Lanthanide-supported molybdenum-vanadium oxide clusters: Syntheses, structures and catalytic properties

Article abstract of DOI:10.1039/c4ra16237d

Three new lanthanide-supported molybdovanadates [Ln(H₂O)₅]₂Mo₆V₂O₂₆·8H₂O (Ln = La 1; Ce 2; Nd 3) have been synthesized by the reaction of the [Mo₆V₂O₂₆

Full text of DOI:10.1039/c4ra16237d

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Lanthanide-supported molybdenum–vanadium
oxide clusters: syntheses, structures and catalytic
properties†
Cite this: RSC Adv., 2015, 5, 18796
Fei Fei, Haiyan An,* Changgong Meng, Lin Wang and Huilong Wang
Three new lanthanide-supported molybdovanadates [Ln(H
2
O)
5
]
2
Mo O (Ln ¼ La 1; Ce 2; Nd 3)
6
V
2
O
26$8H
2
6
ꢀ
have been synthesized by the reaction of the [Mo
V
6 2
O
26
]
3 3
anion with Ln(NO ) in aqueous solution and
characterized by elemental analysis, IR, solid state UV-vis spectroscopy, thermal gravimetric analysis,
powder X-ray diffraction and single-crystal X-ray diffraction. Compounds 1–3 are obtained at the same
pH value (3.0) and have similar structural units which consist of one molybdovanadate unit supported by
two lanthanide cations. Then, these bi-supporting subunits are joined together by strong hydrogen-
bonding interactions between polyoxoanions and coordinated water molecules to form
a 3D
supramolecular framework. To the best of our knowledge, they represent the first examples of inorganic
species constructed from molybdenum–vanadium polyoxometalates and lanthanides. The UV-vis diffuse
reflectivity spectra of 1–3 show that they can be regarded as a wide gap semiconductor. Photocatalytic
studies indicate that compounds 1–3 are not only active photocatalysts for degradation of rhodamine B,
but very stable and easily separated from the photocatalytic system for reuse as well. Moreover, the new
materials were tested as catalysts in cyanosilylation reactions of aldehydes under solvent-free conditions.
The experimental results indicate that all three compounds 1–3 can act as Lewis acid–base catalysts
through a heterogeneous manner to prompt cyanosilylation with good efficiency.
Received 12th December 2014
Accepted 27th January 2015
DOI: 10.1039/c4ra16237d
www.rsc.org/advances
clusters, will not only build new topologic structures, but also
tune the chemical and physical properties of polyoxometalate
Introduction
Polyoxometalates (POMs) as anionic metal oxide clusters, bear precursors, thus, always is the subject of intense study. So far,
many properties that make them attractive for applications in limited success has been achieved in directly introducing metal
1
catalysis, medicine, electronics, magnetism and optics. Most moieties into molybdenum–vanadium polyoxometalate frame-
6
,7
attention has been given to polyoxotungstates, poly- works,
including 1D chain-like complexes such as
42][Co(Phen) ][Hpy]$3H O$H and [PMo
][Him] $2H
2–4
oxomolybdates or polyoxovanadates in this eld, however, it [PMo
is becoming clear that the molybdenum–vanadium poly- [Co(Phen)
V
O
O
V O ]
8 6 42
8
6
2
3
2
O$3H
O (phen ¼ 1,10-phenanthroline,
2
2
3
2
8
oxometalate systems which fuse the merits of molybdenum and py ¼ pyridine, Him ¼ imidazole), 2D layer complexes such as
vanadium elements, can be specially interesting in catalysis [Co(en)
elds. Indeed, some papers have been devoted to the study {PMo
2
][Co(bpy)
2
]
2
[PMo
8
V
8
O44]$4.5H
2
O
and [Cu(en)
2 2
(H O)]

8
V
6
O
42[Cu(en)
2
][Cu0.5(en)]
3 2
}$5.5H O
(en ethylenedi-
¼
0
9
of homogeneous and heterogeneous catalytic reactions amine, bpy ¼ 2,2 -bipyridine), and the 3D framework complexes
0
by the molybdenum–vanadium polyoxometalates, such as such as {AsMo12
O
40(VO)[VO(H
2
O)]}$[Cu(4,4 -bipy)]
5
$H
2
O, and
5
10
{PV
x
Mo12ꢀx
O
40} (x ¼ 0–3). Introducing additional metal Na
5
Cu(Hmorph)
3
[V
2
Mo16 58]$15H
O
2
O, which have focused on
subunits into molybdenum–vanadium polyoxometalate 3d metal cations.
Interest in lanthanides with high coordination numbers and
exible coordination geometries has grown signicantly in
College of Chemistry, Dalian University of Technology, Dalian 116023, P. R. China.
E-mail: anhy@dlut.edu.cn; Tel: +86-411-84657675
recent years, because lanthanide-based species have emerged as
11
excellent candidates for catalytic studies. Lanthanides incor-
porating into the lacunary polyoxometalates, have been demon-
strated as effective Lewis acids for activation of substrates.
†
Electronic supplementary information (ESI) available: The coordination modes
3+
3+
of La cations in 1 and Nd cation in 3, 3D supramolecular topology structure for
, IR, TG curves and PXRD patterns for 1–3, UV-vis-NIR diffuse reectance
1
spectrum for K
5
NaMo
6
V
2
O
26$4H
2
O, UV-vis absorption spectra of RhB solutions For example, several lanthanide-containing polyoxometalates
6ꢀ
4ꢀ
3+
without catalyst and with K
5
6 2 2
NaMo V O26$4H O as catalyst, effect of number of
[
a-Ln(H O) P W O ] and [a-Ln(H O)PW O ] (Ln ¼ Y ,
2
4
2
17 61
2
11 39
recycling cycles on the decolorization, kinetic proles in the aldehyde
cyanosilylation reaction for compounds 1–3, IR spectra for as-synthesized and
recovered catalyst 3 are available. Study on recycling of catalyst 3 is listed in
Table S1. See DOI: 10.1039/c4ra16237d
3+ 3+ 3+ 3+
La , Eu , Sm , or Yb ) have been reported to act as Lewis acid
12
catalysts for aldol and imino Diels–Alder reactions. To date only
two examples of lanthanide-containing molybdenum–vanadium
18796 | RSC Adv., 2015, 5, 18796–18805
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polyoxometalates determined by X-ray single-crystal diffraction
are known, i.e. [Ln (pdc) (H O) ][HPMo V O ]$H O (Ln ¼ La,
Experimental section
Materials and measurements
4
4
2
16
10
2
40
2
13
Ce, Pr, Nd, Sm; H pdc ¼ pyridine-2,6-dicarboxylic acid), and
2
[
Dy(HL) (L)1 (H O) ][PMo VO ]$8H O (L ¼ 1,4-bis(pyridinil-4- La(NO ) $6H O, Ce(NO ) $6H O and Nd(NO ) $6H O were
.5
2
3
11
40
2
3 3
2
3 3
2
3 3
2
14
carboxylato)-l,4-dimethylbenzene). Such cases show the diffi- commercially purchased. The precursor K NaMo V O $4H O
5
6
2
26
2
culty in obtaining the high-quality single crystals containing both was prepared as described by Nenner and characterized by IR
19
constituents. Therefore, it is quite appealing and challenging to spectroscopy. All chemicals were commercially purchased and
directly combining lanthanides with electronically interesting used as received without further purication. A Perkin-Elmer
molybdenum–vanadium polyoxometalates to make new mate- CNHS analyzer 2400 was employed for elemental analysis. IR
ꢀ
1
rials featuring catalytic functions.
spectra were recorded in the range 400–4000 cm on an Alpha
Cyanosilylation reaction provides a convenient route to Centaur FT/IR Spectrophotometer using KBr pellets. TG anal-
cyanohydrins, which are key derivatives in the synthesis of ne yses were performed on a Perkin-Elmer TGA7 instrument in
15
ꢁ
ꢀ1
chemicals and pharmaceuticals. In order to obtain cyanohy- owing N with a heating rate of 10 C min . PXRD patterns of
2
drin molecules, the use of a catalyst with Lewis acid or base the samples were recorded on a Rigaku Dmax 2000 X-ray
character through which both the carbonyl substrate and the diffractometer with graphite monochromatized Cu-Ka radia-
ꢁ
cyanide precursor are activated is necessary. There are very rare tion (l ¼ 0.154 nm) and 2q varying from 5 to 50 . Diffuse
examples of cyanosilylation reaction catalyzing by poly- reectivity spectra were collected on a nely ground sample
oxometalates up to now. An yttrium-containing silicotungstate with a Cary 500 spectrophotometer equipped with a 110 mm
1
0ꢀ
dimer, [{Y(H O) } (g-SiW O ) ] , was rstly reported as a diameter integrating sphere, which were measured from 200 to
2
2 2
10 36 2
Lewis acid–base catalyst by Mizuno's group at 2012, showing 800 nm. UV-vis spectra were acquired on an Analytic Jena
remarkable catalytic performance for cyanosilylation of ketones SPECORD S600 spectrophotometer.
16
and aldehydes with trimethylsilyl cyanide (TMSCN). Then, a
series of isostructural lacunary silicotungstate dimers with
Synthesis of [La(H
2 5 2 6 2 2
O) ] Mo V O26$8H O (1)
1
0ꢀ
lanthanide cations, [{Ln(H O) (acetone)} (g-SiW10O ) ]
2 2 2 36 2
3
+
3+
3+
3+
3+
3+
In a typical synthesis procedure for 1, K NaMo V O $4H O
5
6
2
26
2
(
Ln ¼ Y , Nd , Eu , Gd , Tb , or Dy ) have been synthesized
3 3 2
(138.4 mg, 0.1 mmol) and La(NO ) $6H O (86.6 mg, 0.2 mmol)
by them and effectively promoted cyanosilylation of carbonyl
were dissolved with stirring in 20 mL deionized water. The pH
value of the mixture is about 3.50. Aer the pH value was
compounds with trimethylsilyl cyanide as homogeneous
17
catalysts. Recently, Niu and coworkers have successfully
8
ꢀ
adjusted to 3.00 with 1 M HNO , the solution was heated at
3
isolated one [H
[Cu (bpy)(H
2
W
11
O
38
]
-based metal–organic framework
38]$3H O$0.5CH CN}, which can
2 3
ꢁ
18
80 C for 1 h. When the solution was cooled to room tempera-
{
2
2
O)5.5
]
2
[H
2
W
11
O
ture, a small amount of pale yellow precipitate was ltered off,
and then the ltrate was sealed with pierced paralm for
facilitating slow evaporation at room temperature. The ltrate
was kept for one week at ambient conditions, and yellow block
crystals of 1 were obtained in about 40% yield (based on V).
be as a Lewis acid catalyst through a heterogeneous manner to
drive cyanosilylation with good efficiency. The development of
efficient heterogeneous catalysts for cyanosilylation of carbonyl
compounds with trimethylsilyl cyanide still is a signicant work
in current research. It is thus of considerable interest to inves-
tigate whether lanthanide-containing molybdenum–vanadium
polyoxometalates can effectively catalyze the cyanosilylation
reaction as heterogeneous catalysts.
Anal. calcd (%) for [La(H
2 5 2 6 2 2
O) ] Mo V O26$8H O: Mo, 33.96; La,
1
6.39; V, 6.01; O, 41.52; H, 2.12. Found (%): Mo, 33.59; La, 17.96;
ꢀ
1
V, 6.28; O, 40.16; H, 2.01. FTIR data (cm ): 3377(s), 1626(m),
47(s), 918(s), 890(s), 796(m), 647(s), 563(m), 533(m), 431(w).
9
Herein, we study the interactions between the molybdenum–
6
ꢀ
vanadium oxide cluster [Mo
6
V
2
O
26
]
and different lanthanide
cations. Three new compounds were obtained, namely Synthesis of [Ce(H O) ] Mo V O $8H O (2)
2
5 2
6
2
26
2
[
Ln(H O) ] Mo V O $8H O (Ln
¼
La 1; Ce 2; Nd 3).
2
5 2
6
2
26
2
Compound 2 was synthesized by the same procedure as 1 except
for replacement of La(NO $6H O with Ce(NO $6H O (86.8
mg, 0.2 mmol) and pH ¼ 2.96. The ltrate was kept for one week
at ambient conditions, and then yellow block crystals of 2 were
isolated in about 30% yield (based on V). Anal. calcd (%) for
6
ꢀ
Compounds 1–3 are constructed from [Mo V O ]
supported by two lanthanide cations, representing the rst
examples of inorganic species based on molybdenum–vana-
dium polyoxometalates and lanthanides. The [Mo
cluster consists of six [MoO ] and two [VO ] octahedra con-
nected by shared edges, isostructural with the octamolybdate
anions
6
2 26
3
)
3
2
3
)
3
2
6
ꢀ
V O ]
6 2 26
6
6
[
Ce(H O) ] Mo V O $8H O: Mo, 33.92; Ce, 16.49; V, 6.01; O,
2 5 2 6 2 26 2
4
1.46; H, 2.12. Found (%): Mo, 34.01; Ce, 16.87; V, 5.32; O, 41.81;
6
ꢀ
anion b-[Mo
8
O
26
]
. Noteworthy, all three compounds as
ꢀ1
H, 1.99. FTIR data (cm ): 3375(s), 1627(m), 948(s), 919(s),
heterogeneous Lewis acid–base catalysts are very efficient in the
cyanosilylation of carbonyl compounds and allow the reaction
to be carried out under solvent-free conditions using only a low
loading of the catalyst, which can be recovered and reused
887(s), 797(m), 649(s), 559(m), 527(m), 429(w).
Synthesis of [Nd(H O) ] Mo V O $8H O (3)
2
5 2
6
2
26
2
without displaying any signicant loss of activity. Furthermore, Compound 3 was synthesized by the same procedure as 1 except
compounds 1–3 show good activity in photocatalyzing degra- for replacement of La(NO ) $6H O with Nd(NO ) $6H O (87.6
3
3
2
3 3
2
dation of organic dye RhB.
mg, 0.2 mmol) and pH ¼ 3.02. The ltrate was kept for one week
at ambient conditions, and then yellow block crystals of 3 were
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isolated in about 30% yield (based on V). Anal. calcd (%) for Table 2 Selected bond lengths ( A˚ ) and angles ( ) for 1 and 3
[
Nd(H O) ] Mo V O $8H O: Mo, 33.76; Nd, 16.88; V, 5.98; O,
2 5 2 6 2 26 2
Compound 1
Mo(2)–O(2)
Mo(5)–O(26)
Mo(4)–O(13)
Mo(2)–O(11)
Mo(4)–O(16)
Mo(2)–O(1)
4
4
9
1.27; H, 2.11. Found (%): Mo, 33.12; Nd, 17.37; V, 6.26; O,
1.689(4)
1.707(4)
1.899(4)
2.206(4)
2.281(4)
2.343(4)
2.530(4)
2.623(4)
2.518(4)
2.580(4)
76.63(13)
151.86(15)
Mo(5)–O(20)
Mo(2)–O(14)
Mo(3)–O(4)
Mo(4)–O(4)
V(2)–O(7)
V(2)–O(1)
La(2)–O(9)
La(2)–O(7)
La(2)–O(7W)
La(2)–O(8W)
O(6)–V(1)–O(18)
O(6)–V(1)–O(16)
1.719(4)
1.748(4)
1.963(4)
2.237(4)
1.625(4)
2.227(4)
2.535(4)
2.656(4)
2.496(4)
2.593(4)
106.10(19)
170.29(17)
ꢀ1
1.08; H, 2.17. FTIR data (cm ): 3369(s), 1622(m), 943(s),
18(s), 882(s), 797(m), 634(s), 563(m), 531(m), 428(w).
X-ray crystallography
The crystallographic data of three compounds were collected La(1)–O(17)
La(1)–O(6)
on the Bruker Smart CCD diffractometer with Mo Ka radiation
La(1)–O(1W)
La(1)–O(4W)
O(16)–V(2)–O(1)
O(19)–V(1)–O(4)
˚
(
l ¼ 0.71073 A) by u and q scan modes. Empirical absorption
correction was applied. The structures of 1–3 were solved by the
2
direct method and rened by the Full-matrix least squares on F
20
using the SHELXTL-97 soware. All of the non-hydrogen
atoms were rened anisotropically in 1–3. The hydrogen
atoms attached to water molecules were not located in 1–3. The
detailed crystal data and structure renements for 1–3 are given
in Table 1. Selected bond lengths and angles of 1 and 3 are
respectively listed in Table 2. Crystallographic data for the Mo(3)–O(11)
structures reported in this paper have been deposited. CSD
reference number: 428893 for 1, 428894 for 2 and 428895 for 3.
a
Compound 3
Mo(2)–O(7)
Mo(1)–O(6)
Mo(2)–O(13)
Mo(3)–O(5)
1.680(10)
1.695(10)
1.965(9)
1.969(10)
2.280(10)
2.336(9)
2.534(9)
2.632(10)
76.8(4)
Mo(3)–O(3)
Mo(2)–O(1)
Mo(3)–O(2)
Mo(1)–O(4)
V(1)–O(10)#1
V(1)–O(11)
Nd(1)–O(1W)
Nd(1)–O(4W)
O(10)#1–V(1)–O(9)
O(10)#1–V(1)–O(11)
1.713(9)
1.723(9)
1.887(9)
1.933(10)
1.634(10)
2.211(9)
2.510(12)
2.586(12)
105.8(5)
169.9(4)
Mo(1)–O(11)
Nd(1)–O(3)
Nd(1)–O(10)
O(11)#1–V(1)–O(11)
O(5)#1–V(1)–O(13)#1
General procedure for cyanosilylation reactions
152.2(4)
a
The detailed reaction conditions are shown in the captions of
Table 3. A typical procedure for the cyanosilylation reaction of
aldehydes is as follows: 1 mol% catalyst (1–3) was added to a
Symmetry transformations used to generate equivalent atoms: #1 ꢀx,
y + 1, ꢀz.
ꢀ
3 3
mixture of aldehyde (0.5 mmol) and (CH ) SiCN (1.5 mmol), in
the absence of solvent. The reaction mixture was stirred at room trimethylsilyl ethers) were conrmed by a comparison of their
1
temperature under a N atmosphere. The progress of the reac- GC retention times, GC-MS spectra, and/or H spectra with
2
tion was monitored by GC analysis. Aer the reaction was those of authentic data. GC analysis was performed using
completed, the catalyst was removed by ltration and centrifu- HP6890/5973MS with
a cross-linked (95%)-dimethyl-(5%)-
gation of the reaction mixture. All products (cyanohydrin diphenylpolysiloxane column of 30 m length.
Table 1 Crystal data and structure refinement for 1–3
Complex
1
2
3
Formula
H
36La
2
Mo
6
O
44
V
2
H36Ce
2
Mo
6
O
44
V
2
H
2 6 44 2
36Nd Mo O V
Formula weight
T (K)
1695.63
293(2)
Triclinic
P1ꢀ
10.0427(4)
10.1151(4)
19.7105(8)
77.558(2)
82.638(2)
78.650(2)
1698.05
296(2)
Triclinic
P1ꢀ
10.0173(3)
10.0913(3)
19.6748(7)
77.372(2)
82.719(2)
78.668(2)
1706.29
296(2)
Triclinic
P1ꢀ
10.038(5)
10.049(6)
10.118(6)
106.967(8)
101.480(9)
91.502(8)
952.8(9)
1
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
ꢁ
a ( )
ꢁ
b ( )
ꢁ
g ( )
3
˚
U (A )
1909.29(13)
2
4.677
1895.62(10)
2
4.859
Z
ꢀ1
m (mm
)
5.169
Reections collected
Independent reections
R (int)
10 896
6693
0.0195
1.044
0.0287
0.0687
0.0362
0.0745
9084
6581
4586
3293
0.0189
1.027
0.0373
0.0937
0.0549
0.1087
0.0412
1.028
0.0568
0.1454
0.0917
0.1661
2
GOF on F
a
R
1
[I > 2s(I)]
[I > 2s(I)]
b
wR
2
1
R (all data)
wR
2
(all data)
P
P
P
P
a
b
2
2 2
2 2 1/2
R
1
¼
kF
0
| ꢀ |F
C
k/ |F
0
|. wR
2
¼
[w(F
0
ꢀ F
C
) ]/ [w(F
0
) ]
.
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a
ꢁ
Table 3 Cyanosilylation of aldehydes catalyzed by compounds 1–3
The optimal reaction temperature of the reaction is about 80 C.
ꢁ
At higher temperature (such as 100 C), crystals with poor
ꢁ
quality can be obtained; at lower temperature (such as 25 C),
3
+
3+
no crystals were observed. Furthermore, when Eu , Gd or
3
+
3+
3+
3+
Tb ions were used instead of La , Ce or Nd ions in
compounds 1–3 under the similar conditions, only yellow
precipitates were got. This phenomenon is attributed to the
lanthanide contraction effect, and has been observed in other
b
Compound
Aldehyde
Time (h)
Yield (%)
1
2
3
9
9
7
93.5
90.3
96.2
21
lanthanide–POM-based compounds.
Crystal structures of 1–3
3
3
9
7
78.6
92.8
Single-crystal X-ray crystallographic analyses reveal that all
compounds 1–3 are composed of [Mo V O ] polyoxoanion to
form a bi-supporting structure. The polyoxoanion [Mo V O ]
6
ꢀ
6
2 26
6ꢀ
6
2 26
was rst prepared and structurally determined as its pentapo-
19
tassium sodium salt by Nenner in 1985. The structure of
6
ꢀ
3
9
90.2
[Mo
V
O
26
]
anion consists of six [MoO
] and two [VO
] octa-
6
2
6
6
3
ꢀ
hedra connected by shared edges, with two [Mo
3
VO13]
subunits stacking together, which is isostructural with the
4
ꢀ
octamolybdate anion b-[Mo
8
O
26
]
(shown in Fig. S1†).
3
9
70.5
Isostructural compounds 1 and 2 crystallize in triclinic space
group P1 ꢀ. Herein compound 1 is described as an example in
detail. The asymmetric unit in the structure of 1 consists of one
3
6
7
96.6
16.8
6ꢀ
crystallographically independent [Mo
6
V
2
O
26
]
polyoxoanion,
two La units and eight free water molecules (Fig. 1a). In the
polyoxoanion, each [MoO ] octahedron has two terminal oxygen
atoms and each [VO ] octahedron has one terminal oxygen atom.
According to the manner of oxygen coordination, ve kinds of
3
+
Blank
6
6
K
5
NaMo
6
V
2
O26$4H
2
O
9
58.4
6ꢀ
oxygen atoms exist in the b-[Mo V O ] cluster, that is six
6
2 26
a
3+
0
Reaction conditions: catalyst 1 mol%, aldehyde 0.5 mmol, TMSCN 1.5 terminal oxygen Ot, eight terminal oxygen Ot linked to La , six
ꢁ
b
mmol, without solvent, room temperature (25 C) under N
were determined by GC analysis using naphthalene as an internal
standard.
2
.
Yields
double-bridging oxygen Ob, four central oxygen Oc (m
a V and two Mo atoms), two central oxygen Od (m -O atom of two
V and three Mo atoms). Thus the Mo–O bond lengths fall into
3
-O atom of
5
˚
0

ve classes: Mo–Ot 1.689(4)–1.707(4) A, Mo–Ot 1.719(4)–1.748(4)
˚
˚
˚
A, Mo–Ob 1.899(4)–2.206(4) A, Mo–Oc 1.963(4)–2.237(4) A, and
˚
Mo–Od 2.281(4)–2.343(4) A in 1. The two independent vanadium
Results and discussion
Synthesis
centers V(1) and V(2) possess distorted octahedral environ-
ments. The V–O distances vary from 1.625(4) to 2.227(4) A. The
The reaction of the b-[Mo V O ] polyoxometalate and three bond angles of O–V–Ocis range from 76.63 to 106.10 , and
˚
ꢁ
6
ꢀ
6
2 26
3
+
3+
3+
ꢁ
different trivalent lanthanide cations (Ln ¼ La , Ce , Nd ) has O–V–Otrans range from 151.86 to 170.29 . All bond lengths and
19
been investigated. Compounds 1–3 were synthesized under the bond angles are within the normal ranges.
6
ꢀ
same reaction conditions (i.e., the same Ln/[Mo
6
V
2
O
26
]
ratio
Different from compound 1, the asymmetric unit in the
(
2 : 1), same temperature, same concentration of starting poly- structure of 3 consists of half one crystallographically inde-
6
ꢀ
3+
oxometalate, and same pH) by the conventional solution pendent [Mo V O ] polyoxoanion, one Nd unit and four
6
2 26
method which possesses reasonably good solubility and are free water molecules (Fig. 1c). Also, ve kinds of oxygen atoms
environmentally friendly. Many parallel experiments indicate exist in the cluster according to the manner of oxygen coordi-
that stoichiometry, pH value and reaction temperature can nation. Then, the Mo–O bond lengths fall into ve classes:
˚
0
˚
affect the quality of crystals. The products are obviously affected Mo–Ot 1.680(1)–1.695(1) A, Mo–Ot 1.713(9)–1.723(9) A, Mo–Ob
3
+
6ꢀ
˚
˚
6 2
by the stoichiometry of Ln /[Mo V O ] . Compounds 1–3 can 1.887(9)–1.933(9) A, Mo–Oc 1.965(9)–1.969(1) A, and Mo–Od
26
3
+
6ꢀ
˚
be well isolated by the 2 : 1 Ln /[Mo
6
V
2
O
26
]
ratio. When the 2.280(1)–2.336(9) A in 3. The V–O distances are 1.634(1)–
ratio increased from 2 : 1 to 3 : 1, 4 : 1 or 2.211(9) A, and the O–V–O angles are in the range of 76.8(4)–
decreased to 1 : 1, the crystal quality of compounds 1–3 was 169.9(4) . All bond lengths and bond angles are similar to
poor, and the yields were reduced. The optimal pH range of the compound 1.
3
+
6ꢀ
˚
Ln /[Mo
6
V
2
O
26
]
ꢁ
reaction is from 2.7 to 3.2. For a low pH (pH < 2.5) no reaction
occurs; for a high pH (pH > 3.5) the hydrolysis of the lanthanide display a similar coordination sphere in 1 (Fig. S2†). La(1) cation,
There are two crystallization-independent La(III) cations
6ꢀ
cations makes a mass of precipitates rather than the crystals. coordinating to [Mo V O ] polyoxoanion, has a monocapped
6
2 26
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6
ꢀ
6 2 26
Fig. 1 (a) ORTEP drawing of 1 with thermal ellipsoids at 50% probability. (b) Polyhedral and ball-stick representation of one b-[Mo V O ]
2 6
polyoxoanion supported by two {La(H O) } units in 1. (c) ORTEP drawing of 3 with thermal ellipsoids at 50% probability. (d) Polyhedral and ball-
6ꢀ
stick representation of one b-[Mo
6
V O
2 26
]
2 6
polyoxoanion supported by two {Nd(H O) } units in 3 (color code: V, yellow; Mo, blue; La, green; Nd,
pink; O, red).
square antiprismatic coordination geometry, dened by four
6
ꢀ
terminal oxygen atoms from one [Mo
6 2 26
V O ]
unit [La–O
2.518(4)–
˚
2
2
.530(4)–2.623(4) A] and ve water molecules [La–OH
.580(4) A]. The nine-coordinate La(2) also links to four terminal
2
˚
6ꢀ
6 2 26
oxygen atoms from [Mo V O ]
molecule [La–O 2.535(4)–
˚
2
.656(4) A] and ve water molecules [La–OH 2.496(4)–2.593(4)
2
A˚ ] to complete a monocapped square antiprismatic coordination
˚
geometry. The average La–O bond length is 2.562 A, which is
comparable to the sum of La–O ionic radii for nine-coordinate
3
+
2ꢀ
22
La and two-coordinate O ions (2.56 A˚ ). In 3, one crystallo-
3
+
graphically unique Nd ion bonded to the polyoxoanion as
supporter, is coordinated by four oxygen atoms from the
6ꢀ
˚
6 2 26
[Mo V O ]
POM [Nd–O 2.534(9)–2.632(1) A] and ve water
˚
molecules [Nd–OH 2.510(1)–2.586(1) A] to form a monocapped
2
square antiprismatic coordination geometry (Fig. S3†). The
˚
average Nd–O bond length is 2.566 A, which is closed to the
2
1a
Nd–O bond distances in K0.5Nd0.5[Nd
2
(SiW11
O
39) (H
2
O)11
]
and
23
2 2 6 2 2 4
[NdK(H O)12][Nd(H O) ] [(H O) NdBW11
O39H] .
2
6ꢀ
Interestingly, in compounds 1 and 3 each [Mo V O ]
6 2 26
3
+
polyoxoanion is linked by two [Ln(H
2
O)
5
]
cations to construct
a similar bi-supporting structure (Fig. 1b and d). Then, theses
bi-supporting subunits are rstly joined up together by the
˚
strong hydrogen-bonding interactions (O26/O4W 2.802 A and
˚
O2/O6W 2.851 A in 1) between terminal oxygen atoms of
polyoxoanions and coordinated water molecules to form a 1D
supramolecular chain (shown in Fig. 2a). Then adjacent chains
are linked together by other hydrogen-bonding interactions
Fig. 2 (a) A view of the 1D supramolecular chain in 1, showing the
hydrogen bonds between the terminal oxygen atoms of poly-
oxoanions and coordinating water molecules with green color. (b) 2D
˚
˚
˚
(
O12/O2W 2.821 A, O24/O3W 2.765 A, O25/O10W 2.857 A
in 1) to generate an interesting 2D supramolecular wavelike supramolecular layer in 1, showing the hydrogen bonds between
network (see Fig. 2b). Furthermore, 2D supramolecular polyoxoanions and coordinated water molecules with green color. (c)
3
D supramolecular framework in 1, displaying the interbedded
layers are connected to yield a 3D supramolecular channel
framework via the strong hydrogen-bonding interactions
hydrogen bonds between polyoxoanions and coordinated water
molecules with black color (color code: V, yellow; Mo, blue; La, green;
O, red). Free water molecules are omitted for clarity.
˚
˚
˚
(
O18/O1W 2.761 A, O26/O4W 2.914 A and O11/O9W 2.829 A
in 1) (Fig. 2c). Free water molecules are lled in the cavities of
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3D supramolecular structures and participate in the extensive scattering coefficient. The band gap was determined as the
hydrogen-bonding interactions with the polyoxoanion frame- intersection point between the energy axis at a/S ¼ 0 and the
work. A better insight into the nature of such 3D supramolec- line extrapolated from the linear portion of the adsorption edge
ular framework can be achieved by reducing the structure to in a plot of a/S against energy E. The a/S versus E plots for
simple node and linker net. According to the simplication compounds 1–3 are shown in Fig. 3. For compounds 1–3, the
2 5 2 6 2
principle, each bi-supporting cluster [La(H O) ] Mo V O26 is
linked with eight such units by strong hydrogen bonds. There-
fore, the resulting 3D supramolelar frame is a rare uninodal 8-
connected net, as shown in Fig. S4.†
24
Bond valence sum calculations show that all Mo atoms are
in the +6 oxidation state, V atoms are in the +5 oxidation state,
and lanthanide sites are in the +3 oxidation state. These results
are consistent with the charge balance considerations.
FT-IR spectra
The IR spectra of compounds 1–3 display similar characteristic
6ꢀ
6 2 26
patterns of the [Mo V O ]
structure (see Fig. S5a–S5c†). The
characteristic bands of n(M]Ot) vibrations are observed at 947,
ꢀ
1
ꢀ1
ꢀ1
9
18 cm for 1, 948, 919 cm for 2 and 943, 918 cm for 3; the
characteristic bands of n(M–Ob–M) and n(M–Oe–M) are at 890,
ꢀ
1
ꢀ1
7
96, 647 cm for 1, 887, 797, 649 cm for 2 and 882, 797, 634
ꢀ
1
cm for 3 (M ¼ Mo or V; Oc – corner shared oxygen; and Oe –
10b,25
ꢀ1
edge shared oxygen).
Below 600 cm , the spectrum is rather
difficult to analyze, the bending of the M–Ot and M–Ob bonds
can be mixed with the M–Oe stretching vibrations. These char-
acteristic bands are nearly identical to those of the reported
octamolybdate compounds except for slight shis of some peaks.
ꢀ
1
ꢀ1
A broad band around 3377 cm of 1, 3375 cm of 2 and 3369
ꢀ
1
cm of 3 is attributed to the absorptions of water molecules.
TG analysis
Thermal gravimetric (TG) curve of 1 is shown in Fig. S6a.† The
TG curve possesses a two-step continuous weight loss process in
ꢁ
the range of 40–600 C and gives a total loss of 18.5%, which
arises from the loss of all lattice water molecules and coordi-
nated water molecules and agrees with the calculated value of
19.1%. The TG curves of 2 and 3 exhibit similar weight loss
stages to those of 1 (see Fig. S6b and S6c†), and give a total
ꢁ
weight loss of 18.4% and 18.9% in the range of 40–600 C,
which agree with the calculated value of 19.1% and 19.0%.
Powder X-ray diffraction characterization
The PXRD patterns for 1–3 are presented in Fig. S7.† The
diffraction peaks of both calculated and experimental patterns
match well, indicating the phase purities of these compounds.
These conclusions are in agreement with the results of the
single crystal X-ray analysis.
Optical band gap
To investigate the conductivity potentials of compounds 1–3,
the diffuse reectivity for solid samples of 1–3 were performed
to obtain the band gap (E ). The absorption (a/S) data were
g
calculated from the reectivity using the Kubelka–Munk func-
2
tion: a/S ¼ (1 ꢀ R) /2R where R is the reectivity at a given
Fig. 3 UV-vis diffuse reflectivity spectra of K–M functions vs. energy
wavelength, a is the absorption coefficient, and S is the (eV) of compounds 1–3.
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well-dened optical absorption associated with band gaps (E
g
)
77.5% for 1, 75.6% for 2 and 74.3% for 3 aer 9 h. In compar-
can be assessed at 2.87, 2.91 and 2.94 eV, respectively. The ison, the absorption peaks of RhB without any catalyst show
reectance spectra imply that compounds 1–3 are potential no obvious change (Fig. S9†). The results indicate that
semiconductors with wide band gaps. In addition, the optical compounds 1–3 show good photocatalytic activities for the
band gap of the precursor K
measured (Fig. S8†). The E value of the precursor is 3.17 eV,
which is higher than those of 1–3. Therefore, the optical band catalyst was used to photocatalytically decompose the RhB,
5 6 2 2
NaMo V O26$4H O has been degradation of RhB. Under the same condition, the precursor
g
K
5
NaMo 26$4H O of the same weight as homogeneous
6
V
2
O
2
6ꢀ
gaps of 1–3 are relevant to the POM b-[Mo
6 2 26
V O ]
and lantha- resulting in drop of the RhB concentrations by 53.4% aer 9 h
nide cations, revealing that the optical band gaps of POM-based (Fig. S10†), so the introduction of lanthanide cations can play
compounds could be tuned effectively via metal cations.
the role of improving the photocatalytic activity. Niu's group has
synthesized many new compounds based on lanthanides and
lacunary Keggin POMs, and studied their photocatalytic activi-
26
Photocatalytic activity
ties on degrading the RhB under UV irradiation. Their exper-
imental results indicate that the lanthanide cations may play an
important role in inhibiting the photodegradation of RhB.
Compounds 1–3 represent the rst examples that lanthanide-
ꢀ1 based POMs as photocatalysts enhance the degradation of
organic dye up to now, although their photocatalytic conver-
sions are moderate under UV irradiation. According to the
The evaluation of photocatalytic activities of three samples 1–3
for the photodegradation of rhodamine B (RhB) was performed
ꢁ
at ambient temperature (25 C). The procedure was as follows: a
ꢀ5
sample (10.0 mg) was mixed with 50 mL of 2 ꢂ 10 mol L
RhB solution, followed by magnetically stirring in the dark for
about 30 min. The solution was then exposed to UV irradiation
from a 300 W Hg lamp in the photocatalytic reaction instru-
ment, kept stirring during irradiations. A sample was taken out
every 1 h for analysis. The evolution of the spectral changes
taking place during the photodegradation of RhB over 1–3 is
shown in Fig. 4. It can be clearly seen that the absorbance peaks
of RhB are decreased obviously for 1–3 with increasing reaction
time. The calculations reveal that the conversions of RhB are
27
literatures, POM-based compounds have a similar electronic
attribution to that of semiconductors such as TiO . Both POM
have d transition metal atoms and oxygen atoms. UV
2
0
and TiO
2
irradiation of 1–3 resulted in the formation of an O / M
charge-transfer excited state (M ¼ Mo, V), producing consider-
able holes and electrons, which can be considered as a parallel
process to the promotion of an electron from the highest
Fig. 4 (a) Absorption spectra of the RhB solution during the decomposition reaction under UV irradiation in the presence of compound 1. (b)
Absorption spectra of the RhB solution during the decomposition reaction with catalyst 2. (c) Absorption spectra of the RhB solution during the
decomposition reaction with catalyst 3. (d) Photocatalytic decomposition rate of the RhB solution under UV irradiation with the use of K
NaMo 26$4H O, compounds 1–3 and only RhB.
5
-
6
2
V O
2
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occupied molecular orbital (HOMO) to the lowest unoccupied
28
molecular orbital (LUMO). The photoinduced holes can
oxidize RhB molecules in the solution. Thus, compounds 1–3
became a powerful oxidant by UV irradiation. Furthermore,
compounds 1–3 as heterogeneous catalysts can be easily iso-
lated from the reaction suspension by simple ltration
(Fig. S11†). To demonstrate their recyclability, successive reac-
tions were performed, showing that the activity of the recovered
catalyst does not decrease aer four cycles at least (Fig. S12†).
Cyanosilylation study
The catalytic properties of three compounds 1–3 as heteroge-
neous catalysts in the cyanosilylation reactions of aldehydes
have been investigated under solvent-free condition at room
temperature. Firstly, the cyanosilylation of benzaldehyde with
trimethylsilyl cyanide (TMSCN) acts as a model reaction to
investigate the catalytic nature of compounds 1–3. The reaction
was carried out with 1 mol% of catalyst under the conditions
described in Table 3. All three compounds effectively catalyzed
Fig. 5 Powder X-ray diffraction (PXRD) patterns of 3: calculated
pattern from crystal data (blue line); experimental pattern before
catalysis (red line); recovered catalyst 3 after 3 catalytic runs of the
the cyanosilylation to afford the corresponding cyanohydrin cyanosilylation of benzaldehyde (black line).
trimethylsilyl ether, and a small difference in activity was found
among them (see Fig. S13†). The yield of Nd-based compound 3
is 96.2%, followed by La-based compound 1 (93.5%) and Ce- shi of 10.4 cm from 1702.8 cm of the free benzaldehyde
based compound 2 (90.3%). In a comparison of the activity (Fig. S16†). This experiment unambiguously demonstrated the
ꢀ
1
ꢀ1
3
+
of our catalysts with that of some previously reported possible activation of the substrate by the Nd cation as Lewis
1
8
POM-based compounds ([Cu
2
(bpy)(H
2
O)5.5
]
2
[H
2
W
11
O
38], and acid site in 3. Furthermore, the control experiment for cyano-
NaMo 26$4H O precursor
16
TBA
8
H
2
[(SiYW10
O
36
)
2
]), it was found to be higher than the rst silylation of benzaldehyde with K
5
6
V
2
O
2
one (benzaldehyde, 24 h, and yield ¼ 98.1%), which uses solvent in a homogeneous manner gave 58.4% yield, which is far lower
conditions, and less than the second one, which is a homoge- than that of compound 3 in the heterogeneous manner. The
neous catalyst and uses solvent conditions (benzaldehyde, 15 higher yield with compound 3 is possibly attributed to the
3
+
min, and yield ¼ 94%).
synergistic effect of the Lewis acid (Nd cation) and the Lewis
6
ꢀ
Subsequently, the cyanosilylation reaction of various kinds base (surface oxygen atoms of the [Mo
V
O
26
]
POM) that
6
2
of aromatic and aliphatic aldehyde substrates in the presence of activate respectively aldehydes and TMSCN at the same time.
Nd-based catalyst 3 was further studied (Fig. S14†). The effect of The coexistence of activated coupling partners on the same
6
ꢀ
the nature of the aldehyde was evidenced that the activity of POM molecules in which [Mo
6
V
2
O
26
]
polyoxoanion activates
3
+
aliphatic aldehyde such as heptaldehyde is higher than the the nucleophile TMSCN and Nd cation is responsible for the
aromatic aldehydes like 2-hydroxybenzaldehyde, 4-uo- activation of the electrophile aldehydes, results in the effective
robenzaldehyde, or 4-methylbenzaldehyde (Table 3). When promotion of cyanosilylation, which is consistent with the
16,17
sterically more hindered 1-napthaldehyde is used as the research conclusion of Mizuno and co-workers.
The possible
substrate, the yield is even poor suggesting that the aldehyde mechanism for the cyanosilylation reaction in the case of
here is not accommodated as per the transition-state geometry compound 3 is shown in Scheme S1.†
requirement for driving the reaction to the product according to
15
the Lewis acid effect of lanthanides.
To probe the heterogeneity of the catalysts, aer the catalytic
reactions, the catalysts were recovered from their reaction In summary, three bi-supporting structures based on
Conclusions
6
ꢀ
media. Solid of 3 could be easily isolated from the reaction b-[Mo V O ]
anion and lanthanide cations, are reported,
6
2 26
suspension by a simple ltration and reused at least three times which represent the rst examples of inorganic species con-
without an appreciable loss of its high catalytic performance structed from molybdenum–vanadium POMs and lanthanides.
(from 96.2% to 93.5% yield see Table S1†). The PXRD patterns of These inorganic materials not only exhibit good photocatalytic
the retrieved catalyst 3 were identical with those of the fresh activities for the degradation of organic dye RhB under the
catalyst (shown in Fig. 5). The IR spectrum of the recovered irradiation of UV light, but also act as efficient heterogeneous
compound 3 was also identical to that of the freshly prepared Lewis acid–base catalysts for cyanosilylation of various aldehyde
sample (Fig. S15†). These observations suggested that 3 is a true compounds under solvent-free conditions. The successful
heterogeneous catalyst. In addition, infrared spectroscopy of isolation of these species certainly provokes researchers'
the catalyst 3 impregnated with benzaldehyde exhibited one interest to develop new heterogeneous catalytic materials con-
ꢀ1
broad C–O stretch at 1692.4 cm . The v(C–O) stretch had a red taining molybdenum–vanadium POMs and lanthanides.
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9
(a) C. M. Liu, D. Q. Zhang, M. Xiong and D. B. Zhu, Chem.
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Acknowledgements
The authors thank the National Natural Science Foundation of
China (21371027, 21271037, 20901013) and Fundamental
Research Funds for the Central Universities (DUT12LK01) for 10 (a) J. Q. Sha, J. Peng, H. S. Liu, J. Chen, A. X. Tian and

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P. P. Zhang, Inorg. Chem., 2007, 46, 11183; (b) F. Y. Li,
F. Z. Meng, L. F. Ma, L. Xu, Z. X. Sun and Q. Gao, Dalton
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