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A Journal of
Accepted Article
Title: A {As4Cu4[Cu(H2O)]12} Cluster Incorporated within Four
[Nb7O22]9− Units
Authors: Jingyang Niu, Li Li, Kaili Dong, Pengtao Ma, Chao Zhang, and
Jingping Wang
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201704071
Link to VoR: http://dx.doi.org/10.1002/chem.201704071
Supported by
10.1002/chem.201704071
Chemistry - A European Journal
COMMUNICATION
A {As4Cu4[Cu(H2O)]12} Cluster Incorporated within Four [Nb7O22]9−
Units
Abstract: A gigantic 16-copper-containing 28-niobate cluster,
Na14K7H5{As4Cu4[Cu(H2O)]12Nb28O109}∙37.5H2O (1), has been
prepared by a one-pot reaction, which is the first example of arsenic
copper clusters-containing polyoxoniobate and the first tetrahedral
arrangement of both the Cu cluster and the [Nb7O22]9− subunits.
Polyoxometalates (POMs) are anionic molecular metal
oxide clusters with a vast range of potential applications
including catalysis, magnetism, material science and
medicine.1-4 Polyoxoniobates (PONbs), as a burgeoning
subclass, have attracted increasing interest because of
their unique characteristics (high charge/surface ratio and
basicity) and potential applications in nuclear-waste
treatment and virology.5-8 Due to rich surface oxygen atoms,
PONbs can act as excellent inorganic multidentate O-donor
ligands.9 They can bind with most of the transition metal
(TM) cations, leading to a family of compounds with a huge
diversity of structures. Their morphology and electronic,
electrochemical and acid–basic properties can be finely
tuned, making them useful in various fields ranging from
catalysis to magnetochemistry. To date, some polynuclear
3d TM clusters with unique structure have been
synthesized by employing PONbs as ligands. In 2008,
Casy’s group reported a titanoniobate ion [Ti12Nb6O44]10−
which is a giant octahedral shaped ion with Ti in all the sites
and Nb at the vertexes of the octahedron.10 In 2015, a
octavanadium-substituted PONb [Nb48V8(OH)30 O130]18− was
found by Su et al.11 In recent years, our group reported a
series
of
TM-substituted
PONbs
[Co14(OH)16(H2O)8Nb36O106]20−,12
[H2Co8(Nb6O19)4]18−,13
[Cu24(Nb7O22)8H23NaO8]16−, [H9Cu25.5O8(Nb7O22)8]28−,14 and
[H4Ni10(H2O)8Nb32O102]20−,15 which are constructed by
[Nb6O19]8− or [Nb7O22]9− units and TM clusters.
Combining different 3d TM cations and non-metal
heteroatoms to the typical PONbs will not only increase the
variety of PONb chemistry, but also can supply new approaches
for designing and synthesizing of functional materials.
Nevertheless, none of the framework contains both polynuclear
TM clusters and non-metal heteroatoms, except for several
vanadium-containing PONbs. The main limitation in the
development of PONb chemsitry lies with the great sensitivity
between synthetic conditions and the overall cluster framework
[a]
L. Li, K. Dong, Dr. P. Ma, Dr. C. Zhang, Prof. J. Niu, Prof. J. Wang
Key Laboratory of Polyoxometalate Chemistry of Henan Province
Institute of Molecular and Crystal Engineering
College of Chemistry and Chemical Engineering
Henan University, Kaifeng, Henan 475004 (P. R. China)
Fax: (+ 86)371-238-868-76
E-mail: jyniu@henu.edu.cn
jpwang@henu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
thus formed. For instance, many possible PONb fragments are
present in reaction systems, where many factors such as subtle
changes in pH, ionic strength and cation size and type have a
significant effect on the isolation of products. Commonly, the
construction based on PONbs incorporating high-nuclearity TM
clusters demands the participation N-donor ligands, which not
only play the roles of stabilizing and bridging metal ions, but also
act as structure-directing agent to induce the formation of
aggregates with diverse nuclearities and structural features. At
the present, we focus on the reaction system of [Nb6O19]8−,
As2O3, copper salts with 4,4′-bipy ligand under conventional
aqueous methods because of the fewer research of
arsenicniobate chemistry.16,19,21
Herein,
we
report
the
synthesis,
structure,
characterization and magnetic study of the unprecedented
16-copper(II)-containing PONb cluster, Na14K7H5{As4Cu4
[Cu(H2O)]12Nb28O109}∙37.5H2O (1), which is the first
example of encapsulating both copper cluster and arsenic
atoms in PONb framework. Compound 1 was synthesized
in a simple one-pot reaction of Cu(OAc)2·H2O and As2O3
with K7HNb6O19·13H2O in aqueous solution at pH value of
12.3–12.5 and crystallized as a hydrated sodium–potassium
salt (scheme 1). The current synthetic method
demonstrates that As2O3, 4,4′-bipy ligand and pH value can
dramatically affect the isolation of final product. As2O3 was
added in the reaction system to isolate the crystal, however,
no crystal of 1 was found when As2O3 was replaced by
NaAsO2 and Na3AsO4. Notably, although 4,4′-bipy was used
as a staring material in the system, no 4,4′-bipy ligand was
observed in the compound. In view of that, we removed it from
the preparation recipe, the experimental result showed that
compound 1 was not obtainable. We also tried using various
kinds of N-containing organic ligands as substitutes for 4,4′-bipy
in the synthesis of compound 1, however, no crystal of 1 was
obtained. 4,4′-bipy may play a role of protecting copper atoms
against being precipitated as hydroxides in alkaline mediums in
the reaction and the similar phenomenon has been previously
reported.22-25 Besides the factor, pH value can also affect for the
isolation of crystals. For a lower pH (pH < 12.3), {Cu24Nb56}
cluster was formed;14 for a higher pH (pH > 13), only precipitate
was formed. Hence, 4,4′-bipy ligand and pH value play crucial
roles in the synthesis of the target compound.
Single crystal X-ray diffraction analysis indicates that
compound 1 crystallizes in the monoclinic symmetry, P21/n
space
group,
and
possesses
the
cluster
of
{As4Cu4[Cu(H2O)]12Nb28O109} (1a) along with 14 Na+ cations, 7
K+ ions based on elemental analysis, 37.5 lattice water
molecules and 5 protons for the charge balance. 1a contains a
core of {As4Cu4[Cu(H2O)]12} cluster, which is arranged in four
[Nb7O22]9− clusters (Fig. 1a). The {As4Cu4[Cu(H2O)]12} core
(abbreviated to {As4Cu16}) can be viewed as the fusion of four
{AsCu[Cu(H2O)]3} (abbreviated to {AsCu4}) moieties by sharing
one central μ4-O (O7) oxygen atom (scheme 2). In each {AsCu4}
unit, four copper atoms are linked together by a μ4-O atom and
three Cu–O–Cu bridges forming a {Cu4} cluster, at the same
This article is protected by copyright. All rights reserved.
Accepted Manuscript
Li Li, Kaili Dong, Pengtao Ma, Chao Zhang, Jingyang Niu* and Jingping Wang*
10.1002/chem.201704071
Chemistry - A European Journal
Fig. 1 (a) polyhedral/ball-and-stick view of 1a; (b) ball-and-stick view of the
linking model of arsenic atoms and PONbs. All hydrogens, potassium ions,
sodium ions, and lattice water molecules are omitted for clarity. Colour code:
NbO6 (red), Cu (yellow), As (green), and O (red) respectively.
Scheme 1 The schematic synthetic process of compound 1.
time, arsenic atom is grafted on the {Cu4} cluster through three
As–O–Cu bridges forming the {AsCu4} unit. 16 Cu2+ ions in
{As4Cu16} are coordinated by five oxygen atoms in tetragonal
pyramidal geometry with Cu–O distances in the range
1.899(12)–2.110(14) Å and O–Cu–O angles in the range
80.0(5)–179.3(5)°. Four of them (Cu1, Cu2, Cu5 and Cu9) are
coordinated by two oxygen atoms from the {AsCu4} unit and
three oxygen atoms from three different [Nb7O22]9− units,
whereas the other twelve copper atoms are ligated by two
oxygen atoms from the {AsCu4} unit, two oxygen atoms from the
same [Nb7O22]9− unit, and terminal water molecule. The
presence of the four capping arsenic groups is crucial for the
formation of {As4Cu16} and they adopt four-coordinated
tetrahedron geometry, which is completed by one terminal
oxygen and three μ2-oxygen atoms from the {AsCu4} unit.
In addition, the fundamental building block [Nb 7O22]9− is a
derivative of the Lindqvist-type hexaniobate anion
[Nb6O19]8− (Fig. S3), the three adjacent µ2-oxygen atoms of
which are combined with the seventh Nb atom, Nb(7). More
interestingly, the Nb(7)O6 octahedron in the [Nb7O22]9−
fragment has three terminal oxygen atoms with higher
coordination ability than that of the other six NbO 6 octahedra,
which provides an inevitable condition for the structural
architectures of compound 1.14,26,27 Moreover, if each [Nb7O22]9−
unit is regarded as a node, four [Nb7O22]9− units are located in
the four vertices of a tetrahedron, the As atoms reside outside
the tetrahedron to present another tetrahedron (Fig. 1b). On the
other hand, every [Nb7O22]9− subunit donates two terminal
oxygen atoms to form a {AsCu4Nb7} unit, one oxygen atoms
from the seventh NbO6 octahedron and another one from the
neighboring NbO6 octahedron, then four {AsCu4Nb7} units share
the O7 and other oxygen atoms forming the polyanion 1. From
the above-mentioned structural analysis, the three terminal
oxygen atoms from the [Nb7O22]9− subunit and the presence of
four arsenic atoms play an important role in the formation and
architecture of the 1a cluster.
Noteworthily, 1a resembles structurally the {Cu24Nb56} cluster
previously reported by our group.14 In the latter case, the
structure is composed of eight [Nb7O22]9− blocks and a {Cu24}
cluster. Both 1a and {Cu24Nb56} contain copper cores and
Scheme 2 Representation of the assembly of polyanion 1a. Colour code: NbO6 (red), Cu (yellow), As (green), and O (red) respectively. All hydrogens, potassium
ions, sodium ions, and lattice water molecules are omitted for clarity.
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/chem.201704071
Chemistry - A European Journal
building block [Nb7O22]9−, with the difference that the former
contains both Cu2+ and As5+ ions and the latter contains only
Cu2+ ions (Fig. S4). In addition, the types of coordination
geometry for Cu2+ ions are markedly different; all the Cu2+
cations in 1a give the five-coordinate tetragonal pyramidal
geometry, while the Cu2+ cations in {Cu24Nb56} cluster adopt the
four-coordinate square-planar geometry. Comparing the
structure of 1 with {Co8Nb24} cluster which was discovered by
our group, they are tetrahedral arrangement of both the TM
core and the PONb subunits, 1 was arranged in the capped
copper clusters and [Nb7O22]9− blocks while the subunits of
{Co8Nb24} cluster are cobalt cluster and [Nb6O19]8− anion.
The phase purity of 1 was confirmed by X-ray powder
diffraction. The XRD pattern is in good agreement with the
simulated XRD patterns resulted from single-crystal X-ray
diffraction (Fig. S5), confirming the phase purity of sample. The
bond-valence sum (BVS) calculations indicate that all Cu and As
atoms are in the +2 and +5 oxidation states, respectively. 28
Additionally, all the oxygen atoms on POM fragments are also
discussed based on the BVS calculations. (Table S2 and S3).
Negative electrospray ionization mass spectrometry (ESI-MS)
allowed us to identify intact cluster {As 4Cu4[Cu(H2O)]12Nb28O109}.
The single-crystalline samples of 1 were dissolved in deionized
water. As shown in Fig. S9, the spectrum consists of two
dominant series of multiply charged peaks for −6 and −5
charged ions attributed to [{Na7H13As4Cu4[Cu(H2O)]12Nb28O109}6−
and
[Na6K3H12As4Cu4[Cu(H2O)]12Nb28O109]5‒
polyanions
respectively, on the basis of their m/z values. Prominent
envelopes centered at m/z 1008.90 and 1229.06 are clear
proofs for the existence of the intact cluster anion in solution. In
order to further study the influences of the pH values on
structure, the ESI-MS spectrometry of 1 at different pH values
ranging between 7.20 and 12.42 were observed. When the
sample of compound was dissolved into deionized water at 0.12
mM concentration, the solution reaches a natural pH of 10.40.
Adding HCl or NaOH to this solution leads to decomposition of
the cluster, which is evident in the decreased abundance of
peaks assignable to clusters. When the pH value was adjusted
in the range of 7.41–12.20, the peaks of intact polyanion 1 were
clearly present at m/z 1008.90(−6) and 1229.06(−5) (Fig. 2).
Once increasing the pH value above 12.20 by 2 M NaOH, peaks
assignable to the clusters significantly decrease in abundance,
which shown that the structure of 1 may be destroyed. On
decreasing the pH value below 7.41 by 1 M HCl, the peaks of
intact polyanion 1 were also disappeared, which also indicates
the decomposition of the cluster. Hence, the intact polyanion 1
was stable in a wide pH value range between 7.41 and 12.20 in
aqueous solutions. Furthermore, the extended peaks at around
m/z 1008.90 are provided in the ESI (Tables S4) .
The temperature dependent magnetic susceptibilities of 1
were measured at 1.8–300 K in a dc magnetic field of 1000
Oe. The magnetic properties of 1 in the form of χM and χMT
versus T plots are presented in Fig. 3. Upon cooling, the
χMT decreases to 0.93 emu K mol−1 at 1.8 K from 3.90 emu
K mol−1 at 300 K. This behaviour suggests the presence of
a dominant antiferromagnetic interaction between the
copper centers in the compound.29–31 Besides, curve fitting
for 1/χM versus T plots with Curie-Weiss law χM = C/(T–Ɵ)
in the range of 100–300 K gives the Curie constants C = 4.80
emu K mol−1 and the negative Weiss constants Ɵ = −73.03 K for
1 (Fig. S10). The negative Ɵ value also further supports the
presence of antiferromagnetic interactions in compound 1.
Fig. 3 Temperature dependence of the molar magnetic susceptibility χM and
the product χMT for 1 between 1.8 and 300 K.
Fig. 2 ESI-MS of compound 1 during titration in different pH among the m/z =
920–1320.
In summary, an unprecedented arsenic-containing PONb
cluster Na14K7H5{As4Cu4[Cu(H2O)]12Nb28O109}∙37.5H2O was
synthesised via conventional aqueous method. Compound 1 is
the first example of polyoxoniobate which is arranged in the
capped copper clusters and [Nb7O22]9− blocks and also the first
example of encapsulating both copper cluster and arsenic atoms
in PONb framework. The synthesis of 1 demonstrates that
conventional aqueous method and the use of simple metal salts
can lead to the formation of high-nuclearity magnetic cores
stabilized by POM units, which provides a new synthetic
This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/chem.201704071
Chemistry - A European Journal
pathway for neotype heteropolyoxoniobates, instead of
commonly
employed
hydrothermal
reaction
in
the
heteropolyoxoniobate chemistry. Furthermore, compound 1 was
stable in a wide pH value range of 7.41–12.20, suggesting a
robust use in applications. In future work, we will focus on
investigating the interaction of other 3d metal ions with
[Nb6O19]8‒ precursor to construct more fascinating clusters with
new physical properties.
[7]
[8]
[9]
[10]
[11]
[12]
Experimental Section
[13]
4,4′-bipy (0.237 g) was added into acetonitrile (2 mL) under stirring.
When the solid is dissolved, 5 ml distilled water was added into it, and
then Cu(OAc)2·H2O (0.280 g, 1.41 mmol) was added to obtain the
emulsion solution. Then the resulting solution was added dropwise to the
solution of K7HNb6O19·13H2O (0.822 g, 0.60 mmol), As2O3 (0.189 g, 0.95
mmol) and tetramethylammonium hydroxide (0.274 g, 3.01 mmol) in
distilled water (120 mL) under stirring for 20 mins. Subsequently, the
mixture was adjusted to pH 12.3‒12.5 using NaOH (2 mol L−1) solution
and then stirred for about 20 min. The mixture was heated at 90 °C for 3
h, and then cooled to room temperature and filtered, a cloudy green
solution was obtained, after about one week, the cloudy green solution
became clear after the second filtration, then two months later, viridis
crystals of 1 were obtained. Yield: 4 % (based on K7[HNb6O19]·13H2O).
Elemental analysis (%) calcd for Na14K7H5{As4Cu4[Cu(H2O)]12
Nb28O109}∙37.5H2O, Na 4.50, K 3.82, Cu 14.21, As 4.19, Nb 36.36; found:
Na 4.48, K 3.81, Cu 14.19, As 4.18, Nb 36.02. IR (KBr pellet): 3388,
1642, 872, 664 and 537 cm−1.
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
Acknowledgements
[24]
The authors gratefully acknowledge support from the National
Natural Science Foundation of China (grant number 21371048,
21573056).
[25]
[26]
Keywords: one-pot reaction • copper cluster • magnetism •
polyoxometalate • polyoxoniobate
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This article is protected by copyright. All rights reserved.
Accepted Manuscript
COMMUNICATION
10.1002/chem.201704071
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Li Li, Kaili Dong, Pengtao Ma, Chao
Zhang, Jingyang Niu* and Jingping
Wang*
A {As4Cu4[Cu(H2O)]12} Cluster
Incorporated within Four [Nb7O22]9−
Units
[a]
Accepted Manuscript
A gigantic 16-copper-containing 28niobate cluster,
Na14K7H5{As4Cu4[Cu(H2O)]12Nb28O109}
∙37.5H2O (1), has been prepared by a
one-pot reaction, which is the first
example of arsenic copper clusters
containing polyoxoniobate.
L. Li, K. Dong, Dr. P. Ma, Dr. C. Zhang, Prof. J. Niu, Prof. J.
Key Laboratory of Polyoxometalate Chemistry of Henan Pro
Institute of Molecular and Crystal Engineering
College of Chemistry and Chemical Engineering
Henan University, Kaifeng, Henan 475004 (P. R. China)
Fax: (+ 86)371-238-868-76
E-mail: jyniu@henu.edu.cn
jpwang@henu.edu.cn
Supporting information for this article is given via a link at the
the document.
This article is protected by copyright. All rights reserved.
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