A new porous organic polymer containing Tr?ger's base units: Evaluation of the catalytic activity in Knoevenagel condensation reaction

Article abstract of DOI:10.1016/j.reactfunctpolym.2021.104998

The classic Tr?ger's base polymerization of a diamine and dimethoxymethane with trifluoroacetic acid as catalyst generated a Tr?ger's base-type polymer (TBP), which exhibited the absolute insolubility in a variety of organic solvents because of its highly aggregated model. A new porous organic polymer was obtained by a simple Tr?ger's base polymerization reaction between a diamine and formaldehyde in the form of acetal in the presence of trifluoroacetic acid as catalyst. Tr?ger's base-type polymer (TBP) resulted insoluble in a wide range of organic solvents due to its rigid and aromatic structure. TBP was characterized spectroscopically (FT-IR), thermally and morphologically. As result, a thermostable and amorphous polymer bearing pores ranging between 50 and 300 nm and macro-voids of up to 12 μm was obtained. Due to the insolubility of the TBP, it was tested as a metal-free heterogeneous catalyst in the Knoevenagel condensation reaction, showing a high efficiency. For this, the optimal catalyst load, reaction time and reuse of the catalyst were studied using benzaldehyde and malononitrile as substrates. Furthermore, aldehydes with variable chain sizes and ethyl cyanoacetate replacing malononitrile were tested as substrate with a high percent of conversion (97–99%).

Full text of DOI:10.1016/j.reactfunctpolym.2021.104998

Reactive and Functional Polymers 167 (2021) 104998
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Perspective Article
¨
A new porous organic polymer containing Troger’s base units: Evaluation
of the catalytic activity in Knoevenagel condensation reaction
a
a
a
a
g
´
´
Fidel E. Rodríguez-Gonzalez , Vladimir Niebla , M.V. Velazquez-Tundidor , Luis H. Tagle ,
,
d
c
e
,
f
Rudy Martin-Trasancob, Deysma Collc , Pablo A. Ortiz , Nestor Escalona , Edwin Perez ,
´
´
Ignacio A. Jessoph, Claudio A. Terrazaa *, Alain Tundidor-Cambaa
,
i,
,i,*
a
´
Research Laboratory for Organic Polymers (RLOP), Department of Organic Chemistry, Pontificia Universidad Catolica de Chile, Santiago, Chile
b
´
Departamento de Química, Universidad Tecnologica Metropolitana, J. P. Alessandri 1242, Santiago, Chile
c Centro de Nanotecnología Aplicada, Facultad de Ciencias, Universidad Mayor, Santiago, Chile
d Núcleo de Química y Bioquímica, Facultad de Estudios Interdisciplinarios, Universidad Mayor, Santiago, Chile
e
´
˜
Departamento de Ingeniería Química y Bioprocesos Escuela de Ingeniería Pontificia Universidad Catolica de Chile, Av. Vicuna Mackenna 4860, Macul, Santiago, Chile
f ANID – Millennium Science Initiative Program-Millennium Nuclei on Catalytic Process towards Sustainable Chemistry (CSC), Santiago, Chile
g
´
Department of Organic Chemistry, Faculty of Chemistry and of Pharmacy, Pontificia Universidad Catolica de Chile, Santiago, Chile
h
´
Laboratory of Organic and Polymeric Materials, Faculty of Chemistry, Universidad de Tarapaca, P.O. Box 7-D, Arica, Chile
i
´
UC Energy Research Center, Pontificia Universidad Catolica de Chile, Santiago, Chile
A R T I C L E I N F O
A B S T R A C T
Keywords:
¨
The classic Troger’s base polymerization of a diamine and dimethoxymethane with trifluoroacetic acid as
¨
Troger’s base
¨
catalyst generated a Troger’s base-type polymer (TBP), which exhibited the absolute insolubility in a variety of
Porous organic polymers
Knoevenagel condensation reaction
Catalytic activity
organic solvents because of its highly aggregated model. A new porous organic polymer was obtained by a simple
¨
Troger’s base polymerization reaction between a diamine and formaldehyde in the form of acetal in the presence
¨
of trifluoroacetic acid as catalyst. Troger’s base-type polymer (TBP) resulted insoluble in a wide range of organic
Quantitative 1H NMR
solvents due to its rigid and aromatic structure. TBP was characterized spectroscopically (FT-IR), thermally and
morphologically. As result, a thermostable and amorphous polymer bearing pores ranging between 50 and 300
nm and macro-voids of up to 12 μm was obtained. Due to the insolubility of the TBP, it was tested as a metal-free
heterogeneous catalyst in the Knoevenagel condensation reaction, showing a high efficiency. For this, the
optimal catalyst load, reaction time and reuse of the catalyst were studied using benzaldehyde and malononitrile
as substrates. Furthermore, aldehydes with variable chain sizes and ethyl cyanoacetate replacing malononitrile
were tested as substrate with a high percent of conversion (97–99%).
1. Introduction
thermal resistance, permanent porosity, low density, various functional
groups on the structure and most of them are insoluble in many common
Porous organic polymers (POPs) have gained attention from both the
academic and industrial world due to their recognized applications in
gas separation [1–5], storage [6,7], adsorption [8,9], bio-related ap-
plications [10], water treatment [11], catalysis [12–17], and so on.
Although POPs include hypercrosslinked polymers (HCPs), polymers of
intrinsic microporosity (PIM), conjugated microporous polymers
(CMPs), porous aromatic frameworks (PAFs) and covalent organic
frameworks (COFs), here, we used the term POPs to refer to amorphous
porous organic polymer materials.
organic solvents. These two last characteristics make POPs excellent
candidates as heterogenous catalysts due to their ease separation from
the medium reaction and the possibility of recycling the catalyst
[18,19].
Chen et al synthesized two POPs (specifically COFs) bearing amide
functional groups, which were insoluble in conventional solvents and
stable under strong acid/basic conditions. Both materials were tested as
heterogeneous catalysts exhibiting high catalytic activity towards
Knoevenagel condensation reaction, and with an excellent recyclability
[20]. Li et al designed and synthesized a novel POP with thiourea
POPs exhibit high specific surface area, good chemical stability, high
* Corresponding authors.
E-mail addresses: cterraza@uc.cl (C.A. Terraza), atundido@uc.cl (A. Tundidor-Camba).
https://doi.org/10.1016/j.reactfunctpolym.2021.104998
Received 10 May 2021; Received in revised form 23 July 2021; Accepted 24 July 2021
Available online 27 July 2021
1381-5148/© 2021 Published by Elsevier B.V.

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
linkages, which effectively catalysed the Michael reaction of trans-
β-nitrostyrenes to diethyl malonate and also exhibited good recyclability
and reusability [21]. Moorthy et al prepared a heterogeneous catalyst via
Fridel-Crafts polyalkylation followed by sulfonation to yield a POP
AVANCE III HD-400) using DMSO‑d6 and CDCl3 as solvents. Thermog-
ravimetric analysis (TGA) was performed using a thermogravimetric
balance TGA-7 Perkin Elmer under a nitrogen atmosphere with a heating
rate of 20 ◦C/min from 25 ◦C to 850 ◦C. Elemental analyses were made
on a Fisons EA 1108-CHNS-O equipment. The morphology of the poly-
mer was studied using a scanning electron microscope (Zeizz, model
EVO MA 10, Oberkochen, Germany) to guarantee the conduction of
sample. The polymer was coated with gold using a Cressington-108 auto
sputter coater (Zeizz, Oberkochen, Germany). The analysis of SEM mi-
crographs was performed using the free ImageJ (version 1.46 J/Fiji)
software from the National Institute of Health, Bethesda, MD, USA [31].
Brunauer–Emmett–Teller (BET) surface area was determined from CO2
adsorption at 273 K using a Micromeritics 3Flex.
¨
grafted with sulfonic acid groups. That polymer with Bronsted acid
moieties was able to catalyze the synthesis of a variety of photochromic
diarylbenzopyrans/napththopyrans and pharmacologically relevant
triazoles at room temperature [22].
¨
Troger’s base (TB) is a bridged bicyclic compound obtained from the
reaction between an aromatic amine (p-toluidine) and formaldehyde in
the presence of a strong acid [23]. TBs have been investigated in het-
erogeneous catalysis in the form of additive [24], modifier [25], ligand
[26] or functionalized on nanoporous organic polymers [27]. In the
latter case, TB structure was incorporated in a dihalogenated monomer,
which was polymerized by the Sonogashira–Hagihara cross-coupling
reaction using 1,3,5-triethynylbenzene as a linker.
2.3. Intermediate, monomer and polymer synthesis
2.3.1. 2,7(8)-dinitrodibenzo[b,e][1,4]dioxine (3)
When an aromatic diamine is chosen and equivalent amounts of
¨
formaldehyde are used, a Troger’s base polymerization reaction takes
¨
place generating a rigid POP. However, the Troger’s base polymeriza-
b
a
tion reaction have never been employed to synthesize a polymer till
N
O2
O
O
c
¨
Mckeown et al prepared a POP (specifically a PIM) derived from Troger’s
NO2
base polymerization, which showed a high BET surface area greater than
d
1000 m2 gꢀ 1 [28]. Recently, the same authors prepared another polymer
f
e
¨
derived from Troger’s base polymerization reaction, which showed a
BET surface area of 743 m2 gꢀ 1 and was highly selective for gas sepa-
ration [29]. The above two examples yielded polymers soluble in com-
mon organic solvents like chloroform and THF. That high solubility was
due to the height and number of bridges present in the polymer chain.
So, applications in heterogeneous catalysis are not possible due to the
good solubility of those materials.
A mixture of 4-nitrobenzene-1,2-diol (1, 3.0 g, 19.3 mmol), 1,2-
difluoro-4-nitrobenzene (2, 3.1 g, 19.3 mmol), KOH (3.2 g, 58.0
mmol) and anhydrous DMSO (40 mL) in a 100 mL two-necked round-
bottomed flask was stirred under nitrogen atmosphere at 100 ◦C for 24
h, and then it was poured into 200 mL of water with stirring. The yellow
solid was filtered off, washed first with water and then with methanol,
and dried at 80 ◦C for 12 h. The isomeric mixture 3 was recrystallized
from EtOH to give an yellow powder solid (4.2 g, 80%). Mp: 263–265 ◦C.
¨
In the aforementioned reports the Troger’s base units were incor-
porated to a solid matrix to make it insoluble for heterogenous catalysis
, cmꢀ 1): 3090, 3050 (C H arom.); 1595, 1499 (C C arom.); 1525,
–
IR (υ
–
¨
or in some cases the polymers bearing Troger’s base were soluble
–
1360 (NO2), 1280 (C-O-C). 1H NMR (400.13 MHz, DMSO‑d6, δ, ppm):
7.88 (d, J = 8.5 Hz, 2H, Hd); 7.77 (s, 2H, Hb); 7.23 (d, J = 8.8 Hz, 2H,
He). 13C NMR (100.62 MHz, DMSO‑d6, δ, ppm): 146.12 (Cf); 144.31 (Cc);
140.9 (Ca); 1213 (Cd); 117.7 (Cb); 112.3 (Ce). Elem. Anal. Calcd. for
whereby its use must be for homogeneous catalysis. Herein, we have
¨
presented the synthesis of a new POP from Troger’s base polymerization
reaction using an inexpensive and easy-to-obtain diamine. The absence
of bulky groups in the structure allowed efficient chain packing,
generating an insoluble material, which was tested as a metal/matrix-
free heterogeneous organic catalyst in the Knoevenagel condensation
reaction. It is common the use of pyridine as catalyst in this kind of
reaction. Pyridine is a hazard compound and chronic exposure may
cause serious harm [30]. Our new POP would be a good candidate to
replace pyridine in this kind of reactions.
C
12H6N2O6 (274.19): C, 52.57%; H, 2.21%; N, 10.22%. Found: C,
52.21%; H, 2.13%; N, 10.11%.
2.3.2. dibenzo[b,e][1,4]dioxine-2,7(8)-diamine (4)
g
H2N
b
a
f
O
O
c
2. Experimental section
NH2
d
e
2.1. Materials
Propionaldehyde (97%), butyraldehyde (≥ 96%), isobutyraldehyde
(≥ 99%), acetaldehyde (≥ 99.5%), 1-naphthaldeyde (95%), 1,3,5-trime-
thoxybenzene (Standard for quantitative NMR, TraceCERT®), malono-
nitrile (≥ 99%), ethyl cyanoacetate (≥ 98%), benzaldehyde (≥ 99.5%),
dimethoxymethane (DMM, 99%), trifluoroacetic acid (TFA, 99%),
anhydrous dimethyl sulfoxide (DMSO, ≥ 99.9%), anhydrous potassium
hydroxide (≥ 99.95%), hydrazine monohydrate (80%) and Pd/C acti-
vated (10%) were purchased from Sigma-Aldrich-Merck (Milwaukee,
WI, USA). 4-Nitrobenzene-1,2-diol and 1,2-difluoro-4-nitrobenzene
were obtained from AK Scientific Inc. Company (San Francisco, USA).
All other reagents and solvents were purchased commercially as
analytical grade and used without further purification.
A mixture of 2,7(8)-dinitrodibenzo[b,e][1,4]dioxine (3, 4.0 g, 14.6
mmol), 10% palladium on activated carbon (200 mg) and 100 mL of
absolute ethanol was stirred under a nitrogen atmosphere at reflux
temperature. Dropwise, hydrazine monohydrate (80%, 10.0 mL, 200
mmol) was added to the mixture and then was left under refluxing for 6
h. The hot solution was filtered through a Celite plug and the solvent was
removed under reduced pressure until half volume. The solution was
cooled in an iced-water bath and the product crystallized from ethanol.
The solid was filtered, washed with cold ethanol, and dried at 70 ◦C for
12 h to afford 2.6 g of a pale orange solid. The product was sublimated
–
before used. Mp: 244–246 ◦C. IR (
υ
, cmꢀ 1): 3450, 3365 (N H); 3072,
–
–
–
3043 (C H arom.); 1619 (N H flexion); 1590, 1500 (C C arom.);
–
1288 (C-O-C). 1H NMR (400.13 MHz, DMSO‑d6, δ, ppm): 6.63 (d, J =
8.5 Hz, 2H, He); 6.14 (m, 4H, Hb, d); 4.90 (s, 4H, Hg). 13C NMR (100.62
MHz, DMSO‑d6, δ, ppm): 145.77 (Cc); 142.62 (Ca); 132.19 (Cf); 116.80
(Ce); 108.71 (Cd); 101.90 (Cb). Elem. Anal. Calcd. for C12H10N2O2
(214.22): C, 67.28%; H, 4.71%; N, 13.08%. Found: C, 67.11%; H, 4.58%;
N, 13.01%.
2.2. Instrumentation and measurements
FT-IR spectra (KBr pellets) were recorded on a Nicolet 8700 Thermo
Scientific FTIR spectrophotometer over the range of 4000–450 cmꢀ 1. 1H
and 13C NMR spectra were recorded on a 400 MHz instrument (BRUKER
2

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
2.4. Synthesis of TBP
3. Results and discussion
3.1. Synthesis and characterization
N
O
Intermediate dinitro 3 was easily prepared by a double aromatic
nucleophilic substitution reaction between an in situ generated nucleo-
phile, potassium 4-nitrobenzene-1,2-bis(olate), and an ortho-difluor-
oarene compound bearing an electron-withdrawing group as
electrophile, 1,2-difluoro-4-nitrobenzene (Scheme 1).
N
O
TBP
n
The product labeled as 3 was obtained as an isomeric mixture with
the nitro groups at 2,7 or 2,8 positions, albeit according to the electronic
properties of the reactants isomer 2,7 should be obtained in major
proportion. The inductive and resonance effects of the nitro group in
compound 1 strongly diminish the basicity/nucleophilic power of the
two phenolic groups in catechol [33], but OH at position 2 is a stronger
nucleophile than OH at position 1. So, kinetically, OH at position 2
quickly attack to the most electrophile carbon atom in 1,2-difluoro-4-
nitrobenzene (C bonded to F at position 1). In addition, the obtained
melting point for 2,7-isomer was similar to that previously reported in
the literature [34]. However, the minor product, 2,8-dinitrodibenzo[b,e]
[1,4]dioxine was also obtained. 1H and 13C NMR spectra (Fig. 1) for
compound 3 showed the presence of a majority product as well as a
minority one, which had similar chemical shifts.
In a round bottom flask equipped with a magnetic stirrer bar and
under nitrogen atmosphere dimethoxymethane (2.1 mL, 23.3 mmol)
was added to dibenzo[b,e][1,4]dioxine-2,7(8)-diamine (4, 1.000 g, 4.6
mmol). The flask was cooled to 0 ◦C in an iced-water bath and tri-
fluoroacetic acid (10.0 mL) was added dropwise until most of the solid
was dissolved. At that point, more trifluoroacetic acid (25 mL) was then
added slowly and the mixture was stirred for 12 h. Then, the orange
mixture was poured slowly into a cold ammonia aqueous solution to
precipitate the polymer. The stirring was remained for 1–2 h and the
polymer was then filtered, washed with acetone and then with ethanol.
Polymer was dried at 100 ◦C for 12 h (0.825 g, 70%). IR (
υ
, cmꢀ 1): 3022
–
–
–
(C H arom.); 2957, 2875 (C H aliph.); 1499 (C C arom.); 1370
–
–
(C N bending), 1264 (C-O-C).
Compound 3 was reduced in the presence of hydrazine and Pd/C to
obtain the required dibenzo[b,e][1,4]dioxine-2,7(8)-diamine (4) as an
isomeric mixture as well (Scheme 2).
2.5. Method of catalysis
In 1H and 13C NMR spectra for diamino compound 4 was possible to
see again the signals belonging to the minor isomer (Fig. 2).
The TB polymerization procedure [28] was conducted to obtain the
polymer labeled as TBP. Since there is no control over the regiochem-
istry due to the presence of both isomers in the monomeric diamine and
the various orientations of the methylene bridges of the TB linking
group, the growth of the polymer chains will be random with various
spatial conformations. (Scheme 3). TBP was only spectroscopically
characterized by FT-IR technique because it was insoluble in any organic
solvent.
The general catalysis reaction was performed by mixing benzalde-
hyde (3 equiv.) with malononitrile (1 equiv.) in the presence of TBP
(10% mmol regarding to malononitrile) at room temperature and at-
mospheric conditions under constant stirring for 24 h. The percentage of
conversion was measured by taken off an aliquot from the reaction
mixture, dissolved in CDCl3 and analyzed by quantitative 1H NMR
spectroscopy. Eq. (1) was used to calculate the percentage of conversion
according to the relative method from the integrated proton peaks of the
product and the limiting reactant [32].
IP/NP
IR/NR + IP/NP
%Conversion =
⋅100
(1)
The reaction yield was also measured by quantitative 1H NMR using
a known quantity of an internal standard that was added to the reaction
just before to the NMR sample was prepared. The reaction yield was
calculated using the eq. 2.
3.2. Surface area, solubility, TGA and SEM
In Table 1 is summarized the surface area value, solubility and TGA
results. The adsorption of CO2 was carried out instead of N2 because the
measurements with N2 the surface BET values were very low, suggesting
that the pores of POP are contracted or that the adsorption kinetics of N2
is very low at 77 K. The isotherm carbon dioxide adsorption revealed a
BET surface area of 256 m2 gꢀ 1. As the material adsorbs CO2, this sug-
gests that the material has pores in the microporous and mesopore re-
gion close to 2 nm. Most likely, it has pores in the micro to mesopore
region, which makes sense so that at 77 K it absorbs almost nothing.
Also, SEM images show pores between 50 and 350 nm. Those pores are
only the mouth of the pore, and they must be connected to the meso-
pores and later micropores towards the internal structure of the mate-
rial. These results confirm that the material has a low microporous
contribution.
IP⋅Nstd⋅mstd⋅νR⋅M
%Yield =
R⋅100
(2)
NP⋅Istd⋅Mstd⋅νP⋅mR
In these equations, I is the integrated area, N is the number of pro-
tons, m is the mass, ν is the reaction stoichiometric number and M is the
molar mass of the product (P). The internal standard (std) was 1,3,5-tri-
methoxybenzene and (R) refers to limiting reactant.
The initial reaction rate, r0 (mol gꢀ 1 sꢀ 1) was calculated from the
slope (b) according to the formula:
b⋅n0
mcat⋅100
r0 =
(3)
Where n0 (mol) is the initial mol used and mcat (g) is the mass of the
catalyst.
TBP was tested in several common organic solvents to obtain in
qualitative way the solubility of the material. For that, 10–30 mg of the
N
O2
O
N
NO2
O2
O
O
N
OH
OH
O2
F
F
NO2
KOH
DMSO, 100 °C
NO2
O
2,7-isomer
2,8
iso
(1)
(2)
mer
(3)
Scheme 1. Synthetic route to 2,7(8)-dinitrodibenzo[b,e][1,4]dioxine (3).
3

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
Fig. 1. 1H and 13C NMR spectra for compound 3 in DMSO‑d6.
Scheme 2. Synthetic route to dibenzo[b,e][1,4]dioxine-2,7(8)-diamine (4).
Fig. 2. 1H and 13C NMR spectra for compound 4 in DMSO‑d6.
Scheme 3. Synthetic route to TBP and different possibilities of chain growth and some examples of the regiochemistry within the repeat unit.
4

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
Table 1
BET surface area, thermal analysis, and solubility results.
a
Polymer
BET
Td10% (◦C)
Rb
Solubility
DMSO
ꢀ
(m2 gꢀ 1
)
(%)
c
NMP
DMF
DMAc
THF
CHCl3
H2SO4
TBP
a
b
c
256
530
63
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
+
Thermal decomposition temperature at which 10% weight loss.
Residual yield. Solubility: (ꢀ ) Insoluble even under heating; (+) Soluble.
Concentrated sulfuric acid: solution turned dark reddish.
material was placed into a glass tube and 1 mL of each solvent was
added. The mixture was stirred at room temperature and at the boiling
temperature of the solvents. In any cases, the material was soluble. Even
the solvents remain colourless. Only in the presence of concentrated
sulfuric acid the solution turned dark reddish.
Table 1 (spectra 2–4) and one reaction under the same experimental
conditions used in entry 2 but employing the internal standard 1,3,5-tri-
methoxybenzene (spectrum 5), which was added just before to the NMR
measurement. The catalytic efficiency was defined by varying the molar
ratio of TBP regarding to malononitrile. The signal of the olefinic proton
of benzylidenemalononitrile (product P) at 7.78 ppm and the signal of
the protons of malononitrile (limiting reagent R) at 3.68 ppm were used
to determine the percentage of conversion in each case using eq. 1.
Table 2 shows that the percentage of conversion increase while
increasing the catalyst amount. Entry 4 revealed the highest catalytic
efficiency for 10% mmol of catalyst with a conversion of 98%. In
addition, with the aim to determine whether the conversion percentage
was similar to the reaction yield, a known amount of 1,3,5-trimethoxy-
benzene as NMR standard reference was added to one reaction just
before to the NMR measurement. The signal of the aromatic protons of
this reference at 6.09 ppm was used to calculate the reaction yield
through eq. 2. As it appears, practically all malononitrile consumption
was converted to the product due to the high similarity between the
calculated conversion percentage and reaction yield (Entry 5, Table 1).
Once established the optimal amount of catalyst, the optimal time
reaction was studied using the same substrates. Every hour a small
amount of the reaction mixture was taken out and analyzed by NMR.
Each sample was taken from different vials. As seen in Table 3, the
conversion was increasing in time, reaching 62% after 5 h of reaction. In
order to increase this parameter, the experiment was left overnight (24
h) registering 98% of conversion (Entry 6).
Thermogravimetric analysis showed a thermally stable material that
it began to lose weight at ~400 ◦C with a 10% loss of mass below 530 ◦C.
In the DTGA graph (Fig. 3a, inside), was possible to see two mainly
decomposition peaks, one at ~400 ◦C probably due to the thermal
degradation of the aliphatic TB liking groups and a second at~630 ◦C
related to the decomposition of the main chain of the polymer. In order
to evaluate the morphology of the polymer and the presence of pores the
SEM micrographs were recorded to the TBP polymer. Accordingly, a
roughness surface with distributed pores with sizes diameter ranging
from 50 to 300 nm were observed on it (arrows in Fig. 3b). Additionally,
larger macro-voids (10–12 μm) were also observed (Fig. 3c). Irregular
pores of different sizes and shapes influence capillary flow through
them, allowing the solvent to move through the channels, transporting
the reactant molecules to the sites of catalysis. The understanding of this
spontaneous phenomenon has become increasingly important for many
authors, who have proposed fractal models for capillary flow [35–38].
3.3. Catalysis performance
Catalytic study for TBP was conducted in Knoevenagel condensation
reaction [39], which requires a base as a catalyst, usually pyridine. Due
The graph Conversion (%) vs Reaction time (s) (Fig. 5) showed a
linear behavior, suggesting a pseudo first order dependence on the rate
of malononitrile conversion. The value of the initial reaction rate was
¨
to the basic nature of the Troger’s base, which has a basic strength su-
perior to the aromatic amines like anilines [40], it is reasonable to study
the use of TBP as a free-metal heterogeneous catalyst in replacement of
pyridine.
1.36 ⋅ 10ꢀ 6 mol gꢀ 1 sꢀ 1
.
The optimal amount of catalyst and time reaction were high
Various parameters in this reaction were studied. At first, the reac-
tion conditions were defined, in which benzaldehyde acted both as a
reagent and as a solvent. The molar ratio between benzaldehyde and
malononitrile was 3:1, and the reaction was made at room temperature
¨
compared with other reports, where materials bearing Troger’s base in
its structures were tested in the catalysis of the Knoevenagel conden-
sation reaction [41,42]. That would be due to the low surface area found
for POP in comparison with the values near to 1000 m2 gꢀ 1 reported in
these previous works.
1
and atmospheric conditions. Fig. 4 shows the H NMR signals for the
mixture of benzaldehyde and malononitrile without catalyst (spectrum
1), the reaction crudes using different catalyst percentages defined in
The reusability of the catalyst was also studied for the same
Fig. 3. a) TGA and DTGA to TBP. b) Spongelike pores. c) Macro-voids in TBP.
5

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
Fig. 4. 1H NMR to determine % conversion and yield for catalytic reaction. See Table 1 for more details.
Table 2
Optimization of the catalyst amount according to the percentage of conversion.
Entry
Electrophile
(3 equiv)
Nucleophile
(1 equiv)
TBP
Time (h)
Conversion
(%)
Yield
(%)
(% mmol)1
1
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
none
3
24
24
24
24
24
0
–
2
86
94
98
88
–
3
5
–
–
87
4
10
3
5*
1
Regarding to malononitrile. *Internal standard: 1,3,5-trimethoxybenzene. All experiments were developed at room temperature and atmospheric conditions.
Table 3
Optimization of the time reaction according to the percentage of conversion.
Entry
Electrophile
(3 equiv)
Nucleophile
(1 equiv)
TBP
Time (h)
Conversion
(%)
(% mmol)1
1
2
3
4
5
6
7
8
9
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
10
10
10
10
10
10
10
10
10
1
12
24
36
54
65
78
89
95
98
2
3
4
5
6
7
8
24
1
Regarding to malononitrile. All experiments were developed at room tem-
perature and atmospheric conditions.
Fig. 5. Conversion (%) vs Reaction time (s).
6

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F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
substrates. In this sense, 10% mmol of TBP was continuously reusing.
Each time the catalyst was washed with chloroform, and then with
acetone, dried at 80 ◦C and used again. Table 4 summarizes the behavior
of the catalyst after each cycle in relation to the conversion. As it seems,
after the fifth cycle, the percentage of conversion was considerably
diminished from 94% in the fourth cycle to 85% in the last one. The
decrease in catalytic activity may be due to the decrease in surface area
and as well as in pore volume after each test [43]. In order to corroborate
that result adsorption of CO2 was carried out again to TBP after the fifth
catalytic cycle, showing a decrease of 22% in surface area (256 to 198
m2 gꢀ 1). Probably, some pores collapsed or closed during the catalytic
process due to the amorphous character of the material.
Table 5
Results of the versatility study for the reaction catalysed by TBP.
Entry
Electrophile
(3 equiv)
Nucleophile
(1 equiv)
TBP
Time
(h)
Conversion
(%)
(%
mmol)1
1
1-naphthaldeyde
propionaldehyde
butyraldehyde
isobutyraldehyde
acetaldehyde
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
ethyl
10
10
10
10
10
10
24
24
24
24
24
24
97
99
98
97
98
99
2
3
4
5
6*
benzaldehyde
cyanoacetate
1
Regarding to nucleophile. *Reaction was heated at 50 ◦C, while entry 1–5
The versatility of the reaction using TBP as catalyst was tested with
other substrates, mainly aldehydes, employing the optimal conditions
that were defined previously. Benzaldehyde was replaced by a series of
four aliphatic aldehydes and a naphthyl derivative aldehyde, all of them
playing an electrophilic role, while malononitrile was replaced as
nucleophile agent by a mono-cyane ester compound with less acidic
hydrogens. Table 5 shows the exact nature of the tested compounds and
their associated percentage of conversion.
were made at room temperature and atmospheric conditions.
efficient ranged between 60 and 70 ◦C, while our catalyst works well at
room temperature. Another interesting work was the reported by the
same authors but this time they prepared a robust microporous frame-
work bearing TmIII as metal by employing a ligand-directed synthetic
strategy [46]. The catalyst was labeled as NUC-25. Again, the amount of
catalyst was lower than the used by us (0.04 mmol against 10 mmol) but
the temperature employed by them was 80 ◦C during the same time
reaction (24 h). However, when the authors prepared the catalyst named
NUC-27, they obtained a high yield (99%) using 0.03 mmol of the
For the tested aldehydes, the percentage of conversion ranged be-
tween 97 and 99% demonstrating a high catalytic activity against sub-
strates with different steric hindrances. In the anionic mechanism of the
Knoevenagel condensation, four steps are proposed [44]: 1) acid-base
reaction between malononitrile and catalyst (fast step); 2) nucleo-
philic attack of the malononitrile carbanion to carbonyl carbon (fast
step); 3) proton exchange (fast step); 4) elimination of hydroxide to yield
the condensation product (slow step or determining step). Regarding to
this, the electrophilicity of the aldehyde is not decisive but the elimi-
nation step produces a more thermodynamically stabilized product
when an aromatic aldehyde is used (caused by an increase of the con-
jugate system). Nevertheless, this last step must occur with the assis-
tance of the protonated N atom in the POP surface (inside or outside the
pores), so, from kinetically point of view aliphatic aldehydes with less
steric hindrance would react faster. In conclusion, both type of alde-
hydes reacts with malononitrile in presence of our new catalyst in good
yields under conditions described in Table 5. In the case of entry 6, the
reaction was not developed at room temperature, but by heating at 50 ◦C
the conversion was quantitative. Fig. 6 shows the 1H NMR spectra for
these reactions.
◦
catalyst at 60 C in just one hour of reaction [47]. Obviously, hetero-
geneous metal catalysts show better catalytic performance even at low
catalyst load and moderate reaction conditions, although obtaining the
porous framework can be a more complex synthetic process.
From the point of view of free-metal heterogeneous catalysts, the
obtained results in our research proved that TBP could be employed as
an ideal substitute for the conventional catalyst of pyridine on the
Knoevenagel condensation reaction because of its versatilities including
high catalytic efficiency, recyclability and chemical stability.
4. Conclusions
A mixture of isomeric diamines was obtained from the double aro-
matic nucleophilic substitution reaction between 4-nitrobenzene-1,2-
diol and 1,2-difluoro-4-nitrobenzene followed by a catalytic reduction
reaction using the hydrazine and Pd/C system. TBP was synthesized
from a TB polymerization reaction in 70% yield. Due to its aromatic
nature and rigid structure, the POP obtained was insoluble and ther-
mostable. A BET surface area of 256 m2 gꢀ 1 and the presence of pores
and macro-cavities were observed by SEM revealed the porous nature of
The olefinic proton signal of each product was used to evaluate the
corresponding conversion. In the spectrum related to entry 4, the signal
of the limiting reactant (ethyl cyanoacetate) at 3.50 ppm was used to
calculate the percentage of conversion and, the signal of malononitrile at
3.70 ppm was used for others. Seemingly, the signal corresponding to
the limiting reactant is not observed in the spectra, only if the intensity
of the signals is increased it is possible to see the small peaks at the
mentioned positions (highlighted in the spectra, Fig. 6).
¨
the material. Considering the basic nature of the Troger’s base moieties,
the polymer was performed as a heterogeneous catalyst in the Knoeve-
nagel condensation reaction, showing good catalytic efficiency when a
load of 10% mmol of the polymer was used, achieving a percentage of
conversion greater than 96%. The efficiency of the catalytic process was
maintained for several substrates showing high versatility respect to
steric hindrances effects. Additional measurements about the
morphology (PXRD and SEM) of the catalyst must be carried out to know
how the size and shape of the pores changes after being used in various
catalytic cycles.
In order to compare our results with the metal-containing hetero-
geneous catalysts we look at the work reported by Zhang et al., in which
a dinuclear [CaZn(CO2)6(OH2)]-based nanoporous host framework was
prepared [45]. That heterogeneous catalyst displayed an excellent cat-
alytic activity for the Knoevenagel condensation reaction. The amount
of catalyst that was used by them was one thousand time less than the
employed in our work, however, the temperature to reach the maximum
Table 4
Reusability study.
Entry
Electrophile
(3 equiv)
Nucleophile
(1 equiv)
TBP
Time (h)
Cycle
Conversion
(%)
(% mmol)1
1
2
3
4
5
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
benzaldehyde
malononitrile
malononitrile
malononitrile
malononitrile
malononitrile
10
10
10
10
10
24
24
24
24
24
1st
97
99
98
94
85
2nd
3rd
4th
5th
1
Regarding to malononitrile. All experiments were developed at room temperature and atmospheric conditions.
7

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F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
Fig. 6. 1H NMR spectra for obtained products in the reaction catalysed by TBP.
[7] Y. Kou, Y. Xu, Z. Guo, D. Jiang, Supercapacitive energy storage and electric power
Data availability
supply using an Aza-fused π-conjugated microporous framework, Angew. Chem.
Int. Ed. 50 (2011) 8753–8757, https://doi.org/10.1002/anie.201103493.
[8] S. Wang, Y. Liu, Y. Ye, X. Meng, J. Du, X. Song, Z. Liang, Ultrahigh volatile iodine
capture by conjugated microporous polymer based on: N, N, N ′, N ′-tetraphenyl-
1,4-phenylenediamine, Polym. Chem. (2019), https://doi.org/10.1039/
c9py00288j.
The raw data required to reproduce these findings are included in the
article.
[9] S. Bhowmik, M. Konda, A.K. Das, Light induced construction of porous covalent
organic polymeric networks for significant enhancement of CO2 gas sorption, RSC
Adv. (2017), https://doi.org/10.1039/c7ra09538d.
Declaration of Competing Interest
[10] H. Zhang, G. Li, C. Liao, Y. Cai, G. Jiang, Bio-related applications of porous organic
frameworks (POFs), J. Mater. Chem. B 7 (2019) 2398–2420, https://doi.org/
10.1039/C8TB03192D.
The authors declare no competing financial interests.
[11] S. Bhowmik, R.G. Jadhav, A.K. Das, Nanoporous conducting covalent organic
polymer (COP) nanostructures as metal-free high performance visible-light
Photocatalyst for water treatment and enhanced CO 2 capture, J. Phys. Chem. C
122 (2018) 274–284, https://doi.org/10.1021/acs.jpcc.7b07709.
[12] C.Y. Lin, D. Zhang, Z. Zhao, Z. Xia, Covalent organic framework Electrocatalysts for
clean energy conversion, Adv. Mater. (2018), https://doi.org/10.1002/
adma.201703646.
Acknowledgements
´
This work was funded by the Agencia Nacional de Investigacion y
Desarrollo (ANID) through a Fondecyt project number 1190772 and
FONDEQUIP EQM 160070.
[13] S. Kramer, F. Hejjo, K.H. Rasmussen, S. Kegnæs, Silylative Pinacol coupling
catalyzed by nitrogen-doped carbon-encapsulated nickel/cobalt nanoparticles:
evidence for a Silyl radical pathway, ACS Catal. 8 (2018) 754–759, https://doi.
org/10.1021/acscatal.7b02788.
References
[14] P. Kaur, J.T. Hupp, S.T. Nguyen, Porous organic polymers in catalysis:
opportunities and challenges, ACS Catal. (2011), https://doi.org/10.1021/
cs200131g.
[1] C.H. Lau, K. Konstas, A.W. Thornton, A.C.Y. Liu, S. Mudie, D.F. Kennedy, S.
C. Howard, A.J. Hill, M.R. Hill, Gas-separation membranes loaded with porous
aromatic frameworks that improve with age, in: Angew. Chemie - Int. Ed, 2015,
https://doi.org/10.1002/anie.201410684.
[15] Y. Zhang, S.N. Riduan, Functional porous organic polymers for heterogeneous
catalysis, Chem. Soc. Rev. (2012), https://doi.org/10.1039/c1cs15227k.
[16] Q. Sun, Z. Dai, X. Meng, L. Wang, F.S. Xiao, Task-specific design of porous polymer
heterogeneous catalysts beyond homogeneous counterparts, ACS Catal. (2015),
https://doi.org/10.1021/acscatal.5b00757.
[2] G. Yu, H. Rong, X. Zou, G. Zhu, Engineering Microporous Organic Framework
Membranes for CO2 Separations, Mol. Syst. Des. Eng, 2017, https://doi.org/
10.1039/c7me00017k.
[3] S. Yuan, X. Li, J. Zhu, G. Zhang, P. Van Puyvelde, B. Van Der Bruggen, Covalent
organic frameworks for membrane separation, Chem. Soc. Rev. (2019), https://
doi.org/10.1039/c8cs00919h.
[17] Q. Sun, Z. Dai, X. Liu, N. Sheng, F. Deng, X. Meng, F.S. Xiao, Highly efficient
heterogeneous hydroformylation over rh-metalated porous organic polymers:
synergistic effect of high ligand concentration and flexible framework, J. Am.
Chem. Soc. (2015), https://doi.org/10.1021/jacs.5b02122.
[4] N.B. McKeown, P.M. Budd, Exploitation of intrinsic microporosity in polymer-
based materials, Macromolecules. 43 (2010) 5163–5176, https://doi.org/10.1021/
ma1006396.
[18] Y. Zhang, J.Y. Ying, Main-chain organic frameworks with advanced catalytic
functionalities, ACS Catal. 5 (2015) 2681–2691, https://doi.org/10.1021/
acscatal.5b00069.
[5] P.M. Budd, N.B. McKeown, Highly permeable polymers for gas separation
membranes, Polym. Chem. (2010), https://doi.org/10.1039/b9py00319c.
[6] P. Bhanja, S.K. Das, K. Bhunia, D. Pradhan, T. Hayashi, Y. Hijikata, S. Irle,
A. Bhaumik, A new porous polymer for highly efficient capacitive energy storage,
ACS Sustain. Chem. Eng. (2018), https://doi.org/10.1021/
[19] O.S. Taskin, S. Dadashi-Silab, B. Kiskan, J. Weber, Y. Yagci, Highly efficient and
reusable microporous Schiff Base network polymer as a heterogeneous catalyst for
CuAAC click reaction, Macromol. Chem. Phys. 216 (2015) 1746–1753, https://doi.
org/10.1002/macp.201500141.
acssuschemeng.7b02234.
8

´
F.E. Rodríguez-Gonzalez et al.
Reactive and Functional Polymers 167 (2021) 104998
[20] Y. Li, W. Chen, R. Gao, Z. Zhao, T. Zhang, G. Xing, L. Chen, 2D covalent organic
frameworks with built-in amide active sites for efficient heterogeneous catalysis,
Chem. Commun. 55 (2019) 14538–14541, https://doi.org/10.1039/C9CC07500C.
[21] Q. Wu, W. Gong, G. Li, Porous organic polymers with Thiourea linkages (POP-TUs):
effective and recyclable Organocatalysts for the Michael reaction, ACS Appl. Mater.
Interfaces (2020), https://doi.org/10.1021/acsami.0c01280.
[35] B. Xiao, Q. Huang, H. Chen, X. Chen, G. Long, A fractal model for capillary flow
through a single tortuous capillary with roughened surfaces in fibrous porous
media, Fractals. 29 (2021) 2150017, https://doi.org/10.1142/
S0218348X21500171.
[36] B. Xiao, Y. Zhang, Y. Wang, G. Jiang, M. Liang, X. Chen, G. Long, A fractal model
for kozeny–carman constant and dimensionless permeability of fibrous porous
media with roughened surfaces, Fractals. 27 (2019) 1950116, https://doi.org/
10.1142/S0218348X19501160.
[22] C. Yadav, V.K. Maka, S. Payra, J.N. Moorthy, De Novo Access to SO 3 H-Grafted
Porous Organic Polymers (POPs@H): Synthesis of Diarylbenzopyrans/
Naphthopyrans and Triazoles by Heterogeneous Catalytic Cyclocondensation and
Cycloaddition Reactions, ACS Appl. Polym. Mater (2020), https://doi.org/
10.1021/acsapm.0c00225.
[37] A. Tamayol, M. Bahrami, Transverse permeability of fibrous porous media, Phys.
Rev. E 83 (2011), 046314, https://doi.org/10.1103/PhysRevE.83.046314.
[38] D. Shou, J. Fan, F. Ding, Hydraulic permeability of fibrous porous media, Int. J.
Heat Mass Transf. 54 (2011) 4009–4018, https://doi.org/10.1016/j.
ijheatmasstransfer.2011.04.022.
¨
[23] J. Troger, Ueber einige mittelst nascirenden Formaldehydes entstehende Basen,
J. Für Prakt. Chemie (1887), https://doi.org/10.1002/prac.18870360123.
[24] M.J. Schultz, C.C. Park, M.S. Sigman, A convenient palladium-catalyzed aerobic
oxidation of alcohols at room temperature, Chem. Commun. (2002), https://doi.
org/10.1039/b209344h.
¨
[39] E. Knoevenagel, Condensation von Malonsaure mit aromatischen Aldehyden durch
Ammoniak und Amine, Berichte Der Dtsch. Chem. Gesellschaft (1898), https://doi.
org/10.1002/cber.18980310308.
[25] B. Minder, M. Schürch, T. Mallat, A. Baiker, Chiral nitrogen compounds as new
modifiers for the enantioselective hydrogenation of ethyl pyruvate, Catal. Lett.
(1995), https://doi.org/10.1007/BF00808828.
[40] B.M. Wepster, Steric effects on mesomerism: XI, the ultra-violet absorption
¨
spectrum of troger’s base and the stereochemistry of aromatic amines, Recl. Des
Trav. Chim. Des Pays-Bas (1953), https://doi.org/10.1002/recl.19530720806.
[41] E. Poli, E. Merino, U. Díaz, D. Brunel, A. Corma, Different routes for preparing
¨
[26] Y. Goldberg, H. Alper, Transition metal complexes of troger’s base and their
¨
catalytic activity for the hydrosilylation of alkynes, Tetrahedron Lett. (1995),
https://doi.org/10.1016/0040-4039(94)02272-D.
Mesoporous Organosilicas containing the Troger’s base and their textural and
catalytic implications, J. Phys. Chem. C 115 (2011) 7573–7585, https://doi.org/
10.1021/jp2002854.
¨
[27] X. Du, Y. Sun, B. Tan, Q. Teng, X. Yao, C. Su, W. Wang, Troger’s base-
functionalised organic nanoporous polymer for heterogeneous catalysis, Chem.
Commun. 46 (2010) 970–972, https://doi.org/10.1039/B920113K.
[42] M. Carta, M. Croad, K. Bugler, K.J. Msayib, N.B. McKeown, Heterogeneous
organocatalysts composed of microporous polymer networks assembled by
¨
[28] M. Carta, R. Malpass-Evans, M. Croad, Y. Rogan, J.C. Jansen, P. Bernardo,
F. Bazzarelli, N.B. McKeown, An efficient polymer molecular sieve for membrane
gas separations, Science (2013), https://doi.org/10.1126/science.1228032.
Troger’s base formation, Polym. Chem. 5 (2014) 5262, https://doi.org/10.1039/
C4PY00608A.
[43] A. Samui, N. Kesharwani, C. Haldar, S.K. Sahu, Fabrication of nanoscale covalent
porous organic polymer: an efficacious catalyst for knoevenagel condensation,
Microporous Mesoporous Mater. (2020), https://doi.org/10.1016/j.
micromeso.2020.110112.
ˇ
[29] R. Williams, L.A. Burt, E. Esposito, J.C. Jansen, E. Tocci, C. Rizzuto, M. Lanc,
M. Carta, N.B. McKeown, A highly rigid and gas selective methanopentacene-based
¨
polymer of intrinsic microporosity derived from Troger’s base polymerization,
J. Mater. Chem. A 6 (2018) 5661–5667, https://doi.org/10.1039/c8ta00509e.
[30] Safety Data Sheet: Pyridine, Reagent. https://beta-static.fishersci.com/content
/dam/fishersci/en_US/documents/programs/education/regulatory-documents
/sds/chemicals/chemicals-p/S25762A.pdf, 2021.
[44] E.V. Dalessandro, H.P. Collin, M.S. Valle, J.R. Pliego, Mechanism and free energy
profile of base-catalyzed Knoevenagel condensation reaction, RSC Adv. 6 (2016)
57803–57810, https://doi.org/10.1039/c6ra10393f.
[45] H. Chen, L. Fan, X. Zhang, Highly robust 3s–3d {CaZn}–organic framework for
excellent catalytic performance on chemical fixation of CO 2 and Knoevenagel
condensation reaction, ACS Appl. Mater. Interfaces 12 (2020) 54884–54892,
https://doi.org/10.1021/acsami.0c18267.
[31] C.A. Schneider, W.S. Rasband, K.W. Eliceiri, NIH image to ImageJ: 25 years of
image analysis, Nat. Methods (2012), https://doi.org/10.1038/nmeth.2089.
[32] S.K. Bharti, R. Roy, Quantitative 1H NMR spectroscopy, TrAC Trends Anal. Chem.
35 (2012) 5–26, https://doi.org/10.1016/j.trac.2012.02.007.
[46] H. Chen, T. Hu, L. Fan, X. Zhang, One robust microporous tm III –organic
framework for highly catalytic activity on chemical CO 2 fixation and Knoevenagel
condensation, Inorg. Chem. 60 (2021) 1028–1036, https://doi.org/10.1021/acs.
inorgchem.0c03134.
[33] V.M. Nurchi, T. Pivetta, J.I. Lachowicz, G. Crisponi, Effect of substituents on
complex stability aimed at designing new iron(III) and aluminum(III) chelators,
J. Inorg. Biochem. (2009), https://doi.org/10.1016/j.jinorgbio.2008.10.011.
[34] C.A. Ramsden, The Smiles rearrangement of 2-aryloxy-5-nitrophenoxides.
Attempted routes to benzoxirens and tribenzo[b,e,h]trioxonins, J. Chem. Soc.
Perkin Trans. 1 (1981) 2456, https://doi.org/10.1039/p19810002456.
[47] H. Chen, L. Fan, T. Hu, X. Zhang, 6s-3d {Ba 3 Zn 4 }–organic framework as an
effective heterogeneous catalyst for chemical fixation of CO 2 and Knoevenagel
condensation reaction, Inorg. Chem. 60 (2021) 3384–3392, https://doi.org/
10.1021/acs.inorgchem.0c03736.
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