Oxidation-triggered ring-opening metathesis polymerization

Article abstract of DOI:10.1002/chem.201300868

Eight new N-Hoveyda-type complexes were synthesized in yields of 67-92 % through reaction of [RuCl₂(NHC)(Ind)(py)] (NHC=1,3-bis(2,4,6- trimethylphenylimidazolin)-2-ylidene (SIMes) or 1,3-bis(2,6- diisopropylphenylimidazolin)-2-ylidene (SIPr), I

Full text of DOI:10.1002/chem.201300868

FULL PAPER
Latent Catalysts
R. Savka, S. Foro, M. Gallei,
M. Rehahn, H. Plenio* ...... &&&& — &&&&
Oxidation-Triggered Ring-Opening
Metathesis Polymerization
Turn me on: The iron-centered oxida-
tion of ferrocenyl-substituted ruthe-
nium-based olefin metathesis precata-
lysts by chemical oxidants or by an
electrode at the oxidizing potential
converts a latent catalyst into a catalyt-
ically active complex for ROMP reac-
tions (see scheme).
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DOI: 10.1002/chem.201300868
Oxidation-Triggered Ring-Opening Metathesis Polymerization
Roman Savka,[a] Sabine Foro,[c] Markus Gallei,[b] Matthias Rehahn,[b] and
Herbert Plenio*[a]
Abstract: Eight new N-Hoveyda-type
complexes were synthesized in yields
of 67–92% through reaction of [RuCl2-
centered oxidation reaction occurs at
potentials close to E= +0.5 V. The
crystal structures of the reduced and of
the respective oxidized Hoveyda-type
complexes were determined and show
that the oxidation of the ferrocene unit
has little effect on the ruthenium envi-
ronment. Two of the eight new com-
plexes were found to be switchable cat-
alysts, in that the reduced form is inac-
tive in the ring-opening metathesis pol-
ymerization of cis-cyclooctene (COE),
whereas the oxidized complexes pro-
duce polyCOE. The other complexes
are not switchable catalysts and are
either inactive or active in both re-
duced and oxidized states.
ACHTUNGTRENNUNG(NHC)ACHTUNGTRENNUNG(Ind)(py)] (NHC=1,3-bis(2,4,6-
trimethylphenylimidazolin)-2-ylidene
(SIMes) or 1,3-bis(2,6-diisopropylphe-
nylimidazolin)-2-ylidene (SIPr), Ind=
3-phenylindenylid-1-ene, py=pyridine)
with various 1- or 1,2-substituted ferro-
cene compounds with vinyl and amine
or imine substituents. The redox poten-
tials of the respective complexes were
determined; in all complexes an iron-
Keywords: iron · olefin metathesis ·
oxidation · polymerization · ring-
opening metathesis polymerization ·
ruthenium
Introduction
Latent or switchable catalysts require physical or chemical
stimuli to convert a catalytically inactive species into an
active one.[1] This principle has been put to good use in poly-
mer chemistry since it allows spatial and temporal control
over polymerization reactions.[2,3] Various stimuli, such as
light-induced catalyst activation, have been employed in
ring-opening polymerization[4] and ring-opening metathesis
polymerization (ROMP) reactions (Scheme 1).[5] Buchmeis-
er et al. reported a ROMP catalyst that is activated upon ir-
radiation with UV light.[6] Later, Grubbs et al. described an
alternative approach using a pH-responsive catalyst that, in
combination with a photo acid, also turns out to be a photo-
Scheme 1. Light- or redox-switchable polymerization catalysts (tren=
tris[2-(dimethylamino)ethyl]amine).
sensitive ROMP catalyst.[7] The same group reported a dif-
ferent pH-sensitive catalyst in which an NHC ligand bound
to ruthenium is decomposed by protons resulting in catalyst
activation.[8] Slugovc et al. reported on pyridine/chloride-in-
duced ROMP reactions.[9]
[a] M. Sc. R. Savka, Prof. Dr. H. Plenio
Organometallic Chemistry, Technische Universitꢁt Darmstadt
Petersenstrasse 18, 64287 Darmstadt (Germany)
E-mail: plenio@tu-darmstadt.de
[b] Dr. M. Gallei, Prof. Dr. M. Rehahn
Ernst–Berl Institute for Chemical Engineering
and Macromolecular Science
The modulation of ligand donor ability by the protonation
of basic groups attached to a ligand enables control over the
stereochemistry of ROMP polymers because the electron
density at the catalytic site has been shown to have an influ-
ence on the E/Z ratio of polynorbornenes.[10] Alternatively,
activation of olefin metathesis catalysts is also possible by
utilizing ultrasound-induced shear forces.[11] Recently, Maty-
jaszewski et al. demonstrated redox-controlled atom-transfer
radical polymerization (ATRP) by electrochemically shut-
tling between a deactivated Cu2+ and an activated Cu+
state.[12] Ferrocene-based redox switches[13] attached to a cat-
alytically active complex modulate the electron donation of
Technische Universitꢁt Darmstadt
Petersenstrasse 22, 64287 Darmstadt (Germany)
[c] S. Foro
Clemens–Schçpf-Institut for Organic Chemistry and Biochemistry
Technische Universitꢁt Darmstadt
Petersenstrasse 22, 64287 Darmstadt (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300868. It contains the gen-
eral experimental details, ROMP procedures, and additional experi-
mental data, including NMR spectra, cyclic voltammograms, SEC
traces, mass spectra, and crystal structure analyses.
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Ring-Opening Metathesis Polymerization
FULL PAPER
the respective ligand, which then oscillates between a high-
and low-activity polymerization catalyst state.[14] In another
example, ring-closing metathesis reactions were controlled
through a redox-switched solubility change since the oxida-
tion of a ferrocene-tagged olefin metathesis catalyst led to
the precipitation of the salt from the nonpolar solvent, fol-
lowed by redissolution upon reduction of the catalyst.[15]
Herein, we demonstrate oxidation-activated catalysts for
ROMP reactions employing chemical, as well as electro-
chemical, stimuli.
Results and Discussion
Synthesis of ligands and Hoveyda-type ruthenium com-
plexes: The synthesis of new ferrocene-containing ligands is
described in Scheme 2. The bidentate ligands are character-
ized by a nitrogen donor and a vinyl group bonded to a fer-
rocene group. The nitrogen donor is either directly attached
to the ferrocene unit (5, 6, and 7) or in conjugation with the
electrochemically active group (3) to allow a large change in
the donor ability of this nitrogen donor upon oxidation of
the ferrocene unit.[16] The vinyl group is required for the for-
mation of a carbene complex
Scheme 2. Synthesis of ferrocenyl-based ligands (Fc=ferrocenyl; Mes=
mesityl). Reagents and conditions: a) 2,4,6-trimethylaniline, TsOH (Ts=
tosyl), toluene, reflux, overnight; b) nBuLi, THF, À788C, 1 h then DMF,
À788C to RT; c) MePPh3I, KOtBu, THF, À108C to RT, overnight; d) mo-
lecular sieves, CH2Cl2, RT; e) molecular sieves, toluene, 1008C.
with a suitable ruthenium pre-
cursor complex.
The reactions of the new li-
gands, 3, 5, 6, and 7, with the
commercially available [RuCl2-
ACHTUNGTRENNUNG(NHC)ACHTUNGTRENNUNG(Ind)(py)] (Ind=3-phe-
nylindenylid-1-ene; py=pyri-
dine; NHC=1,3-bis(2,4,6-trime-
thylphenylimidazolin)-2-ylidene
(SIMes) or 1,3-bis(2,6-diisopro-
pylphenylimidazolin)-2-ylidene
(SIPr)) result in the facile for-
mation of the respective Hov-
eyda-type complexes in yields
of 67–92% (Scheme 3). All
complexes were characterized
by NMR spectroscopy and
mass spectrometry. Complexes
with organometallic fragments
in the benzylidene ligand are
rare, although recently a Hov-
eyda-type complex with
a
{Cr(CO)3} unit was reported.[17]
Complexes, closely related to 8,
9, 10, and 11, with phenyl or
1,2-benzenediyl groups instead
of ferrocenyl groups, have been
reported in the literature before
and several were shown to be
latent catalysts in ROMP or
ring-closing metathesis (RCM)
reactions.[18]
Scheme 3. Synthesis of the new ferrocenyl-substituted Hoveyda complexes 8a, 8b, 9a, 9b, 10a, 10b, 11a, and
11b (yields given in brackets), and the structure of the known complex 10c.
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H. Plenio et al.
The complexes 8–11 can be oxidized, thereby converting
an electron-donating ferrocene into an electron-withdrawing
ferrocenium unit. To better understand the properties of the
oxidized species, complexes 11a and 11b were reacted with
the oxidation of RuII/III and of FeII/III. The similarity of the
redox potentials in the series of complexes studied raised
the question of whether the observed redox events are iron
or ruthenium centered. On comparing the redox potentials
of iron in nitrogen-substituted ferrocenes (typically close to
E=0.0 V)[19] with those of ruthenium in Grubbs–Hoveyda-
type complexes (E= +0.45–+1.1 V)[20] the redox potentials
observed in the Hoveyda-type complexes 8–11 appear to be
closer to the RuII/III potentials than to the typical FeII/III po-
tentials. However, it is known that, upon coordination of
metal ions to donor atoms located in the vicinity to the fer-
rocene unit, the FeII/III redox potentials can be shifted anodi-
cally by several hundred millivolts.[21] To firmly assign the
iron- or ruthenium-centered nature of the oxidation reac-
tion, the redox potential of the closely related and known[18c]
complex 10c, in which the ferrocenyl group is replaced by a
phenyl group, was determined. Based on the oxidation
curve, the RuII/III redox potential for complex 10c was esti-
mated as approximately 0.75 V.[22] This is clearly higher than
the FeII/III redox potentials reported for complexes 8–11, but
is a typical RuII/III value for such complexes.[20a] Based on
this experiment, the redox potentials reported in Table 1
À
acetyl ferrocenium BF4 to afford 11a+ and 11b+ in good
yields (Scheme 4). The oxidized complexes are stable spe-
Scheme 4. Synthesis of oxidized ferrocenyl-substituted Hoveyda com-
plexes.
cies and single crystals of 11a+ were obtained without spe-
cial precautions. The NMR signals of the paramagnetic com-
plexes 11a+ and 11b+ are broadened (especially for the
protons close to the paramagnetic iron), but nonetheless the
benzylidene proton can be identified at d=16.2 (11a+) and
16.9 ppm (11b+), which corresponds to a nearly 2 ppm shift
of this resonance in comparison with the neutral complexes
11a and 11b. The identification of the oxidized complexes is
based on high-resolution mass spectra and a crystal structure
analysis of 11a+.
correspond to FeII/III
.
X-ray crystal structure analysis of ferrocenyl-substituted
Hoveyda-type complexes: The crystal structures[23] of com-
plexes 8a, 11a, and of the oxidized complex 11a+ were de-
termined (Figures 1, 2, and 3). Selected bond lengths and
Determination of redox potentials: The redox potentials for
the eight new complexes, 8–11, were determined (Table 1).
Table 1. Redox potentials of complexes 8–11.[a]
Complex
Redox potential [V]
(EaÀEc [mV])
AHCTUNGTRENNUNG
8a
8b
9a
9b
10a
10b
11a
11b
10c
0.518 (64)
0.513 (64)
0.483 (96)
0.511 (78)
0.496 (81)
0.482 (68)
0.501 (72)
0.485 (62)
0.783 (–)[b]
Figure 1. X-ray crystal structure of complex 8a. Selected bond lengths
o
À
À
[pm] and angles [ ]: Ru=CH 181.3(1), Ru C
(NHC) 202.6(11), Ru Cl
À
À
237.0(3), 237.8(3), Ru N 217.6(8), Fe C average 203.1; Cl-Ru-Cl
173.1(3), N-Ru-C(NHC) 168.8(3). CCDC-916243 contains the supple-
AHCTUNGTRENNUNG
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
[a] Solvent: CH2Cl2, scan rate: 100 mVsÀ1, supporting electrolyte: 0.1m
NBu4PF6, potentials versus FcMe8: E1/2 =À0.01 V. [b] Oxidation potential
only, irreversible oxidation.
angles are summarized in the legend of the respective fig-
ures. In all complexes studied, ruthenium displays a nearly
square-pyramidal coordination geometry, which is typical
for such complexes.[24] Complex 8a crystallizes as a racemic
mixture of the two planar chiral isomers. In the crystal of
8a, the two chloro ligands are located trans to each other;
there is no evidence (NOESY NMR spectroscopy) that a
different structure (cis-chloro) occurs in solution. On com-
For all complexes a single reversible redox event is observed
and all potentials are located in a narrow range of 0.48–
0.52 V. This is not surprising for the structurally closely re-
lated complexes 10a, 10b, 11a, and 11b, but is unexpected
for the four complexes 8a, 8b, 9a and 9b. In principle, two
redox reactions have to be considered in such complexes:
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Ring-Opening Metathesis Polymerization
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cative of iron-centered oxidation of the bimetallic com-
plexes. The eclipsed orientation of the two cyclopentadienyl
rings in 11a+ is as normal as the staggered orientation of
those rings in the neutral complex 11a.[25] These structural
changes reconfirm the iron-centered nature of the electro-
chemical redox processes.
Redox-switched ring-opening metathesis polymerization:
The five complexes 8a, 9a, 9b, 10a, and 11a were evaluated
in the ROMP of cis-cyclooctene, which is a popular mono-
mer in ROMP reactions (Scheme 5).[26] To be useful as
Figure 2. X-ray crystal structure of complex 11a. Selected bond lengths
o
À
À
[pm] and angles [ ]: Ru=CH 182.6(3), Ru C
(NHC) 207.0(3), Ru Cl
À
À
237.1(1), 237.5(1), Ru N 213.1(3), Fe C average 204.6; Cl-Ru-Cl
168.2(3), N-Ru-C(NHC) 170.1(1). CCDC-916235 contains the supple-
ACHTUNGTRENNUNG
Scheme 5. ROMP reaction of cis-cyclooctene.
mentary crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
switched catalysts, the complexes need to be latent catalysts
in the reduced state and display sufficient polymerization
activity in the oxidized state.
Complexes 9a and 10a were found to rapidly polymerize
cis-cyclooctene at room temperature and cannot be consid-
ered to be latent. The respective SIPr-based complexes 9b
and 10b initiate the reaction more slowly, but still show sig-
nificant ROMP activity with cis-cyclooctene under the same
reaction conditions and thus are not suitable for switched
ROMP reactions (Figure 4). The nonlatent behavior of com-
Figure 3. X-ray crystal structure of oxidized complex 11a+ (cation only,
À
BF4 omitted). Selected bond lengths [pm] and angles [o]: Ru=CH
À
À
À
182.8(8), Ru C
G
À
213.2(6), Fe C average 207.5; Cl-Ru-Cl 161.5(1), N-Ru-C
170.8(1). CCDC-916218 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
paring the bond parameters of 8a to those of the related
complex by Slugovc et al.,[18a] which has a 1,2-phenylendiyl
group instead of the ferrocene group, only two significant
Figure 4. Conversion–time data for the ROMP reaction of cis-cyclooctene
(0.2 molLÀ1
)
at 208C in toluene/CH2Cl2 with complexes 9a
(
*
;
&
0.1 mol%) and 10a ( ; 0.1 mol%).
À
differences are found: the Ru N bond in 8a is almost 10 pm
longer, and the Ru=CH bond in 8a is approximately 5 pm
shorter. The crystal structures of 11a and 11a+ were of par-
ticular interest in order to determine whether oxidation
leads to characteristic changes in the structure of the oxi-
dized complex compared to the neutral species (11a). On
comparing the geometric parameters around ruthenium,
only minor changes are seen; the iron-centered oxidation
plex 10a is closely related to that of 10c (Ph instead of Fc),
which displays room temperature activity in the ROMP of
dicyclopentadiene.[18c] The nonlatency of complexes 9 recon-
firms the previous observation that five-membered-ring
Hoveyda-type chelates initiate reactions much faster than
six-membered-ring chelates.[18a] In this vain, complex 11a
shows negligible ROMP activity at room temperature; un-
fortunately the same holds true for the oxidized complex
11a+. This is in line with the behavior of the related com-
À
appears to have little influence. The Fe C bond lengths in
11a+ show modest elongation (11a Fe C average=
À
204.6 pm and 11a+ Fe C average=207.5 pm), which is indi-
À
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H. Plenio et al.
Figure 5. Conversion–time data for the ROMP reaction of cis-cyclooctene
(0.2 molLÀ1) at 208C in toluene/CH2Cl2 (12:1) with the complexes 8a
Figure 6. Conversion–time data for the ROMP reaction of cis-cyclooctene
(0.2 molLÀ1) at 208C in CH2Cl2 with the complex 8a ( ; 0.1 mol%) and
*
&
&
* *
*
*
( ,&; 0.1 mol%) and 8b (
,
; 0.2 mol%) with ( , ) or without (&, )
&
electrochemically oxidized 8a ( ; 0.1 mol%).
added oxidizer (acetylferrocenium tetrafluoroborate).
0.385 C of charge had been passed through the solution. The
electrochemical oxidation of 8a generates a catalytically
active species that converts 95% of the monomer into a pol-
ymer over the following 5 h. To make sure that the polymer-
ization of the monomer is induced by oxidation of the ruthe-
nium complex, the experiment was repeated in the absence
of ruthenium complex 8a, under otherwise identical condi-
tions. Polymerization of cis-cyclooctene was not observed.
This clearly shows that the oxidation of precatalyst 8a to
form 8a+ is responsible for the redox-switched catalysis.
The molar masses of the polycyclooctene (polyCOE)
compounds produced with complexes 9a and 10a were de-
termined by using size-exclusion chromatography (SEC)
against polystyrene standards (Table 1). Unfortunately, the
polymer obtained from the switchable catalysts 8a and 8b
was insoluble in THF[27] and SEC data could not be ob-
tained.[28] This insolubility of polyCOE in various solvents is
not uncommon and has been reported previously.[29] None-
theless, the question remains, whether the insolubility of pol-
yCOE is due to very long polymer chains or whether an un-
known (oxidation-induced) process leads to significant
cross-linking. To clarify this, the oxidation-triggered poly-
merization of 8b was repeated for a much higher ratio of
COE/Ru (100:1) to obtain shorter polymer chains. The poly-
mer formed in this experiment is soluble and (according to
NMR spectroscopy) is a normal linear polyCOE. SEC data
(Figure SI42 in the Supporting Information) reveal the for-
mation of a polymer with Mn =88000 gmolÀ1 and Mw =
129000 gmolÀ1, indicative of a relatively low initiation effi-
ciency of the switched catalyst. The insolubility of the
(“switched”) polymer at COE/Ru=1000 appears to be due
to the length of the (linear) polymer chains.
plex with Ph instead of Fc, which was previously found to
be a very reluctant ROMP catalyst.[18a]
In contrast, complexes 8a and 8b appear to be useful in
redox-switched ROMP (Figure 5). The reaction of cis-cyclo-
octene with 8a gives only 4% conversion after 5 h. Adding
an oxidizer (acetylferrocenium tetrafluoroborate) to com-
plex 8a immediately generates 8a+, which is a much more
active catalyst in ROMP reactions. Even better results are
obtained with the more slowly initiating complex 8b in the
same reaction; with 8b, after 24 h, less than 1% of the mon-
omer is consumed, whereas 8b+ converts 96% of the mono-
mer into a polymer during the same period of time
(Figure 5).
We do not know with certainty, why complexes 11 are in-
active following oxidation and why complexes 8 are switcha-
ble catalysts. The main difference between the two com-
plexes is that the oxidation of the ferrocene unit in com-
plexes 11 has a smaller influence on the electron density at
the ruthenium atom than in complexes 8. This becomes ap-
parent when comparing the redox potentials of related Hov-
eyda-type complexes bearing substituents on the 2-OPh or
on the 1,2-benzenediyl unit.[20b,c] In complexes 8, oxidation
of the ferrocene unit acts on the nitrogen donor and on the
ruthenium. In complexes 11, the oxidation of the ferrocenyl
unit weakens nitrogen donation, but has little influence on
the ruthenium atom. It is known that weak donors trans to
the NHC ligand and electron deficiency at the ruthenium
center both enhance precatalyst activation.[20b] Both of these
effects are active in complexes 8+ and this likely contributes
to their better switchability in comparison with 11.
Next, we were interested in whether the ROMP activity
can also be switched electrochemically. Analogous experi-
ments by using complex 8a were done in CH2Cl2 as the sol-
vent and in the presence of a supporting electrolyte,
Bu4NPF6 (Figure 6). Again, complex 8a was found to be a
latent catalyst and, over 5 h, only about 1% of the monomer
underwent a ROMP reaction. In the next experiment, a sol-
ution of complex 8a with cis-cyclooctene in CH2Cl2 was oxi-
dized electrochemically on a platinum mesh electrode. A
constant current of 1 mA was applied to the cell until
1
The H NMR spectra of the various polyCOE compounds
obtained (or the CDCl3 soluble fraction) display two peaks,
at d=5.34 and 5.38 ppm, which can be integrated to provide
the trans/cis ratio (data are listed in Table 2). This parameter
depends very much on catalyst design and has an influence
on Tm.
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Ring-Opening Metathesis Polymerization
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Table 2. Data for polyCOE synthesis obtained with complexes 8a, 9a,
and 10a.
Complex 8b: Complex 8b was obtained as a brown powder (158 mg,
67% yield). H NMR (500 MHz, CDCl3): d=17.49 (s, 1H; Ru=CH), 8.12
1
(s, 1H; N=CH), 7.63 (t, J=7.7 Hz, 1H; aromatic p-HSIPr), 7.52 (d, J=
7.1 Hz, 1H; aromatic m-HSIPr), 7.43–7.37 (m, 2H; aromatic HSIPr), 7.16 (t,
J=7.8 Hz, 2H; aromatic HSIPr), 6.65 (s, 2H; aromatic HMes), 4.81–4.79 (m,
1H; HFc), 4.47 (t, J=2.6 Hz, 1H; HFc), 4.44–4.36 (m, 1H; NCHxHy),
4.34–4.32 (m, 1H; HFc), 4.23–4.10 (m, 8H; 5HFc, 2CH(Me)2, NCHxCHy),
4.09–3.97 (m, 2H; NCH2), 3.05 (sep, J=6.8 Hz, 1H; CH(Me)2), 3.00 (sep,
J=6.8 Hz, 1H; CH(Me)2), 2.22 (s, 3H; p-MeMes), 1.93 (s, 3H; o-MeMes),
1.75 (s, 3H; o-MeMes), 1.38 (d, J=7.0 Hz, 3H; MeiPr), 1.36 (d, J=6.6 Hz,
3H; MeiPr), 1.32 (d, J=6.5 Hz, 3H; MeiPr), 1.17 (d, J=6.9 Hz, 3H; MeiPr),
1.14 (d, J=6.8 Hz, 3H; MeiPr), 1.09 (d, J=6.8 Hz, 3H; MeiPr), 0.92 (d, J=
6.6 Hz, 3H; MeiPr), 0.58 ppm (d, J=6.5 Hz, 3H; MeiPr); 13C NMR
(126 MHz, CDCl3): d=309.77, 220.16, 169.42, 150.34, 149.55, 149.18,
148.62, 147.32, 138.69, 135.23, 134.90, 132.47, 130.42, 129.83, 129.18,
129.08, 128.47, 124.83, 124.41, 123.75, 97.38, 76.53, 75.47, 72.71, 70.27,
66.43, 55.09, 53.66, 29.54, 28.72, 28.41, 28.23, 28.02, 27.05, 26.38, 26.34,
24.59, 23.90, 22.72, 22.36, 21.16, 20.97, 20.72 ppm; HRMS (EI): m/z calcd
for C48H59N3Cl2FeRu: 905.2470 [M]+; found: 905.2472.
Catalyst Ratio
COE/Ru
Mn
[gmolÀ1
PDI trans/cis Time
Conversion
[%]
[c]
N
]
ratio[e]
8a+[a]
8a+[b]
8b+[a]
9a
1000
1000
500
1000
1000
–
–
–
–
–
–
3.2:1
3.1:1
5 h
5 h
24 h
96
95
96
[d]
[d]
[d]
2.8:1[f]
55760
66990
1.16 4.3:1
1.22 4.3:1
10 min 99
30 min 99
10a
[a] Chemical oxidation. [b] Electrochemical oxidation. [c] Solvent: THF.
[d] Insoluble in THF. [e] Determined by NMR spectroscopy in CDCl3.
[f] Same ratio observed for the soluble polyCOE obtained at COE/Ru=
100.
Conclusion
Synthesis of complexes 9a and 9b: A Schlenk flask containing [RuCl2-
AHCTUNGTER(GNNNU SIMes)ACHUTTNGREN(NGU Ind)(py)] (200 mg, 0.26 mmol) or [RuCl2ACHTUNGERTNGN(UN SIPr)ACHTUNGTRENNUNG(Ind)(py)]
Eight new Hoveyda-type complexes with ferrocenyl sub-
stituents were synthesized. Two of these complexes (8a and
8b) were found to be latent catalysts for ROMP reactions in
the reduced state, but are able to polymerize cis-cyclooctene
following chemical or electrochemical oxidation of the fer-
rocenyl group. The oxidative stimulus by using mild chemi-
cal oxidizers is orthogonal to most organic functional
groups. The electrochemical initiation of the polymerization
through electrodes adjusted to an oxidative potential is un-
precedented for ROMP reactions and could enable micro-
structure control of the polymerization process. Further-
more, control over the cross-linking process, might provide
access to thermally triggered shape-memory polymers.
(216 mg, 0.26 mmol) was evacuated and back-filled with nitrogen three
times. Toluene (3 mL) and 5 (75 mL, 0.29 mmol) were added under an at-
mosphere of nitrogen and the resulting mixture was heated for 1.5 h at
658C. After evaporation of the volatile compounds, methanol (5 mL) was
added to the remaining solid, and the mixture sonicated and kept in the
fridge at À358C for 2 h. The precipitate was collected by filtration,
washed with cold methanol, and dried in vacuo.
Complex 9a: Complex 9a was obtained as a pink powder (131 mg, 70%
yield). 1H NMR (500 MHz, C6D6): d=16.43 (d, J=0.8 Hz, 1H), 6.94 (s,
2H), 6.91 (s, 2H), 4.26 (s, 5H), 4.02–4.00 (m, 1H), 3.86 (t, J=2.6 Hz,
1H), 3.74 (dd, J=2.7, 1.1 Hz, 1H), 3.45 (s, 4H), 2.68 (s, 9H), 2.64 (s,
9H), 2.19 ppm (s, 6H); 13C NMR (126 MHz, C6D6): d=291.81, 212.61,
129.79, 129.67, 115.15, 101.18, 69.91, 68.87, 61.11, 57.58, 51.72, 21.15,
19.99 ppm (brs); HRMS (EI): m/z calcd for C34H41N3Cl2FeRu: 719.1061
[M]+; found: 719.1056.
Complex 9b: Complex 9b was obtained as a pink powder (182 mg, 87%
yield). 1H NMR (300 MHz, CDCl3): d=16.45 (d, J=0.8 Hz, 1H), 7.47 (t,
J=7.7 Hz, 2H), 7.39–7.27 (m, 4H), 4.50 (dt, J=2.5, 0.8 Hz, 1H), 4.23 (t,
J=2.7 Hz, 1H), 4.20 (s, 5H), 4.09 (s, 4H), 3.80 (dd, J=2.6, 1.1 Hz, 1H),
3.75 (sep, J=6.8 Hz, 4H), 2.80 (brs, 3H), 2.63 (brs, 3H), 1.40 (d, J=
6.6 Hz, 6H), 1.28 ppm (t, J=6.9 Hz, 18H); 13C NMR (75 MHz, CDCl3):
d=294.43, 214.13, 148.65, 137.78, 129.32, 124.43, 124.35, 114.01, 100.96,
69.82, 69.17, 62.09, 57.33, 54.72, 53.65 (brs), 28.70, 28.59, 26.65, 26.56,
24.38, 24.19 ppm; HRMS (EI): m/z calcd for C40H53N3Cl2FeRu: 803.2000
[M]+; found: 803.1999.
Experimental Section
Synthesis of complexes 8a and 8b: A Schlenk flask containing [RuCl2-
ACHTUNGTRENNUNG(SIMes)ACHTUNGTRENNUNG(Ind)(py)] (200 mg, 0.26 mmol) or [RuCl2ACHTUNGERTNGN(UN SIPr)CAHTNUGTRENN(UGN Ind)(py)]
(216 mg, 0.26 mmol) was evacuated and back-filled with nitrogen three
times. Next, a stock solution of 3 (0.1m, 3.1 mL) in dry and degassed tol-
uene was added under an atmosphere of nitrogen. The resulting solution
was heated for 1.5 h at 658C. The volatile compounds were removed in
vacuo and the residue purified by column chromatography (cyclohexane/
acetone/NEt3, 100:20:1 v/v/v). After evaporation of the volatile com-
pounds, n-pentane (5 mL) was added to the remaining solid, and the mix-
ture was sonicated and kept in a fridge at À358C for 2 h. The precipitate
was collected by filtration and dried in vacuo.
Synthesis of complexes 10a and 10b: A Schlenk flask containing [RuCl2-
AHCTUNGTER(GNNNU SIMes)ACHUTTNGREN(NGU Ind)(py)] (200 mg, 0.26 mmol) or [RuCl2ACHTUNGERTNGN(UN SIPr)ACHTUNGTRENNUNG(Ind)(py)]
(216 mg, 0.26 mmol) was evacuated and back-filled with nitrogen three
times. Next, a solution of 6 (92 mg, 0.31 mmol) in dry and degassed tol-
uene (3 mL) was added under an atmosphere of nitrogen. The resulting
mixture was heated for 1 h at 608C. After evaporation of the volatile
compounds, methanol (10 mL) was added to the remaining solid, and the
mixture was sonicated. The precipitate was collected by filtration,
washed with methanol, and dried in vacuo.
Complex 8a: Complex 8a was obtained as a brown-orange powder
(156 mg, 73% yield). 1H NMR (500 MHz, CDCl3): d=17.45 (s, 1H; Ru=
CH), 8.06 (s, 1H; N=CH), 7.12 (s, 1H; aromatic HSIMes), 7.01 (s, 1H; aro-
matic HSIMes), 6.84 (s, 1H; aromatic HSIMes), 6.75 (s, 1H; aromatic HSIMes),
6.56 (s, 2H; aromatic HMes), 4.80–4.77 (m, 1H; HFc), 4.47 (t, J=2.5 Hz,
1H; HFc), 4.40–4.37 (m, 1H; HFc), 4.16 (s, 5H; HFc), 4.09–4.02 (m, 2H;
NCH2), 3.91–3.84 (m, 2H; NCH2), 2.68 (s, 3H; o-MeSIMes), 2.50 (s, 3H; o-
MeSIMes), 2.45 (s, 3H; p-Me), 2.27 (s, 3H; p-Me), 2.18 (s, 3H; o-MeSIMes),
2.15 (s, 3H; p-Me), 1.99 (s, 3H; o-MeSIMes), 1.87 (s, 3H; o-MeMes),
1.81 ppm (s, 3H; o-MeMes); 13C NMR (126 MHz, CDCl3): d=314.13,
217.03, 169.40, 146.80, 139.30, 139.17, 138.75, 138.71, 138.62, 138.13,
137.92, 134.76, 134.46, 132.01, 129.51, 129.14, 128.62, 99.30, 76.65, 75.15,
72.77, 70.22, 65.00, 51.92, 50.98, 21.39, 21.23, 20.90, 20.76, 20.35, 20.03,
19.19, 18.33 ppm; HRMS (EI): m/z calcd for C42H47N3Cl2FeRu: 821.1531
[M]+; found: 821.1511. Single crystals of 8a were obtained by slow evap-
oration of CH2Cl2/cyclohexane at ambient temperature under air.
Complex 10a: Complex 10a was obtained as
a brownish powder
(156 mg, 79% yield). 1H NMR (500 MHz, CDCl3): d=18.72 (t, J=
5.2 Hz, 1H), 8.08 (s, 1H), 7.03 (s, 4H), 4.04 (s, 9H), 3.92 (t, J=1.9 Hz,
2H), 3.90 (t, J=1.9 Hz, 2H), 2.96 (d, J=5.2 Hz, 2H), 2.71–2.23 (m,
18H), 1.05 ppm (s, 6H); 13C NMR (126 MHz, CDCl3): d=343.65, 218.31,
173.29, 140.10, 138.39, 129.57, 103.11, 70.29, 66.40, 63.74, 51.59 (brs),
42.10, 26.85, 21.39, 20.28 (brs), 18.42 ppm (brs); HRMS (EI): m/z calcd
for C37H45N3Cl2FeRu: 759.1374 [M]+; found: 759.1414.
Complex 10b: Complex 10b was obtained as
a brownish powder
(202 mg, 92% yield). 1H NMR (500 MHz, CDCl3): d=18.80 (t, J=
4.8 Hz, 1H), 8.21 (s, 1H), 7.47 (t, J=7.7 Hz, 2H), 7.32 (d, J=7.7 Hz,
4H), 4.10 (s, 4H), 3.97 (d, J=2.9 Hz, 7H), 3.88 (t, J=1.9 Hz, 2H), 3.60
(d, J=12.6 Hz, 4H), 2.70 (d, J=4.8 Hz, 2H), 1.22 (d, J=6.8 Hz, 24H),
Chem. Eur. J. 2013, 00, 0 – 0
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H. Plenio et al.
1.02 ppm (s, 6H); 13C NMR (126 MHz, CDCl3): d=220.03, 175.24,
165.53, 149.33 (brs), 129.32, 124.55, 104.13, 70.14, 66.70, 64.80, 63.00,
54.43, 41.31, 28.77, 26.71, 26.63, 23.64 ppm; HRMS (EI): m/z calcd for
C43H57N3Cl2FeRu: 843.2313 [M]+; found: 843.2331.
General procedure for the ROMP reaction of cis-cyclooctene utilizing
oxidized complex 8a and electrolysis at constant current: A three-necked
electrochemical cell equipped with a magnetic stirring bar, argon inlet,
platinum mesh anode, and silver spiral cathode was evacuated and back-
filled with argon three times. Next, a stock solution of cis-cyclooctene in
CH2Cl2 (20 mL) and complex 8a (3.3 mg, 4.0 mmol) dissolved in CH2Cl2
(300 mL) were added under a stream of argon. A constant current of
1 mA was applied to the cell until 0.385 C of charge had been passed
through the solution (t=385 s). For the determination of substrate con-
version, samples (60 mL) were taken after the specified times under a
stream of argon and injected into vials containing ethyl vinyl ether
(50 mL) and methanol (500 mL). The mixture was filtered through a clean
pad of cotton to remove insoluble polymer and the solution analyzed by
GC.
Synthesis of complexes 11a and 11b: A Schlenk flask containing [RuCl2-
ACHTUNGTRENNUNG(SIMes)ACHTUNGTRENNUNG(Ind)(py)] (200 mg, 0.26 mmol) or [RuCl2ACHTUNGERTNGN(UN SIPr)CAHTNUGTRENN(UGN Ind)(py)]
(216 mg,0. 26 mmol) was evacuated and back-filled with nitrogen three
times. A stock solution of 7 (3.1 mL, 0.1m) in dry and degassed toluene
was added under an atmosphere of nitrogen. The resulting mixture was
heated for 1 h at 608C. After evaporation of the volatile compounds,
methanol (5 mL) was added to the remaining solid, the mixture was soni-
cated and cooled to À408C. The precipitate was collected by filtration,
washed with methanol, and dried in vacuo.
Complex 11a: Complex 11a was obtained as a dark-green powder
1
(180 mg, 89% yield). H NMR (500 MHz, CDCl3): d=18.65 (s, 1H), 9.10
(s, 1H), 7.67 (td, J=7.5, 1.3 Hz, 1H), 7.50 (d, J=7.5 Hz, 1H), 7.45 (td,
J=7.5, 1.3 Hz, 1H), 7.15 (s, 4H), 6.71 (d, J=7.7 Hz, 1H), 4.21 (t, J=
1.9 Hz, 2H), 4.20 (s, 5H), 4.14 (s, 4H), 4.07 (t, J=1.9 Hz, 2H), 2.74–
2.31 ppm (m, 18H); 13C NMR (75 MHz, CDCl3): d=310.84, 310.75,
214.52, 164.58, 150.13, 141.62, 140.43 (brs), 138.72 (brs), 134.65, 133.06,
129.81, 128.56, 127.92 (brs), 127.07, 126.37 (brs), 123.62, 104.04, 70.68,
67.24, 64.72, 53.57, 52.46 (brs), 51.07 (brs), 21.46, 20.53 (brs), 18.53 ppm
(brs); HRMS (EI): m/z calcd for C39H41N3Cl2FeRu: 779.1061 [M]+;
found: 779.1072. Single crystals of 11a were obtained by slow evapora-
tion of CH2Cl2/cyclohexane at ambient temperature under air.
Acknowledgements
The authors would like to thank the Landesoffensive zur Entwicklung
wissenschaftlich-çkonomischer Exzellenz (LOEWE Soft Control) for fi-
nancial support of this work. Umicore AG & Co KG is thanked for the
generous donation of ruthenium complexes.
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Complex 11b: Complex 11b was obtained as a dark-green powder
(175 mg, 78% yield). H NMR (500 MHz, CDCl3): d=18.65 (s, 1H), 9.16
1
(s, 1H), 7.64 (td, J=7.5, 1.3 Hz, 1H), 7.60 (brs, 2H), 7.50–7.45 (m, 2H),
7.43 (d, J=7.8 Hz, 4H), 6.62 (d, J=7.7 Hz, 1H), 4.30 (t, J=1.9 Hz, 2H),
4.19 (brs, 4H), 4.10 (s, 5H), 4.05 (t, J=1.9 Hz, 2H), 3.86–3.45 (brm,
4H), 1.45–1.20 (brm, 18H), 1.02 ppm (brs, 6H); 13C NMR (126 MHz,
CDCl3): d=303.56, 215.75, 164.64, 139.23, 133.87, 132.39, 128.34, 128.00,
127.19, 126.77, 125.55, 123.76 (brs), 123.39, 104.04, 69.27, 67.52, 66.12,
65.95, 64.41, 28.42–25.57 ppm (several broad signals); HRMS (EI): m/z
calcd for C45H53N3Cl2FeRu: 863.2000 [M]+; found: 863.2000.
Synthesis of oxidized complexes 11a+ and 11b+; A Schlenk flask con-
taining complex 11a (40 mg, 0.051 mmol) or 11b (44 mg, 0.051 mmol)
and acetylferrocenium tetrafluoroborate (13.9 mg, 0.051 mmol) was evac-
uated and back-filled with nitrogen three times. CH2Cl2 (2 mL) was
added under an atmosphere of nitrogen and the resulting solution was
stirred for 20 min at room temperature. The solution was filtered and the
filtrate was concentrated in vacuo. Next, diethyl ether (50 mL) was
added and the precipitate was collected by filtration. The precipitate was
washed with diethyl ether and toluene and dried in vacuo.
Complex 11a+: Paramagnetic complex 11a+ was obtained as a brownish
powder (31 mg, 71% yield). 1H NMR (300 MHz, CD2Cl2): d=16.21 ppm
(s, 1H; Ru=CH); HRMS (EI): m/z calcd for C39H41N3Cl2FeRu: 779.1061
[MÀBF4]+; found: 779.1060. Single crystals of 11a+ were obtained by
slow evaporation of CH2Cl2/benzene at ambient temperature under air.
Complex 11b+: Paramagnetic complex 11b+ was obtained as a brown
powder (33 mg, 69% yield). 1H NMR (300 MHz, CD2Cl2): d=16.86 ppm
(s, 1H; Ru=CH); HRMS (EI): m/z calcd for C45H53N3Cl2FeRu: 863.2000
[MÀBF4]+; found: 863.1959.
General procedure for the ROMP reaction of cis-cyclooctene by utilizing
the oxidized complexes: A Schlenk tube (10 mL) charged with complexes
8, 9, 10, or 11 (0.0025 mmol) and acetylferrocenium tetrafluoroborate
(0.8 mg, 0.0025 mmol) was evacuated and back-filled with argon three
times. CH2Cl2 (500 mL) was added under a stream of argon and the mix-
ture was stirred for 10 min at room temperature. Next, a stock solution
of cis-cyclooctene in toluene (6.0 mL (corresponding to a catalyst loading
of 0.10 mol%) or 3.0 mL (catalyst loading of 0.2 mol%)) was added to
the mixture. For the determination of substrate conversion, samples
(60 mL) were taken after the specified times under a stream of argon and
injected into vials containing ethyl vinyl ether (50 mL) and methanol
(500 mL). The mixture was filtered through a clean pad of cotton to
remove the insoluble polymer and the solution was analyzed by GC. The
same procedure was applied for polymerization reactions utilizing com-
plexes 8a, 8b, 9a, 10a, and 11a.
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Ring-Opening Metathesis Polymerization
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Received: March 6, 2013
Revised: May 15, 2013
Published online: && &&, 0000
Chem. Eur. J. 2013, 00, 0 – 0
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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