New iminophosphonamide chromium(II) complexes as highly active polymer-free ethylene oligomerization catalysts

Article abstract of DOI:10.1021/om2008474

The reactions of the deprotonated form of cis-{[(μ-N)(t-Bu)] ₂PN(H)(o-OMeC₆H₄)]₂} (a) with either CrCl2(THF)₂ or CrCl3(THF)3 afforded the corresponding dimeric Cr(II) and monomeric Cr(III) complexes {

Full text of DOI:10.1021/om2008474

ARTICLE
pubs.acs.org/Organometallics
New Iminophosphonamide Chromium(II) Complexes as Highly
Active Polymer-Free Ethylene Oligomerization Catalysts
Khalid Albahily,† Zeeshan Ahmed,† Sandro Gambarotta,*,† Ece Koc-,‡ Robbert Duchateau,*,‡ and
Ilia Korobkov§
†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
§X-ray Core Facility, Faculty of Science, Ottawa, Ontario K1N 6N5, Canada
S Supporting Information
b
ABSTRACT: The reactions of the deprotonated form of cis-{[(μ-N)(t-Bu)]2PN(H)
(o-OMeC6H4)]2} (a) with either CrCl2(THF)2 or CrCl3(THF)3 afforded the correspond-
ing dimeric Cr(II) and monomeric Cr(III) complexes {cis-[(μ-N)(t-Bu)]2[PN
(o-MeOC6H4)]2Cr}2 (1a) and cis-[(μ-N)(t-Bu)]2[PN-2-MeOC6H4]2CrCl (2a). By replacing
the ligand’s o-OMeC6H4 groups with less-crowded CH2CH2N(i-Pr)2 functionalities and
reacting the deprotonated form of cis-{[(μ-N)(t-Bu)]2[PN(H)CH2CH2N(i-Pr)2)]2} (b)
with CrCl2(THF)2, the dimetallic, divalent {cis-[(μ-N)(t-Bu)]2[PNCH2CH2N(i-
Pr)2)]2Cr}2 (1b) was obtained in crystalline form. Upon activation with MAO, both 1a
and 1b afforded highly active, polyethylene-free ethylene oligomerization catalysts. Com-
plex 2a is instead catalytically inactive.
’ INTRODUCTION
used for feeding conveniently the metal into the catalytic cycle,1ꢀ4 it
is, therefore, equally important to be able to control the extent of
reduction of the metal center bonded to the NP-based ligand
depending on the type of oligomerization that is being pursued.4d
This is not easily achieved with an alkyl aluminum reagent as a
reductant, while stronger reducing agents, such as alkali metals, pose
other problems. An alternative approach to reach a desired oxidation
state consists of the destabilization of the unwanted ones through
the imposition of unfavorable ligand fields. For example, tridentate
ligands, with geometries unable to accommodate the square-planar
and square-pyramidal coordination, as dictated by the d4 electronic
configuration of Cr(II), might be able to destabilize the divalent
state, if undesirable, in favor of the selective monovalent state.
For this reason, we recently studied the (μ-NR)2(PN(H)R0)2
ligand system that was believed to be unable to accommodate
square-planar Cr(II) species. To our surprise, we observed that
the ligand’s P2N2 core was readily cleaved to afford monomeric
NPN anions that, on their turn, could accommodate square-
planar coordination.5h The thus obtained noninnocent (t-Bu)
NPN(t-Bu) ancillary ligand produced chromium-based catalysts
that, depending on the activator, switched selectivity from
ethylene polymerization to nonselective oligomerization and
even to selective trimerization.8 The switchability of this catalytic
Chromium plays an important role in polyolefin chemistry
because it provides some of the best catalysts for commercial
ethylene polymerization,1 nonselective oligomerization,2 and
selective tri-3 and tetramerization.4 Therefore, its derivatives
are ideal substrates for studying the relationship between cata-
lytic behavior, ligand features, and metal oxidation state. For
example, the isolation of self-activating ethylene trimerization
catalysts has allowed linking the selectivity of a chromium-based
trimerization process to the monovalent state.5 While this
information may be of use for targeting the preparation of new
and more performing catalysts, the major remaining challenge is
how to control the metal redox dynamism.6 For example, in the
family of NP derivatives, the three catalytically active oxidation
states of chromium (III, II, and I) may be readily interconverted
by activators via reductions and disproportionations.7 Because, in
this family of catalyst precursors, each oxidation state has its own
distinctive catalytic behavior,5c chromium catalysts producing
mixtures of polymers and oligomers, occasionally enriched with
1-hexene or 1-octene, are often encountered. While targeting
NP-based oligomerization catalysts, it is, therefore, of primary
importance that the ligand system is capable of preventing
disproportionation to the zero- and divalent states if selective
oligomerization is being targeted. Vice versa, if a statistical
distribution of oligomers is sought, it is then the divalent state
that should be stabilized. Because the trivalent state is normally
Received: September 8, 2011
Published: October 13, 2011
r
2011 American Chemical Society
6022
dx.doi.org/10.1021/om2008474 Organometallics 2011, 30, 6022–6027
|

Organometallics
ARTICLE
Scheme 1
Scheme 2
system, in turn, speaks for the ability of this ligand to stabilize
certain oxidation states and to prevent disproportionation reac-
tions. To improve our understanding of this versatile ligand
system, we have attempted with the current study to assess some
of the factors affecting the stability of the divalent state, an
oxidation state often responsible for high activity and poor
selectivity. In particular, we were interested in studying the
influence of the ligand’s denticity and steric bulk vis-a-vis the
tendency of the ligand to cleave and catalyst behavior.
replacing the ortho-i-Pr groups in cis-{[(μ-N)(t-Bu)]2PN(H)-
(2,6-(i-Pr)2C6H3)]2}8 with an ortho-methoxy functionality in a
prevents the ligand from being cleaved upon reaction with Cr(II).
This can be ascribed to the presence of the additional pendant donor
functionality allowing the ligand to accommodate a distorted
square-planar coordination environment that is highly desired for
divalent chromium: [N(4)ꢀCr(1)ꢀN(3) = 96.33(11)°, N(4)ꢀ
Cr(1)ꢀN(3A) = 168.74(12)°, N(3)ꢀCr(1)ꢀN(3A) = 88.71(11)°,
N(4)ꢀCr(1)ꢀO(1A) = 95.83(10)°, N(3)ꢀCr(1)ꢀO(1A) =
155.48(11)°, N(3A)ꢀCr(1)ꢀO(1A) = 75.94(10)°] (Figure1).
Each ligand adopts a bridging-chelating bonding mode. The P2N2
square core is capping one chromium center with none of the
four atoms forming a bonding contact with the metal
[Cr(1) N(1) = 3.579 Å, Cr(1) N(2) = 2.572 Å]. Instead,
the two nitrogen atoms of the two pendant “arms” are both
bonding the same chromium atom [Cr(1)ꢀN(4) = 2.056(3) Å,
Cr(1)ꢀN(3) = 2.098(3) Å]. One of the two nitrogen atoms also
bridges the second metal center [N(3)ꢀCr(1A) = 2.118(3) Å].
The ortho-methoxy group of the phenyl ring attached to the
bridging nitrogen also bonds the second chromium center [O(1)ꢀ
Cr(1A) = 2.208(2) Å]. The second OMe group of the same ligand is
located on an equatorial vertex of an ideal trigonal bipyramid
centered on the chromium atom [Cr(1)ꢀO(2) = 2.450 Å],
but without forming a bonding contact The structure of 2a
shows a monomeric complex with the chromium atom in a dis-
torted octahedral geometry (Figure 2) [O(1)ꢀCr(1)ꢀN(3) =
149.93(18)°, N(3)ꢀCr(1)ꢀCl(1) = 104.09(14)°, N(1)ꢀ
Cr(1)ꢀN(3) = 73.3(2)°]. The coordination octahedron is de-
fined by the two ortho-oxygen atoms [Cr(1)ꢀO(1) = 2.098(4) Å,
’ RESULTS AND DISCUSSION
In this work, we have separately examined two variations of the
(μ-NR)2(PN(H)R)2 scaffold. We have introduced an aromatic
ring carrying a methoxy donor group in the ortho position and, as
a second possibility, a branch terminating with a coordinating
amino group. The two ligands used for this work, cis-{[(μ-N)(t-
Bu)]2PN(H)(o-OMeC6H4)]2} (a) and cis-{[(μ-N)(t-Bu)]2-
[PN(H)CH2CH2N(i-Pr)2)]2} (b), were prepared via reaction
of [(t-Bu)NPCl]2 with 2-methoxyaniline or (i-Pr)2NCH2CH2NH2,
respectively (Scheme 1).
The reactions of the deprotonated form of a with both
CrCl2(THF)2 and CrCl3(THF)3 were carried out in THF at
room temperature, affording the corresponding {cis-[(μ-N)(t-
Bu)]2[PN(o-MeOC6H4)]2Cr}2 (1a) and cis-[(μ-N)(t-Bu)]2-
[PN-2-MeOC6H4]2CrCl (2a), respectively (Scheme 2).
The structures of the two compounds were elucidated by X-ray
analysis. Complex 1a is a symmetry-generated dimer with two
dianionic ligands surrounding the dimetallic unit. Interestingly,
3 3 3
3 3 3
6023
dx.doi.org/10.1021/om2008474 |Organometallics 2011, 30, 6022–6027

Organometallics
ARTICLE
Scheme 3
Figure 1. Thermal ellipsoid plot of 1a with ellipsoids drawn at 50%
probability. Bond distances (Å) and angles (°) for 1a: Cr(1)ꢀN(4) =
2.056(3), Cr(1)ꢀN(3) = 2.098(3), Cr(1)ꢀN(3)A = 2.118(3), Cr-
(1)ꢀO(1)A = 2.208(2), N(3)ꢀCr(1)A = 2.118(3), O(1)ꢀCr(1)A =
2.208(2); N(4)ꢀCr(1)ꢀN(3) = 96.33(11), N(4)ꢀCr(1)ꢀN(3)A =
168.74(12), N(3)ꢀCr(1)ꢀN(3)A = 88.71(11), N(4)ꢀCr(1)ꢀO(1)A =
95.83(10), N(3)ꢀCr(1)ꢀO(1)A = 155.48(11), N(3)AꢀCr(1)ꢀ
O(1)A = 75.94(10), Cr(1)ꢀN(3)ꢀCr(1)A = 91.29(11).
Figure 3. Thermal ellipsoid plot of 1b with ellipsoids drawn at 50%
probability. Bond distances (Å) and angles (°) for 1b: Cr(1)ꢀN(5) =
2.081(4), Cr(1)ꢀN(3) = 2.114(4), Cr(1)ꢀN(6) = 2.232(5), Cr-
(1)ꢀCl(1) = 2.3208(17), Cr(1)ꢀCr(2) = 2.6354(13), Cr(2)ꢀN(3) =
2.071(4), Cr(2)ꢀN(5) = 2.144(5), Cr(2)ꢀN(4) = 2.245(5), Cr-
(2)ꢀCl(2) = 2.3241(17); N(5)ꢀCr(1)ꢀN(3) = 84.97(17), N(5)ꢀ
Cr(1)ꢀN(6) = 82.32(17), N(3)ꢀCr(1)ꢀN(6) = 161.76(18), N(5)ꢀ
Cr(1)ꢀCl(1) = 179.73(14), N(3)ꢀCr(1)ꢀCl(1) = 95.23(12), N-
(6)ꢀCr(1)ꢀCl(1) = 97.53(12), N(5)ꢀCr(1)ꢀCr(2) = 52.49(13),
N(3)ꢀCr(1)ꢀCr(2) = 50.25(12), N(6)ꢀCr(1)ꢀCr(2) = 111.51(13),
Cl(1)ꢀCr(1)ꢀCr(2) = 127.78(6), N(3)ꢀCr(2)ꢀN(5) = 84.47(17),
N(3)ꢀCr(2)ꢀN(4) = 81.37(17), N(5)ꢀCr(2)ꢀN(4) = 158.66(17),
N(3)ꢀCr(2)ꢀCl(2) = 176.96(13), N(5)ꢀCr(2)ꢀCl(2) = 94.97(13),
N(4)ꢀCr(2)ꢀCl(2) = 99.98(13), N(3)ꢀCr(2)ꢀCr(1) = 51.71(13),
N(5)ꢀCr(2)ꢀCr(1) = 50.35(12), N(4)ꢀCr(2)ꢀCr(1) = 108.38(12),
Cl(2)ꢀCr(2)ꢀCr(1) = 129.84(6).
Figure 2. Thermal ellipsoid plot of 2a with ellipsoids drawn at 50%
probability. Bond distances (Å) and angles (°) for 2a: Cr(1)ꢀN(1) =
1.912(5), Cr(1)ꢀN(2) = 1.987(5), Cr(1)ꢀO(1) = 2.098(4), Cr(1)ꢀ
N(3) = 2.111(5), Cr(1)ꢀO(2) = 2.156(4), Cr(1)ꢀCl(1) = 2.326(2);
N(1)ꢀCr(1)ꢀN(2) = 98.2(2), N(1)ꢀCr(1)ꢀO(1) = 78.88(19), N-
(2)ꢀCr(1)ꢀO(1) = 98.45(18), N(1)ꢀCr(1)ꢀN(3) = 73.3(2), N-
(2)ꢀCr(1)ꢀN(3) = 74.93(19), O(1)ꢀCr(1)ꢀN(3) = 149.93(18),
N(1)ꢀCr(1)ꢀO(2) = 169.8(2), N(2)ꢀCr(1)ꢀO(2) = 77.70(18),
O(1)ꢀCr(1)ꢀO(2) = 92.37(16), N(3)ꢀCr(1)ꢀO(2) = 114.06(18),
N(1)ꢀCr(1)ꢀCl(1) = 94.58(17), N(2)ꢀCr(1)ꢀCl(1) = 166.26(16),
O(1)ꢀCr(1)ꢀCl(1) = 89.01(13), N(3)ꢀCr(1)ꢀCl(1) = 104.09(14),
O(2)ꢀCr(1)ꢀCl(1) = 90.54(13).
The reaction of the ligand b with CrCl2(THF)2 also afforded a
dinuclear complex {[(μ-N)(t-Bu)]2[PNCH2CH2N(i-Pr)2)]2Cr}2
(1b) with one ligand wrapping the dimetallic unit (Scheme 3).
A chlorine atom was also retained by each of the two chro-
mium atoms.
Because unreacted ligand salt was invariably present in the
reaction mixture even under prolonged reflux, we decided to
carry out the reaction with the stoichiometry as advised by the
formula obtained from X-ray analysis. As expected, the reaction
afforded a better yield of the same complex.
Cr(1)ꢀO(2) = 2.156(4) Å] of the two ligand’s methoxy groups,
one terminally bonded Cl atom [Cr(1)ꢀCl(1) = 2.326(2)Å],
and three of the four nitrogen donor atoms of the ligand system
facially oriented [Cr(1)ꢀN(1) = 1.912(5A) Å, Cr(1)ꢀN(2) =
1.987(5) Å, Cr(1)ꢀN(3) = 2.111(5) Å]. The fourth N atom,
not engaged in bonding interaction with the metal center,
adopted a trigonal planar geometry [C(19)ꢀN(4)ꢀP(2) =
128.5(5)°, C(19)ꢀN(4)ꢀP(1) = 128.1(5)°, P(2)ꢀN(4)ꢀP(1) =
101.6(3)°].
6024
dx.doi.org/10.1021/om2008474 |Organometallics 2011, 30, 6022–6027

Organometallics
ARTICLE
Table 1. Ethylene Oligomerization Resultsa
cat
cocat
Al/Cr
PE, g(%)
olig, mL
activity, g/mmol Cr/h
C4, %
C6, %
C8, %
C10, %
C12‑18, %
1a
1a
1a
1a
1a
2a
1b
1b
1b
1b
MAO
MAO
TIBA
TEAL
TIBAO
MAO
MAO
MAO
TIBAO
TIBA
500
1000
500
0
33
71
0
9240
traces
1
25
23
0
23
22
0
18
17
0
34
37
0
0
19 900
0
0
0
20
0
0
0
0
0
0
0
0
500
5.1(63)
3(37)
0
2900
<1
0
52
0
41
0
6
0
1000
500
0
0
0
0
0
0
0
0
24
56
0
6700
<1
<1
0
32
30
0
26
25
0
16
18
0
25
26
0
1000
500
15 700
0
2000
0
0
0
0
0
0
0
a Conditions: 100 mL of toluene; 35 bar of ethylene; reaction time, 30 min; reaction temperature, 70 °C; catalyst loading, 5 μmol.
Even in this case, the P2N2 ligand core remained intact as a
result of the pendant donor functionality supporting a stable
square-planar metal coordination environment. The structure of
1b (Figure 3) shows the P2N2 core of the ligand symmetrically
capping a Cr2 unit, but without forming bonding contacts with
the metal centers. Each of the two ligand’s “arms” N atoms
instead bridges the two metal centers [Cr(1)ꢀN(5) = 2.081(4) Å,
Cr(1)ꢀN(3) = 2.114(4) Å, Cr(2)ꢀN(3) = 2.071(4) Å, Cr(2)ꢀ
N(5) = 2.144(5) Å], forming an intermetallic distance [Cr(1)ꢀ
Cr(2) = 2.6354(13) Å] as expected for a folded Cr2N2 core. The
aliphatic amino pendants attached to each of the two arm’s
nitrogen atoms bond each of the two metal centers [Cr(1)ꢀ
N(6) = 2.232(5) Å, Cr(2)ꢀN(4) = 2.245(5) Å]. With one
terminally bonded chlorine atom [Cr(1)ꢀCl(1) = 2.3208(17) Å,
Cr(2)ꢀCl(2) = 2.3241(17) Å] on the fourth coordination site,
each chromium atom displays a distorted square-planar environment
[N(3)ꢀCr(1)ꢀN(6) = 161.76(18)°, N(5)ꢀCr(1)ꢀCl(1) =
179.73(14)°, N(3)ꢀCr(1)ꢀCl(1) = 95.23(12)°].
1-octene, and a minor amount of heavier oligomers was
obtained.8 In all the event and especially when a SꢀF distribution
of oligomers was obtained, the amount of 1-butene (expected to
be the largest component) was in fact around or smaller than 1%.
We take this as a possible indication that the Cossee mechanism
is not being followed in this case.
Complex 2a does not give an active catalyst regardless of the
cocatalyst used. Whether this is due to the lack of reduction
toward the di- or monovalent state or simply because the
products formed after reaction with the cocatalyst lack the
necessary features for catalysis is not clear.
In conclusion, we have herein reported divalent chromium
catalyst precursors that are producing statistical mixtures of
oligomers with high activity. High activity and lack of PE
formation is what makes these complexes potentially interesting
from the application standpoint. The absence of selectivity in
these systems is likely to be ascribed to the ability of the ligand
system to well accommodate the highly favorable square-planar
crystal field of divalent chromium. By doing so, the ligand
hampers subsequent reduction toward the selective monovalent
state. This high selectivity for a single oxidation state might also
be the origin of the absence of polyethylene, generally observed
as a side product. When a reducing agent stronger than MAO,
such as TIBAO, is used to activate 1a, small amounts of 1-hexene
and 1-octene are formed, indicating partial reduction to the
monovalent state. It is clear that the [(μ-NR)2(PNR0)2]2ꢀ ligand
system strongly favors divalent chromium complexes, especially
when pendant donor functionalities are present.
Attempts to form the trivalent analogue by treating CrCl3-
(THF)3 with the dilithium salt of b only afforded intractable
materials of borderline catalytic behavior.
The three complexes described above have been tested for
ethylene oligomerization under standard conditions and activa-
tion with a few alkyl aluminum reagents (Table 1). MAO seems
to be the only effective activator in all cases leading the reactivity
toward SchulzꢀFlory distributions of oligomers. The activities
were, in general, quite high for the two divalent complexes 1a and
1b, and even more interestingly, the production of LAO was not
accompanied by the usual formation of waxes or polymer. Other
activators gave no catalytic activity, with the only exception of
TIBAO that, in the case of 1a, gave a substantial amount of
polymer aside a mixture of mainly 1-hexene and 1-octene. No
such behavior was observed for 1b.
The lack of selectivity of 1a and 1b speaks for the stability of
the divalent state in these two complexes that are unlikely to
undergo further reduction toward the monovalent state. In the
case of the more reducing TIBAO, the presence of small amounts
of 1-hexene and 1-octene aside large quantites of polymer
indicates that some reduction occurs, but not to such an extent
to produce a selective catalytic system. The fact that polyethylene
is the main product suggests that the major part of 1a reacts with
TIBAO to yet another species than that generated by MAO. A
similar behavior was observed for the related [(t-Bu)NPN(t-
Bu)]2Cr. Upon activation with MAO, this species afforded a
highly active, but nonselective, ethylene oligomerization catalyst.
With TIBAO, a combination of polyethylene, 1-hexene and
’ 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 ꢁ 0.32 mm i.d.,
capillary CP volamine column and an FID detector. All single-point
experiments were performed in duplicate. The yield was determined by
1H NMR spectroscopy (Varian Mercury 400 MHz spectrometer).
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 PerkinElmer 2400 CHN analyzer. Data for X-ray crystal structure
determination were obtained with a Bruker diffractometer equipped
with a 1K Smart CCD area detector. The ligand precursor cis-ClP[(μ-N)
(t-Bu)]2PCl was prepared according to a published procedure.9 TIBA
(Strem), MAO (10 wt %, Aldrich), TEAL (Strem), and TIBAO (10 wt %,
Aldrich) were used as received.
6025
dx.doi.org/10.1021/om2008474 |Organometallics 2011, 30, 6022–6027

Organometallics
ARTICLE
Preparation of cis-{[(μ-N)(t-Bu)]2PN(H)(o-OMeC6H4)]2} (a).
Neat 2-methoxyaniline (7.16 g, 58.2 mmol) and Et3N (11.9 g, 116.4
mmol) were added to a stirred solution of cis-ClP[(μ-N)(t-Bu)]2PCl
(8.0 g, 29.1 mmol) in THF (100 mL). The reaction mixture was refluxed
for 1 day. After filtration, all the volatile compounds were removed by
vacuum and hexane (25 mL) was added to the residue. The solution was
stored at ꢀ30 °C for 3 days, and after filtration and removal of the
solvent, the product was obtained as white crystals (11.2 g, 24.9 mmol,
86%). 1H NMR (CDCl3, 300 MHz, 300 K): δ 1.28 (s, 18H, C(CH3)3),
3.86 (s, 6H, OCH3), 5.65 (s, 2H, NH), 6.8ꢀ7.0 (m, 4H, Ph), 7.48ꢀ7.6
(m, 4H, Ph). 13C NMR (CDCl3, 300 MHz, 300 K): δ 30.95 (C(CH3)3),
50.47 (C(CH3)3), 55.55 (OCH3), 110.39, 119.78, 120.88, 133.22,
147.90 (Ph). 31P{1H} NMR (CDCl3, 300 MHz, 300 K): δ 97.93 (s).
MS (EI) m/z (M)+ = 448.22. El. Anal. Calcd. (found) for
C22H34N4O2P2: C, 58.92 (58.84); H, 7.64 (7.58); N, 12.49 (12.47).
Preparation of cis-{[(μ-N)(t-Bu)]2[PN(H)CH2CH2N(i-Pr)2)]2}
(b). Neat 2-(diisopropylamino)ethylamine (14.68 g, 101.7 mmol) and
Et3N (20.80 g, 203.5 mmol) were added to a stirred solution of cis-
ClP[(μ-N)(t-Bu)]2PCl (14.0 g, 50.9 mmol) in THF (100 mL). The
reaction mixture was refluxed for 1 day. After filtration, all volatile
compounds were removed in vacuo, affording the product as a light
yellow oil (22.6 g, 46.0 mmol, 91%). 1H NMR (CDCl3, 300 MHz, 300
K): δ 0.975 (d, J = 6.6 Hz, 24H, CH(CH3)2), 1.24 (s, 18H, C(CH3)3),
2.44 (t, J = 6.9 Hz, 4H, NHCH2CH2N(i-Pr)2), 2.8ꢀ3.1 (m, 4H,
CH(CH3)2), 2.9 (m, 4H, NHCH2CH2N(i-Pr)2). 13C NMR (CDCl3,
300 MHz, 300 K): δ 20.90 (CH(CH3)2), 30.98 (C(CH3)3), 39.16
(NHCH2CH2N(i-Pr)2), 46.62 (CH(CH3)2), 47.90 (NHCH2CH2N(i-
Pr)2), 50.9 (C(CH3)3). 31P{1H} NMR (CDCl3, 300 MHz, 300 K): δ
90.32 (s). MS (ESI) m/z (M + H)+ = 491.29. El. Anal. Calcd. (found) for
C24H56N6P2: C, 58.75 (58.70); H, 11.50 (11.52); N, 17.13 (17.15).
Preparation of {cis-[(μ-N)(t-Bu)]2[PN(o-MeOC6H4)]2Cr}2 (1a).
A solution of n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes) was added to a
solution of a (0.45 g, 1.00 mmol) in THF (5 mL) at 0 °C. The resulting
mixture was stirred at room temperature for 18 h and the resulting solution
added to a suspension of CrCl2(THF)2 (0.27 g, 1.0 mmol) in THF (5 mL).
The reaction mixture was allowed to stir at room temperature overnight. The
solvent was removed in vacuo and the residue redissolved in toluene (10 mL).
The suspension was centrifuged, and the supernatant was reduced to 3 mL
and stored at ꢀ30 °C for 5 days. The resulting green crystals of 1a were
filtered, washed with cold hexanes (10 mL), and dried in vacuo (0.40 g,
0.4 mmol, 80%). μeff = 4.97 μB. El. Anal. Calcd. (found) for C44H64Cr2-
N8O4P4: C, 53.01 (53.11); H, 6.47 (6.51); N, 11.24 (11.31).
days. The resulting green crystals of 1b were filtered, washed with cold
hexanes (10 mL), and dried in vacuo (0.46 g, 0.74 mmol, 74%). μeff
=
4.01 μB. El. Anal. Calcd. (found) for C24H54Cl2Cr2N6P2: C, 43.44
(43.40); H, 8.20 (8.18); N, 12.66 (12.61).
Polymerization and Oligomerization Results. Catalytic runs
were carried out in 200 mL high-pressure B€uchi reactors containing a
heating/cooling jacket. A preweighed amount of catalyst was dissolved
in 100 mL of toluene 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. Reaction mix-
tures were quenched after releasing the pressure by adding mixtures of
EtOH and HCl. The polymers obtained were isolated by filtration,
sonicated with a 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 venting and quenching with EtOH and HCl.
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 j
= 0.00° (600 frames), at j = 120.00° (600 frames), and at j = 240.00°
(600 frames). Initial unit-cell parameters were determined from 60 data
frames collected at different sections of the Ewald sphere. Semiempirical
absorption corrections based on equivalent reflections were applied. The
systematic absences and unit-cell parameters were consistent for the
reported space groups. The structures were solved by direct methods,
completed with difference Fourier syntheses, and refined with full-
matrix least-squares procedures based on F2. All non-hydrogen atoms
were refined with anisotropic displacement parameters. All hydrogen
atoms were treated as idealized contributions. All scattering factors and
anomalous dispersion factors are contained in the SHELXTL 6.12
program library.10 Relevant crystal data and bond distances and angles
are given in the Supporting Information.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data (CIF)
b
for the complexes reported in this paper and an extensive listing
of geometrical parameters. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: sgambaro@uottawa.ca (S.B.); R.Duchateau@tue.nl (R.D.).
Preparation cis-[(μ-N)(t-Bu)]2[PN-2-MeOC6H4]2CrCl (2a). A
solution of n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes) was added to a
solution of a (0.45 g, 1.0 mmol) in THF (5 mL) at 0 °C, and the resulting
mixture was stirred at room temperature for 18 h. The resulting solution
was added to a suspension of CrCl3(THF)3 (0.37 g, 1.0 mmol) in THF
(5 mL). The reaction mixture was allowed to stir at room temperature
overnight. The solvent was removed in vacuo and the residue redissolved
in toluene (10 mL). The suspension was centrifuged, and the resulting
supernatant was concentrated to about 3 mL and then stored at ꢀ30 °C for
5 days. The resulting green crystals of 2a were filtered and washed with cold
hexanes (10 mL) and dried in vacuo (0.20 g, 0.37 mmol, 37%). μeff = 3.89
μB. El. Anal. Calcd. (found) for C22H32ClCrN4O2P2: C, 49.49 (49.45); H,
6.04 (6.01); N, 10.49 (10.40).
’ ACKNOWLEDGMENT
This work was supported by the Natural Science and
Engineering Council of Canada (NSERC) and the Technische
Universiteit Eindhoven (The Netherlands).
’ REFERENCES
(1) Groppo, E.; Lamberti, C.; Bordiga, S.; Spoto, G.; Zecchina, A.
J. Catal. 2006, 240, 172.
(2) (a) Lappin, G. R.; Sauer, J. D. Alpha Olefins Application Hand-
book; Marcel Dekkers, Inc.: New York, 1989; Vol. 37, pp 1ꢀ3. (b) Vogt,
D. Oligomerization of ethylene to higher linear R-olefins. In Applied
Homogeneous Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W. A., Eds.; Wiley-VCH: Weinheim, Germany, 2000;
Chapter 2, pp 245ꢀ258. (c) Alpha Olefins (02/03-4), PERP Report,
Mexant Chem Systems. (d) Lutz, E. F. J. Chem. Educ. 1986, 63, 202.
(e) Weissermel, K.; Arpe, H. -J. Industrial Organic Chemistry, 3rd ed.;
Wiley: New York, 1997. (f) Wittcoff, H.; Reuben, B. G.; Plotkin, J. S.
Industrial Organic Chemicals, 2nd ed.; Wiley-Interscience: New York,
Preparation of {cis-[(μ-N)(t-Bu)]2[PNCH2CH2N(i-Pr)2)]2Cr}2
(1b). A solution of n-BuLi (0.84 mL, 2.1 mmol, 2.5 M in hexanes)
was added to a solution of b (0.49 g, 1.0 mmol) in THF (5 mL) at 0 °C,
and the resulting mixture was stirred at room temperature for 18 h. The
solution was added to a suspension of CrCl2(THF)2 (0.54 g, 2.0 mmol)
in THF (5 mL). The reaction mixture was stirred at room temperature
overnight. The solvent was removed in vacuo, and diethylether (5 mL)
was added. The suspension was centrifuged, and the volume of the
resulting solution was reduced to about 3 mL and stored at ꢀ30 °C for 3
6026
dx.doi.org/10.1021/om2008474 |Organometallics 2011, 30, 6022–6027

Organometallics
ARTICLE
2004. (g) Kuhn, P.; Semeril, D.; Matt, D.; Chetcuti, M. J.; Lutz, P. Dalton
Trans. 2007, 515. (h) Albahily, K.; Licciulli, S.; Gambarotta, S.;
Korobkov, I.; Chevalier, R.; Schuhen, K.; Duchateau, R. Organometallics
2011, 30, 3346.
(6) For a discussion, see, for example: Albahily, K.; Shaikh, Y.;
Ahmed, Z.; Korobkov, I.; Gambarotta, S.; Duchateau, R. Organometallics
2011, 30, 4159.
(7) See, for example: (a) Thapa, I.; Gambarotta, S.; Duchateau, R.;
Kulangara, S. V.; Chevalier, R. Organometallics 2010, 29, 4080. (b)
Temple, C.; Jabri, A.; Crewdson, P.; Gambarotta, S.; Korobkov, I.;
Duchateau, R. Angew. Chem., Int. Ed. 2006, 45, 7050. (c) Albahily, K.;
Al-Baldawi, D.; Savard, D.; Gambarotta, S.; Burchell, T. J.; Duchateau, R.
Angew. Chem., Int. Ed. 2008, 47, 5816. (d) Jabri, A.; Crewdson, P.;
Gambarotta, S.; Korobkov, I.; Duchateau, R. Organometallics 2006,
25, 715. (e) Jabri, A.; Temple, C.; Crewdson, P.; Gambarotta, S.;
Korobkov, I.; Duchateau, R. J. Am. Chem. Soc. 2006, 128, 9238.
(8) (a) Albahily, K.; Koc-, E.; Al-Baldawi, D.; Savard, D.; Gambarotta,
S.; Burchell, T. J.; Duchateau, R. Angew. Chem., Int. Ed. 2008, 47, 5816.
(b) Albahily, K.; Al-Baldawi, D.; Gambarotta, S.; Duchateau, R.; Koc-, E.;
Burchell, T. J. Organometallics 2008, 27, 5708. (c) Albahily, K.; qAl-
Baldawi, D.; Gambarotta, S.; Koc-, E.; Duchateau, R. Organometallics
2008, 27, 5943.
(3) (a) McDermott, J. X.; White, J. F.; Whitesides, G. M. J. Am.
Chem. Soc. 1976, 98, 6521. (b) Manyik, R. M.; Walker, W. E.; Wilson,
T. P. J. Catal. 1977, 47, 197. (c) McDaniel, M. P. Adv. Catal. 1985, 33, 47.
(d) Briggs, J. R. J. Chem. Soc., Chem. Commun. 1989, 674. (e) Meijboom,
N.; Schaverien, C. J.; Orpen, A. G. Organometallics 1990, 9, 774. (f)
Emrich, R.; Heinemann, O.; Jolly, P. W.; Kruger, C.; Verhovnik, G. P. J.
Organometallics 1997, 16, 1511. (g) K€ohn, R. D.; Haufe, M.; Kociok-
K€ohn, G.; Grimm, S.; Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed.
2000, 39, 4337. (h) Wang, M.; Shen, Y.; Qian, M.; Li, R.; He, R.
J. Organomet. Chem. 2000, 599, 143. (i) Carter, A.; Cohen, S. A.; Cooley,
N. A.; Murphy, A.; Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (j)
McGuinness, D. S.; Wasserscheid, P.; Keim, W.; Hu, C.; Englert, U.;
Dixon, J. T.; Grove, C. Chem. Commun. 2003, 334. (k) McGuinness,
D. S.; Wasserscheid, P.; Keim, W.; Morgan, D. H.; Dixon, J. T.;
Bollmann, A.; Maumela, H.; Hess, F. M.; Englert, U. J. Am. Chem. Soc.
2003, 125, 5272. (l) Morgan, D. H.; Schwikkard, H.; Dixon, J. T.; Nair,
J. J.; Hunter, R. Adv. Synth. Catal. 2003, 345, 939. (m) Blok, A. N.;
Budzelaar, P. H. M.; Gal, A. W. Organometallics 2003, 22, 2564. (n) Yu,
Z.; Houk, K. N. Angew. Chem., Int. Ed. 2003, 42, 808. (o) Rensburg, W. J.;
Grove, C.; Steynberg, J. P.; Stark, K. B.; Huyser, J. J.; Steynberg, P. J.
Organometallics 2004, 23, 1207. (p) McGuinness, D. S.; Wasserscheid,
P.; Morgan, D. H.; Dixon, J. T. Organometallics 2005, 24, 552. (q)
Bluhm, M. E.; Walter, O.; D€oring, M. J. Organomet. Chem. 2005,
690, 713. (r) Nenu, C. N.; Weckhuysen, B. M. Chem. Commun.
2005, 1865. (s) Tomov, A. K.; Chirinos, J. J.; Jones, D. J.; Long, R. J.;
Gibson, V. C. J. Am. Chem. Soc. 2005, 127, 10166. (t) Bluhm, M. E.;
Walter, O.; Do, M. J. Organomet. Chem. 2005, 690, 713. (u) Elowe, P. R.;
McCann, C.; Pringle, P. G.; Spitzmesser, S. K.; Bercaw, J. E. Organome-
tallics 2006, 25, 5255. (v) Agapie, T.; Labinger, J. A.; Brecaw, J. E. J. Am.
Chem. Soc. 2007, 129, 14281. (w) McGuinness, D. S.; Suttil, J. A.;
Gardiner, M. G.; Davies, N. W. Organometallics 2008, 27, 4238. (x)
Zhang, J.; Braunstein, P.; Hor, T. S. A. Organometallics 2008, 27, 4277.
(y) Zhang, J.; Li, A.; Hor, T. S. A. Organometallics 2009, 28, 2935. (z) Peitz,
S.; Peulecke, N.; Aluri, B. R.; Hansen, S.; M€uller, B. H.; Spannenberg, A.;
Rosenthal, U.; Al-Hazmi, M. H.; Mosa, F. M.; W€ohl, A.; M€uller, W. Eur. J.
Inorg. Chem. 2010, 1167. (aa) Hey, T. W.; Wass, D. F. Organometallics
2010, 29, 3676. (ab) K€ohn, R. D.; Haufe, M.; Mihan, S.; Lilge, D. Chem.
Commun. 2000, 1927.
(9) (a) Cristau, H.-J.; Garcia, C. Synthesis 1990, 315. (b) Cristau,
H.-J.; Jouanin, I.; Taillefer, M. J. Organomet. Chem. 1999, 584, 68.
(10) Sheldrick, G. M. SHELXTL; Bruker AXS, Madison, WI.
(4) (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, 14712. (b) Han, T.-K.; Ok, M. A.; Chae, S. S.;
Kang, S. O. (SK Energy Corporation), Patent WO 2008/088178, 2008.
(c) Dulai, A.; McMullin, C. L.; Tenza, K.; Wass, D. F. Organometallics
2011, 30, 935. (d) Licciulli, S.; Thapa, I.; Albahily, K.; Korobkov, I.;
Gambarotta, S.; Duchateau, R.; Chevalier, R.; Schuhen, K. Angew. Chem.,
Int. Ed. 2010, 49, 9225.
(5) (a) Bowen, L. E.; Haddow, M. F.; Orpen, A. G.; Wass, D. Dalton
Trans. 2007, 1160. (b) Rucklidge, A. J.; McGuinness, D. S.; Tooze, R. P.;
Slawin, A. M. Z.; Pelletier, J. D. A; Hanton, M. J.; Webb, P. B.
Organometallics 2007, 26, 2782. (c) Jabri, A.; Mason, C. B.; Sim, Y.;
Gambarotta, S.; Burchell, T. J.; Duchateau, R. Angew. Chem., Int. Ed.
2008, 47, 9717. (d) Vidyaratne, I.; Nikiforov, G. B.; Gorelski, S. I.;
Gambarotta, S.; Duchateau, R.; Korobkov, I. Angew. Chem., Int. Ed. 2009,
48, 6522. (e) Dulai, A.; de Bod, H.; Hanton, M. J.; Smith, D. M.;
Downing, S.; Mansell, S. M.; Wass, D. F. Organometallics 2009, 28, 4613.
(f) Albahily, K.; Fomitcheva, V.; Shaikh, Y.; Sebastiao, E.; Gorelsky, S. I.;
Gambarotta, S.; Korobkov, I.; Duchateau, R. Organometallics 2011,
30, 4201. (g) Albahily, K.; Fomitcheva, V.; Gambarotta, S.; Korobkov,
I.; Murugesu, M.; Gorelsky, S. I. J. Am. Chem. Soc. 2011, 133, 6380. (h)
Albahily, K.; Shaikh, Y.; Sebastiao, E.; Gambarotta, S.; Korobkov, I.;
Gorelsky, S. I. J. Am. Chem. Soc. 2011, 133, 6388. (i) Licciulli, S.;
Albahily, K.; Fomitcheva, V.; Korobkov, I.; Gambarotta, S.; Duchateau,
R. Angew. Chem., Int. Ed. 2011, 50, 2346.
6027
dx.doi.org/10.1021/om2008474 |Organometallics 2011, 30, 6022–6027