Heterogeneous Organocatalysts Based on a Triazine-Triazole Silane Ligand

Article abstract of DOI:10.1002/ejic.201800885

Robust and non-air-sensitive metal-free triazine-triazole heterogeneous organocatalysts were prepared. The synthetic procedure involved the co-condensation of a bridged tris silane precursor with tetraethyl orthosilicate (TEOS) in the presence of cetyltrimethylammonium bromide (CTAB). In addition, the silane precursor was anchored post-synthesis onto mesoporous silica nanoparticles and used to stabilize magnetic nanoparticles. The organocatalysts were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), N₂ adsorption-desorption, transmission electron microscopy (TEM), solid-state nuclear magnetic resonance spectroscopy (CP-MAS-NMR), Fourier-transform infrared spectroscopy (FT-IR), and UV/Vis diffuse reflectance spectroscopy (DRUV/Vis). All materials, recoverable and recyclable, possess good catalytic activity in Knoevenagel condensation and in the one-pot three-component reaction for the synthesis of 2-amino-chromene derivatives under mild conditions.

Full text of DOI:10.1002/ejic.201800885

A Journal of
Accepted Article
Title: Heterogeneous Organocatalysts Based on a Triazine-Triazole
Silane Ligand
Authors: Isabel del Hierro, Paula Cruz, and Yolanda Pérez Cortes
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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Heterogeneous Organocatalysts Based on a Triazine-Triazole Silane Ligand
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Paula Cruz a, Dr. Yolanda Pérez a*, Prof. Isabel del Hierro a*.
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Departamento de Biología y Geología, Física y Química Inorgánica. Escuela Superior de
Ciencias Experimentales y Tecnología. Universidad Rey Juan Carlos. 28933 Móstoles
(Madrid), Spain. Tel.: (+34) 916647444 Fax: (+34) 914887022
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E-mail: isabel.hierro@urjc.es, yolanda.cortes@urjc.es
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https://www.urjc.es/investigacion/989-grupo-de-compuestos-quimicos-y-ma-teriales-
nanoestructurados-con-aplicaciones-avanzadas-quinanoap
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Abstract
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Robust and non-air sensitive metal-free Triazine-Triazol heterogeneous organocatalysts
have been prepared. The synthetic procedure involved the co-condensation of a bridged
tris silane precursor with tetraethyl orthosilicate (TEOS) in the presence of
cetyltrimethylammonium bromide (CTAB). In addition, the silane precursor was
anchored post-synthesis onto mesoporous silica nanoparticles and used to stabilize
magnetic nanoparticles. Organocatalysts have been characterized by X-ray diffraction
(XRD), X-ray fluorescence (XRF), N2 adsorption-desorption, transmission electron
microscopy (TEM), solid-state nuclear magnetic resonance spectroscopy (CP-MAS-
NMR), Fourier-transform infrared spectroscopy (FT-IR) and UV-vis diffuse reflectance
spectroscopy (DRUV-vis). All materials, recoverable and recyclable, possess good
catalytic activity in the Knoevenagel condensation and in one-pot three-component
reaction for the syntheses of 2-amino-chromene derivatives under mild conditions.
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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Introduction
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Ordered organic-inorganic hybrid materials, in which the organic units are
homogenously distributed into the silica framework, are a class of organically modified
mesoporous silicas. They are synthesized by using precursors of the type (OR’)3Si-R-
Si(OR’)3, (R’ = Me, Et) and R as the organic bridging group, in the presence of a surfactant
as structure-directing agents and a silicon source to guarantee a sufficient porosity for
the mesoporous material. In literature, it is common to find this type of materials named
as periodic mesoporous organosilicas (PMOs); however, PMOs are synthesized by
hydrolysis and condensation reactions using bridged silane precursor (OR’)3Si-R-Si(OR’)3
acting as a unique source of silica in the presence of the surfactant as a structure-
directing agent.[1] Mesoporous silica materials with organo-bridged framework possess
unique physicochemical and biocompatible properties due to the combination of a
framework of strong Si-O bonds and the presence of organic functionalities. A huge
amount of literature is available on the synthesis of conventional mesoporous silica
nanoparticles and its functionalization by different procedures.[2,3] Among them, there
are numerous works, which cover the preparation of modified materials with nitrogen,
or oxygen based heterocycles. The use of click chemistry coupling of tethered azide onto
mesoporous silicas with different alkyne derivatives is a common approach to introduce
triazol moieties into silica mesoporous materials.[4] As an additional example, Bhaumik
and coworkers have prepared a highly ordered 2D-hexagonal SBA-15 material via the
thiol–ene click reaction of 3-mercaptopropyl previously grafted with 2,4,6-triallyloxy-
1,3,5-triazine.[5] Nevertheless, works dealing with the preparation of PMOs or organo-
bridged MSNs are less frequent due to the difficulties inherent to their synthesis. The
charge, the functional groups, the size and solubility in water of the bis, tris, or even
higher, bridged silane precursors may influence the self-assembly of the silicon source
and surfactant. This in turn, controls the morphology and mesostructure of the material.
As example, isocyanurate-based periodic hexagonally ordered mesoporous
organosilicas have been prepared by Fischer and co-workers for embedding Metal
Nanoparticles.[6] Durand et al. have synthesized nanospheres by co-condensation of
TEOS with a tetra alkoxysilylated two-photon electron donor obtained via click-
chemistry; this material has been used to prepare cargo-loaded multi responsive
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nanogates.[7] In addition, they have demonstrated the possibility of tuning the porous
structure from radial to worm-like and hexagonal MCM-41 types of organization by
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modifying the solvent and temperature during the synthetic procedure.[8] This group has
also used a similar approach to prepare microparticles with porphyrin covalently
entrapped in ethenylene-bridged mesoporous organosilica by using a zinc porphyrin
derivative possessing eight triethoxysilyl groups.[9]
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Herein, we report the direct one-pot synthesis of a new robust and non-air sensitive
metal-free Triazine-Triazol mesoporous organocatalyst. The synthetic procedure
involves the co-condensation of a bridge tris-silane precursor with tetraethyl
orthosilicate (TEOS) in the presence of cetyltrimethylammonium bromide (CTAB) as
structure directing agent. For comparison purposes, the bridge tris-silane precursor has
been anchored onto pristine mesoporous silica nanoparticles and used to stabilize
magnetic nanoparticles. The catalytic activity of the so-synthesized materials has been
studied in Knoevenagel condensation and in one-pot three-component reactions. Multi-
component condensation reactions play an important role in the production of fine
chemicals and biologically relevant compounds.[10,11] The synthesis of 2-amino-
chromene derivatives, present in many natural products, has attracted much attention
because of their biological and pharmacological applications.[12] Some of them can also
be employed as cosmetics, pigments and as potential biodegradable agrochemicals.[13]
The production of these complex molecules through optimized and environmental
friendly transformations is one of the greatest challenges in the last years. In this regard,
the development of new heterogeneous systems, which allow easy separation, recovery
and reuse of the catalyst, has become one of the major issues.[14] Modified mesoporous
silica nanospheres are a promising choice in preparing new heterogeneous catalysts due
to their high surface area and nano-size which allows a quasi-homogeneous behavior
during the catalytic process.[15] In addition, modified magnetic metallic nanoparticle
catalytic systems have aroused much interest in recent years, because of the
straightforward recuperation through the application of a magnetic field.[16]
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Results and Discussion
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Synthesis and Characterization
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Organo-bridged mesoporous silica nanoparticles (MSNs) synthetic procedures are well-
documented.[17] In this work the co-condensation of tetraethyl orthosilicate (TEOS), as
additional silica precursor, and the bridged organosilica derivative allow the formation
of organo-bridged MSNs via the cooperative self-assembly of cetyltrimethylammonium
bromide (CTAB), as structure-directing agent in the sol gel process in basic media and
ethyl acetate as co-solvent (Scheme 1A). A bridged organosilica precursor, consisting of
an organic multifunctional linker placed between three silicon atoms with a structure
R[Si(OEt)3]3, is synthesized by a click chemistry reaction.[18] The reaction catalyzed by
Cu(I) is performed through the cycloaddition of 3-azidopropyltriethoxysilane and
propargyl alcohol as alkyne substrate (Scheme S1). Although the addition of TEOS, as
silica precursor, decreases the total organic content in the framework, it can control the
hydrolysis and condensation reactions of the bridged organosilane, especially when
using complex and bulky organic groups, and hence control the structure and particle
size distribution of the nanoparticles obtained. In addition, the organosilane precursor
was anchored post-synthesis onto mesoporous silica nanoparticles (Scheme 1B) and
used to stabilize magnetic nanoparticles (Scheme 1C).
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Scheme 1. Preparation of organic-inorganic hybrid materials
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The characterization of the organo-bridged MSNs synthesized, after surfactant removal,
confirms the monodispersity and mesoporosity of silica nanoparticles. Nevertheless,
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poor ordered worm-like pore structures were obtained as observed in the TEM images
reported in Figure 1.
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Figure 1. TEM image of organo-bridged TTS-MSNs-Co, synthesis by co-condensation of
organo-bridged tris-silane with TEOS in basic media in the presence of CTAB.
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The XRD pattern of organo-bridged TTS-MSNs-Co synthesized by this method, defined
as co-condensation method, exhibits a poorly defined single diffraction peak at low 2
angle, corresponding to a pore–pore correlation distance of 4.5 nm (see Figure 2) and
to ao = 2d100(3)-1 = 5.2 nm. These are typical patterns for wormhole-like
mesostructures and are similar to those of MSU-X-type mesoporous silica materials
synthesized with a PEO-based surfactant as template in acidic medium.[19] Durand and
co-workers have also obtained similar results when using bis-silane ligands at high
dilution, basic media and short reaction times[20] or when using tetra-silane ligands and
ethanol as co-solvent.[21] For comparison purposes, the surface grafting of this ligand on
pristine mesoporous silica nanoparticles (MSNs) has also been carried out. After
functionalization, the 2-dimensional (2D) hexagonal channel mesostructure (p6mm
symmetry) observed for pristine MSNs (see Figure S5) remains unaltered as can be
deduced from the intense peak observed at low 2 angle indexed as (100) and two
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additional weak peaks indexed as (110) and (200) reflections. The unit cell parameter
was calculated to be 4.7 nm.
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Figure 2. DRX of pristine MSNs and organo-bridged TTS-MSNs-Co and TTS-MSNs-G.
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The physical parameters such as the surface area (SBET), total pore volume and BJH
average pore diameter have been measured by nitrogen adsorption experiments (Figure
3). The characteristic type IV BET isotherms for the prepared materials show the
presence of mesoscale pores. The sharp N2 condensation step at P/Po = 0.2-0.4 for MSNs
indicates that the supports have a highly ordered hexagonal pore system. The parent
MSNs material possesses SBET (1018 m2 g-1), pore volume of 0.96 cm3 g-1 and a BJH pore
diameter of 36 Å. The material bridge silane synthesized by functionalization of this
materials with the bridge silane ligand, denoted in this work as TTS-MSNs-G, presents
an important decrease in the SBET, (519 m2 g-1) pore volume (0.29 cm3 g-1) and average
BJH pore diameter (21 Å) in comparison with the parent support. That is due to the
presence of tris-silane bridge ligand anchored to the channels partially blocking the
adsorption of nitrogen molecules. As previously explained, experimental conditions
influence the morphology and physicochemical properties of MSNs. TTS-MSNs-Co silica
nanoparticles with mesoporous wormhole-like structure show N2 adsorption–
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desorption isotherm characteristic of mesoporous materials and an additional capillary
condensation step at P/P0 > 0.9, which confirms the formation of interparticle porosity
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and increase aggregation.[22] In comparison with the grafted catalyst, the material bridge
silane TTS-MSNs-Co prepared in a one step co-condensation procedure, possesses
higher SBET (637 m2 g-1), higher pore volume (0.85 cm3 g-1) and a BHJ pore size diameter
(53 Å) (see Figure S6). During the grafting process of bridged tris-silane ligand performed
in organic media, the reaction of ethoxy groups with surface silanol takes place.
Nevertheless under these experimental conditions, the reaction of some silane
molecules with each other and simultaneous reaction with silanol groups onto surface
nanoparticles forming aggregates cannot be discarded.[23]
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Based on the nitrogen content obtained by elemental analysis, the quantity of TTS
molecules attached to the mesoporous silica nanoparticles with highly ordered
hexagonal pore system (TTS-MSNs-G) was calculated to be 0.29 mmol g-1. Taking into
account Lo and SBET of the parent mesoporous silica, the average surface density, d, of
attached molecules and the average intermolecular distance were calculated to be 0.17
molecules/nm2 and 2.41 nm, respectively. Similarly, the data calculated for bridge silane
MSNs co-condensation TTS-MSNs-Co are 0.56 mmol g-1 ligand content, 0.53
molecules/nm2 average surface density of attached molecules and 1.38 nm
intermolecular distance. As expected, the ligand content increases, but this does not
mean that all the functionalities are accessible to chemical reactants since they can be
embedded into the inner structure.
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Figure 3. Nitrogen adsorption-desorption isotherms of pristine MSNs and bridge silane
MSNs synthesized by co-condensation (TTS-MSNs-Co) and grafting (TTS-MSNs-G).
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Solid-state CP-MAS 13C NMR spectra of organo-bridged MSNs synthesized by co-
condensation and grafting are shown in Figure 4. For the first material, TTS-MSNs-Co,
the resonances due to the carbon atoms of the propyl alkyl chain, -Si-CH2-, -CH2-CH2-
CH2- and -CH2-CH2-CH2-N, appear at  7.5, 22.7 and 51.2 ppm; this last overlapped with
that belonging to free MeOH. The signals attributed to the carbon atoms of the triazole
unit appears at  123.7 and 140.5 ppm; and the signals due to carbon atoms of the
methylene group bonded to the triazine moiety at 60.4 ppm. Finally, the resonance peak
at ca. 172 ppm indicates the presence of the carbon atoms in the aromatic ring. For the
second material, TTS-MSNs-G, synthesized by reaction of silane ligand with available
silanol groups on the silica surface, in the presence of an organic solvent; the most
relevant difference is the presence of two additional signals at 15.5 and 57.1 ppm
attributed to methyl (CH3)2-CH2-O and methylene (CH3)2-CH2-O groups of the ethoxy
ligands attached to silicon that remains unreacted upon functionalization of the MSNs
silica surface.
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Figure 4. 13C-CP-NMR of bridge silane MSNs synthesized by co-condensation and grafting
(* MeOH impurity from washing).
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A standard solution for the recyclability issue is the immobilization of catalysts onto solid
insoluble supports such as the silica nanoparticles or, as a more desirable approach,
magnetic nanoparticles since their magnetic properties facilitates the catalyst
separation using an external magnet. In this work, superparamagnetic magnetite
nanoparticles with a size of around 8.0 nm have been used as an alternative material.
These MNPs have been synthesized by a classical co-precipitation method[24] and lately
they have been successfully coated with the bridge tris-silane ligand. Condensation of
the precursors leads to the formation of a covalent attachment of the functionalization
agent around the particles; that in turn, effectively avoids the removal during
polymerization.
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XRD analysis of the uncoated Fe3O4 samples (see Figure 5) show the characteristic
diffraction pattern of magnetite, where the presence of broad and intense peaks
confirms the formation of crystalline nanoparticles. After functionalization with the
bridge tris-silane ligand the MNPs did not structurally change. The crystallite size of
MNPs and stabilized MNPs were calculated by applying the Debye-Scherrer equation
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(dhkl = 0.9λ/(βcosθ)), in which β stands for the half-width of the XRD diffraction line, θ is
the peak position in angles and λ is the CuKα radiation wavelength, that is 1.54056 Å, to
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the reflection peak (311). The average size of MNPs was 8.4 nm (see Figure S7) and 8.5
nm for uncoated and functionalized magnetite nanoparticles. This data and the low
amount of ligand, 0.06 mmol g-1 MNPs, as calculated on the basis of elemental analysis,
suggest that the silanization procedure is not highly efficient.
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Figure 5. X-ray powder diffraction patterns of the uncoated magnetic nanoparticles
sample and the organo-bridge silanized magnetic nanoparticles (TTS-MNPs).
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The FTIR spectra of the hexagonal ordered mesoporous nanospheres before and after
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functionalization
are
shown
in
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Figure 6 and Figure S8; in addition, with that corresponding to the organo-bridged TTS-
MSNs-Co. The FTIR spectrum of pristine MSN shows characteristic bands at 3441 cm-1
assigned to O-H stretching vibrations and the remaining physisorbed water molecules,
at 1629 cm−1 due to deformation vibrations of the adsorbed water molecules and at
1082, 959 and 806 cm−1 attributed to the Si-O stretching vibrations. The Triazine-Triazol
functionalized materials displays additional bands at 2979 and 2941 cm-1 due to the ν(C–
H) stretching vibrations, a strong band appears also at 1578 assigned to the skeleton
stretching vibration of (C–N) and at 1457, 1419 cm-1 to the stretching vibrations (N=N,
C=C, C-C) of the triazine rings. Finally, two bands were observed at 1364, 1323 cm-1
which are due to (C-N) stretching vibrations of the triazole units. The FTIR spectra of
bridge silane coating magnetic nanoparticles confirms the success of the silanization
reaction and the coating of the magnetite surface. The characteristic absorption bands
of the Fe–O bond of the grafted MNPs shifted to higher wavenumbers, 628 and 579
cm−1, compared to the uncoated MNPs (in 571 cm−1). This phenomenon can be
explained by the formation of Fe–O–Si bonds. In addition to the above mentioned
bands, those due to the Triazine-Triazol ligand were observed in the range of 1323-1574
cm-1.
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1
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Figure 6. FTIR spectra of A) pristine MSNs, TTS-MSNs-Co and TTS-MSNs-G and B) pristine
MNPs and functionalized TTS-MNPs
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6
7
Since the tris-silane bridged organosilica ligand used as precursor provides chromophore
groups to the siliceous materials, the DRUV-Vis spectra was also recorded. As can be
seen in Figure 7, bridge silane MSNs prepared by grafting and by co-condensation
12
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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methods show an absorption band at 217 nm, due to weak pp* transitions. Similarly,
the DRUV-vis spectra of MNPs and stabilized MNPs with the bridged silane were
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recorded. In the range measured, from 200-800 nm, magnetite nanoparticles spectrum
showed a strong absorption band envelope centered at 345 nm, which represents
transitions between the oxygen non-bonding and metal d-orbitals (VB*to CF) of both
octahedral and tetrahedral ions. The silanization process does not provide any extra
absorption bands in the stabilized material; probably the expected electronic transitions
for triazine and triazol groups overlaps with this broad band.
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Figure 7. DRUV-Vis spectra of pristine MNPs, and functionalized TTS-MNPs and TTS-
MSNs-Co and TTS-MSNs-G.
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13
Catalysis
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17
The catalytic activity of the nano-structured mesoporous silicas and magnetic
nanoparticles were tested in Knoevenagel condensation and multi condensation
reactions. In the beginning, benzaldehyde and malononitrile were used as model
reactants for the Knoevenagel condensation in order to find the optimum reaction
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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2
conditions. Initially the reaction was carried out with ethanol as solvent. No reaction was
observed in absence of nano catalysts. The reaction was started from 50 mg of catalyst
1
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3
3
and stoichiometric amount of reactants (11 mmol each). We observed that the
conversion was practically quantitative at room temperature after half an hour by using
as catalysts the three materials synthesized in this work: TTS-MNPs, TTS-MSNs-Co and
TTS-MSNs-G. To differentiate the catalytic activity of the materials, the less activated
ethyl cyanoacetate was also tested. With this reactant, the experimental conditions are
harder; the temperature and the reaction time increased (see Table 1) and the
maximum conversion value, 40 %, was obtained with the material synthesized by
grafting with a ligand loading of 0.29 mmol g-1. This result is in agreement with the lower
loading ligand possessed by the modified magnetic nanoparticles (0.06 mmol g-1) and
supports the fact that not all the nitrogen containing functional groups are accessible to
the reactants in the bridge silane MSNs materials synthesized by co-condensation, which
shows the highest ligand content, 0.56 mmol g-1. Optimization of the solvent used in the
catalytic reaction was carried out by using this last material. The results show a slight
improvement of the conversion using water instead of ethanol and no difference when
working in the absence of solvent.
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The catalytic activity of this family of materials was also tested for a three-component
multicondensation reaction to synthesize 2-amino-chromene derivatives. As can be
seen in Table 2, the reaction with malonotrile (pKa = 11.1) was practically quantitative
o
for all the three catalysts after half an hour when increasing the temperature to 60 C.
When the solvent used was water instead of ethanol, similar yields were obtained.
However, when ethyl cyanoacetate (pKa = 13.1) was used, the compound 2,2'-
(phenylmethylene)bis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one) was obtained as
reported in Table 3.
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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Table 1. Knoevenagel condensation reaction catalysed by TTS-MNPs, TTS-MSNs-Co and
TTS-MSNs-Ga.
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5
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10
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20
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22
23
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31
32
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Catalyst
Substrate
T(oC)
25
Solvent
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
N/A
Time (h)
% Conversion
≈99b
TTS-MSNs-G
TTS-MNPs
0.5
0.5
0.5
2/4
2
25
≈99b
≈99b
20/40d
35c
25c
TTS-MSNs-Co
TTS-MSNs-G
TTS-MNPs
25
60
60
TTS-MSNs-Co
TTS-MSNs-Co
60
2
60
2
26c
29c
TTS-MSNs-Co
60
Water
2
a
o
3
4
5
6
7
8
Reaction conditions: 50 mg of catalyst, at room temperature or 60 C, 11 mmol of
b
RCH2C≡N, 11 mmol of benzaldehyde and ethanol as solvent (5 mL final volume).
1
c
Isolated yield, compound identified by H NMR spectroscopy. Conversion vs ethyl
d
cyanoacetate (GC values).
Selectivity values are 100% towards 2-
benzylidenemalononitrile and 99% towards ethyl 2-cyano-3-phenylacrylate (traces of
benzoic acid are observed).
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Table 2. One pot multicondensation reaction catalysed by TTS-MNPs, TTS-MSNs-Co and
TTS-MSNs-G to synthesized 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-
4H-chromene-3-carbonitrilea.
Catalyst
Solvent
Time (h)
% Yielda
TTS-MSNs-G
Ethanol
0.5
≈97
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
TTS-MNPs
Ethanol
Ethanol
Water
≈97
≈97
≈97
1
2
3
TTS-MSNs-Co
TTS-MSNs-Co
2h
4
a
o
5
6
7
8
1
2
3
4
5
Reaction conditions: 20 mg of catalyst, at 60 C, 5 mmol of C≡NCH2C≡N, 5 mmol of
benzaldehyde, 5 mmol of 5,5-dimethylcyclohexane-1,3-dione and ethanol as solvent (10
b
1
c
9
mL final volume). Isolated yield, compound identified by H NMR spectroscopy.
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Selectivity value is 100% towards 2-amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-
tetrahydro-4H-chromene-3-carbonitrile
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Table 3. One pot multi-condensation reaction catalysed by TTS-MNPs, TTS-MSNs-Co and
TTS-MSNs-G
to
synthesized
2,2'-(phenylmethylene)bis(3-hydroxy-5,5-
dimethylcyclohex-2-en-1-one)a.
Catalyst
Solvent
Ethanol
Ethanol
Ethanol
Water
Time (h)
Yield %
95
TTS-MSNs-G
TTS-MNPs
2
2
2
2
93
TTS-MSNs-Co
TTS-MSNs-Co
95
37
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10
11
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a Reaction conditions: 20 mg of catalyst, at 60 oC, 5 mmol of benzaldehyde, 10 mmol of
b
5,5-dimethylcyclohexane-1,3-dione and ethanol as solvent (10 mL final volume).
Isolated yield vs dimedona identified by 1H NMR spectroscopy. c Selectivity value is 100%
towards 2,2'-(phenylmethylene)bis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one).
The reusability of the catalysts was studied for Knoevenagel condensation of
benzaldehyde and malononitrile and for the condensation reaction of benzaldehyde and
5,5-dimethylcyclohexane-1,3-dione. In the first reaction, the catalyst was easily
recovered by direct centrifugation or by the action of an external magnet in the case of
the functionalized magnetic nanoparticles, TTS-MNPs. In the case of the condensation
reaction, the high amount of product obtained make it necessary its complete
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European Journal of Inorganic Chemistry
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dissolution in acetone before the recovery of the solid catalyst by simple centrifugation.
As representative examples, the results of malononitrile conversion and selectivity
towards 2-benzylidenemalononitrile and the achievement of 2,2'-
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(phenylmethylene)bis(3-hydroxy-5,5-dimethylcyclohex-2-en-1-one) for six consecutive
catalytic runs are shown in Figure 8. It is observed that both parameters remain
practically unaltered with identical conversion values in comparison to fresh catalysts.
The reusability value of this family of catalysts is consistent with good chemical and
mechanical resistance of the functionalized mesostructured materials against the
reaction conditions.
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A) TTS-MSNs-G Reutilization in Malononitrile-Benzaldehyde
Knoevenagel Reaction
100
80
60
40
20
0
fresh
1º
2º
3º
4º
5º
Run
%Conversion
%selectivity
10
B)TTS-MSNs-Co Reutilization in dimedona-Benzaldehyde
condensation Reaction
100
80
60
40
20
0
fresh
1º
2º
3º
4º
5º
Run
% Yield
%selectivity
11
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
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Figure 8. Catalytic activity of A) TTS-MSNs-G in consecutive reaction cycles of
Knoevenagel condensation and B) TTS-MSNs-Co in consecutive reaction cycles of
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multicomponent condensation.
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4
5
Several and active basic catalysts are found in bibliography as useful catalysts in this type
of condensation reactions. However, the range of reactants and experimental
conditions used are so wide, that it is difficult to get direct conclusions. That is why in
order to compare the activity of the materials synthesized in this work with those
previously published we have looked for or calculated, when needed, the TON values
for several of them. The term TON is more appropriate for homogeneous catalysis (or
enzymatic catalysis from which this term is borrowed), in which well-defined catalyst
molecules are present. For heterogeneous catalysts, this term is not so clear since the
activity depends also on the size of the catalyst surface and the determination of active
centers is not straightforward. As we have observed in this work when the conversion
values obtained with catalysts TTS-MSNs-Co and TTS-MSNs-G, with different
accessibility, were compared. Anyway, with these limitations in mind and assuming that
the active catalytic centers are the nitrogen donor atoms present in the ligands used as
basic functionalities (or appropriate basic functionalities). We have calculated this
parameter for the Knoevenagel condensation reaction with benzaldehyde and
malononitrile or ethyl cyanoacetate and included significant data like temperature,
solvent and reaction time (see Table 4). These results mostly follow the utilization of
experimental “green” conditions, such as the use of green solvents (water and ethanol)
or none solvent at all; grinding or ball milling as mechanical stirring conditions, low
temperature and time values and minimal amount of recoverable catalyst. As reported
in Table 4, chitosan,[25] ionic liquids,[26,27] molecular metallic catalyst,[28] the combination
of both factors in the form of bromozincate ionic liquids[29] or tungsten immobilized
heterogeneous catalysts[30] reach moderate to good TON values in the selective
synthesis of 2-benzylidenemalononitrile at the cost of some green principles, such as,
high temperatures or long reaction times. In comparison, the materials synthesized in
this work, TTS-MSN-Co, TTS-MSN-G and TTS-MNPs, show excellent TON values at room
temperature and low reaction times. When, the less activated ethyl cyanoacetate is
used as reactant; organic solvents, higher temperatures and/or times values were
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required to achieve good conversion results by using amine,[31,32] alanine,[33] imidazole
or dyhydroimidazol[34] functionalized heterogeneous active catalysts. A similar situation
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is observed when mesoporous uncalcined MCM-41 is tested as catalyst to exploit the
structure directing agent CTBA as basic functionality; in this reaction toluene is used as
solvent to improve the catalytic results.[35] In this regard, the prepared material TTS-
MSN-G show a very competitive TON value under mild reaction conditions in agreement
with an environmentally friendly catalytic process.
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
1
Table 4. Screening of different catalyst sources for Knoevenagel condensation of benzaldehyde with malononitrile or ethyl cyanoacetate.
Reactant
Malononitrile
Catalyst
Conversion /Yield (%)
TON
5
183
36
8
4
--
20
15
13
715
751
390
3772
882
812
1288
23
28
69
71
120
5
Time (h)
0.116
24
0.5
6
T (oC)
RT
RT
RT
40
Solvent
Water
Water
Ref
[26]
[Nmm-PDO][OAc]
[Nmm-PDO][OAc]
[TBA-Leucyne]
99
73
89
99
79
86
93
90
80
97
99
99
99
63
58
92
32
36
57
40
96
96
92
94
89
40
25
32
[26]
[27]
[25]
[28]
[29]
[30]
[30]
[30]
[33]
Water
Chitosan
Ethanol
Grinding
[C2MIM]Br/ZnBr2
Water
Ethanol
Ethanol
None
Ethanol
Ethanol
Ethanol
None
[CdNa2(µ-L2)]n·6.34H2O
[C2MIM]Br/ZnBr2
Fe3O4@SiO2@NH-NH2-H3PW12O40
Fe3O4@SiO2@NH-NH2-H3PW12O40
Fe3O4@SiO2@NH-NH2-H3PW12O40
Alanine-SBA-15
0.33
--
0.2
18
2
0.5
0.5
0.5
0.5
1
1
4
1.5
1.5
3
RT
95
Reflux
RT
Reflux
100
RT
RT
RT
90
90
90
RT
RT
30
30
30
RT
Reflux
RT
TTS-MSNs-G
TTS-MSNs-Co
TTS-MNPs
This work
This work
This work
[34]
Ethyl cyanoacetate Im-SBA-15
Im-Nb-SBA-15
[34]
[34]
[31]
[34]
[35]
[35]
[31]
[26]
[30]
[32]
[33]
None
None
Im-Nb-SBA-15
APTES-SBA-15
Toluene
Toluene
Toluene
Toluene
Toluene
Water
Water
None
None
DHIm-SBA-15
[CTA+]–Si–MCM-41
APTES–MCM-41
APTES-SBA-15
3
1
[Nmm-PDO][OAc]
Fe3O4@SiO2@NH-NH2-H3PW12O40
APTES-GO-3 S
10
0.4
10
0.5
2
20
168
654
304
99
Alanine-SBA-15
TTS-MSNs-G
TTS-MSNs-Co
100
60
60
Ethanol
Ethanol
Ethanol
This work
This work
This work
2
2
TTS-MNPs
1219
60
2
20
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10.1002/ejic.201800885
European Journal of Inorganic Chemistry
1
Conclusions
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3
In summary, we have synthesized a bridged organosilica precursor, consisting of an
organic multifunctional linker placed between three silicon atoms with a structure
R[Si(OEt)3]3 by a click chemistry reaction. This organosilica precursor has been used to
prepare a Triazine-Triazol modified mesoporous silica ordered organic-inorganic hybrid
material with wormhole like structure and; additionally, to functionalize by post
synthetic procedures pristine mesoporous silica nanoparticles and magnetic
nanoparticles. The silanization procedure of magnetic nanoparticles is straightforward.
However, the degree of functionalization is lower in comparison to that obtain with
mesoporous silica nanoparticles which influences its lower catalytic performance. The
post synthetic method used to prepare TTS-MSNs-G is developed in two steps in
comparison to the one pot synthesis procedure for the preparation TTS-MSNs-Co.
Nevertheless, TTS-MSNs-G structure guarantees that all the nitrogen containing
functional groups are accessible to the reactants which improves its catalytic activity. In
general; it can be said that these materials, with Triazol-Triazine functionalities, have
demonstrated to be robust and non-air sensitive metal-free organocatalysts, very active
and reusable, in Knoevenagel and multicomponent condensation reactions.
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18 Experimental Section
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30
Reagents and solutions: The chemicals used in this study hexadecyltrimethylammonium
bromide 99% (CTAB), tetraethyl orthosilicate 98% (TEOS), (3-
chloropropyl)triethoxysilane 95% (CPTS), sodium azide (NaN3), tetrabutylammonium
bromide 99% (TBAB), propargyl alcohol 99%, cyanuric chloride (99%) were purchased
from Sigma-Aldrich and used as received without further purification. Sodium hydroxide
97% and hydrochloric acid 35% were purchased from Panreac. Iron(II) sulphate
heptahydrate 99.5% and iron(III) chloride hexahydrate 95% were purchased from
Scharlau. Malononitrile (M = 66.06 g mol-1, d = 1.019 g mL-1), ethyl cyanoacetate (M =
111.13 g mol-1, d = 1.063 g mL-1), benzaldehyde (M = 106.12 g mol-1, d = 1.05 g mL-1) and
5,5-dimethylcyclohexane-1,3-dione (M = 140.18 g mol-1) were purchased from Sigma
Aldrich and used as received. Ethanol was purchased from VWR. Toluene and
chloroform were purchased from Scharlau and VWR respectively, distilled and dried
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European Journal of Inorganic Chemistry
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before use according to conventional literature methods. Water was obtained from a
Millipore Milli-Q system (Waters, USA). Mesoporous silica nanoparticles (spherical)
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3
4
5
(MSNs),[36] [CuBr(PPh3)3],[37] 3-azidopropyltriethoxysilane[38] and 2,4,6-tris(prop-2-
ynyloxy)-1,3,5-triazine[39] were prepared according to reported procedures.
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5
6
Characterization: X-ray diffraction (XRD) patterns of the silica were obtained on a
Phillips Diffractometer model PW3040/00 X’Pert MPD/MRD at 45 KV and 40 mA, using
Cu-K radiation ( = 1.5418 Å). N2 gas adsorption-desorption isotherms were obtained
using a Micromeritics TriStar 3000 analyzer, and pore size distributions were calculated
using the Barret-Joyner-Halenda (BJH) model on the adsorption branch. Infrared spectra
were recorded on a Nicolet-550 FT-IR spectrophotometer (in the region 4000 to 400 cm-
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1) as nujol mulls between polyethylene pellets and KBr disks. C-CP MAS NMR (4.40 μs
900 pulse, spinning speed of 6 MHz, pulse delay 2 s) spectra were recorded on a Varian-
Infinity Plus Spectrometer at 400 MHz operating at 100.52 MHz proton frequency. X-Ray
Fluorescence Spectroscopy (XRF, Philips Analytical) was applied to determine the
titanium and platinum amount of the functionalized materials. Scanning electron
micrographs and morphological analysis were carried out on a XL30 ESEM Philips. The
DRUV-Vis spectroscopic measurements were carried out on a Varian Cary-500
spectrophotometer equipped with an integrating sphere and polytetrafluoroethylene
(PTFE) as reference, with d = 1 g cm-3 and thickness of 6 mm. Elemental analyses were
carried out by the Microanalytical Service of the Universidad Complutense de Madrid.
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Synthesis of 2,4,6-tris((1-(3-(triethoxysilyl)propyl)-1H-1,2,3-triazol-4-yl)methoxy)-
1,3,5-triazine (labelled as TTS): A 10 mL THF solution of 3-azidopropyltriethoxysilane
(3.05 g, 12.3 mmol) was added to a 5mL THF solution of 2,4,6-tris(prop-2-ynyloxy)-1,3,5-
triazine (1.0 g, 4.12 mmol) in the presence of [CuBr(PPh3)3] (1%, 38 mg) as catalyst and
trimethylamine (2 mL, 14.0 mmol) as base. The obtained pale yellow mixture was stirred
for 24 h at room temperature and the solvent then removed in vacuum. The crude
reaction was washed with hexane and extracted with toluene. The orange solution was
filtered to remove any remaining copper catalyst. The clear solution was dried under
vacuum to remove the volatiles obtaining a high boiling point dark orange liquid. Yield:
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85 %.1H NMR (300 MHz, CDCl3, 25 C)  (ppm) = 0.58 (m, 6H, -CH2-Si-), 1.21 (t, 27H, -
Si(OCH2CH3)3), 2.02 (m, 6H, -O-CH2-CH2-CH2-Si-), 3.80 (c, 18H, -Si(OCH2CH3)3), 4.35 (m,
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6H, -O-CH2-CH2-CH2-Si-), 5.58 (s, 6H, C3N3O-CH2-C2N3H2), 7.67 (s, 6H, C2N3H2
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(heterocycle)). C NMR (300 MHz, CDCl3, 25 C)  (ppm) = 0.64 (-CH2-Si-), 17.24 (-
Si(OCH2CH3)3), 23.22 (-O-CH2-CH2-CH2-Si-), 57.48 (-Si(OCH2CH3)3), 51.49 (-O-CH2-CH2-
CH2-Si-), 60.53 (C3N3O-CH2-C2N3H2), 122.82 and 140.94 (C2N3H2 (triazol heterocycle)),
171.14 (C3N3 (triazine)). FTIR (see Figure S1).
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Synthesis of TTS-MSNs-G (Grafting). The MSNs were dehydrated at 150 C in vacuum
for 6 h and thereafter handled under a nitrogen atmosphere. In a typical experiment 1
g of MSNs was suspended in 30 mL of dry toluene and the freshly prepared silane TTS
(2.0 mmol) previously diluted in 20 mL of toluene was added. The mixture was stirred
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for 48 h at 80 C and the resulting light cream solid, labelled as TTS-MSNs-G, was
separated by centrifugation and washed with THF and dichloromethane (3  25 mL). The
solid material was dried under vacuum and stored.
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Synthesis of TTS-MSNs-Co (Co-condensation). TTS-MSNs-Co were synthesized by
modifying a previously described procedure.[40] Cetyltrimethylammonium bromide
(CTAB, 0.2 g, 0.55 mmol) and NaOH (0.7 mL, 2M) were added to 100 mL of ultra-pure
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water. The mixture was stirred for 15 min and then heated to 70 C. When the
temperature was reached, TEOS (1 mL, 0.45 mmol) was added dropwise and,
immediately after, 1 mL solution of ethyl acetate containing the silane TTS (0.45 g, 0.45
mmol) was also added. The mixture was vigorously stirred for 90 additional seconds and
then aged for 4 h. After that the solid was collected and washed with water, ethyl
acetate and methanol. The surfactant was removed by refluxing extraction with
methanol for 48 h. Then the solid was dried at 70 oC for 2 h under vacuum and stored.
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Synthesis of iron oxide magnetic nanoparticles (MNPs)
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400 mL of NaOH (0.5 M) were deoxygenated by bubbling nitrogen for 30 min. FeCl3·6H2O
(1.89 g, 7 mmol) and FeSO4·2H2O (1.94 g, 7 mmol) were dissolved in 30 and 45 mL water,
respectively. The two solutions were mixed and added into the alkaline solution
previously prepared dropwise under vigorous stirring. The reaction was heated to 80 oC
and stirred for 30 min under nitrogen atmosphere. The black solid obtained was washed
with milliQ water (3 × 50 mL) and dried under vacuum.
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Silanization of MNPs with TTS
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The MNPs obtained by co-precipitation (0.25 g) were dispersed in water (50 mL) and an
aqueous solution of the silane compound previously synthesized (2 mmol) was added
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under mechanical stirring. The reaction was stirred and heated at 80 C for 2h and the
stabilized magnetic nanoparticles separated by centrifugation or magnetic decantation
and washed with ethanol. The samples were dried under vacuum and stored.
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Knoevenagel condensation reaction
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In a typical experiment, 11 mmol of benzaldehyde, 11 mmol of ethyl cyanoacetate and
50 mg of catalyst in ethanol (5 mL) were mixed in a 50 mL tube and the mixture heated
at 60 oC for 120 min. After reaction completion, 10 mL of dichloromethane were added
to dilute and solubilize the organic compounds. The solid catalyst was precipitated by
centrifugation or magnetic decantation and separated from the liquid phase. To analyze
the sample by gas chromatography, 1mL of sample was withdrawn and diluted in
ethanol to 5 mL and a fixed amount of dodecane as an internal standard. The sample
was analyzed by GC-FID (Agilent 6890N, DB-Wax capillary column 30 m, 0.53 mm). In
addition, the corresponding compounds obtained after C-C condensations reactions was
recrystallized from dichloromethane or by simple solvent evaporation and checked by
1H NMR (Figure S2). The separated catalysts were reused without further treatment
several times under the experimental conditions previously described.
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Multicomponent condensation reaction
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In a typical experiment, 11 mmol of benzaldehyde, 11 mmol of malononitrile, 11 mmol
of 5,5-dimethylcyclohexane-1,3-dione and 50 mg of catalyst in ethanol (10 mL) were
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mixed in a 50 mL tube and the mixture heated at 60 C for 120 min. After reaction
completion, 20 mL of acetone were added to solubilize the organic compounds. The
solid catalyst was precipitated by centrifugation or magnetic decantation and separated
from the liquid phase. The 2-amino-3-cyano-4H-pyran derivatives obtained after one
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pot condensations reaction was recrystallized from acetone and checked by H NMR
(Figure S3 and Figure S4). The separated catalysts were reused without further
treatment several times under the experimental conditions previously described.
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Acknowledgements
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We gratefully acknowledge financial support from the former Ministerio de Economía y
Competitividad Spain – FEDER (Grant no. CTQ2015-66164-R) and (CTQ2017-90802-
REDT). We would also like to thank Universidad Rey Juan Carlos and Banco de Santander
for supporting our Research Group of Excellence QUINANOAP.
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Keywords: Knoevenagel condensation reactions, magnetic nanoparticles,
mesoporous silica nanoparticles, multicomponent condensation reactions, organic-
inorganic hybrid materials
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