Exploring the reactivity of nickel pincer complexes in the decomposition of formic acid to CO2/H2 and the hydrogenation of NaHCO3 to HCOONa

Article abstract of DOI:10.1002/cctc.201402716

The nickel-catalyzed decomposition of formic acid to yield molecular hydrogen and the nickel-catalyzed hydrogenation of bicarbonate as a carbon dioxide mimic have been examined. Well-defined nickel complexes modified by a PCP-pincer ligand, especially nickel hydride and nickel formate complexes, revealed catalytic activity with turnover numbers of up to 626 (decomposition) and 3000 (hydrogenation). Thus, a formal hydrogen storage and release cycle performed by a well-defined nickel catalyst was accomplished.

Full text of DOI:10.1002/cctc.201402716

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DOI: 10.1002/cctc.201402716
Exploring the Reactivity of Nickel Pincer Complexes in the
Decomposition of Formic Acid to CO2/H2 and the
Hydrogenation of NaHCO3 to HCOONa
Stephan Enthaler,*[a] Andreas Brꢀck,[a] Anja Kammer,[b] Henrik Junge,*[b] Elisabeth Irran,[c] and
Samet Gꢀlak[b]
The nickel-catalyzed decomposition of formic acid to yield mo-
lecular hydrogen and the nickel-catalyzed hydrogenation of bi-
carbonate as a carbon dioxide mimic have been examined.
interesting alternative for the convenient, reversible and envi-
ronmental-friendly storage of hydrogen, allowing an easy
transport, handling and refueling compared to other processes.
Recently the evolution of hydrogen by catalytic decomposition
of formic acid has been extensively studied in the presence of
homogeneous catalysts.[6,7] Especially ruthenium based cata-
lysts have been demonstrated to be highly active for that pur-
pose. In more detail, ruthenium complexes modified by amine
or phosphane ligands have been successfully applied in the
evolution of hydrogen. However, the selection of the catalyst
metal is of great importance, because modern process-require-
ments focus on the substitution of expensive transition metals.
Indeed, during the last years the chemistry of cheap and abun-
dant “bio”metals as catalyst core (e.g., iron) has been investi-
gated in the catalytic decomposition of formic acid.[8] Besides,
the application of nickel complexes can also be an alternative
to replace more expensive ruthenium catalysts.[9] For the de-
composition/production of formic acid several heterogeneous
nickel based catalysts have been presented, while well-defined
homogeneous catalysts have only been reported for the hy-
drogenation or photocatalytic reduction of carbon dioxide, re-
spectively, so far.[10] In this regard, the application of well-de-
fined complexes as homogeneous catalysts for hydrogen stor-
age and evolution potentially allows for a better understand-
ing of the reaction mechanism. Interestingly, several research
groups reported recently the characterization and isolation of
the nickel hydride complex 1 and the nickel formate complex
2, which can be potential intermediates in the catalytic cycle
for the formation and decomposition of formic acid
(Scheme 1).[11] For instance complex 1 can react with formic
acid to produce dihydrogen and the formate complex 2, which
can further release carbon dioxide to regenerate the hydride
complex. On the other hand, both complexes can be involved
in the opposite process, the hydrogenation of carbon dioxide.
However, the catalytic abilities of the complexes 1 and 2 have
not been demonstrated so far. Based on that, we report herein
on our initial studies on the application of nickel pincer com-
plexes as dual catalysts in hydrogen storage and hydrogen
evolution processes.
Well-defined nickel complexes modified by
a PCP-pincer
ligand, especially nickel hydride and nickel formate complexes,
revealed catalytic activity with turnover numbers of up to 626
(decomposition) and 3000 (hydrogenation). Thus, a formal hy-
drogen storage and release cycle performed by a well-defined
nickel catalyst was accomplished.
The establishment of a hydrogen economy has been envis-
aged as a promising substitution for the current fossil fuel
based energy feedstock.[1] In this regard, the application of mo-
lecular hydrogen as a secondary energy carrier yields after de-
composition in e.g., a fuel cell, electric energy and water as
the only environmental-friendly side product.[2] However, so far
various issues of the hydrogen economy are unsolved. One of
the major problems is the development of suitable hydrogen
storage systems, which currently display some difficulties (e.g.,
tank systems: need for high-pressure or low temperature
equipment; metal hydrides, polymers, amino boranes: low re-
versibility; metal organic frameworks: low temperature; bio-
mass: social issues).[3] Promising chemical hydrogen storage
systems, as studied recently, involve the transformation of
carbon dioxide to formic acid and vice versa, as well as carbon-
ate or bicarbonate to formate.[4,5] Formic acid (HCO2H) can be
produced by catalytic hydrogenation of carbon dioxide with
molecular hydrogen in the presence of base and catalysts,
while the catalytic decomposition of formic acid releases the
stored hydrogen and carbon dioxide in the presence of base.
As such formic acid in combination with a suitable base is an
[a] Dr. S. Enthaler, Dr. A. Brꢀck
Department of Chemistry, Technische Universitꢁt Berlin
Cluster of Excellence “Unifying Concepts in Catalysis”
Str. des 17. Juni 115/C2, Berlin (Germany)
E-mail: stephan.enthaler@tu-berlin.de
[b] A. Kammer, Dr. H. Junge, Dr. S. Gꢀlak
Leibniz-Institut fꢀr Katalyse e. V. an der Universitꢁt Rostock
Albert-Einstein-Straße 29a, Rostock (Germany)
E-mail: henrik.junge@catalysis.de
To access the potential catalytic active nickel hydride com-
plex 1, we first synthesized the (tBuPCP)NiÀBr 3 in accordance
to methods reported in the literature.[12] To convert the
[c] Dr. E. Irran
Institute of Chemistry: Metalorganics and Inorganic Materials
Technische Universitꢁt Berlin
Straße des 17. Juni 135/C2, Berlin (Germany)
(
tBuPCP)NiÀBr 3 to the corresponding (tBuPCP)NiÀH 1, equimolar
amounts of 3 and LiBH4 were dissolved in [D8]THF under an at-
mosphere of dinitrogen (Scheme 2).[11b,e] After 10 min at ambi-
ent temperature NMR measurements were carried out of the
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402716.
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tion of 3 with an excess of LiBH4
at ambient temperature for 48 h
it was possible to isolate the in-
termediate
( 4
tBuPCP)Ni(H2BH2)
after work-up.[15] The solid-state
structure of 4 has been charac-
terized by single-crystal X-ray dif-
fraction analysis. Thermal ellip-
soid plots, selected bond lengths
and angles are shown in
Figure 1. Based on these results,
to convert 4 to 1 elevated tem-
Scheme 1. Carbon dioxide/formic acid based hydrogen storage system.
peratures are necessary to
remove the BH3. To the reaction
mixture of (tBuPCP)NiÀH 1 (ob-
tained after 12 h at 508C in THF with LiBH4) 10 equiv of NEt3
were added, but no effect on the chemical shift of the
(
tBuPCP)NiÀH was monitored. Next 10 equiv of formic acid were
added and immediately a gas evolution was observed. A
1H NMR spectra recorded after 10 min at ambient temperature
showed the disappearance of the signals for the hydride 1.
New sets of signals appeared for the CH2P (d=3.19 ppm, t,
2JPÀH =3.84 Hz) and for the tert-butyl groups (d=1.44 ppm, t,
3JPÀH =6.46 Hz). In addition, a triplet (4JPÀH =2.15 Hz) was ob-
served with a chemical shift of 8.08 ppm, which was assigned
as the formate complex (tBuPCP)NiÀOC(=O)H 2 in accordance
4
to literature (d=8.62 ppm, JPÀH =2.24 Hz).[11a] The complex 2
was furthermore confirmed by recording a 31P{1H} NMR peak
with a chemical shift of 65.9 ppm. After heating the mixture
for 12 h at 508C the formic acid was completely decomposed,
which corresponds to 10 turnovers and reveals the possibility
for catalysis. 1H and 31P{1H} NMR measurements showed the
reformation of the hydride complex 1. Based on that, complex
(
tBuPCP)NiÀH 1 and (tBuPCP)NiÀOC(=O)H 2 are presumably
active species of the catalytic cycle. In addition, the reaction of
3 with HCOOH/NEt3 to form the formate complex 2 was stud-
ied. To a solution of 3 in [D8]THF 10 equiv of HCOOH/NEt3
(ratio: 1:1) were added and the mixture was heated for 24 h at
Scheme 2. Nickel-catalyzed hydrogen evolution.
clear yellow solution and the formation of a new species was
1
1
observed (yield: ꢀ30%). In more detail, H NMR measurements
508C. The investigation of the mixture by H and 31P{1H} NMR
showed the formation of a triplet (2JPÀH =52.90 Hz) at d=
À10.29 ppm, which is in agreement to the (tBuPCP)NiÀH 1.
Moreover, new sets of signals appeared for the CH2P (d=
spectroscopy showed no formation of the formate complex 2,
while 3 was stable under these conditions. After these initial
promising results we tested the nickel complexes as (pre)cata-
lyst in the hydrogen generation from formic acid (Table 1).
While without an amine for 1 no activity has been observed, in
the presence of dimethyl-n-octylamine (molar ratio
HCO2H:amine=11:10) at 608C turnover numbers of 91 after
2 h and 122 after 3 h, respectively, were obtained with propyl-
ene carbonate (PC) as solvent (Table 1, entries 1 and 2).[16] Low-
ering the temperature to 408C lead to significantly reduced
productivity. In contrast, at 808C 481 and 626 turnovers and
turnover frequencies of 240 hÀ1 and 209 hÀ1 were obtained
after 2 and 3 h, respectively (Table 1, entries 3 and 4 and
Figure SI 1). However, the increased activity went along with
strongly increased blank volumes especially after longer reac-
tion times (Figure SI 2). Careful analysis of the resulting prod-
uct gases resulted in a hydrogen to carbon dioxide ratio of 1:2
after 20 h, due to partial cleavage of the propylene carbonate.
2
3.43 ppm, t, JPÀH =3.65 Hz) and for the tert-butyl groups (d=
3
1.35 ppm, t, JPÀH =6.34 Hz). In addition, the 31P{1H} NMR spec-
trum revealed a new signal with a chemical shift of 99.7 ppm,
which is in accordance to the reported chemical shift for the
(
tBuPCP)NiÀH complex. Interestingly, along with the chemical
shifts for (tBuPCP)NiÀBr and (tBuPCP)NiÀH a signal at 73.3 ppm
was noticed, which can be probably assigned as (tBuPCP)Ni(BH4)
complex as reported by Peruzzini[11b] and Shaw[13]. Heating the
reaction mixture for 12 h at 508C showed the disappearance
of the signal for (tBuPCP)NiÀBr and the intermediate (tBuPCP)Ni-
(H2BH2)[14] and the complex (tBuPCP)NiÀH was the only phos-
phorous containing compound. Notably, the group of Peruzzini
applied NaBH4 for the synthesis of 1, but at ambient tempera-
ture longer reaction times were necessary, e.g., small amounts
of 1 were formed after 8 h. Interestingly, performing the reac-
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Table 1. Catalytic hydrogen production from a HCO2H/nOctNMe2 mixture
with (tBuPCP)Ni-complexes.[a]
Entry
(Pre)-catalyst Temp. Volume H2 TON Volume H2 TON
[8C]
2 h [mL]
2 h
3 h [mL]
3 h
1[b]
2
3
4
5[c]
6[d]
7[e]
8
1
1
1
1
1
1
1
2
3
4
1
–
60
60
40
80
60
60
60
60
60
60
60
60
<1.0
24
3.9
125
16
–
<1.0
32
5.4
162
22
18
11
12
27
–
91
15
481
62
47
33
34
77
31
60
–
122
21
626
83
68
44
48
104
44
84
–
12
8.5
8.8
20
8.1
157
1.3
9
10
11[f]
12
11
220
1.8
[a] Experimental conditions: 5.3 mmol [Ni], HCO2H/nOctNMe2 (ratio: 11:10,
5.0 mL), propylene carbonate (PC, 5.0 mL). The gas volumes were deter-
mined by automatic gas burettes and were corrected by blank volumes.
The gases were analyzed by GC. CO contents between <10 and 460 ppm
were detected. TON=n(H2)/n(Ni). All experiments were performed at least
twice. The differences between the measurements are in between 1 and
20%, except for entry 8 (22%). [b] HCO2H (2.0 mL), PC (5.0 mL), without
amine. [c] Diglyme (5.0 mL) instead of PC. [d] 1,4-Dioxane (5.0 mL) instead
of PC. [e] Without solvent. [f] 53 mmol [Ni] and 1,4-dioxane as solvent, 432
TON after 80 h, 100% conversion of HCO2H.
compared to PC (Table 1, entries 5 and 6, Figure SI 3). More-
over, in the absence of any solvent a further decrease of cata-
lyst activity was monitored (Table 1, entry 7).
The Ni complexes 2, 3 and 4 were also tested applying PC
as solvent. While 85% of the productivity of the nickel hydride
complex 1 was achieved with the bromide derivative 3, which
is in contrast to the application of [D8]THF as solvent (Table 1,
entry 9), only up to 40% of the best productivities were ob-
served either applying the formate or the borane adduct 4.
(Table 1, entries 8 and 10). Typical gas evolution curves for
complexes 1–4 are depicted in Figure 2. Without any catalyst
Figure 1. Molecular structures of complexes 3 (a), 1[14] (b) and 4 (c). Thermal
ellipsoids are drawn at the 50% probability level. Most hydrogen atoms are
omitted for clarify. Selected bond lengths (ꢂ) and angle (8): 3: Br-Ni:
2.3533(6), NiÀC(1): 1.921(3), NiÀP(1): 2.2041(10), NiÀP(2): 2.2019(10), C(1)-Ni-
P(1): 84.76(11), C(1)-Ni-P(2): 84.25(11), P(2)-Ni-P(1): 168.28(4), C(1)-Ni-Br:
175.81(11), P(2)-Ni-Br: 96.34(3), P(1)-Ni-Br: 94.91(3), 1: NiÀC(1): 1.925(4),
NiÀP(1): 2.1336(4), C(1)-Ni-P(1): 86.48(2), P(1)#1-Ni-P(1): 172.96(4), 4: B-Ni:
2.2470(14), NiÀC(1): 1.9410(13), NiÀP(1): 2.2262(4), NiÀP(2): 2.2164(4), C(1)-Ni-
P(1): 84.22(4), C(1)-Ni-P(2): 83.22(4), P(2)-Ni-P(1): 166.209(16), C(1)-Ni-B(1):
170.50(6), P(2)-Ni-B(1): 98.31(4), P(1)-Ni-B(1): 95.02(4).
Figure 2. Typical gas evolution curves for the dehydrogenation of formic
acid applying nickel complexes 1 (c), 2 (c), 3 (c), and 4 (c). Con-
ditions: 5.3 mmol [Ni], HCO2H/nOctNMe2 (ratio: 11:10, 5.0 mL), PC (5.0 mL),
608C, gas volumes were determined by automatic gas burettes without
correction by blank volumes.
Therefore, diglyme (diethylene glycol dimethyl ether) and
1,4-dioxane have been applied as alternative solvents. Al-
though these solvents were stable under applied reaction con-
ditions, the productivity of the catalyst dropped to 50–70%
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future studies will be directed to improve the catalytic per-
formance, e.g., by tuning the properties of the ligand, and to
further extend the understanding of the reaction mechanism.
Acknowledgements
Financial support from the Cluster of Excellence “Unifying Con-
cepts in Catalysis” (EXC 314/1; www.unicat.tu-berlin.de; funded
by the Deutsche Forschungsgemeinschaft and administered by
the Technische Universitꢁt Berlin) and the state of Mecklenburg-
Vorpommern is gratefully acknowledged.
Figure 3. Recycle-experiment for the dehydrogenation of formic acid with
nickel complex 1. Conditions: 53 mmol [Ni], HCO2H/nOctNMe2 (ratio: 11:10,
5.0 mL), 1,4-dioxane (5.0 mL), 608C, gas volumes were determined by auto-
matic gas burette without correction by blank volumes; first run (c) and
restart by addition of 0.87 mL formic acid (c).
Keywords: formic acid · homogeneous catalysis · hydrogen
generation · hydrogen storage · nickel
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In summary, we presented the synthesis, characterization
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complex modified by a PCP pincer ligand. The reactivity of the
complexes was studied in the catalytic decomposition of
formic acid revealing that the nickel hydride complex
(
tBuPCP)NiÀH 1 and the nickel formate complex (tBuPCP)NiÀ
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[10] See for instance for heterogeneous catalysis: a) M. W. Farlow, H. Adkins,
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Bloom, Appl. Phys. Lett. 1978, 33, 381–383; c) G. Rienꢅcker, H. Bade, Z.
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[13] C. J. Moulton, B. L. Shaw, J. Chem. Soc. Dalton Trans. 1976, 1020–1024.
1
[14] The complex (tBuPCP)Ni(BH4) was not observed by H NMR spectroscopy.
[15] S. Chakraborty, J. Zhang, Y. J. Patel, J. A. Krause, H. Guan, Inorg. Chem.
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[16] Dimethyl-n-octylamine was applied to avoid contamination of the
gases with base.
[11] a) T. J. Schmeier, N. Hazari, C. D. Incarvito, J. A. Raskatov, Chem.
Commun. 2011, 47, 1824–1826; b) A. Rossin, M. Peruzzini, F. Zanobini,
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Received: September 9, 2014
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COMMUNICATIONS
S. Enthaler,* A. Brꢀck, A. Kammer,
H. Junge,* E. Irran, S. Gꢀlak
A nickel for your hydrogen economy:
The catalytic abilities of nickel com-
plexes modified by a PCP pincer ligand
in the decomposition of formic acid to
produce dihydrogen and carbon dioxide
have been studied.
&& – &&
Exploring the Reactivity of Nickel
Pincer Complexes in the
Decomposition of Formic Acid to CO2/
H2 and the Hydrogenation of NaHCO3
to HCOONa
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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