Synthesis and structure of group 4 symmetric amidinate complexes and their reactivity in the polymerization of α-olefins

Article abstract of DOI:10.1021/om4006998

The steric properties of various nitrogen substituents on amidines were tuned in order to obtain group 4 mono- and bis(amidinate) dimethylamido or chloride complexes. The amidinate dimethylamido and chloride complexes were prepared, and their solid-state

Full text of DOI:10.1021/om4006998

Article
pubs.acs.org/Organometallics
Synthesis and Structure of Group 4 Symmetric Amidinate Complexes
and Their Reactivity in the Polymerization of α‑Olefins
Tatyana Elkin,† Naveen V. Kulkarni,† Boris Tumanskii,† Mark Botoshansky,†,§ Linda J. W. Shimon,‡
and Moris S. Eisen*,†
†Schulich Faculty of Chemistry, Technion−Israel Institute of Technology, Haifa 32000, Israel
‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76100, Israel
S
* Supporting Information
ABSTRACT: The steric properties of various nitrogen
substituents on amidines were tuned in order to obtain
group 4 mono- and bis(amidinate) dimethylamido or chloride
complexes. The amidinate dimethylamido and chloride
complexes were prepared, and their solid-state as well as
their solution-state structures were studied. After the activation
by MAO, these complexes were tested in the polymerization of
propylene and ethylene. A noticeable influence of the amidine
carbon and nitrogen substituents on the activity of the catalyst
and properties of the obtained polymer was observed. Further,
a plausible mechanism for the ethylene polymerization process
is presented taking into account a combination of ESR-C60 and
MALDI-TOF experiments, shedding light on the nature of the catalytic species.
active species and yields an isotactic fraction. The second route
involves the formation of a cationic monoamidinate dialkyl
complex, resulting from ligand dissociation of the bis-
(amidinate) complex and its migration to an aluminum moiety
in MAO; the resulting mono(benzamidinate) entity produces
the elastomeric polypropylene due to the open coordination
site.14b,17
In addition, we have demonstrated that after the mono-
(benzamidinate) species are formed, the remaining amidinate
ligand may undergo a rearrangement, producing a κ6
coordination through the ipso-phenyl substituent, inducing
steric hindrance between the para-substituent of the ring and
the growing polymer chain. This interaction will prevent the
polymeric chain termination and correspondingly aid the
formation of polymers with higher molecular weights.
Changing the substituents at the para-position of the aryl
group revealed that bulkier substituents lead to the formation of
polymers with higher molecular weights and correlates with a
linear free energy relationship of the para-substituent with the
Taft steric parameter (Scheme 1).18
INTRODUCTION
■
The design and synthesis of the well-defined homogeneous
catalysts for the polymerization of α-olefins remains an area of
immense interest in academic as well as industrial applica-
tions.1−3 A large number of catalysts developed in this area of
research are metallocenes1c−f,2g−j,4−7 and half-metalloce-
nes;2b,e,f,j,8−11 however, an extensive number of post-metal-
locene complexes containing chelating ancillary ligands have
received considerable attention in recent years.2e,f,k,m,n,9b,12−15
The amidinate ligands are considered to be sterically
equivalent to the cyclopentadienyl ligands, varying most
significantly in their electronic properties.8h,9a,10d The amidi-
nate group [R1-C(NR2)2]− is a four-electron donor, as
compared to the six electrons of the cyclopentadienyl ligand,
in turn offering a higher electrophilicity at the metal center in
such complexes.13a−d,14a,b,15b,16 Amidinate ligands are partic-
ularly attractive owing to the ease with which they may be
modified, producing ligands with specific steric and electronic
constraints. The combination of these factors as well as the
facile synthetic protocols for this class of ligands has allowed for
the production of various organometallic complexes useful for
the polymerization of α-olefins.13−15
Group 4 complexes containing amidinate ligands with an
isopropyl substituent at the ipso-position, instead of an aryl
group, do not form stereospecific polypropylenes when
activated by MAO. This is due to the dynamic behavior of
the complexes, which leads to the complete loss of the C2-
symmetry around the metal center. Interestingly, introduction
of fluorine atoms into different positions of an amidinate N-
Previous research in our group has revealed that group 4
bis(benzamidinate) dichloride and dialkyl complexes, when
activated by methylaluminoxane (MAO), form catalytically
active species, which can polymerize propylene, affording a
mixture of isotactic and elastomeric polypropylenes.17 The
mechanistic studies have suggested that the activation of the
complexes takes place via two pathways. The first route involves
the formation of a cationic bis-amidinate alkyl complex as the
Received: July 16, 2013
© XXXX American Chemical Society
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Scheme 1. Plausible Mechanism for the Formation of a Mixture of Isotactic and Elastomeric Polypropylene by Titanium
Bis(amidinate) Complexes
phenyl substituent revealed that the meta-fluorinated ligand aids
in the suppression of the rate of termination of the growing
chain, whereas a para-fluorine accelerates the rate of insertion
of the monomer.18
mesityl, cyclohexyl, and o-methoxyphenyl groups as the
amidinate N-substituents. In order to minimize possible steric
effects or π bonding of the ipso-carbon substituent, this position
has been occupied by proton, methyl, or ethyl moieties.
The corresponding alkyl amidines were prepared in high
yields following literature procedures by heating the suitable
amine with the respective ortho-esters in a 2:1 ratio, in the
presence of catalytic amounts of HCl or AcOH (eq 1).20Ligand
The research presented in this work investigates the effect of
electron-donating functionalities at the amidinate nitrogen.
Furthermore, we were interested in studying the effect of an
additional chelating site containing an electron-donating group
in the ortho-position of the N-aryl substituent. In order to avoid
the κ6 coordination through the ipso-phenyl substituent, the
ipso-position of the amidinate ligand has been substituted with
hydrogen, methyl, or ethyl moieties. Herein we report the
synthesis of various titanium and hafnium amidinate complexes,
their crystallographic structures, and their catalytic performance
in the polymerization of ethylene and propylene. We present a
combination of ESR, C60 radical trapping, and MALDI-TOF
studies describing the formation of the active species, trapping
some unique features of the complexes and shedding light on
the polymerization mechanism.
RESULTS AND DISCUSSION
■
Synthesis and Structure of Ligands. As has been
previously demonstrated, the presence of the fluorine atoms
on the N-phenyl rings reduces the electron density over the
amidinate HN-CN backbone and induces dissociation and
rearrangement of the ligand about the metal center.19 We
expect that placing electron-donating groups should increase
the electron density over the coordination cavity, increasing the
ligand-to-metal bond strength, and further inhibiting ligand
migration. Furthermore, the introduction of additional
coordinating groups to the ligand structure potentially increases
the denticity of the resulting ligand in the metal complex. In
consideration of these factors, a series of amidinate ligands have
been prepared, which incorporate 2,6-diisopropylphenyl,
7 has not been reported in the literature and was obtained in
92% yield. Ligand 7 was crystallized from an ethanol/water
(70:30) solution, and its solid-state structure is presented in
Figure 1. (The crystal data and refinement details for ligand 7
are presented in the Supporting Information.)
In the solid state, ligands 6 (Supporting Information) and 7
are isostructural, indicating that the ipso-C substitution does not
affect the amidine motif.21 In both ligands, one of the phenyl
rings is disposed in the same plane as that of the amidine N
C-NH backbone, whereas the other is situated nearly
orthogonal (torsion angles, 92.52° and 87.74° for ligands 6
and 7, respectively). Although the groups at the ipso-carbon are
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the strong intramolecular hydrogen bonding between the o-
OMe group and the amidine N−H group.24
In solution, the amidine ligands generally exhibit various
tautomeric structures, making the NMR studies complex for the
study of these forms (Scheme 2).25,26 1H NMR solution studies
of ligand 7 show as expected two different methoxy groups at
room temperature corroborating with the solid-state structure.
Synthesis and Structure of Mono(amidinate) Com-
plexes. In the preparation of the Ti complexes 8−14,
Ti(NMe2)4 was used as a metal precursor. The titanium
complexes bearing amidinate ligands with different steric bulk
were chosen to investigate the steric effect on their
coordination behavior. We have found that when one
equivalent of Ti(NMe2)4 was treated with either one, two, or
three equivalents of ligand 1, the pentacoordinative mono-
(amidinate) tri(dimethylamido) complex 8 was always obtained
(eq 2). Heating in toluene and/or increasing the reaction time
Figure 1. Mercury presentation of the molecular structure of ligand 7
showing the hydrogen bond interaction between the OMe moiety and
the N−H amidine group (50% probability ellipsoids). Hydrogen
atoms (besides the amidine N−H) were removed for clarity.
Representative bond lengths (Å) and angles (deg): C(5)−N(1) =
1.3743(16); C(5)−N(2) = 1.2731(16); C(4)−N(1) = 1.4023(17);
C(6)−N(2) = 1.4099(16); C(7)−O(1) = 1.3681(16); C(8)−O(1) =
1.4206(18) Å; N(2)−C(5)−N(1) =120.66(12); C(7)−O(1)−C(8) =
117.70(11); C(5)−N(2)−C(6)−C(12) = 87.74°.
small, it seems that they are sterically bulky enough to force one
of the N-phenyl rings to an anti-position, leading to a trans-syn
tautomeric form (III in Scheme 2). The OMe group at the
ortho-position forms a hydrogen bond with the amidine N−H
due to the favorable distance for a typical intramolecular
hydrogen bond22 (2.176 and 2.164 Å for ligands 6 and 7,
respectively) and brings the corresponding phenyl ring and the
amidine backbone into the same plane, serving to stabilize the
resulting structure. Interestingly, in a similar system when the
substituent at the ipso-carbon position is a hydrogen (ligand 5),
both phenyl rings are coplanar with the amidine moiety.23
Hafelinger and Kuske24 have defined the parameter ΔCN
=
did not yield any other product. Single crystals of complex 8
were obtained by slow evaporation of a hexane solution, and
the solid-state structure is presented in Figure 2. (The crystal
data and refinement details for complex 8 are provided in the
Supporting Information.)
The mono(amidinate) complex 8 (Figure 2) exhibits a five-
coordinative geometry and is best described as three-legged
piano stool where the amidinate moiety is at the apical (bench)
position.27 From the crystal structure it is evident that once the
̈
d(C−N) − d(CN) (where d is the bond length in Å) for the
central N−C−N linkage of amidines. This parameter ranges
from 0 (in a trans-anti hydrogen-bonded dimer (V in Scheme
2)) to 0.178 Å (in an amidine where conjugation is minimized
due to bulky substituents on both nitrogen and carbon atoms).
In our case, the parameter ΔCN values are found to be 0.113
and 0.101 Å for ligands 6 and 7, respectively, corroborating a
localization of the proton on a particular nitrogen atom due to
Scheme 2. Plausible Tautomeric Structures for Amidines in Solution
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Figure 3. Mercury presentation of the molecular structure of complex
9 (50% probability ellipsoids). Hydrogen atoms are omitted for clarity.
Representative bond lengths (Å) and angles (deg): Ti(1)−N(1) =
2.3087(12); Ti(1)−N(2) = 2.1372(12); Ti(1)−N(3) = 1.8838(13);
Ti(1)−N(4) = 1.9115(13); T(1)−N(5) = 1.9197(13); C(10)−N(1)
= 1.3055(19); C(10)−N(2) = 1.3251(19) Å; N(2)−Ti(1)−N(1) =
59.56(4); N(2)−Ti(1)−N(5) = 89.51(5); N(2)−Ti(1)−N(4) =
124.29(5); N(1)−C(10)−N(2) = 114.55(13); N(2)−Ti(1)−N(1)−
C(13) = 5.74°.
Figure 2. Mercury presentation of the molecular structure of complex
8 (50% probability ellipsoids). Hydrogen atoms are omitted for clarity.
Representative bond lengths (Å) and angles (deg): Ti(1)−N(1) =
2.2926(13); Ti(1)−N(2) = 2.1847(13); Ti(1)−N(3) = 1.9087 (15);
Ti(1)−N(4) = 1.9002(14); Ti(1)−N(5) = 1.9130(14); C(13)−N(1)
= 1.310(2); C(13)−N(2) = 1.328(2) Å; N(2)−Ti(1)−N(1) =
60.20(5); N(2)−Ti(1)−N(3) = 123.28(6); N(3)−Ti(1)−N(5) =
98.64(6); N(4)−Ti(1)−N(3) = 106.77(6); N(1)−C(13)−N(2) =
116.87(15); N(2)−Ti(1)−N(1)−C(13) = 3.69°.
first ligand has been coordinated to the metal center, due to its
large cone angle (208°), an additional amidinate ligand will not
be able to insert, hence yielding specifically the corresponding
mono(amidinate) complex.
Decreasing the steric bulk of the ligands from the
diisopropylphenyl motif (ligand 1) to the slightly less sterically
demanding mesityl substituents (ligands 2 and 3) did not allow
the insertion of a second coordinating ligand, also producing
always the corresponding monoamidinate complexes, 9 and 10,
respectively (eq 2), regardless of the amount of the ligands and
the reaction conditions.
The X-ray diffraction studies performed in single crystals of
complexes 9 (Figure 3) and 10 (Figure 4) show also
pentacoordinative complexes like that observed in complex 8.
(The crystal data and refinement details for complexes 9 and 10
are presented in the Supporting Information.)
From the crystal structures we can learn that ligands 2 and 3
are still bulky enough, inducing large cone angles at the metal
center (complex 9 = 181°; complex 10 = 184°), preventing the
insertion of a second ligand, thereby producing only the
corresponding monoamidinate complexes.
Interestingly, in all three mono(amidinate) titanium
complexes (8−10), the M−N bond lengths of the coordinated
amidinates are nonsymmetric (complex 8: Ti(1)−N(1) =
2.2926(13); Ti(1)−N(2) = 2.1847(13) Å, complex 9: Ti(1)−
N(1) = 2.3087(12); Ti(1)−N(2) = 2.1372(12) Å, and complex
10: Ti(1)−N(1) = 2.181(4); Ti(1)−N(2) = 2.232(4) Å), due
to the different dispositions of the amidinate nitrogen relative
to the amido groups. One nitrogen atom of the amidinate
ligand is almost located trans (∼150−160°) to one of the three
amido groups, whereas the second amidinate nitrogen is
situated between two amido groups (∼120−130°). Small
torsion angles of the amidinate ligand in complexes 8, 9, and 10
(N(2)−Ti(1)−N(1)−C(13) = 3.69°, N(2)−Ti(1)−N(1)-
C(13) = 5.74°, and N(2)−Ti(1)−N(1)−C(10) = 0.2(2)o,
Figure 4. Mercury presentation of the molecular structure of complex
10 (50% probability ellipsoids). Hydrogen atoms are omitted for
clarity. Representative bond lengths (Å) and angles (deg): Ti(1)−
N(1) = 2.181(4); Ti(1)−N(2) = 2.232(4); T(1)−N(3) = 1.904(4);
Ti(1)−N(4) = 1.901(4); Ti(1)−N(5) = 1.939(4); C(10)−N(1) =
1.327(6); C(10)−N(2) = 1.317(6) Å; N(2)−Ti(1)−N(1) =
59.92(14); N(1)−Ti(1)−N(5) = 91.63(16); N(5)−Ti(1)−N(4) =
99.73(19); N(1)−Ti(1)−N(4) = 128.07(16); N(1)−C(10)−N(2) =
113.0(4); N(2)−Ti(1)−N(1)−C(10) = 0.2(2)°.
respectively) indicate that the ancillary ligand is connected in a
σ-fashion.
The Ti−N bond lengths of the mono(amidinate) complexes
8 and 9 are found to be longer as compared to all other
reported amidinate complexes. According to the literature,
most of the Ti−N (amidine) bond lengths (>95%) fall in the
range 2.030−2.275 Å28 with the longest bond reported as
2.312(4) Å in a silyl-linked bis(amidinate)-cyclopentadienyl
mixed complex.29 The monoamidinate complexes 8 and 9
presented here are the third and second longest Ti−N bonds
reported for chelating amidinate complexes, respectively.
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Interestingly, in complex 10, the Ti−N (amidine) bond lengths
fall within the literature reported range. It seems plausible that
the methyl group at the ipso-carbon of the amidinate moiety in
complex 10 donates electron density into the amidinate moiety,
resulting in stronger metal−nitrogen bonds as compared to a
hydrogen atom in a similar position in complex 9.
Synthesis of Bis(amidinate) Complexes. When two
equivalents of ligand 4, which bears a cyclohexyl group on the
amidinate nitrogens, was reacted with Ti(NMe2)4 in toluene,
the corresponding bis(amidinate) complex 11 (eq 3) was
obtained in high yield. This result indicates that the cyclohexyl
groups are flexible enough to allow the formation of
bis(amidinate) complexes.30
methoxide group, producing the observed polymeric material.
It is important to point out that in complexes 13 and 14 the
steric interaction of the ipso-carbon substituents restricts the
rotation of N-phenyl rings (based on the X-ray of complex 14),
thus preventing the interaction of the o-methoxy groups with
other metal centers.
The solid-state structure of the titanium bis(amidinate)
complex 14 (Figure 5) exhibits a pseudo-octahedral geometry.
The ortho-methoxy motif (ligands 5, 6, and 7) is considered
to be less bulky as compared to the diisopropyl and mesityl
substituent groups31 and may be used in order to prepare the
corresponding bis(amidinate) complexes having an additional
chelation and an electron-donating group.
Figure 5. Mercury presentation of the molecular structure of complex
14 (50% probability ellipsoids). Hydrogen atoms are omitted for
clarity. Representative bond lengths (Å) and angles (deg): Ti(1)−
N(1) = 2.103(3); Ti(1)−N(2) = 2.221(3); Ti(1)−N(3) = 2.186(3);
Ti(1)−N(4) = 2.107(3); Ti(1)−N(5) = 1.883(3); Ti(1)−N(6) =
1.901(3); C(1)−N(1) = 1.343(4); C(1)−N(2) = 1.311(4) Å; N(2)−
Ti(1)−N(1) = 60.49(10); N(5)−Ti(1)−N(2) = 91.82(12); N(2)−
Ti(1)−N(1)−C(1) = 5.54; N(4)−Ti(1)−N(3)−C(4) = 0.03.
The reaction of ligand 5 with Ti(NMe2)4 in a 2:1 ratio
exhibits a very peculiar behavior. At room temperature in
toluene, the corresponding bis(amidinate) complex 12 was
obtained when the product was isolated within the first two
hours (eq 3), while if the reaction mixture was stirred for
additional periods of time (beyond this two hours), an
insoluble deep dark red precipitate was obtained. When the
same reaction was performed in hexane, the corresponding
complex 12 could be isolated within the first four hours;
however leaving the reaction mixture for longer periods of time
resulted in the formation of the same precipitate. When the
reaction was carried out in THF, the isolation of the complex
12 was achieved only in the first hour of the reaction.
Interestingly, when analytically pure complex 12 was allowed to
stand in a toluene solution overnight (15 mg/mL), the
formation of the same dark red precipitate was observed; it is
likely that the resulting precipitate is polymeric in nature due to
its insolubility in various polar solvents. These results indicate
that complex 12 is probably a kinetic product of the reaction,
whereas the precipitate seems to be the thermodynamic one.
On the other hand, the reaction of ligands 6 and 7 with
Ti(NMe2)4 in a 2:1 ratio yields only the expected monomeric
bis(amidinate) complexes 13 and 14, respectively, in high
yields, regardless of the reaction conditions (eq 3). Interest-
ingly, the different behavior of complex 12, as compared to 13
and 14, relates to the substitution at the ipso-carbon of the
amidinate moiety. In complex 12, the presence of the amidinate
(The crystal data and refinement details for complex 14 are
presented in the Supporting Information.) The dimethylamido
groups are disposed in a cis-position, N(5)−Ti(1)−N(6) =
100.48(13)°, providing a pseudo-C2-symmetry for the complex.
Each dimethylamido group is disposed opposite one of the
amidinate nitrogen atoms. The Ti−N bond lengths of the
dimethylamido groups (1.883(3) and 1.901(3) Å) and the Ti−
N bond lengths of the coordinated amidinate ligands (2.103(3),
2.221(3), 2.107(3), and 2.186(3) Å) are comparable to
previously reported amidinate complexes.18,19 Both of the
amidinate ligands are connected in a σ-fashion (torsion angles
N(2)−Ti(1)−N(1)−C(1) = 5.54°; N(4)−Ti(1)−N(3)−C(4)
= 0.03°).
The observed asymmetry of the amidine N−Ti bond lengths
are due to the trans influence of the corresponding amido
moieties (angles, N(6)−Ti(1)−N(2) = 156.22(11)°, N(5)−
Ti(1)−N(3) = 152.26(11)o).18
It is important to note that our attempts to prepare
mono(amidinate) complexes with ligands 5, 6, and 7, under
various conditions, were unsuccessful. In all cases the
bis(amidinate) complexes were always obtained (complex 12
1
hydrogen, which is acidic (appearing in the H NMR at δ =
9.36 ppm in the complex, as compared to δ = 8.25 ppm in the
free ligand), presumably forms a hydrogen bond with a
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forms and then precipitates as described above). Interestingly,
when each of the ligands 5, 6, and 7 were reacted with each of
the mono(amidinate) complexes 8, 9, and 10, under various
conditions, the corresponding bis(amidinate) complexes 12,
13, and 14, respectively, were obtained; however no mixed
ligand bis(amidinate) complexes were obtained. This is likely
due to the strong coordination of ligands 5, 6, and 7 to the
metal center, displacing the former ligands. This result indicates
that the o-methoxide-containing amidinate ligands are more
nucleophilic than the amidinate ligands containing the mesityl
and diisopropylphenyl moieties.
respectively (eq 4). Interestingly, when we tried to synthesize
the corresponding zirconium complexes by reacting the lithium
salts of the ligands (6a or 6b) with ZrCl4·2THF, we were
unable isolate the desired bis(amidinate) complex. When the
same reaction was performed between three or one equivalent
of the ligand 6a and HfCl4, either the corresponding tris- or the
mono(amidinate) complex was obtained (complex 17 and 18,
respectively) (Scheme 3). However the latter complex was
always obtained with some amount of the corresponding
tris(amidinate) complex (∼10%). This result indicates that the
energy of activation to replace the second chloride of the metal
precursor after the first metathesis of the ligand must be higher
than that of the corresponding third chloride.
In order to obtain the corresponding dichloride complexes,
the lithium salts of the ligands were produced by the reaction of
n
the neutral ligands with BuLi, followed by the reaction with
All the bis(amidinate) complexes in this study show fluxional
behavior in solution at room temperature. This fluxionality may
involve various processes including opening (κ2 → κ1) and
closing (κ1 → κ2) of the amidinate ligand,19 a Bailar rotation,30
or a methoxide interaction with the metal center. In the case of
complex 14, the thermodynamic parameters (ΔH⧧ = 41.02 kJ
TiCl4·2THF. When ligand 5 was treated with nBuLi, in toluene
or THF, an insoluble lithium complex was formed. Our
attempts to react this precipitate, in situ, with the metal
precursor under the various reaction conditions were not
successful in obtaining the desired bis(amidinate) dichloride
complex and resulted in the formation of a myriad of products.
The lithium salts of ligands 6 and 7 were easily prepared in situ
(followed by 1H NMR) to obtain the corresponding complexes
6a and 7a (eq 4). When two equivalents of the lithium salts 6a
K
−1 mol−1 and ΔS⧧ = −36.88 J K−1) calculated for the dynamic
process from the line-broadening analysis suggest that the
Bailar mechanism is predominant.30
Polymerization of Propylene. The polymerization of
propylene was performed at room temperature for 3 h in
toluene with 30 mL of liquid propylene after the activation of
the precatalysts with methylalumoxane as the cocatalyst in a
ratio of 1:1000 (M:Al) (Table 1).18b Lower MAO amounts did
not activate the complexes for the polymerization of propylene
(for ethylene vide infra).
Table 1. Polymerization of Propylene Promoted by the
Amidinate Complexes Activated by MAO
a
complex
mass of polymer (g)
activity (×104)
8
0.86
2.92
1.95
1.12
0.42
0.57
0.41
0.74
0.75
1.56
4.48
3.03
2.53
0.90
1.33
0.79
1.62
1.72
9
10
11
12
13
14
15
16
a
Activity (g of polymer (mol of catalyst)−1 h−1).
When the polymerization reaction was carried for 1 h, only a
trace amount of polymer was obtained, indicating the slow rate
of activation.
and 7a were reacted with one equivalent of TiCl4·2THF, the
bis(amidinate) dichloride complexes 15 and 16 were obtained,
Scheme 3. Reaction of 6a with HfCl4 Showing the Formation of Mono- or Tris(amidinate) Complexes
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Table 2. Polymerization of Propylene: Heptane-Insoluble Fraction
a
b
c
d
e
f
g
complex
mass (g)
Mw
Mn
PD
Ri
Rt
mmmm (%)
mp (°C)
11
12
13
14
15
16
0.42
0.22
0.14
0.12
0.34
0.47
196 000
821 000
225 000
270 000
246 000
497 000
65 300
391 000
72 600
90 000
98 400
160 300
3.0
2.1
3.1
3.0
2.5
3.9
1.8
1.2
0.5
2.7
3.8
2.5
0.2
0.7
0.2
1.2
1.0
40
27
54
40
42
43
143.4
140.3
149.9
150.1
145.4
142.8
3.1
a
b
c
d
e
Molecular weight (g/mol). Number average molecular weight (g/mol). Polydispersity. Rate of monomer insertion (mmol/h). Rate of chain
f
g
termination (μmol/h). Pentad analysis measured by 13C NMR. Melting point of polymer from the second DSC curve.
After activation with MAO, the catalytic species obtained
from the complexes 11−16 polymerized propylene, forming
two polymeric fractions, which were separated following a
Soxhlet extraction in heptane.32 The heptane-soluble fraction
gave an elastomeric polymer, whereas the heptane-insoluble
polymer was a stereoregular material. We have shown that the
stereoregular fraction is produced by a cationic pseudo-C2-
symmetric bis(κ2-amidinate) alkyl species, whereas the
elastomeric fraction is produced by the corresponding cationic
mono(amidinate) dialkyl species.17 The production of these
two fractions will be discussed further as a function of the
different active catalysts.
Complex 12 was the unique complex that gives only one
insoluble polymeric fraction with a narrow polydispersity,
indicating the formation of only one active species. It is
important to point out that when MAO was added to a toluene
solution of the complex, no precipitate was formed.
14 this ratio is larger as compared to complex 13 (2.5 × 103
and 1.7 × 103 for complexes 14 and 13, respectively).
For the corresponding titanium dichloride complexes 15 and
16, containing a methyl or an ethyl moiety at the ipso-carbon,
respectively, the same trend regarding molecular weights is
observed; however the trend for the insertion rates is reverse.
This result indicates that for complex 16 the ratio Ri/Rt (3.8 ×
103) is higher than that of complex 15 (2.3 × 103). For the
complexes that are discussed in this article, the reactivity of the
amidinate titanium dichloride complexes is higher than that of
the corresponding dimethylamido analogues. This result is
likely due to a slower activation for the latter as compared to
the former complexes. It is worth mentioning that both types of
complexes will yield the same cationic entity and consequently
similar isotactic polymer fractions. Moreover since there is a
large excess of MAO, a minimal effect for the different
counterions could be expected. In the dimethylamido
complexes 13 and 14, the polymer termination rate was
found to be slower than that of the corresponding dichloride
complexes 15 and 16, indicating the importance of the
counterion.1b,2o,18b,34
The rates of monomer insertion and chain termination for
each of the complexes were calculated using eqs 5 and 6,
respectively,18 and were used to compare the catalytic
polymerization activity of the complexes.
Complex 11, which bears a hydrogen on the ipso-carbon and
a cyclohexyl group on each of the amidinate nitrogen atoms,
exhibits higher Ri and Rt values as compared to the N-phenyl-
substituted complexes (12, 13, and 14), producing a polymer
with a lower molecular weight.
Monomer insertion rate (Ri)
m(polymer) [g]
=
g
⎡
⎤
Mw(monomer)
× time [h]
⎣
⎦
(5)
mol
Heptane-Soluble Fraction. As we have shown above, the
stereoirregular elastomeric fraction is produced by the
corresponding cationic mono(amidinate) dialkyl species
formed during the activation with MAO.17 When comparing
the elastomeric fraction obtained by complexes 13 and 14, we
observed that the complex with the ethyl-substituted ligands at
the ipso-position (complex 14) induced the formation of a
polymer with a higher molecular weight as compared to the
polymer obtained with complex 13 (Table 3). In addition, the
polymerization activity, rate of monomer insertion, and the rate
of chain termination exhibited by complex 13 are larger than
that of complex 14. The same trend of a polymer with a higher
molecular weight, a lower activity, and lower rates for monomer
insertion and chain termination is observed for the ethyl-ipso-
substituted complex 16 as compared to the methyl-ipso-
substituted complex 15. The larger coordinative unsaturation
of the cationic mono(amidinate) active species plausibly
induces the larger activation rates as compared to the
corresponding bis(amidinate) species. When comparing the
complexes presented in Tables 2 and 3, the mono(amidinate)
complex that is generated from complex 11 exhibits the larger
activity and the higher insertion and termination rates plausibly
due to its large coordinative unsaturation.
m(polymer) [g]
Mn(polymer) × time [h]
(6)
Chain termination rate (Rt) =
Heptane-Insoluble Fractions. All the polymers that were
heptane-insoluble (Table 2) exhibit higher molecular weights as
compared to the elastomeric fractions with narrow poly-
dispersities, indicating that these fractions are plausibly
obtained by a single-site catalytic species.33 The percentage of
the isotactic pentad domains (mmmm) ranges from 27% to
54% and melting points from 140.3 to 150.1 °C.
Among the dimethylamido titanium compounds, complex
14, bearing an ethyl group at the ipso-carbon, has the lower
monomer insertion rate (Ri) as compared with complexes 12
and 13, bearing a hydrogen atom and a methyl group at the
ipso-carbon, respectively. This result indicates that for a series of
similar complexes the less bulky substituent at the ipso-carbon
induces a faster monomer insertion rate. Although the rate of
insertion for complex 13 is higher than that of complex 14, the
molecular weight Mn of the polymer obtained with complex 14
is higher than that obtained with complex 13. Since the
molecular weight of a polymer depends on the ratio between
the rate of insertion (monomer insertion) and the rate of
termination (formation of chains), it is clear that for complex
Polymerization of Ethylene. The polymerization of
ethylene was performed in toluene after the activation of the
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It is interesting to point out that the presence of a bulkier
substituent at the ipso-position of the amidinate ligand, in all
presented bis(amidinate) complexes, induced lower activities
and lower insertion rates (H > Me > Et). This result is
intriguing, as the ethyl group is not expected to be in close
proximity to the cationic active site. A close investigation of the
X-ray structure of complex 16 shows that the N-phenyl rings
are restricted in their rotation and will consequently dispose the
aromatic ring in such a manner that the methoxy group will
point toward the metal either in the catalyst or at the MAO.
This interaction is presumed to inhibit the rapid insertion as
was found for similar complexes containing the H/Me groups
at the same position or as compared with the cyclohexyl moiety
in complex 11.
The titanium mono(amidinate) complexes (8, 9, and 10)
exhibit good catalytic activity after their activation with MAO in
the polymerization of ethylene and propylene, producing high-
density polyethylene or elastomeric polypropylene (soluble in
hot heptane; mmmm = 12−15%), respectively.17,18 Interest-
ingly, in these complexes, a profound steric influence of the
amidine carbon and N-phenyl substituents on the rate of
monomer insertion and rate of chain termination was observed.
The independently prepared hafnium mono- and tris-
(amidinate) chloride complexes were tested for their catalytic
activity in polymerization reactions. The mono(amidinate)
complex 17 was found to be highly active (16.2 × 103 and 27.0
× 103 g mol−1 h−1 atm−1 for ethylene and propylene,
respectively), whereas the corresponding tris(amidinate)
complex 18 was inactive.
Table 3. Polymerization of Propylene: Heptane-Soluble
Fraction
f
mass
(g)
mmmm
(%)
a
b
c
d
e
complex
Mw
Mn
PD
Ri
Rt
8
0.86
2.92
1.95
0.90
0.43
0.29
0.40
0.28
105 000
55 000
67 700
32 500
16 800
23 800
19 800
41 500
17 200
12 800
20 900
13 000
7000
6.1
4.3
3.2
2.5
2.4
2.7
2.2
2.8
6.8
23.0
15.4
8.8
16.5
76.1
31.1
28.3
21.5
9.1
15
12
13
10
11
15
10
18
9
10
11
13
14
15
16
3.6
8800
1.9
9000
3.2
15.0
6.2
14 800
2.2
a
b
Molecular weight (g/mol). Number average molecular weight (g/
c
d
e
mol). Polydispersity. Rate of monomer insertion (mmol/h). Rate of
f
chain termination (μmol/h). Pentad analysis measured by 13C NMR.
precatalysts with MAO in a 1:1000 (M:Al) ratio, under
constant pressure of ethylene (9.86 atm) at room temperature
for 0.5 h,14f and the results are presented in Table 4.
The generated cationic species, in all cases, produced high-
density polyethylene with narrow polydispersities. Precatalyst
12, with the hydrogen atom at the ipso-position of the
amidinate ligand, induces the formation of a high molecular
weight polyethylene due to the larger rate of insertion as
compared to the rate of chain termination. When comparing
the polymerization activity obtained using complexes 13 and 14
or 15 and 16 as precatalysts, the complexes possessing a methyl
substituent at the ipso-position of the amidinate were found to
be slightly more active than those with the corresponding ethyl
moieties. Regarding the molecular weights, the ratio of Ri/Rt for
complex 13 (Ri/Rt = 4.8 × 103) is lower than that of complex
14 (Ri/Rt = 12.6 × 103), resulting in a lower molecular weight
polyethylene. The opposite was observed for the dichloride
complexes 15 (Ri/Rt = 12.2 × 103) and 16 (Ri/Rt = 2.4 × 103),
which resulted in the formation of a low molecular weight
polymer for the latter complex, due to the high rate of
termination.
Radical Trapping Experiments. In order to understand
the polymerization mechanism and the nature of the active
catalytic species formed during the reaction, complex 15 was
mixed with MAO (1:100) in a sealed NMR tube with toluene-
d8; a precipitate was promptly formed, and the ESR data of the
samples were measured. A broad asymmetrical ESR signal was
observed, indicating the formation of agglomerates containing
paramagnetic Ti(III) species.17b
Only the precipitate was found to be EPR active, whereas the
mother liquor did not produce any ESR signal, indicating the
absence of Ti(III) species in solution.
To elucidate the nature of the Ti(III) in the reaction mixture,
the reaction of complex 15 and MAO (1:100) was performed
with the addition of C60 as a radical trapping agent.35
As it is expected that a methyl radical formed during the
reaction will be trapped by C60, visible light (λ > 500 nm) was
used at the ESR cavity to observe the reversible dissociation of
the dimer of the fullerenyl radical (eq 7).
When the polymerization of ethylene was performed with
complex 15 in a 1:500 and 1:100 ratio of catalyst to MAO, at
the same experimental conditions, smaller amounts of polymer
were obtained (activity: 6.6 × 103 and 0.81 × 103 (g mol−1 h−1
atm−1), respectively), indicating the dependence of the activity
of the complexes on the cocatalyst concentration.
Similarly, in the formation of polyethylene, the activity and
the rate of monomer insertion obtained using the precatalyst 11
were found to be the largest, as was also observed in the
formation of polypropylene.
Table 4. Data for the Polymerization of Ethylene Promoted by the Amidinate Complexes Activated by MAO
a
b
c
d
e
f
g
complex
mass (g)
A
(× 103)
Mw
Mn
PD
Ri
Rt
mp (°C)
8
0.29
0.96
0.87
0.90
0.13
0.53
0.53
0.86
0.46
3.19
8.95
8.36
11.79
1.78
7.49
6.25
11.49
6.40
261 000
391 000
236 600
690 400
1016 000
406 000
745 000
687 000
203 100
104 400
150 400
43 000
2.5
2.6
5.5
1.5
1.8
3.0
2.1
2.0
3.0
20.4
68.5
62.0
75.2
9.7
5.5
12.8
40.4
4.6
138.4
137.2
137.3
139.2
138.2
140.3
137.8
132.3
135.0
9
10
11
12
13
14
15
16
460 300
564 400
135 300
354 800
343 500
67 700
0.5
39.0
31.2
61.5
32.8
8.1
2.5
5.0
13.6
a
b
c
d
Activity (g of polymer) × (mol of catalyst−1 h−1 atm−1). Molecular weight (g/mol). Number average molecular weight (g/mol). Polydispersity.
e
f
g
Rate of monomer insertion (mmol/h). Rate of chain termination (μmol/h). Melting point of polymer from the second DSC curve.
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Figure 6. MALDI-TOF analysis of the sample prepared from complex 15 and MAO using fullerene as radical trapping agent. Catalyst added in a
ratio of complex 15:MAO 1:100; (a) MALDI-TOF spectrum of the reaction precipitate; (b) ESR spectrum of the reaction mixture at high resolution
•
(aH(3H) = 0.035 G) under visible light irradiation (λ > 550 nm); (c) simulated spectrum of C60Me.
Figure 7. MALDI-TOF analysis of sample prepared from complex 15 and MAO using fullerene as radical trapping agent after backfilling with
ethylene. Catalyst added in a ratio of complex 15:MAO 1:100. (a) MALDI-TOF spectrum of the solid part of catalytic slurry. (b) ESR spectrum of
the reaction mixture shows stable adduct of multiple additions of Me-radical to C60 (g = 2.0022) and paramagnetic agglomerate of Ti(III) and MAO
(g = 1.972).
I
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Figure 8. MALDI-TOF analysis of sample prepared from complex 15 and MAO using fullerene as radical trapping agent after backfilling with
ethylene. Catalyst added in a ratio of complex 15:MAO 3:100. (a) MALDI-TOF spectrum of the solid part of the catalytic slurry. (b) Enlargement of
the signal that corresponds to the highest mass. (c) ESR spectrum of the reaction mixture.
MeC60−C60Me ⇌ 2•C60Me
Ti(IV) and producing the corresponding complex c bearing a
radical alkyl chain. Additional ethylene insertions will form
(7)
complex d. An expected β-H elimination of the radical chain
will form the Ti(IV)-hydride complex (e), and the oligomeric
alkyl radical chain will be trapped by C60 (Figure 8b and c). The
Ti(IV) hydride complex plausibly undergoes a metathesis with
MAO, regenerating the Ti(IV) catalyst a. The regeneration of
complex a is proposed based on the additional methyl radicals
that can be trapped by one molecule of C60, which is observed
only after the addition of ethylene (Figure 8a).
The ESR spectrum showed a characteristic signal for the
•C60Me radical (Figure 6), which was further supported by
the respective ions at 735 for the C60Me radical and 736 for
•
HC60Me in MALDI-TOF analysis, indicating the formation of
the methyl radical via the reduction of the dimethyl Ti(IV)
complex with MAO, producing the Ti(III) species. It is
important to note that the reaction between MAO and C60
•
does not produce the C60Me radical.
The parallel reaction of complex a with MAO forms the
corresponding cationic complex f, which will be the active
species for the observed polymerization of ethylene.
When an aliquot of ethylene was added to the catalytic
mixture, unlike in the previous experiments,17b,36 no decrease of
the Ti(III) species was observed, even after multiple additions
of ethylene. However, it was seen that multiple methyl radical
fragments were trapped by a single fullerene molecule (Figure
7), but the trapping of polyethylene chains was not seen at the
ratio of catalyst to MAO of 1:100. Interestingly, when the
catalyst loading was increased to 3:100, a decrease in the Ti(III)
species was observed after multiple additions of ethylene. In
addition, multiple additions of methyl radicals on fullerene were
observed, as well as a polyethylene chain containing 17
monomer units attached to C60 (Figure 8). Furthermore, we
see that if the catalyst loading is increased further, reaching the
ratio of 10:100, no polymer chain-substituted fullerene was
observed.
On the basis of the results presented above and by
comparison to similar studies reported earlier,17b,36 a catalytic
cycle is proposed (Scheme 4). The starting Ti(IV) complex a is
reduced by MAO to form the corresponding Ti(III) complex b
producing a methyl radical, which was trapped by C60 (Figure
7). Complex b reacts with ethylene, reoxidazing the Ti(III) to
CONCLUSIONS
■
Various group 4 complexes were prepared with amidines
bearing electron-donating and/or -coordinating substituents at
the nitrogen moiety. The steric properties of the ligands were
tuned in order to prepare the mono- and bis(amidinate)
complexes. In solution, the complexes exhibit fluxional
behavior. The polymerization of propylene, promoted by
bis(amidinates), after their activation with MAO, provided two
polymeric fractions. All the obtained heptane-soluble polymers
were elastomeric in nature with low molecular weights, whereas
the insoluble polymers were slightly to moderately stereo-
regular with mmmm ≈ 27−54%. In the polymerization of
ethylene, all the complexes yield a high-density polyethylene.
Interestingly, a noticeable influence of the amidine ipso-
substituent on the activity of the catalyst and properties of the
obtained polymer was observed. The presence of a bulkier
substituent at the ipso-position of the amidinate ligand induced
J
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Scheme 4. Plausible Mechanism for Polymerization of Ethylene Catalyzed by a Titanium(IV) Complex
lower activities and lower monomer insertion rates (H > Me >
Et). Considering the distant location of the ipso-substituent
from the active metal center in the κ2-bonded amidinate, this
result is quite remarkable. By close investigation of the
structural features of the ipso-ethyl-substituted complex (14)
it is envisaged that, due to the steric hindrance induced by the
ethyl group, the N-phenyl rings are not able to freely rotate.
Hence, the aromatic rings will be posed with the methoxy
group pointing toward either the metal center or MAO and
presumably impeding the rapid monomer insertion as
compared to similar complexes containing the smaller H/Me
groups at the same position.
The highest activity observed by complex 11 as compared to
all the bis(N-anisyl)-substituted titanium complexes indicates
the plausible methoxide interaction with the metal center,
reducing the polymerization activity of the latter complexes.
All the mono(amidinate) complexes exhibit good activity in
the polymerization of ethylene and propylene, producing high-
density polyethylene and elastomeric polypropylene. The steric
effect of the amidine ipso-carbon and N-phenyl substituents on
the polymerizations was observed and follows the same trend as
in the corresponding bis(amidinates).
EXPERIMENTAL SECTION
■
All manipulations of air-sensitive materials were performed with the
exclusion of oxygen and moisture in flamed Schlenk-type glassware on
a dual-manifold Schlenk line or interfaced to a high-vacuum (10−5
Torr) line, or in a nitrogen-filled M-Braun glovebox with a medium-
capacity recirculator (1−2 ppm O2).
Argon and nitrogen gases were purified by passage through a MnO
oxygen-removal column and a Davison 4 Å activated molecular sieve
column. All the common and deuterated solvents (THF, toluene,
hexane, and toluene-d8) were distilled and stored over Na/K alloy.
The NMR spectra were recorded on Bruker AM 300 and AM 500
spectrometers. Chemical shifts for 1H and 13C were referenced to
internal solvent resonances and are reported relative to TMS. NMR
experiments for the air-sensitive metal complexes were conducted on
Teflon valve-sealed tubes (J-Young) after vacuum transfer of the
solvent in a high-vacuum line. The NMR experiments for
polypropylene (in tetrachloroethylene-d2 (TCE)) and polyethylene
(in 1,2,4-trichlorobenzene with DMSO-d6 capillary) were carried at
363 K on a 300 MHz or 500 MHz NMR spectrometer.
The single-crystal material was immersed in Paratone-N oil and was
quickly fished with a glass rod and mounted on a Kappa CCD
diffractometer under a cold stream of nitrogen. Data collection was
performed using monochromated Mo Kα radiation using φ and ω
scans to cover the Ewald sphere.37 Accurate cell parameters were
obtained with the amount of indicated reflections (Supporting
Information).38 The structure was solved by SHELXS-97 direct
methods39 and refined by the SHELXL-97 program package.40 The
atoms were refined anisotropically. Hydrogen atoms were included
using the riding model. Mercury 3.141 software was used for molecular
graphics. The cell parameters and refinement data are presented in the
Supporting Information.
In addition, a plausible mechanism for the polymerization,
indicating the nature of the catalytic species, is presented using
a combination of ESR, C60 trapping radical, and MALDI-TOF
experiments.
K
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Melting points of the polymers were measured by DSC (Polymer
Laboratories, UK) from the second heating thermogram (heating
rate:10 °C/min). Molecular weights and polydispersities of polymers
were determined by the GPC method on the Waters-Alliance 2000
instrument using 1,2,4-trichlorobenzene as a mobile phase at 160 °C.
Polystyrene standards were used for the standard calibration curve of
the GPC. Elemental analysis of all the compounds was carried out on a
Flash 2000 CHNS analyzer.
MALDI-TOF LD+ and LD− experiments were performed on a
Waters MALDI Micromass MX spectrometer using the standard
Micromass 96-well matrix along with fullerene, which has a higher
ability for light absorption and ionization than Ag. Mass analyzing was
performed in the reflectron mode in the region between 300 and 3000.
ESR spectra were recorded on a Bruker EMX-10/12 X-band (ν = 9.4
GHz) digital ESR spectrometer. All spectra were recorded at a
microwave power of 10−1 mW and a 100 kHz magnetic field
modulation of 1.0−0.5 G amplitude. Digital field resolution was 2048
points per spectrum, allowing all hyperfine splitting to be measured
directly with accuracy better than 0.2 G. Spectral processing and
simulation were performed with Bruker WIN-EPR and SimFonia
software. A standard TEMPO solution (10−3 M) was used for an
estimation of the number of radicals in the catalytic mixture (no. of
radicals = 1.7 × 1018).
All of the anilines, orthoesters, and ZrCl4 were purchased from
Sigma-Aldrich; HfCl4 was purchased from Strem Chemicals. All the
chemicals were used without further purification unless otherwise
stated. TiCl4·2THF,42 ZrCl4·2THF,42 Ti(N(CH3)2)4,43 and ligands
[(2,6-(CH(CH3)2)2C6H3)NC(H)NH(2,6-(CH(CH3)2)2C6H3)] (1),
[(2,4,6-(CH3)3C6H2)NC(H)NH(2,4,6-(CH3)3C6H2)] (2),26 [(2,4,6-
(CH3)3C6H2)NC(CH3)NH(2,4,6-(CH3)3C6H2)] (3), [(C6H11)NC-
(H)NH(C6H11)] (4),20 [(2-OCH3C6H5)NC(H)NH(2-OCH3C6H5)]
(5),44 and [(2-OCH3C6H5)NC(CH3)NH(2-OCH3C6H5)] (6)20 were
prepared by the literature procedures.
[(2-OCH3C6H5)NC(C2H5)NH(2-OCH3C6H5)] (7). Ligand 7 was
prepared by following a method similar to the procedure reported
by Taylor.20 A mixture of 8.81 g (0.05 mol) of triethylorthopropionate,
12.3 g (0.10 mol) of o-anisidine, and 3.0 g (0.05 mol) of glacial acetic
acid was heated and refluxed at 140 °C for 4 h. After that period of
time the temperature was raised to 150 °C, and ethanol was distilled
off. The remaining viscous liquid was treated with an aqueous solution
of sodium carbonate (10%, 100 mL), then extracted with diethyl ether
(50 mL). The aqueous layer was washed with three additional portions
of ether (20 mL × 3). Organic fractions were combined, dried over
Na2SO4, and evaporated under reduced pressure to yield a crude
compound as a white solid. Recrystallization from EtOH/H2O (70:30,
100 mL) yielded pure amidine 7 as colorless crystals. Yield: 92%. Mp:
74−76 °C. 1H NMR (CDCl3, 400 MHz, 296 K): δ 8.79 (1H, s, NH),
7.06−6.85 (8H, m, aromatic), 3.92 (3H, s, OCH3), 3.81 (3H, s,
OCH3), 2.28 (2H, q, J = 7.6 Hz, CH2CH3), 1.17 (3H, t, J = 7.6 Hz,
CH2CH3). 13C NMR: 157.5 (NCN), 149.6 (C-OMe), 139.2 (C-
NCN), 122.7, 121.1, 119.2, 111.2 (C-aromatic), 55.72 (OCH3). 25.8
(CH2CH3), 11.7 (CH2CH3). Anal. Calcd for C17H20N2O2: C, 71.81;
H, 7.09; N, 9.85. Found: C, 71.92; H, 6.98; N, 9.96.
toluene solution of 9. Yield: 92%. 1H NMR (toluene-d8, 300 MHz, 298
K): δ 7.33 (1H, s, NCHN), 6.84 (4H, s, aromatic), 3.15 (18H, s,
N(CH3)2), 2.28 (12H, s, o-CH3), 2.22 (6H, s, p-CH3). 13C NMR:
167.5 (NCN), 144.4 (C-NCN), 136.9, 132.0, 128.7 (C-aromatic), 45.2
(N(CH3)2), 19.9 (o-CH3), 19.3 (p-CH3). Anal. Calcd for C25H42N5Ti:
C, 65.20; H, 9.19; N, 15.21. Found: C, 64.50; H, 9.82; N, 14.81.
[CH3C(N(2,4,6-(CH3)3C6H2))2]Ti[N(Me2)]3 (10). The same proce-
dure as for complex 8 was employed. X-ray quality crystals were
obtained by slow evaporation of a hexane solution of 10. Yield: 90%.
1H NMR (toluene-d8, 300 MHz, 298 K): δ 6.98 (4H, s, aromatic), 3.24
(18H, s, N(CH3)2), 2.36 (12H, s, o-CH3), 2.32 (6H, s, p-CH3), 1.35
(CH3). 13C NMR: 170.5 (NCN), 145.4 (C-NCN), 136.4, 131.0, 127.9
(C-aromatic), 45.6 (N(CH3)2), 20.1 (o-CH3), 19.5 (p-CH3), 19.1
(CH3). Anal. Calcd for C26H44N5Ti: C, 65.81; H, 9.35; N, 14.76.
Found: C, 65.41; H, 9.82; N, 14.92.
[HC(N(C6H11))2]2Ti[N(Me2)]2 (11). A toluene solution of Ti-
(NMe2)4 (0.23 g, 0.001 mol) was added dropwise to a stirred solution
of 4 (0.41 g, 0.002 mol) in toluene (30 mL) at room temperature. The
dark red solution was allowed to stir overnight at room temperature,
followed by removal of toluene in vacuo to yield a dark red residue.
The residue was dissolved in hexane (10 mL) and evacuated again to
1
obtain the pure complex 11 as dark red solid. Yield: 88%. H NMR
(toluene-d8, 300 MHz, 296 K): δ 7.95 (1H, s, NCHN), 3.43 (12 H, s,
N(CH3)2), 3.12 (4H, m, Cy-Hipso), 1.95−1.08 (40 H, m, Cy-H). 13C
NMR: 161.1 (NCN), 60.7 (C-NCN), 48.3 (N(CH3)2), 35.7, 26.0 (C-
Cy). Anal. Calcd for C30H60N6Ti: C, 65.19; H, 10.94; N, 15.21.
Found: C, 64.37; H, 10.63; N, 14.80.
[HC(N(2-OCH3C6H5))2]2Ti[N(Me2)]2 (12). The same procedure as
described above was followed, but the reaction time was reduced to 1
h. Yield: 92%. 1H NMR (toluene-d8, 300 MHz, 296 K): δ 9.40 (1H, s,
NCHN), 7.48 (4H, d, J = 6 Hz, o-H aromatic), 6.92−6.83 (8H, m,
aromatic), 6.57 (4H, d, J = 6 Hz, m-H aromatic), 3.39 (12H, s, OCH3),
3.18 (12H, s, N(CH3)2). 13C NMR: 164.9 (NCN), 152.2 (C-NCN),
137.5, 124.7, 122.3, 120.9, 111.4 (C-aromatic C), 54.5 (OCH3), 46.7
(N(CH3)2). Anal. Calcd for C34H44N6O4Ti: C, 62.96; H, 6.84; N,
12.96. Found: C, 61.55; H, 6.18; N, 11.66.
[CH3C(N(2-OCH3C6H5))2]2Ti[N(Me2)]2 (13). The same procedure
for the preparation of 11 was followed. Yield: 90%. 1H NMR (toluene-
d8, 300 MHz, 298 K): δ 7.28 (4H, d, J = 5.8 Hz, o-H aromatic), 7.01−
6.69 (8H, m, aromatic), 6.65 (4H, d, J = 5.8 Hz, m-H aromatic), 3.39
(12H, s, OCH3), 3.32 (12H, s, N(CH3)2), 1.74 (6H, s, CH3). 13C
NMR: 171.6 (NCN), 153.4 (C-NCN), 138.3, 136.9, 123.4, 120.4,
111.1 (C-aromatic), 54.3 (OCH3), 46.8 (N(CH3)2), 14.4 (CH3). Anal.
Calcd for C36H48N6O4Ti: C, 63.90; H, 7.15; N, 12.42. Found: C,
63.25; H, 6.97; N, 11.81.
[C2H5C(N(2-OCH3C6H5))2]2Ti[N(Me2)]2 (14). The same procedure
for the preparation of 11 was followed. X-ray quality crystals were
obtained by slow evaporation of a hexane/toluene (1:1) solution.
1
Yield: 90%. H NMR (toluene-d8, 300 MHz, 298 K): δ 7.30 (4H, d, J
= 6.3 Hz, o-H aromatic), 6.97−6.86 (8H, m, aromatic), 6.62 (4H, d, J
= 6.3 Hz, m-H aromatic), 3.37 (12H, s, OCH3), 3.27 (12H, s,
N(CH3)2), 2.20 (2H, q, J = 7.5 Hz, CH2CH3), 0.78 (3H, t, J = 7.5 Hz,
CH2CH3). 13C NMR: 174.9 (NCN), 153.44 (C-NCN), 138.3, 136.9,
123.4, 120.2, 110.7 (C-aromatic), 54.18 (OCH3), 46.7 (N(CH3)2),
22.1 (CH2CH3), 10.1 (CH2CH3). Anal. Calcd for C38H52N6O4Ti: C,
64.76; H, 7.44; N, 11.93. Found: C, 64.11; H, 7.77; N, 11.53.
[HC(N(2,6-(CH(CH3)2)2C6H3))2]Ti[N(Me2)]3 (8). A toluene solu-
tion of Ti(NMe2)4 (0.23 g, 0.001 mol) was added dropwise to a stirred
solution of 1 (0.364 g, 0.001 mol) in toluene at room temperature, and
the resulting red-orange solution was stirred overnight. All volatiles
were evaporated to yield complex 8 as an orange solid. X-ray quality
crystals were obtained by slow evaporation of a hexane solution of 8.
n
[CH3C(N(2-OCH3C6H5))2]2TiCl2 (15). A BuLi solution (1.25 mL,
1.6 M in hexane) was added dropwise to a solution of ligand 6 (0.54 g,
0.002 mol) in THF at −78 °C under nitrogen flow. The solution was
allowed to slowly warm to room temperature and stirred overnight to
obtain the lithium salt 6a. This solution was cooled to −78 °C, and a
slurry of TiCl4·2THF (0.34 g, 0.001 mol) in toluene was added
dropwise. The reaction mixture was brought to room temperature and
stirred overnight. The solvents were removed under vacuum to obtain
a solid residue. To this residue was added 30 mL of toluene, and the
resulting suspension was filtered. The filtrate was concentrated, and
hexane was added slowly to the solution, causing a precipitation of 15.
The heterogeneous solution was filtered, and the resulting solid was
washed with hexane (3 × 10 mL) and dried under vacuum to afford
pure complex as a dark brown solid. Yield: 80%. 1H NMR (toluene-d8,
1
Yield: 91%. H NMR (toluene-d8, 300 MHz, 298 K): δ 7.63 (1H, s,
NCHN), 7.10 (6H, m, aromatic), 3.49 (4H, sept, J = 6.6 Hz,
CH(CH3)2), 3.12 (18H, s, N(CH3)2), 1.23 (24H, d (J = 6.6 Hz),
CH(CH3)2). 13C NMR: 165.7 (NCN), 143.5 (C-NCN), 136.9, 124.9,
122.9 (C-aromatic), 45.4 (N(CH3)2), 27.3 (CH(CH3)2), 24.3
(CH(CH3)2). Anal. Calcd for C31H54N5Ti: C, 68.36; H, 9.99; N,
12.86. Found: C, 67.83; H, 9.51; N; 12.76.
[HC(N(2,4,6-(CH3)3C6H2))2]Ti[N(Me2)]3 (9). Similar to the proce-
dure described for 8, 0.28 g (0.001 mol) of 2 and 0.23 g (0.001 mol)
of Ti(NMe2)4 were reacted in toluene to yield complex 9 as an orange
solid. X-ray quality crystals were obtained by slow evaporation of a
L
dx.doi.org/10.1021/om4006998 | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
500 MHz, 298 K): δ 7.52 (4H, d, J = 8 Hz, o-H aromatic), 7.00−6.83
(8H, m, aromatic), 6.61 (4H, d, J = 8 Hz, m-H aromatic), 3.37 (12H, s,
OCH3), 1.63 (6H, s, CH3). 13C NMR: 173.1 (NCN), 146.7 (C-NCN),
131.7, 123.8, 122.4, 115.6, 106.7 (C-aromatic), 50.1 (OCH3), 9.44
(CH3). Anal. Calcd for C32H36Cl2N4O4Ti: C, 58.28; H, 5.50; N, 8.50.
Found: C, 57.84; H, 5.41; N, 8.30.
[C2H5C(N(2-OCH3C6H5))2]2TiCl2 (16). Similar to the above
procedure, the lithium salt 7a was prepared in situ by reacting 0.568
g (0.002 mol) of 7 with 1.25 mL of nBuLi (1.6 M in hexane) in THF.
The resulting solution of 7a was then reacted with 0.34 g (0.001 mol)
of TiCl4·2THF to obtain complex 16 as dark brown solid. Yield: 80%.
1H NMR (toluene-d8, 300 MHz, 298 K): δ 7.46 (4H, J = 6 Hz, o-H
aqueous NaOH (1 M), water, and acetone and then dried in a vacuum
oven at 65 °C.
ESR Studies of the Active Species. Preliminary Experiments. A
Teflon J. Young valve-sealed NMR tube was charged with complex 15
(5 mg), 0.5 mL of deuterated toluene, and MAO (M:Al = 1:100)
inside a glovebox. The mixture was shaken thoroughly, and the EPR
experiment carried out immediately.
Preparation of Samples for the Experiments with Radical
Trapping Reagent (ref 17b). Preparation of each sample was
performed inside a glovebox. Samples were first prepared in glass
vials and then transferred to NMR tubes.
First sample: A mixture of complex 15 (5 mg, 7.58 × 10−6 mol) and
MAO (Ti:Al = 1:100) was dissolved in 0.5 mL of a 2 mg/mL solution
of C60 in toluene. The mixture was shaken thoroughly, and ESR was
measured immediately. This sample imitates the activation stage in
which the complex of Ti(IV) transforms to the paramagnetic complex
of Ti(III) with the concomitant formation of a methyl radical, which is
trapped by C60. The sample was then subjected to MALDI-TOF mass
spectroscopy experiments.
Second sample: This imitates the final stage of the activation,
initiated by propylene, in which the cationic complex of Ti(IV) is
formed. This stage is predicated by the formation of dimers or
oligomers from propylene via intermediate radicals, which are expected
to be trapped by C60.
For the preparation of the second sample, a mixture of complex 15
(5 mg, 7.58 × 10−6 mol) and MAO (Ti:Al = 1:100) was dissolved in
0.5 mL of toluene, and only after the completion of this reaction was
0.5 mL of a 2 mg/mL toluene solution of fullerene added. The NMR
tube was connected to a vacuum line, frozen, and then evacuated,
followed by backfilling with ethylene at atmospheric pressure. The
reaction mixture was shaken well, and the ESR spectrum measured
immediately. The sample was then subjected to MALDI-TOF mass
spectroscopy experiments.
aromatic), 7.12−6.79 (8H, m, aromatic), 6.57 (4H, d, J = 6 Hz, m-H
aromatic), 3.38 (12H, s, OCH3), 2.12 (4H, q, J = 7.6 Hz, CH2CH3),
0.69 (6H, t, J = 7.6 Hz, CH2CH3). 13C NMR: 181.2 (NCN), 151.8 (C-
NCN), 136.9, 127.4, 125.8, 120.4, 111.5 (C-aromatic), 55.1 (OCH3),
22.2 (CH2CH3), 9.27 (CH2CH3). Anal. Calcd for C34H40Cl2N4O4Ti:
C, 59.40; H, 5.86; N, 8.15. Found: C, 59.69; H, 6.15; N, 7.86.
[CH3C(N(2-OCH3C6H5))2]HfCl3 (17). Similar to the above
procedure, the lithium salt 6a was prepared in situ by reacting 0.54
g (0.002 mol) of 6 with 1.25 mL of nBuLi (1.6 M in hexane) in THF
overnight. To the resulting solution of 6a was added slowly a slurry of
0.32 g (0.001 mol) of HfCl4 in toluene at −78 °C. The mixture was
stirred overnight at room temperature to obtain complex 17 as a light
yellow solid. 1H NMR (C6D6, 300 MHz, 298 K): δ 7.15 (2H, d, J = 6
Hz, o-H aromatic), 6.72−6.62 (4H, m, aromatic), 6.19 (2H, d, J = 6
Hz, m-H aromatic), 3.61 (6H, s, OCH3), 1.65 (3H, s, CH3). Anal.
Calcd for C16H16Cl3HfN2O2: C, 34.68; H, 3.09; N, 5.06; Cl, 19.19.
Found: C, 33.26; H, 3.78; N, 4.44; Cl, 21.78.
[CH3C(N(2-OCH3C6H5))2]3HfCl (18). Similar to the above
procedure, the lithium salt 6a was prepared in situ by reacting 0.54
g (0.002 mol) of 6 with 1.25 mL of nBuLi (1.6 M in hexane) in THF
overnight. The resulting suspension of 6a was then added dropwise to
a stirring suspension of 0.32 g (0.001 mol) of HfCl4 in THF at −78
°C. The mixture was allowed to stir overnight at room temperature.
After that period of time all volatiles were removed under reduced
pressure, and then toluene was added to the resulting residue, the
obtained suspension filtered, and the resulting filtrate evaporated
under reduced pressure to yield 18 as a yellow solid. 1H NMR (C6D6,
300 MHz, 294 K): δ 7.10 (6H, d, J = 6 Hz, o-H aromatic), 6.90−6.75
(12H, m, aromatic), 6.55 (6H, d, J = 6 Hz, m-H aromatic), 3.26 (6H, s,
OCH3), 1.49 (3H, s, CH3). Anal. Calcd for C48H51ClHfN6O6: C,
56.25; H, 5.31; N, 8.20; Cl, 3.46. Found: C, 52.36; H, 4.17; N, 9.58;
Cl, 8.24.
General Procedure for the Polymerization of Propylene (ref
18). In a glovebox a mixture of 10 mg of the appropriate precatalyst
and the appropriate amount of the MAO (1:1000, 1:500, or 1:100
metal:Al ratio) in 6 mL of toluene was loaded into a stainless steel
reactor. The reactor was connected to a high-vacuum line, and the
catalytic mixture was allowed to stir for 5 min at room temperature.
The reactor was then frozen in liquid nitrogen, and 30 mL of
propylene was condensed into the reactor. The reactor was then
warmed to room temperature and stirred for 3 h. After this time, the
reactor was opened in a well-ventilated hood to exhaust any excess of
propylene gas, and the reaction was quenched by adding a 10% HCl
solution in methanol to the mixture. The resulting polymer was
washed with methanol followed by water, aqueous NaOH (1 M),
water, and acetone and dried in a vacuum oven at 65 °C. The resulting
polymer was fractionalized with heptane in a Soxhlet apparatus.32
General Procedure for the Polymerization of Ethylene (ref
14f). Similar to the polymerization of propylene, 10 mg of the
precatalyst and the appropriate amount of MAO (1:1000, 1:500, or
1:100 metal:Al ratio) in 6 mL of toluene were loaded into a stainless
steel reactor in a glovebox. The catalytic mixture was stirred for 5 min
at room temperature, and ethylene gas was introduced. The pressure
of the reactor was maintained at 9.86 atm (10 bar) during the entire
polymerization (0.5 h). After this time, the reactor was opened in a
well-ventilated hood to exhaust any excess of ethylene, and the
reaction was quenched by adding a 10% HCl solution in methanol to
the mixture. The resulting polymer was washed with methanol, water,
Third and fourth sample: The sample preparation and experiment
were performed in a similar manner to that of the second sample
above with 15 mg (2.27 × 10−5 mol) or 50 mg (7.58 × 10−5 mol) of
complex 15.
ASSOCIATED CONTENT
* Supporting Information
■
S
The details of the dynamic NMR studies of complexes 13 and
14 and the calculation of thermodynamic parameters for
complex 14, 13C NMR spectra of the heptane-insoluble and
heptane-soluble polypropylenes obtained from complexes 15
and 16 and crystallographic data for ligands 6 and 7 and
complexes 8, 9, 10, and 14. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chmoris@tx.technion.ac.il.
■
Notes
The authors declare no competing financial interest.
§Deceased on September 25, 2013.
ACKNOWLEDGMENTS
■
This research was supported by the USA-Israel Binational
Science Foundation under Contract 2008283. B.T. is grateful
for the support to the Center for Absorption in Science,
Ministry of Immigrant Absorption, State of Israel.
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