A highly active non-precious transition metal catalyst for the hydrogenation of carbon dioxide to formates

Article abstract of DOI:10.1039/c8sc05230a

Herein a highly active non-precious transition metal catalyst system for homogeneous hydrogenation of carbon dioxide to formate is presented. The application of selected nickel(ii) salts in combination with tailored multidentate ligands enabled the effective transformation of carbon dioxide with an exceptional TON of up to 4.65 × 10⁶. This unprecedented productivity based on the novel nickel catalyst not only outmatches that of existing systems containing first row transition metals, but also established catalysts based on precious transition metals.

Full text of DOI:10.1039/c8sc05230a

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A highly active non-precious transition metal
catalyst for the hydrogenation of carbon dioxide to
Cite this: DOI: 10.1039/c8sc05230a
formates†
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
*
Benjamin G. Schieweck, Niklas F. Westhues
and Jurgen Klankermayer
¨
Herein a highly active non-precious transition metal catalyst system for homogeneous hydrogenation of
carbon dioxide to formate is presented. The application of selected nickel(II) salts in combination with
tailored multidentate ligands enabled the effective transformation of carbon dioxide with an exceptional
TON of up to 4.65 ꢀ 106. This unprecedented productivity based on the novel nickel catalyst not only
outmatches that of existing systems containing first row transition metals, but also established catalysts
based on precious transition metals.
Received 23rd November 2018
Accepted 21st May 2019
DOI: 10.1039/c8sc05230a
rsc.li/chemical-science
The utilization of carbon dioxide (CO2) as versatile chemical as an effective system, resulting in a TON of 4400 aer 216
feedstock has fascinated chemists for decades, but only very hours at 50 ꢁC.21 The Beller group developed Fe and Co catalysts
recently have important developments in molecular catalysis with a tetradentate ligand and in their experiments a TON of up
paved the way towards the creation of novel chemical processes to 1300 (M ¼ Co) could be achieved in the hydrogenation of CO2
using this important resource.1–3 Especially, the reduction of to dimethyl formamide (DMF).22,23 An improved iron system,
CO2 with molecular hydrogen has been investigated in great developed by the same group, based on another tetradentate
detail in the last few years and effective catalysts for the ligand resulted in an even higher TON of 5100 for DMF.24
synthesis of formic acid, dimethoxymethane and methanol Linehan and co-workers presented a remarkable active in situ
could be developed.4–9 In 1935, Farlow and Adkins had already cobalt system with a bidentate ligand (dmpe ¼ 1,2-bis(dime-
discovered the formation of formates from H2 and CO2 using thylphosphino)ethane) in combination with Verkade's base,
a heterogeneous RANEY®-nickel catalyst in the presence of resulting in TOFs of up to 74 000 hꢂ1 and TONs of up to 9400 at
amines.10 Later, CO2 hydrogenation to formic acid was reported room temperature.25 Furthermore, the Linehan group reported
by Inoue et al. in 1976 for the rst time with a molecular cata- of two nickel catalyst systems capable of hydrogenating CO2.
lyst, and platinum group transition metal complexes modied The rst system showed feasibility of CO2 hydrogenation in
with mono- and bisphosphine ligands proved to be especially water with a homogeneous nickel catalyst.26 Recently, another
effective.11 As the hydrogenation of CO2 to formic acid is reported catalyst achieved high TONs for CO2 hydrogenation,
endergonic, a stabilizing agent, a base or a Lewis basic solvent, again relying on the strong Verkade's base. A bimetallic nickel–
is needed to actually yield formates instead of free formic gallium catalyst was able to perform the reaction at room
acid.2,12
Based on this important development, precious metal cata-
temperature with TONs of up to 3150 (Fig. 1).27
lysts achieving TONs of up to several million moles of formate
per metal centre could be obtained. In this transformation,
molecular iridium and ruthenium catalysts showed by far the
highest activity with impressive TONs of up to 3.5 ꢀ 106.13–20
However, important progress could also be achieved with the
development of molecular catalysts based on nonprecious
metals. In 2003, the Jessop group screened a series of metal
salts and found the catalyst NiCl2/dcpe (dcpe ¼ Cy2PCH2CH2-
PCy2) with the base DBU (DBU ¼ 1,8-diazabicycloundec-7-ene)
¨
Institut fur Makromolekulare und Technische Chemie, RWTH Aachen University,
Worringerweg 2, 52074 Aachen, Germany. E-mail: jklankermayer@itmc.
rwth-aachen.de
† Electronic supplementary information (ESI) available. See DOI:
Fig. 1 Selected non-precious metal catalysts for the hydrogenation of
CO2 to formate.11,22–27
10.1039/c8sc05230a
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Table 1 Screening of various ligands for the reduction of CO2 with Nia
In summary, the use of rst row transition metals has
already been demonstrated, but the achieved TON and TOF
values are typically one or two orders of magnitude below those
of the best platinum group systems, and highly active and
effective systems with non-precious transition metals remain
largely elusive.28,29 Based on the important results achieved with
molecular nickel-based catalysts, improved catalyst perfor-
mance was envisaged with a tailored multidentate ligand
structure.
Entry
Ligand
TONb
Yield [%]
1c
2
3
4
5
6
7
dmpe
triphos
N-triphos
TPA
36
79
184
14
41
3.6
8
18
1.4
4
PP3
NP3
NP3
846
384
85
38
Cy
In the rst set of CO2 hydrogenation experiments a screening
of carefully chosen multidentate ligands in combination with
Ni(BF4)2 and DBU as the base was performed (Scheme 1).
Selected results of this initial investigation are summarized
in Table 1. In the rst experiment with the bidentate phosphine
ligand dmpe (1,2-bis(dimethylphosphanyl)ethane) and Ni(BF4)2
a low TON of 36 could be obtained (Table 1, entry 1). Using the
tridentate ligand triphos (1,1,1-tris(diphenylphosphinomethyl)
ethane) an increased TON of 79 was achieved and with the
related N-triphos (N,N,N-tris((diphenylphosphanyl)methyl)
amine) a higher TON of 184 was obtained (Table 1, entry 2 and
3). The remarkable difference between these two ligands
suggests the importance of an additional coordination site with
nickel based molecular systems. However, applying ligands
with an additional Lewis basic center and a more exible ligand
backbone showed only a very low catalytic activity. In detail, the
reaction with the TPA-based catalyst (TPA ¼ trispyridylmethyl-
amine) resulted in a TON of 14 and with PP3 (tris(2-(diphenyl-
phosphino)ethyl)phosphine) a TON of only 41 could be
achieved (Table 1, entry 4 and 5).
a
Ni(BF4)2$6H2O (5 mmol), 1 eq. of ligand, nDBU ¼ 5 mmol (1000 eq.),
VMeCN ¼ 2 mL, T ¼ 100 ꢁC, t ¼ 20 h, p(H2/CO2) [bar/bar] ¼ 60/30 at r.
b
t.
TON
¼
nformate/ncat based on integration of the 1H-NMR
resonances of [DBUH][HCO2] with mesitylene as the internal
standard. 2 eq. of ligand used.
c
Consequently, further investigations focused on the appli-
cation of the versatile NP3 ligand in combination with Ni(BF4)2.
In the rst set of experiments, variation of temperature
between 80 and 140 ꢁC revealed 120 ꢁC as the optimal reaction
temperature. In detail, at 80 ꢁC a TON of 650 was achieved aer
20 h, which corresponds to an acid-to-amine-ratio (AAR ¼
nformate/nDBU) of approximately 0.33 (Table 2, entry 1). This ratio
could be further increased to 0.43 at a temperature of 100 C
(Table 2, entry 2). A nearly full conversion of DBU with a higher
AAR of 0.99 was accomplished at 120 C, resulting in a TON of
1970 (Table 2, entry 3). The reaction at 140 C led to a signi-
ꢁ
ꢁ
ꢁ
cantly increased TON of 1250 aer 20 h, but to a maximum AAR
of only 0.65 as well due to reduced catalyst stability under these
extreme conditions (Table 2, entry 4).
Remarkably, using the tetradentate ligand NP3 (NP3 ¼ tris(2-
(diphenylphosphino)ethyl)amine) showed a high productivity
with a TON of 846 (Table 1, entry 6). Using the sterically more
Cy
Cy
Table 2 Hydrogenation of CO2 with the developed Ni(BF4)2/NP3
strained equivalent NP3
(NP3
¼
tris(2-(dicyclohex-
systema
ylphosphino)ethyl)amine) a decreased turnover number of 384
was obtained (Table 1, entry 7). Based on these results, the NP3
ligand showed an excellent performance, and the interplay of
the bridgehead nitrogen atom with the nickel center has
exceptional importance in the catalyst activity. Recently pub-
lished results by Mondal et al. calculating the energy prole of
molecular cobalt and iron catalysts bearing a tetradentate
ligand with varying bridgehead atoms corroborate this
conclusion.30
n(Ni(BF4)2)
[mmol]
TONb
TOFavg [hꢂ1
]
AARd
c
Entry
T [ꢁC]
1
2
3
4
5
6
7
8
5.0
5.0
5.0
5.0
0.5
0.2
0.1
0.04
0.02
0.002
5.0
5.0
—
80
650
850
33
43
99
52
0.33
0.43
0.99
0.65
1.05
1.08
0.93
0.91
0.89
0.93
—
0.02
—
0.27
0.32
0.01
100
120
140
120
120
120
120
120
120
120
120
120
120
120
120
1970
1250
20 900
54 219
93 270
226 630
444 610
4 650 710
0
30
0
548
653
290
753
1336
3148
6175
64 593
—
0.4
—
27
9
10
11e
12f
13g
14h
15i
16j
5.0
5.0
5.0
32
2
37
a
Ni(BF4)2$6H2O, 1 eq. of NP3, VMeCN ¼ 2 mL, nDBU ¼ 10 mmol (2000 eq.),
p(H2/CO2) [bar/bar] ¼ 60/30 at r. t., entries 1–4, 14–16: t ¼ 20 h, entries
5–13: t ¼ 72 h. b TON ¼ nformate/ncat based on integration of the 1H-NMR
resonances of [DBUH][HCO2] with mesitylene as the internal standard.
c
Given as average TOF, TOFavg ¼ TON/t over the whole reaction period.
d
e
AAR ¼ nformate/nDBU
.
Without DBU. f Without NP3. g nNP ¼ 5 mmol. h
3
5 eq. of PCy3. i One additional eq. of NP3. j 1100 eq. of H2O (0.1 mL, 5.5
mmol) added.
Scheme
1 Nickel-catalyzed hydrogenation of CO2 with selected
multidentate ligands.
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In the next set of experiments, the catalyst loading was
systematically reduced and correlated to the respective TONs
(Table 2, entries 5–9). Decreasing the catalyst loading resulted
in an increased TON and with 0.002 mmol nickel catalyst an
outstanding TON of approximately 4.65 ꢀ 106 could be achieved
(Table 2, entry 10), surpassing that of established systems based
on rst row transition metals and even systems based on
precious metals.
Chemical Science
Table 3 Hydrogenation of CO2 with the developed Ni2+/NP3 system
in the presence of selected Lewis acidic additivesa
c
Entry
Lewis acid (LA)
TONb
TOFavg [hꢂ1
]
AARd
1
2
3
4
5
6
7
8
9e
—
1545
1560
1707
130
5440
4590
5140
400
77
78
85
7
272
230
260
20
0.31
0.31
0.34
0.03
1.09
0.97
1.08
0.09
0.63
B(C6F5)3
Zn(OTf)2
Al(OTf)3
LiBF4
LiI
LiCl
NaBF4
LiBF4
Control experiments revealed the tailored combination of
Ni(BF4)2, NP3 ligand and DBU as the basic requirement for the
exceptionally high activity (Table 2, entries 11–13). Recently,
Zall et al. observed the coordination of the DBU base to the
metal center as a crucial factor for a high catalytic turnover in
the hydrogenation of CO2 with copper catalysts and a compa-
rable pathway may thus be present in the established NP3/Ni/
DBU system.29 Thus, to gain further insight into the complex
reaction mechanism, experiments with a dened organome-
tallic compound were performed. Using the molecular nickel(0)
complex NP3Ni31 comparable results and a TON of 2180 could
be obtained, thus corroborating the formation of a molecular
species during the in situ procedure. Furthermore, a mercury-
poisoning test showed no inhibition of the catalytic activity.
Additionally, an excess of phosphine ligands such as tricyclo-
hexylphosphine (PCy3) to a 1 : 1 mixture of Ni2+/NP3 resulted in
a highly decreased TON and a low AAR (Table 2, entry 14). Even
the addition of just one additional equivalent of NP3 results in
reduced activity and a low TON/AAR (Table 2, entry 15). High
amounts of water hinder the catalytic hydrogenation of CO2.
The addition of approx. 1000 equivalents of water leads to
inhibition of catalysis (Table 2, entry 16) and the formation of
a precipitate. The exact molecular structure of the catalyst
remains currently unknown. However, further spectroscopic
experiments and mass analysis showed the formation of
multiple compounds possibly active in the CO2 hydrogenation
(see the ESI† for further explanation).
6260
313
a
Ni(BF4)2$6H2O (2 mmol) and 1 eq. of NP3 used, nLA ¼ 200 mmol, nDBU
¼
10 mmol (5000 eq.), VMeCN ¼ 2 mL, p(H2/CO2) [bar/bar] ¼ 60/30 at r. t., T
¼ 120 ꢁC, t ¼ 20 h. b TON ¼ nformate/ncat based on integration of the 1H-
NMR resonances of [DBUH][HCO2] with mesitylene as the internal
c
standard. Given as average TOF, TOFavg ¼ TON/t over the whole
e
reaction period. d AAR ¼ nformate/nDBU
.
nNi ¼ 1 mmol.
Without any additive, aer 20 h a TON of approximately
1500 could be obtained (Table 3, entry 1). Lewis acids such as
tris(pentauorophenyl)borane (B(C6F5)3) or zinc triate
(Zn(OTf)2) did not enhance the activity of the established
system (Table 3, entries 2 and 3). Aluminium triate (Al(OTf)3)
inhibits catalysis and nearly no formate could be observed
(Table 3, entry 4). Using lithium tetrauoroborate (LiBF4)
a clear enhancement in the catalytic activity was seen achieving
a TON of up to 5440 and an AAR of 1.09 (Table 3, entry 5). The
tested lithium halides, lithium iodide (LiI) and lithium chlo-
ride (LiCl), exhibited a similar behavior in catalysis leading to
only slightly decreased values of TONs and AARs (Table 3,
entries 5 and 6). Switching from lithium to sodium tetra-
uoroborate (NaBF4) has a clearly inhibiting effect resulting in
only a poor yield of formate in a TON of 400 (Table 3, entry 8).
By using LiBF4 and additionally lowering the catalyst amount,
an AAR of only 0.63 aer 20 h with an excellent TON of 6260
could be achieved. Consequently, longer reaction times are
required to obtain full conversion of the amine substrate
(Table 3, entry 9). To gain further insights into the molecular
species formed during catalysis, detailed NMR spectroscopic
and mass spectrometric experiments were performed. In detail,
31P-NMR-spectroscopy revealed the sole formation of a NP3/
Ni2+ complex by mixing the ligand and Ni(BF4)2. Analysis aer
the catalytic experiment revealed a reduction of the initially
formed NP3Ni(II) complex, leading to the formation of a major
species with a chemical shi of 21 ppm. Conrmation of the
dened species NP3Ni(0) could be obtained with a further
control experiment. Consequently, a mixture of NP3/Ni2+, DBU
and molecular hydrogen (p ¼ 3 bar) resulted in a 31P-NMR
spectrum comparable to that of a post-reaction mixture (for
details see the ESI†). However, the isolated NP3Ni complex did
not show evidence of distinct reactivity towards molecular
hydrogen and even aer prolonged heating at 80 ꢁC, no
formation of a hydride species could be observed by NMR
spectroscopy. Additional HR ESI-MS measurements conrmed
the formation of a NP3Ni2+ species from the NP3/Ni2+ mixture
during the catalytic experiment (see the ESI†).
A sequential reaction set-up of subsequently relling the
autoclave with DBU as well as CO2 and molecular hydrogen
leads to outstanding synthesis of the respective formate on
a multigram scale (Scheme 2).
Analysis of the reaction mixture aer a course of ve reac-
tions showed quantitative conversion of the free base resulting
in a good TON of 6324 (AAR ¼ 1.05). This productivity denotes
an overall yield of roughly 6.30 g (31.62 mmol) of isolable
[DBUH] formate and further shows the stability of the nickel
catalyst system in this study.
Towards the development of a more effective system and
inspired by recent results,32–35 selected Lewis acidic additives
were investigated in combination with the nickel NP3 system
(Table 3).
Scheme 2 Sequential hydrogenation of CO2 by the established nickel
catalyst system. 10 mmol of DBU instead of 5 mmol was added in the
last reaction step.
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In conclusion, a non-precious transition metal catalyst system
could be developed for homogeneous hydrogenation of CO2 to
formate. The application of the versatile multidentate NP3
ligand in combination with selected nickel(II) salts enabled the
effective transformation of CO2 in the presence of molecular
hydrogen in acetonitrile solution. Under optimized conditions,
using 0.002 mmol nickel catalyst, an exceptional TON of
approximately 4.65 ꢀ 106 was achieved. This unprecedented
productivity based on the novel nickel catalyst not only
outmatches that of existing systems containing rst row tran-
sition metals, but also established catalysts based on precious
transition metals. Further development and detailed mecha-
nistic investigations on the catalyst system are ongoing in our
laboratories.
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the stock solution was further diluted (see the ESI† for detailed
information). DBU was added and the solution was transferred
to a stainless steel autoclave equipped with a glass inlet under
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saturation, followed by hydrogen to a total pressure of 90 bar at
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Conflicts of interest
There are no conicts to declare.
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