The Chemistry of Catalyst Activation: The Case of Group 4

Organometallics 2010, 29, 4711–4740 DOI: 10.1021/om1004447

4711

The Chemistry of Catalyst Activation: The Case of Group 4 Polymerization Catalysts† Manfred Bochmann‡ Wolfson Materials and Catalysis Centre, School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K. Received May 9, 2010

Organometallic chemistry has provided the foundation for the development of a wide range of new olefin polymerization catalysts over the last two to three decades. The unraveling of the mechanisms of activation and the mode of action of polymerization catalysts has transformed these former “black-box” systems into some of the best understood catalysts to date. In recent years insight into mechanistic steps such as ligand exchange with main-group-metal alkyls has had particular farreaching consequences for materials design and new industrial products. This review attempts to summarize the advances made over the last 9-10 years in our understanding of the chemistry of catalyst activation and associated fundamental mechanistic aspects.

1. Introduction The development of well-defined polymerization catalysts, in particular those based on group 4 metal complexes, must rank as one of the most prominent achievements of organometallic chemistry over the last 20 years. Polyolefin production worldwide is of the order of 108 metric tons per year; enough, it has been suggested, to build about 44 polyolefin pyramids the size of Khufu’s great pyramid at Giza every year.1 Polyolefins have become indispensable to modern society, and while most of these materials continue to be produced with heterogeneous catalysts, the well-defined systems provided by organometallic complexes have opened routes to a wide range of polymer structures that were previously inaccessible. There are two aspects that make organometallic catalysts particularly attractive: the ease of ligand modification and the control of catalyst activation and dynamics. Used imaginatively, these aspects are key to new materials and applications. The developments of new ligands for soluble

polymerization catalysts has continued apace,2-9 as documented by a succession of reviews that have concentrated in particular on the effects of ligand structure.2-4,10-18 Following an in-depth survey of the mechanistic aspects of metallocene catalyst activation, activator effects, and selectivity in 2000,19,20 significant advances have been made both in our understanding of activation processes, living polymerization,21,22 and the structures of active sites and the

† Part of the Dietmar Seyferth Festschrift. To Dietmar, with affection and in recognition of his tremendous contributions to organometallic chemistry and to Organometallics. ‡ E-mail: [email protected]. (1) Severn, R. J.; Chadwick, J. C.; Duchateau, R.; Friederichs, N. “Bound but not gagged” - immobilizing single-site R-olefin polymerization catalysts. Chem. Rev. 2005, 105, 4073. (2) Gladysz, J. A., Ed. Frontiers in Metal-Catalyzed Polymerization. Chem. Rev. 2000, 100, 167-1604. (3) Resconi, L.; Chadwick, J. C.; Cavallo, L. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Amsterdam, 2007; Vol. 4, pp 1005-1166. (4) Fujita, T.; Makio, H. In Comprehensive Organometallic Chemistry III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier: Amsterdam, 2007; Vol. 11, pp 691-734. (5) Tuchbreiter, A.; M€ ulhaupt, R. The Polyolefin Challenge: Catalyst and Process Design, Tailor-made Materials, High-throughput Development and Data Mining. Macromol. Symp. 2001, 173, 1. (6) Kaminsky, W., Ed. Special Issue: Olefin Polymerization. Macromol. Symp. 2006, 236, 1-258. (7) Alt, H. G., Ed. Metallocene Complexes as Catalysts for Olefin Polymerization. Coord. Chem. Rev. 2006, 250, 1-272.

(8) Li, X.; Hou, Z. Organometallic catalysts for the copolymerization of cyclic olefins. Coord. Chem. Rev. 2008, 252, 1842. (9) Busico, V. Metal catalyzed olefin polymerizations: A perspective and outlook. Dalton Trans. 2009, 8794. (10) M€ ohring, P. C.; Coville, N. J. Group 4 metallocene catalysts: quantification of ring substituents effects. Coord. Chem. Rev. 2006, 250, 18. (11) Razavi, A.; Thewalt, U. Site selective ligand modification and tactic variation in polypropylene chains with metallocene catalysts. Coord. Chem. Rev. 2006, 250, 155. (12) Cobzaru, C.; Hild, S.; Boger, A.; Troll, C.; Rieger, B. “Dualside” catalysts for high and ultra-high molecular weight polypropylene elastomers and plastomers. Coord. Chem. Rev. 2006, 250, 189. (13) Nomura, K.; Liu, J.; Padmanabhan, S.; Kitiyanan, B. Nonbridged half-metallocenes containing anionic ancillary donor ligands: New promising candidates as catalysts for precise olefin polymerization. J. Mol. Catal. A: Chem. 2007, 267, 1. (14) Stephan, D. W. The road to early transition metal phosphinimide olefin polymerization catalysts. Organometallics 2005, 24, 2548. (15) Gibson, V, C.; Spitzmesser, S. K. Advances in non-metallocene olefin polymerization catalysis. Chem. Rev. 2003, 103, 283. (16) Gibson, V. C.; Redshaw, C.; Solan, G. R. Bis(imino)pyridines: Surprisingly reactive ligands and a gateway to new families of catalysts. Chem. Rev. 2007, 107, 1745. (17) (a) Matsugi, T.; Fujita, T. High-performance olefin polymerization catalysts discovered on the basis of a new design concept. Chem. Soc. Rev. 2008, 37, 1264. (b) Makio, H.; Fujita, T. Development and application of FI catalysts for olefin polymerization: Unique catalysts and distinctive polymer formation. Acc. Chem. Res. 2009, 42, 1532. (18) Busico, V.; Cipullo, R.; Pellecchia, R.; Ronca, S.; Roviello, G.; Talarico, G. Design of stereoselective Ziegler-Natta propene polymerization catalysts. Proc. Natl. Acad. Sci. 2006, 103, 15321. (19) Chen, E. Y. X.; Marks, T. J. Cocatalysts for metal-catalyzed olefin polymerization: Activators, activation processes, and structureactivity relationships. Chem. Rev. 2000, 100, 1391. (20) Resconi, L.; Cavallo, L.; Fait, A.; Piemontesi, F. Selectivity in propene polymerization with metallocene catalysts. Chem. Rev. 2000, 100, 1253.

r 2010 American Chemical Society

Published on Web 06/29/2010

pubs.acs.org/Organometallics

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Organometallics, Vol. 29, No. 21, 2010

Bochmann

Scheme 1. Principal Activation Pathways for Metal Methyl Catalyst Precursors

dynamic behavior of catalyst systems23,24 and in the application of these more fundamental mechanistic insights in order to generate remarkable new polymer types.25 This review attempts to summarize some of these advances since about 2001, with emphasis on the studies of activators, catalyst activation, and the chemistry of the active species.

2. Catalyst Activation with Borane and Borate Reagents

counteranions. Among the last class of activators, anilinium salts such as [HNR2Ph][B(C6F5)4] (R = Me, Et) have been widely used. There are also several series of adducts of B(C6F5)3 with E-H-containing compounds (E = C, N, O) which are sufficiently acidic to protolyze metal-alkyl bonds; in particular the C-H acidic heterocycles 1-3 and related structures have been explored as convenient alternatives to the highly hygroscopic B(C6F5)3.27

The activation of polymerization catalysts based on coordination complexes consists of generating an equilibrium concentration of a coordinatively unsaturated species, usually cationic, that contains a reactive metal-alkyl bond and is capable of binding an olefin in such a manner that transfer of the alkyl ligand to the monomer can occur (eq 1).

As is now well established,3,19,23 this activation can be achieved by reacting metal dialkyl complexes, usually the metal dimethyls LnMMe2 (M = Ti, Zr, Hf), with (a) suitable Lewis acids such as B(C6F5)3 and its congeners,26 (b) triphenylmethyl (“trityl”) salts of noncoordinating anions, or (c) Brønsted acids capable of generating weakly coordinating (21) Coates, G. W.; Hustad, P. D.; Reinartz, S. Catalysts for the living polymerization of alkenes: access to new polyolefin architectures using Ziegler-Natta chemistry. Angew. Chem., Int. Ed. 2002, 41, 2236. (22) Domski, G. J.; Rose, J. M.; Coates, G. W.; Bolig, A. D.; Brookhart, M. Living alkene polymerization: New methods for the precision synthesis of polyolefins. Prog. Polym. Sci. 2007, 32, 30. (23) Bochmann, M. Kinetic and mechanistic aspects of metallocene polymerization catalysts. J. Organomet. Chem. 2004, 689, 3982. (24) Bochmann, M. Use of spectroscopy in metallocene-based polymerization catalysis. In Mechanisms in Homogeneous Catalysis; Heaton, B., Ed.; Wiley-VCH: Weinheim, Germany, 2005; p 311. (25) Hustad, P. D. Frontiers in olefin polymerization: reinventing the world’s most common synthetic polymers. Science 2009, 325, 704. (26) Piers, W. E. Adv. Organomet. Chem. 2004, 52, 1.

In most cases of catalytic relevance, it is convenient to replace preformed metal alkyls by the corresponding metal chlorides mixed with an excess of an aluminum trialkyls as in situ alkylating agents. The aluminum trialkyls also act as scavengers for background impurities, such as traces of moisture, that could poison part of the catalyst. Thus, mixtures of CPh3þX- salts with AlMe3 or, much more effectively, AlBui3 are often used. However, the addition of aluminum alkyls certainly complicates the chemistry of generating the active species, i.e. its structure and reactivity, and the R2AlCl byproduct has to be considered. All these functions;alkylating agent, scavenger, cation generating agent;are also fulfilled by methylalumoxane (MAO), which is the most widely employed activator in (27) (a) Focante, F.; Mercandelli, P.; Sironi, A.; Resconi, L. Coord. Chem. Rev. 2006, 250, 170. For further B(C6F5)3 adducts see: (b) Mountford, A. J.; Lancaster, S. J.; Coles, S. J.; Horton, P. N.; Hughes, D. L.; Hursthouse, M. B.; Light, M. E. Inorg. Chem. 2005, 44, 5921. (c) Fuller, A. M.; Mountford, A. J.; Scott, M. L.; Coles, S. J.; Horton, P. N.; Hughes, D. L.; Hursthouse, M. B.; Lancaster, S. J. Inorg. Chem. 2009, 48, 11474.

Review


4713

Scheme 2. Representative Weakly Coordinating Anions

industry of group 4 polymerization catalysts. The chemistry of MAO-activated systems is summarized in section 9. The general activation pathways are shown in Scheme 1. The reaction of L2MMe2 with Lewis acids such as B(C6F5)3 is in principle reversible, and although zwitterions of type B are almost invariably observed as thermodynamic products, this reversibility can affect the kinetic behavior of these compounds. Whether or not the binuclear intermediate A1 is detectable or is the main product depends largely on the nature of the counteranion.19 The product of the reaction with [HNR2Ph][B(C6F5)4] is the aniline complex D, and although the assumption is usually made that such complexes are very labile (if they exist in solution at all), this may or may not be true depending on the metal and the ligands L. Finally, while the majority of precatalysts investigated form heterobinuclear adducts with AlMe3 of type E, closer inspection shows that this, too, is not invariably the case. Thus, although Scheme 1 represents the general reactivity pattern followed by most group 4 catalyst precursors, whether they are based on metallocene structures or not, there are notable deviations from the general principles. The reasons for this behavior are often far from clear. Although these reactions are usually referred to as “catalyst activation”, none of the ion pairs A-E represent the actual catalytically active species; they are all best considered as (often isolable) resting states, albeit states that can usually be converted into active species via quite low energy barriers. The main factor that determines the Lewis acidity and olefin binding capacity of a given cationic metal alkyl complex is the counteranion. “Anion engineering” has led to remarkable

activity improvements which can rival or exceed those achievable by changes to the ligand design. The main design principle is to increase the delocalization of the negative charge over as large a volume as possible. A second consideration is to ensure that the anion is not polarizable and has either no or a minimal dipole moment. Scheme 2 shows various counteranions that have proved particularly useful for the activation of metallocene catalysts. The determination of propene polymerization productivities of (SBI)ZrMe2 (SBI = rac-Me2Si(1-indenyl)2) with trityl salts of various anions, in the absence of aluminum alkyls, as a function of zirconium concentration and extrapolation to [Zr] = 0 allowed the quantification of anion effects and the determination of the contribution of various anions to the reaction barrier.28-30 The stability of the ion pairs [(SBI)ZrMeþ 3 3 3 X-] decreased in the order X- = [B(C6F5)4]- > [Ni{CNB(C6F5)3}4]2- > [CN{B(C6F5)3}2]-, and for SBI . Cp2. Although in stoichiometric reactions the cyano-bridged anion [CN{B(C6F5)3}2]- (4) led most readily to isolable deactivation products such as Cp2Zr(Me)CNB(C6F5)3, under catalytic conditions it proved to be remarkably stable; for example, the ethylene polymerization productivity of (SBI)ZrMe2 activated with AlBui3/CPh3[CN{B(C6F5)3}2] at 60 °C was about 30-40 times higher than that of the same catalyst activated with MAO. Although this high (28) Zhou, J.; Lancaster, S. J.; Walker, D. A.; Beck, S.; Thornton-Pett, M.; Bochmann, M. J. Am. Chem. Soc. 2001, 123, 223. (29) Bochmann, M.; Lancaster, S. J.; Hannant, M. D.; Rodriguez, A.; Schormann, M.; Walker, D. A.; Woodman, T. J. Pure Appl. Chem. 2003, 75, 1183. (30) Hannant, M. H.; Wright, J. A.; Lancaster, S. J.; Hughes, D. L.; Horton, P. N.; Bochmann, M. Dalton Trans. 2006, 2415.

4714


Bochmann

Table 1. van der Waals Volumes of Representative Ions30 ion þ

(SBI)ZrMe MeB(C6F5)3B(C6F5)4[H2N{B(C6F5)3}2][N{CN-PBB}2]-a a

van der Waals vol (A˚ ) 3

313 277 349 538 1013

PBB = B(C6F4-2-C6F5)3.

activity led to mass transport limitations, the catalytic activity amounted to ca. 8 108 g of PE (mol of Zr)-1 h-1 bar-1 and a remarkable monomer insertion rate on the order of 105 s-1 for ethene, one of the most active polymerization catalysts ever reported.28 The size of the counteranions correlates with catalytic activity, provided the anion is not polar and contains no sterically accessible donor heteroatoms. In most cases the volumes of the anions are substantially larger than the volumes of the metallocene cations. Some representative figures are given in Table 1.

3. Reactivity of Group 4 Metal Methyl Cations The ease of activation of metallocene complexes depends on a combination of steric and electronic factors. Good methods for assessing steric factors and substituent effects now exist.10 Electronic characteristics are more difficult to quantify. An elegant method employed by Brintzinger consists of probing the electron density of the metal center by ligand exchange equilibria: e.g., eq 2.33

The electron density in SBI complexes was reduced by substituents and bridge effects; the silyl bridge makes the metal center more electrophilic. There was a significant difference between indenyl and Cp complexes. The ΔGexc values paralleled the ionization energies determined by XPS.33 Another way to measure electrophilicity is the determination of the displacement equilibria of the anion (here MeB(C6F5)3-) by donor ligands. Whereas in Cp complexes L2ZrMe(μ-Me)B(C6F5)3 (L2 = Cp2, 1,2-C2Me4Cp2, Me2SiCp2) the anion is partially displaced by dimethylaniline, the more electrophilic indenyl analogues required stronger donors such as Bun2O to show measurable exchange.34 Rather surprisingly, the crystal structure of a dimethylaniline adduct of a metallocene cation has only recently been determined: [(IPCF)ZrMe(NMe2Ph)][B(C6F5)4] shows a Zr-N bond length of 2.424(5) A˚ (IPCF = Me2C(C5H4)(9-fluorenyl)). DFT calculations showed that the Zr-N bond is some 23 kJ mol-1 stronger than the bridging Zr-methyl bond in [{(IPCF)ZrMe}2(μ-Me)]þ. Relatively strong Me2NPh coordination, with diastereotopic N-Me groups, was also evident in toluene solutions of (SBI)ZrMe2 activated with [HNMe2Ph]þ[B(C6F5)4]-. In contrast, the reaction of the hafnium analogue (IPCF)HfMe2 with (31) Abramo, G. P.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 13966. (32) Chen, M. C.; Roberts, J. A. S.; Seyam, A. M.; Li, L.; Zuccaccia, C.; Stahl, N. G.; Marks, T. J. Organometallics 2006, 25, 2833. (33) Wieser, U.; Babushkin, D.; Brintzinger, H. H. Organometallics 2002, 21, 920. (34) Schaper, F.; Geyer, A.; Brintzinger, H. H. Organometallics 2002, 21, 473.

the anilinium salt proved more complex and proceeded via the μ-Me cation, which was only converted to the mononuclear aniline complex after several hours at room temperature. In the zirconium reaction the μ-Me intermediate is only detectable at -40 °C and reacts much more quickly to the NMe2Ph complex (Scheme 3).35 It is a characteristic of many metallocenium salts of [B(C6F5)4]- to form oily deposits in low-polarity solvents such as toluene, which can lead to problems with NMR spectroscopy. As a preliminary to a series of investigations into the species formed from metallocene dimethyls with B(C6F5)3, CPh3[B(C6F5)4], and MAO, the mono- and binuclear reaction products [Cp2ZrMeþ 3 3 3 B(C6F5)4-], [(Cp2ZrMe)2(μ-Me)][B(C6F5)4], and [Cp2Zr(μ-Me)2AlMe2][B(C6F5)4], corresponding to ion pairs C, A2, and E in Scheme 1, respectively, have been studied at low and high concentrations. Solutions containing molecularly dissolved complexes as well as droplets of oily aggregates show separate signals for these two phases.36 Analogous methyl-bridged structures are formed for a variety of non-metallocene complexes. For example, the pyridine diamido complex 12 give triply bridged cations 13 (Scheme 4).37 In contrast, (CPN)MMe3 complexes (M = Zr, Hf) give the binuclear cations 14 with a single methyl bridge.38 The titanium imidophosphoranyl complex 15 shows evidence for the involvement of the dichloromethane solvent in its reaction with tri-o-tolylphosphine, which leads to the chloro-bridged cations 16 (Scheme 5).39 There is no direct information on the structure of the metal-olefin preinsertion complexes in L2ZrMeþ cations generated by the activation pathways discussed above, or about the position of the monomer association equilibria. Aspects of this have been explored using chelate complexes such as 17-20 as surrogates (Scheme 6).23,40-43 Non-ansa complexes such as 17 show barriers of alkene dissociation ΔGq of about 11 kcal/mol.40 On the other hand, the doubly bridged complex 19 gave no indication for olefin dissociation until 273 K; in C6D5Br at 283 K, ΔGq = 15.6 kcal/mol. There was no detectable anion influence on this process. These complexes proved stereochemically surprisingly stable, with no sign of site epimerization even on heating to 75 °C in bromobenzene or dichloromethane solvents. The rates were also unaffected by excess [NBu4][B(C6F5)4].42 Jordan has studied complexes of nontethered alkenes as more realistic models for the alkene preinsertion complex in polymerization catalysts, using noninserting ligands such as OBut and C6F5 in cis positions. At low temperatures complexes of structure 21 were found to bind a wide variety of 1-alkenes, (35) Wilson, P. A.; Wright, J. A.; Oganesyan, V.; Lancaster, S. J.; Bochmann, M. Organometallics 2008, 27, 6371. (36) Talsi, E. P.; Eilertsen, J. L.; Ystenes, M.; Rytter, E. J. Organomet. Chem. 2003, 677, 10. (37) Mehrkhodavandi, P.; Schrock, R. R.; Pryor, L. L. Organometallics 2003, 22, 4569. (38) Said, M.; Thornton-Pett, M.; Hughes, D. L.; Bochmann, M. Inorg. Chim. Acta 2007, 360, 1354. (39) Cabrera, L.; Hollink, E.; Stewart, J. C.; Wei, P.; Stephan, D. W. Organometallics 2005, 24, 1091. (40) (a) Casey, C. P.; Carpenetti, D. W. Organometallics 2000, 19, 3970. (b) Casey, C. P.; Carpenetti, D. W.; Sakurai, H. Organometallics 2001, 20, 4262. (41) Carpentier, J. F.; Wu, Z.; Lee, C. W.; Str€ omberg, S.; Christopher, J. N.; Jordan, R. F. J. Am. Chem. Soc. 2000, 122, 7750. (42) Brandow, C. G.; Mendiratta, A.; Bercaw, J. E. Organometallics 2001, 20, 4253. (43) Martinez, G.; Royo, P. Organometallics 2005, 24, 4782.

Review


4715

Scheme 3. Stepwise Activation of Metallocenes with [HNMe2Ph][B(C6F5)4]

Scheme 4. Activation of Pyridine Diamido Complexes

Scheme 5

with significant polarization of the CdC bond as expected for d0 metals.44 The C6F5 complex 22 binds allylsilane.45 More recently Baird, using the hindered 2,4-dimethylpent1-ene, provided spectroscopic evidence for the formation of the zirconocene alkene complex 23 in detectable concentrations, as an intermediate in the formation of the corresponding zirconocene allyl cation via C-H activation (Scheme 7). This complex only represents about 1% of the total zirconocene sample. DFT calculations showed that the alkene complex is stabilized in CH2Cl2 by 19 kcal/mol relative to Cp2ZrMe(μ-Me)B(C6F5)3. The calculated distances to the alkene C atoms of 2.61 and 3.36 A˚ show an unusually large difference for a coordinated alkene and underline the tendency of 2,2-disubstituted alkenes to coordinate in an η1 fashion.46,47 The C-H activation of a 2,2-disubstituted alkene depicted in Scheme 7 is a common occurrence and leads to the formation of allyl complexes as a product of catalyst deactivation. This (44) Stoebenau, E. J.; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 8162. (45) Stoebenau, E. J.; Jordan, R. F. J. Am. Chem. Soc. 2006, 128, 8638. (46) Vatamann, M.; Stojcevic, G.; Baird, M. C. J. Am. Chem. Soc. 2008, 130, 454. (47) Sauriol, F.; Wong, E.; Leung, A. M. H.; Donaghue, I. E.; Baird, M. C.; Wondimagegn, T.; Ziegler, T. Angew. Chem., Int. Ed. 2009, 48, 3342.

Scheme 6

was demonstrated for the polymerization of propene with the borane-activated C1-symmetric complex 24, which reacts with vinylidene end groups of the produced olefins. Similarly, the reaction with isobutene led to a crystallographically characterized zirconium allyl cation and neopentane (Scheme 8).48 The

4716


Bochmann Scheme 7

Scheme 8

though in this case as the result of methane elimination.54 A similar reaction sequence had previously been observed for the fulvalene complex 31; here C-H activation and methane elimination was fast even at -60 °C (Scheme 10).55 A methane elimination pathway is also involved in some B(C6F5)3-activated systems: for example, in the reaction sequence shown in Scheme 11, which includes C6F5 transfer to the metal, a not unfamiliar decomposition pattern in zwitterionic half-sandwich complexes.56

reaction of such allyl species with alkenes is very slow, at least 1 or 2 orders of magnitude slower than typical alkene insertion rates. Allylic species are, however, responsible for the incorporation of unsaturated linkages of the type -CH2C(dCH2)CH2- in polypropenes.20 Cp2Zr(Me)(allyl) complexes are alkylated by Al2Me6 or MAO to give the more reactive Cp2ZrMe2 and thus re-enter the catalytic cycle.49 Sterically Congested Half-Sandwich Complexes. Zirconium and hafnium Cp amidinato complexes are of interest because of their precise steric requirements and their ability to mediate the living polymerization of a variety of 1-alkenes. For example, activation of 25 (Scheme 9, R1 = R2 = Pri, Cy; R1 = But, R2 = Cy) gave a mononuclear cation which in chlorobenzene at -10 °C polymerizes vinylcyclohexane to a highly isotactic polymer with Mw/Mn ≈ 1.05, via a chain-end control mechanism.50 In 1-hexene polymerizations these catalysts show remarkable selectivity: while the Cp* complex 26 (R = Me) is highly active and gives isotactic poly(1-hexene) in a living fashion, analogues with R = H, Ph are nonliving and offer poor stereocontrol and the R = But complex is inactive.51 Important for the control of polymer structure, the neutral precursors 26 are configurationally flexible and can racemize at a rate commensurable with chain propagation, whereas the corresponding cations [Cp*ZrMe(N-N)]þ are more congested due to shorter Zr-N bonds and are stereochemically rigid.52 On recrystallization from chlorobenzene/pentane at -10 °C the bis-methyl-bridged dications 27 were isolated (as B(C6F5)4- salts) and crystallographically characterized; they show an unusual form of R-agostic CH3 bonding. At room temperature these dications are deprotonated to the μ-CH2, μ-CH3 cation 28 (Scheme 9).53 A similar reaction sequence is found for the transformation of 29 into 30 on warming (as a mixture of rac and meso isomers),

4. Activation of Metal Diene Complexes

(48) Al-Humydi, A.; Garrison, J. C.; Mohammed, M.; Youngs, W. J.; Collins, S. Polyhedron 2005, 24, 1234. (49) Lieber, S.; Prosenc, M. H.; Brintzinger, H. H. Organometallics 2000, 19, 377. (50) Keaton, R. J.; Jayaratne, K. C.; Henningsen, D. A.; Koterwas, L. A.; Sita, L. R. J. Am. Chem. Soc. 2001, 123, 6197. (51) Zhang, Y.; Reeder, E. K.; Keaton, R. J.; Sita, L. R. Organometallics 2004, 23, 3512. (52) Jayaratne, K. C.; Sita, L. R. J. Am. Chem. Soc. 2001, 123, 10754. (53) Keaton, R. J.; Jayaratne, K. C.; Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2000, 122, 12909.

The activation of titanium and zirconium diene complexes with B(C6F5)3 has been reviewed.57 Electrophilic attack on the diene terminus gives allylic or allyl-like zwitterionic complexes. The crystallographically characterized complex 32 provides an example for a new type of zwitterionic structure derived from a butadiene precursor and adopts a structure halfway between a σ- and a π-allyl. Zwitterionic zirconocene allyls have provided evidence for the controlled first insertion of an alkene and allowed the energetics of this process to be determined.58 A number of zwitterionic allylic constrained-geometry complexes, such as 33 and 34, have been structurally characterized (Scheme 12).59-61

5. Complexes without Metal-Alkyl Bonds A number of reports have appeared of catalyst activation processes that lead apparently to active species which do not contain metal-alkyl σ bonds. Most notable among those are the doubly bridged compounds 35 (Scheme 13), which at 70 °C/4 bar are reasonably active ethene polymerization catalysts. The mechanistic reasons for this activity have not been elucidated, although one cannot exclude that under these conditions there is B(C6F5)3 exchange between the benzyl and an amido ligand which would lead to a metal-alkyl species capable of inserting olefins.62,63 (54) Zhang, S.; Piers, W. E. Organometallics 2001, 20, 2088. (55) Bochmann, M.; Cuenca, T.; Hardy, D. T. J. Organomet. Chem. 1994, 484, C10. (56) Fenwick, A. E.; Phomphrai, K.; Thorn, M. G.; Vilardo, J. S.; Trefun, C. A.; Hanna, B.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2004, 23, 2146. (57) Erker, G.; Kehr, G.; Fr€ ohlich, R. J. Organomet. Chem. 2005, 690, 6254. (58) Dahlmann, M.; Erker, G.; Bergander, K. J. Am. Chem. Soc. 2000, 122, 7986. (59) Hannig, F.; Fr€ ohlich, R.; Bergander, K.; Erker, G.; Petersen, J. L. Organometallics 2004, 23, 4495. (60) Hair, G. S.; Jones, R. A.; Cowley, A. H.; Lynch, V. Inorg. Chem. 2001, 40, 1014. (61) Yue, N.; Hollink, E.; Guerin, F.; Stephan, D. W. Organometallics 2001, 20, 4424. (62) Cano, J.; Royo, P.; Lanfranchi, M.; Pellinghelli, M. A.; Tiripicchio, A. Angew. Chem., Int. Ed. 2001, 40, 2495. (63) Cano, J.; Sudupe, M.; Royo, P. J. Organomet. Chem. 2007, 692, 4448.

Review


4717

Scheme 9

Scheme 10

Scheme 11

Scheme 13

Scheme 12. Structures of Allylic Zwitterions

The zirconium complexes 36, when activated with MAO at 120 °C (Al/Zr = 500), give high activities for ethene/octene copolymerizations, while the titanium analogues are almost (64) Jin, J.; Wilson, D. R.; Chen, E. Y. X. Chem. Commun. 2002, 708.

inactive. Here, too, the conditions are such that Cp/alkyl exchange is a possibility.64 Another example is the bis-Cp amido complex 37, which reacts with B(C6F5)3 or CPh3[B(C6F5)4] to give 38, 39, or 40 (Scheme 14), depending on the reagent ratio. While all these products by themselves are inactive toward ethene, the addition of aluminum alkyls generates active catalysts. The reported polydispersities are variable and indicate some multisite behavior in some cases. AlEt3 gave the highest activities and the narrowest molecular weight distribution, and the activity increased with increasing [AlEt3]. There was extensive chain transfer to aluminum, and the product prior

4718


Bochmann Scheme 14

to hydrolysis was mainly Al(polymeryl)3.65 The spectroscopically identified reaction product of 40 with AlMe3 to give 41 may offer an insight into the working of this apparently Zr-C free catalyst, and one may speculate that opening the Zr(μ-N)(μ-Me)Al ring may provide the required entry to the active Zr-Meþ species.

6. Activation of Hafnium Amido Pyridine Complexes Hafnium complexes with cyclometalated amido pyridine ligand frameworks of structures 42A and 42B are excellent examples of catalyst discovery by high-throughput technology.66 They show high performance in terms of activity, high temperature stability, and polymer molecular weight and polymerize propene with high isotacticity.67 They readily copolymerize ethene with 1-octene and are one of the components for the high-volume preparation of hard-soft olefin block copolymers by chain shuttling polymerization (see section 13).68 These catalysts do, however, give ethene/octene copolymers with rather broad polydispersities and evident multisite behavior. Catalysts of type 42 have, in principle, two possibilities of reacting with alkenes, insertion into the Hf-Me as well as the Hf-aryl bonds. With Brønsted acids such as [HNMe2Ph][B(C6F5)4] the Hf-naphthyl bond is protolyzed but the resulting cation 43 is catalytically inactive. Reaction of B(C6F5)3-activated 42A with 13C2H4 gave a single insertion product. DFT calculations on a Hf-butyl model showed that the barrier to insertion into the Hf-aryl bond is lower than that into the Hf-alkyl bond. The uninserted cation 44 has strong β-agostic interactions with the alkyl ligand. Ethene insertion into the Hf-aryl bond generates the less constrained cations 45; because of the asymmetry of the (65) (a) Wang, C.; Luo, H. K.; van Meurs, M.; Stubbs, L. P.; Wong, P. K. Organometallics 2008, 27, 2908. (b) Wang, C.; van Meurs, M.; Stubbs, L. P.; Tay, B. Y.; Tan, X. J.; Aitpamula, S.; Chacko, J.; Luo, H. K.; Wong, P. K.; Ye, S. Dalton Trans. 2010, 807. (66) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Lund, C.; Murphy, V.; Shoemaker, J. A. W.; Tracht, U.; Turner, H.; Zhang, J.; Tetsuo, U.; Rosen, R. K.; Stevens, J. C. J. Am. Chem. Soc. 2003, 125, 4306. (67) Boussie, T. R.; Diamond, G. M.; Goh, C.; Hall, K. A.; LaPointe, A. M.; Leclerc, M. K.; Murphy, V.; Shoemaker, J. A. W.; Turner, H.; Rosen, R. K.; Stevens, J. C.; Alfano, A.; Busico, V.; Cipullo, R.; Talarico, G. Angew. Chem., Int. Ed. 2006, 45, 3278. (68) Arriola, D. J.; Carnahan, E. M.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. Science 2006, 312, 714.

ligand framework there are two possible isomers of ethene insertion, only one of which is shown in Scheme 15, and eight for octene.69 The resulting ring in 45 is more flexible; the modeling also suggested a π interaction of the metal with the naphthyl ring but no agostic bonding of the alkyl chain. Although not all of the possible insertion isomers are likely to be realized or kinetically competitive, it is evident that such insertion processes in the presence of both ethene and octene are capable of generating a multiplicity of active sites which contribute to the polymer molecular weight distribution. The protolysis product 43 reacts with amines such as dimethylaniline and trialkylamines via C-H activation pathways to give products where the Hf-naphthyl bond is re-formed, such as 46-48 (Scheme 16). The naphthyl moiety plays a significant part therefore in the reactivity pattern of these catalysts. The phenyl analogues 42B react similarly but more quickly.70 NMR studies showed that activation of 42A with B(C6F5)3 gives only one diastereomer, 49, within detection limits, whereas the reaction with CPh3[B(C6F5)4] generates both possible diastereomers in about equal amounts. In toluene solution the B(C6F5)4- complexes are stable at room temperature for several days but decompose rapidly on exposure to light or polar solvents such as chlorobenzene.70 The reaction with propene or 1-hexene gives the Hf-aryl insertion product 50 as the kinetic product. The formation of 50 is accompanied by polyhexene, although at -56 °C the reaction was slow enough to identify the structure and stereochemistry of 50 (R = Bun) by NMR spectroscopy. The stereochemistry at the metal, i.e. the relative positions of Me and MeB(C6F5)3, remains the same in 49 and 50 (Scheme 17). The insertion into the Hf-naphthyl bond always proceeds with 1,2-regiochemistry. The formation of 50 takes place in the presence of excess 1-hexene. Under such conditions the rapid formation of polyhexene is also seen, and the monomer is consumed while the Hf-Me bond of 50 remains intact. These results leave the question open whether the formation of 50 does indeed

(69) Froese, R. D.; Hustad, P. D.; Kuhlman, R. L.; Wenzel, T. T. J. Am. Chem. Soc. 2007, 129, 7831. (70) Zuccaccia, C.; Macchioni, A.; Busico, V.; Cipullo, R.; Talarico, G.; Alfano, F.; Boone, H. W.; Frazier, K. A.; Hustad, P. D.; Stevens, J. C.; Vosejpka, P. C.; Abboud, K. A. J. Am. Chem. Soc. 2008, 130, 10354.

Review


4719

Scheme 15

Scheme 16

represent the catalyst activation step or is possibly a side product that is not relevant to the catalytic cycle. In an effort to model the polymer chain more realistically and to probe the relative rates of alkene insertions into the Hf-aryl and Hf-alkyl bonds, the dibutyl complex 51 was activated with CPh3[B(C6F5)4]. This gives a stoichiometric amount of 1-butene in the immediate vicinity of the metal center. A diastereomeric mixture of 52 was produced, alongside the polymer. This is consistent with a very low proportion of active species. Since it had been estimated that the rate of alkene insertion into the Hf-alkyl bond is about 1 order of magnitude slower than insertion into the Hf-aryl bond, it appears that the polymerization proceeds along the path 52 f 53 f 54, with catalyst initiation much slower than propagation.71 (71) Zuccaccia, C.; Busico, V.; Cipullo, R.; Talarico, G.; Froese, R. D. J.; Vosejpka, P. C.; Hustad, P. D.; Macchioni, A. Organometallics 2009, 28, 5445.

However, this activation path is unlikely to be the whole story. Propene polymerizations with these catalysts show multimodal molecular weight distributions which change as a function of reaction times. Calculations have suggested that although 55 may be the kinetic product, other structures are energetically very close, such as 56-58 (Scheme 18). It is likely therefore that with this system there are several kinetically competitive active species.72 That these catalysts are very susceptible to steric influences, with small changes resulting in rather different polymerization characteristics, was demonstrated by Coates, who showed that while 59A gives living polyhexene, 59B is nonliving with broad polydispersities and 59C is inactive.73 (72) Busico, V.; Cipullo, R.; Pellecchia, R.; Rongo, L.; Talarico, G.; Macchioni, A.; Zuccaccia, C.; Froese, R. D. J.; Hustad, P. D. Macromolecules 2009, 42, 4369. (73) Domski, G. J.; Lobkovsky, E. B.; Coates, G. W. Macromolecules 2007, 40, 3510.

4720


Bochmann Scheme 17

Scheme 18

7. Reactivity of LnM-Rþ with Long-Chain Alkyl Ligands It is well recognized that the methyl ligand is a poor model for the growing polymer chain: it has much lower steric requirements and therefore can be expected to allow much tighter anion binding in ion pairs than would be possible in the active species, it is unable to form β- or γ-agostic interactions, and it carries no chiral β-carbons as many polymer chains do. Nevertheless, studies on species that more realistically resemble well-defined catalytically active ion pairs have remained the exception rather than the rule in polymerization catalysis.

Table 2. Bond Dissociation Enthalpies D of M-Alkyl Bonds74 complex

D (kcal/mol)

complex

D (kcal/mol)

Cp2TiMe2 Cp2ZrMe2 Cp*2TiMe2 Cp*2ZrMe2 Cp*2HfMe2 Cp*2HfBun2

298 ( 6 285 ( 2 281 ( 8 284 ( 2 (mean) 306 ( 7 274 ( 10

Cp*ZrMe3 Cp*HfMe3 Ti(CH2CMe3)4 Zr(CH2CMe3)4 Hf(CH2CMe3)4

276 ( 10 294 ( 10 198 249 266

One reason for this is the instability of many group 4 alkyl complexes carrying β-H atoms. Some M-C σ-bond dissociation enthalpies are shown in Table 2, mainly based on reaction solution calorimetry.74 The bond strength trend is Ti e Zr < Hf, and it is therefore not surprising that most examples of cationic long-chain alkyl species are based on hafnium (one example being shown in Scheme 17). Hafnium alkyls also have a lower tendency to decompose by β-H elimination. Another reason for the relative paucity of studies is the difficulty of activating homoleptic long-chain dialkyls L2MR2 with borane or trityl borate activators. The most stable alkyl complexes are those not containing β-H atoms, such as CH2CMe3 and CH2SiMe3, but these are generally unreactive. This necessitates the synthesis of mixed alkyls L2M(Me)R, which adds a level of complexity. The best model for the growing alkyl chain, in terms of steric and electronic properties combined with relative stability, would appear to be the neopentyl ligand. However, even these are not sufficiently stable for structural studies, and their decomposition by a β-methyl elimination pathway is well established.75,76 For the Hf complex 60 the (74) Martinho-Sim~ oes, J. A.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629. (75) (a) Eshuis, J. J. W.; Tan, Y. Y.; Teuben, J. H. J. Mol. Catal. 1990, 62, 277. (b) Horton, A. D. Organometallics 1996, 15, 2675. (c) Resconi, L.; Piemontesi, F.; Fraciscono, G.; Abis, L.; Fiorani, T. J. Am. Chem. Soc. 1992, 114, 362. (76) Beswick, C. L.; Marks, T. J. J. Am. Chem. Soc. 2000, 122, 10358.

Review


4721

Scheme 19

Scheme 20

Scheme 21

process shows an activation enthalpy of 21.3 kcal/mol in toluene which is reduced to 17.0 kcal/mol in chlorobenzene: i.e., it is assisted by increased anion dissociation in the more polar solvent (Scheme 19).77 The analogous zirconium reaction has been shown to proceed by γ-agostic assistance in the transition state.78 Nonmetallocene Alkyl Cations. The Hf isobutyl cation 61 is accessible from the corresponding LHfBui2 and trityl borate in C6D5Br solution at -10 °C or below; it is mononuclear. It decomposes above 0 °C but inserts 1-hexene to give an identifiable first 1,2-insertion product (Scheme 20). The polymerization is first order in [1-hexene], with the propagation rate constant kp = 0.16 L mol-1 s-1at 5 °C. Activation with [HNMe2Ph][B(C6F5)4] leads to ortho metalation of the dimethylaniline byproduct. Activation with B(C6F5)3 gave the corresponding hydridoborate ion pair. The Zr and Hf isobutyl cations were also generated. These complexes catalyze the living polymerization of 1-hexene.37 The activation

of the analogous ethyl complex (NNN)HfEt2 with CPh3þ affords a 1:1 mixture of [(NNN)HfEt]þ and its ethene insertion product, [(NNN)HfBun]þ.79 A series of sterically congested cationic zirconium halfsandwich complexes 62 have been prepared as models for the living Ziegler-Natta polymerization with these catalysts (Scheme 21).80 Even though zirconium alkyls carrying β-hydrogen atoms are usually thermally very unstable, these cations can be used at -10 °C for the living isotactic polymerization of 1-hexene. The crystal structure of the Zr-Bui derivative;a very rare case of a structurally characterized zirconium alkyl B(C6F5)4- salt;shows a β-agostically bonded isobutyl ligand, the absence of coordinated dimethylaniline, and the lack of any close Zr 3 3 3 F contacts to the anion. The Zr-N distances are shorter than those of the neutral precursors, a reflection of increased electrophilicity of the metal center, which has the consequence that the cations are configurationally stable. This lack of fluxionality

(77) Lin, M.; Spivak, G. J.; Baird, M. C. Organometallics 2002, 21, 2350. (78) Chirik, P. J.; Dalleska, N. F.; Henling, L. M.; Bercaw, J. E. Organometallics 2005, 24, 2789.

(79) Mehrkhodavandi, P.; Schrock, R. R. J. Am. Chem. Soc. 2001, 123, 10746. (80) Harney, M. B.; Keaton, R. J.; Fettinger, J. C.; Sita, L. R. J. Am. Chem. Soc. 2006, 128, 3420.

4722

Organometallics, Vol. 29, No. 21, 2010 Scheme 22

is responsible for the observed stereoselectivity of these catalysts. At 0 °C the Zr-2-ethylbutyl cation decomposes twice as quickly as Zr-Bui, while under such conditions the Zr-Prn and Zr-Pri species are stable. Deuterium and 13C labeling revealed that at 0 °C the Zr-Bui complex undergoes extensive chain-end epimerization by a succession of β-H elimination and reinsertion equilibria (Scheme 22); this evidently does not affect the living character of the polymerization under such conditions. The results support the view that one does not necessarily need to avoid β-H elimination in order to observe living olefin polymerizations; however, it is essential that the thus eliminated CdC terminated olefin does not leave the coordination sphere of the metal and remains available for reinsertion. The hafnium isobutyl complex 63 is prepared in a manner analogous to that for the zirconium species and also catalyzes the living polymerization of 1-hexene, but at a rate about 60 times slower than that for zirconium. Rather strangely, and in contrast to the zirconium analogue as well as bis-Cp hafnium compounds (see below), NMR studies revealed no evidence for β-agostic bonding of the alkyl ligand; there may be chlorobenzene coordination. In the presence of excess neutral methyl complex [Hf](Me)(Bui) there is rapid methyl ligand exchange between the stereorigid catalytically active species and the fluxional methyl complexes [Hf](Me)(polymeryl), with the result that the polymer structure changes from isotactic to atactic.81 For stability and fluxionality studies of metal alkyl ion pair catalysts the solvent is of prime importance. Here the complexes were studied in a relatively polar solvent, chlorobenzene (ε = 5.69), which may coordinate. It also provides a decomposition pathway, and both 62 and 63 decompose on standing or warming to the chloro-bridged ions 64. Metallocene Cations with Long-Chain Alkyls. For the reasons outlined above there are few examples of cationic metallocene complexes with alkyl ligands other than methyl or benzyl, unless they are stabilized by donor ligands.82 Mixed-ligand nonbridged metallocene zwitterions (1,2-Me2C5H3)2M(R)MeB(C6F5)3 (M = Zr, Hf) show a marked decrease in ion pair reorganization enthalpies with increasing steric demand of R = Me, CH2CMe3, CH2SiMe3, CH(SiMe3)2.76 The (trimethylsilyl)methyl complexes of ansametallocenes (L)M(Me)CH2SiMe3 (65: M = Zr, Hf; L = SBI, IPCF) react with either B(C6F5)3 or CPh3[B(C6F5)4] selectively under methyl abstraction, to give inner-sphere (ISIPs) and outer-sphere ion pairs (OSIPs) 66-69, respectively (81) Kissounko, D. A.; Zhang, Y.; Harney, M. B.; Sita, L. R. Adv. Synth. Catal. 2005, 347, 426. (82) Guram, A. S.; Jordan, R. F. In Comprehensive Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K., 1994; Vol. 4, p 589. (83) Song, F.; Cannon, R. D.; Bochmann, M. Chem. Commun. 2004, 542.

Bochmann

(Scheme 23).83-86 The ion pairs are stable for several days at room temperature in toluene solution or in mixtures of toluene with 10 vol % 1,2-difluorobenzene. The latter is sometimes required to improve the solubility of the B(C6F5)4- salts without coordinating to the metal or interfering with their solution structures (adding 10% difluorobenzene to toluene increases the dielectric constant of the medium from ε = 2.38 to 3.47). Modeling of the SBI complexes 66 and 68 (M = Zr) by DFT methods has confirmed a preference for an R-CH agostic interaction with one of the methyl hydrogens of the MeB(C6F5)3 anion in 66 and a γ-interaction (or β-Si-C interaction) in the case of 68.87 The γ-agostic metal-alkyl interaction is strongest for the hafnium ion pair 68-Hf, such that the three different Si-Me environments are becoming resolved below -20 °C (the ΔGq value for SiMe3 rotation is 12.6 kcal/mol at 10 °C), while for 68-Zr there is only broadening of the SiMe3 signal even at -80 °C. NOE experiments showed that in solution the B(C6F5)4- anion is located near the open wedge of the SBI ligand, as indicated by interactions of borate-F atoms with the SiMe3 and CH2 moieties of the alkyl ligand. The calculated structure of the ISIP 66-Zr agrees well with the crystal structure reported for the Hf complex.84 Both 66 and 68 are excellent single-component catalysts for the polymerization of propene and 1-hexene.83 The stability and long-lived nature of these compounds is in agreement with the concept that they are best regarded as resting states which can be converted into active species by approach of the alkene. The ligand dynamics, site epimerization, and polymerization kinetics with these and related welldefined metallocene catalysts are discussed in section 12. The different ion pair structures of 66 (ISIP) and 68 (OSIP) and the different conformations of the alkyl ligands do have consequences for the chain termination process: whereas in 1-hexene polymerizations the MeB(C6F5)3 ISIP leads predominantly to vinylidene end groups through β-H transfer to the metal due to the congested transition state in the presence of a closely associated anion, termination for the B(C6F5)4OSIP takes place after a 2,1-misinsertion and gives vinylene end groups. That is, the loosely associated anion permits β-H transfer to monomer via a more voluminous transition state (Scheme 24).85 The synthesis of mixed-alkyl metallocenes is not entirely trivial and depends on the Cp ligands employed.88 The synthesis of hafnium mixed alkyls requires rather more forcing conditions and a careful choice of solvents.84,86 Modeling of the Active Site. Over the last 10 years or so computational methods have developed to such an extent that they are becoming more and more an accompaniment of synthetic and catalytic reports and as such are rather specific to the particular ligands in question. It is beyond the scope of this review to present a full account of this aspect of catalysis research. However, a number of reports should be mentioned here, because they address more fundamental aspects (84) Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey, S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics 2005, 24, 1315. (85) Wilson, P. A.; Hannant, M. H.; Wright, J. A.; Cannon, R. D.; Bochmann, M. Macromol. Symp. 2006, 236, 100. (86) Alonso-Moreno, C.; Lancaster, S. J.; Wright, J. A.; Hughes, D. L.; Zuccaccia, C.; Correra, A.; Macchioni, A.; Cavallo, L.; Bochmann, M. Organometallics 2008, 27, 5474. (87) Ducere, J. M.; Cavallo, L. Organometallics 2006, 25, 1431. (88) Klamo, S. B.; Wendt, O. F.; Henling, L. M.; Day, M. W.; Bercaw, J. E. Organometallics 2007, 26, 3018.

Review


4723

Scheme 23

of well-defined polymerization catalysts. Most of these have concerned themselves with the bis-Cp system, using MeB(C6F5)3- as the anion, which is preferred in computational modeling since it has a more precisely determined position relative to the cation and fewer degrees of freedom. Early studies suggested an approach of ethene from the direction trans to the anion, in the manner of an SN2 reaction.89-91 However, since this would also necessitate a chain swinging step (site epimerization) prior to every insertion, this scenario now seems unlikely. A more complete consideration of the first and second insertion steps showed (a) that ethene binding to give Cp2ZrMe(C2H4)þ is highly exothermic relative to the separated Cp2ZrMeþ þ C2H4 and (b) that while in the first insertion into Cp2ZrMeþ the trans approach is favored, in the product Cp2Zr-propylþ the cis pathway is preferred, with an ethene uptake barrier of 9.5 kcal/mol, followed by a lower insertion barrier of 6.8 kcal/mol, so that ethene uptake becomes rate limiting. These studies also showed the importance of including solvation effects, since for gas-phase calculations the insertion step appears rate limiting, while in solution it is ethene uptake. After each insertion the anion recoordinates, in line with kinetic results.92 Calculations of the ethene insertion process with nonmetallocenes such as (N-N)TiMe(μ-Me)B(C6F5)3 also showed that ethene approach cis to the anion has the lower barrier (N-N = phenyliminopyrrolate).93 These transitionstate models have more recently been used to evaluate the importance of polymeryl chain bonding and the relative energies of chain transfer reactions as a function of ligand design, in an effort to optimize polymer molecular weights in ethene/propene copolymerizations with Cs-symmetric metallocenes.94 (89) Chan, M. S. W.; Ziegler, T. Organometallics 2000, 19, 5182. (90) Vanka, K.; Chan, M. S. W.; Pye, C. C.; Ziegler, T. Macromol. Symp. 2001, 173, 163. (91) Lanza, G.; Fragala, I. L.; Marks, T. J. Organometallics 2002, 21, 5594. (92) Ziegler, T.; Vanka, K.; Xu, Z. T. C. R. Chim. 2005, 8, 1552. (93) Vanka, K.; Xu, Z.; Ziegler, T. Organometallics 2004, 23, 2900. (94) (a) Wang, D. Q.; Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2008, 27, 2861. (b) Wondimagegn, T.; Wang, D.; Razavi, A.; Ziegler, T. Organometallics 2008, 27, 6434. (c) Wondimagegn, T.; Wang, D. Q.; Razavi, A.; Ziegler, T. Organometallics 2009, 28, 1383. (d) Tomasi, S.; Razavi, A.; Ziegler, T. Organometallics 2009, 28, 2609.

Scheme 24

The activation enthalpy for the insertion of ethene into the M-Me bond increases in the order Cp2TiMeþ Zr>Hf and rac-C2H4(Ind)2 (EBI) > Cp > rac-C2H4(tetrahydroindenyl)2. The rate can be correlated with the ability of these complexes to catalyze alkene carboaluminations. In the heterobinuclear cation there is no exchange between terminal and bridging Me groups in the presence or absence of excess AlMe3.107 For [(EBI)Zr(μ-Me)2AlMe2]þ, koff = 0.17 s-1; this compound is an effective catalyst for the stereoselective carboalumination of allylbenzene.108 The reverse, kon, is unknown; therefore, the AlMe3 dissociation equilibrium has yet to be quantified. The activation of ansa-titanocenes 75 and 76 (Scheme 28) either with AlMe3/CPh3[B(C6F5)4] or with MAO has been followed by a combined UV/vis, 1H NMR, and EPR spectroscopic study. The cations [(L)Ti(μ-Me)2AlMe2]þ dominate in (106) (a) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. Chem. Commun. 2005, 3313. (b) Bolton, P. D.; Clot, E.; Cowley, A. R.; Mountford, P. J. Am. Chem. Soc. 2006, 128, 15005. (107) Petros, R. A.; Norton, J. R. Organometallics 2004, 23, 5105. (108) Petros, R. A.; Camara, J. M.; Norton, J. R. J. Organomet. Chem. 2007, 692, 4768.

4726


Bochmann Scheme 29

each case; below room temperature they act as catalysts for the polymerization of propene. However, above 0 °C the cations are reduced to (L)TiIII(μ-Me)2AlMe2. In fact, the reduction to Ti(III) is faster in the presence of monomer.109 Not every zirconium catalyst benefits from stabilization by AlMe3 or indeed forms an adduct of type E. For example, the activity of 77 at 40 °C increases in the presence of AlMe3-free MAO, whereas 78 shows an almost total loss of activity if AlMe3 is absent. Since in this case the AlMe3 was removed by the addition of 2,6-di-tert-butylcresol, the deactivation could be due to the presence of the cresol or, more probably, to the inability of bulky MAO clusters to interact effectively with the sterically very hindered zirconocene.110 In fact, zirconocene 78 has been shown to generate an AlMe3 adduct of a different structure, 79. On MAO addition, the electronic spectrum of 78 (λ 318 and 384 nm) first changes to (L)ZrMeCl (λ 310 and 352 nm) and then, at Al/Zr = 155-1800, to a species with λ 400 nm, typical of the MeMAO- contact ion pair. There is no further shift that would indicate a Zr(μ-Me)2AlMe2þ species. This was confirmed by 1 H NMR studies and activations with CPh3[B(C6F5)4], [HNMe2Ph][B(C6F5)4], or MAO, which give the ion pairs [(L)ZrMeþ 3 3 3 B(C6F5)4-] and [(L)ZrMeþ 3 3 3 Me-MAO-] only.111 In this context it may also be recalled that in some instances, such as in half-sandwich complexes, AlMe3 has been found to be capable of remarkably facile C-H activation reactions which contribute to catalyst deactivation (Scheme 29).112

9. Catalyst Activation with Methylalumoxane Metallocenes. Methylalumoxane activates metallocene catalysts by virtue of Lewis acidic sites on some of the complex structures present in MAO. This can be quantified by adding a stable radical, TEMPO (=2,2,6,6-tetramethylpiperidine-N-oxyl), as a spin probe which binds to the Lewis acid sites. MAO contains two types of acidic sites, I (triplet, g0 = 2.0047, aN = 18.6 G) and II (triplet of sextets, g0 = 2.0045, aN = 19.6 G, and aAl = 1.7 G), in about equal concentration. MAO contains one site of each type per 100 ( 30 aluminum atoms. The adducts of TEMPO with sites I are less stable than those with sites II. The acidic sites I and II were attributed to coordinatively unsaturated aluminum atoms in -OAlMe2and -Al(O)2Me- environments. From the EPR spectra the average radius of MAO oligomers (AlOMe)n was evaluated to be 5.8 A˚ at 20 °C, which corresponds to the value of n = 15-20. (109) Bryliakov, K. P.; Babushkin, D. E.; Talsi, E. P.; Voskoboynikov, A. Z.; Gritzo, H.; Schr€ oder, L.; Damrau, H. R. H.; Wieser, U.; Schaper, F.; Brintzinger, H. H. Organometallics 2005, 24, 894. (110) Tynys, A.; Eilertson, J. L.; Rytter, E. Macromol. Chem. Phys. 2006, 207, 295. (111) Schr€ oder, L.; Brintzinger, H. H.; Babushkin, D. E.; Fischer, D.; M€ ulhaupt, R. Organometallics 2005, 24, 867. (112) Stephan, D. W. Macromol. Symp. 2001, 173, 105. (113) Talsi, E. P.; Semikolenova, N. V.; Panchenko, V. N.; Sobolev, A. P.; Babushkin, D. E.; Shubin, A. A.; Zakharov, V. A. J. Mol. Catal. 1999, 139, 131.

Scheme 30

Scheme 31

This would suggest that the major part of MAO contains not more than one Lewis acidic site per one oligomeric (AlOMe)n molecule.113 A number of cationic Al compounds have been made with MAO-like structures: for example, Al2Me4(μ-Me)(μ-OSiMes3), [Me2Al(NMe2Ph)2][B(C6F5)4],114 and 80. The last species reacts with Cp2ZrMe2 to give, surprisingly, a cyclometalation product (Scheme 30).115 A rare example of a crystallographically characterized MAO-like anion is 81 (Scheme 31).116 From NMR diffusion coefficient measurements, the ion pair [Cp2Zr(μ-Me)2AlMe2]þ[Me-MAO]- has a mean effective hydrodynamic radius of 12.2-12.5 A˚. The ion pair remains associated down to [Zr] = 4.8 10-5 mol/L. The size of the anion suggests an aggregate of ca. 150-200 Al atoms.117 The activation of a series of zirconocenes Cp0 2ZrCl2 (Cp0 = 1,2-Me2Cp, 1,2,3-Me3Cp, 1,2,4-Me3Cp, C5HMe4, C5Me5, BunCp, ButCp) with MAO has been shown to lead to a similar series of species I-IV as shown in Scheme 26. At Al/Zr ratios of 50-200 species IV dominated, while at Al/Zr = 500-1000 the heteronuclear cation III was the major product in all systems. (EBI)ZrCl2 and (SBI)ZrCl2/MAO show very similar behavior. However, whereas the ethene polymerization activity for metallocenes with C5H4R ligands (R = H, Me, Bun, But) depended strongly on the Al/Zr ratio in the range of 200-1000, it remained almost constant over the same range for catalysts with more highly substituted Cp ligands, as well as (SBI)ZrCl2. The data were interpreted assuming that species III is the main (114) Wrobel, O.; Schaper, F.; Brintzinger, H. H. Organometallics 2004, 23, 900. (115) Wrobel, O.; Schaper, F.; Wieser, U.; Gregorius, H.; Brintzinger, H. H. Organometallics 2003, 22, 1320. (116) Richter, B.; Meetsma, A.; Hessen, B.; Teuben, J. H. Angew. Chem., Int. Ed. 2002, 41, 2166. (117) Babushkin, D. E.; Brintzinger, H. H. J. Am. Chem. Soc. 2002, 124, 12869.

Review


4727

Scheme 33

Scheme 34

precursor for the formation of active centers in MAO systems.118 The interaction of MAO with metallocenes can be conveniently monitored by UV/vis spectroscopy, providing that the metallocene is a suitably good chromophore. Thus (SBI)ZrCl2 (λmax 447 nm) reacts with solid MAO to generate first a species with λmax 458 nm, which on increasing the Al/ Zr ratio to 1000 is converted to III (λmax 493 nm). However, at lower Al/Zr ratios two different species could be identified: the more strongly bound ion pair [(SBI)ZrMeþ 3 3 3 MeMAOB-] which, on addition of further MAO, is converted to the more easily separated ion pair [(SBI)ZrMeþ 3 3 3 MeMAOA-]. It is the latter which reacts with AlMe3 to give the OSIP III and is required for the reaction with olefins.119 The reaction of the 13C-labeled zirconocene 82 with MAO led to similar conclusions: there are at least two different ion pairs, with Me-MAOA- and Me-MAOB- counteranions (Scheme 32). At the reagent ratios employed, the [III]/[IVA] ratio was estimated to be ∼5:1 and [III]/[IVB] < 1:2. It is likely that a similar reactivity ratio also applies to the displacement of the MeMAO anion by an olefin substrate: i.e., IVA is the more reactive ion pair. The mole fractions of the less reactive ion pairs (and likely resting states) IVCl and IVB decrease from >75% at Al/Zr ≈ 100 to AlEt3, while the catalytic activity increases.125 It seemed likely therefore that the presence of metal-bound alkyl chains would preclude the formation of AlR3 adducts, (122) Miller, S. A.; Bercaw, J. E. Organometallics 2004, 23, 1777. (123) Irwin, L. J.; Reibenspiess, J. H.; Miller, S. A. J. Am. Chem. Soc. 2004, 126, 16716. (124) Busico, V.; Cipullo, R.; Pellecchia, R.; Talarico, G.; Razavi, A. Macromolecules 2009, 42, 1789. (125) Bochmann, M.; Lancaster, S. J. J. Organomet. Chem. 1995, 497, 55.

4728


Bochmann Scheme 35

Scheme 36

so that such species would only exist as unstable intermediates during the alkyl transfer from the transition metal to aluminum. This aspect was probed in the case of ethene/norbornene copolymerizations with MAO-activated (IPCF)ZrCl2 or racC2H4(1-Ind-2-OSiMe2But)2ZrCl2, by adding AlMe3, AlEt3, or AlBui3. While AlMe3 had little effect on Mn, AlEt3 gave a reduced molecular weight due to more facile chain transfer, presumably via μ-alkyl species such as 85 (Scheme 34). ZnMe2 and ZnEt2 also strongly reduced Mn and also inhibited the catalysis.126 Very recently, Babushkin and Brintzinger were able to provide direct spectroscopic support for bridging polymeryl species. Activating (SBI)ZrMe2 with MAO generates about equal amounts of [(SBI)ZrMeþ 3 3 MeMAO-] and [(SBI)Zr(μ-Me)2AlMe2þ 3 3 3 MeMAO-]; the signal for the latter decreases sharply on 13C-1-hexene addition, only to re-establish itself once the monomer is consumed. Characterization of the intermediates and comparison with the Zr-CH2SiMe3 model compound 86 produced the first evidence for the formation of the μ-polymeryl species 87.127 Non-Metallocenes. An unusual case of “synergistic activation” has been discovered in the case of the benzamidinato complex 88. Whereas this is polymerization-inactive on addition of CPh3[B(C6F5)4] and has only low activity with MAO, the combination of trityl and MAO gives a 100-fold increase in activity. EPR spectroscopy revealed the presence of Zr(III), which disappeared on adding propene. Adding C60 as a radical trap provided evidence of C60Me• and (126) Bhriain, N. N.; Brintzinger, H. H.; Ruchatz, D.; Fink, G. Macromolecules 2005, 38, 2056. (127) Babushkin, D. E.; Brintzinger, H. H. J. Am. Chem. Soc. 2010, 132, 452.

MeC60-C60Me. Propagation via a radical cation was proposed (Scheme 35).128 The addition of 2,6-di-tert-butyl cresol (BHT, 2,6-But2-4MeC6H2OH) to (SBI)ZrCl2/MAO or (O-N)2TiCl2 (89)/ MAO depletes the MAO of AlMe3 by forming Me2Al(OAr) and MeAl(OAr)2; this doubles the activity of zirconium and increases the living character and syndiospecificity of the Ti catalyst.129 The titanium catalyst 89/MAO differs markedly in its chemistry from the examples discussed above. Makio and Fujita identified two species assigned as [(N-O)2TiMe]þ and (N-O)AlMe2, indicating facile ligand transfer to aluminum.130 A more detailed study by Bryliakov et al. confirmed these findings (Scheme 36). In particular, it is evident that, unlike the vast majority of group 4 catalysts, 89 does not form a heterobimetallic AlMe3 adduct of type III. This was confirmed by the reaction of 89 with AlMe3/CPh3[B(C6F5)4], which also gives the methyl cation 90. The failure of apparently unhindered cationic half-sandwich complexes of titanium to form AlMe3 adducts had been noted earlier: e.g., with Cp*TiCl3/MAO and Cp*TiMe3/B(C6F5)3.131 The Me-MAO ion pair is unstable and decomposes at 20 °C in toluene with a half-life of 1 h by two pathways: ligand transfer to Al (minor component, ca. 30%) and reduction to Ti(III) (major pathway). The addition of 16 equiv of 13 C2H4 leads to the disappearance of signals due to 90 and the formation of a Ti-polymeryl complex. This species (128) Volkis, V.; Tumanskii, B.; Eisen, M. Organometallics 2006, 25, 2722. (129) Busico, V.; Cipullo, R.; Cutillo, F.; Friederichs, N.; Ronca, S.; Wang, B. J. Am. Chem. Soc. 2003, 125, 12402. (130) Makio, H.; Fujita, T. Macromol. Symp. 2004, 213, 221. (131) Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. J. Organomet. Chem. 2003, 683, 23.

Review


4729

Scheme 37

shows an equivalent ligand set and relatively sharp signals for an outer-sphere Me-MAO anion.132 An analogous reaction sequence is followed on MAO activation of the bis(pyrrolaldiminato)titanium complex 91 and the mixed-ligand species 92 (Scheme 37).133 These mixed-ligand complexes show improved copolymerization characteristics.134 While 91 deactivates by ligand transfer to Al and formation of an as yet not fully characterized Ti(IV) species, as well as by reduction to Ti(III), there was no evidence for ligand transfer to Al in the case of 92, and reduction to Ti(III) is the main mode of deactivation.133 The interaction of the zirconium “phenoxy-imine” catalysts 93 with MAO or AlMe3/CPh3[B(C6F5)4] gives different species depending on the o-R substituents (Scheme 38). Unlike the titanium analogue, the heterobinuclear AlMe3 adduct is stable and acts as a catalyst precursor. Ligand transfer to Al is dominant for R = H and is the main route to deactivation for R = But.135

10. Catalyst Activation Involving Triisobutylaluminum Triisobutylaluminum (TIBA) as a part of catalyst activator systems has usually remarkably beneficial effects. The reasons for this are not always entirely clear. Fink et al. showed that, in MAO-activated ethene/norbornene copolymerizations with ansa-zirconocenes, TIBA has low or no tendency to undergo exchange of Zr-polymeryl chains. However, TIBA rapidly exchanges ligands with trimethylaluminum, and it was suggested that one reason for the beneficial effect in MAO systems containing TIBA may be (a) the removal of AlMe3 from the reaction by the equilibrium

Al2 Me6 þ 4AlBui 3 ¼ 3Bui 2 Alðμ-MeÞ2 AlBui 2 (132) Bryliakov, K. P.; Kravstov, E. A.; Pennington, D. A.; Lancaster, S. J.; Bochmann, M.; Brintzinger, H. H.; Talsi, E. P. Organometallics 2005, 24, 5660. (133) Bryliakov, K. P.; Kravstov, E. A.; Broomfield, L.; Talsi, E. P.; Bochmann, M. Organometallics 2007, 26, 288. (134) (a) Pennington, D. A.; Coles, S. J.; Hursthouse, M. B.; Bochmann, M.; Lancaster, S. J. Chem. Commun. 2005, 3150. (b) Pennington, D. A.; Coles, S. J.; Hursthouse, M. B.; Bochmann, M.; Lancaster, S. J. Macromol. Rapid Commun. 2006, 27, 599. (135) Kravtsov, E. A.; Bryliakov, K. P.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Organometallics 2007, 26, 4810.

Scheme 38

and (b) methyl exchange with Me-MAO- would give a possibly less coordinating Bui-MAO-.126 The activation of (2-PhInd)2ZrCl2 with MAO and TIBAmodified MAO (MMAO), as well as with AlMe3/[CPh3][B(C6F5)4], allowed the spectroscopic identification of the ions III, IIIBui, IIIBui2, and IV (Scheme 39). With MMAO, only the OSIPs IIIBui and IIIBui2 are seen. At -50 to 20 °C the rate of the indenyl ligand rotation was found to be faster than that of propene insertion; only in the ion IIIBui2 are the rates comparable.136 Mixtures of MAO with AlR3 (R = Bui, Et) as well as commercial MMAO seem to have Lewis acid sites that are stronger than those of ordinary MAO, as judged by EPR spectroscopy on addition of TEMPO. With (SBI)ZrCl2 these activators increased the concentration of IV while the formation of III was suppressed. Again, the less-tight anion binding of a Bui-modified MAO anion seems a likely contributor to the observed increase in catalytic activity.137 In mixtures of the hindered zirconocenes 94-96 (Scheme 40) with MAO and TIBA, the mixed-alkyl aluminum dimers Al2(μ-Me)2Me4-xBuix were found to predominate. The metallocenes give an equilibrium mixture of the ions III, IIIBui, and IIIBui2 in which the isobutyl group is found exclusively on Al in a terminal position; there was no evidence for (136) Lyakin, O. Y.; Bryliakov, K. P.; Semikolenova, N. V.; Lebedev, A. Y.; Voskoboynikov, A. Z.; Zakharov, V. A.; Talsi, E. P. Organometallics 2007, 26, 1536. (137) Bryliakov, K. P.; Semikolenova, N. V.; Panchenko, V. N.; Zakharov, V. A.; Brintzinger, H. H.; Talsi, E. P. Macromol. Chem. Phys. 2006, 207, 327.

4730


Bochmann Scheme 39

Scheme 40

give a similar hydrido-bridged species, 100. There was no evidence for the formation of a cationic Zr allyl complex.142 Somewhat different hydride resting states were found in the reactions of the hafnocenes 101-104. The cationic AlMe3 adducts of type III proved thermally very stable and much less reactive than zirconocene analogues; on addition of ethene they gave only traces of polymer. The reactions with TIBA/CPh3[B(C6F5)4] in toluene gave spectroscopically identified hydrido species (Scheme 42); those derived form 102 were unstable. On addition of 10 equiv of 2,4,4-trimethyl1-pentene the hydrido signals disappeared slowly, although it did not prove possible to positively identify the Hf-alkyl insertion products.143 In spite of the strongly activating effect of TIBA and MMAO, these reagents are able to contribute to catalyst deactivation by facilitating reduction, as the formation of hydrido species suggests. Monitoring the reactions of (2PhInd)2ZrCl2 or (SBI)ZrCl2 with MMAO shows that MMAO is a much stronger reducing agent than MAO. A number of Zr(III) products were identified (Scheme 43). The reduction of (SBI)ZrCl2 can be masked by the formation of EPRsilent ZrIII-ZrIII dimers. In the presence of TIBA and pyridine at 20 °C over 2 h, about 20% of the initial (SBI)ZrCl2 is present as Zr(III).144

bridging isobutyl. It was possible to determine the equilibrium constants for the interchange of III, IIIBui, and IIIBui2. Complex 96/MAO is unusual in this series in that it eliminates methane readily, to give (L)Zr(μ-CH2)(μ-Me)AlMe2 (cf. section 3). Furthermore, IIIBui and IIIBui2 are prone to isobutene elimination, giving zirconocene hydride species.138 The fate of such hydride products was investigated by reacting zirconocenes with TIBA or HAlBui2. Nonbridged complexes (C5H5-nRn)2ZrCl2 gave the hydride-bridged clusters 97, while ansa-zirconocenes produced adducts 98 with only one HAlBui2 unit (Scheme 40).139 The interaction of TIBA with group 4 metallocenes under catalyst activation conditions but in the absence of monomer has led to the isolation of a number of species that can be regarded as resting states, or possibly deactivation products, for cationic metal alkyl species. G€ otz et al. found that Cp2ZrCl2 gives TIBA adducts of zirconocene hydrides [Cp2Zr(μ-H)(HAlBui3)]2,140 probably via Cp2ZrBui2,141 whereas the reaction of 77 with TIBA/[HNMe2Ph][B(C6F5)4] at an Al/Zr ratio of 100 gave the unusual hydrido-bridged cation 99 (Scheme 41).140 The reaction of 94 with TIBA/CPh3[B(C6F5)4] in toluene proceeds in a similar fashion, possibly via the structurally characterized dication [(SBI)Zr(μ-Cl)]22þ, to

Group 4 polymerization catalysts follow essentially the kinetic model developed by Fink for Cp2TiCl2 ethylene oligomerization catalysts (Scheme 44). Common features are (i) spectroscopically unobservable active species in equilibrium with observable resting states;the so-called intermittent growth mechanism;and (ii) alkene insertion into the metal-methyl bond being about 2 orders of magnitude slower than insertion into longer chain metal-alkyl bonds. This tends to lead to the situation that the addition of monomer to pregenerated LnM-Meþ species produces polymer while the concentration of the metal-methyl complex is apparently unchanged.23,145 The kinetics of 1-hexene polymerization show first-order dependence on [monomer] for both metallocene146 and

(138) Babushkin, D. E.; Brintzinger, H. H. Chem. Eur. J. 2007, 13, 5294. (139) Baldwin, S. M.; Bercaw, J. E.; Brintzinger, H. H. J. Am. Chem. Soc. 2008, 130, 17423. (140) G€ otz, C.; Rau, A.; Luft, G. J. Mol. Catal. A: Chem. 2002, 184, 95. (141) Carr, A. G.; Dawson, D. M.; Thornton-Pett, M.; Bochmann, M. Organometallics 1999, 18, 2933. (142) Bryliakov, K. P.; Talsi, E. P.; Semikolenova, N. V.; Zakharov, V. A.; Brand, J.; Alonso-Moreno, C.; Bochmann, M. J. Organomet. Chem. 2007, 692, 859.

(143) Bryliakov, K. P.; Talsi, E. P.; Voskoboynikov, A. Z.; Lancaster, S. J.; Bochmann, M. Organometallics 2008, 27, 6333. (144) Lyakin, O. Y.; Bryliakov, K. P.; Panchenko, V. N.; Semikolenova, N. V.; Zakharov, V. A.; Talsi, E. P. Macromol. Chem. Phys. 2007, 208, 1168. (145) (a) Fink, G.; Zoller, W. Makromol. Chem. 1981, 182, 3265. (b) Fink, G.; Schnell, D. Angew. Makromol. Chem. 1982, 105, 31. (c) Mynott, R.; Fink, G.; Fenzl, W. Angew. Makromol. Chem. 1987, 154, 1. (d) Fink, G.; Fenzl, W.; Mynott, R. Z. Naturforsch., B 1985, 40b, 158. (146) (a) Liu, Z.; Somsook, E.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 2915. (b) Liu, Z.; Somsook, E.; White, C. B.; Rosaaen, K. A.; Landis, C. R. J. Am. Chem. Soc. 2001, 123, 11193.

11. Polymerization Kinetics of Well-Defined Catalysts

Review


4731

Scheme 41

Scheme 42

Scheme 43

non-metallocene147 complexes. Higher and broken-order dependence on [monomer] has also been observed, though this is explicable on the basis of first-order kinetics at a catalytic species that interconverts between propagating and resting states.148 (147) Goodman, J. T.; Schrock, R. R. Organometallics 2001, 20, 5205. (148) Busico, V.; Cipullo, R.; Cutillo, F.; Vacatello, M. Macromolecules 2002, 35, 349.

Scheme 44. Fink’s Intermittent Polymerization Mechanisma

a Legend: C, activated complex; M, monomer; C*, catalytically active species; CPi (i = 1-z), resting states carrying polymer chains of i monomers. The model assumes that after the first activation step (i.e. the first monomer insertion) the propagation rate constants for subsequent insertions are identical.

4732


Bochmann Scheme 45

Table 3. Active Species Concentrationa as a Percentage of Total Metallocene154 active species (%)b precursor complex

[Zr]/[CPh3þ] = 1:1

[Zr]/[CPh3þ] = 1:3

(SBI)ZrCl2 (SBI)ZrMe2 Me2Si(C5Me4)(NBut)TiCl2

13 15 15

22 16 34

a Conditions: activator AlBui3/CPh3[B(C6F5)4], toluene, 25 °C. =100 kpapp/kp.

b

Quenched-flow kinetics of the 1-hexene polymerization with (EBI)ZrMe(μ-Me)B(C6F5)3 in toluene (EBI = racC2H4(1-Ind)2) at T = -10 to þ50 °C confirmed the rate law R = kp[C][M] but showed that the rate is independent of excess B(C6F5)3. In this scheme, all species arising from the first monomer insertion into the Zr-Me bond are considered as active species (i.e., under this definition the active species contains a coordinated anion). In line with this premise, quenching with MeOD gives an active species count of close to 100%. Chain propagation was ca. 30 faster than initiation, with kp ≈ 6 L mol-1 s-1 (at 20 °C). There was no accumulation of dormant states arising from 2,1-misinsertions.146 The lack of influence of [B(C6F5)3] is in contrast with the observation of a strong enhancing effect on increasing the borane activator concentration in the case of the C1-symmetric zwitterion 24 (Scheme 8). Here, as well as for (IPCF)ZrMe2, there was a sharp rise in activity on increasing the B/Zr ratio from 1:1 to 10:1. This increase was due to faster initiation rather than subsequent propagation.149 The reaction of (EBI)ZrMe(μ-Me)B(C6F5)3 with 1-hexene at -40 °C results in partial (60-70%) conversion of the methyl complex into a Zr-poly(1-hexene) (Zr-PH) species.150 Under these conditions the first insertion step was 400 times slower than subsequent insertions; propagation was 40 000 times faster than termination. The polymerization is living under these conditions. The kinetic data are consistent with the process shown in Scheme 45 and follow the Fink model: i.e., after each insertion step the anion recoordinates to give an observable resting state. Kinetic isotope measurements using different catalyst activators (B(C6F5)3, Al(C6F5)3, [HNMe2Ph][B(C6F5)4], and (149) Al-Humydi, A.; Garrison, J. C.; Youngs, W. J.; Collins, S. Organometallics 2005, 24, 193. (150) Landis, C. R.; Rosaaen, K. A.; Sillars, D. R. J. Am. Chem. Soc. 2003, 125, 1710.

MAO) showed that with this catalyst the nature of the transition state was little affected by the counteranion.151 A different kinetic approach was taken by Song et al., who used quenched-flow techniques to measure the kinetics of propene polymerization with (SBI)ZrMe2 in toluene activated either with AlBui3/CPh3[CN{B(C6F5)3}2] at 25 °C or with MAO at 40 °C (Al/Zr = 2400). The MAO catalyst is almost 30 times slower than the trityl borate system but show the same kinetic behavior. Under these conditions the half-life of catalyst initiation was comparable to the half-life of chain propagation (non-steady-state conditions). The number-average molecular weight Mn was determined by size exclusion chromatography. The reaction showed the expected first-order dependence on [propene]. Analysis of the time dependence of monomer conversion on the one hand and of the growth rate of Mn on the other produced two sets of propagation rate constants: kp (from Mn) and kpapp (from polymer yield). Since the kinetic analysis of the time dependence of Mn makes no assumptions about the catalyst concentration (whereas the rate law of polymer mass vs time does), the ratio kpapp/kp provides a measure of the mole fraction of total [Zr] that is actively engaged in chain growth at any given time. This leads to an active species concentration of ca. 8% of total catalyst precursor.152,153 The kinetics of other metallocenes, including Me2Si(C5Me4)(NBut)TiCl2, gave similar results, with active species concentrations of about 10-15% of the total precursor complex (Table 3).154 There is of course an equilibrium between all Zr species, and all except permanently deactivated species will be carrying polymeryl chains. Isotopic labeling techniques such as deuterium quenching measure the total concentration of Zr-C bonds and are therefore unable to distinguish between active and resting states, whereas the kinetic approach does. Unlike the (EBI)ZrMe(μ-Me)B(C6F5)3 catalyst, the (SBI)ZrMe2-based catalyst leads to 2,1-misinsertions that are enchained; 2,1-insertions were found to be about 500 times slower than 1,2-insertions and contribute to chain termination. The kinetic mechanism developed on this basis is shown in Scheme 46. (151) Landis, C. R.; Rosaaen, K. A.; Udin, J. J. Am. Chem. Soc. 2002, 124, 12062. (152) Song, F.; Cannon, R. D.; Bochmann, M. J. Am. Chem. Soc. 2003, 125, 7641. (153) Song, F.; Hannant, M. D.; Cannon, R. D.; Bochmann, M. Macromol. Symp. 2004, 213, 173. (154) Song, F.; Cannon, R. D.; Lancaster, S. J.; Bochmann, M. J. Mol. Catal. A: Chem. 2004, 218, 21.

Review Scheme 46. Kinetic Mechanism of Propene Polymerizationa


4733

Scheme 47

Scheme 48

a Legend: C* = active species for 1,2-propagation; C0 = species after 2,1-misinsertion; kt and kt0 = termination rate constants after 1,2- and 2,1-insertion, respectively.152.

The data also gave an estimate for the hypothetical equilibrium between a Zr-sec-alkyl and its 1,2-insertion product: K = k21/kp0 ≈ 12 (Scheme 47). This may be seen as a quantitative measure for the relative stabilities of n- and sec-alkyls and the steric repulsion of two adjacent Me groups in a polypropylene chain.152 The polymerization kinetics of propene and 1-hexene in batch reactions with (SBI)Zr(Me)CH2SiMe3 activated with either B(C6F5)3 or CPh3[B(C6F5)4], 66-Zr and 68-Zr (Scheme 23), showed a strong anion influence on the rate for propene but hardly any effect for 1-hexene. The same was found using (SBI)ZrMe2 as catalyst precursor in the presence or absence of excess AlBui3.83 It appears that with this catalyst system propene polymerization involves an early transition state where anion dissociation is important, while the TS for hexene is later along the reaction coordinate and is no longer influenced by the anion displacement equilibrium. This would suggest that while the propene polymerization would benefit from “anion engineering”, the 1-hexene system could only be improved by modifying the ligand. These findings are in contrast to reports on (EBI)ZrMe2 activated with borane or trityl borate, where hexene polymerization rates increased for MeB(C6F5)3- , B(C6F5)4-.155 Monitoring hexene polymerizations with (SBI)Zr(Me)(CH2SiMe3) as a precatalyst by NMR spectroscopy suggested slow initiation and the formation of inactive cationic allyl species.156 However, given that batch reactions containing excess TIBA (i.e., alkyl exchange conditions) had shown the same differences in propene vs hexene rates and essentially identical kp values, deactivation or poor initiation seems unlikely as the reason for the observed discrepancies. It should be remembered, however, that although the EBI and SBI ligand frameworks seem sterically similar, they do differ in their conformational rigidity and can behave rather differently where steric demand by the monomer plays a role. For example, in the copolymerization of propene with vinylnorbornene (VNB), the EBI catalyst shows much greater affinity for VBN than for propene, whereas the opposite is (155) Landis, C. R.; Liu, Z.; White, C. B. Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 2002, 43, 301. (156) Landis, C. R.; Christianson, M. D. Proc. Natl. Acad. Sci. U.S. A. 2006, 103, 15349. (157) Sarazin, Y.; Fink, G.; Hauschild, K.; Bochmann, M. Macromol. Rapid Commun. 2005, 26, 1208.

true for the SBI complex.157 On that basis one would indeed expect EBI and SBI catalysts to show different responses to 1-hexene. This does, unfortunately, underline the need to assess the kinetics of each specific catalyst individually and warns against generalizations. The low concentrations of active species demonstrated by the kpapp/kp ratios raises the question about the nature of the dormant state(s). The accumulation of Zr-sec-alkyl species produced from 2,1-misinsertions is ruled out in the case of (EBI)ZrMe(μ-Me)B(C6F5)3, since 2,1-insertion is too rare and they react almost as fast as Zr-n-alkyls.158 DFT calculations have supported the notion that the differences in monomer uptake and insertion barriers are too small for the accumulation of substantial concentrations of Zr-CHMeR species.159 On the other hand, this would not explain the strongly activating effect of H2 in some catalyst systems which is thought to be associated with reactivating sec-alkyl species by hydrogenolysis. Kinetic and quenching studies have shown that in the system 105/MAO/propene the fraction of zirconium sec-alkyl species is about 20%, with ksp/kps = 4 (Scheme 48). Propene insertion into an n-alkyl bond was about 30 times faster than into a sec-alkyl bond, a sufficient difference to allow sec-alkyl accumulation.160 However, in view of the known structures of cationic metal complexes bearing long-chain alkyls (section 7) and the kinetic results, kinetically dormant states are likely to include agostically bonded n-alkyls, as well as sec-alkyls, AlR3 adducts, and ion pair associations, all of which require an activation barrier and/or conformational changes to be converted into active species. More recently, kinetic models have been developed, on the basis of batch polymerizations of propene, that include the effects of chain transfer to Al, β-H transfer to monomer and to the metal, and propene partial pressure, on the molecular weight distribution and polymer end groups. Transfer to Al after a 2,1-misinsertion was found to be much faster then 1,2propene insertion after 2,1-insertion.161 Batch polymerizations (158) Landis, C. R.; Sillars, D. R.; Batterton, J. M. J. Am. Chem. Soc. 2004, 126, 8890. (159) Flisak, Z.; Ziegler, T. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15338. (160) Busico, V.; Cipullo, R.; Romanelli, V.; Ronca, S.; Togrou, M. J. Am. Chem. Soc. 2005, 127, 1608. (161) Quevedo-Sanchez, B.; Nimmons, J. F.; Coughlin, E. B.; Henson, M. A. Macromolecules 2006, 39, 4306.

4734


Bochmann

Scheme 49. Changes in Relative Ion Positions in Site Epimerization and Monomer Insertion Processes

of 1-hexene with (EBI)ZrMe(μ-Me)B(C6F5)3 based on “multiresponse” data have provided a quantitative kinetic model of all chemical species involved, to predict molecular weight distributions. This model assumes that the number of active species is identical with the number of Zr-alkyl complexes in the system. Nevertheless, some 43% of the catalyst was found to have undergone a “catalyst event” that has rendered them inactive.162 Unfortunately, quantification of background impurity levels as required for kinetics with all scavenger-free catalyst systems83 was apparently not carried out; therefore, the source of this partial inactivity remains a matter of conjecture.

12. Site Epimerization and Ion Aggregation Site epimerization, also referred to as ion pair symmetrization or “chain skipping”, is an important parameter of polymerization catalysts, for two main reasons: it can influence the stereoselectivity of the insertion process and, hence, polymer properties, and it can be used to probe ion-ion interactions relevant to catalysis, i.e. processes that under real catalytic conditions are not accessible to investigation. Key to the following considerations is the realization that, in the type of low-polarity solvents used in alkene polymerizations, the ionic catalysts described here exist as distinct ion pairs, and unlike ionic systems in aqueous media there is no independent diffusion of free ions. Monovalent ions in lowdielectric solvents show remarkably high association constants, and attractive interionic forces can extend to g100 A˚.28,163 In agreement with this, there is no “common ion effect”; for example, it was shown that 20 equiv of [PhCH2NEt3][B(C6F5)4] had no effect on the rate of propene polymerization with (SBI)ZrMe2/[CPh3][B(C6F5)4] catalysts.152 Both site epimerization and monomer insertion are subject to similar energetic considerations, since both require the displacement of the anion from its equilibrium position relative to the cation, which costs electrostatic energy (Scheme 49). In monomer insertion there is energy gain from (162) Novstrup, K. A.; Travia, N. E.; Medvedev, G. A.; Stanciu, C.; Switzer, J. M.; Thomson, K. T.; Delgass, W. N.; Abu-Omar, M. M.; Caruthers, J. M. J. Am. Chem. Soc. 2010, 132, 558. (163) Gordon, J. E. The Organic Chemistry of Electrolyte Solutions; Wiley: New York, 1975; p 372ff.

forming new C-C bonds, while site epimerization may be solvent-assisted and, with few exceptions, is 1-2 orders of magnitude slower than the insertion process. Thiel and coworkers have calculated the reaction trajectory of site epimerization of [H2C(Cp)2Zr-R]þ cations (R = Prn, Bui) in the absence of an anion. The process represents the interconversion of β-agostically stabilized states into their mirror images via a trigonal-planar (at the metal) transition state. The inclusion of MeB(C6F5)3- in this model changes the relative stability of R- and β-agostic states: e.g., R-agostic bonding becomes favored.164 In stereoselective polymerizations site epimerization can lead to stereoerrors and, in line with the discussion above, has been shown to be modulated by the activator system.165 In some catalysts, such as the C1-symmetric 106, site exchange is much faster than chain propagation and the polypropene microstructure is unaffected by the counteranion (Scheme 50). Such catalysts are comparatively slow, with the polymerization rate decreasing in the order B(C6F5)4- ≈ MeMAO- >MeB(C6F5)3-.166 In chiral zirconocenes, control of the relative rates of site epimerization and propagation by adjusting the monomer concentration allows tacticity control and kinetic resolution of chiral olefins.167 Propene polymerizations with other C1-symmetric catalysts such as 107 follow predominantly the general alternating insertion mechanism: i.e., each chain swinging event is accompanied by monomer insertion. Site epimerization leading to increased mmmm pentads only kicks in at high temperatures and low [C3H6], or when the steric hindrance by the ligand framework is greatly increased, as is the case for 108, which was found to favor site epimerization after each insertion step.168 These processes can be described with a statistical model for a one-directional process; a bidirectional site exchange may take place with fluorenyl-based zirconocenes only at high temperatures.169 (164) Graf, M.; Angermund, K.; Fink, G.; Thiel, W.; Jensen, V. R. J. Organomet. Chem. 2006, 691, 4367. (165) Busico, V.; Cipullo, R.; Cutillo, F.; Vacatello, M.; van Axel-Castelli, V. Macromolecules 2003, 36, 4258. (166) Mohammed, M.; Nele, M.; Al-Humydi, A.; Xin, S.; Stapleton, R. A.; Collins, S. J. Am. Chem. Soc. 2003, 125, 7930. (167) Min, E. Y. J.; Byers, J. A.; Bercaw, J. E. Organometallics 2008, 27, 2179. (168) Miller, S. A.; Bercaw, J. E. Organometallics 2006, 25, 3576. (169) Miller, S. A. J. Organomet. Chem. 2007, 692, 4788.

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4735

Scheme 50

Scheme 51

The ion pair symmetrization rates of [1,2-Me2Cp)2ZrMeþ 3 3 3 109-] have been measured.170 Monitoring the site epimerization with well-defined alkyl models, e.g. 66-69, by NMR spectroscopy shows that the chain-swinging event involves slightly different motions in C2 and Cs-symmetric ligand frameworks: i.e. 180 and 120°, respectively (Scheme 51).84-86 The equilibrium position of the anion close to the alkyl ligand was demonstrated by NOE measurements, as was the absence of direct cation-anion coordination. Insofar as this process models the dynamics and structures during the chain growth process, Scheme 49 resembles the monomer insertion process depicted in Scheme 45, with the exception that at no stage is there anion dissociation and recoordination, but merely displacement of the anion from its equilibrium position in a rather shallow energy potential in the vicinity of the cation. The implications for the monomer insertion and the chain growth step with these catalysts are that the solution structures support Fink’s “intermittent” chain growth model, with the equilibrium that is reached after each monomer insertion/ chain swinging event representing a resting state. In conventional reaction mechanism terms, the reaction sequence of anion displacement/monomer coordination/anion reassociation represents an associative interchange (Ia) mechanism.153 The rates of site epimerization with these catalyst models were found to increase with increasing ion pair concentration. This is also the case for the Cs-symmetric ISIP 67-Zr (Scheme 23).86 This differs from the behavior of the Zr-Me analogue (IPCF)ZrMe(μ-Me)B(C6F5)3, where ion pair (170) Henderson, L. D.; Piers, W. E. J. Organomet. Chem. 2007, 692, 4661.

Scheme 52

symmetrization rates were found to be independent of [Zr],171 a reflection of CH2SiMe3 as a more realistic model for the polymeryl chain, plus the fact that in 67 site epimerization is not complicated by B(C6F5)3 dissociation/reassociation. For the [(SBI)ZrCH2SiMe3þ 3 3 3 A-] ion pairs, the site epimerization rates in toluene/1,2-difluorobenzene (9:1) at 25 °C are on the order of 27-107 s-1 for A = MeB(C6F5)3 and 500-1700 s-1 for A = B(C6F5)4. Site epimerization for SBI complexes (66, 68) was about 1.5-2.5 times faster than for IPCF compounds (67, 69). Because of stronger agostic and cation-anion interactions, the rates for the Hf analogues are about 1/4-1/2 of the values for Zr. These chain swinging rates represent upper limits for site epimerization under catalytic conditions, where [Zr] is typically 100 times lower; they are thus at least 1 order of magnitude slower than the rate of chain propagation (e.g., kp[propene] ≈ 104 s-1 at [propene] = 0.59 M).84 The site epimerization of the C1-symmetric Hf complex 110 proceeds between the two different sites with slightly different rates (Scheme 52). This catalyst is of interest because of its ability to generate high-molecular-weight elastomeric polypropylene with controllable degrees of stereoerrors due to a favorable back-skip during polymerization.172 This is only possible if the rate of site epimerization is comparable to the rate of propagation, which was indeed found to be the case.143 In early studies on zirconocenium ion pairs in toluene Brintzinger had found that anion exchange rates were (171) Chen, M. C.; Roberts, J. A. S.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 4605. (172) (a) Rieger, B.; Troll, C.; Preuschen, J. Macromolecules 2002, 35, 5742. (b) Hild, S.; Cobzaru, C.; Troll, C.; Rieger, B. Macromol. Chem. Phys. 2006, 207, 665. (c) Cobzaru, C.; Rieger, B. Macromol. Symp. 2006, 236, 151.

4736


Bochmann

Scheme 53. Anion and Site Exchange in an Ion Quadruple

Figure 2. Dependence of propene polymerization activity (in 106 g of PP (mol of Zr)-1 h-1 [C3H5]-1, at 25 °C) on the CPh3þ/Zr ratio.154.

accelerated by excess LiMeB(C6F5)3 and that in such media ion quadruples are formed,173 although Marks was able to demonstrate that zwitterionic complexes (L)MMe(μ-Me)B(C6F5)3 (M = Ti, Zr) exist as mononuclear species.174 Further diffusion coefficient measurements on the ion pairs 66 and 68 showed that, while the MeB(C6F5)3 compounds are indeed mononuclear, the B(C6F5)4- complexes exist mainly as ion quadruples; in fact, within the concentration range amenable to NMR techniques the presence of simple ion pairs could not be confirmed.84,86 The existence of ion quadruples (or, at higher concentration, hextuples) would certainly explain the acceleration of site epimerization with increasing ion pair concentration. As stated before, in the low-polarity solvents employed in these studies there are no freely diffusing anions, and conventional notions such as ionic strength of the medium are not meaningful. Site exchange, like monomer insertion, involves a transition state with increased cation-anion distance, which costs electrostatic energy. In a simple ion pair there is no (173) Beck, S.; Lieber, S.; Schaper, F.; Geyer, A.; Brintzinger, H. H. J. Am. Chem. Soc. 2001, 123, 1483. (174) (a) Stahl, N.; Zuccaccia, C.; Jensen, T. R.; Marks, T. J. J. Am. Chem. Soc. 2003, 125, 5256. (b) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M. C.; Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448.

compensation. In an ion quadruple, however, the energy cost of stretching the Mþ 3 3 3 A- distance can be partially compensated by a closer approximation of A- to another cation (Scheme 53). The formation of ion aggregates may also explain the observation that the activity of some metallocene catalysts can be greatly increased by the addition of excess CPh3[B(C6F5)4], over and above the 1:1 stoichiometric ratio that is usually employed. This is a nontrivial effect; for example, the propene polymerization activity of high-activity catalysts such as 95 increases from 456 to 1187 tons of PP (mol Zr)-1 h-1 [C3H5]-1 on increasing the CPh3þ/Zr ratio from 1:1 to 10:1. For CGCTiCl2 there is an even more dramatic 10-fold increase, from 85 to 835 tons of PP (mol of Zr)-1 h-1 [C3H5]-1 (Figure 2). At the same time, the highest concentration of active species, as determined by kinetic methods, was only 34% and could therefore not account for the steep activity increase.154 Pulsed-field gradient spin-echo experiments showed that mixtures of zirconocene ion pairs [(L)ZrRþ 3 3 3 B(C6F5)4-] with excess CPh3[B(C6F5)4] also form ion quadruples. It seems plausible that such aggregates form composite anions [X- 3 3 3 CPh3þ 3 3 3 X-] which further reduce the coordinative tendency of perfluoroarylborates, with the result that under catalytic conditions such ion aggregates kinetically outperform ion pairs.175 These studies also showed interesting differences in the dynamic behavior of apparently very similar catalyst types. For example, whereas for the SBI catalyst 68 the site epimerization increases linearly with [Zr], the IPCF complex 69 reaches a plateau at kex = 400 s-1 (Figure 3). Apparently at that rate a barrier is encountered which prevents faster ligand mobility.86,175 Interestingly, 69 is also one of the catalysts that do not show a “trityl effect”: i.e., no activity increase with increasing CPh3þ/Zr ratios.154 Any barrier to alkyl ligand mobility would obviously limit the rate of polymer chain growth. This seems to be the case here; the concentration dependence of site epimerization acts therefore as a very sensitive tool for probing such barriers. The principle of changing the course of a polymerization reaction through ion aggregation has been exploited by designing tethered binuclear catalysts 111-113 in combination with dianions such as 114 (Scheme 54).176,177 Due to their open structures and excellent ability to incorporate (175) Alonso-Moreno, C.; Lancaster, S. J.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. J. Am. Chem. Soc. 2007, 129, 9282. (176) Li, H.; Marks, T. J. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15295. (177) (a) Li, L.; Metz, M. V.; Li, H.; Chen, M.-C.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2002, 124, 12725. (b) Abramo, G. P.; Li, L.; Marks, T. J. J. Am. Chem. Soc. 2002, 124, 13966. (c) Li, H.; Li, L.; Marks, T. J.; Liable-Sands, L.; Rheingold, A. L. J. Am. Chem. Soc. 2003, 125, 10788. (d) Li, H.; Li, L.; Marks, T. J. Angew. Chem., Int. Ed. 2004, 43, 4937. (e) Wang, J.; Li, H.; Guo, N.; Li, L.; Stern, C. L.; Marks, T. J. Organometallics 2004, 23, 5112.

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4737

Figure 3. Site epimerization rate constants kex as a function of the ion pair concentration at 20 °C: (9) [(SBI)ZrCH2SiMe3þ 3 3 3 B(C6F5)4-]; (2) [(IPCF)ZrCH2SiMe3þ 3 3 3 B(C6F5)4-].154. Scheme 54

R-olefin comonomers, complexes of the constrained-geometry type have proved to be particularly useful for the study of nuclearity effects. In ethylene polymerizations the polymers show increased branch content, such as an order of magnitude increase in ethyl side chains. Since these polymerizations involve β-H elimination, chain walking, and chain termination followed by reinsertion, the catalyst productivities are lower than those of mononuclear constrainedgeometry catalysts and of comparable ansa-zirconocenes. The combination of binuclear catalysts with dianions is much more effective than using monoanions such as B(C6F5)4-. On the other hand, the binuclear phenoxy-imine complex 113 activated with MAO gives higher activities than its mononuclear counterpart.178 It is proposed that one metal center stabilizes a terminated polymer chain via agostic interactions, while chain growth takes place at the second metal (Scheme 55). Such a scenario looks realistic according (178) (a) Salata, M. R.; Marks, T. J. J. Am. Chem. Soc. 2008, 130, 12. (b) Salata, M. R.; Marks, T. J. Macromolecules 2009, 42, 1920. (179) Motto, A.; Fragala, I. L.; Marks, T. J. J. Am. Chem. Soc. 2009, 131, 3974.

to DFT modeling (using a methylborate dianion).179 The tethered binuclear complex 115 catalyzes the living polymerization of propene. As the length of the tether decreases (n = 8 > 6 > 4) the mr stereoerror content decreases as site epimerization becomes more hindered by the other metal center.180

13. Ligand Transfer and Chain Shuttling Reactions Chain transfer to a main-group metal, via the mechanism discussed above, is an important means of influencing the polymer molecular weight and the nature of the end group and can be used to produce aluminum- and zinc-terminated polymers. As mentioned earlier, addition of AlR3 or ZnR2 (R = Me, Et) to ethene/norbornene copolymerizations with ansa-zirconocenes lowers molecular weights and, after hydrolysis of the Al or Zn polymeryls thus produced, leads to saturated polymer chain ends.126 Propene polymerization with rac-Me2Si(2-MeInd-4-naphthyl)ZrCl2 activated with MAO containing 13 mol % of AlMe3 could be adjusted to (180) Zhang, W.; Sita, L. R. Adv. Synth. Catal. 2008, 350, 439.

4738


Bochmann Scheme 55

Scheme 56

give essentially quantitative aluminum polymeryl products, Al(iPP)3.181 In situ trapping of ethylene oligomers with carbodiimides and mass spectrometric analysis have suggested that in Cp2ZrCl2/MAO systems the rate of transfer to Al exceeds the rate of propagation.182 Chain transfer from living bis(phenoxy-imine)titanium catalysts to diethylzinc gives multiple production of nearly monodisperse zinc-terminated polyethylene chains.183 kex . kp: Polymerizations under Rapid Alkyl Transfer Conditions. Very fast reversible transfer of polymeryl chains to and from a main-group metal constitutes a transitionmetal-catalyzed “Aufbaureaktion” and amounts to a living chain growth process on the main-group metal (Al or Zn). This reaction principle has become known as coordinative chain transfer polymerization, CCTP (Scheme 56). In these cases the main-group polymeryl species act as catalyst resting states. This process gives well-controlled polymerizations and polymers with very narrow molecular weight distribution (Mw/Mn < 1.1) without end-group unsaturations.184 As described in section 3, the sterically congested Cp amidinato complexes 26 catalyze the living polymerization of 1-alkenes. Propagation is comparatively slow. These catalysts are unusual in that the cationic zirconium alkyl species are conformationally rigid, whereas the corresponding neutral zirconium dialkyls are flexible (Scheme 57). The process involves methyl, not polymeryl, exchange.185 Since the catalyst configuration changes more rapidly than the polymer grows, the resulting polymer is atactic. On the other hand, if Cp*(NN)ZrMeCl is used instead of Cp*(NN)ZrMe2, both the neutral and the cationic species are rigid, and the resulting (181) Fan, G.; Dong, J. Y. J. Mol. Catal. A: Chem. 2005, 236, 246. (182) Quintanilla, E.; de Lena, F.; Chen, P. Chem. Commun. 2006, 4309. (183) Mitani, M.; Mohri, J.; Furuyama, R.; Ishii, S.; Fujita, T. Chem. Lett. 2003, 32, 238. (184) Kempe, R. Chem. Eur. J. 2007, 13, 2764. (185) Sita, L. R. Angew. Chem., Int. Ed. 2009, 48, 2464. (186) Zhang, Y.; Sita, L. R. J. Am. Chem. Soc. 2004, 126, 7776.

polyhexene is isotactic.186 By adjusting the amount of neutral versus cationic species, the microstructure of the resulting polymers can be controlled by means of ligand exchange equilibria.187 Complex 26 activated with AlMe3-depleted MAO gives iso-rich poly(1-alkene)s by enantiomorphic site control; the stereoselectivity increases in the series propene < 1-butene < 1-hexene.188 The combination of the active cation 116 with a reagent that is inactive even in its cationic state, 117, allows the synthesis of atactic-isotactic multiblock polypropylenes (Scheme 58).189 Mixtures of 118, [HNMe2Ph][B(C6F5)4], and Et2Zn overcome the “one chain per metal” limitation of living polymerization and produce polymers of tunable molecular weight with very narrow polydispersities.190 Since transfer with Al is much slower than with Zn, the addition of zinc dialkyls facilitates the generation of well-controlled Al(Pn)3 species in a process that is characterized by kCT(Zn,Hf), kCT(Zn,Al) . kCT(Al,Hf) > kp. Transfer also takes place with AlR3 alone, but the polydispersities are broader.191 kp . kex: Polymerizations under Slow Alkyl Transfer Conditions. One area where alkyl ligand exchange between the active species and a main-group metal has been exploited with particular impact is the preparation of block copolymers via chain shuttling between two different catalysts. This is the basis of Dow’s INFUSE olefin block copolymer technology. The principle resembles the CCTP mechanism, except that in this case the polymer chains are exchanged between two different catalysts and chain transfer is slower than chain propagation (Scheme 59).24 If catalyst 1 produces “soft” copolymers and catalyst 2 produces “hard” linear polyethylene, and if chain transfer is on the order of 102 times slower than propagation, a polymer with hard-soft-hard multiblock structure will result. In the absence of a chain shuttling agent, the two catalysts will produce a reactor blend. (187) Harney, M. B.; Zhang, Y.; Sita, L. R. Angew. Chem., Int. Ed. 2006, 45, 6140. (188) Busico, V.; Carbonniere, P.; Cipullo, R.; Pellecchia, R.; Severn, J. R.; Talarico, G. Macromol. Rapid Commun. 2007, 28, 1128. (189) Harney, M. B.; Zhang, Y.; Sita, L. R. Angew. Chem., Int. Ed. 2006, 45, 2400. (190) (a) Zhang, W.; Sita, L. R. J. Am. Chem. Soc. 2008, 130, 442. (b) Zhang, W.; Wei, J.; Sita, L. R. Macromolecules 2008, 41, 7829. (191) Wei, J.; Zhang, W.; Sita, L. R. Angew. Chem., Int. Ed. 2010, 49, 1768.

Review


4739

Scheme 57

Scheme 58

Scheme 59

The catalysts employed in this process are the Hf complex 119, which produces ethene/1-octene copolymers, and 120 (Scheme 60), which is a poor incorporator and gives linear polyethylene. High-throughput techniques were used to optimize the catalysts with respect to their kinetic parameters. ZnEt2 proved to be the ideal chain shuttling agent. The process involves the matching of at least six rate constants: the propagation rates of catalysts 1 and 2, the chain transfer rates from catalysts 1 and 2 to Zn, and the transfer rates from Zn to catalysts 1 and 2. If hydrogen is used for molecular weight control, the hydrogen response for each

catalyst needs to be added to the list of parameters. A kinetic model of Mn and Mw/Mn as a function of chain transfer rates and monomer consumption has been developed.192 The process can be operated in two reactors. Reactor 1 carries out an ethylene homopolymerization and contains Cat-HDPE and CSA-HDPE; this is then transferred to reactor 2 for copolymerizations, to give LLDPE-HDPE diblocks (HDPE = high-density polyethylene; LLDPE = linear (192) Hustad, P. D.; Kuhlman, R. L.; Carnahan, E. M.; Wenzel, T. T.; Arriola, D. J. Macromolecules 2008, 41, 4081.

4740


low-density PE).193 Using 119/[HNMe2Ph][B(C6F5)4] in combination with Al(octyl)3 as chain transfer agent gives ethene/ 1-hexene copolymers with polydispersities Mw/Mn < 2 which decrease linearly with [Al]. However, only half of the available Al sites are occupied by a polymeryl chain, indicating that in this system chain shuttling (polymeryl-polymeryl exchange) is competitive with chain transfer (polymeryl-octyl exchange). The chain transfer to Al was the dominant termination mechanism. Less hindered chains transfer fastest: i.e., those following the insertion of ethene rather than hexene.194

Conclusion Over the last 9-10 years the understanding of the mechanistic and kinetic details of soluble polymerization catalysts (193) Hustad, P. D.; Kuhlman, R. L.; Arriola, D. J.; Carnahan, E. M.; Wenzel, T. T. Macromolecules 2007, 40, 7061. (194) Kuhlman, R. L.; Wenzel, T. T. Macromolecules 2008, 41, 4090.

Bochmann

has progressed from fundamentals to remarkable applications in new polymer architectures. In particular, an understanding of activation processes and the structures of the active species and resting states has enabled polymers with precisely determined properties to be made that were previously inaccessible. The input of high-throughput technology has been extended from catalyst discovery to the matching of complex sets of rate constants, with the effect that new block copolymers with unique properties are now being commercialized. At the heart of these developments is the intriguing interplay between catalytically active transition metals and main-group-metal alkyls which act as protectors, as chain terminators, and as transfer agents, a particularly vivid example of the importance of fundamental organometallic concepts as the basis for new industrial processes and materials.

Acknowledgment. I gratefully acknowledge the contributions and helpful discussions of many colleagues: Drs. S. J. Lancaster, C. Alonso-Moreno, L. Broomfield, K. P. Bryliakov, S. J. Coles, M. D. Hannant, D. L. Hughes, D. A. Pennington, A. Rodriguez, M. Schormann, Y. Sarazin, F. Song, D. A. Walker, P. Wilson, and C. Zuccaccia and Profs. H. H. Brintzinger, V. Busico, R. D. Cannon, L. Cavallo, G. Fink, A. Macchioni, B. Rieger, and E. P. Talsi. This work was supported by the Engineering and Physical Sciences Research Council, the European Commission, the Royal Society, and BP Chemicals Ltd.

The Chemistry of Catalyst Activation: The Case of Group 4

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