A novel dumbbell-like polyoxometalate assembled of copper(II)-disubstituted monovacant Keggin polyoxoanions with a tetranuclear copper cluster

Article abstract of DOI:10.1021/ic5000827

A dimeric Keggin polyoxometalate, [Cu(bpy)(μ₂-OH)] ₄[(H₂O)(bpy)₂HPW₁₁Cu ₂O₃₉]₂·2CH₃CH ₂OH·10H₂O (1), constructed from two dicopper(II)-substituted monovacant Keggin polyoxoanions bridged by a Cu ₄ cluster, has been hydrothermally synthesized. Magnetic analysis indicates predominantly an antiferromagnetic interaction between copper(II) centers. Compound 1 also shows very high catalytic activity for the esterification of phosphoric acid with equimolar lauryl alcohol to monoalkyl phosphate ester.

Full text of DOI:10.1021/ic5000827

Communication
pubs.acs.org/IC
A Novel Dumbbell-like Polyoxometalate Assembled of Copper(II)-
Disubstituted Monovacant Keggin Polyoxoanions with a
Tetranuclear Copper Cluster
Hao Miao, Xiao Xu, Wei-Wei Ju, Hong-Xiang Wan, Yu Zhang, Dun-Ru Zhu, and Yan Xu*
†
†
†
†
†
†
,†,‡
†College of Chemistry and Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing
University of Technology, Nanjing 210009, People’s Republic of China
‡
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, People’s
Republic of China
*
S Supporting Information
polyoxoanions remains mostly unexplored. To the best of our
knowledge, few studies on the ditransition-metal-substituted
monovacant Keggin polyoxoanions have been reported.
Recently, we began the research in this filed, and fortunately
we have successfully prepared a novel dumbbell-like transition-
metal coordination fragment derivative of the monovacant
K e g g i n P O M [ C u ( b p y ) ( μ - O H ) ] [ ( H O ) -
ABSTRACT: A dimeric Keggin polyoxometalate, [Cu-
(
bpy)(μ -OH)] [(H O)(bpy) HPW Cu O ] ·
2 4 2 2 11 2 39 2
CH CH OH·10H O (1), constructed from two
3 2 2
2
dicopper(II)-substituted monovacant Keggin polyoxoan-
ions bridged by a Cu cluster, has been hydrothermally
4
synthesized. Magnetic analysis indicates predominantly an
antiferromagnetic interaction between copper(II) centers.
Compound 1 also shows very high catalytic activity for the
esterification of phosphoric acid with equimolar lauryl
alcohol to monoalkyl phosphate ester.
2
4
2
(
bpy) HPW Cu O ] ·2C H OH·10H O (1; bpy = bipyridine)
2 11 2 39 2 2 5 2
through a conventional hydrothermal method. As far as we know,
compound 1 is the first dicopper(II)-substituted monovacant
Keggin polyoxoanion that bridged with a Cu cluster to construct
4
a dimeric Keggin polyoxoanion. Magnetic analysis indicates
2
+
predominantly an antiferromagnetic interaction between Cu
olyoxometalates (POMs), as a fascinating family of metal−
ions in 1. More interestingly, compound 1 shows very high
catalytic activity for the esterification of phosphoric acid with
equimolar lauryl alcohol to monoalkyl phosphate ester.
P
oxygen clusters, have been extensively researched because of
their unmatched structural versatility and potential applications
in fields as diverse as catalysis, magnetism, luminescence,
̅
Compound 1 crystallizes in the triclinic space group P1. The
1
materials science, molecular electronics, etc. The massive
fundamental building unit of 1 contains two dicopper(II)-
range of applications has led to many deliberate assembly tactics
for the design and synthesis of novel POM-based hybrids by
introducing transition-metal-coordinated complexes as linkers
and modifiers with consideration of the structure direction or
substituted monovacant Keggin polyoxoanions [(H O)-
2
2
−
(bpy) HPW Cu O ] , a Cu cluster, two ethyl alcohol
2
11
2
39
4
molecules, and 10 lattice water molecules (Figure S1 in the
Supporting Information, SI). In {HPW Cu O }, the central
11
2
39
2
charge compensation. As an advance in this field, many POM-
phosphorus atom is surrounded by four oxygen atoms (O4, O12,
O18, and O26). Average distance of the P−O bonds is in the
usual region of 1.528 Å, and the O−P−O bond angle is in the
range of 107.1(4)−111.5(4)°. The distances of the W−O bonds
based compounds with variable topologies and dimensionalities
3
have been prepared hydrothermally. As an important branch of
POMs, Keggin-type POMs, represent an important place at the
forefront of the POM field because of their unusual structural
can be divided into three groups: W−O
W−O , 1.771(8)−2.047(8) Å; W−O , 2.396(7)−2.509(7) Å,
which is in the range of reported monovacant Keggin
t
, 1.705(8)−1.748(7) Å;
4
diversity and electronic versatility. The system containing
b
a
transition-metal cations and monovacant Keggin-type polyox-
oanions has been paid more and more attention since
9
−11
12
polyoxoanions.
Bond-valence-sum calculations show that
7
−
monovacant Keggin species [PW O ] was reported by
all of the tungsten sites exhibit a 6+ oxidation state and all of the
copper ions have a 2+ oxidation state in the disubstituted
1
1
39
5
Kehrmann et al. in 1892. In 1995, Coronado et al. found the
{
HPW Cu O } units. In compound 1, it is interesting that
poly-Keggin compound based on the manganese-monosubsti-
11 2 39
6
PW11 is formed from the saturated Keggin cluster PW under
tuted Keggin unit {PW MnO }. Recently, Gutier
́
rez-Zorrilla
12
1
1
39
8
hydrothermal conditions and two copper(II) (Cu1 and Cu2)
et al. acquired and characterized a series of hybrids constructed
from monocopper(II)-substituted Keggin polyoxoanions by
means of monovacant Keggin precursors in acetic acid. We
cations chelated by the 2,2′-bpy ligands respectively physically
7
occupy the vacant position of PW to generate an anionic
11
2
−
[
[
(H O)(bpy) HPW Cu O ] unit. To our knowledge,
can discover that the condensation of POMs is a charge-driven
progress because all of these polymeric structures are based on
transition-metal-substituted POM units whereby a low-valence
2 2 11 2 39
2−
(H O)(bpy) HPW Cu O ] in 1 is the first dicopper(II)-
2
2
11
2
39
substituted monovacant Keggin polyoxoanion. Cu1 exhibits a
8
metal substitutes a high-valence early transition metal.
However, the progress of this research field is still laggardly,
and the system including transition-metal cations and lacunary
Received: January 13, 2014
Published: February 24, 2014
©
2014 American Chemical Society
2757
dx.doi.org/10.1021/ic5000827 | Inorg. Chem. 2014, 53, 2757−2759

Inorganic Chemistry
Communication
distorted square-planar geometry defined by two oxygen atoms
from one polyoxoanion [Cu−O = 1.901(7)−1.961(4) Å] and
b
two nitrogen atoms from one 2,2′-bpy ligand [Cu−N =
1
.957(10)−2.023(9) Å], while Cu2 displays a square-pyramidal
geometry in which the basic plane is the same as that of Cu1 and
the axial vertex is occupied by a oxygen atom from one water
molecule.
A remarkable structure feature of compound 1 is the presence
4
4
+
of the chair form [Cu(μ -OH)] unit comprising three four-
2
membered Cu O rings (Figure 1). Each copper ion is chelated
2
2
Figure 3. 3D supramolecular architecture built by the dimers. Hydrogen
atoms and lattice water and ethyl alcohol molecules are omitted for
clarity.
1
5,16
performance.
The luminescence studies of compound 1 and
the 2,2′-bpy ligand are explored in the solid state at room
temperature. The spectrum exhibits fluorescent emission bands
at ca. 466 nm upon excitation at ca. 357 nm, which are associated
with ligand-centered π*−π electronic transitions (Figure 4). The
F i g u r e 1 . D i m e r i c { [ C u ( b p y ) ( μ - O H ) ] [ ( H O ) -
2
4
2
4
+
(
bpy) HPW Cu O ] } unit and the chair form [Cu(μ -OH)] unit
2 11 2 39 2 2 4
in 1.
4+
by one 2,2′-bpy ligand to form [Cu(bpy)(μ -OH)] cations.
2
4
Two crystallographically independent copper(II) (Cu3 and
Cu4) cations are in the cluster: both Cu3 and Cu4 display a
square-pyramidal geometry, with the basic plane defined by two
oxygen atoms from two hydroxyl groups and two nitrogen atoms
from one 2,2′-bpy ligand, while the axial vertex of Cu3 and Cu4
are occupied by one oxygen atom from one hydroxyl group and a
Figure 4. Excitation and emission spectra of compound 1 in the solid
state at room temperature.
13
terminal oxygen atom from one polyoxoanion, respectively.
Interestingly, two dicopper(II)-substituted [(H O)-
2
2
−
(
[
bpy) HPW Cu O ] are bridged through the chair form
2 11 2 39
emission spectra of compound 1 red-shifted with the free 2,2′-
4
+
Cu(bpy)(μ -OH)]
cations to give a dimeric Keggin
2
4
bpy ligand display an emission at 408 nm (λ = 337 nm), which
ex
p o l y o x o a n i o n { [ C u ( b p y ) ( μ - O H ) ] [ ( H O ) -
2
4
2
may be assigned to the charge-transfer transitions between
14−16
(
bpy) HPW Cu O ] }, as shown in Figure 1. With all of the
2 11 2 39 2
ligands and metal-ion centers.
Compared with the free
bpy ligands and two water molecules omitted, the dimer can be
better appreciated as a dumbbell, as presented in Figure 2, in
organic ligands, the emission intensity decreases, which may be
assigned to the hydrogen-bonding interactions between the
guest water molecules and organic ligands.
On the basis of the superinteraction through the tetracopper-
II) nucleus in compound 1, which may potentially possess
(
magnetic features, the magnetic behavior of 1 has been
investigated at 500 Oe in the temperature range 2−300 K
−
3
(
Figure 5). The value of χ slowly increases from 9.53 × 10
M
3 3
−1
−2
−1
cm mol at 300 K to 3.18 × 10 cm mol at 25 K and then
3
−1
exponentially to the maximum of 0.32 cm mol at 2 K. The χ T
M
3
curve displays a continuous decrease upon cooling from 2.88 cm
−1
mol K at 300 K, which is a little lower than the calculated value
Figure 2. Dumbbell-like structure of 1.
which the overall length is 28.43(4) Å and the stick is 12.80(3) Å;
this ratio compares well with the scale of the real dumbbell. The
dimers are parallelly packed and interact with each other through
hydrogen bonds and π···π interactions, which leads to the
formation of a 3D supramolecular architecture (Figure 3).
Powder X-ray diffraction (XRD) measurements for 1 were
determined at room temperature (Figure S2 in the SI), and the
diffraction peak positions of the experimental XRD patterns of 1
are in agreement with that of simulated XRD patterns, which
indicate the phase purity of compound 1.
POM-based hybrids with copper(II) ions and conjugated
ligands are of interest owing to their excellent luminescence
Figure 5. Temperature dependence of χ T (blue) and χ (black)
(inset, χM ) for compound 1.
M
M
−1
2
758
dx.doi.org/10.1021/ic5000827 | Inorg. Chem. 2014, 53, 2757−2759

Inorganic Chemistry
Communication
3
−1
2+
of 2.99 cm mol K for the eight uncoupled Cu centers (S =
/2
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(
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(
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ASSOCIATED CONTENT
■
*
S
Supporting Information
X-ray crystallographic data in CIF format, experimental section,
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details, IR, TGA, XRD, and selected bond length tables. This
material is available free of charge via the Internet at http://pubs.
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AUTHOR INFORMATION
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Corresponding Author
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*
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The authors declare no competing financial interest.
(
2
(
4
ACKNOWLEDGMENTS
■
We thank the Natural Science Foundation of Jiangsu Province,
China (Grant BK2012823), and the Project of Priority Academic
Program Development of Jiangsu Higher Education Institutions.
2
759
dx.doi.org/10.1021/ic5000827 | Inorg. Chem. 2014, 53, 2757−2759