Selective Ruthenium-Catalyzed Transformation of Carbon Dioxide: An Alternative Approach toward Formaldehyde

Article abstract of DOI:10.1021/jacs.8b10233

Formaldehyde is an important precursor to numerous industrial processes and is produced in multimillion ton scale every year by catalytic oxidation of methanol in an energetically unfavorable and atom-inefficient industrial process. In this work, we present a highly selective one-step synthesis of a formaldehyde derivative starting from carbon dioxide and hydrogen gas utilizing a homogeneous ruthenium catalyst. Here, formaldehyde is obtained as dimethoxymethane, its dimethyl acetal, by selective reduction of carbon dioxide at moderate temperatures (90 °C) and partial pressures (90 bar H₂/20 bar CO₂) in the presence of methanol. Besides the desired product, only methyl formate is formed, which can be transformed to dimethoxymethane in a consecutive catalytic step. By comprehensive screening of the catalytic system, maximum turnover numbers of 786 for dimethoxymethane and 1290 for methyl formate were achieved with remarkable selectivities of over 90% for dimethoxymethane.

Full text of DOI:10.1021/jacs.8b10233

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Article
Selective Ruthenium-Catalyzed Transformation of Carbon
Dioxide – an Alternative Approach towards Formaldehyde
Max Siebert, Max Seibicke, Alexander Siegle, Sabrina Kräh, and Oliver Trapp
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b10233 • Publication Date (Web): 09 Dec 2018
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Journal of the American Chemical Society
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Selective Ruthenium-Catalyzed Transformation of Carbon
Dioxide – an Alternative Approach towards Formaldehyde
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Max Siebert, Max Seibicke, Alexander F. Siegle, Sabrina Kräh, and Oliver Trapp*
Department Chemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13, 81377 München, Germany.
ABSTRACT: Formaldehyde is an important precursor to numerous industrial processes and is produced in multi-million
ton scale every year by catalytic oxidation of methanol in an energetically unfavorable and atom inefficient industrial
process. In this work, we present a highly selective one-step synthesis of a formaldehyde derivative starting from carbon
dioxide and hydrogen gas utilizing a homogeneous ruthenium catalyst. Here, formaldehyde is obtained as dimethoxy-
methane, its dimethyl acetal, by selective reduction of carbon dioxide at moderate temperatures (90°C) and partial
pressures (90 bar H2 / 20 bar CO2) in the presence of methanol. Beside the desired product, only methyl formate is
formed, which can be transformed to dimethoxymethane in a consecutive catalytic step. By comprehensive screening of
the catalytic system, maximum turnover numbers of 786 for dimethoxymethane and 1290 for methyl formate were
achieved with remarkable selectivities of over 90% for dimethoxymethane.
Introduction
reduction of CO2 using a slightly modified polyhydride
ruthenium complex. In the presence of a primary amine,
the corresponding imine was formed, which could be
Carbon dioxide is considered to play an important role
in the climate change due to its greenhouse properties
and continuous accumulation in the atmosphere. Since
1750, the atmospheric concentration of CO2 has
dramatically risen from 278 ppm to 390 ppm in 2011.1 In
the following five years, the atmospheric concentration
even exceeded 400 ppm, strengthening the assumption
that the emission of CO2 will further increase within the
next decades, thus strongly impacting the world’s
climate.2, 1b Utilization of CO2 as an alternative feedstock
for the chemical industry is an opportunity to rebalance
the carbon cycle and to reduce CO2 emissions, thereby
providing a renewable resource for the anthropogenic
value chains of generations to come.3
hydrolyzed yielding formalin solution.11
A
similar
hydroboration of CO2 was achieved using iron, cobalt and
copper complexes.12 In 2015, Oestreich and Metsänen
reported a ruthenium-catalyzed hydrosilylation of CO2 to
bis(silyl)methylene acetal using triethylsilane.13 Likewise,
López-Serrano and Rodríguez obtained the same acetal
using a nickel catalyst under similar conditions.14 The
groups of Berke and Piers utilized frustrated Lewis pairs
derived from rhenium and scandium complexes in
combination with B(C6F5)3 for the selective hydrosilylation
of CO2.15 These reports are of high academic value but do
not meet industrial needs due to the stoichiometric use of
reducing reagents. The first catalytic conversion of CO2 to
the formaldehyde oxidation level using molecular
hydrogen and a homogeneous catalyst was described by
Klankermayer et al. in 2016. In the presence of alcohols,
CO2 was transformed into dialkoxymethane ethers using a
ruthenium complex and acidic co-catalysts (Scheme 1).
In the last decades, impressive progress has been made
in the catalytic transformation of CO2 towards chemically
relevant products.4 The challenge of such transformations
is the high kinetic barrier resulting from the low energy
level of molecular CO2.5 There are many examples for the
synthesis of functional molecules starting from carbon
dioxide and hydrogen gas, as for instance formic acid or
The best result was obtained using
a molecular
ruthenium-triphos catalyst and aluminum triflate yielding
dimethoxymethane (DMM) with a turnover number
(TON) of 214. Beside the product, methyl formate (MF)
was formed with a TON of 104.16
formate salts6, alkyl formates,7 formamides,7a,
or
7b,
8
methanol.9 The catalytic transformation towards the
formaldehyde oxidation level remains challenging since
the resulting aldehyde group is susceptible to further
reduction.4f
Scheme 1. Acetalization approach towards the form-
aldehyde oxidation level.16
Only
a few reports describe the homogeneously
catalyzed synthesis of formaldehyde or derivatives
thereof. In 2012, Bontemps and Sabo-Etienne used
O
C
Cat.
H2
C
Ru
CO2
H2
H3C
H3C
CH3
O
H
O
O
Ph2P
PPh2
pinacolborane as reducing agent in
a ruthenium-
Additive
MeOH
PPh2
catalyzed reduction of CO2 to trap formaldehyde as
bis(boryl)methylene acetal.10 Free formaldehyde was
observed via NMR analysis in the borane-mediated
MF
DMM
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In 2017, Klankermayer and Schieweck used the triphos
elimination of formaldehyde yielding the initial reactant
dimesitylphosphine. Ligand 2d was synthesized by
reaction of the lithiated phosphine with
1
2
3
4
5
6
7
8
ligand and derivatives thereof in combination with cobalt
salts and an acidic co-catalyst for the same catalytic
transformation. The selectivity for DMM was improved
compared to the previously reported ruthenium system
with TONs for DMM and MF reaching 157 and 37,
respectively.17
tris(chloromethyl)amine with 38% yield (Scheme 3B).
Using the phosphonium salt 1a, the ligand N-py-diphosPh
(2e) was prepared by changing the amine of the
previously used protocol to 2-aminopyridine (50% yield,
Scheme 3C). The analogue py-diphosPh (2f) was
synthesized in an optimized procedure, derived from the
work by Doherty et al., starting with 2-ethylpyridine.21
The ligand was isolated with 7% yield after the four-step
synthesis (Scheme 3D). The tridentate phosphinite ligand
triphosOPPh2 (2g) was obtained according to a procedure
We envisaged that variation of the electronic and steric
properties of the triphos ligand as well as of the
coordinating units would lead to a significant increase in
stability and activity of the catalytic systems, thus yielding
higher TONs for DMM and MF. Herein, we report a
highly optimized ruthenium-catalyzed transformation of
carbon dioxide to DMM and MF, thereby providing an
attractive approach for the synthesis of formaldehyde in
an energetically favorable and atom efficient industrial
process.
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by
Marchand-Brynaert
et
al..22
Reaction
of
1,1,1-tris(hydroxymethyl)ethane with chlorodiphenyl-
phosphine in the presence of triethylamine yielded 2g
with 34% yield (Scheme 3E).
Scheme 3. Synthetic procedures towards the tripodal
ligands.
Results and Discussion
O
For the comprehensive screening of potential catalysts
for the transformation of CO2 to formaldehyde
derivatives, we synthesized structurally flexible
ruthenium catalysts to tune steric and electronic
properties (Scheme 2). All catalysts were based on
tripodal ligands and the doubly charged trimethylene-
methane anion as counter-ion. First, we varied the
electronic and steric properties of the ligand by changing
the backbone of the widely used triphos ligand in
combination with increasing sterical demand in the
coordinating phosphine units. We chose the N-triphos
ligand as basic ligand motive due to its versatile
application and the simplicity of synthesis.18 Secondly, we
exchanged one of the phosphine units with a hemi-labile
pyridyl ligand to increase activity of the resulting
catalysts. Last, we changed the linker from methylene to
oxymethylene groups to vary the coordinating properties
of the ligands.
A
NEt3
NH4Cl
H
H
R
R
R2P
PR2
HCl
OH
OH
PR2
P
HPR2
H2O
rt, 20 min - 24 h
MeOH
rt, 24 - 28 h
Cl-
1a-c
N
2a-c
R =
a
b
c
B
R2P
PR2
1. n-BuLi, THF, -78°C
rt (30 min)
PR2
HPR2
2. N(CH2Cl)3, THF, -78°C
rt
rt (24 h)
60°C (2 h)
N
2d
R =
N
O
C
H2N
H
H
Ph
Ph
N
Ph2P
HCl
NEt3
OH
OH
PPh2
P
HPPh2
Scheme 2. Ruthenium complexes for the catalytic
transformation of CO2 to DMM.
H2O
rt, 20 min
MeOH
Cl-
1a
N
60°C, 5 d
2e
O
D
N
N
Ru
Ru
Ru
HO
R =
OH
N
TsO
TsCl
H
H
Ph2P
PPh2
O
OTs
R2P
PR2
Ph2P
N
PPh2
PR2
PPh2
O
O
H2O
135°C, 67 h
pyridine
N
X
0°C
rt (16 h)
X = N, C(CH3)
DMF
70°C, 72 h
LiBr
Br
The synthesis of tripodal N-triphos ligands bearing a
nitrogen atom in the backbone was achieved according to
a modular two-step procedure published by Gade et al..19
The respective phosphines were transformed into the air-
and moisture-stable phosphonium salts 1a-c by reaction
with formaldehyde in the presence of acid with 54 to 95%
yield (Scheme 3A).20 The ligands N-triphosPh (2a),
N-triphoso-Tol (2b) and N-triphosp-Tol (2c) were prepared by
reaction of 1a-c with triethylamine and ammonium
chloride with good to excellent yields (74 to 90%,
Scheme 3A). The ligand N-triphosMes (2d) could not be
synthesized according to this procedure, as the addition
of triethylamine in the second step favored the
1. HPPh2 + n-BuLi, THF
-78°C rt (30 min)
N
N
Ph2P
PPh2
Br
2. H3CC(CH2Br)2(py), THF
0°C
65°C (18 h)
2f
E
HO
ClPPh2
NEt3
Ph2P
PPh2
PPh2
OH
OH
O
O
O
DCM
40°C, 2 h
2g
Subsequently, the ruthenium complexes were
synthesized according to the protocol of Klankermayer
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The ligands were heated with the
and Leitner.9c,
23
1
ruthenium precursor [Ru(2-methylallyl)2(COD)] in
toluene to 110°C (Scheme 4). The ruthenium complexes
3a,c were isolated after 3 – 4 days with 44 and 21% yield,
whereas the respective complexes 3b and 3d of the
ligands 2b and 2d could not be isolated purely. Therefore,
the complexes were formed in situ in later catalytic
investigations. Complexes 3e-g were isolated after heating
the respective ligand with the ruthenium precursor in
toluene to 110°C for 3 – 8.5 hours with 33 – 64% yield.
Astonishingly, complex 3e did not eliminate isobutene at
all after heating for 3 h. The two phosphine groups of
ligand 2e coordinated to the ruthenium center while the
pyridyl ligand was unable to do so.
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Next, all catalysts were investigated in the catalytic
transformation of CO2 to DMM and MF (Scheme 1,
Table 1). Initial screening conditions were chosen
according to the system described by Klankermayer et
al..16 The catalyses were performed over 18 hours at 80°C
with Al(OTf)3 as Lewis acid utilizing 60 bar H2 and 20 bar
CO2.
The X-ray crystallographic structure of 3c confirmed
the expected distorted octahedral molecular geometry
(Figure 1). The obtained structure explains the difficulties
in accessing complexes 3b and 3d. Due to the spatial
proximity of the o-methyl groups to the phosphine, steric
and repulsive forces become too strong for effective
complexation (see ESI for molecular structure of ligand
2d).
Already at the initial screening conditions, the catalysts
3a and 3c yielded higher TONs for DMM and MF
compared to the [Ru(triphos)(tmm)] system (Table 1,
Entries 1-3+5).16 The complex [Ru(N-triphosPh)(tmm)] (3a)
showed the best results with a TON for DMM of 292 and
a TON for MF of 112 (Table 1, Entry 3). The modification of
the aryl groups of the phosphines in the complex
[Ru(N-triphosp-Tol)(tmm)] (3c) resulted in slightly
decreased activity compared to 3a (Table 1, Entry 5). The
low TONs obtained with [Ru(N-triphoso-Tol)(tmm)] (3b)
and [Ru(N-triphosMes)(tmm)] (3d) can be explained by the
in situ strategy for the formation of the complexes
(Table 1, Entries 4+6). Apparently, the complexation of
the ligands is not feasible under these catalytic
conditions.
The complexes [Ru(N-py-diphosPh)(2-methylallyl)2] (3e)
and [Ru(py-diphosPh)(tmm)] (3f) showed low activity in
the formation of DMM, whereas the TON of MF was
moderate (Table 1, Entries 7+8). Especially for 3f, the TON
of MF with 105 was noteworthy high (Table 1, Entry 8).
Interestingly, complex 3e showed activity at all even
though ligand 2e coordinated only in a bidentate manner.
The complex [Ru(triphosOPPh2)(tmm)] (3g) showed
reasonable activity for both products (Table 1, Entry 9).
Scheme 4. Complexation of the synthesized ligands.
R2P
PR2
Ru
[Ru(2-methylallyl)2(COD)]
PR2
R2P
PR2
PR2
toluene
N
110°C, 3 - 4 d
N
2a,c
3a,c
R =
a
c
Ru
N
Ph2P
[Ru(2-methylallyl)2(COD)]
PPh2
Ph2P
N
PPh2
toluene
N
110°C, 3 h
3e
2e
N
N
Ph2P
Ru
[Ru(2-methylallyl)2(COD)]
PPh2
Ph2P
N
PPh2
toluene
110°C, 8.5 h
2f
Table 1. Initial screening results of catalysts in the
selective synthesis of DMM starting from CO2 and
H2.a,b,c
3f
Ph2P
O
PPh2
O
PPh2
Ru
[Ru(2-methylallyl)2(COD)]
Ph2P
PPh2
O
O
PPh2
Entry
Cat.
[Ru]16
[Co]17
3a
TONDMM
214
TONMF
104
toluene
O
O
110°C, 4.5 h
1
2g
2
3
4
5
6
7
8
9
157
37
3g
292 ± 25
2 ± 0
112 ± 4
23 ± 0
90 ± 25
6 ± 1
Figure 1. Molecular structure of complex 3c. Ellipsoids are
shown at the 50% probability level. Hydrogen atoms and the
solvent molecule are omitted for clarity.
3bd
3c
221 ± 40
3 ± 2
3dd
3e
7 ± 4
39 ± 4
105 ± 6
83 ± 7
3f
5 ± 0
3g
32 ± 10
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aCatalysis conditions: 1.50 µmol catalyst, 6.25 µmol Al(OTf)3,
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21
15
25
30
40
313 ± 38
361 ± 21
329 ± 18
275 ± 14
68 ± 3
101 ± 7
108 ± 7
118 ± 2
0.5 mL MeOH, 80°C, 60 bar H2, 20 bar CO2, 18 h. bTONs were
90
90
1
2
3
4
5
6
7
8
1
determined by H NMR spectroscopy using mesitylene as
c
internal standard. All data are the average of two runs and
the standard deviation is indicated. dThe catalyst was formed
in situ: 1.50 µmol [Ru(2-methylallyl)2(COD)], 1.50 µmol ligand
2b or 2d, respectively.
aCatalysis conditions: 1.50 µmol catalyst, 6.25 µmol Al(OTf)3,
0.5 mL MeOH, 18 h. bTONs were determined by H NMR
1
c
Based on the highly promising results obtained with
complex 3a, we optimized the catalysis conditions with
regard to temperature, partial pressure of H2 and CO2,
reaction time, additive as well as catalyst and Lewis acid
concentration.
spectroscopy using mesitylene as internal standard. All data
are the average of three runs and the standard deviation is
indicated.
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Hereafter, the influence of the partial pressure of CO2
was analyzed by changing the CO2 pressure between 5
and 40 bar (Table 2, Entries 16-21, Figure 2C). The results
indicate a strong dependency of the catalyst activity on
the CO2 pressure. Both TON of DMM and MF increased
with increasing CO2 pressure. Whereas the TON for
DMM reached its maximum at 20 bar CO2 (Table 2,
Entry 14), the TON for MF increased until 40 bar CO2 was
applied (Table 2, Entry 21). Due to the very low MF
formation at low CO2 pressures, the selectivity at 5 bar
CO2 even exceeded 90% for DMM (Table 2, Entry 16).
We started with investigating the influence of the
temperature in a range between 20 and 120°C (Table 2,
Entries 1-9, Figure 2A). The TON for both DMM and MF
is strongly temperature-dependent with the highest TON
for DMM between 85 and 90°C (Table 2, Entries 4+5) and
the highest TON for MF at 70°C or probably lower
(Table 2, Entry 2). The TON for DMM even exceeded 300
for 85 and 90°C. Remarkably, the catalyst showed activity
even at 20°C (Table 2, Entry 1). We chose 90°C as the best
result for further optimization, thereby balancing activity
and selectivity (TONDMM 310 and TONMF 85).
At this point, we were intrigued by the high selectivities
that were achieved using low CO2 pressures. In published
work, it was stated that the catalytic hydrogenation of
CO2 terminates selectively at the formaldehyde level
using the [Ru(triphos)(tmm)] system.16 Consequently, we
investigated the influence of time on the product
distribution at 5 bar CO2. We envisioned to achieve high
TONs with high selectivities by increasing the reaction
time to several days (Table 3, Entries 1-8, Figure 2D).
Surprisingly, the TONs did not increase continuously. For
DMM, the TON reached a maximum after 12 hours
(Table 3, Entry 3), whereas in the case of MF the TON
decreased continuously. After 168 hours, nearly no
product but also no degradation products could be
Next, the partial pressure of H2 was varied between 40
and 100 bar while all other parameters were kept constant
(Table 2, Entries 10-15, Figure 2B). The TON of DMM was
highest at 90 bar H2 (Table 2, Entry 14), whereas the TON
of MF barely changed at pressures higher than 50 bar
prompting us to use 90 bar H2 for all further experiments
(TONDMM 363 and TONMF 92).
Table 2. Optimization of catalysis conditions with
regard to temperature as well as partial pressure of
H2 and CO2. a,b,c
T
(°C)
pH2
(bar)
pCO2
(bar)
Entry
TONDMM
TONMF
1
detected in H NMR spectra (Table 3, Entry 8, see ESI).
1
20
70
8 ± 5
51 ± 7
161 ± 5
112 ± 4
93 ± 3
85 ± 5
73 ± 4
66 ± 6
52 ± 5
42 ± 1
69 ± 2
76 ± 4
82 ± 4
83 ± 5
92 ± 9
86 ± 6
8 ± 1
Most likely, the catalyst decomposes in the course of the
reaction and the resulting degraded catalyst is still able to
deplete the formed DMM and MF.
2
164 ± 15
292 ± 25
324 ± 9
310 ± 12
271 ± 8
208 ± 9
119 ± 9
3
80
85
Table 3. Optimization of catalysis conditions with
regard to time of catalysis at 5 and 20 bar CO2. a,b,c
4
5
90
95
60
20
pCO2
(bar)
t
(h)
6
Entry
TONDMM
TONMF
7
100
110
120
1
2
1
6
21 ± 0
97 ± 4
106 ± 21
97 ± 18
85 ± 14
42 ± 14
20 ± 11
4 ± 1
35 ± 3
16 ± 4
13 ± 4
8 ± 1
8
9
56 ± 8
3
12
18
24
48
72
168
1
10
11
12
13
14
15
16
17
40
50
250 ± 14
279 ± 21
319 ± 24
326 ± 31
363 ± 18
332 ± 27
97 ± 18
213 ± 10
4
5
5
7 ± 1
70
6
7
4 ± 1
90
20
80
90
100
3 ± 2
8
9
10
11
0 ± 0
63 ± 8
63 ± 3
83 ± 15
17 ± 2
5
6
159 ± 4
248 ± 49
10
35 ± 2
12
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24
48
72
363 ± 18
92 ± 9
77 ± 2
61 ± 4
55 ± 1
47 ± 2
20
1
2
3
4
5
6
7
8
9
317 ± 19
293 ± 19
243 ± 17
177 ± 13
168
aCatalysis conditions: 1.50 µmol catalyst, 6.25 µmol Al(OTf)3,
b
0.5 mL MeOH, 90°C, 90 bar H2. TONs were determined by
1H NMR spectroscopy using mesitylene as internal standard.
cAll data are the average of three runs and the standard
deviation is indicated.
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Figure 2. Summarized bar plots of the TONs in the catalytic transformation of CO2 to DMM and MF in dependence of the
investigated reaction parameters.
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Therefore, we examined the dependency of the product
(Table 3, Entry 12), which is why this reaction time was
chosen for further optimization.
distribution on the time of catalysis at 20 bar CO2
(Table 3, Entries 9-16, Figure 2E). Here, the maximum for
both TON of DMM and MF was reached after 18 hours
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Subsequently, the influence of the Lewis acid was
thus be returned to the catalytic process leading to higher
TONs for DMM in the next reaction cycle.
1
2
3
4
5
6
7
8
investigated (Table S3.1 in ESI, Figure 2F). In general, the
employed metal triflates led to higher activity of the
catalytic system than the boron based Lewis acids.
Al(OTf)3 gave the best results for the former (Table S3.1 in
ESI, Entry 1), whereas BF3(OEt2) yielded the highest TONs
for the latter (Table S3.1 in ESI, Entry 8).
After catalysis, DMM can be easily hydrolyzed to
formaldehyde and methanol yielding the desired product
by distillation.24 For an industrial application, complex 3a
has the additional advantage that it is air-stable for over
two months and stable in solution under inert conditions
for over two weeks. Further, the synthesis of the ligand
can easily be scaled up (see ESI). Based on these results,
an industrial process for the energetically favorable and
atom efficient conversion of CO2 to formaldehyde using
the presented catalytic system in a batch process can be
envisioned. Also, the immediate product, DMM, exhibits
the potential for direct application. It can be used as a fuel
additive in biofuels due to its ability to reduce soot
formation during the combustion in diesel engines and its
origin in a renewable feedstock. Further, DMM is a
building block for the synthesis of higher order
oxymethylene ethers, which can be implemented as
synthetic fuels providing an alternative for diesel.4g, 25
An immense increase in activity could be achieved by
varying the concentration of the catalyst while the
catalyst to Lewis acid ratio remained unchanged (Table 4,
Entries 1-6, Figure 2G). The maximum TON for DMM was
reached at a catalyst loading of 0.38 µmol (Table 4,
Entry 2), whereas the TON of MF increased continuously
with decreasing catalyst loading. At a catalyst loading of
0.19 µmol, the TON for MF even exceeded 1000 (Table 4,
Entry 1). For further experiments, we chose 0.38 µmol of
catalyst loading.
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60
Last, we investigated the influence of changing the
catalyst to Lewis acid ratio while the catalyst loading was
kept constant (Table 4, Entries 7-11, Figure 2H). The best
results were still achieved using 1.56 µmol Lewis acid with
TONs for DMM and MF of 786 and 533, respectively
(Table 4, Entry 2). By increasing the amount of Lewis acid
beyond this, the TON of DMM did not change
significantly, whereas the TON of MF decreased slightly
compared to the best result (Table 4, Entry 9-11).
Reducing the amount of Lewis acid below 1.56 µmol led to
a strong decrease in the TON of DMM and an increase in
the TON of MF (Table 4, Entry 7+8).
Conclusion
We present optimized catalysis conditions for the
homogeneously catalyzed transformation of CO2 to DMM
and MF using the ruthenium-triphos complex 3a with
regard to temperature, partial pressure of H2 and CO2,
reaction time, additive as well as catalyst and Lewis acid
concentration. Compared to the results using the initial
parameters (TONDMM 292 and TONMF 112), the catalytic
system was significantly improved leading to a nearly
threefold increase of the TON of DMM to 786 (with it:
TONMF 533). The activity could also be optimized in favor
of MF yielding a TON of 1290 with only a slight decrease
in DMM formation (TONDMM 728). Further, the selectivity
for DMM shows a strong dependency on the catalysis
parameters and can be increased to above 90% (TONDMM
97 and TONMF 8). Thus, the catalytic conditions can be
adjusted depending on the desired product distribution.
Using molecular hydrogen and a homogeneous catalyst,
this catalytic system is unique in its activity and ease to
adjust, converting CO2 to the formaldehyde oxidation
level. The present results exceed by far literature data and
showcase the possibility for an atom efficient, alternative
pathway towards formaldehyde in the near future.
Table 4. Optimization of catalysis conditions with
regard to the amount of the Lewis acid. a,b,c
ncat
(µmol)
nadd
(µmol)
Entry
TONDMM
TONMF
1
2
0.19
0.38
0.75
1.50
2.25
3.00
0.78
1.56
3.13
728 ± 102 1290 ± 151
786 ± 62
528 ± 94
363 ± 18
201 ± 20
122 ± 7
533 ± 17
206 ± 42
92 ± 9
3
4
5
6.25
9.38
12.50
0.94
1.25
47 ± 3
6
7
33 ± 2
447 ± 93
532 ± 131
698 ± 51
650 ± 22
511 ± 67
415 ± 80
8
9
10
11
0.38
1.88
2.19
2.50
756 ± 134 446 ± 98
748 ± 45 444 ± 107
Experimental Section
The complete detailed description of all experiments, the
spectroscopic data of compounds as well as the NMR spectra of
compounds and catalysis samples can be found in the
supporting information. In order to improve comprehensibility,
simplified names were used in some cases rather than using
exact IUPAC names.
aCatalysis conditions: Al(OTf)3, 0.5 mL MeOH, 90°C,
90 bar H2, 20 bar CO2, 18 h. bTONs were determined by 1H
NMR spectroscopy using mesitylene as internal standard.
cAll data are the average of three runs and the standard
deviation is indicated.
All novel ligand precursors (1b, 1c), ligands (2b, 2c and 2d)
and complexes (3c, 3e and 3f) were completely characterized by
NMR spectroscopy, HR-MS, IR spectroscopy and elemental
analysis. The novel complex 3g exhibited too broad signals in the
NMR spectra and thus was characterized by HR-MS, IR
spectroscopy and elemental analysis. Furthermore, X-ray
diffraction analysis from a single crystal was obtained for
In further experiments, we investigated the
transformation of MF to DMM by using MF instead of
CO2 as starting material for the hydrogenation reaction.
After 18 hours, MF was selectively transformed to DMM.
Following the separation of DMM, the formed MF can
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compounds 1b, 2c (in its phosphine oxide form), 2d and 3c (see
ESI).
NMR,
nuclear
magnetic
resonance;
tmm,
1
trimethylenemethane dianion; TON, turnover number.
2
3
4
5
6
7
8
9
The catalyses were performed in stainless steel high-pressure
autoclaves using an NMR tube as inset and a small stirring bar
for mixing. The catalyst and the additive were suspended in a
Schlenk tube in MeOH and the mixture was stirred for 1 h. The
autoclave was evacuated and flushed with nitrogen three times
before 0.5 mL reaction mixture was transferred into the NMR
tube inset. The carbon dioxide gas line was purged with CO2 and
the hydrogen gas line was purged with H2 at least seven times.
After the autoclave was pressurized with CO2, the CO2 was
deposited by cooling the autoclave to -78°C for 5 min. Then, the
autoclave was further pressurized with H2. The closed autoclave
was allowed to warm to room temperature for 10 min. The
reaction mixture was stirred at the mentioned temperature for
the defined time. The autoclave was cooled to 0°C for 15 min
before the gas was discharged. The TONs of the catalyses were
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determined by H NMR spectroscopy. A sample of 50 µl of the
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The catalysis was also performed in the absence of a catalyst, a
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observed (see ESI). All catalyses with catalyst 3a were performed
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standard deviation of the experiments (see ESI).
ASSOCIATED CONTENT
Supporting Information
The supporting information is available free of charge on the
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Crystallographic data for 2cox (CIF)
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Crystallographic data for 3c (CIF)
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AUTHOR INFORMATION
Corresponding Author
*oliver.trapp@cup.uni-muenchen.de
Funding Sources
This research was funded by the German Federal Ministry of
Education and Research (BMBF) within the funding initiative
‘CO2Plus - Stoffliche Nutzung von CO2 zur Verbreiterung
der Rohstoffbasis’.
Notes
The authors declare no conflict of interest.
ACKNOWLEDGMENT
Generous financial support by the German Federal Ministry
of Education and Research (BMBF) within the funding
initiative ‘CO2Plus - Stoffliche Nutzung von CO2 zur
Verbreiterung der Rohstoffbasis’ is gratefully acknowledged.
ABBREVIATIONS
COD, cyclooctadiene; DCM, dichloromethane; DMM,
dimethoxymethane;
HR-MS,
high-resolution
mass
spectrometry; IR, infrared radiation; MF, methyl formate;
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56
57
58
59
60
10
ACS Paragon Plus Environment