Switchable chromium(II) complexes of a chelating amidophosphine (N-P) for selective and nonselective ethylene oligomerization

Article abstract of DOI:10.1021/om1006006

Treatment of the lithium salt of t-BuN(H)PPh₂ in THF with CrCl₂ · (THF)₂ afforded a dinuclear, [(t-BuNPPh₂)Cr₂(μ-t-BuNPPh₂)₃] · (toluene)_1.5 (1), and a tetrameric cluster, [(t-BuNPPh ₂)Cr(μ-t-BuNPPh₂)₂Cr(μ-Cl)]₂ · (toluene)₂ (5), depending on the reagents' stoichiometric ratio. Complex 1 is dimeric with a long intermetallic distance and the ligands adopting an asymmetric and distorted bridging-chelating bonding mode. Complex 5 is instead a symmetry-generated tetramer with two identical dimetallic units, each closely related in geometry to 1, linked by two bridging chlorine atoms. The reactions of 1 with alkyl aluminum activators afforded a series of divalent complexes, [{(μ-AlMe₃)(t-BuNPPh₂)}₂Cr] · (toluene) (2), [{(μ-AlMe₂Cl)(t-BuNPPh₂)} ₂Cr] · (toluene) (3), and [{(μ-AlEt₂Cl)(t- BuNPPh₂)}₂Cr] · (toluene) (4), containing organo-aluminum residues. Similarly, reaction of 5 with AlMe₃ gave [{(μ-AlMe₂)(t-BuNPPh₂)₂}Cr(μ-Cl)] ₂ · (toluene)_1.9 (6), also characterized by an X-ray crystal structure. Finally, the trivalent complex [(t-BuNPPh ₂)₃Cr] (7) was readily prepared via reaction of the lithiated ligand with CrCl₃(THF)₃. Upon treatment with AlMe₃ or Et₂AlCl, complexes 2 and [{(μ-AlEtCl ₂)(t-BuNPPh₂)}₂Cr] · (toluene) (8) were isolated and fully characterized. In turn, this indicated that reduction to the divalent state is the primary stage of the activation process of the trivalent species. The catalytic behavior of all of these complexes has been assessed in the presence and absence of cocatalyst and with different solvents. The result showed a pronounced solvent effect, allowing switching from nonselective oligomerization to selective trimerization.

Full text of DOI:10.1021/om1006006

4080 Organometallics 2010, 29, 4080–4089
DOI: 10.1021/om1006006
Switchable Chromium(II) Complexes of a Chelating Amidophosphine
(N-P) for Selective and Nonselective Ethylene Oligomerization
Indira Thapa,† Sandro Gambarotta,*,† Ilia Korobkov,† Robbert Duchateau,*,‡
Shaneesh Vadake Kulangara,‡ and Reynald Chevalier§
†Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada, ‡Eindhoven
University of Technology, 5600 MB, Eindhoven, The Netherlands, and §Lyondellbasell Industries,
Polyolefine GmbH Industriepark Hoechst, D-65926 Frankfurt, Germany
Received June 21, 2010
Treatment of the lithium salt of t-BuN(H)PPh2 in THF with CrCl2 (THF)2 afforded a dinuclear,
3
[(t-BuNPPh2)Cr2(μ-t-BuNPPh2)3] (toluene)1.5 (1), and a tetrameric cluster, [(t-BuNPPh2)Cr(μ-t-
3
BuNPPh2)2Cr(μ-Cl)]2 (toluene)2 (5), depending on the reagents’ stoichiometric ratio. Complex 1 is
3
dimeric with a long intermetallic distance and the ligands adopting an asymmetric and distorted
bridging-chelating bonding mode. Complex 5 is instead a symmetry-generated tetramer with two
identical dimetallic units, each closely related in geometry to 1, linked by two bridging chlorine atoms.
The reactions of 1 with alkyl aluminum activators afforded a series of divalent complexes, [{(μ-AlMe3)-
(t-BuNPPh2)}2Cr] (toluene) (2), [{(μ-AlMe2Cl)(t-BuNPPh2)}2Cr] (toluene) (3), and [{(μ-AlEt2Cl)-
(t-BuNPPh2)}2Cr] (toluene) (4), containing organo-aluminum residues. Similarly, reaction of 5 with
AlMe3 gave [{(μ-AlMe2)(t-BuNPPh2)2}Cr(μ-Cl)]2 (toluene)1.9 (6), also characterized by an X-ray
crystal structure. Finally, the trivalent complex [(t-BuNPPh2)3Cr] (7) was readily prepared via reac-
tion of the lithiated ligand with CrCl3(THF)3. Upon treatment with AlMe3 or Et2AlCl, complexes 2 and
3
3
3
3
[{(μ-AlEtCl2)(t-BuNPPh2)}2Cr] (toluene) (8) were isolated and fully characterized. In turn, this indicated
3
that reduction to the divalent state is the primary stage of the activation process of the trivalent species. The
catalytic behavior of all of these complexes has been assessed in the presence and absence of cocatalyst
and with different solvents. The result showed a pronounced solvent effect, allowing switching from
nonselective oligomerization to selective trimerization.
Introduction
very different mechanisms. The selective ethylene trimeriza-
tion (and tetramerization) is a redox process where a two-
electron oxidative addition of ethylene forms an expandable
chroma-metallocycle followed by a two-electron reductive
elimination.1-13,17,27-34 The ethylene reductive coupling is
one of the key steps of this catalytic cycle, in turn requiring a
sufficiently reactive oxidation state of the metal center.27-34
Polymerization is typically a nonredox process where the
Lewis acidity of the metal center is the drive for ethylene
precoordination, to be followed by insertion and chain
growth.35 However, evidence has been brought about that,
at least in some case, polymerization may also follow a redox
ring expansion mechanism.15 Nonselective oligomerization
is believed to proceed via the redox mechanism.15,17 Con-
trolling the selectivity of the ring expansion (e.g., toward
either formation of 1-hexene versus 1-octene or statistical
oligomerization) remains a formidable challenge.
Chromium is the feature component of potent ethylene
tri-1-10 and tetramerization,11-13 nonselective oligomeriza-
tion,14-18 and polymerization19-26 catalysts. This versatility
raises interesting questions about this metal’s unique beha-
vior. In principle, the three processes could be regarded as
belonging to the same category of C-C bond forming
reactions, although, in reality, they involve conceptually
*To whom correspondence should be addressed. E-mail: sgambaro@
uottawa.ca.
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Published on Web 08/26/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 18, 2010 4081
of catalytic performance.8c,d The isolation of intermediates
capable of acting as single-component self-activating cata-
lysts has pointed to the monovalent state as responsible for
selective ethylene trimerization and to divalent chromium
for polymerization and nonselective oligomerization. Cata-
lysts based on the neutral PNP and related PCCP ligands are
the only existing systems capable of producing a sizable
excess of 1-octene.11-13 The active species of this selective
system is believed to contain a cationic chromium(I)
center.33,36
One of the major difficulties in generalizing this crucial
information consists of the remarkable redox dynamism
existing between mono-, di-, and trivalent organo-chromium
species, readily interconverting via reduction and dispro-
portionations.8,12,37 Therefore, we are currently preparing
series of chromium complexes in different valence states
aiming at clarifying their interaction with aluminum alkyl
ꢀ
species vis-a-vis the redox behavior of this particular
metal.8,12,37 For this work, we have selected the anionic amido-
phosphine (NP) ligand to assess the role of the ligand’s negative
charge and number of donor atoms within the PNP ligand
framework in determining the type of catalytic behavior in
relation to the metal oxidation state. Furthermore, recent work
by Rosenthal38 on the closely related NPNP ligand system has
identified an excellent selectivity for the production of 1-hexene,
thus reiterating the potential provided by the combination of
these donor atoms.
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4082 Organometallics, Vol. 29, No. 18, 2010
Thapa et al.
Herein we report our observations about single-compo-
nent catalytic behavior and selective ethylene trimerization
of this system.
(cm-1, Nujol mull): 3052s, 2955s, 2924s, 2854s, 1584m, 1457s,
1429m, 1376m, 1352m, 1191s, 1086m, 1055m, 1018s, 819m,
803m, 739, 733s, 724s, 695s. [μeff = 2.75 μB].
Preparation of [{(μ-AlMe3)(t-BuNPPh2)}2Cr] (toluene) (2).
3
Method A. A solution of 1 (0.20 g, 0.16 mmol) in toluene (5 mL)
was cooled to -35 °C for 10 min and treated with neat AlMe3
(0.056 g, 0.78 mmol) dropwise while stirring. After 15 min at
room temperature, the resulting solution was allowed to stand
for 5 days at -35 °C. Blue paramagnetic crystals of 2 formed,
which were washed with cold hexane and dried in vacuo (0.12 g,
0.15 mmol, 47%). Anal. Calcd (found) for C45H64Al2CrN2P2: C
67.48 (67.35), H 8.05 (7.99), N 3.50 (3.46). IR (cm-1, Nujol mull):
3051s, 2922s, 2857s, 2716, 2667, 2032, 1960s, 1884s, 1812s,
1781s, 1607s, 1588s, 1466s, 1436s, 1375m, 1360s, 1303 m, 1192b,
1097s, 1044m, 1014m, 991m, 919m, 839s, 778s, 744m, 696m
[μeff = 4.93 μB].
Experimental Section
All reactions were carried out under a dry nitrogen atmo-
sphere unless otherwise stated. Solvents were dried using an
aluminum oxide solvent purification system. CrCl2(THF)2 and
CrCl3(THF)3 were prepared according to
a published
procedure.39 Aluminum and lithium reagents were purchased
from Aldrich and used as received. Infrared spectra were re-
corded on an ABB Bomen FT-IR instrument from Nujol mulls
prepared in a drybox except in the case of air-stable compounds.
Samples for magnetic susceptibility were preweighed inside a
drybox equipped with an analytical balance and measured on a
Johnson Matthey 2400 CHN analyzer. VT magnetic measure-
ments were carried out with a Quantum Design SQUID mag-
netometer MPMS-XL. NMR data were collected on a Bruker
300 spectrometer and referenced to SiMe4. Data for X-ray
crystal structure determination were obtained with a Bruker
diffractometer equipped with a 1K Smart CCD area detector.
Elemental analyses were carried out with a Perkin-Elmer 2400
CHN analyzer. Molecular weight and molecular weight distri-
butions of the resulting polymers were determined by size
exclusion chromatography on a PL-GPC210 equipped with a
RI and viscosity detector and a 3 Â PLgel 10 μm Mixed-B
column set at 135 °C using 1,2,4-trichlorobenzene as solvent.
The molecular weight of the polyethylenes was referenced to
linear polyethylene standards. Results of the oligomerizations
were assessed by 1H NMR spectroscopy for activity and GC-MS
for reaction mixture composition. Gas chromatography of
oligomerization products was conducted on a Varian 450-GC
equipped with an autosampler and a Factor Four capillary
column VF-5 ms, 25 m  0.25 mm. A gradient oven temperature
program starting from 50 °C (for 2 min) to 280 °C at a rate of
10 °C/min and holding at the final temperature for 3 min was
employed. The chromatograms were obtained via flame ioniza-
tion detector. As a transport gas helium was used.
Method B. A solution of t-Bu-N(H)PPh2 (0.25 g, 0.97 mmol)
in THF (10 mL) was treated with a solution of n-BuLi in hexane
(0.44 mL, 1.10 mmol, 2.5 M) at -10 °C. The reaction mixture
was stirred overnight at room temperature and then combined
with a suspension of CrCl2 (THF)2 (0.13 g, 0.49 mmol) in THF
3
(5 mL). Stirring was continued for 18 h. The volatile materials
were removed in vacuo, and the solid residue was redissolved in
toluene. Neat AlMe3 (0.36 g, 5.00 mmol) was added dropwise,
and the reaction mixture was stirred at room temperature for
10 min. A small amount of colorless, insoluble material was
separated by centrifugation. The resulting green-blue solution
was allowed to stand for 7 days at -35 °C. Blue paramagnetic
crystals of 2 were filtered, washed with cold hexane, and dried in
vacuo (0.20 g, 0.24 mmol, 48%).
Method C. A solution of 7 (0.20 g, 0.22 mmol) in toluene
(5 mL) was cooled to -35 °C for 10 min. Neat AlMe3 (0.16 g,
2.19 mmol) was added dropwise to the cold solution under
stirring. Stirring was continued for 24 h at room temperature,
and the resulting solution was allowed to stand for 5 days at
-35 °C. Blue paramagnetic crystals of 2 were filtered, washed
with cold hexane, and dried in vacuo (0.10 mg, 0.12 mmol, 54%).
Preparation of [{(μ-AlMe2Cl)(t-BuNPPh2)}2Cr] (toluene) (3).
3
Neat AlMe3 (0.29 g, 4.02 mmol) was added dropwise to a solu-
tion of t-Bu-N(H)PPh2 (0.25 g, 0.97 mmol) in toluene (5 mL) at
room temperature during a period of 10 min. The resulting
Preparation of t-BuN(H)PPh2. A solution of t-BuNH2 (20 mL,
173 mmol) in dry diethyl ether (150 mL) was cooled to -78 °C and
treated dropwise with chlorodiphenylphosphine (16 mL, 86 mmol)
under nitrogen. A white precipitate of t-BuNH3Cl immediately
separated. The reaction mixture was allowed to warm to room
temperature and stirred for 48 h. The ammonium salt was sepa-
rated by filtration, and the resulting solution was concentrated to a
small volume under reduced pressure. The title compound was
isolated from the resulting solution as analytically pure crystals
upon standing at room temperature overnight (36 g, 141.7 mmol,
81%). 1H NMR δ: 7.49 (4H, m), 7.37 (6H, m), 2.11 (1H, d, J =
11.4), 1.37 (9H, s). 13C NMR δ: 144.31, 144.15, 131.53, 131.26,
128.67, 128.58, 77.539, 77.12, 51.94, 51.69, 32.79, 32.67. 31P NMR
δ: 23.27.
solution was then combined with a suspension of CrCl2 (THF)2
3
(0.13 g, 0.49 mmol) in toluene (5 mL). Stirring was continued for
1 h, and during this period additional trimethylaluminum was
added (0.34 g, 2.98 mmol). Stirring was continued for an addi-
tional 48 h at room temperature. A small amount of insoluble
material was separated by centrifugation, and the resulting
blue solution was allowed to stand overnight at -35 °C. Blue
paramagnetic crystals of 3 were collected by filtration, washed
with cold hexane, and dried in vacuo (0.34 g, 0.40 mmol, 82%).
Anal. Calcd (found) for C43H58Al2Cl2CrN2P2: C 61.36 (61.19),
H 6.95 (6.88), N 3.33 (3.29). IR (cm-1, Nujol mull): 3205b,
2908b, 1961b, 1899, 1774, 1607b, 1459b, 1381s, 1354m, 1307m,
1190s, 1093s, 1042m, 995m, 905m, 850s, 785s, 737s, 689s, 672b
[μeff = 4.86 μB].
Preparation of [(t-BuNPPh2)Cr2(μ-t-BuNPPh2)3] (toluene)1.5
3
(1). A solution of t-BuN(H)PPh2 (0.50 g, 1.96 mmol) in THF
(15 mL) was treated with a solution of n-BuLi in hexane (0.82
mL, 2.05 mmol, 2.5 M) at -10 °C. The reaction mixture was
stirred overnight at room temperature and then combined with a
suspension of CrCl2(THF)2 (0.27 g, 1.01 mmol) in THF (5 mL).
Stirring was continued for 18 h. Solvent was evaporated in
vacuo, and the solid residue was redissolved in toluene. A small
amount of colorless, insoluble material was separated by cen-
trifugation, and the resulting brown solution was allowed to
stand for three days at -35 °C. Brown paramagnetic crystals
of 1 were filtered, washed with cold hexane, and dried in vacuo
Preparation of [{(μ-AlEt2Cl)(t-BuNPPh2)}2Cr] (toluene) (4).
3
The complex was prepared and isolated in a similar manner and
using the same quantities as complex 3 by using CrCl2(THF)2
(0.13 g, 0.49 mmol), AlEt3 (0.11 g, 0.96 mmol), and t-Bu-
N(H)PPh2 (0.25 g, 0.97 mmol) in toluene (5 mL). Blue para-
magnetic crystals of 4 were collected by filtration, washed with
cold hexane, and dried in vacuo (0.37 g, 0.41 mmol, 84%). Anal.
Calcd (found) for C47H66Al2Cl2CrN2P2: C 62.87 (63.01), 7.41
(7.03), N 3.12 (3.08). IR (cm-1, Nujol mull): 3052s, 2908, 1961,
1899, 1774, 1607, 1459s, 1381s, 1354s, 1307s, 1190s, 1093s,
1042s, 995m, 905m, 850s, 785s, 737s, 697s, 672s [μeff = 4.87 μB].
(0.39 g, 0.32 mmol, 64%). Anal. Calcd (Found) for C74.50
H88Cr2N4P4: C 70.60 (69.99), H 7.00 (6.94), N 4.42 (4.38). IR
-
Preparation of [(t-BuNPPh2)Cr(μ-t-BuNPPh2)2Cr(μ-Cl)]2
3
(toluene)2 (5). A solution of t-Bu-N(H)PPh2 (0.25 g, 0.97 mmol)
in THF (5 mL) was treated with a solution of n-BuLi in hexane
(0.44 mL, 1.10 mmol, 2.5 M) at -10 °C. The reaction mixture
€
(39) (a) Kohler, F. H.; Prossdorf, W. Z. Naturforsch. 1977, 32, 1026.
(b) Kern, R. J. J. Inorg. Nucl. Chem. 1962, 24, 1105.

Article
Organometallics, Vol. 29, No. 18, 2010 4083
was stirred overnight at room temperature, and a suspension of
CrCl2(THF)2 (0.27 g, 1.01 mmol) in THF (5 mL) was added.
Stirring was continued for an additional 18 h. The solvent was
evaporated in vacuo and the solid residue redissolved in toluene.
A small amount of colorless, insoluble material was separated
by centrifugation, and the resulting greenish-brown solution
was allowed to stand for three days at -35 °C. Green paramagnetic
crystals of 5 were filtered, washed with cold hexane, and dried
in vacuo (0.17 g, 0.08 mmol, 32%). Anal. Calcd (found) for
Scheme 1
C
110H130Cl2Cr4N6P6: C 66.03 (65.97) H 6.55 (6.46), N 4.20
(4.19). IR (cm-1, Nujol mull): 3054m, 1952m, 1895m, 1819m,
1770m, 1588s, 1479s, 1432s, 1361s, 1220s, 1093m, 1025s, 978s,
839b, 734b, 696b [μeff = 8.15 μB per tetrameric unit].
Preparation of [{(μ-AlMe2)(t-BuNPPh2)2}Cr(μ-Cl)]2 (toluene)1.9
3
(6). A cooled solution of 5 (0.20 g, 0.10 mmol) in toluene (5 mL) was
treated at -35 °C with AlMe3 (0.014 g, 0.20 mmol). The reaction
mixture was allowed to stir for 15 min at room temperature, while the
initial greenish-brown color turned green. Green crystals of 6 (0.08 g,
0.054 mmol, 54%) separated from the resulting solution upon
standing at -35 °C for 5 days. Anal. Calcd (found) for C81.30
103.20Al2Cl2Cr2N4P4: C 65.57 (65.49), H 6.98 (6.96), N 3.76 (3.71)
[μeff = 3.31 μB per dimeric unit].
-
H
Preparation of [(t-BuNPPh2)3Cr] (toluene) (7). A solution of
3
t-Bu-N(H)PPh2 (1.01 g, 3.93 mmol) in THF (15 mL) was treated
with a solution of n-BuLi in hexane (1.64 mL, 4.1 mmol, 2.5M)
at -10 °C. The reaction mixture was stirred overnight at room
temperature and combined with a suspension of CrCl3(THF)3
(0.49 g, 1.31 mmol) in THF (5 mL). Stirring was continued for
18 h. The solvent was evaporated in vacuo and the resulting solid
residue redissolved in toluene. A small amount of colorless,
insoluble material was removed by centrifugation, and the
resulting green solution was allowed to stand at -35 °C for
5 days. Green paramagnetic crystals of 7 were filtered, washed
with cold hexane, and dried in vacuo (1.1 g, 1.2 mmol, 90%).
Anal. Calcd (found) for C55H65CrN3P3: C 72.35 (72.29), H 7.18
(7.16), N 4.60 (4.57). IR (cm-1, Nujol mull): 3058, 2849b, 2724,
1960s, 1892b, 1816b, 1588, 1464s, 1434s, 1377s, 1357s, 1303,
1265, 1193bm, 1093m, 1063s, 1025s, 979m, 911, 843b, 820, 740s,
696s [μeff = 3.83 μB].
for Lorentz and polarization effects, with an applied multiscan
absorption correction (SADABS). Crystal structures were
solved and refined using the SHELXTL program suite. Direct
methods yielded all non-hydrogen atoms, which were refined
with anisotropic thermal parameters. All hydrogen atom posi-
tions were calculated at their idealized positions riding on their
respective carbon atoms. Full crystallographic data, including
bond lengths and angles, may be found in the Supporting
Information. Crystal Data collection and structure solution
are given in Table S1 of the Supporting Information.
Results and Discussion
Preparation of [{(μ-AlEtCl2)(t-BuNPPh2)}2Cr] (toluene)0.5
3
The reaction of the lithium salt of t-BuN(H)PPh2 in THF
with half an equivalent of CrCl2(THF)2 afforded the di-
(8). A solution of 7 (0.30 g, 0.33 mmol) in toluene (5 mL) was
cooled to -35 °C for 10 min. Neat AlEt2Cl (0.20 g, 1.66 mmol)
was added dropwise to the cold solution under stirring. Stirring
was continued for 5 min at room temperature, and the resulting
solution was allowed to stand for seven days at -35 °C. Blue
paramagnetic crystals of 8 were filtered, washed with cold
hexane, and dried in vacuo (0.19 g, 0.21 mmol, 64%). Anal.
Calcd (found) for C39.5H53Al2Cl4CrN2P2: C 54.87 (54.76), H
6.06 (6.00), N 3.24 (3.21) [μeff = 4.86 μB].
nuclear [(t-BuNPPh2)Cr2(μ-t-BuNPPh2)3] (toluene)1.5 (1) as
3
dark brown paramagnetic crystals (Scheme 1). The dimeric
formulation of the complex was elucidated by a crystal
structure (Figure 1) and confirmed by satisfactory combus-
tion analysis data.
The structure of 1 shows four t-BuNPPh2 anions asymme-
trically located around a dichromium unit. Three ligands adopt
the bridging mode with the nitrogen atoms connected to the
Oligomerization Reactions. A 200 mL high-pressure steel
€
Buchi reactor containing a heating/cooling jacket was dried at
˚
same chromium [Cr2-N4 = 2.007(3) A, Cr2-N2 = 2.009(3)
130 °C for 3 h and then placed under vacuum for 30 min. The
reactor was then charged with the solvent (100 mL) and coca-
talyst, pressurized with 600 psi of ethylene, and heated to 60 °C
using a thermostatic bath with stirring. After 15 min the pressure
was released and the catalyst solution injected into the reactor
under a stream of ethylene. The reactor was immediately repre-
ssurized, and the pressure was maintained constant throughout
the run. The reaction was allowed to run for 30 min, after which
the temperature was rapidly lowered to 0 °C, the reactor was
slowly depressurized, and a mixture of MeOH/HCl (10%) was
injected to quench the reaction. The organic and aqueous phases
were then separated. The samples for NMR and GC-MS
analyses were collected immediately.
˚
˚
A, Cr2-N3 = 2.033(3) A] and their corresponding phos-
˚
phorus bonding the second metal [Cr1-P(4) = 2.5319(12) A,
Cr1-P(3) = 2.5583(12) A, Cr1-P2 = 2.6698(12) A]. The
fourth ligand chelates solely the second chromium atom
˚
˚
˚
˚
[Cr1-P1 = 2.3756(12) A, Cr1-N1 = 2.088(3) A]. As a result,
the coordination of the chromium atom exclusively surrounded
by the nitrogen atoms is trigonal planar with a minor deviation
of the metal from planarity [N4-Cr2-N2 = 117.34(14)°, N4-
Cr2-N3 = 110.28(14)°, N2-Cr2-N3 = 132.37(14)°]. The
second chromium shows instead a severely distorted trigo-
nal-bypyramidal geometry with the trigonal equatorial plane
defined by two P atoms of two bridging ligands and the
phosphorus of the chelating ligand [P1-Cr1-P2 = 106.27(4)°,
P3-Cr1-P2 = 104.13(4)°, P1-Cr1-P3 = 140.18(4)°]. The
axial positions are occupied by one P atom of the third bridging
ligand and the N atom of the chelating ligand [N1-Cr1-P4 =
140.14(9)°,P4-Cr1-P3=98.02(4)°,P1-Cr1-P4=100.23(4)°].
X-ray Crystallography. Single crystals suitable for X-ray diff-
raction measurement were mounted on a glass fiber and placed
under the cold stream of the diffractometer cooling system. Unit
cell measurements and intensity data collections were performed
at low temperature using graphite-monochromated Mo KR
˚
radiation (λ = 0.71073 A). Data reduction included a correction

4084 Organometallics, Vol. 29, No. 18, 2010
Thapa et al.
Figure 2. Partial thermal ellipsoid drawing of 2 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
Scheme 2
Figure 1. Partial thermal ellipsoid drawing of 1 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
˚
The Cr-Cr distance [Cr(1) Cr(2) = 2.7688(9)A] is remark-
3 3 3
ably long for this type of dimeric structure, somewhat reminis-
cent of the paddlewheel complexes of divalent chromium and
outside the M-M bonding range.40
The strange asymmetry observed in the dimeric structure
and the completely different coordination geometries of the
two Cr atoms are probably the result of steric repulsions, as
also emphasized by the folding of the Cr1-Cr2-N-P plane.
The steric encumbrance of the three large t-Bu groups is
likely responsible for the trigonal-planar arrangement of one
of the two chromium atoms.41 The possibility for a termin-
ally bonded hydride to be present in the seemingly void space
opposed to the Cr-Cr vector was ruled out by degradation
experiments showing no evolution of hydrogen gas under
hydrolytic conditions. The value of the magnetic moment
at room temperature [μeff =2.75 μB] was as expected for a
dinuclear structure of divalent chromium without direct
metal-metal ligation.
As described in the section below, complex 1 acts as a
moderately active ethylene polymerization catalyst precur-
sor. Depending on the activation conditions, 1-hexene was
observed in addition to the formation of polyethylene. Thus,
in an attempt to discriminate between the factors leading to
each of the two catalytically active species (respectively for-
ming polymer and 1-hexene) and with the hope of improving
activity and selectivity of the system, we have embarked on a
study to assess the interaction of 1 with alkyl aluminum
reagents. Regrettably, the reaction with MAO (the most
preferred activator) afforded only oily intractable materials.
In the case of the reaction with five equivalents of trimethy-
laluminum (TMA) instead, the crystalline and paramagnetic
and fully characterized. The complex could also conveni-
ently be prepared in a one-pot synthesis (Scheme 1). The
chemical connectivity was clarified by an X-ray crystal
structure (Figure 2), and the magnetic moment was in
agreement with the high-spin configuration of a d4 Cr(II)
complex in a square-planar environment.
Complex 2 is monomeric (Figure 2) and features two
ligands and two AlMe3 units surrounding the chromium
metal center. Each ligand adopted the bridging-chelating
bonding mode with the N atom bonding the organoaluminum
˚
residue [N1-Al1 = 1.896(3) A] and the P atom connected to
˚
chromium [P1-Cr1 = 2.536(1) A]. In turn, one of the three
methyl groups on each Al reaches a bonding contact with
˚
chromium [Cr1-C1= 2.283(5) A]. The square-planar environ-
ment of the transition metal [C1-Cr1-P1 = 86.38(13)°] is
provided by two methyl groups and the two P atoms in trans
position to each other [C1-Cr1-C1a = 179.99(1)°; P1-Cr1-
P1a = 180.0(1)°]. Two other methyl groups, each from one Al,
are oriented on the axial position of a severely distorted octahe-
dron centered on chromium. However, the Cr-C distances with
these two methyl groups are well beyond the bonding range
˚
[Cr1-C2=2.960(5) A; C2-Cr1-C2a =180.00(0)°].
As mentioned above, complex 2 could conveniently be
prepared in a one-pot synthesis by treating the lithiated
phosphineamine ligand with half an equivalent of CrCl2
3
(THF)2 followed by treatment with TMA. When the se-
quence of addition was reversed and the ligand was used in its
protonated rather than lithiated form, two new species,
[{(μ-AlMe3)(t-BuNPPh2)}2Cr] (toluene) (2) was isolated
3
(40) (a) Cotton, F. A.; Walton, R. A. Metal-metal Multiple bonds in
dinuclear clusters. Struct. Bonding (Berlin) 1985, 62, 1. (b) Horvath, S.;
Gorelsky, S. I.; Gambarotta, S.; Korobkov, I. Angew. Chem., Int. Ed. 2008,
47, 9937, and references therein.
(41) Ruppa, K. B. P.; Feghali, K.; Kovacs, I.; Aparna, K.; Gambarotta,
S.; Yap, G. P. A.; Bensimon, C. J. Chem. Soc., Dalton Trans. 1998, 1595, and
references therein.
formulated as [{(μ-AlMe2Cl)(t-BuNPPh2)}2Cr] (toluene)
3
(3) and [{(μ-AlEt2Cl)(t-BuNPPh2)}2Cr] (toluene) (4), were
3
isolated in good and reproducible yield while using TMA
and triethylaluminum (TEAL), respectively (Scheme 2).
Both complexes have the magnetic moments as expected

Article
Organometallics, Vol. 29, No. 18, 2010 4085
Figure 3. Partial thermal ellipsoid drawing of 3 and 4 with carbon atom ellipsoids omitted for clarity (50% probability level).
Scheme 3
for the high-spin d4 electronic configuration of monomeric
divalent chromium in square-planar environments.
The two complexes display very similar geometries (Figure 3),
the only difference consisting of the presence of a Me versus Et
group attached to the Al residue.
As in complex 2, the metal center is surrounded by two
ligands bonding the chromium center with the P atom [3:
˚
˚
Cr-Cl = 2.3429(9) A,andCr-P = 2.5733(8) A;4:Cr1-Cl1 =
Figure 4. Partial thermal ellipsoid drawing of 5 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
˚
˚
2.3321(18) A ;Cr1-P1 = 2.5572(13) A] and retains an Al group
with the N atom [3: N1-Al1 = 1.889(3) A; 4: N1-Al1 =
1.874(5) A]. Different from 2, the Al residues retained one
˚
˚
try-generated tetramer composed by two identical dimetallic
units each surrounded by three P-N ligands and linked by
chlorine atom that is used to bridge the central chromium [3:
˚
˚
˚
Cl1-Cr1 = 2.3429(9) A; Al1-Cl1 = 2.3274(14) A; Al1-
two bridging chlorine atoms [Cr2-Cl1 = 2.4993(16) A;
˚
˚
Cl1-Cr(1) = 90.10(4)°; 4: Cl1-Cr1 = 2.3321(18) A; Al1-
Cr2-Cl1a = 2.6917(16) A]. The three ligands of the dime-
tallic unit adopted an arrangement similar to that observed
˚
Cl1 = 2.282(3) A; Al1-Cl1-Cr1 = 99.26(10)°]. No further
interaction is visible for the chromium center with the organoa-
luminum fragments. As a result, in both cases the coordination
geometry around the metal center is square-planar and defined
by the two trans-located P and Cl atoms [3: Cl1-Cr-Cl1a =
180.00(3)°; P1-Cr1-P1a = 180.0°; 4: Cl1-Cr-Cl1a =
180.000(1)°; P1-Cr1-P1a = 180.0°].
in 1. Two ligands bridge the two metals by orienting the P
˚
atoms toward the same chromium [Cr1-P3 = 2.4637(14) A;
˚
Cr1-P2 = 2.4867(14) A] and the nitrogens toward the
˚
˚
second [Cr2-N3 = 2.019(3) A; Cr2-N2 = 2.020(3) A].
The third instead simply chelates the chromium atom placed
˚
at the periphery of the cluster [Cr1-N1 = 2.020(3) A; Cr1-
P1=2.3780(14) A] and surrounded by the phosphorus atoms.
˚
When the reaction affording 1 was carried out with a 1:1
ratio of ligand to CrCl2 (THF)2, the new tetrameric cluster
3
The coordination geometry might be regarded as severely
distorted square-planar [N1-Cr1-P1=42.70(10)°; N1-Cr1-
P3=143.77(10)°; N1-Cr1-P2=113.73(11)°; P2-Cr1-P3 =
99.31(5)°; P1-Cr1-P3=101.94(5)°; P1-Cr1-P2=139.43(5)°].
The chlorine atom bonded to the second chromium and used for
the bridging interaction with a second dimetallic unit has a
[(t-BuNPPh2)Cr(μ-t-BuNPPh2)2Cr(μ-Cl)]2 (toluene)2 (5) was
3
formed (Scheme 3). As usual for polynuclear chromium
complexes, the magnetic moment was lower than theoreti-
cally expected for a high-spin d4 system, which most likely is
the consequence of antiferromagnetic exchange between the
metal centers.
short nonbonding contact and is located on the apex of an ideal
˚
The molecular structure was elucidated by X-ray diffrac-
tion analysis (Figure 4). The structure consists of a symme-
square pyramid [Cr1 Cl1 = 2.8223(16) A, Cl1-Cr1-P1 =
3 3 3
109.63(5)°;Cl1-Cr1-P2=104.42(4)°;Cl1-Cr1-P3=90.16(4)°;

4086 Organometallics, Vol. 29, No. 18, 2010
Thapa et al.
Scheme 4
Figure 5. Partial thermal ellipsoid drawing of 6 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
Figure 6. Partial thermal ellipsoid drawing of 7 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
Cl1-Cr1-N1 = 95.37(10)°]. The coordination geometry of
the second chromium atom is very distorted square-planar,
or flattened tetrahedral, and is defined by the two nitrogens
of the two bridging ligands [N2-Cr2-N3 = 111.43(13)°]
and the two bridging chlorine atoms [Cl1-Cr2-Cl1a =
79.80(4)°; Cl1-Cr2-N2 = 133.37(10)°; Cl1-Cr2-N3 =
Scheme 5
103.84(10)°]. The Cr Cr distances within each dimetallic
3 3 3
unit are long and outside the bonding range [Cr1 Cr2 =
˚
2.7670(13) A].
3 3 3
As usual, the reactions with AlR3 proved to be difficult,
mostly affording intractable materials. Only in the case of
TMA was it possible to obtain a crystalline product for-
˚
[C33-Cr1 = 2.805(6) A ] is outside the normal bonding
range. The magnetic moment of 6 was in line with that of
the other polynuclear complexes reported in this work with a
room-temperature value lower than expected as a possible
result of an antiferromagnetic interaction.
As previously mentioned, recent work on chromium
pyrrolide8c,d has established a link between the metal oxida-
tion state and catalytic behavior (selective ethylene trimer-
ization versus polymerization and nonselective oligomeriza-
tion). Therefore, in addition to the divalent species described
above, we have also attempted to prepare and test the
trivalent derivative of the same ligand system. Treatment
of three equivalents of the lithium salt of the aminophosphine
mulated as dinuclear, [{(μ-AlMe2)(t-BuNPPh2)2}Cr(μ-Cl)]2
(toluene)1.7 (6), on the basis of the X-ray crystal structure
(Scheme 4).
The structure indicated that the reaction has taken a
surprisingly different pathway with respect to those leading
to 2 and 3. The symmetry-generated dimer (Figure 5) has two
chromium-containing units linked by two bridging chlorides
3
˚
˚
[Cr1-Cl1 = 2.3866(17) A; Cr1-Cl1a = 2.3795(16) A]. Each
unit consists of a chromium atom bonded to two phos-
˚
phorus of two ligands [Cr1-P1=2.4471(17) A; Cr1-P2=
2.5193(17) A] and which in turn use the N atoms to link one
˚
˚
˚
AlMe2 unit [Al1-N2=1.916(5) A; Al1-N1=1.929(5) A].
The coordination geometry around the chromium center is
therefore square-planar and is defined by two chlorine atoms
placed cis [Cl1-Cr1-Cl1a = 86.45(6)°] and the two P atoms
of the two ligands [P1-Cr1-P2 = 95.52(6)°, Cl1-Cr1-
P2 = 172.37(7)°]. Once again, one of the two methyl groups
of the Me2Al unit is oriented toward the metal center and
occupies the axial position of an ideal square pyramid based
ligand with one equivalent of CrCl3 (THF)3 (Scheme 5) af-
3
forded green-colored, X-ray quality paramagnetic crystals
of [(t-BuNPPh2)3Cr] (toluene) (7). The room-tempera-
3
ture magnetic moment was as expected for the high-spin d3
electronic configuration of trivalent chromium in an octahedral
field.
The structure of 7 (Figure 6) confirmed the monomeric
nature of the complex with three ligands in a propeller-type
on the chromium atom. However, the Cr CH3 distance
3 3 3

Article
Organometallics, Vol. 29, No. 18, 2010 4087
Table 1. Effect of Temperature on Catalytic Behavior of 1a
mol %
C12
temp (°C)
activity (g/mmol h)
3
polymer (g)
Mw (g/mol)
PDI
3.8
oligomers (mL)
C6
C8
C10
C14
C16
C18
25
50
75
100
50b
0
1580
5400
6560
320
0
5.5
11.3 (wax)
6.0 (wax)
0
0
2.4
15.7
26.8
1.6
42 180
99.9
27.9
23.9
99.9
23.4
33.2
20.5
26.6
18.2
20.1
12.5
15.7
8
11.7
2
3.5
a Conditions: MAO activator 1000 equiv, 10 μmol catalyst loading, 40 bar ethylene, 30 min reaction time, total volume 100 mL in toluene. bActivated
with TEAL (100 equiv).
Scheme 6
arrangement surrounding the metal center in a distorted
octahedral geometry [P1-Cr1-N1 = 41.54(10)°; P1A-
Cr1-N1A=41.54(10)°, P1A-Cr1-N1A=41.54(10)°]. Of
˚
interest is that the three phosphorus [Cr1-P1=2.4151(12) A;
Figure 7. Partial thermal ellipsoid drawing of 8 (50% prob-
ability level) with carbon atom ellipsoids omitted for clarity.
˚
˚
Cr1-P1A=2.4151(12) A; Cr1-P1A = 2.4151(12) A] and
˚
˚
nitrogen atoms [Cr1-N1=2.024 A; Cr1-N1a = 2.024(3) A;
Cr1-N(1A) = 2.024(3) A] are facially oriented [N1-Cr1-
˚
produced. The overall activity remained, however, rather
low even under a variety of reaction conditions.
N1A = 111.67(10)°; N1-Cr1-N1A =111.67(10)°; N1A-
Cr1-N1A = 111.67(9)°; P1-Cr1-P1A = 108.96(4)°; P1-
Cr1-P1A = 108.96(4)°; P1A-Cr1-P1A = 108.96(4)°].
The catalytic behavior of 7 is by any means closely related
to that of all the other divalent complexes reported in this
work (vide infra). Therefore, it is conceivable that one of the
preliminary steps of the catalyst activation is the reduction of
the trivalent species to a Cr(II) derivative. To conclusively
demonstrate this point, 7 was treated with the same alumi-
num alkyls described earlier in this work (Scheme 6). Reac-
tion with AlMe3 under the usual reaction conditions
afforded 2, while the reaction with AlEt2Cl gave the new
complex 8, whose structure is closely related to those of 3 and
4 with comparable bond distances and angles (Figure 7). The
value of the magnetic moment of 8 was in the expected range.
Increasing the temperature had a rather strong effect on
both the activity and selectivity of 1 (Table 1). With rising
temperature, the activity increased, as did the amount of
olefinic product. Interestingly, the yield of polyethylene
shows a clear optimum around 75 °C, above which it dropped
significantly. The selective trimerization as observed at 50 °C is
clearly lost at higher temperatures. This implies that another
catalytically active species is generated at a temperature of 75 °C
and higher. This high sensitivity to the temperature is also
responsible for the dependence on the amount of catalyst
employed. By increasing the loading of 1 (Table 2) the produc-
tion of polymer was more than doubled, while a moderate
amount of 1-hexene was replaced by a much larger amount of a
S-F distribution of oligomers. This is clearly a temperature
effect that could no longer be properly controlled when activity
increased as a result of the larger amount of catalyst employed.
Complex 5 is closely related to 1, from which it differs only
in the nuclearity and the presence of one chlorine atom per
dimeric unit. In spite of displaying a very similar ligand
arrangement, 5 shows a substantially better catalytic perfor-
mance, giving a large amount of S-F distribution of oligo-
mers, interestingly, with 1-octene as the major component
(Table 2).
Catalytic Behavior
The catalytic behavior of all the complexes described
ꢀ
above was tested vis-a-vis their ability to oligo- or polymerize
ethylene with and without alkyl aluminum activators. Upon
activation with MAO, complex 1 behaves as a moderately
active ethylene polymerization catalyst (Table 1). What
makes this catalyst potentially interesting is that, aside from
the formation of polyethylene, a visible amount of 1-hexene
was formed as the only byproduct. In turn, this indicates that
the activator converts 1 into two separate catalytically active
species respectively responsible for ethylene polymerization
and selective trimerization. Attempts to activate 1 with
different activators such as TMA and TIBA afforded only
a small amount of 1-hexene, but with no polymer formation
whatsoever. Similar behavior was observed with TEAL,
although a substantially larger amount of 1-hexene was
Among the complexes containing an AlR2 residue, only 2
acts as a single-component self-activating catalyst (Table 2).
Upon exposure to ethylene at 100 °C, 2 produced a mixture
of 1-butene (∼75%), 1-hexene, and 1-octene with a minor
amount of polyethylene. While inactive as single-component
catalysts, complexes 3, 4, 6, and 8 showed outstanding acti-
vity as nonselective oligomerization catalysts, producing a
large amount of a S-F distribution of oligomers and some

4088 Organometallics, Vol. 29, No. 18, 2010
Thapa et al.
Table 2. Catalytic Behavior of 1-7a
mol %
C8 C10 C12 C14 C16 C18
cat (μmol) MAO (equiv) activity (g/mmol h) polymer (g) Mw (g/mol) PDI oligomers (mL) C6
3
1 (10)
1 (25)
2 (50)
2 (25)
3 (50)
3 (10)
3 (25)
4 (50)
4 (25)
5 (5)
1000
1000
0
1000
0
1000
1000
0
1000
1000
1000
1000
0
1560
2800
236
4616
0
7200
6200
0
6280
14 000
19 200
5384
20
5. 5
13.0
1. 2
2.7
0
8.0
9. 5
0
8. 5
2.0
5.0
12.7
0. 5
42 180
3.8
2.4
22.0
4.7b
55.0
99.9
37.9 20.0 14.0 10.9
25.0 10.0
14.1 26.4 23.4 19.8 15.6 12.8 0.7
7.6
5.6 0.5
72 150
2.5
28.0
68.0
18.9 24.9 20.2 15.8 11.3
20.0 27.2 16.7 10.3
6.6 2.2
2.9 1.1
969 950
561 380
17.6
8.6
6.1
70.0
33.0
43.0
54.6
35.6 22.2 16.7 10.3
6.1
2.9 1.1
8.1 5.6
9.6 7.5
7.9 6.1
12.7 26.3 19.8 15.9 11.5
22.1 27.7 22.9 17.7 13.1
15.4 23.3 18.1 14.0 10.4
5 (5)
5 (25)
6 (50)
6 (10)
7 (10)
8 (10)
blank
1 225 580
2500
7.1
2.5
1000
1000
1000
1000
10 400
3688
0.2 (wax)
6.4
1.0 (wax)
0.1
51.0
12.0
68.0
0.0
15.0 21.5 18.5 14.5 11.0
37.8 20.6 14.4 11.0 8.1
22.2 23.9 19.2 14.9 11.2
8.2 6.3
5.5 2.3
8.0 6.4
a Conditions: T = 50 °C, 40 bar ethylene, 30 min reaction time, total volume 100 mL in toluene. b Large amount of 1-butene (∼65%) not reliably
quantifiable due to volatility.
Table 3. Effect of the Solventa
cat. (μmol)
MAO (equiv)
activity g/mmol (cat h)
3
oligomers (mL)
1-C6 (%)
polymer (g)
Mw (g/mol)
PDI
1 (25)
2 (25)
3 (10)
3 (25)
3 (10)b
4 (25)
4 (10)b
5 (25)
5 (25)b
6 (11)
7 (25)
8 (10)
150
150
1000
150
1000
150
1000
150
500
720
1760
2660
1880
860
2160
2560
1120
920
4.0
6.0
5.3
4.5
2.3
9.0
7.4
5.0
3.5
1.5
8.0
5.5
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
5. 0
16.0
8. 0
19.0
2. 0
18.0
5. 4
9. 0
8. 0
27.0
19.0
20
42 000
90 040
3.1
2.9
241 880
1 645 820
501 150
8.5
3.9
58
150
150
150
5700
2160
2120
237 280
134 690
551 790
32
6.5
10.1
a Conditions: MAO activator, T = 60 °C, 40 bar ethylene, 30 min reaction time, total volume 100 mL in methylcyclohexane. b MMAO activator.
polymer. Complex 5 gave the highest activity, producing a
S-F distribution of oligomers in addition to a substantial
amount of polymer. Complex 6 also displayed high acti-
vity, and unlike 1-5 no high molecular weight polyethy-
lene was formed. The traces of solid produced were waxy
higher R-olefins as part of the S-F distribution. The
trivalent 7 produced a very large amount of high molecular
weight polyethylene and a S-F distribution of R-olefins
(Table 2).
As mentioned above, treatment of 7 with TMA and DEAC
afforded 2 and 8, respectively. Complex 8 is not a single-
component catalyst and gave a catalytic activity similar to
those of 3 and 4 (Table 2). Its desirable feature is that, in
addition to the outstanding activity as a nonselective cata-
lyst, no high molecular weight polymer was produced. The
small amount of polymer formed is in fact the high molecular
weight fraction of the S-F distribution.
work. It is therefore unfortunate that the change of solvent
also resulted in an increase in the amount of polymer formed.
Interestingly though, complex 6 switched its selectivity from
a S-F distribution to waxy high molecular weight linear
R-olefins (Table 3). Given the trend, it is rather strange that 1
is the only one of this class of complexes to give a selective
trimerization catalyst in toluene. This can be tentatively
ascribed to the low productivity with consequent low
exothermicity that prevented thermal degradation of the
catalyst (see Table 1). With the only exception of 2 as single-
component catalyst (which also seems to give some selec-
tivity), all the other compounds produced a substantially
larger amount of S-F distribution of products. This reiter-
ates the prominent role played by the temperature in deac-
tivating the trimerization catalyst when toluene is used as
solvent.
Finally, the ability of the solvent to switch the catalytic
outcome from a S-F distribution to selective formation of
1-hexene clearly indicates that the same catalytically active
species responsible for the formation of 1-hexene in methyl-
cyclohexane cannot be responsible for the S-F distribution
observed in toluene. Further ring expansion leading to
heavier R-olefins in toluene cannot possibly be hindered by
methylcyclohexane. Therefore, we propose that either a
different mechanism or a different active species should be
operational for the efficient nonselective oligomerization in
toluene.
By changing the solvent to methylcyclohexane,8c all the
complexes display an interesting behavior, as they all give
selective ethylene trimerization upon activation with MAO
(Table 3). Interestingly, this is a complete turnaround with
respect to the behavior of Rosenthal’s PNPN systems,38
which produced high selectivity in toluene. This may tenta-
tively be explained with the presence of an additional P atom
in PNPN ligands, preventing the reoxidation of the mono-
valent intermediates via disporportionation. In the case of 4
the formation of 1-hexene was the highest observed in this

Article
Organometallics, Vol. 29, No. 18, 2010 4089
Conclusions
species. Instead, the switching of catalytic behavior from
trimerization to an occasionally potent nonselective oligo-
merization catalyst is likely the result of diproportionation
toward the trivalent state. Even though this seems to be
contradicted by the ease of the reduction of the trivalent
organochromium derivatives as discussed above, previous
mechanistic work on a highly performing trimerization
system37a has clearly indicated the existence of the redox
dynamism via continuous reduction and reoxidation of the
chromium catalytic center during the catalytic cycle. The
almost ubiquitous presence of polymer besides either 1-hex-
ene or the S-F distribution of oligomers is again an indica-
tion that the three oxidation states, responsible for each of
these three processes, may coexist or rapidly interconvert in
the catalytic mixture.
In conclusions, the anionic N-P ligand system has shown
some interesting behavior by allowing the preparation of a
variety of organochromium complexes, assembling unpre-
cedented molecular and polynuclear structures, and enabling
the elucidation of the retention of AlRx residues. In the series
of complexes isolated and characterized in this work, all
species showed catalytic behavior. The single-component
catalytic activity observed in the case of 2 is particularly
attractive given that this sort of behavior is exceedingly
rare in chromium chemistry.8 The lack of single-component
behavior for 3 and 4 is likely to be due to the presence of
chlorine, building in these complexes robust bridges between
the Al and Cr atoms. The outstanding activity observed for
some of these species as S-F oligomerization catalysts is
useful especially when, as in the cases of 6 and 8, no high
molecular weight polymeric byproducts are formed.
The limited formation of 1-hexene in some of the runs, as
the only oligomer detectable besides the polymer, is clearly
due to the predominant stability of the divalent oxidation
state, which allows in this case only limited accessibility to
the monovalent state responsible for the selective trimeriza-
tion. The transformation of the trivalent 7 into the divalent 3,
4, and 8 upon exposure to the aluminum alkyl activators also
reiterates the greater stability of the divalent state for these
The change of solvent from toluene to methylcyclohexane
transformed these catalytic systems into selective trimeriza-
tion catalysts, producing in some case significant amounts of
1-hexene.
Acknowledgment. Dr. Glenn Yap and Dr. Ilia Korobkov
are gratefully acknowledged for solving crystal structures.
Supporting Information Available: Crystallographic data for
this work are available free of charge via the Internet at http://
pubs.acs.org.