Catalytic Hydrogenation of Cyclic Carbonates using Manganese Complexes

Article abstract of DOI:10.1002/anie.201808676

Catalytic hydrogenation of cyclic carbonates to diols and methanol was achieved using a molecular catalyst based on earth-abundant manganese. The complex [Mn(CO)₂(Br)[HN(C₂H₄PⁱPr₂)₂] 1 comprising commercially available MACHO ligand is an effective pre-catalyst operating under relatively mild conditions (T=120 °C, p(H₂)=30–60 bar). Upon activation with NaO^tBu, the formation of coordinatively unsaturated complex [Mn(CO)₂[N(C₂H₄PⁱPr₂)₂)] 5 was spectroscopically verified, which confirmed a kinetically competent intermediate. With the pre-activated complex, turnover numbers up to 620 and 400 were achieved for the formation of the diol and methanol, respectively. Stoichiometric reactions under catalytically relevant conditions provide insight into the stepwise reduction form the CO₂ level in carbonates to methanol as final product.

Full text of DOI:10.1002/anie.201808676

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Accepted Article
Title: Catalytic Hydrogenation of Cyclic Carbonates using Manganese
Complexes
Authors: Walter Leitner, Akash Kaithal, and Hölscher Markus
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808676
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Catalytic Hydrogenation of Cyclic Carbonates using Manganese
Complexes
Akash Kaithal[a], Markus Hölscher[a], and Walter Leitner*[a][b]
Abstract: Catalytic hydrogenation of cyclic carbonates to diols and
methanol was achieved using a molecular catalysts based on earth
abundant manganese. The complex [Mn(CO)2(Br)[HN(C2H4PiPr2)2] 1
comprising commercially available MACHO ligand is an effective
pre-catalyst operating under relatively mild conditions (T=120°C,
p(H2)=30-60 bar). Upon activation with NaOtBu, the formation of
coordinatively unsaturated complex [Mn(CO)2[N(C2H4PiPr2)2)] 5 was
spectroscopically verified, which proved a kinetically competent
intermediate. With the pre-activated complex, turnover numbers up
to 620 and 400 were achieved for the formation of the diol and
methanol, respectively. Stoichiometric reactions under catalytically
relevant conditions provide insight into the stepwise reduction form
the CO2 level in carbonates to methanol as final product.
carbonates using Ru-MACHO pincer catalyst with TONs up to
87000 (Scheme 1).[9] The diols formed as stoichiometric co-
product are valuable products, making this transformation in
particular also as an attractive synthetic approach to these
compounds.[10] In fact, the carbonate group has been suggested
as valuable protecting group for diols in organic synthesis.[11]
Currently, there is a strong interest in the hydrogenation of CO2
to methanol for the chemical supply chain and within the context
of novel energy carriers.[1,2] The synthesis of methanol by
hydrogenation of CO2 using molecular catalyst systems has
been successfully addressed by various working groups in
recent years. The field was pioneered in 2011 by Milstein and
coworkers who demonstrated the possibility to reduce CO2
indirectly via hydrogenation of isolated derivatives such as
cabonates, carbamates and formates using Ru pincer complex
under mild conditions (Scheme 1).[3] Seminal contributions in this
field were made by the groups of Sanford,[4] Prakash and Olah,[5]
and Klankermayer and Leitner.[6] The latter work demonstrated
that there are also direct pathways that do not require a stable
organic intermediate.[6a] While these studies used noble metals
as active catalytic centers, few examples employing non-noble
metal catalyst systems have also been described. The group of
Beller reported a catalytic system based on Cobalt reaching
turnover numbers (TONs) of up to 78.[7] Recently, Prakash et. al.
reported a Manganese catalyzed sequential hydrogenation of
CO2 to methanol via a pre-formed formamide reaching TONs up
to 36.[8]
Scheme 1: Molecular homogeneous catalysts for the hydrogenation of
carbonates (n = 0, 1; R1-R4 = H or alkyl).
The potential of hydrogenation catalysts based on
abundant and cheap third-row metal Manganese and its pincer
complexes[12] has been pointed out by the groups of Milstein[13]
and Beller[14] in 2016. In particular, complexes of this type have
been successfully employed in the hydrogenation of CO2 to
formic acid derivatives.[5,15] Encouraged by these reports, we
Cyclic carbonates are particularly attractive as
intermediate CO2 derivatives in this context, as they can be
formed readily from the reaction of CO2 with the corresponding
epoxides or oxetanes. In 2012, Ding et. al. reported a highly
efficient catalytic system for the hydrogenation of cyclic
were able to develop
hydrogenation of cyclic carbonates to diols and methanol using
manganese complex comprising the well-known and
a catalytic protocol for catalytic
1
commercially available PNP pincer ligand MACHO. While this
manuscript was in preparation, we became aware of two
independent parallel studies employing newly synthesized PNN-
pincer ligands.[16,17]
Initial studies were conducted using ethylene carbonate 6a
as a benchmark substrate (Table 1). Reactions were carried out
with 1 mol% of catalyst in presence of NaOtBu as basic co-
catalyst (2 equiv.) in THF (0.7 mL) at 120 oC under 30 bar of H2.
While complex 2 - 4 showed only low activity, the Mn-MACHO
complex 1 gave 86% conversion already under screening
[a]
[b]
M.Sc. A. Kaithal, Dr. M. Hölscher, Prof. Dr. W. Leitner
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Worringer Weg 2, 52074 Aachen, Germany
Prof. Dr. W. Leitner
Max Planck Institute for Chemical Energy Conversion
Stiftstraße 34-36, 45470 Mülheim a.d. Ruhr, Germany
E-Mail: leitner@cec.mpg.de
Supporting information for this article is given via a link at the end of
the document.
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conditions. Ethylene glycol (7a) was detected by standard
analytical techniques in quantities corresponding to nearly
perfect selectivity, while the yield of methanol was consistently
somewhat lower. Formate esters were observed as side
products in small amounts to account for the difference at least
partly. Complex 1 was still quite active at 100 oC, reaction giving
74% conversion with 71% yield to 7a and 62% yield to methanol.
Next, we investigated the influence of catalyst and co-
catalyst loading on catalyst performance with the most promising
pre-catalyst 1 (Table 1, entries 6-9). Increasing the loading of 1
to 2 mol% lead to almost full conversion of 6a and 90% yield of
7a. Reducing the Mn loading to 0.5 mol% lead to 70%
conversion and 66% yield of 7a corresponding to a TON of 132.
At a loading as low as 0.2 mol% for complex 1 and 0.5 mol% for
NaOtBu and extended reaction time of 40 h, TONs of 240 for 7a
and 175 for methanol were obtained, respectively.
Table 1. Manganese catalyzed hydrogenation of 6a: Influence of catalyst
precursors and reaction conditions.a,b
Interestingly, the reaction showed significantly higher
TONs to the corresponding products, when complex 1 was pre-
activated with NaOtBu in THF for 30 min at room temperature.
After removal of all volatiles, the nearly quantitative formation of
1
the unsaturated complex 5 was verified by H and 31P{1H} NMR
spectroscopy (Scheme 2).[18] The remaining solid was re-
dissolved in THF and used directly without further purification. At
a 0.2 mol% of manganese loading, the use of 5 at 60 bar
pressure of hydrogen after 14 h gave 38% conversion with 187
TONs to diol and 129 TONs to MeOH (table 1, entry 10).
Gratifyingly, when the reaction time was increased to 40 h, 70%
conversion was achieved with 336 TONs to diol and 220 TONs
to methanol (Table 1, entry 11). The best results were found with
0.1 mol% manganese, leading to 66% conversion corresponding
to TONs of 620 for 7a and 400 for methanol, respectively (Table
1, entry 12).
[Mn]
No.
[Mn]
(mol%)
NaOtBu
(mol%)
H2
(bar)
t
(h)
X
(%)
7a(%)
(TON)
8 (%)
(TON)
entry
1
86
(86)
62
(62)
1
2
1
1
2
2
30
30
30
30
30
30
30
30
60
60
60
60
26
26
26
26
26
26
26
40
40
14
40
40
86
23
23
10
74
98
70
56
60
38
70
66
10
(10)
4
(4)
2
14
(14)
3
(3)
3
3
1
2
11
(11)
5
(5)
4
4
1
2
The substrate scope of the reaction was assessed for
complex 1 using the standard protocol without pre-activation for
the hydrogenation of cyclic 5-membered and 6-membered
carbonates. Reaction of 5-membered cyclic carbonates in THF
71
(71)
62
(62)
5$
6
1
1
2
90
(45)
75
(38)
o
1
2
3
(0.7 mL) at 120 C under 30 bar of H2 using 1 (1-2 mol%) and
NaOtBu (1.1 equiv.) revealed conversion up to >99% with yield
up to 97% for diol and 74% to methanol (Table 2, entry 1-7). The
activity of complex 1 for the hydrogenation of six membered ring
carbonates was equally high and even exceeded that observed
for the five ring substrates under the same conditions.
Hydrogenation of six membered cyclic carbonates under
standard conditions showed conversion up to >99% with yield up
to 98% for diol and 94% for methanol. (Table 2, entry 8-11).
66
(132)
46
(92)
7
1
0.5
0.2
0.2
0.2
0.2
0.1
1
48
(240)
35
(175)
8#
9#
10#
11#
12#
1
0.5
0.5
-
52
(260)
30
(147)
1
37
(187)
26
(129)
5*
5*
5*
67
(336)
44
(220)
-
62
(620)
40
(400)
-
aConditions: 6a (44 mg, 0.5 mmol), H2, Mn complex, NaOtBu, temp. (120 oC),
THF (0.7 mL). bYield was calculated using gas chromatography, ethyl
heptanoate (25 μL, 0.15 mmol) was used an internal standard. $T=100 oC. #6a
(440 mg, 5 mmol), H2, Mn complex , NaOtBu, temp. (120 oC), THF (2 mL).
*Pre-catalyst 1 was treated with NaOtBu in THF for 30 min at r.t.; reaction
mixture was passed through celite, after removal of all volatiles, the formation
of 5 was verified by 31P{1H} NMR spectroscopy and it was used directly without
further work-up.
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Table 2. Hydrgenation of cyclic carbonates using Mn complex 1 as a pre-
catalyst.a, b
and methanol (see Figure S3, S4 and S5, Scheme 2, entry d).
These results confirm the high reactivity of the Mn-MACHO
framework for the sequential transfer of H2 formally as H- and H+
to the C=O units of CO2-derivatives along the reduction path to
methanol.
Diol
(7, %)
[Mn]
(mol%)
Conv.
(%)
Methanol
(8, %)
31P{1H} NMR: δ 112.97 ppm
Entry
Carbonate
1
2
3
4
1
2
1
2
89
>99
86
75
82
82
97
66
75
65
74
31P{1H} NMR: δ 109.21 ppm
1H NMR: δ -5.63 ppm
6b
>99
6c
5
1
82
76
66
6d
6e
31P{1H} NMR: δ ( ppm)
1H NMR: δ (ppm)
6*
7*
8
1
2
1
2
67
77
79
97
59
71
58
80
43
60
41
75
6f
9
Scheme 2: Formation of presumed Mn(I) intermediates and their reactivity
towards organic substrates; P = PiPr2.
10
11
1
1
>99
80
98
73
94
57
6g
The observed species can be related directly to a plausible
mechanism for the catalytic cycle in line with previous reports for
other metals (Scheme 3). Activation of complex 1 leads to the
catalyst species I, which corresponds to the experimentally
observed complex 5. Heterolytic cleavage of the H2 molecule
across the Mn=N double bond leads to Intermediate II detected
as the mono-hydride complex 9. The carbonyl group of the
substrate interacts with the protic N-H hydrogen activating the
C=O unit for nucleophilic attack by Mn-bound hydride. This leads
to an ortho-ester as first organic reduction product. Cyclic ortho-
esters are typically more stable in the isomeric open form of the
formate ester and hydroxyl functionality, in line with the detection
of this species in the control reactions. This principle of H-H
addition to the C=O unit can be re-iterated until the methanol
stage is reached. Coordination of CH3OH to species I would
explain the presence of complex 10.
6h
aConditions: 6 (0.5 mmol), H2 (30 bar), complex 1, NaOtBu (1.1 equiv. with
respect to 1), THF (0.7 mL), temp. (120 oC), 26 h. bYield was calculated using
gas chromatography, ethyl heptanoate (25 μL, 0.15 mmol) was used as an
internal standard. * 1, 3-Propanediol (25 μL, 0.34 mmol) was used as an
internal standard.
A series of control reactions were performed to get insight
into possible intermediates resulting from pre-catalyst 1 and their
reactivity towards the organic substrates. Reaction of complex 1
with NaOtBu leads to the formation of complex 5 (Scheme 2,
entry a), which proved a kinetically competent intermediate in
the catalytic experiments summarized in table 1. In presence of
hydrogen (5 bar), the Mn-monohydride complex 9 is formed
The catalytic cycle and the observed intermediates are in
accord with the DFT calculations reported by the Rüping group
for their PNN-ligand system. Their data associated the highest
activation barrier with the initial hydride transfer to 6a.[17] This in
line with the observation of organic products at all reduction
levels down to methanol upon reaction with the hydride complex
within 30 minutes in toluene-d8 as confirmed by 1H NMR and 31
P
NMR spectrum (1H NMR: δ -5.63 ppm (t, J = 51 ppm, Mn-H);
31P{1H} NMR: δ 109.21 ppm, Scheme 2, entry b). Adding 3
equivalents of ethylene carbonate 6a to this solution
regenerated the unsaturated Mn complex 5 according to 31P{1H}
NMR spectra together with the smaller amounts of the methanol
coordinated manganese complex 10 (see Figures S2).[8] The
formate ester of ethylene glycol 11a and free formaldehyde 12
were observed as organic products in the 1H NMR spectrum
(11a:12 = 75:25) (see Figure S1; Scheme 2, entry c). Treating
complex 9 with methyl formate lead also to formation of
manganese complex 5 and 10 together with free formaldehyde
9. However,
a
concerted H2 activation and transfer
mechanism[19] as suggested for the ligand framework introduced
by Milstein cannot be excluded.[16] The two pathways follow
basically the same bond breaking and bond formation trajectory
with similar energy spans, differentiated by the existence of
structures akin to 9 as intermediates or off-loop species. Further
studies are required to elucidate these alternatives for
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10.1002/anie.201808676
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manganese catalyzed CO2 hydrogenation, which may well
depend on the individual ligand framework given their close
similarity.
Experimental Section
Standard protocol for the catalytic hydrogenation of cyclic
carbonates to diols and methanol using Mn catalyst 1: The catalytic
reactions were carried out in externally heated 20 mL stainless steel
reactors equipped with a glass inlet and a magnetic stir bar. Mn-complex
1 and NaOtBu were weighed into the glass inlet inside a glovebox. The
glass inlet was closed with a septum and transferred into the bottom part
of the steel autoclave, where it was opened under a stream of argon.
After sealing, the autoclave was purged with argon three times. The
required amounts of carbonates 6 and THF were added by syringe
through a needle valve under argon flow at room temperature. The
autoclave was sealed and pressurized with hydrogen gas and heated to
the reaction temperature. After the given reaction time, the autoclave was
cooled to room temperature and carefully vented under continuous
stirring. Ethyl heptanoate was added as an internal standard and the
resulting solution was analyzed by gas chromatography.
Protocol with pre-activated complex 5: A solution of complex 1 (0.1
mmol) and NaOtBu (0.12 mmol) in THF (0.5 mL) was stirred for 30 min at
r.t. and the formation of 5 was checked by 31P{1H} NMR spectroscopy.
After 30 minutes, the reaction mixture was passed through a short pad of
celite. After thorough removal of all volatiles under vacuum, the solid was
re-dissolved in THF. This solution was used for catalysis following the
standard protocol.
Acknowledgements
This work was carried out as part of our activities in the Cluster
of Excellence “Tailor-Made Fuels from Biomass” funded by the
Excellence Initiative by the German federal and state
governments to promote science and research at German
universities (EXC 236) and within the Kopernikus Project “P2X:
Flexible use of renewable resources – exploration, validation
and implementation of ‘Power-to-X’ concepts” supported by the
German Federal Ministry of Education and Research (BMBF)
(Grant Number 03SFK2A). A.K. thanks the the Erasmus Mundus
Action 1 Programme (FPA2013-0037) “SinChem” for a stipend.
We thank Prof. Dr. Magnus Rüping for sharing the results of his
group prior to publication.
Scheme 3: Possible mechanism for catalytic hydrogenation of cyclic
carbonates using the Mn-MACHO complex
1 as pre-catalyst; starting
compounds and products are shown in blue, experimentally detected
complexes and intermediates are highlighted in red.[19]
In
conclusion,
the
manganese
was identified as efficient
complex
[Mn(CO)2(Br)[HN(C2H4PiPr2)2]
1
catalyst for the hydrogenation of cyclic carbonates. The complex
is readily accessible from commercially available starting
materials comprising the PNP pincer ligand MACHO. High yields
for the diols and for methanol were achieved under relatively
mild conditions, corresponding to up to 620 TON and 400 TON,
respectively. These data match or even surpass the
performance of the PNN-ligand based catalysts developed in
parallel studies.[16,17] The formation of unsaturated complex 5
and the hydride complex 9 was validated under relevant reaction
conditions, corresponding to two crucial intermediates of a
potential catalytic cycle for this reaction. In particular, their
reactivity towards H2 and the organic substrates substantiated
Keywords: carbon dioxide • hydrogenation • carbonates
•methanol • manganese
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