Four Strandberg-type polyoxometalates with organophosphine centre decorated by transition metal-2,2'-bipy/H2O complexes

Article abstract of DOI:10.1016/j.jssc.2017.05.027

Four inorganic-organic hybrid compounds composed of Strandberg-type organophosphomolybdate anion [(C₆H₅PO₃)₂Mo₅O₁₅]^4? (abbreviated as (PhP)₂Mo₅) and transition metal (TM)?2,2'-bipy/H₂O complex units, namely [(TM(H₂O)(bipy))₂(C₆H₅PO₃)₂Mo₅O₁₅]_n (TM = Co, (1); Ni, (2)), [(Cu(bipy)₂)₂(C₆H₅PO₃)₂Mo₅O₁₅]?2H₂O (3) and [(Zn(bipy)(μ-OH))₂(Zn(bipy)₂(C₆H₅PO₃)₂Mo₅O₁₅)₂]?3H₂O (4) (bipy = 2,2'-bipyridine), were successfully constructed under hydrothermal conditions, and their structures were determined by single crystal X-ray diffraction analysis and spectroscopic methods. The central heteroatoms in these polyoxometalates (POMs) are all organophosphine (RP) groups. Compound 1 and compound 2 are isostructural (PhP)₂Mo₅-based TM-coordination polymers with the two-dimensional layer frameworks. In compound 3, the bi-supporting structure containing one (PhP)₂Mo₅ unit and two [Cu(bipy)₂]²⁺ cations. For compound 4, it can be regarded as a dimer of two bi-supporting {Zn(bipy)₂(PhP)₂Mo₅Zn(bipy)} clusters that were connected by two μ-OH groups. The acid-catalytic activities and fluorescence properties of the four hybrids have been investigated.

Full text of DOI:10.1016/j.jssc.2017.05.027

Journal of Solid State Chemistry 253 (2017) 52–57
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Four Strandberg-type polyoxometalates with organophosphine centre
decorated by transition metal-2,2'-bipy/H2O complexes
Ting Lu, Shu-Li Feng, Zai-Ming Zhu⁎, Xiao-Jing Sang, Fang Su, Lan-Cui Zhang⁎
School of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Four inorganic-organic hybrid compounds composed of Strandberg-type organophosphomolybdate anion
4−
Polyoxometalate
Strandberg-type
Organophosphomolybdate
Crystal structure
Catalytic property
[(C6H5PO3)2Mo5O15
namely [(TM(H2O)(bipy))2(C6H5PO3)2Mo5O15
]
(abbreviated as (PhP)2Mo5) and transition metal (TM)−2,2'-bipy/H2O complex units,
(TM = Co, (1); Ni, (2)), [(Cu(bipy)2)2(C6H5PO3)2Mo5O15]
]
n
·2H2O (3) and [(Zn(bipy)(µ-OH))2(Zn(bipy)2(C6H5PO3)2Mo5O15)2]·3H2O (4) (bipy = 2,2'-bipyridine), were
successfully constructed under hydrothermal conditions, and their structures were determined by single crystal
X-ray diffraction analysis and spectroscopic methods. The central heteroatoms in these polyoxometalates
(POMs) are all organophosphine (RP) groups. Compound 1 and compound 2 are isostructural (PhP)2Mo5-
based TM-coordination polymers with the two-dimensional layer frameworks. In compound 3, the bi-
supporting structure containing one (PhP)2Mo5 unit and two [Cu(bipy)2]2+ cations. For compound 4, it can
be regarded as a dimer of two bi-supporting {Zn(bipy)2(PhP)2Mo5Zn(bipy)} clusters that were connected by two
µ-OH groups. The acid-catalytic activities and fluorescence properties of the four hybrids have been
investigated.
1. Introduction
they obtained a new Strandberg anion with phosphonocarboxylate
centre, in which the terminal carboxylate groups were firstly coordi-
Inorganic-organic hybrid materials based on polyoxometalates
(POMs) have always been the focus of POM chemistry and the related
subjects, which may be due to their diverse structures, special proper-
ties, and attractive applications in the fields of medicine, magnetic
materials and catalytic materials [1–3]. Strandberg-type POMs, as one
kind of small molecules in POM family, have been found to exhibit
many excellent behaviors, such as high electron density, good stability
and catalytic activity [4–6]. But they have not attracted enough
attention by researchers compared with other types of POMs. The
polyoxoanions of Strandberg-type POMs are capable of coordinating
with transitional metal ions by octahedral and tetrahedral oxygen
atoms, forming many fascinating structures [4,6b,7]. Especially, the
centre inorganic phosphorus atoms of the polyoxoanion can be
replaced by the organophosphine (RP) groups, and thus results in
new inorganic-organic hybrid POMs. Zubieta's group has reported a
series of Strandberg-type hybrid organophosphomolybdate compounds
since 2001 [7a-b,8], and they investigated the magnetic properties of
some compounds containing transition metal (TM) ions [7a-b,8c-f]. In
these compounds, the centre RP groups include phosphonocarboxylate
groups, alkylphosphonic acids, diphosphonic acids, etc. In recent years,
Chen's group further researched the pentamolybdodiphosphates [9],
nated with TM ions [9b]. In 2016, Yang's group separated (PhP)2Mo5,
6–
4–
[Mo7O24
]
and [Mo8O26
]
anions through capillary zone electro-
phoresis in the synthetic process of (PhP)2Mo5, and investigated the
solution stability and equilibria of the three polyoxoanions, the results
showed that the synthetic conditions of Strandberg-type POMs con-
taining RP centre groups are relatively harsh comparing to
6–
[P2Mo5O23
]
with the inorganic phosphorus centre [10]. This study
perhaps provides valuable information for the synthesis of these POM
compounds. Recently, our group successfully obtained two new
Strandberg-type organophosphomolybdates and one organophospho-
tungstate modified by Cu-4,4'-bipyridine/H2O complex units [6b-c],
and further explored their acid-catalytic performances in the synthesis
of cyclohexanone ethylene ketal. It is worth noting that the catalytic
activities of RP-based compounds are higher than those with central
inorganic phosphorus atoms [6a]. In addition, the three compounds
displayed good photoluminescence property. As a continuation of our
previous work, we wish to design and synthesize a series of new
compounds containing (PhP)2Mo5 cluster and TM-bipy subunits
through hydrothermal methods. In the present work, Co(II), Ni(II),
Cu(II) and Zn(II)-bipy/H2O complexes were successfully introduced
into the skeletons of (PhP)2Mo5 polyoxoanions by controlling the
⁎
Corresponding authors.
E-mail addresses: chemzhu@sina.com (Z.-M. Zhu), zhanglcui@126.com (L.-C. Zhang).
http://dx.doi.org/10.1016/j.jssc.2017.05.027
Received 25 March 2017; Received in revised form 7 May 2017; Accepted 20 May 2017
Available online 21 May 2017
0022-4596/ © 2017 Elsevier Inc. All rights reserved.

T. Lu et al.
Journal of Solid State Chemistry 253 (2017) 52–57
reaction temperature, reaction time, pH and the ratio of starting
materials, forming four new (PhP)2Mo5-based compounds, i.e. com-
pounds 1–4. Besides, the acid catalytic activities for the synthesis of
cyclohexanone ethylene ketal and solid-state fluorescence properties of
the four POMs have also been studied.
distilled water according to the order, the pH of the mixture was finally
adjusted to 3–4 with 4 mol L−1 HCl, and the reaction temperature was
increased to 170 °C. Blue block crystals were isolated (yield: ca. 81%
based
on
Mo).
Elemental
analysis
calculated
for
C52H46O23N8Cu2P2Mo5: C 34.32, H 2.55, N 6.16, P 3.40, Cu 6.98,
Mo 26.37%, Found: C 34.40, H 2.58, N 6.25, P 3.35, Cu 6.91, Mo
26.28%. IR (KBr pellet, ν/cm−1): 3452 (s), 3079 (m), 2927 (w), 1605
(m), 1442 (m), 1317 (w), 1251 (w), 1127 (m), 1042 (m), 977 (m), 904
(s), 773 (m), 694 (s), 564 (m).
2. Experimental
2.1. Material and methods
All chemicals were of reagent grade as received from commercial
sources and used without further purification. Elemental analyses were
performed on a Vario El cube elemental analyzer for C, H, N, and a
Prodigy XP emission spectrometer for P, Co, Ni, Cu, Zn and Mo. Single
crystal X-ray diffraction data were collected on a Bruker Smart APEX II
X-diffractometer equipped with graphite monochro-mated Mo Kα
radiation (λ = 0.71073 Å). The infrared spectra (IR) were recorded
on KBr pellets with a Bruker AXS TENSOR-27 FTIR spectrometer in
the range of 4000–400 cm−1. The X-ray powder diffraction (XRPD)
data were obtained on a Bruker AXS D8 Advance diffractometer using
Cu Kα radiation (λ = 1.5418 Å) with a step size of 0.02° in the 2θ range
from 5 to 60°. Thermogravimetric analysis (TGA) were carried out on a
Pyris Diamond TG thermal analyzer in air at a heating rate of
10 °C min−1. The solid UV spectra were recorded on a Agilent Cary-
300 UV–visible spectrophotometer. The photoluminescence properties
were determined on a Hitachi F-7000 fluorescence spectrophotometer
in the solid state at room temperature. The yield of cyclohexanone
ethylene ketal was confirmed on a JK-GC112A gas chromatograph.
2.2.4. Synthesis
(C6H5PO3)2Mo5O15)2]·3H2O (4)
of
compound
[(Zn(bipy)(µ-OH))2(Zn(bipy)2
Compound 4 was synthesized in the same way as that of compound
3, with the exception of the amount of C6H5PO3H2 (0.05 g, 0.3 mmol),
ZnSO4·7H2O (0.10 g, 0.3 mmol) and bipy (0.05 g, 0.3 mmol), mean-
while the reaction time was changed to 4 days. Colorless transparent
bulk crystals were obtained in ca. 13% yield (based on Mo). Elemental
analysis calculated for C84H78O47N12Zn4P4Mo10: C 30.10, H 2.35, N
5.01, P 3.70, Zn 7.80, Mo 28.62%, Found: C 30.02, H 2.40, N 5.09, P
3.62, Zn 7.92, Mo 28.53%. IR (KBr pellet, ν/cm−1): 3440 (s), 3132 (m),
2925 (w), 1603 (m), 1442 (m), 1393 (m), 1317(w), 1247 (w), 1143 (w),
1101 (m), 983 (w), 906 (s), 697 (s), 544 (m).
2.3. Single crystal X-ray crystallography
The structures were solved by direct methods and refined by full-
matrix least-squares fitting on F2 using SHELXTL-97 [11,12]. An
empirical absorption correction was applied using the SADABS pro-
gram. Crystal data and structure refinement parameters of compounds
1–4 are listed in Table 1. In compounds 1–4, H atoms on C atoms
were added in calculated positions. The selected bond lengths and
angles, and the hydrogen bonds are listed in Tables S1–S5. CCDC
reference numbers: 1527719–1527722.
2.2. Synthesis of compounds 1–4
2.2.1. Synthesis of compound [(Co(H2O)(bipy))2(C6H5PO3)2Mo5O15 n
]
(1)
C6H5PO3H2 (0.08 g, 0.5 mmol) and Na2MoO4·2H2O (0.15 g,
0.6 mmol) were dissolved in 20 mL of distilled water under stirring
at room temperature for 30 min. Then bipy (0.06 g, 0.4 mmol) was
added to the above solution under stirring, and the pH of the mixture
was adjusted to 3–4 with 4 mol L−1 HCl, and CoCl2·6H2O (0.20 g,
0.8 mmol) was added simultaneously. The resulting mixture was
transferred to a 30 mL Teflon-lined autoclave and kept at 150 °C for
3 days, and then brick red block crystals were obtained (yield: ca. 71%
2.4. Catalytic experiment
According to the catalytic experimental method described in our
previous work [6], the catalytic activities for the synthesis of cyclohex-
anone ethylene ketal of the title compounds were evaluated and
compared to those of the parent compounds. The main procedures
based
on
Mo).
Elemental
analysis
calculated
for
Table 1
Crystal and refinement date for compounds 1–4.
C
32H30O23N4Co2P2Mo5: C 25.66, H 2.02, N 3.74, P 4.14, Co 7.87,
Mo 32.03%; Found: C 25.70, H 2.05, N 3.65, P 4.08, Co 7.76, Mo
32.21%. IR (KBr pellet, ν/cm−1): 3417 (m), 3065 (w), 2927 (w), 1605
(m), 1442 (m), 1317 (w), 1250 (w), 1147 (w), 1101 (s), 1055 (w), 977
(w), 937 (s), 904 (s), 694 (s), 597 (w), 551 (m).
1
2
3
4
Formula
C32H30O23N4
Co2P2Mo5
1498.10
296(2)
0.71073
C32H30O23N4
Ni2P2Mo5
1497.66
296(2)
0.71073
C52H46O23N8
Cu2P2Mo5
1819.69
296(2)
0.71073
Monoclinic
C2/c
20.697(2)
12.8566(13)
22.387(2)
92.502(2)
5951.3(10), 4 5245.3(16), 2
2.031, 3584
1.014
C84H78O47N12
Zn4P4Mo10
3352.34
296(2)
0.71073
Monoclinic
P21/n
11.790(2)
22.631(4)
20.095(3)
101.971(3)
Formula weight
T/K
Wavelength/Å
Crystal system
Space group
a/Å
b/Å
c/Å
β/°
V/Å3, Z
Dc/g cm−3, F000
GOF
Reflections
collected
Unique data,
Rint
2.2.2. Synthesis of compound [(Ni(H2O)(bipy))2(C6H5PO3)2Mo5O15 n
]
(2)
Monoclinic
C2/c
Monoclinic
C2/c
The detailed procedure for the preparation of compound 2 was
17.441(5)
10.679(5)
24.081(9)
97.998(9)
4442(3), 4
2.240, 2912
1.015
17.3800(15)
10.8393(10)
24.017(2)
97.0220(10)
4490.6(7), 4
2.215, 2920
1.037
similar to that of compound 1 except using NiCl2·6H2O (0.20 g,
0.8 mmol) instead of CoCl2·6H2O. Green block crystals of compound
2 in the yield of ca. 61% based on Mo were isolated from the filtrate.
Elemental analysis calculated for C32H30O23N4Ni2P2Mo5: C 25.66, H
2.02, N 3.74, P 4.14, Ni 7.84, Mo 32.04%; Found: C 25.71, H 2.11, N
3.68, P 4.09, Ni 7.92, Mo 31.92%. IR (KBr pellet, ν/cm−1): 3325 (m),
3058 (w), 2927 (w), 1605 (m), 1442 (m), 1311 (w), 1251 (w), 1140 (w),
1107 (s), 1055 (m), 977 (m), 931 (s), 694 (s), 597 (w), 547 (m).
2.123, 3284
1.023
26734
11220
11255
15031
3920, 0.0390
3943, 0.0193 5238, 0.0248
9216, 0.0508
θ Range (°)
R1(I > 2σ(I))a
wR2 (all data)a
1.71 to 25.00 1.71 to 25.00 1.82 to 25.00 1.80 to 25.00
2.2.3. Synthesis of compound [(Cu(bipy)2)2(C6H5PO3)2Mo5O15]·2H2O
(3)
The dosages of C6H5PO3H2 and Na2MoO4·2H2O were the same as
those of compound 1. The amount of CuCl2·2H2O was decreased to
0.3 mmol (0.05 g), while the amount of bipy was increased from 0.4 to
0.5 mmol (0.08 g), after the above materials were added into 20 mL of
0.0299
0.0614
0.0200
0.0469
0.0243
0.0561
0.0426
0.0948
R1 = ∑||F0| − |FC||/∑|F0|; wR2 = ∑[w(F0 −FC2)2]/∑[w(F02)2]1/2
a
2
53

T. Lu et al.
Journal of Solid State Chemistry 253 (2017) 52–57
are as follows: the catalyst was added to a mixture of cyclohexanone
and glycol, and 10 mL of cyclohexane was used as a water-carrying
agent in the reaction process. After completing the reaction, the
heterogeneous catalyst could be easily separated from the reaction
mixture through decantation or filtration, and washed and dried, and
then used directly for the next reaction cycle. The reusability of the
catalysts were investigated under the optimum reaction conditions.
and keeping other conditions unchanged, no crystals were obtained.
Interestingly, the adding order of TMs has a significant effect on the
formation and crystallization of the compounds. TM should be added
after adjusting the pH for the synthesis of compound 1 or 2, while for
the synthesis of compound 3 or 4, TM must be added before adjusting
the pH. In addition, the pH also plays a key role in the synthesis of
these Strandberg-type POMs with RP. The range of pH from 3 to 4 is
suitable for acquiring reasonable yields of these crystalline compounds.
3. Results and discussion
3.2. Structural analysis
3.1. Synthesis
Single crystal X-ray diffraction analysis reveals that compounds 1 and
2 are isostructural tetra-supporting POM-based inorganic-organic hy-
brids. Compound 1/2 is considered as a Co(II)/Ni(II) coordination
polymer, which consists of (PhP)2Mo5 polyanions and [Co/Ni(H2O)
(bipy)]2+ cations (Fig. 1a, Fig. S1a and Fig. S1b). As shown in Fig. 1a,
the (PhP)2Mo5 unit acts as a hexadentate ligand, connecting four Co(II)/
Ni(II) ions via four terminal oxygen atoms (O5, O5', O9, O9') and two
bridging oxygen atoms (O1, O1'). Co1/Ni1 coordinates with two N atoms
(N1, N2) from one bipy ligand, and two terminal oxygen atoms (O5, O9)
and one bridge oxygen atoms (O1) of the polyoxoanion, and one terminal
water ligand (O1W). The bond lengths of Co–O/OW and Co–N are 2.042
(3)–2.206 (3) and 2.078 (4)–2.086 (4) Å, respectively. The bond lengths
of Ni–O/OW and Ni–N are 2.0311 (18)–2.1596 (19) and 2.032 (2)–2.038
(2) Å, respectively. The Mo–O distances are in the range of 1.678 (3)–
2.410 (3) and 1.690 (2)–2.4203 (18) Å for 1 and 2, respectively (Tables
S1 and S2). As shown in Fig. 1b, the adjacent two (PhP)2Mo5 units are
linked by a Co(II)/Ni(II) ion, and each (PhP)2Mo5 is surrounded by six
(PhP)2Mo5 units, forming a plane, and the adjacent planes are parallel to
each other to further form a 3-D framework through hydrogen bonding
interactions (Fig. 1c). The hydrogen bonds of O–H···O are 2.706(4)–
2.772(4) Å and 2.708(2)–2.761(2) Å for 1 and 2, respectively (Table 5).
The 2-D network topology of compound 1/2 can be further described as
in Fig. 2b. In this simplification, the (PhP)2Mo5 unit act as one kind of
four-node, the Co(II)/Ni(II)-bipy subunit is simplified into a straight line
(Fig. 2a). From the topological point of view, the 2-D structure of
compound 1/2 can be simplified to a unique four-connected 4-c net with
stoichiometry (4-c), and the point symbol for the net is (44·62). In
compound 3, as we can see from Fig. 3 and S2a, each molecule contains
one (PhP)2Mo5 unit, two [Cu(bipy)2]2+ and two lattice water molecules, in
which two [Cu(bipy)2]2+ fragments support on the two sides of
Strandberg-type anion through two terminal oxygen atoms of MoO6
octahedra, forming a bi-supporting structure. All Cu(II) ions adopt five-
coordinated square-pyramidal coordination geometries, and the Cu1–O10
bond length is 2.065 (3) Å, and the bond lengths of Cu–N are 1.957 (4)–
2.119 (5) Å. The Mo–O distances are in the range of 1.682 (3)–2.518 (3)
Å (Table S3). A 3-D architecture is also constructed through hydrogen
bonding and electrostatic interactions (Fig. S3). Compound 4, as a dimer
of (PhP)2Mo5, contains both the mononuclear and binuclear zinc complex
units, and three lattice water molecules. As viewed from Fig. 4 and S2b,
two bi-supporting {Zn(bipy)2(PhP)2Mo5Zn(bipy)} clusters are bridged by
two µ-OH groups coordinating with two Zn2+ ions of the {Zn(bipy)} units,
it also can be regarded as two mono-supporting {Zn(bipy)2(PhP)2Mo5}
RP groups can be introduced into the interior of the Strandberg-
type POM skeleton, forming very interesting inorganic-organic hybrid
compounds, e.g. (RP)2Mo5 and (RP)2W5-based compounds. However,
these kinds of compounds are usually difficult to obtain because their
synthetic conditions including reaction temperature, molar ratio and
adding order of raw materials, pH, etc. are very harsh. In order to
synthesize this sort of compounds, Na2MoO4·2H2O, C6H5PO3H2 and
bipy as well as different TMs (Co(II), Ni(II), Cu(II) or Zn(II)) were used
as the starting materials. Based on our recent work [6b], the synthetic
methods of these compounds were designed, and the synthetic path-
ways are illustrated in Scheme 1. By groping various experiment
conditions, two new crystalline (PhP)2Mo5-based coordination poly-
mers (isostructural compound 1 and compound 2) containing Co(II)
and Ni(II)-bipy/H2O complexes were finally obtained with the molar
ratio of Mo: P: TM: bipy being 1.5: 1.25: 2.0: 1.0 at 150 °C after 3 days.
However, no crystals of Cu(II) or Zn(II)-based compounds were
obtained under the same conditions (Scheme 1(i)). Therefore, the
synthetic and crystallization conditions of this type of POMs varied
with different TMs. When changing the molar ratio of Mo: P: Cu: bipy
to 2.0: 1.7: 1.0: 1.7 and other conditions remain unchanged, only a
small amount of impure crystals of Cu(II)-(PhP)2Mo5 complex (com-
pound 3) were generated (Scheme 1(ii)a). In pathway (ii), when
increasing the temperature to 170 °C, crystalline compound 3 with
81% yield was achieved successfully (Scheme 1(ii)b), which means the
higher temperature is critical to obtain a relatively pure compound with
high yield. Unfortunately, still no Zn(II)-based crystals were obtained
under the identical conditions. To obtain Zn(II)-(PhP)2Mo5 complex,
large amounts of experiments were carried out. The experimental
results showed that when the molar ratio of Mo: P: Zn: bipy was
adjusted to 2.0: 1.0: 1.0: 1.0, and the reaction time was further
increased to 4 days at 170 °C, pure crystalline compound 4 can be
got (Scheme 1(iii)). But the product yield needs to be further increased,
and the optimum conditions need further improving. As can be seen
from Scheme 1(iii), remarkably, if using ZnCl2 instead of ZnSO4·7H2O
clusters are linked by
a binuclear zinc complex [Zn(bipy)(µ-
OH)2Zn(bipy)] through four oxygen atoms of the two polyoxoanions,
forming a symmetrical structure. In the single crystal structure of
compound 4, Zn(II) ions exists in two different coordination environ-
ments, the asymmetric Zn1 is hexa-coordinated by four nitrogen atoms
(N1, N2, N3, N4) from two bipy molecules (Zn–N, 2.080(6)–2.168(5) Å),
one terminal oxygen atom (O5) from a MoO6 octahedron group (Zn–O5
2.240(4) Å), and one bridge oxygen atom (O4) from corner-sharing MoO6
octahedron group and PO4 tetrahedron (Zn–O4 2.108(4) Å), forming
ZnN4O2 octahedron. Zn2 centre is also in an octahedral coordination
environment, it coordinates with two nitrogen atoms (N5, N6) from one
bipy molecule, one terminal oxygen atom (O21) from a MoO6 octahedron,
one bridge oxygen atom (O16) from corner-sharing MoO6 octahedron and
Scheme 1. Schematic representation of the synthetic pathways and conditions of
compounds 1–4.
54

T. Lu et al.
Journal of Solid State Chemistry 253 (2017) 52–57
Fig. 1. (a) Structural representation of the coordination environment of four Co(II)/Ni(II) ions coordinating with the (PhP)2Mo5 cluster in compound 1/2; (b) The infinite 2-D layers of
compound 1/2; (c) The packing view of the infinite 3-D network for 1/2.
Fig. 2. (a) Perspective views of the connected (Co(II)/Ni(II) ions and (RP)2Mo5
polyanion) nodes in compound 1/2; (b) the topology framework of 1/2 with (44·62).
Fig. 4. Combined polyhedral and ball-and-stick representation for compound 4.
3.3. IR spectra of compounds 1–4
The bands in the IR of compounds 1–4 (Fig. S5) at 3452–
2925 cm−1 are attributed to the characteristic vibrations of O–H and
C–H. The bands in the region of 1605–1247 cm−1 are associated with
the pyridine ring of a bipy molecule [6b-c]. The bands of 1147–
1042 cm−1 are due to the v(P–O) of the organophosphonate ligands,
and bands at 983–977, 937–694 cm−1 belong to v(Mo–Oterminal) and
v(Mo–Obridging) of polyoxoanion, respectively [9].
Fig. 3. Combined polyhedral and ball-and-stick representation for compound 3.
3.4. XRPD
PO4 tetrahedron, and two bridge oxygen atoms (O22 and O22′) of two µ-
OH groups, forming ZnN2O4 octahedron. The bond lengths of Zn2–N5,
Zn2–N6, Zn2–O16, Zn2–O21 and Zn–O22/O22′ are 2.085(6), 2.080(6),
2.062(4), 2.194(5) and 2.209(5) Å, respectively. The Mo–O distances are
in the range of 1.685(4)–2.481(4) Å) (Table S4). Fig. S4 exhibits a 3-D
stacked graph of compound 4 formed by the hydrogen bonding and
electrostatic interactions.
The XRPD of the crystalline samples were tested at room tempera-
ture in order to evaluate the purity of as-synthesized compounds. As
shown in Fig. S6, all major peaks of the simulated and experimental
XRPD patterns are in agreement with each other, indicating their
reasonable crystalline phase purity for compounds 1–4. The differ-
ences in intensity may be due to the preferred orientation of the
crystalline powder samples.
55

T. Lu et al.
Journal of Solid State Chemistry 253 (2017) 52–57
3.5. TGA
Table 2
Catalytic performances of 1–4 compared with the reference catalysts for the synthesis of
cyclohexanone ethylene ketal.
The thermal behaviors of compounds 1–4 were investigated with
the thermal analysis methods, and their TGA curves of are given in Fig.
S7. It is found that the four compounds all display a three-continuous
weight loss. As shown in Fig. S7a and Fig. S7b, in the temperature
range of 30–650 °C, the total weight losses of 32.94% and 33.02% for
compound 1 and compound 2 respectively are due to the removal of
two coordinate water molecules, two bipy molecules and two phenyl
groups, which are consistent with the calculated values (calcd. 33.55%
and 33.57%). For compound 3, the total weight loss of 44.94% (calcd.
44.79%) in the temperature range of 30–640 °C corresponds to the loss
of two crystalline water molecules, four bipy molecules and two phenyl
groups (Fig. S7c). The TG curve of compound 4 exhibits the total
weight loss of 40.38% (calcd. 39.84%) at 30–550 °C, which matches to
the loss of three crystalline water molecules, six bipy molecules, four
phenyl groups and two µ-OH groups (Fig. S7d).
Entry
Catalyst
Solubility
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
Compound 1
Compound 2
Compound 3
Compound 4
NH4-(PhP)2Mo5 [13]
4,4'-bipy-Cu-(PhP)2Mo5
CoCl2·6H2O
NiCl2·6H2O
CuCl2·2H2O
ZnSO4·7H2O
–
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
soluble
soluble
soluble
soluble
–
4.0
4.0
4.0
4.0
4.0
2.5
4.0
4.0
4.0
4.0
4.0
90
88
89
89
81
91
98
98
99
98
10
[6b]
9
10
11
Reaction conditions: the molar ratio of cyclohexanone to glycol was 1:1.4 (0.1 mol of
cyclohexanone); water-carrying agent: 10 mL of cyclohexane; reaction temperature: 95–
100 °C. The molar ratio of the catalyst to cyclohexanone is 1:200. 4,4'-bipy-Cu-
(PhP)2Mo5=[(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15
]
n
3.6. Solid UV–vis absorption and fluorescence spectra
In order to further understand the optical properties of the four
compounds, the diffuse reflectivity spectra in absorption mode of the
parent (NH4)4[(C6H5PO3)2Mo5O15] (NH4-(PhP)2Mo5), bipy and com-
pounds 1–4 from 200 to 800 nm are presented in Fig. S8. As seen from
Fig. S8, the parent NH4-(PhP)2Mo5 has a strong band (292 nm) (O →
Mo) only in the UV region [13]. The absorption at 216 and 260 nm of
bipy in the UV region is ascribed to the π→π* transition [14]. While the
four title inorganic–organic hybrid compounds show the strong bands
(302, 308, 305, 310 nm) in the UV and visible region (505, 619,
764 nm). Consequently, the absorption bands in the UV range of
compounds 1–4 were assigned to a charge-transfer transition from the
ligand to metal (LMCT) of O → Mo and the characteristic absorption of
bipy [6a], while the absorption at 500–800 nm are due to the d–d
transition coming from the introduction of TM (Co(II)/Ni(II)/Cu(II))
ions.
The luminescence properties of compounds 1–4 and the parent
NH4-(PhP)2Mo5 in the solid state at room temperature are depicted in
Fig. S9. As shown in Fig. S9, the parent NH4-(PhP)2Mo5 displays
luminescence with two main emission peaks at 398 and 470 nm with
λex = 305 nm, which should be assigned to ligand-to-metal charge
transfer (LMCT, O → Mo) [9]. Compounds 1–4 all show emission
peaks at 398 and 470 nm (λex = 305 nm), indicating that they have a
similar structure. Compared with the emission of the parent NH4-
(PhP)2Mo5, emissions of compounds 1–4 are mainly caused by the
(PhP)2Mo5 cluster, furthermore, the emission intensities of compounds
1–4 have changed, that is, the coordination of (PhP)2Mo5 to Co(II)/Ni/
pounds 1–4, the parent NH4-(PhP)2Mo5 and metal salts are listed in
Table 2. As seen from Table 2, compared with the parent NH4-
(PhP)2Mo5, the yields of ketal for compounds 1–4 are all increased.
It is also found that their catalytic activity approaches that of
[(Cu(H2O)2)2(μ-bipy)(C6H5PO3)2Mo5O15]n reported previously by our
group [6b]. While in the absence of catalyst, the yield of ketal is very
low (10%), which shows that the title compounds display higher acid-
catalytic activities. In addition, the inorganic salts, CoCl2·6H2O,
NiCl2·6H2O, CuCl2·2H2O and ZnSO4·7H2O, exhibit high catalytic
activity, but they act as the homogeneous catalysts, which were not
able to be recycled. From the above analysis, it can be inferred that
Co(II)/Ni(II)/Cu(II)/Zn(II) ions as well as Mo(VI) moieties as the
catalytically active centres, playing important roles in the catalytic
reaction [6b]. Moreover, the reusability of the title compounds was also
examined (Fig. 5). As shown in Fig. 5, we found that they maintained
the catalytic activities after being reused for 3 times, the reason for the
slight decrease in catalytic yield may be the loss of some catalyst during
the washing process with ether. To further confirm the stability of these
compounds after the catalytic reactions, the catalysts before and after
used four runs were characterized by IR and XRPD (Figs. S13 and
S14). As shown in Fig. S13, the IR characteristic peaks of the powder
samples after the catalysis are consistent with those of crystalline
compounds 1–4 before the catalysis. As shown in Fig. S14, the main
diffraction peaks positions are consistent with the simulated values of
compounds 1–4, and the different intensity of peaks may be caused by
the diverse preferred orientation of the powder samples. These results
indicate that the structures of compounds 1–4 are intact after the
catalytic process. Based on the above results, it is concluded that
(II)Cu(II)/Zn(II) ions quenches partially LMCT (O
→ Mo). The
emission peaks at about 390 nm and 409 nm for bipy and 293 nm
for C6H5PO3H2 (λex = 305 nm) should be assigned to π*→π transition
[6b-c].
3.7. Catalytic activities of compounds 1–4
The title compounds were applied as catalysts in the synthesis of
cyclohexanone ethylene ketal (Scheme S1) [6b-c,15]. In order to
investigate the optimal catalytic reaction conditions for the four
compounds, the effects of various factors including reaction time, the
amount of catalyst and molar ratio of cyclohexanone to glycol were
systemically investigated and selected. Taking compound 1 as an
example, the catalytic performances are shown in Figs. S10–S12, and
the optimum conditions for the synthesis of cyclohexanone ethylene
ketal were as follows: (1) the reaction time was 4.0 h, reaction
temperature was 95–100 °C and 10 mL of a cyclohexane water-
carrying agent; (2) the molar ratio of cyclohexanone and glycol was
1:1.4; (3) the molar ratio of the catalyst to cyclohexanone was 1:200.
Under the optimum conditions, the acid-catalytic activities of com-
Fig. 5. The catalytic activities of compounds 1–4 reused for three cycles.
56

T. Lu et al.
Journal of Solid State Chemistry 253 (2017) 52–57
compounds 1–4 are stable and can be used as efficient heterogeneous
acid catalysts for the synthesis of cyclohexanone ethylene ketal.
(g) H. Sun, L.Y. Guo, J.S. Li, J.P. Bai, F. Su, L.C. Zhang, X.J. Sang, W.S. You,
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4. Conclusion
In summary, four inorganic-organic hybrids based on (PhP)2Mo5
clusters and Co(II)/Ni(II)/Cu(II)/Zn(II)-(bipy)/H2O units enriches the
structural diversity of the Strandberg-type POMs with organopho-
sphine centres. Many factors, such as the pH, reaction temperature,
reaction time, and adding order and proportion of the starting
materials are all very important for the formation and crystallization
of them, which also show that their synthetic conditions are very harsh.
Compared with their parent NH4-(PhP)2Mo5, the title compounds
exhibit better acid catalytic activities and can be easily recovered and
reused on the synthesis of cyclohexanone ethylene ketal. Further
research on other organophosphomolybdates is underway in our group.
(i) Y.L. Ren, L. Li, B. Mu, C.X. Li, R.D. Huang, J. Solid State Chem. 249 (2017)
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (nos. 21671091 and 21503103); the
Youth scientific research project of Liaoning Normal University (nos.
LS2014L018 and LS2015L008).
(b) T.M. Smith, K. Perkins, D. Symester, S.R. Freund, J. Vargas, L. Spinu,
J. Zubieta, CrystEngComm 16 (2014) 191–213;
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Appendix A. Supplementary material
(b) R.C. Finn, R.S. Rarig Jr., J. Zubieta, Inorg. Chem. 41 (2002) 2109–2123;
Supplementary data associated with this article can be found in the
online version at doi:10.1016/j.jssc.2017.05.027.
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(d) E. Burkholder, V. Golub, C.J. O’Connor, J. Zubieta, Inorg. Chem. 43 (2004)
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