Grubbs-Hoveyda type catalysts bearing a dicationic N-heterocyclic carbene for biphasic olefin metathesis reactions in ionic liquids

Article abstract of DOI:10.3762/bjoc.11.178

The novel dicationic metathesis catalyst [(RuCl₂(H2ITapMe₂)(=CH-2-(2-PrO)-C₆H₄))²⁺(OTf^-)₂] (Ru-2, H₂ITapMe₂ = 1,3-bis(2',6'-dimethyl-4'-trimethylammoniumphenyl)-4,5-dihydroimidazol-2-ylidene, OTf- = CF₃SO₃^-) based on a dicationic N-heterocyclic carbene (NHC) ligand was prepared. The reactivity was tested in ring opening metathesis polymerization (ROMP) under biphasic conditions using a nonpolar organic solvent (toluene) and the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [BDMIM⁺][BF₄^-]. The structure of Ru-2 was confirmed by single crystal X-ray analysis.

Full text of DOI:10.3762/bjoc.11.178

Grubbs–Hoveyda type catalysts bearing a dicationic
N-heterocyclic carbene for biphasic olefin metathesis
reactions in ionic liquids
Maximilian Koy1, Hagen J. Altmann1, Benjamin Autenrieth1, Wolfgang Frey2
and Michael R. Buchmeiser*1,3
Full Research Paper
Open Access
Address:
Lehrstuhl für Makromolekulare Stoffe und Faserchemie, Institut für
Beilstein J. Org. Chem. 2015, 11, 1632–1638.
1
doi:10.3762/bjoc.11.178
Polymerchemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569
Stuttgart, Germany, Fax: +49 (0)711-685-64050, 2Institut für
Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55,
D-70569 Stuttgart, Germany and 3Institut für Textilchemie und
Chemiefaser (ITCF) Denkendorf, Körschtalstr. 26, D-73770
Denkendorf, Germany
Received: 14 June 2015
Accepted: 25 August 2015
Published: 15 September 2015
This article is part of the Thematic Series "Progress in metathesis
chemistry II".
Email:
Michael R. Buchmeiser* - michael.buchmeiser@ipoc.uni-stuttgart.de
Guest Editor: K. Grela
*
Corresponding author
© 2015 Koy et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
biphasic catalysis; ionic initiators; recycling; ROMP; ruthenium
Abstract
The novel dicationic metathesis catalyst [(RuCl2(H2ITapMe2)(=CH–2-(2-PrO)-C6H4))2+ (OTf−)2] (Ru-2, H2ITapMe2 =
1
,3-bis(2’,6’-dimethyl-4’-trimethylammoniumphenyl)-4,5-dihydroimidazol-2-ylidene, OTf− = CF3SO3−) based on a dicationic
N-heterocyclic carbene (NHC) ligand was prepared. The reactivity was tested in ring opening metathesis polymerization (ROMP)
under biphasic conditions using a nonpolar organic solvent (toluene) and the ionic liquid (IL) 1-butyl-2,3-dimethylimidazolium
tetrafluoroborate [BDMIM+][BF4−]. The structure of Ru-2 was confirmed by single crystal X-ray analysis.
Introduction
Ionic metathesis catalysts offer access to metathesis reactions in second (organic) phase. This offers access to metathesis reac-
either aqueous solution [1-10] or under biphasic conditions [11- tions in which the products have a low ruthenium contamina-
1
4]. Particularly the latter aspect is of utmost relevance in case tion [11]. Equally important, reactions can be run under
of ionic liquids (ILs) can be used as the phase in which the cata- biphasic, continuous conditions applying supported ionic liquid
lyst is dissolved. The ionic character of both the IL and the phase (SILP) technology [11]. We recently reported on different
ionic catalyst effectively block any crossover of catalyst into the Ru-based ionic metathesis catalysts that can be used for these
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Beilstein J. Org. Chem. 2015, 11, 1632–1638.
purposes. In these systems, the charge is either located directly
at the ruthenium [11,12] or at the 1-methylpyridinium-4-
carboxylate ligands that are introduced via anion metathesis
[
13,14]. These novel catalytic systems have successfully been
used under SILP conditions [11,15]. Furthermore, they allow
running ring-opening metathesis polymerization (ROMP) reac-
tions under biphasic conditions, an approach that offers access
to both ROMP-derived polymers with unprecedented low Ru
contamination (typically 25–80 ppm) and to a regeneration of
the initiator [14]. With all that systems at hand it also became
apparent that reactivity of a certain catalyst strongly depends on
the location of the charge. In principle, ionic Ru-based
metathesis catalysts can also be prepared with the aid of N-hete-
rocyclic carbenes (NHCs) that bear pendant ionic groups
(
Figure 1) [10,16-19]. We addressed that issue by preparing a
novel ionic Ru-NHC-alkylidene using NHCs with ionic groups.
Here we report our results.
Results and Discussion
Catalyst synthesis
We were attracted by NHC ligands containing a diamino func-
tion at the aromatic ring as realized in 1 [20-22] since such
ligands can be permanently quaternized to the corresponding
dicationic species via double alkylation. Additionally, they
Figure 1: Catalysts synthesized by post-assembly tagging (Mes =
mesitylene).
remain structurally closely related to mesitylene-based NHC To ensure the solubility in common organic solvents until the
ligands. Attempts to synthesize ionic Ru-based olefin last step of the synthesis, the neutral precursor Ru-1 [21] was
metathesis catalysts using imidazolinium salts bearing two prepared in an improved one-step synthesis in 82% yield. Quar-
quaternary ammonium groups turned out to be unsuccessful, ternization using 2.05 equiv of methyl trifluoromethanesul-
probably due to their insolubility in common organic solvents. fonate gave the dicationic ruthenium alkylidene Ru-2 in 90%
However, quaternization of RuCl2(H2ITap)(=CH–(2-(2-PrO- isolated yield (Scheme 1). This is to the best of our knowledge
C6H4))) (Ru-1, Scheme 1) [21] turned out to be successful.
the first example of a ruthenium alkylidene bearing an NHC
Scheme 1: Improved synthesis of Ru-1 and quarternization with methyl trifluoromethanesulfonate to Ru-2.
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Beilstein J. Org. Chem. 2015, 11, 1632–1638.
ligand with permanent dicationic charge. Crystals of Ru-2
suitable for single-crystal X-ray analysis were obtained from
DMF/diethyl ether. Catalyst Ru-2 crystallizes in the triclinic
Table 1: Selected bond lengths (pm) for Ru-2 and Ru-1 [21].
Ru-2 Ru-1
198.2(4)
space group
, a = 1398.08(6) pm, b = 1399.41(7) pm, c =
Ru1–C1
Ru1–C29
Ru1–O1
Ru1–Cl1
Ru1–Cl2
196.6(7)
173.5(9)
226.0(5)
233.0(2)
233.9(2)
1
9
750.26(13) pm, α = 106.079(3)°, β = 112.209(3)°, γ =
182.8(4)
225.2(3)
9.300(2)°, Z = 2 (Figure 2, Supporting Information File 1).
233.89(12)
233.50(12)
Biphasic ring-opening metathesis polymer-
ization (ROMP) reactions
To test the reactivity of Ru-2, various ROMP reactions were
run under biphasic conditions using [BDMIM+][BF4−] [24] as
IL and toluene as the organic phase. The structure of monomers
M1–M6 that were used are shown in Figure 3 [14]. Results are
summarized in Table 2. At this point it is worth stressing that
the purity of the IL used is of utmost importance, the more since
imidazolium-based ILs can contain substantial amounts of free
base [25], which in turn can negatively affect catalyst perfor-
mance.
Figure 2: Single crystal X-ray structure of Ru-2. Co-solvent and disor-
dered triflates have been omitted for clarity.
Selected bond lengths are summarized in Table 1. For purposes
of comparison, the corresponding distances of the parent system
Ru-1 are provided, too. As can be seen, the dicationic charge
does influence the binding situation in Ru-2, though not
dramatically. Interestingly only a slight increase in the
Figure 3: Monomers used for biphasic ROMP reactions.
Ru–NHC bond length is observed, accompanied by a very As can be seen, M1–M6 can be polymerized via ROMP under
minor decrease in the Ru–O bond. The Ru–Cl bonds remain biphasic conditions in good yields, except for M6. This low
unaffected. The most dramatic effect is observed in the yield is attributed to the comparable low ring strain in M6.
Ru–alkylidene bond, which is about 9 pm longer in Ru-2 than Polydispersity indices (PDIs) were in the range of 1.2 to 3.8.
in Ru-1. This increase in the alkylidene’s length points towards Together with the high molecular weights, this is indicative for
a substantially reduced polarization of the Ru=C bond and substantial chain transfer and potentially incomplete initiation
accounts for a reduced activity of Ru-2 compared to standard of the Ru–alkylidene, particularly with unsubstituted
Grubbs- and Grubbs–Hoveyda catalysts. Thus, Ru-2 delivers norbornene (M1) and cis-cyclooctene (M6) but also with M4.
only turn-over numbers well below 100 in the biphasic ring- However, in turn it allows for the synthesis of high molecular
closing metathesis (RCM) of 1,7-octadiene, diethyl diallyl- weight polymers. The most striking feature, however, is related
malonate and N,N-diallyl p-toluoenesulfonamide using to the Ru content of the resulting polymers, which were all
[
BDMIM+][BF4−] as IL and toluene as the organic phase (see obtained as white powders. Unlike in many other Ru–alkyl-
Supporting Information File 1). It is thus also in line with the idene-triggered metathesis-based polymerizations, Ru contami-
fact that Ru–alkylidenes based on electron-rich NHCs, e.g., nation was very low (<2.5 ppm) and even outrivals earlier
based on tetrahydropyrimidin-2-ylidenes [23], strongly promote reported systems bearing two pyridinium carboxylates by at
olefin metathesis.
least a factor of 10 [14]. Clearly, both the initiator and any
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Beilstein J. Org. Chem. 2015, 11, 1632–1638.
Table 2: ROMP reactions under biphasic conditions.a,b
Monomer
T [°C]
Time [h]
Yield [%]c
Mtheo
Mn
PDId
[g/mol]
[g/mol]d
M1
M2
M3
M4
M5
M6
50
50
50
50
50
70
2
2
2
2
1
3
93
89
6,600
14,700
16,800
20,600
6,500
258,000
94,500
15,800
907,000
–e
3.8
2.3
1.2
2.8
–e
80
86
100
36
7,700
186,000
1.50
aRu-2, toluene, [BDMIM+][BF4−], 50−70 °C, 1−3 h. bRu content (measured by ICP–OES) was lower than the limit of detection, which allows for calcu-
lating a Ru content <2.5 ppm. cDetermined after precipitation in methanol. dMeasured by GPC in THF. eInsoluble because of crosslinking.
Ru-containing decomposition products selectively stay in the IL
Table 3: ROMP of M1 under biphasic conditions with recycling.a
phase while after termination, the polymer stays selectively in
the organic (toluene) phase. Notably, polymers were simply
precipitated from methanol and not subjected to any further
purification steps. We believe that particularly for biomedical
applications such virtually Ru-free polymers will be of utmost
interest.
Cycle
Yield [%]b
Mn [g/mol]c
PDIc
1
2
3
89
83
75
1,340,000
230,000
120,000
2.1
3.0
3.0
a1. Ru-2, M1, toluene, [BDMIM+][BF4−], 50 °C, 1.5 h; 2. 2-(2-PrO)-
styrene, toluene, 50 °C, 1 h; 3. M1, toluene, 50 °C, 1.5 h; following
cycles: repeat 2 + 3; last cycle: quenched with ethyl vinyl ether.
bDetermined after precipitation in methanol. cMeasured by GPC in
THF, Mn, theor: 6,600 g/mol.
Recycling experiments carried out with M1 revealed that with
the aid of 2-(2-PrO)-styrene, Ru-2 could be used in three
consecutive cycles (Scheme 2). Over these three cycles, the
number-average molecular weight, Mn, significantly decreased
while PDIs increased from 2.1 to 3.0. Again, Ru leaching into solvent. While Ru-2 showed low RCM activity, it turned out to
the product was below the limit of detection, i.e., <2.5 ppm. The be active in ROMP reactions of strained cyclic olefins like
results are summarized in Table 3.
norbornenes, 7-oxanorbornenes, norbornadiene and cis-
cyclooctene allowing for the synthesis of the corresponding
polymers with unprecedented low metal contamination
(
<2.5 ppm) without any additional purification steps.
Experimental
General: Unless noted otherwise, all manipulations were
performed in a Labmaster 130 glovebox (MBraun; Garching,
Germany) or by standard Schlenk techniques under N2 atmos-
phere. CH2Cl2 and toluene were purchased from J. T. Baker
(
Devender, Netherlands) and were dried by using an MBraun
Scheme 2: Recycling of Ru-2 for continuous ROMP reactions.
SPS-800 solvent purification system. Hexane was purchased
from VWR and distilled from sodium/benzophenone under N2.
Starting materials were purchased from ABCR, Aldrich, Alfa
Conclusion
The first dicationic Ru–alkylidene catalyst based on an N-hete- Aesar, Fluka and TCI Europe and used without further purifica-
rocyclic carbene bearing two quaternary ammonium groups, tion. KOCMe2Et was purchased from Alfa Aesar as a 25 wt %
[
(
(RuCl2(H2ITapMe2)(=CH–2-(2-PrO)-C6H4))2+ (OTf−)2] solution in toluene. Toluene was co-evaporated with pentane in
Ru-2), was prepared from the neutral precursor vacuo prior to use.
RuCl2(H2ITap)(=CH–2-(2-PrO)-C6H4)) (Ru-1) and methyl
trifluoromethanesulfonate. Also, an improved, high-yield syn- NMR spectra were recorded on a Bruker Avance III 400 spec-
thesis of Ru-1 has been presented. Ru-2 was tested for trometer in the indicated solvent at 25 °C and are listed in parts
its reactivity in ROMP under biphasic conditions using per million downfield from tetramethylsilane as an internal
[
BDMIM+][BF4−] as the ionic liquid and toluene as the organic standard. IR spectra were measured on a Bruker ATR/FT-IR
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Beilstein J. Org. Chem. 2015, 11, 1632–1638.
IFS 128. GPC measurements were carried out on a system, Hz), 76.4 (CH(CH3)2), 57.9 (N(CH3)3), 52.6, 21.9, 20.3
consisting of a Waters 515 HPLC pump, a Waters 2707 (CH(CH3)2, Ar-CH3); 19F NMR (DMF-d7) δ −78.5; FTIR
autosampler, Polypore columns (300 × 7.5 mm, Agilent tech- (ATR, cm−1) : 1589 (s), 1491 (m), 1251 (m), 1152 (m), 1114
nologies, Böblingen, Germany), a Waters 2489 UV–vis and a (m), 1028 (s), 923 (m), 840 (s), 801 (s), 754 (s), 637 (s), 572 (s),
Waters 2414 refractive index detector. For calibration, poly- 517 (s) cm-1; MS (ESI) m/z: calcd. for C35H50Cl2N4ORu (dica-
styrene standards with 800 < Mn < 2,000,000 g/mol were used. tion, z = 2): 357.1199, found: 357.1214; m/z calcd. for
ICP–OES measurements were carried out using a Spectro Acros C36H50F3Cl2N4O4RuS: 863.1923, found: 863.1905. Crystals
device (Ametek GmbH; Meerbusch, Germany). Calibration was suitable for X-ray diffraction were obtained by layering diethyl
done with Ru standards containing 0.1, 0.5, 1.0 and 5.0 ppm. ether over a solution of Ru-2 in anhydrous DMF.
Mass spectra were recorded on a Bruker Daltonics Microtof Q
mass spectrometer at the Institute of Organic Chemistry at the General ROMP-procedure: Ru-2 (5.6 mg, 5 µmol or
University of Stuttgart. 1,3-Bis(2,6-dimethyl-4-dimethylamino- 11.13 mg, 10 µmol) and [BDMIM+][BF4−] (400 mg) were
phenyl)-4,5-dihydroimidazol-2-ylidene (1) [21], 2-(2-PrO)- placed inside a flame-dried Schlenk tube (25 mL) equipped
styrene [23,24], M3 [26] and M4 [27] were prepared according with a magnetic stir bar. The reaction mixture was heated to the
to the literature.
indicated temperature. The monomer (350 µmol or 700 µmol)
and toluene (2 mL) were added to a separate flame-dried
RuCl2(H2ITap)(=CH-2-(2-PrO)-C6H4-O) (Ru-1) [27,28]: Schlenk tube. The monomer solution was added via syringe in
Inside a glovebox, 1 (281 mg, 0.70 mmol), KOCMe2Et (88 mg, one portion and the reaction mixture was allowed to stir at the
0
.70 mmol) and hexane (9 mL) were added to a 50 mL Schlenk indicated temperature for the indicated time. After cooling to
flask equipped with a magnetic stir bar. The reaction mixture room temperature, ethyl vinyl ether (1 mL) was added and the
was allowed to stir for 1 h at room temperature, during which reaction mixture was allowed to stir for another 30 min. Finally,
time a brownish orange suspension formed. The 1st-generation the reaction mixture was poured into methanol. The polymer
Grubbs–Hoveyda catalyst (400 mg, 0.67 mmol) dissolved in was obtained as a white or off-white solid.
hexane (6 mL) was added to the reaction mixture. The reaction
mixture was removed from the glovebox and was heated to General ROMP-procedure with recycling: Ru-2 (11.1 mg,
6
0 °C for 4 h. The formation of a green solid was observed. 10 µmol) and [BDMIM+][BF4−] (400 mg) were placed inside a
After cooling to room temperature, all solids were filtered flame-dried Schlenk tube (25 mL) equipped with a magnetic stir
off and washed with pentane (3 × 10 mL) and diethyl ether bar. The reaction mixture was heated to the indicated tempera-
(
3 × 10 mL); then the product was redissolved in CH2Cl2. ture. M1 (65.9 mg, 700 µmol) and toluene (5 mL) were added
Purification was accomplished by chromatography using silica to a separate flame-dried Schlenk tube. The monomer solution
G60 and CH2Cl2/hexane. Drying in vacuo gave the product as a was added via syringe in one portion and the reaction mixture
dark green solid (375 mg, 0.55 mmol, 82%). Analytical data was allowed to stir at 50 °C for 1.5 h. Then a solution of 2-(2-
were in accordance with the literature [20].
PrO)-styrene in anhydrous toluene (1 mL, 1 M) was added. The
reaction mixture was stirred at 50 °C for 1 h. The two phases
[
(
(RuCl2(H2ITapMe2)(=CH–2-(2-PrO)-C6H4))2+ (OTf-)2] were allowed to separate. The organic phase was poured into
Ru-2): At −30 °C, methyl trifluormethanesulfonate (99 mg, methanol. The IL phase was extracted with toluene (4 × 2 mL).
01 µmol) dissolved in CH2Cl2 (3 mL) was added to Ru-1 The extracted organic phases were also poured into methanol.
200.5 mg, 293 µmol) dissolved in CH2Cl2 (5 mL). The mix- Poly-M1 was obtained as a white solid. New M1 was added to
6
(
ture was stirred for 18 h at room temperature and then, CH2Cl2 the IL phase and the procedure was repeated. After the last
was removed under reduced pressure. The residue was washed cycle, the reaction was quenched with ethyl vinyl ether (1 mL).
with CH2Cl2 (3 × 3 mL) and ethyl acetate (3 × 3 mL), allowing
for the isolation of the target compound as a light-green solid ICP–OES measurements: The corresponding polymer (20 mg)
(
267 mg, 264 µmol, 90%). 1H NMR (DMF-d7) δ 16.47 (s, 1H, was added to high-pressure Teflon tubes. Digestion was
Ru=CH), 8.17 (s, 4H, NHC-Ar), 7.70–7.66 (m, 1H, C6H4), 7.20 performed under microwave conditions using aqua regia
d, J = 8.4 Hz, 1H, C6H4), 7.06–7.04 (m, 1H, C6H4), 6.96 (t, J = (10 mL). The mixture was cooled to room temperature, diluted
(
7
4
.4 Hz, 1H, C6H4), 5.10 (hept, J = 7.4 Hz, 1H, O-CH-(CH3)2), with deionized water (approx. 40 mL), filtered and subjected to
.42 (s, 4H, N-CH2), 4.00 (s, 18H, N-(CH3)3), 2.66 (s, 12H, ICP–OES for Ru with λ = 240.272 nm ion line and background
NHC-Ar-CH3), 1.27 (d, J = 6.1 Hz, 6H, O-CH-CH3); 13C NMR lines at λ1 = 240.254 nm and λ2 = 240.295 nm.
DMF-d7) δ 292.9 (Ru=CH), 211.6 (N-C=N), 153.5, 148.7,
45.9, 143.1, 141.2, 131.3, 123.6, 123.5, 121.8, 114.5 (C6H2, Poly-M1: 1H NMR (CDCl3) δ 5.35 (s, 1H), 5.21 (s, 1H), 2.79
C6H4), 127.3, 124.1, 120.9, 117.7 (CF3-SO3−, q, JC-F = 322.5 (bs, 1H), 2.44 (bs, 1H), 1.88–1.79 (2bs, 3H), 1.35 (bs, 2H),
(
1
1636

Beilstein J. Org. Chem. 2015, 11, 1632–1638.
1
1
3
.09–1.02 (m, 2H); 13C NMR (CDCl3) δ 134.1, 134.0, 133.9, References
33.3, 133.2, 133.0, 43.6, 43.3, 42.9, 42.2, 41.5, 38.8, 38.6, 1. Gallivan, J. P.; Jordan, J. P.; Grubbs, R. H. Tetrahedron Lett. 2005, 46,
2
577–2580. doi:10.1016/j.tetlet.2005.02.096
3.3, 33.1, 32.5, 32.4; FTIR (ATR, cm−1) : 2941 (s), 2863
2
.
.
Hong, S. H.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 3508–3509.
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(
m), 1446 (w), 1260 (s), 1189 (w), 1081 (s), 1020 (s), 965 (s),
62 (w), 798 (s), 737 (m), 688 (w).
8
3
Jordan, J. P.; Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 119,
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6
1
4
H), 3.13–2.85 (4bs, 4H), 1.89 (2bs, 2H); 13C NMR (CDCl3) δ
72.9, 172.5, 131.9, 131.1, 130.5, 51.7, 51.6, 51.5, 51.4, 51.1,
4.5, 39.2, 38.8, 38.1; FTIR (ATR, cm−1) : 3020 (w), 2951
5
.
.
1
6
Lynn, D. M.; Mohr, B.; Grubbs, R. H.
Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 1998, 39,
278–279.
(
(
w), 1726 (s), 1434 (m), 1386 (w), 1347 (w), 1194 (m), 1169
m), 1153 (m), 1095 (w), 1041 (w), 978 (w), 957 (w), 807 (w),
7
47 (m), 666 (w), 603 (w).
7. Lynn, D. M.; Mohr, B.; Grubbs, R. H.; Henling, L. M.; Day, M. W.
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8
.
.
Mohr, B.; Lynn, D. M.; Grubbs, R. H. Organometallics 1996, 15,
317–4325. doi:10.1021/om9603373
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978–3985. doi:10.1021/ma9701595
Poly-M3: 1H NMR (CDCl3) δ 5.74 (bs, 1H), 5.56 (bs, 1H),
4
4
2
1
4
2
1
.59–4.49 (2bs, 1H), 4.17–4.11 (2bs, 5H), 2.39 (bs, 2H),
9
.04–2.02 (2s, 6H); 13C NMR (CDCl3) δ 170.8, 170.7, 133.3,
3
32.9, 132.4, 131.8, 81.5, 81.2, 62.0, 61.9, 61.8, 46.5, 46.2, 10.Klučiar, M.; Grela, K.; Mauduit, M. Dalton Trans. 2013, 42, 7354–7358.
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6.1, 45.9, 45.6, 21.0, 20.9; FTIR (ATR, cm−1) : 3017 (w),
1
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958 (w), 2902 (w), 1733 (s), 1468 (w), 1434 (w), 1389 (w),
366 (m), 1221 (s), 1119 (w), 1030 (s), 968 (m), 834 (w), 750
1
1
2.Autenrieth, B.; Anderson, E. B.; Wang, D.; Buchmeiser, M. R.
Macromol. Chem. Phys. 2013, 214, 33–40.
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(
m), 667 (w), 604 (m), 506 (w), 471 (w).
Poly-M4: 1H NMR (CDCl3) δ 5.27–5.17 (2bs, 2H), 3.40–3.35
2bs, 8H), 2.69 (bs, 1H), 2.32 (bs, 1H), 1.96 (bs, 3H), 1.55 (bs,
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4
1
4
3
1
H), 1.32–1.13 (2bs, 10H), 0.90 (bs, 6H); 13C NMR (CDCl3) δ
34.0, 133.8, 71.3, 71.2, 70.8, 70.6, 48.1, 47.8, 47.6, 47.0, 45.6,
1
5.3, 41.2, 40.1, 29.7, 28.7, 22.7, 14.2; FTIR (ATR, cm−1)
:
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Acknowledgements
Financial support provided by the Deutsche Forschungsgemein-
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