Bifunctional covalent organic frameworks with two dimensional organocatalytic micropores

Article abstract of DOI:10.1039/c4cc07104b

We report the successful incorporation of bifunctional (acid/base) catalytic sites in the crystalline organocatalytic porous COF (2,3-DhaTph). Due to the presence of acidic (catachol) and basic (porphyrin) sites, 2,3-DhaTph shows significant selectivity,

Full text of DOI:10.1039/c4cc07104b

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Bifunctional covalent organic frameworks with
two dimensional organocatalytic micropores†
Cite this:DOI: 10.1039/c4cc07104b
Digambar Balaji Shinde,‡ Sharath Kandambeth,‡ Pradip Pachfule, Raya Rahul Kumar
and Rahul Banerjee*
Received 9th September 2014,
Accepted 6th November 2014
DOI: 10.1039/c4cc07104b
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We report the successful incorporation of bifunctional (acid/base) Also the large surface area, chemical stability and ordered
catalytic sites in the crystalline organocatalytic porous COF crystalline structure with separated antagonist sites are the
(
(
2,3-DhaTph). Due to the presence of acidic (catachol) and basic essential requirements of a heterogeneous bifunctional catalyst
porphyrin) sites, 2,3-DhaTph shows significant selectivity, reusa- to achieve the highest catalytic activity. The large surface area
bility, and excellent ability to perform the cascade reaction.
will provide greater exposure of the catalytic sites to the incom-
ing reactants. The chemical stability will help to avoid the
1
Covalent organic frameworks (COFs) are a new class of crystal- contamination due to catalyst degradation, whereas high crystal-
line porous materials constructed from light elements like H, B, linity will provide information on the structure and the exact
2
C, N, O, Si, etc. Although research on these materials received position as well as the distance between the antagonist catalytic
immense attention due to their potential applications in gas sites. Although a few polymer based organocatalysts are reported
3
adsorption, charge carrier transport, and optoelectronics,
in the literature for the cascade reaction, due to the amorphous
these porous materials are less explored as catalysts owing to nature of the catalysts, the exact position of the antagonist
4
their limited stability in aqueous, acidic and alkaline media.
catalytic sites and the spatial separation between them were very
8
In principle, COFs can be used as catalysts mainly in two ways: poorly understood. Moreover, the reported post functionalization
1) as a support for catalytically active nanoparticles, and strategy does not ensure 100% incorporation of the catalytic sites
2) direct organocatalysis using catalytically active organic in the framework.
building units within the COF. Among these catalytic applica- geneous catalysts having high crystallinity, porosity and chemical
5
(
(
6b,9
Thus, the synthesis of bifunctional hetero-
6
10
tions, heterogenous organocatalytic COFs are more promising, stability is still a challenging task. The COFs reported in this
because they have a large scope in pharmaceutical and food paper, 2,3-DhaTph and 2,3-DmaTph, were synthesized by rever-
7
industries. Here, we report the successful synthesis of a sible Schiff-base reaction using 2,3-dihydroxyterephthalaldehyde
bifunctional catachol-porphyrin COF, consisting of both acidic (2,3-Dha)/2,3-dimethoxyterephthalaldehyde (2,3-Dma) and a
and basic sites, which can act as a heterogeneous catalyst for 5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphine unit (Tph)
one pot deprotection of acetal groups followed by Knoevenagel in dichlorobenzene (o-DCB,) and dimethylacetamide (DMAc) with
8
condensation reaction. Even though there has been a report of a catalytic amount of 6.0 M acetic acid (Fig. 1) (ESI,† Section 2).
6
b
a porphyrin COF ([Pyr] -H P-COF) used as an organocatalyst,
The PXRD patterns of the as-synthesized COFs showed good
the incorporation of the bifunctionality in the COF backbone crystallinity with a highly intense peak at B3.61 2y corresponding
for catalysis is still unprecedented. to the [100] plane reflections and low intense peaks at B7.31 and
x
2
Bifunctional heterogeneous catalysts are more preferred B19–251 2y, which were assigned to [200] and [001] facets
for cascade/tandem/one-pot synthesis reactions, because of the (d spacing = 4.0 Å). The diffraction patterns of the synthesized
fixed location of the acidic and basic sites, within the framework. COFs are similar to that of eclipsed stacking model of the COF
11
structures built using the SCC-DFTB method (Fig. 2a, S4, ESI†).
From the Pawley refinement, we could conclude that the unit cell
values for 2,3-DhaTph are a = 24.7, b = 24.6, c = 4.0 Å; a = 92.9, b =
Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory,
Dr. Homi Bhabha Road, Pune 411008, India. E-mail: r.banerjee@ncl.res.in;
Tel: +91 2025902535
9
0.2, g = 92.7; P1 space group; and for 2,3-DmaTph are a = 25.2,
b = 24.6, c = 4.1 Å; a = 90.1, b = 89.7, g = 88.4; P1 space group
Section S3, ESI†). The crystal structure analysis of the monomers
†
Electronic supplementary information (ESI) available: Experimental procedures
and additional supporting data. CCDC 1019813 and 1019814. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc07104b
(
of 2,3-DhaTph and 2,3-DmaTph indicated that these COFs
exist only in the enol-imine form (Fig. 1 and Fig. S11, ESI†).
‡
D.B.S. and S.K. contributed equally.
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Fig. 1 The synthesis of 2,3-DhaTph and 2,3-DmaTph by the condensation of Tph and 2,3-Dha/2,3-Dma. The catalytically active porphyrin and
catecholic –OH groups are shown in coral and cyan colors, respectively. An ORTEP diagram of 2,3-DhaTph and 2,3-DmaTph monomer units.
Fig. 2 (a) The PXRD pattern of 2,3-DhaTph compared with simulated eclipsed and staggered. (b and c) Stacking diagram and the eclipsed stacking
model of 2,3-DhaTph. (d) Staggered stacking model of 2,3-DhaTph. (e) PXRD pattern of 2,3-DmaTph compared with simulated eclipsed and staggered.
(f and g) Stacking diagram and the eclipsed stacking model of 2,3-DmaTph. (h) Staggered stacking model of 2,3-DmaTph.
The presence of trans conformation of imine bonds and the FT-IR spectra of 2,3-DhaTph and 2,3-DmaTph confirmed the
13
presence of intramolecular hydrogen bonding [–O–HÁ ÁÁN = C; D = formation of imine –CQN bonds. The solid state C CP-MAS
.579 (2), d = 1.858 (2) Å, and y = 146.11 (3)] in 2,3-DhaTph have NMR spectrum confirmed the formation of imine –CQN bonds,
been confirmed from the monomer crystal structure, which leads as it shows the characteristic signals at d 160.8 (Fig. S7, ESI†) and
2
1
0b,12
to the rigid structure enhancing the crystallinity (Fig. 1).
As a 154.2 (Fig. S8, ESI†) ppm respectively. These chemical shifts of
result, the PXRD pattern of 2,3-DhaTph is much more intense imine –CQN appear closer to the monomer unit values of
than that of 2,3-DmaTph, since the intramolecular H-bonding is 2,3-DhaTph (d 161.3) (Fig. S10, ESI†) and 2,3-DmaTph (d 154.9)
absent in 2,3-DmaTph (Fig. 1). The appearance of a strong (Fig. S12, ESI†). The morphological analysis of 2,3-DhaTph and
characteristic imine –CQN stretching frequency in the FT-IR 2,3-DmaTph done using SEM and TEM imaging indicates that
À1
spectrum at 1612 cm
1
for 2,3-DhaTph (Fig. S5, ESI†) and 2,3-DhaTph is composed of 100–200 nm thin platelet like
À1
608 cm for 2,3-DmaTph (Fig. S6, ESI†) confirmed the for- morphology, whereas 2,3-DmaTph consists of 30–50 nm average
mation of COFs. Also, the disappearance of the stretching sized sheet like morphology (Section S10, ESI†).
À1
frequency of the –CQO group (1661 of 2,3-Dha and 1676 cm
The thermal stability of the activated COFs was confirmed by
À1
of 2,3-Dma) and the –NH
2
group (3100–3400 cm ) of Tph in the TGA analysis, which showed high thermal stability up to 300 1C
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Fig. 3 (a) N
2
adsorption isotherms of 2,3-DhaTph and 2,3-DmaTph. (b) Comparative TEM images, as synthesized and after 3rd cycle of 2,3-DhaTph and
2
,3-DmaTph. (c) The catalytic activity towards acid–base catalyzed reaction with various reactants. (d) Kinetics of the cascade reaction between 1 and
malononitrile. (e) Comparison of the recyclability studies performed for 2,3-DhaTph and 2,3-DmaTph.
(
7
(
Section S9, ESI†). The N
7 K for the activated COFs showcase a typical Type-IV isotherm catalyst using a model reaction (Fig. 3c), wherein benzaldehyde-
Fig. 3a) with the Brunauer–Emmett–Teller (BET) surface area dimethylacetal (1a) (152 mg, 0.1 mmol) reacts with malono-
2
adsorption isotherms collected at 2,3-DhaTph was analyzed in the presence of 10 mg of the COF
2
À1
of 1019 and 668 m g , respectively (Fig. S14 and S17, ESI†). nitrile (72.6 mg, 0.11 mmol) in toluene (1.5 mL) and water
The pore width values calculated using the NLDFT method (0.5 mL) at 80 1C. In the case of 2,3-DhaTph the formation of
were found to be 2.2 and 1.4 nm which are in close agreement the desired product 2-benzylidenemalononitrile (3a) with excel-
with the theoretically predicted pore width (Fig. 1). The surface lent isolated yield (96%) was observed. The detailed kinetic
area and pore size of 2,3-DmaTph are lower than that of study showed that the completion of reaction occurs within
2
,3-DhaTph, which is well justified as twisted conformation 90 min (Fig. 3d). It was understood from the control experiments
of phenyl rings in the former case shows disturbed stacking of that this cascade reaction proceeds through two sequential
D layers, which finally reduces the crystallinity as well as steps: (1) the acid catalyzed deacetalization of benzaldehydedi-
2
porosity. We have analyzed the chemical stability of these COFs methylacetal (1a) to yield benzaldehyde (2a); and (2) the base
in aqueous, acidic (3N HCl) and alkaline (3N NaOH) media. catalyzed Knoevenagel condensation reaction to yield 2-benzyl-
Both COFs showed good aqueous stability for more than 7 days, idenemalononitrile (3a). (Fig. 3c and Section S13, ESI†).
which was confirmed by PXRD, FT-IR and porosity studies. The
In order to analyze the necessity of the catalyst for the
presence of –OH and –OCH groups adjacent to the imine cascade reaction, a model reaction was performed without
3
bonds in these COFs probably helped to improve the hydrolytic the addition of the catalyst. The reactions without a catalyst
stability. Similarly, the stability of these COFs in 3N HCl was further yielded only 5% of product 3a during the estimated time span
10
confirmed by FT-IR, PXRD and SEM studies (Section S12, ESI†).
,3-DhaTph showed higher structural integrity than 2,3-DmaTph,
probably due to the strong intramolecular hydrogen bonding S13, ESI† Table 3, entry 1) under the same conditions yielded
of 90 min.
2
The reaction performed without a catalyst (blank) (Section
1
0b,12
[
–O–HÁÁÁNQC].
Due to the protonation of the inner porphyrin only 25% of intermediate 2a, probably due to the deacetaliza-
core by an acid a decrease in the surface area has been observed tion of 1a by water, which has been used as a solvent. We have
before 1019, after 652 m g for 2,3-DhaTph and before 668, after explored the substrate scope of the 2,3-DhaTph catalyst using
07 m g for 2,3-DmaTph). Both 2,3-DhaTph and 2,3-DmaTph a number of substituted dimethyl acetal reactants, keeping
2
À1
(
3
2
À1
10b
are unstable in basic medium (3N NaOH).
all other reaction conditions the same. In general, excellent
The 2,3-DhaTph COF possesses separate antagonist catalytic conversions (480%) to the desired products were observed
1
3
sites in which catecholic –OH groups act as weak acidic sites,
whereas porphyrin units and imine bonds act as basic sites, an electron donating (–Me and –OMe) or withdrawing group
with high chemical stability in aqueous/acidic media along (–NO ) on the acetal based reactants. As shown in Fig. 3c, the
(Section S13, ESI,† Table 4, entries 2–5); despite the presence of
2
with high crystallinity and porosity. Hence, we have decided reactant size (molecular dimensions) did not affect the reactiv-
to explore the catalytic activity of this COF for the acid–base ity of the starting materials, which emphasizes the utility of
8
catalyzed one-pot cascade reactions. The catalytic activity of 2,3-DhaTph for the broad spectrum of the catalytic reactions.
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The kinetics of the cascade reaction after the addition of the
catalyst shows that as the rate of the reaction increases with
time the amount of reactant 1a starts decreasing along with a
related increase in the corresponding product 3a. As shown in
Fig. 2d, with increasing time, the formation of 3a steadily increases
up to 60 min yielding B90% product for the 2,3-DhaTph catalyst.
The probable reason for the significant activity shown by the
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2,3-DhaTph catalyst may be the fine distribution of acidic and
basic sites in the crystalline COF and periodic arrangement of
these centres distributed over the entire COF matrix. In order to
prove the necessity of acidic and basic sites for catalyzing the
1
4
cascade reaction, we have repeated the same catalytic reaction
in the presence of 2,3-DmaTph as a catalyst, which holds only
basic porphyrin centers but lacks acidic catecholic –OH func-
tionality, which has been replaced by –OMe functionality. In
this case, the reaction proceeds very slowly giving only 52%
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to catalyze the cascade reaction (Fig. 3). The solid catalysts can
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recyclability for more than five catalytic cycles giving yields over
B81% in an estimated time span of 90 min. In the case of
4
5
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459, 41.
In conclusion, we have synthesized a catalytically active COF
9
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2,3-DhaTph with weak acidic and basic sites for catalyzing the
cascade reaction. 2,3-DhaTph showed a large surface area, high
crystallinity as well as porosity than 2,3-DmaTph, which lacks
intramolecular hydrogen bonding within the framework. The
potential of 2,3-DhaTph for catalyzing the cascade reaction is
validated by the good catalytic activity shown towards the
cascade reaction with very high product yield and recyclability
over 5 cycles. The necessity of the basic and acidic sites in a
catalyst for catalyzing the cascade reaction has been further
validated by utilization of methoxy functionalized 2,3-DmaTph.
DBS acknowledges for a CSIR-Nehru science postdoctoral
research fellowship, SK and RB acknowledges CSIR (CSC0122
and CSC0102) for funding.
1
1
1
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4 The concept of necessity of both acidic (quinolic–OH) and basic
porphyrin) groups for catalyzing the aforementioned catalytic
(
Notes and references
reaction was confirmed by using a literature reported DhaTph
catalyst, which has both acidic as well as basic centers, with similar
10b
1
(a) A. P. Cote, A. I. Benin, N. W. Ockwig, M. O’Keeffe, A. J. Matzger
and O. M. Yaghi, Science, 2005, 310, 1166; (b) E. L. Spitler and
stability. Also, in the case of DhaTph, we have observed a similar
yield of B94% of product 3a (ESI,† Section S13).
Chem. Commun.
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