Organoimido-derivatized hexamolybdates with a remote carboxyl group: Syntheses and structural characterizations

Article abstract of DOI:10.1021/ic4005278

Four novel organoimido derivatives of hexamolybdate containing a remote carboxyl group have been synthesized: [Bu₄N]₂[Mo ₆O₁₈(N-C₆H₄-3-COOH)] (1), [Bu ₄N]₂[Mo₆O₁₈(N-C₆H ₄-2-CH₃-4-COOH)] (2), [Bu₄N] ₂[Mo₆O₁₈(N-C₆H₄-2-CH ₃-5-COOH)] (3), and [Bu₄N]₂[Mo ₆O₁₈(N-C₆H₄-2-CH₃-3-COOH) ] (4) with 3-aminobenzoic acid, 4-amino-3-methylbenzoic acid, 3-amino-4-methylbenzoic acid, and 3-amino-2-methylbenzoic acid as the imido-releasing agents, respectively. Their structures have been characterized by IR, UV-vis, ¹H NMR, ESI-MS, and single-crystal X-ray diffraction techniques. Hydrogen bonding interactions play an important role in the supramolecular assemblies of these compounds in the solid state. Although the incorporated organic ligands are similar to each other, their supramolecular assembly behaviors are quite different. For compound 1, the dimer structure is formed via hydrogen bonding between the carboxyl group and the POM cluster of two neighboring cluster anions. For compound 2, the 1D chain structure is formed via hydrogen bonding between the carboxyl groups and the POM clusters of the cluster anions. For compound 3, the 2D plane structure is formed via two types of hydrogen bonding between the aromatic rings and the POM clusters of the cluster anions. For compound 4, the 1D plus 2D structures are formed via three types of hydrogen bonding between the aromatic rings and the POM clusters of the two types of cluster anions with different orientations.

Full text of DOI:10.1021/ic4005278

Article
pubs.acs.org/IC
Organoimido-Derivatized Hexamolybdates with a Remote Carboxyl
Group: Syntheses and Structural Characterizations
Guohui Sima,†,∥ Qiang Li,‡,∥ Yi Zhu,§,∥ Chunlin Lv,† Rao Naumaan Nasim Khan,† Jian Hao,†
Jin Zhang,*,† and Yongge Wei*,†
†Department of Chemistry, Tsinghua University, Beijing 100084, PR China
‡Department of Chemistry, College of Science of Beijing Forestry University, Beijing 100083, PR China
§Department of Chemistry, Jinan University, Guangzhou 510632, PR China
S
* Supporting Information
ABSTRACT: Four novel organoimido derivatives of hexamolybdate contain-
ing a remote carboxyl group have been synthesized: [Bu4N]2[Mo6O18(N-
C6H4-3-COOH)] (1), [Bu4N]2[Mo6O18(N-C6H4-2-CH3-4-COOH)] (2),
[Bu4N]2[Mo6O18(N-C6H4-2-CH3-5-COOH)] (3), and [Bu4N]2[Mo6O18(N-
C6H4-2-CH3-3-COOH)] (4) with 3-aminobenzoic acid, 4-amino-3-methyl-
benzoic acid, 3-amino-4-methylbenzoic acid, and 3-amino-2-methylbenzoic
acid as the imido-releasing agents, respectively. Their structures have been
1
characterized by IR, UV−vis, H NMR, ESI−MS, and single-crystal X-ray
diffraction techniques. Hydrogen bonding interactions play an important role
in the supramolecular assemblies of these compounds in the solid state.
Although the incorporated organic ligands are similar to each other, their
supramolecular assembly behaviors are quite different. For compound 1, the dimer structure is formed via hydrogen bonding
between the carboxyl group and the POM cluster of two neighboring cluster anions. For compound 2, the 1D chain structure is
formed via hydrogen bonding between the carboxyl groups and the POM clusters of the cluster anions. For compound 3, the 2D
plane structure is formed via two types of hydrogen bonding between the aromatic rings and the POM clusters of the cluster
anions. For compound 4, the 1D plus 2D structures are formed via three types of hydrogen bonding between the aromatic rings
and the POM clusters of the two types of cluster anions with different orientations.
attention because of their novel structures and properties.
However, there are some difficulties in the synthesis chemistry
that is used to obtain such POM-based hybrid materials.
Among the different kinds of organically functionalized
POMs, the organoimido derivatives with metal−nitrogen
multiple bonds, which largely modify the electronic structures
and redox properties of the parent POMs, have obtained great
attention in recent decades, and a large number of organoimido
derivatives of the Lindqvist-type hexamolybdates have been
synthesized and studied successfully. From the time that Kang
and Zubieta first described an organoimido derivative,15 the
imidoylization of POMs has been synthetically investigated and
was initially reported by Maatta,16 Proust,17 and Errington.18
Thus, three kinds of reactions for the synthesis of the
organoimido derivatives of hexamolybdates have been
developed, including reactions with phosphinimines, isocya-
nates, and aromatic amines. Subsequently, Peng and Wei
discovered the N,N′-dicyclohexylcarbodiimide (DCC) protocol
in which the reaction of hexamolybdates and amines was
remarkably promoted by DCC to obtain the organoimido
derivatives of POMs.19 Next, Wei discovered that the reaction
INTRODUCTION
■
Polyoxometalates (POMs) are a unique class of metal−oxygen
cluster compounds consisting of early transition-metal ions in
the highest oxidization states that are linked by oxygen anions
as ligands. They present remarkable structural diversity,
chemical reactivity, and self-assembly behaviors as well as
excellent magnetic, photonic, and electronic properties that
have potential applications in diverse fields such as materials,
catalysis, and medicinal sciences.1−10 The realizability of the
processing and modifying of POMs is very important for their
subsequent research and application. An effective method to
achieve this goal is through the organic modification of POMs
to obtain organic−inorganic hybrids.
Since the pioneering work of Klemperer,11 Harlow12, and
Pope,13 organically functionalized POMs with covalently
grafted organic ligands have been studied extensively. As
expected, the resulting organic−inorganic hybrid materials not
only have both the advantages of the organic ligands, such as
fine electronic properties and easy processing, and the
advantages of the inorganic POM components, such as good
structural stability and electron acceptability, but also have
fascinating synergistic effects that originate from the interaction
between the organic ligands and the inorganic clusters.14
Therefore, such hybrid materials have drawn a great amount of
Received: March 1, 2013
© XXXX American Chemical Society
A
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Article
solution to an (NH4)6Mo7O24·4H2O aqueous solution, from which the
product immediately precipitated and was dried before use. The
hydrochlorides of 3-aminobenzoic acid, 4-amino-3-methylbenzoic acid,
3-amino-4-methylbenzoic acid, and 3-amino-2-methylbenzoic acid
were prepared by adding concentrated HCl to the corresponding
compounds and were also dried before use. The acetonitrile was dried
by refluxing in the presence of CaH2 and was distilled prior to use.
DCC, acetone, diethyl ether, and ethanol were directly used without
further purification. Elemental analysis was measured with a
ThermoQuest FLASH-1112 instrument. IR spectra were obtained
using KBr pellets with a PerkinElmer FT-IR spectrometer. UV−vis
spectra were measured in an acetonitrile solution with a Shimadzu UV-
of α-octamolybdates and amine hydrochlorides in the presence
of DCC, in which both the proton and the DCC produce
effects, could afford the organoimido derivatives of hexamo-
lybdates in higher yields.20 Since then, a large number of
organoimido derivatives of hexamolybdates containing different
kinds of remote groups, such as amino,21 nitro,22 hydroxyl,23
ester,24 and halo,25,26 have been synthesized and investigated by
using this DCC protocol.
The organoimido derivatives of hexamolybdates with remote
groups can also serve as building blocks in further
functionalization or fabrication to obtain more complicated
and fascinating POM-based organic−inorganic hybrid materials
via postmodification or self-assembly procedures. In recent
years, several inorganic and organic reactions, including
coordination,27 esterification,28 Sonogashira coupling,29 the
Heck reaction,30 and addition polymerization,31 of organo-
imido-substituted hexamolybdates have been investigated. In
addition, the alkylimido derivatives of hexamolybdates have
been discovered to undergo a C−C coupling reaction to form a
CC double bond via the doubly dehydrogenative coupling
(DDHC) reaction of the two saturated sp3 C−H bonds closest
to the nitrogen of the imido groups that are activated by
POMs,32 providing a novel method for CC double bond
formation besides traditional coupling reactions.
Although a large number of organoimido derivatives of
hexamolybdate containing different kinds of remote groups
have been obtained, reports of organically functionalized
hexamolybdates containing remote carboxyl groups have been
surprisingly rare until now.33,34 The organoimido derivatives of
hexamolybdate containing a remote carboxyl group, as an active
group for further coordination and for some organic reactions,
can undergo postmodification to afford either novel POM-
based organic−inorganic hybrid materials or supramolecular
self-assemblies in addition to the organic−inorganic hybrids
formed via the noncovalent interaction between POMs and
organic ligands.35,36 Mialane reported the hybrid compounds
formed by the rare earth POMs and carboxylate ligands
through coordination,37 which ranged from macromolecular
complexes to open framework systems. The Lindqvist-type
POM-based coordination network with terbium and carbox-
ylate was reported by Hill38 and had good catalytic properties
in the O2-based oxidations. Furthermore, Su and Liu reported
the synthesis of a solid, porous MOF by the reaction of a
Keggin-type POM, copper salt, and a carboxylate ligand39 that
displayed fine nerve gas adsorption. This work indicates that
the POM-based hybrids that are formed via the modification of
the carboxyl group show special structural and application
characteristics. Therefore, in this work we synthesized four
novel organoimido derivatives of hexamolybdates containing a
remote carboxyl group on different relative positions of the
imido nitrogen and methyl groups on a benzene ring. Then we
characterized and investigated the title compounds, especially
the hydrogen bonding interaction in the formation of the
supramolecular assemblies, and we hope to give some
information on the reaction chemistry of such POM derivatives
and the further preparation of novel POM-based organic−
inorganic hybrid materials via reactions with carboxyl groups.
1
2100S spectrophotometer. H NMR spectra were measured using d6-
DMSO as the solvent with a Bruker ARX400 NMR spectrometer.
ESI−MS spectra were measured in an acetonitrile solution with a
ThermoFisher LTQ spectrometer. The negative mode was used for
the experiments.
Synthesis of [Bu4N]2[Mo6O18(N-C6H4-3-COOH)] (1). A mixture
of (Bu4N)4[Mo8O26] (2.15 g, 1.0 mmol), 3-aminobenzoic acid
hydrochloride (0.231 g, 1.33 mmol), and DCC (0.454 g, 2.2 mmol)
was dissolved in 10 mL of anhydrous acetonitrile and heated to reflux
at 110 °C for 3 h. After cooling to room temperature, the reaction
solution was filtered to remove white precipitate 1,3-dicyclohexylurea
(DCU), and the filtrate was poured into 100 mL of ether to remove
some of the unreacted organic compounds, resulting in precipitation.
This precipitate was dissolved in acetone and filtered to remove the
unreacted octamolybdate. After the filtrate had evaporated in the open
air, the remaining solid was washed with ethanol to further remove
more of the unreacted organic compounds. The remaining crude
product was then redissolved in acetonitrile, and the red single crystals
for X-ray diffraction were obtained by the diffusion of ether into the
acetonitrile solution. The yield for the crystalline product after
crystallization (based on Mo) was 12%. Anal. Calcd for
C42H83.5O20.5N3.5Mo6 (1541.26): C, 32.73; H, 5.46; N, 3.18. Found:
C, 31.36; H, 5.12; N, 2.94. IR (KBr pellet, cm−1): 2962, 2874, 1724,
1482, 1382, 977, 953, 889, 794. UV−vis (CH3CN, nm): λmax = 343.
1H NMR (d6-DMSO, 400 MHz): δ 0.93 (t, 24H, −CH3, [Bu4N]+),
1.31 (m, 16H, −CH2−, [Bu4N]+), 1.56 (m, 16H, −CH2−, [Bu4N]+),
3.16 (t, 16H, N−CH2−, [Bu4N]+), 7.42 (d, 1H, ArH), 7.57 (t, 1H,
ArH), 7.67 (t, 1H, ArH), 7.74 (d, 1H, ArH), 13.34 (s, 1H, −COOH).
ESI−MS (CH3CN, m/z): 499.77 (100%, [Mo6O18(N-C6H4-3-
COOH)]2−), 1242.75 (35%, [Bu4N][Mo6O18(N-C6H4-3-COOH)]−).
Synthesis of [Bu4N]2[Mo6O18(N-C6H4-2-CH3-4-COOH)] (2).
The synthesis method was similar to that of compound 1 except
that 4-amino-3-methylbenzoic acid hydrochloride was used instead of
3-aminobenzoic acid hydrochloride, and the reaction time was 9 h.
The red single crystals for X-ray diffraction were obtained by the
diffusion of ether into the acetonitrile solution. The yield for the
crystalline product after crystallization (based on Mo) was 20%. Anal.
Calcd for C40H79O20N3Mo6 (1497.70): C, 32.08; H, 5.32; N, 2.81.
Found: C, 32.32; H, 5.44; N, 2.94. IR (KBr pellet, cm−1): 2961, 2873,
1715, 1480, 1379, 977, 955, 883, 794. UV−vis (CH3CN, nm): λmax
=
358. 1H NMR (d6-DMSO, 400 MHz): δ 0.93 (t, 24H, −CH3,
[Bu4N]+), 1.31 (m, 16H, −CH2−, [Bu4N]+), 1.57 (m, 16H, −CH2−,
[Bu4N]+), 2.57 (s, 3H, Ar−CH3), 3.16 (t, 16H, N−CH2-, [Bu4N]+),
7.25 (d, 1H, ArH), 7.79 (d, 1H, ArH), 7.84 (s, 1H, ArH), 13.03 (s, 1H,
−COOH). ESI−MS (CH3CN, m/z): 507.72 (100%, [Mo6O18(N-
C6H4-2-CH3-4-COOH)]2−), 1255.92 (30%, [Bu4N][Mo6O18(N-
C6H4-2-CH3-4-COOH)]−).
Synthesis of [Bu4N]2[Mo6O18(N-C6H4-2-CH3-5-COOH)] (3).
The synthesis method was similar to that of compound 1 except
that 3-amino-4-methylbenzoic acid hydrochloride was used instead of
3-aminobenzoic acid hydrochloride, and the reaction time was 10 h.
After cooling to room temperature, the reaction solution was filtered
to remove white precipitate DCU. The filtrate was evaporated slowly
in the open air to remove the octamolybdate and hexamolybdate by
filtration because they crystallized first, resulting in a crude oily
product. This crude product was redissolved in acetonitrile, and the
red single crystals for X-ray diffraction were obtained by the diffusion
of ether into the acetonitrile solution. The yield for the crystalline
EXPERIMENTAL SECTION
■
General Methods and Materials. All of the syntheses and the
manipulations were carried out under dry N2 gas. The
(Bu4N)4[Mo8O26] was conveniently synthesized according to an
improved literature method40 by the addition of an Bu4NBr aqueous
B
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Table 1. Crystallographic Data for Compounds 1−4
compound 1
compound 2
compound 3
compound 4
empirical formula
formula weight
temperature, K
wavelength, Å
space group
a, Å
C42H83.5O20.5N3.5Mo6
C40H79O20N3Mo6
1497.70
100
C40H79O20N3Mo6
1497.70
103
C40H79O20N3Mo6
1497.70
104
1541.26
102
0.71073
Pcca (no. 54)
19.9827(6)
24.1195(10)
23.8593(7)
90
0.71073
C2/c (no. 15)
41.278(3)
12.2444(8)
24.4842(14)
90
0.71073
P21/n (no. 14)
12.6439(3)
20.7751(4)
20.4235(4)
90
0.71073
P21/c (no. 14)
37.0279(7)
12.3515(3)
24.2391(5)
90
b, Å
c, Å
α, degrees
β, degrees
γ, degrees
V, Å3
90
115.901(7)
90
94.375(2)
90
92.2844(19)
90
90
11499.5(7)
8
11131.8(12)
8
5349.2(2)
4
11077.0(4)
8
Z
Dcalcd [g cm−3
]
1.780
1.787
1.860
1.796
μ [mm−1
]
1.341
1.381
1.437
1.388
a
R1 [I > 2σ(I)]
0.0481
0.0530
0.0434
0.0538
b
wR2 [I > 2σ(I)]
0.1380
0.1537
0.0832
0.1746
a
b
2
2
R1 = (Σ||Fo| − |Fc||)/Σ|Fo|. wR2 = [Σw(Fo − Fc2)2/Σw(Fo )2]1/2
.
product after crystallization (based on Mo) was 10%. Anal. Calcd for
C40H79O20N3Mo6 (1497.70): C, 32.08; H, 5.32; N, 2.81. Found: C,
32.38; H, 5.48; N, 3.13. IR (KBr pellet, cm−1): 2964, 2874, 1718, 1481,
imido derivatives of hexamolybdates: the reaction of
(Bu4N)2[Mo6O19] with amines and the reaction of
(Bu4N)4[Mo8O26] with amine hydrochlorides. Both of these
reactions were carried out in the presence of DCC as a
dehydrating agent. Thus, both methods were carried out in this
article to synthesize the four target derivatives.
1
1379, 977, 952, 881, 793. UV−vis (CH3CN, nm): λmax = 346. H
NMR (d6-DMSO, 400 MHz): δ 0.93 (t, 24H, −CH3, [Bu4N]+), 1.31
(m, 16H, −CH2−, [Bu4N]+), 1.57 (m, 16H, −CH2−, [Bu4N]+), 2.59
(s, 3H, Ar−CH3), 3.16 (t, 16H, N−CH2−, [Bu4N]+), 7.42 (d, 1H,
ArH), 7.61 (s, 1H, ArH), 7.64 (d, 1H, ArH), 13.18 (s, 1H, −COOH).
ESI−MS (CH3CN, m/z): 506.20 (100%, [Mo6O18(N-C6H4-2-CH3-5-
COOH)]2−), 1013.40 (45%, [HMo6O18(N-C6H4-2-CH3-5-
COOH)]−).
When (Bu4N)2[Mo6O19], an aromatic carboxylic acid with an
amino group, and DCC were used in the reaction, the solution
turned brown or green after some time, which meant that the
hexamolybdate oxidized the organic compound and was itself
reduced, resulting in a brown- or green-colored appearance.
The absence of a red-colored appearance indicated that the
electron exchange occurred between the POM cluster and the
organic group before the formation of the organoimido
product. Additionally, the oxidation state of the organic
group might form a radical on the nitrogen atom that could
undergo further reaction, including the polymerization of the
organic reactant. The experimental parameters were then
changed, including the reactant ratio, reaction temperature, and
reaction time, but they still could not give rise to the
organoimido products. However, when (Bu4N)4[Mo8O26], a
hydrochloride of the aromatic carboxylic acid with an amino
group, and DCC were used in the reaction, the organoimido
products were obtained for every corresponding organic
compound by controlling the different reaction conditions.
The single crystals were obtained by using different processing
approaches for the different compounds. In the imidoylization
of the POMs and the amines with different kinds of groups
other than the carboxyl group, both the hexamolybdate method
and the octamolybdate method can, for the most part, afford
the organoimido products. The difference between the two
methods is that the latter one can afford the organoimido
products faster, in higher yield, and under milder conditions. In
the imidoylization of the POMs and the organic compounds
with a carboxyl group, according to the experimental
investigation, the organoimido products cannot be obtained
with the hexamolybdate method. They can be obtained with
the octamolybdate method, although the yield is not high. The
carboxyl group must have played some role in the formation of
the organoimido product because of its acidity and oxidizability,
which can also react with the amino group to produce an amide
Synthesis of [Bu4N]2[Mo6O18(N-C6H4-2-CH3-3-COOH)] (4).
The synthesis method was similar to that of compound 1 except
that 3-amino-2-methylbenzoic acid hydrochloride was used instead of
3-aminobenzoic acid hydrochloride, and the reaction time was 6 h.
The red single crystals for X-ray diffraction were obtained by the
diffusion of ether into the acetonitrile solution. The yield for the
crystalline product after crystallization (based on Mo) was 14%. Anal.
Calcd for C40H79O20N3Mo6 (1497.70): C, 32.08; H, 5.32; N, 2.81.
Found: C, 32.06; H, 5.42; N, 2.94. IR (KBr pellet, cm−1): 2963, 2874,
1736, 1470, 1380, 975, 954, 882, 796. UV−vis (CH3CN, nm): λmax
=
345. 1H NMR (d6-DMSO, 400 MHz): δ 0.93 (t, 24H, −CH3,
[Bu4N]+), 1.31 (m, 16H, −CH2−, [Bu4N]+), 1.56 (m, 16H, −CH2−,
[Bu4N]+), 2.74 (s, 3H, Ar−CH3), 3.16 (t, 16H, N−CH2−, [Bu4N]+),
7.34 (t, 1H, ArH), 7.38 (d, 1H, ArH), 7.56 (d, 1H, ArH), 13.13 (s, 1H,
−COOH). ESI−MS (CH3CN, m/z): 505.78 (100%, [Mo6O18(N-
C6H4-2-CH3-3-COOH)]2−), 1254.70 (80%, [Bu4N][Mo6O18(N-
C6H4-2-CH3-3-COOH)]−).
Single-Crystal Structure Determinations. Suitable single
crystals were mounted on a glass fiber and transferred directly to an
Agilent Gemini CCD diffractometer at room temperature. The data
collections were performed at 102 K for compound 1, 100 K for
compound 2, 103 K for compound 3, and 104 K for compound 4 by
graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The data
reduction, cell refinement, and absorption correction were processed
with CrysAlisPro software (Agilent, 2011, version 1.171.35.11). The
structures were solved by direct methods and refined by full-matrix
least squares. All the nonhydrogen atoms were refined anisotropically,
and the hydrogen atoms were generated geometrically. All of the
computations were performed with the program package SHELXTL,
version 6.14.41,42
RESULTS AND DISCUSSION
■
Preparation of the Compounds. Two methods have
been developed to synthesize the monosubstituted organo-
C
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Table 2. Selected Bond Lengths (Angstroms) and Angles (Degrees) for Compounds 1−4
Mo1−N1
N1−C1
Mo1−N1−C1
Mo1−O1
Mo6−O1
compound 1
compound 2
compound 3
compound 4
1.732(5)
1.739(5)
1.734(4)
1.736(5)
1.390(7)
1.384(7)
1.391(6)
1.377(8)
171.1(4)
169.3(5)
169.1(4)
166.9(5)
2.197(4)
2.205(4)
2.223(3)
2.209(4)
2.359(4)
2.362(4)
2.359(3)
2.361(4)
Table 3. Summary of Hydrogen Bonding (Angstroms and Degrees) for Compounds 1−4
D−H···A
symmetry transformation
d(D-H)
d(H···A)
d(D···A)
∠(DHA)
3
compound 1
compound 2
compound 3
O20−H20···O7
O20−H20···O11
O20−H20···O16
C3−H3···O10
O20−H20···O7
C5−H5···O11
−x + /2, −y, z
0.82
0.82
0.82
0.93
0.82
0.93
0.82
1.90
1.85
1.93
2.45
1.97
2.48
2.03
2.712(6)
2.646(6)
2.706(5)
3.292(6)
2.745(7)
3.402(8)
2.786(7)
170.8
164.2
158.5
150.5
156.9
172.0
153.6
3
1
1
−x + /2, y − /2, -z + /2
1
1
1
−x + /2, y + /2, −z + /2
−x + 1, −y + 1, −z
1
1
compound 4
−x, y + /2, −z + /2
−x, −y + 2, −z + 1
1
1
O40−H40···O22
x, −y + /2, z + /2
Figure 1. ORTEP drawings of the cluster anions for compound 1 (a), compound 2 (b), compound 3 (c), and compound 4 (d) (30% probability
ellipsoids).
Figure 2. ORTEP drawings of the Mo1 cluster (a) and the Mo7 cluster (b) in compound 4 (30% probability ellipsoids).
under the reaction conditions. Besides this, the presence of
water in the system also affects the yield to some extent,
according to the experimental results. By contrast, the carboxyl
group has more of an effect on the reaction. It completely ruins
the intermediate state in the reaction of (Bu4N)2[Mo6O19] and
the aromatic carboxylic acid with an amino group, making the
formation of the organoimido product impossible. The reaction
of (Bu4N)4[Mo8O26] and hydrochlorides of the aromatic
carboxylic acid with an amino group can finally afford the
title compounds because the carboxyl group can only partially
counteract the formation of the organoimido product.
Structure Description. A summary of the crystallographic
data and the structural determinations for compounds 1−4 are
presented in Table 1. The selected bond lengths and angles for
compounds 1−4 are summarized in Table 2. The summary of
the hydrogen bonding lengths and angles for compounds 1−4
are presented in Table 3.
D
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Crystal Structures of Compounds 1−4. In the
structures of compounds 1−4, there is an asymmetric unit
that contains one crystallographically independent POM cluster
anion, two counter tetrabutylammonium cations, half of an
acetonitrile molecule, and half of an ether molecule for
compound 1; one crystallographically independent POM
cluster anion and two counter tetrabutylammonium cations
for compounds 2 and 3; and two crystallographically
independent POM cluster anions and four counter tetrabuty-
lammonium cations for compound 4. The ORTEP drawings of
the cluster anions for the four compounds are presented in
Figures 1 and 2. They display the typical structure of
monosubstituted organoimido derivatives of hexamolybdate
with one terminal oxygen atom (Ot) of the hexamolybdate
cluster replaced by the organoimido ligand. The relatively short
Mo−N bond length and the nearly linear Mo−N−C bond
angle (Table 2) show that the organic ligand and the
hexamolybdate are linked together covalently via sp hybrid-
ization to obtain the organic−inorganic hybrid. The inorganic
part of the cluster anion retains the parent Lindqvist-type
hexamolybdate framework well. However, the central oxygen
atom (Oc) in the cluster is closer to the imido-bearing Mo atom
(Mo1−O1 bond lengths of 2.197, 2.205, 2.223, and 2.209
(2.197 for Mo7−O21) Å for compounds 1−4) and is farther
from the opposite Mo atom (Mo6−O1 bond lengths of 2.359,
2.362, 2.359, and 2.361 (2.364 for Mo12−O21) Å for
compounds 1−4) because of the different negativity between
the imido ligand and the oxo ligand. It is worth mentioning that
in the structure of compound 4 the orientation of the two
cluster anions is quite different. Facing neither the same
direction nor the opposite direction, they form a certain angle
with a trans mode. Another characteristic of compound 4 is that
in the asymmetric unit the two cluster anions, the Mo1 cluster
(Figure 2a) and the Mo7 cluster (Figure 2b), are conforma-
tional isomers. In the Mo1 cluster, the hydroxyl is close to the
methyl group, and the carbonyl oxygen is farther from the
methyl group, whereas in the Mo7 cluster, the carbonyl oxygen
is close to the methyl group, and the hydroxyl is farther from
the methyl group.
Figure 3. Dimer structure of the cluster anions in compound 1.
neighboring cluster anion as an acceptor. The same kind of
hydrogen bond is formed between the hydroxyl oxygen atom in
the carboxyl group of the neighboring cluster anion as a donor
and one bridge oxygen atom (Ob) in the next neighboring
cluster anion as an acceptor. Thus, the hydrogen-bonding
interactions connect the cluster anions into a 1D zigzagging
infinite chain (Figure 4).
Figure 4. One-dimensional zigzagging chain of the cluster anions in
compound 2.
There are some hydrogen-bonding interactions in compound
3 as well, which are different from those in compounds 1 and 2.
The hydrogen bond (O20−H20···O16, 2.706 Å, 158.5°) is
formed between the hydroxyl oxygen atom in the carboxyl
group of the cluster anion as a donor and one bridge oxygen
atom (Ob) in the neighboring cluster anion as an acceptor,
connecting the cluster anions into a 1D zigzagging infinite
chain (Figure 5a). Furthermore, the existence of relatively weak
hydrogen bond (C3−H3···O10, 3.292 Å, 150.5°) between one
carbon atom on the aromatic ring of the cluster anion in the 1D
zigzagging infinite chain and one bridge oxygen atom (Ob) of
the cluster anion in the neighboring 1D-zigzagging infinite
chain assembles the cluster anions into a 2D planar
supramolecular structure (Figure 5b).
The intermolecular hydrogen-bonding interactions also play
some part in compound 4, although they are different from
those of compounds 1−3. The hydrogen bond (O20−
H20···O7, 2.745 Å, 156.9°) is formed between the hydroxyl
oxygen atom in the carboxyl group of one cluster anion (the
Mo1 cluster, Figures 1d and 2a) in the asymmetric unit as a
donor and one bridging oxygen atom (Ob) in the same kind of
cluster anion in the neighboring asymmetric unit as an
acceptor, connecting the Mo1 cluster anions into a 1D
zigzagging infinite chain (Figure 6a). Another hydrogen bond
(O40−H40···O22, 2.786 Å, 153.6°) exists between the
hydroxyl oxygen atom in the carboxyl group of the other
Supramolecular Assemblies of Compounds 1−4. In
addition to the cluster anion structures, the intermolecular
hydrogen-bonding interaction also plays an important part in
the supramolecular assemblies of the four compounds in the
solid state. For compound 1, the hydroxyl oxygen atom in the
carboxyl group of one cluster anion acts as a hydrogen bond
donor, and one bridging oxygen atom (Ob) in the neighboring
cluster anion acts as a hydrogen bond acceptor to form a
hydrogen bond (O20−H20···O7, 2.712 Å, 170.8°). Likewise,
another of the same kind of hydrogen bond is formed between
one bridging oxygen atom (Ob) in the cluster anion and the
hydroxyl oxygen atom in the carboxyl group of the neighboring
cluster anion, with the donor and acceptor exchanged. Such
interactions form an 18-membered ring between the two
neighboring cluster anions consisting of 18 atoms with two
hydrogen bond donors and two acceptors (Figure 3), which is
2
represented by the symbol R2 (18). The dimer-structure
aggregates are obtained by connecting the neighboring cluster
2
anions via R2 (18) hydrogen bonding.
This hydrogen-bonding interaction also exists in compound
2, which is different from that of compound 1. A hydrogen
bond (O20−H20···O11, 2.646 Å, 164.2°) is formed between
the hydroxyl oxygen atom in the carboxyl group of the cluster
anion as a donor and one bridging oxygen atom (Ob) in the
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Figure 5. One-dimensional zigzagging chain (a) and 2D plane (b) of the cluster anions in compound 3.
cluster anion (the Mo7 cluster, Figures 1d and 2b) in the
asymmetric unit as a donor and one terminal oxygen atom (Ot)
in the same kind of cluster anion in the neighboring asymmetric
unit as an acceptor, making the Mo7 cluster anions into another
1D zigzagging infinite chain (Figure 6b). Furthermore, the
existence of relatively weak hydrogen bond (C5−H5···O11,
3.402 Å, 172.0°) between one carbon atom on the aromatic
ring of the Mo1 cluster anion in the 1D zigzagging infinite
chain and one bridge oxygen atom (Ob) in the same kind of
cluster anion in the neighboring 1D zigzagging infinite chain
makes the Mo1 cluster anions into a 2D planar supramolecular
structure (Figure 6c). The different orientations of the Mo1
cluster anion and the Mo7 cluster anion in the asymmetric unit
result in different intermolecular interactions for each cluster.
The Mo1 cluster anions exhibit two types of hydrogen bonding
to form a 2D planar structure, whereas the Mo7 cluster anions
exhibit one type of hydrogen bonding to form the 1D
zigzagging infinite chain. There are no hydrogen-bonding
interactions between the Mo1 cluster anions and the Mo7
cluster anions. Thus, the 1D chain formed by the Mo7 cluster
anions and the 2D plane formed by the Mo1 cluster anions
exist together to afford the entire structure of anions in
compound 4.
Spectroscopic Characterization. The IR spectra of
compounds 1−4 are similar. All of them show two very strong
absorption peaks near 950 and 790 cm−1 that are assigned to
the vibration of Mo−Ot and Mo−Ob−Mo of the Lindqvist
structure, respectively. The visible shoulder peak near 975 cm−1
is the characteristic absorption peak of a monosubstituted
organoimido derivative of a hexamolybdate and is 977 cm−1 for
compounds 1−3 and 975 cm−1 for compound 4. The spectra
also show relatively strong absorption peaks near 1700 cm−1
because of the existence of carboxyl groups in the compounds:
1724 cm−1 for compound 1, 1715 cm−1 for compound 2, 1718
cm−1 for compound 3, and 1736 cm−1 for compound 4
(Figures S1−S4). Compared to the corresponding organic
ligand, each compound displays a shift to the higher
wavenumber for the carboxyl group. This reflects the
electron-withdrawing nature of the hexamolybdate cluster,
increasing the force constant of the CO bond and resulting
in the shift of the absorption peak. This can also be proven by
the shortening of the CO bond length in the compounds as
compared to that in the pure organic ligand. In addition, the
absorption peaks at around 2900 cm−1 are assigned to the
stretching vibrations of the methyl and methylene groups in the
tetrabutylammonium cations and the methyl group on the
aromatic ring.
The UV−vis spectra of compounds 1−4 show the absorption
characters of the monosubstituted organoimido derivatives of
hexamolybdate. The maximum absorption wavelength of the
parent hexamolybdate is 325 nm, which is derived from the
charge-transfer transition from the oxygen π-type HOMO to
the molybdenum d-type LUMO. In the organoimido products,
the maximum-absorption wavelength red shifts to 343 nm for
compound 1, 358 nm for compound 2, 346 nm for compound
3, and 345 nm for compound 4 (Figure S5), resulting from the
formation of the Mo−N bond and the delocalization system as
a result of the strong d−π interaction between the organic
ligand and the inorganic hexamolybdate cluster. Besides the
bathochromic shift, the difference among the organic ligands
makes some difference in the absorption spectra of the
organoimido products. Compound 2 shows the largest
bathochromic shift, resulting from having the best delocaliza-
tion system with a carboxyl group at the para position relative
to the Mo−N moiety.
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Figure 6. One-dimensional zigzagging chain for the Mo1 cluster (a), 1D zigzagging chain for the Mo7 cluster (b), and 2D plane for the Mo1 cluster
(c) of the cluster anions in compound 4.
The 1H NMR spectra of compounds 1−4 show clear signals
that can be assigned easily for the hydrogen atoms in the
different environments (Figures S6−S9). The integration of the
peaks is in accord with the structures of the title compounds.
Compared to the corresponding organic ligands, the chemical
shifts of all of the hydrogen atoms exhibit some downfield
change, indicating that the shielding effect of the POM cluster
is weaker than that of the amino group. This also reflects the
electron-withdrawing nature of the hexamolybdate cluster. In
addition, the relative positions of the carboxyl and methyl
groups relative to the Mo−N moiety also change the aromatic
hydrogen chemical shift to some extent.
The ESI−MS spectra of compounds 1−4 confirm the
formation of these organoimido products (Figure S10). For
compound 1, the isotopic peaks centered at m/z 499.77
(100%) can be assigned to the cluster anion [Mo6O18(N-C6H4-
3-COOH)]2− (calcd m/z: 499.37), and the isotopic peaks
centered at m/z 1242.75 (35%) can be assigned to the
aggregate of the cluster anion and one [Bu4N]+ cation
[Bu4N][Mo6O18(N-C6H4-3-COOH)]− (calcd m/z: 1241.21).
For compound 2, the peaks around m/z 507.72 (100%) and
1255.92 (30%) can be assigned to the cluster anion
[Mo6O18(N-C6H4-2-CH3-4-COOH)]2− (calcd m/z: 506.39)
and to the aggregate of the cluster anion and one [Bu4N]+
cation [Bu4N][Mo6O18(N-C6H4-2-CH3-4-COOH)]− (calcd
m/z: 1255.24), respectively. For compound 3, the peaks
around m/z 506.20 (100%, [Mo6O18(N-C6H4-2-CH3-5-
COOH)]2− (calcd m/z: 506.39)) and 1013.40 (45%,
[HMo6O18(N-C6H4-2-CH3-5-COOH)]− (calcd m/z:
1013.78)) can be assigned. For compound 4, the peaks around
m/z 505.78 (100%, [Mo6O18(N-C6H4-2-CH3-3-COOH)]2−
(calcd m/z: 506.39)) and 1254.70 (80%, [Bu4N][Mo6O18(N-
C6H4-2-CH3-3-COOH)]− (calcd m/z: 1255.24)) can also be
assigned.
CONCLUSIONS
■
In summary, four novel organoimido derivatives of hexamo-
lybdate containing remote carboxyl groups with 3-amino-
benzoic acid, 4-amino-3-methylbenzoic acid, 3-amino-4-meth-
ylbenzoic acid, and 3-amino-2-methylbenzoic acid as the imido-
releasing agents, denoted as [Bu4N]2[Mo6O18(N-C6H4-3-
COOH)] (1), [Bu4N]2[Mo6O18(N-C6H4-2-CH3-4-COOH)]
(2), [Bu4N]2[Mo6O18(N-C6H4-2-CH3-5-COOH)] (3), and
[Bu4N]2[Mo6O18(N-C6H4-2-CH3-3-COOH)] (4), respectively,
have been synthesized using the DCC protocol. Their
1
structures have been characterized by IR, UV−vis, H NMR,
ESI-MS, and single-crystal X-ray diffraction techniques. The
hydrogen-bonding interactions play an important role in the
supramolecular assemblies of these compounds in the solid
state. Similar organic moieties in the organoimido products
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(18) Clegg, W.; Errington, R. J.; Fraser, K. A.; Holmes, S. A.; Schafer,
A. J. Chem. Soc., Chem. Commun. 1995, 455−456.
(19) Wei, Y. G.; Xu, B. B.; Barnes, C. L.; Peng, Z. H. J. Am. Chem.
Soc. 2001, 123, 4083−4084.
result in different supramolecular assembly behaviors. The
dimer structure is formed via one type of hydrogen bonding for
compound 1. The 1D zigzagging chain structure is formed via
one type of hydrogen bonding for compound 2. The 2D planar
structure is formed via two types of hydrogen bonding for
compound 3. The 1D plus 2D structures are formed via three
types of hydrogen bonding for compound 4 resulting from
different orientations of the cluster anions. In addition, the
chemical reactivity of the carboxyl groups makes these
compounds potentially applicable in the design and synthesis
of higher-level POM-based organic−inorganic hybrid materials,
which is underway in our laboratory.
(20) Wu, P. F.; Li, Q.; Ge, N.; Wei, Y. G.; Wang, Y.; Wang, P.; Guo,
H. Y. Eur. J. Inorg. Chem. 2004, 2819−2822.
(21) Zhu, Y.; Xiao, Z.; Wang, L.; Yin, P.; Hao, J.; Wei, Y.; Wang, Y.
Dalton Trans. 2011, 7304−7309.
(22) Xue, S. J.; Chai, A.; Cai, Z. J.; Wei, Y. G.; Xiang, C. S.; Bian, W.
D.; Shen, J. Dalton Trans. 2008, 4770−4775.
(23) Wang, L. S.; Zhu, L.; Yin, P. C.; Fu, W. W.; Chen, J. K.; Hao, J.;
Xiao, F. P.; Lv, C. L.; Zhang, J.; Shi, L.; Li, Q.; Wei, Y. G. Inorg. Chem.
2009, 48, 9222−9235.
(24) Li, Q.; Yin, P. C.; Shi, L.; Wei, Y. G. Eur. J. Inorg. Chem. 2009,
5227−5232.
ASSOCIATED CONTENT
■
(25) Xue, S. J.; Xiang, C. S.; Wei, Y. G.; Tao, Z. W.; Chai, A.; Bian,
W. D.; Xu, Z. Cryst. Growth Des. 2008, 8, 2437−2443.
(26) Li, Q.; Wu, P. F.; Xia, Y.; Wei, Y. G.; Guo, H. Y. J. Organomet.
Chem. 2006, 691, 1223−1228.
S
* Supporting Information
The IR, UV−vis, 1H NMR, and ESI−MS spectra of the
compounds. Crystallographic data for the compounds in CIF
format. This material is available free of charge via the Internet
at http://pubs.acs.org.
(27) Kang, J.; Xu, B. B.; Peng, Z. H.; Zhu, X. D.; Wei, Y. G.; Powell,
D. R. Angew. Chem., Int. Ed. 2005, 44, 6902−6905.
(28) Zhu, L.; Zhu, Y.; Meng, X.; Hao, J.; Li, Q.; Wei, Y.; Lin, Y.
Chem.Eur. J. 2008, 14, 10923−10927.
AUTHOR INFORMATION
Corresponding Author
■
(29) Xu, B. B.; Wei, Y. G.; Barnes, C. L.; Peng, Z. H. Angew. Chem.,
Int. Ed. 2001, 40, 2290−2292.
(30) Zhu, Y. L.; Wang, L. S.; Hao, J.; Yin, P. C.; Zhang, J.; Li, Q.;
Zhu, L.; Wei, Y. G. Chem.Eur. J. 2009, 15, 3076−3080.
(31) Moore, A. R.; Kwen, H.; Beatty, A. M.; Maatta, E. A. Chem.
Commun. 2000, 1793−1794.
*E-mail: jinjin_eva@163.com (J.Z.), yonggewei@mail.tsinghua.
edu.cn (Y.W.).
Author Contributions
∥These authors contributed equally.
(32) Li, Q.; Wei, Y. G.; Hao, J.; Zhu, Y. L.; Wang, L. S. J. Am. Chem.
Soc. 2007, 129, 5810−5811.
Notes
(33) Zhu, Y. Studies on Syntheses and Properties of Organoimido
Derivatives of Hexamolybdates[D]. Ph.D. Dissertation, College of
Chemistry and Molecular Engineering, Peking University, Beijing,
2009.
(34) Bustos, C.; Hasenknopf, B.; Thouvenot, R.; Vaissermann, J.;
Proust, A.; Gouzerh, P. Eur. J. Inorg. Chem. 2003, 2757−2766.
(35) Knaust, J. M.; Inman, C.; Keller, S. W. Chem. Commun. 2004,
492−493.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is sponsored by NFSC (nos. 21225103 and
91022010), Tsinghua University Initiative Foundation Research
Program 20101081771, and THSJZ.
(36) Han, M.; Hey, J.; Kawamura, W.; Stalke, D.; Shionoya, M.;
Clever, G. H. Inorg. Chem. 2012, 51, 9574−9576.
(37) Mialane, P.; Dolbecq, A.; Secheresse, F. Chem. Commun. 2006,
3477−3485.
REFERENCES
■
(1) Muller, A.; Peters, F.; Pope, M. T.; Gatteschi, D. Chem. Rev.
1998, 98, 239−271.
(2) Rhule, J. T.; Hill, C. L.; Judd, D. A. Chem. Rev. 1998, 98, 327−
(38) Han, J. W.; Hill, C. L. J. Am. Chem. Soc. 2007, 129, 15094−
15095.
357.
(3) Coronado, E.; Gomez-Garcia, C. J. Chem. Rev. 1998, 98, 273−
296.
(39) Ma, F. J.; Liu, S. X.; Sun, C. Y.; Liang, D. D.; Ren, G. J.; Wei, F.;
Chen, Y. G.; Su, Z. M. J. Am. Chem. Soc. 2011, 133, 4178−4181.
(40) Klemperer, W. G.; Shum, W. J. Am. Chem. Soc. 1976, 98, 112−
122.
(41) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467−473.
(42) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112−122.
(4) Yamase, T. Chem. Rev. 1998, 98, 307−325.
(5) Sadakane, M.; Steckhan, E. Chem. Rev. 1998, 98, 219−237.
(6) Judd, D. A.; Nettles, J. H.; Nevins, N.; Snyder, J. P.; Liotta, D. C.;
Tang, J.; Ermolieff, J.; Schinazi, R. F.; Hill, C. L. J. Am. Chem. Soc.
2001, 123, 886−897.
(7) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. 1991, 30, 34−48.
(8) Gouzerh, P.; Proust, A. Chem. Rev. 1998, 98, 77−111.
(9) Kozhevnikov, I. V. Chem. Rev. 1998, 98, 171−198.
(10) Dolbecq, A.; Dumas, E.; Mayer, C. R.; Mialane, P. Chem. Rev.
2010, 110, 6009−6048.
(11) Ho, R. K. C.; Klemperer, W. G. J. Am. Chem. Soc. 1978, 100,
6772−6774.
(12) Knoth, W. H.; Harlow, R. L. J. Am. Chem. Soc. 1981, 103, 4265−
4266.
(13) Zonnevijlle, F.; Pope, M. T. J. Am. Chem. Soc. 1979, 101, 2731−
2732.
(14) Katsoulis, D. E. Chem. Rev. 1998, 98, 359−387.
(15) Kang, H. Y.; Zubieta, J. J. Chem. Soc., Chem. Commun. 1988,
1192−1193.
(16) Du, Y. H.; Rheingold, A. L.; Maatta, E. A. J. Am. Chem. Soc.
1992, 114, 345−346.
(17) Proust, A.; Thouvenot, R.; Chaussade, M.; Robert, F.; Gouzerh,
P. Inorg. Chim. Acta 1994, 224, 81−95.
H
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