High Production of Hydrogen on Demand from Silanes Catalyzed by Iridium Complexes as a Versatile Hydrogen Storage System

Article abstract of DOI:10.1021/acscatal.7b04479

The catalytic dehydrogenative coupling of silanes and alcohols represents a convenient process to produce hydrogen on demand. The catalyst, an iridium complex of the formula [IrCp?(Cl)₂(NHC)] containing an N-heterocyclic carbene (NHC) ligand functionalized with a pyrene tag, catalyzes efficiently the reaction at room temperature producing H₂ quantitatively within a few minutes. As a result, the dehydrogenative coupling of 1,4-disilabutane and methanol enables an effective hydrogen storage capacity of 4.3 wt % that is as high as the hydrogen contained in the dehydrogenation of formic acid, positioning the silane/alcohol pair as a potential liquid organic hydrogen carrier for energy storage. In addition, the heterogenization of the iridium complex on graphene presents a recyclable catalyst that retains its activity for at least 10 additional runs. The homogeneous distribution of catalytic active sites on the basal plane of graphene prevents diffusion problems, and the reaction kinetics are maintained after immobilization.

Full text of DOI:10.1021/acscatal.7b04479

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Letter
High production of hydrogen on demand from silanes catalyzed
by iridium complexes as a versatile hydrogen storage system.
David Ventura-Espinosa, Sara Sabater, Alba Carretero-Cerdan, Miguel Baya, and Jose Antonio Mata
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04479 • Publication Date (Web): 13 Feb 2018
Downloaded from http://pubs.acs.org on February 16, 2018
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High Production of Hydrogen on Demand from
Silanes Catalyzed by Iridium Complexes as a
Versatile Hydrogen Storage System
David VenturaꢀEspinosa,a Sara Sabater,a Alba CarreteroꢀCerdán,a Miguel Baya,b and Jose A.
Mata*a
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aInstitute of Advanced Materials (INAM), Universitat Jaume I, Avda. Sos Baynat s/n, 12006,
Castellón (Spain). Fax: (+34) 964387522; Tel.: (+34) 964387516; eꢀmail: jmata@uji.es
bInstituto de Síntesis Química y Catálisis Homogénea (ISQCH), Departamento de Química Inorgánica,
CSICꢀUniversidad de Zaragoza, C/Pedro Cerbuna 12, Eꢀ50009 Zaragoza (Spain)
Abstract
The catalytic dehydrogenative coupling of silanes and alcohols represents a convenient process to
produce hydrogen on demand. The catalyst, an iridium complex of the formula [IrCp*(Cl)2(NHC)]
containing an NHC ligand functionalized with a pyrene tag, catalyzes efficiently the reaction at room
temperature producing H2 quantitatively within a few minutes. As a result, the dehydrogenative coupling
of 1,4ꢀdisilabutane and methanol enables an effective hydrogen storage capacity of 4.3 wt% that is as
high as the hydrogen contained in the dehydrogenation of formic acid, positioning the silane/alcohol pair
as a potential liquid organic hydrogen carrier for energy storage. In addition, the heterogenization of the
iridium complex on graphene presents a recyclable catalyst that retains its activity for at least ten
additional runs. The homogeneous distribution of catalytic active sites on the basal plane of graphene
prevents diffusion problems and the reaction kinetics are maintained after immobilization.
Keywords: hydrogen production, LOHC, iridium, energy storage, supported catalysis, graphene
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Introduction
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Hydrogen is in a leading position among all possible clean alternative sources of energy that will have
an impact in the near future due to a rising global demand.1–4 The combination of hydrogen with oxygen
produces clean energy where the only byꢀproduct is water. The actual limitation of using hydrogen to
obtain clean energy lies in the development of efficient methods for hydrogen storage.5–8 Among them,
hydrogen storage in the form of organic liquids is becoming a very attractive strategy because the energy
storage density and tractability are similar to actual carbonꢀbased fuels.9–11 Liquid organic hydrogen
carriers (LOHCs) in combination with a “green hydrogen production” method, i.e., the electrolysis of
water or water splitting using eolic and/or solar energies, represent a promising technology for a future
energy supply. However, the direct use of eolic and solar energies is limited due to the intermittent
characteristics of wind and sun.12–14 An increase in these renewable energies is subjected to the
availability of efficient energy storage systems.15,16
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An LOHC is an organic substance that stores hydrogen reversibly in the form of chemical bonds.17–24
From a chemical point of view, H2 loading is a hydrogenation reaction, and the reverse process, is a
dehydrogenation reaction. The bondꢀbreaking and bondꢀformation transformations can be driven
thermally, but the activation energy required makes the process unfeasible.25 The introduction of a
catalyst reduces the temperature of both processes, which considerably decreases the required activation
energy and controls the kinetics.26–28 Herein, a major challenge is to develop efficient methods for
hydrogen release.29–32 H2 unloading must proceed at low temperatures and with a control of the
hydrogen flow. In this regard, we have recently reported ruthenium complexes of type
[Ru(arene)(Cl)2(NHC)] that are effective catalysts for the dehydrogenative coupling of silanes and
alcohols under mild thermal conditions. This process has three main advantages in comparison with
traditional ones: (1) hydrogen is obtained at low temperatures, even below 0 ºC, which facilitates the
potential use of silanes for onꢀboard vehicle applications,33–35 (2) the gas is released in short reaction
times and the kineticꢀcontrol permits the production of a constant flow of hydrogen, and (3) the use of
silanes as LOHCs is a carbon free process that produces highꢀpurity hydrogen. Our previous work in
ruthenium catalysts used in the coupling of silanes with alcohols show low to moderate hydrogen
storage capacity.36 In this work, we have developed an active catalytic system based on iridium
[IrCp*(Cl)2(NHC)] that increases the hydrogen storage capacity up to 4.3 wt% of H2 (Scheme 1).
Scheme 1. Hydrogen production by the dehydrogenative coupling of 1,4ꢀdisilabutane and methanol
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Results and Discussion
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The reaction of hydrosilanes with alcohols was investigated using Cp*Ir(NHC) complexes at a
molecular level or supported on graphene (Scheme 2). The reaction of imidazolium salt 1 with
[Cp*IrCl2]2 under transmetallation conditions using Ag2O produced the orthometallated iridium
complex 2. Carbonꢀhydrogen activation processes are commonly observed in chelationꢀassisted
aromatic Cp*Ir complexes. 37–40 Nonꢀorthometallated iridium complex 3 was obtained by deprotonation
of the imidazolium salt with tBuOK and metalation at room temperature. The addition of a second
equivalent of base promotes orthometallation forming complex 2. The iridium molecular complexes
were characterized by NMR spectroscopy, ESIꢀMS spectrometry and elemental analysis. The
immobilization of iridium complex 3 on graphene was carried out as previously described.41–43 This
methodology allows a controlled grafting of molecular complexes on the surface of reduced graphene
oxide (rGO) by πꢀstaking interactions. We have observed that the pyrene tag forms strong πꢀinteractions
with graphene, which prevents desorption of the molecular complex during the catalytic
experiments.44,45 Direct evidence for πꢀstacking interactions due to the pyrene tag was observed using
singleꢀcrystal Xꢀray diffraction of compound 2 (Figure 1). Suitable single crystals were obtained after
halide exchange using NaBr. The packing diagram shows a strong πꢀstacking interaction between the
pyrene groups of different molecules. The interplanar distance between the polyaromatic groups is 3.5
Å. The tendency of the pyrene tag to form πꢀstacking interactions has previously been observed for
palladium,41 ruthenium44 and gold.46 The formation of πꢀπ interactions at the molecular level leads to the
immobilization of the molecular complexes on the graphene surface. Characterization of the hybrid
material 3-rGO was performed by UV/vis, FTIR, HRTEM, and XPS, and the exact amount of complex
3 on graphene was analyzed by ICP/MS analysis (Supporting Information). The results accounted for
6.5% of iridium complex 3 on the surface of graphene. Analysis by Xꢀray photoelectron spectroscopy
(XPS) provides evidence of the molecular structure of complex 3 on the surface of graphene. A
comparative XPS analysis of complex 3 and the hybrid material 3-rGO shows the characteristic coreꢀ
level peaks of C1s and Ir4f at the same binding energy (Figure 2). XPS analysis shows that the mild
conditions used during the immobilization process preserved the intrinsic properties of both the iridium
complex and graphene.36,46
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Ir
i)Ag2O
Cl
N
ii)[Cp*IrCl2]2
KCl
Br-
N
N
2
+
N
Ir
Cl
Cl
tBuOK
Ir
Cl
Cl
N
Ir
Cl
Cl
1
N
N
N
N
N
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i)tBuOK
Ir
Cl
Cl
rGO
N
ii)[Cp*IrCl2]2
KCl
CH2Cl2
N
3-rGO
3
Scheme 2. Synthesis of the iridium molecular complexes and immobilization on graphene
Figure 1. Packing diagram of complex 2 showing an interplanar distance of 3.5 Å between the pyrenes.
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Figure 2. Comparative XPS analysis of molecular complex 3 (top) and hybrid material 3-rGO (down)
for the coreꢀlevel peaks of C1s and Ir4f.
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The coupling of silanes with alcohols is a wellꢀknown process for the protection of alcohols and the
synthesis of silyl ether derivatives. Transition metal complexes are efficient catalysts for this
transformation.47–50 We used dimethylphenylsilane as a model substrate for the optimization of reaction
conditions in the dehydrogenative coupling of the silanes. In a typical experiment, a solution of iridium
catalysts in the appropriate alcohol was added to a solution of silane in the same alcohol that was used as
the solvent and reagent at 30 ºC. Immediately, the reaction starts bubbling hydrogen that was collected
using an inverted burette or a pressure transducer. At the end of the reaction, the yield was evaluated by
isolation of the silyl ether product. The catalytic activity of the orthometallated iridium complex 2 was
low and was not further investigated. In contrast, the iridium complex 3 is an efficient catalyst for the
dehydrogenative coupling of silanes and alcohols. Using 1.0 mmol of dimethylphenylsilane, MeOH and
a catalyst loading of 0.5 mol%, the reaction was completed in less than 2 min at room temperature. The
reaction kinetics are slightly different when measuring the generated hydrogen with an inverted burette
or a pressure transducer due to a change in the reaction conditions (Figure 3.a). Additionally, at the end
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of the reaction, the silyl ether product was isolated by solvent removal and the yield was evaluated by 1H
NMR spectra using anisole or trimethoxybenzene as a standard. The dehydrogenative coupling of
dimethylphenylsilane works for different alcohols using catalyst 3. The reaction rates rapidly decrease
with an elongation of the alcohol chain, and for secondary alcohols, the process is so slow that
quantitative yields are not achieved in 80 min (Figure 3.b). The control of the reaction kinetics is
important for industrial applications, especially in the case of hydrogen release from an LOHC
connected to a fuel cell.51 Herein, we shown that the dynamics are controlled by the use of an
appropriate alcohol that can be adjusted to the operation dynamics of the fuel cell.
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Figure 3. a) Hydrogen evolution monitoring for the reaction of Ph(Me)2SiH with MeOH (1 mL) at 30
ºC using catalyst 3 (0.5 mol%). b) Dehydrogenative coupling of Ph(Me)2SiH with different alcohols (1
mL) using catalyst 3 (0.5 mol%). Yields obtained using a pressure transducer.
Complex 3 is an active catalyst for the coupling of a variety of silanes and alcohols (Table 1). The
structural versatility of silanes and the variety of alcohols allows the formation of mixtures with
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controlled properties such as the boiling point, density or hydrogen storage capacity. The
dehydrogenative coupling of silanes with alcohols catalyzed by complex 3 is sensitive to silane
bulkiness and the alcohol nucleophilic character. Quantitative yields are obtained in the coupling of
primary, secondary and tertiary silanes with alcohols at low temperature. In all cases, the reaction
kinetics is rapid even at 30 ºC. The reaction rates are very fast for the primary alcohols and decrease
with chain elongation (Table 1, entries 1ꢀ4). Bulky silanes such as diphenylsilane, do not reach
quantitative yields even after a 2 h reaction time (Table 1, entry 5). The hydrogen storage capacity of
silanes containing a single hydride is low. For instance, the hydrogen content of Ph(Me)2SiH/MeOH is
only 1.2 wt% of H2 (Table 1, entry 1). However, the versatility of hydrosilanes increases the hydrogen
storage capacity when using secondary or tertiary hydrosilanes. The effective hydrogen storage capacity
of phenylsilane/MeOH increases up to 3.0 wt% (Table 1, entry 9) and in the case of 1,4ꢀ
disilabutane/MeOH the hydrogen storage capacity is 4.3 wt% of H2. This is as high as the hydrogen
contained using formic acid as an LOHC.
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Table 1. Scope of the dehydrogenative coupling of silanes
and alcohols
.
Entry
Silane
ROH
MeOH
EtOH
effHSC
1.2
t (min)
3
Yield (%)a
100(98)
100(97)
100(98)
100(99)
100
1
2
3
4
3
4
1.1
30
nPrOH
nBuOH
MeOH
EtOH
1.0
40
0.96
1.4
55
20
Et3SiH
1.2
30
100
5
MeOH
1.6
120
85(82)
6
7
MeOH
EtOH
3.0
2.5
2.8
2.3
3.3
2.7
4.3
3.3
2.7
5
100
15
20
35
15
20
12
30
25
100(94)
100(93)
100(90)
100(75)
96(80)
100(96)
100(97)
100
8
MeOH
EtOH
9
10
11
12
13b
14c
MeOH
EtOH
MeOH
EtOH
nPrOH
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15c
nBuOH
2.3
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100
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Reaction conditions: silane (1.0 mmol), iridium catalyst (0.5
mol%), 30 ºC and 1 mL of ROH. [a] Yields determined by
H2 formation using an inverted burette and/or a pressure
transducer. Isolated yields in parenthesis based on the
1
9
amount of silyl ether determined by H NMR spectroscopic
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analysis using 1,3,5ꢀtrimethoxy benzene as an external
standard. [b] Iridium catalyst (1 mol%). [c] Iridium catalyst
(2 mol%).
The immobilization of molecular complex 3 on graphene leads to the formation of a hybrid material
composed of an organometallic complex on the surface of reduced graphene oxide. The catalytic
properties of 3-rGO were evaluated in the dehydrogenative coupling of silanes and alcohols. The
coupling of dimethylphenylsilane and MeOH using catalyst loading based on iridium of 0.5 mol%
produces a 100% yield in 7 min. The comparison of the catalytic activity at a molecular level (100%
yield in 3 min) and the support reveals that the catalytic activity is maintained. In most cases, the
immobilization of molecular catalysts leads to a dramatic decrease in activity due to diffusion problems
and saturation kinetics. The situation is different when using graphene as a support because all the
catalytic centers are located at the surface of the material. The location of the active catalytic species on
the surface of a 2D material allows the direct interaction with substrates and diffusion problems are
negligible. We have previously observed the same results for other molecular catalysts anchored on the
surface of graphene.45 Another interesting effect was observed when decreasing the catalyst loading. At
a molecular level, we observed that when using a catalyst loading of 0.1 mol% the reaction of
dimethylphenylsilane and MeOH practically stopped after reaching a 70% yield after 15 min. In
contrast, quantitative yields are obtained when using the hybrid material 3-rGO. This result is an
indication that the reduced graphene oxide is stabilizing the catalytic active species. Graphene is not
only acting as a mere support but also interacting with the catalytic active species. An enhancement in
the catalytic activity and stability has been observed in the internal hydroamination of alkynes using
gold catalysts immobilized on graphene.46
The recycling properties of the hybrid material 3-rGO were tested using dimethylphenylsilane as
a model substrate. The catalytic activity was evaluated using the general conditions previously described
(Table 1). After each run, the solid catalyst 3-rGO was removed from the solution by decantation
washed with methanol and then reused. The hybrid material 3-rGO was reused up to ten times without a
decrease in activity (Figure 4). The results show that the immobilization on graphene leads to a robust
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catalytic hybrid material that is recyclable. The properties of the hybrid material were analyzed by
HRTEM microscopy after the recycling experiments (Figure 5). The images show that the singleꢀlayer
morphology of graphene is preserved after the catalytic experiments. There is no formation of
nanoparticles and the EDS elemental analysis shows the presence of homogeneously distributed iridium.
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Ir
Cl
N
3-rGO
Cl
N
Me
Ph Si
Me
Me
3-rGO
+
H
MeOH
Ph Si OMe + H2
Me
HO
O
OH
OH
OH
O
Figure 4. Recycling experiment using Ph(Me)2SiH. Conditions: Catalyst loading 0.5 mol%, 1.0 mmol of
silane, 1 mL of MeOH at 30 ºC for 15 min. Yields determined by GC using anisole as the standard.
Figure 5. HRTEM images before (left) and after (right) ten catalytic cycles.
The mechanism of the dehydrogenative coupling of silanes with alcohols by homogeneous iridium
complexes is based on the interaction of the SiꢀH bond with the metal center forming the reactive metal
hydrides. The active hydrides come from the hydrosilanes via oxidative addition or the formation of
electrophilic metal/SiꢀH σ complexes. The oxidative addition of a SiꢀH bond to the iridium (III) center is
unlikely under reductive alcoholic solutions and most probably is the formation of a metalꢀσ complex.
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The formation a metal/SiꢀH σ complex requires a vacant coordination site that is not present in complex
1
3. The possible formation of vacant sites in complex 3 were evaluated using different solvents. The H
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NMR spectra of the iridium complexes of type [Cp*Ir(Cl)2(NHC)] in CDCl3 display one set of sharp
signals. In contrast, the presence of different species were observed when an NMR analysis was
conducted in CD3OD (or D2O). This process is reversible, since the removal of CD3OD and the addition
of CDCl3 provided the initial spectrum. This finding is indicative of an equilibrium between different
metal complexes promoted by MeOH (Scheme 3). A similar process was observed in the case of Ruꢀ
arene complexes.52 In this context, the iridium complexes of general formula [Cp*Ir(Cl)2(NHC)] are in
equilibrium in MeOH generating cationic species and vacant sites. This equilibrium enhances the
electrophilic character of complex 3 in alcohols which points to a mechanism based on the formation of
a SiꢀH σ complex.
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Scheme 3. Equilibrium of [Cp*Ir(Cl)2(NHC)] in a MeOH solution
The dehydrogenative coupling of silanes with alcohols is a fast reaction even at room temperature,
which suggest that the activation energy (Ea) is low. To determine the activation energy, a set of reaction
profiles were measured at different temperatures. The catalytic reaction of dimethylphenysilane with
MeOH was carried out at different temperatures in the range of 0 to 30 ºC. In all cases, quantitative
yields were obtained even at 0 ºC (Figure 6). The activation energy obtained from the Arrhenius
equation plot is 15.3 Kcal/mol. The coupling of silanes with alcohols is a convenient reaction to produce
hydrogen onꢀdemand because it is fast (low activation energy) and the process is thermodynamically
favorable (ꢀG < 0).
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Figure 6. Reaction curves at various temperatures. Conditions: 1.0 mmol of dimethylphenylsilane, 1 mL
of MeOH, and 0.5 mol% catalyst 3. Yields obtained using a pressure transducer.
Evaluation of possible pathways of catalyst deactivation was analyzed by recycling experiments and
sequential addition of fresh substrates (Section S10). In the recycling experiments the determination of
catalyst deactivation was performed under standard conditions but using a 10 mol% catalyst loading.
After the gas evolution stopped, the volatiles were removed under reduced pressure, and the residue was
1
analyzed by H NMR spectra using CDCl3. The spectra show identical signals before and after the
catalytic experiment for catalyst 3 and additional signals that correspond to the formation of silylꢀether.
Addition of fresh substrates resumes the catalytic process. In a parallel experiment, we have observed
that catalyst activity is maintained after the sequential addition of dimethylphenylsilane. These
experiments suggest that catalyst 3 is efficiently recycled.
Density functional theory (DFT) calculations were carried out to evaluate the mechanism of the
dehydrogenative coupling of dimethylphenylsilane and methanol catalyzed by [Cp*Ir(Cl)(NHC)]+. The
proposed mechanism is based on the experimental evidence and theoretical results (Figure 7). Among
the different pathways considered, the most plausible involves a transition metal/SiꢀH σ complex. DFT
analysis indicates that the first step of the reaction is an electrophilic activation of the siliconꢀhydrogen
bond.53 The electrophilic 16 valence electron [Cp*IrCl(NHC)]+ cation interacts with the hydrogen atom
of the silane forming an η1ꢀHꢀSiR3 complex (A, Figure 7). This type of electrophilic SiꢀH σ complex
has been isolated and fully characterized.54 Calculations of the SiꢀH σ bond complex reveals that the η1
coordination prevails over the η2ꢀHꢀSiR3 and the origin is based on steric factors. In fact, no energy
minimum was observed for the η2ꢀHꢀSiR3 coordination, and an oxidative addition is a highly disfavored
process (Section S9). Crabtree et al. introduced the first mechanism proposal based on a transition
metal/SiꢀH σ complex.55 The presence of vacant sites in the iridium catalysts allows the formation of an
η2ꢀHꢀSiR3 coordination.56–59
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The Irꢀη1ꢀHꢀSiR3 interaction increases the electrophilic character of silicon, which undergoes a
nucleophilic attack by the methanol (B, Figure 7). As a result, a neutral iridium hydride and the cation
R3Si(H)OMe+ are generated. In the next step, the cation R3Si(H)OMe+ protonates the iridium hydride
forming a dihydrogen complex that finally releases hydrogen gas. As a summary the iridium catalysts
facilitates the methanol nucleophilic attack to the silane and favors the release of hydrogen from a metalꢀ
dihydrogen intermediate.
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Figure 7. Mechanistic proposal for the dehydrogenative coupling catalyzed by [Ir(η5ꢀ
C5Me5)Cl(NHC)]+.
Conclusions
The dehydrogenative coupling reaction of silanes and alcohols represents a convenient process for the
production of hydrogen onꢀdemand using an iridium catalyst of the general formula [IrCp*(Cl)2(NHC)].
Quantitative yields of hydrogen are obtained in less than five min at 30 ºC by the coupling of
dimethylphenylsilane and MeOH using a catalyst loading of 0.5 mol%. Hydrogen is produced at high
rates, room temperature and without additives, which is convenient for combining hydrogen production
with fuel cells. The iridium catalyst is active for sequential additions of silane and highly efficient
suggesting that the catalyst is stable and robust. The proposed mechanism, based on the experimental
evidence and DFT calculations, reveals the formation of a metal/SiꢀH σ complex. The rateꢀdetermining
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step is the alcohol nucleophilic attack to the iridiumꢀcoordinated η1ꢀHꢀSiR3 species. The immobilization
of the iridium complex on the surface of graphene allows the preparation of a solid catalyst that is easily
isolated from the reaction mixture. The recycling properties show that the solid catalyst can be reused up
to ten times without any decrease in activity. The coupling reaction of 1,4ꢀdisilabutane with MeOH
forms six moles of molecular hydrogen. The effective hydrogen storage capacity of disilabutane/MeOH
is 4.3 wt% of H2, which is similar to the storage capacity of formic acid (4.4 wt%). The low
temperatures for hydrogen production and the moderateꢀtoꢀhigh hydrogen storage capacity, make the
pair of disilabutane/MeOH a potential liquid organic hydrogen carrier for energy storage in mobile
applications.
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Experimental Section
General procedures. Anhydrous solvents were dried using a solvent purification system. The alcohols
used for the catalytic experiments were dried over preactivated molecular sieves of 3 Å. All hydrosilanes
were obtained from commercial suppliers and used as received. Safety warning: 1,4ꢀdisilabutane is a
highly flammable liquid and vapors have been reported to spontaneously ignite on contact with air
(Gelest). We have not experienced any problems working in alcohol dilution. Imidazolium salt 1 was
obtained according to reported procedures.41 Nuclear magnetic resonance (NMR) spectra were recorded
on Bruker spectrometers operating at 300 or 400 MHz (1H NMR) and 75 or 100 MHz (13C{1H} NMR),
and referenced to SiMe4 (δ in ppm and J in Hertz). NMR spectra were recorded at room temperature
with the appropriate deuterated solvent. Highꢀresolution transmission electron microscopy (HRTEM)
and highꢀangle annular darkꢀfield HAADFꢀSTEM images of the samples were obtained using a JEMꢀ
2100 LaB6 (JEOL) transmission electron microscope coupled with an INCA Energy TEM 200 (Oxford)
energy dispersive Xꢀray spectrometer (EDX) operating at 200 kV. Samples were prepared by drying a
droplet of a MeOH dispersion on a carbonꢀcoated copper grid. Xꢀray photoelectron spectra (XPS) were
acquired on a Kratos AXIS ultra DLD spectrometer with a monochromatic Al Kα Xꢀray source (1486.6
eV) using a pass energy of 20 eV. To provide a precise energy calibration, the XPS binding energies
were referenced to the C1s peak at 284.6 eV. Catalytic experiments were monitored using a pressure
transducer “Man on the moon” x102 series kit (www.manonthemoon.com) microreactor with a total
volume of 38.2 mL. Hydrogen identification was carried out by injecting the generated gas of a typical
reaction in in a quadrupole mass spectrometer (Omnistar GSD 320 03 from Pfeiffer Vacuum).
General procedure for the catalytic dehydrogenative coupling of silanes and alcohols. The catalytic
experiments were performed in a 25 mL round bottom flask, using 0.5 to 1 mmol of silane, 1 mL of
alcohol, and a catalyst (0.05 – 1.00 mol %). The flask was heated at 30 ºC under an initial aerobic
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atmosphere. The system was connected to a pressure transducer for monitoring the reaction or was
connected to an inverted waterꢀfilled burette that was used to collect the released gas. The yields and
conversions were determined by GC analysis using anisole as the internal standard. The isolated yields
1
were determined by solvent evaporation and analysis by H NMR spectroscopy using 1,3,5ꢀ
9
trimethoxybenzene as the external standard.
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Synthesis of 2. Under the exclusion of light, imidazolium salt 1 (94.7 mg, 0.25 mmol) and Ag2O (58.5
mg, 0.25 mmol) were mixed in 10 mL of acetonitrile in a roundꢀbottom flask, and the suspension was
stirred for 5 h at room temperature. [IrCp*Cl2]2 (100 mg, 0.125 mmol) and KCl (243 mg, 3.25 mmol)
were added, and the reaction mixture was stirred at room temperature for 15 h. Acetonitrile was
removed under reduced pressure, and the mixture was suspended in dichloromethane. The insoluble
salts were removed by filtration, and the crude product was purified by flash chromatography, producing
2 as a crystalline orange powder. Yield: 80 mg (49%).1H NMR (400 MHz, CDCl3) δ 8.67 (s, 1H, CHpyr),
3
3
8.20 (d, JH,H = 9.4 Hz, 1H, CHpyr), 7.98 (m, 6H, CHpyr), 7.07 (d, JH,H = 2.0 Hz, 1H, CHimid), 6.88 (d,
3
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3JH,H = 2.0 Hz, 1H, CHimid), 5.99 (d, JH,H = 14.5 Hz, 1H, CHH), 5.18 (d, JH,H = 14.5 Hz, 1H, CHH),
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3,94 (s, 3H, NꢀCH3), 1.71 (s, 15H, CH3, Cp*). C NMR (101 MHz, CDCl3): δ 156.3 (CcarbeneꢀIr), [142.3,
141.2, 132.6, 130.9, 130.3, 129.8, 127.8, 126.1, 125.5, 125.2, 125.1, 124.3, 124.0, 123.5, 121.9, 121.6,
121.2, 120.6] (Cpyr, CHimid), 90.5 (Cp*), 51.8 (CH2), 37.8 (NꢀCH3), 9.5 (CH3ꢀCp*). HRMS ESIꢀTOFꢀ
MS (positive mode): [M ꢀ K]+ monoisotopic peak 567.0309; calc. 567.0305, εr: 0.7 ppm.
Synthesis of 3. In a Schlenk, a mixture of imidazolium salt 1 (95 mg, 0.250 mmol) and potassium tertꢀ
butoxide (33 mg, 0.280 mmol) was cooled to 0 ºC, in an ice bath. Freshly distilled tetrahydrofuran (5
mL) was added, and the mixture was stirred for 10 min and allowed to reach room temperature. Then,
[Cp*IrCl2]2 (100 mg, 0.125 mmol) and KCl (243 mg, 3.25 mmol) were added and the reaction mixture
was stirred for 4 h at room temperature. The resulting suspension was filtered through celite and the
solvent was removed under reduced pressure. The crude solid was purified by column chromatography.
An elution with a mixture of dichloromethane and acetone (9:1) produced the separation of a yellow
band containing the desired product. Precipitation from dichloromethane/hexane produced an
3
analytically pure yellow solid. Yield: 111 mg, 64%.1H NMR (300 MHz, CDCl3): δ 8.37 (d, JH,H = 9.2
Hz, 1H, CHpyr), 8.25 – 8.02 (m, 7H, CHpyr), 7.84 (d, 3JH,H = 7.8 Hz, 1H, CHpyr), 6.90 (d, 3JH,H = 2.1 Hz,
1H, CHimid), 6.66, 6.28 (AB, 3JAB = 15.1 Hz, 2H, CH2), 6.65 (d, 3JH,H = 2.1 Hz, 1H, CHimid), 4.07 (s, 3H,
NCH3), 1.62 (s, 15H, C(CH3)5).13C{1H} NMR (75 MHz, CDCl3): δ 157.1 (CcarbeneꢀIr), [131.2, 131.1,
130.7, 129.8, 129.0, 128.6, 127.7, 127.2, 126.2, 125.9, 125.6, 125.5, 124.7, 123.1, 122.5, 122.3] (Cpyr,
CHimid), 88.9 (C(CH3)5), 52.1 (CH2), 38.9 (NCH3), 9.2 (C(CH3)5). Anal. Calcd. for C31H31N2IrCl2
(694.71 g/mol): C, 53.59; H, 4.49; N, 4.03. Found: C, 53.67; H, 4.27; N, 4.37. Electrospray MS. (Cone
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20 V) (m/z, fragment): 659.3 [M ꢀ Cl]+. HRMS ESIꢀTOFꢀMS (positive mode): [M ꢀ Cl]+ monoisotopic
peak 659.1803; calc. 659.1798, εr: 0.8 ppm.
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Synthesis of 3-rGO. A suspension of 90 mg of rGO in 10 mL of CH2Cl2 was immersed in an
ultrasounds bath for 30 min. Then, 10 mg of 3 was added to the mixture and the resulting suspension
was stirred at room temperature for 10 h. The black solid was isolated by filtration and washed with 2 x
15 mL of CH2Cl2 producing the hybrid material as a black solid. The exact amount of supported
complex was determined by ICPꢀMS analysis. The results accounted for 6.5 wt% of complex 3 in the
hybrid material 3-rGO. The hybrid material was characterized by UV/Vis, FTIR, XPS and HRTEM
(See Supporting Information for details).
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General procedure for the catalytic dehydrogenative coupling of silanes and alcohols.
The catalytic experiments were performed in a 25 mL round bottom flask using 0.5 mmol of silane, 1
mL of alcohol, and a catalyst (0.05 – 1.00 mol %). The flash was heated at 30 ºC in an initial nitrogen
atmosphere. The system was connected to a pressure transducer for monitoring the reaction or was
connected to an inverted waterꢀfilled burette that was used to collect the released gas. The yields and
conversions were determined by GC analysis using anisole as internal standard. Isolated yields were
determined by solvent evaporation and analysis by 1H NMR spectroscopy using 1,3,5ꢀ
trimethoxybenzene as the external standard.
Corresponding Author
Jose A. Mata*, aInstitute of Advanced Materials (INAM), Universitat Jaume I, Avda. Sos Baynat s/n,
12006, Castellón (Spain). Fax: (+34) 964387522; Tel.: (+34) 964387516; eꢀmail: jmata@uji.es
Author Contributions
The manuscript was written with contributions from all authors. All authors have approved the final
version of the manuscript.
Supporting information. Experimental details, characterization by NMR spectra and mass
spectroscopy, crystallographic data, DFT calculations details, and HRTEM images. CCDC 1811349
Acknowledgments
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The authors are thankful for the financial support from MINECO (CTQ2015ꢀ69153ꢀC2ꢀ2ꢀR and
CTQ2015ꢀ67461ꢀP), Diputación General de Aragón (Grupo Consolidado E21) and Universitat Jaume I
(P1.1B2015ꢀ09). D. VꢀE thanks the MINECO for the FPU grant (FPU15/03011). The authors are very
grateful to the ‘Serveis Centrals d’Instrumentació Científica (SCIC)’ of the Universitat Jaume I and to
the Instituto de Biocomputación y Física de Sistemas Complejos (BIFI) and the Centro de
Supercomputación de Galicia (CESGA) for the generous allocation of computational resources.
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ABBREVIATIONS
LOHC, liquid organic hydrogen carrier; HSC, hydrogen storage capacity; NHC, Nꢀheterocyclic carbene.
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