Four new compounds based on Keggin polyoxotungstates and transition metal complexes

Article abstract of DOI:10.1016/j.poly.2018.03.010

Four new compounds based on Keggin polyoxotungstates and transition metal complexes have been synthesized and characterized by IR, UV–Vis, XRD, XPS, elemental analysis and X-ray diffraction analysis. [Cd(2,2′-bpy)₃][HPW₁₂O₄₀

Full text of DOI:10.1016/j.poly.2018.03.010

Accepted Manuscript
Four new compounds based on Keggin polyoxotungstates and transition metal
complexes
Li-Na Xiao, Li-Xia Yang, Xiao-Di Du, Qing Lan, Hao Zhang, Xiao-Bing Cui
PII:
S0277-5387(18)30135-9
https://doi.org/10.1016/j.poly.2018.03.010
POLY 13076
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
9 January 2018
12 March 2018
Please cite this article as: L-N. Xiao, L-X. Yang, X-D. Du, Q. Lan, H. Zhang, X-B. Cui, Four new compounds based
on Keggin polyoxotungstates and transition metal complexes, Polyhedron (2018), doi: https://doi.org/10.1016/
j.poly.2018.03.010
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Four new compounds based on Keggin polyoxotungstates and
transition metal complexes
a
a
a
a
a
b
Li-Na Xiao,* Li-Xia Yang, Xiao-Di Du, Qing Lan, Hao Zhang, Xiao-Bing Cui*
a
College of Chemistry and Chemical Engineering, Zhoukou Normal University, Zhoukou, Henan, 466001,
China.
b
College of Chemistry and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin
University, Changchun, Jilin, 130023, China.
1

Abstract: Four new compounds based on Keggin polyoxotungstates and transition metal complexes have
been synthesized and characterized by IR, UV-Vis, XRD, XPS, elemental analysis and X-ray diffraction
3 2 3 2
analysis. [Cd(2,2’-bpy) ][HPW12O40](2,2’-bpy) (1) and [TBA] [Mn(phen) ][HBW12O40]·2H O (2) (bpy =
bipyridine, phen = 1,10-phenanthroline, TBA = tetrabutylammonium) are two multi-component
2
-
2+
compounds, of which compound 1 is formed by [HPW12
O
40] , [Cd(2,2’-bpy)
3
]
and 2,2’-bpy, and
5
-
2+
+
compound
[Co(phen)
complex. {[Cd(2,2’-bpy)
2
is
[HAlW12
[SiW12
constructed
from [BW12
O
40] , [Mn(phen)
3
]
and
TBA
species.
{
2
(H
2
O)]
2
O40]]}·3H
2
O (3) is a Keggin polyoxotungstate bi-supported transition metal
4
-
2
]
2
O
40]} (4) is a 2-D layered framework structure of [SiW12O40] bridge by
2
+
[
2
Cd(2,2’-bpy) ] . The catalytic properties of compounds 1-4 have been investigated and we found that all
the four catalysts are active for the epoxidation of styrene.
Keywords: Polyoxometalate; Keggin; Hydrogen bonds; supramolecular structure; 2-D framework.
2

1
. Introduction
Polyoxometalates (POMs) exhibit a variety of structures and fascinating properties which make them
useful in catalysis, material science, medicine etc [1-4]. POM is formed by groups 5 and 6 transition metals
V, Nb, Ta, Mo and W) and oxygens via condensation and protolytic reactions [1]. The first POM was
(
prepared in 1826 [5], while the first POM structural details were reported in 1933 [6]. From then on, the
number of POMs increased at an exponential rate. A recent advance of POM chemistry is the large
number of compounds constructed from a combination of POMs and transition metal complexes (TMCs)
[
7-14]. These compounds could be categorized into two groups: (1) 1-D, 2-D or 3-D extended structures,
for the spherical surface of POMs gives a better opportunity for forming covalent bonds with TMCs; (2)
supramolecular structures assembled from POMs and TMCs through supramolecular interactions.
Up to now, most of POMs have already been used to be combined with TMCs to form hybrids,
including Keggin, Dawson POMs, etc, doubtless, of which the Keggin POM is the most widely used one.
Thus, it is a popular strategy to employ different TMCs to be combined with the Keggin POMs to form
hybrid materials. Based on this strategy, a number of hybrid compounds have been reported [15-20].
In this paper, we reported four new hybrid compounds based on Keggin polyoxometalates.
2
-
2+
[
Cd(2,2’-bpy)
and 2,2’-bpy. [TBA]
structure formed by [BW12
3 3
][HPW12O40](2,2’-bpy) (1) is a 3-D supramolecular structure of [HPW12O40] , [Cd(2,2’-bpy) ]
2
[Mn(phen)
3
][HBW12
O
40]·2H
2
O (2) exhibits a 1-D supramolecular double-chain
5
-
2+
+
O
40] , [Mn(phen)
3
]
and TBA . {[Co(phen)
2
(H
2
O)]
2
[HAlW12
[SiW12 40]} (4) is a 2-D layered
40] and transition metal complexes.
2
O40]]}·3H O (3) is a
polyoxoanions bi-supported transition metal complex. {[Cd(2,2’-bpy)
2
]
2
O
4
-
framework structure constructed from [SiW12
O
2
2
. Experimental
.1. General procedures
All chemicals used were of reagent grade without further purification. C, H, N elemental analyses were
carried out on a Perkin-Elmer 2400 CHN elemental analyser. Elemental analyses of P, B, Si, Al, W, Cd, Co
and Mn were determined by inductively coupled plasma (ICP) analyses on a Perkin-Elmer Optima 3300DV
ICP spectrometer. Infrared spectra were recorded as KBr pellets on a Perkin-Elmer SPECTRUM ONE FTIR
spectrometer. XPS analyses were performed on a Thermo ESCALAB 250 spectrometer with a Mg-Kα
(
1253.6 eV) achromatic X-ray source. Emission/excitation spectra were recorded on a RF-540 fluorescence
3

spectrophotometer. UV-Vis spectra of saturated dimethyl sulfoxide(DMSO) solutions of compounds 1-4
were obtained on a Shimadzu UV-3100 spectrophotometer. Small-angle X-ray diffraction (XRD) patterns
were obtained on a Siemens D5005 diffractometer using Cu Kα radiation. The TG analysis of compound 2
was carried out in a Netzsch STA449F3 equipment with a nitrogen gas of 20mL/min and a scan rate of
1
2
2
0 °C/min.
.2. Preparations
3
.2.1. Preparation of [Cd(2,2’-bpy) ][HPW12O40](2,2’-bpy) (1)
Compound 1 was synthesized hydrothermally from a mixture of Na
9 9 2
[A-PW O34]·7H O (535.00mg,
0
.21mmol) [21], CdCl ·2.5H O (220.00mg, 0.96mmol), 2,2’-bpy (167.00mg, 1.07mmol) and distilled water
2
2
(
25 ml). After adjusting the pH of the mixture to 4 using HCl solution, the mixture was heated under
autogenous pressure at 160°C for 5 days and then left to cool to room temperature. Yellow block crystals
could be isolated in about 43% yield (based on W). Elemental analyses (%) Calcd: W, 61.02; P, 0.86; Cd,
3
.11; C, 13.29; H, 0.92; N, 3.10. Found: W, 61.17; P, 1.09; Cd, 3.02; C, 13.13; H, 1.08; N, 3.16. FT-IR (KBr,
-
1
cm ): 1593, 1471, 1439, 1319, 1247, 1158, 1080, 978, 888, 816, 749, 508, 387.
2
.2.2. Preparation of [TBA]
Compound 2 was synthesized hydrothermally from a mixture of Na
.00mmol), H BO (124.00mg, 2.00mmol), H ·2H O (257.00mg, 2.04mmol), MnCl
NBr (297.00mg, 0.92mmol) and distilled
2 3 2
[Mn(phen) ][HBW12O40]·2H O (2)
2
WO
4
·2H
2
O (659.00mg,
2
·4H O
2
3
3
2
C
2
O
4
2
2
(
401.00mg, 2.03mmol), phen (160.00mg, 0.50mmol), (C
4
H
9
)
4
water (25ml). After adjusting the pH of the mixture to 5 using HCl solution, the mixture was heated
under autogenous pressure at 160°C for 4 days and then left to cool to room temperature. Red block
crystals could be isolated in about 51% yield (based on W). Elemental analyses (%) Calcd: W, 55.51; B,
0
2
2
.27; Mn, 1.38; C, 20.56; H, 2.46; N, 2.82. Found: W, 55.71; B, 0.16; Mn, 1.48; C, 20.89; H, 2.29; N,
-
1
.72. FT-IR (KBr, cm ): 1624, 1516, 1485, 1425, 1381, 1149, 1103, 992, 949, 899, 823, 719, 528, 388.
.2.3. Preparation of {[Co(phen) (H O)] [HAlW12 40]]}·3H O (3)
Compound 3 was synthesized hydrothermally from a mixture of Na
(254.00mg, 2.02mmol), NaAlO (165.00mg, 2.01mmol), CoCl ·6H
2
2
2
O
2
2
4 2
WO ·2H O (661.00mg, 2.00mmol),
H
2
C
2
O
4
2
2
2
O (476.00mg, 2.00mmol), phen
(
302.00mg, 1.52mmol), 4,4’-bpy (194.00mg, 1.01mmol) and distilled water (25ml). After adjusting the pH
of the mixture to 3.5 using HCl solution, the mixture was heated under autogenous pressure at 160°C for 5
days and then left to cool to room temperature. Red block crystals could be isolated in about 55% yield
(
based on W). Elemental analyses (%) Calcd: W, 58.01; Al, 0.71; Co 3.10; C, 15.16; H, 1.14; N, 2.95. Found:
4

-
1
W, 57.65; Al, 0.78; Co, 3.13; C, 15.16; H, 0.97; N, 3.04. FT-IR (KBr, cm ): 1606, 1516, 1425, 1224, 1145,
1
043, 954, 880, 807, 764, 719, 534, 485, 381, 381.
.2.3. Preparation of {[Cd(2,2’-bpy) 40]} (4)
Compound 4 was synthesized hydrothermally from a mixture of Na
Na SiO ·9H O (284.00mg, 1.00mmol), CdCl ·2.5H O (281.00mg, 1.23mmol), 2,2’-bpy (157.00mg,
.01mmol) and distilled water (25ml). After adjusting the pH of the mixture to 2.5 using HCl solution, the
2
2 2
] [SiW12O
2
4 2
WO ·2H O (500.00mg, 1.52mmol),
2
3
2
2
2
1
mixture was heated under autogenous pressure at 160°C for 5 days and then left to cool to room
temperature. Yellow block crystals could be isolated in about 45% yield (based on W). Elemental analyses
(
%) Calcd: W, 59.25; Si, 0.75; Cd, 6.04; C, 12.90; H, 0.87; N, 3.01. Found: W, 59.17; Si, 0.88; Cd, 6.02; C,
-
1
1
9
3.03; H, 0.98; N, 3.06. FT-IR (KBr, cm ): 1594, 1560, 1474, 1439, 1316, 1246, 1156, 1095, 1014, 974, 953,
19, 884, 794, 644, 534, 385.
Table 1. Crystal data and structural refinements for compounds 1-4.
Compound 1 Compound 2 Compound 3
Compound 4
Empirical formula
Formula weight
Crystal system
space group
a (Å)
b (Å)
c (Å)
α (˚)
β (˚)
40 8 68 8 40 12 48 2 8 45 12 40 2 8
C H33CdN O40PW12 C H97BMnN O W C H43AlCo N O W C H32Cd N O40SiW12
3615.31
monoclinic
P2(1)/c
3938.48
monoclinic
Pc
3802.94
Triclinic
P-1
3723.82
monoclinic
Pn
21.039(3)
14.1762(14)
14.5214(16)
27.102(3)
90.00
100.216(4)
90.00
5490.7(10)
2
2.382
12.695
3598
73626
11.278(2)
13.547(3)
13.593(3)
68.20(3)
75.47(3)
69.37(3)
1787.3(6)
1
3.533
19.787
1702
17669
8104
13.8785(6)
12.5015(5)
18.5517(8)
90.00
92.255(2)
90.00
3216.3(2)
2
3.845
22.125
3292
39282
15361
0.0366
99.8
929
1.076
13.3619(19)
23.136(4)
90.00
97.112(8)
90.00
6454.2(16)
4
3.721
21.736
6400
36095
γ (˚)
Volume (Å )
3
Z
D
-
3
C
(Mg·m )
-1
μ (mm )
F(000)
Reflc collected
Reflc unique
R(int)
11428
0.1175
22683
0.0514
99.7
1100
0.0362
99.5
604
Completeness to θ 99.6
parameters
GOF on F
919
1.071
2
1.078
1.005
a
R [I>2σ(I)]
R
1
= 0.0522
ωR = 0.1513
R
1
= 0.0503
ωR = 0.1400
R
1
= 0.0537
ωR = 0.1149
R
1
= 0.0321
ωR = 0.0657
b
R (all data)
2
2
2
2
2
2 2
2 2 1/2
|. b ωR -F ) ]} .
a R
1
= ∑||F
0
|-|F
c
||/∑|F
0
2 0 c 0
= {∑ [w (F ) ]/∑[w(F
2
.3. X-ray crystallographic analysis
All the data collections of 1-4 were performed on a Bruker Apex II diffractometer with a graphite
monochromated Mo K (λ= 0.71073) radiation. Direct methods were used to solve the structures of 1-4,
a
SHELXS-2014/7[22] and SHELXL-2014/7 [22] were used to refine the anisotropic thermal parameters of the
5

2
data of compounds 1-4 using the full-matrix least-squares on F . All the hydrogen atoms of the ligands
were placed in geometrically calculated positions and refined with fixed isotropic displacement
parameters using a riding model except the lattice water molecules in compound 2. The lattice water
molecules in 2 were removed with the SQUEEZE procedure in PLATON [23]. The number of the water
molecules for 2 was deduced from the TGA and elemental analysis. The crystallographic summary of the
data of 1-4 are given in Table 1. CCDC: 1814668 for 1, 1814669 for 2, 1814670 for 3 and 1814671 for 4.
3
3
3
. Results and Discussion
.1. Crystal structures
.1.1. Crystal structure of compound 1
2
-
2+
Fig. 1 ellipsoid (50% probability)-and-wire representation of [HPW12
O
3
40] , [Cd(2,2’-bpy) ] and
2
,2’-bpy in compound 1.
The asymmetric unit of compound 1 consists of a [HPW12 40] , a [Cd(2,2’-bpy)
40] can be viewed as a shell of {W12
2
-
2+
] and a 2,2’-bpy,
O
3
2
-
3-
as shown in Fig. 1. [HPW12
O
O
36} encapsulating a PO
4
with P-O
distances in the 1.50(2)-1.55(2)Å range. W-O bonds are comparable to those in reported compounds
[
15]. The oxidation states of the tungstens were calculated using the parameters suggested by Brown
24]. Results give the average value 5.96, indicating that the oxidation state of the tungstens is +6. So
[
VI
2-
the formula of {HPW12O40} is [HPW 12O40] . In addition, the assignment of the proton to the POM
but not to 2,2-bpy can be interpreted as follows: firstly, the POM in compound 1 contains
negative charges, so it is more easy to interact with the proton; secondly, if the nitrogens of
2
,2’-bpy is protonated, there should be hydrogen bonds between the nitrogens and some
oxygens, however, it is not so.
2
+
2+
3 3
There is only a TMC in compound 1: [Cd(2,2’-bpy) ] . As shown in Fig. 1, Cd(1) of [Cd(2,2’-bpy) ]
receives contributions from six nitrogen donors coming from three 2,2’-bpy with Cd-N distances of
6

2
.27(2)-2.47(2)Å. Thus, a distorted octahedral geometry around Cd(1) is completed. Except for
2
-
2+
[
3
HPW12O40] and [Cd(2,2’-bpy) ] , there is a 2,2’-bpy in the asymmetric unit (Fig. 1) . The 2,2’-bpy showed
no interactions with any cadmiums, just served as the space filling agent.
2
-
There are weak C-H⋯O hydrogen bonding interactions between oxygens of [HPW12
O
40
]
and
2
+
carbons of [Cd(2,2’-bpy)
3
]
with C⋯O distances of 3.2186(4)-3.2936(3)Å. Via these C-H⋯O hydrogen
bonds and van der Waals interactions, POMs, TMCs and 2, 2’-bpy ligands are connected to form a 3-D
supramolecular structure (Fig. S1).
3
.1.4. Crystal structure of compound 2
4
-
2+
+
The asymmetric unit of compound 2 is composed of a [HBW12
O
3
40] , a [Mn(phen) ] and two TBA
4
-
2-
cations. [HBW12
O
40] in compound 2 is very similar to [HPW12
O
40] in compound 1 except that its
central position is occupied by B. B-O distances are in the range of 1.50(3)-1.56(3)Å. W-O distances of
4
-
[
HBW12O40] are comparable to those of compound 1. The +6 oxidation state for W atoms was
VI
4-
confirmed by bond valence sum calculations [24]. So the formula of {HBW12O40} is [HBW 12O40] .
2
+
+
and TBA in compound 2.
Fig. 2 wire representation of [Mn(phen)
3
]
2
+
2+
There is a TMC in compound 2: [Mn(phen)
3
]
(Fig. 2). Mn(1) of [Mn(phen)
3
]
is six-coordinated in a
distorted octahedral environment by six nitrogens from three phen ligands with Mn-N distances of
+
+
2
.26(1)-2.31(1) Å (Fig. 2). There are two TBA cations in compound 2. The role of the two TBA in
+
compound 2 is different from that of 2,2’-bpy in compound 1. TBA contains a positive charge, therefore,
+
TBA acts as both space filling and charge compensation agents.
4
-
2+
3
There is a hydrogen bond between O(34) of [HBW12O40] and C(18) of [Mn(phen) ] with the C···O
4
-
40
distance of 2.9990(3)Å. In addition, there are also hydrogen bonds between oxygens of [HBW12O ]
+
and carbons of TBA with C···O distances of 3.0915(2)-3.1998(3)Å. Via these C-H···O hydrogen bonds,
+
POMs, TMCs and TBA were joined to form a 1-D supramolecular double-chain structure running
along the b axis (Fig. S2).
7

Both compounds 1 and 2 are new examples of multi-component compounds of POMs, TMCs and
organic species. Our group has synthesized a series of such multi-component compounds of Mo-V
clusters [25]. This kind of compounds is different from the hybrids of POMs and TMCs, e. g. the
dissolving ability of the tri-component compounds is obviously better than that of the bi-component
ones of POMs and TMCs.
3
.1.2. Crystal structure of compound 3
4
-
2+
2 2
The asymmetric unit of compound 3 is composed of halve a [HAlW12O40] , a [Co(phen) (H O)]
2
-
4-
and one and a half of lattice water molecules. In contrast to [HPW12
O
40
]
of 1, [HAlW12
O
40
]
is a
5
-
Pseudo-Keggin anion which contains a disordered 2×AlO
Al-O distances are in the range of 1.63(3)-1.93(2)Å. W-O distances of [HAlW12
comparable to those in compound 1. Bond valance sum calculation results reveal that the oxidation
4
cube with its eight oxygens half-occupied.
4
-
O
40
]
are all
VI
4-
states of all the tungsten atoms are +6. So the formula of {HAlW12O40} is [HAlW 12O40] .
Fig. 3 ellipsoid (50% probability)-and-wire representation of the POM bi-supported complex in
compound 3.
4
-
A feature of 3 is that [HAlW12
O
40
]
serves as a bridging ligand coordinating to two cobalts from
2
+
4-
two [Co(phen)
2
(H
2
O)] which are disposed at the two opposite sides of [HAlW12
O
40] , the Co-O bond
4
-
distance is 2.112(9)Å. Each Co is coordinated by one terminal oxygen from [HAlW12
O40] , four
nitrogens from two phen (Co-N bonds: 2.11(1)-2.14(1)Å) and one oxygen from a water molecule
(
Co-O(3w) bond: 2.09(1)Å). Thus the cobalt exhibits an octahedral coordination geometry. In other
4
-
2+
words, [HAlW12
O
40
]
supports two [Co(phen)
2
(H
2
O)]
via Co-O contacts, forming a POM
+
bi-supported complex {[HAlW12
shown in Fig. 3.
O40][Co(phen)
2
(H
2
O)]
2
} with the Co-Co distance of 14.543(4) Å as
In addition, compound 3 contains a di-nuclear water clusters (H
2
2
O) , which is formed by Ow(2) and
2
Ow(3) of [Co(phen) (H O)] with a distance of 2.7049(5) Å (Fig. 3). The detailed analysis shows that the
+
2 2
water cluster exhibits extensive hydrogen-bonding interactions between oxygens of the water cluster and
8

4
-
4-
oxygens of [HAlW12
with O···O distances of 2.925(1)Å, Ow(3) is linked to O(12a, a: -x, 2-y, -z) from [HAlW12
bond with O···O distances of 2.7933(9)Å. In addition, Ow(1) is hydrogen bonded to O(7a, a: 1-x, 1-y, -z),
O40] . Ow(2) is linked to O(5a, a: -1+x, 1+y, z) from [HAlW12O40] via a hydrogen bond
4
-
O
40] via a hydrogen
4
-
O(11a, a: 1-x, 1-y, -z), O(19a, a: 1-x, 1-y, -z) and O(20a, a: 1-x, 1-y, -z) from two adjacent [HAlW12
O40] via
hydrogen bonds with O···O distances of 2.6977(6)Å-2.963(1)Å. There is only a strong C-H···O hydrogen
4
-
2+
bond between O(6a, a: x, 1+y, z) of [HAlW12
.1584(9)Å. Via these O-H···O and C-H···O hydrogen bonds, the POM supported complexes were joined to
form a 3-D supramolecular structure (Fig. S3).
2 2
O40] and C(10) of [Co(phen) (H O)] with a C···O distance of
3
3
.1.3. Crystal structure of compound 4
Fig. 4 ellipsoid (50% probability)-and-wire representation of the 2-D structure constructed from
2
+
2
POMs and [Cd(2,2’-bpy) ] .
4
-
2+
4-
40 2 40
The asymmetric unit of 4 is constructed from a [SiW12O ] and two [Cd(2,2’-bpy) ] . [SiW12O ]
3
-
in 4 is almost identical to [PW12
O
40] in compound 1, the main difference of the two is that the
3
-
3-
central unit is a PO
4
in 1, but a SiO
4
in 4. Si-O distances are in the range of 1.59(1)-1.65(1)Å. W-O
distances are comparable to those of compound 1. Oxidation states for W atoms were calculated and
the results indicate that the oxidation states of the tungsten atoms are +6. So the formula of
VI
4-
{
SiW12O40} is [SiW 12O40] .
There are two crystallographically independent Cd ions in the asymmetric unit of 4 (Fig. 4). Both Cd(1)
4
-
40
and Cd(2) are bonded to two terminal oxygens belonging to two [SiW12O ] with Cd-O distance of
2
.31(2)-2.57(1)Å and four nitrogens coming from two phen ligands with Cd-N distance of 2.23(2)-2.33(2)Å,
4
-
exhibiting a distorted octahedral geometry. Thus, [SiW12O40] acting as a four-fold monodentate ligand
9

2
+
coordinates to four Cd(1) ions from four [Cd(2,2’-bpy)
Fig. 4). Because of strong steric hindrance of {M(2,2’-bpy)
bridges to join POMs to form extended structures. There was an extended compound of which POM is
bridged by both {M(2,2’-bpy) } and {M(en) } [26]. Very recently, Cui et al also reported a compounds of
which Dawson POMs were linked by {M(phen) } to form a 1-D structure [27]. Very recently, our group
reported an extended compound constructed from Mo-V clusters and {M(2,2’-bpy) } [28]. It should be
noted that extended compounds of {M(2,2’-bpy) } or {M(phen) } are rather scarce, compound 4 are a new
example of such compounds, also still the only extended compound of Keggin POMs and {M(2,2’-bpy) }.
2
] to form a novel 2-D layered framework structure
(
2
}, it is not easy for {M(2,2’-bpy) } to be used as
2
2
2
2
2
2
2
2
Further analysis of compound 4 reveals that there were complex C-H···O hydrogen bonding interactions
with C···O distances of 2.9069(1)-3.1921(1)Å. These C-H···O hydrogen bonds increased the stability of the
2
+
2
crystal of 4 and connect POMs and [Cd(2,2’-bpy) ] to form a 3-D supramolecular structure (Fig. S4).
3
3
.2. Properties
.2.1. XRD analysis
XRD patterns of compounds 1-4 are well consistent with the simulated ones based on the data of the
single-crystal structures, confirming the phase purity of the crystals (Fig. S5). The differences in reflection
intensity are probably associated with preferential orientations in the samples of 1–4.
3
.2.2. IR spectra
In the IR spectrum of compound 1 (Fig. S6), the peak of 1080 cm is associated with the asymmetric
-
1
-
1
P-O vibrations. The peaks of 978 and 888 cm are due to υ(W-O
) and υ(W-O -W), respectively, and those
t b
-
1
-1
of 816 and 749 cm are corresponding to υ(W-O
c
). A series of bands in the range of 1158-1593cm are
characteristic of 2,2’-bpy in compound 1. The IR spectra of compounds 2-4 display similar peaks at 949,
-
1
-1
-1
8
99, 823cm , 954, 880, 807cm , 953, 884, 794cm and, related to υ(W-O
t b c
), υ(W-O -W) and υ(W-O ) of
-
1
Keggin POMs. The IR spectra of phen ligands in compounds 2 and 3 exhibits the bands of 1103-1624cm
-1
and 1145-1606cm , respectively, while IR spectrum of 2,2’-bpy in compound 4 shows bands in the
-
1
095-1594 cm region.
1
3
.2.3. UV-Vis spectroscopy
The UV-vis spectrum of saturated DMSO solution of compound 1 (Fig. S7) displays a peak at 259nm
which is corresponding to O→W charge transfer and a shoulder peaks at 266 nm which is assigned to
n→π* transitions of 2,2’-bpy. The almost identical peaks (255nm for 2, 256nm for 3, and 257nm for 4) of
O→W charge transfer were also observed in the UV-Vis spectra of saturated DMSO solutions of
1
0

compounds 2-4 (Fig. S7), confirming that the POMs of the four compounds are similar [29]. The phen
ligands in compounds 2 and 3 exhibit shoulder peaks at 264 and 268nm in the UV-Vis spectra of DMSO
solutions of compounds 2 and 3, attributed to n→π* transitions of phen ligands. 2,2’-bpy in compound 4
exhibits a shoulder peak at 272 nm in the UV-Vis spectrum of the DMSO solution of compound 4, also due
to n→π* transitions.
3
.2.4. XPS spectra
The oxidation state of tungstens calculated from bond valence sum calculations was confirmed by the
XPS analysis (Fig. S8). two peaks at 36.0 and 38.1 eV were observed in the W4f region of the XPS spectrum
6
+
6+
of compound 1, and can be attributed to W 4f7/2 and W 4f5/2. The XPS spectra of tungstens in 2-4
6
+
give two similar peaks at 35.3, 37.4 eV, 35.5, 37.6 eV, and 35.9, 38.0eV, which can be ascribed to W
6
+
4
f7/2 and W 4f5/2 in compounds 2-4, respectively.
3
.2.5. TG Analysis
1
00
Compound 2
95
90
85
80
75
200
400
600
Temperature (℃)
800
1000
Fig. 5 TG curve of compound 2.
The solvent molecules in compound 2 were removed with SQUEEZE in PLATON. The number of lattice
water molecules of 2 was deduced from the TGA and elemental analysis. TG analyses of compound 2 in
the range 35-1000°C are shown in Fig. 5, which shows that the TG curve of 2 can be divided into three
stages. The first stage is from 35 to 290°C with the weight loss of 0.83%, which is due to the release of the
lattice water molecules. The second stage is from 290 to 469°C with the weight loss of 5.35% (calcd:
+
6
.10%), which is corresponding to the removal of a TBA . The third stage is from 469 to 847°C with the
+
weight loss of 18.94% (calcd: 19.70%), which is ascribed to the release of a TBA and phen ligands. The
whole weight loss is 26.10%, which is consistent with the calculated result (calcd: 26.63%). The final
11

product of the thermal decomposition should be the mixture of WO
percentage of the weight of the final product to the weight of compound 2 added for TG analysis is
3.73%, which is in good agreement with the experimental result (73.90%). Also the WO in the mixture
can be further confirmed by the IR analysis of the final product of the thermal decomposition (Fig. S9).
.2.5. Fluorescence properties
3 2 5
, B O and MnO, and the calculated
7
3
3
3
3
2
2
1
1
50
00
50
00
50
00
160
1
1
1
40
20
00
Emission of phen
Emission of 2,2'-bpy
8
6
4
2
0
0
0
0
0
5
0
0
400
450
500
Wavelength (nm)
550
600
400
450
500 550
Wavelength (nm)
600
650
7
00
00
00
00
00
00
00
500
400
6
5
4
3
2
1
Emission of compound 1
Emission of compound 2
300
200
100
0
4
00
450
500
Wavelength (nm)
550
600
360
380
400
420
440
Wavelength (nm)
460
480
500
2
2
1
1
50
00
50
00
800
600
400
200
0
Emission of compound 3
Emission of compound 4
5
0
0
400
450
500
Wavelength (nm)
550
600
400
450
500
Wavelength (nm)
550
Fig. 6 Fluorescent spectra of DMSO solutions of 2,2’-bpy, phen and compounds 1-4.
We have examined the fluorescent properties of DMSO solutions of 2,2’-bpy, phen and compounds 1-4
1
2

at room temperature, as shown in Fig. 6. The fluorescent spectrum of 2,2’-bpy and phen show emission
peaks at 416nm (λex=342nm) and 432nm (λex=357nm), respectively. Emission peaks at 419nm (λex=365nm)
and 412nm (λex=363nm) were observed in the fluorescent spectrum of compounds 1 and 4 (Fig. 6). The
emission bands of 1 and 4 are similar to that of 2,2’-bpy in terms of position and shape, and should be
assigned to the intra-ligand electronic transfer of 2,2’-bpy. The fluorescent spectrum of compound 3
exhibits an emission peak at 423nm (λex=388nm), which should be assigned to the intra-ligand electronic
transfer of phen. The fluorescent spectrum of compound 2, different from the other three, exhibit an
emission peak at 386nm (λex=339nm) (Fig. 6). Clearly, the emission peak of compound 2 is significantly
blue-shifted by 80nm from the peak of phen observed, due to the complexation of phen with manganese
in compound 2.
3
.2.6. Catalytic properties
The styrene epoxidation to styrene oxide using tert-butyl hydroperoxide (TBHP) as oxidant and
compounds 1, 2, 3 or 4 as catalyst was carried out in a batch reactor. In a typical run, styrene (0.114ml,
1
0
mmol), the catalyst (1 (2.00mg, 0.56μmol), 2 (2.00mg, 0.51μmol), 3 (2.00mg, 0.53μmol) and 4 (2.00mg,
.54μmol), and CH CN (2ml) were added to a 10ml two-neck flask equipped with a stirrer and a reflux
3
condenser. The mixture was heated to 80°C and then TBHP (2mmol) was added to start the reaction. The
organic products were quantified by using a gas chromatograph (Shimadzu, GC-8A) equipped with a flame
detector and an HP-5 capillary column and identified by comparison with authentic samples and GC-MS
coupling. The reaction of the styrene epoxidation in the absence of the catalyst did not take place after 8
h.
Table 2 shows the styrene epoxidation results over catalysts at 80°C. All the catalysts are active for the
TBHP oxidation of styrene. Compound 1 shows an activity with 29.3% conversion and 69.0% selectivity to
styrene oxide. Compound 2 displays the highest activity, its conversion reaches 82.9% with the styrene
oxide selectivity of 62.3%. Compound 3 exhibits a higher activity with the styrene conversion and styrene
oxide selectivity reaching 43.9% and 64.3%. Compound 4 presents the lowest activity among the four with
2
8.7% conversion and 63.9% styrene oxide selectivity.
Table 2 Catalytic activity and product distribution
Catalysts
Product selectivity (%)
Styrene conversion (%)
so
bzdh
29.7
34.6
Others
1.3
Compound 1 29.3
Compound 2 82.9
69.0
62.3
3.1
1
3

Compound 3 43.9
Compound 4 28.7
64.3
63.9
33.8
34.3
1.9
1.8
so = styrene oxide; bzdh = benzaldehyde. others: including benzoic acid and phenylacetaldehyde.
The catalytic results for styrene epoxidation of TBHP over catalysts 1, 2, 3 and 4 indicate that the
catalytic property of compound 2 is the best of the four compounds. Since the four compounds are all
based on Keggin polyoxoanions, we thus speculate that the catalytic difference mainly comes from the
different transition metals; though compounds 1 and 4 contain the identical cadmium ions and similar
Keggin POMs with different centers, the two show similar conversions and selectivities, therefore, we
concluded that the different POM centers exhibit a minor influence on their catalytic properties.
Compounds 1-4 includes similar Keggin POMs with different POM centers, therefore, the catalytic
properties of the four compounds is mainly influenced by the transition metals of them. The catalytic
activity of compound 2 is highest, the reason can be that the transition metal of it is manganese. However,
the mechanism why the manganese shows the highest activity is still elusive.
3
.3. Conclusions
Four new organic-inorganic hybrid compounds have been synthesized and characterized. The Four
compounds are all based on Keggin polyoxoanions. However, the crystal structures of the Four are
thoroughly different. It is obvious that the transition metal complex building blocks are the vital factor for
the structures of the Four. Further research is underway to determine the rules of their synthesis and to
explore their attractive properties.
Acknowledgments
This work is financially supported by the National Natural Science Foundation of China (NSFC) (Grant No.
2
1501203 and 21003056), the Natural Science Research Project of Education Department in Henan
Province (Grant No. 15A150085), the Key Science Research Project of Colleges and Universities in Henan
Province (Grant No. 18A150019) and the High Level Personnel Fund of Zhoukou Normal University (Grant
No. ZKNU2014116).
Notes and references
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1
5

Four new compounds based on Keggin polyoxotungstates and transition metal complexes have been
synthesized and characterized.
1
6