Inorganic Chemistry
Article
structure along the a-axis, which can be seen as a network
structure (Figure 2b). Along the b-axis, the structure is a one-
dimensional (1D) chain (Figure 2c). The size of the hole is
energy were further analyzed by XPS. The full spectrum of
SiW Ni -DPNDI shows the presence of C, O, N, Si, W, and Ni
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elements (Figure 4a). The deconvolution of the C 1s spectrum
into four peaks of binding energy 288.1, 285.6, 284.9, and
284.3 eV correspond to the CO, C−N, C−C, and C−H
1
8.14 Å × 8.43 Å (atom-to-atom distance). The 2D structure
further stacked to form a three-dimensional (3D) network
36
structure by a π···π interaction effect. The accessible pores are
bonds, respectively (Figure 4b). Meanwhile, the two peaks of
nitrogen spectra at 400.4 and 399.6 eV represent the C−N
bond of DPNDI. The other peak at 398.9 eV represents the
3
∼
6883.6 Å (39.3%) calculated from a PLATON analysis,
suggesting the possibility to absorb suitable substrates within
the channels of the SiW Ni -DPNDI. In the structure, the
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37
Ni−N bond (Figure 4c). The signal of Si 2p exhibits one
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4
+
anion···π interactions and coordination bonds between
peak for Si at binding energy 101.7 eV (Figure 4d). The W 4f
SiW Ni and DPNDI are beneficial to the electron separation
signal exhibits two peaks at 37.5 eV (4f ) and 35.4 eV (4f7/2)
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5/2
6
+
38
and transfer. Note that the POMOF containing SiW Ni units
that are assigned to the W environment (Figure 4e). The
peaks centered at 873.3 and 885.5 eV are ascribed to the
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had remained unexplored at the present time.
The phase purity of SiW Ni -DPNDI was confirmed by
2
+
2+
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bivalent state Ni 2p1/2 and Ni 2p3/2 with two satellite peaks
39
powder X-ray diffraction (PXRD) patterns (Figure S2). The
located at 879.8 and 861.6 eV, respectively (Figure 4f).
SiW Ni -DPNDI contains a Ni cluster, which shows a
Fourier transform infrared (FTIR) spectra of SiW Ni -DPNDI
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and DPNDI are shown in Figure S3. For the comparison of
two spectra, the characteristic peak of DPNDI of SiW Ni -
ferromagnetic exchange interaction. This has been explored in
40−42
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similar structures.
Therefore, the ferromagnetic exchange
DPNDI shows a slightly blue shift due to the interaction
between metal ions and DPNDI. Both spectra display strong
interaction might exist in the SiW Ni -DPNDI. In order to
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verify this speculation, the magnetic property of SiW Ni -
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−
1
peaks from 1700 to 1580 cm . They correspond to CO,
DPNDI was preliminary explored. The field-dependent
magnetization curve M(H) of SiW Ni -DPNDI at 2 K
N−H, and C−H bond stretching vibrations of the DPNDI
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−
1
ligand, and the peaks between 1450 and 1175 cm are
represented a continuous increase (Figure 5a). The value of
29.39 Nβ at ca. 70 kOe tends to reach the saturation state. The
magnetic susceptibility (χ ) of SiW Ni -DPNDI was also
ascribed to the C−N bond stretching vibrations of the DPNDI
3
3
ligand. In the spectrum of SiW Ni -DPNDI, the strong peak
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m
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−
1
at 3423 cm corresponds to the bending vibration mode of
measured from 1.8 to 300 K under an external magnetic field
of 1000 Oe. The plots of χ versus T and χ T versus T are
−
1
H O. The peaks at 1041 and 941 cm correspond to Si−O
2
m
m
and W−O stretching vibrations. The peaks at ∼881, 776, and
shown in Figure 5b. At 300 K, the χ value of SiW Ni -DPNDI
t
m 9 6
was 0.0852 emu mol . When cooled, the χ value slowly
m
−
1
34
−1
7
00 cm are ascribed to the W−O −W stretching vibration.
b
−
1
The thermogravimetric analysis (TGA) curve of SiW Ni -
increased to 0.6509 emu mol at 50 K. Afterward, the χ
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m
value quickly reached a maximum of 6.606 emu mol− at 1.8 K.
1
DPNDI is shown in Figure S4. The weight loss in the
temperature range of 30−1000 °C corresponds to the removal
of 2 lattice water molecules, 8 protons, and 18 coordinate
water molecules (observed, 7.73%; calcd, 7.25%). The
remaining stage in the range of 400−920 °C might be
attributed to the loss of four DPNDI ligands and POM anion
−
1
The χ T value of SiW Ni -DPNDI was 25.56 emu mol K at
m
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3
00 K; thereafter, the value gradually increased to a maximum
−
1
of 40.14 emu mol K at 16 K. Then, the value decreased
sharply to 11.89 emu mol K at 1.8 K. The sudden decrease
−
1
might be because of the presence of the zero-field splitting
3
5
43
skeleton decomposition. This result indicates the high
thermal stability of SiW Ni -DPNDI, which plays an important
effect. Moreover, the plot of 1/χm versus T obeyed the
Curie−Weiss equation at the temperature of 1.8−300 K with a
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3
−1
positive Curie constant C = 24.44 cm K mol and Weiss
constant θ = 3.637 K (Figure S6). These results indicated the
2
+
ferromagnetic coupling between the Ni ions of SiW Ni -
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DPNDI.
The UV−vis spectrum of SiW Ni -DPNDI exhibited broad
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and strong absorption peaks between 200 and 800 nm, which
indicated its potential application in photocatalysis (Figure 6a).
The absorption peaks at 365, 390, and 411 nm correspond to
the DPNDI ligand. The absorption peak at 680 nm
44
corresponds to the charge transfer of DPNDI to POMs.
With the white light-emitting diode (LED) irradiation of 20
min, the absorbance peak intensity increased. After pyrrolidine
was added and the sample was irradiated for another 10 min,
the peaks at ∼504, 680, and 765 nm increased clearly, and a
Figure 3. Morphology images of a single crystal of SiW Ni -DPNDI.
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new peak at ∼610 nm appeared. That corresponded to the
(
a) Scanning electron microscopy image. (b−g) Elemental mappings
28
generation of DPNDI* and POM . Meanwhile, the
red
of Ni, O, W, N, and Si, respectively.
fluorescence of SiW Ni -DPNDI changed dramatically (Figure
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c), which should be attributed to the electron transfer from
images of a single crystal of SiW Ni -DPNDI, which shows a
the excited DPNDI ligand. Figure 6d indicated that the
emission intensities of SiW Ni -DPNDI also decreased with
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blocky structure. The corresponding element mapping
indicates the uniform distribution of Ni, O, W, N, and Si of
SiW Ni -DPNDI. These results further confirm the composi-
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the addition of pyrrolidine amounts. Therefore, there might
have been a sequential electron transfer process between the
SiW Ni -DPNDI and pyrrolidine during the irradiation.
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tion of SiW Ni -DPNDI.
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The elemental composition of SiW Ni -DPNDI and
oxidation states of constituent elements along with binding
According to a Tauc plot, the bond-gap value estimated of
SiW Ni -DPNDI was 2.79 eV (Figure S7). To verify the
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1
0024
Inorg. Chem. 2021, 60, 10022−10029