Synthesis and click modification of an azido-functionalized Zr(iv) metal-organic framework and a catalytic study

Article abstract of DOI:10.1039/c4ra09883h

An azido-functionalized Zr(ii) metal-organic framework (MOF), UiO-67-N₃, was synthesized from 2-azidobiphenyl-4,4′-dicarboxylic acid. During the synthesis, the ligand can undergo in situ thermocyclization to give 9H-carbazole-2,7-dicarboxylic acid. It proved that UiO-67-N₃ can be obtained at relatively low temperature without ligand transformation. Post-synthetic modification of UiO-67-N₃ was successfully performed via the click reactions between the azido group and different alkyne compounds to produce new MOFs with different functionalities, UiO-67-Tz-X with X = COOCH₃, OH and NH₂. These clicked MOFs, especially UiO-67-Tz-NH₂, exhibit better stability than the mother material UiO-67-N₃. The catalytic properties of the clicked MOFs were studied using the Knoevenagel condensation reactions between benzaldehyde and different methylene compounds. Only the NH₂-functionalized MOF is active, suggesting that the amino group, rather than the triazole group or any other component of the framework, is the crucial active site. The catalysis is heterogeneous. The MOF is recyclable for the reaction with malononitrile but not for the reaction with ethyl cyanoacetate. The deactivation in the latter case is proposed to be because the amino site reacts with the ester group of ethyl cyanoacetate to form amide. This journal is

Full text of DOI:10.1039/c4ra09883h

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Synthesis and click modification of an azido-
functionalized Zr(IV) metal–organic framework and
a catalytic study†
Cite this: RSC Adv., 2015, 5, 893
*
Xiu-Chun Yi, Fu-Gui Xi, Yan Qi and En-Qing Gao
An azido-functionalized Zr(II) metal–organic framework (MOF), UiO-67–N3, was synthesized from 2-
azidobiphenyl-4,40-dicarboxylic acid. During the synthesis, the ligand can undergo in situ
thermocyclization to give 9H-carbazole-2,7-dicarboxylic acid. It proved that UiO-67–N3 can be
obtained at relatively low temperature without ligand transformation. Post-synthetic modification of
UiO-67–N3 was successfully performed via the click reactions between the azido group and different
alkyne compounds to produce new MOFs with different functionalities, UiO-67–Tz–X with X ¼
COOCH3, OH and NH2. These clicked MOFs, especially UiO-67–Tz–NH2, exhibit better stability than the
mother material UiO-67–N3. The catalytic properties of the clicked MOFs were studied using the
Knoevenagel condensation reactions between benzaldehyde and different methylene compounds. Only
the NH2-functionalized MOF is active, suggesting that the amino group, rather than the triazole group or
any other component of the framework, is the crucial active site. The catalysis is heterogeneous. The
MOF is recyclable for the reaction with malononitrile but not for the reaction with ethyl cyanoacetate.
The deactivation in the latter case is proposed to be because the amino site reacts with the ester group
of ethyl cyanoacetate to form amide.
Received 5th September 2014
Accepted 18th November 2014
DOI: 10.1039/c4ra09883h
www.rsc.org/advances
The copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC)
reaction, a typical ‘click’ reaction, represents an elegant route
for PSM of MOFs. A variety of alkyne-20–24 and azide-tagged25–28
MOFs have been successfully modied by CuAAC reactions. The
metal-free strain-promoted azide–alkyne cycloaddition using
cyclooctyne derivatives has also been employed for PSM of a
mesoporous MOF.29 However, the alkyne- or azide-
functionalized linkers and the stable MOFs applicable to click
PSM are still quite limited in number.
The isoreticular Zr(IV) MOFs of general formula [Zr6O4-
(OH)4(L)6] [L ¼ rigid (quasi-)linear dicarboxylates, such as 1,4-
benzenedicarboxylate, biphenyl-4,40-dicarboxylate and ter-
phenyl-4,400-dicarboxylate for UiO-66, UiO-67 and UiO-68,
respectively] contain face-sharing octahedral and tetrahedral
cages with [Zr6(O)4(OH)4(COO)12] as vertices.30 The thermal and
chemical stability of the Zr MOFs has allowed a number of
recent studies related to absorption/separation,31–34 sensing,35,36
catalysis37–41 as well as PSM.42–48 However, only a few Zr–MOFs
have been subjected to click PSM. Azidomethyl-functionalized
UiO-68 MOFs were clicked with different alkynes to modify
the pore surface for selective CO2 adsorption over N2 or to
transform MOFs to polymer gels,49,50 and very recently, an azido-
functionalized UiO-66 MOF (UiO-66–N3) has been clicked on the
nanocrystal surfaces with alkyne-tagged oligonucleotides to
create the rst nucleic acid–MOF nanoparticle conjugates.51
In this article, we report the synthesis and the click PSM of a
new Zr(II) MOF, UiO-67–N3, followed by a catalytic study with the
Introduction
Metal–organic frameworks (MOFs) have emerged as a new class
of fascinating materials having potential applications in some
elds such as gas adsorption/separation, catalysis, sensing and
drug delivery.1–10 The modication of the pore surfaces with
desired functional groups is important for achieving better
performance in the above applications. Post-synthetic modi-
cation (PSM), which involves a reaction with any component of a
known framework, is a very attractive strategy for modication
of MOFs,11–13 but PSM is challenging because the modication
may lead to undesired and unpredictable changes or destruc-
tion in the frameworks. The most widely studied platforms of
covalent PSM are the MOFs bearing uncoordinated amino
groups, which can readily react with aldehydes, isocyanates,
and anhydrides so that various functional groups can be graed
onto the pore surfaces.14–16 More sophisticated reactions and
stable MOFs suitable for PSM is still being sought not only to
enrich the chemical diversity of MOFs but also to functionalize
the materials with application-oriented groups.17–19
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, P.R. China. E-mail:
eqgao@chem.ecnu.edu.cn; Fax: +86-21-62233404; Tel: +86-21-62233404
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra09883h
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Synthesis of UiO-67–N3
All the solvents and reagents were commercially available and
used as received.
H2BPDC–N3. Dimethyl 2-azidobiphenyl-4,40-dicarboxylate
(Me2BPDC–N3) was synthesized from dimethyl biphenyl-4,40-
dicarboxylate according to the literature,57 via multi-step
procedures involving nitration with HNO3–H2SO4, Pd/C-
catalyzed hydrogenation with H2, diazotization with NaNO2
and substitution with NaN3. H2BPDC–N3 was synthesized by the
following hydrolization procedure. Me2BPDC–N3 (3.1 g, 10
mmol) was dissolved in tetrahydrofuran (THF, 40 mL), to which
0.1 M NaOH(aq.) was introduced until pH z 12–13. The mixture
was reuxed for 24 h. Aer cooling down to room temperature,
additional water was added to the resulting solution until the
solid was fully dissolved, then the solution was acidied with
diluted HCl(aq.) to pH z 2. The resulting precipitate was
separated by ltration, washed with water and air-dried to
afford a white solid. Yield: 2.8 g (99%). Anal. calcd (%) for
Scheme 1 Synthesis and click PSM of UiO-67–N3 and the Knoeve-
nagel condensation reactions studied.
modied MOFs (Scheme 1). UiO-67–N3 was synthesized from 2-
azidobiphenyl-4,40-dicarboxylic acid (H2BPDC–N3), a N3-func-
tionalized linker not yet explored for the construction of MOFs.
As will be shown, we have overcome the difficulty arising from
the in situ thermocyclization of the ligand and successfully
obtained the target N3-functionalized MOF. The PSM of UiO-67–
N3 was performed via CuAAC click reactions with different
alkynes [methyl propiolate (HC^CCOOMe), 3-butyn-1-ol
(HC^CCH2CH2OH) and propargylamine (HC^CCH2NH2)] to
produce new and more stable MOFs with different functional-
ities (UiO-67–Tz–COOCH3, UiO-67–Tz–OH and UiO-67–Tz–NH2).
UiO-67–Tz–NH2 is a potential basic catalyst. The Knoevenagel
condensation between carbonyl and activated methylene is
well-known as a base-catalyzed C–C coupling reaction and has
important applications in ne chemicals and pharmaceuti-
cals. It has been demonstrated that the condensation can be
catalyzed by different amino-functionalized MOFs, such as
IRMOF-3, MIL-101–NH2, and UiO-66–NH2, with amino as
active sites.52–56 Here we chose the reactions of benzaldehyde
with ethyl cyanoacetate and malononitrile as model reactions
to test the catalytic properties of the MOFs modied via click
chemistry.
C
14H9N3O4: C 59.4, H 3.2, N 14.8; found: C 59.8, H 2.9, N 14.6.
1H NMR (DMSO-d6, 500 MHz), d 7.55 (d, J ¼ 7.8 Hz, 1H), 7.63 (d,
J ¼ 8.1 Hz, 2H), 7.82 (d, J ¼ 7.9 Hz, 1H), 7.84 (s, 1H), 8.01 (d, J ¼
8.1 Hz, 2H), 13.39 (s, 2H).
UiO-67–N3. ZrCl4 (0.109 g, 0.444 mmol), H2BPDC–N3 (0.126
g, 0.424 mmol) and acetic acid (0.240 mL, 4.20 mmol) were
dissolved in DMF (3 mL) in a 25 mL ask and heated at 80 ꢁC for
24 h. Aer cooling to room temperature, the mixture was
¨
ltered through a Celite Buchner funnel and the residue was
washed with fresh DMF and dried in air to afford pale yellow
solid. Yield 0.18
g (about 40%). Anal. calcd (%) for
C
126H226N32O87Zr6 ([Zr6O4(OH)4(BPDC–N3)6]$14DMF$41H2O):
C 36.7, H 5.5, N 10.9; found: C 36.8, H 5.6, N 10.5. IR (KBr,
cmꢀ1): 3125br, 2117vs, 1658vs, 1596vs, 1542s, 1493m, 1415vs,
1370s, 1282m, 1255m, 1184w, 1103w, 1058w, 1020w, 1005w,
875w, 790w, 773m, 710w, 655m.
PSM of UiO-67–N3
PSM with methyl propiolate. Methyl propiolate (2 mL) were
added to a mixture of UiO-67–N3 (0.150 g) and CuI (0.010 g) and
in DMF (6.0 mL) in a 25 mL round-bottom ask. The mixture
was stirred at 60 ꢁC for 18 h. The resultant orange solid was
collected by ltration, washed with fresh DMF, and dried in air.
Experimental
Physical measurements
Elemental analyses were performed on an Elementar Vario ELIII The product is labeled as UiO-67–Tz–COOCH3. Yield: 0.12 g,
analyzer. FT-IR spectra were recorded in the range of 500–4000 75% based on UiO-67–N3. Anal. calcd (%) for
cmꢀ1 using KBr pellets on a Nicolet NEXUS 670 spectrometer.
C
123H215N23O104Zr6
([Zr6O4(OH)4(BPDC–Tz–COOCH3)6]$
Thermogravimetric Analysis (TGA) was performed on a Mettler 5DMF$55H2O): C 34.9, H 5.1, N 7.6; found: C 35.2, H 5.2, N
TGALSDTA851e/5FL1100 instrument. Powder X-ray diffraction 7.8%. IR (KBr, cmꢀ1): 3425br, 1730s, 1656vs, 1600vs, 1544s,
(XRD) measurements were performed on a Rigaku Ultima IV 1496m, 1415vs, 1370sh, 1265m, 1233m, 1184w, 1157w, 1105w,
diffractometer equipped with Cu-Ka. Nitrogen adsorption– 1041m, 1006w, 875w, 810w, 775m, 710w, 657m.
desorption measurements were performed at 77 K on a Quan-
PSM with 3-butyn-1-ol. A similar procedure using 3-butyn-1-
cachrome Autosorb-3B instrument aer heating the samples at ol instead of methyl propiolate led to a green product (UiO-67–
certain temperature for 6 h; the specic surface areas were Tz–OH) in a yield of 70%. Anal. calcd (%) for C132H210N26O76Zr6
calculated using the Brunauer–Emmett–Teller (BET) method. ([Zr6O4(OH)4(BPDC–Tz–OH)6]$8DMF$40H2O): C 40.4, H 5.4, N
NMR spectra were recorded on a Bruker Avance 500 MHz NMR 9.3; found: C 40.8, H 5.9, N 9.1. IR (KBr, cmꢀ1): 3400br, 3260sh,
spectrometer. Gas chromatography (GC) analyses were per- 1730s, 1657vs, 1600vs, 1544s, 1498w, 1415vs, 1380sh, 1251w,
formed on a LingHua GC 9890E instrument equipped with a 1236w, 1184w, 1105w, 1049m, 1004w, 873w, 790sh, 777m, 710w,
Flame Ionization Detector (FID) detector.
658m.
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PSM with propargylamine. A similar procedure using prop-
argylamine led to a brown product (UiO-67–Tz–NH2) in a yield
of 68%. Anal. calcd (%) for C120H214N30O86Zr6 ([Zr6O4(OH)4
(BPDC–Tz–NH2)6]$6DMF$48H2O): C 36.0, H 5.4, N 10.5; found:
C 36.3, H 5.7, N 10.7. IR (KBr, cmꢀ1): 3400br, 3260sh, 1730s,
1658vs, 1602vs, 1544s, 1494sh, 1408vs, 1388sh, 1253w, 1234w,
1188w, 1100w, 1045m, 1004w, 875w, 790sh, 778m, 710w, 655m.
The click PSM with the different alkynes caused distinct
changes in colour, which are shown in Fig. S1.†
Catalytic studies
A given amount of as-synthesized UiO-67–Tz–NH2 (48 mg, 0.012
mmol. The amount of amino is about 0.07 mmol, i.e., 7 mol%
with reference to benzaldehyde) was placed in a ask. Aer
three cycles of vacuum pumping and nitrogen injection, DMF (3
ꢁ
mL) was added and the ask was kept at 40 C in an oil bath.
The methylene substrates (2 mmol) and benzaldehyde (1 mmol)
was successively syringed to initiate the reaction, and the
reaction was monitored by GC at different time intervals. For
recyclability study, the solid aer the catalytic reaction was
isolated by centrifugation, washed with DMF several times,
transferred to the reactor and used directly for the next run of
catalytic reaction.
Fig. 1 XRD patterns of the products synthesized at different temper-
ature (a–c) and the simulated pattern for UiO-67 (d, with arbitrary
intensity). Note that the reflection intensity of (c) is much high than that
of (a and b).
Results and discussion
Synthesis and characterization of UiO-67–N3
The reaction temperature is crucially important for the
synthesis of UiO-67–N3. Our initial synthetic attempt using the
solvothermal conditions similar to that for UiO-67 was disap-
pointing. The solvothermal reaction of ZrCl4 and H2BPDC–N3
(1 : 1 molar ratio) in DMF at 140 ꢁC in the presence of acetic acid
(HOAc) as modulator always led to an unidentied and essen-
tially amorphous solid, according to XRD measurements
(Fig. 1a). Worse still is that IR spectrum of the sample did not
show the absorption band characteristic of the azide group
(Fig. 2a). To probe the change of the ligand, the sample was
treated with concentrated HCl(aq.), and the resulting solid was
isolated for IR and 1H NMR studies (Fig. S2†). The 1H NMR
Fig. 2 IR spectra of the products synthesized at different temperature.
spectrum of the solid dissolved in DMSO-d6 is in perfect commingled with some unknown amorphous phase(s) with the
agreement with 9H-carbazole-2,7-dicarboxylic acid (H2CDC). CDC ligand. The results encouraged us to further reduce the
Consistently, the IR spectrum of the solid shows a characteristic synthetic temperature to 80 ꢁC, which gave good results. The IR
n(NH) sharp peak at ꢂ3345 cmꢀ1 with no indication of nas(N3) spectrum of the product shows a very strong nas(N3) band at
absorption. So, it could be concluded that under above ꢂ2117 cmꢀ1 together with a band at 1280 cmꢀ1 attributable to
synthetic conditions the H2BPDC ligand underwent complete ns(N3) (Fig. 2c). The 1H NMR and IR spectra aer digestion show
thermocyclization to give H2CDC (Scheme 2), which formed an no evident signals of H2CDC and are identical to those of
unknown phase with Zr(II).
H2BPDC–N3 (Fig. S4†). These indicate that the thermocycliza-
To avoid the in situ ligand transformation, we sought to tion decomposition of BPDC–N3 was prevented under the mild
synthesize UꢁiO-67–N3 at low temperature. The sample synthe- synthetic conditions. Furthermore, the XRD pattern of the
sized at 110 C shows weak and sharp nas(N3) IR absorption at product is well resolved with sharp and intense peaks and in
ꢂ2117 cmꢀ1 (Fig. 2b), and the IR spectrum of the solid obtained good agreement with that of UiO-67 (Fig. 1c and d), with no
by digesting the sample with concentrated HCl(aq.) shows both evident indications of impure phases. So, UiO-67–N3 was
nas(N3) and n(NH) bands (Fig. S3†), suggesting the coexistence of successfully synthesized at 80 ꢁC as a pure crystalline phase.
BPDC–N3 and CDC in the as-synthesized sample. Consistently, The synthesis is reproducible.
XRD data (Fig. 1. Compare (b) with (a) and (d)) indicates that a
The inuence of auxiliary synthetic modulators, benzoic acid
crystalline phase similar to UiO-67 was generated at 110 ꢁC but (HOBz) and HOAc, on the product was also investigated. In the
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intermediate slow weight loss may be due to the dehydration of
the [Zr6O4(OH)4] cluster and partial decomposition of the
organic component. To probe the integrity of the framework
during thermal treatments, the XRD patterns of the samples
heated at different temperatures were measured (Fig. S6†). The
broadening of the reections is evident even at 60 ꢁC, and only
very broad reections were observed for the sample heated at
110 ꢁC, suggesting collapse of the framework. The 1H NMR
spectrum of the sample heated at 110 ꢁC and dissolved in a 20%
NaOD/D2O solution indicates that most (ꢂ80%) of the BPDC–N3
ligand has transformed into CDC (Fig. S7†). So the poor thermal
stability of UiO-67–N3 is related to the thermocyclization of the
BPDC–N3 ligand. The results also indicate that the resulting
CDC ligand cannot support the framework. Furthermore, our
attempts to synthesize a UiO-67-type framework from H2CDC as
starting material have come out in vain, also implying that the
UiO-67-type framework cannot be formed with CDC. Perhaps
the bent shape of the CDC ligand (the orientations of the two
coordinative carboxylate groups have a angle of about 150ꢁ
(ref. 58)) disfavours the highly symmetric structure of UiO-67
type, which prefers a linear linkage between the 12-connected
[Zr6O4(OH)4(COO)12] clusters.
Scheme 2 Thermocyclization of H2BPDC–N3 to H2CDC.
absence of any modulating agent and under otherwise identical
conditions, the reactions gave precipitates quickly. XRD indi-
cated that the product isolated is also UiO-67–N3 (Fig. 3a) but
the very broad reections suggest that very small crystallites
were obtained. The addition of HOBz into the reaction mixture
slowed the precipitation and decreased the nal yield. As can be
see from Fig. 3b, the product obtained in the presence of 40
equivalents of HOBz shows less broad XRD peaks than that
obtained in the absence of modulators, indicating that the
slowed precipitation favours the crystal growth. Further
increasing the amount of the modulator led to signicant
decrease in yield without apparent improvement in crystallinity.
The reaction in the presence of 60 equivalents of HOBz did not
yield any solid product within 2 weeks. The effect of HOAc on
crystal growth is more pronounced. As shown in Fig. 3c, the
product synthesized in the presence of 10 equivalents of HOAc
shows very sharp and well-resolved reections. The yield is
moderate. No further optimization of the synthetic conditions
was performed.
The thermal behaviour of UiO-67–N3 was studied by TGA
(Fig. S5†). UiO-67–N3 undergoes a rapid weight loss of ca. 43%
when heated to 210 ꢁC and a much slower loss upon further
heating until another ꢁrapid loss appears above 440 ꢁC. The
weight loss above 440 C corresponds to the complete decom-
position of the framework, and the loss below 210 ꢁC can be due
to the release of the guest molecules (DMF and water) enclosed
in the material and the N2 byproduct of the possible thermo-
cyclization of BPDC–N3 (calculated total loss: 47%). The
The solvent resistance of UiO-67–N3 was also studied
(Fig. S6†). Immersing the material in some common solvents
including acetone, dichloromethane, trichloromethane and
ethanol led to the loss of crystallinity, according to XRD
measurements. N2 adsorption experiments revealed that the
ꢁ
samples aer solvent exchange and/or heating at 100 C show
only surface adsorption, also indicating collapse of the frame-
work. The poor thermal and solvent resistance of UiO-67–N3
compared with UiO-67 reect that the incorporation of a reac-
tive group into a framework may cause dramatic decrease in
stability, a challenging issue oen encountered in preparing
MOFs for PSM. Fortunately, however, the structural integrity of
UiO-67–N3 remains intact in DMF: immersing it in DMF for a
prolonged time did not lead to appreciable change in XRD. This
made it possible to modify the MOF via the CuAAC click reac-
tion, for which DMF is a frequently used solvent. It will be
shown below that the thermal and chemical stabilities of the
framework can be enhanced aer the click modication, which
further allows us to perform a catalytic study.
Post-synthetic modication by click chemistry
We have succeeded in post-synthetically transforming UiO-67–N3
into UiO-67–Tz–COOCH3, UiO-67–Tz–OH and UiO-67–Tz–NH2
via the CuAAC click reactions with methyl propiolate, 3-butyn-1-ol
and propargylamine, respectively. The reactions were performed
by stirring the mixture of UiO-67–N3, the alkynes and a catalytic
amount of CuI in DMF at 60 ꢁC. The reaction progress was
monitored by IR spectra. A direct evidence for the occurrence of
the click reaction is the change in the characteristic IR band of
the azide group. As shown in Fig. 4 for the reaction with methyl
propiolate, the nas(N3) band at ꢂ2117 cmꢀ1 signicantly
decreased in intensity aer 10 h and completely disappeared aer
18 h, and so did the weaker ns(N3) band at ꢂ1280 cmꢀ1. This
clearly indicates the complete transformation of the azide group
Fig. 3 XRD patterns of the UiO-67–N3 samples synthesized in the
absence/presence of modulators.
896 | RSC Adv., 2015, 5, 893–900
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