Highly active half-sandwich chromium(iii) catalysts bearing bis(imino)pyrrole ligands for ethylene (co)polymerization

Article abstract of DOI:10.1039/c4ra02039a

A series of half-sandwich Cr(iii) complexes bearing bis(imino)pyrrole ligands, Cp′[2,5-C₄H₂N(CHNAr)₂]CrCl [Cp′ = C₅H₅, Ar = C₆H₅ (2a), 2,6-Me₂C₆H₃ (2b), 2,6-ⁱPr ₂C₆H₃ (2c), C₆F₅ (2d); Cp′ = C₅Me₅, Ar = C₆H₅ (3a), 2,6-ⁱPr₂C₆H₃ (3c)] were synthesized with good yields. The complexes were characterized by FTIR and mass spectrometry in addition to elemental analyses. X-ray structural analyses for 2a-c showed that the Cr complexes have a pseudo-octahedral coordination environment with a three-legged piano stool geometry. One of the imino nitrogen atoms is coordinated with the Cr metal. On activation with methylaluminoxane, the Cp-based complexes showed high catalytic activities for ethylene polymerization. High molecular weight polymers with unimodal molecular weight distributions were obtained, indicating that the nature of the polymerization was single site. The copolymerization of ethylene with norbornene by the pre-catalysts 2a-d was also explored in the presence of methylaluminoxane. The catalytic activity, co-monomer incorporation and the properties of the resultant polymers can be controlled over a wide range by tuning the catalyst structures and reaction parameters. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02039a

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Highly active half-sandwich chromium(III) catalysts
bearing bis(imino)pyrrole ligands for ethylene (co)
polymerization†
Cite this: RSC Adv., 2014, 4, 19433
*
Jing-Yu Liu, Ping Tao, Yong-Xia Wang and Yue-Sheng Li
A series of half-sandwich Cr(III) complexes bearing bis(imino)pyrrole ligands, Cp0[2,5-C4H2N(CH]NAr)2]
CrCl [Cp0 ¼ C5H5, Ar ¼ C6H5 (2a), 2,6-Me2C6H3 (2b), 2,6-iPr2C6H3 (2c), C6F5 (2d); Cp0 ¼ C5Me5, Ar ¼
C6H5 (3a), 2,6-iPr2C6H3 (3c)] were synthesized with good yields. The complexes were characterized by
FTIR and mass spectrometry in addition to elemental analyses. X-ray structural analyses for 2a–c showed
that the Cr complexes have a pseudo-octahedral coordination environment with a three-legged “piano
stool” geometry. One of the imino nitrogen atoms is coordinated with the Cr metal. On activation with
methylaluminoxane, the Cp-based complexes showed high catalytic activities for ethylene
polymerization. High molecular weight polymers with unimodal molecular weight distributions were
obtained, indicating that the nature of the polymerization was single site. The copolymerization of
ethylene with norbornene by the pre-catalysts 2a–d was also explored in the presence of
methylaluminoxane. The catalytic activity, co-monomer incorporation and the properties of the resultant
polymers can be controlled over a wide range by tuning the catalyst structures and reaction parameters.
Received 9th March 2014
Accepted 2nd April 2014
DOI: 10.1039/c4ra02039a
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ligands also showed great potential in catalyzing ethylene
polymerization. For example, half-sandwich salicylaldiminato
Cr(III) catalysts, Cp*Cr[2,4-tBu2-6-(CH]NR)-C6H2O]Cl (R ¼ tBu,
Introduction
Cr-based catalysts for polymerization have an interesting posi-
tion among the transition metal catalysts used for olen poly-
merization. Heterogeneous Cr-based systems such as the silica-
supported Phillips1 and Union Carbide2 catalysts are among the
most important catalysts in the production of polyethylene
materials. However, the ill-dened nature of these para-
magnetic Cr catalysts has resulted in intense debate about the
nature of the active species and the catalytic reaction mecha-
nisms; this has prompted considerable efforts to develop well-
dened homogeneous Cr-based catalysts.3 Among the homo-
geneous Cr catalysts, Cp-based Cr(III) complexes have attracted
particular attention as these complexes have structural models
of the active site close to those proposed for the Union Carbide
heterogeneous catalyst.4–11 The [Cp*CrL2R]+Aꢀ complexes (L ¼
py, 1/2dppe, MeCN, THF; R ¼ Me, Et; A ¼ PF6, BPh4; Cp* ¼ ç5-
pentamethylcyclopentadienyl) were reported to catalyze
ethylene polymerization in the absence of a co-catalyst. The
half-sandwich Cr(III) complexes ligated with bidentate ancillary
Ph, 2,6-iPrC6H3), showed high catalytic activity for ethylene
polymerization and produced linear high molecular weight
polyethylene on activation with only a small amount of AlR3 (Al/
Cr ¼ 25).12 The half-sandwich Cr(III) complexes bearing b-
ketoiminato, b-diketiminato and hydroxyindanimine ligands
showed good catalytic activity for ethylene polymerization in the
presence of triethylaluminium.13
Nitrogen-based polydentate ligands have been widely used in
olen polymerization catalysis over the last few decades. A key
attraction of these ligands is their availability and amenability
to modication via straightforward Schiff-base condensation
procedures.14–23 Our group has reported the synthesis of V(III)
complexes bearing bis(imino)pyrrolyl ligands and their use as
catalyst precursors for olen polymerization.24 Although the
bis(imino)pyrrole ligands actually acted as bidentate ligands
rather than tridentate ligands, these complexes showed much
higher thermal stability than mono(imino)pyrrole-based
analogues under similar conditions due to the increased steric
hindrance and electron-withdrawing effects in the N-aryl moiety
of the ligands. We were attracted by the potential of bis(imino)
pyrrole ligands in homogeneous polymerization catalysis.
Considering that the atomic radii of Cr is similar to that of V, we
prepared a new family of half-sandwich Cr complexes contain-
ing bis(imino)pyrrole ligands. This paper describes the
synthesis and characterization of a number of novel Cr
complexes with bis(imino)pyrrole chelating ligands and
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China.
E-mail: ysli@ciac.ac.cn
† Electronic supplementary information (ESI) available: Crystal data and structure
renements of complexes 2a–c; the structures for 2b and 2c and X-ray diffraction
data for 2a–c as cif; 13C-NMR spectra of E/NBE copolymer with different NBE
incorporations produced by 2c. CCDC 979517–979519. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra02039a
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Table
complexes 2a–c
1
Selected bond distances (A˚) and angles (degrees) for
cyclopentadienyl or pentamethylcyclopentadienyl and explores
their application as catalysts in ethylene polymerization and
ethylene/norbornene (NBE) copolymerization.
2a
2b
2c
˚
Bond distances (A)
Cr(1)–N(1)
Cr(1)–N(2)
Cr(1)–Cl(1)
Cr(1)–Cp(centroid)
N(2)–C(11)
2.0304(16)
2.0766(17)
2.206(2)
1.876
1.435(2)
1.280(3)
2.036(3)
2.070(3)
2.2883(11)
1.889
1.445(4)
1.256(5)
2.017(4)
2.080(4)
2.2885(11)
1.889
1.439(5)
1.274(5)
Results and discussion
Synthesis and characterization of half-sandwich Cr(III)
complexes
A general synthetic route for the half-sandwich Cr(III) complexes
bearing bis(imino)pyrrole ligands is shown in Scheme 1. The
bis(imino)pyrrole ligands 1a–d were deprotonated by 1.0 equiv.
of n-BuLi, followed by treatment with Cp0CrCl2(THF) in THF at
N(3)–C(17)
Bond angles (ꢁ)
N(1)–Cr(1)–Cl(1)
N(1)–Cr(1)–N(2)
N(2)–Cr(1)–Cl(1)
Cp(centroid)–Cr(1)–Cl(1)
Cp(centroid)–Cr(1)–N(2)
Cp(centroid)–Cr(1)–N(1)
N(1)–C(6)–C(17)
97.74(5)
80.95(6)
96.97(5)
122.91
121.94
126.04
92.81(9)
80.59(11)
99.20(8)
122.01
121.81
129.96
92.77(9)
80.60(11)
99.19(8)
122.02
121.85
129.94
ꢁ
ꢀ78 C. The bidentate Cr complex containing the iminopyrro-
lide chelate ligand 2f was also prepared for comparison. The
pure complexes were then isolated by recrystallization from a
mixture of dichloromethane and hexane at room temperature.
The severe line broadening in the 1H-NMR spectra indicates
that these complexes are paramagnetic species.
127.20(18)
126.47(18)
124.0(3)
124.5(4)
122.0(4)
121.8(4)
N(3)–C(17)–C(6)
These complexes were identied by FTIR and mass spec-
trometry as well as by elemental analyses. The N–H stretching at
about 3452 cmꢀ1 disappeared in the IR spectra. The decrease in
the signal intensity of n(C]N) around 1622 cmꢀ1 and the
appearance of a low frequency signal around 1559 cmꢀ1 indi-
cate that one of the imino nitrogen atoms coordinated with Cr
metal. To further conrm the structures of these new
complexes, crystals of 2a–c suitable for X-ray diffraction were
grown from the chilled concentrated mixed toluene–hexane
solution. The ratio of toluene to hexane was adjusted in the
range 1 : 3–1 : 6 according to the solubility of the complexes.
The crystallographic data, together with the collection and
renement parameters, are summarized in Table S1† of the ESI.
Selected bond distances and angles for complexes 2a–c are lis-
ted in Table 1.
The structure of 2a is shown in Fig. 1. Complex 2a adopted a
three-legged “piano stool” geometry with the N, N and Cl atoms
representing the three legs and the Cp ring representing the
Fig. 1 Molecular structure of 2a with thermal ellipsoids at the 30%
probability level. Hydrogen atoms are omitted for clarity.
seat, which is similar to the structures of the half-sandwich
salicylaldiminato12 and b-ketoiminato13 Cr complexes. The Cr–
˚
N(1) bond length is 2.0304(16) A, suggesting that the N atom in
the pyrrole group formed a s-bond with the Cr. The Cr–N(2)
˚
bond distance in 2a is 2.0766(17) A, indicative of signicant
coordination of this imino nitrogen atom with the metal center
˚
in the solid state. The distance from N(3) to Cr is 3.686 A,
therefore the interaction between the N(3) atom and the Cr in 2a
is rather weak.
The structures of 2b and 2c are shown in Fig. S1 and S2,†
with the selected bond distances and angles summarized in
Table 1. These complexes also adopt a pseudo-octahedral
coordination environment with a three-legged piano stool
geometry. The Cr–Cl bond distance in 2b and 2c is longer than
˚
˚
˚
that in 2a (2a, 2.206(2) A; 2b, 2.2883(11) A; 2c, 2.2885(11) A), in
˚
line with the Cr–Cp (centroid) distances in 2a–c (2a, 1.876 A; 2b,
Scheme 1 General synthetic route for the Cr complexes used in this
˚
˚
study.
1.889 A; 2c, 1.889 A). These bond distances might be related to
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the catalytic activity of these complexes towards ethylene poly- for these complexes to show high catalytic activity and produce
merization. The N(1)–Cr(1)–N(2), N(1)–Cr(1)–Cl(1) and N(2)– high molecular weight polyethylenes. This is because a bulky
Cr(1)–Cl(1) bond angles in these complexes occur in the ranges substituent may not only provide efficient protection of the
80.59–80.95, 92.77–97.74 and 96.97–99.20ꢁ, respectively, with metal center from inactivation, but also restrains the chain
the variation in the imino groups. These complexes have similar transfer reaction. The catalytic activity of complex 2d was
dihedral angles between the Cp ring and the plane through Cr, between that of 2a and 2b (entries 1 and 2 versus entry 4). These
N(1) and N(2) atoms in the range 46.30–49.73ꢁ. In comparison, data suggested that the electron-donating effect of the aliphatic
the impact of the imino groups on the dihedral angle between group was not important, but the steric bulk effect played a key
the Cp ring and the pyrrole ring in the bis(imino)pyrrole ligand role in the enhanced catalytic activity.
is much more remarkable for these complexes, which makes the
Compared with 2a, complex 2e containing the mono-
Cp-pyrrolyl ring dihedral angle change over a large range from (iminopyrrolyl) chelate ligand showed a low cꢀa1talytic activity for
44.76 to 60.92ꢁ. In addition to the steric effect of the substitu- ethylene polymerization (2e, 230 kg molCr
hꢀ1 barꢀ1). In
ents, in some cases the packing force may also be an important addition, 2e produced low molecular weight polymers with
factor affecting the bond parameters.
broad molecular weight distributions under the same condi-
tions (entry 5 in Table 2). These results provide further evidence
of the advantage of using these novel half-sandwich Cr(III)
complexes bearing bis(imino)pyrrole ligands for ethylene
polymerization.
The Cp*-based complexes displayed much lower activities
and produced much higher molecular weight polymers than the
Cp analogues. The polymers produced by 3a and 3c were
insoluble in 1,2,4-C6Cl3H3 at 150 ꢁC. Moreover, complex 3a
showed higher activity than 3c. The observed result was an
interesting contrast to that found with 2a and 2c, in which 2c
showed a higher activity than 2a. It may be assumed that the
bulky steric hindrance around the metal center prevented
ethylene insertion in this case. These results suggest that both a
cyclopentadienyl ligand and an anionic donor ligand could
affect the polymerization behavior.
As a result of the better performance observed with the
catalysts containing 2,6-iPr substituents at the imine group, pre-
catalyst 2c was investigated in detail by changing the reaction
parameters, such as the co-catalyst concentration and the
ethylene pressure. It was found that the Al/Cr molar ratio
strongly inuenced the catalytic activity. Note that even at an
extremely low amount of MAO (Al/Cr molar ratio ¼ 60), complex
Ethylene polymerization
All the complexes are inactive for ethylene polymerization in the
presence of alkylaluminium compounds or halogen-containing
alkylaluminium compounds. Interestingly, on activation with a
small amount of methylaluminoxane (MAO), these complexes
showed notable activity towards ethylene polymerization
(Table 2).
ꢀ1
Complex 2a showed high catalytic activity (1170 kg molCr
hꢀ1 barꢀ1) for ethylene polymerization. Analogue 2b, containing
a dimethylphenyl-substituted bis(imino)pyrryl ligand, exhibited
ꢀ1
a slightly higher catalytic activity (1320 kg molCr hꢀ1 barꢀ1
)
than 2a under the same conditions. Further improvement was
observed when complex 2c, with a di(isopropyl)phenyl-substituted
bis(imino)pyrryl ligand was used (1770 kg molCr
ꢀ1
ꢀ1
h
barꢀ1).
The resultant polymers prepared by 2a–c had high molecular
weights and unimodal molecular weight distributions. The
molecular weight of the polymers also increased in the following
order: 2c, 42.7 kg molꢀ1 > 2b, 32.4 kg molꢀ1 > 2a, 23.6 kg molꢀ1
.
These results suggest that the bulky substituents at the
ortho-positions in the N-aryl moiety (iPr > Me > H) are important
Table 2 Ethylene polymerization by Cr complexes/MAO catalytic systemsa
Activity (kg molCr hꢀ1 barꢀ1
)
Mwb (104)
Mw/Mn
ꢀ1
b
Entry
Catalyst
Al/Cr (molar ratio)
Ethylene (atm)
Time (min)
Yield (g)
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
3a
3c
2c
2c
2c
2c
2c
2c
2c
250
250
250
250
250
250
250
60
125
500
1000
250
250
250
5
5
5
5
5
5
5
5
5
5
5
5
5
2
10
10
10
10
10
10
10
10
10
10
10
5
2.93
3.31
4.43
3.23
0.58
0.77
Trace
2.38
3.96
2.66
1.56
2.81
9.14
1.12
1170
1320
1770
1290
230
310
—
950
1580
1060
620
2250
1220
1120
23.6
32.4
42.7
24.6
6.7
2.2
2.5
2.6
2.6
3.8
—
c
—
—
c
—
58.6
52.5
22.9
10.3
25.8
60.1
30.8
2.7
3.0
2.9
2.7
2.3
2.9
2.5
9
10
11
12
13
14
30
10
a
Reaction conditions: 3 mmol catalyst; MAO as the co-catalyst; 25 ꢁC; Vtotal ¼ 80 mL. b Weight-average molecular weights and polydispersity indexes
of the polymer samples determined by high temperature GPC at 150 ꢁC in 1,2,4-C6Cl3H3 versus narrow polystyrene standards. c Insoluble in 1,2,4-
C6Cl3H3 at 150 ꢁC.
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2c still showed very high activity towards ethylene polymeriza- in the presence of MAO, but they displayed high catalytic
tion (entry 8). The molecular weights of the resultant polymers activities on activation with a small amount of AlEt3.12,13 The
decreased with the Al/Cr molar ratio, suggesting that chain formation of a hetero-bimetallic Cr–Al intermediate was thus
transfer to Al occurred during the polymerization (Fig. 2).
assumed to occur. In contrast, the complexes used in this study
Increasing the ethylene pressure resulted in an increase both showed rather low activities on activation with AlEt3. Attempts
in the polymer yields and in the molecular weight of the to isolate an intermediate by the treatment of 2c with 10 equiv.
resultant polymers, which also supported the suggestion that MAO in toluene for 2 h were unsuccessful; no exact information
the dominant chain transfer would be Al transfer from the was obtained from the IR spectrum. Based on these results, it
propagating metal alkyl species. Note that these catalysts has been shown that the catalyst structures strongly affect the
showed a long life for ethylene polymerization. The polymer formation of the active species, although the true active species
yields continued to increase for 30 min, although the activity remains unknown at this time.
decreased with time. The decrease in activity might be caused
by mass transport limitations as large amounts of polymers
were produced during the polymerization (Fig. 3). The obtained
Ethylene/NBE copolymerization
polyethylene samples were all analyzed by differential scanning One of the greatest advantages of these novel pre-catalysts for
calorimetry (DSC) and the melting temperature (Tm) was in the olen polymerization is that they could promote the copoly-
range 134–139 ꢁC, which is the classical Tm for high-density merization of ethylene with NBE. Typical results for this
polyethylene. This is in full agreement with the 13C-NMR copolymerization are summarized in Table 3. It was very
studies of the polyethylenes, which showed no branches on the interesting that the trend of catalytic activity for the copoly-
polymer backbone.
merization was similar to that for ethylene homopolymeriza-
The half-sandwich type b-ketoiminato and b-diketiminate tion. Except for 2d, the catalysts 2a–e showed high activities
Cr(III) catalyst systems are inactive for ethylene polymerization towards ethylene/NBE copolymerization. The resultant poly-
mers were poly(ethylene-co-NBE)s with high molecular weights
and unimodal molecular weight distributions. However, the
incorporation of NBE is dependent on the imino substituent
used (2a > 2b > 2c). The steric bulk of the aryl moiety in the
bis(imino)pyrrole ligands might be responsible for this
property.
The effect of the reaction conditions on the ethylene/NBE
copolymerization were also investigated using catalyst 2c.
Both the observed catalytic activities and the molecular
weights for the resultant copolymers decreased with
increasing initial NBE concentration (entries 3, 5 and 6 in
Table 3). The incorporation of NBE increased at higher NBE
concentrations; the level of NBE incorporation approached
40 mol% in the resultant copolymer. A certain excessive
amount of MAO was required to give a high catalytic activity
(entries 3, 7 and 8) in this catalyst system. In addition, the
increase in the molar ratio of Al/Cr led to a decrease in the
Fig. 2 Plots of the catalytic activity of catalyst 2c and the molecular
weight of the resultant polyethylenes versus the Al/Cr molar ratio.
Reaction conditions: 3 mmol; 10 min; 25 ꢁC; 5 atm ethylene.
molecular weight of the copolymer, indicating that chain
transfer to aluminum alkyls is also the dominant chain
transfer pathway in the E/NBE copolymerization under these
conditions. The NBE contents of the resultant poly(ethylene-
co-NBE)s were independent of the Al/Cr molar ratio.
The microstructures of the ethylene/NBE copolymers were
established by 13C-NMR in o-C6D4Cl2 at 125 ꢁC with the
assignment of the microstructure following previous work.21 As
shown in Fig. S3,† the resultant copolymer predominantly
possessed an isolated NBE inserted unit ([ENE] assigned as 47.7
ppm) among the repeated ethylene insertions when the NBE
incorporation of the copolymer was not high (entry 3). However,
when NBE incorporation reached 38.8 mol.% (entry 6), the
resultant copolymer possessed an isolated NBE inserted unit
([ENE] assigned as 47.7 ppm) among the repeated ethylene
insertions and the alternating sequence ([NEN] assigned as 48.4
and 47.9 ppm) was also present. No resonance ascribed to
repeated NBE insertion was observed.
Fig. 3 Kinetic profiles of ethylene polymerization using catalysts 2a.
Reaction conditions: 3 mmol; Al/Cr ¼ 250; 25 ꢁC; 5 atm ethylene.
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Table 3 Copolymerization of ethylene with NBE using complexes 2a–e with MAOa
Activity
NBE incorporated
(mol%)
Entry
Catalyst (mmol)
Al/Cr
NBE (mol Lꢀ1
)
Yield (g)
(kg molCr hꢀ1 barꢀ1
)
Mwb (104)
Mw/Mn
ꢀ1
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2c
2c
2c
2c
1000
1000
1000
1000
1000
1000
500
0.5
0.5
0.5
0.5
1.0
1.5
0.5
0.5
0.23
0.31
0.56
0.26
0.34
0.11
Trace
1.21
180
250
450
210
2700
90
32.8
26.7
18.1
22.5
31.5
38.8
—
13.4
19.8
25.8
22.1
19.9
9.8
2.4
2.1
2.3
2.6
2.1
2.3
—
—
970
—
16.7
2000
18.8
2.7
a
b
Reaction conditions: 1.5 mmol catalyst; MAO as the co-catalyst; ethylene 5 atm; 25 ꢁC; Vtotal ¼ 50 mL; co-polymerization for 10 min. Weight-
average molecular weights and polydispersity indexes of the copolymer samples determined by high temperature GPC at 150 ꢁC in 1,2,4-
C6Cl3H3 versus narrow polystyrene standards.
calorimetry (DSC) measurements were performed with a Perkin-
Conclusions
Elmer Pyris 1 DSC differential scanning calorimeter at a rate of
10 ꢁC minꢀ1. The molecular weight and the polydispersity
A series of half-sandwich Cr(III) complexes bearing bis(imino)
indices of the polymer samples were determined at 150 ꢁC by a
pyrrole ligands, Cp0[2,5-C4H2N(CH]NAr)2]CrCl [Cp0 ¼ C5H5, Ar
¼ C6H5 (2a), 2,6-Me2C6H3 (2b), 2,6-iPr2C6H3 (2c), C6F5 (2d); Cp0
PL-GPC 220 high-temperature chromatograph equipped with
¼ C5Me5, Ar ¼ C6H5 (3a), 2,6-iPr2C6H3 (3c)] were synthesized,
three PLgel 10 mm Mixed-B LS type columns. 1,2,4-Tri-
chlorobenzene was used as the solvent at a ow-rate of 1.0 mL
minꢀ1. Calibration was carried out using the polystyrene stan-
dard EasiCal PS-1 (PL Ltd). The 2.20 M n-butyllithium solution
in hexane was purchased from Acros. CrCl3(THF)3 was
purchased from Aldrich. Methylaluminoxane was purchased
from Akzo Nobel Chemical Inc. Commercial ethylene was used
without further purication. The ligands used were synthesized
according to previously published methods.23
characterized and investigated as efficient catalysts for olen
polymerization. These complexes adopt a pseudo-octahedral
coordination environment with a three-legged piano stool
geometry in the solid state. The bis(imino)pyrrole ligands acted
as a bidentate ligand, with one of the imino nitrogen atoms
coordinated with Cr metal. Both the cyclopentadienyl group
and the anionic donor ligand affected the polymerization
behavior. High molecular weight polymers with unimodal
molecular weight distributions were obtained, indicating that
the nature of the polymerization was single site. In addition,
complexes 2a–d showed high catalytic activities for ethylene/
NBE copolymerization. The level of NBE incorporation reached
38.8 mol% in the resultant copolymers using catalyst 2c. We
believe that these results provide important information for
designing efficient transition metal catalysts for olen
polymerization.
Synthesis of half-sandwich Cr(III) complexes
A suspension of CpLi (0.15 g, 2.00 mmol) in THF (10 mL) was
slowly added to a purple suspension of CrCl3(THF)3 (0.75 g, 2.00
ꢁ
mmol) in THF (20 mL) at ꢀ20 C. The mixture was warmed to
room temperature and stirred overnight to obtain a blue solu-
tion. In another ask, a solution of n-BuLi (2.20 M, 2.00 mmol)
in hexane was slowly added to a solution of compound 1a (0.55
g, 2.00 mmol) in dried THF (15 mL). The reaction mixture was
allowed to warm to room temperature and stirred for 2.5 h, then
added slowly to the above reaction mixture at ꢀ78 ꢁC. The
Experimental
General procedures and materials
All work involving air- and/or moisture-sensitive compounds obtained reaction mixture was allowed to warm to room
was carried out under a dry nitrogen atmosphere using stan- temperature and stirred overnight. During the reaction, the
dard Schlenk techniques or under a dry argon atmosphere in an color of the reaction mixture changes from blue to green. The
MBraun glove box, unless stated otherwise. All solvents used evaporation of the solvent under reduced pressure yielded a
were puried from an MBraun SPS system. The NMR data of crude product, followed by extraction with toluene (20 mL) to
ligands were obtained on a Bruker 400 MHz spectrometer at remove the insoluble impurities. The ltrate was concentrated
ambient temperature with CDCl3 as the solvent. The NMR and recrystallized in toluene–hexane and yielded 601 mg of the
analyses of the polymer samples were performed on a Bruker pure complex as a dark green block solid of 2a. Complexes 2b–e,
400 MHz spectrometer at 135 ꢁC using o-C6D4Cl2 as the solvent. 3a and 3c were synthesized in the same manner as complex 2a
The IR spectra were recorded on a Bio-Rad FTS-135 spectro- with different starting materials.
photometer. Elemental analyses were carried out on an
Cp[2,5-(CH]NC6H5)2C4H2N]CrCl (2a). Yield: (71%). FTIR
elemental Vario EL spectrometer. Mass spectra were obtained (KBr, cmꢀ1): nC]N 1558, 1621. EI-MS (70 eV): m/z ¼ 423 [M+].
using electron impact mass spectrometry (EI-MS) on an LDI- Analysis calculated for C23H19ClCrN3: C, 65.02; H, 4.51; N, 9.89.
1700 instrument (Linear Scientic Inc.). Differential scanning Found: C, 64.84; H, 4.46; N, 9.83.
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Cp[2,5-(CH]N-2,6-MeC6H3)2C4H2N]CrCl (2b). Yield: (75%). X-ray crystallography
FTIR (KBr, cmꢀ1): nC]N 1561, 1629. EI-MS (70 eV): m/z ¼ 479
Single crystals of complexes 2a–c suitable for X-ray structural
determination were grown from a hexane solution at ꢀ20 ꢁC in
a glove box, thus maintaining a dry O2-free environment. The
intensity data were collected in the u scan mode (186 K) on a
Bruker Smart APEX diffractometer with a CCD detector using
[M+]. Analysis calculated for C27H27ClCrN3: C, 67.42; H, 5.66; N,
8.74. Found: C, 67.27; H, 5.61; N, 8.68.
Cp[2,5-(CH]N-2,6-iPrC6H3)2C4H2N]CrCl (2c). Yield: (64%).
FTIR (KBr, cmꢀ1): nC]N 1559, 1620. EI-MS (70 eV): m/z ¼ 592
[M+]. Analysis calculated for C35H43ClCrN3: C, 70.87; H, 7.31; N,
7.08. Found: C, 71.03; H, 7.39; N, 7.01.
˚
Mo Ka radiation (l ¼ 0.71073 A). Lorentz polarization factors
were calculated for the intensity data and the absorption
corrections were determined using the SADABS program. The
crystal structures were solved using the SHELXTL program and
rened using full matrix least squares. The positions of the
hydrogen atoms were calculated theoretically and included in
the nal cycles of renement in a riding model along with the
attached carbons.
Cp[2,5-(CH]NC6F5)2C4H2N]CrCl (2d). Yield: (66%). FTIR
(KBr, cmꢀ1): nC]N 1561, 1617. EI-MS (70 eV): m/z ¼ 603 [M+].
Analysis calculated for C23H9ClCrF10N3: C, 45.68; H, 1.50; N,
6.95. Found: C, 45.83; H, 1.55; N, 6.89.
Cp[2-tBu-5-(CH]NC6H5)C4H2N]CrCl (2e). Yield: (70%). FTIR
(KBr, cmꢀ1): nC]N 1620. EI-MS (70 eV): m/z ¼ 376 [M+]. Analysis
calculated for C20H22ClCrN2: C, 63.57; H, 5.87; N, 7.41. Found:
C, 63.32; H, 5.93; N, 7.53.
Acknowledgements
Cp*[2,5-(CH]NC6H5)2C4H2N]CrCl (3a). Yield: (77%). FTIR
(KBr, cmꢀ1): nC]N 1545, 1618. EI-MS (70 eV): m/z ¼ 493 [M+].
Analysis calculated for C28H29ClCrN3: C, 67.94; H, 5.91; N, 8.49.
Found: C, 67.74; H, 5.96; N, 8.60.
The authors are grateful for nancial support provided by the
National Natural Science Foundation of China (nos 21234006,
21274144).
Cp*[2,5-(CH]N-2,6-iPrC6H3)2C4H2N]CrCl (3c). Yield: (74%).
FTIR (KBr, cmꢀ1): nC]N 1560, 1621. EI-MS (70 eV): m/z ¼ 661
[M+]. Analysis calculated for C40H53ClCrN3: C, 72.43; H, 8.05; N,
6.33. Found: C, 72.61; H, 8.15; N, 6.54.
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