Additive-free cobalt-catalysed hydrogenation of carbonates to methanol and alcohols

Article abstract of DOI:10.1039/c9cy00951e

Reduction of various organic carbonates to methanol and alcohols can be achieved in the presence of a molecularly-defined homogeneous cobalt catalyst. Specifically, the use of Co(BF₄)₂ in combination with either commercial or tailor-made tridentate phosphine ligands allows for additive-free hydrogenations of carbonates. Optimal results are obtained at relatively mild conditions (120 °C, 50 bar hydrogen pressure) in the presence of xylyl-Triphos L4.

Full text of DOI:10.1039/c9cy00951e

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DOI: 10.1039/C9CY00951E
ARTICLE
Additive-Free Cobalt-Catalysed Hydrogenation of Carbonates to
Methanol and Alcohols
Received 00th January 20xx,
Accepted 00th January 20xx
Francesco Ferretti,a,b Florian Korbinian Scharnagl,a Anna Dall’Anese,a,c Ralf Jackstella,
Sarim Dastgir,d and Matthias Bellera*
DOI: 10.1039/x0xx00000x
Reduction of various organic carbonates to methanol and alcohols can be achieved in the presence of a molecularly-
defined homogeneous cobalt catalyst. Specifically, the use of Co(BF4)2 in combination with either commercial or tailor-
made tridentate phoshine ligands allows for additive-free hydrogenations of carbonates. Optimal results are obtained at
relatively mild conditions (120°C, 50 bar hydrogen pressure) in the presence of xylyl-Triphos L4.
carbamates and formates to methanol using different
ruthenium-PNN-pincer complexes.25-27 Later on, the use of
ruthenium-based NHC-pincer systems has been described.28, 29
The groups of Leitner and Klankermayer extensively studied
Introductions
[Ru(Triphos)(TMM)]
tris(diphenylphosphinomethyl)ethane
(Triphos
=
1,1,1-
TMM =
The catalytic hydrogenation of carbon dioxide to methanol is
of general interest in the context of the so-called “Methanol
Economy”. This concept comprises the capture of carbon
dioxide from the atmosphere and its conversion to methanol
or dimethyl ether, using dihydrogen as pointed out originally
by Asinger1 and Olah2, 3. Although most efforts focused on
heterogeneous catalysts, also the development of suitable
molecularly-defined catalysts is interesting due to the
potentially higher activity. Thus in recent years, notable
contributions in this area have been made by the groups of
and
trimethylene methane) in hydrogenations of carboxylic and
carbonic acid derivatives, including cyclic and acyclic
carbonates in the presence or the absence of the additive
HNTf2.30 So far, the most productive catalyst for this reaction
has been reported by the group of Kuiling Ding. By using the
commercial ruthenium-MACHO pincer complex, they achieved
catalyst TONs up to 87000 for hydrogenation cyclic
carbonates, which is about one order of magnitude higher
compared to the direct conversion of carbon dioxide to
methanol, underlining the possible advantage of this indirect
CO2 reduction route.31 Apart from expensive precious metal
complexes, the first non-noble metal catalysts for this
transformation were reported only very recently. In 2018, the
groups of Leitner,32 Rueping33 and Milstein34 at the same time
reported manganese pincer complexes for the hydrogenation
of organic carbonates under basic conditions.
Milstein,4 Klankermayer and Leitner,5-7 Olah and Prakash,8-13
15
Sanford,14,
Himeda and Laurenczy,16 Wass,17 Martins and
Pombeiro,18 as well as our group.19,
So far, a ruthenium-
20
based PNP pincer complex constitutes the most productive
homogeneous system with a reported turnover number (TON)
of 9900, albeit after 10 days.12 An alternative approach to the
direct hydrogenation of carbon dioxide makes use of cyclic
and/or acyclic organic carbonates. The former derivatives can
be easily synthesised from carbon dioxide and epoxides, as
already done on industrial scale in the so-called OMEGA
process by Shell. The latter carbonates are mainly obtained
from CO2 by indirect methods (i.e alcoholysis of other carbon
dioxide derivatives such as urea or cyclic carbonates)
O
catalyst, H2
MeOH + ROH (+ R'OH)
RO
PtBu2
CO
OR/R'
H
H
H
PtBu2
CO
H
Ph2PPh2
N
P
PPh2
CO
N
Ru
P
N
Ru
Ru
Ru
N
NEt2
P
Ph2
Ph2
Cl
+
nowadays, but can also be directly prepared from CO2 and
+
21-23
KOtBu
Ding et al., 2012
alcohols.
Subsequent hydrogenation leads to methanol
HNTf 2
Milstein et al., 2011
Klankermayer,
Leitner et al., 2014
and the corresponding alcohols.24 This indirect strategy for CO2
reduction was first demonstrated by Milstein in 2011, who
reported the selective hydrogenation of carbonates,
H
NtBu
CO
Br
Br
Br
H
H
P(m-Xyl)2
P(m-Xyl)2
i
N
N
P
PPh2
CO
P Pr2
CO
N
Mn
(m-Xyl)2P
Mn
Mn
N
CO
+
P
CO
iPr2
tBu2
+
CO
+
KOtBu
CO
Co(BF4)2·6H2
O
a. Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße
29a, 18059 Rostock, Germany
+
NaOtBu
Leitner et al., 2018
KH
This work
Milstein et al., 2018
Rueping et al., 2018
b. Dipartimento di Chimica, Università degli Studi di Milano, Via C. Golgi 19, 20133
Milano, Italy
c Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Trieste, Via Licio
Giorgieri 1, 34127 Trieste, Italy
Scheme 1: Overview of selected reported homogeneous systems for the hydrogenation
of organic carbonates to the corresponding alcohols.
d
Qatar Environment and Energy Research Institute (QEERI), Hamad bin Khalifa
University (HBKU), Qatar Foundation, Doha, Qatar
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Modifying a system based on the combination of cobalt and tetradentate ligands (L15 – L19) was tried. Similar to Co-
Triphos, initially reported by Elsevier and de Bruin for the catalysed hydrogenation reactions of carboxylicVcieowmArpticoleuOnndlinse,
DOI: 10.1039/C9CY00951E
35
reduction of carboxylic acid,
our group succeeded in the also in the case of carbonates solely ligands with the Triphos
direct hydrogenation of carbon dioxide with a homogeneous backbone were suitable. All the other ligands tested gave only
cobalt catalyst.19 Later on, improved results have been traces of methanol and ethanol.
obtained by us,20 as well as by Klankermayer and Schieweck.7
Table 2: Hydrogenation of diethyl carbonate: Variation of ligands in combination with
Co(BF4)2∙6H2O.
To the best of our knowledge, the applicability of such
complexes for reduction of organic carbonates has not been
reported yet. In this context, herein we describe the efficient
hydrogenation of cyclic and acyclic carbonates in 2,2,2-
trifluoroethanol (TFE) in the presence of Co(BF4)2·6H2O and a
modified Triphos ligand.
R
P(R)2
P
P
(R)2
P
P(R)2
R'
PPh2
R = Ph
R = tBu
PPh2
R2P
PR2
L7
R = Ph
L1
L2
L9
R = Ph, R' = Ph
R = tBu, R' = Ph
L8
4-(H3C)C6H4
L10
4-(H3CO )C6H4 L3
3,5-(H3C)2C6H3 L4
L5
L6
4-(Me2N)C6H4
Naphtyl
Results and Discussion
PPh2
PPh2
At the beginning of our investigations, different cobalt
precursors in the presence of the ligand Triphos were tested
for the hydrogenation of the model substrate diethyl
carbonate at 120 °C and 50 bar H2 (see
Ph
P
P
P
Ph2P
PPh2
Ph2P
PPh2
Fe
L11
L12
L13
Cy2
P
Table 1). Unfortunately, no active catalyst could be generated
PPh2
Ph2P
PPh2
PCy2
N
in the presence of coordinating anions such as halides (entries
1 and 2) acetyl acetonate (entries 3 and 4), carbonate and
acetate (entries 6 and 7). Also the use of the cobalt
hexafluoroacetylacetone did not show any appreciable yield of
MeOH (entry 5). Finally, we found that Co(NTf2)2 and
Co(BF4)2·6H2O are both suitable for the hydrogenation of
carbonates (entries 8 and 9), even at reduced catalyst loadings
of 2 mol% (entries 10 and 11). Co(NTf2)2 afforded the highest
conversions but a lower selectivity than Co(BF4)2. Although the
use of Co(NTf2)2 gave good activities for CO2 reduction,20 here
the low yield of alcohols is ascribed to the decomposition of
the triflimide-anion which reacted with the substrate. In fact,
we observed by GC-MS the formation of several unidentified
by-products containing fragments derived both from diethyl
carbonate and the NTf2-anion.
P
PPh2
PPh2
L16
N
Ph2P
PPh2
PPh2
Ph2P
PCy2
L17
L14
L15
Ph2P
tBu
Py
PPh2
Ph2P
PPh2
P
P
Cy2
P
PCy2
P
P
tBu
L20
Py
PPh2
L18
PPh2
L19
L21
Entry
1
2
3
4
5
6
7
8
Ligand
Conversion [%]
Yield (EtOH) [%]
Yield (MeOH) [%]
L1
L2
L3
L4
L5
L6
L7
L8
L9
24
37
45
57
30
4
22
32
37
48
27
<1
<1
<1
<1
1
16
26
32
42
22
0
<1
<1
<1
<1
0
<1
<1
<1
<1
<1
0
Table 1: Testing different cobalt precursors for the hydrogenation of diethyl carbonate.
4
19
29
32
17
6
5
4
6
2
4
3
11
6
3
9
L10
L11
L12
L13
L14
L15
L16
L17
L18
L19
L20
L21
10
11
12
13
14
15
16
17
18
19
20
21
22
<1
<1
1
Yield EtOH
Yield MeOH
Entry
Co Precursor
Conversion [%]
[%]
<1
<1
<1
<1
<1
<1
<1
37
31
36
22
[%]
<1
<1
<1
<1
<1
<1
<1
13
25
8
<1
<1
<1
<1
<1
<1
<1
<1
<1
1
2
3
4
5
6
7
8
9
CoCl2
CoF2
Co(acac)2
6
2
4
4
2
1
3
60
36
44
24
Co(acac)3
0
0
0
Co(acacF)2
CoCO3 ∙ 0.33 H2O
Co(OAc)2
Co(NTf2)2
Co(BF4)2∙6H2O
Co(NTf2)2
<1
<1
no ligand
4
General conditions: 1.0 mmol diethyl carbonate, 2 mol% Co(BF4)2·6H2O, 2.4 mol%
ligand, 2 mL THF, 120 °C, 50 bar H2, 18 h. Conversions and yields were calculated via GC
using hexadecane as internal standard.
10a
11a
Co(BF4)2∙6H2O
16
Reaction conditions: General conditions: 1.0 mmol diethyl carbonate, 2 mol% cobalt
precursor, 2.4 mol% L1, 2 mL THF, 120 °C, 50 bar H2, 18 h. Conversions and yields were
calculated via GC using hexadecane as internal standard. acacF =
hexafluoroacetylacetone. a Co precursor = 0.02 mmol (2 mol%), L1 = 0.024 mmol.
Among the Triphos-type ligands, the xylyl-Triphos L4 revealed
the highest productivity, followed by anisyl- (L3) and p-tolyl-
Triphos (L2). The dimethylamino-substituted Triphos L5
showed a slightly better productivity than L1. Thus, the
substitution of the phenyl ring of Triphos with electron-
donating groups seems to be beneficial for the system activity.
However, the results are difficult to rationalize only on the
After identifying an active catalyst system, we tried to improve
the comparably low activity investigating different ligands in
the presence of cobalt tetrafluoroborate hexahydrate. Here, a
variety of bidentate (L20 and L21), tridentate (L1 – L14) and
2 | J. Name., 2012, 00, 1-3
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Table 3: Hydrogenation of carbonates to the corresponding alcohols: Substrate scope
base of the basicity of the phosphorus. The catalytic behaviour
can be better explained taking into account the steric
properties of the ligands. Indeed, Leitner and Klankermayer
recently showed the benefit of using sterically hindered
ligands by comparing the activity of L1, L2 and L4 in the
ruthenium/triphos-catalysed hydrogenation of methyl
benzoate and lactams. The increased activity in the order L4 >
L2 > L1 is ascribed to the suppression of inactive hydride
bridged ruthenium dimers formation.36 The same trend was
View Article Online
DOI: 10.1039/C9CY00951E
Co(BF4)2·6H2O (2 mol%)
L4
(2.4 mol%)
O
2.0 mL TFE
MeOH + ROH + R'OH
RO
OR'
50 bar H2, 120 °C, 18 h
1-15
Entry
Substrate
Conv.
[%]
MeOH
[%]
ROH
[%]
R’OH
[%]
O
not noticed when cobalt was used instead of ruthenium for
1
2
3
4
5
>99
85
98
-
-
-
O
O
O
O
O
7, 20
either the reduction of CO2
or reductive transformation of
1
O
carboxylic acids,37 suggesting a negligible role of dimers in
catalyst deactivation. On the other hand, [(Triphos)2Co2(μ-H3)]+
was found inactive, in the absence of acid co-catalysts, for the
synthesis of dimethoxymethane and methyl formate from
CO2.7 Considering the absence of additives in the present
system, the prevention of dimer formation is a possible
explanation for the order of activity of the ligands.
>99
92
>99
73
2
O
96
-
O
O
3
>99
94
>99
84
>99
n.d.a
-
O
O
4
100
80
60
40
20
0
O
89
O
O
5
O
O
O
6
7
>99
>99
11
94
88
54
-
-
6
7
O
O
O
O
O
O
F
F
O
O
F
F
O
F
8
>99
92
97
91
8
O
9
>99
53
80
5
n.d.a
43
-
-
Scheme 2: Hydrogenation of diethyl carbonate: Solvent screening.
F3C
O
O
CF3
F
9
Reaction conditions: 1.0 mmol diethyl carbonate, 2 mol% Co(BF4)2·6H2O, 2.4 mol% L1
or L4, 2 mL solvent, 120 °C, 50 bar H2, 18 h. Conversions and yields were calculated via
GC using hexadecane as internal standard. The ethanol yield could not be determined
in the case of isopropanol as solvent.
F
F
F
F
F
F
F
O
10
O
O
F
F
10
O
Next, the performance of the in situ-generated catalyst with
commercial ligand L1 has been investigated in different
solvents (Scheme 2). By far the best results were obtained with
2,2,2-trifluoroethanol (TFE) leading to 71% yield of both
methanol and ethanol (conversion: 75%). Quantitative
conversion and GC-yields for both alcohols have been achieved
by combining the best solvent with the best ligand, L4.
Previously, such beneficial effect of fluorinated solvents has
been observed by Elsevier,38 as well as by Klankermayer.7
Interestingly, TFE gave significantly better results compared to
the related solvent 1,1,1,3,3,3-hexafluoro isopropanol (HFIP).
THF showed the best productivity as a non-fluorinated solvent.
The sensitivity of the system towards water was shown by
11
12
>99
>99
85
96
75
90
-
-
O
O
O
O
11
O
12
Reaction conditions: 1.0 mmol diethyl carbonate, 2 mol% Co(BF4)2·6H2O, 2.4 mol% L4,
2 mL TFE, 120 °C, 50 bar H2, 18 h. Conversions and yields were calculated via GC using
hexadecane as internal standard. When R = R’, yields are reported as 2 ROH. a The peak
of ROH overlaps with the one of TFE in the gas-chromatogram.
Dimethyl carbonate 1 gave three equivalents of methanol in a
yield of 85% at full conversion. Almost quantitative yields of
both alcohols have been achieved for di-n-butyl carbonate 2.
Also the aromatic carbonate 3 was effectively converted to
methanol and phenol. The asymmetric aromatic/aliphatic
carbonates 4 and 5 yielded the three corresponding alcohols.
For dibenzyl carbonate 6, a low methanol yield (11%) was
combining THF with 5 L (0.28 mmol) of distilled water. Adding
this small amount dropped the conversion from 24% to 10%
and the yield of ethanol from 22% to 4%. Methanol formation
could not be observed anymore. All other solvents resulted in
low conversions, yields and selectivities. With an optimised
system in hand, the hydrogenation of different organic
carbonates was investigated in more detail (Table 3).
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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observed, even though the conversion and the yield of benzylic
alcohol both have been high (<99%/88%). A similar behaviour
was found for the perfluorinated carbonate 10, although at a
lower conversion. It is worth noticing that the used cobalt
source contains six equivalents of water and both benzyl
alcohol and pentafluorophenol are good leaving groups. Thus,
for 6 and 10 hydrolysis of the substrates would lead to
formation of alcohol, carbon dioxide. This, at least in part,
accounts for the discrepancy of methanol and alcohol yields.
The asymmetric carbonate 8, which bears one
hexafluorophenol unit yielded in 92% of methanol, along with
97% of 9-fluorenyl methanol and 91% of hexafluorophenol.
Also the fluorinated substrate 9 was converted to methanol in
high yield. Noteworthy, also the cyclic carbonates 12 and 13
were readily transformed to the diols and methanol. These
carbonates are of particular interest vide supra, as they are
commercially produced from carbon dioxide and epoxides or
oxetanes.39
Experimental Section
Materials
View Article Online
DOI: 10.1039/C9CY00951E
All chemicals were purchased from commercial sources and
were used as received without additional purification, if not
stated otherwise. Molecular hydrogen was purchased from
Linde. All experiments were carried out under argon
atmosphere by using standard Schlenk-techniques, unless
stated otherwise. Solvents were dried and distilled or directly
used from a solvent purification system (MBraun). THF was
stored over molecular sieves 3 Å. Diethyl carbonate was
distilled prior to use. The ligands L2 – L6,37 L7 – L9,40 L11,40
L15,41 L16,42 L17,43 L19,44 and L2045 have been synthesised
according to literature-reported procedures.
Catalytic experiments were conducted in 4 mL screw cap vials,
closed with a polytetrafluoroethylene (PTFE)/white rubber
septum (Wheaton 13 mm Septa) and phenolic cap and
connected with atmosphere by a needle, inside a 300 mL Parr
autoclave and stirred with a magnetic stirring bar. GC
measurements were carried out on a 7890A GC-System with
HP-5 column (polydimethylsiloxane with 5% phenyl groups,
length 30 m, i.d. 0.32 mm, film 0.25 µm) and with a FID
coupled with a 7693 autosampler from Agilent Technologies.
Argon was used as carrier gas. GC-analyses for methanol
quantification were performed on an Agilent HP-6890
chromatograph with a FID detector and an Agilent HP Ultra 1
column (19091A-105, 50 m, 0.20mm i.d., 0.33 mm film
thickness, 100% dimethylpolysiloxane) using argon as carrier
gas.
H2
H2
H2
OH
O
O
O
H
H
O
O
H
O
H
H
H
EtOH
EtOH
Conversion
>99%
Ethanol
>99%
Methanol
86%
Co(BF4)2·6H2O (2 mol%)
L4 (2.4 mol%)
O
2.0 mL TFE
50 bar H2, 120 °C, 18 h
H
O
1.0 mmol
Co(BF4)2·6H2O (2 mol%)
L4 (2.4 mol%)
O
>99%
-
88%
2.0 mL TFE
50 bar H2, 120 °C, 18 h
H
H
n
1.0 mmol
Scheme 3: Hydrogenation of the potential intermediates ethyl formate and
formaldehyde.
In a typical catalytic experiment, Co(BF4)2·6H2O (6.81 mg,
2.0 mmol) and ligand (2.4 mmol) were fast weighed in the air
and transferred into a 4 mL glass vial. If used, solid substrates
were also weighed in the air and added into the vial. The vial
was subsequently set under argon. 2.0 mL solvent were added
and the mixture stirred for 5-10 min. Then, liquid substrates
were added and the vials were placed in a metal plate inside a
300 mL autoclave. After closing, the reactor was pressurised
with hydrogen (about 20 bar), which was released again. This
procedure was carried out three times, after which 50 bar H2
were introduced. The autoclave was then heated inside an
aluminium block to 120 °C for 18 h. Afterwards the reaction
was quenched with an ice-bath and the reactor vented.
As the hydrogenation of diethyl carbonate potentially occurs
stepwise via ethyl formate and/or formaldehyde, these
compounds have been investigated in separate hydrogenation
experiments (Scheme 3). Both, ethyl formate and para-
formaldehyde were completely hydrogenated, giving
methanol in 86% and 88%, respectively, and ethanol in >99%.
Therefore, although only traces of ethyl formate were
detected by GC for the model substrate, we cannot exclude
that these compounds indeed are intermediates in the
hydrogenation of diethyl carbonate.
Hexadecane (30 L) was added to the reaction as internal
standard for GC, along with 2 mL THF. After proper mixing, GC
was measured of the sample.
Conclusions
In conclusion, we investigated the homogeneous cobalt-
catalysed hydrogenation of organic carbonates for the first
time. The combination of Co(BF4)2·6H2O with a Triphos-derived
ligand L4 resulted in an active catalytic system suitable for
reduction of both cyclic and acyclic carbonates. At relatively
mild conditions, good to very good yields of methanol and the
corresponding alcohols have been obtained using the solvent
TFE.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Financial support from Qatar National Research Fund, a
member of Qatar foundation, Qatar (Grant numbers NPRP9-
212-1-042, & NPRP8-235-1-055), BMBF and the State of
Mecklenburg-Vorpommern,
Germany
is
gratefully
acknowledged. We thank the analytical department of LIKAT
for the support and Prof. Uwe Rosenthal for providing L14.
4 | J. Name., 2012, 00, 1-3
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28. X. Wu, L. Ji, Y. Ji, E. H. M. Elageed and GV. ieGwaAorti,cleCOantlainle.
Commun., 2016, 85, 57-60.
29. J. Chen, H. Zhu, J. Chen, Z.-G. Le and T. Tu, Chemistry – An
Asian Journal, 2017, 12, 2809-2812.
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