Homogeneous Chromium Catalysts for Olefin Polymerization

41

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Homogeneous Chromium Catalysts for Olefin Polymerization K.H.Theopold, R. A. Heintz, S. K. Noh, and B. J. Thomas Department of Chemistry and Biochemistry and Center for Catalytic Science and Technology, University of Delaware, Newark, DE 19716

Chromium-based heterogeneous catalysts are widely used for the polymerization of ethylene. We prepared a class of paramagnetic chromium(III) alkyls, which serve as models for the active sites of commercially used catalysts (Phillips catalyst, Union Carbide catalyst). A wide variety of cationic, neutral, or anionic complexes containing the pentamethylcyclopentadienyl (Me Cp,Cp*) ligand and one, two, or three alkyl groups were prepared and characterized structurally and by magnetic measurements. Cationic Cp*Cr(THF) CH ] BPh (THF is tetrahydrofuran) and neutral Cp*Cr[CH Si(CH ) ] were found to polymerize ethylene at ambient pressure and room temperature or below (-40 °C). The polyethylene exhibited relatively low molecular weights and narrow dispersities. A comparison was made between the reaction of [Cp*(dmpe)Cr CH ] PF and Cp*(dmpe)Cr CH with ethylene (dmpe is 1,2-bis(dimethylphosphino)ethane). The chromium(III) alkyl gave polyethylene, but the chromium(II) alkyl yielded mostly propene. This contrast suggested that +III is the active oxidation state of chromium-based catalysts. Finally, Cp*(py)Cr(CH )(O-t-Bu) catalyzes thering-openingmetathesis polymerization (ROMP) of norbornene. This activity indicates formation of a chromium-methylene complex. 5

2

3

2

+

3

III

4

3

2

3

+

6

II

3

3

T H E C O O R D I N A T I O N P O L Y M E R I Z A T I O N O F S M A L L O L E F I N S (such as ethylene

and propene) is arguably the most important industrial process involving organometallic intermediates (1-3). Despite a large research effort and much practical progress since the original discoveries by Ziegler (4) and Natta (5), 0065-2393/92/0230-0591$06.00/0 © 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

592

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

a detailed understanding of the reaction mechanisms and the factors that determine activity and selectivity of the polymerization catalysts remains elusive. Among the transition metals that catalyze the polymerization of olefins, chromium occupies a prominent position. Broadly speaking, two classes of chromium-based heterogeneous catalysts are used commercially. The socalled Phillips catalyst (6) is prepared by depositing C r 0 on silica, followed by activation with hydrogen. O n the other hand, Union Carbide developed catalysts formed by treatment of silica with low-valent organometallic compounds, most notably chromocene ( C p C r , C p is cyclopentadienyl) (7, 8). Questions about the chemical nature of the active site(s), the oxidation state of the active chromium, and the mechanism of initiation have been the subject of a long-standing debate, which continues to this day.


3

2

The study of organometallic compounds of chromium in solution can make a valuable contribution to the understanding and rational modification of these heterogeneous catalysts. However, much of the known organometallic chemistry of chromium concerns low-valent carbonyl derivatives and diamagnetic complexes with 18-electron configurations. Such molecules are unlikely candidates for modeling highly reactive (coordinatively unsaturated) and oxide-supported chromium alkyls. Open-shell molecules (paramagnetic organometallic compounds or "metallaradicals") may be more reactive and thus more appropriate models for catalytic intermediates. With this possibility in mind, we are exploring the reactivity of a class of paramagnetic chromium(III) alkyls. Herein we summarize our recent results in the synthesis and characterization of paramagnetic chromium alkyls and their reactions with olefins.

Synthesis Chart I shows a juxtaposition of the presumed active site of the Union Carbide catalysts ( C p C r - S i 0 ) with various molecules prepared in our laboratory (9-13). The active site has been described as "an adsorbed, divalent chromium species which is still bonded to one cyclopentadienyl ligand" (7). We will ignore for now the contentious question of oxidation state. The wellcharacterized molecules shown in Chart I are apparently good homogeneous structural models of the catalyst. They have in common the cyclopentadienyl ligand, an alkyl group, and a variety of ancillary ligands that substitute for the attachment to the silica support. All the compounds shown contain chromium in the formal + III oxidation state and are thus paramagnetic. 2

2

Crystal structure determinations and magnetic susceptibility measurements were used extensively to characterize this class of organometallic compounds. For example, Figure 1 shows the crystal structure and the magnetic behavior of [ C p C r ( C H ) ^ - C l ) ] (9). The structural studies generally feature pseudooctahedral coordination of chromium centers. The metal 3

2



41.

TIIEOPOLD ET AL.

Homogeneous Chromium Catalysts

(e.g. R = C H S i ( C H ) ) 2

3

593

(e.g. R = Me)

3

Chart I. Presumed active site of Union Carbide catalysts juxtaposed with molecules prepared in our laboratory.

usually exhibits a 15-electron configuration, but in some cases 13-electron compounds have been isolated or observed. Mononuclear complexes invar iably have temperature-independent effective magnetic moments consistent with three unpaired electrons (d , μ > 3.8 μ ; is the effective magnetic moment and μ is the Bohr magneton). However, in polynuclear complexes with bridging ligands, the chromium centers exhibit antiferromagnetic cou pling and metal-metal bonding. Chromium-chromium pairs with cooperative interactions between neighboring metal atoms have been invoked to explain certain features of the Phillips catalyst. Related molecular complexes may serve as precedent for postulated intermediates and allow testing of their catalytic activity. Scheme I shows a series of conversions of extremely electron-deficient di nuclear chromium complexes with bridging hydrocarbon fragments (I J). The three-center-two-electron methyl bridges lead to metal-metal bonding due to core levels, which is reflected in unusually short C r - C r distances and low magnetic moments. Figure 2 depicts the molecular structures of 3

ν(ϊ

Β

Β


594

H O M O G E N E O U S TRANSITION

METAL CATALYZED

REACTIONS

C2-

C4-

CL

cr

ββ.5* C5

CR


C5'

94.2*

CU

C4

Οδ Cr-Cr « 3.287 I 6.0

4.80

5.0

4.00

4.0

1

3.20

ό X

3.0

2.40

2.0

1.60

1.0

0.80

^

0.00

0.0 0 0

40.0

80.0

120.0

160.0

200.0

240.0

280.0

320.0

Temperature (K) Figure 1. The molecular structure and temperature dependence of the molar magnetic susceptibility (χ„„ open circles) and the effective magnetic moment (μ,η, filled circles) of [Cp(CH,)Cr(\i-Cl)} . 2



41.

T H E O P O L D ET AL.


595

Scheme I.

[ C p * ( C H ) C r ^ - C H ) ] and [ C p * C r ^ - C H ) ] ^ - C H ) . The former catalyzes the polymerization of ethylene, albeit slowly; the latter does not react with ethylene at ambient temperature. These molecules are highly unusual in the sense that they exhibit metal-metal bonds between octahedral Cr(III) ions. 3

3

2

3

2

2

Most of the compounds shown in Chart I actually show little or no catalytic activity for ethylene polymerization. Despite their low electron count, they are coordinatively saturated and must dissociate a ligand to enable binding and subsequent insertion of ethylene. Thus we prepared compounds containing weakly bound ligands. Abstraction of halide from [ C p * C r ( C H ) ^ - C l ) ] in tetrahydrofuran (THF) solution or protonation of C p * ( T H F ) C r ( C H ) with H N E t B P h in the same solvent yielded the cationic alkyl complex [ C p * C r ( T H F ) ( C H ) ] B P h (12, 13). The two T H F ligands are bound strongly enough to allow isolation and indeed structural characterization of the complex (Figure 3). However, some T H F dissociates in C H C 1 solution, leaving a coordinatively unsaturated chromium alkyl in equilibrium with the coordinatively saturated precursor. 3

2

3

2

3

+

4

2

2

3

+

4

2

Polymerization Catalysis [ C p * C r ( T H F ) ( C H ) ] B P h (4.5 m M in C H C 1 ) catalyzed the polymerization of ethylene at room temperature and 1.0-1.5 atm of pressure. Ethylene uptake measurements showed a brief rise in activity, followed by a period of rapid polymerization and eventually deactivation of the catalyst. Typical yields were 1.5 g of polyethylene per 60 mg of catalyst, corresponding to an average of 600 turnovers. The time from the onset of polymerization to complete deactivation was approximately 1 h, and at the point of highest 2

3

+

4

2

2



3

2

3

3

2

Figure 2. The molecular structures of [Cp*(CH3)Cr(\L-CH )] (left) and lCp*Cr(\L-CH3)]2(\L-CH2) (right). Detail on lower right shows hydrogen po sition of the bridging groups in lCp*Cr(p.'CH )]2(\L-CH2). ([Cp*Cr(pCH )]^'CH ) structure is reproduced with permission from reference 11. Copyright 1990 VCH VerlagsgeseUschaft mbH.)


C/5

δ ζ

η Ό 5β η >

s

r Ο > > r

s*

Ζ

3 ο

Ζ

Η ?

ζΛ

G

ζ m Ο

M

ο

C

ο

χ

ΟΙ


41.



Figure 3. [ Cp*Cr(THF) CH ] 2

3

+

597

BPhf (tetraphenylborate countenon omitted for clarity).

activity the turnover frequency was —0.7 s" (assuming all chromium was 1

active). The rate of polymerization was decreased dramatically by addition of T H F , presumably because of a shift of the dissociation equilibrium (eq 1).

The equilibrium constant for this dissociation was estimated by N M R experiments to be K = 1 x 10~ M at 20 ° C . During the course of the polymerization the color of the solution slowly changed from red-brown to purple. Attempts to isolate a chromium-containing complex from spent catalyst solutions were unsuccessful. The polyethylene was isolated by filtration, washed with C H C 1 , and dried under vacuum before characterization. It c q

2

3

2


598


had a slight pink coloration, indicating some level of residual chromium. The IR spectrum of the polymer was indistinguishable from that of authentic high-density polyethylene. Its melting range was 135-140 °C. Table I lists the result of molecular weight determinations by gel permeation chroma tography (GPC) and branching data from

I 3

C N M R spectroscopy. The mo

lecular weights were low and their distributions M / M w

were relatively

n

narrow ( M is weight-average molecular weight; M is number-average mo w

n

lecular weight; M / M


w

is polydispersity).

n

Table I. Polyethylene Characterization ^ethylene

(atm)

Temperature (°C)

Added THF (equiv)

20 10 0 20 20 >25

— — —

1.5 1.5 1.5 1.5 1.5 1.1

1 3

—

Μ ./Μ

M„

Α

17,520 20,160 16,740 14,310 14,970 10,280

33,760 47,240 77,070 23,020 24,370 19,320

Branching (R/1000 CH ) 2

Π

p

1.93 2.34 4.60 1.61 1.63 1.88

4.0 0

p

2.6

p

NOTE: Polyethylene was produced by the reaction of 60 mg of [Cp*Cr(THF) Me] BPh m L o f C H C l , 4 . 5 x 10 M . 2

2

The

+

in 20

4

3

2

cationic nature of [ C p * C r ( T H F ) ( C H ) ] B P h 2

3

+

4

nicely comple

mented the emerging notion that positively charged alkyls comprise the active sites of Ziegler-Natta catalyst preparations based on Group 4 ele ments (14-19). However, cationic nature is apparently not a requirement. We

subsequently found

another

compound (i.e.,

the neutral

dialkyl

C p * C r [ C H S i ( C H ) 3 ] 2 ) that is an even more active catalyst for the polym 3

2

erization of ethylene. Because of the steric bulk of the trimethylsilylmethyl ligands—and by contrast to [ C p * C r ( C H ) ] — t h i s compound is monomeric even in the solid 3

2

2

state. Its effective magnetic moment is temperature-independent, and the value of μ

ϋ { Γ

(4.0 μ ) is consistent with three unpaired electrons. In this Β

complex chromium exhibits a 13-electron configuration and has an open coordination

site

for

binding

of

ethylene.

Pentane

solutions

of

C p * C r [ C H S i ( C H ) ] rapidly polymerized ethylene at temperatures as low 2

3

3

2

as -42 °C. Polymer molecular weights ( M 20,100-143,000) and dispersities w

(M /M w

n

2.98-7.06) were similar to the samples prepared with the cationic

catalyst. Neither of the two catalysts polymerize propene, although some lower

oligomers

are

[Cp*Cr(THF) (CH )] B P h 2

3

+

apparently 4

formed

in

the

reaction

with

.

Active Oxidation State The valence state of the active site of chromium-based catalysts has been the subject of much controversy. Oxidation states between + II and + V


41.

TIIEOPOLD ET AL.

599


have been suggested. Our results prove that chromium in its ubiquitous and very stable -h III oxidation state catalyzes the polymerization of ethylene. However, in light of the credit generally given to divalent chromium on heterogeneous catalysts, we thought it worthwhile to find out what effect reduction would have on our catalysts. Figure 4 depicts the cyclic voltammogram of a T H F solution of [Cp*(dmpe)CrCH ] P F ~ [dmpe is l,2-bis(dimethylphosphino)ethane]. The complex exhibits a reversible reduction wave at ca. -2.0 V vs. the C p F e - C p F e couple. Chemical reduction with N a - H g yielded the neutral Cr(II) alkyl C p * ( d m p e ) C r C H , which could be isolated and structurally char acterized by X-ray diffraction. The availability of constitutionally identical Cr(III) and Cr(II) alkyls allowed a direct comparison of the catalytic activity of chromium in both oxidation states. 3


2

+

+

6

2

3

Reaction of ethylene with [Cp*(dmpe)CrCH ] P F ~ required elevated temperatures (because of the greater coordinating power of the dmpe ligand), but at 90 °C reaction ensued and yielded polyethylene. C p * ( d m p e ) C r C H , on the other hand, reacted with ethylene at room temperature. However, the major product of this reaction was propene; no polyethylene was formed. Apparently ethylene inserts into the chromium-carbon bond, but β-hydrogen elimination of propene is more facile than continued insertions. These results clearly suggest that + III is the more appropriate oxidation state for the active site of chromium-based polymerization catalysts. 3

+

6

3

Ring-Opening Metathesis Polymerization Olefin polymerization catalysts are closely related chemically to metathesis catalysts. The former depend on reactive metal alkyl moieties; the latter feature an interplay between metal alkylidenes and metallacyelobutanes. A long-standing problem of the olefin metathesis reaction has been the incom patibility of the traditional early metal catalysts with heteroatom substituents (especially those containing oxygen) of functionalized olefins. A possible path to the solution of this problem is the search for less oxophilic (i.e., late transition metal) catalysts. We thus began an investigation into the possibility of preparing chromium(III) alkylidenes for use as metathesis catalysts. A well-precedented route to metal alkylidenes utilizes α-hydrogen ab straction from metal alkyls. We found that Cp*(py)Cr(CH )(0-r-Bu) (py is pyridine) catalyzes the ring-opening metathesis polymerization (ROMP) (20-24) of norbornene. Heating of a toluene solution of this complex with a large excess of norbornene to —60 °C gave a high yield of polymer (>85% isolated). Scheme II depicts the proposed mechanism of this reaction. The key feature is the generation of the coordinatively unsaturated chromium methylene complex by deprotonation of a methyl group with the internal base terr-butoxide (i.e., an a-abstraction). 3


In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992. 3

6

3

Figure 4. Cyclic voltammogram of [Cp*Cr(dmpe)CH ]*PF ~ in THF (bottom) and the molecular structure of neutral Cp*Cr(dmpe)CH (top right).


C/3

δ ζ

>

m

50

> p -< Ν M Ό

Η

> p Ο >

Η

S

δ ζ

Η

(Λ

ζ

Η

C/5

G

Ο

PÎ

ο M ζ

Ο

ο


41.



601

Scheme 11.

The evidence for this event includes the observed formation of methane and Cp*(py)Cr(0-r-Bu) (from the reaction of the catalyst precursor with the liberated f erf-butyl alcohol) as well as the catalytic activity. The decomposition of Cp*(py)Cr(CH )(0-r-Bu) to the active catalyst is a slow reaction that continues over the whole course of the polymerization. The polymer would thus be expected to show a broad molecular weight distribution, and G P C measurements confirmed this expectation ( M / M = 7.3). The potential importance of this observation lies in the way the catalyst is generated (i.e., by breaking of a metal-oxygen bond. If chromium-based metathesis catalysts can be developed, they may be expected to be less sensitive to oxygen functionalities. Thus, they may be used in the metathesis and R O M P catalysis of functionalized olefins. 2

3

w

n

References

1. Transition Metal Catalyzed Polymerizations, Quirk, R. P., Ed.; Cambridge Un versity Press: Cambridge, 1988. 2. Karol, F. J. Catal. Rev.-Sci. Eng. 1984, 26, 557. 3. Sinn, H.; Kaminsky, W. Adv. Organomet. Chem. 1980, 18, 99. 4. Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. 1955, 67, 541. 5. Natta, G.; Pino, P.; Carradini, P.; Danusso, F.; Mantica, E.; Mazzanti, G.; Moraglio, G. J. Am. Chem. Soc. 1955, 77, 1708.



602


6. Clark, A. Catal. Rev. 1969, 3, 145. 7. Karol, F. J.; Karapinka, G. L.; Wu, C.; Dow, A. W.; Johnson, R. N.; Carrick, W. L.J.Polym. Sci., Polym. Chem. Ed. 1972, 10, 2621. 8. Karol, F. J.; Brown, G. L.; Davison, J. M. J. Polym Sci., Polym. Chem. Ed. 1973, 11, 413. 9. Richeson, D. S.; Hsu, S.-W.; Fredd, Ν. H.; Van Duyne, G.; Theopold, Κ. H. J. Am. Chem. Soc. 1986, 108, 8273. 10. Noh, S. K.; Sendlinger, S. C.; Janiak, C.; Theopold, Κ. H. J. Am. Chem. Soc. 1989, 111, 9127. 11. (a) Noh, S. K.; Heintz, R. Α.; Janiak, C.; Sendlinger, S. C.; Theopold, Κ. H. Angew. Chem. 1990, 102, 805; (b) Angew. Chem., Int. Ed. Engl. 1990, 29, 775. 12. Thomas, B. J.; Theopold, Κ. H. J. Am. Chem. Soc. 1988, 110, 5902. 13. Thomas, B. J.; Noh, S. K.; Schulte, G. K.; Sendlinger, S. C.; Theopold, Κ. H. J. Am. Chem. Soc. 1991, 113, 893. 14. Eisch, J. J.; Piotrowski, A. M.; Brownstein, S. K.; Gabe, E. J.; Lee, F. L. J. Am. Chem. Soc. 1985, 107, 7219. 15. Jordan, R. F.; Bajgur, C. S.; Willett, R.; Scott, B. J. Am. Chem. Soc. 1986, 108, 7410. 16. Jordan, R. F.; LaPointe, R. E.; Bajgur, C. S.; Echols, S.; Willett, R. J. Am. Chem. Soc. 1987, 109, 4111. 17. Gassman, P. G.; Callstrom, M. R. J. Am. Chem. Soc. 1987, 109, 7875. 18. Taube, R.; Krukowka, L. J. Organomet. Chem. 1988, 347, C9. 19. Hlatky, G. G.; Turner, H. W.; Eckman, R. R.J.Am. Chem. Soc. 1989, 111, 2728. 20. Calderon, N. J. Macromol. Sci., Rev. Macromol. Chem. 1972, C7(1), 105. 21. Katz, T. J.; Lee, S. J.; Shippey, M. A. J. Mol. Catal. 1980, 8, 219. 22. Gilliom, L.; Grubbs, R. H. J. Am. Chem. Soc. 1986, 108, 733. 23. Wallace, K. C.; Schrock, R. R. Macromolecules 1987, 20, 450. 24. Schrock, R. R.; Feldman, J.; Cannizzo, L. F.; Grubbs, R. H. Macromolecules 1987, 20, 1172. RECEIVED for review October 19, 1990. A C C E P T E D revised manuscript June 13, 1991.


Homogeneous Chromium Catalysts for Olefin Polymerization

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