Lewis Acid Enhanced Ethene Dimerization and Alkene Isomerization - ESI-MS Identification of the Catalytically Active Pyridyldimethoxybenzimidazole Nickel(II) Hydride Species

Article abstract of DOI:10.1021/acscatal.5b02003

A cationic methallyl 2-pyridine-4,7-dimethoxybenzimidazole (L₁) nickel precatalyst is highly selective in ethene dimerizations to 1-butene. The same catalyst isomerizes 1-butene and 1-octene to internal olefins. Co-catalytic additives of B(C₆F₅)₃ or BF₃·OEt₂ coordinate to the catalyst and increase the reaction rates of ethene dimerization. ESI-MS was applied identifying a [L₁NiH]⁺ cation as the catalytically active species.

Full text of DOI:10.1021/acscatal.5b02003

Letter
pubs.acs.org/acscatalysis
Lewis Acid Enhanced Ethene Dimerization and Alkene
IsomerizationESI-MS Identification of the Catalytically Active
Pyridyldimethoxybenzimidazole Nickel(II) Hydride Species
†
†
†
‡
Manuel A. Escobar, Oleksandra S. Trofymchuk, Barbara E. Rodriguez, Claudia Lopez-Lira,
‡
§
∥
⊥,¶
,⊥,¶
Ricardo Tapia, Constantin Daniliuc, Heinz Berke, Fabiane M. Nachtigall, Leonardo S. Santos,*
,†
and Rene S. Rojas*
†
Nucleus Millennium Chemical Processes and Catalysis (CPC), Laboratorio de Química Inorgan
ica de Chile, Casilla 306, Santiago-22 6094411, Chile
Laboratorio de Química Organica, Facultad de Química, Pontificia Universidad Catol
Chile
́
ica, Facultad de Química, Pontificia
Universidad Catol
‡
́
́
́
ica de Chile, Casilla 306, Santiago-22 6094411,
§
Organisch-Chemisches Institut, Universitat
̈
Mu
rich, Winterthurerstrasse 190, CH 8057, Zurich, Switzerland
Laboratory of Asymmetric Synthesis, Chemistry Institute of Natural Resources, University of Talca, P.O. Box 747, Talca 3460000,
Chile
̈ ̈
nster, Corrensstrasse 40, 48149 Munster, Germany
∥
̈
Chemisches Institut, Universitat Zu
̈
⊥
¶
Chile Nanobiotechnology Division at University of Talca, Fraunhofer Chile Research Foundation − Center for Systems
Biotechnology, FCR-CSB, P.O. Box 747, Talca 3460000, Chile
*
S Supporting Information
ABSTRACT: A cationic methallyl 2-pyridine-4,7-dimethoxybenzimidazole (L ) nickel
1
precatalyst is highly selective in ethene dimerizations to 1-butene. The same catalyst
isomerizes 1-butene and 1-octene to internal olefins. Co-catalytic additives of B(C F ) or
6
5 3
BF ·OEt coordinate to the catalyst and increase the reaction rates of ethene dimerization.
3
2
+
ESI-MS was applied identifying a [L NiH] cation as the catalytically active species.
1
KEYWORDS: ethene dimerization, nickel catalysis, olefin isomerization, microreactor, mass spectrometry
n the Shell Higher Olefin Process (SHOP), olefins are
catalysis. When the allyl or methallyl ligands are combined with
I
oligomerized via neutral Ni(II) catalysts bearing bidentate
NN, NO, PP, PN coordinated ligands, the resulting nickel
precatalysts are capable to oligomerize ethene (a, c, e, and h,
Scheme 1) or to polymerize ethene (b, d, f, g).
The a−c systems are cationic, whereas nickel compounds d−
h are neutral. The latter form zwitterions via Lewis acid
1
monoanionic ligands. In this context, it is a challenging task to
develop alternative catalytic systems, which selectively provide
ethene dimerization and can “on demand” isomerize the
2
1
terminal butene primary product to an internal butene. Low-
molecular-weight oligomerizations of ethene require the β-
hydride elimination step to be fast with respect to the olefin
insertion step into the nickel alkyl bond. To accomplish this
goal within nickel catalysis, we pursued a strategy of
constructing cationic nickel(II) hydride centers with neutral
NN ligands and to boost their electrophilicity further by
cocatalytic addition of strong Lewis acids attached to
appropriately functionalized ligand sites.
attachment, for instance, with B(C
F ) , coordinating to a
6 5 3
ligand donor atom of d−h (Scheme 1).
1g
Methallyl nickel complexes were seen to work with (a, d−h)
or without (b, c) coactivator but were found to have slow
3
initiation rates. Sometimes, when adding larger amounts of the
coactivator, the kinetics of the ethene oligomerizations (and of
3
,4
polymerizations) get accelerated.
Frequently reported
Nickel precatalysts applied in C−C- and C−H-forming
Received: September 9, 2015
Revised: November 6, 2015
Published: November 9, 2015
2
processes often contain allyl or methallyl moieties, which are
anticipated to be split off in the initiation process of the
©
2015 American Chemical Society
7338
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ACS Catalysis
Letter
Scheme 1. Reported Methallyl and Allyl Nickel Pre-Catalysts
precatalysts for this purpose are allyl or methallyl nickel
complexes bearing neutral NN-type α- or β-diimine ligands.
However, there are only very few examples of methallyl or allyl
nickel complexes combined with other NN-ligands, which are
capable of activating ethene (for example, g).
Electrospray ionization tandem mass spectrometry was
On the basis of this notion to build highly electrophilic nickel
catalysts, we tried to explore synthetic access to methallyl nickel
complexes with the neutral 2-pyridine-4,7-dimethoxybenzimi-
11
dazole (L ) ligand, which in addition was designed to bear
1
2g
methoxy functionalities interacting well with Lewis acidic
cocatalysts to foster the Lewis acidity of the nickel center and
consecutively boost the catalytic activities and the selectivities
previously used to study the mechanism of Brookhart olefin
5
,6
3,4
polymerizations
and also of Ziegler−Natta catalyzes by
for low molecular weight oligomers. The quite stable, yellow
7
3
F
coupling the ESI-MS analytics online with a microreactor and
this combination showed significant advantages over the more
conventional parallel screening approaches. These experiments
provided insight into the ionic reactive species taking a
prominent role in the catalytic cycle. Via these studies, the
catalytically active species of ethene polymerizations could be
methallyl nickel complex, [L
1
Ni(η -methallyl)][B(Ar ) ] (1)
4
3
was prepared by direct reaction of L with [Ni(η -methallyl)-
1
F
3
Cl] in the presence of [NaB(Ar ) ] in 78% yield (Scheme 3).
2
4
Scheme 3. Synthesis of the Nickel Complex 1
5
−7
identified, which appear in the presence of cocatalytic MAO.
Electrospray ionization tandem mass spectrometry seemed in
addition very suited to deepen the mechanistic insight into
ethene oligomerizations (and polymerizations) with methallyl
nickel catalysts but naturally also into the related cases of
ethene dimerization. MS-derived mechanistic information was
sought to support the tuning efforts for higher catalytic
selectivities, particularly when the olefin dimerization is
followed by isomerization processes of the primary terminal
NMR characterization of 1 was consistent with the formation
3
of a single isomer containing the bidentate L and the η -
1
1
methallyl ligand. The H NMR of 1 at room temperature
showed the syn and anti allylic hydrogen atoms at δ 2.8, 3.33,
and 4.22 ppm (was confirmed by low temperature NMR, see SI
page 4). The resonances of the anti allylic hydrogen atoms
appeared as broadened signals compared to the syn ones, thus
suggesting fluxional behavior of the allyl moiety on the NMR
time scale. The 11B and F NMR spectra of 1 revealed
8
product olefins. For more conclusive results in the isomer-
ization process, we intended to supplement the butene studies
by a study of the nickel-catalyzed isomerization of 1-octene.
Recently, R. Breuil et al. put forward that “cationic nickel
active species are electronically unsaturated and highly
19
F
−
9
characteristic singlets for the [B(Ar ) ] counterion at −6.5
electrophilic”. They generally comprise nickel(II) active
4
1/2
(
ν
∼ 11 Hz) and at −62 ppm, respectively. An X-ray
species, which have one bidentate donor ligand incorporated
and possess square planar coordination geometry. However,
these species were often difficult to prepare, since they showed
high reactivity. Despite such difficulties, cationic π-allyl nickel
complexes could already be obtained in the quite early days of
diffraction study confirmed the pseudosquare planar structure
of 1 in the solid state (N1−C21−C23−N3 dihedral angle is
−
4.9°), which apparently coincides with the structure in
solution (see Figure 1).
To explore the catalytic potential of 1 in ethene
oligomerization or dimerization, we first performed NMR-
10
organometallic chemistry by Wilke et al. (see Scheme 2).
scale experiments using dichloromethane-d as solvent at 25
2
Scheme 2. Cationic Model of a Nickel-Based Active Species
and 50 °C with and without the use of a Lewis acidic
coactivators B(C F ) or BF ·OEt . From these studies, we
6
5 3
3
2
12
concluded that 1 presents single component behavior.
In an NMR experiment in the absence of a cocatalyst, 1
turned out to slowly react with ethene producing 1-butene,
which subsequently isomerized in a still slower reaction to 2-
butene.
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Letter
agent (see entries 3−6, Table 1), suggesting in these cases a
rate increase of the β-hydride elimination step relative to the
olefin insertion reaction (ethene insertion, alkene isomer-
ization, and β-hydride elimination steps are depicted in Scheme
4
1
, and the coordinatively highly unsaturated nickel alkyl species
c and 1e are drawn with β-agostic stabilization ). The
16
B(C F ) cocatalytic entries 3 and 4 show highest reaction rates
6
5 3
in TOFs, which may be explained by a strong association
equilibrium of B(C F ) , particularly to the 1c species fostering
6
5 3
the β-hydride elimination step. In addition, there is a
temperature effect: for the B(C F ) addition cases of entries
Figure 1. Structure of 1 with thermal ellipsoids drawn at the 30%
F
−
6 5 3
probability level (side view). Hydrogen atoms and the [B(Ar ) ]
4
3
and 4, there is an increasing selectivity for internal butenes on
counterion are omitted for clarity.
the expense of the amount of 1-butene, which could mean that
with increasing temperature B(C F ) dissociation does not
6
5 3
When B(C F ) or BF ·OEt was introduced as a cocatalyst,
occur to a great extent and all the crucial reaction steps,
including also the olefin insertion step, that is accelerated
because of the temperature increase. This idea is also supported
by the similar TON values with changes in the catalytic activity
merely based on the temperature effect on reaction rates and
not on a changed amount of active species.
6
5 3
3
2
the formation of 1-butene and its subsequent isomerization
product 2-butene was observed, however, in much shorter
reaction time (30 times faster with B(C F ) and 70 times faster
6
5 3
1
with BF ·OEt , H NMR in CD Cl at room temperature).
Adding 5 equiv of BF ·OEt to the reaction solution initiated
3
2
2
2
3
2
immediate ethylene consumption. In order to clarify whether
a) 1-butene was formed first and then underwent reinsertion
and isomerization producing 2-butene, or (b) whether 2-butene
However, for the BF ·OEt addition cases of entries 5 and 6,
3
2
(
the interpretation of the catalytic reaction course seems much
more complex and can in our eyes not be resolved by a simple
rational. In contrast to the B(C F ) cocatalytic reactions, we
13
was directly formed through a chain walking mechanism, an
additional ethene dimerization experiment (C D , ethene
6
5 3
6
6
assume stronger dissociation equilibria with the weaker Lewis
pressure of 12.5 bar) was performed in a Parr reactor. Mainly
-butene was detected by NMR indicating that 2-butene
formation occurs only in absence of ethene or at very low
acid BF occurring for the prominent reactive species of crucial
3
1
reaction steps. The BF3 added reactive species are thus
expected to be present in lower amounts when temperature
increases but Table 1 demonstrates the opposite observation
with an increase in the overall reaction rate in form of the
enhancement of the TOF (Table 1). In addition, the yields of
hexanes increases at the expense of the butenes upon a raise in
temperature, presumably meaning that olefin insertion is
enhanced. This contradicting general situation also motivated
us to conduct a more detailed search for the catalytically active
species in these catalyzes (vide infra).
14
ethene concentrations, an observation which confirmed
notion (a) because of a competitive inhibition effect of higher
ethene concentrations hindering or preventing coordination of
butene. Additionally, to probe the capability of 1 to isomerize
terminal olefins, model type experiments with 1-octene were
carried out at room temperature. Also, in this case, 1 featured
single component behavior and isomerized 1-octene without
use of a cocatalyst. The products of 1-octene isomerization
forming internal olefins were 2-, 3-, and 4-octenes, which were
detected by 13C NMR in an approximate ratio of 4:7:1. In the
The reactivity differences observed between the oligomeriza-
tion reactions carried out in a Parr reactor (toluene) and NMR
presence of B(C F ) and BF ·OEt , the isomerization of 1-
6
5 3
3
2
15
experiment (CD Cl ) can be explained by the effect of solvent,
octene occurred instantaneously. The up-scaling of the
aforementioned ethene dimerization reactions was carried out
in a Parr reactor (12.5 bar of ethene) in toluene leading to
dimerization selectivities >89% for 1- and 2-butenes in the
absence of an activator and with B(C F ) as activator >80%.
2
2
and therefore, when an experiment was carried out in
deuterated toluene as solvent, the reaction rate was slower
1
(
see respective H NMR spectra in SI, page 24). Additionally,
when the reaction time was increased to 30 min (see entry 7,
Table 1), the same butene/hexene and 1-butene/2-butene
relations were observed; however, the TOF value decreased
probably to catalyst decomposition at higher reaction times
(see entry 4 and entry 7 in Table 1).
6
5 3
Low amounts of n-hexenes were formed with trimerization of
ethene (see entries 2−4, Table 1). In the case of BF ·OEt as a
3
2
cocatalyst, the selectivities for ethene dimerization were found
to be higher compared to that of B(C F ) as an activating
6
5 3
a
Table 1. Selected Ethylene Oligomerization Reactions with Complex 1
c
product (%)
b
TOF h−1
entry
co-activator (equiv)
T (°C)
C4
C6
C :1-C H / 2-trans/2-cis
TON (mol product/mol Ni)
4
4
8
1
2
3
4
5
6
-
25
50
25
50
25
50
50
100
89
n. d.
11
80/14/6
5
15
252
3072
3195
88
-
69/19/12
65/15/20
36/21/43
90/7/3
83
B(C F ) (5)
80
20
1014
1064
29
6
5 3
B(C F ) (5)
85
15
6
5 3
BF ·OEt (20)
BF ·OEt (20)
B(C F ) (5)
100
87
n. d.
13
3
2
62/17/21
41/21/38
141
1342
427
2684
3
2
d
7
86
14
6
5 3
a
b
Entries 1−6 were carried out in a Parr autoclave reactor in 30 mL of toluene at an ethene pressure of 12.5 bar, reaction time, 20 min. [Ni] = 1.3 ×
−
4
c
d
1
0
M. Determined by combined gas chromatography (GC). For entry 7, reaction time was 30 min.
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Scheme 4. Mechanistic Sketch of the Ethene and Terminal Olefin Activation and Isomerization by the Methallyl Nickel
Complex 1
As mentioned previously, we sought to have a closer look
into the catalytic reaction course of the ethene dimerization
with 1 by electrospray ionization tandem mass spectrometry
studies to trace the reactive nickel species responsible for
ethene dimerization and the terminal olefin isomerization
processes.
Studies of ethene dimerization and 1-octene isomerization
with 1 were performed by in situ catalysis investigations in the
presence of a small excess of B(C F ) as cocatalyst, using a
6
5 3
microreactor coupled directly to the ESI source of an ion-trap
mass spectrometer (Amazon, Bruker), focusing on the direct
detection and mass spectrometric characterization of the
cationic catalytically active species that are supposed to
accumulate in the catalytic cycle before the rate-determining
step.
An initial test was performed exploring the reaction of ethene
with the methallyl nickel complex 1 in the presence of a small
excess of B(C F ) as cocatalyst and trying to intercept the
6
5 3
active species in toluene as solvent, which revealed activity for
ethene dimerization with predominant formation of 1-butene.
At room temperature, a yellow solution was obtained that was
then analyzed approximately 0.7 s after mixing the reagents by
ESI(+)-MS. In that solution, the nickel ion 1a of m/z 368 was
observed (see Scheme 4 and Figure 2a). Besides the cation 1a,
the bisligated nickel complex Ni(L ) of m/z 567 was observed
1
2
as depicted in Figure 2a. Then, 1 to 6 s after mixing the
reagents, the nickel ion 1b (m/z 314) was detected, a product
witnessing the separation of the methallyl ligand from the
nickel cation 1a via ethene insertion into the σ-methallyl moiety
followed by a β-hydride elimination step (Figure 2b). The
isotopic pattern of all ions matched the calculated ones for the
suggested species, in particular for those containing the Ni-
species (see insert in Figure 2b). Moreover, a species of m/z
5
20 revealed losses of Na[C F ] and Na[OC F ] in the ESI-
6 5 6 5
+
MS/MS experiments that produced [1b + O] of m/z 330 and
+
[1b] of m/z 314, respectively. This species may be one of the
products formed during catalyst decomposition when ethylene
oligomerization occurs.
Then, 6−10 s after mixing of the reagents, the nickel ion 1a
disappeared and the intensity of the nickel cation 1b increased
Figure 2. In situ ESI-MS monitoring of (a, b) ethylene dimerization
and (c) 1-octene isomerization by 1/B(C as the catalytic system:
a) 0.7 s; (b) 1 to 6 s; (c) 20 s after mixing of the reagents.
F )
6 5 3
(
(Figure 2b); thus, it appears very probable that the active
species responsible for ethylene dimerization is cation 1b (see
Figure 2b, 1b, with the matching theoretical isotopic pattern see
below Figure 2b). It is noteworthy to mention that by MS we
have not observed any products of ethene insertion into the
cation 1b, presumably due to a very short lifetime of the follow-
up species.
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Finally, as 1/B(C F ) showed activity in the reaction with 1-
Oligomerization/Polymerization of Ethylene. In Studies in Surface
Science and Catalysis; Keii, T., Soga, K., Eds.; Elsevier: Amsterdam,
6
5 3
octene in condensed phase, we performed an experiment trying
to intercept the active nickel species in the gas-phase. Thus,
approximately 20 s after mixing of the reagents, a yellow
solution was obtained that was analyzed by ESI(+)-MS (see
Figure 2c). In Figure 2c, complete disappearance of 1a and 1b
is witnessed by forming a new cationic species, which was
identified and characterized to be 1d, suggesting that 1b is the
active species in the catalytic cycle. Species 1d can be addressed
to either olefin−hydride species or isomeric agostic n-octyl
species perhaps further stabilized by a β-agostic interaction.
In conclusion, the mechanism of the ethene dimerization
with the highly electrophilic cationic methallyl nickel complex 1
could be made plausible, revealing high selectivity toward
formation of butenes. The isomerization of the terminal olefins
to internal olefins is a slower process, also exemplified by 1-
octene isomerization. The mechanism of the dimerization of
ethene and the isomerization of terminal olefins was further
supported by electrospray ionization tandem mass spectrom-
etry. The cationic nickel hydride 1b is supposed to be the active
species driving the catalytic cycle possessing relatively high
stability due to the fact that subsequent ethene coordination
and insertion are slow and rate determining. During 1-octene
isomerization, the formation of the olefin hydride species of
type 1d could be identified as a relatively stable species of the
interaction of 1b with octene. Octene coordination may be
weaker than ethene coordination, but the given observation can
only be explained assuming that octene insertion is still slower
than ethene insertion into the Ni−H bond.
1
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acscatal.5b02003.
■
(
11) Ryu, C.; Song, E.; Shim, J.; You, H.; Choi, K.; Choi, I.; Lee, E.;
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*
S
(
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Experimental details of the synthesis and characterization
of the catalyst and oligomeric products (PDF)
Crystallographic data (CIF)
White, P. S.; Daugulis, O.; Brookhart, M. ACS Catal. 2015, 5, 631−
6
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AUTHOR INFORMATION
Corresponding Authors
E-mail: lssantos@utalca.cl.
E-mail: rrojasg@uc.cl.
(14) The reaction was carried out in a Parr autoclave reactor (100
mL), loaded inside a glovebox with an appropriate amount of the
■
−4
catalyst ([Ni] = 1.3 × 10 M) and the cocatalysts (20 equiv of BF3·
*
Et O) and benzene-d such that the final volume of the solution was
2
6
*
1
5 mL. The reaction mixture (ethylene pressure 12.5 bar) was stirred
Notes
at 50 °C for 20 min, then the reaction mixture was cooled to 0 °C and
The authors declare no competing financial interest.
the ethene was vented. The reaction mixture was analyzed by NMR.
1
(
15) H NMR spectra and GC data can be found in Supporting
ACKNOWLEDGMENTS
The authors thank prof. Alejandro Duran (CPC) for graphical
abstract design. This work was supported by ICM No. 120082
Information.
16) Leatherman, M. D.; Svejda, S. A.; Johnson, L. K.; Brookhart, M.
J. Am. Chem. Soc. 2003, 125, 3068−308.
■
(
(
1
Nucleus Millenium CPC) and FONDECYT projects Nos.
130077, 1150307, and 11130086. L.S.S. and F.M.N. thank
Anillo ACT 1107, InnovaChile CORFO (Code FCR-CSB
9CEII-6991), and PIEI-UTalca. The authors acknowledge the
0
valuable comments of the referees.
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DOI: 10.1021/acscatal.5b02003
ACS Catal. 2015, 5, 7338−7342