Catalytic Dehydrogenative Coupling of Hydrosilanes with Alcohols for the Production of Hydrogen On-demand: Application of a Silane/Alcohol Pair as a Liquid Organic Hydrogen Carrier

Article abstract of DOI:10.1002/chem.201700243

The compound [Ru(p-cym)(Cl)₂(NHC)] is an effective catalyst for the room-temperature coupling of silanes and alcohols with the concomitant formation of molecular hydrogen. High catalyst activity is observed for a variety of substrates affording quantitative yields in minutes at room temperature and with a catalyst loading as low as 0.1 mol %. The coupling reaction is thermodynamically and, in the presence of a Ru complex, kinetically favourable and allows rapid molecular hydrogen generation on-demand at room temperature, under air, and without any additive. The pair silane/alcohol is a potential liquid organic hydrogen carrier (LOHC) for energy storage over long periods in a safe and secure way. Silanes and alcohols are non-toxic compounds and do not require special handling precautions such as high pressure or an inert atmosphere. These properties enhance the practical applications of the pair silane/alcohol as a good LOHC in the automotive industry. The variety and availability of silanes and alcohols permits a pair combination that fulfils the requirements for developing an efficient LOHC.

Full text of DOI:10.1002/chem.201700243

DOI: 10.1002/chem.201700243
Full Paper
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Hydrogen Generation |Hot Paper|
Catalytic Dehydrogenative Coupling of Hydrosilanes with
Alcohols for the Production of Hydrogen On-demand: Application
of a Silane/Alcohol Pair as a Liquid Organic Hydrogen Carrier
David Ventura-Espinosa,[a] Alba Carretero-Cerdꢀn,[a] Miguel Baya,[b] Hermenegildo Garcꢁa,*[c]
and Jose A. Mata*[a]
Abstract: The compound [Ru(p-cym)(Cl)2(NHC)] is an effec-
tive catalyst for the room-temperature coupling of silanes
and alcohols with the concomitant formation of molecular
hydrogen. High catalyst activity is observed for a variety of
substrates affording quantitative yields in minutes at room
temperature and with a catalyst loading as low as 0.1 mol%.
The coupling reaction is thermodynamically and, in the pres-
ence of a Ru complex, kinetically favourable and allows
rapid molecular hydrogen generation on-demand at room
temperature, under air, and without any additive. The pair
silane/alcohol is a potential liquid organic hydrogen carrier
(LOHC) for energy storage over long periods in a safe and
secure way. Silanes and alcohols are non-toxic compounds
and do not require special handling precautions such as
high pressure or an inert atmosphere. These properties en-
hance the practical applications of the pair silane/alcohol as
a good LOHC in the automotive industry. The variety and
availability of silanes and alcohols permits a pair combina-
tion that fulfils the requirements for developing an efficient
LOHC.
Introduction
on-demand as fuel when combined with O2 (even from air).
The system has a high energy density and the only generated
by-product is water.[8–10] The energy obtained from hydrogen
by means of a combustion motor or a fuel cell could easily be
adapted to portable energy systems.
Energy production based on carbon fossil fuels has direct neg-
ative implications in the climate change due to the formation
of CO2.[1] To tackle this problem, wind and solar based energies
are emerging as potential alternatives for energy produc-
tion.[2,3] However, one of their major limitations is the intermit-
tent character, which in addition relies on meteorological fac-
tors. Herein, the success of renewable energies is ligated to
the efficient development of energy storage systems that
would allow the production of energy on-demand.[4–6] In this
regard, the energy storage in the form of chemical bonds is
very promising.[7] In particular, the “hydrogen economy” is
based on the use of H2 as energy vector. This gas can be used
In an ideal situation, hydrogen should be obtained from the
electrolysis of water using wind or solar energy, safely stored
and transported and used on-demand. However, the low boil-
ing point, its gaseous nature and the reactivity represents cur-
rently a strong limitation to its development.[11,12] Hydrogen
storage in the form of liquid organic hydrogen carriers (LOHC)
is very promising.[13–15] In the present work, we analyse the cat-
alytic dehydrogenative coupling of hydrosilanes with alcohols
as a potential system for hydrogen storage (Figure 1).
A potential LOHC requires a fast dehydrogenation reaction
to control the production of molecular hydrogen on-
demand.[16–19] The dehydrogenation coupling of hydrosilanes
and alcohols is a catalytically controlled process.[20–23] As is
shown herein, the ruthenium complex [Ru(p-cym)(Cl)2(NHC)] is
[a] D. Ventura-Espinosa, A. Carretero-Cerdꢀn, 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
a
very efficient catalyst, controls reaction kinetics, and
[b] Dr. M. Baya
Instituto de Sꢂntesis Quꢂmica y Catꢀlisis Homogꢃnea (ISQCH)
Departamento de Quꢂmica Inorgꢀnica
CSIC-Universidad de Zaragoza
C. Pedro Cerbuna 12, 50009 Zaragoza (Spain)
[c] Prof. H. Garcꢂa
Instituto de Tecnologꢂa Quꢂmica (ITQ)
Universidad Politꢃcnica de Valencia
Avda. Los Naranjos s/n, 46022, Valencia (Spain)
E-mail: hgarcia@qim.upv.es
Supporting information and the ORCID identification number(s) for the au-
thor(s) of this article can be found under https://doi.org/10.1002/
chem.201700243.
Figure 1. Implementation of silane/alcohol as a liquid organic hydrogen car-
rier catalyzed by ruthenium.
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produces hydrogen at high rates. A major drawback found in
most LOHCs is the elevated temperatures required in the dehy-
drogenation process. The coupling reaction of hydrosilanes
and alcohols is carried out at low temperatures and proceeds
even at 08C. The different types of hydrosilanes and alcohols
available make the system a versatile LOHC. The system makes
use of a combination of a hydrosilane/alcohol pairs with suita-
ble melting and boiling points. The efficient storage of hydro-
gen in the form of LOHC is a breakthrough technology for the
next years.[24–26] Different types of organic substances have
been considered as potential LOHCs (Figure 2). Cycloalkanes
alysts developed for the rapid hydrogen release.[41–43] Dehydro-
genation of formic acid or ammonia–borane are both thermo-
dynamically driven processes, which means that the reactions
can progress under mild conditions using appropriate cata-
lysts. The main inconvenience of ammonia–borane as an LOHC
is the regeneration process.[44–46] The hydrolysis of ammonia–
borane forms stable borates difficult to recover.
The potential application of hydrosilanes as LOHC is based
on controlling the dehydrogenation process allowing the pro-
duction of molecular hydrogen on-demand. The formation of
molecular hydrogen from the coupling of hydrosilanes and al-
cohols is a spontaneous thermodynamic process. This is very
convenient for an effective control of the reaction conditions
including temperature and rate of hydrogen production.[47] The
reverse process, that is, the regeneration of the silyl ether, is
a non-spontaneous reaction. These reactions normally require
severe conditions, such as high temperatures and/or pressures,
producing more waste and contaminants. From an environ-
mental point, reactions that produce hazardous substances
should be avoided or if not, should be carried out in industrial
facilities where contaminants and by-products are controlled.
The production of waste can be trapped, treated and reused
making the global process sustainable. The situation is very dif-
ferent when many individuals produce the same amount of
waste without control. In essence, the global process implies
obtaining energy on-demand from renewable sources using
a sustainable energy vector.
In this work, we have evaluated the coupling reaction of
a silane with an alcohol as an efficient LOHC. The reaction can
release several moles of hydrogen depending on the hydrosi-
lane. The reaction is spontaneous and requires a catalyst to
proceed, which controls the overall process. The enthalpy of
the reaction is favoured by the formation of strong SiÀO
bonds and the entropy is favoured by the release of hydrogen
gas. This allows the use of low temperatures for hydrogen pro-
duction on-demand. The reaction is versatile in terms of the
different silanes and alcohols that could be coupled, just avoid-
ing the use of solids and gases for practical purposes. The si-
lanes and alcohols are available in bulk quantities, an impor-
tant issue to consider for developing industrial applications.
The coupling product is molecular hydrogen and a silyl ether
that could be regenerated to the corresponding hydrosilane or
used in the silicone industry. In this manuscript, we describe
the results on the dehydrogenative coupling of hydrosilanes
and alcohols catalysed by a ruthenium complex. The potential
application of the silane/alcohol pair as an LOHC is analysed
underlying the advantages and disadvantages from a chemical
and technological point of view.
Figure 2. Organic compounds used as LOHC. Hydrogen storage capacity
(HSC), dehydrogenation reaction and effective hydrogen storage capacity
(
effHSC).
are good candidates as LOHC due to the high hydrogen ca-
pacity.[17,27,28] The main problem in using cycloalkanes is the
high temperature needed for hydrogen release. N-heterocycles
require lower temperatures without decreasing the storage ca-
pacity.[29,30] The dehydrogenation of cycloalkanes and N-
heterocycles is an endothermic process and requires high tem-
peratures and/or efficient catalysts. In general, molecular hy-
drogen has to be removed from the reaction media for quanti- Results and Discussion
tative conversions. Formic acid is a good LOHC because it is
abundant and easy to obtain.[31–34] The problem in this howev-
Catalytic studies
er, is the production of one mole of CO2 for each mole of mo-
lecular hydrogen.[35–37] Ammonia–borane is considered one of
the most suitable LOHCs.[38–40] The hydrogen storage capacity
is high and even more important is the number of efficient cat-
The catalytic dehydrogenative coupling of silanes and alcohols
was optimized using dimethylphenyl silane as a model sub-
strate. In a typical experiment, the molecular ruthenium com-
plex was dissolved in the appropriate alcohol. The alcohol is
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used as both reagent and solvent for practical purposes
(Table 1). The reaction bath is set at 308C and the hydrosilane
is added with a syringe. Immediately, the reaction starts gener-
ating hydrogen, the amount of which was quantified using an
Table 2. Reaction scope.
Entry
[Ru] [mol%]
Silane
Alcohol
Yield [%][a]
H2 [mL][b]
1
2
3
4
5
6
7
8
1
1
1
1
Ph3SiH
Ph3SiH
Ph3SiH
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
Ph2SiH2
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhSiH3
PhMe2SiH
MeOH
EtOH
nBuOH
EtOH
nBuOH
EtOH
EtOH
100
78
44
100 (99)
100 (94)
55
22.0
16.3
8.7
Table 1. Optimization of reaction conditions in the dehydrogenative cou-
pling of silanes and alcohols.
44.1
43.8
24.6
31.0
36.6
44.5
37.0
43.9
44.2
23.2
15.2
39.4
10.1
31.2
33.5
30.9
28.2
22.2
1
0.1
0.15
0.2
0.3
0.15
0.2
0.5
0.3
0.2
0.3
0.05
0.1
0.5
0.1
0.1
0.5
70
84
EtOH
EtOH
9
100
87
100
100 (95)
54
32
90
34
93
10
11
12
13
14
15
16c
17c
18c
19c
20c
21
MeOH
MeOH
nBuOH
nBuOH
nBuOH
nPrOH
MeOH
MeOH
MeOH
nPrOH
nBuOH
PhCH2OH
Entry
[Ru] [mol%]
T [8C]
ROH
Yield [%][a]
H2 [mL][b]
1
2
3
4
5
6
7
8
–
1
1
1
50
30
30
30
30
30
30
30
30
30
30
30
30
0
MeOH
MeOH
EtOH
nPrOH
nBuOH
MeOH
EtOH
nPrOH
nBuOH
MeOH
EtOH
nPrOH
nBuOH
MeOH
0
100
0
22.2
22.0
21.7
22.3
22.1
22.1
21.8
21.2
22.0
21.1
10.6
2.7
100 (93)
100 (90)
100 (92)
100
100 (94)
100 (86)
100 (89)
100
1
100
92
84
0.5
0.5
0.5
0.5
0.1
0.1
0.1
0.1
0.5
100
9
10
11
12
13
14
Reaction conditions: Silane (1.0 mmol), ruthenium complex, 1 mL of ROH
for 10 min at 308C. [a] Yields of silyl ether determined by GC analysis
using anisole as the internal standard. Isolated yields in parenthesis deter-
mined by 1H NMR spectroscopic analysis using 1,3,5-trimethoxybenzene
as external standard. [b] Volume of hydrogen gas collected using an in-
verted burette. [c] PhSiH3 (0.5 mmol).
94 (93)
45 (37)
10
97
20.4
Reaction conditions: PhMe2SiH (1.0 mmol), ruthenium complex, 1 mL of
ROH for 10 min. [a] Yields of silyl ether determined by GC analysis using
anisole as the internal standard. Isolated yields in parenthesis determined
by 1H NMR spectroscopic analysis using 1,3,5-trimethoxy benzene as ex-
ternal standard. [b] Volume of hydrogen gas collected using an inverted
burette.
tions and are isolated by evaporation of the alcohol. The reac-
tions are also efficient at low catalyst loadings in good yields.
The low catalyst loadings are especially important if one con-
siders that the reactions are carried out without special precau-
tions, employing regular solvents and reagents used as re-
ceived from commercial suppliers.
inverted burette. The [Ru(p-cym)(Cl)2(NHC)] complex is air-
stable and the optimization of the reaction was carried out
under aerobic conditions. Using a catalyst loading of 1 mol%
the reaction is complete in 30 s with MeOH, EtOH, nPrOH, and
nBuOH (Table 1, entries 2–5). We have observed that the reac-
tion is very fast for MeOH and EtOH, taking place in less than
1 min, even at a catalyst loading of 0.1 mol% (Table 1, en-
tries 10–11). The reaction is also very fast even at 08C (Table 1,
entry 14). The reaction profile at different temperatures in the
range of À25 to 308C shows that quantitative yields are ob-
tained in short times (vide infra). Fast catalytic dehydrogena-
tion reactions that operate under air and require low tempera-
tures are very promising candidates for efficient LOHCs. The
most reactive alcohol is MeOH and even at low catalyst load-
ings affords quantitative H2 production yields. The product of
the reaction is the corresponding silyl ether isolated and ana-
Recycling properties and catalyst stability
An important issue to consider for the silane/alcohol pairs as
a potential LOHC is that the catalyst should be recovered and
reused at the end of the reaction. For that purpose, we evalu-
ated the catalytic activity of a ruthenium complex immobilized
on the surface of graphene by p–p interactions using a pyrenyl
tag attached to the NHC ligand (see structure in Table 1 and
Figure 3).[48–50] This supported catalyst has recently been devel-
oped by us and is very efficient in the dehydrogenation of al-
cohols and amines.[51,52] The initial catalytic tests show that
properties of the supported ruthenium catalyst in terms of sta-
bility and recyclability are very promising. The catalytic activity
of the supported ruthenium catalyst in the dehydrogenative
coupling of silanes with alcohols was tested and evaluated
using the general conditions previously described (Table 3).
The supported ruthenium catalyst is active for the coupling of
a variety of silanes and alcohols. An initial experiment using
only the reduced graphene oxide reveals that the presence of
the ruthenium is necessary for the catalytic reaction. In all ex-
periments, quantitative yields were obtained using a catalyst
1
lysed by H NMR spectroscopy by simple evaporation of the al-
cohol. This is a first indication that the system has potential for
recovery and recycling.
The [Ru(p-cym)(Cl)2(NHC)] complex is active in the coupling
reaction of a variety of silanes and alcohols (Table 2). Primary,
secondary and tertiary silanes are completely dehydrogenated
producing three, two, and one moles of hydrogen, respective-
ly. The silyl ether products are stable under the reaction condi-
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Figure 4. HRTEM images before (left) and after (right) ten catalytic cycles.
The morphology of the reduced graphene oxide is preserved after the recy-
cling experiments.
Figure 3. Recycling experiment using dimethyl phenyl silane as model sub-
strate. Reaction conditions: Ru-rGO supported catalyst 0.5 mol%. In each
run Ph(Me)2SiH (1.0 mmol), 3 mL of MeOH at 308C for 10 min. Yields deter-
mined by GC using anisole as the standard.
analysis confirms the observations obtained by microscopy
(Supporting Information Figures S7–S9). The characteristic
Ru3d peak at 281 eV is observed in the molecular complex Ru-
rGO and in the Ru-rGO material after the ten catalytic runs, in-
dicating that the chemical environment of ruthenium is pre-
served. Similar results are observed for nitrogen and chlorine.
Table 3. Dehydrogenative coupling of silanes and alcohols using the
ruthenium supported catalyst Ru-rGO.
Entry
Ru-rGO [mol%]
Silane
Alcohol
Yield [%][a]
Hydrogen storage capacity and regeneration
1
2
3
4
5
6
rGO
0.05
0.05
0.05
0.05
0.05
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhMe2SiH
PhSiH3
MeOH
MeOH
EtOH
nBuOH
MeOH
nBuOH
0
100
100 (93)
95 (90)
90
The effective hydrogen storage capacity of the silane/alcohol
pair is similar to other LOHCs (Figure 2).[24] The maximum stor-
age capacity is 5.0 wt% for the coupling of SiH4 with MeOH, al-
though not a useful system for practical applications due to
the gaseous nature of silane. The structural versatility of hydro-
silanes can increase the hydrogen capacity considerably. For in-
stance, the efficient hydrogen storage capacity for the two
liquid system Ph3SiH/MeOH is only 0.7 wt%. On the contrast,
the use of a primary silane generates three moles of H2 and
the effective hydrogen capacity for the reaction of PhSiH3/
3MeOH is 3.0 wt%. The effective hydrogen capacity of silanes
is increased when using disilanes or methyldisilanes with the
generation of six moles of H2 and still maintaining the liquid
nature of reagents.[53] Nevertheless, the use of the system
PhSiH3/3MeOH is a good starting point for an initial evaluation
of the potential application of the pair silane/alcohol as a prac-
tical LOHC.[54] In order to establish the stability of the rutheni-
um catalyst for real applications, a 10 g scale experiment was
performed. The results show that all dimethylphenyl silane is
converted in less than 15 min to the corresponding silyl ether
in quantitative yield using a catalyst loading of 0.1 mol% at
308C. The amount of silyl ether isolated by solvent evaporation
was 12 g that corresponds to a 98% yield. The Ru complex is
stable and exhibits high catalytic activity along with scalability.
In view of these results, the dehydrogenative silane/alcohol
coupling can be considered suitable for real applications.
A bottle-neck issue in the hydrogen storage using LOHCs is
the regeneration process. The coupling of hydrosilanes and al-
cohols is chemoselective producing the silyl ether as the only
reaction product. The formation of a single product facilitates
the regeneration step. The dehydrogenation of other hydrogen
carriers produce mixtures of products or even polymers that
cause difficulties in the regeneration process. The regeneration
of hydrosilanes from silyl ethers is performed by reduction
using lithium aluminum tetrahydride (LiAlH4) or other reducing
agents.[55–60]
Ph2SiH2
100 (94)
Reaction conditions: Silane (1.0 mmol), Ru-rGO supported catalyst
0.05 mol% based on Ru, 1 mL of ROH at 308C for 10 min. [a] Isolated
yields of silyl ether in parenthesis determined by 1H NMR spectroscopic
analysis using 1,3,5-trimethoxybenzene as external standard.
loading of 0.05 mol% (Table 3). The catalytic activity is superior
in the case of the heterogeneous system when comparing the
molecular and supported ruthenium complexes. Most proba-
bly, the rGO material is stabilizing the catalytic active species.
We have observed a similar effect when using molecular cata-
lysts immobilized on the surface of graphene for different cata-
lytic reactions.[48–52]
The stability of the supported ruthenium catalysts was eval-
uated by recycling experiments using dimethyl phenyl silane
as model substrate (Figure 3). The solid catalyst was separated
by decantation after each run, washed thoroughly with MeOH
and reused. The supported ruthenium catalyst was employed
for ten runs without observing any decrease in activity.
The results in terms of stability and recyclability are excellent
considering the low catalyst amount and that the reactions are
carried out without special precautions. Deactivation processes
are limited or avoided, in part, due to the low temperatures
used to perform the reactions. The mild reaction conditions
used do not affect the properties of the support. In order to
evaluate any change in the reduced graphene oxide, the sup-
ported catalyst was analysed before and after the recycling
studies by HRTEM microscopy and XPS. The results obtained
by HRTEM (Figure 4) show that there is no change in morphol-
ogy. The single-layer nature of the sheets is preserved and the
EDS elemental analysis shows the homogeneous distribution
of ruthenium without the formation of nanoparticles. The XPS
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Mechanistic considerations
The proposed mechanism for the dehydrogenative coupling of
alcohols and hydrosilanes catalysed by ruthenium–arene com-
plexes is based on the ionic mechanism first proposed by
Crabtree[61] in the field of s-bond coordination to metal cen-
tres.[62–65] Initial considerations, computational and experimen-
tal details have been included in the Supporting Information.
The species involved are depicted in the catalytic cycle in Fig-
ure S10 (Supporting Information) and in the energy profile in
Figure 6. The ruthenium complexes [Ru(p-cym)(Cl)2(NHC)] dis-
sociate a ClÀ anion generating the [Ru(p-cym)Cl(NHC)]+ spe-
cies, which is a highly electrophilic ruthenium cation. The equi-
librium between these two species has previously been ob-
served and depends on the solvent, the pH and the presence
1
of nucleophiles.[51,52,66] The H NMR spectrum of the ruthenium
complex [Ru(p-cym)(Cl)2(NHC)] in CDCl3 displays one set of
Figure 5. Reaction profile at different temperatures. Conditions: 1.0 mmol of
dimethyl phenyl silane, 1 mL of MeOH and 3 mg (0.5 mol%) of [Ru] catalyst.
Yield corresponds to the amount of hydrogen collected using an inverted
burette (Supporting Information Figure S1).
sharp signals, in contrast, the presence of different species was
1
observed when the H NMR spectrum was carried in CD3OD.
The process is reversible since the set of sharp signals was ob-
served after the removal of CD3OD and the addition of CDCl3.
This observation is an indication that MeOH promotes the
equilibrium between neutral and cationic ruthenium species
generating the electrophilic ruthenium active catalyst. In order
to determine the catalyst resting state we performed a catalytic
experiment using a 10 mol% catalyst loading. The recovered
species is the [Ru(p-cym)(Cl)2(NHC)] suggesting that the cata-
lyst resting state is out of the catalytic cycle (Supporting Infor-
mation Scheme S5 and Figure S13). These observations are in
agreement with the DFT calculations, which indicate that nu-
cleophilic attack of the methanol is the rate-determining step
(Figure 6). Further experimental evidence supporting the pro-
posed mechanism is the dependence of dehydrogenative cou-
pling on the nucleophilicity of alcohols (Table 1, entries 10–13),
which suggests that alcohol attack to the coordinated silane is
involved in the rate-determining step. The reactions between
hydrosilanes and alcohols are faster when increasing the nucle-
ophilic character of the alcohol. Similar catalytic results were
observed when the metal complex [CpRu(PR3)(MeCN)2]+ was
used as catalyst.[67] Another important consideration related to
the proposed mechanism is the inherent reactivity between
ruthenium–arene complexes and hydrosilanes described in the
literature.[68,69] Ruthenium–arene complexes react with hydrosi-
lanes to give ruthenium hydrides and silyl chlorides using di-
chloromethane as solvent. When using alcohols, the formation
of ruthenium hydrides is not observed due to silylation. Silyl
chlorides react with alcohols to yield silyl ethers and hydrogen
chloride that immediately protonates the ruthenium hydride,
which subsequently releases hydrogen (Supporting Informa-
tion Figure S12). Despite the catalytic dehydrogenative cou-
pling of hydrosilanes with alcohols is a thermodynamically fa-
vourable process it requires a catalyst to proceed. The activa-
tion energy is very low in the presence of the ruthenium cata-
lyst. The reaction profile at different temperatures in the range
of À25 to 308C shows that quantitative yields are obtained in
short times (40 s) and that the initial rates are very fast indicat-
ing that the activation energy is very low (Figure 5). These re-
Figure 6. Abbreviated free energy profile for the dehydrogenative coupling
of dimethyl phenyl silane and methanol catalysed by [Ru(h6-C6H6)Cl(NHC)]+
in kcalmolÀ1 (full profile in Supporting Information Figure S11).
sults are consistent with the theoretical calculations where the
free energy profile shows a maximum energy transition state
of only 8.0 kcalmolÀ1 (Figure 6).
Theoretical calculations
DFT calculations provide a mechanistic overview of the catalyt-
ic reaction coupling based on the electrophilic activation of
the silicon–hydrogen bond.[70–72] The behaviour of the rutheni-
um complex towards dimethyl phenyl silane suggests that the
catalytic
process
occurs
via
a
bridging
hydride
[Ru(h6-C6H6)(NHC)(Cl)(h1-H-SiMe2Ph)]+ intermediate (I, Figure 6).
Analysis of the s bond coordination in the ruthenium–silane
complex reveals that: i) h1 is the most favoured geometry, ii) no
minima is observed for an h2-H-SiR3 coordination and iii) the
oxidative addition is a highly disfavoured process. The origin
of the h1-H-SiR3 coordination is based on steric factors (Sec-
tion S7.2) as has been observed for bulky iridium complexes.[73]
The Ru-H-SiR3 interaction increases the electrophilic character
at the silicon atom, which becomes susceptible of nucleophilic
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attack by the methanol substrate (III, Figure 6). Computational
analysis suggests a back-attack of the alcohol to the coordinat-
ed SiÀH as in the case of a CpFe complex.[74] The alcohol
attack produces the neutral ruthenium hydride complex and
the cation R3Si(H)OMe+ (IV, Figure 6). Subsequently, the cation
protonates the ruthenium hydride, forming a dihydrogen com-
plex that finally releases hydrogen gas (VI, Figure 6). As a sum-
mary, the ruthenium catalyst plays two main roles: (i) triggering
the alcohol nucleophilic attack to the silane, and (ii) enabling
the hydrogen evolution process from a metal–dihydrogen in-
termediate (Supporting Information Figure S10).
Keywords: dehydrogenative
hydrosilanes · hydrogen generation · ruthenium
coupling
·
hydrogen
·
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Conflict of interest
The authors declare no conflict of interest.
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Manuscript received: January 17, 2017
Version of record online: && &&, 0000
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Chem. Eur. J. 2017, 23, 1 – 8
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FULL PAPER
&
Hydrogen Generation
D. Ventura-Espinosa, A. Carretero-Cerdꢀn,
M. Baya, H. Garcꢂa,* J. A. Mata*
&& – &&
Catalytic Dehydrogenative Coupling
of Hydrosilanes with Alcohols for the
Production of Hydrogen On-demand:
Application of a Silane/Alcohol Pair as
a Liquid Organic Hydrogen Carrier
On demand! An effective supported
ruthenium catalyst on the surface of
rGO controls the hydrogen production
from hydrosilanes. The hydrogen stor-
age properties show the potential appli-
cation of the silane/alcohol pair as an
efficient liquid organic hydrogen carrier.
&
&
Chem. Eur. J. 2017, 23, 1 – 8
www.chemeurj.org
8
ꢂ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!