Olefin polymerization behavior of titanium(IV) pyridine-2-phenolate-6- (σ-aryl) catalysts: Impact of py-adjacent and phenolate substituents

Article abstract of DOI:10.1021/om300832q

A series of Ti(IV) post-metallocene bis(benzyl) precatalysts supported by tridentate pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands, featuring various substituents on the σ-aryl (directly adjacent to the pyridine ring: fluoro, trifluoromethyl, benzo [C₄H₄]) and phenolate groups (tert-butyl, trifluoromethyl, cumyl, 1,1-diphenylethyl), have been prepared. Multinuclear (including ¹H, ¹³C, and ¹⁹F) NMR characterizations of the complexes have been performed. The principal purpose of this study was to investigate the impact of these substituents upon ethylene polymerization reactivity and polymer properties. The cumyl-phenolate σ-naphthyl Ti precatalyst, in conjunction with [Ph ₃C][B(C₆F₅)₄], displays good activity and produces polyethylene with exceptionally high MW (M_n = 4 × 10⁶) and an M_w/M_n value (2.5) approaching single-site character at 50 C, but multisite behavior is apparent for other catalysts. DFT calculations have been performed to probe the polymerization behavior and the role of the py-adjacent substituent. These studies revealed the possibility of two distinct polymerization reactions, namely conventional and ethylene-assimilated (comprising initial ethylene insertion into the Ti-C(σ-aryl) bond) chain propagation, and found that the latter is kinetically preferred. Furthermore, the viability of another kinetically competitive pathway, namely the isomerization of the ethylene-assimilated [Ti-CH₂CH₂-aryl] species via β-H elimination and 2,1-reinsertion, was also indicated.

Full text of DOI:10.1021/om300832q

Article
pubs.acs.org/Organometallics
Olefin Polymerization Behavior of Titanium(IV) Pyridine-2-phenolate-
6-(σ-aryl) Catalysts: Impact of “py-Adjacent” and Phenolate
Substituents
Jerry C. Y. Lo,† Michael C. W. Chan,*,† Po-Kam Lo,† Kai-Chung Lau,*,† Takashi Ochiai,§
and Haruyuki Makio‡
†Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, People’s Republic of China
§R&D Center, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan
‡Mitsui Chemicals Singapore R&D Centre Pte. Ltd., 50 Science Park Road, #06-08 The Kendall Singapore Science Park II,
Singapore 117406
S
* Supporting Information
ABSTRACT: A series of Ti(IV) post-metallocene bis(benzyl) precatalysts supported by tridentate pyridine-2-phenolate-6-(σ-
aryl) [O,N,C] ligands, featuring various substituents on the σ-aryl (directly adjacent to the pyridine ring: fluoro, trifluoromethyl,
benzo [C4H4]) and phenolate groups (tert-butyl, trifluoromethyl, cumyl, 1,1-diphenylethyl), have been prepared. Multinuclear
(including 1H, 13C, and 19F) NMR characterizations of the complexes have been performed. The principal purpose of this study
was to investigate the impact of these substituents upon ethylene polymerization reactivity and polymer properties. The cumyl-
phenolate σ-naphthyl Ti precatalyst, in conjunction with [Ph3C][B(C6F5)4], displays good activity and produces polyethylene
with exceptionally high MW (Mn = 4 × 106) and an Mw/Mn value (2.5) approaching single-site character at 50 °C, but multisite
behavior is apparent for other catalysts. DFT calculations have been performed to probe the polymerization behavior and the role
of the py-adjacent substituent. These studies revealed the possibility of two distinct polymerization reactions, namely
conventional and ethylene-assimilated (comprising initial ethylene insertion into the Ti−C(σ-aryl) bond) chain propagation, and
found that the latter is kinetically preferred. Furthermore, the viability of another kinetically competitive pathway, namely the
isomerization of the ethylene-assimilated [Ti−CH2CH2−aryl] species via β-H elimination and 2,1-reinsertion, was also indicated.
and the study concluded by recognizing the possibility of com-
petitive insertion into the respective Zr−C(aryl) and Zr−C-
(alkyl/polymeryl) bonds.3 Indeed, the feasibility of olefin
insertion into the M−C(sp2) bond of a chelating σ-aryl ligand
was subsequently demonstrated by the facile, stoichiometric
reactivity toward ethylene or propylene of a cationic titanium
bis(phenolate) complex supported by a bidentate (σ-aryl)amine
ligand.4 Intramolecular ortho-metalation processes mediated by
group 4 derivatives bearing aryl-substituted auxiliaries to afford
chelating σ-aryl moieties have been described.5,6
INTRODUCTION
■
Amongst a myriad of multidentate ancillary ligands, chelating
σ-aryl auxiliaries have increasingly gained prominence in
postmetallocene catalyst design for olefin polymerization.1
Preceded by studies on non-Cp (cyclopentadienyl) carbon-
based anionic ligands,2 zirconium complexes with tridentate
bis(σ-aryl)amine ligands were developed as precatalysts for
propylene polymerization, but low efficiency and broad
molecular weight distribution were reported.3 Therein, Hessen
noted that aromatic σ-carbanions are principally σ-donors with
minimal π-donation (in contrast to amide and phenolate
moieties) and can potentially confer a more electrophilic metal
center. However, a critical caveat is that the M−C(sp2) bond
must be inert relative to the M−C(sp3) bond, which would
generally be true especially in the absence of steric effects,
Researchers at Symyx and Dow developed a high-performance
class of pyridylamido [N,N,C]-Hf(IV) catalysts containing
Received: August 29, 2012
Published: January 10, 2013
© 2013 American Chemical Society
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ortho-metalated phenyl and naphthyl substituents for olefin
polymerization through the use of high-throughput screening,
which produce isotactic polypropylene materials with extremely
high molecular weights.7 Intriguingly, ethylene-(α-olefin)
copolymers from these catalysts display broad, multimodal
molecular weight distributions (MWD), indicative of multisite
behavior, and a mechanism involving monomer insertion into
the Hf−C(σ-naphthyl) bond was proposed to rationalize this,
on the basis of evidence from DFT calculations, 13C NMR and
GC/MS analysis, and isolation of olefin-inserted products.8 For
propylene polymerization, DFT calculations revealed that
monomer insertion into the Hf−C(σ-aryl) bond can generate
multiple active species with different reactivities, consistent with
the observed multimodal MWD.9 Coates reported living
1-hexene and unexpected isotactic propylene polymerizations
using a Cs-symmetry (σ-phenyl)pyridylamido Hf catalyst.10
Extensive NMR studies were performed by Macchioni to
investigate the activation of [N,N,C]-Hf precatalysts.11 Notably,
Brønsted acids protonate the σ-naphthyl moiety to afford a
Hf···η2-naphthyl interaction, driven presumably by the release
of an eclipsed H···H repulsion between the naphthyl and pyridyl
units, which corresponds to that proposed to account for the
favorable monomer-inserted mechanism.8 Furthermore, detailed
NMR experiments supported by DFT calculations were under-
taken to probe the viability of α-olefin insertion into the Hf−C(σ-
aryl) bond and characterize the resultant species, and evidence
was presented which indicates that such catalytic sites are signifi-
cantly more active than conventional “noninserted” species.12 The
observation of both diastereomers arising from rotation of the
2-isopropylphenyl unit in the ligand backbone of [N,N,C]-Hf
and -Zr precatalysts and a comparison of their ethylene-(1-octene)
and propylene polymerization characteristics were described.13 For
analogous Zr derivatives bearing a σ-phenyl, -furanyl, or -thienyl
moiety, an additional species, formed by metalation of an iso-
propyl group on the amido moiety, was identified from aged
catalyst solutions and proposed to be the active site responsible for
producing high-Mn materials.14
Inspired by the monomer-inserted mechanism to develop
ligand frameworks featuring an ancillary C(sp3) donor, a
pyridylamido ligand bearing a pendant vinyl unit was prepared,
which was found to undergo intramolecular vinyl insertion into
a neutral Hf(IV) trimethyl complex to generate a living iso-
selective propylene polymerization precatalyst containing
the [Hf−CH(Et)−naphthyl] moiety.15 Similarly, metalation
of a vinyl-appended phenolate-amine ligand with Zr- and
Hf(CH2Ph)4 was accompanied by benzyl migration to afford
complexes bearing the corresponding tridentate ligand
incorporating a C(sp3) donor, which catalyze living, isoselective
polymerization of α-olefins.16 The application of chelating (σ-
aryl)-imine-amido Hf complexes to mediate the direct coupling
of internal alkynes and 2-alkylpyridine was recently reported.17
We initially developed a family of pyridine-2-phenolate-6-(σ-
aryl) [O,N,C] group 4 precatalysts bearing fluorinated
substituents adjacent to the metal center,18 and evidence
from multinuclear NMR spectroscopy suggesting the existence
of intramolecular C−H···F−C contacts was supported by a
neutron diffraction study.19 These complexes constitute syn-
thetic models of weak attractive ligand−polymer interactions in
olefin polymerization,20 and it is significant to note that recent
reports concerning late transition metal systems hint at the gen-
erality of this concept for modulating polymerization reactions.21,22
Investigations into the impact of σ-aryl substituents upon olefin
homo- and copolymerizations by Ti precatalysts, in conjunction
with MAO and trityl borate,23,24 revealed that (1) a substituent
adjacent to the metal center is not essential and can adversely
affect efficiency, (2) a “py-adjacent” σ-aryl substituent (Cl, Me,
benzo (C4H4)), in an ortho position directly adjacent to the
pyridyl ring, can improve polymerization activities, and (3)
broad MWD polymers are produced, implying multisite
behavior. Bearing in mind the highly rigid nature of [O,N,C]
ligands, and a crystal structure that contains an eclipsed inter-
action between a py-adjacent chloro substituent and the re-
levant pyridyl proton, we hypothesized that the seemingly
remote py-adjacent substituent may affect the conformation of
the M−ligand chelation and destabilize the Ti−C(σ-aryl) linkage,
thereby facilitating olefin insertion into this bond to afford supple-
mentary olefin-assimilated25 active species with enhanced effi-
ciencies, like that reported for the [N,N,C]-Hf system.7−13 In this
context, systems displaying remote or indirect substituent effects in
olefin polymerization have been described in the literature,26 while
the conformational consequences of the bridging moiety in ansa-
metallocenes upon polymerization reactivity are well-established.27
Several reports on highly active Cp-based and post-metallocene
catalysts for olefin polymerization have recently appeared.28
Our objectives are to develop greater understanding of
substituent effects in the Ti-[O,N,C] system, derive insight into
the olefin polymerization mechanism(s) for improved catalyst
design and performance, and produce narrow-MWD polymers
while also investigating the multisite characteristics of the
polymerization process and the nature of these active species.
We have evaluated the impact of changing both phenolate and
py-adjacent substituents, as well as steric and electronic factors,
upon polymerization behavior. The observed polymeriza-
tion results are rationalized with the aid of DFT calculations,
which provide insight into different reaction pathways and the
generation and reactivity of the postulated olefin-assimilated active
species, and possible explanations for the superior efficiencies of σ-
naphthyl catalysts. Comparisons with the (σ-aryl)pyridylamido Hf
system are also made. The cumyl-phenolate σ-naphthyl Ti pre-
catalyst, in conjunction with [Ph3C][B(C6F5)4], produces excep-
tionally high MW polyethylene (Mn = 4 × 106; Mw/Mn = 2.5;
activity = 2.7 kg mmol−1 h−1) at 50 °C.
RESULTS AND DISCUSSION
■
Synthesis and NMR Characterization. The pyridine-2-
phenolate-6-(σ-aryl) ligands were synthesized by modification
of literature procedures that entail the sequential coupling of
two substituted acetophenones, while taking advantage of
the synthetic accessibility (for trifluoromethyl, cumyl, and 1,1-
diphenylethyl derivatives) and commercial availability of
substituted acetophenones (see the Supporting Information
for details).29,30 Cyclometalations of ligands H2L1−7 with
Ti(CH2Ph)4 were performed in a diethyl ether/n-pentane or
toluene/n-pentane mixture at −78 °C and allowed to proceed
upon warming to 20 °C for 12 h. The titanium complexes
(Scheme 1) were obtained as dark red crystalline solids in
moderate to good isolated yields (40−78%), except for 6 and 7
(ca. 28%); this is partially ascribed to their high solubility in
nonpolar solvents.
All complexes have been fully characterized by 1H (plus 19F)
and 13C NMR spectroscopy and assigned using 135-DEPT,
[1H,1H]-COSY, [13C,1H]-HSQC, and ROESY experiments
(plus [1H,19F]-HMQC for 3). A salient point of interest is
the impact of the py-adjacent substituent (R1) upon the pyridyl
1
group, as indicated by the respective H NMR shifts for the
pyridyl hydrogens H9,10 (Table 1). For both 1 and 2, the
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on steric grounds and reflects their greater electrophilicity, although
differences between substituents are not evident. We note that
all [O,N,C] complexes in which the bis(benzyl) moieties in the
molecular structure protrude toward the pyridyl unit in an “anti,
anti” arrangement (rather than “syn, anti”;32 Figure S1 in the
Scheme 1. Synthesis of Complexes 1−7
1
Supporting Information) consistently exhibit a high-field H
NMR resonance for p-Ph (<6.50 ppm in C6D6),19,23,24 and the
corresponding shifts (6.30−6.41 ppm) for 1−7 also tentatively
indicate this conformation.
Olefin Polymerization Studies Using Trityl Borate
Cocatalyst. The complexes in this work have been evaluated
as ethylene polymerization catalysts in conjunction with
iBu3Al/[Ph3C][B(C6F5)4]. Details of polymerization reactions
at 50 and 100 °C and polymer characterization are given in
Table 2, which show good activities for such elevated
temperatures.
a
Table 2. Ethylene Polymerization Results
temp
yield
Mn
b
c
d
d
catalyst
1 (tBu/F)
(°C)
(mg)
activity
(×106)
Mw/Mn
50
100
50
111
98
1.1 (1.3)
1.0 (2.1)
1.0 (1.2)
0.7 (1.6)
1.6 (1.9)
0.9 (2.0)
2.3 (2.7)
3.6 (7.7)
1.7 (2.1)
1.1 (2.3)
2.7 (3.2)
2.2 (4.7)
4.1 (4.9)
2.3 (5.0)
1.72
0.26
0.87
0.15
0.82
0.06
0.41
0.26
0.57
0.19
3.99
1.52
0.83
0.51
2.8
3.8
2 (tBu/2F)
98
4.9
1
Table 1. Influence of py-Adjacent Substituent (R1) upon H
100
50
74
6.7
a
NMR Shifts for H9,10
3 (tBu/CF3)
155
92
7.5
H10
(ppm)
H9
(ppm)
R1/H10
interaction
100
50
19.2
8.1
complex
R1
Δδ(H9,10
)
4 (tBu/benzo)
226
355
173
107
265
216
412
229
1
2
3
4
5
6
7
F
F
7.60
7.46
7.25
7.12
7.09
7.01
7.04
6.73
6.73
6.73
6.73
6.67
6.64
6.64
0.87
0.73
0.52
0.39
0.42
0.37
0.40
JH,F = 2.9 Hz
100
50
6.7
JH,F = 2.6 Hz
5 (CF3/benzo)
10.8
11.0
2.5
CF3
[1H,19F]-HMQC
NOE (with H20)
NOE (with H20)
NOE (with H20)
NOE (with H20)
100
50
benzo
benzo
benzo
benzo
6 (CMe2Ph/benzo)
7 (CPh2Me/benzo)
100
50
4.6
7.6
100
14.2
a
a
Conditions: 5 mL of toluene, 0.3 μmol of catalyst, iBu3Al/
Conditions: C6D6, 400 MHz, 298 K. See Scheme 1 for labeling.
[Ph3C][B(C6F5)4]/catalyst (50/2/1 equiv, respectively), 7 atm of
ethylene pressure (maintained by continuous supply), 20 min reaction
magnitude of the observed 19F coupling for H10 is consistent
b
time. Phenolate/σ-aryl substituents, respectively, are given in
(rather than JH,F) and suggests weak attraction.19
1h
5
with
J
H,F
c
parentheses. Activities are given in kg of polymer (mmol of
However, 19F coupling with the CF3 group is not discernible for
catalyst)−1 h−1 ( 10%); activities with respect to ethylene
concentration (0.836 mol/L at 50 °C and 0.461 mol/L at 100 °C)
are given in parentheses in kg mmol−1 h−1 (mol/L of ethylene)−1
1
H10 in the H NMR spectrum of 3, although some correlation
is apparent in the more sensitive [1H,19F]-HMQC spectrum.
Exchange peaks observed in ROESY NMR experiments
between H10 and H20 (in the benzo moiety) in complexes
4−7 confirm their proximity. Differences in Δδ(H9,10) values
may be tentatively ascribed to the varying extent of interaction
(attractive or repulsive) between R1 and H10. In contrast to the
benzo group in 4, the capability of the CF3 group in 3 to rotate
and modulate interaction (repulsive or attractive) with H10 is
noted (see DFT calculations for further discussion).
d
( 10%). Determined by GPC at 140 °C in 1,2-dichlorobenzene
using polystyrene standards.
A comparison of py-adjacent substituents (1−4) reveals that
the benzo derivative 4 gives the highest activities, followed by 3
at 50 °C with 1 and 2 the lowest, while the activities for 1−3
are comparable at 100 °C. We previously postulated24 that the
likely repulsion between the pyridyl and benzo groups would
facilitate or accelerate olefin insertion into the Ti−C(σ-aryl)
bond (thus alleviating the aforementioned repulsion), like that
observed by Hessen4a and Rothwell33 (for a Ti−vinyl linkage)
and reported for the Hf-[N,N,C] system,8,9,12 to afford a
supplementary catalytic entity with presumably increased
efficiency. Here, an in-depth evaluation of the steric and
electronic effects of the py-adjacent group has been undertaken
using DFT calculations (see below).
1H and 13C NMR spectroscopy have been employed to
evaluate the distortion and hence coordination of benzyl groups
in 1−7 (Table S1 in the Supporting Information). Using criteria
established in the literature,31 the reduced JH,H (8.0−8.5 Hz)
2
and high 1JC,H values (135.0−137.4 ppm) signify η2 coordination
of benzyl groups in all complexes. In addition, the η2
1
coordination mode can be manifested as a high-field o-Ph H
NMR shift (6.34−6.76 ppm), although the influence of ring
currents from ancillary ligands makes this criterion less reliable,
and this is indeed apparent for the cumyl and 1,1-diphenylethyl
derivatives. In comparison with analogous Zr (weaker η2
coordination) and Hf derivatives (η1 coordination),23,24 the η2-
benzyl coordination for the Ti complexes cannot be rationalized
The polymers formed by 3 and 4 display broad to very broad
MWD (Mw/Mn > 6.7), while complex 1 produces polyethylene
with a reasonably narrow MWD (Mw/Mn = 2.8) at
50 °C. The comparable activities found for 1 and 2 (containing
an additional F atom) may imply that the electrophilicities of
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Scheme 2. Gibbs Free Energy Surface for Initial C2H4 Insertion into Ti−C(σ-aryl) (Pathway a [in black], i.e. C2H4 Assimilation)
and Ti−C(benzyl) Bonds (Pathway b [in blue]) of 4+ at the B97D Level using LanL2DZ (Ti) and 6-311G(d,p) (Nonmetals)
a
Basis Sets
a
Relative energies at 298 K in toluene (kcal/mol) and selected interatomic distances (Å) are given; note that Ti···Cipso interactions are observed in
INT1a, TS1a, INT2a, TS2b, and INT3b.
the respective Ti centers are not dissimilar, since enhanced electro-
philicity at an active site would typically promote olefin binding
and activation for insertion, possibly leading to increased catalytic
activity. The employment of fluorinated moieties as electron-
withdrawing substituents is an appealing design approach.34
The σ-naphthyl moiety proved to be the most active among
1−4 and was employed to evaluate the impact of the phenolate
substituent (5−7) upon polymerization characteristics. This
was motivated by the reported beneficial effects of an
increasingly bulky phenolate substituent upon the performance
of bis(phenoxyimine) group 4 catalysts.35 The highest effi-
ciency of 4.1 kg mmol−1 h−1 at 50 °C is recorded for 1,1-
diphenylethyl (7), although the broad MWD (Mw/Mn = 7.6)
implies the presence of multiple active sites. The polyethylene
produced by the cumyl derivative 6 at 50 °C (Mn = 3.99 × 106;
Mw/Mn = 2.5; activity = 2.7 kg mmol−1 h−1) is noteworthy, with
exceptionally high MW and relatively narrow MWD, implying
close to single-site character (see Figure S2 in the Supporting
Information for GPC traces). Conversely, complex 5 exhibits
the lowest activities and very broad MWD even at 50 °C,
signifying that the steric and possibly electron-withdrawing
effects of the CF3 substituent at the phenolate moiety are
detrimental to the performance.
Broadening of MWD and increases in Mw/Mn values indica-
tive of enhanced multisite behavior are observed at 100 °C,
particularly for 3, 5, and 7 (see below). In general, the Mn
values for 1−7 decline at 100 °C, suggesting increased
tendency to undergo chain termination and deactivation pro-
cesses for the active sites. In this regard, greater steric pro-
tection of the catalytic center by the phenolate substituent
could be beneficial, although the possibility of improved protec-
tion (for example, against chain termination) is necessarily
dependent on the suitable size and positioning of said
substituent. While the 1,1-diphenylethyl moiety in 7 displays
the highest activities but produces polymers with low Mn
(0.83 × 106 at 50 °C), the cumyl group in 6 gives high Mn
values even at 100 °C.
The nature of the active sites in these polymerizations would
be of interest, and extensive attempts have been made to char-
acterize the products upon trityl activation by NMR spectroscopy.
However, the putative Ti benzyl cationic species proved to be
highly unstable and could not be observed spectroscopically.36 In
the following section, the ethylene insertion pathways and reac-
tivity of different catalysts were investigated and compared using
DFT calculations, from which interesting insights may be derived.
Density Functional Theory Calculations. Theoretical
calculations have been performed to probe the ethylene in-
sertion and propagation processes and the feasibility of eth-
ylene assimilation (insertion into the Ti−C(σ-aryl) bond),
and our primary focus was to study the impact of the py-
adjacent substituent in 1−4. The potential energy surfaces,
initial ethylene insertion and propagation mechanisms, and
ligand substituent effects upon polymerization behavior for
the 1+, 2+, 3+, and 4+ benzyl cations (generated by trityl
activation) have been investigated by DFT. The structures
and energies of all molecular species have been calculated at
the B97D level37 with the LanL2DZ basis set38 for transition
metals (Ti) and the 6-311G(d,p) basis set for nonmetal
atoms. The polarizable continuum model (PCM)39 is used
to account for solvent effects in toluene. All calculations were
performed using the Gaussian 09 program package.40
As an illustrative example, the initial ethylene insertion at 4+
is found to undergo two distinct reaction pathways (Scheme 2;
results for 1+−4+ are given in Table S2 in the Supporting
Information): (a) C2H4 assimilation, namely insertion into the
Ti−C(σ-aryl) bond, followed by C2H4 insertion into the
Ti−C(benzyl) bond; (b) conventional C2H4 insertion into
the Ti−C(benzyl) bond. At the start of pathway a, ethylene
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coordination occurs at 4+ to afford a η2-ethylene complex
(INT1a), which then undergoes C2H4 insertion into the
Ti−C(σ-aryl) bond via a four-centered transition state (TS1a)
with an energy barrier (ΔG298⧧) of 7.8 kcal/mol (relative to
4+ + C2H4). In TS1a, the Ti−C(σ-aryl) bond is elongated
slightly from 2.084 to 2.160 Å, while the Ti−C(ethylene)
distance shortens from 2.711 to 2.281 Å. The resultant stable
intermediate INT2a undergoes a second C2H4 insertion (anti
to the [Ti−CH2CH2−aryl] linkage), but on this occasion into
the Ti−C(benzyl) bond through the four-centered TS2a, to
yield the propylphenyl cation INT3a. Relative to 4+ + 2C2H4
(INT2a + C2H4), the second C2H4 insertion proceeds with an
(Ti−C−Cipso = 168.5 (I) and 167.5° (II) in TS2a), while η2-
benzyl coordination is evident in III and IV (Ti···Cipso = 2.543
and 2.545 Å, respectively). Each conformer of 4+ can undergo
C2H4 insertions according to pathways a and b, but only the
⧧
pathways with the lowest ΔG298 values for TS2a and TS2b,
respectively, are shown in Scheme 2.41 Generally, con-
formers I and II are preferred for TS2a of 1+−4+, while III
and IV are more favorable for TS2b (Table 3; see the following
paragraph for further discussion). With the exception of 2+,
pathway a involving C2H4 assimilation requires less activation
energy than the conventional pathway b. In other words, although
both pathways are accessible, there is a kinetic preference for
pathway a over pathway b.
⧧
activation energy ΔG298 of 10.6 (24.9) kcal/mol, signifying
that the rate-determining step for pathway a is ethylene
insertion into the Ti−C(benzyl) bond.
For pathway a, the ΔG298⧧ values for 3+ and 4+ (10.7 (I) and
10.6 kcal/mol (I), respectively) are noticeably lower than those
for 1+ and 2+ (13.2 (II) and 14.5 kcal/mol (II), respectively).
This indicates that ethylene insertion/propagation is kinetically
most favorable for 3+ and 4+, which is consistent with the
superior ethylene polymerization efficiency for catalyst 4, and
to some extent 3. This reactivity trend can be rationalized by
considering the η2-benzyl coordination and Ti···Cipso inter-
action (Table 4). The Ti···Cipso distance in INT2a for 1+−4+
follows the order 1+ (2.481 Å) ≈ 2+ (2.478 Å) < 3+ (2.490 Å) <
4+ (2.507 Å), which is in line with the experimental activity
order of 1 ≈ 2 < 3 < 4 for ethylene polymerization at 50 °C.42
Hence, a longer Ti···Cipso separation in 4+ would correspond to
weaker η2-benzyl coordination, which could reduce the
activation barrier for ethylene insertion. The Ti···Cipso distances
in INT1a are discernibly shorter than those in INT2a (by 0.118
(2+) to 0.152 (4+) Å), indicative of stronger η2-benzyl co-
ordination. Since ethylene insertion into the Ti−C(benzyl)
bond in TS2b anti to the Ti−C(σ-aryl) linkage (conformers I
and II) necessitates cleavage of a stronger Ti···Cipso interaction
in INT1a, it follows that conformers I and II are kinetically less
favorable in pathway b.
The conventional propagation pathway b resembles the latter
reaction in pathway a; C2H4 insertion into the Ti−C(benzyl)
bond of INT1a, via the four-centered TS2b, gives the inter-
mediate INT3b. The molecular structures of TS2b and INT3b
correspond to TS2a and INT3a, respectively, except for the
[Ti−CH2CH2−aryl] linkage throughout pathway a. Impor-
tantly, the ΔG298⧧ value for pathway b is 14.9 kcal/mol (relative
to 4+ + C2H4), which is higher than that for pathway a by
4.3 kcal/mol. Thus, reaction pathway a is found to be
kinetically more favorable than b.
It should be noted that for 4+, there exist two conformers
which differ in the relative orientations of the σ-naphthyl and
phenolate moieties with respect to the central pyridine ring
(this applies to all species bearing a py-adjacent substituent).
Furthermore, the second C2H4 can insert in an anti (I and II)
or syn (III and IV) manner with respect to the [Ti−CH2CH2−
aryl] or Ti−C(σ-aryl) linkage, giving rise to a total of four
conformers (I−IV) for TS2a (depicted in Figure S3 in the
Supporting Information for 4+) and TS2b, respectively.
Regarding conformers I and II in pathway a, the second
C2H4 insertion occurs anti to the [Ti−CH2CH2−aryl] linkage
and the Ti−CH2Ph interaction adopts a more linear configuration
A prominent feature in pathway a for 4+ is the exothermic
formation of INT2a (−14.3 kcal/mol relative to 4+ + C2H4)
after C2H4 assimilation via TS1a. The stability of INT2a for 4+
may be rationalized by the removal of an eclipsed Hpy···Hnaphthyl
repulsive interaction (2.126 Å in INT1a), which was similarly
proposed to provide stabilization for the [N,N,C]-Hf
system.8,12 It is of interest to note the changing geometry of
the Ti center in pathway a; as a consequence of the greater
flexibility of the modified seven-membered N,C(sp3)-chelate
ring, the mer-like [O,N,C(σ-aryl)] is transformed into a fac-
coordinating [O,N,C(C2H4-aryl)] ligand to give a metal geom-
etry that is noticeably closer to tetrahedral. In contrast to the
steric repulsion apparent in 4+, the calculated Hpy···F contacts
in INT1a for 1+−3+ (Table 4) are consistent with attrac-
tive interactions,18,19,43 as supported by the observation of the
corresponding JH,F values in 1 and 2 (Table 1). Moreover,
the potential energy surface for multiple C2H4 insertion and
⧧
Table 3. ΔG298 (kcal/mol) for Reaction Pathways a and b
Mediated by Conformers I−IV of Species 1+−4+ in Toluene
at the B97D Level using LanL2DZ (Ti) and 6-311G(d,p)
(Nonmetals) Basis Sets
⧧
ΔG298
pathway a
pathway b
I
II
III
IV
I
II
III
IV
1+
2+
3+
4+
14.9
15.7
10.7
10.6
13.2
14.5
11.8
12.7
14.8
15.4
15.8
14.2
16.2
17.9
18.2
15.7
19.6
20.5
20.2
21.2
22.1
21.6
a
13.6
13.4
13.6
14.9
14.3
14.0
14.5
15.4
a
a
Not found.
Table 4. Selected Interatomic Distances and Angles for INT1a and INT2a of 1+−4+ at the B97D Level using LanL2DZ (Ti) and
6-311G(d,p) (Nonmetals) Basis Sets
INT1a
Ti−C−Cipso (deg)
INT2a
Ti−C−Cipso (deg)
INT1a: closest Hpy···R1 contact
aryl
complex
Ti···Cipso (Å)
Ti···Cipso (Å)
interatomic dist (Å)
interatomic angle (deg)
1+
2+
3+
4+
2.356
2.360
2.350
2.355
80.5
80.8
80.1
80.3
2.481
2.478
2.490
2.507
86.2
86.1
86.9
87.4
Hpy···F: 2.239
Hpy···F: 2.236
Hpy···F: 2.213
Hpy···Hnaphthyl: 2.126
C−Hpy···F: 116.7
C−Hpy···F: 117.0
C−Hpy···F: 141.9
C−Hpy···Hnaphthyl: 98.5
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Scheme 3. Gibbs Free Energy Surface for β-Hydrogen Elimination and 2,1-Reinsertion (Pathway c) by INT2a of 4+ at the B97D
a
Level using LanL2DZ (Ti) and 6-311G(d,p) (Nonmetals) Basis Sets
a
Relative energies at 298 K in toluene (kcal/mol) and selected interatomic distances (Å) are given.
+
propagation reactions by the 4′ methyl cation, which is a simplified
model of 4+, was also investigated (Scheme S1 in the Supporting
Information). The ΔG298⧧ values for the third (in pathway a) and
this β-H elimination and reinsertion pathway would become more
accessible at elevated temperatures.
A preliminary study of phenolate substituent effects was
performed by comparing 4+ (tBu) with 7+ (CPh2Me). The
⧧
second (in pathway b) C2H4 insertions are smaller than the ΔG298
⧧
respective ΔG298 values for TS1a and TS2a are 3.4 and 4.2
values of TS2a and TS2b in pathways a and b, respectively, and
such a trend would also be expected for 4+. Stabilizing β-agostic
(Cβ−Hβ···Ti) interactions are found for reaction intermediates,
such as INT5a and INT5b, in both pathways.
kcal/mol (relative to 7+ + 2C2H4), which are lower than those
for 4+ by 4.4 and 6.4 kcal/mol, respectively, and entirely
consistent with its superior polymerization activity at 50 °C
(see above). The lower activation energy of TS2a for 7+ can be
correlated with the weaker Ti···Cipso interaction (2.792 Å; Ti−
C−Cipso = 101.5°) in INT2a, which is longer than that in 4+ by
0.285 Å. Hence, for pathway a, C2H4 insertion into the Ti−
C(benzyl) bond for conformers I and II (anti to the
[Ti-CH2CH2-aryl] linkage) proceeds via cleavage of the
Ti···Cipso interaction in INT2a, which is weaker and more
facile for 7+. The dramatic steric influence of the amide
substituent in [N,N,C]-Hf catalysts upon polymerization
behavior has been noted.10 Interestingly, Ti···π(Ph) coordina-
tion5,10,45 (2.751 and 2.984 Å) by the CPh2Me substituent in 7+
was found to be viable (Figure S4 in the Supporting Information).
Comparisons and General Remarks. In this section,
factors affecting activity and rationalization of the effects of the
py-adjacent substituent, the formation of multiple catalytic
species and the likelihood of different reaction pathways, as well
as comparisons with previous work and the [N,N,C]-Hf system,
are addressed.
In addition to pathways a and b, attempts have been made to
study the viability of alternative reactions and further trans-
formations to give additional active species, as would be
anticipated on the basis of the observed multisite behavior of
these catalysts. One possible transformation, the rearrangement
of an ethylene-assimilated species through β-H elimination (to
afford M−H/vinyl species) followed by reinsertion, was
spectroscopically observed at ambient temperature by Hessen4a
for a cationic [Ti−CH2CH2−arylamine] bis(phenolate) com-
plex,44 and considered but not detected for the [N,N,C]-Hf
system.8,9 Coates described the insertion of a pendant vinyl moiety
into the metal−C(alkyl) bond of neutral group 4 complexes to
afford derivatives bearing tridentate ligands with an ancillary
C(sp3) donor.15,16 It is therefore noteworthy that the rearrange-
ment of the ethylene-assimilated species in this work, via β-H
elimination and reinsertion to yield a benzylic-type [Ti−CH-
(Me)−Ar] product, was found to be a feasible pathway (c).
Taking 4+ as an illustrative example (Scheme 3), INT2a can
undergo β-H elimination through TS1c (ΔG298⧧ = 12.0 kcal/mol
relative to 4+ + C2H4) to form a Ti hydride/vinyl [CH2
CH−naphthyl] species (INT1c), followed by 2,1-reinsertion via
The DFT calculations have provided insight for comparing
the py-adjacent substituents and rationalizing the consistently
higher activities for σ-naphthyl complexes. (1) The activation
barrier (ΔG298⧧) to the ethylene-assimilated species (TS2a) is
lowest for the σ-naphthyl derivative (10.6 kcal/mol for 4+
(benzyl); for comparison, the corresponding barriers (ΔH⧧) for
⧧
TS2c (ΔG298 = 17.5 kcal/mol) to give the [Ti−CH-
(Me)−naphthyl] complex (INT2c). Alternatively, rather than
reinsertion, toluene elimination from INT1c (or chain termination
from a polymeryl species to give a saturated chain end) could lead
to a dormant or deactivated species. With respect to pathway a,
−
[N,N,C]-Hf alkyl cations (methyl including a MeB(C6F5)3
anion,12,46 n-butyl8) were calculated to be similar or higher).
In addition, a kinetic preference for pathway a (ethylene
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Organometallics
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⧧
assimilation) over b (normal propagation), namely ΔΔG298
=
A discussion comparing the polymerization results in this
work with related catalysts follows. Under identical conditions,
the ethylene polymerization activities for the σ-naphthyl com-
plex 5 are slightly higher than for the σ-phenyl-3,5-bis-
(trifluoromethyl) analogue (1.4 and 1.0 kg mmol−1 h−1 at
50 and 100 °C, respectively),52 reaffirming the ability of py-
adjacent substituents to increase activity, although the resultant
polymers display similarly low Mn values and broad MW
distributions. With regard to the [N,N,C]-Hf system,
Macchioni proposed12 that the normal “alkyl-inserted” catalytic
species is present in high concentration but displays low or
minimal activity, while the olefin-assimilated species forms
slowly but undergoes fast propagation (i.e., low concentra-
tion but highly active). At first glance, this is broadly consistent
with the observations in this work, although clear differences
are apparent. For example, DFT calculations suggest that ethylene
insertion to form the unassimilated Ti-[O,N,C(σ-aryl)] “alkyl-
inserted” cation (pathway b) is slow in comparison with the olefin-
assimilated species, but this pathway is nevertheless viable and the
unassimilated cation should be catalytically active. More generally,
Ti−C bonds are weaker and more reactive than Hf−C, and lower
activation as well as insertion barriers would be anticipated. The
contrasting characteristics of the py-alkyl-amide and py-phenolate
chelates (five- and six-membered rings, respectively), and the
closer proximity of the amide substituent (compared with
phenolate) to the metal center, are also noted.
TS2b − TS2a (4.3 kcal/mol for 4+), is evident for all deriv-
atives except 2+ (for the Me (nBu) cation of [N,N,C]-Hf,8
ΔΔH⧧ = 7.4 (2.2) kcal/mol). Therefore, the σ-naphthyl com-
plex is kinetically the most likely to undergo ethylene assimila-
tion for the first insertion (pathway a), which proceeds faster
than conventional propagation. (2) The activation barrier for
pathway a is lower than for the conventional propagation
pathway b, suggesting that the former is dominant and faster
and the ethylene-assimilated catalyst is more active. For σ-naphthyl
species, this can be explained by the release of Hpy···Hnaphthyl repul-
sion, while attention is drawn to the more tetrahedral geometry of
the Ti center in all ethylene-assimilated structures. Taken together,
these points help to explain the enhanced polymerization
efficiencies of σ-naphthyl catalysts in comparison with other py-
adjacent substituents.
An important aim of this work was to evaluate substituent
effects against the (tert-butyl)phenolate σ-naphthyl derivative
4.24 On the basis of DFT calculations, we conclude that a major
consequence of a py-adjacent substituent such as the benzo
group in 4−7 is to facilitate ethylene assimilation (insertion
into the Ti−C(σ-aryl) bond) by thermodynamic and kinetic
means, as described above. With regard to the role of the
phenolate substituent, the ethylene polymerization results indi-
cate that a cumyl substituent can provide suitable protection for
the active species and reduce multisite behavior. Improved
catalytic performance was realized for the cumyl-phenolate
complex 6, which displays good activity at 50 °C to give
polyethylene with an extremely high MW and an Mw/Mn value
approaching single-site character.
SUMMARY
■
A family of group 4 precatalysts supported by [O,N,C] ligands
bearing various σ-aryl (py-adjacent) and phenolate substituents
has been designed and synthesized. The complexes have been
characterized by multinuclear NMR spectroscopy, and η2
coordination of the benzyl groups was indicated. Ethylene
polymerization studies have been undertaken in conjunction
with [Ph3C][B(C6F5)4], and good activities have been observed
especially for σ-naphthyl catalysts, which feature a benzo group
as the py-adjacent substituent. Replacement of the benzo
moiety by one or more F atoms resulted in lower efficiencies.
This work has shown that suitable steric protection afforded
by the phenolate substituent can improve catalyst integrity and
afford extremely high MW polymers, and narrow MWD (for
the cumyl derivative 6) indicative of close to single-site char-
acter is attainable. However, multisite behavior was evident for
other catalysts, and attempts to directly observe the active sites
using NMR spectroscopy were hampered by their intrinsic
instability. Hence, DFT calculations have been performed to
probe the polymerization characteristics and especially the
initial ethylene insertion and propagation mechanisms, as well
as the function of the py-adjacent substituent. Saliently, these
studies revealed several kinetically competitive reaction path-
ways. In particular, ethylene assimilation (insertion into the
Ti−C(σ-aryl) bond) was found to be faster than conventional
Ti−C(alkyl) insertion and propagation. In this regard, the
benzo py-adjacent substituent is proposed to play an im-
portant role in facilitating the formation of the ethylene-
assimilated [Ti−CH2CH2−naphthyl] species through ther-
modynamic means (by alleviating steric repulsion to gain
stability), as well as by lowering the activation barrier for
subsequent ethylene insertion/propagation (by weakening
the η2-benzyl coordination).
It is evident from the polymerization and DFT results (and
apparent from previous work and related studies) that distinct
reaction pathways can transpire for the Ti-[O,N,C(σ-aryl)]
system and multiple active sites can be generated, including (i)
conventional ethylene insertion into the Ti−C(alkyl) bond and
propagation (pathway b), and conformers thereof (see I−IV
above), (ii) ethylene-assimilated [Ti−CH2CH2−aryl] species
from insertion into Ti−C(σ-aryl) bond (pathway a), plus sub-
sequent isomerization (pathway c) to give [Ti−H/CH2
CH−aryl] followed by benzylic [Ti−CH(Me)−aryl], and con-
formers thereof, (iii) degradation or deactivation (arising from
e.g. transfer of C6F5 moiety from the cocatalyst to Ti,11,12,47
reduction,48 and ligand abstraction35,49 by iBu3Al), (iv) possible
5
11,14
i
metalation of a phenolate substituent (e.g., Ph and Pr
activation have been reported), (v) additional reaction modes
(apart from alkyl abstraction) by the trityl cocatalyst, such as
trityl attack at C(σ-aryl) and C(σ-aryl)/isobutyl exchange by
iBu3Al. For (v), the former is considered unlikely on the basis of
observations from NMR reactions,36 and the latter would be
unexpected (transmetalation generally requires higher activa-
tion energies than olefin insertion and is more sensitive to steric
factors), but neither of the processes can be discounted
(the activation mode and generation of different species, active
or otherwise, is dependent upon the nature of the
cocatalyst11,50,51). In general, highly active species become
more unstable at elevated temperatures as high-energy
processes become increasingly populated. Hence, at higher
polymerization temperatures, the greater diversity of reaction
pathways and active species, each potentially displaying distinc-
tive chain propagation and termination (as well as deactivation)
rates, could account for the broad MW distributions of poly-
mers and multisite behavior of these catalysts.
Interestingly, the ethylene-assimilated species exhibits a more
tetrahedral Ti center, as a result of the conversion of the mer-like
[O,N,C(σ-aryl)] ligand into a fac-coordinating [O,N,C(C2H4-aryl)]
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NMR (376 MHz, CD2Cl2): δ −114.30. Anal. Calcd for C39H40NOFTi
(605.65): C, 77.34; H, 6.66; N, 2.31. Found: C, 77.33; H, 6.59; N,
2.35.
Complex 2.
chelate, and subsequent isomerization via β-H elimination
and 2,1-reinsertion to give a benzylic-type [Ti−CH(Me)−
aryl] complex was shown to be another viable pathway.
Overall, the considerable complexity of the Ti-[O,N,C(σ-
aryl)] catalyst system has become apparent. The inves-
tigations by DFT have indicated the possibility of multiple
active sites and further transformations into supplementary
active species, each with potentially different chain propaga-
tion and termination (as well as degradation) rates, which
could account for the observed multisite behavior during
polymerization. By considering the insights derived herein,
from the polymerization results in conjunction with DFT
calculations, future work will focus on controlling the multisite
characteristics and the development of well-defined catalysts.
EXPERIMENTAL SECTION
1
■
Yield: 0.19 g, 51%. H NMR (400 MHz, C6D6): δ 1.35 (s, 9H,
General Considerations. All reactions were performed under
a nitrogen atmosphere using standard Schlenk techniques or
in a Braun drybox. All solvents were appropriately dried and
5-tBu), 1.80 (s, 9H, 3-tBu), 3.88 (d, J = 8.5 Hz, 2H, CH2), 3.98 (d,
J = 8.5 Hz, 2H, CH2), 6.41 (t, J = 7.3 Hz, 2H, p-Ph), 6.56 (t, J = 7.7
Hz, 4H, m-Ph), 6.57−6.62 (m, 1H, H16), 6.69 (d, J = 7.3 Hz, 4H,
o-Ph), 6.73 (t, J = 8.1 Hz, 1H, H9), 7.05 (d, J = 7.9 Hz, 1H, H8),
7.35 (d, J = 2.3 Hz, 1H, H6), 7.46 (dd, JH,H = 8.0 Hz, JH,F = 2.6 Hz,
1H, H10), 7.69 (d, J = 2.3 Hz, 1H, H4), 8.13 (dd, JH,F = 5.6 Hz,
JH,H = 2.3 Hz, 1H, H14). 13C NMR (101 MHz, C6D6): δ 30.92 (3-
CMe3), 31.76 (5-CMe3), 34.67 (CMe3), 35.74 (CMe3), 93.68 (CH2
(1JC,H = 135.6 Hz)), 104.02 (virtual t, J = 27.2 Hz, C16), 117.01
(dd, J = 13.7 Hz, 3.4 Hz, C14), 119.75 (d, J = 22.0 Hz, C10),
122.30, 124.12 (p-Ph), 124.68, 126.88, 128.01 (m-Ph), 131.56 (o-
Ph), 139.77, 160.20 (dd, J = 266.3, 8.0 Hz, C17), 163.16 (dd, J =
262.0, 7.6 Hz, C15); 4° carbons: 125.20, 136.22, 136.63, 142.73,
156.81, 157.31, 160.51 (d, J = 7.9 Hz, C12), 202.48 (d, J = 6.7 Hz).
19F NMR (376 MHz, C6D6): δ −108.47 (d, J = 11.3 Hz), −109.17
distilled and then degassed prior to use. H, 13C{1H} and 13C[1H]
1
(referenced to residual solvent peaks), and 19F NMR spectra
(external trifluoroacetic acid reference) were recorded at 298 K on
Bruker 500 and 400 DRX spectrometers (ppm). Peak assignments
were made using DEPT-135 and [1H,1H]-COSY, [13C,1H]-HSQC,
and ROESY (plus [1H,19F]-HMQC for 3) NMR experiments.
Elemental analyses were performed on a Vario EL elemental
analyzer (Elementar Analysensysteme GmbH). For polymer
analysis, gel permeation chromatographs were obtained on a
Waters 150CV instrument using polystyrene standards at 140 °C
in 1,2-dichlorobenzene. Synthetic procedures for the H2Ln ligands
are given in the Supporting Information. Ti(CH2Ph)4 was pre-
pared according to the published method,53 and the synthesis of 4
was described previously.24
General Synthetic Procedure for 1−3 and 5−7. A solution of
H2Ln (ca. 0.50 mmol) in pentane and diethyl ether (10/2 mL) was
added dropwise at −78 °C to Ti(CH2Ph)4 (equimolar to H2Ln) in
pentane and diethyl ether (5/0.5 mL). For H2L5, toluene (5 mL) was
used and added to Ti(CH2Ph)4 in pentane (15 mL). The reaction
mixture was stirred for 1 h at −78 °C and 12 h at room temperature.
Filtration and concentration of the reaction mixture gave a dark red
crystalline solid at −25 or −78 °C.
(d, J = 11.3 Hz). Anal. Calcd for C39H40NOFTi (623.64): C,
75.11; H, 6.30; N, 2.25. Found: C, 75.14; H, 6.13; N, 2.30.
Complex 3.
Complex 1.
Yield: 0.24 g, 78%. 1H NMR (400 MHz, C6D6): δ 1.35 (s, 9H, 5-tBu),
1.84 (s, 9H, 3-tBu), 3.94 (d, J = 8.4 Hz, 2H, CH2), 4.08 (d, J = 8.4 Hz,
2H, CH2), 6.38 (t, J = 7.4 Hz, 2H, p-Ph), 6.52 (t, J = 7.8 Hz, 4H, m-
Ph), 6.62 (d, J = 7.2 Hz, 4H, o-Ph), 6.73 (t, J = 8.0 Hz, 1H, H9), 7.07
(d, J = 8.0 Hz, 1H, H8), 7.14 (t, J = 7.6 Hz, 1H, H15), 7.25 (d, J = 8.0
Hz, 1H, H10), 7.34 (d, J = 2.4 Hz, 1H, H6), 7.52 (d, J = 7.6 Hz, 1H,
1
Yield: 0.25 g, 46%. H NMR (400 MHz, CD2Cl2 (aryl resonances
overlap significantly in C6D6)): δ 1.42 (s, 9H, 5-tBu), 1.77 (s, 9H,
3-tBu), 3.65 (d, J = 8.3 Hz, 2H, CH2), 3.91 (d, J = 8.3 Hz, 2H, CH2),
6.44−6.50 (m, 6H, o and p-Ph), 6.55 (t, J = 7.2 Hz, 4H, m-Ph), 6.91−
6.97 (m, 1H, H16), 7.39−7.41 (m, 2H, H6 and H15), 7.52 (dd, J = 8.1
Hz, 1.0 Hz, 1H, H8), 7.59 (td, J = 7.9 Hz, 0.6 Hz, 1H, H9), 7.61 (d, J =
2.4 Hz, 1H, H4), 7.67 (dd, JH,H = 7.9 Hz, JH,F = 2.8 Hz, 1H, H10), 8.35
(dt, J = 6.8 Hz, 1.2 Hz, 1H, H14). 13C NMR (101 MHz, CD2Cl2): δ
30.77 (3-CMe3), 31.62 (5-CMe3), 34.73 (CMe3), 35.63 (CMe3), 92.44
(CH2 (1JC,H = 135.0 Hz)), 115.69 (d, J = 24.7 Hz, C16), 120.22 (d, J =
22.1 Hz, C10), 122.90, 123.42 (p-Ph), 124.66, 126.93, 127.66 (m-Ph),
130.39 (d, J = 6.1 Hz, C15), 130.60 (d, J = 3.8 Hz, C14), 130.85 (o-Ph),
139.93, 159.62 (d, J = 261.5 Hz, C17); 4° carbons: 127.07, 136.29,
H16), 7.73 (d, J = 2.4 Hz, 1H, H4), 8.61 (d, J = 6.8 Hz, 1H, H14). 13
C
NMR (101 MHz, C6D6): δ 31.53 (3-CMe3), 32.31 (5-CMe3), 35.26
(CMe3), 36.32 (CMe3), 93.27 (CH2 (1JC,H = 135.8 Hz)), 121.64
(q, J = 7.0 Hz, C10), 123.45 (C8), 124.56 (p-Ph), 125.57 (C6),
127.34 (C4), 127.95 (q, J = 6.0 Hz, C16), 128.18 (C15), 128.49
(m-Ph), 131.96 (o-Ph), 138.86 (C14), 139.67 (C9), 142.49 (m, CF3);
4° carbons: 126.26, 129.90, 136.73, 137.38, 143.33, 157.85, 158.86,
163.62, 202.77. 19F NMR (376 MHz, C6D6): δ −54.71. Anal. Calcd
for C40H40NOF3Ti (655.66): C, 73.28; H, 6.15; N, 2.14. Found: C,
73.49; H, 6.19; N, 2.01.
136.88, 142.64, 156.61, 157.26, 161.01 (d, J = 7.5 Hz, C12), 201.80. 19
F
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Complex 5.
Complex 7.
1
Yield: 0.13 g, 40%. H NMR (400 MHz, C6D6): δ 4.05 (d, J =
8.2 Hz, 2H, CH2), 4.15 (d, J = 8.2 Hz, 2H, CH2), 6.33 (t, J =
7.2 Hz, 2H, p-Ph), 6.45 (t, J = 7.4 Hz, 4H, m-Ph), 6.54 (t, J = 7.2 Hz,
1H, H5), 6.67 (m, 2H, H8 and H9), 6.76 (d, J = 7.6 Hz, 4H, o-Ph),
7.08−7.13 (m, 2H, H6 and H10), 7.29 (m, 2H, H18 and H19), 7.56
(d, J = 7.2 Hz, 1H, H4), 7.66 (d, J = 7.6 Hz, 1H, H15), 7.75 (d, J =
8.0 Hz, 1H, H17), 8.04 (d, J = 7.6 Hz, 1H, H20), 8.67 (d, J = 8.0 Hz,
1H, H14). 13C NMR (101 MHz, C6D6): δ 96.98 (CH2 (1JC,H = 137.4
Hz)), 119.67 (C5), 121.49 (C8), 121.65 (C10), 124.64 (p-Ph), 125.12
(C20), 126.44, 127.36, 128.41 (m-Ph), 128.62 (C15), 129.76 (m, CF3),
129.90 (C4), 130.39 (C17), 132.50 (o-Ph), 133.33 (C6 and C14), 139.52
(C9); 4° carbons: 119.67, 135.88, 136.86, 139.94, 156.80, 158.99,
165.52, 204.32. 19F NMR (376 MHz, C6D6): δ −61.50. Anal. Calcd for
C36H26NOF3Ti (593.50): C, 72.85; H, 4.42; N, 2.36. Found: C, 72.93; H,
4.79; N, 2.04.
1
Yield: 0.09 g, 28%. H NMR (400 MHz, C6D6): δ 2.92 (s, 3H, Me),
3.36 (d, J = 8.0 Hz, 2H, CH2), 4.01 (d, J = 8.0 Hz, 2H, CH2), 6.36 (m,
2H, p-CH2Ph), 6.41−6.49 (m, 8H, o- and m-CH2Ph), 6.64 (t, J = 7.9
Hz, 1H, H9), 6.71 (t, J = 7.5 Hz, 1H, H5), 6.94 (d, J = 7.9 Hz, 1H, H8),
7.01−7.08 (m, 3H, H10 and H25), 7.16−7.33 (m, 7H, H6, H18, H19 and
H24), 7.40 (d, J = 7.3 Hz, 5H, H4 and H23), 7.61 (d, J = 7.6 Hz, 1H,
H15), 7.72 (m, 1H, H17), 7.99 (m, 1H, H20), 8.59 (d, J = 7.6 Hz, 1H,
H14). 13C NMR (101 MHz, C6D6): δ 30.00 (CMe), 53.42 (CMe),
94.54 (CH2 (1JC,H = 135.3 Hz)), 120.24 (C5), 121.01 (C10), 121.97
(C8), 124.19 (p-CH2Ph), 125.12 (C20), 125.99, 126.97, 127.16 (C25),
128.19 (m-CH2Ph), 128.91 (C15), 129.08 (C23), 129.22 (C4), 129.65
(C24), 130.16 (C17), 131.87 (o-CH2Ph), 133.24 (C6), 134.76 (C14),
139.18 (C9); 4° carbons: 129.64, 129.81, 135.74, 137.55, 138.53,
139.48, 149.76, 158.59, 160.12, 165.30, 204.62. Anal. Calcd for
C49H39NOTi (705.71): C, C, 83.39; H, 5.57; N, 1.98. Found: C,
83.51; H, 5.36; N, 2.22.
Complex 6.
Polymerization Procedure. Ethylene polymerization tests using
[Ph3C][B(C6F5)4] as cocatalyst were carried out in toluene in a 10 mL
glass reactor equipped with a paddlelike stirrer. Toluene (3.5 mL) was
introduced into the nitrogen-purged reactor and vigorously stirred
(600 rpm). The toluene was thermostated to the required temper-
ature, and the ethylene gas feed (7 atm) was then started and
maintained during the experiment. After 15 min, polymerization was
initiated by consecutively adding toluene solutions of iBu3Al,
cocatalyst, and catalyst into the reactor (total volume 5.0 mL) with
stirring. After the prescribed time, isobutyl alcohol (0.5 mL) was added
to terminate the polymerization, and the ethylene feed was stopped.
The polymer was collected by filtration, washed with methanol
(20 mL) and concentrated HCl (2 mL), and dried in vacuo at 80 °C
for 10 h. Errors for the activity values are estimated to be 10%, on
the basis of previous polymerization tests using the same experimental
procedure and reactor.
1
Yield: 0.10 g, 27%. H NMR (400 MHz, C6D6): δ 1.99 (s, 6H,
CMe2), 2.36 (s, 3H, ArMe), 3.42 (d, J = 8.0 Hz, 2H, CH2), 4.09 (d,
J = 8.0 Hz, 2H, CH2), 6.30 (t, J = 7.2 Hz, 2H, p-CH2Ph), 6.34 (d,
J = 7.2 Hz, 4H, o-CH2Ph), 6.41 (t, J = 7.4 Hz, 4H, m-CH2Ph), 6.64
(t, J = 8.0 Hz, 1H, H9), 6.93 (d, J = 8.0 Hz, 1H, H8), 7.00−7.03
(m, 2H, H10 and H25), 7.21 (m, 1H, H6), 7.23−7.29 (m, 4H, H18,
H19 and H24), 7.56−7.58 (m, 3H, H4 and H23), 7.63 (d, J = 7.6 Hz,
1H, H15), 7.72 (d, J = 8.4 Hz, 1H, H17), 7.98 (d, J = 8.0 Hz, 1H, H20),
8.66 (d, J = 7.6 Hz, 1H, H14). 13C NMR (101 MHz, C6D6): δ 22.05
(Me), 31.39 (CMe2), 43.07 (CMe2), 94.33 (CH2 (1JC,H = 135.3 Hz)),
120.91 (C10), 121.73 (C8), 124.01 (p-Ph), 125.09 (C20), 125.95, 126.78,
126.83 (C23), 126.93, 128.03 (m-Ph), 128.19 (C15), 129.07 (C6), 129.31
(C24), 130.16 (C17), 131.40 (C4), 131.87 (o-Ph), 133.47 (C14), 139.04
(C9); 4° carbons: 129.26, 129.83, 135.73, 137.41, 138.33, 139.74,
152.01, 157.68, 158.51, 165.30, 204.29. Anal. Calcd for C45H39NOTi
(657.71): C, 82.18; H, 5.98; N, 2.13. Found: C, 82.43; H, 5.69;
N, 2.24.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text, figures, and tables giving experimental details and
characterization data for H2L1−7, selected NMR data for 1−7,
GPC traces, and additional results for DFT calculations. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mcwchan@cityu.edu.hk (M.C.W.C.); kaichung@cityu.
edu.hk (K.-C.L.).
Notes
The authors declare no competing financial interest.
457
dx.doi.org/10.1021/om300832q | Organometallics 2013, 32, 449−459

Organometallics
Article
(20) (a) Chan, M. C. W. Chem. Asian J. 2008, 3, 18. (b) Mitani, M.;
Nakano, T.; Fujita, T. Chem. Eur. J. 2003, 9, 2396.
(21) Popeney, C. S.; Rheingold, A. L.; Guan, Z. Organometallics
ACKNOWLEDGMENTS
■
The work described in this paper was supported by the
Research Grants Council of the Hong Kong SAR, China
(CityU 100307); the theoretical work was supported by a
Strategic Research Grant from City University of Hong Kong
(Project No. 7002616). We are grateful to Dr. Ka-Ho Tam
(City University of Hong Kong) and Hiromu Kaneyoshi
(Mitsui Chemicals, Inc.) for assistance in synthetic and
polymerization studies, respectively.
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