Passing the framework skeleton and properties of coordination materials onto organic framework materials

Article abstract of DOI:10.1039/d0cc07091b

A practically applicable strategy for transforming fragile metal-organic frameworks (MOFs) into highly stable and ordered organic framework materials (OFMs) is developed by replacing the labile coordination bonds in MOFs with stable covalent bonds in OFMs, which exhibit hypothetically approximated topology, porosity and properties of the parent MOFs by merging the advantages of MOFs and porous organic materials, thus providing a general pathway for the synthesis of highly ordered OFMs with merged advantages of MOFs and organic polymers.

Full text of DOI:10.1039/d0cc07091b

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
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DOI: 10.1039/D0CC07091B
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Passing the framework skeleton and properties of coordination
material on to organic framework material
Xuan Wang, Ming-Jie Dong, Kai Chen, Zi-Kun Liu and Chuan-De Wu*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
A practically applicable strategy for transforming fragile metal- moieties in MOFs with covalent bond-connected organic
organic frameworks (MOFs) into highly stable and ordered organic synthons that have the same symmetry, which could full take
framework materials (OFMs) is deveoped by replacing the labile the advances of reversible coordination bonds and irreversible
coordination bonds in MOFs with stable covalent bonds in OFMs, organic covalent bonds.
which exhibit hypothetically approximated topology, porosity and
properties of the parent MOFs by merging the advantages of MOFs
and porous organic materials, thus providing a general pathway for
the synthesis of highly ordered OFMs with merged advantages of
MOFs and organic polymers.
Metal-organic frameworks (MOFs) are a class of long-range
ordered porous crystalline materials, constructed from
coordination bond connections between metal nodes and
1
multidentate organic ligands. The systematically tunable
nature of MOFs offers unique advantages over traditional
porous materials in that numerous functional moieties are
readily incorporated into the frameworks for potential
applications by either ligand design or postsynthetic
modification.2 However, this promise has not been fulfilled in
practical applications, mainly because the coordination bonds
in most MOFs are chemically susceptible, even under ambient
environment.4
,3
Porous organic polymers (POPs), connected by robust
covalent bonds, are a new type of porous materials for
Scheme 1 (A) Model reaction between copper benzoate and TBMB. (B) Schematic
illustration of the transformation from fragile MOF material CZJ-6 into stable organic
applications in different fields, which offer superior stability,
5
,6
even under harsh environments. However, the irreversible framework material OFM-1.
nature of most covalent organic bonds generally resulted in
highly disordered connections of the building synthons in POPs,
except for the dynamic covalent bond-connected covalent
organic frameworks (COFs).
Inspired by the reaction between benzyl bromide and
carboxylic acids which has been widely used to protect the
carboxyl groups by forming benzyl esters in organic synthesis,
we conducted the reaction between coordination compound
copper benzoate and 1,2,4,5-tetrakis(bromomethyl)benzene
5
a-c
It has been demonstrated that MOFs could be transformed
into POPs under stuitible conditions; however, the framework
skeletons and properties of MOFs could not be well inherited by
the daughter POPs.6 We report herin an effective strategy for
the synthesis of highly ordered organic framework materials
(
TBMB), which also resulted the corresponding ester under the
identical base-catalyzed nucleophilic reaction conditions
Scheme 1). Since copper benzoate coordination moiety is one
d,6e
(
(OFMs) by replacing the dynamically formed coordination
of the very important SBUs in MOFs, the fragile copper
benzoate coordination moieties in MOFs should be also easily
transformed into stable ester covalent bonds under similar
reaction conditions, which would generate stable OFMs with
inherited framework structures and properties from the parent
a. State Key Laboratory of Silicon Materials, Department of Chemistry, Zhejiang
University, Hangzhou, 310027, P. R. China. E-mail: cdwu@zju.edu.cn
Electronic Supplementary Information (ESI) available: Experimental details relating
to synthesis, characterization and catalysis; additional figures and scheme. See
DOI: 10.1039/x0xx00000x
7
MOFs. As a proof of this concept, we reacted the MOF CZJ-6,
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CZJ-6(t) (t represents the reaction time), were rVeiecwoAvrteicrleeOdnlbinye
2 4 2 2
building from four-connected paddle-wheel [Cu (COO) (H O) ]
(abbreviated as {Cu
2
}) secondary building units (SBUs) and eight centrifugation. UV-Vis absorption spectDrOaI:o1f0.1t0h3e9/Dsu0pCeCr0n70a9te1Bs
branched metalloporphyrin ligands Cu-L (H10L = tetrakis-3,5- show that the maximum absorption peak of TBMB decreases
bis((4-hydroxycarbonyl)-phenyl)phenylporphine), with tetra- gradually when prolonging the reaction time, indicating that
branched TBMB by equivalent symmetry substitution of four- there might occur gradual substitution reaction between {Cu
2
}
connected coordination moieties with tetra-branched organic SBUs and TBMB (Fig. S2†).
synthons under solvothermal reaction conditions, and
successfully targeted a highly stable organic material, named as bond stretching vibration band appeared at 1701 cm (Fig. 1C).
OFM-1. The peak almost disappeared in the FT-IR spectrum of CZJ-6,
FT-IR spectrum of the porphyrin ligand shows a strong C=O
-1
-1
Scanning electron microscopy (SEM) and transmission and there emerged two new peaks at 1538 and 1401 cm ,
electron microscopy (TEM) images show that the regular ascribed to the asymmetric and symmetric stretching vibrations
8
octahedral morphology of CZJ-6 was well reserved by OFM-1, of coordinated carboxylate groups, respectively. The vibration
indicating that the substituent reaction could retain the peaks of CZJ-6(3h) are similar to those of CZJ-6, indicating that
apparent morphology of the parent MOF (Fig. 1 and S1†). This most of the Cu-carboxylate coordination bonds remained
result should be ascribed to the large pore space of CZJ-6 with intact. When the reaction time was extended to 6 h, the
pore cages of 1.4 and 2.3 nm in dimensions, which could endow asymmetric stretching vibration at 1538 cm- for the
1
the substituent reaction occur inside the pores.
coordinated carboxylate groups almost disappeared, and the
symmetric stretching vibration at 1401 cm- was red-shift,
indicating that most of the Cu-carboxylate coordination bonds
were destroyed. Additionally, the C=O stretching vibration
1
-1
bands at 1701 and 1720 cm are highly enhanced, and there
-1
appeared two new peaks at 1268 and 1102 cm , suggesting the
formation of ester moieties. When further extending the
reaction time, these absorption bands were enhanced, which
are almost identical to those of the porphyrin molecular analog
5
,10,15,20-tetrakis-3,5-bis((4-
benzyloxycarbonyl)phenyl)phenylporphine (TBPP). These
results indicate that most of the Cu-carboxylate moieties were
replaced by the ester moieties to result an organic framework
material OFM-1.
Powder X-ray diffraction (PXRD) pattern of OFM-1 shows
that there is no sharp diffraction peak, indicating that the
building synthons are distortedly connected in the structure
(
Fig. S3†). In the Raman spectrum, OFM-1 exhibits two broad
-1
peaks at 1345 and 1580 cm , which are ascribed to the out-of-
phase coupled stretch and asymmetric stretch vibrations of
porphyrin rings, respectively (Fig. S4†). UV-Vis diffuse
9
reflectance spectrum of OFM-1 shows that the characteristic
Soret band of porphyrin locats at 434 nm with two Q bands
centered at 557 and 590 nm, which are similar to those of CZJ-
6
(Fig. S5†).
The solid intermediate samples of CZJ-6(t) could be
partially decomposed by hydrochloric acid aqueous solution
pH = 1) to release the unreacted and low-polymerized
(
moieties, which were thoroughly extracted by THF solvent. The
obtained supernates, denoted as CZJ-6(t)-MS, were analyzed by
matrix-assisted laser desorption/ionization time-of-flight mass
spectrometry (MALDI-TOF MS). As shown in Fig. 2, a strong peak
8
at m/z 1637.3 is very close to the molecular mass of Cu-H L
(exact mass = 1635.3) for CZJ-6-MS, and a new peak at m/z
1
629.3 is ascribed to Cu-L for CZJ-6(3h)-MS. There also appears
Fig. 1 (A) SEM images of CZJ-6. (B) SEM images of OFM-1. (C) FT-IR spectra of porphyrin a weak peak at m/z 3442.8, which is different to the peak at m/z
ligand (H10L), CZJ-6, CZJ-6(t) and TBPP.
3
337.8 in CZJ-6-MS, indicating that there occurred the reaction
2
between {Cu } SBU and TBMB. When the reaction time was
When the reaction was proceeded for a certain time, the prolonged to 6 h, a series of weak peaks appeared at m/z 5335,
reaction was intentionally interrupted by cooling to room 7126 and 8837 for CZJ-6(6h)-MS, indicating that the Cu-O
temperature, and the solid reaction intermediates, denoted as coordination moieties were gradually reacted with TBMB.
2
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Further prolonging the reaction time, the released CuII- is markedly decreased in the XPS survey spectrum, due to
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DOI: 10.1039/D0CC07091B
2
porphyrin is undetectable, suggesting that the transformation removal of the {Cu } SBUs (Fig. S9†). In the high resolution Cu
reaction should be completed, which is in agreement with the 2p XPS spectrum of CZJ-6, each Cu 2p peak can be divided into
UV-Vis and FT-IR spectroscopy analysis results.
934.7 and 935.2 eV for Cu 2p3/2, and 954.6 and 955.1 eV for Cu
II
2
p1/2 with a broad satellite band, ascribed to the porphyrin Cu
and carboxylate-coordinated {Cu
2
} moieties, respectively (Fig.
1
1
3
9
A). In contrast, there are only two XPS signals at 934.7 and
54.6 eV ascribed to the porphyrin Cu in OFM-1, indicating that
II
2
the {Cu } SBUs had been successfully substituted and removed.
In the C 1s XPS spectrum of CZJ-6, the peak at 284.8 eV is
ascribed to the porphyrin and benzoic rings, and the peaks at
2
88.7 and 292.5 eV are attributed to the uncoordinated
carboxyl groups on the surface of CZJ-6 and coordinated
1
2
2
carboxyl groups in {Cu } SBUs, respectively (Fig. 3B).
Compared with CZJ-6, the peak at 292.5 eV disappeared in the
C 1s spectrum of OFM-1, and there appeared a new peak at
2
1
86.5 eV, ascribed to the newly formed C-O moieties. In the N
s XPS spectra, the peaks at 398.5 and 400.4 eV are ascribed to
4
the Cu-N and pyrrolic nitrogen species, respectively, indicating
II
11
that the Cu -porphyrin moieties remain intact (Fig. 3C). In the
high-resolution O 1s XPS spectrum of CZJ-6, the peak at 532.0
eV is corresponding to the Cu-O moieties, whereas the peak at
1
3
5
33.9 eV is ascribed to the coordinated water (Fig. 3D).
Different to that of CZJ-6, the O 1s XPS signal of OFM-1 can be
divided into two peaks at 531.7 and 533.1 eV, assigned to the
1
2
C=O and C-O-C moieties in ester groups, respectively. These
characterization results indicate that MOF CZJ-6 was
successfully transformed into stable OFM-1. In contrast, direct
reaction between H10L and TBMB resulted a highly distorted
organic polymer under the identical reaction conditions, in
which only part carboxylate groups were participated in the
polymerization process (Fig. S10-S13†).
Fig. 2 MALDI-TOF MS spectra of CZJ-6(t)-MS.
Owing to its flexible structure with microporous blocking
effect at very low temperature, OFM-1 could not take up N
7
2
at
7 K. The permanent porosity of OFM-1 was examined by CO
gas adsorption measurement at 195 K (Fig. S6†). The activated
1
0
2
3
-1
sample of OFM-1 takes up 109.7 cm g CO
2
at 195 K and 780
mmHg, resulting a Brunauer–Emmett–Teller (BET) surface area
2
-1
of 316.9 m g and a dominated pore size of about 1.8 nm,
calculated from the CO adsorption profile of OFM-1. The above
2
2
-1
results are very close to the BET surface of 336.0 m g and
dominated pore size of about 1.8 nm for CZJ-6, respectively,
indicating that OFM-1 successfully inherited the porosity of CZJ-
6
. TG-MS curves show that the maximum degradation rate of
o
CZJ-6 was found at 324 C with an increased intensity of MS
2
signal at m/z = 44 for CO , the decarboxylated product of
metalloporphyrin ligands (Fig. S8†). Compared with CZJ-6, the
o
maximum degradation rate of OFM-1 increased to 435 C,
indicating that transformation of CZJ-6 into OFM-1 would
improve the thermal stability. The copper content was
decreased from 14.6 wt% in CZJ-6 to 3.7 wt% in OFM-1, which
is very close to the calculated value (3.5 wt%) for the Cu -
2
porphyrin moieties in OFM-1, indicating that the {Cu }
Fig. 3 XPS spectra of CZJ-6 and OFM-1 referenced to (A) Cu 2p, (B) C 1s, (C) N 1s
and (D) O 1s.
II
coordination SBUs in CZJ-6 were almost fully substituted by
covalently bonded organic moieties in OFM-1.
X-ray photoelectron spectroscopy (XPS) was also used to
monitor the transformation of CZJ-6 into OFM-1 (Fig. 3).
Compared with that of CZJ-6, the Cu 2p XPS intensity of OFM-1
Transformation of MOFs into OFMs should be able to tune
the hydrophobic/hydrophilic properties, which was confirmed
by water contact angle measurements. Compared with the
pristine MOF CZJ-6 with a hydrophilic water contact angle of
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o
o
1
5 , OFM-1 generates a water contact angle of 136 , indicating develop highly stable and ordered organic materials with
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its hydrophobic nature (Fig. S14†). Solvent test results revealed inherited properties from the parentDOMI: O10F.103m9/aDt0eCriCa0ls709fo1Br
that OFM-1 is very stable in water and common organic practical applications.
solvents, even in 6 M hydrochloric acid (Fig. S15-S17†).
The catalytic properties of CZJ-6 were also successfully
inherited by OFM-1. OFM-1 could smoothly prompt aerobic
Conflicts of interest
oxidation of ethylbenzene with 51% acetophenone yield, which
is higher than the literature result of 45% yield under the
identical catalytic conditions. Since OFM-1 is hydrophobic and
There are no conflicts to declare.
7
highly stable, we evaluated its catalytic properties in those Acknowledgements
reactions that cannot be catalyzed by fragile MOFs under
We are grateful for the financial support of the National Natural
Science Foundation of China (grant nos. 21525312 and
1872122).
extreme conditions, such as using water as solvent in the
presence of strong Lewis acid. This consideration prompted us
synthesize a composite catalyst, Pd@OFM-1, by simple post-
treatment of OFM-1 with palladium acetate and followed by
hydrogen reduction (Fig. S18-S22†). Pd@CZJ-6 was also
prepared as a reference catalyst.
Cyclohexanol and cyclohexanone, known as K-A oil, are
readily oxidized into adipic acid, an important intermediate in
the manufacture of condensation polymers, such as Nylon-6
2
Notes and references
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o
of AlCl
3
co-catalyst at 90 C in water, which generated a high
yield of 97.5% for the K-A oil products (Fig. S23†). In contrast,
when Pd@CZJ-6 was used instead of Pd@OFM-1 to catalyze the
reaction, the yield of K-A oil is of 81.2%, and Pd@CZJ-6 was
completely decomposed during catalysis, indicating that
4
6, 3242; (e) M. Zhao, S. Ou and C.-D. Wu, Acc. Chem. Res.,
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observed in the reaction mixture for Pd@OFM-1 after catalysis,
as confirmed by UV-Vis absorption spectroscopy and ICP-OES
analysis (Fig. S24†).
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Pd@OFM-1 can be simply recovered by centrifugation, and
reused in the successive run with almost retained high catalytic
properties. ICP-OES analysis results revealed that the Pd
content in the recovered Pd@OFM-1 is identical to that in the
pristine one. HRTEM image and PXRD pattern indicate no
obvious aggregation of the Pd nanoparticles in the recovered
catalyst (Fig. S18 and S25†). FT-IR spectrum shows that the ester
groups in Pd@OFM-1 remain intact; SEM image shows that the
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7
8
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4
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