Highly fluorinated (σ-Aryl)-chelating titanium(IV) post-metallocene: Characterization and scalar [C-H???F-C] coupling

Article abstract of DOI:10.1021/om3002334

The observation of weak intramolecular C-H???F-C interactions in group 4 (σ-aryl)-chelating complexes using NMR spectroscopic and neutron diffraction studies was recently reported. In this work, a new titanium(IV) catalyst precursor supported by a tridentate pyridine-2-phenolate-6-(σ-aryl) ligand, featuring a metal center surrounded by multiple CF₃ substituents, has been synthesized. The nature of intramolecular interactions in the bis(benzyl) complex in solution was probed using multinuclear NMR spectroscopic experiments (including [ ¹H,¹⁹F]-HMQC and -HMBC), which reveal scalar coupling across C-H???F-C interactions between methylene hydrogens and the proximal CF₃ group on the σ-aryl (but not the phenolate) moiety. High activities are observed for ethylene polymerization at different temperatures, which exceed those for the ^tBu-phenolate congener.

Full text of DOI:10.1021/om3002334

Article
pubs.acs.org/Organometallics
Highly Fluorinated (σ-Aryl)-Chelating Titanium(IV) Post-Metallocene:
Characterization and Scalar [C−H···F−C] Coupling
Cham-Chuen Liu,† Loi-Chi So,† Jerry C. Y. Lo,† Michael C. W. Chan,*,† Hiromu Kaneyoshi,‡
and Haruyuki Makio§
†Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, 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: The observation of weak intramolecular C−H···F−C
interactions in group 4 (σ-aryl)-chelating complexes using NMR
spectroscopic and neutron diffraction studies was recently reported. In
this work, a new titanium(IV) catalyst precursor supported by a
tridentate pyridine-2-phenolate-6-(σ-aryl) ligand, featuring a metal
center surrounded by multiple CF3 substituents, has been synthesized.
The nature of intramolecular interactions in the bis(benzyl) complex
in solution was probed using multinuclear NMR spectroscopic
experiments (including [1H,19F]-HMQC and -HMBC), which reveal
scalar coupling across C−H···F−C interactions between methylene
hydrogens and the proximal CF3 group on the σ-aryl (but not the
phenolate) moiety. High activities are observed for ethylene
polymerization at different temperatures, which exceed those for the
tBu-phenolate congener.
by NMR spectroscopic observation of 19F-coupling for the α-
INTRODUCTION
■
carbon atom of the methyl cation, has been invoked to
The generation of highly electrophilic active sites remains an
rationalize the suppression of chain termination.9 Furthermore,
intrinsic and critical objective in the quest for new post-
with regard to late transition metal catalyst systems, two recent
metallocene olefin polymerization catalysts.1 In general,
reports are of particular relevance. The improved performance
enhanced electrophilicity at the metal center should promote
of Ni(II) and Pd(II) α-diimine catalysts incorporated into a
olefin binding and activation for insertion, leading to increased
highly fluorinated cyclophane framework was attributed to
catalytic activity. In this regard, the incorporation of fluorinated
moieties as electron-withdrawing substituents to reduce the
stabilization of the reactive intermediate by M···F−C
interactions (C−H···F−C interactions are feasible and detected
basicity of the ancillary ligand(s) is an attractive design
by 1H−19F coupling for a Pd+−Me complex),10 while the
observation of suppressed β-H elimination for CF3-substituted
Ni(II) phenoxyimine catalysts (relative to the Me-substituted
congener) was ascribed to stabilization afforded by the presence
of Cβ−Hβ···F−C interactions, based on the appearance of
1H···19F dipolar interactions in [1H,19F]-HOESY NMR experi-
ments.11
strategy.2,3 Moreover, the presence of repulsive fluorine-
containing fragments would be expected to decrease undesir-
able intermolecular interactions, such as ion-pairing4 between
cationic metal alkyl species and perfluorinated cocatalysts,
during the polymerization reaction.
On the other hand, the role of organofluorine groups as
attractive moieties in catalysis has become more widespread,
especially in the form of noncovalent M···F−C coordination.5
The remarkable living olefin polymerization capabilities6 of
fluorine-rich bis(phenoxyimine) group 4 catalysts have been
attributed, based on DFT calculations, to the presence of C−
H···F−C interactions in the active species between a β-
hydrogen atom of the polymer chain and an ortho-F substituent
in the arylimine moiety,7 which is proposed to curtail
termination (β-H elimination/transfer) processes. In contrast,
for the related living o-fluorinated bis(enolatoimine) titanium
catalysts,8 the existence of Ti···F−C interactions, as evidenced
We have described the observation of intramolecular C−
H···F−C interactions in F-functionalized pyridine-2-phenolate-
6-(σ-aryl) [O,N,C] group 4 complexes using NMR spectro-
scopic and neutron diffraction studies,12 which constitute
synthetic models of weak attractive ligand−polymer inter-
actions with potential applicability to polymerization pro-
cesses.7,11,13 In the wider context of the growing and diverse
Received: March 21, 2012
Published: July 25, 2012
© 2012 American Chemical Society
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t
Scheme 1. Synthesis of Ti Complex 1, Shown with Bu Analogue 2
importance of fluorinated substituents, and the debate14 over
the existence and significance of C−H···F−C interactions,15,16
the development of spectroscopic techniques to probe the
environment and connectivity around fluorine atoms is
warranted. Very recently, we published the first investigation
acid, the additions of the chromic and acetic acid reagents were
carefully performed at −50 °C to minimize its removal.
Subsequent deprotection was achieved during flash chromatog-
raphy using mildly acidic silica gel to afford the desired
H2L(CF ) ligand in relatively low yields (ca. 20%). The
3
3
1
to probe noncovalent H−19F coupling using [1H,19F]-HMBC
molecular structure of H2L(CF ) (Supporting Information)
revealed an intramolecular O−H···N hydrogen bond as well as
multiple close intermolecular C−H···F−C interactions (short-
est 2.57 Å; C−H···F 138°) that are similar to those previously
detected in organofluorine molecules.15
3
3
NMR experiments, which confirmed the existence of significant
scalar (J) coupling across C−H···F−C interactions in (σ-aryl)-
2-phenolate-6-pyridyl [O,C,N-RF]-Ti (RF = CF3, F) complexes,
but indicated that 1H−19F coupling occurs predominantly
through M···F−C interactions in the [O,C,N-CF3]-Zr and -Hf
congeners.17
Cyclometalation of the H2L(CF ) ligand with Ti(CH2Ph)4
was performed in a diethyl ether/n-pentane mixture at −78 °C
and proceeded upon warming to 20 °C. After stirring for 12 h,
the reaction mixture was filtered, concentrated, and cooled at
−78 °C to afford the titanium complex 1 as a dark red
crystalline solid in an isolated yield of 30% (the relatively low
yield is partly ascribed to its high solubility in nonpolar
solvents). Preparation of the Zr and Hf analogues has been
attempted several times using different solvents and mixtures
thereof (diethyl ether, THF, n-pentane, toluene); the nonpolar
solvents were used because etherate solvents were subsequently
suspected to promote reduction processes. Nevertheless, the
solids obtained from these reactions failed to give discernible
1H NMR signals, indicating reduced products. The attempted
3
3
Aromatic σ-carbanions are predominantly σ-donors and can
impart greater electrophilicity at metal centers compared with
π-donors.18 The aim of this work is to modify the previously
reported [O,N,C]-Ti complex bearing a bis(trifluoromethyl)-
(σ-aryl) moiety12 by replacing the tert-butyl phenolate
substituent with an additional CF3 group. The possible effects
of such an F-dominated catalytic environment, including
enhanced electrophilicity and separation of cation and
fluorinated cocatalyst, and the consequences of these upon
catalytic and polymer properties will be examined. A variety of
multinuclear NMR experiments have been employed to probe
the environments around the multiple CF3 units and their
participation in intramolecular interactions. In particular,
selective [1H,19F]-HMBC NMR experiments have been
performed to evaluate the scalar coupling for individual CF3
groups.
reaction of Zr(CH2Ph)2Cl2(thf)2 with H2L(CF ) in various
solvents yielded intractable mixtures. Radical mechanisms and
migratory insertion and reduction of imine have been described
for related tetradentate Schiff-base zirconium bis(benzyl)
complexes.22
3
3
RESULTS AND DISCUSSION
Multinuclear NMR Characterization. Complex 1 has
been characterized in C6D6 by 1H, 19F, and 13C NMR
spectroscopy and a range of 2-D NMR experiments. According
■
Synthesis. The fluorinated 2-phenol-6-arylpyridine ligand
was synthesized by modification of a literature procedure19
involving the sequential coupling of two substituted acetophe-
nones (Scheme 1).20 Ortho-directed lithiation21 of protected 2-
trifluoromethylphenol, followed by ethanal addition then
oxidation, gave the required acetophenone (I-2). The prop-1-
ene intermediate (I-3) was treated with a mixture of potassium
tert-butoxide and I-2, followed by ammonium acetate in acetic
acid, to give the protected [O,N,C] ligand. It should be noted
that since the ethoxymethyl protecting group is sensitive to
to conventionally accepted criteria,23 the diminished JH,H (8.0
2
Hz) and large 1JC,H (137.4 Hz) values, plus the high-field ortho-
1
Ph H NMR shift (6.42 ppm) for the benzyl groups, are
strongly indicative of η2-coordination (involving ipso-Ph···Ti),
thus reflecting the high electrophilicity of the Ti center in 1.
Nevertheless, the corresponding parameters for 2 (2JH,H = 8.3
1
Hz, JC,H = 138.0 Hz, 1H NMR (o-Ph): 6.44 ppm) are
comparable and do not exhibit the effects of different phenolate
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1
Figure 1. Methylene region in (a) H and (b) 13C NMR spectra and (c) [1H,19F]-HMQC spectrum of 1 (C6D6,* 298 K, + = residual toluene).
t
substituents (CF3 versus two Bu). The previously reported
molecular structures of [O,N,C] complexes, bearing a σ-aryl
substituent adjacent to the metal, show bis(benzyl) moieties
that point toward the pyridine ring in an “anti, anti”
configuration12,24 (rather than the “syn, anti” arrangement
observed for sterically undemanding [O,N,C] ligands12b and
typical in related post-metallocenes25), and careful scrutiny of
section] are consistent with interaction between one methylene
hydrogen on each benzyl ligand and three equivalent 19F nuclei
of a rotating CF3 unit. Furthermore, the [1H,19F]-HMQC
spectrum (Figure 1c) reveals the expected cross-peaks between
the three CF3 groups and the corresponding aryl hydrogens and
indicates strong coupling between the CH2 fragment and the
proximal σ-aryl CF3 substituent (F19), rather than the phenolate
CF3 moiety (F20).
1
the benzyl H NMR resonances for such derivatives reveals a
Scalar Coupling across C−H···F−C Interactions.
noticeably high-field para-Ph shift (<6.5 ppm in C6D6),
reflecting this configuration.12,24,26 The bis(benzyl) units in 1
1
1
Although the H−19F coupling giving rise to the dq H NMR
resonance for 1 was formally referred to as 1hJH,F, clarification is
1
are also expected to exist as “anti, anti”, and the H NMR
1
necessary because apparent H−19F scalar coupling (J, with
resonance for p-Ph (6.17 ppm) is entirely consistent with this.
Interestingly, one of the two expected doublet resonances for
the diastereotopic methylene hydrogens at 3.98 ppm appears as
a slightly broad, overlapping doublet of quartets, which reverts
to a normal doublet upon 19F-decoupling (Supporting
Information), while the 13C{1H} NMR peak for the methylene
carbons appears as an unusual quartet at 101.25 ppm (Figure
chemical connectivity) through noncovalent interactions can
contain interference from cross-correlation (CR) interactions.
Cross-correlation interactions occur through space without
1
bonding and arise from relaxation interference between H
1
chemical shift anisotropy (CSA) and H−19F dipolar (DD)
interactions (which occur when the magnetic field generated by
one nuclear dipole affects the magnetic field of another
nucleus). The potential applicability of C−H···F−C inter-
1a, b). The observed couplings [formally referred to as “1hJH,F
(∼2 Hz) and “2hJC,F” (5.4 Hz) respectively; see following
”
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actions in catalysis is only feasible if the observed coupling
contains a J component and the noncovalent interaction is
genuine.
Regarding the coupling mechanism, the observation of a
doublet of quartets in the H NMR spectrum for only one of
1
the two diastereotopic methylene hydrogens is consistent with
19F-coupling to a rapidly rotating CF3 group through C−
H···F−C interactions (as is the quartet in the 13C{1H} NMR
spectrum for the methylene carbons). Upon consideration, it is
apparent that the observed NMR resonances and 1H−19F
coupling are not consistent with a C−F···Ti−CH2 mechanism,
for which the 19F-coupling to each methylene hydrogen would
be similar but nonidentical.17 This conclusion is supported by
the reported crystal structures of corresponding CF3-
substituted Ti complexes that exhibit analogous 19F-coupling
(including 2),12b all of which display Ti···F distances of around
or in excess of 3.0 Å, signifying that such interactions are
absent. Furthermore, the downfield “normal” doublet 1H NMR
peak for the other methylene proton (H−C−H···F−C) in 1
also engages to a minor extent in 19F-coupling, as evidenced
from the weaker but discernible cross-peak in the [1H,19F]-
HMQC spectrum (Figure 1c), as well as the scalar coupling in
the [1H,19F]-HMBC spectra (Figure 2c). The substantially
Since the HMBC experiment is the most sensitive NMR
1
pulse sequence for probing 19F and H correlations, a series of
selective [1H,19F]-HMBC experiments have been performed for
complex 1 (see Experimental Section for details) to assess the
relative contributions from scalar coupling (J) and cross
correlation. We recently described the use of [1H,19F]-HMBC
experiments to investigate intramolecular interactions in related
[O,C,N] group 4 derivatives bearing a proximal CF3 group.17
For complexes displaying a single 19F resonance, [1H,19F]-
HMBC experiments were performed without the evolution of
its chemical shifts to provide simple one-dimensional 19F-edited
1H spectra. However, for molecules such as 1, containing
several 19F resonances at discrete chemical shifts, separate
[1H,19F]-HMBC NMR experiments using selective pulses are
required to enable excitation of individual 19F resonances
(Figure 2 and Supporting Information).
1
3h
smaller magnitude of this H−19F coupling (namely
J ) is
H,F
entirely consistent with the postulated H−C−H···F−C
coupling mechanism (since
3h
1h
J
≪
J ) and also implies
H,F
H,F
that Ti···F interactions do not occur. Taken together with the
[1H,19F]-HMBC (J) experiments, these results corroborate the
existence of scalar coupling across C−H···F−C interactions
between the CH2 and proximal σ-aryl CF3 moieties in 1.
It is interesting to note that an approximate relationship of
JC,F ≈ nJH,F (n = 1.7−2.4) is consistently observed in the
literature for organometallic systems assigned to C−H···F−C
[Tp(CF ) PtMe3 (Tp(CF ) = HB{3,5-bis(trifluoromethyl)-
pyrazolyl}3),16c [O,N,C-CF3] complexes,12 [O,C,N-RF]-Ti
(RF = CF3, F) complexes,17 and [(F8-Cyc)PdMe(NCMe)]+
(F8-Cyc = fluorinated cyclophane-based α-diimine)10] as well
as HC−M···F−C [(F-N3)ZrMe (F-N3 = HC{SiMe2N(2-
FC6H4)}3)5d and [O,C,N-CF3]-Zr and -Hf complexes;17
3
2
3 2
supported by crystal structures or analogue thereof] coupling
2h
mechanisms, and this relationship is maintained for both
J
C,F
(for C−H···F−C) and 2JC,F (for HC−M···F−C) values across a
range of 3.9−17.6 Hz. By comparison, a relationship of JC,F
≈
2JH,F was originally established for “long-range through-space”
coupling in organofluorine compounds, in which the strong JC,F
coupling was attributed to the capacity of carbon-centered
orbitals to mediate long-range JH,F coupling by interacting with
Figure 2. (a) 1H NMR spectrum (400 MHz, C6D6,* 298 K) for 1 (+ =
residual toluene) and 1D [1H,19F]-HMBC NMR spectra upon
excitation of the 19F resonance for the proximal σ-aryl CF3 moiety
the fluorine-based orbitals.27 For complexes such as 1 (JC,F
=
1
(F19) at −56.49 ppm, showing signals arising (b) from H−19F scalar
2.7JH,F), in addition to the principal C−H···F−C coupling
mechanism, the possibility that the further enhanced JC,F
coupling may reflect greater involvement of the carbon-
centered orbitals should be considered.
(J) and cross-correlation (CR) interactions; (c) exclusively from scalar
couplings (J); (d) exclusively from dipolar CR.
The structure of 1 was optimized by DFT calculations using
the dispersion-corrected functional developed by Grimme,28,29
and the energy-minimized calculated (Gaussian) structure
shows an anti, anti conformation (Supporting Information)
that is consistent with the results from NMR spectroscopy. The
H···F distances between one of the α-hydrogens on each benzyl
group and the nearest fluorine atom on the proximal σ-aryl CF3
moiety are 2.432 and 2.438 Å, respectively, which are
comparable to those found in the molecular structure of 2
(mean 2.49 Å) and its Zr congener (neutron structure:
2.572(6) and 2.607(5) Å)12b and correspond to the
spectroscopically observed C−H···F−C interactions in 1. The
shortest calculated Ti···F separation of 2.980 Å exceeds the
range expected for a discernible interaction.5
Importantly, the [1H,19F]-HMBC (J) spectrum for complex
1, upon excitation of the 19F resonance for the proximal σ-aryl
CF3 moiety (F19) at −56.49 ppm (Figure 2c), depicts a
significant scalar component for 19F-coupling to the CH2
hydrogens, particularly for the upfield resonance. On the
other hand, excitation of the phenolate CF3 resonance (F20) at
−61.60 ppm revealed dominant scalar coupling with the
phenolate aryl hydrogens, and excitation of the 19F resonance
for the distal σ-aryl CF3 moiety (F19) at −62.34 ppm afforded
the expected responses in the aryl region only (Supporting
Information). These selective [1H,19F]-HMBC (J) experiments
reveal clear differences with regard to scalar coupling for the
three CF3 resonances in 1.
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Ethylene Polymerization Studies Using Borate Coca-
talyst. Investigations into the impact of multiple trifluor-
omethyl moieties upon the olefin polymerization behavior of
the [O,N,C]-Ti system, as well as the influence of the
substituent ortho to the Ti−O(phenolate) bond, were under-
taken. Complex 1 has been evaluated for ethylene polymer-
[O,N,O]-Zr systems,35 and implies that (i) the coordination
space around the catalytic center in 1, between the CF3
substituents on the phenolate and σ-aryl moieties, is highly
congested, and (ii) propylene apparently suppresses the
ethylene polymerization reaction.36
i
ization using Bu3Al/[Ph3C][B(C6F5)4] as cocatalyst, and
comparisons with 2 have been made (Table 1). Primarily, the
SUMMARY
■
The titanium(IV) bis(benzyl) complex 1 supported by a
tridentate pyridine-2-phenolate-6-(σ-aryl) ligand bearing multi-
ple CF3 groups was prepared, and spectroscopic and catalytic
comparisons with the congener containing a tert-butyl
phenolate substituent (2) have been made. Complex 1 was
characterized by a variety of 1- and 2-D multinuclear NMR
a
Table 1. Ethylene Polymerization Results
b
c
c
catalyst temp (°C) yield (mg) activity
Mn
Mw/Mn
1
50
100
50
141
97
1.41
0.97
0.23
0.19
5.36 × 105
3.03 × 105
0.29 × 105
0.90 × 105
8.1
13.6
17.7
30.7
experiments. The H and 13C NMR spectra exhibit coupling
1
d
2
46
between the benzyl CH2 groups and the proximal CF3
100
38
substituent on the σ-aryl moiety, and a significant scalar
a
Conditions: 5 mL of toluene, 0.3 μmol of catalyst, iBu3Al/
1
component for the H−19F coupling was demonstrated by
[Ph3C][B(C6F5)4]/catalyst (50/2/1 equiv, respectively), 7 atm of
selective [1H,19F]-HMBC NMR experiments, thereby confirm-
ing that the C−H···F−C interactions in 1 occur with chemical
connectivity. The focus of this work is the generation of a
catalytic center surrounded by fluorinated moieties, while the
impact of the substituent adjacent to the Ti−O(phenolate)
bond was also investigated. Compared with 2, the ethylene
polymerization activities for 1 are substantially higher and
improved polymer properties are obtained.
ethylene pressure (maintained by continuous supply), 20 min reaction
b
time. Activity in kg(polymer) (mmol catalyst)−1 h−1 ( 10%).
c
Determined by GPC at 140 °C in 1,2-dichlorobenzene using
d
polystyrene standards. 0.6 μmol of catalyst.
catalytic activities for 1 at 50 and 100 °C (1.41 and 0.97 kg
mmol−1 h−1, respectively30) are high and superior to those for
2, while the corresponding Mn and Mw/Mn values for 1 are also
improved. The additional CF3 group should engender greater
electrophilicity at the catalytic center in 1 and facilitate ethylene
binding and insertion to give superior activities, and this should
be reinforced by the less crowded and more accessible Ti core
EXPERIMENTAL SECTION
■
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 distilled, then
t
upon changing the phenolate substituent from Bu to CF3.
1
degassed prior to use. H, 13C (referenced to residual solvent peaks)
Although the enhanced electrophilicity of 1 would convention-
ally afford stronger ion-pairing (possibly leading to lower
activity), this effect should be negated or at least counteracted
by effective fluorine−fluorine repulsion between the F-
surrounded cationic active site and the perfluorinated borate
anion. The observed activities for 1 compare favorably with
those previously reported for catalysts containing fluorinated
substituents,2,3 particularly when discrete cationic2a,i,3c rather
than MAO systems are considered. The higher Mn for 1
presumably arises from the combined effects of faster chain
propagation and improved steric protection against chain
termination processes.31 The high Mw/Mn values are partly
caused by some tailing at low molecular weights, especially at
100 °C, and indicate the generation of multiple active sites or
catalyst degradation (e.g., resulting from olefin insertion into
the Ti−C(σ-aryl) bond,24,32 reduction,22,33 or ligand abstrac-
and 19F (external trifluoroacetic acid reference) NMR spectra were
recorded at 298 K on a Bruker Avance 400 (or 300) FT-NMR
spectrometer (ppm). Peak assignments were based on combinations of
DEPT-135 and 2-D 1H−1H, 13C−1H, and NOE correlation 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 using polystyrene standards at 140 °C in 1,2-dichlorobenzene.
Ti(CH2Ph)4 was prepared according to the published method.37
Proton-detected [1H,19F]-HMBC experiments were performed on a
1
Bruker Avance spectrometer operating at H frequencies of 400.14
MHz with the temperature adjusted to 293.3 K, using a 5 mm probe
equipped with actively shielded three-axis pulsed-field gradient (PFG)
coils. To detect scalar coupling between 1H and 19F resonances
through hydrogen bonds, one-dimensional [1H,19F]-HMBC experi-
ments were carried out using pulse sequences adapted from the
[1H,31P]-HMBC experiments described by Ruterjans.38 The [1H,19F]-
̈
tion34 by Bu3Al).
HMBC pulse sequence for recording the 1D spectrum was
90°ϕ1(1H)−Δ−90°ϕ2(19F)−G1−90°ϕ3(19F)−G2−Aq(1H). Sine-
bell-shaped PFGs were applied with strengths of G1 = 40 G/cm
and G2 = −2.36 G/cm [= G1 × (γF/γH − 1)] and durations of 1.0
ms. Pulse phases were cycled according to ϕ1 = 2(x), 2(−x), ϕ2 = x, −
x, ϕ3 = 4(x), 4(−x), ϕrec = x, 2(−x), x, −x, 2(x), −x. The
corresponding one-dimensional [1H,19F]-HMBC (J) experiment was
performed to detect polarization transfer contributed exclusively by J
coupling, which was achieved by simultaneously applying 180° pulses
on 1H and 19F in the middle of the Δ delay, thus suppressing CSA/DD
cross correlation. To detect exclusive cross-correlation contributions,
the one-dimensional [1H,19F]-HMBC (CR) experiment was per-
formed, which contains an additional 180° pulse in the middle of the
Δ delay on the 1H resonance only. This pulse refocuses scalar
coupling, whereas CSA/DD cross correlation remains active. The
experimental conditions for all one-dimensional [1H,19F]-HMBC
experiments were as follows: spectral width (1H) = 4807.7 Hz, data
matrix = 16384 (complex), 64 scans were measured per transient plus
i
Catalyst 1 was also tested for ethylene−propylene copoly-
merizations at 100 °C [conditions: 5 mL of toluene, 0.6 μmol
i
of catalyst, Bu3Al/[Ph3C][B(C6F5)4]/catalyst (50/2/1 equiv,
respectively), designated propylene pressure out of 7 atm total
pressure (maintained by continuous ethylene supply), 20 min
reaction time]. At 0.5 atm propylene pressure, the activity and
Mn diminish substantially compared with the homopolymeriza-
tion to 0.33 kg mmol−1 h−1 and 151 000, respectively [C3
content = 1.3 mol %; estimated by IR analysis]. The activity
nevertheless compares well with 1 (0.11 kg mmol−1 h−1), but
the low comonomer content is undesirable (at 1 atm propylene
pressure, only a minor improvement to 3.1 mol % is achieved
but Mn drops to 49 000). This contrasts with the superior
propylene incorporation capabilities observed for the analogous
[O,N,C]-Ti (without a proximal σ-aryl substituent)24 and
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8 dummy scans, and the recycling delay was 2 s. The data were
processed with the Bruker TOPSPIN 2.0 software package. An
exponential multiplication window function was applied using 0.2 Hz
line-broadening, followed by zero-filling to 64 K complex data points,
Fourier transformation, and phasing. For selective one-dimensional
[1H,19F]-HMBC NMR experiments, all 19F 90° pulses are replaced
with selective pulses in order to allow excitation of each 19F resonance
individually. The same experimental conditions were employed, except
a Eburp1 selective pulse of 10 ms was used to selectively excite each
19F resonance.
give a dark red solution. A solution of I-3 (3.38 g, 10.86 mmol) in
THF (25 mL) was added, and the mixture was stirred for 36 h at room
temperature. A solution of ammonium acetate (8.37 g, 108.6 mmol) in
acetic acid (30 mL) was slowly added at −50 °C, after which the
reaction was warmed to room temperature. THF was removed by
distillation at 130 °C for 2 h, and the resultant mixture was extracted
with dichloromethane, washed by water and brine, and dried over
magnesium sulfate. Solvents were removed to give a red oil, which was
purified by silica gel flash chromatography using n-hexane and ethyl
acetate (100:1) as eluent to give a pale yellow solid. Yield: 0.88 g, 20%.
1H NMR (400 MHz, CDCl3): 7.03 (t, J = 7.8 Hz, 1H), 7.67 (d, J = 7.6
Synthesis of I-1. A solution of 2-hydroxybenzotrifluoride (4.0 g,
24.7 mmol) in THF (20 mL) was slowly added at 0 °C to a
suspension of NaH (0.89 g, 37.1 mmol) in THF (20 mL) under a
nitrogen atmosphere. The yellow suspension was then stirred at room
temperature for 1 h, and chloromethyl ethyl ether (3.2 mL, 34.5
mmol) was added. After stirring at room temperature for 12 h, ice
water was added to the resultant white suspension, and the product
was extracted with diethyl ether, washed by water and brine, and dried
over magnesium sulfate. After the evaporation of solvents, the product
was purified by silica gel flash chromatography using n-hexane as
eluent to give a colorless oil. Yield: 5.41 g, 99%. 1H NMR (400 MHz,
CDCl3): 1.22 (t, J = 7.0 Hz, 3H, OCH2CH3), 3.76 (q, J = 7.0 Hz, 2H,
OCH2CH3), 5.32 (s, 2H, OCH2O), 7.05 (t, J = 7.6 Hz, 1H), 7.25−
7.27 (m, 1H), 7.48 (t, J = 8.0 Hz, 1H), 7.58 (dd, J = 7.6 Hz, 0.4 Hz,
1H).
Hz, 1H), 7.75 (dd, J = 7.6 Hz, 0.8 Hz, 1H), 8.00−8.09 (m, 4H), 8.33
(s, 2H), 14.77 (s, 1H, OH). 19F NMR (377 MHz, CDCl3): −62.83,
−62.84.
Synthesis of Complex 1 (refer to Figure 2 for labeling/
assignment). A solution of Ti(CH2Ph)4 (0.28 g, 0.68 mmol) in
pentane (20 mL) and diethyl ether (3 mL) was slowly added at −78
°C to a solution of H2L(CF ) (0.30 g, 0.67 mmol) in pentane (10 mL)
and diethyl ether (3 mL). The resultant red solution was stirred for 1 h
at −78 °C and for 12 h at room temperature. Filtration and
concentration of the mixture gave a dark red crystalline solid at −78
3
3
Synthesis of I-2. A solution of n-butyllithium (10.0 mL, 1.6 M in
hexanes, 16.0 mmol) was slowly added at −78 °C to I-1 (3.165 g,
°C. Yield: 0.135 g, 30%. 1H NMR (400 MHz, C6D6): 3.98 (br dq, J =
1h
8 Hz,
J
≈ 2 Hz, 2H, CH2), 4.17 (d, J = 8.0 Hz, 2H, CH2), 6.17 (t, J
H,F
= 7.4 Hz, 2H, p-Ph), 6.24 (t, J = 7.4 Hz, 4H, m-Ph), 6.40−6.43 (m,
5H, o-Ph, H10), 6.50 (t, J = 8.0 Hz, 1H, H5), 6.60 (t, J = 8.2 Hz, 1H,
H9), 6.78 (d, J = 7.6 Hz, 1H, H8), 7.10 (d, J = 7.6 Hz, 1H, H6), 7.51 (d,
J = 7.6 Hz, 1H, H4), 7.63 (s, 1H, H17), 8.08 (s, 1H, H15). 13C NMR
2h
(101 MHz, C6D6): 101.25 (q,
J
= 5.4 Hz [1JC,H = 137.4 Hz],
C,F
1
CH2), 117.51 (C10), 119.11 (q, JC,F = 29.7 Hz, CF3), 119.54 (C5),
122.36 (C8), 122.98 (m, C17), 123.32 (m, C15), 124.16 (p-Ph), 127.47
14.37 mmol) in THF (40 mL) to give an orange solution. The mixture
was stirred at −78 °C for 1 h, and ethanal (2.4 mL, 42.93 mmol) was
then added. The resultant mixture was stirred for 12 h at room
temperature. Diethyl ether (30 mL) was added, and a solution of
sodium dichromate dihydrate (6.4 g, 21.5 mmol) and concentrated
sulfuric acid (6.4 mL) in DI water (20 mL) was slowly added at −50
°C. The black solution was slowly warmed to room temperature and
stirred for 1 h. The organic layer was washed by water and brine and
dried over magnesium sulfate. After the evaporation of solvents, the
product was purified by silica gel flash chromatography using n-hexane
as eluent to give a yellow oil. Yield: 3.00 g, 80%. 1H NMR (300 MHz,
CDCl3): 1.20 (t, J = 7.2 Hz, 3H, OCH2CH3), 2.64 (s, 3H, OCMe),
3.71 (q, J = 7.0 Hz, 2H, OCH2CH3), 5.06 (s, 2H, OCH2O), 7.26−7.31
(m, 1H), 7.70 (dd, J = 7.8 Hz, 1.6 Hz, 1H), 7.74 (dd, J = 7.8 Hz, 1.3
Hz, 1H).
(m-Ph), 129.64 (m, C4), 130.57 (o-Ph), 130.73 (q, JC,F = 33.0 Hz,
1
CF3), 132.30 (C6), 137.65 (q, JC,F = 30.5 Hz, CF3), 139.38 (C9); 4°
1
carbons: 125.80, 126.03, 127.22, 137.15, 144.58, 154.33, 157.26,
161.68, 192.82. 19F NMR (377 MHz, C6D6): −56.49 (F19), −61.60
(F20), −62.34 (F18). Anal. Calcd for C43H43NOTi (679.40): C, 60.11;
H, 3.26; N, 2.06. Found: C, 60.40; H, 3.45; N, 1.93.
Polymerization Procedures. Ethylene polymerization tests were
carried out in toluene in a 10 mL glass reactor equipped with a paddle-
like stirrer. Toluene (3.5 mL) was introduced into the nitrogen-purged
reactor and vigorously stirred (600 rpm). The toluene was
thermostated to the required temperature, 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
Synthesis of I-3. A literature procedure19 was adopted using 3,5-
bis(trifluoromethyl)acetophenone (3.16 g, 12.34 mmol). Yield: 2.80 g,
i
solutions of Bu3Al, 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%, based on previous polymerization tests using
the same experimental procedure and reactor. Ethylene/propylene
polymerization tests were performed by a similar procedure to that for
ethylene polymerization, except propylene gas (0.5 or 1 atm) was
added; then the total pressure was increased to 7 atm and maintained
by continuous ethylene supply. After terminating the polymerization,
concentrated HCl (2 mL) was added to the resultant mixture, which
was washed with water (3 × 30 mL) and concentrated in vacuo. The
resultant polymer was dried in vacuo at 130 °C for 10 h.
73%. 1H NMR (300 MHz, CDCl3): 3.00 (s, 3H, NCH3), 3.22 (s, 3H,
NCH3), 5.67 (d, J = 12.3 Hz, 1H), 7.89−7.93 (m, 2H), 8.32 (s, 2H).
19F NMR (282 MHz, CDCl3): −63.14.
Synthesis of H2L(CF ) . Intermediate I-2 (2.56 g, 9.76 mmol) and
potassium tert-butoxide (2.74 g, 24.42 mmol) were stirred in THF (20
mL) for 12 h at room temperature under a nitrogen atmosphere to
3
3
5279
dx.doi.org/10.1021/om3002334 | Organometallics 2012, 31, 5274−5281

Organometallics
Article
Maron, L.; Carpentier, J.-F. J. Am. Chem. Soc. 2011, 133, 9069.
(k) Plenio, H. Chem. Rev. 1997, 97, 3363.
ASSOCIATED CONTENT
■
S
* Supporting Information
(6) (a) Saito, J.; Mitani, M.; Mohri, J.; Yoshida, Y.; Matsui, S.; Ishii,
S.; Kojoh, S.; Kashiwa, N.; Fujita, T. Angew. Chem., Int. Ed. 2001, 40,
2918. (b) Saito, J.; Mitani, M.; Onda, M.; Mohri, J.; Ishii, S.; Yoshida,
Y.; Nakano, T.; Tanaka, H.; Matsugi, T.; Kojoh, S.; Kashiwa, N.; Fujita,
T. Macromol. Rapid Commun. 2001, 22, 1072. (c) Michiue, K.; Onda,
M.; Tanaka, H.; Makio, H.; Mitani, M.; Fujita, T. Macromolecules 2008,
41, 6289. (d) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev.
2011, 111, 2363.
(7) (a) Mitani, M.; Mohri, J.; Yoshida, Y.; Saito, J.; Ishii, S.; Tsuru, K.;
Matsui, S.; Furuyama, R.; Nakano, T.; Tanaka, H.; Kojoh, S; Matsugi,
T.; Kashiwa, N.; Fujita, T. J. Am. Chem. Soc. 2002, 124, 3327.
(b) Mitani, M.; Furuyama, R.; Mohri, J.; Saito, J.; Ishii, S.; Terao, H.;
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(8) (a) Yu, S.-M.; Mecking, S. J. Am. Chem. Soc. 2008, 130, 13204.
Multinuclear NMR spectra; CIF containing crystallographic
data for H2L(CF ) and perspective view of unit cell; details of
DFT calculation. This material is available free of charge via the
Internet at http://pubs.acs.org.
3
3
AUTHOR INFORMATION
Corresponding Author
*E-mail: mcwchan@cityu.edu.hk.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(b) Non-living Zr congener: Yu, S.-M.; Tritschler, U.; Gottker-
̈
The work described in this paper was supported by the
Research Grants Council of the Hong Kong SAR, China
(CityU 100406). We are grateful to Dr. Kong-Hung Sze
(HKU) for assistance with [1H,19F]-HMBC experiments, and
Schnetmann, I.; Mecking, S. Dalton Trans. 2010, 39, 4612.
(9) (a) Bryliakov, K. P.; Talsi, E. P.; Moller, H. M.; Baier, M. C.;
̈
Mecking, S. Organometallics 2010, 29, 4428. (b) Moller, H. M.; Baier,
̈
M. C.; Mecking, S.; Talsi, E. P.; Bryliakov, K. P. Chem.Eur. J. 2012,
Dr. Shek-Man Yiu for solving the crystal structure of H2L(CF )
.
3
3
18, 848.
(10) Popeney, C. S.; Rheingold, A. L.; Guan, Z. Organometallics
2009, 28, 4452.
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