Two new Keggin-type polyoxometalate-based entangled coordination networks constructed from metal-organic chains with dangling arms

Article abstract of DOI:10.1016/j.inoche.2016.08.013

Two new polyoxometalate(POM)-based entangled coordination networks with chemical formula of [Mn₂(H₂O)₂(BBPTZ)₅][SiW₁₂O₄₀] (1) and [Ni₂(H₂O)₂(BBPTZ)₅

Full text of DOI:10.1016/j.inoche.2016.08.013

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Inorganic Chemistry Communications 72 (2016) 132–137
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Short communication
Two new Keggin-type polyoxometalate-based entangled coordination
networks constructed from metal-organic chains with dangling arms
a,
b
a
b,
Xiu-Li Hao a, , Shi-Fang Jia , Yuan-Yuan Ma , Hong-Yi Wang , Yang-Guang Li
⁎
⁎
⁎
a
School of Chemical and Biological Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, PR China
Key Laboratory of Polyoxometalate Science of Ministry of Education, Faculty of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin 130024, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2016
Received in revised form 25 August 2016
Accepted 27 August 2016
Available online 28 August 2016
Two new polyoxometalate(POM)-based entangled coordination networks with chemical formula of
[Mn2(H2O)2(BBPTZ)5][SiW12O40] (1) and [Ni2(H2O)2(BBPTZ)5][SiMo12O40]·6H2O (2) (BBPTZ = 4.4′-bis(1,2,4-
triazol-1-ylmethyl)biphenyl), were prepared in a hydrothermal reaction system. Compounds 1–2 were charac-
terized by elemental analyses, IR spectroscopy, thermogravimetric analysis, powder X-ray diffraction and
single-crystal X-ray diffraction. In compound 1, dangling arms thread in quadrangular window of the adjacent
2-D layers, thus resulting in a rare 2-D → 3-D polythreading motif. Compound 2 exhibits a rare 2-D → 3-D zip-
per-closing motif. Using the degradation of methylene blue (MB) as the model, the photocatalytic activities of
compounds 1–2 were investigated. Both compounds show efficient catalytic activity for the degradation of MB
with the order of 2 N 1. It is found that the POM species of compounds 1–2 play the main role in the photocatalytic
degradation process.
Keywords:
Polyoxometalate
Entangled coordination networks
Transition metal
Photocatalysis
© 2016 Elsevier B.V. All rights reserved.
The design and synthesis of new organic-inorganic hybrid materials,
especially polyoxometalate(POM)-based compounds modified by dif-
ferent transition-metals (TMs) and organic ligands, have attracted con-
siderable attention over recent years, owing to not only their chemical
and structural diversities but also their promising applications, such as
adsorption, luminescence, catalysis, molecular recognition and elec-
tronic and magnetic materials [1]. In the field of catalysis, uniform dis-
persal of POM units within MOFs skeleton at the molecular level can
improve POMs' specific surface area (SSA) to increase the catalytic activ-
ity, and they can be easily recycled after catalytic reactions [2]. In this as-
pect, POMs, as one type of unique nano-sized metal-oxo clusters, can be
regarded as “building blocks” with their terminal or bridging oxides co-
ordinating with metal cations [3]. Especially, Keggin-type POMs are best
choice because of the following reasons: (i) the best chemically-tunable
clusters with multiple components, negative charges and chemical
modifications; (ii) having excellent catalytic properties, such as strong
Brønsted acidity; (iii) their tuned acidic and redox properties [4]. More-
over, transition metal (TM) cations are important nodes due to their ex-
plicit coordination geometries and strong coordination ability to
connect with POMs and organic ligands. However, the choice and design
of organic ligands are of great importance for exploring new POM-based
coordination networks [5–10]. From viewing coordination modes of
organic ligands, the monodentate N-donor ligands such as pyridine
and imidazole groups have been employed due to their definite coordi-
nation modes with TM ions [5]. By comparison, the multi-dentate li-
gands such as triazole and tetrazole ligands have also been extensively
explored in recent years considering their relatively high coordination
activities with TM ions and various coordination modes [6–8]. Thus,
the introduction of \\(CH2)n\\ and/or phenyl spacers between two
terminal multi-dentate N-donor groups can generate flexible ligands
with various coordination modes so as to construct more complicated
and variable structural topologies [6–8]. Therefore, we chose a rigid
and flexible double-triazole-containing ligand, namely 4,4′-bis(1,2,4-
triazol-1-ylmethyl)biphenyl (BBPTZ) (see Scheme 1), and have success-
fully synthesized three new POM-based coordination networks [9]
and a POM-encapsulating cationic MOF with wavelike channels [10].
As a continuing work of this reaction system, we introduce the
metal ions Mn2+ and Ni2+ by changing pH to isolate two new
Keggin-type polyoxometalate-based entangled coordination networks
with the molecular formulas [Mn2(H2O)2(BBPTZ)5][SiW12O40] (1) and
[Ni2(H2O)2(BBPTZ)5][SiMo12O40]·6H2O (2) [11]. Interestingly, both
compounds contain similar 2-D layers which are formed by Keggin-
type POMs and ladder-like chains with dangling arms. Compound 1 ex-
hibits a rare 2-D → 3-D polythreading network due to dangling arms
threading in quadrangular window of the adjacent 2-D layers, but dan-
gling arms of the adjacent 2-D layers are parallel with each other to dis-
play a rare 2-D → 3-D zipper-closing network for compound 2. Due to
the excellent catalytic property of Keggin-type POMs, the photocatalytic
properties of two compounds were also investigated.
⁎
Corresponding authors.
E-mail addresses: haoxiuli1@163.com (X.-L. Hao), jiashifang@126.com (S.-F. Jia),
liyg658@nenu.edu.cn (Y.-G. Li).
http://dx.doi.org/10.1016/j.inoche.2016.08.013
1387-7003/© 2016 Elsevier B.V. All rights reserved.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
X.-L. Hao et al. / Inorganic Chemistry Communications 72 (2016) 132–137
133
1-D chains to form a ladder-like chain (Fig. 1). Meanwhile, each Mn cen-
ter also connects one S-type Lc ligand acted as “dangling arm” and Lc li-
gands protrude upper and lower sides of the ladder (Fig. 1). Further, the
adjacent ladder-like chains are connected by the terminal O atom of the
polyoxoanions to construct a 2-D network with quadrangle-like cavi-
ties, in which Lc ligands having an effectual length of about 16.3 Å are
alternately appeared (Fig. 2 and Fig. S3). Such a 2-D network possesses
two types of meshes (A and B) (Fig. 2). The A mesh is formed by four Ni
centers, two La-type BBPTZ ligands and two Lb-type BBPTZ (Fig. 2a). The
B mesh is constructed by four Ni centers, two La-type BBPTZ ligands and
two SiW12 polyoxoanions (Fig. 2a). The sizes of two types of meshes are
15.33(1) × 16.31(5) Å and 14.67(1) × 16.31(5) Å, respectively (Fig. 2b).
As a result, each B mesh is threaded by two dangling arms of above and
below 2-D layers, thus resulting in a rare 2-D → 3-D polythreading motif
(Fig. 3). In the packing arrangement, dangling arms of adjacent 2-D nets
are extended in different directions, so that these adjacent 2-D nets are
not parallel with each other, but these interval 2-D layers are parallel
with each other (Fig. 3).
Single-crystal X-ray diffraction analysis [14] reveals compound 2
crystallizes in the triclinic space group P-1, and the crystallographically
asymmetric unit consists of one Keggin-type polyoxoanion
Scheme 1. Two configurations of BBPTZ (4,4′-bis(1,2,4-triazol-1-ylmethyl)biphenyl).
Single-crystal X-ray diffraction analyses [14] shows that com-
[SiMo12O40]
4− (SiMo12), two Ni2+ ion, three BBPTZ bridging ligands,
pound 1 crystallizes in the monoclinic space group P2(1)/c. In 1,
two monoprotonated BBPTZ ligand, two coordinated water molecules
and six lattice water molecule (Fig. 4a and Fig. S4). The cationic metal-
organic fragment [Ni2(H2O)2(BBPTZ)5]4+ contains one crystallographi-
cally independent Ni2+ center, which adopts a hexa-coordinated mode
with four nitrogen atoms derived from four BBTZ ligands, one terminal
oxygen atom originated from SiMo12 polyoxoanions and one coordinat-
ed water molecules (Fig. 4a and Fig. S4). The bond lengths of Ni\\N
range from 1.968(2) to 2.018(2) Å and the N\\Ni\\N bond angles
vary from 86.7(6) to 172.8(7)°. The bond distance of Ni\\O(12) is
2.478(1) Å, which can be viewed as weak coordination bonds between
the basic structural unit contains cationic [Mn2(H2O)2(BBPTZ)5]4+
4−
and the Keggin-type polyoxoanion [SiW12O40
]
(SiW12) and two
lattice water molecules (Fig. 1a and Fig. S1). In the cationic
[Mn2(H2O)2(BBPTZ)5]4+ unit, there is only one crystallographically in-
dependent Mn2+ center, which possesses a six-coordinated mode with
four nitrogen atoms derived from four different BBPTZ ligands, one
terminal oxygen atom originated from SiW12 polyoxoanions, and one
coordinated water molecules (Fig. 1a and Fig. S1). The bond distances
of Mn\\N vary from 2.204(2) Å to 2.244(2) Å, and Mn\\O are
2.192(1) Å and 2.237(1) Å. The bond angles of N(O)\\Mn\\N(O) are
in the range of 84.1(5)–173.9(6)°. It is interesting that the BBPTZ li-
gands can be regarded as three different types labeled with La, Lb and
Lc as shown in Fig. 1a and Fig. S2. Although apical nitrogen atom of
each triazole group on the La and Lb ligands displays a monodentate co-
ordination mode with one Mn center, two rigid phenyl centers of La and
Lb ligands with trans-configuration exhibit two different orientation
(Fig. 1a and Fig. S2). For Lc ligand, apical nitrogen atom of one triazole
group exhibits a monodentate coordination mode with one Mn center
and the other triazole group is not coordinated with any atom (Fig. 1a
and Fig. S2). Based on above coordination mode, the Mn atoms are con-
nected via trans-La ligands to form an undulating 1-D chain and the
trans-Lb can be viewed as the “middle rail” that connects the adjacent
Fig. 1. (a) Ball-and-stick and (b) schematic views of the 1-D ladder-like chain with
dangling arms in 1 formed by one Mn2+ center and BBPTZ ligands with three different
kinds of structural configurations.
Fig. 2. (a) Structural views and (b) schematic views of 2-D network motif based on
Keggin-type POM clusters and the ladder-like chains in 1 with two types of meshes (A
and B).

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
134
X.-L. Hao et al. / Inorganic Chemistry Communications 72 (2016) 132–137
Fig. 3. Schematic illustration of 2-D → 3-D polythreading motif of 1, showing the mesh B
threaded by two arms above and below.
Ni2+ center and the surface O atom of SiMo12. It is noteworthy that the
BBPTZ ligands in this metal-organic cationic moiety can be separated
into three groups which are labeled with La, Lb and Lc (Fig. 4a and Fig.
S5). The first group (La) possesses cis-configuration and links adjacent
two Ni atoms into 1-D chains; the second group (Lb) displays trans-
configuration and can be viewed as the “middle rail” that connects the
adjacent 1-D chains to form a ladder-like chain; the third group (Lc)
adopts trans-configuration and regards as “dangling arm”, protruding
upper and lower sides of the ladder (Fig. 4 and Fig. S5). It is worth noting
that the third group (Lc) is monoprotonated and only one triazole group
coordinate with the Ni center. Furthermore, each POM unit acts as an
inorganic linker to join the adjacent parallel 1-D ladder-like chains
into a 2-D network with square meshes (Fig. 5). Compared with
compound 1, the 2-D layer of compound 2 also contains similar two
kinds of meshes (A and B) with the sizes of 18.52(1) × 16.76(3) Å and
15.29(9) × 16.76(3) Å, respectively (Fig. 5b). Note that the surface of
2-D layer is not flat due to existing the zipper-type structural feature
and the adjacent 2-D layers are parallel with each other (Fig. 6). Thus,
the adjacent 2-D layers are packed together through a “zipper-closing”
mode, thus resulting in a rare 2-D → 3-D zipper-closing motif (Fig. 6
and Fig. S6). Moreover, all the alternate dangling arms are well
Fig. 5. (a) Polyhedral and ball-and-stick and (b) schematic views of the Keggin-type
POMs-connected 2-D network of 2 with two kinds of meshes (A and B).
Fig. 4. (a) Ball-and-stick and (b) schematic views of the dangling arms-containing 1-D
ladder-like chain in 2 based by one Ni2+ center and BBPTZ ligands with three different
groups.
Fig. 6. Schematic illustration of 2-D → 3-D zipper-closing motif of 2, packing together
through dangling arms of the adjacent 2-D layers.

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
X.-L. Hao et al. / Inorganic Chemistry Communications 72 (2016) 132–137
135
separated in 3-D supramolecular framework without obvious intermo-
lecular interactions.
Based on the above discussion, it is discovered that both compounds
contain similar 2-D layers with two types of meshes and dangling arms.
However, the structural features of their 2-D → 3-D display obvious dif-
ferences about the following reasons. Firstly, the dihedral angels be-
tween plane of dangling arms and the surface of 2-D layer are 74.3(6)
and 39.6(8)°, respectively, which reveals that dangling arms of com-
pound 1 trend to plumb the surface of 2-D layer (Fig. S7). In addition,
orientations of dangling arms in adjacent 2-D nets exist the slight
angle (33.8(2)°) for compound 1, but the adjacent 2-D layers containing
dangling arms are parallel with each other in compound 2 (Fig. S8). Fi-
nally, the distance between the surfaces of adjacent 2-D layers is
9.70(2) Å less than the effectual length of dangling arm (16.28(5) Å)
in compound 1 (Fig. S9a). In compound 2, the lengths of adjacent 2-D
layers and dangling arm are 15.56(2) Å and 16.13(8) Å, respectively,
which are extremely approximate (Fig. S9b). Therefore, dangling arms
can thread meshes of the adjacent 2-D layers for compound 1, but dan-
gling arms of compound 2 only can parallely and alternately appear
(Figs. 3 and 6).
Nowadays the degradation of waste organic dyes is one of the cur-
rently significant tasks, since discharge of their waste can lead to serious
water pollution and water eutrophication [16]. In this research field, the
photocatalytic reaction by the use of photocatalysts under UV irradia-
tion has been developed as an effective approach to decompose of or-
ganic materials. During the research process, methylene blue (MB) is
usually employed as a typical model dye contaminant to evaluate the
catalytic activities in purification of waste water. It is well known that
a wide range of Keggin-type POMs have been demonstrated to be one
type of potential photocatalysts in the degradation of organic dyes. It
is worth mentioning that compounds 1–2 are insoluble in aqueous solu-
tion, thus they are employed as heterogeneous catalysts, which were in-
vestigated via the experimental model of MB photodegradation under
UV irradiation. In a typical process, aqueous MB solution (400 mL,
10.0 mg L−1) combining one of compounds 1–2 (50 mg) as the catalyst
was exposed to UV light (125 W). Moreover, the photodegradation pro-
cess of the blank MB aqueous solution without any catalyst was also
monitored as a control experiment. Under UV irradiation, in contrast
with the photodegradation process of MB without any photocatalyst,
an obvious decrease in the absorption peaks of the MB solution moni-
tored by UV–vis spectra was observed for compounds 1–2 (Fig. 7), indi-
cating that both compounds possess excellent photocatalytic properties
for MB degradation. As shown in Fig. 8, the plots of Ct/C0 of MB solutions
versus irradiation time further reveal that the photocatalytic activities
increase from 33.37% (without any catalyst) to 93.22% for 1 and
97.45% for 2 after 90 min of irradiation.
Fig. 7. UV–vis absorption spectra of the MB solutions during the decomposition reaction
under UV light irradiation in the presence of compounds 1 (a) and 2 (b).
patterns of compound 2 before the catalytic process, PXRD pattern
after five-cycle photocatalytic tests is still no obvious changes, further
suggesting that such compounds 1–2 as heterogeneous catalysts are
stable in the catalytic process (Fig. S12). Finally, a litter decrease in the
absorption peaks of the MB solution was observed after 90 min without
In order to explore the active ingredient of compounds 1–2 in the
catalytic process, the catalytic activity of the TM precursors (the mixture
of Mn(OAc)2·4H2O/Ni(OAc)2·4H2O and BBPTZ) and the POM precur-
sors ((Bu4N)4[SiW12O40]/(Bu4N)4[SiMo12O40]) were respectively fur-
ther performed as a series of comparative experiments. It is found that
the catalytic activities of compounds 1–2 are a little higher than those
of (Bu4N)4[SiW12O40] and (Bu4N)4[SiMo12O40] POMs, but the catalytic
activities of the POM precursors exhibit obviously higher than the TM
precursors (Fig. S10). Thus, the POM anions should be the catalytically
active species in the photocatalytic decomposition of MB. This result is
consistent with the reported catalytic mechanism of the POM species.
As shown in Fig. S10, the catalytic activity of (Bu4N)4[SiMo12O40] is
slightly higher than that of (Bu4N)4[SiW12O40]. Similarly, the catalytic
activities of compounds 1–2 are slightly different with the order of 2
(97.45%) N 1 (93.22%). These catalytic differences between compounds
1–2 may be mainly derived from their difference in POM species. Fur-
thermore, compound 2 exhibiting the highest catalytic activity was cho-
sen as a representative catalyst to investigate catalyst lifetime. In our
initial experiment, compound 2 recycled for five times also maintains
the higher catalytic activity (Fig. S11). In comparison to the PXRD
Fig. 8. Plot of Ct/C0 of MB versus irradiation time under UV light in the presence of
compounds 1 and 2, and the black curve is the control experiment without any catalyst.