New insight into the role of the metal oxidation state in controlling the selectivity of the Cr-(SNS) ethylene trimerization catalyst

Article abstract of DOI:10.1021/om700452u

The tri- and divalent complexes of the 2,6-bis(RSCH₂)pyridine [R = Ph, Cy] ligands have been prepared. Upon activation with MAO, both species are catalysts for ethylene oligomerization of moderate activity. However, while the trivalent catalysts produced only 1-hexene, the divalent species gave a statistical distribution of oligomers. This clear difference in catalytic behavior indicates that the two oxidation states are not interconnected during the catalytic cycle as it happens instead with other oligomerization catalytic systems. The trivalent precursor is not reduced and the divalent is not oxidized. Treatment of the trivalent catalyst precursors with either MAO or other R₃Al species afforded intractable materials. Instead, similar reactions with the divalent complexes gave new cationic species, which have been characterized by X-ray analysis. These complexes have preserved the divalent state of chromium during the reaction and still produce, upon further activation, a statistical distribution of oligomers. This reiterates the non-interconvertibility of the di- and trivalent oxidation states and the different degree of selectivity for which they are responsible.

Full text of DOI:10.1021/om700452u

4598
Organometallics 2007, 26, 4598-4603
New Insight into the Role of the Metal Oxidation State in
Controlling the Selectivity of the Cr-(SNS) Ethylene Trimerization
Catalyst
Claire N. Temple,† Sandro Gambarotta,*,† Ilia Korobkov,† and Robbert Duchateau*,‡
Department of Chemistry, UniVersity of Ottawa, 10 Marie Curie, Ottawa ON K1N 6N5, Canada, and
Department of Chemistry, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB, The Netherlands
ReceiVed May 7, 2007
The tri- and divalent complexes of the 2,6-bis(RSCH2)pyridine [R ) Ph, Cy] ligands have been prepared.
Upon activation with MAO, both species are catalysts for ethylene oligomerization of moderate activity.
However, while the trivalent catalysts produced only 1-hexene, the divalent species gave a statistical
distribution of oligomers. This clear difference in catalytic behavior indicates that the two oxidation
states are not interconnected during the catalytic cycle as it happens instead with other oligomerization
catalytic systems. The trivalent precursor is not reduced and the divalent is not oxidized. Treatment of
the trivalent catalyst precursors with either MAO or other R3Al species afforded intractable materials.
Instead, similar reactions with the divalent complexes gave new cationic species, which have been
characterized by X-ray analysis. These complexes have preserved the divalent state of chromium during
the reaction and still produce, upon further activation, a statistical distribution of oligomers. This reiterates
the non-interconvertibility of the di- and trivalent oxidation states and the different degree of selectivity
for which they are responsible.
new catalysts. In fact, there are only three features in these
complexes that may be responsible for the exceptional selectiv-
ity: the metal oxidation state of the catalytically active species,
the N-H function, and the particular nature of the sulfur donor
atoms.
Clarifying the chromium oxidation state in the catalytically
active species is central to the design of new and more potent
catalysts. Recent studies9 on the unique Sasol PNP tetramer-
ization catalyst2 and on other catalytic systems7,8,10 have clearly
Introduction
Research in the area of ethylene oligomerization is currently
aimed at the discovery of trimerization1 and tetramerization2
catalysts capable of combining high activity with selectivity.
In fact, catalytic activity has been found with a large series of
diversified metals and ligands, but selective oligomerization
catalysts remain rare.3-5 The recently reported Sasol trimeriza-
tion catalyst [CrCl3(SNS)] [(SNS) ) RSCH2CH2N(H)CH2CH2-
SR, R ) Me, Et, n-Bu, or n-Dec] has been shown to be not
only extremely active but also 98% selective for 1-hexene.6
Given its simplicity, this remarkable catalyst obviously provides
an excellent substrate for studying the factors responsible for
the unique selectivity that may in turn assist with the design of
(6) (a) McGuinness, D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J.
T. Organometallics 2005, 24, 552. (b) McGuinness, D. S.; Wasserscheid,
P.; Keim, W.; Morgan, D.; Dixon, J. T.; Bollmann, A.; Maumela, H.; Hess,
F.; Englert, U. J. Am. Chem. Soc. 2003, 125, 5272. (c) Dixon, J. T.;
Wasserscheid, P.; McGuinness, D. S.; Hess, F.; Maumela, H.; Morgan, D.
H.; Bollmann, A. (Sasol Technology (Pty) Ltd) WO 03053890, 2001.
(7) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238.
* Corresponding author. Fax: (+1) (613-562-5170). E-mail:
sgambaro@uottawa.ca.
† University of Ottawa.
(8) (a) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.;
Dixon, J. T.; Slawin, A. M. Z. Organometallics 2006, 25, 3605. (b) Blom,
B.; Klatt, G.; Fletcher, J. C. Q.; Moss, J. R. Inorg. Chim. Acta 2007, 360,
2890. (c) Bowen, L. E.; Haddow, M. F.; Orpen, A. G.; Wass, D. F. Dalton
Trans. 2007, 1160. (d) Nenu, C. N.; Lingen, J. N. J.; van de Groot, F. M.
F.; Koningsberger, D. C.; Weckhuysen, B. M. Chem.-Eur. J. 2006, 12,
4756. (e) van Rensburg, W. J.; van den Berg, J. A.; Steynberg, P. J.
Organometallics 2007, 26, 1000. (f) McGuinness, D. S.; Overett, M.; Tooze,
R. P.; Blann, K.; Dixon, J. T.; Slawin, A. M. Z. Organometallics 2007, 26,
1108. (g) McGuinness, D. S.; Rucklidge, A. J.; Tooze, R. P.; Slawin, A.
M. Z. Organometallics 2007, 26, 2561. (h) Wass, D. F. Dalton Trans. 2007,
816. (i) Rucklidge, A. J.; McGuinness, D. S.; Tooze, R. P.; Slawin, A. M.;
Pelletier, J. D. A.; Hanton, M. J.; Webb, P. B. Organometallics 2007, 26,
2782. (j) McGuinness, D. S.; Brown, D. B.; Tooze, R. P.; Hess, F. M.;
Dixon, J. T.; Slawin, A. M. Organometallics 2006, 25, 3605. (k) Janse van
Rensburg, W.; Grove, C.; Steynberg, J. P.; Stark, K. B.; Huyser, J. J.;
Steynberg, P. J. Organometallics 2004, 23, 1207. (l) Agapie, T.; Schofer,
S. J.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2004, 126, 1304.
(m) Small, B. L.; Carney, M. J.; Holman, D. M.; O’Rourke, C. E.; Halfen,
J. A. Macromolecules 2004, 37, 4375. (n) Morgan, D. H.; Schwikkard, S.
L.; Dixon, J. T.; Nair, J. J.; Hunter, R. AdV. Synth. Catal. 2003, 345, 1. (o)
Ko¨hn, R. D.; Smith, D.; Mahon, M. F.; Prinz, M.; Mihan, S.; Kociok-Ko¨hn,
G. J. Organomet. Chem. 2003, 683, 200.
‡ Eindhoven University of Technology.
(1) Dixon, J. T.; Green, M. J.; Hess, F. M.; Morgan, D. H. J. Organomet.
Chem. 2004, 689, 3641.
(2) (a) Bollmann, A.; Blann, K.; Dixon, J. T.; Hess, F. M.; Killian, E.;
Maumela, H.; McGuinness, D. S.; Morgan, D. H.; Neveling, A.; Otto, S.;
Overett, M.; Slawin, A. M. Z.; Wasserscheid, P.; Kuhlmann, S. J. Am. Chem.
Soc. 2004, 126 (45), 14712. (b) 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. (c) Overett, M. J.; Blann, K.;
Bollmann, A.; Dixon, J. T.; Haasbroek, D.; Killian, E.; Maumela, H.;
McGuinness, D. S.; Morgan, D. H. J. Am. Chem. Soc. 2005, 127, 10723.
(3) (a) Deckers, P. J. W.; Hessen, B. J.; Teuben, H. Organometallics
2002, 21, 5122. (b) Deckers, P. J. W.; Hessen, B. J.; Teuben, H. Angew.
Chem., Int. Ed. 2001, 40, 2516-2519. (c) Deckers, P. J. W.; Hessen, B. J.;
Teuben, H.; (Stichting Dutch Polymer Institute) WO 02/066404, August
29, 2002. (d) Deckers, P. J. W.; Hessen, B. J.; Teuben, H. (Stichting Dutch
Polymer Institute) WO 02/066405, August 29, 2002.
(4) (a) Wass, D. F. (BP Chemicals Ltd) WO 02/04119, January 17, 2002.
(b) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy, A.; Scutt, J.; Wass,
D. F. Chem. Commun. 2002, 858.
(5) (a) Reagan, W. K. (Phillips Petroleum Co.) EP 0417477, 1991. (b)
Conroy, B. K.; Pettijohn, T. M.; Benham, E. A. (Phillips Petroleum Co.)
EP 0608447, 1994. (c) Freeman, J. W.; Buster, J. L.; Knudsen, R. D.
(Phillips Petroleum Co.) US 5,856,257, 1999.
(9) Jabri, A.; Gambarotta, S.; Korobkov, I.; Duchateau, R. Organome-
tallics 2006, 25, 715.
10.1021/om700452u CCC: $37.00 © 2007 American Chemical Society
Publication on Web 07/13/2007

Cr-(SNS) Ethylene Trimerization Catalyst
Organometallics, Vol. 26, No. 18, 2007 4599
The organic layer was separated and the aqueous layer extracted
with chloroform (3 × 50 mL). The organic extracts were combined
and dried over MgSO4, and the solvent was removed in Vacuo to
yield 2 as an analytically pure pale oil (5.8 g, 0.017 mol, 79%).
Anal. Calcd (found) for C19H29NS2: C 68.01 (67.99); H 8.71 (8.68);
N 4.17 (4.15). 1H NMR (300.13 MHz, CDCl3, 300 K) δ: 1.25 (m,
10H, CH2-Cy), 1.60 (m, 2H, CH2-Cy), 1.75 (m, 4H, CH2-Cy), 1.97
(m, 4H, CH2-Cy), 2.65 (m, 2H, CH-Cy), 3.80 (s, 4H, CH2), 7.20
(d, 2H, CH-Py), 7.55 (t, 1H, CH-Py). 13C{1H} NMR (75.47 MHz,
CDCl3, 300 K) δ: 26.02, 26.22, 33.61 (3 × s, CH2-Cy), 36.75 (s,
CH2), 43.50 (s, CH-Cy), 121.08, (s, C-Py), 137.38, 158.91 (2 × s,
CH-Py). MS: ES+ 337 (M+ + H+).
Preparation of [(2,6-(PhSCH2)2-py)CrCl3]‚CH3CN (3). To a
suspension of CrCl3(THF)3 (0.127 g, 0.33 mmol) in toluene (10
mL) was added 2,6-bis(phenylthiomethyl)pyridine 1 (0.110 g, 0.34
mmol), and the resulting green suspension was stirred at room
temperature for 30 min. The suspension was centrifuged, and the
green solid was washed with hexanes (3 × 10 mL) and dried in
Vacuo to yield complex 3 as a green microcrystalline solid (0.150
g, 0.29 mmol, 88%). Single crystals suitable for X-ray crystal-
lography were grown from a concentrated acetonitrile solution
stored at -35 °C. Anal. Calcd (found) for C21H20N2S2Cl3Cr: C
48.24 (47.94); H 3.86 (3.72); N 5.36 (5.27). µeff ) 3.96 µB.
Acetonitrile-free samples for catalytic runs were obtained by
exposing samples of crystalline 3 to vacuum overnight and at 60
°C.
shown that the formation of divalent chromium is, at least in
some cases, the first step toward the formation of the catalyti-
cally active species. This, together with the general poor stability
of the trivalent organochromium function,11 suggests that
perhaps reduction to the divalent state might be a trend for
chromium catalysts. In fact, work by Bercaw has clearly shown
that oligomerization may be carried out by an even lower
valence state.12 On the other hand, in the case of the SNS system
it was observed that divalent precursors are more inclined to
lose selectivity, and even further, the divalent species can be
oxidized via disproportionation by an alkyl aluminum to the
original catalytically active trivalent complex.10 The working
hypothesis emerging from these observations was that although
both the divalent and trivalent states can be catalytically active,
only the triValent state is capable of selectiVe trimerization.10
Recent work in our group7 has established that the SNS ligand
is not as readily deprotonated as previously thought.6a,8 On the
other hand, several cationic, organochromium species containing
a non-deprotonated SNS ligand were prepared by the reaction
of [CrCl3(SNS)] [(SNS) ) CySCH2CH2N(H)CH2CH2SCy] with
a range of alkyl aluminum reagents. Catalytic tests confirmed
that cationization and alkylation of the neutral catalyst precursor
is directly linked to catalyst performance.7 Among all the species
related to this catalytic system that have been isolated and tested,
the N-H moiety always remained intact in the presence of a
variety of activators (i.e., MAO, AlMe3, AlEt3, i-BAO).
Herein, we report the results of a further investigation with
both CrIII and CrII complexes containing an “SNS” ligand motif
that does not contain an N-H function and the implications of
these findings with regard to catalyst selectivity.
Preparation of [(2,6-(CySCH2)2-py)CrCl3]‚(CH3CN)0.25 (4).
The same procedure as for complex 3 was followed by replacing
ligand 1 with 2. Complex 4 was isolated as a green microcrystalline
solid (0.73 g, 1.48 mmol, 99%). Single crystals suitable for X-ray
crystallography were grown from a concentrated acetonitrile
solution stored at -35 °C. Satisfactory analytical data were obtained
only for samples dried in Vacuo. Anal. Calcd (found) for C19H29-
Experimental Section
NS2CrCl3: C 46.20 (46.17); H 5.92 (5.86); N 2.84 (2.77). µeff
)
3.88 µB. Acetonitrile-free samples for catalytic runs were obtained
by exposing samples of crystalline 4 to vacuum overnight and at
60 °C.
Preparation of [2,6-(PhSCH2)2-py]CrCl2. A mixture of CrCl2-
(THF)2 (0.054 g, 0.202 mmol) in toluene (5 mL) with 2,6-bis-
(phenylthiomethyl)pyridine 1 (0.065 g, 0.202 mmol) was stirred at
room temperature for 1 h, affording a pale orange solid (0.075 g).
Anal. Calcd (found) for C19H17NS2CrCl2: C 51.13 (51.04); H 3.84-
(3.72); N 3.14 (3.11).
Preparation of [2,6-(CySCH2)2-py]CrCl2. A mixture of CrCl2-
(THF)2 (0.054 g, 0.202 mmol) in toluene (5 mL) with 2,6-bis-
(cyclohexylthiomethyl)pyridine (2) (0.068 g, 0.202 mmol) was
stirred at room temperature for 1 h, affording a pale orange solid
(0.069 g). Anal. Calcd (found) for C19H29NS2CrCl2: C 49.78
(49.64); H 6.38 (6.32); N 3.06 (3.07).
Preparation of {2,6-(CySCH2)2-py]Cr-µ-Cl}2‚2[AlMeCl3]‚
toluene (5). The addition of a solution of AlMeCl2 (0.2 mL, 0.202
mmol) in hexanes (1.0 M) to a suspension of 2,6-(CySCH2)2-py)-
CrCl2 (0.091 g, 0.200 mmol) turned the color of the suspension to
green with formation of an oily green-blue residue. The supernatant
was decanted into a vial and stored at room temperature, yielding
complex 5 as blue single crystals suitable for X-ray crystallography.
The substantial contamination of oily material prevented analytical
characterization in this case.
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification
system. Samples for magnetic susceptibility were preweighed inside
a drybox equipped with an analytical balance and measured on a
Johnson Matthey magnetic susceptibility balance. Elemental analy-
sis 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 1K Smart CCD area detector.
NMR spectra were collected on a Varian Inova 500 MHz instru-
ment. CrCl3(THF)3 and CrCl2(THF)2 were prepared according to
standard procedures. The 2,6-bis(phenylthiomethyl)pyridine 1 ligand
was prepared with a slightly modified literature procedure.13 The
reagents AlMeCl2 (Aldrich) and MAO (Chemtura and Aldrich) were
used as received.
Preparation of 2,6-Bis(cyclohexylthiomethyl)pyridine (2). A
mixture of NaOH (1.87 g, 0.047 mol) and cyclohexyl mercaptan
(5.7 mL, 0.047 mol) in ethanol (30 mL, 99%) was stirred at room
temperature for 1 h. The resulting solution was added dropwise
over 10 min to a solution of 2,6-bis(chloromethyl)pyridine (3.91
g, 0.022 mol) in ethanol (25 mL, 99%) and stirred at room
temperature for 18 h. The solvent was removed in Vacuo, then water
(100 mL) and chloroform (100 mL) were added to the white residue.
(10) Temple, C.; Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. Angew. Chem., Int. Ed. 2006, 45, 7050.
(11) See for example: (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.
(12) (a) Elowe, P. R.; McCann, C.; Pringle, P. G.; Spitzmesser, S. K.;
Bercaw, J. E. Organometallics 2006, 25, 5255. (b) Schofer, S. J., Day, M.
W.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Organometallics 2006,
25, 2743.
(13) Canovese, L.; Chessa, G.; Marangoni, G.; Pitteri, B.; Uguagliati,
P.; Visentin. F. Inorg. Chim. Acta 1991, 186, 79.
Preparation of {2,6-(CySCH2)2-py]CrCl}[AlMe2Cl2]‚toluene
(6). A suspension of CrCl2(THF)2 (0.393 g, 2.95 mmol) in toluene
(5 mL) was treated with 2 (0.494 g, 2.94 mmol) and stirred at room
temperature for 1 h. The addition of a solution of MAO (4.25 mL,
14.75 mmol) in toluene (10 wt %) to the resulting pale orange
suspension produced a green solution with formation of a green
precipitate. The suspension was centrifuged, and the green solid
was washed with hexanes (3 × 10 mL) and dried in Vacuo to yield
complex 6 as a green microcrystalline solid (0.548 g, 0.85 mmol,

4600 Organometallics, Vol. 26, No. 18, 2007
Table 1. Crystal, Structure Solution, and Refinement Data
Temple et al.
3
4
5
6
formula
Mw
C21H20Cl3CrN2S2
522.86
C39H59.50Cl6Cr2N2.50S4
1008.33
C27H40AlCl4CrNS2
663.50
C28H43NS2CrCl3Al
643.08
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
orthorhombic
P2(1)2(1)2(1)
7.783(3)
14.181(5)
20.516(7)
monoclinic
P2/n
22.081(7)
9.809(3)
monoclinic
P2(1)/c
13.871(5)
16.849(6)
14.368(5)
triclinic
P1h
10.91(2)
11.81(2)
14.52(3)
86.87(3)
74.34(3
75.86(3)
1747(6)
2
22.623(7)
90.276(4)
95.566(5)
2264.4(12)
4
4900(3)
4
3342(2)
4
Z
radiation (KR, Å)
0.71073
203(2)
1.534
1.055
1068
0.71073
203(2)
1.367
0.971
2100
0.71073
203(2)
1.319
0.831
1384
0.71073
200(2)
1.222
0.718
676
T (K)
D
µ
calcd (g cm-3
)
calcd (mm-1
)
F000
R, Rw
GoF
2 a
0.0421 0.0995
1.034
0.0764, 0.1921
1.063
0.0643, 0.1410
1.007
0.0664, 0.1517
1.053
2
a R ) ∑|Fo| - |Fc|/∑|F|. Rw ) [∑(|Fo| - |Fc|)2/∑wFo ]1/2
.
58%). The supernatant was decanted into a vial and stored at room
temperature. Green single crystals of complex 6 suitable for X-ray
crystallography were isolated from the decanted mother liquor stored
in a vial at room temperature. Anal. Calcd (found) for C28H43NS2-
Scheme 1
CrCl3Al: C 52.29 (52.19); H 6.74 (6.65); N 2.18 (2.05). µeff
4.40 µB.
)
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. 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 nitrogen, and
then the reactor was immediately repressurized. The reaction was
allowed to run for 30 min, after which the temperature was rapidly
reduced to 5 °C, 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.
Yields of oligomers were obtained by GC by using calibrated
standard solutions. The overall catalytic activity was determined
by both GC and by integrating the intensity of the olefinic NMR
resonances versus the protons of the toluene solvent, always
obtaining consistent results. Samples of 3 and 4 for catalytic runs
were dried in Vacuo, and the complete loss of lattice CH3CN was
monitored by IR.
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 all the compounds data
collection was performed with three batch runs at φ ) 0.00° (650
frames), at φ ) 120.00° (650 frames), and at φ ) 240.00° (650
frames). Initial unit-cell parameters were determined from 60 data
frames collected at different sections of the Ewald sphere. Semiem-
pirical 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 the non-hydrogen atoms in all the structures 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. One fully occupied molecule of acetonitrile was
found per Cr atom in the lattice of 3 and per four Cr atoms in the
lattice of 4. Crystal data are reported in Table 1, and relevant bond
distances and angles in Table 2.
Results and Discussion
Ligands 2,6-(RSCH2)2pyridine [R ) Ph (1), Cy (2)] were
prepared according to a modified literature method.13 Although
2,6-bis(phenylthiomethyl)pyridine 1 is already known in the
literature for its complexes with Pd,14 Ni,15 Cu,16 and Ru,17 its
chemistry with Cr remains unknown to date. The cyclohexyl
derivative 2 was prepared to prevent anticipated solubility
problems and would provide a system directly comparable to
the previous studies on the cyclohexyl-containing SNS ligand.7,10
Complexes [2,6-(RSCH2)2pyridine]CrCl3 [R ) Ph (3), Cy (4)]
were prepared via a simple reaction in toluene of the ligand
with CrCl3(THF)3 (Scheme 1). In both 3 and 4 the tridentate
ligands (Figures 1 and 2) are coordinated to the CrIII centers
through both sulfur atoms and the nitrogen of the pyridine, in
the expected meridional manner.
The results of the catalytic testing for ethylene trimerization
(30 µmol of catalyst at 50 °C, 150 mL of toluene, 35 bar of
ethylene, 30 min reaction time) are summarized in Table 3.
Complexes 3 and 4 are exceptionally selective for the production
of 1-hexene (>99%) in the presence of MAO activator, with
no significant difference in activity or selectivity between the
(14) Canovese, L.; Visentin, F.; Chessa, G.; Uguagliati, P.; Levi, C.;
Dolmella, A.; Bandoli, G. Organometallics 2006, 25, 5355.
(15) Ball, R. J.; Genge, A. R. J.; Radford, A. L.; Skelton, B. W.; Tolhurst
V.; White, A. H. J. Chem. Soc., Dalton Trans. 2001, 2807.
(16) Teixidor, F.; Sanchez-Castello, G.; Lucena, N.; Escriche, L.;
Kivekas, R.; Sundberg; M.; Casabo, J. Inorg. Chem. 1991, 30, 4931.
(17) Vin˜as, C.; Angle`s, P.; Sa´nchez, G.; Lucena, N.; Teixidor, F.;
Escriche, L.; Casabo´, J.; Piniella, J. F.; Alvarez-Larena, A.; Kiveka¨s, R.;
Sillanpa¨a¨, R. Inorg. Chem. 1998, 37, 701.

Cr-(SNS) Ethylene Trimerization Catalyst
Organometallics, Vol. 26, No. 18, 2007 4601
Table 2. Selected Bond Distances (Å) and Angles (deg)
3
4
5
6
Cr(1)-Cl(1) ) 2.2934(18)
Cr(1)-Cl(1) ) 2.325(3)
Cr(1)-Cl(1) ) 2.676(3)
Cr(1)-Cl(1) ) 2.389(5)
Cr(1)-Cl(2) ) 2.2900(19)
Cr(1)-Cl(2) ) 2.283(3)
Cr(1)-S(2) ) 2.446(3)
Cr(1)-S(2) ) 2.541(4)
Cr(1)-Cl(3) ) 2.3226(18)
Cr(1)-Cl(3) ) 2.320(3)
Cr(1)-S(1) ) 2.453(3)
Cr(1)-S(1) ) 2.527(4)
Cr(1)-N(1) ) 2.096(5)
Cr(1)-N(1) ) 2.069(7)
Cr(1)-N(1) ) 2.070(8)
Cr(1)-N(1) ) 2.140(7)
Cr(1)-S(1) ) 2.4458(18)
Cr(1)-S(1) ) 2.424(3)
Cr(1)-Cl(1a) ) 2.356(3)
Cr(1)‚‚‚Cl(2) ) 3.287(8)
Cr(1)‚‚‚Cl(1a) ) 2.813(8)
Cr(1)‚‚‚Cl(2) ) 2.840(9)
Cr(1)-S(2) ) 2.4205(19)
Cr(1)-S(2) ) 2.453(3)
Cl(1)-Cr(1)-Cl(2) ) 91.80(7)
Cl(1)-Cr(1)-Cl(3) ) 176.10(7
Cl(1)-Cr(1)-N(1) ) 87.64(13)
Cl(1)-Cr(1)-S(1) ) 96.16(7)
Cl(1)-Cr(1)-S(2) ) 82.83(7)
Cl(2)-Cr(1)-Cl(3) ) 91.90(7)
Cl(2)-Cr(1)-N(1) ) 179.44(14)
Cl(2)-Cr(1)-S(1) ) 97.69(7)
Cl(2)-Cr(1)-S(2) ) 96.41(7)
Cl(3)-Cr(1)-N(1) ) 88.65(13)
Cl(3)-Cr(1)-S(1) ) 84.53(6)
Cl(3)-Cr(1)-S(2) ) 95.56(7)
N(1)-Cr(1)-S(1) ) 82.42(13)
N(1)-Cr(1)-S(2) ) 83.47(13)
S(1)-Cr(1)-S(2) ) 165.89(7)
Cl(1)-Cr(1)-Cl(2) ) 92.59(10)
Cl(1)-Cr(1)-Cl(3) ) 174.91(11)
Cl(1)-Cr(1)-N(1) ) 88.1(2)
Cl(1)-Cr(1)-S(1) ) 83.92(10)
Cl(1)-Cr(1)-S(2) ) 95.03(10)
Cl(2)-Cr(1)-Cl(3) ) 92.32(11)
Cl(2)-Cr(1)-N(1) ) 177.5(2)
Cl(2)-Cr(1)-S(1) ) 93.95(10)
Cl(2)-Cr(1)-S(2) ) 98.87(10)
Cl(3)-Cr(1)-N(1) ) 87.0(2)
Cl(3)-Cr(1)-S(1) ) 97.08(10)
Cl(3)-Cr(1)-S(2) ) 82.88(10)
N(1)-Cr(1)-S(1) ) 83.8(2)
N(1)-Cr(1)-S(2) ) 83.4(2)
S(1)-Cr(1)-S(2) ) 167.17(10)
Cl(1)-Cr(1)-N(1) ) 97.3(2)
l(1)-Cr(1)-S(1) ) 103.29(11)
CCl(1)-Cr(1)-S(2) ) 84.54(10)
N(1)-Cr(1)-S(1) ) 82.5(2)
N(1)-Cr(1)-S(2) ) 82.7(2)
S(1)-Cr(1)-S(2) ) 164.04(11)
Cl(1)-Cr(1)-N(1) ) 177.81(16)
Cl(1)-Cr(1)-S(1) ) 102.35(10)
Cl(1)-Cr(1)-S(2) ) 96.89(8)
N(1)-Cr(1)-S(1) ) 79.19(17)
N(1)-Cr(1)-S(2) ) 81.47(16)
S(1)-Cr(1)-S(2) ) 160.41(9)
Table 4. CrII Oligomerization Results
product (mol %)
C6 C8 C10 C12 C14 C16
activity
catalyst PE (g) (g/gCr/h)
C4
Pha
Phb
Cya
Cyb
6a
3.0
0.37
1.6
0.5
0.4
5233
6625
5200
660
5.4c 57.8 18.8
8.9 4.9 2.4 1.7
6.4c 51.2 21.6 10.7 5.7 2.9 1.5
3.2c 57.2 20.1 10.0 5.3 2.8 1.5
16.6 83.4
0
0
0
0
0
780
9.9 49.5 20.4 10.3 5.6 2.7 1.6
b
c
a Al:Cr ratio ) 1000. Al:Cr ratio ) 100. Venting of the autoclave was
performed between 0 and 5 °C to minimize loss.
Scheme 2
Figure 1. Complex 3 with 30% thermal ellipsoids (solvent
molecule omitted for clarity).
change in the selectivity for 1-hexene. At 10 bar of ethylene
pressure, the activity was again reduced, but the catalyst was
still >99% selective for 1-hexene. Finally, two runs were
conducted at room temperature. After 30 min, no activity could
be detected, but after 15 h a small amount of 1-hexene was
formed.
Figure 2. Complex 4 with 30% thermal ellipsoids (solvent
To ascertain whether the selectivity of this system is exclusive
to the CrIII oxidation state, attempts to prepare CrII derivatives
were carried out. When CrCl2(THF)2 was treated in the same
manner with ligands 1 and 2, pale orange solids were isolated.
Attempts to recrystallize and structurally characterize these
solids were hindered by a lack of solubility in all common
organic solvents. Therefore, their formulation relies exclusively
on analytical data. Interestingly, when dissolved in dichlo-
romethane, an immediate oxidation reaction was observed
resulting in a color change and the formation of the CrIII
complexes 3 and 4. This phenomenon has been observed before
with the original SNS system.6a When the orange solids were
treated with THF, the pale blue CrCl2(THF)2 starting material
precipitated, leading to the conclusion that the ligand system is
quite labile within these two CrII complexes. This lability has
been observed previously with the Sasol-type SNS CrII complex
[η1-(SNS)CrCl2(THF)], in which the ligand is coordinated
through the nitrogen only and the ligand is easily displaced in
THF solutions.7 When the CrII species were tested for oligo-
merization activity under identical conditions (Scheme 2), a
complete loss of selectivity was observed in comparison to their
CrIII analogues, resulting in a distribution of oligomers centered
at 1-hexene (∼57%, Table 4). The activity in these runs was
molecule omitted for clarity).
Table 3. CrIII Oligomerization Resultsa
product [mol %]
catalyst Al:Cr ratio PE (g) activity (g/gCr/h)
C4
C6
3
4
4
4
4
4
4
4
1000
1000
500
1.8
5263
3883
5177
3451
863
781
0
0.7
0.0
0.4
1.0
0
99.0
99.8
99.6
99.0
99.5
99.3
0.0
1.3
1.3
0.3
trace
0.6
trace
3.4
100
50
500b
500c
500d
0
0
0
301
99.2
a Standard conditions: 30 µmol of catalyst, T ) 50 °C, 150 mL of
b
c
toluene, 35 bar of ethylene, 30 min reaction time. P ) 10 bar. T ) 22
d
°C. T ) 22 °C, time ) 900 min.
two. When complex 4 was treated with increased loadings of
MAO, the general trend (up to 500 equiv) was an increase in
activity along with slightly increased polyethylene formation,
but there was no concomitant change in the selectivity. At 1000
equiv of MAO the catalyst activity drops, but again there is no

4602 Organometallics, Vol. 26, No. 18, 2007
Temple et al.
Figure 3. Complexes 5 and 6 with 30% thermal ellipsoids (solvent molecule omitted for clarity).
Scheme 3
comparable to those obtained with the trivalent complexes 3
and 4. Only in the case of the cyclohexyl derivative with low
MAO loading, the higher oligomers disappear in favor of
1-hexene and 1-butene. On the other hand, even the activity
shows a 10-fold decrease, possibly suggesting that some other
mechanism may become operational under these circumstances.
The distinct difference in catalytic behavior between the
divalent and trivalent complexes clearly indicates that the two
oxidation states are not readily interchanged during the catalytic
cycle. In contrast to the behavior of the CySCH2CH2N(H)CH2-
CH2SCy catalytic system, the triValent species are not reduced
to the diValent state. Therefore, each oxidation state maintains
its integrity during the interaction with the aluminum activator
and the oligomerization reaction.
Although we found no conclusive evidence for the reoxidation
of CrII to the trivalent state (as observed with the regular
CySCH2CH2N(H)CH2CH2SCy system), there is a perplexing
observation that can be made from the product distribution
obtained from the divalent complex. When looking at the R
values of the distributions, it is obvious that an enhanced fit
can be obtained for the C8-C16 (especially C8-C14) fractions
compared to the C6-C16 fraction. It is tempting to suggest that
a small amount of CrIII might indeed be slowly generated from
CrII during the catalytic cycle, possibly via the usual dispro-
portionation mechanism.10 The trivalent species may be respon-
sible for the amount of 1-hexene exceeding the S-F distribution.
In an attempt to better assess the stability of the metal centers
and the difference in behavior of the different oxidation states,
complexes 3 and 4 were treated with various alkyl aluminum
reagents in order to isolate organochromium species. Regret-
tably, all reactions yielded oily materials that could not be
properly characterized. However, crystalline materials were
obtained upon reaction of the corresponding CrII species with
AlMeCl2 and, even more surprisingly, with MAO (the actual
activator of the oligomerization cycle) (Scheme 3).
The molecular structures of the corresponding {[2,6-(Cy-
SCH2)2-py]Cr-µ-Cl}2‚2[AlMeCl3]‚toluene (5) and {[2,6-(Cy-
SCH2)2-py]CrCl}[AlMe2Cl2]‚toluene (6) were elucidated by
X-ray crystallography and are displayed in Figure 3. In both
complexes, the metal center is cationized and the diValent
oxidation state was preserVed during the reaction, thus lending
further support to the proposal that in the case of the trivalent
precursor no reduction occurs in the presence of R3Al. Com-
plexes 5 and 6 are similar in terms of the coordination
environment around the pseudo-octahedral chromium atom and
resemble the {SN(H)SCr(µ-Cl)}2{(i-Bu)2AlCl2}2 earlier re-
ported.7 In both complexes, the SNS ligand occupies three
coordination sites and the fourth is occupied by one chloride.
In the case of 5, the fifth coordination site is occupied by the
bridging chlorine of one identical unit, which builds the
dinuclear frame. The last site is defined by the loose coordination
of one chloride of the [MeAlCl3]- counteranion [Cr(1)‚Cl(2)
) 3.287(8) Å].
The structure of 6 is perhaps better described as monomeric,
containing a square-planar chromium atom, given that the bond

Cr-(SNS) Ethylene Trimerization Catalyst
Organometallics, Vol. 26, No. 18, 2007 4603
with the bridging chlorine of the identical unit is noticeably
stretched [Cr(1)‚‚Cl(2) ) 2.813(9) Å]. The remaining difference
resides in the aluminate counteranion, which appears to be
[Me2AlCl2]-, with the chloride atom oriented toward the sixth
chromium coordination site [Cr(1)‚‚Cl(2) ) 2.840(9)Å], but
substantially closer to Cr than in 5. Thus, the coordination
geometry around the metal center in 6 may also be regarded as
axially very distorted octahedral.
The formation of the divalent 5 and 6 sharply contrasts with
the behavior of the Sasol SNS system,10 which clearly showed
the possibility of reoxidation of the metal center upon treatment
with the aluminum activators. Unfortunately, since complex 5
could not be isolated in an analytically pure form, it could not
be tested in a meaningful manner for catalytic activity. Instead,
complex 6 gave, upon further activation with MAO, the same
product distribution as observed for the ligand/CrCl2 catalytic
system.
resulting in the formation of complex 6. The structure of
complex 6 highlights the cationic nature of the metal center,
while the true nature of the catalytically active species at this
stage can only be speculated upon. Nonetheless, all the divalent
derivatives of this ligand system are clearly active but with a
complete lack of selectivity. These results concur with the
working hypothesis that the stabilization of a trivalent alkyl-Cr
is crucial to obtain selective trimerization. Different from the
SNS Sasol system, where the trivalent catalyst precursor can
be reduced and the trivalent reoxidized in what seems to be
steps of the same complex reactivity pattern, the presence of
the pyridine ring in the present system stabilizes the individual
oxidation states. The fact that the ligand system does not contain
an N-H function in the backbone proves that the N-H moiety
is not responsible for the exceptional selectivity of the SNS class
of ligand system and unequivocally demonstrates that the N-H
function is not required.
In conclusion, we have successfully prepared, characterized,
and tested a pyridine-based SNS ligand system and isolated two
trivalent complexes 3 and 4. These species are selective (>99%)
catalyst precursors for the trimerization of ethylene to 1-hexene.
Unfortunately, the observed activities for these catalysts are only
moderate. Structural characterization of the divalent counterpart
has been possible only after treatment with alkyl aluminum
activators. Of particular interest is the result of the reaction with
MAO, which is the actual activator for the catalytic cycle,
Acknowledgment. This work was supported by the Natural
Science and Engineering Council of Canada (NSERC).
Supporting Information Available: Complete crystallographic
data (CIF) for the complexes reported in this paper. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM700452U