A comprehensive study on the dye adsorption behavior of polyoxometalate-complex nano-hybrids containing classic β-octamolybdate and biimidazole units

Article abstract of DOI:10.3390/molecules24040806

Six new hybrids based on β-[Mo₈O₂₆]^4? polyoxometalates, [Ni(H₂biim)₃]₂[β-Mo₈O₂₆]?8DMF(1); (DMA)₂[M(H₂biim)₂(H₂O)₂][β-Mo₈O₂₆]?4DMF (M = Ni (2), Co (3)), DMA = dimethyl-ammonium, H₂biim=2,2⁰-biimidazole); [M(H₂biim)(DMF)₃]₂[β-Mo₈O₂₆]?2DMF (M = Zn (4), Cu (5)); [(DMA)₂ [Cu(DMF)₄][β-Mo₈O₂₆]?2DMF]_n (6), have been successfully synthesized and characterized. Compounds 2–5 show favorable capacity to adsorb methylene blue (MB) at room temperature, and they can selectively capture MB molecules from binary-mixture solutions of MB/MO (MO = Methyl Orange), or MB/RhB (RhB = Rhodamine B). Compound 3 can uptake up to 521.7 mg g^?1 MB cationic dyes rapidly, which perform the maximum adsorption in an hour among the reported materials as far as we know. The compounds are stable and still work very efficiently after three cycles. For compound 3, the preliminary adsorption mechanism studies indicated that the adsorption is an ion exchange process and the adsorption behavior of polyoxometalate-complex can be benefited from additional exchangeable protons in the complex cations.

Full text of DOI:10.3390/molecules24040806

molecules
Article
A Comprehensive Study on the Dye Adsorption
Behavior of Polyoxometalate-Complex Nano-Hybrids
Containing Classic β-Octamolybdate and
Biimidazole Units
Shuang Liang 1, Yan-Mei Nie 1, Sang-Hao Li 1, Jian-Liang Zhou 1 and Jun Yan 2,
*
1
School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China;
liang324@csu.edu.cn (S.L.); 15367493565@163.com (Y.-M.N.); 172311031@csu.edu.cn (S.-H.L.);
zhoujl@csu.edu.cn (J.-L.Z.)
Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources,
Central South University, Changsha 410083, Hunan, China
2
*
Correspondence: yanjun@csu.edu.cn; Tel./Fax: +86-731-8887-9616
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Academic Editor: Santiago Reinoso
Received: 25 December 2018; Accepted: 11 February 2019; Published: 22 February 2019
Abstract: Six new hybrids based on
(DMA)2[M(H2biim)2(H2O)2][ -Mo8O26]
H2biim=2,20-biimidazole); [M(H2biim)(DMF)3]2[
[Cu(DMF)4][ -Mo8O26] 2DMF]n ( ), have been successfully synthesized and characterized. Compounds
β
-[Mo8O26]4− polyoxometalates, [Ni(H2biim)3]2[
4DMF (M = Ni ( ), Co ( )), DMA = dimethyl-ammonium,
-Mo8O26] 2DMF (M = Zn ( ), Cu ( )); [(DMA)2
β
-Mo8O26]
•
8DMF(1);
β
•
2
•
3
β
4
5
β
•
6
2–5 show favorable capacity to adsorb methylene blue (MB) at room temperature, and they can selectively
capture MB molecules from binary-mixture solutions of MB/MO (MO = Methyl Orange), or MB/RhB
(RhB = Rhodamine B). Compound
the maximum adsorption in an hour among the reported materials as far as we know. The compounds
are stable and still work very efficiently after three cycles. For compound , the preliminary adsorption
3
can uptake up to 521.7 mg g−1 MB cationic dyes rapidly, which perform
3
mechanism studies indicated that the adsorption is an ion exchange process and the adsorption behavior of
polyoxometalate-complex can be benefited from additional exchangeable protons in the complex cations.
Keywords: polyoxometalate; octamolybdate; dye adsorption
1. Introduction
Nowadays, the removal of organic dyes from wastewater is an ongoing hot topic in many
fields, due to their high toxicity and potential carcinogenic effects [
are available for the treatment of the dyes such as an electrochemical technique destroying the
color groups [ ], a bio-degradation process mineralizing the colorless organic intermediates [ ],
chemical oxidation [10 11], photocatalytic degradation [12 14], membrane filtration [15], liquid–liquid
extraction [16], and adsorption [17 19]. Among these methods, solid-liquid adsorption technology
is regarded as one with the most traditional and competitive methods [20 21], because it is easy
1–7]. Several techniques
8
9
,
–
–
,
and safe to perform, has a high efficiency, simplicity of design, ease of operation and resilience
to toxic substances [22], and is comparatively eco-friendly. Various adsorbents, such as activated
carbon [23], nanoporous silica [24], clay materials, solid waste [25], MOFs [26
types of biomaterials [28 31] have been used to remove dyes from sewage. However, most of these
adsorbents suffer from low adsorption capacities [32 33], poor selectively or inconvenient separation
,27], and different
–
,
and recycle procedures and some adsorbents might cause secondary environmental contamination if
they are not fully recovered from the environment. The development of stable dye sorbents that have
Molecules 2019, 24, 806; doi:10.3390/molecules24040806
www.mdpi.com/journal/molecules

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high adsorption capacities, good removal efficiency, and can be easily separated is much anticipated,
both in scientific and in industrial applications [34].
In contrast, polyoxometalate (POM)-based sorbent materials with hierarchical structures have
evoked considerable interest owing to their outstanding features such as: (1) large surface area,
(2) controllable shape and size with good stability, (3) large expandable intermolecular free space,
(4) oxo-enriched surfaces with highly electronegative bonding sites and (5) high separation efficiency
with improved reusability [35
materials, have been considered some of the most promising sorbents for the selective separation of
cationic dyes [50 64]. Su’s group has reported a series of new POMs and POM hybrid with rapids
and large-scale adsorption properties [65]. Wang’s group also recently reported a POM@MOF hybrid
sorbent, and various POMs incorporated into MIL-101 can improve the dye adsorption capacity [66 67].
–49]. Therefore, POMs, especially POM-based organic–inorganic hybrid
–
,
However, the adsorption/desorption mechanism in this system is still not well understood and
normally simply described as an ion-exchange process.
Towards developing POM-based hybrid absorbent materials with improved capacities and
removal efficiency and understanding the adsorption process, we intend to simplify the model and
use the classic β-octamolybdate as basic POM building unit to further explore the correlation between
the adsorption and hybrid structures. The functional cations were used transition metal complexes
about 2,20-biimidazole ligand (H2biim). As an organic ligand, imidazole is a good electron donor
for sp2 hybrid N atom in unsaturated system. It is able to coordinate to transition metals with
three reversible types of binding: neutral (H2biim), monoanion (Hbiim−), and dianion (biim2−
)
types, but few compounds involving transition metals, H2biim ligands, and polyoxoanions have
been reported [68 70]. The N–H bond of the ligand can form supramolecular compounds [71],
–
which is beneficial to the ion-exchange and adsorption process of cationic dye in a wastewater. Herein,
by adjusting the synthetic parameters and transition metal centers, six new polyoxometalate-complex
nano-hybrids with different number of H2biim were isolated and characterized, including
[Ni(H2biim)3]2[
Co (3)) (DMA = dimethyl-ammonium); [M(H2biim)(DMF)3]2[β-Mo8O26]•2DMF (M = Zn (4), Cu (5));
[(DMA)2[Cu(DMF)4][ -Mo8O26] 2DMF]n ( ) Complexes 2–5 show rapid and large-scale adsorption
behaviors and good selective separation of cationic dyes. Meanwhile, the detailed complex structure
indeed affects the adsorption efficiency and stability as expected. Particularly, compound can
β-Mo8O26]
•
8DMF (
1); (DMA)2[M(H2biim)2(H2O)2][
β-Mo8O26]
•
4DMF (M = Ni (2),
β
•
6
3
uptake up to 521.7 mg g−1 MB+ cationic dye from a wastewater with excellent adsorbent efficiency,
and performs the best among all the reported materials as far as we know. The primarily adsorption
mechanism study indicated that the process is an ion-exchange process, and the adsorbed dyes can be
easily desorbed. The compounds are quite stable and still work very efficiently after three cycles.
2. Results and Discussion
2.1. Synthetic Strategy
Accurate control of the synthesis of POM-complex hybrids is not always possible because many
subtle factors of the experimental parameter may affect their assembly. In this work, complexes 1–6
were obtained under similar experimental conditions with different metal salts, resulting in noticeably
different structures. MoO3 was used as Mo source instead of Na2MoO4 or (NH4)6[Mo7O24] to avoid
an additional cation that could affect the self-assembly process and change the solubility of the complex
hybrid. For dye sorbent material design, high temperature reactions in DMF/H2O mixture were used
to form complexes with fewer coordinated water molecules in the final crystal structure. A lattice
DMF with bigger size and weaker H-bond to POM unit may also benefit the adsorption/desorption
process. The DMA cation comes from the thermal decomposition of DMF [72,73]. Further, the H2biim
ligand can not only stabilize the transition metal center of the complex which further tune the
solubility of the crystals, but also plays the role of proton donor and morphology-control-factor
of the supramolecular structures. Additionally, although the H2biim ligands adopt unified bidentate

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ligand mode in compounds 1–5 and the POM units are strictly the same
β
-[Mo8O26]4−, parallel
experiments indicated that the metal ions were crucial for the formation of the title complexes, as
shown in Scheme 1. When NiII ion was used as central metal, the simple [Ni(H2biim)3]2+ cation was
formed (compound
forms [Ni(H2biim)2(H2O)2]2+ (compound
replacing the NiII with CoII and changing the ratio of solvent to ligand, the CoII version analogue
of can be isolated as compound
. Also, when ZnII ions or CuII was used instead of NIII ion, the
metal ion coordinated with a single H2biim ligand, three DMF molecules and the
-[Mo8O26]4− in
compounds and . In the case of compound
, the CuII coordinated with four DMF molecules
and two 5.
-[Mo8O26]4− units, and the compound was isolated as a by-product with compound
1
). As the concentration of NiII and ratio of H2O/DMF increased, the cation
2) with two organic ligands and two water molecules. By
2
3
β
4
5
6
β
Clearly, it is believed that H2O, DMF and H2biim are competitive ligands in this synthetic system,
and the fine-tuning structure of the POM-complex hybrid could be realized by simply adjusting the
concentration of reactant and the reaction solvent.
Scheme 1. The four different types of POM-complex hybrid molecules were obtained in the studied
system. Type I: TM = Ni, VDMF/VH2O = 4/1, MTM/MH2biim=1/3; Type II: TM = Ni or Co, VDMF/VH2O
= 7/3, MCo/MH2biim = 16/3 for compound 3, MNi/MH2biim = 8/3 for compound 2; Type III: TM
= Cu or Zn, VDMF/VH2O = 7/3 and MZn/MH2biim = 3/1 for compound 4; VDMF/VH2O = 4/1 and
MCu/MH2biim = 10/3 for compound 5; Type IV: TM = Cu,VDMF/VH2O = 4/1 and MCu/MH2biim
=
4−
10/3.The
β-[Mo8O26
]
was shown in polyhedron mode and the complex cations were shown ball and
stick mode.
2.2. Description of Crystal Structures of Complexes
The single crystal structure data reveals that compound
with space group P21/c. The asymmetric unit of compound
1
crystallizes in the monoclinic system
1 contains half of the β
-[Mo8O26]4− anion,
one NiII ion adopts a six coordinated octahedron structure by three poly-nitrogen heterocyclic H2biim
ligands with Ni–N bond with distances varying from 2.076(1)–2.121(1) Å (Supplementary Figure S3),
and four lattice DMF molecules.
As depicted in Figure 1 and Supplementary Figure S1, there are two types of strong H-bonds
between POM anion and nearby complex cations with average N–H . . . O distance of 2.85(1) Å. Each
[Ni(H2biim)3]2+ cation forms three H-bond with two
β
-[Mo8O26]4− next to it and therefore they
constructed a 2D supramolecular network. The lattice DMF molecules lay between the layers. The bond
lengths around the NiII atoms are listed in Supplementary Table S3.

Molecules 2019, 24, 806
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Figure 1. The structural view of H-bond between POM and [Ni(H2biim)3]2+ cation in compound
1.
The H-bond was shown in dashed line (colors code: Mo—green; Ni—cyan; O–pink; N–blue; C–black;
H–grey).
Alternatively, two H2biim ligands and two water molecules coordinated NiII in trans- mode and
formed [Ni(H2biim)2(H2O)2]2+ cationic complex in compound . The NiII is six-coordinated with two
2
molecule of H2O through Ni–O (2.062(1), 2.082(1) Å) and two molecules through Ni–N bond varying
from 2.078(2) to 2.125(1) Å. As shown in Figure 2 and Supplementary Figure S2, there are also two
kinds of H-bonding mode between the complex and POM units. The first type is that two H-bonding
links a single H2biim ligand and two terminal oxygen form one {MoO6} units of POM with average
N–H . . . O distance of 2.820(1) Å, which further bond the two parts and form a 1D supramolecular
chain. The other mode shows a single H-bond connected to one bridging oxygen ligand of the POM
from NH group, which also produce a 1D chain. These two types of anionic chains alternately arrange
in the crystal and emerge a 2D layer, and the DMA cations and DMF solvent fill the space between the
layers. Compound 3 shows very similar solid structure compared to compound
replaced by CoII.
2
, except the NiII was
Figure 2. The structural view of the 1D supramolecular chain of POM and [Ni(H2biim)2(H2O)2]2+
cation in compound . The H-bond was shown in dashed line (Colors code: Mo—green; Ni—cyan;
2
O—pink; N—blue; C—black; H—grey).
When ZnII is used to replace NiII in
1
, there are less H2biim ligands coordinated to transition
containing a neutral [Zn(H2biim)(DMF)3]2[Mo8O26] hybrid
is obtained. Single-crystal X-ray diffraction analysis reveals that the asymmetric unit of complex
contains half a β-[Mo8O26]4− anion, one ZnII ion coordinated with one neutral H2biim ligands, three
metal ion cations. As a result, compound
4
4
DMF molecules, and a terminal oxygen of the β
-[Mo8O26]4−, as well as one lattice DMF molecule.
As shown in Figure 3, the ZnII ion is six-coordinate in a distorted octahedral geometry, the bond linking
Zn and POM (2.50(1) Å) is much longer than the other five bonds. All of these bond lengths and angles
are within the normal ranges observed in other ZnII-containing complexes [74]. The H-bonding shows

Molecules 2019, 24, 806
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a different mode compared with compounds
1
and
2. Two parallel H-bonds are formed between the
NH piece of coordinated H2biim ligand and two terminal oxygen atoms from two {MoO6} units of
POM cluster, which produce a 1D supramolecular chain, and the lattice DMF molecules arranged
orderly in the space between the chains.
Figure 3. The structural view of H-bonds between neutral [Zn(H2biim)(DMF)3]2[Mo8O26] molecules
in compound 4. The H-bond was shown in dashed line (Colors code: Mo—green; Zn—light blue;
O—pink; N—blue; C—black; H—grey).
Additionally, when CuII was used in the synthesis, compounds
5 and 6 were obtained in a single
reaction. Compound
Compound with the formula {(DMA)2[Cu(DMF)4][
1D coordination polymer {[Cu(DMF)4]n[Mo8O26]n}2n−
5
, [Cu(H2biim)(DMF)3]2[
β
-Mo8O26]
-Mo8O26]
As shown in Figure 4, the CuII ion is
•
2DMF, is an analogue of compound
4.
6
β
•
2DMF}n is constructed by an anionic
.
six-coordinate in a distorted octahedral geometry with two [Mo8O26]4− and four DMFs in a trans-
mode, and the two bonds linking CuII and POM (2.39(1) Å) is much longer than the other four bonds
due to Jahn-Teller effect. No H2biim ligands were detected in this structure.
Figure 4. The structural view of 1D coordination chain of {[Cu(DMF)4]n[Mo8O26]n}2n− in compound
6.
(Colors code: Mo—green; Cu—orange; O—pink; N—blue; C—black; H—grey).
2.3. PXRD Patterns and Thermal Analyses
Powder X-ray diffraction (XRD) measurements of compounds 1–6 were determined at room
temperature (Supplementary Figures S3–S8), and the diffraction peaks positions of the experimental
XRD patterns is in agreement with this of the simulated XRD patterns, which indicating the phase
purifies of compounds
absorption in the case of
compound . The XRD shows some more peaks than the simulated patterns, which indicated the
exitance of small amounts of compound impurities. The thermal stabilities of the compounds were
1–
4. Some shifted peaks were also detected in the data due to the moisture
2–
5
. In the case of compound , the crystals were separate manually from
5
6
6
investigated by thermogravimetric analysis. The TG curves of compound
at 100–300 C of 19.15%, corresponding to the loss of eight DMF molecules (cal. 19.76 %) and the
compound decomposed at into 600 ◦C (Supplementary Figure S9). There were three steps of weight
1
to reveal the weight loss
◦
loss during the decomposition of compound
weight loss at 100–400 ◦C, a weight loss of 22.45%, corresponding to the loss of four DMF, two DMA
and two waters (cal. 21.76 %). Although the structure of compound is highly similar to that of 2,
2 (Supplementary Figure S10). The TG curve reveals the
3

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the decomposition process is different (Supplementary Figure S11), the TG curve reveals the weight loss
◦
of 21.81 % at 100–300 C corresponding to the loss of four DMF, two DMA and two waters (cal. 21.90%).
In the case of compounds
4
and
5
(Supplementary Figures S12 and S13), the first weight loss at 200–500
(cal. 27.03%), which correspond to the loss of free and
, as shown in Supplementary Figure S14, the weight loss at 100–200
◦C, 26.39% for
4
(cal. 26.99%) and 26.94% for
5
coordinated DMF. For compound
6
◦
C of 10.85%, which correspond to the loss of free DMF and DMA (cal. 10.20 %). There is a weight loss
◦
about 17.01% at 250–550 C that may correspond to the elimination of coordinated DMF (cal. 16.48 %).
2.4. Adsorption of Organic Dyes
Recently, potential applications of POM-based composites as organic dye adsorbents have been
attracting much attention [75
have been considered the major performance index. Therefore, methylene blue (MB) was selected as
a model pollutant to investigate the adsorption behavior of complexes . Liquid UV-Vis spectroscopy
was used for concentration change detection. Compound can easily dissolve in aqueous solution;
,76]. The removal efficiency and the specific capacity of the adsorbent
1–5
6
therefore, its adsorption capacity is not considered here. Figure 5 and Supplementary Figures S15–S19
show the standard diluted MB concentration changes along with the adsorption time in the presence of
compounds
1
–
5
. The removal efficiency is very high for all the reported materials. As showed in Figure 5,
adsorbed over 80% of MB in the first 10 min, and the adsorption process of compounds
2–5 was complete in no more than 2 h. However, compound 1 showed a poor removal efficiency of 10% in
compounds
2–5
an hour due to lack of an exchangeable cation source and steric hindrance. IR spectra of the final recycled
fine crystals confirmed that the process is an adsorption process by showing characteristic peaks of MB
(1105 cm−1 and 1430 cm−1) and 1–5, respectively (Supplementary Figures S20–S24).
−1
Figure 5. The removal efficiency of compounds 1–5 in 10 mg L methylene blue solution during the
first hour.
To fully to evaluate the specific capacity of MB, 15 mg of POMs were added to 100 mL MB solution
−1
at a MB concentration of 10 mg L under stirring at room temperature. The solutions were separated
from the adsorbent by means of centrifugation at regular time intervals, and 5 mL of sample soluti−o1n
was taken out and analyzed on a UV/Vis spectrophotometer. The dye adsorption amount qt (mg g
)
was calculated by:
qt = (C0 − Ct) V/W
(1)
where, C0 and Ct (mg L−1) are the liquid phase concentration of dyes at the beginning and after
a given time t (min), respectively V (L) is the volume of the solution, and W (g) is the mass of the
sample used. Compounds
319.7 mg/g and 425.8 mg/g for the uptake of MB+ (Table 1) in 2 hours, separately. Among these
complex hybrids, compound is the most promising adsorbent candidate with the highest specific
capacity and removal efficiency.
1–5 possesses saturation capacity of 174.3 mg/g, 488.2 mg/g, 521.7 mg/g,
3

Molecules 2019, 24, 806
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Aside from MB, cationic dyes like RhB+ with different sizes and anionic dyes like MO− are
usually the models used for selective adsorption and separation ability evaluations of this type of
adsorbent. As shown in Supplementary Figures S26–S45, the selectivity is quite clear in that the
compounds can selectively capture MB+ molecules from the corresponding binary mixtures. After 2
hours, the characteristic MB+ peak at 665 nm in the UV/Vis absorption spectrum almost disappears,
−
while those of RhB+ (554 nm) or MO (475 nm) remain with slightly decreased intensity. Additionally,
the reusability and stability of these complexes was also investigated. When these complexes were
immersed in mixed solution of dichloromethane and dimethylamine hydrochloride, and the dyes
were gradually released within 0.5 h. As shown in Supplementary Figure S46, from the desorption
process of compounds 2 and 3, we speculated that dimethylammouium ion may have replaced MB+
as cationic part and a small amount of MB+, so at the second and third time, it still maintained
good adsorption performance. When compounds
4 and 5 are desorbed, only a part of MB can be
replaced by dimethylammouium ions, and some MB cannot be desorbed, which results in a decrease
in performance. The structural integrities of the regenerated adsorbents were confirmed by PXRD
(Supplementary Figures S47–S51); and the reasonable consistency of the patterns after three cycles
with the simulated patterns indicates their good stability. Partial major peaks shifted indicated
the adsorption process are not simple molecular guest exchange processes in the cavities and the
complex cations in the framework was also involved may through protonation/deprotonation. Cation
exchanges alternated the component packing mode and form supramolecular nano-aggragates in
the solution.
Table 1. Comparison of MB+ adsorption capacity in various materials.
Adsorption Capacity
(mg/g)
Time
(hrs)
Materials
Ref
Zn-DDQ a
C12H64As2Cu3N12O69W18
135
320.5
42.3
8
1.5
1
[77]
[78]
[79]
b
c
MWCNTs/Fe2O3
graphene oxide sponges
PW11V@MIL-101 d
ErCu–POM (Er-3) e
Compound 1
397
371
1
[80]
[67]
[81]
0.5
0.5
2
2
2
391.3
174.3
488.2
521.7
425.8
319.7
This work
This work
This work
This work
This work
Compound 2
Compound 3
Compound 4
Compound 5
2
2
a [Zn(DDQ)(H2O)2]
[Cu(en)2(OH2)]2{Cu(en)2}{As2W18O61}]
e ErCu-POM = [Cu(en)2]3[Er(H2O)SiW11O39]2(C2O4)[Cu(H2O)(en)2]3.
·
2H2O
·
DMF (H2DDQ = N,N0-dibenzoic acid 2,3-diaminoquinoxaline). b C12H64As2Cu3N12O69W18
=
·
6H2O. c MWCNTs = multi-walled carbon nanotubes. d PW11V = K4PW11VO40.
The adsorption abilities of the compounds are obviously related to the structures and compositions of
both the compounds and the dye molecules. In order to verify the electrostatic attraction between MB+ and
the polyoxometalate units, solid NaCl was added at the beginning of adsorption process and the adsorption
rate and special adsorption capacity decreased significantly (Supplementary Figure S52). This result indicates
that an electrostatic attraction was produced between positively charged MB+ and POMs [82,83].
Interestingly, the significant different dye adsorption ability of the complexes demonstrated the
advantage of these nanohybrids for new complex absorbent applications. Firstly, previous studies
have corroborated this kind of adsorption are ion exchange process. This study also confirms that
the exchangeable cations and available channel are key factors in improving the absorption ability.
Secondly, considering the stability of the adsorbent, the additional releasable proton in the structure
could further increase the absorption capacity. Finally, the transition metals in the complex do affect
the absorption volume, and while they have showed very similar structural coordination behavior,
the thermal analyses of compounds 2 and 3 confirm their stability difference, which could affect the

Molecules 2019, 24, 806
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ion-exchange processes. Consequently, compound 3 has shown the best absorption ability of MB+ so
far, to the best of our knowledge. DMA in compounds and
can be substituted by MB+ cations,
as well as the proton on the complex cations [84]. The final data shows it has an adsorption molar ratio
to dye and adsorbent of about 3:1. In compounds 1, 4 and , only the proton on the imidazole N can be
2
3
5
substituted and some of them are well fixed by H-bonding, and they lack exchangeable DMA cations
which leads to an adsorption molar ratio decrease to 1: 1, 2:1 and 2:1, respectively.
3. Materials and Methods
All reagents purchased were used without further purification. X-ray diffraction (XRD, Rigaku,
SmartLab, Tokyo, Japan) was employed to characterize compounds 1–6 using Cu-Kα in the range
of 10–50◦ with the scanning rate of 10◦ min−1. The IR spectra were recorded with a Thermo Nicolet
Corporation FTIR spectrometer (AVATR-360, Madison, WI, USA) in the 400–4000 cm−1 region with
a KBr pellet. Thermogravimetric analyses (TGA) were performed on a PerkinElmer TG-7 (TA SDT
Q600 instrument, Newcastle, Delaware, USA)) under a flowing Ar atmosphere with a heating rate of
◦
10 C min−1. The UV-vis spectra were obtained on a SHIMADZU UV-VIS spectrophotometer (UV-2600,
Kyoto, Japan).
The crystallographic data and structural refinements for 1–6 are given in Tables S1 and S2. Single
crystal X-ray analysis was performed on a Bruker APEX-II diffractometer (Smart 1000 CCD, Berlin,
Germany) with graphite-monochromated Mo-Kα radiation at 296 K (λ = 0.71073 Å). A multi-scan
technique was used for absorption correction using the Wingx program. Structures were solved
with the ShelXT 2014 structure solution program using intrinsic phasing and refined with the ShelXL
refinement package using least squares minimization. All of the non-hydrogen atoms were refined
anisotropically on F2 by the full-matrix least-squares technique. The hydrogen atoms except those
of solvent molecules were generated geometrically and refined isotropically using the riding model.
For these complexes, some hydrogen atoms attached to solvents were not located, but were included
in the structure factor calculations. In complex 1, the unit cell includes some region of disordered
solvent molecule, which could not be modeled as discrete atomic sites. Thus, the SQUEEZE routine
of PLATON was applied to remove the contributions to the scattering from the solvent molecules.
The reported refinements are of the guest-free structures using the
SQUEEZE routine. In the refinement of complex , the restrains command ISOR and DELU was used
on N14 and C12 to avoid the NDP problem. During the refinement of complex , the commands DFIX,
⁄.hkp files produced by using the
3
5
ISOR and DELU were used to restrain one DMF molecule. The command ISOR and DELU were used
to restrain one aromatic ring of the organic ligand. The amount of solvent molecules of complexes
came from the TG experiment. The details of the crystal parameters, data collection and refinements
for complexes 1–4 are summarized in Tables S1 and S2. CCDC 1866410, 1866412, and 1866416–1866419
contain the supplementary crystallographic data for this paper.
3.1. Synthesis of 2,20-Biimidazole Ligand
The 2,20-biimidazole ligand was synthesized according to a reported procedure [83]. In a typical
reaction, aqueous glyoxal (23.0 mL, 40 wt%) was added dropwise to a vigorously stirred solution of
ammonium acetate (70.00 g) and H2O (23.0 mL) at room temperature over a period of 4 h. After completion
of the reaction, the product was filtered and washed several times of water and acetone.
7.08 (d, 4H), 12.68 (s, 2H).
δ
H ([D6] DMSO)
3.2. Synthesis of [Ni(H2biim)3]2[Mo8O26]•8DMF (1)
MoO3 (3.00 g, 20.83 mmol), H2biim (0.20 g, 1.49 mmol), and Ni(CH3COO)2 4H2O (0.12 g,
•
0.48 mmol) were added to 50.0 mL aqueous DMF solution (H2O/DMF = 10/40), and then the solution
was stirred at 120 ◦C for 4 h and was filtered. Deep purple crystals appeared after about one day of
slow evaporation at room temperature. Yield: 42% Anal. Cal.: C, 26.81; H, 3.4; N, 16.66. Found as

Molecules 2019, 24, 806
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follows: C, 27.15; H, 3.81; N, 16.05. IR (KBr, cm−1): 3446 (br), 2410 (w), 1653 (s), 1508 (w), 1428 (m),
1370 (m),1323 (w), 990 (vs), 950 (vs), 850 (m), 550 (w), 521 (w).
3.3. Synthesis of (DMA)2[Ni(H2biim)2(H2O)2][Mo8O26]•4DMF (2)
MoO3 (3.00 g, 20.83 mmol), H2biim (0.20 g, 1.49 mmol), and Ni(CH3COO)2 4H2O(2.00 g,
•
8.04 mmol) added to 50.0 mL mixture solution (H2O/DMF = 15/35). The solution was stirred at
120 ◦C for an additional 4 h and was filtered. Green crystals appeared after about two days of slow
evaporation at room temperature. Yield: 30%. Anal. Cal.: C, 17.40; H, 3.10; N, 10.15. Found as follows:
C, 17.93; H, 3.21; N, 9.97. IR (KBr, cm−1): 3352 (br), 1657 (vs), 1531 (W), 1457 (w), 1386 (m),1000 (m),
947 (vs), 913 (s), 846 (s), 709 (m), 557 (w), 523 (w).
3.4. Synthesis of (DMA)2[Co(H2biim)2(H2O)2][Mo8O26]•4DMF (3)
MoO3 (2.24 g, 15.56 mmol), H2biim (0.20 g, 1.49 mmol), and Co(CH3COO)2 4H2O(1.04 g,
•
4.17 mmol) added 50.0 mL of aqueous DMF (H2O/DMF = 15/35), and then the solution was stirred at
100 ◦C for 4 h and filtered. Pink crystals appeared after about three days of slow evaporation at room
temperature. Yield: 30% Anal. Cal.: C, 17.40; H, 3.11; N, 10.15, Found as follows: C, 17.68; H, 3.05; N,
10.23. IR (KBr, cm−1): 3358 (br), 2928(w), 1659 (vs), 1530 (s),1387 (m), 1107 (s), 947 (vs), 845 (vs), 770
(s),557 (w),522 (w).
3.5. Synthesis of [Zn(H2biim)(DMF)3]2[Mo8O26]•2DMF (4)
MoO3 (3.00 g, 20.83 mmol), H2biim (0.20 g, 1.49 mmol), and Zn(CH3COO)2 2H2O(1.00 g, 4.56
•
mmol) added to 50.0 mL of aqueous DMF (H2O/DMF = 15/35). The solution was stirred at 100 ◦C
for 4 h and was filtered. Colorless crystals appeared in about one day of slow evaporation at room
temperature Yield: 37%. Anal. Cal.: C, 19.94; H, 3.14; N, 10.34 Found as follows: C, 19.72; H, 3.05; N,
10.05. IR (KBr, cm−1): 3500 (br), 1657 (vs),1438 (w), 1385 (m), 1110 (m), 949 (s), 915 (s), 842 (s), 775 (w),
560 (w), 520 (w).
3.6. Synthesis of [Cu(H2biim)(DMF)3]2[Mo8O26]
•
2DMF (
5) and (DMA)2[Cu(DMF)4][Mo8O26]
•
2DMF (6)
MoO3 (2.24 g, 15.56 mmol), H2biim (0.20 g, 1.49 mmol), and Cu(CH3COO)2
•
H2O (1.00 g,
5.01 mmol) added to 50.0 mL of aqueous DMF (H2O/DMF = 10/40). The solution was stirred
◦
at 100 C for 4 h and was filtered. Green crystals of
5 and blue-green block crystals of 6 appeared after
about two days of slow evaporation at room temperature. The two types of crystals were separated
manually. Yield of 5: 15%. Anal. Cal.: C, 19.97; H, 3.14; N, 10.35 F Found as follows: C, 19.06; H, 3.02;
N, 10.0. IR (KBr, cm−1): 3446 (br),3130 (w), 1648 (vs), 1525 (m),1430 (m), 947 (s),913 (s), 845 (m), 557
(w), 523 (w). yield of 6: 52%. Anal. Cal.: C, 14.85; H, 3.26; N, 6.30. Found as follows: C,14.94; H, 3.30; N,
6.28. IR (KBr, cm−1): 3446 (br),1645 (vs), 1367 (s),1254 (m),1120 (m), 943 (vs), 907 (vs), 715 (s), 557 (m),
523 (m)
4. Conclusions
A series of new POM-complex hybrid compounds based on β
-[Mo8O26]4− cluster and H2biim
organic ligands have been successfully synthesized and characterized. The addition of the organic
ligand affects the solid structure, and therefore changes the dye adsorption properties of these
new compounds. The results indicate that they are good, rapid and effective adsorbents for dye
separation. Compound 3
has the highest capacity to adsorb MB+ reported so far, and it shows excellent
−
reusability, stability and ability to selectively capture MB+ from binary-mixture solutions of MB+/MO
and MB+/RhB+. The discovery on their structure and relevant adsorbent behavior not only adds
a representative architecture to the limited family of POM-complex hybrid, but may also motivate
researchers to develop unprecedented hybrid and improve their adsorbent applications.
Supplementary Materials: Supporting information for this article is given via a link at the end of the document.

Molecules 2019, 24, 806
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Author Contributions: S.L. performed the experiments. S.L., Y.-M.N., S.-H.L., J.-L.Z., J.Y. designed the
experiments and analyses the data. J.-L.Z., J.Y. also provided the materials and reagents. J.Y. writes manuscript.
Funding: This work was supported by the Fundamental Research Funds for the Central Universities of Central
South University [2018zzts010] and the Recruitment Program of Global Youth Experts.
Conflicts of Interest: The authors declare no conflict of interest.
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