Chromium catalysts based on tridentate pyrazolyl ligands for ethylene oligomerization

Article abstract of DOI:10.1021/om070215i

A set of chromium(III) complexes CrCl₃(L) based on tridentate ligands (1, L = bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine; 2, L = bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]benzylamine; 3, L = bis-[2-(3,5-dimethyl- l-pyrazolyl)ethyl] ether; 4, L = bis[2-(3-phenyl-l-pyrazolyl)ethyl] ether; 5, L = bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl] sulfide)) have been prepared and characterized by elemental analysis, ESI-HRMS, and an X-ray diffraction study for 3. Upon activation with methylaluminoxane (MAO), these precatalysts show high activity in ethylene oligomerization (TOF = (3.4-131.0) × 10 ³ (mol ethylene) (mol Cr)^-1 h^-1 at 80 °C), producing α-olefins in the range C₄-C₁₄₊ with high selectively. The catalytic performances are substantially affected by the ligand environment, especially the bridge donor atom, and reaction conditions. Under optimized conditions ([Cr] = 10 μmol, 80°C, 40 bar ethylene, MAO-to-Cr = 300), precatalyst 5 leads to TOF = 131.0 × 10³ h^-1 and selectivity for α-olefins in the range 89-97%.

Full text of DOI:10.1021/om070215i

4010
Organometallics 2007, 26, 4010-4014
Chromium Catalysts Based on Tridentate Pyrazolyl Ligands for
Ethylene Oligomerization
Fernando Junges,† Maria C. A. Kuhn,† Ana H. D. P dos Santos,† Carlos R. K. Rabello,‡
Christophe M. Thomas,§ Jean-Franc¸ois Carpentier,*,§ and Osvaldo L. Casagrande, Jr.*,†
Laborato´rio de Cata´lise Molecular, Instituto de Quimica-UniVersidade Federal do Rio Grande do Sul,
AVenida Bento Gonc¸alVes, 9500, Porto Alegre, RS, 90501-970, Brazil, Petrobras, Centro de Pesquisas e
DesenVolVimento Leopoldo A. Miguez de Mello (CENPES), AVenida Jequitiba´ 950, Ilha do Funda˜o,
Rio de Janeiro, RJ, 21941-598, Brazil, and Catalyse et Organome´talliques, UMR 6226 CNRS-UniVersite´
de Rennes 1, Institut de Chimie de Rennes, 35042 Rennes Cedex, France
ReceiVed March 7, 2007
A set of chromium(III) complexes CrCl3(L) based on tridentate ligands (1, L ) bis[2-(3,5-
dimethylpyrazolyl)ethyl)]amine; 2, L ) bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]benzylamine; 3, L ) bis-
[2-(3,5-dimethyl-l-pyrazolyl)ethyl] ether; 4, L ) bis[2-(3-phenyl-l-pyrazolyl)ethyl] ether; 5, L ) bis[2-
(3,5-dimethyl-l-pyrazolyl)ethyl] sulfide)) have been prepared and characterized by elemental analysis,
ESI-HRMS, and an X-ray diffraction study for 3. Upon activation with methylaluminoxane (MAO),
these precatalysts show high activity in ethylene oligomerization (TOF ) (3.4-131.0) × 103 (mol ethylene)
(mol Cr)-1 h-1 at 80 °C), producing R-olefins in the range C4-C14+ with high selectively. The catalytic
performances are substantially affected by the ligand environment, especially the bridge donor atom,
and reaction conditions. Under optimized conditions ([Cr] ) 10 µmol, 80 °C, 40 bar ethylene, MAO-
to-Cr ) 300), precatalyst 5 leads to TOF ) 131.0 × 103 h-1 and selectivity for R-olefins in the range
89-97%.
catalysis field. Thus, we have previously communicated a new
class of pentacoordinated NiII complexes based on these
tridentate Z-bridged bis(pyrazolyl) ligands (Z ) O, N, S), which
act as highly selective and efficient precatalysts for ethylene
dimerization in the presence of methylaluminoxane (MAO).7
Also, we have used bis[2-(3,5-dimethyl-1-pyrazolyl)ethyl]amine
in the synthesis of Al, Zn, and Mg complexes and used them
as initiators for lactide polymerization.8 Herein, we report the
synthesis and characterization of CrIII complexes bearing NZN
Introduction
The pursuit of ethylene oligomerization catalysts capable of
selectively producing R-olefins has been a major focus of
research in recent decades, due to their importance in a variety
of industrial processes. Among linear R-olefins, special attention
has been devoted to the production of 1-hexene and 1-octene,
which are mostly used as co-monomers in the production of
linear low-density polyethylene (LLDPE).1 For this purpose,
several ethylene tri- and tetramerization catalyst systems have
been developed, most of which are based on chromium catalysts
bearing NNN,2 PNP,3 and SNS4 ligands. Such tridentate ligands
are quite prone to steric and electronic modifications, allowing
the formation of highly active chromium catalytic species
capable of producing 1-hexene5 and 1-octene3c,6 with high
selectivity.
(3) (a) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.;
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M.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling,
A.; Otto, S.; Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann,
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A.; Dixon, J. T.; Kuhlmann, S.; Maumela, M. C.; Maumela, H.; Morgan,
D. H.; Nongodlwana, P.; Overett, M. J.; Pretorius, M.; Ho¨fener, K.;
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Blann, K.; Bollmann, K.; Dixon, J. T.; Kuhlmann, S.; Maumela, M. C.;
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P. J. Catal. 2007, 245, 277. (i) McGuinness, D. S.; Overett, M.; Tooze, R.
P.; Blann, K.; Dixon, J. T.; Slawin, A. M. Z. Organometallics. 2007, 26,
1108. (j) Elowe, P. R.; McCann, C.; Pringle, P. G.; Spitzmesser, S. K.;.
Bercaw, J. E. Organometallics 2006, 25, 5255. (k) Bowen, L. E.; Haddow,
M. F.; Orpen, A. G.; Wass, D. F. Dalton Trans. 2007, 1160. (l) Walsh, R.;
Morgan, D. H.; Bollmann, A.; Dixon, J. T. Appl. Catal. A: Gen. 2006,
306, 184.
In recent years we have been interested in studying the
potential applications of tridentate nitrogen-, oxygen-, or sulfur-
bridged bis(pyrazolyl) ligands (NZN) in the oligo/polymerization
* Corresponding authors. E-mail: jcarpent@univ-rennes1.fr; osvaldo@
iq.ufrgs.br. Fax: (+55) (51) 3308 7304.
† Univesidade Federal do Rio Grande do Sul.
‡ Petrobra´s/CENPES.
§ Universite´ de Rennes 1.
(1) (a) Vogt, D. In Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH: Weinheim,
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10.1021/om070215i CCC: $37.00 © 2007 American Chemical Society
Publication on Web 06/30/2007

Chromium Catalysts Based on Tridentate Pyrazolyl Ligands
Organometallics, Vol. 26, No. 16, 2007 4011
Table 1. Crystal Data and Structure Refinement for
Scheme 1
3‚CH3CN.
empirical formula
fw
temp
C16H25Cl3CrN5O
461.76 g mol-1
100 K
wavelength
cryst syst
0.71073 Å
Triclinic
space group
unit cell dimens
P1h, (no. 2)
a ) 8.5821(2) Å, R ) 110.1040(10)°
b ) 9.5447(2) Å, â ) 90.7560(10)°
c ) 13.1028(3) Å, γ ) 97.3420(10)°
997.76(4) Å3
volume
Z
2
density (calcd)
absorp coeff
F(000)
1.537 Mg m-3
0.991 mm-1
478
tridentate ligands, which, in association with MAO, afford active
catalysts for the selective production of R-olefins in the range
C4-C14+. We also show the effect of the central donor atom
and the pyrazolyl substituents on turnover frequency (TOF),
selectivity for R-olefins, and product distributions.
cryst size
θ range for data collection
index ranges
0.18 × 0.09 × 0.05 mm
3.00 to 31.01°
-12 e h e 12,
-13 e k e 13,
-18 e l e 18
25 620
no. of reflns collected
no. of indep reflns
completeness to θ ) 27.75°
refinement method
no. of data/restraints/params
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
Results and Discussion
6350 [Rint ) 0.0333]
99.7%
full-matrix least-squares on F2
6350/0/239
Synthesis and Characterization of Bis(Pyrazolyl)-Cr(III)
Complexes. The reaction of CrCl3(THF)3 with 1.1 equiv of
tridentate nitrogen-, oxygen-, or sulfur-bridged bis(pyrazoyl)
ligands (NZN)9 in THF at room temperature affords the
corresponding complexes CrCl3(NZN) (1-5) (Scheme 1), which
were isolated as green- or red wine-colored solids in high yields
(typically 87-92%). These complexes show moderate solubility
at room temperature in dichloromethane, THF, and ethyl acetate
and are readily soluble in acetonitrile. The identity of 1-5 was
established on the basis of elemental analysis and ESI-high-
resolution mass spectrometry (which indicated the formation
of [M - Cl]+ or [M - 2Cl]+ ions), and by an X-ray diffraction
study for complex 3.
Crystals of complex 3 suitable for an X-ray diffraction study
were grown from a concentrated acetonitrile solution at -4 °C.
Crystal data and structure refinement for 3 are summarized in
Table 1, and selected bond distances and angles are listed in
Table 2. The molecular geometry and atom-labeling scheme
are shown in Figure 1. The crystal structure of 3 confirms both
the monomeric nature of the complex and κ3-coordination of
the NON ligand. The geometry around the chromium atom is
best described as slightly distorted octahedral with the chlorine
ligands in a mer disposition. The Cl-Cr-Cl and N-Cr-N
angles are 94.84(2)°, 176.27(2)°, and 88.23(2)° and 172.51-
(6)°, respectively. The chelate bite angles of the N-O-N ligand
in 3 [N-Cr-O, 85.54(5)° and 87.06(5)°] compare similarly to
those in CrCl3{HN(CH2CH2PPh2)2}3b [P-Cr-N, 81.08(8)°,
82.07(8)°] and CrCl3{HN(CH2CH2SR)2}4a [S-Cr-N, 83.07-
(5)°, 82.90(5)°] complexes. On the other hand, these bite angles
are smaller than those found in nickel(II) and copper(I)
complexes having similar bis(pyrazolyl) tridentate ligands, i.e.,
NiCl2{HN(CH2CH2PzMe2)2} [N-Cr-N, 95.55(8)°, 94.12(8)°]7
and Cu{O(CH2CH2PzMe2)2}BF4 [N-Cu-O, 94.05(17)°, 95.91-
1.090
R1 ) 0.0293, wR2 ) 0.0903
R1 ) 0.0399, wR2 ) 0.1286
0.777 and -1.370 e‚Å-3
largest diff peak and hole
Table 2. Selected Bond Lengths [Å] and Angles [deg] for
3‚CH3CN
Cr(1)-O(3)
Cr(1)-N(21)
Cr(1)-Cl(2)
2.0757(12)
2.1221(14)
2.3189(5)
Cr(1)-N(11)
Cr(1)-Cl(1)
Cr(1)-Cl(3)
2.1163(14)
2.2864(5)
2.3437(5)
O(3)-Cr(1)-N(11)
N(11)-Cr(1)-N(21) 172.51(6)
85.54(5)
O(3)-Cr(1)-N(21)
O(3)-Cr(1)-Cl(1)
N(21)-Cr(1)-Cl(1)
N(11)-Cr(1)-Cl(2)
Cl(1)-Cr(1)-Cl(2)
N(11)-Cr(1)-Cl(3)
Cl(1)-Cr(1)-Cl(3)
87.06(5)
176.34(4)
95.22(4)
89.73(4)
94.836(17)
92.28(4)
N(11)-Cr(1)-Cl(1)
O(3)-Cr(1)-Cl(2)
N(21)-Cr(1)-Cl(2)
O(3)-Cr(1)-Cl(3)
N(21)-Cr(1)-Cl(3)
Cl(2)-Cr(1)-Cl(3)
92.23(4)
88.06(4)
88.88(4)
88.96(4)
88.72(4)
176.269(18)
88.227(17)
(18)°].9a The Cr-N distances in 3 [2.1221(14), 2.1163(14) Å]
are longer than those in the corresponding copper(I) complex
Cu{O(CH2CH2PzMe2)2} [1.877(5), 1.874(4) Å].9a Conversely,
the Cr-O distance of 2.0757(12) Å in 3 is shorter than that in
the latter copper complex (2.197(4) Å). These differences in
the bond distances between chromium(III) and copper(I) are
mainly associated with the oxidation state and ionic radii of
the metal centers. The average Cr-Clav (2.316 Å) bond distances
are comparable to those found in similar chromium(III) com-
pounds having tridentate ligands such as CrCl3L (L ) bis-
(carbene)pyridine, Cr-Clav ) 2.327 Å)5b and CrCl3{HN(CH2-
CH2SEt)2} (Cr-Clav ) 2.312 Å).4a
Ethylene Oligomerization. The ethylene oligomerization
behavior of complexes 1-5 was investigated in toluene with
MAO activation. Representative results are summarized in Table
3. Initial studies carried out at 80 °C under 20 bar with [Al]/
[Cr] ) 300 showed that these catalytic systems are active for
the linear oligomerization/polymerization of ethylene.
(6) Blann, K.; Bollmann, A.; Dixon, J. T.; Neveling, A.; Morgan, D.
H.; Maumela, H.; Killian, E.; Hess, F. M.; Otto, S.; Pepler, L.; Mahomed,
H. A.; Overett, M. J. (Sasol Technology) WO 04056479, 2004 .
(7) Ajellal, N.; Kuhn, M. C. A.; Boff, A. D. G.; Hoerner, M.; Thomas,
C. M.; Carpentier, J.-F.; Casagrande, O. L., Jr. Organometallics 2006, 25,
1213.
(8) (a) Lian, B.; Thomas, C. M.; Casagrande, O. L., Jr.; Lehmann, C.
W.; Roisnel, T.; Carpentier, J.-F. Inorg. Chem. 2007, 46, 328. (b) Lian, B;
Thomas, C. M.; Casagrande, O. L., Jr; Roisnel, T.; Carpentier, J.-F.
Polyhedron 2007, in press (doi: 10.1016/j.poly.2007.04.022).
(9) (a) Sorrell, T. N.; Malachowski, M. R. Inorg. Chem. 1983, 22, 1883.
(b) Martens, C. F.; Schenning, A. P. H. J.; Feiters, M. C.; Berens, H. W.;
van der Linden, J. G. M, Admiraal G.; Beurskens, P. T.; Kooijman, H.;
Spek, A. L.; Noltela R. J. M. Inorg. Chem. 1995, 34, 4135.
The catalytic activities and selectivities are substantially
affected by the ligand environment. Thus, precatalyst 1 exclu-
sively produces polyethylene (PE) with an activity of 10 373
(mol ethylene) (mol Cr)-1‚h-1. The DSC shows that the PE
produced under these conditions is essentially linear (Tm ) 136
°C). This strongly suggests that no oligomers were produced
during the oligomerization reaction. Attempts to carry out GPC
analysis failed since this PE is insoluble under standard analysis

4012 Organometallics, Vol. 26, No. 16, 2007
Table 3. Ethylene Oligomerization with 1-5/MAO Systemsa
oligomer distribution (wt %)c
Junges et al.
cat.
(amount,
µmol)
total
product
(mg)
MAO
TOFb
oligomers
(wt %)
PE
(wt %)
entry
(equiv) (103‚h-1
)
C4 (R-C4)
C6 (R-C6)
15.2 (100)
C8 (R-C8)
C10 (R-C10) C12 (R-C12) C14+
1
2
3
4
1 (30.0)
2 (30.0)
3 (30.0)
4 (30.0)
5 (30.0)
3 (30.0)
3 (11.8)
3 (10.9)
3 (10.2)
5 (10.0)
5 (10.0)
5 (10.0)
300
0
100
16.2
6.0
10.5
18.7
22.4
8.9
10.1
8.1
14.7
15.1
6.2
2182
2717
9501
6020
7485
3512
292
4037
5414
5437
11865
2107
300
300
300
300
300
100
300
1000
300
300
300
11.4
41.2
26.7
29.0
13.7
3.40
47.4
69.6
10.7 (100)
12.3 (98.0) 16.2 (100)
16.1 (87.2) 13.8 (87.6)
17.9 (91.0) 15.2 (90.4)
11.4 (92.4) 16.8
11.5 (91.4) 20.9
11.9 (92.6) 21.5
83.8
94.0
89.5
81.3
77.6
91.1
89.9
91.9
85.3
84.9
93.8
10.4 (97.4) 15.5 (95.1) 15.9 (98.8) 14.3 (97.5)
15.6 (96.0) 20.1 (95.4) 18.9 (90.1) 11.8 (93.7)
16.2 (94.5) 19.3 (93.5) 17.3 (87.7) 11.4 (89.1)
15.6 (76.8) 17.2 (69.8) 20.2 (66.8) 17.3 (52.2)
14.1 (93.8) 15.6 (91.2) 21.0 (76.1) 15.1 (92.0)
23.8 (92.4) 22.9 (90.5) 18.3 (88.0) 12.2 (86.0)
18.0 (94.3) 18.8 (95.8) 16.4 (93.7) 12.8 (92.0)
9.7 (94.3) 14.3 (95.8) 17.7 (93.7) 16.3 (92.0)
19.1 (96.2) 24.0 (97.4) 21.1 (88.7) 12.8 (95.7)
5
6.9 (91.7)
7.2 (90.3)
12.9 (49.4)
8.0
6.2
7.9
6d
7
8
9
10
11e
12f
11.6 (91.6) 12.5
11.6 (88.5)
10.4 (65.3)
3.1
8.9
66.2
131.0
28.2
12.0 (91.1) 14.9
8.2 (93.1) 8.6
a Reaction conditions: toluene ) 50 mL, oligomerization time ) 15 min, P(ethylene) ) 20 bar, T ) 80 °C. The results shown are representative of at
b
c
least duplicated experiments. Moles of ethylene converted (mol of Cr)-1‚h-1 as determined by quantitative GLC. Cn, percentage of olefin with n carbon
d
atoms in oligomers; R-Cn, percentage of terminal alkene in the Cn fraction, as determined by quantitative GLC. Preactivation of Cr complex with MAO
under ethylene atmosphere (1 bar) for 10 min followed by pressurizing of reactor with ethylene (20 bar) for 15 min (see ref 12). P(ethylene) ) 40 bar.fT
e
) 100 °C.
nitrogen have an influence on the formation of ethylene
oligomers and/or polyethylene.3c,g,h,5a Furthermore, the chelating
ring size of these tridentate nitrogen-, oxygen-, or sulfur-bridged
bis(pyrazolyl) ligands (NZN) (formation of six-membered-ring
complexes) may in part explain why these catalysts (2-5) are
not selective for tri/tetramerization of ethylene (Vide infra).10,11
Along the same line, the selectivity for oligomers is increased
up to 94 wt % on using complex 3, which bears an O-bridged
ligand (entry 2). Varying the central donor atom in tridentate
3,5-dimethyl-substituted bis(pyrazolyl) ligands NZNMe,Me (Z )
N, O, S) also has a significant effect on catalyst activity.
Relatively high turnover frequencies (TOFs) in the range
11 400-29 000 h-1 are obtained with N- and S-based ligands
Figure 1. Molecular structure of compound 3 (thermal ellipsoids
are drawn at the 50% probability level; hydrogen atoms are omitted
(entries 1 and 4), while the catalyst system derived from the
for clarity).
NON complex 3 gives an improved TOF of 41 200 h-1 (entry
2). The introduction of relatively bulky phenyl substituents (as
procedures (trichlorobenzene, 145 °C), a characteristic that likely
calls for very high molecular weights.
The amount of PE produced can be dramatically reduced
using complex 2, which bears an N-benzyl-substituted ligand.
compared to methyl groups) leads to a noticeable decrease in
catalytic activity in (NON)Cr(III) systems based on complexes
3 and 4 (compare entries 2 and 3).
In this case, distributions of oligomers10 constituted mostly of
1-alkenes are produced with up to 84 wt % selectivity, and poly-
ethylene is a minor product (16 wt %) (entry 1). Thus, the N-H
functionality does not appear to be essential for high activity
and selectivity with this ligand system. Quite different results
have been found for CrCl3{RN(CH2CH2PPh2)2} precatalysts,5a
where the substitution of the NH bridge by a methyl or benzyl
group leads to a dramatic decrease in both activity and
selectivity; in these cases, the amounts of PE produced jumped
to 30 and 66 wt %, respectively, instead of virtually no PE for
the NH-bridged catalyst. Other studies carried out using Cr
catalysts based on bis(diphenylphosphino)amine ligands have
demonstrated that the oligomerization product selectivity as well
as the activity tendencies are connected to the structure and size
of the N-R functionality.3c,g,h Our results and the above-
mentioned studies confirm that the nature of the donor group
[PR2, SR, PzR] as well as the substituent on the central bridging
All chromium complexes 2-5 produce oligomers ranging
from C4 to C14+ with a high selectivity for R-olefins. As shown
in Figure 2, the selectivities for 1-alkenes afforded by precata-
lysts 2-4 derived from N- and O-bridged ligands are very
similar. This indicates that the central donor atoms and also
the pyrazolyl R2 substituents have no significant influence in
this series on the product distribution. Similar trends have been
observed for ethylene oligomerization with MAO-activated
nickel complexes bearing analogous tridentate ligands.7 Enriched
fractions in R-C4 (15.0 wt %), R-C6 (19.2 wt %), and R-C8 (17.0
wt %) are obtained using the catalyst system derived from 5
that contains a sulfur-bridged ligand, with a concomitant
decrease in the R-C10 (11.1 wt %), R-C12 (6.3 wt %), and C14+
fractions (8.0 wt %).
On the basis of these encouraging preliminary results,
complexes 3 and 5 were selected for further optimization,
(11) A six-membered-ring chromium catalyst (1c) was reported to
produce 27 wt % of hexenes with a selectivity for 1-hexene of 99%. Bluhm,
M. E.; Walter, O.; Do¨ring, M. J. Organomet. Chem. 2005, 690, 713.
(12) Considering the poor solubility of complex 3 in toluene, a
preactivation procedure was attempted to possibly generate higher amounts
of active species. Thus, complex 3 was first dissolved and preactivated in
a MAO/toluene solution under ethylene atmosphere (1 bar) for 10 min,
and then oligomerization was carried out under 20 bar. However, we
observed that this procedure led to lower TOF values (13 700 h-1; compare
entries 2 and 5), suggesting that catalyst deactivation takes place.
(10) Those Cr-NZN catalysts produce olefins that do not strictly follow
Schulz-Flory distributions. As a matter of fact, a single R value, which
represents the probability of chain transfer [rate of propagation/(rate of
propagation + rate of chain transfer)] for a Schulz-Flory distribution, cannot
accurately describe most of the observed distributions. However, individual
R values determined from the data reported in Table 3 [R ) mol of Cn+2
/
mol Cn] all fall in the range 0.5-0.9 [0.64 ( 0.12]. This observed
inconsistency of the Schulz-Flory parameters hampers an accurate analysis
of the reaction parameters (time, pressure, nature of the complex, ...).

Chromium Catalysts Based on Tridentate Pyrazolyl Ligands
Organometallics, Vol. 26, No. 16, 2007 4013
rates of chain transfer and/or â-H elimination. Furthermore, it
should be pointed out that the PE produced at 100 °C, in contrast
to the PE produced with 1 at 80 °C (vide supra), is highly
branched (BPE), the DSC curve of which displays two endot-
hermic peaks at 74 and 124 °C (see Supporting Information).
In this case, we assume that the formation of BPE arises from
incorporation of the in situ produced R-olefins into the growing
polymer chain.
In summary, a new set of chromium(III) complexes based
on tridentate ligands has been prepared and evaluated for
ethylene oligomerization under MAO activation. The central
atom donor was found to play a significant role in the catalytic
activity and product distribution. Substitution of the central
nitrogen donor by a benzyl group appears to be an essential
attribute of this ligand class for high activity and minimization
of the amount of PE produced. This is in sharp contrast to related
classes of precatalysts such as CrCl3{HN(CH2CH2PPh2)2}3b,5a
and CrCl3{HN(CH2CH2SR)2}.4a Replacement of the central
bridging nitrogen with a sulfur donor atom affords a catalyst
that exhibits higher TOF and higher selectivity for R-C4-C8.
At this stage, we can only speculate that the absence of
significant selectivity of precatalysts 2-5 for tri/tetramerization
of ethylene may be in part associated with the formation of
six-membered-ring complexes. Promising investigations of
related NZN ligands that induce the formation of five-membered-
ring chromium complexes are underway and will be reported
in due course.
Figure 2. Selectivity of 2-5/MAO oligomerization systems for
R-olefins (80 °C, 20 bar, MAO-to-Cr ) 300).
investigating the influence of cocatalyst type, preactivation
period, catalyst concentration, [Al]/[Cr] ratio, temperature, and
ethylene pressure. A series of tests using complex 5 revealed
that low loading of precatalyst in this system led to high activity.
For instance, with 10.0 µmol of chromium complex (i.e., [Cr]
) 0.2 × 10-3 M), a high TOF of 66 200 h-1 was obtained (entry
9). This effect can be associated with an easier solubilization
of the precatalyst in toluene solution;12 however, a decrease of
excessive exotherms during the oligomerization reaction, which
would induce catalyst decay, cannot be ruled out.
Experimental Section
Activation of 3 with other Lewis acid cocatalysts, including
diethylaluminum chloride (DEAC), and trimethylaluminum
(TMA) produced neither oligomers nor polymer in significant
amounts, presumably due to the low Lewis acidity of these
compounds. However, it should be pointed out that the use of
perfluoroaryl boranes, along with alkyl metal complexes or
alkylating agents, has been shown to be a highly effective
alternative to the use of MAO.3c,i
General Procedures. All manipulations were performed using
standard vacuum line and Schlenk techniques under a purified argon
atmosphere. Et2O, THF, and toluene were distilled from sodium
benzophenone ketyl under argon and degassed by freeze-thaw-
vacuum cycles prior to use. CrCl3(THF)3 was prepared as previously
reported.13 The ligands bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine,
[bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]benzylamine], bis[2-(3,5-dim-
ethylpyrazolyl)ethyl)] ether, bis[2-(3,5-dimethylpyrazolyl)ethyl)]
sulfide, and bis[2-(5-phenylpyrazolyl)ethyl)] ether were prepared
by literature procedures.7,9 Ethylene (White Martins Co.) and argon
were deoxygenated and dried through columns of BTS (BASF) and
activated molecular sieves (3 Å) prior to use. PMAO-IP (Akzo,
12.9 wt % Al solution in toluene) was used as received. Elemental
analyses were performed by the Analytical Central Service of the
Institute of Chemistry-UFRGS (Brazil) and by the Microanalytical
Laboratory at the Institute of Chemistry of Rennes and are the
average of two independent determinations. ESI-HRMS of Cr
complexes was performed on a Perkin-Elmer Sciex API-I single
quadrupole spectrometer; the instrument was operated at atmo-
spheric pressure in the positive-ion mode (5 kV). Quantitative gas
chromatographic analysis of the products of oligomerization reac-
tions was performed on a Varian 3400CX instrument with a Petrocol
HD capillary column (methyl silicone, 100 m length, 0.25 mm i.d.,
and film thickness of 0.5 µm) operating at 36 °C for 15 min and
then heating at 5 °C min-1 until 250 °C, using cyclohexane as an
internal standard.
The influence of the MAO loading on the catalyst behavior
has been studied. When activated with 100 equiv of MAO,
complex 3 gave a relatively low activity (TOF ) 3400 h-1
,
entry 6), which was increased upon using 300 equiv (TOF )
47 400 h-1, entry 7) and even further with 1000 equiv (TOF )
69 600 h-1, entry 8). At the same time, increasing the amount
of MAO from 300 to 1000 equiv led to slightly improved
selectivities for the R-C4 (13.2 to 22.0 wt %) and R-C6 (14.2 to
20.7 wt %) fractions.
Increasing the ethylene pressure resulted in much higher TOF
values, most likely as an effect of increased ethylene concentra-
tion in solution. For instance, the activity of the 5/MAO system
is significantly enhanced from 20 to 40 bar, giving a maximal
TOF of 131 000 h-1 (entry 10). The ethylene pressure does not
affect the total amount of PE formed (ca. 15 wt %, compare
entries 9 and 10), at least in the 20-40 bar range.
Taking into account the higher activity of systems based on
5, this complex was chosen to investigate the effect of
temperature on TOF and oligomer distributions. The optimal
operating temperature for this class of precatalysts is 80 °C. At
60 °C, neither oligomers nor PE was detected, while at 100 °C
partial catalyst deactivation was observed. However, the selec-
tivity for 1-alkene remains almost unchanged (compare entries
9 and 11). In fact, even at high temperature (100 °C), the 5/MAO
catalyst system remains highly selective toward R-olefins (up
to 88.7%) and only 6 wt % of the product formed was PE. We
may reasonably assume that favored formation of R-olefins
rather than polyethylene at 100 °C is a consequence of enhanced
CrCl3{bis[2-(3,5-dimethylpyrazolyl)ethyl)]amine} (1). To a
solution of bis[2-(3,5-dimethylpyrazol)ethyl)]amine (120 mg, 0.46
mmol) in THF (10 mL) was added a solution of CrCl3(THF)3 (54.9
mg, 0.14 mmol) in THF (15 mL), and the resulting solution was
stirred for 30 min at room temperature. The solvent was removed
until ca. one-third remained and 10 mL of pentane added to
complete precipitation. The product was collected by filtration,
washed with pentane, and dried in Vacuo. Yield: 52.8 mg (89%).
(13) Boudjouk, P.; So, J.-H. Inorg. Synth. 1992, 29, 108.

4014 Organometallics, Vol. 26, No. 16, 2007
Junges et al.
Anal. Calcd for C14H23N5Cl3Cr (%): C, 40.06; H, 5.52; N, 16.69.
Found (%): C, 39.88; H, 5.56; N, 16.44. ESI-HRMS: 347.0972
[M - 2Cl]+ (calcd for C14H23N5ClCr, 347.0969).
C14H22N4SCl3Cr (%): C, 38.50; H, 5.08; N, 12.83. Found (%): C,
38.01; H, 4.96; N, 12.13. ESI-HRMS: 400.0324 [M - Cl]+ (calcd
for C14H22N4SCl2Cr, 400.0347).
CrCl3[bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl]benzylamine] (2).
This compound was prepared according to the method described
for 1 using CrCl3(THF)3 (83.2 mg, 0.22 mmol) and bis[2-(3,5-
dimethyl-1-pyrazolyl)ethyl]benzylamine (85.2 mg, 0.24 mmol).
Complex 2 was obtained as a green solid (112 mg, 92%). Anal.
Calcd for C21H29N5Cl3Cr (%): C, 49.47; H, 5.73; N, 13.74. Found
(%): C, 49.13; H, 5.61; N, 13.11. ESI-HRMS: 473.1171 [M -
Cl]+ (calcd for C21H29N5Cl2Cr, 473.1189).
CrCl3[bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl] ether] (3). This
compound was prepared according to the method described for 1
using Cr(THF)3Cl3 (101.5 mg, 0.26 mmols) and bis[2-(3,5 dimethyl-
l-pyrazolyl)ethyl] ether (77.5 mg, 0.29 mmol). Complex 3 was
recovered as a red wine-colored solid (98.4 mg, 90%). Anal. Calcd
for C14H22N4OCl3Cr (%): C, 39.97; H, 5.27; N, 13.32. Found
(%): C, 39.24; H, 5.03; N, 12.66. ESI-HRMS: 384.0554 [M -
Cl]+ (calcd for C14H22N4OCl2Cr, 384.0577).
General Oligomerization Procedure. Ethylene oligomerization
reactions were performed in a 100 mL double-walled stainless steel
Parr reactor equipped with mechanical stirring, internal control of
temperature, and continuous feed of ethylene. A Parr reactor was
dried in an oven at 120 °C for 5 h prior to each run and then placed
under vacuum for 30 min. A typical reaction was performed by
introducing in the reactor, under ethylene atmosphere, toluene (40
mL) and the proper amount of MAO cocatalyst. After 15 min the
catalyst solution was injected into the reactor under a stream of
ethylene, and then the reactor was immediately pressurized.
Ethylene was continuously fed in order to maintain the ethylene
pressure at the desired value. After 15 min, the reaction was stopped
by cooling the system at -20 °C, depressurizing, and introducing
1 mL of ethanol. An exact amount of cyclohexane was introduced
(as internal standard), and the mixture was analyzed by quantitative
GLC.
CrCl3[bis[2-(3-phenyl-l-pyrazolyl)ethyl] ether] (4). This com-
pound was prepared according to the method described for 1 using
Cr(THF)3Cl3 (148 mg, 0.39 mmol) and bis[2-(3-phenyl-l-pyrazolyl)-
ethyl] ether (155 mg, 0.43 mmol). Complex 4 was obtained as a
red wine-colored solid (175 mg, 87%). Anal. Calcd for C22H22N4-
OCl3Cr (%): C, 51.13; H, 4.29; N, 10.84. Found (%): C, 50.87;
H, 4.08; N, 10.55. ESI-HRMS: 480.0556 [M - Cl]+ (calcd for
C22H22N4OCl2Cr, 480.0575).
CrCl3[bis[2-(3,5-dimethyl-l-pyrazolyl)ethyl] sulfide] (5). This
compound was prepared according to the method described for 1
using Cr(THF)3Cl3 (298 mg, 0.79 mmol) and bis[2-(3,5-dimeth-
ylpyrazolyl)ethyl)] sulfide (243 mg, 0.87 mmol). Complex 5 was
obtained as a green-gray solid (299 mg, 87%). Anal. Calcd for
Acknowledgment. This research was in part financially
supported by the Petro´bras S.A. (Brazil), CNPq (Brazil), and
CAPES/COFECUB program (grant 382/02). The authors thank
Petro´bras S.A. for the permission to publish this work.
Supporting Information Available: Complete crystallographic
data (CIF file) for 3, typical GLC analyses of oligomerization
mixtures, and DSC curve of branched polyethylene produced at
high temperature. This material is free of charge via the Internet at
http://pubs.acs.org.
OM070215I