Polyoxoniobates as a superior Lewis base efficiently catalyzed Knoevenagel condensation

Article abstract of DOI:10.1016/j.mcat.2018.05.002

The outstanding basicity of negative charged Lindqvist type Polyoxoniobate K₇HNb₆O₁₉·13H₂O (Nb₆) have been proved experimentally as well as by theoretical NBO calculations, the results insights high electron density on terminal and bridging oxygen atoms of niobate anion. The most negative Natural Bond Orbital charge (NBO) of oxygen in Nb₆ is ?1.001, which is a much more negative value than those reported in other polyoxometalates, that corroborates its high basicity thus likely to be employed as a strong base catalyst. Experimental study suggests that Nb₆ can efficiently catalyze Knoevenagel condensation of various carbonyl compounds with active methylene compounds neglecting the steric and electronic effect of aromatic aldehydes under mild conditions. Kinetic test shows that Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate exhibits second-order kinetics in the presence of Nb₆ and the calculated activation energy is 43.3 kJ mol^?1. Meanwhile, a proper mechanism according to the NBO study speculates that the most negative charged terminal oxygens in Nb₆ would be pivotal in this transformation.

Full text of DOI:10.1016/j.mcat.2018.05.002

Molecular Catalysis 453 (2018) 93–99
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Polyoxoniobates as a superior Lewis base efficiently catalyzed Knoevenagel
condensation
⁎
Qiaofei Xu, Yanjun Niu, Guan Wang, Yingguang Li, Yuan Zhao, Vikram Singh, Jingyang Niu ,
⁎
Jingping Wang
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecule and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University,
Kaifeng, Henan 475004, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
7 6 2 6
The outstanding basicity of negative charged Lindqvist type Polyoxoniobate K HNb O19·13H O (Nb ) have been
Polyoxoniobates
Base catalysis
Knoevenagel condensation
Natural population analysis
proved experimentally as well as by theoretical NBO calculations, the results insights high electron density on
terminal and bridging oxygen atoms of niobate anion. The most negative Natural Bond Orbital charge (NBO) of
oxygen in Nb
that corroborates its high basicity thus likely to be employed as a strong base catalyst. Experimental study
suggests that Nb can efficiently catalyze Knoevenagel condensation of various carbonyl compounds with active
6
is −1.001, which is a much more negative value than those reported in other polyoxometalates,
6
methylene compounds neglecting the steric and electronic effect of aromatic aldehydes under mild conditions.
Kinetic test shows that Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate exhibits second-
−1
6
order kinetics in the presence of Nb and the calculated activation energy is 43.3 kJ mol . Meanwhile, a proper
mechanism according to the NBO study speculates that the most negative charged terminal oxygens in Nb
would be pivotal in this transformation.
6
V
V
V
1
. Introduction
MoVI, V , Nb , Ta , etc., and x = 4–7), thus leading to provides novel
building units for the assembly of larger systems [15–17]. Moreover,
POMs have exhibited remarkable performance in many fields, such as
fluorescence [18], magnetism [19], drugs [20], electrochemistry [21],
and especially in catalysis [22–25]. To date, many applications invol-
Olefination is one of the most fundamental reactions where the
products exert enormous importance on organic synthesis [1]. As a kind
of C]C bonds-forming reactions, Knoevenagel condensation reaction is
a crucial method to produce olefins using active methylene group re-
activity with diverse functional groups via the coupling reaction of
carbonyl compounds in the presence of base catalysts along with the
V
VI
VI
ving POMs incorporated V , Mo , and W were investigated in various
types of catalytic reactions including acid catalysis [26–29], oxidation
catalysis [30–34], photocatalysis [35–37] and electro-catalysis [38–41]
mainly due to their redox tunable behavior, low pH stability, high
thermal resistivity and photoelectric behaviors [42]. However, there
were few reports which deals on the basic catalytic performance of
POMs [43–45]. In general, it is well known that POMs possess strong
acidity thus the weak basicity of their conjugate base restricts to sup-
port them as outstanding base catalysts. Actually, outstanding works on
Knoevenagel condensation reaction have shown that polyoxotungstates
(POTs) exhibited fascinating performance in this process. Mizuno et al.
firstly reported the superior catalytic reactivity of alkaline dis-
2
loss of molecular H O in the suitable reaction conditions. This reaction
is of prime importance in many significant areas of research like
synthesis of various reaction intermediates, products for perfumes,
calcium antagonists, polymers and in pharmaceutical industries [2,3].
Traditionally, the solid bases [4,5], Lewis acids [6,7], and organome-
tallic compounds [8,9] were applied as catalysts in the process, while
they are lack in their efficiency, robustness, and recyclability [10].
Therefore, considerable research efforts have been focused on this re-
action by using various catalysts, such as metal-organic frameworks
7
−
[
11,12], ionic liquid [13], molecular complexes [14] and etc.
ilicoicosatungstates [H(γ-SiW10
O
32
)
2
(μ-O)
for Knoevenagel condensation reaction in
the similar reaction conditions[46]. Subsequently, Mizuno and cow-
4
]
than the acidic di [{γ-
4
−
Polyoxometalates (POMs) as a class of anionic metal-oxide clusters
SiW10
O
32(H
2
O)
2
}
2
2
(μ-O) ]
mainly obtained by the condensation reaction of corresponding oxysalts
or metallic oxides. Structurally, POMs are constructed by the corner-,
edge-, or face-shared fashion of metal oxide polyhedral (MO
6−
orkers revealed that [γ-H
catalyst for Knoevenagel condensation [47]. Later, Song et al. also
2
GeW10
O
36
]
can act as an efficient base
VI
x
: M = W ,
⁎
Corresponding authors.
E-mail addresses: jyniu@henu.edu.cn (J. Niu), jpwang@henu.edu.cn (J. Wang).
https://doi.org/10.1016/j.mcat.2018.05.002
Received 9 March 2018; Received in revised form 23 April 2018; Accepted 3 May 2018
2468-8231/ © 2018 Published by Elsevier B.V.

Q. Xu et al.
Molecular Catalysis 453 (2018) 93–99
explored highly charged tri-lacunary polyanoins i.e. Na
8
H[A-PW
9
O
34
]
the NBO charges of oxygen atoms in Nb
6
are even more negative than
2−
and Na
8
H[B-PW
9
O
34] as catalysts for cyanosilylation and Knoevenagel
the one (−0.934) in WO
4
, which imply that the superior basicity of
condensation reactions, respectively [48]. Beside these, it is well
documented that POTs supported on graphene oxide [49] and LDH
Nb is likely to be used as the base catalyst.
6
(
Layered double hydroxides) [50] also show excellent performance as
the catalyst for Knoevenagel condensation. However, polyoxoniobates
PONs) as an important subclass of POMs have been investigated re-
2.2. Catalysis
6
Considering the splendid basicity of Nb has, it was employed as a
(
latively in synthetic chemistry but it is seldom explored in catalysis.
Hence, it is significant to open a new way for their applications in
catalytic direction.
base catalyst for Knoevenagel condensation reaction. Initially, the
condensation reaction of benzaldehyde with ethyl cyanoacetate was
carried out with different catalysts. The results of parallel experiments
have been presented in Table 1, which have shown that there was little
reactivity of this transformation in the absence of catalyst (entry 1).
However, there was a great improvement in yield of product with
NaOH loading in this system (entry 2), which has implied that the
process benefited from basicity. Therefore, the lacunary POTs (entries
5, 7,11, 12) are more active than the plenary ones (entries 8, 9, 10, 13).
According to literatures, more electronegative oxygen atoms pro-
vide higher basicity in POMs, which could be approximately deduced
by comparison of their “charge-density” as: Charge-density (charge/
atom) = (anionic charge of the POM)/(number of non-hydrogen atoms
of the POM) [51,52]. From the charge-density of conventional PONs
8
−
8−
6−
and POTs: [Nb
6
O
19
]
(0.32), [Ti
2
Nb
[(PO
19]2− (0.08), [SiW12
8
O
28
]
28
(0.21), [Nb10O ]
15−
(
[
0.16), [SiNb12
O
40]16−
(0.18), [W
(0.16), [PW
(0.30),
2
)
3
PNb
9
O
34
]
(0.28),
40]4− (0.075),
(0.20), it is obvious that PONs
In addition, precursors of Nb , Nb
6
activity for the reaction. Strikingly, Nb has exhibited the best catalytic
6
(Nb
2
O
5
2 5 2
O ·xH O) have no obvious
1
4−
H
2
Si
4
Nb16
O
56
]
6
O
O
8
−
9−
[
SiW11O
39
]
9
O
34
]
performance for the reaction at the same conditions, which is in good
consistent with its splendid basicity.
possess higher charge-density than POTs. However, to date, only two
works were reported on PONs catalyzed Knoevenagel condensation
reaction: Tsukuda et al. reported that the (TMA)
garded as an efficient base catalyst for this process [51]; Wang et al.
discovered Na16[SiNb12 40] as a base catalyst for catalyzing CO cy-
cloaddition and Knoevenagel condensation [53]. Notably, [Nb
6
[Nb10
O
28] can be re-
2.3. Influence of different factors on the process
O
2
To our best knowledge, reaction temperature, dosage of catalyst,
raw ratio and solvents all play critical roles in this process, which were
investigated in detail via the condensation reaction of benzaldehyde
8−
6 19
O ]
shows the highest charge-density in these compounds including
[
SiNb12
O
40]1
6−
and [Nb10
]
O
28
]
6−
, which suggests the wonderful basi-
which facilitates them as the best candidates for
Knoevenagel condensation reaction. In addition, K HNb 19·13H
Nb ) is easily prepared with high yield, which may provide the po-
tential catalytic use in its application.
Herein, we employed Nb as a base catalyst in Knoevenagel con-
6
with ethyl cyanoacetate as the model reaction catalyzed by Nb in
8
−
city of [Nb
6
O
19
various conditions. As shown in Fig. 2a, when 0.1 mol% catalyst was
employed, 91% of yield for the reaction could be obtained. Moreover,
the increase of yield would continue by the addition of the mass
loading. Obviously, more catalytic dosage would improve the reaction
rate. As to the effect of temperature, the results of condensation of
benzaldehyde with ethyl cyanoacetate were studied at different tem-
peratures shown in Fig. 2b. From 25 °C to 40 °C, there is a remarkable
improvement in the yield of product. With the increase of temperature
going on from 40 °C to 60 °C, there is also an enhancement of product
and when the temperature comes to 60 °C, the highest yield of product
was obtained corresponding to the yield of 98%. Considering the effect
of raw ratio, the results of 0.6: 1, 0.8:1, 1:1 and 1.2:1 (donor: acceptor)
have been presented in Fig. 2c, which showed that any ratio could be
quicker to reach the limitation of the reaction than expected 1:1. Given
that solvents also have an effect on this reaction, consequently, several
contrast experiments were performed in various solvents. As the results
7
6
O
2
O
(
6
6
densation reaction, which exhibited excellent catalytic performance for
the condensation of various aldehydes and ketones at mild reaction
conditions. Furthermore, theoretical calculation for the basicity of
8−
[
Nb
6
O
19
]
and the comparison with other POMs (including PONs and
POTs) was also executed by density functional theory (DFT). On the
basis of DFT calculations results, it can be concluded that the most
8−
negative NBO (Natural Bond Orbital) charge in [Nb
which is much higher than the NBO charge of WO
6
O
19
]
is −1.001
(the index of
2
−
4
strong basicity which is −0.934 [53]) and other POMs. Moreover, the
terminal oxygens possess more negative charge than bridging oxygens
8−
with in [Nb
6
O
19
]
, which can be regarded as the basic catalytic sites
in the catalytic reaction.
shown in Fig. 2d, the catalyst in C
the process where an excellent yield of 98% has been obtained.
Meanwhile, Nb also exhibited good yield although the reaction took
place in various solvents, which suggested that Nb could be regarded
2 5
H OH has the best performance for
2. Results and discussion
6
6
2.1. Basicity study
as a wonderful catalyst for processing the Knoevenagel condensation of
benzaldehyde with ethyl cyanoacetate.
According to the literature, the basic active sites are located at the
oxygen atom on the surface of POMs, and the higher negatively charged
polyanions may possess stronger basicity [51]. Herein, the basicity of
2.4. Kinetic study
Nb
anions (W
6
was investigated by NBO and compared with other three poly-
In order to obtain the kinetic parameters of the reaction, several
catalytic experiments were performed by mixing catalyst, benzaldehyde
and ethyl cyanoacetate in ethanol at different times and temperatures.
Fig. 3a–d are the plots obtained for the yield of product and C/
2−
6−
4−
6
O
19 : W
6
; Nb10
O
28 : Nb10; SiW12
O
40 : SiW12). As
shown in Fig. 1a, the oxygen atoms charge in Nb
divided into five species corresponding to the values −0.809, −0.859,
0.869, −0.975, −1.001, where red rhombuses and blue circles re-
presented the charge of terminal oxygens and bridging oxygens in Nb
respectively. Obviously, the terminal oxygens possess more negative
charge than bridging oxygens with in Nb , which is different from Nb10
(SiNb12) [51,53]. Additionally, there is
the comparison of the most negative charge among Nb and three above
6
by NBO analysis are
−
[C
sponding concentration of benzaldehyde transformed to product at time
t and initial concentration of benzaldehyde is C . As indicated by stars
in Fig. 3a–d, they fall into the quasi-linear plot of reaction time and C/
[C (C − C)], which reveals that the reaction catalyzed by Nb exhibits
second-order kinetics for the Knoevenagel condensation. The different
rate constants (k , k2, ) were determined at corresponding tem-
perature (25 °C, 40 °C, 50 °C, 60 °C, respectively) according to Eqs. (1)
and (2), which have been calculated as follows: k = 0.03551 (mol/
= 0.14981 (mol/
0 0
(C − C)] as the function of reaction time, where C is the corre-
6
,
0
6
1
6−
but the same to SiNb12
O
40
0
0
6
6
polyanions in Fig. 1b, where the data spots presented the most negative
charge in these compounds. The results showed that the most negative
charge of Nb
1
3, 4
k k
2
−
6
is much higher than those of W
(−0.884), SiW12
6
O
19
(−0.719),
1
6−
4−
−1
−1
−1
−1
Nb10
O
28
(−0.750). It must be mentioned that
L) (min) , k
2
= 0.08954 (mol/L) (min)
,
k
3
94

Q. Xu et al.
Molecular Catalysis 453 (2018) 93–99
6 6 6
Fig. 1. (a) The NBO charges of oxygens in Nb ; (b) the comparison of Nb , W , Nb10, SiW12. (Nb: purple, W: blue, Si: green, O: red.).
Table 1
Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate was per-
formed with various catalysts.
b
Entry
Catalyst
Yield /%
1a
2
3
4
5
6
7
8
9
–
Nb
Nb
NaOH
K
K
Na
H
K
H
4.4
6.3
10
93
84
98
86
11
11
14
68
84
9
2
O
O
5
5
2
2
·xH O
8
SiW10
HNb
HPW
PW12
AsW12
18
7
6
O
19
34
40
8
9
O
3
O
3
O
O
40
62
1
1
1
1
0
1
2
3
6
P
2
W
Na12
Na
BW12
P
2
W
15
12
40
O
56
6
P
2
W
O
42
K
5
O
Fig. 2. (a) The polts of amout of catalyst versus time; (b) the polts of tem-
perature versus time; (c) the polts of raw ratio versus time; (d) the histogram of
yield in different solvents GC yield for target product ethyl (E)-acyanocinna-
mate. All of the products were identified by GC–MS spectra and 1H NMR
spectra.
a
Reaction conditions: catalyst 5 μmol, benzaldehyde 1 mmol, ethyl cyanoa-
cetate 1 mmol, ethanol 1 mL, temperature 60 °C, reaction time 2 h.
b
GC yields for target product ethyl (E)-acyanocinnamate were based on
methylbenzene as internal standard. All of the products were identified by
GC–MS spectra and GC spectra.
Knoevenagel condensation of various functional groups (electron-do-
nating and electron withdraw) substituted benzaldehydes with ethyl
cyanoacetate (Table 2, entry 1–14) or malononitrile (Table 3, entry
L)−1(min)
k
4
= 0.21875 (mol/L)−1(min)−1. Moreover, the calcu-
,
−1
lated activation energy (Ea = 43.3 kJ mol ) of the Knoevenagel con-
densation of benzaldehyde with ethyl cyanoacetate was obtained ac-
cording to the Arrhenius equation (Eqs. (3) and (4)), which was shown
in Fig. 3e and f detailed.
1–13). Specifically, yield of ≥95% were achieved for the condensation
of substituted benzaldehydes and hexahydrobenzaldehyde with ethyl
cyanoacetate at the ortho-, meta- and para-positions, respectively
(Table 2, entry 1–12) and completely neglected the steric effect and
electronic effect, which exhibits outstanding universality for substituted
benzaldehydes. Nevertheless, the condensation of ketone with ethyl
cyanoacetate only obtained the yield of 38% and 48% (Table 2, entry
− C)2
dC/dt = k (C
0
C/[C (C − C)] = kt
0
(1)
(2)
(3)
(4)
0
k = Aexp (−Ea/(RT))
lnk = lnA − Ea/RT
1
3–14).
Malononitrile as similar nucleophile shows stronger reactivity than
ethyl cyanoacetate in the present of Nb , leading to the yield of 99% for
6
Knoevenagel condensation of benzaldehyde with malononitrile at room
temperature after 45 min. Moreover, this successful approach towards
Knoevenagel condensation with malononitrile showed that the won-
derful catalysis could be performed with other substituted benzalde-
hydes, hexahydrobenzaldehyde and ketones after prolonging the reac-
tion time (Table 3, entry 13–14). Meanwhile, on the basis of these
results, we can conclude that aldehydes are more active than ketones in
the transformation.
2
.5. Scope of substrate
Encouraged by the aforementioned results, Knoevenagel condensa-
tion of various aldehydes and ketones with ethyl cyanoacetate and
malononitrile catalyzed by Nb have been investigated in C OH. As
can be seen from Tables 2 and 3, various substrates including aldehydes
and ketones were used to investigate the applicability of Nb , whose
results identified that Nb have exhibited remarkable catalysis for
6
2 5
H
6
6
95

Q. Xu et al.
Molecular Catalysis 453 (2018) 93–99
Table 3
Knoevenagel condensation of various substrates with malononitrile in the
presence of Nb
6
.
c
Entry
Acceptor
Product
Yield /%
1a
2
3
4
5
R
R
R
R
R
= Cl
= NO
= H
R
R
R
R
R
= Cl
= NO
= H
100
97
99
100
100
1
2
4
1
2
1
2
4
1
2
2
2
= Cl
= NO
= Cl
= NO
2
2
6
7
8
9
1
1
R
R
R
R
R
1
2
3
4
6
= F
= Cl
= Br
= CH
= NO
R
R
R
R
R
1
2
3
4
6
= F
= Cl
= Br
= CH
= NO
96
100
100
98
99
95
3
3
0
1
2
2
1
1
a
2b
99
88
3
Reaction conditions: catalyst 5 μmol, benzaldehyde 1 mmol, ethyl cyanoa-
cetate 1 mmol, ethanol 1 mL, temperature 25 °C, reaction time 45 min.
b
Reaction conditions: catalyst 5 μmol, benzaldehyde 1 mmol, malononitrile
1
mmol, ethanol 1 mL, temperature 25 °C, reaction time 3 h.
c
GC yield for target product.
Fig. 3. (a)–(d): Kinetic profiles of the Knoevenagel condensation of benzalde-
hyhde with ethyl cyanoacetate catalyzed by Nb
6
in different temperatures: (a)
Table 4
2
5 °C; (b) 40 °C; (c) 50 °C; (d) 60 °C. (e): the different k at corresponding tem-
−
1
The comparison of the efficiency of Nb6 with previously reported catalysts for
the Knoevenagel condensation.
perature; (f): the quasi-linear plot of lnk and T
.
Catalyst
Donor:
Acceptor
Temp./°C Time/h Yield/% Ref.
Table 2
Knoevenagel condensation of various substrateswith ethyl cyanoacetate inthe
presence of Nb6.
Nb
6
1:1
60
2
98
This
work
[50]
[53]
[51]
[10]
[48]
[54]
b
Entry
Acceptor
Product
Yield /%
Mg
Na16SiNb12
TMA) [Nb10
Nmm-PDO][OAc]
3
Al-PW12
1.5:1
1:1
1:1
1:1
1.5:1
1:1
1:1
1:1
1:1
60
70
70
rt
6
2
24
6
6
19
7
19
5
> 99
97
88
96
80
97
97
96
94
1a
O
40
R
R
R
R
R
1
2
3
1
2
= Cl
= NO
= CH
= Cl
= NO
R
R
R
R
R
1
2
3
1
2
= Cl
= NO
2
= CH
= Cl
= NO
2
99
99
99
99
99
(
6
O
28]·6H
2
O
2
3
4
5
2
[
3
O
3
O
8
Na H[A-PW
9
O
34
]
rt
ED-MIL-101
80
80
80
rt
2
ED-MIL-101(D)
APS-MIL-101
UPC-30
aminopropyl-functionalized 1:1
MCM
organic-inorganic hybrid
silica material
DMAN/SiO2
6
7
8
9
R
R
R
R
R
R
1
2
3
4
5
6
= F
= Cl
= Br
= CH
= CH
= NO
R
R
R
R
R
R
1
2
3
4
5
6
= F
= Cl
= Br
= CH
= CH
= NO
2
98
99
99
98
97
98
98
[55]
[56]
82
36
94
3
3
3
1.1:1
130
60
2
6
> 99
> 99
[57]
[58]
10
11
12
3
O
O
2
1:1.14
1
1
3
4
48
38
stoichiometry should be the best choice than any proportion. Therefore,
on the basis of these findings, it has been confirmed that Nb as a robust
basic catalyst has presented high efficiency for catalyzing Knoevenagel
6
condensation.
a
Reaction conditions: catalyst 5 μmol, benzaldehyde 1 mmol, ethyl cyanoa-
2.7. Recyclability
cetate 1 mmol, ethanol 1 mL, temperature 60 °C, reaction time 2 h.
b
GC yield for target product.
6
The reusability of catalyst was evaluated by using Nb (20 mg) to
catalyze Knoevenagel condensation of benzaldehyde (1 mmol) with
ethyl cyanoacetate (1 mmol) at 60 °C in 2 h.
2
.6. Comparison of different catalysts for Knoevenagel condensation
reaction
6
Subsequently, insoluble Nb can be recovered easily from the mix-
ture of solvent/substrates/catalyst by filtration at the end of every
cycle. Besides, traces of Nb in the filtrate have not been detected by
inductively coupled plasma spectrometer(ICP), which also shows it
exhibited as a good heterogeneous catalyst. Organic compounds were
removed completely by solvent evaporation at 60 °C leading to the
catalyst left in the tube then after washed three times by alcohol. The
6
In addition, a comparison of the efficiency of Nb with previously
reported catalysts for the Knoevenagel condensation was presented in
Table 4. The results have shown that although many previously re-
ported catalysts displayed good catalysis for Knoevenagel condensation,
most of the reported catalytic systems need higher temperature and
longer time to obtain excellent catalytic performance, where Nb
6
has a
6
infrared spectrum of Nb after every cycle was used to analyze catalyst
better economic substitute as to save time and resources. Ideally, 1:1
composition, followed by a duplicate test mainly proceeded with the
96

Q. Xu et al.
Molecular Catalysis 453 (2018) 93–99
capture the proton from the protonated catalyst leading to the β-hy-
droxyl compounds and regenerative catalyst. Finally, the product will
be obtained in dependence on the dehydration of β-hydroxyl com-
pounds.
3. Materials and methods
3.1. Materials
All of the reagents used were of analytical grade and obtained from
commercial sources without further purification. K HNb O19·13H O
7 6 2
was prepared using literature methods and characterized by IR spec-
troscopy and X-ray powder diffraction [59]. All of the reactants were
purchased from J&K Chemical.
Fig. 4. The histogram of yield of product for reused catalyst.
unchanged catalyst. The results for recyclability of Nb
6
have been
3.2. Catalyst synthesis
shown in Table S3, which indicated that there is a yield of 90% even at
fourth run. Unfortunately, the infrared spectrum of reused catalyst in
Fig. S2 revealed that Nb has changed after the third run, which is
6
corresponding to the results in Fig. 4 and the changes for the traces of
Nb after the third catalytic recycle (Nb, 54.63%).
K
7
HNb
characterized by IR spectrum (Fig. S1) and X-ray powder diffraction. In
a typical procedure, commercial Nb (13.3 g) was added to the melt
6 2
O19·13H O was prepared according to the literature and
2 5
O
KOH in a nickel crucible. The melt was cooled down to room tem-
perature and added 100 mL deionized water in the nickel crucible after
heating 30 min. Followed by filtration of the above mixture and then
putting the solution in refrigerator at 0 °C after 12 h will lead to the
solid (some crystals) which can be obtained via filtration. Finally, the
solid was washed with 1:1 (V/V) ethanol-water and dried in vacuo
2.8. Mechanism study
Although there is no clear reaction pathway, a proposed plausible
mechanism for Knoevenagel condensation of aldehydes with active
methylene compounds has been presented in Scheme 1 [13]. Terminal
resulting solid was denoted as Nb . Elemental analysis (%) calcd for
6
Nb : Nb, 40.70. Found: Nb, 39.57. The polyanion of one above crystal
6
oxygens in Nb
6
exhibit more negative NBO charges corresponding to
in obtained solid has been identified as Lindqvist Nb6 by X-ray single
the higher basicity, hence, we speculated that the reactions are likely
proceeded by the general base mechanism [44]. As Scheme 1 shows,
the O atom in Nb]O is considered to carry off one acidic proton from
the active methylene compounds to produce the corresponding carba-
nion which is regarded as a nucleophile used to attack C atom in al-
dehyde group to generate new CeC bond. The alkoxide ions would
crystal diffraction, whose polyhedron view has been shown in Fig. S3a.
Furthermore, the XRD pattern in Fig. S3b of the obtained solid is in
coincidence with the simulated XRPD patterns of Nb implying that the
6
sample for catalysis is pure.
Scheme 1. Proposed mechanism for Knoevenagel condensation of aldehydes with active methylene compounds.
97

Q. Xu et al.
Molecular Catalysis 453 (2018) 93–99
3.3. General procedure for the Knoevenagel condensation
Appendix A. Supplementary data
The aldehydes (1 mmol), active methylene compounds (1 mmol),
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.05.002.
catalyst (0.5 mol%) and solvent (1 mL) were added into a tube at room
temperature. The reaction mixture was stirred and heated at the pre-
scribed conditions on the parallel reactor. The product carried on
qualitative detection by GC–MS and the yield was monitored by GC.
Followed by using ethanol to wash the catalyst 3 times and then dried
in vacuo at 60 °C.
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