Cyclophane-based highly active late-transition-metal catalysts for ethylene polymerization

Article abstract of DOI:10.1002/anie.200353226

Better by design: Exploitation of the macrocyclic architecture of a cyclophane-based ligand provides new highly active catalysts with improved thermal stability for ethylene polymerization. The strategic positioning of the metal center at the core of the

Full text of DOI:10.1002/anie.200353226

Angewandte
Chemie
Polymerization Catalysts
weight (MW) polyethylenes (PEs) and the PdII systems were
shown to be able tolerate and incorporate polar olefins such
as methyl acrylate.[4] The branching topology of the PEs
formed by the PdII-a-diimine catalysts can easily be tuned
from linear to hyperbranched to dendritic by simply varying
ethylene pressure.[5] Whereas these catalysts exhibit excellent
properties as described, one limitation is their relatively high
sensitivity to temperature. The catalysts decompose rapidly at
508C for PdII-a-diimine[6] and at 708C for NiII-a-diimine
systems.[7] The MW of the PEs formed by NiII catalysts also
decreases precipitously as the temperature of polymerization
is raised.[7] These issues hindered the commercialization of
these catalysts since gas-phase olefin polymerizations typi-
cally operate at temperatures as high as 80–1008C.[8] Herein,
we report novel PdII and NiII-a-diimine catalysts containing a
cyclophane ligand moiety that demonstrate improved thermal
stability and produce high MW PEs at temperature ranges
suitable for industrial gas-phase olefin polymerization.
Cyclophane-Based Highly Active Late-
Transition-Metal Catalysts for Ethylene
Polymerization**
Drexel H. Camacho, Eric V. Salo, Joseph W. Ziller, and
Zhibin Guan*
Cyclophane chemistry has evolved into an exciting research
area stemming from simple curiosity of the syntheses and
structures to the exploitation of physical properties of cyclo-
phanes for various applications including molecular recog-
nition, supramolecular chemistry, and biomimics.[1] A ple-
thora of literature describing cyclophane–metal host–guest
chemistry indicates the potential application of cyclophanes
as ligands in metal-catalyzed transformations.[1] One major
research effort in our group is to explore the use of cyclo-
phanes as ligands for olefin polymerization catalysis. In our
ligand design, we strategically position metal binding sites at
the core of cyclophanes to chelate transition metals. The
cyclophane framework shields all directions of the catalytic
metal center except leaving two cis coordination sites open in
the front: one for monomer coordination and the other for the
growing polymer chain. The well-defined cavity and sterically
hindered microenvironment of cyclophanes should offer great
opportunities for fine-tuning the catalytic properties. The
rigid cyclophane framework may also enhance the stability of
transition-metal complexes. Despite these promises, however,
the use of cyclophanes as ligands for transition-metal
polymerization catalysis remains mostly unexplored.[2]
Herein, we report the first cyclophane-based late-transition-
metal catalyst that shows high activity and thermal stability
for ethylene polymerization.
In the acyclic catalyst A (Figure 1), the aryl groups are
roughly perpendicular to the coordination plane so the
Figure 1. Comparison of the acyclic (A) with cyclophane-based (B)
NiII–a-diimine complexes.
isopropyl substitutents on the aryls are positioned at the
axial directions to retard associative chain transfer[4] or chain
transfer to ethylene monomer.[9] At elevated temperature,
however, the aryl groups may freely rotate away from the
perpendicular orientation, resulting in increased associative
chain transfer or chain transfer to monomer and a resulting
decrease in MW of the PE formed by the acyclic catalyst A.[7]
Moreover, as the aryl groups rotate towards the coordination
plane, the isopropyl substituents on the aryl rings reach within
close proximity of the metal center giving it an opportunity to
Late-transition-metal olefin polymerization catalysts have
received much attention recently because they can produce
polyolefins with interesting new branching topologies and
have better tolerance to functional groups.[3] One noteworthy
example is the NiII- and PdII-a-diimine complexes reported by
Brookhart and co-workers.[4] The NiII systems were shown to
have comparable activities to those of the early-transition-
metal catalysts in polymerizing ethylene into high molecular
À
À
react with the C H bonds (C H activation) to form metal-
lacycles, which was proposed as one potential deactivation
pathway for this family of catalysts.[6] In our cyclophane-based
complex B, the metal center is positioned at the core of the
ligand so that the macrocycle completely blocks the axial
faces of the metal leaving only two cis-coordination sites for
monomer entry and polymer growth. The rigid framework of
the ligand prohibits free rotation of the aryl–nitrogen bonds,
which should allow the catalyst to make high MW polymers at
elevated temperature. The lack of rotational flexibility
[*] Dr. D. H. Camacho, E. V. Salo, Dr. J. W. Ziller, Prof. Dr. Z. Guan
Chemistry Department
University of California
Irvine, California 92697-2025 (USA)
Fax: (+1)949-824-2210
E-mail: zguan@uci.edu
[**] This work was supported by grants from the ACS Petroleum
Research Fund (36730-G7), Army Research Office (42395-CH-H),
National Science Foundation (DMR-0135233),and the University of
California. We thank Materia Inc. for generous donation of the
Grubbs metathesis catalysts and Chris Popeney for the molecular
modeling. Z.G. gratefully acknowledges an NSF CAREER Award, a
Beckman Young Investigator Award, a DuPont Young Professor
Award, and a 3M New Faculty Award.
À
contributes to the retardation of the C H activation of the
ortho subsitutents, hence, should prevent this potential
catalyst-deactivation pathway from being available. In addi-
tion, it has been observed for other systems that rigid
macrocyclic ligands enhance the coordination stability for
metal complexes.[2b] Based on these considerations, we
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 1821 –1825
DOI: 10.1002/anie.200353226
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designed the cyclophane-based a-diimine
ligand to address the critical thermal sensi-
tivity of the acyclic a-diimine systems. In
more general terms, we envision cyclo-
phanes to be a new family of ligand frame-
works to be used in the design of metal
complexes for polymerization catalysis.
The synthesis of the cyclophane ligand
(Scheme 1) began with the Suzuki coupling
of commercially available 2,6-dibromo-4-
methylaniline (4) with 4-formylphenylbor-
onic acid (5) followed by the conversion of
the dialdehyde into divinyl by using the
Wittig reaction to give the product 6 in 64%
total yield. Condensation of 6 with ace-
naphthenequinone gave the a-diimine 7.
Molecular modeling shows that in a-di-
imine 7 the styryl phenyl rings are perpen-
dicular to the aniline phenyl rings, render-
ing the right conformation for ring closing
metathesis (RCM)[10] to close the ring.
Scheme 2. Synthesis of the cyclophane-based NiII and PdII complexes. Reaction conditions:
a) [NiBr2(dme)], CH2Cl2, 18 h, (95%); b) SnMe4, CH2Cl2, À358C, 3 h; c) 8, À358C to RT,
6 h (81%); d) Pd/C, H2, CH2Cl2-MeOH, (99%); e) methyl acrylate, NaBAF, CH2Cl2, RT, 12 h
(97%).
Indeed, the RCM proceeded very efficiently to form the
cyclophane 8 in 80% yield.[11] Compound 8 was hydrogenated
to give the cyclophane a-diimine 9.
High quality single crystals suitable for X-ray analysis for
the PdII complex 2 was obtained by carefully layering a
slightly concentrated dichloromethane solution of 2 with n-
decane.[14] X-ray crystal structure of complex 2
(Figure 2) shows that the active PdII center is in the
core of the cyclophane ligand, as we initially envi-
sioned. The bond angles and distances in the PdII
coordination plane are very similar to the values of an
acyclic a-diimine-PdMeCl complex reported by
Brookhart and co-workers.[6] The resulting angles of
the planes that run through the aniline moiety
(C13–C18) and the adjacent phenyl groups
(C49–C54) and (C20–C25) are 72.08 and 122.88,
respectively. The planes that run through the aniline
moiety on the opposite side (C34–C39) and the
adjacent phenyl groups (C41–C46) and (C28–C33)
are 52.18 and 128.68, respectively. The large difference
in the torsion angles between aryl rings (52.18 for
C34–C46/C41–C46 verses 72.08 for C13–C18/
C49–C54) is presumably caused by the different
sizes of the chloride and methyl groups. The angles
between the five-membered palladacyclic coordina-
tion plane (Pd1-N2-C2-C1-N1) and the two aniline-
derived phenyl planes (C13–C18) and C34–C39) are
91.58 and 91.38, respectively. A view from the top of
the space filling model of 2 indicates that the axial
sites for the metal center are completely blocked by
the alkane bridge on cyclophane ring, which plays a
critical role in obtaining high-molecular-weight polymers at
elevated temperatures (see below).
Scheme 1. Synthesis of the cyclophane-based ligand. Reaction conditions:
a) [Pd(PPh3)4], 2m Na2CO3, 1,4-dioxane, reflux, (85%); b) Ph3PMeBr, THF,
KOtBu, (75%); c) acenaphthenequinone, benzene, PTSA, azeotrope 5 days,
(79%); d) Grubbs generation 2 catalyst, CH2Cl2, 508C, (80%); e) Pd/C, H2,
CH2Cl2-MeOH, (77%).
Both NiII and PdII complexes with the cyclophane a-
diimine ligand 9 were synthesized and characterized.[12]
Complexation of 9 with [NiBr2(dme)] (dme = dimethoxy-
ethane) in dicholoromethane (Scheme 2) afforded the NiBr2
complex 1 as the precatalyst for the ethylene polymerization
studies. For the synthesis of the Pd complex, the ligand 8 was
complexed with Pd(Me)Cl generated in situ[13] (Scheme 2)
followed by hydrogenation to provide 2, which was reacted
with sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate
(NaBAF) and methyl acrylate (MA) to give the preactivated
cationic PdII complex 3.
Exposure of the complex 1 to moderated methylalumox-
ane (MMAO) in toluene generated a highly active catalyst for
ethylene polymerization. The activated catalyst had polymer-
ization activities similar to the most active early-transition-
metal metallocene catalysts[15] and late-transition-metal cata-
lysts[4a,7,16] with a turnover frequency (TOF) of 1.50 ” 106 hÀ1
(productivity of 42000 kg of PE molÀ1 of NihÀ1). To the best
of our knowledge, this is the first reported cyclophane-based
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which generally show a significantly lower productivity at
elevated temperatures.[7] The calculated TOFs for polymer-
ization at constant temperature but different periods of time
indicate that the active catalyst remained active for a period
of time. At temperatures below 708C, the catalyst maintained
almost constant productivity over 15 minutes. For polymer-
izations at 70 and 908C, the productivity was constant in the
first 10 minutes and then started to decrease, thus suggesting
that the active species starts to decompose with longer periods
of time at higher temperatures (see Table 1).
Besides the high activity and thermal stability, the MWs of
the PEs obtained by using complex 1 did not drop as the
temperature was raised. Even at 908C, the MWs of PEs are
still in the range of 300000 gmolÀ1. This again contrasts to the
acyclic NiII-a-diimine systems, for which MWs of PEs usually
decrease rapidly with increasing temperature.[7] We attribute
the constant MWs at elevated temperatures to the unique
cyclophane ligand that keeps the axial sites for the metal
center fully blocked even at elevated temperatures (see
above). The observed relatively narrow polydispersity indices
(PDI) clearly indicate the single-site nature of the catalyst and
are suggestive of some living character for the polymer-
ization. The PEs formed contain short chain branches with
most being simple methyl branches as revealed by 13C NMR.
The branching density increases as the polymerization
temperature increases, which are consistent with the acyclic
NiII–a-diimine systems.[7] One interesting observation is that
the branching density is considerably higher than the values
for the PEs produced by a very similar acyclic NiII–a-diimine
system reported by Rieger and co-workers,[16] which suggests
that the cyclophane ligand environment significantly influen-
ces the catalytic properties of the complex. The branching was
presumably produced by the chain-walking mechanism pro-
posed by Brookhart[4] and Fink.[17] The significantly increased
branching density may result from enhanced chain-walking
processes caused by the unique cyclophane ligand environ-
ment.
Figure 2. a) X-ray crystal structure of complex 2. b) Top view space fill-
ing model of 2 (red=Pd; green=Cl; black=C; blue=N). ORTEP view
of 2 showing important atoms labeled (50% thermal ellipsoids; the
fragment of solvated dichloromethane and 1,2-dichloroethane was
omitted for clarity). Selected interatomic distances [Š]: Pd1-
C55=2.018(3), Pd1-N1=2.053(2), Pd1-N2=2.218(2), Pd1-
Cl1=2.3149(8), N1-C1=1.285(4), N1-C13=1.440(4), N2-
C2=1.279(4), N2-C35=1.443(4), C1-C2=1.507(4). Selected bond
angles [8]: C55-Pd1-N1=92.93(12), C55-Pd1-N2=171.67(11), N1-Pd1-
N2=78.99(9), C55-Pd1-Cl1=88.15(10), N1-Pd1-Cl1=178.60(7), N2-
Pd1-Cl1=99.90(6), C1-N1-C13=121.5(2), C2-N2-C35=117.4(2), N1-
C1-C2. Selected torsion angles (deg): N1-C1-C2-N2=À2.8(4), C2-N2-
C35-C36=À82.2(3), C1-N1-C13-C14=90.5(3), C1-N1-C13-
C18=À99.1(3).
Polymerization of ethylene with the preactivated PdII
complex 3 was also carried out.[18] The preactivated PdII
complex 3 showed much higher thermal stability for ethylene
polymerization than the acyclic PdII–a-diimine analogues
reported previously.[16] The complex 3 has a half life of more
than three hours for ethylene polymerization at 708C.[18] In
contrast, the acyclic analogues decomposed within minutes
even at room temperature.[16] The branching density of the
PEs formed by complex 3 at room temperature is around 110
branches/1000 carbons, which is significantly higher than the
values for PEs formed by the acyclic PdII–a-diimine catalysts
as estimated from the melting temperatures reported for their
PEs.[16] The contrast in both the catalyst thermal stability and
the PE microstructures between the cyclophane and acyclic
PdII catalysts once again suggests that the unique cyclophane
ligand environment significantly influences the catalytic
properties of the complexes. While it has been reported that
the introduction of aryl substituents onto a different ligand
improved the thermal stability for NiII complexes in ethylene
polymerization in the presence of hydrogen,[19] the big
contrast in thermal stability and PE branching density
between our cyclophane NiII and PdII systems with open
highly active catalyst for olefin polymerization. The polymer-
ization was run at temperatures of 30–908C to probe its
thermal stability. At each temperature, the polymerization
ran at three different time periods ranging from 5 to 15 min to
test the catalyst lifetime. The catalyst remained highly active
at temperatures up to 908C, as evident by consistent catalyst
TOFs. More specifically, as the temperature was increased
from 30 to 708C, the observed TOF decreased only by 10%.
At even 908C, the reduction of TOF for polymerization of
10 min is less than 30%. This is in contrast to the acyclic NiII-
a-diimine counterparts (e.g., complex 4g in reference [7]),
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Communications
Table 1: Summary of ethylene polymerization data with the Ni complex.[a]
K. K. Baldridge, R. B. English, D. M. Ho,
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pp. 15–23.
Entry Moles of T [8C] t [min] Yield [g] TOF[b]
Mn
PDI
Branches per
[c]
¯
catalyst
[”103 hÀ1
]
[”103 hÀ1
]
1000 C atoms[d]
[”106]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
1
1
30
30
30
50
50
50
70
70
70
90
90
90
5
10
15
5
10
15
5
10
15
5
10
15
3.48
6.70
9.25
3.20
6.85
9.00
3.11
6.10
7.35
2.50
4.70
4.62
1491
1436
1321
1371
1468
1286
1333
1307
1050
1071
1007
660
320
288
294
248
652
342
323
619
429
252
462
292
1.29 66
1.30 73
1.31 67
1.23 80
1.28 84
1.23 85
1.45 91
1.43 89
1.41 91
1.72 97
1.64 96
1.40 96
10
11
12
[a] Experimental conditions: in 200 mL of toluene, cocatalyst MMAO (Al:Ni ꢀ3000), 200 psi ethylene
pressure. [b] TOF=turnover frequency, which was calculated as the moles of ethylene per mole of
[3] a) S. D. Ittel, L. K. Johnson, M. Brookhart,
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2000, 287, 460 – 462.
¯
catalyst per hour. [c] Mn, number-average molecular weight measured by gel-permeation chromatog-
raphy with polystyrene standards. [d] Branching determined from 1HNMR spectroscopy.
chain analogues containing aryl substituents[16] clearly indi-
cates that the cyclophane ligand environment has a significant
impact on the catalytic properties.
In summary, we report here the first cyclophane-based
late-transition-metal catalyst that shows high activity and
thermal stability for ethylene polymerization. An efficient
route was developed for the synthesis of a novel cyclophane-
based a-diimine ligand. The NiII and PdII-a-diimine com-
plexes with the cyclophane ligand were successfully synthe-
sized and characterized. The NiII complex exhibits excellent
[4] a) L. K. Johnson, C. M. Killian, M. Brookhart, J. Am. Chem. Soc.
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activity for ethylene polymerization with a productivity of
À1
42000 kg of PEmol
of NihÀ1. Both the NiII and PdII
catalysts show significantly higher thermal stability than the
acyclic analogs. The cyclophane catalysts also form PEs with
significantly higher branching density as compared to the
similar acyclic counterparts.[16] All these data suggest the
novel cyclophane ligand change the catalytic properties
significantly for the NiII and PdII complexes. It should be
noted that the NiII complex 1 has sufficiently high productivity
and thermal stability at temperature ranges suitable for gas-
phase olefin-polymerization processes. The MWs of the PEs
formed by complex 1 are also high and rather constant with
polymerization temperature. Encouraged by these results, we
are currently exploring a family of new cyclophane-based
ligands and investigating the polymerization properties of a
family of their transition-metal complexes.
[9] L. Deng, T. K. Woo, L. Cavallo, P. M. Margl, T. Ziegler, J. Am.
Chem. Soc. 1997, 119, 6177 – 6186.
[10] T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18 – 29.
[11] The ring-closing metathesis afforded the bicyclic compound 8 as
the major product (80% yield) and a small amount of mono-
cyclic by-product with ring closing only on one side.
[12] The synthesis and detailed characterization of the ligand and
complexes will be reported separately (manuscript in prepara-
tion).
[13] E. V. Salo, Z. Guan, Organometallics 2003, 22, 5033 – 5046.
[14] Crystal data for 2: (C55H43ClN2Pd·CH2Cl2·1/2(C2H4Cl2): Mr =
¯
1008.17, T= 168(2) K, triclinic, space group P1, a = 10.4795(5),
b = 12.4398(6), c = 19.0436(9) Š, a = 90.2330(10), b =
90.1900(10), g = 111.5580(10)8, V= 2308.86(19) Š3, Z = 2,
1calcd = 1.450 MgmÀ3 m = 0.675 mmÀ1
l = 0.71073 Š, 2qmax
,
,
=
Received: November 3, 2003 [Z53226]
26.378, 21767 measured reflections, 9220 [R(int) = 0.0318] inde-
pendent reflections, GOF on F2 = 1.057, R1 = 0.0388, wR2 =
0.0990 (I > 2s(I)), R1 = 0.0515, wR2 = 0.1056 (for all data),
largest difference peak and hole 0.549 and À0.862 eŠÀ3. The
intensity data were collected on Bruker CCD platform diffrac-
tometer. The structure was solved by direct methods
(SHELXTL) and refined on F2 by full-matrix least-squares
techniques. Hydrogen atoms were included by using a riding
model. There was one molecule of dichloromethane solvent
present per formula unit. There was another solvent present and
was assigned as dichloroethane. This solvent was disordered and
Keywords: cyclophanes · homogeneous catalysis · macrocyclic
ligands · nickel · olefin polymerization
.
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located about an inversion center. CCDC-222192 contains the
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[18] See Supporting Information. The detailed polymerization results
for the palladium catalyst will be reported separately.
[19] L. S. Moody, P. B. MacKenzie, C. M. Killian, G. G. Lavoie, J. A.
Ponasik, T. W. Smith, J. C. Pearson, A. G. M. Barrett, G. W.
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