The enhanced catalytic performance of cobalt catalysts towards butadiene polymerization by introducing a labile donor in a salen ligand

Article abstract of DOI:10.1039/c3dt52708e

A family of cobalt complexes supported by a tridentate Schiff base ligand with a labile donor (O, S, N) as a pendant arm (Co1-Co12, formulated as CoL ₂) were synthesized by the treatment of the corresponding ligands with cobalt acetate tetrahyd

Full text of DOI:10.1039/c3dt52708e

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The enhanced catalytic performance of cobalt
catalysts towards butadiene polymerization by
introducing a labile donor in a salen ligand†
Cite this: Dalton Trans., 2014, 43,
4169
Dirong Gong,*a Baolin Wang,b Xiaoyu Jiac and Xuequan Zhang*b
A family of cobalt complexes supported by a tridentate Schiff base ligand with a labile donor (O, S, N) as a
pendant arm (Co1–Co12, formulated as CoL2) were synthesized by the treatment of the corresponding
ligands with cobalt acetate tetrahydrate. The resultant complexes were well characterized by elemental
analysis, FT-IR, magnetic moment as well as EI-MS. The solid-state structures of Co7 and Co12 were
determined by X-ray diffraction and both established a distorted octahedron geometry around the cobalt
center. The butadiene polymerization capabilities of the 12 complexes were evaluated and compared in
representative cases. Diethylaluminum chloride (AlEt2Cl) was found to be the compatible activator result-
ing in highly active catalysts for producing polybutadiene of 93.8–98.2% cis-1,4 enchainment with negli-
gible 1,2-structure and trans-1,4 units. It appears that a certain degree of lability of the donor is beneficial
for high catalytic activity, generally following the order of O > S > N, and the high cis-1,4 selectivity.
Moreover, the remarkable thermal stability of these systems has been achieved: the catalytic systems have
the ability of conducting a high level of active and selective polymerization, reaching an upper limit
of polymerization temperature of about 70 °C. The enhanced catalytic performances were further ration-
alized by the established diene polymerization mechanism, which could shed light on developing highly
selective and reactive industrially applicable catalysts with an enhanced thermal stability.
Received 30th September 2013,
Accepted 4th December 2013
DOI: 10.1039/c3dt52708e
www.rsc.org/dalton
research launched by Natta, various rare earth metals and tran-
sition metal catalyst recipes have been considerably developed
Introduction
Metal-catalyzed selectively polymerization of 1,3-butadiene is for initiating the cis-1,4 polymerization,5–24 and the Ziegler-
currently one of the major industrial processes for producing Natta type of catalytic systems of TiCl4/I2/Al(i-Bu)3,23 CoCl2/pyri-
cis-1,4-polybutadiene given that it offers a wealth of raw dine/AlEt2Cl,24,25 Ni(OOCR)2/BF3·OEt2/AlEt3,26,27 and Nd
materials for manufacturing tires, conveyor belts, hose roll (OOCR)3/EASC/Al(i-Bu)2H28 have realized cis-1,4-polybutadiene
covers, shoe soles, seals tube etc.1–3 Cis-1,4 polybutadiene can on a large industrial scale.
be also co-polymerized, blended or terminally functionalized
The simple Ziegler-Natta systems CoCl2, carboxylate or
to prepare materials with unique mechanical or chemical pro- acetylacetonate cobalt, activated with organoaluminium com-
perties such as styrene-butadiene rubber (SBR), high impact pounds are able to slowly initiate the polymerization of 1,3-
polystyrene (HIPS), acrylonitrile-butadiene-styrene (ABS) or butadiene,29–32 however, these multi-site catalysts often give
hydroxyl terminated polybutadiene (HTPB).4 Since the pioneer polymers with broad molecular weight distributions. With the
purpose of tuning the catalyst performance and polymer pro-
perties, and in particular for exploring the factors influencing
the activity and regio- and stereo-selectivity as well as the pro-
perties of the product, recently, major effort has been focused
aSchool of Material Sciences and Chemical Engineering, Ningbo University, Ningbo,
315211, People’s Republic of China. E-mail: gongdirong@nbu.edu.cn,
xqzhang@ciac.jl.cn
on designing and developing well-defined organometallic
cobalt complexes for homogeneous catalyst recipes, which are
marked with the discovery and application of novel auxiliary
ligands. The last two decades have met the significant success
of versatile pincer bisiminopyridine ligand sets in the olefin
area,33–36 and the discovery of such ligand sets for transition
metals in the field of olefin polymerization has also intro-
duced their functional ligand systems based on either their
bChangchun Institute of Applied Chemistry, Chinese Academy of Sciences, State Key
Laboratory of Polymer Chemistry and Physics, Changchun 130022, People’s Republic
of China
cKey Laboratory of Urban Environment and Health, Institute of Urban Environment,
Chinese Academy of Sciences, No. 1799, Jimei Road, Xiamen, Fujian 361021,
People’s Republic of China
†Electronic supplementary information (ESI) available. CCDC numbers 950121
and 950122. For crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52708e
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Chart 1 The reported structures of cobalt compounds supported by anionic ligands.
dentate atoms or coordination mode to transition metal catalytic activities and properties of products.49–52 In this con-
coordination chemistry for diene polymerization catalysis, tribution, we describe the new cobalt(II) complexes supported
though quite a few in number, for example, the neutral pincer by the tridentate Schiff base ligand and explore the feasibility
ligand N^X^N (X = S, O and N)4,20,37–41 have been well of their butadiene catalysis performance at elevated tempe-
described in iron, chromium and cobalt complexes for the cis- ratures by introducing a hetero labile atom to the traditional
1,4 polymer. Our lab has a long-standing interest in the pincer bidentated salen ligand. This attention, on one hand, stems
set as an auxiliary ligand of the late transition metals (Cr, Fe, from stabilizing the active center through reversibly occupying
Co and Ni) in diene polymerization41–47 and has established the vacant site by the neutral O, S and N labile donor, intri-
that the symmetry pincer ligand has positive effects on the cis- gued by the fact that the end double bond of the growing
1,4 selectivity. However the rigid backbone could also slow polymer is able to stabilize the active center by means of
down the butadiene coordination rate and therefore lower the π-electron donation in the Nickel catalyzed butadiene polymer-
catalytic activity of the catalyst. Considering that the neutral ization.2 On the other hand, this attention stems from the
ligand association could be responsible for the decay of selecti- adjustable binding interaction between the metal and the
vity and activity at a higher polymerization temperature, the coordination donor, as well as the flexible opening environ-
recent illustrative trend has been aimed at designing multi- ment near the metal center, which is assumed to provide a
dentated anionic ligands. For example, four-coordinated tunable space for butadiene binding and insertion, thus,
diimine based salen and bis(salicylaldiminate) (Chart 1, A, B) should have a profound effect on the catalytic properties.53–56
have been extended to cobalt(II) for butadiene polymeriz-
ation,9,48 but the catalyst deactivation became more prominent
and the overall catalyst selectivity decreased at elevated temp-
Results and discussion
Synthesis and characterization of ligands and complexes
eratures. Recently, Jie et al. developed multi-coordinated
imino-salicylaldehyde10 and 6-imino(amino)-pyridine-2-metha-
nol ligand38 (Chart 1, C, D and E) coordinated cobalt com-
pounds, which were proved for the high cis-1,4 selective
polymerization of butadiene at ambient polymerization tempe-
ratures. Nevertheless, the attempt to maintain the catalytic
properties at 70 °C was unachievable, since the accompanied
deactivation and variation of the active center became evident.
These results illustrate once again that correlating the catalytic
properties with the ligand structure at elevated polymerization
temperatures remains to be fully understood.
A
general route for synthesizing the cobalt complexes
Co1–Co12 bearing tridentate chelate ligands is outlined
in Scheme 1. The ligands (L1–L12) were prepared via the
Our recent interests are vested on exploring a thermally
stable cobalt candidate for butadiene polymerization based on
a simple salen ligand, on the fact that the ligand backbone
can bear numerous substituents, resulting in steric and elec-
tronic versatility via the framework chosen and carefully modi-
fying the ligand environment can tremendously tune the Scheme 1 The structure of the cobalt complexes selected in this study.
4170 | Dalton Trans., 2014, 43, 4169–4178
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condensation of anilines with aldehydes in the presence of
drops of formic acid in refluxed anhydrous ethanol with accep-
table yields.57 The reaction of cobalt(II) acetate tetrahydrate
with the corresponding ligands in refluxed ethanol afforded
the desired complexes (Co1–Co12) in good yields. All the resul-
tant complexes were measured by elemental analysis and
1
FT-IR as well as EI-MS. The HNMR spectra of all of the com-
plexes are broad and uninformative. The paramagnetic pro-
perties of the four coordinated Co1 and Co2 are probably due
to the high spin characters from three odd electrons calculated
from the magnetic moment values, which are in contrast to
those of the planar square cobalt complex A pictured in
Chart 1. While for six coordinated Co3–Co12, the magnetic
moment value contributed from the both electron spin and
orbital angular momentum indicates that a (t2g)5(eg)2 configu-
ration is adopted. From the FT-IR analysis, the CvN stretching
frequencies of Co1–Co12 are shifted into lower values
(1596–1627 cm−1) compared to those of the free ligands
(1632–1654 cm−1), with a reduced absorption intensity. The
Fig. 2 The ORTEP drawing of complex Co12 with thermal ellipsoids at
the 30% probability level. The hydrogen atoms have been omitted
for clarity.
ESI measurement results show that all of the tridentated and N2 located at the trans-position. The O2, O3, O1 and O4
+
ligand supported complexes are in the form of CoL2 , which atoms could be described as forming the equatorial plane with
implies that the oxidation of the cobalt center could be a mean deviation of 0.2851 Å, while the two axial bonds nearly
involved in the process of ionization, while the peak (M + 92)+ formed a straight line through the metal center (N1–Co1–N2,
for Co1 and Co2 was observed, implying that two ethanol 174.17(7)°). The axial Co1–N1 bond length (2.0707(18) Å) is
molecules could be binding to the electron deficient and longer by 0.023(1) Å than the Co1–N2 bond length (2.0677(17) Å).
+
coordination unsaturated CoL2 species. The elemental analy- The anion oxygen atoms O1 and O4 as well as the two co-
sis confirmed they are in accordance with the CoL2 compo- ordinated neutral O1 and O4 are both in cis-disposition. The
sition for all of the complexes.
bond distances of Co1–O1 (1.9963(16) Å) and Co1–O4 (1.9749
To clarify the molecular structures of the complexes, crys- (16) Å) are slight different, and obviously shorter than the
tals of Co7 and Co12 grown from dichloromethane diffused by neutral coordination bonds of Co1–O2 (2.2204(16) Å) and Co1–
diethyl ether were confirmed by single-crystal X-ray analysis. O3 (2.2528(16) Å), which are also longer than the neutral Co1–
The ORTEP drawings of the molecular structures of Co7 and N1 and Co1–N2 bonds, indicating that Co1–O2 and Co1–O3
Co12 are depicted in Fig. 1 and 2, respectively, and the crystal are more likely to cleavage for monomer binding during the
structure data were also complied in Tables 1S and 2S in the polymerization. The five-membered (N1–Co1–O3–C2–C7) che-
ESI.†
lating ring is nearly planar, with the cobalt atom lying
Complex Co7 possesses a six-coordinated cobalt metal 0.3048 Å out of the plane, but the cobalt deviates significantly
center in an approximately octahedral structure with two N1 from the six-membered (N1–Co1–O4–C8–C9–C14) chelating
ring by about 0.7488 Å. The dihedral angles between the six-
membered chelating ring and the aromatic ring attached to
the anionic oxygen atoms (O1 and O4) are 90.4° and 91.1°,
and the dihedral angles between the five-membered chelating
ring and aromatic ring attached to the imine nitrogen atoms
(N1 and N2) are 90.3° and 91.0°, respectively. The imino CvN
bonds in the complex retains a double-bond character, being
1.294(3) Å and 1.297(3) Å for N2–C22 and N1–C8, respectively.
The crystal structure of Co12 confirms its geometry also as
a slightly distorted octahedron with two tridentate N^N^N
ligands. The two ligands are situated nearly perpendicular to
each other. The coordination atoms O1, O2, N2 and N4, locate
in an equatorial plane with N1 and N3 occupying two apical
positions and the cobalt atom lying 0.0148 Å out of the plane.
All the bond angles in the equatorial plane are very close to a
right angle (N1–Co1–O2, 95.96(11)°; N4–Co1–O2, 87.09(12)°;
N2–Co1–N4, 91.94(12)°; O2–Co1–O1, 89.53(11)°). The two axial
Fig. 1 The ORTEP drawing of complex Co7 with thermal ellipsoids at
bonds nearly form a straight line, with an angle of 177.45(12)°.
The bond distances of cobalt and the anionic oxygen are quite
the 30% probability level. The hydrogen atoms have been omitted
for clarity.
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Table 1 The butadiene polymerization results using complex Co10 under various polymerization conditionsa
Microstructure (%)
Entry
Cocatalyst
[Al]/[Co]
Time (min)
Yield (%)
Cis-1,4
Trans-1,4
1,2
Mn (104)
Mw/Mn
1
2
3
4
5
6
7
8
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
AlEt2Cl
EASC
10
20
40
60
80
40
40
40
40
60
60
60
60
60
5
10
20
40
60
60
60
60
27.1
77.4
94.5
87.0
84.0
22.9
48.1
74.6
87.0
90.2
51.4
71.5
65.6
95.7
96.1
96.5
95.6
95.3
96.5
96.5
96.1
96.1
97.0
95.0
94.9
94.4
2.2
2.0
2.3
2.7
2.9
1.7
1.9
2.1
2.1
2.5
2.5
2.8
3.2
2.1
1.9
1.2
1.7
1.8
1.1
1.3
1.4
1.4
0.5
1.5
1.3
2.3
5.6
15.6
12.7
11.6
11.6
3.6
2.5
2.1
2.1
2.3
2.3
1.7
1.7
2.0
2.1
1.6
2.1
2.1
2.1
7.6
11.0
12.0
12.4
14.4
13.4
12.6
9
11
12
13
14
40
MAO
MAO
MAO
100
200
400
a Polymerization conditions: toluene, 5 mL; temperature, 25 °C; butadiene, 0.04 mol; cobalt precatalyst, 0.02 mmol.
equal, with Co1–O1 being 1.856(2) Å and Co1–O2 being 1.863(2) maximal polymer yield by using Co10 (94.5%) was reached
Å, but both shorted by 0.12 Å than those found in Co7. The when the Al/Co ratio was 40. When the Al/Co ratio was mono-
four coordination Co–N bonds all are contracted with respect tonously increased to 60 and 80, the yield decreased (entries 4
to those of Co–O, and the potential labile characters can be and 5, Table 1). The active species were probably over reduced
observed from the elongated Co1–N4 and Co1–N2. The more by excess AlEt2Cl and this led to the partial deactivation of the
congested environment around the cobalt center can be also catalytic centers. The selectivity was retained in the range of
anticipated from the shortened cobalt-nitrogen distances 96.6% to 95.3% in whole monomer conversion, and decreased
which are included in Table 1 and the decreased open space slightly with the increased amount of AlEt2Cl, which is in line
judged from the smaller O1–Co1–O2 angle. These structural with the reported results.42,44 The molecular weight of poly-
discrepancies between complexes Co7 and Co12 might be butadiene was found to be sensitive to the concentration of
responsible for their subtle catalytic behaviors for butadiene Et2AlCl. A reduction in the molecular weight of the resultant
polymerization.
polymer produced by the tridentate cobalt(II) catalysts was
observed as the Al/Co ratio raises, suggesting that the occur-
rence of the chain transfers to the aluminum compounds in
these systems and this regular influence on the molecular
weight of polymers can be verified from the general trend of
molecular weight distribution of the resultant polymer. With
the optimal Al/Co = 40, EASC was also examined as a cocata-
lyst, and the resulted system shown similar catalytic perform-
ances in terms of the selectivity and the polymer yield as well
as the polymer properties. However, the MAO activated system
led generally to lower polymer yields compared to those acti-
vated with alkylaluminum chlorides, indicating that the
neutral propagating centers are more active than the cationic
alkyls, which is reminiscent of a similar observation in the
butadiene polymerization10,41,46 and in the chromium cata-
lyzed olefin polymerization.58–61
The butadiene polymerization behavior of the complexes
The choice of cocatalyst. In the transition metal catalyzed
polymerization process, the cocatalyst plays an important role
for generating the active species, which is mainly established
as a cation–anion ion pair formed through the activation of
the catalyst. The polymerization activity, molecular weight,
stereoregularity of polymer, and chain transfer pathways are
intimately connected with the nature of the ion pairing. Thus
a judicial selection of cocatalyst is of prime importance in
designing the product architecture. Herein, complex Co10 is
chosen as a typical precatalyst for selecting a suitable cocata-
lyst and the optimum amount, and the polymerization results
are summarized in Table 1.
Our studies commenced with by choosing AlEt2Cl as the
cocatalyst, because it usually serves as the best candidate in
cobalt catalyzed butadiene polymerization, whether in terms
of activity, selectivity or the effectiveness of AlEt2Cl as a
Butadiene polymerization catalyzed with various cobalt
complexes in combination with AlEt2Cl
cheaper cocatalyst from a commercial point of view. In The butadiene polymerization behaviors under room tempera-
general, the Al/Co molar ratio does not affect the stereospecifi- ture varied among the cobalt complexes and the results are
city. The cis-1,4 selectivity was kept around 96% and the summarized in Table 2. The bis(salicylaldiminato) cobalt
remaining 4% is attributable to isolated 1,2 and trans-1,4 complex Co1 and butyl substituted Co2, in combination with
units. However, the Al/Co molar ratio has some regular influ- AlEt2Cl were both moderately active in the butadiene polymeri-
ences on the polymer yield and molecular weight of polymer. zation. With the introduction of a labile donor nitrogen,
Increasing the Al/Co molar ratio from 10 to 40 led to mono- oxygen, or sulfur, the formed catalysts Co3, Co5, Co7 and Co9
tonously increased polymer yields (entries 1–3, Table 1). The displayed obvious enhanced polymer yields in comparison to
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nitrogen derivatives, indicating the fact that more labile
donors can depart efficiently from the position proximate to
the cobalt center, leaving vacant site more available for
monomer access. Thereby, to more extent the equilibrium will
be turned to the “Co-butadiene” side where the chain propa-
gation takes place. However for structures Co3, Co9 and Co11
such a disassociation was somewhat difficult due to the inti-
mate binding. In general, the different analogues shown the
order of catalytic activity as represented by O > S > N in terms
of the heteroatoms. The effect of the complex structure on Mn
was not so conspicuous, demonstrating that the chain transfer
and/or termination reactions occurred in a similar way for all
catalysts combined with the same cocatalyst.
The introduction of such a pendant arm benefits the
selectivity as well, as shown in Table 2, the selectivity was
increased to 95.1%–98.2% (see Fig. S1 in the ESI,† 13C NMR
spectrum of polybutadiene obtained with Co7 in entry 7,
Table 2), together with small amounts of trans-1,4 and 1,2-
units. However, the subtle and discernable ligand heteroatom
effect was not easily rationalized at this stage and the struc-
tural variation imparts not only electronic but steric pertur-
bation on the active center. The heteroatom of the ligand (only
one is supposed to remain bonded to the metal in the active
site since otherwise, in the case of two ligands, we would have
an electron in excess with respect to the noble gas (18e) rule)
is assumed to influence the reciprocal orientation of the anti-
allylic unit and the monomer, and this ligand set obviously
favor the kinetic intermediate butadiene-coordinated species
“Co(alkyl)(butadiene)” to a greater extent, leading to higher cis-
1,4 structures.
Table 2 The butadiene polymerization results with various cobalt
complexesa
Microstructure (%)
Yield
Entry Precatalyst (%)
Cis-
1,4
Trans-
1,4
Mn
Mw/
Mn
1,2 (104)
1
2
3
4
5
6
7
8
Co1
Co2
Co3
Co4
Co5
Co6
Co7
Co8
Co9
Co10
Co11
Co12
86.7
69.1
95.4
89.7
87.1
79.5
95.6
85.5
96.1
94.5
84.1
76.5
94.7
95.1
93.8
96.9
95.9
97.5
98.2
93.6
96.4
96.5
93.5
95.5
3.0
2.6
4.2
2.6
2.1
1.3
0.3
3.3
2.1
2.3
2.3
3.3
2.3
7.9
3.1
2.9
2.7
2.6
1.9
1.5
2.5
1.7
2.6
2.1
2.9
1.6
2.4 11.2
2.0 11.3
1.5 15.4
2.0
1.2 14.6
1.5 7.4
3.1 19.6
1.5 7.5
1.2 12.7
4.2 7.7
1.2 22.7
8.6
9
10
11
12
a Polymerization conditions: toluene, 5 mL; temperature, 25 °C; time,
1 h; butadiene, 0.04 mol; cobalt precatalyst, 0.02 mmol; AlEt2Cl,
4.0 mmol.
Scheme 2 The proposed stabilization of the active species by labile
coordination.
The Schiff ligands were also modified with the introduction
of the tert-butyl groups on the 2,4-positions, and the new
formed complexes show a low monomer conversion but a
higher cis-1,4 selectivity than the corresponding unsubstituted
catalysts.48 The molecular weight of the polymers varied irre-
gularly within 11.2 to 19.6 × 104 for the bulky analogues, and
unanimously higher than their corresponding counterparts,
accompanied with a narrowed molecular weight distribution,
implying that the bulky ligand favors the chain propagation
rate over the chain transfer rate. Only the systems Co7 and Co8
make an exception to this rule, probably, we have to consider
that these catalysts are in some way different from the others
since Co8 reached a surprising lower level.
their analogue Co1 under the same conditions. These results
may be relevant to the labile heteroatom as a coordination
switch engaged in the polymerization. As shown in Scheme 2,
the weak coordination can occupy the vacancy released after
the monomer insertion to impede the deactivating species poi-
soning active sites, and such a coordination is also easily
removed from the metal by the monomer and reopened for its
coordination. The labile feature also offer flexibility in the
binding mode of the salen-based ligand, which is believed to
cooperate with the different binding modes of the incoming
monomer and the growing polymer chain, in such a way as to
increase the overall rate of propagation and stabilize the active
species. As such, among the three complexes bearing a nitro-
gen donor, the N,N-dimethylethyl derivative Co3 manifested
the polymer yield of 95.4%, which is probably due to the pres-
ence of a wider open active site formed by a planar ligand
structure with no adjacent rigid moiety near the cobalt center,
followed by Co5 (87.1%) and Co11 (84.1%), which is in line
with the explanation that the more opened and flexible
environment of the cobalt center is favorable for the butadiene
molecule.
A study of catalyst stability
Elevated reaction temperatures commonly resulted in the de-
activation of the catalytic systems of late-transition-metal pre-
catalysts which has been a critical deficiency because of the
highly exothermic reaction of the butadiene polymerization,
notable for the iron and cobalt systems. Therefore, enhancing
the stabilities of the precatalysts has been targeted, for
example, through modifying ligands with functional systems
based on their dentate atoms and coordination modes for
certain potential precatalysts in order to improve their catalytic
activities as well as the realization of useful products at higher
polymerization temperatures.62,63 Herein, the better performed
The catalysts Co5 and Co7, which bear a weakly coordinated
methylsulfur and methoxyl moiety, respectively, exhibited a
relatively higher polymer yield than those obtained from the
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steric bulky complexes were selected and the influence of the
reaction temperature on the butadiene polymerization was
thus investigated to explore their thermal stability at a higher
temperature. Firstly, the tetradentate complex Co2 was investi-
gated for comparison. The butadiene polymerization behavior
with the Co2/AlEt2Cl catalytic system was sensitive to the reac-
tion temperature. The conversion was gradually reduced from
69.1% at 25 °C to 62.2% at 50 °C, and was further decreased to
55.4% at 70 °C, which can be explained on the basis of the
decay of the number of active species. The selectivity also
suffered by 10%, dropping from 95.0% to 84.1%. The mole-
cular weights of the polymer decreased and the molecular
weight distributions became broader at the elevated reaction
temperature, which is in agreement with the fact that the
reduction of the molar mass with the increasing polymeri-
zation temperature was caused by an increase of the irreversible
chain transfer reactions mainly with aluminum alkyl.
In the presence of a labile donor, the Co4, Co6, Co8, Co10
and Co12/AlEt2Cl catalytic systems all showed a remarkable
thermal stability affording a higher polymer yield than the
corresponding values with Co2. As shown in Fig. 3, the
polymer yield did not suffer significantly with the increased
temperature, and in some cases, we even observed a somewhat
positive influence. In the case of Co12 polymer yields of
76.5%, 77.9% and 82.3% were obtained as the temperature
increased. The selectivity was also maintained at a level that
was as high as the original, which usually suffered seriously as
reported literatures.9,10,38,48 For example, the polymer obtained
by Co12 is in the range of 93.0% to 95.5% (Fig. 4). The for-
mation of a slightly increased trans-1,4 content at elevated
temperature is probably ascribed to the facilitated anti–syn iso-
merization because the higher reaction temperature might
supply the needed energy requirement.2 Raising the tempera-
ture also displayed a limited influence on the PDI of the poly-
butadiene produced by bulky catalysts, the molecular weight
distributions were hardly broaden, and the polydispersity
indexes remained in the range of 1.9–2.6 and kept the mono-
modal, suggesting that the single site catalysts remained even
at a higher polymerization temperature, though the fact that
Fig. 4 The cis-1,4 selectivity of complexes Co2, Co4, Co6, Co8, Co10
and Co12 toward butadiene polymerization at various temperatures.
Polymerization conditions: toluene,
5 mL; time, 1 h; butadiene,
0.04 mol; cobalt precatalyst, 0.02 mmol; AlEt2Cl, 4.0 mmol.
chain transfer responsible for the decreased molecular weight
could not be neglected. We can infer that the resultant species
could reinitiate the polymerization which is assumed to main-
tain the high monument in reactivity. On further increasing
the temperature to 90 °C, however, the significant decay in the
activity and selectivity became severe, suggesting the partial
loss of the stabilization effects.
The Co10/AlEt2Cl catalyst system was selected and evaluated
by conducting the polymerization experiments at various time
durations to examine the possible deactivation of catalytic
species. The kinetic curves of conversion of butadiene and ln
([BD]0/[BD]t) vs. the reaction time initiated by the system under
the reaction condition of the Al/Co molar ratio of 40 with
0.02 mmol precatalyst loading at room temperature, are
depicted in Fig. 5, where [BD]0 and [BD]t are the initial
monomer concentration and the unconsumed monomer con-
centration at time designated t, respectively. A good linear
relationship between ln([BD]0/[BD]t) and the reaction time was
observed from Fig. 5, indicating a first-order dependency of
polymerization rate on the monomer concentration, from
which a constant concentration of the active species and no
Fig. 3 The polymer yield of complexes Co2, Co4, Co6, Co8, Co10 and
Co12 toward butadiene polymerization at various temperatures.
Polymerization conditions: toluene,
0.04 mol; cobalt precatalyst, 0.02 mmol; AlEt2Cl, 4.0 mmol.
5
mL; time,
1
h; butadiene,
Fig. 5 The polymer yield and ln(M0/Mt) as a function of polymerization
time in the Co10/AlEt2Cl catalyzed butadiene polymerization.
4174 | Dalton Trans., 2014, 43, 4169–4178
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process is highly cis stereoselective. This explanation is consist-
ent with the mechanism proposed in Ni catalyzed butadiene
polymerization, where the experimental and theoretical
studies of the Ni catalysts revealed the existence of the back-
biting coordination of the double bond of the penultimate
inserted monomer of the growing polymer chain.64–66 In the
case of MAO in our system, however, the mechanism operated
might be slightly different, as there is an increased formation
of the trans-1,4 and 1,2 inserted units.
Conclusion
In summary, we have developed a new type of air and moisture
stable cobalt(II) complexes supported by tridentate Schiff base
ligands with a labile pendant donor. The formed catalysts
shown have improved catalytic properties, in terms of cis-1,4
incorporation and activity with respect to those of cobalt com-
plexes with two chelated Schiff base ligands. The control of the
molecular weight of polybutadiene was also obtained with
Co10/AlEt2Cl. The new catalysts represent a rare example
which are capable of stabilizing highly active and cis-1,4 selec-
tive cobalt species up to 70 °C. This study could shed light on
the design and development of potential industrial applicable
cobalt catalysts for the cis-1,4 polymerization of butadiene.
Fig. 6 The Mn of the polymer as function of polymer yield obtained
with Co10/AlEt2Cl.
significant deactivation during the propagation period can be
also concluded. The molecular weights tended to increase with
the polymer yields, and a pseudo-living nature can also be
found, indicating that controlling the molecular weight of the
resultant polymer can be partially achieved, though the trans-
fer reaction became prominent at the later phase of polymeri-
zation (Fig. 6). The formation of polymers with constantly high
level of kinetically controlled cis-14 stereoregularity indicated
that the active center was effectively stabilized by the tridentate
ligands.
Experimental
Mechanism consideration
General methods
The reaction models which describe the mechanism of diene All manipulations of air and/or moisture sensitive compounds
polymerization by cobalt catalysts have been adopted from were carried out under a dry and oxygen-free argon atmo-
models developed for Ti and Ni-based catalysts systems. In our sphere by using Schlenk techniques or under a nitrogen atmo-
previous reports involving cobalt catalysts, we have discussed sphere in a glovebox. Co(OAc)2·4H2O, 2-methoxyethanamine,
the possible reaction pathways involved in the cis-1,4, trans-1,4 (pyridin-2-yl)methanamine, quinolin-8-amine, N,N,-dimethyl-
and 1,2 formation regulated by a Co metal center46 and the ethane-1,2-diamine, 2-methoxybenzenamine, 2-(methylthio)-
polymerization results herein indicate the formation of identi- benzenamine, 2-hydroxybenzaldehyde and 3,5-di-tert-butyl-2-
cal active centers and the operation of similar reaction modes hydroxybenzaldehyde were purchased from Alfa Aesar. Methyl-
in the present catalyst system. In presence of alkyl chloride, aluminoxane was commercially available from AkzoNoble.
the mono-ligated cobalt center is presumably produced with a Diethylaluminium chloride and ethylaluminum sesquichlo-
subsequent opening for monomer coordination. The spatial ride were purchased from Acros Chemicals. Toluene was freshly
arrangement of the Co orbital and energetics leads to a cis- distilled in the presence of sodium and benzophenone prior to
1,4 mode of coordination with the incoming BD monomer. use. Polymerization grade butadiene was donated from
This monomer inserts into the growing chain forming an allyl Jinzhou Petro-China and purified by passing through four
1
coordinated terminal group which takes preferably an anti-con- columns packed with 4 Å molecular sieves and KOH. H NMR
formation in turn affording a cis stereoregulated growing (400 MHz) and 13C NMR (100 MHz) were recorded on a Varian
polymer chain. After monomer insertion, the released vacant Unity spectrometer in CDCl3 at room temperature. The IR
orbital is then reoccupied by the labile donor, thus expelling spectra were performed on BRUKE Vertex-70 FT-IR spectrophoto-
the competing deactivation species. The anti-conformer could meter. The elemental analyses were recorded on an elemen-
also isomerize to the thermodynamically more stable syn-con- tal Vario EL spectrometer. The EI-MS of the samples (dissolved
former, consequently forming the trans product, which is in ethanol–dichloromethane) was performed at THERMO
efficiently inhibited by the employment of the labile donor in SCENTIFIC ACELA-LCQ Fleet with isopropanol (containing
our system. The energetically demanded anti–syn isomeriza- 0.1% acetic acid) as the eluent. Solid state magnetic moments
tion as it involves a conformational rearrangement of the last were recorded on Quantum Design SQUID magnetometer
monomeric unit of the polymer chain renders monomer inser- running MPMS software. The effective magnetic moment
tion faster than the anti–syn-rearrangement and thus the was calculated by using the method for transition metal
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complexes. The proportion of 1,2, cis-1,4 and trans-1,4 units of
[OC6H4(2,4-tBu)CHvNCH2CH2NMe2]2Co (Co4). Yield: 57%.
polymer were determined by 1H NMR and 13C NMR. The mole- Dark red. IR (KBr, cm−1): 2952, 2906, 2867, 1623 (vCvN), 1547,
cular weights (Mn) and molecular weight distributions (Mw/Mn) 1461, 1439, 1384, 1361, 1312, 1258, 1170, 1166, 780. Anal.
of the polymer were measured at 30 °C by VISCOTEK GPC1000 Calcd for C38H62CoN4O2: C, 68.54; H, 9.39; N, 8.41. Found:
with TDA305 (Triple Detector Array) as the detector. THF was C, 68.29; H, 9.52; N, 8.67. EI-MS for C38H62CoN4O2 (m/z):
used as the eluent at a flow rate of 1.0 mL min−1 against poly- 665.24 (M+), (μeff = 4.19μB).
styrene as the calibration.
[OC6H4CHvNCH2C5H4N]2Co (Co5). Yield: 73%. Dark red.
IR (KBr, cm−1): 954, 2867, 1596 (vCvN), 1577, 1524, 1446, 1415,
1387, 1362, 1168, 836, 761. Anal. Calcd for C26H22CoN4O2:
X-ray structure determinations
Crystals for X-ray analysis were obtained as described in the C, 64.87; H, 4.61; N, 11.64. Found: C, 65.07; H, 4.40; N, 11.98.
Experimental section. The data collections were performed at EI-MS for C26H22CoN4O2 (m/z): 481.42 (M+).
−88.5 °C on a Bruker SMART APEX diffractometer with a CCD
[OC6H4(2,4-tBu)CHvNCH2C5H4N]2Co (Co6). Yield: 67%.
area detector, using graphite monochromated Mo K radiation Dark red. IR (KBr, cm−1): 2956, 2905, 2868, 1632, 1602 (vCvN),
(λ = 0.71073 Å). The determination of the crystal class and unit 1597, 1362, 1257, 1170, 1052, 779, 756. Anal. Calcd for
cell parameters was carried out by the SMART program C42H54CoN4O2: C, 71.47; H, 7.71; N, 7.94. Found: C, 71.47;
package. The raw frame data were processed using SAINT and H, 7.91; N, 7.89. EI-MS for C42H54CoN4O2 (m/z): 705.85 (M+),
SADABS to yield the reflection data file. The structures were (μeff = 4.14μB).
solved by using the SHELXTL program. The refinement was
[OC6H4CHvNC6H4OCH3]2Co (Co7). Yield: 79%. Dark red.
performed on F2 anisotropically for all non-hydrogen atoms by IR (KBr, cm−1): 3040, 2939, 2837, 1607 (vCvN), 1582, 1528,
the full-matrix least-squares method. The hydrogen atoms 1493, 1464, 1440, 1386, 1329, 1238, 1167, 1146, 1118, 1017,
were placed at the calculated positions and were included in 919, 855, 762, 744. Anal. Calcd for C28H24CoN2O4: C, 65.76;
the structure calculation without further refinement of the H, 4.73; N, 5.48. Found: C, 65.55; H, 4.74; N, 5.88. EI-MS for
parameters.
CCDC reference numbers 950121 and 950122 are for com-
plexes Co7 and Co12 respectively.
C28H24CoN2O4 (m/z): 511.22 (M+), (μeff = 4.23μB).
[OC6H4(2,4-tBu)CHvNC6H4OCH3]2Co (Co8). Yield: 67%.
Dark red. IR (KBr, cm−1): 2960, 2926, 2868, 1627 (vCvN), 1587,
1502, 1465, 1370, 1259, 1230, 1057, 1019, 853, 814, 774. Anal.
Calcd for C44H56CoN2O4: C, 71.82; H, 7.67; N, 3.81. Found:
The syntheses and characterization of the complexes
To a stirred solution of the ligand (2.0 mmol) at room C, 71.59; H, 7.79; N, 3.99. EI-MS for C44H56CoN2O4 (m/z):
temperature, an ethanol solution containing 0.5 equiv. of 736.71 (M+), (μeff = 4.21μB).
Co(OAc)2·4H2O (1.0 mmol, 0.249 g) was added. The reaction
[OC6H4CHvNC6H4SCH3]2Co (Co9). Yield: 66%. Dark red.
mixture was refluxed for 6 h and cooled down to room temp- IR (KBr, cm−1): 2927, 1607 (vCvN), 1562, 1537, 1462, 1426,
erature. The resultant precipitate was filtered, subsequently 1396, 1312, 1177, 1155, 859, 758, 670. Anal. Calcd for
washed with diethyl ether and hexane, and dried in vacuo at C28H24CoN2S2O2: C, 61.87; H, 4.45; N, 5.15. Found: C, 62.09;
40 °C overnight finally affording a dark or red powder. For H, 4.31; N, 5.55. EI-MS for C28H24CoN2S2O2 (m/z): 543.07 (M+),
Co1–Co4, the resultant reaction solution was concentrated and (μeff = 4.37μB).
diethyl ether or hexane was added to precipitate the product.
The other cobalt complexes were prepared in a similar Dark red. IR (KBr, cm−1): 2968, 2928, 1621 (vCvN), 1599, 1473,
manner. 1447, 1354, 1247, 1051, 756. Anal. Calcd for C44H56CoN2S2O2:
[OC6H4(2,4-tBu)CHvNC6H4SCH3]2Co (Co10). Yield: 72%.
[OC6H4CHvNC6H5]2Co (Co1). Yield: 79.5%. Light yellow. C, 68.81; H, 7.35; N, 3.65. Found: C, 69.19; H, 7.12; N, 3.87.
IR (KBr, cm−1): 3058, 1609 (vCvN), 1576, 1533, 1487, 1475, EI-MS for C44H56CoN2S2O2 (m/z): 767.44 (M+), (μeff = 4.28μB).
1464, 1438, 1383, 1297, 1181, 1149, 852, 791, 752. Anal. Calcd
[OC6H4CHvNC9H6N]2Co (Co11). Yield: 83%. Dark red.
For C26H20N2O2Co: C, 69.18; H, 4.47; N, 6.21. Found: C, 69.68; IR (KBr, cm−1): 3057, 1607 (vCvN), 1582, 1527, 1508, 1461,
H, 4.20; N, 6.29. EI-MS for C26H20N2O2Co (m/z): 543.52 1432, 1383, 1318, 1215, 1155, 1088, 1033, 832, 747. Anal. Calcd
(M + 2C2H5OH+), (μeff = 3.95μB).
for C32H22CoN4O2: C, 69.44; H, 4.01; N, 10.12. Found: C, 69.55;
[OC6H2-2,4-(C4H9)2CHvNC6H5]2Co (Co2). Yield: 64.1%. H, 3.79; N, 9.89. EI-MS for C32H22CoN4O2 (m/z): 553.25 (M+),
Light yellow. IR (KBr, cm−1): 2955, 2867, 1613 (vCvN), 1596, (μeff = 4.30μB).
1576, 1524, 1389, 1361, 1253, 1268, 836, 752. Anal. Calcd For
[OC6H4(2,4-tBu)CHvNC9H6N]2Co (Co12). Yield: 76%. Dark
C42H52N2O2Co: C, 74.64; H, 7.76; N, 4.15. Found: C, 75.18; red. IR (KBr, cm−1): 2954, 2905, 2867, 1605 (vCvN), 1580, 1523,
H, 7.90; N, 4.09. EI-MS for C42H52N2O2Co (m/z): 767.38 1507, 1418, 1386, 1329, 1250, 1232, 1166, 1090, 842, 831, 782,
(M + 2C2H5OH+) (μeff = 4.12μB).
761. Anal. Calcd for C48H54CoN4O2: C, 74.11; H, 7.00; N, 7.20.
[OC6H4CHvNCH2CH2NMe2]2Co (Co3). Yield: 51%. Dark Found: C, 74.55; H, 7.02; N, 6.91. EI-MS for C48H54CoN4O2
red. IR (KBr, cm−1): 2972, 2928, 2775, 1625 (vCvN), 1599, 1538, (m/z): 777.53 (M+), (μeff = 4.21μB).
1469, 1452, 1351, 1316, 1200, 1148, 1128, 1020, 913, 755, 737.
The procedure for butadiene polymerization
Anal. Calcd for C22H30CoN4O2: C, 59.86; H, 6.85; N, 12.69.
Found: C, 59.89; H, 7.02; N, 12.91. EI-MS for C22H30CoN4O2 A typical procedure for the polymerization is as follows (entry
(m/z): 441.08 (M+) (μeff = 4.36μB).
3 in Table 2): a toluene (5 mL) solution of butadiene (2.16 g,
4176 | Dalton Trans., 2014, 43, 4169–4178
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