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University of Groningen Non Flory-Schulz ethene oligomerization with titanium-based catalysts Deckers, Patrick Jozef Wilhelmus IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2002 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Deckers, P. J. W. (2002). Non Flory-Schulz ethene oligomerization with titanium-based catalysts Groningen: s.n. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 11-05-2017 General introduction 1.1 Introduction Catalysis is the key to many chemical transformations. Catalytic processes are responsible for about 80% of all chemicals produced, and form the basis of nearly all processes with a throughput of more than 10,000 ton per annum1. For successful industrial implementation of a catalyst certain prerequisites have to be fulfilled. The ideal catalyst has to combine high efficiency (i.e. effective use of starting materials, and minimal waste emission2), high selectivity (i.e. optimal conversion to the desired product3), and high total turnover (i.e. amount of product formed per given amount of catalyst) with durability (e.g. low toxicity4) and low overhead expenditure (i.e. cheap catalyst, and little maintenance). Understanding how catalyst structure and properties can affect these parameters, combined with chemical curiosity, is and will be the driving force for the future improvement and development of catalysis. Mutual efforts from industry and academia have made that the field of catalysis has grown from mere trial-and-error catalyst development to judicious design based on the rationalization of previous knowledge and theoretical background (when possible)5. 1.2 Transition metal catalysis Transition metals are at the core of a wide range of catalyst systems6. In comparison with main group metals, transition metals have more orbitals available for interactions, and with different symmetry (d-orbitals). The possibility to distribute its valence electrons in nine valence shell orbitals that can interact with other groups simultaneously, allows the formation of both σ- and π-bonds with, for example, reactive substrates. Additionally, transition metals can combine specific functions, e.g. Lewis acidity, substrate activation, redox chemistry and polar bonds. This diversity is a key factor in imparting catalytic properties to transition metals and their complexes. The ability to accommodate inert spectator ligands, in addition to reactive moieties, forms the basis for innovative transition metal catalyst design. These ancillary ligands primarily control the configuration and conformation of reactive complexed intermediates, and thus catalyst selectivity. Variation of the properties, either steric or electronic or both, of such ligands offers the opportunity to tune catalyst properties. The wide range of possible spectator ligands, metal 1 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ oxidation states and coordination numbers provides a near infinite array of potential catalysts. The unique character of the metal-carbon bond is another important element of (transition) metal-based catalysts, that has been exploited in many catalytic (and stoichiometric) metal-mediated transformations7. Carbon is more electronegative than any (transition) metal and, hence, metal-carbon bonds principally have a polarity Mδ+-Cδ-, making the carbon atom susceptible to electrophilic, and the metal center to nucleophilic attack. The polarity of the M-C bond can be easily tuned by (1) the choice of metal, (2) the oxidation state of that metal, and (3) the properties of its spectator ligands. The wide range of available organometallic metal-alkyl species offers a broad spectrum of reactivity with various substrates to allow interesting opportunities for C-C, C-H, C-O, C-N, and C-X bond formation. A very important application of C-C bond formation with organometallic catalysts is the polymerization and copolymerization of olefins, and, closely related, the oligomerization of olefins. Both processes will be discussed in more detail in the following sections. 1.3 Olefin polymerization 1.3.1 Polyolefins In 1955 Ziegler found that the polymerization of ethene to high molecular weight linear polyethene could be achieved with a TiCl4-AlClEt2 catalyst system8. At about the same time Natta showed that it is possible to stereoselectively polymerize propene to isotactic polypropene with a closely related catalyst system9. Shortly after these findings, mixtures of CrO3 and SiO2 were reported to effectively polymerize ethene to HDPE (High Density Polyethene, ‘Phillips process’)10. Since these initial discoveries almost half a century ago, the quest for new catalysts which allow the cheaper manufacture of existing polyolefins, or afford new and/or better olefin polymers has never ceased11. The production of polyolefins has witnessed a 100% increase in production volume over the past decade only12. The growth has recently slowed down at an estimated worldwide annual production of 80 million tons (in 2000) for the olefin polymerization industry. With the current state of the art in polyolefin production13, the properties of polyolefins can be varied within wide limits to provide not just substitutes but alternatives to currently available polymers, such as polyvinylchloride or polyacetates. The accessible range of stereo- and regioregularities and molecular weights of polyolefins allows for the manufacture of tailored polyolefinic materials with predetermined properties (e.g hardness, toughness, stiffness or transparency). Stereoselective homopolymerization of α-olefins (e.g. propene, 1-hexene and styrene) affords a wide range of polymers (isotactic, syndiotactic, hemiisotactic and block polymers), each with typical properties. Isotactic polypropene is a stiff but brittle polymer, in contrast to syndiotactic polypropene which is a robust transparent material14. 2 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Also copolymers are of great practical interest, and their total production volume matches that of homopolymers15. Flexible linear polyethene with short-chain branches (Linear Low Density Polyethene, LLDPE) is obtained by copolymerization of ethene with C4-C8 α-olefins16, and is predominantly applied in packaging and as thin films. Copolymerization of propene with ethene yields materials with lowered crystallinity17 to give access to EP-rubbers and heterophasic materials18. Copolymerization of ethene or propene with cyclic monomers (e.g. cyclopentene or norbornene) affords amorphous highly transparent materials that can be used in specialty optical and medical applications19. Terpolymerization of ethene, propene and dienes yields EPDM (Ethene Propene Diene Monomer) elastomers with high resistance to light and solvents20. Equally interesting are catalysts with living polymerization characteristics, which should be able to allow (elastomeric) block copolymers21. 1.3.2 Mechanisms of olefin polymerization and active species In classical heterogeneous Ziegler-Natta catalysts8,9 polymerization takes place at the dislocations and edges of TiCl3 crystals22. Cossee and Arlman proposed a mechanism of polymer chain growth by cis-insertion of the olefin into a Ti-C bond (Scheme 1)23, which is now generally accepted, and has been supported convincingly by fundamental studies (vide infra). Et Cl Cl Ti Cl Et AlEt3 Al Cl Cl Cl Cl Ti Et Cl Et Cl - AlClEt2 Cl Cl Cl C2 H4 Ti Cl Cl . . Et Et Cl CH2 Cl Ti CH2 Cl Cl Cl Ti Cl CH2 CH2 Cl Cl Cl Cl Ti CH2 Cl Cl CH2 Et to Ti center Scheme 1: Cossee-Arlman mechanism for polymerization Based on Natta's early ideas about the role of chiral surface sites in the formation of isotactic polyolefins24, models were proposed to explain the induction of stereoregular polymer growth by the chiral environment of the catalytic centers on the crystal edges25. In Figure 1 the incoming propene adopts the enantiofacial orientation which places the methyl substituent trans to the polymer chain at the 3 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ incipient C-C bond (left; the Cl* atom determines the orientation of the growing polymer chain, trans to Cl*). The cis orientation (right) is disfavored. P P y . . x z Ti Cl Model for the stereospecific polymerization of propene at a chiral titanium center on TiCl3 edges Figure 1: Due to the non-uniformity of the active sites in heterogeneous catalysts, and the limited experimental access to structural details there is no direct proof for the structures proposed. Homogeneous olefin polymerization catalysts should eventually allow more direct observations of the catalytically active species, and hence, on the mechanism of chain growth and stereocontrol26. Shortly after the synthesis of the first group 4 metallocenes27, the use of these compounds as polymerization catalysts was investigated28. Mixtures of Cp2TiCl2 and AlClEt2 were found to polymerize ethene with moderate activity, but propene and higher α-olefins were not polymerized at all. Breslow and coworkers29 and Chien30 proposed that ethene polymerization proceeds via olefin insertion into the Ti-C bond of a Cp2Ti-R electron-deficient species. Initially, ligand exchange between aluminum and titanium gives Cp2Ti(Cl)Et. Coordination of the Lewis acidic aluminum to the chloride (Ti-Cl) polarizes the Tiδ+-Clδ- bond to create an electrondeficient titanium Cp2Tiδ+R species, which readily inserts ethene (Scheme 2). This mechanism incorporates several features of the Cossee-Arlman mechanism: (a) the Lewis acidity of the metal center, and (b) the mutual cis orientation of the metalalkyl bond and the incoming monomer. δ- δ- . δ+ Cl AlRCl2 Ti R Scheme 2: 4 Cl C2H4 δ+ AlRCl2 CH2 Ti R CH2 δ- Cl AlRCl2 δ+ Ti CH2 R CH2 Proposed pathway for the polymerization of ethene by Cp2TiRCl⋅AlCl2R via cis-insertion General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ There was considerable debate about the nature of the active species. Various research groups advocated bimetallic halide-bridged titanium-aluminum intermediates as the active species involved31. Crystal structures containing both titanium and aluminum32 were obtained from reaction mixtures, but they are not conclusive as such since they represent degradation products. The possibility of the involvement of a cationic titanium active species [Cp2TiR]+ was suggested by Shilov and coworkers33, but the idea of ionic intermediates did not find widespread acceptance as the active species in olefin polymerization. Interest in the cationic intermediate was revived by the discovery of Eisch and coworkers that Ph-C≡C-SiMe3 reacts with Cp2TiCl2 in the presence of AlMeCl2 to give a cationic titanium vinyl complex [Cp2Ti-C(Ph)=C(Me)SiMe3]+, formally the product of alkyne insertion in the Ti-C bond of [Cp2TiMe]+ 34. A year later Jordan and coworkers reported the synthesis of the ionic species [Cp2ZrMe(L)][BPh4]35. This system is active in ethene polymerization, and provided the first experimental evidence that well-defined, cationic metallocene species are capable of olefin polymerization in the absence of aluminum alkyl activators. This and related findings36 established (alkyl)metallocene cations as crucial intermediates in homogeneous polymerization catalysis, and they are now fully accepted as the catalytically active species. 1.3.3 Activation of metallocene catalysts The moderate productivity of homogeneous catalyst systems in ethene polymerization almost meant the end of research into and development of homogeneous olefin polymerization catalysts. In the early 1980's, this drawback was overcome by the discovery of methylaluminoxanes (MAO) by Sinn and Kaminsky37. Although water is a poison for Ziegler-Natta catalysts, addition of small amounts of water to the catalyst system may dramatically enhance the activity. This water-effect has been observed for the system Cp2TiEtCl/AlEtCl238, and also for the normally inactive Cp2TiCl2/AlMe2Cl system39. Sinn and Kaminsky observed that the normally inactive halogen-free Cp2ZrMe2/AlMe3 system could also be activated tremendously by addition of water37. The presumed formation of methylaluminoxane (MAO) by partial hydrolysis of AlMe3, to give oligomeric mixtures of [MeAlO]n, was supported by its direct synthesis. Activation of Cp2ZrR2 (R = Cl, Me) with the isolated MAO led to exceedingly active catalysts for ethene polymerization. Furthermore, metallocene catalysts activated with MAO were capable of polymerizing propene and higher α-olefins in contrast to aluminum halide activated systems40. MAO is supposed to initially methylate the metal-halide bonds to give a Cp2MMe2 species (Scheme 3, reaction 1). Some of the aluminum centers in MAO are assumed to have a high tendency to abstract a methyl anion from these (dimethyl)metallocenes to give [Cp2MMe]+ species (Scheme 3, reaction 1), that are most likely stabilized by the [Me-MAO]- counteranion. These coordinative contacts are appropriately weak to give way, in the presence of (substituted) olefins, to olefin5 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ separated ion pairs that are required for polymer chain growth. Since its discovery, MAO has become the most commonly used activator for catalytic olefin polymerization. The realization that a weakly coordinating anion is crucial for high catalytic olefin polymerization activity led to the development of other (non-aluminum-based) activators capable of generating weakly coordinating anions. The catalyst systems thus generated were called ‘single site’ metallocene catalysts. Large anions as [B(C6H4R)]- (R = H41, 3-Et36g, 4-F42) and carboranes, such as [C2B9H12]- 36g,43, were used as counterions, but they showed fairly strong interactions44 with the [Cp2MMe]+ cations. Although active in ethene polymerization, most of these catalyst systems did not polymerize propene or higher α-olefins. A breakthrough was the introduction of the very weakly coordinating45 perfluorinated tetraphenylborate, [B(C6F5)4]-, as a counterion46. . M Cl Cl MAO M Me Me [PhNMe2][B(C6F5)4] MAO M Me M Me B(C F ) 6 5 4 [Me-MAO]- (1) + PhNMe2 + MeH (2) . M Me Me M Me Me Scheme 3: M Me Me [Ph3C][B(C6F5)4] B(C6F5)3 M Me B(C F ) 6 5 4 M Me Me B(C6F5)3 + Ph3CMe (3) (4) Different routes for the activation of metallocene precursors for olefin polymerization Reaction of Cp2ZrMe2 with [PhNMe2H][B(C6F5)4] afforded ethene polymerization catalysts with exceptionally high activities and gave the first active ‘single site’ zirconocene catalysts for propene and higher α-olefin polymerization (Scheme 3, reaction 2). To avoid the presence of the amine, alkyl-anion transfer with [Ph3C][B(C6F5)4] could be used to generate the same ion pair (Scheme 3, reaction 3)47. 6 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ As an alternative to the generation of cationic complexes, the strong Lewis acid B(C6F3)3 could be used to abstract a methyl anion from Cp2MMe2 to give [Cp2MMe][MeB(C6F5)3] (Scheme 3, reaction 4)48. Residual coordinative contacts between the cationic metal center and the anion via the abstracted methyl group have been identified48bc,49, but appear weak enough to allow olefin coordination to yield highly active polymerization catalysts. An excellent review on cocatalysts for metal-catalyzed olefin polymerization was recently written by Marks and Chen50. 1.3.4 Development of stereoselective metallocene catalysts The use of MAO and perfluorophenyl boron species as activator in metallocene catalysis gave access to active propene (and higher α-olefin) polymerization catalysts. As Natta discovered, heterogeneous polymerization catalysts, formed from TiCl3 and AlClxEt3-x, can polymerize propene with high stereoselectivity (to isotactic polypropene)9. In general, simple homogeneous metallocene catalysts, e.g. Cp2MMe2/MAO, afford atactic polypropene51. Designing ligands that allow one enantiofacial coordination of the incoming monomer over the other52, Ewen and Brintzinger reported the first homogeneous metallocene precatalysts for stereoselective propene polymerization (Figure 2)51,53. Due to conformational constraints the ethylenebis(indenyl) and ethylenebis(tetrahydroindenyl) ligands give the rac form of the corresponding so-called ansa-metallocenes54 chiral structures with C2 symmetry that are contained under catalysis conditions (note: the meso form is Cs symmetric and affords atactic polypropene). . M Cl Cl X M Cl Cl . MM = Ti, Zr X = C2H4, SiMe2 Figure 2: Structures of rac-(en)(thind)2MCl2 and rac-(X)(ind)2MCl2 Stereocontrol in these systems is similar as proposed for heterogeneous ZieglerNatta catalysts (vide supra)25. The indenyl* ligand determines the orientation of the growing polymer chain, and the incoming monomer adopts the enantiofacial orientation (trans) that minimizes the steric interactions between the polymer chain and the monomer (Figure 3)55. Via this ligand-induced stereoselectivity, these ansabisindenyl group 4 catalysts give isotactic polypropene. 7 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ P P y . . x z Ti Cl Figure 3: Stereocontrol in heterogeneous and homogeneous polymerization catalysts The stereoselectivity of these catalysts could not compete with that of heterogeneous Ziegler-Natta catalysts56, especially at elevated reaction temperatures, but the introduction of α- and β-substituents on the indenyl ligand has afforded homogeneous catalysts with similar stereoselectivity and molecular weight as heterogeneous Ziegler-Natta catalysts, albeit much more active57. The structure-properties relationships in metallocene catalysts, and the underlying mechanisms have been extensively investigated58, and have led to homogeneous metallocene catalysts for syndiotactic59, hemiisotactic60 and stereoblock61 polypropene. 1.3.5 Chain termination processes Two main mechanisms62 (Scheme 4) for chain termination in metallocenecatalyzed olefin polymerization are generally assumed: β-H transfer to metal57bc and β-H transfer to monomer63. When β-H transfer to monomer prevails, the rate of chain termination increases with increasing olefin concentration, but so does insertion, and hence, molecular weights will be independent on monomer concentration. For β-H transfer to metal, the rate of chain termination is olefin independent, and molecular weights will increase with increasing monomer concentration. Another way to release the growing polymer chain is via chain transfer to an added transfer agent. Transfer agents can be used to control the molecular weight of the polymer. The most common transfer agent for that purpose is molecular hydrogen that has been successfully employed in Ziegler-Natta catalysis64. Other, but less frequently occuring, chain release processes are β-methyl transfer65 (quite common for metallocene catalysts with highly substituted cyclopentadienyl ligands) and transalkylation to the aluminium cocatalyst66 (in catalyst systems with high Al/metal ratio and low productivity). 8 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ H M M CH2CHR(P) H CH2 H M CR(P) CR(P) M - CH2=CR(P) H2C β-H transfer to metal + CH2=CHR . CH2CH2R . M - CH2=CR(P) β-H transfer to monomer H2C H2C M CHR M CH2CHR(P) Scheme 4: CHR H CR(P) H2C CH2CH2R M CR(P) H2C Chain termination via β-H transfer to metal and to monomer 1.3.6 Alternative ligand systems and group 4 catalysts Group 4 polymerization catalyst research has mainly been focused on metallocenes, CpR2MCl2. Nevertheless, numerous efforts have been made to explore the potential of other ligands and catalyst systems. Nowadays a wide range of non-metallocene group 4 catalysts has been prepared and tested in catalytic olefin polymerization (Figure 4)13d. One particular modification of the cyclopentadienyl ligands can be achieved by insertion of a BY unit into the Cp ring to afford monoanionic boratobenzene ligands, (C5R5BY)-. The corresponding zirconium complexes (A) are highly active in ethene polymerization when activated with MAO67. By changing the Y substituent on boron the relative rates of propagation and β-H elimination can be controlled and a range of polymer molecular weights is available with these versatile catalyst systems68. Also substitution of a CH unit for a P atom has been investigated by various research groups69. The resulting catalysts (B) show activities comparable to metallocene dichloride species. For monocyclopentadienyl group 4 complexes (C) moderate activities have been reported70, especially for the syndiospecific polymerization of styrene with [CpRTiR'2]+ species. An important class of olefin polymerization catalysts is formed by the so-called ‘constrained geometry catalysts’ (‘CGC’, D) developed by Dow and Exxon71, based on Cp-amido ligands72. The MAO activated procatalysts are highly active in homopolymerization of olefins as well as in copolymerization of ethene and higher α-olefins73. Various groups have investigated the scope of the Cp-amido framework and its effect on catalysis74. Replacing the NR group by an oxygen atom 9 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ affords Cp-alkoxide ligands that have also been successfully applied as ligands for olefin polymerization active group 4 metal species (E)75. B Y R R P Zr Cl Cl B Y Zr Cl Cl P A Cl B Cl N Cl O C R' N N M N Cl Cl N M N R N Cl Cl M R N N N Ar F Cl S Cl R I N M Cl R'' O Cl R'' O K M R' R Cl Cl N O R' Figure 4: Cl M Cl N N O R J Cl Cl N R' R O N H R' M . R R' G O M Cl Cl E D Ar . M Cl Cl Si M R L Non-metallocene group 4 procatalysts for olefin polymerization The high activity of the ‘constrained geometry catalysts’ prompted the development of other catalysts containing amide or other nitrogen based ligands. Monodentate amido76 and bidentate bisamido77 (F) group 4 complexes have been reported. Olefin polymerization activities for the bidentate complexes vary significantly and depend strongly on the chelate ring size and the nitrogen substituents78. Also tridentate bisamido ligand systems containing a third neutral additional donor atom (N, O) have been prepared and introduced on group 4 metals (G). The resulting catalysts again vary strongly in olefin polymerization activities79. The combination of an amido and an imine function gives a monoanionic six-electron donor ligand, isostructural and isoelectronic with a cyclopentadienyl ligand. These so-called 10 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ amidinate ligands (H) yield group 4 catalysts with moderate activity in olefin polymerization80. Amidinate ligands have also been employed in combination with other ligands such as Cp81 and amine ligands82. The latter systems show remarkably high activity for propene and 1-butene polymerization. Also β-diketimates, formally a higher homolog of amidinate ligands, have been introduced on group 4 metals (I), but only give moderately active catalysts83. Besides nitrogen-based ligands, oxygen donor ligands have been used in group 4 olefin polymerization catalysis. Chelating bisphenoxide ligands (J) have given moderately to highly active catalysts for olefin homopolymerization84 and copolymerization of α-olefins with ethene85. Bisalkoxide ligands with one (K)86 or more (L)11d,87 additional donor atoms (N, O, S) have been shown to improve the olefin polymerization activity dramatically. 1.3.7 Group 3 metal and lanthanide catalysts Neutral group 3 (Sc, Y) and lanthanide alkyl complexes are isoelectronic with cationic group 4 alkyl complexes. This analogy has been used in the design of group 3 and lanthanide catalysts for olefin polymerization. The potential advantage would be that they could be active as such, and would not require addition of a cocatalyst. Ligand development for organoscandium and organoyttrium olefin polymerization catalysts has been following the general trends of group 4 catalysts, although the former are much less well explored88. Both scandium65c,72,89 and yttrium90 complexes containing Cp-based ligands have been prepared, but they exhibit low to moderate ethene polymerization activity. For yttrium also benzamidinate ligands have been employed affording catalysts with very low activity91. R N N Sc Ph Figure 5: R Y N M N N SiMe3 N Cationic group 3 catalysts for olefin polymerization Lanthanide-based olefin polymerization catalysts have invariably been stabilized with substituted Cp analogs. The catalysts have been reported to show at best moderate activity92, although high initial activities have been observed93 (note: [Cp*2LnH]2 is the ethene polymerization catalyst with the highest initial activity reported). 11 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Recent investigations in this area have afforded highly active cationic group 3 and lanthanide catalyst systems94, such as [(η5-C5Me4CH2CH2NMe2)Sc(CH2Ph)]+ (Figure 5, M) and {[N,N’-R2-tacn-N’’-CH2CH2N(t-Bu)]Y(CH2SiMe3)}+ (N, R = Me, i-Pr; tacn = 1,4,7-triazacyclononane). 1.3.8 Late transition metal catalysts for olefin polymerization The impetus for late transition metal95 olefin polymerization catalysis is their decreased oxophilicity and greater functional group tolerance. These features would make such catalysts ideal for copolymerization of olefins with polar comonomers. The current advances in late transition metal chemistry have been recently reviewed96. This section will be limited to a discussion of the most important catalyst systems (Figure 6). Group 6 heterogeneous catalysts play an important role in the commercial manufacture of polyethene. The Phillips catalyst consists of a silica support treated with CrO3, which is subsequently reduced to an, as yet elusive, low-valent chromium species10. This system is extremely active and offers the advantage of not requiring a cocatalyst. Homogeneous chromium catalysts have recently been reviewed97. The most active homogeneous chromium catalyst thus far is the Cp-amino Cr(III) compound (O) reported by Jolly and coworkers98. Recently, BASF patented some highly active chromium systems based on a substituted neutral 1,3,5triazacyclohexane (tac, P) ligand on Cr(III)99. By varying the substitution pattern on the tac-ligand the catalyst system can shuttle between a polymerization100 or trimerization101 catalyst. R N R N N Cr X X N R' N R'' X R' X R'' X R' M = Fe, Co Q . Ar Ph Ph N Ni R' L O P Ph O Ni Ph L Ar R N M R N Ar R R Figure 6: 12 R N M X X R N R Cr P O . R' S X Y M = Ni, Pd T Late transition metal procatalysts for olefin polymerization General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Few active group 8 and 9 catalysts have been reported. The most active polymerization catalysts are iron(II) species stabilized with a 2,6-bis(imino)pyridine ligand (Q) with highly substituted aryl substituents on the imino nitrogens102. By decreasing the size of the ortho substituents the system can be converted into a very active olefin oligomerization catalyst102b,103 The corresponding cobalt(II) compound is the most active group 9 catalyst reported thus far, but its activity is a magnitude smaller than that of the iron(II) compound. Group 10 catalysts with anionic [P,O] ligands (R), well-known for the Shell Higher Olefin Process (see section 1.4.3)104, can polymerize ethene under certain conditions105. Crucial to the switch from oligomerization to polymerization is the removal of the phosphine ligand, or the use of more labile stabilizing ligands, such as pyridine or ylides. Recently, Grubbs reported nickel complexes with anionic [N,O] ligands (S) that are highly active in homopolymerization of ethene and copolymerization of ethene with polar comonomers11c. Another important class of group 10 polymerization catalysts is formed by nickel(II) and palladium(II) complexes with bulky diimine ligands (T)106. These catalysts are very versatile in their polymerization behavior. Block polymers107, chain-end-controlled isotactic polypropene108 and ethene copolymers with polar comonomers109 belong to the possibilities. The diimine group 10 systems have been subject of both theoretical110 and experimental111 investigations. 1.4 Ethene oligomerization to higher linear α-olefins 1.4.1 Linear α-olefins Apart from ethene and propene, higher (C4-C20) linear α-olefins have become increasingly interesting as monomer112 (to give poly-α-olefins) and as comonomer113 (to form LLDPE16 and HDPE114) in catalytic olefin polymerization. Besides the use as polyolefin building blocks, higher linear α-olefins find their main applications as starting material for plasticizers (C6-C10) and surfactants (C10-C20). The C6-C8 α-olefins can be hydrocarboxylated to give predominantly the linear carboxylic acids, which can be used as additives for lubricants together with their esters. Hydroformylation of C6-C10 α-olefins affords odd-numbered linear primary alcohols. They can, among others, be converted to polyvinylchloride (PVC) plasticizers. Epoxidation of C10-C12 α-olefins opens a route to bifunctional derivatives or ethoxylates as nonionic surfactants. The C14-C16 α-olefins can be alkylated with phenols to yield alkylphenols, which can be employed as surfactants as well as lubricant oil additives. Several processes are known to obtain α-olefins: wax-cracking of paraffins, oligomerization of ethene, dehydrogenation of paraffins, dimerization and metathesis of olefins, dehydration of alcohols and electrolysis of straight-chain carboxylic acids. Especially the second process, oligomerization of ethene115, has been industrially 13 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ utilized to manufacture large amounts of linear α-olefins in the C4-C20 range due to the abundance of ethene and the high product quality. In 1999, the worldwide annual production of higher α-olefins was about 2.5 million tons116. The increasing need for α-olefins, especially as comonomer (C4-C8), causes the market still to expand, as illustrated by expansion plans of all major α-olefin producing companies. 1.4.2 Aluminum-catalyzed ethene oligomerization The first ethene oligomerization process was developed by Ziegler (Alfen process) in the early 1950's. During extensive studies on organoaluminum compounds Ziegler discovered the Aufbaureaktion117, a series of sequential repetitive olefin insertions into the Al-C bond (Scheme 5, equation 1), under high ethene pressure (100-400 bar) and moderate temperatures (100-150 °C). Relevant for the production of linear αolefins was the observation that the insertion of ethene into an Al-H bond is reversible (Scheme 5, equation 2)118, indicating that olefin elimination can occur. By varying pressure, temperature and reaction time Ziegler was able to induce displacement of the olefin (Verdrängung) affording triethyl aluminum (AlEt3) and three equivalents of α-olefins (Scheme 5, equation 3)119 at high temperature (250400 °C) and low ethene pressure (10 bar). . ( )n AlEt3 + 3n CH2=CH2 Al Al-H + RCH=CH2 Al-CH2CH2R Al ( )n + 3 CH2=CH2 3 Scheme 5: AlEt3 + 3 (1) 3 (2) ( )n-1 . (3) Key steps in aluminum catalyzed ethene oligomerization Unfortunately, the separation of the α-olefins from the AlEt3 is extremely difficult, since the boiling points of AlEt3 and 1-dodecene are close together, and most industrially interesting product mixtures contain significant quantities of 1-dodecene. The problem is often referred to as the Ziegler dilemma, since his group never found a commercially acceptable solution to this problem. Under the right conditions, e.g. at 200-250 °C, chain-growth and displacement can be carried out simultaneously120. The AlEt3 is continuously regenerated via displacement while chain-growth is proceeding. Small amounts of AlEt3 (0.4 wt%) are needed which are hydrolyzed after reaction to facilitate separation. By reinsertion of the displaced higher α-olefins β-branched α-olefins are produced, but by utilizing 14 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ high ethene pressure (250 bar) formation of these branched olefins can be suppressed. This principle forms the basis of the Gulf/Chevron120,121 process, which is responsible for about 15% of the annual worldwide production116. In contrast to the Gulf/Chevron process, Ethyl Corporation122 uses a two-stage (one catalytic, one stoichiometric) α-olefin synthesis to overcome separation problems. The first stage is comparable to the Gulf process yielding predominantly C4-C10 αolefins. These olefins are transalkylated with long-chain aluminum alkyls, liberating the higher α-olefins, whereas via chain-growth the remaining short-chain branched aluminum alkyls are converted into long-chain branched analogs in the stoichiometric part of the process. Via this two-stage α-olefin synthesis, Ethyl Corporation furnishes about 30% of the annual worldwide production of α-olefins116. 1.4.3 The Shell Higher Olefin Process (SHOP) The most important discovery made by Ziegler in 1952117a,119 was the observation that in the presence of nickel salts the alkylaluminum-catalyzed Aufbaureaktion is directed to yield mainly butenes. This phenomenon is often referred to as the ‘nickel effect’123. These findings initiated an intensive investigation into organonickel chemistry. Wilke and coworkers were the first to observe the ligand effects on selectivity in nickel-catalyzed reactions124. At Shell, ligand variations were extended to bidentate [P,O] chelates (Figure 7)125, and are now the basis for the Shell Higher Olefin Process (SHOP)104,115, which accounts for about 35% of the annual worldwide production of α-olefins made via ethene oligomerization116. The SHOP-process is one of the larger industrial applications of homogeneous catalysis. In Table 1, the product distribution of α-olefin products manufactured via SHOP is compared with both aluminum-based processes (vide supra) and wax-cracking. P . R Ni . L O Figure 7: Generalized structure of SHOP catalyst precursors Table 1: Comparison of product distributions (wt%) of C6-C18 α-olefins126 α-Olefins Branched olefins Paraffins Dienes Monoolefins Wax-cracking 83-89 3-12 1-2 3-6 92-95 Gulf/Chevron 91-97 2-8 1.4 99 Ethyl 63-98 2-29 0.1-0.8 >99 SHOP 96-98 1-3 0.1 99.9 15 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ 1.4.4 Neutral nickel(II) complexes for ethene oligomerization Nickel(II)-based catalysts with monoanionic [P,O] chelating ligands for the oligomerization of ethene have been extensively studied127,128, most notably by Keim and coworkers. It turned out that the [P,O] chelating ligand controls the selectivity of the ethene oligomerization127a, while the other ligands stabilize the complex. The importance of the chelate interactions on selectivity, was elegantly illustrated by Braunstein and coworkers127h. By inducing hydrogen bond interactions with the oxygen atom of the [P,O] ligand, β-H elimination became dominant over propagation, thus affording lower α-olefins. Ph Ph P Ph Ni Ph O P CH2=CH2 / 70 °C PPh3 Ph P Ni O F3C F3C C Figure 8: . Ph Ph P O PPh3 O B Ph Ph H Ni - styrene A . Ph Ph H Ni O PCy3 D Evidence for nickel(II) hydride species It is largely accepted that the active species in ethene oligomerization is a nickel(II) hydride. The mechanism for the hydride formation is supported by the reaction of the nickel(II)-phenyl species (A) with ethene to yield the corresponding hydride intermediate (B) and styrene (Figure 8, top)127d, which could be detected by GC. In the case of complex C, in situ NMR studies indicated the existence of a nickel hydride127c, whereas the stable nickel hydride D could be isolated and characterized127d. Multiple ethene insertions into the Ni-H bond (followed by β-H elimination) gives a Flory-Schulz distribution of linear α-olefins (Scheme 6). Over the years many other monoanionic chelating ligands, e.g. [P,N]129, [P,S]130, [S,O]131, [N,O]132, [As,O]133,[O,O]134, [N,N]135 and [S,S]136 chelates, have been investigated, mainly by Keim, Braunstein and Cavell. 16 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ P R Ni L O CH2=CH2 P Ni H O P P . `Rn Ni O Ni O Rn P n-2 CH2=CH2 Scheme 6: CH2=CH2 . CH2=CH2 Ni O Postulated catalytic cycle for Ni-catalyzed ethene oligomerization 1.4.5 Cationic group 10 ethene oligomerization catalysts In general, the neutral catalytically active species require high reaction temperatures and pressure for the oligomerization of ethene. SHOP operates typically at 80-120 °C and 70-140 bar. Since the late 1960's, cationic Ni(II) allyl complexes were known to oligomerize and dimerize ethene137. Later on, also cationic Pd(II)138 complexes (as well as PdCl2139) were found to oligomerize ethene. Keim and Tkatchenko found that neutral [P,O] chelates, which afford cationic nickel complexes140, allow ethene oligomerization at much lower temperature and pressure141. Similar palladium compounds were found to dimerize ethene to butenes142. This discovery led to development of group 10 complexes with neutral chelating ligands, such as [N,N] (diimine143, pyridylimine144, and diamine145) and [P,N]146, to provide highly active ethene oligomerization catalysts. 17 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ 1.4.6 Group 8 and 9 based catalysts for ethene oligomerization Very few active group 8 and 9 oligomerization catalysts have been reported in the literature, in contrast to the well-documented wealth of group 10 catalysts. In the early days of homogeneous catalysis, simple transition metal salts, such as RhCl3139bc,147, were studied and found to be active in the dimerization of ethene to butenes. For the Co(acac)3/AlEt3 system a selectivity of up to 99.5% 1-butene was reported148. While the organometallic chemistry developed, new well-defined hydride and alkyl species of group 8149 and group 9150 were tested in catalytic ethene oligomerization. Only dimerization activity was found, indicating fast chain transfer in these catalyst systems. Until recently, the only active group 8 or 9 catalyst (Figure 9, U) for ethene oligomerization was reported by Braca and Sbrana151 albeit with low activity. Cl . R' CO Ru CO CO Figure 9: R N N R' N . M R'' U R R' X R'' X R' M = Fe, Co Q Group 8 and 9 ethene oligomerization catalysts In 1998, Brookhart103 and Gibson102b reported highly active catalysts for ethene oligomerization based on iron and a neutral tridentate 2,6-bis(arylimino)pyridine ligand (Q). By varying the substitution pattern, especially on the ortho position, of the aryl groups the activity and selectivity can be tuned152. Analogous Co systems have been reported to show similar reactivity but less active153. 1.4.7 Early transition metal catalyzed ethene oligomerization In the late 1950's, titanium precursors, e.g. RnTiCl4-n and Ti(OR)4, were modified with organohalide aluminum compounds, RnAlCl3-n, and found to be active in ethene oligomerization154 and dimerization155. The active species and the mechanistic aspects of these titanium catalysts are not well-understood156. Similar to ethene polymerization (see section 1.3.2), an empty coordination site at an electrondeficient titanium center, and a titanium-alkyl bond are fundamental requirements for ethene oligomerization, but different proposals have been put forth concerning the structure of the Ziegler-Natta catalyst active centers (Figure 10). They consist of either bimetallic systems157, in which titanium is bonded to aluminum through halide or halide-alkyl bridges, but also monometallic titanium systems23,158, and truly ionic species159. Both the bimetallic160 and monometallic161 systems are backed up by 18 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ theoretical calculations. Recently however, it has been suggested that active species similar to those found in polymerization systems are present (vide infra, see section 1.3). Chain termination in these systems is presumed to occur via β-H transfer from the oligomer chain to coordinated ethene (see Scheme 4)162. Reduction of Ti(IV) species to Ti(III) deactivates the oligomerization catalyst, and generates catalytically active polymerization centers (see section 1.3.2). Cln Ti P M P Ti Ti . X Cl Al . Cl P Ti Figure 10: Al P Cl Ti Cl Cl P Bi- and monometallic types of active centers The TiR4/XnAlR'3-n systems can be improved by addition of ancillary ligands, such as ketones, amines and phosphines. These additives increase the selectivity to linear α-olefins163 by increasing the electron density on titanium, which allows ethene to coordinate dominantly. The incorporation of higher α-olefins is inhibited and, in consequence, suppresses the production of branched olefins. In addition to these electronic effects, steric effects of these ligands are able to increase the linearity of the olefins formed. In the early 1970's, comparable zirconium/aluminum systems were tested in catalytic ethene oligomerization164. The zirconium catalysts are, in general, more active and more selective than the titanium catalysts. However, a constant ethene supply is necessary to reach high selectivities to linear α-olefins. As an advantage to the titanium catalysts, catalyst deactivation via reduction of zirconium does not seem to occur115a. Deactivation of the zirconium species is assumed to proceed via disproportionation of two Zr-alkyl species to give inactive bimetallic Zr-alkyl-Zr species and alkane. Similar to titanium-based systems, donor ligands improve the performance of the zirconium/aluminum catalysts. These zirconium/aluminum/donor ligand catalyst systems were used by Idemitsu in a relatively small industrial process for α-olefin manufacture165. The role of the added ligands, like the nature of the active species, is subject of debate. After the initial Idemitsu patents appeared, a vast number of donor ligands have been used, most notably by Idemitsu itself, in zirconium-based ethene oligomerization catalysts166. Based on the similar activities for ZrCl4⋅ester adducts 19 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ and Zr(CH2Ph)4 in ethene oligomerization studies, Young reasoned that the ancillary ligands primarily serve to solubilize the ZrCl4 to allow rapid interaction between the zirconium and the aluminum cocatalyst167. However, recently zirconium-based systems (Figure 11) have been prepared in which catalyst activity168 and product distribution169 can be controlled by steric and electronic ligand variation suggesting active participation of the ancillary ligand. A number of zirconocene-based catalysts have been found active in ethene oligomerization68,170, including cationic species formed in situ in the absence of aluminum cocatalyst171. The latter observation argues that an active cationic species, similar to that suggested for polymerization catalysts, is highly likely. R' B OEt . Zr Cl Cl B OEt N Zr Cl Cl N N R'' X X R'' N X = O, S V Figure 11: W Zr R Cl . Cl R R' X Zirconium-based ethene oligomerization catalysts 1.4.8 Inorganic heterogeneous catalysts for ethene oligomerization Materials such as nickel oxide and nickel salts supported on silica and/or alumina have been reported to be active catalysts for the oligomerization of ethene172. The activities and selectivities of these catalyst systems are strongly affected by product adsorption (on the support), and by isomerization and cooligomerization to produce branched olefins. Increasing acid strength of the silica-alumina support achieves higher activity per nickel site173. Nickel oxide on an alumina support modified with sulfates proved to be a highly active and selective catalyst for ethene oligomerization174, and variations of this system have been thoroughly investigated in the last five years175. Also nickel-exchanged zeolites176 as well as zeolites as such177 have been tested for ethene oligomerization activity. But these systems show poor selectivity to linear αolefins. Recently, Jacobs and coworkers reported the successful use of zeolites as support for active and selective metallocene-based oligomerization catalysts178. 1.4.9 Selective olefin oligomerization All ethene oligomerization catalyst systems described in the foregoing part of this section afford a distribution of linear α-olefins (Flory-Schulz or Poisson). Very few oligomerization catalysts for selective olefin oligomerization, to a single olefin, except dimerization (of ethene or α-olefins), have been reported179, some of which 20 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ have been employed industrially, most notably in the DIMERSOL process180. Dimerization can either occur via an insertion-type pathway, i.e. two insertions followed by β-H elimination181, or via a metallacyclic pathway involving oxidative coupling182 (Scheme 7). 1-butene M H M M M H β-H . . 1-butene M [M] Scheme 7: M β-H or 1,3-H shift [M] Different pathways for ethene dimerization to 1-butene A similar metallacyclic pathway is proposed for the selective ethene trimerization to 1-hexene by chromium-based catalyst systems (mixtures of Cr salts with alkylaluminum compounds, and added ligands, especially imidazoles, or coordinating solvents, such as 1,2-dimethoxyethane)183. A low-valent Cr species couples two ethene molecules to generate a chromacyclopentane, which subsequently inserts a third ethene molecule to give a chromacycloheptane (Scheme 8)184. From the latter, 1-hexene is formed via β-H elimination (or 1,5-H shift) and reductive elimination to regenerate the low-valent Cr species98. Recently, triazacyclohexane-based chromium precursors activated with MAO101, TaMe2Cl3185, and (arene)2VX (X = Cl, Br, [BF4]-)186 systems were also found to effectively affect this unique and intriguing reaction, presumably via the same pathway. 1-hexene 2 ethene [Cr] Cr . Cr H . Cr Cr ethene Scheme 8: Proposed catalytic cycle for selective ethene trimerization by Cr-based catalysts 21 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ 1.5 Switching from ethene polymerization to oligomerization activity In the previous two sections, 1.3 and 1.4, we have encountered various systems that can be tuned to either ethene polymerization or oligomerization activity (1) by controlling the reaction conditions or (2) via subtle, easily accessible ligand variations. For example, Ziegler discovered that the system TiCl4-AlClEt2 polymerizes ethene to high molecular weight polyethene8, but by controlling the reaction conditions (temperature, ethene pressure) the same system can be switched to a moderately active ethene oligomerization154 or dimerization catalyst155. Ligand variation, potentially, offers a more elegant route to govern interconversion between polymerization and oligomerization active species. The key to a successful switch is control over the relative rates of chain propagation (kp) and chain termination (kt). Increasing kt over kp will lower the molecular weight of the polymer, and eventually lead to the production of α-olefins. Recently, a range of transition metal catalysts have been developed that readily alternate between ethene polymerization and oligomerization activity by changing the steric or electronic properties of the ancillary ligand (Figure 12). The diimine Ni(II) and Pd(II) catalysts (A) can be easily tuned by the ortho substituents on the aryl rings. Bulky substituents, such as iso-propyl, effectively block the axial positions110, and suppress the chain termination transition state, to afford polyethene106. The sterically undemanding ortho methyl groups are not adequately bulky, and β-H elimination can readily take place to give a distribution of α-olefins144. The ortho aryl substituents in the bis(imino)pyridyliron(II)187 and cobalt(II) complexes (B) exert a similar steric control on the rate of chain termination, and thus on product formation102,103. The substituents on the 1,3,5-triazacyclohexane ligand in the tac-chromium systems (C) presumably perform steric control in reverse fashion. Linear alkyl chains (C1C12) on the nitrogen atoms lead to moderately active ethene polymerization catalysts100. Substituents on the 2-position of the alkyl chain increase the steric congestion at the metal center and convert the catalyst into a selective ethene trimerization catalyst (note: α-olefins are exclusively trimerized, also with straight chain alkyl nitrogen substituents101). In these systems, activity can also be tuned by the reaction conditions. Similar effects are observed in (η5C5H4CH2CH2PR2)CrCl2/MAO ethene oligomerization systems188, in which the molecular weight of the α-olefins produced decreases with increasing steric bulk of the phosphor substituents189. As a last example, the relative rates of chain propagation and chain termination can be controlled by electronic substituent effects (although steric effects cannot be completely excluded). By changing the boron substituent of di-iso-propylamine to ethoxide (or phenyl) in boratobenzene zirconium complexes (D), the rate of β-H elimination increases, which results in the formation of lower molecular weight oligomers68 instead of polyethene67. 22 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Oligomerization catalysts R R N N Polymerization catalysts R R N N M M Y X Y X N . N N N M X R N Cl N (= R) R N Cl Cl B OEt Zr Cl Cl B OEt 1.6 B (M = Fe, Co) R Cr Figure 12: . N X X R' N Cl M X N R N A (M = Ni, Pd) N C Cr Cl Cl B N(i-Pr)2 Zr Cl Cl B N(i-Pr) D 2 Effects of ligand variation on ethene conversion characteristics Aim of this investigation Group 10 compounds with anionic [P,O] ligands are well-known to oligomerize ethene and play a central role in SHOP104. There have also been reports that SHOPtype oligomerization catalysts can polymerize ethene under certain conditions105. Crucial to the formation of high molecular weight polymers rather than oligomers is the use of labile groups L, such as pyridine or ylides, or the effective removal of phosphorus-based L groups190, such as PPh3. In other words, the catalytic activity of the Ni(II) species can be controlled by the nature of the neutral ancillary group L. 23 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ Ph Polymerization Oligomerization pyridine, ylide PPh3, PCy3 Ph P Ph O Ni Ph L L . . M Me L C4H8S, toluene PhOMe, PhNMe2 L Figure 13: Reactivity control via neutral π-donor ancillary ligands Similarly, the presence of weak donor ligands, such as aromatic ethers and amines, can convert group 4 propene polymerization catalysts [Cp2MMe]+ into oligomerization catalysts, albeit with a significant reduction in activity, since these donor ligands effectively compete with the olefinic substrate for the coordination site required for chain growth36e,f. We became interested in the possibilities to tune the properties (activity, selectivity and stability) of catalytically active species by neutral donor moieties. We decided to combine this idea with the concept of ligand-controlled catalyst reactivity, and set out to design ancillary ligands substituted with neutral donor-functionalized side chains that provide the opportunity of intramolecular coordination. The chemistry of cyclopentadienyl complexes of this type191 (e.g. Cp-N192, Cp-O193, Cp-P, Cp-As and Cp-S194) and their potential application as catalysts present a rapidly growing area of research. As the cyclopentadienyl-based catalyst systems of interest, we choose cationic monocyclopentadienyl titanium species, [CpTiR2]+. These half-sandwich titanium complexes show interesting catalytic activity, most notably in the syndiospecific polymerization of styrene. Ishihara and coworkers were the first to report that syndiotactic polystyrene, a new macromolecule at that time195, could be produced with high stereoregularity and yield under conventional conditions using half-sandwich titanium compounds activated with MAO196. Syndiotactic polystyrene has become an important engineering plastic since197, and many publications and patents have appeared on the subject198. Various monocyclopentadienyl titanium complexes with chelating Cp ligands with neutral oxygen199 and nitrogen200 donor moieties have been prepared (Figure 14), and the effect of the π-donor on catalyst activity was studied. The presence of the tethered amino groups has clear, but not spectacular, effects on olefin polymerization reactivity of these complexes (lower activity and stereoselectivity for styrene polymerization, and higher activity in ethene and propene polymerization, compared to unsubstituted CpTiCl3). 24 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ R N . Cl Ti O Cl Cl Figure 14: N Cl Ti Cl Cl Cl Ti Cl Cl . Half-sandwich titanium compounds with intramolecular π-donor groups Recently, the catalyst [Cp*TiMe2][MeB(C6F5)3] was found to display interesting olefin polymerization activity with ethene and propene in toluene. Bochmann and coworkers reported the living polymerization of propene to high molecular weight atactic polypropene201, and Pellecchia and coworkers showed that polyethene produced by this catalyst contains considerable amounts of n-butyl side chains202. It was suggested that the catalyst is partly converted to a species that trimerizes ethene to 1-hexene, which is then incorporated into the polymer. The latter observation is very interesting, because it indicates that [CpTiR2]+ species can be transformed, from polymerization into oligomerization (i.c. trimerization) catalysts under certain conditions. We sought ways of stabilizing electron-deficient half-sandwich titanium cations in order to study the catalytically active species, but also their reactive properties. To this purpose, we introduced (neutral) aromatic substituents on the cyclopentadienyl ligand, which have the potential of acting as weakly and reversibly coordinating ligands to the titanium center. The labile character of aromatic coordination to titanium has been previously demonstrated. Baird and coworkers showed that [Cp*MMe2]+ cations (M = Ti, Zr, and Hf) can bind arene solvents (benzene, toluene, xylenes, mesitylene, styrene) reversibly203, and that this interaction is significantly weaker for Ti than for Zr and Hf204. The feasibility of intramolecular arene coordination was suggested by observations of Chien and Rausch on the (η5C5Me4CH2CH2Ph)TiMe3/[Ph3C][B(C6F5)4] system, where NMR evidence was obtained for η6-coordination of the arene moiety in the thermally labile cationic dimethyl species205. For our studies on cyclopentadienyl-arene systems, we decided to use benzylsubstituted Cp ligands of general formula [C5H3R-CR’2-Ar]-, which can be easily prepared by reaction of the appropriate 3-R-6,6-R’2-fulvene with aryl lithium to afford lithium salts of the desired ancillary cyclopentadienyl ligands. The wide range of suitable fulvenes and aryl lithium salts gives access to a readily available library of ancillary ligands. Introduction of the ligands on titanium(IV) and subsequent derivatization are straightforward. During the course of this investigation, other groups reported the preparation of related bis-206 and monocyclopentadienyl207 group 4 metal complexes and their properties in catalytic olefin polymerization208. 25 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ 1.7 Survey of the thesis In the foregoing part of Chapter 1, olefin polymerization and ethene oligomerization with transition metal-based catalysts is discussed. A synopsis of catalyst systems that can switch from olefin polymerization to oligomerization activity is given together with the aim of the investigation described herein. In Chapter 2, the synthesis and characterization of (η5-C5H4CMe2Ar)TiR3 (Ar = Ph, 3,5-Me2C6H3; R = Cl, Me, CH2Ph, CH2CMe3, CH2SiMe3) complexes are described. In addition, the thermolysis of the trialkyl complexes (η5C5H4CMe2Ar)Ti(CH2R’)3 (R’ = Ph, CMe3, SiMe3), leading to ortho cyclometalation of the ancillary cyclopentadienyl ligand, and a kinetic study of this process are presented. The thermal decomposition shows simple first-order kinetics in the starting material. The respective rates of thermolysis for the different alkyl compounds show some aberrations from previously reported data for similar compounds, and a tentative explanation is provided. Deuterium labeling studies indicate a pathway involving an alkylidene intermediate, and the selectivity of alkylidene formation and the subsequent C-H addition are discussed. Chapter 3 covers the generation of the cationic complexes from the neutral alkyl precursors activated with B(C6F5)3 and [Ph3C][B(C6F5)4] to give initially cationic ansa-[(η5,η6-C5H4CMe2Ar)TiR2]+ species (Ar = Ph, 3,5-Me2C6H3; R = Me, CH2Ph). For [(η5,η6-C5H4CMe2-3,5-Me2C6H3)TiR2][RB(C6F3)3] in bromobenzene, thermal decomposition gives a dimeric Ti(III) dicationic species. The pathway involves titanium-boron-alkyl scrambling, reduction from Ti(IV) to Ti(III) and solvent C-Br bond activation. The Ti(III) dimer is the first structurally characterized example of ansa-η5-cyclopentadienyl-η6-arene ligand coordination. The cationic species [(η5,η6C5H4CMe2Ph)Ti(CH2Ph)2]+ displays rapid ortho cyclometalation of the ligand at ambient temperature to yield [(η5,η1-C5H4CMe2C6H4)Ti(CH2Ph)]+. Deuterium labeling studies show that in the cations ortho cyclometalation proceeds via direct σbond metathesis, in contrast to the pathway via an alkylidene intermediate identified for the neutral species. Subsequently (Chapter 4), the catalytic olefin conversion properties of the cationic dimethyl species, described in the previous chapter, are investigated. Interestingly, ethene is found to be trimerized with high selectivity to 1-hexene. No significant polymerization activity for propene and styrene is observed. The catalyst system (η5C5H4CMe2Ph)TiCl3/MAO is tested under various conditions in selective ethene trimerization. This catalyst shows >95% trimerization selectivity affording mainly 1hexene and a C10 fraction (cotrimers of ethene and 1-hexene, mainly 5-methylnon-1ene). The effect of the aryl group (Ar = Ph, 4-MeC6H4, 3,5-Me2C6H3) on activity and selectivity is discussed (extra aromatic methyl groups lead to a decrease in activity), together with the influence of solvent and activator. The selective ethene trimerization can be explained by assuming a mechanism involving titanacycloalkane intermediates. Oxidative coupling of two ethene molecules to a formal Ti(II) intermediate gives a titanacyclopentane. Subsequent insertion of ethene 26 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ followed by β-H elimination affords 1-hexene and regenerates the Ti(II) species. The presence of the aryl group attached to the cyclopentadienyl ligand is deemed crucial in the formation of the species active in trimerization. In Chapter 5, the effect of ligand variations on the activity and the selectivity of the selective ethene trimerization process is presented. The results indicate that the bridging moiety between the cyclopentadienyl and arene group is of crucial importance for selective ethene trimerization, since only CR2 groups (R ≠ H) afford trimerization catalysts (the most active with a cyclohexyl-based bridge [(CH2)5]C). Additional substituents on the cyclopentadienyl ligand (CMe3, SiMe3) can improve both activity and selectivity. The substituents are also effective in increasing the 5methylnon-1-ene content in the C10 fraction, giving a higher overall α-olefin content. The effect of the bridging group combined with that of cyclopentadienyl substituent is found to be roughly additive, and leads to activity improvements of 61% for the catalyst {η5-(3-SiMe3)C5H3C[(CH2)5]Ph}TiCl3/MAO. Surprisingly, combinations of cyclopentadienyl and aryl substituents, the latter previously found to decrease activity, give very active catalysts with a 93% improved activity for [η5-(3SiMe3)C5H3CMe2-3,5-Me2C6H3]TiCl3/MAO with respect to its unsubstituted analog. The introduction of a second pendant arene group on the cyclopentadienyl ring eventually gives the catalyst system [η5-C5H3-1,3-(CMe2Ph)2]TiCl3/MAO, which, interestingly, displays a constant trimerization activity in time over at least 2 h. 1.8 References and notes (1) For an overview of catalysis in industrial processes, see: Catalysis: An Integrated Approach, Second, Revised and Enlarged Edition (Eds. Van Santen, R.A., Van Leeuwen, P.W.N.M., Moulijn, J.A., Averill, B.A.), Elsevier, Amsterdam, 1999 (a) Trost, B.M., Science 1991, 254, 1471, (b) Trost, B.M., Angew. Chem. Int. Ed. Engl. 1995, 34, 259 Davis, M.E., Scrib, S.L., Selectivity in Catalysis, American Chemical Society, Washington, DC, 1993 For recent examples, see: (a) Freemantle, M., Chem. Eng. News 2001, September 3, 41, (b) Freemantle, M., Chem. Eng. News 2000, May 15, 37 (a) Jacoby, M., Chem. Eng. News. 2001, September 3, 35, (b) Rouhi, A.M., Chem. Eng. News. 2001, July 23, 33, (c) Thayer, A.M., Chem. Eng. News. 2001, May 21, 27, (d) Jacoby, M., Chem. Eng. News. 2001, February 26, 11, (e) Jacoby, M., Chem. Eng. News. 1999, August 2, 6 (a) Masters, C., Homogeneous Transition-Metal Catalysis, Chapman and Hall, London, UK, 1981, (b) Parshall, G.W., Ittel, S.D., Homogeneous Catalysis, Wiley & Sons Inc., New York, USA, 1992, (c) Applied Homogeneous Catalysis with Organometallic Compounds (Eds. Cornils, B., Herrmann, W.A.), VCH, Weinheim, 1996, (d) Herrmann, W.A., Cornils, B., Angew. Chem. Int. Ed. Engl. 1997, 36, 1048, (e) Organometallic Chemistry and Catalysis (Eds. Kirchner, K., Weissensteiner, W.), Springer-Verlag, Wien, 2001 (a) Organic Syntheses via Metal Carbonyls (Eds. Wender, I., Pino, P.), John Wiley & Sons, New York, 1968, (b) Maitlis, P.M., The Organic Chemistry of Palladium, Academic Press (2) (3) (4) (5) (6) (7) 27 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) 28 Inc., London, 1971, (c) Homogeneous Catalysis with Metal Phosphine Complexes (Ed. Pignolet, L.H.), Plenum Press, New York, 1983, (d) Brandsma, L., Vasilevsky, S.F., Verkruijsse, H.D., Applications of Transition Metal Catalysts in Organic Synthesis, Springer Verlag, Berlin, 1997, (e) Metal-catalyzed Cross-coupling Reactions (Eds. Diederich, F., Stang, P.J.), Wiley-VCH, Weinheim, 1998, (f) Transition Metals for Organic Synthesis (Eds. Beller, M., Bolm, C.), VCH, Weinheim, 1998, (g) Li, J.J., Gribble, G.W., Palladium in Heterocyclic Chemistry, Elsevier Science Ltd., Oxford, 2000 (a) Ziegler, K., Holzkamp, E., Breil, H., Martin, H., Angew. Chem. 1955, 67, 541, (b) Ziegler, K., Angew. Chem. 1964, 76, 545 (a) Natta, G., Angew. Chem. 1956, 68, 393, (b) Natta, G., Angew. Chem. 1964, 76, 553 (a) Hogan, J.P., Banks, R.L., US 2,825,721 (1958) to Phillips Petroleum Co., (b) Hogan, J.P., J. Polym. Sci. A 1970, 8, 2637 For recent examples, see: (a) Tullo, A.H., Chem. Eng. News 2000, August 7, 35, (b) Wilson, E., Chem. Eng. News 2000, January 24, 14, (c) Younkin, T.R., Connor, E.F., Henderson, J.I., Friedrich, S.K., Grubbs, R.H., Bansleben, D.A., Science 2000, 287, 460, (d) Matsui, S., Mitani, M., Saito, J., Tohi, Y., Makio, H., Matsukawa, N., Takagi, Y., Tsuru, K., Nitabaru, M., Nakano, T., Tanaka, H., Kashiwa, N., Fujita, T., J. Am. Chem. Soc. 2001, 123, 6847, (e) Organometallic Catalysts and Olefin Polymerization. Catalysts for a New Millennium. (Eds. Blom, R., Follestad, A., Rytter, E., Tilset, M., Ystenes, M.), Springer-Verlag, 2001 Chem. Eng. News 2001, June 25, 44 For reviews, see: (a) Möhring, P.C., Coville, N.J., J. Organomet. Chem. 1994, 479, 1, (b) Bochmann, M., J. Chem. Soc. Dalton Trans. 1996, 255, (c) Brintzinger, H.H., Fischer, D., Mülhaupt, R., Rieger, B., Waymouth, R.M., Angew. Chem. Int. Ed. Engl. 1995, 34, 1143, (d) Britovsek, G.J.P., Gibson, V.C., Wass, D.F., Angew. Chem. Int. Ed. 1999, 38, 428 (a) Aulbach, M., Küber, F., Chem. Unserer Zeit 1994, 28, 197, (b) Horton, A.D., Trends Polym. Sci. 1994, 2, 158 For copolymerization with polar monomers, see: (a) Yasuda, H., Yamamoto, H., Yokota, K., Miyake, S., Nakamura, A., J. Am. Chem. Soc. 1992, 114, 4908, (b) Yasuda, H., Furo, N., Yamamoto, H., Nakamura, A., Miyake, S., Kibino, N., Macromolecules 1992, 25, 5115, (c) Yasuda, H., Yamamoto, H., Yamashita, M., Yokota, K., Nakamura, A., Miyake, S., Kai, Y., Tanchisa, N., Macromolecules 1993, 26, 7134, (d) Boffa, L.S., Novak, B.M., Chem. Rev. 2000, 100, 1479 and references cited therein Hennico, A., Leonard, J., Forestiere, A., Glaize, Y., Hydrocarbon Process 1990, 69, 73 (a) Guidetti, G.P., Busi, P., Giulianelli, I., Zannetti, R., Eur. J. Polym. Sci. 1983, 19, 757, (b) Busico, V., Corradini, P., De Rosa, C., Di Benedetto, E., Eur. Polym. J. 1985, 21, 239, (c) Avella, M., Martuscelli, E., Volpe, G.D., Segre, A., Rossi, E., Simonazzi, T., Makromol. Chem. 1986, 187, 1927, (d) Zimmermann, J., J. Macromol. Sci. 1993, B32, 141 (a) Mirabella Jr., F.M., Polym. Mater. Sci. Eng. 1992, 67, 303, (b) D'Orazio, L., Mancarella, C., Martuscelli, E., Sticotti, G., Massari, P., Polymer 1993, 34, 3671 (a) Kaminsky, W., Spiehl, R., Makromol. Chem. 1989, 190, 515, (b) Kaminsky, W., Bark, A., Arndt, M., Makromol. Chem. Macromol. Symp. 1991, 47, 83, (c) Brekner, M., Osan, F., Rohrmann, J., Antberg, M., EP 485 893 (1992) to Hoechst AG Encyclopedia of Polymer Science and Engineering, Vol. 6 (Eds. Mark, H.F., Bikales, N.B., Overberger, C.G., Menges, G., Kroschwitz, J.I.), John Wiley & Sons, New York, 1986, 522 Turner, H.W., Hlatky, G.G., WO 91/12285 (1991) to Exxon (a) Natta, G., Farina, M., Peraldo, M., Chim. Ind. 1960, 42, 255, (b) Zambelli, A., Giongo, M.G., Natta, G., Makromol. Chem. 1968, 112, 183 General Introduction ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37) (38) (39) (40) (41) (a) Cossee, P., J. Catal. 1964, 3, 80, (b) Arlman, E.J., J. Catal. 1964, 3, 89, (c) Arlman, E.J., Cossee, P., J. Catal. 1964, 3, 99 Natta, G., Pino, P., Mazzanti, G., Lanzo, R., Chim. Ind. 1957, 39, 1032 (a) Corradini, P., Barone, V., Fusco, R., Guerra, G., Eur. Polym. J. 1979, 15, 1133, (b) Corradini, P., Guerra, G., Fusco, R., Barone, V., Eur. Polym. J. 1980, 16, 835, (c) Corradini, P., Barone, V., Fusco, R., Guerra, G., J. Catal. 1982, 77, 32, (d) Corradini, P., Barone, V., Guerra, G., Macromolecules 1982, 15, 1242 Patat, F., Sinn, H., Angew. Chem. 1958, 70, 496 Wilkinson, G., Pauson, P.L., Birmingham, J.M., Cotton, F.A., J. Am. Chem. Soc. 1953, 75, 1011 (a) Breslow, D.S., US 537,039 (1955) to Hercules, (b) Breslow, D.S., Newburg, N.R., J. Am. Chem. Soc. 1957, 79, 5072, (c) Natta, G., Pino, P., Mazzanti, G., Giannini, U., Mantica, E., Peraldo, M., Chim. Ind. 1957, 39, 19, (d) Natta, G., Pino, P., Mazzanti, G., Giannini, U., J. Am. Chem. Soc. 1957, 79, 2975 (a) Breslow, D.S., Newburg, N.R., J. Am. Chem. Soc. 1959, 81, 81, (b) Long, W.P., Breslow, D.S., J. Am. Chem. Soc. 1960, 82, 1953 Chien, J.C.W., J. Am. Chem. Soc. 1959, 81, 86 (a) Natta, G., Mazzanti, G., Tetrahedron 1960, 8, 86, (b) Sinn, H., Patat, F., Angew. Chem. 1963, 75, 805, (c) Reichert, K.H., Schubert, E., Makromol. Chem. 1969, 123, 58, (d) Reichert, K.H., Angew. Makromol. Chem. 1970, 13, 177, (e) Henrici-Olivé, G., Olivé, S., Angew. Chem. Int. Ed. Engl. 1967, 6, 790, (f) Henrici-Olivé, G., Olivé, S., J. Organomet. Chem. 1969, 16, 339 (a) Natta, G., Corradini, P., Bassi, I.W., J. Am. Chem. Soc. 1958, 80, 755, (b) Kaminsky, W., Kopf, J., Sinn, H., Volmer, H.J., Angew. Chem. Int. Ed. Engl. 1976, 15, 629 Dyachkovskii, F.S., Shilova, A.K., Shilov, A.E., J. Polym. Sci. C 1967, 16, 2333 Eisch, J.J., Piotrowski, A.M., Brownstein, S.K., Gabe, E.J., Lee, F.L., J. Am. Chem. Soc. 1985, 107, 7219 Jordan, R.F., Bajgur, C.S., Willet, R., Scott, B., J. Am. Chem. Soc. 1986, 108, 7410 (a) Bochmann, M., Wilson, L.M., Hursthouse, M.B., Short, R.L., Organometallics 1987, 6, 2556, (b) Bochmann, M., Wilson, L.M., Hursthouse, M.B., Motevalli, M., Organometallics 1988, 7, 1148, (c) Jordan, R.F., LaPointe, R.E., Bajgur, C.S., Echols, S.F., Willet, R., J. Am. Chem. Soc. 1987, 109, 4111, (d) Taube, R., Krukowka, L., J. Organomet. Chem. 1988, 347, C9, (e) Eshuis, J.J.W., Tan, Y.Y., Teuben, J.H., J. Mol. Catal. 1990, 62, 277, (f) Eshuis, J.J.W., Tan, Y.Y., Meetsma, A., Teuben, J.H., Renkema, J., Evens, G.G., Organometallics 1992, 11, 362, (g) Hlatky, G.G., Turner, H.W., Eckman, R.R., J. Am. Chem. Soc. 1989, 111, 2728, (h) Hlatky, G.G., Eckman, R.R., Turner, H.W., Organometallics 1992, 11, 1413 (a) Andresen, A., Cordes, H.G., Herwig, J., Kaminsky, W., Merck, A., Mottweiler, R., Pein, J., Sinn, H., Volmer, H.J., Angew. Chem. Int. Ed. Engl. 1976, 15, 630, (b) Sinn, H., Kaminsky, W., Volmer, H.J., Angew. Chem. Int. Ed. Engl. 1980, 19, 396 Reichert, K.H., Meyer, K.R., Makromol. Chem. 1973, 169, 163 Long, W.P., Breslow, D.S., Liebigs Ann. Chem. 1975, 463 (a) Kaminsky, W., Nachr. Chem. Tech. Lab. 1981, 29, 373, (b) Herwig, J., Kaminsky, W., Polym. Bull. 1983, 9, 464, (c) Kaminsky, W., Miri, M., Sinn, H., Woldt, R., Makromol. Chem. Rapid Commun. 1983, 4, 417 (a) Bochmann, M., Jaggar, A.J., Nicholls, J.C., Angew. Chem. Int. Ed. Engl. 1990, 29, 780, (b) Bochmann, M., Jaggar, A.J., J. Organomet. Chem. 1992, 424, C5, (c) Horton, A.D., Frijns, J.H.G., Angew. Chem. Int. Ed. Engl. 1991, 30, 1152, (d) Bochmann, M., Karger, G., Jaggar, A.J., J. Chem. Soc. Chem. Commun. 1990, 1038 29 Chapter 1 ▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬▬ (42) (43) (44) (45) (46) (47) (48) (49) (50) (51) (52) (53) (54) (55) (56) (57) (58) (59) (60) (61) 30 Horton, A.D., Orpen, A.G., Organometallics 1991, 10, 3910 (a) Turner, H.W., EP 277 004 (1988) to Exxon, (b) Turner, H.W., Hlatky, G.G., EP 277 003 (1988) to Exxon Bochmann, M., Angew. Chem. Int. Ed. Engl. 1992, 31, 1181 Indications for M-F interactions have been identified for [Cp*2ThMe][B(C6F5)4] (Yang, X., Stern, C.L., Marks, T.J., Organometallics 1991, 10, 840) and [Cp*2ZrH][B(C6F5)4] (Yang, X., Stern, C.L., Marks, T.J., Angew. Chem. Int. Ed. Engl. 1992, 31, 1375) Hlatky, G.G., Upton, D.J., Turner, H.W., US 459,921 (1990) to Exxon (a) Ewen, J.A., Elder, M.J., EP 426 637 (1991) to FINA, (b) Ewen, J.A., Elder, M.J., EP 426 638 (1991) to FINA, (c) Chien, J.C.W., Tsai, W.M., Rausch, M.D., J. Am. Chem. Soc. 1991, 113, 8570, (d) Bochmann, M., Lancaster, S.J., J. Organomet. Chem. 1992, 434, C1 (a) Ewen, J.A., Elder, M.J., US 419,017 (1989) to FINA, (b) Yang, X., Stern, C.L., Marks, T.J., J. Am. Chem. Soc. 1991, 113, 3623, (c) Yang, X., Stern, C.L., Marks, T.J., J. Am. Chem. Soc. 1994, 116, 10015 Bochmann, M., Lancaster, S.J., Hursthouse, M.B., Malik, K.M.A., Organometallics 1994, 13, 2235 Chen, E.Y.-X., Marks, T.J., Chem. Rev. 2000, 100, 1391 The catalyst system Cp2TiPh2/MAO was found to produce isotactic PP at low reaction temperatures (-50 °C), see: Ewen, J.A., J. Am. Chem. Soc. 1984, 106, 6355 (a) Schnutenhaus, H., Brintzinger, H.H., Angew. Chem. Int. Ed. Engl. 1979, 18, 777, (b) Wild, F.R.W.P., Zsolnai, L., Huttner, G., Brintzinger, H.H., J. Organomet. Chem. 1982, 23, 233 Kaminsky, W., Külper, K., Brintzinger, H.H., Wild, F.R.W.P., Angew. Chem. Int. Ed. Engl. 1985, 24, 507 The prefix ansa means 'handle' in Latin, and was adopted to describe metallocenes with an interannular bridge (Smith, J.A., Von Seyerl, J., Huttner, G., Brintzinger, H.H., J. 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