A Stable Manganese Pincer Catalyst for the Selective Dehydrogenation of Methanol

Article abstract of DOI:10.1002/anie.201610182

For the first time, structurally defined manganese pincer complexes catalyze the dehydrogenation of aqueous methanol to hydrogen and carbon dioxide, which is a transformation of interest with regard to the implementation of a hydrogen and methanol economy. Excellent long-term stability was demonstrated for the Mn-PNPiPr catalyst, as a turnover of more than 20 000 was reached. In addition to methanol, other important hydrogen carriers were also successfully dehydrogenated.

Full text of DOI:10.1002/anie.201610182

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201610182
German Edition: DOI: 10.1002/ange.201610182
Homogeneous Catalysis
A Stable Manganese Pincer Catalyst for the Selective Dehydrogenation
of Methanol
Marꢀa Andꢁrez-Fernꢂndez+, Lydia K. Vogt+, Steffen Fischer, Wei Zhou, Haijun Jiao,
Marcel Garbe, Saravanakumar Elangovan, Kathrin Junge, Henrik Junge, Ralf Ludwig, and
Matthias Beller*
Abstract: For the first time, structurally defined manganese
pincer complexes catalyze the dehydrogenation of aqueous
methanol to hydrogen and carbon dioxide, which is a trans-
formation of interest with regard to the implementation of
a hydrogen and methanol economy. Excellent long-term
stability was demonstrated for the Mn-PNPiPr catalyst, as
a turnover of more than 20000 was reached. In addition to
methanol, other important hydrogen carriers were also suc-
cessfully dehydrogenated.
T
he long-term transition from a fossil-based energy supply to
renewable energies requires the development of efficient
technologies for energy storage. In this respect, the focus has
been increasingly on hydrogen as an environmentally benign
energy carrier.[1] Nevertheless, it is a highly volatile gas and
features very low gravimetric energy density. Hence, other
chemical energy carriers, which can be reversibly (de)hydro-
genated are also considered as viable options. Among these,
methanol is promising because of its comparably high hydro-
gen content and its liquid state under ambient conditions,
which significantly simplifies transportation and handling.[2]
In the last decade, a variety of catalysts have been successfully
developed for the dehydrogenation of methanol, as well as its
use in hydrogen-borrowing reactions.[3] While heterogeneous
systems in general require harsh reaction conditions
(> 2008C), molecularly defined complexes based on ruthe-
nium and iridium show good activities at temperatures below
1008C. Besides these precious catalysts, in recent years non-
noble metals have attracted increasing attention. However, so
far only a few iron-containing complexes were reported for
this transformation (Scheme 1).[4] This year, manganese
pincer catalysts were successfully employed for the first
time in both hydrogenation and dehydrogenation reactions,
though this metal is commonly used in oxidations.[5] Clearly,
manganese features a range of favorable properties as it is low
Scheme 1. A selection of ruthenium, iridium and iron catalysts
reported for the dehydrogenation of aqueous methanol.
toxic, earth abundant, and inexpensive because of its use for
the iron and steel production.[6]
Inspired by these recent achievements, herein we report,
for the first time, the low-temperature methanol reforming in
the presence of a highly stable manganese PNP-pincer
complex. Initially, a range of different manganese catalysts
and precursors were tested following a standard protocol
developed for todayꢀs state-of-the-art homogenous methanol
reforming catalyst (Table 1).[5a,7] Interestingly, the iPr com-
plex 1, the corresponding pincer complex bearing ethyl
groups (3), and the precursor [Mn(CO)5Br] with an additional
10 equivalents of the HN(CH2CH2)P(CH(CH3)2)2 ligand
(from here on named PNPiPr ligand) showed reasonable
activity for methanol dehydrogenation, thus achieving turn-
over numbers (TONs) of 54, 65, and 68, respectively
(entries 1, 3, and 8). During these preliminary tests, high
sensitivity to light irradiation was noted in the case of the
dissolved manganese complex 1, which caused a rapid decline
in its activity. This phenomenon stands in contrast to the
reported stability of this complex in its solid state and is
ascribed to the light-triggered cleavage of the PNPiPr ligand
(see section SI6.2 in the Supporting Information).[5a] Hence,
all described experiments were carried out under rigorous
exclusion of light.
[*] M. Andꢀrez-Fernꢁndez,[+] L. K. Vogt,[+] W. Zhou, H. Jiao, M. Garbe,
S. Elangovan, Dr. K. Junge, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz-Institut fꢂr Katalyse an der Universitꢃt Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
S. Fischer, Prof. Dr. R. Ludwig
Institut fꢂr Chemie, Universitꢃt Rostock
Dr. Lorenz-Weg 1, 18059 Rostock (Germany)
[+] These authors contributed equally to this work.
A variation of Milsteinꢀs catalyst (5) was less active and
catalyst performance fluctuated significantly. The pincer
catalysts 2 and 4, as well as the commercially available
manganese porphyrine complex 6 and [Mn2(CO)10] did not
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201610182.
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Table 1: Different manganese catalysts for the dehydrogenation of
aqueous methanol.[a]
Figure 1. Long-term experiment. Reaction conditions: MeOH/H2O
(20 mL, ratio 9:1), triglyme (20 mL), 8m KOH, 1 (2.1 mmol, 0.05 mm),
10 equiv. PNPiPr ligand; Tset: 928C.
(CO)5Br] precursor plus additional PNPiPr ligand showed
a decline in activity after 100 hours and the activity ceased
entirely after 300 hours (see SI5).
For a better understanding of possible catalytic inter-
mediates, ex-situ IR investigations under reaction conditions
were performed. To address the aspect of the role of the base
in the mechanistic cycle, KOH was added to a 9:1 MeOH/
H2O solution of 1 and samples were measured. Under non-
basic conditions, two bands at 1925 and 1843 cmÀ1 were
Entry
Catalyst
V5h [mL]
TON5h
1[b]
2
1
2
3
4
5
6
11
–
13
–
9
–
54
–
65
–
41
–
–
3[b]
4
5
6
7
[Mn2(CO)10]
[Mn(CO)5Br] + 10 equiv PNPiPr ligand
–
14
8[b]
68
À
detected and assigned to the symmetric and asymmetric C O
[a] Reaction conditions: MeOH/H2O (5 mL, ratio 9:1), 8m KOH
frequencies of 1. Already when adding 0.5 equivalents of
KOH, additional bands appeared at slightly lower wave-
numbers (1910 and 1825 cmÀ1). At 10 equivalents of KOH full
conversion into this new species was reached (Figure 2). To
identify this complex, NMR investigations were carried out
(see SI6): Dissolving 1 in a basic MeOH/H2O solution lead to
the appearance of two species [31P NMR (additional MeOD-
d4): d = 84.7 (bs, major species) and 86.5 ppm (bs, minor
species]. The formation of the hydride complex 12 could be
excluded as neither a hydride signal was detected in the
1H NMR spectrum nor did the 31P signals match the
previously reported shift of d = 109.6 ppm for the hydride
complex.[5a] Next, the amido complex 8 was generated by
(molarity based on total MeOH + H2O volume), catalyst (8.4 mmol,
1.68 mm); Tset: 928C. Gas volumes were determined using manual or
automatic gas burettes and were analyzed by GC. Each molecule of
hydrogen is counted as one turnover. All gas measurements were
performed at least twice and corrected by blank values. [b] Standard
deviation is less than 15%.
show measurable activities and gas evolution did not exceed
blank values (Table 1, entries 2, 4, 6 and 7).
Further investigations of critical reaction parameters
concentrated on the catalyst 1 and its precursor [Mn(CO)5Br]
because of their promising activities and easy availability
compared to the manganese ethyl complex 3 (see SI3).
Replacing KOH by other bases, such as tBuOK or LiOH, led
to either a significant decrease in activity or to complete
deactivation of the catalyst, as in the case of the latter.
Analogous to the recent findings reported by Boncella and
co-workers, the addition of LiBF4 completely shut down
catalytic activity.[5d] On the other hand, catalytic activity was
improved by the addition of 10 equivalents of the PNPiPr
ligand. A long-term stabilizing effect for a reaction time of
more than ten hours was achieved by using triglyme as
solvent.
To gain better insight into the catalytic systems based on
the iPr complex and the precursor [Mn(CO)5Br], and to test
their stability, long-term experiments were performed under
optimized reaction conditions (Figure 1). Remarkably, the
combination of 1 in the presence of additional PNPiPr ligand
proved to be stable for longer than a month, thus reaching
a TON of more than 20000. Notably, this manganese catalyst
exceeded the stability of the related Fe-iPr complex, which
was only stable up to five days.[4e] In contrast, the [Mn-
Figure 2. IR measurements of the active catalytic species in depend-
ence on base molarity. Reaction conditions: MeOH/H2O (1 mL, ratio
9:1), 1 (10 mmol, 10 mm), RT.
2
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detected after five minutes and its concentration gradually
increased during the reaction. This latter band was assigned
by adding formic acid to the basic reaction solution. Based on
all these findings and in agreement with DFT calculations,[5a,9]
we propose the mechanistic cycle shown in Scheme 2.
Initially, the active amido species 8 is generated by either
base-mediated dehydrobromination of 1 or by dehydration of
the hydroxide complex 7. By coordination of methanol, the
À
methoxide species 9 is formed, and it undergoes C H
cleavage to form formaldehyde and 12. In the last step
hydrogen is produced and the active catalyst is regenerated.
Finally, we were interested in the performance of our
novel dehydrogenation catalyst for other important C1 and C2
substrates. Thus, reactions of paraformaldehyde, formic acid
and ethanol were investigated in the presence of 1.
To our delight, the catalyst was active for all these
substrates and activity was still linear after 14 hours. For
EtOH as a substrate a TON of 163 after five hours (Table 2,
Scheme 2. Proposed catalytic cycle for the manganese-catalyzed aque-
ous-phase reforming of methanol.
adding base to the catalyst in benzene-d6 to prove the
occurrence of possible intermediates in the catalytic cycle
such as the methoxide 9, hydroxide 7 or formate 11 complexes
(Scheme 2).
Table 2: Dehydrogenation of different substrates with 1.
Entry
Substrate
V5h [mL]
TON5h
1[a]
EtOH
paraformaldehyde
formic acid
33
32
73
163
79
283
2[b,d]
3[c,d]
Addition of water confirmed that the minor peak in the
31P NMR spectra can be attributed to the hydroxide species 7.
The analogous experiment of adding methanol and formic
acid resulted in the appearance of broad peaks in the
31P NMR spectrum with very similar chemical shifts to the
major peak. Hence, it was not possible to unambiguously
differentiate between the methoxide and formate complex by
NMR experiments.[8] However, calculated IR shifts support
the assumption that the prevalent species is the methoxide
complex 9 and that the corresponding hydroxide complex 7 is
present to a minor extent (see SI8 and Figure SI14). The
bands of the latter are the reason for the very broad tailing of
[a] Reaction conditions: EtOH/H2O (5 mL, ratio 9:1), KOH (0.04 mol,
8m), 1 (8.4 mmol, 1.68 mm); Tset: 928C. [b] Reaction conditions: tBuOH
(36 mL), H2O (4 mL), KOH (0.05m), paraformaldehyde (4 mmol, 0.1m),
1 (8.28 mmol, 0.21 mm); Tset: 818C. [c] Reaction conditions: PC (5 mL),
11 molFA/10 mol DMOA (5 mL), 1 (5.3 mmol, 0.53 mm); Tset: 608C.
[d] Concomitant production of CO2 (ratio H2/CO2 1:1, analyzed by GC).
entry 1) was obtained. Dehydrogenation of paraformalde-
hyde in a slightly basic tBuOH solution at 818C resulted in
a TON of 80. Following a formic acid dehydrogenation
protocol,[10] the manganese catalyst showed significantly
higher activity, thus reaching a TON of 283 (entries 2 and 3).
In conclusion, we have demonstrated for the first time that
pincer complexes based on nontoxic and easily available
manganese successfully promote the selective dehydrogen-
ation of aqueous methanol. Under optimized reaction con-
ditions, an impressive TON of more than 20000 was attained
and 1 was still active after one month. Although for practical
purposes the general activity still has to be improved, these
promising results show that such molecularly defined non-
noble metal catalysts can be extremely stable and robust,
which is also of importance for future applications.
the bands, which can be observed at 1910 and 1825 cmÀ1
.
To clarify the temperature influence on the catalytic
species, the reaction solution containing 10 mm of the catalyst
was heated from room temperature to 908C (Figure 3).
Similar to the previous investigations, both methoxide and
hydroxide species were observed. Additionally, formate was
Experimental Section
All reactions were carried out under argon atmosphere with
exclusion of air and light using Schlenk techniques. Specific reaction
procedures can be found in the Supporting Information.
Acknowledgments
We thank Dr. E. Alberico, for her excellent support and very
helpful discussions. We also thank P. Bartels for her out-
Figure 3. Reaction monitoring by IR. Reaction conditions: MeOH/H2O
(10 mL, ratio 9:1), 8m KOH, 1 (100 mmol, 10 mm), RT À908C.
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Keywords: dehydrogenation · homogeneous catalysis ·
ligand design · manganese · methanol
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Manuscript received: October 18, 2016
Final Article published: && &&, &&&&
4
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These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Homogeneous Catalysis
M. Andꢁrez-Fernꢂndez, L. K. Vogt,
S. Fischer, W. Zhou, H. Jiao, M. Garbe,
S. Elangovan, K. Junge, H. Junge,
R. Ludwig, M. Beller*
&&&& — &&&&
Just a pinch: A molecularly defined
manganese PNP pincer complex selec-
tively dehydrogenates methanol to
hydrogen and carbon dioxide at low
temperature. Remarkable stability and an
excellent turnover (TON) of more than
20000 was achieved.
A Stable Manganese Pincer Catalyst for
the Selective Dehydrogenation of
Methanol
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