[Cp*Cr(C6F5)(Me)(Py)] as a living chromium(III) catalyst for the aufbaureaktion

Article abstract of DOI:10.1021/om049675r

The reaction of [Cp*Cr(C₆F₅)(μ-Cl)] ₂ (1) with 2 equiv of MeLi in THF yields the methyl-bridged Cr(III) dimer [Cp*Cr(C₆F₅)(μ-Me)]₂ (2). This dinuclear compound is very soluble in hydrocarbon solvent and has been isolated in low yield (6%). Compound 2 reacts with pyridine to afford [Cp*Cr(C ₆F₅)(Me)(Py)] (3), which has been isolated in a 67% yield. Compound 3 is a 15-electron, coordinatively saturated chromium(III) species that has been characterized by NMR, magnetometry, and EA. The structures of 1, 2, and 3 have been determined by single-crystal X-ray diffraction. Compounds 1 and 2 exist in the form of centrosymmetrical dimers with bridging chloride and methyl ligands, respectively. Mononuclear compound 3 adopts the expected three-legged piano stool geometry. Compound 2 polymerizes ethylene in toluene under 1 atm of ethylene at room temperature in the absence of any activators. Compound 3 is not catalytically active by itself. Yet, the addition of excess AlEt₃ to a solution of 3 in toluene leads to a catalytic system that readily oligomerizes ethylene. Oligomerization experiments carried out with [3] = 10^-3 M and [AlEt₃] = 4.5 × 10^-2 M for 15 min lead to the production of ethylene oligomers with an activity of 221 kg mol Cr^-1 h^-1. As indicated by gas chromatography, the Poisson distribution formula accounts for the molecular weight distribution of the growing ethylene oligomers during this reaction, which is indicative of a living polymerization system. Experiments carried out at higher AlEt₃ concentrations ([3] = 10^-3 M and [AlEt₃] = 9 × 10^-2 M) lead to a lower activity (150 kg mol Cr^-1 h ^-1) but still present the characteristic features of a living polymerization system. These results are interpreted on the basis of a catalytic cycle in which the chain grows at chromium and is transferred to aluminum via an alkyl-bridged chromium-aluminum bimetallic intermediate.

Full text of DOI:10.1021/om049675r

4608
Organometallics 2004, 23, 4608-4613
[Cp *Cr (C6F 5)(Me)(P y)] a s a Livin g Ch r om iu m (III)
Ca ta lyst for th e “Au fba u r ea k tion ”
Mani Ganesan and Franc¸ois P. Gabba¨ı*
Chemistry Department, Texas A&M University, 3255 TAMU,
College Station, Texas 77843-3255
Received May 6, 2004
The reaction of [Cp*Cr(C6F5)(µ-Cl)]2 (1) with 2 equiv of MeLi in THF yields the methyl-
bridged Cr(III) dimer [Cp*Cr(C6F5)(µ-Me)]2 (2). This dinuclear compound is very soluble in
hydrocarbon solvent and has been isolated in low yield (6%). Compound 2 reacts with pyridine
to afford [Cp*Cr(C6F5)(Me)(Py)] (3), which has been isolated in a 67% yield. Compound 3 is
a 15-electron, coordinatively saturated chromium(III) species that has been characterized
by NMR, magnetometry, and EA. The structures of 1, 2, and 3 have been determined by
single-crystal X-ray diffraction. Compounds 1 and 2 exist in the form of centrosymmetrical
dimers with bridging chloride and methyl ligands, respectively. Mononuclear compound 3
adopts the expected three-legged piano stool geometry. Compound 2 polymerizes ethylene
in toluene under 1 atm of ethylene at room temperature in the absence of any activators.
Compound 3 is not catalytically active by itself. Yet, the addition of excess AlEt3 to a solution
of 3 in toluene leads to a catalytic system that readily oligomerizes ethylene. Oligomerization
experiments carried out with [3] ) 10-3 M and [AlEt3] ) 4.5 × 10-2 M for 15 min lead to the
production of ethylene oligomers with an activity of 221 kg mol Cr-1 h-1. As indicated by
gas chromatography, the Poisson distribution formula accounts for the molecular weight
distribution of the growing ethylene oligomers during this reaction, which is indicative of a
living polymerization system. Experiments carried out at higher AlEt3 concentrations ([3]
) 10-3 M and [AlEt3] ) 9 × 10-2 M) lead to a lower activity (150 kg mol Cr-1 h-1) but still
present the characteristic features of a living polymerization system. These results are
interpreted on the basis of a catalytic cycle in which the chain grows at chromium and is
transferred to aluminum via an alkyl-bridged chromium-aluminum bimetallic intermediate.
In tr od u ction
witnessed the emergence of an alternative strategy that
relies on the synergy of a transition metal catalyst and
a main group alkyl derivative.3-7 In such systems, the
chain effectively grows at the transition metal centers
and undergoes reversible transfer to the main group
center. Typically, this will involve alkyl-aluminum,
-magnesium, and -zinc derivatives as the main group
alkyl derivative and an olefin polymerization catalyst.3-7
If the rate of chain transfer between the transition metal
and the main group metal is fast and â-H processes do
not occur, the molecular weight of the resulting ethylene
oligomers follows a strict Poisson distribution. The latter
is economically very attractive because it provides the
narrowest molecular weight distribution available for
any polymerization processes. For this reason, the
Linear primary alcohols constitute important elemen-
tary building blocks that serve for the preparation of
detergents, lubricants, and cosmetic products among
others. These alcohols are typically produced by pro-
cesses based on the “Aufbaureaktion”,1 which involves
the multiple insertion of ethylene molecules in the
aluminum-carbon bond of a trialkylaluminum at el-
evated temperature and pressure. While this reaction
is successfully utilized in several industrial plants,
important drawbacks remain. In particular, selectivity
is often affected by the formation of olefin and associated
branching. Moreover, a number of safety problems
linked to elevated temperatures, pressures, and possible
runaway reactions are encountered when dealing with
the current technology. For these reasons, much focus
has been placed on the development of processes that
allow for the oligomerization of ethylene under milder
conditions. In this domain, Ni-based catalysts, also
referred to as SHOP catalysts, hold a prominent place
in the production of R-olefins.2 The past few years have
(3) (a) Gotz, C.; Rau, A.; Luft, G. Macromol. Mater. Eng. 2002, 287,
16-22. (b) J aniak, C.; Lange, K. C. H.; Marquardt, P. J . Mol. Catal.
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1993.
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Nagel, K.; Sauer, H.; Zosel, K. Angew. Chem., Int. Ed. Engl. 1952, 64,
323-329.
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10.1021/om049675r CCC: $27.50 © 2004 American Chemical Society
Publication on Web 09/02/2004

Living Cr(III) Catalyst for the “Aufbaureaktion”
Organometallics, Vol. 23, No. 20, 2004 4609
Sch em e 1
Poisson distribution supplants the Schultz-Flory dis-
tribution usually obtained when â-H processes are
operative.
ing [Cp*Cr(C6F5)(Me)(Py)] (3), which can be used as a
catalyst for the living “Aufbaureaktion”.
Resu lts a n d Discu ssion
Following the discovery of heterogeneous chromium
catalysts such as the Phillips catalyst (CrO3/SiO2)8 or
the Union Carbide catalyst (Cp2Cr/SiO2),9 the study of
homogeneous chromium species for the oligomerization
and polymerization of ethylene has attracted a great
deal of attention. In addition to promoting the trimer-
ization of olefins,10 a number chromium complexes also
catalyze olefin polymerization reactions. In this cat-
egory, those featuring a cyclopentadienide chromium-
(III) moiety have received the most attention and have
served as homogeneous models for the Union Carbide
catalyst.11-16 While the exact molecular structure of
such cyclopentadienide chromium(III) catalysts can be
varied, Theopold has clearly established that the pres-
ence of a Cr-Calkyl σ-bond as well as a vacancy in the
coordination sphere of the metal constitute the basic
prerequisites for catalytic activity toward ethylene.17
Although ethylene polymerization reactions have re-
ceived the most attention, Bazan showed that catalytic
systems such as [Cp*Cr(Me)2(PMe3)]/B(C6F5)3 and
[Cp*Cr(Me)2(PMe3)]/MAO are also competent for eth-
ylene oligomerization reactions when combined with a
trialkylaluminum.18 As part of our contribution to this
research area, we recently communicated a pentafluo-
rophenyl chromium(III) complex, [Cp*Cr(C6F5)(η3-Bz)],
that also promotes the oligomerization of ethylene when
combined with triethylaluminum.19 Unlike other sys-
tems, [Cp*Cr(C6F5)(η3-Bz)] does not necessitate the use
of an activator and produces linear alkyl chains that
strictly follow the Poisson distribution. Herein, we would
like to report the synthesis and characterization of
several pentafluorophenyl-chromium complexes includ-
Treatment of [Cp*CrCl2]2 with 1 equiv of C6F5Li at
-78 °C affords [Cp*Cr(C6F5)(µ-Cl)]2 (1) as previously
reported.19 Unlike [CpCr(X)(µ-X)]2 (X ) F, Cl, Br, I),20,21
compound 1 fails to react with pyridine and does not
afford the corresponding monomeric adduct. This inert-
ness indicates that the chloride-bridged dimer is very
stable. Possibly, the stability of this dimer results from
the electron-withdrawing nature of the C6F5 group,
which increases the Lewis acidity of the chromium
center. The reaction of 1 with 2 equiv of MeLi in THF
yields the methyl-bridged chromium(III) dimer [Cp*Cr-
(C6F5)(µ-Me)]2 (2) (Scheme 1). Although this compound
appears to form in high yield, it is difficult to isolate
because of its high solubility in hydrocarbon solvents.
It was obtained in less than 10% yield by recrystalli-
zation from pentane. Compound 2 is very air-sensitive
and readily reacts with pyridine to afford [Cp*Cr(C6F5)-
(Me)(Py)] (3) (Scheme 1). This reaction is analogous to
that observed in the case of [Cp*Cr(Me)(µ-Me)]2,22 which
converts into [Cp*Cr(Me)2(Py)] upon treatment with
pyridine. Analytically pure 3 was obtained in a 67%
yield by addition of pyridine to the reaction mixture of
1 and MeLi. Compound 3 is a 15-electron, coordinatively
saturated, chromium(III) species. The room-tempera-
ture magnetic moment (µeff) of 3.65 µB is close to the
value expected for three unpaired d-electrons (3.87 µB).
The structures of 1-3 have been determined by
single-crystal X-ray diffraction. Pertinent crystallo-
(15) Rogers, J . S.; Bu, X.; Bazan, G. C. Organometallics 1997, 16,
1511-1513.
(16) (a) Enders, M.; Ferna´ndez, P.; Ludwig, G.; Pritzkow, H.
Organometallics 2001, 20, 5005-5007. (b) Esteruelas, M. A.; Lo¨pez,
A. M.; Me¨ndez, L.; Oliva¨n, M.; On˜ate, E. Organometallics 2003, 22,
395-406.
(17) (a) Heintz, R. A.; Leelasubcharoen, S.; Liable-Sands, L. M.;
Rheingold, A. L.; Theopold, K. H. Organometallics 1998, 17, 5477-
5485. (b) Liang, Y.; Yap, G. P. A.; Rheingold, A. L.; Theopold, K. H.
Organometallics 1996, 15, 5284-5286. (c) Bhandari, G.; Kim, Y.;
McFarland, J . M.; Rheingold, A. L.; Theopold, K. H. Organometallics
1995, 14, 738-745. (d) Bhandari, G.; Rheingold, A. L.; Theopold, K.
H. Chem. Eur. J . 1995, 1, 199-203.
(8) (a) Hogan, J . P.; Banks, R. L. (Phillips Petroleum) U.S. Patent
2825721, 1958. (b) Hogan, J . P. J . Polym. Sci. A 1970, 8, 2637.
(9) (a) Karapinka, G. L. U.S. Patent 3,709,853, 1973. (b) Karol, F.
J .; Karapinka, G. L.; Wu, C.; Dow, A. W.; J ohnson, R. N.; Carrick, W.
L. J . Polym. Sci. A 1972, 10, 2621-2637.
(10) For the trimerization of R-olefins and ethylene, see: (a) Ko¨hn,
R. D.; Haufe, M.; Kociok-Ko¨hn, G.; Grimm, S.; Wasserscheid, P.; Keim,
W. Angew. Chem., Int. Ed. 2000, 39, 4337-4339. (b) McGuinness, D.
S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert, U.; Dixon, J . T.; Grove,
C. Chem. Commun. 2003, 334-335. (c) Carter, A.; Cohen, S. A.; Cooley,
N. A.; Murphy, A.; Scutt, J .; Wass, D. F. Chem. Commun. 2002, 858-
859.
(18) (a) Bazan, G. C.; Rogers, J . S.; Fang, C. C. Organometallics
2001, 20, 2059-2064. (b) Rogers, J . S.; Bazan, G. C. Chem. Commun.
2000, 1209-1210.
(19) Mani, G.; Gabbai, F. P. Angew. Chem., Int. Ed. 2004, 43, 2263-
2266.
(11) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2003, 103, 283-
315, and references therein.
(12) Theopold, K. H. Eur. J . Inorg. Chem. 1976, 15, 5-24, and
references therein.
(13) (a) White, P. A.; Calabrese, J .; Theopold, K. H. Organometallics
1996, 15, 5473-5475. (b) Thomas, B. J .; Noh, S.-K.; Schulte, G. K.;
Sendlinger, S. C.; Theopold, K. H. J . Am. Chem. Soc. 1991, 113, 893-
902.
(14) (a) Doehring, A.; J ensen, V. R.; J olly, P. W.; Thiel, W.; Weber,
J . C. Organometallics 2001, 20, 2234-2245. (b) J ensen, V. R.; Anger-
mund, K.; J olly, P. W.; Borve, K. J . Organometallics 2000, 19, 403-
410. (c) Emrich, R.; Heinemann, O.; J olly, P. A.; Kru¨ger, C.; Verhovnik,
G. P. J . Organometallics 1997, 16, 1511-1513.
(20) Braeunlein, B.; Koehler, F. H.; Strauss, W.; Zeh, H. Z. Natur-
forsch., B: Chem. Sci. 1995, 50, 1739-1747.
(21) Grohmann, A.; Kohler, F. H.; Muller, G.; Zeh, H. Chem. Ber.
1989, 122, 897-899.
(22) (a) Noh, S. K.; Sendlinger, S. C.; J aniak, C.; Theopold, K. H. J .
Am. Chem. Soc. 1989, 111, 9127-9129. (b) Richeson, D. S.; Mitchell,
J . F.; Theopold, K. H. Organometallics 1989, 8, 2570-2577.
(23) Kohler, F. H.; Lachmann, J .; Muller, G.; Zeh, H.; Brunner, H.;
Pfauntsch, J .; Watchter, J . J . Organomet. Chem. 1989, 365, C15-C18
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Theopold, K. H. J . Am. Chem. Soc. 1986, 108, 8273-8274.

4610 Organometallics, Vol. 23, No. 20, 2004
Ganesan and Gabba¨ı
Ta ble 1. Cr ysta l Da ta , Da ta Collection , a n d Str u ctu r e Refin em en t for 1, 2, a n d 3
1
2
3
Crystal Data
formula
Mr
C
32H30Cl2Cr2F10
C
34H36Cr2F10
C
22H23CrF5N
779.46
738.62
448.41
cryst size (mm3)
cryst syst
space group
a (Å)
0.4 × 0.21 × 0.11
monoclinic
P2(1)/c
14.113(4)
15.617(4)
29.042(7)
90.00
95.474(5)
90.00
6372(3)
16, 8
0.5 × 0.5 × 0.3
monoclinic
P2(1)/n
9.6564(18)
11.053(2)
14.406(3)
90.00
98.197(3)
90.00
1521.9(5)
4, 2
0.27 × 0.10 × 0.09
triclinic
P1h
11.229(4)
13.255(5)
16.119(6)
105.477(7)
109.348(7)
102.192(7)
2059.8(13)
2, 4
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z, Z′
F
calc (g cm-3
)
1.625
0.931
3152
1.612
0.800
756
1.446
0.607
924
µ(Mo KR) (mm-1
F(000)
)
Data Collection
110
T/K
110(2)
110
scan mode
hkl range
no. of measd reflns
ω
ω
ω
-16f13, -17f14,-33f33
-11f11, -12f13,-17f9
-11f13, -15f15,-19f14
29 874
7770
10 716
no. of unique reflns, [Rint
no. of reflns used for refinement
]
9981 [0.0581]
9981
2685 [0.0456]
2685
7084 [0.0870]
7084
Refinement
no. of refined params
829
220
535
R1,a wR2 [I > 2σ(I)]
0.0652, 0.1459
1.593, -0.514
0.0327, 0.0867
0.359, -0.255
0.0697, 0.1374
0.675, -0.379
b
F
fin(max/min) (e Å-3
)
R1 ) (Fo - Fc)/Fo. wR2 ) {[w(Fo - Fc2)2]/[w(Fo2)2]}1/2; w ) 1/[σ2(Fo2) + (ap)2 + bp]; p ) (Fo + 2Fc2)/3; for 1, a ) 0.0600, b ) 0; for
a
b
2
2
2, a ) 0.0505, b ) 0; for 3, a ) 0.00, b ) 0.0494.
graphic data have been assembled in Table 1. Com-
pound 1 crystallizes in the P2(1)/c monoclinic space
group. The asymmetric unit contains four independent
Cp*Cr(C6F5)(Cl) moieties. Two of these moieties are
associated in a [Cp*Cr(C6F5)(µ-Cl)]2 dimer. The other
two moieties constitute the halves of two crystallo-
graphically centrosymmetrical [Cp*Cr(C6F5)(µ-Cl)]2
dimers. Altogether, the structures of the three indepen-
dent [Cp*Cr(C6F5)(µ-Cl)]2 are analogous to that of other
Cp*Cr chloro complexes (Figure 1).17a,c,23 The coordina-
tion geometry about the chromium center is pseudooc-
tohedral. The average Cr-Cl of 2.375 Å is comparable
to the Cr-Cl distances observed in other chromium(III)
chloride-bridged complexes such as [CpCr(Me)(µ-Cl)]2
(av 2.356 Å),24 [Cp*Cr(CH2SiMe3)(µ-Cl)]2 (av 2.394 Å),17a
or [Cp*Cr(Bz)(µ-Cl)]2 (av 2.383 Å).17c The two C6F5
groups are coordinated to the chromium centers trans
to each other. The average length of the Cr-C bond
(2.106 Å) formed between the chromium center and the
ipso-carbon of the pentafluorophenyl group is virtually
identical to that found in [Cp*Cr(C6F5)(η3-Bz)] (Cr(1)-
C(11) ) 2.109(3) Å).19 In 2, each chromium atom is
bonded to one Cp* ligand, one terminal C6F5 group, and
two bridging methyl ligands, thus completing the fa-
miliar pseudooctahedral coordination environment of
the three-legged piano stool geometry (Figure 2). The
lengths of the Cr-C bonds formed by the bridging
methyl group (Cr(1)-C(1A) ) 2.156(2) and Cr(1)-C(1)
) 2.205(2) Å) are similar to those found in other methyl-
bridged chromium species such as [Cp*Cr(Me)(µ-Me)]2
(Cr-C ) 2.206(4), 2.170(5) Å).22a As in 1, the two C6F5
groups are coordinated to the chromium centers trans
to each other and form a Cr(1)-C(12) bond of 2.1375(19)
Å, which is similar to 1. The Cr-Cr distance of 2.6971-
(7) Å in 2, which is slightly longer than that found in
F igu r e 1. Molecular structure of [Cp*Cr(C6F5)(µ-Cl)]2 (1).
Selected interatomic distances (Å) and angles (deg) for one
of the independent molecules. The distances and angles
corresponding to the other two independent molecules can
be found in the Supporting Information. Cr(1)-C(41)
2.109(6), Cr(1)-Cl(2) 2.3701(18), Cr(1)-Cl(1) 2.3801(18),
Cr(2)-C(47) 2.099(6), Cr(2)-Cl(1) 2.3708(18), Cr(2)-Cl(2)
2.3779(18), C(41)-Cr(1)-Cl(2) 96.45(17), C(41)-Cr(1)-
Cl(1) 95.33(17), Cl(2)-Cr(1)-Cl(1) 89.55(6), C(47)-Cr(2)-
Cl(1) 93.99(17), C(47)-Cr(2)-Cl(2) 97.33(17), Cl(1)-Cr(2)-
Cl(2) 89.58(6), Cr(2)-Cl(1)-Cr(1) 90.39(6), Cr(1)-Cl(2)-
Cr(2) 90.46(6).
[Cp*Cr(Me)(µ-Me)]2 (2.606(2) Å), indicates the presence
of a weak metal-metal bond.22 Compound 3 crystallizes
with two molecules in the asymmetric unit. Both
independent molecules, arbitrarily denoted as molecule
A and molecule B, adopt the expected three-legged piano
stool geometry and feature very similar structures

Living Cr(III) Catalyst for the “Aufbaureaktion”
Organometallics, Vol. 23, No. 20, 2004 4611
F igu r e 4. GC traces of the alkanes produced after hy-
drolysis from the reaction of ethylene with 3 and AlEt3 ([3]
) 10-3 M, [AlEt3]/[3] ) 90; P ) 1.1 atm, T ) 25 °C).
Sch em e 2a
F igu r e 2. Molecular structure of [Cp*Cr (C6F5)(µ-CH3)]2
(2). Selected interatomic distances (Å) and angles (deg):
Cr(1)-C(12), 2.1375(19); Cr(1)-C(1A), 2.156(2); Cr(1)-C(1),
2.205(2); Cr(1)-Cr(1A), 2.6971(7); C(12)-Cr(1)-C(1A),
98.33(8); C(12)-Cr(1)-C(1), 96.28(7); C(1A)-Cr(1)-C(1),
103.61(6); C(12)-Cr(1)-Cr(1A), 101.85(5); C(1A)-Cr(1)-
Cr(1A), 52.62(5); C(1)-Cr(1)-Cr(1A), 50.99(5); Cr(1A)-
C(1)-Cr(1), 76.39(6).
a
m, n, o ) integers
sociated with its isolation as well as its high air
sensitivity impeded a detailed investigation of its cata-
lytic properties. For these reasons, we turned our
attention to compound 3. Unlike for 2 and [Cp*Cr(C6F5)-
(η3-Bz)], pure 3 is not catalytically active when dissolved
in toluene and exposed to ethylene. This lack of activity
is to be expected in the case of coordinatively saturated
derivatives such as 3. Fortunately, the simple addition
of excess AlEt3 to a solution of 3 in toluene leads to a
catalytic system that readily oligomerizes ethylene
(Scheme 2). Presumably, the Lewis acidic AlEt3 is able
to abstract a ligand, thereby generating an unsaturated
chromium center. For example, when a solution of 3 in
toluene (10-3 M) containing a 45-fold excess of AlEt3 is
exposed to ethylene (1.1 atm) at room temperature,
rapid consumption of ethylene is observed (2.76 g after
15 min, activity ) 221 kg mol Cr-1 h-1). While no
precipitation occurs during the first 13-14 min, a small
amount of precipitate is typically observed after 14-15
min. The precipitate most likely consists of the trialkyl-
aluminum species bearing the longer alkyl chains. After
15 min exposure to ethylene, hydrolysis of the reaction
mixture leads to the formation of linear alkanes with
an average chain length of 32 (1 carbon units, as
determined from 1H NMR spectroscopy. This number
corresponds almost exactly to the ratio of ethylene
consumed per ethyl group (31.2). When the same
experiment is carried out with a 90-fold excess of AlEt3,
1.88 g of ethylene are consumed in 15 min, indicating a
lower activity (150 kg mol Cr-1 h-1). Under these
conditions, shorter ethylene oligomers are produced
(average chain length ) 12 ( 1 carbon), and the
formation of a precipitate toward the end of the reaction
is not observed. Gas chromatography analysis of the
longer oligomers (gC10) shows that the chain length
distribution closely matches that predicted by the Pois-
F igu r e 3. Molecular structure of [Cp*Cr(C6F5)(Me)(Py)]
(3). Selected interatomic distances (Å) and angles (deg) for
molecule A, the distances and angles corresponding to the
other independent molecule (B) are indicated in square
brackets: Cr(1)-C(1), 2.073(7) [2.075(7)]; Cr(1)-N(1),
2.116(6) [2.114(6)]; Cr(1)-C(12) 2.126(6) [2.116(6)]; C(1)-
Cr(1)-N(1), 92.2(2) [92.1(2)]; C(1)-Cr(1)-C(12), 95.2(3)
[94.3(3)]; N(1)-Cr(1)-C(12), 98.0(2) [99.0(2)].
(Figure 3). As expected, the bond formed between the
chromium center and the terminal methyl group (Cr-
(1)-C(1) 2.073(7) Å for A and Cr(2)-C(18) 2.075(7) Å
for B) is shorter than those observed in 2 for the
bridging methyl group (Cr(1)-C(1A) ) 2.156(2) Å and
Cr(1)-C(1) ) 2.205(2) Å). The pentafluorophenyl ligand
is terminally ligated to the chromium center and forms
a Cr-C bond (Cr(1)-C(12) 2.126(6) Å for A and Cr(2)-
C(29) 2.116(6) Å for B) comparable to that found in the
mononuclear complex [Cp*Cr(C6F5)(η3-Bz)] (Cr(1)-C(11)
) 2.109(3) Å).
(25) Flory, P. J . J . Am. Chem. Soc. 1940, 62, 1561-1565.
(26) (a) Bochmann, M.; Lancaster, S. J . Angew. Chem., Int. Ed. Engl.
1994, 33, 1634-1637. (b) Bochmann, M.; Lancaster, S. J . J . Organomet.
Chem. 1995, 497, 55-59.
Although compound 2 polymerizes ethylene in toluene
under 1 atm of ethylene at room temperature as
observed for [Cp*Cr(C6F5)(η3-Bz)],19 the difficulties as-

4612 Organometallics, Vol. 23, No. 20, 2004
Ganesan and Gabba¨ı
Sch em e 3. P r op osed Ca ta lytic Cycle for th e
Oligom er iza tion Rea ction (R ) oligom er ,
L ) liga n d s)
son distribution formula (eq 1, Figure 4) and is indica-
tive of a living polymerization system.25
øn ) mole fraction of C2xH4x+2 ) (ν(x-1)‚e-ν)/(x - 1)!
F igu r e 5. Experimental and calculated chain length
distribution of the alkanes formed, after hydrolysis of the
reaction mixture obtained at 3 min intervals, by the
reaction of ethylene with 3 and AlEt3 ([3] ) 10-3 M, [AlEt3]/
[3] ) 45; P ) 1.1 atm, T ) 25 °C). ([: 3 min, ν ) 2.5. 9:
6 min, ν ) 5.4. b: 9 min, ν ) 8.8. 2: 12 min, ν ) 11.6). The
inset (top right) shows a plot of the average number of
added ethylene (ν) versus time.
(1)
(ν ) av number of added ethylene, x )
number of ethylene units, n ) 2x)
These observations parallel those made previously
with [Cp*Cr(Me)2(PMe3)]/B(C6F5)3, [Cp*Cr(Me)2(PMe3)]/
MAO, or [Cp*Cr(C6F5)(η3-Bz)] as catalysts.18,19 As previ-
ously suggested, the inhibitory effect observed at higher
AlEt3 concentration most likely results from saturation
of the transition metal center through the formation of
a heterobimetallic complex of type C (Scheme 3), whose
concentration increases at higher AlEt3 concentrations.
Cationic examples of such bimetallic intermediates have
been postulated by Bazan18 and are also known in
zirconium chemistry.26 These observations are in agree-
ment with a proposed catalytic cycle in which the
growing alkyl chain is transferred from chromium to
aluminum (Scheme 3). While the exact nature of the
active chromium species is unknown, we propose that
this exchange reaction involves a bridged chromium-
aluminum complex of type C.
While the methyl derivative [Cp*Cr(C6F5)(µ-Me)]2 (2)
is very air sensitive and somewhat difficult to isolate,
its corresponding pyridine adduct, namely, [Cp*Cr-
(C6F5)(Me)(Py)] (3), can be isolated in high yield and
stored for an extended amount of time under a dry, inert
atmosphere. In the presence of excess AlEt3, this
compound promotes the oligomerization of ethylene. Its
activity is comparable to that of [Cp*Cr(C6F5)(η3-Bz)].19
Remarkably, 3 is a neutral catalyst that promotes the
“Aufbaureaktion” in the absence of MAO or B(C6F5)3.
In contrast to the conventional “Aufbaureaktion”, which
requires elevated temperatures and ethylene pressures,
the use of 3 allows for the living oligomerization of
ethylene at room temperature and under 1 atm of
ethylene.
To get more detailed insight of the progress of the
oligomerization reaction as a function of time, the
reaction mixture of a typical oligomerization experiment
([3] ) 10-3 M, [AlEt3] ) 45/[3]) was sampled every 3
min, hydrolyzed, and analyzed by gas chromatography
(Figure 5). The molecular weight distribution observed
at each time interval closely matches the Poisson
distribution. A similar observation was made by Gibson
et al. in the case of an iron-catalyzed ethylene oligo-
merization on zinc dialkyls.27 In the present case, the
average numbers of added ethylene (ν) that provides the
best fit for each experimental distributions are ν ) 2.5,
5.4, 8.8, and 11.6 after 3, 6, 9, and 12 min, respectively.
A plot of the average number of added ethylene (ν)
versus time indicates that ν increases linearly with time.
This feature is in agreement with the existence of a
living polymerization system; it also shows that, in the
fairly narrow window of the growth reaction, the oligo-
merization rate remains essentially constant and is not
influenced by the increasing length of the chains.
Exp er im en ta l Section
Gen er a l P r oced u r es. All manipulations were performed
under an inert atmosphere using standard glovebox and
vacuum-line techniques. MeLi (1.5 M diethyl ether solution)
was obtained from Aldrich. Pyridine, toluene, THF, hexanes,
and pentane were dried over sodium and distilled prior to use.
1H NMR spectra were recorded on a Varian VXR-300 instru-
ment at 298 K. The chemical shifts are reported in ppm
relative to SiMe4. Elemental analyses were performed by
Atlantic Microlab, Inc., Norcross, GA. Gas chromatography
analyses were performed in the temperature range 70-290
°C using Hewlett-Packard, HP 6890 Series, column specifica-
tions: Agilent Technologies Inc. HP5, 30 m × 0.25 mm;
phase: 5% phenyl poly(dimethylsiloxane), 0.25 µm. The GC
response was calibrated with a standard containing C14H30
15H32, and C16H34 in 0.033 wt % in hexane. Magnetic
,
C
susceptibility measurements were carried out with a Quantum
Design SQUID MPMS-XL magnetometer. DC magnetic mea-
surements were performed with an applied field of 1000 G in
the 2-300 K temperature range. Data were corrected for the
diamagnetic contributions calculated from the Pascal con-
stants.28 The magnetic moment was derived from the suscep-
tibility at room temperature. [Cp*Cr(C6F5)(µ-Cl)]2 (1) was
synthesized as previously reported.19
Con clu sion
The synthetic results presented herein indicate that
chromium(III) complexes bearing a Cp* ligand as well
as a C6F5 ligand can be readily prepared and isolated.
(27) Britovsek, G. J . P.; Cohen, S. A.; Gibson, V. C.; Maddox, P. J .;
Van Meurs, M. Angew. Chem., Int. Ed. 2002, 41, 489-491.
(28) Theory and Applications of Molecular Paramagnetism; Bou-
dreaux, E. A., Mulay, L. N., Eds.; J ohn Wiley & Sons: New York, 1976.

Living Cr(III) Catalyst for the “Aufbaureaktion”
Organometallics, Vol. 23, No. 20, 2004 4613
Cr ysta llogr a p h y. Crystals of 1 were grown from CH2Cl2
at -30 °C. Crystals of 2 and 3 were obtained upon cooling of
pentane solutions (vide infra). The crystals were mounted onto
a glass fiber with Apiezon grease. The crystallographic mea-
surements were performed on a Bruker SMART 1000 X-ray
three-circle diffractometer. The goniometer was controlled
using the SMART software suite, version 5.625. The sample
was optically centered with the aid of a video camera such that
no translations were observed as the crystal was rotated
through all positions. The detector was set at 5 cm from the
crystal sample (CCD-PXL-KAF2, SMART 1000, 512 × 512
pixel). The X-ray radiation employed was generated from a
Mo sealed X-ray tube (KR ) 0.70173 Å with a potential of 50
kV and a current of 40 mA) and filtered with a graphite
monochromator in the parallel mode (175 mm collimator with
0.5 mm pinholes). For all three measurements, dark currents
were obtained for the appropriate exposure time of 10 s and a
rotation exposure was taken to determine crystal quality and
the X-ray beam intersection with the detector. The beam
intersection coordinates were compared to the configured
coordinates, and changes were made accordingly. The rotation
exposure indicated acceptable crystal quality, and the unit cell
determination was undertaken. Sixty data frames were taken
at widths of 0.3° with an exposure time of 10 s. Over 100
reflections were centered, and their positions were determined.
These reflections were used in the autoindexing procedure to
determine the unit cell. A suitable cell was found and refined
by nonlinear least squares and Bravais lattice procedures. The
unit cell was verified by examination of the hkl overlays on
several frames of data, including zone photographs. The cell
was verified and refined using at least 500 reflections obtained
after data collections. After careful examination of the unit
cell, a standard data collection procedure was initiated. This
procedure consists of collection of one hemisphere of data
collected using omega scans, involving the collection of 1265
0.3° frames at fixed angles for φ, 2θ, and ø (2θ ) -28°, ø )
54.73°), while varying omega. Each frame was exposed for 10
or 15 s and contrasted against a 10 or 15 s dark current
exposure. The total data collection was performed at 110 K.
No significant intensity fluctuations of equivalent reflections
were observed. Integrated intensity information for each
reflection was obtained by reduction of the data frames with
the program SAINT V6.63. The integration method employed
a three-dimensional profiling algorithm, and all data were
corrected for Lorentz and polarization factors, as well as for
crystal decay effects. Finally the data were merged and scaled
to produce a suitable data set. The structures were solved by
direct methods, which successfully located most of the non-
hydrogen atoms. Subsequent refinement on F2 using the
SHELXTL/PC package (version 6.1) allowed for the location
of the remaining non-hydrogen atoms. All non-hydrogen atoms
were refined with anisotropic thermal parameters. The hy-
drogen atoms bound to carbon were placed in idealized
positions [C-H ) 0.96 Å, Uiso(H) ) 1.2Uiso(C)].
Syn th esis of [Cp *Cr (C6F 5)(Me)(P y)] (3). MeLi (0.54
mmol) and subsequently pyridine (0.1 mL) were added to a
solution of [Cp*Cr(C6F5)(µ-Cl)]2 (1) (0.21 g, 0.27 mmol) in THF
(30 mL) at room temperature. The solution was stirred for 18
h, during which time the color of the solution turned to dark
brown. The solvent was removed under vacuum, and the
resulting residue was extracted with pentane (30 mL). The
pentane solution was filtered and concentrated to 15 mL. The
solution was cooled to 5 °C for 2 days to give dark red crystals
of 3 (0.162 g, 0.36 mmol, 67%). Anal. Calcd for C22H23CrF5N:
C, 58.93; H, 5.13. Found: C, 58.42; H, 5.21. 1H NMR (benzene-
d6) δ: 2.2 (br, Me-Cp*), 22.65 (br). µeff ) 3.65 µB.
Gen er a l Eth ylen e Oligom er iza tion Exp er im en ts. The
catalyst (50 µmol) was placed in a 200 mL Schlenk flask and
dissolved in toluene (50 mL). Following addition of the ap-
propriate amount of AlEt3, the flask was weighed, placed in a
room-temperature water bath, and connected to an ethylene
manifold. Before each experiment, the flask was evacuated for
5 s and refilled with 1.1 atm of ethylene, to which it remained
exposed for 15 min. The ethylene feed was then discontinued,
and the flask was weighed for activity measurements. The
reaction was quenched using water (10 mL) at 0 °C. The
toluene layer was separated for 1H NMR and gas chromatog-
raphy analysis.
Eth ylen e Oligom er iza tion Usin g 3 a s a Ca ta lyst w ith
AlEt3. A Schlenk flask was charged with a toluene solution
(50 mL) of 3 (0.022 g, 0.050 mmol) and AlEt3 (0.513 g, 4.50
mmol). The ethylene oligomerization reaction was carried out
as described above. After 15 min, 1.88 g of ethylene had been
consumed, giving an activity of 150 kg mol Cr-1 h-1. Following
quenching, gas chromatography analysis of the alkanes in the
crude toluene phase provided an average chain length of 12 (
1 carbon units. The same experiment carried out with 3 (0.022
g, 0.050 mmol) and AlEt3 (0.256 g, 2.25 mmol) led to an
ethylene consumption of 2.76 g and an activity of 221 kg mol
Cr-1 h-1. Following quenching, 1H NMR analysis of the alkanes
(δ (ppm): 1.30 (br, CH2, 62 H), 0.91 (br, CH3, 6 H)) in the crude
toluene phase provided an average chain length of 32 ( 1
carbon units. The average molecular weight of the sample
exceeded the range that can be reliably analyzed with our gas
chromatograph. In a separate experiment carried out with 3
(0.022 g, 0.050 mmol) and AlEt3 (0.256 g, 2.25 mmol), aliquots
(1 mL) were taken by syringe at 3, 6, 9, and 12 min. Each
aliquot was quenched with water (1 mL) and analyzed by gas
chromatography. The experimental distributions were fitted
using the Poisson formula, which gave the following average
numbers of added ethylene (ν): ν ) 2.5, 5.4, 8.8, and 11.6 after
3, 6, 9, and 12 min, respectively.
Ack n ow led gm en t. This work was supported by
Sasol North America, Inc. We thank Kurt McWilliams,
Don Wharry, and David Pope for helpful discussions.
The purchase of the X-ray diffractometer was made
possible by a grant from the National Science Founda-
tion (CHE-98 07975). We also would like to thank Curtis
Berlinguette and Andrey Prosvirin for the magnetic
measurements. We thank J oe Reibenspies for his help
with the crystallography.
Syn th esis of [Cp *Cr (C6F 5)(µ-Me)]2 (2). MeLi (0.51 mmol)
was added to a solution of [Cp*Cr(C6F5)(µ-Cl)]2 (1) (0.20 g, 0.25
mmol) in THF (30 mL) at room temperature, upon which the
color of the solution changed from dark blue to dark bluish-
brown. Following stirring for 18 h, the solvent was removed
under vacuum, and the residue was extracted with pentane.
The resulting solution was filtered and the filtrate concen-
trated to 15 mL. Cooling to -30 °C for 7 days gave dark purple
Su p p or tin g In for m a tion Ava ila ble: Molecular weight
distribution analysis. X-ray crystallographic data for 1, 2, and
3 in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
1
crystals of 2 (0.012 g, 0.016 mmol, 6.4%). H NMR (benzene-
d6) δ: 0.28 (br, CH3), 2.27 (br, Cp*). Satisfactory analysis could
not be obtained because of the high air-sensitive nature of the
compound.
OM049675R