Role of the metal oxidation state in the SNS-Cr catalyst for ethylene trimerization: Isolation of Di- and trivalent cationic intermediates

Article abstract of DOI:10.1021/ja0623717

The reaction of the highly selective [CySCH₂CH ₂N(H)CH₂CH₂SCy]CrCl₃ catalyst precursor with alkyl aluminum activators was examined with the aim of isolating reactive intermediates. Reaction with Me₃Al afforded a cationic trivalent chromium alkyl species {[CySCH₂CH₂N(H)CH ₂CH₂SCy]CrMe(μ-Cl)}₂{(AlMe₃) ₂(m-Cl}₂·(C₇H₈)₂ (1a). Although it was not possible to obtain crystalline samples of sufficient quality from the reaction with MAO (the most preferred activator), the near-to-identical EPR spectra indicated a very close structural similarity with 1a. Ethylene oligomerization tests clearly revealed that 1 and other cationic trivalent dimeric complexes {[CySCH₂CH₂N(H)CH ₂CH₂SCy]CrCl(M-Cl)}₂{AlCl₄} ₂·(C₇H₈)_1.5 (2), monomeric [(CySCH₂CH₂N(H)CH₂CH₂SCy)CrCl ₂ (THF)][AlCl₄] (3), and {[CySCH₂CH ₂N(H)CH₂CH₂SCy]Cr(η²-AlCl ₄)}{Al₂Cl₇} (4) adducts display the same catalyst selectivity as the [CySCH₂CH₂N(H)CH ₂CH₂SCy]CrCl₃ complex and, therefore, are probably all precursors to the same catalytically active species. 2, 3, and 4 were obtained upon treatment of [CySCH₂CH₂N(H)CH ₂CH₂SCy] CrCl₃ with different stoichiometric ratios of AlCl₃. When i-BAO activator was used, reduction of the metal center occurred readily, affording {([CySCH₂CH ₂N(H)CH₂CH₂S Cy]Cr)(μ-Cl)] ₂}{(i-Bu)₂AlCl₂}₂ (5). 5 is also a selective catalyst, thus indicating that trivalent species are most probably precursors to a divalent catalytically active complex. Reaction of CrCl ₂(THF)₂ with the ligand afforded the labile divalent adduct [CySCH₂CH₂N(H)CH₂CH₂SCy] CrCl₂(THF) (6), also catalytically active and selective. Instead, deprotonation of the ligand with n-BuLi followed by reaction with CrCl ₂(THF)₂ gave the dinuclear complex [(μ-CySCH ₂CH₂NCH₂CH₂SCy)CrCl]₂ (7), which did not produce oligomers.

Full text of DOI:10.1021/ja0623717

Published on Web 06/27/2006
Role of the Metal Oxidation State in the SNS-Cr Catalyst for
Ethylene Trimerization: Isolation of Di- and Trivalent Cationic
Intermediates
Amir Jabri,† Claire Temple,† Patrick Crewdson,† Sandro Gambarotta,*,†
Ilia Korobkov,† and Robbert Duchateau*,‡
Contribution from the Department of Chemistry, UniVersity of Ottawa,
Ottawa, Ontario K1N 6N5, Canada, and Department of Chemistry,
EindhoVen UniVersity of Technology, P.O.Box 513, 5600 MB, The Netherlands
Received April 6, 2006; E-mail: sgambaro@science.uottawa.ca
Abstract: The reaction of the highly selective [CySCH2CH2N(H)CH2CH2SCy]CrCl3 catalyst precursor with
alkyl aluminum activators was examined with the aim of isolating reactive intermediates. Reaction with
Me3Al afforded a cationic trivalent chromium alkyl species {[CySCH2CH2N(H)CH2CH2SCy]CrMe(µ-Cl)}2-
{(AlMe3)2(m-Cl}2‚(C7H8)2 (1a). Although it was not possible to obtain crystalline samples of sufficient quality
from the reaction with MAO (the most preferred activator), the near-to-identical EPR spectra indicated a
very close structural similarity with 1a. Ethylene oligomerization tests clearly revealed that 1 and other
cationic trivalent dimeric complexes {[CySCH2CH2N(H)CH2CH2SCy] CrCl(µ-Cl)}2{AlCl4}2‚(C7H8)1.5 (2),
monomeric [(CySCH2CH2N(H)CH2CH2SCy)CrCl2 (THF)][AlCl4] (3), and {[CySCH2CH2N(H)CH2CH2SCy]-
Cr(η2-AlCl4)}{Al2Cl7} (4) adducts display the same catalyst selectivity as the [CySCH2CH2N(H)CH2CH2-
SCy]CrCl3 complex and, therefore, are probably all precursors to the same catalytically active species. 2,
3, and 4 were obtained upon treatment of [CySCH2CH2N(H)CH2CH2SCy] CrCl3 with different stoichiometric
ratios of AlCl3.. When i-BAO activator was used, reduction of the metal center occurred readily, affording
{([CySCH2CH2N(H)CH2CH2S Cy]Cr)(µ-Cl)]2}{(i-Bu)2AlCl2}2 (5). 5 is also a selective catalyst, thus indicating
that trivalent species are most probably precursors to a divalent catalytically active complex. Reaction of
CrCl2(THF)2 with the ligand afforded the labile divalent adduct [CySCH2CH2N(H)CH2CH2SCy]CrCl2(THF)
(6), also catalytically active and selective. Instead, deprotonation of the ligand with n-BuLi followed by
reaction with CrCl2(THF)2 gave the dinuclear complex [(µ-CySCH2CH2NCH2CH2SCy)CrCl]2 (7), which did
not produce oligomers.
Introduction
Natta conditions, provide promising catalytic oligomerization
activity.3 However, in the vast majority of cases, these reactions
The interest for ethylene oligomerization is increasingly
focusing on the preparation of selective catalysts for the
formation of 1-hexene and 1-octene (the most industrially
relevant oligomers) and improving catalytic activity.1 The
starting point for the rational design of new catalysts is obviously
based on the understanding of the mechanism of the catalytic
cycle. It is, therefore, not surprising that considerable debate is
focused on the initiation step of the catalytic cycle, the metal
oxidation state, the type of chain growth and termination, and,
most importantly, about how these factors may affect the
selectivity of the reaction.2 Although chromium appears to be
the most preferred metal,1a it is not uncommon to find transition-
metal complexes across the periodic table that, under Ziegler-
afford a statistical distribution of oligomers (Schulz-Flory).4
Catalysts capable of producing 1-hexene with high selectivity
have been found in only two cases,2l,m,5 and a sole case of a
catalyst that predominantly forms 1-octene has only recently
been reported.6
Trivalent chromium is by far the most commonly encountered
species in these catalytic systems,4-6 and only a few cases
(2) (a) de Bruin, T. J. M.; Magna, L.; Raybaud, P.; Toulhoat, H. Organo-
metallics 2003, 24, 3404. (b) Tobisch, S.; Ziegler, T. Organometallics 2005,
26, 256 and referencess therein. (c) Blok, A. N. J.; Budzelaar, P. H. M.;
Gal, A. W. Organometallics 2003, 22, 2564. (d) van Rensburg, W. J.;
Grove, C.; Steynberg, J. P.; Stark, K. B.; Huyser, J. J.; Steynberg, P. J.
Organometallics 2004, 23, 1207. (e) Tobisch, S.; Ziegler, T. J. Am. Chem.
Soc. 2004, 126, 9059. (f) Jolly, P. W. Acc. Chem. Res. 1996, 29, 544. (g)
Morgan, D. H.; Schwikkard, S. L.; Dixon, J. T.; Nair, J. J.; Hunter, R.
AdV. Synth. Catal. 2003, 939. (h) Yang, Y.; Kim, H.; Lee, J.; Paik, H.;
Jang, H. G. Appl. Catal. A 2000, 193, 29. (i) Bercaw, J. E. The 15th
International Symposium on Olefin Metathesis and Related Chemistry,
(ISOM XV) July 28-Aug 1, 2003, Kyoto, Japan. (j) Theopold, K. H. Eur.
J. Inorg. Chem. 1998, 15. (k) Meijboom, N.; Schaverien, C. J.; Orpen, A.
G. Organometallics 1990, 9, 774. (l) Fang, Y.; Liu, Y.; Ke, Y.; Cuo, C.;
Zhu, N.; Mi, X.; Ma, Z.; Hu, Y. Appl. Catal. A. Gen. 2002, 235, 33. (m)
Ko¨hn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem. Commun. 2000, 1927.
(n) Agapie, T.; Schofer, S. J.; Labinger, J.; Bercaw, J. E. J. Am. Chem.
Soc. 2004, 126, 1304.
† University of Ottawa.
‡ Eindhoven University of Technology.
(1) For a recent review see: (a) Dixon, J. T.; Green, M.; Hess, F. M.; Morgan,
D. H. J. Organomet. Chem. 2004, 689, 3641. (b) Manyik, R. M.; Walker,
W. E.; Wilson, T. P. US 3300458, Union Carbide Corporation, 1967. (c)
Reagan, W. K. EP 0417477, Phillips Petroleum Company, 1991. (d)
Lashier, M. E. EP 0780353A1, Phillips Petroleum. (e) Conroy, B. K.;
Pettijohn, T. M.; Benham, E. A. EP 0608447, Phillips Petroleum Company,
1994. (f) Freeman, J. W.; Buster, J. L.; Knudsen, R. D. US 5,856,257,
Phillips Petroleum Company, 1999.
9
9238
J. AM. CHEM. SOC. 2006, 128, 9238-9247
10.1021/ja0623717 CCC: $33.50 © 2006 American Chemical Society

SNS−Cr Catalyst for Ethylene Trimerization
A R T I C L E S
describing interesting activity from divalent chromium com-
pounds have been documented.1c,4h,7 However, given the well-
established ability of trivalent organo-chromium to give spon-
taneous reduction to the divalent state upon alkylation,8 it is
conceivable that reduction of the metal center may occur in the
early stage of the activation. On the other hand, cationization
of a trivalent chromium alkyl via treatment with strong Lewis
acids has also been shown to afford oligomerization.2n On the
contrary, the possibility that the chromium center might be
reduced to an even lower monovalent state has also been
demonstrated to be, at least in two cases, a viable possibility.4f,h
systems for oligomerization involves the use of pyrrole as a
ligand system,1b we have also examined a highly active catalyst
of a s/p-bonded tripyrrole ligand, finding that, again, the trivalent
complex acts as a precursor to a divalent species.10
Having observed in two cases that reduction of the metal
center readily occurs without modification of the catalyst per-
formance, we have herein examined one of the most remarkable
catalytic system, [RSCH2CH2N(H)CH2CH2SR]CrCl3, which
provides a rare case of an extremely selective and highly active
catalyst for 1-hexene formation.5d,e In particular, the questions
posed and so far unanswered by this exceptional system are:
(1) the metal oxidation state in the catalytically active species
or catalyst precursor; and (2) the nature of the complex generated
upon the reaction of the trivalent complex with the activator;
and (3) the role, if any, of the fairly acidic N-H bond. In
addition, will cationization of the metal center occur similarly
to the case of the [PNP] tetramerization catalyst6 and would a
divalent [RSCH2CH2N(H)CH2CH2SR]Cr complex provide the
same outstanding selectivity as the starting Cr(III) precursor?
In an attempt to address these points and hopefully to assist
future catalyst design, we have now reacted the trivalent
[CySCH2CH2N(H)CH2CH2SCy]CrCl3 catalyst precursor and
divalent analogues with three different alkyl aluminum activa-
tors. The resulting complexes were isolated and tested to assess
both activity and selectivity. In addition, we have also examined
the effect of the deprotonation of the ligand system.
To address the important point of the chromium oxidation
state in the catalytically active species and of how this may
affect the selectivity of the oligomerization process, we have
embarked on a project aimed at isolating and characterizing
complexes arising from the reaction of trivalent or divalent
chromium complexes with alkyl aluminum derivatives. To this
end, we have analyzed the remarkable [PNP]CrCl3 [PNP )
Ph2PN(R)PPh2; R ) Cy, Ph] catalytic tetramerization system,6
finding that the primary interaction of the catalyst precursor
with the MAO activator is reduction to the divalent state and
cationization.9 Because one of the most efficient and selective
(3) (a) Reagan, W. K.; Freeman, J. W.; Conroy, B. K.; Pettijohn, T. M.;
Benham, E. A.; US 5,451,645, Phillips Petroleum Company, 1995. (b)
Sugimura, K.; Nitabara, T. S.; Fujita, T. JP 10,324,710, Mitsui Chemicals
Incorporated, 1998. (c) Ban, K.; Hayashi, T.; Suzuki, Y. JP 11,060,627,
Mitsui Chemicals Incorporated, 1999. (d) Salo, V. B.; Guan, Z. Organo-
metallics 2003, 22, 5033. (e) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988,
27, 185. (f) Andes, C.; Harkins, S. B.; Murtuza, S.; Oyler, K.; Sen, A. J.
Am. Chem. Soc. 2001, 123, 7423. (g) Romano, A. M.; Proto, A.; Santi, R.;
Sommazzi, A.; Grande, M.; Masi, F. PCT Int. Appl. WO 0168572, 2001.
(h) McGuinness, D. S.; Gibson, V. C.; Wass, D. F.; Steed, J. W. J. Am.
Chem. Soc. 2003, 125, 12716. (i) Ruther, T. H.; Braussaud, N.; Cavell, K.
J. Organometallics 2001, 20, 1247. (j) Deckers, P. J. W.; Hessen, B.;
Teuben, J. H. Angew. Chem., Int. Ed. 2001, 40, 2516. (k) Deckers, P. J.
W.; Hessen, B.; Teuben, J. H. Organometallics 2002, 21, 5122.
(4) (a) Flory, P. J. J. Am. Chem. Soc. 1936, 58, 1877. (b) Oguri, M.; Mimura,
H.; Aoyama, H.; Okada, H.; Koie, Y. JP 11,092,407, Tosoh Corporation,
1999. (c) Murakita, S.; Yamamoto, T.; Okada, H.; Osamu, Y. JP
2,001,149,788, 2001. (d) Briggs, J. R. US-A 4,668,838, Union Carbide,
1987. (e) Araki, Y.; Nakamura, H.; Nanba, Y.; Okano, T. US-A 5,856,612,
Mitsubishi Chemicals, 1997. (f) Kohn, R. D.; Haufe, M.; Kociok-Kohn,
G.; Grimm, S.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000,
39, 4337. (g) Briggs, J. R. J. Chem. Soc. Chem. Commun. 1989, 674. (h)
Emrich, R.; Heineman, O.; Jolly, P. W.; Kruger, C.; Verhovnik, G. P. J.
Organometallics 1997, 16, 1511.
(5) (a) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scott, J.; Wass,
D. F. Chem. Commun. 2002, 858. (b) Blann, K.; Bollmann, A.; Dixon, J.
T.; Hess, F. M.; Killian, E.; Maumela, H.; Morgan, D. H.; Neveling, A.;
Otto, S.; Overett, M. J. Chem. Commun. 2005, 620. (c) Overett, M. J.;
Blann, K.; Bollmann, A.; Dixon, J. T.; Hess, F.; Killian, E.; Maumela, H.;
Morgan, D. H.; Neveling, A.; Otto, S. Chem. Commun. 2005, 622. (d)
McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Morgan, D. H.; Dixon,
J. T.; Bollmann, A.; Maumela, H.; Hess, F.; Englert, U. J. Am. Chem. Soc.
2003, 125, 5272. (e) McGuinness, D. S.; Wasserscheid, P.; Morgan, D.
H.; Dixon, J. T. Organometallics 2005, 24, 552. (f) McGuinness, D. S.;
Wasseshield, P.; Keim, W.; Hu, C.; Englert, U.; Dixon, J. T.; Grove, C.
Chem. Commun. 2003, 334.
(6) (a) Bollmann, A.; Blann, K.; Dixon, T. J.; Hess, F. M.; Killian, E.; Maumela,
H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.; Overett,
M. J.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem.
Soc. 2004, 126, 14712. (b) Overett, M. J.; Blann, K.; Bollmann, A.; Dixon,
J. T.; Haalsbroek, D.; Killian, E.; Maumela, H.; McGuinness, D. S.; Morgan,
D. H. J. Am. Chem. Soc. 2005, 127, 10723.
Experimental Section
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification
system. The reaction mixtures were analyzed using a CP 9000 gas
chromatograph (GC) fitted with a 30 m × 0.32 mm i.d. capillary CP
volamine column and with an FID detector. Infrared spectra were
recorded on an ABB Bomem FTIR instrument from Nujol mulls
prepared in a drybox. Samples for magnetic susceptibility were
preweighed inside a drybox equipped with an analytical balance and
measured on a Johnson Matthey Magnetic Susceptibility balance.
Elemental analysis was carried out with a Perkin-Elmer 2400 CHN
analyzer. Data for X-ray crystal structure determination were obtained
with a Bruker diffractometer equipped with a 1 K Smart CCD area
detector. CrCl3(THF)3 and CrCl2(THF)2 were prepared according to
standard procedures. The CySCH2CH2N(H)CH2CH2SCy ligand was
prepared using a slightly modified literature procedure.5d Complex
[CySCH2CH2N(H)CH2CH2SCy]CrCl3 was prepared by adapting the
procedure reported in the literature for the phenyl derivative.5d
Aluminum activators, i-BAO (Aldrich), TMA (Strem), MAO (Chem-
tura) were used as received.
Preparation of {[CySCH2CH2N(H)CH2CH2SCy]CrMe(µ-Cl)}2-
{(AlMe3)2(m-Cl)}2‚(C7 H8)2 (1a). A mixture of [CySCH2CH2N(H)CH2-
CH2SCy]CrCl3 (0.503 g, 1.1 mmol) and TMA (2.2 mL in hexane 2.0
M, 4.4 mmol) in toluene (10 mL) was stirred at 22 °C resulting in a
dark-green suspension. After centrifugation and decantation, the result-
ing green solution was concentrated in vacuo and stored at -35 °C for
1 day. Green crystals of analytically pure 1a (0.279 g, 0.20 mmol, 36%
(7) (a) Oguri, M.; Mimura, H.; Aoyama, H.; Okada, H.; Koie, Y. JP 11,092,407,
Tosoh Corporation, 1999. (b) Murakita, S.; Yamamoto, T.; Okada, H.;
Osamu, Y. JP 2,001,149,788, 2001. (c) Small, B. L.; Carney, M. J.; Holman,
D. M.; O’Rourke, C. E.; Halfen, J. A. Macromolecules 2004, 37, 4375.
(8) (a) Bhandari, G.; Kim, Y.; McFarland, J. M.; Rheingold, A. L.; Theopold,
K. H. Organometallics 1995, 14, 738. (b) MacAdams, L. A.; Buffone, G.
P.; Incarvito, C. D.; Golen, J. A.; Rheingold, A. L.; Theopold, K. H. Chem.
Commun. 2003, 1164. (c) Kirtley, S. W. ComprehensiVe Organometallic
Chemistry; Wilkinson, G., Ed.; Pergamon Press: Oxford, 1978. (d) Winter,
M. J. ComprehensiVe Organometallic Chemistry, 2nd ed.; Wilkinson, G.,
Ed.; Pergamon Press: Oxford, 1995. (e) Schulzke, C.; Enright, D.;
Sugiyama, H.; LeBlanc, G.; Gambarotta, S.; Yap, G. P. A.; Thompson, K.
K.; Wilson, D. R.; Duchateau, R. Organometallics 2002, 21, 3810. (f)
Sugiyama, H.; Aharonian, G.; Gambarotta, S.; Yap, G. P. A.; Budzelaar,
P. H. J. Am. Chem. Soc. 2002, 124, 12268.
based on Cr) were isolated from the solution. IR (Nujol, cm-1) νN-H
:
3181. Anal. Calcd (found) for C30H60NS2CrCl2Al2: C 53.32 (53.21);
H 8.95 (8.83); N 2.07 (2.01). µeff ) 3.94µB.
Reaction of [CySCH2CH2N(H)CH2CH2SCy]CrCl3 with MAO
Formation of 1b. A mixture of [CySCH2CH2N(H)CH2CH2SCy]CrCl3
(9) Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.; Duchateau, R.
Organometallics 2006, 25, 715.
(10) Crewdson, P.; Gambarotta, S.; Djomann, M. C.; Korobkov, I.; Duchateau,
R. Organometallics 2005, 24, 5214.
9
J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006 9239

A R T I C L E S
Jabri et al.
Table 1. Crystal Data and Structure Analysis Results
1a
2
3
4
5
6
7
formula
C
29.85H59.55
-
C
26.50H43N-
C20H39Al-
Cl6CrNOS2
665.32
C16H30Al3-
Cl12CrNS2
858.87
C24H49Al-
Cl3CrNS2
601.09
C20H39Cl2-
CrNOS2
495.53
C16H30Cl-
Al2Cl2.15CrNS2
678.83
P2(1)/c
13.782(12)
12.776(11)
21.606(19)
S2CrCl6Al
731.42
P2(1)/n
10.8236(18)
29.167(5)
11.750(2)
CrNS2
Mw
387.9(7)
P1h
space group
a (Å)
b (Å)
c (Å)
R
P2(1)/c
P1h
P1h
P2(1)/n
7.4228(16)
25.919(6)
16.281(3)
9.467(5)
11.999(7)
17.895(10
83.573(10)
81.026(10)
67.885(9)
1857.1(18)
2
10.987(4)
12.816(5)
13.475(5)
67.230(6)
83.158(6)
66.385(6)
1601.7(10)
2
11.198(7)
20.051(15)
12.262(9)
8.019(3)
8.794(4)
14.224(6)
79.18(3)
78.75(6)
85.58(4)
965.5(7)
2
â
γ
96.037(16)
104.261(3)
92.681(4)
115.98(3)
V (Å3)
Z
3783(6)
4
3595.1(10)
4
3128.9(12)
4
2475(3)
4
radiation
T (K)
0.71073
208(2)
1.192
0.630
1457
0.71073
198(2)
1.351
0.923
1520
0.71073
198(2)
1.412
1.054
1380
0.71073
207(2)
1.536
1.367
864
0.71073
208(2)
1.246
0.778
640
0.71073
208(2)
1.330
0.857
1052
0.71073
209(2)
1.335
0.941
412
D
µ
calcd (g cm-3
)
calcd (mm-1
)
F000
R, Rw2a
GoF
0.0734, 0.1659
1.042
0.0684, 0.1643
1.027
0.0676, 0.1262
1.040
0.0969, 0.2271
1.069
0.0734, 0.1328
1.033
0.0642, 0.1173
1.012
0.0397, 0.0671
1.012
a R ) ∑jFoj - jFcj/∑jFj. Rw ) [∑(jFoj - jFcj)2/∑wFo2]1/2
.
(0.475 g, 1.03 mmol) and a 10% MAO solution (3.4 mL toluene, 5.15
mmol) in toluene (15 mL) was stirred at 22 °C, affording a green
precipitate (0.599 g). IR (Nujol, cm-1) νN-H: 3191. Anal. Calcd (found):
C (50.91); H (7.63); N (2.31)
Degradation of 1b and Formation of CrCl3(pyridine)3. A suspen-
sion of 1b (0.250 g) in THF (10 mL) was treated with neat pyridine (2
mL). The insoluble solid partly dissolved, and after centrifugation, the
resulting solution was allowed to stand at room temperature for 3 days.
Dark-green X-ray quality crystals of CrCl3(pyridine)3 separated (0.098
g). Anal. Calcd (found) for C15H15N3CrCl3: C 45.54(45.41); H 3.82
(3.79); N 10.62 (10.61). µeff ) 3.87 µB. Crystallographic cell parameters
were identical to those reported in the literature.11
mmol) in toluene (15 mL) was stirred at 22 °C, forming a blue-green
suspension. Pale green-blue crystals of 5 (0.333 g, 0.28 mmol, 37%
based on chromium) were grown from a centrifuged solution during
storage at -35 °C for 1 day. IR (Nujol, cm-1) νN-H: 3169. Anal. Calcd
(found) for C24H49AlCl3CrNS2: C 47.95 (47.91); H 8.22 (8.19); N 2.33
(2.28). µeff ) 4.60 µB.
Preparation of [CySCH2CH2N(H)CH2CH2SCy]CrCl2(THF) (6).
A mixture of CrCl2(THF)2 (0.507 g, 1.9 mmol) and CySCH2-
CH2N(H)CH2 CH2SCy (0.570 g, 1.9 mmol) in toluene (30 mL) was
stirred at 22 °C, forming a blue solution. Crystals of 6 (0.870 g, 1.8
mmol, 95%) formed over the course of 1 h at -35 °C. IR (Nujol, cm-1
)
ν: 3173. Anal. Calcd (found) for C20H39Cl2CrNOS2: C 48.38 (48.31);
Preparation of {[CySCH2CH2N(H)CH2CH2SCy]CrCl(µ-Cl)}2-
{AlCl4}2‚(C7H8)3 (2). A mixture of CrCl3(THF)3 (0.498 g, 1.3 mmol),
CySCH2CH2N(H)CH2CH2SCy (0.402 g, 1.3 mmol), and AlCl3(1.729
g, 13.0 mmol) in toluene (15 mL) was refluxed for 5 min, forming a
dark-green solution, which was filtered. Dark-green crystals of analyti-
cally pure 2 (0.639 mg, 0.43 mmol, 66% based on chromium) were
grown during storage at 22 °C for 1 day, followed by storage at -35
°C for 1 day. IR (Nujol, cm-1) ν: 3188. Anal. Calcd (found) for
H 7.92 (7.88); N 2.82 (2.79). µeff ) 4.86 µB.
Preparation of [(µ-CySCH2CH2NCH2CH2SCy)CrCl]2 (7). To a
stirring solution of CySCH2CH2N(H)CH2CH2SCy (0.500 g, 1.6 mmol)
in THF (15 mL) n-BuLi (0.67 mL of a 2.5 M solution, 1.7 mmol) was
added at -35 °C, and the resulting mixture was allowed to warm to
room temperature. The addition of CrCl2(THF)2 (0.427 g, 1.6 mmol)
to this solution afforded a light-blue solution, which was evaporated
to dryness under vacuum. The solid residue was recrystallized from
hot toluene to yield blue crystals of 7 (0.560 g, 0.58 mmol, 87% based
on chromium) at room temperature. Anal. Calcd (found) for C16H30-
ClCrNS2: C 49.53 (49.49); H 7.79 (7.73); N 3.61 (3.57). µeff ) 3.95µB.
X-ray Crystallography. Suitable crystals were selected, mounted
on a thin, glass fiber with paraffin oil, and cooled to the data collection
temperature. Data were collected on a Bruker AXS SMART 1 k CCD
diffractometer. For 1a and 6, 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). For 5 and 7, data
collection is performed with four batch runs at phi ) 0.00 deg (600
frames), at phi ) 90.00 deg (600 frames), at phi ) 180.00 deg (600
frames), and at phi ) 270.00 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 reflections were applied. The systematic absences
and unit-cell parameters 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
displacement parameters. All hydrogen atoms were treated as idealized
contributions. All scattering factors and anomalous dispersion factors
are contained in the SHELXTL 6.12 program library.
C26.50H43NS2CrCl6Al: C 43.51 (43.49); H 5.93 (5.88); N, 1.91 (1.86).
[µeff ) 3.94µB.]
Preparation of {[CySCH2CH2N(H)CH2CH2SCy]CrCl2(THF)}-
{AlCl4} (3). A mixture of CrCl3(THF)3 (0.510 g, 1.3 mmol), CySCH2-
CH2N(H)CH2CH2SCy (0.402 g, 1.3 mmol), and anhydrous AlCl3 (0.355
g, 2.7 mmol) in toluene (15 mL) was refluxed for 5 min, forming a
blue solution, which was filtered. Blue-green crystals of analytically
pure 3 (0.752 g, 1.13 mmol, 87%) were grown during storage at -35
°C for a few days. IR (Nujol, cm-1) ν: 3178. Anal. Calcd (found) for
C20H39Cl6CrNOS2Al: C 36.10 (36.09); H 5.91 (5.88); N 2.11 (2.06).
µeff ) 3.78µB.
Preparation of {[CySCH2CH2N(H)CH2CH2SCy]Cr(η2-AlCl4)}-
{Al2Cl7} (4). A mixture of CrCl3(THF)3 (0.503 g, 1.3 mmol), CySCH2-
CH2N(H)CH2CH2SCy (0.402 g, 1.3 mmol), and AlCl3 (3.46 g, 26
mmol) in toluene (15 mL) was refluxed for 5 min, forming a red
solution, which was filtered. Red crystals of 4 mixed together with
unknown blue-colored materials (0.726 mg,) were separated during
storage at -35 °C for a few days. Regrettably, the purification of 4
proved impossible and prevented meaningful analytical determination
and catalytic activity testing.
2
Preparation of {[([CySCH CH2N(H)CH2CH2SCy]Cr)(µ-Cl)]2}-
{(i-Bu)2AlCl2}2 (5). A mixture of [CySCH2CH2N(H)CH2CH2SCy]CrCl3
(0.693 g, 1.5 mmol) and a 10% IBAO solution (7 mL toluene, 2.0
Complex 1 has one molecule of toluene per chromium atom in the
(11) Howard, S. A.; Hardcastle, K. I. J. Cryst. Spectrosc. Res. 1985, 15, 643.
lattice whose methyl group was disordered over two positions with
9
9240 J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006

SNS−Cr Catalyst for Ethylene Trimerization
A R T I C L E S
Table 2. Selected Bond Distances (Å) and Angles (deg)
1a
2
3
4
Cr(1)-C(17) ) 2.049(5)
Cr(1)-Cl(1) ) 2.233(3)
Cr(1)-Cl(1) ) 2.295(2)
Cr(1)-Cl(1) ) 2.498(4)
Cr(1)-N(1) ) 2.088(4)
Cr(1)-N(1) ) 2.057(7)
Cr(1)-Cl(2) ) 2.292(2)
Cr(1)-Cl(2) ) 2.409(3)
Cr(1)-Cl(1) ) 2.339(2)
Cr(1)-Cl(2) ) 2.364(3)
Cr(1)-N(1) ) 2.068(6)
Cr(1)-Cl(3) ) 2.229(3)
Cr(1)-S(1) ) 2.445(2)
Cr(1)-S(1) ) 2.436(3)
Cr(1)-S(1) ) 2.440(2)
Cr(1)-N(1) ) 2.058(10)
Cr(1)-S(2) ) 2.446(2)
Cr(1)-S(2) ) 2.447(3)
Cr(1)-S(2) ) 2.443(2)
Cr(1)-S(1) ) 2.447(4)
Cr(1)-Cl(1a) ) 2.519(2)
Cl(1)-Cr(1)-S(1) ) 93.41(11)
Cl(1)-Cr(1)-Cl(2) ) 94.34(10)
Cl(1)-Cr(1)-N(1) ) 91.7(2)
Cl(1)-Cr(1)-S(2) ) 93.45(10)
Cl(1)-Cr(1)-Cl(2a) ) 179.10(11)
S(1)-Cr(1)-S(2) ) 166.94(11)
S(2)-Cr(1)-Cl(2) ) 95.35(9)
S(1)-Cr(1)-Cl(2) ) 95.20(10)
N(1)-Cr(1)-S(1) ) 84.4(2)
N(1)-Cr(1)-S(2) ) 84.3(2)
Cr(1)-O(1) ) 2.034(5)
Cr(1)-S(2) ) 2.464(4)
Al(1)-Cl(2) ) 2.412(3)
O(1)-Cr(1)-Cl(1) ) 91.92(15)
O(1)-Cr(1)-Cl(2) ) 90.64(15)
O(1)-Cr(1)-S(1) ) 95.50(15)
O(1)-Cr(1)-S(2) ) 95.38(15)
O(1)-Cr(1)-N(1) ) 179.0(2)
N(1)-Cr(1)-Cl(1) ) 89.08(18)
N(1)-Cr(1)-S(1) ) 84.31(17)
Cl(1)-Cr(1)-Cl(2) ) 177.12(9)
S(1)-Cr(1)-S(2) ) 169.12(8)
Al(1)-Cl(1) ) 2.218(5)
C(17)-Cr(1)-N(1) ) 91.8(2)
C(17)-Cr(1)-Cl(1) ) 94.03(16)
N(1)-Cr(1)-Cl(1) ) 174.09(12)
C(17)-Cr(1)-S(1) ) 94.67(15)
N(1)-Cr(1)-S(1) ) 83.63(13)
Cl(1)-Cr(1)-S(1) ) 95.09(7)
S(1)-Cr-S(2) ) 164.87(6)
Al(1)-Cl(2)-Al(2) ) 114.28(9)
Al(1)-Cl(2) ) 2.247(5)
N(1)-Cr(1)-Cl(2) ) 170.6(4)
Cl(1)-Cr(1)-Cl(3) ) 178.20(13)
S(1)-Cr(1)-S(2) ) 166.16(12)
Cr(1)-Cl(1)-Al(1) ) 90.91(14)
Cl(1)-Cr(1)-Cl(2) ) 82.89(11)
Cl(1)-Al(1)-Cl(2) ) 93.36(16)
5
6
7
Cr(1)-N(1) ) 2.134(5)
Cr(1)-O(1) ) 2.098(6)
Cr(1)-N(1) ) 2.071(3)
Cr(1)-Cl(1) ) 2.3486(19)
Cr(1)-S(2) ) 2.511(2)
Cr(1)-N(4) ) 2.142(7)
Cr(1)-N(1a) ) 2.093(3)
Cr(1)-Cl(2) ) 2.372(3)
Cr(1)-Cl(1) ) 2.3581(16)
Cr(1)-S(2) ) 2.5359(17)
Cr(1)-S(1) ) 2.6948(17)
Cr(1)-Cr(1)#1 ) 2.9796(18)
N(1)-Cr(1)-N(1a) ) 88.61(11)
N(1)-Cr(1)-Cl(1) ) 176.29(8)
N(1a)-Cr(1)-Cl(1) ) 94.68(8)
N(1)-Cr(1)-S(2) ) 80.26(8)
N(1a)-Cr(1)-S(2) ) 154.52(8)
Cr(1)-Cl(1a) ) 2.714(2)
N(1)-C(2) ) 1.454(8)
N(1)-Cr(1)-Cl(1) ) 178.79(16)
N(1)-Cr(1)-S(2) ) 82.45(17)
Cl(1)-Cr(1)-S(2) ) 97.07(7)
N(1)-Cr(1)-Cl(1a) ) 187.57(16)
Cr(1)-Cl(1) ) 2.405(3)
Cr (1)-S (1) ) 2.846(6)
Cr (1)-S (2) ) 2.840(6)
O(1)-Cr(1)-N(4) ) 175.4(3)
O(1)-Cr(1)-Cl(2) ) 90.49(18)
N(4)-Cr(1)-Cl(2) ) 92.0(2)
O(1)-Cr(1)-Cl(1) ) 90.00(18)
N(4)-Cr(1)-Cl(1) ) 87.6(2)
Table 3 a
c
MAO
alkenesb
(mL)
PE
(g)
activity
activity
C6
C8
C10
C12
C14
C16
catalyst
(SNS)CrCl3
1a
(equiv)
(g/mol/h)
(g/gCr/h)
(mol %)
(mol %)
(mol %)
(mol %)
(mol %)
(mol %)
1000
13.5
0.08
302 850
5824
>98
traces
traces
traces
traces
traces
2000
1000
500
3.5
16.0
18.5
4.36
0.8
0.4
78.516
358 933
415 016
1510
6903
8024
31.98
>98
>98
28.19
traces
traces
18.37
traces
traces
10.98
traces
traces
6.20
traces
traces
4.28
traces
traces
1b
2000
1000
4.5
5.1
3.2
2.3
100 950
114 410
1941
2200
96.69
>98
0.74
traces
0.74
traces
0.74
traces
0.64
traces
0.44
traces
2
3
5
1000
1000
10.5
32.2
0.18
1.01
235 550
723 475
4530
>98
>98
traces
traces
traces
traces
traces
traces
traces
traces
traces
traces
13913
2000
1000
500
3.8
6.0
8.2
10.5
1.47
0.86
0.78
0.70
85 247
1639
2588
3559
4530
53.37
70.79
96.53
>98
17.90
9.23
0.99
0.36
12.26
8.61
0.87
0.39
8.37
5.66
0.72
0.29
5.04
3.47
0.49
0.19
3.06
2.24
0.41
0.20
134 600
185 075
235 550
300
6
1000
1000
1000
1000
6.7
0
2.6
151 425
2912
0
>98
0
traces
0
traces
traces
traces
traces
7
0.12
0.08
0.42
0
0
0
0
0
0
0
0
0
0
0
0
0
blank
0
0
0
0
0
CrCl2(THF)2
0
n/a
n/a
traces
traces
a T ) 50 °C, V ) 150 mL, P ) 35 bar, catalyst ) 30m mole, time ) 60 min. b By integration of the NMR olefinic resonances with respect to the Me
of the toluene solvent. c By G.C.; values of C4 are not given due to volatility.
equal occupancy. The (Me3Al)2(m-Cl) counteranion had one of the six
methyl groups carbon atom not properly behaving during the refinement.
By arbitrarily attributing 15% chlorine character and the remaining as
carbon character, it was possible to obtain a satisfactory refinement.
Complex 2 has one and a half molecule of toluene per chromium
containing unit. Relevant crystal data and bond distances and angles
are given in Tables 1 and 2, respectively.
General Oligomerization Procedure. A 250-mL steel Bu¨chi reactor
was dried in an oven at 120 °C for 3 h prior to each run and then
placed under vacuum for 30 min and purged with three cycles of Ar/
vacuum. The reactor was then preheated, charged with toluene and the
desired amount of MAO, pressurized with 35 bar of ethylene, and stirred
at 50 °C. After 15 min, the pressure was momentarily released to allow
injecting the catalyst solution into the reactor under a stream of ethylene,
and then the reactor was immediately repressurized. The reaction was
allowed to run for 45 min, after which the temperature was rapidly
reduced, the reactor was depressurized, and a mixture of EtOH/HCl
was injected to quench the reaction. The organic and aqueous phases
were then separated from the polymer. Precautions were taken to
maintain the temperature as low as possible during the workup to
minimize loss of volatiles. Polymeric materials were sonicated with
an aqueous solution of HCl and dried at 60 °C for 18 h under reduced
pressure before the final mass was weighed. Ratios (or selectivity) of
oligomers were obtained by GC by using calibrated standard solutions.
The overall catalytic activity was determined by integrating the intensity
of the olefinic NMR resonances versus the Me group of the toluene
solvent. Results are summarized in Table 3.
9
J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006 9241

A R T I C L E S
Scheme 1
Jabri et al.
Results and Discussion
the protonated nitrogen atom of the ligand [Cr(1)-N(1) )
2.088(4) Å]. The sulfur atoms of each ligand are placed on the
two axial positions [Cr(1)-S(1) ) 2.445(2) Å; Cr(1)-S(2) )
2.446(2) Å], showing substantial deviation from linearity [S(1)-
Cr(1)-S(2) ) 164.87(6)°]. The presence of the hydrogen on
the ligand nitrogen atom is clearly indicated by the deviation
of the nitrogen coordination geometry from planarity [C(2)-
N(1)-Cr(1) ) 113.0(3)°; C(3)-N(1)-Cr(1) ) 112.4(3)°;
C(2)-N(1)-C(3) ) 111.7(4)°]. Two [(Me3Al)2(m-Cl)] anions
unconnected to the dicationic chromium unit and two molecules
of interstitial toluene complete the structure.
The reaction of the catalyst precursor [CySCH2CH2N(H)CH2-
CH2 SCy]CrCl3 with excess Me3Al (10 equiv) afforded a good
yield of {[CySCH2 CH2N(H)CH2CH2SCy]Cr(Me)(m-Cl}2-
{(Me3Al)2(µ-Cl)}2 (1a) (Scheme 1), which was isolated as an
X-ray quality dark-green crystalline sample.
The structure of 1a consists of a symmetry generated
dicationic dimer with an overall edge-sharing bi-octahedral
structure (Figure 1) counterbalanced by two aluminate [(Me3Al)2-
(m-Cl)]- anions. The two slightly distorted octahedral chromium
atoms in the cationic unit are connected by two bridging chlorine
atoms [Cr(1)-Cl(1) ) 2.339(2) Å; Cr(1)-Cl(1a) ) 2.519(2)
Å] forming a Cr2Cl2 planar core [Cl(1)-Cr(1)-Cl(1a) )
85.54(6)°; Cr(1)-Cl(1)-Cr(1a) ) 94.46(6)°]. The equatorial
plane [Cl(1)-Cr(1)-N(1) ) 174.09(12)°; Cl(1)-Cr(1)-C(17)
) 94.03(16)°; N(1)-Cr(1)-C(17) ) 91.8(2)°] around each
chromium atom is defined by the two bridging chlorines, the
terminally bonded Me group [Cr(1)-C(17) ) 2.049(5) Å], and
The fact that the two cationic chromium atoms have retained
the original trivalent state is in striking contrast with the
Ph2PN(R)PPh2 [PNP]9 and tripyrrolide chromium10 catalytic
systems where reduction to the divalent state occurred rapidly
at room temperature upon mixing the trivalent precursor with
an alkyl aluminum. Furthermore, 1a is thermally robust and
showed signs of partial decomposition only after 1 h of reflux
in toluene. However, the rate of the decomposition was greatly
accelerated by the presence of excess MAO or Me3Al. Complete
decomposition occurred at room temperature within 1 h upon
treatment with 100 equiv of alkyl aluminum reagent. The
cationization of the metal center is in line with the behavior of
the [PNP] system9 and provides another case of cationic
organochromium species.2i,n,9,12 The second surprising feature,
as indicated by both the structural parameters and the IR
spectrum [νN-H ) 3181 cm-1], is that the N-H function of the
ligand remained apparently unmodified in the presence of the
alkylating agent.
The same reaction of [CySCH2CH2N(H)CH2CH2SCy]CrCl3
carried out with MAO (the most preferred activator) afforded
also a dark-green homogeneous crystalline substance (1b), which
regrettably was of insufficient quality for X-ray structural
determination. Furthermore, its very low solubility in hydro-
carbon solvents prevented recrystallization. However, the very
close similarity of the colors of 1a and 1b suggested the
possibility that the two species might both possess trivalent
chromium with similar coordination environments. To substanti-
ate these points, we have recorded EPR spectra of both
complexes (Figure 2), because the d3 and d4 electronic con-
figurations of Cr(III) and Cr(II) provide distinctively different
Figure 1. Drawing of the dicationic part of 1a. The two [(Me3Al)2(m-Cl)]
counteranions and two interstitial toluene molecules have been omitted for
clarity. Thermal ellipsoids are drawn at the 30% probability level.
(12) See for example: (a) Foo, D. M.; Sinnema, P. J.; Twamley, B.; Shapiro,
P. J. Organometallics 2002, 21, 1005. (b) Thomas, B. J.; Theopold, K. H.
J. Am. Chem. Soc. 1988, 110, 5902. (c) MacAdams, L. A.; Buffone, G. P.;
Incarvito, C. D.; Rheingold, A. L.; Theopold, K. H. J. Am. Chem. Soc.
2005, 127, 1082. (d) Kohn, R. D.; Haufe, M.; Kociok-Kohn, G.; Grimm,
S.; Wasserschield, P.; Heim, W. Angew. Chem., Int. Ed. 2000, 39, 4337.
(e) Bazan, G. C.; Rogers, J. S.; Fang, C. C. Organometallics 2001, 20,
2059. (f) Rogers, J. S.; Bazan, G. C. Chem. Commun. 2000, 1209.
Figure 2. Powder EPR spectrum of Cr(III) of 1a and 1b at -160 °C.
Frequency ) 9118.995 and 9119.687 MHz, respectively. X axis in mT.
9
9242 J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006

SNS−Cr Catalyst for Ethylene Trimerization
A R T I C L E S
Scheme 2
spectral features. As can be observed in Figure 2, the spectra
of 1a and 1b display only minor differences. The complexity
of the spectra would require an in-depth magnetic study to
enable interpretation and simulation of the spectral features that
are beyond the scope of this study. However, for the purpose
of this work, it will suffice to observe the close spectral
similarity, which substantiates the conclusion that even in the
case of 1b, two trivalent chromium species are part of a
dinuclear species with an overall integer spin configuration. The
room-temperature magnetic moment of 1a [µeff ) 3.94 µBM per
chromium atom] is in the range expected for the high-spin d3
electronic configuration of an octahedral Cr(III). 1b has a
comparable magnetic moment [µeff ) 3.93 µBM per chromium
atom] once the same formulation is assumed with the only
difference consisting of the replacement of the Me3AlCl
counteranion with 1/n[MeAlOCl]n. We suspect that the poly-
meric nature of the counteranion is, in fact, most likely
responsible for the lack of suitable crystallinity and very poor
solubility. Finally, treatment of 1b in THF with an excess of
pyridine afforded a significant amount of CrCl3(pyridine)3
(Scheme 1). Although other ill-defined byproducts were also
formed during this degradation, this experiment qualitatively
confirms that complex 1b also contains trivalent chromium. The
IR spectrum of 1b indicated that, even in this case, the ligand
was not deprotonated at the nitrogen atom [νN-H ) 3191 cm-1].
The only significant spectral difference between the two species
consists of the presence of a large and intense absorption at
580 cm-1 in the spectrum of 1b, attributable to the Al-O-Al
groups.
consists of a symmetry-generated ionic dimer very similar to
1a (Figure 3). The dicationic unit is formed by two slightly
distorted octahedral chromium atoms connected by two bridging
chlorine atoms [Cr(1)-Cl(2) ) 2.364(3) Å; Cr(1)-Cl(2a) )
2.391(3) Å] forming a planar Cr2Cl2 core [Cl(2)-Cr(1)-Cl(2a)
) 84.81(9)°; Cr(1)-Cl(2)-Cr(1a) ) 95.19(9)°]. The equatorial
plane [Cl(2)-Cr(1)-N(1) ) 173.9(2)°; Cl(2)-Cr(1)-Cl(1) )
94.34(10)°; N(1)-Cr(1)-Cl(1) ) 91.7(2)°] around each chro-
mium atom is defined by three chlorine atoms (two bridging
and one terminally bonded) [Cr(1)-Cl(1) ) 2.233(3) Å] and
the ligand nitrogen atom [Cr(1)-N(1) ) 2.057(7) Å]. The sulfur
atoms of each ligand are placed on the two axial positions
[Cr(1)-S(1) ) 2.436(3) Å; Cr(1)-S(2) ) 2.447(3) Å], and the
The behavior of [CySCH2CH2N(H)CH2CH2SCy]CrCl3 with
AlCl3 (Scheme 2) indicates that cationization of the trivalent
center is a trend in the reactivity of this catalyst precursor with
Lewis acids. The reaction afforded three different cationic
complexes, either monomeric or dimeric, depending on the
amount of AlCl3 employed. 2 (Figure 3) has the same structure
as 1a except for the replacement of the Me groups by chlorine
in both the cationic and anionic moieties. The structure of 2
Figure 3. Drawing of the cationic part of 2. The two [AlCl4]- anions have
been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level.
9
J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006 9243

A R T I C L E S
Jabri et al.
Figure 4. Drawing of the cationic part of 3. The AlCl4 counteranion has
been omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level.
Figure 5. Drawing of the cationic part of 4. The [Al2Cl7]- anion has been
omitted for clarity. Thermal ellipsoids are drawn at the 30% probability
level.
protonation of the ligand nitrogen atom is emphasized by its
pyramidal geometry [C(9)-N(1)-C(8) ) 113.8(8)°; Cr(1)-
N(1)-C(8) ) 113.5(6)°; Cr(1)-N(1)-C(9) ) 114.5(6)°]. Two
tetrahedral [AlCl4]- anions unconnected to the dicationic unit
complete the structure.
Scheme 3
3 is ionic with the cationic moiety consisting of an octahedral
chromium atom counterbalanced by one [AlCl4]- tetrahedral
anion. The cationic unit (Figure 4) has the octahedral coordina-
tion geometry of the chromium atom defined by the ligand
meridionally arranged [S(1)-Cr(1)-N(1) ) 84.31(17)°; S(1)-
Cr(1)-S(2) ) 169.12(8)°; N(1)-Cr(1)-S(2) ) 84.81(17)°]. The
three residual coordination sites are occupied by the two trans-
located chlorine atoms [Cr(1)-Cl(1) ) 2.295(2) Å; Cr(1)-Cl(2)
) 2.292(2) Å; Cl(1)-Cr(1)-Cl(2) ) 177.12(9)°] and the
oxygen atom of one THF molecule [Cr(1)-O(1) ) 2.034(5)
Å]. Despite the close similarity with the structure of 6, the Cr-S
distances [Cr(1)-S(1) ) 2.440(2) Å; Cr(1)-S(2) ) 2.443(2)
Å] are substantially shorter in the bonding range as a result of
the higher oxidation state in combination with the cationic
charge for 3 compared to 6. The nitrogen donor atom of the
ligand [Cr(1)-N(1) ) 2.068(6) Å] is sp3 hybridized, as indicated
by the out-of-plane arrangement with respect to the two
neighboring carbon and chromium atoms [C(8)-N(1)-Cr(1)
) 113.8(7)°, C(9)-N(1)-Cr(1) ) 111.6(6)°; C(8)-N(1)-C(9)
) 115.6(9)°].
4 is a heterobimetallic cationic complex formed by one
chromium atom in a regular octahedral coordination environ-
ment bridged by two chlorine atoms to a tetrahedral AlCl3
residue (Figure 5). A heavily disordered [Al2Cl7]- anion
completes the structure. The coordination environment of the
chromium atom [Cl(3)-Cr(1)-Cl(1) ) 178.20(13)°; Cl(2)-
Cr(1)-N(1) ) 170.6(4)°; Cl(2)-Cr(1)-Cl(3) ) 95.41(12)°;
N(1)-Cr(1)-Cl(1) ) 87.7(4)°] is defined by the chlorine atom
bridging the aluminum center [Cr(1)-Cl(1) ) 2.218(5) Å;
Cr(1)-Cl(2) ) 2.247(5) Å], one terminal chlorine atom [Cr(1)-
Cl(3) ) 2.229(3) Å], and the N atom of the ligand system
[Cr(1)-N(1) ) 2.058(10) Å]. The two sulfur atoms are located
on the axial positions [Cr(1)-S(1) ) 2.447(4) Å; Cr(1)-S(2)
) 2.464(4) Å; S(1)-Cr(1)-S(2) ) 166.16(12)°; S(1)-Cr(1)-
N(1) ) 85.0(3)°; S(1)-Cr(1)-Cl(1) ) 85.00(11)°]. The alu-
minum moiety displays the typical tetrahedral coordination
geometry [Cl(1)-Al-Cl(2) ) 93.36(16)°); Cl(4)-Al-Cl(5) )
118.7(2)°] with two terminally bonded chlorine atoms [Al(1)-
Cl(4) ) 2.088(5) Å; Al(1)-Cl(5) ) 2.080(5) Å] and the other
two bridging the chromium center [Al(1)-Cl(1) ) 2.218(5) Å;
Al(1)-Cl(2) ) 2.247(5) Å]. The CrAlCl2 core is coplanar with
the N atom and terminally bonded chlorine atom bonded to
chromium [torsion angles: Al(1)-Cl(2)-Cr(1)-Cl(1) )
-3.80(14)°; Al-Cl(2)-Cr-N(1) ) 2(2)°]. The protonation of
the nitrogen atom is evident from the pyramidal geometry of
the nitrogen atom [Cr(1)-N(1)-C(1) ) 114.8(9)°; Cr(1)-
N(1)-C(3) ) 116.1(11)°; C(1)-N(1)-C(3) ) 122.0(13)°].
Unfortunately, 4 could not be isolated in analytically pure
form, thus preventing its full characterization and meaningful
catalytic activity testing.
The remarkable thermal stability of 1a and 1b in pure toluene
encourages the idea that, different from the PNP9 and tripyrrolide
catalysts,10 the oligomerization might be initiated by a trivalent
organo-chromium species2i,n before the catalyst decomposes in
the presence of the large excess of activator required by the
catalytic cycle. However, this idea is in contrast with the result
of the reaction of [CySCH2CH2N(H)CH2CH2SCy]CrCl3 catalyst
precursor with isobutyl alumoxane [i-BAO]. The reaction did
not produce the analogue of 1 but yielded instead the new
diValent species 5 (Scheme 3).
The structure of 5 appears to be very similar to 1a, the only
difference arising from the absence of the terminally bonded
methyl groups (Figure 6). As a result, the coordination geometry
around each of the two metal centers is square pyramidal
[Cl(1)-Cr(1)-S(1) ) 97.66(7)°; Cl(1)-Cr(1)-S(2) ) 97.07(7)°;
N(1)-Cr(1)-S(1) ) 82.74(17)°; N(1)-Cr(1)-S(2) ) 82.45(17)°]
with the basal plane defined by one of the two bridging chlorines
[Cr(1)-Cl(1) ) 2.3486(19) Å], the ligand nitrogen [Cr(1)-
N(1) ) 2.134(5) Å], and the two sulfur atoms [Cr(1)-S(1) )
2.519(2) Å, Cr(1)-S(2) ) 2.511(2) Å]. The second bridging
chlorine is located on the axial position [Cr(1)-Cl(1a) )
2.714(2) Å; Cl(1a)-Cr(1)-N(1) ) 87.57(16)°; Cl(1a)-Cr(1)-
S(1) ) 87.05(7)°; Cl(1a)-Cr(1)-S(2) ) 88.74(7)°; Cl(1a)-
Cr(1)-Cl(1) ) 91.30(6)°]. The chromium atom shows little
9
9244 J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006

SNS−Cr Catalyst for Ethylene Trimerization
A R T I C L E S
Figure 6. Drawing of 5 with thermal ellipsoids drawn at the 30%
probability level.
deviation from the basal plane [Cl(1)-Cr(1)-N(1) ) 178.79(16)°;
S(1)-Cr(1)-S(2) ) 164.76(7)°]. Even in this case, the presence
of the proton on the nitrogen atom is indicated by the deviation
from the planarity of the N atom [C(2)-N(1)-C(3) ) 114.8(6)°;
C(2)-N(1)-Cr(1) ) 112.8(4)°; C(3)-N(1)-Cr(1) ) 111.7(4)°].
Two [i-Bu2AlCl2] anions complete the structure. One of the two
chlorine atoms of the aluminate unit is loosely coordinated to
the chromium atom [Cr(1)-Cl(2) ) 2.794(2) Å] occupying the
sixth position of an ideal octahedron centered on chromium.
The dicationic, dinuclear structure of 5 is closely reminiscent
of that of 1a apart from the absence of the alkyl group attached
to chromium, the lower oxidation state, and the loose coordina-
tion of the [i-Bu2AlCl2] counteranion. The fact that, similar to
other catalytic systems,9,10 a divalent species has been formed
suggests that 1a and 1b may also follow the same reduction
pathway during the catalytic cycle. On the other hand, the
isobutyl group is more susceptible to reductive elimination than
the Me group. Therefore, the spontaneous room-temperature
reduction leading to 5 is not particularly surprising. Even in
this case, the ligand system was not deprotonated [νN-H: 3169
cm-1].
Due to the their similarity in terms of ligand arrangement,
cationic nature, and dinuclear structure, 1a and 5 provide a
suitable system to compare the effect of the metal oxidation
state on catalytic activity and selectivity. While in combination
with MAO and under the usual reaction conditions (1000 equiv
MAO), 2 and 3 display the same selectivity as 1a, 1b, and
[CySCH2CH2N(H)CH2CH2SCy]CrCl3, thus suggesting that they
can all be precursors to the same catalytically active species
(Table 3). The substantial fluctuation of activity observed among
these complexes can be ascribed to a combination of different
solubility and poor stability in the presence of an excess of
MAO. In other words, a less soluble complex would generate
a lower concentration of catalyst in solution, given the rather
Figure 7. Drawing of 6. Thermal ellipsoids are drawn at the 30%
probability level.
rapid degradation that occurs in the presence of high concentra-
tion of cocatalyst. The decrease of activity observed with higher
loadings of MAO in the case of 1a, 1b, and 5 is most probably
the result of catalyst decomposition. As shown in Table 3, the
catalytic activity of 5 is somewhat between 1a and 1b. Given
the fluctuation of activity observed among the trivalent species,
the lower activity is not particularly informative. The most
striking feature is that, in the presence of a lower loading of
MAO, 5 has the same selectiVity as all the other trivalent
precursors. Furthermore, an increase of MAO loading affords
for both 1a and 5 a similar loss of selectivity in favor of a
Schulz-Flory distribution. These observations strongly suggest
that both Cr(III) and Cr(II) complexes are precursors to the
same catalytically actiVe species most likely containing chro-
mium in the diValent state. Decomposition or ligand dissociation
prior to decomposition (vide infra) could be responsible for the
loss of selectivity. 1b is instead more resilient. The fact that
selectivity is preserved with a higher loading of MAO (possibly
only the first sign of selectivity loss starts to show at 2000 equiv)
can only be ascribed to the nature of the counteranion. In other
words, the alumoxane counteranion, though not preventing
reduction of the metal center to the divalent state, may somehow
improve the robustness of ligand coordination and the stability
of the catalytically active species.
In an attempt to substantiate these working hypotheses, the
noncationic Cr(II) complex [CySCH2CH2N(H)CH2CH2SCy]-
CrCl2(THF) (6) was prepared and fully characterized (Scheme
4).
Scheme 4
9
J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006 9245

A R T I C L E S
Scheme 5
Jabri et al.
6 is a monomer (Figure 7) with the chromium atom in the
center of a regular square plane [N(1)-Cr(1)-Cl(1) ) 87.6(2)°;
N(1)-Cr(1)-Cl(2) ) 92.0(2)°; O(1)-Cr(1)-Cl(1) ) 90.00(18)°;
O(1)-Cr(1)-Cl(2) ) 90.49(18)°] defined by the nitrogen atom
of the ligand [Cr-N(1) ) 2.142(7) Å], two chlorines in trans
to each other [Cr-Cl(1) ) 2.405(3) Å; Cr-Cl(2) ) 2.372(3)
Å; Cl(1)-Cr(1)-Cl(2) ) 178.49(11)°], and one coordinated
molecule of THF [Cr(1)-O(1) ) 2.098(6) Å]. The two sulfur
atoms are located on the axis of the ideal octahedron centered
on chromium. However, the two Cr-S distances [Cr(1)]..S(1)
) 2.846(6) Å, Cr(1)..S(2) ) 2.840(6) Å] are beyond the normal
bonding range. The nitrogen atom of the ligand appears also to
carry the proton, as indicated by its pyramidal arrangement
[C(8)-N(1)-C(9) ) 110.5(6)°; C(8)-N(1)-Cr(1) ) 114.3(5)°;
C(9)-N(1)-Cr(1) ) 112.2(5)°].
Besides the expected square-planar coordination environment
expected for the d4 electronic configuration [µeff ) 4.86 µB], 6
shows a curious positioning of the two sulfur donor atoms,
which are placed on the axial positions of the ideal octahedral
geometry centered on chromium. However, there is a very strong
axial elongation that places the S-Cr distances outside the
normal bonding range. The fact that the ligand coordination in
this neutral divalent chromium may be labile is further suggested
by the fact that, during the preparation of 6, unreacted
CrCl2(THF)2 was always present in the reaction mixtures, unless
using a very large excess of ligand. Conversely, in the divalent
5, the ligand appears to be fully coordinated as a probable result
of the cationic nature. 6 shows comparable activity and the same
selectivity as 1b, thus reiterating that the trivalent species is
simply a precursor to a divalent complex. The substantially
better selectivity of 6 at 1000 equiv of MAO compared to 5
(when 5 has already lost some of its selectivity) is in agreement
with the hypothesis that MAO-based counteranions are benefi-
cial for ligand retention.
7 is also a symmetry-generated dimer (Figure 8) where the
bridging interaction is realized through the deprotonated nitrogen
atoms of two ligands forming a planar Cr2N2 core [Cr(1)-N(1)
) 2.071(3) Å; Cr(1a)-N(1) ) 2.093(3) Å; N(1)-Cr(1)-N(1a)
) 88.61(11)°; Cr(1)-N(1)-Cr(1a) ) 91.39(11)°]. The coor-
dination geometry of each of the two identical chromium atoms
is regular trigonal bipyramidal. The equatorial plane is defined
by the two sulfur atoms [Cr(1)-S(1) ) 2.6948(17) Å; Cr(1)-
S(2) ) 2.5359(17) Å] of the same ligand and the bridging
nitrogen atom [Cr(1)-N(1a) ) 2.093(3) Å] of the second ligand
attached to the other chromium atom [ S(1)-Cr(1)-N(1a) )
104.93(8)°; S(1)-Cr(1)-S(2) ) 96.20(5)°; S(2)-Cr(1)-N(1a)
) 154.52(8)°]. The terminally bonded chlorine [Cr(1)-Cl(1)
) 2.3581(16) Å] and the nitrogen atoms [Cr(1)-N(1) )
2.071(3) Å] of the ligand coordinated to the chromium atoms
are located on the axial positions [N(1)-Cr(1)-Cl(1) )
176.29(8)°].
The dinuclear structure of 7 with the two high-spin coordi-
natively saturated Cr(II) centers in a trigonal bipyramidal
coordination environment is probably very robust and, by not
being disrupted by the MAO activator, accounts for the observed
complete lack of oligomerization actiVity under the usual reaction
conditions. Accordingly, no decomposition or color change was
observed when 7 was treated with a large excess of MAO and
TMA.
In conclusion, we have attempted to isolate intermediate
species formed by the reaction of tri- and divalent chromium
CySCH2CH2N(H)CH2CH2SCy complexes with aluminum ac-
tivators. Unfortunately, this work did not unveil the structure
of the catalytically active species, which would have been the
starting point to understanding the marvelous selectivity of this
system. Nonetheless, it was possible to obtain some useful
information. Our findings indicate that, in line with the other
catalytic systems analyzed before,9,10 the triValent oxidation state
is a precursor to a Cr(II) species which, in turn, is a precursor
to the catalytically actiVe species. Cationization of the metal
Another important parameter of this remarkable catalytic
system is whether the possible deprotonation of the ligand may
play any part in the catalytic system. As clearly indicated by
the crystal structures and the IR spectra of all the complexes,
the N-H function of the SNS ligand remained unreacted during
the reaction with the aluminum alkylating agents. This suggests
that the Al-R function does not have sufficient basicity to
deprotonate the ligand. Nonetheless, the possibility that depro-
tonation may occur in the presence of a large excess of
alkylating agent cannot be ruled out a priori. Because the
CySCH2CH2N(H)CH2CH2SCy ligand can be readily depro-
tonated by MeLi or n-BuLi, and given that the divalent state is
the precursor to the catalytically active divalent state, we have
reacted the deprotonated ligand with CrCl2(THF)2 and obtained
the dinuclear complex 7 (Scheme 5).
Figure 8. Drawing of 7. Thermal ellipsoids are drawn at the 30%
probability level.
9
9246 J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006

SNS−Cr Catalyst for Ethylene Trimerization
A R T I C L E S
center seems to be a leit-motif in the reactivity of these Cr(III)
and Cr(II) complexes of this neutral ligand system with
aluminum-based Lewis acids. In turn, the cationization is
certainly beneficial to improve the retention of the ligand system,
which is not deprotonated as a result of the interaction with the
cocatalyst. At this stage, it is tempting to speculate that a cationic
Cr(II) species, probably carrying an alkyl group and a partly
coordinating alumoxane anion, is the active catalyst. Further
synthetic efforts in this direction are in progress.
Acknowledgment. This work was supported by the Natural
Science and Engineering Council of Canada (NSERC).
Supporting Information Available: Complete crystallo-
graphic data (CIF) for the complexes reported in this paper.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA0623717
9
J. AM. CHEM. SOC. VOL. 128, NO. 28, 2006 9247