Ionothermal Synthesis of Five Keggin-Type Polyoxometalate-Based Metal-Organic Frameworks

Article abstract of DOI:10.1021/acs.inorgchem.8b02277

Five unique Keggin-type polyoxometalate (POM)-based metal-organic frameworks (POMFs), namely [CH₂L1]₂[(CuL1₂)(PMo^VI₉Mo^V₃O₄₀)] (1), [Zn_0.5(H₂O)(L1

Full text of DOI:10.1021/acs.inorgchem.8b02277

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Ionothermal Synthesis of Five Keggin-Type Polyoxometalate-Based
Metal−Organic Frameworks
†
‡
†
‡
‡
,†,‡
́
Kun Lu, Alex Liebman Pelaez, Luo-cheng Wu, Yu Cao, Chen-hui Zhu, and Hai Fu*
†Institute of Chemistry and Life Science, Changchun University of Technology Changchun, Jilin 130012, P. R. China
‡Advanced Light Source, Lawrence Berkeley National Laboratory Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: Five unique Keggin-type polyoxometalate
(POM)-based metal−organic frameworks (POMFs), namely
[CH2L1]2[(CuL12)(PMoVI9MoV O40)] (1), [Zn0.5(H2O)-
3
(L1)][Zn(L1)1.5Cl][H2L1]0.5[PMo12O40]·1.25H2O (2),
(TBA)[Cu(H2O)2L12][PMo12O40] (3), [Cu2(H2O)2(L1)3]-
[PMVI11MoVO40] (4), (H2L2)0.5[(CuIL2)2(PMo12O40)]·H2O
(5), L1 = 4,4′-bis((1H-1,2,4-triazol-1-yl)methyl)-biphenyl, L2
= 1,4-bis((1H-1,2,4-triazol-1-yl)methyl)-benzene, have been
synthesized under ionothermal conditions. According to
single-crystal data, 1 exhibits a novel mechanically interlocked
molecular architecture (MIMA) constructed by two-dimen-
sional (2D) interpenetrating polyrotaxane layers with unique
cyclophanes (tetra-cationic viologen macrocycle cyclobis-
(paraquat-p-phenylene) (CBPQT4+ system)), resulting in an H-bonded 3D supermolecule, and is the first synthesis of self-
assembled cyclophane-PMOFs under ionothermal conditions. 2 shows a novel 2D three-fold interpenetrating polyrotaxane host
and guest network. 1 and 2 are presented as the first MIMA polyrotaxane structures to have been synthesized under
ionothermal conditions. Electrochemical impedance spectroscopy (EIS) reveals that 2, 4, and 5 show high proton conductivity
owing to their unique structural properties. Solid-state diffuse reflection UV−vis-NIR measurements show the title compounds
are potentially semiconductor materials. Photocatalytic investigations indicate that 1−5 possess high and stable photocatalytic
H2 evolution and rhodamine B (RhB) degradation under visible-light irradiation.
overcome the challenge, as it is now regarded as a more
effective and environmentally friendly synthetic method. We
posit that introducing highly soluble ionic liquids (ILs) to the
reaction system as the solvent can successfully promote the
solubility of the POMs and metal−organic phases, resulting in
an enhancement of the reactivity between organic and
inorganic components.26−29
Modern room-temperature ionic liquids (RTILs) can be
used as solvents or templates in preparing a wide variety of
materials under ionothermal conditions. The two main reasons
are as follows: (1) The RTILs used for ionothermal synthesis
are often liquids below a certain temperature (often below 200
°C), as traditionally used in hydro/solvothermal synthesis. (2)
RTILs often show a series of useful properties for preparing the
hybrids, such as high solubility for most organic/inorganic
reactants, tunable pH values, and lower vapor pressure, etc.
Until now, lots of coordination polymers obtained under
ionothermal conditions are two-/three-dimensional (2D/3D)
structures, highlighting the promise of such a synthetic method
for the preparation of PMOFs. It is very clear that changing the
INTRODUCTION
■
Because of the properties such as discrete, mobile ionic
structure and tunable electronic band structure, polyoxometa-
lates (POMs) have been regarded as attractive candidates for
proton conduction and photocatalysis, both important for
addressing the global energy shortage and environmental
pollution.1−4 However, the high solubility of POMs makes
them difficult to be recycled as photocatalysts. Conversely,
metal−organic frameworks (MOFs) are a kind of attractive
hybrid material that can be easily recycled as heterogeneous
catalysts, which have already been widely used in recent years.5
Hence, polyoxometalate-based metal−organic frameworks
(PMOFs) could offer a wonderful opportunity to combine
the unique properties of their organic and inorganic
substituents through the interaction of POMs and metal−
organic components.6,7 Inspired by the applications of POMs
and MOFs to fuel cells and photocatalysis, the hybrids based
on POMs and metal−organic phases as proton conductors and
photocatalysts are model systems for high proton conductivity
and photocataclytic activity.8−25 Due to the distinct solubility
of POMs and MOFs, it is extremely difficult to prepare
PMOFs under traditional hydro/solvothermal conditions,
resulting in one of the most pressing challenges in the field
of synthetic chemistry. We introduce ionothermal synthesis to
Received: August 14, 2018
© XXXX American Chemical Society
A
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of Mo and Cu was measured on Thermo ESCALAB 250. The thermal
stability of the five compounds was measured on TA TGAQ500. The
IR spectra were carried out on the ThermoFisher IS50. The powder
X-ray diffraction (PXRD) was carried out on the Rigaku Smartlab.
solvent from traditional water or organic solvents to RTILs
leads to great successes in preparing the PMOFs.29
In previous work, organic−inorganic hybrids based on
Wells−Dawson and isopolymolybdate have already been
obtained under ionothermal conditions.30 However, the
hybrids based on Keggin in the same system have not been
reported yet. Meanwhile, among the rapidly increasing number
of PMOFs, mechanically interlocked molecular architecture
(MIMA) polyrotaxane frameworks with characteristic “string”
threading “loop” structures have seldom been observed.31−41
For the purpose of enriching the family of PMOFs with Keggin
anions and MIMA polyrotaxane structures obtained under
ionothermal conditions, we chose the IL 1-butyl-3-methyl-
imidazolium bromide ([Bmim]Br) as the solvent and utilized
two different kinds of semirigid/flexible N-donor organic
ligands (4,4,-bis((1H-1,2,4-triazol-1-yl)methyl)biphenyl (short
for BTMB) (L1); 1,4-bis((1H-1,2,4-triazol-1-yl)methyl)-
benzene (short for BBTZ) (L2)), Keggin-type POM
H3PMo12O40·nH2O as the second building blocks (SBU),
and transition-metal (TM) cations to assemble POM-based
hybrid networks. Five unique POMFs based on classic Keggin-
[CH2L1]2[(CuL12)(PMoVI9MoV O40)] (1). [Bmim]Br (2.000 g,
3
4.600 mmol), H3PMo12O40·nH2O (0.400 g, about 0.200 mmol),
CuCl2·2H2O (0.040 g, 0.200 mmol), BTMB (0.016 g, 0.050 mmol),
one drop of dichloromethane (DCM, about 0.050 mL), and a little Vc
were stirred for 0.5 h at room temperature. After that, the pH value of
the mixture was adjusted to nearly 4.5 with HCl (conc.) and NaOH
(solid) sealed in a 23 mL Teflon-lined stainless steel autoclave reactor
at 170 °C for 5 d. After cooling to room temperature at a rate of 10
°C h−1, it was kept for 1 d. Brown block crystals of 1 were filtered off,
washed with acetone, and dried in air (the crystals could be repeated
easily and yield 65% based on Mo). Anal. calcd for 1 (%): C, 27.96;
H, 2.16; N, 10.57; P, 0.97; Mo, 36.21; Cu, 2.00. Found: C, 27.06; H,
2.15; N, 10.01; P, 1.12; Mo, 36.01; Cu, 2.14. The IR spectrum of 1 is
shown in Figure S20.
[Zn0.5(H2O)(L1)][Zn(L1)1.5Cl][H2L1]0.5[PMo12O40]·1.25H2O (2).
Except for ZnCl2·3H2O (0.100 g, 0.450 mmol), without DCM and
Vc, others were the same with 1 (the crystals could be repeated easily
and yield about 58% based on Mo). Anal. calcd for 2 (%): C, 22.01;
H, 1.83; N, 8.56; P, 1.05; Mo, 39.07; Zn, 3.33. Found: C, 22.38; H,
1.63; N, 8.90; P, 1.21; Mo, 39.24; Zn, 3.15. The IR spectrum of 2 is
shown in Figure S20.
type anions [CH2L1]2[(CuL12)(PMoVI9MoV O40)] (1),
3
[Zn0.5(H2O)(L1)][Zn(L1)1.5Cl][H2L1]0.5[PMo12O40]·
(TBA)[Cu(H2O)2L12][PMo12O40] (3). Except for BTMB (0.012 g,
0.040 mmol), a little TBAB, no DCM and Vc, and the pH value 4.0,
others were the same with 1 (the crystals could be repeated easily and
yield 60% based on Mo). Anal. calcd for 3 (%): C, 22.33; H, 2.60; N,
6.51; P, 1.11; Mo, 41.16; Cu, 2.27. Found: C, 22.05; H, 2.43; N, 6.44;
P, 1.04; Mo, 41.66; Cu, 2.48. The IR spectrum of 3 is shown in Figure
S20.
1.25H2O (2), (TBA)[Cu(H2O)2L12][PMo12O40] (3),
[Cu2 (H2 O)2 (L1)3 ][PMoV I 1 1 MoV O4 0
] (4), and
(H2L2)0.5[(CuIL2)2(PMo12O40)]·H2O (5) have been synthe-
sized. Their structures were determined by single-crystal X-ray
diffraction. According to the final data, 1 exhibits a novel
MIMA formed by 2D interpenetrating polyrotaxane layers with
unique cyclophane and is the first synthesis of self-assembled
cyclophane-PMOFs under ionothermal conditions. 2 exhibits a
novel 2D three-fold interpenetrating polyrotaxane host and
guest network. 1 and 2 are both presented as the first MIMA
polyrotaxane structures to have been synthesized under
ionothermal conditions. 3 is a 3D supramolecular host and
guest framework constructed by (4,4) topological monolayers
H-bonded from POMs anions and TBA+ cations. 4 possesses a
3D supramolecular porous structure constructed by ladder-
shaped topological monolayers, resulting in two kinds of
channels, A and B, along the b-axis, where the cavities of
channel B result in an open framework. 4 is shown to be
porous, as confirmed by N2 adsorption measurements of the
fully activated crystals and the result of the solvent accessible
volume by SQUEEZE model of the PLATON. 5 represents a
3D supramolecular structure constructed by (6,3) honeycomb
monolayers, resulting in channels along the b-axis filled with
dissociative protonated organic ligands.
[Cu2(H2O)2(L1)3][PMoVI11MoVO40] (4). Except for CuCl2·2H2O
(0.090 g, 0.450 mmol), no DCM, and the pH value 3.7, others were
the same with 1 (the crystals could be repeated easily and yield 44%
based on Mo). Anal. Calcd for 4 (%): C, 22.10; H, 1.79; N, 8.59; P,
1.06; Mo, 39.23; Cu, 4.33. Found: C, 22.29; H, 1.65; N, 8.65; P, 1.14;
Mo, 39.88; Cu, 4.19. The IR spectrum of 4 is shown in ESI in Figure
S20.
(H2L2)0.5[(CuIL2)2(PMo12O40)]·H2O (5). Except for CuCl2·2H2O
(0.095 g, 0.500 mmol), BBTZ (0.012 g, 0.050 mmol), no DCM and
the pH value 4.0, others were the same with 1 (the crystals could be
repeated easily and yield 61% based on Mo). Anal. calcd for 5 (%): C,
14.03; H, 1.30; N, 8.18; P, 1.21; Mo, 44.81; Cu, 4.95. Found: C,
14.35; H, 1.24; N, 8.06; P, 1.18; Mo, 44.77; Cu, 4.57. The IR
spectrum of 5 is shown in Figure S20.
Single-crystals (1−5) were on the top of a 20 mm long glass fiber,
and collected using a single-crystal diffractometer (BRUKER D8
Venture) with graphite monochromatic Mo-Kα radiation (λ =
0.71073 Å) and SMART CCD diffractometer at 293 (K). The
structures of 1−5 were solved and refined using SHELXTL-2018 by
direct method and full-matrix least-squares on F2 summarized in
Table S1.44,45 During the refinements, not all the non-hydrogen atoms
in the compounds were refined with anisotropic thermal parameters.
The details of the refinement process are attached in the CIF files;
CCDC 1834889, 1040684, 1834890, 1040685, and 1040686 for 1−5
contain the crystal-lographic data. The CIF files can be obtained free
of charge, by contacting CCDC, 12 Union Road, Cambridge CB2
1EZ, UK via fax (+44 1223 336033) or by e-mail (data_request@
ccdc.cam.ac.uk).
MATERIALS AND METHODS
■
The organic and inorganic reagents and solvents were all purchased
from the Aladdin and used without additional purification. The
organic ligands L1, L2, and Keggin-type POM (H3PMo12O40·nH2O)
in this paper were all prepared based on previous literatures.42,43 UV−
vis absorption spectra and solid-state UV−vis-NIR diffuse reflectance
spectra were both carried out on a cary 5000 UV−vis-NIR
spectrophotometer. The N2 adsorption measurement was carried on
an ASAP 2020 HD88 BET surface analyzer. The single-crystal X-ray
crystallography was collected on a BRUKER D8 Venture single-crystal
diffractometer. EIS measurements were performed on a PARSTAT
2273 Advanced electrochemical impedance analyzer. A PerkinElmer
240C elemental analyzer was used to analyze the C, H, and N. A
Leaman inductively coupled plasma (ICP) spectrometer was used to
determine the Mo, Cu, and Zn. The produced H2 was analyzed by a
GC 7890T instrument. The X-ray photoelectron spectroscopy (XPS)
RESULTS AND DISCUSSION
■
Structural Description. Structural Description of 1.
According to the single-crystal X-ray diffraction analysis, in
the asymmetry unit, 1 is composed of one Keggin-type
[PMoVI9MoV O40]6− anion, one [CuIIL12]2+ metal−organic
3
cation fragment, and one free [(CH2L1)2]4+ segment (Figure
S1a). In 1, each CuII center is in a hexa-coordinated
environment, defined by four N atoms from four L1 organic
B
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Figure 1. (a) The representation of 1D interpenetration chain in 1 with the CBPQT4+ fragments (in yellow color). (b) The representation of the
2D topological layers in 1. The POMs short for red sticks, and the Cu centers short for blue nodes, and the organic ligands short for yellow and
dark blue rings. (c) The topological view of the 3D supermolecule polyrotaxane network of 1.
ligands and two terminal O atoms from two different
procedure illustrates a new, environmental friendly and
relatively easy strategy to obtain MIMA MOFs with a
CBPQT4+ system. Ring A (continuous metal−organic ring)
chains pass through the parallel arranged ring B (tetrakis-
imidazolium macrocycle, as shown in Figure 1a) and resulted
1D infinite interpenetrating polyrotaxane chains. The adjacent
chains are connected by CuI I centers through
[PMoVI9MoV O40]6− anions (with the bond distances of
3
Cu1−N1 2.0041(77), Cu1−N4 1.9953(87), Cu1−O11
2.5517(91), Figure S1b). The organic ligands L1 coordinated
with the CuII centers are Z-type configurations (Figure S2b),
resulting in 1D connected metal−organic ring chains (Figure
1a). Additionally, there are two kinds of rings, A and B (Figure
S2a) in 1, which ring A chains interpenetrate through parallel
ring B. Ring A is composed of Z-type organic ligands L1 and
CuII centers. However, ring B (tetrakis-imidazolium macro-
cycle), called cyclophane (34-membered cyclophane), is
formed by two organic ligands (L1 in ring B is U-type
configuration) linked by two −CH2− groups through C−N
bond with four positive charges. Scheme 1 shows the reaction
[PMoVI9MoV O40]6− anions to form 2D infinite monolayers
3
(Figure 1b). The 2D layers are a representation of 2D infinite
interpenetration polyrotaxane structure. They will approx-
imately be (4,4) connected layers where CuII centers act as the
4-connection nodes, and organic ligands and the Keggin-type
anions act as the linkers (Figure S3). The layers are packed
along the c-axis in an eclipsed geometry, with a distance of 14.3
Å between the two adjacent sheets. These 2D sheets ultimately
develop into a 3D supermolecule interpenetrated polyrotaxane
framework by the interaction of H-bonds (Figures 1c and S4).
The CBPQT4+ rings are alternating throughout the 2D layers.
We note that A and B are present in a 1:1 ratio in the crystal
structure of 1.
Scheme 1. Simple Process for the Reaction of the Organic
Ligand with DCM Forming the CBPQT4+
Structural Description of 2. According to the single-crystal
X-ray diffraction analysis, in the asymmetry unit, 2 is composed
of one Keggin-type [PMo12O40]3− anion, one [Zn0.5(H2O)-
L1]+ metal−organic cation fragment, one [Zn(L1)1.5Cl]+
metal−organic cation fragment, half a dissociative protonated
organic ligand [H2L1]2+ segment, and one and a quarter
dissociative water molecules (Figure S5a). The ZnII centers in
the compound can be divided into two groups, Zn1 and Zn2.
Each Zn1 center has a four-coordinated tetrahedral geometry
defined by three N atoms from three organic ligands L1 and
one Cl atom (with the distance of the bonds are Zn1−N3
1.9721(210), Zn1−N5 1.9290(105), Zn1−N9 2.0257(131),
Zn1−Cl1 2.1924(48), as shown in Figure S5b); each Zn2
center has a six-coordinated octahedral geometry which is
defined by four N atoms from organic ligands L1 and two O
atoms from coordinated H2O molecules (with the bond
distance of Zn2−N18 2.2002(230), Zn2−N8 2.0216(203),
Zn2−O2W 2.2073(200), Figure S5b). The organic ligands,
which coordinated with ZnII centers, are divided into two
groups: one is Z-type, and the other is U-type (Figure S6e,f).
The two U-type organic ligands are connected through Zn1
and Zn2 centers resulting in a two-connected metal−organic
ring structure (A and B, in Figure 2a) by sharing one Zn2
process between the organic ligand L1 and dichloromethane
(DCM) (step 1), including the reaction intermediate (step 2)
and the final cyclophane structure (step 3).34−41 This reaction
represents the first demonstration of a self-assembled MIMA
PMOF with cyclophane [tetra-cationic viologen macrocycle
cyclobis(paraquat-p-phenylene) (CBPQT4+ system)]. In pre-
vious works, MOFs with CBPQT4+ systems were obtained by
the method of stepwise synthesis in organic solvent.34−41 We
stress, however, that here the unique framework of 1 was
obtained by a one-step process through self-assembly synthesis
under ionothermal conditions from the original L1 organic
ligand, Keggin-type anions, DCM, and TM. This novel
C
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Figure 2. (a) The representation of the 1D chain of the dimerization
of rotaxanes in 2. (b) The topological view of the 1D chain of the
dimerization of rotaxanes in 2. (c) The representation of the 3D
supermolecule triple interpenetrated polyrotaxane network of 2. (d)
Topological view of the 3D supermolecule triple interpenetrated
polyrotaxane network of 2.
center. The adjacent two-ring fragments are connected by Zn1
centers through Z-type organic ligands resulting in 1D infinite
polyrotaxanes chains (Figure 2a,b). The 1D chains, which
could be considered as a two-ring polyrotaxanes, are
constructed by three contiguous metal centers Zn1−Zn2−
Zn1 connected with four U-type organic ligands (Figure 2a),
and a metal center Zn2 through one Z-type organic ligand
generates dimeric rotaxanes chains (Figure 2a). As shown in
the Figure 2b, the two polyrataxanes that are in the 1D chain of
the dimerization of polyrotaxanes are labeled A and B. The
alkane parts of one rotaxane in the three adjacent dimerization
rotaxanes thread A and B rings of the other two polyrotaxanes,
resulting in three-fold interpenetrating polyrotaxanes layers (as
shown in Figure 2c,d). The 2D interpenetrating network with
cavities can hold nearly two Keggin-type [PMo12O40]3− anions
in the holes (Figure 3a,b). These 2D structures ultimately
develop into a 3D supermolecule triple interpenetrated
polyrotaxane network by the interaction of H-bonds. The
dissociative protonated organic ligand [H2L1]2+ segments were
located between the adjacent 2D layers.
Figure 4. (a) The representation of the unit call of 3. (b) The
representation of the 2D metal−organic layers with the TBA+ cations
and the Keggin-type anions between them. (c) The space packing
representation of adjacent metal−organic layers. (d) The representa-
tion of the metal−organic (4,4) topological layer. (e) Topological
view of the adjacent (4,4) metal−organic layers.
ligands L1 and two O atoms from coordinated water molecules
(with the bond distance of Cu1−N1 2.0173(65), Cu1−O1W
2.2374(123), and Cu1−O2W 2.3132(457), as shown in Figure
4a). And then the CuII centers expand to 2D layers through the
coordinated Z-type organic ligands L1, resulting in (4,4)
topology layers with 11.5 × 11.5 Å sized holes in it (Figure
4d). The packing period of the adjacent 2D (4,4)-layers is two
in the 3D space in Figure 4e. In the two adjacent 2D layers, the
linkers (organic ligands L1) from different layers show cross-
shaped structures along c-axis in Figure 4e. On each side of the
two adjacent 2D layers, free Keggin-type [PMo12O40]3− anions
reside in the holes of the layers and the opposite side of the
CuII centers. However, the free TBA+ cations are on the
opposite side of the organic ligands L1, as shown in Figure
4b,c. On the other side of the two adjacent 2D layers, the space
packing mode of the free Keggin-type [PMo12O40]3− anions
and the TBA+ cations is the same. The free Keggin-type anions
and TBA+ cations interacted with the 2D metal organic layers
Structural Description of 3. According to the single-crystal
X-ray diffraction analysis, in the asymmetry unit, 3 crystallizes
in the tetragonal space group P4/n and is composed of a
classical α-Keggin-type anion [PMo12O40]3−, one [Cu-
(H2O)2L12]2+ metal−organic cation fragment and one
dissociative TBA+ cation as shown in Figure 4a. Each CuII
center is six-coordinated by four N atoms from the organic
Figure 3. (a) The 2D polyrotaxane-type network with the guests Keggin-type POMs in the holes of 2. (b) The topological representation of 2D
polyrotaxane-type triple interpenetrated network with the guests Keggin-type POMs in the holes of 2.
D
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through the H-bonds and result in 3D supermoleculars, as
shown in Figures 4b,c and S7.
8.5 × 7.5 Å (Figure S10). The SQUEEZE module of the
PLATON analysis for the structure without the contribution of
solvent shows that the effective free volumes in 4 are 22.1% of
the whole crystal volume.46 In order to confirm its porosity, N2
adsorption measurements were subsequently implemented for
the fully activated crystals (4) as shown in Figure S27.47−51 As
a result, it is a characteristic type II behavior for microporous
materials with a slight hysteresis between the adsorption and
desorption at 77 K. The phenomenon can be interpreted by
the dynamic feature and cage effect of the framework. The
Langmuir surface areas and Brunauer−Emmett−Teller (BET)
were 714 m2 g−1 and 476 m2 g−1, respectively. The maximum
N2 uptake reaches 204 cm3 g−1 (at standard temperature and
pressure, STP) at 1 atm for 4.
Structural Description of 4. According to the single-crystal
X-ray diffraction analysis, in the asymmetry unit, 4 is composed
of one [PMoVI11MoVO40]4− anion and two [Cu(H2O)-
(L1)1.5]2+ metal−organic cation fragments (Figure S8a). In
4, each CuII center is in a distorted tetrahedral coordination
environment, defined by three N atoms from three organic
ligands L1 and one O atom from the coordinated water
molecule (with the bond distance of Cu1−N1 2.1120(77),
Cu1−N4 2.0108(102), Cu1−N8 2.0492(33), and Cu1−O1W
2.0007(37), as shown in Figure S8a). The organic ligands
coordinated with CuII centers show two different kinds of
coordination configurations called Z- and U-type (Figure S8b).
As shown in Figure 5a,b, the adjacent CuII centers are
Structural description of 5. According to the single-crystal
X-ray diffraction analysis, in the asymmetry unit, 5 crystallizes
in the triclinic space group P-1 and is composed of a classical
α-Keggin-type anion [PMo12O40]3−, two metal−organic cation
fragments (CuIL2)+, one dissociative water molecule, and half
of a dissociative protonated organic ligand (H2L2)2+ (Figure
S11a). In 5, there are two crystallographically independent CuI
centers (Cu1 and Cu2). Both Cu1 and Cu2 centers are three-
coordinated by two N atoms from two organic ligands L2 and
one terminal O atom from one [PMo12O40]3− anion in a
slightly distorted T-type coordination configuration (with the
bond distance of Cu1−N3 1.8711(110), Cu1−N4
1.8822(128), Cu1−O15 2.6857(116), Cu2−N9 1.8583(111),
Cu2−N15 1.8709(122), and Cu2−O24 2.8232(98), Figure
S11b). The metal Cu1 and Cu2 centers are linked by Z-type
organic ligands L2, resulting in 1D infinite helix chains (Figure
6a,b). These helix chains (left:right = 1:1) formed a racemic
compound without chiral in 3D space (Figure 7b,c), because
the adjacent two chains connected to Keggin-type anions are
inversion symmetry (Figure 7a) in the space. The 2D (6,3)
honeycomb monolayers are constructed through Cu1−Cu1−
Cu1−Cu2−Cu2−Cu2 connecting alternately by the Keggin-
type anions and organic ligands L2 (Figure 6c,d). The size of
the honeycomb is about 16 × 17 Å, which could nearly hold
two Keggin-type anions. The adjacent layers are further linked
into a 3D supermolecule framework with channels along the b-
axis, and the channels are filled by dissociative protonated
organic ligands L2 (Figure S12a).
Synthesis Discussion. Under ionothermal conditions,
RTILs can act as solvents, potential templates, or structural
directing agents to aid the assembly of solids, because of their
weak coordination, good solubility, low competition between
solvents and frameworks, and tunable acidity.29 1−5 were all
based on the system of Keggin-type POMs anions/TM (Cu
and Zn)/BTMB or BBTZ organic ligands. According to the
structural properties of the five compounds discussed above,
they could be divided into two groups: One is the polyrotaxane
MIMAs, and the other is 3D supramolecular layer structures
without rotaxane (Scheme 2). In the first group, 1 and 2 both
possess MIMAs constructed by the 2D interpenetrating
polyrotaxane layer structures. The organic ligands BTMB in
both 1 and 2 adopt the U- and Z-type coordination modes
(the U-type BTMB organic ligands in 1 were used to form the
CBPQT4+ system) inclined to a formation of rotaxane
structures because of their long chain structure and flexibility.
In addition, the six-coordinated TM centers promote the
formation of 1 and 2 as well (there are two kinds of
coordination numbers (4 and 6) in 2). As a result, the high
coordination number of the TM centers and the flexibility
Figure 5. (a) The representation of the ladder-shape 1D chain in 4.
(b) Topological ladder-shape structure in 4. (c) The representation of
the adjacent ladder-shaped structures packing in the space existing
two kinds of windows (A and B) along the b-axis. (d) Topological
views of adjacent ladder-shaped structures of 4. (e) The
representation of the 3D supermolecules with POM-encapsulated in
the windows of A. (f) Topological views of the 3D supermolecules of
4.
connected through the U-type organic ligands generating 1D
infinite chains, and the adjacent chains then extend to ladder-
shaped structures with large holes in it by Z-type organic
ligands L1 through the CuII centers. These ladder-shaped 1D
structures then pack to 2D ladder-shaped layers through H-
bonds from the organic ligands L1 (Figure 5c). These adjacent
layers then are further connected with H-bonds to form 3D
porous supermolecules through Keggin-type anions. It can be
observed that 4 is composed of two types of tubes (A and B)
along the b-axis (Figure 5c,d). The channels (A) are filled by
the Keggin-type [PMoVI11MoVO40]4− anions (Figure 5e,f)
interacted with the MOF via H-bond. Intriguingly, the tubes B
are completely open cavities, and the size of the holes is about
E
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Figure 6. (a) The representation of 1D helix chain in 5. (b) The topological view of the 1D chain. (c) 2D (6,3) honeycomb monolayers in 5. (d)
The topological view of the (6,3) honeycomb monolayer (the POMs and the organic ligands short for blue sticks and the Cu centers short for blue
nodes).
configurations of the CuII centers are T-type in fact. Hence, the
two configurations of the organic ligands and T-type
coordinated CuII centers create 1D infinite ladder-shape
structures instead of polyrotaxane MIMAs. Compared to 4,
the configuration of the organic ligands in 3 is simply Z-type
resulting in (4,4) topological monolayers. However, compared
to 3 and 4, the shorter organic ligand BBTZ was applied in 5
instead of BTMB, which only exhibits Z-type configuration.
The single configuration of the organic ligands BBTZ (Z-type)
results in the (6,3) honeycomb monolayer for 5. According to
the discussion above, the long and flexible organic ligands,
more configurations of the organic ligands and the high TM
coordination number are the main factors forming the
polyrotaxane MIMAs.
As we all know, several factors can influence the structures
under ionothermal conditions. In this paper, 1 and 2 were
absolutely obtained under the same pH value (about 4.5) and
reaction temperature (170 °C). With the same mole ratio and
Figure 7. (a) The representation of the inversion symmetry of the
two adjacent helical chains in 5. (b) The representation of the racemic
temperature to 2, changing the pH value to 3.7 and adding a
small quantity of Vc, 4 (without polyrotaxane MIMAs) was
obtained. As the same mole ratio and temperature as 1, 3
(without polyrotaxane MIMAs) was synthesized by decreasing
the pH value to about 4.0. With the same pH value and the
temperature as 3, 5 was obtained by changing the mole ratio
and adding a small quantity of Vc. The different oxidation
states of Mo and Cu cations in 1, 4, and 5 were due to the
addition of the small amount of Vc as the reducing agent. In
addition, the charge of valence states of Mo and Cu cations in
1, 4, and 5 was measured by XPS as shown in Figures S28−
S30. According to the discussion above, it is shown that the pH
value of the reaction system is the main factor to obtain the
polyrotaxane MIMAs.
left- and right-handed single-helical chains.
Scheme 2. Keggin-Based PMOFs with Polyrotaxane or 2D
Layer Supermolecule Structures Were Obtained Under ILs
Conditions
Proton Conductivity. It is clearly shown that 2, 4, and 5
all possess 2D layered structures with dissociative Keggin-type
anions or protonated organic molecule ligands interacting with
neighboring metal−organic layers or channels. According to
the structural properties, they are potentially proton con-
ductors. Hence, we measured the water-assisted proton
conduction of 2, 4, and 5 under the conditions of 65 °C and
different RH.
From Figures 8−10, the electro-conductibility of 2, 4, and 5
increases with the increasing temperature under the conditions
of 45% RH, 70% RH, and 95% RH, respectively. According to
other reports, the increasing RH can increase the up-take of
the water molecules into the hydrophilic channels or layers as
well as facilitating higher proton transportation.52 Compared
with the three different RHs, the conductivity under the
(two configurations of U and Z-type) of the long organic
ligands result in the polyrotaxane 2D layer structures 1 and 2.
In the other group, 3−5 all emerge 3D supramolecular layer
structures without rotaxane constructed by (4,4) topological
monolayers, 1D infinite ladder-shaped structures, and (6,3)
honeycomb monolayers, respectively. Even though the
configurations of the BTMB in 4 are also shown in U- and
Z-type, the coordination number of the CuII centers is low to
4. More importantly, one of the coordination atoms is O from
the coordinated water molecule, so that the coordinated
F
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Figure 8. Nyquist plot of proton conduction for 2 at 65 °C. (a) 45% RH, (b) 70% RH, (c) 95% RH. (d) Arrhenius plots of proton conductivities
for 2 at different RH.
Figure 9. Nyquist plot of proton conduction for 4 at 65 °C. (a) 45% RH, (b) 70% RH, (c) 95% RH. (d) Arrhenius plots of proton conductivities
for 4 at different RH.
condition of 95% RH is the highest for 2, 4, and 5. The high
conductivity for 2 (2.2 × 10−4 S cm−1 under the conditions of
65 °C and 95% RH, Figure 8) was carried out by EIS
measurements. According to Arrhenius equation, the activation
energy is 0.39, 0.41, and 0.44 eV for 95% RH, 70% RH, and
45% RH in 2, respectively. In addition, the proton conductivity
G
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Figure 10. Nyquist plot of proton conduction for 5 at 65 °C. (a) 45% RH, (b) 70% RH, (c) 95% RH. (d) Arrhenius plots of proton conductivities
for 5 at different RH.
of 2 (65 °C and 95% RH) is much higher than that of classic
MIL-53-based MOFs (ca. 10−6 to ca. 10−9 S cm−1 under the
condition of 25 °C and 95% RH by Kitagawa et al.), and it is
nearly the same with the graphene oxide (GO) nanosheets (ca.
10−4 S cm−1 under the conditioin of 25 °C and 60% RH by
Matsumoto et al.).53 The same measurements show that the
activation energy of 4 is 0.40, 0.42, and 0.45 eV (95% RH, 70%
RH, 45% RH) and 0.43, 0.45, and 0.48 eV (95% RH, 70% RH,
45% RH) for 5. Meanwhile, the high conductivity is 2.8 × 10−4
S cm−1 (65 °C and 95% RH) for 4, in Figure 9 and 1.9 × 10−4
S cm−1 (65 °C and 95% RH) for 5, in Figure 10. According to
the results above, it can be concluded that the three
compounds exhibit little differences in their proton con-
ductivity and activation energy. This is likely observed because
the similar structures of the three compounds are all 3D
supermolecules constructed by 2D infinite layers interacting
through H-bonds from the POMs anions and organic
fragments.
The vehicular mechanism is consistent with the results of
proton conduction under the condition of different RH and
the architectural features of 2, 4, and 5. In the channels or
layers, there are a limited number of water molecules and
organic ligands acting as vehicles for proton conductivity.54
The additional protons can be incorporated by the water
molecules lost H-bond from the networks and directly diffuse
in the channels and layers under higher RH.55
visible-light irradiation.6,7,16−23 Introducing the POMs into
MOFs, the PMOFs inherit advantages such as high thermal
stability, easy recyclability, definite structures and chemical
components, porosity, large surface area, high metal content,
and flexible active sites. The advantages discussed above and
structural properties make the organic−inorganic hybrid
materials ideal representations to organize light-harvesting
and catalytic sites achieving solar energy conversion.23
As the photocatalyst, the optical band gap (Eg) is a main
influence for the decomposition efficiency of the small
molecular organic dyes.13,14 First, the low-energy adsorption
bands of 1−5 are similar over about 460 nm (2.71 eV) as
shown in Figure S13. Then, solid-state diffuse reflectivity
measurements were carried out to obtain the Eg to investigate
the semiconductor properties of 1−5 with dry crystalline state
powder at room temperature (Figure S14). The Eg values are
2.20, 2.30, 2.18, 2.71, and 2.35 eV for 1−5, respectively. Due to
the results of the UV−vis-NIR, it is shown that 1−5 are
potentially semiconductor materials for visible-light photo-
catalysts. Because of the layer-structure properties and narrow
optical gaps Eg of 1−5, they were chosen to be photocatalysts
to investigate the RhB photodegradation under visible-light
irradiation. Due to the five undissolved compounds (1−5),
they can only be used as heterogeneous catalysts. Figures 11
and S15 clearly show that the absorption peak of RhB
dramatically decreases with increasing reaction time under
visible-light irradiation, which means RhB rapidly degraded. In
the presence of 1, the RhB degrades from 100% to 7.69%
during the first 120 min with a degradation rate of 46.16% per
hour. This result demonstrates that 1 shows a great
photocatalytic activity for the photodegradation of RhB. For
2−5, the RhB degrades from 100% to 26.67% during 180 min
PHOTOCATALYTIC ACTIVITY
■
Photocatalytic RhB Degradation. Due to the similar
light absorption and electrochemical band-edge positions to
TiO2, POMs show prominent photocatalytic activity (espe-
cially the organic small molecule dye photodegradation) under
H
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Figure 11. Conversion of the RhB with reaction time for 1−5 and the
blank experiment (without catalyst).
Figure 12. Representation of the time course of photocatalytic H2
evolution from 100 mg 1−5, respectively, loaded with 1.2% Pt, in 100
mL (80 mL water and 20 mL methanol) solution under visible-light
irradiation and the blank experiment (without catalyst).
with a degradation rate of 24.44% per hour, from 100% to 40%
during 180 min with a degradation rate of about 20.00% per
hour, from 100% to 18.75% during 210 min with a degradation
rate of about 23.21% per hour, and from 100% to 28.57%
during 180 min with a degradation rate of about 23.81% per
hour, respectively. To compare with 1−5, the RhB photo-
degradation experiments with H3PMo12O40 as catalysts and
without any catalysts were both analyzed as shown in Figures
11, S15, and S16, respectively (the photodegradation of RhB
with H3[PMo12O40] or without any catalyst was examined with
the conversion rates 69% and <10% in 6 h). As a
photodegradation catalyst, 1 is among the best PMOFs with
high photocatalytic activity under visible-light irradiation.23
The PMOFs, as heterogeneous photocatalysts, have the
benefit of simultaneously adsorbing the reactants (organic
substrates and dioxygen) on the catalyst surface and suitable
light energy.56,57 Meanwhile, the modifications in 1−5 to
Keggin-type [PMo12O40]3− anions by the metal−organic
fragments possibly enhance the organic molecules photo-
degradation activity under visible-light irradiation, because Cu/
Zn-BTMB/BBTZ metal−organic subunits as linkers, hosts,
and layers could promote the POM to POM electron transfer
or prevent the POMs deactivation and conglomeration.23
Moreover, before the visible-light RhB photodegradation, the
thermal stability of the five compounds was confirmed by TGA
curves in Figure S23. It is shown that the five compounds
could be stable below 200 °C. After the RhB photodegradation
experiment, the PXRD and IR spectra of the five compounds
as catalysts under visible-light irradiation were compared with
the data before the experiment and the simulated ones as
shown in Figures S18−S21. From the result of long-term
stability, it can be concluded that the five compounds are
stable and reusable for the visible-light RhB photodegradation
at least after three cycles.
catalysts, such as phosphoniobate-based/amino-functionalized
TiIV/dye-sensitized Pt@UiO-66(Zr) frameworks, post-syn-
thesis modification of MOF-253-Pt, as well as some high
active POMs based PMOFs.18−23 Compared with 1, the blank
experiment (without catalyst) was also performed with the
total amount of H2 producing <0.1 μmol in 6 h. The
photocatalytic H2 evolution activity of 1 was also studied in
methanol (100%) and water (100%), respectively. The total
amount of the H2 in methanol was similar with that in 100 mL
20% methanol solution and much higher than that in water
(100%), as shown in Figure S25. Additionally, the same
experiments were also taken to investigate the photocatalytic
H2 evolution activities of 2−5 and were shown to be a little
less active than 1 (Figure 12). From the result of photocatalytic
H2 evolution, it is suggested that 1−5 is highly active for
photocatalytic H2 evolution under visible-light irradiation. The
little difference of the H2 evolution rate can be caused by the
different Eg values and different structures among the five
compounds. In addition, long-tern stability for H2 evolution of
1−5 as the catalysts under visible-light irradiation was studied
as well. In Figure S24, it is shown that 1−5 are very stable and
reusable for at least three times. After the three cycles of H2
evolution, the PXRD of the five compounds matched well with
the ones before and simulated from the structural data, as
shown in Figures S18 and S26.
CONCLUSIONS
■
Five remarkable classic Keggin-type PMOFs have been
successfully synthesized under ionothermal conditions. It is
the first report of self-assembled cyclophane structures with
tetra-cationic viologen macrocycle cyclobis(paraquat-p-phenyl-
ene) CBPQT4+ system via ionothermal synthesis. It is also the
first report of Keggin-type PMOFs with MIMAs (polyrotaxane
structures) (1 and 2) synthesized under ionothermal
conditions. According to the structural properties, the EIS
measurements of 2, 4, and 5, show a high conductivity (2.2 ×
10−4 S cm−1, 2.8 × 10−4 S cm−1 and 1.9 × 10−4 S cm−1 under
the condition of 65 °C and 95% RH), with an activation
energy of 0.44, 0.45, and 0.48 eV for proton conduction,
respectively. Proton conduction for the three compounds is
Hydrogen Evolution. The RhB photodegradation as a
catalytic test is not enough for the research of photocatalytic
activity for 1−5. Hence, the photocatalytic activity of H2
evolution under visible-light irradiation for 1−5 has also
been investigated and compared to the previous litera-
tures.58−64 In Figure 12, H2 is continuously produced under
visible-light radiation in this catalytic system. The total amount
of H2 was 12.2 μmol (at a rate of 29.6 μmol g−1 h−1) in 6 h for
1. It almost possesses the same activity as that of well-known
I
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aluminium porous coordination polymers with high proton
conductivity. Nat. Mater. 2009, 8, 831−836.
proved to be through a vehicular mechanism. Photocatalytic
investigations indicate that the five compounds are active and
stable for photocatalytic H2 evolution and RhB degradation
under visible-light irradiation. As a result, this work presents a
new strategy from which to synthesize PMOFs with entangled,
cyclophane, polyrotaxane, and 2D layered structures with high
proton conductivity and photocatalytic activity by an
ionothermal method.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02277.
■
S
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Accession Codes
CCDC 1040684−1040686, 1834889, and 1834890 contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/
data_request/cif, or by emailing data_request@ccdc.cam.ac.
uk, or by contacting The Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44
1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: Mr.fuhai@163.com.
ORCID
Hai Fu: 0000-0003-1503-9402
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
(17) Gao, C.; Wang, J.; Xu, H.; Xiong, Y. Coordination chemistry in
the design of heterogeneous photocatalysts. Chem. Soc. Rev. 2017, 46,
2799−2823.
This work was financially supported by the National Nature
Science Foundation of China (nos. 21301019 and 21503022),
and the Nature Science Foundation of the Technology
Department of Jilin Province, China (nos. 20160101299JC
and 20160520136JH), the National Science Foundation for
Postdoctoral Scientists of China (no. 2013M530135), China
Scholarship Council (no. 201708220042), and Science
Research Project of the Education Department of Jilin
Province, China (no. jjkh20170546kj).
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