Dehydrogenation of Formic Acid Catalyzed by Water-Soluble Ruthenium Complexes: X-ray Crystal Structure of a Diruthenium Complex

Article abstract of DOI:10.1002/ejic.201801501

Dehydrogenation of formic acid over various Ru-arene complexes containing N-donor chelating ligands was investigated in H₂O and isolated and characterized several important catalytic intermediate species to elucidate the reaction pathway for formic acid dehydrogenation. Among the studied complexes, Ru-arene complexes, namely [(η⁶-C₆H₆)Ru(κ²-N_pyNH₂-AmQ)Cl]⁺ (C-2), [(η⁶-C₁₀H₁₄)Ru(κ²-N_pyNH₂-AmQ)Cl]⁺ (C-3) and [(η⁶-C₆H₆)Ru(κ²-N_pyNHMe-MAmQ)Cl]⁺ (C-4) [AmQ = 8-aminoquinoline and MAmQ = 8-(N-methylamino)quinoline] were proved to be the efficient catalysts for formic acid dehydrogenation at 90 °C, even in the absence of base. With an initial TOF of 940 h^–1, complex C-4 displayed the highest catalytic activity for formic acid dehydrogenation in H₂O and it can be recycled up to 5 times with a TON of 2248. Effect of temperature, pH, formic acid and catalyst concentration on the reaction kinetics were also investigated in detail. Extensive mechanistic investigations using mass spectrometry and NMR evidenced the formation of a coordinatively unsaturated species [(η⁶-C₆H₆)Ru(κ²-N_pyNH-AmQ)]⁺ (C-2A)/[(η⁶-C₆H₆)Ru(κ²-N_pyNMe-MAmQ)]⁺ (C-4A) as the active component during the catalytic dehydrogenation of formic acid. We further characterized the dimer-form of C-2A, possibly the catalyst resting state, by single-crystal X-ray crystallography.

Full text of DOI:10.1002/ejic.201801501

A Journal of
Accepted Article
Title: Dehydrogenation of Formic Acid Catalyzed by Water-Soluble
Ruthenium Complexes: X-ray Crystal Structure of a Diruthenium
Complex
Authors: Soumyadip Patra, Mahendra K. Awasthi, Rohit K. Rai,
Hemanta Deka, Shaikh M. Mobin, and Sanjay Kumar Singh
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201801501
Link to VoR: http://dx.doi.org/10.1002/ejic.201801501

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Dehydrogenation of Formic Acid Catalyzed by Water-Soluble Ruthenium
Complexes: X-ray Crystal Structure of a Diruthenium Complex
Soumyadip Patra,[a] Mahendra K. Awasthi,[a] Dr. Rohit K. Rai,[a] Dr. Hemanta Deka,[a] Dr.
[a]
[a]
Shaikh M. Mobin and Dr. Sanjay K. Singh*
[
a]
Discipline of Chemistry, Indian Institute of Technology Indore, Simrol, Indore 453552,
India
E-mail: sksingh@iiti.ac.in
URL: http://people.iiti.ac.in/~sksingh/
ABSTRACT
Dehydrogenation of formic acid over various Ru-arene complexes containing N-
donor chelating ligands was investigated in H O and isolated and characterized several
2
important catalytic intermediate species to elucidate the reaction pathway for formic acid
dehydrogenation. Among the studied complexes, Ru-arene complexes, namely [(ƞ6-
2
+
6
2
+
6
C
C
6
6
H
6
6
)Ru(κ -NpyNH
2
-AmQ)Cl] (C-2), [(ƞ -C10
H
14)Ru(κ -NpyNH -AmQ)Cl] (C-3) and [(ƞ -
2
2
+
H
)Ru(κ -NpyNHMe-MAmQ)Cl] (C-4) (AmQ = 8-aminoquinoline and MAmQ = 8-(N-
methylamino)quinoline were proved to be the efficient catalysts for formic acid
°
-1
dehydrogenation at 90 C, even in the absence of base. With an initial TOF of 940 h ,
complex C-4 displayed the highest catalytic activity for formic acid dehydrogenation in H
2
O
and it can be recycled up to 5 times with a TON of 2248. Effect of temperature, pH, formic
acid and catalyst concentration on the reaction kinetics were also investigated in detail.
Extensive mechanistic investigations using mass spectrometry and NMR evidenced the
6
2
+
formation of a coordinatively unsaturated species [(ƞ -C
6
H
6
)Ru(κ -NpyNH-AmQ)] (C-2A) /
6
2
+
[
(ƞ -C
6
H
6
)Ru(κ -NpyNMe-MAmQ)] (C-4A) as the active component during the catalytic
dehydrogenation of formic acid. We further characterized the dimer-form of C-2A, possibly
the catalyst resting state, by single crystal X-ray crystallography.
Keywords: dehydrogenation, formic acid, sodium formate, reaction mechanism, 8-
aminoquinoline.
1
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
INTRODUCTION
In the modern era, there prevails a huge demand to supply gap concerning the energy
resources and hence the quest for green and renewable energy sources to meet the global
energy demand is one of the key challenges of the society. In this context, hydrogen (H
2
) is
emerging as a clean alternative source of energy, as only H
2
O is produced by its reaction in
fuel cells.[1,2] However, the extremely explosive nature of H
and the difficulty in its
2
transportation lead to the search for suitable hydrogen storage compounds, which are safer to
under mild conditions.[3] In this regard,
handle, can be transported easily and release H
2
formic acid being a liquid under ambient conditions has attracted considerable interest, as it
can be handled, stored and transported easily and safely.[4] In the presence of a suitable
catalyst, formic acid generates H
2
and CO , under relatively mild reaction condition.
2
After one of the first reports on formic acid dehydrogenation over Ir-phosphine
complex in 1967 by Coffey,[5] this reaction has been investigated extensively only in the
[
6]
[7]
[8,9]
recent past. Several transition metal complexes based on iron, rhodium, ruthenium
and
iridium[10-14] have been particularly proven to be active catalysts for formic acid
dehydrogenation. Notably, ancillary ligands play an important role in tuning the catalytic
activity of these metal catalysts for formic acid dehydrogenation. For instance, a strong σ-
donor ligand enhances the electron density at the metal center, and such electron-rich metals
are found to be the favorable site for efficient activation of formic acid to CO
2
and H
.[4j,10s]
2
Moreover, the protic ligands are of particular importance in formic acid dehydrogenation
reactions, as these ligands enhance the dehydrogenation pathway by involving in hydrogen
bond interactions with the solvent or formic acid.[4k,10c,10d,10g,10m-o] For example, iridium
complexes bearing proton responsive ligands have been found to be particularly active for
formic acid dehydrogenation in H
2
O.[11-13] Himeda et al. reported the catalyst [Cp*Ir(4,4′-
2
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
DHBP)(H
2
O)]SO
4
with the TOF of 14000 h-1.[13] The same group afterwards reported the
with a TOF of 34000 h-1.[10e] In 2015, Himeda et al. further
catalyst [Cp*Ir(TMBI)H
2
O]SO
4
improved the results by developing the catalyst [Cp*Ir(pyrimidylimidazoline)H
2
O]SO which
4
-
1
[10q]
gave a TOF of 322000 h in HCOOH/HCOONa aqueous solution.
Li et al. developed the
5
+
-1 [11]
catalyst [Ir(η -C
5
Me )Cl(2,2’-bi-2-imidazole)] with the TOF of 487500 h . Very recently,
5
this group has reported a new iridium-based catalyst bearing a dioxime derived ligand which
gave a record TON of 5020000 at 70 °C.[10l] Joo et al. reported an iridium hydride complex
which gave a TOF of 298000 h-1 at 100 °C in formic acid dehydrogenation.[10t] Some of the
other premier results obtained for formic acid dehydrogenation especially in aqueous medium
have been listed in the supporting information (Table S1). Although iridium-based catalysts
have been proven to be very active in formic acid dehydrogenation, its price prompts us to
look for a more economical option. Literature reports revealed that ruthenium-based catalysts
6
+
except a few such as [Ru(η -C10
H
14)Cl(2,2’-bi-2-imidazole)] and [Ru(H
O)
2 6
](tos) /TPPTS
2
(tos = p‐toluene sulfonate and TPPTS = m‐trisulfonated triphenylphosphine) have all been
tested in organic solvents for formic acid dehydrogenation.[8a,8g-8j,9,15,16] Higher activities of
5
Me H14)Cl(2,2’-bi-2-
)Cl(2,2’-bi-2-imidazole)]+ and [Ru(η -C10
6
the complexes [Ir(η -C
5
5
+
imidazole)] for formic acid dehydrogenation suggest the importance of N-H moieties on the
catalytic activity.[9,11]
In one of our recent findings, we observed that Ru-arene complexes containing
ethylenediamine (en) and 8-aminoquinoline (AmQ) based chelating ligands efficiently
catalyzed the furfural to levulinic acid (LA) transformation with the aid of formic acid. We
demonstrated that the N-H moieties of en and AmQ ligands play a crucial role in the initial
activation of formic acid to facilitate the transfer hydrogenation of furfural to furfuryl
alcohol, which eventually transformed to LA.[17,18] Very recently, Oro et al. reported Ir-NHC
3
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
(NHC = N-heterocyclic carbene) based complexes for formic acid dehydrogenation in DMF
and H
2
O.[14] Though these complexes exhibit only moderate activity in H
2
O, the initial TOF
-1
is highest (790 h ) for the Ir-NHC-AmQ complex. Moreover, a significant quenching of the
catalytic activity of Ir-NHC complex was observed when AmQ was replaced with 8-(N, N-
dimethylamino)quinoline (DMAmQ), suggesting the crucial role of NH moiety of AmQ in
enhancing the catalytic activity for formic acid dehydrogenation in H O. Envisioned by our
2
previous observations, herein, we investigated in detail the dehydrogenation of formic acid
over Ru-arene complexes containing various nitrogen-donor chelating ligands and
demonstrated the crucial role of these ligands on the formic acid dehydrogenation process.
Moreover, mechanistic insights were elaborated by identifying several catalytic intermediates
involved in various steps of the catalytic dehydrogenation of formic acid under the catalytic
and controlled reaction conditions. Most importantly, the structure of a diruthenium species,
possibly the catalyst resting state, was established by X-ray crystallography.
RESULTS AND DISCUSSION
6
2
-en)Cl]+ (C-1), [(ƞ -C
6
2
Complexes [(ƞ -C
6
H
6
)Ru(κ -NH
2
NH
2
6
H
6
)Ru(κ -NpyNH -8-
2
+
6
2
+
AmQ)Cl] (C-2) and [(ƞ -C10
H
14)Ru(κ -NpyNH
2
-8-AmQ)Cl] (C-3) shown in Scheme 1 were
6
synthesized from the precursor [(ƞ -arene)RuCl
2
] (C-6 and C-7) and the suitable ligands
2
following our previously reported method.[17,18] Complex [(ƞ -C
6
H
6
)Ru(κ -NpyNHMe-8-
2
6
+
6
AmQ)Cl] (C-4) was obtained at room temperature from the reaction of the precursor [(ƞ -
benzene)RuCl
2
2
] (C-6) and the ligand 8-(N-methylamino)quinoline (MAmQ) (Scheme 1 and
6
2
+
Schemes S1-S2). Complex [(ƞ -C
6
H )Ru(κ -bpy)Cl] (C-5) was also synthesized from the
6
6
reaction of [(ƞ -benzene)RuCl
]
2 2
(C-6) and 2,2’-bipyridine (bpy), to evaluate the role of NH
in formic acid dehydrogenation.
4
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
For the initial screening of active catalysts, formic acid dehydrogenation over
complexes C-1  C-5 (1 mol %) and the precursor C-6 – C-7 (0.5 mol %) was investigated
using 0.4 M formic acid (aq.) solution (2.5 mL) and sodium formate (5 mol%) at 90 °C
(Table 1, entries 1-7). The amount of the evolved gases was measured by water displacement
method, and the release of H and CO gas was confirmed by GC-TCD (Figure S1).
2
2
Preliminary results inferred that the complexes C-2 – C-4 containing 8-aminoquinoline based
ligands were much more efficient than the complex C-1 containing ethylenediamine ligand
for formic acid dehydrogenation reaction (Table 1, entries 1-4 and Figure S2). The only
marginal increment in the TOF of complex C-2 over C-3 suggests that the arene-ring offer no
significant enhancement in formic acid dehydrogenation. Among the related complexes, the
-1
TOF for formic acid dehydrogenation over C-4 (TOF 364 h ) was found to be 4-fold higher
-1
6
than the complex C-2 (TOF 83 h ) (Table 1, entries 2 and 4). On the other hand, the [(ƞ -
2
+
C
6
H
6
)Ru(κ -bpy)Cl] (C-5) display only poor activity (Table 1, entry 5). Notably, the
precursor complexes C-6 and C-7, lacking any nitrogen-based ligands, are also poorly active
Table 1, entries 6 and 7). These findings suggest the importance of protic ligands,
(
availability of NH moiety and the stability of the catalyst for the catalytic formic acid
dehydrogenation reaction.
5
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Scheme 1. Complexes C-1 – C-7 explored for the dehydrogenation of formic acid
[
a]
Table 1. Catalytic dehydrogenation of formic acid over various catalysts .
-1 [d]
Entry
Catalyst
C-1
Conv. (%)[b]
TON[d]
90
TOF (h )
1
2
3
4
5
6
7
90
91
90
97
30
39
39
53
83
75
364
26
18
19
C-2
91
C-3
90
C-4
97
C-5
30
C-6[c]
C-7[c]
39
39
[
a] Reaction conditions: catalyst (0.01 mmol), formic acid (0.4 M, 2.5 mL),
HCOONa (0.05 mmol), 90 °C. [b] Based on the total gas released. [c]
Catalyst (0.005 mmol). [d] TOF/TON per Ru atom.
6
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Further, dehydrogenation of formic acid (0.4 M, 2.5mL) was performed over the
active complexes C-2 and C-4 (1 mol%), using a varying amount of sodium formate (0.05 to
3
equivalents) (Table S2, entries 1-14 and Figure 1). Results inferred that the TOF could be
-1
enhanced to 514 h for complex C-4 using [HCOONa]/[HCOOH] ratio of 2:1 (Table S2,
-1
entry 12). Further, a maximum initial TOF of 940 h was achieved with C-4 for formic acid
2.0 M, 2.5 mL) and [HCOONa]/[HCOOH] ratio of 2:1. Moreover, the complex C-4 was
(
recycled for five consecutive catalytic runs for formic acid dehydrogenation reaction, with a
TON of 2248, at 90 °C [HCOONa]/[HCOOH] ratio of 2:1 (Figure 2). These results suggest
that the catalyst C-4 was quite stable and does not lose its catalytic activity even after using it
continuously for 15 hours under the catalytic reaction condition.
-1
Figure 1. TOF (h ) vs [HCOONa] (mmol) plot for the catalytic dehydrogenation of formic
acid over complexes C-2 and C-4. Reaction conditions: formic acid (0.4 M, 2.5 mL), catalyst
(0.01 mmol), 90 °C. [a] formic acid (2.0 M, 2.5 mL).
7
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Figure 2. TON of the catalytic recyclability (1-5 catalytic runs) experiment for the catalytic
dehydrogenation of formic acid over the complex C-4. Reaction conditions: formic acid (2.0
M, 2.5 mL), C-4 (0.01 mmol), [HCOONa]/[HCOOH] = 2:1, 90 °C. (5 mmol of formic acid
was added to the reaction mixture after each run).
8
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Figure 3. (a) Temperature dependent formic acid decomposition over C-4 (1 mol %), formic
acid (2M, 2.5 mL), HCOONa/ HCOOH = 2:1, T = 60  90 °C. (b) Arrhenius plot of initial
TOF values for formic acid (2M, 2.5 mL) decomposition over C-4 (1 mol%), HCOONa/
HCOOH = 2:1.
Further, the dependence of the rate of formic acid dehydrogenation on temperature,
catalyst, and formic acid concentration was studied. The initial rates for the formic acid
dehydrogenation over complex C-4 followed Arrhenius behavior in the temperature range of
6
0 – 90 °C (Figure 3). The obtained apparent activation energy of 87.9 kJ/mol is in line with
the activation energies reported for formic acid dehydrogenation over the analogous
system.[9] Subsequently, the dependence of the rate of formic acid dehydrogenation on the
catalyst concentration was determined by varying the catalyst concentration between 0.005
mmol and 0.03 mmol, while keeping the formic acid concentration constant (2.0 M, 2.5 mL)
at 90 °C. The double logarithmic plots of the initial reaction rates against the concentration of
C-4 catalyst follows a linear dependence on catalyst concentration. The obtained order of 0.8
with respect to catalyst concentration (Figure S3) suggests that C-4 is converted into the
9
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
active monomeric species during the induction period and presumably the monomeric species
is only involved in the formic acid dehydrogenation reaction. Further, the reaction order with
respect to formic acid concentration, varied between 0.4 M and 2 M with a constant catalyst
concentration of 0.01 mmol in 2.5 mL of aqueous solution at 90 °C, was found to be 0.31
-
(
Figure S4). This result most probably indicates that only one formic acid or HCOO is
interacting with the Ru center to form an [HCOO-Ru] intermediate species.[11,19] Also, no
change in formic acid dehydrogenation efficiency over complex C-4 was observed in the
presence of excess metallic mercury, suggesting that the reaction follows a homogeneous
pathway.
Based on the observations of the kinetic studies, attempts were made to systematically
investigate and identify various organometallic intermediates presumably involved in the
catalytic dehydrogenation of formic acid over Ru-catalyst using mass spectrometry. Stirring
catalyst C-4 in H
2
O at room temperature showed few prominent mass peaks at m/z = 169.0
and 178.0, which were assigned to C-4G and C-4E, respectively (Scheme 2 and Figure S5).
6
2
The coordination of a H
2
O molecule to the dicationic species, [(ƞ -C
6
6
H
6
6
)Ru(κ -NpyNHMe-
MAmQ)]2+ (C-4G; m/z = 169.0) gives a dicationic species [(ƞ -C
6
H
)Ru(κ -NpyNHMe-
2
2
+
MAmQ)(H
2
O)] (C-4E; m/z = 178.0) in aqueous solution. Upon stirring an aqueous solution
of C-4 in the presence of sodium formate at room temperature, prominent mass peak at m/z =
6
2
+
3
37.0 was observed, which was assigned to [(ƞ -C
6
H )Ru(κ -NpyNMe-MAmQ)] (C-4A)
6
species (Figure S6). The analogous, mass profile was also observed for C-2 under identical
6
2
reaction conditions (Figure S7). An H
2
O coordinated dicationic [(ƞ -C
6
H
6
)Ru(κ -NpyNH -
2
2
+
AmQ) (H
2
O)] (C-2E; m/z = 171.0) species was observed upon stirring the complex C-2 in
6
2
+
H
2
O. Further, an intense peak corresponding to C-2A ([(ƞ -C
6
H
6
)Ru(κ -NpyNH-AmQ)] ; m/z
=
323.0), a species similar to C-4A, was also observed upon stirring the complex C-2 in the
1
0
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
presence of sodium formate. Mass study of the dehydrogenation reaction with complex C-2
6
2
+
in the presence of sodium formate also revealed C-2A ([(ƞ -C
6
H )Ru(κ -NpyNH-AmQ)] as
6
the main metallic species formed in the reaction mixture (Figure S8). These observations,
suggest that the presence of formate species facilitated the deprotonation of the N-H of the 8-
aminoquinoline based ligands to form a coordinatively unsaturated C-2A/C-4A species.
Scheme 2. Various intermediate species observed during the mass investigation of an
aqueous solution of complex C-4 under varying controlled reaction condition.
The equilibria between the species C-4G and C-4A (for catalyst C-4) and C-2G and
C-2A (for catalyst C-2) observed during the mass investigation (Scheme 2 and Figures S5
and S7) are pH sensitive and hence a pH-dependent study on the initial TOFs was conducted.
The results showed that the highest TOF was achieved at pH = 4.0 (Figure 4), where the
initial concentration of [HCOONa]/[HCOOH] was fixed at 2:1 and hence the concentration
1
1
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
of such coordinatively unsaturated species C-4A/C-2A is expected to be high indicating that
it may have a vital role in the catalytic cycle. The highest TOF obtained at moderately acidic
+
-
condition indicates the importance of both H
dehydrogenation reaction.
3
O ions as well as HCO
2
species in the
Figure 4. Dehydrogenation of formic acid under varying pH values, where pH values are
altered by changing the [HCOONa]/[HCOOH] ratios. Reaction condition: catalyst C-2/C-4
(0.01 mmol), formic acid (0.4 M, 2.5 mL), T = 90 °C.
To further confirm, the formation of the unsaturated species C-2A and C-4A in the
presence of HCOONa, controlled reactions were performed. Treating complex C-2 with an
excess of sodium formate in methanol under reflux condition also led us to obtain C-2A.
Further, orange colored block-shaped single crystals suitable for X-ray diffraction analysis
1
2
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
were obtained by slow diffusion of diethyl ether into the methanolic solution of C-2A. It is
evident that the treatment of C-2/C-4 with sodium formate led to the formation of the species
C-2A/C-4A, which eventually dimerize to the stable diruthenium complex C-2Aʹ/C-4Aʹ,
plausibly the catalyst resting state (Scheme 3).[10g,20-22] Analogous unsaturated species [Ru(η6-
+
6
C
10
H
14)(Imd-H)] (Imd = 2,2’-bi-2-imidazole), generated in situ from [Ru(η -C10H14)Cl(2,2’-
+
bi-2-imidazole)] in the presence of sodium formate, was reported to play an important role
in the formic acid dehydrogenation process.[9] Interesting to note here, that literature also
revealed that such coordinatively unsaturated species are unstable and may undergo
dimerization.[20-22] Similar dimeric species are also reported by Carmona et al., where
6
2
+
treatment of acetone solution of the complex [(ƞ -C10
H
14)Ru(κ -NpyOH)Cl] with Na
2
CO
3
6
2
]2+.[20]. Valerga et al. also reported the
afforded dimer [{(ƞ -C10
H
14 )Ru(κ N,O-µ-O-NO}
2
2
crystal structure of a dinuclear Cp* ruthenium (III) complex [{Cp*Ru(κ -N,S-μ-S-
SC
5
H
4
N)}
]2+ while attempting to crystallize hydrido(alkoxo) derivative.[21] Nishibayashi et
2
al. reported similar species which are particularly relevant in the context of catalytic
propargylation reaction of ketones.[22]
Scheme 3. A plausible route to the diruthenium species (C-2Aʹ/C-4Aʹ), the catalyst resting
state, via the coordinatively unsaturated species C-2A/C-4A.
The NMR spectra of C-2Aʹ are consistent with the presence of deprotonated NH
2
ligand, and only one set of resonances was observed in the H and 13C NMR spectra
1
1
3
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
indicating the chemical equivalence of the two halves of the molecule (Figure S9). The single
crystal X-ray structural determination showed that the asymmetric unit of C-2Aʹ [{(η6-
2
]2+ is formed by two units of {(η -benzene)Ru(κ -NpyNH-
6
2
benzene)Ru(κ -NpyNH-AmQ)}
2
+
6
AmQ)} (C-2A), where two (η -benzene)Ru units are doubly bridged by -NH groups of two
6
8
-AmQ ligands (Figure 5 and Table S3-S5). The ruthenium bound η -benzene ligands are
placed at the mutually cisoid position. The center Ru-N-Ru-N core has a bent butterfly-like
structure, with the dihedral angle of 151.44 between the two RuN planes, presumably due
2
6
to the steric crowding of the cisoid positioned η -benzene rings. Moreover, the Ru1-N2-Ru2
bond angle is 97.28°, which is slightly larger than the reported values, indicating a weak
interaction between two Ru centers having a Ru- Ru distance of 3.2 Å. The Ru-Namido bond
lengths of the two ruthenium units in the complex C-2Aʹ are not same, suggesting the
asymmetry in the coordination of Namido with the two ruthenium centers. In particular, the
Ru1-N4 (2.157 Å) and Ru2-N2 (2.158 Å) bonds (distances between the ruthenium center and
the bridging Namido from the other ruthenium unit) are significantly longer than the Ru1-N2
(
2.106 Å) and Ru2-N4 (2.094 Å) bonds, further suggesting that two units of {(η6-
2
+
benzene)Ru(κ -NpyNH-AmQ)} are weakly interacted via the two bridging Namido groups.
Notably, the Ru-Npy bond lengths in C-2Aʹ are analogous to that observed in the
monometallic complex C-2, while the Ru-Namido bond lengths are quite short in comparison
to Ru-Namine bond in the monometallic complex C-2.[19] Though the carbon atoms in the η6-
benzene rings show some distortion due to the occupancy factors, the overall structure of the
diruthenium complex C-2Aʹ is well in accordance with literature reported analogous dimeric
complexes.[20-22]
1
4
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
-
Figure 5. Single crystal X-ray structure of the diruthenium C-2Aʹ. Counter anions (Cl ) and
all hydrogen atoms of C-2A’, except those on nitrogen, are omitted for clarity. Selected bond
lengths (Å) and angles (º): Ru1-N4 2.157 (4), Ru2-N4 2.094(4), Ru1-N2 2.106(4), Ru2-N2
2
.158(4), Ru1-N2-Ru2 97.28(16), Ru1-N4-Ru2 97.67(16), N2-Ru1-N4 77.94(15), N4-Ru2-
N2 78.18(15).
Interesting to note that during the catalytic reaction, at first instance, a visible color
change from a deep green solution of the catalysts C-2/C-4 to a brown solution was observed
during the initial minutes of the base-assisted catalytic dehydrogenation of formic acid
(Figure S10). It should be noted that the observed brown color of the catalytic reaction was
quite stable and we could not observe the initial green color of the solution even after the
complete dehydrogenation of formic acid. Mass investigation of the brown solution showed
the presence of C-2A/C-4A species. Further, the addition of an excess of dilute HCl in the
obtained brown solution led to the regeneration of the initial green color. Mass spectral
1
5
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
analysis of the obtained green solutions depicted the presence of prominent peaks at m/z
59.0 and m/z 373.0, corresponding to C-2 and C-4, respectively (Figure 6 and Figure S10).
3
Figure 6. Mass investigation of the green-colored solutions obtained by the addition of an
excess of dilute HCl to the brown solution after the catalytic reaction for C-2 and C-4.
The above findings inferred that, upon treatment with HCOONa, C-2/C-4 in situ
transformed to the species C-2A/C-4A, which further reacted with formic acid/sodium
formate to form formate coordinated Ru species (C-2B/C-4B).[23] A Ru-hydride species C-
2
C/C-4C is expected to be formed, upon extrusion of CO from C-2B/C-4B. This step is
2
most probably the rate determining step in the catalytic cycle as indicated by kinetic isotope
effect studies, where the deuterated formic acid (DCOOD) substrate (KIE: 2.9, Table 2, entry
3
) is more influential than deuterated H
2
O (D O) (KIE: 1.9, Table 2, entry 2) on the reaction
2
rate, indicating that C-H bond cleavage of formic acid is the rate determining step. [9,11]
1
6
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Table 2. KIE in the dehydrogenation of formic acid using complex C-2 and C-4[a]
-1 [b]
KIE[c]
Entry
Catalyst
C-4
Substrate
HCOOH
HCOOH
DCOOD
DCOOD
Solvent
TOF (h )
1
2
3
4
H
2
D
2
H
2
D
2
O
O
O
O
75
40
26
22
-
C-4
1.9
2.9
3.4
C-4
C-4
[a]Reaction conditions: Catalyst (0.01 mmol), formic acid (0.4 M, 2.5 mL), 90 °C;
[b]TOF was calculated in the initial 1 hour; [c]KIE = TOF (entry 1)/ TOF (entry n)
(n = 2,3,4)
Further, upon the release of H from the Ru-hydride species (C-2C/C-4C), the
2
intermediate C-2A/C-4A was regenerated (Scheme 4). All our attempts to detect and isolate
these Ru-formate or Ru-hydride species were unsuccessful. The indirect evidence for the
formation of Ru-hydride species was gathered by utilizing the Ru-hydride species (C-2C) in
the transfer hydrogenation of p-anisaldehyde to obtain corresponding alcohol under identical
reaction condition (Figure S11).[24] On the other hand, during base-free catalytic
dehydrogenation of formic acid, a H
2
O coordinated dicationic ruthenium species (C-2E/C-
E) was possibly generated, as also evidenced by the appearance of a mass peak at m/z =
71.0 ( C-2E) and m/z = 178.0 (C-4E) (Figures S5 and S7). Further, formate replaces H O
4
1
2
from C-2E/C-4E resulting in the formation of the species C-2F/C-4F. Hydride species C-
D/C-4D is formed after the decarboxylation of Ru-formate species (C-2F/C-4F). The H
coordinated dicationic ruthenium species (C-2E/C-4E) will be regenerated, upon the proton-
assisted release of H from C-2D/C-4D. Based on the above mass studies and x-ray crystal
2
2
O
2
structure, we propose a plausible pathway for both the base-free and base-assisted
dehydrogenation of formic acid over complexes C-2 and C-4 (Scheme S3).[4k,9]
1
7
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Scheme 4. A plausible catalytic pathway for the base assisted formic acid dehydrogenation
over complexes C-2 and C-4.
Conclusions
In summary, we employed several Ru-arene complexes for the catalytic dehydrogenation of
formic acid in H O, where the complex C-4 containing 8-(N-methylamino)quinoline ligand
2
outperformed over others to achieve a TON of 2248. The high activity of C-4 can be
attributed to the high aqueous stability of the catalyst and availability of -NH moiety. Further,
we employed extensive mass and NMR investigations and kinetic studies to evidence the
formation of several important organometallic intermediate species, such as H
2
O coordinated
6
2
O)]2+ (C-4E) and the
dicationic ruthenium species [(ƞ -C
6
H
6
)Ru(κ -NpyNHMe-MAmQ)(H
2
1
8
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
6
2
+
active coordinatively unsaturated species [(ƞ -C
6
H )Ru(κ -NpyNMe-MAmQ)] (C-4A) during
6
the catalytic dehydrogenation of formic acid over complex C-4, and hence, established their
important role in the catalytic dehydrogenation of formic acid. Most importantly, we
succeeded in isolating the plausible catalyst resting state and characterized it as a dicationic
diruthenium species (C-2Aʹ), by single crystal X-ray diffraction.
Experimental Section
Materials and instrumentation. All reactions were performed in aerobic conditions using
1
high- purity chemicals purchased from Sigma- Aldrich, and Alfa-Aesar. H NMR (400
MHz), 13C NMR (100 MHz) spectra were recorded at 298 K using D
O and CDCl as
2
3
solvents on a Bruker advance 400 spectrometer. ESI-mass spectra were recorded on a
micrOTF-Q II mass spectrometer. The GC-TCD analysis was performed on a Shimadzu GC-
2
014 system using shin carbon-ST packed column.
Synthesis of the ligand 8-(N-methylamino)quinoline. To the freshly prepared solution of
NaOMe (20 mmol) in MeOH (20 mL), 8-aminoquinoline (4 mmol) and paraformaldehyde
(
20 mmol) were added. The mixture was allowed to reflux for 4 hours, and NaBH (6 mmol)
4
was added portion wise at 0 °C and the mixture was refluxed for an additional 2 hours. The
completion of the reaction analyzed by TLC, the mixture was cooled, and the solvent was
evaporated on a vacuum pump to give a residue that was treated with an aqueous layer (20
mL). The compound was extracted with dichloromethane (2 × 15 mL). The organic residue
was dried over anhydrous Na
2
SO , and the solvent was removed using vacuum pump and
4
residue that was purified through silica gel column chromatography with hexane/ethyl acetate
1
(
9.5: 0.5 v/v) to obtain the pure product. Yellow color. H NMR (400 MHz, CDCl
3
): δ (ppm)
=
8.70 (d, 1H, J= 4 Hz), 8.06 (d, 1H, J= 8 Hz), 7.42- 7.35 (m, 2H), 7.05 (d, 1H, J= 8 Hz),
6
.65 (d, 1H, J= 8 Hz), 6.12 (br, 1H), 3.04 (d, 3H, J= 4Hz).
1
9
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Synthesis of the complex C-4. The ligand 8-(N-methylamino)quinoline (1.1 mmol) was
dissolved in 30 mL MeOH taken in a 100 mL round-bottomed flask and [(ƞ6-
benzene)RuCl
2
]
2
(0.5 mmol) was added to it. The mixture was stirred for 24 h at room
temperature after which the volume of solvent was reduced on a vacuum pump. The complex
was precipitated out with diethyl ether, washed several times to get rid of the excess ligand. It
1
was then dried and collected. Colour: Deep green. Yield 68%. H NMR (400 MHz, D
2
O): δ
(ppm) = 9.38 (dd, J = 30.3, 5.1 Hz, 1H), 8.25 (t, J = 7.1 Hz, 1H), 7.63 (d, J = 7.6 Hz, 2H),
7
.43 (dd, J = 10.4, 8.3 Hz, 2H), 5.96 – 5.75 (m, 6H), 3.06 (s, 3H). 13C NMR (100 MHz,
O): δ (ppm) = 129.34, 128.25, 127.75, 127.36, 126.51, 125.57, 124.27, 123.97, 123.71,
D
2
6
H
)Ru(κ -NpyNHMe-MAmQ)Cl]+:
2
8
3
5.48, 85.35, 48.81. MS (ESI) m/z calculated for [(ƞ -C
6
6
73.0 [M] , found 373.0 [M]+
+
Controlled experiment for the synthesis of C-2Aʹ. Complex C-2 (0.2 mmol) and HCOONa
0.4 mmol) in methanol (25 mL) were refluxed for 1 h till a visible color change from green
(
to brown was observed. The resulting solution was filtered, and the solvent was removed
1
under reduced pressure to obtain C-2Aʹ. Colour: Brown. Yield 73%. H NMR (400 MHz,
2
D O): δ (ppm) = 8.12 (d, 1H, J = 8 Hz), 7.82 (d, 2H, J = 8 Hz), 7.51 (t, 1H, J = 6 Hz), 7.16
1
3
(
d, 1H, J = 8 Hz), 6.87-6.84 (m, 1H), 5.74 (s, 6H). C NMR (100 MHz, D
2
O): δ (ppm) =
1
55.32, 153.91, 144.06, 137.44, 128.06, 127.99, 122.96, 122.20, 120.75, 85.27.
Single-Crystal X-ray Diffraction Studies. A Single crystal was obtained by diffusion of
diethyl ether into a methanol solution of C-2Aʹ. X-ray structural studies of C-2Aʹ were
executed on a CCD Agilent Technologies (Oxford Diffraction) SUPERNOVA
diffractometer. Using graphite-monochromated Cu Kα radiation (λ = 1.54184 Å)-based
diffraction, data were collected at 293(2) K by the standard “phi-omega” scan techniques and
were scaled and reduced using CrysAlisPro RED software. The extracted data were evaluated
2
0
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
using the CrysAlisPro CCD software. The structures were solved by direct methods using
SHELXL-2018/1, and refined by full-matrix least squares method, refining on F2.[25] The
positions of all of the atoms were determined by direct methods. All non-hydrogen atoms
were refined anisotropically. The remaining hydrogen atoms were placed in geometrically
constrained positions. The CCDC number 1810973 contains the supplementary
crystallographic data for C-2Aʹ. These data are freely available at www.ccdc. cam.ac.uk (or
can be procured from the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB21 EZ, UK; fax: (+44) 1223-336-033; or deposit@ccdc.cam.ac.uk).
The General process for formic acid dehydrogenation reactions. To an aqueous solution (2.5
mL) of complex C-4/C-2 (0.01 mmol) was added HCOONa (0.05 mmol) in a two-necked
reaction tube fitted with a condenser and a gas burette. Further, HCOOH (1 mmol) was added
to the reaction mixture and was heated at 90 °C. The release of gas was measured as the
displacement of H
The process for recycling experiments. Formic acid (2.0 M, 2.5 mL), C-4 (0.01 mmol),
HCOONa]/[HCOOH] = 2:1 was taken in a two-necked test tube and heated at 90 °C. The
2
O in the burette. The release of H
2
gas was confirmed by GC-TCD.
[
gas released was calculated by the water displacement method. After the release of gas
stopped, 5 mmol of formic acid was added to the reaction mixture after each run, and the
release of gas monitored.
The General process for the mechanistic studies. NMR and mass spectral studies were
performed for the identification of various catalytic species involved in the catalytic
dehydrogenation of formic acid over the complex C-2 and C-4. The species C-2A/C-4A and
C-2E/C-4E was identified by mass spectrometry from an aqueous solution of C-2/C-4 under
varying reaction conditions.
2
1
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Acknowledgments
Authors gratefully acknowledge the financial support from IIT Indore, CSIR and SERB-DST.
SIC, IIT Indore and Discipline of Chemistry are acknowledged for providing instrumentation
facility. S.P., R.K.R, and S.K.S designed the project, S.P. conducted the experiments, H.D.
and S.M.M. helped in X-ray refinement, S.P., M.K.A. and S.K.S. wrote the article. S.P. and
M.K.A. thank MHRD and INSPIRE-DST, respectively for their fellowships. R.K.R thanks
CSIR and IIT Indore for his fellowship. H.D. thanks SERB-DST for his N-PDF fellowship.
Conflict of interest
The authors declare no conflict of interest.
References
[
[
[
1] H. Zhang, P. K. Shen, Chem. Rev. 2012, 112, 2780–2832.
2] M. Pourbaba, S. Zirakkar, Procedia Eng. 2011, 21, 1088–1095.
3] a) A. F. Dalebrook, W. Gan, M. Grasemann, S. Moret, G. Laurenczy, Chem. Commun.
2
013, 8735-8751. b) M. Yadav, Q. Xu, Energy Environ. Sci. 2012, 5, 9698-9725. c) H.
L. Jiang, S. K. Singh, J. M. Yan, X. B. Zhang, Q. Xu, ChemSusChem. 2010, 3, 541-
49.
4] a) D. Mellmann, P. Sponholz, H. Junge, M. Beller, Chem. Soc. Rev. 2016, 45,
5
[
3
8
1
954−3988. b) M. Grasemann, G. Laurenczy, Energy Environ. Sci. 2012, 5, 8171−
181. c) S. Enthaler, J. von Langermann, T. Schmidt, Energy Environ. Sci. 2010, 3,
207−1217. d) T. C. Johnson, D. J. Morris, M. Wills, Chem. Soc. Rev. 2010, 39, 81−88.
e) F. Joꢀ, ChemSusChem. 2008, 1, 805−808. f) J. Eppinger, K. W. Huang ACS Energy
Lett. 2017, 2, 188−195. g) A. K. Singh, S. Singh, A. Kumar, Catal. Sci. Technol. 2016,
6
, 12-40. h) N. Onishi, G. Laurenczy, M. Beller, Y. Himeda, Coordination Chemistry
Reviews. 2018, 373, 317-332. i) N. Onishi, M. Iguchi, X. Yang, R. Kanega, H.
2
2
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Kawanami, Q. Xu, Y. Himeda, Adv. Energy Mater. 2018, 1801275. j) K. Sordakis, C.
Tang, L. K. Vogt, H. Junge, P. J. Dyson, M. Beller, G. Laurenczy, Chem. Rev. 2018,
1
18, 372-433. k) M. Iglesias, L. A. Oro, Eur. J. Inorg. Chem. 2018, 2125–2138.
[
[
5] R. Coffey, Chem. Commun. 1967, 923−924.
6] a) I. Mellone, N. Gorgas, F. Bertini, M. Peruzzini, K. Kirchner, L. Gonsalvi,
Organometallics 2016, 35, 3344−3349. b) E. A. Bielinski, P. O. Lagaditis, Y. Zhang,
B. Q. Mercado, C. Wꢁrtele, W. H. Bernskoetter, N. Hazari, S. Schneider, J. Am. Chem.
Soc. 2014, 136, 10234−10237. c) T. Zell, B. Butschke, Y. Ben-David, D. Milstein,
Chem. - Eur. J. 2013, 19, 8068−8072.
[
7] a) S. Fukuzumi, T. Kobayashi, T. Suenobu, ChemSusChem 2008, 1, 827−834. b) Y.
Himeda, S. Miyazawa, T. Hirose, ChemSusChem 2011, 4, 487−493. c) D. Jantke, L.
Pardatscher, M. Dress, M. Cokoja, W. A. Herrmann, F. E. Kuhn, ChemSusChem 2016,
9
2
, 2849-2854. d) Z. Wang, S. M. Lu, J. Wu, C. Li, J. Xiao, Eur. J. Inorg. Chem 2016,
016, 490-496.
[
8] a) Y. Pan, C. L. Pan, Y. Zhang, H. Li, S. Min, X. Guo, B. Zheng, H. Chen, A. Anders,
Z. Lai, J. Zheng, K.-W. Huang, Chem. - Asian J. 2016, 11, 1357−1360. b) M. Czaun,
A. Goeppert, J. Kothandaraman, R. B. May, R. Haiges, G. K. S. Prakash, G. A. Olah,
ACS Catal. 2014, 4, 311−320. c) A. Guerriero, H. Bricout, K. Sordakis, M. Peruzzini,
E. Monflier, F. Hapiot, G. Laurenczy, L. Gonsalvi, ACS Catal. 2014, 4, 3002−3012. d)
S. Fukuzumi, T. Kobayashi, T. Suenobu, J. Am. Chem. Soc. 2010, 132, 1496−1497. e)
A. Boddien, B. Loges, H. Junge, F. Gꢂrtner, J. R. Noyes, M. Beller, Adv. Synth. Catal.
2
2
009, 351, 2517−2520. f) C. Fellay, P. J. Dyson, G. Laurenczy, Angew. Chem., Int. Ed.
008, 47, 3966−3968. g) B. Loges, A. Boddien, H. Junge, M. Beller, Angew. Chem.,
Int. Ed. 2008, 47, 3962−3965. h) C. Fellay, N. Yan, P. J. Dyson, G. Laurenczy, Chem. -
Eur. J. 2009, 15, 3752-3760. i) A. Boddien, C. Federsel, P. Sponholz, D. Mellmann, R.
2
3
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Jackstell, H. Junge, G. Laurenczy, M. Beller, Energy Environ Sci. 2012, 5, 8907-8911.
j) P. Sponholz, D. Mellmann, H. Junge, M. Beller, ChemSusChem. 2013, 6, 1172-1176.
9] C. Guan, D. Zhang, Y. Pan, M. Iguchi, M.J. Ajitha, J. Hu, H. Li, C. Yao, M. H. Huang,
S. Min, J. Zheng, Y. Himeda, H. Kawanami, K. W. Huang, Inorg Chem. 2017, 56, 438-
[
[
4
45.
10] a) J. J. A. Celaje, Z. Lu, E. A. Kedzie, N. J. Terrile, J. N. Lo, T. J. A. Williams, Nat.
Commun. 2016, 7, 11308−11313. b) M. Czaun, J. Kothandaraman, A. Goeppert, B.
Yang, S. Greenberg, R. B. May, G. A. Olah, G. K. S. Prakash, ACS Catal. 2016, 6,
7
1
475−7484. c) A. Matsunami, Y. Kayaki, T. Ikariya, Chem. Eur. J. 2015, 21,
3513−13517. d) S. Oldenhof, M. Lutz, B. de Bruin, J. I. van der Vlugt, J. N. H Reek,
Chem. Sci. 2015, 6, 1027-1034. e) Y. Manaka, W.-H. Wang, Y. Suna, H. Kambayashi,
J. T. Muckerman, E. Fujita, Y. Himeda, Catal. Sci. Technol. 2014, 4, 34-37. f) E.
Fujita, J. T. Muckerman, Y. Himeda, Biochim. Biophys. Acta, Bioenerg. 2013, 1827,
1
1
031-1038. g) J. H. Barnard, C. Wang, N. G. Berry, J. L. Xiao, Chem. Sci. 2013, 4,
234−1244. h) S. Oldenhof, B. de Bruin, M. Lutz, M. A. Siegler, F. W. Patureau, J. I.
van der Vlugt, J. N. H. Reek, Chem. - Eur. J. 2013, 19, 11507-11511. i) R. Tanaka, M.
Yamashita, L. W. Chung, K. Morokuma, K. Nozaki, Organometallics 2011, 30, 6742-
6
750. j) C. Fink, G. Laurenczy, Dalton trans. 2017, 46, 1670- 1676. k) M. Iguchi, H.
Zhong, Y. Himeda, H. Kawanami, Chem. - Eur. J. 2017, 23, 1-6. l) S. M. Lu, Z. Wang,
J. Wang, J. Li, C. Li, Green Chem. 2018, 20, 1835-1840. m) A. Matsunami, S. Kuwata,
Y. Kayaki, ACS Catal. 2017, 7, 4479−4484. n) W.-H. Wang, S. Xu, Y. Manaka, Y.
Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, ChemSusChem. 2014, 7,
1
976−1983. o) J. Li, J. Li, D. Zhang, C. Liu, ACS Catal. 2016, 6, 4746-4754. p) J. F.
Hull, Y. Himeda, W. H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T.
Muckerman, E. Fujita, Nat. Chem. 2012, 4, 383-388. q) W. H. Wang, M. Z. Ertem, S.
2
4
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
Xu, N. Onishi, Y. Manaka, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y.
Himeda, ACS Catal. 2015, 5, 5496-5504. r) M. Iguchi, Y. Himeda, Y. Manaka, H.
Kawanami, ChemSusChem. 2016, 9, 2749-2753. s) R. Kanega, N. Onishi, L. Wang, K.
Murata, J. Muckerman, E. Fujita, Y. Himeda, Chem. - Eur. J. 2018, 24, 1-5. t) G. Papp,
G. Olveti, H. Horvath, A. Katho, F. Joo, Dalton trans. 2016, 45, 14516-14519.
11] Z. Wang, S. Lu, J. M. Li, J. Wang, C. Li, Chem. - Eur. J. 2015, 21, 12592−12595.
12] J. F. Hull, Y. Himeda, W.-H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, T. James,
J. T. Muckerman, E. Fujita, Nat. Chem. 2012, 4, 383-388.
[
[
[
[
13] Y. Himeda, Green Chem. 2009, 11, 2018-2022.
14] A. Iturmendi, L. R- Perez, J J. Perez- Torrente, M. Iglesias, L. A. Oro,
Organometallics. 2018, DOI: 10.1021/acs.organomet.8b00289.
[
[
[
[
[
15] G. A. Filonenko, R. van Putten, E. N. Schulpen, E. J. Hensen, E. A. Pidko,
ChemCatChem. 2014, 6, 1526−1530.
16] R. E. Rodrguez- Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso- Quinones, H.
Gritzmacher, Nat. Chem. 2013, 5, 342- 347.
17] K. Gupta, D. Tyagi, A. D. Dwivedi, S. M. Mobin, S. K. Singh, Green Chem. 2015, 17,
4
618-4627.
18] A. D. Dwivedi, K. Gupta, D. Tyagi, R. K. Rai, S. K. Singh, ChemCatChem. 2015, 7,
050- 4058.
4
19] A. Boddien, D. Mellmann, F. Gꢂrtner, R. Jackstell, H. Junge, P. J. Dyson, G.
Laurenczy, R. Ludwig, M. Beller, Science 2011, 333, 1733-1736.
[
20] D. Carmona, P. Lamata, A. Sanchez, P. Pardo, R. Rodriguez, P. Ramirez, F. J. Lahoz,
P. Garcia-orduna, L. A. Oro, Organometallics 2014, 33, 4016-4026.
21] M. J. Tenorio, M. C. Puerta, P. Valerga, Inorg. Chem. 2011, 50, 12399-12401.
[
2
5
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
[
[
[
[
22] Y. Nishibayashi, H. Imajima, G. Onodera, Y. Inada, M. Hidai, S. Uemura,
Organometallics 2004, 23, 5100− 5103.
23] I. Mellone, M. Peruzzini, L. Rosi, D. Mellmann, H. Junge, M. Beller, L. Gonsalvi,
Dalton trans. 2013, 42, 2495-2501.
24] Z. Yang, Z. Zhu, R. Luo, X. Qiu, J. T. Liu, J. K. Yang, W. Tang, Green Chem. 2017,
1
9, 3296- 3301.
25] G. M. Sheldrick, Acta crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112-122.
2
6
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201801501
(Table of Content)
Efficient catalytic dehydrogenation of formic acid was achieved over water-soluble Ru-arene
complexes containing 8-aminoquinoline based ligands, where the role of NH moiety and the
stability of catalytic species in H
2
O plays important role in achieving high catalytic activity
6
2
+
over [(ƞ -C
6
H
6
)Ru(κ -NpyNHMe-MAmQ)Cl] (C-4). Moreover, extensive mass studies infer
the involvement of a coordinatively unsaturated species C-2A/C-4A as the active component,
which was isolated and characterized (by X-ray crystallography) as a diruthenium complexes.
2
7
This article is protected by copyright. All rights reserved.