Accurate Regulating of Visible-Light Absorption in Polyoxotitanate-Calix[8]arene Systems by Ligand Modification

Article abstract of DOI:10.1021/acs.inorgchem.0c00330

With use of a macrocyclic polyphenol, tert-butylcalix[8]arene (TBC[8]), as ligands, a series of TBC[8]-stabilized {Ti₄O₂}clusters, containing penta- and hexacoordinated Ti centers, were synthesized. Such complexes are "core-shell" shaped containing a {Ti₄O₂} core arranged in a zigzag fashion. While outer walls of the clusters are decorated by deprotonated TBC[8], their upper and lower surfaces can be modified by various O- or N-donor ligands, and the ratio of the penta- and hexacoordinated Ti(IV) centers in the {Ti₄O₂} core can be precisely regulated from 4:0, to 3:1, to 2:2, to 1:3, and finally to 0:4. The combined coordination of different ligands in the axial direction shows significant influence on the adsorption of the TBC[8]-Ti₄ system in the visible-light region, and their absorption edge can be precisely regulated from 600 to 700 nm. The above structural functionalization in the TBC[8]-Ti₄ system also tunes their photocatalytic H₂ production activities and oxidative desulfurization ability. Thus, for the first time, by confining the polyoxotitanium cluster in macrocyclic molecules, we provide an example of understanding the structure-property relationship of titanium-oxygen materials by ligand modification.

Full text of DOI:10.1021/acs.inorgchem.0c00330

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Accurate Regulating of Visible-Light Absorption in Polyoxotitanate−
Calix[8]arene Systems by Ligand Modification
Xin-Xue Yang,† Wei-Dong Yu,† Xiao-Yi Yi, and Chao Liu*
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ABSTRACT: With use of a macrocyclic polyphenol, tert-butylcalix[8]-
arene (TBC[8]), as ligands, a series of TBC[8]-stabilized {Ti4O2}-
clusters, containing penta- and hexacoordinated Ti centers, were
synthesized. Such complexes are “core−shell” shaped containing a
{Ti4O2} core arranged in a zigzag fashion. While outer walls of the
clusters are decorated by deprotonated TBC[8], their upper and lower
surfaces can be modified by various O- or N-donor ligands, and the ratio
of the penta- and hexacoordinated Ti(IV) centers in the {Ti4O2} core
can be precisely regulated from 4:0, to 3:1, to 2:2, to 1:3, and finally to
0:4. The combined coordination of different ligands in the axial direction
shows significant influence on the adsorption of the TBC[8]-Ti4 system
in the visible-light region, and their absorption edge can be precisely
regulated from 600 to 700 nm. The above structural functionalization in
the TBC[8]-Ti4 system also tunes their photocatalytic H2 production activities and oxidative desulfurization ability. Thus, for the
first time, by confining the polyoxotitanium cluster in macrocyclic molecules, we provide an example of understanding the
structure−property relationship of titanium−oxygen materials by ligand modification.
The reduction in the band gap when doped with
heteroatoms,21,22 or decorated with chromophore moieties23
has led to far ranging applications of TOCs. However, since
most of the TOCs are synthesized in solvent phase by a one-
pot process, the structures and nuclearities of the clusters
inevitably change during the modification process. In response
to this, outstanding research come from Zhang’s group has
been presented.24 They selected a phosphonate-stabilized Ti6
cluster as the research objects. Keeping the {Ti6P2} moieties
unchanged, various O-donor organic ligands can be
introduced, and their absorption band can be greatly regulated
from the ultraviolet region to the visible region.
Despite those above works, how to adjust the band gap of
TOCs without changing their Ti−O core still remains
challenging. We hypothesize that if a TOC can be stably
confined to a macrocyclic molecule, we can further introduce
different auxiliary ligands into the Ti−O core through the
labile surface sites. Because the TOC is tightly wrapped by a
macrocycle, the Ti−O core structure is not easy to change. To
realize this idea, the choice of size-matched macrocyclic
INTRODUCTION
■
Nanosized inorganic clusters modified with a functional
organic moiety have been a hot research topic for hybrid
materials in recent years.1 Titanium−oxygen clusters (TOCs)
are one of the most active research objects in the field of
inorganic chemistry and are favored because of their unique
structural diversity and performance controllability.2 Crystal-
line polyoxotitanium materials with precise crystallographic
information can bridge the molecules and photocatalytic TiO2
nanomaterials, which can help us to understand how the
sensitizer is bound to the titanium−oxygen surface.3−6 Those
materials can be applied in the fields of solar energy
conversion,7 degradation of environmental pollutants,8 and
catalysis9 by re-post-modification or self-assembly.10−12 To
date, various TOCs with the nuclearity varied from two to fifty-
two have been identified. Prominent examples include the
[Ti28O40(OR)20(OOCR′)12] nanocage,13 phosphonate-substi-
tuted [Ti26O26(OEt)39(O3P-Phen)6]Br (O3P-Phen = phenyl
phosphonate),14 fullerene-like {Ti42} cluster,15 boat-like {Ti52}
cluster with 3.6 nm length,16 and the noble-metal-doped
clusters {Ag6@Ti16}17 and {Ag10Ti28}.18 But although the
photocatalytic titanium oxide materials have a wide range of
applications in clean energy,19 due to their large band gap, they
can only be excited by ultraviolet light, and the utilization
efficiency of solar energy is low.20 Therefore, how to regulate
their band gap to enhance visible-light absorption has been a
key issue in this field. Previous work on band gap regulation
has focused on the effects of ligand binding and ion doping.
Received: February 1, 2020
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were magnetically stirred for 30 min and then transferred to a
preheated oven at 100 °C for 3 days. After the oven was cooled to
room temperature, yellow crystals were obtained in a yield of ∼65%
(based on TBC[8]). Elem. Anal. Calcd for C100H124O14Ti4 (%): C,
68.96; H, 7.17. Found: C, 66.71; H, 7.48.
molecule and titanium cluster core is critical. On the one hand,
this macrocyclic molecule needs a large cavity size and enough
donor atoms to encapsulate a TOC inside; on the other hand,
after being wrapped, there needs to be enough space to
accommodate the coordination of auxiliary ligands. After the
detailed investigation of the related literature, we found that
the macrocyclic polyphenol, tert-butylcalix[8]arene (TBC[8],
Scheme 1), which is composed of eight phenol units linked via
Synthesis for Complex {TBC[8](Ti4O2)(iPrO)4(CH3CN)}·CH3CN
(TBC[8]-Ti4-2). The synthesis of TBC[8]-Ti4-2 followed the same
protocol described for -1, except the reaction was performed at 80 °C.
Yellow colored crystals were obtained after cooling to 25 °C in a yield
of ∼55% (based on TBC[8]). Elem. Anal. Calcd for
C104H138N2O14Ti4 (%): C, 68.19; H, 7.59; N, 1.529. Found: C,
67.21; H, 7.821; N, 1.756.
Scheme 1. Structure of tert-Butylcalix[8]arene Molecule
Synthesis for Complex {TBC[8](Ti4O2)(iPrO)4(DMF)2} (TBC[8]-
Ti4-3). This complex was synthesized in the same way as that of -2,
except using the DMF (5 mL) replacing CH3CN as reaction solvents.
Yellow crystals were obtained (yield ∼ 35%, based on TBC[8])).
Elem. Anal. Calcd for C106H146N2O16Ti4 (%): C, 67.156; H, 7.76; N,
1.48. Found: C, 68.36; H, 7.13; N, 1.65.
Synthesis for Complex {TBC[8](Ti4O2)(iPrO)(PA)3} (TBC[8]-
Ti4-4). TBC[8] (42 mg, 0.032 mmol) and pivalic acid (HPA, 100 mg,
0.98 mmol) were added in a reaction vessel with 5 mL isopropanol.
Ti(OiPr)4 (200 μL, 0.65 mmol) was added dropwise. The resulting
mixtures were then transferred to a preheated oven at 80 °C for 3
days. A small amount of red crystals of TBC[8]-Ti4-4 and colorless
crystals [Ti8(μ2-O)8(OOCCH3)12] were obtained after cooling to 25
°C (yield, about 45% in total with regard to Ti(OiPr)4). Elem. Anal.
Calcd for C112H153O19Ti4 (%): C, 67.43; H, 7.73. Found: C, 66.42; H,
6.12.
methylene bridges connected to the ortho positions of the
phenol rings, is an appropriate carrier to host TOCs.25 The
existence of oxygen atoms within this conical molecule
provides a valuable platform for the synthesis of Ti−O
materials. In this study, we found that a ladder-shaped {Ti4O2}
cluster, which exactly matches the size of the lumen of
TBC[8], can just stick in the center of the TBC[8] ligand from
the equatorial plane, thus resulting in a TBC[8]-Ti4 core−shell
nanostructure. Due to the variety of conformers that the
TBC[8] molecule can adopt, different O- or N-donor ligands,
including isopropanol, acetonitrile, DMF, acetic acid, propionic
acid, and pivalic acid, can further modify the {Ti4O2} cluster
from the axial direction. The axial ligand shows influence on
the visible-light adsorption of the TBC[8]-Ti4 complexes,
which causes the band gap to gradually decrease, with the
adsorption edge being precisely regulated from 600 to 700 nm.
The above structural functionalization also tunes the photo-
catalytic H2 production activities of the TBC[8]-Ti4 systems.
In addition, the catalytic properties of the TBC[8]-Ti4
complexes toward the oxidative desulfurization (ODS)
reaction were also studied.
Synthesis for Complexes {TBC[8](Ti4O2)(PA)2(DMF)2(iPrO)2}
(TBC[8]-Ti4-5) and {TBC[8](Ti4O2)(PA)2(DMF)2(iPrO)2}{Ti6(μ3-
O)6(PA)6(iPrO)6} (TBC[8]-Ti4-6). TBC[8] (28 mg, 0.021 mmol),
Ti(OiPr)4 (200 μL, 0.65 mmol), and pivalic acid (100 mg, 0.98
mmol) were mixed in the solvent of isopropanol and DMF (3 mL).
The resulted mixtures were sealed in a 20 mL vessel and transferred to
a preheated oven at 80 °C for 3 days. Three different crystals,
including orange crystals of TBC[8]-Ti4-5, minute quantities of
yellow crystals of TBC[8]-Ti4-6, and colorless crystals [Ti6(μ3-
O)6(PA)6(iPrO)6] were obtained (yield, about 50% in total with
regard to Ti(OiPr)4). Elem. Anal. Calcd for C110H130N2O18Ti4
(TBC[8]-Ti4-5) (%): C, 67.42; H, 6.68; N, 1.43. Found: C, 66.37;
H, 6.19; N, 1.71. Elem. Anal. Calcd for C158H226O42N2Ti10 (TBC[8]-
Ti4-6) (%): C, 57.43; H, 6.89; N, 0.847. Found: C, 56.27; H, 6.12; N,
1.21.
Synthesis for Complex {TBC[8](Ti4O2)(AC)4}·4CH3CN
(TBC[8]-Ti4-7). A mixture of TBC[8] (28 mg, 0.021 mmol),
Ti(OiPr)4 (200 μL, 0.65 mmol), 200 μL of acetic acid (HAC), and
5 mL of CH3CN were sealed in a 20 mL vessel and transferred to a
preheated oven at 80 °C for 3 days. Deep red crystals were obtained
after cooling to 25 °C (yield ∼ 65%). Elem. Anal. Calcd for
C100H128N4O18Ti4 (%): C, 64.38; H, 6.91; N, 3.00. Found: C, 63.15;
H, 6.02; N, 2.55.
Synthesis for Complex {TBC[8](Ti4O2)(PC)4}·4CH3CN (TBC[8]-
Ti4-8). The synthesis of TBC[8]-Ti4-8 followed the same protocol
described for -7, except using the propionic acid (HPC) replacing
acetic acid. Deep red crystals were obtained after cooling to 25 °C.
(yield ∼ 75%). Elem. Anal. Calcd for C104H136O18Ti4N4 (%): C,
65.00; H, 7.13; N, 2.92. Found: C, 64.29; H, 6.86; N, 2.51.
Synthesis for Complex {TBC[8][Ti2(AC)(MT)4]2} (TBC[8]-Ti4-
9). A 100 mg (0.053 mmol) amount of crystalline samples of
TBC[8]-Ti4-7 was dissolved in methanol. The resulted solution was
transferred to a preheated oven at 120 °C for 1 day. After the oven
was cooled to room temperature, orange crystals of TBC[8]-Ti4-9
were obtained in a yield of ∼10%. Elem. Anal. Calcd for
C104H136O18Ti4N4 (%): C, 63.28; H, 7.55. Found: C, 65.49; H, 7.12.
H2 Production Experiment. Photocatalytic hydrogen production
tests were investigated in a closed gas circulation system (Beijing
Perfect Light Co. Labsolar-III (AG)). Typically, 50 mg of cluster
sample was dispersed in 90 mL of H2O with 10 mL of methanol as a
sacrificial agent, and then 33 μL of 1.0 wt % H2PtCl6 was added. A
300 W Xe lamp was used as the UV−vis light source. Light passed
through a UV cutoff filter (λ > 420 nm), and then the filtered light
EXPERIMENTAL SECTION
■
Materials and Characterization. All reagents were purchased
commercially and were not further purified when used. Powder X-ray
diffraction (PXRD) analyses were performed on a Rigaku Mini Flex II
diffractometer at a 2θ range of 5−50° (5 deg min−1) with Cu Kα
radiation (λ = 1.54056 Å). Elemental analyses were collected by a
VarioEL analyzer. The solid-state UV/vis spectra data of the cluster
samples were obtained on a UV-4000 spectrophotometer. We
collected the Fourier transform infrared spectroscopy (FTIR) data
(4000−500 cm−1) on a PerkinElmer Spectrum 100 FT-IR
spectrometer. Crystallographic data for this article were obtained on
a Bruker Apex II CCD diffractometer with graphite-monochromated
Mo Kα radiation. The structures are solved by direct methods and
refined on F2 by full matrix least-squares using new the SHELXL
program.26 All of the non-hydrogen atoms are located from Fourier
maps and are refined anisotropically. Due to the rotation disorder of
tert-butyl groups in the TBC[8] ligand, in all cases ISOR constraints
were necessary to achieve convergence. CCDC 1973738−1973745
and 1988956 contain the crystallographic information herein. The
crystallographic data for the reported clusters are listed in Table S1.
Synthesis for Complex [TBC[8](Ti4O2)(iPrO)4] (TBC[8]-Ti4-1).
To a 25 mL vessel was loaded Ti(OiPr)4 (200 μL, 0.65 mmol),
TBC[8] (42 mg, 0.032 mmol), and acetonitrile (5 mL). The mixtures
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was focused onto the reactor. During irradiation, the headspace gas of
the reactor was intermittently sampled every 30 min and H2 from the
headspace gas was monitored by a gas chromatograph (Shimadzu
GC9860) equipped with a thermal conductivity detector, a 5A
molecular sieve column, and Ar as carrier gas.
system. Indeed, when the reaction temperature slightly
reduced to 80 °C, we found the axial environment of the
{Ti4O2} core in the TBC[8]-Ti4-2 has a slight change. An
additional CH3CN participates in coordination from the axial
direction, thus making one Ti site in hexacoordinated mode
(Figure 2A). Interestingly, we found that DMF molecules can
Oxidative Desulfurization Tests. A 1 μmol (0.14 mol %)
amount of the catalyst was added into the solution containing 0.7
mmol of sulfide and 105 μL of H2O2 (30%). The mixture was reacted
at 60 °C for 2 h. After reaction, the mixture cooled to room
temperature, and the mixture (0.3 mL) was added in an NMR tube
1
with CDCl3 (2.5 mL). The NMR conversion was analyzed by H
NMR analysis (for details see the Supporting Information). The
catalysts were recycled by filtration and washed with ethanol several
times.
RESULTS AND DISCUSSION
■
Syntheses and Structures. Through the solvothermal
reaction of TBC[8] ligand and excess Ti(OiPr)4 in the solvents
of acetonitrile at 100 °C for 3 days, yellow crystals of TBC[8]-
Ti4-1 were isolated. The synthesis does not require the use of
an ultradry solvent or an inert atmosphere and is highly
reproducible. Structural analysis reveals that this complex has a
chemical formula of [Ti4(μ3-O)2(TBC[8])(OiPr)4]. Four Ti
sites are linked by two μ3-O to form the {Ti4O2} core (Figure
1A), which is just stuck in the center by a deprotonated
Figure 2. Crystal structures of TBC[8]-Ti4-2 (A) and TBC[8]-Ti4-3
(B). The penta- (yellow) and hexacoordinated (green) Ti centers can
be regulated from 4:0 to 3:1, to 2:2.
also bind to the TBC[8]-Ti4 core when the reaction was
performed in DMF(Figure 2B). In TBC[8]-Ti4-3, four
isopropanol and two DMF have been found in the axial, and
this time, two Ti centers in the end are in hexacoordinated
environments, each coordinated with a DMF and an
isopropanol ligand.
Considering the unsaturated coordination environment of
Ti1 and Ti2 centers and also the weak axial bonding around
Ti3 and Ti4, the {Ti4O2} core should still have potential for
further ligand addition. By introducing different carboxylate
ligands into the reaction, similar TBC[8]-Ti4 clusters can be
readily obtained. The red crystals of TBC[8]-Ti4-4 appear as
byproducts in the reaction of TBC[8], Ti(OiPr)4, and excess
pivalic acid in isopropanol. In this complex, the pinched conic
TBC[8] ligand also acts as a octanucleating ligand that
involves a similar ladder-shaped {Ti4O2} cluster core. Three
hexacoordinated and one pentacoordinated Ti sites were found
in TBC[8]-Ti4-4. Here the {Ti4O2} core is slightly different
from those in previous cases; one μ3-O and one μ2-O were
found as bridges between the four Ti atoms. The coordination
sphere of three hexacoordinated Ti1, Ti2, and Ti3 atoms,
which are bridged by a μ3-O to form a triangular {Ti3O} motif,
were completed by three pivalic acid and one isopropanol.
Each pivalic acid are bridged to two Ti atoms in the form of μ2-
η1:η1 fashions. The remaining pentacoordinated Ti4 atoms are
coordinated by an isopropanol, which are connected with the
{Ti3O} units with μ2-O atoms (Figure 3).
Figure 1. Ball-and-stick (A) and polyhedron (B) views for the
structure of TBC[8]-Ti4-1; (C) “core−shell” shape of the TBC[8]-Ti4
system; (D) cluster core of TBC[8]-Ti4-1, containing four
pentacoordinated (yellow) Ti centers.
TBC[8] ligand from the equatorial plane, resulting in a “core−
shell”-shaped TBC[8]-Ti4 core with the diameter being about
1.5 and 2.2 nm (Figure 1C). The eight phenoxide groups are in
the cone conformation, six of which each coordinate with one
Ti atom with monodentate mode (u1-η1) and the remaining
two each bridge two Ti atoms. The upper and lower surfaces of
the TBC[8]-Ti4 core are protected by four isopropanol
molecules. It should be noted that all Ti atoms in the axial
direction are only in pentacoordinated environments, which
indicates that the coordination of the {Ti4O2} is unsaturated
and it should be possible to capture more terminal ligands.
Thus, it would provide a good platform for us to study the
ligand-dependent band gap engineering of the TBC[8]-Ti4
The {Ti4O2} core in TBC[8]-Ti4-4 still does not reach
coordination saturation. When the reaction is performed in a
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Figure 3. Crystal structures of TBC[8]-Ti4-4, which contain three
hexacoordinated and one pentacoordinated Ti centers.
mixed solvent of isopropanol and DMF, we found that the
DMF molecule will also participate in coordination, and in this
time, all of the Ti atoms are saturated and hexacoordinated. In
the structure of TBC[8]-Ti4-5, three different O-donor ligands,
including two pivalic acid, two DMF, and two isopropanol,
have been found in the axial. Two phenoxide groups, one
DMF, two μ3-O, and one carboxylate O define the
coordination spheres of Ti1 and Ti2, whereas that of Ti3 and
Ti4 is the same except that the DMF are replaced by the
isopropanol (Figure 4). Interestingly, in the same reaction, the
Figure 5. Crystal structures of TBC[8]-Ti4-7 and -8. All titanium sites
are in a six-coordinate environment.
O vibrations (around 600−650 nm) and a series of
characteristic peaks due to calixarene (Figures S19−S25).
The difference in the Ti−O vibrations may be attributed to
various surrounding factors such as the different axial ligands,
different configuration of calixarene, and so on. Thermogravi-
metric analysis unequivocally revealed the two-step weight
losses, with an initial loss consistent with the loss of axial
ligands, and the TBC[8]-Ti4 bracket can stand up to >390 °C
in air (Figures S26−S31). Furthermore, we take TBC[8]-Ti4-8
as a representative and study the stability of TBC[8]-Ti4
clusters in aqueous solution with pH varying from 1 to 13.
No obvious crystallinity loss can be detected within 24 h from
the PXRD date (Figure S18). The high stability of the
TBC[8]-Ti4 clusters can be attributed to the stronger bonding
between the {Ti4O2} core and eight surrounding phenoxide
groups in the TBC[8] ligand. In addition, in order to study the
stability of these complexes in solution, we dissolved the
crystalline samples of TBC[8]-Ti4-7 in methanol (MT) and
analyzed its composition with ESI-MS. The negative ion mode
spectra revealed a series of strong signals with the general
formula of {[TBC[8](Ti4O2)(AC)x(MT)y(H2O)z-
(CH3CN)e]−1, indicating the axial ligand of TBC[8]-Ti4 can
be gradually replaced by the solvent molecules (Figure S32).
Interestingly, after heating this solution at 120 °C for 1 day,
orange crystals of TBC[8]-Ti4-9 can be recrystallized from
methanol. Here the TBC[8] ligand features a double-cone
conformation and two opposite TBC[4]-like units, each using
four phenolic hydroxyl to bridge one binuclear {Ti2} subunit.
Two Ti centers are further bridged by one acetate and two
methanol (Figure S11).
Figure 4. Crystal structures of TBC[8]-Ti4-5. All Ti sites are in a six-
coordinate environment.
cluster in the TBC[8]-Ti4-5 can also be co-crystallized with the
[Ti6(μ3-O)6(PA)6(iPrO)6] cluster in a 1:1 ratio, resulting in
the yellow crystals of TBC[8]-Ti4-6 (Figure S6). The two
crystals are very different in color and shape and can be
separated manually. Furthermore, when using a smaller
carboxylate, such as acetic acid and propionic acid, we found
that the axial labile coordination sites can be completely
replaced by four carboxylate groups. In those two structures,
TBC[8]-Ti4-7 (Figure 5A) and TBC[8]-Ti4-8 (Figure 5B), the
central two Ti sites were bridged by two carboxylate groups
with a μ2-η1:η1 di-monodentate fashion, while the two end Ti3
and Ti4 atoms are chelated by one carboxylate group,
respectively. All of the four Ti atoms are hexacoordinated in
those structures.
Up to now, by confining the {Ti4O2} clusters to the
macrocyclic TBC[8] ligand, and introducing different O- or N-
donor ligands to modify the upper and lower surfaces of the
{Ti4O2} clusters, the ratio of the penta- and hexacoordinated
Ti sites in the {Ti4O2} core can be precisely regulated from 0:4
to 1:3, to 2:2, to 3:1, and finally to 0:4. The phase purities of
those crystals were affirmed by powder X-ray diffraction
(Figures S12−S17). The FTIR spectra display the typical Ti−
Band Gap Investigation. The TBC[8]-Ti4 system gives
us a perfect model to study band gap regulation. We have
found that the color of these crystals strongly depends on the
axial ligands (Figure 6). The crystals of TBC[8]-Ti4-1 appear
yellow in color. As expected, the diffuse reflectance UV/vis
spectrum of TBC[8]-Ti4-1 reveals significant absorption in the
vision region up to ∼600 nm with a maximum around 426 nm.
The band gap estimated by the Kubelka−Munk function
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Figure 6. Crystal pictures of the TBC[8]-Ti4 clusters.
(F(R) = (1 − R)2/2R, where R is the reflection) is
approximately 2.16 eV (Figure S33), which was significantly
red-shifted compared with TiO2. As we know, the O 2p → Ti
3d charge-transfer transitions in TOCs can only induce
ultraviolet-light absorption. Thus the extended visible
absorption in the TBC[8]-Ti4 cluster reveals the generation
of new charge-transfer states induced by the phenolic ring from
the calixarene. Continuing to modify a CH3CN or DMF on the
TBC[8]-Ti4 system, the penta- and hexacoordinated Ti centers
in the {Ti4O2} core can be regulated from 0:4, to 1:3, and to
2:2. The crystal colors of TBC[8]-Ti4-2 and -3 also appear
yellow, but their absorptions exhibit slight red shifts of about
10 nm compared to TBC[8]-Ti4-1, with their band gaps being
reduced to 2.13 and 2.02 eV, respectively. When pivalic acid is
applied, the penta- and hexacoordinated Ti centers in the
{Ti4O2} core can further be regulated to 3:1 and 4:0; the
colors of the crystals of TBC[8]-Ti4-4 and -5 appear orange
and red, respectively, and the absorption maximum in TBC[8]-
Ti4-5 has been red-shifted to 454 nm, with light absorption up
to ∼675 nm. The calculated band gap for TBC[8]-Ti4-5 is
approximately 1.89 eV. Finally, when the axially active sites are
completely replaced by the carboxylate ligands, the crystals of
TBC[8]-Ti4-7 and -8 appear in deep red, and the absorption
maximum has been largely red-shifted to 530 nm, with
significant vision absorption up to ∼700 nm. The calculated
band gaps of TBC[8]-Ti4-7 and -8 are about 1.82 and 1.80 eV.
Apparently, the incorporation of carboxylate groups in the
TBC[8]-Ti4 system has a significant effect on the their
absorption (Figure 7).
Figure 7. Band gap (upper panel) and adsorption spectra (lower
panel) of the TBC[8]-Ti4 clusters.
H2 Evolution Study. The strong visible-light adsorption of
the TBC[8]-Ti4 system means it can act as a potential
photocatalyst in the solar energy driven hydrogen evolution. In
order to investigate their photocatalytic activity, we used
different TBC[8]-Ti4 clusters as catalyst to perform the
photocatalytic hydrogen production experiments. Typically,
50 mg of TBC[8]-Ti4 samples were dispersed in 90 mL of
water; then, 10 mL of CH3OH was added as sacrificial
reagents,27 and 33 μL of 1.0 wt % H2PtCl6 was used as co-
catalyst agent.28 The whole reaction was performed in a closed
gas circulation system. As shown in Figure 8, despite having
similar structures, those materials exhibit different H2
production activities. The carboxylate-stabilized clusters
TBC[8]-Ti4-5, -7, and -8, give rise to a stable H2 evolution
efficiency at rates of about 23.67, 18.54, and 21.39 μmol g−1
h−1, which are almost twice the rates of 9.80, 12.48, and 8.59
μmol g−1 h−1 given by the TBC[8]-Ti4-1, -2, and -3. Cycle
experiments revealed that those materials displayed sustained
Figure 8. Photocatalytic H2 production experiment of the TBC[8]-
Ti4 system. Conditions: 300 W Xe light (420 nm cutoff).
photoactivity and have almost equal H2 release rates (Figure
S39). The PXRD spectrum shows that the materials separated
after the reactions also retain the basic characteristic signals.
To illustrate the possible photocatalytic mechanism of our
TBC[8]-Ti4 system, we performed the electron paramagnetic
resonance (EPR) experiments for the TBC[8]-Ti4 materials.
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a
The EPR spectra show a sharp signal at the position of g =
1.924 after visible-light illumination, which indicates that the
photoexcited electron transfer from the TBC[8] ligands to the
Ti−O core cause the reduction of Ti4+ to Ti3+ (Figures S42−
S47). Ti3+ can further serve as a reaction center for reduction
reactions. Thus, the possible mechanism for the photocatalytic
H2 evolution process of TBC[8]-Ti4 is as follows: Under
visible-light irradiation, the phenolic rings absorb light and
transfer electrons to the {Ti4O2} core through the LMCT
mechanism, reducing Ti4+ to Ti3+ and producing holes in the
organic moiety. Then, the H2PtCl6 co-catalysts separate the
photogenerated electrons and reduce H+ to generate H2, while
the holes in the phenolic rings obtain electrons from the
sacrificial reagent CH3OH (Figure S40).29
Table 1. Sulfoxidation of Thioethers to Sulfone
Oxidative Desulfurization. Sulfide oxidation is an
important aspect to solve environmental problems at present.30
However, the oxidation products of sulfides are not single, so
how to prepare an efficient catalyst to singulate the catalytic
products is the key to solving the problem. Although a large
number of catalysts have been reported to catalyze the sulfide
to sulfoxide, there are few reports on how to catalyze sulfoxide
into sulfone with high efficiency and energy savings. Titanium-
based catalysts such as (Ti)MIL-125 have been extensively
applied for ODS by using hydrogen peroxide (H2O2) as the
green oxidant.9,31,32 Thus, the catalytic properties of those
reported TBC[8]-Ti4 complexes toward sulfide oxidation
reaction were investigated. To begin with, methyl phenyl
sulfide was chosen as a representative sulfur compound for
catalytic sulfoxidation reactions under heterogeneous con-
ditions, and the data were shown in Table S2. The optimum
conditions were to use CH3CN as the solvent and 60 °C as the
temperature under H2O2. To explore the catalytic properties of
TBC[8]-Ti4 complexed for most the sulfides, diverse sulfides
were used as the substrates and the reaction took place at the
optimal conditions. As shown in Table 1, different complexes
showed different catalytic effects on phenyl methyl sulfide.
TBC[8]-Ti4-1 showed the best conversion and selectivity,
while -7 and -8 showed high conversion and lower selectivity.
Furthermore, both alkyl sulfides and substituted methyl phenyl
sulfides can be converted into sulfone compounds efficiently.
Similar to phenyl methyl sulfide, TBC[8]-Ti4-1 showed the
best catalytic effect among these five complexes. This may be
caused by the coordination number of the center metals. As
mentioned before, four titanium atoms in TBC[8]-Ti4-1 were
pentagonal, leaving an additional coordinated site to combine
with the H2O2. As for TBC[8]-Ti4-7 and -8, the octahedral
coordinate titanium led to the poor binding ability of the
compounds with H2O2, which results in insufficient oxidation.
As the consequence, the selectivity of sulfone is lower than
when TBC[8]-Ti4-1 is used as the catalyst. In addition, the
dynamic study was conducted at the optimum condition. As
shown in Figure S48, at the beginning of the reaction, sulfoxide
and sulfone were directly generated, while the yield of sulfone
increased with the time increasing, resulting in a single
product. This dynamic process is different from most previous
literature, which reported that the sulfoxide was formed and
further oxidized to sulfone. Meanwhile, a leaching test was also
used to verify the heterogeneity of the catalytic reaction. As
shown in Figure S49, when hot filtered after 1 h, the reaction
did not carry out, indicating this was a heterogeneous catalytic
reaction and TBC[8]-Ti4-1 played a crucial role in the catalytic
reaction.
a
Reaction conditions: CH3CN, 5 mL; substrate, 0.7 mmol; H2O2, 1.7
mmol; catalyst, 0.1 mmol; reaction time, 2 h. Conversion (Conv.) was
determined by 1H NMR analysis. Selectivity (Sel.) = sulfone/
(sulfoxide + sulfone) × 100%.
CONCLUSION
■
In conclusion, by confining the {Ti4O2}cluster in the
macrocyclic tert-butylcalix[8]arene molecule, we provide a
new example of band gap engineering of nanoscopic
polyoxotitanate by ligand modification. A variety of O- or N-
donor ligands can be introduced to the TBC[8]-Ti4 clusters
from the axial direction, and the axial ligand shows significant
influence on the visible-light adsorption of the TBC[8]-Ti4
complexes, with the adsorption edge being precisely regulated
from 600 to 700 nm. The above structural functionalization
also can tune the photocatalytic H2 production activities of the
TBC[8]-Ti4 systems. In addition, all of the complexes showed
high oxidative desulfurization convention and selectivity, and
the coordination environment also affected the catalytic
efficiency. Therefore, the TBC[8]-Ti4 system provides a
good platform for us to study the structure−property
relationship of titanium−oxygen materials. Our work has also
pioneered the synthesis of calixarene-based polyoxotitanate
clusters, which to our knowledge are very limited. Further
studies of the structural chemistry of calix[n]arene−polyox-
otitanate are underway.
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00330.
PXRD pattern, ESI-MS, NMR, IR spectroscopy,
thermogravimetric, and EPR spectrum (PDF)
Accession Codes
CCDC 1973738−1973745 and 1988956 contain the supple-
mentary crystallographic data for this paper. These data can be
F
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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
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AUTHOR INFORMATION
Corresponding Author
■
Chao Liu − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, Hunan, People’s
Republic of China; orcid.org/0000-0002-2289-3095;
Email: chaoliu@csu.edu.cn
Authors
Xin-Xue Yang − College of Chemistry and Chemical
Engineering, Central South University, Changsha 410083,
Hunan, People’s Republic of China
Wei-Dong Yu − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, Hunan, People’s
Republic of China
Xiao-Yi Yi − College of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, Hunan, People’s
Republic of China; orcid.org/0000-0001-5482-9496
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c00330
Author Contributions
†X.-X.Y. and W.-D.Y. 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. 21901256). C. L. acknowl-
edges the support from Central South University Science
Research Initial Fund.
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