesis.
Table 2.10. Summary of the cyclic voltammetric data on the dialkyl complexes, 3a, 3c-3d.a
E1/2 - Peak 1 (mV)
E1/2 - Peak 2 (mV)
3a 447 108 3c 425 95 3d 334 170 (Epa)
aConditions: 10-3 M complex solution in dichloromethane, 10-1 M [nBu4N]+[PF6]- as supporting electrolyte and scan rate of 250mV⋅s-1.
54
Figure 2.13. Cyclic voltammogram of 3a. Conditions: 10-3 M complex solution in dichloromethane, 10-1 M [nBu4]+[PF6]- as supporting electrolyte and scan rate of 250mV⋅s-1.
Figure 2.14. Cyclic voltammogram of 3c. Conditions: 10-3 M complex solution in dichloromethane, 10-1 M [nBu4N]+[PF6]- as supporting electrolyte and scan rate of 250mV⋅s-1.
In conclusion, the cyclic voltammetry results show a quasi-reversible oxidation of
FeII/FeIII couple in the dichloride and the dialkyl complexes under the tested
-40
-30
-20
-10
0
10
20
-600-400-20002004006008001000Potential (mV) vs. Fc/Fc+
Cur
rent
(A)
250 mV/s
-20
-10
0
10
-680-480-280-80120320520720920Potential (mV) vs. Fc/Fc+
Cur
rent
(A)
250 mV/s
55
electrochemical conditions. This suggests that the oxidized FeIII species undergoes a
chemical reaction or rearrangement and may be unstable for long periods. Moreover,
the oxidation at the iron center is not exclusive and there may be oxidation at the
nitrogen atom. Isolation of the products from chemical oxidation may aid in further
understanding these results.
2.2.3 Polymerization Results
Ethylene polymerization catalysis was carried out with the dichloride precursors
using MAO as the cocatalyst. The dialkyl precursors were tested using the cocatalysts,
B(C6F5)3 and [Ph3C][B(C6F5)4].
Polymerization activities are compared against a standard catalyst instead of
literature sources since activities are highly dependent on the exact polymerization
protocol. Highly active Cp2ZrCl2 was used as the standard catalyst since amidinates are
considered analogous to cyclopentadienyl ligands. Schlenk line polymerizations were
done and identical conditions were used for all polymerizations. All polymerization trials
were repeated twice.
2.2.3.1 Ethylene Polymerization using Dichloride Precatalysts
Schlenk line polymerizations were conducted using 10μmol of precatalyst, 1000
equivalents of MAO as cocatalyst in 50 mL of toluene. The polymerization trial was run
for 20 minutes at 22°C under 1 atm of ethylene. Polymerization results for dichloride
precatalysts (2a-2d) are summarized in Table 2.11.
56
Table 2.11. Ethylene polymerization results of dichloride precursors using MAO as the cocatalyst.a
Precatalyst Average Activity (g mmol-1h-1atm-1)
Cp2ZrCl2 627 2a 66 2b 27 2c 36 2d 8
aPolymerization conditions: precatalyst = 10μmol; 50mL of toluene; ethylene pressure = 1 atm; temperature = 22° C; cocatalyst = 10 mmol of MAO (Al/M ratio = 1000); Average of two trials.
Catalysts derived from complex 2a, 2b and 2c showed moderate polymerization
activity of 66 g.(mmol.hr. atm)–1, 27 g.(mmol.hr. atm)–1 and 36 g.(mmol.hr. atm)–1,
respectively, whereas, 2d showed low polymerization activity of 7.8 g.(mmol.hr. atm)–1.
The activities of all complexes were significantly lower when compared to Cp2ZrCl2
standard. Comparing the bis(amidinate) complexes, the zirconium analog (2a) showed
about twice the activity compared to the titanium analog (2b). This trend has been
observed before for benzamidinate complexes, [Me3SiNC(C6H5)NSiMe3]2MCl2 (M = Ti,
Zr) and acetamidinate complexes, [CyNC(Me)NCy]2MCl2 (M = Ti, Zr).79,86
A surprising result was observed for the half sandwich mono(amidinate)
complexes. The derivatization of the ligands on the metal center usually affects
polymerization activity by either changing the rate of olefin insertion or affecting the
catalyst decomposition rate. Generally, the Cp* derivatives compared to analogous Cp
derivatives show higher activity due to the greater steric bulkiness which protects the
metal center, thus, decreasing the rate of catalyst decomposition. However, in these
mono(amidinate) complexes, the Cp derivative (2c) showed approximately five times
higher activity than the Cp* derivative (2d).
The higher activity of 2c may be due to the formation of highly active Cp2ZrMe2
by ligand redistribution during polymerization, as observed in the alkylation of 2c. A
57
large excess of MAO (1000 equivalents) was used during the polymerization trial, so the
process of methyl abstraction is faster than ligand redistribution. As a result, only a
small amount of 2c is converted to Cp2ZrMe2 and this accounts for the large difference in
activity compared to the standard. This effect due to ligand distribution can be
investigated further by changing the Zr:Al ratio during the polymerization trial and
comparing the activities.
Generally, activation with MAO is less controlled and results in a lower activity
when compared to perfluorophenyl substituted borane activators. Rearrangement via
ligand redistribution may produce different catalytically active species than assumed.
Eisen and coworkers have studied the catalytically active species formed by titanium
benzamidinate based complexes upon activation with MAO in propylene
polymerization.107 They noted that titanium mono(amidinate) trichloride and
bis(amidinate) dichloride complexes showed very similar polymerization activity, and
also the resulting polymers had similar characteristics such as molecular weight
distribution. After conducting a series of experiments, they concluded that the observed
similarities were due to the formation of the same active species by ligand
rearrangements via an aluminum transfer mechanism. Moreover, Hessen and
coworkers have also showed that some aminopyridinato based zirconium complexes
readily undergo ligand transfer from zirconium to aluminum.108 Hence, ligand
redistribution may result in formation of aluminum amidinate complexes. These species
may undergo decomposition or form active polymerization catalysts, which have been
previously reported by Jordan and coworkers.109
In an another study, Eisen and coworkers reported that the catalytic activity is
strongly dependent on the Al:Zr ratio used during ethylene polymerization for zirconium
bis(amidinate) complexes, [Me3SiN(RC6H4)CNSiMe3]2ZrCI2 (R = H; CH3).86 They
showed that an increase in Al:Zr ratio resulted in a decrease in the catalytic activity,
58
which is consistent with the observation that an increased cocatalyst concentration leads
to other deactivation pathways for amidinate complexes.86
2.2.3.2 Ethylene Polymerization using Dialkyl Precatalysts
Similar to the dichloride precatalysts (2a-2d), Schlenk line polymerizations were
conducted using 10μmol of the respective precatalyst (3a-3d), one equivalent of
cocatalyst, twenty equivalents of iBu3Al as a solvent scrubber in 50 mL of toluene. The
polymerization trial was run for 20 minutes at 22°C under 1 atm of ethylene.
Polymerization results for the dialkyl complexes (3a-3d) activated with B(C6F5)3 (BCF)
and [Ph3C][B(C6F5)4] (TB) are presented in Table 2.12.
Table 2.12. Ethylene polymerization results of dialkyl precursors activated with B(C6F5)3 or [Ph3C][B(C6F5)4].a
Precatalyst Activator Average Activity (g mmol-1 h-1 atm-1)
3a B(C6F5)3 45 3b B(C6F5)3 Trace 3c B(C6F5)3 0 3cb B(C6F5)3 0 3d B(C6F5)3 43
3a [Ph3C][B(C6F5)4] 87 3b [Ph3C][B(C6F5)4] 12 3c [Ph3C][B(C6F5)4] 0 3cb [Ph3C][B(C6F5)4] 0 3d [Ph3C][B(C6F5)4] 57
aPolymerization conditions: precatalyst = 10μmol; 50mL of toluene; ethylene pressure = 1 atm; temperature = 22° C; cocatalyst = 10 μmol; iBu3Al = 200 μmol; Average of two trials unless otherwise specified. bPolymerization was done at 60°C.
Complex 3a and 3d activated with BCF and TB showed higher activities than the
corresponding dichloride complexes activated with MAO. Moreover, 3a and 3d showed
higher activity upon activation with TB compared to BCF. This trend has been observed
previously for other group 4 based polymerization catalysts and the difference in activity
is due to the relative coordination of the counteranion to the catalytically active cationic
59
metal center. NMR spectroscopy studies and structural dynamics show the relative
coordinative ability of the anions in the following order [B(C6F5)4]- < [MeB(C6F5)3]- <
[MeMAO]-.110 Hence, the highest activities were observed upon activation with
[Ph3C][B(C6F5)4] since [B(C6F5)4]- is the least coordinating anion in the resulting ion pair.
Lastly, 3a and 3d activated with BCF showed very similar activities within experimental
error, however, 3a showed much higher activity than 3d upon activation with TB. The
higher activity of 3a compared to 3d was also observed when the corresponding
dichloride complexes were activated with MAO.
The activities of 3b could not be measured with reproducibility due to the purity of
the sample and inherent instability, as discussed previously. Activation of 3b with TB
showed low activity with relatively high percentage difference (40%) between the two
trials, whereas, activation with BCF resulted in traces of polymer in both trials.
Unexpected polymerization results were observed for 3c. Activation of 3c with
BCF or TB resulted in no activity, whereas, the corresponding dichloride complexes
activated with MAO showed moderate activity under the same polymerization conditions.
Gibson and coworkers have shown that ethylene polymerization using
bis(phosphanylphenoxide) zirconium dibenzyl complexes activated with TB at 25°C
resulted in no activity, but an activity of 3900 g⋅(mmol.hr.bar)-1 at 60°C was observed.111
They propose that the difference in activity is due to coordination mode of the benzyl
group in the catalytically active species. At low temperatures, an η2 benzyl coordination
blocks the ethylene from binding to the metal center and deactivates the catalyst. The
high activity at elevated temperatures is attributed to the change in coordination mode of
the benzyl group from η2 to η1. The polymerization trial for 3c was repeated at 60°C,
however, no activity was observed. These results suggest that there might be η2
coordination of the benzyl group at 60°C in amidinate complexes.
60
Low to moderate polymerization activities observed for ferrocenyl amidinate
complexes may be explained by the highly electron donating ferrocenyl group rendering
the metal center less electrophilic and thus less reactive. Although this may explain the
increased stability of the monoalkyl cationic complexes, more evidence is needed to
support this hypothesis. Sita and coworkers have crystallographically characterized
binuclear monocationic and dicationic half sandwich mono(amidinate) complexes with μ-
CH3 and μ-CH2 groups.112 The formation of these dimerized cationic species as
alternate termination pathways may also account for the moderate polymerization
activities in amidinate complexes.
It is noteworthy that ethylene polymerization trials conducted on a Schlenk line
introduce more variability as opposed to a controlled polymerization reactor. This factor,
in addition to the low activity of the catalyst, resulted in a 20%-30% discrepancy between
the two trials. It is also important to consider that the polymerization trials were
conducted at room temperature under 1 atm of ethylene. At higher temperatures, an
increase in activity is observed caused by an increase in rate of propagation.113 For
instance, the zirconium bis(amidinate) complexes, [Me3SiN(RC6H4)CNSiMe3]2ZrCI2 (R =
H; CH3), show a six fold activity difference upon increasing the polymerization
temperature from 5°C to 60°C.86 Hence, optimal activity of these catalysts should be
investigated.
2.2.4 Summary
A series of novel group 4 metal complexes containing ferrocenyl amidinate
ligands have been synthesized. Titanium and zirconium bis(amidinate) dichloride
complexes (2a, 2b) were synthesized in good yields by amine elimination reaction. The
half sandwich zirconium mono(amidinate) dichloride complexes (2c, 2d) were
synthesized in moderate yields by salt metathesis. Attempts to synthesize the half
61
sandwich titanium mono(amidinate) complexes were unsuccessful. The dichloride
complexes activated with MAO show low to moderate ethylene polymerization activity.
The dimethyl derivatives (3a, 3b, 3d) of the corresponding dichloride complexes
were also synthesized. Complex 3b was difficult to isolate cleanly due to its thermal
instability. The dibenzyl derivative (3c) of Cp zirconium mono(amidinate) was
synthesized since the corresponding dimethyl complex undergoes disproportionation.
The dialkyl derivatives activated with B(C6F5)3 and [Ph3C][B(C6F5)4] also show low to
moderate ethylene polymerization activities. Cyclic voltammetry studies on the
dichloride and dialkyl complexes show a quasi reversible redox couple for the FeII
oxidation under different electrochemical conditions.
62
Chapter 3 Experimental Details
3 Heading
The experimental details pertinent to chapter 2 can be found here.
3.1 General Considerations
All preparations were performed in a dry, oxygen and moisture free, nitrogen
atmosphere using either standard Schlenk line techniques or Vacuum Atmospheres
glove box unless otherwise stated. All glassware used for air and/or moisture sensitive
materials were stored in an oven (150°C) for 12 hours prior to their use.
3.1.1 Solvents
Reagent grade solvents including toluene, pentane, hexanes, diethyl ether,
tetrahydrofuran, and dichloromethane were dried using a commercially available
Innovative Technologies PureSolv solvent system, or a commercially available Vacuum
Atmospheres solvent system. The solvents were thoroughly degassed using the freeze-
pump-thaw technique and stored over activated 4 Å molecular sieves.
Deuterated NMR solvents, obtained from Cambridge Isotopes Laboratories, were
dried over sodium/benzophenone (benzene, toluene) or calcium hydride (chloroform,
dichloromethane), distilled, degassed by the freeze-pump-thaw technique and stored
over 4 Å molecular sieves prior to use.
3.1.2 Materials
Hyflo Super Cel ® (Celite) and 4 Å molecular sieves were purchased from Aldrich
Chemical Co., and dried at 150°C in vacuo for 24 hours prior to use.
63
3.2 Instrumentation
3.2.1 Nuclear Magnetic Resonance Spectroscopy (NMR)
NMR spectra were recorded on a Bruker Avance 400 MHz, Varian Mercury 300
and Mercury 400 spectrometers. 1H and 13C{1H} spectra were referenced internally
relative to the residual protio-solvent resonances (δ 7.30 ppm for downfield signal of
C6D5Br, δ 5.32 ppm for CD2Cl2, δ 7.16 ppm for C6D6; δ 122.4 ppm for Cipso of C6D5Br, δ
54 ppm for CD2Cl2, δ 128.4 ppm for C6D6). The 19F NMR spectra were referenced using
80% CFCl3 in CDCl3 as an external standard. The 11B{1H} NMR spectra were referenced
using BF3·Et2O as an external standard. Chemical shifts are expressed in ppm and
coupling constants (J) in Hz.
3.2.2 Electrochemistry
Cyclic voltammetry experiments were performed with a three electrode system in
a BASi RDE-2 cell stand using an aqueous Ag/AgCl reference electrode, a glassy
carbon working electrode with a disk diameter of 3.0 mm, and a Pt wire auxiliary
electrode. The working electrode was polished with alumina (0.05 μm) and rinsed with
deionised water prior to each experiment. [NBu4][PF6] and [NBu4][BF4] was used as the
supporting electrolytes (0.1 M solutions). All the potentials were referenced to the
ferrocene/ferrocenium couple. The electrochemical data was acquired using the Epsilon
EC software with a computer controlled BASi Epsilon EC potentiostat. All
electrochemical experiments were run under nitrogen at room temperature.
3.2.3 Other Instrumentation
Elemental analyses were performed using a Perkin-Elmer 2400 C/H/N analyzer.
In the elemental analysis for some of the complexes, the actual carbon value is lower
than the calculated value due to carbide formation during the combustion process.
64
3.3 Synthesis and Characterization
3.3.1 Reagents and Starting Materials
MeLi (1.6M in ether), MeMgBr (1.4 M in toluene/tetrahydrofuran 7:1),
dicyclohexylcarbodiimide and methylaluminoxane (10% w/w in toluene) were purchased
from Aldrich Chemical Company and used without further purification. [NBu4][PF6] and
[NBu4][BF4], purchased from Aldrich, was recrystallized three times from absolute
ethanol and ethyl acetate, respectively and dried overnight at 100°C in vacuo. Cp2Fe,
CpZrCl3 and Cp*ZrCl3 were purchased from Strem Chemicals. Ti(NMe2)2Cl2114,
Zr(NMe2)2Cl2(THF)2115, C5H5FeC5H4Li74 and KCH2Ph116 were prepared from literature
methods.
3.3.2 Organic and Organometallic Syntheses
[CyNC(Fc)NCy]H (1a). Method A. The reaction was scaled down
and the modified procedure71 was followed. A 500 mL Schlenk
flask was charged with ferrocene (31.5 g, 169 mmol), hexanes
(100 mL) and THF (100 mL). The orange suspension was cooled
to 0°C and tBuLi (100 mL, 169 mmol, 1.7M in pentanes) was added
over a 25 minute period. The solution was stirred for 3.5 hours at room temperature
(This stirring time was optimized for maximum yield and changes to this time reduce the
overall yield). A solution of dicyclohexyl carbodiimide (35 g, 169 mmol) in THF (35 mL)
was cannula transferred dropwise over a 10 minute period. The reaction was stirred for
4 hours at room temperature and quenched with excess, degassed water (5 mL, 278
mmol). The solution was filtered and the solvent was pumped off under reduced
pressure resulting in a dark orange oil, which solidified to yellow solid overnight. The
crude product was sublimed at 65°C under reduced pressure to remove the excess
65
ferrocene and dicyclohexyl carbodiimide (39.9 g, 61% yield). Recrystallization from
boiling hexanes afforded pure orange crystalline solid.
Method B. A 50mL Schlenk flask was charged with monolithioferrocene (0.20 g,1.04
mmol) and THF (10 mL). The flask was cooled to 0°C and a solution of dicyclohexyl
carbodiimide (0.21 g, 1.04 mmol) in THF (10 mL) was added dropwise. The reaction
was warmed to room temperature and stirred for 1 hour before quenching with water (5
mL). The aqueous portion was washed with hexanes (2 x 5 mL), and the combined
organic layers were washed with water and dried over MgSO4. The volatiles were
pumped off resulting in yellow solid without requiring further purification (0.31 g, 85%).
The procedure can be scaled up or down without any noticeable effect on yield.
The NMR data is in agreement with the literature. 1H NMR (C6D6): δ 4.64 (d, 3JH-H = 6.7
Hz, 1H, NH), 4.28 (t, 3JH-H = 1.8 Hz, 2H, H2,5-C5H4), 4.26-4.13 (m, 1H, α-H Cy), 4.01 (s,
5H, C5H5), 3.94 (t, 3JH-H = 1.8 Hz, 2H, H3,4-C5H4), 3.62-3.50 (m, 1H, α-H Cy), 2.28-2.16
(m, 2H, Cy), 1.87-1.12 (m, 18H, Cy).
[iPrNC(Fc)NiPr]H (1b). A solution of bromoferrocene (0.4 g, 1.51
mmol) in THF (10 mL) was cooled to -78°C. nBuLi (0.94 mL, 1.51
mmol, 1.6M in hexanes) was added dropwise and the solution was
stirred for 1 hour as it underwent a colour change from yellow to
orange. A second solution of isopropyl carbodiimide (0.19 g, 1.51
mmol) in THF (10 mL) was cannula transferred dropwise into the first solution over a 5
minute period. The solution was warmed to room temperature and stirred for 1 hour.
The mixture was quenched with degassed water (5 mL) and concentrated to 10 mL
solution under reduced pressure. The aqueous portion was washed with hexanes (2 x 5
mL), and the combined organic layers were washed with water and dried over MgSO4.
The solution was further concentrated and left in the freezer (-15°C) overnight, where the
66
product crystallized. The mother liquor was decanted quickly before the solid melted at
room temperature. The resulting yellow liquid was dried under vacuum (0.38 g, 79%).
1H NMR (C6D6): δ 4.51 (br, 1H, NH), 4.39 (m, 1H, CH(CH3)2), 4.23 (t, 3JH-H = 1.8 Hz, 2H,
H2,5-C5H4), 3.96 (s, 5H, C5H5), 3.93 (t, 3JH-H = 1.8 Hz, 2H, H3,4-C5H4), 3.86 (m, 1H,
CH(CH3)2), 1.27 (d, 3JH-H = 5.9 Hz, 6H, CH(CH3)2), 1.22 (d, 3JH-H = 5.9 Hz, 6H,
CH(CH3)2). 13C{1H} NMR: δ 151.5 (Cipso(NiPr)2), 81.5 (Cipso-C5H4), 69.5 (C5H5-Fe ), 69.4
(C3,4-C5H4), 68.2 (C2,5-C5H4), 50.0(CH(CH3)2), 42.0 (CH(CH3)2), 26.1 (CH(CH3)2), 23.2
(CH(CH3)2).
Li[CyNC(Fc)NCy] (1d). The following modified literature
preparation was followed.76 A bright orange solution of 1a (2.00 g,
5.1 mmol) in hexanes (50 mL) was cooled to 0°C and treated with
MeLi (3.27 mL, 5.1 mol, 1.6M in ether). The resulting suspension
was warmed to room temperature and stirred overnight. The
solution was concentrated to 30 mL and filtered to provide a yellow solid. The solid was
washed with hexanes and dried under vacuum (1.7 g, 84%). The NMR data is
consistent with the literature preparation. 1H NMR (C6D6): δ 4.90-4.10 (overlapping
broad m, 9H, C5H5-Fe-C5H4), 2.6-1.2 (m, 22H, Cy).
[CyNC(Fc)NCy]SiMe3 (1c). To a solution of 1d (0.20 g,
0.50 mmol) in THF (10 mL), excess trimethylsilyl chloride
(0.19 mL, 1.50 mmol) was added dropwise at -30°C. The
reaction was stirred for 3 hours and the solution was filtered
through a pad of Celite. Volatiles were removed under
reduced pressure resulting in a sticky oil. The oil was extracted with hexanes (4 mL)
and over the period of several days, some of the product crystallized at -30°C. For some
subsequent reactions, the TMS-amidine was prepared in situ and not isolated (Isolated
yield: 0.054 g, 23%). 1H NMR (C6D6): δ 4.40 (t, 3JH-H = 3.8 Hz, 2H, H2,5-C5H4), 4.07 (s,
67
5H, C5H5), 4.02 (t, 3JH-H = 1.9 Hz, 2H, H3,4-C5H4), 3.93-3.65 (br, 1H, α-H Cy), 1.93-1.82
(m, 4H, Cy), 1.79-1.46 (m, 10H, Cy), 1.31-1.05 (m, 6H, Cy), 0.46 (s, 9H, SiMe3).
[CyNC(Fc)NCy]2ZrCl2 (2a). A 100 mL Schlenk flask was charged
with Zr(NMe)2Cl2(THF)2 (0.50 g, 1.3 mmol) and slight excess of 2
equiv of 1a (1.05 g, 2.7 mmol). Toluene (50 mL) was added and the
orange solution was heated to reflux overnight. The volatiles were
removed under reduced pressure. The solid was washed with ether
(1 x 5mL) and hexanes (3 x 10 mL) until the washings turned from orange to pale yellow
indicating the complete removal of excess 1a. The orange powder was dried under
vacuum (1.02 g, 85%). Crystals suitable for X-ray diffraction analysis were grown from
slow evaporation of toluene solution at room temperature. 1H NMR (CD2Cl2): δ 4.54 (t,
3JH-H = 1.7 Hz, 4H, H2,5-C5H4), 4.37 (t, 3JH-H = 1.8 Hz, 4H, H3,4-C5H4), 4.29 (s, 10H, C5H5-
Fe), 4.17-4.05 (m, 4H , α-H Cy), 2.09-1.92 (m, 8H, Cy), 1.90-1.80 (m, 8H, Cy), 1.80-1.70
(m, 8H, Cy), 1.69-1.61 (m, 4H, Cy), 1.38-1.18 (m, 12H, Cy). 13C{1H} (CD2Cl2): δ 179.6
(Cipso(NCy)2), 71.2 (Cipso-C5H4), 70.7 (C3,4-C5H4), 70.4 (C5H5-Fe ), 70.1 (C2,5-C5H4), 57.3
(α-C Cy), 35.8 (C2,6-Cy), 26.5 (C3,5-Cy), 26.2 (C4-Cy). Anal. Calcd. for C46H62Cl2Fe2N4Zr:
C, 58.48; H, 6.61; N, 5.93. Found: C, 57.40; H, 6.41; N, 5.90.
[CyNC(Fc)NCy]2TiCl2 (2b). A 100 mL Schlenk flask was charged
with Ti(NMe)2Cl2 (0.25 g, 1.2 mmol) and slight excess of 2 equiv of
1a (1.0 g, 2.5 mmol). Toluene (50mL) was added and the brown
solution was heated to reflux overnight. The volatiles were removed
under reduced pressure resulting in a sticky purple solid. The solid was triturated with
hexanes followed by stirring in hexanes (20 mL) for several hours. The solid was
filtered, then washed with ether (1 x 5mL) and pentanes (3 x 10 mL) until the washings
turned from dark yellow to pink indicating the removal the excess 1a. The purple powder
68
was dried under vacuum (0.84 g, 77%). Crystals suitable for X-ray diffraction analysis
were grown from slow diffusion of hexanes into a saturated solution of 2b in CH2Cl2. 1H
NMR (CD2Cl2): δ 4.56 (br, 4H, H2,5-C5H4), 4.41 (m, 2H, α-H Cy), 4.35 (br, 4H, H3,4-C5H4)
, 4.29 (s, 10H, C5H5-Fe ), 3.99 (m, 2H, α-H Cy), 2.70-2.53 (m, 2H, Cy), 2.42-2.28 (m, 2H,
Cy), 2.22-1.05 (m, 36H, Cy). 13C{1H} (CD2Cl2): δ 177.9 (Cipso(NCy)2), 72.0 (Cipso-C5H4),
70.5 (C5H4), 70.6 (C5H4), 70.4 (C5H5-Fe), 70.0 (C5H4), 69.5 (C5H4), 61.3 (α-C Cy), 59.1
(α-C Cy), 35.5, 35.4, 35.0, 33.5, 26.9, 26.7, 26.3, 26.1. Anal. Calcd. for
C46H62Cl2Fe2N4Ti: C, 61.29; H, 6.93; N, 6.22. Found: C, 59.22; H, 7.31; N, 6.48.
CpZr[CyNC(Fc)NCy]Cl2 (2c). A 100 mL Schlenk flask was charged
with 1d (1.0 g, 2.5 mmol) and THF (25 mL), and cooled to -78°C. The
red solution of 1d was cannula transferred dropwise into a second
100 mL Schlenk flask containing a solution of CpZrCl3 (0.66 g, 2.5
mmol) in THF (40 mL) at -78°C. The reaction mixture was stirred at -78°C for 2 hours,
warmed to room temperature and stirred overnight. The solvent was removed under
reduced pressure and the solid was extracted with CH2Cl2. The solution was filtered
through a plug of Celite on a sintered glass frit followed by removal of volatiles under
reduced pressure. The resulting orange residue was washed with hexanes (3 x 10 mL)
and ether (2 x 5 mL), and dried in vacuo (0.93 g, 61%). The solid was recrystallized
from CH2Cl2 solution layered with hexanes at room temperature (0.59 g, 38%). 1H NMR
(CD2Cl2): δ 6.68 (s, 5H, C5H5-Zr), 4.53 (t, 3JH-H = 1.9 Hz, 2H, H2,5-C5H4), 4.40 (t, 3JH-H =
1.9 Hz, 2H, H3,4-C5H4), 4.30 (s, 5H, C5H5-Fe), 4.17-4.25 (m, 2H, α-H Cy), 1.89-1.72 (m,
8H, Cy), 1.64-1.53 (m, 6H, Cy), 1.33-1.07 (m, 6H, Cy). 13C{1H} NMR: δ 174.4
(Cipso(NCy)2), 116.0 (C5H5-Zr), 71.0 (Cipso-C5H4), 70.8 (C3,4-C5H4), 70.6 (C5H5-Fe), 70.1
(C2,5-C5H4) , 57.8 (α-C Cy) , 35.2(C2,6-Cy), 26.4 (C3,5-Cy) ,25.9 (C4-Cy). Anal. Calcd. for
69
C28H36Cl2FeN2Zr·0.2CH2Cl2: C, 53.29; H, 5.77; N, 4.41. Found: C, 53.05; H, 5.83; N,
4.65.
Cp*Zr[CyNC(Fc)NCy]Cl2, (Cp* = C5Me5), (2d). A 100 mL Schlenk
flask was charged with 1d (0.50 g, 1.26 mmol) and THF (15 mL), and
cooled to -78°C. The red solution of 1d was cannula transferred
dropwise into a second 100 mL Schlenk flask containing a pale
yellow solution of Cp*ZrCl3 (0.42 g, 1.26 mmol) in THF (20 mL) at -78°C. The reaction
mixture was stirred at -78°C for 1 hour, warmed to room temperature and stirred
overnight. The solvent was removed under reduced pressure and the solid was
extracted with CH2Cl2 (40 mL). Filtration through a plug of Celite on a sintered glass frit
was followed by removal of volatiles under reduced pressure. The resulting orange
residue was washed with hexanes (3 x 5mL) and dried in vacuo (0.48 g, 55%). The
solid was recrystallized from a saturated CH2Cl2 solution layered with hexanes at -30°C
(0.30 g, 34%). Crystals suitable for X-ray diffraction analysis were grown from hexanes
solution at room temperature. 1H NMR (CD2Cl2): δ 4.50 (t, 3JH-H =1.8 Hz, 2H, H2,5-C5H4),
4.35 (t, 3JH-H = 1.8 Hz, 2H, H3,4-C5H4), 4.28 (s, 5H, C5H5-Fe), 4.14-4.24 (m, 2H, α-H Cy),
2.21 (s, 5H, C5Me5-Zr), 1.94-1.43 (m, 12H, Cy), 1.37-0.95 (m, 8H, Cy). 13C{1H} NMR: δ
177.2 (Cipso(NCy)2), 126.4 (C5Me5), 73.8(C5H4), 70.8 (C5H4), 70.6(C5H5-Fe), 69.6 (C5H4),
58.3 (α-C Cy), 36.1, 33.1, 26.7, 26.3, 13.6 (C5Me5). Anal. Calcd. for
C33H46Cl2FeN2Zr⋅CH2Cl2: C, 52.78; H, 6.25; N, 3.62. Found: C, 52.68; H, 5.90; N, 3.58.
[CyNC(Fc)NCy]2ZrMe2 (3a). Methyllithium (0.40 mL, 0.64 mmol,
1.6M in ether) was added dropwise to an orange suspension of 2a
(0.30 g, 0.32 mmol) in toluene (18 mL) at -30°C and stirred for 6
hours at room temperature. The solution was concentrated to 10mL
under reduced pressure and filtered through a plug of Celite on a sintered glass frit. The
70
volatiles were removed under reduced pressure and the orange residue was washed
with ether (3 x 1 mL) and dried in vacuo (0.22 g, 75%). Crystals suitable for X-ray
diffraction analysis were grown from slow evaporation of toluene solution at -30°C. 1H
NMR (C6D6): δ 4.31 (t, 3JH-H = 1.7 Hz, 4H, H2,5-C5H4), 4.28-4.19 (m, 4H, α-H Cy), 4.15 (s,
10H, C5H5-Fe), 4.07 (t, 3JH-H = 1.8 Hz, 4H, H3,4-C5H4), 2.19-2.05 (m, 8H, Cy), 2.02-1.92
(m, 8H, Cy), 1.87-1.76 (m, 8H, Cy), 1.70-1.59 (m, 4H, Cy), 1.41-1.23 (m, 12H, Cy), 1.12
(s, 6H, Zr-Me2). 13C{1H} (C6D6): δ 178.1 (Cipso(NCy)2), 73.5 (Cipso-C5H4), 70.2 (C3,4-C5H4),
70.0 (C5H5-Fe ), 69.1 (C2,5-C5H4), 56.7 (α-C Cy), 45.3(Zr-Me2), 36.3 (C2,6-Cy), 26.6 (C3,5-
Cy), 26.3 (C4-Cy). Anal. Calcd. for C48H68FeN2Zr: C, 63.77; H, 7.58; N, 6.20. Found: C,
62.18; H, 7.34; N, 6.31.
[CyNC(Fc)NCy]2TiMe2 (3b). All glassware was covered with
aluminum foil due to the light sensitive nature of the product. A 100
mL Schlenk flask was charged with 2b (0.3 g, 0.33 mmol) and
toluene (50 mL). The flask was cooled to -78°C and methyl
magnesium bromide (0.48 mL, 0.67 mmol, 1.4 M in toluene/tetrahydrofuran 7:1) was
added to the suspension dropwise over a 5 minute period. A colour change from pink to
dark yellow was observed immediately. After stirring for 5 minutes, the solution was
concentrated to 25 mL under reduced pressure while it was warmed to room
temperature. The solution was filtered through a plug of Celite on a sintered glass frit
and the volatiles were removed under reduced pressure. The yellow solid was washed
with hexanes (3 x 4 mL) resulting in 90% pure product (0.086 g, 30%). Dilute conditions
and stirring time are crucial to minimize the decomposition products. 1H NMR (CD2Cl2):
δ 4.47 (t, 3JH-H = 1.8 Hz, 4H, H2,5-C5H4), 4.36 (t, 3JH-H = 1.8 Hz, 4H, H3,4-C5H4), 4.26 (s,
10H, C5H5-Fe), 4.19-4.08 (m, 4H, α-H Cy), 2.05-1.58 (m, 30H, Cy), 1.38-1.17 (m, 16H,
Cy), 1.23 (s, 6H, Ti-Me2). 13C{1H} (CD2Cl2, partial): δ 178.6 (s, Cipso(NCy)2), 69.2 (C5H4),
71
64.7 (Ti-Me2), 70.3 (C5H5-Fe), 70.4 (C5H4), 35.7 (br, Cy), 26.7-26.4 (overlapping peaks,
Cy). Sufficient elemental analysis could not be obtained due to the thermal instability
and the light sensitive nature of the product.
CpZr[CyNC(Fc)NCy](CH2Ph)2 (3c). A 50mL Schenk flask was
charged with 2c (0.25 g, 0.40 mmol) and KCH2Ph (0.11 g, 0.80
mmol), and cooled to -30°C. Toluene (20 mL), precooled to -
30°C, was added to the Schlenk flask and the suspension was
stirred for 1 hour at -30°C. Then, the suspension was warmed to room temperature and
stirred overnight. The red coloured suspension changed to yellow coloured solution over
time. The solution was filtered through a plug of Celite on a sintered glass frit and the
volatiles were removed under reduced pressure. The resulting yellow solid was washed
with hexanes (3 x 2 mL) and ether (3 x 2 mL) and dried in vacuo (0.14 g, 47%). Crystals
suitable for X-ray diffraction analysis were grown from slow evaporation of hexanes
solution at room temperature. 1H NMR (CD2Cl2): δ 7.15 (t, 3JH-H = 7.7 Hz, 4H, m-Ph),
6.89 (d, 3JH-H = 7.3 Hz, 4H, o-Ph), 6.81 (t, 3JH-H = 7.3 Hz, 2H, p-Ph), 6.01 (s, 5H, C5H5-
Zr), 4.52 (t, 3JH-H = 1.7 Hz, 2H, H2,5-C5H4), 4.40 (t, 3JH-H = 1.8 Hz, 2H, H3,4 -C5H4), 4.31
(s, 5H, C5H5-Fe), 4.19-4.10 (m, 2H, α-H Cy), 2.35 (d, 3JH-H = 10.4 Hz, 2H, CH2Ph), 1.87
(d, 3JH-H = 10.6 Hz, 2H, CH2Ph), 1.81-1.48 (m, 16H, Cy), 1.36-1.07 (m, 4H, Cy). 13C{1H}
NMR (CD2Cl2): δ 176.6 (s, Cipso(NCy)2), 151.7 (Cipso-Ph), 128.6 (m-Ph), 125.9 (o-Ph),
121.0 (p-Ph), 114.4 (C5H5-Zr), 73.3 (Cipso-C5H4), 71.9 (CH2Ph), 70.7 (C3,4-C5H4), 70.5
(C5H5-Fe), 69.5 (C2,5 -C5H4), 58.3 (α-C Cy), 36.2 (C2,6-Cy), 26.6 (C3,5-Cy), 26.1 (C4-Cy).
Anal. Calcd. for C42H50FeN2Zr: C, 69.11; H, 6.90; N, 3.84. Found: C, 68.56; H, 6.65; N,
3.67.
72
Cp*Zr[CyNC(Fc)NCy]Me2, (3d). Methyllithium (0.54 mL, 0.87
mmol, 1.6M in ether) was added dropwise to an orange solution of
2d (0.30 g, 0.44 mmol) in toluene (18 mL) at -30°C. The solution
was stirred for 5 hours and filtered through a plug of Celite on a
sintered glass frit. The volatiles were removed under reduced pressure and the orange
residue was washed with hexanes (3 x 1 mL) and dried in vacuo (0.20 g, 70%). The
solid was recrystallized from saturated CH2Cl2 solution at -30°C (0.13 g, 32%). 1H NMR
(CD2Cl2): δ 4.42 (t, 3JH-H = 1.8, 2H, H2,5 -C5H4), 4.28 (t, 3JH-H = 1.8, 2H, H3,4-C5H4), 4.23
(s, 5H, C5H5-Fe), 4.09-3.98 (m, 2H, α-H Cy), 2.07 (s, 15H, C5Me5), 1.75-0.94 (m, 20H,
Cy), -0.10 (s, 6H, Zr-Me2). 13C{1H} NMR (CD2Cl2): δ 176.7 (Cipso(NCy)2), 120.2 (C5Me5),
74.9 (Cipso-C5H4), 70.7 (C3,4-C5H4), 70.3 (C5H5-Fe), 68.9 (C2,5-C5H4), 57.8 (α-C Cy), 46.9
(Zr-Me2), 35.9 (br, C2,6-Cy), 26.6 (C3,5-Cy), 26.2 (C4-Cy), 12.5 (C5Me5). Anal. Calcd. for
C35H52FeN2Zr⋅0.5CH2Cl2: C, 61.76; H, 7.74; N, 4.06. Found: C, 61.08; H, 7.72; N, 4.18.
NMR tube scale synthesis of cationic monoalkyl derivatives. The following
compounds were prepared similarly, thus, a single preparation is described. B(C6F5)3
(5.7 mg, 11 μmol) was added to a solution of 3a (10 mg, 11 μmol) in C6D5Br at -35°C.
The solution was transferred to Teflon stopcock equipped NMR tube. NMR data support
the quantitative formation of the cationic metal monoalkyl species paired with borate
anions.
[{CyNC(Fc)NCy}2ZrMe][MeB(C6F5)3] (4a). 1H (C6D5Br): δ 4.55-3.70 (m, 22H, α-H Cy
and C5H5FeC5H4), 4.39 (br, 4H, H2,5-C5H4-Fe), 4.33 (br, 4H, H3,4-C5H4-Fe), 4.13 (s, 10H,
C5H5-Fe), 2.05-0.60 (m, 43H, Cy and Zr-Me), 0.75 (s, 3H, B-Me). 19F (C6D5Br): δ -132.6
(br, 6F, o-C6F5), -162.2 (br, 3F, p-C6F5), -165.3 (br, 6F, m-C6F5) ∆δ(m,p-F) = 3.07. 11B
(C6D5Br): δ -14.4.
73
[{CyNC(Fc)NCy}2ZrMe][B(C6F5)4] (5a). 1H (C6D5Br): δ 7.18-7.04 (m, 15H, Ph3CMe),
4.53-3.97 (m, 22H, α-H Cy and C5H5FeC5H4), 4.42 (br, 4H, H2,5-C5H4Fe), 4.33 (br, 4H,
H3,4-C5H4Fe), 4.12 (s, 10H, C5H5Fe), 2.04 (s, 3H, Ph3CMe), 1.94-1.02 (m, 43H, Cy and
Zr-Me), 0.85 (br, 3H, Zr-Me). 19F (C6D5Br): δ -132.1 (br, 6F, o-C6F5), -161.5 (br, 3F, p-
C6F5), -165.5 (br, 6F, m-C6F5) ∆δ(m,p-F) = 3.97. 11B (C6D5Br): δ -16.1.
[{CyNC(Fc)NCy}2TiMe][MeB(C6F5)3] (4b). 1H (C6D5Br): δ 4.65-4.03 (m, 22H, α-H Cy
and C5H5FeC5H4), 2.04 (s, 3H, Ph3CMe), 2.06-0.95 (m, 46H, Cy, Ti-Me and B-Me). 19F
(C6D5Br): δ -131.5 (d, 3JF-F = 21 Hz, 6F, o-C6F5), -163.9 (t, 3JF-F = 20 Hz, 3F, p-C6F5), -
166.3 (br, 6F, m-C6F5) ∆δ(m,p-F) = 2.40. 11B (C6D5Br): δ -14.5.
[{CyNC(Fc)NCy}2TiMe][B(C6F5)4] (5b). 1H (C6D5Br): δ 7.18-7.04 (m, 15H, Ph3CMe),
4.33-4.12 (overlapping m, 12H, α-H Cy and C5H4-Fe), 4.10 (s, 10H, C5H5Fe), 1.94-1.62
(m, 20H, Cy), 1.60 (s, 3H, Ti-Me),1.41-0.97 (m, 20H, Cy). 19F (C6D5Br): δ -131.6 (br, 6F,
o-C6F5), 161.9 (t, 3JF-F = 21 Hz, 3F, p-C6F5), -165.8 (t, 3JF-F = 18 Hz, 6F, m-C6F5) ∆δ(m,p-
F) = 3.9. 11B (C6D5Br): δ -16.2.
[CpZr{CyNC(Fc)NCy}CH2Ph][PhCH2B(C6F5)3] (4c). 1H (C6D5Br): δ 7.25-6.77
(overlapping m, 8H, m,p-ZrCH2C6H5 and BCH2C6H5 ), 6.49 (d, 3JH-H = 6.9 Hz, 2H, o-
ZrCH2C6H5), 5.90 (s, 5H, C5H5-Zr), 4.42 (br, 2H, H2,5-C5H4-Fe), 4.32 (br, 2H, H3,4-C5H4-
Fe), 4.13 (br, 5H, C5H5-Fe), 3.37 (s, 2H, BCH2), 2.90 (s, 2H, ZrCH2), 1.97-0.67 (m, 20H,
Cy). 19F (C6D5Br): δ -130.0 (d, 3JF-F = 22 Hz, 6F, o-C6F5), -163.2 (t, 3JF-F = 21 Hz, 3F, p-
C6F5), -166.0 (t, 3JF-F = 21 Hz, 6F, m-C6F5) ∆δ(m,p-F) = 2.8. 11B (C6D5Br): δ -12.2.
[CpZr{CyNC(Fc)NCy}CH2Ph][B(C6F5)4] (5c). 1H (C6D5Br): δ 7.23-6.85 (overlapping m,
23H, PhCH2CPh3 and m,p-ZrCH2C6H5), 6.49 (d, 3JH-H = 6.5 Hz, 2H, o-CH2C6H5-Zr), 5.91
(s, 5H, C5H5-Zr), 4.42 (br, 2H, H2,5-C5H4-Fe), 4.33 (br, 2H, H3,4-C5H4-Fe), 4.13 (br, 5H,
C5H5-Fe), 3.84 (s, 2H, PhCH2CPh3), 2.89 (s, 2H, ZrCH2), 1.89-0.77 (m, 20H, Cy). 19F
74
(C6D5Br): δ -131.6 (d, 3JF-F = 10 Hz, 6F, o-C6F5), -161.7 (t, 3JF-F = 21 Hz, 3F, p-C6F5), -
165.7 (t, 3JF-F = 18 Hz, 6F, m-C6F5) ∆δ(m,p-F) = 4.0. 11B (C6D5Br): δ -16.2.
[Cp*Zr{CyNC(Fc)NCy}Me][MeB(C6F5)3] (4d). 1H (C6D5Br): δ 4.40 (br, 2H, H2,5-C5H4-
Fe), 4.28 (br, 2H, H3,4-C5H4-Fe), 4.21-4.07 (overlapping m, 7H, α-H Cy and C5H5Fe),
4.10 (s, 5H, C5H5-Fe), 1.94 (br, 15H, C5Me5), 1.82-0.70 (overlapping m, 23H, Cy and Zr-
Me), 1.13 (br, 3H, Zr-Me), 0.64 (s, 3H, BMe). 19F (C6D5Br): δ -132.1 (d, 3JF-F = 24 Hz,
6F, o-C6F5), -163.5 (t, 3JF-F = 21 Hz, 3F, p-C6F5), -166.0 (t, 3JF-F = 19Hz, 6F, m-C6F5)
∆δ(m,p-F) = 2.53. 11B (C6D5Br): δ -14.4.
[Cp*Zr{CyNC(Fc)NCy}Me][B(C6F5)4] (5d). 1H (C6D5Br): δ 7.18-7.04 (m, 15H, Ph3CMe),
4.43 (br, 2H, H2,5-C5H4-Fe), 4.32 (br, 2H, H3,4-C5H4-Fe), 4.20-4.06 (overlapping m, 7H, α-
H Cy and C5H5Fe), 4.09 (s, 5H, C5H5-Fe), 2.04 (br, 3H, Ph3CCH3), 1.90 (s, 15H, C5Me5),
1.79-0.93 (m, 20H, Cy), 0.61 (s, 3H, Zr-Me). 19F (C6D5Br): δ -131.7 (d, 3JF-F = 9 Hz, 6F,
o-C6F5), -161.9 (t, 3JF-F = 21 Hz, 3F, p-C6F5), -165.7 (t, 3JF-F = 19 Hz, 6F, m-C6F5)
∆δ(m,p-F) = 3.8. 11B (C6D5Br): δ -16.2.
3.4 Polymerization Protocol
3.4.1 Schlenk Line Polymerization
The following method was used for ethylene polymerization of dichloride
precatalysts using MAO as the cocatalyst. Solutions of precatalyst and cocatalyst were
prepared inside a glovebox and stored at -30°C freezer. A 250 mL Schlenk flask was
charged with toluene (50 mL) and MAO (1000 equivalents of precatalyst). The flask was
connected to a Schlenk line, briefly evacuated and refilled with dry ethylene gas
(repeated 5 times). A solution of dichloride precatalyst (10 μmol) in toluene (3mL) was
added to the stirring solution (500 rpm) in the Schlenk flask. The mixture was stirred for
20 minutes at room temperature and quenched with 10% HCl (v/v) in methanol solution.
The polymer was filtered, washed with methanol and dried under vacuum overnight.
75
The following modifications to the above procedure were made when the
dimethyl precatalysts were tested. Initially, the Schlenk flask was only charged with
toluene (50 mL). After placing it under ethylene atmosphere, iBu3Al (10% w/w in
toluene, 0.5 mL, 200 μmol) was added and the solution was stirred for 5 minutes. The
precatalyst (10 μmol in 1 mL toluene) was added to the solution immediately followed by
the cocatalyst solution (10 μmol in 1 mL toluene). The mixture was quenched after
stirring for 20 minutes and the polymer was collected as described above.
3.5 Crystallography
3.5.1 X-ray Data Collection and Reduction
Crystals were manipulated and suspended in Paratone inside a glovebox,
mounted on a MiTegen Micromount, and placed under a N2 stream, thus maintaining a
dry, O2-free environment for each crystal at a temperature of 150(2)K. The data for
crystals were collected on a Bruker Apex II diffractometer with MoKα radiation (λ =
0.71069 Å). The frames were integrated with the Bruker SAINT software package using
a narrow-frame algorithm. Data were corrected for absorption effects using the empirical
multi-scan method (SADABS). Subsequent solution and refinement were performed
using the SHELXTL solution package.
3.5.2 Structure Solution and Refinement
Non-hydrogen atomic scattering factors were taken from literature tabulations.117
The heavy atom positions were determined using direct methods or Patterson
techniques. The remaining non-hydrogen atoms were located from successive
difference Fourier map calculations. The refinements were carried out by using full-
matrix least squares techniques on F, minimizing the function ω(⏐Fo⏐-⏐Fc⏐)2 where the
weight ω is defined as 4Fo2/2σ(Fo
2) and Fo and Fc are the observed and calculated
76
structure factor amplitudes. In the final cycle of each refinement, all non-hydrogen
atoms were assigned anisotropic temperature factors except where noted otherwise.
Carbon-bound hydrogen atom positions were calculated and allowed to ride on the
carbon to which they are bonded, assuming a C-H bond length of 0.95 Å. Hydrogen
atom temperature factors were fixed at 1.2 times the isotropic temperature factor of the
carbon atom to which they are bonded. The hydrogen atom parameters were
calculated, but not refined.
Table 3.1. Table of crystallographic parameters for [iPrNC(Fc)NiPr]H (1b), [CyNC(Fc)NCy]SiMe3 (1c) and [CyNC(Fc)NCy]2ZrCl2 (2a).
1b 1c 2a Molecular Formula C17H24FeN2 C26H40FeN2Si C46H62Cl2Fe2N4Zr Formula Weight 312.24 464.54 944.82 A (Å) 10.4629(6) 12.5781(2) 11.0872(3) B (Å) 16.2335(10) 11.0826(2) 18.5813(7) C (Å) 10.0270(6) 18.2179(3) 21.3216(6) α (°) 90 90 90 β (°) 109.469(3) 92.510(1) 95.434(2) γ (°) 90 90 90 Crystal System Monoclinic Triclinic Monoclinic Space Group Cc P-1 P2(1)/n Volume (Å3) 1605.70(17) 2537.10(7) 4372.8(2) Dcalc (gcm-3) 1.292 1.216 1.435 Z 4 4 4 T (K) 150(2) 150(2) 150(2) Abs coeff, μ, mm–1 0.932 0.656 1.048 Data Collected 7042 21995 40303 Rint 0.0161 0.0199 0.0685 Data Fo
2 > 3σ(Fo2) 3421 10003 10062
Parameters 189 547 496 R(%) 0.0224 0.0274 0.0413 Rw(%) 0.0566 0.0736 0.0951 Goodness of Fit 1.060 1.027 1.006
77
Table 3.2. Table of crystallographic parameters for [CyNC(Fc)NCy]2TiCl2 (2b), CpZr[CyNC(Fc)NCy]Cl2 (2c) and Cp*Zr[CyNC(Fc)NCy]Cl2 (2d).
2b 2c 2d Molecular Formula C46H62Cl2Fe2N4Ti
⋅CH2Cl2 C28H36Cl2FeN2Zr C66H92Cl4Fe2N4Zr2⋅
C6H14 Formula Weight 986.42 618.56 1463.55 A (Å) 10.7232(5) 19.1426(13) 19.7051(12) B (Å) 22.1321(11) 18.5720(14) 14.5964(10) C (Å) 19.1676(9) 7.4148(5) 24.6497(16) α (°) 90.00 90.00 90.00 β (°) 93.048(2) 96.285(4) 90.00 γ (°) 90.00 90.00 90.00 Crystal System Monoclinic Monoclinic Orthorhombic Space Group P2(1)/n P2(1)/c Pna21 Volume (Å3) 4542.6(4) 2620.2(3) 7089.8(8) Dcalc (gcm-3) 1.442 1.568 1.371 Z 4 4 4 T (K) 150(2) 150(2) 150(2) Abs coeff, μ, mm–1 1.075 1.175 0.880 Data Collected 75328 62648 113245 Rint 0.0720 0.0408 0.0434 Data Fo
2 > 3σ(Fo2) 10329 8715 12881
Parameters 523 307 769 R(%) 0.0408 0.0269 0.0341 Rw(%) 0.0912 0.0690 0.0786 Goodness of Fit 1.008 1.021 1.008
,/ 0∑∑ −= FFFR co ( )
5.022 / ⎥⎦⎤
⎢⎣⎡ −= ∑ ∑ oco FFFR
w
78
Table 3.3. Table of crystallographic parameters for [CyNC(Fc)NCy]2ZrMe2 (3a), CpZr[CyNC(Fc)NCy](CH2Ph)2 (3c) and [CyNHC(Fc)NHCy][B(C6F5)4] (6).
3a 3c 6 Molecular Formula C48H68Fe2N4Zr C42H54FeN2Zr C47H33BF20FeN2 Formula Weight 903.98 733.94 1072.41 A (Å) 18.6543(10) 11.313(3) 11.6192(5) B (Å) 11.9951(6) 11.313(3) 11.8682(5) C (Å) 19.4642(10) 14.851(3) 17.9676(11) α (°) 90.00 85.849(5) 98.152(3) β (°) 90.00 85.849(5) 100.738(3) γ (°) 90.00 69.22(1) 113.356(2) Crystal System Orthorhombic Triclinic Triclinic Space Group Pca21 P-1 P-1 Volume (Å3) 4355.3(4) 1770.2(7) 2170.03(19) Dcalc (gcm-3) 1.379 1.362 1.641 Z 4 2 2 T (K) 150(2) 150(2) 150(2) Abs coeff, μ, mm–1 0.930 0.736 0.473 Data Collected 36674 11628 65359 Rint 0.0916 0.0379 0.0532 Data Fo
2 > 3σ(Fo2) 9775 6026 8257
Parameters 498 415 640 R(%) 0.0533 0.0632 0.0640 Rw(%) 0.1099 0.1889 0.1712 Goodness of Fit 0.981 1.108 1.199
,/ 0∑∑ −= FFFR co ( )
5.022 / ⎥⎦⎤
⎢⎣⎡ −= ∑ ∑ oco FFFR
w
79
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