Three molybdophosphates based on Strandberg-type anions and Zn(ii)-H 2biim/H2O subunits: Syntheses, structures and catalytic properties

Article abstract of DOI:10.1039/c3dt53023j

Three new inorganic-organic hybrid compounds based on Strandberg-type anions and Zn(ii)-H₂biim/H₂O subunits, namely {H ₄(H₂biim)₃}[Zn(H₂biim)(H ₃biim)(H₂O)(HP₂Mo₅O ₂₃)]₂·3H₂O (1), {H₉(H ₂biim)₇}[(μ-biim){(Zn(H₂O)₂) _0.5(HP₂Mo₅O₂₃)}₂] ·7H₂O (2) and {H₇(H₂biim) ₇}[Zn(H₂biim)(H₂O)₂(HP ₂Mo₅O₂₃)][H₂P₂Mo ₅O₂₃]·8H₂O (3) (H₂biim = 2,2′-biimidazole), have been synthesized in aqueous solutions and characterized. They were also used as efficient and reusable catalysts for the protection of carbonyl compounds. Their fascinating structural features are that mono Zn(ii)-supporting biphosphopentamolybdate ({P₂Mo₅}) clusters exist in their crystal structures, and the nitrogen donor ligand H ₂biim exhibits three different coordination modes in these three compounds, respectively: for 1, two 2,2′-biimidazole molecules, as mono- and bidentate ligands coordinate to the same Zn(ii) ion; for 2, one bi-negative tetradentate ligand μ-biim bridges two Zn(ii) ions, while for 3, one neutral bidentate H₂biim ligand links one Zn(ii) ion. Most importantly, compounds 1-3 represent the first example where Strandberg-type POMs are used as acid-catalysts in an organic reaction.

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Three molybdophosphates based on Strandberg-
type anions and Zn(II)-H2biim/H2O subunits:
syntheses, structures and catalytic properties†
Cite this: Dalton Trans., 2014, 43,
5840
Zi-Liang Li, Ying Wang, Lan-Cui Zhang,* Jian-Ping Wang, Wan-Sheng You and
Zai-Ming Zhu*
Three new inorganic–organic hybrid compounds based on Strandberg-type anions and Zn(II)-H2biim/
H2O subunits, namely {H4(H2biim)3}[Zn(H2biim)(H3biim)(H2O)(HP2Mo5O23)]2·3H2O (1), {H9(H2biim)7}-
[(μ-biim){(Zn(H2O)2)0.5(HP2Mo5O23)}2]·7H2O (2) and {H7(H2biim)7}[Zn(H2biim)(H2O)2(HP2Mo5O23)][H2P2Mo5O23]·
8H2O (3) (H2biim = 2,2’-biimidazole), have been synthesized in aqueous solutions and characterized.
They were also used as efficient and reusable catalysts for the protection of carbonyl compounds. Their
fascinating structural features are that mono Zn(II)-supporting biphosphopentamolybdate ({P2Mo5})
clusters exist in their crystal structures, and the nitrogen donor ligand H2biim exhibits three different
coordination modes in these three compounds, respectively: for 1, two 2,2’-biimidazole molecules, as
mono- and bidentate ligands coordinate to the same Zn(II) ion; for 2, one bi-negative tetradentate ligand
μ-biim bridges two Zn(II) ions, while for 3, one neutral bidentate H2biim ligand links one Zn(II) ion. Most
importantly, compounds 1–3 represent the first example where Strandberg-type POMs are used as acid-
catalysts in an organic reaction.
Received 26th October 2013,
Accepted 30th January 2014
DOI: 10.1039/c3dt53023j
www.rsc.org/dalton
research results will help us design, synthesize and utilize this
type of POM-based material. It is well known that POMs can be
Introduction
Progress in polyoxometalate (POM) chemistry has always used as photocatalysts6 and electrocatalysts,7 and they are
been closely related to the functionalized POM materials. most widely used as catalysts in organic synthesis, such as
The Strandberg-type POMs, as an important subclass, have esterification reaction,8 oxidation reaction9 and protection of
attracted significant attention due to their own structural char- carbonyl compounds.10 Protection of carbonyls as acetals or
acters, as well as their higher charges. They have been used to ketals is one of the most important organic reactions, and
construct inorganic–organic hybrid materials. In recent years, there are many methods reported in the literature.11,12 In
many new POMs containing {P2Mo5} skeleton, organic group 2011, Rahimizadeh’s group reported that the Preyssler-type
and metal ions have been successfully synthesized.1,2 More- POM is a mild and efficient catalyst for the protection of carbo-
over, some important properties of these compounds are nyl compounds.10 To the best of our knowledge, there is no
being gradually recognized. For example, Su’s group investi- report on the catalytic activity of the Strandberg POM used as a
gated the electronic properties, redox properties, and relative catalyst in organic synthesis. In addition, 2,2′-biimidazole, as a
basicity of the external oxygen atoms of the Strandberg POMs. polydentate ligand, is able to coordinate to transition metals
They pointed out that the changes in the electronic properties with three reversible types: neutral (H2biim), monoanion
can modify the redox properties of POMs.3,4 In 2012, (Hbiim−), and dianion (biim2−) types,13 but few compounds
Banerjee’s group first reported the proton conductivity of the involving transition metals, H2biim ligands, and polyoxo-
Strandberg POM-based MOFs in the solid state.5 These anions have been reported. In our previous work, we have syn-
thesized some transition metal (TM)-H2biim complex-
supported Strandberg-type molybdophosphates, octamolyb-
dates, and Preyssler-type tungstophosphates.1d,14,15 The cataly-
School of Chemistry and Chemical Engineering, Liaoning Normal University,
Dalian 116029, China. E-mail: zhanglancui@lnnu.edu.cn, chemzhu@sina.com;
tic activities of the above Preyssler-type POMs for the oxidation
of cyclohexanol are preliminarily explored. As can be seen
from the above examples, though the nitrogen donor ligand
Fax: +86 411 82156550; Tel: +86 411 82158559
†Electronic supplementary information (ESI) available: ORTEP views and
packing views of 1–3; infrared spectra and TG-DTA of 1–3. The optimal catalytic
H2biim has a weak reduction ability, it can be introduced into
the POM skeletons for assembling hybrid materials with a
curves. CCDC 962343, 962344 and 962345. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3dt53023j
5840 | Dalton Trans., 2014, 43, 5840–5846
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versatile coordination type. H2biim is a valuable building {P4Mo6} can be mutually transformed. Meanwhile, the concen-
block for constructing inorganic–organic hybrid materials by tration of the reactants is an important factor affecting the
hydrogen bonding and π–π stacking interactions. So, as an structures of the products and the coordinate modes of
extension of the work, we endeavored to construct new in- H2biim ligands.
organic–organic hybrid compounds based on transition
metals and the {P2Mo5} clusters, and further to explore their
Structural analysis
potential catalytic activities in organic synthesis. Herein, we
have synthesized three Zn(II)-H2biim/H2O-supporting Strand-
berg-type POMs, and for the first time, reported {P2Mo5}-based
catalysts used in organic synthesis, i.e. these POMs were used
as catalysts for the protection of carbonyl compounds with
glycol.
As a common feature, each crystal molecule contains two
{P2Mo5} units. The {P2Mo5} unit may be described as a nearly
planar {Mo5} ring of five edge- and corner-sharing MoO6 octa-
hedra capped on either face by two PO4 tetrahedra in a corner-
sharing mode. In the single crystal structures, each Zn(II)-
H2biim/H2O fragment supports on one side of the Strandberg-
type anion linked through one terminal oxygen atom of a PO4
tetrahedron, i.e. the {P2Mo5} unit acts as a monodentate
ligand. Though all Zn(II) ions adopt five-coordinated square-
pyramidal coordination geometries, their coordination
environments are different and distorted. The asymmetric unit
of 1 contains one mono-supporting {P2Mo5} unit, and one and
a half free H2biim molecules, and one and a half lattice water
(Fig. S2(a), ESI†). As shown in Fig. 1, the Zn1 center coordi-
nates with one oxygen atom (O8) from a capped PO4 tetrahe-
dron group (Zn1–O8 1.932(3) Å), three nitrogen atoms (N1, N2,
N5) from two H2biim ligands (Zn1–N 2.029(5)–2.206(5) Å), and
one terminal water ligand O1W (Zn1–O1W 2.251(3) Å). The
two H2biim molecules, as bidentate and monodentate ligands,
provide two and one pairs of electrons, respectively. In one of
the H2biim molecules, two imidazole rings are not co-planar
and distort along the C–C bond.
The N6–C7–C8–N8 torsion angle is 43.2(9)°, and the di-
hedral angle between the planes C12_C11_N7_C8_N8 and
C10_C9_N5_C7_N6 is 46.8°. Bond valence sum (BVS)19,20 cal-
culations show that the oxidation states for P, Zn and Mo
atoms are +5, +2 and +6, respectively. The BVS value of the
tetrahedral oxygen atom O13/O13′ is 1.24, indicating that the
oxygen atom is mono-protonated. As viewed from the packing
diagram in Fig. S3 (ESI†), a 3-D supramolecular framework is
formed via strong hydrogen bonding (e.g. N–H⋯O/OW 2.566(7)–
3.276(7) Å) between H2biim ligands and the polyoxoanions
Results and discussion
Synthesis
Strandberg-type POMs, as typical oxygen-containing mono- or
polydentate ligands, can coordinate with transition metals
through the tetrahedral or octahedral oxygen atoms. Further
introduction of organic groups will construct new hybrid struc-
tures. The nitrogen donor ligand H2biim has versatile coordi-
nation behavior;16 however, the synthesis of the inorganic–
organic hybrid materials based on the Strandberg-type poly-
anion and TM/H2biim has been a great challenge. One of the
main reasons is that the Strandberg-type polyanion has higher
charge, and it can easily precipitate when it encounters
cations. Furthermore, the H2biim ligand is easily protonated
in acidic solution resulting in the decrease of its coordinating
ability. During the syntheses of compounds 1–3,
Na2MoO4·2H2O, H2biim and ZnSO4·7H2O were used as starting
materials, and excess H3PO4 was added. The effect of phospho-
ric acid is not only to ensure the required amounts of P for the
reaction process, but also to control the pH. The synthetic
strategy for compounds 1–3 is presented in Scheme 1. In the
synthetic process, we found that the control of the reaction
conditions played a crucial role in constructing different struc-
tures. Experimental results indicate that reasonable yields of
crystalline products can be obtained at pH = 3–4. The pH of
the reaction system is the key factor for the formation of the
Strandberg-type structures. Lower pH values resulted in
forming Keggin structures, while higher pH gave only powders
containing {P4Mo6} POM, which can be tested by the UV
spectra (Fig. S1, ESI†). The phenomenon is similar to the lit-
erature results,17,18 and also indicates that {P2Mo5} and
Fig. 1 Combined polyhedral and ball-and-stick representation for com-
pound 1 (hydrogen atoms and free water molecules have been omitted
for clarity).
Scheme 1 Schematic representation of pathways and experimental
conditions for the formation of 1–3.
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Fig. 2 Combined polyhedral and ball-and-stick representation for
compound 2 (hydrogen atoms and free water molecules have been
omitted for clarity).
or crystal water molecules, and π–π interactions between the
imidazole rings of biimidazole ligands.
Fig. 3 Combined polyhedral and ball-and-stick representation for
compound 3 (hydrogen atoms and free water molecules have been
omitted for clarity).
The unit cell of 2 contains a binuclear zinc complex based
on two {P2Mo5} units and one dehydrogenated 2,2′-biimidazole
ligand. Fig. S2(b) (ESI†) shows the asymmetric unit of 2. As
viewed from Fig. 2, two mono-supporting Zn-{P2MO5} subunits
are bridged by a bi-negative tetradentate ligand μ-[biim]2−
forming a symmetrical structure. As far as we know, such a
coordinate mode of 2,2′-biimidazole has not been reported in
POM systems. The asymmetric Zn1 center is five-coordinated
by two nitrogen atoms (N15, N16A) from a μ-[biim]2− ligand
(Zn1–N 2.221(7), 2.008(5) Å), and one oxygen atom (O18) from
a capped PO4 tetrahedron group (Zn1–O18 2.126(2) Å), and two
terminal water ligands (Zn1–O1W 2.055(6) Å; Zn1–O2W 1.998(4)
Å), exhibiting a strongly distorted square pyramidal geo-
metry. Furthermore, the Zn1 shows half-occupancy. BVS calcu-
lations show that the oxidation states for P, Zn and Mo atoms
are +5, +2 and +6, respectively. The BVS value of the tetrahedral
atom O13/O13′ is 1.33, indicating that the oxygen atom is
mono-protonated. There also exists a 3-D supramolecular
framework through π–π stacking and strong hydrogen bonding
interactions (e.g. N–H⋯O/OW 2.574(3)–3.002(3) Å) (Fig. S4,
ESI†).
mono-protonated. Fig. S5 (ESI†) exhibits a spatial arrangement
of the polyoxoanions. In their packing arrangement, the
[Zn(H2biim)(H2O)2(HP2Mo5O23)]3− and [H2P2Mo5O23]4− units
(namely A and B) are alternately arranged to form 2D layers
with ABAB pattern. The polyoxoanions in adjacent layers are
situated face to face or back to back, constructing a 3D archi-
tecture via extensive strong hydrogen bonding interactions
(e.g. N–H⋯O/OW 2.560(6)–3.248(7) Å) among the polyanions,
crystal water molecules and H2biim units, and π–π stacking of
H2biim molecules (see Fig. S6, ESI†).
It can be seen from the above structures that the intramole-
cular and intermolecular hydrogen bonds play a key role in
building up and stabilizing these solid frameworks; that is to
say, 2,2′-biimidazole, as an interesting building block, has the
important function of controlling crystal structures, and the
polyoxoanion clusters make the structures more innovative
and more stable. Therefore, we can also say that hydrogen
bonded 2,2′-biimidazolate transition metal complexes can be
used as a tool of crystal engineering.13
In the crystal structure of 3, each molecule also contains
two {P2Mo5} units, but 3 is not a symmetrical structure when
compared with 1 and 2. The structural unit of 3 is composed
of one mono-supporting [Zn(H2biim)(H2O)2(HP2Mo5O23)]3−
,
Characterization
one isolated [H2P2Mo5O23
]
4−, seven free H2biim and eight
FT-IR spectroscopy. As shown in Fig. S7 (ESI†), the IR
lattice water molecules (see Fig. S2(c), ESI† and Fig. 3). The spectra of 1–3 are similar. The bands at 2781–3437 cm−1 are
Zn1 atom is coordinated by two nitrogen atoms from one associated with C–H, N–H and O–H bending and stretching
H2biim ligand, one oxygen atom from a capped PO4 tetra- vibrations. Absorption peaks in the range of 1216–1647 cm−1
hedron group, and two oxygen atoms from two water molecules. can be assigned to imidazole ring stretching of H2biim.13–15,17
The distances around Zn1 are 2.177(6) and 2.063(6) Å for Furthermore, all the compounds exhibit characteristic peaks
Zn1–N1 and Zn1–N2, 2.094(4) Å for Zn1–O10, 2.004(5) and of the {P2Mo5} polyoxoanion: the bands at 1027–1106 cm−1 are
2.014(5) Å for Zn1–O1W and Zn1–O2W. The Zn1 atom adopts attributed to v(P–O), and strong bands at 680–694, 759–773
a tiny distorted square-pyramidal coordination geometry. and 912–918 cm−1 are attributed to the v(Mo–Obridging) and
BVS calculations showed that the values for P, Zn, and Mo v(Mo–Oterminal) modes of the polyoxoanion.1e Small shifts in the
atoms are +5, +2 and +6, respectively, and the values of the wavelengths of the {P2Mo5} characteristic peaks may be due
tetrahedral oxygen atoms O27, O39 and O44 are 1.32, 1.35 to the different coordination environments of the three
and 1.32, respectively, indicating that the oxygen atoms are compounds.
5842 | Dalton Trans., 2014, 43, 5840–5846
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which corresponds to loss of phosphorus oxide and molyb-
denum trioxide. In short, thermogravimetric analyses show
that the title compounds have similar and high thermal stabi-
lities; the decomposition temperatures of the polyoxoanions
are about 700 °C, showing that the transition metal complexes
help to stabilize the inorganic–organic framework.
Catalytic activities of three compounds
The acid-catalytic protection of the carbonyl group is a key
reaction in multi-step synthesis of many important organic
compounds (Scheme 2).21,22 It is well known that some proto-
nic acids: anhydrous HCl, H2SO4, etc. were generally used as
catalysts for protection of carbonyl groups. These acids cannot
be reused or recycled, and produce large amounts of waste and
pollution, especially may cause serious corrosion of equip-
ment. Therefore, the development of more efficient, recyclable,
and environmental pollution-free catalysts is a long-term goal.
POMs, as a kind of multifunctional catalysts, have been
applied to many organic reactions, mainly due to their strong
redox and acidic properties. How about 1–3 based on the
{P2Mo5} units? Firstly, we used them including the parent
{P2Mo5} as heterogeneous catalysts in the oxidation of cyclo-
hexanol to cyclohexanone to evaluate their oxidative catalysis
activities. It was found that the cyclohexanone yield was much
lower (<15%) compared with those of our reported POMs.15,23
Then, we tested them in the synthesis of cyclohexanone ethy-
lene ketal. Delightedly, they exhibit better catalytic activities in
this acid-catalyzed reaction. For compound 2, 99% of yield was
obtained (Table 1, entry 4). For compounds 1 and 3, 86% and
95% of yields were obtained, respectively (Table 1, entry 3/5).
The parent {P2Mo5} and ZnSO4 were also checked as a control
experiment; they showed low catalytic activities (Table 1, entry
1/2). The reaction conditions, such as reaction time, the molar
Fig. 4 Comparison of UV-visible spectra for the parent {P2Mo5}, ZnSO4,
H2biim and 1–3 (5 × 10−5 mol L−1) at pH = NaAc–HAc (pH = 4) buffer
solution.
UV-Visible spectroscopy. The UV-vis absorption behaviours
of the parent {P2Mo5}, ZnSO4, H2biim and 1–3 were analyzed
using NaAc–HAc (pH = 4) buffer solution. The obtained
spectra between 200 and 500 nm are presented in Fig. 4. In the
visible region, no absorption bands were observed. In the UV
region, the UV spectrum of the parent {P2Mo5} has a strong
band at ca. 241 nm in the range of 200–355 nm, which is
attributed to the characteristic absorption of {P2Mo5}: the
ligand-to-metal pπ–dπ charge transfer transitions (O→Mo).18
The electronic spectrum of ZnSO4 shows no absorption band.
A strong band at 278 nm corresponds to π–π charge transfer
transitions of H2biim. The UV spectra of 1–3 also reveal one
strong absorption band centred at ca. 276 nm in the range of
230–320 nm, which is assigned to the absorption behaviours
of the inorganic–organic hybrid compounds based on POM
and H2biim, and the characteristic absorption of {P2Mo5} are
overlapped by the broad band of H2biim.
Thermal analysis. In order to examine the thermal stabi-
lities of 1–3, thermal gravimetric (TG) analyses were carried
out. As seen in Fig. S8 (ESI†), the TG curves of 1–3 exhibit
some obvious similarities from 75 to 1000 °C. In the tempera-
ture range of 75–568 °C, each compound shows a continuous
weight loss step. The observed total weight losses for 1–3 are
39.14%, 44.36% and 42.93%, respectively. It is obvious that
the weight loss is higher than the theoretical value for the
removal of all water and H2biim molecules (calc. 36.52%,
39.75% and 40.13%, respectively), which may be due to partial
loss of phosphorus oxide resulting from the partial decompo-
sition of the polyoxoanion.1d The exothermal peaks observed
respectively at 494, 531 and 534 °C in the DTA curves of 1–3
are further proofs of the collapse of the polyoxoanion skele-
tons. From 300 to 360 °C, weak and strong exothermal peaks
are observed at 318 and 457 °C for 1, 330 and 449 °C for 2 and
3, respectively, which are assigned to combustions of the
H2biim organic fragments. Additionally, three compounds
Scheme 2 Ketalization of cyclohexanone with glycol.
Table 1 Catalytic performance of the catalysts (1–3) for the synthesis
of cyclohexanone ethylene ketala
Entry
Catalyst
Solubility
Time (h)
Yield (%)
1
2
3
4
5
{P2Mo5}
ZnSO4
Compound 1
Compound 2
Compound 3
Insoluble
Insoluble
Insoluble
Insoluble
Insoluble
2
2
2
2
2
50
10
86
99
95
a Reaction conditions: the molar ratios of the catalyst (based on Mo/Zn)
to cyclohexanone was 1 : 300, and cyclohexanone to glycol was 1 : 1.4
(0.05 mol of cyclohexanone); water-carrying agent: 10 mL of
continuously lost weight at temperatures higher than ∼690 °C, cyclohexane; reaction temperature: 95–100 °C.
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TENSOR-27 FTIR spectrometer in the range of 4000–400 cm−1
UV spectra were performed on a Lambda 35 UV-Visible spec-
trophotometer. Single crystal X-ray diffraction data were col-
lected on a Bruker Smart APEX II X-diffractometer equipped
.
Table 2 Synthesis of cyclohexanone ethylene ketal by catalysts 1–3 for
different cyclesa
Yield (%)
Catalyst
Fresh
Cycle 1
Cycle 2
Cycle 3
with graphite-monochromated Mo Kα radiation (λ
=
0.71073 Å). X-ray powder diffraction data were collected on a
Bruker AXS D8 Advance diffractometer using Cu Kα radiation
(λ = 1.5418 Å) in the 2θ range of 5–60° with a step size of 0.02°.
TG analyses were performed on a Pyris Diamond TG-DTA
thermal analyzer in air with a heating rate of 10 °C min−1. The
yield of cyclohexanone ethylene ketal was confirmed on a
JK-GC112A Gas Chromatograph.
Compound 1
Compound 2
Compound 3
86.0
98.8
95.4
84.2
95.6
92.1
82.5
90.2
88.7
80.4
84.8
82.4
a Reaction conditions: the molar ratio of the catalyst (based on Mo) to
cyclohexanone was 1 : 300; n(cyclohexanone) : n(glycol) 1 : 1.4
(0.05 mol of cyclohexanone); water-carrying agent: 10 mL of
=
cyclohexane; reaction time: 2 h; reaction temperature: 95–100 °C.
Syntheses of compounds
Compound 1. ZnSO4·7H2O (0.575 g, 2.0 mmol) and H2biim
(0.805 g, 6.0 mmol) were dissolved in 70 mL of water, and the
mixture was stirred for 30 min. Then Na2MoO4·2H2O (1.210 g,
5.0 mmol) was added with vigorous stirring. To this solution
concentrated H3PO4 was added dropwise until pH = 3–4 under
continuous stirring. The resulting solution was stirred at
80–90 °C for an additional 2 h and was filtered, and the colour-
less filtrate was kept under ambient conditions for ten days.
Colorless block crystals were obtained. Yield: 76% based on
Mo. Elemental analysis, Found: C, 16.8; H, 2.1; N, 13.1; Zn,
4.3; P, 4.2; Mo, 32.2. Calc. for C42H60Mo10N28O51P4Zn2: C, 16.9;
H, 2.0; N, 13.1; Zn, 4.4; P, 4.15; Mo, 32.1%. FT IR (KBr pellet),
cm−1: 3437(m), 3149(s), 2971(m), 2923(m), 2781(m), 1647(w),
1548(w), 1428(w), 1316(w), 1097(m), 1055(m), 916(s), 773(m),
694(s), 580(w), 540(w).
Compound 2/3. The syntheses of compounds 2 and 3 were
the same as above, except that the reactions were carried out
in a molar ratio of 2 : 3 : 5 and 2 : 2 : 5, i.e. H2biim were reduced
to 0.403 g (3.0 mmol) and 0.268 g (2.0 mmol), respectively.
After slowly cooling to room temperature, the resulting solu-
tions were filtered and the filtrates were maintained at room
temperature. The colorless plate crystals for 2 and faint yellow
plate crystals for 3 were isolated after 2–3 days. Yields of 2 and
3 are 65% and 72% (based on Mo), respectively.
Elemental analyses for 2, Found: C, 17.9; H, 2.5; N, 13.8;
Zn, 2.2; P, 3.7; Mo, 31.4. Calc. for C48H74Mo10N32O55P4Zn: C,
18.4; H, 2.4; N, 14.3; Zn, 2.1; P, 4.0; Mo, 30.7%. FT IR (KBr
pellet), cm−1: 3413(m), 3139(s), 3017(m), 2932(m), 1642(m),
1581(w), 1538(w), 1496(w), 1417(w), 1320(w), 1216(w), 1100(m),
1033(s), 918(s), 759(m), 680(s).
Elemental analyses for 3, Found: C, 18.3; H, 2.6; N, 14.1;
Zn, 2.1; P, 3.9; Mo, 30.4. Calc. for C48H78Mo10N32O56P4Zn: C,
18.3; H, 2.5; N, 14.2; Zn, 2.1; P, 3.9; Mo, 30.5%. FT IR (KBr
pellet), cm−1: 3413(m), 3139(s), 3024(m), 2944(m), 2798(m),
1642(m), 1587(w), 1538(w), 1450(w), 1423(w), 1320(w), 1216(w),
1106(m), 1027(s), 912(s), 759(s), 680(s).
ratio of starting material, and the amount of catalyst or water-
carrying agent, were systemically explored (Fig. S9–S12, ESI†).
For the three compounds, similar catalytic performances were
present. As shown in Fig. S9 and S10 (ESI†), the yields of ketals
increased quickly with increasing time within 2 hours, and the
cyclohexanone–glycol molar ratio of 1 : 1.4 is a suitable sub-
strate molar one. The amount of catalyst was one of the impor-
tant affective factors; when the catalyst was 1/300 of
cyclohexanone for compounds 1, 2 and 3, the maximum yields
of cyclohexanone ethylene ketal were 86%, 99% and 95%,
respectively (Fig. S11, ESI†). Taking compound 2 as an
example, we have investigated the amount of water-carrying
agent; it can be seen that 10 mL of cyclohexane water-carrying
agent is the best dosage (Fig. S12, ESI†). The optimum con-
ditions for the synthesis of cyclohexanone ethylene ketal using
compound 1/2/3 as a catalyst were as follows: (1) the cyclohexa-
none–glycol molar ratio is 1 : 1.4; (2) the molar ratio of the
catalyst to cyclohexanone is 1 : 300; (3) a reaction temperature
of 95–100 °C, and a reaction time of 2 h, and 10 mL of a cyclo-
hexane water-carrying agent. Moreover, we found that these
POM catalysts maintain the catalytic activities during the later
cycles, and did not significantly change after cycles (Table 2),
which indicates that the {P2Mo5} skeleton has higher stability,
and can be further proved by IR and PXRD. For example, the
IR spectra and PXRD of the fresh and the used catalysts are
shown in Fig. S13 (ESI†) and Fig. S14 (ESI†): the IR character-
istic peaks of the {P2Mo5} also maintained, and the diffraction
peaks on the pattern correspond well in peak positions of
PXRD, which illustrated that the skeletons of these catalysts
still keep before and after reaction compared with the parent
{P2Mo5} polyanion.
Experimental
Materials and methods
All chemicals were of reagent grade as received from commer-
cial sources and used without further purification. C, H and N
Single-crystal X-ray diffraction
elemental analyses were performed on a Perkin-Elmer 2400 The structures were solved by direct methods and refined by
CHN elemental analyzer and P, Zn and Mo were analyzed on a full-matrix least-squares fitting on F2 using SHELXTL-97.24,25
Plasma-Spec-II ICP atomic emission spectrometer. The infra- An empirical absorption correction was applied using the
red spectra were recorded on KBr pellets with a Bruker AXS SADABS program. Cell parameters were obtained by the global
5844 | Dalton Trans., 2014, 43, 5840–5846
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Table 3 Crystal and refinement data for compounds 1–3
1
2
3
Formula
Formula weight
T/K
Wavelength/Å
Crystal system
Space group
a/Å
b/Å
c/Å
α/°
β/°
C
42H60Mo10N28O51P4Zn2
C48H74Mo10N32O55P4Zn
3128.04
296(2)
C48H78Mo10N32O56P4Zn
3148.09
298(2)
2987.24
298(2)
0.7107
Orthorhombic
Pnn2
20.086(2)
21.185(2)
10.5333(12)
90.00
90.00
90.00
4482.1(8), 2
2.213, 2911
1.036
0.7107
0.7107
Triclinic
Triclinic
ˉ
ˉ
P1
P1
11.1120(16)
11.3558(16)
21.988(3)
100.062(2)
93.345(2)
116.935(2)
2405.5(6), 1
2.159, 1536
1.098
11.1295(8)
20.2353(15)
21.9751(16)
76.875(1)
86.542(1)
87.499(1)
4808.6(6), 2
2.174, 3095
1.051
γ/°
V/Å3, Z
Dc/g cm−3, F000
GOF
Reflections collected
Unique data, Rint
θ Range (°)
R1(I > 2σ(I))a
wR2 (all data)a
25 544
12 541
26 194
9641, 0.0424
2.16 to 27.00
0.0344
8809, 0.0174
2.15 to 25.50
0.0335
18473, 0.0199
1.91 to 26.00
0.0430
0.0774
0.0892
0.1293
a R1 = ∑||F0| − |FC||/∑|F0|; wR2 = ∑[w(F02 − FC2)2]/∑[w(F02)2]1/2
.
refinement of the positions of all collected reflections. Hydro- maintained at 1 : 300, and these catalysts were pretreated by
gen atoms on C or N atoms were added in calculated positions. being dried at 200 °C for 2 h), respectively.
Crystal data and structure refinement parameters of com-
pounds 1–3 are listed in Table 3. Selected bond lengths and
angles are given in Tables S1–S3 (ESI†). Hydrogen bonds are
listed in Tables S4–S6 (ESI†). During the refinement of com-
pounds 1–3, protonated N atoms on the H2biims were deter-
mined by the charge-balance considerations. All hydrogen
atoms on water molecules were directly included in the final
features. The successful syntheses of three compounds show
molecular formula. CCDC reference numbers: 962343, 962344
and 962345.
Conclusions
In summary, compounds 1–3 display the transition metal–
organic complex mono-supporting Strandberg-type structural
that the zinc–H2biim/H2O complex can be assembled with
polyoxoanion, forming new interesting structures. Our experi-
mental results show that these Strandberg-type POMs, as cata-
lysts, showed better catalytic performances in an acid-catalyzed
organic reaction. The recycled catalyst was used without the
observation of appreciable loss in its catalytic activities. In
addition, 1–3 show higher catalytic activity compared with the
catalytic activities of the raw material ZnSO4 and their parent
{P2Mo5}, which proved that the synergistic effect of {P2Mo5}
Catalytic experiment
Protection for the carbonyl group. Acid-catalyzed synthesis
of cyclohexanone ethylene ketal was used as a model reaction
to evaluate the catalytic performances of 1–3. A typical pro-
cedure of the catalytic activity test is as follows: the catalyst
(compound 1, 0.050 g, 0.017 mmol) was added to a mixture of
and transition metal complexes in the inorganic–organic
cyclohexanone (4.91 g, 0.05 mol), glycol (4.34 g, 0.07 mol) and
hybrid compounds leads to the enhanced catalytic properties.
cyclohexane (10 mL) in a 50 mL three-necked round-bottom
Further research on the introduction of other metals as well as
flask fitted with a Dean–Stark apparatus to remove the water
organonitrogen ligands is in progress in our group.
continuously from the reaction mixture. After completion of
the reaction, the heterogeneous catalyst remained at the
bottom of the reaction vessel and was easily separated from
the organic phase containing product by decantation. The
recovered catalyst was dried at 200 °C for 2 h and reused in a
Acknowledgements
new reaction under identical experimental conditions. Each of This work was financially supported by the Natural Science
the procedures was repeated for three cycles. The products Foundation of Liaoning Province (no. 2013020128), and the
obtained were characterized by gas chromatography.
Foundation of Education Department of Liaoning Province
Repeating the above experiment under the same conditions (no. L2013414). We are also thankful to Professor Yan Xu
except that compound 1 was replaced by compound 2/3, the (Nanjing Technology University) and Professor Yang-Guang Li
parent polyoxometalate {P2Mo5}, and raw materials, i.e. ZnSO4 (Northeast Normal University) for assistance with solving the
(note: the molar ratio of Mo or Zn : cyclohexanone was crystal structures.
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5846 | Dalton Trans., 2014, 43, 5840–5846
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