Iridium–PNP Pincer Complexes for Methanol Dehydrogenation at Low Base Concentration

Article abstract of DOI:10.1002/cctc.201700015

A catalytic system based on an iridium–PNP complex was developed for the aqueous-phase reforming of methanol. Investigations revealed higher activity at low base concentrations and higher stability at high base concentrations as an opposing trend relative

Full text of DOI:10.1002/cctc.201700015

Accepted Article
Title: Iridium-PNP Pincer Complexes for Methanol Dehydrogenation At
Low Base Concentration
Authors: Christoph Prichatz, Elisabetta Alberico, Wolfgang Baumann,
Henrik Junge, and Matthias Beller
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201700015
Link to VoR: http://dx.doi.org/10.1002/cctc.201700015
A Journal of
www.chemcatchem.org

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
Iridium-PNP Pincer Complexes for Methanol Dehydrogenation At
Low Base Concentration
Christoph Prichatz,[a] Elisabetta Alberico,[a] [b] Wolfgang Baumann,[a] Henrik Junge[a] and Matthias
Beller*[a]
Abstract: A catalytic system based on an iridium-PNP complex
was developed for the aqueous phase reforming of methanol.
Investigations revealed higher activity at low base concentration
and higher stability at higher base concentration as an opposing
trend compared to known Ru and Fe PNP pincer containing
systems for methanol dehydrogenation. During mechanistic
investigations it was possible to identify a carbonyl species of
our catalyst as deactivation species.
molecules of hydrogen and one molecule of carbon dioxide
(aqueous phase methanol reforming).[9] Noteworthy, in this
process the dehydrogenation of formic acid is also involved as
one of the reaction steps.
The world energy demand constantly increases and still
around 80 % of the consumed energy is based on fossil fuels.
Hence, the need for more sustainable alternatives is one of the
central challenges for the coming decades.[1] Among the various
forms of renewable energy, wind and sun light are essential
sources. However, due to their intermittent character,
applications are limited.[2] Hence, the development of energy
storage systems is indispensable. While batteries are well
established on small scale with comparable short operation
times, chemical energy storage with hydrogen as carrier avoids
these limitations.[3] Hence, a surplus of renewable electricity can
be used for the straightforward generation of hydrogen by water
electrolysis.[4] Later on, the stored energy can be released on
demand by combustion of hydrogen both directly or in a fuel cell,
which is a clean process with just water as side product.
Hydrogen can be physically stored only in high pressure tanks
(350-700 bar) or as liquefied gas (-253 °C). Unfortunately, it is a
flammable gas, able to diffuse through most commonly used
materials. Chemical storage overcomes these disadvantages by
reversible conversion of H2 into (liquid) hydrogen enriched
molecules. In this respect, methanol,[5] formic acid, methane as
well as aromatic hydrocarbons (LOHC) are investigated as
promising storage compounds.[6] Among these materials,
especially methanol has a high gravimetric hydrogen content of
Scheme 1. General pathway for aqueous MeOH dehydrogenation (blue) and
CO2 hydrogenation to methanol (red).
In addition to high activity and stability, the selective
generation of hydrogen without forming carbon monoxide is
crucial for fuel cell applications.[10] In this respect, the low
temperature methanol dehydrogenation processes which can be
achieved with homogeneous catalysts are interesting. Already in
1985, Saito’s group published the first example of
a
homogenously catalyzed dehydrogenation of aqueous methanol,
albeit with a low turnover number (TON = 34).[11] More recently,
significantly more efficient molecularly-defined complexes were
disclosed for selective dehydrogenation of aqueous methanol at
low temperatures and ambient pressure (Figure 1).
[8]
12.6 % [7] and can be generated by hydrogenation of CO2 or
from natural sources.
As shown in Scheme 1, the dehydrogenation of
methanol/water mixtures in the presence of
a suitable
homogeneous or heterogeneous catalyst evolves three
[a]
[b]
C. Prichatz, Dr. E. Alberico, Dr. W. Baumann, Dr. H. Junge, Prof. Dr.
M. Beller
Leibniz-Institut für Katalyse e. V. an der Universität Rostock
Albert-Einstein-Straße 29a, 18059 Rostock
E-mail: matthias.beller@catalysis.de
Homepage: catalysis.de
Dr. E. Alberico
Instituto di Chimica Biomolecolare, CNR, Sassari (Italy)
Figure 1. Recently reported homogeneous catalysts for MeOH
dehydrogenation.
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
In 2013, our group reported the ruthenium based complex
[Ru(H)(Cl)(CO)(HN(CH2CH2P(iPr)2)2)] (3) as a highly productive
catalyst reaching TON >350,000.[12] At the same time
Grützmacher and co-workers showed the dehydrogenation of
methanol by utilizing [K(dme)2][RuH(trop2dad)] (1) without any
rate-determining step under identical reaction conditions is
different.
To understand the effect of base in more detail, we
performed reactions at different base concentrations in the
presence of complex 8. All reactions were carried out at the
reflux temperature allowed by the reaction solution composition.
Interestingly, decreasing the amount of base from 2M to 0.1M
gradually increased the catalyst activity from TOF1h = 134 to 525,
respectively (Table 2, entries 3-6). At lower base loading (8.4
µmol KOH; 2 eq. with respect to Ir) a major loss of activity
(TOF1h of 47) was observed. No gas evolution occurred in the
absence of base regardless of the catalyst either the Cl-
containing complex 8 (Table 2, entry 1) or its trihydride analogue
additives in THF (TON
=
540).[13] Later on, the first
system based on
homogeneous bi-metallic
[Ru(H)(BH4)(CO)(HN(CH2CH2P(iPr)2)2)] and [Ru(H)2(dppe)2] (7)
was reported under base-free conditions (TON = 4,286).[14]
Furthermore,
Milstein
and
co-workers
applied
[Ru(H)(Cl)(CO)(NNPtBu)] (6) as catalyst for this reaction and
obtained a high TON (28,661) under optimized conditions.[15]
Although several iridium catalysts are known for alcohol[16] and
formic acid [17] dehydrogenation, interestingly only complex 5 has
been described by Yamaguchi and co-workers to catalyze the
dehydrogenation of aqueous methanol (TON >10,000).[18]
Apart from all these precious metal-based catalysts, only
few iron pincer complexes were successfully applied for
aqueous phase reforming (APR) of methanol. For example,
complex 2 ([Fe(H)(BH4)(CO)(HN(CH2CH2P(iPr)2)2)]) gave a TON
of around 10,000 under strong basic conditions,[19] while
Bernskoetter, Hazari and Holthausen showed that the related
complex 4 reached up to 50,000 turnovers in combination with a
Lewis acid as co-catalyst.[20] Most of these examples show the
general viability of different pincer complexes for the
dehydrogenation of methanol. However, so far all catalysts with
PNP pincer ligands[21] need large amounts of additives like
strong bases[12b] or Lewis acids[20a] to achieve high activity and
productivity. Hence, there is a continuing interest to perform
methanol dehydrogenation reactions under milder conditions.
In this respect, hereby we describe for the first time the
successful application of Ir PNP pincer complexes in the APR of
methanol at low base concentration and the elucidation of
catalyst deactivation pathways.
9
[23a]. Also no hydrogen was detected using complex 8 with 8M
KOH at a temperature of 70 °C (Table 2, entry 8).
When different metal hydroxides were tested, 0.5 M KOH
led to better results (TOF1h = 326; Table 2, entry 4) than NaOH
(TOF1h = 189; Table 2, entry 9) and LiOH (TOF1h = 127; Table 2,
entry 10). Aqueous methanol reforming has been shown to be
favoured also by a Lewis acid co-catalysts.[20a] However in the
presence of LiBF4 (Table 2, entry 11) we did not observe any
activity.
Table 1. Different catalysts tested for methanol dehydrogenation.
Entry
Catalyst
Base [molL-1]
KOH 8.0
KOH 8.0
KOH 8.0
KOH 8.0
KOH 8.0
KOH 8.0
KOH 8.0
KOH 0.5
KOH 0.5
KOH 0.5
T [°C][a]
91
TOF1h [h-1]
205
702
2276
97
TOF3h [h-1]
107
510
2205
57
1
2
3
4
5
6
7
8
9
10
1
At the start of this project different ruthenium, iron and
iridium PNP pincer complexes were compared under previously
optimized reaction conditions (4.2 µmol catalyst, 10 mL
methanol/water (9:1), 8.0 M KOH at reflux, 91°C). The highest
2
91
3
91
8
91
activity was observed when using ruthenium complex 3 (TOF1h
=
2276; Table 1, entry 3), followed by the iron complexes 2 (TOF1h
= 702; Table 1, entry 2) and 10 (TOF1h = 429; Table 1, entry 6).
Surprisingly, the related iron complex 11, which gave improved
performance in hydrogenation reactions [22] exhibited no activity
at all. As shown in Table 1, under these conditions (high base
9
91
95
54
10
11
2
91
429
-
302
-
91
concentration)
both
iridium
complexes
8
and
9
70
10
8.3
([Ir(H)2(Cl)(HN(CH2CH2P(iPr)2)2)]
and
3
70
45
48
[Ir(H)3(HN(CH2CH2P(iPr)2)2)]) were considerably less active
compared to the related ruthenium and iron pincer complexes
(Table 1, entries 1-7), although 8 had been succesfully already
applied in various (de)hydrogenation reactions.[23]
8
70
326
268
Reaction conditions: 10 mL of a 9:1 v:v mixture of MeOH and water, 4.18 µmol
of catalyst, Tset at 94°C (reflux), evolved gas analysed by gas-phase GC, each
molecule of H2 counted as one turnover, [a] inner temperature, reproduction
differed less than 15%.
Next, three of these complexes were tested using a 0.5M
KOH aqueous methanol solution. Unfortunately, previously
identified state-of-the-art catalysts proved to be much less active
in this case (Table 1, entries 8-9). To our surprise, the
[Ir(H)2(Cl)(HN(CH2CH2P(iPr)2)2)] complex 8 was the most active
system (TOF1h = 326; Table 1, entry 10). While the activity of the
Ru complex 3 dropped by a factor of 50 (Table 1; entries 3, 9)
and that of the iron complex 2 by a factor of 70 (Table 1; entries
2, 8), the iridium complex 8 showed a more than threefold
increase of activity (Table 1; entries 4, 10). Clearly, this indicates
that the related catalysts, Ru and Fe on one side and the Ir one
on the other, operate according to different mechanisms, or the
Next, the influence of water amount relative to methanol on
the activity of 8 was investigated. using neat methanol and
mixtures of methanol and water of 9:1, 8:2 and 5:5 (v:v) (Table 2,
entries 4, 12 and 13). Contrary to the behaviour observed for
previously reported ruthenium- and iron-based systems,[12,19] we
observed an increase of activity from using neat methanol
(Table 2, entry 14, TOF = 223) to methanol/water (9:1) (Table 2,
entry 4, TOF = 326). However, increasing the water amount
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
CO2 were detected in the gas phase. The addition of 44.4 mmol
potassium carbonate (20 % with respect to methanol, 10,000
equivalents with respect to Ir) as a potential catalyst poison
showed no significant decrease of activity (TOF1h = 282; Table 2,
entry 15). Interestingly, by addition of 44.4 mmol potassium
formate as simulation of accumulation during reaction, a lower
activity was achieved (TOF1h = 243, TOF3h = 148; Table 2, entry
16) showing that in this case, a high amount of formate could
negatively influence activity. In order to investigate the peculiar
Table 2. Ir-catalyzed APR of methanol: Influence of additives.
Entry
1
Catalyst
Base [molL-1]
-
T [°C][a]
65
TOF1h [h-1]
-
TOF3h [h-1]
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
-
2
8.36 x10-6
KOH 0.1
KOH 0.5
KOH 1.0
KOH 2.0
KOH 8.0
KOH 8.0
NaOH 0.5
LiOH 0.5
LiBF4 0.5
KOH 0.5
KOH 0.5
KOH 0.5
KOH 0.5
KOH 0.5
65
47
deactivated
3
69
525
326
183
134
97
220
268
160
96
4
70
role
of
the
formate
in
more
detail,
complex
[Ir(H)2(OOCH)(HN(CH2CH2P(iPr)2)2)], which constitutes
a
5
71
possible catalytic intermediate, was synthesized and tested for
its activity.[24] With this complex an almost similar activity (4.2
6
73
µmol cat, 10 ml MeOH/H2O, 0.5 M KOH, Tset: 94 °C, TOF1h
=
7
91
57
278, TOF3h = 229) compared to complex 8 (Table 2, Entry 4)
under these conditions was obtained, demonstrating clearly that
the dehydrogenation of formate took place in the reaction.
8
70
-
-
9
70
189
127
-
114
197
-
10
11
12[b]
13[c]
14[d]
15[e]
16[f]
70
70
70
261
135
223
282
243
197
87
70
70
187
232
148
70
70
Reaction conditions: 10 mL of a 9:1 v:v mixture of MeOH and water, 4.18 µmol
of catalyst, Tset at 94°C (reflux), evolved gas analysed by gas-phase GC, each
molecule of H2 counted as one turnover, [a] internal temperature, [b] v:v
(MeOH:H2O) 4:1, [c] v:v(MeOH:H2O) 5:5, [d] neat MeOH, [e] 44.4 mmol K2CO3
added, [f] 44.4 mmol HCOOK added, reproduction differed less than 15%.
Figure 2. Influence of base on stability: reaction conditions: 10 mL of a 9:1 v:v
mixture of MeOH and water, 4.18 µmol of catalyst 8, Tset at 94°C (reflux).
To investigate the reactions with high and low base
content in more detail, we performed NMR experiments in order
to follow the formation of key species during the reaction. More
specifically, we executed two reactions each using 100 mg of 8
(186.9 µmol) in 10 mL of 9:1 methanol/water mixture at reflux,
one containing 0.5 M KOH, the other 8.0 M KOH. During the
course of the reactions, samples were taken at defined times
and NMR spectra recorded at room temperature (SI 3). As
shown in Figure 3, in the presence of KOH 0.5 M the applied
precursor [Ir(H)2(Cl)(HN(CH2CH2P(iPr)2)2)] (8) is immediately
converted to the trihydride complex 9 even in low basic
environment at room temperature. This transformation can be
observed in the hydride region of the 1H and 31P NMR spectra as
the signal(s) of the original complex 8 (31P:  = 51.90 ppm, 1H: 
= -26.42 ppm,  = -20.50 ppm) disappear and a new set of
signals in both the 1H and 31P spectrum emerge that fits the
reference spectra of complex 9 (31P:  = 56.28 ppm, 1H:
 = -12.27 ppm,  = -12.66 ppm,  = -22.83 ppm) (Figure 3 and
SI, Figures SI3_1, SI3_2, SI3_4).
further on, v:v MeOH:H2O 4:1 (Table 2, entry 12) and
MeOH:H2O 5:5 (Table 2, entry 13), the TOF1h dropped from 326
to 261 and 135, respectively.
Scheme 2. Reaction equations in presence and absence of hydroxide.
We explain this behaviour by the need of a minimal amount of
water or hydroxide necessary to achieve APR of methanol
(Scheme 2, equations 1 and 2). As shown in Scheme 1, the
presence of water or hydroxides is essential for the conversion
of formaldehyde and thus for the formation of formic
acid/formate.
Investigations for long term stability of the catalyst system
demonstrated that the base concentration has a dramatic
influence not only on activity but also on stability. For example,
in the reaction with 0.5 M KOH gas evolution ceased after 16
hours to give a TON of 1,400 and the final expected product gas
ratio of 3:1 (H2:CO2) (Figure 2). In contrast, in the presence of
8.0 M KOH the system was more stable (60 h) and a TON of
1,900 was achieved (SI2, Figure SI2b). Within this time only little
drop in activity was observed. Due to the higher base amount
the CO2 was mainly trapped as carbonate and only traces of
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
observed change in activity (SI, Figure SI3_3). As the cis-
hydride species disappeared again, the trans-hydride species
accumulated with ongoing reaction time and stayed present after
gas evolution stopped. Hence, this latter complex seems to be a
deactivated species. To identify the suggested intermediates of
the catalytic cycle, several stoichiometric experiments were
performed. By stepwise addition of 1, 2, 5, 10 and 50
equivalents of methanol to the trihydride complex 9 we expected
the formation of an iridium methoxy species, which is known to
be formed in case of related ruthenium pincer complexes. [12b]
Figure 3. Stacked 1H NMR spectra (hydride region) of reaction mixture,
recorded during after defined times; reaction conditions: 9 mL MeOH, 1 mL
water, 0.5 M KOH, 100 mg cat. 8, Tset 94 °C.
Scheme 3. Proposed reaction mechanism for Ir-catalyzed dehydrogenation of
methanol.
However, in contrast to previous works with ruthenium
1
complexes,[12b] no new species arises in the H NMR spectra
Notably, this trihydride species is the main species in the
strong basic medium (8.0 M KOH, SI3, Figure SI_4) and the gas
evolution rate stayed constant for 24 h (SI3, Figure SI3_6). After
that time, new sets of signals appeared corresponding to three
species, based on 31P NMR. One set showed two trans-hydride
(SI4).
Even
treating
the
amido
complex
[Ir(H)2(N(CH2CH2P(iPr)2)2)] with methanol, no iridium methoxy
species, but instead the trihydride complex 9 is formed (SI5).[25]
These observations indicate the amido complex to be an
important intermediate in the methanol dehydrogenation step to
formaldehyde. Based on the combined results of stoichiometric
and catalytic NMR experiments, we propose the reaction
mechanism shown in Scheme 3.
1
signals, both quartets at  = -8.21 ppm and  = -8.36 ppm in H
NMR (JHH=13.17) and a related singlet at  = 70.44 ppm in the
31P NMR. The other set of signals represents two hydrides
standing cis to each other as triplets at  = -10.06 ppm and  = -
1
17.55 ppm in the H NMR (JHH=3.13) and an associated singlet
As mentioned above, catalyst deactivation in our system
took place accompanied by the formation of three new species
observed in the 31P NMR. On the one hand, one species, not
correlating to any hydride signal in the 1H-31P HMQC NMR (SI 5,
Figure SI5_3; 31P NMR: = 80.94 ppm, SI6), was identified as
[Ir(CO)(N(CH2CH2P(iPr)2)2)]. It could be independently
at  = 52.04 ppm in the 31P NMR spectrum are observed. The
stability of 9 at high base loadings (Table 1, entry 4) supports
the assumption that 9 is indeed an active dehydrogenation
catalyst which was also proposed by Abdur-Rashid and co-
workers for transferhydrogenation reactions.[23a]
synthetized
by
reaction
of
the
amido
complex
The two dihydride complexes described above were also
observed during the reaction in lower basic medium (0.5 M, SI,
Figures SI3_1, SI3_2). Here, they became the main species
already after 1.5 hours of reaction time. Furthermore, the signals
belonging to 9 disappeared even before this. However with
respect to the higher activity (Table 1, entry 10), this observation
indicates the formation of a more powerful catalyst for the
dehydrogenation of methanol in this case. The appearance of
new Ir species during the NMR experiments correlated with the
[Ir(H)2(N(CH2CH2P(iPr)2)2)] with CO (SI 6) and its NMR data
compared with the published ones.[26] When this species was
used
as
catalyst
precursor
in
methanol
aqueous
dehydrogenation no activity was observed. On the other hand, a
new iridium dihydride complex was also formed. Although it was
not possible to identify this iridium complex clearly by NMR
spectroscopy (trans dihydride signals in 1H NMR:  = -8.21 ppm,
 = -8.36 ppm, SI3) because of overlapping and superimposed
signals, based on MS ((M+H)+ = 528.21314, SI, Figure SI5_2)
and IR spectroscopy (carbonyl group, SI, Figure SI5_1) we
propose the formation of an iridium dihydride carbonyl complex
as deactivated species (Scheme 4).
Scheme 4. Proposed deactivation side reaction.
This suggestion of a trans-dihydride carbonyl pincer
complex formed with formaldehyde as CO source is also
supported by reports of Goldberg and co-workers [27] as well as
Fryzuk and co-workers.[28, 29] Apparently, this deactivation occurs
preferentially in low basic media as it does not impact the
reaction under strongly basic conditions. This also explains the
impact of the base amount on catalyst activity and stability in the
methanol dehydrogenation process.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
Conclusions
[14] A. Monney, E. Barsch, P. Sponholz, H. Junge, R. Ludwig, M. Beller,
Chem. Commun. 2014, 50, 707-709.
We developed an iridium-catalyzed dehydrogenation of aqueous
methanol under mild conditions. Contrary to most known
organometallic catalysts for this transformation no strong basic
conditions are necessary for sufficient catalytic activity.
[15] P. Hu, Y. Diskin-Posner, Y. Ben-David, D. Milstein, ACS Catal. 2014, 4,
2649-2652.
[16] a) G. Zeng, S. Sakaki, K.-i. Fujita, H. Sano, R. Yamaguchi, ACS Catal.
2014, 4, 1010-1020; b) K.-i. Fujita, N. Tanino, R. Yamaguchi, Org. Lett.
2006, 9, 109-111; c) Y. Li, P. Sponholz, M. Nielsen, H. Junge, M. Beller,
ChemSusChem 2015, 8, 804-808.
[17] a) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131,
14168-14169; b) S. Oldenhof, M. Lutz, B. de Bruin, J. Ivar van der Vlugt,
J. N. H. Reek, Chem. Sci. 2015, 6, 1027-1034; c) E. Fujita, J. T.
Muckerman, Y. Himeda, Biochim. Biophys. Acta 2013, 1827, 1031-
1038; d) S. Oldenhof, B. de Bruin, M. Lutz, M. A. Siegler, F. W.
Patureau, J. I. van der Vlugt, J. N. H. Reek, Chem. Eur. J. 2013, 19,
11507-11511; e) J. H. Barnard, C. Wang, N. G. Berry, J. Xiao, Chem.
Sci. 2013, 4, 1234-1244; f) Y. Maenaka, T. Suenobu, S. Fukuzumi,
Energy Environ. Sci. 2012, 5, 7360-7367; g) R. Tanaka, M. Yamashita,
L. W. Chung, K. Morokuma, K. Nozaki, Organometallics 2011, 30,
6742-6750; h) Y. Himeda, Green Chem. 2009, 11, 2018-2022; i) S.
Fukuzumi, T. Kobayashi, T. Suenobu, J. Am. Chem. Soc. 2010, 132,
1496-1497; j) M. Iguchi, Y. Himeda, Y. Manaka, K. Matsuoka, H.
Kawanami, ChemCatChem 2016, 8, 886-890.
General procedure
All reactions were performed under argon with exclusion of air. A
solution of 10 ml MeOH and H2O in a given ratio, containing a
defined amount of base, was heated to the desired temperature
and let equilibrate for 30 minutes. The catalyst (4.18 µmol) was
added which set the starting point for measuring the evolved gas
volume. Gas evolution was measured by manual or automatic
gas burettes. The identity of the gas components and their ratio
was determined by gas-phase chromatography. Plots of volume
amount of gas evolved as a function of time for experiments
reported in Tables can be found in the supporting information.
[18] K.-i. Fujita, R. Kawahara, T. Aikawa, R. Yamaguchi, Angew. Chem. Int.
Ed. 2015, 54, 9057-9060.
Acknowledgements
[19] E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H. J. Drexler, W.
Baumann, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 14162-
14166.
This work has been supported by the state of Mecklenburg-
Vorpommern and the BMBF. Further financial support was
provided by the BMWi within the project “Metha-Cycle”
(03ET6071A). In addition, we thank A. Agapova for experimental
support.
[20] a) E. A. Bielinski, M. Förster, Y. Zhang, W. H. Bernskoetter, N. Hazari,
M. C. Holthausen, ACS Catalysis 2015, 5, 2404-2415; b) E. A. Bielinski,
P. O. Lagaditis, Y. Zhang, B. Q. Mercado, C. Wurtele, W. H.
Bernskoetter, N. Hazari, S. Schneider, J. Am. Chem. Soc. 2014, 136,
10234-10237.
References
[21] For leading references in pincer chemistry see: a) The Privileged
Pincer-Metal Platform: Coordination Chemistry & Applications (Eds: G.
Van Koten, R. A Gossage), Topics in Organometallic Chemistry, Vol.
54, Springer, Berlin-Heidelberg, 2016; b) H. A. Younus, W. Su, N.
Ahmad, S. Chen, F. Verpoort, Adv. Synth. Catal. 2015, 357, 283 – 330;
[1]
[2]
N. Armaroli, V. Balzani, ChemSusChem 2011, 4, 21-36.
T. R. Cook, D. K. Dogutan, S. Y. Reece, Y. Surendranath, T. S. Teets,
D. G. Nocera, Chem. Rev. 2010, 110, 6474-6502.
[3]
[4]
F. Schüth, Chem. Ing. Tech. 2011, 83, 1984-1993.
c)
Pincer
and
Pincer-Type
Complexes:
Applications
in
M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, Int. J. Hydrogen Energy
2013, 38, 4901-4934.
Organic Synthesis and Catalysis (Eds: K. J. Szabo, O. F. Wendt),
Wiley-VCH,Weinheim, 2014; d) C. Gunanathan, D. Milstein, Chem. Rev.,
2014, 114, 12024-12087; e) Organometallic Pincer Chemistry (Eds: G.
van Koten, D. Milstein) Topics in Organometallic Chemistry, Vol. 40,
Springer, Berlin-Heidelberg, 2013; e) The Chemistry of Pincer
Compounds (Eds: D. Morales-Morales, C. M. Jensen), Elsevier Science,
The Netherlands, 2007.
[5]
[6]
E. Alberico, M. Nielsen, Chem. Commun. 2015, 51, 6714-6725.
a) C. Gunanathan, D. Milstein, Science 2013, 341, 1229712; b) M.
Trincado, D. Banerjee, H. Grutzmacher, Energy Environ. Sci. 2014, 7,
2464-2503; c) M. Grasemann, G. Laurenczy, Energy Environ Sci 2012,
5, 8171-8181; d) D. Teichmann, W. Arlt, P. Wasserscheid, R.
Freymann, Energy Environ. Sci. 2011, 4, 2767-2773; e) A. Boddien, F.
Gärtner, M. Nielsen, S. Losse, H. Junge, in Comprehensive Inorganic
Chemistry II (Second Edition) (Ed.: K. Poeppelmeier), Elsevier,
Amsterdam, 2013, pp. 587-603.
[22] S. Elangovan, B. Wendt, C. Topf, S. Bachmann, M. Scalone, A.
Spannenberg, H. Jiao, W. Baumann, K. Junge, M. Beller, Adv. Synth.
Catal. 2016, 358, 820-825.
[23] a) Z. E. Clarke, P. T. Maragh, T. P. Dasgupta, D. G. Gusev, A. J. Lough,
K. Abdur-Rashid, Organometallics 2006, 25, 4113-4117; b) S. Bi, Q.
Xie, X. Zhao, Y. Zhao, X. Kong, J. Organomet. Chem. 2008, 693, 633-
638; c) X. Chen, W. Jia, R. Guo, T. W. Graham, M. A. Gullons, K.
Abdur-Rashid, Dalton Trans. 2009, 1407-1410; d) N. Andrushko, V.
Andrushko, P. Roose, K. Moonen, A. Börner, ChemCatChem 2010, 2,
640-643; e) M. Bertoli, A. Choualeb, A. J. Lough, B. Moore, D. Spasyuk,
D. G. Gusev, Organometallics 2011, 30, 3479-3482; f) M. Nielsen, A.
Kammer, D. Cozzula, H. Junge, S. Gladiali, M. Beller, Angew. Chem.
Int. Ed. 2011, 50, 9593-9597; g) T. J. Schmeier, G. E. Dobereiner, R. H.
Crabtree, N. Hazari, J. Am. Chem. Soc. 2011, 133, 9274-9277; h) M.
Nielsen, H. Junge, A. Kammer, M. Beller, Angew. Chem. Int. Ed. 2012,
51, 5711-5713; i) K. Junge, B. Wendt, H. Jiao, M. Beller,
ChemCatChem 2014, 6, 2810-2814; j) P. Sponholz, D. Mellmann, C.
Cordes, P. G. Alsabeh, B. Li, Y. Li, M. Nielsen, H. Junge, P. Dixneuf, M.
Beller, ChemSusChem 2014, 7, 2419-2422; k) S. T. Ahn, E. A. Bielinski,
E. M. Lane, Y. Chen, W. H. Bernskoetter, N. Hazari, G. T. R. Palmore,
Chem Commun. 2015, 51, 5947-5950; l) P. A. Dub, B. L. Scott, J. C.
Gordon, Organometallics 2015, 34, 4464-4479; m) Y. Li, M. Nielsen, B.
Li, P. H. Dixneuf, H. Junge, M. Beller, Green Chem. 2015, 17, 193-198;
n) L. Zhang, Z. Han, X. Zhao, Z. Wang, K. Ding, Angew. Chem. Int. Ed.
2015, 54, 6186-6189.
[7]
[8]
G. A. Olah, Angew. Chem. 2005, 117, 2692-2696.
a) G. A. Olah, Angew. Chem. Int. Ed. 2013, 52, 104-107; b) F. Liao, Z.
Zeng, C. Eley, Q. Luꢀ, X. Hong, S. C. E. Tsang, Angew. Chem. Int. Ed.
2012, 51, 5832-5836; c) S. Wesselbaum, T. vom Stein, J.
Klankermayer, W. Leitner, Angew. Chem. Int. Ed. 2012, 51, 7499-7502;
d) S. Wesselbaum, V. Moha, M. Meuresch, S. Brosinski, K. M. Thenert,
J. Kothe, T. v. Stein, U. Englert, M. Holscher, J. Klankermayer, W.
Leitner, Chem. Sci. 2015, 6, 693-704; e) J. Klankermayer, S.
Wesselbaum, K. Beydoun, W. Leitner, Angew. Chem. Int. Ed. 2016, 55,
7296-7343.
[9]
R. D. Cortright, R. R. Davda, J. A. Dumesic, Nature 2002, 418, 964-967.
[10] R. M. Navarro, M. A. Peña, J. L. G. Fierro, Chem. Rev. 2007, 107,
3952-3991.
[11] S. Shinoda, H. Itagaki, Y. Saito, Chem. Commun. 1985, 860-861.
[12] a) M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S.
Gladiali, M. Beller, Nature 2013, 495, 85-89; b) E. Alberico, A. J. J.
Lennox, L. K. Vogt, H. Jiao, W. Baumann, H.-J. Drexler, M. Nielsen, A.
Spannenberg, M. P. Checinski, H. Junge, M. Beller J. Am. Chem. Soc.
2016, 138, 14890-14904.
[13] R. E. Rodríguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-
Quinones, H. Grützmacher, Nat. Chem. 2013, 5, 342-347.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
[24] Compound [Ir(H)2(OOCH)(HN(CH2CH2P(iPr)2)2)] was prepared as
described in T. J. Schmeier, G. E. Dobereiner, R. H. Crabtree, N.
Hazari, J. Am. Chem. Soc. 2011, 133, 9274–9277.
[25] Compound [Ir(H)2(N(CH2CH2P(iPr)2)2)] was prepared as described in
Ref. 23a.
[26] A. Friedrich, R. Ghosh, R. Kolb, E. Herdtweck, S. Schneider
Organometallics 2009, 28, 708–718.
[27] S. M. Kloek, D. M. Heinekey, K. I. Goldberg, Organometallics 2006, 25,
3007-3011.
[28] M. D. Fryzuk, P. A. MacNeil, Organometallics 1983, 2, 682-684.
[29] Attempts to prepare trans-[Ir(H)2(CO)(N(CH2CH2P(iPr)2)2)] by reaction
of the amido complex with paraformaldehyde afforded instead a mixture
of the desired product, [Ir(CO)(N(CH2CH2P(iPr)2)2)] and a third species
whose signals are consistent with the same cis-hydride species
observed under the reaction conditions of aqueous methanol
dehydrogenation. This prevented its isolation.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700015
ChemCatChem
COMMUNICATION
COMMUNICATION
Christoph Prichatz, Elisabetta Alberico,
Wolfgang Baumann, Henrik Junge and
Matthias Beller*
1 – 5
Iridium-PNP Pincer Complexes for
Methanol Dehydrogenation At Low
Base Concentration
A catalytic system based on an iridium-PNP complex was developed for the
aqueous phase reforming of methanol. Investigations revealed higher activity at low
base concentration and higher stability at higher base concentration as an opposing
trend compared to known Ru and Fe PNP pincer containing systems for methanol
dehydrogenation.
This article is protected by copyright. All rights reserved.