Chromium catalysts supported by a nonspectator NPN ligand: Isolation of single-component chromium polymerization catalysts

Article abstract of DOI:10.1002/anie.200800845

(Chemical Equation Presented) Switchable catalytic activity, from nonselective ethylene oligomerization to trimerization and even polymerization, is shown by 1, depending on the alkyl aluminum compound with which it is activated. AlMe₃ or iBu₂AlCl resulted in highly active single-component polymerization catalysts (see scheme; R = Me, PE = polyethylene), the activity of which can be switched or inhibited by the presence of an additional alkyl aluminum compound.

Full text of DOI:10.1002/anie.200800845

Communications
DOI: 10.1002/anie.200800845
Catalyst Design
Chromium Catalysts Supported by a Nonspectator NPN Ligand:
Isolation of Single-Component Chromium Polymerization Catalysts**
Khalid Albahily, Ece Koç, Danya Al-Baldawi, Didier Savard, Sandro Gambarotta,*
Tara J. Burchell, and Robbert Duchateau*
Ethylene oligomerization and polymerization can be
treating a catalyst precursor with different aluminum alkyls
may reveal the metal oxidation state of the catalytically active
species. In turn, this may provide significant information
À
regarded as belonging to the same type of C C bond-forming
reactions. In fact, the highly desirable selective tri- and
tetramerization processes may be simply considered to be
polymerizations precisely truncated at an early stage. How-
ever, it is commonly accepted that the mechanisms are
completely different (oxidative addition followed by ring
expansion for oligomerization[1] versus migratory insertion for
polymerization[2]), and therefore the two processes are
conceptually distinct. On the other hand, it is not obvious to
which mechanism oligomerization affording a Schultz–Flory
distribution of products belongs. Furthermore, the fact that
even the best chromium-based oligomerization catalysts
invariably produce small amounts of polyethylene (PE) as
an undesirable side product suggests that the two processes
may be closely linked. Addressing these important questions
cannot be dissociated from understanding of the factors which
govern the selectivity of the same catalyst towards one or the
other type of transformation.
À
towards understanding the factors which govern the C C
bond-forming reaction.
Herein we describe the preparation and testing of a
chromium-based catalyst system supported by a nonspectator
ligand. Activation with different cocatalysts gave spectacular
changes in catalyst selectivity, while attempts to isolate the
catalytically active species resulted in the characterization of
two unprecedented single-component polymerization cata-
lysts.
Wass et al., Bercaw et al., and Overett et al. have clearly
demonstrated the potential of the combination of nitrogen
and phosphorous donor atoms in a three-atom array for
obtaining selective trimerization[4f,g,j,k] and tetramerization
catalysts.[5] Our previous mechanistic studies on selective
trimerization led us to believe that a stable cationic organo–
chromium(III) species might be the key to selectivity for the
oligomerization process.[7d] These two arguments prompted us
Chromium is an ideal substrate for this research, since
effective catalysts for both ethylene polymerization[3] and
oligomerization (including selective tri-[4] and even tetrame-
rization[5]) have been reported for this element. All homoge-
neous chromium-based catalyst systems reported to date
invariably consist of a di- or trivalent chromium precursor and
an activator.[3] However, understanding of these systems
remains limited, since even the oxidation state of the metal in
the catalytically active species is still debated. The nature of
the transition metal is not the only important factor, since the
cocatalyst can also have a profound effect on catalyst
selectivity.[6] Thus, obtaining single-component catalysts by
to use the established (tBuNPNtBu)2 dianion,[8] since the
2À
particular geometry of the cage defined by the four-mem-
bered P2N2 ring of the ligand with two additional N donor
atoms was assumed to be unsuited to accommodating the
square-planar geometry of d4 CrII.
In spite of our expectations for facial, octahedral d3
trivalent complexes, the reaction of the dianion, in the form
of the dilithium salt, with either [CrCl2(thf)2] or [CrCl3(thf)3]
produced the same divalent, distorted square-planar complex
[(tBuNPNtBu)2Cr] (1; Scheme 1). In this species the dianion
was cleaved into two (tBuNPNtBu)À monoanions, a trans-
formation occasionally observed but only with main-group
elements.[8h,j,9]
[*] K. Albahily, D. Al-Baldawi, D. Savard, Prof. Dr. S. Gambarotta,
Dr. T. J. Burchell
When activated with methyl alumoxane (MAO), 1 yielded
a statistical distribution of ethylene oligomers (including
waxes) but with unprecedented activity. It proved difficult to
control the reaction temperature with catalyst concentrations
as low as 5 mm. The catalyst was found to be thermally robust,
as tests with higher catalyst loading easily reached and
maintained temperatures above 1108C for prolonged periods
of time. The activity gradually increases with increasing Al:Cr
ratio, but this did not affect the product distribution.
Interestingly, the selectivity of 1 was completely determined
by the nature of the activator (Table 1). While MAO resulted
in oligomerization of ethylene, activation with (iBu2Al)2(m-O)
gave low-molecular-weight polyethylene with a similarly high
activity as when activated with MAO. Notably, small amounts
of a mixture of exclusively 1-hexene and 1-octene (av ratio
85:15) were formed as side product. In an attempt to track the
Department of Chemistry, University of Ottawa
10 Marie Curie, Ottawa ON K1N6N5 (Canada)
Fax: (+1)613-562-5170
E-mail: sgambaro@uottawa.ca
E. Koç, Dr. R. Duchateau
Department of Chemistry, Eindhoven University of Technology
P.O. Box 513, 5600 MB (The Netherlands)
Fax: (+31)402-463-966
E-mail: R.Duchateau@tue.nl
[**] This work was supported by the Natural Science and Engineering
Council of Canada (NSERC) and by the Dutch Polymer Institute.
W. K. Gerritsen (University Eindhoven) is gratefully acknowledged
for the GPC measurements and Dr. T. J. Burchell (University of
Ottawa) for solving the crystal structures.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200800845.
5816
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Angewandte
Chemie
Cl, iBu (3)] residue to afford an [RP(NtBu)2AlX2]À
anion. The difference between the two complexes is
that 3 contains one [RP(NtBu)2AlX2]À ligand with
chromium bonded to a second AliBu2 residue
through two bridging chloride ligands, while com-
plex 2 contains two [RP(NtBu)2AlX2]À ligands.
Coordination of the alkylated phosphorus atom of
the second ligand in 2 to an additional AlMe3
moiety prevents it from binding to chromium like
the other does. Consequently, one of the two
aluminum methyl groups occupies the fourth coor-
dination site of the distorted square-planar chro-
mium atom (Figure 1).
Scheme 1. Synthesis and partial thermal-ellipsoid plot of 1 with ellipsoids drawn at
À
À
50% probability. Selected bond lengths [Š] and angles [8]: Cr1 N1 2.098(3), Cr1
N2 2.103(3); N1-Cr1-N2 70.50(10), N1-Cr1-N1a 158.10(16), N1-Cr1-N2a
114.08(11).
Since the anionic charge of the [RP-
(NtBu)2AlX2]À ligand is located at the aluminum
center that, at least for Al3, is situated relatively far
from the chromium center, the structure can be
origin of the oligomers, it was argued that possibly AliBu3
present in (iBu2Al)2(m-O) was responsible for oligomeriza-
tion. In fact, on activation with AliBu3, complex 1 indeed gave
1-hexene with excellent selectivity (> 99.9 by GC). Although
the activity was substantially lower than that obtained by
activation with MAO, it was still similar to that of the other
existing selective catalysts.[4g,7]
regarded as having a formally dicationic chromium center
stabilized by two anionic [RP(NtBu)2AlX2]À ligands. Inter-
estingly, such a structure resembles the earlier reported
divalent chromium tetramerization catalyst precursor
[{CyN(PPh2)2}2CrClAlMe3]+, in which a formally cationic
chromium center is stabilized by one (Me3AlCl)À fragment
This intriguing diversity of catalytic behavior depending
on the activator clearly indicates that different catalytically
active species are generated from the reaction of 1 with the
alkyl aluminum reagents. Therefore, we attempted to isolate
such species by reaction of 1 with various aluminum alkyls. In
all cases, the reactions were surprisingly clean and produced
crystalline or microcrystalline materials. However, only when
1 was treated with AlMe3 or iBu2AlCl (Scheme 2), was it
possible to elucidate the structures by single-crystal X-ray
diffraction. Both complexes 2 and 3 contain chromium in the
divalent state. In spite of their different appearance, the two
reactions have in fact followed a similar trend, in the sense
that the P atom has been alkylated in both cases. Also the
resulting [tBuNP(R)NtBu]2À dianion uses the two N donor
atoms to bridge the chromium center to an AlX2 [X = Me (2);
Scheme 2. Reaction of 1 with aluminum alkyls.
Table 1: Ethylene oligomerization and polymerization reactions with 1–3 as single-component catalysts or in combination with cocatalysts.[a]
Entry Cat. ccat
[mmol]
Cocat.
Al:Cr PE
[g]
Mn
PDI Activity
[gmmolÀ1 hÀ1
Oligomers[d]
C8 C10 C12 C14 C16
[gmolÀ1
]
]
amount [mL] C6
a
1
2
1
1
1
1
1
1
1
1
2
2
2
2
3
3
3
3
5
5
5
5
5
5
1
1
10
5
5
5
10
5
MAO
MAO
MAO
MAO
AliBu3
AliBu3
(iBu2Al)2(m-O) 1000
(iBu2Al)2(m-O) 500
–
MAO
MAO
AliBu3
–
MAO
MAO
AliBu3
2000
1000
1000 trace
500
1000
500
0.9 310[c]
0.9 420[c]
1.4 23600
1.3 6400
4800
1.4 400
1.3 800
400
3.7 32800
3.0 24400
3.2 2240
1.3 18800
1.2 18000
560
2.8 3300
1.3 34000
1.2 27600
4.3 920
59
16
12
1
2
1
2
2
0
45.2 29.7 11.0 6.6 3.8 2.1 0.58
40.0 23.9 15.4 9.1 5.3 2.8 0.61
41.5 25.1 15.8 9.0 4.8 2.5 0.57
44.1 23.2 12.1 7.9 4.9 4.4 0.62
3[b]
4
–
3.4 440[c]
5
6
7
8
1.5 400[c]
>99.9
>99.9
82.5 17.5
85.0 15.0
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
trace
–
16.4 34940
12.2 12760
11.2 37990
0.2 490[c]
0.1 500[c]
1.4
16.5 26780
1.2 450[c]
0.7 410[c]
3.6 11420
9
–
–
–
10
11
12
13
14
15
16
2000
1000
1000
–
2000
1000
1000
47
45
0
45.3 26.1 14.2 8.3 4.7 2.1 0.57
39.2 26.0 15.3 9.8 6.3 3.4 0.64
–
–
–
–
–
–
-
–
-
–
–
–-
–
–
0
85
69
0
35.2 26.3 15.8 11.5 7.1 4.1 0.67
36.7 26.5 15.9 10.6 6.5 3.8 0.64
5
5
–
–
–
–
–
–
–
[a] Standard conditions: T=508C, 100 mL of toluene, 35 bar of ethylene, 30 min reaction time. [b] 5 bar of ethylene. [c] Waxes. [d] The oligomer
distribution is given in %.
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Communications
catalysts based on chromium have been reported only once
before.[10] This desirable behavior is ascribed to two character-
istics of the [RP(NtBu)2AlX2]À monoanion: 1) it has the
potential to transfer an alkyl group to chromium (either from
P or from the Al residue), which evidently affords robust
chromium alkyl species, and 2) it establishes a structure in
which the Lewis acidity of the aluminum residue enhances the
positive charge on chromium. Complexes 2 and 3 can
reasonably be regarded as representative of the fate of 1 in
the early stage of its activation. Formation of such species,
which possibly survive in small amounts in the presence of the
large excess of activators, might be responsible for the
presence of the polymer commonly observed during oligo-
merization. The intriguing selective trimerization obtained on
activation of 1 with AliBu3 remains unexplained at this stage
and it is the target of current investigation.
À
Figure 1. Selected bond lengths [Š] and angles [8] for 2: Cr1 N1
À
À
À
À
2.135(3), Cr1 N3 2.136(3), Cr1 C14 2.344(5), Cr1 P2 2.3505(12), N1
À
À
À
Al2 1.940(3), N2 Al2 1.839(3), Al2 C14 2.025(5), P1 N1 1.736(3),
À
À
À
À
P1 N2 1.667(3), Al1 P1 2.5549(15), Al3 N3 1.993(3), Al3 N4
À
À
1.876(4), P2 N3 1.710(3), P2 N4 1.648(4); N1-Cr1-N3 167.32(12),
N1-Cr1-C14 89.40(14), N3-Cr1-C14 103.24(14), N1-Cr1-P2 122.95(9),
N3-Cr1-P2 44.50(9), C14-Cr1-P2 145.67(12), Al2-C14-Cr1 78.08(16).
À
Selected bond lengths [Š] and angles [8] for 3: Cr1 N2 2.1256(12),
Cr1 N1 2.1305(13), Cr1 Cl3 2.4041(5), Cr1 Cl2 2.4063(5), Al3 Cl3
2.2764(7), Al3 Cl2 2.2765(7), Al2 N1 1.9209(13), Al2 N2 1.9283(13),
Al2 C13 1.9370(16), P1 N1 1.7651(14), P1 N2 1.7730(14); N2-Cr1-
N1 65.26(5), N2-Cr1-Cl3 104.24(4), N1-Cr1-Cl3 169.42(4), N2-Cr1-Cl2
169.26(4), N1-Cr1-Cl2 104.67(4), Cl3-Cr1-Cl2 85.713(17), Cl3-Al3-Cl2
91.88(2), N1-Al2-N2 73.20(6), N1-P1-N2 80.88(6).
À
À
À
À
À
À
À
À
À
À
Experimental Section
Samples were tested in a 200-mL high-pressure Büchi reactor
containing a heating/cooling jacket. A preweighed amount of catalyst
was dissolved in 10 mL of toluene under N2 and injected into the
preheated reactor already charged with cocatalyst and toluene (total
volume 100 mL). Solutions were heated with a thermostatic bath and
charged with ethylene, maintaining the pressure throughout the run.
Polymerizations were quenched by addition of EtOH and HCl. The
resulting polymer was isolated by filtration, sonicated with an
acidified ethanol solution, rinsed, and thoroughly dried prior to
mass determination. Molecular weight and molecular-weight distri-
butions of the polymers were determined by means of gel permeation
chromatography on a PL-GPC210 equipped with refractive-index and
viscosity detectors and a 3 ” PLgel 10 mm MIXED-B column set at
1358C with 1,2,4-trichlorobenzene as solvent. The molecular weight
of PE was referenced to polystyrene (Mw = 65500, PDI = 1.02)
standards. All reactions were carried out under a dry nitrogen
atmosphere. Solvents were dried by using an aluminum oxide solvent
purification system. Samples for magnetic susceptibility were pre-
weighed inside a drybox equipped with an analytical balance and
measured on a Johnson Matthey magnetic susceptibility balance.
Data for X-ray crystal structure determination were collected with a
Bruker diffractometer equipped with a 1K Smart CCD area detector.
[CrCl2(thf)2] was prepared according to standard procedures. The
ligand cis-{tBu(H)NP[(m-N)tBu]2PN(H)tBu} was prepared according
to a literature procedure.[6b] The reagents iBu2AlCl (Aldrich), AlMe3
(Aldrich), (iBu2Al)2(m-O), and MAO (Chemtura and Aldrich) were
used as received. Mass spectra were recorded on a Micromass
Quattro-LC Electrospray Triple Quadrupole Mass Spectrometer. All
the experiments were done in the negative mode with toluene as the
main solvent by using 2.5% of toluene in acetonitrile, capillary
voltage 4.00–4.20 kV, cone voltage 40 kV, and a desolvation temper-
ature of 2208C. Diamagnetic corrections were applied to the
calculations of the magnetic moments.
and two diphosphinoamine ligands.[7b] In contrast to this
complex, which required a cocatalyst for any catalytic activity
to be observed, 2 is a single-component catalyst. Simple
exposure of 2 to ethylene (35 bar, 508C) yielded polyethylene
with moderate activity. Treatment of 2 with MAO again led to
a drastic switch in selectivity towards a statistical distribution
of ethylene oligomers (including some waxes) with a similar
remarkable activity to that of the 1/MAO system (compare
entries 1–4 with entries 10 and 11 in Table 1). Surprisingly,
attempts at activating both 1 and 2 with an excess of AlMe3 in
fact had the opposite effect of totally deactivating the system.
Complex 3 also acts as a single-component ethylene
polymerization catalyst with similar activity to 2. Different
from 2, there are no free coordination sites around chromium
À
in 3, nor are Cr C bonds present which could possibly act as
polymerization initiators. In analogy with 2, attempts to
activate 1 and 3 with an excess of iBu2AlCl did not produce
any catalytic transformation. Treatment of 3 with MAO
instead afforded an ethylene oligomerization catalyst even
more active than 1 with a nearly constant activity over a
period of at least 30 min. The most probable explanation for
the single-component activity of 3 is migration of an alkyl
group
from
the
AlR
residue
leading
to
“[iBuP(NtBu))2AliBuCl]CriBu”, which again can be
regarded as an ionic structure containing a cationic chromium
alkyl. Alkyl shuttling from the alkylated P atom is another
possibility which cannot be ruled out at this stage. Since 2, 3,
and (iBu2Al)2(m-O)-activated 1 yield polymers of similar
molecular weight, it is probable that closely related active
species are generated in the three systems.
The unexpected behavior of the initially intended
(tBuNPNtBu)22À ligand system gave access to a nonspectator
(tBuNPNtBu)À anion, which readily reacts with aluminum
alkyls. The resulting [RP(NtBu)2AlX2]À anions afford ther-
mally stable, single-component chromium catalysts. To the
best of our knowledge, homogeneous single-component
1: A solution of cis-{tBu(H)NP[(m-N)tBu]2PN(H)tBu} (0.348 g,
1 mmol) in THF (20 mL) was treated with nBuLi (0.84 mL, 2.1 mmol,
2.5m in hexanes) at 08C. The mixture was stirred at room temperature
for 18 h. The resulting solution was added to a suspension of
[CrCl2(thf)2] (0.268 g, 1 mmol) in THF (10 mL). The reaction mixture
was stirred at room temperature overnight. The solvent was
evaporated in vacuo and the residue redissolved in toluene. The
resulting suspension was centrifuged and the solution was stored at
À308C. Brown crystals of 1 separated over two days and were filtered,
washed with cold hexanes (10 mL), and dried in vacuo to give
analytically pure compound (0.294 g, 74%). meff = 4.98 mB; ESI-MS
(assignment, rel. intensity): m/z: 397.0 ([MÀH]À, 42), 341.1
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Chemie
([MÀC4H9]À, 98), 324.0 ([MÀC4H9ÀNH3]À, 100); C,H,N analysis (%)
found (calcd for C16H36CrN4P2): C 48.23 (48.25), H 9.11 (9.17), N14.06
(13.91).
Organometallics 1995, 14, 738; d) A. Döhring, J. Göhre, P. W.
Jolly, B. Kryger, J. Rust, G. P. J. Verhovnik, Organometallics
2000, 19, 388; e) J. S. Rogers, X. H. Bu, G. C. Bazan, Organo-
metallics 2000, 19, 3948; f) M. Enders, P. Fernandez, G. Ludwig,
H. Pritzkow, Organometallics 2001, 20, 5005; g) V. C. Gibson,
P. J. Maddox, C. Newton, C. Redshaw, G. A. Solan, A. J. P.
White, D. J. Williams, Chem. Commun. 1998, 1651; h) L. A.
MacAdams, W. K. Kim, L. M. Liable-Sands, I. A. Guzei, A. L.
Rheingold, K. H. Theopold, Organometallics 2002, 21, 952;
i) V. C. Gibson, S. Mastroianni, C. Newton, C. Redshaw, G. A.
Solan, A. J. P. White, D. J. Williams, J. Chem. Soc. Dalton Trans.
2000, 1969.
2: A solution of 1 (0.398 g, 1 mmol) in toluene (20 mL) was
treated with trimethylaluminum (0.288 g, 4 mmol) at room temper-
ature and stirred for 10 min. The solvent was removed in vacuo and
then hexanes (10 mL) were added. Storing the resulting solution at
À308C for 3 d afforded small purple crystals of 2, which were
collected by filtration, washed with cold hexanes, and dried in vacuo
(0.386 g, 63%). meff = 4.97 mB; ESI-MS (assignment, rel. intensity):
m/z: 613.9 ([MÀH]À, 48), 703.2 (multiple signals, [M+tolueneÀH]À,
100; experiments with [D8]toluene gave the expected isotopic
pattern), 571.0 ([MÀC3H7]À, 88); C,H,N analysis (%) found (calcd
for C25H63Al3CrN4P2): C 48.93 (48.77), H 10.18 (9.98), N 9.13 (9.15).
3: A solution of 1 (0.398 g, 1 mmol) in toluene (20 mL) was
treated with diisobutylaluminum chloride (0.704 g, 4 mmol) at room
temperature and stirred for 10 min. The solvent was removed in vacuo
and then hexanes were added. The solution was stored in a À308C
freezer for 3 d. The resulting product was collected by filtration,
washed with cold hexanes, and dried in vacuo to give 3 as blue crystals
(0.282 g, 46%). meff = 5.24 mB; ESI-MS (assignment, rel. intensity):
m/z: 613.8 (multiple signals, [MÀH]À, 20), 690.0 (multiple signals,
[M+tolueneÀCH2]À, 28; experiments with [D8]toluene gave the
expected isotopic pattern), 577.0 ([MÀCl]À, 100); C,H,N analysis
found (calcd for C24H54Al2Cl3CrN2P): C 46.95 (46.68), H 8.87 (8.71),
N 4.56 (4.63).
[4] a) W. K. Reagan (Phillips Petroleum Company), EP 0417477,
1991; b) H. Mimura, T. Aoyama (Tosoh Corporation), T.
Yamamoto, M. Oguri, Y. Koie, JP 09268133, 1997; c) H.
Mohamed, A. Bollmann, J. Dixon, V. Gokul, L. Griesel, C.
Grove, F. Hess, H. Maumela, L. Pepler, Appl. Catal. A 2003, 255,
355; d) J. J. C. Grove, H. A. Mohamed, L. Griesel (Sasol
Technology (Pty) Ltd), WO 03/004158, 2002; e) T. Yoshida, T.
Yamamoto, H. Okada, H. Murakita (Tosoh Corporation), US
2002/0035029, 2002; f) D. F. Wass (BP Chemicals Ltd), WO 02/
04119, 2002; g) A. Carter, S. A. Cohen, N. A. Cooley, A. Murphy,
J. Scutt, D. F. Wass, Chem. Commun. 2002, 858; h) D. S.
McGuinness, P. Wasserscheid, W. Keim, D. H. Morgan, J. T.
Dixon, A. Bollmann, H. Maumela, F. M. Hess, U. Englert, J. Am.
Chem. Soc. 2003, 125, 5272; i) J. T. Dixon, P. Wasserscheid, D. S.
McGuinness, F. M. Hess, H. Maumela, D. H. Morgan, A.
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Bercaw, Organometallics 2006, 25, 2733 – 2742; k) S. J. Schofer,
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metallics 2006, 25, 2743 – 2749.
Crystal data for 1: C16H36CrN4P2, formula weight 398.43, ortho-
rhombic, Pccn, Z = 4, a = 16.166(4), b = 11.929(3), c = 11.814(3) Š,
V= 2278.1(11) Š3, D = 1.162 MgmÀ3; m = 0.647 mmÀ1; F(000) = 856;
R1 = 0.0446, wR2 = 0.1054; GoF = 1.118; for 2: C25H63Al3CrN4P2,
¯
formula weight 614.67, triclinic, P1, Z = 2, a = 9.3177(13), b =
11.7323(16), c = 17.301(2) Š, a = 90.963(2), b = 95.876(2), g =
94.465(2)8; V= 1875.0(4) Š3, D = 1.087 MgmÀ3
;
m = 0.479 mmÀ1
;
[5] A. Bollmann, K. Blann, J. T. Dixon, F. M. Hess, E. Killian, H.
Maumela, D. S. McGuinness, D. H. Morgan, A. Neveling, S.
Otto, M. Overett, A. M. Z. Slawin, P. Wasserscheid, S. Kuhl-
mann, J. Am. Chem. Soc. 2004, 126, 14712.
F(000) = 766; R1 = 0.0568, wR2 = 0.1327; GoF = 0.993; for 3:
C24H54Al2Cl3CrN2P, formula weight 613.97, orthorhombic, P212121,
Z = 4,
a = 8.6978(14),
b = 16.930(3),
c = 23.134(4) Š,
V=
3406.6(10 Š3, D=1.197 MgmÀ3; m = 0.685 mmÀ1; F(000) = 1312; R1 =
0.0275, wR2 = 0.0622; GoF = 1.041.
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CCDC 676890 (1), 676891 (2), 676892 (3) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif
Received: February 20, 2008
Revised: April 10, 2008
Published online: June 24, 2008
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Keywords: chromium · N,P ligands · oligomerization ·
polymerization · single-component catalysts
.
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