Synthesis, structure, and catalytic activity of Keggin-type polyoxometalate coordinated Cu(I): {[Cu(py)2]4[SiW12O 40]}, via hydrothermal decarboxylation

Article abstract of DOI:10.1080/00958972.2012.664818

A Keggin-type polyoxometalate [SiW₁₂O₄₀]^4- supported transition metal complex, {[Cu(py)₂]₄[SiW ₁₂O₄₀]} (py=pyridine), has been synthesized by hydrothermal decarboxylation and characterized by elemental analyses, IR spectra, thermal stability analyses, and singlecrystal X-ray diffraction. The compound is a 1-D chain containing an inorganic-organic backbone with alternating [SiW₁₂O₄₀]^4- clusters and [Cu(py)₂]⁺ along the c-axis, and with both three- and four-coordinate copper. The luminescence properties indicated that the emission peak is emission of pyridine, assigned to the Cu(I) to pyridine charge transfer. This air-stable Cu(I) complex can efficiently catalyze the O-arylation of both phenols and aryl halides.

Full text of DOI:10.1080/00958972.2012.664818

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Synthesis, structure, and
catalytic activity of Keggin-type
polyoxometalate coordinated
Cu(I): {[Cu(py) ] [SiW O ]}, via
2
4
12 40
hydrothermal decarboxylation
a
a
a
a
Rui Yang , Shu-Xia Liu , Qun Tang , Shu-Jun Li & Da-Dong
a
Liang
a
Key Laboratory of Polyoxometalates Science of Ministry of
Education, College of Chemistry, Northeast Normal University,
Changchun City, JiLin 130024, P.R. China
Version of record first published: 24 Feb 2012.
To cite this article: Rui Yang , Shu-Xia Liu , Qun Tang , Shu-Jun Li & Da-Dong Liang (2012):
Synthesis, structure, and catalytic activity of Keggin-type polyoxometalate coordinated Cu(I):
{
[Cu(py) ] [SiW O ]}, via hydrothermal decarboxylation, Journal of Coordination Chemistry, 65:5,
2 4 12 40
891-897
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Journal of Coordination Chemistry
Vol. 65, No. 5, 10 March 2012, 891–897
Synthesis, structure, and catalytic activity of Keggin-type
polyoxometalate coordinated Cu(I): {[Cu(py) ] [SiW O ]},
via hydrothermal decarboxylation
2
4
12 40
RUI YANG, SHU-XIA LIU*, QUN TANG, SHU-JUN LI
and DA-DONG LIANG
Key Laboratory of Polyoxometalates Science of Ministry of Education, College of
Chemistry, Northeast Normal University, Changchun City,
JiLin 130024, P.R. China
(
Received 13 September 2011; in final form 4 January 2012)
4
ꢀ
A
Keggin-type polyoxometalate [SiW12
]
O
40
]
supported transition metal complex,
{
[Cu(py)
2
4
[SiW12
O
40]} (py ¼ pyridine), has been synthesized by hydrothermal decarboxylation
and characterized by elemental analyses, IR spectra, thermal stability analyses, and single-
crystal X-ray diffraction. The compound is a 1-D chain containing an inorganic–organic
4
ꢀ
þ
backbone with alternating [SiW12
O
40
]
clusters and [Cu(py) ] along the c-axis, and with both
2
three- and four-coordinate copper. The luminescence properties indicated that the emission
peak is emission of pyridine, assigned to the Cu(I) to pyridine charge transfer. This air-stable
Cu(I) complex can efficiently catalyze the O-arylation of both phenols and aryl halides.
Keywords: Cu(I) compound; Polyoxometalates; Hydrothermal; Decarboxylation
1. Introduction
Polyoxometalates (POMs) have discrete structures of definite sizes and shapes, such as
the Keggin, Dawson, Lindqvist, and Waugh [1]. As a rich and diverse class of inorganic
cluster systems, POM ions are characterized by fascinating structural, electrochemical,
catalytic, magnetic, medicinal, and photophysical properties [2]. POMs with Keggin
structure continue to have an important place at the forefront of the POM field as a
consequence of their unusual electronic versatility and structural diversity. A series of
compounds made of Keggin POMs associated with transition metal coordination
complex cations have been described [3–5]. Transition metal complexes having only
three bonds or low coordination numbers are interesting because orbital energies, spin
states, and reaction pathways can be different from those in traditional complexes [6].
Coordination number three for the late elements of the first transition series, notably
1
0
those of Cu(I) with d electron configuration, is well-documented [7–9]. Copper ions
play important roles in biology [10] and catalysis [11], and great effort has been
*
Corresponding author. Email: liusx@nenu.edu.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.664818

8
92
R. Yang et al.
expended to uncover mechanistic information and, in particular, to identify possible
copper–oxygen intermediates [12].
Here a Keggin unit supported transition metal Cu(I) complex {[Cu(py) ] [SiW O ]}
2
4
12 40
(
py ¼ pyridine) has been obtained by hydrothermal decarboxylation reaction. The
4
backbone of 1 is a 1-D linear chain comprising alternating [SiW O ] clusters and
ꢀ
1
2
40
Cu(I) ions along the c-axis. The luminescence and catalytic activities of this active and
air-stable Cu(I) complex are described.
2
2
. Experimental
.1. Materials and measurement
All solvents and reagents are purchased from commercial sources. Elemental analyses
are performed on a Perkin-Elmer 2400 CHN elemental analyzer. Infrared (IR)
ꢀ
1
spectrum is recorded from 400 to 4000 cm on an Alpha Centaurt FT/IR spectro-
photometer using KBr pellet. Powder X-ray diffraction measurements are performed on
a Rigaku D/MAX-3 instrument with Cu-Kꢀ radiation in the angular range 2ꢁ ¼ 3–60ꢁ
at 293 K. Thermal stability analysis is performed on a Perkin-Elmer TGA7 instrument
ꢁ
ꢀ1
in flowing N2 with a heating rate of 10 C min . Photoluminescence spectra are
measured using an FLSP 920 Edinburgh instrument (Eng) with 450 W Xenon lamp
monochromated by double grating.
2
.2. Synthesis of 1
ꢀ
3
ꢀ3
A mixture of CuCl (0.268 g, 0.2 ꢂ 10 mol), isonicotinic acid (0.158 g, 0.2 ꢂ 10 mol),
2
H SiW O ꢃ nH O (0.2 g), and H O (10 mL) is stirred for 30 min at room temperature,
4
12 40
2
2
and the solution is adjusted to pH ¼ 5.0–6.0 with triethylamine. Then the mixture is
transferred and sealed in a 20 mL Teflon-lined stainless steel container. After being
ꢁ
ꢁ
ꢁ
heated to 180 C under autogenous pressure for 5 days, the bomb is cooled to 100 C at
ꢀ1
the rate of 5 C h and maintained for 10 h. After that the bomb is cooled to room
temperature naturally and crystals of 1 are obtained. Compound 1 is collected as
colorless leaf-shaped crystals with a yield of 45% (based on W). Elemental analysis for
C H Cu N O SiW (%): H, 1.05 (Calcd 1.06); C, 12.64 (12.67); N, 2.99 (2.95); Si,
4
0
40
4
8
42
12
0
.71 (0.74); Cu, 6.69 (6.71); W, 58.19 (58.16).
2
.3. X-ray crystallography
Crystal data for 1 are collected on a Rigaku R-AXIS RAPID IP diffractometer
˚
(
graphite monochromated Mo-Kꢀ radiation: ꢂ ¼ 0.71073 A) at 298 K. The linear
absorption coefficients, scattering factors for the atoms, and anomalous dispersion
corrections are taken from International Tables for X-ray Crystallography [13]. The
structure of 1 is solved by the direct method and refined by full-matrix least-squares on
F using the SHELXTL crystallographic software package [14]. All non-hydrogen
atoms are refined anisotropically. Hydrogen atoms for py are fixed in ideal positions.
The crystal data and structure refinement results are summarized in table 1.
2

Polyoxometalate coordinated Cu(I)
893
Table 1. Crystal data and structural refinement for 1.
Empirical formula
Formula weight
Crystal system
Space group
40 4 8
C H40Cu N O42SiW12
3793.17
Triclinic
P1ꢀ
˚
ꢁ
Unit cell dimensions (A,
)
a
b
c
12.123(5)
13.302(5)
13.366(5)
ꢀ
ꢃ
ꢄ
108.049(5)
115.446(5)
103.088(5)
1680.4(11), 1
3.748
21.795
1690
3.16–29.27
12,635
9184
3
˚
Volume (A ), Z
Calculated density (g cm
ꢀ3
)
Absorption coefficient (mm
ꢀ
1
)
F(000)
ꢁ
ꢁ
range for data collection ( )
Total data collected
Unique data
R
Goodness-of-fit on F
int
0.0455
0.930
0.0491
0.0996
2
a
R
wR
1
[I 4 2ꢅ(I)]
b
2
(all data)
P
P
¼ {P
P
a
R
b
|; wR
[w(F o2 ꢀ F ) ]/ [w(F ) ]}
2
2
2
2 1/2
1
¼
||F
o
| ꢀ |F
c
||/ |F
o
2
c
o
.
Figure 1. The XRPD patterns of 1: simulated (bottom) and experimental (top).
The experimental and simulated X-ray powder diffraction patterns (XRPD) are shown
in figure 1. The main diffraction peaks on experimental and simulated patterns match
well in position, indicating its phase purity.
3
3
. Results and discussion
.1. Synthesis
Mixture of CuCl , isonicotinic acid, and H SiW O ꢃ nH O in water under hydro-
2
4
12 40
2
thermal conditions gave crystals of 1 for X-ray diffraction. In the reaction pH,

8
94
R. Yang et al.
Figure 2. (a) The coordination environment of Cu(I) in 1; (b) polyhedral representation of the 1-D chain
running along the c-axis of 1.
temperature and reaction time are important. Compound 1 can be obtained in pH range
5
.0–6.0. All Cu’s in the resultant compound are reduced to univalent. We hypothesize
that reduction of Cu(II) may be due to triethylamine. This process is usual under
hydrothermal conditions. Similar reduction of Cu(II) to Cu(I) is observed in POMs
bridged by copper complexes [15–17]. Removal of a carboxyl group from a compound,
with hydrogen replacing it, can be performed by catalysis with Cu(I) [18, 19]. The
decarboxylation reaction of the isonicotinic acid to pyridine is interpreted as the
following equation. The space between the copper ions increases, because steric
hindrance is reduced by decarboxylation. Thus, having available large number
of exposed coppers will enhance catalytic activity, compared with similar
complexes [20, 21].
3.2. Structure description
Experimental details and single-crystal X-ray diffraction analysis data reveal that 1
4
consists of one [SiW O ] and four [Cu(py) ] (figure 2a). The backbone of 1 is a 1-D
ꢀ
þ
2
1
2
40
4
ꢀ
linear chain comprising alternating [SiW O ] clusters and Cu(I) along the c-axis
(
1
2
40
4
ꢀ
figure 2b). The heteropolyanion [SiW O ] possesses the classical ꢀ-Keggin-type
12 40
structure. Each Si(IV) is tetrahedrally coordinated and surrounded by four oxygen
0
0
atoms (the O /O , O /O with disorder) with Si–O bonds ranging from 1.592 to
1
1
11
20
20
˚ ˚
.663 A (mean 1.628 A). All tungsten atoms have {WO } octahedral environments. The
6
1
W–O bond distances are divided into three groups: W–Oa (center oxygen), 2.327–
˚
˚
.453 A; W–Ob/c (bridging oxygen), 1.778–2.063 A; W–Od (terminal oxygen), 1.647–
2
˚
.715 A. Each ꢀ-SiW O cluster as a four-tooth ligand covalently bonds to four
1
Cu(I)(py) complexes through O6 and O19 of W O (W2, W6). The bond lengths
1
2
40
2
4
22
˚
around the four Cu(I) ions are 2.611 A (Cu1–O19), 2.559 A (Cu2–O6), 1.906–1.916 A
˚
˚

Polyoxometalate coordinated Cu(I)
895
Figure 3. The excitation (left) and emission (right) spectra of 1.
˚
Cu1–N), 1.888–1.913 A (Cu2–N), while the N–Cu–N angles are 168.26–174.58 and
ꢁ
(
the N–Cu–O angles are 88.52–99.08 . One [SiW O ]
ꢁ
4ꢀ
connects the other
SiW O ] by Cu1 and O12 to form the 1-D chain structure. The distance of Cu
1
2
40
4
ꢀ
[
ions in adjacent chains is about 9 A.
1
2
40
˚
3
.3. FTIR spectroscopy and thermogravimetric analysis
IR spectra exhibit characteristic peaks for the ꢀ-Keggin structure at 947, 898, and
ꢀ1
ꢀ1
7
attributed to ꢆ (W–Od), ꢆ (Si–O), and ꢆ (W–Oc–W) bands, respectively. Peaks at
85 cm for 1 (Supplementary material). The signals at 956, 906, and 786 cm can be
as
as
as
ꢀ
1
800–1100 cm are characteristic of pyridine.
1
The thermal gravimetric curve of 1 is shown in ‘‘Supplementary material.’’ It shows a
ꢁ
ꢁ
ꢁ
total weight loss of 18.34% from 50 C to 600 C. Weight loss of 12.55% at 50–310 C
corresponds to loss of all non-coordinated and coordinated water molecules (Calcd
ꢁ
1
2%). The weight loss of 5.79% at 360–410 C arises from decomposition of organic
molecules (Calcd 18.33%). The bond valence sum calculations [22] indicate that all
tungsten atoms are in the highest oxidation state (þ6), while all coppers are in the þ1
oxidation state.
3
.4. Photoluminescence properties
The luminescent properties of 1 in the solid state at room temperature upon excitation
at 367 nm are depicted in figure 3. A broad intense emission at 460 nm can be found. In
order to understand the nature of such emission, the luminescence of free pyridine is
also measured, which shows an emission at 465 nm upon excitation at 280 nm
(Supplementary material). The emission peaks at 460 nm for 1 may be assigned to
metal-to-ligand charge transfer [23]. According to emission spectra of other Cu(I)-
compounds, the 460 nm peak observed in 1 should be attributed to pyridine emission,
which means that energy transfer to Cu(I)-based states competes with emission from

8
96
R. Yang et al.
Cu(I) ion [24]. Based on the emission spectra of 1, there is 5 nm blue-shift from the
emission of pyridine, and increased emission intensity in comparison with the emission
intensity of pyridine. There is about 5 nm blue-shift from the emission of pyridine in the
emission spectra of 1, and a stronger emission intensity than emission spectra of
pyridine.
3
.5. Catalysis active
The catalytic activity of 1 has been assessed in catalyzing the Ullmann diaryl ether
synthesis [25]. Benzene chloride (2 mmol), phenol (2.5 mmol), K PO4 (425 mg,
3
ꢁ
2
.0 mmol), 1 (1 mol %), and DMF (0.5 mL) are mixed. The system is stirred at 100 C
under an argon environment for 48 h, cooled to room temperature and then diluted
with dichloromethane (15 mL). The mixture is filtered and sediment washed with 15 mL
1
dichloromethane. ( H NMR (ppm): ꢇ 7.32–7.36, 7.10, 7.01. C NMR (ppm): ꢇ 157.1,
13
1
8
28.9, 123.3, 118.5. MS m/z 170(Mþ), 140, 77, 66, 51, 38. Anal. Calcd for C H O: C,
1
2
10
4.67; H, 5.91. Found: C, 84.57; H, 5.90). The catalytic activity of 1 is higher than CuBr
catalyst under the same conditions.
4. Conclusions
A POMs-based Cu(I) organic–inorganic hybrid compound {[Cu(py) ] [SiW O ]} has
2
4
12 40
been synthesized from hydrothermal decarboxylation. The Cu(II) is reduced to Cu(I) by
the reductant triethylamine. The luminescence of 1 indicates that the emission can be
assigned to Cu(I) to pyridine charge transfer. The 1-D chain complex efficiently
catalyzes the O-arylation of both phenols and aryl halides because of the presence of
active and air-stable Cu(I). Significantly, introducing of reductant and decarboxylation
reaction is crucial for the synthesis of 1, which provides a new route for synthesis of
organic–inorganic hybrid compounds.
Supplementary material
CCDC-843099 for 1 contains supplementary crystallographic data for this article. This
data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: t44-1223/336-033; Email: deposit@ccdc.cam.ac.uk).

Polyoxometalate coordinated Cu(I)
897
Acknowledgments
This work is supported by the National Nature Science Foundation of China (Grant
Nos 20973035 and 21171032) and Fundamental Research Funds for the Central
Universities (Grant No. 09ZDQD0015).
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