Symposium on Catalysis and Organometallic Chemistry
Introduction This collection of articles was taken from presentations at a State-of-he-Art Svmuosium on "Chemistry at the lnterface: Catalysis a n d - ~ r ~ a n o m e t a l l ichemistry" c that was sponsored by the ACS Division of ChemicalEducation a t the 194th National ACS Meeting (New Orleans), August 31, 1987. As background, readers should note the excellent erouo of articles assembled bv C. P. Casev and numerous contributing authors from a similar symposium on "Industrial A~olicntionsof Oreanometallic rhemistrv and Caralssis" [J:?hem. Educ. 19k,63,188-2251. The uroceedines in this issue (uage 000-000) feature chemisrry and ca'ialytic reactions tliaiare very much in a development stage. The article by J o r d a n (Washington State Uni\,ersity, page 000) deals with cationic zirconium olefin polymerization catalysts. Olefin polymeri7ation is of course widely practiced in industry, hut Jordan's new catalysts are exceptionally well characterized and provide much new mechanistic insieht. The studies bv Rrown and Crsbtree (Yale University, page 000) present a facinating marriage of an "old" physical process, mercury photosensitization, with a modern chemical problem, alkane activation. Thenovel carbon-carbon bond formina reactions discovered are beginning to attract industrial igterest. The research presented by Trogler (University of California, San Diego,
.. ~~.
~
~
-
page 000) shows how platinum metals can catalyze a variety of transformations, including the hydration and amination of unsaturated organic linkages. Contrast his data with the corresponding "volcanic" industrial hydration reactions frequently covered in sophomore organic chemistry. Also, I would like to thank my co-worker, Yo-HsinHuang (University of Utah), for helping to prepare an article (page 000) on the interface between aldehyde and ketone complexes and catalysis. This work points the way towards the development of new organometallic reactions for use in organic synthesis. We hope that this series of articles will he useful to educators, researchers, and undergraduates considering graduate research. Organometallic chemistry is a young, exciting, and dynamic area for study and is infrequently accompanied by undue formality-attendees a t the symposium quickly came to know the speakers on a first-name basis. Additional hackground material is available in the recent text, Principles and Applications of Organotransition Metal Chemistry [Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. University Science Books: Mill Valley, CA, 19871. J. A. Gladysz University of Utah
Salt Lake City, UT 841 12
Cationic Metal-Alkyl Olefin Polymerization Catalysts Richard F. Jordan Washington State University, Pullman. WA 99164 Transition metal catalysts are used extensively in the chemical industrv for the ~olvmerizationof ethvlene. oro' pblyolefins ( I ) . ~ w d o thLimf pylene, and othe; olefins & portant classes of catalysts are Ziegler-Natta catalysts, such as TiCldAIEt3, TiC13IAlEt3,etc., and Phillips catalysts, such as CrOdSi02 (2). Olefin polymerization by these heterogeneous catalysts is believed toinvolve a number of import& reactions (Fig. 1) including (1)generation of an active metal-alkyl site, (2) repetitive insertion of olefin into the metaalkyl bond (propagation), and (3) chain termination/transfer hv B-H elimination or bv bvdroeenolvsis with added H7. direct detection and stud; of t i e active metal sites in these catalvst svstems is hindered hv the inherent complexities of heterogeneous (colloidal or surface) systems. For manv years oraanometallic chemists have used soluble Ti and Zr Gmp~exeHas model systems for the study of the key reactions in catalvtic olefin polymerization. The most intensely studied systems are bk(iyclopentadieny1) complexes CpzMC12 (Cp- = q5-CsH5-, M = Ti, Zr), which in the nresence of A1 alkvl cocatalvsts nolvmerize ethvlene (eo 1) (3-5). Recent versions of this catalyst system incorporating chiral Zr complexes and alumoxane cocatalysts polymerize
propylene to highly isotactic polypropylene with high activity (eq 2) (6).
ow ever,
u
A
.
Figure 1. Key steps in catalytic ethylene polymerization. The metal-alkyl centers +M-R generate polymer chains by repetitive ethylene Insertion. The metal-hydrides 'M-H can start new chains by reaction with ethylene. P = polymer chain = -(CH2CH2)lr,-R.
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Anattractivemechanism (Fig. 2) forthese CpzMClz-based catalyst systems involves cationic do alkyl complexes CpzM(R)+ as key intermediates (7). It is postulated that such species form by alkylation of Cp2MClz by the Al alkyl cocatalyst, followed by halide abstraction. The 14-electron Cp,M(R)+ ion is highly coordinatively and electronically unsaturated and as a result can coordinate and insert olefins into the M-R hond. Indirect support for this basicmechanistic picture is provided by a number of results including (1) the recent isolation of a cationic Ti alkenyl complex from a Ti/Al system (eq 3) (8)and (2) the observation that neutral lanthanide metal complexes such as (C5Me5)2Lu(CH3), which are isoelectronic with CpzM(R)+ (neglecting f electrons), react with ethylene and propylene by insertion processes (9).
Figure 3. Structure of the Cp2Zr(CH&THF)+ cation. Hydrogen atoms have been omitted for clarity. The B P K structure is normal. Bond distances (angstroms): Zr-C(35) 2.258(10); Zr-0 2.122(14).
Several years ago we initiated a program to isolate and characterize Cp,Zr(R)+ complexes in the absence of Al cocatalysts and to investigate their role in olefin polymerization directly. This article reviews our recent progress and current work in these areas. Synthesis and Structure of Cp2Zr(R)(L)+ Complexes
However, direct detection of CpzM(R)+ ions in active catalyst systems is precluded by (1) their high reactivity, (2) ligand exchange, reduction of the metal center, and other side reactions that can generate a mixture of compounds in solution, and, in some cases, (3) the required large excess of A1 alkyl (often >loo-fold), which interferes with spectroscopic studies.
1
alkylation
Figure 2. Formation of Cp2M(R)+ in a soluble Ziegier-Natta system.
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Wediscovered that cationicCp,Zr(R)(L)'romplexes (L = CHXN. - .THF.etc., are formed in high vield bv the reaction of neutral dialkyl 'complexes ~ p , ~ with i ~Ag[BPhr] j ~ in CH3CN (eq 4), or with [CpnFe][BPha]in a variety of solvents (eq 5). T o date, R- = CH3-, Ph-, PhCHz- (10-12).
These reactions are believed to proceed by one-electron oxidation of do CpzZr(R)z, which occurs with loss of R' radical and formation of Cp2Zr(R)+.This rather unusual type of oxidation is facile because the alkyl ligands in C ~ , Z I ( R ) ~ complexes are distinctly carbanionic in character. We have found t h a t BPha- i s t h e counterion of choice for Cp,Zr(R)(L)+ complexes. Other anions such as PFs-, BF4-, A1C14-, CF3S03-, etc., coordinate strongly or react with Cp*Zr(R)(L)+ cations (13). In contrast, the bulky BPhacounterion does not react with or form strong, site-specific ion pairs in these systems. T h e X - r a y d i f f r a c t i o n s t r u c t u r e (14) of t h e CpzZr(CH,)(THF)+ cation is shown in Figures 3 and 4. Note that the T H F ligand is nearly perpendicular to the plane between the two Cp- ligands. This result, along with bond length data, strongly suggests that there is a significant a component to the Zr-0 bond. In this orientation, the bl (p) orbital on the T H F oxygen can overlap with the LUMO on the 16 electron Zr center, as shown in Figure 5 (15). Chemistry of Cp2Zr(CH3)(THF)+( I ) : The Prototypical Cationic Alkyl Use of the BPh4- counterion bas allowed us to study the reactions of CpzZr(R)(L)+ complexes with a variety of ligands and substrates. In general these complexes are highly
Olefln Polymerlzatlon
We have demonstrated that a variety of CpzZr(R)(THF)+ complexes (R- = H-, CH3-(1). PhCH2-, Ph-, etc.) polymerize ethylene (in CHzCl2and other solvents) in the absence of Al cocatalysts or oxide supports (14). High-molecularweight, linear polyethylene is produced by these catalysts, and polymerization activity is high in nonpolar solvents. These exciting results provide strong support for the proposa l t h a t cationic, metal-alkyls are active species in (CsHs)zMClz/A1R,C13-n and related olefin polymerization catalyst systems. Moreover, because the CpzZr(R)+catalysts are simple and soluble, their chemistry can be studied in detail by NMR spectroscopy. We have shown that ethylenepolymerization by 1 is inhibited by THF, and that 1 does not coordinate a second T H F ligand. These results suggest that the active species is the "naked alkyl" Cp2Zr(CHs)+ formed by T H F dissociation from 1 (eqs 6 and 7). This species can be generated in the absence of T H F (by reaction of CpzZr(CH& with CpzFe+ in CH2C12)and is a highly active polymerization catalyst, but it is too reactive to isolate. Detailed spectroscopic and kinetic studies of the insertion reactions of Cp2Zr(R)(THF)+complexes with olefins and acetylenes are in progress. Snunm of h Cp2Zr(CH,HTHF)+ catoon viewea down the 0 - Z r oom. me 9 , CarDons of tne TrlF ligano have been removed for clarity. F gum 4.
CHzCIZ
C~,Z~(CH,)(THF)+ * Cp,Zr(CH,)+ + THF 1
-
Cp2Zr(CH3)+ + H2C=CH2 polyethylene excess
(6) (7)
Observation of a "Naked Metal-Alkyl Complex
By taking advantage of the multidentate bonding capability of the PhCH2- ligand, we have succeeded in observing one of the key "naked" metal-alkyl cations, Cp2Zr(q2CHZPh)+ (2) (11).
Figure 5.Zr-0 r banding in Cp*Zr(CH3XTHF)+.
reactive. Some reactions of the simplest member of this series, CpzZr(CHs)(THF)+ (I), which illustrate the key chemical properties of cationic doalkyls are shown in Figure 6. Several conclusions are apparent from Figure 6. (1) TheTHFligand of Cp2Zr(CH3)(THF)+(1) islahile and is easily
replaced by other ligands and substrate molecules including ethers, pyridines, phosphines,nitriles, and ketones (all of which are also lahile). (2) The CpzZr(CH3)+ion is a strong and hard Lewis acid as indicated by its ahility to abstract F- from PFs- and BFI- and its tendency to form 5-coordinate complexes with small ligands (CHBCN,PMe3, etc.). (3) The Zr-C bond of 1undergoes facile insertion of polar unsaturated substrates such as CO. RgCO. and RCN. (4) Complex 1polymerizeseth;leiein the absence of an A1 coeotolyst or oxide upp port. The high reactivity of 1 results from high lability of the THF ligand and the enhanced Lewis acidity of the cationic metal center. This combination promotes coordination and actiuation of substrates for nucleophilic attack by the carbanionic R- group (i.e., insertion), and opens up reaction pathways which are not auailable to related neutral alkyl complexes. These properties are the key to the olefin polymerization activity of cationic, do metal-alkyl centers.
Figure 6. Representative reactions of Cp2Zr(CH3XTHF)+ (I).
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Figure 7. Chemistry of Cp2Zr(CH2PhXL)' complexes Figure 9. Ethylene polymerization by the "naked" benzyl complex (C6H,Me),Zr(q2~H,Ph)+. The presumed intermediates in brackets are not observed by low T NMR indicating that propagation is faster than initiation.
CIlBI
Cllll Figure 8. Struchrre of the Cp2Zrh2-CH9PhXCH&N)+ cation. Hydrogen atoms have been omitled for clarity. The BPhli structure is normal. Bond lengths (angstroms) and angler (deg): Zr-C(0) 2.34418); Zr-C(1) 2.64816); Zr-C(0)C(1) 84.9.
We first prepared [CpzZr(q2-CHzPh)(CHaCN)][BPh4] (3) by reaction of CpzZr(CHzPh)z with AgfBPha] (Fig. 7). The X-ray structure of 3 (Fig. 8) reveals the presence of a weak Zr-ring bonding interaction, that is, an q2 or 7" Zr-henzyl bond (16). We hypothesized that replacement of CH3CN with a more labile ligand would promote ligand dissociation to produce a "naked benzyl" species which would be stahilized bv the weak Zr-rine interaction. w e . p r e p a r e d t h k analogous T H F complex Cmi!r(CH,l'h)('rHF)' 1 hv the reaction of CpzZr(CHrPh)> and ~ & ~ in e fTHF. complex 4 has heen ch&acterized by 'H NMR and several derivatization reactions as shown in Figure 7. As a result of increased steric crowding, complex 4 undergoes significant dissociation of T H F in CHzClz solvent to produce the "naked henzyl" complex CpzZr(q2-CHzPh)+ 2, which can be directly observed by low T 'H NMR spectroscopy. The NMR results establish that 2 is indeed stabilized by q2 bonding similar to that of 3. The naked benzyl complex 2, and the more soluble analogue (CsH4Me)zZr(q2-CH2Ph)+, which can he generated in the absence of THF, polymerize ethylene under very mild conditions in CH2C12 solvent. When these reactions are monitored by low T 'H NMR, the rapid formation of polyethyl288
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Figure 10. Reactions of Cp2Zr(CH&L)+ complexes with HI.
ene can be observed, but no intermediates are detected. Thus the initial insertion (initiation) is slow, probably because the 72-CH2Ph ligand must rearrange to an q1 bonding mode prior to ethylene insertion, while subsequent insertions (propagation) are fast (Fig. 9)(17). Molecular-Weight Control in Olefln Polymerizatlons: Reactions of CpnZr(CH3)(L),,+ Complexes with H2
Addition of Hz to olefin polymerization systems is an important means of controlling polymer molecular-weight distributions (1). The Hz cleaves the bond between the metal and the growing polymer chain to give a polymer with a saturated end group and a metal hydride species (Fig. 1). The metal hydride can initiate a new chain by reaction with olefin (eq 8).
new polymer chain (8) We have s t u d i e d t h e r e a c t i o n s of a s e r i e s of CpzZr(CH3)(L)+complexes with Hz in order to determine
which factors influence the rate of this reaction for catalytically active metal-alkyl complexes (18).Our results are summarized in Figure 10. Five-coordinate, 18-electron complexes from which ligand d i s s o c i a t i o n i s i n h i b i t e d by m a s s a c t i o n (e.g., Cp2Zr(CH3)(CH3CN)2+in CH3CN) or precluded by chelation (e.g., CpzZr(CH3)(dmpe)t)do not react with HZunder mild conditions. This establishes that a low-lying, empty, metal-centered orbital (LUMO) is required for H Zreactivity. The 4-coordinate complex CpzZr(CH8)(THF)+ 1 does react with Hz under mild conditions to produce a cationic hydride complex Cp2Zr(H)(THF)+and CHa. This reaction is about four times faster in CHCI? than in THF. sueeestine t h a t t h e "naked" 3 - c o o r i i n a t e m e t h y i c&plex C D ~ Z ~ ( C Hwhich ~ + , exists in much ereater concentration in C H ~ C than I ~ in T H F and has two low-lying empty metalcentered orbitals, is more reactive with Hz than is 1. The donor characteristics of the ancillarv lieand L exert a profound influence on the hydrogenolysis~ate~ Thereaction of Hz with Cp2Zr(CH3)(PMe3)+(5) is -lo5 times faster than the reaction with Cp2Zr(CH3)(THF)+.This spectacular acceleration may be rationalized by noting that the o, *-donor T H F ligand in 1 ties up the Zr-based LUMO required for reaction with H Z(see Fig. 51,while the a-donor PMes ligand in 5 leaves this orbital unperturbed. We thus conclude that coordinarive and elecrronic unsaturation promote the reaction of cationic do complexes with Hz. The Hz must be activated by initial interaction with an empty metal-based orbital for R-H elimination to take place. This is consistent with results for neutral CpzZr(R)X complexes (19). Chaln Termlnatlon and Transfer
As illustrated in Figure 1,p-H elimination to give a polymer that contains an unsaturated end group and a catalvtically arrive mctal-hydride species (see eq.8) is also a key chain terminationltransfer process in ulefin polymerization. Analysis of t h e p o l y e t h y l e n e p r o d u c e d b y t h e Cp,Zr(CH3KTHF)+ catalyst (1 atm CH2=CH2, 25 "C, CHCI. solvent) reveals that -25% of the end eroum are uns&iated and 75% are saturated. This is conskeni with 50%chain termination bv" .B-Helimination and 50%termination by a protonolysis process. Our current working hypothesis is that the primary protonolysis pathway involves C1abstraction from the CHzC12 solvent (eq 9). This formally generates a proton equivalent (CH2Clf) and ultimately results in cleavage of the polymer chain from another Zr-R center (eq 10). Cp2Zr(R)++ CH,C12 active "Ht"
-
where R = H, CH3, CH2CH3, CH2CH2Phetc. Initial indications are that the ethyl complex undergoes 0H elimination much more slowly than the other members of thisseries. One explanation for this observation is that the 8H is transferred to the metal with partial negative charge (i.e., as H-). The resulting partial positive change on the 0 carbon in the transition state is stabilized by alkyl or phenyl suhstituents on that carbon (21). Summary
We have developed methods for t h e synthesis of CpzZr(R)(THF)+ complexes, and we have demonstrated that these compounds polymerize ethylene in the absence of A1 cocatalysts or oxide supports under very mild conditions. These results provide strong support for the theory that cationic, do metal-alkyl centers are active species in soluble Ziegler-Natta catalyst systems (22). The reactivity studies described here are providing a coherent molecular-level understanding of active species generation and structure, polymer chain-growth mechanisms, chain terrninationltransfer reactions, catalyst deactivation processes, and ligand effects for the Cp2Zr(R)(THF)+catalysts. We are also investigating the olefin polymerization chemistry of these catalysts under more vigorous conditions, as well as the properties of the polymer products. Our long-term goal is to combine the results of these parallel programs to develop a molecularlevel understanding of the relationships between catalyst structure and polymer properties. Acknowledgment
This work was conducted in collaboration with the graduate students and postdoctoral fellows named in the references and was supported by the Research Carp., the donors of the Petroleum Research Fund, the National Science Foundation, and the Washington State University Research and Arts Committee. Literature Clted 1. Boar. J. ZieelerNoffo C a t d v r h and Polymzrktions. Academic: New York. 1979.
s. 1.57.
1960,81.1953.
+
Cp,Zr(R)CL "CH2C1+" inactive
+ Cp&(R)+
with substituted olefins (eq 12), and we are investigating their 8-H elimination rates (20).
-
RH
12,3265. Id, M. L.: Grubbs, R. H. J. Am. Cham.
(9)
(10)
We have observed this type of C1- ahstraction reaction for simple cationic alkyl complexes in the absence of ethylene (eq 11). We are currently in the process of identifying the organic products of this reaction in order to determine the mechanism.
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6. Kerninsky. ~ . ; ~ i i e r , ~ . ; ~ r i n t z i n gH.: e rWi1d.P.R. ,~. W. P. Angow. Chem.Int.Ed. Eng. 1985.24.547. I. Dmchkovskii, F.S.;Shilova.A.K.:Shilov,A.E. J,Polym. Sci.,Parf C L961.16, 2333. 8. Eineh, J. J.:Piotmvaki,A. M.;Bmpnstsin.S.K.;Cebe,E.J.;Ls,F.L.J . A m . Chem.
1987,109,4111. 12. Jordan, R. F.: EEhols, S. P. Inorg. Chrm. 1987,26,381. 13. For example, see Jordan,R. F. J . Orgonomet, Chem. 1985,295,321.
14. Jordan,R.F.;Bajg~ajgr, C.S.: Willett,R.;Seott,B. J Am. Chem.Sor. 1986,108 7410.
In model studies we are beginning to probe structure/ reactivity relationships for 8-H elimination reactions. We have prepared a homologous series of cationic alkyl complexes (C5HaMe)zZr(R)(THF)+ (R = E t , n-Pr, n-Bu, CHzCHzPh) by the reaction of (C5H4Me)zZr(H)(THF)+
15. Lauher, J. W.:Hoffmann.R,J.Am. Chem.Soc. 1976.98,1729. I 6 h t s s k y , S. L.; McMullen, A. K.:Niecolai, G.P.: Rothwell, I. P. O?gonomelolliea 1985, 4,902 and references therein. 17. Jordan, R. F.: LsPointe. R. E., unpublished rpeults. 18. J0rdan.R. F.; Bajgur, C. S.:Dssher,W. E.;Rheingold,A.L. O~gonomefollies1987.6,
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