α-olefin polymerization with ether-coordinated chromium(III) alkyls

Article abstract of DOI:10.1021/om960697q

Extremely substitution labile chromium(III) alkyls of the type [Cp*Cr(OR₂)_nCH₂SiMe₃] ⁺[B(3,5-(CF₃)₂-C₆H₃) ₄]^- (R = Me, Et, ⁱ

Full text of DOI:10.1021/om960697q

Organometallics 1996, 15, 5473-5475
5473
r-Olefin P olym er iza tion w ith Eth er -Coor d in a ted
Ch r om iu m (III) Alk yls
Paul A. White,† J oe Calabrese,‡ and Klaus H. Theopold*,†
Department of Chemistry and Biochemistry, Center for Catalytic Science and Technology,
University of Delaware, Newark, Delaware 19716, and Central Research & Development
Department, E. I. du Pont de Nemours and Company, Experimental Station,
Wilmington, Delaware 19898
Received August 14, 1996X
Summary: Extremely substitution labile chromium(III)
alkyls of the type [Cp*Cr(OR2)nCH2SiMe3]+[B(3,5-(CF3)2-
C6H3)4]- (R ) Me, Et, iPr; n ) 1, 2) have been prepared;
they are homogeneous catalysts for the polymerization
of ethylene and, for the first time, R-olefins (propene,
1-hexene).
°C, compromised the structure sufficiently to render
individual bond distances and angles unreliable, the
chemical nature of the compound is not in doubt; the
molecular structure of 1b is depicted in Figure 1. The
fluorinated tetraarylborate lives up to its reputation as
a “noncoordinating” anion,5 being well separated from
the chromium complex. The latter adopts the familiar
three-legged piano-stool geometry, which has proven
characteristic of the Cp*CrIII fragment. Two coordina-
tion sites of the pseudooctahedral chromium complex
are occupied by diethyl ether molecules. In keeping
with the steric saturation of the metal ion, 1 shows no
structural indication of any agostic interactions. Due
to the poor ligand properties of acyclic ethers few transi-
tion-metal alkyls containing such ligands have proven
sufficiently stable for structural characterization.6 1b
is a rare example containing two such labile ligands.
The chemistry of 1a ,b reflects the weakness of their
Cr-OR2 bonds. Addition of donor solvents (e.g. THF,
DME) led to immediate replacement of both dialkyl
ethers and produced the comparatively robust, purple
alkyls [Cp*Cr(THF)2CH2SiMe3]+BAr′4- (1d) and [Cp*Cr-
(DME)CH2SiMe3]+BAr′4- (1e). However, while solid 1a
could be kept under vacuum without ligand loss and
maintained its purple color when dissolved in noncoor-
dinating solvents, 1b displayed a greater tendency to
extrude one of its ligands. When exposed to vacuum,
purple crystals of 1b (UV/vis/near-IR: solid, 536, 700
nm; Et2O, 531 (ꢀ ) 1100), 703 (ꢀ ) 360) nm) were rapidly
transformed into an orange powder (UV/vis/near-IR:
solid, 506, 746 nm), concomitant with a weight loss of
5.5% (theoretical 6% for loss of 1 equiv of Et2O).
Exposure of the orange solid to Et2O vapors restored
the purple color and original mass of 1b. Dissolution
of 1b in noncoordinating solvents gave orange solutions
(UV/vis/near-IR: CH2Cl2, 507 (ꢀ ) 825), 739 (ꢀ ) 475)
nm; o-C6H4F2, 503 (ꢀ ) 760), 742 (ꢀ ) 470) nm). On
the basis of these observations, we assign the formula
[Cp*Cr(OEt2)CH2SiMe3]+BAr′4- (2b) to the orange com-
pound. The thermal stability of solutions of 2b (and
hence 1b) in noncoordinating solvents (CH2Cl2, o-C6H4F2)
is limited (see below), and to date, it has resisted all of
our attempts to determine its crystal structure. 2b
showed no spectroscopic evidence of any agostic interac-
tions, though, and we suggest that it is a coordinatively
unsaturated pseudo-5-coordinate chromium alkyl with
a 13-electron configuration.
Despite the commercial importance of chromium-
based ethylene polymerization catalysts, investigations
of their chemistry on a molecular level remain grossly
outnumbered by studies of Ziegler-Natta catalysts
containing d0, group 4 elements,1 due in part to the
paramagnetism of relevant chromium compounds. (Pen-
tamethylcyclopentadienyl)chromium(III) alkyls now rep-
resent a well-characterized homogeneous model sys-
tem for heterogeneous ethylene polymerization catalysts
containing that metal.2 As part of our continuing effort
to map the reactivity of paramagnetic chromium alkyls,
we are aiming to increase their electrophilicity and co-
ordinative unsaturation, in the hopes ofsinter alias
facilitating polymerization of R-olefins and assessing the
importance of agostic interactions in chromium cataly-
sis. Herein we report a series of cationic chromium
alkyls with very weakly bound dialkyl ether ligands,
which polymerize propene and 1-hexene.
Addition of 1 equiv of [H(OiPr2)2]+[B(C6H3(CF3)2)4]- 3
to a cold diethylether solution of Cp*Cr(CH2SiMe3)2,
followed by addition of pentane, yielded purple crystals
-
of [Cp*Cr(OEt2)2CH2SiMe3]+BAr′4 (1b ) in 94% yield
(see Scheme 1). Its recrystallization from dimethyl
-
ether solvent provided [Cp*Cr(OMe2)2CH2SiMe3]+BAr′4
(1a ). 1a ,b were characterized by standard spectroscopic
techniques (see Supporting Information), and the struc-
ture of 1b was determined by X-ray diffraction.4 While
loss of ether and disorder in the CF3 groups of the
counterion, even at a data collection temperature of -69
* To whom correspondence should be addressed. E-mail: theopold@
udel.edu.
† University of Delaware.
‡ E. I. du Pont de Nemours and Co.
X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(2) (a) Thomas, B. J .; Theopold, K. H. J . Am. Chem. Soc. 1988, 110,
5902. (b) Noh, S.-K.; Heintz, R. A.; J aniak, C.; Sendlinger, S. C.;
Theopold, K. H. Angew. Chem., Int. Ed. Engl. 1990, 29, 775. (c)
Thomas, B. J .; Noh, S.-K.; Schulte, G. K.; Sendlinger, S. C.; Theopold,
K. H. J . Am. Chem. Soc. 1991, 113, 893. (d) Bhandari, G.; Kim, Y.;
McFarland, J . M.; Rheingold, A. L.; Theopold, K. H. Organometallics
1995, 14, 738. (e) Bhandari, G.; Rheingold, A. L.; Theopold, K. H. Chem.
Eur. J . 1995, 1, 199.
(3) [H(OEt2)2]+[B(C6H3(CF3)2)4]- was prepared according to:
Brookhart, M.; Grant, B.; Volpe, A. F., J r. Organometallics 1992, 11,
3920. The isopropyl etherate was obtained by recrystallization from
iPr2O.
(4) 1b: purple cubes from Et2O/pentane; monoclinic, P21/n; a )
12.573(1) Å, b ) 12.829(1) Å, c ) 37.210(1) Å, â ) 95.991(2)°, T ) -69
°C, V ) 5969.2 Å3, Z ) 4, R ) 0.146, Rw ) 0.140.
(5) Strauss, S. H. Chem. Rev. 1993, 93, 927.
(6) (a) Behm, F.; Lotz, S. D.; Herrmann, W. A. Z. Anorg. Allg. Chem.
1993, 619, 849. (b) Solari, E.; Musso, F.; Gallo, E.; Floriani, C.; Re, N.;
Chiesi-Villa, A.; Cizzoli, C. Organometallics 1995, 14, 2265. (c) Yi, C.
S.; Wo´dka, D.; Rheingold, A. L.; Yap, G. P. A. Organometallics 1996,
15, 2.
S0276-7333(96)00697-8 CCC: $12.00 © 1996 American Chemical Society

5474 Organometallics, Vol. 15, No. 26, 1996
Communications
Sch em e 1a
a
i
Identity of L and/or R2O: a , Me2O; b, Et2O; c, Pr2O; d , THF; e, DME.
temperature, yielded 585 mg of polyethylene within 20
min (Mw ) 217 000, Mw/Mn ) 9.65, mp ) 133 °C). We
also note that Et2O solutions of 1b were completely
inactive, in accord with the notion of a coordinatively
unsaturated 13-electron species as the active catalyst.
Exposure of solid 2b to 1.0 equiv of ethylene (room
temperature, 5 min), followed by quenching with D2O,
yielded a mixture of hydrocarbons of the type Me3-
1
SiCH2(CH2CH2)nD (n ) 0-4, detected by H NMR and
GC-MS), providing direct evidence for ethylene inser-
tion into the chromium-alkyl bond.
The high activity in the polymerization of ethylene
encouraged us to try R-olefins. Monitoring a solution
1
of 1a and 10 equiv of propene in CD2Cl2 at -8 °C by H
NMR showed the gradual disappearance of the reso-
nances of the olefin over the course of 2 h and the rise
of several new resonances in the 0.8-2.0 ppm region,
consistent with the formation of polypropene.7 A small
pair of resonances (δ 4.72, 4.65 ppm), characteristic of
a gem-dialkyl olefin such as might be formed by
â-hydrogen elimination from a growing polypropene
chain, was also observed. In a larger scale polymeri-
zation, exposure of a solution of 1a (100 mg in 30 mL of
CH2Cl2, 2.7 mM, -6 °C) to 0.8 atm of propene produced
180 mg of an oily residue (>50 turnovers). GC-MS
analysis of this oil showed it to be a mixture of propene
oligomers (i.e. CH2dC(Me)(CH2C(H)Me)n-1H) ranging
from C9 to C51 in size (n ) 3-17), with a maximum at
C18/C21. Curiously, raising the propene pressure to 5.8
atm did not significantly increase the yield or the
molecular weight of the polymer. Similarily, monitoring
a reaction of 1a with 15 equiv of 1-hexene (7 °C) by NMR
showed complete conversion of the olefin to polymer
after 1 h, and GC-MS analysis revealed a distribution
of molecular weights ranging from C12 to C66 (n ) 2-11),
with a maximum at C24. We note that the mass spectra
did not show any evidence for (trimethylsilyl)methyl end
F igu r e 1. Molecular structure of the chromium cation of
[Cp*Cr(OEt2)2CH2SiMe3]+[B(C6H3(CF3)2)4]- (1b). Disorder
in the ethyl groups of the Et2O ligands and the CF3 groups
of the anion (not shown) made determination of accurate
interatomic distances and angles impossible. Best esti-
mates are as follows: Cr-O1, 2.09(2) Å; Cr-O2, 1.95(2)
Å; Cr-C1, 2.09(2) Å.
As expected, 1a ,b were extremely active ethylene
polymerization catalysts. For example, exposure of a
cold solution of 1b (50 mg in 18 mL of CH2Cl2, -78 °C)
to ethylene (1.2 atm) quickly resulted in precipitation
of polyethylene (30 min, yield 950 mg, Mw ) 73 620, Mw/
Mn ) 2.74, mp ) 133 °C). Indeed, stirring a suspension
of solid 1b in liquid ethylene for 20 min (ambient
pressure, T < -104 °C) produced small amounts of
polymer. At ambient temperature the polymerization
reactions became noticeably hot and rapid deactivation
and high polydispersities indicated thermal decomposi-
tion of the catalyst. Thus, 50 mg of 1b dissolved in 0.5
mL of o-difluorobenzene, diluted with 29.5 mL of toluene
and exposed to ca. 0.8 atm of ethylene beginning at room
(7) Bovey, F. A. Nuclear Magnetic Resonance Spectroscopy; Academic
Press: San Diego, CA, 1988; pp 365-370.

Communications
Organometallics, Vol. 15, No. 26, 1996 5475
groups; thus, we assume that chain transfer by â-hy-
drogen elimination rapidly generates a chromium hy-
dride (i.e. Cp*Cr(OMe2)nH]+BAr′4) as the majority cata-
lyst. Attempts to prepare this hydride by hydrogenolysis
of 1a have been unsuccessful to date, yielding a com-
plicated mixture of decomposition products.
(yielding ethylene and a chromium ethoxide). This
decomposition pathway may limit the general utility of
ethers as ancillary ligands for highly electron deficient
metal alkyls.
While we have long held pseudo-5-coordinate inter-
mediates with 13-electron configurations responsible for
the polymerization activity of Cp*CrIII alkyls, the present
results show that such molecules can be prepared (i.e.
2b,c). Their reactivity is high and for the first time
includes the polymerization of R-olefins. Nevertheless,
the comparatively high selectivity of this chromium
system for ethylene is remarkable, and the current
strategy for overcoming the reticence of R-olefins has
pushed the system to the limits of stability. Our search
for other chromium-based homogeneous R-olefin polym-
erization catalysts is continuing.
On the basis of the apparent stabilization of the
coordinatively unsaturated 13-electron alkyl by steri-
cally more demanding ligands, we hoped to prepare, and
isolate, such a complex using diisopropyl ether. How-
i
ever, when 1b was dissolved in Pr2O, the orange color
of the initially formed [Cp*Cr(iPr2O)CH2SiMe3]+ ion (2c)
rapidly (<30 s) changed to green. Adding pentane and
cooling the solution at this point yielded green crystals
-
of the isopropoxide complex [Cp*Cr(OiPr2)OiPr]+BAr′4
(3c). NMR experiments showed that SiMe4 and propene
were also formed during this reaction. 3c itself was not
i
stable in the presence of excess Pr2O; its green solution
Ack n ow led gm en t. This research was supported by
research grants from Chevron Chemical Co. and the
NSF (Grant No. CHE-9421802).
eventually turned purple, and we have not identified
any of the ultimate decomposition products. Addition
of better ligands to the green solution of 3c yielded blue
-
[Cp*Cr(OEt2)2OiPr]+BAr′4 (4b), [Cp*Cr(THF)2OiPr]+-
Su p p or tin g In for m a tion Ava ila ble: Text giving a sum-
mary of the crystal structure determination and tables giving
the positional and thermal parameters of 1b and tables of
spectroscopic data for all complexes (7 pages). Ordering
information is given on any current masthead page.
-
-
BAr′4 (4d ), and [Cp*Cr(DME)OiPr]+BAr′4 (4e), re-
spectively. Like 1b, 4b reversibly lost 1 equiv of diethyl
ether under vacuum to form green [Cp*Cr(OEt2)OiPr]+-
-
BAr′4 (3b). Cleavage of ethers by strong bases (here
the CH2SiMe3 group) is precedented,8 and the extreme
electrophilicity of the cationic, 13-electron chromium
center probably enhances the leaving-group tendency
of the alkoxide moiety. We attribute the limited ther-
mal stability of 1b to a similar elimination reaction
OM960697Q
(8) (a) Ko¨brich, G. Angew. Chem., Int. Ed. Engl. 1962, 1, 382. (b)
March, J . Advanced Organic Chemistry; McGraw-Hill: New York,
1977; pp 925-926.