Role of electron-withdrawing remote substituents in neutral nickel(II) polymerization catalysts

Article abstract of DOI:10.1021/om400757f

The novel neutral κ²-N,O-salicylaldiminato Ni(II) complex, [κ²-N,O-{2,6-(3′,5′-R₂C ₆H₃)₂C₆H₃-N=C(H)-(3,5- I₂-2-O-C₆H₂)}NiCH₃

Full text of DOI:10.1021/om400757f

Note
pubs.acs.org/Organometallics
Role of Electron-Withdrawing Remote Substituents in Neutral
Nickel(II) Polymerization Catalysts
Anna Osichow, Inigo Gottker-Schnetmann, and Stefan Mecking*
̈
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitatsstrasse 10, D 78457 Konstanz,
̈
Germany
S
* Supporting Information
ABSTRACT: The novel neutral κ2-N,O-salicylaldiminato Ni-
(II) complex, [κ2-N,O-{2,6-(3′,5′-R2C6H3)2C6H3-NC(H)-
(3,5-I2-2-O-C6H2)}NiCH3(pyridine)] (1a-pyr, R = NO2),
with four nitro substituents on the N-terphenyl motif is a
catalyst precursor for ethylene polymerization to yield linear
higher molecular weight polyethylene (e.g., Mn 2.1 × 105 g
mol−1 and only 2 methyl branches per 1000 carbon atoms). A
comparison with other known catalyst precursors at various
polymerization conditions shows that the catalytic properties in
terms of linearity and molecular weight are similar to the
fluorinated catalyst precursor with R = CF3, showing that the
latter is not singular, but rather suppression of chain transfer
and branch formation by β-hydride elimination can also be
brought about by nonfluorinated electron-withdrawing remote substituents.
Catalytic polymerization of olefins by complexes of d8 metals
has been studied intensely due to several unique features by
comparison to the more established catalysts based on early
transition metals.1 Due to their lower oxophilicity, they are
more tolerant toward functional groups.2 Also, ethylene3 and 1-
olefins3,4 can be converted to unique branching patterns. On
the other hand, some late transition metal catalysts display a
very high selectivity for linear ethylene chain growth even in the
presence of 1-olefins, which is of practical relevance in
oligomerization5 and polymerization6 catalysis.
different materials obtained.12b However, these considerations
are based essentially on only one type of electron-withdrawing
substituent that can yield linear high molecular weight polymer,
namely, the trifluoromethyl group. Consequently, CF3 groups
have been suggested to be unique in that the formation of linear
high molecular weight polyethylenes with these catalysts is due
to an F···H interaction with the growing chain.14 Note that a
similar interaction has been considered to occur in living
ethylene polymerization by fluorinated salicylaldiminato Ti
catalysts with nonremote F-substituents.15 In structurally
related enolatoimine Ti catalysts a Ti···F interaction with the
very electrophilic metal center directly observed in solution
provides a plausible alternative explanation for the living
character of polymerization with these Ti catalysts.16
Of the different major cationic and neutral catalyst systems
studied,1,7 neutral salicylaldiminato Ni(II) catalysts8 stand out
in their tolerance toward aqueous reaction media, enabling
catalytic emulsion polymerizations.9 We have previously
reported that in N-terphenyl-substituted catalysts (1) sub-
stituents (R) on the peripheral aromatic rings have a
remarkable effect on the catalytic properties, despite their
remoteness from the active sites.10 Depending on the
substituents, high molecular weight linear polyethylene or low
molecular weight, highly branched oligomers are formed.
Studies of a range of patterns of different substituents in the
3′,5′- and/or 4′-position (e.g., CF3, Me, tBu, OMe) showed
that the catalytic properties correlate with their electronic
character.10−12 More electron-donating substituents favor
branch formation and chain transfer, which both occur through
β-hydride elimination as the underlying step. The observed
effect of substituents can be related to very similar barriers of β-
hydride elimination (ΔG‡β‑elim) and ethylene insertion chain
In order to further probe this issue, we sought to study a
representative of 1 with nonfluorinated electron-withdrawing
remote 3′,5′-substituents.
Bromination of 1,3-dinitrobenzene in sulfuric acid at 80−85
°C by addition of N-bromosuccinimide in small portions
growth (ΔG‡ins).13 Small relative changes in ΔG‡
and
β‑elim
ΔG‡ exerted by the electronics of R can then alter the ratio
Received: July 30, 2013
Published: September 11, 2013
ΔG‡iinnss/ΔG‡
to a noticeable extent, resulting in entirely
β‑elim
© 2013 American Chemical Society
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Organometallics
Note
Scheme 1. Synthesis of the Nitro-Substituted Salicylaldimine 5
yielded 1-bromo-3,5-dinitrobenzene 2.17 Borylation of 2,6-
dibromoaniline with bis(pinacolato)diboron in the presence of
[Pd(dppf)2Cl2·CH2Cl2] in DMF at 90 °C afforded 2,6-
bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)aniline 3
(Scheme 1).18 2,6-Bis(3′,5′-dinitrophenyl)aniline (4) was
prepared by Suzuki coupling of 3 with 2.1 equiv of 2.12a
Acid-catalyzed condensation of this nitro-substituted terphenyl-
amine 4 and 3,5-diiodosalicylaldehyde afforded the desired
salicylaldimine 5 (for characterization data of all compounds
see the Supporting Information, SI).
The targeted NO2-substituted nickel(II) pyridine complex
was generated by reaction of salicylaldimine 5 with 1.05 equiv
of [(tmeda)Ni(CH3)2] and 10 equiv of pyridine in THF
solution at −50 °C under strict exclusion of oxygen (eq 1), to
Figure 1. X-ray crystal structure of complex 1a-PPh3 drawn with 50%
probability ellipsoids. All hydrogen atoms and pentane molecules are
omitted for clarity.
precursors for ethylene polymerization in toluene solution at
various temperatures and ethylene pressures (Table 1).
Moderate activities up to 2.0 × 104 TO h−1 were observed
with 1a-pyr (entries 1 to 3). 1a-pyr requires higher
polymerization temperatures to enhance pyridine dissociation,
compared to the CF3-substituted analogue 1b-pyr. Possibly, a
stronger electron-withdrawing character of the nitro groups
increases the electron deficiency of the Ni(II) center and
enhances pyridine binding. However, when 1a-pyr was
combined with B(C6F5)3 as a pyridine scavenger, polymer-
ization activity increased significantly (4.2 × 104 TO h−1, entry
4) under otherwise identical conditions. The activity of the
catalyst 1a-pyr/B(C6F5)3 decreased only slightly over time, as
concluded from mass-flow traces reflecting ethylene con-
sumption at 50 °C (40 bar ethylene pressure), and the catalyst
is active for at least ca. 2 h. By comparison, a previously studied
3-NO2 monosubstituted Ni(II) catalyst was deactivated
completely within 20 min (and yielded slightly branched PE,
entry 13).10
yield crystalline, deep red 1a-pyr in 90% yield. The observation
of a single resonance for the Ni-CH3 group at −0.56 ppm (1H
NMR) and −6.5 ppm (13C), respectively, shows that only a
single isomer of 1a-pyr is present. The observation of only a
single well-resolved (4JHH = 1.6 Hz) resonance for the four
protons in the 2′,6′-positions evidences an unhindered rotation
of the aryl groups. Since the crystals of 1a-pyr obtained were
not suitable for X-ray diffraction analyses, crystals of the
corresponding triphenylphosphine complex 1a-PPh3 (gener-
ated by replacing pyridine for PPh3) were grown. Single-crystal
X-ray crystallography (Figure 1) shows all four NO2 groups to
be in the plane of the phenyl rings in the solid-state structure of
the square planar complex. The coordination geometry with the
methyl group trans to the oxygen donor agrees with other
salicylaldiminato complexes.8b,10,14 While the quality of the data
does not allow for a detailed discussion of bond lengths and
angles, overall there are no indications of unusual strong
devations of the structure vs known terphenyl analogues.
1a-pyr and the known analogs 1b-pyr (R = CF3) and 1c-pyr
(R = CH3) were studied as single-component catalyst
Both 1a-pyr and 1b-pyr produce high molecular weight
linear polyethylene with very few branches (Table 1). As
expected, degrees of branching increase with temperature, while
molecular weights decrease, due to an increased β-hydride
elimination at higher temperature. In detail, 1a-pyr produced
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Organometallics
Note
a
Table 1. Ethylene Polymerization Results with Complexes 1a-pyr, 1b-pyr, and 1c-pyr as Precatalysts
b
c
c
d
d
e
entry
precatalyst
T [°C]
yield [g]
TOF
4.8
Mn [103 g/mol]
Mw/Mn
Tm [°C]
crystallinity [%]
branches/1000 C
1
2
3
1a-pyr
1a-pyr
1a-pyr
1a-pyr
1a-pyr
1b-pyr
1b-pyr
1b-pyr
1b-pyr
1c-pyr
1c-pyr
1c-pyr
R = NO2/H
30
50
70
50
50
30
50
70
50
30
50
70
50
0.90
1.61
3.69
7.76
5.94
5.24
14.11
26.21
4.24
2.19
6.04
3.34
3.0
214.9
47.7
9.6
1.5
3.2
2.2
2.6
4.1
1.8
4.0
2.1
2.6
n.d.
n.d.
n.d.
2.6
132
121
108
113
110
131
123
115
120
n.d.
n.d.
n.d.
106
65
2
11
15
14
15
3
8.6
19.8
41.6
31.8
28.1
75.6
140.4
22.7
11.7
32.4
17.9
5.4
55
55
f
4
g
17.6
24.6
174.6
31.6
9.6
53
5
53
6
7
8
60
64
8
64
11
8
g
9
16.7
63
h
h
10
2.0
n.d.
n.d.
n.d.
n.d.
80
h
h
11
1.4
80
h
h
12
1310
1.0
85
i
11.0
26
a
b
Polymerization conditions: 10 μmol of precatalyst, 100 mL of toluene, 40 bar of C2H4, 40 min. 103 × mol [C2H4] × mol−1 [Ni] × h−1.
c
d
e
f
g
Determined by GPC at 160 °C. Determined by DSC. Determined by 13C NMR spectroscopy. 1 equiv of B(C6F5)3 added. 1 equiv of B(C6F5)3
h
i
1
added; 8 bar of C2H4 pressure. Determined by H NMR spectroscopy. 40 μmol of precatalyst.
Figure 2. Polymer molecular weights (Mn, left) and methyl branches per 1000 carbon atoms (right, from 13C NMR spectroscopy) for complexes 1a-
pyr, 1b-pyr, and 1c-pyr at various polymerization temperatures (40 bar of ethylene pressure).
polyethylenes with slightly higher molecular weights (215 × 103
g mol−1 at 30 °C and 48 × 103 g mol−1 at 50 °C) vs 1b-pyr
(174 × 103 g mol−1 at 30 °C and 32 × 103 g mol−1 at 50 °C).
At 70 °C molecular weights are essentially identical (9.6 × 103
g mol−1). Also, branching is virtually identical with both
catalysts (2 vs 3, 11 vs 8, and 15 vs 11 methyl branches/1000C,
entry 1 vs 6, entry 2 vs 7, and entry 3 vs 8, Figure 2). At a
polymerization temperature of 70 °C, in addition to methyl
branches also small amounts of higher branches are observed
for both catalysts (Figures S3 and S6).
Overall, with both types of electron-withdrawing substituents
(Hammett substituent constant σm(NO2) = 0.71 and σm(CF3)
= 0.43)19 a similar higher molecular weight linear polymer
microstructure is obtained. This is also most clearly underlined
by comparison to catalysis with the electron-donating
substituted analogue (1c-pyr,10 R = Me; σm(CH3) =
−0.07).19 Under otherwise identical conditions of polymer-
ization temperature and ethylene concentration, in all cases
highly branched (80−85 branches/1000 carbon atoms) low
molecular weight polyethylene ((1−2) × 103 g mol−1, entries
10−12) is formed (Figure 2).
the electronic character of the remote substituents is decisive
for the control of the polymer microstructure. Given that this
class of catalysts is, for example, advantageously compatible
with polar reaction media including water, and here the control
of microstructure is decisive, for example, for potentially
resolving general issues of polymer processing, this is a relevant
guideline for further development of such polymerization
catalysis.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text giving experimental details, NMR data, CIF files, and
analytical data for the compounds prepared in this work. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: stefan.mecking@uni-konstanz.de.
Notes
In conclusion, these findings show that the prototypical 3′,5′-
trifluoromethyl-substituted terphenyl motif10,11,12a,14 is not
singular in suppressing chain transfer and branching in this
type of catalysts. The nonfluorinated 3′,5′-nitro-substituted
analogue reported here likewise affords higher molecular weight
linear polyethylene. This is consistent with the observation that
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Lars Bolk for DSC and GPC analyses and Anke
Friemel and Ulrich Haunz for support with NMR measure-
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Note
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Mecking, S. Organometallics 2010, 29, 4428−4430. (b) Moller, H. M.;
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Baier, M. C.; Mecking, S.; Talsi, E. P.; Bryliakov, K. P. Chem.Eur. J.
2012, 18, 848−856.
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