Synthesis and Reactivity of 1,2-Bis(di- iso-propylphosphino)benzene Nickel Complexes: A Study of Catalytic CO2-Ethylene Coupling

Article abstract of DOI:10.1021/acs.organomet.8b00260

The utilization of renewable carbon dioxide resources in the production of commercial chemicals has been at the center of efforts to improve the sustainability of many widely used consumer products. The catalytic coupling of CO₂ with ethylene to generate acrylates is one such avenue of research. Despite decades of investigations, catalytic CO₂-ethylene coupling has only recently been experimentally demonstrated. The development of a new nickel-based catalyst, 1,2-bis(di-iso-propylphosphino)benzene nickel(0) 1,5-cyclooctadiene, for acrylate production is detailed here. The stoichiometric reactivity of the catalyst toward the CO₂, ethylene, and base reagents, as well as the coordination chemistry of likely catalytic intermediates has been examined. Comparative catalytic experiments were used to identify the influence of commonly employed additives, such as metallic zinc and Lewis acidic salts, on the catalytic CO₂-ethylene coupling reaction. In addition, catalytic comparisons of the 1,2-bis(di-iso-propylphosphino)benzene nickel(0) 1,5-cyclooctadiene species with a similar previously reported catalyst under newly developed conditions afforded turnover numbers greater than 400, an approximately 4-fold increase over the highest activity reported to date. Significantly, the catalytic trials reveal that small changes in the catalytic conditions and additives can have substantial effects on productivity and that these influences are not uniform with respect to catalyst platforms.

Full text of DOI:10.1021/acs.organomet.8b00260

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis and Reactivity of 1,2-Bis(di-iso-propylphosphino)benzene
Nickel Complexes: A Study of Catalytic CO −Ethylene Coupling
2
†
‡
†
,†
Melissa N. Hopkins, Kenichi Shimmei, Katherine B. Uttley, and Wesley H. Bernskoetter*
†
The Department of Chemistry, The University of Missouri, Columbia, Missouri 65211, United States
‡
Sekisui Chemical Co. Ltd., Osaka 530-8565, Japan
*
S Supporting Information
ABSTRACT: The utilization of renewable carbon dioxide resources
in the production of commercial chemicals has been at the center of
efforts to improve the sustainability of many widely used consumer
products. The catalytic coupling of CO with ethylene to generate
2
acrylates is one such avenue of research. Despite decades of
investigations, catalytic CO −ethylene coupling has only recently
2
been experimentally demonstrated. The development of a new
nickel-based catalyst, 1,2-bis(di-iso-propylphosphino)benzene nick-
el(0) 1,5-cyclooctadiene, for acrylate production is detailed here. The stoichiometric reactivity of the catalyst toward the CO2,
ethylene, and base reagents, as well as the coordination chemistry of likely catalytic intermediates has been examined.
Comparative catalytic experiments were used to identify the influence of commonly employed additives, such as metallic zinc
and Lewis acidic salts, on the catalytic CO −ethylene coupling reaction. In addition, catalytic comparisons of the 1,2-bis(di-iso-
2
propylphosphino)benzene nickel(0) 1,5-cyclooctadiene species with a similar previously reported catalyst under newly
developed conditions afforded turnover numbers greater than 400, an approximately 4-fold increase over the highest activity
reported to date. Significantly, the catalytic trials reveal that small changes in the catalytic conditions and additives can have
substantial effects on productivity and that these influences are not uniform with respect to catalyst platforms.
INTRODUCTION
Carmona was among the pioneers in developing a transition
■
8
metal mediated route for acrylates via CO -olefin coupling. In
2
Expanding recognition of the deleterious effects of anthro-
pogenic CO2 production has brought carbon capture,
utilization, and storage (CCUS) technology development to
1
985, his laboratory reported the first well-defined example of
this reaction, using a zero-valent molybdenum ethylene
complex to reduce CO into a bimetallic acrylate complex.
2
8a
1
the forefront of chemical and engineering research. Given the
scale of anthropogenic CO production, it is doubtful that the
2
CO utilization alone will ever directly offset carbon emissions.
2
However, the upconverison of CO into value added chemicals
2
could play a significant role in the overall economics of CCUS
activities as well as enhance the sustainability of many
2
commercially significant chemicals. One avenue toward this
Although subsequent research indicates that group VI metals
are challenging targets for catalyst development due strong
product inhibition from of the metal−oxygen bond, this
seminal work opened the field to the first catalytic processes
goal has been the investigation of transition metal catalyzed
reverse combustion” processes to generate molecular energy
sources, such as methane and methanol, from CO . Due to
the potentially immense scale for the utilization of such
technologies, these studies have been a major focus of CO
upconversion. However, there are also numerous other
9
“
3
2
developed nearly three decades later.
2
Modern reports of catalytic acrylate production from CO −
2
4
ethylene coupling first appeared in 2012 from the joint
laboratories of BASF-University of Heidelburg using a zero-
valent nickel source along with bis(ditert-butylphosphino)-
attractive targets for chemical CO utilization which, though
2
smaller in commodity scale, have more favorable thermody-
namic energy profiles. One such target that has been of
interest to our laboratory is the catalytic synthesis of acrylates
5
10
ethane. Shortly thereafter followed several reports of closely
related nickel catalyzed coupling reactions using a variety of
bidentate diphosphine ligands, in addition to a few less
catalytically active systems described with other late transition
from coupling CO and ethylene.
2
Acrylic acid and its derivatives are primarily produced via
propylene oxidation methods for applications in water
adsorbing polymers and polyester fibers, as well as other
materials. The use of the nonrenewable and relatively
expensive propylene feedstock makes a CO −ethylene derived
path to acrylates a potentially attractive alternative. Ernesto
6
Special Issue: In Honor of the Career of Ernesto Carmona
Received: April 24, 2018
2
7
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00260
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Article
1
1
metals (Figure 1). Acrylate production at nickel has been
widely proposed to proceed via an oxidative coupling reaction
acrylate from CO and ethylene, in addition to stoichiometric
reactions and comparative catalysis trials which highlight the
complex role of base and additives in catalytic performance.
2
RESULTS AND DISCUSSION
■
Catalysts Synthesis and Stoichiometric Reactivity.
One of the leading nickel catalyzed reactions for CO2
functionalization to acrylate reported by Limbach and co-
workers utilizes a combination of Ni(COD) (COD = 1,5-
2
cyclooctadiene) and (R,R)-(+)-1,2-bis(tert-butylmethylphos-
1
1b
phino)benzene (BenzP*) (Figure 4).
While providing an
active catalytic system, BenzP* also incorporates P-stereogenic
sites which complicate ligand synthesis while providing little
Figure 1. General scheme for nickel promoted CO −ethylene
2
coupling to yield acrylate.
benefit to the achiral production of acrylate from CO −
2
ethylene coupling. Given this factor, our laboratory targeted a
i
more easily prepared and symmetric Pr P analog of BenzP*,
2
with CO and ethylene to generate an undesirably stable
2
1
,2-bis(di-iso-propylphosphino)benzene (i-PrBenzP, Figure 4),
12
intermediate nickel−lactone species (Figure 2). Many such
in order to more efficiently investigate the coordination
nickel−lactone complexes have been isolated and characterized
chemistry associated with CO −ethylene coupling as well as
2
since the first CO derived examples described by Heinz
2
1
3
examine the influence of commonly used additives on catalytic
acrylate formation.
Hoberg in 1983. In fact, the stubborn stability of these
species toward the β-hydride elimination reaction required to
14
The i-PrBenzP ligand was coordinated to zero-valent nickel
form acrylate is a significant obstacle in catalyst development.
by stirring in a tetrahydrofuran solution with Ni(COD) at
2
In the nascent catalytic reactions reported to date, this barrier
to acrylate formation has been overcome by the use of strong
ambient temperature to afford (i-PrBenzP)Ni(COD) (1-
3
1
10,11
COD) as a yellow solid upon isolation (Figure 5). The
NMR spectrum of 1-COD exhibited a single resonance at
5.89 ppm in benzene-d , consistent with a symmetric
P
bases and/or Lewis acid additives (Figure 3).
When a
strong base, such as NaOt-Bu, is employed, the exogenous base
directly removes the mildly acidic β-hydrogen with little or no
involvement of the metal. However, the use of strong bases are
6
6
coordination environment about the nickel center. The
corresponding H NMR spectrum displays a distinctive
1
typically incompatible with efficient one-pot CO −ethylene
2
resonance at 4.43 ppm indicative of nickel coordinated 1,5-
COD methine protons, along with a set of resonances assigned
to the i-PrBenzP ligand. Diphosphine nickel(0) species similar
to 1-COD are frequently proposed as the active precatalyst
coupling catalysis, as undesirable reactions between the base
10
and CO2 compete with the β-hydrogen deprotonation.
Although a mildly basic environment is required to overcome
the slightly unfavorable thermodynamics of acrylate production
species in CO −ethylene coupling, motivating closer study of
from CO −ethylene coupling in the gas phase or most organic
2
2
1
2
the reactivity of 1-COD toward CO and ethylene. Treatment
solvents, the use of a strong base has clear limitations. Lewis
acidic additives provide an alternative method to assist in the
β-hydride elimination reaction from the nickel−lactone
complexes. Prior studies from our laboratory suggest that
even weak Lewis acids, such as sodium cation sources, permit
transient opening of the five-membered lactone ring, providing
the geometric flexibility and the open coordinative site that are
2
of 1-COD with 2 atm of a 1:1 mixture of CO and ethylene in
2
an NMR tube resulted in rapid and complete conversion to a
new organometallics species, as judged by NMR spectroscopy.
Additionally, treatment of 1-COD with only ethylene
1
31
produced an identical set of H and P NMR resonances,
indicating the new species, (i-PrBenzP)Ni(C H ) (1-C H ),
2
4
2
4
15
did not arise from a reaction with CO . 1-C H species proved
necessary for efficient β-hydride elimination. Vogt and co-
workers have successfully employed this effect to enhance the
2
2
4
unstable to the removal of the ethylene atmosphere (reverting
quickly back to 1-COD), so it was characterized solely by in
catalytic performance of (DCPE)/Ni(COD) (DCPE = 1,2-
2
bis(dicyclohexylphosphino)ethane) through the addition of
situ NMR spectroscopic methods. The nickel(0) ethylene
11a
31
cocatalytic amounts of LiI salt.
complex exhibits a singlet resonance at 71.70 ppm in the
P
The recent development of the first catalytic reactions for
NMR spectrum. Similar nickel(0) ethylene species have been
acrylate synthesis from CO and ethylene, as well as the
crystallographically characterized and bear only one olefin
2
1016
apparent role of Lewis acids in assisting those reactions, has
motivated our investigations to new nickel/Lewis acid
combinations for this reaction. Herein we described the
preparation for a new nickel catalyst of the production of
bound to the nickel center.
Due to dynamic exchange
between the free and bound ethylene in the solution of 1-
C H , definitive assignment of the stoichiometry of bound
2
4
ethylene could not be made from the data in hand, so 1-C H4
2
Figure 2. Proposed reaction steps for the synthesis of acrylate at diphosphine nickel(0) species.
B
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Figure 3. Methods for inducing elimination from nickel−lactone complexes.
exchange of the ancillary ligands and isolation of 1-C H O
2
3
4
17
31
in good yields following recrystallization (Figure 5). The P
NMR spectrum of 1-C H O displays a characteristic pair of
3
4
2
doublet resonances at 62.92 and 70.53 ppm (J = 9.0 Hz),
consistent with two inequivalent P-environments created by
1
3
the unsymmetric metal−lactone ring. The C NMR spectrum
exhibited a resonance at 188.50 ppm assigned to the nickel−
1
3
Figure 4. Benzene annulated diphosphine ligands for CO −ethylene
lactone carbonyl moiety, which was confirmed by CO
2
2
coupling.
isotopic labeling experiments starting from 1-C H as well as
2 4
1
13
by H− C HMBC and HSQC NMR analyses. The 2D NMR
experiments also provided definitive assignment of the α- and
is depicted with one ethylene ligand on the basis of prior
1
0,16
β-CH resonances of the nickel−lactone ring, which were
2
precedent.
Significantly, prolonged exposure of 1-C H to a mixture of
present at 1.10 and 2.84 ppm, respectively. Solutions of 1-
C H O exhibited minimal decomposition at ambient temper-
2
4
3
4
2
ethylene and CO at 60 °C did afford a nickel−lactone species,
2
ature over several days, but heating at 60 °C for 2 h in the
(
i-PrBenzP)Ni(κC,κO−CH CH COO) (1-C H O ), over 2
2
2
3
4
2
18
presence of COD resulted in substantial reversion to 1-COD.
These observations suggest that the poor yields from the
days. Conversion to the CO −ethylene coupling product was
2
only ca. 20% complete at this time, and extended reaction time
or further heating produced a notable degree of sample
degradation with observation of metallic nickel precipitate and
free ligand. In order to determine if 1-C H O formation was
CO −ethylene coupling synthesis were likely due to both slow
2
rates of reaction under these pressure and temperature
conditions and a weak thermodynamic preference for coupling.
With the i-PrBenzP substituted nickel−lactone complex in
hand, our examination turned toward conversion of the
metallacycle to an acrylate complex. Given the absence of
CO in the isolated samples of 1-C H O , the method of
3
4
2
simply kinetically slow (relative to decomposition of 1-C H )
2
4
or thermodynamically disfavored, an alternative preparation of
the nickel−lactone was targeted. Addition of i-PrBenzP to the
previously reported (2,2′-bipyridine)Ni(κC,κO−
CH CH COO) nickel−lactone complex resulted in clean
2
3
4
2
strong base addition (vide supra) was selected for the ring-
2
2
Figure 5. Reactivity of [(i-PrBenzP)Ni] complexes toward ethylene and CO2.
C
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Article
Figure 6. Molecule structure of 1-acrylate·NaHMDS at 30% ellipsoids. At the left, the full tetrameric unit is illustrated with hydrogen atoms and i-
Prr substitutents removed for clarity. Elements labeled by color as Na (yellow), O (red), Ni (green), N (blue), C (gray), Si (tan), and (P) purple.
2
At the right, a single (i-PrBenzP)Ni(η -CH CHCO Na) unit is illustrated.
2
2
opening reactions. Addition of sodium hexamethyldisilazide
NaHMDS) to a tetrahydrofuran solution of 1-C H O
a coordination geometry and metrical parameters typical of this
class of complexes, it does provide definitive characterization
(
3
4
2
immediately produced a color change from yellow-orange to
pale green. The corresponding 31P NMR spectrum displayed
two new doublet resonances at 67.69 and 72.97 ppm (J = 45
for one of the key intermediates in nickel catalyzed CO
ethylene coupling.
−
2
Perhaps just as significant as the isolation and character-
ization of 1-acrylate, its reactivity toward ethylene provided
further encouragement regarding the application of this system
in catalysis. Treatment of 1-acrylate with even 0.25 atm of
ethylene afforded near instantaneous liberation of sodium
acrylate and concurrent generation of the nickel(0) ethylene
complex, 1-C H , as judged by NMR spectroscopy. This
2
Hz), consistent with formation of an η -sodium acrylate nickel
2
complex, (i-PrBenzP)Ni(η -CH CHCO Na) (1-acrylate).
2
2
Once again, 2D NMR spectroscopy was used to assign the key
H and C resonances of 1-acrylate. The H NMR resonances
1
13
1
for the geminal methylene protons appeared at 2.03 and 2.40
ppm, with a corresponding signal at 34.00 ppm for the
2
4
methylene carbon in the 13C NMR spectrum. The vinylic
methine resonances were observed at 3.16 and 49.70 ppm in
conversion establishes the competency of the [(i-PrBenzP)Ni]
system to complete all of the necessary steps in a catalytic cycle
1
13
for CO −ethylene coupling to acrylate. Using this fundamental
2
the H and C NMR spectra, respectively. Crystals of 1-
acrylate suitable for X-ray diffraction study were obtained by
chilling a pentane solution of the crude synthetic reaction
mixture at −35 °C for several days. The complete molecular
structure (Figure 6; left) indicates a tetramer of 1-acrylate in
the solid state. This is the first crystallographic characterization
insight, our study moved on to an examination of the catalytic
behavior of [(i-PrBenzP)Ni] under conditions commonly
utilized for other transition metal catalyzed acrylate formation
reactions.
Catalytic Acrylate Formation. The current state-of-the-
art nickel catalyzed CO −ethylene coupling reactions have
2
2
of an η -coordinated nickel sodium acrylate complex, though
evolved quickly from the first report by Limbach and co-
the structure of a closely related nickel acrylic acid complex has
10
workers. The development of these reactions has primarily
10
been previously reported. The aggregate is centered on a
core of eight sodium cations and four acrylate anions, along
with four [N(SiMe ) ] anions originating from the excess base
been directed by the empirical testing of reaction additives and
the judicious selection of bases to drive the thermodynamics of
acrylate extrusion. Limbach’s first report employed a bis(ditert-
3
2
2
used in the synthesis. Each acrylate fragment is bound in an η
butylphosphino)ethane/Ni(COD) system along with NaOt-
2
fashion to an [(i-PrrBenzP)Ni] moiety. Some disorder is
present in the structure, owing to partial occupation of one of
the acrylic olefins across two positions. This disorder is also
carried to the [(i-PrrBenzP)Ni] unit bound to that acrylate.
However, a suitable model was created for this disorder and
the data satisfactorily refined. The coordination environments
about the nickel centers are quite similar, so a single 1-acrylate
unit has been depicted in Figure 6 (right). The geometry about
the nickel is best described as square planar with a moderate
distortion arising from the constrained angles of the olefin and
diphosphine ligands. The olefinic C(19)−C(20) bond distance
of 1.43(1) Å is notably longer than that of free acrylic acid
Bu as a base to achieve a turnover number (TON) of 10.
However, the use of NaOt-Bu proved incompatible with CO2
in a single-stage reaction, requiring the repeated sequential
purging of gases and addition of base to make the system
function. Later, the same laboratory examined the use of
various substituted sodium phenoxide bases and screened a
collection of commercially available diphosphine ligands to
achieve a marked improvement in the nickel catalyzed
11b
reaction.
The use of less nucleophilic phenoxide bases
permitted a single batch reaction process with all reaction
components present at the same time. Sodium 2-fluorophen-
oxide (2-F) was found to be a particularly effective base when
(
1.29 Å), suggesting substantial bond order reduction due to
paired with the BenzP*/Ni(COD) catalyst, producing a TON
2
π-backbonding with the nickel and from deprotonation of the
of 39 under 15 bar total pressure CO /ethylene (2:1) at 100
2
carboxylic acid. The two C−O bond lengths of 1.278(9) and
°C for 20 h. The system was further improved by increasing
the pressure and adding an excess of Zn dust to the reaction
mixture to achieve a TON of 107 (Figure 7), the highest
1
.253(6) Å are consistent with delocalization of the C−O
double bond, as is the symmetric coordination of the sodium
to the carboxylate group. While structure of 1-acrylate exhibits
productivity reported to date for the CO −ethylene coupling
2
D
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Article
our own hands using (BenzP*)Ni(COD) under these
conditions. However, the moderate activity of the [(i-
PrBenzP)Ni] system did indicate that it would make a
reasonable platform for assessing the influence of reaction
additives. Additionally, use of the nickel lactone complex, 1-
C H O , afforded a turnover of 55, comparable to that
3
4
2
obtained with 1-COD, consistent with the two species
generating acrylate via a common pathway.
Figure 7. [BenzP*Ni] catalyzed sodium acrylate synthesis reported by
Given our prior research into the role of Lewis acidic salts in
the ring-opening of stable nickel−lactone intermediates in
acrylate formation reactions, we were keen to test the effect of
Limbach and co-workers.
1
1b
reaction. The use of Zn dust to enhance catalytic acrylate
production was first described by Vogt and co-workers, though
the role of Zn in these reactions has not been fully
15
adding alkali metal salts to the catalytic mixture. Computa-
+
tional studies by Vogt and co-workers suggest that Li should
+
1
1a
be considerably more effective than harder salts such as Na in
elucidated. One possibility is that Zn acts as a reducing
agent to reactivate stable nickel(II) salts formed during
catalysis. This hypothesis is partially supported by the
observation that only zero-valent nickel complexes have thus
mediating β-H elimination reactions from nickel−lactone
11a
intermediates.
Lithium iodide was selected for initial
experiments, given its relatively high solubility in THF and
prior precedent in enhancing CO −ethylene coupling
Unfortunately, addition of 1 mmol (100 equiv
1
2
2
far been reported to mediate CO −ethylene coupling.
11a
2
catalysis.
Our investigations into [(i-PrBenzP)Ni] catalyzed CO −
2
per Ni) of LiI to the catalytic mixture reduced the TON to 14.
It has been suggested by other reports that while the presence
of iodide may aid some steps of the proposed catalytic cycle it
could also act as a deactivating agent by trapping the catalyst as
ethylene coupling began with the use of 1-COD in conjunction
with a base (sodium 3-fluorophenoxide, 3-F), which has
previously been described for screening ancillary ligand
1
1b
effects. The use of isolated samples of 1-COD for catalysis
was selected over the more common in situ catalyst generation
19
nickel(II) species. In an effort to reactivate any nickel(II)
species, 2 mmol (200 equiv per Ni) of Zn dust was added as a
potential reducing agent; however, the resulting TON of 30
showed only partial recovery of the prior catalytic activity.
Replacing the LiI with NaI provided a TON of 35. A similar
influence using NaI as also observed with (BenzP*)Ni(COD)
as catalyst. However, conducting catalysis with only Zn dust as
the additive produced a TON = 84, a substantial improvement
from the unadulterated reaction and the Lewis acid/Zn
combinations. A trial using (BenzP*)Ni(COD) also showed
a modest enhancement to a TON = 109. The origin of this
improved productivity is not entirely clear with the limited
data in hand, but the enhancement is qualitatively similar to
technique of mixing ligand/Ni(COD) due to prior observa-
2
tions that such mixing can also produce (ligand) Ni complexes,
2
which appear catalytically inactive. A tetrahydrofuran solution
of catalyst 1-COD was treated with excess 3-F under 30 bar of
CO /ethylene (1:2) for 20 h at 110 °C and produced a TON
2
of 45 (Table 1). This is less than the TON of 82 achieved in
Table 1. Nickel Catalyzed CO −Ethylene Coupling to
2
Acrylate
11b
that observed with the (BenzP*)/Ni(COD) system.
2
Prior studies of CO −ethylene coupling to acrylate have
2
noted the great importance of base selection in balancing the
need to provide a thermodynamic driving force for the reaction
and avoiding excessive side reactions with CO and other
2
11
reaction components. With this factor in mind, we explored
the use of a slightly less basic and more sterically encumbered
phenoxide base, sodium 2-chlorophenoxide (2-Cl). Our initial
experiments employed 2-Cl under the conditions which had
afforded the highest TON with 3-F, including 2 mmol of Zn
dust. This produced a rather disappointing TON of 30.
Removal of the Zn additive from 1-COD also a limited TON
of 21, suggesting that for the [(i-PrBenzP)Ni], 2-Cl holds little
to no advantage over 3-F base. However, catalytic trials using
additives (equiv/Ni)
Zn
a
catalyst
base
LiI
NaI
dust TON
(
(
1
1
1
1
(
1
(
1
1
i-PrBenzP)Ni(COD) (1-COD)
BenzP*)Ni(COD)
3-F
3-F
3-F
3-F
3-F
3-F
3-F
3-F
45
82
55
14
30
35
64
84
109
30
21
-C H O
3
4
2
-COD
-COD
-COD
100
100
(
BenzP*)Ni(COD) with the Zn and 2-Cl afforded a notably
200
200
200
200
200
200
active TON of 135, suggesting that the influence of the base is
not uniform across catalysts bearing even quite similar ancillary
ligands. Likewise, inclusion of cocatalytic amounts of NaI and
Zn did little to improve the activity of 1-COD with 2-Cl base
100
100
BenzP*)Ni(COD)
-COD
BenzP*)Ni(COD)
3-F
(
TON = 30), but this additive combination proved quite
-COD
-COD
2-Cl
2-Cl
2-Cl
2-Cl
2-Cl
potent for (BenzP*)Ni(COD) (TON = 404). The productiv-
ity of (BenzP*)Ni(COD) under these conditions is the highest
(
1
(
BenzP*)Ni(COD)
-COD
BenzP*)Ni(COD)
200
200
200
135
30
404
TON for CO −ethylene coupling reported to date, though the
100
100
2
[(BenzP*)Ni] system was already a leading catalyst platform.
Intriguingly, there appears to be a vastly different influence of
the reductant and Lewis acid additives between the two similar
(diphosphino)benzene nickel(0) catalysts, with Lewis acids
a
TON values reported per Ni equivalent and are the average of two or
more trials.
E
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Organometallics
Article
proving far more enhancing for the chiral BenzP* system.
There is insufficient data at present to draw any meaningfully
hypothesis as to the origin(s) of these variations, but it
certainly gives pause to the generalization of findings from one
Ni catalyst system to the next.
Synthesis of (i-PrBenzP)Ni(COD) (1-COD). A 20 mL
scintillation vial was filled with 141 mg (0.45 mmol) of Ni(COD)2
and dissolved in THF. Then, 109 mg (0.40 mmol) of i-PrBenzP was
dissolved in THF and added dropwise to the nickel solution over a
period of 10 min producing a dark brown solution. The reaction was
stirred for 24 h at ambient temperature. The THF solvent was then
removed under vacuum, and the dark residue was extracted with
pentane, filtered through a Pasteur pipet, and dried under reduced
pressure. The resulting solid was recrystallized in a minimal amount of
pentane at −35 °C to afford 95 mg (50%) of 1-COD as yellow
crystals. Anal. Found (calcd) for C H P Ni: C, 64.18 (64.54); H,
CONCLUDING REMARKS
■
The utilization of the CO toward the synthesis of acrylates is
2
an endeavor which is just beginning to gain promise with the
aid of transition metal catalyst development. The reactivity of
the newly prepared catalyst, 1,2-bis(di-iso-propylphosphino)-
benzene nickel(0) 1,5-cyclooctadiene, indicates that the widely
2
5
44 2
1
9.13 (9.53). H NMR (C D ): δ 0.90 (dd, J = 6.1, 12 Hz, 12H,
6
6
isopropyl CH ), 0.97 (dd, J = 6.1, 12 Hz, 12H, isopropyl CH ), 2.34
3
3
(m, 4H, isopropyl CH), 2.49 (m, 4H, COD CH
), 2.65 (m, 4H, COD
2
CH ), 4.43 (s, 4H, COD CH), 7.09 (m, 2H, C H ), 7.40 (m, 2H,
proposed stepwise process for CO −ethylene (Figure 2) does
2
6
4
2
3
1
1
13
1
C H ). P { H} NMR (C D ): δ 65.89 ppm (s). C { H} NMR
appear to be a viable mechanism. However, most catalyst
productivity improvements to date have relied on screening of
ancillary ligands, optimization of reaction conditions, and the
empirical use of cocatalytic additives. While these are
undoubtedly effective techniques, our findings suggest that
the influence of the base/reductant/Lewis acid mixtures in
acrylate syntheses may not translate intuitively across even very
similar catalyst structures. From an optimistic perspective, it is
clear that relatively minor changes in the base or additives can
result in significant improvements to catalytic performance.
However, comparative catalysis experiments with 1-COD and
6
4
6
6
(
(
1
C D ): δ 18.66 (isopropyl CH ), 19.07 (isopropyl CH ), 26.31
6 6 3 3
isopropyl CH), 32.81 (COD CH ), 80.38 (COD CH), 128.45 (Ar),
2
30.10 (Ar), 146.91 (Ar).
Generation and Spectral Data for (i-PrBenzP)Ni(C H ) (1-
2
4
C H ). A J. Young NMR tube was charged with 10 mg of 1-COD and
2
4
approximately 500 μL of C D . The tube was cooled to −196 °C,
6
6
degassed, and 1 atm of ethylene was admitted via calibrated gas bulb.
Upon thawing, 1-C H4 forms immediately (as judged by NMR
2
spectroscopy), although the sample’s visual appearance remains
unchanged. The product was not stable in the absence of ethylene,
and quickly reverted to 1-COD upon remove of the volatiles. NMR
spectra indicate essentially complete conversion to1-C H under
2
4
(
BenzP*)Ni(COD) indicated the application of an optimized
1
these conditions. H NMR: δ 0.82 (m, 12H, isopropyl CH ), 1.14 (m,
3
formula for one catalyst architecture to other systems maybe
misleading. Likely, there exist multiple interactions between
the Zn reductant, Lewis acidic salts, phenoxide bases, and
nickel species employed in these catalytic systems. Creating a
more rational based approach to catalyst system development
will require a much better understanding of the chemical
intricacies of these complex reaction mixtures. Our laboratory
is currently endeavoring to use the easily prepared 1-COD
catalyst to investigate the role of each additive in the reaction
and elucidate how these additives and bases interact to
influence catalytic acrylate production.
1
2H, isopropyl CH ), 2.20 (m, 4H, isopropyl CH), 7.12 (m, 2H, Ar),
3
7.43 (m, 2H, Ar). The resonance for ethylene was not located due to
rapid exchange with free ethylene. P { H} NMR (C D ): δ 71.70
(s).
Synthesis of (i-PrBenzP)Ni(κC,κO−CH CH COO) (1-C H O ).
A 20 mL scintillation vial was charged with 100 mg (0.64 mmol) of
3
1
1
6
6
2
2
3
4
2
2
,2′-bipyridine, 35 mg (0.35 mmol) of succinic anhydride, and
approximately 5 mL of THF. This mixture was then added dropwise
to a solution of 176 mg (0.64 mmol) of Ni(COD) and 5 mL of THF
2
in a second 20 mL scintillation vial. The reaction solution was stirred
at ambient temperature for 4 h and then filtered through a fine frit.
The resulting red powder (bipyNi-lactone) was collected and washed
with pentane and diethyl ether. Then, 113 mg (0.40 mmol) of the red
powder was combined with 122 mg (0.40 mmol) of i-PrBenzP and
dissolved in approximately 10 mL of methylene chloride. The reaction
was stirred at ambient temperature for 24 h, the solvent removed
under vacuum, and the residue extracted with diethyl ether and
filtered through Celite. The filtrate solution was dried under reduced
pressure, and the resulting orange solid material was recrystallized in
minimal diethyl ether at −35 °C to afford 130 mg (75%) of 1-
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were carried out
using standard vacuum, Schlenk, cannula, or glovebox techniques.
Carbon dioxide (laser-grade) and ethylene (dry) were purchased from
Airgas and used as received. i-PrBenzP was prepared as previously
described, with the exception that a commercially available Grignard
20
1
reagent was employed. BenzP* was obtained as racemic mixture as
C H O as an orange precipitate. H NMR δ 0.78 (dd, J = 6.4, 12 Hz,
6H, isopropyl CH ), 0.86 (dd, J = 6.4, 12 Hz 6H, isopropyl CH ),
0.97 (dd, J = 6.4, 12 Hz, 6H, isopropyl CH ), 1.10 (m, 2H, α-CH ),
3
4
2
2
1
previously described. All other chemicals were purchased from
Aldrich, Fisher, VWR, Strem, or Cambridge Isotope Laboratories.
Phenoxide bases were prepared from the sodium hydride deproto-
nation of commercially available phenols. All other nonvolatile solids
were dried under vacuum at 50 °C overnight. Solvents were dried and
3
3
3
2
1.32 (dd, J = 6.4, 12 Hz, 6H, isopropyl CH ), 1.92 (m, 2H, P−CH),
3
2.18 (m, 2H, P−CH), 2.84 (m, 2H, β-CH ), 6.96 (m, 2H, Ar), 7.01
2
1
3
1
(m, 1H, Ar), 7.07 (m, 1H, Ar). C { H} NMR (C D ): δ 12.10 (α-
6
6
22
1
13
31
deoxygenated using literature procedures. H, C, and P NMR
spectra were recorded on Bruker 300 MHz DRX, 500 MHz DRX, or
CH ), 18.32 (isopropyl CH ), 18.71 (isopropyl CH ), 19.14
(isopropyl CH ), 25.30 (isopropyl CH), 26.19 (isopropyl CH),
37.11 (β-CH ), 129.99, 130.40, 131.31 (Ar), 188.50 (lactone
carbonyl). P { H} NMR (C D ): δ 62.92 (d, J = 9.0 Hz), 70.53
2
3
3
3
6
00 MHz spectrometers at ambient temperature, unless otherwise
2
1
13
31
1
noted. H and C chemical shifts are referenced to residual solvent
signals; 31P chemical shifts are referenced to an external standard of
H PO . Probe temperatures were calibrated using ethylene glycol and
6 6
−1
(d, J = 9.0 Hz). IR (KBr): νC−O = 1629 cm .
3
4
Synthesis of (i-PrBenzP)Ni(η2-CH CHCO Na) (1-acrylate).
2
2
23
methanol as previously described. X-ray crystallographic data were
collected on a Bruker CMOS diffractometer with Mo Kα radiation.
Samples were collected in inert oil and quickly transferred to a cold
gas stream. The structures were solved from direct methods and
Fourier syntheses and refined by full-matrix least-squares procedures
with anisotropic thermal parameters for all non-hydrogen atoms.
Crystallographic calculations were carried out using SHELX
programs. High-pressure catalytic hydrogenation reactions were
performed using a Parr 5500 series compact reactor with glass insert.
A 20 mL scintillation vial was charged with 45 mg (0.10 mmol) of 1-
C H O , 37 mg (0.20 mmol) of sodium hexamethyldisilazane, and
3
4
2
approximately 10 mL of THF. The reaction was stirred for 30 min at
ambient temperature and the volatiles removed under vacuum. The
residue was washed with pentane followed by extraction with diethyl
ether. Concentration of the diethyl ether solution and cooling to −35
°C for several days afforded 42 mg of 1-acrylate as a yellow-green
solid. X-ray diffraction and NMR analysis indicated some contami-
nation with sodium hexamethyldisilazane, which cocrystallized with 1-
F
DOI: 10.1021/acs.organomet.8b00260
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
acrylate. H NMR: δ 0.81 (m, 6H, isopropyl CH ), 1.00 (m, 6H,
Dioxide to Methanol and Derived Products - Closing the Loop. Chem.
Soc. Rev. 2014, 43, 7995−8048.
(4) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazabal, G. O.;
3
isopropyl CH ), 1.28 (m, 6H, isopropyl CH ), 1.38 (m, 6H, isopropyl
CH ), 1.93 (m, 2H, isopropyl CH), 2.03 (s, 1H, acrylate CH ), 2.40
3
3
3
2
(
s, 1H, acrylate CH ), 3.16 (1H, acrylate CH) 2.51 (m, 2H, isopropyl
Perez-Ramirez, J. Status and perspectives of CO conversion into fuels
and chemicals by catalytic, photocatalytic and electrocatalytic
processes. Energy Environ. Sci. 2013, 6, 3112−3135.
2
2
CH), 6.82 (m, 1H, Ar), 7.09 (m, 1H, Ar), 7.35 (m, 1H, Ar), 8.93 (m,
1
(
(
quaternary carbonyl carbon not located. P { H} NMR (C D ): δ
6
13
1
H, Ar). C { H} NMR (C D ): δ 19.42 (isopropyl CH3), 19.61
6 6
isopropyl CH3), 24.53 (isopropyl CH), 26.28 (isopropyl CH) 34.00
(5) Bhanage, B. M.; Arai, M., Eds. Transformation and Utilization of
Carbon Dioxide; Springer-Verlag: Berlin Heidelburg, 2014.
(6) Ohara, T.; Sato, T.; Shimizu, N.; Prescher, G.; Schwind, H.;
Weiberg, O.; Marten, K.; Greim, H. Acrylic Acid and Derivates. In
Ullmann’s Encyclopaedia of Industrial Chemistry; Wiley-VCH:
Weinheim, 2011; pp 1−18.
O CCHCH ), 49.70 (O CCHCH ) 118.14, 127.77 (Ar)
2
2
2
2
3
1
1
6
6
7.69 (d, J = 45 Hz), 72.97 (d, J = 45 Hz).
General Procedure for Catalytic Acrylate Production
Experiments. In a typical experiment, a stainless-steel autoclave
reactor fitted with glass insert was charged with catalyst (0.01 mmol),
sodium phenoxide base (8 mmol), zinc dust (2 mmol), Lewis acid salt
(
7) Arpe, H.J.; Hawkins, S. Industrial Organic Chemistry; Wiley-
VCH: Weinheim, 2010.
8) (a) Alvarez, R.; Carmona, E.; Cole-Hamilton, D. J.; Galindo, A.;
(
1 mmol), and 25 mL of THF. The vessel was sealed under an inert
(
atmosphere and removed from the glovebox. The reactor was
pressurized with 20 bar of ethylene and 10 bar of carbon dioxide,
which were added sequentially. The reactor was then heated with
stirring to 110 °C for 20 h. Following the reaction time period, the
vessel was removed from the heating element, cooled in an ice water
bath, and the atmosphere slowly vented. The reaction residue was
extracted with D O, and a standard of sodium sorbate was added as
an internal standard. The organic soluble species were removed with
diethyl ether washing. Quantitation of the acrylate salt was then
Gutierrez-Puebla, E.; Monge, A.; Poveda, M. L.; Ruiz, C. Formation
of acrylic acid derivatives from the reaction of carbon dioxide with
ethylene complexes of molybdenum and tungsten. J. Am. Chem. Soc.
1985, 107, 5529−5531. (b) Alvarez, R.; Carmona, E.; Galindo, A.;
Gutierrez, E.; Marin, J. M.; Monge, A.; Poveda, M. L.; Ruiz, C.;
Savariault, J. M. Formation of carboxylate complexes from the
reactions of carbon dioxide with ethylene complexes of molybdenum
and tungsten. X-ray and neutron diffraction studies. Organometallics
1989, 8, 2430−2439. (c) Galindo, A.; Pastor, A.; Perez, P.; Carmona,
E. Bis(ethylene) complexes of molybdenum and tungsten and their
reactivity toward carbon dioxide. New examples of acrylate formation
by coupling of ethylene and carbon dioxide. Organometallics 1993, 12,
4443−4451.
2
1
determined by integration of the H NMR spectrum.
ASSOCIATED CONTENT
■
*
S
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00260.
(9) (a) Collazo, C.; del Mar Conejo, M.; Pastor, A.; Galindo, A.
Synthesis and reactivity of bis(ethylene)-phosphite complexes of
molybdenum(0). Inorg. Chim. Acta 1998, 272, 125−130. (b) Schubert,
G.; Papai, I. Acrylate Formation via Metal-Assisted C-C Coupling
between CO2 and C H : Reaction Mechanism as Revealed from
Selected NMR Spectra (PDF)
2
4
Density Functional Calculations. J. Am. Chem. Soc. 2003, 125, 14847−
Accession Codes
1
4858. (c) Bernskoetter, W. H.; Tyler, B. T. Kinetics and Mechanism
CCDC 1838682 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
of Molybdenum-Mediated Acrylate Formation from Carbon Dioxide
and Ethylene. Organometallics 2011, 30, 520−527. (d) Wolfe, J. M.;
Bernskoetter, W. H. Reductive functionalization of carbon dioxide to
methyl acrylate at zero-valent tungsten. Dalton Trans. 2012, 41,
10763−10768. (e) Zhang, Y.; Hanna, B. S.; Dineen, A.; Williard, P.
G.; Bernskoetter, W. H. Functionalization of Carbon Dioxide with
Ethylene at Molybdenum Hydride Complexes. Organometallics 2013,
3
2, 3969−3979. (f) Hanna, B. S.; MacIntosh, A. D.; Ahn, S.; Tyler, B.
AUTHOR INFORMATION
Corresponding Author
E-mail: bernskoetterwh@missouri.edu.
ORCID
Wesley H. Bernskoetter: 0000-0003-0738-5946
Notes
The authors declare no competing financial interest.
■
T.; Palmore, G. T. R.; Williard, P. G.; Bernskoetter, W. H. Ancillary
Ligand Effects on Carbon Dioxide-Ethylene Coupling at Zerovalent
Molybdenum. Organometallics 2014, 33, 3425−3432.
(10) Lejkowski, M. L.; Lindner, R.; Kageyama, T.; Bodizs, G. E.;
Plessow, P. N.; Muller, I. B.; Schafer, A.; Rominger, F.; Hofmann, P.;
Futter, C.; Schunk, S. A.; Limbach, M. The First Catalytic Synthesis of
*
an Acrylate from CO and an AlkeneA Rational Approach. Chem. -
2
Eur. J. 2012, 18, 14017−14025.
(
11) (a) Hendriksen, C.; Pidko, E. A.; Yang, G.; Schaffner, B.; Vogt,
̈
ACKNOWLEDGMENTS
D. Catalytic Formation of Acrylate from Carbon Dioxide and Ethene.
Chem. - Eur. J. 2014, 20, 12037−12040. (b) Huguet, N.; Jevtovikj, I.;
Gordillo, A.; Lejkowski, M. L.; Lindner, R.; Bru, M.; Khalimon, A. Y.;
Rominger, F.; Schunk, S. A.; Hofmann, P.; Limbach, M. Nickel-
catalyzed direct carboxylation of olefins with CO : one-pot synthesis
of α, β-unsaturated carboxylic acid salts. Chem. - Eur. J. 2014, 20,
■
The Curators of the University of Missouri are acknowledged
for financial support of this work. M.N.H thanks the National
Science Foundation (CHE-1350047) for summer support. K.S.
was supported by the Sekisui Chemical Co., Ltd. W.H.B is a
fellow of the Alfred P. Sloan Foundation.
2
1
6858−16862. (c) Stieber, S. C. E.; Huguet, N.; Kageyama, T.;
Jevtovikj, I.; Ariyananda, P.; Gordillo, A.; Schunk, S. A.; Rominger, F.;
Hofmann, P.; Limbach, M. Acrylate formation from CO and
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DOI: 10.1021/acs.organomet.8b00260
Organometallics XXXX, XXX, XXX−XXX