Latent ruthenium initiators containing fluoro aryloxide ligands

Article abstract of DOI:10.1016/j.jorganchem.2011.01.014

A new ligand, methyl 2,3,5,6-tetrafluoro-4-oxybenzoate (C₈H ₃F₄O₃), combining an electron withdrawing group (C₆F₄) to tune the reactivity with an anchor group (CO₂Me) for immobilization on supports, was used to prepare four new ruthenium initiators, viz. Ru(C₈H₃F₄O ₃)₂(CHPh)(3-Br-C₅H₅N)(H ₂IMes) and Ru(C₈H₃F₄O ₃)₂XL, where X = C,N-(CHCH₂CH ₂-2-C₅H₄N) and L = PⁱPr₃, PCy₃ or H₂IMes. The new ligand greatly reduced the reactivity of the ruthenium centre at room temperature. The ¹H NMR and DSC investigation for the ROMP of norbornene dicarboximide monomers clearly demonstrated that the Ru(C₈H₃F₄O ₃)₂XL initiators were inactive at room temperature and required elevated temperatures for their activation.

Full text of DOI:10.1016/j.jorganchem.2011.01.014

Journal of Organometallic Chemistry 696 (2011) 1591e1599
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Latent ruthenium initiators containing fluoro aryloxide ligands
,
,
,
b
,
,
Zhanru Yu a 1, Yulia Rogan a 2, Ezat Khosravi a , Osama M. Musa 3, Lois Hobson c 4, Andrei S. Batsanov a
*
a Durham University, Chemistry Department, South Road, Durham DH1 3LE, UK
b International Specialty Products (ISP), 1361 Alps Road, NJ07470, USA
c ICI, STG, Wilton, Teesside, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
A new ligand, methyl 2,3,5,6-tetrafluoro-4-oxybenzoate (C8H3F4O3), combining an electron withdrawing
group (C6F4) to tune the reactivity with an anchor group (CO2Me) for immobilization on supports, was
used to prepare four new ruthenium initiators, viz. Ru(C8H3F4O3)2(]CHPh)(3-BreC5H5N)(H2IMes) and
Ru(C8H3F4O3)2XL, where X ¼ C,N-(]CHCH2CH2-2-C5H4N) and L ¼ PiPr3, PCy3 or H2IMes. The new ligand
greatly reduced the reactivity of the ruthenium centre at room temperature. The 1H NMR and DSC
investigation for the ROMP of norbornene dicarboximide monomers clearly demonstrated that the Ru
(C8H3F4O3)2XL initiators were inactive at room temperature and required elevated temperatures for their
activation.
Received 18 October 2010
Received in revised form
20 December 2010
Accepted 11 January 2011
Keywords:
Thermally switchable ruthenium initiators
Ring opening metathesis polymerization
(ROMP)
Ó 2011 Elsevier B.V. All rights reserved.
Norbornene dicarboximides
Latent catalysts
1. Introduction
on the search for effective thermally switchable ruthenium
complexes suitable for ROMP of norbornene monomers [17e28].
Ring opening metathesis polymerization (ROMP) using well-
defined initiators based on molybdenum [1e5] and ruthenium
[6e10] is now well-established as a powerful polymerization tool
allowing the synthesis of materials with desired properties
[8,11e17]. These well-defined initiators polymerize norbornene
and its derivatives efficiently at room temperature. However, for
some processes, it is desirable to control the initiation step in order
to allow adequate mixing of monomer and initiator before poly-
merization occurs. For these applications, complexes that are
inactive at room temperature and initiate polymerization only
upon heating would be desirable. Development of well-defined
latent ruthenium initiators considerably expands the range of
applications, particularly in industry. The latent initiators allow
storing, transporting and polymerizing materials at desirable
temperatures. During the last few years efforts have been focused
Related to the work presented here, Ciba reported initiators
1aef (Fig. 1) which were claimed to be inactive at room tempera-
ture but active at elevated temperatures for the polymerization of
dicyclopentadiene (DCPD) [18,19,29]. At the fixed polymerization
temperature of 60 ꢀC, both complexes 1b and 1f showed the onset
temperatures of 40 ꢀC with the exotherms reaching 165 ꢀC and
180 ꢀC, respectively, within 2 min. Only 1a polymerized DCPD at
90 ꢀC and with an exotherm at 120 ꢀC.
Initiators 2aec containing H2IMes ligand (Fig. 1) instead of iPr3P
ligand were also reported [20]. ROMP of DCPD using 2a, showed an
exotherm within 3 min at 30 ꢀC. The complexes 2c and 2b were
reported to have similar behaviour to 2a. For ROMP of DCPD with
complex 3a, an exotherm was observed after 1 min at 30 ꢀC [21].
For initiators 3b and 3c, under similar conditions, the introduction
of iPr and Cy (Cyclohexyl) groups on the nitrogen atom resulted in
the appearance of exotherms after 18 and 28 min at 30 ꢀC,
respectively [21]. These results indicate the unsuitability of these
initiators as latent. Moreover, it should be noted that the thermal
switchability of the initiators 1e3 has not been investigated for
ROMP of functionalized norbornene monomers.
The characteristics of the ligands in ruthenium initiators play
a crucial role in their overall activity, particularly in ROMP of nor-
bornene derivatives. The chelation effect [18,19,29e35] and the
presence of electron withdrawing and electron donating groups in
ligands surrounding Ru centre [27,36e41] can influence the
* Corresponding author. Tel.: þ44 191 334 2014; fax: þ44 191 334 2051.
E-mail address: ezat.khosravi@durham.ac.uk (E. Khosravi).
1
Present address: The Weatherall Institute of Molecular Medicine, University of
Oxford, Headington, Oxford OX3 9DS, UK.
2
Present address: School of Chemistry, Cardiff University, Cardiff CF10 3XQ, UK.
Previous address: Henkel Corporation, Bridgewater, NJ, USA.
Present address: Centre for Process Innovation, Wilton Centre, Teesside TS10
3
4
4RF, UK.
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.01.014

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Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
Fig. 1. Previous examples of ROMP catalysts containing N-chelating alkylidene ligands.
behaviour of the ruthenium initiators in ROMP of norbornene
derivatives.
reaction with Boc-protected diaminodiethylamine followed by
a deprotection reaction. The reaction of amine functional groups
with 2,3,5,6-tetrafluoro-4-aceto benzoic acid would be followed by
the conversion of the acetate groups to hydroxyl groups. The
hydroxyl groups would then be reacted with thallium ethoxide to
form thallium salt of ligand 6 (Tl-6) which upon reactions with
appropriate ruthenium complexes would be anticipated to lead to
immobilized ruthenium initiators 7e10.
The synthesis and activity of ruthenium complexes containing
perfluorophenoxide ligand (4) or perbromophenoxide ligand (5)
(Fig. 2) were reported and they were shown to be efficient initiators
for ROMP of cyclooctene and norbornene monomers at room
temperature, with the rate of propagation being faster than that of
the initiation [36e39].
It is generally accepted that in the case of the first generation,
RuCl2(]CHPh)(PCy3)2, and also second generation, RuCl2(]CHPh)
(PCy3)(H2IMes), Grubbs ruthenium initiators dissociation of the
donor ligand (PCy3) is necessary to start the ROMP reaction [42]. In
contrast, for oxygen chelated GrubbseHoveyda type complexes, it
is believed that initially 14-electron intermediate is formed through
the dissociation of the benzylidene ether chelating group [33,43].
Coordination of the alkene substrate, followed by metathesis,
would then lead to the formation of the catalytically active species
2. Results and discussions
2.1. Synthesis of the ligand 6
The ligand 6 was prepared by a two-step procedure shown
in Scheme 1. The esterification reaction of 2,3,5,6-tetrafluoro-
4-hydroxybenzoic acid hydrate in methanol in the presence of
concentrated sulphuric acid gave white solid intermediate product,
methyl 2,3,5,6-tetrafluoro-4-hydroxybenzoate, in high yield (90%).
The reaction of the intermediate product with thallium ethoxide in
anhydrous THF gave the white solid product, the thallium salt of 6
(Tl-6), in high yield (90%). The structures of the intermediate
product and Tl-6 were confirmed by 1H, 13C and 19F NMR.
and
a molecule of isopropoxy sytrene. When the olefin is
completely consumed, the catalyst is then believed to return to its
resting state by re-binding the isopropoxy styrene (release/return
mechanism). The dissociation mechanism for HoveydaeGrubbs
complexes and the influence of substituents on the benzene ring on
RueO bond length and hence on alkoxy-dissociation have also been
investigated [34,35]. Recently, both dissociation and association
mechanism as interchange mechanism have been reported for
olefin metathesis involving GrubbseHoveyda and Grela type
complexes, however, it was difficult to decide which mechanism
was dominant [44].
It should be noted that these studies concern oxygen chelated
complexes and to the best of our knowledge there are no reports of
either dissociative or associative mechanisms for nitrogen chelated
complexes. For the ruthenium complexes 1e3, the dissociation of
chelating nitrogen to the metal centre is believed to be necessary
for initiating the ROMP reaction. This dissociation readily takes
place at room temperature, in the presence of a monomer, making
the initiator active for ROMP. Therefore, the dissociation process, at
room temperature, must be slowed down or even prevented in
order to render these ROMP initiators latent.
We recently started a programme of work to develop a series of
thermally switchable ruthenium initiators for various applications.
Herein, we describe the synthesis of the designed ligand 6, methyl
2,3,5,6-tetrafluoro-4-oxybenzoate, and the new ruthenium initia-
tors 7e10 containing ligand 6, Fig. 3. The design of the ligand 6 is
novel and is based on the incorporation of an electron withdrawing
group (eC6F4) and an anchor group (eCO2Me). The electron with-
drawing nature of eC6F4 group is expected to make the nitro-
generuthenium chelation stronger and therefore rendering these
ruthenium initiators less reactive at room temperature. The anchor
group is to provide the means for immobilization of the initiators
on Merrifield-type supports for combinatorial processes. The
immobilization process is not presented here and it will be the
subject of the future publications. However, the proposed process
for the immobilization would involve the conversion of carboxylic
functional groups, on Merrifield-type resin, to diamine by the
2.2. Synthesis and characterization of the new ruthenium initiators
7e10
The new ruthenium initiator 7 was prepared via exchange
reactions between ruthenium complex 1a and two equivalents of
Tl-6, Scheme 2. The course of the ligand exchange reaction was
followed by 1H NMR. Disappearance of the alkylidene proton
resonance at 19.40 ppm and appearance of a new alkylidene proton
resonance at 19.86 ppm indicated the complete conversion of
complex 1a to 7, which was characterized by single-crystal X-ray
diffraction, Fig. 4. The Ru coordination is square-pyramidal with C
(39) in the apical position and the fluorophenoxide ligands trans to
each other. Thus 7 can be regarded as an analogue of 1a [18] with
both chloro ligands replaced by fluorophenoxide ligands. The O
(1)eRueO(2) angle of 148.2(1)ꢀ is smaller than PeRueN(1) of
173.58(7)ꢀ, indicating a distortion towards trigonal-bipyramidal
Ph
Ph
OC Br
6
OC F
6
IMes
IMes
5
5
Ru
Ru
OC F
6
Cl
5
N
N
R
R
4a.
5a.
R = H
R = H
5b.
4b.
R = Br
R = Br
Fig. 2. Structures of ruthenium initiators with two eOC6F5 (4) and one eOC6Br5 (5)
ligands.

Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
1593
EWG
PiPr
L
L
PCy
Ru
L
IMesH
Ru
L
Ru
L
F
F
L
IMesH
Ph
L
O
O
N
O
BrPy
Ru
N
N
CH
L
F
F
Anchor Group
10
8
9
7
6
L =6
Fig. 3. The structures of the new ligand 6 and the new ruthenium initiators 7e10.
coordination stronger than in 1a (CleRueCl 155.27(3) and PeRueN
172.80(6)ꢀ).
than the sum of van der Waals radii (2.17 Å [46] to 2.05 Å [47] for Ru
and 1.10 Å for H [48]). However, the corresponding Ru.H distance
in molecule A is 0.46 Å longer. In molecule B, the bromopyridine
ligand is disordered by a ca. 180ꢀ rotation around the Ru0eN(20)
bond [i.e. the bromine atom is disordered between two positions,
Br(10) and Br(20)], in a 93:7 ratio; the minor orientation is identical
with the only orientation observed in molecule A. In both molecules
the apical benzylidene ligand shows a small twist (16.1ꢀ in A, 15.1ꢀ
in B) between the Ru]C(20) bond and the phenyl ring which has
a staggered orientation between the bromopyridine and a fluo-
rophenoxide ligands.
Thus, 10 has the same coordination geometry as 4a [36] (Table
1) but a rather different molecular conformation. In 10, the
pyridine ring is stacked with the N(1)-bonded mesityl group
(dihedral angles 7.5ꢀ in A, 17.9ꢀ in B). Interestingly, although the
two fluoroarene rings are oriented differently in molecules A and
B, in both they are stacked face-to-face with small interplanar
angles of 11.9ꢀ (A) or 8.9ꢀ (B). In 4a, the pyridine/mesityl stacking
is only marginal, and that between perfluoroarenes is absent
altogether. In 10, as in 4a, the RueO bond opposing the H2IMes
ligand is substantially longer than that opposing the bromopyr-
idine ligand, in accordance with the relative trans influence of
these ligands. However, the effect is much smaller than in an
octahedral complex, t-BuC]C]RuCl2(bipy)(H2IMes) [49], where
the RueCl bonds opposite to RueC(H2IMes) and RueN(bipy)
measure 2.49 and 2.39 Å, respectively.
The new ruthenium initiator 8 could not directly be obtained
from 7. Therefore, initially complex 11 was obtained by two routes
outlined in Scheme 2. Either the ruthenium complex 1a was reac-
ted with an excess of tricyclohexylphosphine or Grubbs first
generation ruthenium complex 12 was reacted with an excess of 13
to give (in both cases) complex 11, which was then converted into
the new ruthenium initiator 8 by treatment with two equivalents of
Tl-6. In the 1H NMR spectrum, the resonance due to the proton of
the alkylidene moiety was observed at 19.91 ppm. Attempts to
prepare crystals of 8 suitable for X-ray analysis were unsuccessful.
The synthetic route to the new ruthenium initiator 9 is shown in
Scheme 3. The ruthenium complex 14 [20] was reacted with 2
equivalents of Tl-6 to give 9. In the 1H NMR spectrum of 9 the
corresponding resonance due to the proton of the alkylidene
moiety was observed at 19.47 ppm. Attempts to prepare high
quality crystals of 9 for X-ray analysis were unsuccessful.
The ligand exchange reactions of the first generation (12) and
also the second generation Grubbs ruthenium complexes with Tl-6,
either in toluene or in THF, gave no new initiators, as indicated by
1H NMR investigations.
The new ruthenium initiator 10 was prepared via exchange
reactions between the bromopyridine modified second generation
ruthenium complex 15 and 2 equivalents of Tl-6, Scheme 3.
Disappearance of the alkylidene proton resonance at 19.40 ppm
and appearance of a new alkylidene proton resonance at 18.77 ppm
indicated the complete conversion of complex 15e10. The latter, in
the form of 10$½CH2Cl2 solvate (crystals grown from a mixture of
dichloromethane and hexane) was characterized by single-crystal
X-ray diffraction (Fig. 5, Table 1). The asymmetric unit comprises
two molecules, A and B, besides one disordered molecule of
dichloromethane. The Ru atom has a distorted square-pyramidal
coordination, with the apical benzylidene ligand and the basal
positions occupied by the H2IMes, bromopyridine and two fluo-
rophenoxide ligands in cis arrangement. The conformations of
molecules A and B differ significantly (Fig. 5). As usual [45], steric
overcrowding causes an overall tilt of the H2IMes ligand towards
the vacant coordination site of the Ru (which is located trans to the
benzylidene ligand), as indicated by the unsymmetrical RueC(2)eN
angles. In molecule B, the (mesityl) methyl group C(17)H3 provides
an agostic Ru.H interaction at this site. The Ru0.H(176) distance
of 2.61(3) Å for the refined and 2.56 Å for the idealised (assuming
CeH distance of 1.08 Å) hydrogen position, is substantially shorter
2.3. Stability of the new ruthenium initiators 7e10
The new ruthenium initiators 7e10 were dissolved in CDCl3 in
sealed NMR tubes, kept at 50 ꢀC and 1H NMR spectra were taken at
different intervals over the period of up to 24 h. The investigation
revealed no detectable decrease in the intensity of the alkylidene
protons. This indicated the stability of these initiators in solution at
50 ꢀC, over the times-scale of the ROMP reactions.
2.4. Thermal switchability of the new ruthenium initiators 7e10 in
ROMP
2.4.1. Investigation by 1H NMR analysis
The reactivity and thermal switchability of the new ruthenium
initiators 7e10 for ROMP of norbornene dicarboximide monomers,
shown in Fig. 6, were investigated by 1H NMR and the results are
summarised in Table 2. The ROMP reactions, under similar
F
F
F
F
F
F
TlOEt or t-BuOK
MeOH, H SO (cat)
2
4
O
O
O
O
THF, rt, 3 hrs
O
reflux, overnight
90%
TlO
HO
HO
90%
CH
CH
3
OH
3
F
Thallium salt of 6 (Tl-6)
Scheme 1. Preparation of the thallium salt of the ligand 6 (Tl-6).
F
F
F
F
F

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Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
Cl
PCy
3
Cl
PCy
Cy P Ru
3
3
Cl Ru
Ph
Cl
Cl
PiPr
3
(0.1 eq)
Cl Ru
N
N
PCy (10 eq)
3
12
N
rt, DCM, overnight
rt, DCM,
13
overnight
11
1a
Tl-6
rt, DCM,
overnight
Tl-6
rt, DCM,
(2 eq)
(2 eq)
overnight
L
PiPr
1
3
L
PCy
1
3
L
Ru
1
L
Ru
1
PCy (10 eq)
3
N
X
N
rt, DCM, overnight
L =6
1
7
8
Scheme 2. Preparation route for the new ruthenium complexes 7 and 8.
conditions, were also carried out on these monomers using
ruthenium complex 1a to provide comparison. The conversions of
monomers to polymers were determined by comparing the inte-
gration of the vinylic protons of monomers at 6.30 ppm to that of
the polymers at 5.75 ppm in the 1H NMR spectra of the ROMP
reaction mixtures. The error in the integration was dependent on
the amount of unreacted monomer present in the mixture and it is
estimated to be about 10%.
The ROMP of norbornene dicarboximide monomers with initi-
ator 8, under similar condition, gave the same results to those for
initiator 7, Table 2.
The 1H NMR spectra of the reaction mixture for the ROMP of
N-hexyl norbornene dicarboximide (HNB) monomer using initiator
9 showed a conversion of monomer to polymer of 6% after 1 h at
room temperature, Fig. 8a. However, conversions of 38% and 95%
were obtained after 6 h and 24 h, respectively, at 55 ꢀC, Fig. 8b, c.
Initiator 10 was found to be an efficient initiator for the ROMP of
norbornene dicarboximide monomers at room temperature, based
on solution 1H NMR results. The 1H NMR spectra of the reaction
mixture for the ROMP of 2EHNB, HNB and N-decyl norbornene
dicarboximide (DecNB) monomers with initiator 10 showed
conversions of monomer to polymer of 99% after 24 h at room
temperature.
The 1H NMR spectrum of the reaction mixture for the ROMP of
N-2-ethylhexyl norbornene dicarboximide (2EHNB) monomer
using initiator 7 showed no detectable conversion of monomer to
polymer after 1 h (Fig. 7a) and a conversion of 3% after 24 h
(Fig. 7b), at room temperature. However, a conversion of 80% was
observed after 24 h at 55 ꢀC (Fig. 7c). Similar results were obtained
for the ROMP of other norbornene dicarboximide monomers,
shown in Fig. 6, using initiator 7 under similar conditions (Table 2).
The solution 1H NMR results showed that the new ruthenium
complexes 7e10 were good initiators for ROMP of norbornene
dicarboximide monomers. Moreover, it was established that initi-
ators 7 and 8 were inactive ROMP initiators at room temperature
giving conversion of monomers to polymers of less than 5% after
Cl
Cl Ru
IMesH
L
2
IMesH
1
2
L
Ru
1
N
N
Tl-6 (2 eq)
rt, DCM, overnight
14
9
IMesH
L
2
1
BrPy
IMesH
2
BrPy Ru
Cl Ru
Cl Ph
Fig. 4. X-ray molecular structure of new ruthenium initiator 7 (thermal ellipsoids at
the 30% probability, H atoms omitted), showing the disorder of i-Pr groups. Selected
bond distances (Å) and angles (ꢀ): RueP 2.3317(9), RueO(1) 2.019(2), RueO(2) 2.015
(2), RueN(1) 2.157(2), Ru]C(39) 1.821(3), C(39)eRueP 93.5(1), C(39)eRueO(1) 104.0
(1), C(39)eRueO(2) 107.7(1), C(39)eRueN(1) 91.65(12), O(1)eRueO(2) 148.2(1),
PeRueN(1) 173.58(7).
Ph
L
1
BrPy
10
L =6
1
15
Scheme 3. Synthesis route for the new ruthenium initiators 9 and 10.

Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
1595
Fig. 5. Independent molecules A and B in the crystal of 10$½CH2Cl2 (thermal ellipsoids at 50% probability, hydrogen atoms omitted) and their superposition (A dark, B light).
24 h and that they required elevated temperatures for their acti-
vation. We believe this is due to the electron withdrawing nature of
the ligand 6 which strengthens the chelation of nitrogen to the
ruthenium centre. Therefore, elevated temperatures are necessary
to overcome the chelation and hence initiate ROMP reactions. It
should be noted that the ROMP of these monomers using initiator
O
O
1a, which was reported to be latent for the ROMP of DCPD, gave
conversions of 99% after 24 h at room temperature. This clearly
N
N
showed that the initiator 1a is active at room temperature and
O
O
HNB
2EHNB
Table 1
Selected bond distances (Å) and angles (ꢀ) in 10 and 4a.
O
O
10, mol. A
10, mol. B
4a
RueO(1)
RueO(2)
RueN(2)
RueC(2)
2.094(2)
2.063(2)
2.073(2)
2.027(2)
1.834(2)
89.17(9)
109.87(9)
95.46(9)
96.6(1)
174.87(7)
153.52(8)
117.5(2)
135.1(2)
2.089(2)
2.074(2)
2.060(2)
2.029(2)
1.836(3)
96.25(10)
104.45(9)
94.4(1)
2.110(3)
2.076(3)
2.085(3)
2.040(4)
1.843(3)
96.7(1)
103.1(1)
92.6(1)
91.2(1)
171.2(1)
164.0(1)
127.6(2)
130.5(2)
N
N
O
O
PhNB
CyNB
Ru]C(20)
C(20)eRueO(1)
C(20)eRueO(2)
C(20)eRueN(2)
C(20)eRueC(2)
O(1)eRueN(2)
O(2)eRueC(2)
RueC(2)eN(1)
RueC(2)eN(3)
O
N
98.0(1)
169.06(8)
157.52(8)
120.4(2)
132.0(2)
O
DecNB
Fig. 6. Structures of the norbornene dicarboximide monomers.

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Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
When the DSC was run and the temperature was kept at 180 ꢀC for
30 min, complete conversion of monomer to polymer was achieved
and no endotherm was observed, Fig. 9c. The Fig. 9c clearly shows
that no polymerization was observed at room temperature and that
the polymerization started at 75 ꢀC with an exotherm peak
maximum at 127 ꢀC. The DSC thermograph for the ROMP of HNB
monomer using initiator 8, showed no sign of polymerization at
room temperature and that the polymerization started at 60 ꢀC
with an exotherm peak maximum at 100 ꢀC (Fig. 10). The 1H NMR
spectrum of the polymerization mixtures retrieved from the DSC
pans after the DSC measurements revealed that complete conver-
sions of monomers to polymers were achieved in all cases.
In contrast to results obtained for initiators 7 and 8, the DSC
thermograph for the ROMP of HNB monomer using ruthenium
complex 1a showed that the polymerization started at 40 ꢀC with
an exotherm peak maximum at 55 ꢀC. This further confirmed the
unsuitability of 1a as a thermally switchable initiator, as it was
shown by the 1H NMR investigations.
Table 2
The ROMP of norbornene dicarboximide monomers with the new ruthenium
initiators 7e10 and 1a.
Monomer
Initiators 7
and 8
Initiator 9
Initiator 10
1a
% Conversion
% Conversion
% Conversion
% Conversion
RT
55 ꢀC
24 h
RT
55 ꢀ
C
RT
RT
24 h
1 h
24 h
24 h
24 h
2EHNB
HNB
DecNB
CyNB
PhNB
3
2
3
2
4
80
85
86
78
79
8
6
95
95
99
99
99
99
99
99
99
99
therefore it is not a thermally switchable initiator for ROMP of
functionalized norbornene monomers.
The DSC investigation results for the ROMP of norbornene
dicarboximide monomers clearly confirmed the results obtained by
1H NMR that the new ruthenium initiators 7 and 8 were inactive at
room temperature and that they required elevated temperatures
for their activation.
2.4.2. Investigation by DSC analysis
The thermal switchability of the new ruthenium initiators 7 and
8 for the ROMP reactions, in bulk, of norbornene dicarboximide
monomers was further investigated by DSC analysis and the results
are shown in Table 3. The DSC studies were only performed on
2EHNB and HNB monomers as they were liquid monomers allow-
ing ROMP reactions in bulk.
3. Conclusions
The DSC thermographs for the ROMP of 2EHNB monomer using
initiator 7 are shown in Fig. 9. When the DSC was run at 20e300 ꢀC
at 10 ꢀC/min, an exotherm peak was observed with a maximum at
135 ꢀC along with an endotherm peak at 210 ꢀC, Fig. 9a. The analysis
of the retrieved sample after running DSC revealed a conversion of
monomer to polymer of 90%. The DSC analysis of the neat 2EHNB
monomer, run under the same conditions, showed an endotherm at
about 200 ꢀC due to a retro DielseAlder reaction, Fig. 9b. Therefore,
the endotherm peak in Fig. 9a was attributed to the retro Diel-
seAlder of the unreacted monomer. Initiator 7 was deactivated to
such an extent, due to the presence of ligand 6, that the ROMP
reactions were slow. Therefore, on the time-scale of the DSC runs
the conversion of monomer to polymer was not complete and the
unreacted monomer went through a retro DielseAlder reaction.
The new ligand 6 was designed to incorporate an electron
withdrawing group (eC6F4) group to tune the reactivity of the
ruthenium initiators and an anchor group (eCO2Me) for immobi-
lization of the ruthenium initiators on supports. The ligand 6 was
prepared in high yield and its structure was confirmed by 1H, 13
C
and 19F NMR.
A new range of ruthenium initiators, 7e10, were prepared
bearing the new ligand 6; two of these (7 and 10) were charac-
terized by single-crystal X-ray diffraction.
The 1H NMR investigation results for the ROMP of norbornene
dicarboximide monomers clearly demonstrated that initiators 7
and 8 were inactive for ROMP at room temperature giving
conversions of monomer to polymer of less than 5% after 24 h but
became active at elevated temperatures giving high conversions. It
was also found that initiator 9 was more active than initiators 7 and
8 and that initiator 10 was active for ROMP at room temperature.
The results of 1H NMR and DSC investigations for the ROMP of
norbornene dicarboximide monomers clearly demonstrated that
the new ruthenium initiators 7 and 8 were inactive at room
temperature and that they required elevated temperatures for their
activation, due to the increased strength of chelation of nitrogen to
the ruthenium centre upon the inclusion of ligand 6.
These observations confirmed the new ruthenium initiators 7
and 8 as excellent thermally switchable ruthenium initiators.
4. Experimental section
4.1. General
All reactions involving metal complexes were conducted in
oven-dried glassware using standard Schlenk, drybox techniques
and anhydrous solvents. Dry dichloromethane (DCM), tetrahydro-
furan (THF) and toluene were supplied by Durham Solvent Purifi-
cation System, degassed and stored under nitrogen. The first
generation ruthenium 12 and the second generation ruthenium
complexes were purchased from Aldrich. 2,3,5,6-tetrafluoro-
4-hydroxybenzoic acid was purchased from TCI Europe and used
as supplied. Compound 13 [18], and ruthenium initiators 1a [18], 14
[20] and 15 [10] were synthesized according to literature procedure.
Fig. 7. 1H NMR spectra of the vinylic region for ROMP of 2EHNB monomer with the
new ruthenium initiator 7; a) at room temperature after 1h; b) at room temperature
after 24 h; c) at 55 ꢀC after 20 h.

Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
1597
1
Fig. 8. H NMR spectra of the vinylic region for ROMP of HNB monomer with the new ruthenium initiator 9; a) at room temperature after 1h; b) at 55 ꢀC after 6 h; c) at 55 ꢀC after
18 h.
N-2-ethylhexyl norbornene dicarboximide (2EHNB), N-hexyl nor-
bornene dicarboximide (HNB), N-decyl norbornene dicarboximide
(DecNB), N-Cyclohexyl norbornene dicarboximide (CyNB) and N-
phenyl norbornene dicarboximide (PhNB) monomers were
synthesized according to literature procedures [50,51].
148.70e138.12 (m, Ar CF), 53.16 (s, CH3). 19F NMR, CD3OD: ꢁ144.12
(m, 2F), ꢁ165.99 (m, 2F)
To a solution of methyl 2,3,5,6-tetrafluoro-4-hydroxybenzoate
(1.02 g, 4.55 mmol) in anhydrous THF (10 ml) was added a solution
of thallium ethoxide (1.19 g, 4.78 mmol, 1.05 eq) in anhydrous THF
(5 ml). The white solid was formed and the suspension was stirred
for further 3 h. The white solid was collected, washed with anhy-
drous THF, dried under reduced pressure, to give product Tl-6
(1.82 g, yield 90%). 1H NMR, CD3OD: 3.92 (s, 3H, COOCH3). 13C NMR,
The 1H, 13C and 31P NMR spectra were recorded at room
temperature on Bruker Avance 400 (1H: 400.13, 13C: 100.61) or
Varian Mercury 400 Spectrometers (1H: 399.96, 13C: 100.98 31P:
161.91 and 19F: 376.29 MHz). All chemical shifts are reported in
parts per million (d, ppm) with reference to Me4Si (TMS,
d
0.0).
CD3OD: 162.7 (s, COO), 148.70e138.12 (m, Ar CF), 53.16 (s, CH3). 19
NMR, CD3OD: ꢁ144.12 (m, 2F), ꢁ165.99 (m, 2F).
F
4.2. Synthesis of thallium salt of methyl 2,3,5,6-tetrafluoro-4-
hydroxybenzoate (Tl-6)
8
6
To a solution of 2,3,5,6-tetrafluoro-4-hydroxybenzoic acid hy-
drate (5.2 g, 22.8 mmol) in methanol (400 ml) concentrated sul-
phuric acid (98%, 2 ml) was added. The solution was heated to reflux
for 16 h. TLC was used to monitor the reaction. After the complete
conversion, methanol was removed, and then water (100 ml) was
added to the residual. Dichloromethane was used to extract the
product from aqueous solution. The extract was dried over anhy-
drous magnesium sulphate, and the solvent was removed under
reduced pressure to give white solid methyl 2,3,5,6-tetrafluoro-
4-hydroxybenzoate (5.01 g, yield 90%). 1H NMR, CD3OD: 4.92(br s,
1H, OH), 3.92 (s, 3H, COOCH3). 13C NMR, CD3OD: 162.7 (s, COO),
4
c
2
a
0
b
-2
-4
Table 3
Summary of DSC results for the new ruthenium initiators 7e9 and 1a.
Initiator
Monomer
DSC
0
50
100
150
200
250
300
Temperature, oC
Onset T, ꢀ
C
Max. T, ꢀ
C
Offset T, ꢀC
Delta H, J/g
7
8
1a
2EHNB
HNB
HNB
75
60
40
127
100
55
170
125
65
ꢁ119
ꢁ105
Fig. 9. DSC thermographs; (a) ROMP of 2EHNB monomer using initiator 7 run at
20e300 ꢀC, (b) pure 2EHNB run at 20e300 ꢀC, (c) ROMP of 2EHNB monomer using
initiator 7 kept at 180 ꢀC.

1598
Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
under reduced pressure. The resulting solid was washed with
0.15
0.10
0.05
0.00
-0.05
-0.10
dichloromethaneehexane (1:5 v/v) to afford a brown powder
product 8 (0.0862 g, yield 64%). 1H NMR, CD2Cl2: 19.91 (t,1H, [Ru]]
CH), 8.54 (d, 1H, pyridine), 7.68 (t, 1H, pyridine), 7.22 (m, 2H, pyri-
dine), 3.87 (s, 6H, COOCH3) 3.48 (t, 2H, ]CHCH2CH2), 2.28 (m, 5H, ]
CHCH2CH2 and P(CHMe2)3), 1.32 (m, 18H, P(CHMe2)3). 31P NMR,
CD2Cl2,: 45.3. 19F NMR, CDCl3: ꢁ145.19 (dd, 4F), ꢁ161.53 (t, 4F).
4.6. Synthesis of new ruthenium initiator 9
Tl-6 (0.095 g, 0.22 mmol) and ruthenium complex 14 (0.066 g,
0.11 mmol) were mixed in dichloromethane (3 ml) in a glove box.
The resulting mixture was stirred at ambient temperature for 16 h.
The mixture was filtered and the filtrate was reduced to dryness
under reduced pressure. The resulting solid was washed with
dichloromethaneehexane (1:5 v/v) to afford
a green powder
20
40
60
80
100
120
140
product 9 (0.0704 g, yield 66%). 1H NMR, CDCl3: 19.47 (t, 1H, [Ru]]
CH), 7.82 (d, 1H, pyridine), 7.35 (td, 1H, pyridine), 7.21 (d, 1H,
pyridine), 6.94 (d, 4H, Mes), 6.82 (td, 1H, pyridine), 4.06 (s, 4H,
sIMes), 3.83 (s, 2H, ]CHCH2CH2), 3.79 (s, 6H, COOCH3), 2.41 (s, 12H,
Mes CH3), 2.27 (s, 6H, Mes CH3),1.70 (d, 2H, ]CHCH2CH2). 19F NMR,
CDCl3: ꢁ145.19 (dd, 4F), ꢁ161.53 (t, 4F).
Temperature, oC
Fig. 10. DSC thermograph for ROMP of HNB monomer using ruthenium initiator 8.
4.3. Synthesis of new ruthenium initiator 7
Tl-6 (0.383 g, 0.90 mmol) and ruthenium complex 1a (0.198 g,
0.44 mmol) were mixed in dichloromethane (5 ml) in a glove box.
The resulting mixture was kept to stir overnight at ambient
temperature. After the removal of the solid, the filtrate was reduced
to dryness. Precipitation from dichloromethaneehexane afforded
a brown powder product 7 (0.29 g, yield 80%). 1H NMR, CD2Cl2:
19.86 (t, 1H, [Ru]]CH), 8.28 (d, 1H, pyridine), 7.48 (t, 1H, pyridine),
7.11 (d, 1H, pyridine), 6.96 (t, 1H, pyridine), 3.79 (s, 6H, COOCH3)
3.48 (t, 2H, ]CHCH2CH2), 2.52 (m, 5H, ]CHCH2CH2 and P
(CHMe2)3), 1.34 (m, 18H, P(CHMe2)3). 31P NMR, CD2Cl2: 45.3. 19F
NMR, CDCl3: ꢁ145.19 (dd, 4F), ꢁ161.53 (t, 4F).
4.7. Synthesis of new ruthenium complex 10
Tl-6 (0.218 g, 0.51 mmol) and ruthenium complex 15 (0.218 g,
0.25 mmol) were mixed in dichloromethane (5 ml) in a glove box.
The resulting mixture was stirred at ambient temperature for 16 h.
The mixture was filtered and the filtrate was reduced to dryness
under reduced pressure. The resulting solid was washed with
dichloromethaneehexane (1:5 v/v) to afford
a green powder
product 10 (0.21 g, yield 77%). 1H NMR, CD2Cl2: 18.77 (s, 1H, [Ru]]
CH), 7.93 (d, 1H, pyridine), 7.69 (m, 1H, pyridine), 7.67 (m, 1H, Ph
CH), 7.47 (m, 1H, pyridine), 7.28 (d, 2H, Ph CH), 7.22 (br. s, 2H, Ph
CH), 7.13e7.09 (t, 2H, Mes CH), 6.79 (s, 1H, pyridine), 6.76(m, 2H, Ph,
Mes CH), 4.07 (m, 4H, sIMes), 3.77 (s, 3H, COOCH3), 3.69 (s, 3H,
COOCH3), 2.57 (s, 6H, Mes CH3), 2.41 (s, 6H, Mes CH3), 1.82 (s, 6H,
Mes CH3). 19F NMR, CDCl3: ꢁ145.19 (dd, 4F), ꢁ161.53 (t, 4F).
4.4. Synthesis of ruthenium complex 11
4.4.1. Method A
In a glove box, ruthenium complex 1a (0.045 g, 0.1 mmol) and
tricyclohexylphosphine (2.8 g, 10 mmol) were mixed in dichloro-
methane (5 ml). The reaction was kept at room temperature for
overnight. The volatiles were removed under reduced pressure and
the residue triturated with hexanes. The solid was collected,
washed with cold hexanes (3 ꢂ10 ml) and dried under reduced
pressure to give complex 11 as a pale green solid (0.033 g, yield
57%).
4.8. ROMP reactions for solution 1H NMR experiments
All solution 1H NMR experiments were carried out at a mono-
mer to initiator ratio of 20:1 in NMR tubes equipped with Young’s
tap. In a typical reaction the ruthenium initiator (10 mg) was
weighed into a sample vial and dissolved in CDCl3 (0.4 ml). Nor-
bornene dicarboximide monomer was weighed into another
sample vial and CDCl3 (0.5 ml) was added. The vial containing
monomer was added to the vial containing initiator and the
mixture stirred for 5 min. The reaction mixture was transferred into
an NMR tube. The reactions were carried out at room temperature
and at 55 ꢀC. The course of the reactions was followed by 1H NMR.
4.4.2. Method B
In a glove box, compound 13 (4.0 g, 30 mmol) and ruthenium
complex 12 (0.244 g, 0.3 mmol) were mixed in dichloromethane
(5 ml) and the reaction was stirred at room temperature for 16 h.
The volatiles were removed under reduced pressure and the
residue triturated with hexanes. The solid was collected, washed
with cold hexanes (3 ꢂ 5 ml) and dried under reduced pressure to
give ruthenium complex 11 as a pale green solid (0.11 g, yield 65%).
1H NMR, CDCl3: 17.75 (s, 1H, [Ru]]CH), 7.48 (m, 2H, pyridine), 7.38
(m, 2H, pyridine), 2.97 (m, 4H, ]CHCH2CH2), 1.29 (m, 18H, P
(C6H11)3). 31P NMR, CDCl3: 45.3.
4.9. ROMP reactions for DSC experiments
All ROMP reactions for DSC experiments were carried out at
a monomer to initiator ratio of 50:1. The catalysts was weighed into
the mixing vessel and dissolved in a minimum of deuterated
chloroform (5 drops). The monomer was added and mixed by dual
asymmetric centrifuge (DAC) mixer for 5 min. The sample was then
placed in a vacuum chamber, equipped with an Edward 5 Vacuum
pump, at room temperature for 30 min to ensure all solvent was
removed. Approx. 5 mg of the material was transferred into
a standard PerkineElmer pan. The open pan was then placed into
the TA Instruments Q100 DSC System. Specimens were heated from
25 ꢀC to 300 ꢀC at a rate of 10 ꢀC/min in a nitrogen atmosphere.
4.5. Synthesis of new ruthenium initiator 8
Tl-6 (0.122 g, 0.29 mmol) and ruthenium complex 11 (0.0813 g,
0.14 mmol) were mixed in dichloromethane (3 ml) in a glove box.
The resulting mixture was stirred at ambient temperature for 16 h.
The mixture was filtered and the filtrate was reduced to dryness

Z. Yu et al. / Journal of Organometallic Chemistry 696 (2011) 1591e1599
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Table 4
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4035e4037.
[11] T.C. Castle, E. Khosravi, L.R. Hutchings, Macromolecules 39 (2006) 5639e5645.
[12] A.M. Tayer, Chem. Eng. News (February 12, 2007) 37e47.
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[14] I. Czelusniak, E. Khosravi, A.M. Kenwright, C.W.G. Ansell, Macromolecules
40 (2007) 1444e1452.
[15] R.H. Grubbs, Handbook of Metathesis. Wiley-VCH, Weinheim, Germany, 2003.
[16] C. Slugovc, Macromol. Rapid Commun. 25 (2004) 1283e1297.
[17] A. Leitgeb, J. Wappel, C. Slugovc, Polymer 51 (2010) 2927e2946.
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2255e2258.
Crystal data and X-ray experiment details.
Compound
10
7
Empirical formula
Formula weight
T, K
C49H42BrF8N3O6Ru$½CH2Cl2
1144.30
120
C33H36F8NO6PRu
826.67
120
Crystal system
Space group (No.)
a, Å
b, Å
c, Å
Monoclinic
P21/c (# 14)
21.027(2)
20.353(2)
23.900(2)
90
112.78(1)
90
9431(1)
8
Triclinic
P-1 (# 2)
9.9036(10)
11.1367(11)
16.1893(15)
89.72(1)
82.75(1)
82.52(1)
1756.1(3)
2
1.563
0.58
20,920
9281
0.045
0.048
0.101
ꢀ
ꢀ
ꢀ
a
,
b
,
g
,
V, Å3
Z
r
m
(calc.), g/cm3
1.612
1.32
(Mo K
a
), mmꢁ1
Reflections collected
Independent reflections
R(int)
111,493
24,998
0.051
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Soc. Rev. 38 (2009) 3360e3372.
R [I > 2
s
(I)]
0.038
0.090
wR(F2) [all data]
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121 (1999) 791e799.
4.10. X-ray crystallography
Experiments (Table 4) were carried out on a Siemens 3-circle
diffractometer with a SMART 1000 CCD area detector, using
graphite-monochromated Mo K
ꢀ
a
radiation (
l
¼ 0:71073 A) and
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(2000) 8168e8179.
a
Cryostream (Oxford Cryosystems) open-flow N2 cryostat.
Diffraction data were measured using 0.3ꢀ
u
-scans and corrected
[32] S. Gessler, S. Randl, S. Blechert, Tetrahedron Lett. 41 (2000) 9973e9976.
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J.S. Kingsbury, J.P.A. Harrity, Org. Biomol. Chem. 2 (2004) 8e23.
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K. Grela, Chem. Eur. J. 14 (2008) 9330e9337.
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22 (2003) 3634e3636.
for absorption by Gaussian integration based on crystal face-
indexing [52]. The structures were solved by direct methods and
refined by full-matrix least squares against F2 of all reflections,
using SHELXTL software [53].
[37] J.C. Conrad, J.L. Snelgrove, M.D. Eeelman, S. Hall, D.E. Fogg, J. Mol. Catal.
A Chem. 254 (2006) 105e110.
Acknowledgements
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[39] S. Monfette, D.E. Fogg, Organometallics 25 (2006) 1940e1944.
[40] A. Michrowska, R. Bujok, S. Harutyunyan, V. Sashuk, G. Dolgonos, K. Grela,
J. Am. Chem. Soc. 126 (2004) 9318e9325.
We thank ICI for their financial support of the project. We also
thank Mr. Douglas Carswell for performing the thermal analyses.
[41] L. Gulajski, A. Michrowska, R. Bujok, K. Grela, J. Mol. Catal. A Chem. 254 (2006)
118e123.
Appendix A. Supplementary material
[42] M.S. Sanford, J.A. Love, R.H. Grubbs, J. Am. Chem. Soc. 123 (2001) 6543e6554.
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5533e5536.
[45] A. Furstner, P.W. Davies, C.W. Lehmann, Organometallics 24 (2005)
4065e4071.
CCDC 751991 for 10 ½CH2Cl2, and 751992 for 7 contain the
supplementary crystallographic data for this paper. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Center via www.ccdc.cam.ac.uk/data_request/cif.
[46] S. Nag, K. Banerjee, D. Datta, New J. Chem. 31 (2007) 832e834.
[47] S.S. Batsanov, Inorg. Mater. 37 (2001) 871.
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