One-step synthesis of a highly homogeneous SBA-NHC hybrid material: En route to single-site NHC-metal heterogeneous catalysts with high loadings

Article abstract of DOI:10.1039/c3dt52982g

The one-step synthesis of a mesoporous silica of SBA type, functionalized with a 1-(2,6-diisopropylphenyl)-3-propyl-imidazolium (iPr₂Ar-NHC- propyl) cation located in the pore channels, is described. This material was obtained by the direct hydrolysis and co-condensation of tetraethylorthosilicate (TEOS) and 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)propyl]-imidazolium iodide in the presence of Pluronic P123 as a non-ionic structure-directing agent and aqueous HCl (37%) as an acid catalyst. Small-angle X-ray diffraction measurements, scanning and transmission electron microscopies, as well as dinitrogen sorption analyses revealed that the synthesized material is highly mesoporous with a 2D hexagonal arrangement of the porous network. ¹³C and ²⁹Si CP-MAS NMR spectroscopy confirmed that the material contains intact iPr₂Ar-NHC-propyl cations, which are covalently anchored via silicon atoms fused into the silica matrix. Moreover, comparison of the latter data with those of an analogous post-synthetic grafted SBA-NHC material allowed us to establish that, as expected, (i) it is most probably more homogeneous and (ii) it shows a more robust anchoring of the organic units. Finally, elemental mapping by energy dispersive X-ray spectroscopy in the scanning electron microscope demonstrated a very homogeneous distribution of the imidazolium units within the one-pot material, moreover with a high content. This study thus demonstrates that a relatively bulky and hydrophilic imidazolium unit can be directly co-condensed with TEOS in the presence of a structure-directing agent to provide in a single step a highly ordered and homogeneous mesoporous hybrid SBA-NHC material, possessing a significant number of cationic NHC sites.

Full text of DOI:10.1039/c3dt52982g

Dalton
Transactions
View Article Online
View Journal | View Issue
PAPER
One-step synthesis of a highly homogeneous
SBA–NHC hybrid material: en route to single-site
NHC–metal heterogeneous catalysts with high
loadings†
Cite this: Dalton Trans., 2014, 43,
3722
Mansuy Rocquin,a Mickaël Henrion,a Marc-Georg Willinger,b Philippe Bertani,c
Michael J. Chetcuti,a Benoît Louis*d and Vincent Ritleng*a,e
The one-step synthesis of a mesoporous silica of SBA type, functionalized with a 1-(2,6-diisopropyl-
phenyl)-3-propyl-imidazolium (iPr2Ar-NHC-propyl) cation located in the pore channels, is described.
This material was obtained by the direct hydrolysis and co-condensation of tetraethylorthosilicate (TEOS)
and 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)propyl]-imidazolium iodide in the presence of Pluronic
P123 as a non-ionic structure-directing agent and aqueous HCl (37%) as an acid catalyst. Small-angle
X-ray diffraction measurements, scanning and transmission electron microscopies, as well as dinitrogen
sorption analyses revealed that the synthesized material is highly mesoporous with a 2D hexagonal
arrangement of the porous network. 13C and 29Si CP-MAS NMR spectroscopy confirmed that the material
contains intact iPr2Ar-NHC-propyl cations, which are covalently anchored via silicon atoms fused into the
silica matrix. Moreover, comparison of the latter data with those of an analogous post-synthetic grafted
SBA–NHC material allowed us to establish that, as expected, (i) it is most probably more homogeneous
and (ii) it shows a more robust anchoring of the organic units. Finally, elemental mapping by energy
dispersive X-ray spectroscopy in the scanning electron microscope demonstrated a very homogeneous
distribution of the imidazolium units within the one-pot material, moreover with a high content. This
study thus demonstrates that a relatively bulky and hydrophilic imidazolium unit can be directly co-
condensed with TEOS in the presence of a structure-directing agent to provide in a single step a highly
ordered and homogeneous mesoporous hybrid SBA–NHC material, possessing a significant number of
cationic NHC sites.
Received 22nd October 2013,
Accepted 16th December 2013
DOI: 10.1039/c3dt52982g
www.rsc.org/dalton
like typical strong σ-donor ligands with non-negligible π-acceptor
Introduction
abilities.3,4 These electronic characteristics are similar to those
Since the first isolation of
a
stable imidazol-2-ylidene,1 of tertiary phosphines, and they show similar abilities to stabilize
N-heterocyclic carbenes (NHCs) have become a very important the various oxidation states and coordinatively unsaturated
class of ligands in organometallic chemistry.2 NHCs behave intermediates that appear in catalytic reactions.2,5 However,
NHCs show superior performance in many aspects over
traditional phosphine ligands, including versatility, simple
preparation, and thermal, air and moisture stability. In
aLaboratoire de Chimie Organométallique Appliquée, UMR CNRS 7509,
Ecole européenne de Chimie, Polymères et Matériaux, Université de Strasbourg,
particular, NHCs exhibit superior qualities regarding ligand
dissociation and degradative cleavage,3,4 both of which are
more unlikely compared to tertiary phosphines.6 In addition,
carbene complexes have shown unprecedented catalytic
activity under homogeneous conditions in a very broad palette
of organic reactions.2,7
Despite the high activity and selectivity, homogeneous cata-
lysis has been of somewhat limited use in industry,8 mainly
because of the difficulty of separating the soluble catalyst from
the reaction product, which in turn may lead to economical/
environmental problems, especially in the case of expensive or
25 rue Becquerel, 67087 Strasbourg, France. E-mail: vritleng@unistra.fr;
Tel: +33 3 6885 2797
bDepartment of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society,
Faradayweg 4-6, Berlin 14195, Germany
cLaboratoire de RMN et Biophysique des Membranes, Institut de Chimie,
UMR CNRS 7177, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg, France
dLaboratoire de Synthèse, Réactivité Organiques et Catalyse, Institut de Chimie,
UMR CNRS 7177, Université de Strasbourg, 1 rue Blaise Pascal, 67000 Strasbourg,
France. E-mail: blouis@unistra.fr
eInstitut d’Etudes Avancées de l’Université de Strasbourg (USIAS), 5 allée du Général
Rouvillois, 67083 Strasbourg, France
†Electronic supplementary information (ESI) available: NMR spectra of 1–4, and
powder XRD pattern of 2. See DOI: 10.1039/c3dt52982g
3722 | Dalton Trans., 2014, 43, 3722–3729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
toxic metal catalysts. Among the possible strategies for separ- established.20 However, examples of such direct syntheses for
ating and recycling active catalysts,9 the immobilization of the introduction of mono-silylated imidazolium salts on the
homogeneous catalysts on a solid support appears particularly inner pore surface of ordered mesoporous silica are extremely
attractive because it should combine the advantages of conven- scarce,21 probably because imidazolium cations are believed to
tional heterogeneous catalysis with the versatility of homo- be too hydrophilic to enter the core of the micelle and too
geneous catalysis. Compared to expensive organic polymers,10 bulky to avoid its perturbation. As a result, Thieuleux et al.
inorganic silica materials are common supports for the hetero- recently developed an elegant two-step approach in which they
genization of molecular catalysts due to their excellent thermal (i) introduce a hydrophobic organic group (that can be readily
and chemical stability. They have rigid structures that are not further functionalized22) during the co-condensation synthesis
deformed by solvent swelling during catalytic reactions, and of the material, and (ii) react the thus introduced organic func-
can be used at both high and low temperatures and at high tionalities with imidazole units to obtain the desired highly
pressures.11 In addition, they possess high surface areas and organized hybrid mesostructured imidazolium-containing
organic groups can be robustly anchored to the surface silanol materials with NHC loadings varying between 0.2 and
groups.
Among the variety of silica materials available, ordered
0.5 mmol g−1 23
Herein we describe: (i) a one-step procedure for the syn-
.
mesoporous silicas such as SBA-1512 present advantageous thesis of a mesoporous silica of the SBA type functionalized
properties due to their narrow pore size distribution, large with 1-(2,6-diisopropylphenyl)-3-propyl-imidazolium (iPr2Ar-
specific surface area, pore volume and large number of surface NHC-propyl), as a model imidazolium cation, and (ii) its full
silanol groups. Thanks to their thick framework walls, they characterization and the comparison of its solid state NMR
exhibit very high thermal and hydrothermal stability. This data with those of an analogous post-synthetic grafted
high strength renders them well suited for use as a catalyst material. The obtained results demonstrate that the one-pot
support in catalytic processes where thermal treatments and material is, as expected, of better quality than the latter,
repeated regeneration are frequently encountered.
showing a highly ordered mesoporous 2D hexagonal arrange-
The above described versatility, unique catalytic properties ment of the porous network with a high content (0.77 mmol
and stability of NHC–metal catalysts make them perfect candi- g−1) of robustly anchored and homogeneously distributed
dates for single-site heterogeneous catalysis. The stronger iPr2Ar-NHC-propyl cations.
binding of the NHC ligands to the metal center and their
ability to remain bound to it throughout many catalytic cycles2
should allow one to avoid the limited lifetimes and levels of
recyclability resulting from metal leaching, which are prevalent
for phosphine-based complexes. Thus, this research area is
Experimental section
General information
rapidly increasing and NHC ligands have now been grafted Organic syntheses were carried out using standard Schlenk
onto different supports ranging from soluble polymers to techniques under an atmosphere of dry argon. Material synth-
monolithic supports.13,14
eses were carried out under air. Commercial compounds were
However if one considers NHCs bound to the inner pore used as received. In experiments requiring dry free oxygen sol-
surface of ordered mesoporous materials, it is striking that the vents, solvents were distilled from appropriate drying agents
chosen immobilization procedure generally involves conden- under argon prior to use. Solution NMR spectra were recorded
sation of Si(OR)3-substituted NHC species with the surface at 298 K on a FT-Bruker Ultra Shield 300 spectrometer operat-
1
silanols of preformed hexagonal SBA-1515 or MCM-41,16 or ing at 300.13 MHz for H and at 75.47 MHz for 13C{1H}. The
cubic SBA-1617 materials. The main advantages of this post- assignment of the 13C{1H} NMR spectrum of 1-(2,6-diisopro-
synthetic grafting process lie in its simplicity and in the preser- pylphenyl)-3-[3-(triethoxysilyl)propyl]imidazolium iodide 1 was
vation of the initial texture of the material (despite a slight performed with the aid of a DEPT 13C spectrum. The chemical
decrease in the specific surface area). The main drawback is shifts are referenced to residual deuterated solvent peaks.
that it typically yields materials with an ill-controlled anchor- Chemical shifts (δ) and coupling constants (J) are expressed in
ing mode, loading and spatial distribution of the organic ppm and Hz, respectively. N2 sorption isotherms were
units.18
obtained at 77 K with a Micromeritics sorptometer Tri Star
An alternative approach would consist in the direct intro- 3000 apparatus. Prior to measurements, the samples were out-
duction of trialkoxy-substituted imidazolium salts or NHC gassed under vacuum (0.01 mbar) for 4 h at 250 °C. The
complexes during the synthesis of the material by co-conden- Brunauer–Emmet–Teller (BET) surface area was determined from
sation with tetraethylorthosilicate (TEOS) in the presence of a data in the relative pressure range 0.05–0.30. The pore volume
structure-directing agent (SDA).19 In this case, the NHC was calculated using the nitrogen volume adsorbed when the
species would be regularly distributed on the pore surfaces, capillary condensation was ended. Small-angle X-ray powder
and robustly anchored via a silicon atom fused into the silica diffraction patterns were recorded on a Bruker D8 Advance
matrix. Inclusion of bis-silylated imidazolium salts in the diffractometer, with a Ni detector side filtered Cu Kα radiation
framework walls of periodic mesoporous organosilica by (1.5406 Å) over a 2θ range of 0.7–1.8° and a position sensitive
similar co-condensations in the presence of a SDA is well detector using a step size of 0.02° and a step time of 2 s. For
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 3722–3729 | 3723

View Article Online
Paper
Dalton Transactions
TEM investigations, the samples were dispersed in CHCl3, 1.16 (d, 3J = 6.9, 6H, CHMe2), 0.67 (m, 2H, CH2Si). 13C{1H}
ultrasonificated for a few seconds and drop-deposited on a NMR (CDCl3, 75.47 MHz): δ = 145.5 (o-CAr), 137.6 (NCHN),
copper TEM grid with a holey carbon film. Bright field TEM 132.1 (p- or ipso-CAr), 130.0 (ipso- or p-CAr), 124.8 (m-CAr),
and HAADF STEM images were recorded using an image Cs 124.3 and 123.5 (NCH), 58.8 (OCH2), 52.4 (NCH2), 28.9
corrected FEI Titan instrument operated at 300 kV. For solid- (CHMe2), 24.8 (NCH2CH2), 24.6 (CHMe2), 24.3 (CHMe2), 18.4
state NMR spectra, the samples were packed in a 4 mm ZrO2 (OCH2CH3), 6.8 (CH2Si). MS (ESI+): m/z [M − H+] calcd for
rotor under air. The CP-MAS experiment spectra were recorded C24H42N2O3Si 433.296, found 433.292.
at 298 K on a Bruker Solid State DSX 300 MHz NMR spectro-
One-step synthesis of the hybrid SBA–NHC (2)
meter operating at 75.52 MHz for 13C and at 69.66 MHz for
29Si and equipped with a Bruker 4 mm 1H/X CP-MAS probe. Pluronic P123 (1.36 g, 0.23 mmol) was dissolved in a HCl
A shaped cross-polarization pulse sequence with tangential (37%) (8.9 mL, 108 mmol) solution in water (33 mL, 1.83 mol)
modulation on both channels was used with the following and stirred vigorously at 40 °C for 30 min to give a clear solu-
parameters: the spinning speed was set to 10 kHz, the spectral tion. TEOS (3.40 mL, 15.2 mmol) was then added dropwise.
width to 30 kHz, the contact time was in the range of 2 ms for After 30 min, 1 (303 mg, 0.541 mmol) was poured into the
13C and 1 ms for 29Si, proton RF field was around 55 kHz for reaction mixture (composition:
1 TEOS : 0.036 (1) : 0.015
decoupling (using the SPINAL 64 sequence) and 40 kHz for P123 : 7.1 HCl : 121 H2O), which was stirred at 40 °C for 20 h.
contact, with a recycle delay of 3 s for 13C and 5 s for 29Si. The The obtained slurry was then maintained static and heated in
29Si MAS spectra (see ESI†) were recorded on the same spectro- a Teflon autoclave at 130 °C for 24 h. The resulting solid
meter and under the same conditions as the CP-MAS using a was filtered and washed with water (2 × 50 mL), ethanol
single pulse experiment with high power proton decoupling, a (2 × 50 mL) and diethylether (2 × 50 mL). The surfactant was
29Si 90° pulse of 5 microseconds and a recycling delay of removed by extraction with ethanol using a Soxhlet for 24 h.
30 s. The spectra were calibrated with respect to an external Drying in air at 80 °C for 24 h afforded 2 as a very pale yellow
adamantane sample (38.2 ppm) for 13C and to an external PDMS powder (1.06 g). 13C CP-MAS NMR: δ = 146, 137, 131, 124, 70,a
sample (−35.1 ppm) for 29Si. The microstructure of the hybrid 59,a 58,a 52, 28, 23, 16,a 9. 29Si CP-MAS NMR: δ = −60 (T2),
SBA–NHC 2 was investigated using a Hitachi S4800 SEM −68 (T3), −93 (Q2), −102 (Q3), −111 (Q4).
Genesis 4000 EDX detector. The X-rays emitted upon electron
(a) Peaks at δ = 70, 59, 58 and 16 ppm correspond to the
irradiation were acquired in the range 0–20 keV. Quantification residual template agent present in micropores and to Si-OEt
was done by the standard-less ZAF correction method using signals that arise from surfactant removal by extraction with
the Genesis software from Bruker.
EtOH; see 13C CP-MAS NMR data of pristine SBA-15 3 below,
and the spectrum (Fig. S3a, ESI†).
Reagents
Synthesis of pristine SBA-15 (3)
Pluronic P123 (M ∼ 5800 g mol−1), tetraethylorthosilicate
(TEOS) and (3-chloropropyl)triethoxysilane were purchased Pluronic P123 (1.36 g, 0.23 mmol) was dissolved in a HCl
from Sigma-Aldrich. 1-(2,6-Diisopropylphenyl)-1H-imidazole (37%) (8.9 mL, 108 mmol) solution in water (33 mL, 1.83 mol)
was synthesized according to a published procedure.24
and stirred vigorously at 40 °C for 30 min to give a clear solu-
tion. TEOS (3.40 mL, 15.2 mmol) was then added dropwise
Synthesis of 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)-
propyl]imidazolium iodide (1)
and the reaction mixture (composition:
1 TEOS : 0.015
P123 : 7.1 HCl : 121 H2O) was stirred at 40 °C for 20 h. The
A
suspension of 1-(2,6-diisopropylphenyl)1-H-imidazole obtained slurry was maintained static and heated in a Teflon
(1.90 g, 8.32 mmol), 3-chloropropyltriethoxysilane (2.00 mL, autoclave at 130 °C for 24 h. The resulting solid was filtered
8.31 mmol) and KI (2.07 g, 12.47 mmol) in DME (42 mL) was and washed with water (2 × 50 mL), ethanol (2 × 50 mL) and
vigorously stirred at reflux for 5 days in the dark. The mixture diethylether (2 × 50 mL). The surfactant was removed by extrac-
was cooled to room temperature and the solvent was removed tion with ethanol using a Soxhlet for 24 h. Drying in air at
under vacuum. The orange residue was extracted with aceto- 80 °C for 24 h afforded 3 as a white powder (0.945 g).
nitrile (20 mL) and filtered through Celite, which was rinsed 13C CP-MAS NMR: δ = 75, 70, 59, 58, 16. 29Si CP-MAS NMR:
with acetonitrile (3 × 5 mL). The resulting solution was then δ = −93 (Q2), −102 (Q3), −112 (Q4).
evaporated to dryness. The residue was dissolved in
minimum of CH2Cl2 and triturated with a diethylether–
a
Synthesis of grafted SBA–NHC (4)
pentane mixture to afford 1 as a pale yellow solid (3.96 g, 3 (100 mg) was suspended in toluene (15 mL) and stirred for
7.06 mmol, 85%) after filtration and washings with a diethyl- 30 min at 50 °C to achieve a homogeneous dispersion. para-
ether–pentane mixture (3
× 5
mL). 1H NMR (CDCl3, Toluenesulfonic acid (5 mg, 0.029 mmol) and 1 (33.6 mg,
300.13 MHz): δ = 10.05 (t, J = 1.8, 1H, NCHN), 7.84 (t, J = 1.8, 0.060 mmol) were then added, and the mixture was stirred at
1H, NCH), 7.55 (t, 3J = 7.8, 1H, p-H), 7.32 (d, 3J = 7.8, 2H, m-H), reflux for 2 h. The resulting solid was filtered and washed with
3
7.19 (s, J = 1.8, 1H, NCH), 4.83 (t, J = 6.9, 2H, NCH2), 3.84 (q, ethanol (3 × 15 mL). Drying in air at 100 °C for 16 h afforded 4
3
3J = 7.0, 6H, OCH2), 2.31 (hept, J = 6.9, 2H, CHMe2), 2.14 (m, as a very pale yellow powder (0.083 g). 13C CP-MAS NMR:
2H, NCH2CH2), 1.27–1.20 (d + t, 15H, CHMe2 and OCH2CH3), δ = 146, 138, 131, 125, 70,a 59,a 53, 28, 23, 17,a 10. 29Si CP-MAS
3724 | Dalton Trans., 2014, 43, 3722–3729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
NMR: δ = −49 (T1), −59 (T2), −67 (T3), −91 (Q2), −101 (Q3),
−110 (Q4).
(a) Peaks at δ = 70, 59 and 17 ppm correspond to the
residual template agent present in micropores and/or to Si-OEt
signals that arise from surfactant removal by extraction with
EtOH; see 13C CP-MAS NMR data of pristine SBA-15 3 above
and the spectrum (Fig. S3a, ESI†).
Scheme 2 One-step synthesis of the hybrid SBA–NHC 2.
Results and discussion
Choice and preparation of the imidazolium salt (1)
Copéret, Thieuleux et al. recently demonstrated, in an elegant
study, that a flexible tether may provide significant advantages
during a catalytic reaction compared to a rigid linker, by allow-
ing an eventual low-coordinated catalytic intermediate to have
stabilizing metal–surface interactions.25 With that in mind, we
chose the 1-(2,6-diisopropylphenyl)-3-[3-(triethoxysilyl)propyl]-
imidazolium iodide 1 as a model compound to demonstrate
that imidazolium salts can be introduced directly during
the co-condensation synthesis of functionalized ordered
mesoporous silica. The latter was synthesized according to our
published method for the preparation of N-alkyl-N′-arylimida-
zolium iodides from inexpensive chloro-alkyls (Scheme 1).26
Thus, the reaction of 1-(2,6-diisopropylphenyl)-1H-imidazole24
with (3-chloropropyl)triethoxysilane in the presence of excess
potassium iodide in DME at reflux for 5 days afforded 1 as a
pale yellow solid in 85% yield after work-up. 1 was charac-
terized by 1H and 13C{1H} NMR spectroscopy and mass
spectroscopy.
Scheme 3 Two-step synthesis of the grafted SBA–NHC 4.
For comparison purposes, a grafted SBA–NHC analogue of
2 was also prepared. Its synthesis was achieved by conden-
sation at reflux in toluene in the presence of para-toluenesulfo-
nic acid (PTSA) of the imidazolium salt 1 with the surface
silanols of a preformed SBA-15 material 3 (Scheme 3). Such an
acid-catalyzed condensation on preformed SBA-15 being well
documented,28 no particular comment will be made, except
that in order for both materials to be really comparable: (i) the
pristine SBA-15 3 was synthesized using a molar composition
strictly similar to that used for the synthesis of the hybrid
SBA–NHC 2, i.e. 1 TEOS : 0.015 P123 : 7.1 HCl : 121 H2O, and
(ii) the surfactant was removed by ethanolic extraction simi-
larly to what was done for the synthesis of 2.
Characterization of the hybrid SBA–NHC (2) and comparison
with the grafted SBA–NHC (4)
One-step synthesis of the hybrid SBA–NHC (2) and two-step
synthesis of the grafted SBA–NHC (4)
The structure of the as-synthesized hybrid SBA–NHC silica 2
was first investigated by small-angle powder X-ray diffraction.
In contrast to small-angle diffraction patterns of pristine
SBA-15, which generally display three well-resolved reflections
that can be indexed as the (1 0 0), (1 1 0) and (2 0 0) diffrac-
tions of the ordered p6mm hexagonal space group,12 the latter
only exhibits the intense diffraction peak corresponding to the
d100 spacing (Fig. S2a, ESI†). No long-range ordering could be
resolved. This result is in line with earlier studies reporting the
synthesis of hexagonal-type mesoporous materials containing
either organic functionalities22,23a,b or metal oxides within
their channels.29 Nevertheless, similar single reflection
materials have been shown to still exhibit a high degree of
mesoscopic organization and local hexagonal symmetry.30
This is confirmed by the scanning electron microscopy (SEM)
and transmission electron microscopy (TEM) images of the
hybrid SBA–NHC 2 shown in Fig. 1 and 2, respectively, as well
as by the high angle annular dark-field images (HAADF)
recorded in scanning transmission electron microscopy
(STEM) mode shown in Fig. 3. The nanometer-scaled SEM,
TEM and HAADF STEM images (Fig. 1c, d, 2 and 3) indeed
clearly evidence the hexagonal arrangement of the porous
network within the material and confirm a hexagonal structure
The one-step synthesis of the hybrid SBA–NHC material 2 was
derived from that reported for the preparation of a periodic
mesoporous organosilica containing a covalently bonded
organic ion pair, and featuring a SBA-15 structure27 using a
more concentrated and more acidic medium than is usually
the case for the direct synthesis of SBA materials functiona-
lized with hydrophobic molecules on the inner pore walls.22,23
Thus, the ordered mesoporous hybrid material was obtained
by co-condensation at 40 °C under acidic conditions of 1 with
TEOS in the presence of Pluronic P123 as the SDA, with a
molar composition of the reaction mixture corresponding to: 1
TEOS : 0.036 (1) : 0.015 P123 : 7.1 HCl : 121 H2O (Scheme 2).
After proper ageing of the SBA–NHC gel precursor, the surfac-
tant was removed by hot ethanol extraction in a Soxhlet appar-
atus affording the expected hybrid SBA–NHC material 2 as a
very pale yellow powder.
Scheme 1 Synthesis of the imidazolium salt 1.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 3722–3729 | 3725

View Article Online
Paper
Dalton Transactions
width of less than 1 μm is also obtained (Fig. 1b). The presence
of the NHC salt 1, and hence of additional organic material
within the synthesis mixture, thus induces a double-type self-
assembly mode. This observation is in line with previous
studies,31 and can be partly explained by the modulation of
surfactant/pore wall interactions by co-adsorption of the NHC
cation, thus playing the role of a surface modifying agent.31b
However, one cannot completely rule out the existence of
ligand–ligand interactions at such a high content of NHC
ligand (vide infra) anchored to the surface. The latter inter-
actions could induce vicinal ligand molecules to fall next to
the walls and thus reduce their accessibility for further metala-
tion. Besides, micrometer-sized mesoporous silica SBA-15
spheres with a high dispersivity were also observed by Zhao
et al. when starch was added in the medium.31a Anyway, as
indicated above, these two morphologies, obtained by distinct
aggregation models, exhibit the rod-like morphology of their
P123-templated mesopores with unchanged hexagonal sym-
metry (Fig. 1c–d and 2). The synthesis conditions are therefore
suitable with an appropriate self-assembling rate of the surfac-
tant and the silica species that provides enough time for the
equilibrium-assembly of hundreds of nanometer-sized rod-like
silica.
Fig. 1 SEM images of the hybrid SBA–NHC 2.
The N2 adsorption–desorption isotherm of the hybrid SBA–
NHC 2 is shown in Fig. 4. It is of type IV with H1 hysteresis,
following the IUPAC classification.32 This profile is character-
istic of mesoporous materials possessing one-dimensional
cylindrical channels. Moreover, the presence of a sharp
adsorption step in the P/P0 region from 0.6 to 0.8 and of a hys-
teresis loop supports the existence of a well-defined array of
regular mesopores. Regarding the textural properties of the
hybrid material 2, it is worth noting that a specific surface
area and a pore volume of, respectively, SBET = 555 m2 g−1 and
Vp = 0.83 cm3 g−1 were measured. These values are in line with
those reported for a similar SBA–NHC material prepared in
two steps by Thieuleux et al. (vide supra),25 and nearly 30%
and 15% lower compared to the classical values of a non-func-
tionalized SBA-15 (SBET = 785 m2 g−1 and Vp = 0.98 cm3 g−1),12
thus strongly suggesting that the iPr2Ar-NHC-propyl cation is
present within the pores, as expected. Therefore, though its
presence affects the textural properties of the mesoporous
Fig. 2 TEM images of the hybrid SBA–NHC 2.
Fig. 3 HAADF STEM images of the hybrid SBA–NHC 2.
over a large area, along with an extremely uniform pore size
distribution with an average diameter of 6.7 nm and a wall
thickness of 3.3 nm (Fig. 3a). The unit cell dimension in the
a and b directions (considering a hexagonal cell) measures
10–10.2 nm. The values were abstracted from the Fourier trans-
form shown as an inset in Fig. 2a.
Nevertheless, it is noteworthy that the low-magnification
SEM images of SBA–NHC 2 reveal two kinds of morphologies
for these particles (Fig. 1a and b). Some of the particles aggre-
gate into micrometer-sized spheres of about 2–3 μm (Fig. 1a).
In addition, a classical worm-like morphology with a particle Fig. 4 N2 adsorption–desorption isotherm of the hybrid SBA–NHC 2.
3726 | Dalton Trans., 2014, 43, 3722–3729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
silica, the ordering of regular hexagonal mesopores is kept
within the organic–inorganic hybrid 2.
The high degree of mesoscopic organization and local hexa-
gonal symmetry of the hybrid SBA–NHC material 2 being thus
established, the presence of intact iPr2Ar-NHC-propyl function-
alities was confirmed by 13C CP-MAS NMR. The signals at 146,
137, 131, 124, 52, 28, 23, and 9 ppm indeed closely match with
those of the 1-(2,6-diisopropylphenyl)-3-propyl-imidazolium
part of 1 in its 13C solution NMR spectrum (Fig. S1b and S2b,
ESI†). Moreover, these peaks are relatively sharp when com-
pared to the corresponding signals of the 13C CP-MAS NMR
spectrum of the grafted SBA–NHC 4, the peaks of 2 showing a
ca. 35% smaller full-width at half-maximum compared to
those of 4: for instance 270 Hz (2) vs. 400 Hz (4) for the peak at
145 ppm or 170 Hz (2) vs. 270 Hz (4) for the peak at 28 ppm.
This suggests that the one-pot hybrid material 2 is more homo-
geneous than the latter,33,34 and thus has a more regular distri-
bution of the organic units, as expected (Fig. 5).18
Fig. 6 29Si CP-MAS NMR spectra of the hybrid SBA–NHC 2 (blue) and
of the grafted SBA–NHC 4 (green).
Moreover, the 29Si MAS NMR spectrum of 2 demonstrated a
high degree of condensation of the hybrid material with
signals at −93, −102, −111 ppm associated with Q2, Q3 and Q4
(major)35 substructures, respectively (Fig. S2d, ESI†). In
addition, the 29Si CP-MAS NMR spectrum of 2 reveals signals
at −67 and −59 ppm, associated with T3 and T2 substructures,
which shows that the iPr2Ar-NHC-propyl units are bonded to
the matrix via two or three Si–O bonds (Fig. 6 and S2c, ESI†).
This contrasts with the 29Si CP-MAS NMR spectrum of the
grafted SBA–NHC 4, which in addition to T3 and T2 signals dis-
plays a signal at −49 ppm, associated with a T1 substructure,
and thus suggests a lower degree of condensation of the
grafted iPr2Ar-NHC-propyl units within the preformed silica
matrix,36 as expected (Fig. 6 and S4b, ESI†).18
The high homogeneity of the one-pot hybrid material 2
suggested by the relatively sharp signal observed in its 13C
CP-MAS NMR spectrum was confirmed by elemental mapping
in the scanning electron microscope (SEM) using energy dis-
persive X-ray spectroscopy (EDX). EDX mapping of Si, O, C and
N elements within one SBA–NHC 2 particle indeed supports a
very homogeneous distribution of all these elements through-
out the mesoporous silica material (Fig. 7). As the nitrogen
Fig. 7 SEM-EDX elemental mapping of the hybrid SBA–NHC 2.
atoms can only arise from the iPr2Ar-NHC-propyl units, this
confirms that the latter are well distributed all over the
material. Furthermore, based on EDX analysis, the N-content
in 2 was quantified to 2.17 wt%, which led us to sample esti-
mate the degree of NHC incorporation within its inorganic
host to 0.77 mmol g−1 (by taking into consideration that
2 N-atoms are incorporated into one NHC ligand). Such values
are notably higher than those reported by Thieuleux et al. for
similar SBA–NHC hybrids prepared via their two-step approach
(vide supra),23 and further strengthen the importance of our
strategy to develop one-pot synthesized SBA–NHC inorganic–
organic hybrids.
Finally, based on previous surface characterizations of pris-
tine SBA-15 materials, where the total number of O–H groups
present in the mesoporous silica was shown by the H/D
isotope exchange technique to be 2.70 mmol g−1 37
one can
,
thus roughly estimate the degree of NHC grafting onto its in-
organic host as approximately 30%. All this makes this single-
step-synthesized hybrid SBA–NHC material 2 an excellent can-
didate for further functionalization with any catalyst precursor
to give highly desirable single-site heterogeneous NHC–metal
catalysts with high catalyst loadings.
Conclusions
In summary, we demonstrate in this study that, a contrario
to common belief, a relatively bulky and hydrophilic tri-
ethoxysilyl-functionalized imidazolium salt can be directly
Fig. 5 13C CP-MAS NMR spectra of the hybrid SBA–NHC 2 (blue) and
of the grafted SBA–NHC 4 (green).
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 3722–3729 | 3727

View Article Online
Paper
Dalton Transactions
co-condensed with TEOS in the presence of a structure-
directing agent to provide in one step a highly mesoporous
hybrid silica material with a 2D hexagonal arrangement of the
porous network, functionalized into the pore channels with
regularly distributed and robustly anchored NHC pro-ligands.
8 (a) J. Hagen, Industrial Catalysis: A practical Approach,
Wiley-VCH, Weinheim, 1999; (b) J. G. de Vries,
Can. J. Chem., 2001, 79, 1086; (c) C. E. Tucker and J. G. de
Vries, Top. Catal., 2002, 19, 111.
9 D. J. Cole-Hamilton, Science, 2003, 299, 1702.
In addition to the fact that it is direct, this approach is particu- 10 (a) N. E. Leadbeater and M. Marco, Chem. Rev., 2002, 102,
larly interesting in that it provides a remarkably homogeneous
material with a high content of NHCs. Provided that they are
3217; (b) C. A. McNamara, M. J. Dixon and M. Bradley,
Chem. Rev., 2002, 102, 3275.
accessible, their metalation should indeed give access to het- 11 C. Copéret, M. Chabanas, R. Petroff Saint-Arroman and
erogeneous NHC–metal catalytic materials that should poten- J. M. Basset, Angew. Chem., Int. Ed., 2003, 42, 156.
tially show remarkable efficiencies due to the high number of 12 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson,
catalytic sites present within the material.
B. F. Chmelka and G. D. Stucky, Science, 1998, 279, 548.
13 (a) W. J. Sommer and M. Weck, Coord. Chem. Rev., 2007,
251, 860; (b) C. S. J. Cazin, C. R. Chim., 2009, 12,
1173; (c) A. Monge-Marcet, R. Pleixats, X. Cattoën and
M. Wong Chi Man, Catal. Sci. Technol., 2011, 1, 1544;
(d) K. V. S. Ranganath, S. Onitsuka, A. K. Kumar and
J. Inanaga, Catal. Sci. Technol., 2013, 3, 2161.
14 A. M. Oertel, V. Ritleng and M. J. Chetcuti, Organometallics,
2012, 31, 2829.
15 (a) P. Li, W. A. Herrmann and F. E. Kühn, ChemCatChem,
2013, 5, 3324; (b) L. Li and J.-l. Shi, Adv. Synth. Catal., 2005,
347, 1745.
Acknowledgements
We are grateful to the Université de Strasbourg and the CNRS
for their financial help. The Agence Nationale de la Recherche
is also acknowledged for its support to V.R. and the doctoral
fellowship of M.H. (ANR 2010 JCJC 716 1; SBA-15-NHC-NiCat).
16 (a) S. Dastgir, K. S. Coleman and M. L. H. Green, Dalton
Trans., 2011, 40, 661; (b) C. del Pozo, M. Iglesias and
F. Sánchez, Organometallics, 2011, 30, 2180.
17 H. Yang, X. Han, G. Li and Y. Wang, Green Chem., 2009, 11,
1184.
Notes and references
1 A. J. Arduengo, R. L. Harlow and M. Kline, J. Am. Chem.
Soc., 1991, 113, 361.
2 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290;
(b) L. H. Gade and S. Bellemin-Laponnaz, Coord. Chem. 18 (a) D. J. Macquarrie, D. B. Jackson, J. E. J. Mdoe and
Rev., 2007, 251, 718; (c) F. E. Hahn and M. C. Jahnke,
Angew. Chem., Int. Ed., 2008, 47, 3122; (d) J. C. Y. Lin,
J. H. Clark, New J. Chem., 1999, 23, 539; (b) A. Walcarius
and C. Delacôte, Chem. Mater., 2003, 15, 4181.
R. T. W. Huang, C. S. Lee, A. Bhattacharyya, W. S. Hwang 19 (a) F. Hoffmann, M. Cornelius, J. Morell and M. Fröba,
and I. J. B. Lin, Chem. Rev., 2009, 109, 3561; (e) P. L. Arnold
and I. J. Casely, Chem. Rev., 2009, 109, 3599; (f) S. Díez-
Angew. Chem., Int. Ed., 2006, 45, 3216; (b) A. Mehdi, C. Reyé
and R. Corriu, Chem. Soc. Rev., 2011, 40, 563.
González, N. Marion and S. P. Nolan, Chem. Rev., 2009, 20 (a) T. P. Nguyen, P. Hesemann, P. Gaveau and
109, 3612; (g) M. Poyatos, J. A. Mata and E. Peris, Chem.
Rev., 2009, 109, 3677.
J. J. E. Moreau, J. Mater. Chem., 2009, 19, 4164; (b) H. Yang,
G. Li, Z. Ma, J. Chao and Z. Guo, J. Catal., 2010, 276, 123;
(c) H. Yang, X. Han, G. Li, Z. Ma and Y. Hao, J. Phys. Chem.
C, 2010, 114, 22221; (d) T. P. Nguyen, P. Hesemann and
J. J. E. Moreau, Microporous Mesoporous Mater., 2011, 142,
292; (e) L. Wang, S. Shylesh, D. Dehe, T. Philippi, G. Dörr,
A. Seifert, Z. Zhou, M. Hartmann, R. N. Klupp Taylor,
M. Jia, S. Ernst and W. R. Thiel, ChemCatChem, 2012,
4, 395.
3 (a) G. Frenkin, M. Solà and S. F. Vyboishchikov, J. Organomet.
Chem., 2005, 690, 6178; (b) U. Radius and F. M. Bickelhaupt,
Coord. Chem. Rev., 2009, 253, 678; (c) H. Jacobsen,
A. Correa, A. Poater, C. Costabile and L. Cavallo, Coord.
Chem. Rev., 2009, 253, 687.
4 (a) T. Strassner, Top. Organomet. Chem., 2004, 13, 1;
(b) L. Cavallo, A. Correa, C. Costabile and H. Jacobsen,
J. Organomet. Chem., 2005, 690, 5407; (c) S. Díez-González 21 (a) G. Liu, M. Hou, T. Wu, T. Jiang, H. Fan, G. Yang and
and S. P. Nolan, Coord. Chem. Rev., 2007, 251, 874.
5 N. M. Scott and S. P. Nolan, Eur. J. Inorg. Chem., 2005,
1815.
B. Han, Phys. Chem. Chem. Phys., 2011, 13, 2062;
(b) A. Monge-Marcet, R. Pleixats, X. Cattoën and M. Wong
Chi Man, claimed that they achieved such a synthesis
without giving experimental details, see ref. 13c.
6 Phosphines are often subject to P–C bond cleavage (a) or
orthometalation (b); (a) P. E. Garrou, Chem. Rev., 1985, 85, 22 For a seminal example, see: J. Alauzun, A. Mehdi, C. Reyé
171; (b) J. P. Collman, L. S. Hegedus, J. R. Norton and and R. Corriu, New J. Chem., 2007, 31, 911.
R. G. Finke, Principles and Applications of Organotransition 23 (a) T. K. Maishal, J. Alauzun, J.-M. Basset, C. Copéret,
Metal Chemistry, University Science, Mill Valley, CA, 2nd
edn, 1987.
7 W. A. Herrmann, M. Elison, J. Fischer, C. Kocher and
G. R. J. Artus, Angew. Chem., Int. Ed. Engl., 1995, 34, 2371.
R. J. P. Corriu, E. Jeanneau, A. Mehdi, C. Reyé, L. Veyre and
C. Thieuleux, Angew. Chem., Int. Ed., 2008, 47, 8654;
(b) I. Karamé, M. Boualleg, J.-M. Camus, T. K. Maishal,
J. Alauzun, J.-M. Basset, C. Copéret, R. J. P. Corriu,
3728 | Dalton Trans., 2014, 43, 3722–3729
This journal is © The Royal Society of Chemistry 2014

View Article Online
Dalton Transactions
Paper
E. Jeanneau, A. Mehdi, C. Reyé, L. Veyre and C. Thieuleux, 31 (a) D. Zhao, J. Sun, Q. Li and G. D. Stucky, Chem. Mater.,
Chem.–Eur. J., 2009, 15, 11820; (c) T. K. Maishal,
M. Boualleg, M. Bouhrara, C. Copéret, E. Jeanneau, L. Veyre
and C. Thieuleux, Eur. J. Inorg. Chem., 2010, 5005;
2000, 12, 275; (b) B. Bharti, M. Xue, J. Meissner,
V. Cristiglio and G. H. Findenegg, J. Am. Chem. Soc., 2012,
134, 14756.
(d) M. Baffert, T. K. Maishal, L. Mathey, C. Copéret and 32 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou,
C. Thieuleux, ChemSusChem, 2011, 4, 1762.
24 J. Liu, J. Chen, J. Zhao, Y. Zhao, L. Li and H. Zhang,
Synthesis, 2003, 2661.
R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure
Appl. Chem., 1985, 57, 603.
33 J. Pauli, B. van Rossum, H. Förster, H. J. M. de Groot and
H. Oschkinat, J. Magn. Reson., 2000, 143, 411.
25 M. K. Samantaray, J. Alauzun, D. Gajan, S. Kavitake,
A. Mehdi, L. Veyre, M. Lelli, A. Lesage, L. Emsley, 34 The different line broadening between the spectra of 2 and
C. Copéret and C. Thieuleux, J. Am. Chem. Soc., 2013, 135,
3193.
26 A. M. Oertel, V. Ritleng and M. J. Chetcuti, Synthesis, 2009,
1647.
27 A. El Kadib, P. Hesemann, K. Molvinger, J. Brandner,
C. Biolley, P. Gaveau, J. J. E. Moreau and D. Brunel, J. Am.
Chem. Soc., 2009, 131, 2282.
28 N. García, E. Benito, J. Guzmán, P. Tiemblo, V. Morales and
R. A. García, Microporous Mesoporous Mater., 2007, 106,
129.
4 could, in theory, also result from different mobilities (T2).
However such a difference in T2 would imply faster move-
ments for the sample with the sharpest peaks, i.e. for the
hybrid SBA–NHC 2. This would go against the fact that the
imidazolium units are expected to be more embedded into
the silica matrix than in the grafted material.
35 With the CP-MAS technique, nuclei that are far away from
protons are difficult to detect. That is why Q4 peaks appear
so small in the 29Si CP-MAS spectra displayed in Fig. 6,
whereas it is the major peak in the 29Si MAS spectra
(see Fig. S2d and S4c†).
29 (a) Ch. Subrahmanyam, B. Louis, F. Rainone,
B. Viswanathan, A. Renken and T. K. Varadarajan, Appl. 36 As the CP-MAS dynamics of 2 and 4 are not the same, no
Catal., A, 2003, 241, 205; (b) Y. Sun, S. Walspurger,
J.-P. Tessonnier, B. Louis and J. Sommer, Appl. Catal., A,
2006, 300, 1; (c) G. Laugel, J. Arichi, H. Guerba, M. Moliere,
A. Kiennemann, F. Garin and B. Louis, Catal. Lett., 2008,
125, 14; (d) G. Laugel, J. Arichi, M. Moliere,
A. Kiennemann, F. Garin and B. Louis, Catal. Today, 2008,
138, 38.
conclusion regarding the respective amounts of the
different T groups can be drawn. Nevertheless the fact that,
in contrast to the spectrum of 4, that of 2 does not show T1
signals allows us to think with reasonable confidence that
the iPr2Ar-NHC-propyl unit is more robustly anchored in
2 than in 4.
37 (a) Z. El Berrichi, B. Louis, J. P. Tessonnier, O. Ersen,
L. Cherif, M. J. Ledoux and C. Pham-Huu, Appl. Catal., A,
2007, 316, 219; (b) S. Walspurger and B. Louis, Appl. Catal.,
A, 2008, 336, 109.
30 (a) P. T. Tanev, M. Chibwe and T. J. Pinnavaia, Nature,
1994, 368, 321; (b) P. T. Tanev and T. J. Pinnavaia, Science,
1995, 267, 865.
This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 3722–3729 | 3729