Vinyl and ring-opening metathesis polymerization of norbornene with novel half-sandwich iridium(III) complexes bearing hydroxyindanimine ligands

Article abstract of DOI:10.1039/b803382j

Half-sandwich iridium complexes bearing hydroxyindanimine ligands were synthesized and employed as catalysts for the ROMP and vinyl-type polymerization of norbornene in the presence of methylaluminoxane (MAO). The Royal Society of Chemistry.

Full text of DOI:10.1039/b803382j

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Vinyl and ring-opening metathesis polymerization of norbornene with
novel half-sandwich iridium(III) complexes bearing hydroxyindanimine
ligandsw
Xia Meng, Guang-Rong Tang and Guo-Xin Jin*
Received (in Cambridge, UK) 27th February 2008, Accepted 10th April 2008
First published as an Advance Article on the web 8th May 2008
DOI: 10.1039/b803382j
Half-sandwich iridium complexes bearing hydroxyindanimine
ligands were synthesized and employed as catalysts for the
ROMP and vinyl-type polymerization of norbornene in the
presence of methylaluminoxane (MAO).
Norbornene polymerization has been widely used in industrial
productions due to the special optical and mechanical proper-
ties of the polymers.1 It has been established that different
Chart 1 ROMP and vinyl polymerization of norbornene.
methods of polymerization produce polynorbornene with
different microstructures. Ring opening metathesis polymer-
ization (ROMP) produces polynorbornene with double bonds
remaining in the polymer backbone. Up to now, there are two
major types of catalysts used for ROMP of norbornene: one is
tungsten, molybdenum, rhenium and ruthenium catalysts as
metal halides or metal oxides in combination with alkylating
agents, the other is metal–carbene complexes based on tung-
sten, molybdenum or ruthenium, etc. Another type of poly-
merization is vinyl-type polymerization. It opens the
p-component of the double bond in norbornene, keeping the
bicyclic unit in the polymer chain. Catalysts containing tita-
nium, zirconium, cobalt, chromium, nickel and palladium
have been reported for vinyl-type polymerization of norbor-
nene and copolymerization of norbornene with acyclic
olefins.1 Recently, our group has also developed some high
activity late transition metal catalysts for vinyl-type polymer-
ization.2 Although the catalysts for ROMP and vinyl poly-
merization have been widely developed, there are only a few
iridium complexes involved in this field (Chart 1).1
vinyl-type polymers under different ratios of cocatalyst
(MAO) to iridium.
The half-sandwich iridium complexes 1–3 were synthesized
by the reaction of [Cp*IrCl(m-Cl)]2 (Cp* = Z5-pentamethyl-
cyclopentadienyl) with the corresponding lithium salts of the
[C6H3-i-(R1)2-2,6]NQCC2H3(CH3)C6H(CH3)(R2)OH ligands
in THF (Scheme 1).z Complexes 1–3 are air stable orange–red
solids. Single crystals of complex 1 and 3 were obtained by
recrystallization from hexane–toluene solution at low tem-
perature. The 1H NMR resonance of complexes 1–3 is well
consistent with the expected hydroxyindanimine structure. A
sharp single resonance of Cp* appears at about
1.32–1.28 ppm.
X-Ray crystallographic analysis shows that complexes 1 and
3 have similar configurations (Fig. 1(a, b)). Both of them adopt
three-legged piano stool geometry, containing a hydroxy-
indanimine ligand, a Cp* ligand and chloride. The six-mem-
bered chelating rings Ir(1)–O(1)–C(1)–C(6)–C(7)–N(1) locate
in the same plane. Although well-refined crystallographic
analysis data for complex 2 were not achieved, the DFT
optimised molecular structure based on complex 1 and 3
proved to have a similar configuration and bonding character.
On the basis of our previous experience, iridium complexes
can probably initiate olefin polymerization in an efficient way.5
As a trial, norbornene polymerization tests were carried out.y
The first attempt consisted of activating complex 1, 2 and 3
with 1000 eq. methylaluminoxane (MAO) to form an active
In addition, there are several reports on catalysts which can
initiate both ROMP and vinyl-type polymerization, such as
titanium3 and cobalt,4 depending on the cocatalyst ratio or the
type of cocatalyst. However, the polymers obtained by those
catalysts are only mixtures of the two types, and cannot be
separated effectively.
Herein we report half-sandwich iridium complexes contain-
ing hydroxyindanimine ligands, which could be used as nor-
bornene polymerization catalysts, producing pure ROMP and
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, and Key Lab of Molecular
Engineering of Polymers of Chinese Ministry of Education,
Department of Macromolecular Science, Fudan University, Shanghai
200 433, China. E-mail: gxjin@fudan.edu.cn; Fax: +86-21-65641740;
Tel: +86-21-65643776
w Electronic supplementary information (ESI) available: Experimental
procedures; crystallographic data in cif format, CCDC numbers
679281 and 679282. See DOI: 10.1039/b803382j
Scheme 1 Synthesis of complexes 1, 2, 3.
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Fig. 2 Catalytic behaviour of complex 2.
Fig. 1 Thermal ellipsoid plot of 1 (a) and 3 (b) (30% probability
thermal ellipsoids). Selected bond lengths [A] and angles [1]:
1, Ir(1)–O(1) = 2.081(6), Ir(1)–N(1) = 2.091(6), Ir(1)–Cl(1) =
2.419(3), O(1)–Ir(1)–N(1) = 90.0(2), O(1)–Ir(1)–Cl(1) = 84.75(19),
N(1)–Ir(1)–Cl(1) = 82.67(19). 3, Ir(1)–O(1) = 2.080(3), Ir(1)–N(1) =
2.101(4), Ir(1)–Cl(1) = 2.4201(17), O(1)–Ir(1)–N(1) = 88.38(13),
O(1)–Ir(1)–Cl(1) = 86.57(10), N(1)–Ir(1)–Cl(1) = 83.30(11).
suggesting two different mechanisms might occur with the
increase in amount of cocatalyst (MAO).
Surprisingly, IR, 1H NMR and 13C NMR studies of the
polymers indicate that: low-MAO catalyst produces the
ROMP polymer, while the high-MAO catalyst initiates
vinyl-type polymerization. In FT-IR spectra, the low-MAO
polymer shows broad bands at 2900 cmꢂ1 (C–H stretch), 1680
cmꢂ1 (CQC stretch), and several bands in the fingerprint
region especially at 966 cmꢂ1 and 733 cmꢂ1 (trans and cis
form of CQC bonds).6 On the contrary, high-MAO polymer
shows no absorption at 1600–1700 cmꢂ1, 966 cmꢂ1 and 733
cmꢂ1. But a peak appears at about 942 cmꢂ1 assigned to the
ring system of bicycle [2.2.1] heptane suggesting vinyl-type
polymerization.7
species and employing them in norbornene polymerization.
Successively, white precipitate of polymer product was ob-
tained after the system was stirred for 1 hour and terminated
with acidic methanol. Besides, a notable color change during
polymerization was observed. Especially for complex 2, when
MAO was added into the catalyst (in chlorobenzene) drop by
drop, the color of the mixture would change from light orange
to dark red gradually. This color would suddenly diminish to
light yellow if more MAO was charged. The remarkable color
change suggested that some stable intermediatez might be
formed by employing a little amount of cocatalyst but easily
destroyed when an excessive amount of MAO was added.
Exploration of this intermediate and its catalytic behavior
was carried out by adding a small amount of MAO (about
10 eq.) into the precatalyst (complex 2) so that the mixture
maintained the dark red color. Interestingly, white polynor-
bornene product could also be obtained. If the temperatures
rose up to 60 1C within 15 minutes, the viscosity of the
solution increased distinctively and made it hard for magnetic
stirring, indicating a high activity of this low-MAO system.
Due to a more distinct color change than complex 1 and 3,
complex 2 was chosen as precatalyst for the study of the
polymerization in detail.
The 1H NMR spectra could also prove that low-MAO
polymer (Fig. 3) is a pure ROMP product. The QCH reso-
nances appear at 5.22 ppm (trans) and 5.04 (cis), and the a-H
at 2.65 ppm (cis) and 2.27 ppm (trans). The ratio of cis/trans is
0.7. The 13C NMR shows resonance at 134.0–133.1 ppm (C2,
C3), 43.0–42.0 ppm (C1, C4), 38.8 ppm (C7), 32.7 ppm (C5, C6),
consistent with the previous data reported for polymers of
1
NBE prepared via ROMP.8 In H NMR spectra of the high-
MAO polymer (Fig. 4), several broad resonances show at
2.19–2.16 ppm, 2.11–2.07 ppm, 1.64–1.61 ppm, 1.35 ppm,
1.07–0.66 ppm.9 And resonances at 52–45 ppm (C2, C3),
39–38 ppm (C1, C4), 36 ppm (C7), 30 ppm (C5, C4) in 13C
NMR are all in good agreement with the reported vinyl type
polymer.9
Temperature greatly affects the activity of ROMP in the
low-MAO system. At 30 1C, the yield of ROMP polymer was
only 1.4% (norbornene : catalyst = 1000 : 1) after 6 hours, but
it rose up to 30% at 60 1C. However, for the high-MAO
system, the temperature effect is negligible.
In order to get a full image of the catalytic behavior of this
low- (10 eq.) and high- (1000 eq.) MAO/iridium complex
system, a series of experiments was carefully performed: a
determined amount of MAO (range from 5 eq. to 3000 eq.)
was charged into the catalyst–norbornene chlorobenzene solu-
tion at 60 1C, and stirred for certain time. The polymer was
precipitated from acidic methanol, washed with a great
amount of methanol and dried under vacuum. The activities
were measured and are shown in Fig. 2. As it demonstrates,
the highest activity appears at 15 eq. of MAO which is 4.8 ꢁ
104 g PNB molꢂ1 hꢂ1. When more MAO is added, the activity
decreases distinctively and 30 eq. of MAO makes this system
inactive for norbornene polymerization. After this point, the
activity increases gradually with the increase of MAO, and the
highest point is observed at 2000 eq. of MAO which is 2.8 ꢁ
104 g PNB molꢂ1 hꢂ1. It is important to mention that the zero
point of activity coincides with the changing point of color,
Fig. 3 1H NMR of the low-MAO polymer initiated with complex 2.
Chem. Commun., 2008, 3178–3180 | 3179
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ꢂ30 1C to give a red solid. Red crystals were obtained through
recrystallization from hexane–toluene solution at ꢂ30 1C. Crystal
data: for complex 1: C27H31ClIrNO, M = 613.18, triclinic, space
group P-1, a = 8.659(5), b = 9.353(8), c = 16.720(10) A, a =
84.110(12)1, b = 85.694(9)1, g = 63.243(7)1, V = 1202.2(14) A3, Dc =
1.694 g cmꢂ3 (Z = 2) at 20 1C, R1 = 0.0585 (all data), Rw = 0.1228
(all data), reflections collected/unique = 5039/4150, Rint = 0.0347.
For complex 3: C32.5H42.5ClIrNO, M = 690.83, monoclinic, space
group P2(1)/c, a = 18.612(14), b = 23.534(12), c = 14.195(9) A, a =
901, b = 103.526(8)1, g = 901, V = 6045(6) A3, Dc = 1.518 g cmꢂ3
(Z = 8) at 20 1C, R1 = 0.0531 (all data), Rw = 0.0943 (all data),
reflections collected/unique
CCDC 679281 and 679282.
=
29594/13144, Rint
=
0.0281.
Fig. 4 1H NMR of the high-MAO polymer initiated with complex 2.
y Typical procedure for ROMP: 5.0 mmol of complex 2 in 1.0 mL of
chlorobenzene, 1.3 g of norbornene in 2.0 mL of chlorobenzene and
2.0 mL of chlorobenzene were added into a polymerization bottle
under nitrogen atmosphere at different temperatures. Certain amount
of MAO (0.017–0.17 mL, 1.5 mmol mLꢂ1) was charged into the
system to initiate polymerization. Sometime later, acidic methanol
(Vmethanol : Vconcd.HCl = 20 : 1) was added to terminate the reaction.
The polymer was isolated by filtration, washed with a large amount of
methanol and dried under vacuum for 48 h. Typical procedure for
vinyl polymerization: 1.0 mmol of complex 2 in 1.0 mL of chloroben-
zene, 1.3 g of norbornene in 2.0 mL of chlorobenzene and 2.0 mL of
chlorobenzene were added into a polymerization bottle under nitrogen
atmosphere at different temperatures. The total volume of the system
was 5 mL. Certain amount of MAO (0.06–2 mL, 1.5 mmol mLꢂ1) was
Attempts to isolate the dark red intermediate were unsuccessful
because the addition of MAO brought great complexity and
sensitivity to the system. Neither the single component of MAO
nor complexes 1–3 in the absence of MAO show any activity for
ROMP or vinyl-type polymerization of norbornene. GPC analysis
revealed that the molecular weight distributions (Mw/Mn) of
typical low-MAO (complex 2, 15 eq. of MAO, 60 1C) and high-
MAO (complex 2, 1000 eq. of MAO, 60 1C) polymers are 2.63
(Mw = 2.90 ꢁ 104) and 3.49 (Mw = 3.35 ꢁ 106), respectively,
suggesting a single-site Ir species exists in polymerization. Accord-
ing to the literature, we infer that the catalytically active inter-
mediate might be an iridium carbene complex generated from the
alkalization of MAO and subsequent a-hydrogen abstraction.10
With the increase of cocatalyst, this carbene complex decomposed,
so the activity decreased to zero. After that, another active species
formed with greater amount of MAO and the system exhibited
vinyl-type polymerization. The mechanism of this catalytic process
is still under investigation.
added into the system. One hour later, acidic methanol (Vmethanol
:
Vconcd.HCl = 20 : 1) was added to terminate the reaction. The polymer
was isolated by filtration, washed with methanol and dried at 80 1C
under vacuum for 48 h.
z The intermediate is stable enough to keep in the Schlenk tube in the
absence of air and water for a long time (more than 3 days).
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In summery, novel half-sandwich iridium complexes 1–3
were synthesized and employed for norbornene polymeriza-
tion. Pure ROMP polymer and vinyl-type polymer were
obtained depending on the amount of MAO (0–30 eq. for
ROMP and 430 eq. for vinyl-type polymerization). This
interesting behavior not only provides a promising iridium
catalyst for ROMP and vinyl polymerization of norbornene
for the first time, but also shed light on copolymerization of
norbornene by ROMP and vinyl-type polymerization by
changing the cocatalyst ratio.
This work was supported by the National Science Founda-
tion of China (20531020, 20721063, 20771028), by Shanghai
Leading Academic Discipline project (B108), by the National
Basic Research Program of China (2005CB623800) and by
Shanghai Science and Technology Committee (06XD14002).
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Notes and references
z General procedures for the synthesis of complexes 1, 2 and 3: a
solution of n-BuLi (1.6 M, 0.28 mL, 0.45 mmol) in hexane was added
dropwise to a stirred solution of ligand L1–L3 (0.41 mmol) in THF
(10 mL) at ꢂ78 1C. The mixture was slowly warmed to room
temperature and stirred for 3 h, then channeled to a suspension of
[Cp*IrCl2]2 (0.16 g, 0.2 mmol) in THF (10 mL) and continuously
stirred overnight. The solvent was removed under vacuum and the
residual solid was extracted with toluene and filtered to remove LiCl.
The red solution was concentrated to about 5 mL, and cooled to
10. J. L. Brumaghim and G. S. Girolami, Organometallics, 1999, 18,
192.
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3180 | Chem. Commun., 2008, 3178–3180