Highly active ethylene oligomerization catalysts

Article abstract of DOI:10.1021/om2002359

Reaction of RN(H)PBr(Ph)₂N(H)R [R = t-Bu, i-Pr, Ph] with two equivalents of n-BuLi followed by reaction with CrCl₂(THF) ₂ afforded the divalent chromium complexes [(t-Bu)NP(Ph) ₂N(t-Bu)]Cr(μ²-Cl)₂Li(THF)₂ (1), [(i-Pr)NP(Ph)₂N(i-Pr)]Cr(μ²-Cl)₂Li(THF) ₂ (2), and [{[(Ph)NP(Ph)₂N(Ph)]Cr}₂(μ ²-Cl)₃][Li(DME)₃] (3). The trivalent analogue of 1, {[(t-Bu)NP(Ph)₂N(t-Bu)]Cr(μ²-Cl) ₃(μ³-Cl)₂}{Li (THF)₂} (4), was obtained in a similar manner via treatment of the double deprotonated ligand with CrCl₃(THF)₃. Both reactions of the divalent1 or trivalent4 with AlMe₃ yielded the trivalent and cationic complex {[(t-Bu)NP(Ph)₂N(t-Bu)]₂Cr}{(Me₃Al) ₂Cl}?toluene (5). Upon activation with MAO, 1-3 produced unprecedented and potentially useful catalytic systems for nonselective ethylene oligomerization devoid of polymer. Divalent chromium is clearly the species responsible for the catalytic behavior, ruling out that nonselective oligomerization proceeds via a redox metallacycle mechanism. The absence of polymer in combination with the record activity makes 1 competitive with the best performing industrially used systems.

Full text of DOI:10.1021/om2002359

ARTICLE
pubs.acs.org/Organometallics
Highly Active Ethylene Oligomerization Catalysts
Khalid Albahily,† Sebastiano Licciulli,† Sandro Gambarotta,*,† Ilia Korobkov,† Reynald Chevalier,‡
Kathrine Schuhen,‡ and Robbert Duchateau§
†Department of Chemistry, University of Ottawa, Ottawa, Ont K1N 6N5, Canada
‡Basell Polyolefine GmbH, Industriepark Hoechst, D-65926 Frankfurt am Main, Germany
§Department of Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherland,
S Supporting Information
b
ABSTRACT: Reaction of RN(H)PBr(Ph)2N(H)R [R = t-Bu, i-Pr, Ph] with two equivalents of n-BuLi
followed by reaction with CrCl2(THF)2 afforded the divalent chromium complexes [(t-Bu)NP(Ph)2-
N(t-Bu)]Cr(μ2-Cl)2Li(THF)2 (1), [(i-Pr)NP(Ph)2N(i-Pr)]Cr(μ2-Cl)2Li(THF)2 (2), and [{[(Ph)-
NP(Ph)2N(Ph)]Cr}2(μ2-Cl)3][Li(DME)3] (3). The trivalent analogue of 1, {[(t-Bu)NP(Ph)2-
N(t-Bu)]Cr(μ2-Cl)3(μ3-Cl)2}{Li (THF)2} (4), was obtained in a similar manner via treatment of the
double deprotonated ligand with CrCl3(THF)3. Both reactions of the divalent 1 or trivalent 4 with AlMe3
yielded the trivalent and cationic complex {[(t-Bu)NP(Ph)2N(t-Bu)]2Cr}{(Me3Al)2Cl} toluene (5).
3
Upon activation with MAO, 1ꢀ3 produced unprecedented and potentially useful catalytic systems for
nonselective ethylene oligomerization devoid of polymer. Divalent chromium is clearly the species
responsible for the catalytic behavior, ruling out that nonselective oligomerization proceeds via a redox
metallacycle mechanism. The absence of polymer in combination with the record activity makes 1
competitive with the best performing industrially used systems.
’ INTRODUCTION
Alkylating agents can then reduce the metal center to either the
di- or monovalent state, each with its own distinctive catalytic
behavior.14 What is remarkable is that the original trivalent state
may also be regenerated during the catalytic cycles via simple
disproportionation.15 Further reduction to the zerovalent state
may also occur during the catalytic cycle, eventually affording
metallic chromium or inert complexes with consequent catalyst
deactivation. The easy interconversion of the three catalytically
active oxidation states (þI, þII, and þIII) and their occasional
coexistence within the catalytic cycle explains the frequent
occurrence of much unwanted polymer during the production
of LAO and/or the occasional enrichment of the SꢀF distribu-
tion of R-olefins in 1-hexene and/or 1-octene.11oꢀs,12ꢀ16
The nature of the ligand plays a central role in stabilizing a
particular oxidation state and consequently in determining the
catalytic behavior. Previous work from our lab on an anionic
ligand system based on the NPN framework of trivalent phos-
phorus has clearly indicated that this ligand frame is versatile for
catalytic purposes, having allowed isolation of rare species and
switchable catalytic systems.17 In addition, we observed that it
provides a reversal of the PNP motif of the unique Sasol catalytic
tetramerization system.16b,c,17,18 However, its chemistry ap-
peared to be distinctively different. During the activation pro-
cess of its chromium derivatives, the ligand-P atom was always
attacked by the alkylating agent used for catalyst activation.17
The alkylation at the P atom in turn caused retention of alkyl
A considerable amount of research effort has been dedicated
both recently and in the past to nonselective ethylene oligomer-
ization with the aim of improving the comprehension of this
industrially relevant catalytic process.1ꢀ5 The mixtures of linear
alpha olefins (LAO) produced by this process are in fact valu-
able commodity chemicals for a range of industrial and house-
hold applications depending on their molar mass distribution
(detergents, surfactants, cosmetics, etc.).6
A nonselective oligomerization is closely reminiscent of a
polymerization randomly truncated at the early stages of the chain
growth (CosseeꢀArlman mechanism).7 The commonly accepted
CosseeꢀArlman chain-growth mechanism does not require a
change of the metal oxidation state during the catalytic cycle. In
some cases however, the redox metallacycle mechanism proposed
for selective tri- and tetramerization8 may also be ensued by
nonselective systems.9 This is in the event that the two crucial
steps, further ring expansion and reductive elimination, possess
comparable activation barriers.10 Obviously, the possibility of dis-
criminating between the two steps is central to obtain a general
statistical oligomerization versus a selective process.
Trivalent chromium catalyst precursors constitute catalytic
systems for both selective11ꢀ15,16a and nonselective ethylene
oligomerization12 and also account for an important class of
ethylene polymerization catalysts.13 The versatility of this ele-
ment is likely to be ascribed to the large number of oxidation
states available but most of all to the redox dynamism of the
organochromium derivatives.14 More specifically, the trivalent
state is normally used to feed the metal into the catalytic cycle.
Received: March 17, 2011
Published: May 27, 2011
r
2011 American Chemical Society
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Organometallics
ARTICLE
Table 1
1
2
3
4
5
formula
Mw
C
35H52Cl2CrLiN2O2P
C28H44Cl2CrLiN2O2.5
609.46
P
C64H80Cl3Cr2LiN4O8P2
1312.55
C48H72Cl5Cr2LiN4O2P2
1087.23
C53H82Al2ClCrN4P2
978.58
693.60
space group
a (Å)
monoclinic, P2(1)/n
9.911(2)
16.784(3)
22.967(5)
90
triclinic, P1
11.552(3)
12.374(3)
14.246(3)
67.528(4)
75.972(5)
70.253(4)
1756.1(7)
2
monoclinic, C2/c
28.944(3)
14.4854(15)
22.613(2)
90
triclinic, P1
12.2348(19)
12.872(2)
18.296(3)
71.893(3)
85.406(3)
87.256(3)
2729.1(7)
2
monoclinic, C2/c
13.8861(12)
23.091(2)
18.7679(16)
90
b (Å)
c (Å)
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
91.307(3)
90
128.525(2)
90
105.589(2)
90
3819.7(13)
4
7417.4(13)
4
5796.4(9)
4
radiation
0.71073
201(2)
0.71073
0.71073
0.71073
0.71073
T (K)
200(2)
200(2)
200(2)
200(2)
Dcalcd (g cmꢀ3
)
1.206
1.153
1.175
1.323
1.121
μcalcd (mmꢀ1
)
0.512
0.549
0.493
0.741
0.364
F000
1472
644
2752
1140
2100
R, wR2
GoF
0.0769, 0.2267
1.037
0.0774, 0.1621
1.022
0.0738, 0.1651
1.011
0.0839, 0.1859
0.989
0.0866, 0.2002
1.082
aluminum residues, in the end enabling the formation of single-
component and switchable ethylene oligomerization and poly-
merization catalysts. It was argued that eliminating the possibility
of alkylation at the phosphorus atom might afford a different cata-
lytic behavior. By oxidizing the P atom to the pentavalent state we
have now obtained the first polymer-free highly active oligomer-
ization system. Herein we describe our findings.
65%). μeff = 4.89 μB. Anal. Calcd (found) for C28H44Cl2CrLiN2O2P: C
55.91 (55.88), H 7.37 (7.21), N 4.66 (4.64).
Preparation of [(i-Pr)NP(Ph)2N(i-Pr)]Cr(μ2-Cl)2Li(THF)2 (2).
A solution of i-PrN(H)PBr(Ph)2N(H)i-Pr (0.38 g, 1.0 mmol) in THF
(10 mL) was treated with n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes)
at 25 °C. After stirring for 24 h at room temperature, the clear yellow
solution was added to a suspension of CrCl2(THF)2 (0.27 g, 1.0 mmol)
in THF (5 mL). Stirring was continued at room temperature overnight,
affording a green solution. The solvent was removed in vacuo, and the
residue redissolved in toluene (10 mL). After centrifugation, the super-
natant was concentrated to 4 mL and stored at ꢀ35 °C for 3 days. The
resulting blue crystals of 2 were filtered and washed with cold hexanes
(10 mL) and dried in vacuo (0.30 g, 0.49 mmol, 51%). μeff = 4.91 μB.
Anal. Calcd (found) for C26H40Cl2CrLiN2O2P: C 54.46 (54.39), H 7.03
(7.07), N 4.89 (4.93).
’ EXPERIMENTAL SECTION
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification
system. The liquid reaction mixtures were analyzed by using a CP
9000 gas chromatograph equipped with a 30 mL ꢁ 0.32 mm i.d.,
capillary CP-Volamine column and a FID detector. All single-point
experiments were performed in duplicate. The yields were determined
Preparation of [{[(Ph)NP(Ph)2N(Ph)]Cr}2(μ2-Cl)3][Li(DME)3]
(3). A solution of PhN(H)PBr(Ph)2N(H)Ph (0.45 g, 1.0 mmol) in
dimethoxyethane (10 mL) was treated with n-BuLi (0.84 mL, 2.1 mmol,
2.5 M in hexanes) at 25 °C. The mixture was stirred at room temperature
for 24 h, affording a clear yellow solution. A suspension of CrCl2(THF)2
(0.27 g, 1.0 mmol) in dimethoxyethane (5 mL) was then added. The
reaction mixture was stirred at room temperature overnight, affording a
green solution. The solvent was removed in vacuo, and the residue
redissolved in toluene (10 mL). After centrifugation, the supernatant
was evaporatedto4 mLand stored atꢀ35 °C forthreedays. The resulting
blue crystals of 3 were filtered and washed with cold hexanes (10 mL) and
dried in vacuo (0.35 g, 0.27 mmol, 55%). μeff = 4.97 μB. Anal. Calcd
(found) for C60H70Cl3Cr2LiN4O6P2: C 58.95 (58.90), H 5.77 (5.68), N
5.58 (5.51).
1
by H NMR spectroscopy (Varian Mercury 400 MHz spectrometer).
Samples for magnetic susceptibility were preweighed inside a VAC
drybox equipped with an analytical balance and measured on a Johnson
Matthey magnetic susceptibility balance. Elemental analyses were
carried out with a Perkin-Elmer 2400 CHN analyzer. Data for X-ray
crystal structure determination were obtained with a Bruker diffract-
ometer equipped with a 1K Smart CCD area detector. Methylaluminox-
ane (MAO, 10% in toluene) was purchased from Aldrich. MMAO and
PMAO were purchased from Akzo-Nobel. Ligands (t-Bu)N(H)PBr-
(Ph)2N(H)(t-Bu), (i-Pr)N(H)PBr(Ph)2N(H)(i-Pr), and PhN(H)PBr-
(Ph)2N(H)Ph were prepared according to literature procedures.19
Preparation of [(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ2-Cl)2Li(THF)2 (1).
A solution of t-BuN(H)PBr(Ph)2N(H)t-Bu (0.41 g, 1.0 mmol) in THF
(10 mL) was treated with n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes)
at 25 °C, and the mixture was stirred at room temperature for 24 h. The
resulting clear yellow solution was added to a suspension of CrCl2(THF)2
(0.27 g, 1.0 mmol) in THF (5 mL), and stirring was continued at room
temperature overnight, affording a green solution. The solvent was
removed in vacuo, and the residue redissolved in toluene (10 mL). The
mixture was centrifuged, and the supernatant reduced to 4 mL and stored
at ꢀ35 °C for 2 days. The resulting blue crystals of 1 were filtered and
washed with cold hexanes (10 mL) and dried in vacuo (0.39 g, 0.56 mmol,
Preparation of {[(t-Bu)NP(Ph)2N(t-Bu)]Cr}2(μ2-Cl)3(μ3-Cl)2{Li-
(THF)2} (4). A solution of t-BuN(H)PBr(Ph)2N(H)t-Bu (0.41 g,
1.0 mmol) in THF (10 mL) was treated with n-BuLi (0.84 mL, 2.1
mmol, 2.5 M in hexanes) at 25 °C. The mixture was stirred at room
temperature for 24 h, affording a clear yellow solution. After addition of a
suspension of CrCl3(THF)3 (0.37 g, 1.0 mmol) in THF (5 mL) the
mixture was stirred at room temperature overnight, giving a purple solution.
The solvent was removed in vacuo, and the residue redissolved in dimethox-
yethane (10 mL). After centrifugation, the supernatant was concen-
trated to 4 mL and stored at ꢀ35 °C for 3 days. Purple crystals of 4 were
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Organometallics
ARTICLE
Scheme 1
filtered and washed with cold hexanes (10 mL) and dried in vacuo
(0.40 g, 0.36 mmol, 36%). μeff = 3.82 μB. Anal. Calcd (found) for
C48H72Cl5Cr2LiN4O2P2: C 53.02 (53.00), H 6.67 (6.59), N 5.15 (5.09).
reflections were applied. The systematic absences and unit-cell para-
meters were consistent for the reported space groups. The structures
were solved by direct methods, completed with difference Fourier
syntheses, and refined with full-matrix least-squares procedures based
on F2. All non-hydrogen atoms were refined with anisotropic displace-
ment parameters with the exception of uncoordinated solvent lattice
molecules. Complex 1: the toluene lattice molecule was refined iso-
tropically due to the low quality of the data. Complex 2: the THF solvent
molecules in the lattice were refined isotropically due to the quality of
data and solvent partial occupancy coupled with disorder. Complex 3:
the DME solvent molecules in the lattice were refined isotropically due
to the quality of data and partial occupancy. Complex 4: one of the two
THF molecules coordinated to lithium showed disorder of two vicinal C
atoms, producing an artificially short CꢀC distance. The disorder
unfortunately could not be modeled, and therefore the geometry was
restrained. Complex 5: the toluene solvent molecules in the lattice were
refined isotropically due to the quality of data and solvent partial
occupancy coupled with disorder. The [(Me3Al)2Cl] counteranion
was also disordered over two positions with equal occupancy. All
hydrogen atoms were treated as idealized contributions. All scattering
factors and anomalous dispersion factors are contained in the SHELXTL
6.12 program library. Relevant crystal data are given in Table 1, while
bond distances and angles are given in the Supporting Information.
Preparation of {[(t-Bu)NP(Ph)2N(t-Bu)]2Cr}{(Me3Al)2Cl} tolu-
3
ene (5). MethodA. A solutionof t-BuN(H)PBr(Ph)2N(H)t-Bu (0.41 g,
1.0 mmol) in THF (10 mL) was treated with n-BuLi (0.84 mL, 2.1 mmol,
2.5 M in hexanes) at 25 °C. The mixture was stirred at room temperature
for 24 h, affording a clear yellow solution. After the addition of a
suspension of CrCl2(THF)2 (0.27 g, 1.0 mmol) in THF (5 mL), the
reaction mixture was stirred at room temperature overnight, affording a
green solution. The solvent was removed in vacuo, and the residue
redissolved in toluene (10 mL). Neat AlMe3 (0.36 g, 5 mmol) was added
slowly at ꢀ35 °C, and the resulting mixture stirred for 1 h at room
temperature. The suspension was centrifuged, and the supernatant con-
centrated to 4 mL and stored at ꢀ35 °C for 3 days. Brown-red crystals of 5
were filtered and washed with cold hexanes (10 mL) and dried in vacuo to
give 0.15 g (0.17 mmol, 18%). μeff = 3.89 μB. Anal. Calcd (found) for
C53H82Al2ClCrN4P2: C 64.99 (63.40), H 8.38 (8.01), N 5.72 (5.87).
Method B. The same procedure as method A was followed except
using CrCl3(THF)3 (0.37 g, 1 mmol), producing 0.59 g of the product
(73%).
Polymerization and Oligomerization Results. Catalytic runs
were carried out in 200 mL high-pressure B€uchi reactors containing a
heating/cooling jacket. A preweighed amount of catalyst was dissolved
in 100 mL of toluene under N2 prior to loading the reaction vessel.
Solutions were heated using a thermostatic bath and charged with
ethylene, maintaining the pressure throughout the run. The reaction
mixtures of the oligomerization runs were cooled to 0 °C prior to
releasing the overpressure and quenching with EtOH and HCl. Activity
was measured by NMR by integrating the resonances of the vinyl
protons against the toluene solvent methyl group. Results of catalytic
runs are given in Table 1.
X-ray Crystallography. Suitable crystals were selected, mounted
on a thin, glass fibers with paraffin oil, and cooled to the data collection
temperature. Data collection was performed with three batch runs at
phi = 0.00 deg (600 frames), at phi = 120.00 deg (600 frames), and at
phi = 240.00 deg (600 frames). Initial unit-cell parameters were
determined from 60 data frames collected at different sections of the
Ewald sphere. Semiempirical absorption corrections based on equivalent
’ RESULTS AND DISCUSSION
The three ligands RN(H)PBr(Ph)2N(H)R (R = t-Bu, i-Pr, Ph),
bearing pentavalent phosphorus, were prepared according to an
existing literature procedure.19 These species were subjected to
double deprotonation with n-butyllithium and reacted with an
equimolar amount of CrCl2(THF)2 (Scheme 1). The structures of
the corresponding divalent complexes [(t-Bu)NP(Ph)2N(t-Bu)]-
Cr(μ2-Cl)2Li(THF)2 (1), [(i-Pr)NP(Ph)2N(i-Pr)]Cr(μ2-Cl)2Li-
(THF)2 (2), and [{[(Ph)NP(Ph)2N(Ph)]Cr}2(μ2-Cl)3][Li-
(DME)3] (3) were authenticated by crystal structure analyses
(Figure 1). Complexes 1 and 2 showed closely related geometries
with the metal in the standard square-planar coordination geome-
try, the ligand chelating the chromium atom in a bidentate fashion
and with two chlorine atoms bridging a THF-solvated lithium
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ARTICLE
Figure 1. Selected bond distances (Å) and angles (deg): 1 Cr1ꢀN1 2.053(4), Cr1ꢀN2 2.052(4), Cr1ꢀCl2 2.4363(14), Cr1ꢀCl1 2.4556(15);
N2ꢀCr1ꢀN1 72.44(18), N2ꢀCr1ꢀCl1 156.66(14), N2ꢀCr1ꢀCl2 103.18(14), Cl2ꢀCr1ꢀCl1 90.41(5); 2 Cr1ꢀN1 2.038(8), Cr1ꢀN2 2.061(8),
Cr1ꢀCl1 2.385(3),Cr1ꢀCl2 2.399(3), N1ꢀCr1ꢀN2 71.5(3), N1ꢀCr1ꢀCl1 98.4(2), N1ꢀCr1ꢀCl2 169.7(3), Cl1ꢀCr1ꢀCl2 91.09(11); 3
Cr1ꢀN1 2.065(5), Cr1ꢀN2 2.088(6), Cr1ꢀCl1 2.434(2), Cr1ꢀCl2 2.688(2),Cr1ꢀP1 2.749(2); N1ꢀCr1ꢀN2 70.9(2), N1ꢀCr1ꢀCl1
97.53(17), N2ꢀCr1ꢀCl1 160.84(17), Cl1ꢀCr1ꢀCl2 82.23(6).
Figure 3. Selected bond distances (Å) and angles (deg) for the cationic
part of 5: Cr(1)-N(1a) 1.966(4), Cr(1)ꢀN(2a) 1.969(5), N(1a)ꢀCr-
(1)ꢀN(1) 73.9(3), N(1a)ꢀCr(1)ꢀN(2) 132.34(18), N(1)ꢀCr-
(1)ꢀN(2a) 126.8(2).
thus attempted the isolation of the catalytically active species by
reacting the complexes with MAO. The reactions proved to be
difficult, invariably affording intractable materials. Therefore,
treatment with simple AlMe3 was considered being the next step
to obtain at least some information about the outcome of the
interaction of the transition metal with the activator. The
Figure 2. Selected bond distances (Å) and angles (deg) of 4: Cr1ꢀN2
1.995(8), Cr1ꢀCl1 2.343(3), Cr1ꢀCl3 2.440(3); N2ꢀCr1ꢀN1
73.4(3), N2ꢀCr1ꢀCl 198.2(2), N2ꢀCr1ꢀCl3 94.5(2), Cl1ꢀCr1ꢀ
Cl2 87.86(11), Cl1ꢀCr1ꢀCl3 163.64(10), Cr2ꢀCl2ꢀCr1 83.72(10),
Cr1ꢀCl3ꢀCr2 82.99(9).
reaction of the divalent 1 with five equivalents of AlMe3 afforded
the trivalent and cationic complex {[(t-Bu)NP(Ph)2N(t-Bu)]2-
countercation. Complex 3 is instead dimeric, with three chlorine
atoms bridging the two transition metals. The chromium atom
Cr}{(Me3Al)2Cl} (toluene) (5) as a crystalline product (Scheme 1).
3
displays a square-pyramidal coordination geometry that, albeit not
very common, is precedented.20 A solvated lithium countercation
unconnected with the dimeric unit is also present in the lattice.
The trivalent analogue of 1 was obtained in a similar manner via
treatment of the double-deprotonated ligand with CrCl3(THF)3.
The corresponding trivalent {[(t-Bu)NP(Ph)2N(t-Bu)]Cr(μ2-
Cl)3(μ3-Cl)2}{Li(THF)2} (4) was obtained as a purple crystalline
mass (Scheme 1). The structure was elucidated by X-ray diffraction
(Figure 2), showing the dinuclear complex containing two trivalent
chromium atoms with an overall face-sharing bioctahedral structure.
The bridging interaction between the two metal centers is realized
through the facially oriented chlorine of the distorted octahedral
geometry of each chromium center. One partially solvated lithium
cation is retained as a part of a trimetallic cluster structure being
bridged to the two transition metals by four chlorine atoms.
As described below, all the complexes display an interesting
catalytic activity upon activation by methylaluminoxane. We have
The same species could also be obtained upon treatment of the
trivalent 4 with AlMe3, thus providing two alternate syntheses for
the same species.
The connectivity of 5 was revealed by an X-ray crystal
structure and comprises a trivalent chromium center in a dis-
torted tetrahedral environment defined by two ligand systems
(Figure 3). One [(Me3Al)2Cl](ꢀ) counteranion, unconnected to
the chromium-containing unit, is also present in the lattice, thus
conclusively assigning the trivalent state to the metal. The
magnetic moment of 5 was in the expected range for the high-
spin d3 electronic configuration of trivalent chromium.
The formation of 5 from 1 implies two separate processes:
ligand scrambling and oxidation of the metal center. The acqui-
sition by the chromium center of a second ligand during the
reaction (ligand scrambling) in turn indicates that a second
chromium-containing species must have been generated.
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Table 2. Ethylene Oligomerization Resultsa
co-catalyst
(equiv)
co-cat:
Cr
alkenes
(mL)
activity
C6
C8
C10
C12
C14
C16
C18
catalyst
(g/mmolCr/h)
(mol %)
(mol %)
(mol %)
(mol %)
(mol %)
(mol %)
(mol %)
1
MAO
100
500
1000
500
500
500
500
500
1000
2000
500
500
500
500
80
208
120
14
5600
14 560
8400
980
33
40
37
51
23
0
25
31
26
40
21
0
18
15
18
6
11
9
9
4
3
1
2
0
8
0
4
3
3
1
4
3
4
0
1
0
1
0
4
0
1
1
1
0
2
1
2
0
1
MAO
1
MAO
11
2
5
1b
1c
1d
1
MAO
1
MAO
20
1400
0
18
0
15
0
11
0
MMAO
PMAO
DMAOe
DMAOe
DMAOe
MAO
0
144
240
348
420
112
100
16
10 080
16 800
24 360
29 400
7840
7000
1120
0
37
36
33
42
31
35
25
0
23
26
30
30
30
25
27
0
17
17
16
14
16
18
20
0
11
11
11
9
7
1
6
1
6
1
4
2
10
11
14
0
7
3
MAO
7
4
MAO
8
5
MAO
0
0
a Conditions: 100 mL of toluene, loading 10 μmol of complex, 35 bar of ethylene, reaction temperature 60 °C, reaction time 60 min. b T = 20 °C.
c T = 100 °C. d 100 mL of methylcyclohexane. e DMAO = AlMe3-depleted MAO.
Instead, the oxidation of the metal center during the reaction
with an alkyl aluminum reagent is, at least in principle, surprising
since these activators are known to act as reducing agents14 and
certainly not as oxidants. On the other hand, recent work15,21 has
indicated that the reoxidation of the chromium center in the
presence of alkyl aluminum reagent is indeed possible via
disproportionation as one of the distinctive aspects of the redox
dynamism of chromium. This process is the direct result of the
interaction of the metal center with the alkyl aluminum activator.
In the present case, the additional ligand necessary for the
formation of 5 can simply be provided by the low-valent partner
of the disproportionation of 1 on the way to form metallic
chromium. Instead, the formation of 5 from 4 has a more
straightforward explanation since the oxidation state remained
unchanged. However, given the acquisition of the additional
ligand and that the reduction of the trivalent center to the divalent
state duringthe reaction with activators iswidely documented,14 it
is tempting to suggest that the reaction proceeds with initial
reduction of 4 to a divalent congener followed by similar
disproportionation to that observed in the case of 1.
When activated with methylaluminoxane, 1 yielded a very
large amount of pure oligomers with a minor excess of 1-hexene
and 1-octene (Table 2). Interestingly, 1-butene was observed
only in traces. Increasing the Al:Cr ratio resulted in a tremendous
increase of catalytic activity as well as in a significant increment of
the selectivity toward 1-hexene and 1-octene. The optimization
was reached with an Al:Cr ratio of about 500 since larger excess
decreased the activity as well as the selectivity. A similar rise in
catalytic activity with increasing Al:Cr ratios was also observed
for the original NPN system containing a trivalent phosphorus.17a
In that case the selectivity was, however, unchanged. Tempera-
tures higher or lower than 60 °C visibly decreased the activity. At
low temperature, the lower activity was accompanied by a
considerable increase of selectivity (91% 1-hexene/1-octene
and 9% heavier oligomers). Outstanding features of this catalytic
system are the complete absence of polymer and the purity of the
oligomers, which are both highly desirable from an industrial
standpoint. The use of different aluminoxane activators afforded
variable activities. The most productive run was obtained with
AlMe3-depleted MAO (DMAO), which afforded an activity
comparable to the highest ever observed for a LAO catalyst
obtained with late transition metals22 (420 mL of polymer-free
mixture of oligomers from 10 μmol of catalyst per hour).
Interestingly, the high activity was also accompanied by a
significant enrichment in 1-hexene and 1-octene, the two most
important R-olefinic oligomers (Table 2).
The replacement of the t-Bu substituents by i-Pr or Ph groups
gave a noticeable decrease of both activity and selectivity,
although the overall catalytic activity still remained quite accep-
table. The fact that the large steric bulk of the t-Bu groups may
play a central role in significantly enriching the oligomeric
mixture in 1-hexene and 1-octene is intriguing. If the excess of
these two oligomers is generated by a Cr(I) intermediate via a
ring expansion mechanism, this would be of course in addition to
the CosseeꢀArlman mechanism, presumably operated by Cr(II)
and which produces the large amount of oligomers. We therefore
speculate that the bulky substituents of 1 prevent dimeric
aggregation of the monovalent chromium, thus hindering its
disproportionation but also decreasing the catalytic activity. In
other words, the ligand’s steric bulk might lend some stability to
highly reactive Cr(I) intermediates. However, the absence of
1-butene in significant amount encourages the idea that the
oligomer distribution may be generated by a ring-expansion
mechanism.9 In such an event, one must assume that the
presence of t-Bu groups somehow renders faster the reductive
elimination of the seven- and nine-membered rings with respect
to that of larger rings.
We have also attempted to utilize methylcyclohexane as
solvent for the catalytic run since toluene is known to occasion-
ally have a poisoning effect on trimerization catalysts.14b In turn
this has required the usage of MMAO for solubility reasons. The
observed lack of catalytic behavior is likely to be ascribed to the
strongly reducing i-Bu groups present in this particular activator
most probably decomposing the catalyst to metallic chromium.
The trivalent 4 displays only marginal catalytic activity compared
to the divalent 1.
In conclusion, in this work we have discovered an unprece-
dented and potentially useful catalytic system for nonselective
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Organometallics
ARTICLE
ethylene oligomerization devoid of polymer. This feature, in
combination with the near to record activity of 1, makes the
catalyst competitive with the best performing industrial systems.
The general lack of selectivity and the chemical behavior of the
complexes reported above altogether suggest that the most active
catalytically active species may contain divalent chromium. In
this event, the nonselective oligomerization would not proceed
through a ring-expansion mechanism. It is worth reminding that
divalent chromium, according to the current status of its chem-
istry, does not possess the reducing potential sufficient for
coupling two ethylene molecules into a metallacycle. At this
stage it is tempting to propose that the excess of 1-hexene and
1-octene observed in a few instances is likely to be generated by a
monovalent chromium species produced in parallel to Cr(II) as a
result of the redox dynamism. The subtle effect of ligand
substituents in determining the appearance of the excess of these
two oligomers, together with the exceedingly high activity,
encourages further attempts to transform these systems into
highly active and truly selective oligomerization catalysts.
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data for the complexes reported in this paper. This material is
available free of charge via the Internet at http://pubs.acs.org
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sgambaro@uottawa.ca.
’ ACKNOWLEDGMENT
This work was supported by NSERC and Basell Polyole-
fine GmbH.
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