Chromium complexes with acenaphthene imine derivative ligands synthesis and catalysis on diene polymerization

Article abstract of DOI:10.1021/om200733e

Treatment of the bis(arylimino)acenaphthene (BIAN) ligands with CrCl ₂(THF)₂ in THF affords mononuclear complexes of ^ArBIANCrCl₂(THF)₂ (3a, Ar = 2- ⁱPrC₆H₄; 3b, Ar = 2

Full text of DOI:10.1021/om200733e

ARTICLE
pubs.acs.org/Organometallics
Chromium Complexes with Acenaphthene Imine Derivative Ligands
Synthesis and Catalysis on Diene Polymerization
Bo Gao, Wei Gao,* Qiaolin Wu, Xuyang Luo, Jingshun Zhang, Qing Su, and Ying Mu*
State Key Laboratory for Supramolecular Structure and Materials, School of Chemistry, Jilin University, 2699 Qianjin Street,
Changchun 130012, People’s Republic of China
S Supporting Information
b
ABSTRACT: Treatment of the bis(arylimino)acenaphthene
(BIAN) ligands with CrCl2(THF)2 in THF affords mononuclear
complexes of ArBIANCrCl2(THF)2 (3a, Ar = 2-iPrC6H4; 3b, Ar
= 2,6-Me2C6H3) and chlorine-bridging dinuclear complexes
ArBIANCrCl(μ-Cl)3Cr(THF)ArBIAN (3c, Ar = 2,6-Et2C6H3;
3d, Ar = 2,6-iPr2C6H3). The molecular structures of 3bꢀ3d were
characterized by X-ray diffraction analysis, and all the chromium atoms are in octahedral geometries. Similar reactions of
N-(arylimino)acenaphthenones with CrCl2(THF)2 afford the dinuclear chromium complexes with 1,10-bis(2-aryliminoace-
naphthene-1-olate) tetradentate ligands ArBIAOCr2Cl4(THF)2 (4b, Ar = 2,6-Me2C6H3; 4c, Ar = 2,6-Et2C6H3) via a pinacol
cross-coupling reaction. Upon activation with MAO, these complexes show moderate-to-high activities in butadiene and isoprene
polymerization, affording a cis-1,4-enriched polymer.
In the past decades, the bis(arylimino)acenaphthene (BIAN)
ligands have received considerable attention in coordination
chemistry and were widely used in catalysis, such as hydrogena-
tion of alkynes,17 CꢀC18 and CꢀSn19 bond formation, and
especially in olefin oligomerization/polymerization.20 BIAN has
the unique properties of combining both the diimine and the
naphthalene p systems and can act as stronger σ-donors toward
the metal. In addition to being used as neutral ligands, BIAN
ligands might form stable mono- and di- and possibly even tri-
and tetra-anions by electron transfer to their mixed 14-electron
imine-naphthalene p system.21 Some transition-metal complexes
supported by BIAN ligands in radical-anion or enediamine form
were extensively investigated.22 Recently, Fedushkin has re-
ported the dinuclear chromium complexes that were prepared
by reaction of the CrCl3 with the Ar-BIAN enediamide magne-
sium and, subsequently, elimination of the MgCl2. The bisligated
chromium complexes can be prepared by reaction of (dpp-
BIAN)Na2 with CrCl3.23 In these reactions, the CrCl3 was
reduced to Cr(II) by the enediamide ligands. It is reported that
the 1,4-diazabutadiene (DAB) can be reduced by CrCl2, forming
Cr(III) complexes bearing radical-anion ligands.24 So far no such
Cr(III) BIAN complex and reactivity was reported. Moreover,
despite the extensive studies of the complexes based on diimine
BIAN ligands, the complexes with monoimine ligands still remain
less investigated, although the ligands were reported previously.25
In this regard, herein we wish to report the synthesis of radical-
anion BIAN Cr(III) complexes by the one-pot reaction of CrCl2-
(THF)2 with BIAN ligands. Similar reactions of CrCl2(THF)2 with
monoimine ligands afford dinuclear chromium complexes with
NNOO tetradentate ligands via the pinacol cross-coupling reaction.
1. INTRODUCTION
Polybutadiene (PB) and polyisoprene (PIP) have been the
most widely used synthetic rubbers.1 In the 1950s, the diene
polymers were produced with Alfin catalysts, and the polymer
features a high molecular weight and low regularity.2 The
high regularity of the polymer was achieved by stereospecific
polymerization of conjugated dienes with transition-metal-based
coordination catalysts.3 Homogeneous ZieglerꢀNatta catalyst
systems composed of complexes of rare earth metals4 or transi-
tion metals, such as Ti,5,7d V,6 Fe,7 Co,8,7d and Ni,9,7d and the
coactivators aluminum alkyls or the aluminum alkyl chloride have
been extensively investigated. The chromium-based catalysts have
received special attention due to their specific polymerization of
dienes.10 Some chromium(III) systems, such as Cr(acac)3/AlEt3,
Cr(allyl)3/MAO, Cr(allyl)2Cl/MAO, and Cr(acac)3/MAO, have
been reported to polymerize butadiene to PB with predominant
1,2- or cis-1,4 units.5f,6a,11 The chromium(III) complexes
with bis(pyrazolyl)pyridine12 and bis(benzimidazolyl)amine13
exhibit high activities and high trans-1,4 selectivities in butadiene
polymerization. Some Cr(II) complexes supported by phosphine
ligands, such as CrCl2(dmpe)2 (dmpe = 1,2-bis(dimethyl-
phosphino)ethane)14 and Cr(CH3)2(dmpe)2,14 were found
to show high catalytic activity for butadiene polymerization
in the presence of MAO, affording 1,2-enriched polybutadiene.
The isoprene polymerization initiated by chromium complexes
14
is rare so far. Some Cr-allyl11bꢀd derivatives and CrCl2(dmpe)2
were known to show low activities for isoprene polymerization,
producing PIP with predominant 3,4-units, and some chromium
complexes supported by N,N-bis(diarylphosphino)amine15 ex-
hibit moderate activitiesfor isoprene trimerization. More recently,
we have reported that NCN pincer Cr(III) complexes activated
by alkylaluminum and borate show high activities in isoprene
polymerization, affording a trans-1,4-enriched polymer.16
Received: August 4, 2011
Published: September 26, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Route for the BIAN Chromium(III) Complexes
Figure 1. Perspective view of 3b with thermal ellipsoids drawn at
the 30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity. The selected bond lengths (Å) and angles (deg):
Cr(1)ꢀCl(1) 2.3255(13), Cr(1)ꢀCl(2) 2.3346(12), Cr(1)ꢀN(1)
2.013(3), Cr(1)ꢀN(2) 2.025(3), Cr(1)ꢀO(1) 2.110(3), Cr(1)ꢀO(2)
2.124(3), C(1)ꢀN(1) 1.331(5), C(12)ꢀN(2) 1.335(5), C(1)ꢀC(12)
1.419(6); Cl(1)ꢀCr(1)ꢀCl2 172.27(5), N(1)ꢀCr(1)ꢀN(2) 81.96-
(14), O(1)ꢀCr(2)ꢀO(2) 91.44(12).
Figure 2. Perspective view of 3c with thermal ellipsoids drawn at the
30% probability level. Hydrogens are omitted for clarity. The selected
bond lengths (Å) and angles (deg): Cr(1)ꢀN(1) 2.006(5), Cr(1)ꢀN-
(2) 2.001(5), Cr(1)ꢀCl(1) 2.3978(14), Cr(1)ꢀCl(2) 2.3682(13),
Cr(1)ꢀCl(3) 2.3789(14), Cr(2)ꢀN(3) 2.000(4), Cr(2)ꢀN(4) 2.002-
(4), Cr(2)ꢀCl(2) 2.4645(13), Cr(2)ꢀCl(3) 2.4184(14), Cr(2)ꢀCl-
(4) 2.2902(13), Cr(1)ꢀO(1) 2.075(3); N(1)ꢀCr(1)ꢀN(2) 82.5(2),
N(3)ꢀCr(2)ꢀN(4) 82.78(16).
The catalytic behaviors of these complexes for diene polymerization
were also investigated.
the selected bond distances and angles. The crystallographic data
is summarized in STable 1 (in the Supporting Information).
X-ray structural analysis reveals that 3b was a mononuclear
complex coordinated with a BIAN ligand, two chlorides, and two
THF molecules. The geometries around the chromium atom can
be best described as octahedral with the two chlorides arranged in
trans positions (bite angle = 172.27(5)°). The two CrꢀCl bond
lengths are almost the same length (2.3255(13) and 2.3346(12)
Å), which are similar to those of 2.2915(8)ꢀ2.3205(7) Å in the
NCN-pincer Cr(III) complexes16 and those of 2.2841(11)ꢀ
2.3063(12) Å in bis(oxazolinyl)pyridine chromium(III) com-
plexes.27 The Cr atoms are essentially coplanar with the BIAN
coordination plane with the deviation of 0.2892(37) Å. The
NꢀCr bond lengths of 2.013(3) and 2.025(3) Å are comparable
to those of 1.9873(3)ꢀ2.158(3) Å in the 2-imino-1,10-phenan-
throline chromium complexes28 and those of 1.981(3)ꢀ2.141(3) Å
in bis(imino)pyridyl chromium(III) complexes.29 Careful inspec-
tion of the bond lengths within the BIAN moiety is a useful method
for evaluating the reduction state of the ligands and the value state of
the central chromium atoms.23 The bond lengths of C(1)ꢀN(1)
and C(12)ꢀN(2) in 3b (1.331(5) and 1.335(5) Å) are very
similar to that in the Cr(II) complex with the radical-anion BIAN
ligand23 and lie in the range between that (1.281(5)ꢀ1.284(5) Å)
’ RESULTS AND DISCUSSION
The BIAN ligands were efficiently synthesized according to the
literature.26 Refluxing the acenaphthenequinone with the corre-
sponding substituted aniline in acetonitrile and acetic acid mixed
solution afforded the corresponding BIAN ligands in moderate
yields. The monoimine ligands were synthesized by treatment of
the acenaphthenequinone with substituted aniline in ethanol and
catalytic formic acid.25
Initial attempts to synthesize the simple Cr(III) complexes
with neutral BIAN ligands by stirring the CrCl3(THF)3 and
ligands in various solvents failed. No obvious color change was
found, and the ligand was recovered in high yield. Reactions of
the BIAN ligands (1a and 1b) with the CrCl2(THF)2 in THF at
room temperature result in immediate changes of color form
orange to deep brown. Evaporation of the solvent and recrys-
tallization of the residue in THF/hexane mixed solution afforded
3a and 3b as a brown solid in moderate-to-high yield (Scheme 1).
No informative NMR spectra of these complexes were obtained
due to their paramagnetic nature. Crystals of 3b suitable for X-ray
diffraction determination were grown from THF/hexane solu-
tion. The molecular structure is shown in Figure 1 together with
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Figure 3. Perspective view of 3d with thermal ellipsoids drawn at
the 30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity. The selected bond lengths (Å) and angles (deg):
Cr(1)ꢀN(1) 1.999(5), Cr(1)ꢀN(2) 2.009(5), Cr(1)ꢀCl(1) 2.4002-
(19), Cr(1)ꢀCl(2) 2.3768(17), Cr(1)ꢀCl(3) 2.3392(19), Cr(2)ꢀN-
(3) 2.016(5), Cr(2)ꢀN(4) 1.990(5), Cr(2)ꢀCl(2) 2.4288(19), Cr-
(2)ꢀCl(3) 2.4298(18), Cr(2)ꢀCl(4) 2.2946(19), Cr(1)ꢀO(1)
2.092(5); N(1)ꢀCr(1)ꢀN(2) 82.3(2), N(3)ꢀCr(2)ꢀN(4) 83.3(2).
Figure 4. Perspective view of 4b with thermal ellipsoids drawn at
the 30% probability level. Hydrogens and uncoordinated solvent are
omitted for clarity. The selected bond lengths (Å) and angles (deg):
Cr(1)ꢀCl(1) 2.3206(18), Cr(1)ꢀCl(2) 2.3355(17), Cr(1)ꢀO(1)
2.019(4), Cr(1)ꢀO(2) 1.931(4), Cr(1)ꢀN(1) 2.070(5), Cr(1)ꢀO(3)
2.036(4), Cr(2)ꢀCl(2) 2.3833(17), Cr(2)ꢀCl(3) 2.3029(18), Cr(2)ꢀ
Cl(4) 2.2892(18), Cr(2)ꢀO(1) 2.015(4), Cr(2)ꢀN(2) 2.133(5),
Cr(2)ꢀO(4) 2.044(4); Cl(1)ꢀCr(1)ꢀCl(2) 91.41(6), Cl(3)ꢀCr(2)ꢀ
Cl(4) 90.76(7), N(1)ꢀCr(1)ꢀO(1) 80.71(17), N(2)ꢀCr(2)ꢀO(1)
88.72(17).
for neutral BIAN complexes30 and that (1.401(6)ꢀ1.378(7) Å) for
the enediamide complex.22f The C(1)ꢀC(12) bond distance of
1.419(6) Å is also similar to that of 1.418(4) Å in the radical-anion
BIAN chromium(II) complex.23 All these structural features strongly
suggest that the Ar-BIAN in 3b was reduced by the Cr(II), forming
the radical-anion ligand, and the central chromium atoms should be
Cr(III).24
Scheme 2. Pinacol Cross-Coupling Reaction of Monoamine
Ligands with CrCl2(THF)2
Complexes 3c and 3d with more steric-demanding ligands
were also prepared in a similar procedure as brown powders.
Crystals of 3c and 3d suitable for an X-ray structure determina-
tion were grown from THF solution. The molecular structures of
3c and 3d are depicted in Figures 2 and 3 together with the
selected bond lengths and angles. 3c and 3d are unsymmetrical
chlorine-bridging dinuclear complexes with structures similar to
that of [(iPrDBA)Cr]2(μ-Cl)3(Cl)(THF),24 although the whole
structure of 3c is in a somewhat bent model and 3d is a little
twisted. The bulky ortho substitutes favor the formation of
dinuclear complexes completing the coordination sphere with
less bulky bridging chlorine atoms. In both complexes, the
chromium atoms are six-coordinated and exhibit octahedral
geometries. The bond distances of bridging chlorides to Cr(1)
of 2.3682(13)ꢀ2.3978(14) Å in 3c and 2.3392(19)ꢀ2.4002-
(19) Å in 3d are slightly shorter than that to Cr(2) (2.4184-
(14)ꢀ2.4645(13) Å in 3c and 2.4288(19)ꢀ2.4390(19) Å in 3d).
The terminal CrꢀC1 bond lengths (2.2902(13) Å in 3c and
2.2946(19) Å in 3d) are shorter than exception, but also similar
to that of 2.279(1) Å in [(iPrDBA)Cr]2(μ-Cl)3(Cl)(THF).24 All
the structural characters within the BIAN moieties in 3c and 3d
are comparable to those in 3b and also consistent with the
radical-anion nature of the ligands.
determination were grown from a dichloromethane and hexane
mixed solution. The molecular structures are shown in Figure 4.
The structure reveals that, in the reaction, the two monoimine
ligands were reductive dimerized in a piancol cross-coupling
mechanism, forming a new dinuclear chromium(III) complex
bearing [NOON]2ꢀ tetradentate ligands. Similar pinacol cross-cou-
pling reactions of two carbonyl compounds can be catalyzed by
Sm(II),31a Al,31bꢀd Mg,31d Zn/ZnCl2,31e,f Zn/InCl3,31h and Al/
InCl331g systems. Some CrCl2-based catalytic systems also show
excellent activities in aliphatic carbonyl pinacol cross-coupling
reactions.32 It is well known that the first step of the piancol
Intrigued by the successful preparation of the Cr(III) BIAN
complexes by the simple one-pot reaction of the BIAN ligands
with CrCl2(THF)2, we set out to investigate the reaction of
CrCl2(THF)2 with monoimine ligands. Stirring the mixture of
monoimine ligands and CrCl2(THF)2 in CH2Cl2, evaporation of
the solvent, and recrystallization of the residue in CH2Cl2/
hexane mixed solution afforded complexes 4b and 4c in high
yields as a green solid. Crystals of 4b suitable for an X-ray structure
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Table 1. Polymerization of Isoprene and Butadiene under Various Conditionsa
microstructurec (%)
entry
cat
monomer
Tp (°C)
Al/Cr
time (min)
conv (%)
Mnb (ꢁ104)
PDI
cis-1,4-
3,4- or 1,2-
1
2
3a
3b
3c
3d
4b
4c
3d
3d
3d
3a
3b
3c
3d
4b
4c
3d
3d
3d
IP
25
25
500
500
500
500
500
500
200
800
1000
500
500
500
500
500
500
500
500
500
30
30
95.7
73.8
77.9
82.6
60.0
63.8
48.1
88.2
89.6
91.7
89.5
88.8
90.2
86.3
81.7
33.4
30.8
24.2
0.86
3.04
1.86
7.65
0.71
3.41
9.84
7.83
4.42
0.42
0.70
1.06
1.69
0.20
0.58
1.84
1.10
0.85
1.53
1.72
1.63
1.20
3.62
2.57
1.07
1.13
1.39
2.77
2.35
1.90
1.61
1.81
1.72
1.29
1.15
1.13
56.2
68.9
73.4
80.0
79.8
79.0
81.0
76.4
77.6
70.4
77.9
82.9
89.3
87.0
88.5
92.1
94.3
95.2
43.8
31.1
26.6
20.0
21.2
21.0
19.0
23.6
22.4
29.6
22.1
17.1
10.7
13.0
11.5
7.9
IP
3
IP
25
30
4
IP
25
30
5
IP
25
30
6
IP
25
30
7
IP
25
30
8
IP
25
30
9
IP
25
30
10
11
12
13
14
15
16
17
18
BD
BD
BD
BD
BD
BD
BD
BD
BD
25
30
25
30
25
30
25
30
25
30
25
30
0
360
360
360
ꢀ20
ꢀ40
5.7
4.8
a The polymerization reactions: toluene (10 mL), cat (20 μmol based on Cr), isoprene (10 mmol), monomer/Cr = 500. b Determined by gel permeation
chromatography (GPC) with respect to a polystyrene standard. c Determined by NMR spectrum and IR.
cross-coupling reaction is the generation of a radical enolate chro-
mium complex formed via transfer of one electron from Cr(II) to
aldehyde or keton.32,33 In our case, the conjugated frames of the
ligand favor the formation of radical species, and we assume that a
radical-anion intermediate complex (see Scheme 2) was formed
first, which undergoes a fast intermolecular cross-coupling reac-
tion. Similar radical-anion chromium complexes can be stabilized
by phenanthrene monoimine ligands. Figure 4 shows that the two
anolate oxygen atoms are arranged in a trans position with respect
to the two acenaphthene moieties and both chromium atoms are
six-coordinated and the geometry around the chromium atoms
could be described as a distorted octahedron. The environments
aroundCr(1) and Cr(2) arealmostthe sameexceptthatthe Cr(1)
coordinated with an additional terminal anolate oxygen atom,
while Cr(2) coordinated with anadditional terminal chloride atom
instead. The CrꢀCl bond distances of 2.2892(18)ꢀ2.3892(18) Å
in 4b are comparable to those in 3c and 3d. The C(1)ꢀN(1) and
C(21)ꢀN(2) bond lengths of 1.281(7) and 1.303(8) Å are
obviously short than that of 1.322(8)ꢀ1.342(8) Å in 3c and 3d
and have double bond character.
complexes bearing ortho disubstitute ligands (3bꢀ3d) were
used, the bulkiness of ortho substitutes shows a slightly influence
on the activities and cis-1,4 selectivities, and the conversions and
cis-1,4 selectivities gradually increased with the increase of the
size of the ortho substitutes (entries 2ꢀ4, Table 1). Activated
with MAO, the generated cationic metal center lies in an opening
environment (the bite angles of the two aryl rings in 3b are nearly
172.5°), and we think the difference in selectivity may be
attributed to the electronic factor rather than the steric factor.
It can be seen that the molecular weights of resulting polymers
have an essentially increasing tendency with respect to the
increase of the number and bulkiness of ortho substitutes
(entries 1ꢀ4, Table 1). This may be due to the difference in
the stabilities of resulting active species during the polymeriza-
tion. The ortho substitutes may donate the electrons to the metal
center and make the active center more stable and able exist for a
long time. The tetradentate complexes 4b and 4c show moderate
activities in isoprene polymerization, affording the PIP with
similar cis-1,4 selectivity (79.8% and 79.0% in entries 5ꢀ6,
Table 1). The catalytic activity was also affected by the ratio of
Al/Cr used. When 3d was used as a precatalyst (entries 4 and
7ꢀ9, Table 1), the conversion increased with the increasing Al/
Cr ratios and an 82.6% conversion was achieved at Al/Cr = 500.
A further increase in Al/Cr molar ratios shows no obvious effects
on the productivities. Upon activation with MAO, these com-
plexes also show high activities in butadiene polymerization at
room temperature. It is worth to note that, unlike other
chromium complexes that afforded the 1,2-enriched polymer,
these complexes show moderate-to-high cis-1,4 selectivity in
butadiene polymerization. The diimine complexes 3aꢀ3d show
similar activities, and an 88.8ꢀ91.7% conversion was achieved,
whereas with the increase of the number and size of the ortho
substitutes, the cis-1,4 selectivtieis gradually increased from
70.4% to 89.3%, accompanying with the gradual increase of the
’ BUTADIENE AND ISOPRENE POLYMERIZATION
All the complexes were investigated as catalysts for butadiene
and isoprene polymerization. The polymerization data are sum-
marized in Table 1. All the complexes were almost inert in diene
polymerization when only AlR3 was used as cocatalysts. When
activated with MAO, these complexes show medium-to-high
activities toward isoprene polymerization with varied cis-1,4
selectivity at room temperature and the diimine complexes show
relatively higher activities than that with 4b and 4c. Complex 3a
with one ortho-isopropyl substitute at imine groups show the
highest activity; a conversion of 95.7% was achieved in 30 min,
affording the PIP with only 56.2% cis-1,4 selectivity. When those
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Scheme 3. Proposed Mechanism for Diene Polymerization by BIAN Cr(III) Complexes
molecular weight (entries 10ꢀ13, Table 1). 4b and 4c show
similar activities and selectivities. It is worth to note that the
selectivity is influenced by the polymerization temperature and
the cis-1,4 selectivity gradually increased from 89.3% at 25 °C to
95.2% at ꢀ40 °C.
6.36. 1H NMR (300 MHz, CDCl3, 25 °C): δ 1.13 (t, JHꢀH = 9.0 Hz, 12H,
CH2CH3), 2.49 (m, 4H, CH2CH3), 2.60 (m, 4H, CH2CH3), 6.71 (d,
2H), 7.22 (m, 6H), 7.37 (t, 2H), 7.87 (d, 2H). 13C NMR (75 MHz,
CDCl3, 25 °C): δ 8.74, 19.58, 117.77, 118.81, 121.28, 122.94, 123.68,
124.41, 125.55, 125.86, 135.46, 143.43, 155.61 ppm.
((2,6-Diethylphenyl)imino)-acenaphthenone (2c). ((2,6-Diethylph-
enyl)imino)-acenaphthenone was synthesized with a similar procedure
to that for ((2,6-diisopropylphenyl)imino)-acenaphthenone. Acenaph-
thenequinone (3.2 g, 18 mmol) and Na2SO4 (15 g) were stirred at 45 °C
in 500 mL of methanol. 2,6-Diethylaniline (1.94 g, 16 mmol) and formic
acid (0.60 mL) in 100 mL of methanol were added dropwise to the
acenaphhenequinone solution for 3 h. After 10 h, the solvent was
removed by a rotary evaporator. The crude product was purified by
column chromatography (20% ethyl acetate/hexane, silica gel) to afford
3.9 g of a red-orange powder (yield: 78%). Anal. Calcd for C22H19NO-
(%): C, 84.31; H, 6.11; N, 4.47. Found: C, 84.27; H, 6.08; N, 4.40. 1H
NMR (300 MHz, CDCl3, 25 °C): δ 1.06 (t, JHꢀH = 9.0 Hz, 12H,
CH2CH3), 2.35 (m, 4H, CH2CH3), 2.48 (m, 4H, CH2CH3), 6.68 (d,
1H), 7.21 (s, 3H), 7.43 (t, JHꢀH = 6.0 Hz, 1H), 7.84 (t, JHꢀH = 9.0 Hz,
1H), 8.01 (d, JHꢀH = 6.0 Hz, 1H), 8.19 (d, JHꢀH = 3.0 Hz, 1H), 8.21 (s,
1H) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ 8.71, 19.21, 116.82,
117.65, 119.42, 121.08, 122.43, 122.99, 123.09, 124.16, 125.14, 125.53,
125.70, 126.93, 137.67, 142.43, 155.00, 184.25 ppm.
On the basis of the data mentioned above, we assumed a
cationic mechanism (Scheme 3). The activation of the BIAN
chromium complexes with MAO affords a cationic metal center.
The opening environment around the cationic metal center
favors the coordination of diene in a cis η4 manner and affords
the cis-1,4-enriched polymer.
’ EXPERIMENTAL SECTION
General Considerations. All manipulations involving air- and
moisture-sensitive compounds were carried out under an atmosphere of
dried and purified nitrogen using standard Schlenk and vacuum-line
techniques. Toluene and hexane were dried over sodium metal and
distilled under nitrogen. Elemental analyses were performed on a Varian
EL microanalyzer; infrared spectra were recorded as KBr disks with a
Nicolet Avatar 360. NMR spectra were carried out on Varian 300 Hz
instrument at room temperature in CDCl3 solution for ligands and
polymers. The molecular weight and molecular weight distribution of
the polymers were measured by a TOSOH HLC 8220 GPC at 40 °C
using THF as the eluent against polystyrene standards. The ligands were
synthesized according to the literature. Acenaphthenequinone, 2,6-di-
methylaniline, 2,6-diethylaniline, 2,6-diisopropylaniline, and 2-isopropy-
laniline were bought from Aldrich Chemical Co. and used without further
purification. (2,-iPrC6H4-BIAN) (1a),24 (2,6-Me2C6H3-BIAN) (1b),24
(2,6-iPr2C6H3-BIAN) (1d),24 and N-(2,6-dimethylphenylimino)-ace-
naphthenone (2b)25a were synthesized according to the literature.
1,2-Bis[(2,6-diethylphenyl)imino]-acenaphthene (2,6-Et2C6H3-BIAN)
(1c). 1,2-Bis[(2,6-diethylphenyl)imino]-acenaphthene was synthesized
with a similar procedure to that for 1,2-bis[(2,6-diisopropylphenyl)
imino]-acenaphthene. Acenaphthenequinone (1.35 g, 7.4 mmol) in
65 mL of acetonitrile was heated under reflux for 30 min, and then
12 mL of acetic acid was added and heating was continued until the
acenaphthenequinone had completely dissolved. To this hot solution,
2.39g (16mmol) of2,6-diethylaniline wasaddeddirectly, andthesolution
was heated under reflux for a further 1.5 h. It was then cooled to room
temperature, and the solid was filteredoff to give a yellow product that was
washed with hexane and air-dried. Yield: 2.73 g (83%). Anal. Calcd for
C32H32N2 (%): C, 86.44; H, 7.25; N, 6.30. Found: C, 86.45; H, 7.29; N,
(2-iPrC6H4-BIAN)CrCl2(THF)2 (3a). A mixture of 1a (0.416 g, 1.00
mmol) and CrCl2(THF)2 (0.266 g, 1.00 mmol) in 20 mL of THF was
stirred for 6 h at 25 °C under a nitrogen atmosphere. The solution was
concentrated to 6 mL to give a brown powder, from which the mother
liquor was decanted, and the product was washed with 5 mL of ether and
dried in vacuum. The product was isolated as a brown solid (0.561 g,
82% yield). Anal. Calcd for C38H44Cl2CrN2O2 (%): C, 66.76; H, 6.49;
N, 4.10. Found: C, 66.76; H, 6.52; N, 4.13. IR(KBr): ν (cmꢀ1) 3233 w,
3053 w, 2963 s, 2853 w, 1727 w, 1676 w, 1625 m, 1567 m, 1520 w, 1470
s, 1424 w, 1361 w, 1180 w, 1141 w, 1111 w, 1045 m, 1000 w, 930 w, 859
w, 807 w, 769 s, 678 w, 588 w, 569 w, 537 w.
(2,6-Me2C6H3-BIAN)CrCl2(THF)2 (3b). The synthesis of 3b was car-
ried out according to that of complex 3a, using 0.389 g of ligand 1b
(1.00 mmol) and CrCl2(THF)2 (0.266 g, 1.00 mmol). Yield: 0.518 g
(79%) of a brown powder. Crystals of 3b suitable for X-ray structural
determination were grown in THF/hexane mixed solution. Anal.
Calcd for C36H40Cl2CrN2O2 (%): C, 65.95; H, 6.15; N, 4.27. Found:
C, 65.87; H, 6.10; N, 4.25. IR(KBr): ν (cmꢀ1) 3387 w, 3059 w, 2963 s,
2879 w, 2821 w, 1618 m, 1579 s, 1476 s, 1354 w, 1290 m, 1026 w, 962
w, 897 w, 827 w, 769 s, 665 w, 608 w.
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Organometallics
ARTICLE
(2,6-Et2C6H3-BIAN)CrCl(μ-Cl)3Cr(2,6-Et2C6H3-BIAN)(THF) (3c). The
synthesis of 3c was carried out according to that of complex 3a, using
ligand 1c (0.444 g, 1.00 mmol) and CrCl2(THF)2 (0.266 g, 1.00 mmol).
Yield: 0.459 g (76%) of a brown powder. Crystals of 3c suitable for X-ray
structural determination were grown in THF/hexane mixed solution.
Anal. Calcd for C68H72Cl4Cr2N4O (%): C, 67.66; H, 6.01; N, 4.64.
Found: C, 67.59; H, 5.98; N, 4.57. IR(KBr): ν (cmꢀ1) 3291 w, 3053 w,
2963 s, 2918 m, 2867 m, 1683 w, 1643 w, 1618 m, 1580 s, 1457 m, 1431
m, 1361 w, 1290 m, 1232 w, 1180 w, 1129 w, 1051 w, 948 w, 878 w, 839
m, 769 s, 743 m, 547 w.
’ CONCLUSIONS
In summary, we have reported the synthesis of the radical-
anion Cr(III) complexes by reducing the BIAN ligands with
CrCl2(THF)2. The monoimine ligands undergo a pinacol cross-
coupling reaction, affording the tetradentate dinuclear Cr(III)
complexes. These Cr(III) complexes show high activities in
isoprene and butadiene polymerization, affording the cis-1,4-
enriched polymer.
(2,6-iPr2C6H3-BIAN)CrCl(μ-Cl)3Cr(2,6-iPr2C6H3-BIAN)(THF) (3d). The
synthesis of 3d was carried out according to that of complex 3a, using
ligand 1d (0.501 g, 1.00 mmol) and CrCl2(THF)2 (0.266 g, 1.00
mmol). Yield: 0.574 g (87%) of a brown powder. Crystals of 3d suitable
for X-ray structural determination were grown in THF/hexane mixed
solution. Anal. Calcd for C76H88Cl4Cr2N4O (%): C, 69.19; H, 6.72; N,
4.25. Found: C, 69.21; H, 6.81; N, 4.31. IR(KBr): ν (cmꢀ1) 3060 w,
2963 s, 2867 m, 1734 w, 1618 m, 1573 s, 1451 s, 1424 m, 1354 w, 1322
w, 1290 m, 1245 w, 1180 w, 1129 w, 1038 w, 935 w, 832 m, 814 m, 788
m, 756 s, 600 w, 537 w.
’ ASSOCIATED CONTENT
S
Supporting Information. X-ray crystallographic data and
b
refinements for complexes 3bꢀ3d and 4b in CIF format,
summary of crystallographic data, and typical 13C NMR spectra
of polybutadiene and polyisoprene. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
2,6-Me2C6H3-BIAO(k3N,O,O0-CrClTHF)(μ-Cl)(μ-O)(k2N0,O-CrCl2THF)
(4b). A mixture of 2b (0.285 g, 1.0 mmol) and CrCl2(THF)2 (0.266 g, 1.0
mmol) in 20 mL of CH2Cl2 was stirred for 6 h at 25 °C under a nitrogen
atmosphere and concentrated to 5 mL to give a pale green powder, from
which the mother liquor was decanted, and the product was washed with
2 mL of hexane and dried in vacuum. The product was isolated as a pale
green solid (0.409 g, 85%yield). Crystalsof4bsuitable for X-raystructural
determination were grown in CH2Cl2 /hexane mixed solution. Anal.
Calcd for C48H48Cl4Cr2N2O4(%): C, 59.88; H, 5.03; N, 2.91. Found: C,
59.86; H, 5.00; N, 2.87. IR(KBr): ν (cmꢀ1) 3240 w, 3066 w, 2970 s, 2873
m, 1721 w, 1637s, 1592m, 1464 m, 1424 w, 1341 w, 1270w, 1187 w, 1045
w, 955 w, 904 w, 827 w, 781 s, 620 w.
Corresponding Author
*E-mail: gw@jlu.edu.cn (W.G.) and ymu@jlu.edu.cn (Y.M.).
’ ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
for financial support (Project Nos. 20904013 and 21074043.)
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