Hydrogen Production from Formic Acid and Formaldehyde over Ruthenium Catalysts in Water

Article abstract of DOI:10.1021/acs.inorgchem.9b02882

Water-soluble ruthenium complexes [(??⁶-arene)Ru(κ²-L)]ⁿ⁺ (n = 0,1) ([Ru]-1-[Ru]-9) ligated with pyridine-based ligands are synthesized, and the molecular structure of the representative complex [Ru]-2 is confirmed by X-ray crystallography. The studied complexes are employed for the catalytic dehydrogenation of formic acid in water. Screening of these complexes inferred that [Ru]-1 [(??⁶-C₁₀H₁₄)Ru(κ²-N_pyOH-L1)Cl]⁺ (L1 = pyridine-2-ylmethanol) outperformed others with an initial turnover frequency of 1548 h^-1. Complex [Ru]-1 also exhibited high stability in water and can be recycled up to seven times with a total turnover number of 6050. In addition to formic acid dehydrogenation, [Ru]-1 also catalyzed the conversion of formaldehyde to hydrogen gas in water under base-free conditions. The effects of temperature, pH, formic acid, and catalyst concentration on the reaction kinetics are investigated in detail. Mass and NMR based mechanistic investigations inferred the presence of several important intermediate species, such as ruthenium-formate species [Ru]-1B and ruthenium-hydride species [Ru]-1C, involved in the catalytic dehydrogenation reaction. Moreover, the molecular structure of a diruthenium species [Ru]-1A′ is also authenticated by single-crystal X-ray crystallography.

Full text of DOI:10.1021/acs.inorgchem.9b02882

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Hydrogen Production from Formic Acid and Formaldehyde over
Ruthenium Catalysts in Water
Soumyadip Patra and Sanjay K. Singh*
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ABSTRACT: Water-soluble ruthenium complexes [(η6-arene)Ru(κ2-L)]n+ (n = 0,1) ([Ru]-1−[Ru]-9) ligated with pyridine-based
ligands are synthesized, and the molecular structure of the representative complex [Ru]-2 is confirmed by X-ray crystallography. The
studied complexes are employed for the catalytic dehydrogenation of formic acid in water. Screening of these complexes inferred that
[Ru]-1 [(η6-C10H14)Ru(κ2-NpyOH-L1)Cl]+ (L1 = pyridine-2-ylmethanol) outperformed others with an initial turnover frequency of
1548 h−1. Complex [Ru]-1 also exhibited high stability in water and can be recycled up to seven times with a total turnover number
of 6050. In addition to formic acid dehydrogenation, [Ru]-1 also catalyzed the conversion of formaldehyde to hydrogen gas in water
under base-free conditions. The effects of temperature, pH, formic acid, and catalyst concentration on the reaction kinetics are
investigated in detail. Mass and NMR based mechanistic investigations inferred the presence of several important intermediate
species, such as ruthenium-formate species [Ru]-1B and ruthenium-hydride species [Ru]-1C, involved in the catalytic
dehydrogenation reaction. Moreover, the molecular structure of a diruthenium species [Ru]-1A′ is also authenticated by single-
crystal X-ray crystallography.
development of an effective catalyst is essentially required for
the selective production of H2 from formic acid by suppressing
this side reaction. Beller et al.21 and Laurenczy et al.22
independently reported efficient and selective ruthenium
catalysts for the production of hydrogen gas from formic
acid (Scheme 1 and Table S1). Laurenczy et al. performed
formic acid (HCOOH/HCOONa = 9:1) dehydrogenation in
aqueous solution using [Ru(H2O)6](tos)2/TPPTS (tos is p-
toluene sulfonate and TPPTS is m-trisulfonated triphenyl-
phosphine; 1 in Scheme 1), to achieve a turnover frequency of
460 h−1 at a temperature of 120 °C.22 Since then, many
efficient homogeneous catalysts for formic acid dehydrogen-
INTRODUCTION
■
Sustainable development of our society requires the use of
renewable energy and reduction of the global dependency over
depleting fossil fuel resources. Hydrogen gas is one of the most
promising alternative sources of energy for the next
generation,1 but its explosive nature limits the safe storage
and transportation of hydrogen.2−4 In this context, worldwide
scientific efforts are concentrated on the liquid organic
hydrogen carriers (LOHCs)5,6 such as formic acid (4.4 wt %
H2),7−12 formaldehyde (8.4 wt % H2 HCHO-H2O),13−18 and
methanol (12.5 wt % H2),11 which are not only stable and safe
to handle and transport but also release hydrogen under
relatively mild conditions in the presence of a suitable catalyst.
Moreover, formic acid can be produced either from biomass or
directly by the catalytic hydrogenation of CO2.19,20 Most
importantly, hydrogen generation from formic acid has a low
reaction enthalpy and is thermodynamically favorable, allowing
the dehydrogenation reaction to occur under mild reaction
conditions.9 However, the dehydration of formic acid to CO
gas is also thermodynamically favorable,11 and hence the
Received: September 27, 2019
https://dx.doi.org/10.1021/acs.inorgchem.9b02882
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Scheme 1. Literature Available Ruthenium-Based Catalysts for Formic Acid Dehydrogenation in Aqueous Medium
ation have been reported,7−11,23−25 but most of the catalysts
explored so far either require organic additives (e.g., amines)
or are active in organic solvents (Table S1).26−31
Cl)(μ-H)]+ (5 in Scheme 3) in water at 95 °C.13,14 Later,
several (η5-C5Me5)Ir(III) and Ru(II) based catalysts were
explored for formaldehyde dehydrogenation in water.15−17 For
instance, Grutzmacher et al. investigated the ruthenium
catalyst for formaldehyde dehydrogenation and achieved a
turnover number of 1787 and initial TOF > 20 000 h−1, albeit
reaction required strongly alkaline conditions in H2O/THF
solution.17 Very recently, Himeda et al. reported a water-
soluble arene−ruthenium complex (6 in Scheme 3) for an
efficient additive-free dehydrogenation of aqueous form-
aldehyde with a turnover number of 24 000.49 Hence, it is
evident that dehydrogenation of formic acid and formaldehyde
in water can be achieved over a suitably designed catalytic
system.
Herein, we report water-soluble arene−ruthenium com-
plexes [(η6-arene)Ru(κ2-L)]n+ (n = 0,1; [Ru]-1−[Ru]-9)
bearing pyridine-based ligands as active catalysts for hydrogen
production from formic acid in water. Molecular structure of a
representative complex [Ru]-2 is authenticated by X-ray
crystallography. In addition, the roles of reaction temperature,
pH, formic acid, and catalyst concentration on the reaction
kinetics are investigated in detail. Moreover, mechanistic
insights are elaborated by identifying several catalytic
intermediates involved in various steps of the catalytic
dehydrogenation of formic acid under the catalytic and
controlled reaction conditions. The structure of a diruthenium
species, possibly the catalyst resting state, is also established by
X-ray crystallography. Encouraged by the results obtained in
formic acid dehydrogenation, we also employed the most
active catalyst [Ru]-1 for the base-free hydrogen production
from formaldehyde in water under moderate reaction
conditions.
Notably, the use of organic additives or solvents is not
suitable for PEM fuel cells, and hence the development of
aqueous catalytic systems is highly sought after.32 Until now,
only a limited number of catalysts, mostly iridium-based
complexes, have been found to be very effective for formic acid
dehydrogenation in water in the absence of any organic
additives (Table S1) .7−11,33−39 Himeda et al. reported a family
of (η5-C5Me5)Ir(III) complexes bearing N,N-donor bidentate
ligands as active catalysts for formic acid dehydrogenation in
an aqueous medium, where the electron-donating ability of the
ligands to the metal center is found to be a crucial parameter to
achieve higher catalytic activity.40−46 Although (η5-C5Me5)Ir-
(III) based catalysts have shown very promising results for
hydrogen production from formic acid, development of other
less expensive alternative catalysts such as Ru based complexes
are also gaining attention. For instance, Huang et al.
investigated formic acid dehydrogenation over [(η6-C10H14)-
RuCl(2,2′-bi-2-imidazolin)]+2 (2 in Scheme 1) at 90 °C in
water.24 On the other hand, Gonsalvi et al. investigated RuCl3·
3H2O, in the presence of different water-soluble phosphines
bearing sulfonated groups as ligands (3 in Scheme 1), for
formic acid dehydrogenation in water at 90 °C.47 Recently, we
have also investigated arene−ruthenium catalyst [(η6-C6H6)-
Ru(κ2 -Np y NHMe-MAmQ)Cl]+ (MAmQ is 8-(N-
methylamino)quinoline; 4 in Scheme 1) for formic acid
dehydrogenation in water at 90 °C. Our findings including
detailed mechanistic investigations inferred that the 8-amino-
quinoline ligands played a crucial role in the observed
enhanced catalytic activity.48
Analogous to formic acid, formaldehyde is also emerging as
an attractive liquid organic hydrogen carrier (LOHC), as
formaldehyde exhibits long-term stability (as methanediol in
water), is nonflammable, and a has a hydrogen weight
efficiency of 8.4 wt % (HCHO-H2O).13 In addition to this,
the dehydrogenation of a formaldehyde/water system is
exothermic (ΔHr = −35.8 kJ mol−1) in nature and hence is
thermodynamically favorable.13 The catalytic dehydrogenation
of formaldehyde involves a two-step dehydrogenation pathway,
(i) the first being water-assisted dehydrogenation of form-
aldehyde to formic acid and one equivalent of H2 and (ii)
finally dehydrogenation of formic acid to release the second
equivalent of H2 and one equivalent of CO2.13−18 In this
direction, Prechtl et al. first reported hydrogen production
from formaldehyde using the diruthenium catalysts [(η6-p-
cymene)RuCl2]2 and [{Ru(η6-p-cymene)}2(μ-HCOO)(μ-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Catalysts. Mono-
cationic water-soluble arene−ruthenium complexes [Ru]-1−
[Ru]-4 bearing pyridine-2-ylmethanol (L1) and 1-(pyridin-2-
yl)ethanol (L2) are synthesized in good yields by treating the
respective ligands with the arene−ruthenium precursors [(η6-
arene)RuCl2]2 (arene = C6H6 and C10H14) in methanol under
refluxing conditions (Scheme 2, see the Experimental Section).
The complexes [Ru]-5−[Ru]-8 are synthesized in accordance
with the previously reported procedures, and the character-
ization data of these complexes corroborate well with their
established molecular structures.50,51 Analogous monocationic
arene−ruthenium complex [Ru]-9 is also synthesized by
treating pyridine-2ylmethanamine (L3) with [(η6-C10H14)-
B
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Scheme 2. Complexes Explored for the Dehydrogenation of
Formic Acid in Water
Figure 1. Single crystal X-ray molecular structure of [Ru]-2. Selected
bond lengths (Å) and angles (deg): Ru(1)−N(1) 2.1035(16),
Ru(1)−O(1) 2.1307(14), Ru(1)−Cl(1) 2.4060(5), N(1)−Ru(1)−
O(1) 76.08(6), N(1)−Ru(1)−C(11) 123.45(11), O(1)−Ru(1)−
C(11) 159.86(11).
O, and Ct−Ru−Cl are 133.23°, 130.94°, and 128.15°,
respectively), which is consistent with the piano stool
geometry of [Ru]-2.6,50,51 The centroid (Ct) to Ru distance
(1.422 Å) and Ru−Cl bond length (2.406 Å) are also
consistent with analogous (η6-C6H6)Ru(II) complexes.51 The
ligand pyridine-2-ylmethanol (L1) is coordinated with the
ruthenium metal in a bidentate fashion, involving the nitrogen
atom (Npy) of the pyridine ring and the oxygen atom of the
hydroxyl group. The Ru−Npy and Ru−O bond lengths are
2.104 and 2.131 Å, respectively, which is consistent with the
sp2 hybridized Npy as observed for analogous arene−ruthenium
complexes.51 The important crystallographic details and
selected bond parameters are summarized in Tables S2−S4
(Supporting Information).
Ruthenium Catalyzed Dehydrogenation of Formic
Acid in Water. Preliminary investigation for screening the
most active catalyst among the complexes [Ru]-1−[Ru]-9 (1
mol %) inferred that the complexes [Ru]-1−[Ru]-8 bearing
pyridine-based N,O donor ligands outperformed [Ru]-9
containing pyridine-2yl-methanamine (L3) for hydrogen
generation from formic acid (0.4 M, 2.5 mL) in the presence
of sodium formate (5 mol %) at 90 °C in water (Table 1 and
Figure 2a). Moreover, the η6-arene ring exerted a significant
effect on the catalytic activity, where the electron rich (η6-
C10H14)Ru(II) complexes ([Ru]-1, [Ru]-3, [Ru]-5, [Ru]-6,
and [Ru]-8) exhibited higher activity than the (η6- C6H6)Ru-
(II) complexes ([Ru]-2, [Ru]-4, and [Ru]-7). Among these
catalysts, [Ru]-1 displayed the highest turnover frequency
(TOF) of 660 h−1 under the optimized reaction conditions
(Table 1, entry 1). Analogous to [Ru]-1, the 1-(pyridin-2-
yl)ethanol ligated (η6-C10H14)Ru(II) complex [Ru]-3 also
exhibited similar activity (TOF 653 h−1). It is interesting to
note that the cationic (η6-C10H14)Ru(II)-pyridyl-ethanone
complex [Ru]-5 also displayed appreciably good activity
(TOF 535 h−1). On the other hand, [Ru]-6 bearing a strongly
chelating κ2-hydroxyquinoline ligand exhibited lower activity
(TOF 475 h−1). Moreover, ruthenium−picolinate complex
[Ru]-8 exhibited the lowest activity (TOF 237 h−1). The
observed trends in the catalytic activities are associated with
the involvement of oxygen atoms in the facile de/protonation
pathway during the dehydrogenation reaction and the
coordination behavior of the ligands in the complexes [Ru]-
1−[Ru]-9 (Table 1 and Figure 2a). Sodium formate
concentration exerted a crucial role in enhancing the rate of
formic acid dehydrogenation over [Ru]-1. Results inferred that
TOF gradually increased with the increase in sodium formate
RuCl2]2 in methanol under refluxing conditions.52 The
obtained yellow to brown colored complexes are characterized
using various spectro-analytical techniques, which corrobo-
rated well with the proposed structures. ESI-mass of the
obtained complexes showed prominent mass peaks corre-
sponding to the cationic mononuclear arene−ruthenium
complexes with general formula [(η6-C6H6)RuCl(κ2-L)]+ (L
= L1, L2) for the complexes [Ru]-1−[Ru]-4 (see the
Experimental Section).
1H NMR spectra of the complexes [Ru]-1−[Ru]-4
displayed the downfield shift in the chemical shift for the
protons of the ligands as compared to that of respective free
ligands, which is consistent with the coordination of these
ligands with the (η6-arene)Ru(II) moiety. In [Ru]-1, pyridine
protons of L1 resonated in the range of 7.39−9.17 ppm as
compared to that observed in the free ligand L1 (7.36−8.54
ppm). Analogously for [Ru]-2, pyridine protons of L1
resonated in the downfield region of 7.38−9.22 ppm. Pyridine
protons of L2 in [Ru]-3 also resonated in the downfield region
of 7.36−9.21 ppm as compared to that observed for the free
ligand L2 (7.19−8.36 ppm). In addition, the CH3 protons of
L2 appeared as a doublet at 1.52 ppm for [Ru]-3. A similar
trend in the aromatic and methyl protons of ligand L2 is also
observed for [Ru]-4. In addition, methylene (−CH2−)
protons of L1 in complexes [Ru]-1 and [Ru]-2 resonated in
their usual chemical shift value.53 The 1H NMR resonances for
the protons corresponding to the η6-C10H14 ring coordinated
to ruthenium in [Ru]-1 and [Ru]-3 are observed in the
expected region.51 Moreover, the ruthenium bound η6-C6H6
ring protons resonated as a singlet in the range of 5.92 and
5.95 ppm respectively for [Ru]-2 and [Ru]-4, suggesting the
equivalence of a ruthenium coordinated benzene ring in [Ru]-
2 and [Ru]-4. Further, the molecular structure of the
representative complex [Ru]-2 is also confirmed by X-ray
crystallography using X-ray suitable crystals of [Ru]-2 grown
by slow diffusion of diethyl ether in a methanol solution of
[Ru]-2 at room temperature (Figure 1 and Tables S2−S4).
[Ru]-2 crystallized in a triclinic crystal system with the P1
̅
space group. The geometry around the ruthenium metal center
is pseudo-octahedral with a piano-stool geometry of [Ru]-2,
where the η6-C6H6 ring occupied the top of the piano stool and
the legs of the stool are occupied by the κ2-L1 and chloro
ligands. The bond angles from the η6-C6H6 ring centroid (Ct)
to each of the legs are more than 120° (Ct−Ru−Npy, Ct−Ru−
C
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hydronium (H3O+) and formate (HCO2 ) ions in formic acid
dehydrogenation. At 60 °C, dehydrogenation of formic acid is
found to be sluggish, while with the increase in the reaction
temperature TOF also increased (Figure S15a in Supporting
Information). The apparent activation energy (Ea) of 25.4 kcal
mol−1 (Figure S15b in Supporting Information) revealed that
the catalyst [Ru]-1 is inherently active for formic acid
dehydrogenation in water.24,37,48
−
Table 1. Dehydrogenation of Formic Acid in Water Using
[Ru]-1−[Ru]-9
a
T
n (formic acid)/n (sodium
TOF
b
b
entry catalyst (°C)
formate)
TON
(h−1
)
1
2
3
4
5
6
7
8
[Ru]-1
[Ru]-2
[Ru]-3
[Ru]-4
[Ru]-5
[Ru]-6
[Ru]-7
[Ru]-8
[Ru]-9
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
90
1/0.05
1/0.05
1/0.05
1/0.05
1/0.05
1/0.05
1/0.05
1/0.05
1/0.05
1/−
99
95
98
95
98
95
91
91
95
95
660
380
653
380
535
475
366
237
95
[Ru]-1 also exhibited appreciably good stability in water and
in the presence of a higher concentration of formic acid
([HCOOH]/[cat] up to 7000; Figure 3 and Figure S16 in the
9
10
11
12
13
14
15
16
17
18
146
−/1
1/0.02
1/0.04
1/0.5
5/5
5/10
5/15
95
98
95
450
450
450
880
228
490
1141
1241
1446
1262
1548
c
c
c
c
10/20
a
Reaction conditions: formic acid (0.4−4.0 M, 2.5 mL), catalyst (0.01
b
Figure 3. Recyclability experiment for the catalytic dehydrogenation
of formic acid over [Ru]-1. Reaction conditions: formic acid (4.0 M,
2.5 mL), [Ru]-1 (1 mol %), n(formic acid)/n(sodium formate) = 1:2,
90 °C (10 mmol of formic acid is added to the reaction mixture after
each run).
mmol), 90 °C. Each reaction is repeated twice. TONs and TOFs are
determined after completion of the reaction unless specified
otherwise. Initial TOFs (15 min).
c
content from 0.02 to 0.5 equiv with respect to formic acid. An
initial TOF of 1548 h−1 is achieved for the dehydrogenation of
formic acid (4 M, 2.5 mL) with 2 equiv of sodium formate over
[Ru]-1, under the optimized reaction conditions (Table 1, and
Table S5 and Figures S1 and S2 in the Supporting
Information). The produced gas is analyzed as a mixture of
H2 and CO2 (H2/CO2 1:1) using GC-TCD, which is
consistent with the expected H2/CO2 ratio for formic acid
dehydrogenation (Figures S3−S14 in the Supporting In-
formation). Figure 2b displayed the pH dependent TOF for
[Ru]-1 catalyzed formic acid dehydrogenation. As inferred
from the graph, TOF increased with the decrease in pH from
7.5 to 4.0, and a highest TOF of 1548 h−1 is achieved at pH ≈
4.0. Moreover, with a further decrease in pH < 4.0, TOF also
decreases. These observations suggest the important role of
Supporting Information). The double logarithmic plots of the
initial reaction rates of formic acid dehydrogenation (1.0 M,
2.5 mL) with the concentration of [Ru]-1 (0.01 mmol −0.04
mmol) followed a linear dependence (Figure S17 in
Supporting Information). The obtained order of 0.97 with
respect to the catalyst concentration suggests that presumably
the monomeric species of the [Ru]-1 catalyst generated during
the initial period of the catalytic reaction is the most active
species involved in formic acid dehydrogenation reaction. It is
noteworthy to mention that a turnover number up to 6050 is
achieved for the dehydrogenation of formic acid using the
[Ru]-1 catalyst for seven consecutive catalytic runs at 90 °C,
evidencing the higher stability of [Ru]-1 in water (Figure 3).
Figure 2. (a) Comparative catalytic activity of [Ru]-1−[Ru]-9 for formic acid dehydrogenation. Reaction conditions: formic acid (0.4 M, 2.5 mL),
catalyst (1 mol %), sodium formate (0.05 mmol), 90 °C. (b) pH dependent dehydrogenation of formic acid over [Ru]-1, where pH is altered by
tuning n(formic acid)/n(sodium formate) ratio. Reaction condition: [Ru]-1 (1 mol %), formic acid (4.0 M, 2.5 mL), 90 °C.
D
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Figure 4. Various intermediate species observed during the mass investigation for formic acid dehydrogenation reaction performed over [Ru]-1
under varying controlled reaction conditions and X-ray molecular structure of the dimeric ruthenium species [Ru]-1A′.
Further, the reaction order of 0.43 for the dehydrogenation of
varying concentrations of formic acid (0.2−0.8 M) over 0.01
mmol of [Ru]-1 catalyst at 90 °C evidenced the equimolar
interaction of formic acid or formate ions with a Ru center to
form [HCOO-Ru] species during the catalytic dehydrogen-
ation of formic acid (Figure S18 in the Supporting
Information).24,37,48
172), and [(η6-10H14)Ru(κ2-NpyO-L1)]+ ([Ru]-1A, m/z 344;
pH of the solution was 2.1; Figure 4 and Figure S23 in the
Supporting Information). Further, a weak mass peak at m/z =
412 ([Ru]-1B + Na+, Figure S24 in the Supporting
Information) corresponding to formate coordinated ruthenium
species [Ru]-1B is also observed when formic acid−sodium
formate (10:1) was added to [Ru]-1 in water at room
temperature (pH of the solution was 2.6; Figure S24 in the
Supporting Information). However, [Ru]-1B is not observed
during the mass investigation of the sodium formate assisted
dehydrogenation of formic acid over [Ru]-1 at 90 °C (Figures
S20 and S21 in the Supporting Information). It has been
observed that in the presence of sodium formate, an intense
mass peak at m/z 344 corresponding to [Ru]-1A appeared as
the only most populated species in the reaction aliquot, while
the species [Ru]-1D, [Ru]-1E, and [Ru]-1B are not observed
(Figure 4, Figure S20 and Figure S25 in the Supporting
Information). Further, in an attempt to isolate [Ru]-1A via
crystallization, we eventually obtained a dimeric form of [Ru]-
1A, [{(η6-C10H14)Ru(κ2 N,O-μ-O-L1}2]2+ designated as [Ru]-
1A′ (Figure 4). The X-ray molecular structure of [Ru]-1A′ is
displayed in Figure 4. While the crystallographic refinement
data for [Ru]-1A′ is not satisfactory for reporting, the obtained
molecular structure is consistent with the proposed structure of
[Ru]-1A′ (Figure S26 and Tables S6−S8 in the Supporting
Information). It is evident that coordinatively unsaturated
[Ru]-1A species dimerized to form [Ru]-1A′, where an oxygen
atom of pyridine-2-ylmethanol (L1) bridged two units of
[Ru]-1A to form a stable dicationic [Ru]-1A′. Similar
dimerization of analogous unsaturated metallic species is also
available in the literature.48,54 Further, we noted that the mass
spectra of the isolated dimeric species [Ru]-1A′ also shows an
intense peak at m/z = 344 (Figure S27 in the Supporting
Mechanistic Study. Mass investigations of formic acid
dehydrogenation reaction under catalytic and controlled
reaction conditions evidenced the presence of several
important ruthenium species. At first instance, a visible color
change from the initial yellow solution of the catalyst [Ru]-1 to
a red-orange solution is observed during the initial minutes of
the catalytic dehydrogenation of formic acid, and this color
remains stable even after the completion of the catalytic
reaction (Figure S19 in the Supporting Information). A
prominent mass peak at m/z 344 corresponding to [(η6-
C10H14)Ru(κ2-NpyO-L1)]+ [Ru]-1A is observed in the
reaction aliquot obtained after the catalytic dehydrogenation
of formic acid (0.4 M, 2.5 mL) over [Ru]-1 (1 mol %) in the
presence of sodium formate (0.05 mmol) at 90 °C (Figures
S20 and S21 in the Supporting Information). Further, the
addition of an excess of dilute HCl in the red-orange solution
obtained after the catalytic reaction resulted in the
regeneration of the yellow color. Mass analysis of this solution
exhibited the presence of a prominent mass peak at m/z 380.0,
suggesting the regeneration of [Ru]-1 (Figure S22 in
Supporting Information). Mass analysis of the reaction aliquot
obtained from the reaction of [Ru]-1 with formic acid (in the
absence of sodium formate) in water at room temperature also
inferred the presence of several ruthenium species including
[(η6-C10H14)Ru(κ2-NpyOH-L1)(H2O)]2+ ([Ru]-1D, m/z
181), [(η6-C10H14)Ru(κ2-NpyOH-L1)]2+ ([Ru]-1E, m/z
E
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Information). Therefore, it might be possible that the signals in
the mass spectra for [Ru]-1A are a fragment of [Ru]-1A′
formed by dissociation in the gas phase, and in solution [Ru]-
1A may be present as the coordinatively saturated compound
[Ru]-1A′.
Further, indication of the involvement of Ru-hydride species
[(η6-C10H14)Ru(κ2-NpyO-L1)(H)] ([Ru]-1C) in formic acid
dehydrogenation reaction is obtained by the presence of the
corresponding 1H NMR signals at δ = −10.56 to −10.98 ppm
during the reaction of [Ru]-1 (0.01 mmol) with HCOONa
(0.05 mmol) in D2O (0.6 mL) at room temperature (Figure
S28 in the Supporting Information). Moreover, kinetic isotope
effect (KIE) studies indicated that decarboxylation is the rate-
determining step and not the proton assisted release of
hydrogen gas in the catalytic cycle of formic acid dehydrogen-
ation (Table 2). Results inferred that the deuterated formic
Figure 5. Time course plot for the catalytic dehydrogenation of
formaldehyde over the catalyst [Ru]-1. Reaction conditions: [Ru]-1
(0.01 mmol), formaldehyde (0.4−4.0 M, 2.5 mL) in water at 90 °C.
Table 2. KIE in the Dehydrogenation of Formic Acid over
a
[Ru]-1
Table 3. Catalytic Dehydrogenation of Formaldehyde over
[Ru]-1 in Water
b
c
entry
catalyst
substrate
solvent
TOF (h−1
)
KIE
a
1
2
3
4
[Ru]-1
[Ru]-1
[Ru]-1
[Ru]-1
HCOOH
HCOOH
DCOOD
DCOOD
H2O
D2O
H2O
D2O
146
80
51
[Ru]-1
(mmol)
aq. formaldehyde
(mmol)
1.8
2.9
3.5
b
c
entry
TON
TOF (h−1
)
1
2
3
0.01
0.01
0.01
1
5
10
82(246)
317(961)
618(1838)
131(247)
328(807)
327(1072)
42
a
Reaction condition: [Ru]-1 (0.01 mmol), formic acid (0.4 M, 2.5
mL), 90 °C. Initial TOF at 30 min. KIE = TOF (entry 1)/ TOF
b
c
a
(entry n; n = 2, 3, 4).
Reaction conditions: All the reactions are performed at 90 °C. The
b
initial volume of the reaction is 2.5 mL. TON is determined at the
completion of the reaction (values given in parentheses are
determined by n(H2 + CO2)/n(catalyst)). Initial TOF (10 min),
c
acid (DCOOD) is more influential than D2O (Table 2, entries
2,3) in the reaction rate for the catalytic reaction performed
over [Ru]-1 with HCOOH in D2O or DCOOD in H2O under
the optimized reaction conditions. Therefore, these findings
inferred the crucial role of [Ru]-1A′ in the sodium formate
assisted formic acid dehydrogenation reaction, where initially
the coordination of formate to [Ru]-1A′ generated [Ru]-1B,
which further undergoes decarboxylation to form [Ru]-1C,
and finally [Ru]-1A′ is regenerated after the proton assisted
hydrogen gas release from [Ru]-1C.
Catalytic Dehydrogenation of Formaldehyde in
Water. The dehydrogenation pathway for complete con-
version of formaldehyde to H2 also involves in situ generation
of formate ions, which subsequently dehydrogenate to yield
two equivalents of H2 per molecule of formaldehyde in water.
Therefore, it is evident that most of the active catalytic species
involved in formic acid dehydrogenation may also be active for
formaldehyde dehydrogenation.
The time course plot for [Ru]-1 catalyzed gas generation
from varying concentrations of aqueous formaldehyde (0.4,
2.0, and 4.0 M) is displayed in Figure 5. The release of H2 and
CO2 gas from [Ru]-1 catalyzed dehydrogenation of form-
aldehyde is confirmed by GC-TCD analysis (Figure S29 in the
Supporting Information). To our delight, [Ru]-1 is also active
for the catalytic hydrogen production from formaldehyde (aq.)
at 90 °C in the absence of any additive or base. The [Ru]-1
catalyst performed well even with a higher concentration of
formaldehyde (4.0 M, 2.5 mL), to achieve a turnover number
of 1838 with an initial turnover frequency of 1072 h−1 (Table
3). Though the catalyst reported by Himeda et al. (6 in
Scheme 3) displayed the highest activity for formaldehyde
dehydrogenation under base-free conditions in water,49 the
observed catalytic performance of [Ru]-1 is promising.
Notably, the activity of [Ru]-1 is still several fold higher
values given in parentheses are determined by {n(H2 + CO2)/
n(catalyst)}/t.
than the earlier reported Cp*Ir(III) catalysts active under
alkaline conditions.15,16 Moreover, the turnover number of
[Ru]-1 is higher than the diruthenium complex (5 in Scheme
3) reported by Prechtl et al. for formaldehyde dehydrogen-
ation; however, we could achieve a turnover frequency of 1072
h−1 only using [Ru]-1 catalyst.13,14
In order to gain further insights into the reaction pathway of
formaldehyde dehydrogenation in water, composition of the
gas released was analyzed by GC-TCD and the pH of the
reaction medium was monitored at different time intervals of
the reaction progress (Figures S29 and S30 in Supporting
Information). By measuring the composition of the gas
released (showing a gradual increase in CO2:H2 ratio) and
the pH values, we could confirm that the reaction follows the
same pathway as described by Prechtl et al.13 Notably, time
dependent mass investigation of the catalytic reaction aliquots
inferred the presence of several ruthenium species ([Ru]-1A′,
[Ru]-1D, [Ru]-1E; Figures S31−S34 in the Supporting
Information). Moreover, the mass investigations also revealed
the presence of an m/z peak at 551.6 corresponding to the
formate bridged diruthenium species [{Ru(η6-p-cymene)}2(μ-
HCOO)(μ-Cl)(μ-H)]+ (5 in Scheme 3), as earlier reported by
Prechtl et al.13,14 The time dependent mass study for
formaldehyde dehydrogenation reaction also inferred the
presence of this diruthenium species along with few other
diruthenium species [{Ru(η6-p-cymene)(HCOO)}(μ-
HCOO)(μ-Cl){Ru(η6-p-cymene)(H2O)}]+ (m/z = 615.7),
which are presumably formed due to ligand dissociation.
Noticeably, mass investigation inferred that the mass peak at
F
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Scheme 3. Literature Available Ruthenium-Based Catalysts for Hydrogen Production from Formaldehyde in Water
m/z 344 corresponding to [Ru]-1A′ always remains the most
intense peak during the formaldehyde dehydrogenation
reaction (Figures S32−S34 in the Supporting Information).
Further, we also investigated the catalytic activity of [Ru]-1 for
formaldehyde dehydrogenation under the reaction conditions
as reported by Prechtl et al. (0.026 mmol (0.2 mol %) [Ru]-1,
13.55 mmol, formaldehyde (37 wt % aq. formaldehyde), 95
°C). Results inferred the release of 246 mL of gas (H2 and
CO2) in 200 min for [Ru]-1 catalyzed dehydrogenation of
formaldehyde (Figure S35 in the Supporting Information) in
comparison to the release of 180 mL of gas released in 200 min
using [{Ru(η6-p-cymene)}2(μ-HCOO)(μ-Cl)(μ-H)]+ (5 in
Scheme 3) under analogous base-free conditions as reported
by Prechtl et al.13 On the basis of our findings, we can
conclude that during formaldehyde dehydrogenation, initially
the coordination of methanediol to [Ru]-1A′ resulted in the
formation of Ru-methanediol species ([Ru]-1F), which
subsequently upon dehydrogenation to an equivalent of H2
(via a Ru−H intermediate) form a ruthenium-formate species
([Ru]-1B). Consequently, the Ru-formate species ([Ru]-1B)
undergoes decarboxylation to generate a Ru−H species ([Ru]-
1C). Further, an equivalent of H2 molecules is released from
Ru−H species via a proton assisted dehydrogenation to
regenerate the active catalytic species [Ru]-1A′. A plausible
reaction pathway for the catalytic dehydrogenation of form-
aldehyde over [Ru]-1 is displayed in Scheme 4.
Scheme 4. A Plausible Pathway for Dehydrogenation of
Aqueous Formaldehyde and Formic Acid over [Ru]-1
species [(η6-C10H14)Ru(κ2-NpyO-L1)(H)] ([Ru]-1C) during
the catalytic dehydrogenation of formic acid over [Ru]-1, and
hence, established the crucial role of these species in the
catalytic dehydrogenation of formic acid. We also isolated and
determined the X-ray molecular structure of the dicationic
diruthenium species [Ru]-1A′, the plausible catalyst resting
state. In addition, [Ru]-1 is also active for the hydrogen
production from formaldehyde in water under additive-free
and base-free conditions. Mass and GC-TCD investigation
support the two-step dehydrogenation pathway for form-
aldehyde over the [Ru]-1 catalyst, where several of the reaction
intermediates involved in formic acid dehydrogenation are also
identified during formaldehyde dehydrogenation. Hence, the
present catalytic system highlighting the integration of formic
acid dehydrogenation with formaldehyde dehydrogenation
over the same catalyst may seek broad scientific attention for
the development of other active catalysts for analogous
dehydrogenation reactions in water.
CONCLUSIONS
■
In summary, we employed several arene−ruthenium complexes
[Ru]-1−[Ru]-9 containing N,O/N,N donor ligands for the
catalytic dehydrogenation of formic acid in water. Results
inferred that the complex [(η6-C10H14)Ru(κ2-NpyOH-L1)Cl]+
[Ru]-1 outperformed other complexes with a turnover
frequency of 1548 h−1. The catalyst [Ru]-1 also exhibited
high stability in water, and a total turnover number of 6050 is
achieved while using the catalyst [Ru]-1 for seven consecutive
catalytic runs. The higher activity of [Ru]-1 can be associated
with the involvement of oxygen atoms in a facile protonation
deprotonation step during the dehydrogenation process. To
gain mechanistic insights, we probed extensive mass, NMR,
and kinetic investigations to evidence the formation of several
important organometallic intermediate species, such as the
diruthenium species [{(η6-C10H14)Ru(κ2 N,O-μ-O-L1}2]2+
([Ru]-1A′), the formate coordinated species [(η6-C10H14)Ru-
(κ2-NpyO-L1)(HCO2)] ([Ru]-1B), and the ruthenium hydride
G
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Synthesis of [(η6-C6H6)Ru(κ2-NpyOH-L2)Cl]+ ([Ru]-4). [{(η6-C6H6)-
RuCl2}2] (0.250 g, 0.5 mmol) is suspended in 30 mL of HPLC
methanol and stirred for 30 min at room temperature. Subsequently,
1-(pyridin-2-yl)ethanol (135 mg, 1.1 mmol) is added to it. The
reaction mixture is refluxed for 12 h, and then the volume of the
reaction mixture is reduced to 3 mL under reduced pressure. An
excess of diethyl ether is poured into the above methanolic solution to
precipitate a brown solid. Yield: 68% (0.254 g). 1H NMR (400 MHz,
MeOH-d4): 9.26 (d, J = 4.0 Hz, 1H), 7.94 (t, J = 8.0 Hz, 1H), 7.48 (t,
8.0 Hz 1H), 7.36 (d, J = 8.0 Hz, 1H), 5.95 (s, 6H), 4.98 (m, 1H), 1.53
(d, J = 8.0 Hz, 3H). 13C NMR (100 MHz, MeOH-d4): δ (ppm)
163.75, 155.14, 141.29, 126.61, 122.69, 85.01, 76.54, 21.84. ESI-MS
calcd. for [M]+ [C13H15ClNRuO]: 338.0. Found: 338.0.
Single-Crystal X-ray Diffraction Studies. Single crystals are
obtained by slow diffusion of diethyl ether into a methanolic solution
of [Ru]-2 and [Ru]-1A′. X-ray structural studies of [Ru]-2 and [Ru]-
1A′ are executed on a CCD Agilent Technologies (Oxford
Diffraction) SUPERNOVA diffractometer. Using graphite-monochro-
mated Cu Kα radiation (λ = 1.54184 Å)-based diffraction, data are
collected at 293(2) K by the standard “phi-omega” scan techniques
and are scaled and reduced using CrysAlisPro RED software. The
extracted data are evaluated using the CrysAlisPro CCD software.
The structures are solved by direct methods using SHELXL-2018/1
and refined by full-matrix least-squares methods, refining on F2.55The
positions of all of the atoms are determined by direct methods. All
non-hydrogen atoms are refined anisotropically. The remaining
hydrogen atoms are placed in geometrically constrained positions.
The CCDC number 1564211 contains the supplementary crystallo-
graphic data for [Ru]-2.
EXPERIMENTAL SECTION
■
Materials and Instrumentation. All reactions are performed
without any inert gas protection using high-purity chemicals
purchased from Sigma-Aldrich (Merck) and Alfa-Aesar. 1H NMR
(400 MHz) and 13C NMR (100 MHz) spectra are recorded at 298 K
using MeOH-d4 and D2O as solvents on a Bruker advance 400
spectrometer. The pH values were measured on a Eutech pH meter,
Model Eco Testr pH2. ESI-mass spectra are recorded on a micrOTF-
Q II mass spectrometer. The GC-TCD analyses are performed on a
Shimadzu GC-2014 system using a shin carbon-ST packed column.
Synthesis of the Ligand 1-(Pyridine-2-yl)ethanol (L2). In a
100 mL round-bottom reaction vessel, 1-(pyridine-2-yl)ethenone (10
mmol) is dissolved in 25 mL of methanol, and an excess of NaBH4
(12 mmol) is added to it. The reaction mixture is stirred at room
temperature for 4 h. After completion of the reaction, all volatiles are
removed under reduced pressure. The obtained solid residue is
dispersed in water (20 mL), and the organic product is extracted with
dichloromethane (3 × 15 mL). Further, the combined organic
fractions are dried over anhydrous Na2SO4, and the volatiles are
removed under reduced pressure to obtain the purified product as a
1
colorless liquid. Yield: 86% (1.06 g). H NMR (400 MHz, MeOH-
d4): δ (ppm) 8.36 (d, J = 4 Hz, 1H), 7.74 (t, J = 8.0 Hz, 1H), 7.49 (d,
J = 8 Hz, 1H), 7.19 (t, J = 8.0 Hz, 1H), 1.37 (d, J = 8 Hz, 3H).
Synthesis of Arene−Ruthenium Complexes ([Ru]-1−[Ru]-4).
[(η6-C10H14)Ru(κ2-NpyOH-L1)Cl]+ ([Ru]-1). [(η6-C10H14)RuCl2]2
(0.306 g, 0.5 mmol) is dissolved in 30 mL of HPLC methanol, and
pyridine-2-ylmethanol (101 μL, 1.1 mmol) is added to it. The
reaction mixture is refluxed for 12 h, and then a volume of the
reaction mixture is reduced to 3 mL under reduced pressure. An
excess of diethyl ether is poured into the above methanolic solution to
precipitate a yellow solid. Yield: 73% (0.305 g). 1H NMR (400 MHz,
MeOH-d4): δ (ppm) 9.16 (d, J = 8.0 Hz, 1H), 7.92 (t, J = 8.0 Hz,
1H), 7.50 (t, J = 8.0 Hz, 1H), 7.39 (d, J = 8.0 Hz, 1H), 5.89 (d, J = 4.0
Hz, 1H), 5.81 (d, J = 4.0 Hz, 1H), 5.66 (d, J = 8.0 Hz, 1H), 5.60 (d, J
= 8.0 Hz, 1H), 4.83−4.94 (m, 2H), 2.83−2.80 (m, 1H), 2.17 (s, 3H),
1.20 (d, J = 8.0 Hz, 6H). 13C NMR (100 MHz, MeOH-d4): δ (ppm)
160.51, 154.90, 141.04, 126.80, 121.92, 104.52, 100.42, 84.28, 82.98,
82.73, 81.01, 69.68, 32.32, 22.50, 22.22, 18.67. ESI-MS calcd. for
[M]+ [C16H21ClNRuO]: 380.0. Found: 380.0.
General Process for Formic Acid Dehydrogenation Reac-
tion. An aqueous solution (2.5 mL) containing the catalyst (as
specified), HCOONa (as specified), and formic acid (as specified) in
a two-necked 10 mL reaction tube, fitted with a condenser and a gas
buret, is stirred at 90 °C over a preheated oil bath. The volume of gas
produced is measured as the displacement of water in the buret with
respect to time. The identity of the produced gas is confirmed by GC-
TCD. The turnover number (TON) was calculated by the formula
[(substrate/catalyst) × (conversion/100)].48 The turnover frequency
(TOF) was calculated as TON/time.
Mechanistic Investigations for Formic Acid Dehydrogen-
ation over [Ru]-1 under Catalytic and Controlled Reaction
Conditions. Formic acid (1 mmol) and [Ru]-1 (0.01 mmol) were
dissolved in 2.5 mL of water and stirred at room temperature for 10
min. The resulting solution was analyzed by mass spectrometry to
identify several arene−ruthenium species (such as [Ru]-1A, [Ru]-1E,
and [Ru]-1G). Formic acid (1 mmol), sodium formate (0.05 mmol),
and [Ru]-1 (0.01 mmol) were dissolved in 2.5 mL of water and
heated at 90 °C. A reaction aliquot was collected from the reaction
mixture after 5 min and end of the reaction and analyzed by ESI-MS.
A controlled reaction is also performed to identify a formate
coordinated arene−ruthenium species ([Ru]-1B) by stirring [Ru]-1
(0.01 mmol), formic acid (0.5 mmol), and HCOONa (0.05 mmol)
for 10 min in water (2.5 mL) at room temperature. To identify the
ruthenium-hydride species, [Ru]-1 (0.01 mmol) was dissolved in 0.6
mL of D2O in an NMR tube, and HCOONa (0.05 mmol) was added
Synthesis of [(η6-C6H6)Ru(κ2-NpyOH-L1)Cl]+ ([Ru]-2). [{(η6-C6H6)-
RuCl2}2] (0.250 g, 0.5 mmol) is suspended in 30 mL of HPLC
methanol and stirred for 30 min at room temperature. Subsequently,
pyridine-2-ylmethanol (101 μL, 1.1 mmol) is added to it. The
reaction mixture is refluxed for 12 h, and then the volume of the
reaction mixture is reduced to 3 mL under reduced pressure. An
excess of diethyl ether is poured into the above methanolic solution to
precipitate a yellow solid. Yield: 72% (0.258 g). 1H NMR (400 MHz,
MeOH-d4): δ (ppm) 9.22 (d, J = 4.0 Hz, 1H), 7.92 (t, J = 8.0 Hz,
1H), 7.51 (s, 1H), 7.38 (d, J = 8.0 Hz, 1H), 5.92 (s, 6H). 13C NMR
(100 MHz, MeOH-d4): δ (ppm) 160.68, 155.27, 141.17, 126.77,
121.94, 85.07, 70.28. ESI-MS calcd. for [M]+ [C12H13ClNRuO]:
324.0. Found: 324.0
Synthesis of [(η6-C10H14)Ru(κ2-NpyOH-L2)Cl]+ ([Ru]-3). [{(η6-
C10H14)RuCl2}2] (0.306 g, 0.5 mmol) is dissolved in 30 mL of
HPLC methanol, and 1-(pyridin-2-yl)ethanol (135 mg, 1.1 mmol) is
added to it. The reaction mixture is refluxed for 12 h, and then the
volume of the reaction mixture is reduced to 3 mL under reduced
pressure. An excess of diethyl ether is poured into the above
methanolic solution to precipitate a brown solid. Yield: 80% (0.343
1
and analyzed by H NMR.
Recycling Experiments. Formic acid (4.0 M, 2.5 mL) with
n(sodium formate)/n(formic acid) = 2:1 is stirred at 90 °C in the
presence of [Ru]-1 (0.01 mmol) in a two-necked 10 mL reaction tube
fitted with a condenser and a gas buret. The volume of gas produced
is measured as the displacement of water in the buret with respect to
time. After completion of each catalytic run, 10 mmol of formic acid is
added to the reaction mixture, and the release of gas is monitored.
General Process for Formaldehyde Dehydrogenation
Reaction. An aqueous formaldehyde (0.4−4.0 M, 2.5 mL) solution
is stirred at 90 °C in the presence of [Ru]-1 (0.01 mmol) in a two-
necked 10 mL reaction tube fitted with a condenser and a gas buret.
The volume of gas produced is measured as the displacement of water
in the buret with respect to time. Further, the identity of the produced
gas is confirmed by GC-TCD. The turnover number (TON) was
1
g). H NMR (400 MHz, MeOH-d4): δ (ppm) 9.21 (d, J = 4.0 Hz,
1H), 7.96 (t, J = 8.0 Hz, 1H), 7.51 (t, J = 8.0 Hz, 1H), 7.36 (d, J = 8.0
Hz, 1H), 5.88 (d, J = 8.0 Hz, 1H), 5.80 (d, J = 8.0 Hz, 1H), 5.66 (d, J
= 8.0 Hz, 1H), 5.60 (d, J = 4.0 Hz, 1H), 4.93 (d, J = 8.0 Hz, 1H),
2.78−2.81 (m, 1H), 2.18 (s, 3H), 1.52 (d, J = 4.0 Hz, 3H), 1.19−122
(m, 6H). 13C NMR (100 MHz, MeOH-d4): δ 163.32, 154.84, 141.24,
126.71, 122.90, 104.16, 100.30, 84.39, 83.09, 82.77, 80.98, 75.66,
32.21, 22.46, 22.17, 18.65. ESI-MS calcd. for [M]+ [C17H23ClNRuO]:
394.1. Found: 394.4.
H
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calculated by the formula [n(H2 + CO2)/n(catalyst)].13 The turnover
frequency (TOF) was calculated as TON/time.
(3) Armaroli, N.; Balzani, V. The Hydrogen Issue. ChemSusChem
2011, 4, 21−36.
Mechanistic Investigations for Formaldehyde Dehydrogen-
ation over [Ru]-1. Formaldehyde (0.4 M, 2.5 mL) is stirred with
[Ru]-1 (0.01 mmol) in water at 90 °C. Reaction aliquots are collected
at every 5 min from the reaction mixture and analyzed by ESI-MS.
Gas Composition Analysis. The identification of gaseous
products during the decomposition of formic acid and aqueous
formaldehyde was confirmed as H2 and CO2 with no detectable level
of CO using a Shimadzu GC-2014 system. The chromatograph was
equipped with a shin carbon-ST packed column with thermal
conductivity detector (TCD) using argon as a carrier gas. Parameters
were set for the program to detect H2 and CO2 (detector temperature,
200 °C; oven temperature program, 90 °C (hold time: 1 min), 90−
200 °C (rate: 15 °C per minute)). The H2/CO2 molar ratio during
formic acid and aqueous formaldehyde dehydrogenation was found to
be approximately 1:1 and 2:1, respectively, based on the calibration
curve using standard H2 and CO2 gas.
(4) Dalebrook, A. F.; Gan, W.; Grasemann, M.; Moret, S.;
Laurenczy, G. Hydrogen storage: beyond conventional methods.
Chem. Commun. 2013, 49, 8735−8751.
(5) Jiang, H. L.; Singh, S. K.; Yan, J. M.; Zhang, X. B.; Xu, Q. Liquid-
Phase Chemical Hydrogen Storage: Catalytic Hydrogen generation
under Ambient Conditions. ChemSusChem 2010, 3, 541−549.
(6) Awasthi, M. K.; Tyagi, D.; Patra, S.; Rai, R. K.; Mobin, S. M.;
Singh, S. K. Ruthenium Complexes for Catalytic Dehydrogenation of
Hydrazine and Transfer Hydrogenation Reactions. Chem. - Asian J.
2018, 13, 1424−1431.
(7) Grasemann, M.; Laurenczy, G. Formic acid as a hydrogen source
− recent developments and future trends. Energy Environ. Sci. 2012, 5,
8171−8181.
(8) Singh, A. K.; Singh, S.; Kumar, A. Hydrogen energy future with
formic acid: a renewable chemical hydrogen storage system. Catal. Sci.
Technol. 2016, 6, 12−40.
(9) Mellmann, D.; Sponholz, P.; Junge, H.; Beller, M. Formic acid as
a hydrogen storage material − development of homogeneous catalysts
for selective hydrogen release. Chem. Soc. Rev. 2016, 45, 3954−3988.
(10) Eppinger, J.; Huang, K.-W. Formic Acid as a Hydrogen Energy
Carrier. ACS Energy Lett. 2017, 2, 188−195.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b02882.
■
sı
(11) Sordakis, K.; Tang, C.; Vogt, L. K.; Junge, H.; Dyson, P. J.;
Beller, M.; Laurenczy, G. Homogeneous Catalysis for Sustainable
Hydrogen Storage in Formic Acid and Alcohols. Chem. Rev. 2018,
118, 372−433.
(12) Zhong, H.; Iguchi, M.; Chatterjee, M.; Himeda, Y.; Xu, Q.;
Kawanami, H. Formic Acid-Based Liquid Organic Hydrogen Carrier
System with Heterogeneous Catalysts. Adv. Sustainable Syst. 2018, 2,
1700161.
Optimization data, selected bond parameters, GC
analysis and calibration curve, kinetics data, and
mechanistic investigations (PDF)
Accession Codes
CCDC 1564211 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
̈
(13) Heim, L. E.; Schlorer, N. E.; Choi, J.-H.; Prechtl, M. H. G.
Selective and Mild Hydrogen Production Using Water and Form-
aldehyde. Nat. Commun. 2014, 5, 3621.
(14) Heim, L. E.; Vallazza, S.; van der Waals, D.; Prechtl, M. H. G.
Water Decontamination with Hydrogen Production Using Micro-
wave-formed Minute-made Ruthenium Catalysts. Green Chem. 2016,
18, 1469−1474.
(15) Suenobu, T.; Isaka, Y.; Shibata, S.; Fukuzumi, S. Catalytic
Hydrogen Production from Paraformaldehyde and Water Using an
Organoiridium Complex. Chem. Commun. 2015, 51, 1670−1672.
(16) Fujita, K.-I.; Kawahara, R.; Aikawa, T.; Yamaguchi, R.
Hydrogen Production from a Methanol−Water Solution Catalyzed
by an Anionic Iridium Complex Bearing a Functional Bipyridonate
Ligand under Weakly Basic Conditions. Angew. Chem., Int. Ed. 2015,
54, 9057−9060.
AUTHOR INFORMATION
Corresponding Author
Sanjay K. Singh − Catalysis Group, Discipline of Chemistry,
Indian Institute of Technology Indore, Simrol, Indore 453552,
India; orcid.org/0000-0002-8070-7350; Email: sksingh@
iiti.ac.in
■
Author
Soumyadip Patra − Catalysis Group, Discipline of Chemistry,
Indian Institute of Technology Indore, Simrol, Indore 453552,
India
(17) Trincado, M.; Sinha, V.; Rodriguez-Lugo, R. E.; Pribanic, B.; de
Bruin, B.; Grutzmacher, H. Homogeneously catalysed Conversion of
Aqueous Formaldehyde to H2 and Carbonate. Nat. Commun. 2017, 8,
14990.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.9b02882
(18) Li, R. H.; Zhu, X. H.; Yan, X. Q.; Kobayashi, H.; Yoshida, S.;
Chen, W. X.; Du, L. L.; Qian, K. C.; Wu, B.; Zou, S.; Lu, L.; Yi, W.;
Zhou, Y.; Fan, J. Oxygen-Controlled Hydrogen Evolution Reaction:
Molecular Oxygen Promotes Hydrogen Production from Form-
aldehyde Solution Using Ag/MgO Nanocatalyst. ACS Catal. 2017, 7,
1478−1484.
(19) Wang, W.-H.; Himeda, Y.; Muckerman, J. T.; Manbeck, G. F.;
Fujita, E. CO2 Hydrogenation to Formate and Methanol as an
Alternative to Photo- and Electrochemical CO2 Reduction. Chem. Rev.
2015, 115, 12936−12973.
(20) Klankermayer, J.; Wesselbaum, S.; Beydoun, K.; Leitner, W.
Selective Catalytic Synthesis Using the Combination of Carbon
Dioxide and Hydrogen: Catalytic Chess at the Interface of Energy and
Chemistry. Angew. Chem., Int. Ed. 2016, 55, 7296−7343.
(21) Loges, B.; Boddien, A.; Junge, H.; Beller, M. Controlled
generation of hydrogen from formic acid amine adducts at room
temperature and application in H2/O2 fuel cells. Angew. Chem., Int. Ed.
2008, 47, 3962−3965.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support from
IIT Indore, CSIR (01(2885)/17/EMR-II), and SERB-DST
(EMR/2016/005783). SIC, IIT Indore, and Discipline of
Chemistry are acknowledged for providing the instrumentation
facility. S.P. thanks IIT Indore for his fellowship.
REFERENCES
■
(1) Schlapbach, L.; Zuttel, A. Hydrogen-storage materials for mobile
applications. Nature 2001, 414, 353−358.
(2) Eberle, U.; Felderhoff, M.; Schuth, F. Chemical and Physical
̈
Solutions for Hydrogen Storage. Angew. Chem., Int. Ed. 2009, 48,
6608−6630.
I
https://dx.doi.org/10.1021/acs.inorgchem.9b02882
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
(22) Fellay, C.; Dyson, P. J.; Laurenczy, G. A viable hydrogen-
storage system based on selective formic acid decomposition with a
ruthenium catalyst. Angew. Chem., Int. Ed. 2008, 47, 3966−3968.
(23) Iguchi, M.; Himeda, Y.; Manaka, Y.; Matsuoka, K.; Kawanami,
H. Simple Continuous High-Pressure Hydrogen Production and
Separation System from Formic Acid under Mild Temperatures.
ChemCatChem 2016, 8, 886−890.
(24) Guan, C.; Zhang, D.-D.; Pan, Y.; Iguchi, M.; Ajitha, M. J.; Hu,
J.; Li, H.; Yao, C.; Huang, M.-H.; Min, S.; Zheng, J.; Himeda, Y.;
Kawanami, H.; Huang, K.-W. Dehydrogenation of Formic Acid
Catalyzed by a Ruthenium Complex with an N,N′-Diimine Ligand.
Inorg. Chem. 2017, 56, 438−445.
(25) Iguchi, M.; Chatterjee, M.; Onishi, N.; Himeda, Y.; Kawanami,
H. Sequential hydrogen production system from formic acid and H2/
CO2 separation under high-pressure conditions. Sustainable Energy
Fuels 2018, 2, 1719−1725.
(26) Pan, Y.; Pan, C. L.; Zhang, Y.; Li, H.; Min, S.; Guo, X.; Zheng,
B.; Chen, H.; Anders, A.; Lai, Z.; Zheng, J.; Huang, K.-W. Selective
Hydrogen Generation from Formic Acid with Well-Defined
Complexes of Ruthenium and Phosphorus−Nitrogen PN3-Pincer
Ligand. Chem. - Asian J. 2016, 11, 1357−1360.
(27) Boddien, A.; Federsel, C.; Sponholz, P.; Mellmann, D.;
Jackstell, R.; Junge, H.; Laurenczy, G.; Beller, M. Towards the
development of a hydrogen battery. Energy Environ. Sci. 2012, 5,
8907−8911.
(28) Sponholz, P.; Mellmann, D.; Junge, H.; Beller, M. Towards a
Practical Setup for Hydrogen Production from Formic Acid.
ChemSusChem 2013, 6, 1172−1176.
(29) Filonenko, G. A.; van Putten, R.; Schulpen, E. N.; Hensen, E. J.
M.; Pidko, E. A. Highly Efficient Reversible Hydrogenation of Carbon
Dioxide to Formates Using a Ruthenium PNP-Pincer Catalyst.
ChemCatChem 2014, 6, 1526−1530.
(30) Rodriguez-Lugo, R. E.; Trincado, M.; Vogt, M.; Tewes, F.;
Santiso-Quinones, G.; Grutzmacher, H. A homogeneous transition
metal complex for clean hydrogen production from methanol−water
mixtures. Nat. Chem. 2013, 5, 342−347.
(31) Boddien, A.; Loges, B.; Junge, H.; Gartner, F.; Noyes, J. R.;
Beller, M. Continuous Hydrogen Generation from Formic Acid:
Highly Active and Stable Ruthenium Catalysts. Adv. Synth. Catal.
2009, 351, 2517−2520.
(32) Muller, K.; Brooks, K.; Autrey, T. Hydrogen Storage in Formic
Acid: A Comparison of Process Options. Energy Fuels 2017, 31,
12603−12611.
(33) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. Efficient catalytic
decomposition of formic acid for the selective generation of H2 and
H/D exchange with a water-soluble rhodium complex in aqueous
solution. ChemSusChem 2008, 1, 827−834.
(34) Fukuzumi, S.; Kobayashi, T.; Suenobu, T. Unusually Large
Tunneling Effect on Highly Efficient Generation of Hydrogen and
Hydrogen Isotopes in pH-Selective Decomposition of Formic Acid
Catalyzed by a Heterodinuclear Iridium−Ruthenium Complex in
Water. J. Am. Chem. Soc. 2010, 132, 1496−1497.
(35) Maenaka, Y.; Suenobu, T.; Fukuzumi, S. Catalytic inter-
conversion between hydrogen and formic acid at ambient temperature
and pressure. Energy Environ. Sci. 2012, 5, 7360−7367.
(36) Horvath, H.; Papp, G.; Szabolcsi, R.; Katho, A.; Joo, F. Water-
Soluble Iridium-NHC-Phosphine Complexes as Catalysts for
Chemical Hydrogen Batteries Based on Formate. ChemSusChem
2015, 8, 3036−3038.
(37) Wang, Z. J.; Lu, S. M.; Li, J.; Wang, J. J.; Li, C.
Unprecedentedly High Formic Acid Dehydrogenation Activity on
an Iridium Complex with an N,N′- Diimine Ligand in Water. Chem. -
Eur. J. 2015, 21, 12592−12595.
(40) Himeda, Y. Highly efficient hydrogen evolution by decom-
position of formic acid using an iridium catalyst with 4,4′-dihydroxy-
2,2′-bipyridine. Green Chem. 2009, 11, 2018−2022.
(41) Wang, W.-H.; Xu, S.; Manaka, Y.; Suna, Y.; Kambayashi, H.;
Muckerman, J. T.; Fujita, E.; Himeda, Y. Formic Acid Dehydrogen-
ation with Bioinspired Iridium Complexes: A Kinetic Isotope Effect
Study and Mechanistic Insight. ChemSusChem 2014, 7, 1976−1983.
(42) Hull, J. F.; Himeda, Y.; Wang, W.-H.; Hashiguchi, B.; Periana,
R.; Szalda, D. J.; Muckerman, J. T.; Fujita, E. Reversible hydrogen
storage using CO2 and a proton-switchable iridium catalyst in aqueous
media under mild temperatures and pressures. Nat. Chem. 2012, 4,
383−388.
(43) Manaka, Y.; Wang, W.-H.; Suna, Y.; Kambayashi, H.;
Muckerman, J. T.; Fujita, E.; Himeda, Y. Efficient H2 generation
from formic acid using azole complexes in water. Catal. Sci. Technol.
2014, 4, 34−37.
(44) Onishi, N.; Ertem, M. Z.; Xu, S.; Tsurusaki, A.; Manaka, Y.;
Muckerman, J. T.; Fujita, E.; Himeda, Y. Direction to practical
production of hydrogen by formic acid dehydrogenation with Cp*Ir
complexes bearing imidazoline ligands. Catal. Sci. Technol. 2016, 6,
988−992.
(45) Wang, L.; Onishi, N.; Murata, K.; Hirose, T.; Muckerman, J. T.;
Fujita, E.; Himeda, Y. Efficient Hydrogen Storage and Production
Using a Catalyst with an Imidazoline-Based, Proton-Responsive
Ligand. ChemSusChem 2017, 10, 1071−1075.
(46) Lu, S. M.; Wang, Z.; Wang, J.; Li, J.; Li, C. Hydrogen
generation from formic acid decomposition on a highly efficient
iridium catalyst bearing a diaminoglyoxime ligand. Green Chem. 2018,
20, 1835−1840.
(47) Guerriero, A.; Bricout, H.; Sordakis, K.; Peruzzini, M.;
Monflier, E.; Hapiot, F.; Laurenczy, G.; Gonsalvi, L. Hydrogen
Production by Selective Dehydrogenation of HCOOH Catalyzed by
Ru-Biaryl Sulfonated Phosphines in Aqueous Solution. ACS Catal.
2014, 4, 3002−3012.
(48) Patra, S.; Awasthi, M. K.; Rai, R. K.; Deka, H.; Mobin, S. M.;
Singh, S. K. Dehydrogenation of Formic Acid Catalyzed by Water-
Soluble Ruthenium Complexes: X-ray Crystal Structure of a
Diruthenium Complex. Eur. J. Inorg. Chem. 2019, 2019, 1046−1053.
(49) Wang, L.; Ertem, M. Z.; Kanega, R.; Murata, K.; Szalda, D. J.;
Muckerman, J. T.; Fujita, E.; Himeda, Y. Additive-Free Ruthenium-
Catalyzed Hydrogen Production from Aqueous Formaldehyde with
High Efficiency and Selectivity. ACS Catal. 2018, 8, 8600−8605.
(50) Gemel, C.; John, R.; Slugovc, C.; Mereiter, K.; Schmid, R.;
Kirchner, K. Synthesis and characterization of ruthenium quinolin-8-
olate complexes. Unexpected formation of a κ1-hydrotris(pyrazolyl)-
borate complex. J. Chem. Soc., Dalton Trans. 2000, 2607−2612.
(51) Binnani, C.; Rai, R. K.; Tyagi, D.; Mobin, S. M.; Singh, S. K.
Ligand-Tuned C−H Bond Activation/Arylation of 2-Arylpyridines
over Pyridine-Based N,O/N,N Ligated Ruthenium−Arene Com-
plexes. Eur. J. Inorg. Chem. 2018, 2018, 1435−1445.
(52) Gunnaz, S.; Ozdemir, N.; Dayan, S.; Dayan, O.; Cetinkaya, B.
Synthesis of Ruthenium(II) Complexes Containing Tridentate
Triamine (NNN) and Bidentate Diamine Ligands (NN): as Catalysts
for Transfer Hydrogenation of Ketones. Organometallics 2011, 30,
4165−4173.
(53) Dwivedi, A. D.; Sahu, V. K.; Mobin, S. M.; Singh, S. K.
Cyclopentadienyl-Ru(II)- Pyridylamine Complexes: Synthesis, X-ray
Structure and Application in Catalytic Transformation of Bio-Derived
Furans to Levulinic Acid and Diketones in Water. Inorg. Chem. 2018,
57, 4777−4787.
(54) Carmona, D.; Lamata, P.; Sanchez, A.; Pardo, P.; Rodriguez, R.;
Ramirez, P.; Lahoz, F. J.; Orduna, P. G.; Oro, L. A. Chiral Brønsted
Acid Catalysts. Activation of Methyl 3,3,3-Trifluoropyruvate by
Hydroxymethylpyridine-Containing Half-Sandwich Complexes. Orga-
nometallics 2014, 33, 4016−4026.
(55) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr.,
Sect. A: Found. Crystallogr. 2008, 64, 112−122.
(38) Montandon-Clerc, M.; Dalebrook, A. F.; Laurenczy, G.
Quantitative Aqueous Phase Formic Acid Dehydrogenation Using
Iron(II) Based Catalysts. J. Catal. 2016, 343, 62−67.
(39) Fink, C.; Laurenczy, G. CO2 as a hydrogen vector − transition
metal diamine catalysts for selective HCOOH dehydrogenation.
Dalton Trans. 2017, 46, 1670−1676.
J
https://dx.doi.org/10.1021/acs.inorgchem.9b02882
Inorg. Chem. XXXX, XXX, XXX−XXX