Crystallinity after decarboxylation of a metal-carboxylate framework: indestructible porosity for catalysis

Article abstract of DOI:10.1039/d0dt02075c

We report a curious case study of a Zr(iv)-carboxylate framework, which retains significant crystalline order after cascade thermocyclization of its linker components, and - more notably - after the crucial carboxylate links were severed by heat. Vigorous heat treatment (e.g., 450 °C and above) benzannulates the multiple alkyne groups on the linker to generate linked nanographene blocks and to afford real stability. The resultant Zr oxide/nanographene hybrid solid is stable in saturated NaOH and concentrated H₃PO₄, allowing a convenient anchoring of H₃PO₄into its porous matrix to enable size-selective heterogeneous acid catalysis. The Zr oxide components can also be removed by strong hydrofluoric acid to further enhance the surface area (up to 650 m²g^?1), without collapsing the nanographene scaffold. The crystallinity order and the extensive thermal transformations were characterized by X-ray diffraction, scanning transmission electron microscopy (STEM), IR, solid state NMR and other instrumental methods.

Full text of DOI:10.1039/d0dt02075c

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Crystallinity after decarboxylation of a metal–
carboxylate framework: indestructible porosity
for catalysis†
Cite this: Dalton Trans., 2020, 49,
11902
a
b
c
d,e
a
Shengxian Cheng, Peter Tieu, Wenpei Gao, Jieying Hu, Weijin Feng,
d
c,f,g
a
Jun He, * Xiaoqing Pan*
and Zhengtao Xu
*
We report a curious case study of a Zr(IV)-carboxylate framework, which retains significant crystalline
order after cascade thermocyclization of its linker components, and – more notably – after the crucial
carboxylate links were severed by heat. Vigorous heat treatment (e.g., 450 °C and above) benzannulates
the multiple alkyne groups on the linker to generate linked nanographene blocks and to afford real stabi-
lity. The resultant Zr oxide/nanographene hybrid solid is stable in saturated NaOH and concentrated
3 4 3 4
H PO , allowing a convenient anchoring of H PO into its porous matrix to enable size-selective hetero-
Received 11th June 2020,
Accepted 24th July 2020
geneous acid catalysis. The Zr oxide components can also be removed by strong hydrofluoric acid to
further enhance the surface area (up to 650 m g−1), without collapsing the nanographene scaffold. The
crystallinity order and the extensive thermal transformations were characterized by X-ray diffraction, scan-
ning transmission electron microscopy (STEM), IR, solid state NMR and other instrumental methods.
2
DOI: 10.1039/d0dt02075c
rsc.li/dalton
polymerization and the formation of amorphous precipitates.
As PAH molecules often stack tight together to frustrate the
Introduction
9
–12
Post-synthetic annulation of organic linker molecules is a assembly of open structures, such a two-step approach
also
promising yet under-explored area in the study of metal– serves to integrate PAHs and their highly polarizable
organic frameworks (MOFs) as a topical class of crystalline π-electrons into the MOF matrix, in order to afford better
1
–7
porous materials.
by the groups of Hupp, Farha and Stoddart, annulation of the crystal-to-single-crystal transformation,
spatially separated linker molecules in the rigid MOF scaffold applied Scholl oxidation/benzannulation to the elegant hexa-
As pioneered in a cross-cutting synthesis photochemical and electronic properties. In a recent single-
8
13
the Zhou group
1
4
allows their conversion into unique and discrete polycyclic aro- phenylbenzene-based pbz-MOF-1 system and successfully
matic hydrocarbons (PAHs) that cannot be accessed from the achieved the large, fused PAH motif of hexabenzocoronene for
2
free-flowing solution medium, for the latter leads to stochastic enhanced photocatalytic reduction of CO .
Besides the intramolecular annulations, this group explored
more complex cyclization processes that also covalently bridge
the linker molecules in the MOF scaffold, in order to boost
a
15
Department of Chemistry, City University of Hong Kong, 83 Tat Chee Avenue,
Kowloon, Hong Kong, China. E-mail: zhengtao@cityu.edu.hk
stability and charge transport, and to ultimately reach the field
b
Department of Chemistry, University of California-Irvine, Irvine, CA, 92697, USA
16–18
of porous carbon crystals (i.e., 3D graphene).
The chal-
c
Department of Materials Science and Engineering, University of California-Irvine,
lenges, however, are obvious. Unlike the intramolecular reac-
tions that preserve the well-defined connectivity of the coordi-
nation host, cyclizations involving intermolecular bond for-
mation are highly dynamic, and often disruptive to the host
lattice; and the amorphous products, besides being hard to
characterize, would be deprived of the regular pore features
vital for selectivity and reactivity control.
Irvine, CA, 92697, USA
d
School of Chemical Engineering and Light Industry, Guangdong University of
Technology, Guangzhou, 510006, China
e
Department of Materials Science and Engineering, City University of Hong Kong,
KowloonHong Kong, China
f
Department of Physics and Astronomy, University of California-Irvine, Irvine, CA,
USA
g
Irvine Materials Research Institute, University of California-Irvine, Irvine, CA, USA
†
Electronic supplementary information (ESI) available: General procedure, X-ray
That is why our earlier tests on the alkyne-equipped linkers
single-crystal structure of ZrL3, PXRD pattern for ZrL1, N (77 K) sorption iso-
2
15,19
of H
2
L1 and H
4
L2 (Fig. 1a) had been instructive.
In the
13
therms, CO
2
(195 K) sorption isotherms, solid-state C-NMR spectra, TGA
Zr(IV)-based MOF solids formed therefrom, the contiguous
alkyne units readily underwent thermocyclization (e.g., above
200 °C). While the tetratopic and more rigid L2-based ZrL2
1
spectra, EDX spectra, photographs of TLC plates, H-NMR data of products from
acetalization reaction and Pawley refinement results. See DOI: 10.1039/
D0DT02075C
11902 | Dalton Trans., 2020, 49, 11902–11910
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Fig. 1 (a) Three carboxylic linker molecules featuring backfolded side arms and conjugated alkyne groups; (b) scheme for the thermally induced
linker benzannulation and decarboxylation of ZrL3 crystal (simplified as an octahedron-like unit). The resultant network contains linked nanogra-
phene blocks.
lost crystallinity upon thermocyclization, the linear and more Zr O (OH) (H O) (L3) (DEF) , merits
a
recap: its iconic
6
4
4
2
6
5
2
2
4
supple L1-based ZrL1 afforded an ordered polycrystalline black Zr
6
O
4
(OH)
4
cluster is linked to 8 L3 carboxyl groups in the
covalent solid at up to 320 °C. Further covalent modification regular, bidentate/straddling mode, and two in a monodentate
on ZrL1 (e.g., cross-linking and ring fusion) at higher tempera- mode, with other open sites taken by water or DEF molecules
tures, however, resulted in an amorphous solid. It therefore (Fig. S1†). Compared with the weakly diffracting and unsolva-
remains desirable to develop MOF systems that can maintain ble sulfur analog ZrL1, the crystals of ZrL3 allow single crystal
crystallinity at higher temperatures, in order to further fuse X-ray diffraction studies to locate the linker molecules of L3.
the rings to emulate the fully conjugated 3D graphene—and to The structural order of ZrL3 is perhaps enhanced by its more
bridge the two fields of coordination and covalent solids.
3 3
polar CH O– groups (vs. the thioether CH S– of ZrL1) that
Reported here is an advancement in this area: i.e., the MOF impose stronger intermolecular forces to order the molecules
crystal ZrL3 was found to remain ordered after undergoing in the MOF matrix. In the dynamic, thermally induced cycliz-
extensive linker cyclization and significant fracturing at ing and splintering process, the chemically harder ether O
4
00–600 °C (Fig. 1b). Surprisingly, the crystallinity of the host atoms in ZrL3, swinging close to the Zr(IV) ion, offer a stronger
net persists even after the carboxylate strut of L3 was thermally binding with the latter, to help lock in the L3 components and
ruptured, highlighting robust covalent bridges successfully the host lattice; as a result, ZrL3 exhibits a higher temperature
established among the linker molecules. The resulting 3D gra- (600 °C) at which crystallinity persists, relative to the tempera-
phene analog was stable in broad pH conditions from satu- ture (320 °C) of ZrL1 (Fig. S2†).
rated NaOH to strong H
heterogeneous catalysis
systems.
3
PO
4
solutions, allowing size-selective,
to proceed in its uniform pore tion of temperature is revealing (Fig. 2). The as-made sample
of ZrL3 displays a tetragonal cell (a = 24.68 Å, c = 32.37 Å;
The evolution of powder X-ray diffraction patterns as a func-
2
0–22
Fig. 2b; see Pawley refinement results in the ESI†) as is consist-
ent with the X-ray single crystal structure. As for ZrL3-ac, i.e.,
ZrL3 activated by exchanging with CH
at 100 °C (formula: Zr (OH) ·(L3) (H
ened peaks that preclude unambiguous indexing, and the
3
CN and then evacuating
Results and discussion
Crystallinity retention after being heated at 450 °C or above
6
O
4
6
5
2
O) ), it exhibits broad-
2
The synthesis and characterization of molecule H
L3 and the peaks are right-shifted to indicate unit cell contraction
2
ZrL3 crystal (P4/mnc; a = 24.758 and c = 31.932 Å) were recently (Fig. 2c). The peak broadening (e.g., due to partial framework
2
3
reported.
The X-ray structure of the ZrL3 framework, collapse) and cell contraction can be ascribed to the dynamic
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Fig. 2 X-ray powder patterns (Cu Kα, λ = 1.5418 Å): (a) calculated from
the single-crystal structure of ZrL3; (b) as-made ZrL3; (c) activated ZrL3
(
ZrL3-ac); (d) ZrL3-ac heated at 250 °C for 2 h (ZrL3-250); (e) ZrL3-ac
heated at 450 °C for 2 h (ZrL3-450); (f) ZrL3-ac heated at 500 °C for 2 h
ZrL3-500) (g) ZrL3-ac heated at 600 °C for 2 h (ZrL3-600); (h) ZrL3-450
soaked in a saturated water solution of NaOH overnight; (i) ZrL3-450
soaked in 85 wt% water solution of H PO overnight; ( j) ZrL3-450-P
ZrL3-450 soaked in the mixture of acetonitrile and 85 wt% H PO
4
(
Fig. 3 HAADF-STEM images of (a) ZrL3-450 and (b) ZrL3-600 to show
the lattice fringes of two samples with d-spacing; larger amplified
images of (c) ZrL3-450 and (d) ZrL3-600 to exhibit individual clusters
3
4
(
3
(around 0.8 nm; partially circled in red circles) and merged clusters at
aqueous solution with heating at 60 °C) after being used for 8 catalytic
rounds.
sizes of around 1.6 nm (partially circled in blue circles) and 2.0 nm (par-
tially circled in green circles).
bonding around the Zr–O cluster, as also documented
2
5–27
elsewhere.
Heating the activated sample ZrL3-ac at 250 °C 450 and ZrL3-600 did not exhibit additional evolution, such as
for 2 hours affords the black powder of ZrL3-250, with IR and further growth of the Zr–O clusters, during imaging.
NMR (Fig. 4b and S3†) indicating full alkyne cyclization (but Spectroscopic analysis inside the microscope does induce
with retention of the carboxyl links).
aggregation of the Zr–O clusters, but the scanning time for
ZrL3-250 gives a well-defined PXRD pattern (Fig. 2d) index- spectroscopy is hundreds of times longer than that of
able onto a tetragonal cell (a = 21.74 Å, c = 31.92 Å; see the imaging.
ESI† for the Pawley refinement). Heating ZrL3-ac at a higher
As shown in Fig. 3, the ZrL3-450 solids display ordered
temperature of 450 °C for 2 hours (to form the ZrL3-450 solid) lattice fringes (arising from the ordered distribution of the
induces decarboxylation and further linker fusion (IR and heavy Zr–O clusters). The interplanar spacing shown is about
NMR, Fig. 4c and S3†), but retains significant crystalline order: 1.3 nm, which is comparable with that of the (0 0 2) plane
the remaining PXRD peaks (Fig. 2e) suggest a tetragonal (a = (1.4 nm) calculated from the powder pattern e (Fig. 2). In
2
8.92 Å, c = 29.07 Å) or cubic symmetry (a = 28.99 Å, see the accordance with the lower crystallinity observed by PXRD, the
ESI† for the refinements) with the ambiguity due to limited sample of ZrL3-600 displays fainter lattice fringes with a
diffraction data. Some crystalline order was maintained even d-spacing of ca. 1.68 nm (Fig. 3b), which is close to the value
after ZrL3-ac was heated at 500 °C for two hours, with two dis- determined from the main peak that featured in the PXRD
tinct peaks suggestive of a cubic lattice in ZrL3-500 (a = (Fig. 2g). More notably, the STEM micrographs are dominated
2
8.26 Å; Fig. 2f). After treatment at 600 °C for 2 hours, the by the aggregated Zr–O clusters. As shown in Fig. 3c, cluster
number of peaks further decreased, i.e., with the only lowest- aggregation is found to be already extensive in the sample of
angle peak featuring in the powder pattern of the ZrL3-600 ZrL3-450: i.e., only a few individual, original Zr-clusters
sample (Fig. 2g). Taken together, the gradual and continuous (around 0.8 nm) can be identified, while the majority feature
changes in the PXRD patterns, indicating no abrupt structural larger sizes, around 1.6–2 nm, which result from several clus-
collapse over the broad temperature range, serve to highlight ters being merged together. Similarly, ZrL3-600 exhibits clus-
the resilience of the crystalline lattice originally imposed by ters ranging from 0.8 nm to 2.0 nm, with the small, intact
the ZrL3 scaffold.
ones (0.8 nm) being even harder to spot.
The ZrL3 samples treated at 450 °C and 600 °C were also
examined by high angle annular dark field scanning trans-
Characterization of linker cyclization and decarboxylation
mission electron microscopy (HAADF-STEM). MOF systems are The extensive aggregation of the Zr–O clusters points to their
generally sensitive to the electron beam. By comparison, ZrL3- large displacement and migration during the thermal treat-
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1
3
ment, and such mobility may likely arise from the cleavage of the C for carboxyl is distinctly featured at around 160 ppm.
the Zr-carboxylate links that had originally served to anchor In addition, the peaks of alkyne, generally occurring between
the Zr–O clusters. As decarboxylation is a commonly occurring 70 and 90 ppm, disappear for both ZrL3-450 and ZrL3-600, as
organic reaction, we wonder if this indeed happened in the a result of thermal benzannulation. Instead, the ZrL3-450 solid
samples of ZrL3-450 and ZrL3-600.
displays two dominant signals at the aromatic area (the princi-
The decarboxylation and cyclization of the MOF scaffolds pal one at 128 ppm with a shoulder at 137 ppm), which can be
e.g., at 450 °C or higher) are evidenced in the various charac- largely rationalized by the proposed structure of Fig. S3.†
(
terization studies. First, the IR spectra point to drastic changes Specifically, the latter signal (around 137 ppm) can be ascribed
in functionalities. Besides, the disappearance of the alkyne to the 14 edge quaternary carbons and 6 oxygen-bonded
−
1
stretching at 2204 cm (cf. spectra a, b and c of Fig. 4) is con- carbons, since these have been found to shift to a relatively
3
4
sistent with the cascade cyclization of ZrL1 that was previously low field in polycyclic aromatics; the main peak at 128 ppm
reported. Also, the sharp peaks at 1600 cm−1 and 1398 cm
1
5
−1
can be assigned to the remaining aromatic C atoms. By com-
in spectra a and b, arising from the asymmetric and symmetric parison, the shouldering feature (at 137 ppm) is largely sup-
2
8,29
stretching vibrations of carboxylate groups,
are absent in pressed in ZrL3-600, leaving a broad and more symmetrical
spectra c and d. Instead, a new peak appears at around peak centered at 128 ppm, which is consistent with more
1
1
580 cm− , indicating the existence of aromatic CvC groups extensive carbonization throughout the solid state under
3
0,31
associated with graphitic skeleton.
In addition, the C–H higher temperatures, to render a structure more akin to gra-
stretching from the CH
3
O– groups (at 2836 cm− ) is greatly phene (see Fig. S3c;† as is also revealed in the evolution of the
1
diminished in ZrL3-450 and disappear in ZrL3-600, indicating IR features observed above). Expectedly, the extension of the
the cleavage of the CH -O bond. With the departure of the π-conjugated system significantly increases electrical conduc-
3
methyl groups, the remaining oxygen atoms can annulate with tivity from 10− S m for ZrL3-450 to 10 S m for ZrL3-600
5
−1
−1
−1
the benzenoid carbons to form heterocycles, which might be (preliminarily measured by a two-probe method), both of
−
1
responsible for the peak at 1259 cm (C–O–C asymmetric which also contrast the poor electrical conductivity (i.e., below
3
1
−12
−1
stretching) in spectrum c; or the O atoms can bond to the 10
S m ) of the as-made and activated samples of ZrL3.
Thermal analysis (TG-DSC) indicates that the cyclization of
Zr(IV) centers as an aryloxide group (Fig. S3b†). As for ZrL3-600,
−
1
the peak around 1259 cm disappears, which probably corre- linkers was triggered at around 250 °C with a significant
lates with further loss of the oxygen atoms from the organic release of heat without any obvious loss of weight (Fig. S7†).
grid. The higher-temperature treatment (e.g., at 600 °C) thus Around 0.7% of weight was lost at 100 °C due to the removal
serves to further carbonize and aromatize the organic matrix, of 2 water molecules (coordinated guests). The onset decompo-
with its IR features having a closer resemblance to those of gra- sition occurred at about 385 °C followed by a rapid weight loss
3
2,33
phene or nanoribbons (Fig. S5†).
The C-NMR results also indicate decarboxylation and ation and demethoxylation of the linkers. To trace this
extensive cascade cyclization of the thermally treated samples. decomposition process, stationary thermal treatments were
As seen in Fig. S3,† the carboxyl C peaks are absent in both then studied. The sample of ZrL3-ac was heated at fixed temp-
the ZrL3-450 and ZrL3-600 samples, whereas in the sample eratures, i.e., 450 °C and 600 °C, for 2 hours to afford black
ZrL3-250 and a reported carboxyl-containing sample (Fig. S6†), solid ZrL3-450 and ZrL3-600, respectively. ZrL3-450 displays a
weight loss of 17% (Fig. S8†), which is attributed to dehydra-
till 600 °C, which was most likely caused by the decarboxyl-
1
3
1
3
tion (2 H O from the guest molecules and 3 H O from the
2
2
cluster; totally 5 H
2
O accounting for 1.9 wt%), decarboxylation
(
2 CO
methylation (4 CH3 per link; totally 20 -CH3 equivalent to
.2 wt%). As for ZrL3-600, 7% additional weight loss occurred
relative to ZrL3-450, Fig. S9†), which may be caused by a
2 2
per link; totally 10 CO accounting for 9.2 wt%) and de-
6
(
partial shedding of the remaining O atoms (6.7 wt%, e.g., in
the form of CO); these O atoms are possibly fused with the aro-
matic grid or bonded to Zr(IV) in ZrL3-450 (Fig. S3b†).
Compared with the sulfur analog of ZrL1, such chemically
hard O donors might better bond with the Zr(IV) centers, so as
to help fix the Zr–O clusters in an ordered array at higher
temperatures. The departure of these O anchors, e.g., at
600 °C, unfortunately, led to a degradation of the structural
order (with only one peak in the PXRD pattern and very faint
lattice fringes in the STEM image, as mentioned above). It
therefore remains a challenge to achieve a crystalline, ordered
and carbonized organic grid from the MOF precursor. For
future studies, the design of the linker shape and functional-
Fig. 4 FT-IR spectra for (a) ZrL3-ac; (b) ZrL3-250 (heated at 250 °C); (c)
ZrL3-450; (d) ZrL3-600; (e) ZrL3-450-F (ZrL3-450 treated by HF solu-
3 4
tion); (f) ZrL3-450-P (ZrL3-450 treated by H PO solution).
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ity, as well as the choice of the MOF topology, should be porous carbon framework? Indeed, heating a sample of ZrL3-
closely coordinated, so that the thermocyclization of the linker 450 in HF (48 wt%) at 80 °C for two days results in a solid
pieces smoothly transit into a carbon crystal grid that carries product (ZrL3-450-F) depleted of Zr, as indicated by EDX ana-
over the crystalline order laid out by the MOF precursor.
lysis (Fig. S12†). Incidentally, the treatment by strong HF does
not give rise to ν(COOH) peaks in the FT-IR spectrum of ZrL3-
Surface area, chemical stability, and cluster shedding
−1
450-F (Fig. 4e; the feature around 1580 cm
remains
From a positive perspective, however, the degradation of crys- unchanged), further confirming the absence of carboxylate
tallinity does not reduce the surface area—i.e., the thermally functions in the ZrL3-450 sample. The Zr-depleted structure
treated samples remain highly porous. In particular, the CO2 ZrL3-450-F is non-crystalline as revealed by PXRD (Fig. S13†)2,
sorption isotherms at 195 K (Fig. 5 and Fig. S10†) indicate ever but the specific surface area is greatly increased from 434 m
−
1
2
−1
greater Langmuir surface areas from the as-activated (room
temperature) sample to the 450 and 600 °C-treated samples: amorphous yet highly porous aromatized framework. In
g
(ZrL3-450) to 648 m g (ZrL3-450-F, Fig. 5), indicating an
2
−1
2
−1
i.e., 385 m
g
for ZrL3-ac; 434 m
g
for ZrL3-450; and general, 3D graphene-like carbon scaffolds attract great atten-
64 m g for ZrL3-600. The thermocyclized solids therefore tion from theorists and synthetic chemists,
2
−1
17,36–38
4
with a
maintain the structural openness of the MOF precursor, and popular preparative strategy being based on carbonization
their increased surface areas are also consistent with the shed- and/or chemical vapor deposition onto zeolites as
3
9–41
ding of significant mass from the heated host grid. The N2 templates.
uptake, by comparison, is not significant for all three samples molecules, using ZrL3 and other cyclizable MOF solids as crys-
Fig. S11†), indicating the ultramicroporous (e.g., pore size: talline precursors offers more gradual and controllable
7.0 Å) nature of these materials. In general, the preferred passage onto the prospective carbon scaffolds. The diverse
sorption of CO stems from its smaller kinetic diameter (330 MOF topologies, together with the modifiable linker systems,
Unlike such drastic Aufbau from individual
(
<
2
pm), relative to N
stronger thermal motion at 195 K (relative to 77 K for N2).
2
(364 pm), its quadruple moment and the not only allow various organic functions to be built into the
carbon grid, but also offer hope for eventually achieving the
The thermally treated samples exhibit extraordinary stabi- coveted porous carbon crystals.
lity. For example, the ZrL3-450 solid retains the distinct PXRD
profile (Fig. 2h) after being immersed in saturated NaOH solu-
Heterogeneously catalyzed acetalization reaction
tions overnight. This compares favorably with the reported More imminent uses are also open for the robust thermocy-
ZrL1-320 (ZrL1 heated at 320 °C for 3 hours under argon), clized solids. For example, ZrL3-450 readily anchors H
3 4
PO
whose crystallinity is largely destroyed even in a less concen- guests onto its Zr(IV) sites to make for a porous heterogeneous
4
2,43
trated 1 M solution of NaOH (Fig. S2†). Also, after being acid catalyst.
soaked in the highly corrosive H PO (85 wt%, due to the high
PO solution (1.0 mL of 85 wt% H
affinity of its dense O donors for the densely charged Zr
of acetonitrile) affords the acidified sample ZrL3-450-P. The
ions), ZrL3-450 remains crystalline (Fig. 2i); by comparison, FT-IR spectrum (Fig. 4f) verified the loading of H PO with a
phosphoric acid has proved especially detrimental to Zr(IV)– broad band around 1109 cm− indicative of the stretching
Specifically, soaking the ZrL3-450 solid in a
H
3
4
3
PO diluted with 2.0 mL
4
3
4
4
+
3
4
1
2
3,35
carboxylate networks.
The stability to strong bases and acids prompts the ques- about 1/4.5 by EDX analysis (Fig. S14†), corresponding to 1.3
tion: can one extricate the Zr(IV) oxide clusters to make for a
PO molecules per Zr -cluster.
The solid acid ZrL3-450-P efficiently catalyzes the acetaliza-
tion of both aldehydes and ketones, an important reaction
vibration of P–O–Zr. The P/Zr molar ratio was found to be
H
3
4
6
4
4–46
widely used for masking carbonyl groups.
The reaction
can be done simply by heating the ZrL3-450-P catalyst and sub-
strates (e.g., P/substrate molar ratio: about 0.006/1) in toluene.
Firstly, the benzaldehyde was chosen as the model substrate
and ethanedithiol as the reagent. The reaction near-quantitat-
ively was completed within 2 hours, while no product was
detected when ZrL3-450 was used as the catalyst (indicated by
TLC; Fig. S15†). The reaction most likely proceeded in a proto-
nic acid catalyzed mechanism, with the substrate being acti-
vated for nucleophilic addition by protonating the carbonyl
group with phosphorus acid anchored on the ZrL3-450-P
scaffold. Acetophenone and other benzaldehyde derivatives
substituted with either an electron-donating group or electron-
withdrawing group were also smoothly converted into desired
products with good yield, even though a prolonged reaction
time was needed for 4-cyanobenzaldehyde (entry 4, Table 1).
To illustrate that the catalyzation process was operating within
Fig. 5 CO
2
(195 K) adsorption and desorption isotherms for the
samples of ZrL3-ac, ZrL3-450, ZrL3-600 and ZrL3-450-F.
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Table 1 ZrL3-450-P catalyzed acetalization reaction between alde- species into the supernatant. Moreover, the recyclability of the
hydes/ketones and ethanedithiol/glycol
crystalline ZrL3-450-P is highlighted in the constant yields
observed in the 8 cycles tested: no obvious trend of catalytic
activity loss was observed (entry 6, Table 1), and the crystalli-
nity of the host framework persisted (as seen in the PXRD
pattern ( j), Fig. 2).
a
a
ZrL3-450-P also displays interesting chemoselectivity. For
example, when aliphatic ketones were used as substrates, no
corresponding S,S-acetals were obtained – with the starting
materials unreacted (entries 6–8). However, when ethylene
glycol was used, the selectivity reversed, with only aliphatic
O,O-acetals obtained in high yield; while most of the aromatic
ketones/aldehydes listed in Table 1 did not react (the benz-
aldehyde was sluggishly converted). This type of selectivity
differed from reported cases, for example in cases where alde-
Entry
1
Substrates
S,S-acetal (yield)
O,O-acetal (yield)
2
3
—
—
4
7,48
hydes were thioacetalized over ketones
and aliphatic alde-
49
hydes were reacted preferentially over aromatic aldehydes.
Conclusions
4
5
—
Taken together, hybrid porous solids with extraordinary stabi-
lity were achieved by using the cascade-cyclizing alkyne units
of the linker molecules of metal–organic frameworks. A couple
of key observations can be noted in this study. (1) The persist-
ent porosity feature withstands not only the decarboxylation of
the carboxylate linker, but also the extrication of the Zr-oxo
blocks, highlighting the extensive bonding across the nanogra-
phene units generated from the thermocyclization process. (2)
The crystallinity of ZrL3 can also survive the decarboxylation
step (e.g., at 450 °C), but appears to be quite sensitive to the
order of the Zr–O components: for example, a higher tempera-
ture (e.g., 600 °C) drastically moves and merges the Zr–O clus-
ters, and diminishes the X-ray diffraction intensity as a result.
Looking ahead, opportunities for functionalization abound.
For example, selective oxidization of the Zr-depleted nanogra-
phene grid should afford a porous 3D analog of graphene
oxide. As the nanographene matrix is derived from the well-
defined alkyne linkers of an ordered MOF scaffold, more
uniform local features can be retained in the annulated solid,
even in the absence of global crystallinity. Various O, S and
halogen functions can therefore be affixed to the side arms, so
—
Not tested
Not tested
d
6
7
8
—
—
9
—
Reaction conditions: aldehydes or ketones (0.20 mmol), ethanedithiol as to be carried over into the thermocyclized framework.
or glycol (0.60 mmol), ZrL3-450-P (4.0 mg; the molar ratio of H
3
PO
4
to
One ultimate goal, however, lies in the appealing carbon
crystals known as carbon schwarzites. For this, a carefully
a
substrates is 0.006/1), toluene (0.5 mL), 110 °C, 2 hours. Determined
1
b
c
by H-NMR. Reaction time was 20 hours. Reaction time was 4 hours.
d
e
The 8th cycle. Acetone was used as the solvent, reflux, 20 hours; “—” chosen MOF topology will have to work in close concert with
stands for no reaction.
the linker molecules of the fitting size and function. Besides the
fundamental and practical importance of the carbon schwar-
zite, exploration towards this object will surely sharpen our
skills in synergizing molecular design and solid-state synthesis.
the pores of the host net, a bulkier substrate, 3,5-bis(benzy-
loxy)-benzaldehyde (entry 5, Table 1), was used. No reaction
proceeded even after 20 hours (as per TLC tests; Fig. S16†), in
contrast to the smooth reaction homogeneously catalyzed by
p-toluenesulfonic acid to form the expected dithioacetal
product. Also, a test to examine the catalytic activity of the
Experimental
Crystallization and activation of ZrL3
supernatant yielded a negative result (as shown by the NMR The preparation and activation of the sample of ZrL3 was
2
3
data in Fig. S17†), which indicates no leaching of the catalytic done according to the reported method. Specifically, mole-
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cule H
2
L3 (20 mg, 0.025 mmol) and a N,N-diethylformamide molar ratio being 0.006 : 1), 1,2,4,5-tetramethylbenzene
(
DEF, 0.75 mL) solution of ZrCl4 (5.8 mg, 0.025 mmol) and (internal standard) and toluene (0.5 mL) were loaded into a
acetic acid (300 mg, about 200 molar equivalents to ZrCl 3 mL glass vial. A gentle stream of N gas outflowing from a
were added in a Pyrex glass tube (soda lime, 10 mm OD, Pasteur pipet was then placed slightly above the reaction
mm ID). The tube was flame-sealed and heated at 120 °C mixture in the vial for about 30 seconds (this was for flushing
4
)
2
6
in an oven for 48 hours, followed by programmed cooling to the air in the vial); the pipet was then taken out, and the vial
room temperature over 18 hours to afford orange truncated immediately capped. The reaction mixture was stirred by using
octahedron-shaped crystals (0.6–0.8 mm) with a yield of a magnetic bar at 110 °C for several hours. After the reaction
3
2
5% based on H L3. For elemental analysis, the crystals was completed (monitored via TLC), ZrL3-450-P was removed
were washed with DMF (3 × 1.5 mL) and soaked in aceto- by centrifugation, and the supernatant was evaporated under
1
nitrile (5 × 5 mL, replaced by fresh acetonitrile every reduced pressure, and the residue was examined by H-NMR
4
1
hours). The resulting crystals were then evacuated at with the already added 1,2,4,5-tetramethylbenzene as the
00 °C for 5 hours.
internal standard.
Preparation of the samples heated at various temperatures
General procedure for recycling the catalyst ZrL3-450-P
The solid of ZrL3-ac was placed into a crucible, which was After one round of the acetalization reaction between benz-
then transferred into a tube furnace. Afterward, the sample aldehyde and ethanedithiol, the catalyst of ZrL3-450-P was
−
1
was heated to 250/450/500/600 °C at a rate of 5 °C min under recovered by simple centrifugation and washed with toluene
−
1
the protection of argon gas flow (40 mL min ) and main- three times. After decanting the toluene, the solid was loaded
tained at this temperature for 2 hours, followed by cooling into a vial together with a new batch of reactants and solvent
down to room temperature over 3 hours.
for another catalytic cycle. The ZrL3-450-P heterogeneous cata-
lyst was thus cycled 8 times, and no reduction in catalytic
activity was found.
Preparation of the samples of ZrL3-450-P
The sample of ZrL3-450 (e.g., 30 mg) was soaked in a mixture
Catalytic activity test of the reaction supernatant
of H
0 °C for 24 hours. Then the solid was isolated by centrifu-
gation and thoroughly washed by heating in 2.0 mL of a
mixture of CH CN and pure water (1 : 1, v/v) at 60 °C for
hours, with the solvent being replaced by a new batch every
hour. Afterward, the sample was soaked in CH CN for
–4 hours to remove the water molecules present inside the
solids, followed by drying under dynamic vacuum at 100 °C for
hours.
3 4 3
PO (85 wt%; 1.0 mL) and CH CN (2.0 mL) and heated at
6
After running at 110 °C for one hour, a reaction mixture (using
benzaldehyde and ethanedithiol; set up as above) was filtered
for isolating the supernatant from the catalyst solid. Two
3
3
3
drops of the filtrate/supernatant were added to CDCl (0.5 mL)
1
3
for H NMR measurement. The rest of the supernatant was
2
stirred at 110 °C for one additional hour, after which 2 drops
1
were added to another 0.5 mL CDCl
3
for H NMR measure-
7
ment. The two NMR spectra thus obtained indicate the same
ratio between benzaldehyde and 2-phenyl-1,3-dithiolane (the
target product); in other words, no additional reaction was
found in the supernatant after the isolation of the catalyst
solid.
Preparation of the sample of ZrL3-450-F
The sample of ZrL3-450 (50 mg) was heated in an HF aqueous
solution (48 wt%; 3.0 mL) at 80 °C with stirring for 48 hours.
Then the solid was isolated by centrifugation followed by
heating in pure water (3.0 mL) at 60 °C for 3 h (replaced by p-Toluenesulfonic acid catalyzed acetalization reaction
fresh water every hour). Subsequently, the solid was solvent- between 3,5-bis(benzyloxy)-benzaldehyde and
exchanged with CH CN before being dried under dynamic 1,2-ethanedithiol
3
vacuum at 100 °C for 7 h.
The reaction was similarly set up as in the above hetero-
geneous catalysis, using 3,5-bis(benzyloxy)-benzaldehyde
Preparation of the samples for HAADF-STEM image collection
(
31 mg, 0.10 mmol), 1,2-ethanedithiol (12 mg, 0.20 mmol) and
The sample (ZrL3-450 or ZrL3-600) was ground with mortar
and pestle to afford fine particles, which were then dispersed
into 1 mL of ethanol in an Eppendorf tube. The resultant
suspension was centrifuged at a relative centrifugal force
of 3000 g for 5 minutes. The supernatant was collected and
p-toluenesulfonic acid (0.40 mg, dissolved in 0.4 mL of
toluene). The reaction mixture was stirred at 110 °C for
18 hours (TLC indicating a strong new spot above the faint
spot of the substrate) and then cooled back to room tempera-
ture and directly loaded onto a (microscale) pipet column
dispersed onto
measurement.
a holey carbon grid for HAADF-STEM
(
silica; eluted by 1 : 1 CHCl /hexane, v/v). The yield thus
3
obtained was 23 mg (58%).
Typical conditions for heterogeneously catalytic reactions for
the acetalization reaction
Conflicts of interest
An aldehyde or ketone substrate (0.20 mmol), ethanedithiol or
glycol (0.60 mmol), ZrL3-450-P (4.0 mg; the H PO /substrate There are no conflicts to declare.
3
4
11908 | Dalton Trans., 2020, 49, 11902–11910
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Paper
1
2 S. M. Cohen, The Postsynthetic Renaissance in Porous
Solids, J. Am. Chem. Soc., 2017, 139, 2855–2863.
Acknowledgements
The work was supported by a GRF grant from the Research 13 J. S. Qin, S. Yuan, L. Zhang, B. Li, D. Y. Du, N. Huang,
Grants Council of HKSAR (CityU 11306018), the National
Natural Science Foundation of China (21871061), Local
Innovative and Research Teams Project of Guangdong Pearl
River Talents Program and Science and Technology Program of
Guangzhou (2017BT01Z032).
W. Guan, H. F. Drake, J. Pang, Y. Q. Lan, A. Alsalme and
H. C. Zhou, Creating Well-Defined Hexabenzocoronene in
Zirconium Metal-Organic Framework by Postsynthetic
Annulation, J. Am. Chem. Soc., 2019, 141, 2054–2060.
14 D. Alezi, I. Spanopoulos, C. Tsangarakis, A. Shkurenko,
K. Adil, Y. Belmabkhout, M. O’Keeffe, M. Eddaoudi and
P. N. Trikalitis, Reticular Chemistry at Its Best: Directed
Assembly of Hexagonal Building Units into the Awaited
Metal-Organic Framework with the Intricate Polybenzene
Topology, pbz-MOF, J. Am. Chem. Soc., 2016, 138, 12767–
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