Lanthanide-Based Metal-Organic Frameworks Containing "v-Shaped" Tetracarboxylate Ligands: Synthesis, Crystal Structures, "naked-Eye" Luminescent Detection, and Catalytic Properties

Article abstract of DOI:10.1021/acs.inorgchem.9b02177

Three lanthanide-based metal-organic frameworks, [Tb(HMDIA)(H₂O)₃]·H₂O (Tb-MDIA), [Ho(HMDIA)(H₂O)₃]·(H₂O)₂ (Ho-MDIA), and [Nd(HMDIA)(H₂O)₃]·(H₂O)

Full text of DOI:10.1021/acs.inorgchem.9b02177

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Lanthanide-Based Metal−Organic Frameworks Containing “V-
Shaped” Tetracarboxylate Ligands: Synthesis, Crystal Structures,
“Naked-Eye” Luminescent Detection, and Catalytic Properties
Pengyan Wu, Lingling Xia, Mengjie Huangfu, Fubin Fu, Mengqiu Wang, Bingxin Wen, Ziyun Yang,
and Jian Wang*
School of Chemistry and Materials Science and Jiangsu Key Laboratory of Green Synthetic Chemistry for Functional Materials,
Jiangsu Normal University, Xuzhou, Jiangsu 221116, People’s Republic of China
*
S Supporting Information
ABSTRACT: Three lanthanide-based metal−organic frameworks, [Tb-
(
(
HMDIA)(H O) ]·H O (Tb-MDIA), [Ho(HMDIA)(H O) ]·(H O)
2 3 2 2 3 2 2
Ho-MDIA), and [Nd(HMDIA)(H O) ]·(H O) (Nd-MDIA) from
2
3
2
2
the same V-shaped ligand 5,5′-methylenediisophthalic acid (H MDIA),
4
were prepared by mixing Ln3 and H MDIA under solvothermal
+
4
conditions. The crystal structures of the three complexes were determined
by single-crystal X-ray diffraction. The different coordination modes of
the organic ligands resulted in different framework structures among the
three complexes. The luminescent properties of Ln-MDIA in the
ultraviolet−visible region were also studied. Interestingly, the bright-
green emitter Tb-MDIA showed high selectivity and sensitivity to allow
the naked-eye visualization of Fe3 ions and picric acid (PA) explosive,
and both sensing mechanisms were revealed. Finally, Ho-MDIA and Nd-
MDIA were shown to work as heterogeneous catalysts for the cyanosilylation reaction of aromatic aldehydes, and the catalysts
could be recycled at least three times without any decrease in activity.
+
INTRODUCTION
molecules typically found in common explosive structures
possess electron-deficient nitro groups along with hydrogen-
bonding donor−acceptor hydroxyl groups; thus, hydrogen-
bonding interactions will benefit for PA-related explosive
detection.
■
3
+
Iron(III) (Fe ), which is one of the most important elements
for humans and other living organisms, plays a key role in
1
,2
3+
environmental and biological systems. Deficiency in Fe or
3
+
excess Fe concentrations can lead to various biological
Metal−organic frameworks (MOFs) have attracted increas-
disorders including diabetes, Parkinson’s disease, liver damage,
anemia, heart failure, and hemochromatosis. Many methods
and particularly fluorescence-based methods have been
explored for the detection of Fe ions because of their high
selectivity and sensitivity, fast response, capacity for real-time
3
ing interest because of their intriguing structures and potential
1
2
13−17
applications in biomedical analysis, catalysis,
diagnostics
18
3
+
and potentially therapeutic medicine, molecule/ion recog-
19−29
30−32
nition,
and optics and magnetics.
Compared to the
4
,5
many MOF structures constructed from d-block transition-
metal ions, lanthanide MOFs (Ln-MOFs) are more difficult to
design and control because lanthanide ions typically have
unspecific coordination properties. Taking advantage of the
high affinity of lanthanide ions for oxygen atoms, a number of
Ln-MOFs with various dimensionalities and topologies have
been synthesized via the self-assembly reactions of lanthanide-
monitoring, and noninvasiveness. However, most reported
3
+
fluorescent Fe sensors are time-consuming, suffer from
interference from other metal ions, resulting in decreased
selectivity, and require mixed water/organic solvents. Thus,
developing chemosensors for Fe3+ detection with high
selectivity/sensitivity and fast response in aqueous environ-
ments remains a challenge. Nitro aromatic compounds
NACs) are well-known explosives that are widely used in
the agrochemical industry. NACs cause serious groundwater
and soil pollution along with security concerns because of their
high toxicity and explosivity.
33−35
(
III) salts with multicarboxylate ligands.
Among the
(
various aromatic multicarboxylic acid ligands, 5,5′-methylene-
diisophthalic acid (H MDIA) is a V-shaped tetracarboxylate
4
6
−8
ligand in which multiple connection modes to metal centers
Hence, highly efficient and
36−38
can be introduced to impart abundant structural motifs.
convenient detection techniques for NAC explosives have
attracted considerable attention from chemists and environ-
mentalists. On the basis of fluorescent methods, photoinduced
donor−acceptor electron transfer is the main mechanism for
Meanwhile, the −CH − spacer in this ligand can rotate and
2
Received: July 21, 2019
9
−11
nitro explosive sensing.
However, the picric acid (PA)
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b02177
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Inorganic Chemistry
Article
bend freely once it is coordinated by the metal centers; this
flexible linker enriches the MOF structure and endows
structural peculiarity.
Fluorescent Spectral Detection. For cation detection, a
suspension of Tb-MDIA powder in 3.00 mL of water as a detection
host was placed in a 1 cm quartz cuvette for fluorescence
measurements. Solutions of related metal ions in water with a high
Luminescent Ln-MOFs are excellent chemosensor candi-
dates on account of their high color purity, relatively long
−
2
concentration of 2.0 × 10 M were prepared. For nitro aromatic
detection, related nitro analyst solutions with concentrations of 2.0 ×
39−41
fluorescence lifetime, and large Stokes shift.
However,
−2
10
M were prepared in ethanol, and a Tb-MDIA emulsion was
developing MOFs with good stability in aqueous solution,
including structural stability, along with the bright and stable
luminescence remains a challenge because lanthanide(III)
fluorescence is usually quenched by high-energy vibrations
prepared in 3.00 mL of ethanol. Excitation was carried out at 305 nm.
The slit widths of excitation and emission were both 2.5 nm. The
relative fluorescence intensity was used to normalize the fluorescence
intensity to eliminate the influence of the initial fluorescence on the
measurement.
42
(
typically X−H oscillators, where X = C, N, and O).
Catalysis Experiments. A typical cyanosilylation procedure was
performed as follows. A catalyst (0.01 mmol, 2% mol; 5.9 mg of Ho-
MDIA or 5.7 mg of Nd-MDIA) was suspended in dry dichloro-
methane (2 mL), followed by the addition of the aldehyde (0.5
mmol) and trimethylsilyl cyanide (1.2 mmol). The mixed solution
Therefore, it is critical to design and construct water-stable Ln-
MOFs for environmental-related molecule/ion sensing. On the
other hand, Ln-MOFs show great potential as heterogeneous
catalysts because lanthanide ions have a flexible coordination
sphere and can create coordinatively unsaturated metal centers.
1
was then stirred at room temperature. H NMR spectroscopy was
Herein, we report three lanthanide MOFs based on H MDIA:
used to determine the reaction yield, which was calculated based on
the carbonyl substrate.
Single-Crystal XRD. A Bruker SMART APEX CCD diffrac-
4
[
(
(
Tb(HMDIA)(H O) ]·H O (Tb-MDIA), [Ho(HMDIA)-
2 3 2
H O) ]·(H O) (Ho-MDIA), and [Nd(HMDIA)(H O) ]·
2
3
2
2
2
3
tometer equipped with a graphite-monochromated Mo Kα (λ =
H O) (Nd-MDIA). The MOFs were characterized by
2
2
0
.71073 Å) source was applied to collect the intensities using the
elemental analysis and single-crystal X-ray diffraction (XRD),
and the luminescent properties of the Ln-MDIA complexes
SAINT and SMART programs. Direct methods using SHELXTL,
version 6.1, were employed to refine the structure on F using full-
2
3
+
were examined. Tb-MDIA allowed naked-eye detection of Fe
matrix least-squares methods. The refinement of non-hydrogen atoms
in the ligand backbones was performed in an anisotropic manner. The
fixation of hydrogen atoms within the ligand geometrically at
calculated positions was confirmed, making them ride on the parent
non-hydrogen atoms. The hydrogen atoms of the carboxylate moieties
were refined with the isotropic parameters fixed at 1.5 times those of
the attached oxygen atoms and found from the difference Fourier
maps and with the O···H distance fixed at 0.85 Å (Table 1).
in pure water and explosives in ethanol, and the sensing
mechanisms for both Fe3 and PA were investigated. In
addition, with the Lewis acidic metal sites exposed within the
open channels and the different sizes of confined cavities, Ho-
MDIA and Nd-MDIA exhibited comparable efficiencies for
heterogeneous catalysis of the cyanosilylation reaction.
+
EXPERIMENTAL SECTION
■
Table 1. Crystal Data and Refinement Information for
Complexes Tb-MDIA, Ho-MDIA, and Nd-MDIA
Materials and Instrumentation. All reagents and solvents were
of analytical-reagant grade and were used without further purification
unless otherwise noted. Metal salts were purchased from J&K
Scientific. Nitro aromatics were purchased from J&K Scientific and
Tb-MDIA
Ho-MDIA
Nd-MDIA
formula
fw
C H O Tb
566.18
C H O Ho
590.20
C H O Nd
568.51
17
11 12
17 13 13
17 12 13
1
Xiya Reagent Company (China). H NMR spectra were measured on
a Bruker-400 spectrometer with tetramethylsilane as the internal
standard. The powder XRD patterns of Ln-MDIA were recorded on a
Rigaku D/max-2400 powder X-ray diffractometer (Japan) using Cu
Kα (λ = 1.5405 Å) radiation. Elemental analyses (C, H, and O) were
performed on an Elementar Vario EL analyzer. Fourier transform
infrared (FT-IR) spectra were recorded using KBr pellets on a JASCO
FT-IR-430 spectrometer. The fluorescence spectra of the solutions
were obtained using an F-4600 spectrometer (Hitachi). The contents
of the metal ions in water were measured by inductively coupled
plasma mass spectrometry (ICP-MS; Leeman PROFILE SPEC).
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/c
15.6284(13)
14.1153(12)
8.4801(8)
90.00
104.355(6)
90.00
1812.3(3)
293(2)
monoclinic
P2(1)/c
11.1720(8)
8.4729(6)
21.1375(13)
90.00
101.107(4)
90.00
1963.4(2)
293(2)
4
triclinic
P1
̅
9.716(4)
10.923(4)
11.176(7)
107.114(6)
95.657(6)
113.853(4)
1004.0(9)
293(2)
α (deg)
β (deg)
γ (deg)
V (Å3)
Synthesis of Tb-MDIA. In a glass vial (10 mL), H MDIA (0.05
4
temp (K)
mmol, 17 mg; see Figure S1 for the chemical structure) and
Z
D
4
2
Tb(NO ) ·6H O (0.1 mmol, 45 mg) were placed in a mixture of 2
3
3
2
(g cm−3
)
2.075
3.971
0.0373
0.0253
1.997
4.100
0.0213
0.0221
1.880
2.654
0.0290
0.0466
calcd
mL of water and 1 mL of ethanol and then heated at 110 °C for 2
μ (mm−1)
days. When the vial was cooled to room temperature, colorless
Rint
crystals were collected (yield = 85% based on H MDIA). Elem anal.
4
Calcd for [Tb(HMDIA)(H O) ]·H O: C, 35.68; H, 2.99; O, 33.55.
R
1
[I > 2σ(I)]
2
3
2
Found: C, 35.59; H, 3.05; O, 33.38.
wR [I > 2σ(I)]
2
0.0672
0.0636
0.1173
Synthesis of Ho-MDIA. In a glass vial (10 mL), H MDIA (0.075
4
mmol, 26 mg) and Ho(NO ) ·5H O (0.15 mmol, 48 mg) were
3
3
2
placed in a mixture of 2 mL of water and 1 mL of ethanol and then
heated at 110 °C for 2 days. When the vial was cooled to room
temperature, colorless crystals were collected (yield = 73% based on
H MDIA). Elem anal. Calcd for [Ho(HMDIA)(H O) ]·(H O) : C,
4.24; H, 3.21; O, 34.88. Found: C, 34.32; H, 3.25; O, 34.90.
Synthesis of Nd-MDIA. The synthetic method was similar to that
of Tb-MDIA. Colorless crystals were similarly found and collected
RESULTS AND DISCUSSION
■
Description of the Crystal Structures. [Tb(HMDIA)-
H O) ]·H O (Tb-MDIA). Single-crystal XRD analysis indicated
4
2
3
2
2
(
2
3
2
3
that Tb-MDIA crystallized in the monoclinic crystal system
with space group P2 /c. Each asymmetric unit contained one
1
3−
terbium(III) ion, one HMDIA anion, three coordinated
water molecules, and one solvated water molecule. As shown in
Figure 1a, each terbium(III) ion was nine-coordinated by four
(
(
3
yield = 75% based on H MDIA). Elem anal. Calcd for [Nd-
4
HMDIA)(H O) ]·(H O) : C, 35.47; H, 3.33; O, 36.14. Found: C,
2
3
2
2
5.52; H, 3.45; O, 36.21.
B
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Article
[
Ho(HMDIA)(H O) ]·(H O) (Ho-MDIA). By changing the
2
3
2
2
3
+
3+
metal centers from Tb in Tb-MDIA to Ho , Ho-MDIA in
the monoclinic space group P2 /c was obtained. The
1
asymmetric unit of Ho-MDIA consisted of one holmium(III)
3−
ion, one HMDIA anion, three coordinated water molecules,
and two solvated water molecules (Figure 2a). The crystallo-
graphically independent holmium(III) ion was eight-coordi-
nated by five oxygen atoms from four bidentate/bimonoden-
3−
tate carboxylate groups of four different HMDIA anions and
three oxygen atoms of three different coordinated water
molecules in a dodecahedral geometry. The coordination
3−
2
1
1
1
mode of the HMDIA ligand was μ -η :η :η :η , different
4
from that of Tb-MDIA. The binuclear holmium clusters were
3−
connected to each other by HMDIA ligands, resulting in a
three-dimensional MOF. Rhombic channels with window sizes
2
of 8.17 × 8.84 Å were viewed along the b axis (Figure 2b).
The solvent-accessible volume calculated using the PLATON
3
program was 460.1 Å , which was 23.4% of the unit cell
volume. Considering the [Ho (COO) ] units as a four-
Figure 1. (a) Coordination environment of the Tb1 center in Tb-
MDIA with a labeling scheme. All hydrogen atoms are omitted for
clarity. (b) View of the three-dimensional structure of Tb-MDIA
along the c axis. The hydrogen atoms and isolated water molecules are
omitted for clarity. (c) Enlargement showing one carboxylic group of
every uncoordinated ligand on the b axis.
2
2
3
−
connected node and the HMDIA group as a four-connected
node, the overall structure of Ho-MDIA can be simplified as a
four-connected sra net, similar to Tb-MDIA.
[
Nd(HMDIA)(H O) ]·(H O) (Nd-MDIA). When the metal
2 3 2 2
3+
nodes were replaced by Nd , the structure changed
significantly. Nd-MDIA crystallized in the triclinic space
oxygen atoms of two bidentate carboxyl groups from two
̅
group P1. Each neodymium(III) ion was coordinated by six
3−
3−
HMDIA anions, two oxygen atoms of two dimonodentate
carboxylate oxygen atoms from four different HMDIA
ligands and three water molecules, similar to Tb-MDIA.
3
−
carboxyl groups from two different HMDIA anions, and
3
−
three oxygen atoms from three coordinated water molecules.
Four carboxylate groups from different HMDIA groups
coordinated to a pair of neodymium(III) ions [Nd−Nd
The TbO unit with dodecahedral coordination geometry was
9
3
−
linked by carboxylate groups to produce a dimeric unit with a
distance = 5.171(6) Å]. Each HMDIA ligand coordinated to
four neodymium(III) ions through four carboxylate groups
Tb···Tb distance of 5.225(3) Å. Each HMDIA3− anion acted as
1
1
1
1
1
1
a μ -bridge to link four terbium(III) ions; two of the
adopting μ -η :η ,μ -η :η and μ -η :η coordination modes.
4
2 1 1
1
1
carboxylate groups adopted the μ -η :η mode, while one
was coordinated in the μ -η :η mode and one was
The location distribution was different from those in Tb-
MDIA and Ho-MDIA. The dinuclear [Nd (COO) ] secon-
1
1
1
2
2
2
3−
uncoordinated (Figure 1c). This dimeric unit was further
connected to its neighbors through six HMDIA3 anions,
forming a three-dimensional network structure. Notably, one-
dary building units were bridged by the HMDIA ligands to
form infinite two-dimensional sheets. These sheets were linked
to each other to produce a three-dimensional architecture via
π−π interactions between the benzene rings from the
−
dimensional rhombus channels with window sizes of 9.62 ×
2
3−
3
.55 Å existed in Tb-MDIA along the c axis (Figure 1b). From
neighboring HMDIA ligands of different sheets with a
a topological viewpoint, the entire framework formed a
uninodal four-connected sra net with point symbol 4 6 8 and
vertex symbol 4.6.4.6.6.82.
distance of 3.28 Å in three directions (Figure 3c). As viewed
from the [001] direction, there were one-dimensional channels
with sizes of 3.5 × 9.7 Å .
2
3
2
Figure 2. (a) View of the coordination environment of the Ho1 center in Ho-MDIA with a labeling scheme. All hydrogen atoms are omitted for
clarity. (b) View of the three-dimensional structure of Ho-MDIA with the Connolly surface along the b axis. The hydrogen atoms and isolated
water molecules are omitted for clarity.
C
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Figure 3. (a) View of the coordination environment of the Ho1 center in Nd-MDIA with a labeling scheme. All hydrogen atoms are omitted for
3
−
clarity. (b) View of the binuclear neodymium clusters and the coordination mode of the HMDIA ligand. (c) View of the three-dimensional
structure of Nd-MDIA along the c axis. The isolated water molecules are omitted for clarity.
3
+
Luminescent Properties of Ln-MDIA. Ultraviolet−
visible (UV−vis) absorption examinations revealed an intense
displayed the characteristic Tb emissions assignable to the
5
7
5
7
5
7
transitions of D → F , D → F , and D → F at 493,
4
6
4
5
4
4
absorption band centered at 305 nm for solid H MDIA. This
548, and 587 nm, respectively. The characteristic luminescence
of Tb-MDIA led us to further study the sensing behavior of
this MOF.
4
band originated from the π−π* transition of H MDIA.
4
Because the aromatic acid acted as an efficient group to
sensitize the lanthanide, the solid-state emission spectra of the
Thermal/Chemical Stability of Tb-MDIA and Its
Performance in the Detection of Metal Ions. Thermog-
ravimetric analyses were carried out to determine the thermal
stability of Tb-MDIA. Tb-MDIA exhibited a 12.3% weight loss
from room temperature to 190 °C, which was attributed to the
loss of three coordinated water molecules and one uncoordi-
nated water molecule (calcd 12.58%); the remaining structure
was maintained until 380 °C (Figure S8). The chemical
resistance of Tb-MDIA was tested by placing it in boiling
water for 1 day, an aqueous HCl solution at pH 3.3 for 1 day,
an aqueous NaOH solution at pH 12 for 1 day, and a water
solution for 1 month. Powder XRD was then used to evaluate
the structures of the samples (Figure 5a). Remarkably, Tb-
MDIA was stable in all of the tested environments, confirming
its structural chemical integrity under different pH values.
These experimental results demonstrate that Tb-MDIA has a
high thermal stability along with excellent chemical stability.
The high coordination number and strong coordination
between the carboxylic oxygen atoms and terbium ions
might have contributed to the high stability of Tb-MDIA.
The pH-dependent fluorescence response of Tb-MDIA was
also evaluated. As shown in Figure 5b, the emission intensity of
Tb-MDIA remained almost unchanged in the pH range of
free ligand H MDIA and complexes Tb-MDIA, Ho-MDIA,
4
and Nd-MDIA in the UV−vis region were measured at room
temperature. As shown in Figure 4, the emission spectrum of
Ho-MDIA mainly consisted of ligand-centered fluorescence,
while no emission was observed in the UV−vis region for Nd-
MDIA. Moreover, when excited at 305 nm, Tb-MDIA
3
.0−12.0, suggesting its applicability as a fluorescent sensor in
Figure 4. Room temperature emission spectra for the free ligand and
complexes Tb-MDIA, Ho-MDIA, and Nd-MDIA.
various aqueous environments.
D
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Figure 5. (a) XRD patterns of Tb-MDIA after treatment in water, acid, and base under different conditions. (b) pH-dependent fluorescence of Tb-
MDIA (0.33 g/L) in a 0.02 M NaCl buffer solution with the pH ranging from 3.0 to 12.0.
−
1
3+
Figure 6. (a) Luminescence spectra of Tb-MDIA (0.33 g L ) upon Fe addition. The inset shows that the visible green light emitted from the
3
+
original Tb-MDIA disappeared after the addition of Fe (0.4 mM) under 365 nm UV radiation. (b) Fluorescence responses of Tb-MDIA (0.33 g
−
1
L ) to common metal ions in water. The fluorescence intensities of Tb-MDIA after the addition of metal ions (0.4 mM) are shown by the red bars
for Ca2+ and Mg , the concentrations were 4.0 mM). The fluorescence intensities of Tb-MDIA after the addition of Fe ions (0.4 mM) are
2+
3+
(
shown by the green bars (excitation at 305 nm).
Because Tb-MDIA showed good water stability under harsh
conditions and the pH did not affect the fluorescent emission
of the terbium(III) ion, we further studied the sensing
intensity after the addition of metal ions, KSV is the Stern−
Volmer constant, and [M] is the molar concentration of the
4
−1
metal ion. A KSV value of 1.6 × 10 M was calculated
3
+
behavior of Tb-MDIA for metal ions in water. When Fe
according to the fluorescence titration data in Figure 6a; this
3
+
ions were added to an aqueous solution of Tb-MDIA (0.33 g/
L), the fluorescence intensity was gradually quenched. The
typical terbium-based fluorescence of >99% was quenched
value is comparable to those of organic sensors for Fe
determination in solution (typical K values of approximately
SV
4
−1 47
10 M ).
3
+
when Fe at a concentration of approximately 0.4 mM was
added to the Tb-MDIA water emulsion. To the naked eye, the
visible green light emitted from the original Tb-MDIA MOF
disappeared after Fe3 was added under UV radiation (Figure
To investigate whether Tb-MDIA exhibited selective sensing
3
+
for Fe , fluorescent detection experiments were performed
with other metal ions, including heavy-metal, alkali, transition-
metal, alkaline-earth, and common interfering trivalent ions. As
shown in Figure 6b, no significant spectral changes were
observed for Tb-MDIA in the presence of alkali-metal cations
+
4
3
6
a, inset). On the basis of the 3δ IUPAC criteria, the
3+
detection limit of Tb-MDIA toward Fe was calculated to be
+
+
+
2+
2+
2+
below 90 nM, which is much less than the national standard
(Li , Na , and K ). The alkaline-earth cations (Ba , Sr , Ca ,
3
+
2+
2+
2+
2+
2+
for Fe in drinking water set by the Ministry of Environmental
and Mg ) and transition-metal ions (Cu , Fe , Ni , Cd ,
44,45
2+
2+
2+
2+
Protection of People’s Republic of China.
Furthermore, we
Co , Hg , Mn , and Zn ) led to a weak decrease in the
3
+
observed an excellent linear relationship between the
fluorescence intensity of Tb-MDIA and the Fe3 concentration
in the range of 0−34 μM, indicating that Tb-MDIA has good
florescence intensity. The addition of trivalent cations Cr and
+
3+
Al respectively resulted in 18.0% and 9.0% fluorescence
quenching of the typical terbium(III)-based emission of Tb-
3
+
potential as a sensor to quantitatively determine Fe . The
luminescence titration results could also be treated with the
MDIA. These results demonstrate that Tb-MDIA can
3
+
selectively sense Fe . To further confirm the selectivity of
4
6
3+
Stern−Volmer equation: I /I = 1 + K [M], where I is the
Tb-MDIA for Fe in the presence of other metal ions,
0
SV
0
original emission intensity of Tb-MDIA, I is the emission
competition experiments were conducted. The addition of
E
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Inorganic Chemistry
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3
+
different metal ions did not significantly affect the fluorescence
intensity of Tb-MDIA, and strong quenching was observed
with the peaks of Tb-MDIA treated with Fe for 24 h. These
3
+
results confirm that the sensing of Fe by Tb-MDIA was
attributed to exchange of the central framework cations by the
target cations, resulting in a change in the luminescence.
Detection of Nitro Explosives Based on Tb-MDIA. The
luminescence responses of Tb-MDIA to various aromatic
compounds were investigated by the addition of PA,
nitrobenzene (NB), 2,4-dinitrotoluene (DNT), aniline (A),
2,4,6-trinitrotoluene (TNT), toluene (T), and benzene (B) at
concentrations of 0.33 mM in ethanol. As shown in Figure 8b,
3+
2+
2+
after the addition of Fe ; even 10 equiv of Ca and Mg did
3+
not interfere with the Fe -induced fluorescence response. We
further performed sensing experiments with Tb-MDIA under
simulated physiological conditions (20 mM HEPES aqueous
48−50
3+
buffer solution, pH 7).
As the Fe concentration in the
HEPES buffer solution increased, the luminescence intensity of
Tb-MDIA decreased dramatically, as observed in an aqueous
4
−1
solution; the calculated KSV was 1.73 × 10 M (Figure S11).
To determine why Tb-MDIA could detect iron(III) ions
with high selectivity based on florescence quenching, the XRD
3
+
patterns of Tb-MDIA were monitored at different Fe
exchange times. As shown in Figure 7a, the XRD pattern of
Figure 7. (a) XRD patterns of Tb-MDIA simulated from single-
3
+
crystal data, freshly prepared Tb-MDIA, Tb-MDIA treated with Fe
for 5 and 24 h, and as-synthesized Fe-MDIA. (b) ICP-MS results for
Tb-MDIA after treatment with Fe for 5 min and 5 h.
Figure 8. (a) Luminescent emission spectra of Tb-MDIA in ethanol
3
+
−1
(0.33 g L ) with added PA (0.33 mM). The inset shows that the
visible green light emitted from the original Tb-MDIA complex
disappeared after PA (0.33 mM) incorporation under 365 nm UV
radiation. (b) Fluorescence responses of Tb-MDIA to different
selected analytes (0.33 mM). The inset shows the molecular
structures of the analytes. Excitation was at 305 nm.
Tb-MDIA treated with Fe3+ for 24 h was significantly different
from the pattern of untreated Tb-MDIA, implying a possible
change in the crystal structure. Meanwhile, 5 mg of Tb-MDIA
was dispersed in 10 mL of a Fe3 solution with a concentration
+
of 0.3 mM, and the concentrations of Tb3 and Fe in the
+
3+
filtrate were determined by ICP-MS after 5 min and 5 h. The
the fluorescence intensity of a Tb-MDIA suspension hardly
changed with the addition of A, T, and B. However, a
significant quenching of the emission intensity was observed in
the presence of PA, TNT, DNT, and NB, with the quenching
efficiency decreasing in the following order PA > TNT > DNT
> NB. In this case, the fluorescence quenching of Tb-MDIA
originated from photoinduced electron transfer from the
excited state of the complex to the NAC. Because PA
exhibited the most intense quenching effect, the sensing
performance of Tb-MDIA for PA was further investigated
through fluorescence titration experiments. When up to 0.33
mM PA was added to an ethanol suspension of Tb-MDIA
3+
Tb concentration in the filtrate increased from 3.2 μM (5
3+
min) to 67.2 μM (5 h), whereas the added Fe concentration
increased from 0.28 mM (5 min) to 24.8 μM (5 h; Figure 7b).
3+
Obviously, the amount of Fe was decreased in solution; on
the contrary, large amounts of the Tb3 component were
+
3
+
monitored in solution. These results indicate that Fe was
adsorbed on/interacted with the ligands via uncoordinated
carboxyl groups. Thus, the cation exchange of central cations
with target cations in the framework was the main factor in the
fluorescence quenching for Fe3 detection. To confirm this
cation-exchange mechanism, an iron-based solid (Fe-MDIA)
was synthesized using the same methods as those used to
prepare Tb-MDIA but with Fe(NO ) ·9H O instead of
+
−
1
(0.33 g L ), the quenching efficiency reached >98% (Figure
8a). The limit of detection for PA was approximately 5 × 10−
M according to the 3δ IUPAC criteria, indicating that Tb-
MDIA has good sensitivity for PA as a turn-off luminescent
7
3
3
2
Tb(NO ) ·6H O as the raw material. Interestingly, the new
3
3
2
peaks in the powder XRD pattern of Fe-MDIA matched well
F
DOI: 10.1021/acs.inorgchem.9b02177
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
sensory material. Fitting the data to the linear Stern−Volmer
−
1
equation I /I = 1 + K [PA] gave K = 63708 M ,
0
SV
SV
51
comparable to the values of known organic polymers. The
visible green light emitted by the original Tb-MDIA complex
disappeared after the addition of PA under UV radiation.
Importantly, the XRD pattern of bulk Tb-MDIA after PA
detection matched that of the original complex, further
confirming the stability of Tb-MDIA during PA detection
(Figure S13). The low detection concentration, naked-eye
visualization, and high quenching coefficient for PA indicate
that Tb-MDIA is capable of easily identifying trace amounts of
PA explosives in solution.
Interestingly, the Tb-MDIA framework exhibited different
detection performances for PA and TNT; the quenching
efficiency for PA was nearly twice that for TNT. In the
structures of PA and TNT, the only difference is that PA has a
hydroxyl substituent, whereas TNT has a methyl substituent.
These different fluorescence quenching mechanisms for TNT
and PA might originate from supramolecular interactions (e.g.,
hydrogen bonding) between the Tb-MDIA framework and
hydroxyl groups in the PA molecules. To confirm the specific
function of the hydroxyl groups in PA sensing, other hydroxyl-
containing nitro aromatic moieties with similar structural
characteristics, o-nitrophenol (o-NP), m-nitrophenol (m-NP),
and p-nitrophenol (p-NP), were chosen for fluorescence
titration experiments. The quenching effect for p-NP was
almost the same as that for PA, while m-NP and o-NP led to
much smaller fluorescence changes than p-NP. These results
indicate that only NPs with suitable configurations facilitate
the formation of hydrogen bonds between the hydroxyl groups
Figure 9. FT-IR spectra of p-NA (top), Tb-MDIA after absorbing p-
NA (middle), and Tb-MDIA (bottom).
evidence for the selective detection of PA based on the
combined effects of hydrogen-bonding interactions, electron
transfer, and size matching.
Catalytic Properties Based on Ho/Nd-MDIA. Because
Ho-MDIA had large channels and the holmium ions with
removable coordinated water molecules were well positioned
within the channels, the substrate molecules could easily
approach the unsaturated metal sites. Thus, the catalytic
performance of Ho-MDIA was evaluated for the cyanosilyla-
tion reaction of aldehydes. The Ho-MDIA catalyst (2 mol %),
selected aromatic aldehyde, and cyanotrimethylsilane in a 1:2.4
molar ratio were employed at room temperature in CH Cl
2
2
and framework. Furthermore, nitro aromatics with the −NH
2
through a heterogeneous manner. As shown in Table 2, almost
(
the potential hydrogen-bond donor) and −CH moieties
3
were chosen to replace the −OH groups, the amino
substituent molecular p-nitroaniline (p-NA), m-nitroaniline
Table 2. Conversion for Cyanosilylation of the Carbonyl
Substrates in the Presence of Ho-MDIA and Nd-MDIA in a
(m-NA), and o-nitroaniline (o-NA) and the methyl substituent
a
Heterogeneous Manner
molecular p-nitrotoluene (p-NT), m-nitrotoluene (m-NT), and
o-nitrotoluene (o-NT) were analyzed as guest molecules in the
same fluorescence titration condition. The NAs and NPs
exhibited similar responses, and the NT moiety had a
quenching effect similar to that of NB. These results further
demonstrate that the formation of hydrogen bonds at the para
sites of the nitro groups was critical for the highly sensitive
detection of explosives.
entry
Ar
Ho-MDIA
Nd-MDIA
1
2
3
4
5
phenyl
4-nitrophenyl
4-methoxyphenyl
1-naphthyl
3,5-bis(tert-butyl)phenyl
97.8
93.6
87.3
40.9
4.2
59.1
35.4
34.1
26.6
n.d.
In contrast, the bulky NP HNBP, which has a larger channel
compared to Tb-MDIA, exhibited a fluorescence quenching of
only 5% under the same conditions. This size-selective
detection indicates that the sensing of PA took place mostly
in the MOF channel rather than on its external surface. To
further confirm this result, Tb-MDIA after immersion in a p-
NA ethanol solution was analyzed by FT-IR spectroscopy,
showing that some peaks of p-NA were red-shifted. As shown
in Figure 9, the N−H asymmetric stretching vibration of p-NA
a
Reaction conditions: (CH ) SiCN (1.2 mmol), aldehyde (0.5
3
3
mmol), and Ho-MDIA and Nd-MDIA (2 mol %) at room
1
temperature for 6 h in 2 mL of CH Cl . Determined by H NMR
2
2
based on the carbonyl substrate.
complete conversion of 2-phenyl-2-(trimethylsilyloxy)-
acetonitrile was observed corresponding to benzaldehyde. In
−
1
−1
34
from 3558.1 cm (free p-NA) to 3540.3 cm , the N−H
contrast to previously reported Yb-MOFs (yield = 57%, 5 h),
−
1
stretching vibration of PA from 3481.2 cm (free p-NA) to
our MOF achieved a higher conversion of benzaldehyde, and
the catalytic efficiency (97.8% after 6 h at room temperature)
was comparable to those of the previously reported MOF-
based catalysts for the same reaction under similar conditions
(Table S4). In the case of 3,5-bis(tert-butyl)benzaldehyde,
which is larger in size, the yield was lower. The conversion
decreased in the following order: benzaldehyde > 4-nitro-
benzaldehyde > 4-methoxybenzaldehyde > 1-naphthaldehyde
> 3,5-bis(tert-butyl)benzaldehyde. This ordering may be
related to the significant size-selectivity effect. To confirm
−
1
3
461.5 cm , and the N−H scissoring vibration from 1634.8
−1 −1
cm (free p-NA) to 1615.7 cm confirmed that p-NA was
absorbed in the channels of Tb-MDIA and that hydrogen
5
2,53
bonds were formed between p-NA and Tb-MDIA.
Because hydrogen bonding is important for PA detection, p-
aminophenol (p-AP), which has rich hydrogen-bonding sites,
was also evaluated. Under the same experimental conditions,
almost no luminescence changes in Tb-MDIA were observed
upon the addition of p-AP. These results provide strong
G
DOI: 10.1021/acs.inorgchem.9b02177
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
that the catalytic performance was, in fact, attributable to the
size selectivity, Nd-MDIA, which possesses small one-dimen-
sional channels, was evaluated as a catalyst for comparison. As
shown in Table 2, the catalytic ability of Nd-MDIA for the
cyanosilylation reaction of aldehydes was much lower than that
of Ho-MDIA, indicating that possible activation of the
substrate occurred within the channels of Ho-MDIA with
size selectivity. In particular, the reaction was completely shut
down when Ho-MDIA was removed by filtration, and only 7%
additional conversion was afforded upon continued stirring for
another 4 h (Figure S15). These results support the
heterogeneous catalytic nature of Ho-MDIA. Reutilization of
Ho-MDIA was realized easily through its simple centrifugation
and filtration from the reaction suspension, resulting in only a
slight decrease in the reactivity after three reuses (Figure S16).
The powder XRD patterns of the recycled catalysts were nearly
identical with those of the fresh material; therefore, their
original structural frameworks were retained during the
catalytic process (Figure S17).
AUTHOR INFORMATION
■
*
Corresponding Author
Email: wjian@jsnu.edu.cn.
ORCID
Jian Wang: 0000-0002-0282-3911
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Natural Science
Foundation of Jiangsu Province for Outstanding Youth (Grant
BK20180105), National Natural Science Foundation of China
(Grants 21401087 and 21401086), and Postgraduate Research
& Practice Innovation Program of Jiangsu Province.
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