Ethylene oligomerization promoted by a silylated-SNS chromium system

Article abstract of DOI:10.1021/om200505a

The ethylene trimerization SNS ligand has been modified by replacing the methylene carbons flanking the nitrogen atom with dimethyl silyl groups. Three ligands, CySCH₂Si(CH₃)₂N(H)Si(CH ₃)₂CH₂SCy (a), (t-Bu)SCH₂Si(CH ₃)₂N(H)Si(CH₃)₂CH₂S(t-Bu) (b), and PhSCH₂Si(CH₃)₂N(H)Si(CH ₃)₂CH₂SPh (c), have been prepared. Ligand a in either protonated or deprotonated forms was reacted with CrCl ₃(THF)₃ to afford the corresponding monomeric [CySCH ₂Si(CH₃)₂N(H)Si(CH₃) ₂CH₂SCy]CrCl₃ (1a) or dimeric {[CySCH ₂Si(CH₃)₂NSi(CH₃)₂CH ₂SCy]CrCl(μ-Cl)}₂ (2a). One-pot reaction of a in the presence of Et₂AlCl with either Cr(III) or Cr(II) chlorides afforded in either case the divalent {[CySCH₂Si(CH₃) ₂N(H)Si(CH₃)₂CH₂SCy]Cr{(μ-Cl) Al(CH₂CH₃)₂Cl}₂ (3a). To deprotonate the N-H function of the Si-SNS ligand, n-BuLi was used for the purpose of preparing the divalent chromium analogue. The reaction afforded in the case of both a and b the two nearly isostructural divalent complexes {[CySCH ₂Si(CH₃)₂NSi(CH₃)₂CH ₂SCy]Cr(μ-Cl)}₂ (4a) and {[(t-Bu)SCH ₂Si(CH₃)₂NSi(CH₃)₂CH ₂S(t-Bu)]Cr(μ-Cl)}₂ (4b) in crystalline form. To further clarify the interaction of 4 with aluminate species, we have carried out in situ complexation in the presence of either AlCl₃ or AlMe ₃ and using divalent instead of trivalent chromium salts. In the cases of ligands a and c and AlCl₃, two isostructural complexes, {[CySCH₂Si(CH₃)₂N(H)Si(CH₃) ₂CH₂SCy]Cr{(μ-Cl)AlCl₃}₂ (5a) and {[PhSCH₂Si(CH₃)₂N(H)Si(CH₃) ₂CH₂SPh]C{(μ-Cl)AlCl₃}₂ (5c), have been obtained. The reaction with AlMe₃ afforded {[CySCH ₂Si(CH₃)₂N(Al(CH₃) ₂-μ-Cl)Si(CH₃)₂CH₂SCy]Cr{(μ- Cl)Al(CH₃)₃} (6a). Its structure was informative, showing a possible catalyst deactivation pathway. To better evaluate the role of the N-H function, we have also methylated ligand a at the N atom. The complexation to chromium was successful only in the presence of Me₂AlCl and if a divalent chromium precursor was used. The reaction afforded the catalytically inactive divalent {[CySCH₂Si(CH₃)₂N(CH ₃)Si(CH₃)₂CH₂SCy]Cr(μ-Cl)} ₂{(Al(CH₃)₂Cl)₂)(μ-Cl)} ₂ (7d). Most of these species showed good catalytic activity upon activation but produced only statistical distributions of oligomers.

Full text of DOI:10.1021/om200505a

ARTICLE
pubs.acs.org/Organometallics
Ethylene Oligomerization Promoted by a Silylated-SNS
Chromium System
Khalid Albahily,† Sandro Gambarotta,*,† and Robbert Duchateau*,‡
†Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada
‡Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, P.O. Box 513, Eindhoven 5600 MB,
The Netherlands
S Supporting Information
b
ABSTRACT: The ethylene trimerization SNS ligand has been modified by
replacing the methylene carbons flanking the nitrogen atom with dimethyl silyl
groups. Three ligands, CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy (a), (t-Bu)SCH2-
Si(CH3)2N(H)Si(CH3)2CH2S(t-Bu) (b), and PhSCH2Si(CH3)2N(H)Si(CH3)2-
CH2SPh (c), have been prepared. Ligand a in either protonated or deprotonated
forms was reacted with CrCl3(THF)3 to afford the corresponding monomeric
[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]CrCl3 (1a) or dimeric {[CySCH2Si-
(CH3)2NSi(CH3)2CH2SCy]CrCl(μ-Cl)}2 (2a). One-pot reaction of a in the
presence of Et2AlCl with either Cr(III) or Cr(II) chlorides afforded in either case
the divalent {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr{(μ-Cl)Al(CH2CH3)2-
Cl}2 (3a). To deprotonate the NÀH function of the Si-SNS ligand, n-BuLi was used
for the purpose of preparingthe divalent chromium analogue. The reaction afforded in
the case of both a and b the two nearly isostructural divalent complexes
{[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]Cr(μ-Cl)}2 (4a) and {[(t-Bu)SCH2Si(CH3)2NSi(CH3)2CH2S(t-Bu)]Cr(μ-Cl)}2 (4b)
in crystalline form. To further clarify the interaction of 4 with aluminate species, we have carried out in situ complexation in the
presence of either AlCl3 or AlMe3 and using divalent instead of trivalent chromium salts. In the cases of ligands a and c and AlCl3, two
isostructural complexes, {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr{(μ-Cl)AlCl3}2 (5a) and {[PhSCH2Si(CH3)2N(H)Si-
(CH3)2CH2SPh]C{(μ-Cl)AlCl3}2 (5c), have been obtained. The reaction with AlMe3 afforded {[CySCH2Si(CH3)2N(Al(CH3)2-
μ-Cl)Si(CH3)2CH2SCy]Cr{(μ-Cl)Al(CH3)3} (6a). Its structure was informative, showing a possible catalyst deactivation pathway. To
better evaluate the role of the NÀH function, we have also methylated ligand a at the N atom. The complexation to chromium was
successful only in the presence of Me2AlCl and if a divalent chromium precursor was used. The reaction afforded the catalytically inactive
divalent {[CySCH2Si(CH3)2N(CH3)Si(CH3)2CH2SCy]Cr(μ-Cl)}2{(Al(CH3)2Cl)2)(μ-Cl)}2 (7d). Most of these species showed
good catalytic activity upon activation but produced only statistical distributions of oligomers.
’ INTRODUCTION
reductive coupling of two ethylene molecules, require the inter-
mediate formation of a sufficiently reducing chromium species.
Although the oxidation state was long debated,3a,6,7 today there is
a general agreement that the ability of a trivalent precursor to
generate a monovalent catalytically active intermediate is a
prerequisite for the initial ethylene reductive coupling, ultimately
responsible for the selectivity of the catalytic cycle.3aÀc,4l,6
The McGuinnessÀWasserscheid SNS-Cr catalyst4b,c is one of
the most performing trimerization systems. Our contribution to
understand this highly selective catalyst was to identify the role of
the chromium redox dynamism.8 In other words, the three
critical states (+I, +II, +III) readily interconvert through series
of reductions8a,b,9 and disproprotionations,4o,9c possibly favored
by dimeric aggregation and triggered by the alkyl aluminum
activators. The possibility for the ligand to either remain neutral
Ethylene oligomerization for the production of R-olefins is an
active field of research in organometallic chemistry and catalysis.1
The several industrial applications of R-olefins—particularly as
comonomers for the synthesis of linear low-density polyethylene,
PVC plasticizers, household detergents, shampoos, and lubricants2—
have spurred a resurgence of interest for finding new catalytic
systems for their production.1,3 The focus has been on develop-
ing more efficient catalysts with the ultimate goal of improving
the activity and, most of all, the selectivity.
The most remarkable results with respect to selective ethylene
oligomerization have been obtained through the use of trivalent
chromium catalyst precursors.4 Although this process can also be
promoted by other transition metal complexes,5 chromium-
based catalytic systems remain the most performing.4 The cur-
rent mechanistic hypothesis for the chromium-promoted selec-
tive formation of 1-hexene and 1-octene is a redox ring-expansion
pathway.3b,5,4l,6 The two critical steps, oxidative addition and
Received: June 10, 2011
Published: August 16, 2011
r
2011 American Chemical Society
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|

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Scheme 1
or be deprotonated and the presence of soft donor atoms in the
ligand scaffold altogether play a particularly delicate role in
determining the selectivity of the cycle. In an attempt to further
understand the role of the unique features of this ligand in
stabilizing the different chromium oxidation states, we have
modified the ligand framework by replacing the methylene
carbons, flanking the nitrogen atom of the SNS system, with
dimethyl silyl groups (Scheme 1). This simple modification had
profound effects on the catalytic activity of the SNS system.
Herein we describe our findings and their implications.
SiCH3), 1.19À1.39 (m, 10H, Cy), 1.58À1.65 (m, 2H, Cy), 1.78À1.85
(m, 4H, Cy), 1.77 (s, 4H, SiCH2S), 1.9À2.05 (m, 4H, Cy), 2.07 (s, 1H,
NH), 2.42À2.6 (m, 2H, Cy). 13C{1H} NMR (CDCl3, 75 MHz, 300 K):
δ 0.60 (SiCH3), 17.60 (SiCH2S), 25.95, 26.18, 32.97, 46.13 (Cy). MS
(ESI) m/z (M + H)+ = 390.3. Anal. Calcd (found) for C18H39NS2Si2: C
55.46 (55.42), H 10.08 (10.07), N 3.59 (3.57). IR (Nujol): νNÀH
=
3358 cmÀ1
.
Preparation of (t-Bu)SCH2Si(CH3)2N(H)Si(CH3)2CH2S(t-Bu)
(b). A solution of 2-methyl-2-propanethiol (8.57 g, 95.0 mmol) in THF
(150 mL) was cooled to 0 °C and treated with a suspension of potassium
hydride (4.0 g, 100 mmol) in THF (50 mL). The reaction was stirred for
90 min at room temperature and then treated with neat 1,3-bis-
(chloromethyl-1,1,3,3-tetramethyl)disilazane (10.93 g, 47.5 mmol) at
0 °C. The reaction was stirred overnight at room temperature. After
filtration, the solvent was removed in vacuo, affording b (15.1 g, 44.7 mmol,
94%) as a colorless oil. 1H NMR (CDCl3, 300 MHz, 300 K): δ 0.15 (s,
12H, SiCH3), 1.27 (s, 18H, C(CH3)3), 1.73 (s, 4H, SiCH2S), 2.07 (s, 1H,
NH). 13C{1H} NMR (CDCl3, 75 MHz, 300 K): δ 0.67 (SiCH3), 15.73
(SiCH2S), 29.99 (C(CH3)3), 41.40 (C(CH3)3). (ESI) m/z (M + H)+ =
338.090. Anal. Calcd (found) for C14H35NS2Si2: C 49.79 (49.77), H
’ EXPERIMENTAL SECTION
All reactions were carried out under a dry nitrogen atmosphere.
Solvents were dried using an aluminum oxide solvent purification
system. The liquid product mixtures were analyzed by using a CP
9000 gas chromatograph (GC) equipped with a 30 mL Â 0.32 mm i.d.,
capillary CP volamine column and a FID detector. All single-point
experiments were performed in duplicate. The yield was determined by
1H NMR spectroscopy (Varian Mercury 400 MHz spectrometer).
Infrared spectra were recorded on an ABB Bomem FTIR instrument
from Nujol mulls prepared in a VAC 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 determina-
tion were obtained with a Bruker diffractometer equipped with a 1K
Smart CCD area detector. Diethyl aluminum chloride, trimethyl alumi-
num, aluminum trichloride, and dimethyl aluminum chloride were
purchased from Strem and were used as received. Methylaluminoxane
(MAO, 10% in toluene) was purchased from Aldrich. ESI-MS were
obtained from THF solutions using a MicroMass Quattro-LC instrument.
Preparation of CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy (a). A
solution of cyclohexylthiol (15.0 g, 129.1 mmol) in THF (150 mL) was
cooled to 0 °C and treated with a suspension of potassium hydride (5.7 g,
142.5 mmol) in THF (50 mL). The reaction mixture was stirred for
90 min at room temperature when 1,3-bis(chloromethyl-1,1,3,3-tetra-
methyl)disilazane (15.0 g, 65.1 mmol) was syringed in at 0 °C. The
reaction was stirred overnight at room temperature and filtered. The
solvent was removed in vacuo to give a as a colorless oil (20.3 g,
52.2 mmol, 80%). 1H NMR (CDCl3, 300 MHz, 300 K): δ 0.15 (s, 12H,
10.45 (10.44), N 4.15 (4.14). IR (Nujol): νNÀH = 3369 cmÀ1
.
Preparation of PhSCH2Si(CH3)2N(H)Si(CH3)2CH2SPh (c). A
250 mL round-bottom flask was charged with benzenethiol (10 g, 90.7
mmol) and THF (150 mL). The solution was cooled to 0 °C and treated
with a suspension of potassium hydride (3.99 g, 99.7 mmol) in THF
(50 mL). The mixture was stirred for 1 h at room temperature, and then
1,3-bis(chloromethyl-1,1,3,3-tetramethyl)disilazane (10.36 g, 45.0 mmol)
was syringed in at 0 °C. Stirring was continued overnight at room
temperature followed by filtration. The solvent was removed in vacuo,
1
affording c as a colorless oil (16.14 g, 42.7 mmol, 95%). H NMR
(CDCl3, 300 MHz, 300 K): δ 0.42 (s, 12H, SiCH3) 2.37 (s, 4H, SiCH2S)
2.63 (s, 1H, NH) 7.23À7.29 (m, 2H, SPh), 7.38À7.46 (m, 8H, SPh).
13C{1H} NMR (CDCl3, 75 MHz, 300 K): δ 0.64 (SiCH3), 20.15
(SiCH2S), 124.61, 126.10, 128.62, 140.04 (SPh). (ESI) m/z (M + H)+ =
378.101. Anal. Calcd (found) for C18H27NS2Si2: C 57.24 (57.21), H
7.21 (7.20), N 3.71 (3.69). IR (Nujol): νNÀH = 3386 cmÀ1
.
Preparation of CySCH2Si(CH3)2N(CH3)Si(CH3)2CH2SCy (d).
A suspension of potassium hydride (0.62 g, 15.4 mmol) in THF (30 mL)
was added portionwise to a solution of a (5.0 g, 12.8 mmol) in THF
(50 mL) at 0 °C. The mixture was refluxed overnight and then treated
with neat iodomethane (2.19 g, 15.4 mmol) at room temperature. The
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ARTICLE
mixture was stirred overnight and filtered. The solvent was removed
in vacuo, affording d (5.10 g, 12.6 mmol, 98%) as a colorless oil. 1H NMR
(CDCl3, 300 MHz, 300 K): δ 0.17 (s, 12H, SiCH3), 1.18À1.38 (m, 10H,
Cy), 1.57À1.65 (m, 2H, Cy), 1.76À1.83 (m, 4H, Cy), 1.81 (s, 4H,
SiCH2S), 1.9À2.03 (m, 4H, Cy), 2.42À2.58 (m, 2H, Cy), 2.49 (s, 3H,
NCH3). 13C{1H} (CDCl3, 75 MHz, 300 K): δ À0.430 (SiCH3), 16.255
(SiCH2S), 25.95, 26.18, 32.96, 46.13 (Cy), 31.18 (NCH3). MS (ESI)
m/z (M + H)+ = 404.122. Anal. Calcd (found) for C19H41NS2Si2: C
56.51 (56.49), H 10.23 (10.22), N 3.47 (3.46).
Anal. Calcd (found) for C36H76Cl2Cr2N2S4Si4: C 45.39 (45.38), H 8.04
(8.02), N 2.94 (2.95).
Preparation of {[(t-Bu)SCH2Si(CH3)2NSi(CH3)2CH2S(t-Bu)]-
Cr(μ-Cl)}2 (4b). A solution of b (0.337 g, 1.0 mmol) in THF (10 mL)
was treated with n-BuLi (0.44 mL, 12.1 mmol, 2.5 M in hexanes) at 0 °C
and stirred at room temperature for 18 h. The resulting solution was
added to a suspension of CrCl2(THF)2 (0.268 g, 1.0 mmol) in THF
(10 mL). The mixture was stirred at room temperature overnight. The
solvent was removed in vacuo, and the residue suspended in toluene
(10 mL). Following centrifugation, the supernatant was concentrated
and stored at À30 °C for 4 days. The resulting product was filtered,
washed with cold hexanes (10 mL), and dried in vacuo, affording 4b
(0.301 g, 0.71 mmol, 71%) as a blue crystalline material. μeff = 4.80 μB.
Anal. Calcd (found) for C28H68Cl2Cr2N2S4Si4: C 39.64 (39.62), H 8.08
(8.07), N 3.30 (3.28).
Preparation of [CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]CrCl3
(1a). Solid CrCl3(THF)3 (0.375 g, 1.0 mmol) was added to a solution of
a (0.389 g, 1.0 mmol) in toluene (10 mL). The mixture was refluxed for
30 min and then allowed to stir at room temperature for 18 h. Following
centrifugation, the supernatant was concentrated and stored at À30 °C
for 4 days. The resulting dark blue-purple crystals of 1a were filtered
and washed with cold hexanes (10 mL) and dried in vacuo (0.494 g,
90.1 mmol 90%). μeff = 3.84 μB. Anal. Calcd (found) for C18H39-
Cl3CrNS2Si2: C 39.44 (39.42), H 7.17 (7.16), N 2.56 (2.54). IR (Nujol):
Preparation of {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr-
{(μ-Cl)AlCl3}2 (5a). A solution of a (0.389 g, 1.0 mmol) in toluene
(10 mL) was treated with CrCl2(THF)2 (0.268 g, 1.0 mmol). Solid
aluminum trichloride (0.668 g, 5.0 mmol) was added to the correspond-
ing solution cooled to À35 °C. The mixture was stirred for 2 h and
centrifuged, and the supernatant concentrated and stored at À30 °C for
2 days. The resulting product was filtered, washed with cold hexanes
(10 mL), and dried in vacuo, affording 5a (0.420 g, 0.54 mmol, 54%) as a
blue crystalline material. μeff = 4.99 μB. Anal. Calcd (found) for
C18H39Al2Cl8CrNS2Si2: C 27.74 (27.72), H 5.04 (5.03), N 1.80
νNÀH = 3138 cmÀ1
.
Preparation of {[ CySCH2Si(CH3)2NSi(CH3)2CH2SCy]CrCl-
(μ-Cl)}2 (2a). A solution of a (0.778 g, 2.0 mmol) in THF (10 mL) was
treated with n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes) at 0 °C and
stirred at room temperature for 18 h. The resulting solution was added to
a suspension of CrCl3(THF)3 (0.750 g, 2.0 mmol) in THF (5 mL). The
mixture was stirred at room temperature overnight. The solvent was
removed in vacuo, and the residue suspended in toluene (10 mL).
Following centrifugation, the volume of the supernatant was reduced to
4 mL and stored at À30 °C for 6 days. The resulting product was filtered
and washed with cold hexanes (10 mL) and dried in vacuo, affording 2a
(0.317 g, 0.31 mmol, 31%) as a green crystalline material. μeff = 3.89 μB.
Anal. Calcd (found) for C36H76Cl4Cr2NS4Si4: C 42.25 (42.23), H 7.49
(7.50), N 2.74 (2.73).
(1.79). IR (Nujol): νNÀH = 3113 cmÀ1
.
Preparation of {[PhSCH2Si(CH3)2N(H)Si(CH3)2CH2SPh]Cr-
{(μ-Cl)AlCl3}2 (5c). Solid CrCl2(THF)2 (0.268 g, 1.0 mmol) was
added to a solution of c (0.378 g, 1.0 mmol) in toluene (10 mL). Solid
aluminum trichloride (0.668 g, 5.0 mmol) was added at À35 °C to the
corresponding suspension. The reaction mixture was stirred for 4 h and
centrifuged, and the supernatant concentrated and stored in a freezer at
À30 °C for 2 days. The resulting product was filtered and washed with
cold hexanes (10 mL) and dried in vacuo, affording 5c (0.120 g, 0.16
mmol, 15%) as a blue crystalline material. μeff = 4.89 μB. Anal. Calcd
(found) for C18H27Al2Cl8CrNS2Si2: C 28.18 (28.15), H 3.55 (3.56), N
Preparation of {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr-
{(μ-Cl)Al(CH2CH3)2 Cl}2 (3a). Method A. Solid CrCl3(THF)3
(0.375 g, 1.0 mmol) was added to a solution of a (0.389 g, 1.0 mmol)
in toluene (10 mL). The mixture was refluxed for 30 min and allowed to
stir at room temperature for 18 h. Neat diethylaluminum chloride (1.21 g,
10.0 mmol) was then added dropwise. The reaction mixture was stirred
for 2 h at room temperature and centrifuged, and the supernatant
concentrated and stored at À30 °C for 2 days. The resulting product was
filtered, washed with cold hexanes (10 mL), and dried in vacuo, affording
3a (0.346 g, 0.46 mmol, 46%) as a blue crystalline material. μeff = 4.89 μB.
Anal. Calcd (found) for C26H59Al2Cl4CrNS2Si2: C 41.43 (41.41), H
1.83 (1.82). IR (Nujol): νNÀH = 3126 cmÀ1
.
Preparation of {[CySCH2Si(CH3)2N(Al(CH3)2-μ-Cl)Si(CH3)2-
CH2SCy]Cr{(μ-Cl)Al (CH3)3} (6a). Solid CrCl2(THF)2 (0.268 g, 1.0
mmol) was added to a solution of a (0.389 g, 1.0 mmol) in toluene
(10 mL). After cooling to À35 °C, neat trimethyl aluminum (0.72 g,
10.0 mmol) was added dropwise to the mixture. Stirring was continued
for 30 min, followed by centrifugation. The resulting solution was con-
centrated and stored at À30 °C for 10 days. The product separated,
which was filtered and washed with cold hexanes (10 mL) and dried in
vacuo, affording 6a (0.198 g, 0.31 mmol, 31%) as a blue crystalline
material. μeff = 4.91 μB. Anal. Calcd (found) for C23H53Al2Cl2CrNS2Si2:
C 43.11 (43.12), H 8.34 (8.35), N 2.19 (2.20).
Preparation of {[CySCH2Si(CH3)2N(CH3)Si(CH3)2CH2SCy]-
Cr(μ-Cl)}2{(Al(CH3)2Cl)2)(μ-Cl)}2 (7d). Solid CrCl2(THF)2 (0.268g,
1.0 mmol) was added to a solution of d (0.403 g, 1.0 mmol) in toluene
(10 mL). The mixture was cooled to À35 °C, and neat dimethylalumi-
num chloride (0.462 g, 5.0 mmol) was added dropwise. The reaction
mixture was stirred for 4 h and centrifuged, and the supernatant
concentrated and stored at À30 °C for 3 days. The resulting product
was filtered, washed with cold hexanes (10 mL), and dried in vacuo,
affording 7d (0.336 g, 0.24 mmol, 47%) as a blue crystalline material.
μeff = 4.90 μB. Anal. Calcd (found) for C46H106Al4Cl8Cr2N2S4Si4: C
38.81 (38.80), H 7.51 (7.50), N 1.97 (1.95).
7.89 (7.88), N 1.86 (1.86). IR (Nujol): νNÀH = 3098 cmÀ1
.
Method B. Solid CrCl2(THF)2 (0.268 g, 1.0 mmol) was added to a
solution of a (0.389 g, 1.0 mmol) in toluene (10 mL). The mixture was
cooled to À35 °C, and neat diethylaluminum chloride (1.21 g, 10.0 mmol)
was added dropwise. The reaction mixture was stirred for 2 h and
centrifuged, and the resulting solution concentrated and stored at
À30 °C for 3 days. The resulting blue, crystalline 3a was filtered and
washed with cold hexanes (10 mL) and dried in vacuo (0.595 g, 0.789
mmol, 79%).
Preparation of {[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]Cr(μ-Cl)}2
(4a). A solution of n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes) was
added to a solution of a (0.780 g, 2.0 mmol) in THF (10 mL) at 0 °C and
allowed to stir at room temperature for 18 h. The resulting solution was
added to a suspension of CrCl2(THF)2 (0.536 g, 2.0 mmol) in THF
(10 mL). The mixture was stirred at room temperature overnight. The
solvent was removed in vacuo, and the residue suspended in toluene
(10 mL). Following centrifugation, the supernatant was concentrated
and stored at À30 °C for 4 days. The resulting product was filtered and
washed with cold hexanes (10 mL) and dried in vacuo, affording 4a
(0.534 g, 0.56 mmol, 56%) as a blue crystalline material. μeff = 4.93 μB.
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. Data collection was performed with three batch
runs at phi = 0.00 deg (600 frames), at phi = 120.00 deg (600 frames),
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Table 1. Oligomerization Resultsa
MAO
total
activity
1-C6
1-C8
1-C10
1-C12
1-C14
1-C16
catalyst
(equiv)
product (g)
% PE
% 1-Cn
(g/mmolCr/h)
(wt %)
(wt %)
(wt %)
(wt %)
(wt %)
(wt %)
Rav
(SNS)CrCl3
1000
1000
500
9.5
0
8.4
0
91
0
5824
>98
0
traces
0
traces
0
traces
0
traces
traces
1a
1a
2a
2a
3a
3a
3a
4a
4a
4b
5a
6a
6a
6a
7c
0
0
0
0
0
0
8
8
8
9
0
8
0
8
7
0
0
0
0
0
0
7
5
5
6
0
5
0
4
6
0
0
0
0
0
0
0
0
0
0
1000
500
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
500
3.5
18
22
27
0
12
1.4
1.3
0.4
0
88
98
98
99
0
8970
44 900
55 600
68 200
0
29
30
29
28
0
25
26
26
25
0
18
17
18
19
0
13
14
14
13
0
0.75
0.70
0.71
0.73
1000
1000
500
1000
1000
0
6.3
0
3.1
0
96
0
16 200
0
30
0
26
0
19
0
12
0
0.70
500
0.7
18
0
14
2.3
0
86
97
0
1790
44 900
0
31
29
0
27
27
0
19
19
0
11
12
0
0.67
0.74
1000
1000
a Conditions: 100 mL of toluene, loading 15 μmol of catalyst, 35 bar of ethylene, reaction temperature 60 °C, reaction time 60 min. Only traces of
1-butene (<1%) were sometimes visible but which could not properly be quantified.
and at phi = 240.00 deg (600 frames). Initial unit-cell parameters were
determined from 60 data frames collected at different sections of the
Ewald sphere. Semiempirical absorption corrections based on equivalent
reflections were applied. The systematic absences and unit-cell para-
meters were consistent for the reported space groups. The structures
were solved by direct methods, completed with difference Fourier
syntheses, and refined with full-matrix least-squares procedures based
on F2. All non-hydrogen atoms were refined with anisotropic displace-
ment parameters. All hydrogen atoms were treated as idealized con-
tributions. All scattering factors and anomalous dispersion factors are
contained in the SHELXTL 6.12 program library. Relevant crystal-
lographic data are given as Supporting Information.
Oligomerization Results. Catalytic runs were carried out in
300 mL high-pressure Parr reactors containing a heating/cooling jacket.
A preweighed amount of catalyst was dissolved in toluene (100 or
50 mL, respectively, depending on the reactor size) under N2 prior to
loading the reaction vessel. Solutions were heated using a thermostatic
bath and charged with ethylene, maintaining the pressure throughout
the run. Polymerizations were quenched by releasing the pressure and
Figure 1. Drawing of 1a. Thermal ellipsoids are drawn at the 50%
addition of acidified MeOH. The polymers obtained were isolated by
probability level.
filtration, sonicated with an aqueous solution of HCl, rinsed, and
thoroughly dried prior to measuring the mass. The reaction mixtures
of the oligomerization runs were cooled to 0 °C prior to releasing the
The distortion of the octahedral structure is apparent on the ClÀCr
overpressure and quenching with acidified MeOH. Results of catalytic
bond angles [Cl(1a)ÀCr(1)ÀCl(1) = 72.12(19)°; Cl(2)ÀCr(1)ÀCl-
runs are summarized in Table 1. Activities of catalytic runs were assessed
via NMR by integrating the olefinic resonances against the Me toluene
resonance, whose amount was standardized to 100 mL through all
the runs.
(1) = 93.94(10)°].
Complex 2a. The complex is a symmetry-generated dimer (Figure 2)
with two chlorine atoms bridging the two metal centers [Cr(1)ÀCl(1) =
2.4129(11) Å; Cr(1)ÀCl(1a) = 2.4420(11) Å] and forming a square
Cr2Cl2 core. The slightly distorted octahedral geometry of each metal
center [Cl(2)ÀCr(1)ÀCl(1) = 93.10(4)°; Cl(1)ÀCr(1)ÀCl(1a) =
80.37(3)°; Cl(2)ÀCr(1)ÀCl(1a) = 173.20(4)°] is completed by one
terminally bonded chlorine [Cr(1)ÀCl(2) = 2.2930(11) Å] and the
three donor atoms of the deprotonated ligand system. The N atom is
trigonal planar as a result of the deprotonation and forms with the metal
a bonding distance shorter than in 1a [Cr(1)ÀN(1) = 1.984(3) Å] and
in the expected range for a chromium amido derivative. The two sulfur
atoms are located trans to each other with a minor deviation of the
SÀCrÀS vector from linearity [S(2)ÀCr(1)ÀS(1) = 173.84(4)°].
Description of Crystal Structures. Complex 1a. The structure
of 1a (Figure 1) consists of a meridionally arranged CrCl3 unit
[Cr(1)ÀCl(1) = 2.323(2) Å; Cr(1)ÀCl(1a) = 2.323(2) Å; Cr(1)ÀCl-
(2) = 2.269(4) Å], surrounded by the three donor atoms of the ligand
system in an overall distorted octahedral geometry. The long CrÀN
bond distance [Cr(1)ÀN(1) = 2.190(12) Å] along with the pyramidal
geometry of the N atom clearly indicates the presence of a hydrogen
atom on nitrogen. The two sulfur atoms are trans positioned
[Cr(1)ÀS(1)1a = 2.433(2) Å; Cr(1)ÀS(1) = 2.433(2) Å] and exhibit
a minor deviation from linearity [S(1a)ÀCr(1)ÀS(1) = 173.56(13)°].
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Complexes 4a and 4b. The two complexes have a very similar
arrangement (Figure 4) and geometry with only one main difference in
the ligand disposition. Both complexes are symmetry-generated dimers
with the two metals linked by two bridging chlorines of a Cr2Cl2 planar
core. In 4a the two metals adopted square-pyramidal coordination
geometries. The two bridging chlorine atoms are respectively located
in equatorial and axial positions with respect to each of the two metal
centers [Cr(1)ÀCl(1) = 2.3679(11) Å; Cr(1)ÀCl(1a) = 2.7097(14) Å].
The Cr2Cl2 planar core is almost perfectly square-planar [Cl(1)À
Cr(1)ÀCl(1a) = 92.49(3)°; Cr(1)ÀCl(1)ÀCr(1a) = 87.51(3)°]. The
three ligand donor atoms complete, with the equatorial chlorine atom,
the basal square plane [Cr(1)ÀS(1) = 2.4756(12) Å; Cr(1)ÀS(2) =
2.4788(12) Å, S(1)ÀCr(1)ÀS(2) = 178.88(3)°]. The CrÀN distance is
in the expected range [Cr(1)ÀN(1) = 2.051(2) Å].
The coordination geometry of the two chromium atoms in 4b may be
described as trigonal bipyramidal. The two bridging chlorine atoms are
axially [N(1)ÀCr(1)ÀCl(1) = 176.44(8)°] and equatorially [N(1)À
Cr(1)ÀCl(1a) = 95.53(8)°] located, forming comparable CrÀCl dis-
tances [Cr(1)ÀCl(1) = 2.4118(9) Å; Cr(1)ÀCl(1a) = 2.5079(9) Å].
The trigonal equatorial plane is bound by the two sulfur atoms
[Cr(1)ÀS(2) = 2.5151(9) Å; Cr(1)ÀS(1) = 2.6466(9) Å] and one of
the two chlorines [[S(2)ÀCr(1)ÀS(1) = 123.90(3)°, Cl(1a)ÀCr-
(1)ÀS(2) = 134.64(3)°, Cl(1a)ÀCr(1)ÀS(1) = 101.36(3)°]. The
ligand deprotonated nitrogen atom is located in apical position trans
to one of the two chlorine atoms, forming the expected short distance
with the metal center [Cr(1)ÀN(1) = 2.027(3) Å].
Figure 2. Drawing of 2a. Thermal ellipsoids are drawn at the 50%
probability level.
Complexes 5a and 5c. The two complexes possess the same
structure (Figure 5), the only difference consisting of the different
substituents attached to the ligand sulfur atoms (cyclohexyl versus
phenyl). In both cases the coordination geometry of the metal center
is square pyramidal with a minor elevation of the metal center above the
basal plane. The basal square plane [N(1)ÀCr(1)ÀCl(1) = 174.76(13)°,
N(1)ÀCr(1)ÀS(1) = 88.42(12)°, N(1)ÀCr(1)ÀS(2) = 88.60(12)°,
S(2)ÀCr(1)ÀS(1) = 160.54(6)° for 5a; S(2)ÀCr(1)ÀS(1) =
169.20(6)° and N(1)ÀCr(1)ÀCl(1) = 178.96(12)° for 5c] is defined
by the protonated ligand’s three donor atoms [Cr(1)ÀS(1) =
2.4706(16) Å; Cr(1)ÀS(2) = 2.4459(16) Å for 5a; Cr(1)ÀS(1) =
2.4719(15) Å; Cr(1)ÀS(2) = 2.4718(15) Å for 5c] and one chlorine
atom of a formal AlCl4 anion [Cr(1)ÀCl(1) = 2.4371(16) Å for 5a;
Cr(1)ÀCl(1) = 2.4448(12) Å for 5c]. The CrÀN distance is in the
range formed by quaternary nitrogens [Cr(1)ÀN(1) = 2.149(4) Å for
5a; Cr(1)ÀN(1) = 2.170(4) Å for 5c]. A second chlorine atom of a
second AlCl4 anion is located at the apical position [Cl(1)À
Cr(1)ÀCl(2) = 86.93(6)°, S(1)ÀCr(1)ÀCl(2) = 88.15(6)°, S(2)ÀCr-
(1)ÀCl(2) = 111.10(6)°, N(1)ÀCr(1)ÀCl(2) = 90.39(13)° for 5a;
Cl(1)ÀCr(1)ÀCl(2) = 84.64(4)°, S(1)ÀCr(1)ÀCl(2) = 96.94(5)°,
S(2)ÀCr(1)ÀCl(2) = 93.78(5)°, N(1)ÀCr(1)ÀCl(2) = 95.52(11)°
for 5c], forming the usually longer CrÀCl distance [Cr(1)ÀCl(2) =
2.7484(18) Å for 5a; Cr(1)ÀCl(2) = 2.7287(14) Å for 5c].
Figure 3. Drawing of 3a. Thermal ellipsoids are drawn at the 50%
probability level.
Complex 6a. The structure of 6a consists of a divalent chromium
center in an overall distorted square-pyramidal coordination geometry
(Figure 6). The three ligand donor atoms [Cr(1)ÀS(1) = 2.5083(14) Å;
Cr(1)ÀS(2) = 2.4460(14) Å] and one bridging chlorine [Cr(1)ÀCl(2) =
2.3723(14) Å] atom of a Me3AlCl anion define the distorted basal square
plane [S(2)ÀCr(1)ÀS(1) = 152.89(5)°; S(2)ÀCr(1)ÀCl(1) =
114.75(5)°; S(1)ÀCr(1)ÀCl(1) = 92.18(5)°]. The apical position is
Complex 3a. The geometry of the chromium center (Figure 3) is
square pyramidal with the three ligand donor atoms [Cr(1)ÀS(1) =
2.4890(12) Å; Cr(1)ÀS(2) = 2.5187(12) Å] and one chlorine
[Cr(1)ÀCl(1) = 2.4102(12) Å] of an Et2AlCl2 unit defining the basal
plane [S(1)ÀCr(1)ÀS(2) = 166.71(4)° and N(1)ÀCr(1)ÀCl(1) =
176.01(8)°]. The chromium center shows only little deviation from the
basal plane. The protonation of the ligand nitrogen atom is apparent
from the deviation of the N atom from the trigonal-planar geometry and
the long CrÀN distance [Cr(1)ÀN(1) = 2.177(3) Å]. One additional
chlorine from a second Et2AlCl2 unit is located on the apical position
[N(1)ÀCr(1)ÀCl(3) = 91.26(8)°, Cl(1)ÀCr(1)ÀCl(3) = 85.87(3)°,
S(1)ÀCr(1)ÀCl(3) = 99.79(4)°, S(2)ÀCr(1)ÀCl(3) = 93.01(3)°],
forming a substantially longer distance with the metal center [Cr(1)À
Cl(3) = 2.6821(13) Å].
occupied by a second chlorine atom [N(1)ÀCr(1)ÀCl(1)
=
84.15(11)°; Cl(2)ÀCr(1)ÀCl(1) = 93.14(5)°; S(1)ÀCr(1)ÀCl(1) =
92.18(5)°] of a formally neutral Me2AlCl unit [Cr(1)ÀCl(1) =
2.5825(15) Å] and whose Al atom is bonded to the N atom of the
deprotonated ligand [Al(1)ÀN(1) = 1.990(4) Å]. The ligand N atom
forms with chromium [Cr(1)ÀN(1) = 2.179(4) Å] a bond distance as
expected for a quaternary bridging nitrogen.
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Figure 4. Drawing of 4a and 4b (respectively). Thermal ellipsoids are drawn at the 50% probability level.
Figure 5. Drawings of 5a and 5c. Thermal ellipsoids are drawn at the 50% probability level.
Complex 7d. The complex is a symmetry-generated dimer (Figure 7)
with the two metal centers each in a square-pyramidal coordination
environment. Two aluminate {(Al(CH3)2Cl)2(μ-Cl)} counteranions
unconnected with the dicationic dichromium units complete the
structure. The bonding between the two metal centers is realized by
two bridging chlorine atoms equatorially [N(1)ÀCr(1)ÀCl(1a) =
161.4(2)°] and axially placed [Cl(1a)ÀCr(1)ÀCl(1) = 90.31(12)°;
N(1)ÀCr(1)ÀCl(1) = 108.3(2)°, S(2)ÀCr(1)ÀCl(1) = 89.73(10)°,
S(1)ÀCr(1)ÀCl(1) = 90.67(11)°] with respect to each metal atom.
The two distances formed by the equatorial and axial chlorine are
markedly different [Cr(1)ÀCl(1) = 2.655(3) Å; Cr(1)ÀCl(1a) =
2.349(4) Å], as is typical for the square-pyramidal coordination geo-
metry of divalent chromium. The basal plane is formed by the two sulfur
atoms [Cr(1)ÀS(1) = 2.476(3) Å; Cr(1)ÀS(2) = 2.475(3) Å], one
quaternary N atom [Cr(1)ÀN(1) = 2.127(9) Å], and one of the two
chlorine atoms. The CrÀN distance is in the usual range for a quaternary
N atom bonded to chromium. Two {(Al(CH3)2Cl)2(μ-Cl)} anions
unconnected to the dicationic, dimeric unit complete the structure.
’ RESULTS AND DISCUSSION
The three ligands CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy
(a), (t-Bu)SCH2Si(CH3)2N(H)Si(CH3)2CH2S(t-Bu) (b), and
PhSCH2Si(CH3)2N(H)Si(CH3)2CH2SPh (c) used in this work
were readily prepared via direct reaction of the 1,3-bis(chloro-
methyl-1,1,3,3-tetramethyl)disilazane with the appropriate thio-
late anion. The N-methylated derivative of a, CySCH2Si(CH3)2-
N(CH3)Si(CH3)2CH2SCy (d), was prepared via deprotonation
followed by reaction with MeI. All the ligands have been reacted
with either CrCl2(THF)2 or CrCl3(THF)3, but only in the cases
where crystalline materials were obtained was further investiga-
tion and testing pursued. Oily or ill-characterized organometallic
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Scheme 2
these two species is surprising. The SNS analogues of 1a are in
fact excellent catalyst precursors invariably producing under the
same conditions cationic and trivalent [SNS-CrRCl]2(+) deriva-
tives, also catalytically active.8 Furthermore, these very same
trivalent species may be also obtained from divalent precursors
and only slowly decompose to metallic chromium, eventually
leading to catalyst deactivation. The decomposition of 2a was
also unexpected given that the deprotonated forms of SNS
strongly stabilize chromium species, not even allowing further
reactivity with alkyl aluminum.8 Instead, a one-pot reaction of a
in the presence of Et2AlCl with either Cr(III) or Cr(II) chlorides
did not produce metallic chromium but rather afforded the
divalent {[CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]Cr{(μ-
Cl)Al(CH2 CH3)2Cl}2 (3a) (Scheme 2). The noticeable fea-
tures of these two reactions were that the ligand was not
deprotonated by the aluminum alkyl function and that the
divalent state was either formed or preserved with no sign of
further decomposition to metallic chromium. The formation of
3a from a trivalent precursor seems to be a typical one-electron
reductive elimination of an unstable trivalent organochromium
intermediate.7c,10 Accordingly, preparation of 3a from the diva-
lent precursors afforded substantially higher yield.
Figure 6. Drawing of 6a. Thermal ellipsoids are drawn at the 50%
probability level.
The key to understanding the remarkable selectivity of the
standard SNS catalytic systems is the ability of this ligand to
effectively support the redox dynamism of the metal center. In
other words, the trivalent catalyst precursors may be reduced to
Cr(II) and eventually to Cr(I). When further reduction to Cr(0)
occurs, metallic chromium separates and the catalyst is deacti-
vated. An enhanced tendency of Cr(II) to return to Cr(III) via
disproportionation prevents the formation of SÀF distribution
of oligomers. Therefore, the overall excellent trimerization
activity indicates that during the rapid interconversion of the
three oxidation states, Cr(I)/Cr(III) is the dominant redox
couple. In this scenario, our results seem to indicate that the
introduction of the silyl moiety in the ligand scaffold effectively
blocks the redox dynamism. In the case of 3a, Cr(III) was
irreversibly reduced to Cr(II), which, as suggested by the SÀF
catalytic behavior, seems to indicate that this is the dominant
oxidation state of the catalytic cycle. This also suggests that no
further significant reduction to the monovalent state occurs in
this case. When it does, as for 1a and 2a, further rapid decom-
position to metallic chromium leads to catalyst deactivation. It is
worth noting that Cr(III) species are in general reduced very
easily to Cr(II), suggesting that Cr(I) and Cr(III) might be less
stable/accessible in the present case than with the nonsilylated
ligand.
Figure 7. Drawing of 7d. Thermal ellipsoids are drawn at the 50%
probability level. The two {(Al(CH3)2Cl)2(μ-Cl)} anions have been
omitted for clarity.
compounds (e.g., 1, 2, 3, 6bÀd; 4c,d; 5b,d; 7aÀc) have been
discarded due to the lack of reproducibility of their analytical
data. Ligand a in either protonated or deprotonated forms
reacted with CrCl3(THF)3 to afford the corresponding mono-
meric [CySCH2Si(CH3)2N(H)Si(CH3)2CH2SCy]CrCl3 (1a)
or dimeric {[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]CrCl(μ-
Cl)}2 (2a) (Scheme 1).
Both complexes displayed a disappointing noncatalytic beha-
vior in spite of the close structural relationship of 1a to that of the
nonsilylated SNS analogue.4aÀc Thus, in an attempt to isolate
species that could possibly clarify this comportment, both
complexes were reacted with stoichiometric amounts of several
alkyl aluminums. In the case of R3Al [R = Me, Et], the reactions
afforded separation of metallic chromium, thus indicating that
catalyst decomposition occurred during the attempted activation
process. The extensive reductive decomposition observed for
The NÀH function of the Si-SNS and SNS ligands also plays a
significant role. While in the case of SNS ligands deprotonation
afforded stable divalent complexes producing SÀF distribution,
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Scheme 3
Scheme 4
in the case of the trivalent 2a, it gave only extensive decomposi-
tions. We have therefore prepared the divalent chromium
analogue. The reaction was successful in the case of both a and
b, affording the two nearly isostructural divalent complexes
{[CySCH2Si(CH3)2NSi(CH3)2CH2SCy]Cr(μ-Cl)}2 (4a) and
{[(t-Bu)S CH2Si(CH3)2NSi (CH3)2CH2S(t-Bu)]Cr(μ-Cl)}2
(4b) in crystalline form (Scheme 3).
As anticipated, the two complexes gave a very good activity as
nonselective ethylene oligomerization catalysts, indicating that
the divalent state was preserved in these cases during the
activation process. Therefore, the absence of the amino hydrogen
atom seems to stabilize divalent derivatives of the Si-SNS ligand
and inhibit further reduction to the seemingly unstable, and yet
catalytically active, monovalent state.
To further clarify the interaction of 4 with aluminate species,
we have carried out in situ complexation in the presence of either
AlCl3 or AlMe3 and using divalent instead of trivalent chromium
salts. As mentioned above, trivalent complexes afforded metallic
Cr in the case of AlMe3. In the cases of ligands a and c and AlCl3,
two isostructural complexes, {[CySCH2Si(CH3)2N(H)Si(CH3)2-
CH2SCy]Cr{(μ-Cl)AlCl3}2 (5a) and {[PhS CH2Si(CH3)2N-
(H)Si(CH3)2CH2SPh]C{(μ-Cl)AlCl3}2 (5c), have been obtained
(Scheme 4). The structures of these two species indicate that no
disproportionation occurred. The result of the reaction with
AlMe3 affording {[CySCH2Si(CH3)2N(Al(CH3)2-μ-Cl)Si(CH3)2-
CH2SCy]Cr{(μ-Cl)Al(CH3)3} (6a) was particularly informa-
tive, showing ligand deprotonation and retention of an aluminate
residue bridged to the CrÀN moiety. This structure suggests a
possibly general pathway for ligand abstraction when catalyst
decomposition occurs. The fact that in the case of nonsilylated
SNS the aluminates are unable to deprotonate the ligand (due to
lower acidity) prevents coordination of Al to the N atom and
ligand abstraction from the monovalent chromium intermediate
becomes more difficult, hence the longer lifetime of the mono-
valent intermediate and the very high selectivity of that particular
catalytic system. In the case of 6a the original divalent state has
been preserved during its formation. Given the instability of 2a
with the same activator, we must necessarily conclude that,
during the attempted activation of trivalent precursors, the
divalent state is bypassed when the catalyst is decomposed to
metallic chromium.
To further evaluate this point, we have also methylated the
ligand’s a N atom. The complexation to chromium was successful
only in the presence of Me2AlCl and if a divalent chromium
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Scheme 5
precursor was used. The reaction afforded the catalytically
inactive divalent {[CySCH2Si(CH3)2N(CH3)Si(CH3)2CH2SCy]-
Cr(μ-Cl)}2{(Al(CH3)2Cl)2)(μ-Cl)}2 (7d), which was isolated
in crystalline form and fully characterized (Scheme 5). In the case
of trivalent chromium once again neither clearly defined species
nor catalytic activity was obtained.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sgambaro@uottawa.ca.
’ ACKNOWLEDGMENT
This work was supported by the Natural Science and En-
gineering Council of Canada (NSERC), the University of
Ottawa, and the Technical University of Eindhoven (The
Netherlands).
’ CONCLUSIONS
In conclusion, we have herein reported the results of an
exploratory study on a modification of the ethylene trimerization
SNS ligand system. The introduction of two Me2Si units in the
ligand scaffold and in the positions contiguous to the nitrogen
atom resolved in a loss of redox dynamism. This effect can
possibly be ascribed to electronic features since the introduction
of the bulky Si(Me)2 groups hardly affects the geometry of the
complexes and the coordination sphere of the chromium atom.
Thus, the consequent disappearance of selectivity has to be
ascribed to the instability of both the tri- and monovalent states,
both readily decomposing by activators to metallic chromium.
Instead, depending on the conditions it is possible to stabilize
divalent species that, by no longer possessing redox dynamism,
simply produce the expected SÀF distribution of oligomers. On
the other hand, we must also admit that, given the similarity of
reactivity among complexes 3a, 4a, 5a, and 6a, conclusions about
the relationship between metal oxidation state and catalytic
behavior cannot be drawn with complete confidence in this case.
On the basis of our current understanding of the redox
mechanism for chromium-catalyzed tri- and tetramerization,
the role of the ligand is of stabilizing the doubly alkylated trivalent
chromium species prior to its reductive elimination to the
catalytically active monovalent chromium state. This is a crucial
step for the effective progression through the catalytic cycle.
Without this important effect from the ligand, the alkylated
trivalent chromium center will not undergo reductive elimination
to form the active monovalent chromium species. Instead, it will
initially reduce to the divalent state, affording a SchultzÀFlory
distribution of oligomers, or further decompose to the inactive
zerovalent chromium state. The similar selectivity displayed by
complexes 3a, 4a, 5a, and 6a is an indication that these species all
afford the same deprotonated Cr-SiSNS active species.
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’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data (CIF)
b
for the complexes reported in this paper and extensive listing
of geometrical parameters. This material is available free of
charge via the Internet at http://pubs.acs.org
4663
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