A supramolecular tetradecanuclear copper(II) polyoxotungstate

Article abstract of DOI:10.1002/anie.200351486

Four {SiW₉O₃₄} units enclose a 14 Cu^II cluster obtained through halide supramolecular assembly, leading to the polyanion containing the highest number of divalent metal cations (see picture; W blue, Si yellow, Br orange, Cu green, O red).

Full text of DOI:10.1002/anie.200351486

Angewandte
Chemie
Polyoxometalates
A Supramolecular Tetradecanuclear Copper(ii)
Polyoxotungstate
Pierre Mialane,* Anne Dolbecq, JØrôme Marrot,
Eric Rivi›re, and Francis SØcheresse
Polyoxometalates (POMs) can be perceived as discrete
fragments of extended metal oxide lattices.[1] They continue
to attract much attention with respect to their widespread
applications.[2] To date, the two most studied POM types have
been the so-called Keggin and Dawson–Wells families, which
can be formulated [XM12O40]nꢀ and [X2M18O62]nꢀ (X = PV,
SiIV; M = WVI,V, MoVI,V, VIV,V) respectively. These species are
diamagnetic when the metals M are in their highest oxidation
state. Mono, di and trivacant species of the Keggin type
polyoxotungstates can be easily prepared,[3] and then act as
nucleophilic ligands for transition-metal (TM) or rare-earth
cations. Such inclusions can then lead to the formation of
magnetic clusters. Trivacant POMs are the best candidates to
obtain compounds with the largest number of interacting
magnetic cations. Recently, compounds with a number (from
one to nine) of paramagnetic TM centers of the first row
ranging were reported.[4] Among the numerous complexes
reported to date, the most often encountered contain three or
four MII or MIII centers. Trimeric MII or MIII compounds
consist of either complexes in which the TM centers complete
the free sites of the POMs to form [XW9O37M3(H2O)3]nꢀ (X =
SiIV, M = CrIII, CoII, NiII, CuII),[5] or sandwich complexes of
general formula [(XW9Om)2M3(H2O)3]nꢀ (m = 37, X = PV,
M = MnII, FeII, NiII, CuII, ZnII; m = 33, X = AsIII, M = MnII,
NiII, CuII).[6] Similarly, complexes containing four paramag-
netic TM centers have been characterized: the [H2PW9O34-
Ni4(OH)3(H2O)6]2ꢀ POM, in which a nickel cubane caps the
{PW9O34}
entity,[7]
and
the
sandwich complexes
[(XW9O34)2M4(H2O)2]nꢀ (X = PV, M = MnII,III, FeII,III, CoII,
NiII, CuII, ZnII; X = SiIV, M = CrIII, MnII, CuII, ZnII).[8] In
1995, the hexameric CrIII complex [{SiW9O34Cr3-
(OH)3}2(OH)3]11ꢀ was isolated,[9] and finally, Clemente-Juan
et al. reported in 1999 the nonanuclear NiII complex
[(PW9O34Ni3)3(H2O)6(HPO4)2(OH)3]16ꢀ [10]
An increase in
.
the number of magnetic centers encapsulated in diamagnetic
polyoxometalate matrices remains a challenge. We report
herein the characterization of tetradecanuclear CuII com-
[*] Dr. P. Mialane, Dr. A. Dolbecq, Dr. J. Marrot, Prof. Dr. F. SØcheresse
Institut Lavoisier, IREM, UMR 8637
UniversitØ de Versailles Saint-Quentin
45 Avenue des Etats-Unis, 78035 Versailles (France)
Fax : (+33)1-39-25-43-81
E-mail: mialane@chimie.uvsq.fr
Dr. E. Rivi›re
Laboratoire de Chimie Inorganique, URA CNRS 420
Institut de Chimie MolØculaire d’Orsay
UniversitØ Paris-Sud
91405 Orsay (France)
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3523 –3526
DOI: 10.1002/anie.200351486
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3523

Communications
pounds, constructed through supramolecular assembly:
Na19K4[(SiW9O34)(SiW9O33(OH))(Cu(OH))6Cu]2X·nH2O
(X = Cl, n = 160 2Cl ; X = Cl, n = 90 2Cldhyd ; X = Br, n = 160
2Br ; X = Br, n = 90 2Brdhyd; 2Cldhyd and 2Brhdyd are the
dehydrated forms of 2Cl and 2Br, respectively).
These compounds represent, to the best of our knowledge,
the molecular POMs with the greatest number of di or
trivalent TM ions of the first row.
Compounds 2Cl and 2Br were synthesized in good yield
starting from the same crude precursor 1. Compound 1 is an
azido complex of general formula H5K6[(SiW9O37)-
Cu3N3]·19H2O obtained by mixing in water at room temper-
ature stoichiometric amounts of [A,a-SiW9O34]10ꢀ, CuCl2, and
NaN3. The addition of an excess of azide has no influence on
the composition of the final product. The IR spectrum of the
product shows the presence of two single bands relative to the
n˜as(N vibration at 2082 and 1282 cmꢀ1, thus indicating that
¼
N)
the azido group acts as an end-on bridging ligand.[11] Huge
efforts were made to obtain single crystals of this compound
suitable for X-Ray diffraction without success. A solution of 1
dissolved in molar NaCl or NaBr aqueous media turns from
yellowto green upon heating, and crystals of 2Cl or 2Br,
respectively, appear after cooling to room temperature within
minutes. This phenomenon was not observed when the
precursor was dissolved in a 1m NaF solution, and reduction
followed by the decomposition of the POM occurs when a
NaI solution was used. Similarly, the dissolution in nonhalide
sodium salt solutions (Na2SO4, Na2SO3, NaNO3) did not lead
to the formation of the expected CuII complex, and compound
1 was systematically recovered. Even if 2Cl or 2Br do not
contain any azido ligand, its presence during the synthetic
process seems crucial since it has not been possible to obtain
the title compound in absence of this reactant. The role of this
bridging ligand remains unclear. Sodium azide may act as a
base, as a preassembling agent, or may simply favor the
crystallization process.
Figure 1. Polyhedral representation showing a) the four building blocks
constituting the tetradecanuclear CuII cluster b) complex [{(SiW9O34)-
{SiW9O33(OH)}(Cu(OH))6Cu}2X]23ꢀ (X=Cl in 2Cl; X=Br in 2Br,
depicted). Bounds between CuII centers and oxygen atoms from
another {SiW9O34} subunit are shown as dashed lines.
Single-crystal X-ray diffraction analyses were performed
on compounds 2Cl and 2Br.[12] The protonated oxo bridges
were localized by valence bound summations.[13] Complex 2Cl
is formed from two [(SiW9O34)(Cu(OH))3]7ꢀ subunits
(denoted {Cu3}), and two [(SiW9O33(OH))(Cu(OH))3Cu]4ꢀ
subunits (denoted {Cu4}; Figure 1). Each subunit contains a
CuII center in a square pyramidal environment, the remaining
CuII centers are in a highly distorted octahedral geometry.
Within the tetradecanuclear CuII subunit, the four divalent
metals capping the [SiW9O34]10ꢀ entity form a distorted
trigonal pyramid. Connections between the subunits inside
the tetradecanuclear CuII complex occur a) between one CuII
cation of each subunit and an oxygen atom of an adjacent
subunit (Figure 1a), and b) through bridging OH ligands
shared by the copper centers (Figure 2). Finally, a Clꢀ anion is
locked in the center of the cluster, surrounded by six CuII
ꢀ
Figure 2. Ball-and-stick structural representation of the {Cu14} cluster.
Cu=Gray crosshatched circles, O=white circles, X=black circles,
(X=Cl in 2Cl; X=Br in 2Br). For clarity, only oxygen atoms linking the
CuII ions are represented. Edges of the {Cu3} and {Cu4} sub-units are
shown as bold lines.
cations with long Cu Cl distances (dCu Cl = 2.834(11)–
connecting modes inside the yttrium, niobium and copper
ꢀ
3.003(11) Š). Compound 2Cl approaches C2 symmetry. The
C2 pseudo-axis passes through the chloride anion and is
associated with the plane defined by the four CuII centers of
two {Cu4} units linked to the central halide. Assembly of
trivacant POMs through YIII[14] or Nbv[15] centers leading to
diamagnetic tetrameric species were recently reported. The
tetramers are radically different. In the first case,
a
[Y8W7O30]6+ cluster connects the subunits, while a Nb4O6
8ꢀ
unit ensures the connection in the second case. In 2Cl, the
formation of the high nuclearity cluster is driven by supra-
molecular assembly around a templating halide. The structure
of 2Br is similar to that of 2Cl, the Brꢀ replacing the Clꢀ ion,
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Angewandte
Chemie
with, as expected, longer metal–halide distances (dCu
2.918(15)–3.071(9) Š).
=
5.79 Kcm3 molꢀ1 for fourteen noninteracting S = 1= centers
assuming g = 2.1; S = spin angular momentum, g = Lande
ꢀ
Br
2
The well defined experimental X-ray diffraction powder
pattern performed in air on crystals of 2Cl is not similar to the
simulated powder pattern calculated from the data obtained
by single-crystal analysis performed on the same compound in
a Lindeman tube. Complexes 2Cl and 2Br are highly
hydrated; exposure to air of these compounds lead to a
spontaneous loss of water molecules of crystallization. Single-
crystal X-ray diffraction analysis was performed on a
dehydrated crystal of 2Cl (i.e. 2Cldhyd). Surprisingly, despite
the poor quality of the crystal, it has been possible to refine
factor) clearly indicates that antiferromagnetic interactions are
predominant in 2Brdhyd. The magnetization study at 2 K
showed that M tends to a value of 3 MB at high field (M =
magnetization). As a half-spin ground state is excluded, this
indicates that excited states are populated at 2 K. In the
absence of data at very lowtemperature, the strong decrease
observed under 4 K (Figure 4, inset) cannot be interpreted
unambiguously. It can be attributed to the presence of a singlet
ground state, but also to a triplet, the decrease being then a
consequence of the zero-field splitting effect on the S = 1 state.
In conclusion, tetradecanuclear CuII polyoxometalates
the diffraction data to obtain an acceptable structural model
[12]
ꢀ
for 2Cldhyd
,
thus showing that dehydration lead to a large
have been synthesized and characterized. The role of the N3
compression along the a axis of the lattice (Figure 3).[16]
Simulated and experimental powder patterns of 2Cldhyd are
in good agreement. Even in the dehydrated form, the
magnetic clusters are well isolated from each other in
ligand during the synthetic process is not clearly understood
but seems crucial. Structural characterization of azido–POM
complexes must be performed. The recrystallization of the
POM in high halide concentration allowed the formation of a
supramolecular assembly. Now, it has to be checked whether
the synthetic strategy described herein can be extended to
other divalent metals and/or trivacant POMs systems. This
work is in progress.
ꢀ
2Cldhyd, with the shortest Cu Cu distances of 7.439 Š
(dCu-Cu = 9.119 Š in 2Cl).
Experimental Section
1: A sample of Na10[A,a-SiW9O34]·20H2O[3] (2.8 g, 1 mmol) was
dissolved at room temperature in stirring water (40 mL) by adding
solid CuCl2·2H2O (0.5 g, 3 mmol). After 5 minutes, NaN3 (0.2 g,
3 mmol) in water (20 mL) was added dropwise to the reaction
mixture, and the green solution turned instantaneously to yellow.
Solid potassium chloride (1.5 g, 20 mmol) was then added. After 2 h,
the resulting yellow precipitate was filtered, washed with a KCl
solution (20 mL, 1m), dried with ethanol and diethyl ether. Yield:
2.4 g (80%, based on Cu). Elemental analysis calcd (%) for
H5K6[(SiW9O37)Cu3N3]·19H2O: W 54.41, Cu 6.27, K 7.71, N 1.38;
found: W 55.11, Cu 6.11, K 7.56, N 1.58, Na 0.08. IR (KBr pellet):
n˜ = 2082(s), 1282(w), 1001(m), 940(s), 905(s), 887(s), 783(s), 682(s),
548(m), 527(m), 364(m), 328(m)cmꢀ1. UV/Vis (H2O): lmax(e) = 747
(90).
2Cl and 2Br: Compound 1 (0.3 g) was dissolved in a hot (808C)
aqueous solution of NaCl (1m, 5 mL). The green solution was cooled
down to room temperature, and after one hour, large and long green
needles of 2Cl were obtained. Isolation of the product by filtration led
to the dehydrated complex 2Cldhyd. Yield: 190 mg, 55% based on Cu.
IR (KBr pellet): n˜ = 1006(m), 943(s), 910(s), 894(s), 774(s), 735(s),
670(s), 525(m), 357(m) cmꢀ1. UV/Vis (NaCl 1m): lmax(e) = 757 (310).
Elemental analysis calcd (%) for H194ClCu14W36O238Si4Na19K4:
W 54.09, Cu 7.27, Na 3.76, K 0.96, Cl 0.29; found: W 53.98,
Cu 7.32, Na 4.08, K 1.26, Cl 0.55.
Figure 3. Polyhedral representation of 2Cl and 2Cldhyd showing the con-
traction of the a axis when dehydration occurs.
The magnetic behavior of a microcrystalline sample of
compound 2Brdhyd has been studied in the range of 2 to 300 K,
and the result is shown in Figure 4 under the form cMT versus
T (cM = molar magnetic susceptibility). Due to the topological
complexity of the {Cu14} cluster, it is not possible to fit the
magnetic behavior of 2Brdhyd with a current theory. Indeed, by
using reasonable approximations, up to nine exchange
parameters would be required.[17] Nevertheless, the continu-
ous decrease of the cMT= f(T) curve and the cMT value at
300 K (cMT= 4.33 cm3 molꢀ1 K for a calculated cMT value of
2Br and 2Brdhyd were obtained following the procedure
described for 2Cl and 2Cldhyd in the same yield, but dissolving 1
in a 1m NaBr solution instead of a 1m NaCl solution. Infrared
and UV/Vis spectra are similar to that found for compound
2Cldhyd. Elemental analysis calcd (%) for H194BrCu14W36O238
-
Si4Na19K4: W 53.83, Cu 7.24, Na 3.55, K 1.27, Br 0.65; found:
W 53.53, Cu 7.26, Na 3.78, K 1.57, Br 1.06.
X-ray powder diffraction data were collected in air on a
Siemens D5000 diffractometer (Cu Ka radiation).
TGA was carried out on compounds 2Cldhyd and 2Brdhyd
under nitrogen flow(60 mLmin ꢀ1) with a Perkin-Elmer electro-
balance TGA-7 at a heating rate of 18Cminꢀ1 up to 6008C,
confirming the degree of hydration of these compounds.
Magnetic susceptibility measurements were carried out
with a Quantum Design SQUID Magnetometer with an applied
Figure 4. Thermal dependence of cMT for 2Brdhyd a) in the 2–300 K range,
(B=10 kG) b) in the 2–40 K range (B=1 kG) (inset).
Angew. Chem. Int. Ed. 2003, 42, 3523 –3526
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field of 1000 and 10000 G. The independence of the susceptibility
parameters, R1 = 0.0649, wR2 = 0.1443. Crystal data and struc-
ture refinements for 2Cldhyd: a green large needle crystal (0.440 ”
0.08 ” 0.04 mm) was analyzed. Monoclinic, space group C2/c, a =
52.5809(10), b = 18.2519(2), c = 39.89370(10) Š, b = 110.35(1)8,
value with respect to the applied field was checked at room
temperature. The susceptibility data were corrected from the
diamagnetic contributions of 2Brdhyd as deduced by using Pascal's
constant tables. The M = f(B) curve at 2 K is given as supplementary
material.
V= 35897.6(8) Š3, Z = 8, 1calc = 3.805 gcmꢀ3
,
m(MoKa) =
24.748 mmꢀ1, F(000) = 35612, 80012 reflections measured, of
which 25916 were independent, 1135 refined parameters, R =
0.1816, wR2 = 0.4297. For 2Cl, 2Br and 2Cldhyd, data reduction
was performed with the SAINT software. The absorption
correction was based on multiple and symmetry-equivalent
reflections in the data set by using the SADABS program based
on the method of Blessing. The structures were solved by direct
methods and refined by full-matrix least-squares by using the
SHELX-TL package. Crystallographic data for the structure
reported in this paper have been deposited at the Fachinforma-
tionszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Ger-
many (fax: (+ 49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.
de), on quoting the depository number numbers CSD-413064
(2Cl) and -413063 (2Br).
Received: March 26, 2003 [Z51486]
Keywords: copper · magnetism · polyanions ·
.
solid-state structures · tungsten
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volume between 2Cl and 2Cldhyd
.
[17] Considering the overall pseudo C2 symmetry of the magnetic
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[12] Crystal data and structure refinements for 2Cl: a large green
needle (0.35 ” 0.15 ” 0.07 mm) was analyzed with a Siemens
SMART three-circle diffractometer equipped with a CCD
bidimensional detector with MoKa monochromatized radiation
(l = 0.71073 Š). Monoclinic, space group C2/c, a =
63.5191(16) Š, b = 18.5238(5) Š, c = 41.1156(10) Š, b =
107.395(1)8, V= 46165.2(2) Š3, Z = 8, 1calc = 3.273 gcmꢀ3
,
m(MoKa) = 19.353 mmꢀ1
,
F(000) = 39920, 100177 reflections
measured, of which 32701 were independent, 2343 refined
parameters, R1 = 0.0772, wR2 = 0.1863. Crystal data and struc-
ture refinements for 2Br: a green large needle crystal (0.25 ”
0.08 ” 0.06 mm) was analysed. Monoclinic, space group C2/c, a =
63.4405(9) Š,
107.514(1)8, V= 46288.6(16) Š3, Z = 8, 1calc = 3.270 gcmꢀ3
m(MoKa) = 19.462 mmꢀ1
F(000) = 39968, 103640 reflections
b = 18.4975(4) Š,
c = 41.3627(9) Š,
b =
,
,
measured, of which 33148 were independent, 2379 refined
3526
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Angew. Chem. Int. Ed. 2003, 42, 3523 –3526