Invisible Silver Guests Boost Order in a Framework That Cyclizes and Deposits Ag3Sb Nanodots

Article abstract of DOI:10.1021/acs.inorgchem.1c00012

The infusion of metal guests into (i.e., metalating) the porous medium of metal-organic frameworks (MOFs) is a topical approach to wide-ranging functionalization purposes. We report the notable interactions of AgSbF6 guests with the designer MOF host ZrL1 [Zr6O4(OH)7(L1)4.5(H2O)4]. (1) The heavy-atom guests of AgSbF6 induce order in the MOF host to allow the movable alkyne side arm to be fully located by X-ray diffraction, but they themselves curiously remain highly disordered and absent in the strucutral model. The enhanced order of the framework can be generally ascribed to interaction of the silver guests with the host alkyne and thioether functions, while the invisible heavy-atom guest represents a new phenomenon in the metalation of open framework materials. (2) The AgSbF6 guests also participate in the thermocyclization of the vicinal alkyne units of the L1 linker (at 450 °C) and form the rare nanoparticle of Ag3Sb supported on the concomitantly formed nanographene network. The resulted composite exhibits high electrical conductivity (1.0 S/cm) as well as useful, mitigated catalytic activity for selectively converting nitroarenes into the industrially important azo compounds, i.e., without overshooting to form the amine side products. The heterogeneous/cyclable catalysis entails only the cheap reducing reagents of NaBH4, ethanol, and water, with yields being generally close to 90%.

Full text of DOI:10.1021/acs.inorgchem.1c00012

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Article
Invisible Silver Guests Boost Order in a Framework That Cyclizes and
Deposits Ag3Sb Nanodots
Shengxian Cheng, Yinger Xin, Jieying Hu, Weijin Feng, Dohyun Ahn, Matthias Zeller, Jun He,*
and Zhengtao Xu*
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ABSTRACT: The infusion of metal guests into (i.e., metalating)
the porous medium of metal−organic frameworks (MOFs) is a
topical approach to wide-ranging functionalization purposes. We
report the notable interactions of AgSbF6 guests with the designer
MOF host ZrL1 [Zr6O4(OH)7(L1)4.5(H2O)4]. (1) The heavy-
atom guests of AgSbF6 induce order in the MOF host to allow the
movable alkyne side arm to be fully located by X-ray diffraction,
but they themselves curiously remain highly disordered and absent
in the strucutral model. The enhanced order of the framework can
be generally ascribed to interaction of the silver guests with the
host alkyne and thioether functions, while the invisible heavy-atom
guest represents a new phenomenon in the metalation of open framework materials. (2) The AgSbF6 guests also participate in the
thermocyclization of the vicinal alkyne units of the L1 linker (at 450 °C) and form the rare nanoparticle of Ag3Sb supported on the
concomitantly formed nanographene network. The resulted composite exhibits high electrical conductivity (1.0 S/cm) as well as
useful, mitigated catalytic activity for selectively converting nitroarenes into the industrially important azo compounds, i.e., without
overshooting to form the amine side products. The heterogeneous/cyclable catalysis entails only the cheap reducing reagents of
NaBH4, ethanol, and water, with yields being generally close to 90%.
INTRODUCTION
■
Metal−organic frameworks (MOFs)1−6 of alkyne-equipped
linkers have recently been found to enable versatile metalation
and other postsynthetic modifications, e.g., thermally induced
(200−600 °C) cascade cyclization of the alkyne units to
generate π-rich nanographene surfaces in the porous MOF
medium.7−10 The cyclization is a vigorous, complex process
involving extensive bond reorganization of the linker backbone,
which also offers opportunities to convert the metal guests into
catalytically active species associated with the partially
Figure 1. Cyclizable alkyne linker H2L1 and the bare-bones molecule
graphitized MOF solid. Along this line of exploring the
interaction between metal guests and MOF hosts (e.g., as a
topical area of MOF metalation11−15), we have treated the newly
synthesized MOF solid ZrL1 [Zr6O4(OH)7(L1)4.5(H2O)4; see
Figure 1 for linker molecule] with AgSbF6 guests. Compared
with the symmetrically backfolded analogues reported,9 the Z-
shaped L1 linker here is less bulky (with fewer side arms, e.g., to
allow for more free space in the prospective MOF), and it is
easier to make. Recounted below are two serendipitous and
somewhat surprising findings from this study.
The first one is about the enhancement of the structural order
of the MOF crystal by the AgSbF6 guest. Because of the slender
linker and its mobile side arms, as well as the variable carboxyl
connections on the Zr−O joint, severe disorder prevails in the
native crystals of ZrL1, with the linker molecule hardly resolved
bis(4-carboxylphenyl)butadiyne (H2L2).
from the X-ray diffraction data. Similar, low-resolution
diffraction from big crystals is no surprise to researchers on
flexible open structures, as exemplified by proteins and a
growing number of complex metal−organic frameworks.16,17 In
other words, the long-range order underlying the shapely crystal
Received: January 4, 2021
Published: March 31, 2021
https://doi.org/10.1021/acs.inorgchem.1c00012
© 2021 American Chemical Society
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diffract X-ray better than the as-formed, native sample, see Figure S15).
A saturated toluene solution (0.5 mL) of AgSbF6 was then added into
the tube to immerse the crystals for 2 days, yielding the ZrL1-AgSbF6
crystals for X-ray diffraction study. Using AgBF4 or AgOTf (triflate:
CF3SO3Ag) instead of AgSbF6 and with this same procedure, crystals of
ZrL1-AgBF4 and ZrL1-AgOTf can be obtained, which also feature
stronger X-ray diffraction to yield the same structure solution: with the
linker molecule fully resolved in a disordered model but with no silver
salt species located from the electron density map (similar to the X-ray
structure solution of ZrL1-AgSbF6).
Preparation of ZrL1-AgSbF6-450. A solid sample of ZrL1 (20
mg), which was prewashed with toluene and then soaked in toluene
overnight for further solvent exchange, was further soaked in a saturated
toluene solution (1.0 mL) of AgSbF6 for 24 h. Afterward, the solid was
isolated by centrifugation and washed with toluene (1.5 mL × 3 times)
to remove the AgSbF6 physically trapped on the surface; the resultant
solid was then immersed in hexane for 4 h for solvent exchange.
Afterward, the solid was isolated by centrifugation and dried under
dynamic vacuum at 100 °C for 5 h. The loading of AgSbF6 in thus-
obtained solids was determined by Inductively Coupled Plasma Optical
Emission Spectrometer (ICP-OES) analysis, which indicates a silver
content of 14.8 wt % corresponding to a Zr/Ag molar ratio of 1/1.5.
Using AgBF4 or AgOTf (triflate: CF3SO3Ag) instead of AgSbF6, and
following this same procedure, bulk samples of ZrL1-AgBF4 and ZrL1-
AgOTf can be also obtained, which feature silver contents of 17.3 wt %
(Zr/Ag 1/1.7) and 11.5 wt % (Zr/Ag 1/1.2), respectively (ICP-OES
results). To obtain ZrL1-AgSbF6-450, the dried sample was heated to
450 °C at a rate of 5 °C/min and maintained at this temperature for 3 h
followed by natural cooling down to room temperature under the
protection of an argon flow. The silver content in the resultant sample
was calculated according to the weight loss after heating, which is about
22.5 wt %. For comparison of catalytic activity, the sample of ZrL1-450
was made according to the identical procedures without presence of
AgSbF6.
Electrocatalysis. A ground catalyst solid of ZrL1-AgSbF6-450 (5.0
mg) and a Nafion aqueous solution (50 μL, 5 wt %, Dupont) were
added to ethanol (450 μL), and the resultant mixture was ultra-
sonicated for 30 min to afford a homogeneous suspension. The mixture
(10 μL) was then transferred onto a glassy carbon electrode (GCE,
0.19625 cm2), which was prewashed with distilled water and ethanol
and dried in a high-purity nitrogen steam, with a catalyst loading of 510
μg/cm2. The glassy carbon electrode was prewashed adequately. The
catalyst-coated glassy carbon electrode, Ag/AgCl, and Pt wire were
used as the working, reference and counter electrode, respectively.
Electrochemical measurements were conducted in an O2 saturated 0.1
M KOH aqueous solution with an electrode rotation speed of 1600
rpm. The sweep rate of linear sweep voltammetry (LSV) curves was 10
mV/s for oxygen reduction reaction (ORR) and was 5 mV/s for oxygen
evolution reaction (OER). The experimental potentials were later
converted to the reversible hydrogen electrode (RHE) scale, for Ag/
AgCl reference in alkaline condition, VRHE = VAg/AgCl + VAg/AgClθ + 0.059
pH = VAg/AgCl + 1.0095.
Heterogeneous Catalysis of Nitroarenes Reduction toward
Azo Compounds. A nitroarene substrate (0.10 mmol), NaBH4 (0.40
mmol), ZrL1-AgSbF6-450 (2.4 mg; the Ag/substrate molar ratio being
0.05:1), EtOH (0.9 mL), and H2O (0.6 mL) were loaded into a 3 mL
glass vial. The reaction mixture was stirred with a Teflon coated
magnetic bar at 60 °C for several hours (specified in Table 1). After
being cooled down to room temperature, the reaction mixture was
bubbled with O2 for around 10 min. Afterward, ZrL1-AgSbF6-450 was
removed by centrifugation, and the supernatant was extracted with
DCM (0.5 mL × 3 times) and washed with H2O (0.5 mL × 3 times).
The organic layer was purged by a gentle nitrogen gas flow to remove
solvent, and the residue was loaded onto a (microscale) pipet column
(silica; eluted by 1:4 CH2Cl2/hexane, v/v) to isolate the pure product
for yield calculation and NMR measurements.
is beset with local atom swings that blur the positions. To
facilitate structural study, heavy atom guests were occasionally
inserted to serve as strong scattering centers (e.g., as a phasing
method in protein crystallography) and to enhance the
structural order.18 In the general practice of X-ray crystal
analysis, the heavy atoms were the first to be located from the
electron density map. In a formal sense, it would be unusual to
have light atoms located instead of the heavy ones from the X-ray
diffraction data of a crystal.
Such an unusual case is found in the crystal of ZrL1 in which
the uptake of AgSbF6 guests led to stronger diffraction to resolve
the backbone carbon atom positions, while the heavy guest
atoms of Ag and Sb are not located; they are invisible to the X-
ray analysis. The abnormality, counterintuitive as it is, is easy to
explain. Namely, the AgSbF6 uptake restricts the motion (e.g.,
rotations along the alkynyl axes) of the linkers of the host net,
and their increased order makes for stronger diffraction. The
AgSbF6 species, by comparison, is less tethered: sitting in the
liquid-like open domain, the Ag+ can take various positions
around the sulfur atom or the organic π clouds of the linker. In
other words, the Ag(I)-linker interactions in this case do not fix
the Ag/Sb positions for them to figure in the X-ray structure.
The second finding concerns the rarely observed intermetallic
nanodots of Ag3Sb19 generated in the process of themocylizing
the alkynyl units of the MOF host. The resultant composite
consists of the Ag3Sb nanodeposit and the partially graphitized
scaffold, and exhibits efficient cyclable/heterogeneous catalytic
activities for making azo compounds from cheap nitro
precursors, with yields being comparable to that of Pd-/Au-
catalytic systems.20,21 It is worth noting that much efforts have
been devoted to MOFs/MOFs derived catalysts for making
aniline products from nitroarenes,22−24 but studies on
modulating their catalytic activity for azo synthesis have been
rare. Besides helping to solve the challenging structure of a
porous framework, the AgSbF6 guests in the present case take on
some importance in the broader context of metalating MOFs for
functionalization and application. For the metalation of metal−
organic frameworks (MOFs) is topical for catalysis,13,25−27 gas
separation,14,28,29 heavy metal removal,30,31 and luminescence
properties.32−34
EXPERIMENTAL SECTION
The general procedures (including the single-crystal X-ray diffraction)
are included in the SI.
■
Crystallization and Activation of ZrL1. Molecule H2L1 (20 mg,
0.034 mmol) and a N,N-diethylformamide (DEF, 0.75 mL) solution of
ZrCl4 (8 mg, 0.034 mmol) and acetic acid (453 mg, 7.5 mmol) were
added in a Pyrex glass tube (soda lime, 10 mm OD, 6 mm ID). The tube
was flame-sealed and heated at 120 °C in an oven for 48 h, followed by
programmed cooling to room temperature over 12 h to afford orange
octahedron-shaped crystals (0.6−0.8 mm) with a yield of 52% based on
H2L1. For elemental analysis, the crystals were washed with N,N-
dimethylformamide (DMF, 3 × 1.5 mL) and soaked in acetone (5 × 5
mL, replaced by fresh acetone every 4 h). The resulting crystals were
then evacuated at 100 °C for 5 h. Elemental analyses (57.0% C, 3.30%
H, 8.20% S) on the thus-activated sample indicates a formula of
Zr6O4(OH)7(L1)4.5(H2O)4, (calculated: 56.9% C, 3.10% H, 8.10% S).
FT-IR (KBr pellet, v/cm−1): 3700−3000 (bb), 2919 (w), 2200 (m),
1708 (m), 1589 (s), 1535 (s), 1494 (s), 1423 (s), 1390 (s), 1253(m),
1182 (w) 1143 (m), 1083 (m), 1035 (m), 1014 (m), 956 (m),919 (w),
817 (s), 779 (s), 727 (w), 653 (s), 528 (w).
Entering AgSbF6 into ZrL1 crystals. After opening the Pyrex glass
tube containing the as-formed ZrL1 crystals, the mother liquor was
carefully pipetted out and the crystals were rinsed with toluene (ca. 1
mL) 3 times (incidentally, crystals of ZrL1 soaked in toluene do not
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Table 1. Formation of Azo Compounds from Reduction of
Nitrobenzenes Catalyzed by ZrL1-AgSbF6-450
Figure 2. (A) Photographs of single crystals of ZrL1 and the AgSbF6-
loaded ZrL1-AgSbF6 and (B) X-ray single crystal structure of ZrL1-
AgSbF6 as represented by an octahedral unit featuring Zr6O4(OH)4
clusters at the corners. Atom colors: cyan, Zr; red, O; yellow, S; gray, C.
Disorders are omitted for clarity.
e
Reaction conditions: substrate (0.1 mmol), NaBH4 (0.4 mmol) and
ZrL1-AgSbF6-450 (2.4 mg, Ag/substrate molar ratio: 0.05/1), EtOH
(0.9 mL) and H2O (0.6 mL); after being stirred at 60 °C, the reaction
mixture was bubbled with O2 for 10 min at room temperature. a: All
molar yields are isolation yield based on nitrobenzenes; b: the solvent
is pure EtOH (1.5 mL); c: EtOH/H2O = 0.3/1.2, v/v; d: the fifth
round.
smeared out. Long exposure times (even 5 min per degree) did
not help even with the already sizable crystals, which points to
intrinsic low resolution due to severe disorder.
We then took a page from previous work in which insertion of
heavy metal guests led to a full X-ray structure including even the
floppy thioether side chains.18 We soaked the native ZrL1
crystals in a saturated toluene solution of AgSbF6. The X-ray
diffraction from the AgSbF6-loaded crystal (ZrL1-AgSbF6)
offered higher resolution and located the entire framework
(Fm3m; no. 225). The side arm is disordered over both sides of
the backbone (by crystal symmetry), and it also splits on each
side into two slightly splaying parts. The side arms are confined
within the octahedral cavities of the fcu net (Figure 2B). The
structure is porous and contains about 51% solvent accessible
void space, not counting void space that overlaps with partially
occupied ligand side arms. The residual electron density peaks in
the void space are weak and not arranged in an interpretable
pattern, less than 1.5 e−/Å3, indicating AgSbF6 and other guests
are too disordered to be modeled from the diffraction data.
The impregnation by the AgSbF6 guests, however, is
evidenced otherwise. The yellow, fluorescent (with dim yellow
emission, Figure S4) ZrL1 crystals, after being soaked in
AgSbF6/toluene, become brown and nonemissive (e.g., due to
heavy atom quenching of fluorescence from AgSbF6). Moreover,
the resulted ZrL1-AgSbF6 crystal sample contains 14.8 wt % Ag
(1.5:1 Ag/Zr atom ratio) as per inductively coupled plasma-
optical emission spectrometry (ICP-OES) analysis. The AgSbF6
RESULTS AND DISCUSSION
■
A solvothermal reaction of H2L1 and ZrCl4 in DEF (N,N-
diethylformamide) yielded yellow octahedral ZrL1 crystals
(Figure 2A). Powder X-ray diffraction (PXRD) reveals a lattice
similar to the UiO-type PCN-111 (see Figure 1 for its
linker),35,36 featuring an fcc (face-centered cubic) array of
Zr6O4(OH)4 clusters (Figure S2) and linear linkers. Elemental
1
analyses (57.0% C, 3.30% H, 8.20% S) and solution H NMR
(Figure S3) on the activated sample indicates a formula of
Zr6O4(OH)7(L1)4.5(H2O)4, (calcd: 56.9% C, 3.10% H, 8.10%
S). The ligand deficiency (i.e., 4.5 L1 linkers instead of 6 for full
occupancy) indicates the Zr6O4(OH)4 node to average 9
connections, with the remaining coordination sites filled in by
OH−/H2O (or capping species such as HCOO−). Open metal
sites as such are common in Zr(IV)-MOF structures and can be
exploited for topical catalysis applications.37,38
With the big (0.6−0.8 mm), octahedral crystals at hand, we
proceeded with X-ray data collection. The diffraction data,
however, only revealed the Zr−O cluster in the cubic cell (a =
33.8 Å), with the backbone atoms of the linker molecule all
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uptake also changed the PXRD intensity profile: e.g., it makes
the (200) peak stronger than the (111) peak (cf. patterns b and
c, Figure 3), consistent with increased electron density around
Figure 4. CO2 (195 K) adsorption and desorption isotherms for the
samples of activated ZrL1 and ZrL1-AgSbF6-450.
solids,42−44 alkyne cyclization can be initiated more mildly (ca.
250 °C, without the need of AgSbF6; see spectrum c of Figure S7
for the IR), so as to preserve the chemical functions of the MOF
precursor. The extensive cascade annulations can also result in
covalent bridging of the linker molecules, thereby enhancing
stability. Recent studies also indicated that the porosity and
surface area can be largely improved by thermocyclization, even
if the crystallinity may be lost.7
Figure 3. X-ray powder patterns (Cu Kα, λ = 1.5418 Å): (a) calculated
from the single-crystal structure of PCN-111 (ZrL2); (b) as-made
ZrL1; (c) as-made ZrL1-AgSbF6; (d) activated ZrL1; (e) activated
ZrL1-AgSbF6; (f) sample of (e) heated at 450 °C for 3 h (ZrL1-
AgSbF6-450): the indices are for Ag3Sb; g) sample of e) heated at 250
°C for 3 h (ZrL1-AgSbF6-250).
The ZrL1-AgSbF6 solid was heated at 450 °C for 3 h in an
argon atmosphere to forge extensive conjugation/ring fusion
throughout the solid state. The resultant black solid (ZrL1-
AgSbF6-450) retains 70% of the original mass, with the loss
attributable to the departure of H2O and carboxyl/sulfur groups,
and AgSbF6 decomposition (see Figure S8 for more details).
The alkyne stretching at 2204 cm−1 (IR, Figure S7) disappeared
after heating, consistent with extensive cyclization (see Figure
S9 for a possible reaction scenario). PXRD (pattern f, Figure 3)
reveals no peaks at 2θ < 20°, indicating the thermocyclized
matrix to be amorphous; in the higher angle region, however,
appeared peaks of the Ag3Sb phase (derived from the AgSbF6
guest; no such peaks were seen at 245 °C, as shown in Figure
3g). Scanning electron microscopy (SEM) reveals the Ag3Sb to
exist as smooth nanodots (30−80 nm in diameter, Figure 5a; 3:1
Ag/Sb atom ratio as per analysis by local energy dispersive X-ray,
EDX, Figure S10) evenly dispersed on the organic matrix (this
latter retains the octahedral form of the parent MOF crystals,
Figure 5b). The corresponding EDX elemental map of Ag and
Sb atoms (Figure 5c,d) indicated Ag3Sb to be evenly distributed.
The average EDX results indicate a 4:3 Ag/Sb atom ratio
(Figure S11); the higher Sb content (relative to Ag3Sb) here
suggests that Sb also exist in some amorphous, unidentified
form.
In spite of the amorphous nature of the thermocylized matrix,
ZrL1-AgSbF6-450 exhibits enhanced CO2 sorption and a larger
Langmuir surface area of 388 m2/g (Figure 4): over 3.5 times
that of activated ZrL1 (109 m2/g). ZrL1-AgSbF6-450 also shows
good electrical conductivity (ca. 1.0 S/cm in a two-probe setup
on a compressed powder at room temperature). The
combination of porosity and conductivity (the Ag3Sb nanodots
being also possibly useful) prompted us to explore electro-
catalysis properties. Unfortunately, the oxygen reduction
reaction (ORR) and oxygen evolution reaction (OER) results
the face center (center of the octahedral cage) of the fcc cell. As
it were, the face centers sit midway between the (111) planes of
the heavy (strongly diffracting) Zr clusters and in the (200)
planes of the Zr clusters; electron density around the face center
therefore weakens (111) and strengthens (200) peak. For
further illustration, the PCN-111 (based on H2L2, Figure 1)
structure,35 having no side arms to occupy the octahedral voids,
features a strongest (111) peak relative to the (200) peak
(pattern a, Figure 3).
After solvent exchange (with acetone) and solvent removal,
both ZrL1 and ZrL1-AgSbF6 showed broader PXRD features
(patterns d and e, Figure 3) indicative of framework
degradation. In particular, the peaks of the AgSbF6-loaded
sample appear to be slightly sharper and they are shifted to
higher angles (i.e., indexable onto a cubic lattice with a = 28.2
Å), indicating some lattice contraction which was also observed
in a framework based on the same strut units but with twice the
side arms to occupy the voids.9 In spite of their enhanced
chemical stability (e.g., in water), degradation of crystalline
order upon activation is common in Zr(IV)-carboxyl net-
works,39−41 which is in line with the lability arising from the
ionic coordination links. The framework degradation/collapse
reduces the surface area and compromises their application as
porous materials. For example, the activated ZrL1 sample, being
partially collapsed, exhibits little N2 uptake (77 K; Figure S5),
while its CO2 sorption (195 K; Figure 4 and Figure S6a) reveals
a modest Langmuir surface area of 109 m2/g.
To improve stability, thermal treatment to cascade-cyclize the
alkynyl units (as featured in L1) is helpful.9 Unlike the high-
temperature (e.g., 800 °C), all-breaking carbonization of MOF
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Substituted nitrobenzenes were also tested. All three methyl-
nitrobenzene isomers afford azo products in good yields (entries
4−6, Table 1). Fluoro substitution does not affect the reaction
(entry 7, Table 1). The conversion is also compatible with
alkene groups, which were unaffected (entries 8−9, Table 1).
After five catalytic cycles, no significant decrease in silver loading
(Figure S14) and catalytic activity were observed, showcasing
ZrL1-AgSbF6-450 as a reusable heterogeneous catalyst with
efficiency comparable to precious metal (e.g., Pd, Au)
systems.20,21,50
SUMMARY
■
To conclude, two major instances of serendipity have been
observed from the alkyne-rich framework ZrL1. (1) The first
instance included an unusual, inverted order in the X-ray
structure with regard to the heavy AgSbF6 (or AgBF4/AgOTf)
guests and lighter side arms: the impregnation with the Ag(I)
salts was found to enhance the diffraction intensity as well as the
structural order of the host framework of ZrL1 to allow full
resolution of the linker molecule but not the Ag(I) salt species
(i.e., these heavy atom centers remain curiously invisible to the
X-ray diffraction analysis, unlike the common observation of
heavy metal ions being the first to come out in the electron
density map). (2) The AgSbF6 guests were converted into the
rarely reported Ag3Sb nanoparticles under the energetic (450
°C) condition of linker thermocylization/partial graphitization,
with the resultant Ag3Sb/nanographene composite enabling a
reusable heterogeneous catalyst for selectively reducing nitro
compounds into azo products. The reactive, carbon/alkyne-rich
backbone of the linker molecules apparently acted as a reducing
agent as well as a promoter in forging the intermetallic
nanophase of Ag3Sb. One can also imagine the alkyne-decked
pores of the MOF host as some working nano foundry in which
various other metal salts can be incorporated, so as to be
thermally converted into functional metal/alloy species closely
composited and synergized with the concomitantly formed
nanographene grid. Such molecular metallurgy, as is coupled
and integrated with the thermal/chemical reactivity of the
porous host, opens new avenues to novel metal-based nano
species (e.g., the Ag3Sb particles discovered herein) otherwise
difficult to access.
The luck, on the other hand, is rooted in the broad design
exercise on cyclizable alkyne building blocks and in the rich and
under-explored interactions between metal guests and the
dynamic medium of open frameworks. Such interactions,
however, can be challenging to study: for example, it remains
unclear how the “invisible” AgSbF6, AgBF4, and AgOTf guests
bond with the host net, while guests like Co2(CO)8 and HgBr2
(Figures S16 and S17) were found not to enhance the structural
resolution of the native crystals. Perhaps, the heavy, thermally
sedate Ag+ ion and the counterions electrostatically aggregate
together to better restrict the side chain motions, relative to the
nonionic, molecular species of Co2(CO)8 and HgBr2.
Figure 5. SEM image of ZrL1-AgSbF6-450 with (a) showing Ag3Sb
nanoparticles on the surface and (b) octahedral shape inherited from
MOF precursors; EDX elemental maps of (c) Ag (Inset: image of the
selected mapping area) and (d) Sb (green refers to Ag and blue Sb).
are unimpressive, with onset potentials around 0.8 V for ORR
and 1.6 V for OER and small current densities for both (Figure
S12 and S13), indicating weak electrocatalytic activity of the
AgSbF6-derived species. More active metal (e.g., Pd or Co)
guests can be tested in the future.
We then turned to the selective reduction of nitro compounds
into value-added, technologically important azo products.45,46
One challenge is to specifically target the intermediate azo
product,47 as many metal catalysts tend to overshoot and form
amines instead. Other methods for azo syntheses include amine
oxidation but can involve toxic oxidants (e.g., Hg2+).48,49 To
mitigate the catalytic activity for selective reduction, precious
metals (e.g., Au) have been confined in meticulously crafted
organic cages;50,51 and Ag species loaded into a zeolite exhibited
some selectivity but with low conversion and entailing 25 atm H2
pressure at 115 °C.52 In this context, the Ag3Sb-strewn ZrL1-
AgSbF6-450 composite was examined for potentially moderated
activity (as compared with the overactive Ag nanoparticles53)
for azo production, as a convenient and reusable heterogeneous
catalyst.54,55
The reaction involved the addition of nitroarenes (0.1 mmol),
NaBH4 (0.4 mmol), ZrL1-AgSbF6-450 (2.4 mg; Ag+ is 0.05
equiv to substrate), and 1.5 mL of a solvent mixture of H2O and
EtOH into a 4 mL vial, and the mixture was heated at 60 °C for
several hours followed by bubbling with O2 for 10 min. Water
affects the reaction: nitrobenzene was completely converted
within 4 h with H2O (0.6 mL) and EtOH (0.9 mL), while over 8
h was needed in pure EtOH (1.5 mL). But higher water content
(e.g., 1.2 mL) stops the reaction. Under optimized conditions
(EtOH/H2O = 0.9/0.6, v/v; 4 h) the reaction selectively
generated the azo and hydrazo compounds, while the latter can
be easily oxidized to the azo product by bubbling O2 for 10 min.
If NaBH4 is increased to 8 equiv to substrate or if the reaction
time is extended to 9 h, the hydrazo compound dominates. As a
control, no reaction was detected without ZrL1-AgSbF6-450 or
with using ZrL1 or ZrL1-450 instead; and very low yield was
obtained when the reaction was catalyzed by ZrL1-AgSbF6 or
ZrL1-AgSbF6-250 (see Table S2), highlighting the indispens-
ability of silver loading and higher-temperature (e.g., at 450 °C)
treatment for improving catalytic activity.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge via the
Internet at ACS Publications Web site at DOI: xxxx. . The
Supporting Information is available free of charge at https://
pubs.acs.org/doi/10.1021/acs.inorgchem.1c00012.
Single crystal structure determination, TGA, BET plots,
EDX results and NMR data (PDF)
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Accession Codes
CCDC 2012402 2069047 2069048 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Authors
Zhengtao Xu − Department of Chemistry, City University of
Hong Kong, Kowloon, Hong Kong, China; orcid.org/0000-
0002-7408-4951; Email: zhengtao@cityu.edu.hk
Jun He − School of Chemical Engineering and Light Industry,
Guangdong University of Technology, Guangzhou 510006,
China; orcid.org/0000-0001-7062-4001; Email: junhe@
gdut.edu.cn
Authors
Shengxian Cheng − Department of Chemistry, City University
of Hong Kong, Kowloon, Hong Kong, China
Yinger Xin − Department of Chemistry, City University of Hong
Kong, Kowloon, Hong Kong, China
Jieying Hu − School of Chemical Engineering and Light
Industry, Guangdong University of Technology, Guangzhou
510006, China; Department of Materials Science and
Engineering, City University of Hong Kong, Kowloon, Hong
Kong, China
Weijin Feng − Department of Chemistry, City University of
Hong Kong, Kowloon, Hong Kong, China
Dohyun Ahn − Department of Chemistry, City University of
Hong Kong, Kowloon, Hong Kong, China
Matthias Zeller − Department of Chemistry, Purdue University,
West Lafayette, Indiana 47907, United States; orcid.org/
0000-0002-3305-852X
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00012
Notes
The authors declare no competing financial interest.
(17) Chen, X.; Peng, Y.; Han, X.; Liu, Y.; Lin, X.; Cui, Y. Sixteen
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ACKNOWLEDGMENTS
■
This work was supported by a GRF grant from the Research
Grants Council of HKSAR (CityU 11306018) and the National
Natural Science Foundation of China (21871061). The single-
crystal X-ray diffractometer was funded by the US National
Science Foundation through the Major Research Instrumenta-
tion Program under grant no. CHE 1625543.
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