N-Phosphanyl- and N,N′-Diphosphanyl-Substituted N-Heterocyclic Carbene Chromium Complexes: Synthesis, Structures, and Catalytic Ethylene Oligomerization

Article abstract of DOI:10.1021/acs.organomet.5b00547

The chromium(II) complexes [CrCl₂(^t-BuNHC,P-κC)₂] (1), [CrCl₂(^MesNHC,P-κC)₂] (2), [CrCl₂(^DippNHC,P-κC)₂] (3), and [CrCl₂(P,NHC,P-κC)₂] (4) containing the N-phosphanyl- or N,N′-diphosphanyl-substituted N-heterocyclic carbene (NHC) hybrid ligands ^t-BuNHC,P (1-(di-tert-butylphosphino)-3-tert-butylimidazol-2-ylidene), ^MesNHC,P (1-(di-tert-butylphosphino)-3-mesitylimidazol-2-ylidene), ^DippNHC,P (1-(di-tert-butylphosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene), and P,NHC,P (1,3-bis(di-tert-butylphosphino)imidazol-2-ylidene), respectively, were prepared from Cr^II ([CrCl₂(thf)₂]) or Cr^III ([CrCl₃(thf)₃] or [Cr(Me)Cl₂(thf)₃]) precursors. The solid-state structures of these four complexes show square-planar Cr^II centers, with two trans chloride and two monodentate C_NHC donors. Alkylation of 3 and 4 with [Mg(benzyl)₂(thf)₂] led to the formation of the σ complexes [Cr(benzyl)₃(^DippNHC,P-κC,κP)] (5) and [Cr(benzyl)₃(P,NHC,P-κC,κP)] (6), respectively, with five-coordinate distorted-square-pyramidal Cr^III coordination, comprising a chelating ligand through the C_NHC and one P donor and three benzyl groups. These complexes were used as precatalysts in ethylene oligomerization, and it was found that the nature of the cocatalyst used and the metal oxidation state have a remarkable influence on the catalytic properties. The Cr^III/MAO systems displayed superior catalytic performance (TOF values up to 16320 mol of C₂H₄/((mol of Cr) h) for 6) and gave mostly oligomers. Interestingly, the oligomers obtained with complex 3 were almost exclusively 1-hexene and 1-butene when the reaction was initiated at 30 °C. The overall activities and selectivities were also affected by the initial reaction temperature and the nature of the solvent. With AlEtCl₂ (EADC) as cocatalyst, polyethylene was predominately formed. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00547

Article
pubs.acs.org/Organometallics
N‑Phosphanyl- and N,N′‑Diphosphanyl-Substituted N‑Heterocyclic
Carbene Chromium Complexes: Synthesis, Structures, and Catalytic
Ethylene Oligomerization
Pengfei Ai, Andreas A. Danopoulos,* and Pierre Braunstein*,†
†
,†,‡
†
‡
Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), and Institute for Advanced Study, USIAS,
de Strasbourg, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France
Universite
́
*
S Supporting Information
ABSTRACT: The chromium(II) complexes [CrCl (t‑BuNHC,P-
2
Mes
Dipp
κC) ] (1), [CrCl ( NHC,P-κC) ] (2), [CrCl ( NHC,P-κC)2]
2
2
2
2
(
3), and [CrCl (P,NHC,P-κC) ] (4) containing the N-phosphan-
2 2
yl- or N,N′-diphosphanyl-substituted N-heterocyclic carbene
t‑Bu
(
NHC) hybrid ligands
NHC,P (1-(di-tert-butylphosphino)-3-
Mes
tert-butylimidazol-2-ylidene),
no)-3-mesitylimidazol-2-ylidene),
NHC,P (1-(di-tert-butylphosphi-
Dipp
NHC,P (1-(di-tert-butyl-
phosphino)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene), and
P,NHC,P (1,3-bis(di-tert-butylphosphino)imidazol-2-ylidene), re-
II
III
spectively, were prepared from Cr ([CrCl (thf) ]) or Cr
2
2
(
[CrCl (thf) ] or [Cr(Me)Cl (thf) ]) precursors. The solid-state
3 3 2 3
II
structures of these four complexes show square-planar Cr centers,
with two trans chloride and two monodentate C donors. Alkylation of 3 and 4 with [Mg(benzyl) (thf) ] led to the formation
of the σ complexes [Cr(benzyl)3( NHC,P-κC,κP)] (5) and [Cr(benzyl) (P,NHC,P-κC,κP)] (6), respectively, with five-
NHC
2
2
Dipp
3
III
coordinate distorted-square-pyramidal Cr coordination, comprising a chelating ligand through the C
and one P donor and
NHC
three benzyl groups. These complexes were used as precatalysts in ethylene oligomerization, and it was found that the nature of
III
the cocatalyst used and the metal oxidation state have a remarkable influence on the catalytic properties. The Cr /MAO systems
displayed superior catalytic performance (TOF values up to 16320 mol of C H /((mol of Cr) h) for 6) and gave mostly
2
4
oligomers. Interestingly, the oligomers obtained with complex 3 were almost exclusively 1-hexene and 1-butene when the
reaction was initiated at 30 °C. The overall activities and selectivities were also affected by the initial reaction temperature and the
nature of the solvent. With AlEtCl (EADC) as cocatalyst, polyethylene was predominately formed.
2
1
0
INTRODUCTION
homogeneous catalysis, they have not yet found wide
applications in olefin oligomerization and polymerization
reactions, in particular with electropositive metals. This may
be due to the small metal−NHC bond dissociation energies but
also due to the decomposition of the active species with alkyl−
NHC−metal bonds via alkylimidazolium reductive elimina-
■
Metal-catalyzed transformations for converting ethylene to
linear α-olefins (LAO) or polyethylenes are of continuing
interest, due to the industrial importance and value of the end
1
products. For example, LAOs are highly demanded industrial
intermediates, in particular as comonomers for the production
11
tion. It is conceivable that the side reaction could be
suppressed through the donor functionalization and chelation
of the NHC functionality, by the introduction of steric bulk and
the protection at the NHC, and/or by using early transition
metals that are less susceptible to this type of deactivation (e.g.
due to unavailable oxidation states etc.). Presently, only a few
monodentate, bidentate (N,C; C,S; C,C), and tridentate
(N,C,N; C,N,C; C,S,C) functionalized NHC ligands have
been employed to prepare Cr complexes as precatalysts for
of linear low-density polyethylene (C4−C8), surfactants
(
C12−C20), and plasticizers (C6−C10). Chromium is at the
core of these transformations, being the active center of the
2
3
Phillips and Union Carbide SiO -supported Cr catalyst,
2
responsible for 40%−50% of the annual world production of
4
polyethylene, or of a number of selective homogeneous
oligomerization catalysts affording mainly 1-hexene or 1-octene
5
6
7
8
(
Phillips, Mitsubishi, BP Chemicals, Sasol ). Since the
influence of the Cr oxidation state, the ancillary ligands, and
the cocatalyst on the oligomer selectivity has not yet been fully
elucidated, considerable efforts have been devoted to the
development of well-defined, catalytically active molecular
systems, which lend themselves to mechanistic studies and
11,12
ethylene polymerization or oligomerization.
Among them,
the “pincer” pyridylbis(carbene)chromium(III) complexes of
type A are the most active precatalysts for the oligomerization
5
9a
of ethylene, with activities up to 7.8 × 10 g/((g of Cr) h).
1b,f,9
rational tailoring.
Although N-heterocyclic carbenes (NHCs) have been
Received: June 22, 2015
established as efficient ancillary ligands in diverse topics of
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.5b00547
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
II
III
a
Scheme 1. Synthesis of Cr and Cr Complexes
a
Reagents and conditions: (i) [CrCl (thf) ], [CrCl (thf) ] or
2
2
3
3
[
[
Cr(Me)Cl (thf) ], THF, −78 °C to room temperature; (ii)
2
3
Mg(benzyl) (thf) ] and 1,4-dioxane, THF, −40 °C to room
2
2
temperature; (iii) [CrCl (thf) ], [Mg(benzyl) (thf) ], and 1,4-dioxane,
3
3
2
2
THF, −78 to −40 °C to room temperature.
1
3
Recently, N-phosphanyl-functionalized NHC ligands have
received much attention, since they feature vicinal, strong σ
donors in rigid, easily tunable sterically and electronically
coordination environments. The chemistry that has been
explored to date involves coinage, platinum-group, or late
transition metals, where the ligands adopt bridging or chelating
modes, in the last case giving distorted chelate rings and small
(
(
b e n z y l ) 3 ( D i p p N H C , P - κ C , κ P ) ] ( 5 ) a n d [ C r -
benzyl) (P,NHC,P-κC,κP)] (6), respectively. Whereas sub-
3
stitution of the chlorides by the benzyl moieties was expected,
we do not know at which stage the oxidation of Cr to Cr
II
III
took place. One may speculate that disproportionation of an
1
4
bite angles. In contrast, the coordination chemistry with the
early and 3d metals is unknown.
Of relevance to the origin of the present work were the
versatile ethylene trimerization catalyst PNP-Cr (B, Ar = o-
MeO-Ph), developed by BP Chemicals, and the analogous
ethylene tetramerization catalyst PNP-Cr (B, Ar = Ph),
developed by Sasol, both based on a bis(phosphanyl)amine-
type (PNP) ligand. The recognition that the nature of the
ancillary ligand clearly plays a major role in determining the
led us to consider the
synthesis of complexes with a hybrid ligand containing NHC
and phosphine donor groups, in view of the widely accepted
and the often unique
properties brought about by hybrid ligands. Herein, we report
the synthesis of Cr and Cr complexes based on a family of P-
functionalized NHC ligands of types C and D and their
evaluation in the catalytic oligomerization of ethylene.
II
unstable Cr species has resulted in the formation of 5 and 6.
Subsequently, a more convenient and rational one-pot method
was developed to access 5 and 6, by the reaction of
Dipp
[
CrCl (thf) ] with
NHC,P and P,NHC,P, respectively,
3
3
7
a
and excess [Mg(benzyl) (thf) ] under a carefully controlled
2
2
temperature regime (see the Experimental Section). The
8
c
Dipp
reaction of [Cr(benzyl) (thf) ] with the ligand
NHC,P
3
3
also led to 5, and this procedure could probably be extended to
the other NHC,P ligands. However, the difficulties associated
with the synthesis of [Cr(benzyl) (thf) ] made this route less
1
e−g
catalytic activity and selectivity
3
3
attractive. Isolation and purification of the new benzyl
complexes were carried out by extraction of the complex into
toluene and layering the solution with pentane for crystal-
lization; however, this resulted in lower isolated yields of 5 (8%
based on Cr), which has reduced solubility in toluene, and 6
1
0b,g,15
similarity of their donor abilities
16
II
III
(
56%). Attempts to isolate the complexes by extraction into
more polar solvents, such as THF, gave impure products. The
paramagnetic 5 and 6 are black, extremely air- and moisture-
sensitive crystalline materials that were characterized by single-
crystal X-ray diffraction.
Solid-State Structures of the Cr and Cr Complexes.
The structures of 1−4 are shown in Figures S1 and S2 in the
Supporting Information for 1 and 2 and Figures 1 and 2 for 3
and 4, respectively. Selected bond lengths and angles are given
in Table 1.
RESULTS AND DISCUSSION
■
Synthesis of the Chromium Complexes. The new
complexes [CrCl ( NHC,P-κC) ] (1), [CrCl ( NHC,P-
NHC,P-κC) ] (3), and
CrCl (P,NHC,P-κC) ] (4) were prepared by the addition of
t‑Bu
Mes
II
III
2
2
2
D i p p
κC) ] (2), [CrCl (
2
2
2
[
2 2
t‑Bu
the known N-phosphanyl-substituted NHC ligands NHC,P,
Mes
Dipp
13c,14a,17
NHC,P,
NHC,P, and P,NHC,P, respectively,
to
II
a suspension of 0.5 equiv of [CrCl (thf) ] in THF at −78 °C
In all four complexes, the Cr center, at a center of molecular
2
2
(
Scheme 1).
and crystallographic symmetry, features a perfect square-planar
geometry with mutually trans CNHC and chlorine donors. The
coordination plane and the imidazole ring form a dihedral angle
of 85.51° in 1, 87.37° in 2, 88.30° in 3, and 85.58° in 4. The
They were obtained as purple, paramagnetic, air-sensitive
solids in good yields and characterized by X-ray diffraction (see
below). Initial attempts to use the same methodology to
prepare Cr complexes, i.e. by reacting excess ligand with
III
Cr−C
and Cr−Cl bond distances are in the ranges
NHC
[
CrCl (thf) ] or [Cr(Me)Cl (thf) ], led unexpectedly to the
2.152(6)−2.205(2) and 2.3432(6)−2.3525(5) Å, respectively,
3
3
2
3
III
II
12a,b,19
reduction of Cr to Cr and the isolation of 1−4. Similar redox
and are comparable with those in similar complexes.
No
III
reactions between a Cr complex and N,N′-diisopropylimida-
interaction between the phosphine groups and the chromium
can be deduced on the basis of the observed set of metrical data
and conformations.
zol-2-ylidene have been observed previously, but their
18
mechanisms were not fully elucidated.
Further derivatization of 3 and 4 aiming at the introduction
of reactive Cr−C σ bonds was attempted by the reaction with
The structures of 5 and 6 are shown in Figures 3 and 4,
respectively, and selected bond lengths and angles are given in
Table 2.
[
Mg(benzyl) (thf) ]. This led to the isolation of [Cr-
2 2
B
DOI: 10.1021/acs.organomet.5b00547
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 3. Thermal ellipsoid representation (30% probability level) of
the structure of 5. H atoms are omitted for clarity.
Figure 1. Thermal ellipsoid representation (30% probability level) of
the structure of 3. H atoms are omitted for clarity.
Figure 4. Thermal ellipsoid representation (30% probability level) of
the structure of 6. H atoms are omitted for clarity.
II
the previously described Cr species, the phosphanyl NHC
ligands in 5 and 6 adopt a chelating bonding mode that results
in a strained four-membered ring with an acute bite angle of
Figure 2. Thermal ellipsoid representation (30% probability level) of
the structure of 4. H atoms are omitted for clarity.
6
0.64(3)° and 61.84(11)°, respectively. The sum of the bond
angles within the chelate ring (358.8° for 5, 360° for 6)
indicates planarity. The short-bite nature of the chelating ligand
results in a significant value for the angle θ (Scheme 2), defined
as the angle between the axis going through the carbene donor
atom C1 and the middle of the imidazole CC double bond
and the C1−Cr bond (18.61 and 21.20°, respectively). This
definition is slightly different from that of the yaw distortion
Table 1. Selected Bond Lengths (Å) and Angles (deg) in the
Solid-State Structures of 1−4
1
2
3
4
Cr1−C1
Cr1−Cl1
C1−N1
C1−N2
N2−P1(P2)
N1−P1
C1−Cr1−Cl1
C1−Cr1−Cl1′
Cr1- C1−N1
Cr1- C1−N2
N1−C1−N2
C1−N2−P1(P2)
C1−N1−P1
2.205(2)
2.3525(5)
1.347(2)
1.382(2)
1.739(2)
2.169(2)
2.3432(6)
1.347(3)
1.370(3)
1.755(2)
2.176(4)
2.346(1)
1.351(5)
1.358(5)
1.748(3)
2.152(6)
2.349(2)
1.363(8)
1.370(8)
1.753(5)
1.755(5)
87.2(1)
22
angle used by Crabtree (Scheme 2), which is based on a
symmetrical imidazole ring (although the two N substituents
are not the same), but the θ values obtained according to these
two definitions are almost the same.
The Cr−C
bond distances in 5 and 6 (2.095(5)−
89.97(5)
90.03(5)
133.3(1)
122.6(1)
104.0(2)
120.9(1)
89.98(6)
90.02(6)
133.0(2)
123.0(2)
103.9(2)
115.8(2)
87.40(9)
92.60(9)
131.7(3)
123.1(2)
104.6(3)
117.6(2)
benzyl
18,23
2
.149(1) Å) fall within the expected range
and are
92.8 (2)
127.4(4)
127.5(4)
104.4(5)
116.9(4)
116.8(4)
significantly shorter than the Cr−C
bonds (2.216(1) Å
NHC
for 5 and 2.227(4) Å for 6). In both 5 and 6, the three angles
subtended at Cbenzylic show considerable differences: the C32−
C31−Cr1 angle (107.20(8)° in 5, 99.8(3)° in 6) is more acute
than the other two (C39−C38−Cr1 121.57(8), 124.7(3)° and
C25−C24−Cr1 117.07(9), 118.1(3)° in 5 and 6, respectively).
Even more acute angles of 97.9(2) and 76.6(2)° have been
II
Complexes 5 and 6 constitute rare examples of five-
found recently in a Cr complex of the type [Cr(NHC)-
III
20
coordinate Cr organometallics. In both, the geometry at
(benzyl) ], the angular distortion at the benzylic carbon having
2
III
21
Cr is slightly distorted square pyramidal (τ 0.41 and 0.36 for
and 6, respectively), with the two Cbenzyl (C31 and C38), the
CNHC, and the P donors forming the basal plane. In contrast to
been ascribed to intramolecular polarization-induced metal−
18
5
arene interactions. In the structure of 6 there is no interaction
between the second P(t-Bu) group and the metal. The lone
2
C
DOI: 10.1021/acs.organomet.5b00547
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Organometallics
Article
Table 2. Selected Bond Lengths (Å) and Angles (deg) in the
Solid-State Structures of 5 and 6
pair at P2 is oriented toward the benzylic carbons C24 and
C31.
Catalytic Ethylene Oligomerization and Polymer-
ization. The complexes 1−6 were evaluated as precatalysts
for the oligomerization of ethylene at 10 bar of ethylene with
5
6
Cr1−C1
Cr1−P1
Cr1−C24
Cr1−C31
Cr1−C38
C1−N1
C1−N2
N2−P1
2.216(1)
2.8014(3)
2.100(1)
2.080(1)
2.149(1)
1.358(1)
1.376(1)
1.7517(9)
2.227(4)
2.725(1)
2.095(5)
2.102(4)
2.137(4)
1.371(5)
1.369(5)
1.740(3)
1.775(4)
4
00 equiv of methylaluminoxane (MAO) as cocatalyst. The
results are compiled in Table 3. At 30 °C, 1, 2, and 4 with
MAO as cocatalyst were only poorly active (less than 350 g/((g
III
of Cr) h)) in comparison to the pyridylbis(carbene) Cr
5
9a
species of type A (7.8 × 10 g/((g of Cr) h)) and mostly
polyethylene (PE) was formed (entries 1, 2, and 5 in Table 3).
The 3/MAO system showed higher activity (1040 g/((g of Cr)
h)) and gave smaller amounts of polymers (32 wt %) and
mostly octenes (51%), of which 97% was 1-octene. The C6
fraction (17%) contained almost exclusively 1-hexene; only
very small amounts of 3-cis-hexene and 2-trans-hexene were
found in the liquid phase, but no methylcyclopentane and
methylenecyclopentane, which are usually produced with other
ethylene tetramerization catalysts. When the initial reaction
temperature was set at 80 °C, the 3/MAO system was more
than twice as active (2390 g/((g of Cr) h)) and only oligomers
were produced (entry 4 in Table 3). Under similar conditions,
N1−P2
C1−Cr1−P1
C31−Cr1−C24
C31−Cr1−C38
C31−Cr1−C1
C24−Cr1−C1
C38−Cr1−C1
C31−Cr1−P1
C24−Cr1−P1
C38−Cr1−P1
Cr1−P1−N2
Cr1−C1−N2
Cr1−C38−C39
Cr1−C31−C32
Cr1−C24−C25
C1−N2−P1
60.64(3)
96.41(5)
96.22(5)
96.34(5)
94.99(4)
164.02(4)
139.57(4)
116.98(4)
103.40(4)
77.84(3)
109.58(7)
121.57(8)
107.20(8)
117.07(9)
110.74(7)
61.8(1)
94.6(2)
94.0(2)
100.0(2)
95.4(2)
160.6(2)
139.2(1)
121.8(2)
98.8(2)
79.1(1)
107.5(3)
124.7(3)
99.8(3)
118.1(3)
111.6(3)
120.0(3)
103.9(3)
III
II
the Cr complexes 5 and 6 performed better than the Cr
complexes, with activities of up to 8790 g/((g of Cr) h)) for 6/
24
MAO (entry 7 in Table 3). The molecular mass distribution
of the oligomerization products is of the Schulz−Flory type
with a K value of 0.73 (K = k /(k
+ k
) = (mol of
prop
prop
ch transfer
Cn+2)/(mol of C ); Figure 5). The nature of the ligand N
n
substituent can significantly affect the nature of the products
C1−N1−P2
N1−C1−N2
obtained from catalytic ethylene oligomerization, as found for
103.85(9)
9a
example in Cr complexes with the pyridine bis(carbene) and
21
pyridine bis(imine) ligands. The major oligomer produced by
the 5/MAO system was 1-octene, and 4 wt % of polymer was
formed, whereas in the case of 6/MAO, the major oligomers
were 1-hexene and 1-octene and 9 wt % waxes were produced.
Subsequently, the influence of the reaction conditions on the
catalytic performance of 6/MAO was investigated. The latter
system was found to be still active after 35 min and lasted for
Scheme 2. Distortion Angle θ Resulting from the Short-Bite
Character of the NHC,P Chelating Ligands
a
9
0 min, giving an activity of 5360 g/((g of Cr) h)) (entry 8 in
Table 3). Methylcyclohexane has been reported to improve the
III
catalytic activity of Cr complexes with a P,N donor ligand in
9
e
ethylene oligomerization, whereas an adverse effect was noted
a
In our case (right), the dashed line goes through the middle of the
III
25
recently with Cr systems containing dipyrrole-based ligands.
In our case, using methylcyclohexane as solvent for the 6/MAO
double bond between C4 and C5 and the CNHC atom.
a
Table 3. Oligomerization of Ethylene for Complexes 1−6 with MAO as Cocatalyst
overall selectivity (wt %)
b
c
entry
cat.
C4 (1-C4)
C6 (1-C6)
C8 (1-C8)
C10 (1-C10)
> C10
PE
productivity
TOF
1
2
3
1
2
3
3
4
5
6
6
6
6
1 (97)
0
10 (100)
3 (89)
19 (65)
33 (71)
51 (97)
15 (86)
2 0(96)
18 (84)
15 (91)
16 (90)
16 (90)
16 (87)
0
0
0
0
70
64
32
0
150
190
280
350
0
17 (99)
18 (91)
2 (92)
0
0
1040
2390
310
1930
4440
580
d
4
15 (99)
0
16 (82)
1 (12)
15 (76)
13 (85)
14 (83)
14 (85)
14 (80)
36
2
5
6
7
75
4
10 (99)
14 (95)
16 (100)
15 (100)
12 (99)
15 (91)
16 (95)
18 (95)
17 (95)
15 (84)
38
33
36
37
33
6110
8790
5360
6040
6060
11350
16320
9950
11220
11250
h
9
e
8
f
0
h
9
1
g
1
0
10
a
−5
Conditions unless specified otherwise: temperature 30 °C, 10 bar of C H , reaction time 35 min, 4 × 10 mol of catalyst, solvent 9.33 mL of
2
4
−3
b
c
chlorobenzene and 10.66 mL of the cocatalyst solution (1.5 × 10 M) for 400 equiv of MAO. In (g of C H )/((g of Cr) h). In (mol of C H )/
2
4
2
4
d
e
f
g
h
(
(mol of Cr) h). Temperature 80 °C. Reaction time 90 min. In methylcyclohexane. Temperature 45 °C. Waxes.
D
DOI: 10.1021/acs.organomet.5b00547
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Organometallics
Article
observed when EADC (entries 3, 5, and 7 in Table 4) was used
as a cocatalyst, at variance with the MAO system.
CONCLUSION
■
Motivated by their donor characteristics and bite angles, we
studied the phosphanyl- and diphosphanyl-substituted NHC
t‑Bu
Mes
Dipp
ligands
NHC,P,
NHC,P,
NHC,P, and P,NHC,P as
mimics of the N,N′-bis(diarylphosphino)amines (PNP) used in
Cr complexes for the catalytic oligomerization or polymer-
ization of ethylene. In contrast to the chelating behavior of
bis(phosphanyl)amine-type ligands, the functionalized NHC
ligands behaved as monodentate CNHC donors with dangling,
II
coordination-inactive PR groups in their Cr complexes 1−4,
2
II
III
obtained from either Cr or Cr chlorido precursors. The
mechanism of the redox reaction responsible for the formation
of the Cr complexes has not been elucidated but may be
II
relevant to transformations occurring in situ in catalytic
II
systems. In contrast, benzylation of either well-defined Cr
Figure 5. Oligomer distribution with precatalysts 6 (entry 7 in Table
complexes or [CrCl (THF) ]/ligand mixtures with [Mg-
3
3
2
3) using MAO as cocatalyst.
(benzyl) (thf) ] led to [Cr(NHC)(benzyl) ] species with κ -
2 2 3
phosphanyl-NHC ligand coordination. Upon activation with
MAO, the Cr complexes were active in ethylene oligomeriza-
III
system also had an adverse effect on the activity, which may be
ascribed to the poor solubility of 6 in this solvent, but led to a
decreased formation of waxes (entry 9 in Table 3). After
injection of the catalyst and cocatalyst, 6 consumed ethylene
very slowly during the first 7 min. Acceleration of ethylene
uptake only started when the reaction temperature reached 45
tion, displaying catalytic performances superior to those of the
II
Cr precursors. However, it is noteworthy that the oligomers
obtained with complex 3 were almost exclusively 1-hexene and
1-butene when the reaction was initiated at 30 °C. At 80 °C, a
broader mass distribution of oligomers was observed (from C4
to >C10) and polyethylene formation was suppressed. The use
of EADC as cocatalyst led predominantly to the formation of
polymers. The tri- and tetramerization observed in some cases
validates further the idea that NHC-P can mimic the PNP
ligands, although each ligand family possesses its own specific
features. The significant differences in activities and selectivities
observed with catalytic species generated from seemingly
similar ligand designs demonstrate further the subtlety of the
parameters that determine the catalytic features of metal
complexes.
°
C. Subsequently, the 6/MAO system was tested at an initial
temperature of 45 °C, but this resulted in a decreased activity
entry 10 in Table 3). This could be attributed to the rapid
(
formation of polymers instead of waxes, which can encapsulate
the catalyst and hamper further reaction.
The use of AlEtCl (EADC) as cocatalyst for all of the
2
complexes (1−6) led predominantly to the formation of
polyethylene and less than 5% of oligomers (Table 4). When
Table 4. Polymerization of Ethylene for Complexes 1−4 and
a
6
with AlEtCl as Cocatalyst
2
b
c
EXPERIMENTAL SECTION
entry cat. amt of PE (g) amt of PE (wt %) productivity
TOF
■
1
2
3
4
5
6
7
1
2
3
3
4
4
6
0.36
1.37
2.09
0.26
2.05
0.49
1.64
100
98
380
1370
1650
490
710
2540
3060
910
General Methods. All manipulations involving organometallics
were performed under nitrogen or argon in a Braun glovebox or using
standard Schlenk techniques. Solvents were dried using standard
procedures and distilled under nitrogen prior to use or passed through
columns of activated alumina and subsequently purged with nitrogen
100
96
d
e
97
1690
460
3140
850
t‑Bu
Mes
Dipp
or argon. The ligands
NHC,P,
NHC,P,
NHC,P, and
95
13c,14a,17
P,NHC,P were prepared according to the literature.
[Mg-
benzyl) (THF) ] was prepared as described, and [CrCl (THF) ]
100
1670
3100
26
(
2
2
2
2
a
Conditions unless specified otherwise: temperature 30 °C, 10 bar of
and [CrCl (THF) ] were prepared by continuous Soxhlet extraction
3 3
−5
C H , time 35 min, 4 × 10 mol of catalyst, solvent 10 mL of
of commercial anhydrous CrCl with THF under argon for 24−48 h.
2
4
2
−
5
chlorobenzene and 5 mL of the cocatalyst solution (8 × 10 M) for
Synthesis of the Chromium(II) Complexes 1−4. Method a. To
b
c
1
0 equiv of AlEtCl2. In (g of C H )/((g of Cr) h). In (mol of
C H )/((mol of Cr) h). Temperature 80 °C. 1.5 mL of the
cocatalyst solution (8 × 10 M) for 3 equiv of AlEtCl and 13.5 mL of
a supension of [CrCl (THF) ] (0.133 g, 0.50 mmol) in THF (8 mL)
2
4
2
2
d
e
2
4
was added a solution of the corresponding ligand (1.05 mmol) in THF
(8 mL) at −78 °C. The reaction mixture was warmed slowly to room
temperature and stirred overnight. After evaporation of the solvent
under reduced pressure, the residue was washed with pentane (5 mL)
to give a purple, air-sensitive powder. X-ray-quality crystals were
obtained by slow diffusion of pentane into THF solution of the
corresponding complex.
−
5
2
chlorobenzene.
II
the R group was changed from t-Bu to P(t-Bu) in the Cr
complexes 1−4, the activity gradually increased from 380 to
690 g/((g of Cr) h). Increasing the reaction temperature to 80
C for 3/EADC (entry 4 in Table 4) and reducing the amount
of cocatalyst (3 equiv) for 4/EADC (entry 6 in Table 4) led to
a dramatic decrease in activity. No obvious differences between
2
Complex 1: yield 0.274 g, 83%. Anal. Found (calcd for
1
°
C H Cl CrN P ): C, 54.97 (54.62); H, 8.97 (8.86); N, 8.18 (8.49).
30
58
2
4 2
Complex 2: yield 0.337 g, 86%. Anal. Found (calcd for
C H Cl CrN P ): C, 61.22 (61.29); H, 8.15 (7.97); N, 6.64 (7.15).
40
62
2
4 2
Complex 3: yield 0.380 g, 88%. Anal. Found (calcd for
C H Cl CrN P ): C, 63.52 (63.65); H, 8.73 (8.59); N, 6.37 (6.46).
III
II
the Cr complex 6 and the Cr complexes 3 and 4 were
46
74
2
4 2
E
DOI: 10.1021/acs.organomet.5b00547
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Complex 4: yield 0.309 g, 74%. Anal. Found (calcd for
slowly rise to 18 °C. The gaseous phase was then transferred into a 10
L polyethylene tank filled with water. An aliquot of this gaseous phase
was transferred into a Schlenk flask, previously evacuated, for GC
analysis. The amount of ethylene consumed was thus determined by
differential weighing of the ethylene bottle (accuracy of the scale 0.1
g). From this amount of ethylene was subtracted the remaining
ethylene (calculated using the GC analysis) in the gaseous phase.
Although this method is of limited accuracy, it was used throughout
and gave satisfactory reproducibility. The reaction mixture in the
reactor was quenched in situ by the addition of ethanol (1 mL),
transferred into a Schlenk flask, and separated from the metal
complexes by trap-to-trap evaporation (20 °C, 0.8 mbar) into a second
Schlenk flask previously immersed in liquid nitrogen in order to avoid
loss of product for GC analysis. Each catalytic test was performed
twice to ensure the reproducibility of the results. For GC analyses, 1-
heptene was used as an internal reference.
C H Cl CrN P ): C, 54.35 (54.60); H, 9.03 (9.16); N, 6.68 (6.70).
3
8
76
2
4 4
Method b. To a suspension of [CrCl (THF) ] (0.093 g, 0.25
3
3
mmol) or [Cr(Me)Cl (THF) ] (0.088 g, 0.25 mmol) in THF (8 mL)
2
3
was added a solution of the corresponding ligand in excess (0.70
mmol) in THF (8 mL) at −78 °C. The reaction mixture was warmed
slowly to room temperature and stirred for 4 h. The resulting brown
suspension was filtered, and layering the filtrate with pentane afforded
crystals of the corresponding Cr(II) complex 1−4.
Synthesis of Chromium(III) Complexes 5 and 6. Method a.
Solid [CrCl (THF) ] (0.068 g, 0.183 mmol), the corresponding
3
3
carbene ligand (0.210 mmol), and [Mg(benzyl) (THF) ] (0.150 g,
2
2
0
.428 mmol) were placed in a Schlenk tube in the glovebox. The tube
was connected to the vacuum line and cooled to −78 °C. At this
temperature, precooled THF (15 mL) and dioxane (2 mL) were
added and the reaction mixture was stirred at −40 °C for 1 h. Then
the dark brown reaction mixture was warmed to room temperature
and was further stirred for 1 h. After evaporation of the solvent under
reduced pressure, the residue was extracted with toluene (30 mL) and
the solution was filtered. The filtrate was layered with pentane (60
mL) for 3 days to give a black and air-sensitive crystalline material. X-
ray-quality crystals were obtained by slow diffusion of pentane into a
dilute toluene solution of the complex.
X-ray Crystallography. A summary of the crystal data, data
collection and refinement for structures of 1−6 are given in Table S1
in the Supporting Information. For 1 and 3−5, X-ray diffraction data
collection was carried out on a Bruker APEX II DUO Kappa-CCD
diffractometer equipped with an Oxford Cryosystem liquid N
using Mo Kα radiation (λ = 0.71073 Å). The crystal−detector distance
device,
2
27
was 38 mm. The cell parameters were determined (APEX2 software)
Complex 5: yield 0.010 g, 8% based on Cr.
from reflections taken from 3 sets of 12 frames, each at 10 s exposure.
Complex 6: yield 0.070 g, 56% based on Cr. Due to the extreme
sensitivity of 5 and 6, satisfactory elemental analyses could not be
obtained.
The structure was solved by direct methods using the program
2
8
SHELXS-97. The refinement and all further calculations were carried
2
9
out using SHELXL-97. The H atoms were included in calculated
positions and treated as riding atoms using SHELXL default
parameters. The non-H atoms were refined anisotropically, using
Method b. To a solution of 3 (0.174 g, 0.200 mmol) or a
suspension of 4 (0.167 g, 0.200 mmol) in THF (10 mL) was added
dioxane (2 mL), and the reaction mixture was cooled to −40 °C. At
this temperature, a precooled solution of [Mg(benzyl) (THF) ]
2
weighted full-matrix least squares on F . A semiempirical absorption
2
7
correction was applied using SADABS in APEX2.
2
2
(
0.210 g, 0.60 mmol) in THF (6 mL) was added and the reaction
For 2 and 6, X-ray diffraction data collection was carried out on a
Nonius Kappa-CCD diffractometer equipped with an Oxford
mixture was stirred at −40 °C for 1 h. The dark brown reaction
mixture was then warmed to room temperature and was further stirred
for 1 h. After evaporation of the solvent under reduced pressure, the
residue was extracted with toluene (40 mL) and the solution filtered.
The filtrate was layered with pentane (60 mL) for 3 days to give a
black, air-sensitive crystalline material.
Cryosystem liquid N device, using Mo Kα radiation (λ = 0.71073
2
Å). The crystal−detector distance was 36 mm. The cell parameters
3
0
were determined (Denzo software) from reflections taken from one
set of 10 frames (1.0° steps in ψ angle), each at 20 s exposure. The
structures were solved by direct methods using the program SHELXS-
2
8
Complex 5: yield 0.008 g, 6% based on Cr.
Complex 6: yield 0.030 g, 22% based on Cr.
General Procedure for the Catalytic Ethylene Oligomeriza-
97. The refinement and all further calculations were carried out using
29
SHELXL-97. The H atoms were included in calculated positions and
treated as riding atoms using SHELXL default parameters. The non-H
atoms were refined anisotropically, using weighted full-matrix least
tion. All catalytic reactions were performed in a magnetically stirred
2
(1200 rpm) 145 mL stainless steel autoclave. A 125 mL glass container
squares on F . A semiempirical absorption correction was applied
3
1
was used to avoid corrosion of the autoclave walls. The preparation of
the catalytic solution of the precatalyst was dependent on the nature
and the amount of the cocatalyst.
using MULscanABS in PLATON.
−5
ASSOCIATED CONTENT
■
* Supporting Information
With MAO, 4 × 10 mol of Cr complex was dissolved in 9.33 mL
of chlorobenzene or methylcyclohexane and the solution was injected
into the reactor under an ethylene flux. Then 8 mL of a 1.5 × 10
cocatalyst toluene solution, corresponding to 400 equiv of MAO, was
added to the reactor to reach a total volume of 20 mL with the
precatalyst solution.
S
−3
M
This material is also available free of charge via the Internet at
http://pubs.acs.org/. The Supporting Information is available
free of charge on the ACS Publications website at DOI:
10.1021/acs.organomet.5b00547. The Crystallographic infor-
mation files (CIF) have been deposited with the CCDC, 12
Union Road, Cambridge CB2 1EZ, U.K., and can be obtained
on request free of charge, by quoting the publication citation
and deposition numbers 1055511−1055516.
−5
With AlEtCl (EADC), 4 × 10 mol of Cr complex was dissolved
2
in 13.5 or 10 mL in chlorobenzene and the solution was injected into
the reactor under an ethylene flux, followed by 1.5 or 5 mL of a 8 ×
1
respectively. The total volume of the solution inside the reactor was 15
mL.
The catalytic reaction was started at 30, 45, or 80 °C. No cooling of
the reactor was applied during the reaction. After injection of the
catalyst and cocatalyst solutions under a constant low flow of ethylene,
which is considered as the t0 time, the reactor was immediately
pressurized to 10 bar of ethylene. The temperature increased, owing
solely to the exothermicity of the reaction. The 10 bar working
pressure was maintained through a continuous feed of ethylene from a
bottle placed on a balance to allow continuous monitoring of the
ethylene uptake. At the end of each test (35 or 90 min to facilitate
−
5
0
M cocatalyst solution corresponding to 3 or 10 equiv of EADC,
Structures of 1 and 2 and crystal data for 1−6 (PDF)
Crystal data for 1−6 (CIF)
AUTHOR INFORMATION
■
Corresponding Authors
*
*
E-mail for A.A.D.: danopoulos@unistra.fr.
E-mail for P.B.: braunstein@unistra.fr.
1c,9d
comparisons with previous studies from this laboratory
bath was used to rapidly cool the reactor. When the inner temperature
), a dry ice
Notes
reached 0 °C, the ice bath was removed, allowing the temperature to
The authors declare no competing financial interest.
F
DOI: 10.1021/acs.organomet.5b00547
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
M.; Maumela, H.; Morgan, D. H.; Bollmann, A. (Sasol Technology
ACKNOWLEDGMENTS
■
(Pty) Ltd) WO2003053890, 2003.
This paper is dedicated to Professor Ekkehardt Hahn on the
occasion of his 60th birthday, with our sincere congratulations
(9) (a) McGuinness, D. S.; Gibson, V. C.; Wass, D. F.; Steed, J. W. J.
Am. Chem. Soc. 2003, 125, 12716−12717. (b) Wass, D. F. Dalton
Trans. 2007, 816−819. (c) McGuinness, D. S.; Suttil, J. A.; Gardiner,
M. G.; Davies, N. W. Organometallics 2008, 27, 4238−4247. (d) Liu,
S.; Pattacini, R.; Braunstein, P. Organometallics 2011, 30, 3549−3558.
and best wishes. The USIAS, CNRS, Reg
Communaute Urbaine de Strasbourg are acknowledged for the
award of fellowships and a Gutenberg Excellence Chair (2010−
1) to A.A.D. We are grateful to the China Scholarship Council
́
ion Alsace, and
́
(e) Yang, Y.; Gurnham, J.; Liu, B.; Duchateau, R.; Gambarotta, S.;
1
Korobkov, I. Organometallics 2014, 33, 5749−5757. (f) Yang, Y.; Liu,
for a Ph.D. grant to P.A. We thank Marc Mermillon-Fournier
for technical assistance, the CNRS and the MESR (Paris) for
funding, and the Service de Radiocrystallographie (UdS) for the
determination of the crystal structures. We thank Drs. H.
Olivier-Bourbigou and P.-A. R. Breuil (IFP Energies nouvelles,
Solaize) for discussions and the reviewers for valuable
comments.
Z.; Cheng, R.; He, X.; Liu, B. Organometallics 2014, 33, 2599−2607.
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McGuinness, D. S.; Wierenga, T.; Young, C. ACS Catal. 2015, 5,
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