Homogeneous Catalytic Formic Acid Dehydrogenation in Aqueous Solution using Ruthenium Arene Phosphine Catalysts

Article abstract of DOI:10.1002/zaac.201800107

In this work, we studied catalytic formic acid dehydrogenation with a homogeneous RAPTA type precatalyst in aqueous solution. The effects of an amino group, attached to the arene, and the impact of a second equivalent of 1,3,5-triaza-7-phosphaadamantane (PTA) added in situ to the reaction were evaluated. The initial compound, [(η⁶-benzyldimethylamine)Ru(PTA)Cl₂]Cl (3), was synthesized and proved to be moderately active for selective formic acid dehydrogenation. The addition of a second equivalent of PTA at the beginning of the reaction significantly improved the catalytic performance. The activation energies for both catalytic systems were assessed via an Arrhenius plot. ¹H -and ¹³C-NMR spectroscopy was used to follow the course of the reaction online and verified complete consumption of substrate. A triplet signal was observed in the hydride region along with its corresponding coupling counterpart in ³¹P spectra.

Full text of DOI:10.1002/zaac.201800107

Journal of Inorganic and General Chemistry
DOI: 10.1002/zaac.201800107
ARTICLE
Zeitschrift für anorganische und allgemeine Chemie
Homogeneous Catalytic Formic Acid Dehydrogenation in Aqueous
Solution using Ruthenium Arene Phosphine Catalysts
Cornel Fink,*[a] Lu Chen,[a] and Gábor Laurenczy*[a]
This paper is dedicated to Prof. Dr. Peter Comba on the occasion of his 65th birthday
Abstract. In this work, we studied catalytic formic acid dehydrogena-
tion. The addition of a second equivalent of PTA at the beginning
tion with a homogeneous RAPTA type precatalyst in aqueous solution. of the reaction significantly improved the catalytic performance. The
The effects of an amino group, attached to the arene, and the impact
of a second equivalent of 1,3,5-triaza-7-phosphaadamantane (PTA)
activation energies for both catalytic systems were assessed via an
Arrhenius plot. 1H -and 13C-NMR spectroscopy was used to follow
added in situ to the reaction were evaluated. The initial compound, the course of the reaction online and verified complete consumption
[(η6-benzyldimethylamine)Ru(PTA)Cl2]Cl (3), was synthesized and of substrate. A triplet signal was observed in the hydride region along
proved to be moderately active for selective formic acid dehydrogena-
with its corresponding coupling counterpart in 31P spectra.
ogy for large-scale application has been developed. Compres-
sion and liquefaction of gaseous hydrogen require significant
amounts of energy and in both methods are inherently unsafe
processes since they involve the handling of highly pressurized
containers or liquefied hydrogen.[4]
Introduction
In 2017, “The Guardian” released an article entitled “Want
to fight climate change? Have fewer children”, stating that
“…by far the biggest ultimate impact is having one fewer
child, which the researchers calculated equated to a reduction
of 58.6 tonnes of CO2 for each year of a parent’s life.”[1] As a
matter of facts, the world population is continuously growing,
and rising living standards become apparent in many societies.
Both factors contribute to an ever-increasing demand for en-
ergy. Currently, the primary energy sources are fossils fuels,
namely coal, gas, and petrol, which release carbon dioxide
(CO2) upon their combustion with oxygen. In the earth atmo-
sphere, carbon dioxide acts as a greenhouse gas, contributing
to phenomena known as global warming and in further extend,
climate change.[2] A possible solution to break this vicious cy-
cle could be to switch to renewable energy sources by harvest-
ing energy from natural occurrences such as wind or sun. One
significant disadvantage of solar and aeolian driven energy
generation is their fluctuating nature and in most cases incom-
patibility with a mobile application. A system needs to be put
in place to compensate for the gaps in primary production. The
electrolysis of water to hydrogen and oxygen allows a simple
transition from electrical to chemical energy, in which
hydrogen becomes an energy carrier.[3] Fuel cell technology
converts hydrogen efficiently on demand back to electrical en-
ergy. As of now, no fully satisfying hydrogen storage technol-
Chemical storage of hydrogen in the form of liquid organic
hydrogen carriers (LOHC)[6] is an attractive solution, offering
a range of advantages. Formic acid unites some wanted proper-
ties such as a high hydrogen content of 53 g·L–1, equal to a
700 bar pressurized cylinder, inflammable (85% in water), no
bioaccumulation, posing, therefore, no environmental risk and
is easy to handle since it is a liquid at ambient temperature.[7]
The production of formic acid in a sustainable way can be
accomplished by CO2 hydrogenation in the presence of appro-
priate catalysts. Hydrogen delivery from formic acid requires
as well a catalyst, providing then hydrogen with an overall
storage efficiency of 100%. Both reactions are the two halves
of the formic acid/carbon dioxide cycle for reversible hydrogen
storage (Figure 1), which is a schematic representation of a
hydrogen battery.[8]
* Dr. C. Fink
Figure 1. The formic acid/carbon dioxide cycle for reversible
Fax: +41-21-693-9780
hydrogen storage[5]
.
E-Mail: cornel.fink@epfl.ch
* Dr. G. Laurenczy
E-Mail: gabor.laurenczy@epfl.ch
[a] Institut des Sciences et Ingénierie Chimiques
École Polytechnique Fédérale de Lausanne (EPFL)
1015 Lausanne, Switzerland
Both, the homogeneous catalytic formic acid dehydrogena-
tion and the carbon dioxide hydrogenation can take place in
different solvents, including water, organic solvents, and ionic
liquids.[9] The effect, the solvents exert on chemical reactions
is profound, and additives are used to shift reaction equilib-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/zaac.201800107 or from the au-
thor.
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Zeitschrift für anorganische und allgemeine Chemie
ria.[10] Suitable transfer hydrogen catalysts comprise numerous on the other hand, provides steric protection by its bulky cage-
platinum group metal complexes with Ru[11] and Ir[12] as a like structure, and the phosphine donates electron density
central ion. Often the ligands contain phosphorus,[5] nitrogen towards the center to render it less prone to reduction by for-
donor atoms,[13] η6-aromatic structures,[14] or being pincer- mic acid or dihydrogen. Also, PTA increases the solubility in
type complexes.[15] Recently, Himeda and co-workers reported aqueous media, especially in a protonated state in an acidic
on a Cp*Ir catalyst with an N-phenylpicolinamide ligand, milieu.
reaching a TOF of 118,000 h–1 at 60 °C and a constant dehy-
drogenation rate of 35,000 h–1 over six hours at 50 °C (TON
Results and Discussion
1,000,000).[16] However, for industrial application, first-row
transition metal catalysts are desirable and this for many
reasons, but mainly because they are cheap and abundant. Es-
pecially for iron catalysts, the recent developments in the field
are remarkable.[17] There are recommendable review papers
available.[17] Heterogeneous catalysts, heterogenized com-
plexes, nanoparticles, nanostructures have also been proven to
be active in the hydrogen storage/delivery in the formic acid/
carbon dioxide couple, as reviewed by Himeda et al.[18]
Ruthenium-arene complexes equipped with 1,3,5-triaza-7-
phosphatricyclo-[3.3.1.1]decane ligand (PTA), commonly
known as RAPTA, became famous for their cytostatic proper-
ties.[19] Particularly well known are RAPTA-C[20] (cymene)
and RAPTA-T.[21] The compound depicted in Figure 2 was ini-
tially studied by Dyson et al. in 2006 upon its cytotoxicity.[22]
Besides their cytostatic properties, it was discovered that η6-
arene-ruthenium(II) complexes, among them RAPTA[23] and
related structures,[24] own catalytic properties for several types
of reactions, including transfer hydrogenation.[25]
Compound 3 was synthesized, characterized by NMR and
mass spectroscopy and subsequently studied to explore the po-
tential towards formic acid dehydrogenation. The catalytic ac-
tivity was assessed by dehydrogenating a defined amount of
formic acid and following the process with two orthogonal
techniques. During dehydrogenation, formic acid releases
hydrogen and carbon dioxide in equal amounts, causing a pres-
sure increase in a closed tube, which can, when recorded over
time, be correlated with the reaction progress. The complex,
which is added to the water-substrate mixture is considered as
a precatalyst and undergoes ligand exchange of labile chloride
ligands to become the catalytically active species, as
was described by Guan et al. for the similar complex
[(p-cymene)Ru(2,2Ј-biimidazoline)Cl]Cl.[14b] At the end of the
reaction, pressure increase levels off and finally stops. The in-
crease of pressure in a closed vessel can be correlated with the
progress of the reaction since the two only products from FA
dehydrogenation are carbon dioxide and hydrogen (Figure S2,
Supporting Information). The kinetic curves shown in Figure 3
were obtained when dehydrogenating FA under constant con-
ditions except for the temperature (70–100 °C).
Figure 2. RAPTA type precatalyst 3.
Herein, we investigated a catalytic system for formic acid
dehydrogenation in aqueous media with [(η6-benzyldimethyl-
amine)Ru(PTA)Cl2]Cl (3) as a precatalyst, which is synthe-
sized from dimeric [(η6-benzyldimethylamine)RuCl2]2Cl2 (2)
and one equivalent of PTA per central Ru atom. The dimeth-
ylated amino group connects via a single CH2 linker to the
arene. The amino group introduces a range of new possibilities
and features to the molecule compared to purely aliphatic aro-
matics. In the first place, the solubility in aqueous media is
Figure 3. Conversion of FA (dehydrogenation) vs. time; [(η6-benzyldi-
methylamine)Ru(PTA)Cl2]+ to CO2 and H2 at 70–100 °C; 222 mg FA
in 1778 mg water and catalyst load 0.0165 m.
The activation parameters were assessed with an Arrhenius
considerably increased but also from a mechanistic point of plot (Figure 4), and the activation energy (Ea) was determined
view, the in acidic media positively charged trialkylammonium to be 95.59Ϯ4 kJ·mol–1. The found Ea is higher than other in
functionality, offers interesting options. An important aspect literature reported values for the same reaction with iridium
when selecting this arene was that the amino group could help (77.94Ϯ3.2 kJ·mol–1, [Cp*Ir(1,2-diaminocyclohexane)Cl]Cl)[12c]
to coordinate substrate molecules via hydrogen bonding and or iron (+76.05Ϯ7 kJ·mol–1, [Fe(PP3TS)], PP3TS = m-tri-
also stabilize intermediates during the dehydrogenation pro- sulfonated-tris[2-(diphenylphosphanyl)ethyl]phosphine
so-
cess through its charge via an outer-sphere mechanism.[12b,26] dium salt).[27] The catalyst dehydrogenated the formic acid en-
It should be mentioned that the linker measures only one car- tirely, but when attempting recycling experiments, the catalyst
bon atom to prevent the amino moiety from interacting directly showed signs of degradation and the catalytic activity broke
with the central ruthenium atom via its lone pair.[21a] The PTA, in. From the formation of black debris and metallic depositions
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on the walls of the tube was deduced that reduction of the
central RuII atom is the most plausible cause for inactivation
and further reasoned that a second PTA could provide im-
proved compound stability.
Figure 5. Conversion of FA (dehydrogenation) vs. time at different
temperatures (70–100 °C); catalyst: in-situ generated catalyst.
Figure 4. Arrhenius plot to determine the Ea of the catalyst 3; the
necessary kinetic data was derived from pressure vs. time curves; only
the linear part of the curves was taken into consideration for the extrac-
tion of kinetic data.
In 2004, Kathó et al. reported on
a water-soluble
(η6-arene)ruthenium(II)-phosphine complexes (arenes are
benzene and p-cymene) and their catalytic activity in hydrogen
carbonate hydrogenation in aqueous solution.[28] Of particular
interest to us was the described mode of catalyst formation,
which was accomplished by in-situ combination of the dinu-
clear metal precursor and the desired amount of PTA in the
reaction vessel. Among other mutually catalytically active spe-
cies, they suggest [(η6-arene)Ru(PTA)2H]+ as intermediate
Figure 6. Arrhenius plot to determine the Ea of the in-situ formed
species; the necessary kinetic data was derived from pressure measure-
ments (Figure 5).
structure.
A
structurally related compound, [(η6-p-
cymene)Ru(PTA)2Cl]BPh4, was described by Peruzzini et al.
in 2008.[29] Accordingly, we combined the precatalyst 3 in
water with one equivalent PTA, added substrate and started the
reaction. First, we observed a color change from shades of
red to an orange-yellow solution, then an increased catalytic
activity.
acid is dehydrogenated. Meanwhile, in the 13C NMR, a carbon
dioxide peak is emerging at δ = 125.1 ppm (see Figure S1,
Supporting Information).
The curves in Figure 5 were obtained under the same experi-
mental conditions as outlined for Figure 3. The dehydrogena-
tion progresses significantly faster in with the modified cata-
lyst, therefore the time axis is no longer in logarithmic scale.
The activation energy was assessed by an Arrhenius plot (Fig-
ure 6) and the apparent activation energy (Ea) was determined
to be 88.67Ϯ4 kJ·mol–1, a value lower than the one of the
initial compound and thus providing a possible explanation for
the increased activity at equal temperatures.
The second technique, real-time NMR spectroscopy, enabled
us to observe the reaction progress on a molecular level at all
times. For this purpose, the reaction occurred in sealed me-
dium pressure sapphire tubes within the NMR instrument. Fig-
ure 7 shows the proton spectrum of 13C labeled formic acid
dehydrogenation by the in-situ generated catalyst. Formic acid
appears as a doublet (7.75 and 8.30 ppm, J = 218.8 Hz) since
the carbon-13 induced splitting occurs almost quantitatively,
while the small peak in the center (δ =8.03 ppm) results from
Figure 7. Formic acid dehydrogenation at 80 °C. 1H NMR (400 MHz)
spectrum with 13C label FA; the conditions were identical as for the
unlabeled formic acid. Between spectrum 5 and 6, all formic pressure measurements.
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1H NMR measurements ensured that the catalyst is capable first coordination sphere where phosphorus and one hydride
of completely consuming all formic acid, an important aspect are present.
when using pressure measurements to follow the reaction to
completion.
Conclusions
Transition metal hydrides are crucial intermediates in many
catalytic cycles, especially when transfer hydrogenation,
hydrogen evolution or hydrogenation is observed.[30] For this
reason, we examined our compound towards metal hydrides
and identified a triplet in the hydride region at –11.91 ppm
(J = 40.2 Hz, Figure 8). The next logical step was to look for
a corresponding nucleus, which was coupling with the hydride.
We synthesized and evaluated a RAPTA-type compound
towards its catalytic properties for selective formic acid dehy-
drogenation. The compound proved to be active, but the over-
all performance was below our expectations. The activity be-
came significantly improved when introducing a second PTA
in-situ, obtaining the mutually active structure [(η6-benzyldi-
methylamine)Ru(PTA)2H]2+, on which was reported earlier.
The activation energies for both systems were determined, and
NMR spectroscopy was employed to confirm the complete de-
hydrogenation of formic acid in the reaction mixture.
n
31P spectra, a doublet at δ = –23.68 (Figure 9) featured an
almost identical coupling constant (J = 39.2 Hz). The two sig-
nals are not only linked by their almost identical coupling con-
stant but also phosphorus decoupled measurement caused the
triplet in the hydride region to collapse into a singlet signal
(Figure S3 A/B, Supporting Information). Likewise, 31P{1H}
scans afforded a singlet in the 31P NMR spectrum at the same
place as the doublet appeared before (Figure S3 C/D, Support-
ing Information). These findings serve as confirmation for a
Experimental Section
Materials: Solvents and chemicals were purchased from commercial
suppliers and used without further purification. RuCl3(H2O)n (Ru: 40–
43%) was obtained from Precious Metals Online (PMO Pty Ltd). All
solvents had at least HPLC grade (for synthesis) or analytical grade
(p.a.).
Instruments: Microwave syntheses were carried out with a Biotage
Initiator 2.0 microwave synthesizer (400 W) in 20 mL microwave vials
and equipped with magnetic stirring bars. NMR experiments were per-
formed with a Bruker AV-400 (5 mm) for verification of the synthe-
sized precatalysts while the kinetic measurements were recorded with
a Bruker AVIII-400 in 10 mm medium pressure sapphire tubes.[31]
MestReNova 11.0.2 was used for spectra analysis and evaluation.
Synthesis of 1-(Cyclohexa-2,5-dien-1-yl)-N,N-dimethylmethan-
ammonium Chloride (1): A Birch reduction of N,N-dimethyl-1-phen-
ylmethanamine (11.7 mL; density = 0.9; 10.5 g, 0.0776 mol) with lith-
ium
afforded
1-(cyclohexa-2,5-dien-1-yl)-N,N-dimethylmethan-
ammonium chloride as white crystalline powder (61% yield). 1H
NMR (400 MHz, deuterium oxide): δ = 5.96 (s, 1 H), 5.71 (s, 2 H),
3.60 (s, 2 H), 2.77 (s, 6 H), 2.71 (d, J = 6.7 Hz, 2 H), 2.62 (d, J =
8.7 Hz, 2 H) ppm.
Figure 8. Signals in the hydride region were detected; triplet at δ =
–11.91 ppm (t, J = 40.2 Hz) in H2O ([D6]benzene inlay).
Synthesis
of
[(η6-Benzyldimethylamine)RuCl2]2Cl2
(2):
RuCl3(H2O)n (300 mg, 1.147 mmol) was dissolved in a mixture of iso-
butanol, water, and acetone (6:2:2.2 mL) in a 20 mL microwave tube,
and 1 (3.155 mmol, 548 mg, excess 2.75) was added. Flushing with
nitrogen did not result in higher yields. The sample was placed in a
microwave oven and heated up to 145 °C for 5 min. After cooling to
0 °C, the product was collected by filtration as dark red crystals and
washed with methanol, pentane and diethyl ether. The crystals were
dissolved in water/MeOH (80:20), filtered through celite to remove
traces of unreacted metal. Finally, the filtrate was evaporated to obtain
323 mg of a bright red crystalline substance (yield 82%). 1H NMR
(400 MHz, [D6]DMSO): δ = 10.77 (s, 1 H), 6.39–5.92 (m, 5 H), 4.07
(s, 2 H), 2.83 (s, 6 H) ppm.
Synthesis of [(η6-Benzyldimethylamine)Ru(PTA)Cl2]Cl (3): The di-
nuclear compound 2 (150 mg, 0.218 mmol) was dissolved in water
(15 mL, previously degassed by bubbling N2 through for 15 min) and
warmed to 90 °C in a nitrogen atmosphere. One equivalent of PTA
(68.6 mg, respective to Ru atoms) was added. An immediate color
change was observed. After keeping the reaction mixture at the same
Figure 9. 31P NMR revealed a doublet at δ = –23.68 ppm (d, J =
39.2 Hz) in H2O ([D6]benzene inlay).
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pound collected, washed consecutively with 2-propanol, ether, and
dried in vacuo. The compound was isolated in 71% yield (155 mg) 1H
NMR (400 MHz, deuterium oxide): δ = 6.11–5.78 (m, 5 H), 4.54 (s,
6 H), 4.27 (s, 6 H), 4.11 (s, 2 H), 2.93 (s, 6 H). 31P NMR (162 MHz,
deuterium oxide): δ = –29.95 ppm.
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1
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Acknowledgements
École Polytechnique Fédérale de Lausanne (EPFL), Swiss National
Science Foundation (Grant 200020_162351), Swiss Competence Cen-
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Keywords: Formic acid; Carbon dioxide utilization; Formic
acid dehydrogenation; Hydrogen storage; Homogeneous
catalysis; RAPTA; PTA; Ruthenium catalyst; Sapphire tubes;
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