Catalytic Hydrogen Production by Ruthenium Complexes from the Conversion of Primary Amines to Nitriles: Potential Application as a Liquid Organic Hydrogen Carrier

Article abstract of DOI:10.1002/chem.201603423

The potential application of the primary amine/nitrile pair as a liquid organic hydrogen carrier (LOHC) has been evaluated. Ruthenium complexes of formula [(p-cym)Ru(NHC)Cl₂] (NHC=N-heterocyclic carbene) catalyze the acceptorless dehydrogenation of primary amines to nitriles with the formation of molecular hydrogen. Notably, the reaction proceeds without any external additive, under air, and under mild reaction conditions. The catalytic properties of a ruthenium complex supported on the surface of graphene have been explored for reutilization purposes. The ruthenium-supported catalyst is active for at least 10 runs without any apparent loss of activity. The results obtained in terms of catalytic activity, stability, and recyclability are encouraging for the potential application of the amine/nitrile pair as a LOHC. The main challenge in the dehydrogenation of benzylamines is the selectivity control, such as avoiding the formation of imine byproducts due to transamination reactions. Herein, selectivity has been achieved by using long-chain primary amines such as dodecylamine. Mechanistic studies have been performed to rationalize the key factors involved in the activity and selectivity of the catalysts in the dehydrogenation of amines. The experimental results suggest that the catalyst resting state contains a coordinated amine.

Full text of DOI:10.1002/chem.201603423

DOI: 10.1002/chem.201603423
Full Paper
&
Supported Catalysis
Catalytic Hydrogen Production by Ruthenium Complexes from
the Conversion of Primary Amines to Nitriles: Potential
Application as a Liquid Organic Hydrogen Carrier
David Ventura-Espinosa, Aida Marzꢀ-Beltrꢀn, and Jose A. Mata*[a]
Abstract: The potential application of the primary amine/ni-
trile pair as a liquid organic hydrogen carrier (LOHC) has
been evaluated. Ruthenium complexes of formula [(p-cym)-
Ru(NHC)Cl2] (NHC=N-heterocyclic carbene) catalyze the ac-
ceptorless dehydrogenation of primary amines to nitriles
with the formation of molecular hydrogen. Notably, the reac-
tion proceeds without any external additive, under air, and
under mild reaction conditions. The catalytic properties of
a ruthenium complex supported on the surface of graphene
have been explored for reutilization purposes. The rutheni-
um-supported catalyst is active for at least 10 runs without
any apparent loss of activity. The results obtained in terms
of catalytic activity, stability, and recyclability are encourag-
ing for the potential application of the amine/nitrile pair as
a LOHC. The main challenge in the dehydrogenation of ben-
zylamines is the selectivity control, such as avoiding the for-
mation of imine byproducts due to transamination reactions.
Herein, selectivity has been achieved by using long-chain
primary amines such as dodecylamine. Mechanistic studies
have been performed to rationalize the key factors involved
in the activity and selectivity of the catalysts in the dehydro-
genation of amines. The experimental results suggest that
the catalyst resting state contains a coordinated amine.
Introduction
future of liquid organic hydrogen carriers (LOHC) depends on
a reversible chemical reaction based on the release and uptake
of hydrogen.[23–27] In this work, we have evaluated the potential
of the amine/nitrile pair as a liquid organic hydrogen carrier, as
suggested by Grellier and Sabo-Etienne in a frontier-type arti-
cle (Scheme 1).[28]
The use of molecular hydrogen as an alternative energy source
is a highly desirable process.[1–3] The key is the production of
energy with the formation of water as the only byproduct.
From environmental and sustainability points of view, the ad-
vantages versus other technologies are notorious.[4–6] Neverthe-
less, there are still some limitations to overcome that should
be addressed by technological and scientific approaches:
1) hydrogen production and 2) hydrogen storage.[7–9] The pro-
duction of hydrogen is well-known, the problem arises from
the bulk production. In an ideal situation, industrial hydrogen
production would require the use of renewable energy sources
to obtain hydrogen from the electrolysis of water. Many efforts
have pursued this goal and the achievements in the last few
years have been considerable.[10,11] The success of hydrogen as
an alternative energy source relies on the development of effi-
cient hydrogen storage and transport systems.[12–14] Nowadays,
hydrogen storage represents a technological challenge.[15,16]
Among all the possibilities, chemical hydrogen storage in the
form of organic liquids represents an attractive route for the
development of hydrogen carrier systems.[17–20] In this sense,
the technological problems are minimized as the actual infra-
structure is based in liquids for storage and transport.[21,22] The
Scheme 1. (De)hydrogenation of the amine/nitrile pair.
The acceptorless dehydrogenation of primary amines gene-
rates the corresponding nitrile and two moles of hydrogen.
The reaction is not favored at room temperature and an effec-
tive catalyst is needed for practical purposes. Efficient catalysts
for the acceptorless dehydrogenation of amines are scarce and
were first developed by the group of Szymczak based on
a ruthenium pincer complex.[29] The viability of the amine/ni-
trile pair as a potential organic hydrogen carrier also lies in the
effectiveness of the reverse process, that is, the hydrogenation
of nitriles. Fortunately, the hydrogenation of nitriles is a well-
documented catalytic reaction and efficient catalysts have
been described in the literature.[30–35] Nitriles are versatile inter-
mediates in organic synthesis owing to their direct conversion
into a variety of functional groups. Conventional organic pro-
cedures require nucleophilic displacement of suitable leaving
groups producing stoichiometric amounts of waste and with
[a] D. Ventura-Espinosa, A. Marzꢀ-Beltrꢀn, Dr. J. A. Mata
Institute of Advanced Materials (INAM)
Universitat Jaume I, Avda. Sos Baynat s/n, 12006, Castellꢁn (Spain)
E-mail: jmata@uji.es
Supporting information and authors’ ORDIDs for this article are available
on the WWW under http://dx.doi.org/10.1002/chem.201603423.
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limited selectivity.[36,37] A novel and green method for the syn-
thesis of nitriles consists in the acceptorless dehydrogenation
of primary amines. This method does not require the use of
stoichiometric oxidants and the only byproduct is molecular
hydrogen.
and imine (entry 2). The use of different bases slightly in-
creased the selectivity towards the nitrile compound without
altering the conversion, except in the case of tBuOK. The use
of such a strong base was detrimental for the dehydrogena-
tion of benzylamine and only 34% conversion was reached
(entries 3–5). The best results were achieved in the absence of
any external additive (entries 6–8). The reaction of benzylamine
with catalyst 2a in toluene without any additive afforded full
conversion after 8 h. The reaction selectivity favored the forma-
tion of the nitrile product. The ruthenium catalysts 2a–c with
different electronic properties displayed no significant differ-
ence in terms of catalytic activity or product selectivity. We ob-
served that the selectivity in the dehydrogenation of amines
was largely dependent on the reaction conditions. When the
reaction of benzylamine using catalysts 2a–c was carried out
in a closed vessel, the only product formed was the imine (the
acceptorless dehydrogenative coupling or transamination
product). The homocoupling of primary amines by using cata-
lyst 2a had previously been described in a closed system
showing the sole formation of the imine product.[44] The dehy-
drogenation of amines is a nonspontaneous thermodynamic
process. The release of hydrogen from the reaction media pro-
duces an equilibrium shift towards product formation. We thus
conclude that vigorous refluxing conditions favor the reaction
by facilitating the release of hydrogen. When the reaction is
carried out with intense solvent bubbling, the conversion is in-
creased. Solvent optimization was carried out for catalyst 2c
(entries 9–13). The best results were obtained using p-methyl-
benzylamine as the substrate and toluene as the solvent. The
use of acetonitrile or alcohols as the solvent decreased the re-
action conversion and selectivity of dehydrogenation. Two dif-
ferent experiments served to evaluate whether oxygen from
air is the final oxidant in the dehydrogenation of amines (en-
tries 14–15). The reaction of benzylamine was carried out
under intense nitrogen bubbling. The results suggest that
oxygen is not needed in the dehydrogenation process. Confir-
mation was obtained when carrying out the same experiment
under an oxygen atmosphere. The presence of oxygen afford-
ed less than 5% conversion. This result implies that oxygen is
not the final oxidant in the dehydrogenation of amines; in
fact, the results show that the catalyst is inactive under catalyt-
ic conditions in the presence of oxygen.
In this work, we describe the production of hydrogen from
primary amines by using ruthenium catalysts, showing the po-
tential application of the primary amine/nitrile pair as a liquid
organic hydrogen carrier. Studies on the catalytic activity and
selectivity have allowed us to propose a plausible mechanism
for the dehydrogenation of amines.
Results and Discussion
Synthesis of ruthenium complexes
A series of ruthenium complexes (2a–c) were obtained in
good yield from the corresponding imidazolium salts (1a–c) by
transmetallation using silver oxide (Scheme 2). The complexes
were fully characterized by NMR, electrospray mass ionization,
and elemental analysis. The ruthenium complexes[38–40] differ in
the substituents at the 4- and 5-positions of the N-heterocyclic
carbene ligand (NHC). Fine-tuning of the electronic properties
of the NHC ligands is achieved by the introduction of electron-
donating (ÀCH3) or -withdrawing groups (ÀCl).[41] Modifying
the substituents at the backbone of the azol ring does not
alter significantly the steric properties of the NHC ligands,
which are controlled by the substituents at the nitrogen posi-
tions 1 and 3 (n-butyl groups).[42,43] The catalytic properties of
ruthenium complexes of general formula [(p-cym)Ru(NHC)Cl2]
have been described previously in the dehydrogenation of al-
cohols and homocoupling of amines.[44,45]
The reaction scope and limitations of the catalytic dehydro-
genation of benzylamines were evaluated using complexes 2c
and 3 as catalyst precursors (Table 2). The reaction conditions
consisted of toluene as the solvent at 1108C and a catalyst
loading of 2 mol%. Complex 3 was used for comparative rea-
sons. The immobilization of complex 3 on graphene has been
previously described and we wanted to assess the catalytic
properties of a supported molecular ruthenium catalyst in the
dehydrogenation of amines.[45] The evaluation of the catalytic
properties revealed that both ruthenium complexes are active
in the dehydrogenation of a variety of amines containing dif-
ferent substituents. In all cases, full conversions were achieved
in approximately 8 h. Only in the case of bulky amines, such as
1-naphtylmethylamine, was the dehydrogenation process
slower (Table 2, entries 12 and 13). The best results in terms of
Scheme 2. Synthesis of ruthenium complexes.
Catalytic properties
In a first set of experiments, the dehydrogenation of benzyl-
amine was chosen as the model reaction for the optimization
of the reaction conditions (Table 1). One equivalent of benzyl-
amine and 2 mol% of catalyst were dissolved in 3 mL of sol-
vent and heated at vigorous reflux. A control experiment
showed that the dehydrogenation reaction did not proceed in
the absence of catalyst (Table 1, entry 1). When the reaction
was carried out in the presence of a halide abstractor additive,
benzylamine was converted to an equimolar mixture of nitrile
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Table 1. Optimization parameters for the catalytic dehydrogenation of
primary amines.[a]
Table 2. Reaction scope and limitations of the catalytic dehydrogenation
of primary amines.[a]
Entry
Cat.
R
Additive
Solvent Conv.
[%][b]
Selectivity [%]
nitrile imine
Entry
Cat.
R
Conv.
[%][b]
Selectivity [%]
1
2
3
4
5
6
7
8
–
H
H
H
H
H
H
H
H
CH3
CH3
CH3
CH3
CH3
H
–
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene 100
toluene 100
MeCN
iPrOH
0
92
34
88
97
98
95
–
49
68
70
60
74
73
75
74
50
68
65
64
71
n.d.[c]
–
51
32
30
40
26
27
25
26
50
32
35
36
29
n.d.[c]
nitrile
imine
2a
2a
2a
2a
2a
2b
2c
2c
2c
2c
2c
2c
2c
2c
AgOTf
tBuOK
Cs2CO3
NaOAc
–
–
–
–
–
–
–
–
1
2
3
4
5
6
7
8
2c
3
H
H
H
Cl
Cl
Me
Me
OMe
OMe
CF3
CF3
naphtyl
naphtyl
100
99
100[c]
95
83
100
93
99
97
95
78
75
82
65
50
57
74
70
69
68
62
52
55
57
25
18
35
50
43
26
30
31
32
38
48
45
43
NHC–Ru–rGO (4)
2c
3
2c
3
2c
3
2c
3
2c
3
9
10
11
12
13
14
15
24
54
75
39
98
9
10
11
12
13
tBuOH
nBuOH
80
82
N2(bubbling) toluene
O2 (1 atm) toluene
H
<5
[a] Reaction conditions: substrate (0.3 mmol), catalyst (2 mol%), 2 mL of
toluene at 1108C for 8 h. [b] Conversion determined by GC analysis using
anisole as the internal standard. [c] After 6 h of reaction.
[a] Reaction conditions: substrate (0.3 mmol), catalyst (2 mol%), 3 mL of
solvent at reflux for 8 h. [b] Conversion determined by GC analysis using
anisole as the internal standard. [c] n.d.=not determined.
catalytic activity were obtained in the case of benzylamine
using catalyst 3 and in the case of 4-methoxybenzylamine
using catalyst 2c. A comparative reaction profile was carried
out using 4-methoxybenzylamine as a model substrate and
complexes 2c and 3 as the catalyst precursors (Figure S1, Sup-
porting Information). In general, the differences observed in
catalytic activity of complexes 2c and 3 are negligible. The for-
mation of molecular hydrogen was confirmed by injection of
the gases released in the reactions described in entries 7 and 8
directly into a gas chromatograph. The challenge in the dehy-
drogenation of benzylamines is to control the selectivity of the
reaction. In general, it has been found that controlling the fac-
tors that govern the formation of the different products is diffi-
cult. The use of ruthenium complexes with different ligands
has proved inefficient to control the selectivity and further ef-
forts to design highly selective catalysts are thus required. The
best results in terms of selectivity were obtained for benzyl-
amine using complex 3 as the catalyst (entry 2).
Figure 1. Recycling experiments on the dehydrogenation of benzylamine
using the hybrid material NHC–Ru–rGO (4). Reaction profile for each run de-
termined by GC analysis at a maximum of approximately 50% conversion.
The catalytic properties and recycling studies of the hybrid
material NHC–Ru–rGO (4) were carried out using benzylamine
(entry 3 and Figure 1). The results show that immobilization of
complex 3 on the graphene surface increases the catalytic ac-
tivity. Full conversion was achieved in 6 h, in contrast to the
8 h required with molecular complex 3. These results support
our previous catalytic studies using molecular complexes anch-
ored on graphene.[45,46] The immobilization of molecular com-
plexes on graphene by noncovalent interactions shows a cata-
lytic benefit compared to the molecular complexes most prob-
ably due to the increased stability of the catalytically active
species.
the ruthenium complexes [(p-cym)Ru(NHC)Cl2] are active in the
dehydrogenation of alkylamines under the same reaction con-
ditions as described for the benzylamines. In the case of alkyla-
mines, we observed a better control of the selectivity towards
nitrile formation. Increasing the number of carbon atoms of
the alkyl chain inhibited the formation of the transamination
product. For example, dehydrogenation of heptylamine yielded
78% of nitrile product, octylamine afforded 89% and, in the
case of dodecylamine, the nitrile product was obtained in
100% yield (Table 3, entries 1–3).
The formation of the transamination product is thus inhibit-
ed for long-chain alkylamines. In the case of tetradecylamine,
hexadecylamine, and octadecylamine, the only product ob-
The reaction scope of the catalytic dehydrogenation of alkyl
primary amines was evaluated (Table 3). The results show that
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phene.[48–50] The catalytic results improve when the metal com-
plexes are anchored on graphene and the hybrid materials can
be recycled up to 10 times due to their enhanced stability. In
view of the good results obtained, we decided to assess the
catalytic activity of a similar ruthenium-based hybrid material
derived from complex 3 in the hydrogen production from pri-
mary amine dehydrogenation (Scheme 4).
Table 3. Dehydrogenation of alkyl primary amines.[a]
Entry
Amine
n
Conv.
[%][b]
Selectivity [%]
nitrile
imine
The catalytic properties of molecular complex 3 and its de-
rived hybrid material 4 were analyzed under the same reaction
conditions (Table 2, entries 2 and 3). The results show that the
catalytic activity of the molecular complex is maintained after
immobilization. In general, one of the drawbacks of supported
molecular catalysis is the decrease in the reaction rate due to
diffusion problems. The catalytic activity of 3 was not altered
after immobilization on graphene most probably because of
the 2D character of graphene that facilitates the accessibility
to all catalytic active centers. The same behavior has been pre-
viously observed when comparing the catalytic activity of
other molecular complexes and their corresponding immobi-
lized hybrid materials.[45]
1
2
3
4
5
6
heptylamine
octylamine
dodecylamine
tetradecylamine
hexadecylamine
octadecylamine
5
6
10
12
14
16
95
99
100
95
97
95
78
89
100
100
100
100
22
11
0
0
0
0
[a] Reaction conditions: substrate (0.3 mmol), catalyst (2 mol%), 2 mL of
toluene at 1108C for 24 h. [b] Conversion determined by GC analysis
using anisole as the internal standard.
served was the nitrile compound. The use of long-chain alkyla-
mines to control the selectivity in the oxidation of primary
amines has recently been described.[47] Control over the selec-
tivity is an important issue when considering the amine/nitrile
pair as a promising candidate for LOHC. It has been described
that N-heterocycles are considered good LOHCs and control of
the selectivity can be easily achieved. For comparative purpos-
es, we studied the activity of the ruthenium complexes in the
dehydrogenation of indoline (Scheme 3). The ruthenium com-
plexes 2c and 3 catalyzed the dehydrogenation of indoline
quantitatively in 6 h at 1108C.
Recycling experiments were carried out using benzylamine
as the model substrate. The reactions were performed using
a catalyst loading of 2 mol% (based on ruthenium), and 3 mL
of toluene at 1108C. The reaction profile for each run was
monitored by GC until a maximum of approximately 50% con-
version. After each run, the mixture was allowed to cool down
to room temperature and the solid catalyst was separated by
decantation, washed thoroughly with toluene, and used in the
next run (Figure 1).
The hybrid material NHC–Ru–rGO (4) was reused up to
10 times without any significant decrease in activity. There is
a slight decrease of the catalytic activity in run 9 that is main-
tained in run 10. The reaction profile for each run allows com-
parison of the catalytic activity for different reaction times. The
results show that the catalytic activity is maintained for the dif-
ferent runs. The morphology of the catalytic hybrid material
NHC–Ru–rGO (4) was analyzed by HRTEM after the recycling
experiments. Comparison of the images before and after the
recycling experiments reveal a well-dispersed ruthenium distri-
bution without the presence of metal nanoparticles (Figure 2).
The morphology of graphene does not display significant
changes apart from an increase in the number of wrinkles.
To establish the mode of action of the hybrid material NHC–
Ru–rGO (4) during catalysis, a hot filtration experiment was car-
ried out. This experiment rules out the possibility of release-
and-return of the molecular complex from the surface of gra-
Scheme 3. Dehydrogenation of indoline.
Recycling studies
We have recently described a methodology for obtaining sup-
ported catalyst on the surface of graphene by means of non-
covalent interactions.[45,46] The catalytic properties of the hybrid
materials in terms of activity and stability have demonstrated
the potential of transition metals immobilized on gra-
Scheme 4. Synthesis of the hybrid material NHC–Ru–rGO (4).
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tion. In order to confirm the presence of hydrogen, a similar
experiment was carried out using a sacrificial hydrogen accept-
or. The reaction was carried out with benzylamine, complex
2c, and two equivalents of tert-butyl ethylene (tbe) in deuter-
ated toluene. The 1H NMR recorded after the experiment
showed the formation of products but no ethylene hydrogena-
tion. This result suggests that our ruthenium complex is a poor
hydrogenation catalyst. Further evidence is that the imine
product was not hydrogenated to the corresponding amine.
The formation of H2 in the dehydrogenation of amines was an-
alyzed in a different experiment by gas chromatography (GC).
A solution of complex 2c and benzylamine in toluene was
heated under catalytic conditions. A sample of the generated
gas was collected and injected directly into the GC using
a gas-syringe. The results confirmed the presence of molecular
hydrogen generated from the dehydrogenation of amines.
Further mechanistic investigations were carried out by
single-stage ESI-MS (Figure 4). An equimolar mixture of com-
plex 2c and p-methylbenzylamine was dissolved in toluene,
heated at 708C for 10 min, and analyzed by direct injection
into ESI-MS. The results show the presence of two species in
solution. The peak at m/z: 521.1 was assigned to complex 2c
with a vacant coordination site, formulated as [MÀCl]+. A
second and more intense peak at m/z: 642.1 was assigned to
complex 2c, which contains a coordinated p-methylbenzyl-
amine [MÀCl+H2NCH2PhMe]+. The proposed molecular com-
position was further confirmed by the simulated isotopic pat-
tern distribution. When the same experiment was reproduced
at 1108C instead of 708C, neither the formation of this reaction
intermediate nor any other ruthenium species was detected.
The results suggests that 1) at low temperatures, a chloride
ligand in complex 2c is replaced by a p-methylbenzylamine
and 2) at high temperatures, the amine is converted to the ni-
trile product and no other ruthenium intermediates are detect-
ed. These results support that complex 6 is the resting state in
the dehydrogenation of amines. Metal–halide bond-breaking
and coordination of an L-type ligand yielding cationic species
has previously been reported for ruthenium p-cymene spe-
cies.[51] A further experiment was performed to confirm the co-
ordination of p-methylbenzylamine to ruthenium and discard
the possibility that this product is only formed under ESI-MS
conditions. Ruthenium complex 5 and p-methylbenzylamine
were dissolved in CH2Cl2/MeOH (3:2 v/v). Addition of sodium
tetrafluoroborate afforded the amine-coordinated product 6 in
high yield (Figure 5). Ruthenium complex 6 is stable in solution
and in the solid state. The molecular structure of complex 6
was confirmed by single-crystal X-ray diffraction.
Figure 2. HRTEM images of PhF-Ru-rGO (4): a) before, and b) after ten cata-
lytic runs.
phene. If the molecular complex is retained on the surface of
graphene, the catalysis is heterogeneous in nature; however, if
the molecular complex is released, the catalysis is homogene-
ous. The hot filtration experiment was carried out using NHC–
Ru–rGO (4) (2 mol%) as the catalyst and benzylamine as the
substrate. The catalytic hybrid material NHC–Ru–rGO (4) was
removed from the reaction medium by filtration at high tem-
perature (1108C) when the conversion reached 50% (ca. 2 h).
The filtrate was stirred for 2 h more under identical conditions.
After this time, GC analysis indicated that no further amine de-
hydrogenation had occurred (GC conversion remained at
50%). The catalytic hybrid material NHC–Ru–rGO (4), previously
removed from the reaction medium, was then treated with
more benzylamine. After 2 h at 1108C, we observed that the
reaction had proceeded, indicating that the hybrid material
NHC–Ru–rGO (4) was still active. The hot filtration experiment
confirmed the absence of catalytic species in solution due to
catalyst desorption from the graphene surface and demon-
strated the heterogeneous nature of the catalytic process. The
recycling properties of the hybrid material NHC–Ru–rGO (4) are
excellent and most probably due to the stabilization of molec-
ular ruthenium complex 3 on the surface of graphene.
Mechanistic considerations
Preliminary studies for the mechanism in the acceptorless de-
hydrogenation of amines were performed by monitoring the
1
reaction profile by H NMR spectroscopy (Figure 3). The catalyt-
ic dehydrogenation of p-methoxybenzylamine was carried out
using complex 2c in deuterated toluene using NMR tubes
closed with a perforated septum using a needle. The results
show that signals corresponding to the nitrile product start to
appear after 2 h, suggesting that dehydrogenation towards ni-
trile is fast. However, the reaction needed four days to achieve
full conversion. In general, the kinetics of catalytic reactions
carried out in NMR tubes is slow due to diffusion problems.
We have observed that the reaction conditions have an impor-
tant influence in the kinetics in regard to the facile release of
hydrogen. The reaction is slowed down when there is not a vig-
orous bubbling as occurs in the NMR tube. Importantly, the
only products observed during all the catalytic reactions are
the nitrile and imine, no other products were detected. In gen-
eral, it should be possible to detect the presence of dissolved
A mechanism for the dehydrogenation of primary amines is
proposed in Figure 6 based on experimental evidence. Mecha-
nistic reports for the dehydrogenation of primary amines are
scarce. Brookhart et al. proposed a reaction mechanism for an
iridium complex using a hydrogen acceptor.[52] The only mech-
anistic cycle for the acceptorless amine dehydrogenation reac-
tion was proposed by Szymczak et al.[53] More mechanistic in-
formation is available for the reverse reaction, the hydrogena-
tion of nitriles, but still, a complete catalytic cycle for all the
steps involved in the process is needed.[54–56] Starting from
1
molecular hydrogen by H NMR spectroscopy. In our case, we
did not observe the formation of molecular hydrogen. Most
probably, hydrogen needs to be released for reaction evolu-
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Figure 3. Catalytic dehydrogenation profile of p-methoxybenzylamine by using complex 2c in deuterated toluene showing the full conversion and the forma-
tion of nitrile and imine products.
Figure 4. ESI mass spectrum of complex 2c in the presence of p-methylbenzylamine under stoichiometric conditions. Schematic drawings of the species de-
tected: i) with a vacant coordination site [MÀCl]+, and ii) with a coordinated p-methylbenzylamine [MÀCl+H2NCH2PhMe]+. Inset: experimental and simulated
isotopic pattern of [MÀCl+H2NCH2PhMe]+.
complex 5, the first step is the substitution reaction of a chlo-
ride by benzylamine. The cationic complex 6 was prepared,
isolated, and fully characterized using a sodium tetrafluorobo-
rate salt. Complex 6 is the catalyst resting state and is in acid/
base equilibrium with the amido complex (I). A second mole-
cule of benzylamine promotes this equilibrium. Compound (I)
is converted into the ruthenium hydride–imine complex (II) by
b-hydride elimination. The attack of a second benzylamine to
complex II generates the imine byproduct. This transformation
has been proposed for the acceptorless dehydrogenative cou-
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Conclusions
In this manuscript, we have explored the potential application
of the amine/nitrile pair as a LOHC. Molecular ruthenium com-
plexes of general formula [(p-cym)Ru(NHC)Cl2] are active in the
dehydrogenation of primary amines to nitriles with the simul-
taneous formation of two moles of molecular hydrogen. The
reaction proceeds without any external additive under mild re-
action conditions. Both catalyst activation and product forma-
tion are temperature dependent. The acceptorless dehydro-
genation of primary amines is a nonspontaneous process. Hy-
drogen removal from the reaction medium is required for ef-
fective nitrile formation. Mechanistic investigations have
shown that the catalyst is activated by chloride/amine substi-
tution. We have observed that the catalyst resting state con-
tains a coordinated benzylamine. We have assessed the catalyt-
ic properties of an analogous ruthenium complex supported
on the surface of graphene in the dehydrogenation of benzyl-
amine. The results are promising as the catalytic system can be
recycled up to 10 times without significant loss of activity. The
main challenge in the dehydrogenation of benzylamines is the
control of the selectivity. To be suitable for applications in the
field of hydrogen storage, the transamination reaction that
leads to imine formation should be suppressed. We have ach-
ieved selectivity control using long-chain aliphatic amines. The
nitrile species is the sole product. Although the gravimetric
percentage of hydrogen content in primary amines is not high
compared to other systems, the versatility of amine-type com-
pounds and their easy access makes the amine/nitrile pair a po-
tential candidate for LOHCs. In addition, the reverse process,
Figure 5. Synthesis and molecular diagram of compound 6. Ellipsoids are at
the 50% probability level. Hydrogen atoms and counter anion (BF4À) have
been omitted for clarity.
pling of amines (transamination). A similar species has been
detected by ESI-MS in the dehydrogenative coupling of alco-
hols.[57] Nitrile ruthenium complex (IV) is proposed based on
the catalytic results on which nitrile was used as the solvent,
inhibiting the reaction (Table 1, entry 10). The two hydrogen
release steps are the key points of this catalytic cycle. We pro-
pose that H2 is released from ruthenium hydride species with
either coordinated imine (II) or benzylamine (IV) ligands, and
most probably through dihydrogen species. The proposed
mechanism is one of the possible scenarios for the acceptor-
less dehydrogenation of primary amines. Additional mechanis-
tic data are required to understand all the elementary catalytic
steps of the reaction.
Figure 6. Proposed mechanism for the dehydrogenation of primary amines by using [(p-cym)Ru(NHC)Cl2] complexes.
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product was purified by column chromatography. The pure com-
pound 2b was eluted with dichloromethane/acetone (9:1) and pre-
cipitated in a mixture of dichloromethane/hexane to give an
nitrile hydrogenation, is a well established reaction and very
active and effective catalysts have been described in this
regard. Future research in the field is needed, in particular, to
develop industry compatible catalysts, but the preliminary re-
sults shown here are very encouraging.
1
orange solid (44 mg, 13.1%). H NMR (500 MHz, CDCl3): d=5.37 (d,
3J=5.9 Hz, 2H, CHp-cym), 5.00 (d, 3J=5.9 Hz, 2H, CHp-cym), 4.40 (m,
2H, CH2,n-Bu), 3.98 (m, 2H, CH2,n-Bu), 2.93 (m, 1H, CHiPr,p-cym), 2.17 (s,
6H, CH3,imid), 1.96 (s, 3H, CH3,p-cym), 1.96 (m, 2H, CH2,n-Bu), 1.48 (m,
3
4H, CH2,n-Bu), 1.31 (m, 2H, CH2,n-Bu), 1.26 (d, J=7.0 Hz, 6H, CH3,iPr p-
cym), 0.93 ppm (t, 3J=7.1 Hz, 6H, CH3,n-Bu); 13C {1H} NMR (75 MHz,
CDCl3): d=169.9 (CNHC-Ru), 126.9 (C=Cimid), 107.9 (Cp-cym), 98.7 (Cp-cym),
85.9 (Cp-cym), 85.9 (Cp-cym), 82.3 (Cp-cym), 82.3 (Cp-cym), 49.8 (CH2,n-Bu),
33.9 (CH2,n-Bu), 30.7 (CHiPr, p-cym), 22.7 (CH3,iPr p-cym), 20.3 (CH2,n-Bu), 18.6
(CH3, p-cym), 14.0 (CH3,n-Bu), 9.8 ppm (CH3,imid); electrospray MS (cone
20 V; m/z, fragment): 479.3 [MÀCl]+; HRMS ESI-TOF-MS (positive
mode): [MÀCl]+ monoisotopic peak: 479.1770; calcd: 479.1770, er:
0 ppm.
Experimental Section
General procedures
Anhydrous solvents were obtained using a solvent purification
system (SPS M BRAUN) or purchased from commercial suppliers
and degassed prior to use by purging with dry nitrogen and keep-
ing over molecular sieves. All other reagents were used as received
from commercial suppliers. The imidazolium salts 1,3-bis(n-butyl)-
imidazolium iodide (1a),[58] 4,5-dimethyl-1,3-bis(n-butyl) imidazoli-
um chloride (1b),[59] 4,5-dichloro-1,3-bis(n-butyl) imidazolium
iodide (1c),[60] [RuCl2(p-cymene)]2,[61] and complexes 2a,[62] 3,[45] and
5[63] were obtained according to published methods.
Synthesis of 2c
The reaction was carried out by following the same procedure as
the one described for 2b, with 4,5-dichloro-1,3-bis(butyl)imidazoli-
um iodide (1c) (138 mg, 0.37 mmol) and Ag2O in CH2Cl2. Subse-
quent reaction with [Ru(p-cymene)Cl2]2 (200 mg, 0.36 mmol) afford-
ed 2c as an orange solid (67 mg, 37.7%).1H NMR (300 MHz, CDCl3):
NMR spectra were recorded on Varian Innova spectrometers oper-
ating at 300 or 500 MHz (1H NMR) and 75 or 125 MHz (13C NMR).
ESI-MS were recorded on a Micromass Quatro LC instrument, and
nitrogen was employed as the drying and nebulizing gas. HRMS
were obtained in a QTOF I (quadrupole-hexapole-TOF) mass spec-
trometer with an orthogonal Z-spray-electrospray interface (Micro-
mass, Manchester, UK). The drying and nebulizing gas was nitrogen
at a flow of 400 and 80 LhÀ1, respectively. The temperature of the
source block was set to 1208C and the desolvation temperature to
1508C. A capillary voltage of 3.5 KV was used in the positive scan
mode and the cone voltage was set to 30 V. The mass calibration
was performed using a solution of sodium iodide in isopropanol/
water (50:50) from m/z: 150 to 1000 a.m.u. Sample solutions (ca.
1ꢂ10À4 m) were infused via syringe pump directly connected to
the interface at a flow of 10 mLminÀ1. A 1 mgmLÀ1 solution of 3,5-
diiodo-l-tyrosine was used as the lock mass. Hydrogen analysis
was carried out by gas chromatography (GS-MOL 15 meters,
column ID 0.55 mm, TCD from J&W Scientific).
3
3
d=5.45 (d, J=6.0 Hz, 2H, CHp-cym), 5.08 (d, J=6.0 Hz, 2H, CHp-cym),
4.53 (m, 2H, CH2,n-Bu), 4.19 (m, 2H, CH2,n-Bu), 2.95 (m, 1H, CHiPr, p-cym),
2.02 (s, 3H, CH3,p-cym), 2.02 (m, 2H, CH2,n-Bu) 1,74 (m, 2H, CH2,n-Bu),
3
1.41 (m, 4H, CH2,n-Bu), 1.29 (d, J=6.9 Hz, 6H, CH3,iPr p-cym), 0.96 ppm
3
(t, J=7.3 Hz, 6H, CH3,n-Bu); 13C {1H} NMR (75 MHz, CDCl3): d=175.2
(CNHCÀRu), 117.8 (CHimid), 108.4 (Cp-cym), 98.9 (Cp-cym), 86.4 (Cp-cym), 82.8
(Cp-cym), 51.4 (CH2,n-Bu), 33.2 (CH2,n-Bu), 30.6 (CHiPr, p-cym), 22.5 (CH3,iPr
p-
cym), 20.0 (CH2,n-Bu), 18.7 (CH3,p-cym), 13.7 ppm (CH3,n-Bu); electrospray
MS (cone 20 V; m/z, fragment): 519.1 [MÀCl]+; HRMS ESI-TOF-MS
(positive mode): [MÀCl]+ monoisotopic peak: 519.0671; calcd:
519.0672, er: 0.2 ppm.
Synthesis of 6
Complex 5 (10 mg, 0.0248 mmol) and sodium tetrafluoroborate
(26 mg, 0.23 mmol) were suspended in 3 mL of dichloromethane
and 2 mL of methanol. The mixture was stirred for 5 min. Then,
0.024 mmol of 4-methylbenzylamine was added and the solution
was stirred for 1 h. After solvent removal, complex 6 was obtained
as a pale-yellow solid. 1H NMR (500 MHz, CDCl3): d=7.35 (d, J=
7.9 Hz, 2H, CHamine), 7.17–6.99 (m, 4H, CHamine; CHimid), 5.81 (d, J=
6.0 Hz, 1H, CHp-cym), 5.76 (d, J=6.0 Hz, 1H, CHp-cym), 5.68 (d, J=
6.0 Hz, 1H, CHp-cym), 5.45 (d, J=6.0 Hz, 1H, CHp-cym), 4.09 (m, 2H,
Crystal structure determination
Single crystals of complex 6 were mounted on a polymer tip in
a random orientation. Data collection was performed on a SuperNo-
va dual source equipped with a CCD Atlas detector diffractometer.
The structure was solved using Olex2[64] with the SIR2004[65] struc-
ture solution program using direct methods and refined with the
ShelXL[66] refinement package using least squares minimization.
Crystal data: orthorhombic; space group: Pna21 (no. 33); a=
13.5393(8), b=16.4138(11), c=11.4752(8) ꢃ; V=2550.1(3) ꢃ3; Z=4;
T=200.00(14) K; m(MoKa)=0.765 mmÀ1; 1calcd =1.497 gcmÀ3; 13725
CH2), 3.99 (s, 3H, NÀCH3), 3.75 (s, 3H, NÀCH3), 2.84 (m, 1H, CHiPr,
p-
cym), 2.27 (s, 3H, CH3,amine), 2.06 (s, 3H, CH3,p-cym), 1.29 (d, J=7.0 Hz,
3H, CH3,iPr p-cym), 1.09 ppm (d, J=6.8 Hz, 3H, CH3,iPr p-cym); 13C NMR
(126 MHz, CDCl3): d=172.56 (CNHC-Ru), 138.03, 136.62, 129.78, 129.11
(Camine), 125.07, 124.29 (CHimid), 112.18, 99.96 (Cq,p-cym), 85.42, 84.18,
83.39, 82.81 (CHp-cym), 53.62 (Ph-CH2-NH2), 39.66, 38.39 (NÀCH3),
31.07 (CHiPr, p-cym), 23.98, 21.29 (CH3), 20.89, 18.43 ppm (CH3,iPr p-cym).
reflections measured (5.8048ꢀ2Vꢀ51.988), 4498 unique (Rint
=
0.0679, Rsigma =0.0628), which were used in all calculations. The
final R1 was 0.0448 (I>2s(I)) and wR2 was 0.1140 (all data).
CCDC 1509771 contains the supplementary crystallographic data
for this paper. These data are provided free of charge by The Cam-
bridge Crystallographic Data Centre.
Synthesis of 2b
Catalytic dehydrogenation reactions
Silver oxide (126 mg, 0.54 mmol) was added to a solution of 4,5-di-
methyl-1,3-bis(butyl)imidazolium
chloride
(1b)
(178 mg,
Catalytic runs were performed in a round-bottomed flask by using
one equivalent of amine, 2 mol% of catalyst, and 3 mL of toluene,
and heating for a suitable period of time at 1108C. The yields and
conversions were determined by GC analysis using anisole as the
internal standard.
0.73 mmol) in CH2Cl2. The mixture was stirred at room temperature
overnight and filtered through Celite. Then, [Ru(p-cymene)Cl2]2
(200 mg, 0.36 mmol) was added to the solution and it was stirred
at room temperature overnight. After solvent removal, the crude
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All glassware was carefully cleaned with Milli-Q water and rinsed
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50% conversion), a 1.5 mL sample of the generated gas was col-
lected and the hydrogen content was qualitatively analyzed by gas
chromatography (GS-MOL 15 meters, column ID 0.55 mm, TCD
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Acknowledgements
The authors thank the financial support from MINECO
(CTQ2015-69153-C2-2-R), Generalitat Valenciana (AICO/2015/
039), and the Universitat Jaume I (P1.1B2015-09). The authors
are very grateful to the “Serveis Centrals d’Instrumentaciꢄ Cien-
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Keywords: dehydrogenation · hydrogen production · nitrile
synthesis · ruthenium · supported catalysis
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Received: July 19, 2016
Published online on && &&, 0000
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FULL PAPER
&
Supported Catalysis
D. Ventura-Espinosa, A. Marzꢀ-Beltrꢀn,
J. A. Mata*
&& – &&
Catalytic Hydrogen Production by
Ruthenium Complexes from the
Conversion of Primary Amines to
Nitriles: Potential Application as
a Liquid Organic Hydrogen Carrier
A system of support: The potential ap-
plication of the primary amine/nitrile
pair as a liquid organic hydrogen carrier
(LOHC) has been evaluated. Molecular
ruthenium complexes, [(p-cym)-
bene; see scheme), are active in the de-
hydrogenation of primary amines to ni-
triles with the simultaneous formation
of two moles of molecular hydrogen.
The reaction proceeds without any ex-
ternal additive under mild conditions.
Ru(NHC)Cl2] (NHC=N-heterocyclic car-
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Chem. Eur. J. 2016, 22, 1 – 10
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ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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