Catalytic insertion of ethylene into Al-C bonds with pentamethylcyclopentadienyl-chromium(III) complexes

Article abstract of DOI:10.1021/om001048o

Addition of ethylene to mixtures of Cp*CrMe₂(PMe₃) (1) and access methylaluminoxane (MAO), followed by aqueous workup of the resulting solutions, results in the formation of a distribution of odd-carbon alkanes. When B(C₆F₅)₃ is added to 1, followed by THF, one obtains dark crystals. A single-crystal X-ray diffraction study revealed that the product of the reaction is [Cp*CrMe(PMe₃)(THF)][MeB(C₆ F₅)₃] (2). Polyethylene is obtained when using 1 and 2 equiv of B(C₆F₅)₃. The reaction of ethylene with a mixture of 1 + 2 B(C₆F₅)₃ + 380 AlMe₃ produces a distribution of odd-carbon alkanes. If triethylaluminum is used, instead of trimethylaluminum, the product is a distribution of even-carbon alkanes. The product distribution shifts to lower molecular weight product, and lower activities are attained with increasing triethylaluminum concentration. Use of more dichoromethane instead of toluene or hexane increases the ethylene consumption. Triisobutylaluminum and trioctylaluminum result in considerably lower activities. A mechanism for the oligomerization of ethylene involving fast transmetalation reactions between chromium and aluminum is proposed.

Full text of DOI:10.1021/om001048o

Organometallics 2001, 20, 2059-2064
2059
Ca ta lytic In ser tion of Eth ylen e in to Al-C Bon d s w ith
P en ta m eth ylcyclop en ta d ien yl-Ch r om iu m (III) Com p lexes
Guillermo C. Bazan,* J onathan S. Rogers, and Cindy C. Fang
Departments of Chemistry and Materials, University of California,
Santa Barbara, California 93106
Received December 7, 2000
Addition of ethylene to mixtures of Cp*CrMe2(PMe3) (1) and excess methylaluminoxane
(MAO), followed by aqueous workup of the resulting solutions, results in the formation of a
distribution of odd-carbon alkanes. When B(C6F5)3 is added to 1, followed by THF, one obtains
dark crystals. A single-crystal X-ray diffraction study revealed that the product of the reaction
is [Cp*CrMe(PMe3)(THF)][MeB(C6F5)3] (2). Polyethylene is obtained when using 1 and 2
equiv of B(C6F5)3. The reaction of ethylene with a mixture of 1 + 2 B(C6F5)3 + 380 “AlMe3”
produces a distribution of odd-carbon alkanes. If triethylaluminum is used, instead of
trimethylaluminum, the product is a distribution of even-carbon alkanes. The product
distribution shifts to lower molecular weight product, and lower activities are attained with
increasing triethylaluminum concentration. Use of more dichoromethane instead of toluene
or hexane increases the ethylene consumption. Triisobutylaluminum and trioctylaluminum
result in considerably lower activities. A mechanism for the oligomerization of ethylene
involving fast transmetalation reactions between chromium and aluminum is proposed.
In tr od u ction
(PMe3)8 (1) with methylaluminoxane (MAO) reacts with
ethylene to give an alkylaluminoxane product with the
chain growth product attached to the aluminum sites.9
Hydrolysis of this product gives a distribution of odd-
carbon, straight chain alkanes, as shown in eq 1.
Trialkylaluminum reagents find extensive practical
use in large-scale industrial processes.1 For example,
they are used for the preparation of R-olefins via a
displacement reaction.1 The oxidation with oxygen2 or
amine oxides3 to give linear alcohols is also practiced.
Commercial processes to trialkylaluminum compounds
typically begin with the reaction of triethylaluminum
with ethylene under elevated temperature and pressure
or in the presence of suitable transition metal reagents
under moderate reaction conditions.4 Nickel compounds
are of notable historical importance in this respect.5
Catalysts based on actinide metallocenes are also
noteworthy for generating a product with a Poisson-type
chain length distribution.6 The chain length distribution
in these processes is determined by the rates of reaction
for ethylene insertion, for direct elimination of the
growing chain at the metal, and for the transmetalation
exchange between the metal and aluminum.7
In this contribution, we report on further details of
the reactions of 1 with MAO, and we report on our
studies of the reaction using 1 and the borane activator
B(C6F5)3. As will be shown, this combination of reagents
allows for the use of well-defined homogeneous chro-
mium reagents to create a process that catalyzes the
insertion of olefins into the Al-C bond. In view of the
rising interest in homogeneous organochromium cata-
lysts for olefin polymerization reactions, these results
give an additional useful handle for controlling polymer
molecular weight.10
In connection to the transmetalation reactions, we
recently reported that the combination of Cp*CrMe2-
* Corresponding author: E-mail: bazan@chem.ucsb.edu.
(1) Zietz, J . R.; Robinson, G. C.; Lindsay, K. L. In Comprehensive
Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W.,
Eds.; Pergamon Press: Oxford, 1982; Vol. 7, pp 365-464.
(2) (a) Davies, A. G.; Roberts, B. P. J . Chem. Soc. B 1968, 1074. (b)
Krusic, P. J .; Kochi, J . K. J . Am. Chem. Soc. 1969, 91, 3942.
(3) Kabalaka, G. W.; Newton, R. J . J . Organomet. Chem. 1978, 156,
65.
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Kirk-Othmer Encyclopedia of Chemical Technology; Kroschwitz, J . I.,
Howe-Grant, M., Eds.; J ohn Wiley & Sons: New York, 1996; Vol. 17,
p 839. (b) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.;
J ohn Wiley & Sons: New York, 1992. (c) Freitas, E. R.; Gum, C. R.
Chem. Eng. Prog. 1979, 75, 73.
(5) (a) Ziegler, K.; Gellert, H.-G.; Holzkamp, E.; Wilke, G.; Duck, E.
W.; Kroll, W.-R. J ustus Liebigs Ann. Chem. 1960, 629, 172. (b)
Elschenbroich, C.; Salzer, A. Organometallics; VCH: New York, 1989.
(6) Samsel, E. G.; Eisenberg, D. C. U.S. Patent 5,276,220, 1994.
(7) Weslau, H. Annal. Chem. 1960, 629, 198.
Resu lts a n d Discu ssion
Rea ction s of 1/MAO/C2H4 w ith Tr ia lk yla lu m i-
n u m Rea gen ts. A consideration of the experimental
details is of importance for determining the efficiency
(8) Grohmann, A.; Kohler, F. H.; Muller, G.; Zeh, H. Chem. Ber.
1989, 122, 897.
(9) Rogers, J . S.; Bazan, G. C. Chem. Commun. 2000, 1209.
10.1021/om001048o CCC: $20.00 © 2001 American Chemical Society
Publication on Web 04/07/2001

2060 Organometallics, Vol. 20, No. 10, 2001
Bazan et al.
of transmetalation/oligomerization processes. In our
previous communication,9 the reactions were carried out
by introducing a solution containing the appropriate
ratios of 1, MAO, and the AlR3 into a round-bottom flask
inside a glovebox. After attaching a Teflon needle valve
adapter with suitable ground glass joints, the flask was
attached to a double manifold line and then evacuated
for a brief period of time (∼5 s). Ethylene was then
introduced over the resulting solution. To maintain a
constant 1 atm of ethylene over the reaction, the
ethylene was allowed to exit via an oil-filled bubbler.
Under these “open” conditions, the activity was mea-
sured at 87 kg product/(mol Cr‚hr), and only soluble
products were obtained.
For the reactions described herein, the MAO and 1
were introduced into a flat bottom glass reactor inside
the glovebox. The reactor was then connected to the
ethylene line directly, by using a quick-connect adapter.
Ethylene was then introduced at 15 psig. This setup,
referred to as “closed” conditions, enables better stirring
and a more efficient mass transfer of ethylene into the
solution. These conditions lead to a higher consumption
of ethylene, 278 kg product/(mol Cr‚h), and the forma-
tion of a solid product (30% of total product), which
precipitates as the reaction proceeds. Thus, the condi-
tions reported in this paper, relative to our previous
work, favor propagation processes on the basis of
increased ethylene concentration.
F igu r e 1. ORTEP view of 2. Atoms are shown at the 30%
probability level. Hydrogen atoms and the [MeB(C6F5)3]
anion are not shown for clarity.
paramagnetic nature of chromium(III), it is not possible
1
to characterize the resulting products using H NMR
spectroscopy. Efforts to isolate crystalline material from
this reaction failed in all cases. However, addition of
THF followed by crystallization from Et2O/iPr2O gives
black crystals of the cation/anion pair, [Cp*CrMe(PMe3)-
(THF)][MeB(C6F5)3] (2 in eq 2). The molecular structure
of 2 was determined using single-crystal X-ray crystal-
lography, and the results are illustrated in Figure 1.
The phosphine and one methyl ligand remain bound to
the metal upon reaction of 1 with B(C6F5)3. The isolation
of 2 suggests that the active species for ethylene
insertion using 1 in conjunction with B(C6F5)3 or MAO
is [Cp*CrMe(PMe3)]+ (vide infra) with a loosely bound,
or solvent-separated, borate counteranion ([MeB(C6-
F5)3]-).
Oligom er ization -Tr an sm etalation Reaction s Us-
in g Well-Defin ed Bor a n e Activa tor s. On the basis
of Theopold’s studies on mono(cyclopentadienyl)chro-
mium(III) catalysts for ethylene polymerization11 and
the reactivity of isoelectronic boratabenzene com-
14
plexes,12,13 we postulated that 1 with B(C6F5)3 would
generate a single-site catalyst for fast insertion of
ethylene into the Cr-C bond, followed by Cr-C/Al-C
exchange. Addition of 1 to solutions of B(C6F5)3 results
in a color change from purple to dark green. Due to the
(10) (a) Espelid O, B. K. J . J . Catal. 2000, 195, 125. (b) Kohn, R. D.;
Haufe, M.; Mihan, S. Chem. Commun. 2000, 1927. (c) Ajjou, J .; Scoott,
S. L. J . Am. Chem. Soc 2000, 122, 8968. (d) Siemeling, U.; Kolling, L.;
Stammler, A. Chem Commun. 2000, 1177. (e) Thune, P. C.; Loos, J .;
de J ong, A. M. Top. Catal. 2000, 67. (f) Voges, M. H.; Romming, C.;
Tilset, M. Organometallics 1999, 18, 529. (g) Kasani, A.; McDonald,
R.; Cavell, R. G. Chem. Commun. 1999, 1993. (h) Duchateau, R.;
Cremer, U.; Harmsen, R. J . Organometallics 1999, 18, 5447. (i) J ensen,
V. R.; Angermund, K.; J olly, P. W. Organometallics 2000, 19, 403. (j)
Dohring, A.; Gohre, J .; J olly, P. W. Organometallics 2000, 19, 388. (k)
Matare, C. J .; Foo, D. M.; Kane, K. M. Organometallics 2000, 19, 1534.
(l) Kim, W. K.; Fevola, M. J .; Liable-Sands, L. M. Organometallics 1998,
17, 4541.
(11) (a) Thomas, B. J .; Theopold, K. H. J . Am. Chem. Soc. 1988,
110, 5902. (b) Theopold, K. H. Acc. Chem. Res. 1990, 23, 263. (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) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold,
K. H. Organometallics 1996, 15, 5284. (f) White, P. A.; Calabrese, J .;
Theopold, K. H. Organometallics 1996, 15, 5473. (g) Theopold, K. H.
Chemtech 1997, 27, 26, and references therein.
Addition of trimethylaluminum (1.4 g, 19 mmol) to a
solution of 1 with 2 equiv of B(C6F5)3 in toluene changes
the color from green to red.15 Reaction with ethylene
under “open” conditions ([Cr] ) 1 × 10-3 M; 1 atm C2H4;
30 min reaction time; 23 °C maintained by an external
water bath) results in rapid monomer consumption, and
no polymer precipitation is observed. When “closed”
conditions were used ([Cr] ) 1 × 10-3 M; 15 psig C2H4;
30 min reaction time; 23 °C maintained by an external
water bath), the precipitation of low molecular weight
polyolefin occurs. The solid material accounts for ap-
proximately 50% of the total mass of ethylene consumed.
After hydrolysis and workup with aqueous base, the
organic layer was analyzed by GC/MS, and the results
of these reactions are summarized in Table 1. As shown
in Figure 2a, the reaction product is a distribution of
(12) (a) Rogers, J . S.; Bu, X.; Bazan, G. C. J . Am. Chem. Soc. 2000,
122, 730. (b) Rogers, J . S.; Bu, X.; Bazan, G. C. Organometallics 2000,
19, 3948.
(13) For representative examples of the use of boratabenzene
complexes in catalytic reactions see: (a) Bazan, G. C.; Rodriguez, G.;
Ashe, A. J ., III; Al-Ahmad, S.; Mu¨ller, C. J . Am. Chem. Soc. 1996, 118,
2291. (b) Bazan, G. C.; Rodriguez, G.; Ashe, A. J ., III; Al-Ahmad, S.;
Kampf, J . W. Organometallics 1997, 16, 2492. (c) Rogers, J . S.; Bazan,
G. C.; Sperry, C. K. J . Am. Chem. Soc. 1997, 119, 9305. (d) Barnhart,
R. W.; Bazan, G. C.; Mourey, T. J . Am. Chem. Soc. 1998, 120, 1082.
(e) Rogers, J . S.; Lachicotte, R. J .; Bazan, G. C. J . Am. Chem. Soc.
1999, 121, 1288.
(14) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1991,
113, 3623.
(15) When only 1 equiv of borane is added, the reactivity toward
ethylene is considerably reduced.

Catalytic Insertion of Ethylene
Organometallics, Vol. 20, No. 10, 2001 2061
Ta ble 1.
Ta ble 2.
entrya
activator
MAO
2 B(C6F5)3
2 B(C6F5)3
[Ph3C][B(C6F5)4]
2 B(C6F5)3
2 B(C6F5)3
MAOb
AlR3, R ) [Al]/[Cr]c solvent activityb
entrya
solvent
activityb
K
1
2
3
4
5
6
7
8
1000
380
380
380
380
380
1380
0
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
278
152
154
94
48
17
1
2
3
toluene
hexane
CH2Cl2
154
117
210
0.55
0.60
0.49
Me
Et
Et
isoBu
octyl
octyl
Reaction conditions: [Cr] ) 1 × 10-3 M; 23 °C; 30 min reaction
a
time; 15 psig C2H4; [B(C6F5)3]/[Cr] ) 2; [AlEt3]/[Cr] ) 380.
b
Activity reported in kg[product]/mol Cr‚h.
29
2 B(C6F5)3
139c
Reaction conditions: [Cr] ) 1 × 10-3 M; 23 °C; 30 min reaction
time; 15 psig C2H4; toluene solvent. b Activity reported in kg[prod-
uct]/molCr‚h. c Product is polyethylene (>95%).
a
F igu r e 3. GC/MS traces of the alkane products obtained
from the reaction of ethylene with (a) [1 + 2 B(C6F5)3 +
C2H4] with triethylaluminum (380 Al/Cr) in hexane; (b) [1
+ 2 B(C6F5)3] with triethylaluminum (380 Al/Cr) in CH2-
Cl2. The first three peaks are labeled according to molecular
weight.
reaction with trimethylaluminum (Table 1, entry 3). In
this case however, only even-carbon alkanes are detected
(Figure 2c). Other cocatalysts are suitable for activating
1. For example, 1 equiv of [Ph3C][B(C6F5)4]17 and 1
provide a catalyst (Table 1, entry 4) with slightly lower
activity compared to [1 + 2 B(C6F5)3].
The choice of solvent influences both the overall
activity and the chain length distribution. Keeping the
reaction variables constant (C2H4 pressure, tempera-
ture, and triethylaluminum concentration) and using
the less polar solvent hexane reduces ethylene uptake
and shifts product distribution to longer chains, as
judged by the observation of product precipitate (Table
2, entry 2, and Figure 3a) and the larger K factor of the
soluble fraction.18 By use of the more polar dichloro-
methane, one obtains a higher consumption rate and a
shift to shorter chains, as shown by the smaller K factor
(Table 2, entry 3, and Figure 3b).
F igu r e 2. GC/MS traces of the alkane products obtained
from the reaction of ethylene with (a) [1 + 2 B(C6F5)3] with
trimethylaluminum (380 Al/Cr); (b) MAO (1000 Al/Cr); and
(c) 2 B(C6F5)3 with triethylaluminum (380 Al/Cr). The first
three peaks are labeled according to molecular weight.
(>98%)16 odd-carbon alkanes (Table 1, entry 2) (eq 3).
For comparison, the distribution of alkanes using 1/MAO
(1000 overall Al/Cr) is provided in Figure 2b (Table 1,
entry 1). Note that the activity of 1 + 2 B(C6F5)3
+
trimethylaluminum toward ethylene consumption is
about half that obtained with 1/MAO under similar
reaction conditions (Table 1, entries 1 and 2).
Figure 4 shows the distribution of the mole fractions
of C10 through C22 alkanes as a function of triethyl-
aluminum concentration, as determined by using GC.
At higher aluminum concentrations, a shift to lighter
alkanes is observed. In the absence of triethylaluminum,
Performing the reaction using [1 + 2 B(C6F5)3] in the
presence of triethylaluminum under “closed” conditions
results in the exclusive formation of soluble products.
The total amount of ethylene consumed is similar to the
(17) Chien, J . C. W.; Tsai, W. M.; Rausch, M. D. J . Am. Chem. Soc.
1991, 113, 8570.
(16) The commercially available source of trimethylaluminum used
in these experiments was 97% pure. It is likely that the impurities in
this reagent are responsible for the minor distribution of even-number
alkanes.
(18) The K factor is defined by the molar ratio of the Cn+2 and Cn
products. This ratio also corresponds to the rate of propagation over
the sum of the rate of propagation and the rate of chain transfer: K )
(moles Cn+2 olefin)/(moles Cn olefin) ) RP/(RP + RCT).

2062 Organometallics, Vol. 20, No. 10, 2001
Bazan et al.
a 30 min period of time (Table 1, entry 6). The GC result
shows that the majority of the product from entry 6 in
Table 1 is C10H22 (72 mol %); in other words, the single
insertion product is predominant. For the reactions with
triisobutylaluminum, the ethylene uptake is also con-
siderably depressed, compared to triethylaluminun
(Table 1, entry 5). In the GC/MS trace of the products,
one observes a distribution of 2-methylalkanes (>98%).
These end groups arise from the isobutyl fragments
originally on aluminum.
It is possible to measure the rate of ethylene uptake
at a constant pressure by using a mass flow controller
attached to the reaction flask intake. Figure 5 shows
the total weight of ethylene incorporated into the
reaction as a function of time for different aluminum
alkyl groups. There appears to be no induction period.
In the case of trimethylaluminum and triethylalumi-
num, the consumption is close to linear with time,
indicating no decomposition of the catalytically active
species within 30 min. The dependence on triisobutyl-
aluminum is interesting. Within the first 10 min of the
reaction an activity similar to those of the trimethyl-
aluminum and the triethylaluminum reactions is ob-
served. However, after approximately 15 min the con-
sumption ceases. At this stage, approximately 2 equiv
of ethylene, relative to aluminum, have been incorpo-
rated into the reaction. Finally, for trioctylaluminum
only a sluggish uptake takes place.
F igu r e 4. Graph of the mole fractions of the C10 through
C22 products found using [1 + 2 B(C6F5)3 + C2H4] and
triethylaluminum at Al/Cr ratios of (a) 73; (b) 146; (c) 730;
and (d) 1460.
Ta ble 3.
entrya
[AlEt3]/[Cr]
activityb
K
1
2
3
4
5
73
146
380
730
1460
124
174
154
60
0.88
0.59
0.55
0.47
0.34
2
a
Reaction conditions: [Cr] ) 1 × 10-3 M; 23 °C; 30 min reaction
b
time; 15 psig C2H4; [B(C6F5)3]/[Cr] ) 2; toluene. Activity reported
in kg[product]/mol Cr‚h.
Ta ble 4.
d
entrya
activityb
mixing timec
Ceven/Codd
Su m m a r y Discu ssion
1
2
3
260
255
259
15 min
1.5 h
17 h
10
5
5
Thepold’s work,11 the isolation of 2, and our work with
[(C5H5B-Me)CrMe(PMe3)2][MeB(C6F5)3]12 suggest that
the product forms as shown in Scheme 1. Ethylene
insertion occurs at the cationic chromium alkyl species
of type I, where [Cr] ) [Cp*(Me3P)Cr]. Chain growth
at II is restricted by fast transmetalation reactions with
Al-R. This step produces the distribution of trialkyl-
aluminum compounds represented by III. Hydrolysis of
III gives the alkane product.
Reaction conditions: [Cr] ) 1 × 10-3 M; 23 °C; 30 min reaction
a
time; 15 psig C2H4; [B(C6F5)3]/[Cr] ) 2; toluene; [Al]/[Cr] ) 380;
b
initial [“AlMe3”]/[“AlEt3”] ) 1. Activity reported in kg[product]/
mol Cr‚h. c Time between the mixing of trimethylaluminum with
triethylaluminum and the reaction with 1, B(C6F5)3, and ethylene.
d
Determined by GC analysis.
one obtains polyethylene (Table 1, entry 8). Decreasing
the triethylaluminum concentration also increases the
activity toward ethylene (Table 3, entries 2-5). Entry
1 in Table 3 is an exception to this trend. Under these
conditions ([Al]/[Cr] ) 73) a substantial amount of
precipitate is observed. It is likely that catalyst entrap-
ment within the polymer and isolation from the mono-
mer may be responsible for the activity decrease.
Mixtures of trialkylaluminum reagents were also
examined. Table 4 shows the results for reactions that
contained an equimolar mixture of trimethylaluminum
and triethylaluminum. The oligomerization/transmeta-
lation reactions were done after the two reagents were
mixed neat for specific periods (∼15 min, 1.5 h, and 17
h). There is no obvious change in the overall reactivity
as a function of mixing time. However, the ratio of even-
carbon alkanes to odd-carbon alkanes changes from 10:1
in entry 1 to 5:1 in entries 2 and 3. The ethyl function-
ality is thus more prone to transmetalation, relative to
methyl.
The effect of triethylaluminum concentration on the
chain distribution of the product and the activity of the
catalytic species suggests an equilibrium between spe-
cies I (same I as in Scheme 1) and IV in eq 4. It is
La r ger Alk yl Ch a in s a n d Con su m p tion P r ofile.
Two additional commercially available trialkyaluminum
complexes were examined, trioctylaluminum and tri-
isobutylaluminum. For trioctylaluminum, the total eth-
ylene uptake is approximately an order of magnitude
lower than that observed with triethylaluminum, over
anticipated that ethylene insertion occurs at I, while
species such as IV are responsible for chain transfer to
aluminum. That the activity decreases with increasing
triethylaluminum concentration (Table 3) suggests that

Catalytic Insertion of Ethylene
Organometallics, Vol. 20, No. 10, 2001 2063
F igu r e 5. Graph of total ethylene mass consumption versus time with [1 + 2 B(C6F5)3/C2H4] using (a) trimethylaluminum;
(b) triethylaluminum; (c) triisobutylaluminum; and (d) trioctylaluminum.
Sch em e 1
It may be that the effectively higher concentration of
the reactive monomer for the ethyl-containing species
accounts for their increased participation in transmeta-
lation reactions. Trioctylaluminum is monomeric in
solution, and despite the increased steric interference
expected in species such as IV, these are generated more
readily, accounting for the low ethylene consumption
rates. There is no obvious explanation as to why the
reactions with triisobutylaluminum shut down after a
given time.
Exp er im en ta l Section
Gen er a l Rem a r k s. All manipulations were performed
under inert atmosphere using standard glovebox and vacuum
line techniques.24 The synthesis of 1 has been described
previously.8 Methylaluminoxane was obtained from Akzo
Chemical, and B(C6F5)3 was purchased from Boulder Scientific
Co. and used after sublimation. GC/MS analyses were done
using a Shimadzu GCMS-QP5000. GC analyses were done
using a Shimadzu GC-17A. Ethylene was obtained from
Matheson Co and passed through an oxygen/moisture trap.
Isola tion of [Cp *Cr Me(P Me3)(THF )][MeB(C6F 5)3] (2). A
solution of 1 (19 mg, 65 mmol) in 1 mL of toluene was added
to a solution of B(C6F5)3 (33 mg, 64 mmol), resulting in a color
change from purple to dark green. Addition of THF (∼1 mL)
produces a color change to dark brown. The solvent was
removed in vacuo, and the resulting sticky solid was dissolved
in Et2O/iPr2O (1:1). Cooling to -35 °C results in brown-black
crystals of 2. This solid is unstable at room temperature for
extended periods of time and is exceptionally air and moisture
sensitive.
IV does not undergo ethylene insertion. Bochmann and
co-workers have shown that species similar to IV can
be found in trimethylaluminum19 and triethylalumi-
num20 adducts of group 4 metallocene cations.21 In their
study, a similar trend in activity and molecular weight
with added trialkylaluminum was observed for the
polymerization of propylene.
Several observations are interesting and warrant
further study. For example, it is not clear at this stage
why the even-carbon products are preferentially ob-
tained when mixtures of triethylaluminum and tri-
methylaluminum are present (Table 3). In solution,
these species are predominantly in the form of dimers,
with the methyl species held together more tightly.22,23
(19) Bochmann, M.; Lancaster, S. J . Angew. Chem., Int. Ed. Engl.
1994, 33, 1634.
(20) Bochmann, M.; Lancaster, S. J . J . Organomet. Chem. 1995, 497,
Gen er a l Oligom er iza tion P r oced u r e. In a nitrogen-filled
glovebox, 50 µmol of 1 was weighted to the nearest 0.1 mg
and dissolved in toluene. This solution was combined with the
55.
(21) Bochmann, M.; Lancaster, S. J .; Robinson, O. B. J . Chem. Soc.,
Chem. Commun. 1995, 2081.
(22) Laubengayer, A. W.; Gilliam, W. F. J . Am. Chem Soc. 1941,
63, 477.
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2064 Organometallics, Vol. 20, No. 10, 2001
Bazan et al.
appropriate amount of activator (MAO, 10.3 wt % Al, 1000 Al/
Cr, or B(C6F5)3, 100 µmol, or [Ph3C][B(C6F5)4], 50 µmol) and
placed inside a 500 mL flat-bottom glass reactor with a
magnetic stir bar. Sufficient toluene was added to bring the
total volume to 50 mL. The glass reactor was fitted with a
reactor head and removed from the glovebox. The apparatus
was connected to an ethylene line and a pressure gauge via a
quick-connect adapter. The glass reactor was immersed in a
water bath at room temperature. Ethylene was introduced to
the glass reactor for approximately 1 min before activating a
mass flow controller. The ethylene pressure was maintained
at 15 psig for 30 min. The total ethylene consumption was
recorded every minute. After 30 min, the ethylene feed was
stopped and the reaction was quenched using 5-10 mL of
2-propanol at 0 °C, followed by same amount of water. Any
alumina salts were dissolved in aqueous base. Separation of
the toluene layer provides the product. The quantity of
ethylene consumed was determined by use of a flow mass
regulator or by weighing the entire reaction assembly before
and after ethylene addition; the two techniques are in agree-
ment to within (5%.
Ack n ow led gm en t. The authors are grateful to the
Department of Energy for financial assistance.
Su p p or tin g In for m a tion Ava ila ble: Complete details for
the crystallographic study of 2 (PDF). The material is available
free of charge via the Internet at http://pubs.acs.org.
OM001048O