Ruthenium(IV) complexes for ethylene insertion polymerization

Article abstract of DOI:10.1021/om5001343

The synthesis, characterization, and ethylene polymerization behavior of novel Ru^IV(η³:η³-C₁₀H ₁₆)(OPO) (OPO = bis(arenesulfonato)phosphine) complexes is reported here. Upon activation with AlMe₃-depleted methylaluminoxane (dMAO), the Ru(IV) precursors were able to produce polyethylene with activities up to 1182 h^-1 turnover frequency (TOF). The polymers were highly linear with a low degree of branching (<12 methyl branches per 1000 C) and had high molecular weights (up to M_p = 289 kg mol^-1) with a bimodal molecular weight distribution. The polymerization activity increased with decreasing donor strength of the OPO ligand.

Full text of DOI:10.1021/om5001343

Communication
pubs.acs.org/Organometallics
Ruthenium(IV) Complexes for Ethylene Insertion Polymerization
Tobias Friedberger, Joseph W. Ziller, and Zhibin Guan*
Department of Chemistry, University of California, 1102 Natural Sciences 2, Irvine, California 92697-2025, United States
S
* Supporting Information
ABSTRACT: The synthesis, characterization, and ethylene poly-
merization behavior of novel RuIV(η3:η3-C10H16)(OPO) (OPO =
bis(arenesulfonato)phosphine) complexes is reported here. Upon
activation with AlMe3-depleted methylaluminoxane (dMAO), the
Ru(IV) precursors were able to produce polyethylene with activities
up to 1182 h−1 turnover frequency (TOF). The polymers were
highly linear with a low degree of branching (<12 methyl branches
per 1000 C) and had high molecular weights (up to Mp = 289 kg
mol−1) with a bimodal molecular weight distribution. The polymerization activity increased with decreasing donor strength of the
OPO ligand.
illions of pounds of polyolefins are produced annually to
and co-workers15 first reported the synthesis of high-molecular-
Bserve various applications, including plastics, elastomers, weight polyethylene with a RuII(pybox) complex. However,
and fibers.1 The design of novel olefin polymerization catalysts
Brookhart and co-workers later showed that an analogous
remains at the frontier of polyolefin technology.2 Traditionally,
the heterogeneous Ziegler−Natta catalysts3 and homogeneous
metallocene catalysts2a,d,4 for olefin insertion polymerization are
based on early-transition-metal (early-TM) complexes with high
oxidation states.5 More recently, the potential of late-TM
catalysts for olefin polymerization has been explored. While
Ni-based oligomerization catalysts have been employed
industrially in the Shell Higher Olefins Process (SHOP),6 the
introduction of bulky diimine ligands to Ni(II) and Pd(II)
complexes resulted in the first successful late-TM polymerization
catalysts in the mid-1990s.7 This discovery by Brookhart and co-
workers has sparked tremendous interest in the olefin polymer-
ization community and led to the development of several highly
active late-TM catalyst systems. For example, Fe and Co
complexes of bulky diiminopyridine ligands were shown to
exhibit polymerization activities comparable to those of the most
active Ziegler−Natta catalysts.8 Active neutral
NiII(salicylaldiminato) complexes were also reported for ethyl-
ene homo- and copolymerizations.9 In addition, late-TM
catalysts are less sensitive toward polar functionalities. This
tolerance has been exploited to obtain copolymers of various
polar olefins with neutral palladium catalysts coordinated to o-
phosphinobenzenesulfonate ligands.10
RuII(diiminopyridine) system was inactive for ethylene polymer-
ization. Furthermore, they were able to obtain the putative active
species of their system, the [RuII(diiminopyridine)Me(C2H4)]+
cation, which was inactive toward migratory insertion of
ethylene.16 The authors proposed that the meridional coordina-
tion geometry of the ligand results in distinctly different
coordination sites for the coordinated ethylene and alkyl group
(cis and trans to pyridine), which makes alkyl migration
energetically unfavorable. A computational study calculated the
insertion barriers for this system to be >25 kcal mol−1, which is
conceivably too high for active ethylene polymerization.17 More
recently, a neutral Ru(II) complex containing two o-
phosphinobenzenesulfonate ligands was reported to exhibit
relatively low activity toward ethylene polymerization.18
Surprisingly, the obtained polymer was found to be chemically
cross-linked. Similar to Ru, reports on Rh complexes for olefin
polymerization are also rare.19
Recently, our laboratory has unambiguously demonstrated for
the first time the insertion polymerization of ethylene by a
ruthenium metal center.20 The thioether-tethered RuII(arene)
complex produced polyethylene with relatively low activity. We
reasoned that Ru complexes with higher oxidation states should
lead to an increase in the catalyst activity analogous to early TM.
Ru(IV) was chosen due to its diamagnetic nature, facilitating
NMR spectroscopic characterization. We employed bis-
(arenesulfonato)phosphine (OPO) ligands to stabilize the
Ru(IV) center. The OPO ligand motif was first reported by
Jordan and co-workers21 as a bidentate ligand for Pd(II)
complexes for ethylene polymerization. The dianionic ligand
was envisioned to chelate the Ru(IV) center, which upon
activation could afford a monocationic RuIV−alkyl complex as
Ruthenium complexes have shown great versatility as catalysts
for various transformations, most notably for hydrogenation11
and olefin metathesis reactions.12 The high affinity for olefin
binding combined with good functional group tolerance for Ru
complexes13 makes them desirable for developing new olefin
polymerization catalysts. Nevertheless, despite the high activity
of FeII(diiminopyridine) catalysts,8 surprisingly few Ru-based
olefin insertion polymerization catalysts have been reported. In
addition to sporadic uses of RuII−hydrides for olefin polymer-
ization13 and direct and indirect demonstrations of ethylene
insertion into RuII−hydride, −aryl, and −alkyl bonds,14 Nomura
Received: February 5, 2014
Published: April 16, 2014
© 2014 American Chemical Society
1913
dx.doi.org/10.1021/om5001343 | Organometallics 2014, 33, 1913−1916

Organometallics
Communication
the active catalytic species for olefin insertion polymerization. In
this report we describe the synthesis and characterization of a
series of RuIV(OPO) complexes and their ethylene polymer-
ization behavior upon activation with aluminum alkyl cocatalysts.
The ligand 1a was synthesized following the literature
procedure.21 A series of novel Li2(OPO) ligand variants (1b−
e; Scheme 1) were synthesized by treating the respective
360.0° in both cases. The OPO fragment was coordinated
meridionally with the phosphorus and the dienyl group
occupying the equatorial positions and the sulfonates residing
in the axial positions. The allyl methyl groups adopted an anti
configuration with respect to the equatorial plane. In 2e the Ru−
P bond is elongated to 2.493 Å in comparison to 2.392 Å for 2d,
and the Ru−P−Calkyl angle widened from 123.8° to 131.4° due to
the larger steric profile of the tert-butyl group. The solid-state
structures agreed with the solution structures as determined by
NMR spectroscopy. In 2, the allyl groups gave rise to
nonequivalent NMR resonances. For example, in 2d the allyl
methyl signals showed chemical shifts of 1.42 and 2.24 ppm,
respectively. Presumably, the ring current effect of the
arenesulfonates experienced by the allyl group facing away
from the phosphine alkyl moiety leads to an upfield shift of its
resonance. A 1D NOE NMR experiment enabled the absolute
assignment of the dienyl resonances for 2d. When P-Me was
irradiated, we only observed NOE enhancement for the terminal
methylene protons of the downfield-shifted allyl group (see
Figure S26 in the Supporting Information).
Scheme 1. Synthesis of (A) Li2(OPO) Ligands 1 and (B)
Ru(IV) Polymerization Precatalysts 2
Complexes 2 were active for ethylene insertion polymerization
when treated with AlMe3-depleted methylaluminoxane (dMAO)
in toluene at elevated temperatures under optimized conditions
(Table 1 and Table S1 in the Supporting Information).
Activation with normal MAO gave lower activity in comparison
to dMAO while producing polymers with similar properties (run
9 vs 11). The catalyst system 2/dMAO showed relatively high
thermal stability with optimal activities at polymerization
temperatures between 55 and 85 °C. The highest TOF (1182
h−1) was obtained with 2c (run 7), which, to the best of our
knowledge, represents the most active Ru-based ethylene
polymerization catalyst system reported to date. While this
activity is significantly lower than those of highly active Ni(II)
and Fe(II) complexes (vide supra), it is one order of magnitude
higher than those for other previously reported Ru-based
systems.15,20
A consistent trend was observed for the donor strength of
OPO ligands on catalyst activity. Comparing 2a−c (runs 1, 3, and
8) showed an increase in activity with decreasing donor strength
of the ligand. The most electron-rich species, 2b, produced only
trace amounts of polymer. However, it is difficult to directly
compare the aryl-substituted (2a−c) and alkyl-substituted
(2d,e) complexes, as the sterics for the two systems are
significantly different. For example, alkyl-substituted 2d,e,
containing electron-rich phosphines, gave higher activities than
2a,b. The steric profile of 2e had a pronounced influence on the
polymer properties such as molecular weight (vide infra).
The effects of polymerization time (runs 6−9) and temper-
ature (runs 4, 5, 9, and 10) were investigated in more detail with
the most productive 2c/dMAO system. In experiments with
varying reaction time under otherwise identical conditions, the
significantly higher productivity (TOF of 1182 h−1) for the 2 h in
comparison to that for the 1 h polymerization (runs 6 and 7)
suggests a slow activation of 2c. The decrease in TOF for much
longer times (runs 8 and 9) was presumably due to catalyst
deactivation with time. For temperature dependence, under
otherwise identical conditions, the highest activity was observed
at 85 °C. Presumably, initial increase of temperature (runs 4, 5,
and 9) accelerated the catalyst activation and increased the
insertion rate until catalyst deactivation became more dominant
at overly high temperature (105 °C). The optimal temperature
for maximum productivity is also ligand dependent; while 2c
phosphine dichloride with 2 equiv of o-lithioarenesulfonate (see
the Supporting Information for all ligand syntheses and
characterizations). To tune the catalytic properties of the Ru(IV)
complexes, the substituent on the phosphine was systematically
changed, including three phenyl variants with different electronic
properties (1a−c) and two alkyl substituents with different steric
bulk (1d,e). The structures of all the ligands were fully
established by 1H and 13C NMR, ESI-MS, and elemental analysis
(see the Supporting Information). The solid-state structure of 1c
was determined by X-ray diffraction analysis (see Figure S40 in
the Supporting Information).
Treating the Ru(IV) dimer [{Ru(η3:η3-C10H16)(μ-Cl)Cl}2]
with 1 in the presence of AgBF4 resulted in the formation of
neutral [Ru(η3:η3-C10H16)(OPO)] complexes 2 in 55−73%
yield after purification (Scheme 1). All complexes were
1
characterized by H and 13C NMR, ESI-MS, and elemental
analysis (see the Supporting Information). High-quality single
crystals of 2d,e were obtained, enabling us to determine their
solid-state structure by X-ray diffraction analysis (see Figure 1).
Both complexes show a trigonal-bipyramidal coordination of the
Ru center. The sum of the bond angles of the phosphorus and the
central allyl carbon atoms around the ruthenium center was
Figure 1. Ortep plots of 2d (left) and 2e (right). Hydrogen atoms are
omitted for clarity. Selected bond distances (Å) and angles (deg) for 2d:
Ru1−O6 2.1132(11), Ru1−O3 2.0953(11), Ru1−P1 2.3919(4); Ru1−
P1−C15 123.81(6), O3−Ru1−P1 93.05(3), O6−Ru1−P1 89.50(3).
Selected bond distances (Å) and angles (deg) for 2e: Ru1−O6
2.1124(17), Ru1−O3 2.0998(17), Ru1−P1 2.4931(6); Ru1−P1−C25
131.42(8), O3−Ru1−P1 92.42(5), O6−Ru1−P1 89.43(5).
1914
dx.doi.org/10.1021/om5001343 | Organometallics 2014, 33, 1913−1916

Organometallics
Communication
a
Table 1. Ethylene Homopolymerization Results
b
c
d
e
g
run
catalyst
T (°C)
t (h)
yield (mg)
TOF
64
Tm (°C)
X (%)
Mp (kg/mol)
Nbr
f
1
2
3
4
5
6
7
8
9
10
2a
80
60
80
40
60
85
85
85
85
105
85
55
85
85
95
85
85
85
4
4
72
129
n.d.
n.d.
128
134
135
133
135
133
128
135
122
134
131
129
N/A
N/A
N/A
63
257, 0.9
11
2b
2b
2c
tr
N/A
N/A
25
n.d.
n.d.
37
n.d.
n.d.
n.d.
n.d.
6
4
tr
f
17
17
1
120
788
99
28, 0.8
f
2c
165
353
1182
744
196
24
57
289, 0.9
3
f
2c
56
179, 0.7
2
f
2c
2
663
835
934
115
519
790
228
72
62
153, 0.7
5
2c
4
59
214
1
f
2c
17
17
17
17
4
60
190, 1.1
5
f
2c
49
155, 0.8
11
5
h
f
11
2c
109
166
203
64
58
174, 0.6
f
12
13
14
15
2d
2d
2e
2e
2e
1c
i
250, 0.6
9
f
58
219, 0.7
2
4
42
250
n.d.
12
N/A
N/A
N/A
4
201
0
179
0
37
250
j
16
17
17
17
N/A
N/A
N/A
N/A
N/A
N/A
17
18
0
0
1cH22
0
0
a
b
Conditions: 10 μmol of catalyst, 1000 equiv of dMAO, 600 psi of C2H4, 100 mL of toluene. In units of (mol of C2H4)/((mol of [Ru]) h).
c
d
e
Determined by DSC. Crystallinity measured from DSC traces. A melting enthalpy of 293 J/g was used for 100% crystalline PE.23 Peak molecular
f
g
weights (Mp) determined by GPC in 1,2,4-trichlorobenzene at 140 °C vs polyethylene standards. Bimodal molecular weight distribution. Methyl
branches per 1000 C. Determined by H NMR in tetrachloroethane-d4 at 130 °C. MAO as cocatalyst (Al/Ru = 1000). Very broad melting
endotherm hampers calculation of crystallinity. Without dMAO cocatalyst.
h
i
1
j
exhibited maximal productivity at 85 °C, 2d showed the highest
productivity at 55 °C (runs 5 and 9 vs 12 and 13).
cocatalyst loading, and precatalyst. The formation of two
different types of polyethylene suggests the existence of two
different catalytic species in the polymerization solution. This
could potentially result from different catalyst activation
pathways or complicated interactions of the active metal species
with the aluminum cocatalyst (e.g., formation of different ion
pairs or heterobimetallic species) which has been reported for
other catalysts.20,26 Chain transfer to dMAO is another potential
pathway to give bimodal MWD which was investigated by
varying the cocatalyst loading (see Table S1 in the Supporting
Information).8a The polymerization activity increased with
increasing cocatalyst loading and leveled off at an Al/Ru ratio
of ∼1000. The polymer obtained with Al/Ru = 500 was of low
molecular weight (Mp = 1.3 kg mol−1), while 2000 equivalents of
dMAO gave mainly high molecular weight polymer (Mp = 404 kg
mol−1). This suggests that chain transfer to Al cocatalyst is
unlikely to play a significant role, since it is expected that an
increase in Al/Ru ratio leads to a decrease in MW.
All polymers obtained were fully soluble in tetrachloroethane
at elevated temperatures. The 1H NMR spectra revealed that the
polymers were highly linear with low degrees of branching (<12
Me branches/1000 C) and contained unsaturated end groups
(see Figures S30 and S31 in the Supporting Information). The
high terminal olefinic content suggests that β-hydride elimi-
nation was a major mode of chain transfer during the
polymerization.
The cis diallyl groups in 2 are ideally suited to be activated to
form two active coordination sites in cis positions. Nevertheless,
numerous attempts to use solution NMR to elucidate the
activation mechanism for 2 proved to be elusive due to severe
solubility issues encountered when species of 2 were activated by
dMAO. On the other hand, small-molecule activators, such as
AlMe2Cl, Al(iBu)3, B(C6F5)3, and [Ph3C] [B(C6F5)4]/Al(iBu)3,
were ineffective for activating 2. Solution NMR experiments
showed no reaction of 2 with excess B(C6F5)3 or [Ph3C][B-
(C6F5)4]. This agrees with other reports of Ru(IV) allyl
complexes that show no nucleophilic reactivity.22 Regardless,
on the basis of our control experiments (Table 1, runs 16−18),
the observed activity was due to the Ru metal center involved in
the catalytic process. Polymerization runs without Ru source or
cocatalyst yielded no polymer under otherwise identical
conditions. Similarly, when ligand salt 1c or protonated ligand
1cH24 was employed with dMAO, no polymer was obtained.
This also indicates that the observed polymerization activity was
not due to Al/ligand complexes possibly present in the
polymerization solution. However, it is possible that the Al
cocatalysts undergo redox processes with transition metals,
giving active species that differ in their oxidation state from the
precatalyst.25
The molecular weight and distribution for the polyethylenes
obtained in this study were characterized by gel permeation
chromatography (GPC). Interestingly, most polymers show
bimodal molecular weight distributions (MWD) (see Figures
S34−S39 in the Supporting Information for representative
traces) with a low- (Mp = 0.6−1.1 kg mol−1) and high-molecular-
weight fraction (Mp = 28−289 kg mol−1). The relatively narrow
polydispersity index (Mw/Mn = 1.3−3.2) of the monomodal
distributions suggests that each active species acts as a single-site
catalyst. The molecular weight and the ratio of the two different
mass fractions changed with polymerization temperature, time,
The melting temperature (Tm) and crystallinity of the
polyethylenes were measured by differential scanning calorim-
etry (DSC, see Figures S32 and S33 in the Supporting
Information). All of the polymers were semicrystalline solids
with high Tm values ranging from 129 to 135 °C (except for run
12 with 122 °C and a very broad melting transition).
In conclusion, we report the first Ru(IV) complexes that are
active for ethylene insertion polymerization upon activation with
dMAO. The polymerization activity for the 2c/dMAO system is
one order of magnitude higher than that for the most active Ru
1915
dx.doi.org/10.1021/om5001343 | Organometallics 2014, 33, 1913−1916

Organometallics
Communication
J.; Claverie, J.; Goodall, B.; Jordan, R. F.; Mecking, S.; Rieger, B.; Sen, A.;
van Leeuwen, P. W. N. M.; Nozaki, K. Acc. Chem. Res. 2013, 46, 1438−
1449.
(11) (a) Noyori, R. Angew. Chem., Int. Ed. 2002, 41, 2008−2022.
(b) Warner, M. C.; Casey, C. P.; Baeckvall, J.-E. Top. Organomet. Chem.
2011, 37, 85−125.
(12) Vougioukalakis, G. C.; Grubbs, R. H. Chem. Rev. 2009, 110,
1746−1787.
(13) (a) James, B. R.; Markham, L. D. J. Catal. 1972, 27, 442−451.
(b) Komiya, S.; Yamamoto, A.; Ikeda, S. Bull. Chem. Soc. Jpn. 1975, 48,
101−107.
insertion polymerization catalyst reported previously. The
polymerizations produce highly linear, semicrystalline poly-
ethylenes with a bimodal molecular weight distribution. It was
observed that the catalyst productivity increased with decreasing
donor strength of the OPO ligand. Efforts on the development of
other ligands and Ru complexes to further improve the catalyst
performance are currently underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
(14) (a) Faller, J. W.; Chase, K. J. Organometallics 1995, 14, 1592−
1600. (b) Umezawa-Vizzini, K.; Lee, T. R. Organometallics 2004, 23,
1448−1452. (c) Foley, N. A.; Lee, J. P.; Ke, Z.; Gunnoe, T. B.; Cundari,
T. R. Acc. Chem. Res. 2009, 42, 585−597.
Figures, tables, text, and CIF files giving general consideration
and materials for the experiments, ethylene polymerization
procedure, ligand and complex syntheses, NMR spectra, polymer
characterization, X-ray data collection, structure solution, and
refinement details and crystallographic data for 1c and 2d,e. This
material is available free of charge via the Internet at http://pubs.
acs.org.
(15) Nomura, K.; Warit, S.; Imanishi, Y. Macromolecules 1999, 32,
4732−4734.
(16) Dias, E. L.; Brookhart, M.; White, P. S. Organometallics 2000, 19,
4995−5004.
(17) Heyndrickx, W.; Occhipinti, G.; Minenkov, Y.; Jensen, V. R. J.
Mol. Catal. A: Chem. 2010, 324, 64−74.
AUTHOR INFORMATION
Corresponding Author
*E-mail for Z.G.: zguan@uci.edu.
■
(18) Piche, L.; Daigle, J.-C.; Claverie, J. P. Chem. Commun. 2011, 47,
7836−7838.
(19) (a) Wang, L.; Flood, T. C. J. Am. Chem. Soc. 1992, 114, 3169−
3170. (b) Wang, L.; Lu, R. S.; Bau, R.; Flood, T. C. J. Am. Chem. Soc.
1993, 115, 6999−7000. (c) Hou, X.-F.; Cheng, Y.-Q.; Wang, X.; Jin, G.-
X. J. Organomet. Chem. 2005, 690, 1855−1860.
(20) Camacho-Fernandez, M. A.; Yen, M.; Ziller, J. W.; Guan, Z. Chem.
Sci. 2013, 4, 2902−2906.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Jordan F. Corbey for help with the X-ray diffraction
analysis. This work was supported by the U.S. National Science
Foundation (CHE-1012422). GPC analyses were performed by
the Cornell Center for Materials Research Shared Facilities,
which are supported through the NSF MRSEC program (DMR-
1120296).
(21) Shen, Z.; Jordan, R. F. J. Am. Chem. Soc. 2009, 132, 52−53.
(22) Renaud, J.-L.; Demerseman, B.; Mbaye, M. D.; Bruneau, C. Curr.
Org. Chem. 2006, 10, 115−133.
(23) TN 48, “Polymer Heats of Fusion”, TA Instruments, New Castle,
DE.
(24) 1cH was obtained by treatement of 1c with Amberlite ion-
exchange resin in water with subsequent removal of the solvent after
filtration.
(25) Britovsek, G. J. P.; Clentsmith, G. K. B.; Gibson, V. C.;
Goodgame, D. M. L.; McTavish, S. J.; Pankhurst, Q. A. Catal. Commun.
2002, 3, 207−211.
(26) (a) Corker, J. M.; Evans, J. J. Chem. Soc., Chem. Commun. 1991,
1104−1106. (b) Fernando de Souza, R.; Simon, L. C.; Alves, M. d. C. M.
J. Catal. 2003, 214, 165−168. (c) Bryliakov, K. P.; Talsi, E. P.;
Semikolenova, N. V.; Zakharov, V. A. Organometallics 2009, 28, 3225−
3232.
REFERENCES
■
(1) Vasile, C. Handbook of Polyolefins; Marcel Dekker: New York, 2000.
(2) (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev. 2002, 103, 283−
316. (b) Makio, H.; Terao, H.; Iwashita, A.; Fujita, T. Chem. Rev. 2011,
111, 2363−2449. (c) Delferro, M.; Marks, T. J. Chem. Rev. 2011, 111,
2450−2485. (d) Brintzinger, H. H.; Fischer, D.; Mulhaupt, R.; Rieger,
̈
B.; Waymouth, R. M. Angew. Chem., Int. Ed. 1995, 34, 1143−1170.
(e) Alt, H. G.; Koppl, A. Chem. Rev. 2000, 100, 1205−1222.
̈
(f) Camacho, D. H.; Guan, Z. Chem. Commun. 2010, 46, 7879−7893.
(g) Hustad, P. D. Science 2009, 325, 704−707. (h) Guan, Z. Metal
Catalysts in Olefin Polymerization; Springer: Berlin, 2009.
(3) Ziegler, K.; Holzkamp, E.; Breil, H.; Martin, H. Angew. Chem. 1955,
67, 541−547.
(4) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed.
1999, 38, 428−447.
(5) (a) Margl, P.; Deng, L.; Ziegler, T. Organometallics 1998, 17, 933−
946. (b) Schmid, R.; Ziegler, T. Organometallics 2000, 19, 2756−2765.
(6) Keim, W. Angew. Chem., Int. Ed. 2013, 52, 12492−12496.
(7) (a) Johnson, L. K.; Killian, C. M.; Brookhart, M. J. Am. Chem. Soc.
1995, 117, 6414−6415. (b) Johnson, L. K.; Mecking, S.; Brookhart, M. J.
Am. Chem. Soc. 1996, 118, 267−268.
(8) (a) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc.
1998, 120, 4049−4050. (b) Britovsek, G. J. P.; Gibson, V. C.; McTavish,
S. J.; Solan, G. A.; White, A. J. P.; Williams, D. J.; Kimberley, B. S.;
Maddox, P. J. Chem. Commun. 1998, 849−850.
(9) (a) Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S. K.;
Grubbs, R. H.; Bansleben, D. A. Science 2000, 287, 460−462.
(b) Connor, E. F.; Younkin, T. R.; Henderson, J. I.; Hwang, S.;
Grubbs, R. H.; Roberts, W. P.; Litzau, J. J. J. Polym. Sci., Part A: Polym.
Chem. 2002, 40, 2842−2854.
(10) (a) Drent, E.; van Dijk, R.; van Ginkel, R.; van Oort, B.; Pugh, R. I.
Chem. Commun. 2002, 744−745. (b) Nakamura, A.; Ito, S.; Nozaki, K.
Chem. Rev. 2009, 109, 5215−5244. (c) Nakamura, A.; Anselment, T. M.
1916
dx.doi.org/10.1021/om5001343 | Organometallics 2014, 33, 1913−1916