Hydrogenation of CO2-Derived Carbonates and Polycarbonates to Methanol and Diols by Metal–Ligand Cooperative Manganese Catalysis

Article abstract of DOI:10.1002/anie.201805630

The first base-metal-catalysed hydrogenation of CO₂-derived carbonates to alcohols is presented. The reaction proceeds under mild conditions in the presence of a well-defined manganese complex with a loading as low as 0.25 mol %. The non-precious-metal homogenous catalytic system provides an indirect route for the conversion of CO₂ into methanol with the co-production of value-added (vicinal) diols in yields of up to 99 %. Experimental and computational studies indicate a metal–ligand cooperative catalysis mechanism.

Full text of DOI:10.1002/anie.201805630

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Accepted Article
Title: Hydrogenation of CO2-Derived Carbonates and Polycarbonates
to Methanol and Diols via Metal Ligand Cooperative Manganese
Catalysis
Authors: Magnus Rueping
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COMMUNICATION
Hydrogenation of CO -Derived Carbonates and Polycarbonates to
2
Methanol and Diols via Metal Ligand Cooperative Manganese
Catalysis
[
a]
[b]
[b]
[b]
[a]
[a,b]
Viktoriia Zubar, Yury Lebedev, Luis Miguel Azofra, Luigi Cavallo* Osama El-Sepelgy,* Magnus Rueping*
Abstract: The first example of base-metal catalysed hydrogenation
of the CO -derived-carbonates to alcohols is presented. The reaction
future in which methanol might play the central role as a
[8]
2
hydrogen storage material and C1 building block. This reaction
has been studied with heterogeneous catalysts. Nevertheless,
these catalytic systems have to operate at elevated temperature
(>200 °C) and suffer from the formation of several side-
operates under mild conditions using a well-defined manganese
complex with loading as low as 0.25 mol %. The nonprecious
homogenous catalytic system provides an indirect route to convert
[
9]
CO
2
to methanol with the co-production of the value-added vicinal
products. In contrast, well-defined homogenous catalysts are
potentially more active and can be tuned by mechanistic studies.
In that regard only recently, seminal reports outlined preliminary
diols with yields up to 99%. Experimental and computational studies
indicate a metal ligand cooperative catalysis mechanism.
[
10]
[6,11]
results on the direct and indirect
methanol.
2
hydrogenation of CO to
Transition metal catalysed hydrogenation of polar bonds is
a key technology in modern industrial chemistry. So far most of
these catalytic systems rely on the use of precious metal
However, an efficient catalytic system based on an earth-
abundant metal remains an elusive goal. Thus a development of
a base metal catalyst which could be used at low catalyst
[1]
catalysts. However, the replacement of the rare-earth metal
catalysis by the earth-abundant alternatives is a topic of current
2
loadings for the reduction of CO derived organic carbonates to
value added alcohols would be an important advancement in
achieving the requirements of an ecologically and economically
benign process (Scheme 1a). Besides, the successful
development of such a process may also be extended to the
recycling of polycarbonates with the simultaneous formation of
valuable diols and methanol (Scheme 1b).
[2]
interest. Accordingly, significant advances have been made to
[
3]
[3]
[4]
the hydrogenation of aldehydes, ketones, esters, carboxylic
[
4d]
[5]
acids and amides using base-metal catalysts. On the other
hand, the hydrogenation of the carbonic acid derivatives is
significantly more difficult due to the resonance stabilisation
effect of the adjacent alkoxy groups which lower the
electrophilicity of the carbonyl group. To the best of our
knowledge, catalytic hydrogenation of carbonic acid derivatives
has never been reported using a homogenous non-precious
[
6]
metal catalyst.
Among the carbonic acid derivatives, the hydrogenation of
cyclic organic carbonates (COCs) to alcohols is of particular
interest, because COCs are industrially synthesised by the direct
coupling between carbon dioxide and oxiranes or oxetanes.
Hence, developing mild hydrogenation of the COCs would lead
to a practical two–step route to convert CO to methanol, in
2
addition to the production of the value added diols (Scheme 1).
More specifically, the industrial production of ethylene glycol
(
EG) involves the use of the so called “omega process” in which
CO is inserted into the ethylene oxide to produce ethylene
carbonate, followed by catalytic hydrolysis of the former
2
Scheme 1. Unprecedented base-metal catalysed hydrogenation of organic
carbonates.
[
7]
carbonate to (EG) and CO
hydrolysis by catalytic hydrogenation would lead to the formation
of methanol instead of CO , thus giving a great advantage in
terms of sustainability. The catalytic production of methanol from
CO is an elegant alternative option for the recycling of carbon
and could lead to “methanol economy” which is a suggested
2
.
The replacement of the catalytic
2
[12]
Inspired by the recent progress on manganese catalysis
2
and our interest on developing sustainable transformations using
inexpensive base-metals stabilised by stable non-innocent
[
13-14]
ligands,
we herein present a new manganese complex that
reduces COCs as well as recycles polycarbonates into methanol
and vicinal diols under mild reaction conditions. For the first
time, a combined experimental and computational study
provides insight into the reaction mechanism and explains the
role of the non-innocent ligand in the cascade hydrogenation of
2
the CO -derived COCs.
[
a]
b]
M.Sc. V. Zubar, Dr. O. El-Sepelgy Prof. Dr. M. Rueping
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
E-mail: Magnus.Rueping@rwth-aachen.de
Dr. Y. Lebedev, Dr. L. M. Azofra, Prof. Dr. L. Cavallo, Prof.
Dr. M. Rueping
[
KAUST Catalysis Center (KCC)
King Abdullah University of Science and Technology
In order to accomplish a practical reduction method for the
rather challenging cyclic carbonates, we started our studies with
the synthesis of a new bench stable PNN-Mn complex Mn-1,
(KAUST)
Ph
Thuwal 23955-6900 (Saudi Arabia)
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Angewandte Chemie International Edition
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COMMUNICATION
Ph
which is supported by an air and moisture stable PNN ligand
2). This result highlights the crucial role of the free N-H group in
the pincer ligand. Further screening of different manganese
complexes lead to unsatisfactory results (see Table S4 for
details). Subsequently, the influence of different solvents and
[
15]
L1. Following the typical route for the synthesis of the pincer
[12]
manganese carbonyl complexes,
Mn-1 was readily prepared
Br as metal precursor in
THF at 80 C for 16 h. The pale yellow complex was isolated in
4% yield and characterised by multinuclear NMR, IR, and mass
by treatment of L1 with 1 eq. of Mn(CO)
5
o
bases was investigated. Employing K as a base did not
2
CO
3
8
provide improve the results while Cs CO was similar effective
2
3
spectrometry as well as single crystal X-ray diffraction study. In
order to study the role of the N-H group, we also synthetized and
the corresponding N-methylated ligand and complex Mn-2 in
as KOtBu (Table 1, entries 3, 4). Furthermore, testing various
solvents such as THF, 2-methyl-THF and toluene did not lead to
better results (Table 1, entries 5-7). Hence, 1,4-dioxane was
chosen as appropriate solvent. The reduction of the catalyst
loading to 0.5 mol % resulted in >99% of EG and 89% of
methanol (Table 1, entry 8). Pleasingly, the reaction proceeded
equally well when the catalyst loading was reduced 0.25 mol%
and the substrate concentration was increased to 1.46 M (Table
88% yield (Scheme 2)
1, entries 9-11).
In order to demonstrate the potential and applicability of
the newly developed method, a range of COCs 1a-1m were
tested under the optimised reaction conditions (Table 2). An
array of mono-substituted 5-membered 1,3-dioxolan-2-one
bearing different alkyl and aryl substituents such as Me, Et, n-Bu,
Pent, t-Bu and Ph could be efficiently and selectively
hydrogenated to the corresponding vicinal 1,2-diol and methanol
in very good yields (Table 2, entries 1-6). The reaction tolerates
the benzyloxymethyl and methoxymethyl derivatives 1h and 1i
and the desired alcohols were produced in excellent yields
Scheme 2. Synthesis of two Mn-complexes and the single crystal X-ray
structure of Mn-1 at 50% probability level.
[
a]
Table 1. Optimisation of the reaction conditions.
(
Table 2, entries 7,8). Noteworthy, the disubstituted cyclic
carbonate 1j was successfully converted to methanol and 2,3-
butylene glycol (2j) in very good yields (Table 2, entries 9).
Under the same reaction conditions, different unsubstituted and
substituted 6-membered COCs 1k-1m were reacted in excellent
yields to give methanol and the corresponding vicinal 1,3-diols
yield
of
EG
yield
of
MeOH
[
Mn]
base
(mol%)
solvent
(conc.)
entry
(
mol%)
(
table 2, entries 10-12).
Additionally, our catalytic system was found to catalyse the
hydrogenative depolymerisation of polycarbonates to the
corresponding diol and methanol. An example is shown in Table
1
2
Mn-1 (1)
Mn-2 (1)
KOtBu (2.5)
KOtBu (2.5)
1,4-dioxane
1,4-dioxane
>99
14
92
0
3
4
5
6
7
8
9
Mn-1 (1)
Mn-1 (1)
K
2
CO
3
(2.5)
(2.5)
1,4-dioxane
1,4-dioxane
THF
>99
>99
>99
90
86
92
77
80
52
89
36
92
20
2
, entry 13, poly(propylene carbonate) which can be prepared by
Cs
2
CO
3
[17]
copolymerisation of propylene oxide and CO
2
,
was
Mn-1 (1)
KOtBu (2.5)
KOtBu (2.5)
KOtBu (2.5)
KOtBu (1.25)
KOtBu (0.63)
KOtBu (0.63)
KOtBu (0.25)
hydrogenated using 1 mol% of Mn-1 to produce methanol (87%
yield) and 1,2-propyleneglycol (2b, 99% yield). This result might
open new avenues for developing efficient base metal catalysts
for the recycling of widely used polycarbonate plastic materials.
In order to gain more insight into the reaction mechanism,
2
Mn-1 (1)
2-Me-THF
Toluene
Mn-1 (1)
61
Mn-1 (0.5)
Mn-1 (0.25)
Mn-1 (0.25)
Mn-1 (0.1)
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
>99
54
we performed deuterium-labelling studies by using D instead of
[
c]
c]
H (see SI for details). The manganese catalysed deuteration of
ethylene carbonate results in the production of methanol with
95% deuterium incorporation in the methyl group, whereas the
1
0
1
99
2
[
1
40
>
[
1
a] Reaction conditions: 1a (1 mmol), Mn-1, base, solvent (0.25 M) at
40 °C under 50 bar of hydrogen for 16 h. [b] Determined by the GC
analysis using m-xylene as internal standard. [c] 1a (5.7 mmol, 500 mg)
ethylene glycol was produced with no deuterium incorporation.
Thus, the deuteration of ethylene carbonate is much faster than
the dehydrogenation-deuteration of EG. This result provides a
practical alternative method for the production of the widely used
deuterated methanol. On the other hand, the treatment of EG
with deuterium at 140 °C caused significant deuterium
substitution in the carbon atoms which indicates the potential of
the manganese catalyst in the dehydrogenative deuteration
reactions.
in 1,4-dioxane (1.46 M), .
With the aim to find the right reaction conditions, ethylene
carbonate was selected as a benchmark substrate for catalytic
hydrogenation of COCs. To our delight, the our complex Mn-1
showed excellent reactivity and provided EG in >99% yield and
methanol in 92% yield when the reaction was performed under
50 bar hydrogen pressure in 1,4-dioxane (Table 1, entry 1).
Importantly, the application of Mn-2 resulted in the formation of
only 14% of EG and no methanol was detected (Table 1, entry
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COMMUNICATION
[
a]
Table 2. Manganese-catalysed hydrogenation of COCs.
The mechanism of the Mn(I)-PNN-catalysed hydrogenation of
[
17]
COCs substrates was also investigated by DFT calculations.
The overall hydrogenation of ethylene carbonate to EG and
methanol can be separated into three independent C═O
hydrogenation events, each with a corresponding catalytic cycle,
see Figure 1.
The first is the hydrogenation of ethylene carbonate (1a) into 2-
hydroxyethyl formate (3); the second is the hydrogenation of 3
into EG (2a) plus formaldehyde (4); and the third is the
yield of diol
yield of
methanol (%)
entry
substrate
[b]
[c]
(%)
3
hydrogenation of 4 into CH OH. In the following, we discuss the
first hydrogenation cycle only, 1a to 3 (Figure 1). The other two
catalytic cycles, composed of similar steps, are shown in the
Supporting Information (Figure S3). Calculations were
performed using the most active Mn-1 catalyst.
During the initialisation process concerning the H
Mn active site (steps A to C, where A is a 16-electron species),
coordination to Mn is a non-spontaneous process demanding
2
addition to the
H
2
–
1
1
4.6 kcal mol
TZVP(Mn) computational level in 1,4-dioxane as solvent and
relative to A. The heterolytic cleavage of H , via transition state
at M06/TZVP//B97XD/SVP(H,C,N,O,P)-
2
–1
B-C (at 21.3 kcal mol relative to A), leads to the hydrogenation
of the catalytic species by hydridation of the Mn centre and
protonation of the non-pyridinic N atom.
The resulting [Mn]-H
2
species C (zero energy reference)
promotes the reduction of the C═O bond of the substrates (1a, 3
and 4) in the three catalytic cycles. The C═O hydrogenation step
is characterised by two main mechanistic events. On the one
hand, the nucleophilic character of the hydride on Mn promotes
2
the hydride transfer from Mn to the sp C atom of the substrate
(
steps D to E). On the other hand, in agreement with literature
proton transfer is achieved through cleavage of the H–H bond of
2
[18]
2
a H molecule η -coordinated to the metal (steps F to H). This
prevents formation of the 16-electron species
A
via
deprotonation of the N–H functionality.
In more detail, the first step is the hydride transfer from the
catalyst to the substrate via transition state D-E at 16.4 kcal
-1
mol , leading to the Mn-alkoxide intermediate E, laying at –3.9
–
1
kcal mol . It can be seen that, as consequence of such hydride
transfer, 1,3-dioxolan-2-olate is transformed into 2-
formyloxy)ethan-1-olate by charge reorganisation in the
substrate. The second step is proton transfer from a coordinated
molecule, releasing the product. In excess of methanol, which
is expected as the reaction evolves, metathesis of E with CH OH
can lead to the methoxide species F. Coordination of H leads to
G and triggers rehydration of the Mn active site and proton
(
H
2
3
2
–
1
transfer to methanolate via transition state G-H, 22.1 kcal mol .
This entails the regeneration of CH OH as well as the catalytic
3
species. Based on calculations, the rate-determining TS along
the cycle is this proton transfer, with an overall activation barrier
of 25.0 kcal mol–1 from E to G-H. Of course, this proton transfer
can also occur without involving initial metathesis of CH OH with
3
E, with very similar reaction steps involving the Mn-aloxide bond
of E.
The impact of the phosphine substituents was analysed by
comparing Mn-1 with catalysts presenting aliphatic and bulky
tBu substituents on the P atom and the less-sterically impeded
iPr.
t
[
a] Reaction conditions: 1 (1 mmol), Mn-1 (0.01 mmol) and KO Bu (0.025
mmol) in dioxane (4 mL) at 140 °C under 50 bar of hydrogen for 16 h. [b]
Isolated yields [c] Determined by the GC analysis using m-xylene as
internal standard. [d] Mn-1 (0.012 mmol) and KO Bu (0.03 mmol).
t
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Figure 1. Proposed reaction mechanism for the three-cascade hydrogenation of ethylene carbonate into methanol plus EG catalysed by Mn-1 (P = PPh
2
). Steps
2
A to C refer to the initialisation by catalyst hydrogenation, while steps D to H concern the substrate hydrogenation and catalyst regeneration by MeOH and  -H
2
–1
assistance. Calculated Gibbs free reaction and activation energies, at 140 °C and 50 bar reaction conditions, are shown in kcal mol at the
M06/TZVP//B97XD/SVP(H,C,N,O,P)-TZVP(Mn) computational level in 1,4-dioxane as solvent. Note: H–N–Mn–H species (C, Cycle 1) is relative zero in energy.
Hydrogenation during the initialisation process of both the
Acknowledgements
alkyl substituted catalysts, via transition state B-C, exhibits
–1
larger barriers, 25.3 and 23.7 kcal mol , compared to Mn-1,
L.M.A. and L.C. acknowledge King Abdullah University of
Science and Technology (KAUST) for support. Gratitude is also
due to the KAUST Supercomputing Laboratory using the
supercomputer Shaheen II for providing the computational
resources.
–
1
2
1.3 kcal mol and relative to A. In this sense, our DFT
predictions are in agreement with experiments suggesting that
Ph
the PNN ligand confers to the manganese catalyst superior
behaviour over those functionalised with aliphatic phosphines.
In conclusion, a new hydrogenation of the CO
2
-derived
COCs to alcohols using a homogenous base metal complex is
[
19]
2
Keywords: manganese • reduction • CO • carbonates •
reported. The reaction is catalysed by a new bench stable Mn-
PNN complex, proceeds with high selectivity under mild
conditions, without generation of any waste or side products. A
variety of 5-and 6-membered COCs were employed to furnish
methanol and vicinal diols in very good to quantitate yields,
providing a milder and indirect route for the production of
methanol
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[17] Optimised geometries were located using the B97XD functional
together with the SVP basis set for main group atoms and the TZVP
basis set for Mn. In order to obtain more accurate energy values,
single-point refinement calculations were done using the M06 functional
and TZVP basis set on all atoms. Solvent effects (1,4-dioxane) were
included via the PCM model at both steps. All calculations have been
performed through the facilities provided by the Gaussian09 package.
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