One-Pot Synthesis of Schiff Bases by Defect-Induced TiO2- x-Catalyzed Tandem Transformation from Alcohols and Nitro Compounds

Article abstract of DOI:10.1021/acs.inorgchem.1c01406

Schiff bases that are generally formed from condensation reactions of aldehydes (or ketones) and amino groups could also be produced by a photodriven one-pot tandem reaction between alcohols and nitro compounds, in our case. Herein, TiO2-x porous cages derived from NH2-MIL-125 by a self-sacrificing template route are used to study the organic transformation and exhibit 100% conversion efficiency of nitrobenzene and 100% selectivity for Schiff bases in the system of benzyl alcohol (5 mL) and nitrobenzene (41 μL) upon light irradiation, but hydrogen by dehydrogenation of benzyl alcohol cannot be detected. Successful occurrence of the organic transformation is mainly attributed to Ti(III)-oxygen vacancy associates. Surface oxygen vacancy-related Ti(III) sites are responsible for binding with nitro groups, and low-coordinated Ti5c sites selectively adsorb hydroxyl groups of benzyl alcohol. The Ti(III) and oxygen vacancy associates capture photogenerated electrons for achievement of multielectron reduction of nitrobenzene and the subsequent Schiff base condensation reaction with the as-formed benzaldehyde.

Full text of DOI:10.1021/acs.inorgchem.1c01406

pubs.acs.org/IC
Article
One-Pot Synthesis of Schiff Bases by Defect-Induced TiO2−x-
Catalyzed Tandem Transformation from Alcohols and Nitro
Compounds
Jing Tong,∥ Jinfeng Wang,∥ Xiaoshuang Shen, Hui Zhang, Yao Wang, Qiang Fang, and Liyong Chen*
Cite This: Inorg. Chem. 2021, 60, 10715−10721
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Schiff bases that are generally formed from condensation reactions of aldehydes (or
ketones) and amino groups could also be produced by a photodriven one-pot tandem reaction between
alcohols and nitro compounds, in our case. Herein, TiO2−x porous cages derived from NH2-MIL-125 by a
self-sacrificing template route are used to study the organic transformation and exhibit 100% conversion
efficiency of nitrobenzene and 100% selectivity for Schiff bases in the system of benzyl alcohol (5 mL)
and nitrobenzene (41 μL) upon light irradiation, but hydrogen by dehydrogenation of benzyl alcohol
cannot be detected. Successful occurrence of the organic transformation is mainly attributed to Ti(III)−
oxygen vacancy associates. Surface oxygen vacancy-related Ti(III) sites are responsible for binding with
nitro groups, and low-coordinated Ti5c sites selectively adsorb hydroxyl groups of benzyl alcohol. The
Ti(III) and oxygen vacancy associates capture photogenerated electrons for achievement of multielectron
reduction of nitrobenzene and the subsequent Schiff base condensation reaction with the as-formed
benzaldehyde.
xploration of green synthetic strategies for preparation of
fine chemicals is the fundamental need for supporting the
bases under light irradiation.7−10 However, some fundamental
E
issues related to selectivity and activity are still required to
further clarify: (i) how to suppress H2 generation from
photolysis of alcohols and facilitate proton transfer for coupling
of multielectron reduction of nitro compounds in the
photoredox reaction and (ii) the role of Lewis acidity of
metal ions in the Schiff base condensation reaction.
sustainable development of society. In contrast to traditional
organic synthesis that is carried out under harsh conditions,
such as high temperature and high pressure, resulting in too
much energy consumption and massive pollutant release,
photodriven organic transformation under mild conditions is
considered as an environmentally friendly process. Schiff base
reactions are one of the classical organic reactions that occur
between aldehydes (or ketones) and amino groups to produce
imine groups1,2 and are widely used in chemistry and materials
science to prepare fine chemicals, metal complexes, covalent
organic frameworks, or modify surfaces of materials.3−6
However, aldehydes and amino compounds are generally
produced by thermal reactions of the corresponding alcoholic
and nitro compounds with transition metal-based oxidants/
catalysts, leading to the release of heavy-metal pollutants.
Hence, development of a photodriven alternative strategy by
direct utilization of alcohols and nitro compounds as feedstock
for generation of Schiff base compounds via a one-pot process
possibly brings some comparative advantages of saving energy
and reducing pollution. To achieve the synthetic strategy, an
eligible catalytic system that involves in a one-pot tandem
reaction, including redox reactions between alcohols and nitro
compounds to aldehydes and amino compounds, respectively,
and subsequent Schiff base condensation reactions of
aldehydes and amino compounds, is needed to exploit.
Previously, some efforts have been devoted to the study of
inorganic semiconductors for achievement of the organic
transformation from alcohols and nitro compounds to Schiff
Based on its high chemical stability, nontoxicity, and low
price, TiO2 is considered as one of the sophisticated
photocatalysts, but the photophysical properties of rapid
recombination of photogenerated charge carriers greatly
lower its photocatalytic activity.11,12 Construction of surface
defect states becomes an effective way for electron trapping to
prolong the excited electron lifetime.13−15 TiO2 with oxygen
vacancy-related point defects (TiO2−x) is generally prepared by
thermal treatment of its precursors under a reducing
atmosphere or direct introduction of Ti(III) species, but
there are few reports on the synthesis of TiO2−x in ambient
atmosphere.16−19 In this research, we attempt to develop a
novel, two-step route for the synthesis of porous cages of
TiO2−x(PC-TiO2−x_T) with bridge-bonded oxygen vacancies
by thermal treatment of titanium oxide porous cages derived
Received: May 8, 2021
Published: June 29, 2021
https://doi.org/10.1021/acs.inorgchem.1c01406
© 2021 American Chemical Society
Inorg. Chem. 2021, 60, 10715−10721
10715

Inorganic Chemistry
pubs.acs.org/IC
Article
from NH2-MIL-125(Ti) with an fcu topological structure in
air.20 In PC-TiO2−x_T, T represents the temperature of
thermal treatment. PC-TiO2−x_590, a typical PC-TiO2−x
sample, exhibited the enhanced catalytic performance for the
organic transformation in the reaction system of benzyl alcohol
(5 mL) and nitrobenzene (41 μL) under UV−vis light
irradiation with 100% selectivity to the Schiff base at 100%
conversion of nitrobenzene. The average conversion rate is
about 22.2 mmol h−1 g−1, which is superior to those of other
catalysts reported previously, and thus more favorably lowers
energy consumption and reduces pollutant release during
preparation of Schiff base-related fine chemicals.7−10 This is
the reason for the existence of Ti(III)−oxygen vacancy
associates with appropriate concentrations,21−23 facilitating
capture of photogenerated electrons for nitrobenzene reduc-
tion, as well as porous hollow structures with a high mass
transfer rate. A good understanding of the catalytic process
contributes significantly to the design and construction of high-
performance catalysts for the synthesis of Schiff base materials
by the organic transformation of alcohols and nitro
compounds.
Figure 1. (a, b) and (d, e) TEM and SEM images of anatase PC-
TiO2−x_590 with different magnifications, respectively, (c) HRTEM
image of a TiO2−x nanoparticle and a corresponding fast-Fourier
transform diffraction pattern in the inset of (c), and (f) HAADF-
STEM image/elemental mappings of PC-TiO2−x_590 and the
crystalline structure of anatase PC-TiO2−x_590 in the lower left
corner of (f).
A typical method was used to prepare polyhedral NH2-MIL-
125(Ti) nanocrystals with diameters of ∼800 nm, which were
characterized by X-ray diffraction (XRD) patterns (Figure
S1c), and scanning electron microscopy (SEM) and trans-
mission electron microscopy (TEM) images (Figure
S1a,b).20,24 The as-made NH2-MIL-125(Ti) was dispersed
into an ethanolic solution of L-alanine to prepare porous cages
via a solvothermal route. The porous cages assembled with
nanosheets are clearly observed from TEM and SEM images
(Figure S2), and these nanosheets correspond to titanium
oxide that was confirmed by the XRD pattern (Figure S3, black
curve). The similarity in shape and size between NH2-MIL-
125(Ti) and titanium oxide revealed that the morphology was
inherited by a dissolution and recrystallization process.20 To
prepare TiO2, titanium oxide porous cages were thermally
treated at 590 °C in air for 0.5 h and were completely
converted into anatase PC-TiO2−x_590 corresponding to the
tetragonal crystal structure with TiO6 octahedrons sharing the
edges, according to the XRD pattern (Figure S3, dark yellow
curve). From TEM and SEM images (Figure 1a−d), TiO2−x
nanoparticle-assembled porous cages were clearly observed,
signifying a small impact on the morphology in the thermal
treatment process. The magnified images revealed that the size
of nanoparticles was about 20−30 nm. As shown in Figure 1c,
lattice fringes with interplanar spacings of 0.35 and 0.19 nm
were assigned to the Eg(1), B1g, A1g, and Eg(3) modes of the
anatase phase (Figure 2a), respectively, and other Raman peaks
related to the rutile phase were not found, further signifying
that the sample consisted of anatase TiO2.20,25 X-ray
photoelectron spectroscopy (XPS) enable identification of
the type and the oxidation state of elements on the surfaces. A
high-resolution O 1s XPS spectrum revealed the presence of
lattice oxygen with a binding energy of 529.8 eV and an oxygen
vacancy at 531.65 eV with a small peak area (Figure 2c).26
A
high-resolution Ti 2p XPS spectrum clearly showed binding
energies of Ti 2p3/2 and Ti 2p1/2 peaked at 458.5 and 464.3 eV,
respectively, corresponding to Ti(IV) in PC-TiO2−x_590
(Figure 2b). Besides, weak peaks at lower binding energy
assigned to Ti(III) species were also found in the spectrum,
and the reason was possibly attributed to electron transfer from
oxygen vacancies to neighboring Ti(IV) atoms in PC-
TiO2−x_590.27,28 To achieve more information on the
structure of PC-TiO2−x_590, the electron paramagnetic
resonance (EPR) spectrum was carefully collected at 150 K.
A weak EPR signal at g = 1.946 and a remarkably strong EPR
signal at g = 2.002 corresponding to Ti(III) and oxygen
vacancies, respectively, were detected (Figure 2d), further
revealing the presence of Ti(III)−oxygen vacancy associates.29
A previous report on oxygen-vacancy-related point defects
were also produced via an aerobic annealing process although
most of the cases involved in oxygen vacancies are generally
formed under a reducing atmosphere through a thermal
treatment process.16−18
̅
with an interfacial angle of 90° correspond to the (101) and
(020) facets of anatase TiO2, respectively, in the high-
resolution TEM (HRTEM) image. A corresponding fast-
Fourier transform diffraction pattern (inset of Figure 1c)
further confirmed the results. The crystalline structure (Figure
1f, lower right) illustrated the atomic arrangement on the
The solid UV−vis spectrum of PC-TiO2−x_590 reveals that
it possesses the strong absorption ability in the wavelength
range of less than 400 nm (Figure 2e), which is similar to
optical properties of most of the other TiO2 reported
previously.11,30 A Tauc plot was used to estimate the optical
band gap of TiO2 by the equation (αhν)1/2 = A(hν − Eg)
according to the allowed indirect transition of anatase TiO2.
The optical band gap of PC-TiO2−x_590 was about 2.76 eV
(inset of Figure 2e). The edge of the maximum energy in the
valence-band photoemission spectrum from PC-TiO2−x_590
was 2.25 eV below the Fermi level (Figure 2f).
̅
(010)/(101) facets along the [101] axis. The high-angle
annular dark-field scanning transmission electron microscopy
(HAADF-STEM) image (Figure 1f, upper left) provided
important information on sample shapes because it is highly
sensitive to the thickness of materials. The difference of
contrast in the image between the edge and center clearly
demonstrated the formation of cagelike shapes. Elemental
mappings of Ti and O revealed that both elements were
homogeneously dispersed into the entire cages (Figure 1f).
To further validate the composites, Raman spectroscopy was
carried out. The Raman peaks at 146, 395, 515, and 636 cm−1
Hence, photoredox catalytic organic transformation of
alcohols and nitro compounds was performed under the full
10716
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 2. (a) Raman spectrum, high-resolution (b) Ti 2p and (c) O 1s XPS spectra, (d) EPR spectrum at 150 K, (e) UV−vis spectrum, and (f)
valence-band photoemission spectrum of PC-TiO2−x_590. The inset of (e) corresponds to the Tauc plot.
Figure 3. (a) Time-dependent catalytic performance of PC-TiO2−x_590 toward benzyl alcohol oxidation and nitrobenzene reduction for
subsequent Schiff base condensation, (b) catalytic performance of PC-TiO2−x_590 toward different nitrobenzene substrates and benzyl alcohol,
and (c) tandem organic transformation from benzyl alcohol and nitrobenzene to Schiff bases over different catalysts.
spectrum of Xe lamp (300 W) irradiation. In detail, PC-
TiO2−x_590 (1 mg) was added into a quartz glass reactor
containing benzyl alcohol (5 mL) and nitrobenzene (41 μL,
∼0.4 mmol) and then bubbled with Ar for 30 min to remove
the air completely before irradiation under a Xe lamp (300 W).
In the research, we chose the conversion of nitrobenzene as the
criterion to gauge the photocatalytic activity of TiO2. When
the irradiation time reached 18 h, PC-TiO2−x_590 gave a 100%
conversion efficiency of nitrobenzene based on quantitative gas
chromatography using an internal standard of toluene (Figure
S4). Time-dependent photocatalytic experiments revealed a
gradual increase of the conversion yield with prolonged
irradiation time. The average conversion rate was about 22.2
mmol h−1 g−1 (Figure 3a). The selectivity for Schiff bases was
as high as 100% in the final product. In the conversion process,
the product was nearly Schiff bases at different irradiation time
stages, and other side products were not formed. The excess
benzyl alcohol was reused for the Schiff base condensation
reaction without further purification, and a similar catalytic
result was obtained. If the amount of nitrobenzene in the
catalytic system was increased from 41 to 164 μL, the porous
cages of TiO2−x still exhibited high selectivity (95%) for Schiff
bases but the conversion efficiency of nitrobenzene was
gradually decreased to ∼80% (Table S1).
Owing to full spectra of the Xe lamp including the deep UV
light with high energy that possibly induces the direct
occurrence of the redox reaction between benzyl alcohol and
nitrobenzene, a parallel experiment without PC-TiO2−x_590
was performed to study Schiff base condensation. The
conversion efficiency of nitrobenzene was ∼18.3% for 18 h
under Xe lamp irradiation, but the imine compound was
almost not found. Hence, PC-TiO2−x_590 played an
indispensable role in the transformation process. The light
source with a 365 nm band-pass filter was applied in the
tandem reaction, and finally, a nearly 100% conversion yield of
nitrobenzene with 100% selectivity for the imine compound
was detected while TiO2_590 was introduced in the
photocatalytic system for 18 h. The quantum efficiency for
nitrobenzene conversion at 365 nm was about 1.99%.31
To further examine catalytic performance of PC-
TiO2−x_590, substrates were expanded to other nitro
compounds, including 2-nitrotoluene, 3-nitrotoluene, and 4-
nitrotoluene. In the catalytic system of benzyl alcohol (5 mL)
and the nitro compound (0.4 mmol), PC-TiO2−x_590 also
exhibited high activity (83, 91, and 100%, respectively) toward
10717
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721

Inorganic Chemistry
pubs.acs.org/IC
Article
Figure 4. PC-TiO2_630: (a) SEM and (b) TEM images, (c) XPS spectrum, (d) EPR spectrum, (e) UV−vis spectrum, and (f) valence-band
photoemission spectrum. To clearly compare the difference between PC-TiO2−x_590 and _630, the EPR spectrum of PC-TiO2−x_590 is also
shown in (d). The inset of (e) is a Tauc plot to estimate the optical band gap of PC-TiO2_630.
these tandem transformations with high selectivity (99, 100,
and 100%, respectively) for Schiff bases (Figure 3b).
catalytic activity to ∼76% conversion efficiency of nitro-
benzene (Figure S5).
The stability and reactivity of PC-TiO2−x_590 was studied
because it was a key index for the assessment of photocatalytic
performance of heterogeneous catalysts. PC-TiO2−x_590 was
directly reused without any treatment after catalysis toward the
photoredox between nitrobenzene and benzyl alcohol. The
experimental result showed that the conversion efficiency for
nitrobenzene was decreased to ∼88% under UV−vis light
irradiation (Figure S5). Furthermore, after six cycling runs, the
catalytic activity of PC-TiO2−x_590 was decreased to ∼25%
according to nitrobenzene conversion although the selectivity
for Schiff bases was maintained more than 95%. Structural
characterization revealed that PC-TiO2−x_590 was not
obviously damaged (Figure S6), and thus we speculated that
the experimental observation was possibly attributed to
shielding of catalytic sites by other guest species in the
catalytic process, leading to the loss of catalytic activity. Raman
spectroscopy of the used TiO2−x_590 was performed to
identify these guest species on the surfaces of PC-TiO2−x_590,
however, obvious Raman peaks assigned to −CN, −CO,
−C−OH, or −NO2 groups were absent in the Raman
spectrum (Figure S7a). Hence, the scenario for the decrease
of catalytic activity was ruled out. The used PC-TiO2−x_590
via six cycling runs showed weak EPR signals in the intensity
corresponding to oxygen vacancies or Ti(III) ions as compared
to the original PC-TiO2−x_590 (Figure S7b); even in the high-
resolution O 1s XPS spectrum obvious oxygen vacancies were
found with much difficulty (Figure S7c). The absence of
oxygen vacancy-related signals would be due to the self-healing
of surface lattice oxygen.22 On this basis, Ti(III)−oxygen
vacancy associates have an important effect on the main-
tenance of PC-TiO2−x_590 catalytic activity for the tandem
transformation. After the used PC-TiO2−x_590 (six times) was
thermally treated at 450 °C in a H2/Ar atmosphere, the EPR
signals of Ti(III) ions and oxygen vacancies appeared (Figure
S8a,b). As expected, the treated PC-TiO2−x_590 recovered its
The concentration of oxygen vacancies was affected by
temperature. If porous cages of titanium oxides were thermally
treated in air at a higher temperature of 630 °C, high-
crystalline porous cages of TiO2 (named PC-TiO2_630;
Figures 4a,b and S9) possessed lower concentration of oxygen
vacancies compared to PC-TiO2−x_590, which was confirmed
by XPS and EPR spectra (Figure 4c,d). PC-TiO2_630 showed
a slightly different solid UV−vis spectrum and valence-band
photoemission spectrum from PC-TiO2−x_590 (Figure 4e,f).
The optical band gap of PC-TiO2_630 was broadened to
∼2.87 eV (inset of Figure 4e) and the valence-band edge of the
maximum energy was increased to 2.32 eV below the Fermi
level (Figure 4f), suggesting that surface oxygen vacancies can
affect electronic structures of TiO2.32−34 Under the otherwise
identical conditions, PC-TiO2_630 displayed an inferior
photocatalytic performance with a nitrobenzene conversion
efficiency of 82% and nearly 99% selectivity for Schiff bases
(Figure 3c). The difference in catalytic activity between PC-
TiO2−x_590 and PC-TiO2_630 further reveals the importance
of oxygen vacancies in the tandem transformation. Note that a
very high concentration of oxygen vacancies was not favorable
for catalysis of the organic transformation, and thus if TiO2 was
prepared by thermal treatment of porous cages of titanium
oxide in H2/Ar, it showed inferior catalytic activity. The
possible reason is attributed to excessive oxygen vacancies that
can block the charge separation.22
In addition, by direct annealing of NH2-MIL-125(Ti) in air
at 590 °C for 30 min, solid TiO2 nanocrystals with both
anatase and the rutile phase (named TiO2_d; Figure S10) were
formed. TiO2_d gave a 63% average conversion efficiency of
nitrobenzene and 99% average selectivity for Schiff bases in the
catalytic organic transformation reaction of nitrobenzene and
benzyl alcohol (Figure 3c). Similarly, photocatalytic activity of
commercial P25 was inferior to that of PC-TiO2−x_590
(Figure 3c).
10718
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721

Inorganic Chemistry
pubs.acs.org/IC
Article
As for the organic transformation, nitrobenzene was reduced
into aniline by the acceptance of six electrons, and benzyl
alcohol was oxidized into benzaldehyde by the loss of two
electrons before the Schiff condensation reaction. Hence,
benzaldehyde (∼0.8 mmol) was still obtained after con-
densation (Figure S4). In contrast, H2 was not detected as the
gas product. The experimental observation reveals that
photocatalytic dehydrogenation of benzyl alcohol to produce
benzaldehyde and H2 over PC-TiO2−x_590 cannot occur. To
further confirm the assertion, a parallel experiment involving
the photocatalytic system of PC-TiO2−x_590 and benzyl
alcohol upon Xe lamp irradiation for 18 h was carried out,
and the product of benzaldehyde and H2 was not generated.
On these bases, the synergistic effect of nitrobenzene and
benzyl alcohol provided a reliable guarantee for the successful
achievement of the photoredox reaction that was a prerequisite
for the subsequent Schiff base condensation reaction.
In comparison with other semiconductors reported
previously, TiO2 in the research exhibited ultrahigh selectivity
for Schiff bases in the photoredox reaction between nitro-
benzene and benzyl alcohol. We speculated that Ti(VI) ions
could serve as Lewis acids to facilitate the reaction of Schiff
condensation. For this purpose, aniline (0.4 mmol) and
benzaldehyde (1.2 mmol) were added to benzyl alcohol (5
mL) with gentle stirring. The average conversion rate for Schiff
bases was about 0.24 mmol h−1. If PC-TiO2−x_590 (1 mg) was
introduced into the system, the average conversion rate for
Schiff bases from aniline and benzaldehyde was nearly 0.4
mmol h−1. The result suggests that Ti(IV) could serve as Lewis
acid sites to facilitate the condensation reaction. Hence, the
organic transformation is mainly controlled by the photoredox
reaction of benzene alcohol and nitrobenzene due to the
sluggish reaction rate compared to the Schiff base con-
densation reaction.
To provide insight into the catalytic activity of TiO2, the
photocurrent response of different TiO2 electrodes was
collected at an applied potential of 0.8 V (vs Ag/AgCl)
under Xe lamp (100 W) irradiation with an on/off switch. The
photoresponsive current was detected from all electrodes of
PC-TiO2−x_590, PC-TiO2_630, TiO2_d, and P25 with the
same loadings (Figure S11a). All of these electrodes showed
the photocurrent density and a more noticeable photocurrent
was measured from the PC-TiO2−x_590 electrode. Electro-
chemical impedance spectroscopy (ESI) is an important tool
for determination of charge transport behavior occurring at
electrode/electrolyte interfaces. The Nyquist plots, represent-
ing impedance spectra, were performed in 1 M KCl at a
potential of 1.1 V (vs Ag/AgCl). Generally, a part of the high-
frequency semicircle is shown in the Nyquist plot, correspond-
ing to charge transport at electrode/electrolyte interfaces
(Figure S11b), and thus the PC-TiO2−x_590 electrode showed
the lowest resistance of electron transfer among all electrodes.
On these basis of these facts, we proposed a defect-induced
catalytic mechanism that mainly involved in four steps in the
catalytic process (Figure 5). First, substrates of benzyl alcohol
and nitrobenzene preferentially adsorbed on unsaturated Ti5c
sites from the (101) facets by the coordination interaction
between O with lone-paired electrons and Ti with empty
orbitals.35,36 Moreover, bridge-bonded oxygen vacancies led to
higher activity of neighboring Ti5c for adsorption of substrate
molecules. Second, the bound hole−electron pairs were
generated during light irradiation. The excited electrons that
originated from the bound hole−electron pairs were easily
Figure 5. Proposed catalytic mechanism for generation of Schiff bases
from benzyl alcohol and nitrobenzene under UV−visible light
irradiation.
captured by Ti(III) sites due to surface defect states with lower
energy in terms of the conduction band.28,37,38 Third, holes
were transferred to the adsorbed benzyl alcohol, and electrons
were transferred from Ti(III) to the adsorbed nitrobenzene
molecules. The hydrogen-bonding interaction between oxygen
atoms from nitro groups of nitrobenzene molecules and
hydroxyl groups of benzyl alcohol molecules facilitated
cleavage of hydrogen−oxygen bonds and the subsequent
proton transfer from hydroxyl groups to nitro groups.8 At this
step, the formed hydroxyl groups or water molecules could be
immobilized on the bridge-bonded oxygen vacancies, resulting
in self-healing of defects. Finally, the formed aniline and
benzaldehyde were further catalyzed by Lewis acid Ti(VI) to
selectively generate Schiff bases.
In summary, we have developed a two-step synthetic
strategy for preparation of TiO2−x porous cages with oxygen
vacancies by thermal treatment of precursors of titanium oxide
nanosheet-assembled porous cages derived from NH2-MIL-
125(Ti) nanocrystals. The TiO2−x porous cages exhibit high-
performance catalytic activity for the organic transformation of
benzyl alcohol and nitrobenzene to Schiff bases with a 100%
conversion efficiency of nitrobenzene. Other nitrobenzene-
related products cannot be detected in the catalytic system.
The organic transformation mainly involves in two successive
reactions of the photoinduced redox reaction between benzyl
alcohol and nitrobenzene and subsequent Schiff base
condensation reactions of benzaldehyde and aniline. Achieve-
ment of the photoinduced redox reaction is determined by
Ti(III)−oxygen vacancy associates that act as “electron sinks”
to capture photogenerated electrons. The catalytic systems are
expanded to other nitro compounds and TiO2−x porous cages
still show high catalytic activity and high selectivity toward
Schiff base reactions. The present research offers a possibility
that some typical organic transformations can be achieved via a
novel green synthetic route to produce fine chemicals.
10719
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721

Inorganic Chemistry
pubs.acs.org/IC
Article
(5) Mavrogiorgou, A.; Baikousi, M.; Costas, V.; Mouzourakis, E.;
Deligiannakis, Y.; Karakassides, M. A.; Louloudi, M. Mn-Schiff base
modified MCM-41, SBA-15 and CMK-3 NMs as Single-Site
Heterogeneous Catalysts: Alkene Epoxidation with H2O2 Incorpo-
ration. J. Mol. Catal. A: Chem. 2016, 413, 40−55.
(6) Sulpizio, C.; Breibeck, J.; Rompel, A. Recent Progress in
Synthesis and Characterization of Metal Chalcone Complexes and
Their Potential as Bioactive Agents. Coord. Chem. Rev. 2018, 374,
497−524.
(7) Wu, Y.; Ye, X.; Zhang, S.; Meng, S.; Fu, X.; Wang, X.; Zhang, X.;
Chen, S. Photocatalytic Synthesis of Schiff Base Compounds in the
Coupled System of Aromatic Alcohols and Nitrobenzene Using
CdXZn1‑XS Photocatalysts. J. Catal. 2018, 359, 151−160.
(8) Yang, X.; Tao, H.; Leow, W. R.; Li, J.; Tan, Y.; Zhang, Y.; Zhang,
T.; Chen, X.; Gao, S.; Cao, R. Oxygen-Vacancies-Engaged Efficient
Carrier Utilization for the Photocatalytic Coupling Reaction. J. Catal.
2019, 373, 116−125.
(9) Zhang, Q.; Wang, J.; Ye, X.; Hui, Z.; Ye, L.; Wang, X.; Chen, S.
Self-Assembly of CdS/CdIn2S4 Heterostructure with Enhanced
Photocascade Synthesis of Schiff Base Compounds in an Aromatic
Alcohols and Nitrobenzene System with Visible Light. ACS Appl.
Mater. Interfaces 2019, 11, 46735−46745.
(10) Ye, X.; Chen, Y.; Ling, C.; Ding, R.; Wang, X.; Zhang, X.; Chen,
S. One-pot synthesis of Schiff base compounds via photocatalytic
reaction in the coupled system of aromatic alcohols and nitrobenzene
using CdIn2S4 photocatalyst. Dalton Trans. 2018, 47, 10915−10924.
(11) Meng, A.; Zhang, L.; Cheng, B.; Yu, J. Dual Cocatalysts in TiO2
Photocatalysis. Adv. Mater. 2019, 31, No. 1807660.
(12) Guo, Q.; Ma, Z.; Zhou, C.; Ren, Z.; Yang, X. Single Molecule
Photocatalysis on TiO2 Surfaces. Chem. Rev. 2019, 119, 11020−
11041.
(13) Zhou, Y.; Zhang, Z.; Fang, Z.; Qiu, M.; Ling, L.; Long, J.; Chen,
L.; Tong, Y.; Su, W.; Zhang, Y.; Wu, J. C. S.; Basset, J.-M.; Wang, X.;
Yu, G. Defect Engineering of Metal-Oxide Interface for Proximity of
Photooxidation and Photoreduction. Proc. Natl. Acad. Sci. U.S.A.
2019, 116, 10232−10237.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01406.
■
sı
Experimental methods and characterization; additional
details of the structure and morphology analysis;
photocatalytic product analysis; and photocatalytic
performance characterization (word) (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Liyong Chen − Department of Pharmaceutical Engineering,
Bengbu Medical College, Bengbu, Anhui 233030, P. R.
China; State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, P. R. China;
orcid.org/0000-0002-6735-9195; Email: lychen@
bbmc.edu.cn
Authors
Jing Tong − Department of Pharmaceutical Engineering,
Bengbu Medical College, Bengbu, Anhui 233030, P. R. China
Jinfeng Wang − State Key Laboratory of Fine Chemicals,
Dalian University of Technology, Dalian 116024, P. R.
China
Xiaoshuang Shen − School of Physical Science & Technology,
Yangzhou University, Yangzhou 225002, P. R. China
Hui Zhang − State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, P. R. China
Yao Wang − State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Dalian 116024, P. R. China
Qiang Fang − Department of Pharmaceutical Engineering,
Bengbu Medical College, Bengbu, Anhui 233030, P. R. China
(14) Di, J.; Xia, J.; Chisholm, M. F.; Zhong, J.; Chen, C.; Cao, X.;
Dong, F.; Chi, Z.; Chen, H.; Weng, Y.-X.; Xiong, J.; Yang, S.-Z.; Li,
H.; Liu, Z.; Dai, S. Defect-Tailoring Mediated Electron-Hole
Separation in Single-Unit-Cell Bi3O4Br Nanosheets for Boosting
Photocatalytic Hydrogen Evolution and Nitrogen Fixation. Adv.
Mater. 2019, 31, No. 1807576.
(15) Wang, M.; Wang, J.-Q.; Xi, C.; Cheng, C.-Q.; Zou, C.-Q.;
Zhang, R.; Xie, Y.-M.; Guo, Z.-L.; Tang, C.-C.; Dong, C.-K.; Chen, Y.-
J.; Du, X.-W. A Hydrogen-Deficient Nickel-Cobalt Double Hydroxide
for Photocatalytic Overall Water Splitting. Angew. Chem., Int. Ed.
2020, 59, 11510−11515.
(16) Wan, J.; Chen, W.; Jia, C.; Zheng, L.; Dong, J.; Zheng, X.;
Wang, Y.; Yan, W.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Defect
Effects on TiO2 Nanosheets: Stabilizing Single Atomic Site Au and
Promoting Catalytic Properties. Adv. Mater. 2018, 30, No. 1705369.
(17) Yi, D.; Lu, F.; Zhang, F.; Liu, S.; Zhou, B.; Gao, D.; Wang, X.;
Yao, J. Regulating Charge Transfer of Lattice Oxygen in Single-Atom-
Doped Titania for Hydrogen Evolution. Angew. Chem., Int. Ed. 2020,
59, 15855−15859.
(18) Zhang, W.; Cai, L.; Cao, S.; Qiao, L.; Zeng, Y.; Zhu, Z.; Lv, Z.;
Xia, H.; Zhong, L.; Zhang, H.; Ge, X.; Wei, J.; Xi, S.; Du, Y.; Li, S.;
Chen, X. Interfacial Lattice-Strain-Driven Generation of Oxygen
Vacancies in an Aerobic-Annealed TiO2(B) Electrode. Adv. Mater.
2019, 31, No. 1970367.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c01406
Author Contributions
∥J.T. and J.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grant No. 21671030), Research
Foundation of BBMC for Talented Scholars (Grant No.
15210001), and Natural Science Foundation of the Higher
Education Institutions of Anhui Province (Grant No.
KJ2020A1241).
REFERENCES
■
(1) Liu, X.; Manzur, C.; Novoa, N.; Celedon, S.; Carrillo, D.;
Hamon, J.-R. Multidentate Unsymmetrically-Substituted Schiff Bases
and Their Metal Complexes: Synthesis, Functional Materials
Properties, and Applications to Catalysis. Coord. Chem. Rev. 2018,
357, 144−172.
(19) Xu, M.; Yao, S.; Rao, D.; Niu, Y.; Liu, N.; Peng, M.; Zhai, P.;
Man, Y.; Zheng, L.; Wang, B.; Zhang, B.; Ma, D.; Wei, M. Insights
into Interfacial Synergistic Catalysis over Ni@TiO2‑x Catalyst toward
Water-Gas Shift Reaction. J. Am. Chem. Soc. 2018, 140, 11241−
11251.
(20) Gu, Z.; Chen, L.; Li, X.; Chen, L.; Zhang, Y.; Duan, C. NH2-
MIL-125(Ti)-derived porous cages of titanium oxides to support Pt-
Co alloys for chemoselective hydrogenation reactions. Chem. Sci.
2019, 10, 2111−2117.
(2) Jia, Y.; Li, J. Molecular Assembly of Schiff Base Interactions:
Construction and Application. Chem. Rev. 2015, 115, 1597−1621.
(3) Iscen, A.; Brue, C. R.; Roberts, K. F.; Kim, J.; Schatz, G. C.;
Meade, T. J. Inhibition of Amyloid-beta Aggregation by Cobalt(III)
Schiff Base Complexes: A Computational and Experimental
Approach. J. Am. Chem. Soc. 2019, 141, 16685−16695.
(4) Beuerle, F.; Gole, B. Covalent Organic Frameworks and Cage
Compounds: Design and Applications of Polymeric and Discrete
Organic Scaffolds. Angew. Chem., Int. Ed. 2018, 57, 4850−4878.
10720
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721

Inorganic Chemistry
pubs.acs.org/IC
Article
(21) Tang, H.; Cheng, Z.; Dong, S.; Cui, X.; Feng, H.; Ma, X.; Luo,
B.; Zhao, A.; Zhao, J.; Wang, B. Understanding the Intrinsic Chemical
Activity of Anatase TiO2(001)-(1 ×4) Surface. J. Phys. Chem. C 2017,
121, 1272−1282.
(22) Zhang, Y.; Xu, Z.; Li, G.; Huang, X.; Hao, W.; Bi, Y. Direct
Observation of Oxygen Vacancy Self-Healing on TiO2 Photocatalysts
for Solar Water Splitting. Angew. Chem., Int. Ed. 2019, 58, 14229−
14233.
(23) Henrich, V. E.; Dresselhaus, G.; Zeiger, H. J. Observation of 2-
Dimensional Phases Associated with Defect States on Surface of TiO2.
Phys. Rev. Lett. 1976, 36, 1335−1339.
(24) Khaletskaya, K.; Pougin, A.; Medishetty, R.; Rosler, C.; Wiktor,
C.; Strunk, J.; Fischer, R. A. Fabrication of Gold/Titania Photo-
catalyst for CO2 Reduction Based on Pyrolytic Conversion of the
Metal-Organic Framework NH2-MIL-125(Ti) Loaded with Gold
Nanoparticles. Chem. Mater. 2015, 27, 7248−7257.
(25) Ohsaka, T.; Izumi, F.; Fujiki, Y. Raman-Spectrum of Anatase,
TiO2. J. Raman Spectrosc. 1978, 7, 321−324.
(26) Lei, F.; Sun, Y.; Liu, K.; Gao, S.; Liang, L.; Pan, B.; Xie, Y.
Oxygen Vacancies Confined in Ultrathin Indium Oxide Porous Sheets
for Promoted Visible-Light Water Splitting. J. Am. Chem. Soc. 2014,
136, 6826−6829.
(27) Fang, L.; Chen, J.; Zhang, M.; Jiang, X.; Sun, Z. Introduction of
Ti3+ ions into heterostructured TiO2 nanotree arrays for enhanced
photoelectrochemical performance. Appl. Surf. Sci. 2019, 490, 1−6.
(28) Kang, Q.; Cao, J.; Zhang, Y.; Liu, L.; Xu, H.; Ye, J. Reduced
TiO2 Nanotube Arrays for Photoelectrochemical Water Splitting. J.
Mater. Chem. A 2013, 1, 5766−5774.
(29) Wang, S.; Pan, L.; Song, J.-J.; Mi, W.; Zou, J.-J.; Wang, L.;
Zhang, X. Titanium-Defected Undoped Anatase TiO2 with p-Type
Conductivity, Room-Temperature Ferromagnetism, and Remarkable
Photocatalytic Performance. J. Am. Chem. Soc. 2015, 137, 2975−2983.
(30) Henderson, M. A. A Surface Science Perspective on TiO2
Photocatalysis. Surf. Sci. Rep. 2011, 66, 185−297.
(31) Supporting information.
(32) Kuznetsov, V. N.; Krutitskaya, T. K. Nature of Color Centers in
Reduced Titanium Dioxide. Kinet. Catal. 1996, 37, 446−449.
(33) Sekiya, T.; Ichimura, K.; Igarashi, M.; Kurita, S. Absorption
Spectra of Anatase TiO2 Single Crystals Heat-Treated under Oxygen
Atmosphere. J. Phys. Chem. Solids 2000, 61, 1237−1242.
(34) Woicik, J. C.; Nelson, E. J.; Kronik, L.; Jain, M.; Chelikowsky, J.
R.; Heskett, D.; Berman, L. E.; Herman, G. S. Hybridization and
Bond-Orbital Components in Site-Specific X-Ray Photoelectron
Spectra of Rutile TiO2. Phys. Rev. Lett. 2002, 89, No. 077401.
(35) Xu, C.; Yang, W.; Guo, Q.; Dai, D.; Chen, M.; Yang, X.
Molecular Hydrogen Formation from Photocatalysis of Methanol on
Anatase-TiO2(101). J. Am. Chem. Soc. 2014, 136, 602−605.
(36) He, Y.; Tilocca, A.; Dulub, O.; Selloni, A.; Diebold, U. Local
Ordering and Electronic Signatures of Submonolayer Water on
Anatase TiO2(101). Nat. Mater. 2009, 8, 585−589.
(37) Wendt, S.; Sprunger, P. T.; Lira, E.; Madsen, G. K. H.; Li, Z.;
Hansen, J. O.; Matthiesen, J.; Blekinge-Rasmussen, A.; Laegsgaard, E.;
Hammer, B.; Besenbacher, F. The Role of Interstitial Sites in the Ti3d
Defect State in the Band Gap of Titania. Science 2008, 320, 1755−
1759.
(38) Selim, S.; Pastor, E.; Garcia-Tecedor, M.; Morris, M. R.;
Francas, L.; Sachs, M.; Moss, B.; Corby, S.; Mesa, C. A.; Gimenez, S.;
Kafizas, A.; Bakulin, A. A.; Durrantt, J. R. Impact of Oxygen Vacancy
Occupancy on Charge Carrier Dynamics in BiVO4 Photoanodes. J.
Am. Chem. Soc. 2019, 141, 18791−18798.
10721
https://doi.org/10.1021/acs.inorgchem.1c01406
Inorg. Chem. 2021, 60, 10715−10721