Tetrazine-Based Metal-Organic Frameworks as Scaffolds for Post-Synthetic Modification by the Click Reaction

Article abstract of DOI:10.1002/ejic.201901230

Metal-organic frameworks are one of the most active research fields today. Despite the progress in recent years, synthesis of highly porous structures bearing functional groups for specific applications remains a challenge. Here, we describe synthesis of Zr(IV) and Hf(IV)-based tetrazine containing metal-organic frameworks, ZrTz-68 and HfTz-68 with pore size of 2.1 nm, specific surface area up to 4217 m² g^–1, and a UiO-68 topology. The synthesis can be done on a multi-gram scale from inexpensive linker 4,4'-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (H₂TzDB). Moreover, because of the π-conjugated tetrazine groups these MOFs can be easily post-synthetically modified by a “click” reaction maintaining the crystallinity of the framework. Due to the diversity of “clickable” substituents, this strategy represents highly versatile tool for a wide range of applications.

Full text of DOI:10.1002/ejic.201901230

A Journal of
Accepted Article
Title: Tetrazine-Based Metal-Organic Frameworks as Scaffolds for
Post-Synthetic Modification by Click Reaction
Authors: Madhan Vinu, Kulandaivel Sivasankar, Samikannu Prabu,
Jeng-Liang Han, Chia-Her Lin, Chun-Chuen Yang, and Jan
Demel
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201901230
Link to VoR: http://dx.doi.org/10.1002/ejic.201901230

10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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Tetrazine-Based Metal-Organic Frameworks as Scaffolds for
Post-Synthetic Modification by Click Reaction
Madhan Vinu,*[a,b,c] Kulandaivel Sivasankar,[c] Samikannu Prabu,[c] Jeng-Liang Han,[c] Chia-Her Lin,*[c]
Chun-Chuen Yang,[d] and Jan Demel*[a]
Abstract: Metal-organic frameworks are one of the most active
research fields today. Despite the progress in recent years,
synthesis of highly porous structures bearing functional groups for
Post-synthetic modification (PSM) is a powerful tool that
converts as-synthetized MOFs to topologically identical
materials with diverse functionalities or enhanced stability.[18,19]
In recent years, PSM evolved so that it allows metal-to-metal
exchange in SBUs,[20] covalent modifications on linker
molecules,[21] exchange of linker molecules, or exchange of
terminal ligands at the SBUs.[22]
Alternatively, the modification can be done through chemical
modification of the linker by “click chemistry” to introduce large
reactive species into MOFs.[23-27] In this respect [4+2]
cycloaddition of 1,2,4,5-tetrazines and various dienophiles,
referred as inverse electron demand Diels-Alder (IEDDA)
reaction, represent an attractive reaction that allows reaction of
tetraazines with wide range of unsaturated compounds.[28]
Surprisingly, only few tetrazine based MOFs have been
described in the literature: Calahorro and co-workers developed
tetrazine functionalized potassium 3D MOF for luminescence
and cytotoxicity studies,[29] alternatively Nickerl et al. synthetized
dihydro-1,2,4,5-tetrazine-3,6-dicarboxylate Zr-MOF by ligand
exchange reaction to create an optical sensor for the detection
of oxidative agents such as nitrous gases.
specific applications remains
a challenge. Here, we describe
synthesis of Zr(IV) and Hf(IV)-based tetrazine containing metal-
organic frameworks, ZrTz-68 and HfTz-68 with pore size of 2.1 nm,
specific surface area up to 4217 m2 g-1, and a UiO-68 topology. The
synthesis can be done on a multi-gram scale from inexpensive linker
4,4’-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (H2TzDB). Moreover,
because of the л-conjugated tetrazine groups these MOFs can be
easily post-synthetically modified by a “click” reaction maintaining
the crystallinity of the framework. Due to the diversity of “clickable”
substituents, this strategy represents highly versatile tool for a wide
range of applications.
Introduction
Metal-organic frameworks (MOFs) are crystalline porous
materials formed by organic multi-dentate ligands with metal
centers or metal oxide clusters.[1,2] Because of the high variability
of porosity, surface area, topology, and tunable chemical nature,
this class of materials is a rapidly growing area of research.[3-5]
As a result, MOFs are investigated for a variety of potential
applications including, gas storage[6] and separations,[7]
heterogeneous catalysis,[8-10] sensing,[11] and drug delivery.[12] It
has been shown, that for certain catalyzed reactions the catalytic
centers need to be isolated in order to achieve high catalytic
activity.[13,14] It is very difficult to obtain MOFs equipped with
large functional groups in their pore structures by direct
solvothermal reactions. However, post-synthetic modification of
MOFs have proved to enable addition of functional groups,[15,16]
increasing the hydrophobicity of the pore structure, or the
stability.[17]
In this paper, we report on the direct synthesis of three-
dimensional tetrazine based zirconium and hafnium MOFs that
can be subject to “click reaction” to introduce bulky functional
molecules into the pore structure, see Scheme 1. The linear
12+
tetrazine dicarboxylate linker forms with Zr6O4(OH)4
and
12+
Hf6O4(OH)4 SBUs MOFs denoted ZrTz-68 and HfTz-68, their
structures contain mesoporous channels with 2.1 nm in diameter
and specific surface area up to 4217 and 2773 m2 g-1 for ZrTz-68
and HfTz-68, respectively. First, we optimized the synthetic
procedures in terms of modulator,[30] time, temperature,
activation solvent, and concentration of different modulators.
Then, to demonstrate the versatility of
postsynthetic
modification by “click” reaction we modified the MOFs by three
alkene containing organic molecules: Quinine (QV), thioquinine
(QVS),[31-34] and organometallic copper complex CuNONO, see
Scheme 1.[35,36] Finally, on the basis of the original synthetic
procedures, with only subtle modifications, we demonstrate that
both the organic linker and the MOFs can be synthesized on
multi-gram scale.
[a]
Dr. M. Vinu, Dr. J. Demel Department of Materials Science, Institute
of Inorganic Chemistry of the Czech Academy of Sciences, v.v.i.
Husinec-Řež 1001, Řež, 25068 (Czech Republic)
E-mail: vinuchem06@gmail.com, demel@iic.cas.cz
Homepage: https://demel.iic.cas.cz/
[b]
[c]
[d]
Dr. M. Vinu, Department of Chemistry, Karunya Institute of
Technology and Sciences (Deemed-to-be University), Karunya
Nagar, Coimbatore – 641 114, India
Dr. K. Sivasankar, Dr. S. Prabu, Prof. J.-L. Han, Prof. C.-H. Lin,
Department of Chemistry, Chung-Yuan Christian University, Chung
Pei Road, Chungli, Taoyuan City, 32023, Taiwan
Results and Discussion
Two new zirconium and hafnium MOFs (denoted ZrTz-68 and
HfTz-68) were synthesized using novel linear dicarboxylate
Prof. C.-C. Yang, Department of Physics, Chung-Yuan Christian
University, Chung Pei Road, Chungli, Taoyuan City, 32023, Taiwan
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
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10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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linker 4,4’-(1,2,4,5-tetrazine-3,6-diyl)dibenzoic acid (H2TzDB),
the synthetic procedure of the linker is given in Scheme S1. The
MOFs were prepared by solvothermal reactions of H2TzDB and
ZrCl4 or HfCl4. In order to obtain good crystallinity, modulators
have to be employed, for the list of used modulators and the
resulting powder X-ray diffraction patterns (PXRD) see Figure
S1. The optimal synthetic conditions found were using N,N-
give a plausible crystal structure, which was verified by the final
Rietveld refinement, see Figure. 2.
dimethylformamide (DMF) and trifluoroacetic acid as
a
modulator at 120 °C for 48 h. By this procedure we obtained
octahedron-shaped crystalline powders, see Figure 1.
Interestingly, when the as-synthetized MOFs were soaked in
DMF for 2-30 days the one-month soaked samples show better
resolved particles (see Figure 1) with 1-2 µm in size and keeping
the PXRD intact. This is probably due to slow recrystallization
that takes place in DMF even at RT. The synthesis of the MOF
can be readily scaled-up by a factor of 10 and so one batch can
yield 3.5 g of ZrTz-68 and 3.6 g of HfTz-68, respectively.
Figure 2. Rietveld fit of ZrTz-68, black dots represent measured data, red
curve is a calculated profile, blue line is a difference curve and magenta
vertical bars are Bragg’s positions of the hkl reflections.
The ZrTz-68 crystalize in the space group Fm-3m, with
a=32.447 Å (Figure-2 & Tables S1, SI) and fcu net topology in
which the inner M6O4(OH)4(CO2)12 SBU cores are capped by µ3-
O and µ3-OH groups to form the triangular faces, see Figure 3.
This SBUs are connected together by twelve carboxylate groups
from H2TzDB linker to form three-dimensional (3D) structure with
tetrahedral and octahedral cages with sizes of 0.9 and 2.2 nm,
respectively (Figure 3 and 4). The total solvent-accessible
volume for ZrTz-68 is 69% (determined by PLATON).
Scheme 1. Synthesis and Post-synthetic modification of ZrTz-68, HfTz-68
MOFs with QV, QVS and CuNONO.
.
Figure 1. SEM images of (a), as-synthesized ZrTz-68, (b), one month DMF
soaked ZrTz-68, (c), as-synthesized HfTz-68, and (d), one month DMF soaked
HfTz-68.
To verify that ZrTz-68 adopts the expected structure of UiO-68
with the exception of the four nitrogen atoms in the tetrazine
linker, we employed theoretical modelling using the Materials
Studio 5.5, for details see ESI. The unit cell was elongated
according to the experimental lattice constants and the atoms of
the H2TzDB linker without hydrogen atoms were inserted. Then,
we performed the geometry optimizations which converged to
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10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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Figure 3. View of the ZrTz-68 structure (a), the coordination of twelve linkers
around the SBU (b), the formation of tetrahedral cage (c), and octahedral cage
(d). Blue polyhedra are Zr6 SBU, yellow spheres are void spaces, carbon
atoms are black, nitrogen atoms are blue, and hydrogen atoms were omitted
for clarity.
and HfTz-68@CuNONO confirm that the samples after PSM
retain the original structure. The small decrease in intensities of
diffraction can be caused by the disorder of the newly introduced
functional groups, see Figure 5. In Table 1 are compared the
specific surface areas, pore volumes, and pore diameters of
PSM modified samples with the parent MOFs. It is clear that the
PSM modification led to decrease of the specific surface area
and pore volume while keeping the pore diameter intact, see
Figure 8 and S9(SI). The post-synthetic modification of samples
was accompanied by a significant color change from bright pink
(ZrTz-68 and HfTz-68) to light brown (MOF@PSM), see Figure 7.
UiO-67 and 68 are known to be susceptible to hydrolysis on
air,[36] similarly the newly prepared MOFs ZrTz-68 and HfTz-68
have limited stability on humid air. In Figures S2 & S3 is shown
that after 6 days there is distinguishable decrease in crystallinity.
We also studied the thermal stability by PXRD measured at
varied temperature. Interestingly the stability of HfTz-68 is 300°C
whereas for ZrTz-68 it is only 200°C, see Figure S4.
Figure 5. The PXRD patterns of the MTz-68, MTz-68@QV, MTz-68@QVS
and MTz-68@CuNONO.
Figure 4. The BET isotherm (left) and DFT pore size curve (left) of ZrTz-
68(Blue), ZrTz-68(Red).
Direct evidence of the PSM can be obtained from the IR-spectra.
In Figure 6 are compared spectra of parent MOFs and
MOF@QVS. The FTIR spectrum of the parent MOFs display
several characteristic absorptions that are weakened after PSM,
namely 1414 cm-1 from N=N and 1345 cm-1 from C–N=N–C,[40]
simultaneously new C-H stretching vibrations at 2800-3000 cm-1
arise from the alkene molecules. The FTIR spectra of the
MOF@QV and MOF@CuNONO are given in the SI (Figure S6-
S8). After post-synthetic modified samples morphology of
particles were characterized using SEM images given in the SI
(Figure S-10).
The porosity of the prepared MOFs was probed by N2 adsorption
at 77 K, both samples exhibit type I(b) isotherm (see Figure 4),
which is characteristic for materials containing small mesopores.
The Brunauer-Emmett-Teller (BET) specific surface areas, Non-
Localized Density Functional Theory (NLDFT) calculated pore
sizes, and pore volumes are summarized in Table 1. The
specific surface area of ZrTz-68 (4217 m2g-1) is in good
agreement with the specific surface area reported for UiO-68
(4170 m2g-1).[37] The specific surface area of HfTz-68 is lower by
ca. 35% in comparison with the Zr analogue which is similar to
value of 36 % reported earlier for Zr and Hf analogues of UiO-
66.[38] The pore size distribution shows single pore channel
diameter with the maxima at 2.1 nm. Additionally, we measured
CO2 adsorption isotherm at 273 and 298 K for both ZrTz-68 and
HfTz-68, see Figure S5.
The PSM is also accompanied by increased stability of the
resulting MOFs. As discussed above, the as-synthesized MOF
samples lose their crystallinity at room temperature on humid air
in one week, on the other hand, the PSM modified MOF
samples are stable in humid air, see Figure S3 (SI).
Click reactions were developed in 1998 by K. Barry Sharpless
and since then these reactions have gained tremendous interest
due to the simplicity, versatility, and quantitative yield. In this
work, we use the cycloaddition of 1,2,4,5-tetrazines with alkene,
referred as inverse electron demand Diels-Alder (IEDDA)
reaction.[39] The use of the click reaction allows the introduction
of large functional groups into the structure of mesoporous
MOFs. Here we selected bulky alkene molecules, specifically
quinine, thioquinine, and copper complex CuNONO.
We
selected benign conditions: 70 °C for 12 h in diethyl ether
without any catalyst (Scheme 1). After PSM the powder samples
were washed and centrifuged with fresh diethyl ether three times
and activated at 120 °C for 12 h. After the click reaction of MTz-
68@PSM samples were characterized by FTIR, PXRD, SEM
and N2 adsorption. The PXRD patterns of ZrTz-68@QV, ZrTz-
68@QVS, HfTz-68@QV, HfTz-68@QVS, ZrTz-68@CuNONO,
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10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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Figure 6. FT-IR spectra of QVS compared with before and after post-synthetic
modified ZrTz-68 and HfTz-68.
Figure 8. The BET isotherm of (a) ZrTz-68(Black), ZrTz-68@QV(Blue),
ZrTz-68@QVS(Red), ZrTz-68@CuNONO(Green), (b) HfTz-68(Black),
HfTz-68@QV(Blue), HfTz-68@QVS(Red), and HfTz-
68@CuNONO(Green).
Figure 7. The color changes of MOFs compared with PSM samples of first (a)
ZrTz-68, (b)ZrTz-68@QV, (c)ZrTz-68@QVS, (d)ZrTz-68@CuNONO and
second HfTz-68.
Table 1. BET specific surface areas of MOFs compared before and after
post-synthetic modification derived from N2 isotherm.
No.
MOF
BET Surface
Area (m² g-1)
Pore
Volume
(cm³ g-1)
Pore
Diameter
(nm)
1
2
3
4
ZrTz-68
4217
2773
1348
746
1.72
1.24
0.69
0.67
2.1
2.1
2.0
2.2
HfTz-68
ZrTz-68@QV
ZrTz-
68@QVS
5
6
HfTz-68@QV
1224
757
0.59
0.27
2.1
2.0
HfTz-
68@QVS
7
8
ZrTz-
68@CuNONO
1005
909
0.46
0.48
2.1
2.0
HfTz-
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10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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68@CuNONO
acetic acid (60 mL) followed by an aqueous solution of 30 mL NaNO2 (8.2
g, 120 mmol) at 0 °C for 3 h under stirring. The purple colored tetrazine
compound was collected by filtration and washed with water (2 × 50 mL).
The solid was collected and dried in an oven at 90 oC to give a purple
product (yield = 3.0 g, 46.88% based on 4-cyanobenzoic acid).
3. MOF Synthesis
Synthesis of [M6O4(OH)4(C16N4H8O4)], (M= Zr, Hf) (ZrTz-68 and HfTz-
68)
Conclusions
We have demonstrated that tetrazine based linkers are suitable
for the preparation of UiO-68 analogues ZrTz-68 and HfTz-68.
The MOFs display high porosity with specific surface area up to
4217 m2 g-1 and pore diameter of 2.1 nm. The synthesis was
done using simple solvothermal method which can be easily
scaled up. The main advantage of the tetrazine linker is the
possibility of post-synthetic modification via click reaction. We
have demonstrated that it is possible to introduce bulky
functional groups without compromising the structure,
crystallinity, and morphology of MOFs. We believe that post-
synthetic modification of MOFs by click reactions is better suited
for the introduction of bulky organic molecules than other post-
synthetic modification strategies such as ligand exchange. The
main advantages are nonnecessity to synthetize new linker
molecule bearing carboxylic acid groups and overcoming
solubility issues of such bulky molecule.
The preparation of both ZrTz-68 and HfTz-68 was done by solvothermal
method:0.105 g of ZrCl4 or 0.144 g of HfCl4 (both 0.45 mmol), 0.145 g of
H2TzDB (0.45 mmol) and 0.2 mL of trifluoroacetic acid were mixed in 25
mL of DMF in a 100 mL RB flask by 10 min ultra-sonication. The solution
was heated with stirring in oil bath at 120 °C for 2 days. After cooling to
room temperature the powder was centrifuged and washed with fresh
DMF three times. After that, the sample was immersed in DMF and
CHCl3 for 2 days respectively. Then dried under vacuum at 120 °C for 2-
12 h, and collected with the yield of 350 mg. The large scale synthesis of
both MOFs was done by increasing the amount of all components 10
times yielding 3.5 g from one batch. The resulting MOF exhibited
identical PXRD and sorption of N2 as MOF produced by the standard
procedure.
3. Post-Synthetic Modification
Synthesis of MTz-68@QV, MTz-68@QVS and MTz-68@CuNONO
To 0.01 mmol of the as-synthesized ZrTz-68 (0.0275 g, 0.01 mmol) or
HfTz-68 (0.0312 g, 0.01 mmol) was added QV (0.05 mmol), QVS (0.05
mmol), or CuNONO (0.01 mmol) followed by 5 mL of diethyl ether in a 25
mL glass vial. The mixture was stirred at 70 °C for 12 h. After cooling to
room temperature the resulting powder was centrifuged and washed
there times with fresh diethyl ether and dried under vacuum at 120 °C to
give samples denoted: ZrTz-68@QV, ZrTz-68@QVS, HfTz-68@QV,
HfTz-68@QVS, ZrTz-68@CuNONO, and HfTz-68@CuNONO.
Experimental Section
1. Materials and Methods
Chemicals of reagent grade or better were used as received. The
conventional (glass vial) or heat and stirring (round bottom flask; RB)
reactions were carried out at 120 °C for 2 day. Elemental analyses were
carried out to confirm the composition of the organic part of the prepared
samples. FTIR spectra were recorded in the range of 400-4000 cm-1 on a
JASCO FTIR-460 spectrophotometer using KBr pellets. Thermal
gravimetric analyses (TGA) using a DuPont TA Q50 analyzer were
performed on powder samples under flow of N2 with a heating rate of
10 °C min-1. The gas sorption isotherms were measured at 77 K for N2,
and 273 and 298 K for CO2 using 3flex system of Micromeritics. Ultrahigh
purity grade N2 and He gas were used as received. Before the gas
sorption measurements, the sample was initially dehydrated at 453 K for
Acknowledgments
Financial support from Ministry of Science and Technology,
Taiwan and Chung Yuan Christian University are gratefully
acknowledged.
Keywords: Tetrazine • metal-organic framework • functional
groups post-synthetic modification • click reaction
12
h under vacuum. Field-emission scanning electron microscopy
images (SEM) were collected using a JEOL JSM-7600F. Dried powder
samples were deposited on carbon tape and coated with platinum prior to
the measurement.
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10.1002/ejic.201901230
European Journal of Inorganic Chemistry
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Post-Synthetic Modified MOF*
Dr. Madhan Vinu, Dr. Kulandaivel
Sivasankar, Dr. Samikannu Prabu, Prof.
Jeng-Liang Han, Prof. Chia-Her Lin,
Prof. Chun-Chuen Yang and Dr. Jan
Demel.
Page No. – Page No.
Tetrazine-Based Metal-Organic
Frameworks as Scaffolds for Post-
Synthetic Modification by Click
Reaction
* The л-conjugated tetrazine containing metal-organic frameworks, ZrTz-68 and HfTz-68 with pore size of 2.1 nm, specific
surface area up to 4217 m2 g-1 MOFs can be easily post-synthetically modified by a “click” reaction maintaining the crystallinity
of the framework and stability.
This article is protected by copyright. All rights reserved.