A functionalized UiO-66 MOF for turn-on fluorescence sensing of superoxide in water and efficient catalysis for Knoevenagel condensation

Article abstract of DOI:10.1039/c9dt03638e

In the present work, a new MOF material of the UiO-family called Zr-UiO-66-NH-CH₂-Py (1) has been obtained by the solvothermal technique and successfully characterized. The MOF structure was assembled with 2-((pyridin-4-ylmethyl) amino) terephthalic acid (H₂BDC-NH-CH₂-Py) as linker and Zr⁴⁺ ion. The activated form of 1 (called 1′) exhibits considerable thermal and chemical stability. Compound 1′ showed a very rapid and selective response for the fluorometric sensing of superoxide (O₂^·-) in aqueous medium even in the presence of the potentially competitive reactive oxygen species (ROS). The limit of detection value for O₂^·- sensing is 0.21 μM, which is comparable with those of the reported O₂^·- sensors. This is the first MOF based fluorescent sensor for the detection of O₂^·-. The response time of this MOF sensor for O₂^·- is very short (240 s). On the other hand, 1′ was employed as a solid heterogeneous catalyst for Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate at 80 °C in ethanol resulting in a very high yield of the desired product. The effects of the esterified linker ((CH₃)₂BDC-NH-CH₂-Py) and the corresponding metal salt (ZrCl₄) on this catalytic reaction were examined separately. We have also tested the substrate scope elaborately for the catalytic reaction promoted by catalyst 1′.

Full text of DOI:10.1039/c9dt03638e

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A functionalized UiO-66 MOF for turn-on
fluorescence sensing of superoxide in water and
efficient catalysis for Knoevenagel condensation†
Cite this: Dalton Trans., 2019, 48,
17371
Aniruddha Das,a Nagaraj Anbu,‡b Mostakim SK,‡a
a
Amarajothi Dhakshinamoorthy *b and Shyam Biswas
*
In the present work, a new MOF material of the UiO-family called Zr-UiO-66-NH-CH2-Py (1) has been
obtained by the solvothermal technique and successfully characterized. The MOF structure was
assembled with 2-((pyridin-4-ylmethyl) amino) terephthalic acid (H2BDC-NH-CH2-Py) as linker and Zr4+
ion. The activated form of 1 (called 1’) exhibits considerable thermal and chemical stability. Compound 1’
showed a very rapid and selective response for the fluorometric sensing of superoxide (O2·−) in aqueous
medium even in the presence of the potentially competitive reactive oxygen species (ROS). The limit of
·−
detection value for O2·− sensing is 0.21 μM, which is comparable with those of the reported O2 sensors.
This is the first MOF based fluorescent sensor for the detection of O2·−. The response time of this MOF
sensor for O2·− is very short (240 s). On the other hand, 1’ was employed as a solid heterogeneous catalyst
for Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate at 80 °C in ethanol result-
ing in a very high yield of the desired product. The effects of the esterified linker ((CH3)2BDC-NH-CH2-Py)
and the corresponding metal salt (ZrCl4) on this catalytic reaction were examined separately. We have
also tested the substrate scope elaborately for the catalytic reaction promoted by catalyst 1’.
Received 11th September 2019,
Accepted 18th October 2019
DOI: 10.1039/c9dt03638e
rsc.li/dalton
NADH via the NAD(P)H oxidase system.13 In biological
systems, among all the ROS, O2 is the primary ROS, which is
Introduction
·−
Reactive oxygen species, which are generated endogenously responsible for cardiovascular disease and cancer and is also
mainly from the respiration of mitochondria1–3 and from involved in cellular signal transduction.3,9,14–16 Some serious
oxygen species3 and exogenously upon exposure to UV light1,4 issues like oxidative chain reactions,17 hepatitis,18 degenerative
and infectious agents,1,4 are the origin of many biological pro- disorders,3 and ischemia-reperfusion (IR) injury take place in
cesses such as inflammation,5,6 oxidative stress,5 oxidation of surgery19 because of excess dosage of O2·− in living organisms.
DNA and signal transduction,1,7 neurodegenerative injury,8 etc. Hence, development of some analytical probes for the detec-
·−
Superoxide (O2·−) is a one-electron reduction species of mole- tion of endogenous and exogenous O2 with excellent selecti-
cular O2, which damages biological membranes and tissues vity and sensitivity has great impacts on current research. In
directly.9,10 The root of production of all ROS like H2O2, hypo- the last decades, many researchers have tried to prepare some
chlorite (OCl−), singlet oxygen (1O2), t-butyl hydroperoxide probes including some organic probes,20 hydroethidine
(TBHP), the hydroxyl radical (OH•) and the TBHP radical (HE),21 and nanoscale coordination polymers (NCPs)22 for the
·− 11,12
·−
(TBHP•) is O2
.
Again, O2 production is enhanced by fluorometric detection of O2·−. But, the disadvantages associ-
ated with these probes are poor stability, biotoxicity for cells,
poor selectivity, and less efficiency due to their amorphous
aDepartment of Chemistry, Indian Institute of Technology Guwahati, 781039 Assam,
India. E-mail: sbiswas@iitg.ac.in; Fax: +91-3612582349; Tel: +91-3612583309
bSchool of Chemistry, Madurai Kamaraj University, Madurai 625021, Tamil Nadu,
India. E-mail: admguru@gmail.com
nature and smooth surface.23–25
The Knoevenagel condensation reaction involves the for-
mation of a substituted olefin using an aldehyde or a ketone
†Electronic supplementary information (ESI) available: Materials and character- and an active methylene compound in the presence of base as
catalysts.26–30 This reaction is one of the familiar reactions
ization methods, synthesis of the linker, mass, NMR and IR spectra, XRPD pat-
terns, TG curves, N2 sorption isotherms, fluorescence and UV-Vis spectra,
for forming C–C bonds in organic transformations and
GC-MS traces, the reusability plot for catalysis, and tables showing the unit cell
often produces α,β-unsaturated ketones.26,29,31 Base catalyzed
·−
parameters of 1, detection limits and response times for O2 sensing. See DOI:
condensation reactions are some of the important reactions
10.1039/C9DT03638E
‡These authors contributed equally to this work.
involved in the construction of molecules that can be used as
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fine chemicals and drugs.26,32,33 Mostly, these reactions are
performed using conventional homogeneous base catalysts
such as amine, ammonia, pyridine, piperidine and alkali
metal hydroxide which are poisonous and non-reusable and
Experimental section
Synthesis of [Zr6O4(OH)4(BDC-C6H8N2)6]·2.5H2O·1·5DMF (Zr-
UiO-66-NH-CH2-Py, 1)
produce environmental waste.33,34 Henry reactions and In a sealed glass tube, ZrCl4 (35 mg, 0.15 mmol), the
Claisen–Schmidt reactions are also examples of organic con- H2BDC-NH-CH2-Py linker (40 mg, 0.15 mmol), benzoic acid
densation reactions which follow similar mechanistic proto- (538 mg, 4.5 mmol) and 2 mL of N,N-dimethylformamide
cols to Knoevenagel condensation using different types of (DMF) were taken and allowed to sonicate for 10 min.
nucleophiles.35 But, use of these homogeneous catalysts for Afterwards, the sealed glass tube was heated at 120 °C for 24 h
such types of condensation reactions leads to many problems using a block heater and a yellow colored precipitate was
such as undesired side reactions, necessity of higher tempera- obtained. The product was collected by vacuum filtration and
ture, poor loading of catalysts, reduced recyclability, consump- acetone (3 × 2 mL) was used to wash the product. The solid
tion of longer time during work up and problems associated material was dried at 70 °C for 4 h in a conventional oven. The
with catalyst recovery.32,33 In order to overcome these real pro- yield was 49 mg (0.02 mmol, 81%) based on the Zr salt. Elem.
blems, scientists have developed numerous heterogeneous cat- anal. calcd for C88.5H79.5N13.5O36Zr6 (2455.49 g mol−1): C,
alysts like modified zeolites,36 carbon nanotubes,37 ionic 43.28; H, 3.26; N, 7.70%. Found: C, 43.65; H, 2.72; N, 7.28%.
liquids,38 and Cu3TATAT.39 Still researchers are trying to FT-IR (KBr, cm−1): 3365 (br), 3065 (w), 2917 (w), 1688 (w), 1598
develop some new heterogeneous catalysts which would be (s), 1567 (vs), 1504 (m), 1419 (s), 1383 (s), 1307 (m), 1288 (m),
very easy to prepare and have high substrate scope, high stabi- 1257 (vs), 1150 (m), 1068 (m), 1024 (m), 768 (vs), 719 (vs), 661
lity and high recyclability.
(vs), 571 (sh), 473 (s).
Due to their distinctive qualities such as tunable pore
sizes,40 extraordinary surface areas,41 high thermal42 and
Activation of compound 1
chemical stability,43 and tunable functionality,44 metal– In a round bottom flask, 100 mg of compound 1 was stirred in
organic frameworks (MOFs) are being explored vastly for 25 mL of methanol at room temperature. After 24 h, the solid
various applications like gas separation and storage,45 hetero- material was collected by vacuum filtration and dried at 60 °C
geneous catalysis,46 drug delivery,47 light harvesting in a conventional oven for 4 h. In the next step, this compound
materials,48 etc. Luminescent MOF-based chemosensors have was kept at 100 °C under high vacuum for 24 h. Thus, acti-
gained splendid research attention recently for detecting vated compound 1′ was achieved.
ions,49,50 explosives,51–53 pH,54 gas molecules55 and small
Preparation of the suspension of 1′ for fluorescence sensing
experiments
molecules.56 The prime advantages of luminescent MOFs
(LMOFs) are their easy preparation, fast response and high
sensitivity towards various toxic molecules using fluorescence As the fluorescence titration experiments need a stable suspen-
techniques.
sion of the probe, 2 mg of 1′ was taken in a glass vial contain-
Herein, we present the solvothermal synthesis and com- ing 3 mL of water, followed by sonication for 1 h. Then, it was
plete characterization of a Zr(IV)-based UiO-66 MOF (1) con- kept under ambient conditions for 24 h to get a stable suspen-
taining a novel functional moiety attached to the linker. The sion for sensing experiments.
activated compound 1′ was successfully employed for the
·−
Fluorescence sensing experiments
aqueous phase detection of O2 via turn-on fluorescence in
·−
the presence of competitive ROS. Compound 1′ exhibits For performing O2 sensing experiments, 2900 μL of water
similar selectivity and sensitivity (Limit of Detection (LOD) = and 100 μL of an aqueous suspension of 1′ were taken in a
0.21 μM) to those found in the existing reports for the detec- 3 mL quartz cuvette. Afterwards, a 10 mM aqueous solution of
tion of O2·−. The response time of the present MOF for O2
O2 (KO2 in water) was added in incremental volumes (30 μL
·−
·−
sensing is comparable or shorter than the reported at every addition) up to 180 μL. Fluorescence spectra were
probes.5,14,57–62 Probe 1′ is the first example for the MOF based acquired after light excitation at 330 nm.
detection of O2·−. Furthermore, 1′ was successfully employed
Reaction procedure for the catalytic study
for the Knoevenagel condensation between benzaldehyde and
ethyl cyanoacetate at 80 °C with very high yield of the desired To a reaction vessel containing a solution of aldehyde
product. The functional moiety attached at the linker plays a (0.5 mmol) and ethyl cyanoacetate (0.5 mmol) in ethanol
significant role in the Knoevenagel condensation reaction, is (0.3 mL), the catalyst (20 mg, 1.7 mol%) was added. Then, this
quite new compared to the traditional functional groups used mixture was mixed homogeneously and placed in an oil bath
for the MOF based Knoevenagel condensation reaction.27,63–65 maintaining the temperature at 80 °C for 12 h. The samples
The substrate scope and the effects of the metal salt and the were aliquoted into a gas chromatograph (GC) to examine the
free linker on catalysis are described here. The reusability and progress of the reaction at different time intervals. Then, the
heterogeneous characteristics of the material for the catalytic conversion of the substrate to the product and its selectivity
study make this catalyst very important and useful for practical were checked by GC. Agilent GC was used to calculate the per-
applications.
centage yield of the product using the internal standard
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method. The obtained products were characterized by the From 130 to 340 °C, the second weight loss of 3.7 wt% occurs
GC-MS technique. The same procedure was followed during due to the removal of 1.2 occluded DMF molecules per
the reusability experiments except that the catalyst was filtered formula unit (calcd: 3.6 wt%). The third weight loss was
from the reaction mixture, washed two times with dichloro- noticed at 345 °C due to the collapse of the framework and the
methane and dried at 80 °C for 1 h. After drying, the catalyst elimination of organic linkers. Hence, it can be deduced that
was recycled for the reaction in the subsequent cycles with compound 1 was not completely activated. It can be men-
fresh aldehyde and ethyl cyanoacetate.
tioned that 1 and 1′ are well comparable with the reported Zr-
based UiO-66 MOF materials in terms of thermal stability.66–71
XRPD analysis
Results and discussion
The XRPD patterns of 1 and 1′ are shown in Fig. 1. The XRPD
patterns of 1 and 1′ are very similar to that of the simulated Zr-
UiO-66 MOF (Fig. 1). We have also carried out the indexing of
the XRPD pattern of as-synthesized 1 and the obtained unit
cell parameters are given in Table S1 (ESI†). The results of this
indexing are much close to those obtained for the reported
UiO-66 MOF. Furthermore, a Pawley fit (Fig. S6, ESI†) of the
XRPD pattern of 1 was performed, which exhibited excellent
similarity with the simulated XRPD pattern of 1. Therefore, it
is very clear that as-synthesized 1 has the UiO-66 framework
structure.72–74 Besides, it can be noticed that 1′ retains its
structural robustness (as judged by its crystallinity) even after
the activation process (Fig. 1). Therefore, 1 is stable under acti-
vation conditions.
Preparation and activation procedures
A yellow precipitate of 1 was obtained when a mixture of ZrCl4,
the H2BDC-NH-CH2-Py (Fig. S1–S3, ESI†) linker, and benzoic
acid (BA) was kept in a 1 : 1 : 30 molar ratio in a sealed glass
tube and 3 mL of DMF was added to it, followed by heating
the total mixture at 120 °C for 24 h.
To achieve the guest free form of 1, we treated compound 1
with methanol for 24 h at room temperature. Afterwards, this
methanol exchanged material was collected by vacuum fil-
tration and heated at 100 °C under high vacuum for 24 h.
Consequently, activated compound 1′ was achieved.
Infrared spectroscopy
The FT-IR spectra of the H2BDC-NH-CH2-Py linker, as-syn-
thesized (1) and activated (1′) compounds are shown in Fig. S4,
ESI†. In the FT-IR spectrum of the free linker, the strong
bands at 1580 and 1441 cm−1 are observed due to the asym-
metric and symmetric stretching vibrations of the free car-
boxylic acid moiety. Two strong bands at 1567 and 1419 cm−1
are observed in the FT-IR spectra of 1 and 1′ due to the pres-
ence of the asymmetric and symmetric stretching frequencies
of the coordinated carboxylate group of linker molecules,
respectively. A band at 1688 cm−1 is observed in the FT-IR
spectrum of the as-synthesized compound due to the presence
of the carbonyl group of DMF molecules. This band is absent
in the activated compound, which confirms that complete acti-
vation of 1 has been achieved successfully.
Chemical stability
In order to check the chemical stability in water, 1 M HCl and
glacial acetic acid, 1′ was stirred in these three liquids for 14 h.
After that we collected the solid materials by vacuum filtration,
dried them at 60 °C for 1 h and recorded their XRPD data. The
XRPD data (Fig. S7, ESI†) reveal that the peak positions and
intensities remained almost unaltered even after immersing 1′
in these three liquids, which confirmed that 1′ is stable
enough in water, 1 M HCl and glacial acetic acid. The chemical
Thermal stability
Thermogravimetric (TG) analyses were performed from 25 to
700 °C under an argon atmosphere. As per the TG curve
(Fig. S5, ESI†), the thermal stability of 1 is up to 340 °C.
The TG trace of 1 (Fig. S5, ESI†) displays three main steps
of weight loss. From 25 to 130 °C, the first weight loss of
2.2 wt% occurs due to the elimination of 2.5 guest water mole-
cules per formula unit (calcd: 1.8 wt%). From 130 to 340 °C,
the second weight loss of 4.6 wt% occurs due to the removal of
1.5 occluded DMF molecules per formula unit (calcd:
4.4 wt%). The third weight loss was noticed at 340 °C due to
the collapse of the framework and the decomposition of
organic linkers. The weight loss of compound 1′ in the temp-
erature range of 130–340 °C occurs due to the loss of water and
DMF molecules (Fig. S5, ESI†). From 25 to 130 °C, the first
weight loss of 1.2 wt% occurs due to the elimination of
Fig. 1 XRPD patterns of (a) Zr-UiO-66 (simulated), (b) as-synthesized 1
1.5 guest water molecules per formula unit (calcd: 1.1 wt%). (experimental) and (c) activated 1’ (experimental).
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stability of compound 1′ was also checked in aqueous solu-
tions of KOH at pH = 8, 10 and 12. After stirring in these three
KOH solutions having different pH values for 16 h, the solid
materials were collected by vacuum filtration and their XRPD
data were recorded. The XRPD patterns (Fig. S8, ESI†) of these
materials revealed that compound 1′ is moderately stable in
aqueous solutions of KOH at pH = 8 and 10 but it completely
lost its framework structure in the aqueous solution of KOH at
pH = 12. The chemical stability of 1′ is well comparable with
that of the previously reported UiO-66 MOFs.72,73,75
N2 sorption analysis
For investigating the nature and extent of porosity, we acquired
the N2 sorption isotherms of 1′ at −196 °C. The N2 sorption
curves (Fig. S9, ESI†) clearly indicate that 1′ is microporous
(type I shape) in nature. The BET surface area was found to be
642 m2 g−1. The micropore volume at p/p0 = 0.5 corresponded
to 0.40 cm3 g−1. The values of micropore volume and BET
surface area are comparable with those of reported UiO-66
MOF materials.42,56,70,75
Fig. 2 Enhancement of the fluorescence response of 1’ upon stepwise
·−
addition of 10 mM aqueous solution of O2 (λex = 330 nm and λem
=
430 nm). Inset: digital photographs of cuvettes containing aqueous sus-
·−
pension of 1’ before and after the addition of O2 under UV-Vis lamp
light.
Photoluminescence properties
The photoluminescence properties of both the free
H2BDC-NH-CH2-Py linker and 1′ were studied in the solid state
and aqueous medium. The results (Fig. S10 and S11, ESI†)
indicate that the free H2BDC-NH-CH2-Py linker has much
higher fluorescence intensity compared to that of 1′ in both
the solid state and aqueous medium. In the solid-state lumine-
scence study, the emission maxima for 1′ and the free
H2BDC-NH-CH2-Py linker were found at 470 nm and 505 nm,
respectively. In the case of aqueous medium, the emission
maxima for 1′ and the free H2BDC-NH-CH2-Py linker were
observed at 450 nm and 455 nm, respectively.
Fig. 3 Fluorescence enhancement of 1’ (aqueous suspension) upon
treatment with 180 µL of 10 mM aqueous solution of various ROS.
·−
Sensing behavior towards O2
·−
In order to investigate the potential ability of 1′ to sense O2
·−
in aqueous medium, we studied its fluorescence change upon O2 was added. The fluorescence of 1′ was saturated within
·−
treatment with O2 solution. The suspension of 1′ (100 μL) 240 s (Fig. 4).
·−
was added to 2900 μL of water in a quartz cuvette. Afterwards,
The selectivity of 1′ towards O2 in the presence of other
10 mM aqueous solution of O2·− was added incrementally. The potentially competitive ROS is highly required for practical
fluorescence intensity was saturated after the addition of applications. To check this ability, 180 μL of competing ROS
·−
180 μL of O2 solution. There was 100 fold increment in fluo- solution (10 mM) was added to the suspension of 1′ and fluo-
rescence intensity with a blue shift of emission maximum rescence spectra were acquired after 240 s. Thereafter, 180 μL
·−
from 450 to 430 nm (Fig. 2 and 3).
of O2 solution (10 mM) was added and fluorescence data
For a smart sensor, selectivity for desired species is highly were collected after 240 s. It was observed that there was negli-
preferred. For this purpose, we tested the sensing ability of 1′ gible turn-on fluorescence response after the addition of com-
towards other ROS including HOCl, H2O2, OH•, TBHP, TBHP•, peting ROS, whereas there was considerable fluorescence turn-
and 1O2. From Fig. 3 and Fig. S12–S17, ESI†, it is very clear on response after the addition of O2·−. All these results (Fig. 5
·−
that none of the ROS except O2 caused considerable turn-on and Fig. S12–S17, ESI†) suggest that 1′ is highly selective
·−
fluorescence upon addition to the suspension of 1′. Therefore, towards O2 only, even in the presence of other potentially
·−
1′ exhibits high selectivity towards O2
.
competitive ROS.
·−
·−
The sensing proficiency of 1′ towards O2 with respect to
The LOD for O2
was calculated using the well-
time was studied by performing time dependent fluorescence known formula: LOD = 3σ/m (Fig. S18, ESI†).49,51,76 The
sensing experiments. In order to perform these experiments, LOD value was found to be 0.21 μM, which is comparable
·−
we poured 100 μL of the suspension of 1′ into a cuvette con- with those of other reported O2
sensors (Table S2,
taining 2900 μL of water. Afterwards, 180 μL of a solution of ESI†).5,9,14,20,57,77
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ESI†), it was observed that the structure of 1′ was gradually
destroyed upon treatment with increasing concentrations of
·−
O2 in aqueous medium. FT-IR spectroscopy (Fig. S22, ESI†)
also supported the structural collapse of 1′ in the presence of
·−
O2 by showing a decrease in the peak intensity of stretching
·−
vibrations of 1′ with increasing concentration of O2 in
aqueous medium. Afterwards, UV-Vis spectra (Fig. S23, ESI†)
were recorded for the free linker, the supernatant after treating
·−
1′ with O2 and the free linker after treatment with O2·−. As
the UV-Vis spectra of the supernatant after treatment of 1′ with
·−
O2 and the O2·−-treated free linker are almost similar, we can
conclude that the coordinated linker molecules were released
from the framework of 1′ in aqueous medium upon treatment
with O2·−. The absorption maxima of the supernatant collected
·−
after treating 1′ with O2 and the O2·−-treated free linker are
blue shifted (Fig. S23, ESI†) due to the change in the pH value
of the solvent system after the addition of KO2.78,79 In
addition, we recorded the HR-MS spectrum of the filtrate after
Fig. 4 Turn-on fluorescence response of 1’ with time upon treatment
with 180 µL of 10 mM aqueous solution of O2·− (λex = 330 nm and λem
=
430 nm). Inset: fluorescence kinetic spectra of 1’ upon treatment with
10 mM aqueous solutions of KO2 (red) and KOH (violet).
·−
treatment of 1′ with O2 solution. We observed a strong peak
at [m/z + H+] = 273.0864 (+ESI mode) (Fig. S24, ESI†), which
corresponds to the linker molecule (H2BDC-NH-CH2-Py).
Moreover, it is noticed from Fig. S11 (ESI†) that the
H2BDC-NH-CH2-Py linker has higher fluorescence intensity
compared to that of 1′ in water. Hence, it can be inferred that
the organic linker is released into the solution upon treatment
of 1′ with O2·−, which enhances the fluorescence intensity of
the system. Consequently, a remarkable turn-on fluorescence
·−
signal was observed for 1′ in the presence of O2
.
Control experiments for O2·− sensing in the presence of KOH
·−
As KO2 can produce both O2 and KOH in water, we per-
·−
formed control fluorescence experiments for O2 sensing in
the presence of 10 mM aqueous KOH solution. In one set of
·−
experiments, 180 μL of 10 mM O2 solution was added to the
Fig. 5 Fluorescence enhancement of 1’ upon treatment with 180 µL of
10 mM aqueous solution of O2 in the presence of other potentially
competing ROS.
aqueous suspension of 1′ followed by the addition of 180 μL of
10 mM KOH solution (Fig. S25, ESI†). In another set of experi-
ments, 180 μL of 10 mM KOH solution was added to the
aqueous suspension of 1′ followed by the addition of 180 μL of
·−
·−
10 mM O2 solution (Fig. S26, ESI†). In both cases, turn-on
The recyclability experiments were conducted with 1′ up to
three cycles. The compound was acquired by filtration and
washed with water after every cycle of the sensing experiment.
Compound 1′ showed no recyclability after the first cycle
(Fig. S19, ESI†).
fluorescence responses were observed. However, XRPD analysis
showed that the framework structure of 1′ was lost partially
after treatment with 10 mM aqueous KOH solution (Fig. S27,
ESI†), whereas the framework of 1′ was lost completely after
·−
treatment with 10 mM aqueous O2 solution under sensing
conditions (Fig. S21, ESI†). Moreover, the fluorescence kinetics
·−
Mechanism for the sensing of O2
plots of 1′ (Fig. S28, ESI†) after treatment with 10 mM aqueous
·−
·−
To find out the mechanism for O2 sensing in aqueous KOH and O2 solutions showed that the saturation time is
medium, XRPD, FT-IR, UV-Vis, and HR-MS analyses were much faster in the presence of KOH (30 s) than in the presence
·−
carried out. From the recyclability study (Fig. S19, ESI†), it was of O2 (240 s). Hence, from the above results, it can be con-
observed that 1′ was not recyclable, which indicated that the cluded that fluorescence turn-on was observed due to the pres-
·−
·−
reaction between 1′ and O2 in aqueous medium led to the ence of O2 and KOH produced in the sensing medium
·−
fluorescence turn-on properties. For XRPD and FT-IR analyses, during the generation of O2
.
samples of 1′ were treated with various concentrations of
Catalytic activity in Knoevenagel condensation
aqueous solutions of O2·−. Afterwards, the solid materials were
collected by filtration. The XRPD and FT-IR data of these Knoevenagel condensation produces unsaturated compounds
solids were recorded. From the XRPD plots (Fig. S20 and S21, upon nucleophilic addition of a carbanion to an electrophilic
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Table 1 Optimization of reaction conditions for the Knoevenagel con-
densation reaction between benzaldehyde and ethyl cyanoacetate using
MOF 1’ as the heterogeneous solid catalysta
carbonyl group by removal of a water molecule. This is one of
the most classical organic reactions and has received wide
interest due to the importance of condensation products as
therapeutic drugs, natural products, heterocyclic compounds,
and fine chemicals. The Knoevenagel condensation reaction
was reported conventionally using a series of base catalysts
like piperidine, morpholine, triethylamine, KF and K2CO3.
Furthermore, this condensation reaction has been reported
using a series of heterogeneous solid base catalysts with
obvious advantages of recovery and reusability. In recent years,
the Knoevenagel condensation reaction between active methyl-
ene and carbonyl compounds has also been investigated using
MOFs as heterogeneous solid catalysts such as Zn2(tpt)2(2-atp)
I2,80 MOF-NH2,81 Al-MIL-101-NH2,82 IRMOF-3,83 Cu3(BTC)2/Fe
(BTC),84 and Hf-UiO-66-N2H3.78
The catalytic activity of 1′ was investigated by selecting
benzaldehyde and ethyl cyanoacetate as model substrates and
the reaction conditions were optimized in terms of solvent,
reaction temperature and catalyst loading. The obtained pro-
ducts were characterized by the GC-MS technique (Fig. S29–
S44, ESI†). A blank control experiment between benzaldehyde
and ethyl cyanoacetate without a catalyst resulted in 23% yield
at 80 °C in ethanol after 12 h. Furthermore, the reaction of
benzaldehyde and ethyl cyanoacetate was performed using 1′
with different loadings. The observed results are presented in
Fig. 6. This figure clearly indicates that the reaction was sig-
nificantly promoted to provide an optimum yield of 94% with
1.7 mol% catalyst loading compared to other catalyst loadings.
Hence, further reactions were performed with this catalyst
loading. Similarly, the reaction of benzaldehyde and ethyl cya-
noacetate in the presence of 1′ was screened with various
T
Yield
(%)b
Entry Catalyst
(°C) Solvent
1c
2
3
4
5
—
1′
1′
1′
1′
1′
1′
1′
80
80
80
80
80
80
80
80
60
40
80
80
Ethanol
Ethanol
Acetonitrile
Dichloroethane 18
Toluene
Xylene
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
Ethanol
23
94(81)
39
31
38
54
35
56
27
64
85
88
33
63
89
6
7d
8e
9
1′
1′
1′
10
11f
12g
13h
14i
15
16
H2BDC-NH-CH2-Py linker
(CH3)2BDC-NH-CH2-Py linker 80
ZrCl4
Zr-UiO-66
Zr-UiO-66-NH2
80
80
80
a Reaction conditions: benzaldehyde (0.5 mmol), ethyl cyanoacetate
(0.5 mmol), catalyst (20 mg, 1.7 mol%), solvent (0.3 mL), 80 °C, 12 h.
b Yields were determined by GC. The value in parentheses indicates
the catalytic activity of 1. c The blank experiment in the absence of a
catalyst. d Performed with 1 mL of solvent under stirring. e Performed
with 2 mL of solvent under stirring. f Catalyst loading was 10 mg
(0.9 mol%). g 13.5 mg (9.9 mol%) of the linker was used. h 13.5 mg
(8.9 mol%) of the ester linker was used. i 11.5 mg (9.8 mol%) of the
metal salt was used.
organic solvents like acetonitrile, dichloroethane, toluene and Furthermore, the reaction between benzaldehyde and ethyl
xylene. The experimental results are displayed in Table 1. The cyanoacetate using MOF 1′ as the catalyst showed lower yield
catalytic data using 1′ as the catalyst in the above-mentioned when ethanol was used in excess amounts compared to the
solvents indicate that ethanol is the preferred solvent to optimized reaction conditions. In addition, the reaction
achieve higher yield of the Knoevenagel condensation product. between benzaldehyde and ethyl cyanoacetate was performed
in the presence of 1′ at 60 and 40 °C in ethanol and the
respective yields of the desired products were 56 and 27%.
Fig. 7 portrays the effect of reaction temperature on the reac-
tion between benzaldehyde and ethyl cyanoacetate using 1′ as
the solid catalyst. These catalytic data suggest that higher yield
of the product is found only at higher reaction temperature.
Furthermore, the activity of 1′ was compared with those of the
respective metal salts and the organic linker. The experimental
results are shown in Table 1. The reaction of benzaldehyde
with ethyl cyanoacetate in the presence of the
(CH3)2BDC-NH-CH2-Py linker as
a homogeneous catalyst
resulted in a yield which is comparable to 1′, whereas the
H2BDC-NH-CH2-Py linker showed 85% yield at 80 °C in
ethanol. These catalytic data clearly indicate that the pyridine
moiety in the linker acts as a catalytic site and the superior
activity of the (CH3)2BDC-NH-CH2-Py linker is due to the
absence of self-quenching of basic sites in the free linker. In
contrast, this reaction exhibited only 33% yield using ZrCl4 as
a homogeneous control catalyst under identical reaction con-
ditions. On the other hand, the catalytic performance of 1
Fig. 6 The effect of catalyst loading on the Knoevenagel condensation
reaction between benzaldehyde and ethyl cyanoacetate in ethanol.
17376 | Dalton Trans., 2019, 48, 17371–17380
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1′ with the Zr-UiO-66-NH2 MOF under identical conditions
(Fig. 8). These catalytic data clearly indicate that 1′ exhibits a
slightly better activity than Zr-UiO-66-NH2. In contrast, Zr-
UiO-66 without any basic sites showed much lower activity
than 1′ and Zr-UiO-66-NH2 solids under similar conditions.
Hence, these results prove that the basic functionality in the
linker is essential to achieve higher yield of the condensation
product.
Fig. 9 shows the time conversion plot for the Knoevenagel
condensation between benzaldehyde and ethyl cyanoacetate in
the presence of 1′ as the catalyst with minimal amounts of
ethanol as shown in Table 1. One of the important control
experiments in heterogeneous catalysis is to check the hetero-
geneity of the reaction. This control study was performed by
the leaching or hot-filtration experiment. As shown in Fig. 9,
the reaction was started under optimized reaction conditions.
Later, the catalyst was filtered at reaction temperature after 1 h
Fig. 7 The effect of temperature on the Knoevenagel condensation and the reaction of the resulting reaction mixture in the
reaction between benzaldehyde and ethyl cyanoacetate using 1’ as the
solid catalyst at (a) 40 °C, (b) 60 °C and (c) 80 °C.
absence of the solid catalyst was continued further for 11 h.
Samples were periodically collected and analyzed by GC. The
comparison of the kinetic profiles with and without the solid
catalyst indicates that the reaction is heterogeneous in nature.
Recovery and reuse of the catalyst in consecutive reaction
cycles are the key aspects of developing new heterogeneous cat-
(without any activation) and 1′ was compared for the above
reaction under similar conditions to reveal the effect of acti-
vation. The results indicate that the prior activation of 1 has a
alysts. In this aspect, the reusability of 1′ was investigated in
strong influence on the catalytic activity as shown in Fig. 8 and
the reaction between benzaldehyde and ethyl cyanoacetate
Table 1. Comparison of the activity of 1′ with its related metal
salt and the methyl ester of the organic linker shows that MOF
1′ exhibits better activity than the methyl ester of the organic
linker and the metal salt (Fig. 8). Furthermore, it is clearly
under optimized reaction conditions. Fig. S45 (ESI†) displays
the reusability results of 1′ as a solid base catalyst in promot-
ing Knoevenagel condensation up to five cycles. It can be seen
from this figure that the activity of the catalyst slightly
proved that the active site promoting this reaction under opti-
decreased from the first cycle to the fifth cycle and this may be
mized reaction conditions is the basic functionality available
in the organic linker compared to the metal center. This
hypothesis was further confirmed by comparing the activity of
due to the catalyst loss (during filtration) in every cycle.
Furthermore, the XRPD pattern of fresh 1′ was compared with
Fig. 9 The time conversion profile for the Knoevenagel condensation
■
Fig. 8 Time yield plots for the Knoevenagel condensation reaction
reaction between benzaldehyde and ethyl cyanoacetate: ( ) in the pres-
▲
), the
●
between benzaldehyde and ethyl cyanoacetate using 1’
(
ence of the catalyst, ( ) hot filtration experiment upon filtration of the
■
▼
●
(CH3)2BDC-NH-CH2-Py linker ( ), Zr-UiO-66-NH2
(
), 1 ( ), and ZrCl4
catalyst after 1 h from the reaction mixture excluding the contribution
◆
).
▲
(
from blank control and ( ) the catalyst-free control experiment.
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Table 2 (Contd.)
Table 2 Knoevenagel condensation reaction between various alde-
hydes and ethyl cyanoacetate in the presence of 1’ as a heterogeneous
solid base catalysta
Entry
15
Aldehyde
Product
Yield (%)b
70
Entry
1
Aldehyde
Product
Yield (%)b
94
16
44
2
3
93
88
a Reaction conditions: aldehyde (0.5 mmol), ethyl cyanoacetate
(0.5 mmol), 1′ (20 mg, 1.7 mol%), ethanol (0.3 mL), 80 °C, 12 h.
b Yields were determined by GC.
4
5
6
92
89
88
that of 1′ used in five cycles (Fig. S46, ESI†). These XRPD pat-
terns indicate that the crystallinity of 1′ is not altered through-
out the course of the reaction and the catalyst is highly robust
under the present reaction conditions. In addition, the hot fil-
tration experiment also proved the stability of the catalyst.
Moreover, the FE-SEM images of 1 and 1′ before and after cata-
lysis (Fig. S47–S49, ESI†) showed that the morphology of the
nanocrystals of 1 remained unaffected after thermal activation
and catalytic cycles during the recyclability experiments.
Hence, all these available catalytic data, XRPD patterns and
FE-SEM images suggest the stability of 1′ during the
Knoevenagel condensation reaction under the present experi-
mental conditions.
7
8
93
74
These preliminary catalytic data prompted us to examine
the substrate scope of 1′ as a solid base catalyst with a wide
range of substituted benzaldehydes containing electron with-
drawing and electron donating groups. The obtained catalytic
data are presented in Table 2. The reaction of 4-methyl-
benzaldehyde and 4-methoxybenzaldehyde with ethyl cyanoa-
cetate using 1′ as the catalyst provided 93 and 88% yields of
condensation products, respectively. On the other hand,
4-fluoro, 4-chloro, 4-bromo and 4-nitrobenzaldehydes reacted
conveniently with ethyl cyanoacetate in the presence of 1′ to
afford 92, 89, 88 and 93% yields, respectively. The reaction of
4-tert-butylbenzaldehyde with ethyl cyanoacetate in the pres-
ence of 1′ resulted in 74% yield under identical reaction con-
ditions. Similarly, di- and tri-substituted benzaldehydes
reacted with ethyl cyanoacetate, but the yields of the products
reduced significantly under identical conditions. These cata-
lytic data infer that these bulky benzaldehydes may experience
diffusion limitations in reaching the active sites in 1′.
Furthermore, heterocyclic aldehydes like 2-furfural condensed
with ethyl cyanoacetate in the presence of 1′ to afford 92%
yield. On the other hand, salicylaldehyde and phenylacetalde-
hyde reacted with ethyl cyanoacetate to give the desired pro-
ducts in 73 and 48% yields, respectively. An α,β-unsaturated
substrate like cinnamaldehyde also reacted with ethyl cyanoa-
cetate in the presence of 1′ to provide 74% yield. Finally,
9
58
65
92
10
11
12
13
73
48
14
74
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1-naphthaldehyde and 9-anthracenecarboxaldehyde were sub-
jected to Knoevenagel condensation with ethyl cyanoacetate in
the presence of 1′ leading to 70 and 44% yields of the respect-
ive condensed products. From these data, it is undoubtedly
clear that the reaction is highly influenced by the molecular
dimensions of aldehydes. These results clearly show that the
condensation reaction occurs within the pores of 1′ and this is
an interesting example to demonstrate the size-selective cataly-
sis of MOF 1′ convincingly.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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