Group 4 Post-Metallocenes Supported by [OCH2 N,C(σ-aryl)] Auxiliaries Bearing a Seven-Membered Metallacycle: Synthesis, Characterization, and Catalysts for Olefin Polymerization

Article abstract of DOI:10.1021/acs.organomet.9b00307

A series of pyridine-2-phenolate-6-(σ-aryl) [O^CH₂ N,C] group 4 bis(benzyl) precatalysts, featuring a flexible O,N-donor chelate, have been prepared and characterized by multinuclear NMR spectroscopy. These complexes adopt C₁

Full text of DOI:10.1021/acs.organomet.9b00307

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
CH
2
Group 4 Post-Metallocenes Supported by [O N,C(σ-aryl)]
Auxiliaries Bearing a Seven-Membered Metallacycle: Synthesis,
Characterization, and Catalysts for Olefin Polymerization
Cham-Chuen Liu, Qian Liu, Shek-Man Yiu, and Michael C. W. Chan*
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
*
S Supporting Information
ABSTRACT: A series of pyridine-2-phenolate-6-(σ-aryl)
CH
2
[O
N,C] group 4 bis(benzyl) precatalysts, featuring a flexible
O,N-donor chelate, have been prepared and characterized by
multinuclear NMR spectroscopy. These complexes adopt C
symmetry, and variable-temperature H NMR experiments for a
1
1
difluoro-substituted σ-aryl Ti(IV) derivative indicate a fluxional
process involving inversion of the seven-membered chelate ring
accompanied by conformational changes for the benzyl groups
‡
−1
(
ΔG = 62.6 kJ mol ). Scrutiny of the molecular structure of a
Zr(IV) relative, determined by X-ray crystallography, revealed severely distorted “quasi-meridional” coordination for the
CH
2
[O
N,C] ligand and virtually perpendicular orientation of the benzyl moieties. Olefin polymerization studies have been
undertaken in conjunction with methylaluminoxane (MAO) and trityl borate. Using dried MAO (250 equiv) as cocatalyst and 7
−
1
−1
2
atm of ethylene pressure, a CF -substituted Ti(IV) catalyst displays good activity (680 g mmol
h
at 75 °C; M = 3.4 × 10
w
3
3
−
1
kg mol , M /M = 2.6), while the difluoro-substituted analogue produces high molecular-weight polymers (M = 1.34 × 10 kg
w
n
w
−
1
mol at 50 °C; M /M = 2.4). For the latter derivative, a weak C−H···F−C interaction between a CH (benzyl) proton and the
w
n
2
1
19
fluorine atom ortho to the Ti−C(σ-aryl) bond is detected by [ H, F]-HMQC experiments. These results open up the
possibility that such stabilizing interactions could potentially occur for the catalytic species to suppress β-H elimination and
hence chain termination processes.
1
7,18
19,20
INTRODUCTION
into the M−C(σ-aryl) bond of these
and related
Zr
■
derivatives to generate olefin-assimilated (or “monomer-
inserted”) active sites have been reported. Inspired by this
unusual insertion reaction, Coates et al. designed pyridyl-
In the field of post-metallocene (non-cyclopentadienyl)
catalyst design for olefin polymerization, notable advances
have been achieved in the development of σ-aryl and related
carbanion-based chelating ligands. We have previously
developed group 4 precatalysts supported by fluorinated
pyridine-2-phenolate-6-(σ-aryl) [O,N,C] ligands, as well as
σ-aryl)-2-phenolate-6-pyridine [O,C,N] relatives, that dis-
1
21
22
2
amido and phenolate-amine ligands bearing a pendant
23
vinyl moiety, which undergo metalation with group 4 alkyl
3
3
,4
species to afford C(sp )-chelating catalysts that mediate the
living isoselective polymerization of α-olefins.
Anionic C(sp )-based ligands (and C(sp ) congeners) are
predominantly σ-donating and can confer greater electro-
5
(
2
3
play spectroscopically (NMR) and structurally (including
neutron diffraction) discernible intramolecular C−H···F−C
interactions. These complexes can be considered as synthetic
models of “weak attractive ligand−polymer interactions”
2
4,25
philicity at metal centers compared with strong π-donors.
For olefin polymerization, the chelate ring size can stabilize or
enhance the reactivity of the catalytic center as well as promote
or disfavor olefin assimilation processes. We recently reported
a family of pyridine-2-phenolate-6-arylmethine [O,N,CH-
Ar)]-Ti catalysts featuring a four-membered N,C-donor
chelate ring, which produce polyethylene (in conjunction
with [Ph C][B(C F ) ]) with excellent efficiencies and narrow
molecular weight distributions (M /M down to 2.2). The
present study concerns the expansion of the [O,N] metallacyle
in [O,N,C] catalysts to a seven-membered chelate ring, and its
effects upon polymerization behavior. Such high-membered
chelates, if sufficiently adjustable, can nonetheless afford small
6
,7
(
WALPI), a new strategy in polymerization reactions that
utilizes secondary interactions and offers the capability to
manipulate the reactivity of the polymer chain during catalysis,
and reports have appeared that point to the applicability and
(
8
,9
10,11
generality of this concept. Our initial studies
revealed
3
6 5 4
that Ti-[O,N,C] catalysts can produce polyolefins with high
M /M values suggestive of multisite behavior, which was
14
w
n
w
n
subsequently ascribed to facile olefin insertion into the Ti−
C(σ-aryl) bond to afford supplementary “olefin-assimilated”
1
2
active species, and efforts have been undertaken to modify
1
3,14
and ameliorate polymerization characteristics.
The corresponding 2-arylamido [N,N,C]-Hf catalysts were
developed by Symyx and Dow through the use of high-
Received: May 8, 2019
15,16
throughput screening,
and the analogous olefin insertion
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00307
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Organometallics
Article
Scheme 1. Synthesis of Complexes 1−13
a
b
Mixture of isomers obtained for Ti. Followed by heating (60 °C) in toluene; unstable product for Ti.
Scheme 2
bite angles (which would facilitate the approach and
coordination of monomers) as well as support or favor
different coordination geometries. The additional methylene
benzaldehyde with o-lithiated pyridines to give a −CH(OH)−
group, which then undergoes acetylation and reduction, and
the final step involves deprotection of the 2-methoxy
CH
2
n
moiety in the [O N] chelate would enhance flexibility, thus
facilitating the formation of optimal structures for each
insertion and propagation step and minimizing activation
energies. The importance of introducing greater degrees of
ligand flexibility for improved catalyst performance has been
substituent. Treatment of the H L ligand with M(CH Ph)
2
2
4
(M = Ti, Zr, Hf) in n-pentane/diethyl ether at −78 °C,
followed by stirring at 20 °C for 18 h, resulted in tridentate
coordination of the ligand and the formation of the
CH
2
[O N,C]M(CH Ph) complexes 1−9, 12, and 13 (Scheme
2
2
2
6−29
highlighted previously.
Hence, the synthesis of a family of
1
), which were obtained as dark maroon (Ti), yellow (Zr), and
CH
2
group 4 precatalysts containing the [O N,C] ligand and
bearing various σ-aryl substituents is described. The crystal
structure of a Zr(IV) bis(benzyl) complex exhibits an unusual
pale yellow (Hf) solids in moderate to good isolated yields
(
57−82%, except 30% for 6 due to high solubility).
1
19
H and F NMR analyses of the solid product isolated from
4
(
“quasi-meridional”) coordination geometry for the tridentate
the reaction of H L with Ti(CH Ph) signified the formation,
2
2
4
ligand, plus atypical conformation of the benzyl groups. The
in an approximate 1:1 ratio, of two isomers that proved
inseparable (Scheme 2). These results (plus the successful
CH
2
flexibility of the [O N] moiety is signified by the observation
of a fluxional process (using variable-temperature NMR
spectroscopy) for Ti(IV) derivatives. Investigations into their
capabilities as catalysts for olefin polymerization in conjunction
with different cocatalysts, and evaluation of substituent effects,
are presented.
synthesis of 4 and 5) suggest that, for ligands with two
2−4
potential ortho-cyclometalation sites such as H L , the sole
2
2
R substituent must be sufficiently bulky (e.g., Me, CF ) to
3
ensure that the cyclometalation step remains regioselective.
8
Treatment of H L with M(CH Ph) (M = Ti, Zr, Hf) in n-
2
2
4
pentane/diethyl ether resulted in [O,N]-metalation only and
RESULTS AND DISCUSSION
8
■
the generation of L (H)M(CH
2
Ph)
indicated by H NMR spectroscopy (a singlet resonance
with integration of 6, assignable to CH Ph, was observed; see
3
intermediates, as
Synthesis. The H L1 ligands were prepared by multistep
−9
1
2
procedures starting from readily available starting materials
such as 3,5-di-tert-butyl-2-methoxybenzaldehyde (see Support-
ing Information and Schemes S1−S3 for details). For example,
the methylene moiety is formed by treatment of the
2
experimental details). Conversion into the targeted Zr (10)
and Hf (11) σ-naphthyl complexes were achieved by heating
the tris(benzyl) derivatives at 60 °C for ca. 6 h in toluene, but
B
DOI: 10.1021/acs.organomet.9b00307
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Article
Figure 1. Expanded [ H, 19F]-HMQC spectrum of 9 (400 MHz, d -toluene) at 233 K.
1
8
1
the Ti analogue proved to be unstable (upon H NMR
analysis) and no cyclometalation was apparent at temperatures
below 60 °C.
Interestingly, for the zirconium complexes (2, 8, and 10),
2
2
one of the benzyl groups exhibits strong η -coordination ( J
H,H
1
= 8.4−8.8 Hz and J = 136.3−137.8 Hz) while the other
C,H
1
2
1
Spectroscopic and Structural Characterization. All
complexes have been characterized and assigned using H and
resembles η -coordination ( J = 10.4−10.8 Hz and J
=
H,H
C,H
1
118.7−126.3 Hz). These results are unexpected, since the
1
3
1
1
1
13
2
C NMR spectroscopy (including [ H, H]-COSY, [ H, C]-
propensity for (stronger) η -coordination is normally allied
HSQC, and ROESY experiments), plus 19F and [ H, F]-
1
19
with the electrophilicity of the metal (Hf < Zr < Ti), and may
be rationalized here by considering the unusual molecular
structure of 8 with distorted “quasi-meridional” coordination
1
HMQC experiments for 5−9. As illustrative examples, the H
NMR spectra of 1 and 2 have been fully assigned (Figures S1
and S2). For all complexes, the CH (benzyl) and
CH (chelate) protons (denoted H and H respectively) are
CH
2
by the [O N,C] ligand (see below).
2
a
b
1
In general, the H NMR resonances for the benzyl and
2
diastereotopic and appear as six discrete doublets in the alkyl
CH (chelate) protons of the titanium complexes are slightly
2
1
1
region, and can be assigned using [ H, H]-COSY and ROESY
experiments (Figures S3−S5 respectively for 2). The presence
of two singlets in the 13C and DEPT-135 spectra for the
broad, and this is especially conspicuous for 9, which contains
4
a fluorine atom ortho to the Ti−C(σ-aryl) bond (N.B. this R
substituent is adjacent to the benzyl groups and may hinder
their rotation). Variable-temperature H NMR experiments
1
CH (benzyl) groups (Figure S6 for 2), and the observation
2
that the o-, m-, and p-Ph protons of each benzyl unit are
resolved into separate resonances, are suggestive of markedly
different chemical environments for the two benzyl moieties.
These results confirm that, unlike [O,N,C] derivatives that
exhibit C symmetry (due to a virtual mirror plane through the
were performed for 9 in d -toluene (Figure S8), which signify a
8
fluxional process involving inversion of the seven-membered
chelate ring, that is presumably accompanied by conforma-
tional changes for the benzyl moieties. At 363 K, a multiplet
(4.28 ppm) and singlet (2.91 ppm) are observed for the
2v
a
b
meridional ligand), the introduction of a methylene unit into
CH (benzyl) and CH (chelate) protons (H and H ),
2
2
the [O,N] chelate has afforded complexes with C symmetry,
respectively. Lowering the temperature leads to decoalescence
1
a
thus hinting at a coordination mode that deviates substantially
(T ≈ 253 K), to give sharp peaks for the methylene (H and
c
1
b
from meridional (see crystal structure). The H NMR
H ) and o-, m-, and p-benzyl protons. The possibility of weak
4
spectrum of the solid obtained from the reaction of H2L
intramolecular C−H···F−C interactions in 9 (with significance
4
14
with Ti(CH Ph)4 reveals the formation of two isomeric
in catalysis) between the F atom at R (F ) and the
2
1
19
products in a 1:1 ratio (Figure S7).
CH (benzyl) protons was investigated using [ H, F]-HMQC
experiments
2
30
2
5,11
According to established criteria, η -coordination of benzyl
at ambient temperature (293 K), but because
2
1
groups are indicated by reduced J (<10 Hz) and high J
of the fluxionality, no interaction was observed. Importantly,
H,H
C,H
η -
4
,10
1
1
19
(>125 Hz) values. Compared with [O,N,C] congeners,
the [ H, F]-HMQC experiment was also performed at −40
a
benzyl coordination is similarly observed for the hafnium
°C (233 K), at which the four CH (benzyl) resonances (H )
2
2
complexes, but the η -coordination for the titanium complexes
have decoalesced, to reveal a weak cross-peak between the
proximal F atom and one of the four CH (benzyl) protons
1
14
is relatively weak ( J
= 125.8−129.8 Hz; Table S1).
C,H
2
C
DOI: 10.1021/acs.organomet.9b00307
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Figure 1; the expected cross-peak between F and H is also
Article
14
15
(
The molecular structure of 8 was determined by X-ray
crystallography (Figure 2), after orange crystals were obtained
by diffusion of n-pentane into a toluene solution at 20 °C. The
structure is C -symmetric and the Zr center displays an almost
ideal square pyramidal geometry (structural parameter τ =
0.07), with the base occupied by the [O N,C] ligand plus
one C(benzyl) atom (C27) and the remaining C(benzyl) atom
(C34) at the apical position. This is in contrast to the distorted
square pyramidal geometry adopted by previously reported
3
1
shown). The occurrence of weak intramolecular C−H···F−C
interactions in 9 may carry implications for its polymerization
behavior (see below).
1
‡
The activation barrier (ΔG ) for the ring inversion was
calculated to be 62.6 kJ mol− (Scheme 3), which is
1
33
CH
2
Scheme 3. Inversion of [OCH2N] Chelate in 9
4
,10
[
O,N,C]-Zr congeners (τ = 0.27−0.38).
Saliently, the
2
CH
coordination geometry of the tridentate [O N,C] moiety is
observed to deviate significantly from a conventional meri-
dional arrangement (e.g., the O1 atom is displaced from the
N1−Zr1−C1 plane by 1.24 Å). This is manifested in a
surprisingly small O1−Zr1−C1 bite angle (132.62(6)°) and
enlarged O1−Zr1−N1 (82.13(5)°) and C14−O1−Zr1
(
156.2(1)°) angles, when compared with a chlorine-
substituted [O,N,C]-Zr analogue (148.5(1), 77.8(1) and
10
1
44.2(1)° respectively).
Consistent with the solution structure indicated by NMR
spectroscopy, one of the benzyl groups in 8 engages in η -
coordination (Zr1···C35 = 2.634(2)Å, Zr1−C34−C35 =
86.3(1) ) while the other exhibits η -coordination. Attention
is drawn to the conformation of the benzyl moieties, which
typically extend outward and away from the tridentate ligand in
related group 4 postmetallocene complexes (as is the case for
the Ti derivatives herein, according to ROESY experi-
2
comparable to that reported for a Ti(bisphenolate)Br
2
‡
o
1
derivative bearing a nine-membered ring (ΔG = 60.4 kJ
−
1
32
mol ). Such a process is analogous to the enantiomerization
of chiral complexes, and is common in organic heterocyclic
CH
2
compounds. Hence the seven-membered [O N] ring in the
Ti derivatives is conformationally nonrigid at ambient
temperatures, and the likelihood of the ring inversion process
is dependent on the molecular structure, including the steric
4
,14,22,34
ments).
Here, the severely distorted “quasi-meridional”
2
coordination evident in 8 causes the η -coordinated benzyl
group (C34−C40) to “wrap” around the σ-aryl unit (without
π-stacking) so that the Zr−benzyl linkages and the alignment
of the benzyl moieties are virtually perpendicular (torsional
angle between C27−C28 and C34−C35 = 95.8°). Con-
sequently, the shortest distance between the CH (benzyl) and
CH (chelate) protons (H12A···H34B) is only 2.36 Å, which
indeed gives rise to a cross-peak in the ROESY spectrum (as
4
impact of the R and phenolate substituents upon the benzyl
1
units. Interestingly, the benzyl and CH (chelate) H NMR
2
resonances for the Zr and Hf complexes 12 and 13, bearing a
methyl substituent ortho to the O atom, are slightly broader
than for the tert-butyl analogues 2 and 3, respectively,
consistent with the likelihood that a smaller phenolate
substituent would lower the barrier for ring inversion.
2
2
Figure 2. Perspective view of 8 (30% probability ellipsoids). Selected bond lengths (Å) and angles (deg): Zr1−O1 1.986(1), Zr1−N1 2.360(2),
Zr1−C1 2.310(2), Zr1−C27 2.301(2), Zr1···C28 3.106(2), Zr1−C34 2.287(2), Zr1···C35 2.634(2); O1−Zr1−C1 132.62(6), O1−Zr1−N1
82.13(5), C1−Zr1−N1 70.44(6), C14−O1−Zr1 156.2(1), C28−C27−Zr1 108.4(1), C35−C34−Zr1 86.3(1), C11−C12−C13 112.8(2).
D
DOI: 10.1021/acs.organomet.9b00307
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Organometallics
Article
shown in Figure S4 for 2). The C11−C12−C13 angle
intramolecular C−H···F−C interaction (Figure 1). Further-
more, we note that these C−H···F−C interactions have been
(
112.8(2)°) at the CH (chelate) unit is smaller than that
2
(
120.6(1)°) at the C(methine) moiety in the [O,N,CH(Ph)]-
reported for structurally related 2-phenolate-6-pyridine-(σ-
1
4
5,13
Zr analogue.
aryl) [O,C,N] analogues.
These observations open up the
Olefin Polymerization Studies. The fluorinated com-
plexes 6, 7, and 9 were selected as representative examples for
evaluation as ethylene polymerization catalysts at different
temperatures, in conjunction with dried methylaluminoxane
possibility that such stabilizing interactions between the F
4
atom at R (ortho to Ti−C(σ-aryl) bond) and the growing
polymer chain could potentially suppress β-H elimination
(leading to chain termination) to afford polymers with higher
6
,7
(
MAO) and trityl borate at 7 atm of ethylene pressure. Using
M . Such WALPI-type C−H(β)···F−C interactions have
w
dried MAO (250 equiv) as cocatalyst (Table 1), the
been invoked by Delferro and Marks, based on the appearance
1
19
1
19
of H··· F dipolar interactions detected by [ H, F]-HOESY
Table 1. Ethylene Polymerization Results for 6, 7, and 9 at
experiments, to account for the dramatically higher M (plus
w
a
Different Temperatures in Conjunction with Dried MAO
lower degrees of branching) of polyolefins produced by
fluorinated terphenylphenoxyimine-Ni(II) mono- and bimet-
temp
(°C)
yield
(mg)
M
8a,b,35
w
b
(kg mol−1)c
c
allic catalysts.
catalyst
activity
M /Mn
w
i
Using [Ph C][B(C F ) ]/ Bu Al as cocatalyst and 7 atm
3
6
5 4
3
6
25
50
57
68
59
43
7
14
23
22
23
27
500 [420]
570 [680]
680 [1110]
590 [1280]
430 [1260]
70 [60]
140 [170]
230 [375]
220 [180]
230 [270]
270 [440]
660
712
341
191
120
980
788
375
1145
1338
838
2.6
2.8
2.6
8.5
13.2
2.5
5.5
51.9
5.3
2.4
3.5
pressure of ethylene feed (continuously supplied; Table S2),
complexes 6, 7 and 9 exhibit lower catalytic activities than with
dried MAO. For comparison, the observed activity for 6 (0.95
5
7
0
5
1
1
00
25
25
−
1
−1
−1
kg mmol
h
(mol/L of C H ) at 75 °C) is lower than
2
4
12
−1 −1
those for the Ti-[O,N,C(σ-naphthyl)] (2.7 kg mmol
h
7
9
−
1
17b
(
(
mol/L of C H ) at 50 °C) and Hf-[N,N,C(σ-naphthyl)]
2 4
1
5
7
0
5
8.9 kg mmol h−1 [C H ] at 40 °C) relatives. In general,
−
−1
2
4
the polymers produced show high M /M values or are
multimodal, except by 6 at 25 °C (M = 6.3 × 10 kg mol ,
M /M = 2.6). Complexes 6 and 7 were employed in ethylene-
w
n
25
2
−1
w
5
7
0
5
w
n
propylene copolymerization studies (1 atm of propylene gas
was added, then total pressure was increased to 7 atm and
a
Conditions: 0.3 μmol of catalyst, 5 mL of toluene, dried MAO/
catalyst (250/1 equiv), 7 atm of ethylene pressure (maintained by
b
maintained by continuous ethylene supply) in conjunction
continuous supply), 20 min reaction time. Activity given in g of
i
polymer (mmol of catalyst)−
1
−1
with [Ph C][B(C F ) ]/ Bu Al at 25 °C. The activities for 6
h
[parentheses: activity with respect
3
6
5 4
3
−
1
−1
and 7 are low (70 and 10 g mmol
h
respectively) and
to ethylene concentration (1.2, 0.84, 0.61, 0.46, and 0.34 mol/L at 25,
−
1
−1
5
0, 75, 100, and 125 °C respectively) given in g mmol
h
(mol/L
comparable to the homopolymerizations, and preliminary IR
analysis of the resultant polymers indicate low propylene
content (≤2%). The nature of the active sites in these
−
1
c
of ethylene) ] (±10%). Determined by GPC at 160 °C in 1,2,4-
trichlorobenzene versus polystyrene standards.
3
,14,23
polymerizations would be of interest,
and extensive
trifluoromethyl derivative 6 generally shows high catalytic
efficiencies and is more active than 7 and 9. Increasing the
temperature resulted in enhanced efficiencies, and this is
particularly apparent when decreases in ethylene concen-
trations are considered (e.g., from 25 to 75 °C, activity of 6
attempts have been made to characterize the products upon
trityl activation of the bis(benzyl) complexes by NMR
spectroscopy. However, the assumed Ti benzyl cationic species
proved highly unstable and could not be observed
spectroscopically. Reactions of Zr derivatives such as 8 with
−
1
−1
increased from 0.42 to 1.11 kg mmol
h
(mol/L of
[Ph
and presence of d
complicated mixtures were similarly produced, the generation
of significant amounts of Ph CCH Ph were nonetheless
observed (CH singlet appears at 4.04 ppm in C Br [and
4.03 ppm in C Br containing 3 drops of d -THF]),
C][B(C
3
F
6
)
5
4
] in C
6 5
D Br (at −20 and 20 °C, in absence
−
1
C H ) ), while the MW distributions at 25, 50, and 75 °C
8
-THF) were also performed. Although
2
4
remained reasonably narrow (M /M = 2.6−2.8, with slight
tailing at low MW; Figure S9). The highest activity for 6 (0.68
kg mmol
.6) can be compared to those determined for the Ti-
O,N,C(σ-phenyl)] analogue (1.0 kg mmol
w
n
3
2
−
1
−1
2
−1
h
at 75 °C; M = 3.4 × 10 kg mol , M /M =
2
D
6 5
8
w
w
n
2
[
6
D
5
4
−1
−1
h
under
suggestive of benzyl abstraction and benzyl cation formation
identical conditions) and the related Zr-[N(amido),N,N-
(rather than trityl attack at the C(σ-aryl) atom) like that
2
0
−1 −1
−1
12
(
pyrrolyl)] catalyst (0.4 kg mmol
h
atm at 25 °C,
reported for the Zr-[O,N,C(σ-naphthyl)] analogue.
using dried MAO (200 equiv) and 1 atm of C H pressure).
The polymerization behavior of complexes using MAO
solutions (500 equiv) as cocatalyst under 1 atm pressure of
ethylene have also been studied (Table S3). The catalytic
efficiencies at 22 °C are broadly moderate (Ti) to poor (Zr
and Hf), and the polymers produced display generally elevated
M but also high M /M values. The observed activity for 6
2
4
However, at elevated temperatures (≥100 °C for 6; 50 °C for
7
; 75 °C for 9), lower M and higher M /M values were
w
w
n
observed, indicative of multisite behavior (presumably caused
by catalyst degradation).
It is notable that the polymers produced by 9 at different
temperatures gave relatively high Mw values, and the
polymerization at 50 °C gave a polymer with very high MW
w
w
n
−
1
−1
(0.12 kg mmol h ) is lower than for the [O,N,C(σ-
naphthyl)] and [O,N,CH(Ph)] relatives under identical
conditions (0.25 and 0.46 kg mmol
12
14
3
−1
−1 −1
and narrow MW distribution (M = 1.34 × 10 kg mol , M /
n
h
respectively). For
w
w
M = 2.4), implying behavior that approaches single-site. The
both MAO and trityl borate cocatalysts, the observation of
CH
2
incorporation of fluorine atoms adjacent to the catalytic center
lower efficiencies for the Ti-[O N,C] catalysts compared
with [O,N,C] analogues, and in some cases high M /M values
8
,11
can be beneficial to polymerization characteristics.
Sig-
w
n
1
19
nificantly, a cross-peak is evident in the [ H, F]-HMQC
spectrum of 9 at 233 K, signifying the existence of a weak
(especially at elevated temperatures), may be related to the
distorted “quasi-meridional” coordination geometry, and is
E
DOI: 10.1021/acs.organomet.9b00307
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
nonetheless indicative of catalyst instability (as evident from
NMR reactions) and deactivation, as well as the generation of
multiple active sites (e.g., resulting from reduction of Ti or
modification/abstraction of ligand).
were performed on a Vario EL elemental analyzer (Elementar
Analysensysteme GmbH). For polymer analysis, gel permeation
chromatographs were obtained on a PL-GPC 220 instrument (at 1.0
mL/min versus polyethylene standards) at 160 °C in 1,2,4-
trichlorobenzene. Synthetic procedures and characterization for the
1
−9
H2L
ligands and complexes 1−5 and 10−13 are given in the
CONCLUSION
Supporting Information.
■
A series of tridentate [OCH2N,C(σ-aryl)] ligand frameworks
containing a methylene-linked phenolate-pyridine chelate has
been developed, and the corresponding group 4 bis(benzyl)
complexes with various σ-aryl substituents have been
synthesized and characterized by 1H, C and F NMR
13
19
spectroscopy. Unlike [O,N,C] relatives that are C -symmetric,
2v
all complexes in this work adopt C symmetry in solution. The
1
CH
2
flexibility of the [O N] chelate is clearly manifested in
solution and the solid state by variable-temperature H NMR
1
spectroscopy (indicating fluxionality for Ti derivatives) and X-
ray crystallography, respectively. The distorted “quasi-meri-
dional” coordination geometry in the X-ray crystal structure of
Synthesis of Complex 6. A solution of H
L5 (0.175 g, 0.40
2
mmol) in n-pentane (18 mL) and diethyl ether (8 mL) was added
dropwise at −78 °C to Ti(CH Ph) (0.17 g, 0.42 mmol) in n-pentane
8
leads to an unconventional molecular structure, as reflected
2
4
by the orientations of the benzyl moieties, which extend almost
perpendicularly away from the Zr center. Consequently, the
shortest distance between the CH (benzyl) and CH (chelate)
(
5 mL). The reaction mixture was stirred for 1 h at −78 °C and for 18
h at 20 °C. Filtration and concentration of the resultant mixture gave
2
2
a purple solid after 24 h at −25 °C, which was collected and dried
protons is only 2.36 Å, and their proximity is also evident in
the ROESY spectrum, thus indicating a close correlation
between the solution- and solid-state structures for Zr
derivatives.
The capabilities of the fluorinated complexes 6, 7, and 9 as
ethylene polymerization catalysts in conjunction with dried
MAO and trityl borate at 7 atm of ethylene pressure were
examined. The Ti catalysts exhibit good efficiencies with dried
MAO, and the highest activity was observed for 6.
1
under vacuum. Yield: 0.08 g, 30%. H NMR (400 MHz, C D ): δ 1.31
6
6
b
(
s, 9H, 5-t-Bu), 1.62 (s, 9H, 3-t-Bu), 2.99 (d, J = 12.4 Hz, 1H, CH ),
a
b
3.35 (d, J = 8.4 Hz, 1H, CH ), 3.41 (d, J = 12.4 Hz, 1H, CH ), 3.53
a
a
(d, J = 8.4 Hz, 1H, CH ), 3.69 (d, J = 8.8 Hz, 1H, CH ), 4.21 (d, J =
a
8
8.8 Hz, 1H, CH ), 6.16 (d, J = 7.2 Hz, 1H, H ), 6.35−6.44 (br, 2H, o-
1
0
9
Ph), 6.51 (d, J = 7.6 Hz, 1H, H ), 6.58−6.62 (m, 2H, p-Ph and H ),
6
2
.64−6.73 (br, 2H, m-Ph), 6.83−6.93 (br, 1H, p-Ph), 7.03−7.05 (m,
6 17
H, H and H ), 7.11−7.12 (m, 2H, m-Ph), 7.35−7.39 (m, 3H, o-Ph
1
6
4
14 13
and H ), 7.42 (d, J = 2.4 Hz, 1H, H ), 8.64 (s, 1H, H ). C NMR
101 MHz, C D ): δ 30.58 (3-CMe ), 31.76 (5-CMe ), 34.56 and
(
6
6
3
3
Significantly, high molecular-weight polymers (up to M =
b
1
a
1
w
35.60 (CMe ), 41.01 (C ), 94.43 ( J = 128.8 Hz, C ), 101.10 ( J
3 C,H C,H
3
−1
1
9
.34 × 10 kg mol at 50 °C; M /M = 2.4) are produced by
a
10
17
8
w
n
= 128.3 Hz, C ), 115.77 (C ), 122.88 (C ), 123.00 (C ), 123.30
4
6
16
, for which a weak intramolecular C−H···F−C interaction is
(two sets of p-Ph and C ), 124.18 (C ), 124.95 (q, J = 3.5 Hz, C ),
C,F
1
19
detected in the [ H, F]-HMQC spectrum at 233 K, between a
127.36 (o-Ph), 128.09 (m-Ph), 128.59 (o-Ph), 129.27 (m-Ph), 130.54
1
8
14
9
CH (benzyl) proton and the fluorine atom ortho to the Ti−
(q, JC,F = 30.7 Hz, C ), 133.81 (d, JC,F = 3.0 Hz, C ), 140.90 (C );
2
4
1
−
° carbons: 124.32, 127.04, 128.51, 136.72, 144.05, 145.55, 146.72,
C(σ-aryl) bond. These observations tentatively point to the
possibility that such interactions could occur for the catalytic
species to suppress or disfavor β-H elimination (and hence
19
49.10, 158.73, 162.62, 193.67. F NMR (376 MHz, C D ): δ
6 6
61.74. Anal. Calcd for C H F NOTi (669.64): C, 73.54; H, 6.32;
41 42 3
N, 2.09. Found: C, 73.25; H, 6.17; N, 1.92.
chain transfer), and give polymers with higher M . The range
w
of beneficial attributes offered by flexible or enlarged chelate
rings remains unclear, as do the consequences of unusual or
highly distorted ligand coordination geometries upon catalyst
reactivity and polymerization behavior, and further studies are
justified.
EXPERIMENTAL SECTION
■
General Methods. All reactions were performed using standard
Schlenk techniques under an argon atmosphere or in a Braun drybox
under a nitrogen atmosphere. All solvents were appropriately dried
and distilled then degassed prior to use. Methylaluminoxane (MAO,
6
10 wt % solution in toluene; purchased from Aldrich) was pretreated
Synthesis of Complex 7. A solution of H L (0.19 g, 0.48 mmol)
2
by removing all volatiles under vacuum for 12 h to minimize residual
amounts of “free” trimethylaluminum, and stored as dried MAO in
powder form (containing ca. 4% of trimethylaluminum; dried MAO is
stable for longer periods and provides more reproducible
experimental results compared with MAO solutions). Alternatively,
dried toluene was added to give a 10 wt % solution that was freshly
in n-pentane (10 mL) and diethyl ether (4 mL) was added dropwise
at −78 °C to Ti(CH Ph) (0.22 g, 0.53 mmol) in n-pentane (5 mL).
2
4
The reaction mixture was stirred for 1 h at −78 °C and for 18 h at 20
°C. Filtration and concentration of the resultant mixture gave a purple
solid, which was collected and dried under vacuum. Yield: 0.19 g,
1
64%. H NMR (400 MHz, C D ): δ 1.31 (s, 9H, 5-t-Bu), 1.64 (s, 9H,
6
6
1
13
b
prepared before use. H, C (referenced to residual solvent peaks),
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
3-t-Bu), 2.97 (d, J = 13.4 Hz, 1H, CH ), 3.33 (d, J = 8.8 Hz, 1H,
a
b
a
CH ), 3.44 (d, J = 13.4 Hz, 1H, CH ), 3.55 (d, J = 8.8 Hz, 1H, CH ),
a
a
3.62 (d, J = 9.2 Hz, 1H, CH ), 4.17 (d, J = 9.2 Hz, 1H, CH ), 6.09 (d,
J = 7.6 Hz, 1H, H ), 6.42−6.47 (m, 3H, o-Ph and H ), 6.55−6.61
(m, 2H, p-Ph and H ), 6.71−6.77 (m, 3H, m-Ph and H ), 6.87−6.94
6 17
8
10
1
1
13
1
1
19
9
16
of DEPT-135 and 2-D H− H, C− H, and NOE (plus [ H, F]-
HMQC for 5−9) correlation NMR experiments. Elemental analyses
(br, 1H, p-Ph), 7.00−7.03 (m, 2H, H and H ), 7.15−7.23 (m, 2H,
F
DOI: 10.1021/acs.organomet.9b00307
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
4
b
a
m-Ph), 7.42 (d, J = 1.6 Hz, 1H, H ), 7.45−7.52 (br, 2H, o-Ph), 7.96
3.07 (br, 1H, CH ), 4.00−4.30 (br, 1H, CH ), 4.36−4.65 (br, 2H,
a a 10
1
4
13
(
dd, J = 8.0 Hz, 2.0 Hz, 1H, H ). C NMR (101 MHz, C D ): δ
CH ), 4.65−4.99 (br, 1H, CH ), 6.27 (d, J = 7.6 Hz, 1H, H ), 6.33
8 9
6
6
3
(
1
0.64 (3-CMe ), 31.78 (5-CMe ), 34.55 and 35.62 (CMe ), 40.96
(dd, J = 7.4 Hz, 1.0 Hz, 1H, H ), 6.55 (t, J = 7.8 Hz, 1H, H ), 6.60
3
3
3
b
1
a
1
a
17
C ), 93.78 ( J = 129.8 Hz, C ), 101.19 ( J = 129.8 Hz, C ),
(dd, J = 10.0 Hz, 2.0 Hz, 1H, H ), 6.80 (dd, J = 9.8 Hz, 1.8 Hz, 1H,
H ), 7.08 (d, J = 2.4 Hz, 1H, H ), 7.58 (d, J = 2.4 Hz, 1H, H ); o-,
m-, and p-Ph resonances are broad and could not be assigned (see
C,H
C,H
1
0
16
8
1
5
6
4
15.00 (C ), 115.14 (d, J = 23.2 Hz, C ), 121.67 (C ), 123.04 (p-
Ph), 123.12 (p-Ph), 123.18 (C ), 123.56 (d, J = 16.1 Hz, C ),
24.19 (C ), 124.68 (d, J = 7.0 Hz, C ), 127.11 (o-Ph), 128.01 (m-
Ph), 128.59 (m-Ph), 129.28 (o-Ph), 140.79 (C ), 163.77 (d, J
C,F
4
14
C,F
6
17
1
13
C,F
Figure S8). C NMR (101 MHz, C D ): δ 31.41 (3-CMe ), 31.77 (5-
6
6
3
9
=
b
C,F
CMe ), 34.53 and 35.74 (CMe ), 42.13 (C ), 104.22 (dd, J = 38.2
1
5
3 3 C,F
2
55.5 Hz, C ); 4° carbons: 128.68, 128.75, 136.59, 142.01, 143.90,
15
17
10
and 23.1 Hz, C ), 106.0 (d, J = 17.1 Hz, C ), 115.78 (C ),
1
9
C,F
1
58.33, 159.32, 162.71, 196.92. F NMR (376 MHz, C D ): δ
8
4
6
9
6
6
1
1
1
23.47 (C ), 124.09 (C ), 124.55 (C ), 140.54 (C ); 4° carbons:
29.56, 138.11, 143.51, 145.78, 145.94, 160.21, 160.28, 160.58,
62.55, 167.80; some carbon resonances are broad and could not be
assigned. F NMR (376 MHz, C D ): δ −85.06 (F ), −111.57
F ). Anal. Calcd for C H F NOTi (637.62): C, 75.35; H, 6.48; N,
−
110.84. Anal. Calcd for C H FNOTi (619.63): C, 77.53; H, 6.83;
N, 2.26. Found: C, 77.18; H, 6.47; N, 2.01.
40 42
1
9
14
6
6
1
6
(
40 41 2
2.20. Found: C, 75.20; H, 6.21; N, 2.07.
Polymerization Procedures. Schlenk-line ethylene polymer-
ization tests were carried out under atmospheric pressure in toluene
in a 100 mL glass reactor containing a magnetic stir bar. The stirred
toluene solution containing the catalyst was thermostated to the
required temperature, and the ethylene gas feed (1 atm) was then
started and maintained for the duration of the experiment. After 15
min, polymerization was initiated by adding a toluene solution of the
MAO cocatalyst with stirring. High-pressure 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
6
Synthesis of Complex 8. A solution of H L (0.10 g, 0.26 mmol)
2
in n-pentane (10 mL) and diethyl ether (3 mL) was added dropwise
at −78 °C to Zr(CH Ph) (0.12 g, 0.27 mmol) in n-pentane (5 mL).
2
4
The reaction mixture was stirred for 1 h at −78 °C and for 18 h at 20
°
C. Filtration and concentration of the resultant mixture gave a
orange-yellow solid, which was collected and dried under vacuum.
Yield: 0.12 g, 75%. H NMR (400 MHz, C D ): δ 0.60 (d, J = 10.6
Hz, 1H, CH ), 1.35 (s, 9H, 5-t-Bu), 1.47 (s, 9H, 3-t-Bu), 2.01 (d, J =
1
6
6
consecutively adding toluene solutions of the cocatalyst (MAO or
a
i
[
Ph C][B(C F ) ]/ Bu Al) and catalyst into the reactor (total volume
3
6
5
4
3
a
a
b
1
8
2
6
6
7
0.6 Hz, 1H, CH ), 3.28−3.31 (m, 2H, CH and CH ), 3.45 (d, J =
a b
5.0 mL) with stirring. After the prescribed reaction time, methanol
.8 Hz, 1H, CH ), 4.04 (d, J = 13.6 Hz, 1H, CH ), 6.07 (d, J = 7.6 Hz,
(40 mL) was added to terminate the polymerization, the ethylene gas
8
H, o-Ph), 6.38 (d, J = 7.2 Hz, 1H, H ), 6.43 (t, J = 7.2 Hz, 1H, p-Ph),
feed was stopped, and the resultant mixture was allowed to stir for 1 h.
The polymer was collected by filtration, washed with HCl-acidified
methanol (20 mL), and dried under vacuum at 80 °C for 12 h.
Ethylene/propylene polymerization tests were performed by a similar
procedure to that for ethylene polymerization, except propylene gas
(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
under vacuum. The resultant polymer was dried under vacuum at 130
°C for 10 h. Errors for catalytic activities are estimated to be ±10%,
based on previous polymerization tests using the same experimental
procedure.
1
0
9
16
.59−6.63 (m, 3H, m-Ph and H ), 6.67−6.71 (m, 2H, H and H ),
6
17
.99 (t, J = 7.2 Hz, 1H, p-Ph), 7.08−7.17 (m, 4H, m-Ph, H and H ),
4
.22 (d, J = 7.6 Hz, 2H, o-Ph), 7.38 (d, J = 2.0 Hz, 1H, H ), 8.07 (dd,
1
4
13
J = 7.6 Hz, 1.8 Hz, 1H, H ). C NMR (101 MHz, C D ): δ 29.97
6
6
b
(
6
3-CMe ), 31.87 (5-CMe ), 34.49 and 35.38 (CMe ), 40.69 (C ),
3
3
3
1
a
1
a
6.73 ( J = 136.3 Hz, C ), 67.61 ( J = 126.3 Hz, C ), 114.08 (d,
JC,F = 23.1 Hz, C ), 116.16 (C ), 121.59 (C ), 122.31 (p-Ph),
23.35 (C ), 123.72 (C ), 124.78 (p-Ph), 125.38 (d, J = 7.0 Hz,
C ), 125.61 (d, J = 14.1 Hz, C ), 126.79 (o-Ph), 127.24 (o-Ph),
C,H C,H
1
6
10
8
4
6
1
C,F
1
7
14
C,F
9
1
28.98 (m-Ph), 133.05 (m-Ph), 140.66 (C ), 163.83 (d, J = 255.5
C,F
15
Hz, C ); 4° carbons: 136.53, 140.47, 142.04, 142.87, 143.12, 159.07,
1
9
1
59.90, 161.24, 188.52. F NMR (376 MHz, C D ): δ −112.15.
6
6
Anal. Calcd for C H FNOZr (662.99): C, 72.46; H, 6.39; N, 2.11.
40
42
Found: C, 72.30; H, 6.01; N, 2.03.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00307.
Experimental details and characterization data for
1−9
H2L ligands and complexes 1−5 and 10−13; selected
NMR spectra and data; polymerization results and GPC
traces (PDF)
7
Synthesis of Complex 9. A solution of H L (0.18 g, 0.44 mmol)
Accession Codes
2
in n-pentane (15 mL) and diethyl ether (1 mL) was added dropwise
at −78 °C to Ti(CH Ph) (0.20 g, 0.48 mmol) in n-pentane (10 mL).
CCDC 1913381 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
4
The reaction mixture was stirred for 1 h at −78 °C and for 18 h at 20
°
C. Filtration and concentration of the resultant mixture gave a purple
crystalline solid at −25 °C after 4 h, which was collected and dried
1
under vacuum. Yield: 0.16 g, 57%. H NMR (400 MHz, C D ): δ 1.32
6
6
b
(s, 9H, 5-t-Bu), 1.86 (s, 9H, 3-t-Bu), 2.45−2.75 (br, 1H, CH ), 2.75−
G
DOI: 10.1021/acs.organomet.9b00307
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of Thermoplastic Polyolefin Elastomers from Nickel-
Catalyzed Ethylene Polymerization. Macromolecules 2017, 50,
AUTHOR INFORMATION
■
Corresponding Author
E-mail: mcwchan@cityu.edu.hk.
6
(
074−6080.
*
10) Tam, K.-H.; Chan, M. C. W.; Kaneyoshi, H.; Makio, H.; Zhu,
ORCID
N. Indirect Substituent Effects upon the Olefin Polymerization
Reactivity of Titanium(IV) Chelating σ-Aryl Catalysts. Organo-
metallics 2009, 28, 5877−5882.
Michael C. W. Chan: 0000-0002-2997-8577
Notes
The authors declare no competing financial interest.
(11) Liu, C.-C.; So, L.-C.; Lo, J. C. Y.; Chan, M. C. W.; Kaneyoshi,
H.; Makio, H. Highly Fluorinated (σ-Aryl)-Chelating Titanium(IV)
Post-Metallocene: Characterization and Scalar [C−H···F−C]
Coupling. Organometallics 2012, 31, 5274−5281.
ACKNOWLEDGMENTS
■
(12) Lo, J. C. Y.; Chan, M. C. W.; Lo, P.-K.; Lau, K.-C.; Ochiai, T.;
The work described in this paper was wholly supported by the
Research Grants Council of the Hong Kong SAR, China
Makio, H. Olefin Polymerization Behavior of Titanium(IV) Pyridine-
-Phenolate-6-(σ-Aryl) Catalysts: Impact of “py-Adjacent” and
2
(
CityU 11300414). We are grateful to the reviewers for
Phenolate Substituents. Organometallics 2013, 32, 449−459.
(13) Lo, J. C. Y.; So, L.-C.; Chan, M. C. W. Synthesis and
spectroscopic characterization of group 4 post-metallocenes bearing
(σ-aryl)-2-phenolate-6-pyridyl and -isoquinolinyl auxiliaries. Dalton
Trans. 2015, 44, 15905−15913.
insightful comments and suggestions.
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