Zinc titanate nanopowder: An advanced nanotechnology based recyclable heterogeneous catalyst for the one-pot selective synthesis of self-aggregated low-molecular mass acceptor-donor-acceptor-acceptor systems and acceptor-donor-acceptor triads

Article abstract of DOI:10.1039/c2gc16641k

Highly stable, environmentally benign ZnTiO₃ nanopowder has been prepared via a sustainable sol-gel method. The nanopowder (90 nm) has been thoroughly characterized by SEM, XRD, EDS, Laser Raman, photoluminescence, UV and IR. The activity of th

Full text of DOI:10.1039/c2gc16641k

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Green Chemistry
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PAPER
Zinc titanate nanopowder: an advanced nanotechnology based recyclable
heterogeneous catalyst for the one-pot selective synthesis of self-aggregated
low-molecular mass acceptor–donor–acceptor–acceptor systems and
acceptor–donor–acceptor triads†
Paramita Das,a Ray J. Butcherb and Chhanda Mukhopadhyay*a
Received 21st December 2011, Accepted 22nd February 2012
DOI: 10.1039/c2gc16641k
Highly stable, environmentally benign ZnTiO3 nanopowder has been prepared via a sustainable sol–gel
method. The nanopowder (90 nm) has been thoroughly characterized by SEM, XRD, EDS, Laser Raman,
photoluminescence, UV and IR. The activity of the catalyst was probed through one-pot four-component
reaction of aldehydes, ketones and two equivalent propanedinitriles in water without requiring any
additives or anhydrous conditions. The reaction requires two different catalytic functions, i.e., an acidic
one which is given by Ti(IV) ions and a basic one, given by the oxide ion incorporated within the ZnTiO3
metal oxide framework. The advantages of this method lie in its simplicity, cost effectiveness,
environmental friendliness, and easier scaling up for large scale synthesis without using high pressure,
temperature and toxic chemicals. As water was used as a reaction medium and since we are particularly
interested in the isolation of a non-aromatic intermediate, the elimination of poisonous HCN was
prevented by the Lewis acid character of Ti4+ up to a sufficiently high temperature. Thus, this process can
be considered as a “green” process. Elimination of HCN at higher temperatures still maintains “green”
attributes as HCN can be trapped by the basic catalyst under such conditions. Spontaneous generation of
low molecular mass self-aggregated organic materials, their one-dimensional packing, and interesting
photophysical properties are reported.
walls, films and other nanoscaled structural arrays. From this
standpoint, an important challenge in the discovery of novel
Introduction
A “strong marriage” between nanotechnology and the principles
and practices of green chemistry1 holds the key to building an
environmentally sustainable society. Nanotechnology offers us
the opportunity to improve the green aspect of many processes
and products. Organic low-dimensional nanostructures used as
nanoscale building blocks have attracted considerable research
interest in the development of novel nano-devices.2 This interest
is driven by the fact that organic nanostructures may exhibit a
wide range of electrical, magnetic and optical properties which
depend, sensitively, on both their shapes and sizes and thus, is
likely to provide a new method for modifying the optical and
electronic properties of organic functional materials.3 However,
researchers continue to face great challenges in the construction
of well-defined organic compounds that aggregate into larger
molecular materials such as wires,3e tubes,3f rods, particles,3a–d
mesoscopic properties and the development of nanotechnology
is to fabricate functional conjugated organic molecules with
structural features that favor assembly into aggregate nanostruc-
tures with desired shape and size,via weak intermolecular
interactions.
As catalysis lies at the heart of countless chemical protocols,
another aspect of green chemistry to take into consideration is
the development of green processes and pollution abatement cat-
alysts. Recently, metal-based nanoparticles in the form of nano-
catalysts have emerged as viable alternatives to conventional
materials in various fields of chemistry, attracting interest among
chemists. Metal-based nanoparticles are known to be promising
materials for heterogeneous catalysis in a variety of organic
transformations.4 In addition, metal-based nanoparticles have
wide ranging applications in electronic, magnetic, optical, bio-
logical and mechanical materials because of their notable differ-
ences compared to bulk metals.5
aDepartment of Chemistry, University of Calcutta, 92 APC Road,
Kolkata-700009, India. E-mail: cmukhop@yahoo.co.in; Tel: +91-
9433019610
2,6-Dicyanoanilines belong to typical acceptor–donor–accep-
tor (A–D–A) systems, which are the basis for artificial photosyn-
thetic systems, materials possessing semiconducting or non-
linear properties, and molecular electronic devices.6 Further-
more, 2,6-dicyanoanilines are useful intermediates for building
blocks for cyclophanes to create a large molecular cavity and
bDepartment of Chemistry, Howard University, Washington DC 20059,
USA
†Electronic supplementary information (ESI) available. CCDC 812619,
836803 and 855038–855039. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2gc16641k
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known method18 using zinc acetate [(CH3COO2)2Zn], ethylene
glycol (OHCH2CH2OH), titanium butoxide (C16H36O4Ti) and
citric acid anhydrous [C3H9(OH)(COOH)3] (purity of the start-
ing materials greater than 99.9%) (all in equimolar ratios). Even
though the starting materials are not very sensitive to moisture,
both the handling of chemicals and all procedures were carried
out under a N2 atmosphere.
Scanning electron microscopy (SEM) was used to study the
morphology of the surface of ZnTiO3 nanopowders calcinated at
various temperatures: (A) 200 °C, (B) 500 °C and (C) 800 °C
for 6 h (Fig. S2, see ESI†). The analysis of the obtained figure
clearly shows that a higher calcination temperature favors par-
ticles with larger grain sizes (the grain size is about 90 nm for
800 °C, 70 nm for 500 °C and 35 nm for 200 °C) and higher
density compared to a lower calcinated temperature because a
higher temperature enhanced the atomic mobility and caused
grain growth which resulted in better crystallinity. As shown in
the SEM micrographs, the grain sizes seem to increase linearly
with increasing calcination temperatures.
The exact crystal plane and the particle size were estimated by
X-ray diffraction (XRD) analysis of the ZnTiO3 nanopowder cal-
cinated at 800 °C in air (Fig. S3, see ESI†). In the wide angle
study, a characteristic peak—due to the crystal planes—
appeared. The peaks correspond to the 220, 311, 400, 422, 511
and 440 planes of ZnTiO3 which are consistent with the standard
Scheme 1 ZnTiO3 nanopowder catalyzed efficient synthesis of A–D–
A triads and A–D–A–A systems under two different conditions.
host–guest complex.7 In addition, their biaryl unit is represented
in several types of compounds of current interest including
natural products, polymers, advanced materials, liquid crystals,
ligands and medicinal compounds.7,8 Traditionally, the synthesis
of 2,6-dicyanoanilines was catalyzed by a basic catalyst such as
piperidine.9 Many homogeneous catalysts have also been
employed for this condensation including NaOH,10 ethanedi-
amine, and11 Et3N.12 However, most of the reported methods are
cumbersome, require prolonged reaction times,11 have relatively
poor yields, and involve the use of costly reagents13 and toxic
solvents.14 Moreover, the main disadvantage of almost all the
existing methods is that the catalysts are destroyed in the workup
procedure and cannot be recovered or reused.10–14 On the other
hand, no heterogeneous catalysts have been used for this trans-
formation. Therefore, there is still a need for a simpler, more ver-
satile, and environmentally friendly process.
Considering the above points and in continuation of our quest
for developing green protocols for the synthesis of various pro-
ducts such as various nanoparticles, herein we employ ZnTiO3
nanopowder as an efficient and heterogeneous catalyst for the
synthesis of acceptor–donor–acceptor–acceptor (A–D–A–A)
systems (4) as well as its aromatization product (A–D–A triads)
(5) via a one-pot four-component reaction in aqueous media
(Scheme 1).
JCPDS reported values. Applying Sherrer’s formula, Dp
=
0.941λ/β cos θ where Dp is the average grain size, λ is the X-ray
wavelength equal to 1.5406 Å and β is the half-peak width, the
particle sizes are found to be 91 nm. These results are in good
accordance with the SEM observations.
The presence of Zn, Ti and O atoms was also observed in the
EDX spectrum collected from SEM (Fig. S4, see ESI†).
Based on the SEM and XRD observations, the optical proper-
ties of the synthesized ZnTiO3 nanopowder calcinated at 800 °C
were investigated using Laser Raman and Photoluminescence
measurements (Fig. S5, see ESI†). The Raman spectra of calci-
nated powder are in good agreement with the XRD pattern. The
peaks located at about 139, 228, 351, 427, 445, 605 and
709 cm−1 are ascribed to the perovskite structure of ZnTiO3.
These results agreed well with previous reports.19
The photoluminescence (PL) spectrum was acquired at r.t.
with an excitation wavelength of 536 nm from 400 to 700 nm
(Fig. S6, see ESI†). Room-temperature PL spectra of the ZnTiO3
nanopowder show emissions in the visible light regions. It has
been well understood that the deep level (DL) visible light emis-
sions are due to various point defects.20 From the position of the
PL emission band at 536 nm, the optical gap can be calculated
as 2.30 eV. PL depends on the degree of the system disordering,
which leads to the appearance of new energy levels, both in the
valence band and in the conduction band, decreasing the band
gap. In the present case, a low temperature during synthesis
increases the degree of disorder. As explained by Blasse and
Grabmaier,21 the PL arises from a radiative return to the ground
state, a phenomenon that is in concurrence with the non-radiative
return to the ground state, in which the energy of the excited
state is used to excite the vibrations of the host lattice, i.e. to heat
the lattice. The radiative emission process occurs more easily if
trapped holes or trapped electrons exist in the structure. The lit-
erature associated with the PL in titanates indicates that the
Furthermore, the compound 4 is prone to aromatization by the
elimination of hydrogen cyanide affording compound 5. In some
instances, the driving force of aromatization is so enhanced that
the intermediate 4 cannot be isolated and the reaction affords
only 5.15 Yan et al. reported a multicomponent domino reaction
of malononitrile, benzaldehyde and ethyl α-bromoacetate (or
chloroacetonitrile) in refluxing acetonitrile, affording 2,6-dicya-
noaniline (5) in 31–53% yields.16 However, only very few
domino reactions are known for the synthesis of compound 4
with moderate yields.17 Therefore, we wish to disclose the devel-
opment and implementation of a new method for one-pot selec-
tive generation of both the aromatic and non-aromatic
intermediates under two different conditions.
Results and discussion
The synthesis of the nanostructured zinc titanate (ZnTiO3)
(Scheme S1, see ESI†) in powder state has been carried out by a
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phenomenon is related to the presence of [TiO5] clusters, particu-
larly from the local distortion of the Ti4+ ions within the TiO6
octahedrons, oxygen vacancies, or charge transfers via intrinsic
defects inside an oxygen octahedron22 leading to delocalized
electron levels in the optical gap.
To further investigate the optical properties of ZnTiO3 nano-
powder, UV-vis absorption spectroscopy of the solid nanopow-
der catalyst was performed (Fig. S7, see ESI†). Fig. S7† shows
absorbance maxima at 324 nm of ZnTiO3 nanopowder calcinated
at 800 °C. The band gap energy of ZnTiO3 was calculated from
the UV-vis absorption spectra which showed a strong absorption
band (below 350 nm in wavelength) in the UV region. For the
indirect band-gap semiconductor, the relation between the
absorption edge and photon energy (hν) can be written as
follows:
simple addition of oxides is that while individual oxides have
either acidic or basic properties, mixed oxides produce systems
that have enhanced novel acidic as well as basic properties and
by varying the mixing concentration, it is possible to control
their acidic/basic active sites. For example, the basicity of the
ZnO–TiO2 system strongly depends on the ZnO content. More-
over, Bhattacharyya et al. reported that it is not the simple
addition of ZnO and TiO2 but rather, it is the ZnO–TiO2 inter-
face which is the source of catalytic activity.30a This point was
further supported by Gómez et al. who concluded that the inter-
actions between the different crystalline structures coexisting in
the mixed oxide should be responsible for observed activity and
selectivity patterns.30b
Transition metal oxides are extensively employed as catalysts
because they possess an active centre which can adsorb reaction
molecules. Chemical properties of the active sites can be
adjusted by mixing an oxide catalyst with an oxide support, and
so the interaction between the two metal catalysts is highly
crucial for catalytic performance.29 Therefore, these results
strongly suggest that ZnTiO3 materials have potential appli-
cations in catalytic fields.
2
αhν ¼ Aðhν – EgÞ
where α is the absorption coefficient, A is a constant, h is
Planck’s constant, ν is the photon frequency and Eg is the optical
band gap. The optical band gap for ZnTiO3 nanopowder is about
3.78 eV, which is significantly larger than that reported for the
bulk material (3.5 eV).23 These results are in good accordance
with the theoretical observations which suggested that the value
of blue-shifting is inversely proportional to the square of the
crystallite size due to the quantum confinement effect.24 The
band gap of 3.78 eV indicates that the cubic phase ZnTiO3 is an
indirect band gap semiconductor, which is consistent with that
reported in the literature.25
The nanopowder obtained by heating the catalyst at 800 °C
exhibits a trace of absorption band probably due to δ (H2O)
around 1629 cm−1, νS (H2O) ∼ 3430 cm−1 (Fig. S8, see ESI†).
Absorption bands ∼576 cm−1 from ∼400–800 cm−1 can be
ascribed to Ti–O stretching vibrations in ZnTiO3. The broader
absorption bands of the Ti–O stretching vibrations, as compared
with those reported in ref. 26, may arise from the nanocrystalline
nature of the sample.
To optimize the reaction conditions, a series of experiments
were conducted with a representative reaction of 4′-methoxy
acetophenone (1f) (2 mmol), 4-methoxy benzaldehyde (2a)
(2 mmol) and malononitrile (3) (4 mmol), with variation of reac-
tion parameters, such as catalyst, reaction temperature etc., and
the results are summarized in Table 1. Screening of the reaction
Table 1 Optimization of reaction conditions for the multicomponent
coupling reactionsa
From the XRD analysis (Fig. S3, see ESI†), ZnTiO3 nano-
powder was characterised by the cubic perovskite structure. Here
O2− ions along with the larger cations (i.e. Zn2+) are in a ccp or
fcc array and the smaller cations (Ti4+ ions) occupy the octa-
hedral holes formed by the O2− ions. Thus in the unit cell, 8
corners of a cube are occupied by Zn2+, at the centre of each
face there is an O2− ion, and at the centre of the cube, a Ti4+ ion
is found. Thus, the contributions of the lattice points per unit
cell are as follows: one Zn2+ (8 × 1/8 = 1, for corner point),
Yieldb (%)
Entry
Catalyst (0.2 mmol)
Temp. (°C)
Time (h)
4fa
5fa
1
2
3
4
5
6
7
8
NaOH
NaOH
NaOH
NaOAc
NaOAc/AcOH
Piperidine
Et3N
r.t.c
60
120
60
60
60
48
6
3
9
0
0
19
45
35
42
25
43
40
39
38
20
40
72
0
10
10
12
13
15
16
24
16
14
4
4
3
3
15
0
20
12
18
22
28
15
30
0
79
0
93
0
three O2− (6 × = 3, face centered point), one Ti4+ (1 × 1 = 1,
1
2
60
60
for body centered point).27 In this context we mention that ZnO
Pyridine
9
MgO NPs
ZnO NPS
ZnO NPS
ZnO NPS
ZnO NPS/TiO2 NPs
ZnO NPS/TiO2 NPs
ZnTiO3 NPsd
ZnTiO3 NPsd
TiO2 NPs
50
adopts the hexagonal wurtzite structure and TiO2 can exist in the
10
11
12
13
14
15
16
17
r.t.c
50
anatase or rutile phase.28
A short review on the literature depicts that BET surface area
is a function of the Zn/Ti mole ratio. Mixed oxides have greater
surface area, smaller particle sizes and higher generation of new
catalytic active sites than the corresponding oxide homologues.
This is extremely important since higher surface area favors
adsorption of reaction molecules and small particle size is advan-
tageous for minimal internal diffusion resistance of molecules
thereby increasing the catalytic performance of heterogeneous
catalysts.29 Another advantage of using mixed oxides over
70
r.t.c
100
r.t.c
100
50
80
0
95
0
0
a All reactions were carried out in H2O (2 mL) using the substrates
according to the indicated ratio in mmol scale. b Isolated yields. c Here
r.t. corresponds to 30 °C. d Thermally activated ZnTiO3 at 800 °C in air.
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conditions established that the nature of the catalyst and the
temperature of the reaction medium had a significant effect on
the yield of the non-aromatic product (4fa) and, consequently,
the aromatic product (5fa).
4xy is only possible with Ti4+ since it can arrest the elimination
of HCN. However, with other bases we have used, the aromatic
compound 5xy is mostly formed even at r.t. Therefore, selective
generation of the non-aromatic intermediate is not possible with
other catalysts even if it is the desired product (Table 1, entries
10–12). Thus, the entries 10–14 (Table 1) clarify that Ti4+ has a
definite role in the prevention of elimination of HCN, thus allow-
ing the selective generation of non-aromatic 4xy and aromatic
compounds 5xy at different temperatures. Therefore, ZnO/TiO2
though it is a good catalyst for this reaction, leads to a yield that
is not as high as that of ZnTiO3 (Table 1, entries 15–16). Thus,
ZnTiO3 is the best catalyst for this particular reaction in generat-
ing non-aromatic and aromatic compounds. Moreover, when
ZnO and TiO2 were mechanically mixed in a 1 : 1 ratio then the
elemental analysis of the recovered catalyst showed that the 1 : 1
ratio was not maintained after the first run. This may presumably
be due to the loss of some amount of catalyst during filtration
and therefore, it is unrealistic to expect that atom ratio would
remain 1 : 1 after filtration. Table 1, entry 13 showed that the
reaction with ZnO/TiO2 afforded the 4fa in 79% yield and 100%
selectivity. The elemental analysis of the recovered ZnO/TiO2
showed that the atomic ratio was 1.0 : 0.8. When the same reac-
tion was carried out with this recovered ZnO/TiO2, 8% of the
aromatic product 5fa was formed along with 68% of non aro-
It was found that 0.2 mmol of NaOH as the base catalyst
could mainly isolate the aromatic compound (5fa) (19%) along
with very small amount of non-aromatic compound (4fa) (9%)
after 48 h stirring at room temperature (Table 1, entry 1). Rep-
etition of the same experiment at 60 °C, yielded only the aro-
matic product (5fa) in 45% yield (Table 1, entry 2). This could
probably be due to the ease of elimination of HCN at higher
temperatures. The yield of 5fa decreased by up to 35% at a
much higher temperature with NaOH (Table 1, entry 3). This
was presumed to occur due to the competitive Cannizaro reac-
tion,31 a typical reaction for aldehydes in basic media under high
temperatures which yields an acid and consumes aldehyde in the
reaction mixture. This was corroborated by the NMR spectra.
Interestingly, with NaOH we always obtained the aromatic
product (5fa) as the major product even at much lower tempera-
tures. This is because NaOH promotes the elimination of the
acidic component HCN. Therefore, we were in search of a cata-
lytic system which carried out the reaction at room temperature
to prevent the elimination of HCN so that the non-aromatic com-
pound (4fa) could be isolated which would allow us to syn-
thesize the aromatic compound (5fa) when needed. With NaOAc
as a base catalyst, the reaction did not proceed at all at room
temperature. However, increasing the temperature to 60 °C
yielded only the aromatic product (5fa) in 42% yield (Table 1,
entry 4). On the other hand, the use of NaOAc/AcOH as catalyst,
showed a significant improvement in the yield of non-aromatic
product (4fa) (20%) along with 25% of aromatic product (5fa)
at 60 °C (Table 1, entry 5) indicating that the presence of an acid
can prevent the elimination of the other acidic HCN which
confirms our earlier observation—that the acid additive HOAc
functioned to inhibit the elimination of another acidic component
HCN very efficiently.11 Thus, it was found that decreasing the
base strength increases the yield of non-aromatic product (4fa)
along with the overall yield (Table 1, entries 6–8) because of the
lesser extent of HCN elimination with weaker base catalyst and
also the lesser extent of the Cannizaro reaction of aldehydes.
However, an improved yield was obtained with weaker bases
like MgO and ZnO (Table 1, entries 9–12). Interestingly, we
observed that TiO2 was most effective in terms of prevention of
elimination of another acidic component HCN in the presence of
another strong Lewis acid Ti4+.32 Again MgO and ZnO nanopar-
ticles were more effective than bulk MgO and ZnO in terms of
reaction time and temperature. We also found that ZnO nanopar-
ticles were potentially better catalysts than MgO nanoparticles.
Therefore, the dual catalytic activity of ZnO NPs and TiO2 NPs
was essential for the room temperature synthesis of the non-aro-
matic compound (4fa) with a shorter reaction time (Table 1,
entries 13, 14). The compounds obtained were highly pure as
can be seen in the 1H NMR spectra (see ESI †). Significant
enhancement of the product yield and cleaner reaction conditions
were the outcomes of this dual catalytic system. The decreased
extent of the Cannizaro reaction and other side products formed
under low temperatures and short reaction times could be a
plausible reason. Therefore, from the entries 13–14 (Table 1) it is
clear that selective generation of the non-aromatic compounds
1
matic 4fa as is evident from the H NMR spectra of the crude
reaction mixture (see ESI †). This is due to the fact that TiO2
cannot prevent the decomposition of 8% non aromatic 4fa in the
presence of an excess of basic ZnO over TiO2. These results
firmly established that the 1 : 1 ratio must be maintained for the
aforesaid selectivity and catalytic efficacy. To avoid the problem,
mixed oxide ZnTiO3 nanopowder was selected as the hetero-
geneous catalyst. After first run, this recycled catalyst was found
to contain the same proportion of Zn and Ti as that of the orig-
inal catalyst. The non aromatic product 4fa was selectively
formed in 93% yield without the formation of 5fa at room temp-
erature with ZnTiO3 (Table 1, entry 15). Again the aromatic
product 5fa was the only product at 100 °C (Table 1, entry 16).
Starting materials were mostly unaffected with only TiO2 as cata-
lyst (Table 1, entry 17). Therefore, presence of ZnO as a base is
necessary for this reaction to proceed. Thus, the efficacy and
superiority of ZnTiO3 with respect to other catalysts have been
firmly established.
To analyze the temperature dependence on the yield of non-
aromatic product (4fa) to the aromatic product (5fa), a series of
experiments were conducted with a representative reaction of 4′-
methoxy acetophenone (1f) (2 mmol), 4-methoxy benzaldehyde
(2a) (2 mmol), malononitrile (3) (4 mmol) and ZnTiO3 nano-
powder catalyst (0.2 mmol) in H2O (2 mL) for 3 h at various
temperatures with an interval of 10 °C (Fig. 1). Analysis of the
reaction results established that whereas ZnTiO3 can resist the
elimination of HCN up to a sufficiently high temperature, ZnO
alone could afford only a little of the non-aromatic compound
(4fa) even at very low temperature. Again ZnO alone was not an
efficient catalyst for the synthesis of aromatic compound (5fa) at
room temperature. Therefore, ZnTiO3 was the most efficient cata-
lyst for room temperature synthesis of the non-aromatic com-
pound (4fa), whereas the same catalyst afforded the aromatic
compound (5fa) when heated to a sufficiently high temperature
(∼100 °C). However, elimination of the acidic component HCN
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Fig. 1 Temperature dependence on the yield of non-aromatic product
to the aromatic product.
Fig. 2 Yields at different catalyst loadings in the four-component
coupling reaction of aldehydes, ketones and malononitrile.
at higher temperatures still maintains “greenness” as it can be
trapped by the basic catalyst.
To determine the best amount of the catalyst, the synthesis of
the non-aromatic product (4fa) was carried out in the presence
of various amounts of catalyst in water at room temperature.
Fig. 2 shows a remarkable increase in output according to the
increase in the quantity of the catalyst up to 0.2 mmol. With a
higher amount of catalyst no significant increase in the yield was
observed. In order to use the minimum amount of the catalyst,
we have been limited to 0.2 mmol for the continuation of the
study.
Scheme 2 Activated ZnTiO3 nanopowder catalysed one-pot synthesis
of non-aromatized product at r.t.
With these optimized conditions in hand, this multicomponent
reaction can be readily diversified through a combination of a
range of ketones, aldehydes and malononitrile to synthesize the
non-aromatic products (4) as well as aromatic products (5) under
two different conditions and the results obtained are summarized
in Schemes 2 and 3 respectively. Among the ketones, alicyclic
as well as aromatic ketones afforded excellent yields (Scheme 2,
4aa–fa and Scheme 3, 5ac–gm). A variety of aromatic ketones
with electron-withdrawing as well as electron-donating groups
on the aromatic ring were converted into desired products in
good to excellent yields. In addition, the use of aliphatic ketones
was also examined (Scheme 2, 4hm and Scheme 3, 5hm). It is
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Fig. 3 (a) Instability of the carbanion in 7h. (b) Generation of carba-
nion in RCOCH2R′.
Fig. 4 Difference in the structure of all the non-aromatic compounds
derived from alicyclic ketones from all the previous reported structures.
alkyl group (Fig. 3(a)).33 However, when R = Ph in RCOCH2R′
then irrespective of the electron-donating (R′ = CH3) or electro-
withdrawing nature of the R′ group (R′ = Ph) there is no second
choice of generation of carbanion except for 7 [Fig. 3(b)]. There-
fore, irrespective of the steric crowding among the substituents,
fully substituted products were also isolated in good to excellent
yields (Scheme 2, 4ie–jm and Scheme 3, 5io). To further expand
the scope of the reaction the use of heteroaryl methyl ketones
was investigated (Scheme 3). 2-Acetylthiophene was easily
transformed into the desired products in excellent yields
(Scheme 3, 5kb and 5kk).
Despite the difficulty of a Knoevenagel condensation reaction
with electron-rich carbonyl, the reaction occurred successfully
with 4-methoxy benzaldehyde, 3,4-dimethoxy benzaldehyde,
2,5-dimethoxy benzaldehyde, 2,5-dimethyl benzaldehyde and 4-
methyl benzaldehyde (Scheme 2, 4aa–ab, 4ba–bb, 4bh–fa, 4ie,
4je and Scheme 3, 5ac–5ad, 5bh–cb, 5fa, 5kb). To check the
generality of the reaction, aldehydes with electron-withdrawing
substituents on the aromatic ring were also employed (Scheme 2,
4ag–ao, 4bk–hm, 4ih–im, 4jm and Scheme 3, 5af–bk, 5cm–
em, 5gm–io, 5kk). In addition, the use of aliphatic aldehydes
was also examined (Scheme 2, 4bh and Scheme 3, 5bh). To
further expand the scope of the reaction, the use of heteroaryl
aldehyde was investigated. Pyridine-4-aldehyde was easily trans-
formed into the desired products in excellent yields (Scheme 2,
4ao and Scheme 3, 5io).
Scheme 3 Activated ZnTiO3 nanopowder catalysed one-pot synthesis
Again, the novelty of our reaction lies in the fact that all the
non-aromatic compounds (4) derived from alicyclic ketones
(Fig. 4a) have a slightly different structure from all the pre-
viously reported structures (12) (Fig. 4b).11 We have confirmed
our supposition unambiguously by X-ray crystallographic analy-
sis of compound 2-amino-4-(3-nitro-phenyl)-5,6,7,8-tetrahydro-
4H-naphthalene-1,3,3-tricarbonitrile (4aj) (CCDC 836803)
(Fig. 5) and compound 2-amino-6-(2,5-dimethoxy-phenyl)-5-
of aromatized product at 100 °C.
important to mention that the selectivity of this reaction towards
CH3CO– rather than RCH2CO– [R = CH (CH3)2] observed in
the case of products 4hm and 5hm (Schemes 2 and 3 respect-
ively) probably resulted from the instability of the carbanion
(7h) because of the electron donating inductive effect of the
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Fig. 5 ORTEP representation of 4aj showing the crystallographic num-
bering (CCDC 836803).
Fig. 6 ORTEP representation of 5cb showing the crystallographic
numbering (CCDC 812619).
Scheme 4 A plausible mechanistic pathway to explain the ZnTiO3
nanopowder catalyzed formation of 2,6-dicyanoanilines.
methyl-4-phenyl-cyclohexa-2,4-diene-1,1,3-tricarbonitrile (4ie)
(Fig. S9, see ESI†). The final structure of the aromatic com-
pound was confirmed by the X-ray crystallographic analysis of
compound 2-amino-4-p-tolyl-6,7,8,9-tetrahydro-5H-benzocyclo-
heptene-1,3-dicarbonitrile (5cb) (CCDC 812619) (Fig. 6) and
compound 3-amino-6-methyl-5-pyridin-4-yl-biphenyl-2,4-dicar-
bonitrile (5io) (Fig. S10, see ESI†).
increasing the electrophilicity of the carbonyl carbon and thereby
making it possible to carry out the reaction at room temperature
and at short reaction time. Obviously the acid character of Ti4+
in the reaction functioned to efficiently inhibit the elimination of
HCN up to a sufficiently high temperature.32
A plausible mechanism for the formation of 2,6-dicyano ani-
lines catalyzed by ZnTiO3 nanopowder is shown in Scheme 4.
The reaction commences with the formation of the Knoevenagel
condensation product from aldehyde with malononitrile.34 This
is evident from the NMR spectrum of the major product (6, see
ESI †), isolated after few minutes in the reaction of 4′-methoxy
acetophenone (1f) (2 mmol), 4-methoxy benzaldehyde (2a)
(2 mmol), malononitrile (3) (4 mmol). This intermediate (6)
undergoes a Mannich-type reaction with another molecule of
ketone (1), and subsequent elimination of the malononitrile leads
to the intermediate (9). The structure of 9 is confirmed from the
NMR spectrum of the major product (9, see ESI †), isolated by
quenching the reaction after 45 min. Again the attack of malono-
nitrile on intermediate (9), yields intermediate (10). This is
evident from the NMR spectrum of the major product isolated
after 1.30 h (see ESI †). Finally, the ring is produced by the
attack of second molecule of malononitrile on the intermediate
(10) followed by the elimination of HCN.
Here, despite Knoevenagel condensation being a net dehy-
dration of the water molecule, the reaction is favored in aqueous
medium. A plausible explanation is that water forms several
hydrogen bonds with the catalyst and the organic molecule,
thereby acting as a bridge between the homogeneous and hetero-
geneous phases.35 Thus, realizing the environmental concerns,36
as well as the vast utility and scope of reactions36a,37 carried out
in water, we established water to be the preferred solvent.
In order to validate the recyclability of the current ZnTiO3
nanopowder catalyst, the re-use investigation was performed in a
model reaction of 4′-methoxy acetophenone (1f) (2 mmol), 4-
methoxy benzaldehyde (2a) (2 mmol), malononitrile (3)
(4 mmol) and ZnTiO3 nanopowder catalyst (0.2 mmol) in H2O
(2 mL) for 3 h at r.t. producing 4fa (Table 2).
Another recycling experiment on producing 5fa was carried
out and it was found that the catalyst had the potential of
efficient recycling for at least 6 cycles without any loss of cataly-
tic activity. For this purpose, a model reaction of 4′-methoxy
acetophenone (1f) (2 mmol), 4-methoxy benzaldehyde (2a)
Here, oxides of the ZnTiO3 metal oxide framework acting as a
base and Ti4+ coordinate with the carbonyl oxygen, thus
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Table 2 Reusability studies of ZnTiO3 nanopowder catalyst for the
synthesis of 4fa
No. of cyclesa
Yieldb (%)
1
2
3
4
5
6
93
92
93
91
90
90
a Reaction was carried out with recovered catalyst. b Isolated yields.
Table 3 Reusability studies of ZnTiO3 nanopowder catalyst for the
synthesis of 5fa
Fig. 7 SEM image of the nanostructured materials of compound 4ba.
Design and easy access to the low molecular mass self-aggre-
gated organic materials (LMSOM) is now a new challenge in
organic synthesis, wherein macrocyclic compounds or optically
insensitive alkyl chain/steroidal groups substituted compounds
have usually been utilized for the fabrication of 1D organic
nanomaterials.38,39 In this regard, the generation of novel self-
aggregated architectures of well-defined shape and size from 2,6-
dicyanoanilines is important. In a bid to our continuous effort to
design and synthesize significant, diverse, complex and medicin-
ally important organic frameworks and to study their self-assem-
bly properties, we herein report a new class of LMSOM from
4ba and 5bk (Schemes 2 and 3 respectively) which are easily
accessible by the one-pot green synthesis protocol.
However, these small organic molecules spontaneously form
tunable nanomaterials in common organic solvents. To gain an
insight into the aggregation morphology of the nanomaterials
formed by compound 4ba, two different solvents have been sub-
jected to SEM. In THF it forms organic materials of spherical
dimension with a “Kadam flower-like” well-defined shape con-
structed by highly self-aggregated nanosheets about 350 nm in
width (panels a–c, Fig. 7), whereas tube-like nanomaterials are
obtained in chloroform at about 70 nm in width (panels d–e,
Fig. 7). However, an insight into the aggregation morphology
formed by compound 5bk in chloroform, gave flower-like well-
defined shape with each petal about 100 nm in width (panels
a–b, Fig. 8).
No. of cyclesa
Yieldb (%)
1
2
3
4
5
6
95
94
94
92
90
90
a Reaction was carried out with recovered catalyst. b Isolated yields.
(2 mmol), malononitrile (3) (4 mmol) and ZnTiO3 nanopowder
catalyst (0.2 mmol) in H2O (2 mL) for 3 h at 100 °C was carried
out and the results are represented in Table 3.
Nearly quantitative catalyst (up to 98%) could be recovered
from each run. In a test of six cycles, the catalyst could be re-
used without significant loss of catalytic activity. The slight
reduction in yield is probably due to the loss of some catalyst at
the time of filtration. The recovered catalyst after six runs had no
obvious change in structure according to the FT-IR spectrum in
comparison with the fresh catalyst (Fig. S8, see ESI†). The XRD
observation of the recovered catalyst was also made and there
was no obvious change in morphology (Fig. S3, see ESI†).
These results revealed that the catalyst was very stable and could
induce these conditions.
The flaky structure of compound 5kb was further confirmed
from the atomic force microscopy (AFM) analysis in chloroform
(panel c, Fig. 8) where the aspect ratio was found to be 2.2353.
Fig. 8(d) shows the 3D view of the compound in chloroform.
It is interesting for us to screen the polysubstituted 2,6-dicya-
noanilines for their optical properties. The correlation between
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optical properties and the molecular structure can be described
only empirically, since no detailed theoretical predictions are
possible.40 A large number of compounds in a combinatorial
library should correlate to a better understanding of the inherent
structure–property relationships from which specifically tailored
materials can be produced. From the library, two members 4ba
and 5bk with the most intensive fluorescence were selected. The
large red shift in UV, measured in the self-aggregated solid state
of compound 5bk compared to their dilute solutions in three
different solvents is probably due to the J-aggregation of the
compound leading to the formation of the highly-ordered stack-
like architecture (panels a, b, Fig. 9). Similar observations were
found in UV spectra of compound 4ba with an additional
vibrational hump existing around 300 nm in EtOAc and THF
compared to that in CHCl3 (Fig. S11 and S12 see ESI†). The
spectral properties of compounds 4ba and 5bk are further
emphasized in the fluorescence study. Maximum wavelengths
(λem = 403–423 nm) were observed for compound 5bk in
CHCl3, EtOAc and THF. The bathochromic shift observed for
the emission spectra of compound 5bk in EtOAc and THF com-
pared to that in CHCl3 is attributed to the stronger hydrogen-
bonding with the NH2 group of compound 5bk with the oxygen
atoms of the solvents leading to the stabilization of the excited
state, thereby decreasing the energy gap and increasing the emis-
sion wavelength (panel c, Fig. 9). Unusual fluorescence amplifi-
cation is also observed in both dilute and higher concentrations.
The planar aromatic molecules with strong assembly properties
exhibited amplified fluorescent sensing ability because of
growing unidirectional π–π stacking (panel d, Fig. 9). Similar
fluorescence amplification was also observed with compound
4ba. The additional emission wavelength of the compound 4ba
Fig. 8 SEM and AFM images of the LMSOM of compounds 5bk.
Fig. 9 Optical properties (UVand fluorescence) of the compound 5bk.
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in EtOAc and THF compared to that in CHCl3 is probably due
to the structured vibrational spectra (Fig. S13, see ESI†).
In view of possible applications of these compounds (e.g., as
organic light-emitting diodes) the NH2 substituted A–D–A and
A–D–A–A systems are of interest since these compounds can be
transformed into more thermostable derivatives through their
amino groups. Also, the subsequent step of the combinatorial
development process, namely structural optimization should be
possible. The application of combinatorial strategies for screen-
ing and optimization of organic charge-transporting materials in
a spatially addressable library of organic light-emitting diodes
was reported by Schmidt et al.41
than 99.9%). Even though the starting materials are not very sen-
sitive to moisture, both the handling of chemicals and procedures
were carried out under a N2 atmosphere. Equimolar zinc acetate
and titanium butoxide are separately dissolved in deionized
water and ethanol and mixed in a two-neck round bottom flask.
When the precursor was completely dissolved in the solution,
equimolar amounts of citric acid and ethylene glycol were
added. Ethylene glycol (propionic acid) is added to the above as
a stabilizing agent. The contents are refluxed in N2 atmosphere
for 2 h at 25 °C, yielding a homogeneous organic solution. After
refluxing, the contents are double filtered to dispose of any small
particles which may be present from the mixing of the materials.
Obtained powders were heat treated from 200 °C to 800 °C in
air, respectively.
Conclusions
In conclusion, zinc titanate nanopowder has been synthesized
and characterized using an array of sophisticated analytical tech-
niques. It is demonstrated that such ZnTiO3 nanopowder per-
forms efficiently as a recyclable heterogeneous catalytic system
for the one-pot construction of diverse one donor and poly
acceptor systems containing a 2,6-dicyanoaniline moiety invol-
ving four-component condensation of aldehyde, ketone and two
equivalent malononitrile in aqueous media. We have found Ti4+
as an active component for the catalytic process. This versatile
approach provides direct access to the non-aromatic intermediate
containing one donor and three acceptors, thus preventing the
elimination of HCN. The key findings of high significance
described in this work are three-fold. The first one is that ZnTiO3
catalytic system enables “sensitive substances” such as pyridine-
4-aldehyde, 2-acetyl thiophene, highly activated aromatic alde-
hydes (containing –NO2 groups), aliphatic ketones such as iso-
butyl methyl ketones and aliphatic aldehydes such as
butyraldehyde to react under our experimental condition to
obtain corresponding 2,6-dicyanoamines in good to excellent
yields. The second key finding in our work is that all the non-
aromatic compounds derived from alicyclic ketones have slightly
different structures from all the previously reported structures.
The third finding in our work is that no anhydrous conditions
are needed to promote the reaction and a reasonably low
amount of ZnTiO3 nanocatalyst is sufficient enough to catalyze
our reaction. In addition, the route takes advantages of the abun-
dance of aldehydes and ketones available commercially to make
2,6-dicyanoamines. These key findings make the nanotechnol-
ogy based heterogeneous catalysis platform provided herein
inherently advanced, economical, green and therefore, environ-
mentally sustainable for the one-pot synthesis of 2,6-dicyano-
amines. Using this robust green protocol we have synthesized
UV and fluorescent active LMSOM forming tunable 1D stacking
suitable for designing biosensors and optoelectronic
nanodevices.
General procedure for the synthesis of non-aromatic
intermediate A–D–A–A system (4aa–4jm)
Ketone 1 (2 mmol), aldehyde 2 (2 mmol), malononitrile 3
(4 mmol) and 0.2 mmol of activated ZnTiO3 nanopowder cata-
lyst were added to 2 mL water and the reaction mixture was
stirred at r.t. for 3 h. After completion of the reactions (monitored
by disappearance of the starting material in the thin layer chrom-
atography), ethyl acetate (5 mL) was added to the whole reaction
mixture and the solid catalyst was separated from the mixture by
filtering through a sinter funnel. The recovered catalyst was
washed several times with water and acetone, dried in a desicca-
tor and stored for another consecutive reaction run. Pure organic
compounds were afforded by evaporation of the solvent in a
rotary evaporator and the residue was subjected to silica gel
(60–120 mesh) column chromatography using ethylacetate–
petroleum ether as eluents. The products were characterized by
standard analytical techniques such as 1H NMR, 13C NMR,
FTIR, elemental analysis, melting point determination, X-ray crys-
tallographic analysis and all gave satisfactory results (see ESI†).
General experimental procedure for the synthesis of 2,6-
dicyanoanilines (A–D–A triads) catalyzed by ZnTiO3
nanopowder (5ac–5kk)
A mixture of ketone 1 (2 mmol), aldehyde 2 (2 mmol), malono-
nitrile 3 (4 mmol) and activated ZnTiO3 nanopowder catalyst
(0.2 mmol) in water (2 mL) was heated under reflux at 100 °C
for 3 h. After completion of the reactions (monitored by disap-
pearance of the starting material in the thin layer chromato-
graphy), ethyl acetate (5 mL) was added to the whole reaction
mixture and the solid catalyst was separated from the mixture by
filtering through a sinter funnel. The recovered catalyst was
washed several times with hot water and acetone, dried at
300 °C for 4 h and stored in a desiccator for another consecutive
reaction run. Pure organic compounds were afforded by evapor-
ation of the solvent in a rotary evaporator and the residue was
subjected to silica gel (60–120 mesh) column chromatography
using ethylacetate–petroleum ether as eluents. The products were
characterized by standard analytical techniques such as 1H
NMR, 13C NMR, FTIR, elemental analysis, melting point deter-
mination, X-ray crystallographic analysis and all gave satisfac-
tory results (see ESI†).
Experimental
Synthesis of ZnTiO3 nanopowder catalyst
The ZnTiO3 powders were prepared by sol–gel method using
zinc acetate [(CH3CO2)2Zn], ethylene glycol (HOCH2CH2OH),
titanium butoxide (C16H36O4Ti) and citric acid anhydrous
[C3H4(OH)(COOH)3] (purity of the starting materials greater
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Experimental procedure for preparation of intermediate 6
gave satisfactory results. All the other entries incorporated in
Table 1 have been performed in a similar fashion.
Ketone 1 (2 mmol), aldehyde 2 (2 mmol), malononitrile 3
(4 mmol) and 0.2 mmol of activated ZnTiO3 nanopowder cata-
lyst were added to 2 mL water and the reaction mixture was
stirred at r.t. for 5 min. To the reaction mixture, ethyl acetate
(5 mL) was added and the solid catalyst was separated from the
mixture by filtering through sinter funnel. The solvents were
evaporated in rotary evaporator and the residue was subjected to
silica gel (60–120 mesh) column chromatography using ethyl-
acetate–petroleum ether as eluents to give Knoevenagel product
6 (357 mg, 97%) as a light yellow solid.
Detailed experimental procedure to confirm the presence of
Ti4+ in ZnTiO3 prevents the formation of HCN at r.t.
(Table 1, entry 15)
4′-Methoxy acetophenone 1f (2 mmol), 4-methoxy benzal-
dehyde 2a (2 mmol), malononitrile 3 (4 mmol) and 0.2 mmol of
activated ZnTiO3 nanopowder catalyst were added to 2 mL water
and the reaction mixture was stirred at r.t. for 3 h. After com-
pletion of the reactions (monitored by disappearance of the start-
ing material in the thin layer chromatography), the crude reaction
mixture was filtered and washed 10 times with hot water. To the
combined filtrate several drops of AgNO3 was added. No white
precipitate of AgCN was obtained indicating no cyanide in the
medium. Moreover when the same reaction was carried out in
presence of other catalysts, the above test was positive. Thus,
from the above test we can confirm that the presence of Ti4+ in
ZnTiO3 prevents the elimination of HCN at r.t.
Experimental procedure for preparation of intermediate 9
Ketone 1 (2 mmol), aldehyde 2 (2 mmol), malononitrile 3
(4 mmol) and 0.2 mmol of activated ZnTiO3 nanopowder cata-
lyst were added to 2 mL water and the reaction mixture was
stirred at r.t. for 45 min. To the reaction mixture, ethyl acetate
(5 mL) was added and the solid catalyst was separated from the
mixture by filtering through sinter funnel. The solvents were
evaporated in rotary evaporator and the residue was subjected to
silica gel (60–120 mesh) column chromatography using ethyl-
acetate–petroleum ether as eluents to give Aldol product 9
(515 mg, 96%) as a white solid.
Detailed experimental procedure to confirm the formation of
HCN at 100 °C (Table 1, entry 16) and reusability of the catalyst
A mixture of 4′-methoxy acetophenone 1f (2 mmol), 4-methoxy
benzaldehyde 2a (2 mmol), malononitrile 3 (4 mmol) and acti-
vated ZnTiO3 nanopowder catalyst (0.2 mmol) in water (2 mL)
were heated under reflux at 100 °C for 3 h. After completion of
the reactions (monitored by disappearance of the starting
material in the thin layer chromatography), the crude reaction
mixture was just filtered and to the filtrate several drops of
AgNO3 was added. No white precipitate of AgCN was obtained
indicating no cyanide in the medium. However, after thorough
washing the crude mixture (contained both ZnTiO3 and the aro-
matic product) 10 times with hot water when 2 drops of AgNO3
was added to the combined filtrate a white precipitate of AgCN
was obtained. The foregoing results established that the CN− in
the medium came from the trapped HCN which is adsorbed
inside the nano-pores of ZnTiO3 and did not occupy lattice sites
or interstitial position or covalently bonded with either Ti4+ or
Zn2+ which would otherwise result in the shifting of the original
peaks not observed in the XRD pattern. After further heating the
ZnTiO3 to 300 °C for 4 h any residual HCN was removed. The
water extract after further washing the catalyst did not contain
any trace of CN−. The recovered catalyst was stored in a desicca-
tor for another consecutive reaction run.
Experimental procedure for preparation of intermediate 10
Ketone 1 (2 mmol), aldehyde 2 (2 mmol), malononitrile 3
(4 mmol) and 0.2 mmol of activated ZnTiO3 nanopowder cata-
lyst were added to 2 mL water and the reaction mixture was
stirred at r.t. for 1.30 h. To the reaction mixture, ethyl acetate
(5 mL) was added and the solid catalyst was separated from the
mixture by filtering through sinter funnel. The solvents were
evaporated in rotary evaporator and the residue was subjected to
silica gel (60–120 mesh) column chromatography using ethyl-
acetate–petroleum ether as eluents to give intermediate 10
(607 mg, 96%) as a yellow solid.
General procedure for the synthesis of non-aromatic
intermediate and aromatic compound catalyzed by ZnO
nanopowder (Table 1, entry 10)
4′-Methoxy acetophenone 1f (2 mmol), 4-methoxy benzal-
dehyde 2a (2 mmol), malononitrile 3 (4 mmol) and 0.2 mmol of
ZnO nanopowder catalyst were added to 2 mL water and the
reaction mixture was stirred at r.t. for 24 h. After completion of
the reaction two spots—one of aromatic product 4fa and the
other of non-aromatic compound 5fa—were monitored in the
thin layer chromatography. Ethyl acetate (5 mL) was added to
the whole reaction mixture and the solid catalyst was separated
from the mixture by filtering through a sinter funnel. Pure
organic compounds were afforded by evaporation of the solvent
in a rotary evaporator and the residue was subjected to silica gel
(60–120 mesh) column chromatography using ethylacetate–
petroleum ether as eluents. The products were characterized by
standard analytical techniques such as 1H NMR, 13C NMR,
FTIR, elemental analysis, melting point determination and all
Acknowledgements
One of the authors (PD) thanks the Council of Scientific and
Industrial Research, New Delhi for her fellowship (SRF). We
thank the CAS Instrumentation Facility, Department of Chem-
istry, University of Calcutta for spectral data. We also acknowl-
edge grant received from the UGC funded Major project, F. No.
37-398/2009 (SR) dated 11-01-2010.
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