The effect of the central donor in bis(benzimidazole)-based cobalt catalysts for the selective cis-1,4-polymerisation of butadiene

Article abstract of DOI:10.1039/c0dt00402b

A series of bis(benzimidazole)-based cobalt(II) dichloride complexes containing a range of different central donors has been synthesized and characterized. The nature of the central donor affects the binding of the ligand to the cobalt centre and determin

Full text of DOI:10.1039/c0dt00402b

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The effect of the central donor in bis(benzimidazole)-based cobalt catalysts
for the selective cis-1,4-polymerisation of butadiene†
Renan Cariou,a Juan J. Chirinos,a Vernon C. Gibson,*a Grant Jacobsen,b Atanas K. Tomov,a
George J. P. Britovsek*a and Andrew J. P. Whitea
Received 30th April 2010, Accepted 19th June 2010
DOI: 10.1039/c0dt00402b
A series of bis(benzimidazole)-based cobalt(II) dichloride complexes containing a range of different
central donors has been synthesized and characterized. The nature of the central donor affects the
binding of the ligand to the cobalt centre and determines the coordination geometry of the metal
complexes. All complexes have been shown to catalyse the polymerization of butadiene, in combination
with MAO as the co-catalyst, to give cis-1,4-polybutadiene with high selectivity. The nature of the
central donor has a marked influence on the polymerization activity of the catalysts, but does not affect
the polymer microstructure. The addition of PPh3 generally increases the polymerization activity of
these cobalt catalysts and results in predominantly (60–70%) 1,2-vinyl-polybutadiene.
salts such as CoCl2, activated with organoaluminium compounds,
are also able to polymerise 1,3-butadiene to give cis-specific
Introduction
living polymerisation.23–27 Interestingly, the presence of PPh3 or
CS2 during the polymerisation reaction results in the formation
of 1,2-polybutadiene.16–18,28–33 Numerous nickel(II) allyl complexes
have been used as polymerization initiators,34–36 whereby the most
active systems are the “ligand-free” nickel complexes, which
give polybutadiene with 93% cis-1,4 enchainment.37,38 Several
experimental and computational mechanistic studies have been
carried out on these systems.39,40
Industrial interest in synthetic polybutadiene rubbers is driven
by the low cost of the monomer and the wide variety and
versatility of the butadiene-based polymers. Polybutadienes are
formed via 1,4- or 1,2-insertions, which lead to trans-1,4, cis-1,4
or 1,2-vinyl microstructures, whereby the latter can be isotactic,
syndiotactic or atactic. The production of synthetic rubbers
reached 13 million metric tons at the end of 2008 and applications
are mostly in the automotive industry, with nearly 70% used in
the production of tyres.1 Polybutadienes are often co-polymerised
or blended to give materials with improved properties such
as Styrene-Butadiene Rubber (SBR), High Impact PolyStyrene
(HIPS) or Acrylonitrile-Butadiene-Styrene (ABS). Ziegler–Natta
neodymium-based catalysts are the main catalysts used for the in-
dustrial production of polybutadiene.2,3 Because of their ill-defined
nature, these multi-site catalysts often give polymers with broad
molecular weight distributions and therefore poor mechanical
properties. In order to gain better control over molecular weight
and polymer compositions, academic and industrial research has
focused in recent years on well-defined single-site catalysts, mainly
based on rare earth metals and first row transition metals.4,5
For example, lanthanide complexes supported by bidentate or
tridentate ligands,4,6–12 as well as “ligand-free” allyl complexes have
been investigated.13 Ricci et al. have reported Cr(II) and Co(II)
complexes supported by various phosphine ligands that are re-
markably active and selective for the polymerisation of conjugated
dienes.14–19 Furthermore, Endo and co-workers have reported that
Co(acac)3 and Co(Salen) in combination with methyl aluminoxane
(MAO) or ethyl aluminium sesquichloride (EAS) catalyse the cis-
stereospecific polymerisation of 1,3-butadiene.20–22 Simple cobalt
Recently, we have reported the production of polybutadi-
ene using chromium and cobalt catalysts, stabilized by triden-
tate bis(benzimidazolyl)amine (BIMA) ligands (Fig. 1).41 The
chromium BIMA catalyst results in a high trans-1,4 microstruc-
ture, whereas the cobalt BIMA system results in a high cis-1,4
content. Mixing of the two catalyst systems resulted in in-reactor
blending of cis and trans polybutadiene. Around the same time,
Kim et al. reported the polymerization of 1,3-butadiene using
a BIMA cobalt complex in combination with EAS as the co-
catalyst.42 Tridentate ligands have also featured in other catalyst
systems, for example bis(imino)pyridine ligands, in combination
with a range of first row transition metals (Cr, Fe, Co and Ni)43
and substituted terpyridine ligands in combination with Fe,44,45
have been reported. Catalytic activities and selectivities have been
found to be highly dependant on the metal. Iron- and cobalt-based
catalysts have generally shown very high activities, the former
resulting predominantly in trans-1,4-polybutadiene, whereas the
latter produce mainly cis-1,4-polybutadiene. Here, we report a
detailed investigation of the effect of more subtle ligand variations
on the catalytic behaviour of a series of cobalt(II) complexes,
using BIMA ligands with a range of different central donors. The
aDepartment of Chemistry, Imperial College London, Exhibition Road,
South Kensington, London, UK, SW7 2AY. E-mail: g.britovsek@
imperial.ac.uk; Fax: +44-(0)20-75945804; Tel: +44-(0)20-75945863
bIneos Technologies, Rue de Ransbeek 310, B-1120, Brussels, Belgium
† Electronic supplementary information (ESI) available: Experimental and
spectroscopic details for all ligands L1–L13 and complexes 2–13. CCDC
reference numbers 762753–762757. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c0dt00402b
Fig. 1 Chromium and cobalt catalysts with bis(benzimidazolyl)amine
(BIMA) ligands.
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complexes have been applied as catalysts for the polymerization
of 1,3-butadiene in combination with MAO as the co-catalyst. An
interesting correlation between the nature of the central donor
and the polymerisation activity has been established, whereas the
addition of PPh3 to the catalyst systems has a significant effect on
the polymer microstructure.
Results and discussion
The ligands L1–L13 were synthesized via the condensation
reaction of 1,2-phenylenediamine with the appropriate di-acid
or di-ester.46–48 The cobalt(II) complexes were prepared by the
reaction of the ligands with anhydrous cobalt(II) dichloride
in THF (Scheme 1).41 All complexes have been characterized
by elemental analysis, mass spectrometry, IR spectroscopy and
magnetic susceptibility measurements. The magnetic moments
of the complexes are in the range 3.55–4.20 BM, in agreement
with the spin-only value of 3.87 BM expected for three unpaired
electrons in these five-coordinate high spin Co(II) complexes. The
solid state structures for complexes 1, 6, 7, 8 and 12 have been
determined by X-ray diffraction. The variations at the central
donor give rise to interesting variations in the coordination around
the metal centre, which will be discussed in the next section.
Fig. 2 The molecular structure of complex 1.
◦
˚
Table 1 Selected bond lengths (A) and angles ( ) for complex 1
Co–Cl(1)
Co–N(1)
Co–N(14)
2.3420(6)
2.2895(17)
2.0535(17)
Co–Cl(2)
Co–N(4)
2.3049(6)
2.0668(15)
Cl(1)–Co–Cl(2)
Cl(1)–Co–N(4)
Cl(2)–Co–N(1)
Cl(2)–Co–N(14)
N(1)–Co–N(14)
98.35(2)
108.52(5)
175.00(5)
100.51(5)
75.71(6)
Cl(1)–Co–N(1)
Cl(1)–Co–N(14)
Cl(2)–Co–N(4)
N(1)–Co–N(4)
N(4)–Co–N(14)
86.52(4)
122.90(5)
103.68(5)
75.67(6)
118.19(6)
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) for complex 6
Co–Cl(1)
Co–N(1)
Co–N(14)
2.2593(5)
2.7546(14)
2.0293(14)
Co–Cl(2)
Co–N(4)
2.3082(5)
2.0271(14)
Cl(1)–Co–Cl(2)
Cl(1)–Co–N(4)
Cl(2)–Co–N(1)
Cl(2)–Co–N(14)
N(1)–Co–N(14)
102.442(19)
106.72(4)
169.10(3)
100.67(4)
70.58(5)
Cl(1)–Co–N(1)
Cl(1)–Co–N(14)
Cl(2)–Co–N(4)
N(1)–Co–N(4)
N(4)–Co–N(14)
88.31(3)
129.32(4)
106.70(4)
71.28(5)
108.76(6)
Changing the N(1)–H unit in complex 1 to a N(1)–Ph unit in
complex 6 (Fig. 3 and Table 2) results in a very marked lengthening
˚
˚
˚
by ca. 0.5 A of the Co–N(1) bond from 2.2895(17) A in 1 to
2.7546(14) A in 6, suggesting that the amine donor is more weakly
bound in this case. This could be due to the weaker basicity of
the central donor in this case or due to increased steric hindrance.
As a consequence, the geometry at the cobalt centre resembles
a flattened tetrahedron with the N(1) ◊ ◊ ◊ Co vector approaching
via the basal plane. Taking this long interaction into account,
the geometry at the cobalt centre is best described as highly
distorted trigonal bipyramidal, illustrated by the t value of 0.66.
Associated slight shortenings of both the Co–N(4) and Co–N(14)
bond lengths in 6 as compared to those in 1 and 2 are also observed,
along with a ca. 10◦ contraction of the N(4)–Co–N(14) angle
[118.19(6)◦ in 1, 108.76(6)◦ in 6], presumably a consequence of the
reduced steric crowding around the metal.
Complex 7 crystallised with two independent molecules (7-I
and 7-II) in the asymmetric unit, shown in Fig. 4 and Figure S4
in the Supporting Information,† respectively. The two molecules
have similar geometries, but with some notable variations (see
Table 3). The tripodal ligand coordinates in a tetradentate fashion
and the geometry at the cobalt centre is best described as trigonal
Scheme 1
Solid state structures
The geometry at the cobalt centre in complex 1 (Fig. 2 and Table 1)
is best described as distorted trigonal bipyramidal with a t value
of 0.87;49 N(1) and Cl(2) occupy the axial sites with an N(1)–Co–
Cl(2) angle of 175.00(5)◦. The ligand adopts a facial coordination
mode with the two chlorides cis to each other. The cobalt centre
˚
lies ca. 0.40 A out of the {Cl(1),N(4),N(14)} plane in the direction
3
˚
of Cl(2). The axial Co–N(sp ) bond [Co–N(1) 2.2895(17) A] is
significantly longer than the equatorial Co–N(sp2) bonds [Co–
˚
N(4) 2.0668(15), Co–N(14) 2.0535(17) A]. The Co–N(1) bond is
˚
shorter by about 0.12 A compared to the N-methyl-substituted
˚
complex 2 [Co–N(1) = 2.414(4) A], whose solid state structure has
been reported.50
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) for complex 7
7-I
7-II
7-I
7-II
Co–Cl
Co–N(4)
Co–N(24)
2.2899(4)
2.0414(13)
2.0302(12)
2.2842(5)
2.0595(12)
2.0552(12)
Co–N(1)
Co–N(14)
2.3826(12)
2.0342(12)
2.3690(12)
2.0312(13)
Cl–Co–N(1)
179.07(3)
104.16(3)
74.80(4)
75.65(4)
112.69(5)
176.62(3)
106.60(4)
75.55(5)
75.30(5)
117.79(5)
Cl–Co–N(4)
Cl–Co–N(24)
N(1)–Co–N(14)
N(4)–Co–N(14)
N(14)–Co–N(24)
104.79(4)
105.28(4)
75.38(4)
117.25(5)
111.31(5)
104.25(4)
101.99(4)
76.37(5)
115.57(5)
108.97(5)
Cl–Co–N(14)
N(1)–Co–N(4)
N(1)–Co–N(24)
N(4)–Co–N(24)
Fig. 3 The molecular structure of complex 6.
bipyramidal with N(1) and Cl in the axial positions (t values
of 1.03 and 0.98 for 7-I and 7-II respectively). Similar crystal
structures have been reported previously, which were obtained as
a different solvate51 or with a different tetrachlorocobaltate(II)
Fig. 4 The molecular structure of one (7-I) of the two crystallographically
independent cationic complexes present in the crystals of complex 7. The
chloride counteranion has been omitted.
counter-anion.52,53 The cobalt atom lies ca. 0.52 and 0.51 A for
˚
7-I and 7-II respectively, out of the {N(4),N(14),N(24)} plane in
the direction of the chloride ligand. The Co–N(sp3) bond lengths
were grown from a methanol solution. An octahedral cationic
complex with two trans coordinating methanol molecules and a
non-coordinating chloride counteranion was observed.54
˚
˚
of 2.3826(12) A in 7-I and 2.3690(12) A in 7-II compare with
˚
distances of 2.2895(17), 2.4398(18) and 2.7546(14) A for the Co–
N(sp3) linkage in 1, 2 and 6 respectively.
Complex 8 was crystallised from N,N-dimethylformamide
(DMF) and a five-coordinate complex as shown in Fig. 5 was
obtained. A slightly distorted square-based pyramidal geometry
is seen in this case, with one chloride ligand Cl(1) occupying
the apex (t value of 0.02). The neutral ligand with a central sp2
nitrogen donor adopts a meridional coordination mode in contrast
to the bis(benzimidazolyl)amine ligands, which coordinate in a
facial mode in complexes 1 and 2. The cobalt centre lies ca.
˚
0.52 A out of the {Cl(2),N(8),N(1),N(17)} plane in the direction
of Cl(1). All three Co–N bond lengths are very similar, ranging
˚
between 2.1359(11) and 2.1407(11) A (Table 4). The bond to the
˚
central nitrogen donor [Co–N(1) 2.1360(10) A] is, as expected,
significantly shorter than those seen in complexes 1 and 2
˚
[2.2895(17) and 2.414(4) A]. A different crystal structure analysis
of complex 8 has been reported previously where the crystals
Fig. 5 The molecular structure of complex 8.
Dalton Trans., 2010, 39, 9039–9045 | 9041
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◦
˚
Table 4 Selected bond lengths (A) and angles ( ) for complex 8
Co–Cl(1)
Co–N(1)
Co–N(17)
2.3308(4)
2.1360(10)
2.1359(11)
Co–Cl(2)
Co–N(8)
2.2862(4)
2.1407(11)
Cl(1)–Co–Cl(2)
Cl(1)–Co–N(8)
Cl(2)–Co–N(1)
Cl(2)–Co–N(17)
N(1)–Co–N(17)
111.558(16)
100.99(3)
147.04(3)
100.13(3)
74.58(4)
Cl(1)–Co–N(1)
Cl(1)–Co–N(17)
Cl(2)–Co–N(8)
N(1)–Co–N(8)
N(8)–Co–N(17)
101.41(3)
98.37(3)
98.35(3)
74.38(4)
146.01(4)
◦
˚
Table 5 Selected bond lengths (A) and angles ( ) for complex 12
Co–Cl(1)
Co–N(4)
2.2484(4)
2.0135(10)
Co–Cl(2)
Co–N(14)
2.2478(3)
2.0312(10)
Cl(1)–Co–Cl(2)
Cl(1)–Co–N(14)
Cl(2)–Co–N(14)
114.474(14)
107.14(3)
107.98(3)
Cl(1)–Co–N(4)
Cl(2)–Co–N(4)
N(4)–Co–N(14)
106.65(3)
111.06(3)
109.40(4)
Fig. 6 The molecular structure of complex 12.
decrease in the reverse order and the strongest interactions are
observed in the four-coordinate complex 12.
The molecular structure of complex 12 (Fig. 6 and Table 5)
shows the cobalt centre to have a tetrahedral geometry with the
ligand coordinating via the benzimidazolyl nitrogens in a bidentate
fashion. The amine nitrogen N(1) does not coordinate to the metal
centre, which is believed to be a consequence of the steric bulk of
the 2,6-dimethylphenyl substituent. Instead, N(1) lies 4.0216(10)
Butadiene polymerization
Upon activation with MAO, the cobalt complexes 1–13 poly-
merise 1,3-butadiene to give high molecular weight polybutadiene,
with Mn values ranging from 500 to 1,000 kg mol-1 and with
narrow molecular weight distributions. The microstructure was
analysed by NMR and was found to be predominantly cis-1,4-
polybutadiene (> 92%) together with small amounts of trans-
1,4 (< 6%) and 1,2-vinyl-polybutadiene (< 2%) (see Table 6).
This polymer microstructure is observed for many cobalt-based
catalysts, including CoCl2, and it appears that the different ligands
do not affect the microstructure of the polymer.
In contrast, the catalytic activities were found to be strongly
affected by the nature of the ligand, in particular the nature of
the central donor. Simple cobalt salts such as CoCl2 are known to
polymerise 1,3-butadiene upon activation with MAO.24 Under our
conditions, the activity observed for CoCl2 was 480 gmmol-1h-1,
˚
A away from the metal and has a trigonal planar geometry, with
˚
N(1) lying only ca. 0.10 A out of the plane of its substituents. The
Co–N(4) and Co–N(14) bond lengths of 2.0135(10) and 2.0312(10)
˚
A, respectively, are similar to those seen in complex 6 and slightly
shorter than those in complexes 1 and 2.
From these structural studies we can conclude that there appears
to be a direct correlation between the central Co–N distances
and the Co–N(benzimidazolyl) bond lengths. The central Co–N
distances are affected by the steric and electronic properties of
the N-substituent and decrease in the order 12 (N-2,6-Me2Ph)
> 6 (N–Ph) > 2 (N–Me) > 7 (N–CH2Bim) > 1 (N–H) and
the strongest interaction is observed in the pyridine substituted
meridional complex 8. The Co–N(benzimidazole) bond lengths
Table 6 Butadiene polymerization with bis(benzimidazole) cobalt catalysts 1–13a
Microstructure/mol%b
c
Complex (D)
Activity/g mmol-1 h-1
tr-1,4
cis-1,4
1,2-vinyl
Mn/g mol-1
Mw/Mn
1 (N–H)
130
780
750
690
320
1,200
630
70
140
60
720
70
4
4
5
5
3
4
5
2
4
5
6
6
4
4
96
95
94
95
96
95
94
97
95
95
92
94
96
95
0
1
1
0
1
0
1
0
1
1
2
1
1
0
800,000
1010,000
680,000
880,000
660,000
700,000
840,000
760,000
570,000
700,000
760,000
660,000
730,000
580,000
1.3
1.4
1.4
1.4
1.4
1.5
1.4
1.4
1.3
1.3
1.5
1.3
1.4
1.4
2 (N–Me)
3 (N–iPr)
4 (N–Cy)
5 (N–CH2Ph)
6 (N–iPh)
7 (N–CH2Bim)
8 (pyridine)
9 (CHMe)
10 (O)
11 (S)
12 (N-2,6-Me2Ph)
13 (N–Me)d
CoCl2
210
480
a Conditions: catalyst: 10 mmol; co-catalyst: MAO; MAO/Co = 600; solvent: 20 mL toluene; 10 min; T = 298 K, p = 1 bar. b Determined by IR and 13
C
NMR spectroscopy. c Determined by GPC against polystyrene standards and reported uncorrected. d N-Methyl-benzimidazolyl instead of benzimidazolyl
donors.
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which is somewhat higher than the activity of 150 gmmol-1h-1
reported previously, which was obtained at 291 K and with a
MAO/Co ratio of 100. The complexes containing ligands with an
alkyl substituent at the central amine donor, resulted generally in
high activities of ca. 700 g mmol-1 h-1. In the case of pre-catalyst
2, the activity of the catalyst was monitored over time up to three
hours. A linear increase in the percentage conversion of butadiene
with time was observed, indicating no catalyst decomposition
during this period.
AlEt3/H2O/phosphine system decreases with an increase in the
steric bulk of the phosphine.57 Ricci et al. have reported similar
effects in the system CoCl2(PR3)2/MAO.16 The 1,2-vinyl units
are of particular importance as they provide potential sites
for cross-linking within the polybutadiene materials. Therefore,
polymerisation reactions of 1,3-butadiene were carried out in the
presence of one equivalent of PPh3. As can be seen in Table 7,
the addition of PPh3 increases the catalytic activity for all cobalt
catalysts 1–13. Interestingly, also in this series of polymerisation
runs, the highest catalytic activities are observed for complex 6 (N–
Ph), followed by the N-alkyl substituted complexes 2 and 3. The
molecular weight of the resultant polybutadiene (Mn) ranges from
150 to 350 kg mol-1, with molecular weight distributions between
1.5 and 2.0. These polymer molecular weights are somewhat lower
than those obtained in PPh3-free polymerisations, indicating that
PPh3 induces chain transfer reactions. The polymer microstructure
shows an increase in the 1,2-vinyl content upon addition of
PPh3 with approximately 65% 1,2-vinyl units and ca. 35% cis-
1,4 units observed in all cases, whereas the trans-1,4 units are
present in negligible amounts. The cis-1,4 and 1,2-vinyl units are
believed to be randomly distributed along the polymer chain, as
observed in other cobalt/phosphine systems.16,58 The nature of the
benzimidazole ligand does not appear to affect the microstructure
of the polymer.
The pre-catalyst 6 (D = N–Ph) displayed the highest activity
under the conditions used of 1200 gmmol-1h-1. This is the complex
that showed a weak interaction between the central donor and the
metal centre. Complete dissociation of the central N-donor, as in
complex 12 results in a much lower activity, whereas complex 8,
which contains the strongly binding central pyridine donor, also
displays a low activity of 70 gmmol-1h-1. It appears that for high
catalytic activity a certain degree of lability of the central donor is
highly beneficial. Too strongly bound donors, as in complex 8, as
well as non-binding donors, as in complex 12, significantly reduce
the catalytic activity. Indeed, this effect is also seen when using
complex 9 (D = C(H)Me), which bears a bidentate ligand without
a central donor and shows a low activity of 140 gmmol-1h-1.
The effect of the nature of the central donor is further illustrated
by the two catalysts 10 (D = O) and 11 (D = S). In the
case of 11, containing a thioether central donor, an activity of
720 gmmol-1h-1 is observed, comparable to that of complexes
bearing N-alkyl substituted ligands (2–4). For this complex a weak
Co–S interaction is anticipated based on similar observations with
related ligands.55 For catalyst 10 (D = O) a very low activity of
60 gmmol-1h-1 is obtained, which is most likely attributable to the
lack of coordination by the ether donor. Non-coordinating oxygen
donors were also observed in the solid state structure of the cation
[Co(L10)2]2+.56
Mechanistic considerations
The mechanism of transition metal catalysed butadiene poly-
merisation has been extensively studied, in particular for the
“ligand-free” polybutadienyl nickel catalysts.39,40 For the cobalt-
based catalysts studied here, activation of the cobalt dichloride
pre-catalysts with MAO is expected to result initially in a cobalt-
methyl species, with the metal centre being either in a +II or +I
oxidation state. Reduction of Co(II) to Co(I) has been previously
observed in the alkylation of related bis(imino)pyridine cobalt
dichloride complexes.59 Binding of the butadiene monomer can
The addition of Lewis bases such as phosphines or CS2
to cobalt-mediated 1,3-butadiene polymerization reactions has
been previously shown to increase the 1,2-vinyl content of
the resulting polymer.16–18,28–33 Furthermore, studies by Jang
et al. have shown that the activity of the Co(2-ethylhexanoate)2/
2
4
occur either via one (h ) or both (h ) double bonds resulting,
after insertion, in an allyl ligand, which is bound either in an
Table 7 Butadiene polymerization with bis(benzimidazole) cobalt/PPh3 catalystsa
Microstructure/mol%b
c
Complex (D)
Activity/g mmol-1 h-1
tr-1,4
cis-1,4
1,2-vinyl
Mn/g mol-1
Mw/Mn
1 (N–H)
300
1
2
1
1
1
2
2
0
1
1
0
1
2
2
37
33
38
34
34
36
36
36
33
32
39
33
31
30
62
65
61
65
65
62
62
64
66
67
61
66
67
68
330,000
210,000
160,000
300,000
180,000
180,000
280,000
330,000
340,000
290,000
270,000
310,000
320,000
190,000
1.6
1.9
2.3
1.7
1.8
2.0
1.8
1.7
1.5
1.7
1.8
1.7
1.8
1.9
2 (N–Me)
3 (N–iPr)
4 (N–Cy)
2,220
2,100
1,100
1,440
2,400
840
330
360
150
660
5 (N–CH2Ph)
6 (N–Ph)
7 (N–CH2Bim)
8 (pyridine)
9 (CHMe)
10 (O)
11 (S)
12 (N-2,6-Me2Ph)
13 (N–Me)d
CoCl2
390
750
1,680
a Conditions: catalyst: 10 mmol; co-catalyst: MAO; MAO/Co = 600; solvent: 20 mL toluene; 10 min; T = 298 K, p = 1 bar. b Determined by IR and 13
C
NMR spectroscopy. c Determined by GPC against polystyrene standards and reported uncorrected. d N-Methyl-benzimidazolyl instead of benzimidazolyl
donors.
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Table 8 Crystal Data, Data Collection and Refinement Parameters for compounds 1, 6, 7, 8 and 12a
Data
1
6
7
8
12
Formula
solvent
C16H15Cl2CoN5
C3H7NO·0.5(C4H10O)
517.32
blue shards
0.11· 0.11 · 0.05
173
Monoclinic
C2/c (no. 15)
22.6305(3)
8.25098(10)
28.3753(4)
—
112.2954(15)
—
4902.24(12)
8
1.402
Cu-Ka
7.716
C22H19Cl2CoN5
C3H7NO
556.35
blue needles
0.28 · 0.08 · 0.06
173
Monoclinic
P21/n (no. 14)
8.04885(4)
17.64238(8)
18.51859(9)
—
97.2342(5)
—
2608.72(2)
4
1.417
Cu-Ka
7.281
[C24H21ClCoN7](Cl)
2(C3H7NO)·0.25(C4H10O)
702.03
purple blocky needles
0.29 · 0.13 · 0.07
173
C19H13Cl2CoN5
C3H7NO
514.27
green platy needles
0.42 · 0.09 · 0.04
173
Monoclinic
P21/n (no. 14)
14.6250(2)
7.70335(12)
19.6751(3)
—
102.3472(16)
—
2165.36(6)
4
1.578
Mo-Ka
1.068
C24H23Cl2CoN5
2(C3H7NO)
657.50
blue needles
0.30 · 0.13 · 0.06
173
Monoclinic
P21/n (no. 14)
8.94991(11)
17.56344(19)
20.0433(2)
—
96.4273(11)
—
3130.83(6)
4
1.395
Mo-Ka
0.758
Formula weight
colour, habit
Crystal size/mm
T/K
Crystal system
Triclinic
¯
Space group
P1 (no. 2)
˚
a/A
15.3900(4)
15.7696(4)
15.8027(4)
68.049(2)
70.125(2)
77.034(2)
3324.43(16)
4c
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
V/A
Z
Dc/g cm-3
1.403
Mo-Ka
0.721
radiation used
m/mm-1
2q max/deg
no. of unique reflns
measured
143
143
64
64
65
4673
5050
20849
7005
10513
obs, |Fo| > 4s(|Fo|)
no. of variables
R1(obs), wR2(all)b
3714
317
0.0294, 0.0714
4457
326
0.0281, 0.0834
11658
935
0.0344, 0.0706
4932
299
0.0280, 0.0662
7218
393
0.0313, 0.0771
ꢀ
ꢀ
ꢀ
ꢀ
a Details in common: graphite monochromated radiation, refinement based on F2. b R1 = ꢀFo| - |Fcꢀ/ |Fo|; wR2 = { [w(Fo - Fc2)2]/ [w(Fo2)2]}
;
2
1/2
2
w-1 = s (Fo2) + (aP)2 + bP. c There are two crystallographically independent molecules.
1
3
h - or h -fashion to the cobalt centre. In the case of the related
tridentate terpyridine iron/MMAO catalyst system, the sequence
of butadiene coordination, insertion and polybutadienyl ligand
rearrangement has been investigated in a detailed computational
analysis by Tobisch.45 We propose that a similar mechansim is in
operation here, although a cis-1,4 enchainment is observed here,
compared to the trans-1,4 enchainment in the case of iron. The
flexibility in the binding mode of the benzimidazole-based ligand
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 to overall rate of propagation and stabilise the active
species.
Experimental
The synthesis and characterization of complex 1 are given below,
whereas the experimental details for other ligands and complexes
2–13 can be found in the supplementary material.
N,N-bis(1H-benzimidazol-2-ylmethyl)-N-amine]cobalt(II)
dichloride, 1
Equimolar quantities of N,N-bis(1H-benzimidazol-2-ylmethyl)-
N-amine L1 (1.06 g, 3.83 mmol) and anhydrous CoCl2 (0.498 g,
3.83 mmol) were stirred in 30 ml of THF at room temperature for
48 h. The blue solid was filtered, washed twice with THF (2 ¥ 20
ml), once with Et2O (20 ml) and dried under vacuum overnight.
Yield = 1.30 g (83%). Anal. Calcd (%) for C16H15N5Cl2Co (407.16):
C, 47.20; H, 3.71; N, 17.20. Found: C, 47.58; H, 3.23; N, 17.25.
+FAB-MS (m/z): 371 [M-Cl]+, 335 [M-2Cl]+. IR (KBr (s), cm-1):
3326 (s), 3286 (s), 3182 (s), 3071 (s), 2927 (m), 2784 (m), 1664 (m),
1622 (m), 1594 (m), 1542 (m), 1491 (m), 1471 (s), 1456 (s), 1426
(m), 1386 (m), 1369 (m), 1332 (s), 1275 (s), 1243 (m), 1225 (m),
1114 (m), 1071 (m), 1047 (s), 1003 (m), 915 (m), 868 (m), 827 (m),
750 (s), 689 (m), 644 (w), 628 (m). meff = 4.21 BM.
In conclusion, a family of cobalt(II) complexes bearing BIMA-
type ligands with different central donors has been synthesized.
Single crystal X-ray analyses have revealed a variation in the
bond length between the central donor and the cobalt centre,
resulting in either five-coordinate or four-coordinate complexes
or complexes with an intermediate geometry. All complexes,
upon activation with MAO, catalyse the polymerisation of 1,3-
butadiene to give predominantly cis-1,4 polybutadiene with high
selectivity. The resultant polymers are of high molecular weight
and narrow molecular weight distribution. Catalysts containing
a tertiary amine as the central donor were found to be the most
active and the degree of lability of the central amine appears to
be related to the activity of the catalysts. The microstructure of
the polybutadiene obtained from these cobalt complexes could
be modified to predominantly 1,2-vinyl-polybutadiene by the
addition of PPh3 to the polymerization medium. The precise nature
of the active species in these cobalt-based catalysts is not yet known
and will be the subject of future computational and experimental
studies.
1,3-Butadiene polymerisation procedure
In a nitrogen-filled glove-box, a known quantity of pre-catalyst
was transferred into a Schlenk. The pre-catalyst was activated by
adding the desired amount of MAO and stirred for 15 min or
until obtaining a clear solution. When required, 1 ml of a PPh3
solution (10 mmol ml-1 in toluene) was added and the mixture aged
for 30 min. Toluene (20 ml) was added, and the Schlenk tube was
purged with 1 bar of butadiene gas for no more than 5 s. The
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polymerisation was carried out for 10 min and was terminated
by ceasing the 1,3-butadiene supply, venting off the excess 1,3-
butadiene, followed by addition of acidified MeOH containing
0.01% of 2,6-di-tert-butyl-4-methylphenol (BHT) as antioxidant.
After filtration, the polymer was washed with MeOH and dried
under vacuum at 60 ◦C for 12 h.
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