Imidotungsten(VI) complexes with chelating phenols as ROMP catalysts

Article abstract of DOI:10.1016/j.inoche.2011.05.021

Tungsten(VI) complexes of the type [W(NPh)Cl₃(L)] (L = chelating phenolate) were studied as catalyst precursors for ROMP of 2-norbornene, dicyclopentadiene and 5-vinyl-2-norbornene. These compounds form active catalysts when treated by ethyl magnesium bromide. Moreover, polymerisations can be run under ambient atmosphere without complicated inert atmosphere techniques. Synthesis and crystal structure of a new precursor complex [W(NPh)Cl₃(L^S)] (L^S = 2,4-di-tert-butyl-6- (phenylthiomethyl)phenolate) are also described.

Full text of DOI:10.1016/j.inoche.2011.05.021

Inorganic Chemistry Communications 14 (2011) 1362–1364
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
☆
Imidotungsten(VI) complexes with chelating phenols as ROMP catalysts
Juuso Hakala a, Mikko M. Hänninen b, Ari Lehtonen a,
⁎
a
Laboratory of Materials Chemistry and Chemical Analysis, Department of Chemistry, University of Turku, FI-20014, Turku, Finland
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Jyväskylä, FI-40014 Jyväskylä, Finland
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Tungsten(VI) complexes of the type [W(NPh)Cl3(L)] (L=chelating phenolate) were studied as catalyst
precursors for ROMP of 2-norbornene, dicyclopentadiene and 5-vinyl-2-norbornene. These compounds form
active catalysts when treated by ethyl magnesium bromide. Moreover, polymerisations can be run under
ambient atmosphere without complicated inert atmosphere techniques. Synthesis and crystal structure of a
new precursor complex [W(NPh)Cl3(LS)] (LS =2,4-di-tert-butyl-6-(phenylthiomethyl)phenolate) are also
described.
Received 10 February 2011
Accepted 20 May 2011
Available online 27 May 2011
Keywords:
ROMP
Phenolate ligands
Tungsten
© 2011 Elsevier B.V. All rights reserved.
Catalysis
Ring-opening metathesis polymerization (ROMP) of norbornene
derivatives generate polymers with diverse features, which can be
tailored by the functional groups substituted at the backbone. The
easiest way to run olefin metathesis is to use some well-defined
catalysts, such as Mo-based Schrock catalysts or Ru-based Grubbs
catalysts [1,2]. As these catalysts are expensive and highly sensitive to
air and moisture, they are rarely used in the preparation of low-cost
products, such as elastomers or other polymeric materials. For
practical use, the isolation of active catalyst species is not necessary;
for example, tungsten phenoxides [WCl6-x(OAr)x] and [WOCl4-y
(OAr)y] are used on a regular basis as precursors for two-component
catalyst systems, typically in the presence of an organometallic
activator [3–6]. As typical for high-valent metal chlorides, these
catalyst precursors are also rather sensitive towards water and other
nucleophiles. Especially, if the coordination number of the metal
centre is low, these compounds must be handled under an inert
atmosphere using rather tedious practices. The hydrolytic stability of
high-valent tungsten complexes can be improved by coordinative
protection, i.e. by using chelating ligands, which carry a potentially
coordinating neutral group within the molecule [7,8]. Such hemilabile
ligands are important in catalysis because they stabilize inactive states
while freeing up coordination sites when they are needed, thus
adopting the role of a coordinating solvent [9].
We have previously studied the phenoxide chemistry of imido-
tungsten(VI) and prepared compounds 1–3 with chelating phenox-
ides (see Fig. 1) using a simple two-step reaction [8,10]. The first step
comprises a reaction of WOCl4 with a stoichiometric amount of
phenolic ligand precursor in CH2Cl2, whereas the second step involves
a reaction of formed tungsten oxo compound with phenyl isocyanate
in toluene, which reaction produces a final imido complex. These
complexes are stable in air and they can be easily purified by crys-
tallization or by column chromatography. Complex 4 was prepared
similarly using 2,4-di-tert-butyl-6-(phenylthiomethyl)phenol [11] as
a ligand precursor and it was isolated by crystallisation from dry
acetonitrile in a moderate yield.1 It is stable as a crystalline material
under inert atmosphere but decomposes slowly in solutions. The
molecular structure of 4 consists of neutral mononuclear units, in
which the central W(VI) ion is surrounded by one terminal phenyl
imido group, one bidentate phenolate and three chlorides (Fig. 2). As
expected due to the strong structural trans-effect of the imido
nitrogen, the neutral sulphur donor is situated trans to the imido
group with the coordinative W–S bond length of 2.7221(11). The
overall coordination geometry and bonding parameters around the
1
WOCl4 (1.0 mmol, 340 mg) and 2,4-di-tert-butyl-6-(phenylthiomethyl)phenol
(1.0 mmol, 330 mg) were heated in CH2Cl2 (20 ml) at reflux temperature for 2 h.
The intense red solution was evaporated and the residue was dissolved in PhMe
(10 ml) and refluxed with phenyl isocyanate (1.5 mmol, 0.16 ml) for 2 h. Filtered
solution was evaporated and residue was crystallised from acetonitrile (2 ml) at
refrigerator to obtain 4 in a 28% (180 mg) yield as deep purple crystals. NMR δH
(CDCl3, standard SiMe4): 7.57–7.64 (overlapping signals, 4H, ArHthioether and ArHimido),
7.48 (d, J=2.4 Hz, 1H, ArHphenolate), 7.37–7.39 (overlapping signals, 5H, ArHthioether
and ArHimido), 7.17 (d, J=2.4 Hz, 1H, ArHphenolate), 7.09 (m, 1H, ArHimido), 4.77 (2H,
CH2–S), 5.35 (m, 6H), 1.48 (9H, tBu), 1.34 (9H, tBu). Found: C, 46.3; H, 5.0.
2Crystal data for 4: C27H32Cl3NOSW, space group P121/n1, a=13.78090(20),
☆
b=14.04840(29), c=15.22950(29) Å, β=106.8858(11), V=2821.305(91), Z=4. The
structure was refined on F2 to R=0.0281, wR=0.0537 for all 5535 reflections. Single-
crystal data collections, data reduction and subsequent calculations were essentially as
described in earlier papers from our group. [8,10].
⁎
Corresponding author at: University of Turku, Department of Chemistry, FIN-20014
TURKU, Finland. Tel.: +358 2 3336733; fax: +358 2 3336700.
E-mail address: Ari.Lehtonen@utu.fi (A. Lehtonen).
C27H32Cl3NOSW requires C, 45.8; H, 4.6%.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.05.021

J. Hakala et al. / Inorganic Chemistry Communications 14 (2011) 1362–1364
1363
Fig. 1. Catalyst precursors for ROMP of norbornene derivatives.
metal ion are comparable to those found for structurally correspond-
ing aminophenolate and iminophenolate compounds [8,10].
analyses of these polymers indicate that the polymerisation of
dicyclopentadiene and 5-vinyl-2-norbornene occurs through norbor-
nene rings while the additional C=C double bonds remain intact.
Microstructures of the norbornene polymers were analysed by 1H
NMR, 13C NMR and IR spectroscopy to estimate the contents of cis and
trans (−HC=CH–) units in a polymer backbone. The proton spectra
peaks from 5.2 to 5.4 ppm indicate the presence of unsaturated
structures, which arise from ring-opening polymerization chains.
More precisely, in a CDCl3 solution, the cis and trans ethylenic protons
give peaks at 5.24 and 5.38, respectively. Corresponding α-protons
show resonances at 2.8 and 2.45 ppm [12,13]. For ethylenic carbon
atoms, two sets of resonances are seen: one for carbon atoms
positioned in trans RHC=CHR groups (around 133 ppm) and another
for carbon atoms positioned in cis RHC=CHR groups (around
134 ppm) [12,13]. The IR spectra of studied polymers show absorption
bands at ca. 730 and 970 cm−1 for bending modes due to cis and trans
substituted C=C double bonds [14]. The microstructures of poly-
dicyclopentadiene and poly(5-vinyl-2-norbornene) were also studied
by abovementioned spectroscopy although the analysis of NMR
spectra is more complicated due to the additional C=C double bond
in the monomeric units. Nevertheless, IR absorption intensities can be
used to assess the stereochemistry of the polymers. Though it is
difficult to estimate the exact intensity ratio of these IR-bands, in
all spectra their intensity is consistent with the results obtained by
NMR. Accordingly, it was found that in all polymers, the cis-content is
clearly over 80%. The high cis-content can be explained by the steric
hindrance of the catalytically active species. It is known, that the low
coordination number of the metal site favours the trans products,
whereas more crowded metal centre yields predominantly cis double
bonds [Ref 1, p. 241]. In the studied catalyst systems, the supposed
activation process, i.e. dialkylation and α-elimination, leads to the
formation of pentacoordinated carbene species. As studied catalyst
precursors have closely similar structures, they performed essentially
similarly under identical conditions.
We have earlier found that catalyst precursor 1 polymerises
norbornene derivatives when activated by Et2AlCl [8]. In the present
study we have applied precursors 1–4 in two-component catalyst
system using ROMP of 2-norbornene, dicyclopentadiene and 5-vinyl-
2-norbornene (the mixture of endo and exo) as test reactions.
2-norbornene and dicyclopentadiene were polymerised in various
reaction media (toluene, chlorobenzene, 1,2-dichlorobenzene)
whereas 5-vinyl-2-norbornene was polymerised without any solvent.
The overall conversion of monomer to methanol-insoluble polymers
was used as a measure of activity. Imidotungsten(VI) complexes 1–4
were dissolved in solvent/cycloalkene mixtures and were activated
by ethyl magnesium bromide in a 200:5:1 monomer:activator:
tungsten ratio. The conceivable role of the organomagnesium activator
is to replace two chloride ligands on tungsten with alkyl groups;
the resultant organotungsten intermediate would then undergo an
α-elimination reaction to afford the corresponding alkylidene deriv-
ative [1]. Moreover, the excess Grignard reagent can scavenge protic
impurities from the reaction mixture. As a result, the solutions
changed from a red to a yellow and viscosity of the solution increased
with time. The polymeric materials that formed were precipitated by
methanol, dried and weighed. In all experiments, poly(2-norbornene)
and poly(cyclopentadiene) were formed in 85–95% yields as white
solid materials whereas the polymers of 5-vinyl-2-norbornene were
isolated in 40–50% yields as sticky solids. All catalyst precursors
presented practically similar activity in studied reactions. Spectral
In conclusion, imidotungsten(VI) complexes with chelating phe-
nolates can be activated by EtMgBr to catalyse ROMP of norbornene
derivatives. Polymerisations can be run simply under ambient
conditions without inert atmosphere techniques. All solid polymers
have a high cis-content.
Acknowledgement
Turku University Foundation and University of Jyväskylä are
acknowledged for financial support.
Appendix A. Supplementary material
CCDC 812274 contains the supplementary crystallographic data
for compound 4. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Fig. 2. A molecular structure of 4. Thermal ellipsoids are drawn at 30% probability level.
Selected bond distances (Å) and angles (º) N1–W1: 1.736(3), O1–W1: 1.866(2),
S8–W1: 2.7221(11), Cl1–W1: 2.3712(11), Cl2–W1: 2.3209(10), Cl3–W1: 2.3619(11),
N1–W1–S8: 171.20(9), O1–W1–S8: 76.12(7).

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J. Hakala et al. / Inorganic Chemistry Communications 14 (2011) 1362–1364
(b) P.A. van der Schaaf, D.M. Grove, W.J.J. Smeets, A.L. Spek, G. van Koten,
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