A catalyst for the oligomerization of 1-hexene, generated by the activation of a benzimidazolylidene zirconium dibenzyl complex, switches to a polymerization catalyst on addition of a trialkylphosphine

Bercaw, John E.

Despagnet-Ayoub, Emmanuelle

Henling, Lawrence M.

Labinger, Jay A.

Caltech Authors - Main

Dalton
Transactions
COMMUNICATION
Cite this: Dalton Trans., 2013, 42, 15544
Received 26th August 2013,
Accepted 1st September 2013
DOI: 10.1039/c3dt52342j
www.rsc.org/dalton
Addition of a phosphine ligand switches an
N-heterocyclic carbene-zirconium catalyst from
oligomerization to polymerization of 1-hexene†
Emmanuelle Despagnet-Ayoub,*a,b Lawrence M. Henling,c Jay A. Labinger*c and
John E. Bercaw*c
A catalyst for the oligomerization of 1-hexene, generated by
the activation of a benzimidazolylidene zirconium dibenzyl
complex, switches to a polymerization catalyst on addition of a
trialkylphosphine.
Polyolefin production is practiced on an extremely large scale.1
Over the past 20 years there have been significant advances in
the development of homogeneous single-site catalysts. Well-
defined early transition metal metallocene catalysts have facili-
tated studies on the fundamental mechanistic features of poly-
olefin catalysis.2 More recently, “post-metallocene” complexes
bearing multidentate oxygen- and nitrogen-based ligands have
proven to be good alternatives as catalysts for ethylene
polymerization;3 however, α-olefins still present challenges to
non-metallocene catalysts, which often show poor to only mod-
erate activity and/or rapid deactivation, thus aﬀording only
traces of oligomers or polymers.4 Furthermore, the factors that
determine oligomerization vs. polymerization of α-olefins are
unpredictable: subtle changes in the ligand framework, the co-
catalyst, and the reaction conditions can all aﬀect the reactiv-
ities and selectivities of these catalyst systems. As part of our
studies on early transition metal complexes of tridentate NHC
ligands in catalysis,5 especially for olefin oligomerization/
polymerization,6 we report here on a bis(phenolate)benzimid-
azolylidene zirconium based catalyst that can be switched
between oligomerization and polymerization of 1-hexene by
the simple addition of tertiary phosphines. While this work
was in progress, a report appeared of a highly regioselective
1-hexene oligomerization catalyzed by a closely-related NHC-
zirconium complex activated by anilinium; in that case, coordi-
nation of the dimethylaniline byproduct was suggested to
account for the selective behavior.6g
The (OCO) ligand 1 is obtained by cyclization of N,N′-
bis(3,5-di-tert-butyl-2-phenol)-1,2-phenylenediamide7 with
triethylformate in the presence of hydrochloric acid in 74%
yield (Scheme 1). Subsequent reaction of 1 with one equivalent
of KHMDS followed by addition of tetrabenzylzirconium
aﬀords (OCO)ZrBn2 (2) in 88% yield. The
13C NMR spectrum
reveals a characteristic downfield signal (δ 199.9 ppm)
assigned to the Ccarbene–Zr carbon.
Crystals of 2 suitable for X-ray diﬀraction were obtained
from a cold diethyl ether solution. The unit cell contains two
independent molecules exhibiting a distorted trigonal bipyra-
midal geometry about zirconium (only one is shown in Fig. 1)
with very similar structural parameters. The benzimidazolyl-
idene moiety is non-planar, lying outside of the O(1)–Zr–O(2)
plane.8 One of the two benzyl groups adopts a pronounced η2
binding mode (Zr(1)–C(36) = 2.30 Å; Zr(1)–C(37) = 2.64 Å;
Zr(1)–C(36)–C(37) = 86°; cf. Zr(1)–C(43) = 2.27 Å; Zr(1)–C(44) =
2.82 Å; Zr(1)–C(43)–C(44) = 95°), a common feature of electron-
deficient five-coordinate dibenzylzirconium complexes.4c,9
Activation of complex 2 with one equivalent of [Ph3C]-
[B(C6F5)4] generates a catalyst for 1-hexene oligomerization/
polymerization at room temperature. With no additive, oligo-
merization is observed: GC analysis shows a roughly Schultz–
Flory distribution between C12 and C42 with a maximum at the
tetramer (Table 1, entry 1); MALDI-TOF analysis10 reveals that
the distribution extends to at least C78 (n = 13). A second
addition of 1-hexene after 14 h was partially consumed (68%
conversion), demonstrating some remaining activity of the
catalyst.11
The nature of the end-group in the resulting oligomers was
addressed by 1H NMR spectroscopy, revealing the presence of
vinylene R1CH = CHR2 (δ = 5.4 ppm) and vinylidene H2C =
CR1R2 (δ = 4.8 ppm) resonances in an 85 : 15 ratio respectively
(Fig. S6†).11 This result suggests a preference for the formation
of the oligo(1-hexene)s by β-H-elimination following 2,1-
enchainment of the monomer.
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, crystallographic data and catalytic details (GC traces, MALDI-TOF, NMR
of oligomers/polymers). CCDC 942447. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c3dt52342j
aCNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
31077 Toulouse Cedex 4, France
bUniversité de Toulouse, UPS, INPT, LCC, 31077 Toulouse Cedex 4, France.
E-mail: emmanuelle.despagnet-ayoub@lcc-toulouse.fr
cArnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute
of Technology, Pasadena, California 91125, USA. E-mail: bercaw@caltech.edu
15544 | Dalton Trans., 2013, 42, 15544–15547 This journal is © The Royal Society of Chemistry 2013
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When [HNMe2Ph][B(C6F5)4] is used instead as the stoichio-
metric activator, only a very low conversion (∼5%) of 1-hexene
is observed.11 This is likely due to the coordination of the
dimethylaniline byproduct generated on activation, which
blocks the approach of the olefin. Indeed, a similar behavior is
observed when sterically hindered 2,6-lutidine is added to the
2/[Ph3C][B(C6F5)4] system (conv.: <1%). Addition of phosphine
ligands results in a more complex behavior. Triphenylphos-
phine does not perturb the system, giving essentially the same
yield and distribution of oligomers as that when no additive is
used (Table 1, entry 2), while tricyclohexylphosphine comple-
tely inhibits activity. Surprisingly, the addition of one equi-
valent of triethylphosphine or trimethylphosphine results in
the formation of poly(1-hexene) (Table 1, entries 3 and 4); by
13C and 1H NMR spectroscopy, both polymers are atactic with
average molecular weights of 10 545 and 2965 respectively, and
contain a predominance of vinylidene end groups (Fig. S8 and
S10†).11 Addition of two equivalents of triethylphosphine leads
to complete inhibition of catalysis.
NMR studies on this system are consistent with the struc-
tures proposed in Scheme 2. Addition of complex 2 in
chlorobenzene to [Ph3C][B(C6F5)4] at 0 °C
12 results in the clean
and quantitative formation of what appears to be cationic zir-
conium complex 3, as indicated by the downfield shift of the
benzyl CH2 group (δ 2.94 in
1H NMR; 71.3 in 13C NMR), along
with one equivalent of Ph3CCH2Ph. Only a slight diﬀerence in
the carbene chemical shift is observed between the cationic (3)
and the neutral (2) complexes (δ 196.0 for 3 vs. 199.9 for 2).
The benzyl ligand appears still to be coordinated in a η2
fashion, as indicated by the large coupling constant (1JCH =
142 Hz).13 Addition of one equivalent of trimethylphosphine
results in the formation of what appears to be cationic phos-
phine adduct 4, which displays a singlet resonance (δ −20.2) in
the 31P{1H} NMR spectrum. The benzyl group resonances shift
considerably relative to complex 3 (CH2 group
13C: δ 63.2 for
4 vs. 71.3 ppm for 3; phenyl group 1H: δ [7.7–7.2] for 4 vs.
[6.5–6.2] ppm for 3), suggesting η1 coordination in 4. Addition
of a second equivalent of trimethylphosphine gives the bis(tri-
methylphosphine) adduct 5, indicated by the appearance of an
AB quartet in the 31P{1H} NMR (δ −28 and −32 ppm with JPP =
Scheme 1 Synthesis of the benzimidazolium chloride 1 and its zirconium complex 2.
Fig. 1 Molecular structure of complex 2. Selected bond lengths (Å) and angles
(°): Zr(1)–C(1) = 2.3358(17), Zr(1)–O(1) = 1.9957(12), Zr(1)–O(2) = 1.9953(11),
Zr(1)–C(36) = 2.3038(17), Zr(1)–C(37) = 2.64061(17), Zr(1)–C(43) = 2.2737(18),
Zr(1)–C(44) = 2.8246(16), Zr(1)–C(36)–C(37) = 85.91(10), Zr(1)–C(43)–C(44) =
95.49(11).
Table 1 Oligomerization/polymerization of 1-hexene using complex 2a
Entry Additives Conv. (%) C12 (%) C18 (%) C24 (%) C30 (%) C36 (%) C42 (%) Higher oligomers Polymer (g, Mn
b)
1 None 89 7 7 15 13 11 5 31 <0.02
2 PPh3 96 7 6 16 15 13 6 33 <0.02
3 PEt3 99 0 0 0 0 2 6 n.d. 0.48, 2965
4 PMe3 99 0 0 0 0 0 0 n.d. 0.49, 10 545
a Conditions: [2] = [Ph3C][B(C6F5)4] = 0.006 mmol; 1-hexene (0.53 g, 6.3 mmol); additive (1 equiv., 0.006 mmol), room temperature; reaction time:
14 h. Oligomers were analyzed by GC. b Calculated by integration using 1H NMR spectroscopy of olefinic end groups relative to aliphatic groups.
Scheme 2 Proposed species generated by the activation of 2 with [Ph3C]-
[B(C6F5)4] followed by addition of PMe3.
Dalton Transactions Communication
This journal is © The Royal Society of Chemistry 2013 Dalton Trans., 2013, 42, 15544–15547 | 15545
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99 Hz);14 the benzyl CH2
13C resonance (δ 61.2) is close to that
for 4. 19F NMR spectra for complexes 3, 4 and 5 are indicative
of solvent separated cation/anion pairs.11 Addition of 1-hexene
to complex 5 results in no reaction, even after 5 hours at room
temperature.
The oligomerization/polymerization of 1-hexene was fol-
lowed over time in the absence and presence of trimethyl-
phosphine. When no additive is present, conversion of the
1-hexene is approximately 60% complete after 60 minutes, and
reaches its maximum value (∼89%) after 5 hours at room
temperature.11 On the other hand, when one equivalent of
trimethylphosphine is present, the consumption of 1-hexene
is slightly exothermic and rapid (no trace of remaining
1-hexene is observed by GC after 30 minutes).
A model that can account for these ligand eﬀects and is
consistent with all observations is shown in Scheme 3. In the
absence of added ligand, 2,1-insertion of the monomer into a
growing polymer chain is preferred, and termination by β-H
elimination of the resulting α-substituted alkyl is relatively fast
compared to further growth, leading to a predominance of oli-
gomers with vinylene end groups. Addition of a ligand such as
PMe3 or PEt3 increases steric crowding, changing the regio-
preference to 1,2-insertion, giving an α-unsubstituted alkyl
that is less prone to β-H elimination relative to growth, and
hence a predominance of polymers with vinylidene end groups
is obtained. Coordination of a still larger ligand (dimethyl-
aniline, 2,6-lutidine, PCy3) increases crowding so much that
olefin coordination is completely blocked, as does addition of
a second smaller PR3; PPh3 apparently binds more weakly and
does not compete with olefin coordination.
The reason for the acceleration of monomer conversion
with the addition of PR3 is less clear, but the complexity of
kinetics in such systems, where any of a number of steps invol-
ving initiation, propagation or termination may strongly aﬀect
the observed rates, leaves many possibilities open. Examples
of both acceleration and retardation by additional ligation
have been observed previously.4j,6g Mechanistic studies are
ongoing in our labs in the hope of reaching a more complete
and conclusive explanation for the dramatic ligand “toggle”
eﬀect reported here, which would be an important step in the
much larger project of understanding the subtle connections
between the catalyst structure and catalytic behavior.
This work was supported by the USDOE Oﬃce of Basic
Energy Sciences (grant no. DE-FG03-85ER13431). The Bruker
KAPPA APEXII X-ray diﬀractometer was purchased via an NSF
CRIF:MU award to the California Institute of Technology
(CHE-0639094). We thank Dr Mona Shahgholi for the MALDI-
TOF analysis.
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Communication Dalton Transactions
15546 | Dalton Trans., 2013, 42, 15544–15547 This journal is © The Royal Society of Chemistry 2013
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Dalton Transactions Communication
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Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene

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Addition of a phosphine ligand switches an N-heterocyclic carbene-zirconium catalyst from oligomerization to polymerization of 1-hexene

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