Angewandte
Communications
Chemie
Hydrogen Storage
Reversible Interconversion between 2,5-Dimethylpyrazine and
2,5-Dimethylpiperazine by Iridium-Catalyzed Hydrogenation/
Dehydrogenation for Efficient Hydrogen Storage
Abstract: A new hydrogen storage system based on the
hydrogenation and dehydrogenation of nitrogen heterocyclic
compounds, employing a single iridium catalyst, has been
developed. Efficient hydrogen storage using relatively small
amounts of solvent compared with previous systems was
achieved by this new system. Reversible transformations
between 2,5-dimethylpyrazine and 2,5-dimethylpiperazine,
accompanied by the uptake and release of three equivalents
of hydrogen, could be repeated almost quantitatively at least
four times without any loss of efficiency. Furthermore, hydro-
gen storage under solvent-free conditions was also accom-
plished.
a nitrogen atom to organic hydrides would lead to lowering of
the endothermicity of dehydrogenation.[5] Several examples
of existing hydrogen storage systems using nitrogen-contain-
ing organic hydrides are summarized in Scheme 1.[6–12] Pez
et al. have reported a hydrogen storage system involving the
hydrogenation and dehydrogenation of N-ethylcarbazole
(Scheme 1a).[6] This system was highly efficient with a max-
imum hydrogen gravimetric capacity of 5.8 wt%. However,
different metal catalysts have to be employed for the hydro-
genation and dehydrogenation processes. From the stand-
point of practical applications, the use of a single catalyst
which promotes both hydrogenation and dehydrogenation is
desirable. Previous examples in which hydrogenation and
dehydrogenation of nitrogen-containing organic hydrides
were achieved by employing a single catalyst are shown in
Schemes 1b–e.[7–10] However, the following problems still
remain with these systems: 1) a high pressure of hydrogen
(> 49 atm) is required in the hydrogenation process and 2) a
relatively large amount of solvent is always used. The latter
point is a critical problem for the practical development of
nitrogen-containing organic hydrides for hydrogen storage.
To evaluate the amount of the solvent used in each system, we
here introduce the “H/S ratio” which is defined as the ratio of
released or stored hydrogen (with mmol dimensions) to the
amount of solvent (with mL dimensions). In all of the
previous systems shown in Scheme 1b–e, the H/S ratios are
lower than 19, which means that a new system requiring
a much smaller amount of solvent must be developed.[13]
Herein we report a new system for the efficient storage of
hydrogen based on the reversible interconversion between
2,5-dimethylpyrazine and 2,5-dimethylpiperazine by hydro-
genation and dehydrogenation catalyzed by a single iridium
complex. In this new system, the hydrogenation process has
been accomplished under a relatively low pressure (15 or
30 atm) of hydrogen. In addition, a high H/S ratio (over 30)
was achieved. Solvent-free hydrogen storage using a nitrogen-
containing organic hydride has also been realized.
H
ydrogen is regarded as the most promising clean energy
carrier because of its advantageous properties such as: 1) high
mass energy density, 2) easy transformation into electrical
energy by fuel cells, and 3) generation of only water upon
utilization of stored energy.[1] To work toward a “hydrogen
economy,” safety issues arising from the explosive nature of
hydrogen need to be overcome. In this context, an important
challenge is to search for a new way to store hydrogen.
Consequently, a variety of hydrogen storage methods have
been established and investigated, including compressed
hydrogen, liquid hydrogen, metal hydrides, inorganic chem-
ical hydrides (ammonia borane, etc.), and storage in metal–
organic frameworks.[2]
The use of organic hydrides, which store hydrogen in the
covalent bonds of organic molecules, is another promising
method for hydrogen storage.[3] By using this method, hydro-
gen can be stored by hydrogenation, thus yielding dense, safe,
and easy-to-handle organic hydrides. When the hydrogen is
needed, it suffices to dehydrogenate the organic hydrides. To
date, cycloalkanes such as methylcyclohexane or decalin have
been extensively studied as organic hydrides, however, harsh
reaction conditions (usually over 2008C) are required for the
release of hydrogen because of the high endothermicity of the
dehydrogenation of these molecules.[4]
Recently, much attention has been given to nitrogen-
containing organic hydrides because both theoretical and
experimental studies have suggested that the introduction of
The catalysts used in this study are illustrated in Scheme 2.
We have already reported the synthesis and applications of
the catalyst 1.[14,15] The related catalysts 2 and 3, bearing
functional bipyridonate ligands with different electronic
natures, have been newly synthesized in this study (see the
Supporting Information for details of their preparation).
We started our study by searching for new organic
hydrides, paying special attention to nitrogen heterocyclic
compounds. By employing 1 as a catalyst, the dehydrogen-
ation of a variety of piperidine and piperazine derivatives was
surveyed in p-xylene as the solvent (see Table S1 in the
Supporting Information) and under solvent-free conditions
[*] Prof. Dr. K. Fujita, T. Wada, T. Shiraishi
Graduate School of Human and Environmental Studies
Kyoto University
Sakyo-ku Kyoto 606-8501 (Japan)
E-mail: fujita.kenichi.6a@kyoto-u.ac.jp
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
Angew. Chem. Int. Ed. 2017, 56, 1 – 5
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!