Tantalum vs Niobium MCF nanocatalysts in the green synthesis of chromene derivatives

Article abstract of DOI:10.1016/j.cattod.2018.06.038

TaMCF silicas modified with alkaline metals can be considered a novel family of highly efficient bifunctional catalysts involved in the synthesis of chromene derivatives, from salicylaldehyde 2 and acetonitrile compounds, under mild conditions, showing enhanced catalytic performance than their NbMCF analogues. The observed reactivity was mainly attributed to the higher basicity of the Me/TaMCF but also the texture of the samples. The Me/TaMCF silicas showed higher Br?nsted basicity than the Nb ones as indicated by the stronger interaction between alkali metals and Ta in the UV–vis and the test reaction experiments. On the other hand, the basicity of Me/TaMCF together the reactivity degree and steric hindrance of the starting acetonitriles are key factors influencing the reaction selectivity. In conclusion, the basicity of the samples plays an important role initiating the reaction by activation of nucleophile but also a compromise between alkaline cation size and basicity is required.

Full text of DOI:10.1016/j.cattod.2018.06.038

Accepted Manuscript
Title: Tantalum vs Niobium MCF nanocatalysts in the green
synthesis of chromene derivatives
´
´
Authors: Agata Smuszkiewicz, Jesus Lopez-Sanz, Izabela
´
Sobczak, Rosa M. Martın-Aranda, Maria Ziolek, Elena
´
Perez-Mayoral
PII:
S0920-5861(18)30776-4
DOI:
Reference:
https://doi.org/10.1016/j.cattod.2018.06.038
CATTOD 11527
To appear in:
Catalysis Today
Received date:
Revised date:
Accepted date:
15-3-2018
11-6-2018
18-6-2018
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´
Please cite this article as: Smuszkiewicz A, Lopez-Sanz J, Sobczak I, Martın-
´
Aranda RM, Ziolek M, Perez-Mayoral E, Tantalum vs Niobium MCF nanocatalysts
in the green synthesis of chromene derivatives, Catalysis Today (2018),
https://doi.org/10.1016/j.cattod.2018.06.038
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Tantalum vs Niobium MCF nanocatalysts in the green synthesis of chromene derivatives
Agata Smuszkiewicz1, Jesús López-Sanz1,2, Izabela Sobczak2, Rosa M. Martín-Aranda1, Maria
Ziolek2*, Elena Pérez-Mayoral1*
1Departamento de Química Inorgánica y Química Técnica, Universidad Nacional de Educación a
Distancia, UNED, Paseo Senda del Rey 9, E-28040 Madrid, Spain
2Adam Mickiewicz University, Faculty of Chemistry, Umultowska 89b, 61-614 Poznan, Poland
Corresponding authors:
Elena Pérez Mayoral; eperez@ccia.uned.es
Phone (+34) 91 398 9047; Fax (+34) 91 398 6697
Graphical abstract

Highlights





Nb and Ta show shared properties but different interaction with alkaline metals in Me/TMCF.
Me/TaMCF samples showed higher Brönsted basicity than the Me/Nb analogues.
Me/TaMCF are highly efficient bifunctional catalysts in the synthesis of chromenes.
Basicity of the Me/TMCF samples are behind of the catalytic behaviour.
A compromise between basicity and alkaline cation size influences the catalytic performance.
Abstract
TaMCF silicas modified with alkaline metals can be considered a novel family of highly efficient
bifunctional catalysts involved in the synthesis of chromene derivatives, from salicylaldehyde 2 and
acetonitrile compounds, under mild conditions, showing enhanced catalytic performance than their
NbMCF analogues. The observed reactivity was mainly attributed to the higher basicity of the
Me/TaMCF but also the texture of the samples. The Me/TaMCF silicas showed higher Brønsted
basicity than the Nb ones as indicated by the stronger interaction between alkali metals and Ta in
the UV-Vis and the test reaction experiments.
On the other hand, the basicity of Me/TaMCF together the reactivity degree and steric hindrance of
the starting acetonitriles are key factors influencing the reaction selectivity. In conclusion, the
basicity of the samples plays an important role initiating the reaction by activation of nucleophile
but also a compromise between alkaline cation size and basicity is required.
Keywords: Nb and Ta mesoporous silicas, heterogeneous catalysis, multicomponent reactions,
chromene derivatives
1. Introduction
Niobium and tantalum are transition metals belonging to the group V almost always paired
together in nature and showing shared physico-chemical properties. Both elements as their
oxide forms are recognized as catalyst supports or active catalysts, even combined with other
metals, exhibiting interesting catalytic behavior in relevant organic transformations [1-5] and
oxidative processes [6-8].

In the last decades, the development of Nb-containing molecular sieves has undergone a
significant progress [9-10]. Such materials have been widely investigated in the synthesis of
valuable products because of their catalytic properties. Our research groups both in
cooperation have recently contributed to this progress reporting new families of aminopropyl
organosilicas functionalized with transition metals centers, especially niobium, involved in the
synthesis of N-alkylimidazoles under sonochemical conditions [11], Knoevenagel condensation
[12], C=C isomerization reaction of safrole for the synthesis of interesting compounds useful in
perfume manufacture [13] and the preparation of relevant nitrogen heterocyclic systems such
as substituted quinolines and naphtiridines through the Friedländer reaction [14-15]. All of
them can be considered bifunctional catalysts in which both functionalities can act in
cooperation producing an enhancement of the overall reaction yields. The presence of Nb in
the silica framework is responsible of an increasing of the basicity of the amine functions but
also the metal center is involved in the reaction, probably by stabilizing the corresponding
transition structures, as demonstrated by computational methods.
There are also some reports comprising the use of tantalum mesoporous silica catalysts
although as a crushing minority. For instance, silica-supported alkoxo-tantalum complexes
have been used in the deperoxidation of cyclohexyl hydroperoxide [16], Ta/SBA15 for the
olefin epoxidation by using a benign oxidant such as H2O2 aqueous solution [17] or even chiral
ligand modifying Ta/MCM-41 catalysts implied in the enantioselective oxidation of thioanisole
[18]. Ziolek and co-workers have also participated on increasing the knowledge to stablish
differences on the catalytic behavior of Nb or Ta-containing mesoporous silicas by using those
in several organic transformations, highlighting the production of biofuels additives on
sulphonated MCF materials modified with Nb and Ta [19] and acetalation of glycerol [20].
Interestingly, basicity can be also introduced on Nb doped mesoporous silicas by modifying
within alkaline cations. In this sense, we also recently reported a new series of NbMCF

mesoporous silicas modified with alkali metals useful as bifunctional catalysts yielding 2-
amino-4H-chromenes in good to excellent yields, heterocyclic scaffolds of importance because
of their therapeutic uses (Scheme 1) [21]. Our theoretical results indicated that the presence
of alkaline oxides on Me/NbMCF catalysts (where M is Li, Na, K, Rb and Cs), showing strong
basicity, activates the nucleophile initiating the reaction, whereas the Nb center is implied on
the approaching of the reactants also stabilizing the aldolization transition structure.
Scheme 1. Synthesis of 2-amino-4H-chromenes over NbMCF silicas modified with alkaline
metals.
Following our ongoing investigations and considering that Ta nanocatalysts have barely been
explored in the synthesis of valuable compounds, the goal of this paper is aimed to stablish
differences on the catalytic behavior between comparable series of TMCF mesoporous silicas
(where T is Nb or Ta), modified with alkaline metals, in the multicomponent synthesis of 2-
amino-4H-chromenes 1 (Scheme 1). Having in mind our previous results, it would be
reasonable to expect cooperative effects between Ta and alkaline metals but also probably
varying the appropriate basicity in comparison to the catalytic performance for the NbMCF
series.
2. Materials and Methods
2.1. Synthesis of NbMCF and TaMCF mesoporous silicas

MCF materials were prepared by one-pot synthesis method described in Ref. [22]. For the
preparation of NbMCF and TaMCF, the synthesis procedure was modified in order to introduce
niobium and tantalum sources, niobium ethoxide and tantalum ethoxide, respectively. The
source of Nb or Ta was added to the mixture 10 min after TEOS admission (TEOS/Nb and
TEOS/Ta molar ratio = 64). Moreover, 1,3,5trimethylbenzene (TMB) / Pluronic ratio = 0.75 was
applied (8 g, 1.4 mmol of Pluronic and 6 g, 49.92 mmol of TMB).
2.2. Modification of NbMCF and TaMCF materials with alkali metals
NbMCF and TaMCF supports were impregnated by using the incipient wetness method with
aqueous solution of MeCH3COO (Me = Li, Na, K, Rb, Cs). The outgassed NbMCF and TaMCF
were filled in with the appropriate amount of an aqueous solution of alkali metal acetate
(CH3COOLi, CH3COONa, CH3COOK, CH3COOR, CH3COOCs, a volume of the solution was a little
bit higher than the pore volume of the support) and rotated and heated in an evaporator flask
at 373 K. The impregnated powder was dried at 373 K for 18 h and then calcined at 723 K for
10 h. Two series of catalysts were obtained: Me/NbMCF and Me/TaMCF, where Me = alkali
metal. The amount of alkali metal acetate used for the impregnation was calculated for the
loading of metal equal to 0.5 mmol per 1 g of support. Such value gives rise to the following
weight percent of metals: Li-0.35%; Na-1.15%; K-1.96%; Rb-4.27; Cs-6.65%. The surface area of
the modified materials differs from that of the parent one and therefore the mole number of
alkali metal cations calculated per m2g−1 of the supports was calculated and shown in Table 1.
2.3. Characterization of the samples
The materials prepared were characterized using XRF, N2 adsorption/desorption, UV–vis and
test reactions.

X-ray fluorescence (XRF) was applied using MiniPal-Philips. The measurements were done
using calibration curves prepared from mixtures of silica and Nb2O5 and silica and Ta2O5 (Si/Nb
and Si/Ta from 5 to 300).
The N2 adsorption/desorption isotherms at 77 K on materials studied were recorded using a
static volumetric apparatus ASAP 2020 (Micromeritics). Before each sorption measurement
the samples were outgassed at 383–393 K overnight until the residual pressure was lower than
0.7 Pa.
UV–vis spectra were recorded using a Varian-Cary 300 Scan UV–visible Spectrophotometer.
Sample powders were placed into the cell equipped with a quartz window. The spectra were
recorded in the range from 800 to 190 nm. Spectralon was used as a reference material.
2,5-Hexanedione cyclisation and dehydration was performed using a tubular, down-flow
reactor that was carried out at atmospheric pressure, with nitrogen as the carrier gas. 0.05 g of
catalyst was activated for 2 h at 673 K under nitrogen flow (40 cm3 min−1). Subsequently, 0.5
cm3 of 2.5-hexanedione was passed continuously over the catalyst at 623 K. The substrate was
delivered with a pump system and vaporized before being passed through the catalyst with
the flow of nitrogen carrier gas (40 cm3 min−1). The reaction products were collected for 30 min
downstream of the reactor in the cold trap (mixture of 2-propanoland liquid nitrogen) and
analyzed by gas chromatography (CHROM-5, Silicone SE-30/Chromosorb G column).
2.4. Catalytic performance
The reactions were performed in a liquid phase under atmospheric pressure in a multi-
experiment work station StarFish (Radley’s Discovery Technologies UK). In a typical procedure,

2-hydroxybenzaldehyde (2) (2 mmol), and ethyl cyanoacetate (3) (4 mmol) were added to the
3-necked vessel of 10 mL of capacity, equipped with condenser and thermometer and
magnetic stirred (0.8 cm). Subsequently, the catalyst (50 mg) was added and the reaction
mixture was stirred (250 rpm) at room temperature, 303 K, during the appropriated reaction
time shown in figures or tables. The samples were taken at different reaction times by diluting
a small amount of the reaction mixture in dichloromethane (0.5 mL). Subsequently, the
catalyst was filtered off and the solvent evaporated in vacuo.
Activation of the catalyst samples was carried out at 333 K overnight. The reactions were
followed by TLC chromatography performed on DC-Aulofolien/Kieselgel 60 F245 (Merck) using
mixtures of CH2Cl2/ EtOH in 98:2 as eluent. The characterization of the reaction products was
carried out by 1H NMR. NMR spectra were recorded with a Bruker AVANCE DPX-300 (300 MHz
for1H). 1H chemical shifts () in CDCl3 or DMSO are given from internal tetramethylsilane. Mass
spectral data (ESI-MS) was performed with a mass spectrometer MAXIS II (Bruker) equipped
with electrospray ionization (ESI) interface operating in positive ion mode.
1H NMR spectra of chromenes 1 [30], 7 [31] and 10 [31] are in good agreement with those
previously reported.
All reagents and solvents were purchased from Aldrich, Fluka and Alfa-Aesar.
3. Results and discussion
3.1. Structure and Texture characterization of TaMCF materials
The mesoporous structure of TaMCF samples prepared was confirmed by the N2
adsorption/desorption isotherms (Figure 1). The isotherm of TaMCF is typical of
mesostructured cellular foams exhibiting a H1 type hysteresis loop at a relatively high pressure
(p/p0) with parallel adsorption and desorption branches [22]. This type of isotherm is slightly
distorted by modification of TaMCF with alkali metal sources. Similar properties were observed

for niobium containing series [21]. Textural parameters of the samples studied calculated from
N2 adsorption/desorption isotherms are shown in Table 1. Both TMCF supports show large
surface areas (500 - 700 m2/g). These values significantly decrease for the samples modified
with alkali metals, the surface area decreasing with the growth of the alkali metal loading. Pore
volume is changed in the same order.
Figure 1. Nitrogen adsorption/desorption isotherms for TaMCF (blue), Li/TaMCF (red) and
Na/TaMCF (green).
Table 1. Textural parameters and metal loading for M/TMCF materials
b
Catalyst
SBET
Vp [cm3g-1]
Metal Loading
BdB FHH
[mol/m2g-1·10-6]
adsorption
TaMCF
556
301
251
228
257
211
717
331
341
303
250
2.5
1.2
1.2
1.0
1.0
0.9
2.8
1.6
1.5
1.7
1.3
--
Li/TaMCF
Na/TaMCF
K/TaMCF
Rb/TaMCF
Cs/TaMCF
NbMCFa
1.66
1.99
2.19
1.94
2.37
--
1.51
1.47
1.65
2.00
Li/NbMCFa
Na/NbMCFa
K/NbMCFa
Rb/NbMCFa

Cs/NbMCFa
a See ref 21. b Vp = pore volume.
234
1.4
2.14
3.2. The loading and location of Nb and Ta in MCF materials
The loading of niobium and tantalum was estimated on the basis of XRF data and is shown in
the form of Si/Nb and Si/Ta ratios and metal wt. % (Table 2).
Table 2. Nb and Ta loading in MCF materials.
In both materials (NbMCF and TaMCF) the efficiency of metal incorporation in the framework
of MCF
is
Sample
NbMCF
TaMCF
Si/Nb/Ta
as assumed
64
Si/Nb
(XRF)
169
Nb, wt. %
(XRF)
Si/Ta
(XRF)
-
Ta, wt. %
(XRF)
0.8
-
lower
than
64
-
-
98
2.9
assumed. However, it is clear that much higher efficiency of tantalum inclusion than niobium
was achieved. It is in agreement with literature data, which proved that the introduction of
niobium into the framework of molecular sieves is more difficult than tantalum and typically
gives materials with low niobium concentration [23, 24].
The location of niobium and tantalum introduced during the synthesis of MCF material was
studied by UV–vis spectroscopy by identification of metal coordination. The UV-vis spectra of
NbMCF support and samples after modification with alkali metals (Figure 2) show the presence
of two UV bands at ca 225 and 260 nm. The first band is typical of charge transfer in
tetrahedrally coordinated niobium species located in a silica skeleton [25, 26]. The second
should be assigned to penta - coordinated niobium species located in the silica framework as

was proved in [27]. It is worth of notice, that the band corresponding to octahedrally
coordinated polynuclear niobium species (at 330 nm) was not detected in UV-vis spectra of
NbMCF.
3,0
NbMCF: 226&268 nm
Li/NbMCF: 227&257 nm
Na/NbMCF: 227&259 nm
K/NbMCF: 230&260 nm
Rb/NbMCF: 229&263 nm
Cs/NbMCF: 231&262 nm
TaMCF:
224 nm
1,4
1,2
1,0
0,8
0,6
0,4
0,2
0,0
Li/TaMCF: 228 nm
Na/TaMCF: 232 nm
K/TaMCF: 234 nm
Rb/TaMCF: 235 nm
Cs/TaMCF: 237 nm
2,5
2,0
1,5
1,0
0,5
0,0
200
300
400
500
600
200
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Figure 2 UV-vis spectra of NbMCF (A) [21] and TaMCF (B) materials before and after
modification with alkali metals.
In contrast, the UV-vis spectra of TaMCF samples show one symmetric band at 224 nm which is
typical of tretra-coordinated Ta species. These results indicate the location of niobium and
tantalum in the MCF framework. Some variation in the position of the bands in UV–vis spectra
of NbMCF and TaMCF materials after modification indicates the interaction of alkali metal
species with niobium or tantalum in the supports. This interaction is higher for the series of
tantalum containing materials as deduced from the value of red shift of the bands at ca. 225
nm (13 nm for Cs/TaMCF vs 5 nm for Cs/NbMCF).
3.3. Acid-base properties of catalysts—test reaction

The cyclisation of 2,5-hexanedione has been used as Brønsted acid–base test [28]. The
formation of 2,5-dimethylfuran (DMF) occurs on Brønsted acidic centers, whereas in the
production of 3-methyl-2-cyclopentenone (MCP) Brønsted basic centers take part. On the basis
of the ratio of selectivity to MCP/selectivity to DMF, the sequence of the basicity of the
prepared catalysts can be determined. According to literature the basicity of the catalyst is
stated if MCP/DMF >1 [28-29]. When MCP/DMF < 1 the catalyst exhibits acidic properties,
while for MCP/DMF ~1 the acid–base character of catalysts is postulated.
Table 3 summarizes the results of this test reaction. The conversion of 2,5-hexanedione
depends strongly on the composition of the support. The activity is higher for NbMCF than
TaMCF, both materials exhibiting acidic properties (MCP/DMF < 1).
Table 3: The results of 2,5-hexanedione cyclisation at 623 K.
Catalyst
MCP/DMF
Conversion (%)
Selectivity (%)
MCP
DMF
NbMCF
0.2
58
49
90
332
499
72
55
52
87
61
68
84
1.7
2.0
1.1
0.3
0.2
16.0
98.3
98.0
98.9
99.7
99.8
Li/ NbMCF
Na/ NbMCF
K/ NbMCF
Rb/ NbMCF
Cs/ NbMCF
TaMCF
0.2
99
199
499
499
499
44
71
82
71
54
67
82
1.0
0.5
0.2
0.2
0.2
18.0
99.0
99.5
99.8
99.8
99.8
Li/ TaMCF
Na/ TaMCF
K/ TaMCF
Rb/ TaMCF
Cs/ TaMCF
After alkali metal loading the conversion of 2,5-hexanedione slightly decreases on NbMCF with
the exception of K/NbMCF (the maximum of activity). For TaMCF modified samples the
conversion increases reaching the maximum for Na/TaMCF sample. The introduction of alkali
metals leads to the significant increase in the selectivity to MCP (98-99.8%) for all alkali

modified catalysts studied. The results indicates that impregnation with alkali metal sources
generates Brønsted basic centers. Basicity, expressed by MCP/DMF ratio, is higher for the
series of tantalum modified support. This observed behavior point out the role of interaction
between alkali metal and transition metal in the support.
3.4. Catalytic performance
The mesoporous silicas under study were explored in the synthesis of chromene derivatives 1
(Scheme 1), at 303K, under solvent-free conditions. Remarkably, the T/MCF supports were
inactive in the reaction obtaining the unaltered reactant mixture after 4h of reaction time. The
Figure 4 depicts the conversion values of salycilaldehyde 2 in the presence of Me/TMCF
catalysts. A rapid look at the Figure 3 confirms the analogous reactivity trends of the Me/TMCF
catalysts in the reaction as function of the basicity of the samples, in such a manner that the
conversion values notably diminished when increasing the basicity of the samples. In this
sense, the reactivity order for the T/NbMCF silicas is as follow: Li/NbMCF > Na/NbMCF >
K/NbMCF > Rb/NbMCF > Cs/NbMCF [21]. Although this order is also maintained in the case of
using T/TaMCF catalysts, it was observed similar catalytic behavior from Na/TaMCF to
Rb/TaMCF samples leading to intermedia conversion values, probably attributed to the texture
of the samples. Although both series Me/NbMCF and Me/TaMCF showed slight differences
concerning the pore volume, it could be though that the SBET variation can influence the
catalytic behavior of the samples. In this sense, while for the Me/NbMCF serie it was observed
an increase of metal loading but also the decreasing of SBET from Li to Cs samples, the
Me/TaMCF catalysts, where Me is Na, K and Rb, showed similar SBET and Vp and metal loadings
affording the corresponding chromenes 1 with barely different conversion values probably.
However, considerable decreasing of the conversion was obtained for the most basic samples,
Cs/TaMCF, as expected.

Figure 3. Synthesis of 2-amino-4H-chromenes 1 catalyzed by M/TMCF mesoporous silicas, at
303K, under solvent-free conditions.
Comparing both series of catalysts, the highest conversion values were observed for the
M/TaMCF samples, this circumstance being clearly visualized from Figure 4. Our results
demonstrate that the conversion decreased with the basicity of the samples, the alkaline
cation size having a deep impact on the catalytic performance. These results are in accordance
with those previously postulated by us for the NbMCF catalysts. Thus, the reactive specie able
to initiate the reaction should comprise the enolate from ethyl cyanoacetate 3, formed by
abstraction of the proton at -position, involving the alkaline oxides as highly exothermic step
which proceeds without activation barrier [21]. The computational study concerning the
NbMCF catalysts strongly suggested the involvement of Nb in the reaction by stabilizing the

transition structures favoring the reactant interactions. In the same context, our experimental
observations indicate that the presence of Ta in the silica framework clearly influences the
catalytic performance suggesting its participation in the reaction.
It is important to note, that the reaction regioselectivity is maintained for all the investigated
catalysts leading to diastereomeric mixtures of chromenes 1a/1b, selectivity to 1a as the most
thermodynamically stable isomer being up to 60% (Figure 5). Finally, we also carried out
reused experiments using the most efficient catalyst, Li/TaMCF, in the synthesis of chromenes
1 almost maintaining its activity during at least three consecutive cycles (96, 93, 90%; 3h).
Figure 4. Synthesis of 2-amino-4H-chromenes 1 catalyzed by Me/TMCF mesoporous silicas, at
303K, under solvent-free conditions, after 1h of reaction time.

Figure 5. Synthesis of 2-amino-4H-chromenes 1 catalyzed by Me/TMCF mesoporous silicas, at
303K, under solvent-free conditions, after 1h of reaction time.
Considering the above commented results, we decide to investigate some modified TaMCF
catalysts in the reaction when using other cyano components, more and less reactive than
ethyl acetate 3 (Scheme 2).
Scheme 2. Synthesis of 2-amino-4H-chromenes over TMCF silicas modified with alkaline
metals.
Thus, we carried out the reaction using malononitrile 4, operating under the same
experimental conditions, in the presence of Me/TaMCF catalysts (where Me = Li, K and Cs),
providing compound 10 as major reaction product, resulting of the nucleophilic addition of
third malononitrile molecule to the initially formed chromene 7 (Scheme 3) [31]. Considering

these results, we try the reaction in the presence of increased amounts of malononitrile
(reactant ratio 2/4 = 2:6). Changes in the reaction mixture were detected almost
1
instantaneously. At this regard, an analysis of the H NMR spectra of the reaction crude
confirmed the presence of the intermediate compounds 7 and 11, also demonstrated by MS
spectrometry (Scheme 3).
Scheme 3. Reaction between salicylaldehyde 2 and malononitrile 4 catalyzed by Me/TMCF
mesoporous silicas (Me = Li, K, Cs), at 303K, under solvent-free conditions.
As can be seen in Figure 6, the reaction in the presence of Me/TaMCF catalysts takes
place very quickly providing high conversion values close to 80% in only 30 min of reaction
time in the presence of the Li/TaMCF and K/TaMCF samples. The most basic catalyst,
Cs/TaMCF, afforded lower conversion values as compared with the Li or K analogues as
expected. It is noticeable that while the reaction promoted by Li/TaMCF catalyst afforded the
chromene 10 as unique reaction product, selectivity to 10 was diminished when the reaction
was carried out in the presence of K/TaMCF (91%) or Cs/TaMCF (87%) catalysts, after 30 min of
reaction time. In these cases, the basicity of the samples could be also related with the
reaction selectivity. It is important to note that although malononitrile is the most reactive
cyano compound explored herein, the blank experiment yielded the corresponding chromene
10 in almost 5% after 2h of reaction time.

Li/TaMCF
K/TaMCF
Cs/TaMCF
100
80
60
40
20
0
10
20
30
Time (min)
Figure 6. Conversion values for the reaction between salicylaldehyde 2 and malononitrile 4
catalyzed by Me/TMCF mesoporous silicas (Me = Li, K, Cs), at 303K, under solvent-free
conditions.
Finally, we tested the most efficient catalysts in the reaction between salycilaldehyde 2
and two different sulfonyl acetonitriles 5 and 6, in these cases in the presence of
tetrahydrofuran as solvent, leading to the corresponding reaction products although with
decreases conversion values. Remarkably, the blank experiments when using ethyl
cyanoacetate 3 but also sulfonyl acetonitriles 5 and 6 afforded mixtures of unaltered reactants
after 4h of reaction times. More specifically, in Figure 8 it is represented the conversion values
vs time when using tert-butyl sulfonylacetonitrile 6 in the presence of Li/TaMCF and K/TaMCF
exclusively yielding the corresponding erythro-chromene 9 with decreased conversion values
(Scheme 2). In both cases, basicity but also the porosity of the materials and the presence of
tert-butylsulphonyl group as bulky substituent in compound 6 seem strongly to limit the
formation of the less stable isomer, the threo-isomer. These results are in accordance with
those obtained in the reaction between salyciladehyde 2 and methyl sulfonyl acetonitrile 5
catalyzed by K/TaMCF silica leading to a mixture of Erythro/Threo isomers (6:1 ratio) of
chromene 8 in 15% of yield after 4h of reaction time.

Figure 7. Synthesis of compound 9 catalyzed by Li/TaMCF and K/TaMCF catalysts.
4. Conclusions
We report herein for the first time a new family of Ta/MCF silicas modified with alkaline metals
as highly efficient bifunctional catalysts for the synthesis of chromene derivatives, as
pharmacologically interesting compounds, from salicylaldehyde 2 and distinct acetonitrile
compounds, under mild reaction and solvent-free conditions. An enhancement of the catalytic
performance of these TaMCF samples was observed in comparison with those for Nb ones,
conversion values increasing from the Cs to Li modified TaMCF samples with maintained
selectivity. Our experimental observations suggest an optimum basicity of the samples, being
the less basic sample considered the most active catalyst, Li/TaMCF. It is reasonable to think
that this notably augmented catalytic performance for Me/TaMCF silicas could be attributed to
the enhanced Brønsted basicity provided by the stronger interaction between alkali metals and
Ta than between Me-Nb as demonstrated by test reaction and UV-vis experiments.
The basicity of Me/TaMCF catalysts and the reactivity and steric hindrance of the starting
acetonitriles are determinant factors which affect the reaction selectivity. However, it was
found that the basicity plays an important role initiating the reaction but also a compromise
between alkaline cation size and basicity is required.

5. Acknowledgements
This work has been supported in part by Spanish Ministry (projects CTM2014-56668-R) and
National Science Centre in Poland (Project No. 2014/15/B/ST5/00167).
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