Enhanced Hydrogenation of Carbon Dioxide to Methanol by a Ruthenium Complex with a Charged Outer-Coordination Sphere

Article abstract of DOI:10.1021/acscatal.0c02268

We report the hydrogenation of CO2 to MeOH by a Ru(triphos) catalyst containing a cationic tetraalkylammonium moiety in the outer coordination sphere. This catalyst affords higher TON and TOF values for MeOH than isostructural catalysts with neutral phosphine ligands. Kinetic data from operando NMR spectroscopy studies indicate the improvement in MeOH production arises from a 12-fold enhancement in the rate of hydrogenation of the transient formaldehyde intermediate. These results provide insight into the catalyst characteristics that promote MeOH formation.

Full text of DOI:10.1021/acscatal.0c02268

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Letter
Enhanced Hydrogenation of Carbon Dioxide to Methanol by a
Ruthenium Complex with a Charged Outer-Coordination Sphere
Jeremy D. Erickson, Andrew Preston, John C. Linehan, and Eric S. Wiedner
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.0c02268 • Publication Date (Web): 18 Jun 2020
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Enhanced Hydrogenation of Carbon Dioxide to Methanol by a
Ruthenium Complex with a Charged Outer-Coordination Sphere
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Jeremy D. Erickson, Andrew Z. Preston, John C. Linehan, and Eric S. Wiedner*
Catalysis Science Group, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United
States
Keywords: Carbon Dioxide, Methanol, Hydrogenation, Homogeneous Catalysis, Ruthenium, Phosphine
Supporting Information Placeholder
displays enhanced production of MeOH compared to
previously reported Ru(triphos) catalysts. Kinetics
measurements by operando H NMR spectroscopy reveal the
ABSTRACT: We report the hydrogenation of CO
2
to MeOH by
a cationic
a
Ru(triphos) catalyst containing
1
tetraalkylammonium moiety in the outer coordination sphere.
This catalyst affords higher TON and TOF values for MeOH than
isostructural catalysts with neutral phosphine ligands. Kinetic
data from operando NMR spectroscopy studies indicate the
improvement in MeOH production arises from a 12-fold
enhancement in the rate of hydrogenation of the transient
formaldehyde intermediate. These results provide insight into
the catalyst characteristics that promote MeOH formation.
presence of distinct rate-limiting steps in the formation of each
intermediate product formed on the path to MeOH. The
(+)
MeN -triphos ligand was found to increase the rate of MeOH
formation while simultaneously slowing the production of
formaldehyde and formate, leading to an enhancement in the
selectivity of MeOH production.
For this study, we selected catalyst precursors that varied by
the nature of the bridgehead group in the outer-coordination
sphere of the triphos ligand (Figure 1). Substitution at the
bridgehead group allows the electronics of the complex to be
varied without affecting its steric profile (vide infra). In
Carbon dioxide has become the focus of intense study due to
its potential as an alternative carbon source, eschewing the
need for traditional fossil fuels, and its continuing impact on
8
21
addition to the known C-triphos and N-triphos ruthenium
complexes (1 and 2, respectively), we also synthesized a new
1-3
(+)
the environment as a greenhouse gas. A carbon-neutral
production of fuel from CO is a long-sought goal that addresses
these pressing issues. Among the possible C-1 products
derived from CO , MeOH is highly attractive as an energy vector
cationic MeN -triphos complex (3) by treating 2 with
2
[Me O][BF ] (full details are given in the Supporting
3
4
4
-6
Information).
2
due to its high energy density and ease of transport.
Development of energy-efficient catalysts that can produce
MeOH at low temperature and pressure is a key scientific
challenge.
Chart 1. Catalyst precursors used in this study.
Me
PPh2
Ru
Ph2
Me
P
Ph2
BF4
P
C
N
N
Ph2
P
Ph2
P
Ph2
P
Molecular catalysts have been reported to hydrogenate CO
2
Ru
Ru
7-12
to MeOH in the presence of Lewis or Brønsted acids,
or
P
Ph2
P
Ph2
P
Ph2
13-18
amine bases.
Tandem catalysis has also been employed to
1
9-20
assuage deficiencies of individual catalysts.
Nonetheless, a
1
2
3
An acid co-catalyst is typically employed in hydrogenation of
CO by triphos-based Ru catalysts. One role of the acid co-
2
catalyst is to activate the Ru precursor complex by removal of
the trimethylenemethane ligand. Treatment of 3 with one
equivalent of Al(OTf) in ethanol led to gas evolution, and
3
subsequent removal of the solvent afforded a pale yellow solid.
Crystallization from dichloromethane and acetonitrile allowed
us to obtain the X-ray crystallographic structure of
(
+)
29
2 2 3 3 3
[(MeN (CH Ph ) )Ru(NCMe) ][OTf] (4), which confirms
the tetraalkylammonium moiety of the ligand scaffold and
production by this family of catalysts has been studied
shows that 4 is structurally similar to the known acetonitrile
7
, 26-28
30-31
computationally,
the existing experimental kinetic and
adducts of 1 and 2.
For example, the Ru-P and Ru-NCMe
mechanistic data are insufficient to allow the rational design of
catalysts that show high activity at lower temperatures and
pressures.
distances of all three structures are within 0.02 Å of one
another. The bridgehead-Ru distance of 4 is intermediate
between the C-triphos and N-triphos analogues, differing by
0.04 and 0.07 Å, respectively.
In this manuscript, we report a new ruthenium complex
(+)
+
2 2 3
containing a MeN -triphos ligand, MeN (CH PPh ) , that
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Figure 1. Thermal ellipsoid plot (50% probability) of 4.
Hydrogen atoms, phenyl rings (other than the ipso carbon),
three triflate anions, and solvent molecules are omitted for
clarity.
1
Catalysis was monitored by operando H NMR spectroscopy
3
2-33
using polyetheretherketone (PEEK) tubes.
-3 were activated by 4 equiv of Al(OTf)
pressurized to 45 bar H and 15 bar CO , then heated to 90 ° C.
These conditions are similar to previous catalytic studies.
Pre-catalysts
1
3
in dry ethanol,
2
2
7
-8, 21
The catalytic reaction products were ethyl formate (EF),
diethoxy methane (DEM), which is the ethanol acetal of
formaldehyde, and MeOH. An example of the product
2
distribution observed throughout the hydrogenation of CO by
3 is shown in Figure 2a. Selected results of the operando
experiments are shown in Table 1, while comprehensive
results can be found in the SI.
Figure 2. a) Production of EF, DEM, and MeOH over time by 3,
and b) production of MeOH over time by 1-3. Conditions: 0.64
Complexes 1-3 displayed a high initial rate of EF production
over the first 30 min of catalysis, followed by the development
of a quasi-steady-state concentration of EF after 2 h. DEM and
MeOH were produced more slowly throughout the reaction,
and a gradual reduction in reaction rate was observed after 5 h.
The initial turnover frequency (TOF) for production of EF,
measured over the first 30 min, was the same for 1-3 within
the experimental error. The TOF for DEM production was
found to increase in the order of 3 < 2 < 1, while the TOF for
MeOH production increased in the opposite order of 1 < 2 < 3
mM catalyst, 2.6 mM Al(OTf)
3
, and 340 mM mesitylene in
ethanol, 90 ° C, 45 bar H, 15 bar CO .
2
2
1
Table 1. Hydrogenation of CO
NMR Spectroscopy
2
by 1-3 Using Operando H
a,b,c
EtOH
(
Figure 2b). These data demonstrate the product selectivity
o
O
90 C
changes over time. For 3, EF makes up 94% of the product
O
O
CO2
+
H2
Et +
+ MeOH
Et
Et
mixture at 0.5 h, while MeOH makes up 34% of the products at
H
O
EF
15 bar 45 bar
1
6 h and further increases as time continues.
DEM
Hydrogenation tests were also conducted in stainless steel
Catalyst
EF
TON TOF
(16 h) (h )
DEM
MeOH
batch reactors at 60, 90, and 120 ° C under 45 bar H
CO in dry ethanol for 16 h (see SI for tabulation of results). In
comparison to TONs measured by operando
2
and 15 bar
TON TOF
(16 h) (h )
TON TOF
(16 h) (h )
2
−
1
−1
−1
1
H
NMR
spectroscopy, batch reactions of 1-2 displayed a 70% and 90%
reduction in the TONs for DEM and MeOH, respectively, while
1 d
2 d
3 e
200
220
300
180
230
220
520
420
160
56
45
14
98
8.2
12
23
140
240
3
displayed only a 60% reduction in the TON for both DEM and
MeOH (all at 90 °C and 16 h of catalysis). Thus 3 outperforms 1
and 2 in both operando and batch reactions, with batch
reactions affording a larger difference between catalysts. The
observation that the TONs are lower in batch reactor studies
indicates that these data do not report on the intrinsic reaction
a
Conditions: 0.64 mM catalyst, 2.6 mM Al(OTf)
3
, and 340 mM
b
mesitylene in ethanol, 90 ° C, 45 bar H, 15 bar CO
initial turnover frequency. Uncertainties in TON and TOF
values are 10-20%. Values are the average of three runs.
2
2
.
TOF =
c
d
e
3
4
kinetics of 1-3. For example, stirring in a batch reactor could
increase the rate of vaporization of the intermediate products
EF and DEM relative to catalytic hydrogenation, leading to a
slower overall conversion and lower TON.
Values are the average of four runs.
We propose that MeOH is formed via the pathway shown in
Scheme 1. Sequential hydrogenation of CO produces both
formic acid and formaldehyde (H CO), which are rapidly
converted to EF and DEM in acidic ethanol solution. 1 is known
2
2
2
7
to hydrogenate both carboxylic acids and esters, so it is likely
that formic acid and EF are both hydrogenated during CO
2
reduction. In contrast, DEM must first undergo hydrolysis back
2
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to H
2
CO before hydride transfer can occur, as shown by
propose that the RLS for reduction of EF and H
intramolecular hydride transfer and H activation, respectively
(Figure 4a-b). Hydride transfer would be favored by a more
electron-rich catalyst, while H activation would be favored by
an electron-deficient catalyst, consistent with the activity
trends observed for 1-3. A second key insight is that the
tetraalkylammonium moiety of 3 has a much greater influence
2
CO are
2
0
1
2
3
4
5
6
7
8
9
Goldberg and confirmed by our own tests (see SI). Indeed,
DFT calculations of an Ir catalyst suggest the barrier for
hydride transfer to DEM, by analogy of H
higher than that of H
EF and H
conditions, while MeOH is not dehydrogenated.
2
2
C(OH)
CO. Control reactions also indicate that
CO can be dehydrogenated under the catalytic
2
, is much
2
3
5
2
2
on the RLS for hydrogenation of H
the RLS for the first two steps of catalysis (H transfer). Both
2 2
CO (H activation) than on
−
3
6
a
the acidity (pK ) of dihydrogen complexes and the hydricity
Scheme 1. Proposed reaction pathway for reduction of
CO to MeOH.
2
3
7
(ΔGH−) of metal hydrides are affected by molecular charge,
however, it is presently unclear whether both thermodynamic
parameters are affected equally by the cationic bridgehead of
1
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O
3.
H2
H
OH H2 H2O
H2
O
CO2
-H2O
MeOH
a) H Transfer (RLS for EF to H2CO)
c)
Effect of an internal electric
H
O
H
field on Ru H2 back-donation
n+
n+
O
PPh2
PPh2
MeN+
Ph2
P
Ph2
Et
O
-
2 H O
H
H
2
P
Ru
Ru
H
P
Ph
O
P
Ph2
O
O
L
R'
C
2
L
Et
Et
R'
R
R

*
b) H2 Activation (RLS for H2CO to MeOH)
n+
(
H2)
n+
PPh2
H
Ru
PPh2
Favored by
electron-rich catalyst
Favored by
electron-poor catalyst
Ph2
P
Ph2
P
d
H
H
Ru
(Ru)
P
Ph2
O
P
Ph
O
H
lp
(O)
L
2
L
R
R
To obtain additional insight into the trends observed for 1-3,
the catalytic kinetics were modeled according to the reaction
pathway shown in Scheme 1. The results of the kinetic fitting
are shown in Figure 3 as relative rate constants (krel), defined
as the ratio of the rate constants for a given catalyst (kcat) and
−
Figure 4. a) Proposed mechanism of H transfer, b) proposed
mechanism of H activation, and c) illustration of the spatial
proximity between the internal electric field of 3 and the Ru d
orbital that is involved in backdonation to the H ligand.
2
π
2
1
(k
1
). By construct, 1 has krel = 1 for all three hydrogenation
to EF and EF to H CO
steps. The measured krel values for CO
2
2
decrease by 1.3-fold and 3-fold from 1 to 3, which are similar
to the variations in TOF for production of EF and DEM. In
contrast, 3 displays a 12-fold increase in krel for hydrogenation
of H CO to MeOH compared to 1. This increase in krel is much
2
larger than the 3-fold increase in the TOF for MeOH production
by 3, reflecting the lower concentration of DEM (and hence
2
H CO) during catalysis by 3 in comparison to 1-2.
Establishment of the physical underpinnings of the selective
enhancement in H activation by 3 will be important for
2
rational design of improved catalysts. One possible mode of
action is that the tetraalkylammonium group modulates the
electron donor ability of the phosphine ligand via inductive
effects. Alternatively, or perhaps concurrently, the cationic
bridgehead group could create an internal electric field that
3
8-39
stabilizes the molecular orbitals of the complex.
illustrated in Figure 4c, stabilization of the Ru π-symmetry
orbitals (d ) by an electric field will decrease back-donation
into the σ* orbital of an H ligand, thereby increasing the charge
and acidity of the H ligand and lowering the barrier to
As
π
2
4
0
2
intramolecular deprotonation by an adjacent oxygen ligand.
Further studies are needed to delineate the contributions of
inductive and electric field effects on the enhancement of H
2
activation by 3.
In summary, we have shown that the rate and selectivity for
MeOH production from CO hydrogenation by Ru(triphos)
2
catalysts are enhanced by a cationic ammonium group in the
outer-coordination sphere. In contrast, the selectivity for the
formaldehyde acetal is enhanced by electron-rich catalysts.
This divergence in catalyst selectivity reveals the presence of
different rate-limiting steps during the stepwise reduction of
CO . We believe these findings provide critical insights towards
2
the rational design of improved catalysts for the direct
Figure 3. Relative rate constants for 1-3 obtained from kinetic
modeling.
Collectively, these data reveal two key insights for the
hydrogenation of CO
favored by electron-rich catalysts, while the production of
MeOH is favored by electron-deficient catalysts. These
opposing trends in catalyst activity indicate the presence of
different rate-limiting steps (RLS) for reduction of the
hydrogenation of CO to MeOH. Further in-depth studies of the
2
2
to MeOH. First, the production of DEM is
kinetics and mechanism of MeOH production are underway.
ASSOCIATED CONTENT
Supporting Information
intermediate substrates EF and H
observations and previous computational studies of 1,
2
CO. Based on our kinetic
7
, 26-28
we
3
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The Supporting Information is available free of charge on the
ACS Publications website.
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Scharnagl, F. K.; Hertrich, M. F.; Neitzel, G.; Jackstell, R.;
to Methanol –
Experimental details, synthesis, NMR spectra, metric data
from XRD, kinetic data (PDF)
Crystallographic data (CIF)
2
1
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Schneidewind, J.; Adam, R.; Baumann, W.; Jackstell, R.; Beller,
AUTHOR INFORMATION
M. Low-Temperature Hydrogenation of Carbon Dioxide to Methanol
with a Homogeneous Cobalt Catalyst. Angew. Chem., Int. Ed. 2017, 56,
1890-1893.
Corresponding Author
*eric.wiedner@pnnl.gov
12.
Korstanje, T. J.; Ivar van der Vlugt, J.; Elsevier, C. J.; de Bruin,
B. Hydrogenation of Carboxylic Acids with a Homogeneous Cobalt
Catalyst. Science 2015, 350, 298-302.
ORCID
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R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S. Mechanistic Insights into
Ruthenium-Pincer-Catalyzed Amine-Assisted Homogeneous
Hydrogenation of CO to Methanol. J. Am. Chem. Soc. 2019, 141, 3160-
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4. Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO
2
Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury,
Jeremy D. Erickson 0000-0002-2335-1159
Andrew Z. Preston 0000-0002-5461-0563
John C. Linehan 0000-0001-8942-7163
Eric S. Wiedner 0000-0002-7202-9676
2
3
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Notes
Capture and Hydrogenation to Methanol with Reusable Catalyst and
Amine: Toward a Carbon Neutral Methanol Economy. J. Am. Chem. Soc.
The authors declare no competing financial interests.
2018, 140, 1580-1583.
1
5. Kar, S.; Goeppert, A.; Kothandaraman, J.; Prakash, G. K. S.
ACKNOWLEDGEMENTS
2
Manganese-Catalyzed Sequential Hydrogenation of CO to Methanol
via Formamide. ACS Catal. 2017, 7, 6347-6351.
This research was supported by the U.S. Department of Energy
(
DOE), Office of Science, Office of Basic Energy Sciences,
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Ruthenium-Catalyzed Hydrogenation of CO
Soc. 2015, 137, 1028-1031.
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to Methanol. J. Am. Chem.
Division of Chemical Sciences, Geosciences and Biosciences.
Mass spectrometry experiments were performed in the
William R. Wiley Environmental Molecular Sciences
Laboratory, a DOE national scientific user facility sponsored by
the DOE’s Office of Biological and Environmental Research and
located at the Pacific Northwest National Laboratory (PNNL).
The authors thank Dr. William Kew for mass spectrometry
analysis. PNNL is operated by Battelle for DOE.
17.
Ribeiro, A. P. C.; Martins, L. M. D. R. S.; Pombeiro, A. J. L.
Carbon Dioxide-to-Methanol Single-pot Conversion Using
a C-
scorpionate Iron(II) Catalyst. Green Chem. 2017, 19, 4811-4815.
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Everett, M.; Wass, D. F. Highly Productive CO
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Hydrogenation to Methanol – A Tandem Catalytic Approach via Amide
Intermediates. Chem. Commun. 2017, 53, 9502-9504.
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20. Chu, W.-Y.; Culakova, Z.; Wang, B. T.; Goldberg, K. I. Acid-
Assisted Hydrogenation of CO to Methanol in a Homogeneous
Catalytic Cascade System. ACS Catal. 2019, 9, 9317-9326.
21. Siebert, M.; Seibicke, M.; Siegle, A. F.; Kräh, S.; Trapp, O.
Huff, C. A.; Sanford, M. S. Cascade Catalysis for the
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to Methanol. J. Am. Chem. Soc.
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