{Nb288O768(OH)48(CO3)12}: A Macromolecular Polyoxometalate with Close to 300 Niobium Atoms

Article abstract of DOI:10.1002/anie.201804088

A protein-sized (ca. 4.2×4.2×3.6 nm³) non-biologically derived molecule {Nb₂₈₈O₇₆₈(OH)₄₈(CO₃)₁₂} (Nb₂₈₈) containing up to 288 niobium atoms has been obtained, which is by far the largest and the highest nuclearity polyoxoniobate (PONb). Particularly, in terms of metal nuclearity number, Nb₂₈₈ is the second largest cluster so far reported in classic polyoxometalate chemistry (V, Mo, W, Nb, and Ta). Nb₂₈₈ can be described as a giant windmill-like cluster aggregate of six nanoscale high-nuclearity PONb units {Nb₄₇O₁₂₈(OH)₆(CO₃)₂} (Nb₄₇) joined together by six additional Nb ions. Interestingly, the 47-nuclearity Nb₄₇ units generated in situ can be isolated and bridged by copper complexes to form an inorganic–organic hybrid three-dimensional PONb framework, which exhibits effective catalytic activity for hydrolyzing nerve agent simulant of dimethyl methylphosphonate. The unique Nb₄₇ cluster also provides a new type of topology to very limited family of Nb-O clusters.

Full text of DOI:10.1002/anie.201804088

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Accepted Article
Title: {Nb288O768(OH)48(CO3)12}: A Macromolecular
Polyoxometalate with Niobium Atoms Close to 300
Authors: Yan-Lan Wu, Xin-Xiong Li, Yan-Jie Qi, Hao Yu, Lu Jin, and
Shou-Tian Zheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201804088
Angew. Chem. 10.1002/ange.201804088
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http://dx.doi.org/10.1002/ange.201804088

Angewandte Chemie International Edition
10.1002/anie.201804088
COMMUNICATION
{
Nb O (OH) (CO ) }: A Macromolecular Polyoxometalate with
2
88 768
48
3 12
Niobium Atoms Close to 300
Yan-Lan Wu, Xin-Xiong Li, Yan-Jie Qi, Hao Yu, Lu Jin, and Shou-Tian Zheng*
th
Dedicated to Professor Xintao Wu on the occasion of his 80 birthday
3
[6-8]
Abstract: A protein-sized (ca. 4.2 ᵡ 4.2 ᵡ 3.6 nm ) non-biologically
field of polyoxotungstates (POTs).
A handful of giant hetero-
derived molecule {Nb288
O
768(OH)48(CO
3
)
12} (Nb288) containing up to
metallic POTs with more than 100 metal centers have also been
[
8a]
[8b]
[8c]
2
88 niobium atoms has been obtained, which is by far the largest
made, including {Ce16
W
124},
148},
{Sn12
W
108},
{Ce20Ge10
W
100},
224},
[
8d]
[8e]
[8f]
[8g]
and the highest nuclearity polyoxoniobate (PONb). Particularly, in
terms of metal nuclearity number, Nb288 is the second largest cluster
so far reported in classic polyoxometalate chemistry (V, Mo, W, Nb,
and Ta). Nb288 can be described as a giant windmill-like cluster
aggregate of six brand-new, nanoscale high-nuclearity PONb units
{Ce
{Co
9
Mn
6
W
90},
{Gd
8
W
{Ce24Ge12W120}, {Mn40
W
[
8h]
[8i]
8
W200}, and {Dy30Co
8
Ge12W108}.
Polyoxoniobates (PONbs), as an important subclass of POMs,
often have unique stoichiometry and structure with promising
applications in the domains of photocatalysis, nuclear-waste
[
9-11]
{Nb47O
128(OH)
6
(CO
3
)
2
} (Nb47) joined together by six additional Nb
treatment, and base-catalyzed reactions.
However, due to
ions. Interestingly, the in situ generated 47-nuclearity Nb47 units can
be isolated and bridged by copper complexes to form an inorganic-
organic hybrid three-dimensional PONb framework, which exhibits
effective catalytic activity for hydrolyzing nerve agent simulant of
dimethyl methylphosphonate. The unique Nb47 cluster also provides
a new type of topology to very limited family of Nb-O clusters.
synthetic challenges caused by the narrow working pH region,
the slight solubility, and the low reaction activity of niobate
species, the development of PONb chemistry falls far behind
that of POMo or POT chemistry. Especially, even after decades
of research, almost all reported PONbs are in the small-to-
medium nuclearity range (< 100). The development of much
larger PONb materials parallel to the POMos and POTs has long
been an attractive but challenging target in POM chemistry.
The most common macromolecules in nature are biomolecules
composed of thousands of atoms or more, such as proteins,
DNA, and nucleic acids. Compared with biomacromolecules, the
majority of known inorganic molecules are much smaller in size,
nuclearity number, and molecular mass. The creation of giant
inorganic molecules at biomacromolecule size has continuously
been one of the most interesting subjects in synthetic chemistry
Recently, the first PONb {Nb114} with more than 100 Nb atoms
[11d]
has been identified by us,
however it is still considerably
smaller than many reported giant POMo and POT species.
Here, we report an unprecedented inorganic macromolecular
180-
3 12
PONb [Nb288O768(OH)48(CO ) ]
(Nb288) with more than 1000
non-hydrogen atoms per molecular cluster, which was isolated
as alkali salt H O (1). With 288 niobium
Li24Na50K87[Nb288]·132H
7 2
[
1-11]
as it is challenging to achieve on the one hand.
Especially,
metal ceners, Nb288 is by far the largest PONb and contains the
largest number of niobium centers so far reported in a POM. In
particular, in terms of metal nuclearity number, Nb288 already
surpasses any known giant POTs and represents the second
largest molecular POM to date (second only to {Mo368}). Another
fascinating feature of Nb288 is the existance of fundamentally
how large an inorganic molecule that can be synthesized
artificially is a scientific interest. And on the other hand, giant
inorganic molecules have appealing structural features (e.g.,
nanometer dimensions, multiple configurations, and high-
nuclearity metal centers) associated with unusual physical
[
1-11]
properties that do not occur for smaller molecules.
new, high-nuclearity PONb building unit {Nb47
Nb47). What’s more, the novel building unit Nb47 can be isolated
by introduction of copper complexes, giving an inorganic-roganic
hybrid 3D PONb framework H Li Na [Cu(en) [Nb47]·20H
2) (en = ethylenediamine). Both Nb288 and Nb47 enrich the very
6 3 2
O128(OH) (CO ) }
Giant polyoxometalates (POMs) are of special attention, as
they represent the largest inorganic molecules ever made
combined with fascinating structures and manifold applications
(
2
5
5
K
5
]
2 7
2
O
[
3-8]
in catalysis, medicine, and material sciences.
Typically,
(
Müller’s group has achieved great success in creating giant
polyoxomolybdates (POMos) during the past two decades,
establishing a series of incredibly large POMos with hundreds of
limited structural types of Nb-O cluster family. Additionally, 2
shows effective catalytic activity in the hydrolyzation of nerve
agent simulant of dimethyl methylphosphonate (DMMP) under
mild conditions.
[
4a]
[4b]
[4c]
[4d]
Mo centers, such as {Mo132}, {Mo154}, {Mo176}, {Mo248},
[
4e]
and {Mo368}. Especially, {Mo368} with more than 360 Mo atoms
still is the largest POM to date despite many new huge POMos
reported in recent years. In POM chemistry, another significant
progress on the synthesis of giant clusters has been made in the
Macromolecular Nb288 crystallizes in trigonal space group R-3
3
with a very large unit cell (ca. 116000 Å ), which consists of six
[
5]
nanosized PONb building units Nb47 and six additional Nb atoms.
As shown in Figure 1, the building unit Nb47 has a unique W-
shaped configuration formed by three kinds of unusual PONb
fragments {Nb17(OH)
Nb 14} (Nb ). Protonated oxgen atoms are identified by bond
valence sum calculations (Figure S1). The C
fragment can be described as a new-type superlacunary POM.
In 2010, Cronin’s group reported 27-nuclearity PONb
that can be seen as a combination of a α-Keggin
2 2
O53} (Nb17), {Nb10(OH) O31} (Nb10), and
[
a]
Y.L. Wu, Dr. X.X. Li, L. Jin, Y.J. Qi, H. Yu, Prof. Dr. S.T. Zheng
State Key Laboratory of Photocatalysis on Energy and Environment
College of Chemistry
Fuzhou University
Fuzhou, Fujian 350108, China
{
3
O
3
[
12]
s
-symmetric Nb17
a
E-mail: stzheng@fzu.edu.cn
16-
[HNb27
76
O ]
[
10a]
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author
{Nb12
O
40} unit with a bowl-like {Nb15
O
51} unit.
The Nb17
16-
76
fragment can be derived from [HNb27O ]
by the removel of its
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Angewandte Chemie International Edition
10.1002/anie.201804088
COMMUNICATION
three Nb atoms from the α-Keggin unit and seven Nb atoms
from the bowl-like unit (Figure S2). Note that Nb47 has two
inequivalent Nb17 fragments as one of them bonds to two
from Nb17 fragments, forming a W-shaped aggregation {W44}
bearing two bilaterally symmetrical V-type holes. Interestingly,
the symmetrical holes encapsulate different components. One
2-
2-
additional bidentate CO
3
ligands. The coordination of CO
3
ion
3
hole traps a Nb via two corner-sharing and three edge-sharing
2
3- [10a]
to PONb has been observed before in [H10Nb31
3
O93(CO )]
.
O atoms from the middle Nb10 and four corner-sharing O atoms
from one side Nb17 (Figure 1). While, another hole captures two
2-
3
CO
ligands coordinating to the lacunary sites on the other side
Nb17. As a result, a total of 47 Nb atoms are involved to form a
nanoscale W-shaped high-nuclearity PONb building unit Nb47
3
with dimensions of 2.3 × 1.4 ×1.3 nm .
3
Figure 1. Structure of Nb47. Polyhedra key: Nb17, red; Nb10, green; Nb , yellow.
Different from the Nb17, the Nb10 fragment is derived from
8-
Lindqvist PONb [Nb
special dimeric entity composed of two distinct monolacunary
Lindqvist units {Nb 18} and {Nb 19} condensed together via
6
O19] . As shown in Figure 1, Nb10 exhibits a
5
O
5
O
four corner-sharing O atoms in an eclipsed face-to-face fashion,
forming a C2v-symmetric structure with a cavity in the center.
The {Nb
constructed from five edge-sharing NbO
Nb 19} with {Nb 18}, the difference is that the Nb atom
located at the cap site in {Nb 19} doesn‘t bond to a terminal O
5
O18} is a typical monolacunary Lindqvist-type structure
6
octahedra. Comparing
{
5
O
5
O
5
O
atom as usual but two addtional bridging O atoms from two Nb17
fragments (Figure S3), resulting in a 7-coordinate center and
capped trigonal-prismatic coordination geometry. As thus, the
overall configuration of Nb10 is unique thought it is similar to
4
−
known decatungstate ion [W10
O
32
]
formed by two equivalent
The third fragment Nb is a rare Nb-O trimer
comprising two NbO square pyramids corner-sharingly bridged
by a NbO octahedron.
The combination of one Nb
[
13]
5
{W O18} units.
3
5
6
3
, one Nb10, and two Nb17 frag-
ments produces a particular W-like building unit Nb47 wth 47 Nb
centers. Specifically, two co-planar Nb17 fragments represent a
horn-like arrangement with C2v symmetry and lie at an angle of
ca. 81 with respect to each other (Figure 1, S3). Further, the
Nb10 stands between these two Nb17 fragments and acts as a
linker via corner- and edge-sharing O atoms with eight Nb atoms
o
Figure 2. a) Side view of Nb288, showing the six additional Nb linkers (blue)
are in the same plane and form a C
of Nb288, showing a windmill-like strcuture. Polyhedra key: Nb17, red; Nb10
green; Nb , yellow; Nb, blue.
6
-symmetry hexagonal array. b) Top view
,
3
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COMMUNICATION
Besides the six Nb47 units, Nb288 also contains six additional
with Cu-O bond lengths of 2.275(18) - 2.618(2) Å. Specifically,
2+
Nb atoms. The six Nb atoms lie in the same plane forming a C
6
-
each Nb47 is decorated by four [Cu(en)
2
]
cations that are not
symmetry hexagonal array with adjacent Nb⋯Nb distances of
further connected to other Nb47 clusters, forming a tetra-Cu-
supported polyoxoanion (Figure S5). Then, every tetra-Cu-
supported polyoxoanion is further bridged to five adjacent ones
o
6.6 Å and Nb-Nb-Nb angles of 120 C (Figure 2a, S4a). The six
Nb47 are evenly distributed on either side of the ring plane
defined by the six additional Nb atoms, and entire 6-Nb47
2+
by remaining five [Cu(en)
2
]
complexes (Figure 3), forming a 3D
arrangement has S
6
symmetry with the principal axis coinciding
inorganic-organic hybrid, anionic PONb framework charge-
balanced by alkali metal ions and isolated Cu-complexes (Figure
S6). Notably, it is important to develop POMs beyond the Mo-,
with the sixfold axis of the middle hexagonal ring (Figure S4b).
Alternatively, the 6-Nb47 arrangement can be viewed as a
triangular antiprism geometry with a Nb47 at each of its six
vertexes (Figure S4c). In Nb288, all of the six addtional Nb atoms
are 6-coordinate and act as bridges between Nb47 units.
Specifically, each addtional Nb atom links two Nb47 units via four
corner-sharing Nb-O-Nb bonds (Nb-O bond lengths: 1.829(12)-
[
10a]
W-, and V-based building units.
Cluster Nb47 represents a
remarkably different structure from all of known POMs.
The formulas of 1 and 2 were determined by single-crystal X-
ray diffractions and ICP analyses, and their phase purity was
confirmed by PXRD patterns (Figure S7,8). To balance the
charges of polyoxoanions, seven and two protons should be
added to 1 and 2, respectively. These protons can not be
located and are assumed to be delocalized on the overall
2.164(14) Å) whilst each Nb47 unit bonds to two addtional Nb
atoms. The two remaining coordinate sites of each addtional Nb
atom are occupied by two terminal OH groups (Nb-O bond
lengths: 1.802(2)-1.906(13) Å) confirmed by BVS calculations
[
15]
structures, which is common in POMs. Due to high negative
charges and basic surface oxygens, PONbs have been shown
as promising candidates for hydrolyzing chemical warfare
(Figure S1). Thus, the assembly of six Nb47 units and six
bridging Nb atoms generates a fascinating windmill-like PONb
macromolecule Nb288 with a diameter of ca. 4.3 nm and a side
length of ca. 3.6 nm and with a molecular mass over 45000
[
16]
agents in the past a few years. Dimethyl methylphosphonate
(DMMP), as a quite inert nerve agent simulant, was nominated
by the U.S. Army as an ideal model chemical for toxicology and
(Figure 2b), displaying one of the largest non-biologically derived
[
17]
molecules so far synthesized.
carcinogenesis studies.
The brand-new PONbs 1 and 2
Despite classic POM chemistry has a long history of almost
two centuries, known giant macromolecular POMs have
generally been restricted to POMos and POTs. Molecular cluster
Nb288 shows a significant breakthrough in the nuclearity number
and also the size of PONbs. This is the first PONb with size and
nuclearity number comparable to known the largest POMos and
POTs since PONb has been known for over one and a half
triggered us to check if they could display excellent catalytic
performances in the hydrolysis of DMMP.
To examine the water stability, both 1 and 2 were dissolved in
water and then recrystallized from water. IR spectra of 2 before
and after recrystallization proved to be identical, that is, 2 kept
stable and no other PONbs were formed (Figure S9). While, the
IR spectra of 1 are different (Figure S10), indicating Nb288 just
can exist in solid. Note that IR spectrum of 1 after recrystalli-
zation is somewhat similar to that of 2, indicating Nb288 probably
partial degrade into Nb47. Unfortunately, the structure of the
recrystallized phase of 1 could not be determined due to powder
crystallinity. Thus, only 2 was employed for catalytic test.
[
14]
centuries.
Figure 3. View of coordination mode of cluster Nb47 in 2.
The structure of Nb288 indicates its likely formation mechanism
by initial formation of the cluster Nb47 in solution. It is known that
cationic metal complexes are easily coordinated with anionic
POMs, which may hinder further aggregation of in situ generated
POM intermediates. So, to check if Nb47 is actually present in
solution, we investigated the introduction of copper complex
2+
[
Cu(en)
2
]
for the stabilization and isolation of novel cluster Nb47.
The approach resulted in successful formation of a targeted
Figure 4. a) DMMP decomposition using 2. b) Three consecutive runs of
DMMP decomposition using 2. c) and d) IR and PXRD spectra of 2 before and
after 3-run catalytic reactions, respectively. Conditions: [DMMP] = 15.5 mm, 50
2+
compound 2 built from Nb47 clusters and [Cu(en)
2
]
complexes.
In 2, each Nb47 acts as a 9-dentate ligand bonding to nine
2+
2 2
mg 2, 0.6 mL D O and 1.0 mL H O at room temperature.
[
Cu(en)
2
]
2
cations via its three terminal O and six μ -O atoms
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Angewandte Chemie International Edition
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COMMUNICATION
Experiment was performed in a homogeneous system by
dissolving 15.5 mM DMMP and 50 mg 2 (5.8 umol) in a solution
of 1 ml H O and 0.6 ml D O at room temperature and 1 atm. The
2 2
results showed that 2 can convert about 46% of quite inert
DMMP to nontoxic methyl phosphonic acid (MP) over a time
course of 263 h without using additional chemicals (Figure 4a).
Compared with a blank experiment without 2 under the same
conditions, no obvious conversion detected, revealing that 2 is
an efficient DMMP hydrolysis catalyst. Notably, 2 is more active
than many MOFs towards DMMP and comparable with the
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2]
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effective PONb K12[Ti
2 2 2
O ][GeNb12O40]·19H O reported recently
by Hill’s group (56 % conversion under the same reaction
[
16a]
conditions).
The extent of conversion was calculated as the
31
ratio of the integrated P NMR peak for MP (the only hydrolysis
product) to that of DMMP (Figure S11).
In order to check the stability of 2 during the catalytic process,
some experiments were performed by us. On the one hand, the
recyclability of 2 was investigated by directly adding additional
substrate DMMP (15.5 mM) into the reaction mixture when the
previous run was completed. The results show 2 has very good
recyclability with almost the same DMMP conversions for three-
run duplicate operations (Figure 4b). On the other hand, IR and
PXRD measurements were conducted for powder crystallinity
obtained by volatilizing the solution after three-run successive
catalytic reactions at room temperature. Both IR spectrum and
PXRD of 2 were well consistent with those of the powder
crystallinity obtained from the solution after catalytic reactions
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Figure 4c,d), confirming that 2 could maintain its structural
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In summary, for the first time, a PONb Nb288 with hundreds of
niobium centers and over 4 nm in diameter has been artificially
synthesized. It is the largest molecular niobium aggregate and
the second highest nuclearity POM to date. Further, in Nb288, the
W-shapled high-nuclearity PONb building unit Nb47 with three
3
types of new fragments Nb17, Nb10, and Nb has a brand-new
topology to POM chemistry, providing a new member of very
limited Nb-O clluster family. Especially, the novel Nb47 formed in
situ can be stabilized and isolated by copper complexes to form
an extended framework, which is effective hydrolysis catalyst for
nerve agent simulant of DMMP. The results represent a major
step forward in the development of PONb system and open up
new perspectives for the development of diverse and larger
PONb materials parallel to the POMo and POT systems.
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We gratefully acknowledge the financial support from the NSFs
of China (No. 21773029), National 1000 Youth Talents Plan,
and 111 project.
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Table of Contents
Giant Polyoxoniobate An
unprecedentedly huge polyoxoniobate
Yan-Lan Wu, Xin-Xiong Li, Yan-Jie Qi,
Hao Yu, Lu Jin, and Shou-Tian Zheng*
(PONb) with as many as 288 Nb
centers has been made. This is by far
the largest PONb and the second
highest nuclearity polyoxometalate
reported to date. It incorporates a
unique 47-nuclearity PONb building
unit formed in situ, which can be
stabilized by metal complexes to form
an extended PONb framework with
the capability of hydrolyzing dimethyl
methylphosphonate.
Page No. – Page No.
{
3
Nb288O768(OH)48(CO )12}: A
Macromolecular Polyoxometalate
with Niobium Atoms Close to 300
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