Ni(II) phenoxyiminato olefin polymerization catalysis: Striking coordinative modulation of hyperbranched polymer microstructure and stability by a proximate sulfonyl group

Article abstract of DOI:10.1021/cs500114b

The synthesis, structural characterization, and ethylene polymerization properties of two neutrally charged Ni(II) phenoxyiminato catalysts are compared and contrasted. Complex FI-SO₂-Ni features a -SO₂- group embedded in the ligand skeleton, whereas control FI-CH₂-Ni has the -SO₂- replaced by a -CH₂- functionality. In comparison with FI-CH₂-Ni, at 25 C, FI-SO₂-Ni is 18 times more active, produces polyethylene with 3.2 times greater M_W and 1.5 times branch content, and is significantly more thermally stable. The FI-SO ₂-Ni-derived polymer is a hyperbranched polyethylene (148 branches 1000 C^-1, M_W = 3500g mol^-1) versus that from FI-CH₂-Ni (98 branches 1000 C^-1, M_W = 1100g mol^-1). DFT calculations argue that the distinctive FI-SO ₂-Ni catalytic behavior versus that of FI-CH₂-Ni is associated with nonnegligible OSO···Ni interactions involving the activated catalyst.

Full text of DOI:10.1021/cs500114b

Letter
pubs.acs.org/acscatalysis
Ni(II) Phenoxyiminato Olefin Polymerization Catalysis: Striking
Coordinative Modulation of Hyperbranched Polymer Microstructure
and Stability by a Proximate Sulfonyl Group
Casey J. Stephenson,†,§ Jennifer P. McInnis,†,§ Changle Chen,† Michael P. Weberski, Jr.,†
Alessandro Motta,‡ Massimiliano Delferro,*,† and Tobin J. Marks*,†
†Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States
‡Dipartimento di Scienze Chimiche, Universita
̀
di Catania and INSTM, UdR Catania, 95125 Catania, Italy
S
* Supporting Information
ABSTRACT: The synthesis, structural characterization, and
ethylene polymerization properties of two neutrally charged
Ni(II) phenoxyiminato catalysts are compared and contrasted.
Complex FI-SO2-Ni features a −SO2− group embedded in the
ligand skeleton, whereas control FI-CH2-Ni has the −SO2−
replaced by a −CH2− functionality. In comparison with FI-
CH2-Ni, at 25 °C, FI-SO2-Ni is 18 times more active, produces
polyethylene with 3.2 times greater MW and 1.5 times branch
content, and is significantly more thermally stable. The FI-SO2-Ni-derived polymer is a hyperbranched polyethylene (148
branches 1000 C−1, MW = 3500g mol−1) versus that from FI-CH2-Ni (98 branches 1000 C−1, MW = 1100g mol−1). DFT
calculations argue that the distinctive FI-SO2-Ni catalytic behavior versus that of FI-CH2-Ni is associated with nonnegligible
OSO···Ni interactions involving the activated catalyst.
KEYWORDS: hemilabile ligand, sulfonyl group, nickel, hyperbranched polyethylene, DFT calculations
ate transition metal olefin polymerization catalysts1 have
Chart 1. (A) General Representation of Stabilization of
Coordinatively Unsaturated Reactive Intermediates in
Transition Metal Olefin Polymerization Catalysts and (B)
Ni(II)phenoxyiminato Catalyst with a Remote −SO2−
Group in the Ligand Framework
Lgarnered much attention since the discovery of cationic
Ni(II)- and Pd(II)-based α-diimine catalysts by Brookhart2 and
neutral Ni(II) phenoxyiminato catalysts by Grubbs.3 Under
ethylene homopolymerization conditions, these catalysts yield
polyethylenes that range from linear to highly branched, while
also enchaining polar comonomers, stimulating detailed
synthetic and mechanistic studies.3−5 Following olefin
insertion, 14-electron intermediates are formed that can
undergo either ethylene binding and continued chain
propagation or β-H elimination, which then promotes branch
introduction, chain transfer, or catalyst deactivation. In the
latter, β-H elimination leads to formation of a transient Ni−H
species, which reductively eliminate to yield metallic Ni and the
free ligand.1b,5 The stabilization of reactive Ni−H species might
thereby increase catalyst lifetimes and facilitate chain walking,
which in turn could have profound effects on the polymer
microstructure.
Here we report that a Ni(II)phenoxyiminato catalyst
containing a remote ligand −SO2− group exhibits greatly
enhanced olefin polymerization activity and thermal stability
and produces higher MW polyethylenes with remarkably high
levels of chain branching (Chart 1, B) versus a −CH2− control.
Hyperbranched polyethylenes are of interest as blending
additives in polymer extrusion and as lubricant shear-stable
viscosity additives. The performance of such additives should
be further enhanced by lowering the MW, increasing short chain
branch densities, or both.11
Hemilabile ligands have been successful in transition metal-
mediated catalysis in stabilizing coordinatively unsaturated
reactive intermediates via proximate, flexible, weakly coordinat-
ing ligands;6−8 however, such groups in transition metal-
mediated olefin polymerization catalysis have been only
minimally explored.9,10 To this end, we report that introducing
spatially proximate but electronically remote “hard” −SO2−
groups in the vicinity of “soft” d10 catalytic centers offers a new
means to modulate single-site polymerization processes (Chart
1).
Received: January 24, 2014
Revised: January 27, 2014
Published: January 31, 2014
© 2014 American Chemical Society
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The catalyst ligands are synthesized by condensing the
corresponding anilines with 1.2 equiv of 3-tert-butyl-2-hydroxyl
benzaldehyde using a catalytic amount of formic acid (Scheme
1). The reactions proceed in high yield and afford products of
aryl-H. This is consistent with the distorted square−planar
coordination geometry found in the solid state (Supporting
Information (SI) Figures S9 and S10). However, as evidenced
by a bis-chelated (FI-SO2-Ph)2Ni side product (see SI Figure
S25), the −SO2− group can coordinate to a similar Ni(II)
center and under polymerization conditions should be able to
closely approach the Ni(II) center of FI-SO2-Ni (vide infra). In
FI-CH2-Ni, a long Ni···H−C(benzyl) contact is observed at
2.56 Å.
Scheme 1. Synthesis of Ligands HFI-SO2-Ph and HFI-CH2-
Ph, and Single-Crystal XRD Structures of Precatalysts FI-
a
SO2-Ni and FI-CH2-Ni
Ethylene homopolymerizations were performed using Ni-
(cod)2 as a phosphine scavenger in toluene at a constant 8 atm
ethylene pressure, under conditions minimizing mass transport
and exotherm effects (Table 1).16 A 25 °C, FI-SO2-Ni produces
polyethylene with an activity as high as 35 kg of PE mol [Ni]−1
atm−1 h−1 to yield a polymer with MW = 3500 g mol−1 and a
polydispersity indicative of single-site behavior. Furthermore,
1
the H NMR spectrum of the FI-SO2-Ni-derived polyethylene
exhibits 148 branches 1000 C−1, which is consistent with
extensive chain walking17 relative to propagation. The
unsaturation in the FI-SO2-Ni-derived polyethylene consists
of 87% internal olefins (SI Figures S18, S19), which is
consistent with β-H elimination and CC migration.17 Under
the same conditions, FI-CH2-Ni produces far less polyethylene
at an activity of ∼2.2 kg PE mol[Ni]−1 atm−1 h−1, with product
MW = 1100 g mol−1 and 98 branches 1000 C−1. This latter
branch density is comparable to that produced by conventional
Ni(II) phenoxyiminato catalysts.3 It is unlikely that steric effects
alone are responsible for the marked FI-SO2-Ni versus FI-CH2-
Ni polymerization differences because sterically encumbered
ligand substituents tend to suppress β-H transfer/chain-walking
processes in catalysts of this type, yielding nearly linear high-Mw
polyethylenes.1b
a
H atoms are omitted for clarity.
high purity, as confirmed by conventional spectroscopic and
analytical techniques.12 The ligands are then converted to
sodium salts by stirring with excess NaH in THF and were used
without further purification (Scheme 1). Metalated precatalysts
FI-SO2-Ni and FI-CH2-Ni were then prepared by reacting the
respective sodium salts with 1.0 equiv of NiClMe(PMe3)2 in
benzene and were isolated by filtration, followed by removal of
the volatiles in vacuo and recrystallization from n-hexane (84%
and 88% yield, respectively). Diffraction quality crystals were
grown from n-hexane; the molecular structures are shown in
Scheme 1. FI-SO2-Ni and FI-CH2-Ni were characterized by
standard spectroscopic and analytical techniques,12 and the
results are in good agreement with the diffraction data. The
geometrical parameters of FI-SO2-Ni and FI-CH2-Ni are similar
to those of previously reported square−planar Ni(II)
phenoxyiminato complexes with the Ni-PMe3 ligand trans to
the imine functionality (Scheme 1).15 Both diamagnetic
complexes have distorted square−planar coordination geo-
metries, with P1 and C1 of FI-SO2-Ni and FI-CH2-Ni displaced
from the mean O1/Ni1/N1 planes by 0.496 and 0.344 Å and
by 0.315 and 0.361 Å, respectively. In the solid state, the O
atoms of the FI-SO2-Ni sulfonyl group are orientated away
from the Ni center (Scheme 1). The 1H−1H 2D NOESY NMR
spectra of FI-SO2-Ni and FI-CH2-Ni display NOE interactions
between the t-Bu group and the PMe3 ligand, the Ni-CH3
group and the Ni-PMe3 ligand, and the Ni-CH3 group and an
In comparison with FI-CH2-Ni, FI-SO2-Ni is 18 times more
active for ethylene polymerization and produces polyethylene
that has a 3.2 times higher MW with 1.5 times greater branch
density than does FI-CH2-Ni. 13C NMR analysis of the polymer
microstructure produced by FI-SO2-Ni at 25 °C reveals a
variety of branch types, including methyl, ethyl, n-propyl, n-
butyl, sec-butyl, and >C6 (Figure 1), whereas the FI-CH2-Ni
product has predominantly methyl and >C6 branches, with a
small density of n-butyl branches (SI Figures S14 and S15).13
The presence of sec-butyl groups indicates branch-on-branch
formation.1 13b The greater branch densities and varied branch
h,
architectures introduced by FI-SO2-Ni indicate very rapid
chain-walking, likely promoted by sulfonyl group stabilization
of 14-electron intermediates (see more below).
a
Table 1. Ethylene Polymerization Data for FI-SO2-Ni- and FI-CH2-Ni-Derived Catalysts
d
% branch type
T
(°C)
PE
(g)
branches
(×1000 C)
Mn (NMR)
Mw (GPC)
g
b
c
e
f
g
entry
cat.
activity
Me
Et
Pr
Bu Hex+
(kg/mol)
(kg/mol)
PDI
1
2
3
4
5
6
FI-SO2-Ni
FI-CH2-Ni
FI-SO2-Ni
FI-CH2-Ni
FI-SO2-Ni
FI-CH2-Ni
23
23
40
40
60
60
1.9
35
2.2
53
148
98
45
55
44
10
3
4
<1
2
22
11
23
19
31
22
2.1
3.5
1.1
2.8
1.8
1.4
1.9
0.12
2.8
0.66
1.6
145
9
trace
2.7
51
150
41
10
3
23
23
1.5
2.1
1.6
trace
a
Polymerizations carried out in 25 mL of toluene with 10 μmol of catalyst at a constant 8.0 atm ethylene pressure for 40 min and using 2 equiv of
Ni(cod)2 as phosphine scavenger. All polymerizations were performed in duplicate. kg of PE mol [Ni]−1 atm−1 h−1. By 1H NMR.13 By 13C NMR.
b
c
d
e
f
14 g
1
Bu = n-butyl + sec-butyl. By H NMR. By GPC vs polystyrene standards.
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walking to propagation rate ratios remain approximately
constant over this temperature range. At 60 °C, FI-SO2-Ni
polymerization activity remains at 51 kg PE mol[Ni]−1 atm−1
h−1, while the polyethylene MW has decreased to 2100 g mol−1
with the approximately same branch density as at 25 °C. Falling
MW with increased temperature mirrors trends observed in
many other single-site catalyst systems,1 18 and presumably
g,
reflects more favorable β-H elimination. Furthermore, catalyst
activity versus time studies at 60 °C reveal that FI-SO2-Ni
undergoes significantly slower decomposition (SI Table S2,
Figure S22) than FI-CH2-Ni, which quickly decomposes. Note
that the FI-SO2-Ni-derived polymer MW and branch density are
insensitive to polymerization time.
To obtain better insight into the polymerization mechanism,
DFT calculations were performed12 to examine the origin of
the catalytic differences between FI-SO2-Ni and FI-CH2-Ni.19
The geometric, electronic, and energetic properties of all
intermediates and transition states in the propagation, branch-
forming, and termination pathways were investigated, with the
γ-complex of Scheme 2 taken as the reference point. In addition
to the γ-complex, two other stable structures are identified,
related to this structure by rotation of the polymeryl chain.
The first structure involves displacement of the polymer
chain agostic interaction by a ligand SO2 or CH2 group of
activated FI-SO2-Ni or FI-CH2-Ni, respectively (PII). For
activated FI-CH2-Ni, PII formation is slightly endoergic,
whereas it is significantly exoergic for FI-SO2-Ni (Figure 2).
This structure precedes ethylene coordination and the
subsequent insertion step (propagation pathway, red in Scheme
2). Note that there is no energy barrier for ethylene
coordination in FI-SO2-Ni, versus 1.7 kcal mol−1 (formation
of PII) in FI-CH2-Ni (Figure 2). Furthermore, at constant
elevated ethylene pressure, the turnover-limiting propagation/
insertion step has a slightly lower computed barrier for FI-SO2-
Figure 1. 125 MHz 13C{1H}NMR spectrum of the polyethylene
produced by FI-SO2-Ni at 40 °C reaction temperature.
To compare the thermal stability of the catalysts derived
from FI-SO2-Ni and FI-CH2-Ni, ethylene polymerizations were
performed at elevated temperatures. At 40 °C, activated FI-
CH2-Ni rapidly decomposes to yield a black Ni precipitate and
negligible quantities of polyethylene (Table 1, entry 4). In
contrast, at 40 °C, the catalyst derived from FI-SO2-Ni gives no
obvious decomposition over the same reaction time and
produces polyethylene at activities slightly higher than at 25 °C
(Table 1, entry 3). This result further supports the hypothesis
that the sulfonyl group stabilizes the catalyst against
deactivation.
Ni (ΔG‡
= 14.7 kcal mol−1) versus FI-CH2-Ni (ΔG‡
=
The polymer produced by FI-SO2-Ni at 40 °C has MW
=
prop
prop
15.2 kcal mol−1), in accord with the higher measured
polymerization activity of FI-SO2-Ni (Table 1).
2800 g mol−1 with 145 branches 1000 C−1. That the 40 °C
branching is similar to that at 25 °C indicates that the chain-
Scheme 2. Proposed Catalytic Cycle for Ethylene Polymerization, Branch Formation, and Catalyst Deactivation Mediated by
Activated Catalyst FI-SO2-Ni
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Letter
with the increased stability of the FI-SO2-Ni catalyst observed
experimentally.
From the forgoing experimental and theoretical discussion, it
is evident that the hemilabile SO2 interaction with the metal
center in the PII, PII isomer and FI-SO2-Ni(H) structures can
be identified as the primary factor differentiating the reactivities
of the two catalysts. For this reason, a more extensive analysis
of the electronic structure of species PII was carried out. The
greater stabilization of PII in the activated FI-SO2-Ni vs FI-
CH2-Ni catalysts can be ascribed to a monodentate Ni···O
SO interaction, supported by both geometric and electronic
population evidence. An analogous but significantly weaker
Ni···H−CH interaction occurs in FI-CH2-Ni. Note that the
computed FI-SO2-Ni S−O2 versus S−O1 bond elongation (see
SI Figure S24; Δ = +0.042 Å) as well as FI-CH2-Ni Ni···H−C
bond elongation (1.187 Å) vs a normal sp3 C−H bond distance
of 1.10 Å indicate that secondary interactions occur in both
complexes. NBO analysis of PII shows that the former
elongation is due to partial population of σ* antibonding
orbitals localized on the S−O2 and C−H bonds of FI-SO2-Ni
and FI-CH2-Ni, respectively. The NBO distribution also reveals
stabilizing electron donation from the SO2 or CH2 fragments to
the Ni center, with overlap between the lone pair localized on
one O atom sp2 orbital and the empty Ni 4s orbital (SI Figure
S24). Less efficient electron donation occurs between the sp3
C−H orbital and the FI-CH2-Ni Ni center. These interactions
help explain why stabilization by the SO2 group is greater than
by the CH2 group in PII and FI−Ni(H), and differences in
reactivity observed between the FI-SO2-Ni- and FI-CH2-Ni-
derived catalysts can be ascribed primarily to this interaction.
In conclusion, these results indicate that a spatially proximate
but electronically remote, weakly coordinating ligand −SO2−
moiety in a phenoxyiminato-based Ni ethylene polymerization
catalyst significantly enhances activity and thermal stability and
produces product polymers with exceptional branch densities,
indicating that β-H elimination/olefin reinsertion is especially
facile. DFT analysis of the propagation, branch formation, and
chain transfer pathways indicates that the proximate SO2 group
significantly stabilizes the Ni center relative to the control CH2
complex, explaining the marked differences in FI-SO2-Ni vs FI-
CH2-Ni polymerization characteristics. Work is underway to
further probe how sulfonyl group sterics and electronics affect
catalyst function and product polymer microstructure in other
catalysts.
Figure 2. Comparative Gibbs free energy profiles (kcal mol−1)
associated with the propagation (red), branch-forming (blue), and
chain transfer (green) pathways for FI-SO2-Ni and FI-CH2-Ni.
The second structure involves a β-agostic interaction of the
growing polymer with the Ni center (β-complex). For both FI-
SO2-Ni and FI-CH2-Ni, β-agostic formation is exoergic and
proceeds with minimal barrier to the β-H elimination complex,
which in turn precedes olefin reinsertion (branch-forming path)
or olefin dissociation (chain transfer path). The energetic
profiles of the branch-forming pathways (blue in Figure 2)
reveal that for FI-SO2-Ni, the activation barrier primarily
reflects formation of the β-H elimination complex (ΔG‡
=
branc
9.0 kcal mol−1) and, ultimately, the OSO···Ni-stabilized
PII isomer. In contrast, the FI-CH2-Ni barrier reflects β-agostic
interaction dissociation to form the less stable β-complex and
PII isomer, necessary for subsequent ethylene coordination
(ΔG‡
= 11.1 kcal mol−1) and propagation of the branched
branc
chain. The difference between propagation and branching
barriers for FI-SO2-Ni (ΔΔG‡ = 5.7 kcal mol−1) versus FI-CH2-
Ni (ΔΔG‡ = 4.1 kcal mol−1) shows that branching is slightly
more favorable for FI-SO2-Ni, in agreement with experiment.
Ethylene insertion at the PII isomer, necessary to incorporate
the methyl branch into the polymer chain, follows a kinetic
trend comparable to that in the propagation step for both the
FI-SO2-Ni and FI-CH2-Ni systems (FI-SO2-Ni: ΔG‡prop.
isomer = 14.4 kcal mol−1; FI-CH2-Ni: ΔG‡ prop. isomer =
14.6 kcal mol−1; see Figure S23).
ASSOCIATED CONTENT
■
S
* Supporting Information
Olefin dissociation from the β-H elimination structure yields
a Ni−H species {FI−Ni(H)} and free polymer (green in Figure
2). The barrier for this chain transfer process is computed as
Details of ligand and catalyst synthesis and characterization,
polymerization experiments, polymer characterization, DFT
calculations, and X-ray structures. This material is available free
of charge via the Internet at http://pubs.acs.org.
ΔG‡
= 16.8 kcal mol−1 for FI-SO2-Ni, and ΔG‡
= 15.9
term
term
kcal mol−1 for FI-CH2-Ni. The greater difference between
propagation and chain transfer barriers for FI-SO2-Ni (ΔΔG‡ =
2.1 kcal mol−1) versus FI-CH2-Ni (ΔΔG‡ = 0.7 kcal mol−1) is
in accord with the higher polyethylene Mw produced by FI-
SO2-Ni (Table 1).19b Finally, a plausible catalyst deactivation
pathway (pink in Scheme 2), at least at high ethylene
concentrations, would involve the reductive elimination of the
free ligand and Ni(0) from the β-H elimination/olefin
dissociation derived FI−Ni(H).4b,20 Note that the computed
barrier to this first reasonable decomposition step is 1.6 kcal
mol−1 greater for FI-SO2-Ni than for FI-CH2-Ni,12 in accord
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: m-delferro@northwestern.edu.
*E-mail: t-marks@northwestern.edu.
Author Contributions
§C.J.S. and J.P.M. contributed equally to this paper.
Notes
The authors declare no competing financial interest.
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Letter
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ACKNOWLEDGMENTS
■
Financial support by DOE Grant 86ER13511 (M.P.W. and
C.C) on the heterogeneous-homogeneous catalytic interface,
and by NSF Grant CHE-1213235 (C. J. S. and J. P. M.) on
fundamental homogeneous catalysis, is gratefully acknowl-
edged. Use of NMR and X-ray facilities at the IMSERC facility
of Northwestern University was supported by NSF under
Grants CHE-1048773 and CHE-0923236. We acknowledge
CINECA Award N. HP10C9RDDE 2013 for high performance
computing resources and support.
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