Electronic influences in phosphinesulfonato palladium(II) polymerization catalysts

Article abstract of DOI:10.1021/om400297x

To study the influence of electronics on catalytic polymerization properties independent from sterics, phosphinesulfonato Pd(II) complexes bearing remotely located substituents on the nonchelating P-bound aryls [κ²-(P,O)-(4-R-2-anisyl)₂PC₆H ₄SO₂O]Pd(Me)(dmso) (1a-e-dmso: 1a, R = CF₃; 1b, R = Cl; 1c, R = H; 1d, R = CH₃; 1e, R = OCH₃) were prepared. The electron-poor complex 1a-dmso (4-CF₃) undergoes the fastest insertion of methyl acrylate (MA) and is the most active for ethylene polymerization. The polyethylene molecular weight increases by a factor of 2 for the more electron rich complex 1e-dmso (4-OCH₃) (M_n = 17 × 10³ vs 8 × 10³ for 1a-dmso (4-CF ₃)). MA/ethylene copolymerization experiments revealed that the MA incorporation ratio and copolymer molecular weights are largely independent of the electronic nature of the remote substituents. These trends were further confirmed by studies of two mixed P-aryl/-alkyl complexes 1f-dmso ([κ²-(2,4,6-(OMe)₃C₆H₂)( ^tBu)PC₆H₄SO₂O]Pd(Me)(dmso)) and 1g-dmso ([κ²-(C₆H₅)(^tBu) PC₆H₄SO₂O]Pd(Me)(dmso)). In ethylene/MA copolymerization, 1f-dmso affords a significantly higher molecular weight polymer with reasonable MA incorporation (M_n = 12 × 10 ³ and 7.7 mol % MA) and activities similar to those observed for complexes 1a-e-dmso.

Full text of DOI:10.1021/om400297x

Article
pubs.acs.org/Organometallics
Electronic Influences in Phosphinesulfonato Palladium(II)
Polymerization Catalysts
Philipp Wucher, Verena Goldbach, and Stefan Mecking*
Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, Universitatsstrasse 10, 78457 Konstanz,
̈
Germany
S
* Supporting Information
ABSTRACT: To study the influence of electronics on
catalytic polymerization properties independent from sterics,
phosphinesulfonato Pd(II) complexes bearing remotely
located substituents on the nonchelating P-bound aryls [κ2-
(P,O)-(4-R-2-anisyl)2PC6H4SO2O]Pd(Me)(dmso) (1a−e-
dmso: 1a, R = CF3; 1b, R = Cl; 1c, R = H; 1d, R = CH3;
1e, R = OCH3) were prepared. The electron-poor complex 1a-
dmso (4-CF3) undergoes the fastest insertion of methyl
acrylate (MA) and is the most active for ethylene polymer-
ization. The polyethylene molecular weight increases by a
factor of 2 for the more electron rich complex 1e-dmso (4-OCH3) (Mn = 17 × 103 vs 8 × 103 for 1a-dmso (4-CF3)). MA/
ethylene copolymerization experiments revealed that the MA incorporation ratio and copolymer molecular weights are largely
independent of the electronic nature of the remote substituents. These trends were further confirmed by studies of two mixed P-
aryl/-alkyl complexes 1f-dmso ([κ2-(2,4,6-(OMe)3C6H2)(tBu)PC6H4SO2O]Pd(Me)(dmso)) and 1g-dmso ([κ2-(C6H5)(tBu)
PC6H4SO2O]Pd(Me)(dmso)). In ethylene/MA copolymerization, 1f-dmso affords a significantly higher molecular weight
polymer with reasonable MA incorporation (Mn = 12 × 103 and 7.7 mol % MA) and activities similar to those observed for
complexes 1a−e-dmso.
ligands lead to higher molecular weights.12 In this context, it is
INTRODUCTION
■
noteworthy that such steric modifications usually go along with
Insertion polymerization of ethylene and propylene by early-
changes in the ligand acidity and that it is therefore difficult to
transition-metal catalysts is employed on a vast scale.1 Late-
separate steric effects from electronic influences of the ligand.
transition-metal catalysts have also been studied intensively for
Herein we report the synthesis of phosphinesulfonato Pd(II)
olefin polymerization and oligomerization, due to their different
complexes with remotely located electron-donating and
incorporation behavior, branch formation, and generally less
-withdrawing groups in the para position of the nonchelating
oxophilic nature.2−6 The last feature is particularly advanta-
geous with regard to the challenge of insertion polymerization
of polar vinyl monomers.
Thus, acrylates and ethylene can be copolymerized by
anisyl moiety and the effects of these electronic modifications
on insertion and (co)polymerization behavior as well as their
influence on the polymer microstructure.
cationic α-diimine Pd(II) complexes, yielding highly branched
copolymers with acrylate units located at the end of branches.7
RESULTS AND DISCUSSION
■
Ligand and Complex Synthesis. The phosphinesulfonato
system with (P∧O) = {2-(2-anisyl)2PC6H4SO2O} (4c; Scheme
1), first described for ethylene−MA copolymerization by Drent
et al. in their seminal work,8 remains the most well studied
ligand motif. Corresponding catalysts have a high functional
group tolerance as well as reasonable activity, combined with
high comonomer incorporation ratios.8,9 For this reason, we
sought to study purely electronic influences by modifications of
the anisyl groups of this prototypical ligand motif. From
structure analysis it is evident that the most suitable position for
electronic modifications with a minimum steric impact is the 4-
position of the anisyl moieties.11c,13
In contrast, neutral Pd(II) phosphinesulfonato catalysts afford
linear ethylene−acrylate copolymers with the polar repeating
units incorporated into the polymer backbone.8 In the past
decade the scope of functional monomers amenable to
insertion polymerization has been broadened extensively,
even enabling the copolymerization of ethylene with acryloni-
trile, vinyl acetate, or acrylic acid.9,10
In view of these unique catalytic properties, further
substitution patterns of the phosphine aryl sulfonato motif
have been reported.11 So far, however, there is no
comprehensive picture of how desirable properties such as
high polymer molecular weights, high polymerization activities,
and high comonomer incorporation correlate with the catalyst
structure. For example, in some cases the PE molecular weights
decrease with increased steric bulk,11b whereas other bulky
Received: April 8, 2013
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Different Phosphonium Sulfonates
Different 2-bromoanisole derivatives 2 were synthesized by
nucleophilic aromatic substitution of the corresponding 1-
bromo-2-fluorobenzene compounds with sodium methoxide.
From such anisole derivatives, the anionic ligands 4 were either
synthesized in a one-pot synthesis as described by Goodall et
al.11a or by a stepwise synthesis as shown in Scheme 1. The 4-
chloro-substituted ligand 4b was prepared via an in situ
generated Grignard reagent. Protonation of the salts 4 afforded
the zwitterionic phosphonium sulfonates H(P∧O) (5) with the
acidic proton located at the phosphorus atom, as indicated by a
activation with silver salts.14 Hence, no labile ligand (L), which
decreases activity by shifting the equilibrium [(P∧O)Pd-
(polymeryl)L] + ethylene ⇄ [(P∧O)Pd(polymeryl)ethylene]
+ L toward the dormant species, is present in the polymer-
ization mixture.15
To underline the effects of varied electron density at
phosphorus, further electron-rich P-alkyl-substituted phosphi-
nesulfonato catalyst precursors with a weakly coordinated labile
ligand (dmso) were prepared. Such complexes are known to
promote polar olefin copolymerization as well.10b,16,17 The new
P-aryl/-alkyl complex 1f-dmso (Figure 2) was prepared, as well
as a dmso-substituted analogue of the known lutidine complex
1g-lut.17 The methoxy substituents of 1f-dmso represent a
more electron-rich variation of complex 1g-dmso, analoguous
to the electron-rich modification of the prototypical anisyl
motif 1a−e-dmso. The required phosphonium sulfonates 5g,f
were obtained by a stepwise procedure similar to the approach
described above.
1
characteristic coupling constant of about JP−H ≈ 600 Hz. This
is also exemplified by the solid-state structure of the
phosphonium sulfonate 5b (Figure 1). 5b crystallized in the
The identity of the complexes was confirmed unambiguously
by one- and two-dimensional NMR spectroscopy and elemental
analysis (see the Supporting Information) as well as the crystal
structures of [{1a-μLiCl}2], [{1d-μLiCl}2], 1e-dmso, and 1f-
dmso (Figure 3). All complexes crystallized in the monoclinic
space group P21/c and exhibit a square-planar coordination
sphere around the palladium center, with the palladium methyl
group and the phosphorus atom located mutually cis to each
other. The bond lengths around the palladium center are in the
expected range for phosphinesulfonato Pd(II) complexes
(Table 1).18 The complexes [{1a-μLiCl}2] and [{1d-μLiCl}2]
are binuclear, with both Pd centers bridged via LiCl, similar to
the reported structure of [{1c-μNaCl}2].19 One additional
solvent molecule is coordinated to the Li atom. The labile
ligand dmso is κ-S-coordinated to the Pd atom in both 1e-
dmso and 1f-dmso. The factors determining this coordination
mode remain unclear, since both κ-S and κ-O coordination of
dmso to phosphinesulfonato complexes is observed and there is
no obvious correlation between sterics and electronics in the
environment of the Pd(II) center and the coordination
mode.20−22
Figure 1. ORTEP plot of phosphonium sulfonate 5b. Ellipsoids are
drawn at the 50% probability level. All hydrogen atoms, except H23
(P−H), are omitted for clarity.
monoclinic space group C2/c. The position of the hydrogen
atom H23 (P−H) was found in the electron density map at a
distance of 1.293 Å to the phosphorus atom.
The 31P NMR shifts of the isolated Pd-dmso complexes 1a−
e-dmso in CD2Cl2 correlate nicely with the Hammett constants
of the different substituents, as an established measure for the
electronic nature of aromatic substituents (Figure 4).23 The
phosphorus resonances are shifted to high field for complexes
bearing electron-donating groups.
These phosphonium sulfonates could be reacted with
[(tmeda)PdMe2] to yield the binuclear tmeda-bridged
complexes 1a−e-tmeda, from which the dmso complexes
1a−e-dmso are directly accessible (Scheme 2). A comple-
mentary approach is given by the reaction of the anionic ligands
4 with [(COD)PdMeCl] to the chloride-bridged binuclear
palladate complexes [{1a−e-μMCl}2] (M = Na, Li), which are
very active in polymerization and in insertion reactions upon
Insertion of Methyl Acrylate. The reaction with methyl
acrylate is one of the most well studied reactions of such κ2-
(P∧O)-phosphinesulfonato Pd(II) complexes.11c,12,19,24 While it
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Scheme 2. Synthesis of Phosphinesulfonato Pd(II) Complexes
insertion of MA into the Pd−Me bond (kobs = 3.0 × 10−3 s−1),
which is significantly faster than for 1c-dmso (kobs = 6.1 × 10−4
s−1) and even 1 order of magnitude faster than for complexes
1d-dmso (kobs = 3.8 × 10−4 s−1) and 1e-dmso (kobs = 2.1 ×
10−4 s−1). The P-aryl/P-alkyl complex 1f-dmso (kobs = 3.1 ×
10−4 s−1) shows approximately the same net rate constant for
the first insertion as the electron-rich complexes 1d-dmso and
1e-dmso. These results reflect the general reactivity pattern for
migratory insertion reactions into metal−carbon bonds:
namely, a faster insertion for more electron deficient complexes
due to a more effective activation of the olefin.25
Figure 2. Mixed P-aryl-/P-alkyl-substituted Pd(II) complexes 1g-
dmso and 1f-dmso.
Further reactions from the first insertion product 6, such as
β-hydride elimination to methyl crotonate or further MA
insertion to the double-insertion product 7, occur simulta-
neously. Overlapping resonances hamper a detailed kinetic
analysis of these processes. However, the ratio of formation of
methyl crotonate and the double-insertion product 7 can be
compared to determine whether β-hydride elimination is
influenced by electronics. The samples were analyzed after 15
h at 298 K. Under these conditions, all samples still contained
some residual first insertion product 6 (Figure 5). Whereas the
reactivity of this first insertion product for both β-hydride
elimination and further insertion is highest for the electron-
poor complexes 1a-dmso and 1b-dmso, there is no clear
correlation between the tendency for β-hydride elimination and
the electronic nature of the phosphinesulfonato ligand and
complex. A second MA insertion is favored over β-hydride
elimination for all complexes except the 4-chloro-substituted
complex 1b-dmso.
is known that sterics can strongly influence the rate of insertion
and the insertion mode and even reverse the regioselectivity,22
the (κ2-(P∧O)-(2-anisyl)2)phosphinesulfonato Pd(II) complex
1c-dmso is known to clearly favor the electronically preferred
2,1-insertion mode.22a,24c The first 2,1-insertion product 6 can
undergo either β-hydride elimination to methyl crotonate or
insert another MA molecule to form the double-insertion
product 7 (Scheme 3).
The reactions of an excess of MA (ca. 20 equiv) with
complexes 1a−e-dmso ([Pd] = 7 mmol L−1) were monitored
by NMR at 298 K. The first insertion of MA into the
palladium−methyl bond to the 2,1-insertion product 6 was
relatively fast at room temperature for all complexes 1a−e-
dmso and was complete within about 1 h at room temperature
(see the Supporting Information, Figure S5.1). Note that the
observed rate constants are a combination of the preequili-
brium 1a−e-dmso + MA ⇄ 1a−e-MA + dmso and the actual
rate of insertion from 1a−e-MA. Due to overlapping
resonances in some cases, either the decay of the Pd−Me
signal or the decay of proton 6-H (ortho to the SO3 group) was
analyzed. Under these pseudo-first-order conditions, the
electron-poor complex 1a-dmso (4-CF3) undergoes the fastest
A consecutive insertion into product 7 results in two
diastereomers, with characteristic 1H methyl resonances at
around 0.9 ppm. The characteristic chemical shifts of the
aromatic proton 12-H (ortho position of the P-substituted
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Figure 3. ORTEP plots of the Pd(II) complexes [{1a-μLiCl}2], 1e-dmso, and 1f-dmso. Ellipsoids are drawn at the 50% probability level. All
hydrogen atoms and solvent molecules are omitted for clarity. Top right: structure of [{1a-μLiCl}2].
Table 1. Selected Bond Lengths (Å) of Complexes [{1a-
μLiCl}2], [{1d-μLiCl}2], 1e-dmso, and 1f-dmso
Scheme 3. Reaction of Methyl Acrylate with Complexes 1a−
e-dmso to 6, Followed by β-Hydride Elimination or Further
MA Insertion (7)
bond
Pd−P
Pd−O
Pd−C
Pd−Cl/S
[{1a-μLiCl}2]
[{1d-μLiCl}2]
1e-dmso
1f-dmso
2.215(8)
2.214(9)
2.029(2)
2.371(7)
2.224(3)
2.231(8)
2.019(3)
2.399(0)
2.267(1)
2.174(4)
2.039(3)
2.313(9)
2.268(4)
2.151(4)
2.025(7)
2.333(2)
Figure 4. 31P NMR chemical shifts of Pd(II)-dmso complexes 1a−e-
dmso vs Hammett parameter (σp) of the remote substituents.
Figure 5. 1H NMR spectra (400 MHz, CD2Cl2) of the reaction of 1a−
e-dmso with excess methyl acrylate (ca. 20 equiv, [Pd] = 7 mmol L−1)
after 15 h at 298 K.
anisyl moiety) and the γ-proton of complexes 7a,e were
assigned by comparison to reported data.11c A ratio of the two
diastereomers of about 2:1 was found, independent of the
electronic modification (see the Supporting Information, Figure
S5.25).
Polymerization Results. All Pd(II) complexes prepared
are precatalysts for ethylene polymerization at 80 °C (Table 2).
As expected from general reactivity patterns of late-transition-
metal complexes,4a,11b,25b,26,27 the highest activity of 18.4 × 104
(mol of ethylene) (mol of Pd)−1 h−1 (Table 2, entry 2-1) was
observed with the electron-deficient complex 1a-dmso. The
more electron rich complexes 1d-dmso and 1e-dmso exhibit a
maximum activity of about 12.0 × 104 (mol of ethylene) (mol
of Pd)−1 h−1 in a 15 min polymerization experiment (Table 2,
entry 2-10). Polymerization experiments with variable reaction
times as well as mass flow plots (see the Supporting
Information, Figure S6.1−S6.8) also reveal a higher stability
for more electron rich complexes. This may result from the
D
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a
Table 2. Polymerization of Ethylene with Phosphinesulfonato Pd(II) Complexes 1a−g-dmso
b
c
cat.
precursor
time
(min)
yield
(g)
productivity ((g of
polymer)/(mmol of Pd))
TON (104 (mol of E) av TOF (104 (mol of E)
Mn(NMR)
× 103
Mn(GPC)
× 103
M /
wc
Mn
entry
(mol Pd)−1
)
(mol of Pd)−1 h−1
)
2-1
1a-dmso
1a-dmso
1a-dmso
1b-dmso
1b-dmso
1b-dmso
1c-dmso
1c-dmso
1c-dmso
1d-dmso
1d-dmso
1d-dmso
1e-dmso
1e-dmso
1e-dmso
1f-dmso
15
30
60
15
30
60
15
30
60
15
30
60
15
30
60
60
30
30
60
4.50
7.03
9.20
2.13
2.77
3.27
4.09
7.05
10.13
2.95
4.60
8.70
2.32
4.69
7.43
1.80
3.92
1.81
1.19
1288
2016
2632
616
4.6
7.2
9.4
2.2
2.8
3.3
4.2
7.2
10.3
3.0
4.7
8.9
2.4
4.8
7.6
1.8
4.0
1.8
1.2
18.4
14.3
9.4
10.1
10.1
11.2
13.4
14.1
14.6
13.8
14.2
13.5
17.1
17.4
17.5
20.0
19.0
19.6
>100
>100
>100
41.2
7.9
1.9
2.0
1.9
2.3
2.4
2.3
2.2
2.2
2.1
2.3
2.4
2.5
2.5
2.1
2.4
2.2
1.5
n.d.
2.6
2-2
8.1
2-3
8.3
2-4
8.8
13.7
13.2
14.5
9.9
2-5
784
5.6
2-6
924
3.3
2-7
1176
2016
2884
840
16.7
14.4
10.3
12.0
9.4
2-8
9.8
2-9
10.5
14.2
14.1
13.8
14.1
18.4
16.4
122
77.1
n.d.
15.8
2-10
2-11
2-12
2-13
2-14
2-15
2-16
2-17
2-18
2-18
1316
2492
672
8.9
9.5
1344
2128
504
9.6
7.6
1.8
d
1f-dmso
1120
504
8.0
e
1f-dmso
3.6
1g-dmso
336
1.2
a
Reaction conditions: 3.5 μmol of Pd, 100 mL of toluene, 10 bar of ethylene, 80 °C reaction temperature, 250 mL stainless steel reactor.
b
c
d
1
Determined by H NMR spectroscopy at 130 °C in C2D2Cl4. Determined by GPC at 160 °C in 1,2,4-trichlorobenzene vs linear PE. 97 °C
e
reaction temperature. 40 bar ethylene pressure.
different tendencies for reductive elimination from the
intermediately formed Pd hydride complexes, which is an
entry into the most relevant decomposition pathway, as
revealed recently.28 The origin of the relatively low activity of
complex 1b-dmso remains unclear, but it parallels the results of
the stoichiometric MA insertion experiments (vide supra).
A more pronounced trend is found for the molecular weights
of the polyethylenes produced. The influence of electronics,
represented by the Hammett parameter (σp),23 correlates with
the molecular weight of the polyethylenes obtained (Figure 6).
obtained are highly linear, as revealed by high-temperature 13C
NMR spectra, with degrees of branching between one and two
methyl branches per 1000 carbon atoms.
This tendency to produce higher molecular weight poly-
ethylene was further confirmed by studies of 1f-dmso and 1g-
dmso, with an electron-donating alkyl substituent at the
phosphorus. Complex 1g-dmso with the known17 tBu-/Ph-
substituted phosphinesulfonato motif produced polyethylene
with a weight of about 41.2 × 103. In comparison, the polymer
produced by the more electron rich (and more bulky
substituted) complex 1f-dmso shows no quantifiable end
groups in the 1H NMR spectra, which corresponds to a
molecular weight of at least 100 × 103 (Mn(GPC) = 122 × 103)
while the activities are similar (about 104 (mol of ethylene)
(mol of Pd)−1 h−1, as also observed for all other complexes 1a−
e-dmso and 1g-dmso). Note that the activities are not limited
by mass transfer with the mechanically stirred pressure reactor
employed (cf. the Supporting Information)
Copolymerization of Ethylene and Methyl Acrylate.
Complexes 1a−e-dmso are also active in copolymerization of
ethylene and methyl acrylate (Table 3). While the stoichio-
metric MA insertion experiments (vide supra) indicate a faster
MA insertion into electron-poor complexes, in the copoly-
merization experiments, the MA incorporation ratio is
independent of the electronic nature of the phosphinesulfonato
ligand within experimental error (ca. 12 mol % of MA for [MA]
= 0.6 M and p(ethylene) = 5 bar; this corresponds to a relative
reactivity ratio for insertion into the growing polymeryl chain of
ca. 20 in favor of ethylene).29 As expected, MA incorporation
increases with increasing MA concentration in the reaction
mixture. Molecular weights decrease with increasing MA
incorporation, since β-hydride elimination is more pronounced
after the incorporation of an MA repeat unit, as seen by end
group analysis. The ratio of MA vs ethylene derived end groups
increases with higher MA content in the copolymer but does
not correlate with the electronic nature of the catalyst. In
Figure 6. Average molecular weight of polyethylenes vs Hammett
parameter (σp).
That is, the average molecular weight of the samples, as
determined by high-temperature 1H NMR spectroscopy,
increases moderately with more electron donating ability of
the ligands, resulting in a factor of 2 for the 4-OMe substituted
complex 1e-dmso (20.0 × 103) compared to the 4-CF3
substituted complex 1a-dmso (10.1 × 103).
The electronic modifications of the ligand have no obvious
effect on the branching structure of the polymers. All polymers
E
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a
Table 3. Copolymerization of Ethylene and Methyl Acrylate
c
cat.
precursor
MA
(M)
yield
(g)
productivity((g of polymer)/(mmol of
TON
ethylene
TON
b
incorp MA
c
end groups
MA:E
Mn
×
b
entry
Pd))
MA
(mol %)
103
3-1
1a-dmso
1a-dmso
1a-dmso
1b-dmso
1b-dmso
1b-dmso
1c-dmso
1c-dmso
1c-dmso
1d-dmso
1d-dmso
1d-dmso
1e-dmso
1e-dmso
1e-dmso
1f-dmso
1f-dmso
1g-dmso
1g-dmso
0.2
0.6
1.2
0.2
0.6
1.2
0.2
0.6
1.2
0.2
0.6
1.2
0.2
0.6
1.2
0.6
1.2
0.6
1.2
3.42
0.90
0.77
0.54
0.22
0.10
2.08
0.68
0.28
2.73
0.82
0.28
2.07
0.60
0.32
0.45
0.19
0.53
0.33
171
45
38
27
11
5
5600
1200
830
160
130
180
40
3.7
12.2
20.1
5.1
1.6
3.7
8.0
1.8
4.4
42.3
1.4
3.5
5.4
1.0
4.4
5.9
1.4
2.1
5.5
9.6
25.8
3.9
56.8
2.0
2.0
1.5
4.1
2.6
1.0
3.1
2.9
1.5
5.4
2.7
3.9
4.5
3.9
1.5
12.3
5.8
8.7
5.9
3-2
3-3
3-4
860
3-5
290
30
12.5
19.9
5.0
3-6
110
20
3-7
104
34
14
136
41
14
103
30
16
22
10
26
16
3300
900
130
100
70
3-8
12.3
20.8
4.1
3-9
290
3-10
3-11
3-12
3-13
3-14
3-15
3-16
3-17
3-18
3-19
4430
1070
290
150
130
70
12.8
21.0
4.5
3320
790
120
90
12.6
19.4
7.7
360
40
670
40
240
30
14.5
6.1
820
20
460
30
10.5
a
Reaction conditions: 20 μmol of Pd, 50 mL total volume (toluene + MA), 5 bar of ethylene, 200 mg of BHT, 93 °C reaction temperature, 30 min
b
c
reaction time, 250 mL stainless steel reactor. In units of (mol of monomer consumed) (mol of Pd)−1. Determined by H NMR spectroscopy at
1
130 °C in C2D2Cl4.
contrast to ethylene polymerization, the molecular weights of
the copolymers do not correlate that well with the Hammett
parameter of the substituent. This may be due to a larger
number of competing reaction pathways, such as chain growth
by insertion of either olefin or β-hydride elimination after an
acrylate or ethylene repeat unit, respectively, which are all
influenced to some extent by the electronic nature of the ligand.
Additional polymerization experiments with the binuclear
complexes [{1-μMCl}2] after in situ chloride abstraction by
AgBF4 showed the same trends (see the Supporting
Information, Table S7.1).
weight copolymers at reasonable MA incorporation ratios and
activities similar to those of the symmetrically substituted
complexes 1a−e-dmso. Also, they offer additional possibilities
for further electronic and steric modifications. As this study
revealed, the electronic effect of the P-aryl substituents alone is
limited, and combined fine tuning of electronics and especially
sterics is necessary to improve the properties of the
phosphinesulfonato system.
ASSOCIATED CONTENT
■
S
* Supporting Information
The mixed P-aryl/P-alkyl complexes 1f-dmso and 1g-dmso
exhibit similar activities while incorporating less MA than the
symmetrically substituted P-diaryl complexes under identical
conditions (compare e.g. entries 3-16/3-18 and 3-8; estimated
reactivity ratios are 32 and 43 in favor of ethylene vs MA for
1f,g, respectively).29 Both complexes, however, produce
copolymers with significantly higher molecular weights.
Interestingly, the three methoxy groups of complex 1f-dmso
not only promote an increase in molecular weight but also the
MA incorporation is about 50% higher in comparison to the P-
tBu-/PPh-substituted complex 1g-dmso.
Text, figures, tables, and CIF files giving supplemental
polymerization data, crystallographic data, general experimental
procedures, details of the syntheses, additional NMR spectra,
and crystal structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*S.M.: e-mail, stefan.mecking@uni-konstanz.de; fax, +49 7531
88-5152; tel, +49 7531 88-5151.
■
Notes
The authors declare no competing financial interest.
SUMMARY AND CONCLUSION
■
ACKNOWLEDGMENTS
This study of partially novel phosphinesulfonato Pd(II)
complexes with remotely located electron-withdrawing or
-donating substituents shows that electron-rich complexes
produce polyethylene with higher molecular weight at the
expense of activity. More electron rich complexes appear to be
more stable over time under the polymerization conditions.
Stoichiometric insertion experiments with methyl acrylate again
indicate a faster olefin insertion into more electron-deficient
complexes. Copolymerization experiments show no clear
correlation of the acrylate incorporation ratio with the
electronic nature of the ligand. The two mixed P-alkyl-/P-
aryl-substituted complexes 1f,g-dmso produce higher molecular
■
Financial support by the DFG (Me1388/10-1) is gratefully
acknowledged. We thank Lars Bolk for GPC, Anke Friemel and
Ulrich Haunz for support with NMR measurements, and
Hannes Leicht for the synthesis of complex 1g-dmso.
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