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PREPARATION AND REACTIVITY OF PRECURSORS TO COUNTERANIONS IN ZIEGLER-NATTA
a-OLEFIN POLYMERISATION
A thesis submitted to the
Faculty of Graduate Studies and Research in partial fulfilment of the requirements of the degree of
Master of Science
by Virginie Guillemette
Department of Chemistry McGill University. Montdal
November 1997
Q Virginie Guillemette 1997
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ABSTRACT
The present study has for objective to investigate the nature of the interactions
existing between the cationic tirconocene and its counteranion as xt ive species in Ziegler-
Natta a-olefin polymerisation.
Precursors to anions in the fonn of alcohol and amine adducts of
tris(pentduorophenyl)boron [RR'XH@B(C,F,),] (where R, R' = H, alkyl, aryl; X = 0, N)
were synthesised and reacted with dimethylzirconocene (DMZ), leading to the Formation of
an active cationic catalyst [Cp,ZrMe]+[RXB(C,F,),]-. The results of propylene
polymerisation with the simple, achiral [Cp,ZrMe]+[PhNHB(C6F5)3]- system yielded a
lower molecular weight polypropylene compared to the well-known
[Cp,~Me]+~eB(C,F,),]- system, implying tight ion association during polymerisation.
Formation of amine-substituted borates was attempted by reacting various lithium
amides with B(C,F,),. Reaction of N-perfluorophenyl-substituted amides with two
equivalents of B (C6F5), led to the clean formation of LiB(C,F,), and R(C,F,)NB (C,F,),
(where R = CH,Ph, CH(CH,)Ph). This offers a new method to synthesise novel chiral
Lewis acids such as a-C6H5CH(CH3)(C,F,)NB(C6F5)2.
La prksente etude a pour objectif d'elucider la nature des interactions existant entre le
zirconockne cationique et son conue-anion en tant qu'especes actives dans polymkrisation
d'a-olefines selon Ziegler-Natta.
Des pr6curseurs anioniques sous forme de complexes d'alcools et d'amines avec le
tris(pent~uorophknyi)bore WXHmB(C,F,),] (oh R. R' = H. alkyl, aryl; X = 0, N) ont
it6 synthbtises. La &action de ces p rhneurs avec le dimithylzirconoctne a conduit Zt la
formation dun catdyseur cationique actif, [Cp2ZrMe]'[RXB(C,F,),]-. La polyrnkrisation
du prophe avec le systkme simple et achiral de [CpgrMe]+[PhNHB(C6F5)3]- a permis
d'obtenir un polymkre de masse mol&ulaire moins tlevke que Iorsque le systiime de
kfkrence [Cp,BMe]+WeB(C,F,),]- est employt, sugg6rant une plus grande association
des ions lors de la polymkrisation.
La formation de borates amino-substituts a tt6 tentee par Gaction d'une vari6tt5
d'amidures de lithium avec ie B(C6F,),. La rtaction d'amidures N-perfIuorophCny1-
substituds avec deux Cquivalents de B(C,F,), a conduit ;i la fomation de LiB(C,F,), et de
R(C,F,)NB(C,F,), (oh R = CH?Ph, CH(CH,)Ph). Ceci se d d l e une nouvelle rnethode
pour la synthkse d'acides de Lewis chiraux tels que a-C,H,CH(CH,)(C,F,)NB(C6F5)2.
iii
ACKNOWLEDGEMENTS
The work outlined in this thesis could not have been completed without the support,
both personal and professional, of numerous people. So before getting into the chemistry,
let me be human, for one page only.
First and foremost, I would like to thank Dr. Karnyar Rahimian whose great
knowledge of chemistry, witty sense of humour, amazing cuisine, good company on
Sunday and Monday night football games' and friendship changed my life forever. I love you and I thank you from the bottom of my heart, Special K. May our friendship last
forever.
Secondly, the people from our very own Jean-Coutu: Danny Lafrance, Ngiap Kie Lim, Jason Davis (who proof-read this thesis, thanks), Rania Dghaym as well as our
newest acquisitions, David Llewellyn and Joyce Hung; Karin Yaccato, who commented on this thesis and whose friendship I appreciate, thank you: Dan Adamson, thanks.
Some very dear friends of mine: Lili, Francois. Marc, Bibi, Grace, mon duo de
I'UdeM (Seb et Isabelle). Pour ma soeur MyIkne et Pierre. mon piire Jean-Pierre et
Anne-Marie, toute mon affection.
I would also Like to acknowledge those great people I met. who served as role- models, mentors and friends : Dr. Suzanne Black, Dr. Ron Brown and Dr. Ashok Kakkar.
On a professional note, I would like to thank first my supervisor, Dr. Bruce A.
Arndtsen as well as: Dr. Vladirnir Dioumaev, Dr. Kamyar Rahimian, Dr. James L. Gleason and Dr. Michael Baird; for helpful advices, guidance and willingly sharing their chemistry
tricks. I would also like to acknowledge the financial support of FCAR for a scholarship
and NSERC for the funding of the project. Elemental analyses were performed at
Universite de Monwal by Isabelle Dubuc.
I would like to dedicate this work to Kamyar without whom I would not have made it
through ... Thank you @
TABLE OF CONTENTS
CHAPTER ONE . INTRODUCTION TO ZIEGLER-NATTA CATALYSIS
1 . 0 . Perspective ........................................................................... 1
1 . I . Discovery .............................................................................. 1 . .
1 . 2. Mechanism of Polymensation ...................................................... - 2 ... .................................. 1.3. Catalysts in Ziegler-Natta Polymerisation .. 4
1 .3. 1. Metals in Heterogeneous Ziegler-Natta Catalysis .......................... 5
1 .3. 2. Metals in Homogeneous Ziegler-Natta Catalysis .......................... 6
1.3.3. Co-Catalysts: Generation of the Active Catalyst ........................... 6 1.4. Olefin polymerisation by Ziegler-Natta catalysts .................................. 9
......................................................... 1.4.1. PolymerisableOlefins 9
1.4.2. Conditions of Reaction ...................................................... LO 1.5. Tacticity in Poly-a-Olefins ........................................................ 1 1
1 S .1 . Definition ............................. ,,, ................................ I 1 1.5.2. Tacticity Control Mechanisms .............................................. 12
1.5.2.1. Chain-Endcontrol .................................................... 12
1.5 2.2 . Enantiomorphic-Site Control .......................................... 13
.......... 1.6. Ancillary Ligand Design in Homogeneous Ziegler-Natta Catalysis 15
1.7. Non-Coordinating Anions in Ziegler-Natta Catalysis .......................... 18
1 .7.1 . Mechanism of Ion Association ............................................. 18 .................. 1 -7 .2 . Review of Non-Coordinating Anions as CoGitalysu 19
...................................................................... 1.7.2.3. BR, 21
........................................ .................. 1.7.2.4. .BR, ....... 2 2
.................... .......**...........*.......*....**..... 1.7.2.5. .AIR, X ... 23 . .................... 1 . 7 .3 Anions Effects in a-Olefin Polymerisation Activity -24
1 .7.4. Anions Effects on Tacticity ................................................. 26
1.7.5. Importance of Anion Effects in a-Olefin Polymerisation .............. -27 REFERENCES ................................................................................ 28
CHAPTER TWO . PREPARATION OF ADDUCTS OF B(C6F5). AND THEIR REA- WITH
DIME-CON- AS CATALYSTS IN ZiEGLER-NATTA a-OLEFIN POLYMERIS ATION
............................................................................ 2.0. Objectives 34 ..................... 2.1. Approach to Anion Synthesis: Base Adducts of B(C6F, ) , 35
2.1.1. Review of Alcohol Adducts of B(C,F,), as Co-Catalysts ................................................. in a-Olefin Polymerisation 35
............................................................. 2.1.2. Alcohol Adducts 37 ........................ 2.1.2.1. Formation and Stability of Alcohol Adducts 37
................................. 2.1 .2.2. Reactivity with Dimethylzirconocene 38
.................. ................. . A CH,(CH,),CH,OHaB(C,F,),. 27 .. 39 ..................................... . B CH3CH,CH(OH)CH3.B(C6F5)3. 28 41
..................................................... . C Menthol@B(C,F,),. 26 42 ............................................................. 2.1.2.3. Conclusions 43
.............................................................. 2.1 . 3. Amine Adducts -44
.......................... 2 . L .3 . I . Formation and Stability of Arnine Adducts 44
................................. 2.1.3.2. Reactivity with Dirnethylzirconocene 46 .................................................. . A C,H,NH,.B(C,F,),. 40 -46
............................................... B . C,H,CH,NH,@B(C,F,),. 43 48 ................... ................. . C p.CF3(C6H,)NH2.B(C,F,),. 44 ... 48
.................................... . D a- C,H,CH(CH,)NH,eB(C,F ,),. 45 49 .................................................... . E C6F,NH243(C,F,)3. 46 49
. .............*.............. . . . F a.C,H,CH(CH,)N(C,F,)H. 47 ... 50 G . a- C,H&H(CH,)N(2.4.(N02)2(C6Hj))HaB(C6F5)3. 49 ............. 50
............................................................. 2.1.3.3. Conclusions 51 ....................................................... 2.1.4. Olefin Polymerisation 52
.............................................. 2.1.4.1 . 1 -Hexene Polymerisation 52 ............................................. 2.1.4.2. Propy lene Polymerisation 53
............................................................. 2.1.4.3. Conclusions 55 ................................................................................ REFEWCES 56
CHAPTER THREE . FORMATION OF CHIML BORON LEWTS ACIDS
. ....*.................................*..................... 3 .0 Review of Borate Salts - 5 7
3. L . Formation of Lithium Amides ..................................................... 58 3.2. Reactivity of the Lithium Amides with B(C$,), ............................... 59
. ......................................................... A C,HFH(CH3)NHLi. 51 59
. B C,H,NHLi. 52 ................................................................. 59
C . C.H,CH,NHLi. 53 ............................................................... 60 D . C,F,NHLi. 54 ..................................................................... 60
E . Me,NLi. 55 .................................................................. 60
................. F . C,H,CH,N(C6F5)Li. 56; a.C,H,CH(CH,)N(C,F,)Li. 50 - 6 1
3.3. ConcIusions .............................~....~.~.~................................. 6 2 REFERENCES ................................................................................ 64
EXPERIMENTAL SECTION
1 . General .................................................................................. 65
2 . Olefin Polymerisation ................................................................ 65 3 . Experimental Details ................................................................... 66
REFERENCES ............................................................................... -74
LIST OF TABLES
........ . Table 1 Transition metal complexes in heterogeneous Ziegler-Natta catalysis 5 ............................. . Table 2 Examples of co-catalysts in Ziegler-Natta systems 7
. ................................ Table 3 Olefins polymerised by Ziegler-Natta catalysis 10
....................... . Table 4 a-Olefins polymerisation activity of differents anions 24 .......... . Table 5 Isotacticity in propylene polymerisation by zirconium complexes 26
......................................... Table6 . Amine precursors to B(C,F,), adducts 45
Table 7 . Arnides precursors to borate trityl salts ......................................... 58
LIST OF FIGURES
..................................................... . Figure I Possible structures of MA0 -8 ......................................................... Figure 2 . Polymer-chain tacticity 1 1
............................ Figure 3 . Entering olefin with enantiomorphic-site control 14 Figure 4 . Various ligand frameworks leading to different stereoregulation .......... 15
....................... Figure 5 . Concerted transition state of Zr-MeB-Me exchange 19
......................................... Figure 6 . iMeletallocene-PBA anion interaction 24
LIST OF SCHEMES
Scheme 1 . Mechanism of Ziegler-Natta polymerisation ................................ -3
.................. Scheme 2 . Termination mechanisms in Ziegler-Natta polymerisation 3 ................ Scheme 3 . Reaction of trialkylalurninium with metallocene &chloride -7
................................... Scheme 4 . Formation of the active species with M A 0 8
Scheme 5 . Chain-end control in 1 . Zinsertion of propylene ........................... 12
Scheme 6 . a-Agostic interactions between dimethylziroconocene
and B (C,F,), ................................................................... 19
Scheme 7 . Expected influence of an asymmetric anion upon a-olefin ................................................................. polymerisation 34
Scheme 8 . Reactivity of H,O43(C6F5), with 4PtMe. complexes .................... 36 ................................................ Scheme 9. Formation of alcohol adducts 37
Scheme 10 . Decomposition path for DMZ and 27 ....................................... 39
Scheme I I . Decomposition path for DMZ and 28 ...................................... -41
Scheme 12 . Decomposition path for DMZ and 26 ....................................... 42 .................................................. Scheme 13 . Formation of arnine adducts 44
Scheme 14 . Polymerisation of 1 -hexene by the reference system ...................... 52
Scheme 15 . Polymerisation of propylene by the reference system ..................... 53 Scheme 16 . Displacement reaction of a perfluorophenyl group ........................ 62
Scheme 17 . Proposed influence on tacticity induced by the novel mine-borane Lewis acids in a-olefin polymerisation ................................. 63
CHAPTER ONE. INTRODUCTION TO ZIEGLER-NATTA CATALYSIS
1.0. PERSPECTIVE
The Ziegler-Natta polymerisation is a widely used industrial process for olefm polymerisation as both a homogeneous and a heterogeneous system. It produces linear
polyolefins of variable molecular weights and tacticities. Polyolefins are amongst the most
important materials in today's modem world. They find their application in many areas of
domestic, economic and technological interest [ 1, 21. Chemical inertness, lightness,
strength, durability and flexibility allow polyolefins to be shaped in every possible form
thus making them a material of choice for their versatility. With the industrial implementation of metallocene-based olefin polymerisation processes in major production facilities, the Ziegler-Natta catalytic system is undergoing yet another important phase of its
development.
The past few years have revealed many examples of rapid growth in homogeneous-
system applications. DuPont Dow Elastomers recently announced the start-up of a
90-million m.t.lyear ethylene propylene diene monomer plant in Plaquemine, LA (USA).
Dow Chemicals introduced linear low-density polyethylene and plans to build a 500 million
US$, 1.2 1-million m.tJyear production plant by the year 2000. Union Carbide and Exxon
Chemicals recently launched a joint venture, Univation Technologies, and announced their
plan for building a world-scale plant in Texas (USA) to manufacture metaflocene catalysts.
Similarly, Dow Chemical and BP Chemicals have joined their efforts in licensing a
metallocene catalyst/gas-phase process. Such investments in metallocene-based polymerisation technology highlights the importance of this process and the beginning of a
new era in olefin polymerisation [I].
1.1. DISCOVERY
In 1955, Karl Ziegler of the Max Planck Institute for Coal Research in Miilheim,
Germany and Giulio Natta, a consultant for the Montecatini Company in Italy, revealed a
new catalytic pathway to polyrnerise olefins [Z]. Ziegler's contribution lies in the discovery
of metal-catalysed poly-insertions, whereas Natta contributed to the stereoselective polymerisation of a-olefins. Prior to this discovery, olefins were polyrnerised via
conventional cationic, anionic and free radical initiation.
In 1953 Ziegler, using a heterogeneous mixture of TiCl, and AlEt, under a low
pressure of ethylene, successfully synthesised high density polyethylene [3] (Eq. 1). Shortly thereafter, Natta used the same type of catalyst to polymerise other a-olefins (propylene, I -butene and styrene) into isotactic polymers [4].
Other major discoveries emerged from Natta's group such as the synthesis of highly
syndiotactic polypropylene at -78°C with AIEtCI and VCI, 151; the copolymerisation of
cycloolefins (cyclopentene and cycloheptene) and ethylene [6] and the stereospecific
homopolymerisation of cyclopentene [7]. This changed, broadened and enriched the field
of polymer catalytic science in such an indisputable manner that the Royal Academy of
Science of Sweden awarded the 1963 Nobel Prize for Chemistry to both Professors Ziegler
and Natta.
Broadly speaking, the first so-called Ziegler-Natta catalysts consisted of a binary
mixture of metal alkyls or alkyl halides of group I-III base meds and transition metal salts
(halide, alkoxide, alkyl or aryl) of group IV to Vm [2,8].
1.2. MECHANISM OF POLYMERISATION
The mechanism by which Ziegler-Natta catalysts polymerise olefins is still a matter of
debate and a number of mechanisms have been proposed in order to explain this process.
Among these. the most plausible mechanisms feature the propagation step taking place at
the transition metal-carbon bond [2].
Cunent evidence strongly supports the mechanism developed by Cossee in the
1960's [9-141. Cossee's mechanism requires two initial conditions to be fulfilled. First,
the active complex must contain at least one transition metakarbon or transition
metal-hydride bond. Second, an open coordination site must be present or formed at the
transition metal centre, in the course of the reaction. The catalytic system is generally
referred to as the catalyst and the co-catalyst which will, upon reaction, form the active
complex. The polymerisation itself, takes place via a two-step mechanism (Scheme 1)
WI*
H2(3=CH2
step 1 : [Ln.<J+-~ - [ q + J + - A
Step 2 :
Scheme I. Mechanism of Ziegler-Natta polymerisation
The first step involves the complexation of the olefin at the electron-poor transition
metal's vacant coordination site. The second step consists of the migratory insertion of the
complexed olefin into the transition metal-carbon. or hydride, bond. The latter step regenerates a vacant coordination site thus allowing the process to continue, hence formation and growth of the polymer chain. Self-termination of the chain-growth process
can occur via 3 mechanisms (Scheme 2) [8, 15-17]. The use of an added transfer agent,
such as hydrogen, provides a fourth mechanism.
(P = polymer chain)
R R
Scheme 2. Termination mechanisms in Ziegler-Natta polymerisation
The first termination mechanism is b-hydrogen transfer to the metal centre, or 8-H elimination, producing a 1,l -disubstituted olefin terminated polymer. The second one is p- alkyl migration and produces an ally1 terminated polymer. These two mechanisms
regenerate a metal-carbon or metal-hydride bond, thus an active catalytic species. Finally, another termination process is chain transfer onto the co-catalyst (transmetallation). This
leads to a co-catd y st-terminated polymer and the regeneration of a metal-carbon bond.
Presence of hydrogen as a transfer agent is also a known termination path and leads to a saturated end-group and an active metal-hydrogen centre [17]. This is often used as a mean to control molecular weight. These termination processes are important as they determine
the length of the polymer chin which can be expressed as in equation 2.
Chain length a Rate of propagation (Eq. 2) Rate of termination
1.3. CATALYSTS IN ZIEGLER-NATTA POLYMERISATION
The reactions occurring between the catalyst and the co-catalyst in order to create the
active species strongly depend on the species initially present. The Ziegler-Natta
polymerisation reaction can be performed with both heterogeneous and homogeneous
systems. While the first catalytically active systems discovered were heterogeneous, active homogeneous systems were later discovered as a result of the effort to understand the
mechanism of polymerisation through models of the heterogeneous systems of group IV metals and their co-catalysts [ I 81.
Although it has long been suspected that the catalytically active system involves an
active cationic species, Jordan and co-workers [19] isolated the fiat cationic group IV metal complex Cp,ZrMe(THF)I4 BPh,' (1) and demonstrated its ability to polymerise ethylene.
This was the first strong evidence for a cationic metal centre containing an alkyl bond which was catalytically active in Ziegler-Natta catalysis. Since then, many studies confirmed this
condition and it is now generally accepted that the active catalyst involves a metal cation
containing an alkyl group and an empty coordination site [8, 15- 17,20,2 11.
1.3.1. Metals in Heterogeneous Ziegler-Natta Catalysis
The variety of metals explored in heterogeneous Ziegler-Natta catalysis is very broad.
Initial research by Ziegier involved the very active titanium catalysts as well as Lithium and
aluminium alkyls. Similarly, Natta studied vanadium, molybdenum, tungsten, chromium and zirconium. Other heterogeneous catalysts include zirconium oxide, strontium oxide,
thorium oxide, nickel and cobalt as well as copper and iron. An exhaustive investigation of
the reactivity of the transition metals was performed by both Ziegler and Natta in the early
stages of the discovery, rapidly followed by many teams of researchers [22]. Table 1
presents an overview of the non-metallocene transition metal compounds used to
polymerise a-olefins from the early Ziegler-Natta era to the present [2]. While the co-
catalysts will be discussed in a following section, it is important to underline the fact that
not all combinations of metal-compound and cocatalyst are active in olefin polymerisation.
Some are effective only for certain monomers or under certain conditions.
Table 1. Transition metal complexes in heterogeneous Ziegler-Natta catalysis
Halogenated Alkyl r-ally1 acac Oxides/ complexes complexes complexes complexes Metals
VCI, (n=3,4) Vocl ,
VO(0E t)p
FeX, (X=Cl, Br; n=2,3)
Cr(n-ally l),
Mo(lr-all y l)
Ni(x-ally l)X (X=CI, Br)
Zr oxide
Cr oxide
Mo metai
Co metal
Ni oxide Ni metal
1.3.2. Metals in Homogeneous Ziegler-Natta Catalysis
Advances in research have now brought the focus on homogeneous systems such as
the very active group IV metallocenes (Ti, Zr, Hf) and related compounds of group III (Sc,
Y) and lanthanum and actinium [17,21,23]. Even more recently, group VIII metals have been used in olefin polymerisation with quite promising results [24]. The late transition-
metal catalysts show at least one major advantage over group N metallocenes. Their
moderate reactivity toward oxygen allows it and other functional groups, including polar
monomers (e.g. acrylates) to be tailored onto an ethylene backbone thus producing novel
ethylene copolymers [ 1 1.
Various metallocenes of the following transition metals have been used in homogeneous Ziegler-Natta cataiysis : Sc, Y, Ti, Zr, Hf, Nb, Ru, Pd. Rh, La, Nd, Gd,
Dy, Ho, Er, Tm. Yb, Lu, Cr. Group III and IV metallocenes have received a great deal of
attention due to their high activity. Although the variety of ligands on these metallocenes is
very diverse, they all follow the general formula of Cpt2MC12 (2) or Cpl,MR, (3) (where
Cp' is a cyclopentadienyl-related ligand and R is an alkyl group) for group N metals and
above. The few exceptions to this general trend and the effects of the ligands on the
catalysts properties are discussed in section 1.6.
1.3.3. Co-Catalysts: Generation of the Active Catalyst
The co-catalyst is defined as the species that, upon reaction with the metal centre, will
lead to the formation of the active catalpc species. The co-catalysts for Ziegler-Natta polymerisation can be divided into two categories: alkylation/complexation agents and
weakly coordinating anions. Alumoxanes are the most important among
alkylation/cornplexation agents and methylalumoxane (MAO) is by far the most widely
used. Prior to the discovery of MAO, aluminium akyls and alkyl aluminium chlorides acted as co-catalysts [2]. The use of triethylaluminium (AEQ was a key step in Ziegler's
experiments (31. Later on, derived co-catalysts such as AlEGCl, ALEbOEL, Al-i-Bu,, were
used by Natta and co-workers [5 ,25] . Other group I to III base metals -1s or hydrides
have been investigated but to a lesser extent. They are listed in Table 2 [2].
Table 2. Examples of co-catalysts in Ziegler-Natta systems
I Group I Group II Group III I
(R = alkyl, aryl, hydride; X = halide)
L 4
active catalyst
Scheme 3. Reaction of trialkylaluminium with mefallocene dichloride
Trialkyl aluminium co-catalysts react with the metal chloride as described in this typical reaction of an homogeneous catalyst (Scheme 3) [IS]. The complexation step arises
from the Lewis acidity of the aluminium centre. The formation of the adduct Cp,Ti(R)- C W C l leads to the creation of an electron-deficient metal centre which will coordinate an incoming olefin and begin polymerisation (Scheme 1).
Currently, one of the most commonly employed cocatalyst is methylalurnoxane (MAO). MA0 is the product of the hydrolysis of trirnethylaluminiurn. It is a poorly-
defined oligomer containing 6 to 20 units of -0-Al(Me)-. Figure 1 shows two possible
structures of this reagent [8]. The formation of the active centre with MA0 proceeds according to Scheme 4 [26], where MA0 becomes a weakly coordinating anion.
Linear
Figure 1 . Possible structures of MA0
Step 1 : Alkylation
Scheme 4. Formation of the active species with MA0
The second category of co-catalysts further develops this idea of weakly coordinating anions. The well-defined anions can only be used with metal alkyi complexes as they will not alkylate the metal centre. Three procedures have been adopted in the generating of a
non-coordinated anion. Marks and co-workers [27] made use of a strong Lewis acid
B(C,F,),, to abstract a methyl group from the metal centre (Eq. 3).
The second method, developed by Chien and co-workers 1281 involves the reaction
of the metal-alkyl with a salt of the type [Ph,C][B(C,F,)J (Eq. 4).
The third method resembles the previous one as it also involves a perfluorinated
borate salt. Developed by Hlatky, Turner and co-workers [29], it uses a dimethylanilium cation which will protonate off a methyl group thus generating methane as well as the
empty coordination site (Eq. 5).
The use of salts as co-catalysts (Eq. 4 and 5) produces very weakly bonding anions and the desired cationic metal centre thus generating well-defined catalytic species. The concept of these weakly coordinating anions in Ziegler-Natta polymerisation will be
developed in detail in section 1.7.
1.4. OLEFIN POLYMERISATION BY ZIEGLER-NATTA CATALYSTS
1.4.1. Polymerisable Olefins
There has been multiple attempts to polymerise olefins through Ziegler-Natta
catalysis. Some were successful, polyethylene and polypropylene are patent proofs of this
success, while others were disappointingly unreactive. A variety of co-polymers have also been synthesised, mostly ethylene co-polymers. Table 3 illustrates this reactivity probing
over a few olefins including a-olefins, dienes, cycloolefins and substituted olefins.
Table 3. Olefins polymerised by Ziegler-Natta catalysis
Ethylene f -Pentene 2-Pentene
3-Methyl- 1 -pentene Butadiene
Cyclobutene Cyclopen tene
4-'buty ldimethylsiloxy- 1 - pen tene
Propy lene 1 -Hexene Styrene
&Methyl- 1 -pentene 1,5-Hexadiene
Viny lcyclopropane Norbornene
5-N,N-diisopropylamino- 1 - pen tene
I -Butene 2-Butene Isoprene
4trimethylsiloxy- 1,6- hep tadiene
1.4.2. Conditions of Reaction
Besides the choice of catalyst and cocatalyst, a number of experimental factors will
affect the overall outcome of the reaction. All polymerisation reactions require an inert atmosphere of Nz or Ar and the absence of reactive polar impurities. The presence of
oxygen, water or sulphur-containing impurities can h a t i c a l l y afkct the activity of the
catdyst and therefore must be rigorously controlled. Otherwise. most of the catdyst will be
used up as an impurity scavenger.
The choice of solvent depends on the catalyst and the conditions of polymerisation.
In general, aliphatic and aromatic hydrocarbon solvents are preferred. It has been reported [30] that aromatic solvents such as benzene and toluene can sometimes lower the activity of
a cationic catalyst by decreasing its electrophilicity through x-coordination at the vacant
coordination site. Dichloromethane is also commonly used in the study of catalytic system
interactions as it usually does not interact with the cationic centre and favours the charge
separation due to its greater polarity [3 I].
The concentration of catalyst and co-catalyst varies according to the type of
co-catalyst. It has been found that, with MAO, the ratio of [Al]/[Zr] can range from 80 to 75 x lo3 [2] with a widely preferred ratio of 1000. This excess of MA0 is believed to be
necessary to insure complete akylation of the metal [27,30,32], when the ratio of A1 to Zr is lower then 300, no catalytic activity is generally observed. On the other hand, no significant increase in activity is observed when the ratio exceeds 700. The use of Lewis acids (Eq. 3) or weakly coordinating anions (Eq. 4 and 5) allows the use of equimolar ratios of both catalyst and cocatalyst.
1.5. TACTICITY IN POLY-a-OLEFINS
1 . 5 1 Definition
Tacticity is the term used to describe the stereoregularity in a polymer chain structure.
It is best defined when using polypropylene as an example. There are three main types of
structures and they are illustrated in Figure 2 [33,34].
1) Atactic : a random arrangement of methyl groups about the tertiary carbon.
2) Isotactic : the same configuration of the methyl groups, on the same side of the backbone plane, about each tertiary carbon.
3) Syndiotactic : an alternating configuration of the methyl groups about each tertiary
carbon with respect to the backbone plane.
Figure 2. Polymer-chain tacticity : (1) atactic; (2) isotactic and (3) syndiotactic
In both isotactic and syndiotactic configurations, the polymer chains pack more
closely and more uniformly. leading to a high degree of crystallinity. For example,
isotactic polypropylene has a melting range of 16& 170°C whereas syndiotactic
polypropylene melts at 138OC [33!. This leads to a greater strength and rigidity in
polypropylene which can find a wide range of applications. Atactic polypropylene has an
amorphous structure, little strength, is soft and tacky and therefore has a melting point
below room temperature. Commercially, both types of polypropylene, tactic (crystalline)
and atactic (amorphous), have applications. Traditionally however, only resins of a single
form are utilised, and mixed resins, with a large percentage of both amorphous and
crystalline content, have had little commercial value [33].
The various stereo-isomeric configurations, atactic, isotactic and syndiotactic, can be
distinguished and quantitatively analysed by 13c NMR of polymer solutions by the
difference of nuclei coupling with neighbour methyl of the various configurations [34].
The chemical shifts of "C nuclei are sensitive to both the chemical nature and the
geometrical arrangement of the neighbouring atoms over several bonds. Thus in
sufficiently dilute solutions and at sufficiently elevated temperature, separate signals of
carbons in various surroundings can be resolved. Analysis of polymers' pentads, sequences of five carbon atoms, have been performed using such statistical treatment.
Physical methods such as X-ray diffraction, differential scanning calorimetry, optical microscopy and density gradient measurements allow to measure properties related to
crystallinity [34-361.
1.5.2. Tacticity Control Mechanisms
In homogeneous Ziegler-Natta catalysis, tacticity in the polymer is a result of two effects. Chain-end control arises from the growing polymer chain while enantiomorphic-
site control is provided by the catalytic centre and more specifically by the sterics at the
insertion site [B, 16, 171.
1.5.2.1. Chain-End Control
The first stereochemical control mechanism involves the stereochemistry of the chiral Bsarbon of the growing polymer chain at the insertion step. and is known as "chain-end
control" (Scheme 5) [IS]. This type of stereochemical control usually occurs at very low
polymerisation temperatures. at which the polymer chain becomes rigid [8]. Under these conditions, the incoming olefin will minimise the steric interactions with the side-groups of
the Iast added monomer (at the p-carbon for 1.2-insertions) into the polymer chain.
Scheme 5. Chain-end control in 1,2-insertion of propylene
1.5.2.2. Enantiornorphic-Site Control
The second stereochemical control mechanism involves a "catalytic -enantiomorphic-
site control" where the insertion site, the metal centre, defines the stereoregularity of the
polymer chain. This control can be achieved by changing the symmetry of the ancillary
ligands at the metal centre.
Homogeneous organometallic catalysts can be designed to create a stereospecific
control of the polymerisation reaction as a route to produce isotactic polypropylene. Chiral metdlocene catalysts became available around 1980 [37]. First synthesised by Brintzinger
and co-workers and featuring ethylene-bridged ligands, their ch id structure of
Cisymmetry, was retained even under catalytic conditions. These bridged metallocenes
are commonly known as msa-metallocenes [38]. The bridge can also be formed through a
SiMe, unit; a variety of substituted metallocenes has been explored this way.
The origin of the stereochemical control of chid ma-metdlocenes over the polymer
growth is reasonably well-understood [39]. Experiments [40-43] and molecular mechanics
calculations 1441 have shown that repulsive interactions force enantiofacid approach of the
olefin toward the metal-alkyl unit, placing the olefin substituent tram (i.e. away from) to
the p-C atom bound to the metal. This is true for the second olefin insertion. into the
metal-polymer chain bond, the fiat insertion between the metal and methyl bond showing
no stereoselectivity [ 161.
Coordinatioll iif the olefin followed by the migration of the dkyl chain and insertion gives rise to an alternating position of coordination site, on each face of the metal centre.
The growing chain then has to orient itself respectively to the rigid ancillary ligand at its
new position. The metal-alkyl (polymer) chain, or at least its C(a)-C(P) segment, appears
to orient itself in a manner as to avoid interaction with the CS ring substituents. The entering olefin is then forced into an orientation in which the two alkyl substituents at the
incipient C-C bond are trans to each other, Figure 3 represents this situation.
This process repeats itseif as the chain moves back and forth across the metal centre,
thus imposing stereoregularity at each insertion of a monomer. Rigid q-symmetric metallocenes force these alternating stereoregular insertions and thus leads to isotactic
poly-a-olefins.
(P = polymer chain)
Figure 3. Entering olefin with enantiomorphic-site control
Figure 4 depicts how rigid am-metallocenes, by their ligand framework, impose an
insertion stereochemistry and lead to a control in polymer tacticity [ 151. Catalyst ( 1) has a bulky ligand at the bottom and a relatively small Cp at the top, this configuration forces an approaching propylene to orient its methyl group in a manner to avoid interactions with the
bulky lower ligand, thus having it oriented upwards. Since the growing polymer chain
alternates position on the metal centre, this leads to an alternating regularity on the polymer
chain or syndiotacticity. Addition of a methyl group on the Cp ring in catalyst (2) increases
its bulkiness on one side, propylene will then orient itself upwards when approach the other
side and will have no preference when approaching this side because it will have to interact
with one ligand or another. This leads to an hemiisotactic polymer, a polymer that has
stereoregularity at every other tertiary carbon. If the methyl group on the ligand is replaced by a tert-butyl group (catalyst (3)), the propylene will avoid this strong steric interaction by
orienting itself towards the arene ring of the fluorenyl ligand (bottom). This latter conformation will force the monomer to alternate its orientation at every insertion and leads
to isotactic polymer.
Figure 4. Various ligand frameworks leading to different stereoregulation ( I ) syndiotactic, (2) hemiisotactic and (3) isotactic polypropylene
1.6. ANCILLARY LIGAND DESIGN IN HOMOGENEOUS ZIEGLER- NATTA CATALYSIS
The observation that variation of ligand steric and electronic properties on do, group IV metal complexes has triggered investigations in polymerisation activity and tacticity. The first homogeneous catalysts employed in Ziegler-Natta polymerisation made use of the cyclopentadienyl ($-C,H,, Cp) ligand and were of the form CpWCI, (where M = Ti, Zr, Hf) [45,46]. Similarly, the dialkyl substituted metallocene, C p m - was prepared as well
as the pentamethyIcyclopentadieny1 (q '-C,M~,, Cp*) dialkyl metallocene. Electronic and steric influence of the substituted cyclopentadienyl ligands on the polymerisation outcome has been intensively studied [8, 15, 16, 17,20 and references therein].
Ligand systems other than bis-cyclopentadienyl type have also been examined for
their effects on activity, among them are Cp*MMe, 4 [47], chelating diamide complexes 5 [48,49], chelated amide Iigands 6 [SO, 5 11, macrocyles 7, 8 [52], Schiff-base complexes
9 [53], tridentate amide 10 [54] and bidentate aryloxides I1 [55, 561. While 4 and 5 polymerised a-oletins successfully, 6 can only polymerise ethylene with high activities. 7 and 8 present modest catalytic activities even though their geometry is similar to Cp
complexes. The Schiff-base complex 9 is a poor polymerisation catalyst. Complexes such
as 10 and 11 are moderately active in ethylene polymerisation but it is uncertain that these
complexes remain unchanged under reaction conditions. Thus, these systems appear to be
generally less active than the heavily patented bis-Cp complexes [IS].
9 M= Ti, Zr
The synthesis of the chiral, ethylene-bridged, onra-metallocenes using an indenyl
ligand, 12 and 13 [57,58] by Brintzinger and co-workers opened the door to a new type of ancillary ligands, leading to isotactic polymers [36]. When activated by MAO, they
catalyse the polymerisation of propylene with very high activities to isotactic
polypropylene. An onso-metdocene can have either C,, C, or C, symmetry, depending upon the substituents on the two modified Cp' rings and the structure of the bridging unit,
with each of these symmetries inducing varying tacticities in poly-a-olefins (section 1 S.2.2.). 12 and 13 have C,-symmetry.
Besides changing the transition metal (Ti, Zr or Hf), one can use a variety of
substituted Cp ligands (Cp'). Ewen designed a non-chiral, yet syndiospecific. catalyst due
to the very different sizes of its Cp' Iigands (Figure 4 (1)) [59]. The introduction of a
methyl group on the Cp' ring produced hemiisotactic polypropylene (Figure 4 (2)) 1601.
Notable improvements in stereoselectivity have been achieved by replacing the ethylene
bridge with a shorter dimethylsilyl bridge [61] and by placing substituents on the Cp'
ligand (Figure 4) [60,62].
Bridged complexes improve stereoselectivity due to their increased rigidity. The use
of a shorter silyl bridge further increases the activity most Likely because of the wider
coordination gap aperture [63]. The use of three or four-atom bridges have so far been
found to be practically inactive for propylene polymerisation [MI. Similarly, a ligand containing two silyl bridges has been synthesised by Bercaw and co-workers [65] and
found to have various degrees of polypropylene tacticity as a result of changing the
substituents of the Cp' ligands.
A number of recent reviews and papers deal with the trends in activity and
stereospecificity of different ligands [IS, 16, 20, 27, 50, 66-72]. The main conclusions
that can be drawn from these studies are :
I- The electrophilicity of the metal centre, thus its activity, strongly depends on the
nature of the ancillary ligands. Electron-poor metal centre show a greater activity in
a-olefin polymerisation.
2- The steric factors can be translated into accessibility of the metal centre which then
depends on the substituents position and size, on the ligand intrinsic size as well as
on the size of the bridge in ansa-metallocenes. h general, bulky ligands on the metal
centre will decrease the activity but increase the tacticity.
3- Ion pairing seems to strongly affect the reactivity of the cationic catalyst.
1.7, NON-COORDINATING ANIONS IN ZIEGLER-NATTA CATALYSIS
Although the Ziegler-Natta mechanism takes place at a cationic metal centre, it has
been observed that the name of the counteranion can dramatically influence the
polymerisation process [73]. The nature of the anion, its method of generation and its
degree of association with the metal centre are discussed below.
17.1. Mechanism of Ion Association
Formation of the cationic catalytic system through methide abstraction by the organo-
Lewis acid B(C,F,), has been achieved by Marks and co-workers (Eq. 3) [26, 74. 751.
Spectroscopic and crystallographic data support a closely associated ion-pair. Firstly, a cation-anion dissociation/reorganisation exchange process occurs in such compounds
which causes broadening of the Cp' ligand resonance in the 'H NMR spectrum. Secondly,
a Zr-MeB-Me exchange process causes broadening of both Cp' and Zr-Me/B-Me signals
simultaneously. Results of a dynamic behaviour study are consistent with an essentially
concerted transition state (Figure 5) for the Zr-Me/E3-Me exchange process and a simple
intramolecular dissociation/reassociation process, predominantly B(C,F,), dissociative in
nature, for the ion-pair reorganisation process.
Figure 5. Concerted transition state of Zr-MeB-Me exchange
Solid state and solution structure determination for such an ion pair show evidence
for a weak anion-cation interaction through the hydrogen atoms of the bridging methyl
group in non-polar aromatic solvents [67,74,75]. Other studies [47h, 76, 771 support the
existence of a-agostic interactions through two bridging hydrogens (Scheme 6).
Scheme 6. a-Agostic interactions between dimethy lziroconocene and B(C6F,),
Well-defined ion pairs systems are obtained when the cocatdyst acts as an proton-
donor. such as an ammonium species, which will induce the formation of methane via
protonolysis of a methyl group from the metal complex (Eq. 4). Alkyl abstraction by the
co-catalyst, such as uityl salts of B(C6F,),- (Eq. S), also cleanly cleaves a methyl group
from the med complex forming an inactive alkane in solution and a well-defined metal cation-counteranion system in solution. The reactivity of a variety of counteranions with
metallocenes has been examined and is reviewed in the following section.
1.7.2. Review of Non-Coordinating Anions as Co-Catalysts
The existence of a tight ion-pair and its influence in catalytic reactions have brought a
great deal of attention to the choice of non-coordinating anion in many processes [78-821. The anions studied in Ziegler-Natta polymerisation can be divided in the five categories
below.
1.7.2.1. -M X, (M= B, P; X= halide, aryl)
Classic non-coordinating anions such as BF; and PF, are known to undergo
fluorine transfer reactions to metdlocene cations [83] or demonstrate little lability [28, 84,
851. Bulkier -BPh, has been studied and shown to undergo degradation or to show very
little catalytic activity due to q'- or q3- coordination of a phenyl ring to the metal centre (14) [28, 85, 861. Derivatives of -BPh, such as -B(C,H,R), where R= Me, Et, F, as well as
BPh, itself, decompose when reacted with metallocenes, forming a zwitterion of the type 15 (R= H, Me, Et) or a fluorine-coordinated adduct (16) (R= F) whiie generating methane
gas.
1.7.2.2. Carboranes
Other compatible types of anions are the large, chemically inen carboranes. Acidic
C,B,H13 (Eq. 6 ) and (C,B,H,,),M (M= Fe, Co, Ni) (Eq. 7) have been used in combination
with metallocenes and have demonstrated their ability to form a catalytically active species for ethylene polymerisation [28, 871. However, these species are often generated in the
presence of a Lewis base, dimethylaniline, which is coordinated to the metal centre at low
temperature (-80°C). As temperature is increased, an exchange equilibrium between the
free amine and the anion takes place.
Cp*~ZrMez + C2BgH13 - Cp*2ZrMe[C2BgH12] + CH4 (Eq. 6 )
1.7.2.3. BR,
The use by Marks and co-workers of the strong Lewis acid tris(pentafluoropheny1)borane has triggered a number of closely-related research projects.
Acting as an alkyl abstractor, B(C,F,), is now widely used as a reference system in
a-olefin polymerisation.
A similarly electrophilic compound, HB(C,F,), has been synthesised [88]. Although
its reaction with DMZ in benzene leads to a mixture of products, reaction in hexanes results in evolution of methane and isolation of product 17, trapped by trimethylphosphine. Furthermore, if an excess of borane is used in benzene, unusual products such
Modification of the pemuorophenyl substituents was recently achieved by Marks and
co-workers [75,89,90]. The new, sterically encumbered tris(2,2',2"-perfluorobiphenyl)-
borane (PBB) (M), was reacted with group IV metallocene methyls and cleanly formed the
cationic complexes upon abstraction of a single methyl group. Spectroscopic evidence
supports a loose ion-pair in this type of complexes. However, when reacted with group IV metailocene dimethy Is, PBB generates cationic dimeric p-Me complexes (2 1 ), reflecting its
reduced coordinating ability.
Novel weakly coordinating borate anions have similarly demonstrated their ability to
form the catalytically active system when reacted with metallocenes. Modification of
B(C,F,),- with Iipophilic, stericdly shielding, R-electron withdrawing protective groups
formed B (C,F,TBS); (22) and B (C,F,TIPS), (23) where TBS = ten-butyldimeth y M y 1 and TIPS = triisopropylsilyl [75]. These react with dimethylzirconocene to cleanly generate
the alkyl rnetallocene cation. the borate anion and Ph,CCH,.
Another approach to borate complex formation involves the reaction of B(C,F,), adduct of alcohols, silanols, rnercaptans and oximes with triethylamine to form salts as in
(Eq* 8) [9 1 I -
Since the amine serves only as a proton carrier, base-free reaction can be performed directly from the adduct and the metallocene dialkyl (Eq. 9). However, this system readily
undergoes degradation via transmetallation (Eq. 10).
Borane adducts of imidazole [92] have also been shown to react with DMZ (Eq. 1 I )
however no catalytic reactivity study was performed with complex 24.
The perfluoroaryl aluminate anion (PBA-) of composition (C,,F,),A.lF co-exists as a uity 1 salt, Ph,C+ PBA- (25) [go].
Its reaction with metallocene dialkyls cleanly generates cationic complexes which
show some degree of M+---F-Al- interaction (Figure 6), diminishing with increasing
ancillary ligand steric bulk.
gure 6. Metallocene-PB A anion interaction
1.7.3. Anions Effects in a-Olefin Polymerisation Activity
Since polymerisation conditions vary depending upon the systems examined, it is
difficult to compare the activities of one anion over an other unless there has been a reference system used by the authors. Marks and co-workers have had the most success in
synthesizing active anions and their results are discussed below [26.75].
Although the mechanism of association of B(C,F,), with metallocenes is still under
investigation, this co-catalyst acts as a well-defined and very active precursor to Ziegler- Natta catalysts. The same reaction carried out with the trityl salts of B(C,F,),- and of the
modified borate anions B(C,FJBS); (22) and B(C,F,TIPS)c (23) also provides a route
to active catalysts. Typical results of polymerisation reactions at 25'C in toluene with dimethylzirconocene under one atmosphere of a-olefins are presented in Table 4 [26.75].
Table 4. a-Olefins polymerisation activity of differents anions
Activity"
Ethylene (x lo6) Propylene (x 1 6 )
" g of polymer (mol of Zr)" Y1 atrn-l.
24
Comparison of the reactivity of the new borate anions (22 and 23) with MeB(C$J,
showed an increase in thermal stability, solubility and activity for ethylene polymerisation.
The relative coordinating ability of the series of fluoroarylborates follows the approximate order -MeB(C,F,), > -B(C,F,TIPS), = -B(C,F,TBS), > -B(C,F,), 1751. This trend is
representative of the reverse of the activity since the more coordinating the anion will be,
the less active the catalyst is, as the olefin will have to compete with the anion to occupy the
coordination site on the metal centre. The -MeB(C,F,), anion is less active due to both its size and coordinating ability. It is the only anion among the ones listed above to bear an electron-donating alkyl group and is believe to interact via a Zf-Me-B- contact as
mentioned in the previous section. The interactions between the metal centre and anions 2 2 and 23 are believe to occur through a fluorine atom as observed by I9F NMR whereas 'B(C,F,), is relatively labile and coordination to the metal is weak [75].
While many other precursors do not form the active catalytic species,
B(C,F,(2-C,F,))3, PBB, (20) and its catalytically active metallic dimer form
(Cp,ZrMe),Me+MePBB- (21) is at least as active as B(C,F,), in a-olefin polymerisation (4.8 x 10' g of polyethylene (mol of ~ r ) " h'l atm"), and yields higher molecular weight of polyethylene which is believed to be caused by a slow dimer dissociation. Reactivity with
constrained-geometry metal catalysts affords promising results for reactions at room
temperature [89] .
(C,,F,),AlF, PBA- (25) exhibits negligible ethylene pclymerisation activity at 25OC
when reacted with dimethyizirconocene. However, an increase of the steric bulk on the metal centre leads to a dramatic raise in ethylene polymerisation activity [93]. Activities
ranging from 1.33 to 690 x 104 g of polyethylene (mol of catalyst)-' h" atm-' have been
attained when Cp*,ZrMe, is used. Temperature increase also positively affects the catalytic
activity and molecular weight. Finally, propylene was successfully polymerised by 25 and an ansa-metallocene with an activity of 1.63 x lo4 g of polypropylene (mol of catalyst)'' h"
atm-I .
Polymerisation of 1-hexene by dirnethylzirconocene and the longchain alcohol
adduct (C,,H,OHB(C,F,),) at 0°C produced polyhexenes of M, = 8.3 x 10' g/mok. This
value is similar to the one obtained by Cp+QCI, and MAO. Replacing the Cp by bulkier
indenyl or fluorenyl ligands helps to prevent degradation by transmetallation and leads to an
increased molecular weight (8.6 x lo') [9 11.
1.7.4. Anions Effects on Tacticity
Isotactic polymers are obtained by enantiomorphic-site control using chiral ligands or msa-metallocenes. The degree of tacticity in poly-a-olefins shows a dependance with the
anion used in combination with the metal complex. Table 5 illustrates the differents tacticities found when various pairs of catalyst/co-catalyst are used in propylene polymerisation.
Table 5. Isotacticity in propylene polymerisation by zirconium complexes
Catalyst % [mmmm]
5.6
atac tic
atactic
atac tic
0.06
0.06
35
70
90
35
atistic
64
atactic
98
[Ref.] - 27
75
75
75
74
74
67
67
67
67
67
67
90
90
" AU polymerisations in toluene. R* = (lR,2S,5R) cyclo hexy 1.
As these results show, the use of B(C,F,), produces only atactic polypropylene in these systems. Similarly, when -B(C,F,), is used with dimethylzirconocene or roc-Me$i(Ind),ZrM%, no tacticity in propylene is reported. . On the other hand, the use of an other anion (25) leads to a great degree of tacticity even if used with the same
rac-metallocene. Furthermore, the use of Me2Si(Me,C,)(C,H,R*)ZrM~ as the catalyst and
-B(C,F,), as the co-catalyst, produces almost twice as much tacticity in polypropylene than when MA0 is the co-catalyst. Temperature also affects the tacticity, a decrease in
temperature will slow the reaction process and increase the degree of tacticity. Entries with
Me$i(Me,C,)(C,H,R*)ZrC& and MA0 illustrate this trend in tacticity as the temperature is
lowered from 25 to -45°C.
1.7.5. Importance of Anion Effects in a-Olefin Polymerisation
The findings discussed in sections 1.7.3. and 1.7.4. clearly demonstrate the importance of the role played by the anion in the cationic a-olefin polymerisation. Both the
sterics and electronics of the anion associated with the cationic metallocene catalyst have an
effect on the reaction outcome. The sterics partially account for the extent of contact
between the ion pair. Bulky substituents prevent a close approach of the anion, thus freeing the coordination site for the entering olefin and increasing the activity. Similarly, a stable anion bearing electron-withdrawing substituents is less likely to share its electrons with the cationic metal centre thus forming a looser ion pair and a more active metal centre.
Conversely, substituents on the anion can undergo secondary reactions with the metal
centre due to their proximity. Transmetallation, n- or q- coordination and protonation are reactions likely to occur between the metal centre and the anion.
The importance of anion association is clearly highlighted in this last section. However, still very little chemistry has been done in this domain in comparison to ligand
design. More research has to be undertaken in order to understand the details of the ion
pair association mechanism and influence upon the overall Ziegler-Natta polymerisation.
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CHAPTER TWO. PREPARATION OF ADDUCTS OF B(C,F,), AND THEIR REACTIVITY WITH DIMETHYLZIRCONOCENE AS CATALYSTS
IN ZIEGLER-NATTA a-OLEFIN POLYMERISATION
2.0. OBJECTIVES
In light of the observation (sections 1.7.3. and 1.7.4.) that the nature of the anion influences both the activity and the degree of tacticity in aslefin polymerisation, we believe
that the geometry of the anion is also likely to have a similar effect. An asymmetric anion,
i.e. with different substituents, or a c h i d anion, i.e. bearing c h i d substituents, is expected
to adopt a preferred conformation with respect to its cationic counterpart.
Asymmetry in the anion will cause both the steric and electronic factors to re-arrange
in order to maximise the stability of the complex. This last effect can greatly influence the
approach of an entering olefin at the coordination site as it will preferentially choose one
side over another. Scheme 7 represents this proposed "asymmetry effect" upon propylene
polymerisation. One can expect various degrees of tacticity as the symmetry around the anion is changed.
R R R
Scheme 7. Expected influence of an asymmetric anion upon a-olefin polymerisation
The goal of this project is to better understand the interactions existing between the cationic
Ziegler-Natta rnetallocene and its anionic counterpart. More specifically, we want to
determine the extent of the influence of a chid anion over the degree of tacticity induced in
pmpylene polymerisation. The size, the symmetry and the chemical nature of the anion
affect the degree of association and the stability of the catalytic species. A study of the influence of these three factors will be performed and the importance of each one will be
highlighted in a polymerisation perspective.
2.1. APPROACH TO ANION SYNTHESIS: BASE ADDUCTS OF B(C,F,),
2.1.1. Review of Alcohol Adducts of B(C,F,), as Co-Catalysts in amolefin Polymerisation
A. R. Siede and his team at 3M Corporate Research Labs. presented a novel
approach to making counteranions [ 1 ] (see section 1.7.2.4). Using the strong Lewis acid
B(C,F,),, they formed stable adducts which were reacted with dirnethylzirconocene to
generate a catalytic species. Their primary goal was to create lipophilic catalytic species that
would be soluble in neat olefins in order to have the highest concentration of monomer
possible thus reaching high molecular weight polymers. Attempts to modify the Cp ligands
yielded disappointing results and the use of solubilizing anions provided another simple
route to achieve their goal.
Formation of alcohol adducts proceeds in a I: 1 ratio with the exceptions of methanol
which coordinates in a 2: 1 ratio and water which combines in a 3: 1 fashion. These last two
compounds can easily be converted into stoichiometric complexes by adding equivalent
amounts of B(C,F,),. Complexes of silanols, mercaptans and oximes exhibit a
stoichiornetric coordination chemistry with the Lewis acid. The coordination of alcohols to
B(C,F,), greatly enhances the acidity of their proton which can. in turn, react with amines to form salts (Eq. 1) or react directly with dialkylmetallocenes generating methane and the
catalytic species (Eq. 2).
H0R.B (C6F5)3 + Et3N - [Et 3NH]+[ROB(~6;5)3]- (Eq- 1)
The use of large alcohols, for example R = C ,,H,,, formed salts soluble in toluene
and 1-hexene. The reaction product of alcohol adducts with an arnine and its subsequent
reaction with dialkylzirconocene forms a catalyst that polymerises I -hexene. The highest
molecular weight of polyhexene obtained in such manner is 8.3 x lo3 g/mole at O°C. The
use of a bulky fluorenyl ligand on the zirconocene increased the Y, obtained to 3.4 x 1 06.
However, the thermodynamically favoured alkyl-alkoxy exchange, transmetallation,
ultimately takes place and forms an unreactive alkoxymetallocene species (Eq. 3).
Finally, water adducts of borane can also catalyse the polymerisation of 1-hexene
when reacted with a dialkylrnetallocene. Eventually however, a second equivalent of
methane is formed along with a novel zirconoxyborane species [eq. 4, 5 and 61, unreactive
in Ziegler-Natta polymerisation. This species also features a bond between the metal centre
and an o-fluorine atom from a C6F, ring.
Similarly, Puddephatt and co-workers [2] used the water adduct of B(C6F,), to react
with PtMe,(bu,bpy - - ) where bu2bpy = 4.4'-di-ten- butyl-2%bipyridine. When the adduct is used as such, a methyl group is protonated off, forming CH, and
[P~M~(~UQ~~)]'[HOB(C~F~)~]- (Scheme 8) where the oxygen atom coordinates the metal
centre. This cationic platinum complex is an example of the recent advances in Ziegler-
Natta polymerisation by group Vm metal complexes. This complex can then further
coordinate a donor ligand such as CO, C2H, or PPh,. The same metal complex
(PtMe?(bu,bpy)) - also reacts with B(C,F,), where the Lewis acid abstracts a methyl group as it is known with group IV metdlocenedialkyls [3]. An identical behaviour is observed
when the cationic species F-PtMe]+[MeB(C,F,),]' are in presence of CO, C2H4 or PPh,,
coordination at the metal centre occurs.
Scheme 8. Reactivity of tEQ.B(C,F,), with kPtM% complexes
2.1.2. Alcohol Adducts
Alcohol adducts of B(C,F,), have demonstrated their ability to form a catalytically active system when reacted with dialkylmetallocenes, which polymerises a-olefins. We
propose to extend the application of these novel counterions to study the influence of
chirality of the anion upon polymer formation.
Since a variety of alcohols are commercially available, both achiral and ch id alcohols
were used to prepare adducts of the strong Lewis acid. The reactivity of these new adducts
with dimethylzirconocene was studied.
2.1.2.1. Formation and Stability of Alcohol Adducts
The formation of alcohol adducts proceeds via a straightforward complexation
reaction between the Lewis acid, B(C,F,),, and the Lewis basic alcohols, ROH, in pentane
at room temperature (Scheme 9).
Scheme 9. Formation of alcohol adducts
Alcohol adducts are isolated as solids aFter removal of the solvent under vacuum.
They are kept under nitrogen atmosphere as B(C,F,), is known to be hygroscopic [4, 51.
They are stable at -40°C for several weeks. The formation of the adducts is confirmed by
observation of the 'v NMlX upfield shift upon coordination. Free B(C,F,), shows fluorine resonances at 6- 129.1 (o-F), - 142.0 @-F) and - 160.3 (m-F) in C,D,. Typically, ortho-
fluorine signal is shifted by 4 to 5.5 ppm, para-fluorine is shifted by about 12 ppm and
meta-fluorine is shifted by 2 ppm upon coordination with an alcohol. Alcohols studied
indude the achiral 1 -butanol, and the chiral2-butanol and menthol.
Characterisation of the chiral menthol~B(C,F,), (26), a white solid, was performed by 'H, 13c and '? NMR as well as elemental analysis. The 'H NMR of the adduct of menthol.B(C,F,), shows two distinctive resonances at 65.14 (OH) and 3.57 (CHOH).
The 19F NMR shows 3 resonances at 6-133.1 (br, 2F, o-F), -153.6 (br, IF, p-F) and
-161.8 (br, 2F, m-F). The "C NMR shows 10 resonances at 615.0, 20.3, 21.4, 22.7,
25.4, 3 1.3, 32.8, 41.3 and 48.8 ppm corresponding to the carbons of the menthyl moiety. In addition to these, one broad resonance is visible at 682 pprn and five others are visible
between 6135 and 150 ppm which correspond to the perfluorinated phenyl rings. Other
alcohol adducts studied ( 1-butanol(27) and 2-butanol(28)) show analogous properties.
26 27
2.1.2.2. Reactivity with Dimethylzirconocene
The ability of such alcohol adducts (26,27,28) to form a catalytically active system
when reacted with dimethylzirconocene (DMZ) was investigated by 'H and I9F NMR spectroscopy. A stoichiometric amount of DMZ and ROHaB(C,F,), was reacted in C,D, at
room temperature and the spectrum was recorded within 10 minutes of mixing. In all
cases, the solution immediately turned yellow and some gas evolved upon mixing.
Reaction between DMZ and any given alcohol adduct initially gives rise to a cationic
zirconocene species (29) (Eq. 7). Analysis of the 'H NMR spectrum shows a singlet corresponding to methane at 60.15, as well as resonances (60.29, s, 3H) and (65.42, s ,
IOH) corresponding to the methyl and the Cp in 29 respectively. Other species are also
identified in solution and degradation mechanisms are discussed in the following sections.
Reaction of CpJrMe, with 27 in C,D, gives rise to the cationic species (29b) as
detected by 'H NMR ( 13% by NMR) and produces four other species 30 (70%); 3 1 (1 1%); 32 (6%) and 33 (Q%), characterised by their Cp resonance (30: 65.60; 31: 5.87;
32: 5.76 and 33: 6.05) (Scheme 10). The species 32 and 29b disappear within five
minutes while the others are more stable in solution. Identification of species 30 to 33 is discussed below.
Scheme 10. Decomposition path for DMZ and 27
Species (30 - 33) are the result of the degradation of 29b. 32 was identified as the alkoxymethylzirconocene [Cp,ZrMe(O(CH&H,)I species. arising from transmetallation,
believed to be one of the major degradation mechanism. Transmetallation can arise directly from the ionic species (Eq. 8) or from the reaction of the rnetallocene and the alcohol which
might exists in equilibrium both as an adduct and as a free species (Eq. 9).
A control reaction confirmed the nature of 32 as a stoichiometric amount of the
I-butanoi was reacted with DMZ and 32 was formed quantitatively (Eq. 10). The obtained
solution was colourless and evolution of gas was observed. 'H NMR identification of the species was performed and specific resonances for the alkoxymethylzirconocene [Cp,ZrMe(O(CH2)$H3) 1 are found at 65.77 (Cp), 3.80 (RCH,OZr) and 0.33 (CH3Zr).
30 was identified as the product of the reaction of 32 and free B(C,F,),, a novel
cationic alkoxyzirconocene species [Cp,ZrO(CH2),CH,]+[MeB (C,F3,]-. While 3 2 forms
via transmetallation and B(C,F,), is released in solution (Eq. 8 and 9), these two products
readily undergo further reaction leading to the formation of 30 (Eq. I I). NMR spectroscopy supports the existence of this species by a Cp resonance in the 'H NMR situated between the cationic 29b and 32. Furthermore, the "F NMR spectrum of the mixture reveals two types of perfluoroborane species. one free B(C,F,), (6-129, -142, - 160) and one coordinated (6- 133. - 159. - 165) whose shifts are similar to [MeB(C,F,),]-.
We can expect the electron-rich alkoxy group to stabilize the cationic zirconium and
therefore allow the free Lewis acid to abstract the remaining methyl.
33 was identified as a dialkoxyrnetallocene species, [Cp,Zr(O(CH,),CH,)J. Reaction of two equivalents of I-butanol with one equivalent of DMZ (Eq. 12) leads to the
clean formation of 33 in solution. The product is colourless, shows no methylmetal
resonance in the 'H NMR and displays a characteristic resonance for its Cp at 66.03.
Finally, 31 is believed to correspond to some 0x0-rnetallocene species. Reaction of
DMZ with a stoichiometric amount of water leads to the formation of species presenting similar resonances in the ' H NMR. Furthermore, the great oxophilicity of zirconium 0 complexes has been known to lead to products such as 3la and 3lb and other unidentified
Zr(IV) 0x0 complexes [6 - 81.
In conclusion, reaction of DMZ with 27 produces the active cationic alky~zirconocene
species (29b) in minority. These species degrade rapidly and formation of 0x0-
zirconocene (31) and a cationic alkoxyzirconocene species (30) is major. Both are inactive in Ziegler-Natta a-olefin polymerisation.
Reaction of Cp,ZrMez with 28 in C,D, offers a behaviour simiiar to 27 (Scheme 1 1 ) .
The cationic species (29c) is detected by 'H NMR (15% by NMR) as well as the four other
species 34 (64%); 31 (12%); 35 (8%) and 36 (4%) which present Cp resonances comparable to the ones listed in the previous section (A). The species 35 and 29c present the same instability in solution and disappear within five minutes.
Scheme 1 1. Decomposition path for DMZ and 28
A control reaction confirmed the nature of 35 as a stoichiometric amount of the
2-butanol was reacted with DMZ and 35 was formed quantitatively (Eq. 13). 'H NMR identification of the species was performed and specific resonances for the alkoxymethylzirconocene [Cp,ZrMe(OCH(CH3)CHJH3)] are found at 65.77 (Cp), 3.72
(R,EL,CHOZr) and 0.33 (CH,Zr), all comparable to 32.
Although the active cationic catalyst (29c) is initially formed, rapid degradation to inactive species occurs, making this system inactive towards a-olefin polymerisation.
As expected, the reactivity of the menthol adduct (26) with DhdZ presents similarities
with the previous alcohols (27 and 28) (Scheme 12). The cationic species (29a) (27% by
NMR) is identified by 'H NMR as well as 38 (29%) and 39 (44%). While the cationic
species (29a) readily disappears, the two other species are stable. As opposed to the two
previous alcohols, 37 and 31 are not detected in the solution mixture at room temperature.
Scheme 12. Decomposition path for DMZ and 26
Low temperature 'H and '? NMR of the same mixture in C,D, allows us to detect
37 (15% by NMR) and its counterpart, the free B(C,F,), as well as the cationic
&ox yzirconocene 38, [Cp@(menthoxy) ]+[CH3B (C6FJ3]- (8 5% by m).
The alkoxyrnethylzirconocene [Cp,ZrMe(menthoxy)] (37) was formed in a control
reaction between DMZ and menthol, similar to equation 13 and its characteristic 'H NMR resonances are 65.80 (Cp), 0.33 (CH,Zr) and 3.52 (CHOZr). This experiment confinm
the nature of the species formed at low temperature. The instability of this species at room
temperature probably arises from the bulkiness of the menthoxy group thus favoring the
abstraction of the zirconocene-methyl group by the free B(C,F,), and releasing the sterical
restriction. The cationic alkoxyzirconocene (38) so-formed is stable and has been detected
both at low and room temperature.
39 was further identified by a control reaction between two equivalents of menthol
and one equivalent of dimethylzirconocene. This reaction is rather slow as only the
alkoxymethylzirconocene (37) is initially formed. The bis-alkoxyzirconocene (39) does
not form instantly probably due to the steric bulk of the menthol cycle within the molecule
already bearing two cyclopentadirnyl ligand and one menthoxy group. However, within two hours, new 'H NMR resonances appear at 66.10 (Cp) and 3.60 (CHOZr) which can
be attributed to [Cp,2k(menthoxy),].
The presence of the Lewis acid B(C,F,), apparently increases the rate of formation of
the bis-alkoxy species (39). Since the alkoxyrnethylzirconocene (37) is not detected in the
reaction mixture at room temperature, it is believed that the formation of the cationic
alkoxyzirconocene (38) is rapid and this species is more reactive toward the alcohol and
readily forms the bis-alkoxyzirconocene.
Although the catalytically active cationic alkylmetallocene (290) is initially formed in solution, degradation to inactive species occurs rapidly. The decomposition leads to two
major products identified as 3 8, [CpzZr(menthoxy)]'[CH,B(C6F5)3]- and 3 9 , [Cp,Zr(menthoxy),]. Some minor impurities are also present and could not be identified.
2 J 0 2 A Conclusions
As detailed in the previous sections, the catalytically active species are formed upon
reaction between DMZ and an ROH.B(C,F,), adduct. However, its lifetime is very short
and degradation occurs rapidly. The species fonned upon degradation are unreaaive in
Ziegler-Natta polymerisation as they do not contain a metal-alkyI bond and an empty coordination site.
Degradation of the catalytic species yields a mixture of alkoxy-zirconocene
compounds among which two major products are formed; a cationic alkoxyzirconocene and
its counterpart, [CH,B(C,F,),]- as well as om-metallocene species. h the case of the
menthol adduct (26), degradation leads to a majority of bis-alkoxyzirconocene (39).
The rapid degradation apparently occurs via transmetallation. The relatively high acidity of the alcohol proton allows the required first step, i.e. protonation of a methyl
group and departure of methane gas. However, the pendant conjugated base RO- is not
very strong and does not form a strong bond with B(C,F,),. As a result. trammetallation
occurs quite easily. In addition to this, early transition metals are known for their
oxophilicity [9].
In the light of these results. we can conclude that alcohols are not suitable for the
formation of versatile adducts to be used in combination with DMZ to form a catalyst in a-olefin polymerisation. The use of amines, also readily available in both chiral and achiral
forms, could hold an answer to the transmetallation problem. Arnines are less acidic and
their conjugated base will form a stronger bond with B(C6F5),.
2.1.3. Amine Adducts
The need for a Lewis base suitable to form coordination complexes with B(C,F5), while bearing an acidic proton led us to select amines as a versatile precursor to
counteranions. A variety of mines are commercially available. Chiral and achird, aromatic and aliphatic, primary and secondary amines will allow to form numerous adducts
whose reactivity with DMZ will be studied in order to create a novel catalytic system in a-olefin polymerisation.
2.1.3.1. Formation and Stability of Amine Adducts
As for the alcohols, most amines readily undergo
Lewis acid B(C6F,), (Scheme 13).
complexation in presence of the
Scheme 13. Formation of amine adducts
Reaction occurs at room temperature in pentane and the adduct is isolated as a solid
after removal of the solvent under vacuum. The amhe adducts formed were kept under
nitrogen either at -40°C or room temperature and are stable for several weeks but do show a
strong tendency to absorb solvent molecules. The amines studied are listed in Table 6.
Table 6. Amine precursors to B(C,F,), adducts
I Primary Secondary
Formation of the adduct is confirmed by the NH resonance in 'H NMR and the
upfield shift of the borane fluorines in I9F NMR. For example, characterisation of the C6H,NH, adduct (40) was performed by 'H, '9 and "C as well as by elemental analysis. Spectroscopic data are 'H 6: 6.57 (m, 3H), 6.33 (dd, 2H), 5.82(s, 2H, NH,); I9F 6: -132.8(d, 2F, o-F), -154.7(t, IF, p-F), -162.2(m, ZF, m-F); "C 6: 121.9, 128.5, 129.2,
133.9. The 13C NMR spectrum also features 5 broad peaks between 135 and 1 50 ppm (approximately at 135.5, 139.0, 142.3, 146.0 and 149.5) corresponding to the perfluorinated phenyl rings.
Some amines proved unable to form stable and characterisable adducts. When reacted with B(C,F,), from 5 to 60 minutes, N-ethylaniline (C,H,N(CH,CH,)H) never
precipitates out of a cold pentane solution. Removal of the solvent under vacuum leads to an heterogeneous mixture. Analysis by both 'H and I9F NMR spectroscopy confirms the
presence of a minimum of two products. Further purification did not lead to isolation of
identifiable products. A plausible degradation path for this secondary amhe involves acid
catalysed elimination, proceeding via an imine intermediate as observed for another
secondary amine (vidu infio).
The amine a-C6H,CH(CH,)N(C6F,)H never formed the desired adduct with
B(C,F,),. Reaction in either pentane or benzene led to formation of a yellowsrange solid characterised by only one 'H resonance around 65.88 and some impurities between 0.8 to
1.3 ppm. All peaks belonging to the -CH(CH,)- portion had disappeared. The 1 9 ~ NMR
however shows 6 major resonances at 6-132.7, -149.5, -15 1.3, -152.8, -160.3 and 162.0,
none of them corresponding to the free form of either the amine or B(C$,),. Observation of the species formed by 'H NMR showed that no reaction occurs between the two compounds within 30 minutes in deuterated benzene. After 6 hours, some new peaks
appear, after 20 hours the peak identified as the -NH resonance has disappeared and after 4
days less than 20% of the starting material -CH(CH,)- resonances can be identified.
Comparison with C,F,NH, and the product of the reaction of the latter with B(C,F,), did
not allow to observe any similarities with the degradation process. Identification of the
products formed was unfortunately unsuccessful.
These two secondary mines do not form a stable adduct in solution. It is plausible that an equilibrium between the free species and the coordinated one exists and strongly
depends on both the temperature and the solvent. Moreover, the steric bulk of the second
substituent on the nitrogen might cause this equilibrium to be shifted towards the free species. Finally, the electron-withdrawing substituent in a-PhCH(CH,)N(C,F,)H might
decrease the basicity of the amhe and thus its ability to react with the Lewis acid.
2.1.3.2. Reactivity with Dimethylzirconocene
The ability of the novel atnine adducts to form an active species in Ziegler-Natta
catalysis when reacted with DMZ was investigated by 'H and I9F NMR spectroscopy. A
stoichiometric amount of DMZ and mine adduct was reacted in deuterated solvent and the
spectrum was recorded within 10 minutes of mixing unless otherwise stated.
Reaction of 40 with DMZ in C,D, at room temperature leads to a yellow solution and
methane formation (Eq. 14). The 'H NMR shows formation of a new species, with peaks at 60.30 (CH,Zr+) and 5.43 (CpZr'). These shift are similar to those found when DMZ was reacted with the alcohol adducts and correspond to the cationic methylzirconocene (41) (70% by NMR). Two other species can also be detected around 65.46 (9% mct 21%).
In an attempt to identify these other species, DMZ was reacted with aniline in a stoichiometric ratio (Eq. IS). The solution turned yellow but the reaction does not reach
completion even after 10 days. The product formed, Cp,Zr(CH,)NHPh (42), is characterised by its 'H NMR resonances at 60.27 (3H, CH,Zr), 5.62 (IOH, CpZr), 6.43
(lH. ZrNHPh) and phenyl resonances at 86.70.6.85 and 7.19. Apparently. 42 is not one
of the decomposition products found in equation 14. In the reaction mixture, 42 could react with the free B(C6F5), and form a cationic complex, similar to the alcohols.
Low temperature (-20°C) 'H and ' 9 ~ NMR was performed on the catalyst mixture
(DMZ + 40) in order to observe the formation of the cationic methylzirconocene species
(41). The 'H NMR shows unreacted DMZ and 40, methane as well as a new species with resonances at 60.3 1, 1.28 and 5.52. The 1 9 ~ NMR shows the free adduct (40) and the
new species at 6- 132.1. - 163.0 and - 165.8. This new species likely corresponds to the
desired novel counteranion, where the broad resonance in the H NMR spectrum at 6 1.28
can be assigned to the [PhNHB(C,F,),]* proton. while the resonance at 5.52 corresponds to the cationic (Cp-Zr*) and 0.3 1 to (CH,-Zr').
The same solution was slowly warmed and its 'H and 19F NMR spectra were
recorded at regular intervals. When reaching OOC after 1 hour, a dark oil starts forming at
the bottom of the NMR tube and new resonances are detected in both 'H and NMR. Firstly, formation of [CH,-B(C6F,),]- occurs (by "F NMR). The 'H NMR shows new
resonances which could not be identified. Warming up to 40°C increases the degradation
process and leads to unidentified species. Performing the same experiment in other
deuterated solvents (CD& CD,CN) does not lead to significant improvement in the
identification of the species formed by degradation.
The experiment at low temperature supports the formation of the novel counteranion along with the cationic methylzirconocene (41). Existence of these species is observed at
low temperature but degradation occurs rapidly as we reach 0°C. As we increase the
tempemture, the possibility for the adduct to exist both as a complex and as free species
increases which can be held responsible for some of the decomposition products.
Transmetallation is also a possible decomposition path. The presence of [CH3-B(C$,),]- can arise from W e r reaction of [Cp,ZrMeNHPh] with the free Lewis acid as observed
with the alcohol adducts.
Compound 43 is a white solid and is soluble in benzene. When added to DMZ at
room temperature in C,D,, no immediate reaction was observed (Eq. 16). After one day however. some decomposition occurs and the colourless solution turns pale yellow.
Analysis by 'H NMR showed mostly unreacted starting material as well as a variety of unidentified resonances. The "F NMR revealed two additional B(C,F,), species along with the unreacted adduct. Apparently. decomposition leads to multiple products, none of
which were identified.
These results suggest that the adduct is stable in solution at room temperature and the
NH? protons are not acidic enough to readily protonate off one the metallocene methyl
groups. Apparently, the stabilizing aromatic ring on the nitrogen, such as in 40, plays a key role in the ability of the mine to be a precursor to the counteranion or not.
Compound 44 is also a white solid. When reacted with DMZ in C6D6 at room temperature, the solution immediately turns bright red while an orange oil forms at the
bottom and gas evolves. Analysis by 'H and 1 9 ~ NMR shows degradation and no formation of the cationic methylzirconocene.
When CD3CN is used instead of C,D,, the solution tums yellow and the 'H NMR shows a broad singlet at 60.46 and a peak at 80.06 which correspond to [CHI-B(C,F,),]-
and CH,-Zr, respectively. Several Cp resonances are present in the same spectrum. The presence of a coordinating solvent seems to favour the dissociation of the adduct by acting
as a stronger Lewis base than the amine thus coordinating the Lewis acid, B(C,F,),. It also
seems to prevent, or at least slow down, the decomposition process via coordination of the
cationic metallocene . When two equivalents of CH,CN are added to DMZ prior to the addition of 44 in C6D,, the reaction proceeds readily to degradation. A large excess of
acetonitrile is needed to efficiently stabilise the cationic species. Even then, the species
formed arise from the reaction of DMZ and B(C,F,), and the arnine is released is solution. Adduct 44 yields disappointing results as it degrades DMZ in benzene solution or is
not under its adduct form in acetonitrile solution. The presence of the electronwithdrawing para- substituent on the phenyl ring certainly influences the stability of the adduct and the
acidity of the two protons. For these reasons, no further study was done with this arnine.
The adduct of the ch id a-PhCH(CH3)NH2 was formed with an enantiomerically
pure amhe and is a white solid. Since it is an aliphatic amine, its behaviour is expected to
be similar to the reaction of 43 and DMZ.
C6D6 _ CpzZrMez + a-PhCH(CH3)NHpB(C6F~)3 22 no reaction (Eq. 17)
The 'H NMR spectrum of 45 + DMZ shows mostly unreacted starting materials (Eq. 17). After 22 hours, degradation has partially occurred as observed by both 'H and
"F NMR. None of the degradation products can be identified and none corresponds to
either the free B(C$,), or [CH,B (C,F,),]-. No further reaction occurred in the following
24 hours which led us to believe that the reaction arose from some excess mine present in the adduct. Since the reactivity of this adduct is indeed similar to benzylamine, no further
study was done.
Compound 46 forms as a white solid. Upon reaction with DMZ in C,D, at room
temperature, the solution turns yellow and gas evolves. Analysis by 'H NMR reveals the
presence of methane as well as a variety of unidentified species. The 19F NMR presents
two distinct sets of resonances and some unidentified species. Three resonances in the 1 9 ~
NMR correspond to [CH,B(C,F,),]-. The second well-defined set has resonances at 6 152.2, - 16 1.7 and - 162.8. The latter species likely corresponds to some zirconocene-
bound amine, the evolution of methane also supports this option.
The strongly electron-withdrawing C,F, weakens the dative bond between the amine
and the Lewis acid thus making it less stable and more likely to exist under the free forms. The same factor accounts for the high acidity of the amine protons and their reaction with
the metallocene methyls. The reaction of the zirconocene and the free amine being
predominant. this adduct does not represent a suitable precursor to the counteranion.
This new secondary amine, a-PhCH(CH,)N(C,F,)H (47) was prepared by reaction between a-PhCH(CH,)NH, and C,F6 (Eq. 18) [lo]. Reaction with B(C6F,), leads to a
mixture of products as determined by both 'H and "F NMR and reported in a previous
section. When the amhe itself is reacted with DMZ in C6D6 at room temperature, no
reaction occurs within 120 hour.
YH3 P T - N H 2 + C66
A FH3 ,H ' -r\ ,F5
+ HF (Eq. 18) 1) neat, 16h
H 2) 10% Na2C03 H 47
This can be interpreted as being the result of two factors. The strong electron-
withdrawing substituent does not increase the acidity of the proton in a significant manner or the sterically encumbered mine cannot approach the metallocene close enough to allow a
reaction between the metallocene methyl and the mine proton.
The new secondary mine, a-PhCH(CH3)N(2,4-(N07,)2(C6H J))H 48, was prepared by reacting a-PhCH(CH,)NH, and 2,4-dinitrophenylfluoride (Eq. 19) [I[]. Product 48 is
a yellow oil but its reaction with B(C,F,), leads to the formation of a bright red solid (49).
When 49 is further reacted with DMZ in C6D6 at room temperature, an immediate and
vigourus reaction occurs leading to the decomposition of DMZ and 49. Presence of
acetonitrile allows the amhe to be released in solution under its free form. Apparently, the
adduct can decomplex in solution, leaving the Lewis acid to react with DMZ or with
acetonitrile.
2.1.3.3. Conclusions
The results obtained in adduct formation and reactivity with DMZ strongly depend on
the type of amhe used. We can divide the mines in three main categories : aromatic,
aliphatic and secondary amines.
The secondary amines studied included PhN(CH,CH,)H, a-PhCH(CH,)N(C,F,)H
(47) and a-P hCH(CH,)N(2,4-(N02)2(C6HJ))H (48). They each contain an aromatic and
an aliphatic chain on the nitrogen. These amines proved unable to form stable adducts with
B(C,F,),, only the dinitro-su bstituted mine formed an actual isolable adduct. Its reaction with DMZ demonstrated that the metallocene reacts with the separate constituents and not
the adduct itself thus supporting the existence of an equilibrium in solution. Furthermore,
the metallocene species is rapidly and completely degraded when reacted with this adduct.
Degradation is believed to occur via the slightly acidic proton or the nitro groups. Presence
of an electron-withdrawing group makes the amine-nitrogen lone pair less Lewis basic.
thus the adduct formation more difficult. It should, however. make the nitrogen proton
more acidic. Furthermore, sterically encumbered amines will have more dificulty in complexing an already bulky B(C,F,),.
Aliphatic arnines, PhCH,NH, and a-PhCH(CH,)NHI, readily form stable adducts
with B(C,F,), (43 and 45, respectively) however they lack acidic protons to make them
suitable as anion precursors. Extended reaction time of the adduct with DMZ does not lead
to the protonation of the metallocene methyl.
Aromatic primary amines (C,H,NH,, p-CF,(C,HaNH, and C,F,NH,) afforded interesting results. The peduoromethyl-substituted aniline formed a stable solid adduct
(44) but seemed to exist also under its free fonn in benzene solution. The free amine showed great reactivity with DMZ, protonating off both methyl groups. Addition of a coordinating reagent such as acetonitrile in solution slowed down the decomposition but the anion of interest was not detected by either 'H or "F NMR. The peffluoroaniline adduct
(46) exists in equilibrium with its free form in solution and the highly acidic protons of the
free mine react promptly with DMZ. Both of these aromatic amines are not suitable as
precursors of the counteranion. Their electron-withdrawing substituents, although
increasing the acidity of the protons, allow them to exist freely in solution and undergo a rapid reaction with the metallocene.
The aniline adduct (40) proved to be the only one, among the amines studied, to form
the counteranion. Even then, temperatures below OOC are required. Degradation occurs
rapidly above this temperature. The catalytic system formed by CpJrMe, and
PhNH,aB(C,F,), at low temperatures in C,D, is characterised by its 'H NMR resonances at W. 14, 0.3 1. 1.28 and 5.67 and I9F NMR resonances at 6- 132.1, - 163.0 and - 165.8.
In light of these results, it appears that a delicate balance exists between adduct
stability expressed in terms of Lewis basicity of the amine and the acidity of the protons.
The sterics of the ilmine also play an important role in the formation of the adduct. Other
aniline derivatives may be suitable as precursors but the choice of substituent (its nature,
number and position) is likely to be critical in their reactivity with dirnethylzirconocene.
2.1.4. Ole fin Polymer isa tion
In order to probe the reactivity of the new counteranions in Ziegler-Natta catalysis, a-olefin polymerisation reactions were carried out. Simultaneous reactions of the new
systems and a reference system consisting of DMZ and B(C,F,), were performed. Two olefins were studied, a gaseous a-olefin, propylene and a liquid a-olefin, 1-hexene.
2.1.4.1. 1-Hexene Polymerisation
Polymerisation of 1-hexene was performed under an inert atmosphere at room
temperature (Scheme L4) [12]. An equivalent amount of the catalyst and the co-catalyst was mixed in dry toluene for 5 minutes. The a-olefin was then added to the yellow
solution while stirring. The reaction was carried out for a determined period of time at the
end of which the mixture was quenched with acidified methanol.
Scheme 14. Polymerisation of 1-hexene by the reference system
The reference system polymerises 1-hexene in a highly exothermic reaction. The
initially yellow solution turns momentarily green upon addition of the olefin but promptly
returns to its initial colour. The reaction is stopped after 1 hour. The product obtained after the workup of the reaction is a clear, colourless liquid. Analysis by 'H NMR shows that it
is poly(1-hexene), yield = 1.5 g, M, = 344 (by end-group analysis).
A similar experiment was set up and performed with the aniline.B(C,F,), (40) and
DMZ system. The reaction is highly exothermic and a colour change from yellow to green
also occurs. The product obtained after 1 hour is poly(1-hexene) (yield = 1.5 g. Mn = 257) as confumed by 'H NMR. For both of these two reactions, the yields and the molecular weights are similar and the products possess the same physical properties, it is a slightly
viscous, clear and colourless liquid, soluble in benzene and pentane.
Other reactions were attempted with other adducts. No reaction occurs when 49 and
D M are used. Decomposition products can be seen covering the inside of the reaction
flask. Finally, when 45 is reacted with DMZ for 4 hours (for activation purposes) and
1-hexene is added to the mixture, no polymerisation occurs.
2.1.4.2. Propylene Poly merisa tion
The D M and B(C,FJ, catalyst is used as the reference system for the polymerisation of propylene (Scheme 15) [13]. Since propylene is a gas, we have had the choice to use it
under its gaseous or liquid state. Many attempts were made to polymerise propylene, varying several parameters: the temperature of reaction, the amount of gas used and the
duration of the reaction.
Cp2BMe2 + B(GFd3 - toluene
Scheme 15. Polymerisation of propylene by the reference system
The purity of the gas used had to be increased in order to obtain satisfactory results.
The method of purification chosen involves both oxygen and water removal columns as well as freeze-pumpthaw cycles. The propylene is used as a condensed liquid in the
polymerisation reactions. Details of a typical reaction are given in the experimental section.
The typical experiment uses 0.03 rnmoles of catalyst and co-catalyst in 2.0 mL of
toluene and 12.5 mL (4-5 a m ) of condensed propylene at O°C. Under these conditions. the reference system (DMZ + B(C,F,),) yields 7.75 g of a soft tacky white product. This
polymer is totally soluble in pentane and thus is atactic [14]. The gas condensed in the
reaction flask is completely consumed and the solution is yellow. The yellow colour
disappears upon quenching. The polymer produced has approximately M, = 1600 as determined by 'H NMR end-group analysis.
Two conclusions can be put forward from the several trials of polymerisation by the
reference system. Firstly, the catalyst is still active after the complete consumption of the monomer present. Successive additions of the monomer to the reaction flask have allowed to form up to 15.2 g of polypropylene. At this point, the content is highly viscous and
yellow and cannot be magnetically stirred. The high concentration of monomer present
compared to the small amount of solvent leads to a very viscous polymer solution and the
active catalyst is dispersed in a quasi-solid product thus making it nearly impossible for the reaction to continue. The use of more solvent would certainly allow to reach higher yields in polymer.
The second important conclusion also arises from the near-solidification of the
reaction mixture. When low yields of polymer were obtained, for various experimental
reasons, the molecular weight was significantly higher. Up to M, = 25 x 10' was obtained
when the yield was a few milligrams. This can be interpreted as the result of an increased
termination rate relative to the insertion when the viscosity increases and the concentration
of monomer decreases. Again, a greater amount of solvent would lead to increased
molecular weight.
When the anilineaB(C,F,), (40) and DMZ system was used under the same conditions, polypropylene is also formed as a tacky white product, totally soluble in
pentane. Yields obtained range from 4.0 to 6.5 g with an average of 5.2 g of atactic
polypropylene. For all of these experiments, some of the propylene gas was still present at
the end of the reaction time. The average molecular weight as determined by 'H NMR
end-group analysis is M, = 650. The highest molecular weight was M, = 7000 in a
reaction where a very low yield of polymer was obtained.
2.1.4.3. Conciusions
Polymerisation of propylene was achieved with the aniline adduct (40) and DMZ system at O°C, temperature at which the [PhNHB(C,F,),]- anion does exist. Significant
differences are observed in the yield and molecular weight obtained whether the reference system or the aniline-based system was used. The aniline-based system is less active and
produces polypropylene of lower molecular weight than the reference system under the
same experimental conditions.
Although the decomposition pathways were not clearly identified, we observed that [MeB(C,F,),]- was formed at higher temperatures, supporting the decomposition of the
desired anion. A plausible degradation mechanism involves transmetallation and rapid
methyl abstraction by the free Lewis acid which is supported by the presence of the methylborate anion. Degradation of the anion causes a decrease in the yield of polymer
obtained, as observed in the above experiments. As degradation occurs, an insoluble oil
forms, which was not characterised. The reactivity of other amine adducts showed their
tendency to exist in equilibrium in solution as the complexed and the k e form.
Nevertheless, the significant difference in molecular weight obtained when the aniline adduct was used as opposed to the borane, is most probably caused by a different
association of the cationic zirconocene and the counteranion. It can be rationdised that the
electron-rich nature of the amine-borate anion [C,H,NHB(C,F,),]- can account for a
significant increase in association with the metal centre and thus lead to an overall lower
molecular weight when compared to the reference system.
Since the variety of B(C6F,), amine adducts actually suitable to form a counteranion
is much more restricted then it was fiat thought, another approach will be used to achieve
the formation of a B(C6F,), derivative. B(C,F,)< has been used as counteranion in Ziegler-
Natta catalysis, we will try to form the same type of salts by reacting various Lithium amides
and B(C6F,), and proceeding via a simple exchange of cation. This method will dlow us to
create a formal bond between the nitrogen and the boron prior to addition of DMZ. This
will likely reduce the degradation occurring via transmetallation and the reaction of the
metallocene with the free species.
REFERENCES
Siedle, A.R.; Lamanna, W.M.; Newmark, R.A.; Stevens, J.; Richardson, D.E.; Ryan, M . Mnkromol. Chem. Macromol. Symp., 1993-66, 225.
Puddephatt, R.J.; Hill, G.S.; Manojlovic-Muir, L.; Muir, K.W. Organometallics, 1997, 16, 525.
Puddephatt, R.J.; Rendina, L.M.; Hill, G.S. J. Chem. Soc., Dalton Trans., 1996, 1809.
Massey, A.G.; Park, A.J. J. Organomet. Chem., 1964.2. 245.
Massey, A.G.; Park, A.J. J. Organomet. Chem., 1966.5, 218.
Marsella, J.A.; Folting, K.; Huffman, J.C.; Caulton, K.G. 3. Am. Chem. Soc., 1981, 103, 5596.
Hanna, T.A.; Baranger, A.M.; Walsh. P.J.; Bergman, R.G. J. Am. Chem. Soc., 1995, 1 1 7 , 3292.
Hores, LC.; Chien, J.C.W.: Rausch, M.D. Macromolecules, 1996,29, 8030.
Collman, J.P.; Hegedus. L.S.; Norton, J.R.; Finke, R.G. In Principles and Applications of Organotransition Metal Chemistry. University Science Books, Mill Valley CA, 1987.
10. Poe, R.; Schnapp, K.; Young, M.J.T.; Grayzar, I.; Platz, M.S. J. Am. Chem. Soc., 1992, 114, 5054.
1 I . Gleason, J. L., personal communication.
1 2. See for example, ref. 1.
13. See for example, Marks, T.J.; Yang, X.; Stem, C. J. Am. Chem. Soc., 1991, 113, 3623.
14. (a) Welch, M.B.; Hsieh, H.L. In Handbook of Polyolefins; Synthesis and Properties. Vasile C.; Seymour, R.B. Eds. Marcel Deklcer, New York, 1989. (b) Monasse, B .; Haudin, J.M. In Polypropylene: Structure, blends and composites, Vol. 1, Karger-Kocsis, J. Ed. ChapmanBrHall, London, 1995.
CHAPTER THREE. FORMATION OF CHIRAL BORON LEWIS ACIDS
3.0. Review of Borate Salts
Two classes of perfluorinated borate salts have been used in Ziegler-Natta catalysis.
The first one is formed of a dimethylanilinium cation and the borate anion. The ammonium
proton reacts with the metdocene methyl to form methane and an arnine is release in
solution (Eq. 1) [I] . This method has the disadvantage of forming a base in solution
which can coordinate the cationic zirconium species and decrease the catalytic activity.
The second salt is more widely used and involves a bulky carbocation [Ph,C]+ also
known as a uityl cation [2]. This species reacts by abstracting a methyl group from the
metallocene and forming a stable and unreactive species, Ph,CCH, (Eq. 2).
Trityl salts are easily synthesised via an exchange reaction between the trityl chloride
and the lithium borate salt (Eq. 3) [2]. The trityl salts have been used in the recent
syntheses of modified perfluorinated borate anions and aluminate anions (cf. Section
1.7.2.4 - 5) .
Using a variety of amines, we propose to synthesise lithium amides which will be
reacted with the neutral B(C,F,), to form the lithium borate. These novel borate salts will
then incorporate an arnide chain. The lithium cation will then be exchanged with the trityl cation as in equation 3.
3.1. Formation of Lithium Amides
Lithium amides are synthesised by reacting a variety of mines with an exact
equivalent of n-butyilithiurn in cold solvent (Eq. 4). The product obtained after removal of
the solvent is a solid and is purified by pentane washes and dried thoroughly under
vacuum. The side-product formed by this reaction is butane which is gaseous and does not
stay in solution.
It is critical to use an exact equival ent of the n-B uLi solution as any excess will react with other protons present, making the purification nearly impossible. The solid product is
stable for several weeks when stored under an inert atmosphere at -40°C.
The lithium amides prepared are listed in Table 7. Most amides are only slightly
soluble in benzene, making their characterisation by NMR spectroscopy difficult.
Solubilization can be achieved in more polar solvent such as acetone, water or methanol but
the proton exchange occurring in such solvent defeats the purpose of identification.
Moreover, since every amhe was characterised in benzene, it is useful to keep the same
solvent for comparison purposes. 'H, I3C and ''F NMR where applicable, were used to
characterise the species in a C6D, solution.
Table 7. Amides precursors to borate trityi salts
I -
Primary Secondary -1
As an example, a-PhCH(CH,)N(C,F,)Li (50), was characterised by its 'H NMR 6: 6.83 (m, 5H, Ph), 4.80 (q, lH, CH), 1.05 (d, 3H, CH,); 6: -162.9 (m, 2F, m-F),
-166.2 (t, 2F, o-F), -181.3 (m, IF, p-F); "C 6: 22.5, 55.8, 125.3, 127.6, 129.9 and
145.9. The carbons of the pemuorinated phenyl rings could not be identified.
3.2. Reactivity of the Lithium Amides with B(C,F,),
51 + decomposition products
Compound 51 is isolated as a white solid. partially soluble in benzene. Its
characteristic %I NMR resonances appear at 6- 1.00 (NH), 1.18 (CH,) and 4.27 (0. Reaction with one equivalent of B(C6F,), in C6D6 yields a soluble product, isolated as a
white solid after removal of the solvent under vacuum (Eq. 5). This solid shows multiple
'H and 19F NMR resonances, among which we can identify the free mine adduct (ca. 30%) (45). The other resonances could not be identified. The desired lithium amide-
borate species did not form selectively.
52 + decomposition products
The lithium amide 52 is a yellow solid, mostly insoluble in benzene. When reacted
with one equivalent of B(C,F,), in C,D,, some solid forms and some of the initial amide is
present (Eq. 6). The *H NMR shows a strong singlet at 62.73 and a broad singlet at 5.84,
similar to the neutral mine adduct (ca. 18%) (40) . The "F NMR confirms the presence
of the neutral adduct as well as of free B(C,F,), in solution. Other peaks cannot be
identified. It appears that the proton on the lithium arnide can react in a manner to form the
neutral amhe adduct, leading to a mixture of products, difficult to isolate and identify.
Isolated as a pink solid, 53 is only slightly soluble in benzene. When reacted with
one equivalent of B(C,F,), in C,D, or CD,CN, a complex mixture of products is formed,
among which the neutral mine adduct (ca. 30%) (43) is detected (Eq. 7). As in the other
systems, the desired lithium amide-borate species did not form selectively.
53 + decomposition products
54 is insoluble in both benzene and pentane. It was, however, possible to characterise it in CD,CN. In 'H NMR, one resonance is observed at 6-0.06 which likely
corresponds to the N-H proton. The 'T NMR spectrum shows two peaks at 6 169.9 and - 17 1.9, integrating for roughly two and three fluorines respectively.
H C~F~<' + B(C&I3 a decomposition products
* .
When the solid is reacted with one equivalent of B(C,F,), in C6D6, the solution turns
yellow but most of the initial solid is insoluble (Eq. 8). The NMR shows a multitude of compounds but mostly free B(C,F,),. The desired lithium amide-borate species did not
Form selectively.
The aliphatic amide 55 was reacted with one equivalent of B(C,F,), in C6D6. The 'H NMR spectrum of this reaction mixture shows multiple resonances among which an imiw
species, CH,-N=C&, appears to be forming (66.33 (d), 5.40 (d) and 0.96). Analysis by
'T NMR confirms the multiple product formation.
The last two arnides will be treated as a whole. Their structures are similar and their
reactivity toward B(C,F,), is also very much alike. The two lithium amides, 56 and 50, are reasonably soluble in benzene and therefore, were characterised by their 'H, 13c and 'v NMR spectra.
The reaction of RN(C,F,)Li (50.56) with B(C,F,), proceeds slowly. A white solid eventually forms in benzene solution. Careful interpretation of both 'H and "F NMR spectra as well as separation of the products into pentane- and benzene- soluble fractions
confirmed an unexpected reaction.
The product precipitating in the course of the reaction in benzene is Li+B(C,F,);, a
beige solid. While the soluble fraction contains 2 products. Evaporation of the solvent led
to a yellowish solid. When this solid is washed with cold pentane, an insoluble white solid
and a yellow pentane-soluble oil can be recovered. The white solid has been characterised as 57 and 58, whereas the yellow oil has not been identified.
The yellow oil also possesses new resonances in both 'H and 19F NMR and is likely
to be a by-product of the amine. The reaction believed to occur involves the displacement of a perfiuorinated phenyl from one borane to another (Scheme 16).
Scheme 16. Displacement reaction of a perfluorophenyl group
When this reaction is performed with a twofold excess of the borane. the amine residue does not form, consistent with the mechanism illustrated above. An interesting
feature of the 19F NMR of 58 compared to 57 is the double signal for ortho- and metu-
fluorines on the nitrogen perfluorophenyl compared to the achiral molecule where all the
N-C,F, fluorines are equivalent. The formation of this novel compound can be explained
by the stabilisation provided through the nitrogen lone pair.
3,3. Conclusions
While the lithium amides can be formed rather easily, their lack of solubility makes
them difficult to characterise and to react with the peffluorobome species in solution. The
proton-bearing amides 51, 52, 53 and 54 undergo degradation when reacted with B(C,F,),. Among the products formed, the neutral amhe adducts can be detected. NMR spectroscopy analysis demonstrate the formation of what may be the desired species
however it degrades within minutes and cannot be isolated as a pure solid. Thus, while
B(C,F,), adds LiC,F, in a straightforward manner, LiMiR does not selectively form the
borate.
The peffluorophenyl-substituted nitrogen lithium arnides 50 and 56 are more soluble
in benzene and have been characterised by NMR spectroscopy. Reaction of these amides with B(C,F,), however leads to an unexpected product. The transfer of a pedluorophenyl-
substituent from one borane to another leads to the formation of LiB(C$,), and a novel
species, 57 and 58.
If a stoichiometric amount of lithium amide and B(C,F,), is reacted, an
uncharacterised mine product is formed along but if a twofold excess of B(C,F,), is used, the reaction proceeds cleanly. The new borane-amine species, while not being the one
aimed at, still presents novel characteristics. When the amine chain attached to the boron is
chiral, the "F NMR signals are diastereotopic for the -C,F, group on the nitrogen. The
presence of a lone pair on the nitrogen is also likely involved with the Lewis acidic boron.
We can expect a lower overall Lewis acidity of the tris-substituted borane species thus a diminished activity of the borane as a methyl-abstractor when reacted with DMZ to form an active catalytic species. Nevertheless, if this chiral species can, in fact, act as an active precursor to cationic Ziegler-Natta metallocene catalyst, the chid group on the amhe would allow us to observe if the anion influences indeed the tacticity in propyiene
polymetisation (Scheme 17).
F"F5
Scheme 17. Proposed influence on tacticity induced by the novel amine-borane Lewis
acids in a-olefin polymerisation
REFERENCES
I . Hlatky, G.G.; Turner, H.W.; Eckman, R.R. J. Am. Chem. Soc., 1989,111, 2728.
2. Chien, J.C.W.; Tsai, W.M.; Rausch, M.D. J. Am. Chem. Soc., 1991, 11.3, 8570.
7 . Poe, R.; Schnapp, K.; Young, M.I.T.; Grayzar, J.; Platz. M.S. J. Am. Chem. Soc., 1992,114, 5054.
EXPERIMENTAL SECTION
1. General
Standard inert atmosphere techniques were used in all experiments unless otherwise
stated. Dry benzene, diethy1 ether and toluene were distilled under nitrogen from
sodium/benzophenone kety 1. Dry dichloromethane and pentane were distilled under
nitrogen from CaH2. Benzene-d, and toluene-$ were vacuum transferred from
sodiudbenzophenone and CD,Cl, was vacuum transferred from Cali,. All NMR solvents were stored over 3A molecular sieves. All other solvents were purchased in reagent grade and used without further purification.
'H, ')c and "F NMR spectra were recorded on either Jeol 270, Varian XL-200,
Varian XL-300 or Unity 500 spectrometers. Low temperature experiments were performed
on the Jeol270 spectrometer. NMR experiments on air-sensitive samples were conducted
in Teflon valve-sealed tubes. Chemical shifts for 'H and and ')c are referenced to internal
solvent resonances and are reported relative to tetramethylsilane. 1 9 ~ NMR shifts are
referenced to external CFCI,. IR spectrum was recorded on Michelson 100 by Bomem.
Dimethylzirconocene [I], B(C$,), [2] and N-pefluorophenyIbellzylamine [3] were
prepared by literature methods. N-peffluoropheny 1-a-meth ylbenzy lamine (47) was prepared by the adapted literature procedure in [3] using a-methylbenzylarnine and C,F,. Purification of B(C,F,), was achieved by successive washes of the white solid in a
minimum of cold dry pentane (yields 55%).
2. Olefin Polymerisation
In a 25-mL Schlenk-flask equipped with a septum and a magnetic stirbar, 0.03
mmoles of DMZ and co-catalyst wen charged in 2 mL of dry toluene. The catalyst mixture was stirred at room temperature for 5 minutes. I-Hexene (2 to 5 rnL) was syringed in and
allowed to react for a determined period of time. The reaction was stopped by quenching
with acidified methanol (5% HCI). The resulting oily liquid polyhexene was dried under
vacuum. Characterisation was performed by 'H NMR (60.89 (CH,), 1.30 (CH,), 2.00 (CH), 4.84 (=CH& M, was determined by end-group analysis.
Propylene (Matheson, 99.5%) was purified by passing through a oxygen removal
column (BASF R3-11 catalyst) and a water removal column containing 3A molecular sieves
(Aldrich). It was then condensed in a reaction flask and three freeze-pumpthaw cycles
were done. The propylene was kept under pressure until the polymerisation experiment.
In a 50-mL air-tight reaction flask equipped with a magnetic stirbar, 0.03 m o l e s of
DMZ and cocatalyst were charged in 2 rnL of dry toluene. The catalyst mixture was s h e d
at O°C for 5 minutes. The solution was frozen and three freeze-pump-thaw cycles were
done. The reaction flask was finally evacuated and propylene ( 12.5 mL) was then vacuum
transferred into it. The reaction was stirred at PC overnight, period along which the temperature slowly rose to room temperature.
At the end of the reaction time, the flask was carefully opened to atmosphere to allow
the excess propylene to evaporate. The sticky polypropylene was quenched with acidified
methanol and dried in vacuo. Pentane was added to the mixture to dissoIve all of the
polymer and the mixture was allowed to stir for 3 hours. Removal of the pentane led to a sticky, white solid, characterised by 'H NMR (80.88 (CH,), 1.20 (CH, _. ), 1.70 (CH), 4.79
(=CH,). M, was determined by end-group analysis.
3. Experimental Details
Svnthesis of MentholeB(C
Menthol (30.5 mg, 0.20 mmoles) was added to B(C,F,), (100.0 mg, 0.20 mrnoles)
in 2 mL cold pentane while stirring. A white precipitate formed immediately in solution.
After 15 minutes the solvent was removed under vacuum. The white solid was dried in vacuo for an hour to yield 26 (129 mg, 0.19 mrnoies, 99% yield). 9-1 NMR (C6D6): 60.25 (d, 3H), 0.35 (m, 2H), 0.58 (d, 3H), 0.65 (d, 3H), 0.80 (m, 3H), 1.13 (m, 2H), 1.85
(m, ZH), 3.58 (m, 1H), 5.14 (br, 1H); 19F NMR (C6D6): 6-133A (br, 6F), -153.6 (br,
3F), -161.8 (br, 6F); "C NMR (C6Dd; 15.0, 20.3, 21.4, 22.7, 23.9, 25.4, 31.3, 32.8, 41.3, 48.8, 82.5, 135.5, 139.2, 142.6, 146.2, 149.7. Elemental analysis found for
C,HJ3F,,O: C, 50.58%; H, 3.35% (calculated: C, 50.335; H, 3.0245).
Svnthesis of CH11CH2)ZCH~OHaB(C4EI)l, 2 7
I-Butanol(20 @, 0.22 mmoles) was added to B(C6F5)), ((1 12.2 mg, 0.22 mmoles) in
2 rnL cold pentane while stirring. The solution became cloudy and white. After 15 minutes
the solvent was removed under vacuum. The residue was triturated with pentane then dried
in vacuo for an hour to yield 27, a white solid ( 1 18 mg, 0.20 mmoles, 9296 yield). 'H NMR (C6DJ: 60.45 (t, 3H), 0.67 (m, 2H), 0.8 1 (m, 2H), 3.19 (m, 2H), 5.76 (br, 1H); "F NMR (C6D6): 6- 134.4 (d, 6F), - 153.6 (m, 3F), - 16 1.7 (t, 6F). Elemental analysis
found for C,H,@F,,O: C, 44.68%; H, 1 .go% (calculated: C, 45.08%; H, 1.72%).
Svnthesis of C H 1 ~ Z C H r O H K H , 2 8
2-Butanol(2.6 mg, 0.03 mmoles) was added to B(C6F,), (15.0 mg, 0.03 mmoles) in 2 mL cold pentane while stirring. The solution became cloudy and white. After 15 minutes
the solvent was removed under vacuum. The residue was triturated with pentane then dried
in vacuo for an hour to yield 28, a white solid (15.7 mg, 0.026 mmoles, 92% yield). 'H NMR (C,D,): 60.38 (t, 3H), 0.61 (d, 3H), 0.82 (m, 2H), 3.44 (m, lH), 5.20 (br, 1H).
Svnthesis of 0x0-zirconnocene species. 3 1
Water (1 pL, 0.05 mmoles) was added to Cp&rMez (14.0 mg, 0.05 mmoles) in 0.5 mL C,D,. The solution was colourless and gas evolved. The species 31 was not isolated. 'H NMR (C6D,): initially: 60.14 (s, CHd, 0.25 (s, 3H) and 5.73 (s, 4H); after 2
hours: 60.14 (s, CH,), 0.25 (s, 2H), 0.33 (s, IH), 5.73 (s, 3H), 5.88 (s, 3H), 5.94 (s,
2H), 6.21 (s, 3H).
1 Butanol(3.6 w, 0.04 mmoles) was added to Cp2ZrM% (10.0 mg, 0.04 motes)
in 0.5 mL C6&. The solution was colourless and gas evolved. The species 32 was not
isolated ( ~ 9 9 % yield by NMR). 'H NMR (C6DJ: 60.33 (s, 3H), 0.90 (t, 3H), 1.3 1 (m,
4H), 3.80 (t, 2H), 5.77 (s, 10H).
1-Butanol(7.2 pL, 0.04 mmoles) was added to Cp2ZrMe, (10.0 mg, 0.04 mmoles)
in 0.5 mL C,D,. The solution was colourless and gas evolved. The species 33 was not
isolated (25% yield by NMR). 'H NMR (C6D,): 60.81 (t, 3H), 1.7 (m, 4H), 3.3 1 (t, 2H), 6.03 (s, 5H).
2-Butan01 (3.0 mg, 0.04 mmoles) was added to Cp,ZrMe, (10.0 mg, 0.04 mmoles)
in 0.5 mL C,D,. The solution was colourIess and gas evolved. The species 35 was not
isolated (22% yield by MR). 'H NMR (C6D6): 80.14 (s, CH,), 0.33 (s, 3H), 0.82 (t,
3W, 0.95 (d, 3H), 1.24 (rn, 2H), 3.72 (m, IH), 5.77 (s, \OH).
Synthesis of CgZrlMe)menthoxv. 3 7
Menthol (62.1 mg, 0.40 mrnoles) was added to Cp,ZrMel ( 100.0 mg, 0.40 mrnoles)
in 2 mL of dry pentane. The solution was colourless and gas evolved. After 15 minutes the solvent was removed under vacuum to yield 37, a white solid (1 5 1 .O mg, 0.39
mrnoles, 97% yield). 'H NMR (C6D,): 60.33 (s, 3H), 0.80 (m, 4H), 0.95 (m, 9H), 1.24
(br, LH), 1.51 (m, 2H), 1.78 (m, IH), 2.18 (m, IH), 3.52 (m, IH), 5.80 (s, IOH).
Menthol ( 12.4 mg, 0.08 mmoles) was added to CpJrMe, ( 10.0 mg, 0.04 mrnoles)
in 2 mL of dry pentane. The solution was colourless and gas evolved. The species 39 was
not isolated (79% yield by NMR after 22 hours). 'H NMR (C,D,) of characteristic peaks: 80.9 (m), 1 S (m), 1 -7 (m), 2.0 (m),2.3 (m), 2.4 (m), 3.1 (m), 3.60 (m, CHOZr), 6.10
(s9 Cp)*
Aniline (17.8 pL. 0.20 rnmoles) was added to B(C,F,), (100.0 mg, 0.20 mmoles) in 2 mL cold pentane while stirring. A white precipitate formed immediately in solution. After 15 minutes the solvent was removed under vacuum. The white solid was dried in
vacuo for an hour to yield 40 ( 1 15.4 mg, 0.19 mmoles, 98% yield). 'H NMR (C6D6):
65.82 (br, 2H), 6.33 (m, 2H), 6.56 (m, 3H); NMR (C6DJ: 6-132.9 (d, 6F), -154.7 (t,
3F), -162.2 (br, 6F); "C NMR (C6D,); 121.9, 128.5, 129.2, 133.9. IR (KBr pellet): v, = 3283.3, 3324.8 cm-'. Elemental analysis found for q,H,BF,,N: C, 47.8 1%; H, 1.29%; N, 2.3 1 % (calculated: C, 47.64%; H, 1.17%; N, 2.3 1 %).
Aniline (3.6 pL, 0.04 mrnoles) was added to Cp2ZrMe, ( 10.0 mg, 0.04 mmoles) in
0.5 mL C,D,. The solution was colourIess and gas evolved. The species 42 was not
isolated (80% yield by NMR after 7 days). 'H NMR (C6D&: 60.27 (s, 3H), 5.62 (s,
[OH), 6.43 (s, IH), 6.70 (d, 2H), 6.85 (t, IH), 7.20 (m, 2H).
Benzylamine (21.3 pL, 0.20 mmoles) was added to B(C,F,), (100.0 mg, 0.20
mrnoles) in 2 mL cold pentane while stirring. A white precipitate formed immediately in
solution. After 15 minutes the solvent was removed under vacuum. The white solid was
dried in vacuo for an hour to yield 43 (1 14.5 mg, 0.19 mmoles. 95% yield). 'H NMR (C,D6): 63.04 (rn, 2H). 4.4 1 (br, 2H), 6.59 (d, 2H), 6.93 (m. 3H); I9F NMR (C,D,): 6-134.3 (d, 6F), -154.7 (t, 3F), -161.8 (m, 6F); "C NMR (C,DJ; 48.5, 127.9, 129.4,
129.5, 133.4. Elemental analysis found for C,H,BF,,N: C, 48.66%; H, 1 .SO%; N, 2.20% (calculated: C, 48.50%; H, 1.47%; N. 2.26%).
4-Peffluoromethylaniline (24.5 pL, 0.20 mmoles) was added to B(C,FJ3 (KK).O mg,
0.20 mmoles) in 2 mL cold pentane while stirring. A white precipitate formed immediately
in solution. After 15 minutes the solvent was removed under vacuum. The white solid
was dried in vacuo for an hour to yield 44 (130.6 mg, 0.19 mrnoles, 97% yield). 'H NMR (C,D,): 5.80 (br, 2H), 6.14 (d, 2H), 6.76 (d, 2H); ' 9 ~ NMR (C6D6): 662.8 (s, 3F),
-132.7 (d, 6F), -153.9 (t, 3F), -161.8 (m, 6F). Elemental analysis was unsuccessful.
a-Methylbnzylamine (25.2 pL, 0.20 mmoles) was added to B(C6F,), (100.0 mg,
0.20 mmoles) in 2 mL cold pentane while stining. The solution became cloudy and white.
After 15 minutes the solvent was removed under vacuum. The white solid was dried in
vacuo for an hour to yield 45 (1 30.6 mg, 0.19 mmoles, 97% yield). 'H NMR (C6D&
0.81 (d, 3H), 3.54 (m, lH), 4.00 (br, lH), 5.22 (br, lH), 6.17 (rn, 2H), 6.77 (m, 3H); 19 F NMR (C6D6): 6-133.5 (d, 6F9, -155.5 (t, 3F), -162.4 (m, 6F); "C NMR (C6D& 624.2, 57.0, 124.6, 128.4, 128.7, 138.7. Elemental analysis found for C,H,,BF,,N: C,
49.08%; H, 1.4 1%; N, 2.25% (calculated: C, 49.32%; H, 1.75%; N, 2.2 1 %).
Peffluoroaniline (30.0 mg, 0.16 mmoles) was added to B(C6F,), (84.0 mg, 0.16
rnmoles) in 2 rnL cold pentane while stirring. A white precipitate formed immediately in
solution. After 15 minutes the solvent was removed under vacuum. The white solid was
dried in vacuo for an hour to yield 46 (109.0 mg, 0.16 mmoles, 98% yield). 'H NMR (CsDJ: 5.80 (br); 19F NMR (C,D&: 6- 132.5 (d, 6F), -149.2 (br, 2F), -152.2 (s, 3F),
-152.8 (br, IF), -159.8 (t, 2F), -161.5 (t, 6F).
Svnthesis of a-C,W~CH~CH1)N~2.4-rNOZ12GYH~~lH. 4 8
To a stirred solution of C6H,CH(CH,)NH, (0.29 mL, 2.25 mmoles) in 5 rnL of
acetone at room temperature, 5 mL of a saturated solution of NaHCO, in water was added.
To the previous solution, 2.4-dinitrophenylfluoride (0.28 mL, 2.25 mrnoles) was added.
The solution immediately turned yellow and cloudy and gas evolved. The reaction was
allowed to continue for 45 minutes. The acetone was removed under vacuum and the
aqueous layer was extracted with 15 mL of diethyl ether. After drying over MgSO,, the
solvent was removed, leaving 48 as a bright yellow oil (670 mg, 2.3 mrnoles, >99%
yield). 'H NMR (C6D6): 1.06 (d, 3H), 3.94 (q, 1H), 5.92 (d, IH), 6.89 (d, 2H), 7.07
(m, 3H), 7.39 (dd, lH), 8.51 (br, lH), 8.82 (d, 1H); 13C hJMR (C6D& 623.9, 53.3, 114.5, 123.6, 125.4, 128.3, 129.1, 129.3, 130.5, 136.4, 142.3, 146.7. Elemental
analysis found for C,,H,,N,O,: C, 58.38%; H,4.72%; N, 14.38% (calculated: C, 58.5396; H, 4.56%; N, 14.63%).
Compound 48 (1 12.2 mg, 0.40 mmoles) was added to B(C6F,), (200.0 mg, 0.40 mmoles) in 2 mL cold diethyl ether while stirring. The solution immediately turned bright
red. After 15 minutes the solvent was removed under vacuum. The red solid was dried in
vacuo for 4 hours to yield 49 (317 mg, 0.40 mmoles, 99% yield). 'H NMR (C6DJ: 60.96
(d, 3H), 3.83 (q, IH), 5.70 (d, lH), 6.75 (m, 2H), 7.03 (m, 3H), 7.32 (dd, IH), 8.50 (d, lH), 8.78 (br, 1H); 13c NMR (C6D6); 623.3, 53.8, 115.1, 125.2, 126.0, 128.3,
128.4, 129.3, 130.1, 130.9, 140.9, 148.5. Elemental analysis found for
C,,H,,BF,,N30,: C, 48.13%; H, 2.2 1 %; N. 5.17% (calculated: C, 48.09%; H, 1.6446; N,
5.26%).
with dimethvlzirconocene
AU NMR experiments to probe the reactivity of the different B(C,FJ, adducts of
alcohol (26,27,28) and mine (40,43, 44, 45, 46, and 49) with dimethylzirconocene
(DMZ) were carried out according to the following procedures. Room temperature NMR study: to DMZ (7 mg, 0.028 mmoles), the adduct was added (0.028 mmoles) in 0.5 mL of
C,D, in a Teflon valve-sealed tube. Upon mixing, the solution turned yellow and evolution
of gas is observed. The 'H and "F NMR spectra were recorded within 10 minutes of
mixing. Formation of the cationic species leads to specific resonances in the 'H NMR at
60.15 (s, CH,), 0.29 (s, 3H, CH,-Zr') and 5.42 (s, [OH, Cp-Zrf). Low temperature NMR study: DMZ (7 mg, 0.028 mrnoles) and the adduct (0.028 rnmoles) were transferred
as solids in a Teflon valve-sealed NMR tube. The NMR tube was attached to the vacuum
line and evacuated, dry C,D, was vacuum transferred into the tube and was not allowed to
mix with the solids. The tube was kept in liquid nitrogen until the last moment when the
solvent was thawed and the solids were mixed. The sample was inserted in a pre-cooled
NMR instrument and the spectra were recorded within a few minutes.
Determination of the concentration of n-BuLi
To determine its exact concentration, n-BuLi was used to titrate an weighed amount
of diphenylacetic acid. (C,H,)FCOOH (14.7 mg, 0.069 rnmoles) was placed in 3 rnL of
THF and titrated with the solution of n-BuLi in hexanes until the solution turned yellow.
One equivalent of n-Buli reacts with one equivalent of the acid.
Svnthesis of a-Cd&CHCH3)-Flr)Li. 5 0
To a stirred solution of a-C,H$H(CH,)N(C$,)H (47) (300 mg, 1.04 mmoles) in 3
mL cold benzene, n-BuLi (1.44M, 720.3 pL) was added. The solution turned yellow and
was stirred for 30 minutes. The solvent was removed under vacuum and a beige solid was
washed small fractions of cold pentane until the decanted pentane solutions were clear. The
solid residue was dried m vacuo for an hour to yield 50 (198 mg, 0.68 mmoles, 65%
yield). 'H NMR (C6D& 61.05 (d, 3H), 4.80 (q, lH), 6.83 (m, 5H); "F NMR (C6DJ: 6-162.9 (m, 2F), -166.2 (t, 2F), -18 1.3 (m, IF); "C NMR (C,D&: 622.5, 55.8, 125.3,
127.6, 129.9, 145.9. Elemental analysis found for C,,I19F,LiN: C, 57.2396; HT 3.49%;
N, 4.83% (calculated: C, 57.36%; HT 3.09%; N, 4.78%).
Svnthesis of a-CLH,CH(CH$NHLi. 5 1
To a stirred solution of a-C,H,CH(CH,)NH, (47 mg, 0.39 mrnoles) in 2 rnL cold
pentane, n-BuLi (1.60M. 243 pL) was added. A beige solid appeared in solution. The
solvent was immediately removed under vacuum and the beige solid was washed small
fractions of cold pentane until the decanted pentane solutions were clear. The solid residue
was dried in vacuo for an hour to yield 51 (34 mg, 0.27 mmoles, 69% yield). 'H NMR (C,D,): 6 1.00 (br, 1 H), 1.18 (d, 3H), 4.27 (q, 1 H) arid 7.18 (m, 5H). Elemental analysis
was unsuccessful.
To a stirred solution of C,H,NH, (600 mg, 6.44 mmoles) in 5 mL cold benzene, n-BuLi ( 1.68M, 3.83 mL) was added. A yellow solid appeared in solution. The solvent
was immediately removed under vacuum and the yellow solid was washed with small fractions of cold pentane until the decanted pentane solutions were clear. The solid residue
was dried in vacuo for an hour to yield 52 (625 mg, 6.3 1 mmoles, 98% yield). No valid
NMR spectra can be obtained in C6D6 due to the insolubility of 52. Elemental analysis
found for C,H,LiN: C, 72.71%; H, 6.86%; N, 13.83% (calculated: C, 72.75%; H, 6.1 1%; N, 14.14%).
Svnthesis of C6HfCHzNHLi. 5 3
To a stirred solution of C6H,CH,NH, (600 mg, 5.60 mmoles) in 5 rnL cold benzene,
n-BuLi (1.68M, 3.34 mL) was added. A pink solid appeared in solution. The solvent was
immediately removed under vacuum and the pink solid was washed small fractions of cold
pentane until the decanted pentane solutions were clear. The solid residue was dried zh
vacuo for an hour to yield 53 (522 mg, 4.62 mmoles, 82% yield). No valid NMR spectra can be obtained in C,D, due to the insolubility of 53. Elemental analysis found for c,H8LiN: C, 74.53%; HT 7.18%; NT 12.13% (cakulated: C, 74.34%; HT 7.1396; N, 12.39%).
Attem~ted svnthesis of C6FfNHI,i. 5 4
To a stirred solution of C,F,NH, (600 mg, 3.28 mmoles) in 3 mL cold benzene, n-BuLi (1.93M. 1.70 mL) was added. A black solid appeared in solution. The solvent
was removed under vacuum and the black solid was washed with small fractions of cold
pentane until the decanted pentane solutions were clear. The solid residue was dried in vacuo for an hour to yield an unidentified product.
Svnthesis of C6HsCH2NK$&i. 5 6
To a stirred solution of C6HICH,N(C6F,)H (300 mg. 1.10 mmoles) in 3 mL cold
benzene, n-BuLi ( l.54M, 7 1 5 p.L) was added. The solution turned yellow and was stirred
for 30 minutes, a solid formed. The solvent was removed under vacuum and the beige solid was washed with small fractions of cold pentane until the decanted pentane solutions
were clear. The solid residue was dried in vacuo for an hour to yield 56 (261 mg, 0.94
rnmoles. 85% yield). 'H NMR (C6D& 64.00 (s, 2H). 6.90 (m, 5H); 19F NMR (C6DJ:
6-164.5 (br, 2F), -166.2 (t, 2F), -182.1 (m, IF). Elemental analysis found for C,,H,F,LiN: C. 55.96%; H. 2.68%; N. 5.00% (calculated: C, 55.94%; H, 2.53%; N, 5.02%).
Reactivity study of the lithium amides (50. 5 1. 5 2. 5 3. 5 4. 5 5 and 5 61 with BIC&&
All NMR experiments to probe the reactivity of the lithium amides (50, 5 1 , 52, 53, 54-55 and 56) with B(C6F,), were carried out according to the following procedure. At
room temperature, the lithium amide (0.025 mmoles) was added to B(C,F,), (50.0 mg,
0.025 mmoles) in 0.5 mL of deuterated solvent (C6D6 or CD,CN) in a Teflon valve-sealed
tube. The 'H and ' 9 ~ NMR spectra were recorded within 10 minutes of mixing.
Formation of the neutral adduct species was compared to their synthesised analogs (vida
supra).
To 56 (5.58 mg, 0.02 mmoles), two equivalents of B(C6F,), (20.5 mg, 0.04 mmoles)
were added in 0.5 mL C,D,. The species 57 and LiB(C,FJ, are formed cleanly in
solution. 'H NMR (C,D,J of 57: 64.40 (s, 2H), 6.90 (m, 3H), 7.04 (d, 2H); '?F NMR (C,D6) of 57: 6-131.1 (m, 2F), -132.4 (m, 2F), -145.3 (d, 2F), -149.0 (t, IF), -150.0 (t,
IF), -153.0 (t, IF), -160.0 (m, 4F). -161.4 (t, 2F).
Svnthesis of U - C ~ H ~ C H ~ C I I , ) N I C ~ ~ ~ ~ ) ~ , 5 8
To 50 (5.86 mg, 0.02 mmoles), two equivalents of B(C,F,), (20.5 mg, 0.04 mmoles)
were added in 0.5 mL C6D,. The species 58 and LiB(C,F,), are formed cleanly in solution. 'H NMR (C,DJ of 58: 61.3 1 (dd, 3H), 4.94 (q, IH). 6.94 (m, 5H); "F NMR (C6D6) of 58: 6130.6 (m, 2F), -132.2 (m, 2F), -141.9 (m, IF), -142.7 (m, IF), -149.3
(t, IF), -150.4 (t, IF), -152.3 (t, IF), -160.0 (m, 4F), -161.1 (m, IF), -161.4 (m, IF).
REFERENCES
1. Samuel, E.; Rausch, M.D. J. Am. Chem. Soc., 1973,95, 6263.
2. Massey, A.G.; Park, A.J. J. Organornet. Chem., 1964,2, 245.
3 . Poe, R.; Schnapp, K.; Young, M.J.T.; Grayzar, I.; Platz, M.S. J. Am. Chem. Soc., 1992, 114, 5054.